Ionic liquid as a structure-directing agent for Frank-Kasper phase formation in block copolymers.

Accessing Frank-Kasper (FK) phases in block copolymers (BCPs) traditionally requires complex molecular designs or polymer blending strategies. In this work, we demonstrate that incorporating the ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), into a cylinder-forming poly(ethylene oxide)-block-poly(1,2-butadiene) (PEO-b-PB) provides a facile and tunable alternative that significantly expands the phase space for complex spherical micelle packings. A diverse array of structures, including σ, A15, and Laves C15 phases, emerges over broad composition and temperature ranges. Notably, we observe thermotropic A15 → σ → C15 and lyotropic σ → A15 → C15 phase transition sequences. Analysis reveals a dual role for the IL: it enhances the effective Flory-Huggins χ parameter through thermally responsive hydrogen bonding with PEO and, at elevated temperatures, redistributes to induce partially dry-brush segregation and micellar size dispersity. These synergistic effects stabilize the FK lattices, enabling reconfigurable complex spherical assemblies in a simple diblock system.