Molecular-weight factors affecting formation of the OBDD morphology in block copolymer blends

Block copolymers undergo self-organization when the blocks are sufficiently incompatible, and generate a variety of periodic morphologies in the limit of strong segregation. An equilibrium morphology only recently added to the diblock copolymer phase diagram is the ordered bicontinuous double-diamond (OBDD) morphology, which possess a Pn3m space group. It has been observed over a very narrow composition range (ca. 4 vol%), thereby making it difficult to obtain in pure copolymers. This obstacle can, however, be surmounted by blending a copolymer with one of the parent homopolymers. In the present study, several symmetric poly(styrene-b-isoprene) diblock copolymers varying in molecular weight (M) have been blended with homopolystyrene to produce the OBDD morphology. Transmission electron microscopy is employed here to identify the morphologies in cryosections of each blend and reveals that, at intermediate molecular weights, the OBDD morphology is indeed observed. At low M, near the order-disorder transition, however, a lamellar catenoid or disordered morphology is preferred. At the other extreme, high-M blends are frustrated by molecular entanglements and adopt a cylindrical morphology.