Revisiting the radical copolymerization of vinylidene fluoride with perfluoro-3,6-dioxa-4-methyl-7-octene sulfonyl fluoride for proton conducting membranes

The radical copolymerizations of vinylidene fluoride (VDF) with perfluoro-3,6-dioxa-4-methyl-7-octene sulfonyl fluoride (PFSVE) and hPFSVE (hydrolyzed form of PFSVE) are presented. The resulting poly(VDF-co-PFSVE) random copolymers were hydrolyzed then cast into proton conducting membranes. Another strategy was first to hydrolyze PFSVE under basic conditions, then to copolymerize it with VDF. The radical copolymerization of VDF with PFSVE led to homogeneous copolymers in contrast to that of VDF with hPFSVE, as assessed by ¹⁹F NMR spectroscopy. The thermal and electrochemical properties of all copolymers were determined. As expected, the thermal stability of the poly(VDF-co-PFSVE) copolymers bearing -SO₂F groups were higher than that of the hydrolyzed copolymers containing -SO₃H moieties. Membranes processed by casting displayed water uptake of ca. 40%, ion exchange capacity of 1.25 meq.g^?1 and conductivities up to 28 mS cm^?1 at 80 °C and 100% relative humidity.