Measurement of the biaxial permittivities for several smectic C host materials used in ferroelectric liquid crystal devices

Abstract

Dielectric biaxiality plays an important role in the electric field behaviour of ferroelectric liquid crystal devices, providing the necessary driving force for AC field stabilization and playing an important role in device dynamics. The static electric permittivities have been measured as a function of temperature for several mixtures with an N[sbnd]S_A[sbnd]S_C phase sequence. Three permittivities are defined for the biaxial S_C phase, and were determined from measurements of homeotropic and planar cells and from the average permittivity extrapolated from the uniaxial nematic and smectic A phases. The S_C cone angle is assumed to be equivalent at dielectric and optical frequencies. Results are reported for five common ferroelectric liquid crystal hosts and for two racemic mixtures, including the racemic analogue of the commercially available SCE 13. The dielectric biaxiality is sufficiently large below the S_A to S_C transition in each of the studied mixtures to have a strong influence on the electro-optic behaviour. This is consistent with the occurrence of AC stabilization in surface stabilized ferroelectric liquid crystal devices with the chevron layer configuration.