Synthesis of Ultrathin Films of Polyacrylonitrile by Photoinitiated

© Copyright 2002 American Chemical Society

NOVEMBER 12, 2002 VOLUME 18, NUMBER 23

Letters Synthesis of Ultrathin Films of Polyacrylonitrile by Photoinitiated Polymerization from Self-Assembled Monolayers on Gold† Rituparna Paul, Rolf Schmidt, and Daniel J. Dyer* Department of Chemistry, Southern Illinois University, Carbondale, Illinois 62901-4409 Received July 15, 2002. In Final Form: September 16, 2002 We report the synthesis of grafted polyacrylonitrile (PAN) brushes from alkanethiolate self-assembled monolayers (SAMs) on gold. The SAM consists of a dithiol-AIBN initiator that can be activated at room temperature by irradiation at 300 nm. The polymerizations were performed in DMF and yielded PAN films with a maximum thickness of ∼45 nm and a root mean square roughness of 0.17 nm after Soxhlet extraction. The static water contact angle was 55°, and the films were thermally stable after annealing at 105 °C for 3 days. The film thickness increased by 12% after annealing. The polymer films were characterized by reflection absorption infrared spectroscopy, contact angle measurements, and ellipsometry.

Introduction The synthesis and characterization of brush polymers is a growing field in contemporary polymer science.1 Currently, there are two primary strategies for the synthesis of polymer brushes that include the grafting to (GT) technique and the grafting from (GF) technique. In the GT technique a preformed polymer is covalently linked to a reactive substrate via a chemical transformation (e.g., an amide linkage). Thus, the polymer chain is grafted to the surface by a covalent bond, making the film more durable than a spin cast film. However, the GT method is generally limited to low surface densities since grafted chains may initially collapse onto the surface and inhibit diffusion of reactive chains to the surface active groups. Therefore, chemists are developing GF techniques whereby a polymer initiator is covalently linked to a substrate and immersed into a monomer solution. The polymerization * To whom correspondence may be addressed. E-mail: ddyer@ chem.siu.edu. † This paper is dedicated to Professor James W. Neckers on the occasion of his 100th birthday. (1) (a) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677-710. (b) Milner, S. T. Science 1991, 251, 905-914. (c) Ru¨he, J.; Knoll, W. J. Macromol. Sci.: Polym. Rev. 2002, C42, 91-138.

is then initiated, and chains grow out from the surface, generally leading to higher grafting densities than those obtained from the GT technique. Polymer brushes have been grafted from a variety of substrates including gold, silicon, glass, clays, linear polymers, and hyperbranched2 polymers. In addition, a variety of initiating mechanisms have been described including anionic,3 cationic,4 ring-opening,5 ring-opening (2) (a) Bergbreiter, D. E.; Tao, G. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3944-3953. (b) Bergbreiter, D. E.; Liu, M. L. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 4119-4128. (c) Mori, H.; Seng, D. C.; Zhang, M.; Mu¨ller, A. H. F. Langmuir 2002, 18, 3682-3693. (d) Litvinenko, G. I.; Mu¨ller, A. H. E. Macromolecules 2002, 35, 45774583. (3) (a) Jordan, R.; Ulman, A.; Kang, J. F.; Rafailovich, M. H.; Sokolov, J. J. Am. Chem. Soc. 1999, 121, 1016-1022. (b) Fan, X.; Zhou, Q.; Xia, C.; Cristofoli, W.; Mays, J.; Advincula, R. Langmuir 2002, 18, 45114518. (c) Zhou, Q.; Wang, S.; Fan, X.; Advincula, R. Langmuir 2002, 18, 3324-3331. (4) (a) Jordan, R.; West, N.; Ulman, A.; Chou, Y.-M.; Nuyken, O. Macromolecules 2001, 34, 1606-1611. (b) Jordan, A.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 243-247. (5) (a) Mo¨ller, M.; Nederberg, F.; Lim, L. S.; Kånge, R.; Hawker, C. J.; Hedrick, J. L.; Gu, Y.; Shah, R.; Abbott, N. L. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 3529-3538. (b) Husemann, M.; Mecerreyes, D.; Hawker, C. J.; Hedrick, J. L.; Shah, R.; Abbott, N. L. Angew. Chem., Int. Ed. Engl. 1999, 38, 647-649. (c) Choi, I. S.; Langer, R. Macromolecules 2001, 34, 5361-5363.

