Thermodynamics of gas and vapor sorption by amorphous glassy AF teflons

The sorption of gases and vapors in amorphous copolymers of 2,2-bis-trifluoromethyl-4,5-difluoro-1,3-dioxol and tetrafluoroethylene (AF teflons), known as gas-permeable materials possessing a high free volume, were studied by methods of reversed-phase gas chromatography and volumetry. The measurements were performed for a wide circle of sorbates from He to n-C₁₃H₂₈. The gas and vapor solubility coefficient S increases with the content of perfluorodioxol monomer, 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 (e.g., a critical volume V_c) of sorbate molecules exhibits a minimum. The coordinate V_c(min) of this minimum can be considered as estimate of the free volume element of the copolymer. A joint analysis of the volumetric and chromatogtraphic data allowed a new relationship to be established between logS and squared critical temperature of a sorbate.