Gyroid Morphologies in Single-ion Conducting Multiblock Copolymers: A Phase Diagram and Efficient Ion Transport

We investigated the nanoscale morphologies and ionic conductivities of polyethylene-based multiblock copolymers as single-ion conducting polymers. These polymers contain short polar blocks with a single sodium sulfonate group separated by polyethylene blocks of fixed length (PESxNa, x = 10, 12, 18, 23, 48). At room temperature, these polymers exhibit layered ionic aggregates with semicrystalline polyethylene backbones. For PES12Na, PES18Na, and PES23Na, the layered morphologies transition into gyroid morphologies upon melting of the polyethylene blocks, and further transition into hexagonal morphologies at higher temperatures. In contrast, PES10Na and PES48Na exhibit layered to hexagonal transitions at the melting temperature, without an intermediate gyroid morphology. The phase diagram of these PESxNa polymers is well-matched with the conventional diblock copolymers and identifies the presence of gyroid morphologies at the volume fraction of polar blocks 0.27 - 0.41. Ionic conductivities reveal the faster ion transport through bicontinuous gyroid than hexagonal morphologies, and its relationship to the interaggregate spacings. This new phase diagram develops design strategies for single-ion conducting polymers with a bicontinuous ionic aggregate and efficient ion transport.