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metathesis,6 free radical,7 controlled radical,8 enzymes,9 and organometallic10 catalysts. Radical polymerization, whether from a controlled or uncontrolled process, is preferred for many applications due to the tolerance for moisture, air, and a wide variety of organic functional groups. Furthermore, researchers have been particularly interested in free radical initiation from self-assembled monolayers on gold via thiol or disulfide precursors, and silicon, via chlorosilane or alkoxysilane precursors. Our interest in this field is derived from a desire to create densely packed films of polar polymers and also to design functional liquid crystalline polymer composite films.11 Thus, photochemical activation is preferred over thermal activation due to the temperature constraints of liquid crystalline phases. Furthermore, we desire to deposit films on conducting substrates such as gold or indium tin oxide (ITO). In particular, the technique of microcontact printing8c,5e,f,12 offers a convenient method for patterning gold substrates. However, self-assembled monolayers (SAMs) of alkylthiolates on gold are thermally unstable at high temperatures (>70 °C)13 and are generally believed to be unstable to ultraviolet radiation. Furthermore, sulfur compounds are known to participate in chain transfer and therefore could inhibit polymer growth from surface-bound radicals. Nevertheless, we demonstrated (6) (a) Buchmeiser, M. R.; Sinner, F.; Mupa, M.; Wurst, K. Macromolecules 2000, 33, 32-39. (b) Kim, N. Y.; Jeon, N. L.; Choi, I. S.; Takami, S.; Harada, Y.; Finnie, K. R.; Girolami, G. S.; Nuzzo, R. G.; Whitesides, G. M.; Laibinis, P. E. Macromolecules 2000, 33, 2793-2795. (c) Juang, A.; Scherman, O. A.; Grubbs, R. H.; Lewis, N. S. Langmuir 2001, 17, 1321-1323. (d) Weck, M.; Jackiw, J. J.; Rossi, R. R.; Weiss, P. S.; Grubbs, R. H. J. Am. Chem. Soc. 1999, 121, 4088-4089. (e) Skaff, H.; Ilker, M. F.; Coughlin, E. B.; Emrick, T. Polym. Prepr. 2002, 43 (1), 738-739. (7) (a) Prucker, O.; Ru¨he, J. Langmuir 1998, 14, 6893-6898. (b) Fujiki, K.; Tsubokawa, N.; Sone, Y. J. Macromol. Sci.: Chem. 1991, A28, 715731. (c) Tauer, K.; Kosmella, S. Polym. Int. 1993, 30, 253-258. (d) Fujiki, K.; Sakamoto, M.; Yoshida, A.; Maruyama, H. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2121-2128. (e) Zhou, F.; Liu, W.; Chen, M.; Sun, D. C. Chem. Commun. 2001, 2446-2447. (f) Boven, G.; Folkersma, R.; Challa, G.; Schouten, A. J. Polym. Commun. 