THE COMPETITION BETWEEN LONG-RANGE ANTIFERROELECTRIC ORDER AND SHORT-RANGE ANTIFERROELECTRIC FORCES AS A POSSIBLE CAUSE OF REENTRANT BEHAVIOR IN POLAR LIQUID-CRYSTALS

In the present paper one of of possible ways of explanation of reentrant behavior in polar liquid crystals (LC) is proposed. The smectic A-phase (S(Ad)) existing between high temperature nematic phase (N) and reentrant nematic phase (N(re)) is considered as a partially bilayer smectic A-structure (1 < d < 21) with long-range antiferroelectric order. Each layer of such structure consists of two interpenetrating macroscopic sublayers with mutually opposite alignments of molecular dipoles. In addition, it is assumed that each molecule feels not only some molecular mean field (made by all LC molecules) defining macroscopic layer structure with long-range antiferroelectric order, but also interacts with nearest neighbours by means of short-range antiferroelectric forces of Ising's type. By analogy with McMillan's theory in the approximation of ideal orientational order the single-particle distribution functions for the molecules with different dipole orientations are introduced and self consistent equations determining the smectic order parameter are written. The temperatures of the N --> S(Ad) second order transitions together with the temperature dependence of smectic order parameter are determined by the solution of these equations. It is shown that when the increase of short-range antiferroelectric forces with decreasing temperature is sufficiently rapid then the reentrant S(Ad) --> N(re) transition is possible. Thus the appearance of reentrant nematic phase in polar LC can be considered as a consequence of the competition between long-range antiferroelectric order in the S(Ad)-phase and short-range antiferroelectric forces. In the framework of proposed model the reentrant phase diagram very similar to experimental that for some polar LC is obtained. For the same LC the pressure dependences of the heat capacity discontinuities at the N --> S(Ad) and S(Ad) --> N(re) second order phase transitions are determined.