INTERFACE AND SURFACE EFFECTS ON THE GLASS-TRANSITION TEMPERATURE IN THIN POLYMER-FILMS

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.