Dynamic viscoelasticity of gelling and nongelling aqueous mixtures of ethyl(hydroxyethyl)cellulose and an ionic surfactant

Oscillatory shear experiments have been carried out over an extended temperature range (10-45 degrees C) on thermoreversible gelling and nongelling aqueous systems of ethyl(hydroxyethyl)cellulose (EHEC) of different polymer concentrations in the presence of various amounts of sodium dodecyl sulfate (SDS). At moderate ratios (r) of SDS/EHEC, the semidilute systems form gels at Elevated temperatures, whereas no temperature-induced gels are formed at high values of r in this temperature region. The gel point temperature depends on the polymer/surfactant composition, similar to the cloud point. It is shown that phase separation and gelation are phenomena that are closely related. At the gel temperature, a power law frequency dependence of the dynamic moduli (G' similar to G " similar to omega(n)) was constantly observed. Depending on the composition of the system, the viscoelastic exponent assumes values in the approximate range 0.1-0.7. This finding indicates that the structure of the incipient gel is strongly influenced by the concentrations of polymer and surfactant. The dynamic viscosity results suggest 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 moderate 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.