MOLECULAR-WEIGHT FACTORS AFFECTING FORMATION OF THE OBDD MORPHOLOGY IN BLOCK-COPOLYMER BLENDS

Block copolymers undergo self-organization when the blocks are suffici ently incompatible, and generate a variety of periodic morphologies in the limit of strong segregation. An equilibrium morphology only recen tly added to the diblock copolymer phase diagram is the ordered bicont inuous double-diamond (OBDD) morphology, which possesses 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. Thi s obstacle can, however, be surmounted by blending a copolymer with on e of the parent homopolymers. In the present study, several symmetric poly(styrene-b-isoprene) diblock copolymers varying in molecular weigh t (M) have been blended with homopolystyrene to produce the OBDD morph ology. Transmission electron microscopy is employed here to identify t he morphologies in cryosections of each blend and reveals that, at int ermediate molecular weights, the OBDD morphology is indeed observed. A t low M, near the order-disorder transition, however, a lamellar caten oid or disordered morphology is preferred. At the other extreme, high- ill blends are frustrated by molecular entanglements and adopt a cylin drical morphology. (C) 1994 Wiley-Liss, Inc.