Classification of Polymers: A polymer is a natural or synthetic substance made up of large molecules known as macromolecules which are a combination of many subunits called monomers. Naturally, polymers are found all around us; from DNA strands to minerals such as diamonds and quartz and man-made materials like concrete, glass, paper and plastics.

Polymers have various industrial applications and are used in making multiple consumer products. They are used to make plastic bottles, rubber tips of headphones, plastic carry bags, etc. Many proteins in the human body are made of polymers. In this article, we will learn more about polymers, classification of polymers based on structure, occurrence, molecular forces and thermal behaviour and other major uses.

Polymers

The term polymer refers to a large molecule made up of many small constituent units of an identical structure known as monomers. It can occur naturally or be created artificially in a lab. The number of monomer units constituting a polymer may vary in different substances. Polymers with a very large number of monomer units are called high polymers.

Classification of Polymers

Polymers can be classed in various ways due to their large number, diverse behaviour, and ability to be found naturally or synthetically. However, the following are some basic ways in which polymers are classified:

Classification of Polymers Based on Occurrence

Based on source or occurrence, the polymers are classified into two types;

1. Natural Polymers

Polymers that are obtained from animals and plants are known as natural polymers. A few examples of natural polymers are:

a) Polysaccharides: Cellulose and starch are very common examples of polysaccharides. They are the polymers of glucose. The monomer of polysaccharides is a monosaccharide.
b) Proteins: These are the polymers of α-amino acids. They are building blocks of animal cells. They constitute an indispensable part of our food. Wool, natural silk, etc., are proteins.
c) Nucleic acids: These are the polymers of various nucleotides. RNA and DNA are common examples.
d) Natural rubber: The substance obtained from latex is a polymer of isoprene.

2. Synthetic Polymers

The polymer prepared in the laboratory is referred to as synthetic polymers or man-made polymers. Examples are; PVC, polyethylene, polystyrene, synthetic rubber, nylon-\(6\), nylon-\(66\). Etc

3. Semi-Synthetic Polymers

These polymers are mostly derived from naturally occurring polymers by carrying out a chemical modification. For example, Cellulose diacetate is prepared by reacting cellulose and acetic anhydride in the presence of sulphuric acid.

\({\rm{Cellulose}} + {\left( {{\rm{C}}{{\rm{H}}_3}{\rm{CO}}} \right)_2}\mathop \to \limits^{{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}} {\rm{Cellulos ediacetate}}\)

Classification of Polymers Based on Structure

This classification is based on how the monomeric units are linked together. Based on their structure, the polymers are classified as;

1. Linear Polymer

Linear polymers are polymers in which monomeric units are linked together to form long straight chains. The chains are very well ordered in relation to one another, and The structure is densely packed. As a result, linear polymers have high densities, melting points, and tensile strengths. Polyethylene, nylon, and polyester are a few examples.

2. Branch Chain Polymers

Branch chain polymers have a branched-chain of monomeric units. These polymers do not pack well due to the presence of branches. As a result, when compared to a linear polymer, branched-chain polymers have lower melting points, densities, and tensile strength. Both addition and condensation can form branch chain polymers. Examples are amylopectin, glycogen, low-density polyethene and all vulcanised rubbers.

3. Cross-Linked Polymers

In these polymers, the initially formed linear polymeric chains are joined together to form a three-dimensional network structure. These polymers are hard, rigid, and brittle. Cross-linked polymers are always condensation polymers. Resins are an example of cross-linked polymers.

Classification of Polymers Based on Molecular Forces

The mechanical properties of polymers, such as tensile strength, toughness, elasticity, and so on, are determined by intermolecular forces in the macromolecules, such as Van der Waal forces and hydrogen bonds. Polymers are classified into four types based on the magnitude of their intermolecular forces.

1. Elastomers

An elastomer is a plastic that stretches and then reverts to its original shape. It is an amorphous polymer that is randomly oriented. The structure of elastomers has cross-linking to prevent the chains from slipping over one another. The polymeric chains have very weak Van der Waal forces. For example, vulcanised rubber.

2. Fibres

Fibres are linear polymers with hydrogen bonds or dipole-dipole attraction holding the individual chains of polymer together. The polymeric chains in the fibres are highly ordered with respect to one another. Fibres have the highest tensile strength and negligible elasticity due to strong intermolecular forces of attraction and highly ordered geometry. They are crystalline in character and have a high melting point and low solubility. Cellulose, nylon, wool, silk, etc., are some examples of fibres.

3. Thermoplastic Polymers

Thermoplastic polymers have both ordered crystalline regions and amorphous, non-crystalline regions. They are hard at room temperature, but when heated, the individual chains can slip -past one another, and the polymer becomes soft and viscous. This soft and viscous material becomes rigid on cooling.

The heating, softening, and cooling process can be repeated as many times as desired without any change in the plastic’s chemical composition and mechanical properties. As a result, these plastics can be moulded into toys, buckets, telephone and television cases. Some common examples are; Polyethyene polypropylene, Teflon, and polyvinyl chloride.

4. Thermosetting Polymers

Polymers that become hard on heating are called thermosetting polymers. Once heated, the thermosetting polymers turn into a solid, which cannot be remelted by heating. Thermosetting polymers are cross-linked polymers. The greater the degree of cross-linking that exists, the more rigid the polymer.

Cross-linking reduces the mobility of the polymer chains, causing them to be relatively brittle. The hardening on heating is due to the extensive cross-linking between different polymer chains to give a three-dimensional network solid. Examples of thermosetting polymers are phenol-formaldehyde resin, urea-formaldehyde resin, alkyd resin, melamine-formaldehyde resin.

