Aromatic para-linked

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]. 

Coloration in Polyhydrazide-Base Solutions. The wholly aromatic, para-linked polyhydrazides exhibit a deep yellow coloration in aqueous bases. This coloration is not shown in polymers wherein the aromatic rings are all meta-linked, or polyhydrazides derived from monomethylhydrazine (Table VIII). Intermediate color formation is shown by such polymers as alternating O-I-O-T and... 

The single-monomer route (eq. 5) is preferred as it proves to give more linear and para-linked repeat unit stmctures than the two-monomer route. Other sulfone-based polymers can be similarly produced from sulfonyl haUdes with aromatic hydrocarbons. The key step in these polymerisations is the formation of the carbon—sulfur bond. High polymers are achievable via this synthesis route although the resulting polymers are not always completely linear. 

Similar reactions to those used in interfacial polymerizations are sometimes carried out in solution and are employed to prepare some polymers which yield ultra-high-strength high modulus fibers. Tliese species typically contain para-linked aromatic rings and amide or ester linkages in the polymer backbone. 

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 second endothermic process (region B) is observed at the temperature higher 230 °C and ends at 270 °C. And during this process crystalline structure breaks and polyester melts. This transition, as crystallization, is phase. High temperature of PETP melting is described by specific role of aromatic nucleus linked by p - para-position [245]. 

A series of wholly aromatic poly(etherimide)s were synthesised [80,81], one of which was found to contain a smectic A phase in the melt. Its structure is given in Chap. 8 (number 160) and consists of pyromellitic anhydride co-reacted with a five ringed diamine containing a central meta- unit linked via an isopropylene unit to two para- linked ether diphenylamine units. If the central ring is made para-oriented, then an LC phase is not observed. 

Structural effects are best illustrated with respect to polyesters. The vast majority of aromatic polyesters which are fusible and contain a substantial number of para links tend to be liquid crystalline in the melt. Their low solubility means that such materials are not normally found to be lyotropic. 

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. 

Figure 2. Modification concepts for para-linked aromatic polymers including typical monomer structures (a) monomer units of different length, (b) kinked comonomers, (c) double kinked comonomers, (d) crankshaft comonomers, (e) flexible lateral substituents, (f) bulky and stiff lateral substituents, and (g) monomers with non-coplanar conformation.

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. 

Figure 3. Monomers and polymer structures of the unsubstituted para-linked aromatic polyesters, poly-(p-phenylene terephthalate) [poly(TA/HQ)] and poly-(p-hydroxybenzoic acid) [poly(HBA)].

As flexible substituents, linear and branched alkyl, alkoxy, or thioalkyl side chains of different lengths have been utilized to modify para-linked aromatic polyesters. The main difference to the systems discussed above is the limited thermal stability caused by the substitution with alkyl chains. Also, the mechanical poperties are substantially lowered with increasing... 

Noncoplanar aromatic monomers, such as 2,2 -disubstituted biphenylene derivatives and 4,4 -functionalized 1,1 -binaphthyl derivatives (Fig. 14) have been used as comonomers in para-linked aromatic polyesters with remarkable effects on the phase transition temperatures, the crystallinity, and the solubility. The incorporation of these noncoplanar monomers will not initially reduce the chain stiffness. The phenyl rings are forced by the 2,2 -substitution into a noncoplanar conformation which strongly decreases the intermolecular inter-... 

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. 

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... 

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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