Nematic phases alignment

The dielectric constants of an aligned nematic phase are dependent upon both the temperature and the frequency of the applied field at temperatures below the clearing point. The dielectric permitivity, j, measured parallel to all three axes above the clearing point in the isotropic liquid is the same. Therefore, the dielectric anisotropy of the same compound in the liquid state is zero, see Figure 2.10. The sign and magnitude of the dielectric constants and, therefore, the dielectric anisotropy are dependent upon the anisotropy of the induced molecular polarisability, Aa, as well as the anisotropy and direction of the resultant permanent molecular polarisation determined by permanent dipole moments. 

Figure 100 The molecular organization within the aligned nematic phase of Kevlar.

The most straightforward way to produce partially oriented solute molecules is to orient them in anisotropic solvents. Good solute orientation can be achieved in stretched polymer sheets [94, 95, 96]. Homogeneously oriented nematic liquid crystals are perfectly clear and are thus excellently suited as anisotropic solvents for optical polarization experiments. Moreover, the liquid crystal method allows the performance of polarization experiments in fluid media. This unique feature of the liquid crystal method can be exploited for polarization studies of metastable molecular species (e.g. excited complexes) formed by a diffusion-controlled process. The ordered glasses produced by rapid cooling of a uniformly aligned nematic phase can be used for phosphorescence polarization experiments. 

Polarized absorption (or fluorescence) spectra are taken in uniformly aligned nematic phases. The orientation of the optical transition moment in the molecular fixed coordinate system of the solute can be determined from the change in the optical density of the solute produced by the partial solute alignment [7]. 

Figure 13. Diffraction pattern from an aligned nematic phase.

If an aligned nematic phase exhibits more than one wide angle equatorial diffuse peak. 

FIGURE 2.27 These cartoons illustrate the different scattering patterns that may be observed in an x-ray diffraction experiment on a liquid crystal material for (a) an unaligned smectic phase, (b) an aligned smectic A phase, and (c) an aligned nematic phase. 

Which of the following liquid crystal samples will be birefrin-gent when viewed on the polarizing microscope between crossed polarizers (a) homeotropically aligned nematic phase, (b) the isotropic phase, (c) a planar-aligned smectic A phase, (e) the cholesteric phase with a focal conic texture, (d) the isotropic phase of a discotic material ... 

This alignment giving rises to a low viscosity and makes the LCP phases easier in processing. Nematic mesophases has got fluidity characteristics very similar to that of isotropic liquids but they can be easily oriented by external source like electric or magnetic field. Aligned nematic phases have got very... 

Small-angle neutron scattering (SANS) indicated that the aligned nematic phase of tetramethylammonium perfluorononanoate maintains its alignment when it is cooled in a magnetic field to the temperature of the lamellar phase and the magnetic field is withdrawn. 

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