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Rapid Polymer Brush Growth by TEMPO-Mediated Controlled Free-Radical Polymerization from Swollen Plasma Deposited Poly(maleic anhydride) Initiator Surfaces D. O. H. Teare, W. C. E. Schofield, R. P. Garrod, and J. P. S. Badyal* Department of Chemistry, Science Laboratories, Durham University, Durham DH1 3LE, England, UK Received June 13, 2005. In Final Form: August 28, 2005 Pulsed plasma-chemical deposition of poly(maleic anhydride) is shown to be a substrate-independent method for functionalizing solid surfaces with initiator sites for nitroxide-mediated controlled free-radical graft polymerization. Swelling of the initiator film via aminolysis can lead to grafted polymer brushes that are 1 order of magnitude thicker than those obtained by existing methods on solid surfaces.
Introduction The grafting of polymers with controllable molecular weight and polydispersity onto solid surfaces has become an important area of scientific research due to its relevance to biomedical, frictional, protective, and adhesive applications. To this end, controlled, free-radical polymerization from surfaces producing well-defined polymer brushes are of particular interest.1-4 The grafting of polymers has traditionally relied on a “grafting to” approach. This can be achieved mainly by either physisorption of block copolymers (where one section strongly adsorbs onto a surface, leaving the other to form the brush layer)5-8 or covalent attachment of a terminal functional group on the polymer chain to a suitable surface moiety.9,10 An inherent disadvantage of the grafting to approach is that the surface density of polymer brushes is limited by steric hindrance from chains already immobilized.11 The aforementioned drawbacks can be addressed by reverting to a “grafting from” technique, where initiating groups are present on the substrate surface. Polymer brushes are then grown directly from the surface. This can be achieved by anionic12-15 cationic,16-18 ring* To whom correspondence should be addressed. (1) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921. (2) Kickelbick, G.; Schubert, U. Monatsh. Chem. 2001, 132, 13. (3) Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. Rev. 2001, 101, 3661. (4) Tsubokawa, N.; Yoshikawa, S. Rec. Res. Dev. Polym. Sci. 1998, 2, 211. (5) Balastre, M.; Li, F.; Schorr, P.; Yang, J.; Mays, J. W.; Tirrel, M. V. Macromolecules 2002, 35, 9480. (6) Hadziionnou, G.; Patel, S.; Granick, S.; Tirrell, M. J. Am. Chem. Soc. 1986, 108, 2869. (7) Dan, N.; Tirrell, M. Macromolecules 1993, 26, 4310. (8) Belder, G. F.; ten Brinke, G.; Hadziioannou, G. Langmuir 1997, 13, 4102. (9) Jordan, R.; Graf, K.; Riegler, H. Chem. Commun. 1996, 9, 1025. (10) Zhao, W.; Krausch, G.; Rafailovich, H.; Sokolov, J. Macromolecules 1994, 27, 2933. (11) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (12) Jordan, R.; Ulman, A.; Kang, J. F.; Rafailovich, M. H.; Sokolov, J. J. Am. Chem. Soc. 1999, 121, 1016. (13) Zhou, Q.; Fan, X.; Xia, C.; Mays, J.; Advincula, R. Chem. Mater. 2001, 13, 2465. (14) Zhou, Q.; Wang, S.; Fan, X.; Advincula, R.; Mays, J. Langmuir 2002, 18, 3324. (15) Zhou, Q.; Nakamura, Y.; Inaoka, S.; Park, M.-K.; Wang, Y.; Fan, X.; Mays, J. Advincula, R. ACS Symp. Ser. 2002, 804, 39.
