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Blends of a Polystyrene-Block-Poly(ethylene Oxide) Copolymer and Its Corresponding Homopolymers at the Air-Water Interface


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BLENDS OF A POLYSTYRENEBLOCK -POLY(ETHYLENE OXIDE) COPOLYMER AND ITS CORRESPONDING HOMOPOLYMERS AT THE AIR-WATER INTERFACE By SOPHIE BERNARD A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2006

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Copyright 2006 by Sophie Bernard

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iii ACKNOWLEDGMENTS First, I would like to thank my adviso r, Dr. Randolph S. Duran for his guidance troughout the past few years. None of this wo rk could have been executed without the precious help of the Duran group members: Jorge Chvez, Henk Keiser, Brian Dorvel, Danyell Wilson, Dr. Firouzeh Sabri, Eric Greeley, and Aleksa Jovanovi I am highly grateful to Thomas Joncheray and Dr. Jenni fer Logan for our produc tive discussions and for their support during this entire process. My thanks also go to Dr. John R. Reynol ds and Dr. B. Kenneth Wagener. Their trust and encouragement have meant a lot to me. I would also like to thank all my past and present peers at the polymer fl oor for their friendship and advice. Last but not least, very special thanks go to my parents and my two brothers in France for their unconditional l ove and support at all times.

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iv TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iii LIST OF TABLES.............................................................................................................vi LIST OF FIGURES..........................................................................................................vii ABSTRACT....................................................................................................................... ix CHAPTER 1 INTRODUCTION........................................................................................................1 2 EXPERIMENTAL TECHNIQUES..............................................................................9 Langmuir Trough..........................................................................................................9 Isotherm Experiments.................................................................................................11 Langmuir-Blodgett Films...........................................................................................13 Atomic Force Microscopy..........................................................................................14 3 ISOTHERM EXPERIMENTS...................................................................................18 Experimental...............................................................................................................18 Linear Polystyreneblock -Poly(ethylene oxide) (PSb -PEO) Diblock Copolymers..18 Blends of PSb -PEO and a PS homopolymer.............................................................21 Pancake Region (I)..............................................................................................22 Pseudoplateau Region (II)...................................................................................23 Condensed Region (III).......................................................................................24 Blends of PSb -PEO and PEO homopolymer............................................................26 Pancake Region (I)..............................................................................................28 Pseudoplateau Region (II)...................................................................................29 Condensed Region (III).......................................................................................31 4 ATOMIC FORCE MICROSCO PY (AFM) EXPERIMENTS...................................33 Experimental...............................................................................................................33 Qualitative Analysis....................................................................................................35 Linear Polystyreneblock -Poly(ethylene oxide) (PSb -PEO) Diblock Copolymers..37 Blends of PSb -PEO and a PS homopolymer.............................................................41

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v Blends of PSb -PEO and a PEO homopolymer..........................................................45 5 CONCLUSION...........................................................................................................48 LIST OF REFERENCES...................................................................................................49 BIOGRAPHICAL SKETCH.............................................................................................52

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vi LIST OF TABLES Table page 3-1. Characteristics of the PSb -PEO sample investigated................................................19 3-2. The mass ratio of PS between the dibl ock copolymer and the homopolymer as well as the apparent number of styrene units have been calculated for each blend.........................................................................................................................2 2 3-3. Width of the pseudoplateau for each blend................................................................23 3-4. The mass ratio of PEO between the di block copolymer and the homopolymer as well as the apparent number of styrene units have been calculated for each blend.........................................................................................................................2 7 3-5. Pancake areas extrapolated from the -A isotherms..................................................28 3-6. Area for the second transition (described in Figure 2.5) extrapolated for each blend.........................................................................................................................2 9 3-7. Molar ratio of PEO from the homopolymer and the diblock copolymer as well as the total number of EO units is given for each blend...............................................31

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vii LIST OF FIGURES Figure page 1-1 Schematic representation of the pancake to brush transition for PSb -PEO copolymers.................................................................................................................4 1-2 The two conformatins for PE O at the air-water interface..........................................4 2-1 The original Langmuir balance..................................................................................9 2-2 Set up of a typical Langmuir trough.........................................................................10 2-3 Schematic of the Wilhelmy plate.............................................................................10 2-4 Schematic -A isotherm...........................................................................................12 2-5 Schematic -A isotherm showing the different areas that can be determined by extrapolation.............................................................................................................12 2-6 Different types of deposited LB films......................................................................14 2-7 Optical system that de tects cantilever deflection.....................................................15 2-8 AFM scanner tube containing the piezo electric material and metal electrode. The x, y, and z-directional components of the scanner are also indicated...............16 3-1 -A isotherm for the 32,500 g mol-1 PSb -PEO copolymer.....................................19 3-2. Several isotherms are shown, indicati ng the dependence of surface pressure on the mean molecular area fo r different blend ratios...................................................22 3-3 Condensed area per EO unit vers us the number of styrene units.............................25 3-4 Condensed area versus the total number of styrene units........................................25 3-5 Several isotherms are shown, showing the dependence of surface pressure on the mean molecular area for different............................................................................27 3-6 Pancake area versus th e total number of EO units...................................................28 3-7 The area for the second transition depe nds linearly on the mole ratio of PEO from the homopolymer over PEO from the PSb -PEO............................................30

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viii 3-8 The area for the second transition depends linearly on the appa rent total number of EO repeat units.....................................................................................................30 4-1 Sample height image and surface plot (scan area shown is 2 2 m)......................34 4-2 Sample section image (2 2 m)...............................................................................35 4-3 The software allows choosing a dom ain range by varying the minimum and maximum areas........................................................................................................36 4-4 The software gives you a computed im age representing the different domains and the possible angles between domai ns in the presence of chaining....................36 4-5 Error made by the computer can be corrected by the user.......................................37 4-6 AFM images of the pure PSb -PEO for several transfer pressures (scale 2 2 m)..38 4-7 Schematic representation of surface micelles formed by Ab -B diblock copolymers with A strongly adsorbed to the surface...............................................39 4-8 Model of PSb -PEO absorbing at the air/water interface.........................................39 4-9 Dependence of the number of molecules per domain on pressure...........................40 4-10 AFM images of the pure diblock copolymer as well as two of the blends for several transfer pressures (scale 2 2 m).................................................................42 4-11 AFM images for the pure diblock copolym er and Blend 2 (transfer pressure of 4mN/m) as well as the distribution of the domain areas..........................................43 4-12 Computed images for the pure diblock copolymer and Blend 2 (transfer pressure of 10mN/m) as well as the di stribution of the domain areas....................................44 4-13 Magnification of a single domain formed for Blend 2 for a transfer pressure of 10mN/m (scale 150 150nm)....................................................................................45 4-14 AFM images of the pure PSb -PEO diblock copolymer and several blends for transfer pressures of 4 and 9 mN/m (scale 2 2 m).................................................45 4-15 AFM images for Blend 2; (a) from su ccessive spreading, and (b) from the mixed solution(scale 2 2 m)..............................................................................................46

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ix Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science BLENDS OF A POLYSTYRENEBLOCK -POLY(ETHYLENE OXIDE) COPOLYMER AND ITS CORRESPONDING HOMOPOLYMERS AT THE AIR-WATER INTERFACE By Sophie Bernard May 2006 Chair: Randolph S. Duran Major Department: Chemistry The two-dimensional structure of a polys tyrene-block-poly(ethylene oxide) (PS-bPEO) diblock copolymer at th e air-water interface is st udied using Langmuir-Blodgett methods and atomic force microscopy (AFM). M easurements are also made for blends of the PS-b-PEO copolymer with bot h a PS and a PEO homopolymer. When increasing the amount of PS hom opolymer, the isotherms do not show any change in the high surface ar ea region. However, a linear dependence of the condensed area is observed. An increase in the PEO ratio has an effect on the biphasic region of the isotherms but no change is detected for the condensed area. Each of the blends was subsequently studied by AFM. The data indicate a significant effect of the hom opolymers on the monolayer struct ure. In fact depending on the homopolymer added, a change in the chaini ng behavior of the copolymer is observed. Also, when introducing more PEO, a phase separation between the layer of PEO and clusters of two-dimensi onal micelles is detected.

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1 CHAPTER 1 INTRODUCTION Amphiphilic copolymers are widely used becau se of their interfacial properties and their ability to form molecu lar architectures at various interfaces. Amphiphilic diblock copolymers have been observed to self-asse mble into numerous nanoscale and mesoscale structures when spread onto a water substrate, finding potential appl ications in coatings, microelectronics, stabiliz ation, and lubrification.1 Such copolymers are appropriate for surface pressure studies involving Langmuir troughs. This technique provides insight on the monolayer morphologies by controlling th e surface density. For example, Seo et al. 2 showed the formation of st abilized two-dimensional micelles using polystyreneb poly(methyl methacrylate) (PSb -PMMA) diblock copolymers at the air-wate r interface. Once formed, those surface aggregates were kinetically stable, preventing any unimermicelle exchange. Polystyreneb -poly(ethylene oxide) (PSb -PEO) diblock copolymers of various molecular weights and chemical compositions have also been extensively used to study their properties in both th e bulk and in solution. In addition, several groups have described their behavior at the air-water interface.3-15 The choice of PEO as one of the blocks renders the copolymer both biocompa tible as well as amphiphilic. The inclusion of PS provides an anchor at the air/water in terface, preventing the PEO from eventually dissolving into the water subphase. As a result, PSb -PEO films can be further compressed than a film composed simply of PEO homopolymer.

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2 Without PS, PEO can still be spread at th e air/water interface. Shuler and Zisman 16 studied the behavior of such a film. They obser ved a change in the film compressibility as surface density increases leading to a phase change reflecting change in the films structure. The lack of reversibility in the compression and expansion experiments is explained by a structural change in the polymer molecule. A modification in conformation was given to explain the different monomer area observed in the -A isotherms. Kuzmenka and Granick 17 performed the same type of experiment for a wide range of PEO molecular weights. They dete rmined that for PEO chains beyond molecular weights of 100,000g.mol-1, the film attains a co nstant equilibrium surface pressure. This behavior was explained by the difficulty of a high molecular weight PEO to pass into an aqueous substrate due to the amphiphilic char acter of the EO monomer. Lower molecular weight PEO, however, requires a more hydrophob ic anchor in order to quantitatively remain at the air/water interface, generally partitioning between the subphase and the surface, analogous to soluble surfactants. While PEO has been widely studied at th e air-water interface, PS has been studied by only one group. Being hydrophobic, PS is not expected to form any type of morphology when spread onto a wa ter subphase. However, Kumaki 18 detected a change in surface pressure when a dilute solution of PS (2.0 10-5g.mL-1) was spread at the airwater interface. Even if surface pressure main ly represents mechanical force due to the compression, stable monomolecular particles we re observed for molecular weights higher than 50,000g.mol-1. As a result, recent work has focused on the behavior of PSb -PEO at the air-water or solid-water interface, dem onstrating the formation of novel nanostructures. Gonalves

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3 da Silva et al. 5,10 described the utility of diblock amphiphilic copolymers in testing the scaling properties of grafte d polymers. They presented -A isotherms that show several regions referred to as pancake, quasi-bru sh, and brush stages. Within these regions, different morphologies of surface micelles and further micellar aggregates were observed by transmission electron microscopy (TEM) and atomic force microscopy (AFM) depending on the balance between block sizes At the air-water interface, copolymers behave similarly to copolymers in bulk disper sion. Static light scat tering proved that PSb -PEO copolymers aggregate spontaneously in to micelles over th e critical micellar concentration (CMC). The isotherm regions compare to those observed for solution CMC values: (1) below the CMC, surface micellization is observed; (2) at the CMC, the PEO segments are pushed into the substrate in order to decrease the surface area per molecule; and (3) above the CMC, PS-rich regions ex ist in between spaces formed by the PEO chains. The importance of PEO in film behavi or has been recognized by others. For example, Gonalves da Silva et al. 5,10 investigated the effect of the PEO block size on the copolymer behavior at the air-water interface. In this case, the short PS chains are only used as an anchor to prevent the PEO from dissolving completely into the water substrate. Upon compression, they observed a transition of the PEO blocks, from a twodimensional structure floating on the water, to a three-dimensional structure when the PEO streched into the water. The first structure is the one previously termed pancake whereas the second was identified as brush (Figure 1-1). The plat eau displayed in the -A isotherms is an indication of the transi tion between these two states with its span dependent on the relative sizes of the two blocks.

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4 Figure 1-1. Schematic representation of th e pancake to brush transition proposed by Goncalves da Silva et al.5 for PSb -PEO copolymers. Figure 1-2. The two conformatins for PEO at the air-water interface proposed by Shuler and Zisman.16 Devereaux and Baker14 conducted -A isotherms experiments of PSb -PEO copolymers with varying PEO chain lengths One copolymer contained 15% of PEO whereas the other had only 7%. The copolymer with the longest PEO block displayed a plateau around 10mN.m-1, indicating that the copolymer spreads well at the interface. In contrast, the copolymer contai ning only 7% of PEO has no pl ateau, supporting the theory that PS chains interfere with the PEO blocks upon compression. While most groups support the model of a transition from pancake to brush described previously, Cox et al. 12,13 provide a different model to explain the shape of the -A isotherm for a PSb -PEO copolymer. Whereas in the first model the PEO passes into the aqueous subphase, the Cox model suggest s a dehydration of the PEO followed by a

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5 conformational change, similar to that pr eviously described by Shuler and Zisman16 for homopolymer PEO. As shown in Figure 12, conformation (a), more flexible, is compressed into conformation (b) more compact and sterically hindered. This transformation can be explained by an increase in the intramolecular forces in the second conformation. While numerous studies detail the behavior of linear PSb -PEO, advances in polymerization techniques within the past de cade have allowed chemists to design new copolymer architectures. Logan,19 Logan et al.,20 and Francis et al.,21,22 for example, investigated the behavior of a threearm star amphiphilic copolymer, PEO3b -PS3, at the air-water interface. Peleshanko et al.23 observed formation of morphologies when spreading an amphiphilic heteroarm PEOb -PSm. The AFM images showed that the formation of different morphol ogies depends on the pressure used during the transfer. The unusual properties of those architectures allow the formation of more stable morphologies than those formed us ing regular linear copolymers. While different architectures can result in different surface film behavior, the synthesis of such systems can be difficult and time-consuming. In an effort to acquire new properties without the re quired synthesis, surface film s of blended polymers have also been investigated. In the 1980s, the Gabrielli group24,25 examined the behavior of numerous mixtures of polymers and low mol ecular weight materials as binary systems with different degrees of incompatibility. Th ey also quantified the determination of the two-component monolayer miscibility by observing the -A isotherms of their twodimensional blend. Thibodeaux et al.26 studied mixtures of a liquid crystalline copolymer with its corresponding monomer. The films fo rmed by the blend monolayer appeared to

