Poly(aryl ether ketone) containing flexible tetra-sulfonated side chains as proton exchange membranes

Abstract

A new difluoride monomer containing electron-rich tetraphenylmethane was designed and synthesized. Based on this monomer along with 4,4′-(hexafluoroisopropylidene)diphenol (6FBPA) and 4,4′-difluorobenzophenone (DFB), a series of tetra-sulfonated poly(aryl ether ketone)s (TS-PAEK-x) were prepared by nucleophilic polycondensation, followed by a sulfonation reaction using chlorosulfonic acid. Tough, flexible, and transparent membranes were obtained by solvent casing. These membranes with ion exchange capacity (IEC) values ranging from 0.85 to 1.43 meq. g⁻¹ exhibited good mechanical properties, excellent dimensional stability, and suitable proton conductivity. The largest swelling ratio (in-plane direction) with a value of 10.3% was observed from TS-PAEK-25 (IEC = 1.43 meq. g⁻¹) membranes at 100 °C, which was much lower than that of Nafion 117 under the same conditions. The highest proton conductivity of 151 mS cm⁻¹ was obtained from the TS-PAEK-25 membrane at 100 °C in the fully hydrated state. Compared to Nafion 117, TS-PAEK-25 with comparable water content exhibited a superior effective proton mobility (μ) value (up to 1.19 × 10⁻³ cm² s⁻¹ V⁻¹). Under reduced humidity conditions, TS-PAEK-25 showed desired conductivity considering its lower IEC level. The results indicate that the TS-PAEK-x membranes are promising candidates for application as proton exchange membranes.