Dynamics of Micelles of Poly(ethylene oxide)−Poly(propylene oxide)−Poly(ethylene oxide) Block Copolymers in Aqueous Solutions

The dynamics of the micelles of two triblock poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) copolymers, the Pluronics EO13PO30EO13 (L64) and EO73PO27EO73 (PF80), has been investigated using two chemical relaxation methods: the temperature-jump (T-jump) with Joule heating and light scattering detection of the relaxation effects and the ultrasonic relaxation. These two methods are capable of detecting dynamic events with characteristic times between 1 ns and 2 s. The ultrasonic absorption measurements in the frequency range 0.4−85 MHz showed the existence of a large excess absorption with respect to water, occurring at temperatures above the critical micelle temperature (cmT) of the Pluronic solutions investigated. The frequency dependence of the absorption revealed the existence of a relaxation process with a relaxation frequency around 0.5 MHz at 40 °C for L64. This process was assigned to the exchange of copolymer between micelles and intermicellar solution. The T-jump experiments revealed two relaxation processes. The relaxation time of the fast process was dependent on the scattering angle. This process was assigned to the effect of the Pluronic exchange on the fluctuations of the Pluronic micelle concentration. The slow process gave rise to a decrease of scattered intensity and was attributed to the return to equilibrium of the micelle size distribution involving an increase in the number of micelles. Overall, the kinetics of block copolymer micelles follows the scheme proposed by Aniansson and Wall for conventional surfactants (Aniansson, E. A. G.; Wall, S. J. Phys. Chem. 1974, 78, 1024; 1975, 79, 857), with a response of the micellar systems to a fast perturbation characterized by two time constants, one for the fast change of aggregation number of the micelles at nearly constant number of micelles (exchange process) and the other for the adjustment of the micelles numbers to the final micelle size distribution (micelle formation/breakup). The association of the Pluronic L64 to its micelles is nearly diffusion-controlled. The micelle formation/breakup process appears to proceed via reactions of fission/fusion (Kahlweit, M. J. Colloid Interface Sci. 1981 , 90, 92; Pure Appl. Chem. 1981 , 53, 2069), as in the case of nonionic ethoxylated surfactants. These results demand to be extended to other Pluronics in order to assess their degree of generality.