Phase diagram and X-ray studies of mixtures of 8OCB (cyanooctyloxybiphenyl) and 4O8 (butyloxybenzilidene octylaniline)and 8OCB and 6OCB (cyanohexyloxybiphenyl) demonstrate: 1. 4O8 molecules inhibit the pairing of the 8OCB molecules. One 4O8 molecule is attributed with the breakup of one 8OCB pair. 2. The maximum pressure at which the smectic A phase is stable increases as the number of 8OCB pairs decreases. 3. The temperature at which the smectic A phase is stable is maximum when the mixture is most polar [no 8OCB pairs at all]. For these mixtures, there is no nematic phase. 4. There is no smectic A phase when all of the 8OCB molecules are paired. 5. When the nematic-smectic A transition temperature of the mixture approaches the nematic-isotropic transition temperature of the pure compound, the nematic phase is completely suppressed. Assuming that the smectic A order parameter is coupled to the nematic order parameter and the coupling depends on temperature and composition, a Landau description i...

The Re-entrant Nematic, Enhanced Smectic A Phases and Molecular Composition

X-ray diffraction patterns have been made on mesophases of 4-n-octyloxybenzoyloxy-4'-cyano-stilbene. In agreement with microscopic and magnetic studies we have reported the evidence of a thermotropic reentrant polymorphism nematic-smectic A-nematic-smectic A in this pure system. The evolution of the smectic layer thickness d as a function of temperature requires some kind of bilayer structure in the higher temperature S A whereas d corresponds to the molecular length in the lower temperature SA. In terms of local order the diffuse scattering intensity which appears in the higher temperature SA is the usually observed one, but in addition we have also seen an original two-dimensional superlattice consistent with smectic A symmetry in the lower temperature SA. The reentrant nematic phase seems to be a consequence of the competition between both short range smectic A orders.

https://hal.archives-ouvertes.fr/jpa-00209214/document

X-Ray study of the reentrant polymorphism N-SA-N-SA in a pure liquid crystal compound

Since the discovery of liquid crystals one hundred years ago, reentrant phase transitions have transformed from being isolated curiosities to an established phenomenon. A significant influence for this change in point of view, has been the almost overnight discovery of the many reentrant transitions in liquid crystals. In this review, I survey some of the interesting results discovered by different groups studying the reentrant nematic phase since its discovery making contact with current theories whenever possible. To put the reentrant nematic in the context of current understanding and to provide a historical perspective of this phenomenon, I summarize some reentrant transitions in other systems. In particular, I discuss the ferroelectric transition in the molecular crystal, Rochelle salt, because it may be the first observed example of a reentrant transition. Again, whenever possible, similarities and differences to the liquid crystal case are discussed.

A One Hundred Year Perspective of the Reentrant Nematic Phase

A systematic comparative study of several series of chemically neighbouring compounds apt to give a reentrant mesomorphism is proposed. Experimental data concerning the various reentrant phases are summed up. From these observations various factors favourable to the occurence of the reentrance are revealed. Thus, the chemical composition, the sense and the location of dipoles in the molecules, the reduced temperatures, the length of the rigid core are discussed as relevant parameters.

Occurrence of Reentrant Nematic and Reentrant Smectic A Phases in Mesogenic Series

Article de synthese sur les phases smectiques de substances avec des molecules type bâtonnet en structure en couche. Les phases nematiques ne sont considerees que dans le contexte du polymorphisme

Polymorphism in Thermotropic Liquid Crystals

Some derivatives of an homologous series of 4-aIkoxy-benzoyloxy-4′-cyanotolanes are synthesized. Two of them exhibit a stable nematic reentrant phase at atmospheric pressure.

H. Gasparoux

Papers

New Series Exhibiting Pure Enantiotropic Nematic Reentrant Compounds at Atmospheric Pressure