Shear Modulation Force Microscopy Studies on Cross-linked Elastomer Thin Films
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Shear Modulation Force Microscopy Studies on Cross-linked Elastomer Thin Films Y. Zhang1, S. Ge1, M.H. Rafailovich1, J.C. Sokolov1, D.G. Peiffer2, A.J. Dias3 1 Department of Materials Science and Engineering, SUNY at Stony Brook, NY11794, USA. 2 ExxonMobil Research and Engineering Company, Annandale, NJ08801, USA. 3 ExxonMobil Chemical Company, Baytown, TX77520, USA. ABSTRACT The atomic force microscope in the shear force modulation microscopy (SMFM) mode has been used to characterize the surface modulus and cross-link density of elastomer thin films, where standard rheological methods cannot be applied. Brominated poly(isobutylene-co-4methylstyrene) (BIMS) is a synthetic terpolymer which can be stoichiometrically cross-linked by N, N’-dicinnamylidene-1,6-hexanediamine. The results on several types of BIMS elastomers with different bromide content are reported. The cross-linking reaction at the surface is found to be significantly faster than that of bulk. The estimated shear moduli of the thin films were found to be proportional to the cross-link density, as expected from the rubber elasticity theory for bulk materials. INTRODUCTION Elastomeric materials are often used as coatings to provide corrosion protection, to improve adhesion of inks to paper, and to control diffusion in food packaging. Very often the coatings are cross-linked to improve their physical and mechanical properties. The extent and type of cross-links affect the performance of the final product and its processing conditions. Hence it is important to probe the cross-link density and the kinetics of these materials. Traditional methods such as dynamic mechanical analysis and tensile strength tests only measure bulk properties and hence are not applicable if the films are too thin to be self-supporting or if they only modify the surface property of a component. Therefore, the investigation of the cross-link density and kinetics at surfaces and in ultra-thin polymer films, requires a microscopic approach. Shear modulation force microcopy (SMFM) is a new scanning force microscopy (SFM) mode, which was recently developed to study surface elastic properties and the glass transition temperatures of polymer thin films [1,2]. In this paper we further probe this technique by measuring the surface modulus of thin polymer films where the cross-link density can be controlled. We can then test whether the results obtained by this method agree with the predictions of the rubber elasticity theory [3]. We chose to study the surface cross-link behavior of brominated poly(isobutylene-coparamethylstyrene) (BIMS), a synthetic terpolymer of isobutylene (IB), para-methylstyrene (PMS), and para-bromomethylstyrene (BrPMS) [4]. BIMS polymers can be stoichiometrically cross-linked by N,N’-dicinnamylidene-1,6-hexanediamine (DIAK) at 150 ºC in the presence of small amount of water (figure 1). Since both BIMS and DIAK are soluble in toluene, it is possible to spin cast thin films that can be cross-linked directly on a substrate. By varying the bromide content of the BIMS we ca
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