1991, 32, 50-53. (g) Boven, G.; Oosterling, M. L. C. M.; Challa, G.; Schouten, A. J. Polymer 1990, 31, 2377-2383. (h) Carlier, E.; Guyot, A.; Revillon, A.; LlauroDarricades, M. F.; Petiaud, R. React. Polym. 1991, 16, 41-49. (i) Carlier, E.; Guyot, A.; Revillon, A. React. Polym. 1992, 16, 115-124. (j) Sidorenko, A.; Minko, S.; Schenk-Meuser, K.; Duschner, H.; Stamm, M. Langmuir 1999, 15, 8349-8355. (k) Jung, D.-H.; Park, I. J.; Choi, Y. K.; Lee, S.-B.; Park, H. S.; Ru¨he, J. Langmuir 2002, 18, 6133-6139. (l) Hamann, K.; Laible, R.; Horn, J. Polym. Sci. Technol. 1975, 9A, 93-105. (8) (a) Kim, J.-B.; Bruening, M. L.; Baker, G. L. J. Am. Chem. Soc. 2000, 122, 7616-7617. (b) Husseman, M.; Malmstro¨m, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J. Macromolecules 1999, 32, 14241431. (c) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597-605. (d) Huang, X.; Wirth, M. J. Anal. Chem. 1997, 69, 4577-4580. (e) Ejaz, M.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 2000, 33, 28702874. (f) de Boer, B.; Simon, H. K.; Werts, M. P. L.; van der Vegte, E. W.; Hadziioannou, G. Macromolecules 2000, 33, 349-356. (g) Jones, D. M.; Brown, A. A.; Huck, W. T. S. Langmuir 2002, 18, 1265-1269. (h) Luo, N.; Hutchison, J. B.; Anseth, K. S.; Bowman, C. N. Macromolecules 2002, 35, 2487-2493. (i) Blomberg, S.; Osterberg, S.; Harth, E.; Bosman, A. W.; Van Horn, B.; Hawker, C. J. J. Polym. Sci, Part A: Polym. Chem. 2002, 40, 1309-1320. (j) Xiao, D.; Wirth, M. J. Macromolecules 2002, 35, 2919-2925. (k) Ejaz, M.; Yamamoto, S.; Tsujii, Y.; Fukuda, T. Macromolecules 2002, 35, 1412-1418. (l) Tsujii, Y.; Ejaz, M.; Yamamoto, S.; Fukuda, T.; Shigeto, K.; Mibu, K.; Shinjo, T. Polymer 2002, 43, 38373841. (9) Kim, Y.-R.; Paik, H.-J.; Ober, C. K.; Coates, G. W.; Batt, C. A. Polym. Prepr. 2002, 43 (1), 706-707. (10) Ingall, M. D. K.; Joray, S. J.; Duffy, D. J.; Long, D. P.; Bianconi, P. A. J. Am. Chem. Soc. 2000, 122, 7845-7846. (11) Peng, B.; Johannsmann, D.; Ru¨he, J. Macromolecules 1999, 32, 6759. (12) (a) Wilbur, J. L.; Kumar, A.; Biebuyck, H. A.; Kim, E.; Whitesides, G. M. Nanotechnology 1996, 7, 452-457. (b) Tien, J.; Xia, Y.; Whitesides, G. M. Thin Films 1998, 24, 227-254. (c) Everhart, D. S. CHEMTECH 1999, 29, 30-37. (d) Bernard, A.; Renault, J. P.; Michel, B.; Bosshard, H. R.; Delamarche, E. Adv. Mater. 2000, 12, 1067-1070. (e) Crooks, R. M. ChemPhysChem 2001, 2, 644-654. (13) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335.

