Nematic Liquid Crystals N

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. 

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The nematic liquid crystal, N phase, is the only liquid crystal phase without any long range translational order. Nematics are the most important member in the family of the liquid crystals and are widely used in the display industry. 

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. 

Chiral nematic liquid crystal (N -LC) induced by the addition of a chiral dopant into N-LC. Schlieren texture (left) and fingerprint texture (right) are observed for N-LC and N -LCs, respectively, with a polarized optical microscope. 

Chiral Dopants and Chiral Nematic Liquid Crystals (N -LCs)... 

The N -LC is used as an asymmetric liquid reaction field and it is prepared by adding a small amount of a chiral dopant to a nematic liquid crystal (N-LC). The formation of the N -LC can be confirmed under the polarizing microscope (POM) by the change of the characteristic nematic Schlieren texture to a fingerprint-like or striated Schlieren texture of the chiral nematic phase. The distance between the fingerprint lines of the optical pattern is equivalent to half of the helical pitch of the N -LC phase. Therefore, the stronger twisting power of the dopant can be observed by the shorter helical pitch of the optical pattern under the POM (Fig. 9.33). 

An alternative approach to obtain luminescent liquid crystals is to use the host-guest concept a luminescent lanthanide complex is dissolved in a hquid-crystalline matrix. This approach allows one to optimize the luminescence and mesomorphic properties of the liquid-crystal mixture independently. Boyaval et al. investigated the luminescence of lanthanide complexes in a cholesteric hquid crystal mixture (Boyaval et al., 1999, 2001). Birmemans and Moors (2002) showed that a nematic liquid crystal can be an interesting host matrix to study the spectroscopic properties of luminescent lanthanide complexes. These authors doped the /3-diketonate complex [Eu(tta)3(phen)] into the nematic liquid crystals N-(4-methoxybenzyhdene)-4-butylaniline (MBBA) and 4-n-pentyl-4 -cyanobiphet5rl (5CB) and observed narrow-band red photoluminescence with a well-resolved crystal field sphttmg. Later on, these authors extended their studies to lanthanide j8-diketonate complexes emitting in the near-infrared (Ln = Nd, Er, Yb) (Van Deun et al., 2003). 

An aligned monodomain of a nematic liquid crystal is characterized by a single director n. However, in imperfectly aligned or unaligned samples the director varies tlirough space. The appropriate tensor order parameter to describe the director field is then... 

Fig. 29. Schematic representation of a bend deformation (a) changes in the components of the director, n defining the orientation change (b) bend deformation of an oriented layer of a nematic liquid crystal.

Nematic phase this is the simplest structure. It is the most disordered mesophase and therefore very fluid. It is called N. In the nematic phase, the molecules are ordered mainly in one dimension with their long axes parallel, and they are free to move parallel to this axis (there is no long-range order). Nematic liquid crystal mixtures, containing various amounts of different liquid crystal compounds, are used in electro-optic display systems such as flat-panel displays. 

The three-state RIS model of conformer statistics is used to analyze the 16 independent dipole coupling constants measured in a proton NMR study of n-hexane in a nematic liquid crystal solvent. The orientational ordering of the n-hexane molecule is treated in the context of the modular formulation of the potential of mean torque. This formulation gives an accurate description of alkane solute orientational order and conformer probabilities in the nematic solvent. Consequently, substantially more accurate calculated diplar couplings are obtained, and this is achieved without the need to resort to unconventionally high values of the trans-gauche energy difference E(g) in the RIS model. 

Sorai, M. and Seki, S. Heat capacity of N-(o-hydroxy-p-methoxybenzylidene)-p-butylaniline A glassy nematic liquid crystal. Mol. Cryst. Liq. Cryst. 23, 299 (1973)... 

Aizawa and Suzuki (83,84,85,86) utilized, as an ordered system, liquid crystals in which Chi was immobilized. Electrodes were prepared by solvent-evaporating a solution consisting of Chi and a typical nematic liquid crystal, such as n-(p-methoxybenzyl-idene)-p -butylaniline, onto a platinum surface. Chl-liquid crystal electrodes in acidic buffer solutions gave cathodic photocurrents accompanied by the evolution of hydrogen gas (83). This was the first demonstration of photoelectrochemical splitting of water using in vitro Chi. Of particular interest in these studies is the effect of substituting the central metal in the Chi molecule. 

Lopatina and Selinger recently presented a theory for the statistical mechanics of ferroelectric nanoparticles in liquid crystals, which explicitly shows that the presence of such nanoparticles not only increases the sensitivity to applied electric fields in the isotropic liquid phase (maybe also a possible explanation for lower values for in the nematic phase) but also 7 N/Iso [327]. Another computational study also supported many of the experimentally observed effects. Using molecular dynamics simulations, Pereira et al. concluded that interactions between permanent dipoles of the ferroelectric nanoparticles and liquid crystals are not sufficient to produce the experimentally found shift in 7 N/ so and that additional long-range interactions between field-induced dipoles of nematic liquid crystal molecules are required for such stabilization of the nematic phase [328]. 

Kitzerow et al. recently demonstrated that temperature-induced phase transitions (Iso-N) and electric field-induced reorientation of a nematic liquid crystal (5CB in this case) can be used to tune photonic modes of a microdisc resonator, in which embedded InAs quantum dots serve as emitters feeding the optical modes of the GaAs-based photonic cavity [332],... 

Finally, dispersions of MWCNT in chiral nematic liquid crystals were studied as well. These experiments suggested no change in the helical twisting characteristics of the chiral nematic phase. However, the MWCNTs were thought to disrupt the translational order in the SmA phase (decrease of the SmA-N phase transition) yet follow the twist of the nematic director in the chiral nematic phase [498]. 

G. Ayton and G. N. Patey, Phys. Ref. Lett., 76, 239 (1996). Ferroelectric Order in Model Discotic Nematic Liquid Crystals. 

Figure 2.8 The dependence of the refractive indices, n and ng, of the ordinary and extraordinary rays, respectively, on the temperature, T.for a typical nematic liquid crystal. Above the clearing point, Tg, there is no birefringence and only one refractive index,is observed. ...

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)... 

Schematic diagrams of die cell and apparatus for measuring die temperature dependence of die optical rotation of an aligned nematic liquid crystal cell. The birefringence cell is formed from microscope slides and die lines in the cell denote the rubbing direction for the glass plates and thus the direction for the nematic director n.

Xiang Y., Lin Z. Y., Li T., Shi J., Huang J. W. and Ji L. N., A homeotropical Alignment Nematic Liquid Crystal Doped with FeTPPCl will Dramatically Decrease the Critical Magnetic Field for the Bend Reorientation in a Static Magnetic Field. Mol. Cryst. Liq. Cryst. 339 (2000) pp. 139-143. 

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