Nanocomposite materials present specific features, mainly due to their very high interfacial area, and a very short distance between reinforcing particles surface. For polymer based nanocomposites, reinforcing particles are most of the time much stiffer than the matrix. In that case, the percolation of those particles which occurs at a threshold fraction which depends on their shape factor (and on their dispersion characteristics) is shown to play a drastic role, especially if they form a rigid network. Such a network may result from strong interactions between the surface of the dispersed particles, but softer network may appear if the particles are bound together through the interactions of their surface with polymer chains. In order to analyze and to predict the behavior of nanocomposite materials, a modeling of this type of interactions is proposed, and allows to better understand the origin of the high reinforcing effect, generally observed. However, the complete prediction of composite materials with percolating dispersed fillers is still a difficult problem and requires further developments. It exemplifies for the special case of composites a general problem of two phase materials: the effect of scaling down the reinforcement and the properties of entangled structures.
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