We study the nonlinear rheology of a novel class of transient networks, made of surfactant micelles of tunable morphology reversibly linked by block copolymers. We couple rheology and time-resolved structural measurements, using synchrotron radiation, to characterize the highly nonlinear viscoelastic regime. We propose the fluctuations of the degree of alignment of the micelles under shear as a probe to identify a fracture process. We show a clear signature of a brittle-to-ductile transition in transient gels, as the morphology of the micelles varies, and provide a parallel between the fracture of solids and the fracture under shear of viscoelastic fluids.

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 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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We report on a new class of self−assembled transient networks made of surfactant micelles of tunable morphology (from spheres, to rodlike to wormlike) reversibly linked by telechelic polymers. Linear rheological measurements show that three distinct domains can be defined depending on the morphologies of the micelles: a domain where the micelles are isolated and not entangled, an intermediate domain where the micelles are partially entangled and a domain where the micelles are fully entangled. Flow curves of the transient networks of tunable morphology associated with structural informationjs obtained from Rheo−SAXS suggest that one can associate to the three domains distinct failures modes: a brittle mode, an intermediate mode and finally a ductile/shear banding mode, as the micelles grow. The ultimate goal is to elucidate the parameters that drive the transition