DYNAMIC VISCOELASTICITY OF GELLING AND NONGELLING AQUEOUS MIXTURES OFETHYL(HYDROXYETHYL)CELLULOSE AND AN IONIC SURFACTANT

Oscillatory shear experiments have been carried out over an extended t emperature range (10-45 degrees C) on thermoreversible gelling and non gelling aqueous systems of ethyl(hydroxyethyl)cellulose (EHEC) of diff erent polymer concentrations in the presence of various amounts of sod ium dodecyl sulfate (SDS). At moderate ratios (r) of SDS/EHEC, the sem idilute systems form gels at Elevated temperatures, whereas no tempera ture-induced gels are formed at high values of r in this temperature r egion. The gel point temperature depends on the polymer/surfactant com position, similar to the cloud point. It is shown that phase separatio n and gelation are phenomena that are closely related. At the gel temp erature, a power law frequency dependence of the dynamic moduli (G' si milar to G '' similar to omega(n)) was constantly observed. Depending on the composition of the system, the viscoelastic exponent assumes va lues in the approximate range 0.1-0.7. This finding indicates that the structure of the incipient gel is strongly influenced by the concentr ations of polymer and surfactant. The dynamic viscosity results sugges t a strengthening of the association network at moderate values of r, while a gradual disruption of the network occurs at high levels of SDS addition. A temperature increase of semidilute EHEC solutions at mode rate values of r gives rise to a strengthening of the network, whereas a temperature-induced weakening of the network structure is found at higher values of r.