Peter Lindner

Small-angle neutron scattering study of structural changes in temperature sensitive microgel colloids

The structure of temperature-sensitive poly(N-isopropylacrylamide) microgels in dilute suspension was investigated by means of small-angle neutron scattering. A direct modeling expression for the scattering intensity distribution was derived which describes very well the experimental data at all temperatures over an extensive q range. The overall particle form as well as the internal structure of the microgel network is described by the model. The influence of temperature, cross-linking density, and particle size on the structure was revealed by radial density profiles and clearly showed that the segment density in the swollen state is not homogeneous, but gradually decays at the surface. The density profile reveals a box profile only when the particles are collapsed at elevated temperatures. An increase of the cross-linking density resulted in both an increase of the polymer volume fraction in the inner region of the particle and a reduction of the smearing of the surface. The polymer volume fraction inside the colloid decreased with increasing particle size. The structural changes are in good agreement with the kinetics of the emulsion copolymerization used to prepare the microgel colloids.

Colloidal quasicrystals with 12-fold and 18-fold diffraction symmetry

Micelles are the simplest example of self-assembly found in nature. As many other colloids, they can self-assemble in aqueous solution to form ordered periodic structures. These structures so far all exhibited classical crystallographic symmetries. Here we report that micelles in solution can self-assemble into quasicrystalline phases. We observe phases with 12-fold and 18-fold diffraction symmetry. Colloidal water-based quasicrystals are physically and chemically very simple systems. Macroscopic monodomain samples of centimeter dimension can be easily prepared. Phase transitions between the fcc phase and the two quasicrystalline phases can be easily followed in situ by time-resolved diffraction experiments. The discovery of quasicrystalline colloidal solutions advances the theoretical understanding of quasicrystals considerably, as for these systems the stability of quasicrystalline states has been theoretically predicted for the concentration and temperature range, where they are experimentally observed. Also for the use of quasicrystals in advanced materials this discovery is of particular importance, as it opens the route to quasicrystalline photonic band gap materials via established water-based colloidal self-assembly techniques.

SHEAR-INDUCED ISOTROPIC-TO-NEMATIC PHASE TRANSITION IN EQUILIBRIUM POLYMERS

We report on a first-order isotropic-to-nematic phase transition induced by shear in concentrated solutions of elongated flexible wormlike micelles. As shear is applied to a disordered micellar solution, the transition shows up in steady-shear experiments as a kink in the shear stress behaviour. Using small-angle neutron scattering under shear, we show that beyond the transition rate, a nematic phase manifests itself by the growth of crescentlike scattering patterns in the direction perpendicular to the flow. A dynamical phase diagram is finally proposed for this system of equilibrium polymers.

Nanoparticle-Loaded Magnetophoretic Vesicles

Magnetic nanoparticles have been assembled into the bilayer membrane of block copolymer vesicles. The nanoparticles decorate the hydrophobic/hydrophilic interface, which leads to bridging of adjacent bilayers and the formation of oligo-lamellar vesicles. The nanoparticle uptake of the vesicles is sufficiently high to become magnetophoretic in external magnetic fields as shown by video microscopy.

Structure and Morphology of Charged Graphene Platelets in Solution by Small-Angle Neutron Scattering

Solutions of negatively charged graphene (graphenide) platelets were produced by intercalation of nanographite with liquid potassium-ammonia followed by dissolution in tetrahydrofuran. The structure and morphology of these solutions were then investigated by small-angle neutron scattering. We found that >95 vol % of the solute is present as single-layer graphene sheets. These charged sheets are flat over a length scale of >150 Å in solution and are strongly solvated by a shell of solvent molecules. Atomic force microscopy on drop-coated thin films corroborated the presence of monolayer graphene sheets. Our dissolution method thus offers a significant increase in the monodispersity achievable in graphene solutions.

