Fragmentation\nKinetics of Block Copolymer Micelles:\nEffect of Core and Corona Block Lengths

Block copolymer molecular weight is a crucial factor\ninfluencing\nmicelle fragmentation kinetics. In particular, it is not established\nhow block length affects the fragmentation rate, and which block is\nmore important. In this work, we studied the separate dependence of\nmicelle fragmentation kinetics on core and corona block lengths, with\ntemperature-jump experiments by dynamic light scattering. Two series\nof 1,2-polybutadiene-<i>b</i>-poly(ethylene oxide) (PB-<i>b</i>-PEO) copolymers were prepared, both with fixed <i>N</i><sub>PB</sub> (degree of polymerization of the core block)\nand various <i>N</i><sub>PEO</sub>. In all, a total of nine\nnarrow dispersity (<i>D̵</i> < 1.1) polymers were\nused, with PB-<i>b</i>-PEO block molar masses (in kDa) of\n(9–6), (9–9), (9–12), (9–13), (9–19),\n(15–10), (15–14), (15–17) and (15–22).\nMicelles were initially prepared using direct dissolution of PB-<i>b</i>-PEO in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethyl\nsulfonyl)imide [C<sub>2</sub>MIM][TFSI], followed by fragmentation\nduring thermal annealing at 170 °C. The relaxation time (τ)\nfor micelle fragmentation exhibits a power-law correlation with both <i>N</i><sub>PB</sub> and <i>N</i><sub>PEO</sub> as τ\n∼ <i>N</i><sub>core</sub><sup>1.4</sup> × <i>N</i><sub>corona</sub><sup>0.6</sup>. Micelles were also characterized\nbefore and after fragmentation, by small-angle X-ray scattering and\nby liquid-phase transmission electron microscopy. Both analyses confirmed\nthat all micelles were spherical, and the mean aggregation numbers\nbefore and after fragmentation could be extracted. The stronger τ\ndependence on <i>N</i><sub>core</sub> suggests a higher\ncore elastic free energy penalty due to core chain extension in the\ntransition state, compared to the contribution of corona crowding.\nThese results are apparently not captured by any available theory\nof micelle fragmentation.