Anomalous Thermotropic Cylinder-to-Gyroid Transition in Polystyrene- block -poly(ethylene oxide)/Ionic Liquid Blends

Accessing complex morphologies in polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymers at high PEO volume fractions is often hindered by PEO crystallization and relatively weak segregation strength. Here, we report an anomalous thermotropic transition from hexagonally packed cylinders (HEX) to a double gyroid (DG) network in PEO-rich PS-b-PEO/ionic liquid (IL) blends, where a strongly PEO-selective IL suppresses crystallization, swells the PEO domains, and increases the effective segregation strength (χeff) with increasing IL concentration. At low temperatures, strong PEO-IL hydrogen-bonding interactions promote anisotropic conformations of the PEO blocks and stabilize a frustrated HEX morphology under vitrified PS domains. Upon heating above the glass transition temperature of the PS block (Tg,PS), increased segmental mobility enables chain relaxation, while possible weakening of directional PEO-IL interactions may further reduce the tendency toward locally constrained chain extension. The swollen PEO/IL domain is consequently accommodated through an increase in the interfacial area per chain (Σ) and reduced chain stretching. The cylindrical geometry cannot accommodate this enlarged Σ without imposing severe packing constraints on the PS domains. The system therefore undergoes a HEX→DG transition, wherein the bicontinuous DG morphology redistributes interfacial area and chain conformations in three dimensions to relieve packing frustration under incompressibility. The DG structure can be kinetically trapped upon cooling, resulting in a robust three-dimensional ion-conducting network that exhibits higher conductivity than the cylindrical and lamellar phases.