Mesophases calamitic mesogens

The mesophases of calamitic mesogens are classified in two groups nematic and smectic. The nematic mesophase (N) is characterized by an orientational order of the molecules that are aligned along a preferred direction (defined by a director n) (Figure 8.2). The molecules can slide and move in the nematic mesophase (while roughly keeping their molecular orientation) and rotate around their main axis. This is the less ordered mesophase and it is usually very fluid. 

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). 

In contrast to calamitic mesogens, discotic liquid crystals are built from disk-like molecules that can arrange into different structures, such as the discotic nematic mesophase, the discotic columnar mesophase, or the discotic hexagonal meso-phase. 

Based on previous work which showed that backbone rigidity correlates with mesophase behavior for SCLCPs bearing calamitic mesogens [64], the double bonds of the main chain in polybutadienes were hydrogenated, leading to pure saturated alkane main chains, poly-(XXIX-5)a and poly-(XXIX-10)a in Table 18. But in contrast to the studies of calamitic SCLCPs, no dependency of backbone rigidity on mesophase behavior could be estabhshed. 

In the section on calamitic materials, we discussed how, by appropriate consideration of the molecular shape of bis( -diketonato)copper(II) complexes, either nematic and lamellar, or columnar mesophases could be realized. Furthermore, increasing the number of chains on the periphery of calamitic mesogens can lead to molecules which demonstrate both types of mesomorphism, e.g. the phasmids of Malthete... 

Some examples of typical calamitic mesogens are given in Figure 4. Cyanobiphenyls are included in this list due to their special position in the development of LCDs, but it should be noted that in many ways they are not typical as their liquid-crystal mesophases arise from antiparallel molecular correlations. 

The true liquid-crystal mesophases of calamitic mesogens are divided into two classes—the nematic and smectic phases and these will be described in turn. There is, in addition, a series of crystal smectic phases that are not really liquid-crystal phases but which for many years were classified as such these will be mentioned briefly below. 

FIGURE 3 Mesophases exhibited by calamitic mesogens (rod-like shape) versus mesophases exhibited by discotic mesogens (disk-Uke shape). 

Rod-shaped molecules forming liquid crystalline phases are called calamitic . A prominent example of such a calamitic molecule is terephthal-bis-(p-butylaniline) (TBBA) [2]. Its chemical structure is shown in the upper left comer of Fig. 3.1. The molecule possesses a rigid aromatic core as well as flexible alkyl chains. The aromatic core favors a parallel packing of the molecules, while the flexible chains keep them from crystallizing. These intermolecular interactions, as well as entropic effects and steric interactions between the mesogens, promote the formation of mesophases, as discussed by Onsager [3]. The mesophases formed by calamitic mesogens frequently possess a layered structure, but different phase types are also possible. 

In thermotropic liquid crystals, the formation of particular mesophases depends mainly on the temperature. On cooling, the structure of the mesophases becomes more and more ordered and thus less symmetric. For thermotropic mesophases formed by calamitic mesogens a fixed sequence was found [37, 38] ... 

Figure 2 Representative structure of the arrangement of calamitic mesogens in the nematic (N), smectic A (SmA), and smectic C (SmC) mesophases, with the director (n) shown in...

Until 1977, it was believed that only rod-like molecules, having a considerably higher length than width, could show liquid crystalline properties (calamitic mesogens). However, in 1977, Chandrasekhar realized that not only rod-like molecules, but also compounds with a disc-like molecular shape are capable of forming mesophases (Chandrasekhar et al. 1977 Sabine et al. 2007) and these are referred to as discotic liquid crystals (DLCs). The first examples of this kind of mesomorphism were observed in hexa-alkanoyloxybenzenes, hexaalkoxy- and -aUcanoyloxy-triphenylenes (Fig. 16.2), and were established by X-ray studies (Kumar 2010). Generally, the disc shaped molecules have central core as benzene or polyaromatic, such as triphenylene or phthalocyanine. 

Fig. 1.31 These figures trace the development of the diffraction pattern of a sample of a calamitic mesogen as the crystalline solid (a) is heated and passes through a succession of mesophases (b-d), eventually becoming the isotropic liquid (e). In each figure, an edge-on view of the layers is shown on the left and the face-on view on the right. The diffraction patterns correspond to the alignment of the edge-on view on the left, i.e., a vertical line on the figure corresponds to the axial direction in the diffraction pattern. For all mesophases, the sample as a whole is taken to have a random orientation of domains around the layer normal. Note the stepwise way in which...

Note 5 If the mesogenic side-groups are rod-like (calamitic) in nature, the resulting polymer may, depending upon its detailed structure, exhibit any of the common types of calamitic mesophases nematic, chiral nematic or smectic. Side-on fixed SGPLC, however, are predominantly nematic or chiral nematic in character. Similarly, disc-shaped side-groups tend to promote discotic nematic or columnar mesophases while amphiphilic side-chains tend to promote amphiphilic or lyotropic mesophases. 

---