Current-voltage characteristics of a nematic cell oriented by a doped polypyrrole surface layer

The influence of a surface layer of polypyrrole on the electrical response of a nematic cell is investigated. By assuming that the nematic sample can be described by means of a Maxwell-Wagner model, we determine the electrical conductivity and the dielectric constant of the surface layer and of the liquid crystal. From the experimental data, relevant to measurements performed on the sample submitted to an external excitation with zero average value and different amplitudes, we deduce information on the density of ions in the liquid crystal. The obtained values for the electrical conductivity, the dielectric constant and the ionic density of the two media are independent of the amplitude of the external voltage and of the same order of the ones measured by other groups. From this result we infer that a description in terms of Maxwell-Wagner model for linear media represents a good approximation of our system