Molecular dynamics simulation based evaluation of glass transition temperatures of polystyrene in the presence of carbon dioxide

This paper develops a detailed molecular dynamics (MD) simulation model to study the glass transition temperature (T_g) of a finite polystyrene (PS) system in the presence of high pressure carbon dioxide (CO₂). Validated inter-atomic potentials for pure PS and CO₂ are used for these simulations. Specific parameters and combinations rules are introduced to accurately model the intermolecular interactions between PS and CO₂. The intermolecular interaction model has a strong effect on the T_g of the PS-CO₂ system. The MD model comprises PS and CO₂ molecules confined in a finite walled system to manifest the effects of high pressure CO₂. The effectiveness of the simulation model is established by comparison with experimental free-volume data from positronium annihilation lifetime spectroscopy (PALS). An important outcome of this study is the identification of clearly demarcated regions for bulk and surface analysis. Physical properties such as density, free volume, segmental motion across the thickness and end group mobility are also studied to gain insight into the polymer dynamics. As is expected, the simulations show that the presence of high pressure CO₂ reduces the T_g of PS significantly due to an increase in chain mobility. Additionally, the simulation data show a remarkable effect of CO₂ on the extent and characteristics of the surface layer.