Discotic, nematics director

Discotic LC are formed by disk-like molecules with aromatic cores and side chains that are either hydrophobic (i.e., thermotropic) or hydrophilic (i.e., lyotropic). The discotic nematic (No) phase behaves like a normal nematic phase formed by rod-like molecules, and the disk-like molecules are oriented with their short molecular axes parallel to the director but show no positional order. More ordered columnar phases are commonly formed by thermotropic discotics. The two-dimensional structure can pack the columns into a hexagonal or rectangular columnar phase, while within the columns, disks can be... 

In the simplest liquid-crystalline phase, namely the uniaxial nematic, there is at rest a special direction designated by a unit vector n called the director (see Fig. 10-2). In the plane transverse to the director, the fluid is isotropic. The most common nematics are composed of oblong molecules that tend to point in a common direction, which defines the director orientation. Oblate, or disc-like, molecules can also form uniaxial nematics for these discotic nematics, the director is defined by the average orientation of the short axis of the molecule. Lath-like molecules or micelles (shaped like rectangular slabs), in which all three dimensions of the molecule are significantly different from each other, can form biaxial nematics (Praefcke et al. 1991 Chandrasekhar 1992 Fialtkowski 1997). A biaxial... 

Very few data exist for the viscosities or Frank constants of discotic nematics—that is, nematics composed of disc-Uke particles or molecules (Chandrasekhar 1992). One can estimate values of the Leslie viscosities from the Kuzuu-Doi equations (10-20) by setting the aspect ratio p equal to the ratio of the thickness to the diameter of the particles thus /j — 0 for highly anisotropic disks. This implies that R(p) —1, and Eq. (10-20b) implies that the viscosity o 2 is large and positive for discoidal nematics, while it is negative for ordinary nematics composed of prolate molecules or particles. If, as expected, is much smaller in magnitude than 0 2. the director (which is orthogonal to the disks) will tend... 

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... 

Fig. 6.1.2(/) gives a schematic illustration of the structure of the discotic nematic (N ) in contrast to the classical nematic of rod-shaped molecules, the director n now represents the preferred orientation of the short molecular axis (or the disc norm" /. The symmetry of the two kinds of nematics is the same, and identical types of defects - the schlieren texture,... 

This section describes both dynamics studies at the molecular level and also cooperative bulk macroscopic properties as sensed by diffusion studies. The measurements of the proton spin-lattice relaxation time of liquid crystal 4- -octyl-4 -cyanobiphenyl (8CB) confined in randomly oriented untreated porous glass have been presented. The studies are in agreement with the model of mutually independent pores with nematic director parallel to the pore axis in each segment. The local translational diffusion of molecules within the cavities is found to be nearly as fast as in bulk. Orientational relaxation of a model discotic liquid crystal, consisting of dislike molecules... 

In the discotic nematic (No) phase (Fig. 16.3a), molecules have orientationally ordered arrangement of discs with no long-range translational order. This is the least order (usually high temperature) in the disc-like molecules (Kumar 2009). The nematic mesophase can be assimilated to a lamellar nematic liquid crystal, in which the director vector (an optical axis) is perpendicular to the average direction along which the flat molecules are aligned, as illustrated in Fig. 16.3a. 

FIGURE I Schematic rqjresentations [7] of the nematic phase (N, left) of rod-like molecules cwnpared with die discotic nematic (Nq, coitre) and die non-tihed columnar nematic (Ngdi, right) phases of disc-like molecules. D = discotic. Col = columnar, n = director. 

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. 

Mesophase with a helicoidal superstructure of the director, formed by chiral, calamitic or discotic molecules or by doping a uniaxial nematic host with chiral guest molecules in which the local director n precesses around a single axis. 

The directors (long molecular axes) of the constituent molecules in nematic phases are parallel to one another on average. This is the only order present in nematic liquid crystals, which are the most fluid type of liquid-crystalline phase. Molecules that form cholesteric phases must be optically active or contain an optically active dopant. As the phase name implies, the constituent molecules are frequently steroids and most commonly are cholesteric esters or halides. A conceptual model of the cholesteric phase includes layers of molecules in nematic-like positions, each layer being twisted slightly with respect to the ones above and below it. When the phase consists only of optically active molecules, the angle of twist between layers is typically less than one degree. Several subclasses of discotic phases exist. In all, the molecular planes of the constituent molecules are parallel. However, the discs can pack in nematic-like arrangements (ND) or in columns that are internally ordered (D ) or disordered (Dd) and may be stacked vertically,... 

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. 

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... 

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. 

When a discotic liquid crystal is sandwiched between two substrates (or exposed to air), the direction of the uniaxial axis can be controlled by alignment layers, external electric fields, and chiral dopants [46,47]. It is therefore possible to develop discotic compensation films with spatially varied uniaxial axis orientations. For example, Fuji Photo Film Co. developed discotic compensation films for TN LCDs. In both the TN display and discotic compensation film, the liquid crystal directors vary in the vertical direction. Each layer of nematic liquid crystal with a certain director orientation is compensated by a layer of discotic liquid crystal with the same director orientation. 

Fig. 3.3 Structural sketches of nematic phases composed of a calamitic and b discotic mesogens with indicated direction of the director n. In the sketches of the corresponding cholesteric phases of c calamitic and d disoctic mesogens, only the local director njoeai is drawn in...

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. 

Liquid crystals. A. Generic description of select phases. A rod-shaped mesogen can form a smectic or a nematic phase. A disk-shaped m can form a columnar/discotic phase. B. Illu.stration of how the dirertor of a cholesteric phase rotates as one moves through the material, giving rise to a helical pitch. Note that the oval in this part of the figure is not a sinci mesogenic molecule, but rather represents the director of a thin section of the cholesteric phase. 

Nematic liquid crystalline phases can be thought of as smectic phases absent the layering effect (Figure 13.7 A). There is clearly still a preferred orientation, a director, but that is the only type of order. Another type of phase termed the columnar or discotic liquid crystalline phase can form when the mesogen is more disk-shaped rather than cigar-shaped (Figure 13.7 A). 

The nematic phase of nonlinear mesogens may be biaxial - a translationally disordered fluid phase with two directors n and o specifying the orientational order (Fig. 5.3). Biaxial order in a nematic is predicted to occur [29] if the shape anisotropy of the idealized molecule (discoid) representing the nonlinear meso-gen is appropriately intermediate between the prolate shape of calamities and the oblate shape of discotics. Discoid-shaped mesogens lend themselves to a variety of stratified phases. Ferroelectric (Sapp) and antiferroelectric (Sapa) layer motifs in the normal smectic phases of discoid-shaped mesogens are readily envisioned (Fig. 5.4), but less obvious is the possibility of generating chiral supramolecular structures from such achiral discoid-shaped mesogens (Fig. 5.5) [30]. 

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