|UFDC Home||myUFDC Home | Help|
This item has the following downloads:
TWO-DIMENSIONAL SELF-ASSEMBLY OF AMPHIPHILIC BLOCK COPOLYMERS AT
THE AIR/WATER INTERFACE AND NANOPARTICLES FOR DRUG DETOXIFICATION
THOMAS J. JONCHERAY
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
Thomas J. Joncheray
First and foremost, I would like to thank my parents, Dominique Joncheray and Catherine
Stona, my sister, Alice Joncheray, as well as the rest of my family, past and present, for being an
endless source of support in my education and life. In February 2001, I met Emilie Galand in
Bordeaux who came to the University of Florida with me where we spent together almost five
years of graduate school. I could not have achieved this seemingly insurmountable amount of
work without her. We supported each other through these challenging years, and helped each
other through the difficult times.
I would like to acknowledge my research director, Prof. Randolph S. Duran, for his help
and support over the years I spent under his supervision. His experienced advice has been an
integral part of my education and has given me a deeper understanding of what is needed to be
successful as a research chemist. I would like also to thank the other members of my Ph.D.
committee: Prof. Kenneth B. Wagener, Dr. Ronald K. Castellano, Dr. Thomas J. Lyons, and Dr.
Wolfgang M. Sigmund.
I am also very grateful to the many collaborators I have had the chance to interact with
over the years I spent in graduate school. I have had the pleasure of working with Prof. Audebert
from ENS Cachan on the nanocapsule project. He provided very interesting discussions and
ideas, and always made himself available when I needed his help. I also really appreciated the
collaboration on PEO-b-PCL block copolymers I had with Prof. Schubert and particularly with
Dr. Mike Meier from the Eindhoven University of Technology. It was a pleasure to have Mike
and Jutta staying for a few days at the University of Florida, and Emilie and I also enjoyed very
much the time spent in Eindhoven. I express my appreciation to Prof. Gnanou from the
Laboratoire de Chimie des Polymeres Organiques in Bordeaux, France, for his input in the PS-b-
PtBA, PS-b-PAA, and PB-b-PEO proj ects, and for helping me in j oining the graduate program of
the University of Florida.
At the University of Florida, special thanks go to the polymer floor and the Chemistry
Department staff, most notably Lorraine Williams, Sara Klossner, and Lori Clark for their
patience in answering my various questions. I want to show my gratitude to the three professors
of the Butler Polymer Laboratory, Prof. Duran, Prof. Wagener, and Prof. Reynolds, for putting a
lot of effort in providing the polymer floor members with a superior work environment to
conduct research in polymer chemistry. I want to thank all my co-workers from the Duran group
and from the George and Josephine Butler polymer floor. Special thanks need to go to Rachid
Matmour, who made the time spent in the lab really enjoyable. It was a pleasure working with
him, often in a hilarious atmosphere, on several challenging research projects. Other group
members to whom I owe extra thanks for their help along the way are Dr. Aleksa Jovanovic, Dr.
Jennifer Logan, Jorge Chavez, Sophie Bernard, Brian Dorvel, Rita El-Khouri, Kristina
Denoncourt, and Claire Mathieu.
Finally, I would like to thank all the friends I have had the chance to meet and interact with
inside or outside the Chemistry Department, especially Benoit Lauly, Christophe Grenier, Pierre
Beaujuge, and Changhwan Ko who made my days in Gainesville very enj oyable.
TABLE OF CONTENTS
ACKNOWLEDGMENTS .............. ...............3.....
LIST OF TABLES ............ ..... .__ ...............8...
LIST OF FIGURES .............. ...............9.....
AB S TRAC T ........._. ............ ..............._ 15...
1 INTRODUCTION ................. ...............17.......... ......
1.1 Block Copolymers in the Bulk and in Solution ................. ...............17...........
1.2 Block Copolymers at the A/W Interface .............. ...............20....
1.3 Current Status of Drug Detoxification Therapy .............. ...............23....
1.4 Nanoparticle Technology............... .................2
1.5 Microemulsions and Sol-Gel Chemistry .............. .. ...............25...
1.6 Molecular Imprinting and Miniemulsion Polymerization .............. .....................2
2 EXPERIMENTAL TECHNIQUES............... ...............3
2.1 Langmuir Monolayers and Surface Pressure Related Experiments ................ ...............32
2.2 Langmuir-Blodgett Films and Atomic Force Microscopy .............. ....................3
2.3 Transmission Electron Microscopy and Quasi-Elastic Light Scattering ................... .......39
2.4 Cyclic Voltammetry............... ..............4
3 POLYSTYRENE-b-POLY(TERT-BUTYLACRYLATE) AND POLYSTYRENE-b-
POLY(ACRYLIC ACID) DENDRIMER-LIKE COPOLYMERS: TWO-
DIMENSIONAL SELF-AS SEMBLY AT THE AIR-WATER INTERFACE ....................44
3 .1 Introducti on ............ ..... ._ ...............44..
3.2 Results and Discussion ................. .... ...............4
3.2. 1 PS-b-PtBA Dendrimer-Like Copolymer ................. ...............46...............
3.2.2 PS-b-PAA Dendrimer-Like Copolymer ................. ........._.. ....... 52_.._ ...
3.3 Conclusions............... ..............5
3.4 Experimental Methods ..........._._ ......_..._ ...............59....
3 .4. 1 M ateri al s........._.___..... ._ __ ...............59..
3.4.2 Langmuir Films .............. ...............59....
3.4.3 AFM Imaging ................. ...............60..._..._ .....
4 LANGMUIR AND LANGMUIR-BLODGETT FILMS OF POLY(ETHYLENE
OXIDE)-b-POLY(s-CAPROLACTONE) STAR-SHAPED AND LINEAR BLOCK
COPOLY MERS .............. ...............61....
4. 1 Introducti on ................. ...............61._._ ._.....
4.2 Results and Discussion .............. ...............64....
4.2.1 PEO Homopolymers............... ..............6
4.2.2 PCL Homopolymers .................... ....__ ...............67..
4.2.3 Star-Shaped PEO-b-PCL Block Copolymers ......____ ........_ ................73
220.127.116.11 High MMA region............... ...............73.
18.104.22.168 Intermediate MMA region .............. ...............77....
22.214.171.124 Low MMA region .............. ...............79....
126.96.36.199 AFM imaging ............... .... ...............81.
4.2.4 PEO-b-PCL Linear Diblock Copolymers ................. .............. ...._.. ......83
188.8.131.52 Low surface pressure region .............. ...............85....
184.108.40.206 High surface pressure region............... ...............90.
4.3 Conclusions............... ..............9
4.4 Experimental Methods ................. ...............96........... ....
4.4.1 Langmuir Films .............. ...............96....
4.4.2 AFM Imaging ................. ...............97................
5 TWO-DIMENSIONAL POLYMERIC NANOMATERIALS THROUGH CROSS-
LINKING OF POLYBUTADIENE-b-POLY(ETHYLENE OXIDE) MONOLAYERS
AT THE AIR/WATER INTERFACE ................. ...............98................
5 .1 Introducti on ................. ...............98........... ...
5.2 Results and Discussion ................ .......... ...............100 ....
5.2. 1 Hydrosilylated PB Homopolymer ....__ ................ ............... 100 ...
5.2. 1.1 Hydrosilylation reaction ............... ....____ ...............100
220.127.116.11 Cross-linking reaction at the A/W interface............... ...............10
5.2. 1.3 AFM imaging ................. ... ......... ...............107....
5.2.2 Hydrosilylated PB-b-PEO Three-Arm Stars .............. ...............109....
5.2.2. 1 Hydrosilylation reaction ............... ....____ .....__ ..........12
18.104.22.168 Cross-linking reaction at the A/W interface ................. ......................115
22.214.171.124 AFM imaging ................. ...............119......... .....
5.3 Conclusions............... ..............12
5.4 Experimental Methods .................. ...............125...............
5.4. 1 Materials and Instrumentation ......___ ..... ..___ ...............125
5.4.2 Langmuir Films ............... ...... ..............12
5.4.3 Hydrosilylation of the PB Homopolymer.............. ... .... .. .. ............_ ...126
5.4.4 Hydrosilylation of the (PB200-b-PEO326)3 Three-Arm Star Block Copolymer....127
5.4.5 A/W Interfacial Cross-Linking ................. ...............128........... ...
6 ELECTROCHEMICAL AND SPECTROSCOPIC CHARACTERIZATION OF
ORGANIC COMPOUND UPTAKE IN SILICA CORE-SHELL NANOCAPSULES ......129
6. 1 Introducti on ............ ..... ._ ...............129..
6.2 Results and Discussion ............ ..... ._ ...............131..
6.2. 1 Nanocapsule Characterization ........_.._ ... ...._. ....._. ...........13
6.2.2 Uptake Study .............. ....... .. ............13
6.2.2. 1 Optical measurements results ..........._.. ......_. ....._.. ...........3
126.96.36.199 Electrochemical experiments .............. ...............138....
6.3 Conclusions............... ..............14
6.4 Experimental Methods ................. ...............147................
6.4. 1 Nanocapsule Synthesis ................ ...............147...............
6.4.2 Transmission Electron Microscopy ................. ...............147...............
6.4.3 Particle Size Analysis ................ ...............148...............
6.4.4 Spectroscopy Measurements .............. ...............148....
6.4.5 Electrochemistry Experiments ................. ...............149...............
7 TOWARD SPECIFIC DRUG DETOXIFICATION AGENTS: MOLECULARLY
IMPRINTED NANOPARTICLES................ ............15
7. 1 Introducti on ................. ...............151........... ...
7.2 Results and Discussion ................ ...............152...............
7.3 Conclusions............... ..............16
7.4 Experimental Section............... ...............161
7.4. 1 M ateri al s................. .......... ...............161...
7.4.2 Nanoparticle Synthesis ................ ...............162................
7.4.3 FTIR Spectroscopy ............ .......__ ...............162.
7.4.4 Particle Size Analysis ............ ...... ...............162.
7.4.5 AFM Imaging ................. ...............163...............
7.4.6 Uptake Experiments .............. ...............163....
8 CONCLUSIONS AND PERSPECTIVES .............. ...............165....
LIST OF REFERENCES ................. ...............168................
BIOGRAPHICAL SKETCH ................. ...............184......... ......
LIST OF TABLES
4-1 Characteristic values of the star-shaped polymers. ......___ .... ... .__. ........_........63
4-2 Characteristic values of the linear PEO macroinitiator and of the linear diblock
copolym ers. .............. ...............64....
4-3 Collapse pressure values of the PCL homopolymers. ................ ...._ ................69
5-1 Number average molecular weights and polydispersity indexes of the three-arm star
block copolymers ................. ...............110................
7-1 Loading compositions of the miniemulsions. ..........._.....___ .......___........15
LIST OF FIGURES
1-1 Mean-field predication of the morphologies for conformationally symmetric diblock
m elts ................. ...............18.................
1-2 Solution state for amphiphilic diblock copolymers in water for concentrations below
and above the CM C. ............. ...............19.....
1-3 Spherical, rodlike, and vesicular morphologies for PS-b-PAA crew-cut micelles............20
1-4 A schematic illustration showing the components of an amphiphile and the
orientation of this amphiphile adopted at an interface ....._.__._ ... ......__ ........._.....21
1-5 Transmission electron micrographs of LB fi1ms of poly(styrene-b-vinylpyridinium
decyl iodide) AB diblock copolymers .............. ...............23....
1-6 Structure of normal and inverse spherical micelles formed in microemulsion systems....26
1-7 Sol-Gel hydrolysis and condensation reactions. ............. ...............27.....
1-8 Sol-Gel technologies and their products ................. ....__. ......._...........2
1-9 Outline of the molecular imprinting strategy ....._.._.. .... ... .__. ...._.._ .........2
1-10 Examples of commercially available functional monomers and cross-linkers ........._.......29
1-11 Principal of miniemulsion polymerization. ....._ .....___ ........___ ...............3 1
2-1 The Langmuir Teflon trough .............. ...............32....
2-2 A Wilhelmy plate partially immersed in a water subphase .............. .....................3
2-3 Schematic isotherm for small amphiphilic molecules .............. ...............35....
2-4 LB fi1m transfer onto a hydrophilic mica substrate .............. ...............37....
2-5 The AFM tapping mode electronic setup ......... .........._.... ......... ...........3
2-6 Specimen interactions in electron microscopy .............. ...............39....
2-7 Basic shape of the current response for a cyclic voltammetry experiment. ................... ....42
3-1 Schematic sketch of the PS-b-PtBA (Dendl) and the PS-b-PAA (Dend2) samples.........45
3-2 Isotherm of Dend1. (Inset) Static elastic modulus plot versus surface pressure. .............47
3-3 Topographic AFM images of Dendl LB fi1ms transferred at 5, 10, 15, 20, 24 (middle
of plateau, MMA = 20,000 A+2), and 40 mN/m ................. ............ ...................49
3-4 Compression-expansion hysteresis plot of Dend1 (target MMA = 20,000 A+2).................50
3-5 Surface pressure/time isochoric relaxation plot of Dend1 after compression up to 40
m N /m .............. ...............51....
3-6 Compression-expansion hysteresis plot of Dend1 (target pressure = 40 mN/m) ..............52
3-7 Isotherm of PAA250K (pH = 2.5). ................ ......... ......... ........ ..............53
3-8 Topographic AFM images of PAA250K LB fi1ms transferred at 1.5, 3, and 3.5 mN/m. ....54
3-9 Isotherm of Dend2 (pH = 2.5). (Inset) Compressibility plot versus surface pressure. ......55
3-10 Topographic AFM images of Dend2 LB fi1ms transferred at 2, 4, 4.5, 5, 5.5, 6, and 8
m N /m ............. ...............57.....
3-11 Compression-expansion hysteresis plot of Dend2 (target pressure = 5 mN/m, pH =
2.5). ............. ...............57.....
4-1 The star-shaped PEO-b-PCL block copolymers. ............. ...............63.....
4-2 The linear PEO-b-PCL block copolymers. ............. ...............64.....
4-3 Isotherms of the PEO homopolymers. (Inset) Same isotherms normalized with
respect to the number of ethylene oxide units. ............. ...............66.....
4-4 Compression-expansion hysteresis plot of the PEO core (target pressure = 2 mN/m)......66
4-5 Isotherms of the PCL homopolymers. (Inset) Same isotherms normalized with
respect to the number of e-caprolactone units. ............. ...............68.....
4-6 Isotherms of the PCL homopolymers (compression speed = 100 mm/min). ....................70
4-7 Compression-expansion hysteresi s plot of PCL 1250. ................ ....._._ ........_......7 1
4-8 Topographic AFM images of PCL homopolymers LB fi1ms transferred below and
above monolayer collapse ................. ...............72........... ....
4-9 Isotherms of the star-shaped PEO-b-PCL copolymers. ......... ................ ...............73
4-10 Compression-expansion hysteresis plot of Star#6 (target pressure = 9 mN/m). ...............74
4-11 Compression-expansion hysteresis plot of Star#1 (target pressure = 9 mN/m). ...............74
4-12 Plots of MMA versus number of e-caprolactone repeat units for different surface
pressures from the isotherms of Star#3, Star#4, Star#5, and Star#6 .............. ..............76
4-13 Isotherms of the PEO core extrapolated and PEO2000 normalized with respect to the
number of ethylene oxide units. ................. ...............76.._._._ ....
4-14 Compressibility plots of the star-shaped PEO-b-PCL block copolymers versus
surface pressure............... ...............77
4-15 Isobaric relaxation plots of Star#6 at 9, 11, and 13 mN/m ................. .......................78
4-16 Compression-expansion hysteresis plot of Star#6 (target pressure = 15 mN/m) ........._....79
4-17 Compression-expansion hysteresis plot of Star#1 (target pressure = 15 mN/m) ........._....79
4-18 Proposed conformations modeling the adsorption of the star-shaped block
copolymers at the A/W interface versus surface pressure. ............. .....................8
4-19 Topographic AFM images of the star-shaped PEO-b-PCL copolymers LB films
transferred below and above the pseudoplateau .............. ...............82....
4-20 Isotherms of the PEO-b-PCL linear diblock copolymers. ................ ................. .....84
4-21 Compressibility plots of the PEO-b-PCL linear diblock copolymers versus surface
pressure. ............. ...............8 5....
4-22 Isotherms of PEO2670 and PCL2000 binary mixtures. (Inset) Corresponding
compressibility plots versus surface pressure. .............. ...............86....
4-23 Compression-expansion hysteresis plot of the binary mixture with 49 mol % in
PEO2670 (target pressure = 9 mN/m). ............. ...............86.....
4-24 MMA plots versus mole fraction of PCL2000. Dashed lines: theoretical ideal mixing....87
4-25 Compression-expansion hysteresis plot of PEO60-b-PCL11 (target pressure = 18
m N /m ). .............. ...............89....
4-26 Compressibility plots of Figure 4-25 (PEO60-b-PCLll, target pressure = 18 mN/m).......89
4-27 Compression-expansion hysteresis plot of PEO60-b-PCL35 (target pressure = 16
m N /m ). .............. ...............9 1....
