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Surface Restructuring of Polystyrene/Polymethacrylate Blends in Water Studied by Atomic Force Microscopy Sara E. Woodcock,† Chunyan Chen,‡ and Zhan Chen*,†,‡ Department of Chemistry and Department of Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109 Received September 11, 2003. In Final Form: January 5, 2004 Blends of poly(n-butyl methacrylate) (PBMA)/polystyrene (PS) and poly(n-octyl methacrylate) (POMA)/ PS were imaged with atomic force microscopy in both ambient and aqueous environments. The surfaces of the blends were monitored over time under water, and most PBMA/PS and POMA/PS samples showed no surface restructuring. A morphology change, however, was observed for high ratios of POMA and specific ratios of PBMA in the blend. For the blend of 60% PBMA and 40% PS, the surface was pitted in air with depths of 60 nm. In water, as the exposure time increased, islands replaced the holes on the surface. The height of these islands averaged 50 nm. At high ratios of POMA, in POMA/PS blends, the surface structure also changed when contacted with water. The structural changes that were observed for all of these select blends were reversible after the surface was dried and reimaged in air. Sum frequency generation vibrational spectroscopy was used to give insight into the functional groups and surface chemical composition of the blends in air and water.
1. Introduction Polymer materials are widely used as biomedical materials, ranging from artificial organs, pacemakers, and artificial bones to tissue adhesives.1-4 To implant these polymers in the body, the surface properties must be properly customized and their surface structures must be studied in situ in an aqueous environment. Unfortunately, most surface- or interface-sensitive techniques require a high vacuum and are not employable for studying materials in biological conditions. Due to the changes in surface structures in different environments, surface structural information detected from high-vacuum techniques cannot always provide accurate in situ data. We are applying atomic force microscopy (AFM) and sum frequency generation (SFG) vibrational spectroscopy, both of which are in situ surface-sensitive techniques,5-15 to examine surface structures of polymer blends in air and water and monitor their surface restructuring over time. Blending homopolymers is an effective method to design polymer materials with both desired surface and bulk properties. By variation of the bulk concentrations of one or more of the components in a polymer blend, surface chemical structures can be altered systematically.16 Changing the proportions and concentrations of the components in the polymer blends can also tailor the surface morphology and surface domain structures.17-20 Different surface morphologies can affect the biocompatibility by altering the blends’ interactions with their environment. It is, therefore, necessary to study surface morphology and domain structure of polymer blends in a biologically relevant environment. Many of the polymer * To whom correspondence should be addressed. E-mail: zhanc@ umich.edu. Fax: 734-647-4685. † Department of Chemistry. ‡ Department of Macromolecular Science and Engineering. (1) Silver, F. H. Biocompatibility: Interactions of Biological and Implantable Materials; VCH: New York, 1989. (2) Park, J. B.; Lakes, R. S. Biomaterials: An Introduction; Plenum Press: New York, 1992. (3) Ratner, B. D.; Castner, D. G. Surface Modification of Polymeric Biomaterials; Plenum Press: New York, 1996. (4) Feast, W. J.; Munro, H. S.; Richards, R. W. Polymer Surfaces and Interfaces II; Wiley: New York, 1992.
