Ordered states in diblock copolymer blends

When cooled below their order-disorder transition, diblock copolymers adopt an ordered, nanostructured state that optimizes the competition between interfacial tension and chain stretching subject to the constraint of constant density. This presentation will discuss predictions from self-consistent field theory (SCFT) for blends of AB and B'C diblock copolymers, where the prime indicates a (potentially) different degree of polymerization, for the case where the segregation strength between the A and C blocks is strong. In such a system, ordered states that could be produced by blending must outcompete macrophase separation into AB-rich and B'C-rich phases. The first part of the presentation will focus on cases where the diblock copolymers produce a body-centered cubic phase in their neat melt state. We predict the emergence of a Laves phase that appears as a non-ideal line compound state at strong segregation. The complete phase diagram of this "block copolymer alloy" bears a striking resemblance to metallic alloys. The second part of the presentation will focus on systems where the two diblock copolymers form double-gyroid network phases in their neat melt state. An alternating gyroid morphology is predicted to be metastable in such a blend, but may be realizable via solvent annealing owing to the small free energy difference with the macrophase separated state. Addition of a miniscule amount of ABC tetrablock terpolymer stabilizes the alternating gyroid through a surfactant-like interaction that bridges between the A-network and the C-network. A common theme that emerges in this work is a focus on destabilizing the macrophase separated states to produce robust blended morphologies.