Physical Properties of Nematic Liquid Crystalline Mixture (IV) : Effects of Fluorinated Terminal Groups

Abstract

Viscoelasticity of binary liquid crystal systems were studied using a light scattering method which can be applied to the determination of the decay constant associated with the ratio of the viscosity and the elastic constant for the twist deformation. For the mixtures of 7HEF, a terminal-fluorinated liquid crystal, and BCH5 containing three aromatic rings as the core group, the phase diagram shows a wide nematic region in the temperature range of about 60 ℃ to 140 ℃ at 30 mol per cent of 7HEF. The activation energy for the viscosity was evaluated from the temperature dependence of the viscoelastic constant, showing a minimum at 60 mol per cent of 7HEF. It has also been found that the viscoelastic constant is strongly dependent on the molar fraction at a constant reduced temperature, i.e, a constant ratio of the measurement temperature and the isotropic-nematic transition temperature at each molar fraction.