4-28 Compressibility plots of Figure 4-27 (PEO60-b-PCL35, target pressure = 16 mN/m)........92
4-29 Topographic AFM images of the linear PEO-b-PCL diblock copolymers LB films
transferred after crystallization of the PCL segment at the A/W interface (7<= 15
m N/m ) ................. ...............93.......... ......
4-30 Proposed conformations modeling the adsorption of the linear PEO-b-PCL diblock
copolymers at the A/W interface versus surface pressure .............. .....................9
5-1 Hydrosilylation of the pendant double bonds of the PB homopolymer. ........................101
5-2 1H NMR spectrum of the PB homopolymer ................. ...............101.............
5-3 1H NMR spectrum of the hydrosilylated PB homopolymer............._ .........._ .....102
5-4 FTIR spectra of the PB homopolymer before and after hydrosilylation. ........._.............102
5-5 Cross-linking reaction involving hydrolysis and condensation of the triethoxysilane
groups ................. ...............104................
5-6 Surface pressure-Mean Molecular Area isotherms of the hydrosilylated PB carried
out after different reaction times (subphase pH = 3.0). ............. ....................10
5-7 Static elastic modulus-surface pressure curves of the hydrosilylated PB
homopolymer at different reaction times (subphase pH = 3.0). ............. ....................105
5-8 MMA-time isobars of the hydrosilylated PB for various subphase pH values (ri= 10
m N /m ). .............. ...............106....
5-9 Removal of the cross-linked hydrosilylated PB from the water surface ......................107
5-10 AFM topographic images of the LB films transferred onto mica substrates at zi= 10
m N /m ............. ...............109....
5-11 Surface Pressure-MMA isotherms for the (PB200-b-PE0n)3 three-arm star block
copolymers. ........._.._.. ...._... ...............111....
5-12 Isotherm of (PB76-b-PEO444)4 depicting how Apancake, Ao, and AApseudoplateau are
determ ined............... ..............11
5-13 Linear dependence of AApseudoplateau On the total number of ethylene oxide units............1 12
5-14 Hydrosilylation of the pendant double bonds of the (PB-b-PEO)3 three-arm star
block copolymers. ........... ..... .._ ...............113...
5-15 1H NMR spectra of the (PB200-b-PEO326)3 Star block copolymer and the
corresponding hydrosilylated (PB(Si(OEt)3)-b-PEO)3 Star block copolymer. ................114
5-16 FTIR spectra of the (PB200-b-PEO326)3 Star block copolymer and the corresponding
hydrosilylated (PB(Si(OEt)3)-b-PEO)3 Star block copolymer. ................ ................. .114
5-17 Surface Pressure-MMA isotherms of the (PB200-b-PEO326)3 Star block copolymer and
of the corresponding hydrosilylated (PB(Si(OEt)3)-b-PEO)3 Star block copolymer
before and after cross-linking. ........... ......... ...............116.....
5-18 Surface Pressure-MMA isotherms and compressibility-MMA curves of the
hydrosilylated (PB(Si(OEt)3)-b-PEO)3 Star block copolymer at various reaction times
(subphase pH = 3.0). ........... ......_._ ...............117...
5-19 Isobars of the hydrosilylated (PB(Si(OEt)3)-b-PEO)3 Star block copolymer for
various subphase pH values (7<= 5 mN/m) ................. ...............118.............
5-20 Removal of the cross-linked (PB(Si(OEt)3)-b-PEO)3 three-arm star copolymer from
the Langmuir trough surface. ........._.__ ..... .___ ...............119...
5-21 AFM topographic images of the (PB(Si(OEt)3)-b-PEO)3 Star block copolymer LB
film s. ............. ...............122....
5-22 Cross-section analysis of Figures 5-21D and 5-21H, PEO pore size versus surface
pressure plot, and PSD plots of Figures 5-21C, 5-21D, 5-21G, and 5-21H. ...................122
5-23 AFM topographic images and corresponding cross-sections of the (PB(Si(OEt)3)-b-
PEO)3 Star block copolymer LB films cross-linked at 9 mN/m (pH = 3.0, t = 10 h)
and transferred at 9 and 2 mN/m............... ...............123.
5-24 Surface pressure-MMA isotherms of the hydrosilylated (PB(Si(OEt)3)-b-PEO)3 Star
block copolymer cross-linked at 5 and 20 mN/m (pH = 3.0, t = 10 h). ........._................124
6-1 Oil-filled silica nanocapsule synthesis through initial hydrophobic core formation
followed by hydrophilic silica shell formation after TMOS addition..............................132
6-2 Description of the nanocapsule samples prepared using 0.07, 0.28, 0.44, and 0.88 wt
% TM OS. ............. ...............132....
6-3 DLS results for the microemulsion immediately after preparation and the same
solution after TMOS addition (0.07 wt %) and dialysis. ................ .................13
6-4 TEM micrographs of the 0.07 wt % TMOS nanocapsules and of the 0.88 wt %
TM OS nanocapsules. ............. ...............134....
6-5 Chemical structures of ferrocene methanol, ferrocene dimethanol, and Nile Red. .........135
6-6 UV-vis absorption spectra of iodine in water solution, in nanocapsule solution, in
Tween-80 aqueous solution, and in ethyl butyrate solution. ............. .....................3
6-7 Nile Red emission spectra in ethyl butyrate solution, in nanocapsule solution, in
Tween-80 aqueous solution, in crushed Xerogel dispersion in acidic water, on silica
gel, and in acidic water solution. ............. ...............137....
6-8 Typical cyclic voltammogram of ferrocene methanol in water. ..........__.... ................ 138
6-9 Uptake of ferrocene methanol versus time in 0.07, 0.28, 0.44, and 0.88 wt % TMOS
nanocapsule solutions. ............. ...............141....
6-10 Plot of normalized aqueous concentration of ferrocene methanol after uptake in 0.07,
0.28, 0.44, and 0.88 wt % TMO S nanocapsule solutions ..........._... ........._............142
6-11 Uptake of ferrocene dimethanol versus time in 0.07, 0.28, 0.44, and 0.88 wt %
TMOS nanocapsule solutions .............._ ...............144........ .....
6-12 Uptake of ferrocene methanol versus time in 0, 2, 4, 6, and 8 wt % Tween-80
aqueous solutions ................. ...............145................
6-13 Uptake of ferrocene dimethanol versus time in 0, 2, 4, 6, and 8 wt % Tween-80
aqueous solutions ................. ...............146................
7-1 Chemical structures of amitriptyline and bupivacaine. ....._____ ........._ ........._....152
7-2 The molecular imprinting strategy in miniemulsion polymerization. ............. ................154
7-3 IR absorbance spectra of EGDMA, MIP1, and MIP3 ................. ................ ...._..155
7-4 DLS size distribution of MIP6............... ...............156
7-5 Tapping mode topographical AFM images and cross-section analysis of MIP6............. 157
7-6 Uptake of amitriptyline by the non-molecularly imprinted nanoparticles MIP1, MIP2,
and M IP3 .............. ...............158....
7-7 Uptake of amitriptyline by the nanoparticles molecularly imprinted with
amitriptyline: MIP4, MIPS, and MIP6............... ...............159.
7-8 Uptake of bupivacaine by the nanoparticles molecularly imprinted with
amitriptyline: MIP4, MIPS, and MIP6............... ...............160.
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
TWO-DIMENSIONAL SELF-ASSEMBLY OF AMPHIPHILIC BLOCK COPOLYMERS AT
THE AIR/WATER INTERFACE AND NANOPARTICLES FOR DRUG DETOXIFICATION
Thomas J. Joncheray
Chair: Randolph S. Duran
Major Department: Chemistry
The two-dimensional self-assembly at the air/water (A/W) interface of various block
copolymers (dendrimer-like polystyrene-b-poly(tert-butylacrylate) (PS-b-PtBA) and
polystyrene-b-poly(acrylic acid) (PS-b-PAA), linear and five-arm star poly(ethylene oxide)-b-
poly(s-caprolactone) (PEO-b-PCL), and three-arm star triethoxysilane-functionalized
polybutadiene-b-poly(ethylene oxide) (PB(Si(OEt)3)-b-PEO)) was investigated through surface
pressure measurements isothermss, isobars, isochores, and compression-expansion hysteresis
experiments) and atomic force microscopy (AFM) imaging. The PS-b-PtBA and the PS-b-PAA
samples formed well-defined circular surface micelles at low surface pressures with low
aggregation numbers (~ 3-5) compared to linear analogues before collapse of the PtBA chains
and aqueous dissolution of the PAA segments take place around 24 and 5 mN/m, respectively.
The linear PEO-b-PCL samples exhibited three phase transitions at 6.5, 10.5, and 13.5 mN/m
corresponding respectively to PEO aqueous dissolution, PEO brush formation, and PCL
crystallization. The two PEO phase transitions were not observed for the star-shaped PEO-b-PCL
samples because of the negligible surface activity of the star-shaped PEO core compared to its
linear analogue. The PB(Si(OEt)3)-b-PEO sample was cross-linked at the A/W interface by self-
condensation of the pendant triethoxysilane groups under acidic conditions, which resulted in the
formation of a two-dimensional PB network containing PEO pores with controllable sizes.
With a view toward drug detoxification therapy, the encapsulation abilities of oil core-
silica shell nanocapsules and molecularly imprinted nanoparticles were also investigated by
electrochemical (cyclic voltammetry) and optical (fluorescence and UV-vis spectroscopies)
techniques. The core-shell nanocapsules were shown to efficiently remove large amounts of
organic molecules present in aqueous solutions, with the silica shell acting analogously to a
chromatographing layer. The molecularly imprinted nanoparticles were prepared by the non-
covalent approach and by miniemulsion polymerization. Binding studies on the molecularly
imprinted nanoparticles in aqueous solutions under physiological pH conditions indicated that, in
the absence of specific imprinting, the uptake of toxic drugs was mainly driven by non-specific
hydrophobic interactions. As demonstrated with the use of the antidepressant amitriptyline, in the
presence of specific imprinting the uptake significantly increased as the amount of specific
binding sites was increased.
This dissertation aims at summarizing the work realized on two different research domains
of polymer chemistry: the air/water (A/W) interfacial behavior of block copolymers and the
synthesis of nanoparticles for drug detoxification applications. As presented in Chapters 3, 4, and
5, the first three research projects are related to the self-assembly of amphiphilic block
copolymers at the A/W interface, whereas Chapters 6 and 7 describe the investigations carried
out on the possibility for 2 different types of nanoparticulate systems to be used as drug
detoxification agents. While Chapter 2 briefly describes the principal experimental techniques
mentioned in the subsequent chapters, this first introductory chapter serves as literature
background for the different research proj ects and also defines the key concepts used throughout
1.1 Block Copolymers in the Bulk and in Solution
"Block copolymer" is a general term used to define a macromolecule composed of
different polymer chains. The field of block copolymers has attracted a lot of interest in the past
thirty years because the eventual phase separation between immiscible blocks in various
environments, such as in the bulk, often leads to well-defined self-assembled structures with
unique morphologies with characteristic sizes ranging between a few nanometers up to hundreds
of nanometers.1,2 Moreover, the recent emergence of controlled polymerization techniques such
as living anionic polymerization,3 ATRP (atom transfer radical polymerization),4 Or RAFT
(reversible addition fragmentation chain transfer)' has allowed access to a variety of
compositions and architectures (star,6 mikto-arm,7 cyclic .), which significantly increases the
diversity of peculiar and regular patterns obtainable resulting from their self-assembly. A lot of
research still has to be done to better understand the relationships between block copolymer
architecture and self-assembly. Therefore, because of their simple architecture, linear diblock
copolymers are still currently the best-known class of block copolymers. Several theoretical
models have been proposed to describe the behavior of block copolymers such as for instance the
self-consistent field theory (SCFT)9 Or the mean-field theory (MFT)1o where the phase behavior
is dictated by the Flory-Huggins segment-segment interaction parameter, the degree of
polymerization, and the composition. As an example, if the two A and B blocks of a linear AB
diblock copolymer are immiscible, they can adopt in the bulk, as shown in Figure 1-1, various
microphase morphologies such as spheres (S and S'), cylinders (C and C'), double gyroids (G
and G'), or lamellae (L).1"
Figure 1-1. Mean-field predication of the morphologies for conformationally symmetric diblock
melts. Phases are labeled as: S (spheres), C (cylinders), G (double gyroids), L
lamellaee). fA is the volume fraction.1c
When block copolymers are dissolved in a selective solvent, the chains can aggregate to
reversibly form well-defined micelles above the so-called critical micellar concentration (CMC).
For concentrations lower than the CMC, block copolymer molecules are unassociated as
illustrated in Figure 1-2 for amphiphilic diblock copolymers in water aggregating into spherical
t ~ ~Hydrophilic block c 12 J ecc
C < CMC C > CMC
Figure 1-2. Solution state for amphiphilic diblock copolymers in water for concentrations below
and above the CMC.
The critical micellar concentrations of block copolymers are usually very low compared to
low molecular weight surfactants, and therefore the micelles formed have great potential in drug
delivery when used as nanocontainers since they hardly dissociate in the blood stream, even
under highly dilute conditions.1a Depending on the architecture, composition, concentration, or
solvent, block copolymers can aggregate into a variety of micellar structures. Figure 1-3 shows
as an example some peculiar morphologies obtained by Eisenberg and co-workers for "crew-cut"
micelles of linear polystyrene-b-poly(acrylic acid) (PS-b-PAA) diblock copolymers in aqueous
solutions."l Transitions from spheres to rod to vesicles were observed as the length of the PAA
segment was decreased. In semi-dilute or concentrated solutions, gelation normally occurs, and
block copolymer micelles organize into a nanostructure-ordered lyotropic liquid crystal phase.12
Figure 1-3. Spherical (a), rodlike (b), and vesicular (c) morphologies for PS-b-PAA crew-cut
1.2 Block Copolymers at the A/W Interface
The behavior of block copolymers at interfaces is also of great interest since other
parameters such as the surface energies as well as film thickness can strongly influence the
microphase separation.l" Confining polymeric chains in a layer thinner than their natural length
scale (the radius of gyration) considerably alters their conformation and the resulting physical
properties compared to the bulk properties.13 Among the variety of surfaces and interfaces
available, the A/W interface has attracted a lot of attention because it allows the easy formation
of two-dimensional polymeric monolayers (Langmuir monolayers), providing that the block
copolymers studied are surface active by having sufficiently polar functional groups to adsorb at
the interface (without being too much water soluble to avoid their irreversible dissolution in the
aqueous subphase). Similarly as for low molecular weight surfactants, surface active block
copolymers self-assemble at the A/W interface to reduce the surface tension (internal pressure
caused by the attraction of molecules below the surface for those at the surface) with the
hydrophilic segments immersed into the water and the hydrophobic segments desorbed in the air
as illustrated in Figure 1-4 for a low molecular weight fatty acid amphiphile.14
ACir, Gas olr Oil
Figure 1-4. A schematic illustration showing the components of an amphiphile and the
orientation of this amphiphile adopted at an interface.14
As previously shown, one of the great advantages of the A/W interface is that it is possible
and fairly easy to accurately control and adjust the way the chains of the block copolymers self-
assemble simply by varying their surface concentration (amount present at the interface) in
addition to the other usual parameters mentioned before for the bulk or for solutions. This often
leads to peculiar arrangements of the polymer chains with the formation of surface micelles,
which significantly differs from what is commonly observed for low molecular weight
amphiphiles. Control of surface density in the Langmuir films and transfer of the block
copolymer monolayers onto solid substrates for further analysis (Langmuir-Blodgett films) are
experimental procedures commonly done with the use of a Langmuir trough as extensively
described later in Chapter 2. A wide range of experimental techniques can be used for
morphology investigation in Langmuir and Langmuir-Blodgett (LB) monolayers including
neutron and X-ray reflectivity, surface pressure and potential measurements, Brewster angle
microscopy (BAM), atomic force microscopy (AFM), transmission electron microscopy (TEM),
and ellipsometry.15-20 Examples of block copolymers with various architectures previously
studied at the A/W interface include poly(ethylene oxide)-b-poly(propylene oxide) (PEO-b-
PPO), polystyrene-b-poly(ethylene oxide) (PS-b-PEO), polybutadiene-b-poly(ethylene oxide)
(PB-b-PEO), and polystyrene-b-polyacrylate.21-24 Block copolymers based on PS and decylated
poly(4-vinylpyridine) (decylated P4VP) have also been extensively investigated at the A/W
interface by Eisenberg and co-workers, and some of their results are presented here to illustrate
the concept of surface micelle formation. Surface micelle formation results from the presence at
the A/W interface of immiscible blocks that phase separate because of sufficiently different
polarities. In the case of PS and decylated P4VP-based block copolymers, only the hydrophilic
decylated P4VP segments adsorb at the A/W interface with the hydrophobic PS segments
desorbing and aggregating above the interface. This results in the formation of well-defined
surface micelles with architectures evolving from circular to rod-like to planar as the PS % in the
diblock copolymers is increased (Figure 1-5).25 Prospective applications of such well-defined
patterns with feature sizes typically in the nanometer scale order include lithographic masks,
photonic materials, and nanopatterned substrates for microelectronics.26-28
The results presented in Chapters 3, 4, and 5 of this dissertation are all related to the self-
assembly of block copolymers at the A/W interface. While the work presented in Chapters 3 (on
polystyrene-b-poly(tert-butylacrylate) and polystyrene-b-poly(acrylic acid) dendrimer-like block
copolymers) and 4 (on linear and star-shaped poly(ethylene oxi de)-b-poly(e-caprol actone) block
copolymers) describes the interfacial aggregation of several block copolymers on a fundamental
level, the work presented in Chapter 5 (on star-shaped poly(ethylene oxide)-b-hydrosilylated
polybutadiene block copolymers and done in collaboration with Rachid Matmour, graduate
student in the Duran group at the University of Florida) focuses primarily on the use of a post
monolayer formation cross-linking method using Sol-Gel chemistry (Section 1-5) for the
synthesis of novel two-dimensional cross-linked nanomaterials with well-defined morphologies.
