Infrared markers for determining the order parameters of uniaxial liquid crystals

Polarization-dependent absorption coefficients of various molecular vibrational bands (i.e., IR markers) in the 2.5-20-microm spectral region were measured for two nematic liquid crystals (LCs), 5CB and MBBA. Combining the measured absorption dichroism of a dipole moment with a correction factor g, which takes the anisotropy of the local field into consideration, the order parameter S of a uniaxial LC can be determined. This g factor is found to have significant effect on the determination of S. The temperature-dependent g factor is calculated for 5CB, for example. Results on S agree well with those obtained by the NMR technique, which does not require the local field correction. Order parameters of several 5CB-host-LC mixtures were measured at room temperature. Results indicate that the LC mixtures with higher nematic-isotropic phase transition temperature exhibit larger S. This phenomenon can be used to improve the LC materials for IR applications.