The mesogen

Parameters (ii)-(vii) depend on the dielectric, mechanical and optical properties of the mesogens. To optimize a dis compromise between different molecular characteristics is often required and mixtures of liquid crystals are usually commercial displays. 

Mesogenic diols, such as 4,4 -bis( CO-hydtoxyaLkoxy)biphenyls, ate used with 2,4-TDI or 1,4-diisocyanatobenzene (PPDI) to constmct Hquid crystalline polyurethanes (7). Partial replacement of the mesogenic diols by PTMG shows that the use of lower molecular weight flexible spacers form polymers that have a more stable mesophase and exhibit higher crystallinity (8). Another approach to Hquid crystal polyurethanes involves the attachment of cholesterol to the polyurethane chain utilizing the dual reactivity in 2,4-TDI (9). 

Aromatic polyesters constitute an important class of main-chain liquid-crystalline polymers, but present the inconvenience of their reduced solubility and very high transition temperatures (sometimes not detected before the degradation of the sample). Their processability can be improved in several ways [2,3], e.g., reduction of the rigidity of the mesogen, lengthening of the spacer, or introduction of lateral substituents. 

Fig. 29. Observed and calculated 2H NMR spectra for the mesogenic groups of a) the nematic (m = 2), b) the smectic (m = 6) liquid crystalline polymer in the glassy state, showing the line shape changes due to the freezing of the jump motion of the labelled phenyl ring. The exchange frequency corresponds to the centre of the distribution of correlation times. Note that the order parameters are different, S = 0.65 in the frozen nematic, and S = 0.85 in the frozen smectic system, respectively...

In summary the results of our 2H NMR investigation illustrate the spacer model for liquid crystalline polymers, indicating, however, that the decoupling of the mesogenic groups from the main chain, while effective, is not complete. 

Scheme 3. Lowest energy conformations for the mesogen 5CB (4-n-pentyl-4 -cyanobiphenyl)...

Binger and Hanna [76] have considered the alignment of 4-n-octyl-2-flurophenyl-4-n-octyloxybiphenyl-4 -carboxylate (MBF) and 8CB at room temperature on the (110) surface of polyethylene and the (100) surface of nylon. They find that alignment of both molecules is dominated by the behaviour of the flexible tails that prefer to align parallel between two polymer chains. This forces the mesogenic core to straddle one or more chains. This is illustrated in Fig. 4 for the specific case of MBF on the (110) surface of... 

Fig. 5. The essential form of the phase diagram for the mesogen GB(3.0, 5.0, 2, 1) the open circles indicate the approximate coexistence lines and the solid circles show the density of the isotropic liquid in equilibrium with the vapour phase...

Fig. 8. The distance dependence of the bond orientational correlation function gs (r ) found for the mesogen GB(4.4, 20.0, 1, 1) in the smectic A (.) and the smectic B (—) phases...

The alignment of the director at the nematic free surface of real systems is not found to exhibit universal behaviour. Depending on the mesogen, homeotro-pic, tilted and planar anchoring have been observed. Clearly, to study this interface in a simulation a potential which exhibits a nematic phase in co-... 

The crystal structures of the mesogenic compounds CBn were reported by different authors [45-51]. Selected crystallographic and molecular data of the investigated compounds CBn are summarised in Table 1. Only the crystal structures of some compounds will be discussed here. 

In this section, we will describe the crystal structures of the mesogenic A -n-alkoxy-4-cyano-biphenyls (Fig. 1) [53-57]. The compounds have the phase sequences crystal-nematic-isotropic for CB05—CB07 and monotropic nematic for CB01-CB04 crystal-smectic A-nematic-isotropic for CB08. The crystal and molecular data of the investigated compounds CBOn are summarised in Table 2. 

The crystal structure of the mesogenic compound 4 -(4-hydroxy-1-n-butoxy)-4-cyanobiphenyl (CB0(CH2)40H) was described by Gehring et al. [59]. In... 

Hartung and Rapthel [64] described the crystal structure of the mesogenic 2-methylthio-5-(4 -n-butyloxyphenyl)-pyrimidine which forms a monotropic smectic A phase. The chemical structure of this compound is presented in Fig. 5. The compound crystallises in the triclinic space group PI with two molecules per unit cell. The molecules adopt a fully stretched and nearly planar form. The pyrimidine ring is nearly planar. The dihedral angle between the phenyl and the pyrimidine rings is 22.7°. The molecules are arranged parallel to each other. 

The synthesis of the mesogenic trans-4-n-alkyl-(4 -cyanophenyl)-cyclohexanes (PCHn) was described by Eidenschink et al. [65] in 1977. Most of the compounds exhibit a nematic phase close to room temperature. The chemical structure of the mesogenic PCHn is shown in Fig. 6. During the past few years, the crystal structures of some mesogenic phenylcyclohexanes were published [66-70]. Selected crystallographic and molecular data of the investigated compounds PCHn are presented in Table 4. 

In 1977, Eidenschink et al, [65] reported on the new liquid crystalline trans, trans-4 -n-alkylbicyclohexyl-4-carbonitriles (cyclohexylcyclohexanes, CCHn), The general chemical structure of the mesogenic CCHn compounds is presented in Fig, 9,... 

The crystal structure of the mesogenic 4-(3"-pentenyl)-4 -(ethoxy)-l,l -bicyclohexane (PEBCH) was determined by Nath et al. [78]. This compound exhibits a nematic phase. The molecules are packed parallel to each other in an imbricated mode. 

In 1989, Walz et al. [80] reported on the crystal and molecular structures of four mesogenic compounds of the series trans, tran5-4,4 -m-n-dialkyl-(la,l -bicyclohexyl)-4)S-carbonitrile (CCNnm). The chemical structure of the mesogenic CCNnm is presented in Fig. 11. 

Fig. 12. Chemical structure of the mesogenic 4-cyano-4 -n-pentyl-p-terphenyl (T15)...

In this section we will report on the crystal structure analyses of mesogenic 2,5-diphenyl pyrimidines. The crystal structure of 5-phenyl-2-(4 -n-butoxy-phenyl)-pyrimidine (5-PBuPP) and 2-phenyl-5-(4 -n-pentoxyphenyl)-pyrimi-dine (2-PPePP) were determined by Winter et al. [83, 84]. Compound 5-PBuPP forms a monotropic nematic phase, whereas compound 2-PPePP exhibits a smectic A mesophase within a wide temperature range. The chemical structure of the mesogenic 2,5-diphenyl pyrimidines is shown in Fig. 13. 

In this section, we will neglect the crystal structures of the mesogenic perfluorinated phenyl benzoates [23-27], benzoic acids [6, 28-31], cinnamic acids [7, 32, 33], dicarboxylic acids [34, 35], and cinnamate compounds [8, 36-40]. The single crystal X-ray analyses of chiral mesogenic carboxylates are described in Sect. 6. 

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