Biaxiality in nematics

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. 

Another example of a symmetry change is the field-induced biaxiality in nematic liquid crystals with negative dielectric anisotropy [29]. In this case the field is applied perpendicular to the director. The latter is parallel to the z axis. The orientational order at a point r in a nematic liquid crystal is defined by the ordering matrix... 

PHASE BIAXIALITY IN NEMATIC LIQUID-CRYSTALLINE POLYMERS... 

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],... 

Fig. 21 Experimental geometry and schematics of the aligned PF2/6 films. Above-. Uniaxi-ally aligned frozen-in nematic PF2/6 microstructures. Below Biaxially aligned hexagonal PF2/6 microstructures with crystallite types I—III. Assuming chain alignment (i.e., the c axis) along the rubbing direction, then the equatorial and meridional directions may be defined by the (xyO) plane and z-axis, respectively. See [114,115] for details...

Similar results have been obtained by, M. Ebert, O. Herrmann-Schonherr, and J. H. Wendorff, Makromol. Chem. Rapid Comniun., 9, 445 (1988), in their X-ray study of the phase of a sanidic discotic polymer. Evidence of biaxiality in certain nematic polymers has also been reported by Hessel and Finkelmann and by Windle et al. ... 

O. Francescangeli, V. Stanic, S.l. Torgova, A. Strigazzi, N. Scaramuzza, C. Ferrero, S. Orlandi and C. Zannoni, Ferroelectric response and induced biaxiality in the nematic phase of bent-core mesogens, Adv. Func. Mater. 19(16), 2592-2600, (2009). doi 10.1002/adfm.200801865... 

Oxadiazole mesogens have been significandy less investigated than their close relatives 1,3,4-oxadiazoles, but recently there has been growing interest in this moiety due to the claim of ferroelectric (FE) switching and biaxiality in the nematic phase (2009AFM2592). 

A survey of recent studies of biaxial liquid crystals, in the context of the reported biaxial nematic phase in low molecular mass bent-core mesogens, with emphasize on the use of NMR spectroscopy is available. A discussion of orientational order parameters particularly in smectic-C and biaxial nematic phases and their determination by various techniques including NMR has been presented. The relation between molecular dynamics and biaxiality of nematic polymers and elastomers has been studied by H NMR spectroscopy. "" It was observed that the side-on attachment of the mesogens hinder the rotation of the molecules around their long axes and that in nematic polymers and elastomers phase biaxiality... 

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... 

The electrooptical (or magnetooptical) effect corresponding to the geometry in Fig. 4.4(c) is called the T-effect. It is caused by a pure twist deformation and is most amenable to theoretical analysis. Unfortunately, it is less suitable for experimental investigation, since the field induces optical biaxiality in the nematic liquid crystal. The effect, for which the relaxation kinetics were considered in Section 4.1.6, must not be confused with the twist effect which includes combined T, S, and B deformations (to be discussed in Section 4.2). 

Several examples of textures of the rod-like, disk-like, and biaxial chiral nematics are shown and discussed in a paper by Melnik and Saupe [14]. Contact preparations can be used to determine the handedness of the twist if a sample of known handedness is available. The observation of a nonchiral (compensated nematic) region between two twisted parts of the sample is a strong hint to opposite handedness. 

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