The permeability of poly(dimethylsiloxane) [PDMS] to H2, O2, N2, CO2, CH4, C2H6, C3H8, CF4, C2F6, and C3F8, and solubility of these penetrants were determined as a function of pressure at 35 °C. Permeability coefficients of perfluorinated penetrants (CF4, C2F6, and C3F8) are approximately an order of magnitude lower than those of their hydrocarbon analogs (CH4, C2H6, and C3H8), and the perfluorocarbon permeabilities are significantly lower than even permanent gas permeability coefficients. This result is ascribed to very low perfluorocarbon solubilities in hydrocarbon‐based PDMS coupled with low diffusion coefficients relative to those of their hydrocarbon analogs. The perfluorocarbons are sparingly soluble in PDMS and exhibit linear sorption isotherms. The Flory–Huggins interaction parameters for perfluorocarbon penetrants are substantially greater than those of their hydrocarbon analogs, indicating less favorable energetics of mixing perfluorocarbons with PDMS. Based on the sorption results and conventional lattice solution theory with a coordination number of 10, the formation of a single C3F8/PDMS segment pair requires 460 J/mol more energy than the formation of a C3H8/PDMS pair. A breakdown in the geometric mean approximation of the interaction energy between fluorocarbons and hydrocarbons was observed. These results are consistent with the solubility behavior of hydrocarbon–fluorocarbon liquid mixtures and hydrocarbon and fluorocarbon gas solubility in hydrocarbon liquids. From the permeability and sorption data, diffusion coefficients were determined as a function of penetrant concentration. Perfluorocarbon diffusion coefficients are lower than those of their hydrocarbon analogs, consistent with the larger size of the fluorocarbons. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 415–434, 2000

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The solubility and permeability of H2, O2, N2, CO2, CH4, C2H6, C3H8, CF4, C2F6, and C3F8 in TFE/BDD87, a random copolymer prepared from 87 mol % 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole [BDD] and 13 mol % tetrafluoroethylene [TFE], are reported as a function of temperature and pressure. Sorption isotherms of all penetrants except hydrogen are concave to the pressure axis and are well-described by the dual-mode model. Hydrogen exhibits linear sorption isotherms. In contrast to previous results in hydrocarbon-rich polymers, the solubility of perfluorocarbon penetrants is higher in TFE/BDD87 than that of their hydrocarbon analogues. The solubility of all penetrants in TFE/BDD87 decreases with increasing temperature. Enthalpies of sorption become more negative as penetrant size increases. Fluorocarbon enthalpies of sorption at infinite dilution are significantly more exothermic than those of their hydrocarbon analogues, suggesting more favorable interactions between fluorocarbon penetrants and perfluorinated TFE/BDD87 than between hydrocarbon penetrants and this polymer. Perfluorocarbon permeability coefficients are nearly an order of magnitude lower than those of their hydrocarbon analogues due to the larger size of the fluorocarbons and their subsequently lower diffusivities. The permeability of TFE/BDD87 increases with increasing temperature, indicating that activation energies of permeation (E p) are positive. E p values in TFE/BDD87 are smaller than those of conventional glassy polymers. Diffusion coefficients of the lower sorbing gases (O2, N2, CO2, CH4, CF4) exhibit a concentration dependence that is consistent with dual-mode transport in unplasticized glassy polymers. For more strongly sorbing C2H6, C3H8, C2F6, and C3F8, diffusion coefficients increase exponentially with increasing penetrant concentration, suggesting plasticization. Activation energies of diffusion in TFE/BDD87 are positive and increase linearly with penetrant diameter squared. Relative to conventional glassy polymers, E D values in TFE/BDD87 are low. However, |E D| is larger than |ΔH S|. TFE/BDD87 is easily plasticized by the larger, more soluble penetrants and is susceptible to penetrant-induced conditioning. The level of conditioning is highest for the largest, most soluble penetrant examined (C3F8), and the conditioned state gradually relaxes toward that of the as-cast state.