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Figure 1. SAM precursor and photochemical initiator based on AIBN.

that photoinitiated polymerization of styrene is possible from alkylthiolate SAMs on gold.14 Our previous results with polystyrene (PS) suggested that UV irradiation is a viable technique for alkylthiolatebased SAMs as long as care is taken to remove air, in particular ozone, from the reaction vessel. We found that the SAM of initiator 1 in Figure 1 forms an extended structure with a single S-Au bond and a terminal sulfur at the air interface.14a We designed this symmetrical molecule due to the ease of synthesis relative to a more desirable unsymmetrical initiator with a single thiol group. Upon irradiation, 1 decomposes to form a surface-bound radical and a second radical that diffuses into solution. Thus, polymer is formed at the surface and in the bulk. Furthermore, we found that SAMs of 1 could be initiated with 300 nm irradiation to yield films of PS from 7 to 190 nm thick, despite the possibility of inhibition due to chain transfer.14b Thus, we decided to extend this work to polar monomers such as acrylonitrile. Several approaches have been used to synthesize grafted films of polyacrylonitrile (PAN): First, methods have been developed to graft other polymers directly to PAN substrates. For instance, Sheng15 and Tanioka16 have used redox treatments of ferrous ions and hydrogen peroxide to initiate polymerization of various monomers from PAN membranes. In addition, Marzin has used plasma treatments to graft methacrylate monomers onto PAN ultrafiltration membranes.17 In contrast, PAN has been grafted directly to various substrates by electrochemical polymerization including Je´roˆme’s work with carbon and nickel electrodes18 and Bell’s modification of steel.19 Furthermore, Plessier has used γ-irradiation to graft PAN onto polypropylene fibers.20 Alternatively, Liu has used alkoxysilanes to tether a thiol group onto silica and silicon wafers; these substrates were then immersed into a monomer solution and initiated thermally with AIBN.21 Thus, the thiol group acts as a chain transfer agent to generate grafted PAN chains. In contrast, Bianconi and Kunz activated a bromoterminated SAM on ceramic substrates to initiate the anionic polymerization of PAN.22 In addition, Mohanty and co-workers have grafted PAN copolymers to plant fibers.23 Finally, Laible and Hamann have tethered PAN to silica by a GF strategy with radical initiators.24 However, (14) (a) Schmidt, R.; Zhao, T.; Green, J.-B.; Dyer, D. J. Langmuir 2002, 18, 1281-1287. (b) Paul, R.; Schmidt, R.; Feng, J.; Dyer, D. J. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 3284-3291. (c) For photoinitiated polymerization from siloxane monolayers see: Prucker, O.; Schimmel, M.; Tovar, G.; Knoll, W.; Ru¨he, J. Adv. Mater. 1998, 10, 1073. (15) (a) Yuan, X.; Sheng, J.; He, F.; Tang, Y.; Shen, N. J. Appl. Polym. Sci. 1998, 69, 1907-1915. (b) Yuan, X.; Sheng, J.; Shen, N. J. Appl. Polym. Sci. 1998, 69, 1917-1921. (16) Jimbo, T.; Higa, M.; Minoura, N.; Tanioka, A. Macromolecules 1998, 31, 1277-1284. (17) Michel, V.; Marzin, C.; Tarrago, G.; Durand, J. J. Appl. Polym. Sci. 1998, 70, 359-366. (18) Je´roˆme, C.; Geskin, V.; Lazzaroni, R.; Bre´das, J. L.; Thibaut, A.; Calberg, C.; Bodart, I.; Mertens, M.; Martinot, L.; Rodrigue, D.; Riga, J.; Je´roˆme, R. Chem. Mater. 2001, 13, 1656-1664. (19) Zhang, X.; Bell, J. P. J. Appl. Polym. Sci. 1999, 73, 2265-2272. (20) Gupta, B.; Plessier, C. J. Appl. Polym. Sci. 1999, 73, 22932297. (21) Zhou, F.; Liu, W.; Chen, M.; Sun, D. C. Chem. Commun. 2001, 2446-2447. (22) Ingall, M. D. K.; Honeyman, C. H.; Mercure, J. V.; Bianconi, P. A.; Kunz, R. R. J. Am. Chem. Soc. 1999, 121, 3607-3613.

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Figure 2. FT-RAIR spectrum of PAN on gold: (a) spin cast film; (b) polymer brush.

