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

We have measured the thickness dependence of the glass-transition temp erature (T-g) of thin films of poly(methyl methacrylate) (PMMA) by usi ng spectroscopic ellipsometry to detect the discontinuity in thermal e xpansivity occurring at T-g. We studied films on two surfaces: the nat ive oxide of silicon, and evaporated gold. The T-g of PMMA on a gold s urface decreases with decreasing film thickness, in accordance with pr evious results for polystyrene on silicon. We suggest that at the air surface a liquid-like layer exists whose size diverges as T-g is appro ached from below. For films of PMMA on the native oxide of silicon, ho wever, we find a slight increase in T-g with decreasing thickness. We speculate that hydrogen bonding at the interface restricts mobility an d leads to an increase in T-g, outweighing the effect of the free surf ace.