Dynamic light scattering and rheological studies of thermoreversible gelation of a poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymer in aqueous solution

Abstract

Shear relaxation and dynamic light scattering (DLS) experiments have been performed at various temperatures on the thermoreversible gelling aqueous system of Pluronic F68 (35 wt.%). As the gel zone is approached, both the stress relaxation process and the time correlation function indicate three different relaxation modes: a short-time behaviour (exponential profile) followed by a power law at intermediate time and then a long-time tail (stretched exponential profile). At low temperatures the stress relaxation function is described by only two relaxation processes: an exponential at early times followed by a stretched exponential at long time, while the decays of the correlation functions from DLS display three modes at all temperatures. The stress relaxation results reveal that the extension of the power-law regime is most pronounced in the vicinity of the gel point. The value of the power law exponent α_s from stress relaxation measurements (G∞t^–α_s) is 0.5 at all temperatures, except at the lowest temperature where α_s≈ 0.6. In the case of the DLS measurements the power-law exponent is much lower (0.05–0.15) and an angular dependence is observed.