opening,19-21 and free-radical22-28 polymerization. In the case of free-radical polymerization, a variety of “controlled” techniques are available, namely, nitroxide-mediated stable free-radical polymerization (SRP);29-31 atom transfer radical polymerization (ATRP);32-41 and dithioesterbased reversible addition-fragmentation chain transfer (RAFT).42-44 Common to many of these controlled, surfaceinitiated polymerization methods is the prerequisite for (16) Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 243. (17) Jordan, R.; West, N.; Ulman, A.; Chou, Y. M.; Nuyken, O. Macromolecules 2001, 34, 1606. (18) Zhao, B.; Brittain, W. J. Macromolecules 2000, 33, 342. (19) Weck, M.; Jackiw, J. J.; Rossi, R. R.; Weiss, P. S.; Grubbs, R. H. J. Am. Chem. Soc. 1999, 121, 4088. (20) 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. (21) Juang, A.; Scherman, O. A.; Grubbs, R. H.; Lewis, N. S. Langmuir 2001, 17, 1321. (22) Hyun, J.; Chilkoti, A. Macromolecules 2001, 34, 5644. (23) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 592. (24) Ru¨he, J. Macromol. Symp. 1998, 126, 215. (25) Fujiki, K.; Sakamoto, M.; Yoshida, A.; Maruyama, H. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2121. (26) Biesalski, M.; Ru¨he, J. Macromolecules 1999, 32, 2309. (27) Prucker, O.; Habicht, J.; Park, I. J.; Ru¨he, J. Mater. Sci. Eng., C 1999, 8-9, 291. (28) Prucker, O.; Ruehe, J. Langmuir 1998, 14, 6893. (29) Husseman, M.; Malmstro¨m, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoıˆt, D. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J. Macromolecules 1999, 32, 1424. (30) Husseman, M.; Morrison, M.; Benoıˆt, D. G.; Frommer, J.; Mate, C. M.; Hinsberg, W. D.; Hedrick, J. L.; Hawker, C. J. J. Am. Chem. Soc. 2000, 122, 1844. (31) Blomberg, S.; Ostberg, S.; Harth, E.; Bosman, A. W.; Van Horn, B.; Hawker, C. J. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 1309. (32) Bo¨ttcher, H.; Hallensleben, M. L.; Nuss, S.; Wurm, H. Polym. Bull. 2000, 44, 223. (33) Ejaz, M.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 2000, 33, 2870. (34) Zhao, B.; Brittain, W. J. Macromolecules 2000, 33, 8813. (35) Mandal, T. K.; Fleming, M. S.; Walt, D. R. Chem. Mater. 2000, 12, 3481. (36) Pyun, J.; Matyjaszewski, K.; Kowalewski, T.; Savin, D.; Patterson, G.; Kickelbick, G.; Huesing, N. J. Am. Chem. Soc. 2001, 123, 9445. (37) Carlmark, A.; Malmstro¨m, E. J. Am. Chem. Soc. 2002, 124, 900. (38) Kim, J. B.; Bruening, M. L.; Baker, G. L. J. Am. Chem. Soc. 2000, 122, 7616. (39) Kong, X.; Kawai, T.; Abe, J.; Iyoda, T. Macromolecules 2001, 34, 1837. (40) Von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 1999, 121, 7409. (41) Von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 2001, 123, 7497. (42) Tsujii, Y.; Ejaz, M.; Sato, K.; Goto, A.; Fukuda, T. Macromolecules 2001, 34, 8872. (43) Baum, M.; Brittain, W. J. Macromolecules 2002, 35, 610.
10.1021/la051566+ CCC: $30.25 © 2005 American Chemical Society Published on Web 10/05/2005
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surface-bound functionalized initiators. Self-assembled monolayers (SAMs) are widely utilized for this purpose: for instance, silane-based linkers are employed for hydroxyl surfaces (such as silica and silicon), while thiolmetal bonds work well for transition-metal surfaces (usually gold). However, silane- or thiol-group-containing initiator molecules tend not to be commercially available, and consequently, they require either custom synthesis18,32,39,45,46 prior to assembly of the SAM or postassembly functionalization of the SAM.16,29,39,47 Furthermore, rigorous cleaning and moisture exclusion is of paramount importance in order to ensure defect-free surface coverage. Another pertinent factor is the issue that thiol-gold linkages suffer from instability at the higher temperatures associated with nitroxide-mediated SRP.48 Rare examples of controlled grafts that have not relied on SAMs have still only been applicable to specific substrates.37,49,50 Nitroxide-mediated grafting of polymer chains is an important class of controlled surface free-radical polymerization. Examples include grafting from: silane functionalized nitroxide initiators tethered to silica surfaces29,30,31,51-55 magnetite nanoparticles,50 polyacrylate chemisorbed on conducting surfaces,56 and gamma irradiated polymer films.57-59 This type of stable free-radical process permits accurate control of molecular weight and polydispersity of the covalently attached polymer chains as well as providing scope for the formation of block copolymers. However, all previous attempts have been limited to specific substrates and require elaborate multistep wet chemical syntheses of the initiator layer. In this paper, we describe a substrate-independent approach entailing the use of trapped-free-radicalcontaining maleic anhydride pulsed plasma polymer films60 as surface-bound initiators for the nitroxidemediated controlled