Kinetics and Mechanisms of the Sphere-to-Rod and Rod-to-Sphere Transitions in the Ternary System PS310-b-PAA52/Dioxane/Water

The relaxation kinetics and transition mechanisms of the sphere-to-rod and rod-to-sphere transitions occurring in polystyrene-b-poly(acrylic acid) (PS-b-PAA) aggregates have been examined using solution turbidity measurements and transmission electron microscopy. The copolymer PS310-b-PAA52 has been found to self-assemble into aggregates with spherical, rodlike, and vesicular shapes in dioxane−water mixtures. Recently, the morphological phase diagram for this ternary system was constructed in order to determine the concentration boundaries associated with each aggregate architecture and the regions of morphological coexistence. It had previously been found that a sudden alteration of the solvent composition near one of these boundaries could induce a morphological transition. This approach has been used to investigate the kinetics and mechanisms of the sphere-to-rod and rod-to-sphere transitions. The transformation of spherical micelles to rodlike aggregates occurs through a two-step mechanism; it begins with the fast, adhesive collisions of spheres resulting in the formation of irregular “pearl necklace” intermediate structures. This is followed by the reorganization of the necklace intermediates to form smooth rods. The rod-to-sphere transition also involves two steps. A bulb develops quickly on one or both ends of the rod, and then the bulbs are slowly pinched off to release free spheres in solution. The sphere-to-rod transition occurs at comparable rates to the reverse process. The effect of the jump magnitude, the initial solvent content, and the copolymer concentration on the relaxation rates was also investigated.