The structure of ferroelectric liquid crystals in planar geometry and its response to electric fields

Abstract

The review describes possible structures of the chiral smectic C (Sm C*) liquid crystals (DOBAMBC and a particular Sm C + cholesteric mixture) in samples with planar surface orientation of molecules. In two of them (planar helicoidal PH₁ and PH₂ configurations) the helicoidal structure is realized in the centre of the plate-like samples. In the other two (planar twisted PT and planar uniform PU configurations) the helix is unwound. Behaviour of these structures in an electric field is discussed. In the field induced transition PH → PT.motion and mutual annihilation of ±2π twist disclinations plays an important role. On the other hand the transition PT → PU or the switching processes between two PU states (PU⁺ and PU^-) with opposite dipole-moments are governed by ferroelectric domain-like mechanisms (nucleation and growth of domains). Hysteresis loop P(E) is observed at frequencies higher than 1 Hz, when the transition PU⁺ → PU⁻ takes place. The small signal dielectric permittivity of PH or PT configurations is also discussed. It is shown that there is no dielectric anomaly in the Sm A phase. The permittivity behaviour in dependence of temperature, frequency and biasing field is explained on the basis of the field induced changes in the real structure of planar samples.