Polymeric materials/polymers Kevlar

Clearly, the hardnesses of thermoplastic polymers are not intrinsic. They depend on various extrinsic factors. Only trends can be cited. For example, as the molecular weight in polyethylene materials increases, they become harder. And, as the molecular aromaticity increases, a polymeric material becomes harder. Thus, higher molecular weight anthracene is harder than napthalene and more aromatic Kevlar is harder than polymethacrylate. 

Another class of inhomogeneous materials of increasing practical importance is fiber-reinforced composites. Usually the matrix is a polymer, such as epoxy. The reinforcement may be polymeric, such as Kevlar, or nonpolymeric, ie, glass, steel, or graphite. For a unidirectional composite with transverse isotropy, there are five independent elastic constants or moduli Cn, C13, C33, C44, and Cee, (28,29). Absorption measurements are particularly difficult to make on such composites. Frequently, the measured attenuation has a significant contribution as a result of scattering from the reinforcement this is sometimes referred to as geometric dispersion. 

The formation of composites by reinforcing polymers with particulate or fibrous polymeric or inorganic materials has been accepted as a general means in improving the wear resistance of polymers. One of the interesting developments is reinforced thermoplastics, such as carbon fiber-PEEK. The uses of LCPs, e.g., Kevlar, PBT, as reinforcing media for amorphous matrices are also of significant interest. 

In 1965, Stephanie Kwolek, working for DuPont to develop new polymer fibers, noticed an odd cloudy product from a polymerization reaction. Some researchers might have rejected the product, but Kwolek insisted on examining its properties more carefully. The results were astonishing When the polymer was spun into a fiber, it was stronger than any other fiber known before. Kwolek had discovered Kevlar, a material that is pound for pound five times stronger than steel. 

Aramid yarns (Kevlar of DuPont, Twaron of Teijin-Twaron) are produced from poly(p-phenylene terephthalamide), PPTA (2), which is specially developed for fiber spinning and not used in any other application. DuPont had experience with poly(m-phenylene isophthalamide) in a fiber product called Nomex for high-temperature applications. The polymer is produced in dimethylacetamide and the solution is dry-spun. This cannot be done with the stiff-chain para-para analogue PPTA. The polymer does not dissolve in organic solvents. A special polymerization route had to be developed, and the discovery of lyotropic behavior of concentrated solutions in sulfuric acid then led the way to the production of a magnificent new fiber material. 

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