Order versus Temperature in Cholesteric Liquid Crystals from Reflectance Spectra

The temperature dependence of specular-reflectance spectra of obliquely incident light reflected by a single-domain sample of a cholesteric liquid crystal in the wavelength range covering the first- and second-order Bragg reflection bands has been studied. The sample was a mixture of 4, 4′bis$n$-hexyloxy)azoxybenzene and optically active 4, 4′bis (2-methylbutoxy) azoxybenzene. The spectra of this sample are consistent with the Oseen-De Vries spiraling dielectric-tensor model and reveal that local dielectric tensors are ellipsoids of revolution to within our limits of resolution. Increasing the temperature increases the libration of the molecules and hence decreases the order parameter. This causes a decrease in eccentricity of the dielectric tensor which, in turn, produces readily measurable changes in the Bragg reflectance spectra. The observed changes are roughly in agreement with those predicted by assuming that the Maier-Saupe theory for temperature dependence of the order parameter in nematic liquid crystals also applies to cholesterics. We find it necessary to include a thermal-expansion coefficient to fit the data.