Enhancing desalination performance by manipulating block ratios in a polyethylene-based triblock copolymer anion exchange membrane for electrodialysis

Development of improved durable, conductive, and selective anion exchange membranes (AEMs) are critical for electrodialysis. A series of three ABA triblock copolymers derived from polychloromethylstyrene-b-polyethylene-b-polychloromethylstyrene (PCMS-b-PE-b-PCMS), designated as large, 0.8:1, medium, 0.2:1, and small 0.1:1, block AEMs by their PCMS:PE block size were quaternized with methylpiperidine. The water uptake decreases from 100% for the large block AEM to 5% for the small block AEM. At this low water content in liquid water, the small block AEM shows complete ion dissociation, with similar diffusion (2.6 × 10−6 mol m−2 s−1) and Ea (24 kJ/mol) as the larger block AEMs. However, a 34% increase in apparent permselectivity was achieved with the small block AEM. In electrodialysis, the small block AEM improved salt removal by 300% compared with a commercial AEM. Under further investigation, migration was shown to be enhanced by 600% in the small block AEM. The small block AEM has a λ = 5 in liquid water, which is too low to explain the chloride conductivity behavior. Furthermore, the permselectivity in the small block AEM is much higher than expected from its small-angle X-ray scattering morphology, indicating there must be a water-rich region of smaller dimensions where cation/anion pairs are fully solvated. [Display omitted] •A triblock copolymer AEM was developed with decreasing hydrophilic block lengths.•Small block has low water uptake and charge density, but full ion dissociation.•Small block achieves enhanced permselectivity and migrative flux.•Small block improves electrodialysis by nearly 300x compared to large block.