Enhancing ion transport in charged block copolymers by stabilizing low symmetry morphology: Electrostatic control of interfaces

Significance We show the enhancement of ion transport properties for charged block copolymers comprising nonstoichiometric ionic liquids by stabilizing the cubic Frank–Kasper A15 phases. The ionic liquid cations predominantly present near the micellar interfaces to increase stabilization energies of the A15 structures if they have strong attractive electrostatic interactions with the charged polymer chains. Unprecedented reentrant phase transitions between lamellar and A15 structures occur through the electrostatic control of interfaces, resulting in radical changes in the conductivity by an order of magnitude. This study is one of the rare demonstrations of a low symmetry morphology to establish a prospective avenue for advanced polymer electrolytes having tailor-made interfaces. Our findings will have implications for energy storage and transfer devices.
Recently, the interest in charged polymers has been rapidly growing due to their uses in energy storage and transfer devices. Yet, polymer electrolyte-based devices are not on the immediate horizon because of the low ionic conductivity. In the present study, we developed a methodology to enhance the ionic conductivity of charged block copolymers comprising ionic liquids through the electrostatic control of the interfacial layers. Unprecedented reentrant phase transitions between lamellar and A15 structures were seen, which cannot be explained by well-established thermodynamic factors. X-ray scattering experiments and molecular dynamics simulations revealed the formation of fascinating, thin ionic shell layers composed of ionic complexes. The ionic liquid cations of these complexes predominantly presented near the micellar interfaces if they had strong binding affinity with the charged polymer chains. Therefore, the interfacial properties and concentration fluctuations of the A15 structures were crucially dependent on the type of tethered acid groups in the polymers. Overall, the stabilization energies of the A15 structures were greater when enriched, attractive electrostatic interactions were present at the micellar interfaces. Contrary to the conventional wisdom that block copolymer interfaces act as “dead zone” to significantly deteriorate ion transport, this study establishes a prospective avenue for advanced polymer electrolyte having tailor-made interfaces.