Assembly of Multilayered Films Using Well-Defined, End-Labeled Poly

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Langmuir 2007, 23, 8452-8459

Assembly of Multilayered Films Using Well-Defined, End-Labeled Poly(acrylic acid): Influence of Molecular Weight on Exponential Growth in a Synthetic Weak Polyelectrolyte System Bin Sun,† Christopher M. Jewell,‡ Nathaniel J. Fredin,‡ and David M. Lynn*,†,‡ Department of Chemistry and Department of Chemical and Biological Engineering, UniVersity of Wisconsin - Madison, 1415 Engineering DriVe, Madison, Wisconsin 53706 ReceiVed April 13, 2007. In Final Form: May 16, 2007 We report on the influence of polyanion molecular weight on the growth and structure of multilayered thin films fabricated from poly(allylamine) (PAH) and well-defined, end-labeled poly(acrylic acid) (PAA) synthesized by atom transfer radical polymerization. We observed striking differences in the growth of PAH/PAA films fabricated using well-defined PAA compared to films fabricated using higher molecular weight, commercially available PAA. Past studies demonstrate that the thicknesses of PAH/PAA films increase as linear functions of the number of PAH and PAA layers deposited over a broad range of pH (e.g., from pH 2.5 to 4.5). We observed the thicknesses of films fabricated using solutions of PAH and PAA adjusted to pH 7.5 and 3.5, respectively, to increase in a nonlinear manner. Films fabricated using well-defined, low molecular weight samples of PAA under these conditions increased in thickness exponentially. Experiments using samples of PAA having substantially non-overlapping molecular weight distributions demonstrated a clear relationship between the molecular weight of PAA and rates of film growth. We also used confocal microscopy, in combination with fluorescently end-labeled samples of PAA, to characterize the location of PAA in these assemblies. The results of these experiments, when combined, support the general conclusion that PAA is able to penetrate or diffuse into these films over large distances during assembly. The mechanism of growth for these films thus appears similar to that recently reported for the exponential growth of films fabricated using a variety of biologically relevant polyelectrolytes. The use of living/controlled methods of polymerization to synthesize well-defined samples of PAA facilitates an interpretation of these differences in film behavior as arising largely from differences in polymer molecular weight and polydispersity. This work provides insight into the assembly and structure of a well-studied weak polyelectrolyte film system and illustrates the potential of living/controlled methods of polymerization to contribute to the characterization and understanding of the physical properties of these ionically cross-linked materials.

Introduction Layer-by-layer methods for the fabrication of polyelectrolyte assemblies provide a simple and versatile approach to the fabrication of multilayered thin films.1-4 Although the influence of environmental factors such as solution pH and ionic strength on the assembly and properties of these materials has been well studied, the influence of polyelectrolyte molecular weight and polydispersity on the growth and internal structures of these assemblies is less well understood.5-9 Here, we report on the use of well-defined and fluorescently end-labeled poly(acrylic acid) (PAA) synthesized using living/controlled methods of polymerization to investigate the influence of molecular weight on the layer-by-layer assembly of thin films fabricated from PAA and poly(allylamine) (PAH). Several past studies have demonstrated that multilayered polyelectrolyte assemblies can be fabricated using PAA and PAH * Corresponding author. † Department of Chemistry. ‡ Department of Chemical and Biological Engineering. (1) Decher, G. Science 1997, 277, 1232-1237. (2) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319-348. (3) Peyratout, C. S.; Dahne, L. Angew. Chem., Int. Ed. 2004, 43, 3762-3783. (4) Hammond, P. T. AdV. Mater. 2004, 16, 1271-1293. (5) Dubas, S. T.; Schlenoff, J. B. Macromolecules 2001, 34, 3736-3740. (6) Sui, Z. J.; Salloum, D.; Schlenoff, J. B. Langmuir 2003, 19, 2491-2495. (7) Richert, L.; Lavalle, P.; Payan, E.; Shu, X. Z.; Prestwich, G. D.; Stoltz, J. F.; Schaaf, P.; Voegel, J. C.; Picart, C. Langmuir 2004, 20, 448-458. (8) Kujawa, P.; Moraille, P.; Sanchez, J.; Badia, A.; Winnik, F. M. J. Am. Chem. Soc. 2005, 127, 9224-9234. (9) Porcel, C.; Lavalle, P.; Decher, G.; Senger, B.; Voegel, J. C.; Schaaf, P. Langmuir 2007, 23, 1898-1904.

and that films fabricated from these weak polyelectrolytes exhibit functional and potentially useful properties.10-24 For example, Rubner and co-workers have demonstrated that assemblies fabricated from PAA and PAH undergo transformations in response to changes in environmental pH to generate microporous11 or nanoporous16 thin films that could be useful in applications ranging from drug delivery to the development of antireflective and superhydrophobic coatings.11,15,16,18,20,23,24 These and other past studies have generally made use of commercially available polyelectrolytes that are of high molecular weight (e.g., Mn ≈ 90 000 for PAA and Mn ≈ 70 000 for PAH) (10) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 43094318. (11) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017-5023. (12) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213-4219. (13) Xiao, K. P.; Harris, J. J.; Park, A.; Martin, C. M.; Pradeep, V.; Bruening, M. L. Langmuir 2001, 17, 8236-8241. (14) Dai, J. H.; Jensen, A. W.; Mohanty, D. K.; Erndt, J.; Bruening, M. L. Langmuir 2001, 17, 931-937. (15) Chung, A. J.; Rubner, M. F. Langmuir 2002, 18, 1176-1183. (16) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59-63. (17) Kato, N.; Schuetz, P.; Fery, A.; Caruso, F. Macromolecules 2002, 35, 9780-9787. (18) Mendelsohn, J. D.; Yang, S. Y.; Hiller, J.; Hochbaum, A. I.; Rubner, M. F. Biomacromolecules 2003, 4, 96-106. (19) Burke, S. E.; Barrett, C. J. Langmuir 2003, 19, 3297-3303. (20) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349-1353. (21) Tanchak, O. M.; Barrett, C. J. Chem. Mater. 2004, 16, 2734-2739. (22) Fujita, S.; Shiratori, S. Nanotechnology 2005, 16, 1821-1827. (23) Berg, M. C.; Zhai, L.; Cohen, R. E.; Rubner, M. F. Biomacromolecules 2006, 7, 357-364. (24) Zhai, L.; Berg, M. C.; Cebeci, F. C.; Kim, Y.; Milwid, J. M.; Rubner, M. F.; Cohen, R. E. Nano Lett. 2006, 6, 1213-1217.

