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

Amorphous, atactic polypropylene structures, consisting of 125, 729, and 21 97 monomer repeat units folded into periodic cells, were generated to study the effects of simulation size on the transport of small molecules in simu lations 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 agai nst the formation of large cavities in the smaller structures. Surprisingly , this bias does not significantly affect the diffusivities of the penetran ts in these structures despite their jumplike diffusive motion. We also fin d the characteristic length scale for the turnover from the anomalous to th e diffusive regime to be insensitive to the simulation size but inversely d ependent on penetrant size. This insensitivity to simulation size of the di ffusivity and turnover is in contrast to that found for diffusion in system s that are either static or have percolating networks. This difference poin ts to the importance of dynamic coupling between the penetrant motion and t he 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 correlatio n function for analyzing penetrant motion.