Influence of polymer motion, topology and simulation size on penetrant diffusion in amorphous, glassy polymers: Diffusion of helium in polypropylene

The influence of polymer topology and motion on the diffusion of a small gas penetrant (He) in united atom atactic polypropylene models of varying density (0.872-0.971 g/cm(3)) and simulation cell size (27-60 Angstrom), are studied by performing molecular dynamics simulations on rigid polymer structures. A multimode analysis of the self-part of the van Hove space-time autocorrelation function for penetrant diffusion is introduced to investigate the mechanism of diffusion and distinguish simulation box size effects. For the frozen matrix, the diffusion is no longer an activated process but is better described as hindered kinetic motion. The anomalous diffusion regime is identified to be a consequence of the tortuosity in the percolated diffusion pathways in the polymer structure, which itself is connected to the correlation length in the polymer glass. Comparisons with fully mobile matrix simulations demonstrate the influence of polymer motion on the mechanism of molecular diffusion in glassy polymers and are in qualitative agreement with previous studies. The turnover from anomalous to Fickian diffusion shows a marked simulation size dependence not seen in the results for which the polymer is allowed thermal motion.