Density Perturbations in Polymers Near a Solid Substrate: An X-ray Reflectivity Study

We have studied by X-ray reflectometry the variation of electron density of thin polymer films near solid substrates. For most polymers studied, which interact only mildly with the selected substrate (SiO 2 ), a decrease of density was detected over a region spanning two to three statistical segment lengths at the solid interface. For poly(4-vinylpyridine) (P4VP), however, an increase of density was observed. This is related to the existence of strong adsorption interactions between P4VP and SiO 2 . These observations are in good agreement with theoretical predictions and may play a role in explaining the variation of the glass transition of thin supported polymer films as compared to bulk glass transitions.