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6 be more condensed than the pure copoly mer films, proving that two-dimensional mixtures of two or more polymers could enha nce the interfacial behavior and enable the formation of more stable films. In addition, such technique allows the ble nding of different polymer characteristics into a single film. Malzert et al.27 developed a suitable model for understanding the interactions between polymers by mixing poly(ethylene glycol ) and poly(lactideco glycolide), whereas Hottle et al.28,29 studied blends of amphiph ilic poly(dimethylsiloxane) and trisilanolisobutyl-POSS. More recently, Seo et al.30 investigated the structures formed at the air-water interface by blending poly(styreneb -ferrocenyl silane) (PSb -FS) and poly(styreneb -2-vinyl pyridine) (PSb -P2VP). While neither of those copolymers assembles when spread separately at the air-w ater interface, their blends formed ordered structures which appear to be more versatil e, a promising development in the fabrication of polymeric templates for lithography. The most commonly used technique to observe the morphologies formed by compressing a monolayer at a certain pressure is AFM. For soft samples such as polymer films, an appropriate AFM technique is tappi ng mode. Here, the cantil ever is excited to an oscillation near its resona nce frequency. The interactio ns between the tip and the sample give a deviation in the oscillation am plitude, recording the changes in the sample. This mode has been employed for most polymer samples because of its ability to investigate soft materials wit hout further staining and with l ittle or no tip-induced damage or morphology changes. However, Knoll et al.31 highlighted the limitations of this technique, finding that the a pparition of artifacts was relate d to tip-sample interactions. Nevertheless, AFM provides valuable info rmation on film morphology. Bodiguel et al.,32

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7 for example, introduced a method for determ ining the dependence of the phase signal on the thickness of the sample. They corroborated that the origin of the phase signal was adhesive and represented the local elastic properties of the sample. In general, the surface behavior of amphiphilic diblock copolymers is readily examined through Langmuir techniques. Me thods involving film compression and transfer provide both quantitative and qua litative results indicating how surfactant responds to pressure. PSb -PEO proves to be particular ly of interest due to the biocompatibility of PEO. While different ar chitectures of this copolymer have been shown to demonstrate different properties than those of linear analogues, additional characteristics may yet be attained trough blending, both with PS and PEO homopolymers. This work focuses on the la tter part, examining the effects of adding either PS or PEO to a linear PSb -PEO chain. The first chapter thus provides an introduc tion to this study. In the second chapter, a brief review of the techniques used is gi ven, covering Langmuir monolayers, LangmuirBlodgett films, and atomic force microscopy (AFM). The third chapter describes th e behavior of a linear PSb -PEO copolymer at the airwater interface and the formati on of ordered structures at di fferent pressures. Blends of the linear copolymer and its correspo nding homopolymerspolystyrene (PS) and Poly(ethylene oxide) (PEO)are also exam ined to determine the effect of each homopolymer on the interf acial behavior of PSb -PEO. AFM studies of both the linear chain and its blends appear in the fourth chapter, providing insight about the sh ape and the size of the aggregates. A computer program designed by our group helped determine the pr operties of the different morphologies,

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8 such as areas, angles, and aggregation numbers. Those results were compared both to previous and new results involving the se lf-assembly of relate d star copolymers.

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9 CHAPTER 2 EXPERIMENTAL TECHNIQUES Any study involving Langmuir monolayers requires the use of a Langmuir trough set-up for the preparation of Langmuir-Blodg ett films. Irving Langmuir was one of the principal initial scientists to observe the formation of monolayers when a surfactant is spread onto water, developing the Langmuir trough technique (figure 2-1). With this apparatus, he studied floating monolayers on water in the late 1910s and early 1920s. Several years later, Katherine Blodgett gave the first detailed desc ription of sequential monolayer transfer onto solid supports. Figure 2-1. The original Langmuir ba lance as designed by I. Langmuir33 Langmuir Trough A typical Langmuir trough (figure 2-2) is composed of the trough itself, two movable barriers, and a device measuring surf ace pressure. The Wilh elmy technique is the most commonly used and consists of a we ttable thin plate part ially submerged in a subphase and suspended from a balance. Th e force acting on the plate is directly proportional to the surface tension of the liquid.

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10 Figure 2-2. Set up of a t ypical Langmuir trough The plate is usually very thin and made of platinum, but glass, quartz, mica, and filter paper can also be used. The net downward force is given by the equation: F = pglwt + 2 (t + w)cos lgtwh 2-1 where p and l are the densities of the thin plat e material and liquid, respectively, g represents the gravitational constant, is the subphase surface tension, and is the contact angle of the liquid on the solid plate. The plate is also desc ribed by its thickness (t), width (w), and leng th (l) (see figure 2-3). Figure 2-3. Schematic of the Wilhelmy plate When measuring the change in h for a constant applied force: lgtw h = 2 (t + w). 2-2 Since the surface pressure is defined as a negative change in surface tension: = = lgtw h/2(t + w). 2-3 l w h t Dipper Electrobalance Barrier Barrier Heat Exchanger Wilhelmy Plate Substrate Trough

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11 when measuring the change in F for a sta tionary plate between a clean surface and the same surface with a monolayer present. If the plate is completely wetted by the liquid (cos = 1), the surface pressure is then obtained from the following equations: F = 2 (t + w) 2-4 = = F/2(t + w) 2-5 For the Wilhelmy method, the thickness of the plate used is small, giving t << w. So, = F/2w 2-6 Nowadays, electrobalances allo w very little change in the plates movement, improving sensitivity (5 10-2 mN.m-1). Isotherm Experiments Measuring the surface pressure as a functi on of the area of water surface available to each molecule provides insight into m onolayer properties. Such experiments are carried out at constant temperature using a heat exchanger, and are known as isotherm experiments. The data are recorded by comp ressing the film at a constant rate while monitoring the surface pressure (figure 2-4). Distinct regions can be observed defining the different phases of the monolayer. Vari ous monolayer states can be observed, depending on the hydrocarbon chain length; in fa ct, an increase in chain length increases the interactions between the chai ns, leading to a more condensed -A isotherm. For a minimal compression, the monolayer exists in the gaseous phase (G). While compressing, the monolayer undergoes phase transitions to the liquid-expanded state (L1), followed by the liquid-condensed state (L2), and finally the solid state (S). If th e monolayer is further compressed, it will collapse into three-dimensional structures.

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12 Figure 2-4. Schematic -A isotherm (picture bo rrowed from the website http://www.ksvinc.com/LB.htm ) While these various areas can often be f ound in small surfactant molecules, diblock copolymers typically have fewer regions. An example is shown in Figure 2-5. Here, area extrapolations quantify the isotherm, a llowing the surface behavior of different copolymers and their blends to be compared. Figure 2-5. Schematic -A isotherm showing the different areas that can be determined by extrapolation

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13 Langmuir-Blodgett Films Besides Langmuir monolaye rs, a common application of the Langmuir trough is the transfer of monolayer ont o a solid substrate. This is accomplished by dipping the substrate into the subphase, allowing the adsorption of the monolayer. The surface pressure is maintained constant by a comput er controlled feedback system between the electrobalance measuring the surface pre ssure and the barrier moving mechanism. Depending on the number of dippings, severa l successive monolayers can be deposited onto the solid substrate. Numerous substrates have been used. Mica is commonly preferred in LB film transfer due to its low cost, facile cleaning, and easy preparation. However, it possesses a water layer that may affect the film transfer. Other substrates such as silicon wafers can be used; treatment with chro mic sulfuric acid renders them highly hydrophilic. Other materials can be used as hydrophobic substrates including graphite a nd silanized silicon dioxide. LB films can be formed either by pulling or dipping the substrate into the subphase. The upward pass of the substrat e through the subphase is known as an upstroke while the downward dipping refers to the downstroke. Th ree different types of deposition can exist (figure 2-6). The X-type depos ition can be done by a downstroke whereas Z-type refers to an upstroke. The Y-type, the most common, is produced by an upstroke followed by a downstroke. Intermediate structures can some times be observed for some LB multilayers and they are often referred to as XY-type multilayers.

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14 Figure 2-6 Different types of deposited LB films (borrowed from Jennifer Logans dissertation19) Once transferred, these films can be studied by different surface analysis techniques, such as atomic force microscopy (AFM) or transmission electron microscopy (TEM). Atomic Force Microscopy Contrary to its precursor, scanning tu nneling microscopy (STM), which only allows the study of conductive samples, AFM can be applied to both conductors and insulators. The instrument consists of a tip at the end of a cantilever, which bends in response to the force between the tip and the sample (figure 2-7). Since the cantilever obeys Hookes law for small displacements, the interaction force between the tip and the sample can be found: F = -k x 2.7 where x is the cantilever deflection and k the spring constant.

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15 Figure 2-7. Optical system that detects cantilever deflection (Figure adapted from Digital Instruments Training Notebook34) In the early stages of AF M, contact mode was used. This method consists of a tip in close contact with the surface. The deflection of the cantilever is sensed and compared to the desired value of deflection. The voltage needed to restore the desired value of deflection is a measure of height of feat ures on the sample surface. This mode was quickly forgotten for polymer studies because of excessive tracking forces applied by the probe to the sample. To remove these drawbacks, a non-contact mode was developed. In this case, the tip hovers 50-150 Angstrom above the samp le surface. The attractive Van der Waals forces acting between the tip and the sample are detecte d, and topographic images are constructed by scanning the tip above the su rface. This technique was found to be inapplicable to polymer samples. In general, the fluid contaminant layer existing on the sample is substantially thicker than the range of the Van der Waals force gradient and, therefore, all attempts to image the tr ue surface with non-contact AFM fail as the oscillating probe becomes trapped in the fluid layer. Later, a third method was developed in or der to study softer samples. This mode, called tapping mode, consists of alternately pl acing the tip in contac t with the surface to

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16 provide high resolution and then lifting the tip off the surface to avoid dragging the tip across the surface. As the oscillating cantilever begins to intermittently touch the surface, the cantilever oscillation is necessarily re duced due to energy loss caused by the tip contacting the surface. The reduction in osci llation amplitude is used to identify and measure surface features. AFM involves scanne rs made from piezoelectric material, a substance which proportionally contracts and expands, depending on an applied voltage. If a positive voltage elongates the scanner, a negative voltage contracts it. The scanner is made of a piezoelectric material surrounde d by electrodes which control the applied voltage. As scanning occurs in three dimens ions, a scanner tube contains three piezo electrodes for the X, Y, and Z directions (Fig. 2-8). Figure 2-8. AFM scanner tube c ontaining the piezoelectric ma terial and metal electrode. The x, y, and z-directional components of the scanner are also indicated. (Figure adapted from Digital Instrument s Training Notebook.34) The studies described in this work utili ze a Digital Instruments Nanoscope system, and with this system three different sca nners can be used depending on the sample studied. They differ on the scanning size and resolution. For example the J-scanners can scan images up to 125 m, whereas Escanners scan smaller sizes of 10 m or less.

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17 Polymer thin films can thus be characte rized through a combination of Langmuir and AFM techniques. Such methods allow the easy control of surf ace density as well as the facile transfer of surface films onto solid substrates. The results of such analysis will be presented in the subsequent chapters.

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18 CHAPTER 3 ISOTHERM EXPERIMENTS Experimental The PSb -PEO diblock copolymers as well as the blends were characterized as surface films at the air/water interface. The copolymer a nd each of the homopolymers were dissolved in chloroform at a concentr ation of 1mg/mL. Using a Hamilton syringe, the solution was then spread dropwise acro ss a layer of Millipore filtered water ( 18.2 M cm-1) in a Teflon TM Langmuir trough system (KSV Ltd., Finland) After waiting 30 minutes to allow for complete evaporati on of the chloroform, the surface film was compressed at mm min-1 at 25C. Compressing the film generates an isotherm of surface pressure ( ) vs. mean molecular area (MMA). The latter represents the average area each molecule occupies at the air/water interface Linear Polystyreneblock -Poly(ethylene oxide) (PSb -PEO) Diblock Copolymers Linear PSb -PEO copolymers represent a conv enient choice when studying the interface behavior of amphiphi lic compounds, due to the biocompatibility of the PEO block and the low cost and availability of th e PS block. Moreover, they have been widely studied and found to form st able, condensed surface films.3-15 The -A isotherm for a 32,500 g/mol copolymer (see Table 3.1.) displays a plateau at ca. 10mN/m and is shown in Fig. 3-1. The observed plateau results from the hydrophilic part of the copolymer and appear s over the same pressure range as the collapse pressure of a PEO homopolymer.16

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19 020406080100120140 0 10 20 30 40 50 60 70 Surface Pressure (mN/m)Mean Molecular Area (nm2/molecule) Figure 3-1. -A isotherm for the 32,500 g mol-1 PSb -PEO copolymer The shape of the isotherm is independen t of the copolymer solution concentration and the compression speed. In addition, multiple runs confirmed these experiments to be reproducible within 1.0nm2. Within the isotherm, three dist inct regions are observed. At large molecular areas, the surface film is expa nded (Region I); this is usually called the pancake region due to the shape the PE O units form on the water surface. Table 3-1. Characteristics of the PSb -PEO sample investigated As compression continues, a plateau appear s (Region II) over the pressure range of 8 to 10 mN/m. Kuzmenka and Granick17 studied the behavior of PEO homopolymers at the air-water interface with varying molecular weights. They observed a constant equilibrium spreading pressure for po lymers having a molecular weight beyond 100,000g.mol-1. The pseudoplateau detected in the cas e of our copolymer is in the same MW (g/mol) PEO wt% PS wt% Polydispersity MWPEO MWPS NPEO NPS 32,500 32 68 1.05 10,500 22,000 238 211

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20 range of the collapse pressure of a PEO homopolymer and corresponds to the hydration and desorption of these chains from the surface and into the subphase. The appearance of the plateau with an in creasing amount of PEO in the copolymer was considered by Devereaux and Baker14 They studied two PSb -PEO copolymers containing different masses of PEO. The 7% PEO copolymer had no plateau whereas the 15% PEO did. This observation was explained by the long PS interfering with the PEO blocks, preventing the PEO from stretching in to the aqueous subphase. Our results are in agreement with this theory showing a plat eau for a copolymer containing 32% PEO. Considering the affinity of PEO for water, at large molecular areas the films most likely exist as PEO films with globules of PS on top. Region II, however, represents a biphasic phase where aggregates and single po lymer domains coexist. The fact that the pseudoplateau occurs within the same pre ssure range as the co llapse region of PEO homopolymer illustrates the si gnificant influence PEO has on the copolymer surface film. Bijsterbosh et al.4 and Goncalves da Silva et al.5,10 both demonstrated the existence of a pseudo first-order transition from pancak e-like structure to that of a brush upon compression of a series of PSb -PEO copolymers containing a constant PS length and varying amounts of PEO. While this model is prevalent in the literature, Cox et al.12,13 provided a new interpretation for the presence of the pseudoplateau assuming that the formation of brushes is not possible due to PEOs low surface energy. They proposed that PEO instead undergoes a dehydration pro cess and a conformational change upon compression. Contrary to PEO homopolymers, a third region (III) appears beyond the pseudoplateau and shows a sharp increase in su rface pressure, indicati ng the formation of

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21 more rigid films. Here, the PS block serves as an anchor, keeping the PEO at the interface and allowing the films to be compressed to higher surface pressures. Without the PS, PEO would dissolve into the aqueous subpha se at pressures beyond the plateau. In examining Region III, Bijsterbosh et al.4 and Goncalves da Silva et al.5,10 studied a series of copolymers with varying PEO lengths and a constant PS block. While Region III typically reflects PS, they found that the copol ymer interfacial behavi or at high pressures depends slightly on the size of the PEO block. Blends of PSb -PEO and a PS homopolymer The same linear copolymer described in th e previous section was used to study the effect of adding a homopolymer solution on its behavior at the air-water interface. The PS homopolymer used has a molecular weight of 20,000 g/mol, which corresponds to the molecular weight of the PS block in the c opolymer. Different ratios of copolymer and homopolymer were studied in order to de termine the impact on the formation of Langmuir monolayers Langmuir-Blodgett films. The mixed monolayers were performed by se parately spreading solutions of the PS and the PSb -PEO block copolymer. After evapora tion of the solvent, the floating monolayer was symmetrically compre ssed by the two movable barriers. -A isotherms were recorded for several mole ratios of PS within the homopolymer and copolymer (Table 3-2). Fig. 3-2 shows the isotherms data for di fferent copolymer/homopolymer ratios. For all these blends as well as the pure linear c opolymer isotherms, the three regions defined in the previous section were observed.

