Marianne Impéror-Clerc

Liquid-crystalline nematic phase in aqueous suspensions of a disk-shaped natural beidellite clay.

After size-selection and osmotic pressure measurements at fixed ionic strength, the behavior of aqueous colloidal suspensions of anisotropic disklike beidellite clay particles has been investigated by combining optical observations under polarized light, rheological, and small angle X-ray scattering (SAXS) experiments. The obtained phase diagrams (volume fraction/ionic strength) reveal, for ionic strength below 10(-3) M/L, a first-order isotropic/nematic (I/N) phase transition before gel formation at low volume fractions, typically around 0.5%. This I/N transition line displays a positive slope for increasing ionic strength and shifts toward lower volume fraction with increasing particle size, confirming that the system is controlled by repulsive interactions. The swelling laws, derived from the interparticle distances obtained by SAXS, display a transition from isotropic swelling at low volume fractions to lamellar swelling at higher volume fractions. The liquid-crystal properties have then been investigated in detail. Highly aligned nematic samples can be obtained in three different ways, by applying a magnetic field, an ac electric field, and by spontaneous homeotropic anchoring on surfaces. The birefringence of the fluid nematic phase is negative with typical values around 5 x 10(-4) at a volume fraction of about 0.6%. High nematic order parameters have been obtained as expected for well-aligned samples. The nematic director is aligned parallel to the magnetic field and perpendicular to the electric field.

Structure of Micelles of a Nonionic Block Copolymer Determined by SANS and SAXS

The micellar state of Pluronic P123, which is a poly(ethylene oxide)-b-poly(propylene oxide)-b- poly(ethylene oxide) block polymer (EO(20)PO(70)EO(20)), has been investigated using SANS, SAXS, and differential scanning calorimetry under the conditions utilized in the synthesis of ordered mesoporous materials, such as SBA-15. The absolute intensity measurements, both with SANS and SAXS, have provided a detailed quantitative description of the P123 micelles in the framework of a simple core-shell spherical model. The model developed has been used to establish the structure of the copolymer micelles, including their size, shape, aggregation number and detailed composition, as well as the structural changes induced by varying reaction conditions. The effects of temperature, pH, acidic source and the addition of swelling agents (toluene and TMB) are reported and discussed.

Initial stages of SBA-15 synthesis: An overview☆

This work presents an overview of the data obtained for SBA-15 synthesis under the reaction conditions using synchrotron based small angle X-ray scattering and small angle neutron scattering. Three major stages in the synthesis of SBA-15 materials proceeding according to the cooperative self-assembly mechanism have been identified, and the structures of the intermediates species have been established. Our in situ time-resolved neutron scattering experiments demonstrate that only spherical micelles of the templating agent are present in the synthesis mixture during the first stage of the reaction. According to the neutron scattering and X-ray scattering data, in the second stage of the reaction the formation of hybrid organic-inorganic micelles is accompanied with the transformation from spherical to cylindrical micelles, which takes place before the precipitation of the ordered SBA-15 phase. During the third stage, these micelles aggregate into a two-dimensional hexagonal structure, confirming that the precipitation takes place as the result of self-assembly of the hybrid cylindrical micelles. As the synthesis proceeds, the voids between the cylinders are filled with the silicate species which undergo condensation reactions resulting in cross-linking and covalent bonding, leading to the formation of highly ordered SBA-15 mesostructure. This work demonstrates that valuable structural information can be obtained from X-ray and neutron scattering characterisation of complex systems containing periodic phases with d-spacing values up to 30 nm, and that both techniques are powerful means for in situ monitoring of the formation of nanostructured materials.

Smectic polymer micellar aggregates with temperature-controlled morphologies

The morphological control of polymer micellar aggregates is a very important issue in applications such as drug delivery and material science. We report the temperature-controlled formation of nanotubes, nanofibers, ellipsoidal and faceted vesicles, and spherical aggregates by nanoprecipitation of amphiphilic diblock copolymers in dioxane/water mixture. The copolymers used are composed of a cholesterol-based smectic liquid crystal core-forming block and a PEG hydrophilic block. The morphology of the micellar self-assemblies was studied by transmission electron microscopy (TEM), cryo-electron microscopy (cryo-TEM) and atomic force microscopy (AFM). In all these aggregates smectic organization is clearly present in the hydrophobic cores. We propose a smectic “liquid crystallization”-driven self-assembly process for the formation of nanofibers and nanotubes on the basis of small angle X-ray scattering (SAXS) studies during the nanoprecipitation. The temperature dependence of the morphology (from T = 5–55 °C) is explained by the free energy consideration. The different aggregates finally dispersed in water after the removal of dioxane are thermally stable at temperature ≤55 °C and can be preserved for years at room temperature without structural change.

