Aromatic mesogenic units

Polymers with fluorinated segments represent a field on its own which is not considered here [382-384], Polymer chains were also used to interconnect fluorous mesogenic units, either comprising a single aromatic unit [385-389] or a rod-like units which can be attached either terminally [193, 390-394] or laterally [240, 395-397] to the polymer backbone. In most cases the termini of the mesogenic units... 

The effect of substituents on the thermal properties of a series of aromatic azoxy polymers is shown in Table 3, Polymers 10-12, in which it is seen that the introduction of methyl groups into the mesogenic units resulted in a depression in both the melting point and the clearing temperature of the polymer. As can be seen from these data, the substitution position in the mesogenic unit is also important, but it will be necessary to have much more experimental results before the positional effect may be evaluated systematically. 

In the previous section, the mesogenic unit was defined as the part of the polymer chain that is composed of the aromatic or cycloahphatic segment. Simply for convenience, we have defined the remainder of the polymer repeating unit as the flexible spacer. What is important, however, is the different structural nature of the spacer and the mesogenic unit, and unfortunately the separate roles of these two groups are not yet fully understood. 

Among the many different classes of thermotropic polymers, only a limited number of polyesters based on aromatic ester type mesogenic units have been studied by rheological methods, beginning with the publication by Jackson and Kuhfuss of their work on the p-oxybenzoate modified polyethylene terephthalate, PET, copolymers. They prepared a series of copolyesters of p-hydroxybenzoic acid, HBA, and PET and measured the apparent melt viscosity of the copolymers as a function of their composition by use of a capillary rheometer. On inclusion of low levels of HBA into PET, the melt viscosity increased because of partial replacement of the more... 

Further studies by Nishiyama et al. [34-45] showed that when taken in isolation, only one of the aromatic units within a supermolecular system has a propensity to exhibit liquid crystal phases, then the supermolecular material itself could be mesomorphic, see Fig. 5. For example, for the top molecular structure, 5 [45], in Fig. 5, only the biphenyl unit at the center of the structure supports mesophase formation, whereas the benzoate units are too isolated from the biphenyl moiety in order to affect mesomorphic behavior. The second material, 6 [45] has terminal phenyl units, which are only connected by aliphatic chains to the benzoate units. Thus in this case, the material has four aromatic units out of six which are not in positions that can enhance mesophase formation. However, the second material has similar transition temperatures and phase sequences to the first, i.e., both materials exhibit an unidentified smectic phase and a synclinic ferroelectric smectic C phase. If the third material, 7 [38], is examined, it can be seen that the mesogenic unit at the center of the supermolecule is an azobenzene unit which is more strongly supportive of mesophase behavior than the simple biphenyl moiety. Thus the clearing point is higher for this material in comparison to the other two. The attachment of the terminal phenyl unit is by a methylene spacer of odd parity, and as a consequence the smectic C phase has an anticlinic structure rather than synclinic. 

All LC-polyimides discussed in this review possess at least one phthalimide unit in the mesogenic unit and most of them contain an AT-phenyl phthalimide moiety. For a better understanding of the potential mesogenic character of aromatic imide groups it is useful to discuss briefly their stereochemical and conformational properties. 

The substitution effect on liquid crystals has been discussed for low mass and fully aromatic polymers. The comparison of (3.28) with (3.27) shows the effect of substitution in mesogenic units on the properties of polymers with flexible segments in main chains. Again, the substituted polymer has a lower melting point, a lower clearing temperature, and a narrower temperature... 

In (3.65) there is more of an aliphatic component than an aromatic one. Phenylene rings are each isolated by aliphatic components. No mesogenic units of rigid-rods are present in the molecule. However, the thermotropic liquid crystalline phase was observed for many of the polymers although... 

C.Ober, J.I. Jin, and R.W.Lenz, Thermotropic polyesters with either dyad or triad aromatic ester mesogenic units and flexible polymethylene spacers in the main chain, Pol3mier J. 14 9 (1982). 

In this article, in connection with our previous endeavor, we would like to report synthesis and properties of new copolyesters having ordered comonomer sequences consisting of mesogenic units and polymethylene spacers, i.e., thermotropic aromatic-aliphatic copolyesters having sequentially ordered structures ... 

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