The discotic nematic phase

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. 

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... 

The apparent disagreement between simulation results and experimental observations on the the existence of power law relaxation near the isotropic-columnar (I-C) phase boundary needs further attention. This appears to be associated with the existence of nematic-like fluctuations in the experimental system. This can be checked, on the experimental side, by studying the relaxation near the I-C phase boundary located away from the discotic-nematic phase. For simulation, a careful analysis should be done by changing the temperature and density along the phase coexistence line and studying the existence or lack of nematic-like orientational fluctuations. 

Typical mesophases formed by disldike molecules are columnar, wherein the molecules are stacked into columns. The columns are in turn packed together to form two-dimensional arrays. In addition to the columnar arrangements, the molecules can become ordered in a way that is comparable to a heap of coins spread on a flat surface— the discotic nematic 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. 

H. Monobe, S. Mima, T. Sugino, Y. Shimizu, M. Ukon, Alignment behavior of the discotic nematic phase of 2,3,6,7,10,1 l-hexa(4-n-octyloxybenzoyloxy)triphenylene on polyimide and cetyltrimethylammonium bromide coated substrates. Liq. Cryst. 28, 1253-1258 (2(X)1)... 

FIGURE 6 Schematic representation of the discotic nematic phase, Nd (left) and of the columnar nematic phase, Njoi (right). 

Figure 4.4. Molecular arrangements within the discotic nematic phase.

The symmetry of the discotic nematic phase Nq (Figure 12) is the same as that of the classical nematic phase of rod-like molecules. Thus identical types of defects and textures are seen in both cases. 

Even though no tabular data are available for truxene derivatives, inclusion in this Datareview is important because these materials exhibit discotic nematic phases. The truxene phase data presented by Busine and co-workers [11] are for the transition of the discotic nematic phase to the disordered rectangular columnar phase (TABLE 18). Over the phase range studied which was up to 900 bar, the relationship between T and p is most conveniently represented as linear. Once again the magnitudes... 

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