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

Area of Science:

• Materials Science
• Polymer Chemistry
• Soft Matter Physics

Background:

• Accessing complex Frank-Kasper (FK) phases in block copolymers (BCPs) typically involves intricate molecular engineering or polymer blending.
• Existing methods present limitations in achieving diverse FK phase structures and tunability.

Purpose of the Study:

• To demonstrate a facile and tunable method for accessing complex FK phases in BCPs.
• To expand the phase space of spherical micelle packings using ionic liquids.
• To investigate the role of ionic liquids in stabilizing FK lattices.

Main Methods:

• Incorporation of 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) system.
• Exploration of phase behavior across various compositions and temperatures.
• Analysis of the IL's dual role in modifying polymer interactions and morphology.

Main Results:

• Observation of diverse FK phases, including σ, A15, and Laves C15, over broad composition and temperature ranges.
• Identification of thermotropic (A15 → σ → C15) and lyotropic (σ → A15 → C15) phase transition sequences.
• Demonstration of the IL enhancing the effective Flory-Huggins parameter (χ) and inducing dry-brush segregation.

Conclusions:

• The ionic liquid [EMIM][TFSI] acts as a versatile additive for stabilizing complex FK phases in a simple diblock copolymer system.
• This approach provides a tunable and facile route to reconfigurable complex spherical micelle assemblies.
• The findings offer new possibilities for designing advanced materials with ordered nanostructures.