Gelation and long-time relaxation of aqueous solutions of Pluronic F127

We have studied the gelation dynamics of solutions of Pluronic F127, a triblock copolymer, using oscillatory rheometry. As the solutions are slowly heated from 10 to 85°C, they undergo a transition from sol to gel around room temperature, followed by a gel-sol transition at a higher temperature. The sol-gel transition temperature decreases and the width in the temperature of the gel phase broadens with increasing concentration. The reverse transitions occur on cooling. Both transitions are hysteretic, and the temperature range over which the transitions occur is different on cooling than on heating. Near the phase transitions, the rheological relaxation time becomes very long, up to of order τs∼104s. Gelation in Pluronic solutions is due to the arrangement of micelles into an organized large-scale structure, and our results suggest that the rearrangement of micelles into or out of the gel structure is a slow process at temperatures within the phase transitions.