Question

How are polymers classified into different categories on the basis of intermolecular forces? Give an example of a polymer of each of these categories.

Classify polymers on the basis of intermolecular forces. Mention the characteristics of each type.

Answer 1

1. Elastomers: The polymers in which the polymeric chains are held together by the weakest intermolecular forces are termed as elastomers. The elastomers can be stretched due to the presence of extremely weak intermolecular forces. These polymers are composed of randomly coiled polymer chains. Upon the application of force, the randomly coiled chains stretch, resulting in the elongation of the polymer. To facilitate the polymer’s retraction to its original position upon the removal of the applied force, many cross-links are often incorporated between the polymer chains.
   Example: Buna-S, Vulcanized rubber
2. Fibres: The polymers consisting of strong intermolecular forces between the polymeric chains are called fibres. The forces that hold the polymeric chains in fibres are strong intermolecular forces, including hydrogen bonding and dipole-dipole interactions. The presence of strong intermolecular forces produces fibres with increased tensile strength and high modulus. The strong intermolecular interactions facilitate the dense arrangement of polymeric chains within them. This imparts a crystalline quality upon them. This explains why fibres have different melting points.
   Example: Polyesters, Nylon-66, polyacrylonitriles
3. Thermoplastic polymers: The polymers in which the intermolecular forces of attraction are intermediate to those of elastomers and fibres are called thermoplastics. Thermoplastics are linear polymers characterised by the absence of crosslinking among polymer chains. Thermoplastics soften upon heating due to the presence of weak intermolecular forces of attraction. At normal temperature, they are relatively rigid, but upon heating, they transition to a soft and viscous state. However, upon cooling, they harden once again. Heating does not induce cross-linking, allowing for easy moulding with heat application. The operations of thermal softening, moulding, and cooling can be repeated indefinitely without compromising the mechanical properties of the polymer.
   Example: Polythene, Polystyrene, polypropylene, polyvinyl chloride (PVC), Teflon
4. Thermosetting polymers: The polymers which on heating change irreversibly into hard, rigid and infusible materials are called thermosetting polymers. Usually, semi-fluid polymers with a relatively low molecular mass are heated in a mould to make thermosetting polymers. When heated, they stop dissolving and turn into a hard mass that can't be broken up. When heated, the material gets harder because many cross-links form between the different polymeric bands. Because of this, a three-dimensional network of links forms between the polymer chains.
   Example: Phenol-formaldehyde (bakelite) resins, melamine-formaldehyde resins, urea-formaldehyde resins, etc.