Ref HAL: hal-00129044_v1
Ref Arxiv: cond-mat/0702102
DOI: 10.1122/1.2828645
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34 Citations
Résumé:

We survey the linear viscoelasticity of a new type of transient network: bridged wormlike micelles, whose structure has been characterized recently [Ramos and Ligoure, (2007)]. This composite material is obtained by adding telechelic copolymers (water-soluble chains with hydrophobic stickers at each extremity) to a solution of entangled wormlike micelles (WM). For comparison, naked WM and WM decorated by amphiphilic copolymers are also investigated. While these latter systems exhibit almost a same single ideal Maxwell behavior, solutions of bridged WM can be described as two Maxwell fluids components blends, characterized by two markedly different characteristic times, t_fast and t_slow, and two elastic moduli, G_fast and G_slow, with G_fast >> G_slow. We show that the slow mode is related to the viscoelasticity of the transient network of entangled WM, and the fast mode to the network of telechelic active chains (i.e. chains that do not form loops but bridge two micelles). The dependence of the viscoelasticity with the surfactant concentration, phi, and the sticker-to-surfactant molar ratio, beta, is discussed. In particular, we show that G_fast is proportional to the number of active chains in the material, phi beta. Simple theoretical expectations allow then to evaluate the bridges/loops ratio for the telechelic polymers.

Ref HAL: hal-00502754_v1
DOI: 10.1021/ma0621167
WoS: 000244160800065
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49 Citations
Résumé:

The thermodynamics and structural behavior of a new type of transient network are reported. The network is obtained by adding in a solution of entangled surfactant wormlike micelles a telechelic triblock copolymer whose hydrophobic ends anchor into the micelles and whose hydrophilic tails are swollen in the aqueous solvent and reversibly link the entangled cylindrical micelles. For comparison, we have also studied the same surfactant system decorated with an amphiphilic diblock copolymer which corresponds exactly to a triblock telechelic copolymer cut into two identical diblock copolymers. We find that the addition of telechelic polymers induces an effective attraction of entropic origin between the surfactant that may result, in equilibrium, in the coexistence of a dilute phase and a connected network, as predicted by theory. Small-angle neutron scattering experiments show first that the locally cylindrical structure of the micelles is maintained upon copolymer addition over a wide range of copolymer-to-surfactant molar ratio. On the other hand, the addition of telechelic polymer correlates with (i) the emergence of a broad peak in the structure factor at a finite scattering vector, q, due to the short-range steric repulsion induced by the polymer between the cylinders, and (ii) the large rise of the scattered intensity at low q's (for small enough surfactant concentration), which indicates an increase of the osmotic compressibility of the solution due to the effective attractive interaction between cylinders provoked by the telechelic polymers linking them.

Ref HAL: hal-00103848_v1
Ref Arxiv: cond-mat/0610148
DOI: 10.1088/1742-5468/2007/02/P02010
WoS: 000246553000013
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19 Citations
Résumé:

We study experimentally and theoretically the sedimentation of gels made of strongly aggregated colloidal particles, focussing on the long time behavior, when mechanical equilibrium is asymptotically reached. The asymptotic gel height is found to vary linearly with the initial height, a finding in stark contrast with a recent study on similar gels [Manley \textit{et al.} 2005 \textit{Phys. Rev. Lett.} \textbf{94} 218302]. We show that the asymptotic compaction results from the balance between gravity pull, network elasticity, and solid friction between the gel and the container walls. Based on these ingredients, we propose a simple model to account for the dependence of the height loss on the initial height and volume fraction. As a result of our analysis, we show that the static friction coefficient between the gel and the container walls strongly depends on volume fraction: the higher the volume fraction, the weaker the solid friction. This nonintuitive behavior is explained using simple scaling arguments.

Commentaires: 13 pages, 5 figures. Submitted to JSTAT

Ref HAL: hal-00502796_v1
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When telechelic polymers (poly-ethyleneoxide grafted at both ends with an aliphatic chain) are added to a microemulsion of decane droplets in water stabilized by a surfactant film they can link the droplets and thus introduce an effective attractive interaction between the droplets, eventually leading to a phase separation. Starting from a neutral microemulsion we show that this attractive interaction can be offset by adding minute quantities of an ionic surfactant, thus introducing a repulsive Coulombic interaction. This opens up the possibility of 'weighting' an unknown effective attraction against a well known repulsion. The phase behaviour and the small angle neutron scattering spectra of different samples illustrate this point. The spectra are simulated assuming the interaction potentials to be the sum of attractive and repulsive contributions and a good agreement is found between the experimental and simulated spectra in the neutral microemulsion, in the charged microemulsion and in the charged and connected microemulsion.