Phases discotic molecules

Altliough in figure C2.2.2 they are sketched witli rodlike molecules, botli nematic and chiral nematic phases can also be fonned by discotic molecules. 

McMillan s model [71] for transitions to and from tlie SmA phase (section C2.2.3.2) has been extended to columnar liquid crystal phases fonned by discotic molecules [36, 103]. An order parameter tliat couples translational order to orientational order is again added into a modified Maier-Saupe tlieory, tliat provides tlie orientational order parameter. The coupling order parameter allows for tlie two-dimensional symmetry of tlie columnar phase. This tlieory is able to account for stable isotropic, discotic nematic and hexagonal columnar phases. 

Monte Carlo computer simulations of spheres sectioned into a disc [104, 105] show tliat steric interactions alone can produce a nematic phase of discotic molecules. Columnar phases are also observed [104, 105]. 

The prime requirement for the formation of a thermotropic liquid crystal is an anisotropy in the molecular shape. It is to be expected, therefore, that disc-like molecules as well as rod-like molecules should exhibit liquid crystal behaviour. Indeed this possibility was appreciated many years ago by Vorlander [56] although it was not until relatively recently that the first examples of discotic liquid crystals were reported by Chandrasekhar et al. [57]. It is now recognised that discotic molecules can form a variety of columnar mesophases as well as nematic and chiral nematic phases [58]. 

Berardi et al. [66] have also investigated the influence of central dipoles in discotic molecules. This system was studied using canonical Monte Carlo simulations at constant density over a range of temperatures for a system of 1000 molecules. Just as in discotic systems with no dipolar interaction, isotropic, nematic and columnar phases are observed, although at the low density studied the columnar phase has cavities within the structure. This effect was discovered in an earlier constant density investigation of the phase behaviour of discotic Gay-Berne molecules and is due to the signiflcant difference between the natural densities of the columnar and nematic phases... 

A nematic phase of discotic molecules exists where the short molecular axes are correlated directionally but this phase is still rather rare. By far and away the most common behaviour is for the molecules to stack in columns, which are then arranged in a particular way with respect to one another [7]. Examples are given in Fig. 4. 

Thermotropic liquid crystalline (LC) phases or mesophases are usually formed by rod-like (calamitic) or disk-like (discotic) molecules. Spheroidal dendrimers are therefore incapable of forming mesophases unless they are flexible, because this would allow them to deform and subsequently line up in a common orientation. However, poly(ethyleneimine) dendrimers were reported to exhibit lyotropic liquid crystalline properties as early as 1988 [123],... 

In discotic phases the orientation of the molecules is perpendicular to the molecular plane. Here, the columns can be arranged in a nematic or columnar manner. In the nematic phase the molecules possess a centre of gravity randomly ordered, but with the short molecular axis of each molecule more or less parallel. In the columnar phase, beside the preferable orientation of the short molecular axes, the disc-like molecules are ordered forming columns. Depending on the correlation strength between he columns these phases can be subdivided into ordered or disordered. A third possibility is to have a thermodynamically preferable position of the columns in the mesophase, like in a hexagonal cell. Additionally, a tilt of the columns is also possible. 

Nd phase. A phase similar to the rod-like smectic liquid crystal phase is the column phase in which molecules are packed as columns parallel to each other. The columns are arranged in a hexatic or rectangular array. Inside the columns, the spacings of the molecules are either constant or random. The axes of the columns may be tilted with respect to the normal of discotic molecules. Figure 1.12 sketches the chemical formulae of one example of a discotic liquid crystal molecule. 

Discotic liquid crystals came to prominence in the late 1970s when Chandrasekhar, Sadashiva, and Suresh reported the discovery of this new class of liquid-crystalline molecules, which were found to form columnar phases. The first of these, a hexaalkanoate of benzene, is shown in Figure 9. There then followed a rather unfortunate confusion of nomenclature in which the phases formed by discotic molecules were themselves referred to as discotic, carrying the abbreviation D. A liquid-crystal phase must be characterized by its symmetry and organization and not the shape of the molecules of which it is composed this is particularly important in columnar systems as many non-discotic molecules exhibit columnar phases. Indeed, columnar mesophases have been recognized for many years and studies date back to at least the 1960s with the work of Skoulios with various metal soaps. " Therefore, columnar phases take the abbreviation Col followed by some descriptor that describes the symmetry of the phase. 

In contrast to calamitic materials, with discotic molecules, it is the short axis that correlates and the simplest phase formed is a nematic phase. This is usually abbreviated as Nq and referred to as the discotic nematic phase, although for the reasons outlined above, this is somewhat unsatisfactory. In this nematic phase, there is again only orientational order as illustrated schematically in Figure 11(a). Materials showing this phase are relatively rare. 

Discotic molecules with a benzene-1,3,5-tricaboxyamide (BTA) core are used to form a columnar LC phase through threefold intermolecular H-bonding. The BTAs are functionalized with a carboxylic acid moiety, which is able to form an ionic interaction with amines. A well-ordered superlattice is formed by mixing the BTAs... 

DLCs generally exhibit two types of phases nematic phase and columnar phase (Fig. 7.1). Discotic nematic phases are rarely observed while columnar phases are commonly found. In the nematic phase, the molecules possess only orientational ordering however in the columnar phase, the disc-like molecules spontaneously self-assemble into infinite one dimensional (ID) stacks, which in turn self-organize to various two dimensional (2D) liquid crystalline lattices. The hexagonal columnar phase is the most commonly encountered phase in DLCs. Some discotic compounds are also known to exhibit lamellar and cubic phases. 

Cyclic hydrocarbons with six equivalent substituent chains emanating from several carbon atoms about the ring can again adopt conformations in which the molecular shape is disk-like, but the aspect ratio is much larger. If the chains are somewhat flexible and the molecules possess an appropriate balance between order and disorder and attractive and repulsive intermolecular forces, the creation of new mesophase types can be envisioned - and several have been found [102,104]. The two general classes of aggregation in liquid crystalline phases of discotic molecules are lenticular nematic (N ) and columnar discotic (D) [100-104]. Carbonaceous pitch mesophases, discussed in section 1.4.5, resemble phases. Only those discotic mesophases with benzene, cyclohexane and shape-related cores having primarily alkyl chains as substituents will be discussed here. 

---