Crosslinked terpolymers of vinylidene fluoride, perfluoro-3,6-dioxa-4-methyl-7-octene sulfonyl fluoride, and cure site monomers for membranes in PEMFC applications
Radical terpolymerisations of perfluoro(8-cyano-5-methyl-3,6-dioxaoct-1-ene) (CNVE) or 2,2,3,3,4,4,5,5,6,6-decafluoro-6-[(1,2,2-trifluoroethenyl) oxy] hexanenitrile (MV5CN), vinylidene fluoride (VDF) and perfluoro(4-methyl-3,6-dioxaoct-7-ene) sulfonyl fluoride (PFSVE) are presented. Changing the monomer feed compositions of these terpolymerisations enabled the synthesis of different statistical-type poly(VDF-ter-PFSVE-ter-CNVE) terpolymers containing sulfonyl fluoride and nitrile side groups. Yields higher than 11% were reached in all cases. The hydrolysis of the sulfonyl fluoride groups into sulfonic acid functions, using potassium hydroxide (or lithium carbonate) under mild conditions, was achieved quantitatively without affecting the content of VDF, assuming that no dehydrofluorination of the VDF units occurred. These original terpolymers were then thermally crosslinked using cyclisation of the pendant nitrile functions to produce films insoluble in organic solvents such as acetone and dimethylformamide (that completely dissolved uncured terpolymers). The thermal stabilities of the crosslinked materials were higher than those of the uncured terpolymers. Depending on the sulfonic acid content in the terpolymers, the ion-exchange capacities ranged from 0.95 to 1.32 meq H+ per g. The proton conductivity at 80 °C and 100% relative humidity reached 58 mS cm−1 for a membrane crosslinked at 240 °C and having a hydration number of 33 H2O/SO3H, while excessive swelling was observed for membranes crosslinked at lower temperatures.