Polymer means many mers or units, generally a repetition of hundreds or thousands of times a chain-like structure. Most monomers are organic materials in which the carbon atoms are joined in covalent bonds with other atoms, such as hydrogen, oxygen, nitrogen, fluorine, chlorine, silicon and sulfur.
To form macromolecular bonds from each thermoplastic material molecule is done in three ways, namely:
a. Polymerization:
Bringing together several similar molecules to form a large molecule Polymerisate. The monomers in polymers can be linked in repeating units that lengthen and enlarge the molecules by a chemical reaction known as a polymerization reaction. Despite the many variations, the two basic processes are Condentation polymer and Polyaddition.
b. Polycondensation;
The bonding of several molecules to form large macromolecules through the process of separating one of the atoms to bind small molecules from water. From this process, a material called Polycondensate is formed
c. Polyaddition;
Namely the union of several basic molecules through the placement of several molecules without separation of the non-fixed parts. The material formed is called "Polyadducf. In this reaction an initiator is added to break the two bonds between the carbon atoms and start the bonding process by adding more monomers to build the chain. For example the Ethylene monomer bonds to produce a polymer known as Polyethylene .
The sum of the molecular weights of the mer-mers in the polymer chain is the molecular weight of the polymer. The higher the molecular weight in a given polymer, the greater the chain length. Since polymerization is a random event, the resulting polymer chains are not all the same length, but the resulting chain lengths are formed in a traditional distribution curve. We determine and express the average molecular weight of a polymer on a statistical basis by averaging. The distribution of the molecular weight distribution is referred to as the molecular weight distribution (MWD). Molecular weight and MWD have a strong influence on polymer properties. For example, fracture and impact strength, resistance to cracking, and viscosity in the liquid state all increase with increasing molecular weight. Most of the polymers traded have molecular weights between 10,000 and 10,000,000.
In some cases, it is easier to describe the size of a polymer chain in Degrees of Polymerization (DP), defined as the ratio of the molecular weight of the polymer to the molecular weight of the repeating unit. For example, Polyvinyl chloride ( PVC) has a mer weight of 62.5, so the DP of PVC which has a molecular weight of 50,000 will be 50,000 / 62.5 = 800. In the polymerization process, the higher the DP, the greater the polymer viscosity, or flow resistance, thus making it easier to formation and overall costs.
During polymerization the monomers are bonded together in a covalent bond, forming a polymer chain. Because of their strength, covalent bonds are also called primary bonds. In addition, polymer chains hold on to secondary bonds such as van der Waals bonds, hydrogen bonds and ionic bonds. Secondary bonds are weaker than primary bonds. In a polymer, the increase in strength and viscosity with molecular weight is partly due to the fact that the longer the polymer chain, the greater the energy required for secondary bonding.
If the repeating units of the polymer chain are of the same type, we call the molecule a homopolymer. However, two or three types of heavy monomers can be combined to obtain special characteristic advantages, such as improved strength and durability. Copolymer consists of two types of polymers such as Styrene-butadine, widely used for car tires. Terpolymer consists of three types such as ABS used for helmets, telephones.
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