Proton conducting graft copolymers with tunable length and density of phosphonated side chains for fuel cell membranes

Polysulfones functionalized with highly phosphonated poly(pentafluorostyrene) side chains of different lengths were synthesized applying controlled polymerization and modification methods. The graft copolymersʹ thermal properties were evaluated by differential scanning calorimetry and thermal gravimetrical analyses. The proton conductivity of membrane prepared from the graft copolymer with the shortest phosphonated side chains was 134 mS cm−1 at 100 °C under fully immersed conditions. The graft copolymer TEM image shows a nanophase separation of ion-rich segments within the polysulfone matrix. Increasing the ionic groups content in the graft copolymers led to extensive membrane swelling. To improve the dimensional stability the graft copolymers were blended with pyridine-modified polysulfone. The blend membranes were transparent with formation of nano-phase domains as revealed from TEM images. The acid–base blend membranes exhibited a slightly higher thermal stability but lower proton conductivity compared to the membranes formed from pure graft copolymers.