Liquid, definition nematic

Crystalline forms presenting large amounts of disorder of the kind (ii) or (iii) are generally called mesomorphic modifications (Section 3.6), in analogy with the ordered liquids (smectic and nematic). In these cases the lack of periodicities in one or two dimensions (e.g., along the chain axes or along the directions normal to the chain axes) prevents the definition of a unit cell. Typical features in the X-ray diffraction patterns of mesomorphic forms are diffuse halos on the equator or on the layer lines depending on the kind of disorder present. 

FIGURE 5.11 Supramolecular, helical architecture and definition of pitch length p of chiral nematic liquid crystals. 

Polarized light is the must powerful tool for investigating liquid crystals, all of which exhibit characteristic optical properties. A smectic liquid crystal transmits light more slow ly perpendicular to the layers than parallel to them. Such substances are said to be optically positive. Nematic liquid crystals are also optically positive, bui their action is less definite than that of smectic liquid crystals. However, the application of a magnetic field to nematic liquid crystals lines up their molecules, changing their optical properties and even their viscosity. 

Because nematic liquid-crystalline polymers by definition are both anisotropic and polymeric, they show elastic effects of at least two different kinds. They have director gradient elasticity because they are nematic, and they have molecular elasticity because they are polymeric. As discussed in Section 10.2.2, Frank gradient elastic forces are produced when flow creates inhomogeneities or gradients in the continuum director field. Molecular elasticity, on the other hand, is generated when the flow is strong enough to shift the molecular order parameter S = S2 from its equilibrium value 5 . (Microcrystallites, if present, can produce a third type of elasticity see Section 11.3.6.)... 

Here 0 is the angle between the long molecular axis and the optical axis of the liquid. In the same way we can define a degree of order of any molecular axis, whether we consider a molecule of the pure nematic liquid or a molecule dissolved in a nematic solvent. By definition the S values range between 1 and —1/2. S = 1 means that the corresponding axis is always parallel to the optical axis S = —1/2 means that it is always perpendicular to it S = 0 corresponds to a random orientation as in isotropic liquids. The S value of the long molecular axis in pure nematic liquids usually lies between 0.4 and 0.7 (1, 6, 7, 8, 15, 19, 20). 

Until now there was no obvious correlation found between the monomer structure and the resulting pol qner phase. No.theorr retical structural conditions were described which would result in a liquid crystalline polymer with a definite ordered phase e.g. with a nematic a smectic or a cholesteric phase as in conventional liquid crystals. Although previous examples have established (8 9) the existence of enantiotropic liquid crystalline side chain polymers additional considerations are in order for a systematic synthesis of such polymers. 

The films of common amorphous p state are optically isotropic. In c birefringence observed in the unload cates that a definite orientational chain arrangement (in the absence of tial lattice) similar to the nematic liquids. This property is a direct high rigidity and regular structure molecules O favoring the formation o gions with mesomorphic structure. 

Liquid crystals appear to constitute a further step in the orientation of liquids. Although we are unable today to systematize finally the existing experimental material and the somewhat contradictory views of the authors working in this field, there appear to be two definite arrangements of particularly frequent occurrence, named by G. Friedel the smectic and the nematic state. 

A cholesteric phase represents a special case of a nematic phase. The planes of adjacent molecules are also parallel, but the longitudinal axis of adjacent planes are turned about a definite angle. The result is a screw structure of the system. Cholesteric thermotropic liquid crystalline phthalocyanines are also known. ( + )-2,3,9,10,16,17,23,24-octakis[4-(dodecyloxy)-2-oxapentyl]phthalo-cyanine (preparation see Schemes 13 and 14) shows a texture typical for cholesteric phases [155], At 160°C, this compound gives a fluid isotropic phase. Upon cooling an anisotropic phase appears at 153 °C. The texture similar to platelets (blue phase) changes at 66 °C to typical cholesteric fan-shape structure and remains fan-shaped down to room temperature. Except for the discotic... 

The surface polarization can be measured by different means. The most straightforward one is based on the pyroelectric technique [15]. To measure P one has to deal only with one surface of a cell with uniform director alignment, either planar or homeotropic at both interfaces. The main idea is to use a spatially dependent temperature increment in order to separate the contributions to the pyroelectric response coming only from the surface under study and not from the opposite one. By definition, the pyroelectric coefficient is y = dPIdT where P is macroscopic polarization of a liquid crystal and T is temperature. If we are interested only in the polarization originated from the orientational order we can subtract the isotropic contribution to y and calculate P in the nematic or SmA phases by integrating the pyroelectric coefficient, starting from a certain temperature T, in the isotropic phase ... 

Fig. 1. Calculated capacitance C and optical transmission of a nematic liquid-crystal cell of thickness d and area A between parallel polarizers set parallel or perpendicular to the rubbing directions (solid curve) or at 45 to these directions (dashed curve) as a function of the applied RMS voltage V. Definition of Freederlksz threshold optical threshold Vthr> 50 80...

Optically, a nematic phase can be uniaxial or biaxial. The latter is formed by elongated lath-like molecules. Conventional nematic liquid crystals formed by rod-like molecules constitute a uniaxial medium with nonpolar symmetry. The constituent molecules rotate (freely or hindered) around both their short and long axes. Nonoriented samples are analogous in some measure to polycrystalline powders they consist of individual (liquid) crystallites, each of which has as a definite feature, a directed optical axis, which... 

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