International audienceWe investigate the dispersion mechanisms of nanocomposites made of well-defined polymer (polystyrene, PS) grafted-nanoparticles (silica) mixed with free chains of the same polymer using a combination of scattering (SAXS/USAXS) and imaging (TEM) techniques. We show that the relevant parameter of the dispersion, the grafted/free chains mass ratio R tuned with specific synthesis process, enables to manage the arrangement of the grafted nanoparticles inside the matrix either as large and compact aggregates (R \textless 0.24) or as individual nanoparticles dispersion (R \textgreater 0.24). From the analysis of the interparticles structure factor, we can extract the thickness of the spherical corona of grafted brushes and correlate it with the dispersion: aggregation of the particles is associated with a significant collapse of the grafted chains, in agreement with the theoretical models describing the free energy as a combination of a mixing entropy term between the free and the grafted chains and an elastic term of deformation of the grafted brushes. At fixed grafting density, the individual dispersion of particles below the theoretical limit of R = 1 can be observed, due to interdiffusion between the grafted and the free chains but also to processing kinetics effects, surface curvature and chains poly dispersity. Mechanical analysis of nanocomposites show the appearance of a longer relaxation time at low frequencies, more pronounced in the aggregated case even without direct connectivity between the aggregates. Correlation between the local structure and the rheological behavior suggests that the macroscopic elastic modulus of the nanocomposite could be described mainly by a short-range contribution, at the scale of the interactions between grafted particles, without significant effect of larger scale organizations

The present communication reports the first direct measurement of the conformation of a polymer corona grafted around silica nano-particles dispersed inside a nanocomposite, a matrix of the same polymer. This measurement constitutes an experimental breakthrough based on a refined combination of chemical synthesis, which permits to match the contribution of the neutron silica signal inside the composite, and the use of complementary scattering methods SANS and SAXS to extract the grafted polymer layer form factor from the inter-particles silica structure factor. The modelization of the signal of the grafted polymer on nanoparticles inside the matrix and the direct comparison with the form factor of the same particles in solution show a clear-cut change of the polymer conformation from bulk to the nanocomposite: a transition from a stretched and swollen form in solution to a Gaussian conformation in the matrix followed with a compression of a factor two of the grafted corona.In the probed range, increasing the interactions between the grafted particles (by increasing the particle volume fraction) or between the grafted and the free matrix chains (decreasing the grafted-free chain length ratio) does not influence the amplitude of the grafted brush compression. This is the first direct observation of the wet-to-dry conformational transition theoretically expected to minimize the free energy of swelling of grafted chains in interaction with free matrix chains, illustrating the competition between the mixing entropy of grafted and free chains, and the elastic deformation of the grafted chains. In addition to the experimental validation of the theoretical prediction, this result constitutes a new insight for the understanding of the general problem of dispersion of nanoparticles inside a polymer matrix for the design of new nanocomposites materials.3 Soft polymer properties are improved by inclusion of small hard inorganic particles inside the melt matrix. More recently, special attention has been focused on the reduction in filler size down to the nanometer range, to enhance the macroscopic properties of the nanocomposites for the design of new materials for applications in mechanical, optical, fuel cells or gas barrier engineering. The key point is the compatibility between the nano-filler and the polymer matrix, and one active field is chain grafting synthesis to dress the particle with a polymer corona of the same nature as the polymer matrix. Many recent studies present various refined synthesis processes of controlled polymerization to obtain well defined grafted nanoparticles 1 . Depending on the synthesis conditions, it is possible in a controlled way to change the grafting density, the length of the grafted chains or the particles colloidal stability. The resulting grafted objects can be used as fillers by mixing with free chains of the same polymer to create new hierarchical structures inside the matrix, from well dispersed state to large anisotropic or fractal aggregates without direct connectivity, or with f...