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Photoinitiated Polymerization of Styrene from Self-Assembled Monolayers on Gold Rolf Schmidt,† Tongfeng Zhao,† John-Bruce Green,*,‡ and Daniel J. Dyer*,† Department of Chemistry, Southern Illinois University, Carbondale, Illinois 62901-4409 Received September 17, 2001. In Final Form: November 6, 2001 We report the synthesis of grafted polystyrene (PS) films on gold via a surface-initiated polymerization strategy. The polymerization proceeded by a “grafted-from” strategy in which the polystyrene grew by free-radical polymerization from a surface-bound photoinitiator. These experiments exploited the spontaneous formation of alkanethiolate monolayers on gold to immobilize the alkylthiol photoinitiator to the gold substrate. The polymerization was initiated by irradiation with UV light (λ ) 300 nm) and resulted in the formation of a PS film. This film was found to be 25-30 nm thick by contact-mode AFM and ellipsometry. Water contact angles, X-ray photoelectron spectroscopy, and reflection-absorption infrared spectroscopy confirmed the monolayer formation and the presence of polystyrene, and all data was consistent with a grafted polymer film.
Introduction An important scientific and technological challenge in materials science involves the modification of organic and inorganic substrates with polymer films. In particular, there are three primary methods for the modification of planar substrates with organic polymers. First, physisorption involves the noncovalent adsorption of a preformed polymer onto the substrate surface. The durability of these films is dependent on the interaction energy between the substrate and the organic polymer. Typically, physisorbed polymers are deposited by dip-coating or spincoating; consequently, these polymer films are usually not robust and may become desorbed from the surface. Second, organic polymers may be “grafted to” a substrate by attaching a preformed polymer to surface-active functional groups. These tethered polymer films are more robust than physisorbed polymers because the polymer chains are linked to the surface via covalent bonds. However, the “grafting-to” technique may lead to low grafting densities because once a polymer chain is tethered to the surface it inhibits diffusion of additional chains to the surface-active groups. Third, the “grafting-from” technique uses a polymer initiator precursor that is grafted to the surface before initiation is induced. Thus, the polymer chains grow out from the surface and remain tethered to the substrate. Importantly, the “grafting-from” strategy yields high grafting densities relative to those from the “grafting-to” technique. The investigation of these so-called “polymer brushes” poses significant synthetic, theoretical, and technological challenges.1 A variety of initiating mechanisms have been used in the “grafting-from” technique,including uncontrolled free radicals2and living free radicals3 as well as anionic,4 cationic,5 ring-opening,6 and lanthanide-catalyzed poly* Corresponding authors. E-mail:
[email protected]. Telephone: (618) 453-2897. FAX: (618) 453-6408. † Southern Illinois University. ‡ Current address: Department of Chemistry, University of Alberta, Edmonton, Alberta T6G2G2, Canada. (1) (a) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677-710. (b) Milner, S. T. Science 1991, 251, 905-914. (2) (a) Prucker, O.; Ru¨he, J. Langmuir 1998, 14, 6893-6898 (b) Fujiki, K.; Sakamoto, M.; Yoshida, A.; Maruyama, H. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2121-2128.
merization.7 In addition, Bergbreiter and co-workers have synthesized hyperbranched polymers from gold and polyethylene.8 However, free-radical polymerization is the most convenient technique because the reaction is insensitive to moisture and tolerates a large variety of organic functional groups. Recently, several groups have synthesized grafted polymer films via self-assembled monolayers (SAMs) composed of free-radical initiators that are similar in structure to 2,2′-azobisisobutyronitrile (AIBN). These studies focused on organosilicon-initiator precursors and polymerization from silica or oxidized silicon. Because the silicon-oxygen and silicon-carbon bonds are very strong, one can use thermal and photochemical initiation strategies. Although the polymer films formed on silicate surfaces are robust, there are a number of advantages to making similar polymer brushes on gold surfaces. Recently, several groups have successfully polymerized various polymers from surface-bound SAMs on gold.9 However, SAMs adsorbed to gold pose significant (3) (a) Kim, J.-B.; Bruening, M. L.; Baker, G. L. J. Am. Chem. Soc. 2000, 122, 7616-7617. (b) Husseman, M.; Malmstro¨m, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J. Macromolecules 1999, 32, 14241431. (c) Shaw, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597-605. (d) Huang, X.; Wirth, M. J. Anal. Chem. 1997, 69, 4577-4580. (e) Ejaz, M.