Solubility and diffusivity of gases in composites formed by high free volume glassy polymers and fumed silica: experimental behavior and simulations based on the NELF model

The addition of fumed silica nano-particles to high free volume glassy polymers such as amorphous Teflon results in an enhancement of gas solubility, diffusivity and permeability: this behavior is not captured by traditional models for composites, which underestimate the gas transport parameters. In this work we consider that such phenomenon is due to the additional free volume induced by insertion of fumed silica nano-particles in the polymeric phase. In order to evaluate the exact value of polymer free volume after the addition of a certain amount of filler, we model the experimental solubility of one test penetrant in the polymeric phase of the composite material with the NELF theory. The fractional free volume (FFV) calculated in this fashion may then be used to estimate a priori the solubility of other gases in the same composite material and to estimate with one adjustable parameter only the diffusivity and permeability of other gases in the mixed matrix, by means of the free volume theory. This model has been used to predict solubility, diffusivity and permeability of two model vapors (n-C4 and n-C5) in mixed matrix membranes based on Teflon AF2400 and fumed silica: the model predictions are in good agreement with experimental data for both diffusivity and solubility