free-radical graft polymerization of styrene (Scheme 1). The benefits of swelling the maleic anhydride pulsed plasma polymer initiator films via aminolysis or imidization prior to nitroxide-mediated controlled free-radical graft polymerization is highlighted. (44) Lowe, A. B.; Sumerlin, B. S.; Donovan, M. S.; McCormick, C. L. J. Am. Chem. Soc. 2002, 124, 11562. (45) Nuss, S.; Bo¨ttcher, H.; Wurm, H.; Hallensleben, M. L. Angew. Chem., Int. Ed. 2001, 40, 4016. (46) Jones, D. M.; Brown, A. A.; Huck, W. T. S. Langmuir 2002, 18, 1265. (47) Kim, J. B.; Huang, W.; Bruening, M. L.; Baker, G. L. Macromolecules 2002, 35, 5410. (48) Huang, W.; Skanth, G.; Baker, G. L.; Bruening, M. L. Langmuir 2001, 17, 1731. (49) Matsuno, R.; Yamamoto, K.; Otsuka, H.; Takahara, A. Chem. Mater. 2003, 15, 3. (50) Matsuno, R.; Yamamoto, K.; Otsuka. H.; Takahara, A. Macromolecules 2004, 37, 2703. (51) Bartholome, C.; Beyou, E.; Bourgeat-Lami, E.; Chaumont, P.; Zydowicz, N. Macromolecules 2003, 36, 7946. (52) Andruzzi, L.; Hexemer, A.; Li, X.; Ober, C. K.; Kramer, E. J.; Galli, G. Challini, E.; Fischer, D. A. Langmuir 2004, 20, 10498. (53) Andruzzi, L.; Seneratne, W.; Hexemer, A.; Sheets, E. D.; Ilic, B.; Kramer, E. J.; Baird, B.; Ober, C. K. Langmuir 2005, 21, 2495. (54) Bartholome, C.; Beyou, E.; Bourgeat-Lami, E.; Chaumont, P.; Lefebvre, F.; Zydowicz, N. Macromolecules. 2005, 38, 1099. (55) Parvole, J.; Laruelle, G.; Khoukh, A.; Billon, C. Macromol. Chem. Phys. 2005, 206, 372. (56) Voccia, S.; Je´roˆme, C.; Detrembleur, C.; Lecle`re, P.; Gouttebaron, R.; Hecq, M.; Gilbert, B.; Lazzaroni, R.; Je´roˆme, R. Chem. Mater. 2003, 15, 923. (57) Miwa, Y.; Yamamoto, K.; Sakaguchi, M.; Shimada, S. Macromolecules 1999, 32, 8234. (58) Miwa, Y.; Yamamoto, K.; Sakaguchi, M.; Shimada, S. Macromolecules 2001, 34, 2089. (59) Yamamoto, K.; Nakazono, M.; Miwa, Y.; Hara, S.; Sakaguchi, M.; Shimada, S. Polym. J. 2001, 33, 862. (60) Teare, D. O. H.; Schofield, W. C. E.; Roucoules, V.; Badyal, J. P. S. Langmuir 2003, 19. 2398.
Langmuir, Vol. 21, No. 23, 2005 10819 Scheme 1. Controlled Free-Radical Graft Polymerization on Pulsed Plasma Deposited Poly(maleic anhydride) Layers
Experimental Section 2.1. Preparation of Anhydride Surfaces. Briquettes of maleic anhydride (+99%, Aldrich) were ground into a fine powder, loaded into a sealed glass tube, and further purified using several freeze-pump-thaw cycles. Plasma polymerization was carried out in an electrodeless cylindrical glass reactor (5 cm diameter, 520 cm3 volume, base pressure of 2 × 10-3 mbar, with a leak rate better than 1.8 × 10-9 kg s-1) surrounded by an externally wound copper coil (4 mm diameter, 9 turns, spanning 8-15 cm from the gas inlet) enclosed in a Faraday cage. The chamber was fitted with a gas inlet, a Pirani pressure gauge, and a 30 L min-1 twostage rotary pump attached to a liquid nitrogen cold trap. All joints were grease free. An L-C network was used to match the output impedance of a 13.56 MHz radio frequency (RF) generator to the partially ionized gas load. In the case of pulsed plasma deposition experiments, the RF power supply was triggered by a signal generator. The pulse width and amplitude were monitored with an oscilloscope. Prior to each experiment, the reactor was cleaned by scrubbing with detergent and rinsing with water and propan-2-ol, followed by oven drying. The system was then reassembled and evacuated. Further cleaning was carried out by running an air plasma at 0.2 mbar pressure and 30 W power for 30 min. Next, a clean silicon wafer was placed into the center of the reactor. At this stage, maleic anhydride vapor was introduced at a pressure of 0.2 mbar for 5 min prior to ignition of the electrical discharge. Pulsed plasma deposition was carried out for 30 min using a peak power of 5 W in combination with a duty cycle on-time of 20 µs and an off-time equal to 1200 µs.61 Upon completion of each plasma deposition, maleic anhydride vapor was allowed to flow through the reactor for a further 5 min, followed by evacuation to base pressure and venting to atmosphere. For comparative purposes, continuous wave plasma depositions were carried out for 2 min at a power of 5 W in order to produce films of equivalent thickness. 2.2. Aminolysis and Imidization of Anhydride Surfaces. The maleic anhydride plasma polymer functionalized silicon wafers were placed into a glass reactor (pumped by a 30 L min-1 rotary pump attached to a liquid nitrogen cold trap). The system was evacuated to a base pressure of 2 × 10-3 mbar, followed by valving off the pump and exposing the anhydride-coated substrate to either propylamine (+99%, Aldrich) or allylamine (+98%, Aldrich) vapor at a pressure of approximately 200 mbar for 30 min. Upon completion of aminolysis,62 any excess reagent was (61) Ryan, M. E.; Hynes, A. M.; Badyal, J. P. S.; Chem. Mater. 1996, 8, 37. (62) Evenson, S. A.; Fail, C. A.; Badyal, J. P. S. Chem. Mater. 2000, 12, 3038.
