Fillers design and best use: recent trends and basic questions

This paper is a discussion of a few selected topics or trends in recent research literature which raise fundamental issues in our understanding of the way fillers improve the properties of polymers. A first topic is the now well‐established experimental and theoretical evidence that the T g and the molecular mobility in free standing or supported films are modified over a much larger thickness than was previously thought. Considering their enormous interface area, this introduces a new look to the remarkable reinforcement effect obtained in delaminated clay nanocomposites. On the other hand, the reason why, in spite of their extraordinary mechanical properties and their very large surface area, carbon nanotubes, with their fibre‐like morphology, are still far from giving the same reinforcement, is still an open question. A second topic to be mentionned is the development of hybrid materials and biomimetic approaches which, due to very small scale of the organic‐inorganic heterogeneities which can be achieved, raises questions on the phase concept and points to the need for new theoretical approaches of the mechanical properties of these materials. Finally, a third topic is the control of the filler particles distribution in the matrix. The general theory of random diphasic media permits to define several types of “good” dispersions in terms of disordered distributions, according to the correlations of the particles positions in a matrix. This raises two problems, which are briefly discussed: (i) Are we able to define the type of dispersion we want? (ii) Is it relevant for modifying the properties?