Electrostatic Stabilization of Low Symmetry Morphology in Block Copolymer Electrolytes

In present study, we investigated a role of electrostatic interactions in phase behavior of block copolymer electrolytes. We prepared two block copolymers having identical molecular weights and block composition, but tethered with different types of acid functional moieties. With added various nonstoichiometric ionic liquids, we observed rich phase transitions among lamellae, hexagonal cylinder, and A15 structures. Intriguingly, radical phase shifts between lamellar and A15 structures were obtained with small changes in the volume fraction of ionic domains. Key to this stems from the amount of ionic liquid cations in the block copolymer/ionic liquid mixtures upon inducing dissimilar strength of electrostatic interactions. Notably, the type of acid functional group in polymer matrix played a crucial role in determining the stability window of the A15 structures, which becomes widened when the acid function groups are strongly interacting with ionic liquid cations. Computational simulations revealed that the thin ionic shell layers were formed at the micellar interfaces, which is the key to stabilizing anisotropic micelles to develop the low symmetry morphology. The resultant A15 phases of block copolymer electrolytes enhanced ion transport efficiency with 3-dimensionally connected ion-conducting pathways.