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22 Table 3-2. The mass ratio of PS between the diblock copolymer and the homopolymer as well as the apparent number of styren e units have been calculated for each blend. Blend # 1 2 3 4 5 6 7 Mass % of PS 70.2 72.3 75.8 78.6 80.7 84.0 86.3 Mole ratio of PS (homopolymer/copolymer) 0.138 0.275 0.551 0.826 1.102 1.653 2.204 NPS,TOT 236 259 307 355 403 499 595 Figure 3-2. Several isotherms are shown, indi cating the dependence of surface pressure on the mean molecular area fo r different blend ratios Pancake Region (I) The first region, defined by low surface pre ssure and low surface density, can be expressed by its extrapolated area, AP (Figure 2-5). The values for every blend remain constant with varying th e amount of PS (average AP = 89.7 nm2). This is in agreement

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23 with a film of PEO with gl obules of PS on top of it where increasing the amount of PS will not change the area occupied at the interface by the PEO. Faur et al.11 observed the same behavior for pure diblock copolymers at the airwater interface and showed that at low coverage, the interac tion between the EO monomers and the interface is attractive and th erefore leads to the adsorption of the EO at the air-water interface. They a ssume the pressure to be only due to the total number of PEO segments in water. As a result, in creasing the PS should have no effect on the behavior of this region. Logan,19 Logan et al.,20 and Francis et al.,21,22 observed a similar trend for star copolymers of PSb -PEO in which the pancake area did not depend on the number of PS segments. The pa ncake area per EO monomer (0.38 nm2) is in reasonable agreement to the one found by Logan et al. for star copolymers (0.33 nm2) and to the that determined for linear PSb -PEO by Gonalves da Silva et al. (0.27 or 0.31 nm2)5,10 and Bijsterbosch et al.4 (0.31 nm2). Pseudoplateau Region (II) The pseudo-plateau observed in Se ction 3-2 for a pure linear PSb -PEO copolymer is observed for all blends and remains cons tant with varying the total mass of PS. Table 3-3. Width of the ps eudoplateau for each blend Blend # Pure 1 2 3 4 5 6 7 AP (nm2) 22.8 23.0 22.2 21.1 22.3 22.5 20.5 21.1 The width ( AP) of the pseudoplateau can be estim ated as the difference between ATransition 1 and ATransition 2 (Table 3-3, Figure 2-5). The value of AP remains constant for every blend (average AP = 21.9 nm2). Due to its phase transition nature, region II is believed to represent a biphasic region. Th e phase transition is mostly due to the reorganization of the PEO chains from a pan cake to a brush conformation and therefore a

PAGE 33

24 change in the amount of PS does not have a ny effect on the width of the pseudoplateau. In this region, the EO repeat unit occupies 9.2 2 which is smaller than the value found by Logan et al.22,19 (13.3 2) for star copolymers. Condensed Region (III) In Table 3-2, the theoretical area (A0) that a compact surface film would occupy at zero pressure was determined for each ble nd. In agreement with our expectations, the area increases with an increasing mass of PS. The condensed area, representing mostly th e behavior of the PS chains, varies linearly with the total mass of PS chains. This behavior can be compared to the behavior of copolymers presenting PS chains of high molecular weights. Cox et al.6 studied several PSb -PEO copolymers with varying PS molecula r weights, observing a variation in the A0 values. The increase of A0 with increasing PS can be ex plained by the a ggregation of the PS homopolymer with the PS chains of the copolymer. To compare our results with those fro m copolymers of longer PS blocks, a normalization of the total number of styren e units was obtained by using the following equation. Diblocks PS r Homopolyme PS r Homopolyme PS Diblock PS TOT PSn n N N N, , , With NPS,Diblock and NPS, Homopolymer being the number of styrene repeat units in the PS-b-PEO diblock copolymer and the PS homopolymer respectively. nPS, Hompolymer and nPS,Diblocks represent the number of moles of PS in the homopolymer and the copolymer. When plotting the condensed area per EO un it versus the apparent number of styrene units (Figure 3-3), a linear dependence can be observed (R2 = 0.9932) with a trendline of y = 0.0001 x + 0.0753.

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25 y = 0.0001x + 0.0753 R2 = 0.9932 0.08 0.09 0.1 0.11 0.12 0.13 0.14 0.15 0.16 0100200300400500600700 Total number of styrene unitsA0/#EO (nm2) Figure 3-3. Condensed area per EO unit versus the number of styrene units The positive y-intercept shows us that even without any PS present in the monolayer, the PEO occupies 7.5 2/EO units. This value is a lot smaller than the one observed for star copolymers by Logan et al. (16 2/EO). 22,19 y = 0.0288x + 17.914 R2 = 0.993215 20 25 30 35 40 0100200300400500600700 Total number of styrene unitsA0 (nm2) Figure 3-4. Condensed area versus th e total number of styrene units The collapse area plotted vs. the total number of styrene units follows the trendline y = 0.0288 x + 17.914. The area per styrene unit obtain from the slope (2.9 2) is smaller than the one J. Logan described for th e behavior of star copolymers of PS-b-PEO at the

PAGE 35

26 air-water interface (ranging from 6.2-8.32, depending on architecture).22 She reports the results for (PEO26)8-(PS42)8 (an 8 arm PSb -PEO star copolymer with each arm containing 26 EO units and 42 Styrene units) wh ich has a total of 336 repeat units of PS. For Blend 4, the PS homopolymer o ccupies an area equal to 5nm2. This value is in the same range as the area per styrene value found in the literature for an atactic PS in the bulk, calculated from the radius of gyration (38).35 The PS, not covalently bound to the PEO, tends to adopt a random coil confor mation less compact than the conformation produced by the PS segments of the copolymers. This copolymer can be compared to Blend 4 which presents a to tal of repeat units of 355 for the PS (using the formula described previously). J. L ogan obtained a value of 28.6 nm2 which is similar to the value obtained for Blend 4. Blends of PSb -PEO and PEO homopolymer Contrary to PS, PEO is an amphiphilic polymer forming monolayers at the airwater interface. The addition of a PS block as an anchor keeps the PEO from going into the water subphase. This al so allows the formation of more compact films by compressing at higher pressures. The effect that unencumbered PEO has on such films is examined in this part of the discussion wh ere blends of the copolymer and homopolymer PEO were studied (Table 3-4).

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27 Table 3-4. The mass ratio of PEO between the diblock copolymer and the homopolymer as well as the apparent number of styrene units have been calculated for each blend. Blend 1 2 3 4 Mass % of PEO 92.4 95.9 97.9 98.9 Mole ratio of PEO (homopolymer/copolymer) 0.025 0.05 0.101 0.202 NPEO,TOT 295 352 467 697 Fig. 3-5 shows the isotherm data for several blends of the 32,500g.mol-1 PS-b-PEO copolymer and a 100,000 g.mol-1 PEO homopolymer. The choice of the homopolymer was made in agreement with the resu lts published by Kuzmenka et Granick17 on PEO homopolymers who showed a constant equi librium surface pressure for PEO with molecular weights higher than 100,000g.mol-1. 0200400600800100012001400 0 10 20 30 40 Surface Pressure (mN.m-1)MMA (nm2/molecule) Blend 1 Blend 2 Blend 3 Blend 4 Pure PSb -PEO Figure 3-5. Several isotherms are shown, show ing the dependence of surface pressure on the mean molecular area for different

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28 Pancake Region (I) Similar analysis was done for the homopolym er PEO blends as that seen for the PS ones. As with the pure copolymer, the resul ting isotherms displaye d all three regions. AP was obtained for each blend and for the pure PS-b-PEO from the -A isotherms (Table 35). Table 3-5. Pancake areas extrapolated from the -A isotherms Blend # Pure 1 2 3 4 AP (nm2) 86 153 244 352 610 The pancake area depends linearly on the to tal number of ethylene oxide units (R2 = 0.9968) with a trendline of y = 1.1297 x 173.95. The area obtained from the slope (1.13 nm2) is significantly higher than th e one observed by Sauer et al.34 for a PEO homopolymer (0.40-0.48 nm2). Gonalves da Silva et al.5,10 recorded a smaller area for PS-b-PEO diblock copolymers (0.27 and 0.31 nm2). This can be explained by the fact that the PEO chains from the homopolymer pack less closely when in the presence of the PS-b-PEO diblock copolymer. y = 1.1297x 173.95 R2 = 0.99680 100 200 300 400 500 600 700 0100200300400500600700800 Total number of EO unitsPancake area (AP) (nm2) Figure 3-6. Pancake area versus the total number of EO units

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29 Table 3-6. Area for the second tr ansition (described in Figur e 2.5) extrapolated for each blend We can also observe a negative y-intercept indicating that all the EO units are not at the interface. In such a case, a pancake area equal to zero should correspond to zero EO units. Even with a negligible effect on the pancake area, the PS units may trap some of the PEO, leading to a lowe r apparent number of PEO units. Pseudoplateau Region (II) The addition of PEO, however, has an e ffect on the shape of the plateau observed for a pressure around 10mN/m. The more PEO is added to the monolayer, the longer the biphasic region becomes. To illustrate this point, ATransition 2 (Figure 2-5) was recorded for each blend as well as for the pure diblock. The results are given in Table 3-6. A graph of ATransition 2 vs. the ratio of number of moles of homopolymer over the number of moles of dibloc k shows a linear dependence (R2 = 0.994) with a trendline of y = 1082.9 x + 47.532 (Figure 3-7). To be able to compare those results to the one published previously for pure dibl ocks or star copolymers, an identical formula as the one used in the previous part was developed. Presenting a constant ATransition 1 for every blend, an increase in ATransition 2 indicates the presence of a bigger plateau area. Diblock PEO r Homopolyme PEO r Homopolyme PEO Diblock PEO TOT PEOn n N N N, , , The values calculated using this fo rmula, are reported in Table 3-7. Blend # Pure 1 2 3 4 ATransition 2 (nm2) 46 72 112 149 268

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30 A linear dependence (R2 = 0.9942) was observed when the area for the second transition was plotted vs. the tota l number of repeat units of PEO, yielding a trendline of y = 0.4769 x 66.032 (Figure 3-8). y = 1082.9x + 47.532 R2 = 0.9940 50 100 150 200 250 300 00.050.10.150.20.25 moles of PEO in the homopolymer/moles of PEO in the diblock copolymerATransition2 (nm2/molecule) Figure 3-7. The area for the second transition depends linearly on the mole ratio of PEO from the homopolymer over PEO from the PS-b-PEO y = 0.4769x 66.032 R2 = 0.9942 0 50 100 150 200 250 300 0100200300400500600700800 Total number of EO repeat unitsATransition 2 (nm2/molecule) Figure 3-8. The area for the second transiti on depends linearly on the apparent total number of EO repeat units These observations compare to those seen by Faur et al.11 They studied the phase transitions in monolayers of PS-b-PEO copolymer at the air-wa ter interface for different PEO block sizes. Faur et al. observed an incr ease in the length of the pseudoplateau as

PAGE 40

31 the number of PEO units increases. The transi tion from pancake to brush becomes more and more first order as they increase the PEO segment size Table 3-7. Molar ratio of PEO from the homopolymer and th e diblock copolymer as well as the total number of EO units is given for each blend. Blend # Pure 1 2 3 4 nHomo/nBlock 0 0.025 0.05 0.101 0.202 NPEO,TOT 238 295 352 467 697 In addition, one of the diblock copolymers th ey studied consisted of 31 repeat units of PS and 700 of PEO, a PEO amount simila r to that of Blend 4. The Faur copolymer demonstrates a -A isotherm with an almost flat plateau, confirming the first order transition of the copolymer. Similarly, Ble nd 4 displays a plateau representing a strong indication of a first order tr ansition. By adding PEO homopo lymer to our monolayer, we have been able to enhance the copolymer properties without havi ng to increase the PEO block length trough time-consum ing synthetic techniques. Condensed Region (III) This third region appears at higher su rface pressures beyond the pseudoplateau. As demonstrated by Shuler and Zisman,16 such a region does not exist for a PEO homopolymer, as no anchor exis ts to prevent PEO from completely immersing in the water subphase. This region depends only on th e length of the PS blocks and not on PEO, as demonstrated by the -A isotherms of the different blends in Figure 3-5. A0 remains the same regardless of PEO added (23.7 nm2). By blending a PS-b-PEO diblock copolymer with its corresponding homopolymers, we were able to mimic linear chain behavi or by manipulating PS and PEO quantities. On one hand, the addition of PS has proven to have the same effect on the copolymer

PAGE 41

32 behavior than increasing the PS block size. On the other ha nd, raising the amount of PEO only had an effect on the biphasic region of the isotherm. While this technique could be a good a lternative to time-consuming synthetic techniques and expensive samples purchase, ex periments still need to be performed with various molecular weight homopolymers as we ll as hysteresis data in order to better understand the aggregation be havior of those films. Additional analysis continues in chapter 4 in which the blends are transferred as Langmuir-Blodgett films and examined through atomic force microscopy (AFM).

