Morphological interpretation of n-hexane diffusion in polyethylene

The diffusion of small-molecule penetrants in polyethylene is retarded by the detour caused by the crystals and by the segmental constraints imposed by the crystals on the penetrable phase. The earlier reported n-hexane diffusivity data for a series of homogeneous poly(ethylene-co-octene)s showed unexpectedly that the detour was greatest in the low crystallinity polymers. The crystal width-to-thickness ratio and the crystallinity were assessed by electron microscopy and differential scanning calorimetry and used in the Fricke model. The calculations showed that the geometrical impedance factor followed the same trend with increasing crystallinity as the data obtained from n-hexane desorption. The high geometrical impedance factor shown by the low crystallinity samples was due to the presence of crystals with an unusually high crystal width-to-thickness ratio. A unified relationship, including data for both linear and branched polyethylene was found between the fractional free volume and the phase composition of the penetrable phase including the liquid-like, interfacial liquid and the interfacial crystal core.