10.1021/la7010875 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/06/2007

Assembly of Multilayered Films Using PAA

and synthesized using methods that typically yield broad molecular weight distributions (i.e., polydispersity indices, or PDIs, g 2.0). We reasoned that the use of living or controlled methods of polymerization to synthesize structurally well-defined samples of polyelectrolytes such as PAA or PAH would provide new opportunities to investigate the influence of molecular weight and PDI on the assembly and properties of films fabricated from these weak polyelectrolytes. In the context of addressing mechanistic questions and investigating the influence of polymer molecular weight and polydispersity on the assembly of multilayered films, living methods of polymerization offer several potential advantages over conventional methods used to synthesize commercially available polyelectrolytes.25-27 First, living methods of polymerization permit straightforward and precise control over molecular weight by control over the ratio of monomer to the amount of initiator used during synthesis. Second, it is generally feasible using such methods to synthesize polymers having narrow or low PDIs (e.g., from 1.10 to 1.20), thus making possible the synthesis of multiple polymer samples having different average molecular weights and mutually exclusive (i.e., non-overlapping) molecular weight distributions. Finally, living polymerizations permit the synthesis of polymers having precisely defined end groups by selection of appropriately functionalized initiators. The advantages outlined above, when combined, provide opportunities for the synthesis of polyelectrolytes that could simplify the identification of correlations between molecular weight, polydispersity, and the assembly or properties of multilayered films. Two recent reports describe the living/ controlled synthesis of cationic and anionic polystyrene- and poly(acrylamide)-based polymers using reversible additionfragmentation chain transfer (RAFT) polymerization and the incorporation of these well-defined polyelectrolytes into multilayered films.28,29 Here, we describe the synthesis of welldefined and end-labeled PAA derivatives (polymers 1 and 2) using atom transfer radical polymerization (ATRP)30 and report on the use of these polymers to investigate the influence of molecular weight on the layer-by-layer assembly of PAH/PAA films.

(25) Odian, G. Principles of Polymerization, 4th ed.; John Wiley and Sons, Inc.: Hoboken, NJ, 2004. (26) Webster, O. W. Science 1991, 251, 887-893. (27) Quirk, R. P.; Lee, B. Polym. Int. 1992, 27, 359-367. (28) Baussard, J. F.; Habib-Jiwan, J. L.; Laschewsky, A. Langmuir 2003, 19, 7963-7969. (29) Morgan, S. E.; Jones, P.; Lamont, A. S.; Heidenreich, A.; McCormick, C. L. Langmuir 2007, 23, 230-240. (30) Matyjaszewski, K.; Xia, J. H. Chem. ReV. 2001, 101, 2921-2990.

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We report striking differences in the growth profiles of PAH/ PAA films fabricated using well-defined, low molecular weight PAA compared to films fabricated using higher molecular weight, commercially available PAA. Past studies demonstrate that the thicknesses of PAH/PAA films increase as linear functions of the number of PAH and PAA layers deposited over a broad range of pH (e.g., using solutions of PAH or PAA ranging from pH 2.5 to 4.5).10 However, we observed the thicknesses of films fabricated using solutions of PAH and PAA adjusted to pH 7.5 and 3.5, respectively, to increase in a nonlinear manner. The growth of films fabricated using well-defined, low molecular weight samples of PAA under these pH conditions increased in a manner that was substantially nonlinear and similar to the growth behavior reported recently for the exponential growth of multilayered assemblies fabricated using a variety of strong or weak polyelectrolytes.7,9,31-44 Experiments using samples of polymer 1 having substantially non-overlapping molecular weight distributions demonstrated a clear relationship between the molecular weight of PAA and rates of film growth. We also used confocal microscopy, in combination with fluorescently endlabeled polymer 2, to characterize the location of PAA in these assemblies. The results of these experiments, when combined, support the general conclusion that PAA is able to penetrate or diffuse into these multilayered films over large distances during film assembly, and provide insight into the influence of polymer molecular weight and polydispersity on the assembly and structure of films fabricated from PAA and PAH. Materials and Methods Materials. 2-Bromopropionyl bromide, ethylene diamine, and di-t-butyl dicarbonate were purchased from Aldrich Chemical Co. (Milwaukee, WI). tert-Butyl acrylate, trifluoroacetic acid (TFA), and N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) were purchased from Acros Organics (Geel, Belgium). Phthalic anhydride was purchased from Kodak, Inc., and anhydrous hydrazine was purchased from Fisher Biotech (Wembley, Australia). 5-(and 6)carboxytetramethylrhodamine succinimidyl ester (TMR-NHS) was purchased from Invitrogen (Carlsbad, CA). Samples of high molecular weight commercially available PAA used to fabricate multilayered films (PAA, MW ≈ 90 000) were purchased as a 25% aqueous solution in water from Polysciences, Inc. (Warrington, PA). Samples of low molecular weight commercially available PAA (MW ≈ 15 000) were purchased as a 35% aqueous solution in water from Aldrich Chemical Co. (Milwaukee, WI). PAH (MW ≈ 60 000) was purchased from Alfa Aesar (Ward Hill, MA). tert-Butyl acrylate (31) Elbert, D. L.; Herbert, C. B.; Hubbell, J. A. Langmuir 1999, 15, 53555362. (32) Picart, C.; Lavalle, P.; Hubert, P.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C. Langmuir 2001, 17, 7414-7424. (33) McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. Langmuir 2001, 17, 6655-6663. (34) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J. C.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 1253112535. (35) DeLongchamp, D. M.; Kastantin, M.; Hammond, P. T. Chem. Mater. 2003, 15, 1575-1586. (36) Schoeler, B.; Poptoschev, E.; Caruso, F. Macromolecules 2003, 36, 52585264. (37) Boulmedais, F.; Ball, V.; Schwinte, P.; Frisch, B.; Schaaf, P.; Voegel, J. C. Langmuir 2003, 19, 440-445. (38) Garza, J. M.; Schaaf, P.; Muller, S.; Ball, V.; Stoltz, J. F.; Voegel, J. C.; Lavalle, P. Langmuir 2004, 20, 7298-7302. (39) Lavalle, P.; Vivet, V.; Jessel, N.; Decher, G.; Voegel, J. C.; Mesini, P. J.; Schaaf, P. Macromolecules 2004, 37, 1159-1162. (40) Guyomard, A.; Muller, G.; Glinel, K. Macromolecules 2005, 38, 57375742. (41) Zhang, L.; Li, B. Y.; Zhi, Z. L.; Haynie, D. T. Langmuir 2005, 21, 54395445. (42) Porcel, C.; Lavalle, P.; Ball, V.; Decher, G.; Senger, B.; Voegel, J. C.; Schaaf, P. Langmuir 2006, 22, 4376-4383. (43) Jourdainne, L.; Arntz, Y.; Senger, B.; Debry, C.; Voegel, J. C.; Schaaf, P.; Lavalle, P. Macromolecules 2007, 40, 316-321. (44) Ji, J.; Fu, J. H.; Shen, J. C. AdV. Mater. 2006, 18, 1441-1444.