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pumped away and then the chamber vented to atmosphere (Scheme 1). Conversion of amide linkages to imide groups via ring closure62 was achieved by placing the functionalized substrate in a vacuum oven at 120 °C for 1 h (Scheme 1). 2.3. Quantification of Surface Radical Density. The concentration of radical sites contained in the maleic anhydride pulsed plasma polymer and the amine-derivatized and imidefunctionalized films was determined by the DPPH method using 1,1-diphenyl-2-picrylhydrazyl (DPPH, 95%, Aldrich).63 For these experiments, a borosilicate glass coverslip slide (BDH) coated with the plasma polymer film was placed into a glass tube containing 5 mL of DPPH dissolved in toluene solution (1 × 10-4 mol dm-3). The tube was then sealed and subjected to several freeze-pump-thaw cycles prior to heating at 70 °C for 3 h. The DPPH molecules consumed by surface radicals were quantified by measuring the difference in absorbance at 520 nm between a control and the coated sample using a spectrophotometer (PYE Unicam PV8600). 2.4. Thermal Styrene Surface Polymerization. Styrene (5 mL, 43.7 mmol, +99%, Aldrich) was added to a reaction tube containing the functionalized silicon wafers. Any dissolved gases were removed by several freeze-pump-thaw cycles, and then the tube was heated at 120 °C for 3 h. Upon cooling, the solidified reaction mixture was dissolved in dichloromethane, and the silicon wafers were removed. The substrates were then washed continuously in dichloromethane using a Soxhlet extractor for 16 h, in order to remove any loosely bound polymer, and finally dried under nitrogen. 2.5. Stable Free Radical Mediated Styrene Surface Polymerization. A mixture of styrene (5 mL, 43.7 mmol, +99%, Aldrich), recrystallized benzoyl peroxide (0.021 g, 0.087 mmol, 70%, Aldrich, purified by dissolving in chloroform and recrystallized by adding an equivalent amount of methanol), and 2,2,6,6, tetramethylpiperidin-1-oxyl (TEMPO, 0.0177 g, 0.11 mmol) (98%, Aldrich) was loaded into a glass tube containing a plasmapolymer-coated silicon wafer in a molar ratio of 500:1:1.3.64 The tube was then subjected to several freeze-pump-thaw cycles, sealed under vacuum, and heated at 120 °C for 16 h. Upon cooling, the silicon wafer was removed from the solidified reaction mixture by dissolving in dichloromethane and then continuously washed in dichloromethane using a Soxhlet extractor for 16 h prior to finally drying in flowing nitrogen. To demonstrate the continued existence of stable free radicals in the case of TEMPO-mediated versus thermally polymerized polystyrene films, both were placed into separate tubes containing a mixture of vinylbenzyl chloride (5 mL, 35.5 mmol, 97%, Aldrich), benzoyl peroxide (0.021 g), and TEMPO (0.0177 g) and subjected to the aforementioned polymerization procedure in order to produce surface-grafted polystyrene-polyvinylbenzyl chloride block copolymer brushes. 2.6. Patterned Surface Polymerization. Cut pieces of poly(tetrafluoroethylene) (PTFE, 0.25 mm thick, Goodfellow) were embossed with copper grids (mesh size 15 µm, hole size 47 µm, Agar Scientific) using a pressure of 5100 kPa for 10 min. With the embossed grid in place, maleic anhydride pulsed plasma deposition was carried out followed by allylamine derivatization. The copper grid was then removed from the PTFE substrate to leave behind well-defined arrays of initiator. At this stage, nitroxide-mediated controlled free-radical graft polymerization was carried out (as described previously). A Raman microscope system (LABRAM, Jobin Yvon Ltd) was used to chemically map the polymer arrays. This used a He-Ne laser as the excitation source (632.8 nm line, operating at 20 mW). The unattenuated laser beam was focused onto the sample using a microscope objective (×50), and the corresponding Raman signals were acquired by the same microscope objective in a backscattering configuration in combination with a cooled CCD detector system. The 1800 mm-1 grooves diffraction grating was calibrated against the Si-Si stretching band (521 cm-1) from a silicon wafer. The sample was mounted on a computerized X-Y translational mapping stage and the surface scanned (100 µm (63) Suzuki, M.; Kishida, A.; Iwata, H.; Ikada, Y. Macromolecules 1986, 19, 1804. (64) MacLeod, P. J.; Veregin, R. P. N.; Odell, P. G.; Georges, M. K. Macromolecules 1997, 30, 2207.
