Micellar Self-Assembly and Probe Exchange Dynamics of Triblock Copolymer Poly(ethylene oxide)–Poly(propylene oxide)–Poly(ethylene oxide) PEO57PPO45PEO57

The dynamics of micellar solutions of a triblock poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) copolymer, PEO 57 PPO 45 PEO 57 where the subscripts indicate the block length, has been analyzed. This copolymer is characterized by a large monodispersity and hydrophilicity if compared to the previously studied commercially available Pluronic block copolymers. Different experimental techniques were used to characterize its micellar self-assembly, elucidating the existence of a concentration-dependent transition to a “pseudo-percolated” state as the temperature increases in which interconnected polymeric micelles are formed. The kinetics of the micellar assembly and disassembly processes and solute exchange dynamics were monitored by stopped-flow time-scan fluorescence measurements using a fluorescent probe, which points to the involvement of several underlying mechanisms other than simple particle collisions: a slow dynamic regime below 45 °C; a transition regime between 45 and 50 °C; and a fast dynamic one above 50 °C. The slow dynamic regime was associated with fragmentation and fusion processes in competition; the fast one was mainly regulated by a fusion-dominant mechanism in which both concentration and temperature play a role; and finally, a linear increase in the fluorescence decay rate with increasing copolymer concentrations indicated that the transition regime (in which the interconnected micelles are formed) starts to be dominated by micellar fusion, whereas fragmentation becomes less likely. It was observed that the kinetic constant for the fusion process, k fusion, increased ca. twofold as the temperature rises in a concentration-dependent manner, whilst that corresponding to the fragmentation process, k fragmentation, increases until ca. 50 °C (close to the transition temperature, T t ), and then, it decreases independently of the micellar concentration. Above the transition temperature, k fragmentation and k fusion are related to two apparent activation energies of similar magnitude but different sign. Finally, the kinetic constants of the micellar fragmentation and fusion processes were influenced by the PEO/PPO ratio when compared to the previously analyzed structure-related commercial Pluronic copolymers.