to our knowledge PAN has never been grafted directly to gold substrates via a SAM initiator nor has a simple photochemical method been described for the synthesis of PAN brushes. Results and Discussion The polymerization of acrylonitrile is complicated by the fact that PAN is insoluble in the monomer and only slightly soluble in polar aprotic solvents such as DMF. Therefore, our polymerizations were performed in DMF at various monomer concentrations. After careful degassing, the substrates were irradiated for 4 h at 300 nm. The substrates were removed, rinsed with DMF, and cleaned by a Soxhlet extraction with DMF for 24 h in order to remove untethered polymer. From previous results with grafted PS films, we found that extraction times in excess of 12 h are required to remove untethered polymer. Since polymer chains may become entangled within the grafted brush, it is not possible to confirm that all of the untethered polymer is removed; however we believe most is removed by this procedure. Fourier transform reflection-absorption infrared spectroscopy (FT-RAIRS) was used to confirm the presence of the PAN brush, as illustrated in Figure 2. The nitrile band at 2243 cm-1 is clearly evident for both the spin cast film (Figure 2a) and the polymer brush (Figure 2b). The asymmetric methylene stretch at 2939 cm-1 varies slightly from sample to sample but usually lies at 2938 cm-1 for the brush. The strong band at 1670 cm-1 in the spin cast film is typical for swollen films of PAN;25 this band is most likely due to residual DMF or a mixture of DMF and water, as the band intensity decreases slightly after annealing. Interestingly, the brushes did not exhibit a strong band in this region, rather a double band in the region from 1620 to 1690 cm-1 was observed, which has been attributed to a CdN stretch.26 Typically, these bands were weak and (23) (a) Rout, J.; Misra, M.; Tripathy, S. S.; Nayak, S. K.; Mohanty, A. K. J. Appl. Polym. Sci. 2002, 84, 75-84. (b) Mishra, S.; Misra, M.; Tripathy, S. S.; Nayak, S. K.; Mohanty, A. K. Macromol. Mater. Eng. 2001, 286, 107-113. (c) Mohanty, A. K.; Tripathy, P. C.; Misra, M.; Porija, S.; Sahoo, S. J. Appl. Polym. Sci. 2000, 77, 3035-3043. (24) (a) Laible, R.; Hamann, K. Adv. Colloid Interface Sci. 1980, 13, 65-99. (b) Fery, N.; Laible, R.; Hamann, K. Angew. Makromol. Chem. 1973, 34, 81-109. (25) Turska, E.; Grobelny, J. Eur. Polym. J. 1983, 19, 985-990.

resided at 1668 and 1624 cm-1, respectively. We are not sure of the origin of this band splitting, but the intensity does not change significantly after annealing. In addition, there is also a strong band at 1452 cm-1 (not shown) due to the CH2 bending mode;27 thus our spectra are consistent with previous reports in the literature.26,27,28 Finally, the static water contact angle also confirms the synthesis of PAN since it changed from less than 5° for clean gold, to 55 ( 2° for the PAN substrate; this matches a spin cast film and is consistent with literature reports that range from 42° to 57°.17,21,22 FT-RAIRS can be used to monitor the progress of the film growth. In particular, we monitored the absorption intensity of the nitrile band (2243 cm-1) and found that the reaction is terminated after a few hours at high monomer concentrations. Furthermore, the film thickness was always less than 50 nm after Soxhlet extraction, regardless of the reaction conditions. To probe the rate of film growth, we monitored the effect of monomer concentration after 4 h of irradiation and 24 h of Soxhlet extraction. Figure 3a shows that at a concentration of 10 vol % of acrylonitrile in DMF, the film growth is minimal, whereas at 25% the film growth hits a maximum. There is a slight dip in the absorption maximum from 25 to 100% that is most likely an artifact of the measurement since the optical thickness by ellipsometry increases slightly from 43 to 46 nm over this range (Figure 3b). Other than the slight dip, the ellipsometry data closely matches the FT-RAIRS data. In particular, we observe rapid growth from 7 nm at 10% monomer concentration to 43 nm at 25% monomer concentration, at which point the film thickness levels off. No significant increase is observed above 25% concentration with 4 h reaction time. This polymerization has several possible termination mechanisms. First, a grafted chain may be terminated by an active chain from solution. Second, adjacent grafted chains may terminate each other. Third, the thiols from the untethered half of the initiator may participate in chain transfer. (Compound 1 was designed for ease of synthesis whereas the optimal structure would be unsymmetrical and include a single thiol group. Thus, the sulfur atom would remain tethered to the gold at all times. We have begun exploring unsymmetrical derivatives and will report these in due course.) Fourth, grafted chains may become decoupled from the surface and either terminate by recombination or participate in chain transfer. Fifth, the reactive ends of the chain may become imbedded in phase-separated polymer, thus inhibiting diffusion of monomer to the radical site. This last termination mechanism is likely for PAN due to the low solubility in monomer and DMF. Interestingly, Frey et al. have reported improved grafting of PAN with water as the solvent.24b Therefore, further experiments are planned with solvents such as water and N-methylpyrrolidinone; we are also examining the synthesis of copolymers. Literature reports of the rate constant for the radical polymerization of acrylonitrile are inconsistent and range drastically depending on the conditions. The best estimates (26) Adams, D. M.; Davey, L. M.; Tan, T.-K.; Ekejiuba, I. O. C. J. Chem. Soc., Faraday Trans. 2 1982, 78, 1617-1621. (27) (a) Gu, X.; Xue, G. Spectrosc. Lett. 1997, 30, 1313-1323. (b) Gu, X.; Xue, G.; Jin, S.; Li, F. Spectrosc. Lett. 1997, 30, 139-148. (28) (a) Minagawa, M.; Taira, T.; Kondo, K.; Yamamoto, S.; Sato, E.; Yoshii, F. Macromolecules 2000, 33, 4526-4531. (b) Minagawa, M.; Miyano, K.; Takahashi, M.; Yoshii, F. Macromolecules 1988, 21, 23872391. (c) Ko, Y.-G.; Choi, U. S.; Kim, J. S.; Chun, Y. J.; Ahn, D. J. Polym. Prepr. 2002, 43, 619-620. (d) Wu, C. R.; Liedberg, B. J. Polym. Sci., Part B: Polym. Phys. 1988, 26, 1127. (29) The raw data and standard deviations are reported in the Supporting Information.

