Micellization of PS-PMMA diblock copolymers in an ionic liquid

Three polystyrene-block-poly(methyl methacrylate) (PS-PMMA) block copolymers with varying molecular content have been shown to form micelles when dissolved in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM PF₆). The micellar structure was studied via cryogenic transmission electron microscopy and dynamic light scattering, and a morphological transition from spherical to cylindrical micelles was observed upon reduction of the PMMA volume fraction. The possibility of frozen micellar morphology was considered, due to the solution preparation method and high glass transition temperature (Tg) of the PS blocks that form the micellar cores. By comparison of the behavior of a 100 kDa PMMA homopolymer dissolved in both BMIM PF₆ and a known good solvent, acetone, it was determined that BMIM PF₆ behaves as a good solvent for PMMA. It was also observed that extended exposure to the electron beam during cryogenic transmission electron microscopy could damage the copolymer micelles and result in a reversal of contrast.