The movie tends to exaggerate a little bit, but Kevlar is actually a material that features striking properties. In this blog I would like to make a short introduction about the story, structure and properties of Kevlar.
Kevlar was developed in DuPont labs in the 60s, by Stephanie Kwolek. During that time, DuPont was working on a new material featuring the same heat-resistance of asbestos and the stiffness of glass. A nice option in order to develop that kind of material was to use the stiff chains of aromatic polyamides. These polymers tend to be extremely insoluble in several solvents and their processability could be quite difficult.
That was due to the presence of aromatic rings that tend to make the structure quite rigid and poorly flexible in terms of molecular motion. In a polymer chemistry lab, a solution like that is normally thrown away; in fact, proper polymer solutions are transparent and have a quite high-viscosity.
Despite the expected disappointment, the solution obtained was tested in order to get a fiber. The results were amazing. Not only did the fibers spin well but they also were superb in terms of mechanical properties. Figure 1 - Condensation between 1,4-phenylene-diamine and terephthaloyl chloride to give Kevlar.
Keep in mind that the reaction above is that used currently it can involve the use of terephthalic acid instead of the acyl chloride. In fact, the DuPont research team spent years in order to develop the final process. In the beginning, the reaction for obtaining Kevlar was carried out by polymerization of p-aminobenzoic acid. The process was costly though and it did need an understanding of the chemistry behind it as well. Eventually, the DuPont team came up with new procedures along with new challenges.
Even though a new and less costly reaction was developed and carried out in hexamethylphosphoramide HMPA solvent, DuPont found this solvent to be carcinogenic. A new task to accomplish came up: finding a safe solvent able to give a material whose properties were not compromised.
Finally, the combination of N-methylpyrrolidone and calcium chloride was selected as the best solvent. The corrosive properties and the high viscosity of the solutions involved were a concern but, DuPont research scientist Herbert Blades was still able to solve that problem.
Finally, full commercialization took off in the early 80s. In terms of chemistry, Kevlar is an aramid, short for aromatic polyamide. The reason for the name is easily seen by refering back to Figure 1. In fact, its repeating unit features an aromatic ring and an amide bond. The high-performance properties of Kevlar come from its reduced molecular motion even in the solution state.
Macromolecules like nylon or polyester are quite flexible systems. Therefore, they tend to form random coils in the solution, with a certain level of entanglement. Spinning processes actually promote orientation of the chains as well as a partial extension of them. On the other hand, aramide molecules are particularly rigid and they have a rod-like behaviour even at low concentration.
At higher concentrations, the chains are forced to align in parallel giving rise to a liquid crystalline domain. The monomer in this case is made up of an amide group and a phenyl group. Kevlar is a polyamide, a type of synthetic polymer, in which the amide groups are separated by para phenylene groups, meaning that the amide groups are attached to each other on opposite sides of the phenyl group i. Polymer structure. The large phenyl groups separating the amides cause the polymer of Kevlar to nearly always form the trans conformation, where the phenyl groups arrange themselves so that they are on opposite sides of the rigid amide bond:.
This is caused by the phenyl groups to be too large to fit on the same side of the bond, as there would be great steric hindrance between the hydrogen atoms:. The hydrogens are too close together in the cis conformation.
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