Silk has always been a prize to the wealthy. Even today, it’s considered very expensive and a treasure to own. It’s not hard to wonder why. With the soft feel, warmth in cold conditions, and the cool touch it gives in the summer, along with the brilliant shine and ease it takes to dye the material, one hardly thinks about it’s chemical structure, which happens to be the reason all of those properties even exist. Silk is a protein, much like wool or hair. Most proteins are made from 22 amino acids, which is turn are made of amino groups and an organic acid group. The three most common amino acids in silk are glycine, serine, and alanine; these three make up about 85% of the structure. Silk is also a polymer, like cellulose; it’s made up of repeating units of the amino acids. However, silk differs from the norm by having a somewhat variation of the side groups on each of the amino acids.
It’s predicted that 80 to 85% of the silk is a repeating sequence of glycine-serine-alanine-glycine-alanine, all arranged in a zig-zag pattern with the side groups alternating on the sides. Each “chain” of amino acids runs parallel with a chain next to it, patterned in the opposite direction. The molecular strands holding them together eventually create a pleated sheet structure that is resistant to stretching and can be linked to many physical properties such as the smooth feel and the shine. The irregular repetitions in the structure can also be blamed for silk’s ability to absorb colors from both man-made dyes and natural.
The search for a synthetic silk began in the late nineteenth century because of it’s high demand. Unfortunately, the irregularities in silk make it very difficult to duplicate. In truth, silk is a very simple molecule, but to be able to randomly attach the side groups in the random and non-random order is quite difficult. In the end, after all the time spent in the lab, with all the resources used, the synthetic silk was more expensive than the real thing. It wasn’t until the twentieth century that the complexities of the chemical structure of silk was truly understood, and many early efforts to make a synthetic silk were guided by fortunate accidents. For example, in the late 1870’s Hilaire de Chardonnet discovered a solution of the nitrocellulose material used for photographic plates that had set to a sticky mess. From this he was able to extract long, silk-like threads. Chardonnet tried forcing the solution through a set of tiny holes, thus creating the first facsimile of silk fiber. Unfortunately, what Chardonnet had created was artificial silk, which consisted of a different chemical structure from a different compound, not synthetic silk.
This synthetically made artificial silk was in fact soft and shiny, but also highly flammable, which quickly led to it’s downfall. In one incident, cigar ash was flicked onto a dancer’s silk dress, and it burst into flame. By 1895, Chardonnet was using a denitrating agent that was much less flammable. By 1901, Charles Cross and Edward Bevan had developed a different method using viscose. This method is still used today, used to make rayons, or artificial silk in which the threads are composed of cellulose. Unfortunately, this too had it’s setbacks. When wet, the rayon would sag, so a new artificial silk was needed.
In 1938, a noncellulose-based nylon was created by Wallace Carothers. Nylon is a polyamide, meaning that it’s polymer units are held together with amide links. Carothers nylon differed from real silk with it’s monomer units that consisted of two amino acid groups on one chain and two amine groups on the other. The amide link in the nylon is created by eliminating a molecule of water between the ends of the molecules. The amide bond created joins the two different molecules.
The first commercial use of nylon was in 1939 on toothbrush bristles. Nylon stockings, however, were gaining popularity. Nylon was very similar to silk in its properties while it cost far much less. Nylon was also used for fishing lines and nets, strings in rackets, surgical sutures, and electrical wire coatings. Soon the use went from commercial to military. During World War Two, women stopped wearing the nylon stockings so that the material could be used for tire cords, mosquito netting, weather balloons, ropes, and many other uses. After the war, the nylon was once again used for clothing, carpets, and furnishings. Over ten million pounds of it was used for those reasons alone.
I truly believe that silk and nylon impacted the world and her history quite a bit. While it may not have had an impact like explosives or birth control, silk and it’s synthetic counterpart has weaved in and out of our history as much as the next molecule. Silk alone led to imperialism and colonialism by stretching the international trade in ancient China with the Silk Rode, a major trade route that stretched all over China, India, and much of Europe. It also stimulated fashion trends and wartime efforts. Silk alone has lead to great change and growth worldwide.