Effect of an Ionic Liquid Solvent on the Phase Behavior of Block Copolymers

The phase behavior of concentrated mixtures of block copolymers with an ionic liquid has been studied using a large series of block copolymers with varying molecular weight and volume fraction to gain a thorough understanding of the thermodynamics of self-assembly. The lyotropic phase behavior of mixtures of poly(styrene-block-2-vinylpyridine) (S2VP) copolymers with the ionic liquid imidazolium bis(trifluoromethane)sulfonimide ([Im][TFSI]) is reminiscent of block copolymer/selective molecular solvent mixtures, and ordered microstructures corresponding to lamellae, hexagonally close-packed cylinders, body-centered cubic, and face-centered cubic oriented micelles are observed. Scaling analysis reveals that, in contrast to observations of block copolymer/molecular solvent mixtures, the interfacial area occupied by each S2VP chain decreases upon the addition of [Im][TFSI], indicating a considerable increase in the effective segregation strength of the S2VP copolymer with ionic liquid addition.