In situ X-ray reflectivity study of polystyrene ultrathin films swollen in carbon dioxide

Abstract

We report here original in situ X-ray Reflectivity (XRR) studies of thin films isothermally exposed to CO₂ as a function of CO₂ pressure starting from ambient pressure up to its supercritical state. Swelling of polystyrene confined films is investigated by this technique. Cycling through the supercritical state of CO₂, the PS films show an irreversible behavior that is analyzed in terms of CO₂ uptake that depends on P_g(h) (the pressure at which the film crosses thickness dependent T_g(h)). We evidence that this pressure is indeed thickness dependent and that CO₂ can be up taken in the film in a condensed state as soon as P > P_g(h). We also evidence that the content of CO₂ stored inside the film after depressurization can be determined by a careful analysis of the electron density of the film.