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33 CHAPTER 4 ATOMIC FORCE MICROSCOPY (AFM) EXPERIMENTS AFM is a technique that provides the opportunity to study surface morphology and structure at the submicron scale. By inves tigating transferred Langmuir-Blodgett films, AFM can give insight into the behavior of the copolymer blends at various pressures, providing both quantitative a nd qualitative results. Such da ta helps demonstrate the degree of interaction between the copolymer and homopolymers Experimental Surface films of the linear copolymer and th e blends were transferred onto freshly cleaved mica at various pressures (25C). The desired surface pressure was attained at rates of 10 mmmin-1. Once the film had equilibrated at a constant for 30 minutes, the mica was then pulled out at a rate of 1 mmmin-1. The transferred film was air-dried in a dust-free environment for 24 hours and subseque ntly scanned in ta pping mode with a Nanoscope III AFM (Digital Instruments, Inc ., Santa Barbara, CA) using silicon probes (Nanosensor dimensions: T = 3.8-4.5 m, W = 26-27 m, L = 128 m). The hydrophilicity of the substrate allows us to c onsider the hydrophilic PE O to be attached to the mica whereas the hydrophobic PS occupies a higher layer. By consequence, the PEO is represented by the darker (lower) areas wh ereas the PS exists as the brighter (higher) domains. Tapping mode was used, giving a be tter image of a polymer sample without damaging the surface by dragging the tip. This m ode consists of a tip vibrating at its resonance frequency in tapping the surface. As the tip encounters a surface feature, its amplitude of oscillation is decreased from its set-point value. This decrease is noted by

PAGE 43

34 the sensor and the tip is moved up away fr om the sample to re-attain the set-point amplitude. A similar behavior happens wh en the tip moves past the feature. A topographical map of the sample can then be recorded. Figure 4-1. Sample height image and surface plot (scan area shown is 22 m) The AFM software contains several f unctions for image analysis. One method represents the three-dimensional surface plot of the imaged sample, as shown in Figure 4-1. The color shading is a representati on of the height of the sample (7.3nm for Figure 4-1). Precise height data can be obtained for given domains through section analysis. This technique is illustrated in Figure 42. A line is traced acro ss the domain region of interest, giving a cross-sectional view of the sample. In this example, the height difference between the two marked domains is 1.2nm and the difference between the domain at the left and the PEO surface (brown) is 4.5nm.

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35 Figure 4-2. Sample section image (22 m) Qualitative Analysis A program designed by Yves Heckel, an unde rgraduate student from Paris, France, allowed us to define the char acteristics of the aggregates observed in the AFM images. Parameters such as the number of domains as well as the size of those domains were determined in order to better understand th e aggregation behavior of the copolymer blends. This program allows a domain size range to be chosen in which the values of the minimum and the maximum can be varied (Figure 4-3). The image on the left of the screen allows the user to adapt the area range limits using a visual aid. Another attr ibute of this program, is th at it counts the number of domains present in one chain as opposed to c onsidering the chain as a single domain. If such a mistake is made, however, the program can be manually manipulated by the user to define domain separation and number. Wh ile this feature give s a better approximation of the shape and number of domains, the resu lting disadvantage is lower user efficiency, but significantly higher accuracy.

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36 Figure 4-3. The software allows choosing a domain range by vary ing the minimum and maximum areas Figure 4-4. The software gives you a computed image representing the different domains and the possible angles between domai ns in the presence of chaining Once all the domains are counted, the computer gives a computed image representing all the domains present with the different chaining and angles for each domain (Figure 4-4). Computer errors can o ccur, giving wrong angl es and poorly defined Original image Computed image

PAGE 46

37 aggregates, in which case the user can, by c licking on the domain, redo the separations and redefine the domain (Figure 4-5). Figure 4-5. Error made by the computer can be corrected by the user The software allows domain populations to be chosen and analysis error to be manually corrected, permitting the analysis of images with more than one domain population, as in the cas e of the images observed for the blends. Linear Polystyreneblock -Poly(ethylene oxide) (PSb -PEO) Diblock Copolymers AFM is a valuable technique for stud ying morphologies formed by spreading copolymer solutions at an aqueous s ubphase. Bodiguel et al. demonstrate the complementary nature of AFM and TEM in de picting phase separation of two distinct polymer blocks.32 The technique assumes that the morphology of the transferred film represents that of the floa ting monolayer and that transf er is homogeneous. LangmuirBlodgett (LB) films were prepared at severa l surface pressures and then studied using

PAGE 47

38 AFM in tapping mode. For each sample, an average of ten images was taken to ensure reproducibility. Figure 4-6. AFM images of the pure PS-b-PEO for several transfer pressures (scale 22m) The images shown in Fig. 4-6 clearly demonstrate the formation of ordered structures in which the observed morphology depends on surface pressure. In fact, three distinct regions corresponding to those in the isotherm can be seen once again. For pressures of 4 and 7 mN/m (Region I of Fi g. 3-1.), images show a majority of single domains, typical of an expa nded liquid. Two-dimensional mice lles form at the air-water interface with a morphology depending on the ratio of the hydrophobic and hydrophilic block sizes. For pressures unde r 7mN/m, circular micelles are observed like the one described by Potemkin et al.36 (Figure 4-7) where one of the blocks is strongly adsorbed on a planar surface.

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39 Figure 4-7. Schematic representation of surface micelles formed by A-b-B diblock copolymers with A strongly adsorbed to the surface (adapted from Potemkin et al.36) In the case of PS-b-PEO at the air-wate r interface, similar micelles are observed with the PEO extending more and more in the aqueous subphase as the concentration increases (Figure 4-8). Figure 4-8. Model of PS-b-PEO absorbing at the air/water interface (Adapted from Dewhurst et al.8) When compression continues and reaches the pseudoplateau range (Region II of Fig. 3-1), chain formation is detected and con tinues until collapse pressure is occurs. The images also demonstrate the presence of inte rmediate stages in which the single domains begin to aggregate prior to chain formation. Due to the hydrophilicity of the mica, we suppose PEO represents the bottom layer whereas PS occupies the top part of the LB film at a thickness of some nanometers, B B B B B B B A

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40 ranging from about 2 to 10 depending on the blend. In our images, the darker layer represents PEO and the bright domains show the PS blocks. Using a program designed by our group, the number of domains per image was found, allowing the molecules per domain (or aggregation nu mber) to be calculated. 0 200 400 600 800 1000 1200 1400 1600 05101520 Surface Pressure (mN/m)Molecules/domain Figure 4-9. Dependence of the number of molecules per domain on pressure For each given pressure, the aggregation number was determined using Formula 41. = A/Nd. 4-1 where refers to the number of molecules per domain, A the scanned area of the image, Nd the number of domains, and the mean molecular area during transfer. As shown in Figure 4-9, the number of molecule s/domain depends str ongly on the surface pressure. For pressures less than 10mN/m, the number of molecules/domain remains almost constant. However, once the pressure of the pseudoplateau is reached, an increase in aggregation number is observed. As compression continues, aggregation in creases and at the transition between Regions II and III, the aggregation number rises sharply. This behavior is another

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41 indication of the transition between the liquid expanded st ate and the liquid condensed state. Logan et al.19,22 showed that compressi on-induced aggregation occurs when PEO is pushed into the aqueous subphase. However, at higher pressures, some PEO can remain at the interface and separate the PS domains. This situation represents two conflicting forces. The attraction between PEO and the wa ter allows the polymer to spread on the surface, whereas the repulsion of PS with bot h water and PEO drives aggregation. Cox et al.12,13, however, thought the re lative interaction of the two blocks with the subphase and air is a more probable explanation for the existence of aggregation. Blends of PSb -PEO and a PS homopolymer To observe the possible formation of aggregates between copolymer and homopolymer, the blends were studied by AFM fo r different transfer pressures. In Figure 4-10, the AFM images for the pure diblock, Bl end 2, and Blend 5 at several transfer pressures are given. As described previousl y, a chaining of the domains is observed for the pure PS-b-PEO when increasing the pressure. However, in the blend case, the addition of PS homopolymer seems to inhibit the formation of these chains. In Figure 411, the histograms of the domain area are give n for both films at a tr ansfer pressure of 10 mN/m. The pure diblock exhibits larger domai n areas whereas the blend seems to show an increase of the ratio be tween big and small domains. In the pancake region, the pure diblock exhi bits local hexagona l packing with six neighbors for each domain, showing that even at low pressure, the copolymers arrange into surface micelles. In th e case of the blends, the addi tion of PS to the monolayer disrupts this packing by increas ing the size of only some of the domains and enabling a

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42 population of smaller domains to form (Figure 4-11). Figure 4-10. AFM images of the pure diblock copolymer as well as two of the blends for several transfer pressures (scale 22m) This behavior can be compared to that described by Logan,19 Logan et al.,20 and Francis et al.,21,22 for PS-b-PEO star copolymers of vari ous hydrophobicity. For both stars and linear chains in the literature (particularly Devereaux and Baker14), increased PS results in nonuniform films with a greater variety of morphology. As pressure increases, no chaining is obs erved in the case of the blend. In fact instead of chaining, an augmentation in populatio n of the bigger domains compared to the small domains can be observed. This phenomenon can be shown by using the computer program designed by Yves Heckel. Histograms of the domain areas are given in Figure 4-12. AmountofPS = 4mN/m = 7mN/m = 10mN/m PURE BLEND 2 BLEND 5

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43 Figure 4-11. AFM images for the pure diblock copolymer and Blend 2 (transfer pressure of 4mN/m) as well as the di stribution of th e domain areas In the pure diblock, formation of large ch ains that resemble pearl necklace-like strings.occurs when increasing the transfer pressure. This phenomenon continues with the formation of domain dimers or trimers that then keep on chaining with increased pressure. When PS is added to the monolayer, no such domains are observed. An increase in the size of the circular domains is obs erved, indicating the a ggregation of the PS homopolymer within the PS chains of the copolymer (Figure 4-13). 020406080100 0 50 100 150 200 CountsDomain Area (nm2) 0102030405060708090100 0 20 40 60 80 100 CountsDomain Area (nm2)

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44 Figure 4-12. Computed images for the pure diblock copolymer a nd Blend 2 (transfer pressure of 10mN/m) as well as th e distribution of the domain areas In the pure diblock, formation of large ch ains that resemble pearl necklace-like strings.occurs when increasing the transfer pressure. This phenomenon continues with the formation of domain dimers or trimers that then keep on chaining with increased pressure. When PS is added to the monolayer, no such domains are observed. An increase in the size of the circular domains is obs erved, indicating the a ggregation of the PS homopolymer within the PS chains of the copolymer (Figure 4-13). 050100150200250300350 0 20 40 60 80 100 120 140 CountsDomain Area (nm2) 050100150200250300350 0 20 40 60 80 100 120 140 CountsDomain Area (nm2)

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45 Figure 4-13. Magnification of a single domain fo rmed for Blend 2 for a transfer pressure of 10mN/m (scale 150150nm) Blends of PSb -PEO and a PEO homopolymer In a similar way, blends of the c opolymer and a PEO homopolymer were transferred onto a mica substrate in order to study the evolutio n of the morphologies depending on the amount of PEO added. Results are shown in Figure 4-14. Figure 4-14. AFM images of the pure PS-b-PEO diblock copolymer and several blends for transfer pressures of 4 and 9 mN/m (scale 22m) = 4mN/m = 9mN/m Blend 1 Blend 2 Blend 3 Blend 4 Pure AmountofPEO

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46 At low pressure, we can observe the di sappearance of the hexagonal packing when increasing the amount of PEO. This beha vior can be compared to that of PS-b-PEO star copolymers studied by Logan,19 and Francis et al.,22 By increasing the hydrophilicity of the stars, they observed a decrease in the number of domains and an increase in the distance between each domain. While less uniform, this same effect appears in the blends as a result of PEO homopolymer aggregat ion to the PEO chains of the diblock copolymer. To remove the artifacts than could have been formed by spreading successively the pure copolymer and the hom opolymer, a mixed solution was made and this was spread as a comparison. The proporti ons were the same as the one used for Blend 2 and the film was transferred at a pressure of 9mN/m. The AFM images are shown in Figure 4-15. Figure 4-15. AFM images for Blend 2; (a) fr om successive spreading, and (b) from the mixed solution (scale 22m) Those experiments show that independent of the spreading technique, an increase in PEO is followed by the formation of longer chains than for the pure copolymer sample. A second effect of this incr ease is the apparition of a phase separation between pure layers of PEO surrounding clusters of micelles. a b

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47 In addition, the addition of a copolym er to a PEO homopolymer monolayer increases its stability and allows the formation of films at higher pres sures than that of a pure PEO monolayer which collapses at 10mN/m.16 The difference between the morphologies at high pressure for the pure diblock and for th e blends shows that the PEO homopolymer aggregates with the copolymer instead of dissolving into the aqueous subphase.

PAGE 57

48 CHAPTER 5 CONCLUSION When adding the homopolymers to the pur e diblock copolymer at the air-water interface, reproducible isotherms were obtained and displayed the three regions present in a pure diblock copolymer (pancake, pseudoplat eau, and brush). While the increase in the PS amount had an effect only on the condens ed area which varies linearly with the amount of PS added, by combining PEO and the copolymer, no change was observed in the condensed region of the isotherms. Th e pseudoplateau representing the biphasic region gets longer as the amount of PEO increases. This behavior has been observed for pure diblocks when increasing the size of the PEO block as well as for star copolymers.19,22 AFM images were taken and were consistent with the isotherms showing the three regions described previously. On one hand, the addition of PS to the copolymer monolayer inhibited the chaining of th e copolymer domains and enhanced the hydrophobic properties of the Langmuir-Blodgett film. On the other hand, combining the PS-b-PEO diblock copolymer with the PEO ho mopolymer also had an effect on the film morphology, increasing the chaining of the domains as well as favoring the phase separation between clusters of micelles and pure layer of PEO. Future work would include additional ch aracterization studies, such as using a different spreading technique in order to elim inate any artifacts that could be caused by experimental procedures. Different molecular weight PS and PEO could be used in order to determine the influence of the molecular weight on the system.

PAGE 58

49 LIST OF REFERENCES 1. Hadjichristidis, N.; Pispas, S.; Floudas, G. Block Copolymers: Synthetic Strategies, Physical Properties, and Applications; John Wiley & Sons, Inc.: Hoboken, NJ, 2003. 2. Seo, Y.; Paeng, K.; Park, S. Macromolecules 2001 34, 8735. 3. Niwa, M.; Hayashi, T.; Higashi, N. Langmuir 1990 6, 263. 4. Bijsterbosch, H. D.; de Haan, V. O.; de Graaf, A. W.; Mellema, M.; Leermakers, F. A. M.; Cohen Stuart, M. A.; van Well, A. A. Langmuir 1995 11, 4467. 5. Gonalves da Silva, A. M.; Filipe, E. J. M.; dOliveira, J. M. R.; Martinho, J. M. G. Langmuir 1996 12, 6547. 6. Pagac, E. S.; Prieve, D. C.; Solomentsev, Y.; Tilton, R. D. Langmuir 1997 13, 2993. 7. Mortensen, K.; Brown, W.; Almdal, K.; Alami, E.; Jada, A. Langmuir 1997 13, 3635. 8. Dewhurst, P. F.; Lovell, M. R.; Ri chards, R. W.; Webster, J. R. P. Macromolecules 1998 31, 7851. 9. Gragson, D. E.; Jensen, J. M.; Baker, S. M. Langmuir 1999 15, 6127. 10. Gonalves da Silva, A. M.; Simes Gamboa, A. L.; Martinho, J. M. G. Langmuir 1998 14, 5327. 11. Faur, M. C.; Bassereau, P.; Lee, L. T.; Menelle, A.; Lheveder, C. Macromolecules 1999 32, 8538. 12. Cox, J. K.; Yu, K.; Eisenberg, A.; Lennox, R. B. Phys. Chem. Chem. Phys. 1999 1, 4417. 13. Cox, J. K.; Yu, K.; Constantine, B.; Eisenberg, A.; Lennox, R. B. Langmuir 1999 15, 7714. 14. Devereaux, C. A.; Baker, S. M. Macromolecules 2002 35, 1921.

