Binary blends of a lamellar poly(styrene‐b‐isoprene) diblock copolymer and homopolystyrenes of various molecular weights self‐assembled into highly ordered morphologies upon solvent evaporation. The equilibrium blend morphologies were studied by transmission electron microscopy and small angle x‐ray scattering. The type of ordered morphology (lamellae, the ordered bicontinuous double diamond morphology, cylinders on a hexagonal lattice, and spheres on a cubic lattice) observed in each blend was generally dictated by the overall polystyrene volume fraction in the blend, i.e., the homopolymer concentration. The blend morphology also depended on the homopolymer molecular weight at a single fixed composition. The intermaterial dividing surface between the polystyrene and the polyisoprene microdomains was characterized by the interfacial mean curvature (H) and the average area per copolymer junction (σj). H and σj increased as the homopolymer concentration increased and/or as the homopolymer molecular weight decreased. These trends were generally obeyed both between and within ordered morphology types. The size of the microdomains was readily controlled by the homopolymer molecular weight.

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