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Figure 4. Comparison of the actual film growth versus the predicted growth from the rate constant for the propagation (kp) of PAN in the bulk. The predicted growth should be linear with respect to the chain length or degree of polymerization.

Figure 3. Effect of monomer concentration on PAN film growth by (a) FT-RAIRS, where we observe the nitrile stretch at 2243 cm-1, and (b) the optical thickness as determined by ellipsometry. Each data point for the thickness is an average from at least two separate trials with four measurements per substrate. The error in each measurement was (0.05 nm, and the standard deviation was typically less than 1 nm.29

range from 2000 to 1.8 × 106 M-1 s-1.30 In Figure 4 we plot the thickness as measured by ellipsometry along with a fit of the expected degree of polymerization (DP) based on kp ∼ 4800 M-1 s-1, where kp is the rate constant for propagation.30a Clearly these molecular weights are not achieved due to termination; however, the growth is expected to be linear since similar termination and initiation mechanisms should hold for each data point. At low concentrations the increase in thickness is consistent with the expected increase in the kinetic chain length of the polymer (i.e., DP). However, the film growth rapidly terminates at 25% concentration with a film thickness that is seven times greater than that observed at 10% concentration, whereas the predicted growth based on the DP would be 2.5, or ∼15 nm versus the actual thickness of ∼43 nm. Clearly the rate of film growth cannot be explained solely by an increase in the length of the polymer chains; it is likely that as the surface density increases, the chains are pushed away from the surface, and the thickness increases. Experiments are in progress to (30) (a) Zetterlund, P. B.; Busfield, W. K.; Jenkins, I. D. Macromolecules 1999, 32, 8041-8045. (b) Herberger, K.; Fisher, H. Int. J. Chem. Kinet. 1993, 25, 249-263. (c) Capek, I.; Barton, J. React. Kinet. Catal. Lett. 1977, 7, 21-25.

