New Polyethylene Based Anion Exchange Membranes (PE–AEMs) with High Ionic Conductivity

This paper discusses a new class of high performance polyethylene-based anion exchange membranes (PE–AEMs) that contain a wide concentration range of pendant (flexible) ammonium chloride (NR₃⁺Cl^–) groups and with or without a cross-linked PE matrix structure. The chemistry involves a metallocene-mediated polymerization of ethylene, silane-protected α,ω-amino-olefin [C_xN(SiMe₃)₂], with or without styrenic diene (cross-linker), to form ethylene/C_xN(SiMe₃)₂ copolymers and ethylene/C_xN(SiMe₃)₂/diene terpolymers, respectively. The resulting co- and ter-polymers were completely soluble in common organic solvents and were solution-casted into uniform films (thickness, 50–70 μm; without backing material) and then thermal cross-linked in ethylene/C_xN(SiMe₃)₂/diene case, further interconverting the silane-protected amino groups into the desired −NR₃⁺Cl^– groups (R: H, CH₃, and C₃H₇) under solid state conditions. The resulting PE–NR₃⁺Cl^– and cross-linked x-PE–N(CH₃)₃⁺Cl^– membranes were systematically studied to understand how the PE structure (−NR₃⁺Cl^– concentration, R group, cross-linking density, etc.) affects ionic conductivity, water uptake, film stability, and ion selectivity. For comparison, several commercially available AEMs were also examined. Evidently, an x-PE–N(CH₃)₃⁺Cl^– membrane, with 28.1 mol % −N(CH₃)₃⁺Cl^– groups and 0.2 mol % cross-linkers, shows moderate water swelling and outperforms all commercial membranes with exceptionally high ionic conductivities of 119.6 mS/cm in 2 N HCl solution and 78.8 mS/cm in 2 N HCl–0.2N CuCl solution at room temperature.