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.