Sulfonated copoly(phthalazinone ether ketone nitrile)s as proton exchange membrane materials

Sulfonated poly(phthalazinone ether ketone nitrile) copolymers (SPPEKN) were prepared by copolymerization of disodium 3,3′-disulfonate-4,4′-difluorobenzophenone (SDFB-Na), 2,6-difluorobenzonitrile (2,6-DFBN), and 4-(4-hydroxyphenyl)-1(2H)-phthalazinone (DHPZ) at 160 °C in N-methyl-2-pyrrolidione containing anhydrous potassium carbonate. The polymerization reactions proceeded smoothly and in high yield giving high molecular weight SPPEKN copolymers with different sulfonic acid content (SC) values. The polymer structures were defined by 1H NMR and FT-IR. Membrane films of SPPEKN copolymers in both salt and acid forms, prepared at SDFB-Na to 2,6-DFBN feed ratios up to 60/40 mol/mol, were cast from the N,N-dimethylacetamide (DMAc) polymer solutions. The presence of highly polar nitrile groups are expected to increase inter-chain molecular forces, contributing to the observed reduction in water swelling over previously prepared sulfonated poly(phthalazinone ether ketone) (SPPEK) copolymers. Membranes containing nitrile groups are expected to show improved adhesion between polymer and catalyst when utilized in a fuel cell. All SPPEKN copolymers exhibited tensile strength higher than that of Nafion®117. The proton conductivities of acid form SPPEKN copolymers (SPPEKNH)s, prepared at the sulfonated/unsulfonated monomer feed ratio above 0.35 mol/mol, were around 10−1 S/cm at 80 °C, which is close to or higher than that of Nafion®117 under same measurement conditions.