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 cylindrical) and lamellar morphologies, as well as complex morphologies (e.g., gyroid*, lamellar catenoid, and hexagonally perforated lamellae), can be selectively accessed through either tailored molecular synthesis or copolymer/homopolymer blends. In the present work. control over ultimate morphology is achieved through the use of binary copolymer blends composed of two strongly-segregated poly(styrene-b-isoprene) (SI) diblock copolymers of equal molecular weights but different compositions (one 50 wt % S and the other 85 wt % S). Blend morphologies are examined by electron microscopy (including three-dimensional imaging) and small-angle X-ray scattering, and a theoretical framework is proposed to describe strongly-segregated copolymer blends exhibiting the lamellar morphology. Results obtained here indicate that diblock copolymer blends of a given bulk composition may exhibit coexisting morphologies but in general behave as single-phase diblock copolymers of equal molecular composition, suggesting that such blends offer an alternative, and attractive, route by which to generate a desired morphology.