8454 Langmuir, Vol. 23, No. 16, 2007 was washed three times with 5% aqueous NaOH, washed with distilled, deionized water, dried over MgSO4, filtered, and then distilled under vacuum prior to use. All other materials were used as received without further purification unless otherwise noted. Test grade n-type silicon wafers were purchased from Si-Tech, Inc. (Topsfield, MA). Fluorescent microscopy calibration slides containing fluorescein used for confocal microscopy experiments were purchased from Microscopy and Microscopy Education, Inc. (Allen, TX). Deionized water (18 MΩ) was used for the washing steps and to prepare all polymer solutions. Compressed air used to dry films and coated substrates was filtered through a 0.4 µm membrane syringe filter. General Considerations. 1H nuclear magnetic resonance (NMR) spectra were recorded on Bruker AC+ 250 (250.133 MHz) and Bruker AC+ 300 (300.135 MHz) spectrometers. Chemical shift values are given in ppm and are referenced with respect to residual protons from solvent. Gel permeation chromatography (GPC) was performed using a Waters 515 HPLC pump (Waters Corporation, Milford, MA), a Rheodyne model 7725 injector with a 20-µL injection loop, and two Waters Styragel HT 6E columns in series. Tetrahydrofuran (THF) containing 0.1 M triethylamine was used as the eluent at a flow rate of 1.0 mL/min. Data were collected using a Waters 2410 refractive index detector and processed using the Waters Empower software package. Molecular weights are reported relative to monodisperse polystyrene standards. Silicon substrates (e.g., 0.5 × 3.0 cm) were cleaned with methylene chloride, ethanol, methanol, and deionized water, and dried under a stream of filtered compressed air. Surfaces were then activated by etching with oxygen plasma for 5 min (Plasma Etch, Carson City, NV) prior to film deposition. Ellipsometric thicknesses of films deposited on silicon substrates were determined using a Gaertner LSE Stokes ellipsometer (632.8 nm, incident angle ) 70°). Data were processed using the Gaertner Ellipsometer Measurement Program software package. Relative thicknesses were calculated assuming an average refractive index of 1.55 for the multilayered films. Thicknesses were determined in at least 12 different standardized locations on each substrate and are presented as an average (with standard deviation) of measurements for three different films. Laser scanning confocal microscopy (LSCM) was performed using a Bio-Rad Radiance 2100 MP Rainbow laser scanning confocal microscope. Images were processed using the Bio-Rad LaserSharp 2000 processing kit, Image J, and Adobe Photoshop 8.0. All films fabricated on silicon substrates for characterization by ellipsometry or atomic force microscopy (AFM) were dried under a stream of nitrogen prior to measurement. Films used for characterization by LSCM and AFM imaging in liquid were not dried prior to or during imaging. Synthesis of 2-Phthalimido-2-bromo-N-ethyl-propanamide Initiator (3). N-(2-aminoethyl)phthalimide hydrochloride (1.41 g, 5.28 mmol) and triethylamine (1.84 mL, 13.2 mmol) were placed in a 125 mL round-bottomed flask and dissolved in dichloromethane (55 mL). A solution of 2-bromopropionyl bromide (542.2 µL, 5.28 mmol) in dichloromethane (27.5 mL) was added through a dropping funnel at 0 °C with stirring for 1 h. The reaction mixture was allowed to stir for another 6 h at room temperature and was then washed with deionized water (25 mL), saturated sodium bicarbonate (25 mL), and again with deionized water (25 mL). The dichloromethane solution was dried over MgSO4 and the solvent was removed in vacuo. The crude product was recrystallized from hexane/EtOAc and isolated as a white powder in 68% yield. 1H NMR data: (300 MHz, CDCl3): δ (ppm) 1.81 (d, 3H, -CH(CH3)Br), 3.59 (m, 2H, -CH2-NH), 3.92 (t, 2H, -CH2-N), 4.34 (q, 1H, -CH(CH3)Br), 7.75 (m, 2H, CH Ar), 7.88 (m, 2H, CH Ar). 13C NMR (75 MHz, CDCl3): δ (ppm) 23.195, 37.415, 39.968, 45.018, 123.679, 132.127, 134.416, 168.700, 170.011. Synthesis of Phthalamide End-Functionalized Poly(tert-butyl acrylate) (4). ATRP of tert-butyl acrylate was conducted in the following general manner: tert-Butyl acrylate (3.6 g, 0.028 mol), Cu(I)Br (10.0 mg, 0.07 mmol), and phthalamide-functionalized initiator 3 were added to a dry round-bottomed flask. The flask was sealed with a rubber septum, then purged with nitrogen for 15 min, and PMDETA (14.6 µL, 0.07 mmol) was injected into the flask by

