Polymers and their structure

Synthetic Materials (Polymers)

In an effort to achieve new materials, humans have achieved the production of synthetic materials using organic materials (mainly hydrocarbons) found in nature. These materials mainly contain the elements carbon, hydrogen, oxygen, nitrogen and sulfur and are known as polymeric materials. Polymers or synthetic materials have a wide range of applications, including in the manufacture of household appliances, toys, packaging, bags and suitcases, shoes, tables and chairs, hoses and water pipes, coating materials as paints for corrosion protection and decoration, car tires and finally as engineering polymers with high strength even at relatively high temperatures in the manufacture of machine components.

Advantages of polymers

Polymers have relatively good and useful physical and mechanical properties. They have a low specific gravity and good stability against chemicals. Some of them are transparent and can replace glass. Most polymers are electrical insulators. But there are also certain polymers that are electrically conductive to some extent. The insulation of polymers is related to the covalent bonds between atoms in molecular chains. However, research conducted in recent years has shown that it is possible to create electrical conductivity along the axis of the molecules. These types of polymers are basically composed of polystyrene.

Doping polymers

By infiltrating elements such as alkali metals or halogens (doping process) into the molecular chains of polystyrene, N- and P-type polymer semiconductors are obtained, respectively. Adding elements or doping allows electrons to move along the carbon atoms in the chain. Teflon is a polymer material that is used as a coating to prevent food from sticking to cooking utensils due to its low coefficient of friction.

Structure of polymers

Most common polymers are obtained from the polymerization of simple organic molecules called monomers. For example, polyethylene (PE) is a polymer that is obtained by polymerization by adding (combining) several ethylene molecules. Each ethylene molecule is called a monomer. With the right combination of heat, pressure, and catalyst, the double bond between the carbon atoms is broken and replaced by a single covalent bond. The two free ends of this monomer are now converted into free radicals, so that each carbon atom has a single electron that can be added to other free radicals. Therefore, there are two sites (related to the carbon atom) in ethylene where other molecules can be attached.

This reactive molecule is the building block of polymers and is called a (mer) or more repeating unit. The repeating unit is repeated many times along the chain of the polymer molecule. The average length of the polymer depends on the degree of polymerization, or the number of repeating units in the polymer molecule chain.

Therefore, the ratio of the molecular mass of the polymer to the molecular mass of the repeating unit is defined as (degree of polymerization). As the molecular chain becomes larger (if only intermolecular forces bind the molecules together), both the thermal resistance and tensile strength of the polymer materials increase.

Polymerization Process

In general, the polymerization process can be carried out in different ways such as additive, stepwise, etc. In additive polymerization, a number of repeating units are added to each other and produce a larger molecule called a polymer. In this type of polymerization, first in the first stage, free radicals are created by giving energy (thermal, light) to ethylene molecules with a double bond and breaking the double bond.

Then, free radicals are added to the repeating units to form active centers called initiators, and each of these centers is added to other repeating units and the growth of the polymer continues. Theoretically, the degree of addition polymerization can be unlimited, in which case a very long chain molecule is formed by connecting a large number of repeating units to each other.

However, in practice, chain growth does not occur indefinitely. The greater the number of active centers or initiators formed, the greater the number of chains and consequently the shorter the chain length, which is why the properties of polymers change. Of course, the growth rate also affects the length of the chains. When all the repeating units and chains are connected to each other, growth ends.

Another method of polymerization is step polymerization, in which monomers react chemically with each other to form linear polymers. In many step polymerization reactions, a small molecule is formed as a byproduct. This type of reaction is sometimes called condensation polymerization.