The hybridization of two diblock copolymer micelles in mixtures of ionic liquids, 1-ethyl and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM] and [BMIM][TFSI], or EMIM and BMIM in short), was studied by time-resolved dynamic light scattering (DLS) and small-angle X-ray and neutron scattering (SAXS/SANS). Two poly(methyl methacrylate)-block-poly(n-butyl methacrylate) (PMMA-b-PnBMA) copolymers, where PnBMA and PMMA are the core- and corona-forming blocks, respectively, were employed. The two diblocks have the same corona block molecular weight (25 »000 g/mol) but core block lengths differing by a factor of 2.2 (24 »000 and 53 »000 g/mol). Both polymers assemble into spherical micelles in mixed ionic liquid solvents containing 0-30 wt % BMIM, albeit with different sizes. The solvent selectivity decreases with increasing BMIM content. Time-resolved SANS quantified the unimer exchange time for each copolymer as a function of solvent composition. In the most selective solvent (100% EMIM), the longer chains exchanged ∼104 times more slowly than the shorter ones; this difference was reduced to a factor of ca. 50 in 30% BMIM. The two micelle solutions in a given solvent were then mixed in equal proportions, and the structural evolution of the blended micelles was monitored over several months. From previous work, we know that at equilibrium the two copolymers should form a uniform population of mixed ("hybridized") micelles, with size intermediate between the precursor micelles (albeit closer to the larger ones). In the more selective solvents, both light and neutron scattering show that the apparent weight-average molecular weight (Mw) of the micelles initially increases with time, while SAXS shows an increase in the average micelle core size. These observations reflect a net transfer of shorter chains from smaller to larger micelles, as the shorter-chain exchange is much more facile but, in a certain sense, takes the system further away from equilibrium. The long-time evolution monitored by DLS shows that the process of micellar hybridization depends greatly on solvent selectivity. In 100% EMIM, Mw continues increasing even after several months, indicating that equilibrium is not reached within the experimental time scale. For less selective solvents, the micelle size eventually begins to decrease and approaches the equilibrium size, indicating that a unimer exchange of both molecular weights is operative. In the least selective solvent, 30% BMIM, both Mw and the average hydrodynamic size of the micelles start to decrease immediately and ultimately approach the values of the equilibrium micelles. However, this process must involve other relaxation mechanisms, e.g., micelle fusion/fragmentation or micelle creation/annihilation, as the total number of micelles also needs to be adjusted. Overall, this work exposes the possibility of different routes to equilibrium for a given system and thereby underscores the complexity of equilibration in block copolymer micelles.
Bibliographical noteFunding Information:
We acknowledge financial support from the National Science Foundation (DMR-1707578). Portions of this work were performed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by Northwestern University, E.I. DuPont de Nemours & Co., and The Dow Chemical Company. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by the Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Additionally, a portion of this research used resources at the High Flux Isotope Reactor, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. We would like to thank Dr. Lilin He for the assistance in SANS measurements and data reduction and Dr. Yuanchi Ma for preparing the polymers.