Synthesis and properties of novel sulfonated polyimides from 4,4′-(biphenyl-4,4′-diyldi(oxo))bis(1,8-naphthalic anhydride)

Abstract

A series of benzimidazole-groups-containing copolyimides (PIs) have been synthesized by random condensation copolymerization of 4,4′-(biphenyl-4,4′-diyldi(oxo))bis(1,8-naphthalic anhydride) (BPNDA), 1,3-bis(4-aminophenoxy)benzene (BAPBz), and 2-(4-Aminophenyl)-5-aminobenzimidazole (APABI) in m-cresol in the presence of benzoic acid and isoquinoline at 180 °C for 20 h. The resultant PIs were subsequently post-sulfonated in concentrated sulfuric acid at 50 °C for 24 h to yield the desired sulfonated copolyimides (SPIs). No significant polymer degradation occurred during the process of sulfonation and tough membranes (tensile stress: 50–83 MPa) of the SPIs were obtained by solution casting. To reduce the swelling ratio, the SPI membranes were further covalently cross-linked by immersing in polyphosphoric acid at 180 °C for 14 h. The resulting covalently cross-linked membranes (CSPIs) displayed significantly reduced swelling ratio, while high ion exchange capacities (IECs) were maintained. The CSPI membranes exhibited comparable proton conductivities to that of Nafion112® in their fully hydrated state. The Fenton's test results suggest good radical oxidative stability of the CSPI membranes due to the synergic action of the covalent cross-linking and the presence of benzimidazole groups in polymer main chains. The CSPI membranes also showed excellent hydrolytic stability due to the covalent cross-linking as well as the low electron affinity of the BPNDA moiety making them good candidate for fuel cell applications.