The dielectric behaviour of various carbon black polymer composites has been characterized by the critical frequency ωc denoting the crossover from the dc plateau of the conductivity to its frequency dependent ac behaviour. The critical frequency can be related to the dc conductivity using a power law, ωc∝σdcz, with the exponent z. Presently accepted models predict z to be greater than one when varying the filler content. However, in accordance with published experimental results this work shows that z is rather close to one indicating a nearly constant static permittivity. Furthermore, the above power law makes it possible to describe all investigated compounds using a single master curve ranging over ten decades of dc conductivity. These findings are explained by a qualitative percolation model based on electron tunnelling. Increasing the dc conductivity along the percolation curve does not require the establishment of more physical links between carbon black aggregates. Rather, new conduction paths of nearly the same lengths but with higher tunnelling probability due to smaller gaps satisfy the percolation theory. This scenario allows the number of capacitive gaps to be nearly constant making z near one.

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Polyaniline or polypyrrole composites with fir or oak wood have been prepared by in situ polymerization of the corresponding monomers in an aqueous suspension of wood sawdust. The percolation threshold of compressed coated particles is located below 5 wt % of the conducting component and, above this limit, the conductivity of most composites was higher than 10 Ϫ3 S cm Ϫ1 . The conductivity of composites containing ca 30 wt % of conducting polymer was of the order of 10 Ϫ1 S cm Ϫ1 , an order of magnitude lower than that of the corresponding homopolymers, polyaniline and polypyrrole. The conductivity stability has been tested at 175°C. The polypyrrole-based composites generally lasted for a longer time than pyrrole homopolymers, also on account of the improved mechanical integrity of the samples provided by the presence of wood.The reverse order was found with polyaniline composites. The dielectric properties of the composites were determined in the range of 100 MHz-3 GHz, indicating that thick layers of composite material, ϳ 100 mm, are needed for the screening of the electromagnetic radiation below -10 dB level in this frequency range. Nevertheless, considering the potential production cost of composites and their low weight, such composite materials could be of practical interest in the shielding of electromagnetic interference.

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