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Behavior of Several Amphiphilic Copolymers at the Air-Water Interface

Permanent Link: http://ufdc.ufl.edu/UFE0021363/00001

Material Information

Title: Behavior of Several Amphiphilic Copolymers at the Air-Water Interface
Physical Description: 1 online resource (153 p.)
Language: english
Creator: Bernard, Sophie
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: afm, amphiphilic, bam, blends, interface, polymer
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The two-dimensional structure of amphiphilic copolymers is studied at the air-water interface using Langmuir-Blodgett methods, atomic force microscopy (AFM), and Brewster angle microscopy (BAM). Measurements are made for several block copolymers containing polystyrene (PS), poly(ethylene oxide) (PEO), poly(tert-butylacrylate) (PtBA) and polycaprolactone (PCL). Measurements are also made for blends of the PS-b-PEO copolymer with both a PS and a PEO homopolymer. When increasing the amount of PS homopolymer, the isotherms do not show any change in the high surface area region. However, a linear dependence of the condensed area is observed. An increase in the PEO ratio has an effect on the biphasic region of the isotherms but no change is detected for the condensed area. The AFM data indicate a significant effect of the homopolymers on the monolayer structure. In fact depending on the homopolymer added, a change in the chaining behavior of the copolymer is observed. Also, when introducing more PEO, a phase separation between the layer of PEO and clusters of two-dimensional micelles is detected. The PS-b-PtBA copolymers investigated are star-shaped copolymers with a polystyrene (PS) core and a poly(tert-butylacrylate) (PtBA) corona. They were prepared with a constant PS block and three different PtBA block lengths. A transition can be observed for the three copolymers with a plateau?s length depending on the PtBA composition. The images obtained by AFM are in agreement with the isotherms showing the evidence of a phase transition around 24mN/m. In fact for the three copolymers, below the plateau only single domains are observed whereas for pressures higher than 24mN/m, aggregates can be detected. Transfer experiments were performed after several equilibration times. The structure of the film formed seems to be dependant on the time waited before performing the transfer, showing more compact films when the time waited was longer. Poly(ethylene oxide)-block-Polycaprolactone (PEO-b-PCL) copolymers are also studied in this work. Their behavior and the PCL crystallization process at the air-water interface is investigated using BAM. The formation of crystals directly on the water subphase is illustrated and compared to the pictures obtained by AFM.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Sophie Bernard.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Duran, Randolph.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021363:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021363/00001

Material Information

Title: Behavior of Several Amphiphilic Copolymers at the Air-Water Interface
Physical Description: 1 online resource (153 p.)
Language: english
Creator: Bernard, Sophie
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: afm, amphiphilic, bam, blends, interface, polymer
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The two-dimensional structure of amphiphilic copolymers is studied at the air-water interface using Langmuir-Blodgett methods, atomic force microscopy (AFM), and Brewster angle microscopy (BAM). Measurements are made for several block copolymers containing polystyrene (PS), poly(ethylene oxide) (PEO), poly(tert-butylacrylate) (PtBA) and polycaprolactone (PCL). Measurements are also made for blends of the PS-b-PEO copolymer with both a PS and a PEO homopolymer. When increasing the amount of PS homopolymer, the isotherms do not show any change in the high surface area region. However, a linear dependence of the condensed area is observed. An increase in the PEO ratio has an effect on the biphasic region of the isotherms but no change is detected for the condensed area. The AFM data indicate a significant effect of the homopolymers on the monolayer structure. In fact depending on the homopolymer added, a change in the chaining behavior of the copolymer is observed. Also, when introducing more PEO, a phase separation between the layer of PEO and clusters of two-dimensional micelles is detected. The PS-b-PtBA copolymers investigated are star-shaped copolymers with a polystyrene (PS) core and a poly(tert-butylacrylate) (PtBA) corona. They were prepared with a constant PS block and three different PtBA block lengths. A transition can be observed for the three copolymers with a plateau?s length depending on the PtBA composition. The images obtained by AFM are in agreement with the isotherms showing the evidence of a phase transition around 24mN/m. In fact for the three copolymers, below the plateau only single domains are observed whereas for pressures higher than 24mN/m, aggregates can be detected. Transfer experiments were performed after several equilibration times. The structure of the film formed seems to be dependant on the time waited before performing the transfer, showing more compact films when the time waited was longer. Poly(ethylene oxide)-block-Polycaprolactone (PEO-b-PCL) copolymers are also studied in this work. Their behavior and the PCL crystallization process at the air-water interface is investigated using BAM. The formation of crystals directly on the water subphase is illustrated and compared to the pictures obtained by AFM.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Sophie Bernard.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Duran, Randolph.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021363:00001


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BEHAVIOR OF SEVERAL AMPHIPHILIC COPOLYMERS
AT THE AIR-WATER INTERFACE






















By

SOPHIE BERNARD


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2007
































@ 2007 Sophie Bemard






























To my parents Frangoise and Guy and my brothers Romain and Mathieu

for their constant love and support









ACKNOWLEDGMENTS

I would first and foremost like to thank my advisor Dr Randolph S. Duran for making this

experience possible. Throughout my entire PhD, he has been supporting in the good and bad

times and I could not be more thankful. His presence and advices made my stay at the University

of Florida, an experience I will never forget.

I also like to give a special thanks to Dr John Reynolds and Dr Ken Wagener. Without

their support and their caring attention, they made my experience on the polymer floor really

enj oyable. I would also like to show appreciation to the rest of my committee members, Dr

Daniel Talham, Dr Valeria Kleiman, Dr Eric Enholm, and Dr Richard B. Dickinson. Their

constructive criticism and their trust in me did not go unnoticed, and for that I could not be more

grateful .

A special thank you goes to Denise Sharbaugh for all her precious help and patience.

There are a lot of past and present members of the polymer floor that made my work

environment so enj oyable and I will never forget all those times inside and outside the lab. So for

that thank you Florence Courchay, Merve Ertas, Piotr Matloka, Aubrey Dyer, Emilie Galand,

Tim Steckler, and Genay Jones.

None of that could have been possible without the support of the entire Duran' s group,

Jorge Chavez, Henk Keiser, Brian Dorvel, Danyell Wilson, Dr Martin Andersson, Dr Maria

Stj erndahl, Dr. Firouzeh Sabri, Eric Greeley, Kristina Denoncourt and Aleksa Jovanovic. A

special thank goes to Dr Thomas Joncheray and Dr Jennifer Logan for all their support and

constructive conversations during my whole PhD.

Some people were always here in good and bad times. Thank you Vicky for those

wonderful five years that you definitely made interesting. Thanks to.Jordan, Neil, Eric and Brent

for all those times spent at number 23. Thank you to Jamie, Austin and Kristen for all those late









Friday nights. I would also like to acknowledge my classmates, Lisa, Richard and Lindsay for

going through those five years with me. Special thanks goes to Roxy, David, Nicolas and Ashlee

for being the best support a person could ask for during the writing process. Everyone made my

life as a graduate student so much easier and I will never forget all those times spent at GC. I

love them all and will miss them immensely.

Finally, probably the most important, I want to thank my parents Frangoise and Guy as

well as my brothers Romain and Mathieu, who without even being aware of it, have made this

experience possible. Their love and support was the constant in my life that kept me going and

got me through some rough times.











TABLE OF CONTENTS


page

ACKNOWLEDGMENTS .............. ...............4.....


LIST OF TABLES ............ ...... ._ ...............9...

LIST OF FIGURES .............. ...............10....


LIST OF ACRONYMS .............. ...............16....

AB S TRAC T ........._._ ............ ..............._ 17...


CHAPTER

1 INTRODUCTION ................. ...............19.......... ......


1.1 Scope of the Dissertation ................. ...............19........... ..
1.2 Block Copolymers in the Bulk. ................ ....................19
1.3 Block Copolymers at the Air-Water Interface .................. .... ... .... ....... .................. ....2
1.3.1 Polystyrene (PS) and Poly(ethylene oxide) (PEO) at the Air-Water Interface ......22
1.3.2 Polyacrylates at the Air-Water Interface .............. .. ...............25...
1.3.3 Poly(s-caprolactone) (PCL) at the Air-Water Interface .............. ....................2
1.3.4 Previous Studies on Polymer Blends ................. ...............27..............

2 EXPERIMENTAL TECHNIQUES............... ...............3


2.1 Langmuir Trough............... ...............33.
2.2 Equilibrium Spreading Pressure .............. ...............35....
2.3 Langmuir Trough Experiments............... ..............3
2.4 Langmuir-Blodgett Films .............. ...............39....
2.5 Atomic Force Microscopy .............. ...............40....
2.5.1 Instrument Parameters ............ ...... ._._ ...............40..
2.5.2 Limitations ........._.. ..... ._ ...............42....
2.5.3 Parameters Used ........._.._ ..... ._._ ...............43...
2.5.4 Image Analysis .............. ...............44....
2.6 Brewster Angle Microscopy ........._.._ ..... .___ ...............45...

3 BLEND S OF A POLY STYRENE-BL OCK-P OLY(ETHYLENE OXIDE)
COPOLYMER AND ITS CORRESPONDING HOMOPOLYMERS ................. ..............56


3.1 Isotherm Experiment .............. ...............56....
3.1.1 PEO H om opolym er ........................ ........................ ...... .......5
3.1.2 Linear Polystyrene-block-Poly(ethylene oxide) (PS-b-PEO) Diblock
Copolymer. ................... ... ...... ....... ........... .. .. .... .......5
3.1.3 Blends of a PS-b-PEO diblock copolymer and a PS homopolymer .....................58











3.1.3.1 Pancake Region (I) ................. ...............59.............
3.1.3.2 Pseudoplateau Region (II) ................. ......... ...............60. .
3.1.3.3 Condensed Region (III) ................. .. ......... .... ...............60...
3.1.4 Blends of a PS-b-PEO diblock copolymer and PEO homopolymer ......................62
3.1.4. 1 Pancake Region (I) ................. ...............62........... .
3.1.4.2 Pseudoplateau Region (II) ................. ...............63........... ..
3.1.4.3 Condensed Region (III) ............... ...............64...............
3.2 Atomic Force Microscopy (AFM) Experiments.............................. ...... .......6
3.2.1 Linear Polystyrene-block-Poly(ethylene oxide) (PS-b-PEO) Diblock
Copolymer. ................... ... ...... ....... ........... .. .. .... .......6
3.2.2 Blends of a PS-b-PEO diblock copolymer and a PS homopolymer .....................68
3.2.3 Blends of a PS-b-PEO diblock copolymer and a PEO homopolymer....................69
3.3 Conclusions............... ..............7

4 INTERFACIAL BEHAVIOR OF STAR-SHAPED POLYSTYRENE-BLOCK-
POLY(TERT-BUTYLACRYLATE) COPOLYMERS .................... ............... 9

4. 1 Introducti on ............ ..... ._ ...............90..
4.2 Results and Discussion .............. ...............91....
4.2. 1 PtBA Homopolymer ........._._........ ...............92.
4.2.2 PS-b-PtBA Star-Shaped Copolymers ................ ...............94........ ....
4.2.2.1 Isotherm Experiments .............. ...............94....
4.2.2.2 AFM Imaging ................. ............ ...............97......
4.2.2.3 Stability of the Langmuir Films ................. ...............100..............
4.3 Conclusions............... ..............10


5 SURFACE CHARACTERIZATION OF POLY(ETHYLENE OXIDE)-BLOCK-
POLY (s-CAPROLACT ONE) ........._._ ...... .__ ...............119...

5 .1 Introducti on ........._..... .. ...... ..... ....._. ...........11
5.1.1 Crystallization of Semi-Crystalline Polymers ....._._._ .......___ ................119
5.1.2 Crystallization of PCL at the Air-Water Interface ................. ............ .........121
5.2 Results and Discussion ................ ...............122........... ...
5.2.1 Isotherm Experiments................. .......................12
5.2.2 Brewster Angle Microscopy and Comparison to Previous AFM Imaging ..........124
5.3 Conclusions............... ..............12

6 CONCLUSION AND PERSPECTIVES............... .............13


6. 1 Sum m ary ................... ........... ..... ... .... ..... ..... .. ...... .......... 3
6. 1.1 Blends of a PS-b-PEO Copolymer with PS and PEO Homopolymers ................137
6. 1.2 PS-b-PtBA Star Copolymers at the Air-Water Interface ................. ................. 137
6. 1.3 PEO-b-PCL Copolymers at the Air-Water Interface ................. .....................138
6.2 Correlation Between the Different Systems .............. ...............138....
6.3 Future Work............... ...............140.












LIST OF REFERENCES ................. ...............145................

BIOGRAPHICAL SKETCH ................. ...............153......... ......

































































8










LIST OF TABLES


Table page

3-1 Characteristics of the PS-b-PEO sample investigated ................. ......... ................88

3-2 The mass ratio of PS between the diblock copolymer and the homopolymer as well
as the apparent number of styrene units have been calculated for each blend. .................88

3-3 Width of the pseudoplateau for each blend ................. ...............88..............

3-4 The mass ratio of PEO between the diblock copolymer and the homopolymer as well
as the apparent number of styrene units have been calculated for each blend. .................88

3-5 Pancake areas extrapolated from the xn-A isotherms ................. ................ ......... .89

3-6 Area for the second transition (described in Figure 2.5) extrapolated for each blend.......89

3-7 Molar ratio of PEO from the homopolymer and the diblock copolymer as well as the
total number of EO units is given for each blend. ................ ...............89.............

4-1 Characteristics of the copolymers ssed ................. ...............118..............

4-2 Characteristics of the homopolymer used ................. ...............118..............

4-3 Specific areas calculated for the 3 copolymers ................. ...............118.............

4-4 Transfer ratios for different stabilization times before transferring the copolymer PS-
b-PtBA(215) monolayer onto a mica substrate ................. ...............118.............

5-1 Characteristics of the linear and star-shaped copolymers ................. .......................136










LIST OF FIGURES


Figure page

1-1 Morphologies of diblock copolymers: cubic packed spheres (S), hexagonal packed
cylinders (C or Hex), double gyroid (G or Gyr), and lamellae (L or Lam).............._._......30

1-2 Solution states for an amphiphilic diblock copolymer for concentration below and
above the CM C. ............. ...............30.....

1-3 Schematic representation of a "starlike" (1) and a "crew-cut" (2) micelle........................30

1-4 Description of Surface tension showing how the forces on surface molecules differ
from those in the bulk ........._ ...... .... ...............31..

1-5 Schematic representation of the pancake to brush transition for PS-b-PEO
copoly m ers. ........._ ...... .. ............... 1....

1-6 The two conformatins for PEO at the air-water interface: Conformation (a) is loose
and flexible, but compressing it into the compact conformation (b) tends to lock the
chain in one position resulting in a sterically hindered and more rigid structure ..............31

1-7 BAM images obtained during an hysteresis experiment at 22.50C with a compression
and expansion rate of ~ 0.010 nm2.mOnomer- .minl for a PCL homopolymer ................32

2-1 The original Langmuir balance as designed by I. Langmuir ...........__... .......__........48

2-2 Set up of a typical Langmuir trough .............. ...............48....

2-3 Schematic of the Wilhelmy plate ................. ...............48...............

2-4 Schematic Surface pressure-MMA isotherm ................. ...............49...............

2-5 (a) Several representative isotherms are shown, depicting the dependence of surface
pressure (2n) on mean molecular area. ............. ...............49.....

2-6 Different types of deposited LB films .............. ...............50....

2-7 Optical system that detects cantilever deflection............... ...............5

2-8 An atomic force microscope (AFM) (left) and the tapping mode electronic set-up
(right) are shown ................. ...............50........... ....

2-9 AFM scanner tube containing the piezoelectric material and metal electrode. The x,
y, and z-directional components of the scanner are also indicated ........._..._... ........._......51

2-10 Dependance of the piezoelectric ceramic on voltage ................. ...........................5 1

2-11 Imaging limitations in tip sharpness .............. ...............52....










2-12 The software allows choosing a domain range by varying the minimum and
maximum areas .............. ...............53....

2-13 The software gives you a computed image representing the different domains and the
possible angles between domains in the presence of chaining .............. ....................5

2-14 Error made by the computer can be corrected by the user............... ...............54..

2-15 Scheme of the Brewster angle at the air-water interface and in presence of a thin film ...54

2-16 The different components of the BAM2 are shown ................. ................ ......... .55

3-1 Surface pressure-MMA isotherm for the PEO homopolymer (Mn = 100,000g/mol)
recorded for a compression speed of 5mm/min ................. ...............72........... ..

3-2 Surface pressure-MMA isotherm for the 32,500 g-moll PS-b-PEO copolymer
recorded for a compression speed of 5mm/min .....__.....___ ..............__........7

3-3 Several surface pressure-MMA isotherms are shown, indicating the dependence of
surface pressure on the mean molecular area for different blend ratios. All the
isotherm experiments were performed using a compression speed of 5mm/m ........._......74

3-4 Monolayer compressibility plot for the PS-b-PEO copolymer and blend 1 ......................75

3-5 The condensed area per EO unit (Ao/EO) varies linearly with the number of styrene
units present in the blends ................. ...............75........... ...

3-6 The condensed area (Ao) varies linearly with the total number of styrene units
present in the blends............... ...............76.

3-7 Several surface pressure-MMA isotherms are shown, showing the dependence of
surface pressure on the mean molecular area for different amounts of PEO. All the
isotherm experiments were performed using a compression speed of 5mm/min ..............77

3-8 Monolayer compressibility plots for the PS-b-PEO diblock copolymer and blend 1........78

3-9 The pancake area (AP) VarieS linearly with the total number of EO units present in
the blends .............. ...............78....

3-10 The area for the second transition depends linearly on the mole ratio of PEO from the
homopolymer over PEO from the PS-b-PEO .............. ...............79....

3-11 The area for the second transition depends linearly on the apparent total number of
EO repeat units............... ...............79.

3-12 Example of a sample height image and surface plot (scan area shown is 2x2 Clm) ..........80

3-13 Example of a sample section image (2x2 Clm) .............. ...............80....










3-14 Height AFM images from tapping mode of the pure PS-b-PEO for several transfer
pressures (scale 2x2Clm). The transfer ratios are also given for each images. ..................81

3-15 Schematic representation of surface micelles formed by A-b-B diblock copolymers
with A strongly adsorbed to the surface .............. ...............81....

3-16 Model of PS-b-PEO absorbing at the air-water interface .........._.... ....._.... ............82

3-17 Dependence of the number of molecules per domain on pressure ................. ................82

3-18 Height AFM images from tapping mode of the pure PS-b-PEO diblock copolymer as
well as two of the blends for several transfer pressures (scale 2x2Clm) ................... .........83

3-19 Height AFM images (scale 2x2Clm) for the pure PS-b-PEO diblock copolymer and
Blend 2 (transfer pressure of 4mN/m) as well as the distribution of the domain areas.....84

3-20 Computed images for the pure PS-b-PEO diblock copolymer and Blend 2 (transfer
pressure of 10mN/m) as well as the distribution of the domain areas .............. ..... ........._.85

3-21 Magnification of a single domain formed for Blend 2 for a transfer pressure of
10mN/m (scale 150x150nm) .............. ...............86....

3 -22 Height AFM images from tapping mode of the pure PS-b-PEO diblock copolymer
and several blends for transfer pressures of 4 and 9 mN/m (scale 2x2Clm). The height
scale remains constant for the blends............... ...............86.

3-23 Height AFM images from tapping mode for Blend 2; (a) from successive spreading,
and (b) from the mixed solution (scale 2x2Clm) .............. ...............87....

4-1 Schematic of the PS-b-PtBA copolymers with n=48 and m=104,215,445 ..........._.._......104

4-2 Schematic of dendritic initiator (a) and model of the PS-b-PtBA star copolymers (b) ...104

4-3 Surface pressure-MMA isotherm for a 13000g/mol PtBA homopolymer recorded for
a compression speed of 5mm/min. ........._. ....__ ............ ...........10

4-4 Height AFM images in tapping mode for the PtBA Homopolymer after transfer at
three distinct surface pressures (Scale 10x10plm) .............. ...............106....

4-5 Proposed conformations for the PtBA homopolymer at the air-water interface. The
different regions are labeled in Figure 4-3 ....__ ......_____ .......___ ...........0

4-6 Section view of a circular domain formed when the PtBA monolayer is compressed
above the plateau pressure (30mN/m) (image scale lxl ym) ................. .............. .....107

4-7 Scheme of the structures formed for 3 distinct regions of the isotherm when a
monolayer of PtBA homopolymer is compressed at the air-water interface (Scale
10x 10 Cm) ................. ...............108..............










4-8 Compression-Expansion Hysteresis Experiment for the PtBA homopolymer
compressed to a target pressure of 50mN/m (above the plateau). The compression
and expansion rates were both 5mm/min. ......__....._.__._ ......._._. ..........10

4-9 Compression-expansion hysteresis experiment for the PtBA homopolymer
compressed to a target pressure of 20mN/m (below the plateau). The compression
and expansion rates were both 5mm/min. ......__....._.__._ ......._._. ..........10

4-10 Isochoric experiment recorded for the PtBA homopolymer for a maximum pressure
of 50mN/m. the compression speed was 5mm/min. .................... ..............10

4-11 (a) Surface pressure-MMA isotherms plots of the 3 star-shaped copolymers (b)
Isotherms of the 3 copolymers in terms of number of tBA repeat units. All the
isotherm experiments were performed with a compression speed of 5mm/min. ............110

4-12 Plot of Ao versus the number of tBA repeat units ................. ............... 111...........

4-13 Proposed conformations for the PS-b-PtBA copolymers at the air-water interface.
The different regions are defined in Figure 4-3 ................ ...............111.............

4-14 Height AFM images from tapping mode for the three copolymers. Transfer pressures
were chosen to be in region (1) and region (3) of the isotherms. (Scale 10x 10p~m,
inset scale lx l ym) ................. ...............112..............

4-15 Scheme of AFM images for a transfer pressure of 15mN/m when increasing the size
of the PtBA chains ................. ...............112......... .....

4-16 Schematic of the structures formed for 3 distinct regions of the isotherm when a
monolayer of PS-b-PtBA copolymer is compressed at the air-water interface ...............1 13

4-17 Cross section of the PS-b-PtBA(445) copolymer when compressed to a pressure of
30mN/m and studied in tapping mode ................. ...............113..............

4-18 Close-up pictures and schematic of the three layers observed when the PS-b-
PtBA(445) is compressed above the plateau pressure ....._.__._ ........___ .............114

4-19 Plot of the Area occupied by the PtBA blocks (values in Table 4-3) versus the
number of tBA repeat units ................. ...............114........... ...

4-20 Compression-expansion hysteresis experiment for the PS-b-PtBA(215) copolymer
compressed to a target pressure of 30mN/m (above the plateau). The compression
and exp an sion rate s were b oth 5 mm/mi n........._.._ .......___ ..............11

4-21 Compression-expansion hysteresis experiments for the PS-b-PtBA(215) copolymer
compressed to a target pressure of 20mN/m (below the plateau). The compression
and exp an sion rate s were b oth 5 mm/mi n........._.._ .......___ ..............11










4-21 AFM images for a transfer pressure of 30 mN/m for different stabilization times.
Stabilization times before transfer (minutes) for PS-b-PtBA(104): (a) 15, (b) 45 and
(c) 90; PS-b-PtBA(215): (d) 15, (e) 45 and (f) 90; PS-b-PtBA(445): (g) 15, (h) 45
and (i) 90 (Scale 10x10plm) ................ ...............116.............

4-22 Isochoric experiments recorded for the three copolymers after compression to a
target pressure of 50mN/m via a compression rate of 5mm/min ................. ................11 7

5-1 General behavior of thermodynamic variables at the equilibrium melting temperature
Tine (a) Gibbs free energy (b) entropy, enthalpy and volume............... ..................2

5-2 Schematic of perpendicular (a) and parallel (b) chain folding in semicrystalline block
copolymers ........... __..... ._ ...............127....

5-3 Schematic illustration of the morphology formed by blends and copolymers of two
crystallizable polymers .............. ...............128....

5-4 Compression rate dependence of crystal growth in Langmuir monolayers at 22.5
C.49 BAM images were obtained at compression rates of (a) 0.010, (b) 0.013, and
(c) 0.026 nm2 mOnomer -min- for A ~ 0.08 nm2mOnomer- All images are 1.28 x
0.96 m m2 ................ ...............128......... ......

5-5 The linear PEO-b-PCL copolymer used in this study (n = 60, m = 35) ..........................128

5-6 Schematic of the star-shaped PEO-b-PCL copolymer used in this study. Further
characterization data are shown in table 5-1 ........._.._.. ......._ ......._.........12

5-7 Surface pressure-MMA isotherm for the linear PEO60-b-PCL35 linear COpolymer for
a compression rate of 5mm/min. ........._._. ....._. ...............130.

5-8 Compressibility plots of the PEO-b-PCL linear diblock copolymer versus surface
pressure. ............. ...............130....

5-9 Compression-expansion hysteresis plot of the PEO-b-PCL linear copolymer
compressed at a target pressure of 16 mN/m. The compression and expansion rates
used in the experiments were both 5mm/min ........._..__......___ ......_._ .........3

5-10 Surface pressure-MMA isotherm for the PEO-b-PCL star copolymer recorded for a
speed of 5mm/min. ........._.._.. ...._... ...............13 1...

5-11 Compressibility plots of the PEO-b-PCL star diblock copolymer versus surface
pressure. ............. ...............13 2....

5-12 Compression-expansion hysteresis plot of the PEO-b-PCL star copolymer
compressed at a target pressure of 9 mN/m. The compression and expansion rates
used in the experiments were both 5mm/min .............. ...............132....










5-13 Compression-expansion hysteresis plot of the PEO-b-PCL star copolymer
compressed at a target pressure of 15 mN/m. The compression and expansion rates
used in the experiments were both 5mm/min .............. ...............133....

5-14 BAM Images of the PEO-b-PCL linear copolymer at a pressure for 10mN/m (a) and
15mN/m (b)............... ...............133..

5-15 BAM Images of the PEO-b-PCL star copolymer for a pressure of 10mN/m (a) and
20mN/m (b)............... ...............134..

5-16 AFM Images for the linear PEO-b-PCL copolymer ((a) 10mN/m and (c) 15mN/m)
and the PEO-b-PCL star copolymer ((b) 10mN/m and (d) 20mN/m) ................... ..........134

5-17 BAM Images of the PEO-b-PCL star copolymer for pressures of 10 and 20mN/m
during the first (a and b) and the second compression cycle (c and d) ................... .........135

5-18 BAM images for a pressure of 18mN/m (a) and after waiting 15min (b), 20min (c),
35 min (d) and 55min (e) ................. ...............135........... ..

6-1 Morphologies observed at low surface pressures for (a) the pure PS-b-PEO
copolymer, (b) the pure PS-b-PEO copolymer + PS homopolymer, and (c) the pure
PS-b-PEO copolymer + PEO homopolymer (blue = PEO, black = PS) ................... .......143

6-2 Morphologies observed at high surface pressure for (a) PS, (b) PS-b-PEO diblock
copolymer, (c) PS + PS-b-PEO diblock copolymer, and (d) PS-b-PtBA copolymer
(blue = PEO, black = PS, and orange = PtBA) ................ ...............143............

6-3 Morphologies observed at high surface pressure for (a) the PEO homopolymer, (b)
PEO-b-PS diblock copolymer, and (c) the PEO-b-PCL copolymer (blue = PEO,
black = PS, and green = PCL) ................. ...............144........... ..









LIST OF ACRONYMS


AFM

TEM

STM

BAM

ATRP

RAFT

TR

LB

LC

CMC

EO

tBA

St

PEO

PtBA

PCL

PS

PS-b-PEO

PS-b-PtBA

PEO-b-PCL


Atomic Force Microscopy

Transmission Electron Microscopy

Scanning Tunneling Microscopy

Brewster Angle Microscopy

Atom Transfer Radical Polymerization

Reversible Addition Fragmentation Chain transfer

Transfer Ratio

Langmuir Blodgett

Liquid Crystalline

Critical Micellar Concentration

Ethylene Oxide

tert-butyl acrylate

Styrene

Poly(ethylene oxide)

Poly(tert-butyl acrylate)

Polycaprolactone

Polystyrene

Polystyrene-block-Poly(ethylene oxide)

Polystyrene-block-Poly(tert-butyl acrylate)

Poly(ethylene oxi de)-block-Polycaprol actone









Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

BEHAVIOR OF SEVERAL AMPHIPHILIC COPOLYMERS AT THE AIR-WATER
INTERFACE

By

Sophie Bernard

December 2007

Chair: Randolph S. Duran
Major: Chemistry

The two-dimensional structure of amphiphilic copolymers is studied at the air-water

interface using Langmuir-Blodgett methods, atomic force microscopy (AFM), and Brewster

angle microscopy (BAM). Measurements are made for several block copolymers containing

polystyrene (PS), poly(ethylene oxide) (PEO), poly(tert-butylacrylate) (PtBA) and

polycaprolactone (PCL).

Measurements are also made for blends of the PS-b-PEO copolymer with both a PS and a

PEO homopolymer. When increasing the amount of PS homopolymer, the isotherms do not

show any change in the high surface area region. However, a linear dependence of the condensed

area is observed. An increase in the PEO ratio has an effect on the biphasic region of the

isotherms but no change is detected for the condensed area. The AFM data indicate a significant

effect of the homopolymers on the monolayer structure. In fact depending on the homopolymer

added, a change in the chaining behavior of the copolymer is observed. Also, when introducing

more PEO, a phase separation between the layer of PEO and clusters of two-dimensional

micelles is detected.

The PS-b-PtBA copolymers investigated are star-shaped copolymers with a polystyrene

(PS) core and a poly(tert-butylacrylate) (PtBA) corona. They were prepared with a constant PS









block and three different PtBA block lengths. A transition can be observed for the three

copolymers with a plateau's length depending on the PtBA composition. The images obtained by

AFM are in agreement with the isotherms showing the evidence of a phase transition around

24mN/m. In fact for the three copolymers, below the plateau only single domains are observed

whereas for pressures higher than 24mN/m, aggregates can be detected. Transfer experiments

were performed after several equilibration times. The structure of the film formed seems to be

dependant on the time waited before performing the transfer, showing more compact films when

the time waited was longer.

Poly(ethylene oxi de)-block-Polycaprol actone (PEO-b-PCL) copolymers are also studied in

this work. Their behavior and the PCL crystallization process at the air-water interface is

investigated using BAM. The formation of crystals directly on the water subphase is illustrated

and compared to the pictures obtained by AFM.









CHAPTER 1
INTTRODUCTION

1.1 Scope of the Dissertation

The aim of this proj ect is to find fundamental information on a novel series of polymers by

understanding their behavior at the air/water interface. Chapter 2 describes the different

techniques used throughout my doctoral research. A review of these techniques as well as their

multiple applications is given. Chapter 3 and 4 go over the investigation of several amphiphilic

copolymers at the air-water interface as well as their ability to form more complex architectures

when blended with one of their corresponding homopolymers. Atomic Force Microscopy (AFM)

is used as a tool to observe the formation of the morphology once the copolymer solutions are

spread onto an interface and transferred to solids. Chapter 5 describes the behavior of a

poly styrene-b-polycaprol actone (P S-b-PCL) linear copolymer and a P S-b-PCL star-shaped

copolymer. In this investigation, Brewster angle microscopy (BAM) is used in order to

understand the copolymer crystallization process. The goal of Chapter 5 was to obtain a better

understanding about the formation of polycaprolactone crystals when a solution is spread onto a

water subphase.

The reminder of this chapter is a literature review on the behavior of block copolymers in

the bulk and at an interface.

1.2 Block Copolymers in the bulk

Self-assembling copolymer materials are interesting because of their multiple commercial

applications such as thermoplastic elastomers and compatibilizers in polymer blends. In the past

decades, interest in the behavior of block copolymers has grown due to many further potential

applications in nanoscale lithography or electronics.2-4 The emergence of controlled

polymerization techniques such as such as living anionic polymerization,5 ATRP (atom transfer









radical polymerization),6 Or RAFT (reversible addition fragmentation chain transfer)' has

allowed the formation of more complex architectures leading to a wide range of properties.s-1

Block copolymers are composed of different polymer chains. Depending on the number of

different blocks, their composition, and the way they are linked together, they can form a variety

of ordered morphologies with characteristic lengths on the mesoscale .11

The simplest class of block copolymers is linear block copolymers composed of two

immiscible blocks, A and B. This type of architecture can adopt several equilibrium

morphologies depending on the composition of the two blocks: spheres (S), cylinders (C), double

gyroid (G), lamellae (L), and the inverse structures (Figure 1-1).11,12

This phase behavior is dictated by the Flory-Huggins segment-segment interaction

parameter (XAB), the degree of polymerization (N), and the composition (4). The product XABN

gives us an idea on the phase segregation.13 For small values (XABN < 10), the A and B blocks

mix, resulting in a disordered phase. When XABN is larger (XABN > 10), the enthalpic terms

dominate, causing an order-disorder transition (ODT) where the A and B segments segregate into

various microstructures.

When block copolymers are dissolved in a selective solvent that is good solvent for one of

the block and precipitant for the other, the copolymer chains aggregate to form micelles.5 Those

reversible, well-defined micelles form above the critical micellar concentration (CMC). The

CMC can be determined from plots of the surface tension as a function of the logarithm of the

concentration. It is then defined as the concentration at which the surface tension stops

decreasing and reaches a plateau value. For concentrations lower than the CMC, only single

polymer chains are present in solution. Once the CMC is reached, the polymers chains aggregate

into spherical aggregates as shown in Figure 1-2.










The aggregates observed are in conjunction with those seen for low molecular weight

surfactant even if the values of the CMC are much lower in the case of block copolymers.

Depending on the composition of the starting block copolymer, two limiting structures can

be observed: (1) "starlike" micelles with a small core compared to the corona and (2) crew-cut

micelles with a large core and highly stretched coronal chains. Both situations are shown in

Figure 1-3.

1.3 Block Copolymers at the Air-Water Interface

The morphologies described previously represent those formed in a three-dimensional

system, but block copolymers also have the ability to self-assemble as two-dimensional

structures. Such experiments are performed using a Langmuir apparatus, the setup of which will

be further explained in chapter 2. In such case, the polymer solutions are spread onto a subphase

and each block aligns in the phase for which it has an affinity. Amphiphilic diblock copolymers

have been observed to self-assemble into numerous nanoscale and mesoscale structures when

spread onto a water substrate, finding potential applications in coatings, microelectronics,

stabilization, and lubrification.l5 Such copolymers are appropriate for surface pressure studies

involving Langmuir troughs. This technique provides insight on the monolayer morphologies by

controlling the surface density. The force acting on the molecules when spread onto a liquid is

referred as the surface tension, y, which is the measurement of the cohesive (excess) energy

present at a gas/liquid interface. The molecules of a liquid attract each other; the interactions of a

molecule in the bulk of a liquid are balanced by an equally attractive force in all directions.

Molecules on the surface of a liquid experience an imbalance of forces as indicated in Figure 1-









The net effect of this situation is the presence of free energy at the surface. The excess

energy called surface free energy can be quantified as a measurement of energy/area. For a

constant temperature and pressure, surface free energy equals surface tension, which is

quantified as a force/length measurement. The common units for surface tension are dynes/cm or

mN/m. An analogous quantity is the line tension. From a mechanical point of view, line tension

is defined as the operative force along the so-called three-phase line. A three-phase line is the

intersection of three interfaces; for example, the periphery of the contact circle of a liquid drop

that is placed on a solid surface and is surrounded by a vapor phase. Similar to surface tension,

i.e., the tensile force encountered where two bulk phases meet, line tension is a well-defined

thermodynamic property. However, unlike surface tension, it is not a well-quantified property;

experimental values in the literature range from 10-"1 to 10-' N.16-30

1.3.1 Polystyrene (PS) and Poly(ethylene oxide) (PEO) at the Air-Water Interface

Seo et at. 31 Showed the formation of stabilized two-dimensional micelles using

polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers at the air-water

interface. Once formed, those surface aggregates were kinetically stable, preventing any unimer-

micelle exchange. Polystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock copolymers of

various molecular weights and chemical compositions have also been extensively used to study

their properties in both the bulk and in solution.

In addition, several groups have described their behavior at the air-water interface.32-44 The

choice of PEO as one of the blocks renders the copolymer both biocompatible as well as

amphiphilic. The inclusion of PS provides an anchor at the air-water interface, preventing the

PEO from eventually dissolving into the water subphase. As a result, PS-b-PEO films can be

further compressed than a film composed simply of PEO homopolymer.









Without PS, PEO can still be spread at the air-water interface. Shuler and Zisman 45

studied the behavior of such a film. They observed a change in the film compressibility as

surface density increases leading to a phase change reflecting in the fi1m's structure. The lack of

reversibility in the compression and expansion experiments is explained by a structural change in

the polymer molecule. A modification in conformation was given to explain the different

monomer area observed in the n-A isotherms. Kuzmenka and Granick 46 perfOrmed the same

type of experiment for a wide range of PEO molecular weights. They determined that for PEO

chains beyond molecular weights of 100,000g/mol, the fi1m attains a constant equilibrium

surface pressure. This behavior was explained by the difficulty of a high molecular weight PEO

to pass into an aqueous substrate due to the amphiphilic character of the EO monomer. Lower

molecular weight PEO, however, requires a more hydrophobic anchor in order to quantitatively

remain at the air-water interface, generally partitioning between the subphase and the surface,

analogous to soluble surfactants.

While PEO has been widely studied at the air-water interface, PS has only been studied by

one group. Being hydrophobic, PS is not expected to form any type of morphology when spread

onto a water subphase. However, Kumaki 47 detected a change in surface pressure when a dilute

solution of PS (2.0x 10- g/mL) was spread at the air-water interface. Even if in this case, the

measured surface pressure mainly represented mechanical force due to the compression, this

group found stable monomolecular particles were observed for molecular weights higher than

50,000g/mol. However this work remains controversial. In fact, despite its importance, this

remains the only study on pure PS.

As a result, recent work has focused on the behavior of PS-b-PEO at the air-water or solid-

water interface, demonstrating the formation of novel nanostructures. Gongalves da Silva et al.









34,39 described the utility of diblock amphiphilic copolymers in testing the scaling properties of

grafted polymers. They presented n-A isotherms that show several regions referred to as

pancake, quasi-brush, and brush. Within these regions, different morphologies of surface

micelles and further micellar aggregates were observed by transmission electron microscopy

(TEM) and atomic force microscopy (AFM) depending on the balance between block sizes. At

the air-water interface, copolymers behave similarly to copolymers in bulk dispersion. Static

light scattering proved that PS-b-PEO copolymers aggregate spontaneously into micelles over

the CMC. The isotherm regions compare to those observed for solution CMC values: (1) below

the CMC, surface micellization is observed; (2) at the CMC, the PEO segments are pushed into

the subphase in order to decrease the surface area per molecule; and (3) above the CMC, PS-rich

regions exist in between spaces formed by the PEO chains.

Gongalves da Silva et at. 34,39 also investigated the effect of the PEO block size on the

copolymer behavior at the air-water interface. In this case, the short PS chains are only used as

an anchor to prevent the PEO from dissolving completely into the water substrate. Upon

compression, they observed a transition of the PEO blocks, from a two-dimensional structure

floating on the water, to a three-dimensional structure when the PEO stretched into the water.

The first structure is the one previously termed "pancake" whereas the second was identified as

"brush" (Figure 1-5). The plateau displayed in the n-A isotherms is an indication of the transition

between these two states with its span dependent on the relative sizes of the two blocks.

The importance of PEO in film behavior has been recognized by others. For example,

Devereaux and Baker 43 COnducted n-A isotherms experiments of PS-b-PEO copolymers with

varying PEO chain lengths. One copolymer contained 15% of PEO whereas the other had only

7%. The copolymer with the longest PEO block displayed a plateau close to 10mN/m, indicating









that the copolymer spreads well at the interface. In contrast, the copolymer containing only 7%

of PEO has no plateau, supporting the theory that PS chains interfere with the PEO blocks upon

compression.

While most groups support the model of a transition from pancake to brush described

previously, Cox et al. 41,42 prOVide a different model to explain the shape of the n-A isotherm for

a PS-b-PEO copolymer. Whereas in the first model the PEO passes into the aqueous subphase,

the Cox model suggests a dehydration of the PEO followed by a conformational change, similar

to that previously described by Shuler and Zisman40 for homopolymer PEO. As shown in Figure

1-6, conformation (a) (more flexible) is compressed into conformation (b) (more compact and

sterically hindered). This transformation can be explained by an increase in the intramolecular

forces in the second conformation.

