Morphology-Dependent Ionic Conductivity in Block Copolymers Based on Polymer Ionic Liquids.

Block copolymer electrolytes (BCPs) made of nonionic polymers linked to polymer ionic liquids (PILs) are a promising class of electrolytes for energy storage and conversion devices. However, our previous studies have shown that the bulk ionic conductivity of certain lamellae-forming materials is significantly lower than the expected values based on the ionic conductivity of the PIL homopolymer, the composition of the BCP, and the self-assembled morphology of the BCP. We hypothesized that this lower ionic conductivity may be due to poor connectivity among ionic domains in the lamellar phase, diffused block interface, or the difference in glass transition temperature between the PIL homopolymer and PIL block in the block copolymer. To test this hypothesis, we examined the ionic conductivity in highly asymmetric (spherical and cylindrical morphologies) and lamellar BCPs with a majority PIL phase. We found that the normalized ionic conductivity in spherical and cylindrical BCPs met or exceeded the predicted values. Interestingly, the poorly ordered lamellar block copolymer exhibited higher ionic conductivity than well-ordered lamellar block copolymers.