Phase Biaxiality in Nematic Liquid Crystals

R328 V. Domenici, A Brief Overview of NMR Experiments Used to Study the Phase Biaxiality in Nematic Liquid Crystals , Mol. Cryst. Liq. Cryst., [online computer file], 2012, 558, 37. 

Conoscopy is known to be prone to artifacts because of its sensitivity to the symmetry of the refractive index, which might be tampered due to surface effects and flow phenomena. By using deuterium NMR spectroscopy. Severing et al. [11] confirmed phase biaxiality in a polymeric liquid crystal similar to that studied earlier by Leube. To evaluate different parameters that bias the formation of a biaxial nematic phase and gain a more general picture of the phase biaxiality in nematic liquid crystalline polymers, the investigations were expanded to side-chain polymers of different chemical constitutions as well as to mixtures of polymers and low molar mass liquid crystals [12],... 

Conoscopy provides an extremely sensitive method with which to determine the degree of biaxiality. By the early 1990 s, conoscopic measurements had already indicated the presence of phase biaxiality in a nematic side-on liquid crystalline side-chain polymer [9]. However, the method s sensitivity is also its weak point because surface effects may induce optical biaxiality in an actual uniaxial system. For this reason, deuterium NMR was used to confirm phase biaxiality in a liquid crystalline polymer system similar to the one investigated with conoscopy by Leube [11-13]. Due to the fairly high viscosity of the polymeric samples, the tilt experiment, employed by Yu and Saupe to show phase biaxiality in a lyotropic liquid crystal [4], was used. The results obtained in this way are in good agreement with observations of optical textures in a biaxial cholesteric copolymer [16], where phase biaxiality disturbs the smooth optical periodicity of the cholesteric phase structure. 

The Gay-Berne potential has successfully been used for many liquid crystal simulations, and (depending on the parameterisation used and the state points studied) can be used to simulate nematic, smectic-A and smectic-B phases. Variants of the GB potential have also been used to study the biaxial nematic phase (biaxial GB potential) [21] and the smectic C phase (GB with quadrupole) [22]. The GB model has been used also to provide predictions for key material properties, such as elastic constants [23] and rotational viscosities [24], which have an important role in determining how a nematic liquid crystal responds in a liquid crystal display (LCD). 

Optically, a nematic phase can be uniaxial or biaxial. The latter is formed by elongated lath-like molecules. Conventional nematic liquid crystals formed by rod-like molecules constitute a uniaxial medium with nonpolar symmetry. The constituent molecules rotate (freely or hindered) around both their short and long axes. Nonoriented samples are analogous in some measure to polycrystalline powders they consist of individual (liquid) crystallites, each of which has as a definite feature, a directed optical axis, which... 

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... 

The majority of the existing molecular theories of nematic liquid crystals are based on simple uniaxial molecular models like sphe-rocylinders. At the same time typical mes-ogenic molecules are obviously biaxial. (For example, the biaxiality of the phenyl ring is determined by its breadth-to-thick-ness ratio which is of the order of two.) If this biaxiality is important, even a very good statistical theory may result in a poor agreement with experiment when the biaxiality is ignored. Several authors have suggested that even a small deviation from uniaxial symmetry can account for important features of the N-I transition [29, 42, 47, 48], In the uniaxial nematic phase composed of biaxial molecules the orientational distri-... 

The principal elastic constants for a nematic liquid crystal have already been defined in Sec. 5.1 as splay (A , j), twist(/ 22) and bend(fc33). In this section we shall outline the statistical theory of elastic constants, and show how they depend on molecular properties. The approach follows that of the generalised van der Waals theory developed by Gelbart and Ben-Shaul [40], which itself embraces a number of earlier models for the elasticity of nematic liquid crystals. Corresponding theories for smectic, columnar and biaxial phases have yet to be developed. 

Finally, polarized IR spectroscopy is another sensitive method for the study of phase symmetry in liquid crystals. It was recently applied to prove phase biaxiality in organo-siloxane tetrapodes [37] - a supermolecular system forming quasi-flat platelets. Planar homogenously and homeotropically oriented samples were studied in order to derive all relevant order parameters from the three components of the IR absorbance. The presence of both a uniaxial and a biaxial nematic phase was detected, and again, optical textures and conoscopic observations supported these findings. 

Subclass 02, biparallel structures. This class, with nematic structures, was obtained by Hessel and Finkelmann and called biaxial. While the word biaxial is quite appropriate, it is also used in the liquid crystal field in connection with structures of smectic C phases, hence a new name was proposed. A low-molecular-mass 02 compound was synthesized by Malthete and his colleagues. Properties of a polysiloxane with phenyl benzoate mesogens obtained by Finkelmann were studied by Hotz and Strobl. " ... 

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). 

The bent core molecules do not only exhibit spontaneous resolution in smectic phases. One achiral derivative resolves in a nematic phase in this fluid state [ 145], while a substituted oxadiazole which forms a biaxial nematic phase also segregates [ 146]. The bent core clearly has a special stereochemical influence as a result of the effects it induces beyond the molecule, at least for liquid crystals. 

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)... 

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