Sun et al. syringe. The solution was then placed in an oil bath thermostated at 72 °C. After 9 h, the solid or viscous reaction mixture was dissolved in anhydrous THF, stirred over DOWEX 50WX8 ion-exchange resin for 1.5 h, and filtered through a short plug of alumina to remove the copper catalyst. THF was removed by evaporation, and the crude polymer product was dissolved in diethyl ether (10 mL) and precipitated into a MeOH/water solution (100 mL, 50:50 v/v). Precipitated polymer was collected by decanting the supernatant, and the polymer was precipitated twice more using the above procedure. The final product was dried under vacuum to yield phthalamide end-functionalized PAA as a white powder. Representative 1H NMR data (300 MHz, CDCl3): broad backbone proton resonances were at δ ) 1.81 (s) and 2.23 (s) ppm, with a sharp singlet corresponding to the t-butyl group at δ ) 1.45 ppm; resonances arising from phthalimide end groups were observed from 7.7 to 7.9 ppm. Synthesis of Amine End-Functionalized Poly(tert-butyl acrylate) (5). Phthalamide end-labeled poly(tert-butyl acrylate) (200 mg) was dissolved in anhydrous THF (3.0 mL), and anhydrous hydrazine (50 µL) and triethylamine (0.1 mL) were added. The resulting solution was stirred at room temperature overnight. The solvent was removed under vacuum, and the resulting product was redissolved in diethyl ether (1 mL) and precipitated into a MeOH/ water solution (10 mL, 50:50 v/v). Polymer was collected by centrifugation and dried under vacuum. Removal of the phthalimide end groups was confirmed by the disappearance of the resonances at 7.7 to 7.9 ppm in the 1H NMR spectrum of the isolated material. Representative 1H NMR data (300 MHz, CDCl3): broad backbone proton resonances were at δ ) 1.81 (s) and 2.23 (s) ppm, with a sharp singlet corresponding to the t-butyl group at δ ) 1.45 ppm. Synthesis of Fluorescently End-Labeled Poly(tert-butyl acrylate) (6). Conjugation of fluorophore to amine end-labeled poly(tert-butyl acrylate) was conducted in the following general manner: Amine end-functionalized polymer (50 mg) was dissolved in anhydrous THF (800 µL). To this solution was added TMR-NHS (5 mg as a solution in 400 µL of MeOH). Triethylamine (0.1 mL) was added, and the reaction mixture was stirred at 50 °C for 4 days. The solvent was removed under reduced pressure, and the crude product was dissolved in diethyl ether (600 µL) and precipitated into a MeOH/water solution (5 mL, 50:50 v/v) to remove excess unreacted fluorophore. Polymer was collected by centrifugation, dried under vacuum, and used for the synthesis of fluorescently end-labeled PAA without further purification. Synthesis of Fluorescently End-Labeled PAA (2). Fluorophore end-labeled poly(tert-butyl acrylate) was dissolved in TFA (4 mL), and the resulting solution was stirred at ambient temperature overnight. TFA was removed under reduced pressure, and the crude product was dissolved in water and purified by dialysis (Spectra Por, MWCO ) 3500) against deionized water at ambient temperature in the dark for 3 days. The resulting solution of polymer was then freeze-dried to give the desired product as a bright pink solid. Representative 1H NMR data (300 MHz, D2O): broad backbone proton resonances were at δ ) 1.87 (s) and 2.63 (s) ppm. Preparation of Polyelectrolyte Solutions and Fabrication of Multilayered Films. Solutions of PAH and polymers 1 or 2 used for dipping (10 mM with respect to the molecular weight of the polymer repeat unit) were prepared in water. Solutions of PAH and PAA were adjusted to pH values of 7.5 and 3.5, respectively, using 1.0 M NaOH or 1.0 M HCl. Multilayered films were fabricated either manually or by using an automated dipping robot (model DR-3, Riegler & Kirstein GmbH, Berlin, Germany) using an alternating dipping procedure. Briefly, (1) substrates were submerged in a solution of PAH for 10 min, (2) substrates were removed and immersed in a water wash bath for 2 min followed by a second wash bath for 1 min, (3) substrates were submerged in a solution of PAA for 10 min, and (4) substrates were rinsed in the manner described above. This cycle was repeated until the desired number of PAH and PAA layers had been deposited. Films prepared using this procedure were either used immediately or dried under a stream of filtered compressed air and stored in a vacuum desiccator until use. All films were fabricated at ambient room temperature.

Assembly of Multilayered Films Using PAA

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Scheme 1. Synthesis of Fluorescently End-Labeled PAA Derivatives

Characterization of Surface Topography by AFM. Film topography and surface roughness were obtained from height data imaged in tapping mode on a Nanoscope MultiMode atomic force microscope (Digital Instruments, Santa Barbara, CA), using scan rates of 20 µm/s to obtain 256 × 256 pixel images. For imaging in air, silicon cantilevers with a spring constant of 40 N/m and a radius of curvature of less than 10 nm were used (model NSC15/NoAl, MikroMasch USA, Inc., Portland, OR). For imaging in liquid, silicon nitride cantilevers with a spring constant of 0.58 N/m were used (model NP-S, Veeco Probes, Santa Barbara, CA). For each sample, at least two different 10 µm × 10 µm scans were obtained at randomly chosen points near the center of the film. Height data were flattened using a second-order fit. Root-mean-squared surface roughness (Rrms) was calculated over the scan area using the Nanoscope software package (Digital Instruments, Santa Barbara, CA). Characterization of Multilayered Films Using LSCM. For LSCM experiments, films were fabricated on commercially available fluorescent microscopy calibration slides containing fluorescein. Samples were imaged while submerged in water at ambient temperature using a 100×/1.40 NA oil-immersion objective. Fluorescein and tetramethylrhodamine were excited sequentially using laser lines at 488 and 543 nm, respectively. Fluorescence emission signals were collected individually for the green and red channels using direct xz scans (N ) 1, scan speed ) 25 lps) to construct a cross-section fluorescence profile of the film. Image acquisition was carried out using section thicknesses of 0.72 µm collected at intervals of 0.3 µm, and individual channels were processed in Image J and Adobe Photoshop 8.0 to create two-color images.

