Thermodynamics of Sorption in an Amorphous Perfluorinated Copolymer AF1600 Studied by Inverse Gas Chromatography

Abstract

Detailed investigation of infinite dilution sorption of organic vapors (C7−C16 n-alkanes, toluene, octafluorotoluene, p-fluorotoluene, 2,3,4,5,6-pentafluorotoluene, perfluorooctane) in perfluorinated copolymer AF1600 (random copolymer of 35 mol % perfluoroethylene and 65 mol % 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole) was performed by inverse gas chromatography (IGC). C13−C16 n-alkanes were studied in the temperature range including both glassy and rubbery states of the polymer, while sorption of other solutes was investigated only in the glassy state of the perfluorinated copolymer. Retention diagrams of C13−C16 n-alkanes showed breakpoints at ∼150 °C which corresponds to Tg of AF1600. Thermodynamic analysis of IGC data in rubbery and glassy states was performed based on the model developed earlier which included evaluation of Flory−Huggins interaction parameter χ and glassy state structual parameters: excess free volume φV0, polymer cohesion energy ε22, and entropy depression term s12. It was shown that aliphatic and aromatic hydrocarbons were poor solvents for glassy AF1600. Positive χ values substantially increased when the length of the alkane increases. Fluorinated hydrocarbons were, however, good solvents for AF1600, and their quality improved with the increase in fluorination degree. On the basis of the cohesion energy of AF1600 (−17.76 kJ/mol) evaluated by CAChe 7.5 molecular modeling package, excess free volume of AF1600 in the glassy state was found to be close to 0.12 and slightly dependent on temperature. The entropy effect of sorption in glassy polymer was highly negative with entropy depression proportional to molar volume of solute.