Ordered Network Phases in Linear Poly(isoprene-<i>b</i>-styrene-<i>b</i>-ethylene oxide) Triblock Copolymers

Abstract

The equilibrium phase behavior of 43 linear poly(isoprene- b -styrene- b -ethylene oxide) (ISO) triblock copolymer melts, with molecular weights that place these materials near the order−disorder transition, is reported. Ordered phase morphologies were characterized using small-angle X-ray scattering, transmission electron microscopy, dynamic mechanical spectroscopy, and static birefringence measurements. Interpretation of these results was aided by a modeling technique that facilitates resolution of reciprocal and real-space experimental data, leading to definitive three-dimensional morphological structures. Three distinct multiply continuous network morphologies are identified across a range of compositions between 0.1 ≤ f O ≤ 0.3, situated between two-domain and three-domain lamellae, where f O represents the volume fraction of O blocks. Two cubic network phases, Q 230 (core−shell double gyroid, Ia 3̄ d space group symmetry) and Q 214 (alternating gyroid, I 4 1 32 space group symmetry), flank an orthorhombic network phase, denoted O 70 ( Fddd space group symmetry), which is positioned around the isopleth composition f S ≈ f I . These results provide a powerful strategy for designing network phases in linear ABC triblock copolymers when χ AB ≈ χ BC < χ AC, where χ represents the Flory−Huggins interaction parameter.