Mesogenic fragment

The general strategy of reducing the rigidity can be achieved by several synthetic routes. First of all, there is the already discussed method based on the introduction of methylenic groups (spacers) between mesogenic fragments,... 

These studies led to the discovery of a new class of liquid crystals whose molecules contain the dendritic matrix chemically bonded to mesogenic fragments situated both inside the hyperbranched stmaure and as terminal groups of the dendritic core (Figure 25) (Chapters 6.04 and 6.05). 

At present, several hundred LC dendrimers including homo-dendrimers and statistical and block codendrimers have been synthesized their stmcture may be represented as a combination of three basic stractural elements polymer matrix, or core, spacer, and mesogenic fragments located at the periphery of spherical dendritic molecules (Figure 25). " ... 

Poly(propyleneimine), poly(amidoamine), siloxane, carbo-silane, and other derivatives are most often used as initial polymer matrices. Spacers are usually aliphatic chains of various lengths, and mesogenic fragments cover the whole arsenal of classical low-molecular-mass liquid crystals. 

The possibility of synthesizing a large number of comb-shaped thermotropic polymers with a different structure of the main chain [31-35], different mesogenic fragments [36-41], and a different type of connection to the main chain provides broad opportunities for their investigation [1, 2, 19, 31, 43-50]. The presence of chemical groups which cause the formation of the liquid-crystalline phase in their side chains, while the main chain of the molecule can be flexible, is a basic structural feature of these comb-sh )ed mesogenic macromolecules. 

It follows from the data in Table 3.2 and Fig. 3.3 that the rigidity of the main chain increases with an increase in the length of the side chains for both the previously studied comb-shaped macromolecules and for the macromolecules of thermotropic polymers with mesogenic fragments in the side chains. The ob-... 

The combination of comparatively low equilibrium rigidity of the main chain and long side branches with mesogenic fragments is responsible for the unique electrooptical properties of comb-shaped macromolecules. 

The conformational properties of macromolecules with mesogenic groups in the main chain were studied in a series of studies by VJ. Tsvetkov et al. for alkylene aromatic polyethers [21, 100-103] with a different structure and length of the flexible spacers [101-103] and for some copolymers containing mesogenic fragments in the main chain [100]. The synthesis and features of formation of the mesophase in these polymers are described in [104, 105] and in Chapter 5 of the present book. The orientational order was also studied by IR spectroscopy for alkylene aromatic polyethers in [106, 107]. 

In conclusion, it should be noted that the conformational, optical, and electrooptical properties of mesogenic macromolecules reflect the dual nature of their molecular structure the main chain of these molecules is characterized by significant flexibility, but features characteristic of typical rigid-chain polymers are simultaneously manifested in the optical and electrooptical properties. This is due to the presence and interaction of mesogenic fragments in the side or main chains. 

The LC state in polymers was first discovered and studied in solutions of rigid-chain polymers, which satisfy the requirement of asymmetry and rigidity to the greatest degree [2]. One of the approaches to the creation of thermotropic LC polymers is based on the obvious idea of incorporation of mesogenic fragments in the macnomolecule. 

In a study on the example of model reactions of polycondensation of aroyl-bis-4-hydroxybenzoyl chlorides with aliphatic and aromatic dipoles, it was shown in [17] that the competing reactions of alcoholysis and phenolysis do not affect (in conditions identical to polycondensation) the internal aromatic ester bonds in the mesogenic fragment. In contrast to them, the tominal ester bonds in the fragment und go alcoholysis, but this does not result in perturbation of the regularity of the structure of the polyether. The interchain exchange reaction could be the cause of perturbation of the regularity of the structure of the chain, but NMR spectroscopy showed that the perturbations of the structure of the terephthaloyl-bis-4-hydroxybenzoate triad did not take place [17]. 

The presence of substituents in LC polymers (in both flexible and mesogenic fragments) decreases the temperature of the phase transitions, increases the solubility, alters the polarity, and leads to the formation of asymmetric, chiral centers. Substitution of the flexible polyethylene oxide fragment by a polypropylene oxide fragment in the same mesogenic fragment -COC5H4OCOC6H4COOC6H4CO- thus markedly decreases the temperatures of the transition to the LC phase and to an isotropic liquid [10]. 

Only a slight decrease in the region of existence of the LC phase takes place in incorporation of small polar groups (Cl, Br, CN) as substituents in the mesogenic fragment [S]. Incorporation of alkyl substituents in the central nucleus of the aromatic triad... 

Note the very low value of Sj in PHTHB. This could be explained by the fact that in the hexamethylene chain, the central part contains a total of two methylene groups after exclusion of the methylene groups whose motion is limited by the mesogenic fragment (two on each side). Such a short fi gment cannot form an ordered sequence. In the case of PDTHB, the central part is the (-CH2-)5 fragment, and it can assume a stretched conformation. 

The theoretical spectrum was calculated in [57] and compared with the experimental spectrum. Calculation of the nematic order parameter was performed with the equation S = 1/2(3 cos 0 - 1), where 0 is the angle between the nematic director and the long (inertial) axis of the mesogenic fragment. 

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