Explain viscoelasticity property of polymer.

Viscoelasticity is the property of polymers by which they show both elastic (solid-like) and viscous (liquid-like) behaviour when deformed. The response depends on time, temperature and rate of loading.

• An ideal elastic solid obeys Hooke's law: it deforms instantly and recovers fully when the load is removed (σ = Eε).
• An ideal viscous liquid flows: its deformation increases with time and is not recovered (σ = η dε/dt).
• In a polymer, a stress causes an instant elastic strain (bond stretching), then a delayed elastic strain (uncoiling of chains) and then viscous flow (chains slipping past each other). Only part of the strain is recovered when the load is removed.

Effects of viscoelasticity:

• Creep – under constant stress, strain increases slowly with time.
• Stress relaxation – under constant strain, stress decreases with time.
• Hysteresis – energy is lost in each loading–unloading cycle, as in rubber damping.

It is described by spring and dashpot models (Maxwell and Kelvin–Voigt). The behaviour is most marked near T_g. Examples: rubber, PE, nylon.