Polymères associatifs téléchéliques asymétriques (synthèse, structure et rhéologie)

Dans ce manuscrit nous étudions les polymères associatifs (PA) téléchéliques composés d'une chaîne hydrophile poly(oxyde d'éthylène) (POE) modifiée aux extrémités par des groupements hydrophobes de même nature (PA symétriques) ou de structures différentes (PA asymétriques) avec un groupement hydrocarboné à une extrémité et semi-fluoré à l'autre. L'objectif est de corréler la structure chimique des PA avec les propriétés rhéologiques en solution aqueuse. La synthèse des PA asymétriques est réalisée par polymérisation anionique de l'oxyde d'éthylène amorcée par un alcoolate, suivie de l'estérification par un acide semi-fluoré. Cette synthèse permet d'obtenir des PA modèles totalement modifiés avec un faible indice de polymolécularité. Les polymères sont purifiés grâce à un polymère à empreinte moléculaire MIP. En solution aqueuse, les PA s'associent en agrégats et peuvent induire une séparation de phase. Pour homogénéiser les solutions un tensioactif est additionné : le dodécyl sulfate de sodium (SDS) ou le perfluorooctyl sulfonate de lithium (LFOS). L'étude des solutions diluées en présence de SDS par tensiométrie, RMN 13C et DNPA a permis de déterminer la structure des agrégats en fonction des concentrations des deux composés. En solution semi-diluée, les PA forment un réseau transitoire et augmentent la viscosité. L'étude des solutions en rhéologie linéaire a mis en évidence des corrélations entre la structure du PA (longueur et nature des groupements hydrophobes, PA symétrique ou asymétrique) et les caractéristiques rhéologiques du réseau (temps de relaxation t et module élastique G0). De plus l'influence du SDS est comparée à celle du tensioactif perfluoré (LFOS)In this manuscript we study telechelic associative polymers (APs) composed of hydrophilic poly(ethylene oxide) chain (PEO, Mn = 20 000 g/mol) end-capped by hydrophobic groups. Symmetric APs have the same hydrophobic group at each end and asymmetric APs contain an alkyl group on one chain end and a perfluoroalkyl group on the other one. The aim of this work was to establish a relationship between the microscopic structure and the rheological properties of APs in aqueous solution. The asymmetric AP synthesis is carried out by anionic polymerization of ethylene oxide initiated by an hydrocarbon alkoxide, followed by esterification with a perfluoroalkylated carboxylic acid. By this way we can obtain completely end-capped APs with a low polydispersity index. Polymers are purified using a molecularly imprinted polymer MIP. In aqueous solution, APs form aggregates and can lead to a phase separation. To investigate homogeneous solutions a surfactant is added: the sodium dodecyl sulfate (SDS) or the lithium perfluorooctyl sulfonate (LFOS). The dilute solution studies in the presence of SDS, by tensiometry, 13C NMR and SANS, enable to determine the aggregate structure as a function of concentrations of the two components. In semi-diluted solution, APs form a transient network with an increase of viscosity. The aqueous solution study by rheology reveals correlations between the AP chemical structure (hydrophobic group length and nature, symmetric or asymmetric AP) and the network rheological characteristics (relaxation time t and elastic modulus G0). The SDS influence is compared with that of the perfluorinated surfactant LFOSMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Single step hybrid coating process to enhance the electrosteric stabilization of inorganic particles

We report on a single-step coating process and the resulting colloidal stability of silica-coated spindle-type hematite nanoparticles (NPs) decorated with a layer of poly(acrylic acid) (PAA) polyelectrolyte chains that are partially incorporated into the silica shell. The stability of PAA coated NPs as a function of pH and salt concentration in water was compared to bare hematite particles and simple silica-coated hematite NPs, studying their electrophoretic mobility and the hydrodynamic radius by dynamic light scattering. Particles coated with this method were found to be more stable upon the addition of salt at pH 7, and their aggregation at the pH of the isoelectric point is reversible. The hybrid coating appears to increase the colloidal stability in aqueous media due to the combination of the decrease of the isoelectric point and the electrosteric stabilization. This coating method is not limited to hematite particles but can easily be adapted to any silica-coatable particle

Influence of Surfactants on Hydrophobically End-Capped Poly(ethylene oxide) Self-Assembled Aggregates Studied by SANS

International audienceThe rheological properties of associative polymers (APs) are modified by the presence of surfactants, but little is known about the influence of surfactants on the structure of selfassociated aggregates in aqueous media. Using small-angle neutron scattering (SANS), the structure of self-assembled aggregates of a series in concentration of telechelic poly(ethylene oxide) (M ≈ 20 000 g mol 1) end-capped by hydrocarbon or of fluorocarbon groups in the presence of a fixed amount of sodium dodecyl sulfate (SDS) or lithium perfluorooctyl sulfonate (LiFOS) is investigated. In all systems studied, surfactants and telechelic polymers form mixed aggregates. Their aggregation numbers are reduced and evolve with the concentration of each component. For example, with SDS at 8 mM, the mixed aggregates are composed of 1 AP and 15 SDS or 9 APs and 45 SDS molecules for 1 or 3 wt % of polymer, respectively. The addition of a surfactant enhances the number density of aggregates, which causes the increase of the gel viscosity

Asymmetric End-Capped Poly(ethylene oxide). Synthesis and Rheological Behavior in Aqueous Solution.

