Nematic calamitic phase

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

It is very interesting to see that a crossover between the equilibrium flexodomains and the dissipative EC patterns can be observed in the same experiment (planar geometry) by merely increasing the AC frequency co. Inspection of Fig. 4.5 reveals that at very small u) and for suitable material parameter combinations of the nematics, the critical voltages of the equilibrium flexodomains and those of the dissipative electroconvection patterns might approach each other. In fact, very recent theoretical and experimental studies on the calamitic nematic Phase 4, have demonstrated the... 

Figure 6 Structure of the calamitic nematic phase of rod-like molecules.

There are some specific amphiphile systems which form exceptionally stable pancake-like micelles which, as one might expect, give anisotropic solutions analogous to discotic nematic phases. Conversely, there are other lyotropic systems containing elongated stacks of aromatic molecules, i.e., chromonic systems, which are analogous to thermotropic calamitic nematic phases. 

The most mobile and least-ordered mesophase formed by discotic molecules is the Nd phase, sketched in Fig. 1.12. This is the discotic analogue of the calamitic nematic phase. The planes of the molecules lie more or less parallel, but there is no positional order in any direction. In the columnar nematic phase, Ncoi there is some degree of aggregation, with short stacks of molecules floating in a nematic array. Both of these phases are relatively rare and there only are few known examples. 

A similar scheme for discotic systems is shown in Fig. 1.33. The diffraction pattern of the nematic discotic phase is analogous to that of the calamitic nematic phase and contains only diffuse reflections, indicating the liquid-like distribution of face-to-face and edge-to-edge separations. For the columnar phases, the high-angle... 

Other more exotic types of calamitic liquid crystal molecules include those having chiral components. This molecular modification leads to the formation of chiral nematic phases in which the director adopts a natural helical twist which may range from sub-micron to macroscopic length scales. Chirality coupled with smectic ordering may also lead to the formation of ferroelectric phases [20]. 

Many technological applications of liquid crystals, as in electro-optic display devices, are based on multicomponent mixtures. Such systems offer a route to the desired material properties which cannot be achieved simultaneously for single component systems. Mixtures also tend to exhibit a richer phase behaviour than pure systems with features such as re-entrant nematic phases [3] and nematic-nematic transitions possible. In this section, we describe simulations which have been used to study mixtures of thermotropic calamitic mesogens. 

The liquid crystal phases of calamitic mesogens fall into two types - nematic (N) and smectic (Sm). The nematic phase is the most disordered of the liquid crystal phases and possesses only orientational order, so that the long axes of the molecules are correlated in one direction (known as the director, n) while being positioned randomly (Fig. 2A). There are several smectic phases and these differ from the nematic phase in possessing partial posi-... 

Liquid crystals are materials that exist in a state that is intermediate between a liquid state and a solid. They are formed by anisotropic molecules, known as calamitic molecules, which are long and narrow, i.e. have a large length-to-breadth ratio. These rod shaped molecules orientate themselves in different ways as they change from the crystalline to the liquid state at different temperatures, existing in the smectic and nematic phases, as shown in Figure 5.1. 

Upon addition of a terminal crown ether to the calamitic core unit, nematic phases were observed for 2, 3 in agreement with previous studies on substituted cyano biphenyls [25, 26]. 

Chiral monophosphorus ligands, in hydrogenation, 10, 16 Chiral nematic phase, calamitic mesogens, 12, 200 Chiral N,P ligands, in hydrogenation, 10, 17 Chiral phosphines... 

Figure 2.2 Schematic representation of the structures of a solid, a smectic phase (SmA), the nematic phase and the isotropic liquid state formed by calamitic organic molecules with a large length-to-breadth ratio.

Calamitic compounds which exhibit a smectic and/or nematic phase usually consist of a relatively rigid central core containing co-linear six-membered rings, either aromatic rings, such as 1,4-disubstituted-phenylene, 2,5-disubstituted-pyridine, 2,5-disubstituted-pyrimidine, 3,6-disubstituted-pyridazine, and alicyc-lic rings, such as /ra j-l,4-disubstituted-cyclohexane, 1,4-disubstituted-bicy-clo[2.2.2]octane, 2,5-disubstituted-dioxane. Heteroaromatic rings tend to lead to the formation of smectic phases rather than the nematic phase unless combined with a polar terminal function, such as a cyano group. The dependence of the liquid crystalline transition temperatures on the nature of... 

The third electro-optical effect using calamitic nematic liquid crystals makes use of a flexoelectric effect manifested by a curved asymmetrical nematic medium. This corresponds to piezoelectricity in crystals. The existence of flexoelectricity in a nematic phase under certain boundary conditions was predicted in the late 1960s and then confirmed experimentally several years later. However, LCDs using this effect, such as bistable nematic displays are only in the development stage and as such they will not be discussed in this monograph. 

Calamitic mesogens in general form two main types of mesophase the nematic phase (N) and the smectic (S) phases. The nematic phase is the most disordered of the mesophase tjpes in it, the molecules possess one-dimensional orientational order and no positional order (Fig. 22). 

The average orientational direction of the molecules is denoted by the director, n,with the degree of orientational correlation evaluated by the order parameter, S, expressed as in Eq. (8), where 0 is the angle between a molecule and the director. It is the nematic phase of calamitic molecules that is used in liquid crystal displays. 

In calamitic systems it is the long axis of the molecules that is correlated in the mesophases but in discotic systems it is the short axis and different types of organization are seen, although disk-like molecules also form a nematic phase in which a unique axis is orientationally correlated (Fig. 25). However, below the nematic phase is a series of columnar phases (Col) in which the disks are stacked up into columns, which are themselves arranged according to some symmetric pattern. Typical... 

Calamitic phases are essentially defined as a set of mesophases that occur between the breakdown of the long-range periodic, translational ordering of the crystal and the loss of long range orientational order at the transition to the isotropic liquid. In this context six mesophases exist the nematic phase and five smectic phases labelled A, B, C, F and I. Other closely related soft crystal phases, in which the molecules have long-range periodic order but are themselves rotationally disordered, also exist in concert with liquid crystal phases. This second set of mesophases are labelled E, J, G, H, and K, and are essentially the crystal versions of the... 

Figure 4.9 Elastic deformations of calamitic, rod-like liquid crystals in the nematic phase. The corresponding elastic elasticity constants are K, (splay), /<2 (twist), and Kj (bend). K, has the largest influence on the threshold voltage, of TN cells [23f].

Molecules of a disc-like shape form either nematic or columnar phases. Structurally the most simple mesophase is the discotic nematic phase (Nd). Just as in the case of the nematic phase formed by calamitic molecules, the molecules possess orientational order, but no positional order (Figure 2.8). The columnar nematic phase (Nc) consists of short columns of a few molecules that act like the rod-like molecules in the nematic calamitic phase (Figure 2.9). There is no organisation of the columns in a two-dimensional lattice. 

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