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

Abstract

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 E a (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. • 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.