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Polymerization of a Thiol-Bound Styrene Monolayer Joseph F. Ford, Thomas J. Vickers, Charles K. Mann, and Joseph B. Schlenoff* Department of Chemistry, The Florida State University, Tallahassee, Florida 32306 Received October 20, 1995X The polymerization of a self-assembled monolayer of (mercaptomethyl)styrene is monitored in situ with surface enhanced Raman spectroscopy. Chemisorption of this thiol-containing styrene involves the loss of the S-H bond. The vinyl stretching mode at 1625 cm-1, which disappears during both azo- and photoinitiated surface polymerization, is used to follow the kinetics of the process. Laser desorption/ Fourier-transform ion cyclotron resonance/mass spectrometry confirms the transformation of the initiated monomer into a nondesorbable species.
Various systems have been employed to promote polymerization reactions of reduced dimensionality. Most of these have been constrained to surfaces, although some studies on 1-D systems have been reported.1 Typically, monomer is adsorbed to the liquid/solid,2,3 liquid/liquid,4,5 or air/liquid6,7 interface via physical or chemical interactions. Polymerization is then induced, yielding ultrathin films with specialized properties. Alkoxy- and chlorosilanes undergo hydrolysis/condensation polymerization surface reactions without distinct adsorption and polymerization steps.8-12 These silanes are widely used to generate surfaces with enhanced adhesion, chemical resistance, hydrophobicity, or chromatographic behavior. It is also possible to produce polymers with 2-D character using monomers that self-organize in the bulk.13 Recently, a popular strategy for producing thin films of organic material on surfaces has been to use sulfur chemisorption as a point of attachment to metals.6 Selfassembled monolayers (SAM’s) on gold or silver, formed under ambient conditions from alkanethiols, for example, are ordered and densely packed.14-23 Here we employ the SAM approach to produce monolayers of 4-(mercapto* To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, April 1, 1996. (1) Bein, T.; Enzel, P. Angew. Chem., Int. Ed. Engl. 1989, 28, 1692. (2) Higayashi, N.; Mori, T.; Niwa, M. J. Chem. Soc., Chem. Commun. 1990, 225. (3) Tashiro, K.; Matsushima, K.; Kobayashi, M. J. Phys. Chem. 1990, 94, 3197. (4) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem. 1988, 100, 117. (5) Higayashi, N.; Adachi, T.; Niwa, M. Macromolecules 1990, 23, 1475. (6) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, 1991. (7) Rolandi, R.; Paradiso, R.; Xu, S. Q.; Palmer, C.; Fendler, J. H. J. Am. Chem. Soc. 1989, 111, 5233. (8) Moaz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (9) Kessel, C. R.; Granick, S. Langmuir 1991, 7, 532. (10) Wirth, M. J.; Fatunmbi, H. O. Anal. Chem. 1992, 64, 2783. (11) Thompson, W. R.; Pemberton, J. E. Chem. Mater. 1993, 5, 241. (12) Moaz, R.; Sagiv, J. Langmuir 1987, 3, 1034. (13) Stupp, S. I.; Son, S.; Li, L. S.; Lin, H. C.; Keser, M. J. Am. Chem. Soc. 1995, 117, 5212. (14) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (15) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (16) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (17) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (18) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723. (19) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (20) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (21) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (22) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1990, 112, 570. (23) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87.
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Scheme 1
methyl)styrene (4-MMS), which may then be polymerized as in Scheme 1. This method of producing a polymer film on a surface has certain advantages over adsorbing a preformed sulfur-containing polymer. Examples of these polymers in the literature contain thiol (water-24,25 and organic-soluble24,26), thioether,27 and disulfide28 groups. Although introduction of a thiol group induces adsorption in otherwise-nonadsorbing systems, the coverage suggests incomplete or diffuse packing on the surface, with a porous structure. On adsorption, the random coil solution conformation of the polymer is projected onto the surface, reducing the possibility for ordered, dense coverage. Such a structure can be “densified” to promote maximum surface protection by the addition of small molecules that fill gaps.27,29 Small molecules, however, can be displaced by other species with surface affinity,30 and alkanethiol monolayers exhibit limited stability.31 An additional problem discovered with poly(4-mercaptomethyl)styrene) is that the polymer undergoes ready cross-linking, leading to insoluble material.24 Self-assembly of the 4-MMS thiol monomer, made from 4-(bromomethyl)styrene,32 was carried out on a roughened silver surface and followed by surface enhanced Raman spectroscopy (SERS), a technique which has been applied to prior studies of alkanethiol monolayer formation on gold and silver.33-35 SERS on silver, using a 514 nm laser (24) Schlenoff, J. B.; Dharia, J. R.; Xu, H.; Wen, L.; Li, M. Macromolecules 1995, 28, 4290. (25) Niwa, M.; Fukui, H.; Higayashi, N. Macromolecules 1993, 26, 5816. (26) Stouffer, J. A.; McCarthy, T. J. Macromolecules 1988, 21, 1204. (27) Lenk, T. J.; Hallmark, V. M.; Rabolt, J. F.; Haussling, L.; Ringsdorf, H. Macromolecules 1993, 26, 1230. (28) Sun, F.; Castner, D. G.; Grainger, D. W. Langmuir 1993, 9, 3200. (29) Carron, K. T.; Lewis, M. L.; Dong, J.; Ding, J.; Xue, G.; Chen, Y. J. Mater. Sci. 1993, 28, 4099. (30) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192. (31) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528. (32) Okawara, M.; Nakagawa, T.; Inoto, E. Kogyo Kagaku Zasshi 1957, 60, 73. (33) Sandroff, C. J.; Garoff, S.; Leung, K. P. Chem. Phys. Lett. 1983, 96, 547. (34) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 3629. (35) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284.