Figure 1-5. Transmission electron micrographs of LB films of poly(styrene-b-vinylpyridinium
decyl iodide) AB diblock copolymers (surface pressure = 2 mN/m) (a) P(S260-b-
VP120 10oI); % PS = 68.4; (b) P(S260-b-VIP71/Clol); % PS = 78.5; (C) P(S260-b-
WP29 C0I) % PS = 90 .0; (d) P(S480-b-VP34C10I) % PS = 93 .4; (e) P(Slso-b-VP11/Clol)
% PS = 94.2; (f) P(S480-b-VPl3 C10I) % PS 97.4.25
1.3 Current Status of Drug Detoxification Therapy
Drug toxicity is a major health concern, because more than 3 million insecticide
poisonings are reported every year worldwide with, among them, around 200,000 fatal cases.29
In the United States, a 2000-query showed that drug-related complications lead to the hospi-
talization of more than 300,000 patients per year. Other important issues are related to clinical
mistreatment for instance in the area of anesthesiology where it was recently shown that the
intravenous injections of local anesthetics such as bupivacaine are a major cause of cardiac
blocks with a fatal outcome.30,31 Tricyclic antidepressants such as amitriptyline (one of the most
common drugs used in suicideS32) are among the most frequently ingested substances in self-
poisoning.33 COmmon clinical treatments in the case of anesthetic-induced toxicity include rapid
oxygenation, ventilation, seizure control, and cardiovascular support.34 To Eight drug overdose,
several methods have been explored such as the intravenous injection of sodium bicarbonate,35
the use of activated carbon,36 the use of lipid infusions,37 Or even the use of other drugs such as
amiodarone,38 inSUlin,39 Or propofol.40 All these methods have shown some interesting results but
an efficient and specific antidote scavenging the toxicity of the aforementioned antidepressant
and anesthetic drugs by reducing their bio-availability within the body still has to be designed.
1.4 Nanoparticle Technology
Living organisms are built of cells that are usually a few micrometers in size, and their
organelles are even much smaller, in the sub-micron size range. The recent development of
nanotechnologies that deal with nanometer-sized objects (nanoparticles) for instance in
electronics, cosmetics, or catalysis41,42 also opened the possibility to probe the cellular machinery
without introducing too much interference43 and to understand biological processes on the
nanoscale level.44 Recent developments of nanoparticulate systems include for instance protein
detection,45 tissue engineering,46 cancer therapy,47 and drug delivery.48 Because of the absence of
specific antidotes in the case of drug intoxication as mentioned in the previous paragraph, a
multidisciplinary effort is being made at the University of Florida (Particle Engineering Research
Center and Departments of Chemistry, Material Science, Chemical Engineering, and
Anesthesiology) to synthesize a series of novel nano-sized bioparticles that are able to move
freely in the blood stream even in the smallest capillaries (diameter ~ 5 pum) and that are able to
effectively reduce the free blood concentration of toxic drugs. The ideal nanoparticulate system
therefore should have high and fast (within seconds or minutes) encapsulation capacities, should
be biocompatible (non-toxic), biodegradable (slowly enough so the aqueous concentration of the
drug released in the blood stays below toxic levels), and above all, should be specific to the
target drug to avoid side encapsulation of other undesired molecules present in the blood stream.
Oil core-silica shell nanocapsules have been recently synthesized in the Duran group by
combining microemulsion formulation and Sol-Gel chemistry,49 and with a view toward their
application in detoxification media, the work presented in Chapter 6 focused on investigating
their uptake kinetics and capacities of drug-mimicking compounds. In Chapter 7, a different and
novel type of nanoparticles was synthesized and characterized by combining miniemulsion
polymerization and molecular imprinting technology in an attempt to design a nanoparticulate
system with increased selectivity.
1.5 Microemulsions and Sol-Gel Chemistry
A microemulsion is defined as a system comprising oil, water, and surfactants (surface
active agents having both a hydrophilic part and a hydrophobic part) that results in a single-
phase, optically isotropic, and therefore thermodynamically stable liquid solution. This is not the
case for emulsions that are usually only kinetically stable and that coalesce and phase-separate
over time. When water, oil, and surfactants are mixed together to form a microemulsion, the
surfactant molecules rest at the oil-water interface with the hydrophilic groups solubilized into
the aqueous phase and the hydrophobic groups solubilized into the organic phase to avoid the
direct oil-water contact. The sizes of these structures are usually in the range of a few hundred
nanometers, which makes them suitable for the synthesis of nano-sized objects. Depending on
several parameters such as the nature of the oil or of the surfactant and the concentrations or the
relative amounts of oil, surfactant, and water used, it is possible to obtain a variety of internal
structures. They can be spherical (Figure 1-6), spheroidal, or cylindrical "rod" or "worm-like"
micelles, and they may exist in hexagonal, cubic, or lamellar phases. Alternatively, a range of
bicontinuous phases may exist.
Figure 1-6. Structure of normal (left) and inverse (right) spherical micelles formed in
Microemulsions can have characteristic properties such as ultra low interfacial tension,
large interfacial area and capacity to solubilize both aqueous and oil-soluble compounds.'o
Therefore, they can be used in a variety of applications, for example in enhanced oil recovery,"
as fuels,52 as lubricants, cutting oils, and corrosion inhibitors,53 as coatings and textile finishing,54
in detergency," in cosmetics,56 in food,57 in pharmaceuticals,58 or in biotechnology.59 As
presented in Chapter 6, an oil-in-water microemulsion system composed of spherical micelles
was designed in an attempt to selectively encapsulate hydrophobic toxic drugs inside the micellar
oil core and to decrease their free concentration in the blood stream. Even though microemulsion
systems are thermodynamically stable, the high dilution taking place after their injection inside
body fluids would lead to rupture and further coalescence of the micellar structures. This
problem can be avoided by building a solid shell surrounding the microemulsion oil droplets that
will not break upon dilution, for instance by the means of Sol-Gel chemistry (hence the name
The Sol-Gel process allows the easy synthesis of ceramic and glass materials with high
purities and homogeneities by using preparation techniques different from the traditional process
of fusion of oxides. This process occurs in liquid solutions in the presence of organometallic
precursors such as metal alkoxides (tetramethoxysilane, tetraethoxysilane, Zr(IV)-propoxide, or
Ti(IV)-butoxide), which, through acid- or base-catalyzed hydrolysis and condensation reactions
as shown in Figure 1-7, leads to the formation of a new phase (Sol).60
M-O-R + H20 -- M-OH + R-OH (Hydrolysis)
M-OH + HO-M -- M-O-M + H20 (Water condensation)
M-O-R + HO-M M-O-M + R-OH (Alcohol condensation)
( M = S i, Z r, T i)
Figure 1-7. Sol-Gel hydrolysis and condensation reactions.
The Sol is made of solid particles of a diameter of a few hundred nanometers suspended in
a liquid phase. The particles then condense in a new phase (Gel) in which a solid macromolecule
is immersed in a liquid phase (solvent). Drying the Gel by means of low temperature treatments
allows the formation of materials in a wide variety of forms: ultra-fine or spherical shaped
powders, thin film coatings, ceramic fibers, microporous inorganic membranes, monolithic
ceramics and glasses, or extremely porous aerogel materials as illustrated in Figure 1-8.61 Sol-Gel
materials can be used for instance in chemical sensing,62 drug delivery,63 Or electrochromics.64
As further explained in Chapter 6, the formation of a rigid shell to impart stability to the
microemulsion droplets resulting in the formation of core-shell nanocapsules was done by
polycondensation of tetramethoxysilane directly on the microemulsion droplet surface previously
functionalized with trimethoxysilane groups. A similar polycondensation method was also used
as presented in details in Chapter 5 to cross-link in two dimensions block copolymers containing
triethoxysilane-functionalized polybutadiene segments at the A/W interface.
Eglegraulian 'w Oi
Figure 1-8. Sol-Gel technologies and their products.61
1.6 Molecular Imprinting and Miniemulsion Polymerization
The concept of molecularly imprinted polymers (MIPs) was first introduced by Wulff
nearly thirty years ago.65 Since then, the methodology has undergone a number of important
developments, and MIPs have been used in a wide range of techniques (in solid-phase extraction,
as biosensors, as catalysts, and as binding assays) and approaches covalentt, non-covalent, metal
coordination).66-7 MIPs result from the polymerization of monomeric units in the presence of a
template molecule. The imprinting strategy investigated in Chapter 7 is shown in Figure 1-9 and
uses the non-covalent imprinting approach first developed by Mosbach.73,74 Functional
monomers are associated with a template via non-covalent interactions. The complex is then
copolymerized with a cross-linking monomer, followed by removal of the template using
extraction procedures. Removal of the template results in cavities whose shape, size,
functionality, and spatial arrangement are complementary to the imprinted molecule. These
recognition sites with predetermined selectivity enable MIPs to rebind selectively the original
template from a mixture. In principle, MIPs can be made with selectivity for essentially any of a
diverse range of analyte species, such as drug enantiomers, pesticides, hormones, toxins, short
peptides, and nucleic acids.75s
Figure 1-9. Outline of the molecular imprinting strategy.
Many parameters can affect the efficiency of the synthetic molecularly imprinted polymers
such as the porogen (solvent), the polymerization technique, and also the relative concentrations
of the functional monomer, cross-linker, template molecule, and initiator. Figure 1-10 shows
examples of classical functional monomers and cross-linkers commercially available for radical
i~ CHa~ 0 CH
0 CH, OCH,
Figure 1-10. Examples of commercially available functional monomers and cross-linkers.8
The main motivation to investigate the potential of molecularly imprinted materials for
drug detoxification applications was to harvest their remarkable ability to rebind specifically a
variety of compounds including toxic drugs. As described later in further details (Chapter 7), the
synthesis of nano-sized molecularly imprinted particles was attempted through miniemulsion
radical polymerization, and their toxic drug-rebinding capacities were investigated.
Miniemulsions are produced by the combination of a high shear to break up the emulsion into
submicron monomer droplets and a surfactant/costabilizer system to retard monomer diffusion
from the submicron monomer droplets.82 The costabilizer is used to prevent Ostwald ripening.83
Compared to microemulsions that usually lead to the formation of oil droplets ranging from 10 to
100 nm, the formation of miniemulsions, which requires a lower amount of surfactants, usually
leads to oil droplet diameters ranging from 50 to 500 nm. The droplet surface area in these
systems is very large, and most of the surfactants are adsorbed at the droplet surface. Since little
surfactant is present in the form of micelles in a properly formulated microemulsion, particle
nucleation during polymerization primarily takes place inside the monomer droplets. This leads
to a 1:1 correspondence between the initial monomer droplets and the final polymer
nanoparticles. The principal of miniemulsion polymerization is schematically shown in Figure 1-
11. As described in greater details in Chapter 7, the synthesis of molecularly imprinted
nanoparticles was done radically, but the miniemulsion polymerization process is not limited to
radical polymerizations.84-86 The imprinting takes place inside the monomer droplets, and
extraction of the toxic drug molecules results in porous nanoparticles with recognition sites
complementary in shape to the original toxic drug template.
.0 on ome 1
Mon omy Ir Polymer-
Mi ni emul sifi cati on P olymerizati on
(high pressure homogeniz er
Figure 1-11i. Principal of miniemulsion polymerization.
In this chapter, an overview of the principal experimental techniques is presented. The
specific detailed experimental procedures are not described here but are included instead at the
end of each of the following chapters in the Experimental sections.
2.1 Langmuir Monolayers and Surface Pressure Related Experiments
As further detailed in Chapters 3, 4, and 5, investigation of block copolymer self-assembly
at the air/water (A/W) interface in Langmuir monolayers requires the use of a Langmuir trough.
A typical set-up is shown in Figure 2-1. It provides a very simple method to control monolayer
surface density thanks to the movable barriers controlled by a DC motor. The trough is
composed of an inert material such as for instance Teflon polytetrafluoroethylenee, PTFE) that is
resistant to water absorption.
r Block copolymers
Teflonm trough IMilli-Q Water
Figure 2-1. The Langmuir Teflon trough.
To form a Langmuir monolayer, the block copolymer of interest is first dissolved in a
solvent (C 1mg/mL) which is then spread dropwise on the water surface. The solvent must be
hydrophobic to prevent its dissolution in the water subphase and so it doesn't influence the
surface pressure measurements. It should be also volatile enough to quickly evaporate from the
surface after spreading, leaving behind the adsorbed block copolymer molecules. Chloroform
was the solvent of choice in this dissertation except in Chapter 2 where the use of a more polar
chloroform/ethanol mixture was necessary to dissolve the polystyrene-b-poly(acrylic acid)
dendrimer-like block copolymer sample. Other commonly used solvents include n-hexane,
benzene, diethyl ether, as well as mixtures such as for instance hexane-ethanol, benzene-ethanol,
or chloroform-methanol .s After spreading, the surface active block copolymers expand to form
an adsorbed monolayer that quickly covers the entire available area present between the barriers.
In addition to the A/W interface which is the most commonly studied interface, others are
possible such as oil/water interfaces or gas (different from air)/water interfaces.
Figure 2-2. A Wilhelmy plate partially immersed in a water subphase.
The surface pressure measurements were achieved by use of the Wilhelmy plate technique
as shown in Figure 2-2.87 The surface tension (s)~ is defined by Equation 2-1,
ai = To 7 = -A7 (2-1)
where To represents the surface tension of the water subphase in the absence of an adsorbed
monolayer, and 7 represents the surface tension of the water subphase in the presence of an
adsorbed monolayer. Water is a very interesting subphase when investigating monolayers since it
has an exceptionally high surface tension (- 73 mN/m at 20 oC and under atmospheric
pressure"') that allows a wide range of surface pressures to be obtained. When the Wilhelmy
plate is partially immersed into the subphase, the forces acting consist of gravity and surface
tension downward as well as an upward buoyancy due to displaced water. The resulting force F
is monitored and related to the surface tension yas shown in Equation 2-2:
F = p,~g/wf + 27(t +w) cos p ghutl (2-2)
where p, and ps correspond to the densities of the Wilhelmy plate and the subphase, respectively,
g corresponds to the gravitational constant, t (thickness), w (width), and I (length) are the
dimensions of the Wilhelmy plate, B is the contact angle made by the subphase onto the plate (6
= 0 for a completely wetted plate, cos8= 1), and h corresponds to the plate height immersed into
the subphase. When measuring the change in F (AF) for a stationary plate after formation of an
adsorbed Langmuir monolayer, assuming that t is negligible compared to w, the equation can be
AF = 2wA7 (2-3)
and therefore, the surface pressure riis directly related to the change in force and the width of the
x = (2-4)
With the Wilhelmy plate technique, the sensitivity can be as low as 5x10-3 mN/m. Nevertheless,
since very small amounts of impurities can affect surface pressure measurements, the water
subphase consists of Millipore filtered water with resistivities greater than 18.2 MGZ.cm.
Moreover, the trough needs to be carefully cleaned for instance with ethanol before forming a
Langmuir monolayer and rinsed several times with Millipore filtered water. Contamination with
impurities coming from the air can also be minimized by covering the trough or by carrying out
the experiments in a clean room. The subphase temperature can also be regulated with a
circulating water bath.
Once a Langmuir monolayer is formed at the A/W interface, a variety of surface pressure
related experiments are possible. The simplest experiment is the isotherm, where the monolayer
is compressed toward the center of the trough and the surface pressure recorded versus surface
density or mean molecular area (MMA, average area occupied by one molecule at the interface,
calculated from Equation 2-5).
M VA = "(2 -5 )
A is the area between the barriers, Mn is the molecular weight of the sample, NA is the
Avogadro' s number, and C and V are the concentration and the volume of the spreading solution,
respectively. Upon compression, the adsorbed molecules move closer to one another, the surface
pressure increases, and the phase transitions taking place in the monolayer are characterized by
pseudoplateaus or inflection points in the surface pressure versus MMA (or surface density)
isothermal plot as exemplified in Figure 2-3 for small amphiphilic molecules. The monolayer
undergoes phase transitions from the gaseous state (G) to the liquid state (L) and eventually to
the solid state (S) upon high compression.
