High performance quaternary ammonium-functionalized polybenzoxazine-based anion exchange membranes

Two kinds of benzoxazine monomers were synthesized by Mannich reaction with N,N-dimethyl-p-phenylenediamine and N,N-dimethylethylenediamine using polyformaldehyde, 4,4′-dihydroxydiphenylmethane as raw materials. Subsequently, quaternary ammonium functionalization was realized by Menshutkin reaction. The aromatic amine-based quaternary ammonium-functionalized polybenzoxazine (AQPBZ) and aliphatic amine-based quaternary ammonium-functionalized polybenzoxazine (LQPBZ) were synthesized by ring-opening polymerization. Polybenzoxazine anion exchange membrane (AEM) was prepared by solution casting method. The AQPBZ and LQPBZ AEMs exhibit micro-phase separated morphology clearly. LQPBZ AEM displays more large ion clusters and ion conductive channels due to the flexible alkyl connection between QA group and polybenzoxazine skeleton. AQPBZ and LQPBZ AEMs have the ionic conductivity of 75.5 mS/cm and 86.3 mS/cm at 80 °C and 100% relative humidity, respectively. AQPBZ and LQPBZ AEMs show excellent alkaline stability. After treated with 6 M sodium hydroxide solution at 80 °C for 240 h, they manitained the 49.8 mS/cm and 50.2 mS/cm, respectively. They also exhibit good oxidation stability and thermal stability. Their oxidation weight losses are 4.1% and 7.2% through Fenton test, respectively. Their Td5s are 325 °C and 289 °C, respectively.