© 1996 American Chemical Society
Letters
Figure 1. Raman spectra of (A) neat 4-(mercaptomethyl)styrene; (B) neat 4-ethylbenzyl mercaptan; (C) 4-MMS adsorbed onto roughened silver; (D) polymerized 4-MMS on silver; (E) 4-ethylbenzyl mercaptan adsorbed onto silver.
line (Ar+), typically provides a cross-section enhancement factor of 105-106 over bulk spectroscopy.36,37 SERS spectra were acquired with a fiber optic system and a CCD detector.38 The SAM was formed by immersing the roughened silver substrate39 in a dilute solution (10-2 M) of the monomer dissolved in methylene chloride. Binding of the monomer to silver via metal-sulfur interaction, and the absence of physisorbed multilayers40 were confirmed by the disappearance of the 2577 cm-1 S-H stretch. A and C of Figure 1 depict the Raman spectra for neat and adsorbed 4-MMS, respectively. All spectra have been corrected for instrument response and have had the spectrum of silica due to the optical fiber subtracted. The vibrational mode at 1625 cm-1, assigned to the vinyl stretch, is attenuated in the adsorbed monomer, compared with the bulk, due to the surface selection rule that favors transition moments perpendicular to the surface.41 Using the ring breathing mode at 1603 cm-1 as a reference, we infer that the orientation of the double bond is as depicted in Scheme 1: at a small angle to the plane of the surface. We also reason that the molecule does not lie flat on the surface, since this would render the ring mode unobservable. The 1625 cm-1 assignment was confirmed by synthesizing 4-ethylbenzyl mercaptan and adsorbing it to roughened silver under the same conditions. The SERS spectrum, Figure 1E, shows no vinyl stretch. Polymerization of the adsorbed monomer was effected by immersing the substrate in an aqueous solution of azo initiator at 58 °C (Wako VA044) or simply by irradiating the monolayer with the green laser. The laser power used for both polymerization and observation was 160 mW (36) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163. (37) Chang, R. K., Furtak, T. E., Eds. Surface-Enhanced Raman Scattering; Plenum Press: New York, 1982. (38) Raman spectra were acquired with a J-Y HR 640 spectrograph with a 600 grooves mm-1 grating and a 1152 × 298 pixel liquid nitrogen cooled CCD detector from Princeton Instruments. The fiber optic probe was a bifurcated bundle with the incident radiation from a 200 µm central fiber collected by six surrounding fibers. A holographic notch filter was used for laser line rejection. To collect spectra the probe was positioned at approximately 35° to normal, 2 mm from the surface with a stream of argon flowing over the surface. (39) The electrode was 99.9% silver foil roughened with six oxidation/ reduction cycles from -600 to +150 mV at 1 V s-1 in 1 M KCl, holding for 5 and 10 s at the lower and upper potentials, respectively. The electrode area was 1 cm2, the counter electrode was platinum, and the reference electrode was saturated calomel. (40) Kim, Y. T.; McCarley, R. L.; Bard, A. J. Langmuir 1993, 9, 19411944. (41) Pearce, H. A.; Sheppard, N. Surf. Sci. 1976, 59, 205.
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Figure 2. Time-resolved SERS spectra of adsorbed 4-MMS exposed to green light, showing the disappearance of the 1625 cm-1 stretch (CdC).
Figure 3. Plot of monomer spectrum 0; and polymer spectrum 4; obtained by least squares fit, as a function of time for surface polymerization of 4-MMS.
