Molecular Mobility of Perfluorinated Nafion and Aquvion Type Copolymers in the Sulfonyl Fluoride and Sulfonic Acid Forms Depending on the Number of Functionalized Substituents in the Polytetrafluoroethylene Main Chain

Abstract

The interaction of water with air-dried membrane films and precursors of Nafion and Aquvion type membranes was investigated. The differential scanning calorimetry studies demonstrated the presence of free water in the films that was trapped at the stage of sample preparation. The dielectric spectroscopy results revealed two relaxation processes associated with the mobility of functionalized substituents in the polytetrafluoroethylene main chain for the polymers in the sulfonic acid form, with activation energies ranging from 12.9 to 33 kcal/mol, which indicates the “cooperativity” of local processes. The activation energies vary depending on both the length of a substituent and the distance between two neighboring substituents (the polymer chain parameter n). It was demonstrated by wide-angle X-ray scattering that an increase in the polymer chain parameter leads to an increase in the degree of crystallinity of Nafion copolymers in the sulfonyl fluoride form. The presence of “water-polymer” and “water-water” associates, as well as non-associated water molecules, was shown by infrared spectroscopy.