Results and Discussion Synthesis and Characterization of Well-Defined, EndLabeled PAA Derivatives. The synthetic approach reported here builds upon past reports that PAA can be synthesized with control over molecular weight and polydispersity by the copper-catalyzed ATRP of tert-butyl acrylate followed by treatment with acid to remove tert-butyl protecting groups.45-47 Scheme 1 shows our general synthetic approach. We selected phthalamide-functionalized R-bromo amide initiator 3 for use in this initial study for two reasons: (i) removal of the phthalamide group after polymerization would afford a polymer with a primary aminefunctionalized end group48-50 suitable for conjugation of a (45) Davis, K. A.; Matyjaszewski, K. Macromolecules 2000, 33, 4039-4047. (46) Davis, K. A.; Charleux, B.; Matyjaszewski, K. J. Polym. Sci. Polym. Chem. 2000, 38, 2274-2283. (47) Ma, Q. G.; Wooley, K. L. J. Polym. Sci. Polym. Chem. 2000, 38, 48054820.

fluorescent probe, and (ii) this approach would facilitate conjugation of a fluorescent probe through an amide linkage that would be stable upon further chemical transformation of the polymer and would not hydrolyze readily under the aqueous conditions required for layer-by-layer assembly. We note here that past reports demonstrate that R-bromo ester-functionalized initiators typically provide greater levels of control over polymerization than R-bromo amide-functionalized initiators.51,52 However, amide-functionalized initiator 3 yielded levels of control over molecular weight and polydispersity suitable for all investigations described in this study (as described below). The copper-catalyzed ATRP of tert-butyl acrylate using initiator 3 was conducted in the absence of solvent at 72 °C and proceeded to yield poly(tert-butyl acrylate) 4 having a phthalamide-functionalized end group in 65% isolated yield. Characterization of samples of polymer 4 using GPC demonstrated that molecular weight could be controlled by manipulating the ratio of monomer to initiator ([M]/[I]) used for ratios up to 400:1 (see Figure S1, Supporting Information) and that the PDIs of these samples were generally low (e.g., ranging from ∼1.20 to 1.35). We selected two samples of polymer 4 having number-average molecular weights of ∼7000 (PDI ) 1.36; referred to hereafter as polymer 4a) and ∼49 000 (PDI ) 1.27; referred to hereafter as polymer 4b) for further investigation in this study. Estimation of the molecular weights of polymers 4a and 4b by end-group analysis using 1H NMR spectroscopy yielded number average molecular weights similar to those determined using GPC (e.g., ∼7600 and ∼50 300, respectively). Inspection of the GPC traces of these polymers revealed the molecular weight distributions of these two samples to be substantially non-overlapping (see Figure S2, Supporting Information). Treatment of polymers 4a and 4b with TFA overnight resulted in complete removal of the tert-butyl groups (as determined by 1H NMR spectroscopy) and afforded samples of PAA derivatives 1a and 1b, having phthalamide-functionalized end groups in 95% yield after purification by dialysis. (48) Lecolley, F.; Waterson, C.; Carmichael, A. J.; Mantovani, G.; Harrisson, S.; Chappell, H.; Limer, A.; Williams, P.; Ohno, K.; Haddleton, D. M. J. Mater. Chem. 2003, 13, 2689-2695. (49) Postma, A.; Davis, T. P.; Evans, R. A.; Li, G. X.; Moad, G.; O’Shea, M. S. Macromolecules 2006, 39, 5293-5306. (50) Postma, A.; Davis, T. P.; Moad, G.; O’Shea, M. S. React. Funct. Polym. 2006, 66, 137-147. (51) Limer, A.; Haddleton, D. M. Macromolecules 2006, 39, 1353-1358. (52) Venkataraman, S.; Wooley, K. L. Macromolecules 2006, 39, 96619664.

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Figure 1. Plot of ellipsometric thickness vs the number of PAH/ PAA bilayers deposited for multilayered polyelectrolyte assemblies fabricated using three different samples of PAA. Symbols correspond to films fabricated using commercially available PAA (O), lower molecular weight polymer 1a (0), higher molecular weight polymer 1b (]), and a mixture of equal weights of polymers 1a and 1b (∆). Each symbol represents the average value of film thickness calculated from individual measurements of three different films in each case.

Treatment of polymer 4 with anhydrous hydrazine in THF resulted in removal of the phthalamide end group (as determined by 1H NMR spectroscopy) to give primary amine-functionalized poly(tert-butyl acrylate) 5 in 61% isolated yield. Reaction of polymer 5 with an excess of TMR-NHS yielded fluorescently end-labeled poly(tert-butyl acrylate) 6. Characterization by GPC of samples of polymers 6a and 6b, prepared from polymers 4a and 4b, demonstrated that the molecular weights and PDIs of these polymers did not change measurably under the conditions used for these chemical transformations (data not shown). Treatment of polymers 6a and 6b with TFA resulted in removal of the tert-butyl groups and afforded fluorescently end-labeled PAAs 2a and 2b. Aromatic resonances arising from TMR observed in the 1H NMR spectra of these polymers confirmed the presence of the fluorophore in these materials; characterization by agarose gel electrophoresis demonstrated that the TMR fluorophore was linked covalently to the polymer and that these samples were free of unreacted TMR (see Figure S3, Supporting Information). Fabrication and Characterization of Multilayered Films Using Well-Defined Polymers 1 and 2. We used phthalamide end-labeled PAAs 1a and 1b (prepared from 4a and 4b) to fabricate multilayered PAH/PAA films using an aqueous alternate dipping procedure. All films fabricated in these initial studies were fabricated on planar silicon substrates to facilitate characterization of film thickness and growth profiles using ellipsometry. Past studies have demonstrated that the thicknesses of films fabricated from PAH and PAA are influenced significantly by the pH of the solutions of these polyelectrolytes used during fabrication.10,12 The pH of polyelectrolyte solutions used for all experiments reported here was adjusted to 3.5 (for PAA) and 7.5 (for PAH), because these conditions have been reported to (i) lead to thicker films than those fabricated at lower pH values and (ii) lead to assemblies with interesting and potentially useful pH-responsive properties.11 Figure 1 shows a plot of the optical thicknesses of PAH/PAA films fabricated using solutions of either 1a or 1b, or a solution containing a mixture of equal weights of 1a and 1b, as a function of the number of PAH/PAA layer pairs (hereafter termed “bilayers”) deposited. For comparison, Figure 1 also shows the growth profile for PAH/PAA films fabricated using a solution of commercially available PAA similar to that used in previously reported studies (MW ≈ 90 000). Inspection of these data reveals that the growth profiles of films fabricated from polymers 1a