Structural Characterisation of Deposits Formed During Frontal Filtration

Understanding the mechanisms that control the filtration of a complex media is a major challenge for the development of membrane-based processes in bioindustries, agro-industries or sludge treatment, where the complexity of the fluids is seen in terms of composition (liquid-liquid or liquid solid mixtures), or physico-chemical characteristics (rheology, stability,..). This complexity is likely to induce different material organisation within the fluid depending on concentration and hydrodynamics fields. One of the aims of this study is to determine a relation between structural mechanisms and macroscopic properties of cakes formed on filtration of a colloidal suspension. Our investigations were carried out on clays suspensions. Filterability of those suspensions was appreciated throught simple dead en filtration tests and structural characteristics of the deposits formed during filtration were determined by using small-angle neutron scattering (SANS), static light scattering (SLS) and local birefringence techniques, associated with rheometric studies. These led to the conclusion that in the cakes formed from laponite suspensions (volumic fraction= 0.48 %, transmembrane pressure = 0.5 bar), the particles are packed in an anisotropical arrangement parallel to the membrane. Moreover, we show that upon filtration, an aggregation mechanism is promoted , leading to the formation of a porous fractal structure. Adding a peptiser to the suspension changes the structural characteristics of the cake in the sense of a higher fractal dimension in the arrangement. Macroscopic measurements of permeation velocity through the cakes formed with an without a peptiser showed that the initial aggregated state promoted by addition of a peptiser led in this case to a greater permeability.

Calculation of scattering-patterns of ordered nano- and mesoscale materials.

Analytical expressions for the scattering patterns of ordered nano- and mesoscopic materials are derived and compared to measured scattering patterns. Ordered structures comprising spheres (fcc, bcc, hcp, sc, and bct), cylinders (hex and sq), lamellae (lam) and vesicles, as well as bicontinuous cubic structures (Ia3d, Pn3m, and Im3m) are considered. The expressions take into account unit cell dimensions, particle sizes and size distributions, lattice point deviations, finite domain sizes, orientational distributions, core/shell-structures as well a variety of peak shapes. The expressions allow to quantitatively describe, model and even fit measured SAXS and SANS-patterns of ordered or oriented micellar solutions, lyotropic phases, block copolymers, colloidal solutions, nanocomposites, photonic crystals, as well as mesoporous materials.

Unperturbed volume transition of thermosensitive poly-(N-isopropylacrylamide) microgel particles embedded in a hydrogel matrix.

The thermoresponsive behavior of poly-(N-isopropylacrylamide) (PNiPAM) microgels embedded in a covalently cross-linked polyacrylamide hydrogel matrix was investigated using ultraviolet-visible (UV-vis) spectroscopy, small-angle neutron scattering (SANS), and confocal laser scanning microscopy. The hydrogel synthesis was performed at two different temperatures, below and above the volume phase transition temperature of PNiPAM, resulting in highly swollen or fully collapsed PNiPAM microgel particles during the incorporation step. UV-vis spectroscopy experiments verify that the incorporation of thermosensitive microgels leads to temperature-sensitive optical properties of the composite materials. SANS measurements at different temperatures show that the thermosensitive swelling behavior of the PNiPAM microgels is fully retained in the composite material. Volume and structure criteria of the embedded microgel particles are compared to those of the free microgels in acrylamide solution. To visualize the temperature responsive behavior of larger PNiPAM particles, confocal fluorescence microscopy images of PNiPAM beads, of 40-microm size, were taken at two different temperatures. The micrographs also demonstrate the retained temperature sensitivity of the embedded microgels.

The conformation of drag reducing micelles from small-angle-neutron-scattering experiments

Small-angle-neutron-scattering measurements (SANS) were done with a dilute solution of the surfactant n-tetradecyltrimethylammoniumbromide (TTAB) with sodium salicylate (Na Sal) in pipe and channel flow. The solvent used was heavy water. The anisotropy observed in the SANS-curves can be related to the drag reduction behaviour of the surfactant solution. The breakdown of the drag reducing properties above the critical wall shear stress is accompanied by a strong decrease in the scattering anisotropy. However, in this flow region the scattering curves reveal a very similar micellar structure to that found in the fluid at rest. This shows that the micelles exist above the critical wall shear stress.

A new design for the standard pinhole small-angle neutron scattering instrument D11

Monte Carlo simulations have been performed to check ideas for improvements of the collimating guide system of the standard pinhole small-angle neutron scattering (SANS) instrument D11 at Institut Laue–Langevin, which reaches very low Q values by using long detector and collimation distances. According to the simulations, improved guide characteristics can increase the flux at the sample by about 10–15% for short collimation distances. Widening the guide width over the first few metres by 50% will yield a flux gain of about 40% for long and intermediate collimation distances, but a loss for short distances. A gain for all collimation distances can be achieved by a reduction of the guide cross section over the last few metres (in addition to the widening). The best performance was found for a quasi-elliptical exit that reduces width and height to 60% of its nominal size. The effect on divergence of the neutron arriving at the sample has been studied. The changes will be realized in the framework of the ongoing D11 renewal project.