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Figure 1. RAIRS of (a) maleic anhydride pulsed plasma polymer; (b) propylamine-derivatized maleic anhydride pulsed plasma polymer; (c) propylamine-derivatized maleic anhydride pulsed plasma polymer, after heating at 120 °C for 2 h; (d) allylamine-derivatized maleic anhydride pulsed plasma polymer; and (e) allylamine-derivatized maleic anhydride pulsed plasma polymer, after heating at 120 °C for 2 h. × 100 µm) using 2-micrometer steps. Integration of the polystyrene (990-1010 cm-1) band for each pixel provided the overall surface chemical map. 2.7. Film Characterization. Film thicknesses were measured using spectrophotometry (Nanoptix NKD 6000, Aquila Instruments Ltd). The obtained transmittance-reflectance curves spanning the 350-1000 nm range were fitted according to the Cauchy model using a modified Levenburg-Marquardt algorithm.65 Surface infrared spectroscopy was performed using a PerkinElmer Spectrum One FTIR spectrometer equipped with a liquid nitrogen cooled MCT detector operating at 4 cm-1 resolution over the 750-4000 cm-1 range. The instrument was fitted with a variable angle reflection-absorption accessory (Specac) set to an angle of 66° and adjusted for p-polarization for reflectionabsorption spectroscopy (RAIRS). X-ray photoelectron spectroscopy (XPS) analysis was carried out using a VG Microtech surface analysis system. The instrument was equipped with an unmonochromated Mg KR X-ray source (1253.6 eV) and a hemispherical analyzer (VG 100 AX) operating in the CAE mode at a pass energy of 20 eV. XPS core level spectra were fitted using Marquardt minimization computer software assuming a linear background and equal full width at half-maximum (fwhm) for all the component peaks within each core level envelope.66 Elemental concentrations were calculated using the following experimentally determined sensitivity (multiplication) factors: C(1s):O(1s):N(1s):Cl(2p) ) 1.00:0.34:0.69: 0.40. Contact angle measurements were taken with a video capture apparatus (AST Products VCA2500XE) using sessile 2-µL droplets of deionized water.
Results 3.1. Amine Derivatization of Maleic Anhydride Plasma Polymer. Infrared analysis confirmed a high level of structural retention of the anhydride groups in the pulsed plasma deposited layers (Figure 1). The following characteristic cyclic anhydride absorbances were (65) Tabet, F. M.; McGahan, W. A. Thin Solid Films 2000, 370, 122. (66) Evans, J. F.; Gibson, J. H.; Moulder, J. F.; Hammond, J. S.; Goretzki, H. Fresenius J. Anal. Chem. 1984, 319, 841.
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Table 1. XPS, Contact Angle, and Radical Density Analysis of Derivatized Pulsed Plasma Poly(maleic anhydride) Films contact radical angle/ density/ -9 % C % O % N % Si deg 10 mol cm-2 XPS
surface maleic anhydride plasma polymer propylamineplasma polymer propylamineplasma polymer/120 °C allylamineplasma polymer allylamine plasma polymer/120 °C
64
36
0
0
45
3.0
66
21
13
0
21
20
68
22
10
0
65
12
66
21
12
0
16
9.5
68
22
10
0
65
6.4
identified:67-69 asymmetric and symmetric CdO stretching [1861 cm-1 (A1) and 1796 cm-1 (A2)], cyclic conjugated anhydride group stretching [1245 cm-1 (A3)], C-O-C stretching vibrations [1098 cm-1 (A4)], and cyclic unconjugated anhydride group stretching [938 cm-1 (A5)]. Reaction with propylamine and also allylamine gave rise to the appearance of carboxylic acid at 1710 cm-1 (B), amide I at 1670 cm-1 (C), and amide II at 1575 cm-1 (D) bands (Figure 1). In the allylamine-derived spectra only, additional allylic double bond bands [1000 and 935 cm-1 (F)] were present. Heating the amine-derivatized films to 120 °C under vacuum gave rise to cyclic imide group formation [1780 and 1710 cm-1 (E)]. The high-resolution C(1s) XPS spectra of the maleic anhydride pulsed plasma polymer can be fitted to five different carbon environments:70 hydrocarbon (CHx ∼ 285.0 eV), carbon singly bonded to an anhydride group (C-C(O)dO ∼ 285.7 eV), carbon singly bonded to oxygen (-C-O ∼ 286.6 eV), carbon doubly bonded to oxygen (OC-O/-CdO ∼ 287.9 eV), and anhydride groups (OdCO-CdO ∼ 289.4 eV). The optimum plasma deposition conditions (toff ) 1200 µs, ton ) 20 µs, Pp ) 5 W, and deposition rate ) 3.4 ( 0.5 nm min-1) corresponding to the greatest level of structural retention gave rise to 58% of all surface carbon atoms belonging to cyclic anhydride repeat units (on the basis of the C(1s) envelope peak fitting61). XPS confirmed the presence of carbon, oxygen, and nitrogen atoms following amine derivatization of the anhydride functionalized surface (Table 1). The N(1s) peak contained a major component at 399.8 eV (74.3% and 70.5% of N environment for propylamine and allylamine respectively), which can be attributed to the formation of C(dO)-N(H)-C groups70 (Figure 2). The smaller component at 401.5 eV belongs to C-NH3+ centers.70 Upon imidization, there was a small decrease in the total N(1s) signal (due to removal of acid-base interactions), and the N(1s) shoulder at 401.5 eV diminished in intensity relative to the component at 399.8 eV (also attributable to loss of acid-base interactions). The trends were similar for both propylamine and allylamine (Figure 2 and Table 1). Contact angle analysis showed an increase in hydrophilicity (acid-base interactions) upon amine derivatization of the maleic anhydride pulsed plasma polymer films and a rise in hydrophobicity (removal of acid-base (67) Mirone, P.; Chiorboli, P. Spectrochim. Acta 1962, 18, 1425. (68) Silverstein, R. M.; Bassler, G. C.; Morril, T. C. Spectrometric Identification of Organic Compounds, 5th ed.; John Wiley: New York, 1991. (69) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: San Diego, 1991. (70) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers; John Wiley: Chichester, UK, 1992.
