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

Abstract

Abstract Temperature dependences of the average refractive index 〈n〉, refractive indices of the ordinary ray no and the extraordinary ray ne, and the birefringence Δn have been investigated in two cholesteric liquid crystals. Investigations have been carried out for the heating – cooling reversible cycle. Thermal hysteresis for the ne = ne(T), no = no(T) and ∣Δn ∣ = ∣ Δn ∣ (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〉, no, ne and ∣Δn∣ with the thermo-morphologic properties of the heterophase regions of the direct cholesteric mesophase – 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.