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and 1b vary significantly from that of the film fabricated using the commercially available PAA. The thickness of the film fabricated from commercial PAA appears to increase in a manner that is nearly linear (although not strictly linear, as discussed below) with respect to the number of bilayers deposited. However, the growth profiles of films fabricated using polymers 1a and 1b were substantially nonlinear and similar to those reported for films that increase in thickness exponentially.7,9,31-44 The growth profiles of PAH/PAA films fabricated using fluorescently endlabeled polymers 2a and 2b did not vary significantly from those shown in Figure 1 using polymers 1a and 1b (data not shown). Further inspection of the data in Figure 1 reveals that the differences in the rates of nonlinear or exponential growth for films fabricated from polymers 1a and 1b correlate with differences in the molecular weights of these two polymers. Specifically, the rate of growth of the film fabricated from lower molecular weight polymer 1a is greater than that of the film fabricated from higher molecular weight polymer 1b. We note further that the growth profile of the film fabricated from the solution containing a mixture of these two polymers lies between the profiles for the PAH/1a and PAH/1b films. The thicknesses of films fabricated without drying after the deposition of each bilayer were similar to those shown in Figure 1; we return to these observations again below. The optical thicknesses of films fabricated from polymer 1a or 1b that were more than ∼10 bilayers thick could not be measured reliably using ellipsometry. We therefore also measured the physical thicknesses of these films using AFM to characterize the height profiles of films that were intentionally scratched prior to imaging. The average thicknesses of PAH/1a and PAH/ 1b films 10 bilayers thick measured by AFM (555 and 385 nm, respectively) agreed reasonably well with optical thicknesses measured by ellipsometry (562 and 357 nm, respectively; see Figure 1). Measurements of the thicknesses of PAH/1a and PAH/ 1b films 12 bilayers thick by AFM (1360 and 614 nm, respectively) confirmed that film growth continued to increase in a nonlinear manner upon deposition of additional PAH/PAA layers. Many multilayered polyelectrolyte film systems increase in thickness in a manner that is linear with respect to the number of layers of polymer deposited.2 However, several recent reports have identified polyelectrolyte pairs and fabrication conditions for which increases in thickness can also occur exponentially.7,9,31-44 Exponential increases in film growth have been considered to occur by one of two general mechanisms: one attributed to progressive increases in the surface roughness of the films upon successive deposition of each new layer,33,35,36 and one demonstrated to result from the ability of at least one polyelectrolyte component to diffuse into or out of a film during assembly.7,34,37,39,44 To determine whether increases in surface roughness might contribute to the exponential growth profiles shown in Figure 1, we used AFM to characterize the morphology and root-mean-squared roughnesses (Rrms) of films fabricated from low molecular weight polymer 1a and higher molecular weight polymer 1b. The films used in these experiments were fabricated using the methods described above and then transferred immediately (and without drying) to a liquid-filled AFM imaging cell for characterization. Figure 2 shows representative images (10 µm × 10 µm) of the surfaces of two films 12 bilayers thick fabricated using polymer 1a (Figure 2A; Rrms ) 5.5 nm) and polymer 1b (Figure 2B; Rrms ) 6.7 nm). The smooth surfaces of these films suggest that exponential increases in the thicknesses of films fabricated using polymers 1a and 1b do not arise from large increases in surface roughness during fabrication. The

Assembly of Multilayered Films Using PAA

Figure 2. Tapping-mode AFM images (10 µm × 10 µm) and values of Rrms for PAH/PAA films 12 bilayers thick fabricated and then transferred immediately (and without drying) to a liquid-filled AFM imaging cell for characterization: (A) a film fabricated using lower molecular weight polymer 1a, and (B) a film fabricated using higher molecular weight polymer 1b. The scale in the z direction is 100 nm for each image.

surfaces of films that were dried and imaged under air were more rough, consistent with the results of past studies describing the characterization of dried PAH/PAA films.22 As noted above, however, measurement of the thicknesses of films such as those shown in Figure 2 (fabricated without drying after the deposition of each bilayer) demonstrated that the exponential growth profiles shown in Figure 1 did not arise from changes in film structure that could occur upon drying. To consider the potential influence of the diffusion of PAA on the exponential growth of PAH/PAA assemblies, we return to Figure 1. These data demonstrate clearly that the rates of growth for films fabricated from polymers 1a and 1b correlate with differences in the molecular weights of these two polyelectrolytes. Specifically, the thicknesses of films fabricated using lower molecular weight polymer 1a are significantly greater than those fabricated using polymer 1b. Because the mobility of low molecular weight polymers is greater than that of higher molecular weight polymers, these results are consistent with the view that the nonlinear growth of these films could arise from the diffusion of PAA into these materials during assembly. Several recent reports have demonstrated that LSCM can be used to observe the diffusion of fluorescently labeled polyelectrolytes during the assembly of multilayered films that grow exponentially.7,9,34,38,39,42-44 Below, we describe experiments using fluorescently end-labeled polymers 2a and 2b that sought to determine whether PAA was able to penetrate or diffuse into these films during assembly. As noted above, the conjugation of TMR to the ends of polymers 2a and 2b did not alter the growth profiles of PAH/PAA films relative to films fabricated using phthalamide end-labeled polymers 1a and 1b (e.g., Figure 1). All films fabricated for characterization by LSCM were fabricated on commercially available fluorescent microscopy calibration slides containing fluorescein to allow identification of the location of this substrate (and, thus, the bottom of the multilayered film) relative to the location of TMR-labeled PAA. Films used for these experiments were fabricated from 20 bilayers of PAH and PAA to provide films thick enough to permit acquisition of sufficient fluorescence data using section thicknesses of 0.72 µm collected at intervals of 0.3 µm. Figure 3a shows the cross-section of a film having the general architecture (PAH/2a)20 (fabricated entirely from PAH and fluorescently labeled polymer 2a) obtained using LSCM in direct xz scanning mode. Inspection of this image reveals red fluorescence arising from TMR distributed uniformly over the entire cross-section of

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Figure 3. LSCM images (72.8 µm × 19.3 µm) of multilayered PAH/PAA films fabricated using phthalamide end-functionalized polymers 1a,b and TMR (red) end-labeled polymers 2a,b. All films were fabricated on fluorescent microscopy calibration slides containing fluorescein (green) to permit identification of the film/ substrate interface (see text). Images correspond to films having the structure (A) (PAH/2a)20, (B) (PAH/1a)19(PAH/2a), and (C) (PAH/ 1a)19(PAH/2b).

