The effect of polymer density on the diffusion of ethane in polyethylene

Abstract

AbstractDiffusion and solubility coefficient were determined by the time‐lag method for ethane and some other low hydrocarbons in various polyethylenes. The time‐lag method was modified to give significant savings in experimental time and increased precision. Diffusion and solubility coefficients both depend quite strongly on the density and hence on the crystallinity. There is no discernible effect of weight‐average molecular weight, and the effect of polymer chain side branches is found to be small or absent. The thermal and fabrication history of the samples has some effect on the diffusion coefficients. The character of the amorphous regions appears to be independent of the overall density as far as the diffusion process is concerned. The effect of the size of the penetrant molecule is analogous to that found by other authors for rubbers. The findings support the view that small molecule diffusion in polymers requires only short range motions, whereas other mechanisms, such as the dissolution of entanglements, are known to be of importance in melt flow and polymer self‐diffusion.