Mesomorphic and structural properties of some liquid crystals possessing a bicyclohexane core

The physical properties of seven bicyclohexane compounds have been investigated from polarizing optical microscopy, refractive indices, high resolution optical birefringence, density and dielectric permittivity measurements. These materials are characterized by low optical anisotropy and are often introduced as an effective component to reduce the birefringence (?n) of the commercial mixtures. The refractive indices and density data have been used to determine the orientational order parameter as a function of temperature using the standard Vuks method and the results are compared with the mean field values. Optical Transmission (OT) method has also been employed to obtain a high resolution (better than 10-5) measurement of the temperature dependences of the optical birefringence (?n), which provides a macroscopic measure of the anisotropy of the liquid crystalline phase, and can also, be considered as a measure of the orientational ordering. For the investigated compounds, the values of the order parameter critical exponent ? related to the limiting behavior of the nematic order parameter in the vicinity of the nematic–isotropic (N-I) transitions, are found to be close to 0.25, thus are in well accordance with the tricritical hypothesis. Additionally, the dielectric permittivities parallel and perpendicular to the molecular long axis throughout their mesomorphic range of the seven pure liquid crystalline compounds has also been measured.