PHASE-BEHAVIOR OF ORDERED DIBLOCK COPOLYMER BLENDS - EFFECT OF COMPOSITIONAL HETEROGENEITY

Diblock copolymers order into a variety of periodic morphologies when the constituent blocks are sufficiently incompatible. Previous studies have demonstrated that classical dispersion (spherical and cylindrica l) and lamellar morphologies, as well as complex morphologies (e.g., g yroid, lamellar catenoid, and hexagonally perforated lamellae), can b e selectively accessed through either tailored molecular synthesis or copolymer/homopolymer blends. In the present work. control over ultima te morphology is achieved through the use of binary copolymer blends c omposed of two strongly-segregated poly(styrene-b-isoprene) (SI) diblo ck copolymers of equal molecular weights but different compositions (o ne 50 wt % S and the other 85 wt % S). Blend morphologies are examined by electron microscopy (including three-dimensional imaging) and smal l-angle X-ray scattering, and a theoretical framework is proposed to d escribe strongly-segregated copolymer blends exhibiting the lamellar m orphology. Results obtained here indicate that diblock copolymer blend s of a given bulk composition may exhibit coexisting morphologies but in general behave as single-phase diblock copolymers of equal molecula r composition, suggesting that such blends offer an alternative, and a ttractive, route by which to generate a desired morphology.