Polymers - science lessons for life


Sunday, July 1, 2018


Pay your attention to the following pictures.
Let us inquire into the chemical nature of the materials shown in the above pictures that we use frequently in our daily life.
At molecular level, they all have a common special feature. That is, they all are made up of large molecules arranged in the form of long chains. Another speciality is that most of those long chain molecules are composed of repeating small molecular units. Thus, the molecules from which they are made are called polymers. In this lesson, let us discuss about polymers.
Large molecules formed by the joining of a large number of small molecules with one another are known as polymers.
The process of forming polymers is called polymerization. The small molecules forming polymers are known as monomers and the large molecules formed by the polymerization of monomers are referred to as polymers. Pay your attention to the chain formed by joining some paper clips together.
The single paper clips used to form the above chain are analogous to monomers and the chain of clips is equivalent to the polymer. The basic structural units contained in the chain after the formation of the polymer are referred to as repeating units. The molecular mass of monomers is relatively low. However, the relative molecular mass of polymers formed by the polymerization of a large numbers of monomers has a very high value. 
Now, let us investigate into some common polymers.
Polythene (Polyethene)
Consider the ethene molecule we learnt earlier.
Polythene is produced by the polymerization of ethene molecules. What happens here? Of the double bond, one bond breaks and thousands of ethene molecules are added together as shown below.
The above polymerization process may be summarized and indicated as follows.
This means that 'n' number of ethene molecules link with one another and create a polythene molecule with 'n' number of – CH2 – CH2 – repeating units.
Hence, it may be clear to you that polythene is a macromolecule formed by the linking of a large number of ethene molecules in a specific pattern. The polymer, monomer and the repeating unit of polythene are given below.
Polymers - Very large molecules formed from linking together, a large number of small molecules are named polymers. 
Monomers - Small molecules contributing to form polymers are called monomers. 
Repeating unit - The basic structural unit contained in a polymer are known as repeating units.

Polychloroethene (Polyvinyl chloride) 
Polymerization of chloroethene gives polychloroethene. This can be summarized as follows.
Try to identify the monomer, repeating unit and the polymer of polychloroethene.

Polytetrafluoroethene (Teflon)
Polytetrafluoroethene is formed by the polymerization of tetrafluroethene. This can be illustrated as follows.
Identify the monomer, repeating unit and the polymer of tetrafluoroethene.

Classification of polymers based on origin
Recall the polymers you studied earlier. All of them are artificially synthesized polymers. Have you heard about natural polymers Pay attention to the bio - molecules you learnt in grade 10. Molecules like proteins, starch, cellulose and DNA are polymers. They belong to the category of natural polymers. Therefore, by origin, polymers can be classified into two types as natural and artificial. Rubber which is frequently used for technical purposes is also a natural polymer. below Table  lists some examples of natural and artificial polymers.

Rubber is a natural polymer formed by the polymerization of a monomer called isoprene. The structure of an isoprene molecule is given below.
The process leading to the formation of the polymer can be illustrated as follows.
Classification of polymers based on structure
All the polymers so far discussed are made of linear chains. However, all polymers are structurally not linear polymers. By joining polymer molecules laterally to the principle chain of the linear polymers of the type described above, branched polymers are produced. The polymers in which the linear chains are cross-linked are known as cross-linked polymers. Therefore, the polymers can be classified as follows according to the structure.

Have you heard about vulcanized rubber? Because of the elastic property of rubber, it is difficult to be used in some applications. By vulcanization, rubber can be made harder while decreasing its elasticity. During vulcanization natural rubber is reacted with sulphur. Then, cross links are formed among the linear chains of rubber through sulphur.
Vulcanized rubber is used to make tyres, tubes and battery cases.
Importance of polymers 
When taking lunch away from home, natural materials such as a banana leaf or a "Kolapatha" was used for wrapping in the past. But today, what we use for this purpose is a type of polythene which is a synthetic polymer. Like this, at present, artificial polymers are being used as substitutes for materials. Properties such as the ability to synthesize with required characteristics, ease of usage, ability to produce in various shapes, ability to make colourful with any required colour and cheapness have made the items produced with polymers popular. 
Most of the artificial polymers are not subjected to biodegradation. That is, they do not decay through biological process. Therefore they get collected in the environment. This is a big environmental problem. Since the combustion of artificial polymers release poisonous gases, burning is not suitable. Chemists are making attempts to find a solution for this problem by producing degradable polymers. At present, production of biodegradable and photodegradable polymers and water-soluble polymers are underway. 
The clothes produced with artificial polymers such as nylon, terylene and polyester do not absorb sweat and cause discomfort to the body. This can be minimized by mixing natural polymers such as cotton and wool with artificial polymers.

No comments:

Post a Comment