distributed over a spot 1.5 mm in diameter. Although the 514 nm radiation would seem to be of insufficient energy to promote photochemical reactions, there is ample precedent for photochemistry induced by green light at such roughened surfaces.42-46 We discount thermal initiation of polymerization, since the thermal conductivity of silver is very high and we noticed a maximum temperature rise of only 5 °C in our experiments. The polymerization was indicated by a decrease in the vinyl group scattering intensity, shown in Figure 2, normalized to the 1603 cm-1 band. A classical least squares47 fit of the data, plotted in Figure 3, adhered neither to first- nor to second-order kinetics over the whole time span. Complex kinetics are to be expected from our system, which includes continuous initiation and propagation during irradiation. The rate of reaction is expected to decrease significantly as monomer is exhausted and residual monomer becomes isolated by growing chains. It is surprising that nearly all the monomer appears to be consumed by the end of the reaction (Figure 2), given that molecules are bound to the surface via strong chemisorption. A small degree of mobility may allow residual monomer to be incorporated into polymer. The detailed spectra shown in Figure 2 reveal other spectral changes in adsorbed species, such as 9 cm-1 redshifting of the ring breathing mode at 1603 cm-1, consistent (42) Nitzan, A.; Brus, L. E. J. Chem. Phys. 1981, 75, 2205. (43) Cooney, R. P.; Howard, M. W.; Mahoney, M. R.; Mernagh, T. P. Chem. Phys. Lett. 1981, 79, 459. (44) Garoff, S.; Weitz, D. A.; Alverez, M. S. Chem. Phys. Lett. 1982, 93, 283. (45) Parry, D. B.; Dendramis, A. L. Appl. Spectrosc. 1986, 40, 656. (46) Wolkow, R. A.; Moskovits, M. J. Chem. Phys. 1987, 87, 5858. (47) Strong, G. Linear Algebra and Its Applications; Academic Press: New York, 1980; Chapter 3.
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with a small increase in reduced mass on polymerization. To confirm that the first and last spectra in the series depicted in Figure 2 are true representations of individual components, factor analysis48 was performed on the spectra, revealing two major and one minor components. The minor component was attributable to incomplete silica background subtraction. Another test was to subtract the first, last, and silica spectra from various intermediate spectra: zero residuals confirmed that the intermediate spectra were multiples of pure monomer and pure polymer spectra. Our work is the first to describe surface polymerization of thiol-bound vinyl monomers. Prior Raman spectroscopic studies have examined the polymerization of adsorbed and thin films of nitrobenzenes,49 pyrroles,50 anilines,51 and siloxanes.11 A recent study reported the formation and polymerization of self-assembled monolayers of diacetylene thiols on smooth gold,52 where resonance enhanced Raman scattering was used to follow polymerization. In order to confirm consumption of the monomer during reaction, laser desorption experiments with irradiated (polymerized) and unirradiated monolayers were performed. Desorption products were detected using Fourier transform ion cyclotron mass resonance/mass spectroscopy (FT-ICR/MS).53,54 Under identical conditions, using a NdYAG (1064 nm) laser, material with m/z of 117 could be desorbed from unpolymerized areas of a sample, whereas very little could be desorbed from polymerized areas (Figure 4). The 117 m/z peak corresponds to monomer which has been cleaved at the benzyl sulfur, as shown in Figure 4. The 107 and 109 m/z peaks are silver isotopes ablated from the surface. Thus, in addition to producing nondesorbable organic material, polymerization also appears to prevent silver at the surface from desorbing. From Figure 4 there is evidence for only minimal residual monomer following exposure to the 514 nm laser. Scans up to 1000 m/z revealed little evidence for dimers or oligomeric material desorbing. (48) Malinowski, E. R.; Hower, D. G. Factor Analysis in Chemistry; Wiley: New York, 1980. (49) Tsai, W. H.; Boerio, F. J.; Clarson, S. J.; Montaudo, G. J. Raman Spectrosoc. 1990, 21, 311. (50) Fujita, W.; Ishioka, T.; Teramae, N.; Haraguchi, H. Chem. Lett. 1994, 933. (51) Holze, R. J. Electroanal. Chem. 1987, 224, 253. (52) Batchelder, D. N.; Evans, S. D.; Freeman, T. L.; Ha¨ussling, L.; Ringsdorf, H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050. (53) FT-ICR/MS employed a focused Nd-YAG laser at 1064 nm with 7 ns pulses of 100 mJ. The laser was focused to an area of 300 × 600 µm.2 The ICR was an Extrel FTMS 200 instrument equipped with 1.875 in dual cubic traps and a 3 T superconducting magnet. (54) Kim, H. S.; Wood, T. D.; Marshall, A. G.; Lee, J. Y. Chem. Phys. Lett. 1994, 224, 589.
Letters
Figure 4. FT-ICR/MS spectra of material laser desorbed from (A) unpolymerized surface; (B) polymerized surface.
The ability to produce polymer via laser irradiation suggests the possible application of these vinyl monolayers for patterning, where a laser writes a pattern onto a substrate. Unexposed areas, comprised of monomer, could be exchanged with small, hydrophilic monomers, whereas polymer-covered areas are expected to remain hydrophobic and nonexchangeable. This type of “negative resist” behavior complements recent uses of SAM’s in “positive resist” approaches, where areas of monolayer exposed to UV irradiation become oxidized and hydrophilic55 and may be washed away with aqueous solutions.56 Acknowledgment. We wish to thank George Jackson and Alan Marshall at the National High Magnetic Field Laboratory for their help in obtaining FT-ICR/MS data. This work was supported, in part, by a grant from the National Science Foundation (DMR 9414289). LA950913K
(55) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (56) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626-628.
