Langmuir 2007, 23, 667-672
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Effect of Sequential Layer-by-Layer Surface Modifications on the Surface Energy of Plasma-Modified Poly(dimethylsiloxane) Woo-Sung Bae, Anthony J. Convertine, Charles L. McCormick,* and Marek W. Urban* School of Polymers and High Performance Materials, Shelby F. Thames Polymer Science Research Center, The UniVersity of Southern Mississippi, Hattiesburg, Mississippi 39406 ReceiVed August 2, 2006. In Final Form: October 9, 2006 Surface-initiated grafting of N,N-dimethylacrylamide, styrenesulfonate (SS), and (ar-vinylbenzyl)trimethylammonium chloride (VBTAC) from microwave plasma carboxylated, initiator-functionalized poly(dimethylsiloxane) (PDMS) surfaces was accomplished utilizing reversible addition-fragmentation chain transfer (RAFT) polymerization. Surface spectroscopic attenuated total reflectance (ATR) FT-IR analysis and atomic force microscopy (AFM) measurements were utilized to determine surface grafting and morphological surface features. The VBTAC-grafted PDMS provided a smooth, hydrophilic cationic surface for creating layer-by-layer (LBL) surfaces via alternating deposition of welldefined poly(SS) and poly(VBTAC), also prepared via aqueous RAFT. Comparisons of the ATR FT-IR spectra of the LBL assemblies and those of respective anionic poly(SS) and cationic poly(VBTAC) components confirmed strong electrostatic complexation of a fraction of the sulfonate and quarternary ammonium species in the layers as well as the existence of noncomplexed species. AFM images of surface topology indicated the presence of domains, likely phase-separated segments of the respective homopolymers, as well as interlayer mixing. The employed LBL methodology results in formation of stable, highly hydrophilic surfaces on a PDMS substrate. To our knowledge, this is the first study that illustrates surface functionalization of PDMS using microwave plasma and RAFT polymerization, followed by LBL deposition of polyelectrolytes.
Introduction Continuous interest in surface modifications is driven by the ability to create unique surface properties while maintaining bulk polymer characteristics. Such surface modifications may be accomplished by numerous approaches, and surface-initiated living polymerization on a variety of substrates using different monomers has been utilized to achieve specific surface properties.1-15 While polymerization reactions on inorganic substrates have become a popular approach of altering surface chemistry,7-9,13,16-24 relatively limited studies have been con* To whom correspondence should be addressed. (1) Boltau, M.; Walheim, S.; Mlynek, J.; Krausch, G.; Steiner, U. Nature 1998, 391 (6670), 877-879. (2) Bowden, N.; Brittain, S.; Evans, A. G.; Hutchinson, J. W.; Whitesides, G. W. Nature 1998, 393 (6681), 146-149. (3) Brooksby, P. A.; Downard, A. J. Langmuir 2005, 21 (5), 1672-1675. (4) Chen, L.; Zhuang, L.; Deshpande, P.; Chou, S. Langmuir 2005, 21 (3), 818-821. (5) Corcoran, N.; Ho, P. K. H.; Arias, A. C.; Mackenzie, J. D.; Friend, R. H.; Fichet, G.; Huck, W. T. S. Appl. Phys. Lett. 2004, 85 (14), 2965-2967. (6) Hu, Z.; Chen, Y.; Wang, C.; Zheng, Y.; Li, Y. Nature 1998, 393 (6681), 149-152. (7) Huang, X.; Wirth, M. J. Macromolecules 1999, 32 (5), 1694-1696. (8) Husemann, M.; Morrison, M.; Benoit, D.; Frommer, J.; Mate, C. M.; Hinsberg, W. D.; Hedrick, J. L.; Hawker, C. J. J. Am. Chem. Soc. 2000, 122 (8), 1844-1845. (9) Husseman, M.; Malmstroem, 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 (5), 1424-1431. (10) McCarley, R. L.; Vaidya, B.; Wei, S.; Smith, A. F.; Patel, A. B.; Feng, J.; Murphy, M. C.; Soper, S. A. J. Am. Chem. Soc. 2005, 127 (3), 842-843. (11) Pallandre, A.; De Meersman, B.; Blondeau, F.; Nysten, B.; Jonas, A. M. J. Am. Chem. Soc. 2005, 127 (12), 4320-4325. (12) Pan, F.; Wang, P.; Lee, K.; Wu, A.; Turro, N. J.; Koberstein, J. T. Langmuir 2005, 21 (8), 3605-3612. (13) Tu, H.; Heitzman, C. E.; Braun, P. V. Langmuir 2004, 20 (19), 83138320. (14) Xia, Y.; Tien, J.; Qin, D.; Whitesides, G. M. Langmuir 1996, 12 (16), 4033-4038. (15) Yoshikawa, C.; Goto, A.; Tsujii, Y.; Fukuda, T.; Yamamoto, K.; Kishida, A. Macromolecules 2005, 38 (11), 4604-4610. (16) Biesalski, M.; Ruehe, J. Macromolecules 2003, 36 (4), 1222-1227. (17) Fan, X.; Xia, C.; Advincula, R. C. Langmuir 2003, 19 (10), 4381-4389. (18) Hu, S.; Brittain, W. J. Macromolecules 2005, 38 (15), 6592-6597.
