Dielectric Biaxiality in S C Host Systems

AC field stabilisation of ferroelectric SC* liquid crystals with the chevron layer geometry requires that there is a significant dielectric biaxiality.1,2 To investigate this, the electric permittivities of an SC host mixture based on the phenyl pyrimidines and exhibiting an N-SA-SC phase sequence are reported. A large relaxation of ε∥ occurred, which caused a sign reversal of the dielectric anisotropy with a crossover frequency of 25 kHz at 25°C. The host electro-optic behaviour was then characterised using extinction angle measurements as a function of applied voltage at frequencies corresponding to Δ≈ > 0, Δε ≈ 0 and δε < 0, and the results explained using a tilted layer model with a biaxial dielectric tensor. The magnitude of the SC dielectric biaxiality, the layer tilt angle in the planar homogeneous geometry and the cone angle are determined. Measurements for several other hosts commonly used in commercial FLC mixtures are reported and it is shown that in each case the dielectric biaxiality is sufficiently large to cause AC field stabilisation in ferroelectric liquid crystals.