International audienceA method was developed to prepare asymmetric end-capped poly(ethylene oxide)s (PEOs), containing an alkyl group on one chain end and a perfluoroalkyl group on the other one (C18H37-PEO-C2H4-C8F17 and C18H37-PEO-C10H20-C8F17). These new telechelic associative polymers (APs) were synthesized by anionic polymerization of ethylene oxide initiated by an alkoxide (C18H37-O-K+) followed by esterification. The rheological behavior of asymmetric and analogous symmetric APs in aqueous solutions has been investigated as a function of polymer concentration (C), hydrophobic end-cap and surfactant used to homogenize the system: sodium dodecyl sulfate (SDS) or lithium perfluorooctyl sulfonate (LiFOS). A steep increase of the static viscosity η, attributed to the formation of a multiconnected network, is observed for C ≈ 1 wt %. Semidilute solutions present a Maxwellian rheological behavior with only one relaxation time τ associated with the least hydrophobic end group. The viscoelastic response is modified by the interactions between hydrophobic end groups and surfactant inside mixed aggregates

SDS interactions with hydrophobically endcaped poly(ethylene oxide) studied by 13C NMR and SANS

International audienceThe interactions, in aqueous medium, between hydrophobically end-capped poly(ethylene oxide) (PEO) and sodium dodecyl sulfate (SDS) have been investigated by means of tensiometry, 13C NMR spectroscopy and small-angle neutron scattering (SANS). Besides the association of SDS with the ethylene oxide backbone of associative polymer (AP), revealed by 13C NMR, SDS interacts with hydrophobic endcaps leading to the formation of mixed aggregates. At low SDS concentration, the formation of flowerlike mixed aggregates is governed by AP. Despite the poor affinity between hydrocarbon and fluorocarbon chains, uncooperative binding was detected between SDS and fluorinated AP end groups. With a further increase of SDS concentration, the critical aggregation concentration (cac) marks the onset of SDS cooperative binding with AP end-caps. This process driven by SDS leads to the formation of micellar aggregates with a necklacelike structure. Due to the SDS/AP complex formation, the surfactant aggregation number determined by SANS is lower than that observed in pure water

Light and Neutron Scattering Study of PEG-oleate and Its Use in Emulsion Polymerization

Steric stabilization of colloids forms a robust mechanism to obtain colloids that are stable in a variety of environments, and that can be used to study the phase behavior of hard or soft spheres. We report the synthesis of sterically stabilized colloids in an aqueous environment using readily dissolvable surfactants, with an unsaturated hydrophobic tail. We synthesized a new surfactant by esterification of a poly(ethylene glycol) (PEG) chain of 4.1 kg/mol with oleic acid, called PEG4OA. The micellization of PEG4OA was characterized by light and neutron scattering, which yielded values for the aggregation number and the overall size that are in excellent agreement with a comparable surfactant with a saturated octadecane chain, Brij 700. We successfully used PEG4OA in the emulsion polymerization of polystyrene colloids. In comparison with the smaller surfactant Tween 80, PEG4OA yielded smaller colloids with radii around 50 nm, and the addition of 1-dodecanethiol reduced the formation of aggregates during the synthesis. A contrast variation study with small angle neutron scattering (SANS) showed that a dense PEG layer was grafted to the colloid surface

Light and Neutron Scattering Study of PEG-oleate and Its Use in Emulsion Polymerization

Steric stabilization of colloids forms a robust mechanism to obtain colloids that are stable in a variety of environments, and that can be used to study the phase behavior of hard or soft spheres. We report the synthesis of sterically stabilized colloids in an aqueous environment using readily dissolvable surfactants, with an unsaturated hydrophobic tail. We synthesized a new surfactant by esterification of a poly­(ethylene glycol) (PEG) chain of 4.1 kg/mol with oleic acid, called PEG4OA. The micellization of PEG4OA was characterized by light and neutron scattering, which yielded values for the aggregation number and the overall size that are in excellent agreement with a comparable surfactant with a saturated octadecane chain, Brij 700. We successfully used PEG4OA in the emulsion polymerization of polystyrene colloids. In comparison with the smaller surfactant Tween 80, PEG4OA yielded smaller colloids with radii around 50 nm, and the addition of 1-dodecanethiol reduced the formation of aggregates during the synthesis. A contrast variation study with small angle neutron scattering (SANS) showed that a dense PEG layer was grafted to the colloid surface

Anisotropic magnetic particles in a magnetic field

We characterize the structural properties of magnetic ellipsoidal hematite colloids with an aspect ratio ρ ≈ 2.3 using a combination of small-angle X-ray scattering and computer simulations. The evolution of the phase diagram with packing fraction φ and the strength of an applied magnetic field B is described, and the coupling between orientational order of magnetic ellipsoids and the bulk magnetic behavior of their suspension addressed. We establish quantitative structural criteria for the different phase and arrest transitions and map distinct isotropic, polarized non-nematic, and nematic phases over an extended range in the φ-B coordinates. We show that upon a rotational arrest of the ellipsoids around φ = 0.59, the bulk magnetic behavior of their suspension switches from superparamagnetic to ordered weakly ferromagnetic. If densely packed and arrested, these magnetic particles thus provide persisting remanent magnetization of the suspension. By exploring structural and magnetic properties together, we extend the often used colloid-atom analogy to the case of magnetic spins