Tailoring Ionic Conductivity of Polymeric Ionic Liquid Block Copolymers through Morphology Control

Block copolymers containing polymeric ionic liquids (PILs) can potentially combine high ionic conductivity and mechanical robustness. However, recent work demonstrates that the ionic conductivity of a model lamellar material is significantly depressed relative to expectations based on the measured properties of a PIL homopolymer (Coote et al., ACS Polymers Au, 3, 2023). Herein, the factors that control the ionic conductivity of this block copolymer chemistry are interrogated through systematic variations in morphology, where ionic conductivity is measured with a configuration that is insensitive to restructuring of the block copolymer at the electrode surface. The principal reason for the depressed ionic conductivity of lamellar phases at intermediate-to-strong segregation strength is defects that disrupt the long-range continuity of ionic domains, and a secondary reason is the elevated glass transition temperature (Tg) of the PIL domains due to the high-Tg nonionic domains. Transport-blocking defects are reduced by decreasing the molecular weight to achieve a weakly segregated state or by increasing the molecular weight to suppress diffusion and trap a morphology with only short-range order. We further show that transport-blocking defects are largely absent from PIL-rich morphologies having nonionic cylindrical or spherical domains embedded in a PIL matrix. The methodology outlined in this work offers a simple approach to identify the physics that control the bulk ionic conductivity of block copolymeric ionic liquids, providing critical information that can guide the design of such materials for target applications.