Effects of Cyclohexyl Groups and Ortho-Substituents on the Viscosity of Ester Liquid Crystal Components

Structural effects on the relative viscosity of liquid crystal ester components are studied by capillary flow viscosity measurements of each component as an additive (10 to 25%) in a standard nematic mixture of short length 4-aIkoxyphenyl 4-alkylbenzoates. An approximate class viscosity at 25°C is calculated and assigned to each additive component, indicative of its viscosity contribution in the environment of the phenyl benzoate mixture. Studies are made on the viscosity effects of replacing benzene rings with cyclohexane rings, and of using various ortho-substituents on phenyl groups. All of the additive components are esters or thioesters, including several newly synthesized compounds. The effect of a cyclohexyl group is strongly dependent on its position in the structure. The class viscosity values show a large decrease when cyclohexane-carboxylate replaces benzoate, only a modest decrease when cyclohexylbenzoate replaces biphenylcarboxylate, and a slight increase when a cyclohexyl ester replaces a phenyl ester. Although o-groups can be used effectively to alter the dielectric anisotropy and modify the nematic range of a component, they increase the class viscosity values substantially, with the magnitude of the viscosity effect in the following approximate order from several o-groups: Br > CN > CH3 > pyridyl > C1.