0.20 0 23 Oy 50 10 0
a (nmw molcule*)
Figure 2-3. Schematic isotherm for small amphiphilic molecules.14
Other possible experiments include compression-expansion hysteresis, isobaric, or
isochoric experiments. In the hysteresis experiment, the barriers compress the monolayer up to a
target surface pressure (or target MMA) and expand several times. In the isobaric experiment,
the monolayer is compressed up to a desired surface pressure which is then kept constant by
small barriers adjustments, and the evolution of the MMA versus time is recorded. In the
isochoric experiment, the barriers are stopped after reaching the desired surface pressure (or
MMA), and the surface pressure evolution is subsequently recorded versus time. These last 3
experiments are usually used to investigate monolayer stability versus time or its formation
reversibility as further exemplified in Chapters 3, 4, and 5.
2.2 Langmuir-Blodgett Films and Atomic Force Microscopy
One of the interesting properties of the Langmuir trough is that it is easy to transfer a
Langmuir monolayer prepared at the A/W interface onto a solid substrate for further analysis.
The most common technique for monolayer transfer is done by vertically lifting a substrate into a
Langmuir monolayer compressed at the desired surface pressure which is kept constant by small
barrier adjustments (Figure 2-4). The resulting transferred film is called a Langmuir-Blodgett
(LB) monolayer. The substrate used in this dissertation is freshly cleaved hydrophilic mica
because it mimics the water subphase during transfer. It is also possible to prepare LB films
consisting of multilayers by successively transferring several monolayers. This technique
provides thin films of supramolecular assemblies with well-defined molecular arrangement,s
chemical composition, and thickness, and with fewer defects than if prepared directly from the
The transfer ratio measures the degree of monolayer transfer. During LB monolayer
formation, part of the molecules initially adsorbed at the A/W interface are slowly transferred
onto the substrate, which results in a lower surface density in the Langmuir monolayer. To keep
the surface pressure constant during transfer (and therefore the surface density), the barriers have
to move closer together which therefore reduces the available area between them. The transfer
ratio can be subsequently directly calculated by dividing this decrease in surface area between
the barriers by the total surface area of the substrate. Transfer ratio values smaller or larger than
unity indicate that the packing of the transferred molecules is less or more dense than at the A/W
Barrier Wilhelmy plate Barrier
Figure 2-4. LB film transfer onto a hydrophilic mica substrate.
The experimental technique used in this dissertation to examine the morphology of the LB
films in Chapters 3, 4, and 5 is atomic force microscopy (AFM). This technique provides a
topographical map of the LB films by scanning their surfaces with a sharp tip attached at the end
of a flexible cantilever which is deflected as it moves up and down mountains and valleys. A
laser is reflected off the end of the cantilever, and the deflection (measured by a photodiode) is
converted into a topographical image with a resolution of 0.1 nm accuracy. Contact and non-
contact AFM modes were first designed, but while in the contact mode soft samples can be
distorted, in the non-contact mode the tip can easily become stuck in the water layer covering the
surface of samples exposed to the atmosphere. The AFM tapping mode used in this dissertation
overcomes these problems. The tip is vibrated with sufficient energy to overcome the surface
tension of the contaminant water layer, and this vibration also prevents frictional forces from
dragging the sample, which is crucial when working with soft polymeric samples. The tapping
mode electronic setup is shown in Figure 2-5. As scanning occurs in three dimensions, the
scanner tube contains three piezo electrodes (a piezoelectric material is a substance that
proportionally contracts and expands depending on an applied voltage) for the X, Y, and Z
directions. In addition to its convenience in characterizing LB monolayers, AFM was also used
to image the molecularly imprinted nanoparticles after their deposition onto mica substrates as
further described in Chapter 7.
Fe~edr bac op Mai~bntalins
Constant Oscilledon1 Am1Splbituve! NlanoScope Illa
Split Cantl~ever & Tip
Figure 2-5. The AFM tapping mode electronic setup.89
2.3 Transmission Electron Microscopy and Quasi-Elastic Light Scattering
Transmission electron microscopy (TEM) and quasi-elastic light scattering (QELS) were
used in Chapters 6 and 7 to determine the shape and size of the core-shell nanocapsules and of
the molecularly imprinted nanoparticles.
Electron microscopes use a beam of highly energetic electrons to examine objects on a
very fine scale and can yield information such as sample topography, morphology, composition,
or crystallinity.90 The electron source first forms a beam of electrons that are accelerated toward
the specimen with a positive electrical potential. The beam is focused onto the sample, and
various interactions can occur inside the irradiated sample, affecting the electron beam as shown
in Figure 2-6.
Figure 2-6. Specimen interactions in electron microscopy.'"
The electrons of interest in TEM are only the unscattered electrons, even though the other
types of electrons can be of interest for other applications. These unscattered electrons are
Incklant le Beam
Ysi Ft hocdialuml
transmitted through the thin sample without any interaction occurring inside the sample. The
thicker areas will have fewer transmitted unscattered electrons and will therefore appear darker
than the thinner areas. The unscattered electrons strike a screen where light is generated,
allowing the user to see the sample image. Staining agents such as osmium tetroxide, ruthenium
tetroxide, or uranyl acetate can also be used to increase the contrast of thin samples that do not
sufficiently scatter the electron beam.
Quasi elastic light scattering (QELS, also called dynamic light scattering, DLS) is used to
determine the hydrodynamic radius or diameter of nano-sized particles suspended in a solution.
These particles are subject to Brownian motion, with small particles diffusing faster than large
particles according to the Stokes-Einstein equation (2-6) relating diffusion coefficient (D) and
particle radius (R):
D = (2-6)
where kB is the Boltzmann constant, Tis the temperature, and rl is the solution viscosity. A beam
of laser light is first focused on the sample, the particles scatter the incoming light in all
directions, and the scattered photons are measured by a photomultiplier tube. The time variation
of the scattered intensity is then analyzed with a digital correlator used to compute the following
autocorrelation function C(r):
C(r) = Ae-2r "+ B (2-7)
where A and B are instrumental constants. The diffusion coefficient D is then determined from
the following 2 equations:
q = (2-8)
where q is the scattering vector, n is the refracting index, 0 is the scattering angle, Ai is the laser
wavelength, and the particle hydrodynamic radius R is calculated from Equation 2-6. Each
monodisperse population of particle sizes produces its own unique autocorrelation function
(exponential decay). Mixtures of more than one size population produce sums of exponentials,
and therefore available algorithms can be used to extract "true" size distributions from complex
2.4 Cyclic Voltammetry
Cyclic voltammetry was used in Chapter 6 to monitor the uptake versus time of
hydrophobic electrochemical probes inside the oil-core of the core-shell nanocapsules. In cyclic
voltammetry, the potential of a working electrode is continuously changed as a linear function of
time, with the rate of change of potential referred to as the scan rate. The potential is first
scanned in one direction and then reversed at the end of the first scan. The advantage of such a
technique in electrochemistry is that the product of the electron transfer that occurred in the
forward scan can be probed again in the reverse scan. Figure 2-7 shows the basic shape of the
current response for a cyclic voltammetry experiment. The bulk solution first contains only the
reduced form of the redox couple, so for potentials (in A) lower than the redox potential, there is
no net conversion of the reduced form into the oxidized form. As the potential is increased
toward the redox potential, a net anodic current appears and increases exponentially. As the
reduced form is oxidized, concentration gradients appear, and diffusion occurs down these
gradients. At point B anodicc peak) and beyond, the potential is sufficiently high so the reduced
species reaching the electrode are immediately oxidized, and the current therefore now depends
upon the rate of mass transfer which results in an asymmetric peak shape. In C, the potential is
reversed (decreased), and the current continues to decay. As the redox potential is approached, a
net reduction of the oxidized species takes place resulting in the appearance of a peak-shaped
cathodic current (D).
Figure 2-7. Basic shape of the current response for a cyclic voltammetry experiment.92
In the presence of a reversible redox system, that is for fast electron transfer processes at
the electrode (reaction kinetics fast compared to the scan rate), the surface concentrations of the
oxidized and the reduced species are maintained at the values required by the Nernst equation (2-
E = E" + -In( )
where E is the electrode potential, Eo is the standard potential of the redox couple, R is the
universal gas constant (8.314510 J K^1 mol- ), T is the temperature in K, n is the number of
electrons transferred in the half reaction, F is the Faraday constant (9.6485309x104 C mOl-1), and
Co and C, represent the surface concentrations of the oxidized and reduced species, respectively.
The peak potential separation (Epa-Epc) is equal to 57/n, the peak width is equal to 28.5/n, the
peak current ratio (Ipa/Ipc) is equal to unity, and finally the faradaic current (Ip) is given by the
following Randles-Sevcik equation (2-11):
Ip = 2.69 x105 n3/2AD1/2 1/2C' (2-1 1)
where A is the electrode area (cm2), D is the diffusion coefficient of the electroactive species
(cm2/S), V is the scan rate (V/s), and C is the bulk concentration of the electroactive species
(mol/cm3). If A and vare kept constant during successive cyclic voltammetry scans, a decrease in
the bulk concentration of the electroactive species can be simply measured by monitoring the
decrease in the Faradaic current Ip. This strategy was applied as further discussed in Chapter 6 to
measure the encapsulation abilities and kinetics of two electrochemical probes with reversible
electrochemical activities (ferrocene methanol and ferrocene dimethanol) inside the oil core-
silica shell nanocapsules.
POLYSTYRENE-b-POLY(TERT-BUTYLACRYLATE) AND POLYSTYRENE-b-
POLY(ACRYLIC ACID) DENDRIMER-LIKE COPOLYMERS: TWO-DIMENSIONAL
SELF-ASSEMBLY AT THE AIR-WATER INTERFACE
As introduced in Chapter 1, block copolymers are of great interest for nanotechnological
applications because of their potential in forming well-defined morphologies in bulk,93 in
solution,94 Or at interfaces.95 Amphiphilic block copolymers offer good opportunities to control
interfacial properties, and since they self-assemble at hydrophilic/hydrophobic environments
such as the air/water (A/W) interface to form regular arrangements of two-dimensional surface
micelles, they are particularly interesting as precursors for the synthesis of well-defined patterns
with feature sizes typically in the nanometer scale order. The nanosized structures can be easily
controlled by varying various parameters such as the nature of the blocks, the molecular weight,
the relative lengths of the blocks, or even the block copolymer architecture. With a view toward
understanding the relationships between polymer architecture and A/W interfacial aggregation, a
variety of polystyrene-b-poly(ethylene oxide) block copolymers with simple and complex
architectures (linear, star-shaped, mikto-arm, dendrimer-like...) has been synthesized and
extensively investigated in the past few years.22,96
Concerning polystyrene-b-poly(tert-butylacrylate) (PS-b-PtBA) and polystyrene-b-
poly(acrylic acid) (PS-b-PAA) block copolymers, only little work has been reported on their
A/W interfacial behavior. Lennox and co-workers were to our knowledge the only ones to report
on the two-dimensional surface micelle formation for PS-b-PtBA block copolymers,
investigating linear samples with relatively small PtBA chains compared to the PS block.24,97 The
self-assembly of PS-b-PAA block copolymers was also studied almost exclusively for linear
samples,20,98 and it is only very recently that Tsukruk and co-workers described the self-assembly
of a twelve-arm heteroarm star copolymer with alternating PS and PAA arms at the A/W
interface.99 In a recent publication, we described the synthesis of a variety of dendrimer-like
copolymers based on polystyrene and poly(tert-butylacrylate) or poly(acrylic acid), and a
preliminary investigation of their surface properties confirmed that such systems are ideal
candidates for the synthesis of Langmuir and Langmuir-Blodgett (LB) monolayers with well-
In this chapter, we study the A/W interfacial behavior of two dendrimer-like copolymers
composed of an eight-arm star polystyrene core with a sixteen-arm poly(tert-butylacrylate) or
poly(acrylic acid) corona (Figure 3-1) by surface pressure measurements isothermss, isochores,
and compression-expansion hysteresis experiments) and atomic force microscopy (AFM)
imaging. Among the different dendrimer-like copolymer samples previously synthesized, we
deliberately chose to focus here on these 2 samples since they have the largest number of arms
and the largest PtBA (or PAA) weight %. These polymers are significantly different from the
block copolymers studied in the previous literature in terms of architecture and chain length
ratio, which should therefore lead to an interesting and peculiar interfacial behavior.
S callx[8arene ore
S branchinrg pointP~~
polysyrn e o
or porly~crlic cd
Figure 3-1. Schematic sketch of the PS-b-PtBA (Dendl) and the PS-b-PAA (Dend2) samples.
3.2 Results and Discussion
3.2.1 PS-b-PtBA Dendrimer-Like Copolymer (Dendl)
The isotherm of Dend1 is presented in Figure 3-2, and the inset shows the static elastic
modulus plot (e,, Equation 3-1) versus surface pressure.101
The surface pressure in the isotherm increases in the high MMA region with a behavior
characteristic of liquid expanded and liquid condensed phases until a plateau suggesting a
biphasic state is observed at 24 mN/m. Upon further compression in the low MMA region, the
pressure sharply increases up to values as high as 70 mN/m. PS and PtBA are two hydrophobic
polymers, but contrary to PS, the presence of slightly polar ester groups in PtBA allows the
adsorption of PtBA homopolymers and PtBA-based block copolymers at the A/W interface. The
isotherm of Dend1 is very similar in shape to the isotherms of PtBA homopolymers, with only
one phase transition observed at 24 mN/m.102 Moreover, the extrapolation of the linear portion of
the isotherm below the plateau to zero pressure (Apancake 103 yields a value of 29 A+2 per PtBA
repeat unit, which is in good agreement with the previously reported literature.104 These two
observations indicate that Dend1 behaves similarly in terms of surface activity to PtBA
homopolymers, with the PS blocks desorbed in the air on top of the adsorbed PtBA blocks.
Nevertheless, these results are significantly different from those obtained by Lennox and co-
workers for linear PS-b-PtBA diblock copolymers with relatively low PtBA wt %.24,97 They
showed that the presence of the PS block induced 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. As shown in the inset of Figure 3-2, only one local
minimum characteristic of a phase transition is observed at 24 mN/m for the static elastic
modulus versus surface pressure plot. Moreover, Dend1 also aggregates into spherical surface
micelles (Figure 3-3). This indicates that in the case of circular surface micelles, the extra phase
transitions are observable only for small PtBA chain length over PS chain length ratios (high PS
wt %). In the case of Dend1, we believe these transitions might be present but cannot be detected
since they involve only the small amount of PtBA repeat units present in the vicinity of the PS
micellar core, with the maj ority of the PtBA repeat units being too far and behaving similarly as
in PtBA homopolymers.
O 25 50 7
0 25000 50000 75000 100000
Figure 3-2. Isotherm of Dend1. (Inset) Static elastic modulus plot versus surface pressure.
The AFM images of the LB films transferred below the plateau at 5, 10, 15, and 20 mN/m
(Figures 3-3a, 3-3b, 3-3c, and 3-3d) show the circular micellar aggregation of Dend1 mentioned
before with the bright circular domains corresponding to the desorbed PS cores (~ 1-1.5 nm
thick) and the darker background corresponding to the adsorbed PtBA chains. Estimation of the
number of Dend1 molecules per circular micelle by the total area method'os yielded an average
aggregation number of 3, independent of the surface pressure. PS has a high glass transition
temperature (Tg ~ 100 oC for high molecular weight PS),106 and therefore, once a surface micelle
has formed during the solvent spreading process, the micellar cores come closer to each other
during compression but the aggregation number remains unchanged with the chains stuck within
a particular micelle (the temperature of the experiment is well below the Tg of the desorbed PS
cores). Compression-expansion hysteresis experiments were carried out with target surface
pressures below the plateau and all the compression and expansion curves overlapped, which is
indicative of film formation reproducibility and thermodynamic stability. AFM imaging of the
LB films prepared during the second compression step of the hysteresis experiment did not
reveal any significant difference compared to the LB films formed during the first compression.
The aggregation number is much smaller than the value of 95 block copolymers per circular
surface micelle reported by Lennox and co-workers for a linear PS305-b-PtBA222 diblock
copolymer,97 which was expected based on the previous studies investigating the relationship
between aggregation number and block copolymer architecture. For instance, increasing the wt
% of the adsorbed PEO segment in linear PS-b-PEO diblock copolymers resulted in a lower
aggregation number.96c Even lower aggregation numbers were observed for PS-b-PEO three-arm
stars with a PEO corona compared to the linear analogues or compared to the reverse star
architecture with a PS corona.22 Similarly to these examples, the adsorbed PtBA chains of Dend1
are present in the corona and are approximately 10 times longer than the desorbed PS chains.
Moreover, the large number of arms connected to the central calixarene core and the
dendrimer-like structure with 2 PtBA chains attached at the end of each PS arm also heavily
favor the formation of highly curved interfaces,96f therefore also considerably reducing the
0 nm no i
Figure 3-3. Topographic AFM images ofDendl LB films transferred at 5 (a), 10 (b), 15 (c), 20
(d), 24 (middle of plateau, MMA = 20,000 A+2, e and f), and 40 mN/m (g and h).
A LB film of Dend1 was also transferred in the middle of the plateau (MMA = 20,000 A+2,
and the resulting AFM images are shown in Figures 3-3e and 3-3f. In this case, no transfer ratio
could be calculated since the barriers were manually stopped after compression before transfer,
and therefore these two AFM images only qualitatively reflect the A/W interfacial aggregation.
