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Langmuir 2003, 19, 2398-2403
Substrate-Independent Growth of Micropatterned Polymer Brushes D. O. H. Teare, W. C. E. Schofield, V. Roucoules, and J. P. S. Badyal* Department of Chemistry, Science Laboratories, Durham University, Durham DH1 3LE, England, U.K. Received March 21, 2002. In Final Form: November 29, 2002 A substrate-independent method is described for the synthesis of polymer brushes on solid surfaces. This entails pulsed plasma deposition of maleic anhydride to generate a free radical initiator containing thin film followed by styrene polymerization. Polystyrene layers of up to 150 nm thickness can be grown from these initiator surfaces. This reaction can be inhibited by exposure to a free radical scavenger, which is consistent with the proposed free radical initiator mechanism. UV irradiation through a photomask of such capped surfaces leads to localized reactivation of the initiator sites, which in turn provides a method for the microfabrication of patterned polystyrene brush surfaces.
Introduction Modification of surfaces is often necessary to improve their physicochemical properties. One way of achieving this objective is to deposit, attach, or grow thin polymer films by spin-coating,1 physisorption from solution,2 chemical vapor deposition,3 alternating deposition of polyelectrolyte multilayers via electrostatic interactions,4,5 plasma polymerization,6 or grafting end-functionalized polymers to reactive sites located at the substrate surface.7,8 Most of these approaches suffer from some kind of drawback. For instance, spin-coated polymer films may not adhere well to the substrate. In the case of physisorbed polymers, their inherently weak adhesive van der Waals interactions can become thermally unstable, and are liable to displacement by other solutes in solution.9 Electrostatic deposition of multilayer stacks relies on ionic forces, which makes it self-limited to ionic polymers. Chemical vapor deposition and plasma polymerization are restricted to volatile monomers. In principle, “grafting to” methods can potentially circumvent a number of the aforementioned drawbacks via covalent attachment of preformed polymer chains to the substrate surface, thereby ensuring good adhesion. However, end-on grafting of this type tends to produce low levels of surface immobilization due to entropic and steric effects.9 In contrast, “grafting from” methods using surface initiated polymerization offer greater promise. Since in this case the initiator is covalently anchored onto the substrate surface, this enables the in situ addition of monomer units in a sequential fashion so that film * To whom correspondence should be addressed. (1) Chen, W. L.; Shull, K. R. Macromolecules 1999, 32, 6298. (2) Abraham, T.; Giasson, S.; Gohy, J. F.; Je´roˆme, R. Langmuir 2000, 16, 4286. (3) Vaeth, K. M.; Jackman, R. J.; Black, A. J.; Whitesides, G. M.; Jensen, K. F. Langmuir 2000, 16, 8495. (4) Chen, W.; McCarthy, T. J. Macromolecules 1997, 30, 78. (5) Xiao, K. P.; Harris, J. J.; Park, A.; Martin, C. M.; Pradeep, V.; Bruening, M. L. Langmuir 2001, 17, 8236. (6) Oehr, C.; Muller, M.; Elkin, B.; Hegemann, D.; Vohrer, U. Surf. Coat. Technol. 1999, 116, 25. (7) Minko, S.; Patil, S.; Datsyuk, V.; Simon, F.; Eichhorn, K. J.; Motornov, M.; Usov, D.; Tokarev, I.; Stamm, M. Langmuir 2002, 18, 289. (8) Bergbreiter, D. E.; Franchina, J. G.; Kabza, K. Macromolecules 1999, 32, 4993. (9) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993.
