Unstable flow and nonmonotonic flow curves of transient networks

We have measured the nonlinear theological response of a model transient ne twork over a large range of steady shear rates. The system is built up from an oil in water droplet microemulsion into which a telechelic polymer is i ncorporated. The phase behavior which comprises a liquid-gas phase separati on and a percolation threshold is characterized. The theological measuremen ts are performed in the one phase region above the percolation line. Shear thinning is observed for all samples, leading in most cases to an unstable stress response at intermediate shear rates. We built up a very simple mean field model which involves the reduction of the residence time of the stic kers in the droplets due to chain tensions at high shear. The computed flow curves are nonmonotonic with a range where the stress is a decreasing func tion of the rate, a feature that indeed makes homogeneous flows unstable. T he computed the flow curves compare well to the experiments. (C) 2001 The S ociety of Rheology.