While numerous studies detail the behavior of linear PS-b-PEO, advances in

polymerization techniques within the past decade have allowed chemists to design new

copolymer architectures. Logan,48 Logan et al.,49 and Francis et al.,50,51 for example, investigated

the behavior of several star amphiphilic copolymers at the air-water interface. Peleshanko et al.42

observed formation of morphologies when spreading an amphiphilic heteroarm PEO-b-PSm. The

AFM images showed that the formation of different morphologies depends on the pressure used

during the transfer. The unusual properties of those architectures allow the formation of more

stable morphologies than those formed using regular linear copolymers.

1.3.2 Polyacrylates at the Air-Water Interface

Some copolymers, called stimuli-responsive systems, respond by a change of size or shape

to a modification in temperature, ionic strength or pH.









For example, in the Armes group, the synthesis and solution properties of water-soluble

copolymers were studied. The copolymers were composed of different alkyl methacrylates for

the hydrophobic block and poly(2-dimethylamino) ethyl methacrylate, poly(sulfubetaine),

poly(4-vinylbenzoate) and more for the hydrophilic block.53,54,55 Block copolymers containing PS

and poly(acrylic acid) (PAA) are another type of self-assembling copolymers.56-59 Eisenberg et

at. 57,59 Synthesized PS-b-PAA copolymers by anionic polymerization and investigated their

behavior at the air-water interface. They identified a wide range of structures like spheres, rods,

bilayer and bicontinuous architectures, as well as inverted structures. All the aggregates observed

possess a phase-separated insoluble core and a crew-cut soluble corona. The synthesis as well as

the micellar properties in an aqueous media was studied for more complex architectures based on

PS and PAA.60-66 Even if numerous efforts have been done to study the interfacial behavior of

PAA-based copolymer, Poly(tert-butylacrylate) (PtBA) -based copolymers studies remain rare.

Our group was the first to publish results concerning PS and PtBA based copolymers at the air-

water interface.67,68

1.3.3 Poly(s-caprolactone) (PCL) at the Air-Water Interface

PCL is an interesting flexible polymer for monolayer studies. It is a hydrophobic and

crystalline polyester with a glass transition around -600C, a melting point around 550C, good

biocompatibility, and low toxicity. In the past decades, PCL-containing systems have been found

potentially useful for applications such as controlled-release drug delivery and scaffolds for

tissue engineering.69-71 At the air-water interface, previous results indicate that PCL can form a

"2D" close-packed monolayer with a collapse point around A 20 A+2/mOnomer. We found that

very little interest was given to studying the assembly of monolayers of PCL homopolymers and

PCL-based copolymers. Leiva et al. were first to describe the behavior of PCL homopolymers at









the air-water interface.72 Recently, Li et al 73 Studied their crystallization behavior using

Brewster angle microscopy (BAM). The architectures formed are illustrated in Figure 1-7.73

1.3.4 Previous Studies on Polymer Blends

While different architectures can result in different surface fi1m behavior, the synthesis of

such systems can be difficult and time-consuming. In an effort to acquire new properties without

the required synthesis, surface films of blended polymers have also been investigated. In the

1980s, the Gabrielli group74-81 examined the behavior of numerous mixtures of polymers and low

molecular weight materials as binary systems with different degrees of incompatibility. They

also quantified the determination of the two-component monolayer miscibility by ob serving the

n-A isotherms of their two-dimensional blend. Thibodeaux et al8 2 Studied mixtures of a liquid

crystalline copolymer with its corresponding monomer. The films formed by the blend

monolayer appeared to be more condensed than the pure copolymer films, proving that two-

dimensional mixtures of two or more polymers could enhance the interfacial behavior and enable

the formation of more stable films.

In addition, such technique allows the blending of different polymer characteristics into a

single film. Malzert et al8 3-85 developed a suitable model for understanding the interactions

between polymers by mixing poly(ethylene glycol) (PEG) and poly(lactide-co-glycolide). The

difference between the organization of the films resulting from a spreading of a mixture or a

covalently linked copolymer gave the possibility to modulate and control the composition of the

interfaces. Hottle et at 86 Studied blends of amphiphilic poly(dimethyl siloxane) and

trisilanolisobutyl-POSS and observed the formation of uniform monolayers when adding up to

80wt% POSS. By adding poly(lactide-co-glycolide) to the PEG monolayer, Malzert et al were

able to avoid the dissolution of the hydrophilic polymer and therefore control the composition of









the interface. More recently, Seo et al.s investigated the structures formed at the air-water

interface by blending poly(styrene-b-ferrocenyl silane) (PS-b-FS) and poly(styrene-b-2-vinyl

pyridine) (PS-b-P2VP). While neither of those copolymers assemble when spread separately at

the air-water interface, their blends formed ordered structures which appear to be more versatile,

a promising development in the fabrication of polymeric templates for lithography. Experiments

were performed for several temperatures and showed that close to the Tg of one of the block, the

formed micelles retained their surface mobility and could organize in equilibrium structures in

response to changes in surface density or applied electric Hields. In general, the surface behavior

of amphiphilic diblock copolymers is readily examined through Langmuir techniques. Methods

involving fi1m compression and transfer provide both quantitative and qualitative results

indicating how surfactant responds to pressure. PS-b-PEO proves to be particularly of interest

due to the biocompatibility of PEO. While different architectures of this copolymer have been

shown to demonstrate different properties than those of linear analogues, additional

characteristics may yet be attained through blending, both with PS and PEO homopolymers.

While only a few preliminary studies of blending at the air-water interface exist, spin

coated thin films of polymer blends have been investigated.88-94 Mayes et al88,89 Studied the

homopolymer distributions in ordered block copolymers. By blending a PS-b-PMMA diblock

copolymer with PS and PMMA homopolymers, this group was able to illustrate the interactions

between a homopolymer and its corresponding block. A similar system studied by Jeong et al 92-

94, Showed that the miscibility between PMMA homopolymer and PMMA block was higher in

thin films than in the bulk.

The most commonly used technique to observe the morphologies formed by compressing a

monolayer at a certain pressure is AFM. For soft samples such as polymer films, an appropriate









AFM technique is tapping mode. Here, the cantilever is excited to an oscillation near its

resonance frequency. The interactions between the tip and the sample give a deviation in the

oscillation amplitude, recording the changes in the sample. This mode has been employed for

most polymer samples because of its ability to investigate soft materials without further staining

and with little or no tip-induced damage or morphology changes. However, Knoll et al 95

highlighted the limitations of this technique, finding that the appearance of artifacts was related

to tip-sample interactions. Nevertheless, AFM provides valuable information on film

morphology. Bodiguel et al.,96 for example, introduced a method for determining the dependence

of the phase signal on the thickness of the sample. They corroborated that the origin of the phase

signal was adhesive and represented the local elastic properties of the sample. Garcia et al 97

showed that phase shift measurements obtained by AFM in tapping mode could be converted

into energy dissipation values. They suggested that the characterization of material properties is

less sensitive to the interaction regime and more to the tip-surface interactions. For a completely

different application, Dorvel et al.98 USed AFM to characterize the formation of tethered bilayer

lipid membranes on gold substrates.

In the following chapters, three different polymer systems will be investigated. Several PS

or PEO containing amphiphilic systems will be considered. Those copolymers were chosen for

their interesting surface activities, making them good candidates for interfacial investigations.

Those experiments will give us important fundamental information regarding the two-

dimensional self-assembly of amphiphilic copolymers. The blending properties of a copolymer

will also be given, allowing the formation of different morphologies by controlling the amount of

each component. Moreover, the effect of the nature of the more hydrophobic block will be

described using an amorphous (PS) and a crystalline (PCL) polymer.





Figure 1-1. Morphologies of diblock copolymers: cubic packed spheres (S), hexagonal packed
cylinders (C or Hex), double gyroid (G or Gyr), and lamellae (L or Lamn) (Adapted
from Reference 2).


~"~f


~"*~
"Sl,

Zll~pll~


'Lsl~,~


"*I~,,


C< CMC


C> CMC


Figure 1-2. Solution states for an amphiphilic diblock copolymer for concentration below and
above the CMC.


Figure 1-3. Schematic representation of a "starlike" (1) and a "crew-cut" (2) micelle.









Air


Liquid

Figure 1-4. Description of Surface tension showing how the forces on surface molecules differ
from those in the bulk





Waler~d




Figure 1-5. Schematic representation of the pancake to brush transition proposed by Goncalves
da Silva et al.34 for PS-b-PEO copolymers.














Figure 1-6. The two conformatins for PEO at the air-water interface proposed by Shuler and
Zisman.45 COnformation (a) is loose and flexible, but compressing it into the compact
conformation (b) tends to lock the chain in one position resulting in a sterically
hindered and more rigid structure

























Figure 1-7. BAM images obtained during an hysteresis experiment at 22.50C with a compression
and expansion rate of ~ 0.010 nm2.mOnomer- .minl for a PCL homopolymer.
Compression (A/nm l.monomer- ): (a) 0.243, (b) 0.200, (c) 0.170, (d) 0.120, and (e)
0.080; Expansion (A/nm l.monomer- ): (f) 0.082, (g) 0.173, (h) 0.272, and (i) 0.387.
(Adapted from Li et all 37)









CHAPTER 2
EXPERIMENTAL TECHNIQUES

Any study involving Langmuir monolayers requires the use of a Langmuir trough set-up

for quantitative measurements and for the preparation of Langmuir-Blodgett films. Irving

Langmuir was one of the principal scientists to observe the formation of monolayers when a

surfactant is spread onto water, which lead to the development of the Langmuir trough technique

(Figure 2-1).99 With this apparatus, he studied floating monolayers on water in the late 1910s and

early 1920s. Several years later, Katherine Blodgett gave the first detailed description of

sequential monolayer transfer onto solid supports.102

2.1 Langmuir Trough

A typical Langmuir trough (Figure 2-2) is composed of the trough itself, one or two

movable barriers, and a device for measuring surface pressure. The Wilhelmy technique is the

most commonly used and consists of a wettable thin plate partially submerged in a subphase and

suspended from a balance. The force acting on the plate is directly proportional to the surface

tension of the liquid.

At equilibrium the surface tension can be described as:



TV~n; T,P,n, -

where F and G are the Helmholz and Gibbs free energies respectively, and A is the surface

area. An analogous quantity, the line tension is defined as the free energy per unit length

associated with the boundary between two phases on a surface. Because most Langmuir-Blodgett

techniques involve using pure water or aqueous subphases, the surface tension of pure water is

important. Its value at 250C is 72mN/m.









The surface tension can be measured using a Wilhelmy plate technique as shown in Figure

2-1. The plate is usually very thin and made of platinum; however glass, quartz, mica, and filter

paper can also be used. The net downward force is given by the equation:

F = p~glwrt +27(t +w)cos9 pgtwh~l 2-2

where p, and pi are the densities of the thin plate material and liquid, respectively, g represents

the gravitational constant, y is the subphase surface tension, and 6 is the contact angle of the

liquid on the solid plate. The plate is also described by its thickness (t), width (w), and length (1)

(see Figure 2-3).

When measuring the change in h (height of the plate) for a constant applied force, the

change in surface tension can be calculated using equation 2-3.

plgtw~~h= 2Ay(t+w) 2-3

The 2D analogue of pressure at a surface is called surface pressure, n: and is the difference

between the surface tension of a film covered surface, y, and a pure liquid subphase, Yo. A

decrease in the surface pressure will be observed when in presence of a film. This leads the

interesting fact that the maximum surface pressure attainable at an air-water interface will be

72mN/m with pure water at T = 250C.

Therefore surface pressure can be defined as:

xi~= 70 -7 2-4

Since the surface pressure is defined as a negative change in surface tension, the surface

pressure can then be determined using equation 2-5 when measuring the change in F for a

stationary plate between a clean surface and the same surface with a monolayer present.

~z= -Ay= -pgtwr Ah2(t +w1r) 2-5










If the plate is completely wetted by the liquid (cos6 = 1), the surface pressure is then

obtained from the following equations:

AF= 2Ay(t+w) 2-6


xi = -A7 = -2-7
2(t +w)

For the Wilhelmy method, the thickness of the plate used is small, leading to t << w and

the equation 2-8.


r = 2-8
2w

Nowadays, electrobalances allow very little change in the plate's movement, improving

sensitivity (5 x 10-2 mN.m )~.

2.2 Equilibrium Spreading Pressure

Surfactants in general are simply molecules that migrate to the air-water interface and form

a film. Molecules are spontaneously inserted into the film only up to a certain surface pressure,

known as the equilibrium spreading pressure (ESP). Surface pressure can be thought of as the

repulsive force resulting from molecules in a film being in close proximity with each other. At

equilibrium spreading pressure, the molecules are too close to each other to easily allow more

material to be inserted. However, if the surface area of the film is increased, more molecules

from the subphase are inserted into the film to maintain the equilibrium spreading pressure. For

polymers, a way to measure the ESP is to place a solid polymer onto the water subphase. The

polymer chains are inserted in the monolayer up to a certain surface pressure representing the

ESP.









2.3 Langmuir Trough Experiments

Measuring the surface pressure as a function of the area of water surface available to each

molecule provides insight into monolayer properties. Such experiments are carried out at

constant temperature using a heat exchanger and are known as isotherm experiments. The data

are recorded by compressing the film at a constant rate while monitoring the surface pressure

(Figure 2-4).

In a manner analogous to bulk materials, distinct regions can be observed at surfaces,

defining the different phases of the monolayer. These different monolayer states can be

observed, depending on the hydrocarbon chain length, nature of the head group, conditions, and

other factors; for example, an increase in chain length increases the interactions between the

chains in classical low molecular weight amphiphiles, leading to a more condensed xn-A

isotherm. For a minimal compression, and areas typically in the hundreds or thousand of A+2 and

pressures of only a small fraction of 1mN/m, the monolayer exists in the gaseous phase (G).

While compressing, the monolayer undergoes phase transitions to one or more of the following

phases: liquid-expanded state (Ll), followed by the liquid-condensed state (L2), and finally one

or more solid state phases that we will group together with the symbol (S) for simplicity. There

also exists a series of liquid crystalline phases in the L2 region that are important for certain low

molecular weight systems and in particular for biological properties of membrane amphiphiles.

The various LC and solid phases are not relevant to the polymers discussed in this work so we

will not further discuss them. Another important detail is that the sequence of phases observed,

like in the case of cooling bulk 3D materials, does not need to show all the potential phases

possible, though the phases always proceed in such a way that more and more ordered phases are

observed as the pressure increases. If the monolayer is compressed beyond the solid phase, it will









collapse into three-dimensional structures. A way to better observe the different transitions

observed on a surface pressure-MMA (where MMA is the mean molecular area representing the

available area per molecule) plot is to calculate the compressibility of the monolayer using

equation 2-9 (where K is the compressibility and n: the surface pressure).


K = 1dAI)2-9
ARIA d (F)

d(MMA)/d(xn) represents the slope of the surface pressure-MMA isotherm plot. In these

experiments, this value was calculated using for each point on the isotherm by calculating the

instantaneous slope over 2000 points. It is important to note that the MMA does not take into

account the polydispersity of the sample and is based on the number average molecular weight.

Therefore, the area occupied by one polymer chain of average molecular weight is only

calculated.

While the regions described above can often be found in small surfactant molecules,

diblock copolymers typically have fewer regions as noted above. An example is shown in Figure

2-5. Here, extrapolations of selected regions of the isotherm to zero pressure quantify the

isotherm in such a way that the surface behavior of different copolymers and their blends to be

compared independent of differences in pressure. It should also be noted that the surface area in

experiments involving polymers is often expressed in area per repeat unit. This is calculated by

dividing the MMA by the number of repeat units present in one polymer chain. This allows

interpretations and comparisons with low molecular weight standards. Similarly, for block

copolymers the area can be expressed in area per repeat unit in one of the blocks which can be

calculated by dividing the MMA by the total number of repeat units of one of the block in one

copolymer chain. This simplifies comparisons between copolymers consisting of different

segments of varying molecular weights and even different architectures.









In addition to the classic isotherm experiment, numerous experiments such as isobars,

isochores or compression-expansion hysteresis, can be done using the Langmuir trough. In the

isobaric experiment, the monolayer is compressed to a specific target pressure. The pressure is

then maintained constant while the MMA is recorded over time. Isobars provide the ratio of the

current monolayer area to its initial area. The change in area can be correlated with film

properties such as creep. In the isochoric experiment, the monolayer is compressed to a specific

target pressure. Once that pressure is reached, the compression is stopped and the surface

pressure is recorded versus time.

The compression-expansion experiment measures the ability of the monolayer to relax to

its initial expanded state after compression. If upon expansion, the monolayer relaxes following

the same pressure-MMA curve, no hysteresis is detected. If however, the second compression

produces a new curve, the hysteresis can be detected to quantify the amount of irreversibility of

the film formed. Those experiments can give us information about the stability of the film

formed. In fact, for high surface pressures, numerous block copolymers form metastable films,

metastability being described as the ability of a non-equilibrium chemical state to persist for a

long period of time. For example, for PCL homopolymers, Li et al reported the formation of

metastable crystals during the compression cycle with the crystal sizes being dependent on the

compression rate.73 During the expansion cycle, those crystals were re-adsorbed (melted) onto

the water subphase.

Unless otherwise noted, surface pressure measurements were performed using a Teflon

Langmuir trough system (W= 160 mm, L = 650 mm; KSV Ltd., Finland) equipped with two

movable barriers and a Wilhelmy plate Between runs, the trough was cleaned with ethanol and

rinsed several times with millipore filtered water (resistivity > 18.2 MGZ.cm). The samples were










typically prepared by dissolving approximately 1 mg of polymer in 1 mL of chloroform.

Volumes ranging from 10 to 30 CLL were spread dropwise on a Millipore filtered water subphase

with a gastight Hamilton syringe. The chloroform was allowed to evaporate for 30 min to ensure

no residual solvent remained. When not in use, the volumetric flasks containing the polymer

solutions were wrapped with Teflon tape followed by Parafilm and stored at 10 OC in order to

prevent changes in concentration due to chloroform evaporation. In all the experiments, subphase

temperature and barrier speed were kept constant at 25 OC and 5 mm/min, respectively.

2.4 Langmuir-Blodgett Films

Besides Langmuir monolayers, a common application of the Langmuir trough is the

transfer of monolayer onto a solid substrate. This is accomplished by dipping the substrate into

the subphase, allowing the adsorption of the monolayer. The surface pressure is maintained

constant by a computer controlled feedback system between the electrobalance measuring the

surface pressure and the barrier moving mechanism. Depending on the number of dippings,

successive monolayers, from several to hundreds, can be deposited onto the solid substrate.

Numerous substrates have been used. For analytical work involving subsequent imaging,

mica is commonly preferred in LB film transfer due to its low cost, easy cleaning, and easy

preparation. However, mica possesses a water layer that may affect the fi1m transfer. Other

substrates such as silicon wafers can be used; treatment with chromic sulfuric acid renders them

highly hydrophilic. Still other materials can be used as hydrophobic substrates, including

graphite, polymer films, and silanized silicon dioxide.

LB fi1ms can be formed either by pulling out or dipping the substrate into the subphase.

Pulling or the upward pass of the substrate through the subphase is also known as an upstroke

while the downward dipping refers to the downstroke. Three different types of deposition can

exist (Figure 2-6). The X-type deposition can be done by a downstroke whereas Z-type occurs










during the upstroke. The Y-type, the most common, is characterized by deposition on both an

upstroke and a downstroke. Intermediate structures can sometimes be observed for some LB

multilayers and are often referred to as XY-type multilayers.

Once transferred, these fi1ms can be studied by different surface analysis techniques, such

as optical spectroscopy, X-ray diffraction, as well as atomic force microscopy (AFM) or

transmission electron microscopy (TEM).

The quantity and quality of the deposited monolayer on a solid support are measured by a

so called transfer ratio, TR. This is defined as the ratio between the decrease in monolayer area

during a deposition stroke, Al, and the area of the substrate, As. For ideal transfer the TR is equal

to 1. However, experimentally the TR often varies significantly. Low transfer ratios indicate a

that the polymer chains transferred on the substrate are less dense than those on the water

subphase. For transfer ratios greater than 1, the polymer chains on the solid substrate are more

densely packed than those at the air-water interface. Another type of transfer ratio can also be

calculated and recorded in typical LB experiments. The instantaneous transfer ratio is the amount

of monolayer transferred versus the amount of substrate pulled out at a specific time. This type

of transfer ratio gives information on the uniformity of the fi1m transferred. In our experiments,

all the instantaneous transfer ratios observed were homogeneous indicating a good transfer of the

monolayer onto the mica substrate.

2.5 Atomic Force Microscopy

2.5.1 Instrument Parameters

Contrary to its precursor, scanning tunneling microscopy (STM), which only allows the

study of electrically conductive samples, AFM can be applied to both conductors and insulators.

The instrument consists of a tip at the end of a cantilever, which bends in response to the force

between the tip and the sample (Figure 2-7).









Since the cantilever obeys Hooke's law for small displacements, the interaction force

between the tip and the sample can be found:

F =-k x 2-10

where x is the cantilever deflection and k the spring constant.

In the early stages of AFM, contact mode was used. This method consists of a tip in close

contact with the surface. The deflection of the cantilever is sensed and compared to the desired

value of deflection. The voltage needed to restore the desired value of deflection is a measure of

height of features on the sample surface. This mode was quickly forgotten for polymer studies

because of excessive tracking forces applied by the probe to the sample.

To remove these drawbacks, a non-contact mode was developed. In this case, the tip

hovers 50-150 Angstrom above the sample surface. The attractive Van der Waals forces acting

between the tip and the sample are detected, and topographic images are constructed by scanning

the tip above the surface. This technique was also found to be inapplicable to polymer samples.

In general, the fluid contaminant layer existing on the sample is substantially thicker than the

range of the Van der Waals force gradient and, therefore, all attempts to image the true surface

with non-contact AFM fail as the oscillating probe becomes trapped in the fluid layer.

Later, a third method was developed in order to study softer samples (Figure 2-8). This

mode, called tapping mode, consists of alternately placing the tip in contact with the surface to

provide high resolution and then lifting the tip off the surface to avoid dragging the tip across the

surface. As the oscillating cantilever begins to intermittently touch the surface, the cantilever

oscillation is necessarily reduced due to energy loss caused by the tip contacting the surface. The

reduction in oscillation amplitude is used to identify and measure surface features.










AFM involves scanners made from piezoelectric material, a substance which

proportionally contracts and expands, depending on an applied voltage. If a positive voltage

elongates the scanner, a negative voltage contracts it. The scanner is made of a piezoelectric

material surrounded by electrodes which control the applied voltage. As scanning occurs in three

dimensions, a scanner tube contains three piezo electrodes for the X, Y, and Z directions (Figure

2-9). Piezoelectric ceramics are capable of moving a probe very small distances. However, when

a linear voltage ramp is applied, they move in a non-linear motion (Figure 2-10). All AFM must

therefore be calibrated in the X-Y axis so that the images presented on the computer screen are

accurate. Height measurements require that the piezoelectric ceramics in the Z axis of the

microscope be both linear and calibrated. Often the microscope is calibrated at only one height;

however, if the relationship between the measured and the actual Z height is not linear, the

measurements will not be correct.

The studies described in this work utilize a Digital Instruments Nanoscope system. Several

different scanners can be used depending on the sample studied. They differ on the scanning size

and resolution. For example the J-scanners can scan images up to 125 Clm, whereas E-scanners

scan smaller sizes of 10 Cpm or less. In these experiments, the x-y range was calibrated monthly

using a calibration grid with known periodicity. Likewise, z calibration was performed bi-

annually, using standards of different heights. In our case, the standard used is a silicon

calibration grating with a step height of 18.5+1nm (close to the height of our samples).

2.5.2 Limitations

Some limitations exist when using AFM. The tip can easily be damaged and the formation

of debris can lead to dullness of the tip. A blunt tip can not detect narrow valleys or higher peaks










and will therefore create a blurry image. Resolution in AFM images consists of lateral (X,Y) and

vertical (Z) components with the lateral resolution dependent on the tip radius of curvature for

However the fabrication technique of the AFM probes leads to the formation of tip

possessing flat, rectangular ends. This reduces the sharpness and therefore the resolution of the

images as shown in Figure 2-11.

Polymer thin films can thus be characterized through a combination of Langmuir and AFM

techniques. Such methods allow the easy control of surface density as well as the easy transfer of

surface fi1ms onto solid substrates. The results of such analysis will be presented in the

subsequent chapters.

2.5.3 Parameters Used

Surface fi1ms of the copolymers were transferred onto freshly cleaved mica at various

pressures (250C). The desired surface pressure was attained at rates of +10 mm-min- Once the

film had equilibrated at a constant n: for 30 minutes, the mica was then pulled out at a rate of 1

mm-min- The transfer ratios are noted in the chapters and unless specifically noted, the transfer

ratio remained constant over the transfer and even transfer can be assumed. The transferred film

was air-dried in a dust-free environment for 24 hours and subsequently scanned in tapping mode

with a Nanoscope III AFM (Digital Instruments, Inc., Santa Barbara, CA) using silicon probes

(Nanosensor dimensions: T= 3.8-4.5 Clm, W= 26-27 Clm, L = 128 Clm). Experiments were

redone after a few weeks in order to observe if any change occurred in the fi1m structure after

transfer. No difference was detected, indicating the stability of our samples once transferred onto

the solid substrate. Tapping mode was used, giving a better image of a polymer sample without

damaging the surface by dragging the tip. This mode consists of a tip vibrating at its resonance

frequency in tapping the surface. As the tip encounters a surface feature, its amplitude of










oscillation is decreased from its set-point value. This decrease is noted by the sensor and the tip

is moved up away from the sample to re-attain the set-point amplitude. A similar behavior

happens when the tip moves past the feature. A topographical map of the sample can then be

recorded.

2.5.4 Image Analysis

A program designed by Yves Heckel, an undergraduate student from Paris, France,

allowed us to define the characteristics of the aggregates observed in the AFM images.

Parameters such as the number of domains as well as the size of those domains were determined

in order to better understand the aggregation behavior of the copolymers. This program allows a

domain size range to be chosen in which the values of the minimum and the maximum can be

varied (Figure 2-12).

The image on the left of the screen allows the user to adapt the area range limits using a

visual aid. Another attribute of this program, is that it counts the number of domains present in

one chain as opposed to considering the chain as a single domain. If such a mistake is made,

however, the program can be manually manipulated by the user to define domain separation and

number. While this feature gives a better approximation of the shape and number of domains, the

resulting disadvantage is lower user efficiency, but significantly higher accuracy.

Once all the domains are counted, the computer gives a computed image representing all

the domains present with the different chaining and angles for each domain (Figure 2-13).

Computer errors can occur, giving wrong angles and poorly defined aggregates, in which case

the user can, by clicking on the domain, redo the separations and redefine the domain (Figure 2-

14). The software allows domain populations to be chosen and analysis error to be manually

corrected, permitting the analysis of images with more than one domain population, as in the

case of the images observed for the blends.










Additional image analysis was performed with software from Nanoscope instrument.

Notably, this software allowed calculation of domain size and domain height. For this reason, the

calibrations described above are very important in order to obtain absolute values. Precise height

values can be obtained by cross-section analysis. This is done by drawing a line across the

domains of interest, giving a cross-sectional trace of the topography an example is given in

Figure 3-13.

2.6 Brewster Angle Microscopy

The AFM necessitates, as seen previously, appropriate modification (notably transfer to a

solid substrate) in order to study monolayer films, therefore there is an uncertainty of whether the

system under study is in its original state or not. The Brewster angle microscope is a non-

invasive technique that allows the characterization of floating monolayers at the air-water

interface. Ultra thin films on air-water interface or on dielectric substrates can therefore be

directly observed.

For a beam of p-polarized light, there is an angle of incidence 6 at which no reflection occurs.

This is called the "Brewster angle" and is denoted by equation 2-11 where n is the refractive

index of the corresponding phase.


tan B = IlSubphase 2-11


Introducing a thin film in between the two phases changes the optical properties of the

system so that a small amount of the incident intensity is reflected (Figure 2-15).

The principle behind the Brewster Angle Microscope (BAM) makes use of the zero

reflectance of an air-water interface for vertically linearly polarized light at the Brewster Angle

of incidence. As stated in equation 2-11, the Brewster angle is calculated from the refractive

indexes of the two substrates involved (for example, the critical angle of the air-water interface is










53). When a condensed phase of a monolayer with different refractive index is spread or

deposited on the interface of interest a measurable change in reflectivity will occur. The reflected

light can then be used to form a high contrast image of the lateral morphology of the spread or

deposited layer. For example, a monolayer spread on an air-water interface is extremely thin,

approximately 0.5 % of the wavelength of visible light. The relative effect on the electric field

reflected from a water surface is therefore very small and the monolayer is under normal

conditions quite invisible. However, if the water surface is illuminated with pure vertically

linearly polarized light at the Brewster angle before spreading the monolayer at the air-water

interface, there is no reflection from the water surface and the monolayer itself is much more

easily visualized. One of the drawbacks to this method is that due to the use of a small aperture

(the entire interface is not scanned) the lateral resolution is low compare to the AFM: a typical

resolution is only about 2Clm.

The BAM consists of an opto-mechanical, an electronic control unit and a personal

computer (Figure 2-16). The Z lift of the BAM is a linear stage with around 30mm to travel. To

avoid any damage, the vertical distance between the sample and the obj ective should be such that

there is some space left if the Z lift is in its lowest position.

A crystal polarizer allows to produce p-polarized laser beam. To obtain optimal results,

both the angle of incidence and the state of polarization of the incident beam must be set to their

optimal values (Brewster angle and p-polarization).

The BAM also contains a scanner that consists of an obj ective and a mechanical unit. The

obj ective has typically a nominal 10x magnification that images the sample surface onto the

CCD chip and provides a diffraction limited resolution of approx. 2Clm. For optically anisotropic









materials, the reflected light shows both s- and p-polarized components. This anisotropy can be

detected by the analyzer in front of the CCD camera.

All our BAM experiments were performed using a Nanofilm Technologie GmbH

(Goettingen, Germany) BAM2plus system. A polarized Nd:YAG laser (532 nm, 50 mW) was

used with a CCD camera (572 x 768 pixels). The instrument is equipped with a scanner that

allows an obj ective of nominal magnification of 10x or 20x to be moved along the optical axis,

producing a series of small focused image. X-y scanning and subsequent image reconstruction

allows a larger focused image to be acquired. For the 10x obj ective a laser power of 50% and

maximum gain is used. A shutter timing (ST) of 1/50 s, 1/120 s or 1/1000 s is used to obtain

maximum contrast between the monolayer and the COM crystals. For the 20x obj ective a laser

power of 80%, maximum gain and ST of 1/50 s are always used. The incident beam is set at the

Brewster angle in order to obtain minimum signal before spreading the monolayer. A piece of

black glass is placed at the bottom of the trough to absorb the refracted light beam that would

otherwise cause stray light. The polarizer and analyzer are set at 00 for all experiments. The laser

and camera are mounted on an x-y stage that allows examination of the monolayer at different

regions. The reflected light is recorded into the CCD camera. Different systems can be studied

using BAM. Isotropic (same reflection in all directions) or anisotropic (reflection in a

preferential direction) monolayers can be observed. For typical anisotropic thin film systems

such as crystalline monolayers, it is possible to invert the contrast between domains by rotation

of the analyzer in the range of 700-1 100








































t


Figure 2-1. The original Langmuir balance as designed by I. Langmuir76
Dipper Electrobalance


JTroug


t Heat Exchanger


I


Figure 2-2. Set up of a typical Langmuir trough



10I


Figure 2-3. Schematic of the Wilhelmy plate


SubsrateWilhelmy Plate
Barrier Barrier

















(mN/m)I
L1
L1-G




Mean Molecular Area (nm2/mO OCU 6)

Figure 2-4. Schematic Surface pressure-MMA isotherm

















MMrA (mrImolecule)
Figure 2-5. (a) Several representative isotherms are shown, depicting the dependence of surface
pressure (TT) on mean molecular area.












Sulbstrate


0- -D


-0 0-


2-Type


Figure 2-6 Different types of deposited LB films (borrowed from Jennifer Logan' s dissertation48)

$lSatrcat Laser Diode


Lr8
orp~:


Spiri Ptm~ii~ciddre De


Clhartlbr Tap
Figure 2-7. Optical system that detects cantilever deflection (Figure adapted from Digital
Instruments' Training Notebookl03


Fematnk LOap Maimartf i
CPorLnial Oncilulainat Anpl.Lude NlanoSSOPe IIlo
Controller
Elmetronics




2 Sunnner I



Detctor






~mpemple



Figure 2-8. An atomic force microscope (AFM) (left) and the tapping mode electronic set-up~
(right) are shown (Figure adapted from Digital Instruments Training Notebookl0


X-Type


Y-Type














X X Xiu

Figure 2-9. AFM scanner tube containing the piezoelectric material and metal electrode. The x,
y, and z-directional components of the scanner are also indicated (Figure adapted
from Digital Instruments' Training Notebookl03)


B


Figure 2-10. Dependance of the piezoelectric ceramic on voltage (Figure adapted from Digital
Instruments' Training Notebook.103)


Voltage














~/i~;Y??i~,
~OC~t


C9L


Ic)


Figure 2-11. Imaging limitations in tip sharpness: Part (a) shows the dimple that results from two
rigid spheres while (b) illustrates the imaging of two spikes. Part (c) resembles
polymers in depicting two soft spheres that undergo slight deformation. (Figure
adapted from Sheiko.lo01





s easelsewen .) I~mits


Domains selected
b~y the chosen area
range


Figure 2-12. The software allows choosing a domain range by varying the minimum and
maximum areas


Figure 2-13. The software gives you a computed image representing the different domains and
the possible angles between domains in the presence of chaining


O~gB~e~ ~h~eg~


e~rlpv~8b ~B~ge












( Omar~ vOperann :



I rl- 1r l I- **** I

Figure ~ ~ ~ ~ ~ ~ ~ EIOIDP~ 2-4 ro aeb tecmue a ecorce yteue


6


Mr


~


Figure 2-15. Scheme of the Brewster angle at the air-water interface and in presence of a thin
film


Subphase











asrpolarizer BgIg

shuftter




h~amera



r~laserr scanne goni


Figure 2-16. The different components of the BAM2 are shown (Borrowed from the Nanofilm
Surface Analysis website)









CHAPTER 3
BLENDS OF A POLYSTYRENE-BLOCK-POLY(ETHYLENE OXIDE) COPOLYMER AND
ITS CORRESPONDING HOMOPOLYMERS

3.1 Isotherm Experiment

3.1.1 PEO Homopolymer

As previously described 46, PEO homopolymers of sufficiently high molecular weight form

thermodynamically stable Langmuir films at low surface pressures. The isotherm of a PEO

homopolymer (Mn = 100,000g/mol) is given in Figure 3-1. Upon compression, the monolayers

collapse, leading to the irreversible dissolution of the PEO chains into the water subphase. The

collapse pressure value is highly molecular weight dependentl0 and reaches a maximum ~

10mN/m for high molecular weight PEO.

3.1.2 Linear Polystyrene-block-Poly(ethylene oxide) (PS-b-PEO) Diblock Copolymer

Linear PS-b-PEO copolymers represent a convenient choice when studying the interfacial

behavior of amphiphilic compounds, due to the biocompatibility of the PEO block and the low

cost and availability of the PS block. Moreover, they have been widely studied and found to form

stable, condensed surface films.32-44

The n-A isotherm for a 32,500 g/mol copolymer (see Table 3.1.) displays a pseudoplateau

between 8 and 10mN/m and is shown in Figure 3-2. The observed pseudoplateau results from the

hydrophilic part of the copolymer and appears over the same pressure range as the collapse

pressure of a PEO homopolymer.45

The shape of the isotherm is independent of the copolymer solution concentration and the

compression speed. In addition, multiple runs confirmed these experiments to be reproducible

within +1.0nm2. Within the isotherm, three distinct regions are observed. At large molecular

areas, the surface film is expanded (Region I); this is usually called the "pancake" region due to

the shape the PEO units form on the water surface.









As compression continues, a plateau appears (Region II) over the pressure range of 8 to 10

mN/m. Kuzmenka and Granick46 Studied the behavior of PEO homopolymers at the air-water

interface with varying molecular weights. They observed a constant equilibrium spreading

pressure for polymers having a molecular weight beyond 100,000g.mor l. The pseudoplateau

detected in the case of our copolymer is in the same range of the collapse pressure of a PEO

homopolymer and corresponds to the hydration and desorption of these chains from the surface

and into the subphase.

The appearance of the pseudoplateau with an increasing amount of PEO in the copolymer

was considered by Devereaux and Baker43 They studied two PS-b-PEO copolymers containing

different masses of PEO. The 7% PEO copolymer had no pseudoplateau whereas the 15% PEO

did. This observation was explained by the long PS interfering with the PEO blocks, preventing

the PEO from stretching into the aqueous subphase. Our results are in agreement with this

interpretation showing a plateau for a copolymer containing 32% PEO.

Considering the affinity of PEO for water, at large molecular areas the fi1ms most likely

exist as PEO fi1ms with globules of PS on top. Region II, however, represents a biphasic phase

where aggregates and single polymer domains coexist. The fact that the pseudoplateau occurs

within the same pressure range as the collapse region of PEO homopolymer illustrates the

significant influence PEO has on the copolymer surface fi1m. Bij sterbosh et al.33 and Goncalves

da Silva et al.34,39 both demonstrated the existence of a pseudo first-order transition from a

pancake-like structure to that of a brush upon compression of a series of PS-b-PEO copolymers

containing a constant PS length and varying amounts of PEO. While this model is prevalent in

the literature, Cox et al.41,42 prOVided a new interpretation for the presence of the pseudoplateau

assuming that the formation of brushes is not possible due to PEO's low surface energy. They









proposed that PEO instead undergoes a dehydration process and a conformational change upon

compression. This observation is consistent with the one made by Shuler and Zisman45 for a PEO

homopolymer. They explained the lack of reversibility observed in their hysteresis experiments

by a structural change in the polymer molecule during the compression cycle.

Contrary to PEO homopolymers, a third region (III) appears beyond the pseudoplateau and

shows a sharp increase in surface pressure, indicating the formation of more rigid films. Here,

the PS block serves as an anchor, keeping the PEO at the interface and allowing the films to be

compressed to higher surface pressures. Without the PS, PEO would dissolve into the aqueous

subphase at pressures beyond the plateau. In examining Region III, Bij sterbosh et al. I and

Goncalves da Silva et al 34,39 Studied a series of copolymers with varying PEO lengths and a

constant PS block. While Region III typically reflects PS, they found that the copolymer

interfacial behavior at high pressures depends slightly on the size of the PEO block.

3.1.3 Blends of a PS-b-PEO diblock copolymer and a PS homopolymer

The same linear copolymer described in the previous section was used to study the effect

of adding a homopolymer solution on its behavior at the air-water interface. The PS

homopolymer used has a molecular weight of 20,000 g/mol, which corresponds to the molecular

weight of the PS block in the copolymer. Different ratios of copolymer and homopolymer were

studied in order to determine the impact on the formation of Langmuir monolayers.

The mixed monolayers were formed by separately spreading solutions of the PS and the

PS-b-PEO block copolymer. After evaporation of the solvent, the floating monolayer was

symmetrically compressed by the two movable barriers. n-A isotherms were recorded for several

amount of added PS homopolymer (Table 3-2).









Figure 3-3 shows the isotherms data for different copolymer/homopolymer ratios. For all

these blends as well as the pure linear copolymer isotherms, the three regions defined in the

previous section were observed. This can be better detected on the compressibility plot where

two different local maxima are observed (Figure 3-4). Each maximum corresponds to a phase

transition and a rearrangement of the polymer chains in the monolayer. Increasing the amount of

PS has only a small effect on the phase transitions observed, the local maxima in the

compressibility plots show only a small increase when increasing the amount of PS

homopolymer. This suggests the presence of PEO-related phase transitions independent of the

PS content.

3.1.3.1 Pancake Region (I)

The first region, defined by low surface pressure and low surface density, can be quantified

by its extrapolated area, AP (Figure 2-5). The values for every blend remain constant while

varying the amount of PS (average AP = 89.7 nm2). This is in agreement with a film of PEO with

globules of PS on top of it where increasing the amount of PS will not change the area occupied

at the interface by the PEO.

Faure et al 40 Observed the same behavior for pure diblock copolymers at the air-water

interface and showed that at low coverage, the interaction between the ethylene oxide (EO)

monomers and the interface is attractive and therefore leads to the adsorption of the EO at the

air-water interface. They assume the pressure to be only due to the total number of PEO

segments in water. As a result, increasing the PS should have no effect on the behavior of this

region. Logan,48 Logan et al.,51 and Francis et al.,49,50 Observed a similar trend for star

copolymers of PS-b-PEO in which the pancake area did not depend on the number of PS

segments. The pancake area per EO monomer (0.38 nm2) is in reasonable agreement to the one

found by Logan et al. for star copolymers (0.33 nm2) and to the that determined for linear PS-b-









PEO by Gongalves da Silva et at. (0.27 or 0.3 1 nm2 34,39 and Bij sterbosch et at.I (0.3 1 nm2). The

area observed in our case is a little higher; the addition of PS homopolymer increases the

aggregation of the PS segments allowing the PEO blocks to spread more easily.

