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

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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 ≤ fO ≤ 0.3, situated between two-domain and three-domain lamellae, where fO represents the volume fraction of O blocks. Two cubic network phases, Q230 (core−shell double gyroid, Ia3̄d space group symmetry) and Q214 (alternating gyroid, I4132 space group symmetry), flank an orthorhombic network phase, denoted O70 (Fddd space group symmetry), which is positioned around the isopleth composition fS ≈ fI. These results provide a powerful strategy for designing network phases in linear ABC triblock copolymers when χAB ≈ χBC < χAC, where χ represents the Flory−Huggins interaction parameter.

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