Structure and Properties of Bicontinuous Microemulsions from Salt-Doped Ternary Polymer Blends

We have systematically examined the phase behavior of lithium salt-doped A/B/AB ternary polymer blends composed of low-molar-mass poly(ethylene oxide) (PEO) and polystyrene (PS) homopolymers, a symmetric PS-b-PEO block copolymer (SO), and lithium bis(trifluoromethane) sulfonimide (LiTFSI) by a combination of small-angle neutron scattering and small-angle X-ray scattering, bolstered by ionic conductivity measurements. The salt partitions exclusively to the PEO and acts to increase the segregation strength of the blends, leading either to macroscopic or microscopic phase segregation. By constraining the volume fractions of the two homopolymers to be the same (φ_PEO+LiTFSI/φ_PS = 1), a two-dimensional phase diagram along the volumetrically symmetric isopleth of the phase prism has been mapped out, and a well-structured bicontinuous microemulsion (BμE) is found over a wider range of total homopolymer composition (ca. φ_H ≈ 80-86%), compared to the neutral polymer case, where a 1-3% range in φ_H is typical. The characteristics of the BμE are obtained via the Teubner-Strey structure factor and are tunable by φ_H, temperature, and salt concentration. Moreover, the BμE possesses superior ionic transport properties, demonstrating higher conductivity compared to both microphase-separated (lamellar) and nonstructured disordered blends. This work offers a strategy to obtain well-defined microstructured ion-containing polymer systems with tunable co-continuous morphology and favorable conductivity properties, which could help optimize the design of polymer electrolytes.