Nonuniversal percolation exponents and broadband dielectric relaxation in carbon black loaded epoxy composites

In this study we present measurements and data analysis of the dielectric relaxation in carbon black (CB) loaded epoxy composites over a wide range of CB concentration (below and above the percolation threshold) and a broad range of frequency (180-1.5 x 10(7) Hz). We show that our data are in conflict with the expectation that the fit parameters of the much-discussed two exponent phenomenological percolation equation (TEPPE) represent the critical exponents of the standard percolation theory. This result therefore gives new insight to the problem of why the TEPPE has a limited applicability in representing the effective permittivity versus CB volume fraction functional relationship. We believe that the reason for this discrepancy is that the true dependence of the internal length scales of the mesostructure and the elasticity network is too complicated to fulfill a simple unified functional equation. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3253744]