The role of elasticity in the anomalous swelling of polymer thin films in density fluctuating supercritical fluids

In situ neutron reflectivity was used to investigate the effects of density fluctuations on the solubility of supercritical carbon dioxide (scCO₂) in polymer thin films. Deuterated polystyrene, deuterated polybutadiene, and the corresponding random copolymer, deuterated styrene-random-butadiene copolymer, as well as deuterated poly(methyl methacrylate) were investigated. Data were obtained as a function of pressure under two isothermal conditions (T = 36 and 50 °C). All the polymer films used showed anomalous swelling and CO₂ Sorption on the density fluctuation ridge in the P-T phase diagram of CO₂. We found that the magnitude of the swelling was a function of the elasticity of the films rather than the bulk solubility of CO₂. The enhanced miscibility of the rubber/CO₂ systems, which are very poor in bulk, was found to be almost identical to that of the silicon rubber/CO₂ mixture, which is one of the highly miscible polymeric materials under moderate CO₂ conditions.