Dielectric and microstructure properties of polymer carbon black composites

Dielectric and physicochemical properties of a composite material prepared by incorporating carbon black particles into a polymer matrix were investigated. Two types of carbon blacks, having very different structures of aggregates, were used. The volume fraction of the carbon blacks ranged from 0.2% to 7%, i.e. below and above the percolation threshold concentration observed from the measurements of dc conductivity. The composite samples were characterized in terms of: swelling by a compatible solvent, electron paramagnetic resonance (EPR) response, and frequency variation of permittivity. First, the article attempts to evaluate the diffusion coefficient of an appropriate solvent in these materials. Sorption kinetics experiments with toluene indicate that the initial uptake of solvent exhibits a square root dependence in time as a consequence of Fick’s law and permit to evaluate the effective diffusion coefficient in the range 10^-11–10^-12 m² s^-1 depending on the volume fraction of the carbon black in the sample. Second, the analysis of the carbon black concentration dependence of the intensity and linewidth of the EPR signals indicates that EPR is an important experimental probe of the structure of the elasticity network. The most notable feature of the present work is that we find a correlation of the percolation threshold concentration which is detected from the dc electrical conductivity with moments of the EPR lines. The conclusions on the elasticity networks deduced from swelling measurements are confirmed by EPR data carried out on swollen samples. On qualitative grounds the role of the specific surface of carbon black is further analyzed. It is suggested that the elasticity network is mainly controlled by secondary (respectively primary) aggregates for samples containing low (respectively high) specific surface carbon blacks. Last, the article reports precise experimental data on the permitti- vity of these composite materials as a function of frequency. Thanks to a sensitive measurement technique using an impedance analyzer, we are able to measure the complex permittivity and permeability values of the samples in the frequency range from 10⁸ to 10¹⁰ Hz. It is found that the real part of the permittivity is a function of frequency f, via a power law expression ϵ^′=af^-b, where a and b are two parameters depending upon carbon black concentration, in the range of frequency investigated. The data analysis reaffirms the result that percolation threshold is a key parameter for characterizing the topological arrangement in these structures. © 1997 American Institute of Physics.