The orientational order parameters of a dendritic liquid crystal organo-siloxane tetrapode oligomer, determined using polarized infrared spectroscopy.

Abstract

The observed macroscopic anisotropic properties such as the components of infrared (IR) absorbances of liquid crystals are expressed in terms of the order parameters of the long molecular axis, molecular, and phase biaxiality. The order parameters of the organo-siloxane tetrapode liquid crystal of zero dendritic order (G0) in its nematic and smectic phases have been determined using results of the polarized IR spectroscopic measurements on a planar homogenously and hometropic aligned cells. The spatial components of the absorbances for the vibrational bands (in the mesogenic unit, terminal chains, and spacer) have been measured and analyzed. For the laboratory reference system, the apparent orientational order parameter S of the mesogen unit shows a significant drop in the transition from the nematic to the smectic phase while the phase biaxiality order parameter P increases to almost 0.4 in the smectic phase. This result shows that the director is tilted out of the sample plane in the smectic phase. The molecular biaxiality parameter D is found to be positive both for the nematic and smectic phases. This suggests that the carbonyl dipoles are oriented close to the tilt plane. For the vibrational bands in the chains, low values of S and D indicative of their low orientational order are obtained. As a result of the interaction among the molecules in the tilted smectic phases, the transition dipoles show positive correlations for the transversal and negative for the longitudinal dipoles.