Kinetics and Mechanism of the Sphere-to-Rod Transition of Triblock Copolymer Micelles in Aqueous Solutions

Abstract

The kinetics of the sphere-to-rod transition of micelles composed of triblock copolymers of ethylene oxide and propylene oxide (EO(20)PO(70)EO(20)) have been investigated using dynamic light scattering (DLS) and cryogenic electron transmission microscopy (Cryo-EM). Sphere-to-rod transition is induced by a solvent jump, initiated by adding KCl and ethanol to an aqueous micellar solution. The growth process of the wormlike micelles depends on the experimental conditions and has two distinct regions that can be described as initiation period and actual growth to equilibrium. All growth curves exhibit a single relaxation time that represents the lifetime of the micelles. The growth curves collapse into a master curve, when shifted by the relaxation time, indicating that the actual growth process of the micelles in all samples occurs through the same mechanism. The relaxation time decreases with increasing surfactant concentration. Additionally, some of the formed micelles exhibit a caterpillarlike shape in which some of the original spherical species can still be detected. These facts suggest that the micelles grow longer predominantly by random coagulation/fragmentation reactions involving micellar species of different sizes. However, the appearance of a unimer peak is detected with DLS during the growth stage. This implies that unimer exchange may also contribute to the elongation of the micelles.