On the basis of intermolecular forces of attraction," polymers, are classified as elastomers, fibres, thermoplastics and thermosetting polymers. Elastomers have the weakest while fibres have the strongest intermolecular forces of attraction. Polymers which can be melted and reshaped time and again without any change in chemical composition and mechanical strength are called thermoplastics. In contrast, those polymers which can be heated only once when they undergo a permanent change in chemical composition to give a hard, infusible and insoluble mass, are called thermosetting polymers.