Gas and Vapor Sorption, Permeation, and Diffusion in Glassy Amorphous Teflon AF1600

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Abstract

Sorption and permeation parameters of light gases, C1−C12 hydrocarbons, and C1−C7 perfluorocarbons were determined in a random, amorphous, glassy copolymer containing 65 mol % 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (BDD) and 35 mol % tetrafluoroethylene (TFE) (TFE/BDD65 or AF1600). AF1600 results were compared to those of another copolymer, AF2400, which contains 87 mol % BDD. As the amount of bulky, packing-disrupting BDD increases, solubility coefficients increase systematically, primarily due to increases in the nonequilibrium excess volume of the glassy polymer. Permeability and diffusivity also increase with increasing BDD content. AF1600 is easily plasticized by larger, more soluble penetrants and is susceptible to penetrant-induced conditioning. As penetrant size increases, permeability and diffusion coefficients decrease. The rates of decrease of permeability and diffusivity with increasing penetrant size, which characterize permeability and diffusivity selectivity, are intermediate between those of conventional glassy polymers and exceptionally high free volume glassy materials such as poly(1-trimethylsilyl-1-propyne) (PTMSP). Positron annihilation lifetime spectroscopy (PALS) results suggest unusually large free volume elements and a bimodal distribution of free volume element size: this is consistent with similar results obtained earlier for other high free volume glassy polymers such as AF2400 and PTMSP. Inverse gas chromatography and PALS estimates of free volume element size distributions were consistent.

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