Crystalline behaviour

Typical materials that exhibit liquid crystalline behaviour are made up of long, thin molecules. Hence in principle polymers ought to show the basic requirement for liquid crystal behaviour. Conventional polymers, however, are too flexible and tend to adopt random coil configurations in the melt. They are thus not sufficiently anisotropic to exhibit a mesophase. 

Moreover, the new smectic phases with the broken head-to-tail inversion symmetry (of the type of A, A2, A, etc.) and the re-entrant effects (i.e. the reentrance of the less ordered phase at temperatures below those of a more ordered phase) were found among strongly asymmetric mesogens [11-13, 30] (Fig. 2). These observations indicate that with asymmetric molecules, dipolar and steric interactions are important for the liquid crystalline behaviour. 

The effect of incorporating p-hydroxybenzoic acid (I) into the structures of various unsaturated polyesters synthesised from polyethylene terephthalate (PET) waste depolymerised by glycolysis at three different diethylene glycol (DEG) ratios with Mn acetate as transesterification catalyst, was studied. Copolyesters of PET modified using various I mole ratios showed excellent mechanical and chemical properties because of their liquid crystalline behaviour. The oligoesters obtained from the twelve modified unsaturated polyesters (MUP) were reacted with I and maleic anhydride, with variation of the I ratio with a view to determining the effect on mechanical... 

P-chiral dibenzophosphole oxide (52a) (Scheme 14) shows liquid crystalline behaviour [52], a property that is of interest in the area of electro-optical displays [53]. Chiral resolution of (52a) was achieved by column chromatographic separation of the diastereoisomers obtained following coordination of the o -benzophosphole (52b) to chiral cyclometallated palladium(II) complexes [52]. Notably, the presence of a stereogenic P-centre is sufficient to generate a chiral cholesteric phase. 

First, it is important to appreciate that all liquid crystal mesophases exist due to non-covalent interactions between molecules, namely the anisotropic dispersion forces mentioned earlier. However, this section will address more specific non-covalent interactions that have been used either to induce liquid-crystalline behaviour or to generate a new species that is liquid crystalline. 

Polymers formed between a and c, d and e all failed to show any liquid-crystalline behaviour. However, for all a examined (m = 2,4,6 and 8), nematic phases were observed with b-4 (all monotropic) - a further monotropic nematic material was the copolymer of a-6 and b-3. Unidentified crystal smectic mesophases were reported for a further three examples. 

A Cu2+ containing linear polymer [77] exhibiting smectogenic behaviour is obtained by a polycondensation technique (Fig. 27). A homologous set of polycondensates of 4,4 -[l,12 dodecanediyl bisfoxy)] bis benzoic acid with bis[AT-[[2,4 dihydrophenyl]methylene]-alkylamino] Cu(II) exhibits monotropic liquid crystalline behaviour with 4—13 carbon atoms in the alkylamino group [78]. 

Some of the phenomena discussed in earlier sections of this book involve forms of matter which might well be described as smectic liquid crystals but are better dealt with under other headings. However, there are three particular processes which do not fit into these other categories but which certainly make use of liquid crystalline behaviour and will thus be examined here. 

This was realized with derivatives of the complementary heterocyclic groups 2,6-diaminopyridine P and uracil U bearing long aliphatic chains [9.150]. Whereas the pure compounds did not show liquid crystalline behaviour, 1 1 mixtures gave a metastable mesophase of columnar hexagonal type. The existence of the latter was attributed to the formation of a mesomorphic supermolecule 173 via association of the complementary components. 

Arasabenzene, with chromium, 5, 339 Arcyriacyanin A, via Heck couplings, 11, 320 Arduengo-type carbenes with titanium(IV), 4, 366 with vanadium, 5, 10 (Arene(chromium carbonyls analytical applications, 5, 261 benzyl cation stabilization, 5, 245 biomedical applications, 5, 260 chiral, as asymmetric catalysis ligands, 5, 241 chromatographic separation, 5, 239 cine and tele nucleophilic substitutions, 5, 236 kinetic and mechanistic studies, 5, 257 liquid crystalline behaviour, 5, 262 lithiations and electrophile reactions, 5, 236 as main polymer chain unit, 5, 251 mass spectroscopic studies, 5, 256 miscellaneous compounds, 5, 258 NMR studies, 5, 255 palladium coupling, 5, 239 polymer-bound complexes, 5, 250 spectroscopic studies, 5, 256 X-ray data analysis, 5, 257... 

In cases where the substituents X and Y are long-chain alkyl esters or amides, liquid crystalline behaviour is observed (Section 13.3). While the individual components have no mesogenic properties, the aggregate forms a metastable columnar hexagonal type structure. 

Figure 13.12 Fundamental classes of liquid crystalline behaviour (temperature increases downwards).

An extended, structurally rigid, highly anisotropic shape seems to be the main criterion for liquid crystalline behaviour, and, as a result, many liquid crystalline materials are based on substituted... 

Figure 13.18 A ribbon structure of a folic acid derivative self-assembles into a disc-like tetramer based on a G-quartet motif showing liquid crystalline behaviour on the addition of a metal ion such as Na+.

A purely organic chiral nitroxide which shows liquid crystalline behaviour as well as intriguing magnetic properties and a dependence on the enantiomeric nature has been reported [180]. The reason for studying the compounds was to increase the sensitivity of mesophases to magnetic and electric fields. The racemic modification of the radical, which displays a nematic phase, proved to be more sensitive to alignment than the cholesteric phase with the enantiomers present. It was proposed that the compounds may also be used to study the dynamic nature of mesophases by electron paramagnetic resonance spectroscopy. 

The last polymer mentioned in Table 10.9 is poly(p-phenylene terephthalamide). This polymer is spun from its solution in pure sulphuric acid (100%), a dope that exhibits mesomorphic (=liquid crystalline) behaviour it is optically anisotropic and is nematic in character. A number of other polymers, containing rigid elements in the chain have melts of polymer liquid crystals, with a high birefringence and a non-linear optical behaviour in electric fields. 

Neve and co-workers have also made use of hydrogen-bonded perhalometallate ions, but in this case using salts mononuclear ions hnked by weak charge-assisted C-H---X(M) hydrogen bonds from alkylpyridinium cations [128]. The use of cations with long-chain alkyl substituents leads to hquid crystalline behaviour in these salts [128a-c]. 

Mechanical studies have also been performed on another non-LCP system -copolyesters of PET and PEN, where both copolymer units having flexible chain segments and lack a liquid-crystalline behaviour (Santa Cruz et al, 1992). The whole range of PET/PEN copolymers can therefore be prepared in the amorphous state. 

NMR studies it has been observed that the long molecular axis is nearly parallel to the para- x s of the phenyl moiety and hence the first few fragments of the chain attached to the indazole group are not along this axis. This requires that the alkoxy chain attached to the indazole group has at least six carbon atoms to promote liquid crystalline behaviour. The order parameters of 4 -cyanophenyl 4-heptylbenzoate have been obtained by using 2D-NMR spectroscopy.Linear relationships between C-chemical shifts and order parameters have been obtained. Phase behaviour of a mixture of this liquid crystal and its chain-perfluorinated analogue has also been studied. 

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