To what extent is the structure of a random composite compatible with a percolation model?

Recent developments of local field microscopy allow a considerable improvement over more traditional techniques (like transmission electron microscopy) of the knowledge of the actual structure of composites made of conducting particles dispersed in an insulating polymer matrix. The Resiscope is an attachment to an atomic force microscopy (AFM) microscope that provides at the same time a classical image of the sample surface and a novel image representative of the local electrical conduction through the sample, between the tip and the back. Using the thickness of the sample as an independent variable, we have obtained original data on the 3D structure of the finite as well as on the infinite cluster and their relative variations with the particle concentration in a series of carbon black-filled elastomers. We have performed computer simulations on percolation cubic networks aimed at providing similar data when both the filling factor p and the sample "thickness" (number of planes) are allowed to vary. We have found that in spite of a general agreement between experimental and calculated data, there exist significant differences that seem to prove that the actual structure is more complex than the one predicted by a classical percolation model for random composites. (C) 2003 Elsevier B.V. All rights reserved.