EFFECTS OF TEMPERATURE, SURFACTANT, AND SALT ON THE RHEOLOGICAL BEHAVIOR IN SEMIDILUTE AQUEOUS SYSTEMS OF A NONIONIC CELLULOSE ETHER

Oscillatory shear experiments have been carried out on thermoreversibl e gelling and nongelling semidilute aqueous systems of ethyl(hydroxyet hyl)cellulose (EHEC) (at a constant polymer concentration of 1 wt %) i n the presence of various amounts of sodium dodecyl sulfate (SDS) and at some different levels of NaCl addition. Depending on the concentrat ions of surfactant and salt, a temperature-induced sol-gel transition or only a viscosification of the solution was observed. For the gellin g systems, the value of the gel temperature, determined by the observa tion of a frequency independent loss tangent, was found to be dependen t on the composition of the system. At the gel temperature, a power la w frequency dependence of the dynamic storage modulus (G' similar to o mega(n') and loss modulus (G '' similar to omega(n '')) was constantly observed with n' = n '' = n. Values of the viscoelastic exponent n in the range 0.3-0.4 were reported. The value of n, as well as the gel s trength parameter S, was dependent on the composition of the system. T he rheological properties of the nongelling systems were affected by t emperature, surfactant, and salt. In the absence of salt, the network structure is disrupted at high surfactant concentations and the dynami c viscosity decreases. However, if salt is added at this stage an enha nced viscoelastic response is observed and the network structure is re -established. The rheological results of this work indicate that the e ffects of surfactant and salt counteract each other. The present resul ts for both gelling and nongelling systems are analyzed in a model whe re the interplay between swelling (caused by the ionic surfactant) and connectivity (established by ''lumps'' or hydrophobic associations) i s considered.