PAGE 59

50 15. Rivillon, S.; Muoz, M. G.; Monroy, F.; Ortega, F.; Rubio, R. G. Macromolecules 2003 36, 4068. 16. Shuler, R. L.; Zisman, W. A. J. Phys. Chem. 1970 74, 1523. 17. Kuzmenka, D. J.; Granick, S. Macromolecules 1988 21, 779. 18. Kumaki, J. Macromolecules 1988 21, 749. 19. Logan, J. L. PhD Dissertation; University of Florida: Gainesville, FL, 2005 20. Francis, R.; Skolnik, A. M.; Carino, S. R.; Logan, J. L.; Underhill, R. S.;Angot, S.; Taton, D.; Gnanou, Y.; Duran, R. S. Macromolecules 2002 35, 6483. 21. Francis, R.; Taton, D.; Logan, J. L.; Masse, P.; Gnanou, Y.; Duran, R. S. Macromolecules 2003 36, 8253. 22. Logan, J. L.; Masse, P.; Dorvel, B.; Skol nik, A. M.; Sheiko, S. S.; Francis, R.; Taton, D.; Gnanou, Y.; Duran, R. S. Langmuir 2005 21, 3424. 23. Peleshanko, S.; Jeong, J.; Gunawidjaja, R.; Tsukruk, V. V. Macromolecules 2004 37, 6511. 24. Baglioni, P.; Dei, L.; Gabrielli, G. Colloid & Polymer Sci.1986 264, 241. 25. Caminati, G.; Gabrielli, G.; Puggelli, M.; Ferroni, E. Colloid & Polymer Sci. 1989 267, 237. 26. Thibodeaux, A. F.; Rdler, U.; Shashidhar, R.; Duran, R. S. Macromolecules 1994 27, 784. 27. Malzert, A.; Boury, F.; Saulnier, P.; Benot, J. P.; Proust, J. E. Langmuir 2001 17, 7837. 28. Hottle, J. R.; Deng, J.; Kim, H.; Farmer-Creely, C. E.; Viers, B. D.; Esker, A. R. Langmuir, in press. 29. Hottle, J. R.; Kim, H.; Deng, J.; Farmer-Creely, C. E.; Viers, B. D.; Esker Macromolecules 2004 37, 4900. 30. Seo, Y.; Kim, K. S.; Galambos, A.; La mmertink, R. G. H. ; Vansco, G. J.; Sokolov, J.; Rafailovich, M. Nano Lett. 2004 4, 483. 31. Knoll, A.; Magerle, R.; Krausch, G. Macromolecules 2001 34, 4159. 32. Bodiguel, H.; Montes, H.; Fretigny, C. Rev. Sci. Instrum. 2004 75, 2529.

PAGE 60

51 33. Langmuir, I. J. Am. Chem. Soc. 1917 39, 1848. 34. Digital Instruments Nanoscope III Multimode Scanning Probe Microscope Instruction Manual.: Santa Barbara, CA, 2000 35. Konishi, T.; Yoshizaki, T.; Einaga, Y.; Yamakawa, H. Macromolecules 1990 23, 290. 36. Potemkin, I. I.; Kramarenko, E. Y.; K hokhlov, A. R.; Winkler, R. G.; Reineker, P.; Eibeck, P.; Spatz, J. P.; Mller, M. Langmuir 1999 15, 7290.

PAGE 61

52 BIOGRAPHICAL SKETCH Sophie Bernard was born on February 25, 1979, in Bordeaux, France. She graduated from the University of Bordeaux in June 2001 with her B.S. degree in physical chemistry. She then worked as a research assistant under the direction of Dr. Yves Gnanou, in the Laboratoire de Chimie des Polymres Organiques, University of Bordeaux. She received her M.S. degree in physical chemistry of polymers from the University of Bordeaux in July 2002. Sophie enrolled at the University of Florida in September 2002 in pursuit of a Ph.D. degree under the directi on of Dr. Randolph S. Duran. She is currently a Ph.D. candidate in the Chemistry Department of the University of Florida. Her academic interests include polymer and surface chem istry having biomedical applications.


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Title: Blends of a Polystyrene-Block-Poly(ethylene Oxide) Copolymer and Its Corresponding Homopolymers at the Air-Water Interface
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Copyright Date: 2008

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Title: Blends of a Polystyrene-Block-Poly(ethylene Oxide) Copolymer and Its Corresponding Homopolymers at the Air-Water Interface
Physical Description: Mixed Material
Copyright Date: 2008

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BLENDS OF A POLYSTYRENE-BLOCK-POLY(ETHYLENE OXIDE) COPOLYMER
AND ITS CORRESPONDING HOMOPOLYMERS AT THE AIR-WATER INTERFACE















By

SOPHIE BERNARD


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2006





























Copyright 2006

by

Sophie Bernard















ACKNOWLEDGMENTS

First, I would like to thank my advisor, Dr. Randolph S. Duran for his guidance

throughout the past few years. None of this work could have been executed without the

precious help of the Duran group members: Jorge Chavez, Henk Keiser, Brian Dorvel,

Danyell Wilson, Dr. Firouzeh Sabri, Eric Greeley, and Aleksa Jovanovic. I am highly

grateful to Thomas Joncheray and Dr. Jennifer Logan for our productive discussions and

for their support during this entire process.

My thanks also go to Dr. John R. Reynolds and Dr. B. Kenneth Wagener. Their

trust and encouragement have meant a lot to me. I would also like to thank all my past

and present peers at the "polymer floor" for their friendship and advice.

Last but not least, very special thanks go to my parents and my two brothers in

France for their unconditional love and support at all times.
















TABLE OF CONTENTS



A C K N O W L E D G M E N T S ................................................................................................. iii

LIST OF TABLES .............. ............................................. ......... vi

L IST O F FIG U R E S .... ...... ...................... ........................ .. ....... .............. vii

ABSTRACT .............. .......................................... ix

CHAPTER

1 INTRODUCTION ............... .................................. ................... 1

2 EXPERIM ENTAL TECHNIQUES....................................... ........................... 9

Langm uir Trough .................. ................................... ... ...... .. ...... ....
Isotherm Experim ents ........................................................ ... ...................... 11
L angm uir-B lodgett F ilm s ............................................ ......................................... 13
A tom ic Force M icroscopy ................................................ .............................. 14

3 ISOTHERM EXPERIMENTS ............................................................................18

E x p erim en tal .................................................................. ......... ....... .. ................18
Linear Polystyrene-block-Poly(ethylene oxide) (PS-b-PEO) Diblock Copolymers ..18
Blends of PS-b-PEO and a PS hom opolym er ................................... .....................21
P ancake R region (I) ......................... .. .................... ......... ........... 22
Pseudoplateau Region (II) ............................................................................23
C ondensed R egion (III) .............................................. ............................. 24
Blends of PS-b-PEO and PEO homopolymer ....................................... ............... 26
P ancake R region (I) ......................... .. .................... ......... ........... 28
Pseudoplateau Region (II) ............................................................................29
Condensed R egion (III) ......................................................... .............. 31

4 ATOMIC FORCE MICROSCOPY (AFM) EXPERIMENTS.............................. 33

E x p e rim e n ta l ......................................................................................................... 3 3
Q ualitative A naly sis.......... .................. ........................................................... 35
Linear Polystyrene-block-Poly(ethylene oxide) (PS-b-PEO) Diblock Copolymers ..37
Blends of PS-b-PEO and a PS homopolymer .............. ............. ..... ..............41



iv










Blends of PS-b-PEO and a PEO homopolymer................ ..................45

5 C O N C L U SIO N ......... ......................................................................... ......... ........4 8

LIST OF REFEREN CE S ............................................. ........................ ............... 49

B IO G R A PH IC A L SK E T C H ...................................................................... ..................52



















































v
















LIST OF TABLES


Table page

3-1. Characteristics of the PS-b-PEO sample investigated.....................................19

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

3-3. Width of the pseudoplateau for each blend .....................................................23

3-4. The mass ratio of PEO between the diblock copolymer and the homopolymer as
well as the apparent number of styrene units have been calculated for each
b len d ................................................................................ 2 7

3-5. Pancake areas extrapolated from the 7t-A isotherms ...............................................28

3-6. Area for the second transition (described in Figure 2.5) extrapolated for each
b le n d ............................................................................................. 2 9

3-7. Molar ratio of PEO from the homopolymer and the diblock copolymer as well as
the total number of EO units is given for each blend..............................................31
















LIST OF FIGURES


Figure page

1-1 Schematic representation of the pancake to brush transition for PS-b-PEO
cop oly m ers. ......................................................... ................ .. 4

1-2 The two conformatins for PEO at the air-water interface .......................................4

2-1 The original Langm uir balance ........................................... ........................... 9

2-2 Set up of a typical Langmuir trough .......................... ........................ ............. 10

2-3 Schem atic of the W ilhelm y plate ..................................... ........................ .......... 10

2-4 Schem atic rT-A isotherm .......................................................... ............... 12

2-5 Schematic rT-A isotherm showing the different areas that can be determined by
extrapolation ..................................... ............................... ........... 12

2-6 D different types of deposited LB film s............................................... ..................... 14

2-7 Optical system that detects cantilever deflection................. .............................15

2-8 AFM scanner tube containing the piezoelectric material and metal electrode.
The x, y, and z-directional components of the scanner are also indicated..............16

3-1 rT-A isotherm for the 32,500 g-mol1 PS-b-PEO copolymer.............................. 19

3-2. Several isotherms are shown, indicating the dependence of surface pressure on
the mean molecular area for different blend ratios........................................22

3-3 Condensed area per EO unit versus the number of styrene units.............................25

3-4 Condensed area versus the total number of styrene units .....................................25

3-5 Several isotherms are shown, showing the dependence of surface pressure on the
m ean m olecular area for different ..................................... ........................ ......... 27

3-6 Pancake area versus the total number of EO units................................................28

3-7 The area for the second transition depends linearly on the mole ratio of PEO
from the homopolymer over PEO from the PS-b-PEO .......................... .........30









3-8 The area for the second transition depends linearly on the apparent total number
of E O rep eat u n its .......................... ........ ................................................ 3 0

4-1 Sample height image and surface plot (scan area shown is 2x2 [tm) .....................34

4-2 Sam ple section im age (2x2 tm ) ............ ............... ... ...........................35

4-3 The software allows choosing a domain range by varying the minimum and
m axim um areas .......................................................................36

4-4 The software gives you a computed image representing the different domains
and the possible angles between domains in the presence of chaining ..................36

4-5 Error made by the computer can be corrected by the user................... ..............37

4-6 AFM images of the pure PS-b-PEO for several transfer pressures (scale 2x2[tm)..38

4-7 Schematic representation of surface micelles formed by A-b-B diblock
copolymers with A strongly adsorbed to the surface ............................................ 39

4-8 Model of PS-b-PEO absorbing at the air/water interface.............................39

4-9 Dependence of the number of molecules per domain on pressure........................40

4-10 AFM images of the pure diblock copolymer as well as two of the blends for
several transfer pressures (scale 2x2 tm ) ...................................... ............... 42

4-11 AFM images for the pure diblock copolymer and Blend 2 (transfer pressure of
4mN/m) as well as the distribution of the domain areas........................................43

4-12 Computed images for the pure diblock copolymer and Blend 2 (transfer pressure
of 10mN/m) as well as the distribution of the domain areas...............................44

4-13 Magnification of a single domain formed for Blend 2 for a transfer pressure of
lOm N /m (scale 150 x 50nm )........................................................ .......................... 45

4-14 AFM images of the pure PS-b-PEO diblock copolymer and several blends for
transfer pressures of 4 and 9 mN/m (scale 2x2tm) .............................................45

4-15 AFM images for Blend 2; (a) from successive spreading, and (b) from the mixed
solution(scale 2x2Cjm ) .............................................. .... ..... .. ........ .... 46















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

BLENDS OF A POLYSTYRENE-BLOCK-POLY(ETHYLENE OXIDE) COPOLYMER
AND ITS CORRESPONDING HOMOPOLYMERS
AT THE AIR-WATER INTERFACE

By

Sophie Bernard

May 2006

Chair: Randolph S. Duran
Major Department: Chemistry

The two-dimensional structure of a polystyrene-block-poly(ethylene oxide) (PS-b-

PEO) diblock copolymer at the air-water interface is studied using Langmuir-Blodgett

methods and atomic force microscopy (AFM). Measurements are also made for blends of

the PS-b-PEO copolymer with both a PS and a PEO homopolymer.

When increasing the amount of PS homopolymer, the isotherms do not show any

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

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

isotherms but no change is detected for the condensed area.

Each of the blends was subsequently studied by AFM. The data indicate a

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

the homopolymer added, a change in the chaining behavior of the copolymer is observed.

Also, when introducing more PEO, a phase separation between the layer of PEO and

clusters of two-dimensional micelles is detected.

ix














CHAPTER 1
INTRODUCTION

Amphiphilic copolymers are widely used because of their interfacial properties and

their ability to form molecular architectures at various interfaces. Amphiphilic diblock

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

structures when spread onto a water substrate, finding potential applications in coatings,

microelectronics, stabilization, and lubrification.1 Such copolymers are appropriate for

surface pressure studies involving Langmuir troughs. This technique provides insight on

the monolayer morphologies by controlling the surface density. For example, Seo et al. 2

showed the formation of stabilized two-dimensional micelles using polystyrene-b-

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

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

micelle exchange.

Polystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock copolymers of various

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

their properties in both the bulk and in solution. In addition, several groups have

described their behavior at the air-water interface.3-15 The choice of PEO as one of the

blocks renders the copolymer both biocompatible as well as amphiphilic. The inclusion

of PS provides an anchor at the air/water interface, preventing the PEO from eventually

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

compressed than a film composed simply of PEO homopolymer.









Without PS, PEO can still be spread at the air/water interface. Shuler and Zisman 16

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

surface density increases leading to a phase change reflecting change in the film's

structure. The lack of reversibility in the compression and expansion experiments is

explained by a structural change in the polymer molecule. A modification in

conformation was given to explain the different monomer area observed in the 7t-A

isotherms. Kuzmenka and Granick 17 performed the same type of experiment for a wide

range of PEO molecular weights. They determined that for PEO chains beyond molecular

weights of 100,000g.mol-1, the film attains a constant equilibrium surface pressure. This

behavior was explained by the difficulty of a high molecular weight PEO to pass into an

aqueous substrate due to the amphiphilic character of the EO monomer. Lower molecular

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

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

surface, analogous to soluble surfactants.

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

by only one group. Being hydrophobic, PS is not expected to form any type of

morphology when spread onto a water subphase. However, Kumaki 18 detected a change

in surface pressure when a dilute solution of PS (2.0x10-5g.mL-1) was spread at the air-

water interface. Even if surface pressure mainly represents mechanical force due to the

compression, stable monomolecular particles were observed for molecular weights higher

than 50,000g.mol-1.

