Composites materials comprising nanoparticles dispersed in a matrix are of great scientific and technological interest, since nanoparticles can enhance dramatically the matrix properties or even impart new functionalities, and because the matrix can act as a template that structures the particles at the nanoscopic level. However, controlling the three-dimensional spatial distribution of nanoparticles in a molecular or macromolecular matrix is a challenging task, as particle segregation usually depends crucially on the surface chemistry of the particles. Here, we present a model hybrid material, obtained by dispersing nanoparticles in a colloidal crystalline matrix, composed of thermoresponsive micelles. Using confocal microscopy, we show that the nanoparticles segregate in a network of thin sheets, in analogy to impurities confined in the grain boundaries of atomic polycrystals. We demonstrate that the size of the colloidal crystallites is tuned by varying independently the nanoparticle concentration (regardless of their composition and surface chemistry) and the crystallization rate, because they both determine the number of critical nuclei during the nucleation process and we quantify our findings using classical nucleation theory. Remarkably, we find that the efficiency of the segregation of the nanoparticles in the grain-boundaries is dictated solely by the typical size of the crystalline grains, due to the fact that the larger a grain can grow, the higher the concentration of the impurities progressively expelled from the crystallites during their growth and eventually trapped in the grain boundary, as we clearly show. Our method provides a general approach for confining nanoparticles in absence of any external field and in a controlled and tunable fashion in a three-dimensional soft colloidal matrix.

The 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.

We report on a small angle neutron scattering (SANS) study of self-organization of a novel triblock copolymer containing a crystalline poly(ethylene terephthalate) oligomer, a rubbery poly(lauryl acrylate) midblock, and a polyelectrolyte block (quaternized poly(2-(dimethylamino)ethyl methacrylate)) of various length. The rheology of the solutions showed a transition from Newtonian flow at low concentrations, to shear-thinning at intermediate concentrations, and finally a yield behavior at high concentrations. The microstructure of the samples investigated by SANS was found to evolve strongly with the polyelectrolyte block length. The data were analyzed in terms of aggregation numbers, radius of gyration, and aggregate hydration. Complete modeling of the intensity curves was achieved by an inversion method extracting an average aggregate radial volume fraction profile. In both rheology and microstructure, a surprising strong impact of the small PET block was observed.

The structure and rheology of polycrystals is relevant in particular for metals, where post mortem studies are usually performed. In colloidal crystalline systems, the accessible size range makes in-situ investigations possible, under flow and at rest. In this project, termed ‘colloidal metallurgy’, a polycrystalline material is formed spontaneously at high concentrations (34%wt) of spherical Pluronics micelles (F108) in water. The crystalline grains have a local FCC structure (lattice constant 30 nm), and are separated from neighbouring grains of different orientation by grain boundaries (GB), cf. Fig 1, where fluorescent nanoparticles of size comparable to the lattice parameter have been added to visualize the GBs. We have determined the size distribution of grains for different crystallization speeds and nanoparticle contents (typically below 1%). In particular, a universal relationship between grains size and thermal history, and between grain size and amount of nanoparticles has been found, in agreement with theoretical considerations based on classical nucleation theory and constitutional under-cooling. The microscopic structure of silica-containing polycrystals has been studied by contrast-variation small angle neutron scattering, highlighting either the particles or the polymer micelles. The local micellar structure is found to be unaffected by the nanoparticles, which are shown to segregate predominantly in dense zones, presumably GBs. Very small-angle light scattering is currently used to characterize the mesostructure, and the influence of deformation and flow is investigated in a confocal rheo set-up. An alternative view of our findings is that we present here a general approach to confine nanoparticles in colloidal materials in a controlled fashion, independent of the details of the surface chemistry.

Composite materials, comprising nanoparticles dispersed in a matrix, are of great interest, since the nanoparticles can enhance the matrix properties or impart new functionalities and because the matrix can act as a template that structures the particles at the nanoscopic level. However, controlling the spatial distribution of nanoparticles remains a challenging task, as it usually depends crucially on the particles surface chemistry. We present here a general approach to confine nanoparticles in colloidal materials in a controlled fashion, independent of the details of the surface chemistry. We use a nanocomposite material obtained by dispersing small quantities of nanoparticles (at most 2%) in a colloidal crystalline matrix composed of thermosensitive micelles. The volume fraction of the micelles increases with temperature T, until crystallization occurs due to entropic reasons, as in hard sphere colloidal systems. Hence our system allows crystallization to be induced at the desired rate simply by varying T. By analogy with atomic crystals, we expect the nanoparticles, which act as impurities, to be partially expulsed from the growing lattice and to segregate in the grain boundaries. We use small angle neutron scattering to probe the microscopic structure of the composite materials. We show that nanoparticles do not perturb the crystalline structure of the micelles and that different crystallization rates lead to cubic crystals with the same lattice parameter (30 nm) but different nanoparticles rearrangements. In particular, we find that slow crystallization rates induce the formation of regions, up to about tenfold more concentrated in nanoparticles, without macroscopic phase-separation; these regions are the grain boundaries between crystallites. Consistent and complementary results are obtained by confocal microscopy, where the network of grain- boundaries enriched in nanoparticles can be visualized.

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Microemulsion nanocomposites