Thermal Crosslinking of an Alkaline Anion Exchange Membrane Bearing Unsaturated Side Chains

We report a facile new route for the synthesis of self-crosslinked anion exchange membranes (AEM) without the need for any crosslinkers or catalysts. The soluble copolymers bearing flexible side chains, with alkene pendant groups, were synthesized via the Menshutkin reaction. The crosslinked derivatives were then prepared by the thermal crosslinking of the unsaturated side chains during the membrane formation process. 1H NMR was used to determine the content of available alkene groups before crosslinking, while in situ FTIR spectroscopy was used to confirm successful thermal crosslinking. This approach, distinct from the use of classical post-crosslinking processes, installs ionic cross-links (bearing trimethyl quaternary ammonium hydroxide functionality) between the polymer chains. This is to mitigate against excessive water uptakes and dimensional swelling on hydration (extremely low swelling ratio of 2.5% in-plane and 1.2% through-plane 30 oC), while retaining a high concentration of charge carriers (ion-exchange capacity) for target hydroxide conductivities. Additionally, the self-crosslinking strategy, and resulting dense crosslinked network, has the additional advantage of protecting the quaternary ammonium groups from hydroxide ions attack. The strategy produced an AEM that yielded a peak power density of 42 mW cm–2 in a H2/O2 fuel cell at 60 oC.