Kinetics of Polybutadiene-poly(ethyleneoxide) Micelles in an Ionic Liquid

Abstract

Amphiphilic block copolymers can form micellar structures when dissolved in a selective solvent, which are of interest in a range of applications. Once in solution, a dynamic steady state is established; however, in some cases the incompatibility between block and solvent prevents the realization of equilibrium, forming kinetically frozen aggregates. The expectation is that sufficient thermal energy may allow the locked-in micelle system to relax into a more stable conformation. In this experiment, polybutadiene-poly(ethylene oxide) (PB-PEO) micelles were prepared in the ionic liquid 1-butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl) imide [BMI][TFSI] via direct dissolution and cosolvent-aided dissolution using dichloromethane. Measurements of the hydrodynamic radii conducted by dynamic light scattering revealed the formation of micelles with drastically different average sizes in each preparation procedure. The thermally stable environment provided by [BMI][TFSI] then permitted the annealing of both samples at relatively high temperatures. The micelles formed by direct dissolution significantly decreased in size and width of distribution, in contrast to the cosolvent-produced micelles, which retained their smaller size and low polydispersity. Interestingly, the aggregates prepared by the two protocols yielded different micelle sizes even after thermal relaxation, suggesting the formation of a metastable morphology. These findings illustrate the strong path-dependence of micelle formation and relaxation.