Nanocomposite materials filled with nanoparticles currently exhibit two important unsolved experimental challenges: (i) the elaboration of a general strategy allowing to finely tune, for an easy-to-tune range of parameters, the anisotropy of nanoparticle assembly inside a polymer matrix for improved anisotropic mechanical reinforcement and (ii) an experimental demonstration establishing that the macroscopic mechanical properties of the materials are quantitatively controlled by the filler microstructure. We address them both here by showing how the versatile bottom-up organization of spherical magnetic nanoparticles controlled by a moderate external magnetic field during processing, enables to obtain a wide variety of anisotropic structures, from quasi-isotropic up to a homogeneous dispersion of aligned chains of nanoparticles, as shown by a refined structural study combining SAXS and TEM experiments. The resulting anisotropy of the mechanical properties is spectacular relative to the low particle volume fraction. The Young modulus can be more than 3 times higher when the bulk material is stretched parallel as opposed to perpendicular to the chains and correlates quantitatively in a proportional manner with the anisotropy of the microstructure.

We present here the synthesis and structural characterization of new nanocomposites made of spherical magnetic nanoparticles of maghemite (γ-Fe 2 O 3) dispersed in a polystyrene (PS) matrix. The γ-Fe 2 O 3 nanoparticles, synthetized in aqueous media, were first gently transferred by dialysis in dimethylacetamide (DMAc), a polar solvent which is a good solvent for PS. Electrostatic repulsions enable to keep colloidal stability in DMAc. The nanocomposites were then processed by a controlled evaporation of DMAc of binary mixtures of γ-Fe 2 O 3 nanoparticles and PS chains. The size of the nanoparticles ranges from 3.5 to 6.5 nm and can be changed without any modification of the nanoparticles' surface. The structural organization of the nanoparticles inside the polymer was determined as a function of the nanoparticles' size. It was performed by combining very high resolution SAXS measurements which permit to decrease the nanoparticles content down to very low values (Φ mag ~ 10-5) and TEM microscopy. Whatever the size, the nanoparticles are organized with a hierarchical structure that shows that their aggregation has been driven by a two-step process. At low spatial scale, dense primary aggregates composed of some tens of nanoparticles are formed whatever Φ mag , resulting from the first aggregation step. For Φ mag > 10-4 , these primary aggregates underwent a second aggregation step and are organized at larger scale in fractal aggregates of finite size of ~ 200nm of radius, with a dimension of 1.7. The size of the dense primary aggregates is almost constant when changing the nanoparticles radius, i.e the mean aggregation number of primary aggregates decreases with an increase of the radius.