Classification of Polymers Based on Thermal Behaviour

Based on the thermal behaviour, polymers are classified into two groups:

a. Thermoplastic Polymers: The polymer that softens on heating and can be converted into any shape on cooling is called thermoplastics. The process of heating, reshaping, and retaining the shape on cooling can be repeated several times without affecting their properties much. Examples are Polycarbonate, Polytetrafluoroethylene, etc.

b. Thermosetting Polymers: Thermosetting polymers undergo chemical changes and cross-linking when heated and become permanently hard and infusible when cooled. They do not soften on reheating; instead, they undergo degradation. Examples are phenol-formaldehyde, urea-formaldehyde, etc.

Classification of Conducting Polymers

Organic polymers that conduct electricity are known as conductive polymers. These compounds can be metallic conductors or semiconductors. The most significant advantage of conductive polymers is their processability, which is mostly achieved through dispersion.

Conductive polymers are not thermoplastics, which means they are not thermoformable. They are, however, organic compounds similar to insulating polymers. Conducting polymers are classified as follows:

1. Intrinsically Conducting Polymers

It is a polymer with delocalised electron pairs (conjugated \({\rm{\pi }}\)–bonds) or residual charges in the structural frame. Delocalised electrons are activated and moved through the polymer material in the presence of an electric field. Because of overlapping orbital conjugated \({\rm{\pi }}\)– bonds, valence and conduction bands are generated.

2. Doped Conducting Polymers

The polymers are either oxidised or reduced during the doping process, resulting in a resonant charge in the polymer chain. Doped conducting polymers are further classified as:

a. P-doping: It is done by oxidising chemicals such as iodine.

b. N—doping: Treating intrinsically conducting polymers with Lewis base causes a reduction process, which is called \(N—\)doping.  Sodium metal, \({\rm{FeC}}{{\rm{l}}_3}\) and other \(N-\) dopants are commonly employed in this process.

3. Extrinsically Conducting Polymers

The presence of externally added chemicals accounts for the conductivity of these polymers.
They can also be divided into the following subcategories:

a. Conductive Polymer: A conductive polymer is a material that contains conducting components such as carbon black or metallic fibres.
b. Blended Polymer: Blended polymers are obtained by mixing a metal storm with a polydentate ligand and blending ordinary polymers.

Summary

In this article, we discussed various types of polymers and their uses in daily life. We classified them based on their large number, availability, thermal conductivity, molecular forces, and behaviour. You will know that the most common way of classifying polymers is to separate them into groups called thermoplastics, thermosets, and elastomers.

Also, know that the Polymerization reactions classified on the basis of using cations with the reacting monomers are called Cationic Polymerization.

FAQs

Q.1: Classify the polymers based on the source of origin?
Ans: Based on the origin, polymers are classified into natural polymers, synthetic polymers and semi-synthetic polymers.

Q.2: What does it mean if a polymer is classified as synthetic?
Ans: The polymer prepared in the laboratory is referred to as synthetic polymers or man-made polymers.

Q.3: What are polymers? How are they classified? Give examples.
Ans: The term polymer refers to a large molecule made up of many small constituent units. These small molecular units of which the polymer is formed are called monomers.
Polymers can be classed in various ways due to their large number, diverse behaviours, and ability to be found naturally or synthetically. The following are some basic ways in which polymers are classified:
i. Based on occurrence, polymers are classified into natural polymers (ex; polysaccharide, nucleic acid, silk, etc.), synthetic polymers (Ex; PVC, polyethylene, nylon-6, nylon-66), and semi-synthetic polymers (Ex; Cellulose diacetate).
ii. Based on the structure, polymers are classified into a linear polymer (Ex, polyethylene, nylon, and polyester), branch chain polymer (Ex, amylopectin, glycogen, low-density polyethylene), and cross-linked polymers (Ex; Resins).
iii. Based on the molecular forces, polymers are classified into elastomers (Ex; vulcanised rubber), fibres (Ex; Cellulose, nylon, wool, silk), thermoplastics (Ex; Polyethyene polypropylene, Teflon, polyvinyl chloride), and thermosetting plastics (Ex; phenol-formaldehyde resin, urea-formaldehyde resin, alkyd resin).
iv. Based on the thermal behaviour, polymers are classified into thermoplastic polymers (Ex, PE, Polycarbonate, Polytetrafluoroethylene) and thermosetting polymers (Ex, phenol-formaldehyde, urea-formaldehyde).

Q.4: What are polymers? Classify the polymers on a different basis?
Ans: Polymers can be classed in various ways due to their large number, diverse behaviours, and ability to be found naturally or synthetically. The following are some basic ways in which polymers are classified:
1. Based on occurrence, polymers are classified into natural polymers, synthetic polymers, and semi-synthetic polymers.
2. Based on the structure, polymers are classified into linear polymers, branch chain polymers and cross-linked polymers.
3. Based on the molecular forces, polymers are classified into elastomers, fibres, thermoplastics and thermosetting plastics.
4. Based on the thermal behaviour, polymers are classified into thermoplastic polymers and thermosetting polymers.

Q.5: Explain the classification of polymers.
Ans: Polymers can be classed in various ways due to their large number, diverse behaviours, and vast applications. The following are some basic ways in which polymers are classified:
i. Based on occurrence, polymers are classified into natural polymers, synthetic polymers and semi-synthetic polymers.
ii. Based on the structure, polymers are classified into linear polymers, branch chain polymers and cross-linked polymers.
iii. Based on the molecular forces, polymers are classified into elastomers, fibres, thermoplastics and thermosetting plastics.
iv. Based on the thermal behaviour, polymers are classified into thermoplastic polymers and thermosetting polymers.

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