Azide-assisted self-crosslinking of highly ion conductive anion exchange membranes

Abstract

Poly(2,6-dimethyl-phenylene oxide)s (PPO)s with thermally crosslinkable azide groups and “side-chain-type” architecture containing quaternary ammonium and triazole groups were synthesized by Cu-catalyzed “click chemistry”. Tough and flexible anion exchange membranes (AEMs) were prepared by solution casting at 70°C. By simple thermal crosslinking at 135°C for 18h, controllably crosslinked PPO AEMs were obtained. Key properties of the crosslinked AEMs, such as ion exchange capacity, thermal stability, water uptake, hydroxide conductivity and alkaline stability were investigated. The crosslinked membranes maintained high hydroxide conductivities despite their extremely low water uptake (up to 57.5mScm–1 at room temperature, water uptake of 34.5wt%). The unusually low water uptake and good hydroxide conductivity may be attributed to the crosslinked, “side-chain-type” structures of pendent triazole and cationic groups, which facilitate ion transport. Moreover, the crosslinked membranes exhibited excellent alkaline stability, and retained over 65% of hydroxide conductivity in 5M or 10M of NaOH at 80°C for 400h. It is assumed that crosslinking decreases the water uptake by polymer chain compaction, thus reducing the nucleophilic attack of water or hydroxide at the cationic center. Thus, triazole-containing AEMs with “side-chain-type” architecture appear to be very promising candidates with good stability as high performance anion exchange membrane.