Etude in-situ des instabilités plastiques dans les polymères semi-cristallins par balayage SAXS continu

International audienceL'objectif général de ce travail est d'étudier l'évolution de la microstructure de tous les points matériels d'une éprouvette en polyéthylène haute densité (PEHD, polymère semi-cristallin) sollicitée en traction. Du fait du phénomène de striction, plusieurs points matériels de l'éprouvette peuvent parvenir à un même niveau de déformation après avoir subi des histoires de déformation très différentes. Dans cette étude nous analysons l'influence des cinétiques de déformation sur l'évolution de la microstructure de notre PEHD. La caractérisation de la microstructure sur l'ensemble de l'éprouvette a été obtenue in situ sur synchrotron grâce à un balayage SAXS ultra-rapide. En réalisant également des expériences de corrélation d'images 3D, il a été possible de construire un observable quantitatif qui permet d'analyser la dépendance de l'anisotropie de la microstructure pour les différents points matériels (représentation lagrangienne) ou géométriques (représentation eulérienne) d'une même éprouvette en fonction de la déformation locale

Destabilization of montmorillonite suspensions by Ca2+ and succinoglycan

International audienceAggregation of colloidal clay particles (Na-montmorillonite) by CaCl2 and anionic polysaccharide (succinoglycan) in turbulent conditions was investigated using time-resolved size measurements by laser diffraction on diluted (50 mg l(-1)) and stirred suspensions. Excess of Ca2+ ions promotes coagulation of the clay, reducing interparticle repulsions, and allows adsorption of succinoglycan, inducing bridging flocculation. Growth/breakage cycles, characteristic of the turbulent conditions, cause the macromolecules to be incorporated in the innermost of the flocs, where the morphological units are shown by confocal microscopy to be the micrometric Ca-clay particles. Such incorporation results in an increased floc tensile strength, depending on the amount of macromolecules adsorbed, with a maximum at polysaccharide concentrations of 2 wt.% with respect to clay mass

Novel Insights on the Transport of HPAM Solutions in Low Permeability Porous Media: Impacts of Brine and Reservoir Properties

International audienceChemical EOR is now considered as an attractive option for low permeability reservoirs, in particular where lack of gas supply does not allow gas injection processes. However, its application can be challenging for permeabilities below 100 mD as poor injectivity and high chemical retention are frequently observed. This work aimed at investigating the impact of both chemical and mineralogical parameters on the transport of polymer solutions in well-controlled low permeability porous media. Selected polyacrylamide (HPAM) solubilized in brines of variable strengths and hardnesses were injected in granular sand and clays packs having similar petrophysical characteristics (permeability around 60-80 mD) but variable and well controlled mineralogical compositions. The granular packs were characterized in terms of structure (SEM) and specific surface area (BET) before and after polymer injections. The main observables of the coreflood tests were the resistance and residual resistance factors generated by the polymer, the polymer inaccessible pore volume and its irreversible retention. Viscometric analysis showed that the HPAM solutions intrinsic viscosity decreased with increasing total salinity, as expected from charge screening, with a sharp decrease in presence of divalent cations, even at low ionic strength, which was less expected. Coreflood experiments revealed that polymer retention, resistance factor and irreversible resistance factor increased significantly: (a) with increasing ionic strength and hardness for porous media of a given mineralogical composition (this appeared consistent with the outcomes of the viscometric study and confirmed the major impact of hardness); (b) in presence of clays, even at low ionic strength and hardness. The polymer inaccessible pore volume was significantly impacted by the presence of clays, but not by the brine composition. Assuming that polymer retention originated in polymer adsorption, irreversible resistance factors were translated into adsorbed layer thicknesses according to a simple capillary bundle model. This allowed discussing the results in terms of adsorbed layer density, which was showed to increase if brine hardness was increased and to be lower in presence of illite than kaolinite and pure quartz. These findings indicate that complex polymer adsorption/retention mechanisms occur depending on the clay type (layer charge and expandability). This systematic study allowed dissociating the impacts of salinity, hardness and clay contents/types on the transport properties of polymer solutions in low permeability porous media. Its results should be of interest to the chemical EOR industry as they provide guides to help tuning the injection brine composition and polymer concentration to the reservoir properties

Influence of the ionic strength and solid/solution ratio on Ca(II)-for-Na+ exchange on montmorillonite. Part 2: Understanding the effect of the m/V ratio. Implications for pore water composition and element transport in natural media

