Classification of polymers on the basis of intermolecular forces

A polymer's mechanical behaviour -- how stretchy, tough or rigid it is -- is governed mainly by the strength of the intermolecular forces between its chains, giving four classes. Elastomers are polymers with elastic properties: weak van der Waals forces between chains let the material stretch readily under an external force, while a few crosslinks between the chains pull the stretched material back to its original shape once the force is released; vulcanized rubber, buna-S, buna-N and neoprene are examples. Fibres are polymeric solids that can be drawn into threads; strong intermolecular forces such as hydrogen bonding and strong dipole-dipole attraction give them high tensile strength (resistance to breaking under tension) and make them crystalline in nature -- polyamides (nylon 6,6) and polyesters (terylene) are examples. Thermoplastic polymers can be repeatedly softened by heating and hardened again on cooling (a property called plasticity); their intermolecular forces are moderately strong, intermediate between those of elastomers and fibres -- polythene, polystyrene and polyvinyls are examples. Thermosetting polymers are rigid: while being formed they can be shaped by heating, but once they harden while still hot (thro …