Nevertheless, the images clearly confirm the biphasic state of the plateau, with areas exhibiting
circular surface micelles similarly as for lower pressures and other areas with aggregated
domains significantly thicker (~ 3 nm) than the PS micellar cores. It was previously suggested
that at the beginning of the plateau region, the tert-butyl groups reorient from prone to vertical,
with further compression leading to film collapse.10 Since PtBA is a hydrophobic polymer, it
collapses by aggregating on top of the water surface. The aggregated areas observed in Figures
3-3e and 3-3f therefore likely correspond to desorbed Dend1 mono or multilayers. Compression-
expansion hysteresis experiments (3 cycles) were carried out in the plateau region, and the
resulting curves are shown in Figure 3-4. All the compression curves closely overlapped,
indicating that the collapsed films are able to go back to their original adsorbed monolayer state
upon barrier expansion. AFM imaging of the LB films transferred during the second
compression at a surface pressure of 20 mN/m confirmed the presence of adsorbed and circular
surface micelles and the absence of collapsed domains. Nevertheless, a drop in pressure is
observed during barrier expansion. Faster barrier speeds increased the pressure drop, which
suggests that the readsorption kinetics of the collapsed PtBA segments are slower than the barrier
O 25000 50000 75000 100000
Figure 3-4. Compression-expansion hysteresis plot of Dend1 (target MMA = 20,000 A+2.
Further compression in the low MMA region leads to a sharp surface pressure increase.
The films formed in this region of the isotherm should be unstable since the surface pressure is
higher than the PtBA collapse pressure of 24 mN/m. This was checked by carrying out isochoric
experiments. A typical isochore after compression up to 40 mN/m is shown in Figure 3-5. The
surface pressure undergoes a sudden drop within the first seconds of the experiment before
leveling off after a few minutes around 24 mN/m. It is not clear to us whether the sharp surface
pressure increase upon high compression in the isotherm originates from interactions between
the collapsed and desorbed aggregates formed in the plateau region or between eventual
remaining adsorbed PtBA segments. 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 24 mN/m, which can be also seen as the equilibrium spreading
pressure of the PtBA blocks. Compression-expansion hysteresis experiments (3 cycles) were also
carried out in this high surface pressure region, and an example for a target pressure of 40 mN/m
is shown in Figure 3-6. The results are essentially similar to the hysteresis experiments carried
out in the middle of the plateau, with the compression curves overlapping each other, and the
expansion curves exhibiting a drop in pressure that corresponds to the slow readsorption of the
collapsed PtBA chains. AFM imaging of the LB films transferred during the second compression
at a surface pressure of 20 mN/m also confirmed here the presence of circular surface micelles
only. The morphology of the LB film transferred at 40 mN/m is shown in Figures 3-3g and 3-3h,
and aggregated domains in addition to the circular surface micelles are observed similarly as
within the plateau. Nevertheless, the transfer ratio was in this case significantly smaller than one
(~ 0.25), which indicates that the film underwent non-negligible expansion during transfer,
suggesting that the density of desorbed aggregates on the water surface is significantly greater
than as shown in Figures 3-3g and 3-3h.
O 10 20 30
Figure 3-5. Surface pressure/time isochoric relaxation plot of Dend1 after compression up to 40
0 25000 50000 75000 100000
Figure 3-6. Compression-expansion hysteresis plot of Dend1 (target pressure = 40 mN/m).
3.2.2 PS-b-PAA Dendrimer-Like Copolymer (Dend2)
As a preliminary study, we investigated the behavior of a PAA homopolymer (PAA250K>,
since to our knowledge no information is available in the previous literature on the Langmuir and
LB monolayers of PAA homopolymers. PAA250K COuld not be dissolved in chloroform, so a
chloroform/ethanol mixture (1/2 in volume) was used instead as the spreading solvent. PAA is a
weak acid (pKa ~ 5.5),'0s and its degree of ionization and consequently its surface activity are
therefore strongly pH dependent. Under basic conditions (pH = 11i), the acid groups of PAA are
deprotonated, and stable Langmuir monolayers of PAA250K COuld not be prepared, with the
surface pressure remaining close to 0 mN/m during barrier compression. When ionized, PAA is
not surface activel09 and irreversibly dissolves into the aqueous subphase. Under acidic
conditions (pH = 2.5), the acid groups of PAA are protonated, the chains become less
hydrophilic and surface active, and therefore stable Langmuir monolayers of PAA250K COuld now
be prepared. The resulting isotherm is shown in Figure 3-7. The surface pressure slowly
increases in the high MMA region before a plateau is reached with the surface pressure leveling
off around 3.5 mN/m. In the high MMA region, the surface pressure increases with a behavior
characteristic of liquid expanded and liquid condensed phases, and PAA250K is likely adsorbed in
a pancake conformation. The AFM image shown in Figure 3-8a (7<= 1.5 mN/m) confirmed the
presence of a pretty homogeneous PAA250K LB film with the randomly distributed darker holes
possibly forming after transfer as the PAA chains un-swell (dehydrate) during the drying step.
PAA in its protonated form is surface active but it is still highly water soluble, and it has been
previously shown for aqueous solutions of protonated PAA that the surface pressure reaches a
maximum, for instance around 3 mN/m for a molecular weight similar to PAA250K.109a This value
correlates well with the plateau surface pressure, which indicates that, at this pressure, PAA250K
desorbs from the interface and aggregates inside the water subphase. The AFM images obtained
for the LB films transferred at 3 and 3.5 mN/m are shown in Figures 3-8b and 3-8c, respectively.
As the films are compressed within the plateau region, more and more desorbed aggregates (~ 3-
5 nm thick) are observed. This indicates that when the PAA chains collapse by dissolving into
the water subphase, they stay in the vicinity of the interface, probably remaining anchored by
some adsorbed acrylic acid repeat units that have not collapsed yet.
0 50000 100000 150000 200000
Figure 3-7. Isotherm of PAA250K (pH = 2.5).
11nm 10 ~ nm e e
0 nm m 0nm
Figure 3-8. Topographic AFM images of PAA250K LB films transferred at 1.5 (a), 3 (b), and 3.5
The spreading solutions of Dend2 were prepared by using the same solvent mixture as for
PAA250K (chloroform/ethanol mixture, 1/2 in volume). Similarly as for PAA250K, HO stable
Langmuir monolayers could be formed for Dend2 under basic conditions with its irreversible
dissolution in the aqueous subphase. Contrary to the other PS-b-PAA systems reported in the
previous literature,20,98,99 the PS wt % in Dend2 is too low to allow it to act as an anchor and thus
preventing the dendrimer-like aqueous dissolution. Nevertheless, under acidic conditions (pH =
2.5), the PAA segments of Dend2 are now protonated, and, similarly as for PAA250K, Dend2
becomes surface active. The resulting isotherm is shown in Figure 3-9, and the inset shows the
monolayer compressibility plot (K, Equation 3-2) versus surface pressure.96
1 1 d (NLVA )
E, MVIA d(Fir)
In the high MMA region, the surface pressure slowly increases before reaching a pseudoplateau
around 5 mN/m (maximum in monolayer compressibility as seen in the inset of Figure 3-9) and
sharply increasing under high compression up to approximately 10 mN/m. A pseudoplateau was
already observed in the previous literature by Currie et al. only for long PAA chains in linear PS-
b-PAA systems under acidic conditions, where they demonstrated by surface pressure and
ellipsometry measurements the presence of a pancake-to-brush transition in the pseudoplateau
region with the PAA segments dissolving into the water subphase.20 It is not very clear at this
point why the pseudoplateau surface pressure is a little bit higher for Dend2 (5 mN/m) compared
to the PS-b-PAA copolymers studied by Currie et al. or compared to PAA250K (3-4 mN/m). It
might simply be a PAA chain length dependence, or possibly a peculiar influence of the
4- 2 4 6 8
~t ~Ix (mN/m)
0 25000 50000 75000 100000
Figure 3-9. Isotherm of Dend2 (pH = 2.5). (Inset) Compressibility plot versus surface pressure.
We investigated here the morphological evolution by AFM imaging as the Langmuir films
of Dend2 are compressed, and the resulting images of the LB films are shown in Figure 3-10. At
low surface pressure (7< = 2 mN/m, Figure 3-10a), Dend2 aggregates into spherical surface
micelles with bright PS cores (~ 1 nm thick) and a darker background corresponding to the PAA
blocks adsorbed in a pancake conformation similarly as in Figure 3-8a for PAA250K. The average
aggregation number was estimated around 5 by the total area method.'os Such a low value can be
rationalized similarly as for Dend1, with the long PAA chains present in the corona, the
dendrimer-like shape, and the large number of arms emanating from the central calixarene
core all heavily favoring the formation of highly curved interfaces. The images shown in Eigures
3-10b through 3-10j were recorded for surface pressures within the pseudoplateau and ranging
from 4 to 8 mN/m. As the fi1ms are compressed, the PAA segments progressively dissolve in the
aqueous subphase, underneath the PS cores of the surface micelles that aggregate into larger and
thicker (~ 2 nm) domains. At the end of the pseudoplateau, in the region where the surface
pressure sharply increases under high monolayer compression (7[= 8 mN/m, Figures 3-10k and
3-101), all the PAA chains have desorbed in the water subphase and stretch to form a brush
underneath the aggregated PS cores. Contrary to the hydrophobic PtBA segments of Dend1 that
collapsed above the A/W interface, the hydrophilic PAA segments of Dend2 anchored by the PS
segments dissolve into the water subphase. Compression-expansion hysteresis experiments were
also conducted within the pseudoplateau region (target pressure = 5 mN/m), and the results are
presented in Figure 3-11. Very little hysteresis is observed with the pseudoplateau still present
after numerous cycles, which means that the desorbed PAA segments can return to their original
adsorbed state at low pressure after monolayer decompression. This was verified by transferring
a LB fi1m at 2 mN/m during the second compression cycle, and AFM imaging revealed the
presence of circular surface micelles with no aggregated domains. We do not have yet at this
point enough experimental evidence to rationalize the small hysteresis shift, but it could for
instance come from some entanglement of the PAA chains during the pancake-to-brush
transition or from a different arrangement of the expanded PAA chains in terms of hydration and
conformation compared to the one adopted after spreading and before the first compression.110
Figure 3-10. Topographic AFM images of Dend2 LB films transferred at 2 (a), 4 (b), 4.5 (c and
d), 5 (e and f), 5.5 (g and h), 6 (i and j), and 8 mN/m (k and 1).
Figure 3-11. Compression-expansion hysteresis plot of Dend2 (target pressure = 5 mN/m, pH
In this work, the A/W interfacial behavior of two dendrimer-like block copolymers based
on polystyrene and poly(tert-butylacrylate) (Dendl) or poly(acrylic acid) (Dend2) and their LB
film morphologies on hydrophilic mica substrates were investigated. Dend1 formed
thermodynamically stable Langmuir monolayers and self-assembled into circular surface
micelles up to 24 mN/m with a very low aggregation number (~ 3), likely resulting from the high
PtBA wt % and from the peculiar dendrimer-like architecture. At 24 mN/m, the PtBA segments
desorbed and aggregated on top of the water surface, and all the monolayers formed beyond this
threshold were metastable and relaxed down to 24 mN/m. Poly(acrylic acid) is surface active
only under acidic pH conditions below its pKa value (~ 5.5), and a preliminary study on a PAA
homopolymer (PAA250K, Mn = 250,000 g/mol) showed that it is able to form stable monolayers
up to ~ 3.5 mN/m before dissolving in the water subphase. Because of its small PS wt %, Dend2
similarly did not form stable Langmuir monolayers under high pH conditions, and therefore its
self-assembly at the A/W interface was investigated only under acidic conditions. The isotherm
indicated the presence of a pseudoplateau at 5 mN/m characteristic of a phase transition that
corresponds to a pancake-to-brush transition, with the progressive aqueous dissolution of the
PAA segments underneath the anchoring PS cores. For pressures below the pseudoplateau,
Dend2 molecules also aggregated into circular surface micelles with a very low aggregation
number (~ 5). This study confirmed that various parameters such as for instance polymer
architecture, chain length/polarity, surface density, or even subphase pH can all strongly
influence the self-assembly of block copolymers at the A/W interface. Therefore, while this
work gave some hint of the interesting interfacial properties of these two novel PS-b-PtBA and
PS-b-PAA dendrimer-like copolymers, additional investigations with other samples with
different numbers of arms and chain lengths are necessary for a better understanding of the
influence of the dendrimer-like structure.
3.4 Experimental Methods
The synthesis of the PS-b-PtBA (Dendl, 2s = 240,000 g/mol, PDI = 1.26) and PS-b-PAA
(Dend2, 2s = 139,000 g/mol, PDI = 1.28) dendrimer-like copolymers was realized according to
a previously reported procedure.100 These block copolymers consist of an eight-arm PS core (2\%,
= 10,000 g/mol, ~ 12 styrene units/arm) with a sixteen-arm PtBA (2, = 230,000 g/mol, ~ 112
tert-butylacrylate units/arm, PS wt % ~ 4 %) or PAA (2, = 129,000 g/mol, ~ 112 acrylic acid
units/arm, PS wt % ~ 7%) corona. The PAA homopolymer (PAA250K, 2Ms ~ 250,000 g/mol) was
purchased from Aldrich Chemical Co. and was used as received without further purification.
3.4.2 Langmuir Films
Surface pressure measurements were accomplished by use of a Teflon Langmuir trough
system (W= 160 mm, L = 650 mm, KSV Ltd., Finland) equipped with two moving barriers and a
Wilhelmy plate. The PS-b-PtBA dendrimer-like copolymer sample (Dendl) was prepared by
dissolving approximately 1 mg of polymer in 1 mL of chloroform. The PS-b-PAA dendrimer-
like copolymer (Dend2) and the PAA homopolymer (PAA250K) Samples were prepared by
dissolving approximately 1 mg of polymer in 1 mL of a chloroform/ethanol mixture (1/2 in
volume). Volumes ranging from 10 to 30 puL were spread dropwise with a gastight Hamilton
syringe on Millipore filtered water (subphase resistivity > 18.2 MGZ.cm) of the desired pH value,
and the spreading solvents were allowed to evaporate for 30 min. In all the experiments,
subphase temperature and barrier speed were kept constant at 25 OC and 5 mm/min, respectively.
3.4.3 AFM Imaging
The LB films were formed by transferring the Langmuir films of Dendl, Dend2, and
PAA250K Onto freshly cleaved mica at various surface pressures. The desired surface pressure
was attained at compression/expansion rates of +/-5 mm/min. Once the films had equilibrated at
a constant surface pressure for 15 min, the mica substrate was then pulled out of the water at a
rate of 1 mm/min. All the transfer ratios were close to unity unless otherwise stated, which is
indicative of successful transfer. The transferred films were air-dried in a desiccator for 24 h and
subsequently scanned in tapping mode with a Nanoscope III AFM (Digital Instruments, Inc.,
Santa Barbara, CA) using Nanosensors silicon probes (dimensions: T = 3.8-4.5 pum, W = 26-27
pum, L = 128 pum). All the images were processed with a second-order flattening routine.
LANGMUIR AND LANGMUIR-BLODGETT FILMS OF POLY(ETHYLENE OXIDE)-b-
POLY(s-CAPROLACTONE) STAR-SHAPED AND LINEAR BLOCK COPOLYMERS
Poly(s-caprolactone) (PCL) and poly(ethylene oxide) (PEO) are two biocompatible
polymers with PCL biodegradation leading to nontoxic products.111,112 Low molecular weight
PEO is not biodegradable but can be eliminated from the body by the renal system.113 The
introduction of a hydrophilic and highly flexible PEO block at the end of a PCL block allows the
potential to tailor PCL properties such as its high hydrophobicity, high crystallinity, and slow
biodegradation.114 Therefore, synthesis of PEO-b-PCL copolymers with various architectures has
resulted in a growing number of reports citing potential applications including drug delivery
systems, temporary bioabsorbable implants, and tissue engineering.112,1 15,116
The air/water (A/W) interface has received much attention to investigate the self-assembly
of various amphiphilic molecules because of its ability to mimic hydrophilic/hydrophobic
interfaces. When polymers having biological applications are considered, such as PEO and PCL,
the A/W interface is of particular interest because of its presence in many biological systems.
