The permeability coefficients of gases through glassy polymers have been correlated with the fractional free volume (FFV) of the polymers. In general, polymers with high fractional free volumes have high permeabilities while those with low FFV have low permeabilities. This observation is valid for many, but not all materials. This study evaluates the impact of the average size of a free volume element on the permeability of gases through the polymer.
Evaluation of the influence of average free volume element size is only possible by employing model systems in which the chemistry and the total free volume are essentially equivalent. In this study, two stereochemical forms of a methyl-substituted polynorbornene were employed. The isomers are chemically equivalent, with similar total free volumes (0.181 versus 0.188). The average defect size was probed using positron annihilation lifetime spectroscopy. The ortho-positronium lifetimes were measured and it was determined that the difference between the two isomers was approximately 10% with the lower-FFV isomer having the larger average lifetime. For simplicity, the two isomers will be termed Pd and Ni (in reference to the catalysts used in their preparation). The Pd isomer has a slightly lower FFV, but larger average defect size. The Ni isomer has a higher FFV, but smaller average defect size.
Transport evaluation indicates that the Pd-isomer has gas permeabilities two to three times those of the Ni-isomer. In depth analysis indicates that the increase in permeability is a result of an increase in both the diffusivity of gases through the polymer and gas solubility.