JOURNAL OF MATERIALS SCIENCE 34 (1 9 9 9 ) 1753 – 1759 Characterization of the damage in nanocomposite materials by a.c. electrical properties: experiment and simulation L. FLANDIN, J.-Y. CAVAILLE ∗ CERMAV-CNRS Universit´ e Joseph Fourier, BP 53, 38041 Grenoble Cedex, France E-mail: cavaille@gemppm.insa-lyon.fr Y. BRECHET LTPCM-ENSEEG, Rue de la Houille Blanche Du, BP75 38402 Cedex SMH, France R. DENDIEVEL GMP2, Rue de la Houille Blanche Du, BP75 38402 Cedex SMH, France Evolution of a.c. electrical properties under large strain of random nanocomposite materials made of a soft thermoplastic insulating matrix and hard conductive fillers is investigated. The transport properties are directly linked with the macroscopic mechanical strain on the composites during uniaxial tensile test or to the time under relaxation, meaning that the method is suitable for monitoring microstructural evolution of such composites. The real part of the conductivity indicated the breaking of the percolating network, while the imaginary part gave information on the possible “spatial correlation” of the damage events. Two different filler shapes were used, i.e. spherical and stick-like (aspect ratio about 15), leading to quantitatively different results. The microstructural evolution was simulated with the help of a resistance–capacitance (RC) model for the electrical properties and with finite element analysis for the mechanical properties. C  1999 Kluwer Academic Publishers 1. Introduction Intrinsic conducting polymers have received much at- tention during the last decades, because of their inter- esting potential applications [1]. However, their poor mechanical properties constitute a major obstacle for a large-scale industrial use. To overcome this drawback, numerous polymer composites have been processed, in recent years, by mixing polypyrrole (PPy) conducting fillers with different kinds of polymer matrices [2, 3]. The aim being to obtain materials combining both the mechanical properties and processability of the matrix as well as the electrical properties of the fillers. The preparation and characterization of such materials, ob- tained by mixing an insulating latex of a styrene-butyl acrylate copolymer with a colloidal suspensions of PPy particles, have been recently reported [4]. The final composites films were prepared by freeze drying the suspensions and hot pressing the dried product. It was shown that the samples may be considered as model materials, as the fillers have a well-defined geometry and are randomly dispersed within the matrix. Thanks to different polymerization methods [4], the fillers can be chosen with either a spherical shape or an high aspect ratio (15), with nanoscale sizes. The d.c. electrical prop- erties of the composite versus the filler content followed the well-known power law equation predicted by the statistical percolation theory [5, 6] and the percolation ∗ Author to whom correspondence should be addressed. Present address: GEMPPM-CNRS-INSA, 69621 Villeurbanne Cedex, France. thresholds were found to be 3% and 13% for stick-like and spherical fillers, respectively. It is well known electrical properties of binary mix- ture depend strongly on their microstructures. In partic- ular, the dispersion [7], the aspect ratio of the fillers [8] or the filler–filler [9] and filler–matrix [10] interactions affect directly the electrical properties. It has also been shown, with carbon fibre reinforced polymers (CFRP), that electrical transport properties are very sensitive to microstructural changes. For instance, Schulte [11] has proven that electrical measurements can be a very effi- cient way for monitoring damage of CFRP. These au- thors [12] related fibre breaking or delamination with an increase in the electrical resistivity. A similar method has also shown to be relevant for detecting failure of CFRP at high strain rates [13]. Ceysson et al. [14] found good relationships between the evolution of the resis- tivity with the number of events in acoustic emission. Further investigations, dealing with a.c. electrical con- ductivity [15] correlated the dissipation factor with the macroscopic stress on CFRP. Despite of the accuracy of the method demonstrated with CFRP, it appears to have been seldom applied to thermoplastic particulate com- posites. Pramanik et al. [16] and Radhakrishnan et al. [17] reported the evolution of the conductivity under large strain of insulator-conductor composites above the glass transition temperature (T g ) of the matrix. 0022–2461 C  1999 Kluwer Academic Publishers 1753