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

The evolution of linear viscoelasticity during the thermoreversible ge lation of the aqueous systems ethyl(hydroxyethyl)cellulose (EHEC)/cety ltriammonium bromide (CTAB), and EHEC/sodium dodecyl sulfate (SDS) was measured by oscillatory shear. The experiments were carried out at co nstant surfactant concentration but at different EHEC concentrations. The gel point, determined by the observation of a frequency independen t loss tangent, was found to be shifted toward lower temperatures with increasing EHEC concentration for both systems. At the gel point, a p ower law frequency dependence of the dynamic storage modulus (G' simil ar to omega(n') and loss modulus (G'' similar to omega(n'')) was const antly observed with n' = n'' = n. The viscoelastic exponent n was obse rved to decrease slightly (0.43 to 0.38) with increasing EHEC concentr ation for the EHEC/CTAB system, while for the EHEC/SDS system a pronou nced increase (0.24-0.41) was detected. The implications of these obse rvations with respect to the structure of the critical gels are discus sed 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 conce ntration a significant difference in the value of S of the systems was revealed.