Path-Dependent Morphology and Relaxation Kinetics of Highly Amphiphilic Diblock Copolymer Micelles in Ionic Liquids

The role of kinetics in the formation and relaxation of highly amphiphilic diblock copolymer micelles in ionic liquids is analyzed. In particular, we studied the morphology of poly(butadiene)-poly(ethylene oxide) (PB-PEO) block copolymer micelles dispersed in 1-ethyl-3-methylimidazolium bis-(trifluoromethyl sulfonyl)imide [EMI][TFSI] and prepared via two methodologies: direct dissolution (DD) and cosolvent-aided dissolution (CS). Dynamic light scattering (DLS) measurements reveal that the former procedure leads to large and polydisperse aggregates that relax upon annealing at high temperatures into smaller and narrowly distributed, steady-state spherical micelles, while the latter protocol results in smaller, monodisperse micelles that retain their size upon prolonged thermal treatment. Remarkably, the CS micelles are only half the size of the annealed DD micelles. The development of distinct, steady-state micellar distributions through the two preparation protocols is consistently observed when changing the selectivity of the solvent (by changing the cation to 1-butyl-3-methylimidazolium) or by substituting the core-forming PB block for a polystyrene chain. Two different distributions were also observed when changing the solubility of the copolymer through variation of the corona block length. A better understanding of the underlying mechanism of DD-micelle relaxation is obtained by measuring the temperature-dependent relaxation of the micelles using DLS. In all cases the relaxation is, well described by an Avrami-type relaxation function with exponent n = 2. The resulting relaxation times decrease with increasing copolymer concentration, suggesting that fusion/fission is the main mechanism of DD-micelle relaxation, and with annealing temperature. The results are in agreement with complementary small-angle neutron scattering experiments that reveal no single chain exchange between steady-state PB-PEO micelles in the imidazolium-based ionic liquid, up to at least 200 degrees C.