INFLUENCE OF AN IMPENETRABLE INTERFACE ON A POLYMER GLASS-TRANSITION TEMPERATURE

The thermal expansion of polystyrene thin films, supported on hydrogen -terminated silicon substrates, is measured by x-ray reflectivity. Fil ms on the order of 400 Angstrom and thinner show no glass transition u p to at least 60 degrees C above the bulk glass-transition temperature , while a break in the thickness versus temperature curve, signaling t he glass transition and the onset of bulk behavior, is observed for th icker films. This increase in the glass-transition temperature is in c ontrast to similar studies on the silicon native-oxide surface where a decrease in the glass-transition temperature is observed. This illust rates the importance of the character of the substrate surface in dete rmining thin film behavior.