3.1.3.2 Pseudoplateau Region (II)

The pseudoplateau observed in Section 3-2 for a pure linear PS-b-PEO copolymer is

observed for all blends and remains constant for different amount of PS. The width (AAP) Of the

pseudoplateau can be estimated as the difference between ATransition 1 and ATransition 2 (Table 3-3,

Figure 2-5). The value of AAP TemainS constant for every blend (average AAP = 21.9 nm112.

Region II is believed to represent a biphasic region. The phase transition is mostly due to the

reorganization of the PEO chains from a pancake to a brush conformation and therefore a change

in the amount of PS does not have any effect on the width of the pseudoplateau. In this region,

the EO repeat unit occupies 9.2 A+2 which is smaller than the value found by Logan et al 48,51

(13.3 A+2) for star copolymers. This difference can be explained by the fact that the PEO

homopolymer is being dissolved in the subphase as the monolayer is compressed.

3.1.3.3 Condensed Region (III)

In Table 3-2, the theoretical area (Ao) that a compact surface film would occupy at zero

pressure was determined for each blend. In agreement with our expectations, the area increases

with an increasing mass of PS. The condensed area, representing mostly the behavior of the PS

chains, varies linearly with the total mass of PS chains. This behavior can be compared to the

behavior of copolymers presenting PS chains of high molecular weights. Cox et al.41,42 Studied

several PS-b-PEO copolymers with varying PS molecular weights, observing a variation in the

Ao values. The increase of Ao with increasing PS can be explained by the aggregation of the PS

homopolymer with the PS chains of the copolymer.










To compare our results with those from copolymers of longer PS blocks, a normalization

of the total number of styrene units was obtained by using the following equation.

nPS,Homopolymer
NPS,TOT PS,Dlblock PS,Homopolymer -
nPS,Dlblocks

With NPS,Diblock and NPS, Homopolymer being the number of styrene repeat units in the PS-b-

PEO diblock copolymer and the PS homopolymer respectively. nPS, Hompolymer and nPS,Diblocks

represent the number of moles of PS in the homopolymer and the copolymer. When plotting the

condensed area per EO unit versus the apparent number of styrene units (Figure 3-5), a linear

dependence can be observed (R2 = 0.9932) with a trendline of y = 0.0001 x + 0.0753.

The positive y-intercept shows us that even without any PS present in the monolayer, the

PEO occupies 7.5 A+2/EO units. This value is significantly smaller than the one observed for star

copolymers by Logan et al. (16 A+2/EO). 48,51 When PEO homopolymers are compressed at the

air-water interface, no condensed area is observed. Without the presence of PS as an anchor, the

PEO eventually dissolves in the aqueous subphase. In our case, while most of the PEO

homopolymer remains aggregated to the PEO blocks of the copolymer, some of it could be

dissolving in the water, explaining the smaller occupied area.

The collapse area plotted vs. the total number of styrene units follows the trendline y =

0.0288 x + 17.914. The area per styrene unit obtained from the slope (2.9 A2) is Smaller than the

one J. Logan described for the behavior of star copolymers of PS-b-PEO at the air-water

interface (ranging from 6.2-8.3A+2, depending on architecture).5 She reports the results for

(PEO26)8-(PS42)8 (an 8 arm PS-b-PEO star copolymer with each arm containing 26 EO units and

42 Styrene units) which has a total of 336 repeat units of PS. For Blend 4 which has a total

number of PS repeat units of 3 55, the PS homopolymer occupies an area equal to 5nm2. This

value is in the same range as the area per styrene value found in the literature for an atactic PS in









the bulk, calculated from the radius of gyration (3 8A).59 The PS, not covalently bound to the

PEO, tends to adopt a random coil conformation less compact than the conformation produced

by the PS segments of the copolymers. This copolymer can be compared to Blend 4 which

presents a total of repeat units of 3 55 for the PS (using the formula described previously). J.

Logan obtained a value of 28.6 nm2 which is similar to the value obtained for Blend 4.

3.1.4 Blends of a PS-b-PEO diblock copolymer and PEO homopolymer

Contrary to PS, PEO is an amphiphilic polymer forming monolayers at the air-water

interface. The addition of a PS block as an anchor keeps the PEO from going into the water

subphase. This also allows the formation of more compact films by compressing at higher

pressures. The effect that unencumbered PEO has on such films is examined in this part of the

discussion where blends of the copolymer and homopolymer PEO were studied (Table 3-4).

Figure 3-7 shows the isotherm data for several blends of the 32,500g.moll PS-b-PEO

copolymer and a 100,000 g.moll PEO homopolymer. The same transitions can be observed for

all the copolymers independent on the amount of PEO added. When looking at the

compressibility plots in Figure 3-8, one can see that when increasing PEO the local maximum

present at 10mN/m increases. This suggests a PEO-related transition at 10mN/m.

3.1.4.1 Pancake Region (I)

Similar analysis was done for the homopolymer PEO blends as that seen for the PS ones.

As with the pure copolymer, the resulting isotherms displayed all three regions. AP WAS obtained

for each blend and for the pure PS-b-PEO from the n-A isotherms (Table 3-5).

The pancake area depends linearly on the total number of ethylene oxide units (R2

0.9968) with a trendline of y = 1.1297 x 173.95. The area obtained from the slope (1.13 nm2) is

significantly higher than the one observed by Sauer et al. x for a PEO homopolymer (0.40-0.48

nm2). Gongalves da Silva et al 34,39 TOCOrded a smaller area for PS-b-PEO diblock copolymers










(0.27 and 0.31 nm2). This can be explained by the fact that the PEO chains from the

homopolymer pack less closely when in the presence of the PS-b-PEO diblock copolymer.

We can also observe a negative y-intercept indicating that all the EO units are not at the

interface. In such a case, a pancake area equal to zero should correspond to zero EO units. Even

with a negligible effect on the pancake area, the PS units may trap some of the PEO, leading to a

lower apparent number of PEO units.

3.1.4.2 Pseudoplateau Region (II)

The addition of PEO, however, has an effect on the shape of the pseudoplateau observed

for a pressure around 10mN/m. The more PEO is added to the monolayer, the longer the biphasic

region becomes. To illustrate this point, ATransition 2 (Figure 2-5) was recorded for each blend as

well as for the pure diblock. The results are given in Table 3-6.

A graph of ATransition 2 VS. the ratio of number of moles of homopolymer vs the number of

moles of diblock shows a linear dependence (R2 = 0.994) with a trendline of y = 1082.9 x +

47.532 (Figure 3-9). To be able to compare those results to those published previously for pure

diblocks or star copolymers, an identical formula as the one used in the previous part was

developed. While ATransition 1 TemainS constant for every blend, one can observe an increase in

ATransition 2 indicating the presence of a larger pseudoplateau area.


=N +N nPEO,Homopolymer -
NPEO,TOT PEO,Dlblock PEO,Homopolymer -
PEO,Dlblock

The values calculated using this formula, are reported in Table 3-7.

A linear dependence (R2 = 0.9942) was observed when the area for the second transition

was plotted vs. the total number of repeat units of PEO, yielding a trendline of y = 0.4769 x -

66.032 (Figure 3-10). This dependence is detected in the compressibility plots also showing a

higher local maximum when increasing the amount of PEO (Figure 3-8).









These observations compare to those seen by Faure et al 40 They studied the phase

transitions in monolayers of PS-b-PEO copolymer at the air-water interface for different PEO

block sizes. Faure et al. observed an increase in the length of the pseudoplateau as the number of

PEO units increases. The transition from pancake to brush becomes more and more first order as

they increase the PEO segment size.

In addition, one of the diblock copolymers they studied consisted of 3 1 repeat units of PS

and 700 of PEO, a PEO amount similar to that of Blend 4. The Faure copolymer demonstrates a

n-A isotherm with an almost flat pseudoplateau, confirming the first order transition of the

copolymer. Similarly, Blend 4 displays a pseudoplateau representing a strong indication of a first

order transition. By adding PEO homopolymer to our monolayer, we have been able to broaden

the range of the copolymer properties without going trough time-consuming synthetic techniques

in order to increase the size of the PEO block.

3.1.4.3 Condensed Region (III)

This third region appears at higher surface pressures beyond the pseudoplateau. As

demonstrated by Shuler and Zisman,45 Such a region does not exist for a PEO homopolymer, as

no anchor exists to prevent PEO from completely immersing in the water subphase. This region

depends only on the length of the PS blocks and not on PEO, as demonstrated by the n-A

isotherms of the different blends in Figure 3-7. Ao remains the same regardless of PEO added

(23.7 nm2)

By blending a PS-b-PEO diblock copolymer with its corresponding homopolymers, we

were able to mimic linear chain behavior by manipulating PS and PEO quantities. On one hand,

the addition of PS has proven to have the same effect on the copolymer behavior as increasing

the PS block size. We see an increase in the condensed area while adding more PS homopolymer









to the system. On the other hand, raising the amount of PEO only had an effect on the biphasic

region of the isotherm.

While this technique could be a good alternative to time-consuming synthetic techniques

and expensive sample purchases, experiments still need to be performed with various molecular

weight homopolymers as well as hysteresis data in order to better understand the aggregation

behavior of those films.

Additional analysis continues in the next section in which the blends are transferred as

Langmuir-Blodgett films and examined through atomic force microscopy (AFM).

3.2 Atomic Force Microscopy (AFM) Experiments

AFM is a technique that provides the opportunity to study surface morphology and

structure at the submicron scale. By investigating transferred Langmuir-Blodgett films, AFM can

give insight into the behavior of the copolymer blends at various pressures, providing both

quantitative and qualitative results. Such data helps demonstrate the degree of interaction

between the copolymer and homopolymers. The hydrophilicity of the substrate allows us to

consider the hydrophilic PEO to be attached to the mica whereas the hydrophobic PS occupies a

higher layer. By consequence in the AFM images, the PEO is represented by the darker (lower)

areas whereas the PS exists as the brighter (higher) domains.

The AFM software contains several functions for image analysis. One method represents

the three-dimensional surface plot of the imaged sample, as shown in Figure 3-12. The color

shading is a representation of the height of the features on the sample (up to 7.3nm for Figure 3-

12, for example).

Precise height data can be obtained for given domains through section analysis. This

technique is illustrated in Figure 3-13. A line is traced across the domain region of interest,

giving a cross-sectional view of the sample. In this example, the height difference between the









two marked domains is 1.2nm and the difference between the domain at the left and the PEO

surface (brown) is 4.5nm.

3.2.1 Linear Polystyrene-block-Poly(ethylene oxide) (PS-b-PEO) Diblock Copolymer

AFM is a valuable technique for studying morphologies formed by spreading copolymer

solutions at an aqueous subphase. Bodiguel et al. demonstrated the complementary nature of

AFM and TEM in depicting phase separation of two distinct polymer blocks.96 The technique

assumes that the morphology of the transferred film represents that of the floating monolayer and

that transfer is homogeneous. For pressures below the pseudoplateau pressure, transfer ratio (TR)

were found to be close to unity (~1.10) implying a homogeneous transfer. However, once the

pseudoplateau pressure is reached, a decrease in TR is observed (~0.40) suggesting that the films

observed by AFM are less condensed than the floating monolayer. In the case of a TR equal to 1,

the image observed for a pressure of 15mN/m should show a homogeneous layer of PS.

However, we observed large domains surrounded by areas of PEO. The TR of 0.40 leads us to

the conclusion that the monolayer expanded after transfer onto the mica substrate and does not

represent the structure we would observe directly onto the water subphase. Langmuir-Blodgett

(LB) films were prepared at several surface pressures and then studied using AFM in tapping

mode. For each sample, an average of ten images was taken to ensure reproducibility.

The images shown in Figure 3-14 clearly demonstrate the formation of ordered structures

in which the observed morphology depends on surface pressure. In fact, three distinct regions

corresponding to those in the isotherm can be seen once again. For pressures of 4 and 7 mN/m

(Region I of Figure 3 -2.), images show a maj ority of single domains, typical of an expanded

liquid. Two-dimensional micelles form at the air-water interface with a morphology depending

on the ratio of the hydrophobic and hydrophilic block sizes. For pressures under 7mN/m, circular










micelles are observed like the one described by Potemkin et al. os (Figure 3-15) where one of the

blocks is strongly adsorbed on a planar surface.

In the case of PS-b-PEO at the air-water interface, similar micelles are observed with the

PEO extending more and more in the aqueous subphase as the concentration increases (Figure 3-

16).

When compression continues and reaches the pseudoplateau range (Region II of Figure 3 -

2), chain formation is detected and continues until the collapse pressure is attained. The images

also demonstrate the presence of intermediate stages in which the single domains begin to

aggregate prior to chain formation.

Due to the hydrophilicity of the mica, we suppose PEO represents the bottom layer

whereas PS occupies the top part of the LB film at a thickness of some nanometers, ranging from

about 2 to 10 depending on the blend. In our images, the darker layer represents PEO and the

bright domains show the PS blocks. Using a program described above, the number of domains

per image was found, allowing the molecules per domain (or aggregation number) to be

calculated.

For each given pressure, the aggregation number was determined using equation 3-3.

r = A/Nd.o 3-3

where r refers to the number of molecules per domain, A the scanned area of the image,

Nd the number of domains, and a the mean molecular area during transfer. As shown in Figure

3-17, the number of molecules/domain depends strongly on the surface pressure. For pressures

less than 10mN/m, the number of molecules/domain remains almost constant. However, once the

pressure of the pseudoplateau is reached, an increase in aggregation number is observed.









As compression continues, aggregation increases and at the transition between Regions II

and III, the aggregation number rises sharply. This behavior is another indication of the transition

between the liquid expanded state and the liquid condensed state. Logan et al.48,51 Showed that

compression-induced aggregation occurs when PEO is pushed into the aqueous subphase.

However, at higher pressures, some PEO can remain at the interface and separate the PS

domains. This situation represents two conflicting forces. The attraction between PEO and the

water allows the polymer to spread on the surface, whereas the repulsion of PS with both water

and PEO drives aggregation. Cox et al.41,42, however, thought the relative interaction of the two

blocks with the subphase and air is a more probable explanation for the existence of aggregation.

3.2.2 Blends of a PS-b-PEO diblock copolymer and a PS homopolymer

To observe the possible formation of aggregates between copolymer and homopolymer,

the blends were studied by AFM for different transfer pressures. Transfers for pressures equal or

below 10mN/m are considered homogeneous (TR~1.04) while transfers made for pressure above

the pseudoplateau pressure (TR~0.40) indicate that the aggregates observed by AFM are less

packed than those present at the air-water interface. In Figure 3-18, the AFM images for the pure

diblock, Blend 2, and Blend 5 at several transfer pressures are given. As described previously, a

chaining of the domains is observed for the pure PS-b-PEO when increasing the pressure.

However, in the blend case, the addition of PS homopolymer seems to inhibit the formation of

these chains. In Figure 3-19, the histograms of the domain area are given for both films at a

transfer pressure of 10 mN/m. The pure diblock exhibits larger domain areas whereas the blend

seems to exhibit the formation of big and small domains.

In the pancake region, the pure diblock exhibits local hexagonal packing with six

neighbors for each domain, showing that even at low pressure, the copolymers arrange into









surface micelles. In the case of the blends, the addition of PS to the monolayer disrupts this

packing by increasing the size of only some of the domains and enabling a population of smaller

domains to form (Figure 3-19). The PS homopolymer aggregates with some of the PS blocks

from the copolymer increasing the size of the domains observed by AFM. The addition of PS in

the domains increases their attractive forces on the free PS.

This behavior can be compared to that described by Logan,48 Logan et al.,51 and Francis et

al.,49,50 for PS-b-PEO star copolymers of various hydrophobicity. For both stars and linear chains

in the literature (particularly Devereaux and Baker43), increased PS results in nonuniform films

with a greater variety of morphology. As pressure increases, no chaining is observed in the case

of the blend. In fact instead of chaining, an augmentation in population of the bigger domains

compared to the small domains can be observed. This phenomenon can be shown by using the

computer program designed by Yves Heckel. Histograms of the domain areas are given in Figure

3-20.

In the pure diblock, formation of large chains that resemble pearl necklace-like strings

occurs as the transfer pressure is increased. This phenomenon starts with the formation of

domain dimerss" or "trimers" at a pressure of about 7mN/m that then keep on chaining with

increased pressure. When PS is added to the monolayer, no such domains are observed. An

increase in the size of the circular domains is observed, indicating the aggregation of the PS

homopolymer within the PS chains of the copolymer (Figure 3-21).

3.2.3 Blends of a PS-b-PEO diblock copolymer and a PEO homopolymer

In a similar way, blends of the copolymer and a PEO homopolymer were transferred

homogeneously onto a mica substrate (TR~1.07) in order to study the evolution of the

morphologies depending on the amount of PEO added. Results are shown in Figure 3-22.









At low pressure, we can observe the disappearance of the hexagonal packing when

increasing the amount of PEO. This behavior can be compared to that of PS-b-PEO star

copolymers studied by Logan,48 and Francis et al.,49,50 By increasing the hydrophilicity of the

stars, they observed a decrease in the number of domains and an increase in the distance between

each domain. While less uniform, this same effect appears in the blends as a result of PEO

homopolymer aggregation to the PEO chains of the diblock copolymer. To remove the artifacts

than could have been formed by spreading successively the pure copolymer and the

homopolymer, a mixed solution was made and this was spread as a comparison. The proportions

were the same as the one used for Blend 2 and the film was transferred at a pressure of 9mN/m.

The AFM images are shown in Figure 3-23.

Those experiments show that independent of the spreading technique, an increase in PEO

composition results in the formation of longer chains than for the pure copolymer sample. A

second effect of increased PEO content is the apparition of a phase separation between pure

layers of PEO surrounding clusters of micelles.

In addition, the addition of a copolymer to a PEO homopolymer monolayer increases its

stability and allows the formation of films at higher pressures than that of a pure PEO monolayer

which collapses at 10mN/m.45 The difference between the morphologies at high pressure for the

pure diblock and for the blends shows that the PEO homopolymer aggregates with the copolymer

instead of dissolving into the aqueous subphase.

3.3 Conclusions

When adding the homopolymers to the pure diblock copolymer at the air-water interface,

reproducible isotherms were obtained and displayed the three regions present in a pure diblock

copolymer (pancake, pseudoplateau, and brush). While the increase in the PS amount had an

effect only on the condensed area (varies linearly with the amount of PS added), by combining









PEO and the copolymer, no change was observed in the condensed region of the isotherms. The

length of the pseudoplateau representing the biphasic region increases as the amount of PEO gets

more important. This behavior has also been observed for pure diblocks as well as for star

copolymers when increasing the size of the PEO block.48,51 AFM images were taken and were

consistent with the isotherms showing the three regions described previously. On one hand, the

addition of PS to the copolymer monolayer inhibited the chaining of the copolymer domains and

enhanced the hydrophobic properties of the Langmuir-Blodgett film. On the other hand,

combining the PS-b-PEO diblock copolymer with the PEO homopolymer also had an effect on

the film morphology, increasing the chaining of the domains as well as favoring the phase

separation between clusters of micelles and pure layer of PEO.











20

18


E
Z
E


3



a>
o
ca
't
3
V)


10 I IIIIII
0 200 400 600 800 1000 1200 1400 1600

Mean Molecular Area (nm2/mOleCUle)




Figure 3-1. Surface pressure-MMA isotherm for the PEO homopolymer (Mn = 100,000g/mol)
recorded for a compression speed of 5mm/min









70-

E 60-

E 50-

S40-



8 20-

310-


O 20 40 60 80 100 120 140

Mean Molecular Area (nm2/mOleCUle)


Figure 3-2. Surface pressure-MMA isotherm for the 32,500 g-molf PS-b-PEO copolymer
recorded for a compression speed of 5mm/min









70


e ruP Di bl ock (68%)


60-
---Blend 1 (70%)
E ~ Blend 2 (72%)
z 50-Bln3(7%
Blend 4 (79%)
a>40 -Blend 5 (81%)
en~ Blend 6 (84%)
8 30 --C Blend 7 (86%)

o1 20-

a 10-


O 20 40 60 80 100 120 140

Mean molecular area (nm2/mOleCUle)


Figure 3-3. Several surface pressure-MMA isotherms are shown, indicating the dependence of
surface pressure on the mean molecular area for different blend ratios. All the
isotherm experiments were performed using a compression speed of 5mm/m








































Sy = 0.0001x + 0.0753
-R2 = 0.9932


0.6


0.5


0.4


0.3


0.2


0.1


SPure PS-b-PEO
&- Blend 1


5 10


n (m N/m)



Figure 3-4. Monolayer compressibility plot for the PS-b-PEO copolymer and blend 1


0.16
0.15
0.14
0.13
0.12
0.11
0.1
0.09
0.08


"E
c
O
LL
St


200


400


600


800


Total number of styrene units


Figure 3-5. The condensed area per EO unit (Ao/EO) varies linearly with the number of styrene
units present in the blends











40

35

"E30

a 25
y = 0.0288x + 17.914
20 R2 = 0.9932

15
0 200 400 600 800

Total number of styrene units

Figure 3-6. The condensed area (Ao) varies linearly with the total number of styrene units present
in the blends









40


E 30-1 Mass % of PEO
z -~ 32%
S92%
II 1 96%
S20-
99%

o10-




0 200 400 600 800 1000 1200 1400

MMA (nm2/mOleCU 6)


Figure 3-7. Several surface pressure-MMA isotherms are shown, showing the dependence of
surface pressure on the mean molecular area for different amounts of PEO. All the
isotherm experiments were performed using a compression speed of 5mm/min











































1y = 1.12297x 173.95


1.4-

1.2- Pure diblock
1.0-I Blend 1
Z
E 0.8-




0.2-

0.0
8 10 12
TI(mN/m)

Figure 3-8. Monolayer compressibility plots for the PS-b-PEO diblock copolymer and blend 1


a


Q


e


800
700
600
500
400
300

20


200


400


600


800


Total number of EO units

Figure 3-9. The pancake area (AP) VarieS linearly with the total number of EO units present in the
blends





I R2 = 0.9942


0 0.05 0.1 0.15 0.2 0.25

moles of PEO in the homopolymer/moles of PEO
in the diblock copolymer


Figure 3-10. The area for the second transition depends linearly on the mole ratio of PEO from
the homopolymer over PEO from the PS-b-PEO


300 -
2 -1

2 -1


150

100
-0


y = 0.4769x 66.032


Total number of EO repeat units


Figure 3-11. The area for the second transition depends linearly on the apparent total number of
EO repeat units










Pigital Instruments NanoEScpr




Xm~iep,, view ngl




7.293nmI







pspaps4-iq1.000


Figure 3-12. Example of a sample height image and surface plot (scan area shown is 2x2 Clm)


'7 0


1.00


2.00


Figure 3-13. Example of a sample section image (2x2 Clm)















80







































Figure 3-14. Height AFM images from tapping mode of the pure PS-b-PEO for several transfer
pressures (scale 2x2Clm). The transfer ratios are also given for each images.


Figure 3-15. Schematic representation of surface micelles formed by A-b-B diblock copolymers
with A strongly adsorbed to the surface (adapted from Potemkin et at los)





1600
1400
1200
1000
800
600
400
200
0


10
Surface Pressure (mN/m)


Figure 3-17. Dependence of the number of molecules per domain on pressure


"C~tLj* IPEO

CDnoe~ntratiDn
lucreases








Figure 3-16. Model of PS-b-PEO absorbing at the air-water interface (Adapted from Dewhurst et
at 37)








PUTRE BT END 2


BT END 5


H = 4mN/m i Ho,~~PBBI- $~~BBII






H = 7mN/m g:


II



S= 10mN/m




Amount of PS


Figure 3-18. Height AFM images from tapping mode of the pure PS-b-PEO diblock copolymer
as well as two of the blends for several transfer pressures (scale 2x2Clm)





























10 20o 30 40 50 60O 70 ~ 80 90 100

Domain Area (nm2)


200


150
B
r
0 100
O

50


O


Domain Area (nm2)


Figure 3-19. Height AFM images (scale 2x2Clm) for the pure PS-b-PEO diblock copolymer and
Blend 2 (transfer pressure of 4mN/m) as well as the distribution of the domain areas





















o
40

20


0 50 100 150 200 250 300 350

140. -, Domain Area nm24

1 20

100-

S 80-



40

20-


0 50 100 150 200 250 300 350

Domain Area (nm2)



Figure 3-20. Computed images for the pure PS-b-PEO diblock copolymer and Blend 2 (transfer
pressure of 10mN/m) as well as the distribution of the domain areas




























Figure 3-21. Magnification of a single domain formed for Blend 2 for a transfer pressure of
10mN/m (scale 150x150nm)





Pure Blend 1 Blend 2 Blend 3 Blend 4



II = 4mN/m







II = 9mN/m




Amount of PEO


Figure 3-22. Height AFM images from tapping mode of the pure PS-b-PEO diblock copolymer
and several blends for transfer pressures of 4 and 9 mN/m (scale 2x2Clm). The height
scale remains constant for the blends.






















Figure 3-23. Height AFM images from tapping mode for Blend 2; (a) from successive spreading,
and (b) from the mixed solution (scale 2x2Clm)












Table 3-1. Characteristics of the PS-b-PEO sample investigated


MW
(g/mol)


PEO
wt%


PS
wt%


Polydispersity M1WPEO 1Ps


NPEO


32,500 32 68


1.05


10,500 22,000 238


Table 3-2. The mass ratio of PS between the diblock copolymer and the homopolymer as well as
the apparent number of styrene units have been calculated for each blend.


Blend #


1 2 3 4 5 6 7


70.2 72.3 75.8 78.6 80.7 84.0 86.3


Mass % of PS


Mole ratio of PS
(homopolymer/copolymer)


0.138 0.275 0.551


0.826 1.102 1.653 2.204


NPS,TOT 236 259 307 355 403 499 595



Table 3-3. Width of the pseudoplateau for each blend


Blend #


Pure


2 3 4


5 6


AAP (Hm12)


22.8 23.0 22.2 21.1 22.3 22.5 20.5 21.1


Table 3-4. The mass ratio of PEO between the diblock copolymer and the homopolymer as well
as the apparent number of styrene units have been calculated for each blend.


Blend #


Mass % of PEO


92.4

0.025


95.9

0.05

352


97.9

0.101

467


98.9

0.202

697


Mole ratio of PEO
(homopolymer/copolymer)

NPEO,TOT


295


















Table 3-6. Area for the second transition (described in Figure 2.5) extrapolated for each blend


Table 3-5. Pancake areas extrapolated from the xn-A isotherms


Blend #

AP HM112)


Pure

86


352


610


Blend #

ATransition 2 HM112)


Pure

46


268


Table 3-7. Molar ratio of PEO from the homopolymer and the diblock copolymer as well as the
total number of EO units is given for each blend.
Blend # Pure 1 2 3 4

nHomo DBlock 0 0.025 0.05 0.101 0.202

NPEO,TOT 238 295 352 467 697









CHAPTER 4
INTERF ACIAL BEHAVIOR OF S TAR- SHAPED P OLY STYRENE-BLOCK-POLY(TERT -
BUTYLACRYLATE) COPOLYMERS

4.1 Introduction

A system that has been of particular interest in our laboratories is one that consists of a

hydrophilic block as well as a hydrophobic block. These types of copolymers are particularly

interesting due to their ability to self-assemble when spread onto a water subphase. Many

studies, including the previous chapter, have focused on polystyrene-b-poly(ethylene oxide) (PS-

b-PEO) 32-44 COpolymers due to the availability of PS and the biocompatibility of PEO, but very

few have involved copolymers containing polyacrylates.

Homopolymers such has polyesterS106,107 and polyalkylesters spread easily at the air-water

interface and have been widely studied. For example, Mengel et al. investigated the formation of

multilayers of poly(tert-butylacrylate) using the LB film technique.10s The isotherms obtained

were in agreement with previous studies made on linear PtBA.109,110 A plateau was observed for

a pressure around 24mN/m indicating the presence of a transition from liquid-condensed (LC) to

solid phase (S). Transferred onto silicon wafers, those types of polymers were also found to be

very useful after modification forming multilayers of poly(acrylic acid) (PAA). The direct

formation of Langmuir films using PAA homopolymers is impossible due to their high

hydrophilicity .

Depending on the size and the composition of the chains, copolymers present different

properties such as chain density and intermolecular interactions allowing them to adopt different

geometries and form different morphologies. By varying the ratio between the hydrophobic and

hydrophilic segments of the copolymers, researchers have been able to observe different

morphologies of the films transferred from the air-water interface onto a solid substrate. Li et al.

investigated the behavior of Langmuir films of linear PS-b-PtBA."1 For mostly hydrophobic










copolymers, they observe multiple plateaus indicating the presence of more than one phase

transition. This differs from the behavior of the PtBA homopolymer which only shows one

transition corresponding to side-group reorientation towards the interface. They suggest that the

other transitions correspond to some form of backbone condensation or organization.

4.2 Results and Discussion

The PS-b-PtBA diblock star copolymers given in Figure 4-1 were synthesized via atom

transfer radical polymerization (ATRP) of styrene and tert-butylacrylate using the fifth

generation dendritic initiator shown in Figure 4-2. The synthesis procedure has been described

elsewhere.112 The characteristics of the copolymers are given in Table 4-1 as well as their

structures shown in Figure 4-2. The PtBA homopolymer was purchased from Polymer Source

Inc. Its characteristics are given in Table 4-2. Regarding the copolymer, the calculated molecular

weight values assumed the presence of 64 PS-b-PtBA branches. However, the absolute values

obtained by differential viscometry and universal calibration showed that each star is composed

of significantly less branches. The number of branches was obtained by dividing the absolute

molecular weight (shown in Table 4-2) of each copolymer by the calculated molecular weight of

one branch. Based on discussion with Dr. W. Ford and Young Hie Kim who synthesized the

samples, we assumed that each arm is composed of 48 PS repeat units. After calculation, we then

estimated that PS-b-PtBA(104) contains 37wt% PtBA; PS-b-PtBA(215), 47%; and PS-b-

PtBA(445) is composed of 58% of PtBA (Table 4-1). It is important to note that those numbers

are averages and that some of the stars may have a different number of branches as well as some

linear chains might be present in the samples.









4.2.1 PtBA Homopolymer

Figure 4-3 shows the surface pressure-mean molecular area isotherm for the homopolymer.

As described in previous investigations,1l09,110 a phase transition is observed for a pressure around

24mN/m shown by the presence of a plateau on the isotherm plot.

The PtBA homopolymer is amphiphilic, forming stable films when compressed at the air-

water interface. On the isotherm, three distinct regions can be detected: (1) all the PtBA chains

are adsorbed onto the air-water interface, (2) represents a phase transition from a two-

dimensional to a three-dimensional structure, and (3) accounts for the collapse region of the

PtBA. As shown in Figure 4-3, two distinct areas can be extrapolated from the isotherm plot. We

divided those values by the number of tBA units present in the homopolymer in order to simplify

the comparison with other systems. For low pressure, A, is equal to 30A+2/tBA. This value is in

agreement with the one given by Mudgil et at. 113 (35A+2/tBA). In region 3, the extrapolation of

the linear portion of the isotherm to xn=0mN/m yields to the theoretical area Ao. Once again, the

value observed for our PtBA homopolymer (5.4A+2/tBA) is in conformity with the one observed

in the literature (5A+2/tBA). "l

The PtBA homopolymer was transferred onto a mica substrate at three distinct pressures.

The resulting images are given in Figure 4-4. The pressures were chosen in order to investigate

the morphologies formed in the three distinct regions of the isotherm shown in Figure 4-5.

As expected, no morphology exists for a pressure of 15mN/m (Region 1). When

compressed to a pressure corresponding to the plateau pressure (24mN/m), bright domains start

to form (Region 2). Further compression leads to a film collapse observed in region (3).

A scheme of the characteristic Langmuir film structures for the different regions of the

isotherms is presented in Figure 4-6. Region (1), as confirmed by the AFM images (several










images were taken in order to prove the reproducibility of the morphologies formed), shows a

flat monolayer of PtBA completely adsorbed onto the air-water interface. Region (2)

(morphology observed at the plateau) is composed of either single domains or small chains

dimerss or trimers) of PtBA aggregates. These domains are visible due to desorption of the PtBA

from the interface as the total available area is decreased. Each aggregate contains multiple

polymer chains. In the LB film transferred for a surface pressure of 30mN/m (region 3), the

bright domains represent 25% of the total area (assuming the darker area is composed of the

adsorbed PtBA chains). If we assume that the bright domains observed in the AFM picture taken

for a 1VMVA corresponding to the middle of the plateau (Figure 4-4) represent half of the total

desorbed PtBA observed in condensed region, then the aggregation number is about 12000. It is

also important to note that the TR detected during the transfer (1.062) was also taken into

account in the calculations. Region (2) represents the transition between the state where all the

PtBA is adsorbed onto the interface and forms a homogeneous monolayer and that present in

region (3) where the PtBA forms a three-dimensional network. This last structure explains the

presence of the sharp increase in pressure and the dependence of the collapse on the polymer

molecular weight. When transferred at a pressure of 30mN/m (Region 3), the appearance of

bright domains is observed. These domains are believed to represent the PtBA that desorbs from

the interface as the monolayer is compressed. The circular domains are characteristic and

reproducible. They also tend to have a narrow distribution showing diameters of around 0.6Clm

as confirmed by the section view shown in Figure 4-7.

Additional experiments were done in order to determine the stability of the films formed at

different surface pressures. Compression-expansion hysteresis experiments done for a maximum

pressure of 50mN/m, which is located after the collapse pressure of the homopolymer (Figure 4-










8) also support our conclusions, showing a hysteresis as the monolayer was successively

compressed and expanded. After complete expansion, the compression curves again overlap each

other showing that the fi1ms formed can relax to their original monolayer. In contrast, for a

maximum surface pressure of 20mN/m (below the plateau), the experiment shows no signs of

hysteresis on expansion, proving the presence of a thermodynamically stable fi1m for pressure

values below the equilibrium spreading pressure of the PtBA segments (Figure 4-9). During the

expansion, a decrease to a pressure lower than the one observed during the compression is

observed. This can be explained by the fact that the desorbed PtBA chains undergo a faster

desorption during the compression cycle than re-adsorption during the expansion cycle. Another

possible explanation would be an artifact due to the wilhelmy plate. However, this is unlikely

because there is no difference in multiple subsequent compression curves.

To observe the stability of the fi1ms formed at high surface pressures, an isochoric

experiment was also performed. The fi1m was compressed to a pressure of 50mN/m. Once this

pressure was reached, the barriers were stopped and the pressure recorded over time. The plot in

Figure 4-10 illustrates a rapid drop in pressure as soon as the compression stops, then the

observed pressure slowly decreases to the equilibrium spreading pressure of the homopolymer

(24mN/m). The fi1ms formed are thermodynamically unstable and quickly relax to the pressure

observed for the plateau. The reason for the sharp surface pressure increase observed on the

isotherm is still not clear; it could be from interactions between the collapsed and desorbed

aggregates formed in region (2) or between eventual adsorbed PtBA segments.

4.2.2 PS-b-PtBA Star-Shaped Copolymers

4.2.2.1 Isotherm Experiments

Figure 4-11(a) shows the surface pressure-mean molecular area isotherms obtained for the

three copolymers. Similar to the PtBA homopolymer, plateaus can be observed for a pressure of









about 24mN/m. Independent of the molecular weight of the copolymer, the plateau observed is

horizontal, representing a first-order transition where the temperature and pressure stays constant

while the single domains aggregate. This differs from polymers such as polystyrene-b-

poly(ethylene oxide) (PS-b-PEO) presenting a pseudoplateau where a slight change in the

surface pressure is detected upon compression.32-44 As for the PtBA homopolymer, the plateau is

detected for a surface pressure of 24mN/m. We know the desorption of PtBA chains from the

interface has an effect on the length of the plateau; however, the effect of the PS domains

remains unclear.

Our results are significantly different from those obtained by Lennox and co-workers for

linear PS-b-PtBA diblock copolymers with relatively low PtBA wt %."1 They showed that the

presence of the PS block induces two additional phase transitions of the PtBA block below 20

mN/m and suggested that these transitions might originate from the peculiar surface aggregation

into circular micelles. This difference in behavior can be explained by the significant difference

in PS molecular weight between their sample (305 styrene repeat units) and our copolymers

(each arm has 48 styrene repeat units).

The same three regions observed in the homopolymer are observed for the copolymers.

An increase in the surface pressure is observed in the high mean molecular area going from a

liquid expanded phase to a liquid condensed phase until a plateau suggesting a biphasic phase is

reached at xn=24mN/m. PS and PtBA are both hydrophobic homopolymers but contrary to PS,

PtBA possesses slightly polar ester groups that allow its adsorption onto the water subphase.

Upon further compression in the low molecular areas, the apparent surface pressure shows a

sharp increase to values higher than 80mN/m. For this region, extrapolation of the linear portion

of the isotherm to TT = 0 mN/m yields the theoretical area that a most compact surface fi1m









would occupy at zero pressure (Ao). In previous studies on amphiphilic copolymers such as PS-

b-PEO, no dependence of the collapse area on the PEO block length was observed. In our case,

we can see the increase in collapse area as the size of the PtBA chains increases suggesting that

the film collapse depends on both the hydrophilic and the hydrophobic block. Interestingly the

films become more compact as the amount of PtBA increases. Likewise, Li et al. investigated the

behavior of two different PtBA homopolymers, and observed more compact films when

increasing the PtBA molecular weight." This suggests that the area occupied by the longer

chains includes a significantly smaller trapped area than the shorter chains. Throughout the

plateau, the tert-butyl side groups go from a prone to a vertical conformation. Li et al. also

studied the influence of the PtBA molecular weight on the behavior of PS-b-PtBA diblock

copolymers. "l They observed a linear dependence (y = 0. 128x + 16.473) of the collapse area on

the number of tBA repeat units. When the surface pressure is plotted versus the mean unit area

(area available for 1 tBA repeat unit), a strong molecular weight dependence can be observed

throughout the entire isotherm plot (Figure 4-11 (b)). In Figure 4-12, when Ao per tBA repeat

unit is plotted versus the number of tBA repeat units present in one arm, a linear dependence is

detected (y = 0.0021 + 2.982). The value detected for PS-b-PtBA(104) is really close to the one

given for the PtBA homopolymer (5.5 A+2). As the PtBA molecular weight is increased, the star

copolymers become more sterically hindered, forming less packed aggregates, yielding to a

higher area per tBA unit. The areas per tBA unit observed by Li et al. 11 for several copolymers

are significantly more larger (from 12 A+2 to 70 A+2) due to the amount of PS present in their

copolymers: longer chains preventing the PtBA segments from packing more closely.

The area occupied by a styrene repeat unit is constant for the three copolymers and has an

approximate value equal to 12A+2/St (values are given in Table 4-3). This value is larger than the









one observed by Logan et al.48 (8.9A+2/St) for PS-b-PEO star copolymers. The significant

variation between both values can be explained by the difference in interfacial behavior between

PEO and PtBA blocks. For small molecular areas, the PEO chains stretch in the aqueous

subphase while the PtBA desorbs from the interface. Therefore, in the case of the PS-b-PEO

copolymers, the steric hindrance at the interface is lowered, allowing the PS domains to pack

more closely.

4.2.2.2 AFM Imaging

AFM images were obtained for two transfer pressures as seen in Figure 4-14 We can see

that the images obtained for a transfer pressure lower than the plateau value (24mN/m) only

show the presence of single domains. The formation of adsorbed circular surface micelles

without any collapsed domains can be observed. Even though their size decreases as the PtBA

content increase, the size distribution of those surface micelles is rather monodisperse (verified

by the image analysis software introduced in chapter 2) and independent on the molecular weight

of PtBA in the sample suggesting those domains represent aggregates of PS chains. The domain

sizes averaged 4200, 3500, and 3200nm2 for the 3 systems. Li et al. "1 studied the behavior of

linear PS-b-PtBA copolymers and observed domains that were approximately 10 times larger

than the ones detected for our copolymers. However, this difference can easily be explained by

the distinct architectures and the size of the PS blocks (305 styrene repeat units in their

copolymers versus 48 for each arm of our star copolymers). It should be noted that the transfer

ratios varied significantly between the values obtained for pressures below and above the plateau

pressure. In fact for pressures equal to 24mN/m or less, all the transfer ratios were close to unity

suggesting a homogeneous transfer of the Langmuir monolayer onto the mica substrate.

However, when the monolayers were transferred for pressures above the plateau, the transfer

ratios decreased dramatically (TR~0.5 for the 3 copolymers).









