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Notes Morphology of Vapor-Deposited Poly(r-amino acid) Films Nancy H. Lee,† Lisa M. Christensen,‡ and Curtis W. Frank*,† Department of Chemical Engineering, 381 North-South Mall, Stanford University, Stanford, California 94305-5025, and Materials Engineering Department, California Polytechnic State University, San Luis Obispo, California 93407 Received September 16, 2002. In Final Form: January 16, 2003
Introduction Poly(amino acids), which can exist as R-helices, β-sheets, and random coils, have been deposited onto various types of surfaces, which may prove to be important in such applications as biosensors,1 chiral separation membranes,2-4 optical switches,5 and liquid crystal displays.6 Since these applications require the formation of a polymer film on a solid substrate, it is important to understand the molecular conformation and microscopic morphology that these chemically grafted and physisorbed films exhibit. Microscopy techniques have been used to image monolayer and bilayer polypeptide films formed by LangmuirBlodgett (LB) transfer.7-11 For poly(γ-benzyl-L-glutamate) (PBLG), LB deposition produced lacey, fibrillar, and aggregate structures depending on the hydrophobicity of the substrate.7 Fibrillar structures have also been formed on mica surfaces from monodisperse and polydisperse PBLG solutions in dioxane, which is a helix-supporting solvent. However, when trifluoracetic acid (TFA), a helixbreaking solvent, was added to dioxane, PBLG formed globular structures (20-40 nm diameter) on the substrate.12-14 The morphology of R-helical poly(L-leucine), on the other hand, was featureless when chloroform was * To whom correspondence should be addressed. Phone: (650) 723-4573. Fax: (650) 723-9780. E-mail:
[email protected]. † Stanford University. ‡ California Polytechnic State University. (1) Deming, T. J. Adv. Mater. 1997, 9, 299-311. (2) Aoki, T. Prog. Polym. Sci. 1999, 24, 951-993. (3) Lee, N. H.; Frank, C. W. Langmuir, submitted. (4) Lee, N. H.; Frank, C. W. Polymer 2002, 43, 6255-6262. (5) Whitesell, J. K.; Chang, H. K. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1994, 240, 251-258. (6) Machida, S.; Urano, T. I.; Sano, K.; Kato, T. Langmuir 1997, 13, 576-580. (7) Musselman, I. H.; Smith, D. L.; Enriquez, E. P.; Guarisco, V. F.; Samulski, E. T. J. Vac. Sci. Technol., A 1994, 12, 2523-2529. (8) Auduc, N.; Ringenbach, A.; Stevenson, I.; Jugnet, Y.; Duc, T. M. Langmuir 1993, 9, 3567-3573. (9) Yamamoto, S.; Tsujii, Y.; Fukuda, T. Polymer 2001, 42, 20072013. (10) Ulvenlund, S.; Gillgren, H.; Stenstam, A.; Backman, P.; Sparr, E. J. Colloid Interface Sci. 2001, 242, 346-353. (11) Gillgren, H.; Stenstam, A.; Ardhammar, M.; Norden, B.; Sparr, E.; Ulvenlund, S. Langmuir 2002, 18, 462-469. (12) Kitaev, V.; Kumacheva, E. Langmuir 1998, 14, 5568-5572. (13) Kitaev, V.; Schillen, K.; Kumacheva, E. J. Polym. Sci., Part B: Polym. Phys. 1998, 36, 1567-1577. (14) Sohn, D.; Kitaev, V.; Kumacheva, E. Langmuir 1999, 15, 16981702.
Figure 1. Structures of the polypeptides.
