Mesogen-containing liquid crystals

One of the earlier contributions to the revival of metal-containing liquid crystal systems was the synthesis of the ort, ti-metallated palladium complexes of mesogenic azobenzenes 91, 92 by Ghedini and co-workers, which represented the first systematic attempt to coordinate metals to known liquid-crystal systems. 

Liquid crystals may be divided into two broad categories, thermotropic and lyotropic, according to the principal means of breaking down the complete order of the soHd state. Thermotropic Hquid crystals result from the melting of mesogenic soHds due to an increase in temperature. Both pure substances and mixtures form thermotropic Hquid crystals. In order for a mixture to be a thermotropic Hquid crystal, the different components must be completely miscible. Table 1 contains a few examples of the many Hquid crystal forming compounds (2). Much more is known about calamitic (rod-Hke) Hquid crystals then discotic (disk-like) Hquid crystals, since the latter were discovered only recendy. Therefore, most of this section deals exclusively with calamities, with brief coverage of discotics at the end. 

This polymer suffered thermal degradation only above 400 C, before melting. For molecular weights higher than about 20,000, it showed a lyotropic liquid-crystal behaviour in N-metnypyrrolidone containing LiCl. This is the first reported example of a furanic polymer with mesogenic properties. 

Most polymeric liquid 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. 

Table 6 contains data on the transition parameters for a large series of main-chain mesogen macromolecules. Figure 15 shows scanning calorimetry data for poly(oxy-2,2 -dimethylazoxybenzene-4,4 -diyloxydodecanedioyl), entry 8 in Table 6. These data should be compared to data on low molecular mass p-butyl-p -methoxyazoxy-benzene which are shown in Fig. 12. Similar comparisons are available for two benzalazines (entries 14 and 16 of Table 6). Although the entropies of transition from the liquid crystal to the isotropic melt are small for all polymers listed as expected, they are larger than those of the corresponding nematic small molecules [1.6, 3.35, and 3.26 J/(K mol), respectively for the small molecules corresponding to entries 8, 14, and 16]. The main-chain nematics seem to have a somewhat larger entropy of transition than the side-chain nematics for larger flexible spacers.

This review deals with LC polymers containing mesogenic groups in the side chains of macromolecules. Having no pretence to cover the abundant literature related to thermotropic LC polymers, it seemed reasonable to deal with the most important topics associated with synthesis of nematic, smectic and cholesteric liquid crystals, the peculiarities of their structure and properties, and to discuss structural-optical transformations induced in these systems by electric and magnetic fields. Some aspects of this topic are also discussed in the reviews by Rehage and Finkelmann 27), and Hardy 28). Here we shall pay relatively more attention to the results of Soviet researchers working in the field. 

Let us consider some aspects of thermal behaviour of LC polymers 45> (Fig. 3). In case of crystallizable polymers, which are mainly those containing mesogenic groups in the main chain, the LC state is observed from above the melting temperature (Tm) and up to the clearing temperature (Tcl), the melt displays anisotropy and may flow. The polymer thus behaves alike low molecular liquid crystals (Fig. 3 a), the viscosity of the former being, however, essentially higher. 

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