The hydrophilicity of the substrate and results from previous studies on PS-b-PtBA

copolymers allow us to consider the more hydrophilic PtBA to be adsorbed onto the mica and the

hydrophobic PS to occupy the higher layer. Therefore, the PtBA is represented by the darker

(lower) areas whereas the PS exists as the brighter (higher) domains. The diameter of the bright

circular domains observed in the AFM pictures is approximately 100nm and independent of the

surface pressure. Each aggregate consists of multiple chains for example, the aggregation

number for the domains observed in Figure 4-14 in the case of the PS-b-PtBA(104) is equal to

132. Therefore, once a micelle has formed during the solvent spreading process, the micellar

cores come closer to each other during compression. However, the aggregation number remains

unchanged with the chains frozen within the glassy core of a particular micelle (the temperature

of the experiment is well below the Tg of the desorbed PS cores). It should be noted that the

glass transition temperature (Tg) of a pure bulk PS (molecular weight comparable to the PS

studied in this chapter) is 980C.141 In addition, Bliznyuk et at.142 meaSured the surface glass

transition for PS with molecular weights ranging from 3900 to 1,340,000g/mol. Using the model

proposed by their group, the Tg of a PS of 5000g/mol (close to the PS molecular weight in one

arm of the star copolymers) is approximately 500C.

For low surface pressures, a decrease in the size of the circular micelles as well as an

increase in the distance between them is observed when the size of the PtBA segments is

increased (Figure 4-15). This observation is in accord with the interpretation of a PtBA layer

located at the bottom of the film with PS domains on top of it as shown in the Figure 4-13. The

decrease in the circular domain size can be due to the increase in repulsive forces between PS

and PtBA when the fraction of PtBA becomes more significant.









For a pressure of 24mN/m several trials were made to observe the aggregation of the PS

domains and the formation of lower domains representing the PtBA chains. In fact, as seen for

the PtBA homopolymer throughout the plateau, the desorption of the PtBA chains can be

observed leading to the chaining and further aggregation of the single and oligomeric PS

domains observed at lower pressure. The formation of such aggregates is more easily seen on the

copolymer containing the longest chains of PtBA (PS-b-PtBA(445)). When compressed to a

pressure above the plateau (30mN/m), aggregation of these domains is observed for all

copolymers. As shown in Figure 4-14, this aggregation becomes more significant as the number

of tBA repeat units increases.

As demonstrated previously for the homopolymer, three regions can be detected in the

isotherms as well as in the structures observed when the monolayer is transferred onto a solid

substrate. Figure 4-16 gives a scheme of the general structures formed for different regions of the

isotherms. Region (1) contains a homogeneous monolayer of amphiphilic PtBA with the

hydrophobic PS forming single domains aggregated on top of it. Region (2) is similar to the

structure observed for the PtBA homopolymer, the only difference being the presence of PS

domains sitting on top. Finally region (3) is the analogous morphology to the one observed for

the region (3) in the homopolymer. The domains of PtBA form a network, rendering the film

very rigid and increasing the apparent surface pressure. The three-dimensional structure depicted

for regions (2) and (3) can also be explained by the cross section shown in Figure 4-17 showing

the different phases of the Langmuir-Blodgett film at high pressure adsorbedd PtBA, desorbed

PtBA, and PS).The domains formed by the PtBA desorbed from the interface have a size

comparable to those previously observed for the PtBA homopolymer (around 1Clm) in Figures 4-

4 and 4-6. This is explained in pictorial fashion in the scheme shown in Figure 4-18, representing









the three layers discussed in the previous paragraph. We can see the formation of large PtBA

aggregates which are desorbing from the interface and possess smaller aggregates of PS sitting

on top of them.

As discussed in the previous part, contrary to PS-b-PEO block copolymers, for PS-b-PtBA

copolymers, the collapse area (Ao) depends on the PtBA block (Table 4-3). By calculating the

area occupied by PS (APS) USing equation 4-1 (where r is the aggregation number), we found

APs to be constant for the 3 copolymers.

M~ean domain area
APS =4-1


However, when APtBA (area Occupied by the PtBA chains in the 3D structure) is plotted

versus the number of tBA repeat units, a linear dependence is observed (y = 0.0482x + 37.049)

(Figure 4-19). The linear dependence detected by Li et al. 11 exhibited a slope of 0. 128. The

significant difference between this value and the slope observed in the case of our copolymers

could be explained by the difference in architecture (star-shaped versus linear copolymers), as

well as the size of their PS segment which increases the steric hindrance.

4.2.2.3 Stability of the Langmuir Films

As shown in the compression-expansion hysteresis experiment (Figure 4-20) performed in

the low MMA region, we were able to observe the formation of unstable films for pressures

higher than 24mN/m. All the compression curves closely overlap, showing that the collapsed

films are able to return to their original monolayer.

A drop in pressure is observed during the expansion cycle. This is due to the fact that the

re-adsorption of the PtBA chains onto the air-water interface during the expansion cycle is

slower than the desorption during the compression cycle. Additional compression-expansion

hysteresis experiments performed for pressures below the plateau shown in Figure 4-21 allow us










to propose that the monolayers exist as thermodynamically stable fi1ms of PS-b-PtBA for

pressures under 24mN/m. Figures 4-20 and 4-21 only show the results for the PS-b-PtBA(215).

The behavior of the fi1ms being independent on the molecular weight of the PtBA block; the

hysteresis experiments for the two other copolymers gave similar results.

To investigate the stability of the network formed at high pressures, transfers were

performed after different stabilization times (from 15 minutes to 12 hours) for the copolymer PS-

b-PtBA(215). The transfer ratios were recorded (Table 4-4) in order to determine if the fi1ms

observed by AFM correspond to what is formed at the air-water interface. The transfer ratios

being smaller than one even for a short stabilization time shows us the expansion of the fi1m

when transferred from the interface onto the solid substrate. This explains the AFM results

observed in Figure 4-16, where the structure observed is not a complete network. The decrease

of transfer ratio when increasing the annealing time supports once again, as shown in the

compression-expansion hysteresis experiments, that the fi1ms formed at high pressures are

thermodynamically unstable. Figure 4-22 shows the AFM images for the three copolymers

transferred at a pressure of 30mN/m, which corresponds to region 3 of the isotherm. We can see

that by increasing the stabilization time before transfer, the aggregation also increases, proving

once again the instability of the monolayers formed at the air-water interface. A possible

explanation for this phenomenon is the presence of attractive forces between the PtBA chains of

different copolymers that at high surface pressures have the tendency to aggregate. Contrary to

the PS-b-PEO copolymers studied in chapter 3, which showed a less well-defined transition from

a two-dimensional system to a three-dimensional system, for the PS-b-PtBA copolymers, it is not

clear to us whether the sharp surface pressure increase upon high compression in the isotherm is










due to interactions between the collapsed and desorbed aggregates formed in the plateau region

or between eventual remaining adsorbed PtBA segments.

The presence of an increase in aggregation when increasing the stabilization time suggests

the unstability of the Langmuir films above the plateau. In Figure 4-23, isochoric experiments

are reported. The monolayers were compressed up to a pressure of 50mN/m and after the

compression was stopped, values of the pressure were recorded over time. Once stopped, we

observe a drastic drop in surface pressure toward the plateau pressure. This, once again, shows

the unstable behavior of the film above the plateau. All three copolymers present the same

behavior; the only observed difference is the presence of a larger pressure drop as the amount of

PtBA in the copolymer increases. The surface pressure undergoes a sudden drop within the first

seconds of the experiment before leveling off after a few minutes around 24mN/m. In any case,

the isochoric experiments confirmed that the films formed above the plateau are not

thermodynamically stable and relax within minutes down to the plateau surface pressure of

24mN/m, which can also be seen as the equilibrium spreading pressure (ESP) of the PtBA

blocks. ESP analysis (not shown) was also performed by placing the solid copolymer sample

onto the water subphase. The ESP value (24mN/m) observed was in agreement with the

isochoric experiment.

4.3 Conclusions

In this chapter the behavior of several PS-b-PtBA star copolymers was investigated. The

isotherm experiments showed the presence of a plateau for a surface pressure of 24mN/m. The

copolymers studied, were composed of, on average 5 branches containing a constant PS core and

different PtBA molecular weights. Even if the shape of the isotherms does not vary from the one

of a PtBA homopolymer (already amphiphilic), a strong dependence on the molecular weight










was observed with an increase in the length of the plateau as we increase the number of tBA

umits.

AFM images were taken at pressures below and above the transition plateau. For the

homopolymer, as expected, uniform flat fi1ms were observed before the transition. However,

once the pressure reached the plateau pressure, aggregates were formed containing

approximately 92000 PtBA chains, suggesting desorption of PtBA chains from the interface.

When the PS-b-PtBA star copolymers were scanned, a strong molecular weight dependence of

the length of the plateau as well as the collapse area was observed in correlation with the

isotherm experiments. As the number of tBA repeat units increases, an increase in the

aggregation was detected. For low surface pressures, the distance between the PS domains was

greater as the PtBA molecular weight was increased, however, for high surface pressures, we

saw the appearance of three different domains: adsorbed PtBA, desorbed PtBA, and PS domains.

Estimations of the area occupied by the PtBA in the collapse area also showed the influence of

the PtBA segment on Ao.

The Langmuir films formed above the plateau were found to be thermodynamically

unstable as the aggregation increases with the stabilization time. Isochoric experiments were also

performed and once the compression stopped, the monolayers relaxed to their equilibrium

spreading pressure of 24mN/m. Below the plateau however, the copolymers formed elastic fi1ms,

no hysteresis was observed during the compression-expansion experiments and the monolayer

seems to relax to its initial state.











"L


)--~HN


Figure 4-1. Schematic of the PS-b-PtBA copolymers with n=48 and m=104,215,445 (Adapted
from Reference 87)


q'


I I


mmPS
- L~PtBA


(b)


Figure 4-2. Schematic of dendritic initiator (a) (Adapted from Reference 87) and model of the
PS-b-PtBA star copolymers (b)


ny
~n~L

k~llh









80-


Z 60-

S 50-

cn 40 -1 (3)
Q2 30 (2)

e 20-




0 Ao 10 20 30A, 40 50
Molecular Area (nm2/mOlOCUle)


Figure 4-3. Surface pressure-MMA isotherm for a 13000g/mol PtBA homopolymer recorded for
a compression speed of 5mm/min















x=15 mN/m


Sr10 nm


7T= 24 mN/m
(middle of plateau)









n = 30 mN/m


-5 nm


LO nm


~. r t~


Figure 4-4. Height AFM images in tapping mode for the PtBA Homopolymer after transfer at
three distinct surface pressures (Scale 10x10plm)
























Figure 4-5. Proposed conformations for the PtBA homopolymer at the air-water interface. The
different regions are labeled in Figure 4-3


0


0.20 0.40 0.60 0.80


Figure 4-6. Section view of a circular domain formed when the PtBA monolayer is compressed
above the plateau pressure (30mN/m) (image scale lxlylm)


Region (~1)


Regions (2) and (3)





Regin ( )


Regin (1)


Region (3)


Figure 4-7. Scheme of the structures formed for 3 distinct regions of the isotherm when a
monolayer of PtBA homopolymer is compressed at the air-water interface (Scale
10 x 10pm)



50-


E 40-





e 20-





O 10 20 30 40 50 60 70

Mean Molecular Area (nm2/mOlOCUle)


Figure 4-8. Compression-Expansion Hysteresis Experiment for the PtBA homopolymer
compressed to a target pressure of 50mN/m (above the plateau). The compression and
expansion rates were both 5mm/min.
















S10-







O 10 20 30 40 50 60 70

Mean Molecular Area (nm2/mOlOCUIS)



Figure 4-9. Compression-expansion hysteresis experiment for the PtBA homopolymer
compressed to a target pressure of 20mN/m (below the plateau). The compression and
expansion rates were both 5mm/min.


50-

E 45-
z
40-






2 0


100 150


200 250


Figure 4-10. Isochoric experiment recorded for the PtBA homopolymer for a maximum pressure
of 50mN/m. the compression speed was 5mm/min.


Time (minutes)











(a)


80

70

60
E
50

S40

S30

$ 20-

S10


---PS-b-PtBA(104)
SPS-b-PtBA(215)
SPS-b-PtBA(445)


0 100 200 300 400 500 600 700

Mean Molecular Area (nm2/mOleCUle)


80
70
S60
E
50

v 40
-- 30
0 20
S10


-0- PS-b-PtBA(104)
-o PS-b-PtBA(215)
-A PS-b-PtBA(445)


10 20 30 40 50 60 70 80 90 100
Area per tBA unit (100 x nm2/tBA)


Figure 4-11. (a) Surface pressure-MMA isotherms plots of the 3 star-shaped copolymers (b)
Isotherms of the 3 copolymers in terms of number of tBA repeat units. All the
isotherm experiments were performed with a compression speed of 5mm/min.









































Regions (2) and (3)


16
S14


e o 10


c 6-
*** a- 4-
R2 = 0.997
5e 2-
40O
0 200 400 600

Nu mbe r of tBA re peat u nits


Figure 4-12. Plot of Ao versus the number of tBA repeat units.





Reaion (1)


Figure 4-13. Proposed conformations for the PS-b-PtBA copolymers at the air-water interface.
The different regions are defined in Figure 4-3.









PS(48)-b-PtBA(1 04) PS(48)-b-PtBA(215) PS(48)-b-PtBA(445)


S= 15mN/m







n: 24mN/m
Middle Plateau i











Figure 4-14. Height AFM images from tapping mode for the three copolymers. Transfer
pressures were chosen to be in region (1) and region (3) of the isotherms. (Scale
10x10Clm, inset scale lxlylm)


Figure 4-15. Scheme of AFM images for a transfer pressure of 15mN/m when increasing the size
of the PtBA chains


? PtBA




















I I 1 II
1
++ +
I I II
+ 1
r~. ~ r r +
I +I + I
+I I +
L
I +I + r
II +

F~dm (1)


+ 'i. ;






F~dm I;))


Figure 4-16. Schematic of the structures formed for 3 distinct regions of the isotherm when a
monolayer of PS-b-PtBA copolymer is compressed at the air-water interface


PtBA desorbed/
from the interface\


0.10


0.20


0.30 0.40


Figure 4-17. Cross section of the PS-b-PtBA(445) copolymer when compressed to a pressure of
30mN/m and studied in tapping mode


Redm I3)















































'


-8nm


-


-4nm



-Onm


(Scale: 2x2tpm)


Figure 4-18. Close-up pictures and schematic of the three layers observed when the PS-b-
PtBA(445) is compressed above the plateau pressure


"E70
65
60 -
S55 -
aE 50
. 4 5 -
0 40 -
a
o
35
S30


y = 0.0482x + 37.049
R2 = 0.9731


100


200


300


400


500


Num ber of tBA repeat units


Figure 4-19. Plot of the Area occupied by the PtBA blocks (values in Table 4-3) versus the
number of tBA repeat units


~P I

~B~Be














E 25-

20 1

S15-


5-
a 10


Compression


Expansion


0 50 100 150o'l~ 2~00 250 3~00 350

Mean Molecular Area (nm2/mO 8CU 8)


Figure 4-20. Compression-expansion hysteresis experiment for the PS-b-PtBA(215) copolymer
compressed to a target pressure of 30mN/m (above the plateau). The compression and
expansion rates were both 5mm/min.


30


25-
-

E 20

S15-

S10
-


Compression


Exruansion
200 250


300 350 400 450


Mean Molecular Area (nm2/mOleCUle)



Figure 4-21. Compression-expansion hysteresis experiments for the PS-b-PtBA(215) copolymer
compressed to a target pressure of 20mN/m (below the plateau). The compression and expansion
rates were both 5mm/min.













































Figure 4-21. AFM images for a transfer pressure of 30 mN/m for different stabilization times.
Stabilization times before transfer (minutes) for PS-b-PtBA(104): (a) 15, (b) 45 and
(c) 90; PS-b-PtBA(215): (d) 15, (e) 45 and (f) 90; PS-b-PtBA(445): (g) 15, (h) 45 and
(i) 90 (Scale 10x10pim)












116















S45-
-E PS-b-PtBA(104)
z
E PS-b-PtBA(215)
g 4V PS-b-PtBA(445)

a 35-


S30-


25-


20
O 50 100 150 200 250 300

Time (minutes)



Figure 4-22. Isochoric experiments recorded for the three copolymers after compression to a
target pressure of 50mN/m via a compression rate of 5mm/min.










Table 4-1. Characteristics of the Copolymers Used
Polymer Mn* (g/mol)
PS-b-PtBA(104) 131,000


wt% of PtBA PDI*
37 1.28


PS-b-PtBA(215) 155,000 47 1.33
PS-b-PtBA(445) 194,000 58 1.53
* Mn and PDI were determined using differential viscometry detection and universal calibration


Table 4-2. Characteristics of the Homopolymer Used
Polymer # Tert-butylacrylate repeat units (m)
11101 101


Mn (g/mol)
13000


Table 4-3. Specific areas calculated for the 3 copolymers
Copolymer r Ao (nm2) APS HM112) APtBA HM112)


PS-bPtBA(104)

PS-bPtBA(215)

PS-bPtBA(445)


132 68

124 80

105 88


Table 4-4. Transfer ratios for different stabilization times before transferring the copolymer PS-
b-PtBA(215) monolayer onto a mica substrate.
Stabilization Time (min) 15 45 90 720


Transfer Ratio


0.665 0.300 0.199 0.055









CHAPTER 5
SURFACE CHARACTERIZATION OF POLY(ETHYLENE OXIDE)-BLOCK-POLY(s-
CAPROLACTONE)

5.1 Introduction

Extensive work has been done on amphiphilic copolymers at the air-water interface but

surprisingly studies on PCL-containing polymers are hard to Eind. On one hand PCL is an

interesting polymer to use due to its biodegradability leading to non-toxic products.114,115 On the

other hand, low molecular weight PEO can be eliminated from the body by the renal system.116

The addition of a highly flexible PEO chains at the end of a PCL segment can tailor some

inherent properties of pure PCL such as high crystallinity, high hydrophobicity, and slow

biodegradation."'7 Whereas very few investigations have been performed at the air/water

interface, PEO-b-PCL copolymers have been investigated in solution and their micellar

properties render them very useful as drug delivery vehicles.115,118,1 19

5.1.1 Crystallization of Semi-Crystalline Polymers

The crystalline state is characterized as a state exhibiting a melting temperature, Tm (first-

order transition), and an X-ray scattering pattern with sharp reflections. Tm is controlled by

properties such as hydrogen bonding, polarity, molecular weight, etc...

A crystal is in a lower free energy state than the liquid when the temperature is below the

melting point for a large crystal of a very high molecular weight polymer. Figure 5-1 shows

schematically the changes in the Gibbs free energy of liquid and a crystal with temperature.

For any spontaneous transformation, AG needs to have a negative value for a constant

temperature and constant pressure process. At Tine (melting point of an infinitely long crystal of

Einite molecular weight), a condition of equilibrium exists between the crystal and liquid as both

phases have the same value of G and AG=0.









The crystallization properties of block copolymers have been thoroughly investigated.

The block copolymers studied are mostly composed of a semicrystalline block and an amorphous

block. In such cases, the presence of a non-crystalline block enables the modification of the

mechanical and structural properties of a crystalline polymer, through introduction of a rubbery

or glassy component. There are a number of interesting features of these copolymers. Contrary to

homopolymers, equilibrium chain folding can occur and crystallization is therefore controlled by

the size of the non-crystallizable block.120-13 Furthermore, in copolymers containing a

crystallizable component, structural changes occurring due to microphase separation compete

with those resulting from crystallization. Also, selective solvents for the amorphous block can

lead to non-equilibrium morphologies because the crystallizable block can precipitate from

solution and crystallize. As might be expected, the morphologies formed by the crystallization of

block-copolymers therefore depend on both the molecular weight and the crystallization protocol

(i.e. cooling rate). Contrary to homopolymers, where folding of chains occurs such that stems are

always perpendicular to the lamellar interface, a parallel orientation has been observed for block

copolymers crystallized from a lamellar melt phase, and a perpendicular folding has been seen in

a cylindrical microstructure. Both orientations are shown in Figure 5-2.

While the case of block copolymers containing one crystalline block has been studied

extensively, the behavior of block copolymers having two crystallizable blocks is still fairly

unknown. It has been investigated for PCL-PEO-PCL triblocks and it was found that the crystals

of PEO and PCL coexist independently and that there are no mixed crystals containing both PEO

and PCL. In the copolymers, when the crystallizable block represents less than 25% of the minor

component, its crystallinity decreases to zero. In the contrary, in a PEO/PCL binary blend, the

two components crystallize separately even with less than 25% of the minor component.









As shown in Figure 5-3, in the blend PEO and PCL form independent crystallites whereas,

in the block copolymer, chain connectivity leads to PEO and PCL lamellae occupying the same

crystallite when comparable fractions of each are present. In that case, a significant reduction of

the lamellae thickness as well as crystal imperfections was observed.

5.1.2 Crystallization of PCL at the Air-Water Interface

Li et al. studied the behavior of several PCL homopolymers at the air-water

interface.73,137,138 The crystallization processes were captured using BAM and showed an

anisotropic crystal growth at slow compression rates. They observed an increase in the number

of nuclei formed with increasing compression rates like the data shown in Figure 5-4.

Li et al. also studied the dependence 138 Of PCL crystals on the molecular weight. They

used PCL samples with number average molar masses (Mn) ranging from 3500 to 36000 g.mof l,

and found out that the nucleation and growth of crystals as well as their morphologies and

melting properties were dependent on the molar mass. Thomas Joncherayl39 Studied the behavior

of PCL homopolymers at the air-water. At low pressures the PCL chains were completely

adsorbed onto the water subphase with characteristics of liquid expanded and liquid condensed

phases. When an isotherm is recorded, below the collapse pressure, Ivanova et al 140 determined

that the PCL chain packing and orientation was dependent of the surface pressure. The isotherms

recorded showed that the collapse pressure and chain packing was not dependent on the

molecular weight of the PCL homopolymer used and that thermodynamically stable monolayers

were formed for pressures below the collapse. While the films formed for such pressures are

easily studied, the ones formed above the collapse pressure are more complicated to investigate

due to phenomena such as desorption of molecules from the interface or formation of

multilayers. In the case of the PCL, it was observed that the PCL chains aggregate on top of the

water subphase. An interesting phenomenon was also surveyed; after the collapse point, a









decrease in surface pressure was detected corresponding to the crystallization of the PCL

segments directly on the water. Compression-expansion hysteresis experiments also show the

appearance of a pseudo-plateau during the expansion cycle as a result of the re-adsorption or

melting of the PCL chains. AFM images of the PCL homopolymers transferred for a surface

pressure before the crystallization on the water surface were taken and the formation of PCL

crystals was observed. According to the isotherms, for such surface pressures, the PCL

homopolymers are transferred into smooth and hydrated monolayers adsorbed onto the mica

surface. It is assumed that upon drying, during and after transfer, part of the PCL chains

crystallize, which likely results in a mica surface only partially covered with adsorbed or

crystallized PCL chains.

5.2 Results and Discussion

The linear PEO-b-PCL diblock copolymer synthesized from a linear PEO macroinitiator

(PEO2670, Mn = 2,670 g/mol, ~ 60 ethylene oxide repeat units) contains a PCL block with 35 e-

caprolactone repeat units (Figure 5-5 and Table 5-1). The PEO-b-PCL five-arm star-shaped

copolymer consists of a hydrophilic PEO core with 9 ethylene oxide units/arm with hydrophobic

PCL chains at the star periphery. Each star contains PCL block with 18 e-caprolactone units/arm

(Figure 5-6 and Table 5-1).

5.2.1 Isotherm Experiments

The isotherm of the linear PEO-b-PCL copolymer is given in Figure 5-7. Contrary to the

isotherms observed for the PEO or PCL homopolymers,138 preSsures as high as 25mN/m can be

reached. Better shown in the compressibility plot in Figure 5-8, a transition can be observed

around 13.5mN/m. Thomas Joncherayl39 Studied several PEO-b-PCL linear copolymers with a

number of e-caprolactone units ranging from 11 to 35 (the number of ethylene oxide units was









kept constant throughout the entire investigation). The maximum in monolayer compressibility

for the high pressure transition increases as PCL chain length increases. This suggests the

presence of a PCL-related phase transition. For PEO-b-PCL linear copolymers with a smaller

amount of e-caprolactone units, three different phase transitions can be observed for pressures of

6.5, 10.5, and 13.5mN/m. For the copolymer studied in this chapter (PEO60-b-PCL35), Only the

high pressure transition is observed. This transition is known to represent the crystallization of

the PCL blocks.138-140 It was suggested that the transition observed for a pressure of 6.5mN/m

represents the dissolution of the PEO into the aqueous subphase. In our case, no transition is

observed at low pressures indicating that when the PCL block size increases, the dissolution of

PEO is made more difficult. Compression-expansion hysteresis plots of the PEO-b-PCL linear

copolymer are shown in Figure 5-9. A broad melting transition is observed during the first

expansion. This behavior was not observed for PCL homopolymer samplesl38,139 and could be

explained by the properties of the crystals formed during the first compression, such as size,

shape or polydispersity. No plateau was observed during the compression or expansion cycle but

we can detect a re-adsorption of the crystallized PCL chains for pressures under 5mN/m.

Joncheray et all 39 perfOrmed compression-expansion hysteresis experiments for several barrier

speeds and the melting (re-adsorption) and crystallization (desorption) pressure values were

found to be strongly barrier speed dependent. In the case where the barriers were moved

infinitely slowly, the melting and crystallization pressure values would become identical.

The isotherm of the PEO-b-PCL star copolymer is shown in Figure 5-10. It exhibits three

distinct regions corresponding to different conformations of the polymer chains. For high MMA,

the surface pressure slowly increases until it reaches a pseudoplateau. For low MMA, a sharp

increase in surface pressure is observed representing the formation of highly condensed films.









However, for the star copolymer studied, the intermediate MMA region shows an almost

inexistent pseudoplateau. The compressibility plot shown in Figure 5-11 illustrates that absence

even better by exhibiting no transition in the intermediate MMA range. This suggests that the

PEO blocks are not adsorbed onto the interface but are more likely to be in the water subphase.

Compression-expansion hysteresis experiments were performed for target pressures below

(Figure 5-12) and above (Figure 5-13) the pseudoplateau pressure. For surface pressures below

9mN/m, the compression and expansion curves are superposable independent of the target

pressure. This illustrates the reproducibility and stability of the films formed. Due to the size of

the PCL blocks, the block copolymer is hydrophobic enough to avoid the irreversible dissolution

of material in the aqueous subphase. The compression-expansion curves for a target pressure of

15mN/m are shown in Figure 5-13. A pseudoplateau is observed during the expansion cycles

corresponding to the re-adsorption (melting) of the crystallized PCL chains.

In order to further interpret the crystallization behavior as well as the stability of the films

formed, additional analysis using AFM and BAM is going to be discussed in the next part.

5.2.2 Brewster Angle Microscopy and Comparison to Previous AFM Imaging

Joncheray et al. studied the crystallization of PEO-b-PCL linear and star copolymers in the

transferred LB monolayers and observed the formation of crystalline domains independent of the

surface pressure, when investigated by AFM. While AFM gives some hint about the

crystallization behavior of those polymers, BAM allowed us to observe the formation of crystals

directly at the water subphase, subsequently eliminating variables such as affinity to substrate,

transfer pressure, water evaporation and film thickness.

Figures 5-14 and 5-15 show the BAM images for the PEO-b-PCL linear and star

copolymer respectively. Those images were taken at both intermediate and low MMA. Both star

and linear copolymers do not seem to show any crystal formation for pressures below 10mN/m.










By AFM, several crystals were observed at low surface pressures, indicating the importance of

sample preparation, differences in resolution of the two techniques, or differences in polymer

behavior absorbed to a solid compared to the water surface. Generally, it also appears that more

crystals are observed by AFM than what can be observed at the air-water interface. Nonetheless,

above a specific pressure, crystals formed at the water surface exhibit the same morphologies as

those observed by AFM (Figure 5-16). The only noticeable difference is the presence of more

numerous crystals on the AFM images, suggesting again the influence of sample preparation

before AFM imaging. As seen in chapter 2, the lateral resolution of the BAM is significantly

lower than the one observed for AFM. Nonetheless, the BAM can detect features as small as

2Clm which is 5 times smaller than the crystals showed in the AFM pictures.

A compression-expansion experiment was performed and the corresponding images are

given i n F figure 5 17. Anal ogou s to the pre assure re sults of the previ ou s compare ssi on- exp ansi on

hysteresis isotherm plots, BAM reveals that the crystallized PCL chains melt and re-adsorb

during the expansion cycle. When comparing the images for a surface pressure of 10mN/m, one

can notice that some remaining crystals are still present during the second compression. As

discussed earlier in the chapter, the melting (re-adsorption) and the crystallization (desorption)

pressures are strongly dependent on the barrier speed. One can expect the images for different

compressions to be identical for an infinitely slow barrier speed. The barrier speed dependence

as well as the presence of hysteresis between the compression and expansion curves suggests the

formation of thermodynamically unstable films for intermediate and high surface pressures. This

metastability is shown in Figure 5-18 where the monolayer was compressed to a pressure of

18mN/m. The surface pressure was then kept constant and images were taken after several

stabilization times. One can notice that for t = 0, no crystals are observed but as the stabilization









time increases, the amount of crystals increases, suggesting once again the presence of

thermodynamically unstable films.

5.3 Conclusions

In this chapter, the interfacial behavior of a PEO-b-PCL linear copolymer and a PEO-b-

PCL star copolymer were investigated. The morphologies formed were observed by BAM and

compared to previous AFM results. The isotherm of the star-shaped block copolymer indicated

the presence of a single phase transition characterized by a pseudoplateau that corresponds to the

collapse and crystallization of the PCL chains above the water surface. Below that plateau, the

PCL chains are completely adsorbed, anchoring the PEO chains to the interface. In this region,

the compression-expansion hysteresis experiments as well as the structures observed by BAM

showed the formation of thermodynamically stable monolayers.

Once the pseudoplateau is reached, the PCL chains crystallize at the interface. This

phenomenon was highlighted by the BAM pictures. We were able to show that the PCL chains

crystallize directly at the interface and not during the AFM sample preparation. The images

observed by BAM were found to be strongly dependent on the stabilization time and a more

important aggregation was observed over time. Compression-expansion experiments showed that

the melting (re-adsorption) of the PCL chains was slower than their crystallization (desorption).



























II r 1 I


(b)





Tmm


ICrystal
IMelt
Tm


Figure 5-1. General behavior of thermodynamic variables at the equilibrium melting temperature
Tmm (a) Gibbs free energy (b) entropy, enthalpy and volume


(a)


(b)


Figure 5-2. Schematic of perpendicular (a) and parallel (b) chain folding in semicrystalline block
copolymers









PCL Domains
PEOD~mai- PEO + PCL Domains










(a) (b)

Figure 5-3. Schematic illustration of the morphology formed by blends and copolymers of two
crystallizable polymers. (a) a PEO/PCL Blend and (b) a PCL-PEO-PCL triblock. In
the blend, PEO and PCL are phase separated into domains in which each
homopolymer crystallizes in a lamellar texture. In the copolymer, PEO and PCL
blocks crystallize in the same domain due to chain connectivity


Figure 5-4. Compression rate dependence of crystal growth in Langmuir monolayers at 22.5 O
C.49 BAM images were obtained at compression rates of (a) 0.010, (b) 0.013, and (c)
0.026 nm2 mOnomer- -min for A ~ 0.08 nm2mOnomer All images are 1.28 x 0.96
mm2 (Data borrowed from reference 137)









Figure 5-5. The linear PEO-b-PCL copolymer used in this study (n = 60, m = 35)










n=9


n/n

n


m2 mH






~O O







O O



Oi O O O

Hm mH



mH

Figure 5-6. Schematic of the star-shaped PEO-b-PCL copolymer used in this study. Further
characterization data are shown in table 5-1








30

25

20


E

E


05 1

0 2000 4000 6000
Mean Molecular Area (A2)

Figure 5-7. Surface pressure-MMA isotherm for the linear PEO60-b-PCL35 linear COpolymer for a
compression rate of 5mm/min

0.24


0.2

0.16

0.12

0.08


-


Figure 5-8. Compressibility
pressure.


2


4 6 8 10 12 14 16
x (m Nlm)
plots of the PEO-b-PCL linear diblock copolymer versus surface


















1 I


20


* First compression
n First expansion
o Second and third compressions
* Second and third expansions


i-


E

E


5


0


Figure 5-9. Compression-expansion hysteresis plot of the PEO-b-PCL linear copolymer
compressed at a target pressure of 16 mN/m. The compression and expansion rates
used in the experiments were both 5mm/min

40 1 I


30

20


0 3000


Figure 5-10. Surface pressure-MMA isotherm for the PEO-b-PCL star copolymer recorded for a
speed of 5mm/min


2000 4000
Mean Molecular Area (A2)


6000


6000 9000
MMA (AZ2)


12000 15000










0.25



0.21








0.05


r




E

u
rl
rl
q
rl
TR
Q)
k

E
O
U


2 4 68 10 12 14

xn (mN/m)


Figure 5-11. Compressibility
pressure.

25


20







0 ,,


plots of the PEO-b-PCL star diblock copolymer versus surface


10000 12000


Figure 5-12. Compression-expansion hysteresis plot of the PEO-b-PCL star copolymer
compressed at a target pressure of 9 mN/m. The compression and expansion rates
used in the experiments were both 5mm/min


O 2000 4000 6000 8000
MMA (A2Z






















O5



0 2000 4000 6000 8000 10000 12000
MMA (M2)

Figure 5-13. Compression-expansion hysteresis plot of the PEO-b-PCL star copolymer
compressed at a target pressure of 15 mN/m. The compression and expansion rates
used in the experiments were both 5mm/min


Figure 5-14. BAM Images of the PEO-b-PCL linear copolymer at a pressure for 10mN/m (a) and
15mN/m (b)


























Figure 5-15. BAM Images of the PEO-b-PCL star copolymer for a pressure of 10mN/m (a) and
20mN/m (b)


--40nm












75nm


Sr0nm








mr~nm






%0n


Figure 5-16.AFM Images for the linear PEO-b-PCL copolymer ((a) 10mN/m and (c) 15mN/m)
and the PEO-b-PCL star copolymer ((b) 10mN/m and (d) 20mN/m)












10OmN/m


20mN/m


First
Compression


100am 100am





Second
Compression


100um 100am


Figure 5-17. BAM Images of the PEO-b-PCL star copolymer for pressures of 10 and 20mN/m
during the first (a and b) and the second compression cycle (c and d)


Figure 5-18. BAM images for a pressure of 18mN/m (a) and after waiting 15min (b), 20min (c),
35 min (d) and 55min (e)












Table 5-1. Characteristics of the linear and star-shaped copolymers

Name PEO60-b-PCL35 Star

Mn a (g mol) 6,680 13,110

PDIa 1.24 1.44

Avg no. of ethylene oxide units 60 9

Avg no. of -caprolactone units 3 5 18

a Determined by 'H NMR
b Determined by GPC calibrated with linear polystyrene standards.









CHAPTER 6
CONCLUSION AND PERSPECTIVES

In this dissertation, the behavior of various amphiphilic copolymers was studied at the air-

water interface using Langmuir-Blodgett methods as well as AFM and BAM.

6.1 Summary

6.1.1 Blends of a PS-b-PEO Copolymer with PS and PEO Homopolymers

As discussed previously, the addition of free homopolymer chains have an important effect

on the architecture of the films formed (Figure 6-1). The isotherms displayed three different

regions known as "pancake", pseudoplateau, and "brush" areas. While the addition of PS seemed

to affect the brush area of the isotherm, the addition of PEO showed a difference in the transition

area (pseudoplateau). AFM was able to give us insights on the film architectures and showed the

influence of each homopolymers on the chaining observed in the pure PS-b-PEO copolymer. On

one hand the addition of PS increased the amount of chaining of the PS domains enhancing the

hydrophobic properties of the monolayer, on the other hand the accumulation of PEO decreased

the chaining present between the PS domains and favored the phase separation between clusters

of micelles and layers of pure PEO. This chapter gave us fundamental information on copolymer

blends at the air-water exhibiting more complex architectures without having to go through

difficult synthesis techniques.

6.1.2 PS-b-PtBA Star Copolymers at the Air-Water Interface

Three star copolymers as well as a PtBA homopolymer were studied at the air-water

interface. All the isotherms observed had a plateau at a pressure of 24mN/m suggesting the

presence of a first order transition. A dependence on the PtBA molecular weight could be seen

throughout the entire isotherm plot, however, the PS influence still unclear. After performing

AFM experiments on the monolayers formed, we proposed the transition from a two-









dimensional PtBA completely adsorbed onto the interface to a three-dimensional PtBA that starts

desorbing from the interface. An increase in the length of the PtBA chains affected both the

region below and above the plateau. For pressures below the plateau, the addition of PtBA

decreased the size of the copolymer domains as well as increased the distance between those

domains. However, when transferred for pressures above 24mN/m, the aggregation observed in

the monolayer increased with the amount of PtBA present in the copolymer. Compression-

expansion hysteresis experiments showed the formation of thermodynamically stable films for

pressures under 24mN/m. For pressures higher than 24mN/m, isochoric experiments were

performed and exhibited a relaxation of the monolayer to its equilibrium spreading pressure.

6.1.3 PEO-b-PCL Copolymers at the Air-Water Interface

AFM results were compared to those obtained by BAM in order to better understand the

crystallization behavior of the copolymers. The isotherms showed the presence of a

pseudoplateau around 15mN/m. Below that pressure, the PCL chains are completely adsorbed

onto the interface whereas they crystallize for pressures above the pseudoplateau pressure. BAM

illustrated the influence of sample preparation when LB fi1ms were studied by AFM. In fact the

films formed above the pseudoplateau were thermodynamically unstable and the aggregation

was strongly time dependent. BAM pictures were taken for a pressure of 18mN/m after several

stabilization times and the aggregation increased when upon annealing.

6.2 Correlation Between the Different Systems

PS- and PEO-containing systems were studied in order to determine the role played by

each block on the copolymer self-assembling behavior.

Four PS-containing systems were investigated in this work: one completely hydrophobic

(PS), PS-b-PEO diblock copolymer, PS-b-PtBA star copolymers, and blends. It is well-known

that PS is hydrophobic and aggregates into irregular islands instead of spreading into monolayers









(Figure 6-2). We showed that by adding small quantities of an amphiphilic copolymer (PS-b-

PEO), one was able to enhance and vary the interfacial properties of the PS homopolymer. The

PS aggregates observed when in presence of the copolymer were also more regular than those

observed when free PS homopolymer chains were spread at the interface. The films formed by

the blends seem to have similar properties as those observed for a copolymer possessing the

same total number of PS repeat units.

Two distinct PS-containing copolymers were used to investigate the effect of the

hydrophilic block on the architectures formed. PS-b-PEO and PS-b-PtBA copolymers were

studied with PEO being more hydrophilic than PtBA. For low surface pressures, comparable

domains of PS can be detected for both systems (r ~ 100). PS-b-PEO copolymers have been

widely studied at the air-water interface.16-31 At low coverage, the interactions between the EO

monomers and the interface is attractive. However, when the monolayer is compressed, the

repulsive forces between the PS and the PEO blocks promote the desorption of some PEO from

the interface into the water. Upon further compression the PEO is pushed into the water subphase

as shown in Figure 6-2.25 COntrary to the PEO-containing copolymer, the one containing PtBA

as the hydrophilic group presented a completely different conformation. The PtBA is not as

hydrophilic as the PEO and therefore cannot stretch into the water subphase. Instead, once the

plateau pressure (24mN/m) is reached the PtBA started desorbing from the interface. For high

surface pressures, a three-dimensional system was formed presenting 3 different regions:

adsorbed PtBA, desorbed PtBA, and PS domains.

Depicted in Figure 6-3, three different PEO-containing systems were investigated in order

to study the various films formed. High molecular weight PEO homopolymer form monolayers

when spread at the air-water interface however, once the pseudoplateau pressure is reached, the









PEO dissolved in the water subphase. PS was used as an anchor to keep the PEO from

dissolving, and a new region was detected on the isotherm. Contrary to PS, PCL homopolymers

are already surface active and form films when spread at the air-water interface. Compare to the

PS-b-PEO copolymer where the PS was just sitting on top of the PEO layer, the PCL crystallized

directly at the interface forming new and interesting architectures. Even if the PCL

homopolymers already formed monolayers when spread at the air-water interface, the PEO-b-

PCL films could be compressed to higher surface pressures. In the pseudoplateau region, PCL

homopolymers as well as PCL chains in the copolymers crystallized directly at the air-water

interface. It was confirmed that above the pseudoplateau, all the PCL chains have collapsed and

the sharp pressure represents the interactions between the hydrated PEO cores.