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

or solid-water interface, demonstrating the formation of novel nanostructures. Goncalves









da Silva et al. 5,10 described the utility of diblock amphiphilic copolymers in testing the

scaling properties of grafted polymers. They presented 7t-A isotherms that show several

regions referred to as pancake, quasi-brush, and brush stages. Within these regions,

different morphologies of surface micelles and further micellar aggregates were observed

by transmission electron microscopy (TEM) and atomic force microscopy (AFM)

depending on the balance between block sizes. At the air-water interface, copolymers

behave similarly to copolymers in bulk dispersion. Static light scattering proved that PS-

b-PEO copolymers aggregate spontaneously into micelles over the critical micellar

concentration (CMC). The isotherm regions compare to those observed for solution CMC

values: (1) below the CMC, surface micellization is observed; (2) at the CMC, the PEO

segments are pushed into the substrate in order to decrease the surface area per molecule;

and (3) above the CMC, PS-rich regions exist in between spaces formed by the PEO

chains.

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

example, Goncalves da Silva et al. 5,10 investigated the effect of the PEO block size on the

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

used as an anchor to prevent the PEO from dissolving completely into the water

substrate. Upon compression, they observed a transition of the PEO blocks, from a two-

dimensional structure floating on the water, to a three-dimensional structure when the

PEO streched into the water. The first structure is the one previously termed "pancake"

whereas the second was identified as "brush" (Figure 1-1). The plateau displayed in the

7t-A isotherms is an indication of the transition between these two states with its span

dependent on the relative sizes of the two blocks.















Wa:ert M


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






(b)




U Hydrogen
Oy'gen
Carbon
Figure 1-2. The two conformatins for PEO at the air-water interface proposed by Shuler
and Zisman.16

Devereaux and Baker14 conducted 7t-A isotherms experiments of PS-b-PEO

copolymers with varying PEO chain lengths. One copolymer contained 15% of PEO

whereas the other had only 7%. The copolymer with the longest PEO block displayed a

plateau around 10mN.m-1, indicating that the copolymer spreads well at the interface. In

contrast, the copolymer containing only 7% of PEO has no plateau, supporting the theory

that PS chains interfere with the PEO blocks upon compression.

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

described previously, Cox et al. 12,13 provide a different model to explain the shape of the

7t-A isotherm for a PS-b-PEO copolymer. Whereas in the first model the PEO passes into

the aqueous subphase, the Cox model suggests a dehydration of the PEO followed by a









conformational change, similar to that previously described by Shuler and Zisman16 for

homopolymer PEO. As shown in Figure 1-2, conformation (a), more flexible, is

compressed into conformation (b) more compact and sterically hindered. This

transformation can be explained by an increase in the intramolecular forces in the second

conformation.

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

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

copolymer architectures. Logan,19 Logan et al.,20 and Francis et al.,21'22 for example,

investigated the behavior of a three-arm star amphiphilic copolymer, PEO3-b-PS3, at the

air-water interface. Peleshanko et al.23 observed formation of morphologies when

spreading an amphiphilic heteroarm PEO-b-PSm. The AFM images showed that the

formation of different morphologies depends on the pressure used during the transfer.

The unusual properties of those architectures allow the formation of more stable

morphologies than those formed using regular linear copolymers.

While different architectures can result in different surface film behavior, the

synthesis of such systems can be difficult and time-consuming. In an effort to acquire

new properties without the required synthesis, surface films of blended polymers have

also been investigated. In the 1980s, the Gabrielli group24'25 examined the behavior of

numerous mixtures of polymers and low molecular weight materials as binary systems

with different degrees of incompatibility. They also quantified the determination of the

two-component monolayer miscibility by observing the 7t-A isotherms of their two-

dimensional blend. Thibodeaux et al.26 studied mixtures of a liquid crystalline copolymer

with its corresponding monomer. The films formed by the blend monolayer appeared to









be more condensed than the pure copolymer films, proving that two-dimensional

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

formation of more stable films.

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

into a single film. Malzert et al.27 developed a suitable model for understanding the

interactions between polymers by mixing poly(ethylene glycol) and poly(lactide-co-

glycolide), whereas Hottle et al.28'29 studied blends of amphiphilic poly(dimethylsiloxane)

and trisilanolisobutyl-POSS. More recently, Seo et al.30 investigated the structures

formed at the air-water interface by blending poly(styrene-b-ferrocenyl silane) (PS-b-FS)

and poly(styrene-b-2-vinyl pyridine) (PS-b-P2VP). While neither of those copolymers

assembles when spread separately at the air-water interface, their blends formed ordered

structures which appear to be more versatile, a promising development in the fabrication

of polymeric templates for lithography.

The most commonly used technique to observe the morphologies formed by

compressing a monolayer at a certain pressure is AFM. For soft samples such as polymer

films, an appropriate AFM technique is tapping mode. Here, the cantilever is excited to

an oscillation near its resonance frequency. The interactions between the tip and the

sample give a deviation in the oscillation amplitude, recording the changes in the sample.

This mode has been employed for most polymer samples because of its ability to

investigate soft materials without further staining and with little or no tip-induced damage

or morphology changes. However, Knoll et al.31 highlighted the limitations of this

technique, finding that the apparition of artifacts was related to tip-sample interactions.

Nevertheless, AFM provides valuable information on film morphology. Bodiguel et al.,32









for example, introduced a method for determining the dependence of the phase signal on

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

adhesive and represented the local elastic properties of the sample.

In general, the surface behavior of amphiphilic diblock copolymers is readily

examined through Langmuir techniques. Methods involving film compression and

transfer provide both quantitative and qualitative results indicating how surfactant

responds to pressure. PS-b-PEO proves to be particularly of interest due to the

biocompatibility of PEO. While different architectures of this copolymer have been

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

characteristics may yet be attained trough blending, both with PS and PEO

homopolymers. This work focuses on the latter part, examining the effects of adding

either PS or PEO to a linear PS-b-PEO chain.

The first chapter thus provides an introduction to this study. In the second chapter,

a brief review of the techniques used is given, covering Langmuir monolayers, Langmuir-

Blodgett films, and atomic force microscopy (AFM).

The third chapter describes the behavior of a linear PS-b-PEO copolymer at the air-

water interface and the formation of ordered structures at different pressures. Blends of

the linear copolymer and its corresponding homopolymers-polystyrene (PS) and

Poly(ethylene oxide) (PEO)-are also examined to determine the effect of each

homopolymer on the interfacial behavior of PS-b-PEO.

AFM studies of both the linear chain and its blends appear in the fourth chapter,

providing insight about the shape and the size of the aggregates. A computer program

designed by our group helped determine the properties of the different morphologies,






8


such as areas, angles, and aggregation numbers. Those results were compared both to

previous and new results involving the self-assembly of related star copolymers.















CHAPTER 2
EXPERIMENTAL TECHNIQUES

Any study involving Langmuir monolayers requires the use of a Langmuir trough

set-up for the preparation of Langmuir-Blodgett films. Irving Langmuir was one of the

principal initial scientists to observe the formation of monolayers when a surfactant is

spread onto water, developing the Langmuir trough technique (figure 2-1). With this

apparatus, he studied floating monolayers on water in the late 1910s and early 1920s.

Several years later, Katherine Blodgett gave the first detailed description of sequential

monolayer transfer onto solid supports.






C A


T--





Figure 2-1. The original Langmuir balance as designed by I. Langmuir33

Langmuir Trough

A typical Langmuir trough (figure 2-2) is composed of the trough itself, two

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

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

subphase and suspended from a balance. The force acting on the plate is directly

proportional to the surface tension of the liquid.









Dipper Electrobalance




Substrate Wilhelmy Plate

Barrier BarrierTrough





Heat Exchanger

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

The plate is usually very thin and made of platinum, but glass, quartz, mica, and

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

F = ppglwt + 2y(t + w)cosO pigtwh 2-1

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

represents the gravitational constant, y is the subphase surface tension, and 0 is the

contact angle of the liquid on the solid plate. The plate is also described by its thickness

(t), width (w), and length (1) (see figure 2-3).







w t
Figure 2-3. Schematic of the Wilhelmy plate

When measuring the change in h for a constant applied force:

pigtwAh = 2Ay(t + w). 2-2

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

n = Ay = -pigtwAh/2(t + w). 2-3









when measuring the change in F for a stationary plate between a clean surface and the

same surface with a monolayer present. If the plate is completely wetted by the liquid

(cosO = 1), the surface pressure is then obtained from the following equations:

AF = 2Ay(t + w) 2-4

n = Ay = -AF/2(t + w) 2-5

For the Wilhelmy method, the thickness of the plate used is small, giving t << w.

So,

I = -AF/2w 2-6

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

sensitivity (5 x 10-2 mN.m-1).

Isotherm Experiments

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

to each molecule provides insight into monolayer properties. Such experiments are

carried out at constant temperature using a heat exchanger, and are known as isotherm

experiments. The data are recorded by compressing the film at a constant rate while

monitoring the surface pressure (figure 2-4). Distinct regions can be observed defining

the different phases of the monolayer. Various monolayer states can be observed,

depending on the hydrocarbon chain length; in fact, an increase in chain length increases

the interactions between the chains, leading to a more condensed 7t-A isotherm. For a

minimal compression, the monolayer exists in the gaseous phase (G). While compressing,

the monolayer undergoes phase transitions to the liquid-expanded state (LI), followed by

the liquid-condensed state (L2), and finally the solid state (S). If the monolayer is further

compressed, it will collapse into three-dimensional structures.


















(mN m')


\ x\ C
0.20 0.25 050 10.0
o (nmt moLecul )

Figure 2-4. Schematic 7t-A isotherm (picture borrowed from the website
http://www.ksvinc.com/LB.htm )

While these various areas can often be found in small surfactant molecules, diblock

copolymers typically have fewer regions. An example is shown in Figure 2-5. Here, area

extrapolations quantify the isotherm, allowing the surface behavior of different

copolymers and their blends to be compared.







\

S Tararnsition I


'0 APancak:e

MMA (nm2/molecule)
Figure 2-5. Schematic 7t-A isotherm showing the different areas that can be determined
by extrapolation









Langmuir-Blodgett Films

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

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

substrate into the subphase, allowing the adsorption of the monolayer. The surface

pressure is maintained constant by a computer controlled feedback system between the

electrobalance measuring the surface pressure and the barrier moving mechanism.

Depending on the number of dippings, several successive monolayers can be deposited

onto the solid substrate.

Numerous substrates have been used. Mica is commonly preferred in LB film

transfer due to its low cost, facile cleaning, and easy preparation. However, it possesses a

water layer that may affect the film transfer. Other substrates such as silicon wafers can

be used; treatment with chromic sulfuric acid renders them highly hydrophilic. Other

materials can be used as hydrophobic substrates, including graphite and silanized silicon

dioxide.

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

The upward pass of the substrate through the subphase is known as an upstroke while the

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

(figure 2-6). The X-type deposition can be done by a downstroke whereas Z-type refers

to an upstroke. The Y-type, the most common, is produced by an upstroke followed by a

downstroke. Intermediate structures can sometimes be observed for some LB multilayers

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









Substrate







X-Type Y-Type Z-Type


Figure 2-6 Different types of deposited LB films (borrowed from Jennifer Logan's
dissertation9)

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

as atomic force microscopy (AFM) or transmission electron microscopy (TEM).

Atomic Force Microscopy

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

allows the study of conductive samples, AFM can be applied to both conductors and

insulators. The instrument consists of a tip at the end of a cantilever, which bends in

response to the force between the tip and the sample (figure 2-7).

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

force between the tip and the sample can be found:

F = -kx 2.7

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









Sald Sae Laasr tDHdp

--B /






Spild Phludkdie Deiecrla



Carve"r TIp
Figure 2-7. Optical system that detects cantilever deflection (Figure adapted from Digital
Instruments' Training Notebook34)

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

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

the desired value of deflection. The voltage needed to restore the desired value of

deflection is a measure of height of features on the sample surface. This mode was

quickly forgotten for polymer studies because of excessive tracking forces applied by the

probe to the sample.

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

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

forces acting between the tip and the sample are detected, and topographic images are

constructed by scanning the tip above the surface. This technique was found to be

inapplicable to polymer samples. In general, the fluid contaminant layer existing on the

sample is substantially thicker than the range of the Van der Waals force gradient and,

therefore, all attempts to image the true surface with non-contact AFM fail as the

oscillating probe becomes trapped in the fluid layer.

Later, a third method was developed in order to study softer samples. This mode,

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









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

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

the cantilever oscillation is necessarily reduced due to energy loss caused by the tip

contacting the surface. The reduction in oscillation amplitude is used to identify and

measure surface features. AFM involves scanners made from piezoelectric material, a

substance which proportionally contracts and expands, depending on an applied voltage.

If a positive voltage elongates the scanner, a negative voltage contracts it. The scanner is

made of a piezoelectric material surrounded by electrodes which control the applied

voltage. As scanning occurs in three dimensions, a scanner tube contains three piezo

electrodes for the X, Y, and Z directions (Fig. 2-8).



Ele smde
Z Alwerial




z-i wna x
Y






Figure 2-8. AFM scanner tube containing the piezoelectric material and metal electrode.
The x, y, and z-directional components of the scanner are also indicated.
(Figure adapted from Digital Instruments' Training Notebook.34)

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

and with this system three different scanners can be used depending on the sample

studied. They differ on the scanning size and resolution. For example the J-scanners can

scan images up to 125 rpm, whereas E-scanners scan smaller sizes of 10 am or less.






17


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

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

the facile transfer of surface films onto solid substrates. The results of such analysis will

be presented in the subsequent chapters.














CHAPTER 3
ISOTHERM EXPERIMENTS

Experimental

The PS-b-PEO diblock copolymers as well as the blends were characterized as

surface films at the air/water interface. The copolymer and each of the homopolymers

were dissolved in chloroform at a concentration of lmg/mL. Using a Hamilton syringe,

the solution was then spread dropwise across a layer of Millipore filtered water (0 > 18.2

MQO cm-1) in a Teflon TM Langmuir trough system (KSV Ltd., Finland) After waiting

30 minutes to allow for complete evaporation of the chloroform, the surface film was

compressed at 10 mm-min-1 at 250C. Compressing the film generates an isotherm of

surface pressure (n) vs. mean molecular area (MMA). The latter represents the average

area each molecule occupies at the air/water interface

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

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

interface behavior of amphiphilic compounds, due to the biocompatibility of the PEO

block and the low cost and availability of the PS block. Moreover, they have been widely

studied and found to form stable, condensed surface films.3-15

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

at ca. 10mN/m and is shown in Fig. 3-1. The observed plateau results from the

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

collapse pressure of a PEO homopolymer.16












70-

60-


E
50-



30-



10-
S20


0 20 40 60 80 100 120 140
Mean Molecular Area (nm2/molecule)



Figure 3-1. 7t-A isotherm for the 32,500 g-mol-1 PS-b-PEO copolymer

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

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

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

large molecular areas, the surface film is expanded (Region I); this is usually called the

"pancake" region due to the shape the PEO units form on the water surface.


