Hybridization of a Bimodal Distribution of Copolymer Micelles

The hybridization of two diblock micelles was studied by dynamic light scattering and SAXS/SANS. Two PMMA-b-PnBMA copolymers, where PnBMA and PMMA are the core- and corona-forming blocks, respectively, were employed in mixed ionic liquid solvents. Time-resolved SANS quantified the unimer exchange time for each copolymer as a function of solvent composition. The two micelle solutions in a given solvent were then mixed, and the structural evolution of the blended micelles was monitored. In more selective solvents, the apparent weight-average molecular weight (M_w) of the micelles initially increases with time. These observations reflect a net transfer of shorter chains from smaller to larger micelles, a but in a sense takes the system further away from equilibrium. The long-time evolution shows that hybridization depends greatly on solvent selectivity. In more selective solvents, M_w continues increasing even after months, while for less selective solvents, the micelle size eventually decreases and approaches equilibrium. 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.