thickness and polymer chain graft density can be easily controlled. Several methods have been devised for this purpose including cationic,10,11 anionic,12 free radical,13 living free radical,14 and ring opening polymerization15-18 techniques. So far, free radical initiated polymer film growth has proved to be the most popular, since this method provides the greatest versatility on the basis of the wide range of readily available monomers. In many cases, initiator sites are fixed onto the substrate using a coupling reagent to form a self-assembled monolayer (SAM). For instance, silane chemistry is amenable to reaction with surface hydroxyl groups on inorganic substrates,19-21 while thiol terminated molecules react with gold22-24 (or other non-oxide forming transition metal) surfaces. Disadvantages of SAMs include the fact that they require careful substrate preparation to ensure homogeneity; also specific initiator coupling reagents need to be specially synthesized.19,23 Related systems which rely upon the stepwise generation of initiator monolayers via attachment to an anchoring SAM10,22are susceptible to the risk of losing active sites through incomplete reactions, as well as the requirement for additional preparative steps. For the case of polymer substrates, a variant of the “grafting from” approach is already known, where high (10) Ingall, M. D. K.; Honeyman, C. H.; Mercure, J. V.; Bianconi, P. A.; Kunz, R. R. J. Am. Chem. Soc. 1999, 121, 3607. (11) Spange, S.; Hohne, S.; Francke, V.; Gunther, G. Macromol. Chem. Phys. 1999, 200, 1054. (12) Jordan, R.; Ulman, A.; Kang, J. F.; Rafailovich, M. H.; Sokolov, J. J. Am. Chem. Soc. 1999, 121, 1016. (13) Biesalski, M.; Ruhe, J. Macromolecules 1998, 31, 592. (14) Husseman, M.; Malmstrom, 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, 1424. (15) Choi, I. S.; Langer, R. Macromolecules 2001, 34, 5361. (16) Chang, Y. C.; Frank, C. W. Langmuir 1998, 14, 326. (17) Husemann, M.; Mecerreyes, D.; Hawker, C. J.; Hedrick, J. L.; Shah, R.; Abbott, N. Angew. Chem., Int. Ed. 1999, 38, 647. (18) Juang, A.; Scherman, O. A.; Grubbs, R. H.; Lewis, N. S. Langmuir 2001, 17, 1321. (19) Prucker, O.; Ruhe, J. Macromolecules 1998, 31, 592. (20) Fujiki, K.; Sakamoto, M.; Yoshida, A.; Maruyama. H. J. Polym. Sci. A: Polym. Chem. 1999, 37, 2121. (21) Biesalski, M.; Ruhe, J. Macromolecules 1999, 32, 2309. (22) Huang, W.; Skanth, G.; Baker, G. L.; Bruening, M. L. Langmuir 2001, 17, 1731. (23) Schmidt, R.; Zhao, T.; Green, J. B.; Dyer, O. J. Langmuir 2002, 18, 1281. (24) Hyun, J.; Chilkoti, A. Macromolecules 2001, 34, 5644.
10.1021/la020279s CCC: $25.00 © 2003 American Chemical Society Published on Web 02/11/2003
Growth of Micropatterned Polymer Brushes
energy γ-radiation25 or plasma exposure26-29 can directly generate initiation sites at the polymer surface. Free radical polymerization can then proceed from these sites. It is generally accepted that surface peroxide groups30 initiate this variant of free radical polymerization. However, this technique is restricted to surfaces which are organic in nature. As yet no method has been devised which is equally amenable to organic and inorganic substrates. In this article we describe a substrate-independent approach for achieving surface initiated polymerization. It is shown that the pulsed plasma polymerization of maleic anhydride yields a well-adhered free-radicalcontaining plasma polymer film. These free radicals can serve as initiation sites for free radical polymerization. Furthermore, they can be capped with a free radical scavenger. UV photolithography of such capped surfaces is shown to be capable of reactivating the initiator film, thereby making this approach a very simple and effective way for producing micropatterned polymer brush surfaces. Experimental Section Ground maleic anhydride powder (Aldrich 99%) and 4-fluorostyrene (Aldrich 99%) monomers were loaded into glass tubes and further purified by several freeze-pump-thaw cycles prior to use. Plasma polymerization was carried out in an electrodeless cylindrical glass reactor (5 cm diameter, 520 cm3 volume, base pressure of 4 × 10-3 mbar, and with a leak rate better than 1.8 × 10-9 kg s-1) enclosed in a Faraday cage. The chamber was fitted with a gas inlet, a Pirani pressure gauge, a 30 L min-1 two-stage rotary pump attached to a liquid nitrogen cold trap, and an externally wound copper coil (4 mm diameter, 9 turns, spanning 8-15 cm from the gas inlet). 