A polymer can be defined as a macromolecule that is formed by the union of smaller molecules, called monomers, through covalent bonds.
Since they are often used interchangeably, it is important to clarify the difference between a polymer and a plastic. A plastic is a polymer that deforms after mechanical stress and does not subsequently regain its original shape. Accordingly, it can be said that all plastics are polymers; but not all polymers are plastics.
Due to the great diversity of existing polymers, they can be classified in many ways: according to their origin, according to their internal structure or the type of monomers of which they are composed, according to their mechanical or chemical properties, or by their thermal properties, etc.
For example, according to their structure we can distinguish between amorphous and semi-crystalline polymers.
Amorphous polymers are polymers that have no order in their structure, while semi-crystalline polymers are those that have regions with a periodic arrangement.
All polymers that crystallize have an amorphous part and a crystalline part in their structure, so we cannot speak of crystalline polymers but of semi-crystalline polymers.
The glass transition temperature (Tg) is the temperature at which the chains that make up the amorphous part of the polymers become mobile. The melting temperature (Tf or Tm) of a polymer is the temperature at which the crystalline part of a polymer changes from a solid state to a liquid with greater or lesser viscosity.
A second classification can be made on the basis of their chemical structure, being grouped into homopolymers and copolymers. Homopolymers are those that are made up of a single type of monomer, while copolymers are materials whose structure includes different types of monomers.
A third classification of polymers can be made according to their behavior in the presence of heat; thus, a distinction is made between:
Thermoplastic polymers:
This is the group of polymers that, after being heated and melted or softened, are able to recover their original properties, i.e. they do not undergo a chemical change.
These polymers generally have a linear structure and are therefore not very rigid. They are also less resistant and their melting temperature and hardness are low. They are not good thermal or electrical conductors and are usually soluble in organic solvents. Examples of this type of polymers are polyethylene, polypropylene, PVC or polyamides.
Thermosetting polymers:
These are polymers with a cross-linked structure, forming a network based on covalent bonds. This type of polymer is only molded when it is formed, since after this initial cooling its structure cannot be altered again by applying heat or pressure. Their general characteristics are their high thermal stability, rigidity and hardness. In addition, they are usually insoluble and are both thermally and electrically insulating.