Fragmentation of Block Copolymer Micelles in Ionic Liquids

Abstract

Block copolymers self-assemble into various micellar nanostructures in selective solvents and can be applied in a host of diverse technologies including drug delivery, viscosity modification, and nanoreactor design. To fully realize these practical uses, the mechanisms for micellization and equilibration must be elucidated. Micelle fragmentation, fusion, and chain exchange are all possible relaxation mechanisms. To date, fragmentation has not been studied in any detail. Block copolymer micelle fragmentation was studied using temperature-jump dynamic light scattering (T-jump DLS), synchrotron small-angle X-ray scattering (SAXS), and liquid-phase transmission electron microscopy (LP-TEM) to develop a quantitative understanding of micelle fragmentation kinetics in ionic liquids (ILs). The use of non-volatile IL solvents enables high temperature annealing and direct use of TEM. Fragmentation of one molar mass of 1,2-polybutadiene-b-poly(ethylene oxide) (PB-PEO) was studied in five ILs to determine the effect of solvent selectivity. Then, fragmentation of PB-PEO was visualized directly for three molar masses in one IL using LP-TEM. The molar mass dependence of micelle fragmentation kinetics for six molar masses of PB-PEO in one IL were quantified by time-resolved SAXS and T-jump DLS. The effects of PB-PEO molar mass dispersity on micelle fragmentation was explored using hybrid micelles comprised of molar mass blends of PB-PEO. Finally, the swelling behavior of PB-PEO in an IL was explored using high temperature LP-TEM to determine how the solution preparation procedure used to make large, kinetically trapped micelles depends on the morphology of the bulk diblock copolymer. By combining these experimental techniques, a detailed analysis of micelle fragmentation kinetics, along with the direct observation of intermediate structures during fragmentation events, was achieved.