Aromatic LC polyesters

PE 1 was obtained with high molecular weight by melt polycondensation of phenylterephthaloyl dichloride and 2,2 -dimethylbiphenyl-4, 4 -diol following a published procedure [50]. Molecular weight can be controlled by monomer ratio or by fractionation. PE lA is a high molecular weight fraction (M = 75 000) of PE 1 with = 46000. PE IB-IE are non-fractionated samples. 

PE 1 is fairly soluble in a variety of organic solvents such as chloroform and 1,1,2,2-tetrachloroethane. The Mark-Houwink parameters of PE 1 in chloroform at 20°C are [49] 

The comparison of mixtures with 20wt% PE IB and different solvents shows considerable differences in gel-sol transitions and isotropization temperatures. The lowest gel-sol transition temperature was detected with 2-f-butylphenol at 36 C and the highest with 

3-phenoxytoluene at 96 C. Surprisingly, the gel-sol transitions of 3-phenoxytoluene is 53 C higher than the gel-sol transition of the corresponding mixture with diphenylether. A correlation of gel—sol transitions or isotropization temperatures with solubility parameters was not observed. 

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Recent developments in the substitution of completely aromatic LC polyesters have produced polymers which show improved solubilities and reduced transition temperatures (29). The presence of these side groups provides a method for producing polymers that are compatible with other similarly modified polymers. In this way, blends of rigid and flexible polymers can be prepared. Substituents have included alkyl, alkoxy (30) and phenyl alkyl groups (21), some of which lead to mesophases that have been reported as being "sanidic" or board-like. This approach has been used with both polyesters and polyamides and has lead to lyotropic and thermotropic polymers depending on the particular composition used. Some compositions even show die ability to form both lyotropic and thermotropic mesophases (22). 

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. 

Considerable synthetic efforts have been undertaken in order to deerease the melting temperatures of aromatic LC polyesters while retaining LC properties. Melting temperatures below 350 °C are necessary in order to obtain stable, melt-proeessable poly-... 

A reduction of the melting temperatures of aromatic LC polyesters can also be accomplished efficiently by the partial incorporation of kinked comonomers. Suitable monomers are meta- or ortho-functionalized benzene rings, meta- or ortho-functionalized six-member heterocycles, and 2,5-func-tionalized five-member heterocycles. A frequently used alternative is two benzene rings connected by linking groups such as... 

Figure 6. Examples of kinked and double kinked comonomers for aromatic LC polyesters.

In addition to the 2,6-naphthalene based LCPs, an interesting aromatic LC polyester with 2,6-substituted DBD moieties has been disclosed in several patents [15, 16]. 

As already mentioned, positional isomerism is important for the solubility and fusibility of aromatic LC polyesters. Consequently, polyesters made from symmetrical 2,5-disubstituted or 2,3,4,5-tetrasubstituted monomers should result in polymers that are less soluble and less fusible. This is in general the case with short lateral substituents. Ballauff and others reported that the series of poly( 1,4-phenylene-2,5-dialkoxy tereph-thalate)s with long flexible alkoxy side chains at the terephthalic moiety result in tractable LC polyesters [20] (Fig. 12). These polyesters exhibit a high degree of crystallinity with melting temperatures below 300 °C. Polyesters with short side chains (2350°C for m = 2... 

The same structural modification concepts, which were utilized to modify the properties of para-linked aromatic LC polyesters, have also been applied to aromatic polyamides. Para-linked aromatic polyamides are an important class of LC polymers. In contrast to thermotropic LC polyesters, para-linked aromatic polyamides form lyotropic solutions. Due to the formation of intermolecular hydrogen bridges, these polymers are in most cases unable to melt below their thermal decomposition temperature. Infusibility and limited solubility of unsubstituted para-linked aromatic polyamides are characteristic properties which limit synthesis, characterization, processing, and applications. 

The solution behavior has been significantly enhanced by the same structural modifications as reported previously for aromatic LC polyesters. For example poly-(p-phenylene terephthalamide) has been modified by bulky, stiff substituents [32], flexible alkyl side chains [33], the incorporation of kinked and double kinked comonomers, and comonomers of different lengths [34], as well as the use of noncoplanar bipheny-lene monomers [35]. To develop high performance materials, modifications that increase the solubility while maintaining the rod-like character, high glass transition temperatures, and the temperature stability are of particular interest. The solubility and the chain stiffness are critical factors in achieving lyotropic solutions. 

Pam-linked aromatic LC polyesters are semirigid polymers which are typically thermotropic. Although it has only rarely been mentioned in the literature, thermoreversible gelation is a general observation for LC polyesters. An initial study on the thermoreversible gelation has been performed on the polyester shown in Figure 13.6 [48]. 

The increased interest in thermotropic wholly aromatic LC polyesters is related above all to their commercial applications. Their ability to exist as anisotropic melts, combined with high molding and extrusion characteristics, allows very easy processing of these materials into various pats and articles. The fundamental aspect of this interest is associated with the fact that, with these polymers, it is possible to examine the effect of chain stiffness and of the influence of regular chain structure on evolution of macromolecular order in polymers on the whole. 

Another approach to avoid the difficulty in processing of all aromatic LC polyester, a LC poly(aryl ether ketone)-polyester block... 

The chemical resistance of polyester amide glass fibre composite is excellent [126]. A solvent mixture of CF3COOH/CHCI3 was used as a solvent for thermotropic LC polyester, based on 4-chlorocarbonyl phenyl esters of aromatic dicarboxylic acids and phenols or aliphatic diols for viscosity measurement. This indicates thermal stability in various organic solvents. [127]. Unsaturated aromatic LC polyesters, synthesized with the aim to fix the LC state, can be crosslinked by using styrene. The crosslinked matrix can be degraded by refluxing in 3 M aqueous sodium hydroxide solution and methanol in a vol. ratio of 3 2 [128]. 

Because thermotropic wholly aromatic LC polyesters have characteristics such as high strength, low melt viscosity, low shrinkage, ease of processibility, excellent thermal resistance, low water, and gas absorption. They have wide applications in following areas fibers, rods, sheets, composites used in mechanical and chemical industries chip carriers, connectors, switches used in electronics connectors, couplers, buffers used in fiber optics interior components, brackets in aerospace and so on. 

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. 

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