Dynamics of small molecules in dense polymers subject to thermal motion

Abstract

A new transition-state theory model has been proposed that is based on the concept that the dynamics of small molecules dissolved in dense polymers is coupled to the elastic thermal motion of dense polymers but can be treated separately from their structural relaxation. The model has been used to study the dynamics of light gases dissolved in atomistic microstructures of poly(isobutylene) and bisphenol-A-polycarbonate. Short-time scale MD runs have been used to characterize the elastic thermal motion of the host matrix. This information on mobility is then used for a stochastic simulation of solute dynamics up to approximately 10 μs. Three time regimes have been observed: a short-time, high-mobility domain, a time domain of anomalous diffusion, and a diffusive regime at long times. From the long-time data diffusion coefficients for He, H2, Ar, O2, and N2 have been estimated. Comparison with experimental data has resulted in satisfactory agreement indicating that the mechanisms of the motion of small gases in glassy and rubbery polymers are the same.