By adding nanoparticles to polymer melts, nanocomposites with improved mechanical and thermal properties are obtained. For a given polymer-filler couple, the improvement of properties appears related to the filler size, volume fraction and dispersion. Nevertheless no universal model is established at the present time. Whereas the filler size and volume fraction are determined by the formulation, the dispersion is more difficult to control and investigate. In the first part of the talk, we will show how we can tune the particles dispersion in model nanolatex systems (polymethacrylate-silica) using different strategies: the control of the filler charge, the modulation of the latex/filler size ratio (R) and the use of different polymer chain length. Using small angle scattering and TEM analysis we will demonstrate that repulsive aggregates of varying size can be obtained playing with the pH of the casting solution1. A good filler dispersion is obtained for R=1, whereas a network of well-organized silica particles around the latex beads is highlighted for high latex/filler size ratio. Finally, decreasing the polymer molecular weight, fractals aggregates of increasing size are generated. The effect of these various structures on mechanical properties will be discussed. The mechanical properties of nanocomposite materials are controlled to a large extent by the filler-filler interactions, nevertheless, another important contribution - less well understood - is due to the polymer chain-filler interactions. In a second part, we will display results of an investigation of the polymer chains in such nanocomposites2. Small angle neutron scattering experiments performed in zero average contrast conditions (mixing hydrogenated and deuterated latex beads) enabled to follow the latex beads dissolution using an original model. Experimental results demonstrate that the dissolution dynamic of polymer chains is significantly slowed down by the presence of silica nanoparticles.