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, rinsing in water and propan-2-ol, and 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, either a polished silicon wafer (cleaned ultrasonically in a 50/50 propan-2-ol/cyclohexane solvent mixture, and then CO2 snow jet) or a preformed potassium bromide plate was inserted into the center of the reactor, and the system was pumped back down to base pressure. At this stage, monomer vapor was introduced at a pressure of 0.2 mbar for 5 min prior to ignition of the electrical discharge. In the case of pulsed plasma deposition, the peak power (PCW) was set to 5 W, the duty cycle pulse on-time (ton) at 20 µs, and the pulse off-time (toff) equal to 1200 µs,31 and deposition was carried out for 30 min duration. Comparable film thicknesses (ca. 100 nm) were obtained under continuous wave plasma conditions at a power of 5 W for 2 min. Upon completion of plasma polymerization, the monomer was allowed to continue to flow through the reactor for a further 5 min, prior to evacuation to base pressure and then venting to atmosphere. Graft polymerization onto these plasma polymer coated substrates entailed immersion into a sealed glass tube containing approximately 5 mL of styrene (Aldrich 99%) followed by multiple freeze-pump-thaw cycles to remove any dissolved gases. Next, the tube was placed into an oil bath at 120 °C for 3 h to facilitate (25) Bhattacharya, A. Prog. Polym. Sci. 2000, 25, 371. (26) Gupta, B.; Hilborn, J.; Bisson, I.; Frey, P. J. Appl. Polym. Sci. 2001, 81, 2993. (27) Bamford, C. H.; Jenkins, A. D.; Ward, J. C. Nature 1960, 186, 712. (28) Bradley, A.; Fales, J. D. Chem. Technol. 1971, 4, 232. (29) Bradley, A. Chem. Technol. 1973, 3, 507. (30) Suzuki, M.; Kishida, A.; Iwata, H.; Ikada, Y. Macromolecules 1986, 19, 1804. (31) Ryan, M. E.; Hynes, A. M.; Badyal, J. P. S. Chem. Mater. 1996, 8, 37.
Langmuir, Vol. 19, No. 6, 2003 2399 thermal polymerization. Subsequently, any loosely bound polystyrene attached to the substrate surface was removed by Soxhlet extraction in dichloromethane for 16 h. Corresponding control experiments with uncoated substrates were also undertaken. Confirmation of the initiator sites being present in the pulsed maleic anhydride plasma polymer films was demonstrated by reaction with 4-fluorostyrene vapor in the reactor following plasma polymerization (30 min exposure at 20 °C, followed by evacuation to base pressure). Initiator quenching studies employed diphenylpicrylhydrazyl32 (DPPH, Aldrich 95%) as a free radical scavenger. This entailed immersion of each plasma polymer coated substrate into a 5 mL solution of 1 × 10-4 mol dm-3 DPPH dissolved in toluene. The reaction tube was sealed, and any dissolved gases were removed by multiple freeze-pump-thaw cycles. The DPPH solution was then heated in an oil bath at 80 °C for 3 h, followed by removal of the substrate and rinsing in clean toluene. The final step entailed placing DPPH treated plasma polymer films into styrene and exposing them to the previously described styrene polymerization conditions. Plasma polymer and grafted polystyrene film thicknesses were measured using an optical reflectometer (Nanoptix NKD 6000). The obtained transmittance-reflectance curves spanning the 350-1000 nm range were fitted in accordance with the Cauchy model using a modified Levenburg-Marquardt procedure.33 Fourier transform infrared (FTIR) analysis of the polymeric films at each stage of reaction was carried out using a PerkinElmer Spectrum One spectrometer equipped with a liquid nitrogen cooled MCT detector operating at 4 cm-1 resolution across the 750-4000 cm-1 range. Plasma polymer films deposited onto preformed potassium bromide plates were characterized using a diamond attenuated total reflection accessory (Graesby Specac Golden Gate). Polystyrene films grafted on plasma polymer coated silicon wafers were analyzed by reflectionabsorption (RAIRS) spectroscopy using a variable angle accessory (Specac) set to a 66° grazing angle and adjusted for p-polarization. X-ray photoelectron spectroscopy (XPS) analysis of the films 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 constant analyzer energy mode (CAE, pass energy ) 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 Gaussian component peaks. Elemental concentrations were calculated using instrument sensitivity (multiplication) factors determined from chemical standards, C(1s):O(1s):N(1s): F(1s) ) 1.0:0.34:0.69:0.19. The absence of any Si(2p) signal from the underlying silicon substrate was taken as being indicative of continuous polymer film coverage at a thickness greater than the XPS sampling depth (2-5 nm).34 Ultraviolet (UV) patterning of the plasma polymer films was performed by irradiating the film surface through a fine copper mesh photomask (Agar Scientific, grid thickness of 5 µm and open square width of 20 µm) with a Hg-Xe UV lamp (Oriel Corporation, Model 6136) operating at 120 W. Exposure times were varied between 10 min and 2 h. This protocol gave rise to reduced polystyrene growth in the exposed areas (i.e., a negative image). To create a positive image, the maleic anhydride plasma polymer film was capped with DPPH and then UV irradiated for 10 min through the copper grid mask to reactivate initiator sites, followed by styrene polymerization. In each case, the patterned polymer films were viewed by optical microscopy (BX40, Olympus confocal microscope).
