The use of a time resolved Raman microprobe to study the electric field induced Fréedericksz transition in the alkyl cyanobiphenyl homologues

Abstract

The bond selectivity of the Raman technique is exploited with a time resolved Raman microprobe to determine the effective splay viscosity coefficients and elastic constants in the nematic phase of the 4-n-alkyl-4-cyanobiphenyl homologues 5,6,7 and 8CB. The Raman effect, using the signal associated with the core and the flexible alkyl chains, was used to probe the Fréedericksz transition in the limit of small deformations. The data for the cores show good agreement with data published elsewhere. Odd-even effects were found and, in the case of 8CB, pretransitional effects due to the lower temperature S_A phase were observed. The viscosity data for the flexible alkyl chains, at a reduced temperature of 3°C, yield values for the apparent viscosity coefficients that are approximately half that of their respective rigid cores. This is considered to be indicative of the influence of the molecular flexibility on the effective director field. The data illustrate the usefulness of the technique for studying microscopic volumes of multicomponent systems.