Thermotropic nematic phases biaxial

The aggregates created by amphiphiles are usually spherical (as in the case of micelles), but may also be disc-like (bicelles), rodlike, or biaxial (all three micelle axes are distinct) (Zana, 2008). These anisotropic self-assembled nanostructures can then order themselves in much the same way as liquid crystals do, forming large-scale versions of all the thermotropic phases (such as a nematic phase of rod-shaped micelles). 

Fig. 3 Hekates with flexible spacers. Compounds 1, 2 Thermotropic properties as a function of spacer length. Compounds 3, 4 LC properties with unchanging spacer length and variable peripheral chains. Compounds 5, 6 Nematogens claimed to form biaxial nematic phases. Cr crystal, g glass, N nematic, N cholesteric, SmX unidentified smectic phase, Colh columnar hexagonal phase. All temperature are given in °C...

However, in 1986, Chandrasekhar [70] published a derivative (Figure 35) which he claimed to show a biaxial nematic phase. This report was interesting because the biaxial nematic phase (Nb), demonstrated in lyotropic systems [71], had been long sought after in thermotropic materials. Further, the molecules were described as bridging the gap between rod- and disc-like materials (a reference perhaps better reserved for polycatenar liquid crystals—vide infra)... 

The existence of the thermotropic biaxial nematic phase was theoretically predicted almost thirty years ago [119] but such a phase has yet to be unambiguously identified. Indeed, as we saw in the previous section, a biaxial nematic phase has been claimed for a class of discotic dimers [111], Theory predicts that mixtures of rod-like and disc-like molecules should exhibit the biaxial nematic phase [120] but experimentally such systems phase separate... 

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( >). 

In 1970 the existence of a biaxial nematic phase for thermotropic LCs was theoretically predicted by Freiser [73] the first-order transformation from the isotropic state to... 

Acharya, B.R., Primak, A., Kumar, S. Biaxial nematic phase in bent-core thermotropic mesogens. Phys. Rev. Lett. 92(14), 145506 (2004)... 

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]. 

The chemical shifts allowed the local order parameters to be computed which indicated the uniaxial to biaxial nematic phase transition. The nematic phase of a deuterated fiuorenone nematogen has been studied by NMR and X-ray and evidence for biaxial order close to its glass transition temperature has been inferred. The possible symmetries of the biaxial nematic phase have been examined based on experimental results and it is concluded that a monoclinic symmetry rather than an orthorhombic symmetry that is more likely to be the cause for the observed phase biaxiality in thermotropic bent-core and calamitic-tetrapode nematic systems. Density functional theory has been employed in a detailed conformational study of a bent-core mesogen The chemical shielding... 

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. 

Molecules of the lath-like shape can form the thermotropic biaxial nematic phase [23] shown in Fig. 1.9. The point group symmetry of this phase is i 2h the rotation of molecules around their longitudinal axes is considerably hindered. An example of the biaxial nematic is [bis-l-(p-n)-decylbiphenyl)-3-(p-ethoxyphenyl) propane-1,3-dionatocopper] [18]. 

Biaxial thermotropic nematic L.C.s would be of great importance in L.C. display technology. Less than a decade ago, such L.C.s were suggested. The biaxiality of the phases was confirmed using NMR spectroscopy of deuterated probe molecules. An alternate technique that is based on the second order quadrupole shift detectable in Xe NMR spectra of dissolved xenon has been proposed. The method has many advantages, such as the NMR spectra are taken from a static sample and the Xe quadrupole coupling tensor is extremely sensitive to the symmetry of the phase. ... 

Roetting O, Hinrichsen G (1994) Blends of thermotropic liquid crystalline and thermoplastic polymers a short review. Adv Polym Technol 13(l) 57-64 Roth D, Thomas L (1989) Why LCP film. In Abstracts of papers of the American Chemical Society, vol 198, p 3-CMEC. American Chemical Society, Washington, DC Rudko O (2002) Liquid crystalline polymers. Uniaxial-biaxial nematic phase transition Scott CE, Macosko CW (1995) Morphology development during the initial stages of polymer-polymer blcmding. Polymer 36(3) 461-470... 

An accepted experimental proof for the existence of a biaxial nematic phase in a thermotropic liquid crystal remained missing for a very long time. However, in recent years, biaxial nematic phases have been found in liquid crystalline polymers as well as in liquid crystals made of rod-disc mesogens, banana-shaped (bent-core) molecules, and organo-siloxane tetrapodes. Here, some characteristics of these systems and the corresponding experimental procedure for the investigation of phase biaxiality will be introduced. Further details for the individual systems can be found in the cited literature. 

The first thermotropic compound for which a biaxial nematic phase was claimed is the laterally broadened mesogen shown in Figure 5-1. The assignment of the phases was based on the observation of the optical textures and the X-ray diffraction pattern [2]. 

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