Results (a) Styrene Graft Polymerization onto Maleic Anhydride Plasma Polymer Surfaces. Infrared spectroscopy of ca. 100 nm thickness maleic anhydride pulsed (32) Johnson, D. R.; Osada, Y.; Bell, A. T.; Shen, M. Macromolecules 1981, 14, 118. (33) Technical Information, Aquila Instruments, Cambridge, UK. (34) Practical Surface Analysis. Vol. IsAuger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; John Wiley: Chichester, UK, 1990.
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Figure 1. ATR-FTIR spectra of (a) maleic anhydride pulsed plasma polymer, (b) maleic anhydride pulsed plasma polymer following styrene polymerization, (c) DPPH capped pulsed plasma polymer followed by styrene polymerization, and (d) conventional polystyrene film.
Figure 2. C(1s) XPS spectra of (a) maleic anhydride pulsed plasma polymer, (b) DPPH capped maleic anhydride pulsed plasma polymer, (c) DPPH capped pulsed plasma polymer following styrene polymerization, and (d) maleic anhydride pulsed plasma polymer following styrene polymerization.
Table 1. Infrared Spectroscopy Assignments for Maleic Anhydride Pulsed Plasma Polymer (PP), Conventional Polystyrene and Grafted Polystyrene Films
Table 2. Polystyrene Film Growth Rates
peak position/ cm-1
assignment
3100-3000 3000-2900 1850 1780 1715 1600 1492, 1453 1248-1242 1100-1050 965-935 757 700
CH stretch, phenyl ring CH stretch, CHx groups CdO anhydride stretch CdO anhydride stretch (A) CdO acid stretch (B) phenyl ring stretch (C) phenyl ring stretch (D) cyclic anhydride stretch C-O-C stretch cyclic anhydride stretch phenyl ring deformation phenyl ring deformation
PP + polyPP styrene styrene * * * *
* * *
* * * * * * * * *
* *
* *
* *
plasma polymer film revealed characteristic cyclic anhydride stretching peaks;35-37 see Figure 1 and Table 1. Immersion in styrene at 120 °C for 3 h followed by Soxhlet extraction gave rise to the appearance of new phenyl ring absorbances indicative of polystyrene growth onto the plasma polymer surface. This was accompanied by a loss in intensity of the characteristic anhydride peak at 1780 cm-1 and the appearance of a new peak at 1715 cm-1 (carboxylic acid CdO stretching) attributable to anhydride group hydrolysis by atmospheric moisture.38,39 Typical polystyrene film growth rates measured on maleic anhydride pulsed plasma polymer were 19 ( 4 nm h-1 (e.g., (35) Silverstein, R. M.; Bassler, G. C.; Morril, T. C. Spectrometric Identification of Organic Compounds, 5th ed.; John Wiley: New York, 1991. (36) 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. (37) Mirone, P.; Chiorboli, P. Spectomchim. Acta 1962, 18, 1425. (38) Wang, S.; Wang, M.; Lei, Y.; Zhang, L. J. Mater. Sci. Lett. 1999, 18, 2009. (39) Jenkins, A. T. A.; Hu, J.; Wang, Y. Z.; Schiller, S.; Foerch, R.; Knoll, W. Langmuir 2000, 16, 6381.
