Diffusion of gases in fluorocarbon polymers

The diffusion and solubility coefficients and their temperature coefficients of several gases in fluorocarbon polymers were measured by a spring balance and radiotracer method. The activation energies of small molecule diffusion in polytetrafluoroethylene increase with the diffusate molecular diameter squared. The increase is less than that found in many other typical polymers and this is probably due to pre‐existing holes or channels. The latter are most likely to occur in polymers consisting of stiff chain molecules which have a low packing efficiency. Furthermore, the activation energies are not simply proportional to the cohesive energy density. These findings and the general relation between activation energy and solute molecular size can be explained in terms of the model proposed earlier. The apparent frequency factors in the Arrhenius expression for the diffusion coefficients also are low in comparison to other typical polymers presumably because the diffusate molecules are forced to follow a tortuous path through the polymer. The effect of polymer crystallinity and transitions are discussed briefly. Many molecular motions apparently lead to successful diffusion events, again in agreement with the model mentioned above. A simple and rather unambiguous method to derive activation entropies is reported and the diffusion processes in low and high molecular weight materials have been compared.