Figure 2. N(1s) XPS envelopes of maleic anhydride pulsed plasma polymer following (a) propylamine exposure and (b) propylamine exposure and then heating at 120 °C (a similar trend was noted for allylamine-derivatized plasma polymer films).
interactions) upon imide formation (Table 1). This is consistent with the reactions depicted in Scheme 1. Thickness measurements of the pulsed plasma polymer films indicated swelling upon amine derivatization (Table 2). For a typical 100-nm-thick maleic anhydride pulsed plasma polymer film, a 39% increase in thickness was noted following propylamine exposure and a 22% swelling in the case of allylamine. Subsequent imidization caused these films to revert to approximately their original thicknesses. Quantification of the surface radical concentration by the DPPH assay indicated that the pulsed plasma polymer film contains radicals at the surface (Table 1). Amine derivatization gave rise to a large increase in the number of accessible radical centers. Subsequent imidization reduced this number by approximately 33%. Therefore, it would appear that the swelling of the maleic anhydride pulsed plasma polymer film during amine vapor exposure gives rise to greater accessibility to trapped-free-radical sites within the subsurface of the plasma polymer layer. In the case of continuous wave (CW) deposited maleic anhydride plasma polymer films, similar infrared absorbances to those seen for pulsed conditions were detected (except a lot broader due to poor structural retention61), with a dominant peak centered at 1780 cm-1. Exposure to propylamine and allylamine resulted in amide I peaks appearing; however, the efficiency of derivatization was found to be poor, as evident by the absence of any strong perturbation in the 1780-cm-1 absorbance. XPS analysis confirmed this viewpoint, with substantially less nitrogen incorporation (3% for propylamine-treated CW plasma polymerized maleic anhydride compared to 12-13% for the pulsed plasma deposited films). 3.2. Thermal Styrene Polymerization. Thermal polymerization of styrene from the surface of aminederivatized maleic anhydride pulsed plasma polymer films was examined by infrared spectroscopy (Figure 3). Characteristic polystyrene phenyl ring stretching absorbances at 1600, 1494, and 1452 cm-1 (G) were evident. The growth of polystyrene onto the propylamine-functionalized film was greatest (64 ( 29 nm) compared to the native plasma film (27 ( 6 nm) and the allylamine-treated film (19 ( 6.6
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Table 2. Film Thickness and Growth Rate Measurements maleic anhydride pulsed plasma polymer propylamine derivatized
allylamine derivatized
treatment
thickness/ nm
underivatized growth rate/h-1
thickness/ nm
growth rate/h-1
thickness/ nm
growth rate/h-1
no treatment imidization (120 °C) thermal styrene polymerization (3 h): amidea,b thermal styrene polymerization (3 h): imidea,b controlled styrene polymerization (16 h): amidea controlled styrene polymerization (16 h): imidea
100 ( 15 100 ( 15 27 ( 6 36 ( 15 -
9 2.3 -
139 ( 23 90 ( 14 64 ( 29 50 ( 10 611 ( 90 552 ( 81
21.3 16.7 38.2 34.5
122 ( 15 102 ( 15 19 ( 13 5(1 536 ( 79 21 ( 3.1
6.3 1.7 33.5 1.3
a The initial plasma polymer film thickness has been subtracted. b A shorter thermal polymerization time is reported in order to avoid termination reactions distorting the film thickness growth rate value.
Figure 3. RAIRS of thermally polymerized styrene on (a) maleic anhydride pulsed plasma polymer, (b) propylaminederivatized maleic anhydride pulsed plasma polymer, and (c) allylamine-derivatized maleic anhydride pulsed plasma polymer.
nm) (Table 2). The temperature employed for styrene polymerization (120 °C) was found to be sufficient for imide formation. Propylamine-treated films which had been imidized at 120 °C prior to styrene polymerization displayed similar behavior (grafted film thickness of 50 ( 10 nm) to their amide counterparts. In contrast, preimidized allylamine films generated thinner (5 ( 1 nm) polystyrene grafts than their amide and maleic anhydride native film counterparts. XPS and contact angle analysis confirmed the grafting of polystyrene films, i.e., only a single C(1s) XPS signal centered at 285.0 eV (together with its π-π* shake-up satellite),71 and a large rise in water contact angle to ca. 100° (typical of hydrocarbon surfaces).72 Continuous wave maleic anhydride plasma polymer (CW) films were also found to thermally graft polystyrene films from the surface. However, the polymerization rates were found to be significantly lower for both the native (5 ( 1 nm) and propylamine-derivatized (4 ( 1 nm) films. This is consistent with the inherently poor structural retention associated with continuous wave plasma deposition. 3.3. Controlled Radical Polymerization. TEMPOmediated stable free-radical polymerization of styrene (71) Clark, D. T.; Adams, D. B.; Dilks, A.; Peeling, J.; Thomas, H. R. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 51. (72) Zhao, B.; Brittain, W. J. J. Am. Chem. Soc. 1999, 121, 3557.