the film. The average thickness of this film was estimated to be 3.5 µm by computer image analysis. Control experiments using a (PAH/1a)20 film demonstrated that unlabeled polymer 1a did not yield any observable red or green fluorescence when imaged by LSCM (data not shown). We next fabricated films having the general structure (PAH/ 1a)19(PAH/X) consisting of a “base” of film 19 bilayers thick fabricated using unlabeled polymer 1a and a final bilayer fabricated using either fluorescently labeled polymer 2a or 2b. Figure 3b shows an image of a film having the architecture (PAH/1a)19(PAH/2a). This image reveals a broad band of red fluorescence in the topmost portion of the film and a dark band (corresponding to a region of the film devoid of labeled PAA) located between the red band and the green fluorescent substrate. The thickness of the band of red fluorescence extends over a distance representing ∼65% of the overall thickness of this film (e.g., ∼2.3 µm over a total film thickness of ∼3.5 µm). This observation differs somewhat from the behavior reported in past studies of the growth of films fabricated using poly(lysine) and hyaluronic acid (HA), for which fluorescently labeled poly(lysine) was observed to diffuse throughout the entire thickness of the film during fabrication.34 We note, however, that the portion of the film in Figure 3b in which red fluorescence is observed is considerably thicker (e.g., more than half of the thickness of the film) than the increase in thickness that would be expected to arise from the deposition of a single, final layer of polymer 2a. This result demonstrates that fluorescently labeled polymer 2a was able to penetrate into the base of unlabeled (PAH/1a)19 layers over a large distance during the deposition of the final layer of polymer 2a. Figure 3c shows an image of a film having the general structure (PAH/1a)19(PAH/2b). The relative thickness of this film, as estimated by computer image analysis, is similar to that of the film shown in Figure 3b. However, the band of red fluorescence arising from the incorporation of polymer 2b is less intense than the red fluorescence shown in Figure 3b and, in contrast to this previous experiment, the thickness of the red band extends over only ∼40% of the overall thickness of the film (e.g., 1.7 µm over a total film thickness of ∼3.9 µm). This result demonstrates that higher molecular weight polymer 2b is also able to penetrate into the base of unlabeled (PAH/1a)19 layers during the deposition of the final layer, but that it does

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Figure 4. Plot of ellipsometric thickness vs the number of PAH/ PAA bilayers deposited for films fabricated using polymer 1b. All films were fabricated using solutions of PAA at pH ) 3.5. Symbols correspond to films fabricated using solutions of PAH adjusted to pH values of 7.5 (]), 5.5 (0), 3.5 (∆), and 2.0 (O).

not penetrate or diffuse as deeply into the film as lower molecular weight polymer 2a (under otherwise identical conditions). Discussion of the Influence of Molecular Weight and Polydispersity of PAA on Film Growth. Past studies of films fabricated using commercially available samples of PAH and PAA report that film thicknesses increase in a linear manner over a broad range of pH (e.g., from pH 2.5 to 4.5).10 Our measurements of films fabricated using commercially available PAA with molecular weights similar to those used in previous studies (see Figure 1) demonstrate that film growth is almost linear (but clearly not strictly linear) when films are fabricated under the pH conditions used in this study (pH 7.5 for PAH; pH 3.5 for PAA). The nonlinearity of this growth profile is reproducible over at least 10 different trials (not shown), and hints at the complexity of the molecular level processes that may occur during fabrication under these conditions. The results of our experiments using well-defined and lower molecular weight samples of PAA (see Figure 1) suggest that such complexity could arise from (i) the higher molecular weight of this commercially available sample, or (ii) the polydispersity of this sample (and the ability of low molecular weight fractions of PAA in these samples to diffuse into these assemblies during fabrication). We return to these observations in the discussion below. Past studies demonstrate that the thicknesses and internal structures of PAH/PAA films vary considerably depending upon the values of pH at which they are assembled.12 For example, solution conditions that increase the extent to which these polyelectrolytes are ionized (e.g., higher pH for PAA, lower pH for PAH) generally result in thinner films with high degrees of ion pairing and ionic cross-linking. In contrast, films fabricated under conditions that result in lower polymer charge densities (such as the values of pH used in this study) are generally thicker, less ionically cross-linked, and composed of layers of that are more interpenetrated. Thus, one possible explanation for the exponential growth observed using polymers 1a and 1b is that the relatively “loose” internal structures of films fabricated under the conditions used here are sufficient to permit the penetration and diffusion of low molecular weight PAA during assembly. This proposition is supported by the results of additional experiments for which films were fabricated using solutions of PAH adjusted to lower values of pH (Figure 4; pH of PAA solutions held constant at 3.5). The results of these experiments demonstrate that, as the pH of the PAH solution is lowered from 7.5 to 2.0 (conditions that increase the ionization of PAH and promote the growth of thinner, more ionically cross-linked films),10 film growth profiles transition from exponential growth

