Classification of Polymers on the Basis of Molecular Forces

• The practical utility of polymers depends upon their mechanical properties such as, tensile strength, elasticity, toughness, etc.
• These mechanical properties depend upon the nature and strength of forces acting between the polymeric chains.
• As a result, therefore, the polymers may be classified into various categories on the basis of strength and nature of intermolecular forces operating them.
• The strength of intermolecular forces which hold the polymeric chains in polymers follows the order

Fibrous polymers        >                               thermoplastics                  >              elastomers                                      (strong molecular forces)                  (intermediatery between)      (weakest intermolecular forces

A) Elastomers (rubber like)

• The polymer chains are held together by the weakest intermolecular forces in elastomers.
• Elastomers become soft on heating, and can be moulded into any desired shape. Natural rubber is an elastomer.

• The weak intermolecular forces permit the polymer to be stretched by an application of small force.
• During processing, however, a few cross-links may be introduced in between the chains.
• These cross-links between the chains of elastomer help it to come back to its original position after the force is removed.

B) Synthetic fibres (or, fibrous polymers)

• These polymers have long linear chains, with very strong intermolecular forces between the chains.
• The forces which hold the chains together are strong intermolecular forces like, hydrogen bonding or dipole-dipole interactions.
• Due to these strong intermolecular forces, the polymeric chains in fibrous polymers are closely packed.
• The close packing in fibrous polymers imparts the following characteristics to such polymers.

a)Crystalline b) sharp melting points   c) high tensile strength    d) high modulus

• Polyamides such as, nylon-66 show high tensile strength, high modulus and high melting points etc., due to hydrogen bonding between the polymeric chains.

C) Thermoplastic polymers

• These show the following characteristics:

1. These are linear polymers, with no cross-links.
2. These are generally soluble in organic solvents.
3. The intermolecular forces of attraction in thermoplastic polymers are intermediatery to those in elastomers and fibrous polymers.
4. Because of the not so strong intermolecular forces, these polymers soften on heating and harden on cooling. Heating does not cause any cross-linking in these polymers. Therefore, thermoplastics can be easily moulded by heating. The process of heat-softening, moulding, and cooling can be repeated as often as desired without affecting the properties of the polymer.
5. To enhance the workability of thermoplastics at relatively lower temperatures, certain compounds called plasticizers are added during processing. Tricresyl phosphate, dioctylphthalate etc., are good plasticizers.

Examples: typical thermoplastics are polypropylene (PP), polyethene (PE), polyvinyl chloride (PVC), and Perspex, etc.

D) Thermosetting polymers

• These polymers are normally made by heating relatively low molecular mass semi-fluid polymers.
• When heated in a mould, thermosetting polymers become infusible and form an insoluble hard mass.
• As a result, once moulded, thermosetting plastics cannot be re-melted, or reshaped by heating.
• Thus, thermosetting polymers can be processed only once.

When low molecular mass semi-fluid polymers are heated extensive cross-linking between different polymer chains takes place. Formation of these cross-links leads to a three-dimensional network of bonds connecting the polymer chains (e.g., in bakelite). The three-dimensional network structure does not soften on heating. Thus, thermosetting polymers cannot be reprocessed.

• Examples: Phenol-formaldehyde resins (bakelite) and melamine-formaldehyde resins are typical thermosetting polymers.

Thermosetting vs Thermoplastic Polymers

Thermosetting and thermoplastic polymers differ in certain characteristics. Some major differences between these two types of polymers are as follows:

References:

i) https://byjus.com/jee/polymers/

ii) https://selfstudypoint.in/classification-of-polymers/

iii) https://www.kharagpurcollege.ac.in/studyMaterial/43618Lecture-Note-1-Polymer-Chemistry-6th-Sem-SEC-4T-Chem-Hons.pdf

Classification of Polymers on the Basis of Molecular Forces