Figure 4. RAIRS of controlled radical polymerized styrene on (a) maleic anhydride pulsed plasma polymer; (b) propylaminederivatized maleic anhydride pulsed plasma polymer; (c) propylamine-derivatized maleic anhydride pulsed plasma polymer, after heating at 120 °C; (d) allylamine-derivatized maleic anhydride pulsed plasma polymer; and (e) allylaminederivatized maleic anhydride pulsed plasma polymer, after heating at 120 °C.
from the surface of maleic anhydride pulsed plasma polymer films for 16 h resulted in 36 ( 15-nm-thick films of polystyrene (Table 2). Infrared spectroscopy shows the characteristic polystyrene peaks (G) superimposed on the maleic anhydride pulsed plasma polymer background (Figure 4). The absorbance intensity is on a comparable scale to films obtained from the 3-h thermal polymerization. Controlled radical polymerization of polystyrene onto the propylamine- and allylamine-derivatized surfaces resulted in the generation of grafted films of 611 ( 90 and 536 ( 79 nm, respectively (Figure 4 and Table 2). These are significantly thicker than the corresponding thermally grafted polystyrene films. Conversion of the propylaminetreated surfaces to imide groups prior to controlled radical styrene polymerization resulted in grafted films that were of comparable thickness to their amide counterparts (552 ( 81 nm) (Table 2). Interestingly, the allylamine-imidized surface resulted in a grafted film thickness comparable to that of the native maleic anhydride plasma polymer (21 ( 3 nm). Variation of the controlled free-radical polymerization time from 1 to 16 h for the propyl- and allylaminederivatized surfaces yielded a linear plot of film thickness
Controlled Free-Radical Polymerization
Figure 5. Changes in film thickness (a) following thermal or TEMPO-mediated controlled free-radical polymerization of styrene from propylamine-derivatized, 100-nm-thick maleic anhydride pulsed plasma polymer films as a function of time and (b) for varying thickness maleic anhydride pulsed plasma polymer layers derivatized with propylamine and following subsequent TEMPO-mediated controlled free-radical polymerization of styrene (a similar trend was noted with allylamine).
versus time for the resulting polystyrene films, indicating that the polymerization rate is constant, with no significant termination reactions (Figure 5). The nonlinearity at the start is due to concurrent induction of the polymerization reaction and imidization (deswelling) of the amine-derived films. The initial thickness of the maleic anhydride pulsed plasma polymer was found to influence the thickness of the subsequent grafted polystyrene film (Figure 5). Amine derivatization of a range of plasma polymer thicknesses, followed by controlled polymerization on these films, resulted in correspondingly thick polystyrene layers, proving that the graft polymerization is not restricted to the outermost surface. Such polymer brush surfaces could be prepared on a range of substrates, including silicon, gold, glass, PTFE, and polyethylene. Verification of the controlled stable free-radical nature of these systems was demonstrated by subsequent controlled copolymerization of vinylbenzyl chloride onto the polystyrene grafted surfaces. The resultant graft block poly(vinylbenzyl chloride) layer on TEMPO-terminated polystyrene grown from maleic anhydride pulsed plasma polymer had a thickness of 18 nm after 16 h, whereas the increase in film thickness for the TEMPO-terminated polystyrene grown from propylamine-derivatized maleic anhydride pulsed plasma polymer was 120 nm (a significant enhancement). The XPS C(1s) envelope for these copolymer grafts resembled that of polyvinylbenzyl chloride70 (7 ( 1% chlorine was detected at the surface compared to a theoretical value of 10%ssome polymer will be buried in the polystyrene). Also, new infrared absorbances at 1511 cm-1 [(H) phenyl ring stretch] and 1266 cm-1 [(I) -CH2-Cl wag] confirmed the presence of poly(vinylbenzyl chloride) (Figure 6). XPS analysis of conventional thermally grown polystyrene on the amine-derivatized plasma polymer films, which had then been subjected to controlled polymerization conditions for graft polymerization of vinylbenzyl chloride, did not reveal any surface chlorine, and the overall film thickness remained unchanged.
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Figure 6. RAIRS of (a) spin cast polyvinylbenzyl chloride, (b) controlled radical polymerized styrene on propylamine-treated plasma polymer, and (c) controlled radical polymerized vinylbenzyl chloride on surface b.
Figure 7. (a) 100 × 100 µm optical image of controlled radical polymerized styrene on PTFE patterned with a propylaminederivatized pulsed plasma poly(maleic anhydride) array, (b) corresponding Raman image at 999 cm-1; and (c) Raman point spectra taken at points A and B.
3.4. Patterned Controlled Polymerization. Optical microscopy of PTFE films that had been embossed with a copper grid prior to pulsed plasma deposition of poly(maleic anhydride) (with the copper grid subsequently removed) and then subjected to the aforementioned amidecontrolled free-radical polymerization procedure displayed a well-defined pixel pattern (Figure 7). The Raman spectrum from between the square pixels (point A) indicates PTFE, with a prominent peak at 732 cm-1(K), whereas the pixels themselves (point B) show a combination of PTFE (K) and polystyrene [620 cm-1 (L) and 999 cm-1(M)] signals, i.e., where maleic anhydride pulsed plasma polymer was originally deposited.
