Jacqueline Appell

Unstable flow and nonmonotonic flow curves of transient networks

We have measured the nonlinear rheological 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 rheological 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.We have measured the nonlinear rheological 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 rheological 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.

A transient network of telechelic polymers and microspheres: structure and rheology

We study the structure and dynamics of a transient network composed of droplets of microemulsion connected by telechelic polymers. The polymer induces a bridging attraction between droplets without changing their shape. A viscoelastic behaviour is induced in the initially liquid solution, characterized in the linear regime by a stretched-exponential stress relaxation. We analyse this relaxation in the light of classical theories of transient networks. The role of the elastic reorganizations in the deformed network is emphasized. In the non-linear regime, a fast relaxation dynamics is followed by a second one having the same rate as that in the linear regime. This behaviour, in step strain experiments, should induce a non-monotonic behaviour in the elastic component of the stress for a constant shear rate. However, we obtain in this case a singularity in the flow curve very different from the one observed in other systems, that we interpret in terms of fracture behaviour.

Bending elasticity of a curved amphiphilic film decorated with anchored copolymers: a small angle neutron scattering study

Microemulsion droplets (oil in water stabilized by a surfactant film) are progressively decorated with increasing amounts of poly(ethylene oxide) (PEO) chains anchored in the film by the short aliphatic chain grafted at one end of the PEO chain. The evolution of the bending elasticity of the surfactant film with increasing decoration is deduced from the evolution in size and polydispersity of the droplets as reflected by small angle neutron scattering. The optimum curvature radius decreases while the bending rigidity modulus remains practically constant. The experimental results compare well with the predictions of a model developed for the bending properties of a curved film decorated with non-adsorbing polymer chains, which takes into account the finite curvature of the film and the free diffusion of the chains on the film.