blend surfaces have been previously studied by highvacuum surface-sensitive techniques, such as X-ray photoelectron spectroscopy (XPS)17,18,21 and secondary ion mass spectrometry (SIMS),18,22,23 to determine the composition of the surface. AFM has also been widely used to study the morphology of these blends, with the studies primarily carried out in ambient conditions24-28 and not in a biological environment, such as water. (5) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930-933. (6) Magonov, S. N.; Reneker, D. H. Annu. Rev. Mater. Sci. 1997, 27, 175-222. (7) Al-Mawaali, S.; Bemis, J.; Akhremitchev, B. B.; Janesco, B.; Walker, G. C. J. Phys. Chem. B 2001, 105, 3965-3971. (8) Akhremitchev, B. B.; Mohney, B. K.; Marra, K.; Chapman, T. M.; Walker, G. C. Langmuir 1998, 14, 3976-3982. (9) Li, L.; Chen, S.; Oh, S.; Jiang, S. Anal. Chem. 2002, 74, 60176022. (10) Carlo, S. R.; Perry, C. C.; Torres, J.; Wagner, A. J.; Vecitis, C.; Fairbrother, D. H. Appl. Surf. Sci. 2002, 195, 93-106. (11) Wang, J.; Buck, S. M.; Even, M. A.; Chen, Z. J. Am. Chem. Soc. 2002, 124, 13302-13305. (12) Chen, Z.; Shen, Y. R.; Somorjai, G. A. Annu. Rev. Phys. Chem. 2002, 53, 437-465. (13) Shen, Y. R. The Principles of Nonlinear Optics; Wiley: New York, 1984. (14) Shen, Y. R. Nature 1989, 337, 519-525. (15) Shen, Y. R. Annu. Rev. Phys. Chem. 1989, 40, 327-350. (16) Polymer Blends and Composites in Multiphase Systems; Han, C. D., Ed.; Advances in Chemistry Series 106; American Chemical Society: Washington, DC, 1984. (17) Affrossman, S.; Kiff, T.; O’Neil, S. A.; Pethrick, R. A.; Richards, R. W. Macromolecules 1999, 32, 2721-2730. (18) Ton That, C.; Shard, A. G.; Daley, R.; Bradley, R. H. Macromolecules 2000, 33, 8453-8459. (19) Chen, C.; Wang, J.; Woodcock, S. E.; Chen, Z. Langmuir 2002, 18, 1302-1309. (20) Ton That, C.; Shard, A. G.; Teare, D. O. H.; Bradley, R. H. Polymer 2001, 42, 1121-1129. (21) Artyushkova, K.; Wall, B.; Koenig, J.; Fulghum, J. E. Appl. Spectrosc. 2000, 54, 1549-1558. (22) Vanden Eynde, X.; Betrand, P. Appl. Surf. Sci. 1999, 141, 1-20. (23) Feng, J. Y.; Weng, L. T.; Li, F.; Chan, C. M. Surf. Interface Anal. 2000, 29, 168-174. (24) Grandy, D. B.; Hourston, D. J.; Price, D. M.; Reading, M.; Silva, G. G.; Song, M.; Sykes, P. A. Macromolecules 2000, 33, 9348-9359. (25) Karim, A.; Slawecki, T. M.; Kumar, S. K.; Douglas, J. F.; Satija, S. K.; Han, C. C.; Russell, T. P.; Liu, Y.; Sokolov, J.; Rafailovich, M. H. Macromolecules 1998, 31, 857-862. (26) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 1996, 29, 3232-3239.
