(6ci) Understanding Diffusion of Small Molecules in Polymer Glasses

The Free Volume Model (FVM) proposed by Cohen and Turnbull over 50 years ago had provided a simple picture that relates the structural properties of amorphous glasses to the diffusing behaviors of small molecules in them. Albeit the semi-phenomenological nature of the model, FVM has met with great success in matching experimental results and providing guidance in designing new materials for membrane separations. However, accumulating experimental data on diffusion of gases through polymer membranes have increasingly suggested inconsistencies of the model. In attempts to correct these deviations within the framework of FVM, effects of activation energy, cavity connectivity and accessible free volume had been introduced. These modifications are largely phenomenological due to the lack of a systematic understanding of the diffusion process in glassy polymers. In this study, using molecular dynamics simulations we aim to provide a coherent picture of small molecules diffusing in polymer glasses. Our results suggest that there are three distinct diffusion regimes depending on the relative size of the diffusant and “cavities”. The dominant mechanism that controls diffusion in each regime is identified respectively. Based on the identified mechanisms, deviations from the FVM can now be systematically understood and predicted. Our results provide a basis on which further development of more comprehensive diffusion models will be constructed, a work that is undergoing.