Dynamic Viscoelasticity of Gelling and Nongelling Aqueous Mixtures of Ethyl(hydroxyethyl)cellulose and an Ionic Surfactant

Abstract

Oscillatory shear experiments have been carried out over an extended temperature range (10−45 °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' ∼ G'' ∼ ωn) 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.