Microscopic Description of Nematic-Smectic A₂ and Nematic-Smectic A₁-Smectic A₂ Phase Transitions in Binary Mixture of Polar and Nonpolar Liquid Crystals

Abstract

In the present paper, in a mean field approximation, we considered binary mixtures of polar and nonpolar liquid crystals (LC) with ideal orientational order. It is assumed that nonpolar molecules can form only monolayer smectic A-structure whereas polar molecules can form both monolayer smectic A-structure with short-range antiferroelectric order and bilayer smectic A-structure with long-range antiferroelectric order. By analogy with McMillan's theory the self-consistent equations determining order parameters for these smectic A structures are introduced. It is shown that smectic A₁ phase with monolayer smectic A structure and smectic A₂ phase with coexisting monolayer and bilayer smectic-A structures can ocur in the mixture. Phase diagram for mixture and entropy discontinuities at the nematic-smectic A₁ and smectic A₁-smectic A₂ phase transitions obtained by numerical solution of self-consistent equations are similar to experimental ones. Also the temperature and concentration dependences of intensities of X-ray scattering from monolayer and bilayer smectic A structures are obtained.

Key Words:

• NSA₁SA₂ phase diagram
• X-ray scattering