10.1021/la035698j CCC: $27.50 © 2004 American Chemical Society Published on Web 02/05/2004
Surface Restructuring of Polymer Blends
Both polymethacrylates (PMA) and polystyrene (PS) are extensively used in many important biomedical applications.1-4 The polymer blends of PMA and PS also serve as model systems for AFM studies. Previous studies17-20 have shown that the morphology of the blend changes as the ratio of polymeric components is varied. At low concentrations of PMA, the surface is smooth; as the PMA increases, phase segregation is observed and ribbons coalesce to become distinct islands. This trend has been observed with both poly(methyl methacrylate) (PMMA)18,20,26,29-31 and poly(n-butyl methacrylate) (PBMA) blended with PS.19,32 In our previous studies, we have elucidated surface structures and morphologies of PBMA/PS blends in air using SFG and AFM.19 We found that PBMA preferentially segregates to the surface in air, and the surface morphology changes as a function of bulk concentration of PBMA and PS components in the blend. For various bulk concentrations, the surface can either be flat, have holes, or create islands.19,32 In addition, we have detected surface structure changes of PMMA, PBMA, and poly(n-octyl methacrylate) (POMA) in water using SFG. The SFG data indicated that the orientation of the methyl side chains changed upon exposure to water for PMMA and PBMA, and there were backbone changes for POMA.33 The focus of this work is the surface restructuring behaviors of PBMA/PS and POMA/PS blends from air to water, using AFM as a primary technique and supplementing with SFG. The research on surface structures of polymer blends in water should give insight into the surface phase segregation in different media (air versus water). By varying the bulk concentrations of the components, we were able to engineer the surface morphology and image in situ the restructuring behavior of select blends in real time. This information will help explore the surface structures of polymers in biological conditions, on the microscale, and may be used to deduce the biocompatibility for similar materials. 2. Materials and Methods 2.1. Preparation of Samples. The polymers were purchased from Scientific Polymer Products, Inc., and Aldrich and were used as received. The physical properties of all polymers under investigation were compared and listed in Table 1.34,35 Polymer films were prepared by spin coating 2 wt % polymer blend (with different ratios of two components) toluene solution onto microscope cover slides for AFM and onto fused silica substrates for SFG studies. The samples were spun at 3000 rpm for 30 s using a spin coater purchased from Specialty Coating System. All spin-cast samples were oven dried at 80 °C for 24 h to ensure complete evaporation of the solvent before analysis. The film thickness of the samples was determined to be approximately 100 nm with a Dektak profilometer. (27) Affrossman, S.; Henn, G.; O’Neil, S. A.; Pethrick, R. A.; Stamm, M. Macromolecules 1996, 29, 5010-5016. (28) Takahara, A.; Nakamura, K.; Tanaka, K.; Kajiyama, T. Macromol. Symp. 2000, 159, 89-96. (29) Harris, M.; Appel, G.; Ade, H. Macromolecules 2003, 36, 33073314. (30) Morin, C.; Ikeurfa-Sekiguchi, H.; Tyliszczak, T.; Cornelius, R.; Brash, J. L.; Hitchcock, A. P.; Scholl, A.; Nolting, F.; Appel, G.; Winesett, D. A.; Kaznacheyev, K.; Ade, H. J. Electron Spectrosc. Relat. Phenom. 2001, 121, 203-224. (31) Winesett, D. A.; Ade, H.; Sokolov, J.; Rafailovich, M.; Zhu, S. Polym. Int. 2000, 49, 458-462. (32) Affrossman, S.; O’Neill, S. A.; Jerome, R.; Schmitt, T.; Stamm, M. Colloid Polym. Sci. 2000, 278, 993-999. (33) Wang, J.; Woodcock, S. E.; Buck, S. M.; Chen, C.; Chen, Z. J. Am. Chem. Soc. 2001, 123, 9470-9471. (34) Brandrup, J.; Immergut, E. H.; Grulke, E. A. Polymer Handbook; John Wiley & Sons: New York, 1999. (35) Wu, S. Polymer Interface and Adhesion; Marcel Dekker: New York, 1982.
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Figure 1. PBMA/PS blend in 40:60 proportion imaged in (a) air and (b) water. The RMS values are 1.61 and 1.52 nm, respectively, and the image size is 3 µm × 3 µm. The color contrast represents a total range of 5 nm. Table 1. Physical Properties of Investigated Polymers
polymer name
glass transition temp (°C) (ref 34)
molecular weight
surface energy (dyn/cm) (ref 35)
poly(n-butyl methacrylate) poly(n-octyl methacrylate) polystyrene
20 -70 100
332 000 100 000 280 000
31.2 25.0 40.7
2.2. Atomic Force Microscopy. The AFM images were collected using a Molecular Imaging Picoscan. The images were taken with Magnetic AC (MAC) mode with magnetically coated silicon nitride cantilevers. The average resonance frequency of the tips was 75 kHz in air and 30 kHz in water. The tips had an average spring constant of 2.8 N/m. The images were obtained in ambient conditions at room temperature and under water with a Teflon fluid cell and deionized Millipore water. All images were processed with Scanning Probe Imaging Processor software by Image Metrology. 2.3. Sum Frequency Generation. The SFG setup and theory have been previously reported and will not be repeated here.13-15,19,36 The SFG spectra shown in this paper were collected in air with the input beam traveling through the fused silica and to the polymer blend/air interface. The blend was then placed in contact with water, the polymer/air interface was replaced by the polymer/water interface,11,33,36 and SFG spectra were collected from the blend/water interface. The SFG spectra were also collected from the blend surface in air after the water exposure.