used as the spreading agent, but the chains formed aggregates with the addition of TFA.10 However, no aggregation was observed for poly(L-valine), a β-sheetforming polymer, even when TFA was added.10 In addition to imaging micron-scale features, interchain distances of LB monolayers of poly(β-benzyl-L-aspartate) (PBLA) and poly(L-alanine) and helical dimensions of PBLG have also been measured using atomic force microscopy (AFM) and scanning tunneling microscopy.8,15 In all these studies, a preformed polymer was adsorbed unto a substrate, but the morphology of chemically grafted polypeptides has not been extensively studied. Previously, Chang and Frank16 have imaged vapor-deposited PBLG on silicon substrates using AFM, and in this study, we expand upon their work to understand the relationship between the molecular conformation and the morphology of physisorbed and chemisorbed films of PBLG, PBLA, poly(O-benzyl-L-tyrosine) (PBLTyr), and poly(L-tryptophan) (PLTrp). The thermal stability of the physisorbed poly(amino acid) films and the development of supramolecular structures will also be probed. Experimental Section The N-carboxy anhydride (NCA) monomers of PBLG, PBLA, PBLTyr, and PLTrp were synthesized by reacting their respective amino acids with triphosgene in dry tetrahydrofuran.17 NCA monomers of PBLG and PLBA were purified by rephosgenation,18 while PBLTyr and PLTrp monomers were purified by recrystallization. The structures of the polypeptides are shown in Figure 1. The silicon (100) substrate (1.5 cm × 2.5 cm) was prepared by first cleaning the wafer using 30% hydrogen peroxide and concentrated sulfuric acid (30:70 volume ratio) and then reacting it with (γ-aminopropyl)triethoxysilane (APS) to attach amino functional initiating groups to the surface.16 To synthesize the polymer on the substrate, 0.01 g of the NCA monomer was first spread at the bottom of a glass reactor. The amine-modified substrate, supported on a glass holder, was placed 1.5 cm above (15) McMaster, T. J.; Carr, H. J.; Miles, M. J.; Cairns, P.; Morris, V. J. Macromolecules 1991, 24, 1428-1430. (16) Chang, Y. C.; Frank, C. W. Langmuir 1998, 14, 326-334. (17) Daly, W. H.; Poche, D. Tetrahedron Lett. 1988, 29, 5859-5862. (18) Dorman, L. C.; Shiang, W. R.; Meyers, P. A. Synth. Commun. 1992, 22, 3257-3262.
10.1021/la020788u CCC: $25.00 © 2003 American Chemical Society Published on Web 03/07/2003
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Table 1. Summary of the Film Characteristics and the Molecular Conformation of the Polypeptide Films % R-helix polymer
film thickness (nm)
rms roughness (nm)
PBLG, chemisorbed PBLG, physisorbed PBLA, chemisorbed PBLA, physisorbed PBLTyr, chemisorbed PBLTyr, physisorbed PLTrp, chemisorbed PLTrp, physisorbed
20 75 24 84 14 48 23 36
1 10 3 32 1 15 1 12
contact angle advancing receding 87 ( 2 84 ( 1 83 ( 2 88 ( 2 85 ( 3 85 ( 2 83 ( 4 83 ( 2
78 ( 3 75 ( 4 71 ( 5 71 ( 6 79 ( 2 75 ( 2 73 ( 1 70 ( 4
righthanded
lefthanded
% β-sheet parallel antiparallel
% random
100 100 55 42
45 58 100 100
100 100
the monomer, and the chamber was evacuated and backfilled with nitrogen. In most cases, the system was heated to 140 °C for 0.5 h, except for PBLG, in which the reaction temperature was set to 105 °C, and PLTrp, in which the reaction time was increased to 1 h. During the course of the polymerization, the chamber was continuously evacuated using a turbo pump (5 × 10-5 mbar) to remove carbon dioxide that evolved from the polymerization, thus driving the reaction. Afterward, the substrate was divided into two pieces. One piece was placed overnight in dichloroacetic acid/chloroform solution (for PBLG and PBLA films) or N,N-dimethylformamide (DMF) solution (for PBLTyr and PLTrp films) to remove ungrafted oligopeptides adsorbed on the substrate, leaving only covalently bonded (chemisorbed) polymers.19 No additional solvent treatments were performed for the second piece that contained both chemically bonded and ungrafted polymers. For the purposes of this paper, the term physisorbed will be used to describe this sample. Film thickness was measured using ellipsometry (model L116C from Gaertner Scientific Corp.). The refractive index of organic and polymer films typically ranges from 1.45 to 1.50,16,20 and we used a value of 1.46 for our thickness measurements. Polymer conformation was detected using a Perkin-Elmer Spectrum 2000 Fourier transform infrared (FTIR) spectrometer, and for each sample, 1064 scans were taken in transmission mode at a resolution of 4 cm-1. The percentage of R-helical (right- or lefthanded), β-sheet (parallel or antiparallel), and random conformations was calculated by deconvolution of the amide I region (1620-1670 cm-1) using a Lorentzian fit and then taking the ratio of the peak areas. The film morphology was probed using a NanoScope IIIa MultiMode atomic force microscope from Digital Instruments, and the images were captured in the TappingMode using E and J scanners at frequencies of 0.5-1 Hz. The FTA 200 dynamic contact angle system from Camtel Ltd. was used to obtain advancing and receding water contact angles on the polymer film by recording the contact angle of a drop as water was injected into or withdrawn from that drop at 0.1 µL/s. These measurements were performed at two separate locations on the sample.
