Gas sorption, diffusion, and permeation in poly(2,2-bis(trifluoromethyl)-4,5-difluoro-l,3-dioxole-co-tetrafluoroethylene)

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 welldescribed 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 CE_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, ED 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.