RAIRS and TPD Study of the Direct Photopolymerization of Styrene Thin Films in Ultrahigh Vacuum

Abstract

Photoinduced polymerization of styrene monomer thin films condensed on a Ag(110) substrate has been studied using a combination of reflectance absorption infrared spectroscopy (RAIRS) and temperatureprogrammed desorption (TPD). At 100 K, styrene polymerizes upon ultraviolet irradiation. Postirradiation TPD indicates that several species form in the film. These species are identified as styrene dimer, trimer, and tetramer. RAIR spectra as a function of temperature provide evidence for the formation of polystyrene as well. A mechanism for the direct UV photopolymerization of styrene thin films is proposed. IntroductionRecently, there have been reports in the literature on the in-situ formation of polymer films on metal surfaces. Formation of thin polymer films on a metal substrate can be achieved by adsorbing a preformed polymer 1,2 or by forming the polymer in situ. [3][4][5][6][7][8][9][10][11] The motivation for these studies is the desire to form a polymer with a regular two-dimensional morphology that may have novel anisotropic properties. In-situ polymerization has been initiated by photon, electron, and γ-ray irradiation. Radiationinduced chemistry allows for select areas of the substrate to be polymerized, opening the way for the production of positive and negative resists. In addition to the interest in surface modification of metal substrates, in-situ methods for forming polymers in an ultrahigh vacuum allow for further study of the chemistry of the polymer surface and the metal-polymer interface using standard surface analysis techniques.There have been several studies on photoinduced polymerization of adsorbed monomers on metal surfaces. Laser-initiated (514 nm) polymerization of 1,4-dinitrobenzene on Ag colloids was studied by surface-enhanced Raman scattering (SERS). 4 Thick films of 1,4-and 1,3-dinitrobenzene were deposited onto Ag colloids from solution. SERS showed that 1,4-dinitrobenzene polymerized readily on the Ag surface upon CW Ar ion irradiation whereas 1,3-dinitrobenzene did not. Differences in molecular orientation of 1,4-dinitrobenzene compared to 1,3-dinitrobenzene were determined to be the cause of the difference in the photoreactivity of these two isomers on the metal substrate.Ford et al. used a self-assembled monolayer approach to form ordered polymer films by adsorbing 4-(mercap-

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Adsorption and Polymerization of Formaldehyde on Cu(100)

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