Unstable flow and nonmonotonic flow curves of transient networks

We have measured the nonlinear theological response of a model transient network 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 incorporated. The phase behavior which comprises a liquid-gas phase separation and a percolation threshold is characterized. The theological measurements 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 stickers 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 function of the rate, a feature that indeed makes homogeneous flows unstable. The computed the flow curves compare well to the experiments. (C) 2001 The Society of Rheology.