Results A summary of the chemisorbed and physisorbed film characteristics is given in Table 1. The film thickness for chemisorbed polymers was approximately 2-4 times less than for the physisorbed polymers. This decrease in film thickness is due to the removal of physically adsorbed peptides, which were polymerized thermally during the course of the vapor deposition reaction.21 These peptides are comprised of low molecular weight (MW) oligopeptides. For example, we determined the MW of thermally polymerized PBLG (105 °C, 5 × 10-5 mbar, 3 h) to be 2000, which is equivalent to a degree of polymerization of eight.19 Both chemisorbed and physisorbed surfaces were relatively hydrophobic, with advancing contact angles ranging from 83° to 88°. In most cases, the hysteresis between advancing and receding contact angles (19) Lee, N. H.; Frank, C. W. Langmuir, in press. (20) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991. (21) Kricheldorf, H. R. R-Aminoacid-N-Carboxy-Anhydrides and Related Heterocycles, 1st ed.; Springer-Verlag: Berlin, 1987.
Figure 2. FTIR spectra of chemisorbed (a) and physisorbed (b) polymer films.
was greater for physisorbed compared to chemisorbed polymers, which may be attributed to the higher surface roughness of the physisorbed film.22 The FTIR spectra of PBLG, PBLA, PBLTyr, and PLTrp films are shown in Figure 2. The conformation of the polypeptide can be determined from the location of the amide I peak (backbone CdO stretch), which occurs at 1650-1655 and 1668 cm-1 for right- and left-handed R-helices, respectively; 1636-1640 and 1622-1632 cm-1 for parallel and antiparallel β-sheets, respectively; and 1660-1664 cm-1 for random conformations.23-26 The (22) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: New York, 1997. (23) Sen, A. C.; Keiderling, T. A. Biopolymers 1984, 23, 1533-1545. (24) Sen, A. C.; Keiderling, T. A. Biopolymers 1984, 23, 1519-1532. (25) Miyazawa, T.; Blout, E. R. J. Am. Chem. Soc. 1961, 83, 712719. (26) Bonora, G. M.; Moretto, V.; Toniolo, C.; Anzinger, H.; Mutter, M. Int. J. Pept. Protein Res. 1983, 21, 336-343.
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Figure 3. AFM images (5 µm × 5 µm) and the cross sections of covalently bonded poly(amino acids). The height difference (H) of the two arrows is also given.
