Diffusion of small-molecule penetrants in polyethylene: Free volume and morphology

Based on desorption and permeation measurements, the diffusivity and solubility of n-hexane and oxygen have been obtained for a wide range of linear and branched polyethylenes (PEs) with crystallinities between 40 and 97% and mass-average molar masses between 10(3) and 10(6) g mol(-1). The morphology and contents of crystal core (CC), crystal-core-like interfacial (I-CC), liquid-like interfacial (I-L), and liquid (L) components were assessed by transmission electron microscopy, Raman spectroscopy, C-13 cross-polarization/magic-angle spinning nuclear magnetic resonance spectroscopy, differential scanning calorimetry, density measurements and small-angle light scattering. The penetrant solubility in the non-crystalline phases increased with increasing concentration of chain ends and chain branches. This effect was masked at certain crystallinities by the constraining effect of the crystallites. The diffusivity selectivity of oxygen over n-hexane increased strongly with increasing crystallinity and decreasing non-crystalline layer thickness, demonstrating that the crystal-induced constraint on the non-crystalline chains more efficiently retards the diffusion of larger molecules. The fractional free volume of the non-crystalline components decreased strongly with increasing crystallinity in the low-crystallinity range (<60%), above which it remained practically constant. The latter is because the constraining effect of the crystals is compensated for by the plasticizing effect of the chain ends, which leads to a constant free volume in this crystallinity range. A model, based on the Cohen-Turnbull-Fujita (CTF) model, considering the polymers to consist of four components, CC, I-CC, I-L and L, was applied to the diffusivity data. The branched PEs and the majority of the linear PEs could be described by the modified CTF model. However, the lowest-molar-mass linear PEs exhibited a considerably larger interfacial free volume than the other samples. Real-time Raman spectroscopy on CCl4-swollen samples showed that the changes in the CC and I-CC contents during sorption were only small. Copyright (C) 1996 Elsevier Science Ltd.