Structure, rate, and mechanism of methane diffusion in glassy atactic polypropylene as determined by coarse-grained molecular modeling

Michael L. Greenfield, University of California, Berkeley
Doros N. Theodorou, University of California, Berkeley


Using a combination of molecular modeling techniques, methane jumps induced by explicit motions of an underlying polypropylene matrix were simulated. The methyl groups nearest to the penetrant moved slightly during each jump, while chain segments further away were not affected. Jumps near a chain start or end were faster on average, due to an absence of slower jumps. Angles between consecutive jumps were similar to those expected from a random distribution, except that fewer pairs than expected were seen near 0 and 90°. An anomalous region was detected for particles diffusing on a network whose structure matched those of the glassy polypropylene systems, but with jump rates all equal to each other, demonstrating that the geometry of penetrant jump networks plays at least some role in leading to an anomalous diffusion regime.