Those proj ects gave us interesting insights on the fundamental self-assembling behavior

of PS- and PEO-containing amphiphilic copolymers. By using different hydrophilic and

hydrophobic groups, we were able to monitor the formation of various architectures at the air-

water interface. The control of conformations or orientations of the different blocks was easily

achievable by monitoring the copolymer architectures, the amount of homopolymer added (in the

case of the blends), or the surface pressure.

6.3 Future Work

Jeong et al 92-94, Showed that the miscibility between PMMA homopolymer and PMMA

block was higher in thin films than in the bulk. In that case spin coating was used to prepare the

films. However, an interesting study for our blends would be to investigate the miscibility

between PEO homopolymer and PEO block or PS homopolymer and PS block when spread onto

a water subphase, compare to that observe in the bulk. This would allow us to get more

fundamental information about the blend behavior and have a better control on the morphologies

formed.









The condensed area, present for small molecular areas, demonstrates a dependence in PS

in the case of the blends seen in chapter 3, as well as for star-shaped PS-b-PEO copolymers

previously studied in our group33. For the PS-b-PtBA copolymers studied in chapter 4, a

dependence of the condensed area on the PtBA molecular weight was observed. However, the

effect of PS remains unclear. Studying copolymers with varying PS segments would provide

more information on the influence of the hydrophobic block on the architectures formed and

allow a better control on the architectures formed.

Further investigations would need to be done to determine the exact number of arms in

each PS-b-PtBA star copolymer. One method would be to cleave the arms of the stars by base

hydrolysis and then perform viscometry/universal calibration and light scattering experiments in

order to detect the exact molecular weight as well as the mass distribution of the branches. Once

we determine this value, we would be able to calculate the exact number of arms for each star

and therefore have a better understanding of the aggregation properties at the air-water interface.

Mengel et al8 3 Studied the chemical modification of PtBA films into PAA after transfer

onto a solid substrate. The presence of PAA chains at the surface renders the film very useful for

applications in molecular electronic and optical devices. This technique could be applied to our

system in order to obtain PS-b-PAA films with well-defined architectures and surface chemistry.

PS-b-PAA copolymers were shown to spread at the air-water interface. However, contrary to PS-

b-PtBA copolymers, a strong pH-dependence is observed and the structures formed are not as

well defined. By performing the chemical modification after transfer, we would be able to

optimize the formation of those films and control the structures formed.

Various PCL molecular weights could also be investigated in the case of the PEO-b-PCL

copolymers. Knowing the influence of the molecular weight on the crystallization process would









give insights on the transport properties of the polymer chains. The driving force of polymer

crystallization is the degree of undercooling (AT = Tmo Tx), which is directly related to the

crystallization temperature. Therefore, studies over a wide range of temperatures would provide

information about the effect of temperature on the copolymer crystallization. An interesting

investigation would be to study blends of PEO-b-PCL copolymers with PCL homopolymers of

various molecular weights. This would give us information on the crystallization behavior and

also know if the formation of crystals could be controlled by the amount of PCL present.

Finally, for most of the copolymers studied, the non stability of the films formed at high

surface pressure was detected in the compression-expansion hysteresis or the isochoric

experiments. Performing investigations for several compression rates would quantitatively

describe the compression rate dependence of film formation at the air-water interface.


































AA ~O(

L~i~i~i~


+ PS


+ PEO


Figure 6-1. Morphologies observed at low surface pressures for (a) the pure PS-b-PEO
copolymer, (b) the pure PS-b-PEO copolymer + PS homopolymer, and (c) the pure
PS-b-PEO copolymer + PEO homopolymer (blue = PEO, black = PS)


Figure 6-2. Morphologies observed at high surface pressure for (a) PS, (b) PS-b-PEO diblock
copolymer, (c) PS + PS-b-PEO diblock copolymer, and (d) PS-b-PtBA copolymer
(blue = PEO, black = PS, and orange = PtBA)

















(b)


Figure 6-3. Morphologies observed at high surface pressure for (a) the PEO homopolymer, (b)
PEO-b-PS diblock copolymer, and (c) the PEO-b-PCL copolymer (blue = PEO, black
= PS, and green = PCL)










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BIOGRAPHICAL SKETCH

Sophie Bernard was born on February 25, 1979, in Bordeaux, France. She graduated from

the University of Bordeaux in June 2001 with her B.S. degree in physical chemistry. She then

worked as a research assistant under the direction of Dr. Yves Gnanou, in the Laboratoire de

Chimie des Polymeres Organiques, University of Bordeaux. She received her M. S. degree in

physical chemistry of polymers from the University of Bordeaux in July 2002.

Since Sophie enrolled at the University of Florida in September 2002, she has been a

graduate student under the direction of Dr. Randolph S. Duran.





PAGE 1

1 BEHAVIOR OF SEVERAL AMPHIPHILIC COPOLYMERS AT THE AIR-WATER INTERFACE By SOPHIE BERNARD A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2007

PAGE 2

2 2007 Sophie Bernard

PAGE 3

3 To my parents Franoise and Guy and my brothers Romain and Mathieu for their constant love and support

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4 ACKNOWLEDGMENTS I would first and foremost like to thank my advisor Dr Randolph S. Duran for making this experience possible. Throughout my entire PhD, he has been supporting in the good and bad times and I could not be more thankful. His presen ce and advices made my stay at the University of Florida, an experience I will never forget. I also like to give a special thanks to Dr John Reynolds and Dr Ken Wagener. Without their support and their caring atte ntion, they made my experien ce on the polymer floor really enjoyable. I would also like to show appreciation to the rest of my committee members, Dr Daniel Talham, Dr Valeria Klei man, Dr Eric Enholm, and Dr Richard B. Dickinson. Their constructive criticism and their trust in me did not go unnoticed, and for that I could not be more grateful. A special thank you goes to De nise Sharbaugh for all her pr ecious help and patience. There are a lot of past and present members of the polymer floor that made my work environment so enjoyable and I will never forget all those times inside an d outside the lab. So for that thank you Florence Courchay, Merve Erta s, Piotr Matloka, Aubrey Dyer, Emilie Galand, Tim Steckler, and Genay Jones. None of that could have been possible wit hout the support of the entire Durans group, Jorge Chvez, Henk Keiser, Brian Dorvel, Danyell Wilson, Dr Martin Andersson, Dr Maria Stjerndahl, Dr. Firouzeh Sabr i, Eric Greeley, Kristina De noncourt and Aleksa Jovanovi A special thank goes to Dr Thomas Joncheray a nd Dr Jennifer Logan for all their support and constructive conversations during my whole PhD. Some people were always here in good and bad times. Thank you Vicky for those wonderful five years that you definitely made in teresting. Thanks to.Jordan, Neil, Eric and Brent for all those times spent at number 23. Thank you to Jamie, Austin and Kristen for all those late

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5 Friday nights. I would also lik e to acknowledge my classmates, Lisa, Richard and Lindsay for going through those five years with me. Special thanks goes to Roxy, David, Nicolas and Ashlee for being the best support a person could ask fo r during the writing process. Everyone made my life as a graduate student so much easier and I will never forget all thos e times spent at GC. I love them all and will miss them immensely. Finally, probably the most important, I want to thank my parents Franoise and Guy as well as my brothers Romain and Mathieu, who wit hout even being aware of it, have made this experience possible. Their love and support was th e constant in my life that kept me going and got me through some rough times.

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........9 LIST OF FIGURES................................................................................................................ .......10 LIST OF ACRONYMS............................................................................................................... ..16 ABSTRACT....................................................................................................................... ............17 CHAPTER 1 INTRODUCTION..................................................................................................................19 1.1 Scope of the Dissertation..................................................................................................19 1.2 Block Copolymers in the Bulk. ....................................................................................... 19 1.3 Block Copolymers at the Air-Water Interface..................................................................21 1.3.1 Polystyrene (PS) and Poly(ethylene oxi de) (PEO) at the Air-Water Interface......22 1.3.2 Polyacrylates at the Air-Water Interface................................................................25 1.3.3 Poly( -caprolactone) (PCL) at the Air-Water Interface.........................................26 1.3.4 Previous Studies on Polymer Blends......................................................................27 2 EXPERIMENTAL TECHNIQUES........................................................................................33 2.1 Langmuir Trough............................................................................................................ ..33 2.2 Equilibrium Spr eading Pressure.......................................................................................35 2.3 Langmuir Trough Experiments.........................................................................................36 2.4 Langmuir-Blodgett Films.................................................................................................39 2.5 Atomic Force Microscopy................................................................................................40 2.5.1 Instrument Parameters............................................................................................40 2.5.2 Limitations..............................................................................................................42 2.5.3 Parameters Used.....................................................................................................43 2.5.4 Image Analysis.......................................................................................................44 2.6 Brewster Angle Microscopy.............................................................................................45 3 BLENDS OF A POLYSTYRENEBLOCK -POLY(ETHYLENE OXIDE) COPOLYMER AND ITS CORRES PONDING HOMOPOLYMERS..................................56 3.1 Isotherm Experiment........................................................................................................56 3.1.1 PEO Homopolymer................................................................................................56 3.1.2 Linear Polystyreneblock -Poly(ethylene oxide) (PSb -PEO) Diblock Copolymer....................................................................................................................56 3.1.3 Blends of a PSb -PEO diblock copolymer and a PS homopolymer.......................58

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7 3.1.3.1 Pancake Region (I).......................................................................................59 3.1.3.2 Pseudoplateau Region (II)............................................................................60 3.1.3.3 Condensed Region (III)................................................................................60 3.1.4 Blends of a PSb -PEO diblock copolymer and PEO homopolymer......................62 3.1.4.1 Pancake Region (I).......................................................................................62 3.1.4.2 Pseudoplateau Region (II)............................................................................63 3.1.4.3 Condensed Region (III)................................................................................64 3.2 Atomic Force Microscopy (AFM) Experiments...............................................................65 3.2.1 Linear Polystyreneblock -Poly(ethylene oxide) (PSb -PEO) Diblock Copolymer....................................................................................................................66 3.2.2 Blends of a PSb -PEO diblock copolymer and a PS homopolymer.......................68 3.2.3 Blends of a PSb -PEO diblock copolymer and a PEO homopolymer....................69 3.3 Conclusions................................................................................................................ .......70 4 INTERFACIAL BEHAVIOR OF STAR-SHAPED POLYSTYRENEBLOCK POLY(TERT-BUTYLACRYLATE) COPOLYMERS.........................................................90 4.1 Introduction............................................................................................................... ........90 4.2 Results and Discussion.....................................................................................................91 4.2.1 P t BA Homopolymer...............................................................................................92 4.2.2 PSb -P t BA Star-Shaped Copolymers.....................................................................94 4.2.2.1 Isotherm Experiments..................................................................................94 4.2.2.2 AFM Imaging...............................................................................................97 4.2.2.3 Stability of the Langmuir Films.................................................................100 4.3 Conclusions................................................................................................................ .....102 5 SURFACE CHARACTERIZATION OF POLY(ETHYLENE OXIDE)BLOCK POLY( -CAPROLACTONE)..............................................................................................119 5.1 Introduction............................................................................................................... ......119 5.1.1 Crystallization of Semi-Crystalline Polymers......................................................119 5.1.2 Crystallization of PCL at the Air-Water Interface...............................................121 5.2 Results and Discussion...................................................................................................122 5.2.1 Isotherm Experiments...........................................................................................122 5.2.2 Brewster Angle Microscopy and Comp arison to Previous AFM Imaging..........124 5.3 Conclusions................................................................................................................ .....126 6 CONCLUSION AND PERSPECTIVES..............................................................................137 6.1 Summary.................................................................................................................... .....137 6.1.1 Blends of a PSb -PEO Copolymer with PS and PEO Homopolymers................137 6.1.2 PSb -PtBA Star Copolymers at the Air-Water Interface......................................137 6.1.3 PEOb -PCL Copolymers at the Air-Water Interface............................................138 6.2 Correlation Between the Different Systems...................................................................138 6.3 Future Work................................................................................................................ ....140

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8 LIST OF REFERENCES.............................................................................................................145 BIOGRAPHICAL SKETCH.......................................................................................................153

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9 LIST OF TABLES Table page 3-1 Characteristics of the PSb -PEO sample investigated.......................................................88 3-2 The mass ratio of PS between the dibloc k copolymer and the homopolymer as well as the apparent number of styrene units have been calculated for each blend..................88 3-3 Width of the pse udoplateau for each blend........................................................................88 3-4 The mass ratio of PEO between the dibl ock copolymer and the homopolymer as well as the apparent number of styrene units have been calculated for each blend..................88 3-5 Pancake areas extrapolated from the -A isotherms..........................................................89 3-6 Area for the second transition (described in Figure 2.5) extrapolated for each blend.......89 3-7 Molar ratio of PEO from the homopolymer and the dibl ock copolymer as well as the total number of EO units is given for each blend..............................................................89 4-1 Characteristics of the copolymers ssed.......................................................................... 118 4-2 Characteristics of the homopolymer used...................................................................... 118 4-3 Specific areas calculated for the 3 copolymers................................................................118 4-4 Transfer ratios for different stabilizat ion times before transferring the copolymer PSb -P t BA(215) monolayer onto a mica substrate................................................................118 5-1 Characteristics of the lin ear and star-shaped copolymers................................................136

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10 LIST OF FIGURES Figure page 1-1 Morphologies of diblock copol ymers: cubic packed spheres ( S ), hexagonal packed cylinders ( C or Hex ), double gyroid ( G or Gyr ), and lamellae ( L or Lam )........................30 1-2 Solution states for an amphiphilic dibl ock copolymer for concentration below and above the CMC..................................................................................................................30 1-3 Schematic representation of a sta rlike (1) and a crew-cut (2) micelle........................30 1-4 Description of Surface tension showi ng how the forces on surface molecules differ from those in the bulk........................................................................................................31 1-5 Schematic representation of the pancake to brush transition for PSb -PEO copolymers..................................................................................................................... ....31 1-6 The two conformatins for PEO at the ai r-water interface: Conformation (a) is loose and flexible, but compressing it into the co mpact conformation (b) tends to lock the chain in one position resulting in a sterically hindered and more rigid structure..............31 1-7 BAM images obtained during an hysteresis experiment at 22.5 C with a compression and expansion rate of 0.010 nm2.monomer-1.min-1 for a PCL homopolymer................32 2-1 The original Langmuir bala nce as designed by I. Langmuir.............................................48 2-2 Set up of a typical Langmuir trough..................................................................................48 2-3 Schematic of the Wilhelmy plate.......................................................................................48 2-4 Schematic Surface pressure-MMA isotherm.....................................................................49 2-5 (a) Several representative isotherms are shown, depicting th e dependence of surface pressure ( ) on mean molecular area.................................................................................49 2-6 Different types of deposited LB films...............................................................................50 2-7 Optical system that de tects cantilever deflection...............................................................50 2-8 An atomic force microscope (AFM) (lef t) and the tapping mode electronic set-up (right) are shown.............................................................................................................. ..50 2-9 AFM scanner tube containing the piezoel ectric material and metal electrode. The x, y, and z-directional components of the scanner are also indicated....................................51 2-10 Dependance of the piezoelectric ceramic on voltage.........................................................51 2-11 Imaging limitations in tip sharpness..................................................................................52

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11 2-12 The software allows choosing a dom ain range by varying the minimum and maximum areas..................................................................................................................53 2-13 The software gives you a computed image representing the different domains and the possible angles between domains in the presence of chaining..........................................53 2-14 Error made by the computer can be corrected by the user.................................................54 2-15 Scheme of the Brewster angl e at the air-water interface and in presence of a thin film...54 2-16 The different components of the BAM2 are shown...........................................................55 3-1 Surface pressure-MMA isotherm fo r the PEO homopolymer (Mn = 100,000g/mol) recorded for a compression speed of 5mm/min.................................................................72 3-2 Surface pressure-MMA isotherm for the 32,500 g mol-1 PSb -PEO copolymer recorded for a compression speed of 5mm/min.................................................................73 3-3 Several surface pressure-MMA isotherm s are shown, indicating the dependence of surface pressure on the mean molecular ar ea for different blend ratios. All the isotherm experiments were performed using a compression speed of 5mm/m.................74 3-4 Monolayer compressi bility plot for the PSb -PEO copolymer and blend 1......................75 3-5 The condensed area per EO unit (A0/EO) varies linearly with the number of styrene units present in the blends..................................................................................................75 3-6 The condensed area (A0) varies linearly with the total number of styrene units present in the blends.......................................................................................................... .76 3-7 Several surface pressure-MMA isothe rms are shown, showing the dependence of surface pressure on the mean molecular ar ea for different amounts of PEO. All the isotherm experiments were performed using a compression speed of 5mm/min..............77 3-8 Monolayer compressi bility plots for the PSb -PEO diblock copolymer and blend 1........78 3-9 The pancake area (AP) varies linearly with the total number of EO units present in the blends..................................................................................................................... ......78 3-10 The area for the second transition depends linearly on the mole ratio of PEO from the homopolymer over PEO from the PSb -PEO....................................................................79 3-11 The area for the second transition depends linearly on the appare nt total number of EO repeat units................................................................................................................ ...79 3-12 Example of a sample height image and surface plot (scan area shown is 2 2 m)..........80 3-13 Example of a sample section image (2 2 m)..................................................................80

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12 3-14 Height AFM images from tapping mode of the pure PSb -PEO for several transfer pressures (scale 2 2 m). The transfer ratios are also given for each images...................81 3-15 Schematic representation of surface micelles formed by A-b-B diblock copolymers with A strongly adsorbed to the surface............................................................................81 3-16 Model of PSb -PEO absorbing at the air-water interface..................................................82 3-17 Dependence of the number of molecules per domain on pressure....................................82 3-18 Height AFM images from tapping mode of the pure PSb -PEO diblock copolymer as well as two of the blends for seve ral transfer pre ssures (scale 2 2 m)............................83 3-19 Height AFM images (scale 2 2 m) for the pure PSb -PEO diblock copolymer and Blend 2 (transfer pressure of 4mN/m) as we ll as the distribution of the domain areas.....84 3-20 Computed images for the pure PSb -PEO diblock copolymer and Blend 2 (transfer pressure of 10mN/m) as well as th e distribution of the domain areas...............................85 3-21 Magnification of a single domain formed for Blend 2 for a transfer pressure of 10mN/m (scale 150 150nm).............................................................................................86 3-22 Height AFM images from tapping mode of the pure PSb -PEO diblock copolymer and several blends for transfer pressures of 4 and 9 mN/m (scale 2 2 m). The height scale remains constant for the blends.................................................................................86 3-23 Height AFM images from tapping mode for Blend 2; (a) from successive spreading, and (b) from the mixed solution (scale 2 2 m)................................................................87 4-1 Schematic of the PS-b-P t BA copolymers with n=48 and m=104,215,445.....................104 4-2 Schematic of dendritic initi ator (a) and model of the PS-bPtBA star copolymers (b)...104 4-3 Surface pressure-MMA isotherm for a 13000g/mol P t BA homopolymer recorded for a compression speed of 5mm/min....................................................................................105 4-4 Height AFM images in tapping mode for the P t BA Homopolymer after transfer at three distinct surf ace pressures (Scale 10 10 m)...........................................................106 4-5 Proposed conformations for the P t BA homopolymer at the ai r-water interface. The different regions are la beled in Figure 4-3.......................................................................107 4-6 Section view of a circular domain formed when the P t BA monolayer is compressed above the plateau pressure (30mN/m) (image scale 1 1 m)..........................................107 4-7 Scheme of the structures formed for 3 distinct regions of the isotherm when a monolayer of P t BA homopolymer is compressed at the air-water interface (Scale 10 10 m).........................................................................................................................108

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13 4-8 Compression-Expansion Hyst eresis Experiment for the P t BA homopolymer compressed to a target pressure of 50 mN/m (above the plateau). The compression and expansion rates were both 5mm/min.........................................................................108 4-9 Compression-expansion hysteresis experiment for the P t BA homopolymer compressed to a target pressure of 20mN /m (below the plateau). The compression and expansion rates were both 5mm/min.........................................................................109 4-10 Isochoric experiment recorded for the P t BA homopolymer for a maximum pressure of 50mN/m. the compression speed was 5mm/min.........................................................109 4-11 (a) Surface pressure-MMA isotherms pl ots of the 3 star-shaped copolymers (b) Isotherms of the 3 copolymers in terms of number of tBA repeat units. All the isotherm experiments were performed with a compression speed of 5mm/min.............110 4-12 Plot of A0 versus the number of t BA repeat units............................................................111 4-13 Proposed conformations for the PSb -P t BA copolymers at the air-water interface. The different regions are defined in Figure 4-3...............................................................111 4-14 Height AFM images from tapping mode fo r the three copolymers. Transfer pressures were chosen to be in region (1) and region (3) of the isotherms. (Scale 10 10 m, inset scale 1 1 m)...........................................................................................................112 4-15 Scheme of AFM images for a transfer pressure of 15mN/m when increasing the size of the P t BA chains...........................................................................................................112 4-16 Schematic of the structures formed for 3 distinct regions of the isotherm when a monolayer of PS-b-P t BA copolymer is compressed at the air-water interface...............113 4-17 Cross section of the PSb -P t BA(445) copolymer when compressed to a pressure of 30mN/m and studied in tapping mode.............................................................................113 4-18 Close-up pictures and schematic of the three layers observed when the PSb P t BA(445) is compressed above the plateau pressure.....................................................114 4-19 Plot of the Area occupied by the PtBA blocks (values in Table 4-3) versus the number of tBA repeat units..............................................................................................114 4-20 Compression-expansion hysteresis experiment for the PSb -P t BA(215) copolymer compressed to a target pressure of 30 mN/m (above the plateau). The compression and expansion rates were both 5mm/min.........................................................................115 4-21 Compression-expansion hystere sis experiments for the PS-b-P t BA(215) copolymer compressed to a target pressure of 20mN /m (below the plateau). The compression and expansion rates were both 5mm/min.........................................................................115

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14 4-21 AFM images for a transfer pressure of 30 mN/m for different stabilization times. Stabilization times before transfer (minutes) for PSb -P t BA(104): (a) 15, (b) 45 and (c) 90; PSb -P t BA(215): (d) 15, (e) 45 and (f) 90; PSb -P t BA(445): (g) 15, (h) 45 and (i) 90 (Scale 10 10 m).............................................................................................116 4-22 Isochoric experiments recorded for th e three copolymers after compression to a target pressure of 50mN/m via a compression rate of 5mm/min.....................................117 5-1 General behavior of thermodynamic vari ables at the equilibrium melting temperature Tm (a) Gibbs free energy (b) entropy, enthalpy and volume..........................................127 5-2 Schematic of perpendicular (a) and parallel (b) chain folding in semicrystalline block copolymers..................................................................................................................... ..127 5-3 Schematic illustration of the morphol ogy formed by blends and copolymers of two crystallizable polymers....................................................................................................128 5-4 Compression rate dependence of crys tal growth in Langmuir monolayers at 22.5 C.49 BAM images were obtained at comp ression rates of (a) 0.010, (b) 0.013, and (c) 0.026 nm2 monomer-1min-1 for A ~ 0.08 nm2monomer-1. All images are 1.28 0.96 mm2..........................................................................................................................128 5-5 The linear PEOb -PCL copolymer used in this study (n = 60, m = 35)..........................128 5-6 Schematic of the star-shaped PEOb -PCL copolymer used in this study. Further characterization data ar e shown in table 5-1....................................................................129 5-7 Surface pressure-MMA isotherm for the linear PEO60b -PCL35 linear copolymer for a compression rate of 5mm/min.......................................................................................130 5-8 Compressibility plots of the PEOb -PCL linear diblock copolymer versus surface pressure....................................................................................................................... .....130 5-9 Compression-expansion hys teresis plot of the PEOb -PCL linear copolymer compressed at a target pressure of 16 mN/m. The compression and expansion rates used in the experiments were both 5mm/min..................................................................131 5-10 Surface pressure-MMA isotherm for the PEOb -PCL star copolymer recorded for a speed of 5mm/min............................................................................................................131 5-11 Compressibility plots of the PEOb -PCL star diblock copolymer versus surface pressure....................................................................................................................... .....132 5-12 Compression-expansion hys teresis plot of the PEOb -PCL star copolymer compressed at a target pressure of 9 mN/m. The compression and expansion rates used in the experiments were both 5mm/min..................................................................132

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15 5-13 Compression-expansion hys teresis plot of the PEOb -PCL star copolymer compressed at a target pressure of 15 mN/m. The compression and expansion rates used in the experiments were both 5mm/min..................................................................133 5-14 BAM Images of the PEOb -PCL linear copolymer at a pressure for 10mN/m (a) and 15mN/m (b)..................................................................................................................... .133 5-15 BAM Images of the PEOb -PCL star copolymer for a pr essure of 10mN/m (a) and 20mN/m (b)..................................................................................................................... .134 5-16 AFM Images for the linear PEOb -PCL copolymer ((a) 10mN/m and (c) 15mN/m) and the PEOb -PCL star copolymer ((b) 10mN/m and (d) 20mN/m).............................134 5-17 BAM Images of the PEOb -PCL star copolymer for pressures of 10 and 20mN/m during the first (a and b) and the second compression cycle (c and d)............................135 5-18 BAM images for a pressure of 18mN/m (a) and after waiting 15min (b), 20min (c), 35 min (d) and 55min (e).................................................................................................135 6-1 Morphologies observed at low su rface pressures fo r (a) the pure PSb -PEO copolymer, (b) the pure PSb -PEO copolymer + PS homopolymer, and (c) the pure PSb -PEO copolymer + PEO homopolymer (blue = PEO, black = PS)..........................143 6-2 Morphologies observed at high surface pressure for (a) PS, (b) PS -b -PEO diblock copolymer, (c) PS + PSb -PEO diblock copolymer, and (d) PSb -PtBA copolymer (blue = PEO, black = PS, and orange = PtBA)................................................................143 6-3 Morphologies observed at high surface pr essure for (a) the PEO homopolymer, (b) PEOb -PS diblock copolymer, and (c) the PEOb -PCL copolymer (blue = PEO, black = PS, and green = PCL)..........................................................................................144

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16 LIST OF ACRONYMS AFM Atomic Force Microscopy TEM Transmission Electron Microscopy STM Scanning Tunneling Microscopy BAM Brewster Angle Microscopy ATRP Atom Transfer Radical Polymerization RAFT Reversible Addition Fr agmentation Chain transfer TR Transfer Ratio LB Langmuir Blodgett LC Liquid Crystalline CMC Critical Micellar Concentration EO Ethylene Oxide t BA tert -butyl acrylate St Styrene PEO Poly(ethylene oxide) P t BA Poly( tert -butyl acrylate) PCL Polycaprolactone PS Polystyrene PSb -PEO Polystyreneblock -Poly(ethylene oxide) PSb -PtBA Polystyreneblock -Poly(tert-butyl acrylate) PEOb -PCL Poly(ethylene oxide)block -Polycaprolactone

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17 Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy BEHAVIOR OF SEVERAL AMPHIPHILI C COPOLYMERS AT THE AIR-WATER INTERFACE By Sophie Bernard December 2007 Chair: Randolph S. Duran Major: Chemistry The two-dimensional structur e of amphiphilic copolymers is studied at the air-water interface using Langmuir-Blodgett methods, atomic force microscopy (AFM), and Brewster angle microscopy (BAM). Measurements are ma de for several block copolymers containing polystyrene (PS), poly(ethyl ene oxide) (PEO), poly( tert -butylacrylate) (P t BA) and polycaprolactone (PCL). Measurements are also made for blends of the PS-b-PEO copolymer with both a PS and a PEO homopolymer. When increasing the amount of PS homopolymer, the isotherms do not show any change in the high su rface area region. However, a lin ear dependence of the condensed area is observed. An increase in the PEO ratio has an effect on the biphasic region of the isotherms but no change is detected for the cond ensed area. The AFM data indicate a significant effect of the homopolymers on the monolayer structure. In fact depend ing on the homopolymer added, a change in the chaining behavior of th e copolymer is observed. Also, when introducing more PEO, a phase separation between the laye r of PEO and clusters of two-dimensional micelles is detected. The PSb-PtBA copolymers investigated are star-shaped copolymers w ith a polystyrene (PS) core and a poly(tert -butylacrylate) (P t BA) corona. They were prep ared with a constant PS

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18 block and three different P t BA block lengths. A transition can be observed for the three copolymers with a plateaus length depending on the PtBA composition. The images obtained by AFM are in agreement with the isotherms s howing the evidence of a phase transition around 24mN/m. In fact for the three copolymers, be low the plateau only single domains are observed whereas for pressures higher than 24mN/m, aggregates can be detected. Transfer experiments were performed after several equilibration times. The structure of the film formed seems to be dependant on the time waited before performing th e transfer, showing more compact films when the time waited was longer. Poly(ethylene oxide)block -Polycaprolactone (PEOb -PCL) copolymers are also studied in this work. Their behavior and the PCL crystall ization process at the air-water interface is investigated using BAM. The form ation of crystals di rectly on the water s ubphase is illustrated and compared to the pictures obtained by AFM.

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19 CHAPTER 1 INTRODUCTION 1.1 Scope of the Dissertation The aim of this project is to find fundament al information on a novel series of polymers by understanding their behavior at the air/water interface. Chapter 2 describes the different techniques used throughout my doctoral research. A review of these tech niques as well as their multiple applications is given. Chapter 3 and 4 go over the investigation of several amphiphilic copolymers at the air-water interf ace as well as their ability to form more complex architectures when blended with one of their corresponding homopolymers. Atomic Force Microscopy (AFM) is used as a tool to observe the formation of the morphology once the copolymer solutions are spread onto an interface and tran sferred to solids. Chapter 5 describes the behavior of a polystyreneb -polycaprolactone (PSbPCL) linear copolymer and a PSb -PCL star-shaped copolymer. In this investigation, Brewster a ngle microscopy (BAM) is used in order to understand the copolymer crystalliza tion process. The goal of Chap ter 5 was to obtain a better understanding about the formation of polycaprolact one crystals when a solution is spread onto a water subphase. The reminder of this chapter is a literature review on the behavior of block copolymers in the bulk and at an interface. 1.2 Block Copolymers in the bulk Self-assembling copolymer materials are intere sting because of their multiple commercial applications such as thermoplastic elastomers a nd compatibilizers in polymer blends. In the past decades, interest in the behavior of block c opolymers has grown due to many further potential applications in nanoscale lithography 1 or electronics.2-4 The emergence of controlled polymerization techniques such as su ch as living anionic polymerization,5 ATRP (atom transfer

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20 radical polymerization),6 or RAFT (reversible addition fragmentation chain transfer)7 has allowed the formation of more complex architect ures leading to a wide range of properties.8-10 Block copolymers are composed of different polymer chains. Depending on the number of different blocks, their composition, and the way they are linked together, they can form a variety of ordered morphologies with charac teristic lengths on the mesoscale .11 The simplest class of block copolymers is linear block copolymer s composed of two immiscible blocks, A and B. This type of architecture can adopt several equilibrium morphologies depending on the composition of the tw o blocks: spheres (S), cylinders (C), double gyroid (G), lamellae (L), and the inverse structures (Figure 1-1).11,12 This phase behavior is dictated by the Flory-Huggins segmentsegment interaction parameter ( AB), the degree of polymerizati on (N), and the composition ( ). The product ABN gives us an idea on the phase segregation.13 For small values ( ABN < 10), the A and B blocks mix, resulting in a disordered phase. When ABN is larger ( ABN > 10), the enthalpic terms dominate, causing an order-disor der transition (ODT) where the A and B segments segregate into various microstructures. When block copolymers are dissolved in a sel ective solvent that is good solvent for one of the block and precipitant for th e other, the copolymer chains aggregate to form micelles.5 Those reversible, well-defined micelles form above th e critical micellar con centration (CMC). The CMC can be determined from plots of the surface tension as a function of the logarithm of the concentration. It is then defined as the c oncentration at which the surface tension stops decreasing and reaches a plateau value. For c oncentrations lower than the CMC, only single polymer chains are present in solution. Once the CMC is reached, the polymers chains aggregate into spherical aggregates as shown in Figure 1-2.

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21 The aggregates observed are in conjunction with those seen for low molecular weight surfactant even if the values of the CMC are much lower in the case of block copolymers. Depending on the composition of the starting block copolymer, two limiting structures can be observed: (1) starlike micelles with a small core compared to the corona and (2) crew-cut micelles with a large core and highly stretched coronal chains. Both situations are shown in Figure 1-3. 1.3 Block Copolymers at the Air-Water Interface The morphologies described previously repr esent those formed in a three-dimensional system, but block copolymers also have the ab ility to self-assemble as two-dimensional structures. Such experiments are performed using a Langmuir apparatus, the setup of which will be further explained in chapter 2. In such case, the polymer solutions ar e spread onto a subphase and each block aligns in the phase for which it has an affinity. Amphiphilic diblock copolymers have been observed to self-assemble into num erous nanoscale and mesoscale structures when spread onto a water substrate, finding potential applications in coatings, microelectronics, stabilization, and lubrification.15 Such copolymers are appropriate for surface pressure studies involving Langmuir troughs. This technique provide s insight on the monolayer morphologies by controlling the surface density. The force acting on the molecules when spread onto a liquid is referred as the surface tension, which is the measurement of the cohesive (excess) energy present at a gas/liquid interface. The molecules of a liquid attract each other; the interactions of a molecule in the bulk of a liquid are balanced by an equally attractive force in all directions. Molecules on the surface of a liquid experience an imbalance of forces as indicated in Figure 14.

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22 The net effect of this situation is the pres ence of free energy at the surface. The excess energy called surface free energy can be quantif ied as a measurement of energy/area. For a constant temperature and pressure, surface free energy equals surface tension, which is quantified as a force/length measurement. The common units for surface tension are dynes/cm or mN/m. An analogous quantity is th e line tension. From a mechanic al point of view, line tension is defined as the operative force along the so-called three-phase line. A three-phase line is the intersection of three inte rfaces; for example, the periphery of the contact circle of a liquid drop that is placed on a solid surface and is surrounde d by a vapor phase. Simila r to surface tension, i.e., the tensile force encounter ed where two bulk phases meet, lin e tension is a well-defined thermodynamic property. However, unlike surface te nsion, it is not a well -quantified property; experimental values in the literature range from 10-11 to 10-5 N.16-30 1.3.1 Polystyrene (PS) and Poly(ethylene oxi de) (PEO) at the Ai r-Water Interface Seo et al. 31 showed the formation of stabiliz ed two-dimensional micelles using polystyreneb -poly(methyl methacrylate) (PSb -PMMA) diblock copolymers at the air-water interface. Once formed, those surf ace aggregates were kinetically stable, preventing any unimermicelle exchange. Polystyreneb -poly(ethylene oxide) (PSb -PEO) diblock copolymers of various molecular weights and chemical compositions have also been extensively used to study their properties in both the bulk and in solution. In addition, several groups have described their behavior at the air-water interface.32-44 The choice of PEO as one of the blocks renders the copolymer both biocompatible as well as amphiphilic. The inclusion of PS provides an anc hor at the air-water in terface, preventing the PEO from eventually dissolving into the water subphase. As a result, PSb -PEO films can be further compressed than a film comp osed simply of PEO homopolymer.

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23 Without PS, PEO can still be spread at th e air-water interface. Shuler and Zisman 45 studied the behavior of such a film. They obser ved a change in the film compressibility as surface density increases leading to a phase change reflecting in the films structure. The lack of reversibility in the compression and expansion experi ments is explained by a structural change in the polymer molecule. A modification in conf ormation was given to explain the different monomer area observed in the -A isotherms. Kuzmenka and Granick 46 performed the same type of experiment for a wide range of PEO molecular weights. They determined that for PEO chains beyond molecular weights of 100,000g/mol, the film attains a constant equilibrium surface pressure. This behavior was explained by the difficulty of a high molecular weight PEO to pass into an aqueous substrate due to the amphiphilic character of the EO monomer. Lower molecular weight PEO, however, requires a more hydrophobic anchor in or der to quantitatively remain at the air-water interface, generally pa rtitioning between the subphase and the surface, analogous to sol uble surfactants. While PEO has been widely studied at the ai r-water interface, PS ha s only been studied by one group. Being hydrophobic, PS is not expected to form any type of morphology when spread onto a water subphase. However, Kumaki 47 detected a change in surface pressure when a dilute solution of PS (2.0 10-5g/mL) was spread at the air-water in terface. Even if in this case, the measured surface pressure mainly represented mechanical force due to the compression, this group found stable monomolecular particles were observed for molecular weights higher than 50,000g/mol. However this work remains controvers ial. In fact, despite its importance, this remains the only study on pure PS. As a result, recent work has focused on the behavior of PSb -PEO at the air-water or solidwater interface, demonstrating the formation of novel nanostructures. Gonalves da Silva et al.

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24 34,39 described the utility of dibl ock amphiphilic copolymers in test ing the scaling properties of grafted polymers. They presented -A isotherms that show seve ral regions referred to as pancake, quasi-brush, and brush. Within thes e regions, different morphologies of surface micelles and further micellar aggregates were observed by transmission electron microscopy (TEM) and atomic force microscopy (AFM) de pending on the balance between block sizes. At the air-water interface, copolymers behave similarly to c opolymers in bulk dispersion. Static light scattering proved that PSb -PEO copolymers aggregate spont aneously into micelles over the CMC. The isotherm regions compare to t hose observed for solution CMC values: (1) below the CMC, surface micellization is observed; (2) at the CMC, the PEO segments are pushed into the subphase in order to decrease the surface area per molecule; and (3) above the CMC, PS-rich regions exist in between spaces formed by the PEO chains. Gonalves da Silva et al. 34,39 also investigated the effect of the PEO block size on the copolymer behavior at the air-water interface. In this case, the short PS chains are only used as an anchor to prevent the PEO from dissolving completely in to the water substrate. Upon compression, they observed a transition of the PE O blocks, from a two-dimensional structure floating on the water, to a threedimensional structure when the PEO stretched into the water. The first structure is the one previously term ed pancake whereas the second was identified as brush (Figure 1-5). The plateau displayed in the -A isotherms is an indication of the transition between these two states with its span depende nt on the relative sizes of the two blocks. The importance of PEO in film behavior ha s been recognized by others. For example, Devereaux and Baker 43 conducted -A isotherms experiments of PSb -PEO copolymers with varying PEO chain lengths. One copolymer cont ained 15% of PEO wher eas the other had only 7%. The copolymer with the longest PEO block di splayed a plateau close to 10mN/m, indicating

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25 that the copolymer spreads well at the interface. In contrast, the copolymer containing only 7% of PEO has no plateau, supporting th e theory that PS chains interfere with the PEO blocks upon compression. While most groups support the model of a tr ansition from pancake to brush described previously, Cox et al. 41,42 provide a different model to explain the shape of the -A isotherm for a PSb -PEO copolymer. Whereas in the first mode l the PEO passes into the aqueous subphase, the Cox model suggests a dehydration of the PE O followed by a conformational change, similar to that previously described by Shuler and Zisman40 for homopolymer PEO. As shown in Figure 1-6, conformation (a) (more flexible) is compre ssed into conformation (b) (more compact and sterically hindered). This transformation can be explained by an increase in the intramolecular forces in the second conformation. While numerous studies detail the behavior of linear PSb -PEO, advances in polymerization techniques within the past de cade have allowed chemists to design new copolymer architectures. Logan,48 Logan et al.,49 and Francis et al .,50,51 for example, investigated the behavior of several star amphiphilic copol ymers at the air-water interface. Peleshanko et al.42 observed formation of morphologies when spreading an amphiphilic heteroarm PEOb -PSm. The AFM images showed that the formation of diffe rent morphologies depends on the pressure used during the transfer. The unusual pr operties of those architectures allow the formation of more stable morphologies than those form ed using regular linear copolymers. 1.3.2 Polyacrylates at the Air-Water Interface Some copolymers, called stimu li-responsive systems, respond by a change of size or shape to a modification in temperat ure, ionic strength or pH.

