Interface and surface effects on the glass-transition temperature in thin polymer films

Abstract

We have measured the thickness dependence of the glass-transition temperature (T_g) of thin films of poly(methyl methacrylate)(PMMA) by using spectroscopic ellipsometry to detect the discontinuity in thermal expansivity occurring at T_g. We studied films on two surfaces: the native oxide of silicon, and evaporated gold. The T_g 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 T_g is approached from below. For films of PMMA on the native oxide of silicon, however, we find a slight increase in T_g with decreasing thickness. We speculate that hydrogen bonding at the interface restricts mobility and leads to an increase in T_g, outweighing the effect of the free surface.