Para-Linked Aromatic Polyamides

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. 

Already in 1965, the formation of lyotropic liquid crystalline solutions of poly-(p-aminobenzoic acid) in concentrated sulfuric acid was observed. This work was the basis for the commercialization of high strength, high modulus, and heat-resistant poly-(p-phenylene terephthalate) (PPDT) fibers under the trade name Kevlar (Dupont) and Twaron (Akzo) (Fig. 15) [31]. PPDT is typically prepared by the polycondensation of terephthaloyl chloride and 1,4-pheny-lenediamine in tertiary amidic solvents such as A-methylpyrrolidinone or dimethylacet- 

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. 

Depending on the modifications of poly-(p-phenylene terephthalamide) and analogous aromatic polyamides, it is possible to vary the solubility from concentrated sulfuric acid, to nonpolar aprotic solvents with inorganic salts, or to nonpolar aprotic solvents and to common organic colvents. In particular the incorporation of noncoplanar 2,2 -disubstitution has proven to remarkable enhance the solubility and lower the crystallinity. Gaudiana et al. [35] demonstrated that para-linked aromatic polyamides containing noncoplanar 2,2 -bis(trifluorome-thyl)biphenylene units are highly soluble in 

The substituted polyamides with long alkyl- and alkoxychains [33] are highly soluble and form anisotropic melts above their melting temperature. These polyamides are typical examples of sanidic liquid crystalline polymers. Generally, no lyotropic behavior is observed. The temperature stability is obviously substantially lowered due to the substitution with alkyl chains. 

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Yeo, S. D., Debenechi, P. G., Radosz, M., and Schmidt, H. W. (1993) Supercritical anti-sol-vent process for substituted para-linked aromatic polyamides phase equilibrium and morphology study. Macromolecules 26, 6207-6210. 

The results obtained here confirm their findings, and extend them by showing that even a small percent of N-methylation is sufficient to change the hydrodynamic properties of the para-linked aromatic polyamide molecule. The agreement between the molecular weights measured for polymer 7 (50% N-methyl substitutive) in 96% sulfuric acid and the N-methyl pyrrolidone with 5% LiCl and 1% water implies that this polymer forms a molecularly disperse solution in the latter solvent. This observation differs significantly from the report by Tsvetkov et al. (6) and Koton and Nozova (5) that the fully methylated polymer only formed molecularly disperse solutions in sulfuric acid. 

The relative importance of the various factors determining the persistence length in solution and the conformation in the solid state of aromatic polyamides are well illustrated by the differences between the para and the meta form of polyphenylene terephthalamide. Both chains consist of the same planar elements, viz. the phenyl and amide groups. The latter group adopts only the trans conformation in these polymers, whereby the chance of a transition to the cis conformation is extremely small due to a barrier of about 60—80 kJ/mol. Although in both para- and meta-linked chains the same intermolecular interactions exist... 

Figure 4.1 represents the chemical structures of para-linked, poly(p-phenylene terephthalamide) (PPPT) and meta-linked, poly(m-phenylene isoph-thalamide) (PMPI) aromatic polyamides. These commercial aramids can be transformed into flame, cut-resistant, and high-tensile-strength synthetic fibers, which are widely used in the field of coatings. 

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