Coarse-grained molecular dynamics simulation of activated penetrant transport in glassy polymers

K Zhang and D Meng and F Muller-Plathe and SK Kumar, SOFT MATTER, 14, 440-447 (2018).

DOI: 10.1039/c7sm01941f

Membrane separations of gas mixtures strive to maximize the permeability of a desired species while keeping out undesired ones. Permeability vs. selectivity data from many polymer membranes for a given gas pair with diameters d(A) and d(B) are typically collected in a "Robeson plot", and are bound from above by a line with a slope lambda = (d(B)/d(A))(2) - 1. A microscopic understanding of this relationship, especially lambda, is still missing. We perform molecular dynamics simulations of penetrant diffusion using three different coarse-grained polymer models over a wide range of penetrant sizes, temperatures, and monomer densities. The empirically relevant lambda = (d(B)/d(A))(2) - 1 is only found for polymers that are either supercooled liquids with caged segmental dynamics or glasses and when the penetrant size is approximately half the Kuhn length of the chains, for which the penetrant diffusion is an activated process.

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