3. Results and Discussion 3.1. Pure PBMA, POMA, and PS. The surfaces of PBMA, POMA, and PS have been imaged in air, and they are flat and featureless (not shown). AFM has also been applied to image the surfaces of these three polymers in water. For PBMA and PS, the surfaces are still featureless, with a similar roughness to those in air. For POMA, however, the flat surface in air changed to a surface structure of island domains of approximately 25 nm in height. These AFM results are compatible with our previous SFG studies.33 Our SFG results show that the side chain methyl groups on PBMA change orientation in water, but there are no backbone changes. Such side chain reorientation will not affect the surface morphology of PBMA in water. For POMA, the SFG signals disappear after the surface contacted water, showing tremendous backbone changes. The AFM data indicate that surface changes do occur, agreeing with the SFG observation. 3.2. PBMA/PS Blend with Lower than 50% PBMA Bulk Concentration. In our previous studies, the results reflected the segregation of PBMA to the surface in air, due to the lower surface energy. Therefore, even at the surface of a blend with only 4% PBMA bulk concentration, (36) Wang, J.; Paszti, Z.; Even, M. A.; Chen, Z. J. Am. Chem. Soc. 2002, 124, 7016-7023.
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Figure 2. PBMA/PS blend in 50:50 ratio imaged (a) in air, (b) in water, and (c) after water. The image size is 5 µm × 5 µm, and the depths of the pits average 40 nm. The total range for the color contrast is 40 nm.
the PBMA coverage detected by the SFG is substantial.19 The morphologies of PBMA/PS blends with PBMA bulk concentrations lower than 50% imaged are all quite flat in air, in contrast to PMMA/PS blends. For a PMA with a shorter side chain length than PBMA, such as PMMA, the compatibility with PS decreases. When PMMA is blended with PS, domains are seen for surfaces with bulk PMMA concentration as low as 25%.18,20,26 At low bulk concentrations of PBMA, PBMA and PS are still compatible and thus the PBMA/PS surface is flat, indicating no surface domain structures formed.19,32 At 40% PBMA, the root-mean-square (RMS) roughness of the surface in air was 1.61 nm. When we applied MAC mode AFM to image the 40:60 PBMA/PS blend surface in water, the surface was also flat, with a rms of 1.52 nm, similar to that obtained in air (Figure 1). 3.3. 50:50 PBMA/PS Blend. The surface morphology of a 50:50 PBMA/PS blend imaged in air had a pitted structure, with the pits about 40 nm in depth (Figure 2a). The surface domain structure was induced by the incompatibility between PBMA and PS on the surface. As the sample contacted water, no detectable surface domain structural change was observed (Figure 2b). The pitted structure was still evident, and there was no change in the domain size. After allowing the sample to dry, the blend was reimaged and the structure remained as it was initially in air (Figure 2c). 3.4. 60:40 PBMA/PS Blend. The blend of 60:40 PBMA/ PS showed a pitted surface structure in air, similar to that of the 50:50 blend. However, the surface structure of the 60:40 blend changed in water. This blend was imaged over time to monitor the restructuring trend, and it was observed that the restructuring begins almost instantaneously and is complete after a period of approximately 2 h. Figure 3 shows the blend changing from the pitted topography, with depths of 50-60 nm, to a structure that was composed of large islands that had an average height of approximately 50 nm. After the film dried, it was reimaged in air and completely recovered the original topography. The sample was re-exposed to water to see if the restructuring would occur again, and the results were identical to the first trial. The results of surface restructuring of this polymer blend in water were quite reproducible. Figure 4 shows the changes in three dimensions as a function of time and on a larger scale using a different scan head. The results were consistent with those from the smaller scan head, and the surface structure stabilized and completely changed from air to water over a period of about 2 h. The change was reversible, and the surface returned to the pitted structure after drying. 3.5. 80:20 PBMA/PS Blend. Instead of the pitted structure detected on the 50:50 and 60:40 PBMA/PS blend surfaces, an island structure was observed on the 80:20 PBMA/PS surface in air. The islands were 45-50 nm in height (Figure 5a). Our previous research19 using selective
Figure 3. PBMA/PS 60:40 blend imaged (a) in air, (b) in water initially, (c) in water as time progresses, and (d) after water. The images are 5 µm × 5 µm.
