Liquid Crystalline Aromatic Polyamides

The polymers in this category are best represented by poly(l,4-benzamide) (PBA) and poly(p-phenylene terephthalamide) (PPD-T) prepared in high 

For fiber production, the inherent viscosity of PPD-T is normally in excess of 4 dL/g. The molecular weight is in the order of 20,000 which corresponds to a degree of polymerization of 80. A higher tenacity and higher modulus are obtained for the polymer with higher molecular weight. The Mark-Houwink equation for PPD-T in 96% H2SO4 was determined (Aspin and Strazielle, 1977) to be  

Melting point Shrinkage after 250 °C none none 0.09 none 

Kevlar 149 has a substantially higher degree of crystallinity and larger crystal size than for Kevlar 49. The unit cell dimensions for the two also differ significantly. In addition, while there are present skin-core structure in both Kevlar 29 and Kevlar 49 fibers, there is no definable difference between the skin and core for Kevlar 149. 

The proposed end applications of Kevlar (Yang, 1989) and related fibers include ballistic and fire protective apparel, hard armors, reinforced tires and rubber goods, various forms of composites, filament wound pressure vessels, marine ropes, optical cables and industrial gloves. Almost all of them utilize the high tensile properties and high heat-resistance. 

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Liquid crystalline aromatic polyamides were the first chemical class to be commercialised. The best known example is Kevlar fibre which is spun from liquid crystalline solution to obtain the benefit of the high orientation present in the nematic phase. Subsequently, melt-processable main-chain polyesters were developed and brought to the market (Amoco with Xydar, Hoechst-Celanese with Vectra). 

A lyotropic liquid crystalline aromatic polyamide, sold under the trade name of Kevlar, is available commercially ... 

Example 4.12 Synthesis of a Lyotropic Liquid Crystalline Aromatic Polyamide from Terepbtbalic Acid Dicbloride and Silylated 2-chloro-1,4-Phenylenediamine by Polycondensation in Solution... 

S. L. Kwolek, a woman scientist of DuPont, invented the liquid crystal aromatic polyamides which eventually paved the way to the first commercial liquid crystalline polymer product—poly-p-phenyleneterephthalamide under the trade name Kevlar. She recently recalled, When I dissolved the PBA (poly-p-aminobenzamide) polymer at 10% concentration in tetram-ethylurea with 6.5% LiCl, the solution was unusually fluid, turbid, stir-opalescent, and butter-milk-like in appearance. The fiber that was spun turned out to be extremely strong with a modulus of 430 gpd This discovery in 1964 remains a milestone of this field. In recognition of her contribution, the American Society of Chemistry Industry awarded Kwolek the 1997 Perkin Medal. 

Weathering tests, 245 Wholly aromatic liquid crystalline polyesters, degradation of, 38 Wholly aromatic polyamides, 136-137, 139 synthesis of, 184-189 Wholly aromatic polyesters, 25-26, 32 copolymerization and, 35 synthesis of, 71-72... 

Both low molecular weight materials [145] and polymers [146,147] 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. One reason such liquid-crystalline elastomers are of particular interest is the fact that (i) they can be extensively deformed (as described for elastomers throughout this chapter), (ii) the deformation produces alignment of the chains, and (iii) alignment of the chains is central to the formation of liquid-crystalline phases. Because of fascinating properties related to their novel structures, liquid-crystalline elastomers have been the subject of numerous studies, as described in several detailed reviews [148-150]. The purpose here will be to mention some typical elastomers exhibiting liquid crystallinity, to describe some of their properties, and to provide interpretations of some of these properties in molecular terms. 

Unusual properties of fully aromatic polyesters are observed if they have at least partially a rigid planar chain structure. In particular, they can form thermotropic liquid crystalline states (see Example 4-5). As already discussed in Sect. 1.2.4 an important structural prerequisit for LCPs of Type A in order to attain the liquid crystalline state of aromatic polyesters (and aromatic polyamides, see Example 4-14), is a rigid main chain according to the following construction principle ... 

Since Robinson [1] discovered cholesteric liquid-crystal phases in concentrated a-helical polypeptide solutions, lyotropic liquid crystallinity has been reported for such polymers as aromatic polyamides, heterocyclic polymers, DNA, cellulose and its derivatives, and some helical polysaccharides. These polymers have a structural feature in common, which is elongated (or asymmetric) shape or chain stiffness characterized by a relatively large persistence length. The minimum persistence length required for lyotropic liquid crystallinity is several nanometers1. 