ducted on polymeric substrates using surface-initiated living radical (reversible addition-fragmentation chain transfer, RAFT) graft polymerization.25 Although among popular substrates poly(dimethylsiloxane) (PDMS) is often26,27 used owing to its inertness and other useful properties, poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) and poly(ethylene terephthalate) (PET) substrates were utilized for surface-initiated radical polymerization to obtain hydrophilic and/or biocompatible polymeric surfaces.15,28 In this study we will utilize a “grafting from” approach in an effort to create PDMS surfaces with potentially stimulusresponsive characteristics, which may be accomplished by attaching poly(N,N′-dimethylacrylamide) (p-DMA), poly(psytrenesulfonate) (p-PSS), and poly((ar-vinylbenzyl)trimethylammonium chloride) (p-VBTAC). This choice is dictated by the fact that p-DMA is highly biocompatible with living tissues and exhibits low cytotoxicity,29-31 whereas polyelectrolytes such as p-PSS have been utilized for drug microcapsulation due to its (19) Ista, L. K.; Mendez, S.; Perez-Luna, V. H.; Lopez, G. P. Langmuir 2001, 17 (9), 2552-2555. (20) Khan, M.; Huck, W. T. S. Macromolecules 2003, 36 (14), 5088-5093. (21) Kim, D. J.; Lee, K.-B.; Chi, Y. S.; Kim, W.-J.; Paik, H.-j.; Choi, I. S. Langmuir 2004, 20 (19), 7904-7906. (22) Zhao, B.; Brittain, W. J. J. Am. Chem. Soc. 1999, 121 (14), 3557-3558. (23) Zhao, B.; Brittain, W. J.; Zhou, W.; Cheng, S. Z. D. J. Am. Chem. Soc. 2000, 122 (10), 2407-2408. (24) Zhou, F.; Liu, Z.; Li, W.; Hao, J.; Chen, M.; Liu, W.; Sun, D. C. Chem. Lett. 2004, 33 (5), 602-603. (25) Zhenping Cheng, X. Z., E. T. Kang, and K. G. Neoh. Macromolecules 2006, 39, 1660. (26) Bae, W.-S.; Urban, M. W. Langmuir 2004, 20 (19), 8372-8378. (27) Zhao, Y.; Urban, M. W. Langmuir 1999, 15 (10), 3538-3544. (28) Li, Y.; Neoh, K. G.; Kang, E. T. Polymer 2004, 45 (26), 8779-8789. (29) Abraham, G. A.; de Queiroz, A. A. A.; Roman, J. S. Biomaterials 2001, 22 (14), 1971-1985. (30) Tomita, N.; Tamai, S.; Okajima, E.; Hirao, Y.; Ikeuchi, K.; Ikada, Y. J. Appl. Biomater. 1994, 5 (2), 175-81. (31) de Queiroz, A. A.; Castro, S. C.; Higa, O. Z. J. Biomater. Sci., Polym. Ed. 1997, 8 (5), 335-47.
10.1021/la062281f CCC: $37.00 © 2007 American Chemical Society Published on Web 11/17/2006
668 Langmuir, Vol. 23, No. 2, 2007
Figure 1. Schematic diagram of surface-initiated polymerization on the initiator-attached PDMS substrates with (a) DMA, (b) VBTAC, and (c) PSS.
