Polymer electrolyte membranes based on poly(m-phenylene)s with sulfonic acid via long alkyl side chains

Abstract

Polymeric electrolyte membranes (PEM) based on the cross-linked poly(m-phenylene)s with sulfonic acid via long alkyl side chains (c-SPMPs) were successfully obtained by the reaction of the phenyl groups in the sulfonated poly(m-phenylene)s (SPMPs) and 4,4′-methylene-bis[2,6-bis(hydroxyethyl)phenol] (MBHP) as the cross-linker in the presence of methanesulfonic acid. The c-SPMPs produced transparent membranes with a relatively high mechanical property in the dry state, regardless of their high ion exchange capacity (IEC) values. The rigid main chains and the cross-linking structure suppressed the unacceptable water uptake and dimensional change of the c-SPMPs membranes even in the hydrated state. The c-SPMP8-2 membrane with IEC = 2.93 mequiv g⁻¹ showed a higher proton conductivity than Nafion 117 over a wide range of relative humidities (30–95%) at 80 °C. The mean distance between the domain of the hydrophobic main chains and that of the hydrophilic sulfonic acid groups was determined by the wide-angle X-ray diffraction (WAXD) measurements, supporting the formation of proton channels based on phase separation. The c-SPMP membranes might be promising alternatives of the perfluorinated PEM materials based on both advantageous features of a high proton conductive performance and straightforward synthetic method.