remove the polymer in order to determine Mn and the grafting density since the polymer may be removed from the gold by treatment with iodine in DMF. Interestingly, our previous results with polystyrene suggested that grafted PS films were unstable above 60 °C and care must be taken to control the extraction temperature within the Soxhlet apparatus. In contrast, these PAN films are quite stable. For instance, a PAN brush with a thickness of 44 nm (after 24 h of Soxhlet extraction) was immersed into DMF at 75 °C for 56 h in air. After the film was rinsed with DMF and dried under a stream of nitrogen, the thickness was measured again by ellipsometry; we found that the thickness had increased to 49 nm. In addition, the intensity of the nitrile band in the RAIR spectrum had increased by 20% over the initial state. Furthermore, DMF was not observed by FT-RAIRS; thus the changes were not due to solvent swelling. We propose two possible explanations for the increase in film thickness: First, water may have been incorporated in lieu of DMF since the solution was open to the air; our data are collected with an atmospheric suppression algorithm that removes moisture and carbon dioxide. However, Turska and Grobelny25 reported that PAN films swollen with water exhibit a strong band from 1600 to 1630 cm-1. Since our spectrum exhibited the usual small doublet at 1668 and 1630 cm-1, we conclude that it did not incorporate a large amount of moisture while immersed in DMF. Second, the heating may have annealed the polymer and caused the chains to reorient to a more ordered state; thus, the average orientation or tilt of the nitrile group with respect to the substrate normal may have decreased, yielding an increase in absorption. We performed another annealing experiment by heating a PAN brush under vacuum (0.6 Torr) for 77 h at 105 °C. Surprisingly, there was no degradation of the film as measured by FT-RAIRS. Rather, the thickness changed from 44 nm before annealing to 54 nm ((0.2) after annealing. There are two possible explanations for the increase in thickness: First, the chains may rearrange to form a more ordered structure with a larger percentage of chains normal to the gold substrate, and second, the index of refraction (n) may have changed due to loss of residual DMF or water. Since a change in n would require

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a different optical model for ellipsometry, we utilized the Garnet equation to calculate n and compensate for solvent swelling.31 The thickness by ellipsometry would increase from 44 nm, where we assumed n ) 1.52 for a dry film, to 48 and 49 nm for PAN swelled with water and DMF, respectively. Therefore, the increase in thickness due to annealing is probably closer to 6 nm, compared to 10 nm when we assumed both substrates were dry. Furthermore, the RAIR spectrum did not exhibit any obvious loss of water or DMF; the only significant change was an increase in the intensity of the nitrile band relative to the asymmetric methylene band. Finally, the surface roughness, as measured by ellipsometry, and the water contact angle did not change significantly. Additional experiments are planned with atomic force microscopy to elucidate the swelling behavior in more detail. The PAN films in this study are very smooth. As Figure 5 illustrates, we used the ellipsometry data to generate a thickness map of a typical PAN substrate. This sample had an average thickness of 43 nm over an area of 120 × 155 µm2 and a root mean square roughness of 0.2 nm. Furthermore, several spikes were evident with a maximum height of 46 nm, or 3 nm above the average. The root mean square roughness for this sample was similar to a 15 nm substrate, suggesting that the film growth is uniform. It should be noted that the size of the spikes (31) Rehfeldt, F.; Tanaka, M.; Pagnoni, L.; Jordan, R. Langmuir 2002, 18, 4908-4914.

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Figure 5. Thickness map of a PAN brush as measured by ellipsometry over a 120 × 155 µm2 area. The average thickness is 43 nm, the root mean square roughness is 0.2 nm, and the spikes reached a maximum height of 46 nm (i.e., 3 nm above the average).

depicted in Figure 5 are exaggerated by the software for improved viewing. Acknowledgment. We thank the reviewers for some insightful comments and useful references. Funding was provided by the Materials Technology Center at SIUC; the donors of the Petroleum Research Fund, administered by the American Chemical Society; 3M Corporation; and the National Science Foundation under Grant CHE0094195 for partial support of this research. Supporting Information Available: The materials, equipment, and experimental procedures are discussed in more detail. This material is available free of charge via the Internet at http://pubs.acs.org. LA0206413