Discotic nematic

If the molecules are chiral or if a chiral dopant is added to a discotic Hquid crystal, a chiral nematic discotic phase can form. The director configuration ia this phase is just like the director configuration ia the chiral nematic phase formed by elongated molecules (12). Recendy, discotic blue phases have been observed. 

The discotic mesophases are classified in two types columnar, and nematic discotic. The structure of the nematic discotic mesophase (Np, Figure 8.3, left) is similar to that of rod-like molecules, but constituted by disk-like units. In columnar mesophases, the molecules are stacked in a columnar disposition and, depending on the type of columnar arrangement, several columnar mesophases are known. The most common lattices of the columnar phases are nematic discotic (No), columnar nematic (Ncoi), columnar hexagonal (Coin), and columnar rectangular (Col,) mesophases. 

Figure 8.3 Schematic representation of the molecular arrangement in the nematic discotic (Nq), columnar nematic (Ncoi), columnar hexagonal (Coin), and columnar rentangular (Coir) mesophases. A tablet represents a molecule with disk-like shape.

Figure 6.2 Discotic molecules in a (a) nematic state ND, (b) twisted nematic discotic state Np (P/2 is half of cholesteric pitch), (c) columnar state, ordered D0 and disordered Dd, (d) hexagonal ordered columnar state Dho two-dimensional packing arrays for columnar structures in (e) hexagonal, Colh rectangular, Colr oblique, Col0b.

After the discovery of the nematic discotic (No) mesophase,8,9 it was soon... 

Turning now to those molecules whose shape can be approximated by oblate spheroids, one arrives at the discotic phase. Here the average of the normals to planes of the molecules corresponds to the director. A fluid phase in which these normals point in roughly the same direction over a macroscopic distance is said to be discotic. If this factor is the only degree of order, the material is said to be in the nematic discotic phase. If, in addition, the discs stack in regular columns, the material is said to be in the columnar discotic phase. Such structures have been discussed in Section 4.5.1. 

The discotic phases can show also a complex polymorphism. Nematic and cholesteric-like, low viscosity phases have been reported recently. In these, the director vector is perpendicular to the plane of alignment of the flat molecules56) in contrast to the normal nematics and cholesterics where it is parallel to the molecular axis. Most frequently, however, discotics form columnar arrangements as shown in Fig. 10. The order within the columns may change from liquid to quasi-crystalline. The columns are then packed in hexagonal or tetragonal coordination, but are free to slide in the direction parallel to their axes S7). The viscosity of these more ordered discotics is considerably higher than the nematic discotics. 

D. Baalss and S. Hess, The Viscosity Coefficients of Oriented Nematic and Nematic Discotic Liquid Crystals Affine Transformation Model, Z. Naturforsch. 43a (1988) 662. 

The polyaromatic mesophase (PA-MP) is a nematic, discotic, chemotropic liquid crystal. Owing to its high density (about 1.5 gcm ), its high carbon yield of about 90 %, and its thermoplasticity, it is unique as a precursor of structure carbons. An important application is the manufacture of high modulus (HM) and ultra-high modulus (UHM) carbon fibers [1]. By alloying with silicon, physical and chemical properties of the materials, such as strength, hardness and oxidation resistance, can be improved. These modified carbons were available by chemical vapor deposition (CVD) processes only up to now. The preparation by liquid phase pyrolysis is novel, economic, and thus opens a completely new field of applications. 

Earlier, H-shaped orthometallated palladium(II) complexes were discussed which displayed nematic and smectic phases. However, Praefcke has reported related complexes where the number of chains on the periphery of the molecule is increased leading to columnar mesophases. The complex in Figure 91 displays a monotropic nematic discotic phase (No) and the structure has been confirmed as that in the diagram by single-crystal X-ray crystallography [170]. 

The simplest discotic liquid crystal phase is the nematic discotic phase, Nd, in which the normals of the molecular discs tend to align with respect to a preferred direction, i.e., the director, but the mass centers of molecules do not have any positional order. The discs in Figure 1.11 represent the disclike molecules, the molecules are packed in the way a pile of coins is packed randomly. The discotic nematic phase has its chiral counterpart, i.e., the... 

Star-shaped compounds having a 1,3,5-triazine core and stilbenoid arms have been prepared. These compounds form nematic discotic systems <04T6881>. Discotic liquid crystalline... 

Most of what has been aid so far applies to rod-like molecules, but similar phases exist for disk-shaped molecules, such as porphyrins and phthalo-cyanines. These tend to form phases in which the director is now orthogonal to the plane of the molecule. Thus, where the planes of the molecules are more or less parallel, but with no positional ordering into columns, the phase is a nematic discotic. If, however, the macrocycles form themselves into columns as well, we have a columnar smectic phase. A loose analogy of this phase is that it is like stacked coins, with the stacks arranged in a hexagonal lattice. 

An example of this phenomenon is given in Fig. 3.20, which shows the force curve obtained for a nematic solution of surfactants in water (molar concentration water 95%, decylammonium chloride 0.75%, potassium lam-ate 4.25%). The solution forms biaxial micelles and it is a nematic discotic at 29°C, where the micelles freely rotate about their shortest principal axis, defining the director orientation [53]. In bulk nematic there is a short-range... 

The system of Fig. 3.20 has the following bulk phases sequence hexatic calamitic nematic discotic nematic. In the calamitic nematic phase the... 

III. The simplest discotic LC phase is the nematic discotic phase, in which the normal of the discs tends to align to the director. The mass centers of the molecules do not have any positional order. A phase similar to rod-like smectic LC phase is the column phase where molecules are packed in columns parallel to each other (Figure 10). Columns are arranged in a hexagonal or rectangular array. 

Figure 1 General structures of calamitic and discotic thermotropic liquid crystals. (A) Layered calamitic smectic liquid crystal. The structures of the various types depend on the local packing of the molecules, the extent of the packing, and the orientation of the long axes with respect to the layers. (B) Calamitic nematic liquid crystal. The molecules have no long-range order, and are only orientationally ordered. (C) Ordered columnar discotic liquid crystal. Disk-like molecules form ordered or disordered columns different column packings give rise to various mesophase structures. (D) Nematic-discotic liquid crystal phase. The disk-like molecules are only orientationally ordered.

For a uniaxial phase (nematic, discotic nematic, SmA, SmB, etc.) with the symmetry axis along z, all properties along x and y are the same and Xn= X22 X33- The corresponding matrix... 

Compound 18 exhibits solely a chiral nematic discotic phase (N ) phase because the steric effect of the branched chains at the chiral centre disrupts the ability of the molecules to pack in columns. The large size of the planar aromatic core ensures a high clearing point, but the liquid crystal tendency depends critically on the type of chiral peripheral chain. Hexa-substituted phenylacetylenes were discussed in chapter 3 and exhibited the Np phase. Perhaps not surprisingly, when one of the peripheral acetylene units is chiral, the N phase is exhibited (compound 19). 

In the discotic phase, disclike molecules form liquid crystal phases in which the axis perpendicular to the planes of the molecules, orients along a specific direction. The nematic discotic phase has orientational order but no positional order. In the columnar discotic phase, the disclike molecules form columns and therefore exhibit orientational and positional order. In a chiral discotic liquid crystal, the director rotates in a helical path throughout the system. 

The three different shapes (rodlike, disclike, boardlike) and two different mesophases (nematic and smectic) together form six different kinds of crystals. They are rodlike nematic, discotic nematic, boardlike nematic, rodlike smectic, discotic smectic, and boardlike smectic. Figure 6.3 shows these six combinations. 

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