Rheology of Liguid Crystals II. Rheological Properties of a Cholesteric Liguid Crystal Mixture

Abstract

Rheo-optical experiments have been carried out to study the relationship between rheological properties and structure of a typical cholesteric liquid crystal, which is a meltmixed sample of 25 wt-% of cholesteryl chloride and 75 wt-% of cholesteryl oleyl carbonate. The flow properties and the reflection spectrum of the cholesteric mixture have been measured simultaneously by means of a cone and plate type rheometer combined with a spectrometer at room temperature (27°C). The flow curve for the cholesteric mixture has the Newtonian flow region at shear rates ranging from 2×10-1 to 1×102 (sec-1). At shear rates below 2×10-1(sec-1), the logarithmic flow curve is a straight line parallel to the ordinate. At shear rates higher than 1×102 (sec-1), the flow curve concaves upward. The absorbance vs. wave length curve for the cholesteric mixture under shear shows maximum at wave length of around 510 mμ. The absorbance at longer wave lengths increases with increasing shear rate as long as the flow region is Newtonian. The experimental results obtained can be explained well in terms of orientation of Bragg reflection sites under shear.