Phase behavior of lithium perchlorate-doped poly (styrene-b-isoprene-b-ethylene oxide) triblock copolymers

We report on the phase behavior of a system of lithium perchlorate-doped poly(styrene-b-isoprene-b-ethylene oxide) (SIO) triblock copolymers. Two complete phase diagrams are described along the isopleth formed by equal volume fractions of polystyrene and polyisoprene and varying poly(ethylene oxide) contents at ether oxygen-to-lithium ratios of 48:1 and 24:1 between 70 and 200°C. Our results, based primarily on small-angle X-ray scattering (SAXS) data, reveal four phases as a function of PEO and salt content: two-domain lamellae (LAM₂), a pillared lamellar structure (PLS), core-shell cylinders (CSC), and three-domain lamellae (LAM₃). Two microstructures present in the neat material, core-shell gyroid (CSG), and semiperforated lamellae (SPL), are replaced by the CSC phase in the doped systems. In addition, the order-disorder transition temperatures at low PEO content increase dramatically with the addition of salt. Within the CSC phase, LiClO₄ can be dispersed up to a loading of 3:1 without inducing macroscopic phase separation. These findings are rationalized on the basis of the associated modifications to the three segment-segment interaction parameters and PEO chain statistics that accompany the selective mixing of LiClO₄ in the PEO domains, thereby driving microphase separation toward the strong-segregation limit.