Structure–Property Relationships in Single-Ion Conducting Multiblock Copolymers: A Phase Diagram and Ionic Conductivities

Abstract

We investigated the nanoscale morphologies and ionic conductivities of polyethylene-based multiblock copolymers as single-ion conducting polymer electrolytes. These polymers contain short polar blocks with a single sodium sulfonate group separated by polyethylene blocks of fixed length (PESxNa, x = 10, 12, and 18). At room temperature, these multiblock copolymers exhibit layered ionic aggregates with semicrystalline polyethylene backbones. For PES12Na and PES18Na, the layered ionic aggregate morphologies transition into Ia3̅d gyroid morphologies upon melting the polyethylene blocks and further transition into hexagonal morphologies at higher temperatures. With a shorter polyethylene block, PES10Na exhibits a layered to hexagonal transition at the melting temperature, without an intermediate gyroid morphology. The phase diagram of these PESxNa polymers is reminiscent of conventional diblock copolymers and identifies the presence of gyroid morphologies at a polar block volume fraction of ∼0.27 to 0.41, which is broad compared to typical diblock copolymers. Temperature-dependent ionic conductivities reveal faster ion transport through bicontinuous gyroids than hexagonal ionic aggregate morphologies and a relationship between conductivity and the characteristic distance between ionic aggregates. This study presents material design strategies for single-ion conducting polymers with a bicontinuous ionic aggregate and toward efficient ion transport.