Synthesis and characterization of well defined polysulfone-g-poly(styrenesulfonic acid) graft copolymers for proton exchange membrane

A series of amorphous graft copolymers of polysulfone-g-poly(styrenesulfonic acid) (PSF-g-PSSA) with well-defined structures was prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. Graft copolymers of three different graft densities, i.e., 8, 5.5, and 3 grafts per chain, with three different degrees of polymerization (DP) of PSSA for each graft density were prepared. The obtained copolymers were transformed into proton exchange membranes, and well-separated simple two phase morphology was targeted in their hydrated states. They were characterized based on water uptake, ion exchange capacity (IEC), proton conductivity, and thermal stability. Water uptake increased as the wt% of the ionic graft or IEC increased. For a similar value of IEC, the water uptake for the graft copolymer decreased with higher graft density. The proton conductivities of the graft membranes were in the range of 2.1x10(-3) to 1.36x10(-1) S/cm. Proton conductivity increased as the IEC increased. However, when the wt% of PSSA was lower than 20%, very low proton conductivities were observed.