The Columnar 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. 

Fig. 1.1.10. Mesophases of discotic polymers (a) the hexagonal columnar phase. The diagram illustrates intercolumnar binding as well as intracolumnar back-folding of the main chain (Herrmann-Schonherr, Wendorff and Ringsdorf ) (b) the sanidic nematic phase composed of boards stacked parallel to one another (Herrmann-Schonherr et a/. ) (c) the columnar nematic phase (Ringsdorf...

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

Figure 13. Schematic diagram of the structure of the columnar nematic phase induced hy charge transfer interactions between an electron donor, pentakisfphe-nylelhynyl)phenyialkyl ether (in hlack), and an electron acceptor, 2,4,7-trinitrofluorenone (hatched) [37-39].

Figure 33. Mesophases of discotic polymers (a) hexagonal columnar phase (b) the board-like, or sanidic, nematic phase (c) the columnar nematic phase.

The two compositions of the hexaether 4 which exhibits a columnar phase with the non-mesomorphic fluorenone derivative 5c are the known examples [18] for reduced order phase inductions on heating of low molar mass mesogens resulting not in the complete destruction of the previous highly ordered mesophase, but the formation of the columnar nematic phase with its reduced degree of order. Apparently, this process arises from a combination of steric and electron donor-acceptor interactions within the complexes. 

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. 

Fig. 1.12 The nematic discotic, Nd phase, where individual molecules have lateral freedom of movement, and the columnar nematic phase, Ncoi where the molecules are aggregated in short stacks of stacks of three or four molecules...

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. 

Cobalt(II) nitrate induces a columnar nematic phase in the 1 2 complex with 98 (Table 7) as deduced from X-ray scattering which is stable for 30 K. Also, nickel(II) nitrate turned 98 into a liquid crystal with an unknown phase. The complex between 99 and copper(II) nitrate showed the same unknown mesophase. The phase range was 140 K but the samples decompose before clearing. A possible reason for the induction of a mesophase by complexation is the stiffening of the crown and the adjacent flexibly linked groups [112]. 

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. 

Molecules which combine the features of the rod and the disc may be expected to form new types of mesophases. An example is the biaxial nematic phase reported in thermotropic systems (see 6.6). Malthete et a/. have prepared an interesting series of mesogens shaped like stick insects called phasmids (fig. 6.1.5(n)). Some of them form columnar mesophases the structure proposed for the hexagonal phase is shown schematically in fig. 6.1.5( >). 

A second noteworthy series is that of the dirhodium tetraalkanaotes and tetrabenzoates which have been studied extensively as thermotropic mesogens (vide supra). In solution in alkane, columnar nematic phases were formed as evidenced by the schlieren textures observed. 

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