(39c) Cosurfactant Effect On the Emergence of Bicontinuous Phases in Binary Diblock-Copolymer Blends

Bicontinuous phases, exhibiting self-assembled microdomains with two intertwined and non-intersecting labyrinths, are of great technological interest for templating nanoporous materials with large interfacial area and isotropic continuous transport pathways. The phase behavior of binary blends composed of a long asymmetric diblock and a short symmetric diblock with three different size ratios are investigated using a synergistic approach between self-consistent field theory and particle-based simulations. The model diblock copolymer considered has conformational asymmetry between the constituent monomers resembling that of polyisoprene-block-polystyrene. Both, theory and simulations predict a phase diagram in qualitative agreement with experiment. The gyroid morphology is shown to correspond to the bicontinuous phase previously unidentified in experiment; other co- and bicontinuous metastable phases are also obtained spontaneously in the simulations. A detailed analysis of the cosurfactant effect, i.e., distribution of the two different diblock chains across the bicontinuous interface, and its relation to the local interfacial curvature is presented. The cosurfactant effect on the thermodynamic stabilization of the gyroid phase is also analyzed in terms of the the dynamic balance between interfacial fluctuations and competing diffusion (redistribution) of both diblocks at the common interface.