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 2000, 33, 28702874. (f) de Boer, B.; Simon, H. K.; Werts, M. P. L.; van der Vegte, E. W.; Hadziioannou, G. Macromolecules 2000, 33, 349-356. (4) Jordan, R.; Ulman, A.; Kang, J. F.; Rafailovich, M. H.; Sokolov, J. J. Am. Chem. Soc. 1999, 121, 1016-1022. (5) (a) Jordan, R.; West, N.; Ulman, A.; Chou, Y.-M.; Nuyken, O. Macromolecules 2001, 34, 1606-1611. (b) Rainer, J.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 243-247. (6) (a) Buchmeiser, M. R.; Sinner, F.; Mupa, M.; Wurst, K. Macromolecules 2000, 33, 32-39. (b) Kim, N. Y.; Jeon, N. L.; Choi, I. S.; Takami, S.; Harada, Y.; Finnie, K. R.; Girolami, G. S.; Nuzzo, R. G.; Whitesides, G. M.; Laibinis, P. E. Macromolecules 2000, 33, 2793-2795. (c) Juang, A.; Scherman, O. A.; Grubbs, R. H.; Lewis, N. S. Langmuir 2001, 17, 1321-1323. (d) Weck, M.; Jackiw, J. J.; Rossi, R. R.; Weiss, P. S.; Grubbs, R. H. J. Am. Chem. Soc. 1999, 121, 4088-4089. (e) Mo¨ller, M.; Nederberg, F.; Lim, L. S.; Kånge, R.; Hawker, C. J.; Hedrick, J. L.; Gu, Y.; Shah, R.; Abbott, N. L. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 35293538. (f) Husemann, M.; Mecerreyes, D.; Hawker, C. J.; Hedrick, J. L.; Shah, R.; Abbott, N. L. Angew. Chem., Int. Ed. 1999, 38, 647-649. (g) Choi, I. S.; Langer, R. Macromolecules 2001, 34, 5361-5363. (7) Ingall, M. D. K.; Joray, S. J.; Duffy, D. J.; Long, D. P.; Bianconi, P. A. J. Am. Chem. Soc. 2000, 122, 7845-7846. (8) (a) Bergbreiter, D. E.; Tao, G. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3944-3953. (b) Bergbreiter, D. E.; Liu, M. L. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 4119-4128.
10.1021/la011445j CCC: $22.00 © 2002 American Chemical Society Published on Web 01/17/2002
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Figure 1. Thiol initiator based on the structure of AIBN.
problems because of the thermally and UV labile sulfurgold bond. Furthermore, organosulfur compounds are good chain-transfer agents and could inhibit polymer growth from the surface. Thus, attempts at photochemical initiation of free-radical polymerizations from alkanethiolate monolayers on gold have been largely unsuccessful.10 We report the synthesis of grafted polystyrene films by surface-initiated polymerization from self-assembled monolayers. The initiator consisted of an AIBN-type free-radical initiator that was tethered to a planar gold surface by a thiol group (Figure 1). The polymerization was initiated photochemically at room temperature. Experimental Section Materials. Unless otherwise noted, all reagents and solvents were purchased from Acros or Fisher Chemicals and used without further purification. NMR solvents were purchased from Cambridge Isotopes. Column chromatography was performed with silica gel (60-200 mesh), and TLC was performed on 250 µm silica gel 60 glass plates with F254 fluorescent indicator. 4,4′Azobis(4-cyanovaleric acid) (3) was purified by recrystallization from ethanol prior to use. Neat styrene was passed through a small column of alumina prior to use. Silicon 〈100〉 wafers were purchased from waferworld.com, and both the Au and Cr were 99.995+% pure. TEM grids were used as received from Electron Microscopy Sciences. Equipment. 1H and 13C NMR data were collected on a Varian VXR-300 MHz NMR spectrometer. Gel permeation chromatography was performed on a Waters Alliance 2690 HPLC with a column bank of three Waters 7.8 × 300 mm styragel columns (HR3, HR4, HR5). Calibration was performed with narrow polystyrene standards from Polymer Laboratories. Static contact angles were measured on a Tantec contact angle meter (model CAM Micro). Melting points were obtained on a Mettler 821e differential scanning calorimeter with a scanning rate of 10 degrees min-1. Polymerizations were carried out in a Rayonet photochemical reactor (model RMR-600, Southern New England Ultraviolet Co., Branford CT). Deposition of metallic films was performed in an Edwards E13E vacuum evaporator equipped with a Leybold Inficon QCM film-thickness monitor. Substrates were cleaned with a Jelight UVO model 42 ozone cleaner operating at atmospheric oxygen concentrations. Molecular Modeling. Structures were determined using a semiempirical PM3 model and the Polak-Ribiere optimization algorithm for isolated molecules in vacuo. The calculations were performed with HyperChem v. 6.03 (Hypercube, Inc.). Atomic Force Microscopy (AFM). Topographic maps of the surfaces were created with a Nanoscope IIIa scanning probe microscope (Digital Instruments/Veeco) operating as an AFM. All data were collected using a 140 µm tube scanner and a triangular Si3N4 cantilever obtained from Thermomicroscopes. This cantilever had a nominal force constant of ∼25 pN/nm as determined by spectral evaluation of the thermal noise.11 The normal force applied during imaging was