International audienceThe aim of the present paper is to clarify previous results [1] showing that selectivity coefficients determined for the exchange of Na+ for Ca2+ in montmorillonite were dependent on the solid/solution ratio. The organization of montmorillonite suspensions upon Na+/Ca(II) exchange was analyzed by combining optical microscopy, small-angle X-ray scattering and X-ray diffraction. All samples displayed flocculated characteristics, eliminating the possibility of contrasting accessibility of sorption sites with the solid/ solution ratio. Modeling of experimental X-ray diffraction patterns was used to quantify the relative proportions of interlayer Ca2+ and Na+ cations along the exchange isotherm. The results further confirmed the influence of the solid/solution ratio on the degree of interlayer Ca(II)-for-Na+ exchange, and specific selectivity coefficients for interlayer sites were determined. The effect of the solid/solution ratio was finally interpreted by the resulting local changes in the solution chemistry. We demonstrated that by accounting for the Donnan effect, the different data can be interpreted using a single selectivity coefficient. The obtained Kc constant was successfully applied to interpret existing hydrogeochemical data on a natural aquitard. This most likely represents a more constrained and valid approach for the modeling of reactive element transport in natural media than does the poorly defined Kd parameter

Sol/Gel and Isotropic/nematic transitions in aqueous suspensions of natural nontronite clay. Influence of particle anisotropy. Part 1. Features of the I/N transition

The phase behavior of a natural nontronite clay was studied for size-selected particles by combining osmotic pressure measurements, visual observations under polarized light, and rheological experiments. In parallel, the positional and orientational correlations of the particles were analyzed by small-angle X-ray scattering. Aqueous suspensions of nontronite exhibit a true isotropic/nematic (I/N) transition that occurs before the sol/gel transition, for ionic strengths below 10(-3) M/L. In this region of the phase diagrams, the system appears to be purely repulsive. The I/N transition shifts toward lower volume fractions for increasing particle anisotropy, and its position in the phase diagram agrees well with the theoretical predictions for platelets. SAXS measurements reveal the presence of characteristic interparticular distances in the isotropic, nematic, and gel phases. The swelling law (separation distance vs swelling law) exhibits two regimes. For high volume fractions, the swelling law is one-dimensional as in layered systems and reveals the presence of isolated platelets. At lower volume fraction, distances scale as phi(-1/3), indicating isotropic volumic swelling. Finally, the experimental osmotic pressure curves can be satisfactorily reproduced by considering the interparticle distances between two charged planes whose effective charge is around 10% of the structural charge

Adsorption of polyamine on clay minerals

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Iron speciation at the riverbank surface in wetland and potential impact on the mobility of trace metals

International audienceFe oxyhydroxides in riverbanks and their high binding capacity can be used to hypothesize that riverbanks may act as a “biogeochemical filter” between wetlands and rivers and may constitute a major mechanism in the trapping and flux regulation of chemical elements. Until now, the properties of Fe minerals have been very poorly described in riverbanks. The goals of the present work are to identify Fe speciation in riverbanks where ferric deposits are observed and to determine their impact on the metal behavior (As, Co, Cu, Ni, Pb, Zn, etc.).At the surface, Fe speciation is mainly composed of small poorly crystalline Fe phases, i.e. ferrihydrite (~30%), Fe-OM associations (~40%) as well as crystalline Fe phases, i.e. goethite (~35%). At the subsurface, the Fe distribution is dominated by goethite (~35%) and Fe-mica (~35%), the proportion of which increases at the expense of ferrihydrite and the Fe-OM associations.At the riverbank surface, ferrihydrite and the Fe-OM associations are therefore the main Fe hosting phases in response to (i) the fast Fe(II) oxidation induced by the presence of O2 and (ii) the high amount of OM favoring the formation of nano-phases bound to OM (Fe monomers, polymers and nanoparticles) and preventing mineralogical transformation (ferrihydrite into goethite).During the high-water level period (high flow), a strong erosion of the riverbank transfers these ferric deposits into the river. However, the physicochemical parameters of the river (pH 6.6–7.6 and continuous oxic conditions) do not promote the dissolution of Fe oxyhydroxides and OM. Ferric deposits and the associated trace metals are therefore maintained as colloids/particles and are exported to the outlet. All of the results presented here demonstrate that the ferric deposits trap metals on a seasonal basis and are therefore a key factor in the mobilization of metals during riverbank erosion by river flow

Electrokinetics as an alternative to neutron reflectivity for evaluation of segment density distribution in PEO brushes

Unravelling details of charge, structure and molecular interactions of functional polymer coatings defines an important analytical challenge that requires the extension of current methodologies. In this article we demonstrate how streaming current measurements interpreted with combined self consistent field (SCF) and soft surface electrokinetic theories allow the evaluation of the segment distribution within poly(ethylene oxide) (PEO) brushes beyond the resolution limits of neutron reflectivity technique