Particle Packings in Bidisperse Diblock Copolymer Blends*

Recent investigation into the phase behavior of compositionally asymmetric diblock copolymers has resulted in the discovery of a host of previously unanticipated particle packings, including a dodecagonal quasicrystal and several Frank-Kasper (FK) phases. Conformational asymmetry has been shown to favor these fascinating morphologies. However, the number of sufficiently conformationally asymmetric systems available for study is limited. An alternative approach, simply blending two diblocks, has been predicted by self-consistent field theory to allow access to these FK phases, potentially enabling study of these morphologies in a wider range of systems. In this work, we investigated the phase behavior of bidisperse blends of polystyrene-b-poly(1,4-butadiene); the nominally single component diblock copolymer melts show no evidence of FK phase formation. The core block length and relative volume fraction of each polymer was varied, keeping constant the length of the corona block. Small angle X-ray scattering and transmission electron microscopy revealed a rich phase behavior, providing a new means of accessing various particle-packings in diblock copolymer melts.