The dielectric response during the deformation of a rubber composite filled with carbon black (CB) was studied by polarization-resolved terahertz spectroscopy, because the characterization of such responses can be used to reveal strain-induced irreversible changes in the electrical properties of a CB-filled rubber. During the first stretching-and-relaxation cycle, two characteristic behaviors were observed: strain-induced anisotropy and an irreversible reduction of the dielectric constants in the measured frequency range. To understand the origin of this result, we tried to reproduce the strain dependence of the dielectric function with a modified effective medium theory that accounts for the orientational distribution of the CB aggregates. From the dependence of the fitting parameters on the deformation condition, it was found that the reduction of the dielectric constants in the terahertz frequency range is mainly caused by the breakdown of the CB-based conductive network due to a dissociation of CB agglomerates. This finding indicates that polarization-resolved terahertz spectroscopy is a promising technique for the assessment of changes in the internal structure of CB fillers on the nanometer scale.
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