Polymeric aramides

Specific inter- and intramolecular bonding are not necessary for ordered structures to persist in dilute solution. Ordered structures, that lead to highly asymmetric molecules, can be perpetuated by severe steric repulsions of substituents or an inherent restraint to rotations about single bonds. Such structures are known, even among synthetic macromolecules, and they form liquid-crystal systems. Some examples are polymeric aramides, poly(N-alkyl isocyanates) and some cellulose derivatives. 

Another test of the theory is to compare the experimentally determined dependence of Up, the volume fraction at which the nematic phase separates, on the axial ratio." The agreement between theory and experiment is particularly good with the alpha-helical polypeptide poly(y-benzyl-L-glutamate).(94-96) Studies of solutions ofthe polymeric aramides, such as poly(p-benzamide) and poly(/ -phenylene tereph-thalamide) indicate a qualitative accord between theory and experiment. Studies... 

Another factor is breaking up the projectile. This is again done using polymeric materials—here composites such as boron carbide ceramics in combination with aramids, UHMWPE, or fibrous glass. 

The majority of commercially available polymeric composites are reinforced by glass fibers, carbon fibers, aramid fibers (e.g., Kevlar) and, to a lesser degree, boron fibers. In some cases hybrid composites are made that contain combinations of fibers. 

Composite In polymer technology a combination of a polymeric matrix and a reinforcing fiber with properties that the component materials do not have. The most common matrix resins are unsaturated thermosetting polyesters and epoxies, and reinforcing fibers are glass, carbon, and aramid fibers. The reinforcing fibers may be continuous or discontinuous. Some matrix resins are thermoplastics. 

In theory, almost any polymer can be used as a reinforcement in an advanced composite. By far the most common polymer fiber, however, is aramid, better known by its trade name, Kevlar. Developed originally for use in belted radial tires, aramid is an aromatic polyamide in which benzene fragments (C6H4) alternate with peptide groups (NHC = Q) in forming a polymeric structure ... 

Another novel technique of making oxide fibres is called the inviscid melt technique [24], In principle, any material in a molten state can be drawn into a fibre shape. Organic polymeric fibres such as nylon, aramid, etc., as well as a variety of glasses can be routinely converted into fibrous form by passing a molten material, having an appropriate viscosity, through an orifice. The inviscid (meaning low viscosity) melt technique uses this principle to make oxide fibres. 

Reichert and Mathias prepared related branched aramids, to those of Kim,t5-34] from 3,5-dibromoaniline (23) under Pd-catalyzed carbonylation conditions (Scheme 6.7). These brominated hyperbranched materials (24) were insoluble in solvents such as DMF, DMAc, and NMP, in contrast to the polyamine and polycarboxylic acid terminated polymers that Kim synthesized, which were soluble. This supports the observation that surface functionality plays a major role in determining the physical properties of hyperbranched and dendritic macromolecules J4,36 A high degree of cross-linking could also significantly effect solubility. When a four-directional core was incorporated into the polymerization via tetrakis(4-iodophenyl)adamantanc,1371 the resultant hyperbranched polybromide (e.g., 25) possessed enhanced solubility in the above solvents, possibly as a result of the disruption of crystallinity and increased porosity. 

FIGU RE 24.2 TGA curves in air of (a) glass, carbon, and aramid fibers and (b) fiber/epoxy mixtures in 1 1 mass ratio. (From Kandola, B.K. et al., Recent advances of flame retardancy of polymeric materials, in Proceedings of the 17th Conference, Lewin, M. (Ed.), BCC, Stamford, CT, 2006. With permission.)... 

Several types of polymeric fibers are used as thixotropes in both adhesive and sealant formulations. These are cellulose, polyolefin, and aramid fibers. A fiber that exhibits good thixotropy usually has several key characteristics including... 

FIG. 19.1 Morphological models of some polymeric crystalline structures. (A) Model of a single crystal structure with macromolecules within the crystal (Keller, 1957). (B) Model of part of a spherulite (Van Antwerpen, 1971) A, Amorphous regions C, Crystalline regions lamellae of folded chains. (C). Model of high pressure crystallised polyethylene (Ward, 1985). (E) Model of a shish kebab structure (Pennings et al., 1970). (E) Model of paracrystalline structure of extended chains (aramid fibre). (El) lengthwise section (Northolt, 1984). (E2) cross section (Dobb, 1985). 

The mechanical properties of paracrystalline aramides and other high-performance polymeric fibres are shown in Table 19.11. In comparison some other fibres are mentioned in this table (see also Sect. 13.6). Aramide fibres display a very high refractivity (refractive index and strong birefringence // 2.2, n 1.6). 

Poly(p-phenylene terephthalamide) PPD-T, also called para-aramid, can be polymerized to a fiber-forming molecular weight by polycondensation of terephthaloyl chloride and 1,4-phenylene diamine. 

There is, however, a limit to the amount of stretch that can be given to a polymer because the phenomenon of necking can intervene and cause rupture of the fiber. In other words, in a polymeric fiber there is a limit to the modulus enhancement that can be obtained by subjecting it to an ever higher draw ratio. As we shall see in Chapter 4, this led to other means of obtaining high stiffness polymeric fibers such as aramid and polyethylene. 

In what follows, we first describe general processing techniques used to make synthetic polymeric fibers, followed by a description of the processing, structure, and properties of some important low modulus organic fibers. Finally, we describe, in some detail, two commercially important, high-stifihess fibers aramid and polyethylene. 

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