Figure 4. PBMA/PS 60:40 restructuring imaged in water over time in 3D: (a) air, (b) 12 min, (c) 2 h and 30 min, (d) after water. The image size is 10 µm × 10 µm, the pits are 60 nm deep, and the islands are 50 nm high.
solvation showed that these islands are PS rich. The surface imaged in water using the MAC mode showed the same morphology as air (Figure 5b), and no changes in island size or height were detected. The sample was also imaged after drying, and the morphology was unchanged (Figure 5c).
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Figure 5. PBMA/PS blend in 80:20 ratio imaged (a) in air, (b) in water, and (c) after water. The image size is 5 µm × 5 µm, and the domain height is 45-50 nm. The color contrast represents a total range of 50 nm.
Figure 6. SFG spectra of PBMA/PS at the air/polymer and water/polymer interface: (a) PBMA/PS 50:50, (b) PBMA/PS 60:40, and (c) PBMA/PS 80:20.
3.6. SFG Studies. The SFG spectra of these blends were collected over time to correlate chemical information to the changes observed by AFM. Figure 6 shows the spectra in air and water for PBMA/PS blends with 50% and greater PBMA. These spectra showed that the side chain methyl groups of PBMA dominated the surface in both air and water but have very different orientations. The orientation information of a typical surface/interface functional group can be deduced by observing the relative SFG signal intensity from the symmetric and asymmetric
stretches.19 For all three PBMA/PS blends in air, the SFG data were dominated by the symmetric stretch of the ester side chain methyl groups on PBMA at 2875 cm-1 and Fermi resonance at 2940 cm-1. Such spectral features indicate that the ester methyl groups on PBMA tend to orient along the surface normal.36 In water, however, the SFG signals were dominated by the asymmetric stretch, 2960 cm-1, of the ester side chain methyl groups on PBMA, suggesting that the methyl groups lay down upon contact with water.36 This behavior is similar to that we observed with pure PBMA in water.33 Phenyl groups of PS could be detected on the 50:50 PBMA/PS surface at 3055 cm-1, but such signals were quite weak. The surfaces of 60:40 and 80:20 blends were also dominated by side chain methyl groups of PBMA with very weak SFG phenyl group signals detected. Similar to the 50:50 blend, reorientation of the side chain methyl groups occurred when the sample was contacted with water. Therefore, for all three blends, 50:50, 60:40, and 80:20, the surfaces were still dominated by PBMA in air as well as in water. The SFG is sensitive to the chemical surface composition, not the morphology, and the composition of the surface does not appear to change as the topography restructures. We believe that the surface restructuring of the 60:40 blend observed by AFM is due to the change of PBMA structure in water, which will be discussed in detail in the following section. 