Synthesis and properties of side-chain-type sulfonated poly(phenylene oxide) for proton exchange membranes

To obtain side-chain-type proton exchange membranes, bromomethylated poly(phenylene oxide) (BPPO) was modified via etherification reaction under mild conditions to introduce pendant naphthalene sulfonic acid groups. The reaction has been proved facile and efficient, and more than 70% of benzyl bromide groups of BPPO could be substituted. Their solution-cast membranes showed potential application as proton exchange membranes. The membranes had acceptable proton conductivity ranging from 38 to 71 mS/cm (25 °C, 100% relative humidity) in spite of relatively low ion exchange capacity ranging from 0.93 to 1.62 mmol/g. Meanwhile they exhibited outstanding dimensional stability in water (i.e., no more than 32% water uptake and no more than 11% linear expansion ratio in water at 80 °C), which was supposed to be ascribed to unique π–π interaction between pendant naphthalene rings.