Both PEO and PCL homopolymers spontaneously adsorb at the A/W interface to form stable
monolayers in a moderate surface pressure range. PEO homopolymers have been extensively
studied in solutions and at interfaces, and it has been shown that PEO chains irreversibly dissolve
in the water subphase at relatively low surface pressures. Surprisingly, very little interest has
been put into fundamentally understanding the aggregation in Langmuir monolayers of PCL-
based block copolymers and PCL homopolymers. Leiva et al. were to our knowledge the first to
report on the A/W interfacial properties of PCL homopolymers, and they observed collapse
pressure values ranging from 13 to 20 mN/m depending on the molecular weight, and
aggregation of the hydrophobic PCL chains on top of the water surface after collapse." It is only
very recently that Esker and co-workers extensively studied PCL crystallization mechanism in
the collapse region by Brewster angle microscopy and isobaric area relaxation analyseS.101,119
The only example in the previous literature investigating the A/W self-assembly of PCL-based
block copolymers was reported by Lee et al. for an architecture with two linear PCL chains
anchored to a dendritic hydrophilic head.120
Chemically attaching a hydrophilic PEO block to a hydrophobic PCL block enhances the
amphiphilicity and the surface activity of the resulting block copolymer, allowing higher surface
pressures to be reached, as previously shown for other PEO-based amphiphilic block
copolymers.22,23 To our knowledge, no work has been reported on the A/W interfacial behavior
of block copolymers based on PEO and PCL. With a view toward guest encapsulation, we
recently reported the synthesi s of a seri es of poly(ethylene oxi de)-block-poly(s-caprol actone)
copolymers with star-shaped and linear architectures by ring-opening polymerization of e-
caprolactone at the end of hydroxyl-terminated star-shaped or linear PEO macroinitiators.121 In
the present chapter, we study the behavior of these samples as well as the corresponding PEO
and PCL homopolymers at the A/W interface (Langmuir monolayers) by surface pressure
measurements isothermss, compression-expansion hysteresis, and isobaric relaxation
experiments), and we employ AFM to characterize the Langmuir-Blodgett (LB) films'
morphologies after transfer onto hydrophilic mica substrates. The PEO-b-PCL five-arm stars
consist of a hydrophilic PEO core with 9 ethylene oxide units/arm with hydrophobic PCL chains
at the star periphery. Each star contains different amounts of PCL, varying from 0 to 18 e-
caprolactone units/arm (Figure 4-1 and Table 4-1). The linear PEO-b-PCL diblock copolymers
synthesized from a linear PEO macroinitiator (PEO2670, Mn = 2,670 g/mol, ~ 60 ethylene oxide
repeat units) contain different amounts of PCL, varying from 11 to 35 e-caprolactone repeat units
(Figure 4-2 and Table 4-2). The other PEO homopolymer PEO2000 (2r = 2,000 g/mol; PDI =
1.20) and the PCL homopolymers PCL1250 (2r = 1,250 g/mol; PDI = 1.50), PCL2000 (2r =
2,000 g/mol; PDI = 1.45), and PCL10000 (2r = 10,000 g/mol; PDI = 1.40) were commercially
available (purchased from Aldrich Chemical Co.) and were used as received without further
PEO core: n=9, m=0
H, Star#1: n=9, m=3 ,H
m /Yo_ Star#2: n=9, m=6 r m
Star#3: n=9, m=9
Star#4: n=9, m=12
oh c Star#5: n=9, m=15 ? 'o
Star#6: n=9, m=18
Figure 4-1. The star-shaped PEO-b-PCL block copolymers.
Table 4-1. Characteristic values of the star-shaped polymers.
Name PO Star#1 Star#2 Star#3 Star#4 Star#5 Star#6
2Mn a (g/mol) 2, 150 4,200 6,370 8,200 9,630 11,220 13,110
PDIb 1.10 1.25 1.37 1.35 1.45 1.36 1.44
Avg no. of ethylene oxide
99 99 99 9
umits per arm
Avg no. of e-caprolactone 0369 1 51
umits per arm
a Determined by 1H NMR
b Determined by GPC calibrated with linear polystyrene standards.
n = 60, m = 0, 11, 19, 27 and 35
Figure 4-2. The linear PEO-b-PCL block copolymers.
Table 4-2. Characteristic values of the linear PEO macroinitiator and of the linear diblock
PEO60-b- PEO60-b- PEO60-b- PEO60-b-
PCL11 PCL19 PCL27 PCL35
M~n a (g/mol) 2,670 3,940 4,780 5,780 6,680
PDIa 1.05 1.10 1.13 1.11 1.24
Avg no. of ethylene
60 60 60 60 60
Av n. f -0 11 19 27 3 5
a Determined by GPC calibrated with linear poly(ethylene oxide) standards.
4.2 Results and Discussion
4.2.1 PEO Homopolymers
The isotherms of the five-arm PEO core for subphase pH values of 5.5 (Millipore filtered
water) and 13 (0.1 M NaOH) are presented in Figure 4-3. The isotherms of the linear PEO
homopolymers (PEO2000 and PEO2670) are also included. The inset shows the same isotherms
normalized with respect to the number of ethylene oxide repeat units. For PEO2000 and
PEO2670, the surface pressure increases until a pseudoplateau that corresponds to the
irreversible aqueous dissolution of the PEO chains is reached at 5 and 6.2 mN/m, respectively.
As previously described, Langmuir films of PEO homopolymers are thermodynamically stable
for low surface pressures."' Upon compression the monolayers collapse, and the water-soluble
PEO chains irreversibly dissolve in the water subphase for pressures that are molecular weight
dependent (maximum collapse pressure value ~ 10 mN/m for high molecular weight PEO).122
The behavior of the PEO core significantly differs from that of the linear analogue PEO2000. No
pseudoplateau is observed, and the pressure slightly increases only for high compression, as
indicated by the shift toward the low area per monomer region compared to PEO2000. The
Millipore water pH (5.5) is acidic enough to protonate the triamine present in the center of the
PEO core, therefore increasing its water solubility compared to the neutral form. To investigate
the pH influence on the surface activity of the PEO core, a control isotherm was carried out with
a subphase pH value of 13, where the triamine is no longer protonated. The isotherm shape was
very similar to that at pH = 5.5. A slightly increased pressure was obtained for high compression,
but no pseudoplateau was observed. The absence of a pseudoplateau and the small pressure
increase only for high compression indicate a negligible surface activity of the adsorbed
molecules and the presence of a nonstable film. This assumption was verified by carrying out
compression-expansion hysteresis experiments on the PEO core for a target pressure of 2 mN/m
(Figure 4-4). Successive compression/expansion curves shifted toward smaller mean molecular
areas, which is indicative of a loss of material in the aqueous subphase. The pressure increase is
thought to arise from a kinetic effect, where the barrier compression speed is faster than the
dissolution rate of the PEO core molecules into the aqueous subphase, leading to a metastable
monolayer. Compared to the linear analogue, the star-shaped PEO core not anchored to
hydrophobic chains has little or no surface activity at the A/W interface and is therefore
thermodynamically more stable being solvated in the aqueous subphase than being adsorbed at
the interface, independent of the surface pressure. This behavior can be related to the high
polarity (presence of five hydroxyl groups) and the compact architecture (star-shaped) of the
PEO core, which is therefore more likely to dissolve in the water subphase than the linear
. First compression
cI First expansion
o Second compression
* Second expansion
* PEO2000 2
a PEO core pHI=5.5 20
~-oPEO core pH=1 3 15 -
~4 -s- PEO2670 10E~l
E 0 20
10 Area per eth
~t oxide unit
0 500 1000 1500 2000
Figure 4-3. Isotherms of the PEO homopolymers. (Inset) Same
respect to the number of ethylene oxide units.
isotherms normalized with
Figure 4-4. Compression-expansion hysteresis plot of the PEO core (target pressure = 2 mN/m).
LB films of the PEO core could not be prepared because of its Langmuir monolayer
instability, so only PEO2000 and PEO2670 were transferred at 2 mN/m. PEO crystallization
from the bulk and organic solvents has been extensively investigated under various experimental
conditions including in thin films.123,124 The AFM scan of the PEO LB films indicated the
presence of a flat surface with no PEO crystals. PEO is highly hydrophilic, and previous studies
showed that every ethylene oxide unit can be hydrated with up to three water molecules.125
Recent studies in PEO-b-PCL systems indicated that hydrated PEO chains essentially lost their
ability to crystallize.126 The fact that we did not observe any crystallization in PEO2000 and
PEO2670 LB films probably arose from a similar situation where as the films dried, the PEO
chains remained highly hydrated and could not crystallize. PEO also has a high affinity for the
hydrophilic mica substrate, which can be another energy barrier preventing aggregation and
chain folding necessary for crystallization to take place.
4.2.2 PCL Homopolymers
While the main objective of our study was to gain a deeper understanding of the A/W
interfacial behavior of PEO- and PCL-based block copolymers, a preliminary study on PCL
homopolymers involving isothermal and compression/expansion hysteresis experiments with
experimental parameters (barrier speeds and target pressures) similar to the ones used for the
linear and star-shaped PEO-b-PCL samples needed to be done. The isotherms of the PCL
homopolymers introduced earlier are presented in Figure 4-5, with a log scale on the x-axis for
convenient visualization. The inset shows the same isotherms normalized with respect to the
number of e-caprolactone repeat units. At low pressures, the PCL chains are adsorbed at the
interface with a behavior characteristic of liquid expanded and liquid condensed phases. Below
the collapse pressure, theoretical investigations by Ivanova et al. showed that PCL chain packing
and orientation is probably surface pressure-dependent. 127 The inset shows that below the
collapse pressure, chain packing is not molecular weight-dependent. Compression-expansion
hysteresis cycles were performed in this surface pressure range and all the compression-
expansion curves closely overlapped, confirming the thermodynamic stability of the
Area per e-caprolactone
10 unit (A~2)
+ PC L2000)
10 100 1000 10000 100000
Figure 4-5. Isotherms of the PCL homopolymers. (Inset) Same isotherms normalized with
respect to the number of e-caprolactone units.
The collapse region of most amphiphilic molecules is more difficult to quantitatively
analyze because it is a thermodynamically unstable pressure region where various phenomena
such as desorption of molecules from the interface or local formation of multilayers can happen.
Many polymers show viscoelastic behavior in this region, leading to surface properties that are
barrier speed-dependent.130 As shown in Table 4-3, the collapse pressure (defined as the point
where a sudden change in slope is observed) is molecular weight-dependent, with the longer
PCL chains collapsing at lower pressures. PCL is a highly hydrophobic polymer, and its collapse
therefore results in the aggregation of the water-insoluble PCL chains on top of the aqueous
subphase. An interesting feature characteristic of PCL collapse is the obvious decrease in
pressure after the collapse point, a phenomenon that was attributed by Esker and co-workers to
PCL crystallization directly on the water surface. Longer PCL chains have increased
hydrophobicity and crystallinity compared to smaller PCL chains, which explains the lower
collapse pressure and increased pressure drop after the collapse point.
Table 4-3. Collapse pressure values of the PCL homopolymers.
Name PCL1250 PCL2000 PCL10000
Collapse pressure (mN/m) for
14.6 12.9 11.1
compression speed = 5 mm/min
Collapse pressure (mN/m) for
N/A 14.1 11.4
compression speed = 100 mm/min
Further evidence for PCL crystallization came from the isotherms recorded for a barrier
compression speed of 100 mm/min (Figure 4-6). The pressure decrease after the collapse point
almost completely vanishes, and as shown in Table 4-3 the collapse pressure increases. This
indicates that in this case the rate of crystallization is slow compared to the compression speed
and that the resulting monolayers are metastable in this pressure region. Thermodynamic
collapse pressure values could not be easily obtained experimentally with our equipment because
it would require infinitely slow compressions. The collapse pressure for PCL1250 (100 mm/min)
could not be accurately determined because its isotherm does not show a clear collapse, but the
shallower turning point between 12 and 15 mN/m can nevertheless be attributed to the collapse
"point" of this film.
20 -( *PCL1250
10 100 1000 10000 100000
Figure 4-6. Isotherms of the PCL homopolymers (compression speed = 100 mm/min).
Compression-expansion hysteresis cycles were also carried out beyond the collapse point.
Figure 4-7 shows the compression-expansion hysteresis experiment carried out for PCL1250.
The first compression is similar to the isotherm reported in Figure 4-5. As the film is expanded,
the pressure suddenly drops until an expansion pseudoplateau appears at a pressure significantly
lower than the collapse pressure during compression. This expansion pseudoplateau corresponds
to the readsorption/melting of the PCL chains that had previously crystallized. This
pseudoplateau pressure is molecular weight-dependent, with the smaller PCL chains readsorbing
at higher pressure (10, 8, and 4 mN/m for PCL1250, PCL2000, and PCL10000, respectively).
The compression curve of the second cycle is slightly shifted toward smaller areas, and a
decrease is observed for the collapse pressure. This phenomenon is the result of residual PCL
crystals that remained on the water surface from the initial monolayer compression and that act
as nucleation sites for the crystallization of the other adsorbed PCL chainS.101,119 Further
compression-expansion cycles essentially overlapped the second cycle with no noticeable
pressure changes concerning the collapse or the expansion pseudoplateau.
I ci First expansion
S15 .o Second and third compressions
Z Second and third expansions
0 200 400 600 800
Figure 4-7. Compression-expansion hysteresis plot of PCL1250.
Typical AFM images of the PCL homopolymers LB films transferred below monolayer
collapse (7<= 7 mN/m), before crystallization on the water surface takes place, are shown in
Figure 4-8a and 4-8d. Surprisingly, PCL crystals could be observed. According to the isotherms
that are characteristic of liquid condensed phases in this low surface pressure range, the PCL
homopolymers are transferred into smooth and hydrated monolayers adsorbed onto the mica
surface. We believe that upon drying, during and after transfer, part of the PCL chains leave the
surface and crystallize, which likely results in a mica surface only partially covered with
adsorbed or crystallized PCL chains. For comparison, PCL homopolymers were also transferred
beyond monolayer collapse, after crystallization on the water surface takes place (Figure 4-8b, 4-
8c, and 4-8e). The transfer ratios were in this case significantly greater than 1 (~ 2-3), which was
predictable since crystallization and therefore intrinsic MMA decrease take place over time in
this surface pressure range. PCL crystals were also observable, with spherulitic architectures
significantly different from those obtained by Esker and co-workers directly at the A/W interface
using BAM.101,119 This suggests that the AFM images recorded above monolayer collapse are not
only the result of PCL crystallization at the A/W interface but also the result of additional PCL
crystallization taking place during and after transfer. By use of cross-section analysis (Figure 4-
8f), all the PCL crystals were determined to be approximately 7.5 nm thick, which is consistent
with the previously reported literature on PCL lamellae thickness.131 The thickness was
independent of PCL molecular weight, which indicates that the chains stretch perpendicular to
the surface and fold every 7.5 nm, with the crystals probably growing parallel to the surface.
Nevertheless, further comparison between PCL crystallization in LB films and the previously
reported work on PCL crystallization in bulk,132 fTOm an organic solution,133 Or even in thin
filmsl34-136 remains difficult to make because different types of variables are involved.
Figure 4-8. Topographic AFM images of PCL homopolymers LB films transferred below and
above monolayer collapse: PCL2000 at 7 (a) and 13 mN/m (b and c), and PCL10000
at 7 (d) and 11.2 mN/m (e). (f) Cross-section analysis performed at the edge of a
4.2.3 Star-Shaped PEO-b-PCL Block Copolymers
The isotherms of the PEO-b-PCL five-arm stars are presented in Figure 4-9. They exhibit
essentially three different regions that can be attributed to different conformations of the polymer
chains. In the high MMA region, the surface pressure slowly increases as the films are
compressed until a pseudoplateau is reached for intermediate mean molecular areas. As the
compression continues in the low MMA region, the surface pressure sharply increases, reaching
elevated surface pressure values and highly compressed films.
lit I* Star#4
0 3000 6000 9000 12000 15000
Figure 4-9. Isotherms of the star-shaped PEO-b-PCL copolymers.
188.8.131.52 High MMA region
The first step toward understanding the behavior of the stars in this region was to check
monolayer formation reversibility and stability. This was done by carrying out compression-
expansion hysteresis experiments with target pressures up to 9 mN/m. For Star#6, Star#5, Star#4,
and Star#3, all the compression and expansion curves are superimposable independent of the
target pressure, which is indicative of film formation reproducibility and stability. These four
stars have the largest PCL amounts and are therefore hydrophobic enough so the amount of
material adsorbed at the interface remains constant over time, without irreversible dissolution in
the aqueous subphase (Figure 4-10). As the PCL content is decreased, monolayer stability is
reduced. For Star#2, the compression-expansion cycles start shifting toward smaller mean
molecular areas, and this shift is even more pronounced for Star#1 (Figure 4-11). For these two
samples, the small PCL chains are insufficient to overcome the overall star hydrophilicity arising
from the water-soluble PEO core, and irreversible water dissolution takes place over time.
Figure 4-10. Compression-expansion hysteresis plot of Star#6 (target pressure = 9 mN/m).
h First compression
15 -I c First expansion
a Second compression
0 1000 2000 3000 4000
Figure 4-11. Compression-expansion hysteresis plot of Star#1 (target pressure = 9 mN/m).
0 2000 4000 6000 8(
From the initial study on the homopolymers, it was shown that in the low pressure region
e-caprolactone repeat units are adsorbed at the A/W interface independent of the molecular
weight and that the PEO core alone has limited surface activity which leads to its irreversible
dissolution in the aqueous subphase. For the star-shaped block copolymers, the interfacial
behavior of the PEO core might be significantly changed as it is chemically attached to
hydrophobic PCL chains. To estimate the interfacial area occupied by the PEO core of the star-
shaped block copolymers, only the isotherms of Star#6, Star#5, Star#4, and Star#3 were used, as
the corresponding Langmuir monolayers are thermodynamically stable below the pseudoplateau.
