The equation of state of colloidal dispersions.

International audienceWe present a comparison of experimentally and theoretically determined osmotic pressures for various colloidal dispersions. Experimental data is collected from several different silica and polystyrene dispersions. The theoretical pressure determinations are based on the Primitive Model combined with the Cell Model and the physical quantities are calculated exactly using Monte Carlo simulations in the canonical and grand canonical ensemble

Applications et Propriétés des fluides Rhéologiques

International audienc

Synthèse et propriétés électrorhéologiques de matériaux composites silice/silicone

Une synthèse de particules de silice par hydrolyse d'alcoxysilane a été optimisée afin d'obtenir des nanosphères monodisperses. En dispersion dans de l'huile silicone, elles constituent la phase solide d'un fluide électrorhéologique, fluide à viscosité électriquement contrôlable. La description théorique des interactions entre particules hors et sous champ électrique donne les outils nécessaires à la compréhension des effets de champ sur les propriétés d'écoulement des fluides. L'expérience valide ce travail. En effet, la contrainte seuil des fluides à faibles cisaillements augmente avec le taux d'eau adsorbée sur les particules, mais la réversibilité des systèmes est alors médiocre. Elle sera nettement améliorée par adsorption, en surface de la silice, de cations alcalins hydratés. Les propriétés structurales des fluides, déduites des spectres de diffusion de neutrons aux petits angles, révéleront un confinement des particules en colonnes dans la direction du champ électrique.The composite material formed from monodispersed silica nanospheres dispersed in silicone oil form a so-called electrorheological fluid. The silica spheres were synthesized by alkoxysilane hydrolysis that was optimized in order to control the final size and monodispersity of the spheres. The description of the electrorheological phenomena is based on a theoretical approach of the interparticular interactions under the influence or not of an applied electric field. Experimentally, the flow properties of the fluids agreed well with the theory. The yield stress increased with the amount of water adsorbed on the particles at low shear rate, but the reversibility of the system was poor. It was greatly improved when the surface properties were modified by adsorption of hydrated alkaline cations on the silica particles. The fluid structure was deduced from small angle neutron scattering data. The particles were found to be organized in chain-like structures in the electric field direction.BESANCON-BU Sciences Staps (250562103) / SudocSudocFranceF

Agrégats colloïdaux destinés au renforcement des élastomères

BESANCON-BU Sciences Staps (250562103) / SudocSudocFranceF

Les fluidifiants du plâtre

Cette étude a porté sur l effet de différents fluidifiants (plus particulièrement le Polynaphatalène Sulfonate : PNS et PolyCarboxylatePolyéthoxylé : PCP) sur la rhéologie d une suspension de particules de gypse. Cette rhéologie a été caractérisée essentiellement par la contrainte seuil, par la viscosité dynamique et les modules de cisaillement. Nous avons montré, en utilisant différentes tailles de particules et différentes longueurs pour les molécules de fluidifiant, que la contrainte seuil était inversement proportionnelle au diamètre des particules et au carré de la distance entre les surfaces de particules. Un modèle nous a permis de remonter à l épaisseur de la couche de polymère adsorbée sur la surface du gypse. Nous avons observé une diminution de l efficacité du fluidifiant avec l augmentation de la fraction volumique de particules de gypse qui a été associée à une diminution de l écart entre les surfaces des particules. La viscosité dynamique d une suspension de gypse broyé rets importante malgré la présence de fluidifiant et reflète la présence d agrégats contenant plusieurs dizaines de particules. Les impuretés (ions divalents : MG2+, trivalents : A13+ et Fe3+) ainsi que la température (T>45C) entraînent une dégradation de l efficacité du PCP soit en réduisant l interaction entre le polymère et la surface des particules dans le cas des cations, soit en réduisant la solvatation des polymères dans le cas de l élévation de température.We have studied the effect of different fluidizer molecules (especially the Polynaphtalène Sulfonate : PNS and the PolyCarboxylatePolyéthoxylé : PCP) on the rheology of gypsum suspensions. The rheology was mainly characterized by the yield stress, the dynamic viscosity and the shear moduli. We have shown, using different size of particles and different lengths for the size of the fluidizer molecules, that the yield stress was inversely proportional to the diameter of the particles and to the square of the distance of separation between the surfaces of the particles. With the help of a model we were able to deduce the thickness of the polymer layer adsorbed on the surface of the particles. The dynamic viscosity of a suspension of crushed gypsum remains large despite the use of fluidizer ; it indicates the presence of aggregates containing several tens of particles. The impurities (divalent ions like Mg2+ or trivalent ions like AI3+ and Fe3+) as well as temperature decrease the efficiency of the PCP either by reducing the interaction between the polymer and the surface of the particles in the case of cations, or by reducing the salvation of polymer in the case of a rise in temperature.NICE-BU Sciences (060882101) / SudocSudocFranceF

