Dielectric studies of three liquid crystals HBT, E₈ and PCH-1132

Abstract

The dielectric behavior of a Schiff base N-(P-Hexyloxybenzylidene)-P-toluidine (HBT) and two technologically important mixtures, E₈ and PCH-1132, are reported in the radio frequency region. The value of the mean dielectric constant increases by ˜5% on going from the nematic to the isotropic phase in the case of E₈ and PCH-1132. This may perhaps be due to antiferroelectric arrangement of the molecules in the nematic phase. These mesogens show unusual dielectric behavior and exhibit positive anisotropy at lower frequencies and negative dielectric anisotropy above a critical frequency termed as isotropic frequency. The isotropic frequency is found to be temperature dependent. All these nematogens exhibit single relaxation in the radio frequency region which is due to hindered rotation of the molecules under nematic potential. The relaxation frequencies are 3.98 MHz (at 61.4°C), 2.27 MHz (at 34.3°C), 1.87 MHz (at 33°C) in case of HBT, E₈ and PCH-1132 respectively. The activation energy is found to be 16.7 K Cal/mole, 14.6 KCal/mole and 16.1 KCal/mole for HBT, Ea and PCH-1132 respectively. In smectic B phase of HBT, the dipolar orientations are found to be somewhat "locked" similar to that in solids. The inversion of the sign of dielectric anisotropy in S_B phase of HBT is also discussed.