Analysis of carbon black networking in elastomers by dielectric spectroscopy

The dielectric response of various series of cross-linked and non-cross-linked solution-styrene-butadiene-rubber and/or carbon black composites is investigated in the frequency range from 0.10.3em0exHz up to 100.3em0exMHz. Two types of rubber, differing in vinyl content, and two carbon black grades, differing in particle size, are considered. The dielectric spectra are analyzed in the framework of percolation theory, describing the effect of structural disorder of the conducting carbon black network on the diffusive charge transport. Various deviations from the predictions of percolation theory are observed, which are related to a superimposed kinetic aggregation or flocculation process. It is demonstrated that due to the heat treatment during vulcanization, a pronounced flocculation of filler particles takes place in the rubber matrix, especially in the case of a small particle size, leading to a drastic modification of the dielectric spectra. In particular, the percolation threshold decreases significantly and a second high-frequency relaxation transition appears, which is traced back to the tunneling of charge carriers over small gaps between adjacent carbon black particles. The gap size is found to be independent of the carbon black concentration in the range of 2--50.3em0exnm.