Résumé:

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.