stimulus-responsiveness to pH or ionic strength,32 and p-VBTAC is a cationic polyelectrolyte capable of self-assembling as a function of pH.33 The overall scope of these studies is illustrated in Figure 1, which schematically depicts processes implemented to generate PDMS surfaces decorated with p-DMA, p-PSS, and p-VBTAC alternating multilayers. Due to the inertness of PDMS, the first step involves microwave plasma surface reactions of maleic anhydride (MA) with PDMS, followed by hydrolysis leading to carboxylic acid groups. Such surfaces will be exposed to polymerization of N,N′-dimethylacrylamide (DMA) (Figure 1a), (ar-vinylbenzyl)trimethylammonium chloride (VBTAC) (Figure 1b), and p-styrenesulfonate (PSS) monomers (Figure 1c) in the presence of surface-tethered 2,2′-azobis(2-methylpropionamidine) dihydrochloride (ABAH) initiator. Using a p-VBTAC-tethered PDMS,p-PSS/p-VBTACionicself-assemblymultilayerapproach,34-39 well-organized layer-by-layer surfaces will be created with an ultimate goal of generating stable hydrophilic surfaces. Experimental Section PDMS substrate was prepared from a linear trimethylsiloxylterminated vinylmethylsiloxane-dimethylsiloxane copolymer (Mn ) 28000 g/mol; VDT-731, Gelest, Inc.). Reactions between vinyl groups forming cross-linked PDMS networks were initiated by the addition of 0.5 wt % tert-butyl perbenzoate (Aldrich Chemical Co.). The copolymer and the initiator were premixed for 24 h to ensure complete dissolution of the initiator in PDMS. Cross-linked PDMS substrates were prepared by pressure molding the oligomer-initiator mixture for 15 min at 149 °C and post-cross-linking for an additional 5 h at 182 °C. Surface contaminants and residual low-molecularweight species were removed by washing PDMS in methylene chloride for 5 min, followed by slow deswelling and drying in air and vacuum-desiccating at 1.3 Pa for 24 h at room temperature. (32) Zhu, Y.; Shi, J.; Shen, W.; Dong, X.; Feng, J.; Ruan, M.; Li, Y. Angew. Chem., Int. Ed. 2005, 44 (32), 5083-5087. (33) McCormick, C. L.; Lowe, A. B. Acc. Chem. Res. 2004, 37 (5), 312-325. (34) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21 (7), 319-348. (35) Choi, J.; Rubner, M. F. Macromolecules 2005, 38 (1), 116-124. (36) Park, S. Y.; Barrett, C. J.; Rubner, M. F.; Mayes, A. M. Macromolecules 2001, 34 (10), 3384-3388. (37) Park, S. Y.; Rubner, M. F.; Mayes, A. M. Langmuir 2002, 18 (24), 96009604. (38) Buescher, K.; Graf, K.; Ahrens, H.; Helm, C. A. Langmuir 2002, 18 (9), 3585-3591. (39) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32 (24), 81538160.
Bae et al. Plasma reactions were conducted using open reactor conditions, as described elsewhere.26,40 Cross-linked PDMS substrate, with approximate dimensions of 10 × 7 × 2 mm, and 100 mg of solid maleic anhydride (Aldrich) were placed into the reactor chamber and spaced by 8.5 cm. It should be noted that the amount of monomer and the distance between the monomer and substrate are significant as they affect the efficiency of surface reactions. In a typical experiment, the reactor is evacuated to 10 mTorr, followed by purging with Ar gas to reach the steady-state pressure (250 mTorr) with a flow rate of 2.96 mL/min. At this point, a microwave radiation of 600 W of power with an output frequency of 2.45 GHz is turned on to induce plasmons. Under these conditions, the reaction-chamber pressure increases continuously during microwave plasma discharge. Under the same pressure conditions, but without microwave plasma discharge, the pressure in the reaction chamber remains constant, and no sorption of maleic anhydride into the PDMS network is detected. To achieve surface-initiated RAFT polymerization on carboxylic acid moieties of microwave-plasma-modified PDMS surfaces, 10 mM aqueous 2-ethyl-5-phenylisoxazolium 3′-sulfonate (Woodward’s reagent K, Aldrich)41 was utilized as an activator for 30 min. After being rinsed with deionized (DI) H2O, the PDMS specimen was placed into a 200 mM aqueous solution of ABAH (Aldrich) free radical initiator and allowed to react for 2 h. PDMS specimens were rinsed again with DI H2O and used immediately for polymerization. DMA was vacuum distilled immediately prior to use, and VBTAC and 4-styrenesulfonic acid, sodium salt hydrate were used as received (Aldrich). 4-Cyanopentanoic acid dithiobenzoate (CTP) was prepared as a RAFT chain transfer agent (CTA) according to previously reported procedures.42 The monomers, initiator, and CTA were dissolved in DI H2O with an initial monomer concentration ([M]0) of 0.5 M. The initial monomer to CTA ratio ([M]0:[CTA]0) was held constant at 500:1 with a [CTA]0:[I]0 (I ) initiator) ratio of 5:1. Initiator-derivatized specimens were added to the prepared polymerization solution, and the entire content was purged with N2 for 30 min prior to the reaction. All polymerization reactions were performed at 70 °C. The grafted substrate was rinsed three times with DI H2O and stored in DI H2O prior to analysis. Gel permeation chromatography (GPC) analysis was performed on the polymer solution prepared using RAFT surface-initiated polymerization. p-PSS and p-DMA solutions were analyzed by aqueous size exclusion chromatography (ASEC) using an eluent of 20% acetonitrile/80% 0.05 M Na2SO4(aq), a flow rate of 0.5 mL/ min at 25 °C, and Viscotek TSK Viscogel columns (G3000 PWXL (
