Mechanism of charge transport in castor oil-based polyurethane/carbon black composite (PU/CB)

In order to identify and quantify the conduction mechanisms involved in the transport process, castor oil based polyurethane/carbon black composite (PU/CB) was introduced in a statistical model that reproduced the ac conductivity of composite PU/CB measurements by the application of a transfer-matrix technique. The simulation is based on virtual composite sample generation represented by a two-dimensional model of a resistor–capacitor network. The network represents a microstructure that contains both insulating (polyurethane) and conductive regions (carbon black). Drudeʹs formula was applied in the conductance calculation between two neighboring sites in the CB particles or aggregates, and Miller–Abrahams formula in conductance between two neighboring sites of the polymeric matrix. The theoretical–experimental adjustment indicates that, at low frequencies (below 103 Hz), the transport mechanism by charges hopping in the polymeric matrix is dominant. On the other hand, at high frequencies (above 105 Hz) electron conduction in carbon black, given by Drudeʹs formula, is dominant. The ac conductivity in the polymeric matrix is determined by the competition of two processes, one connected to hopping between the nearest sites with large energy difference or to hopping between the sites beyond the nearest neighbors with small energy difference.