Design of polyphosphazene-based graft copolystyrenes with alkylsulfonate branch chains for proton exchange membranes

In order to develop efficient proton exchange membranes used for direct methanol fuel cells, we are focusing on the design and synthesis of polyphosphazene-based copolymers. In the present study, block polystyrenes were grafted onto polyphosphazenes by atom transfer radical polymerization (ATRP) of styrene and 4-acetoxystyrene, and then alkylated with 1,4-butanesultone to yield two series of graft copolymers having branch chains of alkylsulfonic acids. They were further made into the corresponding membranes with the assist of simultaneous cross-linking reaction with 2,6-bis(hydroxymethyl)-4-methylphenol (BHMP) as the cross-linker. High proton conductivities were found for the membrane series of M-PS_x-PSBOS_y in the range of 0.184-0.266S/cm, and for F-PS_x-PSBOS_y in the range of 0.147-0.284S/cm under fully hydrated conditions at 80°C, respectively, and were supported by the nanophase-separated structures by the transmission electron microscopy (TEM) investigations. Also, the membranes showed good methanol resistance (1.60-10.4×10^-7cm²/s) and high oxidative stabilities. These results suggest a potential of the membranes for the DMFCs application.