Calamitic mesogens

Most polymeric Hquid crystals are based on stiff rod-like molecular units which are called calamitic mesogens. There are some unusual polymers (which are not discussed here) that contain flat disk-like molecular units called discotic mesogens in which the disks form columnar arrays like stacks of poker chips. 

Many technological applications of liquid crystals, as in electro-optic display devices, are based on multicomponent mixtures. Such systems offer a route to the desired material properties which cannot be achieved simultaneously for single component systems. Mixtures also tend to exhibit a richer phase behaviour than pure systems with features such as re-entrant nematic phases [3] and nematic-nematic transitions possible. In this section, we describe simulations which have been used to study mixtures of thermotropic 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. 

The liquid crystal phases of calamitic mesogens fall into two types - nematic (N) and smectic (Sm). The nematic phase is the most disordered of the liquid crystal phases and possesses only orientational order, so that the long axes of the molecules are correlated in one direction (known as the director, n) while being positioned randomly (Fig. 2A). There are several smectic phases and these differ from the nematic phase in possessing partial posi-... 

Chiral monophosphorus ligands, in hydrogenation, 10, 16 Chiral nematic phase, calamitic mesogens, 12, 200 Chiral N,P ligands, in hydrogenation, 10, 17 Chiral phosphines... 

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 a subsequent study [20], the authors attached pendant calamitic mesogenic cores to the pyrrolidinium cation, 14,15, and 16. In some cases, a low-ordered SmA... 

Laschat and coworkers have investigated the mesomorphic behavior of guanidinium salts [38, 52], When calamitic mesogenic groups were attached to the guanidinium head, such as 31, SmA phases were observed chloride salts showed a smectic phase from above room temperature up to more than 200°C, depending on the chain length... 

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... 

Figure 2 (a) The rod-like motif of calamitic mesogens (b) the disk-like motif of disootio mesogens. 

---