Chiral nematic nanocomposites with pitch gradient elaborated by filtration and ultraviolet curing of cellulose nanocrystal suspensions.

An innovative method combining frontal filtration with ultraviolet (UV) curing has been implemented to design cellulosic nanocomposite films with controlled anisotropic textures from nanometric to micrometric length scales. Namely, an aqueous suspension containing poly (ethylene glycol) diacrylate polymer (PEGDA) as a photocurable polymer and cellulose nanocrystals (CNCs) at a 70/30 mass ratio was processed by frontal filtration, followed by in-situ UV-curing in a dedicated cell. This procedure allowed designing nanocomposite films with highly oriented and densely-packed CNCs, homogeneously distributed in a PEGDA matrix over a thickness of ca. 500 µm. The nanocomposite films were investigated with small-angle X-ray scattering (SAXS), by raster-scanning along their height with a 25 µm vertically-collimated X-ray beam. The CNCs exhibited a high degree of orientation, with their director aligned parallel to the membrane surface, combined with an increase in the degree of alignment as concentration increased towards the membrane surface. Scanning electron microscopy images of fractured films showed the presence of regularly spaced bands lying perpendicular to the applied transmembrane pressure, highlighting the presence of a chiral nematic (cholesteric) organization of the CNCs with a pitch gradient that increased from the membrane surface to the bulk.

Compressibility and Structural Transformations of Aluminogermanate Imogolite Nanotubes under Hydrostatic Pressure.

We present in situ pressure experiments on aluminogermanate nanotubes studied by X-ray scattering and absorption spectroscopy measurements. Structural transformations under hydrostatic pressure below 10 GPa are investigated as a function of the morphology, organization, or functionalization of the nanotubes. Radial deformations, ovalization for isolated nanotubes, and hexagonalization when they are bundled are evidenced. Radial collapse of single-walled nanotubes is shown to occur, in contrast to the double-walled nanotubes. The effect of the transmitting pressure medium used on the collapse onset pressure value is demonstrated. Axial Young's moduli are determined for isolated (400 GPa) and bundled (600 GPa) single-walled nanotubes, double-walled nanotubes (440 GPa), and methylated single-walled nanotubes (200 GPa).

A new way to apply ultrasound in cross-flow ultrafiltration: application to colloidal suspensions.

A new coupling of ultrasound device with membrane process has been developed in order to enhance cross-flow ultrafiltration of colloidal suspensions usually involved in several industrial applications included bio and agro industries, water and sludge treatment. In order to reduce mass transfer resistances induced by fouling and concentration polarization, which both are main limitations in membrane separation process continuous ultrasound is applied with the help of a vibrating blade (20 kHz) located in the feed channel all over the membrane surface (8mm between membrane surface and the blade). Hydrodynamic aspects were also taking into account by the control of the rectangular geometry of the feed channel. Three colloidal suspensions with different kinds of colloidal interaction (attractive, repulsive) were chosen to evaluate the effect of their physico-chemical properties on the filtration. For a 90 W power (20.5 W cm(-2)) and a continuous flow rate, permeation fluxes are increased for each studied colloidal suspension, without damaging the membrane. The results show that the flux increase depends on the initial structural properties of filtered dispersion in terms of colloidal interaction and spatial organizations. For instance, a Montmorillonite Wyoming-Na clay suspension was filtered at 1.5 × 10(5)Pa transmembrane pressure. Its permeation flux is increased by a factor 7.1, from 13.6 L m(-2)h(-1) without ultrasound to 97 L m(-2)h(-1) with ultrasound.

Electric-field-induced perfect anti-nematic order in isotropic aqueous suspensions of a natural beidellite clay.

We study the electric-field-induced birefringence and orientational order in the isotropic phase of aqueous suspensions of exfoliated natural beidellite clay particles, thin (L = 0.65 nm) flat charged sheets with high aspect ratio, D/L ≈ 300. Our electric birefringence experiment is optimized for aqueous suspensions of colloidal particles, with a high frequency a.c. electric field, ν ≈ 1 MHz, applied by two external electrodes to a thin flat sample, sealed in an optical capillary. In isotropic and biphasic samples, we observed strong field-induced birefringence Δn(E), saturating at moderate E(sat) field to a plateau Δn(sat) proportional to the volume fraction ϕ. The field-induced order parameter S(E) is negative and saturates to S(sat) = -0.5 above E(sat). This corresponds to a perfect "anti-nematic" order, i.e. the normals of the beidellite particles are perpendicular to the field, without any preferred azimuthal direction. The measured specific excess polarizability ΔA(sp) is among the highest data reported for other strongly anisometric dielectric and metal particles. We explain the high ΔA(sp) value with the strong induced polarization of the electric double layer of counterions at the charged particle/electrolyte interface. The estimated equivalent conductivity of the beidellite particle K(eq) = 2 K(σ)/L is several orders of magnitude larger than the bulk conductivity of the electrolyte K(e), resulting in a metal-like behavior of the beidellite disks under field. In the isotropic regions of biphasic nematic/isotropic samples, the excess polarizability is further enhanced by an order of magnitude, indicating collective reorientation of the particles. We propose that this enhancement might be due to pretransitional fluctuations of the spontaneous nematic order S(N) of the colloidal suspension and/or formation of chains of particles, with antinematic order of the beidellite disks in the chains.

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.

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.