Molecular Thermodynamic Model for Sorption and Swelling in Glassy Polymer-CO₂ Systems at Elevated Pressures

A molecular thermodynamic model is presented that can quantitatively predict both gas sorption and polymer swelling in glassy polymer-compressed gas systems. The model is based on a lattice theory of polymer solutions and the concept of order parameters to describe the glass state. Model parameters that characterize molecular interactions between gas molecules and polymer segments are determined by fitting data for gas sorption in the polymer melt, and the order parameters that characterize the glassy polymer are determined from measured gas solubilities at the glass transition. These parameters are then used to predict gas sorption in the polymer glass and swelling behavior for both the liquid and glassy polymer. The predictions are compared to experimental data for poly(methyl methacrylate) and polystyrene in the presence of CO₂ at temperatures from 33 to 65 °C and pressures up to 175 atm. The model is also used to predict the depression in glass transition temperature for poly(methyl methacrylate) as a function of the amount of CO₂ sorbed.