For target pressures ranging from 1 to 11 mN/m, the MMA was plotted versus the number of e-
caprolactone repeat units as shown in Figure 4-12. The resulting curves were analyzed by linear
regression leading to R2 ValUeS greater than 0.99. The linear relationships indicate that the
adsorption of the PCL blocks is not molecular weight-dependent as it is for linear PCL
homopolymers. More interestingly, the y-axis intercepts are significantly different from zero,
indicating the non-negligible interfacial area occupied by the PEO core anchored by the PCL
chains. The surface pressure was then plotted versus the y-axis intercept values to give the
extrapolated isotherm of the PEO core of the block copolymers (Figure 4-13). The experimental
PEO2000 isotherm was also included for comparison. Both curves have been normalized with
respect to the number of ethylene oxide units. For low surface pressures (x < 4 mN/m), the
experimental PEO2000 and the extrapolated PEO core isotherms overlap reasonably, indicating a
similar interfacial area occupied by an ethylene oxide repeat unit of the PEO core compared to
the linear PEO analogue. The two curves stop overlapping above 4 mN/m because PEO2000 is
irreversibly dissolved in the aqueous subphase. It is also very interesting to notice in the
isotherms of the block copolymers, for surface pressures lower than 11 mN/m, the absence of
pseudoplateaus or inflection points characteristic of PEO aqueous dissolution. This absence is
also shown in the compressibility plot K versus surface pressure (Figure 4-14), where no peak
(local maximum) is observed below 11 mN/m (in compressibility plots versus surface pressure,
every phase transition in a Langmuir monolayer results in a local compressibility maximum).
This suggests that the PEO core of the star-shaped block copolymers is probably not adsorbed at
the interface but more likely already solvated in the water subphase in the vicinity of the
0 20 40 60 80 100
Number of E-caprolactone units
Figure 4-12. Plots of MMA versus number of e-caprolactone repeat units for different surface
pressures from the isotherms of Star#3, Star#4, Star#5, and Star#6.
20 I PEO core extrapolated
E 9+ PEO2000
SArea per ethy-lene oxide~ unit (ii) 6
Figure 4-13. Isotherms of the PEO core extrapolated and PEO2000 normalized with respect to
the number of ethylene oxide units.
Z ~- Star#6G
E L IStar#4
2 4 6 8 10 12 14 16
Figure 4-14. Compressibility plots of the star-shaped PEO-b-PCL block copolymers versus
184.108.40.206 Intermediate MMA region
This region is characterized by a pseudoplateau in the isotherms and by a maximum in the
compressibility plot (Figure 4-14). This phase transition occurs between approximately 12 and
15 mN/m, a similar pressure range as for the collapse pressure of PCL homopolymers. The
pseudoplateau length correlates with the PCL chain length in the stars, and this phase transition
is attributed to the PCL segments aggregating and crystallizing above the interface and the PEO
core, which is consistent with the work reported by Lee et al. on PCL-based block copolymers
with a dendritic hydrophilic head.120
Crystallization of the PCL segments was also characterized by carrying out isobaric
experiments for Star#6 with target pressures below and within the pseudoplateau pressure range
(9, 11, and 13 mN/m). The results of MMA decrease versus time are presented in Figure 4-15,
and the y-axis has been normalized to facilitate comparison. No or very little MMA decrease
takes place over time for pressures below the pseudoplateau (9 and 11 mN/m), which is
indicative of thermodynamically stable monolayers. As the target pressure is increased (13
mN/m), PCL quickly collapses and crystallizes as indicated by the sharp initial area decrease.
The MMA levels off around 1000 A+2, a value that correlates well with the MMA value obtained
at the end of the plateau in the isotherm.
eJ0 o 13mN/m
S.6 -i o
0 10000 20000 30000
Figure 4-15. Isobaric relaxation plots of Star#6 at 9, 11, and 13 mN/m.
PCL crystallization in the pseudoplateau region was finally investigated by hysteresis
experiments. The compression-expansion curves for Star#6 with a target pressure of 15 mN/m
are shown in Figure 4-16. As the barriers expand, an expansion pseudoplateau appears that
corresponds to the readsorption (melting) of the PCL chains previously crystallized.
Interestingly, the second and third compression curves overlap each other but do not overlap the
initial compression, with a slight shift toward the low MMA region and a decrease in the
compression pseudoplateau pressure. Similarly as it was observed for PCL homopolymers, this
confirms that crystallization takes place for Star#6 in the pseudoplateau region during
compression. Star#5, Star#4, and Star#3 have a similar behavior with the expansion
pseudoplateau vanishing as the PCL amount is decreased. Because of even lower PCL content in
Star#2 and Star#1, no expansion pseudoplateau was observed, and the subsequent compression-
expansion isotherms shifted toward smaller mean molecular areas because of irreversible
polymer dissolution in the water subphase (Figure 4-17), similarly as for low surface pressures.
a First impression
ti15 -1 r First expansion
I ca Second and third conspressionsa
E I Second and third expansionss
0 2000 4000 6000 8000 10000 12000
MMA (A Z)
Figure 4-16. Compression-expansion hysteresis plot of Star#6 (target pressure = 15 mN/m).
h First compression
15 B First expansion
a Second compression
E I Second expansion
0 500 1000 1500 2000 2500 3000
Figure 4-17. Compression-expansion hysteresis plot of Star#1 (target pressure = 15 mN/m).
220.127.116.11 Low MMA region
In the low MMA region, the surface pressure sharply increases up to values as high as 35
mN/m. Such high surface pressures can be reached because of the increased amphiphilicity of
the block copolymers compared to the homopolymers, even if the collapse pressure of the
individual PEO and PCL blocks is surpassed. The fact that the isotherms of Star#6, Star#5,
Star#4, and Star#3 overlap in this region suggests that all the PCL chains have collapsed and
crystallized above the interface, and that the sharp pressure increase arises mainly from
interactions between the hydrated PEO cores. Therefore, PCL crystallization probably takes
place for these four stars with the PEO core still hydrated in the water subphase, with the PCL
chains stretching and crystallizing away from the interface. The isotherms of Star#2 and Star#1
shift toward smaller mean molecular areas because of the water solubility behavior mentioned
before. A cartoon of the polymer chains' conformations as the Langmuir monolayers are
compressed is proposed in Figure 4-18. It should be mentioned that the experiments discussed
here do not provide enough information to fully understand the aggregation of the star-shaped
PEO-b-PCL block copolymers. For instance, eventual phase separation between the different
blocks leading to the formation of various surface micelles could not be directly determined from
surface pressure measurements.
O e-caprolactone repeat unit
*ethylene oxide repeat unit
High MMLA reon~l~ ~N
Figure 4-18. Proposed conformations modeling the adsorption of the star-shaped block
copolymers at the A/W interface versus surface pressure.
Intermediate MMLA region
Low MMLA region
18.104.22.168 AFM imaging
Typical AFM images of the star-shaped block copolymers LB films are presented in Figure
4-19 at pressures below (ri= 10 mN/m) and above (ri= 30 mN/m) the pseudoplateau. Similarly
as for PCL homopolymers, crystalline domains are observed for all the stars independent of the
surface pressure except for Star#1, which was unable to crystallize and only aggregated into
dewetted domains (~ 1-2 nm thick). Crystallization in PEO-b-PCL systems has been extensively
investigated in the previous literature, but the number of variables influencing crystallization
rates and morphologies is too large to allow any kind of generalization. For instance, Deng et al.
reported that, in PEO-b-PCL four-arm stars, PCL crystallinity for a constant PCL chain length
decreased as the PEO chain length was increased.114 Gan et al. observed that hydrophilic and
highly flexible PEO segments enhance the hydrophilicity and reduce the degree of crystallinity
of the polyester. The PEO block was also shown to provide nucleation sites for the crystallization
of the PCL block.137 CryStallization of PEO-b-PCL block copolymers in thin films resulted in
various types of spherulitic growths with crystallization of both PEO and PCL blocks.138-140
Nevertheless, it was widely demonstrated that PEO and PCL crystallize in well-defined
separated areas after their phase separation.141 Therefore, as PEO crystallization is obviously
difficult in LB films, probably as a result of residual hydration, we can reasonably assume that
the crystals obtained on mica substrates are the result of PCL crystallization only. Section
analysis indicates a constant crystal thickness around 7.5 nm, which is consistent with the
lamellae thickness obtained for PCL homopolymers.
Figure 4-19. Topographic AFM images of the star-shaped PEO-b-PCL copolymers LB films
transferred below and above the pseudoplateau: Star#1 at 10 (a) and 30 mN/m (b);
Star#2 at 10 (c) and 30 mN/m (d); Star#3 at 10 (e) and 30 mN/m (f); Star#4 at 10 (g)
and 30 mN/m (h); Star#5 at 10 (i) and 30 mN/m (j); Star#6 at 10 (k) and 30 mN/m (1).
From the previously reported literature, the PCL crystal unit cell has a length of 1.7297 nm
that corresponds to two e-caprolactone repeat units.142 PCL crystal thickness after transfer is
around 7.5 nm for all the star-shaped block copolymers, and if it is assumed that the PCL chains
orient perpendicularly to the mica substrate, this thickness indicates that the chains fold
approximately every eight e-caprolactone repeat units. This would also support why the stars
with a number of e-caprolactone repeat units per PCL chain greater or close to eight all
crystallized (Star#6, Star#5, Star#4, Star#3, and Star#2), whereas Star#1, with only three repeat
units per PCL chain, did not.
Similarly as for PCL homopolymers, the star-shaped samples exhibited PCL crystals at
low and high pressures, before and after crystallization of the PCL block at the A/W interface.
Additional crystallization of the PCL segments therefore also took place during and after transfer
for the star-shaped block copolymers, which likely leads to a mica surface only partially covered
with adsorbed or crystallized polymer chains. At high surface pressures, dendritic and needlelike
crystals could be seen. These crystal structures are significantly different from those observed for
PCL homopolymers, which indicates that even though the PEO core did not crystallize, it
strongly influenced the crystallization of the PCL blocks in terms of crystal morphology. Many
variables have to be taken into consideration, such as molecular weight, PEO amount, film
thickness, water evaporation, residual water content, transfer pressure, and substrate affinity for
the polymer chains to fully investigate crystallization of PCL homopolymers and PCL-based
block copolymers in LB films. While this study gives some hint of the surface properties of these
interesting star-shaped block copolymers, complete understanding of the system will therefore
require further investigation.
4.2.4 PEO-b-PCL Linear Diblock Copolymers
The PEO segment of the linear diblock copolymers (PEO2670, Mn = 2,670 g/mol) has a
molecular weight in the same range as the one of the PEO core of the star-shaped PEO-b-PCL
samples. Nevertheless, because of its linear architecture, this PEO segment has intrinsic surface
activity as shown in Figure 4-3, which should lead to the appearance of PEO phase transitions
not observable for the star-shaped samples in the low surface pressure region. The isotherms of
the four linear PEO-b-PCL diblock copolymers are presented in Figure 4-20. Compared to the
homopolymers PEO2670 and PCL2000 (PCL homopolymer with a molecular weight in the same
range as the PCL blocks of the linear diblock copolymers), higher surface pressures as high as 25
mN/m can be reached, similarly as for the star-shaped samples. As better shown in the
compressibility plot (Figure 4-21), three phase transitions that correspond to conformational
rearrangements of the polymer chains are clearly observed around 6.5, 10.5, and 13.5 mN/m. The
local maxima in monolayer compressibility for the two low pressure transitions increase as the
PCL chain length decreases, suggesting PEO-related phase transitions as previously observed for
comb-like polymers consisting of a poly(vinyl amine) backbone with 2kDa PEO side chains.143
The maximum in monolayer compressibility for the high pressure transition increases as the PCL
chain length increases, suggesting a PCL-related phase transition. Comparison with the
isotherms of PEO2670 and PCL2000 indicates that the transitions around 6.5 and 13.5 mN/m
arise from dissolution of the PEO block in the water subphase and crystallization of the PCL
block above the water surface, respectively. The transition at 10.5 mN/m was not observed for
PEO2670, but has been previously described as a brush formation of the PEO chains stretching
away from the interface when anchored by hydrophobic blocks for other PEO-based amphiphilic
block copolymers.144,145 In the following, we report our investigations on the behavior of the
linear PEO-b-PCL diblock copolymers in the low (r < 12 mN/m) and high (r > 12 mN/m)
surface pressure regions.
25 -1 \ PEOo-b-PCL,,
E 20 -- PEOo-b-PCL19
7 ~- PEOo-b-PCL,
0 2000 4000 6000
Mean Molecular Area (A2)
Figure 4-20. Isotherms of the PEO-b-PCL linear diblock copolymers.
0.2 PEOso-b-PCL, ~~1~
2 4 6 8 10 12 14 16
x; (mN /m)
Figure 4-21. Compressibility plots of the PEO-b-PCL linear diblock copolymers versus surface
22.214.171.124 Low surface pressure region (ir< 12 mN/m)
For surface pressures lower than the PEO aqueous dissolution around 6.5 mN/m, both the
PEO and the PCL blocks are adsorbed at the A/W interface in a pancake conformation. To
investigate the 2-dimensional miscibility of the two blocks in more details, we prepared
Langmuir monolayers with binary mixturesl46-14 Of PEO2670 and PCL2000, and the resulting
isotherms are presented in Figure 4-22. Contrary to the isotherms of the linear diblock
copolymers, only 2 phase transitions corresponding to PEO2670 aqueous dissolution (around 6.5
mN/m) and PCL2000 crystallization (around 13 mN/m) are observed. The absence of a phase
transition at 10.5 mN/m comes from the fact that PEO2670, which is not chemically anchored by
a hydrophobic PCL segment, is already irreversibly dissolved at this surface pressure in the
water subphase. This aqueous dissolution of PEO2670 (loss of material in the monolayer by
solubilization in the subphase) can be observed more clearly in the hysteresis experiment in
Figure 4-23 (49 mol % of PEO2670), with a target surface pressure of 9 mN/m, where the
successive compression-expansion curves shift toward the low MMA region only below 6.5
mN/m. Above 6.5 mN/m, the curves overlap because the amount of PCL2000 adsorbed at the
A/W interface stays constant, independent of the number of compression-expansion hysteresis
*E 0 mol% PCL2000
E 10 -1 \\ 28 mol% PCL2000
Z 51 mol% PCL2000
0 I 70 mol% PCL2000
"~~ 6 10 86 mol% PCL2000
-100 mol% PCL2000
0 1000 2000 3000 4000
Mean Molecular Area (A2)
Figure 4-22. Isotherms of PEO2670 and PCL2000 binary mixtures. (Inset) Corresponding
compressibility plots versus surface pressure.
8 First expansion
E o Second compression
S 6 *Second expansion
0 1000 2000 3000 4000
Mean Molecular Area (A2)
Figure 4-23. Compression-expansion hysteresis plot of the binary mixture with 49 mol % in
PEO2670 (target pressure = 9 mN/m).
It is also interesting to notice that the collapse surface pressure of PEO2670 slightly
increases (up to 6.7 mN/m for 86 mol % of PCL2000) when increasing the amount of PCL2000
in the mixed monolayers, as shown by the shift of the compressibility maximum toward higher
surface pressures (inset of Figure 4-22). This is a first indication that PEO2670 and PCL2000
are miscible when both blocks are adsorbed at the A/W interface."s Figure 4-24 shows plots of
MMA versus the mole fraction of PCL2000 for three surface pressures below 6.5 mN/m (2, 3,
and 4 mN/m). The data exhibit negative deviations from ideal mixing (dashed lines), which
confirms that in the surface pressure range where they are adsorbed at the A/W interface (7<< 6.5
mN/m), PCL2000 and PEO2670 do not phase-separate and thermodynamically interpenetrate
each other.151,152 From these results, we can reasonably extrapolate that the PEO and PCL
segments of the linear diblock copolymers are miscible as well below 6.5 mN/m, and therefore
that their A/W interfacial adsorption probably does not lead to the formation of surface micelles
previously observed for other amphiphilic block copolymers.97,153,154 These results are in good
agreement with a previous study that demonstrated the miscibility of PEO and PCL
homopolymers in the amorphous phase in blend films prepared by solution casting.'
<(~ As = 2 m Nlm
= 3 mNlm
Q) I a = 4 mNlm
2 0 0.2 0.4 0.6 0.8 1
PCL2000 mole fraction
Figure 4-24. MMA plots versus mole fraction of PCL2000. Dashed lines: theoretical ideal
The reversibility of the two PEO phase transitions of the linear diblock copolymers was
investigated by carrying out compression-expansion hysteresis experimentS110,15 On the block
copolymer sample with the smaller PCL segment (PEO60-b-PCLll) for ai target pressure of 18
mN/m. The resulting x/1I1VA curves and the corresponding compressibility plots are shown in
Figures 4-25 and 4-26, respectively. After the first compression, the curves in Figure 4-25 shift
to the low IV1VA region, which is indicative of some irreversibility in the PEO phase transitions.