Further calculations on aromatic polyesters, polyamides and liquid crystalline systems ... 

Similady, liquid-crystal polymers exhibit considerable order in the liquid 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 (a ram id) with the repeating unit designated as (2). It is spun into... 

Both lyotropic and thermotropic liquid-crystalline synthetic polymers have been widely studied. Aromatic polyamides constitute the most important class forming liquid-crystalline solutions the solvents are either powerfully protonating acids such as 100% sulphuric acid, chloro-, fluoro- or methane-sulphonic acid, and anhydrous hydrogen fluoride, or aprotic dipolar solvents such as dimethyl acetamide containing a small percentage, usually 2-5 %, of a salt such as lithium chloride or calcium chloride. Such solutions constitute a nematic phase within certain limits. Some criteria for formation of a nematic instead of an isotropic phase are ... 

Introduction of ring-opening metathesis as a versatile polymerisation technique (ROMP) by Chauvin and Herisson Nobel Prize Chemistry to Paul J. Flory for his fundamental achievements, both theoretical and experimental, in the physical chemistry of the macromolecules Fully aromatic polyamides developed Aramids, being lyotropic liquid crystalline polymers of high strength, due to extended molecular chains (Morgan and Kwolek)... 

Following the technological breakthroughs which led to the discovery of (1) the liquid crystalline behavior ofpara-oriented aramids26 and (2) a novel method for spinning anisotropic liquid crystalline polymer solutions,27 Kevlar aramid fiber was produced and commercialized by the DuPont company in 1972. Other fibers based on aromatic polyamide compositions, which were produced and commercialized by other companies, were Technora (Teijin, Japan), Teijinconex (Teijin, Japan), andTwaron (Akzo, The Netherlands). Additionally, SVM is a fiber produced in the Former Soviet Union and it was announced in 1990 that a new aramid fiber had been introduced by Hoechst, in Germany. 

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. 

Aromatic polyamides are generally made by low-temperature reactions of aromatic diamines and aromatic diacid chlorides in special solvents such as a 1 3 molar mixture of hexamethylphosphoramide A-methylpyrrolidone, as in reaction (4-50). Intensive stirring is required to attain high molecular weights because the polymer precipitates. These macromolecules are very rigid and rodlike. They form oriented liquid crystalline arrays in solution and require little postspinning orientation to produce extremely strong and stiff fibers. The polymer would not be made in the melt because it is infusible. It must be synthesized and handled in solution, and this requires the use of reactive precursors. 

We confined ourselves only to poly(y-benzyl-L-glutamate) and p-substituted aromatic polyamides, as most of experimental studies on phase equilibrium with the formation of liquid crystalline phase are based on the analysis of the behavior of these... 

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... 

Polymer films have been obtained by plasma polymerization of hexafluorobenzene, N-vinylpyrrolidine, and chloracrylonitrile (Munro). Higuchi et al. have shown that irradiation of an azobenzene-modified poly(Y-methyl-L-glutamate-CO-L-glutamic acid) in bilayer membrane vesicles of distearyldimethylammonium chloride leads to trans-cis isomerization of the polymer this leads to transfer of the polypeptide from the hydrophobic bilayer membrane interior to the hydrophilic surface. As a result, there was a decrease in the ion permeability through the bilayer membrane and the formation of intervesicular adhesion. Eisner and Ritter have prepared photosensitive membranes from an aromatic polyamide and a cinnamate that incorporates a liquid crystalline component. 

Solubilities. Rogers et al. (16-18) have prepared and reported on a large number of aromatic polyesters and polyamides containing the 2,2 -disubstituted 4,4 -biphenylene moiety. Some of the polymers were soluble in common solvents such as tetrahydrofuran and acetone, with solubility as high as 50% in one case. The solubilities of the amide monomers synthesized in this work are shown in Table I. These compounds exhibited virtually no solubility in common solvents such as acetone but displayed fairly high solubilities in several amide solvents, with and without added salts. Lyotropic liquid crystallinity was not observed although solubilities were as high as 40% (w/w) in some solvents. 

A variable-director NMR method was used to investigate the order and dynamics of aromatic polyamides in lyotropic liquid crystalline phases. " In... 

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