Preparation and evaluation of crosslinked sulfonated polyphosphazene with poly(aryloxy cyclotriphosphazene) for proton exchange membrane

Several crosslinked proton exchange membranes with high proton conductivities and low methanol permeability coefficients were prepared, based on the sulfonated poly[(4-fluorophenoxy)(phenoxy)] phosphazene (SPFPP) and newly synthesized water soluble sulfonated poly(cyclophosphazene) (SPCP) containing clustered flexible pendant sulfonic acids. The structure of SPCP was characterized by fourier transform infrared spectroscopy (FTIR) and ¹H NMR spectra. The membranes showed moderate proton conductivities and much lower methanol permeability coefficients when compared to Nafion 117. Transmission electron microscopy (TEM) results indicated the well-defined phase separation between the locally and densely sulfonated units and hydrophobic units, which induced efficient proton conduction. In comparison with SPFPP membrane, the proton conductivities, oxidative stabilities and mechanical properties of crosslinked membranes remarkably were improved. The selectivity values of all the crosslinked membranes were also much higher than that of Nafion 117 (0.74×10⁵ S. s/cm³). These results suggested that the cSPFPP/SPCP membranes were promising candidate materials for proton exchange membrane in direct methanol fuel cells.