Permeation of gases through modified polymer films. V. Permeation and diffusion of helium,nitrogen, methane,ethane, and propane through γ-ray crosslinked polyethylene

The permeation and diffusion of helium, nitrogen, methane, ethane, and propane through γ-irradiated polyethylene films were investigated. These studies were carried out with two objectives in mind: (1) to determine the effect of crosslinking by γ irradiation on permeability and diffusivity using the gas molecules as molecular probes; and (2) to study the plasticizing effects of the low hydrocarbons on the polyethylene film. The γ-ray-induced crosslinking efficiency of polyethylene was investigated in the following irradiation atmospheres: vacuum, acetylene, and nitrogen–acetylene mixtures. Results showed that irradiation in acetylene decreased the crosslinking efficiency while an acetylene–nitrogen atmosphere increased the efficiency compared to irradiation in vacuum. Both the permeation constants and the diffusion coefficients were found to decrease with increasing irradiation dose while the activation energies increased. The permeation constants of the organic gases through polyethylene increased with molecular diameter while the diffusion coefficients decreased. This increase in permeability was attributed to an increase in the solubility due to solubilization of the membrane by the penetrant. For example, the molecular diameter of propane is 4.397 Å compared with 2.807 Å for methane; however, propane permeated the polyethylene film at a rate twice that of methane. Nitrogen and methane have approximately the same molecular diameters—2.7085 and 2.807 Å, respectively—but owing to the plasticizing effect of methane, it permeated the film at a rate three times greater than that of nitrogen. It is interesting to note that the stronger the plasticizing ability of the penetrant, the greater the effect of the irradiation dose. The permeability of propane decreased by 40.7%, while the permeability of helium decreased by 6.4% after an irradiation dose of 50 Mrad.