Molecular dynamics simulation of penetrant diffusion in amorphous polypropylene: Diffusion mechanisms and simulation size effects

Amorphous, atactic polypropylene structures, consisting of 125, 729, and 2197 monomer repeat units folded into periodic cells, were generated to study the effects of simulation size on the transport of small molecules in simulations of amorphous polymers. The diffusion coefficients and solubilities of three particles having different sizes representative of He, Ar, and CO2 are calculated from 4 ns molecular dynamics simulations. A definite system size dependence is observed in the solubilities resulting from a bias against the formation of large cavities in the smaller structures. Surprisingly, this bias does not significantly affect the diffusivities of the penetrants in these structures despite their jumplike diffusive motion. We also find the characteristic length scale for the turnover from the anomalous to the diffusive regime to be insensitive to the simulation size but inversely dependent on penetrant size. This insensitivity to simulation size of the diffusivity and turnover is in contrast to that found for diffusion in systems that are either static or have percolating networks. This difference points to the importance of dynamic coupling between the penetrant motion and the thermal motion of the polymer matrix. A rigorous statistical analysis of different methods of extracting the penetrant tracer diffusivity from the molecular simulations emphasizes the value of using the van Hove correlation function for analyzing penetrant motion.