Coupling of penetrant and polymer motions during small-molecule diffusion in a glassy polymer
Multidimensional transition-state theory was used to simulate methane jump motions in glassy atactic polypropylene at 233 K in the limit of small methane concentrations. Transition states were found with respect to both penetrant and polymer degrees of freedom, using all generalized coordinates associated with atoms interacting with the methane penetrant. Animations followed the multidimensional reaction coordinate for three different jumps. The jump mechanism involved polymer atoms retracting to form a channel, followed by penetrant motion through the channel. Methyl groups within 4 Å of the penetrant transition state location were displaced by 0.9 Å on average, while carbon atoms and methyl groups further than 9 Å from the penetrant transition state location were displaced by less than 0.2 Å. The energy profiles along the diffusion path differed considerably among all jumps simulated, and the jump rate did not correlate simply with changes in particular types of degrees of freedom. Jumps for which the penetrant transition state location was within 5 Å of a chain start or end had average rates of order 60 μs-1, while jumps further from a chain start or end were an order of magnitude slower.