substrate
growth rate/ nm h-1
silicon wafer pulsed maleic anhydride plasma polymer (PP) continuous wave maleic anhydride plasma polymer DPPH capped PP UV exposed DPPH capped PP UV exposed PP
0 19 ( 4.7 1.7 ( 1.0 1.5 ( 0.4 10.3 ( 4 7.3 ( 1.8
57 nm after 3 h); see Table 2. Polymer brush film thickness of up to 150 nm could be achieved by this method. For comparison, ca. 100 nm thick continuous wave maleic anhydride plasma polymer layers gave much thinner (only 5 nm after 3 h) polystyrene films; see Table 2. Another control experiment comprised soaking maleic anhydride pulsed plasma polymer films in toluene for 3 h at 120 °C; this produced negligible change in overall film thickness, thereby confirming that the plasma polymer coatings are not removed or swollen by apolar solvents. XPS analysis of maleic anhydride pulsed plasma polymer film surfaces indicated the presence of only carbon and oxygen; see Table 3. The corresponding high resolution C(1s) envelope could be fitted to five different types of carbon environment: hydrocarbon (CHx ∼ 285.0 eV), carbon adjacent to an anhydride group (C-C(dO)-OC(dO)-C ∼ 285.7 eV), carbon singly bonded to oxygen (C-O ∼ 286.6 eV), carbon doubly bonded to oxygen (CdO ∼ 287.8 eV), and carbon belonging to anhydride groups (OdC-O-CdO ∼ 289.4 eV);40 see Figure 2. Of the total carbon atoms detected, 55.2 ( 2.4% belong to cyclic anhydride rings. This is consistent with high levels of monomer structural retention during pulsed plasma deposition.31 Following styrene polymerization, only car(40) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers; John Wiley: Chichester UK, 1992.
Growth of Micropatterned Polymer Brushes
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Table 3. Elemental XPS Compositions (at. %) surface
%C
%O
%N
%F
maleic anhydride pulsed plasma polymer (PP) styrene polymerized onto PP DPPH treated PP styrene polymerized onto DPPH treated PP PP exposed to 4-fluorostyrene vapor
64.8 ( 1.2 99.3 ( 0.1 63.4 ( 1.2 93.9 ( 1.3 66.8 ( 3.3
35.2 ( 1.2 0.7 ( 0.1 33.4 ( 1.7 6.1 ( 1.0 31.4 ( 1.6
0 0 3.2 ( 0.5 0 0
0 0 0 0 1.7 ( 0.1
bon was detected on these surfaces, and the high resolution C(1s) envelope displayed a narrow peak at 285.0 eV with an additional peak at 7-9 eV higher binding energy corresponding to the phenyl ring π f π* shakeup satellite of polystyrene;41 see Table 3 and Figure 2. In the case of uncoated silicon wafers, no polystyrene film growth was detected by XPS. Vapor phase exposure of 4-fluorostyrene to the maleic anhydride pulsed plasma polymer film indicated the presence of the fluorine tag at the surface; see Table 3. Thereby additional evidence for the existence of free radical initiation sites at the plasma polymer surface was provided. (b) Radical Capping with Diphenylpicrylhydrazyl (DPPH). Reaction of maleic anhydride pulsed plasma polymer with DPPH gave rise to the incorporation of nitrogen at the surface, thereby confirming radical capping; see Table 3. Subsequent styrene graft polymerization followed by Soxhlet extraction attenuated the amount of oxygen present at the surface, and nitrogen was no longer detectable by XPS. The corresponding C(1s) envelope contained a dominant hydrocarbon component at 285.0 eV and some oxygenated carbon species at higher binding energy. On this basis, only a very thin layer of grafted polystyrene must have formed (within the XPS sampling depth of 2-5 nm). Infrared analysis of the DPPH treated maleic anhydride pulsed plasma polymer coating following immersion in styrene for 3 h at 120 °C confirmed very low levels of polystyrene graft polymerization; see Figure 1. Reflectometry data indicated a polystyrene graft layer thickness of only 8.0 ( 2.2% of the corresponding uncapped maleic anhydride pulsed plasma polymer system, thereby confirming that the DPPH molecule caps free radical sites at the pulsed maleic anhydride plasma polymer surface, leading to a drop in polystyrene graft polymerization by a factor of 12.5. (c) Micropatterning. Optical microscopy of the deposited maleic anhydride pulsed plasma polymer films exposed