Sulfonated poly(phenylene oxide) membranes as promising materials for new proton exchange membranes

Poly(phenylene oxide) (PPO) was sulfonated to different ion exchange capacities (IECs) using chlorosulfonic acid as the sulfonating agent. Tough, ductile films were successfully cast from sulfonated PPO (SPPO) solutions in N-methyl-2-pyrrolidone or N,N-dimethylformamide. The obtained membranes had good thermal stability revealed by thermogravimetric analysis (TGA). Compared with an unsulfonated PPO membrane, the hydrophilicity and water uptake of the SPPO membranes were enhanced, as shown by reduced contact angles with water. The tensile test indicated that the SPPO membranes with IEC ranging from 0.77 to 2.63 meq/g were tough and strong at ambient conditions and still maintained adequate mechanical strength after immersion in water at room temperature for 24 hr. The results of wide-angle X-ray diffraction (WAXD) showed amorphous structures for PPO and SPPO while the peak intensity decreased after sulfonation. The proton conductivity of these SPPO membranes was measured as 1.16x10(-2) S/cm at ambient temperature, which is comparable to that of Nafion 112 at similar conditions and in the range needed for high-performance fuel cell proton exchange membranes. Copyright (C) 2006 John Wiley & Sons, Ltd.