(396a) The Molecular Mechanism of Gas Diffusion in Polymers

Separation of gas mixtures using light-weight and low-cost polymeric membranes presents an efficient solution to many emergent energy and environmental challenges. The central goal of membrane design is to enhance the permeability of a desired species while keeping out undesired ones. The permeability vs. selectivity data for a given gas pair within many polymeric membranes seem to be bounded from above by an upper bound limit. However, the position of such an “upper bound” is changing as more data are reported and its existence has mostly been described by simple thermodynamic and kinetic arguments. In order to gain microscopic insight into this problem, we perform coarse-grained molecular dynamics simulations of small penetrant diffusion in large polymer matrix. A wide range of polymer stiffness has been studied by tuning the strength of bond angle and torsion potentials. We find that the effects of polymer properties on gas diffusion, regardless of the model details, can be summarized into two contributions: the static free volume, characterized by the typical cavity size d_f and the chain dynamics, characterized by the cage size u of chain mean-square displacement. At constant pressure, the best separation performance occurs when the polymer chains are stiff enough to exhibit small u, while being not too stiff to give rise to very large d_f. Our work provides a microscopic mechanism for the existence of the Robeson upper bound in the context of polymeric membranes.