Control of Nanostructure in Mixtures of Block Copolymers:  Curvature Control via Cosurfactant Effects

We aim to elucidate the cosurfactant effects, which are expected when junctions of two block copolymers share a common microdomain interface, on morphology and phase behavior of mixtures. Especially this paper addresses the effects involved for binary mixtures composed of polystyrene- block -polyisoprene having about equal molecular weights but complementary compositions, one forming polystyrene (PS) cylinders in polyisoprene (PI) matrix and the other forming PI cylinders in PS matrix. Transmission electron microscopy and small-angle X-ray scattering were used to characterize the phase behavior and domain spacing of the binary mixtures. First, we found an expanding composition range for hexagonally packed cylindrical morphology and a narrowed composition range for lamellae relative to the corresponding composition ranges for neat SI block copolymer under a strong segregation condition. The result indicates that the cosurfactant effects help a block copolymer to take its spontaneous curvature. Second, it was found that the effects enlarged the domain size and interdomain distance of the binary mixtures. Those results were compared with the theory by Birshtein and co-workers, which is proposed to describe the microdomain morphology for strongly segregated binary block copolymers. We found good agreements between experimental and theoretical results in terms of (i) domain size, (ii) interdomain distance, and (iii) the blending compositions where morphological transitions occur.