- 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:
- These are linear polymers, with no cross-links.
- These are generally soluble in organic solvents.
- The intermolecular forces of attraction in thermoplastic polymers are intermediatery to those in elastomers and fibrous polymers.
- 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.
- 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:
||Linear polymer with no cross-linking.
||Low molecular mass, semi-fluid polymers
||Effect of heating and cooling
||Soften on heating and harden on cooling. Heating and cooling process can be carried out any number of times. No cross-links are developed on heating.
||On heating, the low molecular mass polymeric material gets further polymerized due to extensive cross-linking. On cooling, it becomes hard, and infusible. Once thermo-setted, these cannot be remelted/reshaped.
Classification of Polymers on the Basis of Molecular Forces