amide II (C-N stretch and N-H deformation) region is present at 1500-1600 cm-1, but this region also includes the contribution from the aromatic ring and is not as sensitive to the changes in conformation compared to the amide I band.26,27 For PBLTyr, the peak at 1510 cm-1 is due to the para disubstituted benzene in its side chain, and peaks in the region of 1700-1750 cm-1 are from the CdO stretch of the ester side group (PBLG and PBLA) and the acid end group, which is one of the products of the termination reaction.28,29 Both chemisorbed and physisorbed films of PBLG and PLTrp existed in the right-handed R-helical conformation, and PBLTyr films were in a parallel β-sheet conformation. For PBLA, the chemisorbed polymer consisted of lefthanded R-helices and random structures, while the physisorbed PBLA existed as left-handed R-helices and parallel β-sheets. The polypeptide conformations were consistent with those observed in the bulk and in solution.21,26,30-32 The AFM images of the polymer films are shown in Figures 3 and 4. For the chemisorbed films shown in Figure 3, the root-mean-square (rms) roughness of PBLA (3 nm) was larger than for the other polymer films (1 nm). PBLG and PLTrp surfaces had a wispylike appearance, but the surfaces of PBLA and PBLTyr appeared granular. The morphology of the physisorbed polymers was considerably different, as shown in Figure 4. PBLG was composed of fibrillar and networklike structures, but PBLA, PBLTyr, and PLTrp consisted of relatively large irregular-shaped domains, which either coalesced with other structures, as in the case of PBLTyr, or formed distinct boundaries with their neighbors for PBLA and PLTrp films. These structures were thermally stable, and no change in morphology was observed after heating the samples in situ at 120 °C for 2 h. (27) Singh, B. R. Basic Aspects of the Technique and Applications of Infrared Spectroscopy of Peptides and Proteins; Singh, B. R., Ed.; American Chemical Society: Washington, DC, 2000; pp 2-37. (28) Niwa, M.; Morikawa, M.; Higashi, N. Langmuir 1999, 15, 50885092. (29) Sela, M.; Berger, A. J. Am. Chem. Soc. 1955, 77, 1893-1898. (30) Blout, E. R.; Asadourian, A. J. Am. Chem. Soc. 1956, 78, 955961. (31) Hayashi, Y.; Teramoto, A.; Kawahara, K.; Fujita, H. Biopolymers 1969, 8, 403-420. (32) Berger, A.; Noguchi, J.; Katchalski, E. J. Am. Chem. Soc. 1956, 78, 4483-4488.
To determine the change in surface properties with film thickness, we synthesized physisorbed PBLG samples with film thicknesses ranging from 13 to 67 nm by varying the reaction time from 10 to 30 min. The change in film thickness did not influence the ratio between the chemically bound and physically adsorbed PBLG. The AFM images, given in Figures 5 and 6, showed that at low film thickness, the polymer formed circular patches on the substrate; as polymer thickness increased, the patches coalesced with their neighbors, followed by the formation of fibrillar structures. A schematic of the patch-and-fiber morphology is shown in Figure 7, and the dimensions of these features, measured using section analysis, are presented in Figure 8. The height of the patch remained relatively constant (9-11 nm), but the patch diameter and the fiber height and width increased with film thickness. In addition, as the film thickness increased from 13 to 30 nm, the advancing water contact angle also increased (Figure 9), which indicated that the surface became more hydrophobic with patch coverage, but the contact angle hysteresis remained relatively constant with increasing film thickness. Discussion The morphology of physisorbed polymers was vastly different from that of their chemisorbed counterparts. Relatively large domains (1-8 µm diameter) were present for the physisorbed samples compared to the granular or wispy structure of covalently bonded poly(amino acids). The large domains and superstructures may be due to the condensation and coalescence of the patches, as evidenced by the physisorbed PBLG film for varying film thicknesses (13-67 nm). The initial PBLG patches, which were clearly visible at a film thickness of 13 nm and likely composed of physically adsorbed and chemically grafted polymers, increased in size as film thickness increased and eventually coalesced to cover the entire surface. For film thicknesses of 24 nm and greater, fiberlike structures were found on some parts of the film. Fibers of synthetic PBLG with diameters ranging from approximately 0.1 to 0.28 µm have been prepared by the precipitation of PBLG from a DMF solution.33,34 Depending (33) Ishikawa, S.; Kurita, T. Biopolymers 1964, 2, 381-393. (34) Rybnikar, F.; Geil, P. H. Biopolymers 1972, 11, 271-278.
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Notes
Figure 4. AFM images and cross sections of physisorbed poly(amino acids). The height difference (H) of the two arrows and the maximum height of the image are given. Note the scale change between the PBLG and PBLTyr images (10 µm × 10 µm) and the PBLA and PLTrp images (20 µm × 20 µm); 10 µm cross sections of PBLA and PLTrp are also provided.
