Soft water-soluble microgel dispersions: structure and rheology.

The size and structural characteristics of polyacrylamide-based water-soluble microgel dispersions were investigated by optical and rheological methods. Microgel hydrodynamic radii Rh were measured by light scattering and derived from intrinsic shear viscosity [eta]0. The variations of Rh3 and [eta]0 with the crosslink density Nx, follow the scaling law Rh3 congruent withNx(-alpha) with alpha close to 0.63, in good agreement with the simple structural model proposed in this paper showing how the exact value of alpha depends on inner structural details of the microgel. The plateau viscosity versus particle apparent volume fraction shows a monotonous change from hard sphere dispersions (high crosslink density of microgels) to flexible linear polymer solutions. Measurements of the first normal stress difference N1 show that increasing the microgel crosslink density affects the system viscosity more than its elasticity. Under oscillatory shear flow, loss and storage moduli undergo both qualitative and quantitative changes with crosslink density. At moderate concentrations, the elastic modulus is the most affected and its slope in low frequency regime decreases from two to less than one as Nx increases. We discuss the experimental results within the frame of knowledge on linear, branched polymer solutions and soft microgel suspensions.

Surface heterogeneity of passively oxidized silicon carbide particles: vapor adsorption isotherms.

The surfaces of silicon carbide particles subjected to two different passive oxidation treatments have been characterized by immersion calorimetry and vapor adsorption techniques. Surface enthalpies and surface free energies have been computed using semiempirical models and are compared to theoretical estimations. The surface entropy term appears higher than in the case of other solids studied with the same analysis. The definition of the surface entropy term is discussed in order to explain the discrepancy between calculation and experiment. An explanation of results is proposed, which is related to the constitution of silicon oxide layers at the surface of silicon carbide, a fact demonstrated by previous XPS measurements.

Zirconium speciation in lactate solutions and polyacrylate gels.

Controlling gelation kinetics is an important objective for several applications (ceramic and thin film syntheses, reduction in permeability of porous rock, etc). There is a growing interest in studying the gelation of polymers by zirconium, a crosslinker of lower toxicity than the chromium which is still commonly used. XAS at the Zr K-edge was performed at the European Synchrotron Radiation Facility (ESRF) on the BM32 beamline. The fluorescence detection was used to carry out successful in situ speciation at concentrations as low as 36 ppm. The Zr speciation was determined both in ZrLa (where La stands for lactate) aqueous solutions and in gels of a terpolymer of acrylamide having 2% of zirconium reactive acrylate side groups and 2% of sulfonate groups introduced to prevent syneresis. XANES results show that Zr is always in a dodecahedral geometry. In ZrLa solutions. EXAFS results indicate that Zr species grow from a dimer Zr2(La)6 to a tetramer (Zr4(La)x) and then to larger polymers resulting from tetramer associations, as the Zr concentration decreases from 51840 ppm to 36ppm. In polymer gels, Zr species appear to be dimers at pH 6 while tetramers are found when gelation occurred at pH 7. Calculations taking into account multiple scattering effects as well as dynamic molecular calculation confirmed conclusions derived from conventional EXAFS analysis.

Surface Heterogeneity of Passively Oxidized Silicon Carbide Particles: Hydrophobic-Hydrophilic Partition.

The surface of silicon carbide (SiC) particles previously subjected to passive oxidation has been characterized using various techniques such as adsorption of positively and negatively charged surfactants in aqueous solution, immersion microcalorimetry in three probe liquids, and flow microcalorimetry in organic media. Adsorption data show that around one quarter of the surface appears negatively charged and thus hydrophilic, while three quarters appear uncharged and hydrophobic. This is attributed to dissociation of silanols groups. Immersion calorimetry in liquids having well-defined polar and nonpolar components of surface energy shows that the Lifshitz-van der Waals component of SiC is very high and that the acid and basic components are weak. The experimental results appear to be consistent with both computations of surface energy using Lifshitz theory and experimental data previously obtained with other minerals. The three indexes are discussed and it is argued that they represent different terms of the solid surface energy. Copyright 2000 Academic Press.