Anisotropic diffusion of small penetrants in the δ crystalline phase of syndiotactic polystyrene:: A molecular dynamics simulation study

The diffusional behavior of helium and carbon dioxide in crystalline syndiotactic polystyrene, in its nanoporous delta form, has been studied by means of molecular simulation. Crystallographic directions corresponding to prevailing diffusion pathways between crystalline cavities have been determined. Moreover, diffusion coefficients and their anisotropy at different temperatures have been calculated. For both gases at all temperatures, preferential diffusion pathways are parallel to the crystallographic ac planes, corresponding to rows of parallel helices with minimum interchain distances. Diffusion of carbon dioxide is slower and more anisotropic than that of helium. For both gases, the preferred diffusion pathways are along the <101> directions at low temperatures while parallel to the chain axes at high temperatures.