(505g) A Diblock Polymer Alloy Resulting in a C14 Phase Field

Frank-Kasper phases are complex ordered phases with large unit cells that often form in metal alloys. These phases have also been observed as the equilibrium self-assembled structure for packings of spherical micelles in several different types of soft matter. Research identifying new pathways to stabilize Frank-Kasper phases in soft matter and characterizing the driving forces for their formation has led to identification of significant parallels in symmetry breaking behavior between soft and hard matter.

Laves phases, a subset of Frank-Kasper phases, have an AB₂ stoichiometry and tend to form when the A particles are significantly larger than the B particles. Equilibrium Laves phases have been formed in soft matter through techniques that accommodate this particle size asymmetry, like blending with a second component that swells the micelle core. In this work, we sought to stabilize a Laves phase in a diblock polymer system using an approach not yet reported in literature that is more closely analogous to hard matter: blending two micelle-forming diblocks that have incompatible core blocks. We choose diblocks for which the pure component melts form BCC phases with a particle size ratio similar to that of the Laves phase, with the expectation that this particle size ratio will be maintained in the blend because of the core block incompatibility. Using self-consistent mean field theory, we demonstrate that this technique is successful. The eutectic phase diagram for this AB/B’C diblock blend contains a phase field (a non-ideal line compound) for the C14 Laves phase, indicating that this “alloying” approach leads to a large stability window in which C14 is one of two phases in two-phase equilibrium. This computational work suggests that diblock polymer alloying is a simple synthetic route for equilibrium complex phases that may be suitable as a pathway to stabilize heretofore unobserved phases in soft matter.