A Polyethylene-Based Triblock Copolymer Anion Exchange Membrane with High Conductivity and Practical Mechanical Properties

A challenge in anion exchange membrane (AEM) development is simultaneously optimizing alkaline chemical stability and mechanical integrity during thermal and humidity cycling and achieving high anionic conductivity. Here, we report on the hydrogenation of an ABA triblock copolymer polychloromethylstyrene-b-polycyclooctene-b-polychloromethylstyrene (PCMS-b-PCOE-b-PCMS) to yield a polyethylene-based triblock copolymer, polychloromethylstyrene-b-polyethylene-b-polychloromethylstyrene (PCMS-b-PE-b-PCMS). A polydisperse midblock was synthesized with narrowly disperse outer blocks to favor nanoscale phase separation and promote an interconnected morphology. Varying degrees of chemical cross-linking of the PCMS domains were achieved by using different processing temperatures to tune the water uptake and dimensional swelling. Quaternization with either trimethylamine or methylpiperidine resulted in AEMs with improved characteristics, including excellent Cl- and OH- conductivity (119 and 179 mS cm(-1) at 80 degrees C, respectively) and moderate water uptake (33 wt %, lambda = 12). Unexpectedly, extensional testing indicated that the mechanical strength of the film improved upon hydration. Wide-angle X-ray scattering revealed that in the presence of liquid water the PE backbone rearranges and forms larger crystalline domains, which led to the improved stress at break. These fundamental mechanistic insights are of critical importance in designing mechanically robust AEMs for aqueous applications such as electrolysis and reverse electrodialysis. This work demonstrates the applicability of tunable block copolymer systems for developing practical AEM materials for more modest pH liquid applications. Numerous tunable variables including chemical cross-linking and semicrystalline variability highlight how mechanical integrity, water management, and ionic conductivity can be simultaneously achieved.