Thermodynamics of gas and vapor sorption by amorphous glassy AF teflons

The sorption of gases and organic vapors in amorphous copolymers of 2,2-bis-trifluoromethyl-4,5-difluoro-1,3-dioxol and tetrafluoroethylene (AF amorphous teflons), known as gas-permeable materials possessing a high free volume, were studied by methods of inverse gas chromatography and volumetry. The measurements were performed for a wide set of sorbates from He to n-C₁₃H₂₈. The gas and vapor solubility coefficients S increase with the content of perfluorodioxol monomers, reaching very high values for a copolymer containing 87% of this monomer. Analysis of the sorption isotherms showed that this property is related to the Henry solubility coefficients k_D being unusually high as compared to the hydrogen-containing polymers. The excess enthalpy of mixing ΔH_m as function of the size of sorbate molecules (e.g., of their critical volume V_c) exhibits a minimum. The coordinate of this minimum V_c(min) can be considered as an estimate of the free volume element of the copolymer. A joint analysis of the volumetric and chromatographic data allowed a new relationship to be established between log S and squared critical temperature of a sorbate.