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to linear growth. We caution, however, that repeated exposure of the weak polyelectrolytes in these materials to large changes in pH introduces additional levels of complexity to this system (as discussed below), and that our current results thus do not rule out other potential mechanistic explanations. The data in Figure 1 demonstrate that the use of lower molecular weight PAA leads to films that increase in thickness more rapidly than films fabricated using higher molecular weight PAA. In this respect, the behavior of this PAH/PAA system appears to be similar to the results of a recent report on exponentially growing films fabricated from chitosan (CH) and HA,7 which demonstrated that films fabricated using low molecular weight CH were thicker than those fabricated using higher molecular weight CH. The exponential growth of the CH/HA system was shown to involve the diffusion of fluorescently labeled CH, and the extents and dynamics of CH diffusion during assembly were described in terms of changes in the chemical potentials of polymer chains diffusing through a film relative to chains in solution.7 In general, the diffusion of PAA into PAH/PAA films must also be governed by changes in chemical potentials. We note, however, that several aspects of our experimental system complicate the identification of driving forces for PAA diffusion relative to past studies of exponentially growing film systems. For example, whereas past studies of the CH/HA system7 were conducted at constant pH and ionic strength, the pH values of the PAH and PAA dipping solutions used in this study are different (pH 7.5 for PAH; pH 3.5 for PAA); these differences in pH appear to be critical to facilitating the exponential growth of the films studied here (e.g., see Figure 4). Both PAH and PAA are weak polyelectrolytes and, as a result, the ionization of these polymers is influenced strongly by changes in pH experienced during fabrication. Because these assemblies are exposed repeatedly to large changes in pH, and because the charge densities of weak polyelectrolytes in solution could vary from that of a polyelectrolyte in a film at the same pH,53 it is difficult to isolate the origin of driving forces governing the diffusion of PAA in this system (e.g., in comparison to the CH/HA system). Finally, to consider the potential influence of the polydispersity of PAA on the nonlinear growth of PAH/PAA films, we return again to Figure 1. One benefit of the use of living/controlled methods of polymerization to synthesize low polydispersity samples of PAA is the ability to identify and interpret differences in film growth as arising from differences in molecular weight. However, large differences do exist in the film growth profiles associated with polymers 1a and 1b as compared to those associated with films fabricated using commercially available PAA. We also note that a potential inconsistency arises when these large differences in film growth are explained solely in terms of average molecular weight. For example, the sample of commercially available PAA used in this study is of high molecular weight (MW ≈ 90 000) but it is also polydisperse (PDI ≈ 6.2). As a result, samples of this polymer will contain a large mole fraction of low molecular weight PAA. The slight but notable nonlinear increase in the growth of films fabricated using commercially available PAA (Figure 1) is consistent with the view that diffusion of this low molecular weight fraction of PAA chains could also occur in this system. The results of additional experiments investigating the fabrication of PAH/ PAA films using a commercially available sample of lower molecular weight PAA (Mn ≈ 15 000) demonstrated that these films also exhibited nonlinear, exponential growth similar to that observed using polymers 1a and 1b (see Figure S4, Supporting Information). These results suggest that molecular weight, rather (53) Choi, J.; Rubner, M. F. Macromolecules 2005, 38, 116-124.

Assembly of Multilayered Films Using PAA

than polydispersity, plays a dominant role in governing the growth of these assemblies.54 However, the results of our film fabrication experiments conducted using solutions containing equal weights of polymers 1a and 1b (see Figure 1) suggest that the presence of higher molecular weight chains in a polydisperse sample likely also play a significant role in governing the growth and behavior of these assemblies. In this experiment, the polydispersity of PAA is broadened relative to a solution of either polymer alone (albeit artificially, and in a manner that leads to a bimodal molecular weight distribution; see Figure S2, Supporting Information). The growth profile for films fabricated using this mixture of polymers 1a and 1b is also exponential, but it lies between the growth profiles of films fabricated exclusively from either polymer 1a or 1b. The observation that the growth profile of “mixed” films is influenced by, but not dominated by, the addition of lower molecular weight polymer 1a demonstrates that the presence of higher molecular weight polymer 1b plays a role in dictating film growth. One possible explanation for this behavior is that the high molecular weight fraction of PAA in this sample cannot diffuse into these films as readily as lower molecular weight chains (as described above). It is also possible that preferential incorporation of higher molecular weight PAA into these films during the deposition of each PAH/PAA bilayer could influence the internal structures of these assemblies in ways that retard the diffusion of lower molecular weight PAA or reduce the impact of PAA diffusion on film thickness. The results of this present investigation do not directly address issues related to polyelectrolyte adsorption kinetics during film assembly. We note, however, that the use of living/controlled methods of polymerization to synthesize multiple samples of well-defined PAA end-labeled with different fluorophores could offer opportunities to investigate the influence of polymer molecular weight and polydispersity on polyelectrolyte adsorption during the fabrication of these assemblies. The development of facile methods for the living or controlled polymerization of PAH could also provide future opportunities to characterize the potential influence of the molecular weight and polydispersity of this polycationic component on the growth and properties of these weak polyelectrolyte assemblies.

Summary and Conclusions In summary, we have investigated the influence of polyanion molecular weight on the growth and structure of multilayered thin films fabricated from PAH and well-defined samples of (54) We thank an anonymous reviewer for sharing with us the unpublished results from similar investigations using low molecular weight, polydisperse samples of PAA.

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PAA synthesized by ATRP. We observed striking differences in the growth of PAH/PAA films fabricated using these samples of well-defined PAA compared to films fabricated using commercially available, higher molecular weight PAA. For example, whereas past studies have demonstrated the thicknesses of PAH/PAA films to increase as a linear function of the number of PAH/PAA layers deposited over a broad range of pH (e.g., from pH 2.5 to 4.5), we observed that the thicknesses of films fabricated using solutions of PAH and PAA adjusted to pH 7.5 and 3.5, respectively, increase in a nonlinear manner. Films fabricated under these conditions using well-defined samples of PAA increased in thickness in an exponential manner. Experiments using samples of low and high molecular weight PAA having substantially nonoverlapping molecular weight distributions demonstrated a clear relationship between the molecular weight of PAA and rates of exponential growth. Characterization of the surfaces of these films by AFM demonstrated that the exponential growth of these assemblies was not the result of large changes in surface roughness during fabrication. The dependence of film growth on molecular weight and the results of confocal microscopy experiments using films fabricated from fluorescently end-labeled samples support the view that the exponential growth of these assemblies arises from the ability of PAA to diffuse into these films over large distances during assembly. The behavior of this multilayered film system thus appears to be similar to that reported for the exponential growth of films fabricated using a variety of different biologically relevant polyelectrolytes. The work reported here provides insight into the influence of polyanion molecular weight and polydispersity on the assembly and structure of a well-studied weak polyelectrolyte film system and illustrates the potential of living/controlled methods of polymerization to contribute to the characterization and understanding of the properties of multilayered polyelectrolyte assemblies. Acknowledgment. Financial support was provided by the Arnold and Mabel Beckman Foundation and the University of Wisconsin. N.J.F. thanks the NSF for a Graduate Research Fellowship. We are grateful to the NSF (CHE-9208463) and the NIH (NIH 1 S10 RR0 8389-01) for support of the UW NMR spectroscopy facilities, and to the W. M. Keck Center for Biological Imaging at the UW for assistance with confocal microscopy experiments. Supporting Information Available: Details of polymer characterization and results of additional film fabrication experiments. This information is available free of charge via the Internet at http://pubs.acs.org. LA7010875