Synthesis of the sulfonated poly(ether ether ketone ketone) proton exchange membrane with butane-1,4-diylbis(oxy) spacers and sulfopropyloxy pendants through Friedel-Crafts polyacylation

A novel proton exchange membrane composed of the sulfonated poly(ether ether ketone ketone) (SPEEKK) with butane-1,4-diylbis(oxy) spacers in the backbone and sulfopropyloxy pendants is synthesized through the Friedel-Crafts polyacylation of 1,4-diphenoxybutane (DPB), diethyl-3,3’-([1,1′-biphenyl]-2,2′-diylbis(oxy))bis(propane-1-sulfonate) (SBP–OEt), and terephthaloyl chloride (TPC) under mild conditions. The monomer DPB can introduce the flexible aliphatic butane-1,4-diylbis(oxy) spacers into the polymeric backbone and thus promote the formation of proton transport channels. The as-synthesized SPEEKK exhibits a high proton conductivity of 152.5 mS cm−1 at 95 °C and 90% relative humidity (RH), a moderate water uptake of 53.2% at 100 °C and also a high ion exchange capacity of 1.74 mmol g−1. These characteristics are attributed to the enhanced nanophase separation induced by the flexible butane-1,4-diylbis(oxy) spacers and hydrophilic sulfopropyloxy sidechains.