Nematic phase identification

For spin-f nuclei, dipolar interactions may be modulated by intramolecular (DF, reorientation etc.) and/or intermolecular (TD) processes. In general, the intra- and inter-molecular processes can produce quite different Tj frequency dispersion curves. In practice, NMR field cycling experiments are often needed to extend the frequency domain from those employed in conventional spectrometers to a lower frequency range (i.e., the kHz regime) for unambiguous separation (and identification) of different relaxation mechanisms. The proton spin relaxation by anisotropic TD in various mesophases has been considered by Zumer and Vilfan.131 133,159 In the nematic phase, Zumer and Vilfan found the following expression for T ... 

Between crossed polars these defects appear as dark lines or brushes with curved or irregular shapes that correspond to extinction positions of the director and molecular long axes. Thus, the director can be either parallel or perpendicular to the polarizer and analyzer. The brushes tend to cover the specimen in rather a continuous way, indicating the liquid-like nature of the mesophase. The points where the brushes meet are called singularities in the texture (see Figure 3A). For nematic phases two forms of schlieren defect are found, one where two brushes meet at a point and one where four brushes meet. All tilted smectic phases (C, I, F, and ferrielectric C), except for the antiferroelectric phase, exhibit four brush singularities. Therefore, this provides a simple way of distinguishing between smectic and nematic phases. It should be noted that phases such as smectics A and B(hexatic) and crystal phases B(crystal), E, G, H, J, and K do not exhibit schlieren textures and so this narrows down the possibilities for phase identification. 

Liquid crystal polymers exhibit the same liquid crystalline phases and mesophases exhibited by low molar mass mesogens. However, the identification of the mesophases generated by polymers is usually far more difficult than for low molar mass materials (see Chapter 9). Usually, the nematic phase is readily characterised but smectic phases, especially the highly ordered analogues, are often uncharacterised and simply denoted S Many liquid crystal polymers, like conventional polymers, exhibit a glass... 

The identification of the Nc and Nd phases can be archived by polarization microscopy based on their different textures which are presented in Fig. 11.28 a and 11.28 b. H-NMR spectroscopic studies and orientation experiments on the nematic phases in a magnetic field are suitable for their detection, especially if SANS measurements are done on oriented samples. For these experiments the phases have to be prepared with D2O instead of H2O. This... 

For phase identification, therefore, it is essential to be familiar with the different birefringence textures each phase can exhibit. These textures depend on molecular alignment with respect to the optical axis and the polarizers. To illustrate this, we can consider the Schlieren texture of the nematic phase shown in Figure 2.22. 

The identification of the appropriate order parameter for nematic liquid crystals is aided by a consideration of the observed structure and symmetry of the phase. As in any liquid, the molecules in the nematic phase have no translational order i.e., the centers of mass of the molecules are distributed at random throughout the volume of the liquid. Experiments of many varieties, however, do demonstrate that the nematic phase differs from ordinary liquids in that it is anisotropic. The symmetry, in fact, is cylindrical that is, there exists a unique axis along which the properties of the phase display one set of values, while another set of values is exhibited in all directions perpendicular to this axis. The symmetry axis is traditionally referred to as the director . The optical properties of nematics provide an example of how the cylindrical symmetry is manifest. For light passing parallel to the director, optical isotropy is observed, while for all directions perpendicular to the director, optical birefringence is observed. Rays polarized parallel to the director have a different index of refraction from those polarized perpendicular to the director. 

The most common use of X-ray scattering in the field of nematics has been phase identification and measurement of orientational order parameters. Even when a birefringent optical texture is ambiguous (e.g. in some polymers), a nematic can be easily identified by the absence of sharp X-ray reflections. The capability of the X-ray technique to determine the complete orientational distribution function has been enhanced in recent years by the comparatively wide availability of area detectors. Much of the theoretical background work applicable to nematics has been developed for other oriented systems, notably oriented amorphous polymers. They have yet to be fully exploited in the field of liquid crystals, e.g. in determining the molecular conformation. 

G.c.-m.s. with selected ion monitoring provides a very sensitive determination of oestrogens on a scale of picograms. Standards with high specific deuterium labelling (e.g. [ Hgjoestradiol) were prepared for this purpose. Various dimethyl-alkylsilyl ethers (alkyl = Et, Pr", or Pr ) have proved superior to trimethylsilyl ethers for the g.c. separation of bile-acid methyl esters. G.c.-m.s. has been applied to the separation and identification of unsaturated bile acids found in natural extracts,and, with computerized recognition, to a series of sterols and bile-acid esters.G.c. separation of various steroids and bile acids has been affected on the nematic liquid crystal N,N -bis(p-phenylbenzylidene)-a,a -bi-p-toluidine as stationary phase. 

The concept of defects came about from crystallography. Defects are dismptions of ideal crystal lattice such as vacancies (point defects) or dislocations (linear defects). In numerous liquid crystalline phases, there is variety of defects and many of them are not observed in the solid crystals. A study of defects in liquid crystals is very important from both the academic and practical points of view [7,8]. Defects in liquid crystals are very useful for (i) identification of different phases by microscopic observation of the characteristic defects (ii) study of the elastic properties by observation of defect interactions (iii) understanding of the three-dimensional periodic structures (e.g., the blue phase in cholesterics) using a new concept of lattices of defects (iv) modelling of fundamental physical phenomena such as magnetic monopoles, interaction of quarks, etc. In the optical technology, defects usually play the detrimental role examples are defect walls in the twist nematic cells, shock instability in ferroelectric smectics, Grandjean disclinations in cholesteric cells used in dye microlasers, etc. However, more recently, defect structures find their applications in three-dimensional photonic crystals (e.g. blue phases), the bistable displays and smart memory cards. 

It was used for the identification of the smectic B phase in another substance (4-n-hexyloxyphenyl-4 -n-decyloxybenzoate, HOPDOB). Figure 1.16 shows the corresponding phase diagram for mixtures of the two compounds. For the sake of simplicity, all lines for the monotropic transitions are removed. Prom the diagram it is clear that the nematic, smectic A, and smectic B phases (but not the 5c phase) are extended from pure HBAB to pure HOPDOB and the following sequence of phases for HOPDOB was established ... 

After the isotropic to nematic transition, the next step towards more ordered mesophas-es is the condensation of SmA order when the continuous translational symmetry is broken along the director. The theoretical description of the N-SmA transition begins with the identification of an order parameter. Following de Gennes and McMillan [1, 16], we notice that the layered structures of a SmA phase is characterized by a periodic modulation of all the microscopic properties along the direction z perpendicular to the layers. The electron density for instance, commonly detected by X-ray scattering can be expanded in Fourier series ... 

Identification of nematic polymeric mesophases is a more complex problem than identification of polymer smectics. The structural data are usually limited to finding the absence of small-angle reflections in the x-rays of unoriented samples. The low enthalpy of the transition from the anisotropic to the isotropic phase (Table 6.9), close to the corresponding values characteristic of low-molecular nematic liquid crystals, and the absence of layered reflections indicate the one-dimensional type of ordering, although these data are insufficient for a complete description of the structure of nematic polymers, which can be both similar to and (Afferent from low-molecular-weight nematics. 

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