Coupling of Ethylene-Oxide-Based Polymeric Network Structure and Counterion Chemistry to Ionic Conductivity and Ion Selectivity

Polymer networks are important constituents of ion separation membranes and battery electrolytes. In such systems, understanding the coupling of the network structure to ion transport is important to guide network design. However, a comprehensive understanding of how polymer network variations affect segmental relaxation and ion transport is still lacking. A series of single-anion-conducting polymer networks was synthesized with a controlled crosslinking density, ethylene oxide (EO) side chain length, tethered cationic monomer concentration, and mobile counteranion size. From dielectric spectroscopy, segmental relaxation times were obtained and found to vary by orders of magnitude across the investigated crosslinking densities and side chain lengths. Ionic conductivity is found to be coupled with segmental relaxation, which slowed with an increase in the number of crosslinks, whereas Young's moduli of the networks are found to be most coupled with the crosslinking density. Longer side chains provide faster segmental relaxation but do not impede the mechanical strength generated by crosslinks, showing an approach toward designing networks with both high moduli and ionic conductivities. Using a similar network with added lithium salts, Li+ transport selectivity is enhanced by weak interactions between Li+ and large anions, as well as higher crosslinking densities.