Stepwise Fusion Kinetics in Block Copolymer Micelles: Effect of Solvent Quality for the Corona

Abstract

Block copolymer micelles in the strongly segregated regime are often trapped in nonequilibrium states, with large barriers to re-arrangement. Here, we show that micelle fusion can be induced, and its onset tuned, by controlling the solvent quality for the corona block. Small, kinetically trapped polystyrene-block-poly(ethylene oxide) (PS-PEO) micelles were prepared in two ionic liquids ([C4mim][BF4] and [C6mim][BF4]) and their mixtures, in which the PEO corona exhibits lower critical solution temperature phase behavior. Upon heating, synchrotron small-angle X-ray scattering and static light scattering reveal that the micelles grow irreversibly in discrete steps, as the second virial coefficient for PEO in the solvent approaches zero and even changes sign. The characteristic step temperatures vary linearly with the solvent composition, demonstrating that the onset of fusion is dictated by the corona–solvent interaction. A second-order kinetic model with an Arrhenius rate constant quantitatively describes the first fusion step and yields a large apparent activation energy of ca. 185 kJ/mol, independent of solvent composition. These results establish solvent quality for the corona as a key parameter regulating the fusion of kinetically trapped micelles and provide a general strategy for directing block copolymer assemblies toward equilibrium.