Figure 5. AFM images (4 µm × 4 µm) and cross sections of physisorbed PBLG with varying film thicknesses. The height difference (H) of the two arrows and the maximum height of the image are also given.
on the age of the solution, the fibers, which are formed from the linear (head-to-tail) and side-to-side (antiparallel) association of polymer helices, would exhibit a superhelical structure.35 However, the helical sense of the superhelix was not affected by the handedness of the primary helix.33 In addition, PBLG fibers have also been obtained by Kitaev and Kumacheva12 from the adsorption of monodisperse and polydisperse PBLG (MW ) 17 400 and 308 000) onto mica substrates from a dioxane solution. We observed PBLG fibers only when the surface was entirely covered with patches. One possible hypothesis (35) Pyzuk, W.; Krupkowski, T. Makromol. Chem.sMacromol. Chem. Phys. 1977, 178, 817-826.
for the formation of such fibers may be a hydrophobic interaction. As patch coverage increased, the surface became more hydrophobic, and attractive hydrophobichydrophobic interaction between the surface and the incoming oligopeptides could have played a role in the adsorption and organization of the polymer fibers. This possibility may be rejected, however, because for the adsorption of PBLG onto a solid substrate in solution, fibrillar morphology was not observed using a hydrophobized mica substrate but was present when a freshly cleaved (negatively charged) mica substrate was used.13 Thus, electrostatic-dipole or dipole-dipole attractions between the surface and PBLG molecules may be more important in the formation of fibrillar structures.
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Figure 9. Water contact angles of physisorbed PBLG films as a function of film thickness. Figure 6. AFM images (1.25 µm × 1.25 µm) and cross sections of the patch (a) and fiber (b) morphology observed on physisorbed PBLG films. The height difference (H) of the two arrows and the maximum height of the image are also given.
Figure 7. Schematic of the patch-and-fiber morphology on a silicon substrate.
Figure 8. The patch (a) and fiber (b) dimensions as a function of film thickness for physisorbed PBLG films.
Support for the proposal emphasizing dipole interaction is provided by FTIR spectroscopy, which demonstrated that both physisorbed and chemisorbed PBLG films were in the R-helical conformation, which is known to have a relatively large dipole moment (3.5 D per residue) along
its helix.36,37 In the case of PBLG adsorption, fibers were formed on a negatively charged mica substrate, in which electrostatic-dipole interaction would have been present.13 For the adsorption of polydisperse PBLG, dipole-dipole interactions between the low MW PBLG adsorbed in the first layer and the incoming higher MW polymers have led to the formation of fibers in the second layer.12 Similar to the case of the vapor deposition of PBLG, the fibers may be formed from the dipole-dipole attraction of the initial patch layer and the incoming peptides. The fibers formed from the adsorption of polydisperse PBLG have been shown to be thicker and more linear than those from monodisperse PBLG.12 Although we did not measure polydispersity explicitly, vapor deposition polymerization produced PBLG with MW ranging from 2000 to 50 000 (for a 20 nm chemisorbed film).19 Consequently, we obtained linear and relatively long (in the order of 10 µm) fibrillar structures. For the 75 nm physisorbed PBLG film, both coalesced patches and fibers were present, and the conformation of the polymer was R-helical. As mentioned earlier, the formation of fibrils in solution or on a substrate results from the end-to-end and side-to-side association of individual chains along the length of the fibers;13,34,35,38 therefore, the helices in the fibers are believed to lie parallel to the substrate. On the other hand, the orientation of the helices in the patch morphology may be more perpendicular to the surface. For chemisorbed PBLG films, the average tilt angle of the helix is 32° with respect to the substrate.39 However, the tilt angle of an unwashed PBLG film has been found to be greater than that for a chemisorbed film because the physically adsorbed chains form antiparallel pairs with the grafted chains, preferentially aligning the chains perpendicular to the substrate.39,40 The presence of domains has been previously detected in the formation of organic self-assembled monolayers (SAMs).41-48 Islands with diameters of 20-50 nm have been imaged in the assembly of octadecylsiloxane, hexyl (36) Branden, C.; Tooze, J. Introduction to Protein Structure, 2nd ed.; Garland Publishing: New York, 1999. (37) Wada, A. J. Chem. Phys. 1959, 31, 495-500. (38) Gupta, A. K. Biopolymers 1976, 15, 1543-1554. (39) Wieringa, R. H.; Siesling, E. A.; Werkman, P. J.; Angerman, H. J.; Vorenkamp, E. J.; Schouten, A. J. Langmuir 2001, 17, 6485-6490. (40) Chang, Y. C.; Frank, C. W.; Forstmann, G. G.; Johannsmann, D. J. Chem. Phys. 1999, 111, 6136-6143. (41) Resch, R.; Grasserbauer, M.; Friedbacher, G.; Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H. Appl. Surf. Sci. 1999, 140, 168175. (42) Zhang, A. D.; Qin, J. G.; Gu, J. H.; Lu, Z. H. Thin Solid Films 2000, 375, 242-246. (43) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Ishida, T.; Hara, M.; Knoll, W. Langmuir 1998, 14, 3264-3271.
