Rheology of cetyltrimethylammonium p-toluenesulfonate-water system. 3. Nonlinear viscoelasticity

Abstract

The nonlinear viscoelastic behavior of the cetyltrimethylammonium p-toluenesulfonate (CTAT)-water system is investigated in steady and unsteady shear flow as a function of surfactant concentration and temperature. A rheo-optical study which includes measurements of dichroism, birefringence, and turbidity under flow at various shear rates is also discussed. The shear viscosity data in steady shear agree with the complex viscosity in the limit of low deformation rates. For moderate deformation rates, in the shear thinning region, the Cox-Merz rule is not followed. In all cases, a limiting stress or plateau stress was observed at shear rates that exceed one-half of the reciprocal of the main relaxation time [(2τd)-1]. At the stress plateau, the micellar solution most likely undergoes an isotropic-to-nematic phase transition induced by shear. However, our results do not conclusively exclude the possibility of a constitutive instability with respect to shear banding, in which simultaneous shear rates coexist under controlled stress experiments. In unsteady shear flow, CTAT-water micellar solutions exhibit a slow transient behavior in which the system achieves steady state in starting up experiments after tens to hundreds of Maxwell relaxation times. This is consistent with the existence of shear banding. Metastable branches are also observed in thixotropic loops produced under exponential shear. The time scale of this branch coincides with that of the inception of shear flow just before the overshoot peak. Moreover, the system exhibits a quasilinear rheological behavior at long times characterized by an exponential relaxation with a single time constant. A simple model consisting of the co-deformational Maxwell constitutive equation and a kinetic equation for construction and destruction of structure is proposed to predict distinct features of the complex rheological behavior of the elongated micellar solutions. © 1999 American Chemical Society.