The determination of the elastic constants of a series of n-alkylcyanobiphenyls by anisotropy of turbidity

Abstract

Measurement of the anisotropy of turbidity of aligned nematic liquid crystals in three selected geometries can yield the splay, twist, and bend elastic constants (K₁₁, K₂₂, and K₃₃, respectively) which, along with the viscosity coefficients of the medium, govern director fluctuations that lead to a strong light scattering. We have utilized this procedure to determine the temperature dependence of the elastic constants of a homologous series of n‐alkycyanobiphenyls (n‐CB, where n=5, 6, 7, 8) in the nematic range. K₃₃≳K₁₁≳K₂₂ at all temperatures for each member of the series. The absolute values of the moduli are consistently higher than those obtained by magneto‐ and electro‐optic distortion methods (Frederik’s transitions). Odd–even effects are displayed as the number of methylene carbons in the alkyl chain increases. Distinctly larger K₃₃ values for 8CB may be attributed to the short range smectic ordering in this liquid crystal, which has a lower temperature smectic‐A phase.