RHEOLOGICAL BEHAVIOR DURING THERMOREVERSIBLE GELATION OF AQUEOUS MIXTURES OF ETHYL(HYDROXYETHYL)CELLULOSE AND SURFACTANTS

The evolution of linear viscoelasticity during the thermoreversible gelation of the aqueous systems ethyl(hydroxyethyl)cellulose (EHEC)/cetyltriammonium bromide (CTAB), and EHEC/sodium dodecyl sulfate (SDS) was measured by oscillatory shear. The experiments were carried out at constant surfactant concentration but at different EHEC concentrations. The gel point, determined by the observation of a frequency independent loss tangent, was found to be shifted toward lower temperatures with increasing EHEC concentration for both systems. At the gel point, a power law frequency dependence of the dynamic storage modulus (G' similar to omega(n') and loss modulus (G'' similar to omega(n'')) was constantly observed with n' = n'' = n. The viscoelastic exponent n was observed to decrease slightly (0.43 to 0.38) with increasing EHEC concentration for the EHEC/CTAB system, while for the EHEC/SDS system a pronounced increase (0.24-0.41) was detected. The implications of these observations with respect to the structure of the critical gels are discussed qualitatively in the framework of the fractal model of Muthukumar. The critical gel strength parameter S was found, for both systems, to increase with EHEC concentration, but especially at low polymer concentration a significant difference in the value of S of the systems was revealed.