Polyamides lyotropic properties

High modulus fibers from lyotropic aromatic polyamides, poly(p-phenylene terephthalamide) (PPTA), were first conunercialized imder the Kevlar trademark by DuPont [414]. The aromatic polyamides, or aramids, are produced by a dry jet-wet spinning process where the nematic structure in solution is responsible for the high modulus fiber performance [415-419]. Another class of lyotropic fibers, also produced by dry jet-wet spinning, are the rigid rod polymers developed as part of the U.S. Air Force Ordered Polymers Program [420-424]. The most conunon of these ordered polymers, poly(p-phenylene benzobisthiazole) (PBZT), is difficult to process, but it exhibits the highest tensile properties of all the LCP fibers produced to date. 

Similady, hquid-crystal polymers exhibit considerable order in the hquid state, either in solution (lyotropic) or melt (thermotropic). When crystallized from solution or melt, they have a high degree of extended-chain crystallinity, and thus have superior mechanical properties. Kevlar (Du Pont) is an aromatic polyamide (atamid) with the repeating unit designated as (2). It is spun into... 

One of the characteristic properties of rod-like polymers is that their concentrated solutions form lyotropic liquid crystals. Such examples among synthetic polymers are polyamides -and polyisocyanates " which form cholesteric or nematic liquid crystals in selected solvents. 

The range of polymers which were found to be able to form liquid crystalhne systems has been considerably extended. Poly(Y-benzyl-L-glutamate) and its analogs, as well as para-aromatic polyamides, exhibit this property in solutions, which served the basis for relating them to lyotropic liquid crystals (see Sect. 2.1). Subsequently, the classes of polymers were found which exhibited such a transition during a change of temperature thermotropic liquid crystals). 

Liquid crystalline compounds are remarkable because of their ability to show spontaneous anisotropy and readily induced orientation in the liquid crystalline state. When polymers are processed in the liquid crystalline state, this anisotropy may be maintained in the solid state and can readily lead to the formation of materials of great strength in the direction of orientation. A particularly important example of the use of this property for polymers is in the formation of fibers from aromatic polyamides which are spun from shear oriented liquid crystalline solutions Solutions of poly(benzyl glutamate) also show characteristics of liquid crystalline mesophases, and both of these types of polymers are examples of the lyotropic solution behaviour of rigid rod polymers which was predicted by Flory... 

Another important property of polyamide is their lyotropism. Polyamides with higher solubility in common organic solvents are still required. Only a few examples exist in the literature. High solubility in a solvent is one of the key factors but lyotropism and the ability to form and retain a high degree of ordering depends, in particular, on the polymer primary structure. 

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. 

Some other compounds may also have the potential to possess amphotropic behavior, but because of too high melting points these properties can not be observed, for example polyamides such as Kevlar show lyotropic behavior only, and are not discussed here. Furthermore, one has to take into account... 

Among polymer molecules exhibiting lyotropic mesomorphism the properties of synthetic polypeptides, poly-alkylisocyanates, cellulose derivatives and para-aromatic polyamides have been investigated in greatest detail. 

SINCE the discovery of liquid crystalline phenomenon for low molecular weight liquid crystals (LMWLCs) more than 100 years ago, anisotropic ordering behaviors of liquid crystals (LCs) have been of considerable interest to academe [1-8], In the 1950s, Hory postulated the lattice model for various problems in LC systems and theoretically predicted the liquid crystallinity for certain polymers [1-3], As predicted by the Hory theory, DuPont scientists synthesized lyotropic LCPs made of rigid wholly aromatic polyamide. Later, Amoco, Eastman-Kodak, and Celanese commercialized a series of thermotropic main-chain LCPs [2]. Thermotropic LCPs have a unique combination of properties from both liquid crystalline and conventional thermoplastic states, such as melt processibility, high mechanical properties, low moisture take-up, and excellent thermal and chemical resistance. Aromatic main-chain LCPs are the most important class of thermotropic LCPs developed for structural applications [2,4-7]. Because they have wide applications in high value-added electronics and composites, both academia and industry have carried out comprehensive research and development. 

In the phrase liquid-crystalline, the crystalline adjective refers to the faa that these materials are sufSdentiy ordered to diffract an X-ray beam in a way analogous to that of normal crystalline materials. On the other hand, the liquid part specifies that there is frequently sufSdent disorder for the material to flow like a liquid. liquid crystals can be divided into thermotropic, that exhibit a phase transition with change of temperature, and lyotropic, that exhibit phase transition as a function of both temperature and concentration of the LC molecules in a solvent. Both low molecular wdght materials and polymers " can show liquid crystallinity. In the case of polymers, it frequently occurs in very stiff chains such as the Kevlars and other aromatic polyamides. It can also occur with flexible chains, however, and it is these flexible chains in the elastomeric state that are the focus of the present discussion. LC networks of flexible chains have the following three properties (1) they can be extensively deformed (as described for elastomers throughout this book), (2) the deformation produces alignment of the chains, and (3) alignment of the chains is central to the formation of LC phases. Elastomers of this type have been the subject of numerous studies, as described in several detailed reviews. ... 

---