DYNAMIC VISCOELASTICITY OF AN AQUEOUS SYSTEM OF A POLY(ETHYLENE OXIDE)-POLY(PROPYLENE OXIDE)-POLY(ETHYLENE OXIDE) TRIBLOCK COPOLYMER DURINGGELATION

Viscoelastic properties during thermo-induced gelation of a binary pol oxamer (Pluronic F68)/water system have been determined by oscillatory shear and shear stress relaxation measurements. The polymer concentra tion has been kept constant at 35 wt %. A sol-gel transition is observ ed in the range 34-37 degrees C. The oscillatory shear data at tempera tures around the gel temperature reveal a complex pattern of behavior. The results cannot be described by a single Maxwell element behavior as demonstrated by converting the data to Cole-Cole plots. Shear stres s relaxation experiments show that the stress relaxation at the lowest temperatures of measurement (34 and 35 degrees C) can be described in itially by a single exponential followed at longer times by a stretche d exponential profile. At 36 degrees C and higher temperatures, a new relaxation mode in the form of a power law enters at intermediate time s, between the exponential and stretched exponential domains. The powe r law part of the relaxation function has its maximum time window at 3 7 degrees C (incipient gel) where it covers a time region of more than 3 orders of magnitude. The power law exponent is generally close to 0 .5, except at the lowest temperature (36 degrees C) of power law behav ior, where it is close to 0.6. A relaxation exponent of 0.5 can be rat ionalized within a framework of the fractal model for polymer networks . When the oscillatory shear data are transformed into equivalent shea r stress relaxation data, they are shown to be compatible with the exp erimental shear stress relaxation data at all temperatures.