Reflections on the Reentrant Nematic and the S_A-S_A Phase Transitions

Abstract

In the usual classifications of liquid crystals, the nematic phase is said to occur at higher temperature than the smectic A one. Although it is well known that no definite relation between macroscopic symmetry and entropy exists, the general character of this sequence seemed to make sense since smectic A really looks intermediary between the nematics and the crystals. Thus when P.E. Cladis reported the observation of a nematic phase occurring at a lower temperature than the smectic [1], the liquid crystal community was quite interested and puzzled. The experimental findings were as follows: consider a solution of 4-n-hexyloxybenzylidene-4′-cyanoaniline (HBAB) in 4-cyanobenzylidene-4′-n-octyloxyanilinel (CBOOA), with a 15 (mole) % of HBAB in CBOOA, you will observe a rather usual Isotropic-Nematic-Crystal sequence; however with mixtures containing less than 10% HBAB, you will observe upon cooling the sequence Isotropic — Nematic — Smectic A — Nematic again and eventually crystal. The low temperature nematic phase is monotropic, but stable enough to allow the measure of very characteristic nematic properties such as the bent elastic constant K₃₃. This last quantity exhibits the same pre-transitional phenomena when approaching the smectic A phase either from the high or from the low temperature side [1]. This set of observations seemed to be a real challenge to our understanding of the smectic layering.