Process-Directed Self-Assembly of the Frank-Kasper A15 Structure in Linear, Conformationally Symmetric Block Copolymers.

Protracted equilibration times and a multitude of competing periodically modulated structures are common characteristics of complex, self-assembled phases in block copolymer materials. These characteristics highlight the importance of designing processes to reproducibly direct the structure evolution of complex, spatially modulated structures. Using general symmetry considerations, we design a process that deterministically transforms the common, double-gyroid (DG) phase of linear diblock copolymers into a metastable A15 structure by irreversibly switching a conformationally symmetric ABB^{'} diblock copolymer into an ABA^{'} triblock copolymer. Key to the fabrication of the Frank-Kasper A15 structure is the nonmonotonic time evolution of the segregation that allows us to partition the large unit cell of the DG phase into 8 equivalent A15 units. Comparison between dynamic self-consistent field theory and particle-based simulations demonstrates the robustness of the designed pathway against thermal fluctuations and memory effects due to the underlying Rouse dynamics. Metastable structures that can be accessed from the DG phase are systematically explored as a function of molecular asymmetry and incompatibility.