Aramid fibers, liquid crystal polymers

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

Liquid Crystal Polymers (LCP) This novel polymeric family excels in thermal and mechanical performance. The uniqueness of these polymers stems from the extraordinary crystalline stmcture, exhibiting ordered domains even in the liquid state. These are chainlike macromolecules of rigid structure, mainly because of the existence of aromatic rings such as aromatic polyamides and polyesters. There are two major groups—lyotropic and thermotropic. In the former group, the liquid crystals are formed in an appropriate solution, usually forming fibers (like Kevlar , that appeared in 1965 as an aramide). 

Polyamide fibers with much higher softening points than nylon 66 or nylon 6 were produced by a team of DuPont chemists who used aromatic reactants instead of the aliphatic reactants used to produce nylon 66. The aramids which were the first liquid crystal polymers, were spun to produce high tenacity fibers which were sold under the trade names of Kevlar and Nomex. 

Teijin aramid fiber, known as Technora (formerly as HM-50), is made slightly differently from the liquid crystal route described above. Three monomers, terephthalic acid, p-phenylenediamine (PDA), and 3,4-diamino diphenyl ether are used. The ether monomer provides more flexibility to the backbone chain which results in a fiber that has slightly better compressive properties than PPTA aramid fiber made via the liquid crystal route. An amide solvent with a small amount of salt (calcium chloride or lithium chloride) is used as a solvent (Ozawa et al., 1978). The polymerization is done at 0-80 C in 1-5 h and with a polymer concentration of 6-12%. The reaction mixture is spun from a spirmeret into a coagulating bath containing 35-50% CaClj. Draw ratios between 6 and 10 are used. 

Polycondensation of p-phenylenediamine with terephthaloyl dichloride produces aromatic polyamides, which display liquid-crystal characteristics. The spinning of these polymers from hexamethylphosphoramide or sulfuric acid solution yields high-value aramid fibers (Kevlar Du Pont), Twaron Akzo) which are distinguished by their high temperature resistance (see Chapter 7.3.1). 

The spinning solution extruded from the large holes is above 100° C, in order to keep the polymer in solution and to keep the viscosity acceptable. Fiber formation for gel spinning by quenching is relatively rapid solidification is by crystallization, rather than by evaporation or extraction of solvent. Complete removal of solvent from the thick filaments proceeds much more slowly. The quench liquid can simply be water and the spinneret cannot be immersed in the water bath because the spinning solution would then freeze in. An air gap must be used, as in aramid spinning, but without the function of increasing the molecular orientation. 

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