DYNAMIC LIGHT-SCATTERING AND RHEOLOGICAL STUDIES OF THERMOREVERSIBLE GELATION OF A POLY(ETHYLENE OXIDE)-POLY(PROPYLENE OXIDE)-POLY(ETHYLENEOXIDE) TRIBLOCK COPOLYMER IN AQUEOUS-SOLUTION

Shear relaxation and dynamic light scattering (DLS) experiments have b een performed at various temperatures on the thermoreversible gelling aqueous system of Pluronic F68 (35 wt.%). As the gel zone is approache d, both the stress relaxation process and the time correlation functio n indicate three different relaxation modes: a short-time behaviour (e xponential profile) followed by a power law at intermediate time and t hen a long-time tail (stretched exponential profile). At low temperatu res the stress relaxation function is described by only two relaxation processes: an exponential at early times followed by a stretched expo nential at long time, while the decays of the correlation functions fr om DLS display three modes at all temperatures. The stress relaxation results reveal that the extension of the power-law regime is most pron ounced in the vicinity of the gel point. The value of the power law ex ponent alpha(s) from stress relaxation measurements (G proportional to t(-alpha s)) is 0.5 at all temperatures, except at the lowest tempera ture where alpha(s) approximate to 0.6. In the case of the DLS measure ments the power-law exponent is much lower (0.05-0.15) and an angular dependence is observed.