Equilibrium Fusion–Fragmentation Kinetics across Block Copolymer Micelle Morphologies: From Spheres to Rods

Abstract Equilibrium fusion–fragmentation dynamics of block copolymer micelles are investigated across distinct equilibrium morphologies, from spherical to rod-like aggregates. Using temperature-responsive Pluronic triblock copolymers as model systems, fusion and fragmentation rate constants are quantified under near-equilibrium conditions by time-resolved fluorescence probe exchange. Fragmentation rates decrease monotonically with increasing temperature, with no pronounced change across the sphere-to-rod transition. Fusion rates decrease slightly within the spherical regime and then exhibit a pronounced upturn in the rod-like regime. This asymmetric response highlights the role of equilibrium morphology in biasing fusion–fragmentation dynamics. The fragmentation trend in the rod-like regime is rationalized using a scaling-based energetic analysis, which indicates that the fragmentation barrier is governed by the core–solvent interfacial penalty associated with end-cap formation, making it only weakly dependent on micelle length. The same framework attributes the enhancement of fusion in the rod-like regime to a reduced coronal free-energy penalty, primarily controlled by effective steric interactions within the poly(ethylene oxide) (PEO) corona and likewise only weakly dependent on micelle length. These results establish a direct link between equilibrium micelle shape, thermodynamic interaction parameters, and collective dynamic pathways, providing a unified framework to rationalize morphology-dependent fusion–fragmentation dynamics in block copolymer micelles.