Structures biaxial nematics

Note 1 See Fig. 16 for an illustration of the molecular arrangement in a Nb mesophase. Note 2 From a crystallographic point of view, the biaxial nematic structure is characterised by the symbol D2h in the Schoenflies notation (2 m, m in International System). 

Note 5 A biaxial nematic has the same structure as a disordered sanidic mesophase (see Definition 3.4, Note 2) it is recommended that the latter name be discontinued and the name biaxial nematic be used. 

Molecules which combine the features of the rod and the disc may be expected to form new types of mesophases. An example is the biaxial nematic phase reported in thermotropic systems (see 6.6). Malthete et a/. have prepared an interesting series of mesogens shaped like stick insects called phasmids (fig. 6.1.5(n)). Some of them form columnar mesophases the structure proposed for the hexagonal phase is shown schematically in fig. 6.1.5( >). 

The difference between the structures of the uniaxial and biaxial nematics is illustrated schematically in fig. 6.6.4. The N phase is depicted here as an orthorhombic fluid whose preferred molecular orientation is described by an orthonormal triad of director fields. (In principle, nematics of lower symmetry are possible, but none of the phases identified to date have been reported to be other than orthorhombic.) The structure, therefore, gives rise to an additional pair of diffuse (liquid-like) X-ray diffraction peaks -" (fig. 6.6.3(A)). 

There is also a group of the so-called lyotropic nematics. They are intermediate between the isotropic micellar phase and structured (lamellar or hexagonal) phases and can be formed by both discotic and calamitic molecules. The lyotropic nematics can be aligned by an electric or magnetic field and show Schlieren texture as thermotropic nematics. The building blocks of these mesophases are vesicles or similar mesoscopic objects. From the symmetry point of view the nematic phases can be uniaxial or biaxial, as shown in Fig. 4.22. In fact, the biaxial nematics have been found unequivocally only in the lyotropic systems [13]. 

Under just the right conditions, a mixture of a highly polar liquid, a slightly polar liquid, and an amphiphilic molecule form micelles that are not spherical. They can be rodlike, disc-like, or biaxial (all three axes of the micelles are different). These anisotropic micelles sometimes order in the solvent just as liquid crystal molecules order in thermotropic phases. There is a nematic phase of rod-shaped micelles, another nematic phase of disc-shaped micelles, and even a biaxial nematic phase, in which the molecular axes transverse to the long molecular axis partially order. Chiral versions of these phases with the same structure as the chiral nematic phase also form. 

The second publication was a brief communication reporting the first thermotropic biaxial nematic liquid crystal [19]. The thermotropic biaxial nematic liquid crystal was only monotropic. Saupe mentioned that Helmut Ringsdorf advised him to contact me in order to help him transform the monotropic phase into an enantiotropic one. I looked at the structure of the molecule published by Malthete... 

Phase diagram of the potassium laurate (KL)-l-decanol-D20 system, with the Landau point. Nc and Nl are uniaxial nematic phases with cylindrical and lamellar micelle structures, and Nj, denotes the biaxial nematic phase. (Figure reproduced from Reference [8] with permission... 

In 1980, the first justified claim for the discovery of a biaxial nematic phase was made for a lyotropic liquid crystal comprised of the ternary system of potassium laurate-l-decanol-D20 [4], In addition to a uniaxial phase (micelles of a bilayer structure), there were two further nematic phases. One of the phases was found to be uniaxial as well, probably corresponding to a phase with cylindrical micelles. Existing in a temperature range in between these two uniaxial nematic phases was a third phase which was found to be biaxial. The phases were classified by microscopic smdies as well as deuterium NMR measurements. Three years later, Galerne and Marcerou studied the same system by conoscopy, leading to a complete determination of the ordering tensor in all three nematic phases [5]. 

Another class of mesogen, which has proven to be a successful candidate for the formation of a biaxial nematic phase, is made up of the so-called bent-core, or banana mesogens. In this kind of mesogen, two rod-like molecules are chemically linked together via a central unit. The molecular structure of such a banana mesogen is schematically given in Figure 5-19. 

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