Thermotropic cholesteric liquid crystals. Thermo-morphologic, optical refracting and birefringent properties: Direct and reverse phase transitions

Abstract

Temperature dependences of the average refractive index < n >, refractive indices of the ordinary ray n(o) and the extraordinary ray n(e), and the birefringence Delta n have been investigated in two cholesteric liquid crystals. Investigations have been carried out for the heating - cooling reversible cycle. Thermal hysteresis for the n(e) = n(e)(T), n(o) = n(o) (T) and vertical bar Delta n vertical bar = vertical bar Delta n vertical bar (T) and also shift of the reverse isotropic liquid state - cholesteric mesophase phase transition to lower temperatures have been found. Comparative estimations of temperature changes of the < n >, n(o), n(e) and vertical bar Delta n vertical bar with the thermo-morphologic properties of the heterophase regions of the direct cholestericmesophase - isotropic liquid state and reverse isotropic liquid state - cholesteric mesophase phase transitions have been carried out. Results of investigations exhibit connection of temperature behavior of the optical properties and the thermo-morphologic peculiarities of the heterophase regions, and indicate that phase transition between cholesteric mesophase and isotropic liquid state is the first order. (C) 2020 Elsevier B.V. All rights reserved.