English abstract
Low frequency dielectric investigations have been carried out on two thermotropic liquid crystal materials. The first one, viz., E7 exhibited nematic phase while the second one is a ferroelectric liquid crystal which exhibited SmA and SmC* phases. The nematic loss spectrum exhibits a single relaxation, which corresponds to the molecular rotation around the short axis. The activation energy (1.64 eV) estimated from the reduced temperature plots reflects the strong orientational coupling tuned by terminal Cyano-group. The Cole-Cole plots drawn at different temperatures support the increasing effective response of the low frequency (80) end of the spectrum with decreasing temperature and the distribution parameter a confirms the resumption of the nematic orientational order with decreasing temperature. The loss spectrum in the ferroelectric liquid crystal is found to accompany with two relaxation processes (one at -600Hz and another at -6.2MHz in SmA phase and two relaxations in SmC* phase at frequencies ~250Hz and ~4.9MHz. All the relaxations follow the Arrhenius shift from which the activation energy of each mode is calculated. The temperature and voltage variations of the (low-frequency, helix related) Goldstone mode relaxation of SmC* phase are studied. While the SmC* soft mode relaxation frequency in MHz relevant to the tilt of the molecules) is found to increase with decreasing temperature, the SmA soft mode relaxation frequency is found to decrease with decreasing temperature. The temperature variation of the dielectric strength, Aε = [ɛo-Ec] pertaining to the SmA soft mode relaxation inferred the growth of ferroelectric SmC* following Curie Weiss law across AC* transition.
