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The anisotropy of molecular polarizability in liquid crystals is linked to the birefringence in these substances. The classic methods to compute the polarizabilities of liquid crystals assume an average number density of molecules that is equal in all directions. In the present work, a new model is proposed for the anisotropic molar polarization based on a virtual anisotropy of the number density of molecules in the liquid-crystalline material. This new strategy hence allows for the computation of both the anisotropic polarizabilities and the anisotropic thermal-expansion coefficients of liquid crystals. The model is applied to the liquid crystals 4-n-pentyl-4′-cyanobiphenyl and N-(4-methoxybenzylidene)-4-butylaniline, yielding polarizabilities similar to those reported for these materials. For these nematic liquid crystals, the results imply the existence of a positive thermal-expansion coefficient in the direction perpendicular to the director vector throughout the entire nematic temperature range and a negative thermal-expansion coefficient parallel to the director vector near the temperature of the nematic–isotropic transition. At the isotropization temperature, there exists divergent and critical behavior of the anisotropic thermal-expansion coefficients, consistent with the typical discontinuity of volume in first-order transitions. [ABSTRACT FROM AUTHOR]

In the design and development of liquid crystal (LC) devices, it is crucial to determine the characteristic times of optical responses in devices driven with applied electric fields. These characteristic times associated with relaxation processes of the LC upon application (on) or removal (off) of an electric field are commonly calculated through measurement of the interval between 10% and 90% of the total change. Signals with non-monotonic oscillatory patterns originating either from the influence of backflow processes or from the variation in the phase shift between the anisotropic refractive indices in the reorientation dynamics complicate such a determination of the response times of LC devices. To obtain response times in LC devices with non-monotonous relaxation processes, we developed a novel numeric method that suppresses the oscillatory component of the signal and allows a direct determination of the characteristic times. The technique was validated with simulations of smooth and noisy data sets and was also applied to experimental data obtained from LC devices based on the LC 4-n-pentyl-4ʹ-cyanobiphenyl (5CB). The obtained results confirm that the method is reliable and applicable to data sets with complicated oscillatory patterns. [ABSTRACT FROM AUTHOR]