Sorption and transport of gases and vapors in perfluorinated rubbers

Fluorinated polymers are recently being considered in membrane gas separation in combination with highly permeable polymers, in view of their ability to inhibit the plasticization induced by organic vapors, which highly decrease selectivity. The main action is due to the effects on penetrant solubility. In this work, we measured the sorption and transport properties of a series of hydrocarbon and fluorocarbon gases (C2H6, n-C5, n-C6, cyclo-C5, cyclo-C6, CF4, C2F6 and C2HF5) and alcohols (methanol, ethanol) into rubbery copolymers of tetrafluoroethylene (TFE) and perfluoro methylvinylether (PMVE). The logarithm of the solubility coefficient at infinite dilution, log(S0), increases linearly with the critical temperature of each penetrant, TC ; moreover, in the plot log(S0) vs. TC, the data relative to fluorocarbons and to hydrocarbons lie on two separate straight lines with similar slope, where the fluorocarbons line lies above the hydrocarbons line. Interesting empirical correlations have been found between diffusivity and permeability and the molecular dimensions of the penetrant. The solubility data have been compared to predictions of thermodynamic models for gas sorption in rubbery polymers. The model calculations predict correctly the solubility of fluorocarbons in fluoroelastomers, based on the usual first order approximation for the binary interaction parameter, while adjustable binary parameters must be introduced to describe the hydrocarbons solubility isotherms: this behavior is consistent with previous results about solutions formed by hydrocarbon and fluorocarbon compounds, which generally do not follow the geometric mean rule. In any case, the values of binary parameters needed to correctly fit the solubility isotherms of hydrocarbons in fluoroelastomers are very regular and allow to predict the solubility of other vapors not experimentally investigate