Conductivity in Liquid Crystals

Dark and photoconductivities of nematic, smectic and cholesteric materials have been measured with particular attention to the changes in properties at phase transitions. Although a reasonably large photo-response could be found in several liquid crystals at room temperature, only a very small photoresponse occurred in the liquid crystal state. At the transition point solid → liquid crystal the dark conductivity, [sgrave] D and the thermal activation energy, Ea typically show large changes, while in the liquid crystal → liquid transition, [sgrave] D and Ea change only slightly. Both transition temperatures can easily be identified in the plots of log [sgrave] vs I/T, and agree well with microscopic and calorimetric observations. Cholesteric materials show a surprising decrease in dark conductivity at the solid → cholesteric transition during heating, in contrast with the behavior of nematic and smectic materials.