Influence of an impenetrable interface on a polymer glass-transition temperature

Abstract

The thermal expansion of polystyrene thin films, supported on hydrogen-terminated silicon substrates, is measured by x-ray reflectivity. Films on the order of 400 \AA{} and thinner show no glass transition up to at least 60 \ifmmode^\circ\else\textdegree\fi{}C above the bulk glass-transition temperature, while a break in the thickness versus temperature curve, signaling the glass transition and the onset of bulk behavior, is observed for thicker films. This increase in the glass-transition temperature is in contrast to similar studies on the silicon native-oxide surface where a decrease in the glass-transition temperature is observed. This illustrates the importance of the character of the substrate surface in determining thin film behavior.