G. Porte

SHEAR-INDUCED ISOTROPIC-TO-NEMATIC PHASE TRANSITION IN EQUILIBRIUM POLYMERS

We report on a first-order isotropic-to-nematic phase transition induced by shear in concentrated solutions of elongated flexible wormlike micelles. As shear is applied to a disordered micellar solution, the transition shows up in steady-shear experiments as a kink in the shear stress behaviour. Using small-angle neutron scattering under shear, we show that beyond the transition rate, a nematic phase manifests itself by the growth of crescentlike scattering patterns in the direction perpendicular to the flow. A dynamical phase diagram is finally proposed for this system of equilibrium polymers.

Time scales in shear banding of wormlike micelles

We show the existence of three well defined time scales in the dynamics of wormlike micelles after a step between two shear rates on the stress plateau. These time scales are compatible with the presence of a structured interface between bands of different viscosities and correspond to the isotropic band destabilization during the stress overshoot, reconstruction of the interface after the overshoot and travel of a fully formed interface. The last stage can be used to estimate a stress diffusion coefficient.

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.

Entropic Phase Separation in Polymer-Microemulsion Networks

We study theoretically a model system of a transient network of microemulsion droplets connected by telechelic polymers and explain recent experimental findings. Despite the absence of any specific interactions between either the droplets or polymer chains, we predict that as the number of polymers per drop is increased, the system undergoes a first order phase separation into a dense, highly connected phase, in equilibrium with dilute droplets, decorated by polymer loops. The phase transition is purely entropic and is driven by the interplay between the translational entropy of the drops and the configurational entropy of the polymer connections between them. Because it is dominated by entropic effects, the phase separation mechanism of the system is extremely robust and does not depend on the particlular physical realization of the network. The discussed model applies as well to other polymer linked particle aggregates, such as nano-particles connected with short DNA linkers.

Excess Area in Fluctuating-Membrane Systems

Using neutron and high-resolution X-ray scattering, the repeating distances of dilute lyotropic lamellar phases have been measured as a function of the membrane volume fraction, for several ternary or quaternary systems. In the case of systems dominated by undulation forces, a systematic logarithmic deviation is observed. It is attributed to the excess area coming from the undulations. A method for measuring the membrane bending elastic constant κ is proposed.

The shear-induced transition between oriented textures and layer-sliding-mediated flows in a micellar cubic crystal

A series of experiments has been performed in order to analyse the shear-induced structures for a micellar cubic phase using small-angle neutron and x-ray scattering techniques. Steady shear was applied in a Couette cell to the (EO) triblock copolymer system dissolved in water. At rest, the system crystallizes into a long-range ordered mesophase of face-centred symmetry (lattice parameter ). The good resolution of the x-ray technique enables us to study in detail the transition between shearing flows dominated by oriented textures at low shear rates and flows mediated by the mechanisms of layer sliding at higher rates .

Isotropic phases of bilayers

The bending properties of surfactant bilayers are the foundation for the formation and stability of the so-called L 3 sponge phase. During the past two years, improved theoretical interpretations and models of this fascinating multiconnected disordered structure have been worked out. On the experimental side, critical behaviours observed in the very dilute region have been explored, leaving the issue of the topological transition of the L 3 phase towards disconnected micelles unresolved. Comparatively little attention has focused on the dynamics and rheology of the sponge.

Structure and rheological properties of model microemulsion networks filled with nanoparticles.

Abstract.Model microemulsion networks of oil droplets stabilized by non-ionic surfactant and telechelic polymer C18 -PEO(10k)- C18 have been studied for two droplet-to-polymer size ratios. The rheological properties of the networks have been measured as a function of network connectivity and can be described in terms of simple percolation laws. The network structure has been characterised by Small Angle Neutron Scattering (SANS). A Reverse Monte Carlo (RMC) approach is used to demonstrate the interplay of attraction and repulsion induced by the copolymer. These model networks are then used as matrix for the incorporation of silica nanoparticles (R = 10 nm), individual dispersion being checked by scattering. A strong impact on the rheological properties is found for silica volume fractions up to 9%.

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

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