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 (EMIMTFSI), 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. Achieving Frank-Kasper phases in block copolymers is challenging, as is control over these morphologies. Here, the authors report the use of ionic liquids for direction of assembly to give control over micelle packing to obtain range of phases and transitions.