Gas transport through glassy polymer membranes from all-atom molecular dynamics simulations

Polymers are attractive membrane materials owing to their mechanical robustness and relatively inexpensive fabrication. An important feature central to polymer membrane performance is the distribution of connected void spaces created by inefficient packing of bulky groups on the polymer backbone, known as free volume elements (FVEs). FVEs tend to degrade over time as polymer chains reorganize irreversibly; relating local chain dynamics to the distribution of FVEs can help control phenomena like plasticization and aging. In this work, we use atomistic molecular dynamics simulations to study three polymers - polymethylpentene (PMP), polystyrene (PS), and HAB-6FDA thermally rearranged polymer (TRP). These polymers represent a broad range of structures, allowing us to understand the interplay between polymer chemistry and membrane function. We compute FVEs by identifying filled and free regions in the membrane, as well as atoms that lie on the surface separating the two regions. We find that polymer segments near the surface of voids exhibit faster dynamics compared to bulk segments. This dictates the stability of FVE distribution across different polymer chemistries, serving as a predictor for membrane aging and plasticization. To compute the permeation of gas mixtures such as olefins and paraffins through the membrane matrix, we use the non-equilibrium concentration gradient method. The results from this approach are compared against experimental techniques that measure permeation.