Nematic liquid crystals molecular alignment

The use of birefringence to determine the behavior of 5( 7) is a natural choice since the principal characteristic of the nematic phase is optical birefringence i.e., the refractive index differs for light polarized parallel (/ n) or perpendicular (%) to the axis of molecular alignment. Eor a nematic liquid crystal, the director n specifies this optical z axis and / n = and = Dg are called the extraordinary and ordinary refractive indices, respectively. In general, rig > rig and the difference is the refractive index anisotropy (birefringence)... 

D. Baalss and S. Hess, Nonequilibrium Molecular Dynamics Studies on the Anisotropic Viscosity of Perfectly Aligned Nematic Liquid Crystals,... 

K. Aoki, T. Seki, Y. Suzuki, T. Tamaki, A. Hosoki, and K. Ichimura, Factors affecting photoinduced alignment regulation of cyclohexanecarboxylate-type nematic liquid crystals by azobenzene molecular films, Langmuir 8, 1007-1013 (1992). 

K. Ichimura, Y. Hayashi, K. Goto, and N. Ishizuki, Photocontrol of in-plane alignment of a nematic liquid crystal by a photochromic spiropyran molecular layer absorbing linearly polarized light, Thin Solid Films 235, 101-107 (1993). 

Fig. 6.9. The origin of liquid crystal alignment on rubbed polymer surfaces (A) Liquid crystal molecules have highly anisotropic charge distributions with their tt system preferentially oriented perpendicular to their long axis, which gives an ensemble of oriented (nematic) liquid crystals an asymmetric charge distribution (B). This can be characterized by molecular orientation factors (fa,fb,fc) describing the preferential orientation of the tt system. For the experimentally observed alignment directions the anisotropic charge distribution of the liquid crystal is oriented parallel to the one of the rubbed polyimide (C) and polystyrene (D) surface, which optimizes their interaction energy [3].

Cognard J (1982) Alignment of nematic liquid crystals and their mixtures. Molecular Crystals and Liquid Crystals (Special Supplement) 51 1-77. 

Helfrich, W. Molecular theory of flow alignment of nematic liquid crystals. J. Chem. Phys. 50, 100-106 (1969)... 

With the aim of quantitatively predicting the orientational order of rigid solutes of small dimensions dissolved in the nematic liquid crystal solvent, 4-n-pentyl-4 cyanobiphenyl (5CB), an atomistic molecular dynamics (MD) computer simulation has been applied. It is found that for the cases examined the alignment mechanism is dominated by steric and van der Waals dispersive forces. A computer simulation of the deuterium NMR spectra of molecules in a thin nematic cell has been carried out and the director distribution in the cell has been studied. An experiment for the direct estimation of an element of the order matrix from H NMR spectra of strongly dipolar coupled spins that is based on the multiple quantum spin state selected detection of single quantum transitions has been proposed. The experiment also enables obtaining nearly accurate starting dipolar... 

Schematic representation of a nematic liquid crystal phase. The lines represent the molecules and the unit vector , called the director, describes the average direction of the molecular alignment along the uniaxial anisotopic axis.

Figure 11.1 Simplified representations of molecular alignment in (a) smectic, (b) nematic and (c) chiral nematic liquid crystals.

Besides aligning liquid crystals, external electric fields can also change the orientational order and thus the electro-optical properties of liquid crystals. When the long molecular axis of a liquid crystal molecule, whose anisotropy of polarizability is positive, is parallel to the applied field, the potential of the molecule is low. Thus the applied field suppresses the thermal flue-mation and increases the order parameter. Now we discuss how the orientational order of a nematic liquid crystal changes with applied fields. Using the Landau-de Gennes theory, the free energy density of a liquid crystal in an electric field (when the liquid erystal director is parallel to the field) is [4]... 

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