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

MW PEO PS
(g/mol) wt% wt% Polydispersity MWPEO MWps NPEO Nps
32,500 32 68 1.05 10,500 22,000 238 211

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

8 to 10 mN/m. Kuzmenka and Granick17 studied the behavior of PEO homopolymers at

the air-water interface with varying molecular weights. They observed a constant

equilibrium spreading pressure for polymers having a molecular weight beyond

100,000g.mol-1. The pseudoplateau detected in the case of our copolymer is in the same









range of the collapse pressure of a PEO homopolymer and corresponds to the hydration

and desorption of these chains from the surface and into the subphase.

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

was considered by Devereaux and Baker14 They studied two PS-b-PEO copolymers

containing different masses of PEO. The 7% PEO copolymer had no plateau whereas the

15% PEO did. This observation was explained by the long PS interfering with the PEO

blocks, preventing the PEO from stretching into the aqueous subphase. Our results are in

agreement with this theory showing a plateau for a copolymer containing 32% PEO.

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

likely exist as PEO films with globules of PS on top. Region II, however, represents a

biphasic phase where aggregates and single polymer domains coexist. The fact that the

pseudoplateau occurs within the same pressure range as the collapse region of PEO

homopolymer illustrates the significant influence PEO has on the copolymer surface film.

Bijsterbosh et al.4 and Goncalves da Silva et al.5'10 both demonstrated the existence of a

pseudo first-order transition from pancake-like structure to that of a brush upon

compression of a series of PS-b-PEO copolymers containing a constant PS length and

varying amounts of PEO. While this model is prevalent in the literature, Cox et al.12'13

provided a new interpretation for the presence of the pseudoplateau assuming that the

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

PEO instead undergoes a dehydration process and a conformational change upon

compression.

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

pseudoplateau and shows a sharp increase in surface pressure, indicating the formation of









more rigid films. Here, the PS block serves as an anchor, keeping the PEO at the interface

and allowing the films to be compressed to higher surface pressures. Without the PS,

PEO would dissolve into the aqueous subphase at pressures beyond the plateau. In

examining Region III, Bijsterbosh et al.4 and Goncalves da Silva et al.5'10 studied a series

of copolymers with varying PEO lengths and a constant PS block. While Region III

typically reflects PS, they found that the copolymer interfacial behavior at high pressures

depends slightly on the size of the PEO block.

Blends of PS-b-PEO and a PS homopolymer

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

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

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

molecular weight of the PS block in the copolymer. Different ratios of copolymer and

homopolymer were studied in order to determine the impact on the formation of

Langmuir monolayers Langmuir-Blodgett films.

The mixed monolayers were performed by separately spreading solutions of the PS

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

monolayer was symmetrically compressed by the two movable barriers. 7t-A isotherms

were recorded for several mole ratios of PS within the homopolymer and copolymer

(Table 3-2).

Fig. 3-2 shows the isotherms data for different copolymer/homopolymer ratios. For

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

in the previous section were observed.










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

Blend # 1 2 3 4 5 6 7


Mass % of PS 70.2 72.3 75.8 78.6 80.7 84.0 86.3

Mole ratio of PS
0.138 0.275 0.551 0.826 1.102 1.653 2.204
(homopolymer/copolymer)

NPS,TOT 236 259 307 355 403 499 595


- Pure
- Blnd 1
- Blend 2
-7- Blend 3
--- Blend 4
- Blend 5
--- Blend 6
-- Blend 7


MMA (nm 2An ole cule)


Figure 3-2. Several isotherms are shown, indicating the dependence of surface pressure
on the mean molecular area for different blend ratios

Pancake Region (I)

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

expressed by its extrapolated area, Ap (Figure 2-5). The values for every blend remain

constant with varying the amount of PS (average Ap = 89.7 nm2). This is in agreement









with a film of PEO with globules of PS on top of it where increasing the amount of PS

will not change the area occupied at the interface by the PEO.

Faure et al.11 observed the same behavior for pure diblock copolymers at the air-

water interface and showed that at low coverage, the interaction between the EO

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

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

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

behavior of this region. Logan,19 Logan et al.,20 and Francis et al.,21'22 observed a similar

trend for star copolymers of PS-b-PEO in which the pancake area did not depend on the

number of PS segments. The pancake area per EO monomer (0.38 nm2) is in reasonable

agreement to the one found by Logan et al. for star copolymers (0.33 nm2) and to the that

determined for linear PS-b-PEO by Goncalves da Silva et al. (0.27 or 0.31 nm2)5'10 and

Bijsterbosch et al.4 (0.31 nm2).

Pseudoplateau Region (II)

The pseudo-plateau observed in Section 3-2 for a pure linear PS-b-PEO copolymer

is observed for all blends and remains constant with varying the total mass of PS.

Table 3-3. Width of the pseudoplateau for each blend
Blend # Pure 1 2 3 4 5 6 7

AAp (nm2) 22.8 23.0 22.2 21.1 22.3 22.5 20.5 21.1

The width (AAp) of the pseudoplateau can be estimated as the difference between

ATransition 1 and ATransition 2 (Table 3-3, Figure 2-5). The value of AAp remains constant for

every blend (average AAp = 21.9 nm2). Due to its phase transition nature, region II is

believed to represent a biphasic region. The phase transition is mostly due to the

reorganization of the PEO chains from a pancake to a brush conformation and therefore a









change in the amount of PS does not have any effect on the width of the pseudoplateau.

In this region, the EO repeat unit occupies 9.2 A2 which is smaller than the value found

by Logan et al.22'19 (13.3 A2) for star copolymers.

Condensed Region (III)

In Table 3-2, the theoretical area (Ao) that a compact surface film would occupy at

zero pressure was determined for each blend. In agreement with our expectations, the

area increases with an increasing mass of PS.

The condensed area, representing mostly the behavior of the PS chains, varies

linearly with the total mass of PS chains. This behavior can be compared to the behavior

of copolymers presenting PS chains of high molecular weights. Cox et al.6 studied several

PS-b-PEO copolymers with varying PS molecular weights, observing a variation in the

Ao values. The increase of Ao with increasing PS can be explained by the aggregation of

the PS homopolymer with the PS chains of the copolymer.

To compare our results with those from copolymers of longer PS blocks, a

normalization of the total number of styrene units was obtained by using the following

equation.

> N IT i PS,Homopolymer
NpS,TOT NPS,Dblock + PS,Homopolymer "
lPS,Dlblocks

With Nps,Diblock and Nps, Homopolymer being the number of styrene repeat units in the

PS-b-PEO diblock copolymer and the PS homopolymer respectively. nps, Hompolymer and

nPS,Diblocks represent the number of moles of PS in the homopolymer and the copolymer.

When plotting the condensed area per EO unit versus the apparent number of styrene

units (Figure 3-3), a linear dependence can be observed (R2 = 0.9932) with a trendline of

y = 0.0001 x + 0.0753.












0.16
0.15
0.14
0.13
0.12
0.11
0.1
0.09
0.08


0 100 200 300 400 500 600 700
Total number of styrene units



Figure 3-3. Condensed area per EO unit versus the number of styrene units

The positive y-intercept shows us that even without any PS present in the

monolayer, the PEO occupies 7.5 A2/EO units. This value is a lot smaller than the one

observed for star copolymers by Logan et al. (16 A2/EO). 22,19


0 100 200 300 400 500
Total number of styrene units


600 700


Figure 3-4. Condensed area versus the total number of styrene units

The collapse area plotted vs. the total number of styrene units follows the trendline

y = 0.0288 x + 17.914. The area per styrene unit obtain from the slope (2.9 A2) is smaller

than the one J. Logan described for the behavior of star copolymers of PS-b-PEO at the


y = 0.0001x +0.0753
R2 = 0.9932


y = 0.0288x + 17.914
R2 = 0.9932









air-water interface (ranging from 6.2-8.3A2, depending on architecture).22 She reports the

results for (PE026)8-(PS42)8 (an 8 arm PS-b-PEO star copolymer with each arm

containing 26 EO units and 42 Styrene units) which has a total of 336 repeat units of PS.

For Blend 4, the PS homopolymer occupies an area equal to 5nm2. This value is in the

same range as the area per styrene value found in the literature for an atactic PS in the

bulk, calculated from the radius of gyration (38A).35 The PS, not covalently bound to the

PEO, tends to adopt a random coil conformation less compact than the conformation

produced by the PS segments of the copolymers. This copolymer can be compared to

Blend 4 which presents a total of repeat units of 355 for the PS (using the formula

described previously). J. Logan obtained a value of 28.6 nm2 which is similar to the value

obtained for Blend 4.

Blends of PS-b-PEO and PEO homopolymer

Contrary to PS, PEO is an amphiphilic polymer forming monolayers at the air-

water interface. The addition of a PS block as an anchor keeps the PEO from going into

the water subphase. This also allows the formation of more compact films by

compressing at higher pressures. The effect that unencumbered PEO has on such films is

examined in this part of the discussion where blends of the copolymer and homopolymer

PEO were studied (Table 3-4).











Table 3-4. The mass ratio of PEO between the diblock copolymer and the homopolymer
as well as the apparent number of styrene units have been calculated for each blend.
Blend 1 2 3 4


92.4 95.9 97.9 98.9
Mass % of PEO

Mole ratio of PEO 0.025 0.05 0.101 0.202
(homopolymer/copolymer)
295 352 467 697
NPEO,TOT


Fig. 3-5 shows the isotherm data for several blends of the 32,500g.mol-1 PS-b-PEO

copolymer and a 100,000 g.mol1 PEO homopolymer. The choice of the homopolymer

was made in agreement with the results published by Kuzmenka et Granick17 on PEO

homopolymers who showed a constant equilibrium surface pressure for PEO with

molecular weights higher than 100,000g.mol1.


E
z
E
a)
_ 20
1)
U)

0
o_
10
()


-- Blend 1
--- Blend 2
Blend 3
-o- Blend 4
Pure PS-b-PEO


0 200 400 600 800 1000 1200 1400
MMA (nm2/molecule)



Figure 3-5. Several isotherms are shown, showing the dependence of surface pressure on
the mean molecular area for different










Pancake Region (I)

Similar analysis was done for the homopolymer PEO blends as that seen for the PS

ones. As with the pure copolymer, the resulting isotherms displayed all three regions. Ap

was obtained for each blend and for the pure PS-b-PEO from the xt-A isotherms (Table 3-


5).

Table 3-5. Pancake areas extra olated from the xt-A isotherms
Blend # Pure 1 2 3 4


Ap (nm2) 86 153 244 352 610



The pancake area depends linearly on the total number of ethylene oxide units (R2

= 0.9968) with a trendline of y = 1.1297 x 173.95. The area obtained from the slope

(1.13 nm2) is significantly higher than the one observed by Sauer et al.34 for a PEO

homopolymer (0.40-0.48 nm2). Goncalves da Silva et al.5'10 recorded a smaller area for

PS-b-PEO diblock copolymers (0.27 and 0.31 nm2). This can be explained by the fact

that the PEO chains from the homopolymer pack less closely when in the presence of the

PS-b-PEO diblock copolymer.


700

600 y= 1.1297x- 173.95
500 R2= 0.9968

400

300
200
100

0 100 200 300 400 500 600 700 800
Total number of EO units

Figure 3-6. Pancake area versus the total number of EO units









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

Blend # Pure 1 2 3 4

ATransition 2 (nm2) 46 72 112 149 268



We can also observe a negative y-intercept indicating that all the EO units are not

at the interface. In such a case, a pancake area equal to zero should correspond to zero

EO units. Even with a negligible effect on the pancake area, the PS units may trap some

of the PEO, leading to a lower apparent number of PEO units.

Pseudoplateau Region (II)

The addition of PEO, however, has an effect on the shape of the plateau observed

for a pressure around 10mN/m. The more PEO is added to the monolayer, the longer the

biphasic region becomes. To illustrate this point, ATransition 2 (Figure 2-5) was recorded for

each blend as well as for the pure diblock. The results are given in Table 3-6.

A graph of ATransition 2 vs. the ratio of number of moles of homopolymer over the

number of moles of diblock shows a linear dependence (R2 = 0.994) with a trendline of

y = 1082.9 x + 47.532 (Figure 3-7). To be able to compare those results to the one

published previously for pure diblocks or star copolymers, an identical formula as the one

used in the previous part was developed. Presenting a constant ATransition 1 for every blend,

an increase in ATransition 2 indicates the presence of a bigger plateau area.

N +N X PEO,Homopolymer
NpEO,TOT PEO,Dzblock PEO,Homopolymer X
nPEO,Dblock

The values calculated using this formula, are reported in Table 3-7.











A linear dependence (R2 = 0.9942) was observed when the area for the second


transition was plotted vs. the total number of repeat units of PEO, yielding a trendline of


y = 0.4769 x 66.032 (Figure 3-8).


300

250

w 200

E 150 y=1082.9x+47.532
R2 = 0.994
100

50

0
0 0.05 0.1 0.15 0.2
moles of PEO in the homopolymer/moles of PEO in the
diblock copolymer


Figure 3-7. The area for the second transition depends linearly on the mole ratio of PEO
from the homopolymer over PEO from the PS-b-PEO


300

S250

o 200
E
E 150
-

S100

S50
0
0


0 100 200 300 400 500 600
Total number of EO repeat units


700 800


Figure 3-8. The area for the second transition depends linearly on the apparent total
number of EO repeat units

These observations compare to those seen by Faure et al.11 They studied the phase


transitions in monolayers of PS-b-PEO copolymer at the air-water interface for different


PEO block sizes. Faure et al. observed an increase in the length of the pseudoplateau as


y = 0.4769x -66.032
R2= 0.9942









the number of PEO units increases. The transition from pancake to brush becomes more

and more first order as they increase the PEO segment size

Table 3-7. Molar ratio of PEO from the homopolymer and the diblock copolymer as well
as the total number of EO units is given for each blend.
Blend # Pure 1 2 3 4

nHomo/nBlock 0 0.025 0.05 0.101 0.202

NPEO,TOT 238 295 352 467 697


In addition, one of the diblock copolymers they studied consisted of 31 repeat units

of PS and 700 of PEO, a PEO amount similar to that of Blend 4. The Faure copolymer

demonstrates a 7t-A isotherm with an almost flat plateau, confirming the first order

transition of the copolymer. Similarly, Blend 4 displays a plateau representing a strong

indication of a first order transition. By adding PEO homopolymer to our monolayer, we

have been able to enhance the copolymer properties without having to increase the PEO

block length trough time-consuming synthetic techniques.

Condensed Region (III)

This third region appears at higher surface pressures beyond the pseudoplateau. As

demonstrated by Shuler and Zisman,16 such a region does not exist for a PEO

homopolymer, as no anchor exists to prevent PEO from completely immersing in the

water subphase. This region depends only on the length of the PS blocks and not on PEO,

as demonstrated by the 7t-A isotherms of the different blends in Figure 3-5. Ao remains

the same regardless of PEO added (23.7 nm2).

By blending a PS-b-PEO diblock copolymer with its corresponding homopolymers,

we were able to mimic linear chain behavior by manipulating PS and PEO quantities. On

one hand, the addition of PS has proven to have the same effect on the copolymer









behavior than increasing the PS block size. On the other hand, raising the amount of PEO

only had an effect on the biphasic region of the isotherm.

