Length of the Core Forming Block Effect on Fusion and Fission Dynamics at Equilibrium in PEO–PPO–PEO Triblock Copolymer Micelles in the Spherical Regime

The slow kinetics of block copolymer micelles make their dynamical pathways as important as the copolymer architecture in the self-assembly of nanostructures. Two types of dynamical pathways could govern these kinetics: the insertion–expulsion of copolymer chains and the fusion–fission of micelles. Yet, our understanding of the fusion and fission processes remain limited in copolymer micelles, especially at equilibrium. In this work, we use a fluorescence technique to quantify these processes at equilibrium in a series of triblock copolymer micelles of poly­(ethylene oxide)–poly­(propylene oxide)–poly­(ethylene oxide) in aqueous solutions. This technique is based on the randomization of a highly hydrophobic pyrene derivative between micelles. The temperature dependence of fusion and fission is investigated for copolymers with various core block length (N_PPO) in the spherical regime. Fusion and fission rates were found to strongly decrease with increasing N_PPO. The dependence of the fission rate on N_PPO is analyzed using the models of the thin corona and the starlike micelle, allowing to show that fission is mainly dominated by the core interfacial tension as predicted by Halperin et al. The comparison between fission and expulsion kinetics and their dependence on N_PPO is also reported. Finally, fusion is found to follow the same dependence on temperature and on the N_PPO as the fission, which indicates that the interfacial tension plays a relevant role in the fusion kinetic.