Gas and Vapor Sorption, Permeation, and Diffusion in Poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether)

Solubilities of N2, CO2 , C1-C3 saturated hydrocarbons, C1-C3 saturated fluorocarbons, and normal- and cyclo-C5 and -C6 alkanes at 35 °C are reported in an amorphous, rubbery, random copolymer composed of 50.7 mol % tetrafluoroethylene and 49.3 mol % perfluoromethyl vinyl ether, TFE/ PMVE49. Permeabilities of N2, O2, CO2, and C1-C3 saturated hydrocarbons in this polymer at 35 °C are also presented. Solubilities of hydrocarbon vapors in this fluoropolymer are lower than those of their corresponding fluorocarbon analogues due to less favorable interactions of the fluorinated polymer with hydrocarbon vapors than with fluorocarbon penetrants. Polymer density was measured over a wide temperature and pressure range to estimate parameters for equation-of-state modeling of gas sorption. Modeling the sorption isotherms with the Sanchez-Lacombe equation-of-state model requires a reduction of approximately 10% in the cohesive energy density of the gas-polymer mixture from that predicted by the geometric mean mixing rule for all hydrocarbon penetrants other than CH4. Sorption of CH4 and fluorocarbon penetrants is well described with little or no variation in the geometric mean rule estimate of interaction energy. While linear correlations between the natural logarithm of hydrocarbon gas solubility and penetrant critical temperature in hydrocarbon polymers often have slope values of about 0.019 K-1 at 35 °C, this fluoropolymer has a much lower slope value, 0.011 K-1. Hydrocarbon/nitrogen permeability selectivity is much lower in TFE/PMVE49 than in hydrocarbon-based rubbery polymers such as poly- (dimethylsiloxane). This effect is, to a very large extent, a result of hydrocarbon solubility suppression in the fluoropolymer, due to less favorable hydrocarbon-fluorocarbon interactions.