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

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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]SA[sbnd]SC phase sequence. Three permittivities are defined for the biaxial SC 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 SC 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 SA to SC 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.