Aramids isotropic solution

Some aramids are processed from isotropic solutions. Flexible chain homo-polymers like MPDI can be dissolved in solvents like NMP and DMAc [88] to form such solutions but the degree of solubility can be further enhanced by copolymerization [83]. Isotropic solutions can be also obtained with p-aramids but in this case copolymerization is required to enhance solubility. 

A distinctive feature of semirigid polymers such as p-aramids is that their solutions develop molecular orientation under shear or extension with great ease. This results in a unique difference in properties in the direction of shear or extension vs. those perpendicular to the shear direction. There are two classes of materials that have this characteristic lyotropic, which form anisotropic solutions and thermotropic, which form anisotropic melts. As aramids do not melt we will focus here on lyotropic systems. Anisotropic solutions differ from isotropic solutions in many physical characteristics including light depolarization, rheological properties, phase behavior, and molecular orientation. 

Aramid polymers have high melting points and melt with decomposition that makes impractical their processing to yams (fibers) by melt spinning. Among the commercial aramids, only the flexible chain homo- and copolymers of MPDI type can be dissolved in NMP and DMAc [15] to form isotropic solutions transformed into yarns by a dry spinning... 

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. 

Aramid films have been in development since the late 1990s by several Japanese companies including Toray, Teijin, and Asahi. As with fibers, aramid solutions can be extruded through flat dies to form films. The conventional wet process can be employed to produce unidirectional and bi-oriented films from isotropic aramid solutions. Production of films from anisotropic solutions requires unique processes as shown by the example of PPTA film. 

In structural studies, fibres are generally considered to be transversely isotropic. However, it is sometimes found that commercial fibres have a distinct radial differentiation of structure. This is most pronounced in fibres spun from solutions, which show a distinct skin-core effect. A radial nonuniformity has been also found in some melt-spun fibres, e.g. in polyester fibres produced at very high spinning speeds.It should also be noted that in some fibres e.g. aramids" ) there is a preferential radial orientation of certain crystal planes, in contrast to a random radial orientation normally exhibited by conventional commercial fibres. 

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