Degradation of radiation grafted anion exchange membranes tethered with different amine functional groups via removal of vinylbenzyl trimethylammonium hydroxide

Low-density polyethylene (LDPE)-based anion exchange membranes (AEMs) with 65% degree of grafting of vinylbenzyl chloride (VBC) were tethered with different amine functionalities namely, trimethyl amine (TMA), 1,4-diazabicyclo[2.2.2]octane (DABCO), 1-azabicyclo[2.2.2]octane (ABCO) and N-methylpiperidine (NMP), and were subjected to degradation test by immersing the OH− exchanged AEMs in deionised water at 60 °C, a condition analogous to fuel cell and electrolyser environment. All the quaternised membranes, regardless of the tethered amine functional group, exhibited similar degradation loss of ca. 5% IEC per month. Benzylic peroxide was detected in the degradation solution in all the tested AEMs. The observed degradation of the OH− exchanged AEMs was mainly attributed to peroxide and hydroxide radical attacks on the ternary (benzylic) carbon resulting in the release of vinylbenzyl trimethylammonium hydroxide (VBTMA) as a whole which was also detected. The degradation test performed on TMA-functionalised membrane under nitrogen, oxygen and 3 wt% H2O2 showed similar degradation products namely benzylic peroxide and VBTMA suggesting that the degradation mechanisms under these three conditions are similar. The over three-fold increase in the degradation rate under oxygen saturated solution compared to nitrogen is due to the higher peroxide concentration produced from oxygen reduction to superoxide via ylide.