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Discussion Maleic anhydride pulsed plasma polymer films can be employed as initiating surfaces for “controlled” radical polymerization in the presence of nitroxide-stable free radicals. This leads to the formation of polymer grafts of controllable molecular weight and polydispersity, as well as offering scope for the formation of block copolymer brushes. The diphenyl picryl hydrazyl (DPPH) assay results reveal that the number of accessible free radicals in the as-deposited (underivatized) maleic anhydride pulsed plasma polymer films is approximately equivalent to those found in studies of plasma-treated polymer surfaces (4.95.5 nmol cm-2)73,74 (Table 1). These films exhibit polymerization behavior akin to other reported nitroxide freeradical systems.29 Although the rate of polymer growth is relatively slow when compared to grafting without nitroxide present, over a longer period of time (e.g. more than 16 h in this study) controlled radical polymerization yields thicker polymer brush films. This can be attributed to the minimization of termination reactions in the controlled grafting process. Maleic anhydride pulsed plasma polymer films are known to readily react with nucleophilic reagents to form functionalized surfaces.62 Furthermore, they are known to behave as swellable gels in the appropriate solvent.75 In the case of propylamine - and allylamine-derivatized maleic anhydride pulsed plasma polymer layers, XPS, infrared spectroscopy, DPPH assay, water contact angle, and reflectometry all indicate a marked change in the film. The DPPH assay reveals that the number of accessible free radicals has increased by an order of magnitude (Table 1). This is accompanied by a rise in hydrophilicity, due to acid-base interactions at the surface, and film swelling. Such film expansion during aminolysis enables greater access to subsurface trapped radicals. Imidization of these amide films by heating62,76 leads to the disappearance of acid-base interactions (as seen by XPS) and a rise in hydrophobicity. This increase in hydrophobicity should enable apolar solvents to penetrate the film more effectively, thus assisting access to the subsurface free radicals. Propylamide/imide and allylamide functionalized films resulted in polystyrene grafts 1 order of magnitude thicker than the native and allylamine-imidized pulsed plasma poly(maleic anhydride) films. The graft polymerization occurs at available sites (radicals) throughout the film in a fashion similar to preswelling polymerization. This is (73) Lai, J. Y.; Denq, Y. L.; Chen, J. K.; Yuan, L. Y.; Lin, Y. Y.; Shyu, S. S. J. Adhes. Sci. Technol. 1995, 9, 813. (74) Michel, V.; Marzin, C.; Tarrago, G.; Durand, J. J. Appl. Polym. Sci. 1998, 70, 359. (75) Schiller, S.; Hu, J.; Jenkins, A. T. A.; Timmons, R. B.; SanchezEstrada, F. S.; Knoll, W.; Fo¨rch, R. Chem. Mater. 2002, 14, 235. (76) Zhao, M.; Liu, Y.; Crooks, R. M.; Bergbreiter, D. E. J. Am. Chem. Soc. 1999, 121, 923.
Teare et al. Scheme 2. Autoinhibition by Allylic Monomersa
a X and Y are substituent groups contained in the plasma polymer.
supported by the fact that the graft polymerization is not restricted to just the surface but is dependent upon plasma polymer film thickness (Figure 5). Clearly, thicker plasma polymer films contain more trapped radicals, which translates into thicker polymer grafts. Of particular importance is the fact that derivatization of the plasma polymer with a spacer molecule yields grafted polymer brush films 1 order of magnitude thicker than previously reported for nitroxide-mediated surface-controlled freeradical graft polymerizations.29 In the case of the allylamine-derivatized plasma polymer that is not heated to 120 °C prior to TEMPO-mediated radical polymerization, the TEMPO molecules are able to cap the growing radical chains prior to possible termination by allylic radicals (Scheme 2). This ensures minimization of early termination reactions and enables a controlled polymerization to proceed (control of molecular weight and polydispersity). The linear increase in film thickness (hence molecular weight29,46) reveals that the nitroxide-mediated graft process is controlled. Further evidence of the controlled nature of surface polymerization comes from the ability to readily synthesize a copolymer by adding additional monomer with the nitroxide after the initial graft. The thinner films observed for thermal polymerization can be attributed to termination events occurring. As well as the employment of masking during pulsed plasma deposition of the initiator layer, patterned growth of polymer brushes could also potentially be achieved by micro-contact printing or nanopipetting the swelling reagent prior to TEMPO-mediated controlled polymerization. Conclusions Swelling of the pulsed plasma deposited poly(maleic anhydride) initiator layer with a spacer molecule enhances the rate of TEMPO-mediated controlled graft polymerization. Furthermore, in contrast to conventional surfaceinitiated polymer grafts, which rely upon self-assembed monolayers (SAMs), the present approach is amenable to a wide range of substrate materials and geometries. Acknowledgment. J.P.S.B. would like to thank the EPSRC for an Advanced Research Fellowship. LA051566+
