Phases lamellaires dopées avec des polyoxométalates

Les systèmes hybrides organiques-inorganiques sont actuellement l'objet d'une grande attention parce qu'ils peuvent combiner les propriétés électroniques des matériaux inorganiques avec les propriétés d'autoassemblage des molécules organiques. Nous avons formulé un tel système, en associant des nanoparticules de polyoxométallates (POMs) avec une phase lamellaire Iyotrope. La diffusion de rayons X et la microscopie en lumière polarisée ont démontré que ce système hybride, très chargé en POMs, est un nanocomposite de structure lamellaire cristal-liquide (La), avec des propriétés viscoélastiques proches de celles des gels. L'interprétation des données de rayons X suggère fortement que les POMs sont localisés près des groupements -OH terminaux du tensio-actif, dans la couche aqueuse. De plus, le système présente du photochromisme réversible, associé à la photoréduction de l'anion polyoxométallate. D'un point de vue plus fondamental, ces matériaux hybrides peuvent servir comme modèles pour étudier les propriétés statiques et dynamiques des nanoparticules confinées dans des phases lamellaires. Ainsi, la diffusion de rayons X a été utilisée pour étudier l'effet du confinement sur le potentiel d'interaction entre les particules. En outre, nous avons mesuré les coefficients de diffusion des particules, en utilisant la RMN en gradient de champ. Nous avons démontré que le coefficient de diffusion est très réduit dans la phase, ce qui confirme la forte interaction entre les particules et la bicouche. Finalement, des expériences simples ont permis l'étude des propriétés rhéologiques du système.Organic-inorganic hybrid systems have recently attracted a lot of attention. They can potentially combine the electronic properties of inorganic materials with the self-assembly properties of organic molecules. We have developed such a hybrid system, in which polyoxometallate nanoparticles (POMs) are inserted between the bilayers of a Iyotropic lamellar phase. X-ray scattering and polarized light microscopy demonstrate that these hybrid materials, highly loaded with POMs, are nanocomposites of liquid-crystalline lamellar structure (La), with viscoelastic properties reminiscent of those of gels. The interpretation of X-ray scattering data strongly suggests that the POMs are located close to the terminal -OH groups of the nonionic surfactants, within the aqueous sublayers. Moreover, these materials exhibit a reversible photochromism associated to the photoreduction of the polyoxometallate anion. From a more fundamental point ofview, these hybrid materials can serve as models for the study ofboth static and dynamic properties of nanoparticles confined within soft lamellar structures. X-ray scattering was used to study the effect of confinement on the interparticle interaction potential. Furthermore, pulsed-field gradient NMR was used to measure the translational diffusion coefficients of the POMs as a function of the inter-bilayer distance. It was demonstrated that the dynamics of the particles are significantly slowed down, and, thus, that they strongly interact with the bilayer. Finally, sorne simple experiments were done to determine the rheological behaviour of the system in question.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Large amplitude oscillatory shear (LAOS) in model colloidal suspensions and glasses: frequency dependence

We investigate the effect of frequency on the non-linear large amplitude oscillatory shear (LAOS) response of concentrated colloidal suspensions of model soft and hard spheres at various concentrations, both below and above the glass transition. We show that the anharmonic response in the stress increases with frequency for liquid-like samples but decreases with frequency for solid-like samples. We argue that for samples below the glass transition, higher frequencies involving higher maximum shear rates promote shear thinning and increase anharmonicity. On the other hand, solid-like samples deform plastically at low frequencies as they are subjected to low shear rates within the period. Higher frequencies (higher average shear rates) lead to viscous flow over a larger fraction of the period thereby decreasing anharmonic behavior. We also demonstrate that LAOS experiments in strain-controlled rheometry at moderately high frequencies (ω > 5 rad/s) have to be very carefully interpreted, due to the superharmonic instrumental resonance effects