Structural and electrical properties of polystyrene-carbon composites

Conducting polymer composites are of great interest from the practical poin t of view, because their electrical properties can be tailored by properly choosing the components and their relative concentration. In this paper we report the unusual aspects of the behaviour of d.c. conductivity of a polys tyrene (PS) matrix doped with carbon particles (C) and, in another case, ca rbon particles encapsulated (C-enc) by a thin layer of another polymer. We study samples with different concentrations of carbon and try to relate the observed behaviour with the structure. A percolation threshold in the conductivity, as the concentration of carbon particles is varied, was observed in both C and C-enc systems. Additionall y, a sort of ageing effect was observed when the samples were submitted to temperature cycles during the measurement. The ageing effect is, however, o bserved only in the samples whose carbon concentration is less than the per colation threshold and there is a difference in the behaviour of C and C-en c system, which we discuss in the paper. For all the samples, the presence of a percolation threshold indicates that the polystyrene does not cover th e carbon particles uniformly. We study the structure of both the systems by dynamic light scattering, transmission electron microscopy and X-ray diffr action. The d.c. conductivity was measured, in the temperature range 80-300 K, with applied fields of up to 100 kV m(-1). A tentative model is provide d to explain some of the unusual features observed. (C) 1999 Elsevier Scien ce Ltd. Ail rights reserved.