Unraveling Chain Exchange Kinetics in Mixed Micelles through FRET: Impact of Segment Interactions between Hydrophilic Polymers in the Corona

Mixed micelle systems have gained significant attention due to their ability to integrate multifunctionality based on different copolymers. However, an understanding of chain exchange behavior in mixed micelles is limited. In this study, we reported a mixed micelle system where noncovalent cross-linking between the corona blocks enhanced micelle stability. Specifically, we utilized polystyrene-block-poly(2-ethyl-2-oxazoline) (PS-b-PEtOx) and polystyrene-block-poly(methacrylic acid) (PS-b-PMAA) to form the mixed micelle system. Donor- and acceptor-labeled PS-b-PEtOx was employed to investigate the chain exchange kinetics using fluorescence resonance energy transfer (FRET). The addition of PS-b-PMAA facilitated the formation of hydrogen bonds between EtOx and protonated MAA units, resulting in a noncovalent cross-linked corona. As the pH decreases, the percentage of affected PS-b-PEtOx gradually increases, while the extra energy barrier first decreases and then increases. This is mainly due to the conformational changes of PMAA chains, which cause crowding between the core and shell, and the interactions between PMAA and PEtOx, especially hydrogen bonding. This study demonstrates that the hydrogen-bonded cross-linking of the corona in the mixed micelle system can effectively control the chain exchange rate and improve micelle stability.