The diffusion of long-chain molecules through bulk polyethelene

Abstract

This paper describes an experimental study of the centre of mass diffusion of linear hydrocarbon or deuterocarbon molecules, of the type (CH₂)_N or (CD₂)_N, through bulk polyethylene. N was varied between about 15 and 1500. The diffusion coefficient D was measured as a function of temperature, matrix morphology and diflfusant chain length, both in semi-crystalline and molten polyethylene. The technique used involved labelling the diflfusants so that they absorbed radiation in the infrared spectrum in a region relatively free of absorption by the matrix. Infrared microdensitometry was used to measure diffusion broadening of a known diffusant concentration distribution, and hence to evaluate D. The method avoids problems presented by interfacial resistance and surface migration effects and can also account for the effect of diffusant polydispersity. The diffusion was found to be Fickian and essentially independent of concentration, for the diflfusants used, up to 2%. The data indicate that the morphology of the disordered interlamellar region can have an appreciable effect on the mobility of long-chain diffusants. They also suggest that the activation energy for diffusion in the melt becomes independent of diflfusant length beyond a certain lower limit. The results provide support for de Gennes’s (1971) model of snake-like motion (reptation) for polymeric diflfusants in an entangled polymer melt, and suggest that such motion persists down to a diflfusant size of some 30-(CH₂)-backbone units.