Dielectric relaxation of a reentrant nematic-liquid-crystal mixture by time-domain reflectometry

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Abstract
Complex-permittivity measurements of a reentrant nematic-liquid-crystal mixture, 4-n-hexyloxy-4’-cyanobiphenyl and 4-n-octyloxy-4’-cyanobiphenyl, for both the parallel and perpendicular components, have been performed by time-domain reflectometry in the frequency range between 300 kHz and 10 GHz and at temperatures from 58 °C (above the nematic and smectic-A phase-transition temperatures) down to 25 °C (below the smectic-A and the reentrant nematic phase-transition temperatures). It is shown that the molecular rotation around the short axis in the reentrant nematic phase is much hindered compared to that in the nematic phase, but the motion around the long axis is the same for both phases. It is also shown that, for the parallel component, the activation energy for the smectic-A phase is larger than that for the nematic phase but similar to that of the reentrant nematic phase. Finally, the transition between the reentrant nematic and the solid phases is discussed.