Silk is a protein. Like all proteins, it is a polypeptide, and it has a structure something like this:

You'll notice the segment highlighted in blue repeats over and over again. In a real protein molecule, the repeat unit occurs hundreds of times, not just four times as shown here. Scientists call giant molecules such as proteins macromolecules, or polymers. The portion highlighted in blue is called the repeat unit.

Take a closer look at the protein segment over on your right. The portion highlighted in red is what chemists call an amide group. Amide groups have a lot to do with why silk is such a wonderful fiber, as we'll see in just a moment. Macromolecules with amide groups in their repeat units are called polyamides.

Nylon was designed to be a synthetic replacement for silk. So it isn't surprising that the material Wallace Carothers invented has a structure very similar to that of natural silk. Nylon 6-6 has a chemical structure like you see below. This picture shows one repeat unit, but in a real nylon macromolecule, many repeat units are linked together, as shown in the picture on the left side of this page, and most every other page in this exhibit.

Notice the amide groups in the structure. The amide groups are what make nylon such a good fiber. Amide groups are strongly attracted to each other. When nylon is spun into fibers, the long chain-like macromolecules line up parallel to each other. The amide groups on adjacent chains then form strong bonds with each other called hydrogen bonds. These hydrogen bonds hold the adjacent chains together, making nylon yarn strong.