Electro-active elastomer composite based on doped titanium dioxide Electro-active elastomer composite based on doped titanium dioxide

International audienceRecently, electro-active composites have been considered by several researchers because they exhibit an interesting change in their viscoelastic properties under an applied electric field. However, their relative elastic modulus change ΔG'=G'(E)-G'(0) is still low and rarely exceed 100 kPa. In this article, we demonstrated that, by synthesizing mesoporous aggregates of titanium dioxide (TiO2) and by adsorbing acetylacetone dipolar molecule (Acac) onto the TiO2 surface, the TiO2-Acac/PDMS electrorheological elastomer achieved a relative elastic modulus change ΔG' higher than 500 kPa for an applied electric field of 2 kV mm-1. The dependence of the electrorheological response of TiO2-Acac/PDMS on the DC electric field strength, AC electric field frequency and shear strain magnitude was discussed regarding the conductivity ratio and permittivity ratio between doped TiO2 semiconducting particles and the PDMS matrix. The high electrorheological performance of TiO2-doped Acac as semiconducting particles filled in the elastomeric matrix makes this kind of material a promising candidate for application in the automotive industry, robotics, vibration isolators, building applications or electro-active actuators

Crystallization of Polytetrafluoroethylene in a Wide Range of Cooling Rates: Nucleation and Diffusion in the Presence of Nanosilica Clusters

Polytetrafluoroethylene (PTFE) is a polymer that displays exceptional properties. This synthetic fluoropolymer is also known to crystallize very fast upon cooling. The present work highlights for the first time the influence of nanosilica clusters on PTFE crystallization at fast cooling rates (up to 5000 K·s−1). The silica was synthesized from aqueous silicate solution and the surface modification was performed using TriEthoxyFluoroSilane (TEFS). In order to understand the crystallization behavior of PTFE/silica nanocomposite at a fast cooling rate, the measurements were carried out by Fast Scanning Calorimetry (FSC). The data were consequently combined with the measurements performed by conventional Differential Scanning Calorimetry (DSC). Interestingly, the results displayed variation of the crystallization behavior for the nanocomposite at fast cooling rates compared to slow cooling rates. The differences in crystal morphologies were then observed by Scanning Electron Microscopy (SEM) after slow and fast cooling rates. Finally, the effective activation energies (Eα) obtained from the crystallization under various cooling rates were combined in order to obtain one set of Hoffman-Lauritzen parameters. This procedure allowed us to show that the crystallization of PTFE in the presence of silica is promoted or hampered according to the cooling rates employed

Impact of Silica Nanoclusters on Furfuryl Alcohol Polymerization and Molecular Mobility

International audienceNanocomposite materials present attractive properties and are widely employed in various applications. Most of the time, the insertion of nanoparticles in a polymer matrix induces an enhancement of its performances, yet the effect of the filler on the polymerization mechanisms and the glass transition is less often investigated. In the present study, the PFA/silica nanocomposite was studied to highlight the variation of its polymerization behavior, thermomechanical properties and glass transition induced by the presence of a clustered silica nanoparticles network. The structure of nanosilica clusters was studied by thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR). The furfuryl alcohol (FA) polymerization was studied via its activation energy variation in the presence of nanosilica clusters and anhydride maleic (MA) that led to some modifications of the polymerization mechanism. An enhancement of thermal stability and an increase of glass transition temperature have been put in relief by dynamic mechanical analysis (DMA) and were correlated to the presence of silica. Finally, the activation energy associated with the glass transition highlighted a change of the polymer chain motion process in the presence of silica