3.7. Discussion. Using the SFG information and AFM topographic images, we can determine a simple model of segregation for the PBMA/PS blends in air. The SFG data for all ratios indicate that the surface is dominated by PBMA and there is little PS signal. The AFM topography images show distinct morphology and can be modeled as either a pitted or island structure. The ribbons in the pitted structures of 50:50 and 60:40 PBMA/PS blends are PS rich and the pits PBMA rich.32 The island surface structure of 80:20 has PS-rich islands in a PBMA matrix.19,32 We believe that the entire surface however is covered with a thin layer, at least 5 nm in thickness of PBMA. The thin cover of PBMA on top has been confirmed by XPS32 as well as our SFG results. According to the literature,32 PBMA has a higher affinity for the substrate and will tend to segregate to the glass. Since PBMA also covers the surface, it is depleted from the bulk. At low PBMA ratios, for example, when PBMA is less than 50%, PS stays in the bulk and during the spin coating process spreads over the whole PBMA layer on the substrate. The thin top layer of PBMA resembles the PS topography, and thus the sample surface is flat (Figure 7a). When the PS ratio is reduced, it cannot spread over the entire PBMA layer on the substrate and thus segregates into ribbons. This creates the ribbonlike structure or pits on the surface that are seen in air at 50 and 60% PBMA blend surfaces, and pits are mainly PBMA (Figure 7b,c). As the amount of PBMA increases to 80%, the PBMA still wants to occupy the substrate and air and the PS still likes to segregate
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Figure 7. Model of phase segregation in air for PBMA/PS blends: (a) below 50% PBMA, (b) 50% PBMA, (c) 60% PBMA and 40% PS, and (d) 80% PBMA and 20% PS.
Figure 8. Restructuring behavior of the 60:40 PBMA/PS blend in water: (a) initial restructuring of PBMA/PS blend with PBMA going toward the polymer/water interface and PS drifting away from the water; (b) islands begin to grow toward the surface and PS segregates away from water; (c) islands of PBMA dominate the surface and substrate allowing PS to minimize interaction with water and glass.
to itself. There is not enough PS to form connected ribbons, and thus it has to form discrete islands on the surface (Figure 7d).
When the PBMA/PS blends are contacted with water, our AFM results indicated that there was no restructuring behavior for the blends except in the ratio of 60% PBMA and 40% PS. The restructuring was gradual and stabilizes over time, and many hypotheses can be speculated for this change. Swelling is one possibility for the change in morphology. Previous work with water absorption and uptake for PS found that the swelling is negligible and that the water absorption is only 0.1% even at saturation. To investigate PBMA’s swelling potential, we created a thick film of PBMA and submerged it into water for 3 days. We found no substantial difference in mass after the time in water, and we believe that PBMA is also not swelling in our thin films. The option of an air microbubble on the surface due to preparatory conditions was also explored. This hypothesis follows the idea that the sample, when submersed in water, has air that is trapped between the PBMA pit and water layer. The air bubbles increase
Figure 9. POMA/PS blend in air at (a) 20% POMA, (b) 60% POMA, and (c) 80% POMA. The image size is 5 µm × 5 µm, and the domain heights are 40 nm deep and 60 nm high for pits and islands, respectively.
Figure 10. POMA/PS 80:20 blend restructuring in water over time: (a) air, (b) 5 min in water, (c) 10 min in water, (d) 22 min of exposure, (e) 43 min in water, and (f) after water. All images are 5 µm × 5 µm except panel f which is 3 µm × 3 µm. The color contrast represents a total range of 50-60 nm.
