We have measured the thickness dependence of the glass-transition temperature (Tg) of thin films of poly(methyl methacrylate)(PMMA) by using spectroscopic ellipsometry to detect the discontinuity in thermal expansivity occurring at Tg. We studied films on two surfaces: the native oxide of silicon, and evaporated gold. The Tg of PMMA on a gold surface decreases with decreasing film thickness, in accordance with previous results for polystyrene on silicon. We suggest that at the air surface a liquid-like layer exists whose size diverges as Tg is approached from below. For films of PMMA on the native oxide of silicon, however, we find a slight increase in Tg with decreasing thickness. We speculate that hydrogen bonding at the interface restricts mobility and leads to an increase in Tg, outweighing the effect of the free surface.
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