Linear thermotropic polymers structure

In the experimental study of the optical properties of molecules by the method of flow birefringence (FBF), the question of the molecular dispersion of the solution is always important, particularly in the analysis of the properties of mesogenic macromolecules. The complex structure of thermotropic polymers and their tendency to form supermolecular structures require special monitoring of possible association or decomposition of the macromolecules in solution. The linear dependence of An in a wide range of rate gradients (Figs. 3.7 and 3.8) and... 

Random copolymerization is an effective way to disturb the regular molecular structure of the polymer chain. The lack of periodicity along the chain inhibits crystallization and, thus, reduces the crystal size and perfection and depresses Tm. If the chosen comonomer is essentially linear, losses of chain mesogenicity can be minimized. This approach has been followed in designing thermotropic copolyesters, such as Vectra A by Celanese (now Ticona), which continues to enjoy commercial success. Vectra A is the copolyester of p-acetoxybenzoic acid (ABA) and 2,6-acetoxynaphthoic acid (ANA) with a mole ratio of 73/27. The melting point of this LCP is around 280°C, which is much lower than the melting points of the either poly (ABA) or poly (ANA) [6]. 

In contrast to the poly(enamine-ketones) prepared by Imai e al. the methyl substituted poly(enamine ketones), J 2, prepared from the condensation of diamines and bis-acetylacetylarenes, were found to be hydrolytically stable and thermally stable from 257 to 340 C, When a linear and rigid aromatic bis(1,3-diketone) was condensed with alkaline dieamines with ten or more carbons or with rigid aromatic diamines, thermotropic liquid crystalline materials were obtained. The monomers and polymers have much higher solubilities and lower transition temperatures as compared with liquid crystals containing only aromatic hydrocarbon rings of similar structures. 

The aromatic semiflexible polyesters can be formed by varying the dicarboxylic multi phenyl acid structure (naphthalene and diphenol) and non-linear aromatic diol (3,4 -dihydroxybenzophenone). 3,4 -dihydroxybenzophenone can be synthesized from 4-methoxy bromobenzene and m-methoxybenzonitrile via multistep reaction, which can be condensed with respective diacid chloride in o-dichlorobenzene at reflux temperature [43]. It is well known that dimethyl siloxane spacer can make a flexible/semiflexible LC polymer. Thermotropic flexible liquid-crystalline main chain polyesters... 

Most LC polymers in the market are linear, highly aromatic thermotropic polyesters. They combine chemical stability with chain rigidity and retain dimensional stability (remain as glasses) up to 200°C or 300°C and are also very chemically stable (resistant to oxidation). These types of PLCs, such as the commercial Vectra and Xydar, are usually processed in the melt state with conventional fabrication techniques such as extrusion and molding (see Chapter 14). A major advantage of such polymers is that they can be melt processed and form extranely precise molded structures that do not shrink on cooling. They have been used in molded parts for microelectronics and in mounting brackets for optical communications. 

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