Spontaneous Sphere to Vesicle Transition of Block Copolymer Micelles in an Ionic Liquid

Self–assembly of block copolymers in a block–selective solvent into micelles is essential for applications such as viscosity modification and drug delivery. Understanding how micelles evolve via chain exchange, fusion, and fragmentation is necessary for optimizing their properties. We employ time–resolved small–angle X–ray scattering (TR–SAXS) to monitor the spontaneous fusion of polybutadiene–block–poly(ethylene oxide) micelles in an ionic liquid (IL). The TR–SAXS data can be modelled with a sum of spherical micelle populations and show a decrease (increase) in the population of smaller (larger) micelles over time, indicating fusion upon annealing. Fitting the data at later times to a vesicle model reveals vesicle sizes with reduced dispersity, suggesting evolution towards a preferred state for the block copolymer chains in the IL. Changes in micelle size are also characterized by dynamic light scattering before and after annealing. Liquid phase transmission electron microscopy has been performed to image micelles and verify the presence of vesicles after annealing.