Inert gas permeation through homopolymer membranes

New data are reported for the permeation of inert gases through polyethylene, polytetrafluoroethylene, and silicone and natural rubbers. Additional data are compiled from the literature. The relative solubilities of these gases are practically insensitive to chemical variations in the homopolymer. Hence variations in structure at the glass transition (T_g) and melting (T_m) temperatures that affect diffusion also unambiguously affect permcation. Consequently an equivalence results between permeation at a given temperature for different polymers and permeation at different temperatures for a given polymer. Although the diffusion coefficient changes continuously with temperature, the Arrhenius parameters D_o and E_d apparently change discontinuously at T_g and T_m. Their magnitudes and variations with atomic weight reach maxima at about T_g. These data indicate a dependence of the classical correlation between D_o and E_d on polymer properties. A perturbed diameter for the permeant, specific for each polymer, is proposed for correlating the D_o and E_d data. This correlation makes the changes observed at T_g and T_m more perceptible.