A polyamide is a polymer

Polymer

A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 containing monomer

Monomer

A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...

s of amide

Amide

In chemistry, an amide is an organic compound that contains the functional group consisting of a carbonyl group linked to a nitrogen atom . The term refers both to a class of compounds and a functional group within those compounds. The term amide also refers to deprotonated form of ammonia or an...

s joined by peptide bond

Peptide bond

This article is about the peptide link found within biological molecules, such as proteins. A similar article for synthetic molecules is being created...

s. They can occur both naturally and artificially, examples being protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s, such as wool

Wool

Wool is the textile fiber obtained from sheep and certain other animals, including cashmere from goats, mohair from goats, qiviut from muskoxen, vicuña, alpaca, camel from animals in the camel family, and angora from rabbits....

 and silk

Silk

Silk is a natural protein fiber, some forms of which can be woven into textiles. The best-known type of silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori reared in captivity...

, and can be made artificially through step-growth polymerization

Step-growth polymerization

Step-growth polymerization refers to a type of polymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers, then trimers, longer oligomers and eventually long chain polymers. Many naturally occurring and some synthetic polymers are produced by step-growth...

 or solid-phase synthesis

Solid-phase synthesis

In chemistry, solid-phase synthesis is a method in which molecules are bound on a bead and synthesized step-by-step in a reactant solution; compared with normal synthesis in a liquid state, it is easier to remove excess reactant or byproduct from the product. In this method, building blocks are...

, examples being nylon

Nylon

Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...

s, aramid

Aramid

Aramid fibers are a class of heat-resistant and strong synthetic fibers. They are used in aerospace and military applications, for ballistic rated body armor fabric and ballistic composites, in bicycle tires, and as an asbestos substitute. The name is a portmanteau of "aromatic polyamide"...

s, and sodium poly(aspartate)

Sodium poly(aspartate)

Sodium poly is a condensation polymer based on the amino acid aspartic acid.-Polymerization:The polymerization reaction is an example of a step-growth polymerization to a polyamide and in one practical procedure aspartic acid is simply heated to 180 °C resulting in water release and the formation...

. 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:


According to the number of repeating units' types, polyamides can be:

• homopolymers :
  • PA 6 : [NH−(CH₂)₅−CO]_n made from ε-Caprolactam
    Caprolactam
    Caprolactam is an organic compound with the formula 5CNH. This colourless solid is a lactam or a cyclic amide of caproic acid. Approximately 2 billion kilograms are produced annually...
    
     ;
  • PA 66 : [NH−(CH₂)₆−NH−CO−(CH₂)₄−CO]_n made from hexamethylenediamine and adipic acid
    Adipic acid
    Adipic acid is the organic compound with the formula 42. From the industrial perspective, it is the most important dicarboxylic acid: About 2.5 billion kilograms of this white crystalline powder are produced annually, mainly as a precursor for the production of nylon...
    
    ;
• copolymers :
  • PA 6/66 : [NH-(CH₂)₆−NH−CO−(CH₂)₄−CO]_n−[NH−(CH₂)₅−CO]_m made from caprolactam, hexamethylenediamine and adipic acid ;
  • PA 66/610 : [NH−(CH₂)₆−NH−CO−(CH₂)₄−CO]_n−[NH−(CH₂)₆−NH−CO−(CH₂)₈−CO]_m made from hexamethylenediamine, adipic acid and sebacic acid
    Sebacic acid
    Sebacic acid is a dicarboxylic acid with structure 8, and is naturally occurring.In its pure state it is a white flake or powdered crystal...
    
    .

According to their crystallinity

Crystallinity

Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, transparency and diffusion. In a gas, the relative positions of the atoms or...

, polyamides can be:

• semi-crystal
  Crystal
  A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
  
  line:
  • high crystallinity : PA46 et PA 66 ;
  • low crystallinity : PA mXD6 made from m-xylylenediamine and adipic acid;
• amorphous
  Amorphous solid
  In condensed matter physics, an amorphous or non-crystalline solid is a solid that lacks the long-range order characteristic of a crystal....
  
   : PA 6I made from hexamethylenediamine and isophthalic acid
  Isophthalic acid
  Isophthalic acid is an organic compound with the formula C6H42. This colourless solid is an isomer of phthalic acid and terephthalic acid. These aromatic dicarboxylic acids are used as precursors to commercially important polymers, e.g. the fire-resistant material Nomex...
  
  .

According to this classification, PA66, for example, is an aliphatic semi-crystalline homopolyamide.
The amide link is produced from the condensation reaction

Condensation reaction

A condensation reaction is a chemical reaction in which two molecules or moieties combine to form one single molecule, together with the loss of a small molecule. When this small molecule is water, it is known as a dehydration reaction; other possible small molecules lost are hydrogen chloride,...

 of an amino group and a carboxylic acid

Carboxylic acid

Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...

 or acid chloride group.