Role of Interfacial Interactions on the Anomalous Swelling of Polymer Thin Films in Supercritical Carbon Dioxide

ABSTRACT: It has recently been shown that thin polymer films in the nanometer thickness range exhibit anomalous swelling maxima in supercritical CO2 (Sc-Co2) in the vicinity of the critical point of CO2. The adsorption isotherm of CO2 on carbon black, silica surfaces, porous zeolites, and other surfaces, is known to exhibit anoma-lous maxima under similar CO2 conditions. It is believed that because CO2 possesses a low cohesive energy density, there would be an excess amount of CO2 at the surfa-ces of these materials and hence the CO2/polymer interface. This might cause excess CO2 in the polymer films near the free surface, and hence the swelling anomaly. In addition, an excess of CO2 would reside at the polymer/substrate and polymer/CO2 interfaces for entropic reasons. These interfacial effects, as have been suggested, should account for an overall excess of CO2 in a thin polymer film compared to the bulk, and would be responsible for the anomalous swelling. In this study, we use in situ spectroscopic ellipsometry to investigate the role of interfaces on the anoma-lous swelling of polymer thin films of varying initial thicknesses, h0, exposed to Sc-CO2. We examined three homopolymers, poly(1,1 0-dihydroperflurooctyl methacrylate) (PFOMA), polystyrene (PS), poly(ethylene oxide) (PEO), that exhibit very different interactions with Sc-CO2, and the diblock copolymer of PS-b-PFOMA. We show that the anomalous swelling cannot be solely explained by the excess adsorption of CO2 a