As shown in the compressibility plots, the local maximum at 6.5 mN/m disappears after the first
compression whereas the maximum at 10.5 mN/m is still present, independent of the number of
compression-expansion cycles. At 6.5 mN/m, we believe the PEO chains dissolve irreversibly in
the aqueous subphase and adopt a mushroom conformation. Nevertheless, because of the
anchoring effect of the PCL segments, the PEO chains stay in the vicinity of the interface. Upon
further monolayer compression, the PEO chains are compressed against each other and stretch
perpendicularly to the interface to form a compact brush at 10.5 mN/m.22,23,144,157-160 During
monolayer expansion, the PEO brush reversibly relaxes, but the PEO chains do not readsorb and
stay hydrated underneath the interface, which explains the complete absence of phase transition
at 6.5 mN/m after the first compression. The maximum in compressibility corresponding to the
transition at 10.5 mN/m is nevertheless slightly decreased after the first compression, which
suggests that the PEO chains do not completely relax during the subsequent expansions to their
original mushroom conformation of the first compression. It should be noticed that, contrary to
our linear PEO-b-PCL diblock copolymers, only one apparent PEO phase transition around 10
mN/m is usually observed in the isotherms of PEO-based block copolymers with high molecular
weight PEO blocks (Mn > 10,000 g/mol),22 prObably because PEO aqueous dissolution and brush
formation take place simultaneously.
0 1000 2000 3000
Mean Molecular Area (A2)
Figure 4-25. Compression-expansion hysteresis plot of PEO60-b-PCLll (target
pressure = 18
n; (mN Im)
Figure 4-26. Compressibility plots of Figure 4-25 (PEO60-b-PCLll, target pressure = 18 mN/m).
Top curve: first compression. Bottom curve: first expansion, second compression, and
126.96.36.199 High surface pressure region (fr> 12mN/m)
In this surface pressure region, similarly as for the PCL homopolymers and the star-shaped
block copolymers, the phase transition observed in Figures 4-20 and 4-21 at 13.5 mN/m arises
from collapse and crystallization of the PCL segments above the water surface. The
compression-expansion hysteresis experiment carried out on the linear block copolymer sample
with the longest PCL block (PEO60-b-PCL35) for a target pressure of 16 mN/m gives more
insight on the crystallization/melting behavior of the PCL segments. The n/~MMA curves and the
corresponding compressibility plots are shown in Figures 4-27 and 4-28, respectively. During
monolayer compression, the hydrophobic PCL segments collapse and crystallize on top of the
water surface as shown by the inflection points in the 2/MMA curves and the local maxima in
the compressibility plots at 12.5 and 13.5 mN/m. Similarly as for the PCL homopolymers and for
the star-shaped samples, the collapse/crystallization surface pressure in the first cycle is
approximately 1 mN/m higher than in the subsequent compressions (13.5 versus 12.5 mN/m),
and also, after the first compression, the subsequent curves are slightly shifted toward the low
mean molecular area region. After the first hysteresis cycle, the PCL crystals that did not melt
(i.e. did not readsorb at the A/W interface) act as nucleation sites to catalyze the crystallization of
other PCL segments during the subsequent cycles, therefore lowering the crystallization surface
pressure and shifting the compression isotherms toward lower mean molecular areas. During
monolayer expansion, melting of the PCL segments was characterized in the 2/MMA plots for
the star-shaped samples and the PCL homopolymers by an expansion pseudoplateau at a surface
pressure lower than for the crystallization. As shown in Figures 4-27 and 4-28, no pseudoplateau
or sharp local maximum in compressibility are observed during the first expansion, whereas for
the subsequent cycles the PCL segments clearly readsorb around 5 mN/m (sharp local maximum
in the compressibility plot). The presence of a broad melting transition during the first monolayer
expansion is a little bit surprising because this behavior was not previously observed for the star-
shaped samples and the PCL homopolymers, but it is probably related to a particular shape, size,
or polydispersity of the PCL crystals formed during the first monolayer compression. It should
be noticed that the crystallization and melting surface pressures reported here for linear diblock
copolymers (as well as for the star-shaped block copolymers and the PCL homopolymers) are
not thermodynamic values and were shown to be strongly barrier speed dependent. If one could
compress and expand the monolayers infinitely slowly, the crystallization and the melting
surface pressures would respectively decrease and increase, probably leveling off to a common
ai First expansion
15- o Sec~ond and third co~mpressions
E Second and third expansions
O 2000 4000 6000
Mean Molecular Area (A2)
Figure 4-27. Compression-expansion hysteresis plot of PEO60-b-PCL35 (target pressure = 16
1 3 5 7 9 11 13 15
x; (m Nlm)
Figure 4-28. Compressibility plots of Figure 4-27 (PEO60-b-PCL35, target pressure 16 mN/m).
a: PCL crystallization at 13.5 mN/m during the 1st compression. b: PCL
crystallization at 12.5 mN/m during the 2nd and 3rd COmpressions. c: broad PCL
melting transition during the 1st expansion. d: PCL melting transitions during the 2nd
and 3rd expansions.
Crystallization of the linear PEO-b-PCL diblock copolymers in the LB monolayers was
finally evidenced by AFM imaging after transfer onto mica substrates for surface pressures
above 13.5 mN/m, after crystallization of the PCL segments on the water surface took place. As
shown in Figure 4-29 for a surface pressure of 15 mN/m, all the samples show hair-
like/needlelike crystal structures, with a constant crystal thickness around 7.5 nm as determined
from cross-section analysis. This thickness is consistent with the value obtained for the PCL
homopolymers and the star-shaped block copolymer samples. The PEO chains are adsorbed onto
the mica substrate and the PCL segments stretch perpendicularly to the interface, folding
approximately every 8 e-caprolactone repeat units. These results confirm what we observed for
the star-shaped PEO-b-PCL samples, where the PEO block highly influenced crystallization of
the PCL segments in terms of crystal morphology. Nevertheless, it is crucial to emphasize here
once again that this brief crystal structure analysis is done for the LB films only and is not
rigorously valid for Langmuir monolayers, because film drying during LB film formation can
lead to further crystallization of the PCL segments. Evidence for crystallization of the PCL
segments in the star-shaped and the linear block copolymer samples directly at the A/W interface
came from the isotherms, the hysteresis, and the isobaric experiments, but no clear conclusions
can yet be drawn in terms of crystal structures in the Langmuir monolayers. BAM experiments
are currently underway to investigate more deeply the in-situ and real-time PCL crystal
growth/melting directly on the water surface for the star-shaped and the linear PEO-b-PCL block
50n 50n 5nm1
25nm 25nm 25nm
0 nm 20 nm 0 n
S- 20 nm 2u
50nm 50nm 7.5 nm
0 nm 0 nm
Figure 4-29. Topographic AFM images of the linear PEO-b-PCL diblock copolymers LB films
transferred after crystallization of the PCL segment at the A/W interface (7<= 15
mN/m). (a): PEO60-b-PCLll (b): PEO60-b-PCL19 (c): PEO60-b-PCL27 (d) and (e):
PEO60-b-PCL35 (f): CTOss-section analysis on PEO60-b-PCL35
A cartoon summarizing the polymer chains conformations as the Langmuir monolayers of
the linear PEO-b-PCL diblock copolymers are compressed is proposed in Figure 4-30. For
surface pressure values lower than 6.5 mN/m, both the PEO and the PCL segments are adsorbed
at the A/W interface in a pancake conformation (1). As the surface pressure is increased, the
PEO segments irreversibly dissolve in the aqueous subphase and adopt a mushroom
conformation around 6.5 mN/m (2), before forming a brush above 10.5 mN/m upon further
compression (3). Finally, around 13.5 mN/m, the PCL segments collapse and crystallize
perpendicularly to the interface (4).
Ethylene oxide repeat unit
O e-caprolactone repeat unit
(2) 6.5 mN/m < xr < 10.5 mN/m
(1) x < 6.5 mN/m
(3) 10.5 mN/m < 7x7 < 13.5 mN/m
Figure 4-30. Proposed conformations modeling the adsorption of the linear PEO-b-PCL diblock
copolymers at the A/W interface versus surface pressure.
(4) 13.5 mN/m < r
In this chapter, the A/W interfacial behavior of various PEO-b-PCL block copolymers and
their LB film morphologies on hydrophilic mica substrates were investigated, and the results
were compared to PEO and PCL homopolymers. The isotherms of the star-shaped block
copolymers 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 the plateau, the PCL segments are adsorbed, anchoring the water-soluble star-shaped PEO
core in the vicinity of the interface. Compression-expansion hysteresis experiments showed that,
in this region, the spread monolayers are thermodynamically stable except the ones containing
the smallest PCL amounts, which irreversibly dissolved in the water subphase. In the
pseudoplateau region, PCL homopolymers crystallized directly at the A/W interface as well as
the PCL segments of the star-shaped block copolymers. Above the pseudoplateau, the isotherms
of the star-shaped block copolymers with the longest PCL chains overlapped, indicating that all
the PCL chains have collapsed and that the sharp pressure increase mainly arises from
interactions between the hydrated PEO cores. Compression-expansion hysteresis experiments
indicated that the readsorption/melting of the PCL segments takes place at a lower surface
pressure than for the crystallization. AFM imaging of the homopolymers and the star-shaped
block copolymers LB films was complicated by the fact that both PEO and PCL are highly
crystalline polymers that can undergo morphological changes during monolayer transfer. The
PEO homopolymers did not crystallize, probably because residual hydration or large hydrophilic
substrate/PEO monolayer interactions inhibited crystal formation. The PCL homopolymers and
the star-shaped block copolymers crystallized directly at the A/W interface only above the PCL
collapse pressure, but additional crystallization could take place during water evaporation on the
mica substrates. Various crystal morphologies were observed for the star-shaped block
copolymers such as spherulitic, dendritic, and needlelike structures, with the presence of the PEO
core strongly influencing the crystallization of the PCL blocks. The linear diblock copolymers
successfully self-assembled as well at the A/W interface to form stable Langmuir monolayers.
Preliminary investigation on PEO2670 and PCL2000 homopolymers blends showed that these
polymers are non-ideally miscible for low surface pressures when both blocks are adsorbed at the
A/W interface. Nevertheless, the individual collapse surface pressures (PEO2670 aqueous
dissolution around 6.5 mN/m and PCL2000 crystallization above the interface around 13 mN/m)
were not significantly influenced by the presence of the other homopolymer. For the linear PEO-
b-PCL diblock copolymers, an additional PEO phase transition at 10.5 mN/m was observed
corresponding to the formation of a PEO brush underneath the anchoring PCL segments. These
two PEO phase transitions were not observed for the star-shaped PEO-b-PCL block copolymers,
and our investigations consequently confirmed the significant influence of the polymer
architecture on its interfacial properties. AFM imaging of the linear PEO-b-PCL diblock
copolymers LB fi1ms for high surface pressures confirmed the formation of PCL crystals with
hairlike/needlelike architectures. These crystals were significantly different from those obtained
in LB films of PCL homopolymers, confirming the strong influence of the PEO block on the
crystallization of the PCL segments. This fundamental investigation gave interesting insight on
the interfacial self-assembly of PEO-b-PCL copolymers and showed that an accurate and easy
control of the conformations and the orientations of the different blocks at the A/W interface can
be easily achieved by simply varying the polymer architecture or the surface pressure.
4.4 Experimental Methods
4.4.1 Langmuir Films
Surface pressure measurements were accomplished by use of a Teflon Langmuir trough
system (W= 160 mm, L = 650 mm; KSV Ltd., Finland) equipped with two moving 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 puL 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, unless otherwise stated.
4.4.2 AFM Imaging
The LB films were formed by transferring the Langmuir films of the PEO-b-PCL block
copolymers (linear and star-shaped) and the homopolymers onto freshly cleaved mica at the
desired surface pressure which was attained at compression/expansion rates of +/-5 mm/min.
Once the films had equilibrated at a constant surface pressure for 15 min, the mica substrate was
then pulled out of the water subphase at a rate of 1 mm/min. All the transfer ratios were close to
unity unless otherwise stated, which is indicative of successful transfer. The transferred films
were air-dried in a desiccator for 24 h and subsequently scanned in tapping mode with a
Nanoscope III AFM (Digital Instruments, Inc., Santa Barbara, CA) by use of Nanosensors
silicon probes (dimensions: T = 3.8-4.5 pum, W = 26-27 pum, L = 128 pum). All the images were
processed with a second-order flattening routine.
TWO-DIMENSIONAL POLYMERIC NANOMATERIALS THROUGH CROSS-LINKING
OF POLYBUTADIENE-b-POLY(ETHYLENE OXIDE) MONOLAYERS AT THE
The idea of stabilizing amphiphilic self-assemblies by polymerization was introduced at
least thirty years ago for monolayers and about ten years later for bilayer vesicles.161,16 This
approach to bridging the nanoscale world of labile, interfacially driven self-assemblies with the
meso-scale has resulted in several examples of cross-linked 3D structures.163-16 For example,
Bates and co-workers were the first to succeed in retaining the cylindrical morphology formed by
gigantic wormlike rubber micelles of polybutadiene-b-poly(ethylene oxide) (PB-b-PEO) diblock
copolymers in water by chemical cross-linking of the PB cores through their pendant 1,2-double
bonds.163,167,168 However, relatively few groups have shown interest in stabilization by cross-
linking of two-dimensional (2D) polymeric self-assemblies formed at the air/water (A/W)
interface; most studies have involved interfacial polymerization of small molecules in Langmuir
In the early 1970's, Veyssie and co-workerS180,190,191,193 were the first to demonstrate the
formation of 2D cross-linked materials by cross-linking monolayers of dimethacrylates and
several other difunctional reactive amphiphiles under UV irradiation for a constant surface
pressure at the A/W or the oil/water interface. This idea inspired other research groups and
several examples followed. Regen and co-workers introduced the concept of a 2D-network of
molecular pores, i.e. "perforated monolayers" derived from calix[n]arene-based amphiphiles.183-
Iss Cross-linking with malonic acid or via UV irradiation enabled them to synthesize porous and
cohesive "perforated monolayers" with pore diameters in the range 2-6 A+ potentially applicable
for gas permeation selectivity.186,187,189 Michl and co-workers synthesized grids through the
coupling of star-shaped monomers forced to adhere to a mercury surface.194,195 After
polymerization, well-defined covalent 2D square- or hexagonal-grid polymers could be
synthesized,194,195,199 and analogous supramolecular routes were also proposed.200-202 Palacin and
co-workers reported on cross-linking porphyrins through molecular recognition between
oppositely charged monomers at the A/W interface.196-19 Kloeppner and Duranl7 were the first
to demonstrate the possibility to remove from the water surface free-standing fibers of 2D cross-
linked 1,22-bis(2-aminophenyl)docosane polyanilines. Finally, alkylalkoxysilanes have been
widely used, 177-179,203,204 and our group has for instance investigated some of the fundamental
aspects of the A/W interfacial cross-linking of octadecyltrimethoxysilane (OTMS) and
octadecyltriethoxysilane (OTES) under acidic conditions.177-179,203 However, relatively few
groups have shown interest in stabilizing by cross-linking 2D "true" polymeric self-assemblies at
the A/W interface. To our knowledge, only one example based on a lipopolymer was previously
proposed by O'Brien and co-workers involving network formation by photopolymerization.205
Our interest is to cross-link monolayers of block copolymers to achieve porosity at the sub-
micrometer scale. In this chapter, the synthesis of a 2D polymeric nanomaterial consisting of a
continuously cross-linked PB network containing PEO domains of controllable size is illustrated.
This work was done in collaboration with Rachid Matmour, graduate student in the Duran group
at the University of Florida. Such thin films have potential applications in the preparation of
membranes which will show large differences in permeability to water, methanol, and other
polar compounds, depending on the PEO "pore" size.
We report in this chapter the 2D self-condensation, at the A/W interface and under acidic
conditions, of a triethoxysilane-functionalized PB-b-PEO three-arm star block copolymer (PB
core and PEO corona) and of a triethoxysilane-functionalized linear PB homopolymer in a
preliminary investigation. Using PB, which pendant double bonds have been hydrosilylated with
trialkoxysilanes, as the cross-linkable block is a novel approach that can be applicable to a
variety of other polydiene-based block copolymers in order to retain a specific morphology at the
nanoscopic scale. The surface properties of the cross-linked monolayers were characterized by
surface pressure measurements such as surface pressure (7r)-mean molecular area (MMA)
isotherms at different reaction times, and isobaric experiments for various subphase pH values.
The morphologies of the Langmuir monolayers were studied by atomic force microscopy (AFM)
imaging of the corresponding Langmuir-Blodgett (LB) films.
5.2 Results and Discussion
5.2.1 Hydrosilylated PB Homopolymer
188.8.131.52 Hydrosilylation reaction
To demonstrate the viability of the 2D cross-linking method, we chose to first focus on a
commercially available linear PB homopolymer (Mn = 11,050 g/mol, ~ 204 butadiene repeat
units). Many publications and patents can be found in the literature on the hydrosilylation of
polymers.206-21 In mOst cases, the hydrosilylated polydienes were used as precursors to
synthesize macromolecular complex architectures such as arborescent graft polybutadienes,218
multigraft copolymers of PB and polystyrene,219 Or side-loop polybutadienes.220 Triethoxysilane
was used here as the pendant double bond hydrosilylating agent in stoichiometric amount with
the total molar amount of repeat units in the PB homopolymer and in the presence of Karstedt
catalyst (platinum catalyst) as shown in Figure 5-1. The reaction was carried out under argon for
24 h at 80 oC in dry toluene (water free environment). After workup, the hydrosilylated PB was
analyzed by 1H NMR and FTIR spectroscopies (Figures 5-2, 5-3, and 5-4).