Orientational Order Parameters of Binary Mixture 80CB + 60CB Showing Re-entrant Nematic Phase

Abstract

Orientational order parameters have been determined for the mixture of hexyloxy cyanobiphenyl (60CB) and octyloxy cyanobiphenyl (80CB) for three different compositions containing molar concentration 0.289, 0.337 and 0.401 of 60CB over the complete mesomorphic ranges. Both x-ray diffraction and refractive index measurements on aligned samples have been used for order parameter calculations. The experimental order parameter values show a small enhancement with decreasing temperature in the smectic phase, but decrease on entering the re-entrant nematic phase. Smectic fluctuations were observed even in the nematic phase of the mixtures not having any smectic or re-entrant nematic phase. Previous order parameter values obtained from NMR studies are much smaller than ours. However, since this NMR measurement used a rather small molecule (p-xylene) as a probe, the ordering of these probe molecules may be less than that of the liquid crystalline mesophase studied.

Key Words:

• re-entrant nematic
• smectic fluctuations
• order parameters