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in size over time in the water, due to the water’s oxygen content, and this is reflected in the changes that the AFM observes. This is unlikely, however, because we observe such changes only for the 60:40 blend and not the 50:50 blend. If the islands are due to air bubbles, we should observe them in both pitted samples. We performed additional experiments to exclude the possibility of the formation of air bubbles on the surface: The sample was kept in water and sonicated in order to break any bubbles and obliterate any air trapped between the polymer and the water. The sonicated sample showed the same morphology under water as the previous featured samples (Figures 3 and 4). We also imaged under increased force and saw no difference until the force was so large that it began to deform the soft polymer surface. In addition, the SFG experiments on the polymer/water interface did not show any of the scattering effects due to the difference in the refractive indices between air and the polymer that should be apparent if air bubbles were present. The most likely explanation is that the PBMA is restructuring in this blend. For 50% PS, there may be a small layer of PS between the PBMA layer on the glass and the thin overlayer (Figure 7a). The exposure to water for this blend does not promote a change because the water cannot diffuse through both the thin PBMA top layer and PS to reach the PBMA on the substrate on the time scale that we are investigating. As the ratio decreases to 40% PS, the layer of PS is less continuous and thinner and the PBMA layer on the substrate is directly connected to the thin layer that covers the surface (Figure 7b). The diffusion of the water through the upper layer of PBMA is possible due to the permeability of this thin film and affects the PBMA on the substrate (Figure 8a). Both polymers have a high contact angle of 91° 35 and are hydrophobic, but the PBMA is more mobile than PS, with a glass transition temperature below room temperature,34 and has a lower surface free energy for the water/polymer interface.37 The polymer/water interfacial energies of PBMA and PS have been measured to be 26 and 32 mN/m, respectively. To minimize energetics, the PBMA segregates to both the glass substrate and water interfaces. The PBMA segregates to itself in the pits and begins to form islands that slowly dominate the surface (Figure 8b,c). The SFG continues to see PBMA on the surface throughout the changes due to its chemically sensitive nature and not morphology. In addition to the 60:40 PBMA/PS blend, surface restructuring behavior in water was also observed on the (37) Dong, Y.; Sundberg, D. C. J. Colloid Interface Sci. 2003, 258, 97-101.
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surfaces of the POMA/PS blend systems. POMA and PS phase segregated in a manner similar to that seen for the PBMA/PS blends in air. The increased compatibility between POMA and PS compared to PBMA and PS leads to a flat, single phase until 60% of POMA in the blend. Figure 9 illustrates the blend at 20% POMA (Figure 9a), 60% POMA (Figure 9b), and 80% POMA (Figure 9c). In air, the surface was flat at 20% POMA, pitted at 60% POMA, and island like at 80% POMA, similar to that of the PBMA/PS blend. The samples, when placed in water, show restructuring for both 60% and 80% POMA. The 80:20 POMA/PS blend showed the most dramatic change in water and completely recovered when contacted with air again (Figure 10). The surface was covered with island domains in air of a height ranging from 45 to 60 nm. As the surface was contacted with water, a ribbon like topography coalesced over time (Figure 10b-e). After the film dried, it resembled its original structure with the domain size constant (Figure 10f). The model for the restructuring of this blend is still under investigation. The homopolymer, POMA, restructures in water, so the change may be due to the backbone restructuring of this polymer, as seen by SFG, or it may be energetically driven like that seen with 60:40 PBMA/ PS. Another point of consideration is the increased tendency of POMA to absorb water and swell. 4. Conclusions The imaging of polymer blends in water using AFM allows the direct monitoring of surface restructuring. The blends of PBMA/PS at below 50% PBMA, 50% PBMA, and 80% PBMA showed no change in air and water. The surface structure that they established in air was also the most favorable in water. The blend of 60% PBMA was pitted in air and had islands in water. The air surface was similar to that of 50% PBMA, and it resembled that of higher PBMA ratios in water. The change in environment shifted the equilibrium, and the two polymers restructured to the most energetically favorable configuration. After the blend dried, the surface recovered and was once again pitted. POMA/PS blends at higher ratios of POMA also showed restructuring behaviors and are under further investigation. Acknowledgment. This work was supported by the American Chemical Society Petroleum Research Fund (37547-G7), the National Science Foundation (0315857), the Office of Naval Research (N00014-02-1-0832), the Beckman Foundation, and the start-up fund from the University of Michigan. LA035698J
