Enhanced conductivity in morphologically controlled proton exchange membranes: Synthesis of macromonomers by SFRP and their incorporation into graft polymers

A novel series of graft polymers comprising graft chains of a macromonomer poly(sodium styrenesulfonate) (macPSSNa) and a polystyrene (PS) backbone have been prepared using a combination of stable free radical polymerization (SFRP) and emulsion polymerization. Well-defined PSSNa polymers possessing 32 repeat units and a polydispersity index of 1.25 were prepared by SFRP. The pseudo-living chains were terminated with divinylbenzene to afford a macromonomer (macPSSNa), which was subsequently copolymerized with styrene by emulsion polymerization to afford PS-g-macPSSNa graft polymers. The number density of macromonomer graft chains was controlled by the feed ratio of macromonomer to styrene. Films were prepared by compression molding and Na+ ions exchanged for protons to yield proton exchange membranes (PEMs). The graft polymer membranes exhibited lower water uptake but much larger proton conductivity for a given sulfonic acid content compared to membranes prepared from random copolymers of styrenesulfonic acid and styrene (PS-r-PSSA). Transmission electron microscopy showed that the graft polymer membranes exhibit a higher degree of phase separation and enhanced connectivity between ionic domains.