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26 For example, in the Armes group, the synthesi s and solution propertie s of water-soluble copolymers were studied. The copolymers were co mposed of different alkyl methacrylates for the hydrophobic block and poly(2dimethylamino) ethyl methacrylate, poly(sulfubetaine), poly(4-vinylbenzoate) and mo re for the hydrophilic block.53,54,55 Block copolymers containing PS and poly(acrylic acid) (PAA) are anothe r type of self-assembling copolymers.56-59 Eisenberg et al.57,59 synthesized PSb -PAA copolymers by anionic polymeriz ation and investigated their behavior at the air-water interface. They identified a wide range of structures like spheres, rods, bilayer and bicontinuous ar chitectures, as well as inverted st ructures. All the aggregates observed possess a phase-separated insoluble core and a crew -cut soluble corona. The synthesis as well as the micellar properties in an a queous media was studied for more complex architectures based on PS and PAA.60-66 Even if numerous efforts have been done to study the interf acial behavior of PAA-based copolymer, Poly( tert -butylacrylate) (P t BA) -based copolymers studies remain rare. Our group was the first to publish results concerning PS and P t BA based copolymers at the airwater interface.67,68 1.3.3 Poly( -caprolactone) (PCL) at the Air-Water Interface PCL is an interesting flexible polymer fo r monolayer studies. It is a hydrophobic and crystalline polyester with a glass transition around -60 C, a melting point around 55 C, good biocompatibility, and low toxicity. In the past decades, PCL-containing systems have been found potentially useful for applications such as c ontrolled-release drug delivery and scaffolds for tissue engineering.69-71 At the air-water interfac e, previous results indicate that PCL can form a D close-packed monolayer with a collapse point around A 20 2/monomer. We found that very little interest was given to studying the assemb ly of monolayers of PCL homopolymers and PCL-based copolymers. Leiva et al. were first to describe the be havior of PCL homopolymers at

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27 the air-water interface.72 Recently, Li et al.73 studied their crystal lization behavior using Brewster angle microscopy (BAM). The architec tures formed are illustrated in Figure 1-7.73 1.3.4 Previous Studies on Polymer Blends While different architectures can result in di fferent surface film behavior, the synthesis of such systems can be difficult and time-consuming. In an effort to acquire new properties without the required synthesis, surface f ilms of blended polymers have also been investigated. In the 1980s, the Gabrielli group74-81 examined the behavior of numerous mixtures of polymers and low molecular weight materials as binary systems w ith different degrees of incompatibility. They also quantified the determinati on of the two-component monola yer miscibility by observing the -A isotherms of their two-dimensional blend. Thibodeaux et al.82 studied mixtures of a liquid crystalline copolymer with its corresponding monomer. The films formed by the blend monolayer appeared to be more condensed th an the pure copolymer films, proving that twodimensional mixtures of two or more polymers c ould enhance the interfacia l behavior and enable the formation of more stable films. In addition, such technique allows the blending of different polymer ch aracteristics into a single film. Malzert et al.83-85 developed a suitable model for understanding the interactions between polymers by mixing poly(ethylen e glycol) (PEG) and poly(lactideco -glycolide). The difference between the organization of the films resulting from a spreading of a mixture or a covalently linked copolymer gave the possibility to modulate and control the composition of the interfaces. Hottle et al.86 studied blends of amphiphili c poly(dimethylsiloxane) and trisilanolisobutyl-POSS and observed the forma tion of uniform monolayers when adding up to 80wt% POSS. By adding poly(lactideco -glycolide) to the PEG monolayer, Malzert et al were able to avoid the dissolution of the hydrophilic polymer and therefore control the composition of

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28 the interface. More recently, Seo et al.87 investigated the structures formed at the air-water interface by blending poly(styreneb -ferrocenyl silane) (PSb -FS) and poly(styreneb -2-vinyl pyridine) (PSb -P2VP). While neither of those copolymers assemble when spread separately at the air-water interface, their blends formed ordered structures which appear to be more versatile, a promising development in the fabrication of polymeric templates for lithography. Experiments were performed for several temperat ures and showed that close to the Tg of one of the block, the formed micelles retained their su rface mobility and could organize in equilibrium structures in response to changes in surface density or applied el ectric fields. In general, the surface behavior of amphiphilic diblock copolymers is readily examined through Langmuir techniques. Methods involving film compression and transfer provide both quantitative and qualitative results indicating how surfactant responds to pressure. PSb -PEO proves to be part icularly of interest due to the biocompatibility of PEO. While differe nt architectures of this copolymer have been shown to demonstrate different properties than those of linear analogues, additional characteristics may yet be attained through blending, both with PS and PEO homopolymers. While only a few preliminary studies of ble nding at the air-water interface exist, spin coated thin films of polymer bl ends have been investigated.88-94 Mayes et al88,89 studied the homopolymer distributions in ordere d block copolymers. By blending a PSb -PMMA diblock copolymer with PS and PMMA homopolymers, this group was able to illustrate the interactions between a homopolymer and its corresponding block. A similar system studied by Jeong et al 9294, showed that the miscibility between PMMA homopolymer and PMMA block was higher in thin films than in the bulk. The most commonly used technique to obser ve the morphologies formed by compressing a monolayer at a certain pressure is AFM. For soft samples such as polymer films, an appropriate

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29 AFM technique is tapping mode. Here, the cantile ver is excited to an oscillation near its resonance frequency. The interac tions between the tip and the sa mple give a deviation in the oscillation amplitude, recording the changes in the sample. This mode has been employed for most polymer samples because of its ability to in vestigate soft materials without further staining and with little or no tip-induced damage or morphology changes. However, Knoll et al.95 highlighted the limitations of this technique, finding that the appearance of artifacts was related to tip-sample interactions. Nevertheless, AFM provides valuable information on film morphology. Bodiguel et al .,96 for example, introduced a method for determining the dependence of the phase signal on the thickness of the sample. They corroborated that the origin of the phase signal was adhesive and represented the loca l elastic properties of the sample. Garcia et al.97 showed that phase shift measurements obtaine d by AFM in tapping mode could be converted into energy dissipation values. They suggested that the characterizat ion of material properties is less sensitive to the interaction regime and more to the tip-surface interactions. For a completely different application, Dorvel et al.98 used AFM to characterize the formation of tethered bilayer lipid membranes on gold substrates. In the following chapters, three different polymer systems will be investigated. Several PS or PEO containing amphiphilic systems will be c onsidered. Those copolymers were chosen for their interesting surface activities, making them good candidates for interfacial investigations. Those experiments will give us important fundamental information regarding the twodimensional self-assembly of amphiphilic copolym ers. The blending properties of a copolymer will also be given, allowing the formation of di fferent morphologies by c ontrolling the amount of each component. Moreover, the effect of the nature of the more hydrophobic block will be described using an amorphous (PS) a nd a crystalline (PCL) polymer.

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30 Figure 1-1. Morphologies of diblock c opolymers: cubic packed spheres ( S ), hexagonal packed cylinders ( C or Hex ), double gyroid ( G or Gyr ), and lamellae ( L or Lam ) (Adapted from Reference 2). Figure 1-2. Solution states for an amphiphilic di block copolymer for concentration below and above the CMC. Figure 1-3. Schematic representation of a starlike (1) and a crew-cut (2) micelle.

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31 Figure 1-4. Description of Surface tension show ing how the forces on surface molecules differ from those in the bulk Figure 1-5. Schematic representation of the pan cake to brush transition proposed by Goncalves da Silva et al.34 for PSb -PEO copolymers. Figure 1-6. The two conformatins for PEO at th e air-water interface proposed by Shuler and Zisman.45 Conformation (a) is loose and flexible but compressing it into the compact conformation (b) tends to lock the chain in one position resulting in a sterically hindered and more rigid structure Air Liquid

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32 Figure 1-7. BAM images obtained duri ng an hysteresis experiment at 22.5 C with a compression and expansion rate of 0.010 nm2.monomer-1.min-1 for a PCL homopolymer. Compression (A/nm-1.monomer-1): (a) 0.243, (b) 0.200, (c) 0.170, (d) 0.120, and (e) 0.080; Expansion (A/nm-1.monomer-1): (f) 0.082, (g) 0.173, (h) 0.272, and (i) 0.387. (Adapted from Li et al.137)

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33 CHAPTER 2 EXPERIMENTAL TECHNIQUES Any study involving Langmuir monolayers requ ires the use of a Langmuir trough set-up for quantitative measurements and for the prep aration of Langmuir-Blodgett films. Irving Langmuir was one of the principal scientists to observe the formation of monolayers when a surfactant is spread onto water, which lead to the development of the Langmuir trough technique (Figure 2-1).99 With this apparatus, he studied floati ng monolayers on water in the late 1910s and early 1920s. Several years later, Katherine Blod gett gave the first detailed description of sequential monolayer transfer onto solid supports.102 2.1 Langmuir Trough A typical Langmuir trough (Fi gure 2-2) is composed of the trough itself, one or two movable barriers, and a device for measuring su rface pressure. The Wilhelmy technique is the most commonly used and consists of a wettable th in plate partially submerged in a subphase and suspended from a balance. The force acting on the plate is directly propor tional to the surface tension of the liquid. At equilibrium the surface tension can be described as: i in P T n V TA G A F, 2-1 where F and G are the Helmholz and Gibbs free energies respectively, and A is the surface area. An analogous quantity, the line tension is defined as the free energy per unit length associated with the boundary between two phase s on a surface. Because most Langmuir-Blodgett techniques involve using pure water or aqueous subphases, the surface tension of pure water is important. Its value at 25C is 72mN/m.

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34 The surface tension can be measured using a W ilhelmy plate technique as shown in Figure 2-1. The plate is usually very thin and made of platinum; however glass, quartz, mica, and filter paper can also be used. The net downw ard force is given by the equation: gtwh w t glwt Fl P cos 2 2-2 where p and l are the densities of the thin plate mate rial and liquid, respec tively, g represents the gravitational constant, is the subphase su rface tension, and is the contact angle of the liquid on the solid plate. The plat e is also described by its thickne ss (t), width (w), and length (l) (see Figure 2-3). When measuring the change in h (height of the plate) for a constant applied force, the change in surface tension can be calculated using equation 2-3. w t h gtwl 2 2-3 The 2D analogue of pressure at a surface is called surface pressure, and is the difference between the surface tension of a film covered surface, and a pure liquid subphase, 0. A decrease in the surface pressure will be observe d when in presence of a film. This leads the interesting fact that the maximum surface pressure attainable at an air-water interface will be 72mN/m with pure water at T = 25C. Therefore surface pressure can be defined as: 0 2-4 Since the surface pressure is defined as a ne gative change in surf ace tension, the surface pressure can then be determined using equati on 2-5 when measuring the change in F for a stationary plate between a cl ean surface and the same surf ace with a monolayer present. w t h gtwl 2 2-5

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35 If the plate is completely wetted by the liquid (cos = 1), the surface pressure is then obtained from the following equations: w t F 2 2-6 w t F 2 2-7 For the Wilhelmy method, the thickness of the plate used is small, leading to t << w and the equation 2-8. w F 2 2-8 Nowadays, electrobalances allow very little change in the plates movement, improving sensitivity (5 10-2 mN.m-1). 2.2 Equilibrium Spreading Pressure Surfactants in general are simp ly molecules that migrate to the air-water interface and form a film. Molecules are spontaneously inserted into the film only up to a certain surface pressure, known as the equilibrium spreading pressure (E SP). Surface pressure can be thought of as the repulsive force resulting from mol ecules in a film being in close proximity with each other. At equilibrium spreading pressure, the molecules ar e too close to each other to easily allow more material to be inserted. However, if the surface area of the film is increased, more molecules from the subphase are inserted in to the film to maintain the e quilibrium spreading pressure. For polymers, a way to measure the ESP is to pl ace a solid polymer onto the water subphase. The polymer chains are inserted in the monolayer up to a certain surface pressure representing the ESP.

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36 2.3 Langmuir Trough Experiments Measuring the surface pressure as a function of the area of water su rface available to each molecule provides insight into monolayer prope rties. Such experiment s are carried out at constant temperature using a heat exchanger a nd are known as isotherm experiments. The data are recorded by compressing the film at a cons tant rate while monito ring the surface pressure (Figure 2-4). In a manner analogous to bulk materials, dis tinct regions can be observed at surfaces, defining the different phases of the monolayer. These differe nt monolayer states can be observed, depending on the hydrocarbon chain length, nature of the head group, conditions, and other factors; for example, an increase in chai n length increases the interactions between the chains in classical low molecular weight amphiphiles, leading to a more condensed -A isotherm. For a minimal compression, and area s typically in the hundreds or thousand of 2 and pressures of only a small fraction of 1mN/m, th e monolayer exists in the gaseous phase (G). While compressing, the monolayer undergoes phase transi tions to one or more of the following phases: liquid-expanded state (L1) followed by the liquid-condensed state (L2), and finally one or more solid state phases that we will group to gether with the symbol (S) for simplicity. There also exists a series of liquid cr ystalline phases in the L2 region that are important for certain low molecular weight systems and in particular for biological properties of membrane amphiphiles. The various LC and solid phases are not relevant to the polymers discussed in this work so we will not further discuss them. Another important detail is that the sequence of phases observed, like in the case of cooling bulk 3D materials, does not need to show all the potential phases possible, though the phases always proceed in such a way that more and more ordered phases are observed as the pressure increases. If the monolay er is compressed beyond the solid phase, it will

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37 collapse into three-dimensional structures. A wa y to better observe th e different transitions observed on a surface pressure-MMA (where MMA is the mean molecular area representing the available area per molecule) plot is to calcula te the compressibility of the monolayer using equation 2-9 (where K is the compressibility and the surface pressure). ) ( ) ( 1d MMA d MMA K 2-9 d(MMA)/d( ) represents the slope of the surface pressure-MMA isotherm plot. In these experiments, this value was calculated using fo r each point on the isotherm by calculating the instantaneous slope over 2000 points. It is important to note th at the MMA does not take into account the polydispersity of the sample and is based on the number average molecular weight. Therefore, the area occupied by one polymer ch ain of average molecular weight is only calculated. While the regions described above can often be found in small surfactant molecules, diblock copolymers typically have fewer regions as noted above. An example is shown in Figure 2-5. Here, extrapolations of selected regions of the isotherm to zero pressure quantify the isotherm in such a way that the surface behavior of different copolymers and their blends to be compared independent of differences in pressure. It should also be noted that the surface area in experiments involving polymers is often expressed in area per repeat unit. This is calculated by dividing the MMA by the number of repeat units present in one polymer chain. This allows interpretations and comparisons with low molecu lar weight standards. Similarly, for block copolymers the area can be expressed in area per repeat unit in one of the blocks which can be calculated by dividing the MMA by the total number of repeat units of one of the block in one copolymer chain. This simplifies comparisons between copolymers consisting of different segments of varying molecular weight s and even different architectures.

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38 In addition to the classic isotherm experi ment, numerous experiments such as isobars, isochores or compression-expansi on hysteresis, can be done using the Langmuir trough. In the isobaric experiment, the monolayer is compressed to a specific target pressure. The pressure is then maintained constant while the MMA is reco rded over time. Isobars provide the ratio of the current monolayer area to its initial area. The change in area can be correlated with film properties such as creep. In the isochoric experiment, the monolay er is compressed to a specific target pressure. Once that pressure is reach ed, the compression is stopped and the surface pressure is recorded versus time. The compression-expansion experiment measures the ability of the monolayer to relax to its initial expanded stat e after compression. If upon expansio n, the monolayer relaxes following the same pressure-MMA curve, no hysteresis is detected. If however, the second compression produces a new curve, the hysteresis can be detected to quantify the amount of irreversibility of the film formed. Those experiments can give us information about the stability of the film formed. In fact, for high surface pressures, numer ous block copolymers form metastable films, metastability being described as the ability of a non-equilibrium chemical state to persist for a long period of time. For example, for PCL homopolymers, Li et al reported the formation of metastable crystals during the compression cycle with the crystal sizes being dependent on the compression rate.73 During the expansion cycle, those crys tals were re-adsorbed (melted) onto the water subphase. Unless otherwise noted, surface pressure meas urements were performed using a Teflon Langmuir trough system ( W = 160 mm, L = 650 mm; KSV Ltd., Finlan d) equipped with two movable barriers and a Wilhelmy plate Between runs, the trough was cleaned with ethanol and rinsed several times with millipore filtered water (resistivity 18.2 M .cm). The samples were

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39 typically prepared by dissolving approximately 1 mg of polymer in 1 mL of chloroform. Volumes ranging from 10 to 30 L were spread dropwise on a Mi llipore filtered water subphase with a gastight Hamilton syringe. The chloroform was allowed to evaporate for 30 min to ensure no residual solvent remained. When not in use, the volumetric flasks containing the polymer solutions were wrapped with Teflon tape followed by Parafilm and stored at 10 C in order to prevent changes in concentration due to chloroform evaporation. In all the experiments, subphase temperature and barrier speed were kept cons tant at 25 C and 5 mm/min, respectively. 2.4 Langmuir-Blodgett Films Besides Langmuir monolaye rs, a common application of the Langmuir trough is the transfer of monolayer onto a solid substrate. This is accomplished by dipping the substrate into the subphase, allowing the adsorption of the mo nolayer. The surface pressure is maintained constant by a computer controlled feedback sy stem between the electr obalance measuring the surface pressure and the barrier moving mech anism. Depending on the number of dippings, successive monolayers, from seve ral to hundreds, can be deposited onto the solid substrate. Numerous substrates have been used. For an alytical work involving subsequent imaging, mica is commonly preferred in LB film transfer due to its low cost, easy cleaning, and easy preparation. However, mica posse sses a water layer that may aff ect the film transfer. Other substrates such as silicon wafers can be used; tr eatment with chromic sulfuric acid renders them highly hydrophilic. Still other materials can be used as hydrophobic s ubstrates, including graphite, polymer films, and silanized silicon dioxide. LB films can be formed either by pulling out or dipping the substrate into the subphase. Pulling or the upward pass of the substrate through the subphase is also known as an upstroke while the downward dipping refers to the downs troke. Three different types of deposition can exist (Figure 2-6). The X-type deposition can be done by a downstroke whereas Z-type occurs

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40 during the upstroke. The Y-type, the most common, is characte rized by deposition on both an upstroke and a downstroke. Interm ediate structures can sometimes be observed for some LB multilayers and are often referred to as XY-type multilayers. Once transferred, these films can be studied by different surface analysis techniques, such as optical spectroscopy, X-ray diffraction, as well as atom ic force microscopy (AFM) or transmission electron microscopy (TEM). The quantity and quality of the deposited m onolayer on a solid support are measured by a so called transfer ratio, TR. Th is is defined as the ratio betw een the decrease in monolayer area during a deposition stroke, Al, and th e area of the substrate, As. For ideal transfer the TR is equal to 1. However, experimentally the TR often va ries significantly. Low transfer ratios indicate a that the polymer chains transferred on the s ubstrate are less dense than those on the water subphase. For transfer ratios greater than 1, the polymer chains on the solid substrate are more densely packed than those at the air-water interfa ce. Another type of tran sfer ratio can also be calculated and recorded in typical LB experiments. The instan taneous transfer ratio is the amount of monolayer transferred versus the amount of substrate pulled out at a specific time. This type of transfer ratio gives information on the uniformity of the film transferred. In our experiments, all the instantaneous tran sfer ratios observed were homogene ous indicating a good transfer of the monolayer onto the mica substrate. 2.5 Atomic Force Microscopy 2.5.1 Instrument Parameters Contrary to its precursor, s canning tunneling microscopy (STM), which only allows the study of electrically cond uctive samples, AFM can be applied to both conductors and insulators. The instrument consists of a tip at the end of a cantilever, whic h bends in response to the force between the tip and the sample (Figure 2-7).

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41 Since the cantilever obeys Hookes law for small displacements, th e interaction force between the tip and the sample can be found: F = -k x 2-10 where x is the cantilever deflection and k the spring constant. In the early stages of AF M, contact mode was used. This method consists of a tip in close contact with the surface. The defl ection of the cantilever is sensed and compared to the desired value of deflection. The voltage n eeded to restore the desired valu e of deflection is a measure of height of features on the sample surface. This mode was quickly forgotten for polymer studies because of excessive tracking forces applied by the probe to the sample. To remove these drawbacks, a non-contact m ode was developed. In this case, the tip hovers 50-150 Angstrom above the sample surface. The attractive Van der Waals forces acting between the tip and the sample are detected, a nd topographic images are constructed by scanning the tip above the surface. This technique was also found to be inapplicable to polymer samples. In general, the fluid contaminant layer existing on the sample is substantially thicker than the range of the Van der Waals force gradient and, th erefore, all attempts to image the true surface with non-contact AFM fail as the oscillating probe becomes trapped in the fluid layer. Later, a third method was developed in order to study softer samples (Figure 2-8). This mode, called tapping mode, consists of alternatel y placing the tip in cont act with the surface to provide high resolution and then lifting the tip o ff the surface to avoid dr agging the tip across the surface. As the oscillating cantilever begins to intermittently touch the surface, the cantilever oscillation is necessarily reduced due to energy loss caused by th e tip contacting the surface. The reduction in oscillation amplit ude is used to identify and measure surface features.

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42 AFM involves scanners made from piezo electric material, a substance which proportionally contracts and expa nds, depending on an applied vo ltage. If a positive voltage elongates the scanner, a negative voltage contracts it. The scanner is made of a piezoelectric material surrounded by electrodes which control the applied voltage. As scanning occurs in three dimensions, a scanner tube contains three piezo el ectrodes for the X, Y, and Z directions (Figure 2-9). Piezoelectric ceramics are capable of moving a probe very small distances. However, when a linear voltage ramp is applied, they move in a non-linear motion (Fig ure 2-10). All AFM must therefore be calibrated in the X-Y axis so that the images presented on the computer screen are accurate. Height measurements require that th e piezoelectric ceramics in the Z axis of the microscope be both linear and calibrated. Often the microscope is calibrated at only one height; however, if the relationship between the measured and the actual Z heig ht is not linear, the measurements will not be correct. The studies described in this work utilize a Digital Instrume nts Nanoscope system. Several different scanners can be used depending on th e sample studied. They differ on the scanning size and resolution. For example the J-scanners can scan images up to 125 m, whereas E-scanners scan smaller sizes of 10 m or less. In these experiments, the x-y range was calibrated monthly using a calibration grid with known periodicity. Likewise, z calibration was performed biannually, using standards of diffe rent heights. In our case, th e standard used is a silicon calibration grating with a step height of 18.5 1nm (close to the height of our samples). 2.5.2 Limitations Some limitations exist when using AFM. The tip can easily be damaged and the formation of debris can lead to dullness of the tip. A blunt tip can not det ect narrow valleys or higher peaks

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43 and will therefore create a blurry image. Resoluti on in AFM images consists of lateral (X,Y) and vertical (Z) components with th e lateral resolution de pendent on the tip radius of curvature.101 However the fabrication technique of the AFM probes leads to the formation of tip possessing flat, rectangular ends. This reduces the sharpness and therefore the resolution of the images as shown in Figure 2-11. Polymer thin films can thus be characterized through a combination of Langmuir and AFM techniques. Such methods allow th e easy control of surface density as well as the easy transfer of surface films onto solid substrates. The results of such analysis will be presented in the subsequent chapters. 2.5.3 Parameters Used Surface films of the copolymers were transf erred onto freshly cleaved mica at various pressures (25 C). The desired surface pressu re was attained at rates of 10 mm min-1. Once the film had equilibrated at a constant for 30 minutes, the mica was then pulled out at a rate of 1 mm min-1. The transfer ratios are noted in the chapters and unless specifically noted, the transfer ratio remained constant over the transfer and even transfer can be assumed. The transferred film was air-dried in a dust-free environment for 24 hours and subsequently s canned in tapping mode with a Nanoscope III AFM (Digital Instruments, Inc., Santa Barbara, CA) using silicon probes (Nanosensor dimensions: T = 3.8-4.5 m, W = 26-27 m, L = 128 m). Experiments were redone after a few weeks in order to observe if any change occurr ed in the film structure after transfer. No difference was detect ed, indicating the stability of our samples once transferred onto the solid substrate. Tapping mode was used, giving a better image of a polymer sample without damaging the surface by dragging the tip. This mode consists of a tip vibrating at its resonance frequency in tapping the surface. As the tip enco unters a surface featur e, its amplitude of

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44 oscillation is decreased from its set-point value. This decrease is noted by the sensor and the tip is moved up away from the sample to re-attain the set-point amplitude. A similar behavior happens when the tip moves past the feature. A topographical map of the sample can then be recorded. 2.5.4 Image Analysis A program designed by Yves Heckel, an unde rgraduate student from Paris, France, allowed us to define the characteristics of the aggregates observed in the AFM images. Parameters such as the number of domains as we ll as the size of those domains were determined in order to better understand the aggregation behavi or of the copolymers. This program allows a domain size range to be chosen in which the values of the minimum and the maximum can be varied (Figure 2-12). The image on the left of the screen allows the user to adapt the area range limits using a visual aid. Another attribute of this program, is that it counts the number of domains present in one chain as opposed to considering the chain as a single domain. If su ch a mistake is made, however, the program can be manually manipulated by the user to define domain separation and number. While this feature gives a better approximation of the sh ape and number of domains, the resulting disadvantage is lo wer user efficiency, but significantly higher accuracy. Once all the domains are counted, the computer gives a computed image representing all the domains present with the different chai ning and angles for each domain (Figure 2-13). Computer errors can occur, giving wrong angles and poorly defined aggr egates, in which case the user can, by clicking on the domain, redo the separations and redefine the domain (Figure 214). The software allows domain populations to be chosen and analysis error to be manually corrected, permitting the analysis of images with more than one domain population, as in the case of the images observed for the blends.

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45 Additional image analysis was performed w ith software from Nanoscope instrument. Notably, this software allowed calculation of dom ain size and domain height. For this reason, the calibrations described above are very important in order to obtain absolute values. Precise height values can be obtained by cross-section anal ysis. This is done by dr awing a line across the domains of interest, giving a cross-sectional trac e of the topography an example is given in Figure 3-13. 2.6 Brewster Angle Microscopy The AFM necessitates, as seen previously, appr opriate modification (notably transfer to a solid substrate) in order to study monolayer films, therefore there is an unc ertainty of whether the system under study is in its original state or not. The Brewster angl e microscope is a noninvasive technique that allows the characterization of floati ng monolayers at the air-water interface. Ultra thin f ilms on air-water interface or on dielect ric substrates can therefore be directly observed. For a beam of p-polarized light, there is an angle of incidence at which no reflection occurs. This is called the Brewster angle and is de noted by equation 2-11 where n is the refractive index of the corresponding phase. air Subphasen n tan 2-11 Introducing a thin film in be tween the two phases changes th e optical properties of the system so that a small amount of the incide nt intensity is refl ected (Figure 2-15). The principle behind the Brewster Angle Microscope (BAM) makes use of the zero reflectance of an air-water interf ace for vertically linea rly polarized light at the Brewster Angle of incidence. As stated in e quation 2-11, the Brewster angle is calculated from the refractive indexes of the two substrates invol ved (for example, the critical an gle of the air-wat er interface is

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46 53). When a condensed phase of a monolayer w ith different refractive index is spread or deposited on the interface of interest a measurable change in reflectivity will occur. The reflected light can then be used to form a high contrast image of the lateral morphology of the spread or deposited layer. For example, a monolayer spread on an air-water interface is extremely thin, approximately 0.5 % of the wavelength of visibl e light. The relative eff ect on the electric field reflected from a water surface is therefore very small and the monolayer is under normal conditions quite invisible. Howe ver, if the water surface is i lluminated with pure vertically linearly polarized light at the Brewster angle before spread ing the monolayer at the air-water interface, there is no reflection from the water surface and the monolayer itself is much more easily visualized. One of the drawbacks to this me thod is that due to the use of a small aperture (the entire interface is not scanned) the lateral resolution is low compare to the AFM: a typical resolution is only about 2 m. The BAM consists of an opto-mechanical, an electronic control unit and a personal computer (Figure 2-16). The Z lif t of the BAM is a linear stage with around 30mm to travel. To avoid any damage, the vertical di stance between the sample and the objective should be such that there is some space left if the Z lift is in its lowest position. A crystal polarizer allows to produce p-polari zed laser beam. To obtain optimal results, both the angle of incidence and the state of polarization of the incide nt beam must be set to their optimal values (Brewster angle and p-polarization). The BAM also contains a scanner that consists of an objective and a mechanical unit. The objective has typically a nominal 10 magnification that images the sample surface onto the CCD chip and provides a diffractio n limited resolution of approx. 2 m. For optically anisotropic

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47 materials, the reflected light shows both sand p-polarized components. This anisotropy can be detected by the analyzer in front of the CCD camera. All our BAM experiments were perform ed using a Nanofilm Technologie GmbH (Goettingen, Germany) BAM2plus system. A polarized Nd:YAG laser (532 nm, 50 mW) was used with a CCD camera (572 x 768 pixels). Th e instrument is equipped with a scanner that allows an objective of nominal magnification of 10x or 20x to be moved along the optical axis, producing a series of small focu sed image. X-y scanning and s ubsequent image reconstruction allows a larger focused image to be acquired. For the 10x objective a la ser power of 50% and maximum gain is used. A shutter timing (ST) of 1/50 s, 1/120 s or 1/1000 s is used to obtain maximum contrast between the monolayer and th e COM crystals. For the 20x objective a laser power of 80%, maximum gain and ST of 1/50 s are always used. The incident beam is set at the Brewster angle in order to obtain minimum si gnal before spreading the monolayer. A piece of black glass is placed at the bottom of the trough to absorb the refracted light beam that would otherwise cause stray light. The pol arizer and analyzer are set at 0 for all experiments. The laser and camera are mounted on an x-y stage that allo ws examination of the monolayer at different regions. The reflected light is recorded into th e CCD camera. Different systems can be studied using BAM. Isotropic (same reflection in all directions) or anisot ropic (reflection in a preferential direction) monolayer s can be observed. For typical anisotropic thin film systems such as crystalline monolayers, it is possible to invert the contrast be tween domains by rotation of the analyzer in the range of 70 -110

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48 Figure 2-1. The original Langmuir balance as designed by I. Langmuir76 Figure 2-2. Set up of a typical Langmuir trough Figure 2-3. Schematic of the Wilhelmy plate l w h t Dipper Electrobalance Barrier Barrier Heat Exchanger Wilhelmy Plate Substrate Trough

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49 Figure 2-4. Schematic Surface pressure-MMA isotherm Figure 2-5. (a) Several represen tative isotherms are shown, de picting the dependence of surface pressure () on mean molecular area. S L2 I L1 L1-G G (mN/m) Mean Molecular Area (nm2/molecule)

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50 Figure 2-6 Different types of deposited LB film s (borrowed from Jennifer Logans dissertation48) Figure 2-7. Optical system that detects canti lever deflection (Figure adapted from Digital Instruments Training Notebook103) Figure 2-8. An atomic force microscope (AFM) (left) and the tapping m ode electronic set-up (right) are shown (Figure adapted fro m Digital Instruments Training Notebook103)

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51 Figure 2-9. AFM scanner tube c ontaining the piezoelectric materi al and metal electrode. The x, y, and z-directional components of the s canner are also indicated (Figure adapted from Digital Instruments Training Notebook103) Figure 2-10. Dependance of the piezoelectric ce ramic on voltage (Figure adapted from Digital Instruments Training Notebook.103)

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52 Figure 2-11. Imaging limitations in tip sharpness: Pa rt (a) shows the dimple that results from two rigid spheres while (b) illustrates the im aging of two spikes. Part (c) resembles polymers in depicting two soft spheres th at undergo slight deformation. (Figure adapted from Sheiko.101)

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53 Figure 2-12. The software allows choosing a domain range by vary ing the minimum and maximum areas Figure 2-13. The software gives you a computed image representing the different domains and the possible angles between domai ns in the presence of chaining

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54 Figure 2-14. Error made by the computer can be corrected by the user Figure 2-15. Scheme of the Brewster angle at th e air-water interface and in presence of a thin film

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55 Figure 2-16. The different components of the BAM2 are shown (Borro wed from the Nanofilm Surface Analysis website)

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56 CHAPTER 3 BLENDS OF A POLYSTYRENEBLOCK -POLY(ETHYLENE OXIDE) COPOLYMER AND ITS CORRESPONDING HOMOPOLYMERS 3.1 Isotherm Experiment 3.1.1 PEO Homopolymer As previously described 46, PEO homopolymers of sufficiently high molecular weight form thermodynamically stable Langmuir films at lo w surface pressures. The isotherm of a PEO homopolymer (Mn = 100,000g/mol) is given in Figure 3-1. Upon compre ssion, the monolayers collapse, leading to the irrevers ible dissolution of the PEO chains into the water subphase. The collapse pressure value is highly molecular weight dependent102 and reaches a maximum 10mN/m for high molecular weight PEO. 3.1.2 Linear Polystyreneblock -Poly(ethylene oxide) (PSb -PEO) Diblock Copolymer Linear PSb -PEO copolymers represent a convenient choice when studying the interfacial behavior of amphiphilic compounds, due to the bi ocompatibility of the PEO block and the low cost and availability of the PS bl ock. Moreover, they have been widely studied and found to form stable, condensed surface films.32-44 The -A isotherm for a 32,500 g/mol copolymer (see Table 3.1.) displays a pseudoplateau between 8 and 10mN/m and is shown in Figure 32. The observed pseudoplateau results from the hydrophilic part of the copolymer and appears over the same pressure range as the collapse pressure of a PEO homopolymer.45 The shape of the isotherm is independent of the copolymer solution concentration and the compression speed. In addition, multiple runs conf irmed these experiments to be reproducible within 1.0nm2. Within the isotherm, three distinct re gions are observed. At large molecular areas, the surface film is expanded (Region I); this is usually called the pancake region due to the shape the PEO units form on the water surface.

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57 As compression continues, a plateau appears (R egion II) over the pressure range of 8 to 10 mN/m. Kuzmenka and Granick46 studied the behavior of PEO homopolymers at the air-water interface with varying molecular weights. They observed a constant equilibrium spreading pressure for polymers having a molecular weight beyond 100,000g.mol-1. The pseudoplateau detected in the case of our copolymer is in the same range of the collapse pressure of a PEO homopolymer and corresponds to the hydration and desorption of these chains from the surface and into the subphase. The appearance of the pseudoplateau with an increasing amount of PEO in the copolymer was considered by Devereaux and Baker43 They studied two PSb -PEO copolymers containing different masses of PEO. The 7% PEO copolym er had no pseudoplateau whereas the 15% PEO did. This observation was explained by the long PS interfering with the PEO blocks, preventing the PEO from stretching into the aqueous subpha se. Our results are in agreement with this interpretation showing a plateau fo r a copolymer containing 32% PEO. Considering the affinity of PEO for water, at large molecular areas the films most likely exist as PEO films with globules of PS on top. Region II, however, represents a biphasic phase where aggregates and single polymer domains coex ist. The fact that the pseudoplateau occurs within the same pressure range as the co llapse region of PEO homopolymer illustrates the significant influence PEO has on the copolymer surface film. Bijsterbosh et al.33 and Goncalves da Silva et al.34,39 both demonstrated the existence of a pseudo first-order transition from a pancake-like structure to that of a brush upon compression of a series of PSb -PEO copolymers containing a constant PS length a nd varying amounts of PEO. While this model is prevalent in the literature, Cox et al.41,42 provided a new interpretation for the presence of the pseudoplateau assuming that the formation of brushes is not possible due to PEOs low surface energy. They

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58 proposed that PEO instead undergoes a dehydrat ion process and a conformational change upon compression. This observation is consistent with the one made by Shuler and Zisman45 for a PEO homopolymer. They explained the la ck of reversibility observed in their hysteresis experiments by a structural change in the polymer molecule during the compression cycle. Contrary to PEO homopolymers, a third re gion (III) appears beyond the pseudoplateau and shows a sharp increase in surface pressure, indica ting the formation of more rigid films. Here, the PS block serves as an anchor, keeping the PEO at the interface and allowing the films to be compressed to higher surface pressures. Without the PS, PEO would dissolve into the aqueous subphase at pressures beyond the plateau. In examining Region III, Bijsterbosh et al .18 and Goncalves da Silva et al.34,39 studied a series of copolymers with varying PEO lengths and a constant PS block. While Region III typically reflects PS, they found that the copolymer interfacial behavior at high pressures depends slightly on the size of the PEO block. 3.1.3 Blends of a PSb -PEO diblock copolymer and a PS homopolymer The same linear copolymer described in the pr evious section was used to study the effect of adding a homopolymer solution on its beha vior at the air-wate r interface. The PS homopolymer used has a molecular weight of 20,000 g/mol, which corresponds to the molecular weight of the PS block in the copolymer. Differe nt ratios of copolymer and homopolymer were studied in order to determine the impact on the formation of Langmuir monolayers. The mixed monolayers were formed by separate ly spreading solutions of the PS and the PSb -PEO block copolymer. After evaporation of the solvent, the floating monolayer was symmetrically compressed by the two movable barriers. -A isotherms were recorded for several amount of added PS homopolymer (Table 3-2).

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59 Figure 3-3 shows the isotherms data for diffe rent copolymer/homopolymer ratios. For all these blends as well as the pure linear copolymer isotherms, the three regions defined in the previous section were observed. This can be bett er detected on the compressibility plot where two different local maxima are observed (Figur e 3-4). Each maximum corresponds to a phase transition and a rearrangement of the polymer chains in the m onolayer. Increasing the amount of PS has only a small effect on the phase transi tions observed, the local maxima in the compressibility plots show only a small in crease when increasi ng the amount of PS homopolymer. This suggests the presence of PEO -related phase tr ansitions independent of the PS content. 3.1.3.1 Pancake Region (I) The first region, defined by low surface pressu re and low surface density, can be quantified by its extrapolated area, AP (Figure 2-5). The values for ever y blend remain constant while varying the amount of PS (average AP = 89.7 nm2). This is in agreement with a film of PEO with globules of PS on top of it where increasing the amount of PS will not change the area occupied at the interface by the PEO. Faur et al.40 observed the same behavior for pure diblock copolymers at the air-water interface and showed that at lo w coverage, the interaction be tween the ethylene oxide (EO) monomers and the interface is attractive and ther efore leads to the adsorption of the EO at the air-water interface. They assume the pressure to be only due to the total number of PEO segments in water. As a result, increasing the PS should have no effect on the behavior of this region. Logan,48 Logan et al. ,51 and Francis et al. ,49,50 observed a similar trend for star copolymers of PSb -PEO in which the pancake area did not depend on the number of PS segments. The pancake area per EO monomer (0.38 nm2) is in reasonable agreement to the one found by Logan et al. for star copolymers (0.33 nm2) and to the that determined for linear PSb -

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60 PEO by Gonalves da Silva et al. (0.27 or 0.31 nm2)34,39 and Bijsterbosch et al.18 (0.31 nm2). The area observed in our case is a little higher; the addition of PS hom opolymer increases the aggregation of the PS segments allowing th e PEO blocks to spread more easily. 3.1.3.2 Pseudoplateau Region (II) The pseudoplateau observed in Sect ion 3-2 for a pure linear PSb -PEO copolymer is observed for all blends and remains constant for different amount of PS. The width ( AP) of the pseudoplateau can be estimated as the difference between ATransition 1 and ATransition 2 (Table 3-3, Figure 2-5). The value of AP remains constant for every blend (average AP = 21.9 nm2). Region II is believed to represent a biphasic regi on. The phase transition is mostly due to the reorganization of the PEO chains from a pancake to a brush conformation and therefore a change in the amount of PS does not have any effect on th e width of the pseudopla teau. In this region, the EO repeat unit occupies 9.2 2 which is smaller than the value found by Logan et al.48,51 (13.3 2) for star copolymers. This difference ca n be explained by the fact that the PEO homopolymer is being dissolved in the s ubphase as the monolayer is compressed. 3.1.3.3 Condensed Region (III) In Table 3-2, the theoretical area (A0) that a compact surface f ilm would occupy at zero pressure was determined for each blend. In agre ement with our expectations, the area increases with an increasing mass of PS. The condensed ar ea, representing mostly the behavior of the PS chains, varies linearly with the total mass of PS chains. This behavior can be compared to the behavior of copolymers presenting PS ch ains of high molecular weights. Cox et al.41,42 studied several PSb -PEO copolymers with varying PS molecula r weights, observing a variation in the A0 values. The increase of A0 with increasing PS can be explai ned by the aggreg ation of the PS homopolymer with the PS chains of the copolymer.

