A polyamide is a polymer containing monomers of amides joined by peptide bonds. They can occur both naturally and artificially, examples being proteins, such as wool and silk, and can be made artificially through step-growth polymerization or solid-phase synthesis, examples being nylons, aramids, and sodium poly(aspartate). Polyamides are commonly used in textiles, automotives, carpet and sportswear due to their extreme durability and strength.
According to the composition of their main chain, polyamides are classified as follows:
| Polyamide family | Main chain | Examples of polyamides | Examples of commercial products |
|---|---|---|---|
| Aliphatic polyamides | Aliphatic | PA 6 and PA 66 | Nylon from DuPont |
| Polyphthalamides | Semi-aromatic | PA 6T = hexamethylenediamine + terephthalic acid | Trogamid from Evonik Industries, Amodel from Solvay |
| Aramides = aromatic polyamides | Aromatic | Paraphenylenediamine + terephthalic acid | Kevlar and Nomex from DuPont, Teijinconex, Twaron and Technora from Teijin, Kermel from Kermel |
According to the number of repeating units' types, polyamides can be:
According to their crystallinity, polyamides can be:
According to this classification, PA66, for example, is an aliphatic semi-crystalline homopolyamide.
The amide link is produced from the condensation reaction of an amino group and a carboxylic acid or acid chloride group. A small molecule, usually water, or hydrogen chloride, is eliminated.
The amino group and the carboxylic acid group can be on the same monomer, or the polymer can be constituted of two different bifunctional monomers, one with two amino groups, the other with two carboxylic acid or acid chloride groups.
Amino acids can be taken as examples of single monomer (if the difference between R groups is ignored) reacting with identical molecules to form a polyamide:
Aramid (pictured below) is made from two different monomers which continuously alternate to form the polymer and is an aromatic polyamide:
These materials are commonly completely insulating, and generate static electricity, which can cause dangerous sparks that can damage electronics in manufacturing plants and in consumer products. This has resulted in an interest in incorporation of conductive fillers such as carbon black, metals and conducting polymers. The most common conducting fillers are silver and carbon black. Both of these materials have processing deficiencies, while silver is also prohibitively expensive for applications such as antistatics.