Investigation of the Visco-Elastic Properties of Mixtures with Different Birefringence.

The STN switching mechanism is governed by elastic constant ratios K33/K11 and K22/K11 (K11,K22 and K33 are respectively the splay, twist and bend elastic constants). This work explores the limitation of K22/K11 and how this can be translated into material/mixture design. The visco-elastic parameters K11, K22, ηsplay and γ1, were measured by dynamic light scattering (DLS) for three mixtures, based respectively on cyanobiphenyls, phenylbenzoates and dicyclohexyls with birefringence Δn ranging from 0.05 to 0.2. This study confirms that K22 and K11 only show slight variation for these mixtures; therefore, the ratio of K22/K11 remains remarkably constant and cannot be exploited in material design. In contrast, the twist and splay viscosities are directly related to the composition of the mixtures. Strongly polar nematic liquid crystals, such as the cyanobiphenyls, exhibit higher rotational viscosity coefficients. This work also outlines the dependence of the measurement accuracy of the visco-elastic parameters on the birefringence of the material investigated. [ABSTRACT FROM AUTHOR]