Multiscale Tortuous Diffusion in Anion and Cation Exchange Membranes

A fundamental understanding of water transport and morphology is critical for improving ionic conductivity in polymer membranes. In a series of random copolymer anion exchange and cation exchange membranes, we systematically investigate the influence of counterion type, side chain type, and degree of ionic functionalization on water transport using NMR diffusometry. Time-dependent water diffusion measurements reveal micrometer-scale heterogeneity of the hydrophilic network in these random copolymers. We propose a model in which the hydrophilic domain network in these membranes has micrometer-scale distributions of local nanometer-scale dead ends, leading to changes in tortuosity as a function of water content and membrane composition. We furthermore parse tortuosity into two length-scale regimes, one regime from nanometer (local) to bulk and another from micrometer to bulk, offering enhanced discrimination of the multiscale morphological structures that influence bulk transport. This study thus provides new insights into ionic polymer membrane morphology and diffusion behavior, with the ultimate goal of controlling polymeric materials for enhanced fuel cells and other separations applications.