to UV irradiation for 2 h through a copper grid mask gave rise to the appearance of a patterned surface attributable to photooxidative etching (lighter regions); see Figure 3. Dark contrast enhancement of the unexposed regions was observed following styrene graft polymerization (negative image). Thickness measurement of UV exposed versus unexposed regions of maleic anhydride pulsed plasma polymer following styrene graft polymerization at 120 °C for 3 h indicated a factor of 2.5 slower polystyrene growth rate in the UV irradiated areas; see Table 2. To create a positive image, DPPH capped maleic anhydride pulsed plasma polymer was UV exposed through the copper grid photomask for 10 min (just sufficient to dislodge the DPPH molecules attached to the surface), and then immersed in styrene under polymerization conditions. In this case, the darker (polymer growth) regions appeared in the UV exposed regions (i.e., DPPH inhibits graft polymerization); see Figure 3. (41) Clark, D. T.; Adams, D. B.; Dilks, A.; Peeling, J.; Thomas, H. R. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 51.
Scheme 1. Example of Resonance Stabilization of Poly(maleic anhydride) Radical
Polystyrene film growth was a factor of 7 higher in the UV irradiated areas; see Table 2. Discussion It is already well-known that plasma exposure can lead to the formation of radicals on the surface of polymer substrates29 and also plasma polymer films.42 Although graft polymerization is known for the former,26,43-48 polymer chain growth from plasma polymer surfaces is relatively unexplored. In fact, studies have been performed only on continuous wave plasma polymers49-53 (rather than pulsed plasma polymer films), where the nature of the initiating species was uncertain or the thickness/extent of graft polymerization was unknown. In the present study, the graft polymerization of styrene onto maleic anhydride plasma polymer surfaces is found to be significantly greater for pulsed compared to continuous wave deposition conditions. This is due to high levels of anhydride ring incorporation in the former case originating from two distinct reaction regimes (duty cycle on- and off-times31), where the short on-period generates active species (via UV irradiation, ion, or electron bombardment), followed by conventional polymerization reaction pathways proceeding during the long off-period (in the absence of any UV-, ion-, or electron-induced damage). The polystyrene films subsequently grown on these surfaces are homogeneous, are of controllable thickness, and can be easily patterned. On the basis of DPPH capping experiments, the mechanism of graft styrene polymerization is likely to be free radical based. The presence of “trapped radicals”42 in maleic anhydride pulsed plasma polymer will be enhanced by resonance stabilization; see Scheme 1. Radical propagation between monomer units during the pulsed plasma duty cycle off-period is likely. This is analogous to conventional solution-phase free radical synthesis of poly(maleic anhydride), where trapped radi(42) Yasuda, H. Plasma Polymerization; Academic Press: London, 1995. (43) Lin, J. C.; Tiong, S. L.; Chen, C. Y. J. Biomater. Sci. Polym. Ed. 2000, 11, 701. (44) Chen, H.; Belfort, G. J. Appl. Polym. Sci. 1999, 72, 1699. (45) Tan, K. L.; Woon, L. L.; Wong, H. K.; Kang, E. T.; Neoh, K. G. Macromolecules 1993, 26, 2832. (46) Mori, M.; Uyama, Y.; Ikada, Y. J. Polym. Sci., Polym. Chem. 1994, 32, 1683. (47) Hirotsu, T. J. Macromol. Sci., Pure Appl. Chem. 1996, A33, 1663. (48) Chen, Y. J.; Kang, E. T.; Neoh, K. G.; Tan, K. L. J. Phys. Chem. B 2000, 104, 9171. (49) Kuzuya, M.; Kawaguchi, T.; Mizutani, S.; Okuda, T. J. Polym. Sci., Polym. Lett. Ed. 1985, 23, 69. (50) Kuzuya, M.; Kawaguchi, T.; Nakanishi, M.; Okuda, T. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1441. (51) Kuzuya, M.; Kawaguchi, T.; Yanagihara, Y.; Nakai, S.; Okuda, T. J. Polym. Sci., Polym. Chem. 1986, 24, 707. (52) Ji, G.; Fang, J.; Cai, S.; Xue, G. Appl. Surf. Sci. 1994, 81, 63. (53) Yang, M. R.; Chen, K. S. Mater. Chem. Phys. 1997, 50, 11.
