Aromatic LC Liquid Crystal Polyesters

Monomers with noncoplanar conformation are, for example, 2,2 -substituted bi-phenylenes or binaphthyl derivatives. Substitution in the 2,2 -position causes the phenyl units to be in a noncoplanar conformation. This reduces the intermolecular interactions between the chains very effectively. These monomers reduce the chain stiffness far less than the bulky substituents. It is obvious that combinations of these different structural modifications can be used and have been utilized in numerous examples to modify LCPs properties. 

An other important concept for the modification of LCPs is the incorporation of flexible spacers between mesogenic units. These semiflexible LCPs have been extensively investigated and have to be distinguished from the above systems. As a consequence, a separate contribution in this handbook is devoted to this class of LCPs (Sect. 2 of this Chapter). 

In the following sections, the different concepts of structural modifications of LCPs will be discussed in more detail for thermotropic LC polyesters and briefly for lyotropic polyamides. The same structural modifications have been applied to other 

Aromatic para-linked LC polyesters have been investigated extensively and represent the most important class of aromatic LCPs. The historical development, the chemistry, and the physics of aromatic LC polyesters have been summarized in several excellent reviews and book chapters. Some examples are given in the literature [2]. 

Several synthetic routes are known for the synthesis of aromatic LC polyesters. Melt, solution, and slurry polycondensations are mainly used. Most significant are the polycondensation of terephthalic acid diesters and aromatic diols, the polycondensation of terephthalic acids and acetates of aromatic diols with the addition of transesterification catalysts, and the polycondensation of aromatic diols and aromatic diacid dichlorides [2]. A method successfully utilized for laboratory synhesis is the polycondensation of silated aromatic diols and aromatic diacid dichlorides [3]. Molecular weights depend significantly on the reaction conditions and on the solubility as well as the fusibility of the polyesters, which is relatively poor for para-linked unsubstituted aromatic polyesters. 

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The next development in liquid crystal polyesters was the preparation by polycondensation based on terephthalic acid (TPA) and hydroquinone (HQ) or p-hydroxybenzoic acid (HBA). The polyesters are insoluble with very high melting temperatures of 600 °C for poly (TPA/HQ) and 610 °C for poly (HBA), which are by far too high to obtain stable liquid crystalline phases for melt processing. In 1972, Economy and coworkers patented several copolyester compositions, and one of these are the copolymerization of poly (4-hydroxybenzoic acid) (PHB) with 4,4 -dihydroxybiphenyl (BP) and terephthalic acid (TPA) due to the need for lower melting, melt-processable polymers. Considerable synthetic efforts have been attempted in order to decrease the melting temperatures of aromatic LC polyesters while retaining LC properties. The copolyester structure was tailored by partial substitution of TPA with isophthalic acid to produce a melt-spinnable material. 

Second, the ability of completely aromatic polyesters to form mesomorphic structures is interesting. Obviously, in such cases, the appearance of nematic LC states seems most probable. Furthermore, some polybenzoates (3-8) show mesophases, which cannot be described in terms of the conventional classification of liquid crystals, with respect to their arrangement, and which are similar to mesomorphic structures of flexible-chain polyorganosiloxanes (9) and polyphosphazenes (10). Finally, studies of relaxations and phase transitions in rigid-chain LC polyesters, in particular, their molecular mobility in the solid state, i.e., below the melting temperature of crystalline phase, are of great interest. 

Thermotropic LC polyester nanocomposites based on a small quantity of multi-walled carbon nanotubes (M WCNTs) can be prepared by in situ polymerization of l,4-bis(4-hydroxybenzoyloxy) butane and terephthaloyl dichloride. Significant change in the crystal structure of LC polyester cannot be observed even after forming the nanocomposite. The evidence from various instrumentation results indicates interaction of MWCNT with the surrounding liquid crystal molecules, most likely through aromatic interactions (H-stacking), The thermal stability and transition temperature of the hybrid is always better than pure LC polyester [71]. 

The influence of mesomorphic polymer components on the morphological, physical, and rheological properties of blends with polyester, polyamide, polycarbonate, and polyolefin matrix has been investigated in several cases [115]. Studies on blends of PET, PBT, PC, PS with main chain liquid crystal polymers and copolymers of various type (including flexible LC copolyesters and wholly aromatic LC copolyesters) showed that the phase transitions and properties were largely dependent on the chemical structure, molar mass, processing conditions, and concentration of components [132-134]. Generally, these systems displayed phase separation in the molten... 

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