Quantized Fusion Kinetics in Block Copolymer Micelles

The fusion kinetics of block copolymer micelles in dilute solutions have been investigated. As a model system, 1,2-polybutadiene-block-poly(ethylene oxide) micelles in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate have been studied. The ionic liquid is a selective solvent for poly(ethylene oxide), promoting the self-assembly of the block copolymer into spherical micelles. Furthermore, the quality of the solvent for the corona block is near-theta, thereby reducing the large steric barrier to fusion. Small, kinetically trapped micelles were prepared using a cosolvent, and the kinetics of fusion were subsequently monitored via dynamic light scattering at elevated temperatures. Small-angle X-ray scattering and cryo-transmission electron microscopy quantified significant increases in the mean aggregation number after thermal annealing and confirmed the formation of well-defined, larger spherical micelles. For higher annealing temperatures, the process occurs in two steps, with the relaxation time of the second step being at least an order of magnitude longer than the first. Interestingly, the steady-state micelles after the first step had approximately twice the starting aggregation number, and those after the second step had four times the original value. This result strongly suggests a quantization effect, where the rate of fusion is much slower for larger micelles, presumably due to enhanced corona crowding. The relaxation rate is also an increasing function of concentration, consistent with fusion being the dominant mechanism.