Gas transmission in irradiated polyethylene

Abstract

AbstractThe transmission of helium, nitrogen, methane and propane was studied in samples of a high pressure, branched polyethylene (46% crystalline) irradiated to a dose in air of 108 roentgens from a Co60 source. By using a time‐lag apparatus the permeability, diffusion, and solubility constants were measured in the temperature range 0‐55°C. These data were compared with those obtained in samples of the above polymer beforeirradiation. Infrared analysis indicates that oxidation, unsaturation, and croaslinking had resulted from irradiation of the polymer. Because of a possible gradient in chemical composition through the irradiated films, the usual analysis of the time‐lag data may not have been applicable to the present work. The apparent diffusion and solubility constants were, however, qualitatively consistent with the anticipated effects of the chemical and physical changes resulting from irradiation. The apparent solubility constants were uniformly higher by 40% in the irradiated film as compared to the values in the unirradiated films. No reduction in crystallinity was observed upon irradiation, so this increase was ascribed to the effect of chemical composition changes. A thermo‐dynamically consistent increase in the apparent heats of solution was also observed. The apparent diffusion constants were lower in the irradiated samples with the fractional reduction increasing with gas molecular diameter. The reduction for helium was 30%, while that for propane was 54%. The apparent energies of activation for diffusion showed no significant change upon irradiation. Arguments are presented to indicate that the reduced apparent diffusion constants apparently arose from the mild cross‐linking which had occurred in the amorphous phase. Comparisons are made between the gas transmission data obtained in the unirradiated samples with those for other high pressure, branched polyethylenes studied in this laboratory in light of a simple model for gas flow in the crystalline‐amorphous matrix.