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61 To compare our results with those from c opolymers of longer PS blocks, a normalization of the total number of styrene units wa s obtained by using the following equation. Diblocks PS r Homopolyme PS r Homopolyme PS Diblock PS TOT PSn n N N N, , 3-1 With NPS,Diblock and NPS, Homopolymer being the number of styrene repeat units in the PSb PEO diblock copolymer and the PS homopolymer respectively. nPS, Hompolymer and nPS,Diblocks represent the number of moles of PS in the homopolymer and th e copolymer. When plotting the condensed area per EO unit versus the apparent number of styrene units (Figure 3-5), a linear dependence can be observed (R2 = 0.9932) with a trendline of y = 0.0001 x + 0.0753. The positive y-intercept shows us that even without any PS present in the monolayer, the PEO occupies 7.5 2/EO units. This value is significantly sm aller than the one observed for star copolymers by Logan et al. (16 2/EO). 48,51 When PEO homopolymers are compressed at the air-water interface, no condensed area is observed. Without the presence of PS as an anchor, the PEO eventually dissolves in the aqueous s ubphase. In our case, while most of the PEO homopolymer remains aggregated to the PEO bl ocks of the copolymer, some of it could be dissolving in the water, explai ning the smaller occupied area. The collapse area plotted vs. the total number of styrene units follows the trendline y = 0.0288 x + 17.914. The area per styrene unit obtained from the slope (2.9 2) is smaller than the one J. Logan described for the be havior of star copolymers of PS -b -PEO at the air-water interface (ranging from 6.2-8.32, depending on architecture).51 She reports the results for (PEO26)8-(PS42)8 (an 8 arm PSb -PEO star copolymer with each arm containing 26 EO units and 42 Styrene units) which has a total of 336 repeat units of PS. For Blend 4 which has a total number of PS repeat units of 355, the PS homopolymer occupies an area equal to 5nm2. This value is in the same range as the area per styren e value found in the literature for an atactic PS in

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62 the bulk, calculated from the radius of gyration (38).59 The PS, not covalently bound to the PEO, tends to adopt a random coil conformati on less compact than the conformation produced by the PS segments of the copolymers. This c opolymer can be compared to Blend 4 which presents a total of repeat units of 355 for the PS (using the formula described previously). J. Logan obtained a value of 28.6 nm2 which is similar to the value obtained for Blend 4. 3.1.4 Blends of a PSb -PEO diblock copolymer and PEO homopolymer Contrary to PS, PEO is an amphiphilic polym er forming monolayers at the air-water interface. The addition of a PS block as an an chor keeps the PEO from going into the water subphase. This also allows the formation of more compact films by compressing at higher pressures. The effect that unencumbered PEO has on such films is examined in this part of the discussion where blends of th e copolymer and homopolymer PE O were studied (Table 3-4). Figure 3-7 shows the isotherm data for several blends of the 32,500g.mol-1 PSb -PEO copolymer and a 100,000 g.mol-1 PEO homopolymer. The same transitions can be observed for all the copolymers independent on the amount of PEO added. When looking at the compressibility plots in Figure 3-8, one can s ee that when increasing PEO the local maximum present at 10mN/m increases. This sugge sts a PEO-related transition at 10mN/m. 3.1.4.1 Pancake Region (I) Similar analysis was done for the homopolymer PEO blends as that seen for the PS ones. As with the pure copolymer, the resulting isotherms displayed all three regions. AP was obtained for each blend and for the pure PSb -PEO from the -A isotherms (Table 3-5). The pancake area depends linearly on the total number of ethylene oxide units (R2 = 0.9968) with a trendlin e of y = 1.1297 x 173.95. The area obtained from the slope (1.13 nm2) is significantly higher than the one observed by Sauer et al.81 for a PEO homopolymer (0.40-0.48 nm2). Gonalves da Silva et al.34,39 recorded a smaller area for PSb -PEO diblock copolymers

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63 (0.27 and 0.31 nm2). This can be explained by the fact that the PEO chains from the homopolymer pack less closely wh en in the presence of the PSb -PEO diblock copolymer. We can also observe a negative y-intercept indi cating that all the EO units are not at the interface. In such a case, a pa ncake area equal to zero should correspond to zero EO units. Even with a negligible effect on the pancake area, the PS units may trap some of the PEO, leading to a lower apparent number of PEO units. 3.1.4.2 Pseudoplateau Region (II) The addition of PEO, however, has an eff ect on the shape of the pseudoplateau observed for a pressure around 10mN/m. The more PEO is added to the monolayer, the longer the biphasic region becomes. To illustrate this point, ATransition 2 (Figure 2-5) was recorded for each blend as well as for the pure diblock. The results are given in Table 3-6. A graph of ATransition 2 vs. the ratio of number of mole s of homopolymer vs the number of moles of diblock shows a linear dependence (R2 = 0.994) with a trendline of y = 1082.9 x + 47.532 (Figure 3-9). To be able to compare those results to th ose published previously for pure diblocks or star copolymers, an identical form ula as the one used in the previous part was developed. While ATransition 1 remains constant for every blend, one can observe an increase in ATransition 2 indicating the presence of a larger pseudoplateau area. Diblock PEO r Homopolyme PEO r Homopolyme PEO Diblock PEO TOT PEOn n N N N, , 3-2 The values calculated using this fo rmula, are reported in Table 3-7. A linear dependence (R2 = 0.9942) was observed when the area for the second transition was plotted vs. the total number of repeat units of PEO, yielding a trendline of y = 0.4769 x 66.032 (Figure 3-10). This dependence is detected in the compressibility plots also showing a higher local maximum when increasing the amount of PEO (Figure 3-8).

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64 These observations compare to those seen by Faur et al.40 They studied the phase transitions in monolayers of PSb -PEO copolymer at the air-wate r interface for different PEO block sizes. Faur et al. observed an increase in the length of the pseudoplateau as the number of PEO units increases. The transition from pancake to brush becomes more and more first order as they increase the PEO segment size. In addition, one of the diblock copolymers they studied consisted of 31 repeat units of PS and 700 of PEO, a PEO amount similar to that of Blend 4. The Faur copolymer demonstrates a -A isotherm with an almost flat pseudoplatea u, confirming the first order transition of the copolymer. Similarly, Blend 4 displays a pseudopl ateau representing a strong indication of a first order transition. By adding PEO homopolymer to our monolayer, we have been able to broaden the range of the copolymer properties without going trough time-consuming synthetic techniques in order to increase the size of the PEO block. 3.1.4.3 Condensed Region (III) This third region appears at higher su rface pressures beyond the pseudoplateau. As demonstrated by Shuler and Zisman,45 such a region does not exist for a PEO homopolymer, as no anchor exists to prevent PEO from complete ly immersing in the water subphase. This region depends only on the length of the PS blocks and not on PEO, as demonstrated by the -A isotherms of the different blends in Figure 3-7. A0 remains the same regardless of PEO added (23.7 nm2). By blending a PSb -PEO diblock copolymer with its corresponding homopolymers, we were able to mimic linear chain behavior by ma nipulating PS and PEO quan tities. On one hand, the addition of PS has proven to have the same effect on the copolymer behavior as increasing the PS block size. We see an increase in the co ndensed area while adding more PS homopolymer

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65 to the system. On the other hand, raising the am ount of PEO only had an effect on the biphasic region of the isotherm. While this technique could be a good alternat ive to time-consuming synthetic techniques and expensive sample purchases, experiments still need to be performed with various molecular weight homopolymers as well as hysteresis data in order to better unde rstand the aggregation behavior of those films. Additional analysis continues in the next sec tion in which the blends are transferred as Langmuir-Blodgett films and examined through atomic force microscopy (AFM). 3.2 Atomic Force Microscopy (AFM) Experiments AFM is a technique that provides the opportunity to study surface morphology and structure at the submicron scale. By investiga ting transferred Langmuir-Blodgett films, AFM can give insight into the behavior of the copolymer blends at various pressures, providing both quantitative and qualitative results. Such data helps demonstrate the degree of interaction between the copolymer and homopolymers. The hydr ophilicity of the substrate allows us to consider the hydrophilic PEO to be attached to the mica wher eas the hydrophobic PS occupies a higher layer. By consequence in the AFM images the PEO is represented by the darker (lower) areas whereas the PS exists as the brighter (higher) domains. The AFM software contains several functions for image analysis. One method represents the three-dimensional surface plot of the imaged sample, as shown in Figure 3-12. The color shading is a representati on of the height of the features on the sample (up to 7.3nm for Figure 312, for example). Precise height data can be obtained for given domains through section analysis. This technique is illustrated in Figure 3-13. A line is traced across the domain region of interest, giving a cross-sectional vi ew of the sample. In this example, the height difference between the

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66 two marked domains is 1.2nm and the difference between the domain at the left and the PEO surface (brown) is 4.5nm. 3.2.1 Linear Polystyreneblock -Poly(ethylene oxide) (PSb -PEO) Diblock Copolymer AFM is a valuable technique for studying mo rphologies formed by spreading copolymer solutions at an aqueous subphase. Bodiguel et al. demonstrated the complementary nature of AFM and TEM in depicting phase separa tion of two distinct polymer blocks.96 The technique assumes that the morphology of the transferred film represents that of th e floating monolayer and that transfer is homogeneous. For pressures below the pseudoplateau pressure, transfer ratio (TR) were found to be close to unity ( 1.10) implying a homogeneous transfer. However, once the pseudoplateau pressure is reached, a decrease in TR is observed ( 0.40) suggesting that the films observed by AFM are less condensed than the floati ng monolayer. In the case of a TR equal to 1, the image observed for a pressure of 15mN/m should show a homogeneous layer of PS. However, we observed large domains surrounded by areas of PEO. The TR of 0.40 leads us to the conclusion that the monolayer expanded after transfer onto the mica substrate and does not represent the structure we would observe directly onto th e water subphase. Langmuir-Blodgett (LB) films were prepared at several surface pr essures and then studied using AFM in tapping mode. For each sample, an average of ten images was taken to ensure reproducibility. The images shown in Figure 3-14 clearly demons trate the formation of ordered structures in which the observed morphology depends on surface pressure. In fact, three distinct regions corresponding to those in the isotherm can be se en once again. For pressures of 4 and 7 mN/m (Region I of Figure 3-2.), images show a majority of single do mains, typical of an expanded liquid. Two-dimensional micelle s form at the air-water inte rface with a morphology depending on the ratio of the hydrophobic and hydrophilic block sizes. For pre ssures under 7mN/m, circular

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67 micelles are observed like the one described by Potemkin et al.105 (Figure 3-15) where one of the blocks is strongly adsorbed on a planar surface. In the case of PSb -PEO at the air-water in terface, similar micelles are observed with the PEO extending more and more in the aqueous sub phase as the concentration increases (Figure 316). When compression continues and reaches the pseudoplateau range (R egion II of Figure 32), chain formation is detected and continues unt il the collapse pressure is attained. The images also demonstrate the presence of intermediate stages in which the single domains begin to aggregate prior to chain formation. Due to the hydrophilicity of the mica, we suppose PEO represents the bottom layer whereas PS occupies the top part of the LB film at a thickness of some nanometers, ranging from about 2 to 10 depending on the blend. In our im ages, the darker layer represents PEO and the bright domains show the PS blocks. Using a pr ogram described above, the number of domains per image was found, allowing the molecules pe r domain (or aggregat ion number) to be calculated. For each given pressure, the aggregation number was determined using equation 3-3. = A/Nd. 3-3 where refers to the number of molecules per domain, A the scanned area of the image, Nd the number of domains, and the mean molecular area during transfer. As shown in Figure 3-17, the number of molecules/domain depends strongly on the surface pressure. For pressures less than 10mN/m, the number of molecules/doma in remains almost constant. However, once the pressure of the pseudoplateau is reached, an increase in aggregation number is observed.

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68 As compression continues, aggregation increa ses and at the transition between Regions II and III, the aggregation number rises sharply. This behavior is another i ndication of the transition between the liquid expanded state an d the liquid condensed state. Logan et al.48,51 showed that compression-induced aggregation occurs when PEO is pushed into the aqueous subphase. However, at higher pressures, some PEO can remain at the interface and separate the PS domains. This situation represents two conflic ting forces. The attraction between PEO and the water allows the polymer to spread on the surface, whereas the repulsion of PS with both water and PEO drives aggregation. Cox et al.41,42, however, thought the relativ e interaction of the two blocks with the subphase and air is a more probabl e explanation for the exis tence of aggregation. 3.2.2 Blends of a PSb -PEO diblock copolymer and a PS homopolymer To observe the possible formation of aggr egates between copolymer and homopolymer, the blends were studied by AFM for different tran sfer pressures. Transfer s for pressures equal or below 10mN/m are considered homogeneous (TR 1.04) while transfers made for pressure above the pseudoplateau pressure (TR 0.40) indicate that the aggreg ates observed by AFM are less packed than those present at th e air-water interface. In Figure 3-18, the AFM images for the pure diblock, Blend 2, and Blend 5 at several transfer pressures are gi ven. As described previously, a chaining of the domains is observed for the pure PSb -PEO when increasing the pressure. However, in the blend case, the addition of PS homopolymer seems to i nhibit the formation of these chains. In Figure 3-19, the histograms of the domain area are given for both films at a transfer pressure of 10 mN/m. The pure diblock exhibits larger domain areas whereas the blend seems to exhibit the formation of big and small domains. In the pancake region, the pure diblock e xhibits local hexagona l packing with six neighbors for each domain, showing that even at low pressure, the copolymers arrange into

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69 surface micelles. In the case of the blends, the addition of PS to the monolayer disrupts this packing by increasing the size of only some of the domains and enabling a population of smaller domains to form (Figure 3-19). The PS homopolym er aggregates with some of the PS blocks from the copolymer increasing the size of the domains observed by AFM. The addition of PS in the domains increases their attractive forces on the free PS. This behavior can be compared to that described by Logan,48 Logan et al. ,51 and Francis et al. ,49,50 for PSb -PEO star copolymers of va rious hydrophobicity. For both stars and linear chains in the literature (particularly Devereaux and Baker43), increased PS results in nonuniform films with a greater variety of morphology. As pressure increases, no chaining is observed in the case of the blend. In fact instead of chaining, an augmentation in populati on of the bigger domains compared to the small domains can be observe d. This phenomenon can be shown by using the computer program designed by Yves Heckel. Hist ograms of the domain areas are given in Figure 3-20. In the pure diblock, formation of large chains that resemb le pearl necklace-like strings occurs as the transfer pressure is increased. This phenomenon starts with the formation of domain dimers or trimers at a pressure of about 7mN/m that then keep on chaining with increased pressure. When PS is added to the monolayer, no such domains are observed. An increase in the size of the circular domains is observed, indicating the aggregation of the PS homopolymer within the PS chains of the copolymer (Figure 3-21). 3.2.3 Blends of a PSb -PEO diblock copolymer and a PEO homopolymer In a similar way, blends of the copolymer and a PEO homopolymer were transferred homogeneously onto a mica substrate (TR 1.07) in order to study the evolution of the morphologies depending on the am ount of PEO added. Result s are shown in Figure 3-22.

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70 At low pressure, we can observe the di sappearance of the hexagonal packing when increasing the amount of PEO. This beha vior can be compared to that of PSb -PEO star copolymers studied by Logan,48 and Francis et al. ,49,50 By increasing the hydrophilicity of the stars, they observed a decrease in the number of domains and an increase in the distance between each domain. While less uniform, this same effect appears in the blends as a result of PEO homopolymer aggregation to the PEO chains of the diblock copolymer. To remove the artifacts than could have been formed by spreading successively the pure copolymer and the homopolymer, a mixed solution was made and this was spread as a comparison. The proportions were the same as the one used for Blend 2 and th e film was transferred at a pressure of 9mN/m. The AFM images are shown in Figure 3-23. Those experiments show that i ndependent of the spreading te chnique, an increase in PEO composition results in the formation of longer chains than for the pur e copolymer sample. A second effect of increased PEO content is th e apparition of a phase separation between pure layers of PEO surrounding clusters of micelles. In addition, the addition of a copolymer to a PEO homopolymer monolayer increases its stability and allows the formation of films at hi gher pressures than that of a pure PEO monolayer which collapses at 10mN/m.45 The difference between the morphol ogies at high pressure for the pure diblock and for the blends shows that the PEO homopolymer aggregates with the copolymer instead of dissolving into the aqueous subphase. 3.3 Conclusions When adding the homopolymers to the pure dibl ock copolymer at the air-water interface, reproducible isotherms were obtained and displaye d the three regions pres ent in a pure diblock copolymer (pancake, pseudoplateau, and brush) While the increase in the PS amount had an effect only on the condensed area (varies linear ly with the amount of PS added), by combining

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71 PEO and the copolymer, no change was observed in the condensed region of the isotherms. The length of the pseudoplateau representing the biphas ic region increases as the amount of PEO gets more important. This behavior has also been observed for pure diblocks as well as for star copolymers when increasing the size of the PEO block.48,51 AFM images were taken and were consistent with the isotherms showing the three regions described previously. On one hand, the addition of PS to the copolymer monolayer inhibited the chaining of the copolymer domains and enhanced the hydrophobic propert ies of the Langmuir-Blodgett film. On the other hand, combining the PS-b-PEO diblock copolymer with the PEO homopolymer also had an effect on the film morphology, increasing the chaining of the domains as well as favoring the phase separation between clusters of micelles and pure layer of PEO.

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72 02004006008001000120014001600 0 2 4 6 8 10 12 14 16 18 20 Surface Pressure (mN/m)Mean Molecular Area (nm2/molecule) Figure 3-1. Surface pressure-MMA isotherm for the PEO homopolymer (Mn = 100,000g/mol) recorded for a compression speed of 5mm/min

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73 020406080100120140 0 10 20 30 40 50 60 70 Surface Pressure (mN/m)Mean Molecular Area (nm2/molecule) Figure 3-2. Surface pressure-MMA isotherm for the 32,500 g mol-1 PSb -PEO copolymer recorded for a compression speed of 5mm/min

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74 020406080100120140 0 10 20 30 40 50 60 70 Surface Pressure (mN/m)Mean molecular area (nm2/molecule) Pure Diblock (68%) Blend 1 (70%) Blend 2 (72%) Blend 3 (76%) Blend 4 (79%) Blend 5 (81%) Blend 6 (84%) Blend 7 (86%) Figure 3-3. Several surface pre ssure-MMA isotherms are shown, indicating the dependence of surface pressure on the mean molecular ar ea for different blend ratios. All the isotherm experiments were performe d using a compression speed of 5mm/m

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75 510 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Pure PSb -PEO Blend 1K (m/mN) (mN/m) Figure 3-4. Monolayer compre ssibility plot for the PSb -PEO copolymer and blend 1 y = 0.0001x + 0.0753 R2 = 0.9932 0.08 0.09 0.1 0.11 0.12 0.13 0.14 0.15 0.16 0200400600800 Total number of styrene unitsA0/#EO (nm2) Figure 3-5. The condensed area per EO unit (A0/EO) varies linearly with the number of styrene units present in the blends

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76 y = 0.0288x + 17.914 R2 = 0.9932 15 20 25 30 35 40 0200400600800 Total number of styrene unitsA0 (nm2) Figure 3-6. The condensed area (A0) varies linearly with the total number of styrene units present in the blends

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77 0200400600800100012001400 0 10 20 30 40 Surface Pressure (mN/m)MMA (nm2/molecule) Mass % of PEO 32% 92% 96% 98% 99% Figure 3-7. Several surface pre ssure-MMA isotherms are shown, showing the dependence of surface pressure on the mean molecular ar ea for different amounts of PEO. All the isotherm experiments were performed using a compression speed of 5mm/min

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78 81012 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Pure diblock Blend 1K(m/mN)(mN/m) Figure 3-8. Monolayer compre ssibility plots for the PSb -PEO diblock copolymer and blend 1 y = 1.1297x 173.95 R2 = 0.9968 0 100 200 300 400 500 600 700 800 0200400600800 Total number of EO unitsPancake area (AP) (nm2) Figure 3-9. The pancake area (AP) varies linearly with the total num ber of EO units present in the blends

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79 y = 1082.9x + 47.532 R2 = 0.9940 50 100 150 200 250 300 350 00.050.10.150.20.25moles of PEO in the homopolymer/moles of PEO in the diblock copolymerAtransition2 (nm2/molecule) Figure 3-10. The area for the second transition de pends linearly on the mole ratio of PEO from the homopolymer over PEO from the PSb -PEO y = 0.4769x 66.032 R2 = 0.99420 50 100 150 200 250 300 350 0200400600800Total number of EO repeat unitsATransition2 (nm2/molecule) Figure 3-11. The area for the sec ond transition depends linearly on th e apparent total number of EO repeat units

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80 Figure 3-12. Example of a sample height image and surface plot (scan area shown is 2 2 m) Figure 3-13. Example of a sample section image (2 2 m) 7.293nm

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81 Figure 3-14. Height AFM images from tapping mode of the pure PSb -PEO for several transfer pressures (scale 2 2 m). The transfer ratios are also given for each images. Figure 3-15. Schematic representation of surf ace micelles formed by A-b-B diblock copolymers with A strongly adsorbed to the surface (adapted from Potemkin et al.105) B B B B B B B A TR = 1.220 TR = 1.150 TR = 0.405 TR = 0.451

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82 Figure 3-16. Model of PSb -PEO absorbing at the air-water interface (Adapted from Dewhurst et al.37) 0 200 400 600 800 1000 1200 1400 1600 05101520 Surface Pressure (mN/m)Molecules/domain Figure 3-17. Dependence of the number of molecules per domain on pressure

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83 Figure 3-18. Height AFM images from tapping mode of the pure PSb -PEO diblock copolymer as well as two of the blends for several transfer pressures (scale 2 2 m) Amount of PS = 4mN/m = 7mN/m = 10mN/m PURE BLEND 2 BLEND 5

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84 Figure 3-19. Height AFM images (scale 2 2 m) for the pure PSb -PEO diblock copolymer and Blend 2 (transfer pressure of 4mN/m) as we ll as the distribution of the domain areas 020406080100 0 50 100 150 200 CountsDomain Area (nm2) 0102030405060708090100 0 20 40 60 80 100 CountsDomain Area (nm2)

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85 Figure 3-20. Computed images for the pure PSb -PEO diblock copolymer and Blend 2 (transfer pressure of 10mN/m) as well as th e distribution of the domain areas 050100150200250300350 0 20 40 60 80 100 120 140 CountsDomain Area (nm2) 050100150200250300350 0 20 40 60 80 100 120 140 CountsDomain Area (nm2)

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86 Figure 3-21. Magnification of a single domain form ed for Blend 2 for a transfer pressure of 10mN/m (scale 150 150nm) Figure 3-22. Height AFM images from tapping mode of the pure PSb -PEO diblock copolymer and several blends for transfer pressures of 4 and 9 mN/m (scale 2 2 m). The height scale remains constant for the blends. = 4mN/m = 9mN/m Blend 1 Blend 2 Blend 3 Blend 4 Pure AmountofPEO

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87 Figure 3-23. Height AFM images from tapping mode for Blend 2; (a) from successive spreading, and (b) from the mixed solution (scale 2 2 m) a b

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88 Table 3-1. Characteristics of the PSb -PEO sample investigated Table 3-2. The mass ratio of PS between the dibl ock copolymer and the homopolymer as well as the apparent number of styrene units have been calculated for each blend. Blend # 1 2 3 4 5 6 7 Mass % of PS 70.2 72.3 75.8 78.6 80.7 84.0 86.3 Mole ratio of PS (homopolymer/copolymer) 0.138 0.275 0.551 0.826 1.102 1.653 2.204 NPS,TOT 236 259 307 355 403 499 595 Table 3-3. Width of the pseudoplateau for each blend Blend # Pure 1 2 3 4 5 6 7 AP (nm2) 22.8 23.0 22.2 21.1 22.3 22.5 20.5 21.1 Table 3-4. The mass ratio of PEO between the diblock copolymer and the homopolymer as well as the apparent number of styrene units have been calculated for each blend. Blend # 1 2 3 4 Mass % of PEO 92.4 95.9 97.9 98.9 Mole ratio of PEO (homopolymer/copolymer) 0.025 0.05 0.101 0.202 NPEO,TOT 295 352 467 697 MW (g/mol) PEO wt% PS wt% Polydispersity MWPEO MWPS NPEO NPS 32,500 32 68 1.05 10,500 22,000 238 211

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89 Table 3-5. Pancake areas extrapolated from the -A isotherms Blend # Pure 1 2 3 4 AP (nm2) 86 153 244 352 610 Table 3-6. Area for the second tr ansition (described in Figure 2.5) extrapolated for each blend Blend # Pure 1 2 3 4 ATransition 2 (nm2) 46 72 112 149 268 Table 3-7. Molar ratio of PEO from the homopolymer and the di block copolymer as well as the total number of EO units is given for each blend. Blend # Pure 1 2 3 4 nHomo/nBlock 0 0.025 0.05 0.101 0.202 NPEO,TOT 238 295 352 467 697

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90 CHAPTER 4 INTERFACIAL BEHAVIOR OF STAR-SHAPED POLYSTYRENEBLOCK -POLY(TERTBUTYLACRYLATE) COPOLYMERS 4.1 Introduction A system that has been of particular interest in our laboratories is one that consists of a hydrophilic block as well as a hydrophobic block. These types of copolymers are particularly interesting due to their ability to self-asse mble when spread onto a water subphase. Many studies, including the previous chap ter, have focused on polystyreneb -poly(ethylene oxide) (PSb -PEO) 32-44 copolymers due to the availability of PS and the biocompatibility of PEO, but very few have involved copolymers containing polyacrylates. Homopolymers such has polyesters106,107 and polyalkylesters spread easily at the air-water interface and have been widely studied. For example, Mengel et al. investigated the formation of multilayers of poly(tert-butylacrylate) using the LB film technique.108 The isotherms obtained were in agreement with previous studies made on linear P t BA.109,110 A plateau was observed for a pressure around 24mN/m indicating the presence of a transition from liquid-condensed (LC) to solid phase (S). Transferred onto silicon wafers, those types of polymers were also found to be very useful after modification forming multilaye rs of poly(acrylic acid) (PAA). The direct formation of Langmuir films using PAA homopo lymers is impossible due to their high hydrophilicity. Depending on the size and the composition of the chains, copolymers present different properties such as chain density and intermolecula r interactions a llowing them to adopt different geometries and form different morphologies. By varying the ratio be tween the hydrophobic and hydrophilic segments of the copolymers, research ers have been able to observe different morphologies of the films transf erred from the air-water interf ace onto a solid substrate. Li et al. investigated the behavior of Langmuir films of linear PSb -P t BA.111 For mostly hydrophobic

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91 copolymers, they observe multiple plateaus indi cating the presence of more than one phase transition. This differs from the behavior of the P t BA homopolymer which only shows one transition corresponding to side-gro up reorientation towards the interface. They suggest that the other transitions correspond to some form of backbone condensation or organization. 4.2 Results and Discussion The PSb -P t BA diblock star copolymers given in Figure 4-1 were synt hesized via atom transfer radical polymerizati on (ATRP) of styrene and tert -butylacrylate using the fifth generation dendritic initiator s hown in Figure 4-2. The synthesi s procedure has been described elsewhere.112 The characteristics of the copolymers ar e given in Table 4-1 as well as their structures shown in Figure 4-2. The P t BA homopolymer was purchas ed from Polymer Source Inc. Its characteristics are given in Table 4-2. Regarding the c opolymer, the calculated molecular weight values assumed the presence of 64 PSb -P t BA branches. However, the absolute values obtained by differential viscometry and universal calibration showed that each star is composed of significantly less branches. The number of br anches was obtained by dividing the absolute molecular weight (shown in Table 4-2) of each c opolymer by the calculated molecular weight of one branch. Based on discussion with Dr. W. Ford and Young Hie Kim who synthesized the samples, we assumed that each arm is composed of 48 PS repeat units. After calculation, we then estimated that PSb -P t BA(104) contains 37wt% PtBA; PSb -P t BA(215), 47%; and PSb P t BA(445) is composed of 58% of PtBA (Table 41). It is important to note that those numbers are averages and that some of the stars may have a different number of branches as well as some linear chains might be present in the samples.

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92 4.2.1 P t BA Homopolymer Figure 4-3 shows the surface pressure-mean mo lecular area isotherm for the homopolymer. As described in previous investigations,109,110 a phase transition is observed for a pressure around 24mN/m shown by the presence of a plateau on the isotherm plot. The P t BA homopolymer is amphiphilic, forming stab le films when compressed at the airwater interface. On the isotherm, three dist inct regions can be de tected: (1) all the P t BA chains are adsorbed onto the air-water interface, (2 ) represents a phase transition from a twodimensional to a three-dimensional structure, and (3) accounts for the collapse region of the P t BA. As shown in Figure 4-3, two distinct areas can be extrapolat ed from the isotherm plot. We divided those values by the number of t BA units present in the homopol ymer in order to simplify the comparison with other systems. For low pressure, Ap is equal to 302/tBA. This value is in agreement with the one given by Mudgil et al.113 (352/tBA). In region 3, the extrapolation of the linear portion of the isotherm to =0mN/m yields to the theoretical area A0. Once again, the value observed for our P t BA homopolymer (5.42/tBA) is in conformity with the one observed in the literature (52/tBA).111 The P t BA homopolymer was transferred onto a mica s ubstrate at three distinct pressures. The resulting images are given in Figure 4-4. Th e pressures were chosen in order to investigate the morphologies formed in the th ree distinct regions of the isotherm shown in Figure 4-5. As expected, no morphology exists for a pr essure of 15mN/m (Region 1). When compressed to a pressure corresponding to the pl ateau pressure (24mN/m), bright domains start to form (Region 2). Further compression leads to a film collapse observed in region (3). A scheme of the characteristic Langmuir film structures for the diffe rent regions of the isotherms is presented in Figur e 4-6. Region (1), as confirme d by the AFM images (several

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93 images were taken in order to prove the reprodu cibility of the morphologies formed), shows a flat monolayer of P t BA completely adsorbed onto th e air-water interface. Region (2) (morphology observed at the plateau) is composed of either single domains or small chains (dimers or trimers) of P t BA aggregates. These domains are visible due to desorption of the P t BA from the interface as the total available area is decreased. Each aggregate contains multiple polymer chains. In the LB film transferred for a surface pressure of 30mN/m (region 3), the bright domains represent 25% of the total area (assuming the darker area is composed of the adsorbed PtBA chains). If we assume that the bright domains observed in the AFM picture taken for a MMA corresponding to the middle of the plat eau (Figure 4-4) repres ent half of the total desorbed PtBA observed in condens ed region, then the aggregati on number is about 12000. It is also important to note that the TR detected during the transfer (1.062) was also taken into account in the calculations. Region (2) represents the transition between the state where all the P t BA is adsorbed onto the inte rface and forms a homogeneous m onolayer and that present in region (3) where the P t BA forms a three-dimensional network. This last structure explains the presence of the sharp increase in pressure a nd the dependence of the collapse on the polymer molecular weight. When transferred at a pressu re of 30mN/m (Region 3), the appearance of bright domains is observed. These do mains are believed to represent the P t BA that desorbs from the interface as the monolayer is compressed. The circular domains are characteristic and reproducible. They also tend to have a narrow distribution showing di ameters of around 0.6 m as confirmed by the section view shown in Figure 4-7. Additional experiments were done in order to de termine the stability of the films formed at different surface pressures. Compression-expans ion hysteresis experiments done for a maximum pressure of 50mN/m, which is lo cated after the collaps e pressure of the ho mopolymer (Figure 4-

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94 8) also support our conclusions, showing a hysteresis as the monolayer was successively compressed and expanded. After complete expans ion, the compression curves again overlap each other showing that the films form ed can relax to their original monolayer. In contrast, for a maximum surface pressure of 20mN/m (below th e plateau), the experiment shows no signs of hysteresis on expansion, proving the presence of a thermodynamica lly stable film for pressure values below the equilibrium spreading pressure of the P t BA segments (Figure 4-9). During the expansion, a decrease to a pressure lower than the one observed dur ing the compression is observed. This can be explained by the fact that the desorbed P t BA chains undergo a faster desorption during the compression cycle than re-a dsorption during the expansion cycle. Another possible explanation would be an artifact due to the wilhelmy plate. However, this is unlikely because there is no difference in multiple subsequent compression curves. To observe the stability of the films form ed at high surface pressures, an isochoric experiment was also performed. The film was co mpressed to a pressure of 50mN/m. Once this pressure was reached, the barriers were stopped a nd the pressure recorded over time. The plot in Figure 4-10 illustrates a rapid dr op in pressure as soon as the compression stops, then the observed pressure slowly decreases to the equi librium spreading pressure of the homopolymer (24mN/m). The films formed are thermodynamically unstable and quickly re lax to the pressure observed for the plateau. The reason for the shar p surface pressure increase observed on the isotherm is still not clear; it could be from interactions between the collapsed and desorbed aggregates formed in region (2) or between eventual adsorbed P t BA segments. 4.2.2 PSb -P t BA Star-Shaped Copolymers 4.2.2.1 Isotherm Experiments Figure 4-11(a) shows the surface pressure-mean molecular area isotherms obtained for the three copolymers. Similar to the P t BA homopolymer, plateaus can be observed for a pressure of

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95 about 24mN/m. Independent of the molecular wei ght of the copolymer, the plateau observed is horizontal, representing a first-or der transition where the temperature and pressure stays constant while the single domains aggregate. This differs from polymers such as polystyreneb poly(ethylene oxide) (PSb -PEO) presenting a pseudoplateau wh ere a slight change in the surface pressure is detected upon compression.32-44 As for the P t BA homopolymer, the plateau is detected for a surface pressure of 24mN/m. We know the desorption of P t BA chains from the interface has an effect on the length of the pl ateau; however, the effect of the PS domains remains unclear. Our results are significantly different from those obtaine d by Lennox and co-workers for linear PSb -P t BA diblock copolymers with relatively low P t BA wt %.111 They showed that the presence of the PS block induces two additional phase transitions of the P t BA block below 20 mN/m and suggested that these transitions might originate from the peculiar surface aggregation into circular micelles. This difference in beha vior can be explained by the significant difference in PS molecular weight between their sample (305 styrene repeat uni ts) and our copolymers (each arm has 48 styrene repeat units). The same three regions observed in the homopol ymer are observed for the copolymers. An increase in the surface pressure is observe d in the high mean molecular area going from a liquid expanded phase to a liquid condensed phase until a plateau suggesting a biphasic phase is reached at =24mN/m. PS and P t BA are both hydrophobic homopolym ers but contrary to PS, P t BA possesses slightly polar es ter groups that allow its adso rption onto the water subphase. Upon further compression in the low molecular areas, the apparent su rface pressure shows a sharp increase to values higher than 80mN/m. For this region, ex trapolation of the linear portion of the isotherm to = 0 mN/m yields the theoretical area that a most compact surface film

PAGE 96

96 would occupy at zero pressure (A0). In previous studies on am phiphilic copolymers such as PSb -PEO, no dependence of the collapse area on the PEO block length was observed. In our case, we can see the increase in collapse area as the size of the P t BA chains increases suggesting that the film collapse depends on bot h the hydrophilic and the hydrophobi c block. Interestingly the films become more compact as the amount of P t BA increases. Likewise, Li et al. investigated the behavior of two different P t BA homopolymers, and observed more compact films when increasing the P t BA molecular weight.111 This suggests that the area occupied by the longer chains includes a significantly smaller trapped area than th e shorter chains. Throughout the plateau, the tert-butyl side groups go from a prone to a vertical conformation. Li et al. also studied the influence of the P t BA molecular weight on the behavior of PSb -P t BA diblock copolymers.111 They observed a linear dependence (y = 0.128x + 16.473) of the collapse area on the number of t BA repeat units. When the surface pressu re is plotted versus the mean unit area (area available for 1 t BA repeat unit), a strong molecula r weight dependence can be observed throughout the entire isotherm plot (Fi gure 4-11 (b)). In Figure 4-12, when A0 per t BA repeat unit is plotted vers us the number of t BA repeat units present in one arm, a linear dependence is detected (y = 0.0021 + 2.982). The value detected for PSb -P t BA(104) is really close to the one given for the P t BA homopolymer (5.5 2). As the P t BA molecular weight is increased, the star copolymers become more sterically hindered, fo rming less packed aggregates, yielding to a higher area per t BA unit. The areas per t BA unit observed by Li et al .111 for several copolymers are significantly more larger (from 12 2 to 70 2) due to the amount of PS present in their copolymers: longer chains preventing the P t BA segments from packing more closely. The area occupied by a styrene repeat unit is c onstant for the three copolymers and has an approximate value equal to 122/St (values are given in Table 4-3) This value is larger than the

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97 one observed by Logan et al.48 (8.92/St) for PSb -PEO star copolymers. The significant variation between both values can be explained by the difference in interfacial behavior between PEO and P t BA blocks. For small molecular areas, the PEO chains stretch in the aqueous subphase while the P t BA desorbs from the interface. Theref ore, in the case of the PS-b-PEO copolymers, the steric hindrance at the interface is lowered, allowing the PS domains to pack more closely. 4.2.2.2 AFM Imaging AFM images were obtained for two transfer pr essures as seen in Figure 4-14 We can see that the images obtained for a transfer pressu re lower than the plateau value (24mN/m) only show the presence of single domains. The formation of adsorbed circular surface micelles without any collapsed domains can be observed. Even though their size decreases as the PtBA content increase, the size distribution of those surface micelles is rather monodisperse (verified by the image analysis software introduced in ch apter 2) and independent on the molecular weight of P t BA in the sample suggesting those domains re present aggregates of PS chains. The domain sizes averaged 4200, 3500, and 3200nm2 for the 3 systems. Li et al.111 studied the behavior of linear PSb -P t BA copolymers and observed domains that were approximately 10 times larger than the ones detected for our copolymers. Howe ver, this difference can easily be explained by the distinct architectures and the size of the PS blocks (305 st yrene repeat units in their copolymers versus 48 for each arm of our star copol ymers). It should be noted that the transfer ratios varied significantly between the values obtained for pressu res below and above the plateau pressure. In fact for pressures e qual to 24mN/m or less, all the tr ansfer ratios were close to unity suggesting a homogeneous transfer of the Langmuir monolayer onto the mica substrate. However, when the monolayers were transferred for pressures above the plateau, the transfer ratios decreased dramatically (TR 0.5 for the 3 copolymers).

PAGE 98

98 The hydrophilicity of the s ubstrate and results from previous studies on PSb -P t BA copolymers allow us to cons ider the more hydrophilic P t BA to be adsorbed onto the mica and the hydrophobic PS to occupy the higher layer. Therefore, the P t BA is represented by the darker (lower) areas whereas the PS exists as the brighter (higher) domains. The di ameter of the bright circular domains observed in th e AFM pictures is approximately 100nm and independent of the surface pressure. Each aggregate consists of mu ltiple chains for example, the aggregation number for the domains observed in Figure 4-14 in the case of the PSb -PtBA(104) is equal to 132. Therefore, once a micelle has formed duri ng the solvent spreading process, the micellar cores come closer to each other during compre ssion. However, the aggregation number remains unchanged with the chains frozen w ithin the glassy core of a part icular micelle (the temperature of the experiment is well below the T g of the desorbed PS cores). It should be noted that the glass transition temperature (Tg) of a pure bul k PS (molecular weight comparable to the PS studied in this chapter) is 98 C.141 In addition, Bliznyuk et al.142 measured the surface glass transition for PS with molecular weights rang ing from 3900 to 1,340,000g/mol. Using the model proposed by their group, the Tg of a PS of 5000g/m ol (close to the PS molecular weight in one arm of the star copolymers) is approximately 50 C. For low surface pressures, a decrease in the si ze of the circular micelles as well as an increase in the distance between them is observed when the size of the P t BA segments is increased (Figure 4-15). This observation is in accord with the interpretation of a P t BA layer located at the bottom of the film with PS domain s on top of it as shown in the Figure 4-13. The decrease in the circular domain size can be due to the increase in repulsive forces between PS and P t BA when the fraction of P t BA becomes more significant.

