Bicontinuous Double-Diamond Structures Formed in Ternary Blends of AB Diblock Copolymers with Block Chains of Different Lengths

We report on the formation of a bicontinuous double diamond (DD) structure in ternary blends of poly­(styrene-b-isoprene) (SI) diblock copolymers with chains of different lengths merely in a polyisoprene block. Certainly, the materials revealing the DD structure with mesoscopic length scale have strongly been expected due to high potential for applications; however, it is difficult to form owing to thermodynamically evident disadvantages such as wider surface area and larger domain thickness variation, particularly for monodisperse copolymers. The DD structure was successfully formed for ternary blends composed of three parent diblock copolymers; SI-L (M = 190k, φ_s = 0.46), SI-G (M = 151k, φ_s = 0.62), and SI-C (M = 124k, φ_s = 0.73), resulting in covering the overall composition range 0.57 ≤ φ_s ≤ 0.60, where φ_ss denotes volume fractions of polystyrene blocks. Four-branched double network structure with space group symmetry of Pn3̅m was clearly proved by transmission electron microscopy (TEM) observation aided by computer simulation, combined with diffracted data by small angle X-ray scattering. In addition, TEM tomography gave direct information concerning interwoven double networks and four-branching nature of the diamond framework.