Synthesis and proton conductivity of partially sulfonated poly([vinylidene difluoride-co-hexafluoropropylene]-b-styrene) block copolymers

A series of novel, amphiphilic block copolymers comprising of sulfonated poly([vinylidene difluoride-co-hexafluoropropylene]-b-styrene) [P(VDF-co-HFP)-b-SPS] were synthesized. The number-average molecular weights of the fluorous and polystyrene segments were 17 900 and 8100 g/mol, respectively. Sulfonation of the polystyrene segment to different extents provided a series of polymers which were cast into films to yield proton exchange membranes with varying ion exchange capacity (IEC). Proton conductivity of the membranes increased significantly when the IEC was increased from 0.5 to 1.2 mmol/g. For 0.9-1.2 mmol/g IEC membranes, the conductivity was similar to Nafion 117, significantly higher than random copolymers of polystyrene and sulfonated polystyrene, and twice that of nonfluorous block copolymer membranes based on sulfonated poly(styrene-b-[ethylene-co-butylene]-b-styrene) (S-SEBS) and sulfonated hydrogenated poly(butadiene-b-styrene) (S-HPBS) copolymers. TEM revealed a disruption in ordered morphology with increasing degree of sulfonation. Morphological structures of membranes having 0.6-1.2 mmol/g IEC comprised of interconnected networks of ion channels, each of 8-15 nm width.