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azobenzene thiol, and n-alkanethiol monolayers. With increased exposure time, the islands grew and coalesced, but the height of the domains remained constant.41,43,44 These SAM films were formed by the mechanism of nucleation and growth of these islands, which were composed of both grafted and adsorbed molecules.44,47,49 In our case, the nucleation-and-growth mechanism may also be occurring for the vapor-deposited films of physisorbed polymers, as shown by the formation and coalescence of the PBLG patches. In addition, the domains of physisorbed PBLTyr appear to coalesce, reminiscent of the PBLG image at low film thickness (13-20 nm); however, the domains of PBLA and PLTrp did not merge but instead formed boundaries between neighboring domains. The molecular conformation of both the chemisorbed and the physisorbed polymers did not affect the final morphology of the film. For example, granular structures were observed for chemisorbed PBLA (left-handed R-helix/ random) and PBLTyr (parallel β-sheet), while the morphology of physisorbed R-helical polypeptides PBLG (fibers/network) and PLTrp (irregular-shaped domains) was quite different. Even for polymers present only in the R-helical conformation, smooth, lacey, granular, banded, and wormlike morphologies have all been observed from (44) Xu, S.; CruchonDupeyrat, S. J. N.; Garno, J. C.; Liu, G. Y.; Jennings, G. K.; Yong, T. H.; Laibinis, P. E. J. Chem. Phys. 1998, 108, 5002-5012. (45) Ohno, H.; Motomatsu, M.; Mizutani, W.; Tokumoto, H. Jpn. J. Appl. Phys., Part 1 1995, 34, 1381-1386. (46) Tamada, K.; Ishida, T.; Knoll, W.; Fukushima, H.; Colorado, R.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Langmuir 2001, 17, 19131921. (47) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558-1566. (48) Uchihashi, T.; Ishida, T.; Komiyama, M.; Ashino, M.; Sugawara, Y.; Mizutani, W.; Yokoyama, K.; Morita, S.; Tokumoto, H.; Ishikawa, M. Appl. Surf. Sci. 2000, 157, 244-250. (49) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145-1148.
Notes
LB deposition as the substrate and the spreading agent were varied.7,10,11 Furthermore, the morphology of poly(γ-methyl-L-glutamate) films formed by LB deposition exhibited an aggregate network in a monolayer matrix for both R-helical and β-sheet conformations.11 These studies indicate that the association and aggregation of the polypeptide chains, which may result in the formation of patches, fibers, and domains, may have a greater impact on film morphology than the molecular conformation of the polymer. Conclusion We have performed AFM studies on chemisorbed and physisorbed PBLG, PBLA, PBLTyr, and PLTrp films vapor deposited on silicon substrates. The morphology of physisorbed polymers consisted of large domains, while granular or wispy structures were observed for chemisorbed polymers. As film thickness increased for physisorbed PBLG, circular patches on the substrate coalesced and then fibers formed on top of the patches, which may be due to the dipole-dipole interaction of the two layers. The polymer films were thermally stable and exhibited R-helical, β-sheet, and random conformations, depending on the nature of the side group; however, the molecular conformation of both chemisorbed and physisorbed polymers did not have a significant effect on the morphology. Instead, the association of the individual polymer chains may be more important in determining the morphology of vapor-deposited poly(amino acid) films. Acknowledgment. Financial support was provided by the NSF-MRSEC Program through the Center on Polymer Interfaces and Macromolecular Assemblies (NSF-DMR 9808677) and the Stanford Graduate Fellowship program. LA020788U
