Anisotropic liquid crystals

Optically anisotropic liquid-crystal displays consisting of triphenylene, (IV), were prepared by Ushiyama et al. (4) and used in optical compensatory film. 

Figure 7. Close to the PIT value two phases with a lamellar structure exist. One of these, the surfactant phase (S), is an isotropic liquid, the other one, (N), is an optically anisotropic liquid crystal with a lamellar structure.

Static quadrupole effects in NMR are observed in solids (9) and also in anisotropic liquid crystals (10, 11, 12). For nuclei with spin quantum numbers, I, greater than V2, the distribution of positive charge over the nucleus can be nonspherical and the situation can be described in terms of a nuclear electric quadrupole moment. The interaction between the quadrupole moment, eQ and electric field gradients, eq, shifts the energy levels of the nuclear spin states. 

A number of important process parameters were investigated to find out their effect on pitch characteristics and yield of the toluene and quinoline insolubles. The pitches produced were characterized by solvent analysis, NMR, thermal, and elemental analysis. Insolubles in toluene, pyridine and quinoline were used because these fractions represent the fusable and infusable anisotropic liquid crystal fraction formed in the pitch. 

Anisotropy of the Spin-Spin Coupling Tensor. - In NMR experiments performed in anisotropic liquid crystal (LC) phases or in the solid state, the anisotric part of an indirect nuclear spin-spin coupling tensor J appears combined with the direct dipolar coupling D. The NMR spin Hamiltonian appropriate for spin 1/2 nuclei in molecules partially oriented in uniaxial LC solvents can be written in the high field approximation as... 

When a thermotropic liquid crystal is heated, it passes from the crystalline state into the liquid-crystalline state at a point called the melting point. By further heating, the birefringent (anisotropic) liquid crystal is transformed at the clearing point into an isotropic fluid the birefringent fluid becomes clear and all molecular order is lost. 

When an oriented sample is available, such as a drawn fibre, a stretched polymer film, or an anisotropic liquid crystal [1-5, 12, 41-50], it is possible to measure the orientation of the labelled C— H bond vector with respect to the sample axis. When local orientational constraints are collected for several connected groups in a polymer, they can be combined to calculate... 

Huang CH, Zhang Q, Jakli A (2003) Nematic anisotropic liquid-crystal gels-self-assembled nanocomposites with high electromechanical reponse. Adv Funct Mater 13 525... 

FIGURE 1.14 Viscosity as a function of polymer concentration of a solution of a 50 50 random copolymer, poly(w-hexylisocyanate-co-n-propylisocyanate) (M , =41,000 g/mol) in toluene, encompassing the isotropic, biphasic, and anisotropic (liquid crystal) regimes. (Adapted from Aharoni [1980a].)... 

For microemulsions, the main significance of, for example, studies of NMR of the water lies in the fact that a broad phase characterization is needed for any microemulsion system leading proponents of this have been Friberg, Shinoda, and Kahlweit. However, sometimes a given system cannot be easily identified as a single-phase microemulsion, and then NMR work can be helpful. It should be noted, though, that while it is easy to obtain evidence for minute amounts of isotropic phase in a sample of mainly anisotropic liquid crystal, the converse is much more demanding. 

For slow motions, when T -xj (o , the system is in the solid regime and coj contributes to the resonanee frequencies observed. This is the situation encountered in anisotropic liquid crystals where the NMR speetra are usually dominated by static effects. 

The phase transition from isotropic droplet to crystal or liquid crystal depends on the rate of temperature decrease (A). B and C exhibit the hepatic liquid crystal droplets in crossed nicols with 90 degree of angle (B) and 45 degree (C). D and E show the hepatic crystal in crossed nicols at 90 degree of angle (D) and 45 degree (E), which transited from liquid crystals. Anisotropic liquid crystals locate in hepatocytes in the cords and are absent in the blood sinus (bs). Bars, 60 pm. 

Isotropic pitches with a high degree of aromaticity are capable of forming an anisotropic liquid crystal phase, referred to as the mesophase or the carbonaceous mesophase, when heated in the 350-500°C temperature range in an inert atmosphere [13-16]. 

The formation of the mesophase by this route has the disadvantage of requiring rather long processing times. An alternative route is the solvent route [18]. Isotropic aromatic pitches contain a separable fraction which, when heated at 230-400°C, develops an optically anisotropic liquid crystal phase in <10 minutes. This mesophase has been called a neomesophase since it is highly soluble in solvents such as pyridine or quinoline, while the mesophase derived by the thermal route is insoluble. The separable fraction of isotropic pitch is insoluble in solvents like benzene, toluene, or mixtures of toluene and heptane. Thus, it can be separated by solvent extraction from isotropic or heat soaked pitches (Figure 5, b and b ). 

Mesopitch A carbonaceous solid, consisting primarily of a complex mixture of polycyclic aromatic compounds, which can form an anisotropic liquid crystal mesophase. 

After the proof-of-principle that covariance proves robust in the case of sparsely sampled data sets, non-uniformly sampled data were finally subjected to covariance processing for the analysis of anisotropic liquid crystal samples investigated by natural abundance 2D NMR [38]. A natural abundance deuterium (NAD) Q-COSY with a z-gradient filter, Q-COSY Fz, was used for data acquisition [69,70]. The sequence involves cancellation of diagonal peaks. As NUS scheme, exponentially weighted deterministic on-grid sampling was employed [71]. Yet, quantitative direct... 

Our experiments have shown us that nematic phases are anisotropic liquid crystals, that is, their properties are not the same in every spatial direction. The long thin molecules are randomly distributed but tend to line up parallel to one another, as shown in Fig. 9.0b. However, at any given instant, each molecule is inclined at a small angle a to the average direction over all... 

M. Oh-e, M. Yoneya, and K. Kondo, Switching of a negative and positive dielectric anisotropic liquid crystals by in-plane electric field, J. Appl Phys. 82, 528 (1997). 

It should be noted that for anisotropic liquid crystals the NMR signal is split into 21 component signals due to first-order static quadrupole interactions but that, in the absence of second-order quadrupole effects, the width of the central line gives the transverse relaxation rate both for powder samples and for macroscopically aligned samples (see further Chapter 7). 

Calamitic Liquid Crystal An anisotropic liquid crystal with a longer axis in one direction, giving it a rodlike shape (also referred to as prolate). 

Figure 2.22 shows the temperature dependences of In ry (Fig. 2.21) on the inverse temperature 1/T, which are very close to linear functions [68]. As seen from Fig. 2.22 the isotropic viscosity rjs = a4/2 does not undergo considerable change near the phase transition region, while Vi and rj2 vanish in the isotropic phase. Several approaches have been proposed to describe the temperature dependences of the anisotropic liquid crystal viscosity coefiicients [75, 76]. In [75] phenomenological viscosity coefficients in the equations of nematodynamics were found in terms of the order parameter S — S T)... 

It is useful to note that the dielectric anisotropy of anisotropic liquid crystals can be related to the angle p that the molecular dipole moments make witii tire long axis. 

Let 3 1 denote the value of the Landau free energy density of the spatially uniform isotropic phase and denote the analogous quantity for the anisotropic, liquid crystal phase. and 3 o correspond to the values of the two minima in Fig. 4 for T close to Tc, and their difference, ASF = Si, is positive for T > Tc, zero for T = Tc, and... 

Titus, C. M., P. J. Bos, J. R. Kelly, and E. C. Gartland. 1999. Comparison of analytical calculations to finite-difference time-domain simulations of one-dimensional spatially varying anisotropic liquid crystal structures. Jpn. J. Appl Phys., Part 1. 38 1488. 

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