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99 For a pressure of 24mN/m several trials were made to observe the aggregation of the PS domains and the formation of lower domains representing the P t BA chains. In fact, as seen for the P t BA homopolymer throughout the plat eau, the desorption of the P t BA chains can be observed leading to the chaining and further aggregation of the si ngle and oligomeric PS domains observed at lower pressure. The formation of such aggregates is more easily seen on the copolymer containing the longest chains of P t BA (PSb -P t BA(445)). When compressed to a pressure above the plateau (30mN/m), aggrega tion of these domains is observed for all copolymers. As shown in Figure 4-14, this aggr egation becomes more significant as the number of t BA repeat units increases. As demonstrated previously for the homopolym er, three regions can be detected in the isotherms as well as in the stru ctures observed when the monola yer is transferred onto a solid substrate. Figure 4-16 gives a scheme of the genera l structures formed for different regions of the isotherms. Region (1) contains a hom ogeneous monolayer of amphiphilic P t BA with the hydrophobic PS forming single domains aggregated on top of it. Re gion (2) is similar to the structure observed for the P t BA homopolymer, the only differe nce being the presence of PS domains sitting on top. Finally region (3) is the analogous morphology to the one observed for the region (3) in the homopolymer. The domains of P t BA form a network, rendering the film very rigid and increasing the apparent surface pres sure. The three-dimensional structure depicted for regions (2) and (3) can also be explained by the cross section shown in Figure 4-17 showing the different phases of the Langmuir-Blodge tt film at high pressure (adsorbed P t BA, desorbed P t BA, and PS).The domains formed by the P t BA desorbed from the interface have a size comparable to those previously observed for the P t BA homopolymer (around 1 m) in Figures 44 and 4-6. This is explained in pictorial fashion in the scheme shown in Figure 4-18, representing

PAGE 100

100 the three layers discussed in the previous pa ragraph. We can see the formation of large P t BA aggregates which are desorbing from the inte rface and possess smaller aggregates of PS sitting on top of them. As discussed in the previ ous part, contrary to PSb -PEO block copolymers, for PSb -P t BA copolymers, the collapse area (A0) depends on the P t BA block (Table 4-3). By calculating the area occupied by PS (APS) using equation 4-1 (where is the aggregation number), we found APS to be constant for the 3 copolymers. area domain Mean APS 4-1 However, when AP t BA (area occupied by the PtBA chains in the 3D structure) is plotted versus the number of tBA repeat units, a linear depende nce is observed (y = 0.0482x + 37.049) (Figure 4-19). The linear dependence detected by Li et al. 111 exhibited a slope of 0.128. The significant difference between this value and the slope observed in the case of our copolymers could be explained by the differen ce in architecture (star-shaped versus linear copolymers), as well as the size of their PS segment wh ich increases the steric hindrance. 4.2.2.3 Stability of the Langmuir Films As shown in the compression-expansion hystere sis experiment (Figure 4-20) performed in the low MMA region, we were able to observe the formation of unstable films for pressures higher than 24mN/m. All the co mpression curves closely overlap, showing that the collapsed films are able to return to their original monolayer. A drop in pressure is observed during the expansi on cycle. This is due to the fact that the re-adsorption of the PtBA chains onto the air-water inte rface during the expansion cycle is slower than the desorption during the compression cycle. Additional compression-expansion hysteresis experiments performed for pressures below the plateau shown in Figure 4-21 allow us

PAGE 101

101 to propose that the monolayers exist as thermodynamically stable films of PS-b-PtBA for pressures under 24mN/m. Figures 4-20 a nd 4-21 only show the results for the PS-b-PtBA(215). The behavior of the films being indepe ndent on the molecular weight of the PtBA block; the hysteresis experiments for the two ot her copolymers gave similar results. To investigate the stability of the network formed at high pressures, transfers were performed after different stabi lization times (from 15 minutes to 12 hours) for the copolymer PS-b-PtBA(215). The transfer ratios were recorded (Table 4-4) in or der to determine if the films observed by AFM correspond to what is formed at the air-water interface. The transfer ratios being smaller than one even for a short stabiliz ation time shows us the expansion of the film when transferred from the inte rface onto the solid substrate. Th is explains the AFM results observed in Figure 4-16, where the structure obser ved is not a complete network. The decrease of transfer ratio when increas ing the annealing time supports once again, as shown in the compression-expansion hysteresis experiments, th at the films formed at high pressures are thermodynamically unstable. Figure 4-22 shows the AFM images for the three copolymers transferred at a pressure of 30 mN/m, which corresponds to region 3 of the isotherm. We can see that by increasing the stabilization time before tr ansfer, the aggregation also increases, proving once again the instability of th e monolayers formed at the air-water interface. A possible explanation for this phenomenon is the pres ence of attractive fo rces between the PtBA chains of different copolymers that at high surface pressures have the tendency to aggregate. Contrary to the PS-b-PEO copolymers studied in chapter 3, which showed a less well-defined transition from a two-dimensional system to a th ree-dimensional system, for the PS-b-PtBA copolymers, it is not clear to us whether the sharp surface pressure increase upon high compression in the isotherm is

PAGE 102

102 due to interactions between the collapsed and desorbed aggregates formed in the plateau region or between eventual remaining adsorbed PtBA segments. The presence of an increase in aggregati on when increasing the st abilization time suggests the unstability of the Langmuir films above the plateau. In Figure 4-23, isochoric experiments are reported. The monolayers were compressed up to a pressure of 50mN/m and after the compression was stopped, values of the pressure were recorded over time. Once stopped, we observe a drastic drop in surface pressure toward the plateau pressure. This, once again, shows the unstable behavior of the film above the plateau. All three copolym ers present the same behavior; the only observed difference is the presen ce of a larger pressure drop as the amount of PtBA in the copolymer increases The surface pressure undergoes a sudden drop within the first seconds of the experiment before leveling off after a few minutes around 24mN/m. In any case, the isochoric experiments confirmed that th e films formed above the plateau are not thermodynamically stable and relax within minut es down to the plateau surface pressure of 24mN/m, which can also be seen as the equi librium spreading pressure (ESP) of the PtBA blocks. ESP analysis (not shown) was also performed by placing the solid copolymer sample onto the water subphase. The ESP value (24mN/m) observed was in agreement with the isochoric experiment. 4.3 Conclusions In this chapter the behavior of several PS-b-PtBA star copolymers was investigated. The isotherm experiments showed the presence of a plateau for a surface pressure of 24mN/m. The copolymers studied, were composed of, on average 5 branches containing a constant PS core and different PtBA molecular weights. Even if the shape of the isotherms does not vary from the one of a PtBA homopolymer (already amphiphilic), a st rong dependence on the molecular weight

PAGE 103

103 was observed with an increase in the length of the plateau as we increase the number of tBA units. AFM images were taken at pressures below and above the transition plateau. For the homopolymer, as expected, uniform flat films were observed befo re the transition. However, once the pressure reached the plateau pressure, aggregates were formed containing approximately 92000 PtBA chains suggesting desorption of PtBA chains from the interface. When the PS-b-PtBA star copolymers were scanned, a st rong molecular weight dependence of the length of the plateau as well as the coll apse area was observed in correlation with the isotherm experiments. As the number of tBA repeat units increases, an increase in the aggregation was detected. For low surface pressu res, the distance between the PS domains was greater as the PtBA molecular weight was increased, however, for high surface pressures, we saw the appearance of three different domains: adsorbed PtBA, desorbed PtBA, and PS domains. Estimations of the area occupied by the PtBA in the collapse area also showed the influence of the PtBA segment on A0. The Langmuir films formed above the plat eau were found to be thermodynamically unstable as the aggregation increases with the st abilization time. Isochoric experiments were also performed and once the compression stopped, th e monolayers relaxed to their equilibrium spreading pressure of 24mN/m. Below the plateau however, the c opolymers formed elastic films, no hysteresis was observed during the compressi on-expansion experiments and the monolayer seems to relax to its initial state.

PAGE 104

104 Figure 4-1. Schematic of the PS-b-PtBA copolymers with n=48 and m=104,215,445 (Adapted from Reference 87) Figure 4-2. Schematic of dendriti c initiator (a) (Adapted from Reference 87) and model of the PS-b-PtBA star copolymers (b) PS PtBA (b)

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105 01020304050 0 10 20 30 40 50 60 70 80 APA0 (3) (2) (1)Surface Pressure (mN/m)Molecular Area (nm2/molecule) Figure 4-3. Surface pressure-MMA isotherm for a 13000g/mol PtBA homopolymer recorded for a compression speed of 5mm/min

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106 Figure 4-4. Height AFM images in tapping mode for the PtBA Homopolymer after transfer at three distinct surf ace pressures (Scale 1010m) = 15 mN/m = 30 mN/m = 24 mN/m (middle of plateau) 5 nm 0 nm 10 nm

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107 Figure 4-5. Proposed conformations for the PtBA homopolymer at the ai r-water interface. The different regions are la beled in Figure 4-3 Figure 4-6. Section view of a ci rcular domain formed when the PtBA monolayer is compressed above the plateau pressure (30mN/m) (image scale 11m)

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108 Figure 4-7. Scheme of the structures formed fo r 3 distinct regions of the isotherm when a monolayer of PtBA homopolymer is compressed at the air-water interface (Scale 1010m) 010203040506070 0 10 20 30 40 50 Surface Pressure (mN/m)Mean Molecular Area (nm2/molecule) Figure 4-8. CompressionExpansion Hysteresis Experiment for the PtBA homopolymer compressed to a target pressure of 50mN /m (above the plateau). The compression and expansion rates were both 5mm/min.

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109 010203040506070 0 5 10 15 20 Surface Pressure (mN/m)Mean Molecular Area (nm2/molecule) Figure 4-9. Compressionexpansion hysteresis experiment for the PtBA homopolymer compressed to a target pressure of 20mN/m (below the plateau). The compression and expansion rates were both 5mm/min. 050100150200250300 20 25 30 35 40 45 50 Surface Pressure (mN/m)Time (minutes) Figure 4-10. Isochoric experiment recorded for the PtBA homopolymer for a maximum pressure of 50mN/m. the compression speed was 5mm/min.

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110 0100200300400500600700 0 10 20 30 40 50 60 70 80 (a) PSb -P t BA(104) PSb -P t BA(215) PSb -P t BA(445)Surface Pressure (mN/m)Mean Molecular Area (nm2/molecule) 0102030405060708090100 0 10 20 30 40 50 60 70 80 (b) PSb -P t BA(104) PSb -P t BA(215) PSb -P t BA(445)Surface Pressure (mN/m)Area per tBA unit (100 x nm2/tBA) Figure 4-11. (a) Surface pressure-MMA isotherm s plots of the 3 star-shaped copolymers (b) Isotherms of the 3 copolymers in terms of number of tBA repeat units. All the isotherm experiments were performed with a compression speed of 5mm/min.

PAGE 111

111 y = 0.0206x + 3.1145 R2 = 0.997 0 2 4 6 8 10 12 14 16 0200400600 Number of tBA repeat unitsArea per tBA repeat unit (Angstrom2) Figure 4-12. Plot of A0 versus the number of tBA repeat units. Figure 4-13. Proposed conformations for the PS-b-PtBA copolymers at the air-water interface. The different regions are defined in Figure 4-3.

PAGE 112

112 Figure 4-14. Height AFM images from tapping mode for the three c opolymers. Transfer pressures were chosen to be in region (1) and region (3) of the isotherms. (Scale 1010m, inset scale 11m) Figure 4-15. Scheme of AFM images for a transfer pressure of 15mN/m when increasing the size of the PtBA chains P t B A PS(48)-b-P t BA(445) PS(48)-b-P t BA(215) PS(48)-b-P t BA(104) = 15mN/m = 30mN/m = 24mN/m Middle Plateau

PAGE 113

113 Figure 4-16. Schematic of the structures formed for 3 distinct regions of the isotherm when a monolayer of PS-b-PtBA copolymer is compressed at the air-wat er interface Figure 4-17. Cross section of the PS-b-PtBA(445) copolymer when compressed to a pressure of 30mN/m and studied in tapping mode PtBA desorbed from the interface (b) PS (c) PtBA adsorbed onto the interface

PAGE 114

114 Figure 4-18. Close-up pictures and schematic of the three layers observed when the PS-bPtBA(445) is compressed above the plateau pressure y = 0.0482x + 37.049 R2 = 0.973130 35 40 45 50 55 60 65 70 0100200300400500Number of tBA repeat unitsArea occupied by PtBA (nm2) Figure 4-19. Plot of the Area occupied by the Pt BA blocks (values in Table 4-3) versus the number of tBA repeat units P P t B ( Scale: 1 1 m ) ( Scale: 2 2 m ) 8nm 4nm 0nm

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115 050100150200250300350 0 5 10 15 20 25 30 Surface Pressure (mN/m)Mean Molecular Area (nm2/molecule) Figure 4-20. Compression-expansion hysteresis experiment for the PS-b-PtBA(215) copolymer compressed to a target pressure of 30mN /m (above the plateau). The compression and expansion rates were both 5mm/min. 200250300350400450 0 5 10 15 20 25 30 Surface Pressure (mN/m)Mean Molecular Area (nm2/molecule) Figure 4-21. Compression-expansion hys teresis experiments for the PS-b-PtBA(215) copolymer compressed to a target pressure of 20mN/m (b elow the plateau). The compression and expansion rates were both 5mm/min. Ex p ansion Compression Expansion Compression

PAGE 116

116 Figure 4-21. AFM images for a transfer pressure of 30 mN/m for different stabilization times. Stabilization times before transfer (minutes) for PS-b-PtBA(104): (a) 15, (b) 45 and (c) 90; PS-b-PtBA(215): (d) 15, (e) 45 and (f) 90; PS-b-PtBA(445): (g) 15, (h) 45 and (i) 90 (Scale 1010m) (a) (h) (g) (f) (e) (d) (c) (b) (i)

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117 050100150200250300 20 25 30 35 40 45 50 PSb -P t BA(104) PSb -P t BA(215) PSb -P t BA(445)Surface Pressure (mN/m)Time (minutes) Figure 4-22. Isochoric experiments recorded fo r the three copolymers after compression to a target pressure of 50mN/m via a compression rate of 5mm/min.

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118 Table 4-1. Characteristics of the Copolymers Used Polymer Mn (g/mol) wt% of PtBA PDI* PS-b-PtBA(104) 131,000 37 1.28 PS-b-PtBA(215) 155,000 47 1.33 PS-b-PtBA(445) 194,000 58 1.53 Mn and PDI were determined using differential viscometry detection and universal calibration Table 4-2. Characteristics of the Homopolymer Used Polymer # Tert-butylacrylate repeat units (m) Mn (g/mol) H101 101 13000 Table 4-3. Specific areas calculated for the 3 copolymers Copolymer A0 (nm2) APS (nm2) AP t BA (nm2) PS-bPtBA(104) 132 68 27 41 PS-bPtBA(215) 124 80 31 49 PS-bPtBA(445) 105 88 30 58 Table 4-4. Transfer ratios for di fferent stabilization times before transferring the copolymer PS-b-PtBA(215) monolayer onto a mica substrate. Stabilization Time (min) 15 45 90 720 Transfer Ratio 0.665 0.300 0.199 0.055

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119 CHAPTER 5 SURFACE CHARACTERIZATION OF POLY(ETHYLENE OXIDE)-BLOCK-POLY(CAPROLACTONE) 5.1 Introduction Extensive work has been done on amphiphilic copolymers at the ai r-water interface but surprisingly studies on PCL-cont aining polymers are hard to find. On one hand PCL is an interesting polymer to use due to its bi odegradability leading to non-toxic products.114,115 On the other hand, low molecular weight PEO can be eliminated from the body by the renal system.116 The addition of a highly flexib le PEO chains at the end of a PCL segment can tailor some inherent properties of pure PCL such as hi gh crystallinity, high hydrophobicity, and slow biodegradation.117 Whereas very few investigations ha ve been performed at the air/water interface, PEO-b-PCL copolymers have been investig ated in solution and their micellar properties render them very usef ul as drug delivery vehicles.115,118,119 5.1.1 Crystallization of Semi-Crystalline Polymers The crystalline state is characterized as a state exhibiting a melting temperature, Tm (firstorder transition), and an X-ray scatte ring pattern with sharp reflections. Tm is controlled by properties such as hydrogen bonding, polarity, molecular weight, etc A crystal is in a lower free energy state than the liquid when the temperature is below the melting point for a large crysta l of a very high molecular we ight polymer. Figure 5-1 shows schematically the changes in the Gibbs free ener gy of liquid and a crystal with temperature. For any spontaneous transformation, G needs to have a negative value for a constant temperature and constant pressure process. At Tm (melting point of an infi nitely long crystal of finite molecular weight), a condition of equilibri um exists between the crystal and liquid as both phases have the same value of G and G=0.

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120 The crystallization properties of block c opolymers have been thoroughly investigated. The block copolymers studied are mostly composed of a semicrystalline block and an amorphous block. In such cases, the presence of a non-cr ystalline block enables the modification of the mechanical and structural properties of a crys talline polymer, through in troduction of a rubbery or glassy component. There are a nu mber of interesting features of these copolymers. Contrary to homopolymers, equilibrium chain fo lding can occur and crystallizat ion is therefore controlled by the size of the non-crystallizable block.120-136 Furthermore, in copolymers containing a crystallizable component, stru ctural changes occurring due to microphase separation compete with those resulting from crysta llization. Also, selective solv ents for the amorphous block can lead to non-equilibrium morphologies because th e crystallizable block can precipitate from solution and crystallize. As might be expected, the morphologies fo rmed by the crys tallization of block-copolymers therefore depe nd on both the molecular weight a nd the crystallization protocol (i.e. cooling rate). Contrary to homopolymers, where folding of chai ns occurs such that stems are always perpendicular to the lame llar interface, a parall el orientation has been observed for block copolymers crystallized from a lamellar melt phase and a perpendicular folding has been seen in a cylindrical microstructure. Both or ientations are shown in Figure 5-2. While the case of block copolymers containi ng one crystalline bloc k has been studied extensively, the behavior of block copolymers ha ving two crystallizable blocks is still fairly unknown. It has been investigated for PCL-PEO-PC L triblocks and it was f ound that the crystals of PEO and PCL coexist independently and that there are no mixed crysta ls containing both PEO and PCL. In the copolymers, when the crystallizab le block represents less than 25% of the minor component, its crystallinity decreases to zero. In the contrary, in a PEO/PCL binary blend, the two components crystallize sepa rately even with less than 25% of the minor component.

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121 As shown in Figure 5-3, in th e blend PEO and PCL form inde pendent crystallites whereas, in the block copolymer, chain connectivity lead s to PEO and PCL lamellae occupying the same crystallite when comparable fractions of each are present. In that case, a significant reduction of the lamellae thickness as well as crystal imperfections was observed. 5.1.2 Crystallization of PCL at the Air-Water Interface Li et al. studied the behavior of several PCL homopolymers at the air-water interface.73,137,138 The crystallization processes were captured using BAM and showed an anisotropic crystal growth at sl ow compression rates. They observed an increase in the number of nuclei formed with increas ing compression rates like the data shown in Figure 5-4. Li et al. also studied the dependence 138 of PCL crystals on the molecular weight. They used PCL samples with number average mo lar masses (Mn) ranging from 3500 to 36000 g.mol-1, and found out that the nucleation and growth of crystals as well as their morphologies and melting properties were dependent on the molar mass. Thomas Joncheray139 studied the behavior of PCL homopolymers at the air-water. At lo w pressures the PCL chains were completely adsorbed onto the water subphase with charact eristics of liquid expanded and liquid condensed phases. When an isotherm is recorded, below the collapse pressure, Ivanova et al.140 determined that the PCL chain packing and orientation was dependent of the surface pressure. The isotherms recorded showed that the collapse pressure and chain packing was not dependent on the molecular weight of the PCL homopolymer used and that thermodynamically stable monolayers were formed for pressures below the collapse. While the films formed for such pressures are easily studied, the ones formed above the collapse pressure are more complicated to investigate due to phenomena such as desorption of mo lecules from the interface or formation of multilayers. In the case of the PCL, it was observe d that the PCL chains aggregate on top of the water subphase. An interesting phenomenon wa s also surveyed; afte r the collapse point, a

PAGE 122

122 decrease in surface pressure was detected co rresponding to the crystallization of the PCL segments directly on the water. Compression-e xpansion hysteresis experi ments also show the appearance of a pseudo-plateau during the expansi on cycle as a result of the re-adsorption or melting of the PCL chains. AFM images of th e PCL homopolymers transferred for a surface pressure before the crystallization on the wa ter surface were taken and the formation of PCL crystals was observed. According to the isot herms, for such surface pressures, the PCL homopolymers are transferred into smooth a nd hydrated monolayers ad sorbed onto the mica surface. It is assumed that upon drying, during and after transfer, part of the PCL chains crystallize, which likely result s in a mica surface onl y partially covered with adsorbed or crystallized PCL chains. 5.2 Results and Discussion The linear PEO-b-PCL diblock copolymer synthesized from a linear PEO macroinitiator (PEO2670, Mn = 2,670 g/mol, ~ 60 ethylene oxide repeat units) contains a PCL block with 35 caprolactone repeat units (Figur e 5-5 and Table 5-1). The PEO-b-PCL five-arm star-shaped copolymer consists of a hydrophi lic PEO core with 9 ethylene oxide units/arm with hydrophobic PCL chains at the star periphery. Ea ch star contains PCL block with 18 -caprolactone units/arm (Figure 5-6 and Table 5-1). 5.2.1 Isotherm Experiments The isotherm of the linear PEO-b-PCL copolymer is given in Figure 5-7. Contrary to the isotherms observed for the PEO or PCL homopolymers,138 pressures as high as 25mN/m can be reached. Better shown in the compressibility plot in Figure 5-8, a transition can be observed around 13.5mN/m. Thomas Joncheray139 studied several PEO-b-PCL linear copolymers with a number of -caprolactone units ranging fr om 11 to 35 (the number of ethylene oxide units was

PAGE 123

123 kept constant throughout the enti re investigation). The maximum in monolayer compressibility for the high pressure transition increases as PCL chain length increases. This suggests the presence of a PCL-related phase transition. For PEO-b-PCL linear copolymers with a smaller amount of -caprolactone units, three different phase tran sitions can be observ ed for pressures of 6.5, 10.5, and 13.5mN/m. For the copo lymer studied in this chapter (PEO60-b-PCL35), only the high pressure transition is observe d. This transition is known to represent the cr ystallization of the PCL blocks.138-140 It was suggested that the transiti on observed for a pressure of 6.5mN/m represents the dissolutio n of the PEO into the aqueous subpha se. In our case, no transition is observed at low pressures indica ting that when the PCL block size increases, the dissolution of PEO is made more difficult. Compressionexpansion hysteresis plots of the PEO-b-PCL linear copolymer are shown in Figure 5-9. A broad melting transitio n is observed during the first expansion. This behavior was not observed for PCL homopolymer samples138,139 and could be explained by the properties of the crystals form ed during the first compression, such as size, shape or polydispersity. No plat eau was observed during the comp ression or expansion cycle but we can detect a re-adsorption of the crysta llized PCL chains for pressures under 5mN/m. Joncheray et al.139 performed compression-expansion hystere sis experiments for several barrier speeds and the melting (re-adsorption) and crystall ization (desorption) pressure values were found to be strongly barrier speed dependent. In the case where the barriers were moved infinitely slowly, the melting and crystallization pressure values would become identical. The isotherm of the PEO-b-PCL star copolymer is shown in Figure 5-10. It exhibits three distinct regions corresp onding to different conformations of the polymer chains. For high MMA, the surface pressure slowly increases until it reaches a pseudoplateau. For low MMA, a sharp increase in surface pressure is observed represen ting the formation of highly condensed films.

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124 However, for the star copolym er studied, the intermediate MMA region shows an almost inexistent pseudoplateau. The compressibility plot shown in Figure 5-11 il lustrates that absence even better by exhibiting no tran sition in the intermediate MMA range. This suggests that the PEO blocks are not adsorbed onto the interface but are more likely to be in the water subphase. Compression-expansion hysteresis experiments were performed for target pressures below (Figure 5-12) and above (Figure 5-13) the pse udoplateau pressure. For surface pressures below 9mN/m, the compression and expansion curves are superposable independent of the target pressure. This illustrates the re producibility and stability of the films formed. Due to the size of the PCL blocks, the block copolymer is hydrophobi c enough to avoid the i rreversible dissolution of material in the aqueous subphase. The compressi on-expansion curves for a target pressure of 15mN/m are shown in Figure 5-13. A pseudoplat eau is observed during the expansion cycles corresponding to the re-adsorption (melti ng) of the crystallized PCL chains. In order to further interpret the crystallization behavior as well as the stability of the films formed, additional analysis using AFM and BAM is going to be discusse d in the next part. 5.2.2 Brewster Angle Microscopy and Comp arison to Previous AFM Imaging Joncheray et al. studied the crystallization of PEO-b-PCL linear and star copolymers in the transferred LB monolayers and observed the forma tion of crystalline doma ins independent of the surface pressure, when investigated by AFM. While AFM gives some hint about the crystallization behavior of thos e polymers, BAM allowed us to observe the formation of crystals directly at the water subphase, subsequently eliminating variables such as affinity to substrate, transfer pressure, water evaporation and film thickness. Figures 5-14 and 5-15 show the BAM images for the PEO-b-PCL linear and star copolymer respectively. Those images were take n at both intermediate and low MMA. Both star and linear copolymers do not seem to show any crystal formation for pr essures below 10mN/m.

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125 By AFM, several crystals were observed at low surface pressures, indicating the importance of sample preparation, differences in resolution of the two techniques, or differences in polymer behavior absorbed to a solid compared to the wate r surface. Generally, it also appears that more crystals are observed by AFM than what can be ob served at the air-water interface. Nonetheless, above a specific pressure, crysta ls formed at the water surface exhibit the same morphologies as those observed by AFM (Figure 516). The only noticeable difference is the presence of more numerous crystals on the AFM images, suggestin g again the influence of sample preparation before AFM imaging. As seen in chapter 2, the lateral resolution of the BAM is significantly lower than the one observed for AFM. Nonethele ss, the BAM can detect features as small as 2m which is 5 times smaller than the crystals showed in the AFM pictures. A compression-expansion experiment was pe rformed and the corresponding images are given in Figure 5-17. Analogous to the pressure results of the previ ous compression-expansion hysteresis isotherm plots, BAM reveals that the crystallized PCL chains melt and re-adsorb during the expansion cycle. When comparing the images for a surface pressure of 10mN/m, one can notice that some remaining crystals are still present during the second compression. As discussed earlier in the chapter, the melting (re-a dsorption) and the crysta llization (desorption) pressures are strongly dependent on the barrier speed. One can expect the images for different compressions to be identical for an infinitely slow barrier speed. The barrier speed dependence as well as the presence of hysteresis between th e compression and expansion curves suggests the formation of thermodynamically unstable films for intermediate and high surface pressures. This metastability is shown in Figure 5-18 where th e monolayer was compressed to a pressure of 18mN/m. The surface pressure was then kept co nstant and images were taken after several stabilization times. One can notice that for t = 0, no crystals are observed but as the stabilization

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126 time increases, the amount of crystals increa ses, suggesting once ag ain the presence of thermodynamically unstable films. 5.3 Conclusions In this chapter, the inte rfacial behavior of a PEO-b-PCL linear copolymer and a PEO-bPCL star copolymer were investigated. The morphologies formed we re observed by BAM and compared to previous AFM results. The isotherm of the star-shaped block copolymer indicated the presence of a single phase transition characterized by a pse udoplateau that co rresponds to the collapse and crystallization of the PCL chains above the water surface. Below that plateau, the PCL chains are completely adsorbed, anchoring th e PEO chains to the interface. In this region, the compression-expansion hysteresis experiments as well as the structures observed by BAM showed the formation of therm odynamically stable monolayers. Once the pseudoplateau is reached, the PCL ch ains crystallize at the interface. This phenomenon was highlighted by the BAM pictures. We were able to show that the PCL chains crystallize directly at the in terface and not during the AFM samp le preparation. The images observed by BAM were found to be strongly dependent on the st abilization time and a more important aggregation was observed over time. Co mpression-expansion experiments showed that the melting (re-adsorption) of the PCL chains was sl ower than their crystall ization (desorption).

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127 Figure 5-1. General behavior of thermodynamic variables at the equilibrium melting temperature Tm (a) Gibbs free energy (b) entropy, enthalpy and volume Figure 5-2. Schematic of perpendicular (a) and pa rallel (b) chain folding in semicrystalline block copolymers (a) (b) Crystal Melt G T T m T m S or V or H T (a) (b)

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128 Figure 5-3. Schematic illustration of the mor phology formed by blends and copolymers of two crystallizable polymers. (a) a PEO/PCL Bl end and (b) a PCL-PEO-PCL triblock. In the blend, PEO and PCL are phase se parated into domains in which each homopolymer crystallizes in a lamellar texture. In the copolymer, PEO and PCL blocks crystallize in the same domain due to chain connectivity Figure 5-4. Compression rate de pendence of crystal growth in Langmuir monolayers at 22.5 C.49 BAM images were obtained at compression rates of (a) 0.010, (b) 0.013, and (c) 0.026 nm2 monomer-1min-1 for A ~ 0.08 nm2monomer-1. All images are 1.28 0.96 mm2 (Data borrowed from reference 137) Figure 5-5. The linear PEO-b-PCL copolymer used in this study (n = 60, m = 35) PCL Domains PEO Domains (b) (a) PEO + PCL Domains

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129 Figure 5-6. Schematic of the star-shaped PEO-b-PCL copolymer used in this study. Further characterization data are shown in table 5-1

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130 Figure 5-7. Surface pressure-MMA isotherm for the linear PEO60-b-PCL35 linear copolymer for a compression rate of 5mm/min Figure 5-8. Compressibility plots of the PEO-b-PCL linear diblock copolymer versus surface pressure.

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131 Figure 5-9. Compression-expansio n hysteresis plot of the PEO-b-PCL linear copolymer compressed at a target pressure of 16 mN/m. The compression and expansion rates used in the experiments were both 5mm/min Figure 5-10. Surface pressure-MMA isotherm for the PEO-b-PCL star copolymer recorded for a speed of 5mm/min

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132 Figure 5-11. Compressibili ty plots of the PEO-b-PCL star diblock copolymer versus surface pressure. Figure 5-12. Compression-expansio n hysteresis plot of the PEO-b-PCL star copolymer compressed at a target pressure of 9 mN/m. The compression and expansion rates used in the experiments were both 5mm/min

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133 Figure 5-13. Compression-expansio n hysteresis plot of the PEO-b-PCL star copolymer compressed at a target pressure of 15 mN/m. The compression and expansion rates used in the experiments were both 5mm/min Figure 5-14. BAM Images of the PEO-b-PCL linear copolymer at a pressure for 10mN/m (a) and 15mN/m (b) (a) 100m (b) 100m

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134 Figure 5-15. BAM Images of the PEO-b-PCL star copolymer for a pressure of 10mN/m (a) and 20mN/m (b) Figure 5-16.AFM Images for the linear PEO-b-PCL copolymer ((a) 10mN/m and (c) 15mN/m) and the PEO-b-PCL star copolymer ((b) 10mN/m and (d) 20mN/m) 40nm 20nm 0nm 20 m 20 m 75nm 37.5nm 0nm 10 m 50nm 25nm 0nm 10 m 50nm 25nm 0nm (a) (b) (c) (d) 100m(a) 100m(b)

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135 Figure 5-17. BAM Images of the PEO-b-PCL star copolymer for pressures of 10 and 20mN/m during the first (a and b) and the second compression cycle (c and d) Figure 5-18. BAM images for a pressure of 18mN/ m (a) and after waiting 15min (b), 20min (c), 35 min (d) and 55min (e) First Compression Second Compression 10mN/m 20mN/m 100 m (a) 1 00 m (b) 100 m (c) 100 m (d) (a) (b) (c) (d) (e)

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136 Table 5-1. Characteristics of th e linear and star-shaped copolymers a Determined by 1H NMR b Determined by GPC calibrated with linear polystyrene standards. Name PEO60-b-PCL35 Star Mn a (g/mol) 6,680 13,110 PDIa 1.24 1.44 Avg no. of ethylene oxide units 60 9 Avg no. of -caprolactone units 35 18

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137 CHAPTER 6 CONCLUSION AND PERSPECTIVES In this dissertation, the behavior of various amphiphilic copolymers was studied at the airwater interface using Langmuir-Blodge tt methods as well as AFM and BAM. 6.1 Summary 6.1.1 Blends of a PSb -PEO Copolymer with PS and PEO Homopolymers As discussed previously, the a ddition of free homopolymer chains have an important effect on the architecture of the films formed (Figure 6-1). The isothe rms displayed three different regions known as pancake, pseudoplateau, an d brush areas. While the addition of PS seemed to affect the brush area of the isotherm, the a ddition of PEO showed a di fference in the transition area (pseudoplateau). AFM was able to give us insights on the film architectures and showed the influence of each homopolymers on th e chaining observed in the pure PS-b-PEO copolymer. On one hand the addition of PS increased the amount of chaining of the PS domains enhancing the hydrophobic properties of the monolayer, on the ot her hand the accumulation of PEO decreased the chaining present between the PS domains and favored the phase separation between clusters of micelles and layers of pure PE O. This chapter gave us fundamental information on copolymer blends at the air-water exhi biting more complex architectur es without having to go through difficult synthesis techniques. 6.1.2 PSb -PtBA Star Copolymers at the Air-Water Interface Three star copolymers as well as a PtBA ho mopolymer were studied at the air-water interface. All the isotherms observed had a plat eau at a pressure of 24mN/m suggesting the presence of a first order transi tion. A dependence on the PtBA mo lecular weight could be seen throughout the entire isotherm plot, however, th e PS influence still unclear. After performing AFM experiments on the monolayers formed, we proposed the transition from a two-

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138 dimensional PtBA completely adsorbed onto the in terface to a three-dimensi onal PtBA that starts desorbing from the interface. An increase in the length of the PtBA chains affected both the region below and above the plat eau. For pressures below the pl ateau, the addition of PtBA decreased the size of the copolym er domains as well as increased the distance between those domains. However, when transferred for pressu res above 24mN/m, the aggregation observed in the monolayer increased with the amount of PtBA present in the copolymer. Compressionexpansion hysteresis experiments showed the fo rmation of thermodynamically stable films for pressures under 24mN/m. For pressures higher than 24mN/m, isochoric experiments were performed and exhibited a relaxation of the m onolayer to its equilibrium spreading pressure. 6.1.3 PEOb -PCL Copolymers at the Air-Water Interface AFM results were compared to those obtaine d by BAM in order to better unde rstand the crystallization behavior of the copolymers. The isotherms showed the presence of a pseudoplateau around 15mN/m. Below that pressure the PCL chains are completely adsorbed onto the interface whereas they crystallize for pr essures above the pseudoplateau pressure. BAM illustrated the influence of sample preparation wh en LB films were studied by AFM. In fact the films formed above the pseudoplateau were th ermodynamically unstable and the aggregation was strongly time dependent. BAM pi ctures were taken for a pressure of 18mN/m after several stabilization times and the aggregation increased when upon annealing. 6.2 Correlation Between the Different Systems PSand PEO-containing systems were studied in order to determine the role played by each block on the copolymer self-assembling behavior. Four PS-containing systems we re investigated in this work: one completely hydrophobic (PS), PS-b-PEO diblock copolymer, PS-b-PtBA star copolymers, and blends. It is well-known that PS is hydrophobic and aggregates into irregular islands instead of spreading into monolayers

PAGE 139

139 (Figure 6-2). We showed that by adding sma ll quantities of an amphiphilic copolymer (PS-bPEO), one was able to enhance and vary the in terfacial properties of the PS homopolymer. The PS aggregates observed when in presence of the copolymer were also more regular than those observed when free PS homopolymer chains were spread at the interface. The films formed by the blends seem to have similar properties as those observed for a copolymer possessing the same total number of PS repeat units. Two distinct PS-containing copolymers were used to investigate the effect of the hydrophilic block on the architectures formed. PS-b-PEO and PS-b-PtBA copolymers were studied with PEO being more hydrophilic than PtBA. For low surface pressures, comparable domains of PS can be detected for both systems ( 100). PS-b-PEO copolymers have been widely studied at the air-water interface.16-31 At low coverage, the interactions between the EO monomers and the interface is attractive. Howe ver, when the monolayer is compressed, the repulsive forces between the PS and the PEO bloc ks promote the desorption of some PEO from the interface into the water. Upon further compression the PEO is pushed into the water subphase as shown in Figure 6-2.25 Contrary to the PEO-containing copolymer, the one containing PtBA as the hydrophilic group presented a comp letely different conformation. The PtBA is not as hydrophilic as the PEO and therefore cannot stretc h into the water subphase. Instead, once the plateau pressure (24mN/m) is reached the PtBA started desorbing from the interface. For high surface pressures, a three-dimensional system was formed presenting 3 different regions: adsorbed PtBA, desorbed PtBA, and PS domains. Depicted in Figure 6-3, three different PEO-containing systems were investigated in order to study the various films formed. High molecu lar weight PEO homopol ymer form monolayers when spread at the air-water interface however, once the pseudoplateau pressure is reached, the

PAGE 140

140 PEO dissolved in the water subphase. PS was us ed as an anchor to keep the PEO from dissolving, and a new region was detected on the isotherm. Cont rary to PS, PCL homopolymers are already surface active and form films when spr ead at the air-water in terface. Compare to the PS-b-PEO copolymer where the PS was just sitting on top of the PEO layer, the PCL crystallized directly at the interface forming new and in teresting architectures. Even if the PCL homopolymers already formed monolayers when spread at the air-wat er interface, the PEO-bPCL films could be compressed to higher surface pressures. In the pseudoplateau region, PCL homopolymers as well as PCL chains in the copo lymers crystallized directly at the air-water interface. It was confirmed that above the pseudoplateau, all the PCL chains have collapsed and the sharp pressure represents the intera ctions between the hydrated PEO cores. Those projects gave us inte resting insights on the fundament al self-assembling behavior of PSand PEO-containing amphiphilic copoly mers. By using different hydrophilic and hydrophobic groups, we were able to monitor the fo rmation of various architectures at the airwater interface. The control of conformations or orientations of the different blocks was easily achievable by monitoring the copolymer architect ures, the amount of hom opolymer added (in the case of the blends), or the surface pressure. 6.3 Future Work Jeong et al 92-94, showed that the miscibility be tween PMMA homopolymer and PMMA block was higher in thin films than in the bulk. In that case spin coating was used to prepare the films. However, an interesting study for our bl ends would be to investigate the miscibility between PEO homopolymer and PEO block or PS homopolymer and PS bloc k when spread onto a water subphase, compare to that observe in the bulk. This would allow us to get more fundamental information about the blend behavior and have a better control on the morphologies formed.

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141 The condensed area, present for small molecu lar areas, demonstrates a dependence in PS in the case of the blends seen in chapter 3, as well as for star-shaped PS-b-PEO copolymers previously studied in our group33. For the PS-b-PtBA copolymers studied in chapter 4, a dependence of the condensed area on the PtBA molecular weight was observed. However, the effect of PS remains unclear. Studying copolym ers with varying PS segments would provide more information on the influence of the hydroph obic block on the architectures formed and allow a better control on th e architectures formed. Further investigations would n eed to be done to determine the exact number of arms in each PS-b-PtBA star copolymer. One method would be to cleave the arms of the stars by base hydrolysis and then perform viscom etry/universal calibration and light scattering experiments in order to detect the exact molecular weight as we ll as the mass distribution of the branches. Once we determine this value, we would be able to calculate the exact number of arms for each star and therefore have a better unders tanding of the aggregation propert ies at the air-water interface. Mengel et al.83 studied the chemical modification of PtBA films into PAA after transfer onto a solid substrate. The presen ce of PAA chains at the surface re nders the film very useful for applications in molecular electronic and optical devices. This technique co uld be applied to our system in order to obtain PS-b-PAA films with well-defined arch itectures and surface chemistry. PS-b-PAA copolymers were shown to spread at the air-water interface. Howe ver, contrary to PS-b-PtBA copolymers, a strong pH-dependence is obse rved and the structures formed are not as well defined. By performing the chemical modifi cation after transfer, we would be able to optimize the formation of those films and control the structures formed. Various PCL molecular weights could also be investigated in the case of the PEO-b-PCL copolymers. Knowing the influence of the molecu lar weight on the crysta llization process would

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142 give insights on the transport properties of the polymer chains. The driving force of polymer crystallization is the degree of undercooling ( T = Tm 0 Tx), which is directly related to the crystallization temperature. Therefore, studies ov er a wide range of temp eratures would provide information about the effect of temperature on the copolymer crystallization. An interesting investigation would be to study blends of PEO-b-PCL copolymer s with PCL homopolymers of various molecular weights. This would give us information on the crystallization behavior and also know if the formation of crystals could be controlled by the amount of PCL present. Finally, for most of the copolymers studied, th e non stability of the films formed at high surface pressure was detected in the compre ssion-expansion hysteresis or the isochoric experiments. Performing investigations for several compression rates would quantitatively describe the compression rate dependence of film formation at th e air-water interface.

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143 Figure 6-1. Morphologies obs erved at low surface pre ssures for (a) the pure PS-b-PEO copolymer, (b) the pure PS-b-PEO copolymer + PS homopolymer, and (c) the pure PS-b-PEO copolymer + PEO homopolymer (blue = PEO, black = PS) Figure 6-2. Morphologies obs erved at high surface pressure for (a) PS, (b) PS-b-PEO diblock copolymer, (c) PS + PS-b-PEO diblock copolymer, and (d) PS-b-PtBA copolymer (blue = PEO, black = PS, and orange = PtBA) (a) (b) (c) (d) (a) (b) (c) + PS + PEO

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144 Figure 6-3. Morphologies observe d at high surface pressure for (a) the PEO homopolymer, (b) PEO-b-PS diblock copolymer, and (c) the PEO-b-PCL copolymer (blue = PEO, black = PS, and green = PCL) (a) (b) (c)

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PAGE 153

153 BIOGRAPHICAL SKETCH Sophie Bernard was born on February 25, 1979, in Bordeaux, France. She graduated from the University of Bordeaux in June 2001 with he r B.S. degree in physical chemistry. She then worked as a research assistant under the direc tion of Dr. Yves Gnanou, in the Laboratoire de Chimie des Polymres Organiques, University of Bordeaux. She received her M.S. degree in physical chemistry of polymers from th e University of Bordeaux in July 2002. Since Sophie enrolled at the University of Florida in September 2002, she has been a graduate student under the direc tion of Dr. Randolph S. Duran.


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