While this technique could be a good alternative to time-consuming synthetic

techniques and expensive samples purchase, experiments still need to be performed with

various molecular weight homopolymers as well as hysteresis data in order to better

understand the aggregation behavior of those films.

Additional analysis continues in chapter 4 in which the blends are transferred as

Langmuir-Blodgett films and examined through atomic force microscopy (AFM).














CHAPTER 4
ATOMIC FORCE MICROSCOPY (AFM) EXPERIMENTS

AFM is a technique that provides the opportunity to study surface morphology and

structure at the submicron scale. By investigating transferred Langmuir-Blodgett films,

AFM can give insight into the behavior of the copolymer blends at various pressures,

providing both quantitative and qualitative results. Such data helps demonstrate the

degree of interaction between the copolymer and homopolymers

Experimental

Surface films of the linear copolymer and the blends were transferred onto freshly

cleaved mica at various pressures (250C). The desired surface pressure was attained at

rates of +10 mm-min-1. Once the film had equilibrated at a constant 7t for 30 minutes, the

mica was then pulled out at a rate of 1 mm-min-1. The transferred film was air-dried in a

dust-free environment for 24 hours and subsequently scanned in tapping mode with a

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

(Nanosensor dimensions: T= 3.8-4.5 |tm, W= 26-27 |tm, L = 128 [tm). The

hydrophilicity of the substrate allows us to consider the hydrophilic PEO to be attached to

the mica whereas the hydrophobic PS occupies a higher layer. By consequence, the PEO

is represented by the darker (lower) areas whereas the PS exists as the brighter (higher)

domains. Tapping mode was used, giving a better image of a polymer sample without

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

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

amplitude of oscillation is decreased from its set-point value. This decrease is noted by










the sensor and the tip is moved up away from the sample to re-attain the set-point

amplitude. A similar behavior happens when the tip moves past the feature. A

topographical map of the sample can then be recorded.


u of _'jt_ 5-.
fIy'r sam pl 1




.Ugi Dn w










Figure 4-1. Sample height image and surface plot (scan area shown is 2x2 jtm)

The AFM software contains several functions for image analysis. One method

represents the three-dimensional surface plot of the imaged sample, as shown in

Figure 4-1. The color shading is a representation of the height of the sample (7.3nm for

Figure 4-1).

Precise height data can be obtained for given domains through section analysis.

This technique is illustrated in Figure 4-2. A line is traced across the domain region of

interest, giving a cross-sectional view of the sample. In this example, the height

difference between the two marked domains is 1.2nm and the difference between the

domain at the left and the PEO surface (brown) is 4.5nm.









nm











I 0 1.00 2.00
pm

Figure 4-2. Sample section image (2x2 pm)

Qualitative Analysis

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

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

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

determined in order to better understand the aggregation behavior of the copolymer

blends. This program allows a domain size range to be chosen in which the values of the

minimum and the maximum can be varied (Figure 4-3).

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

using a visual aid. Another attribute of this program, is that it counts the number of

domains present in one chain as opposed to considering the chain as a single domain. If

such a mistake is made, however, the program can be manually manipulated by the user

to define domain separation and number. While this feature gives a better approximation

of the shape and number of domains, the resulting disadvantage is lower user efficiency,

but significantly higher accuracy.











lut I -exlu I ,fletn I -m f I -amutl I
.A gtOaclu I .......


I "-,"- ---- -


Domains selected
by the chosen area
range


Figure 4-3. The software allows choosing a domain range by varying the minimum and
maximum areas
















Original image Computed image

Figure 4-4. The software gives you a computed image representing the different domains
and the possible angles between domains in the presence of chaining

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

representing all the domains present with the different chaining and angles for each

domain (Figure 4-4). Computer errors can occur, giving wrong angles and poorly defined


'j X









aggregates, in which case the user can, by clicking on the domain, redo the separations

and redefine the domain (Figure 4-5).


Figure 4-5. Error made by the computer can be corrected by the user

The software allows domain populations to be chosen and analysis error to be

manually corrected, permitting the analysis of images with more than one domain

population, as in the case of the images observed for the blends.

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

AFM is a valuable technique for studying morphologies formed by spreading

copolymer solutions at an aqueous subphase. Bodiguel et al. demonstrate the

complementary nature of AFM and TEM in depicting phase separation of two distinct

polymer blocks.32 The technique assumes that the morphology of the transferred film

represents that of the floating monolayer and that transfer is homogeneous. Langmuir-

Blodgett (LB) films were prepared at several surface pressures and then studied using


(^ : A. P )




cm ar dn An -)

| ,- 1 U r







r Jf 3









AFM in tapping mode. For each sample, an average often images was taken to ensure

reproducibility.


Figure 4-6. AFM images of the pure PS-b-PEO for several transfer pressures (scale
2x22tm)

The images shown in Fig. 4-6 clearly demonstrate the formation of ordered

structures in which the observed morphology depends on surface pressure. In fact, three

distinct regions corresponding to those in the isotherm can be seen once again. For

pressures of 4 and 7 mN/m (Region I of Fig. 3-1.), images show a majority of single

domains, typical of an expanded liquid. Two-dimensional micelles form at the air-water

interface with a morphology depending on the ratio of the hydrophobic and hydrophilic

block sizes. For pressures under 7mN/m, circular micelles are observed like the one

described by Potemkin et al.36 (Figure 4-7) where one of the blocks is strongly adsorbed

on a planar surface.




















Figure 4-7. Schematic representation of surface micelles formed by A-b-B diblock
copolymers with A strongly adsorbed to the surface (adapted from Potemkin
et al.36)

In the case of PS-b-PEO at the air-water interface, similar micelles are observed

with the PEO extending more and more in the aqueous subphase as the concentration

increases (Figure 4-8).



SAggregated
PS
'- PEO

Concentration
Increases





Figure 4-8. Model of PS-b-PEO absorbing at the air/water interface (Adapted from
Dewhurst et al.8)

When compression continues and reaches the pseudoplateau range (Region II of

Fig. 3-1), chain formation is detected and continues until collapse pressure is occurs. The

images also demonstrate the presence of intermediate stages in which the single domains

begin to aggregate prior to chain formation.

Due to the hydrophilicity of the mica, we suppose PEO represents the bottom layer

whereas PS occupies the top part of the LB film at a thickness of some nanometers,










ranging from about 2 to 10 depending on the blend. In our images, the darker layer

represents PEO and the bright domains show the PS blocks. Using a program designed by

our group, the number of domains per image was found, allowing the molecules per

domain (or aggregation number) to be calculated.


1600
1400 *
S1200
1000
E
o 800
S600
S400 *
0
I 200
0
0 5 10 15 20
Surface Pressure (mN/m)

Figure 4-9. Dependence of the number of molecules per domain on pressure

For each given pressure, the aggregation number was determined using Formula 4-

1.

F = A/Nd.o 4-1

where F refers to the number of molecules per domain, A the scanned area of the

image, Nd the number of domains, and cy the mean molecular area during transfer. As

shown in Figure 4-9, the number of molecules/domain depends strongly on the surface

pressure. For pressures less than 10mN/m, the number of molecules/domain remains

almost constant. However, once the pressure of the pseudoplateau is reached, an increase

in aggregation number is observed.

As compression continues, aggregation increases and at the transition between

Regions II and III, the aggregation number rises sharply. This behavior is another









indication of the transition between the liquid expanded state and the liquid condensed

state. Logan et al.19'22 showed that compression-induced aggregation occurs when PEO is

pushed into the aqueous subphase. However, at higher pressures, some PEO can remain

at the interface and separate the PS domains. This situation represents two conflicting

forces. The attraction between PEO and the water allows the polymer to spread on the

surface, whereas the repulsion of PS with both water and PEO drives aggregation. Cox et

al.12,13, however, thought the relative interaction of the two blocks with the subphase and

air is a more probable explanation for the existence of aggregation.

Blends of PS-b-PEO and a PS homopolymer

To observe the possible formation of aggregates between copolymer and

homopolymer, the blends were studied by AFM for different transfer pressures. In Figure

4-10, the AFM images for the pure diblock, Blend 2, and Blend 5 at several transfer

pressures are given. As described previously, a chaining of the domains is observed for

the pure PS-b-PEO when increasing the pressure. However, in the blend case, the

addition of PS homopolymer seems to inhibit the formation of these chains. In Figure 4-

11, the histograms of the domain area are given for both films at a transfer pressure of 10

mN/m. The pure diblock exhibits larger domain areas whereas the blend seems to show

an increase of the ratio between big and small domains.

In the pancake region, the pure diblock exhibits local hexagonal packing with six

neighbors for each domain, showing that even at low pressure, the copolymers arrange

into surface micelles. In the case of the blends, the addition of PS to the monolayer

disrupts this packing by increasing the size of only some of the domains and enabling a










population of smaller domains to form (Figure 4-11).

PURE BLEND 2 BLEND 5



[- = 4mN/m





= 7mN/m





n = 10OmN/m




Amount of PS


Figure 4-10. AFM images of the pure diblock copolymer as well as two of the blends for
several transfer pressures (scale 2x2[tm)

This behavior can be compared to that described by Logan,19 Logan et al.,20 and

Francis et al.,21,22 for PS-b-PEO star copolymers of various hydrophobicity. For both stars

and linear chains in the literature (particularly Devereaux and Baker14), increased PS

results in nonuniform films with a greater variety of morphology.

As pressure increases, no chaining is observed in the case of the blend. In fact

instead of chaining, an augmentation in population of the bigger domains compared to the

small domains can be observed. This phenomenon can be shown by using the computer

program designed by Yves Heckel. Histograms of the domain areas are given in Figure

4-12.













100-


80-


60


o 40


20



0 10 20 30 40 50 60 70 80 90 100
Domain Area (nm2)

200


150


6 100


50


0-1
0 20 40 60 80 100
Domain Area (nm2)



Figure 4-11. AFM images for the pure diblock copolymer and Blend 2 (transfer pressure
of 4mN/m) as well as the distribution of the domain areas


In the pure diblock, formation of large chains that resemble pearl necklace-like


strings.occurs when increasing the transfer pressure. This phenomenon continues with the


formation of domain dimerss" or "trimers" that then keep on chaining with increased


pressure. When PS is added to the monolayer, no such domains are observed. An increase


in the size of the circular domains is observed, indicating the aggregation of the PS


homopolymer within the PS chains of the copolymer (Figure 4-13).



















80-

o 60-

40-

20-

149
0 50 100 150 200 250 300 350
120- Domain Area (nm2)

100

a 80-

0 60

40

0-

0 50 100 150 200 250 300 350
Domain Area (nm2)



Figure 4-12. Computed images for the pure diblock copolymer and Blend 2 (transfer
pressure of 10mN/m) as well as the distribution of the domain areas

In the pure diblock, formation of large chains that resemble pearl necklace-like


strings.occurs when increasing the transfer pressure. This phenomenon continues with the


formation of domain dimerss" or "trimers" that then keep on chaining with increased


pressure. When PS is added to the monolayer, no such domains are observed. An increase


in the size of the circular domains is observed, indicating the aggregation of the PS


homopolymer within the PS chains of the copolymer (Figure 4-13).



























Figure 4-13. Magnification of a single domain formed for Blend 2 for a transfer pressure
of 10mN/m (scale 150xl50nm)

Blends of PS-b-PEO and a PEO homopolymer

In a similar way, blends of the copolymer and a PEO homopolymer were

transferred onto a mica substrate in order to study the evolution of the morphologies

depending on the amount of PEO added. Results are shown in Figure 4-14.

Pure Blend 1 Blend 2 Blend 3 Blend 4


H = 4mN/m





H = 9mN/m




Amount of PEO

Figure 4-14. AFM images of the pure PS-b-PEO diblock copolymer and several blends
for transfer pressures of 4 and 9 mN/m (scale 2x2[tm)









At low pressure, we can observe the disappearance of the hexagonal packing when

increasing the amount of PEO. This behavior can be compared to that of PS-b-PEO star

copolymers studied by Logan,19 and Francis et al.,22 By increasing the hydrophilicity of

the stars, they observed a decrease in the number of domains and an increase in the

distance between each domain. While less uniform, this same effect appears in the blends

as a result of PEO homopolymer aggregation to the PEO chains of the diblock

copolymer. To remove the artifacts than could have been formed by spreading

successively the pure copolymer and the homopolymer, a mixed solution was made and

this was spread as a comparison. The proportions were the same as the one used for

Blend 2 and the film was transferred at a pressure of 9mN/m. The AFM images are

shown in Figure 4-15.













Figure 4-15. AFM images for Blend 2; (a) from successive spreading, and (b) from the
mixed solution (scale 2x2[tm)

Those experiments show that independent of the spreading technique, an increase

in PEO is followed by the formation of longer chains than for the pure copolymer sample.

A second effect of this increase is the apparition of a phase separation between pure

layers of PEO surrounding clusters of micelles.






47


In addition, the addition of a copolymer to a PEO homopolymer monolayer

increases its stability and allows the formation of films at higher pressures than that of a

pure PEO monolayer which collapses at 10mN/m.16 The difference between the

morphologies at high pressure for the pure diblock and for the blends shows that the PEO

homopolymer aggregates with the copolymer instead of dissolving into the aqueous

subphase.














CHAPTER 5
CONCLUSION

When adding the homopolymers to the pure diblock copolymer at the air-water

interface, reproducible isotherms were obtained and displayed the three regions present in

a pure diblock copolymer (pancake, pseudoplateau, and brush). While the increase in the

PS amount had an effect only on the condensed area which varies linearly with the

amount of PS added, by combining PEO and the copolymer, no change was observed in

the condensed region of the isotherms. The pseudoplateau representing the biphasic

region gets longer as the amount of PEO increases. This behavior has been observed for

pure diblocks when increasing the size of the PEO block as well as for star

copolymers.19,22

AFM images were taken and were consistent with the isotherms showing the three

regions described previously. On one hand, the addition of PS to the copolymer

monolayer inhibited the chaining of the copolymer domains and enhanced the

hydrophobic properties of the Langmuir-Blodgett film. On the other hand, combining the

PS-b-PEO diblock copolymer with the PEO homopolymer also had an effect on the film

morphology, increasing the chaining of the domains as well as favoring the phase

separation between clusters of micelles and pure layer of PEO.

Future work would include additional characterization studies, such as using a

different spreading technique in order to eliminate any artifacts that could be caused by

experimental procedures. Different molecular weight PS and PEO could be used in order

to determine the influence of the molecular weight on the system.
















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BIOGRAPHICAL SKETCH

Sophie Bernard was born on February 25, 1979, in Bordeaux, France. She

graduated from the University of Bordeaux in June 2001 with her B.S. degree in physical

chemistry. She then worked as a research assistant under the direction of Dr. Yves

Gnanou, in the Laboratoire de Chimie des Polymeres Organiques, University of

Bordeaux. She received her M.S. degree in physical chemistry of polymers from the

University of Bordeaux in July 2002.

Sophie enrolled at the University of Florida in September 2002 in pursuit of a

Ph.D. degree under the direction of Dr. Randolph S. Duran. She is currently a Ph.D.

candidate in the Chemistry Department of the University of Florida. Her academic

interests include polymer and surface chemistry having biomedical applications.