Glass transition temperature of poly(vinylchloride) from molecular dynamics simulation: Explicit atom model versus rigid CH₂ and CHCl groups model

Glass transition temperature is the most important descriptor of the properties of amorphous polymers. In this study, molecular dynamics (MD) simulation is used to generate volume-temperature (VT) data at constant pressure for poly(vinylchloride) (PVC) over a temperature range that includes the experimental glass transition temperature (T_g) to study the validity of MD simulation in predicting T_g of amorphous polar polymers. PVC contains a polar group (chloride) which induces a partial charge distribution on all atomic sites of the polymer repeat unit. Two types of MD simulation were conducted. In the first type, all atomic sites were explicitly represented in the polymer chain model. In the second type of simulation, the CH₂ and CHCl groups were modeled as rigid units to minimize the computational effort. The T_g values obtained from the MD VT curves were slightly displaced upward relative to the experimental T_g. The rigid unit model tends to under estimate the liquid density compared with the explicit atom model. MD simulation seems to be a practical procedure for predicting the T_g of polar polymers. The rigid unit model provides substantial saving in the computational effort without loss of accuracy.