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

Abstract

This paper develops a detailed molecular dynamics (MD) simulation model to study the glass transition temperature (Tg) 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 Tg 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 Tg 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.