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Teare et al. Scheme 2. Resonance Stabilization of Reaction Intermediate between Maleic Anhydride Radical and Styrene
where resonance and polar effects will help to stabilize the intermediate radical;57 see Scheme 2. The resultant maleic anhydride-styryl radical intermediate can then participate in conventional styrene graft polymerization from the surface until eventually termination occurs by either radical recombination or disproportionation reaction pathways.57 The much lower graft polymerization efficiency observed for the corresponding continuous wave plasma polymerized maleic anhydride surfaces can be attributed to the poor retention of anhydride functionality,31 which is prerequisite for the proposed stabilization of the styryl radical reaction intermediate. Heating the maleic anhydride pulsed plasma polymer surface in the presence of diphenylpicrylhydrazyl (DPPH, a free radical scavenger) gave rise to the capping of trapped radical sites at the surface, thereby inhibiting styrene graft polymerization; see Table 2. The absence of complete prevention of styrene graft polymerization is most likely to be attributable to subsurface trapped radicals migrating to the surface via thermal rearrangement of the plasma polymer. UV photooxidation of these surfaces was found to be effective at selectively removing the surface-bound DPPH species to expose underlying poly(maleic anhydride) plasma polymer capable of initiating styrene graft polymerization. DPPH retained in the nonexposed regions continues to hinder polymer growth. Therefore, in conjunction with a photomask, it is possible to fabricate positive image micropatterned polymer brush surfaces. These are comparable in scale (micrometers) to other photolithographically patterned “grafted from” surfaces.58,59 Yet this method is relatively simple compared to other well-established patterning techniques,17,23,60 which typically involve far more complex multistep procedures.
Figure 3. Optical images of (a) maleic anhydride pulsed plasma polymer following 2 h UV exposure, (b) surface (a) following 3 h styrene polymerization, and (c) DPPH capped maleic anhydride surface, exposed to UV for 10 min followed by 3 h styrene polymerization. (Squares are 20 µm wide and the grids are 5 µm wide.)
cals are known to propagate along the polymer chain.54-56 The graft polymerization of styrene is most likely to be concentrated at radical sites located at or near the surface,
(54) Nakayama, Y.; Hayashi, K.; Okamura, S. J. Appl. Polym. Sci. 1974, 18, 3633. (55) Nakayama, Y.; Kondo, K.; Takakura, K.; Hayashi, K.; Okamura, S. J. Appl. Polym. Sci. 1974, 18, 3653. (56) Nakayama, Y.; Kondo, K.; Takakura, K.; Hayashi, K.; Okamura, S. J. Appl. Polym. Sci. 1974, 18, 3661. (57) Stevens, M. P. Polymer Chemistry, 2nd ed.; Oxford University Press: New York, 1990. (58) Prucker, O.; Habicht, J.; Park, I. J.; Ruhe, J. Mater. Sci. Eng. C 1999, 8-9, 291. (59) Higashi, J.; Nakayama, Y.; Marchant, R. E.; Matsuda, T. Langmuir 1999, 15, 2080. (60) Jeon, N. L.; Choi, I. S.; Whitesides, G. M.; Kim, N. Y.; Laibinis, P. E.; Harada, Y.; Finnie, K. R.; Girolami, G. S.; Nuzzo, R. G. Appl. Phys. Lett. 1999, 75, 4201.
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Conclusions Styrene graft polymerization readily takes place at the surface of maleic anhydride pulsed plasma polymer films. Trapped free radical centers act as the initiation sites and are stabilized by the large concentration of anhydride rings present in the pulsed plasma polymer structure.
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These initiation sites can be capped with diphenylpicrylhydrazyl (DPPH, a free radical scavenger), and reactivated by subsequent exposure to UV radiation. This feature is attractive for the microfabrication of patterned polymer brush surfaces. LA020279S
