The diffusion of CO₂, CH₄, C₂H₄, and C₃H₈ in polyethylene at elevated pressures

Diffusion and solubility coefficients have been determined for the CO 2 − , CH 4 − , C 2 H 4 − , and C 3 H 8 ‐polyethylene systems at temperatures of 5, 20, and 35°C and at gas pressures up to 40 atm. Diffusion coefficients were obtained from rates of gas absorption in polyethylene rods under isothermal‐isobaric conditions by means of a new diffusivity apparatus. The concentration dependence of the diffusion coefficients was represented satisfactorily by Fujita's free‐volume model, modified for semicrystalline polymers, while the solubility of all the penetrants in polyethylene was within the limit of Henry's law. Semiempirical correlations were found for the free‐volume parameters in terms of physicochemical properties of the penetrant gases and the penetrant‐polymer systems. These correlations, if confirmed, should permit the prediction of diffusion and permeability coefficients of other gases and of gas mixtures in polyethylene as functions of pressure and temperature.