Molecular dynamics simulations of gas transport mechanisms in glassy polymer membranes: A review

Glassy polymer membranes offer an energy-efficient platform for gas separation, where performance is governed by gas transport dynamics across the polymer microstructure. Understanding these transport mechanisms at the molecular level is therefore essential for rational membrane design, with molecular dynamics (MD) simulations serving as a direct tool to resolve them. This review first summarizes MD simulation approaches for studying gas transport in glassy polymer membranes, from atomistic membrane construction and equilibration to equilibrium and non-equilibrium simulations. We then investigate the molecular mechanisms governing gas transport in glassy polymers through molecular simulations. Molecular trajectories, spatial gas distributions, and size-dependent permeability indicate that gas transport occurs predominantly through persistent pore-mediated pathways. Depending on pore structure and gas properties, pore-mediated transport is further classified into elementary mechanisms, including surface diffusion, Knudsen diffusion, and viscous flow, as well as distinct transport behaviors such as hopping diffusion, dynamic-pore-mediated transport, and molecular sieving. Characteristic molecular observables obtained from MD simulations are then identified to distinguish among these transport regimes. Finally, we discuss key challenges in improving the accuracy and reliability of MD simulations and highlight the need for predictive frameworks linking molecular-scale pore structure and gas properties to macroscopic gas transport.