Growth and Self-Assembly of Ultrathin Au Nanowires into Expanded Hexagonal Superlattice Studied by in Situ SAXS

We report the self-assembly of gold nanowires into hexagonal superlattices in liquid phase followed by in situ small-angle X-ray scattering and give new insights into their growth mechanism. The unprecedented large interwire distance of 8 nm strongly suggests the stabilization of the ultrathin gold nanowires by a ligands double layer composed of oleylamine and oleylammonium chloride. The one-dimensional growth is discussed, opening perspectives toward the control growth and self-assemblies of metallic nanowires.

New insights into the initial steps of the formation of SBA-15 materials: an in situ small angle neutron scattering investigation.

Time-resolved in situ SANS investigations have provided direct experimental evidence for the three initial steps in the formation of the SBA-15 mesoporous material: an induction period is followed by a shape transformation of the micelles from spherical to cylindrical ones followed by the precipitation of a two-dimensional hexagonal phase.

Liquid–crystalline properties of aqueous suspensions of natural clay nanosheets

Clay minerals, like beidellite or nontronite, spontaneously exfoliate in water and form colloidal suspensions of nanosheets. In a given range of concentration, these suspensions display a nematic liquid–crystalline phase whose structure and properties can be conveniently studied in detail by polarized-light microscopy and small-angle X-ray scattering (SAXS). Moreover, in situ SAXS investigations of sheared clay suspensions provide information about their flow properties, both in the isotropic and nematic phases. The colloidal nematic phase shows the classical properties of usual nematics, such as surface anchoring and electric-field and magnetic-field alignment. Thus, nematic single domains can be produced. The isotropic phase also displays strong electro-optic effects in moderate electric fields. Finally, we describe a few examples of applications of such systems and we show how these studies could be extended to suspensions of other types of nanosheets.

An effective geometrical approach to the structure of colloidal suspensions of very anisometric particles

We show in the present letter that the organization of colloidal suspensions of very anisometric repulsive particles can be understood on the basis of simple geometrical considerations. Using a large set of rod-like and plate-like particles, we first evidence that the experimental inter-particle distances can be accurately predicted from geometrical constraints. We then show that the experimental static structure factors can be satisfactorily fitted using an effective Percus-Yevick structure factor. The fit parameters are then interpreted in terms of the co-excluded volumes of effective ghost particles, which further supports the geometrical representation previously developed.

Facile direct synthesis of ZnO nanoparticles within lyotropic liquid crystals: towards organized hybrid materials

Here we report the use of cubic and hexagonal phases of lyotropic liquid crystals as templates for the synthesis of zinc oxide nanoparticles. The templating effect of the lyotropic phases resulted in the formation of isotropic nanoparticles, whose size and assembly were dictated by the geometry of hydrophobic PPO cores and the nature of the phase.

Rheo-SAXS investigation of shear-thinning behaviour of very anisometric repulsive disc-like clay suspensions

Aqueous suspensions of swelling clay minerals exhibit a rich and complex rheological behaviour. In particular, these repulsive systems display strong shear-thinning at very low volume fractions in both the isotropic and gel states. In this paper, we investigate the evolution with shear of the orientational distribution of aqueous clay suspensions by synchrotron-based rheo-SAXS experiments using a Couette device. Measurements in radial and tangential configurations were carried out for two swelling clay minerals of similar morphology and size, Wyoming montmorillonite and Idaho beidellite. The shear evolution of the small angle x-ray scattering (SAXS) patterns displays significantly different features for these two minerals. The detailed analysis of the angular dependence of the SAXS patterns in both directions provides the average Euler angles of the statistical effective particle in the shear plane. We show that for both samples, the average orientation is fully controlled by the local shear stress around the particle. We then apply an effective approach to take into account multiple hydrodynamic interactions in the system. Using such an approach, it is possible to calculate the evolution of viscosity as a function of shear rate from the knowledge of the average orientation of the particles. The viscosity thus recalculated almost perfectly matches the measured values as long as collective effects are not too important in the system.