Polythene

Polythene is a polymer that does not obey Hooke's law. Thin strips are very easy to stretch and undergo plastic deformation with only a small force.The force-extension graph has only a short straight section. Beyond the low limit of proportionality the curve flattens, and the strip keeps extending with little extra force as the long polymer chains slide past each other and straighten out.When the load is removed, the unloading line does not return to the origin. The extension during unloading is greater than during loading for any given force, and the strip is left permanently longer than it was.The extension that remains at zero force is the plastic deformation. The area enclosed between the loading and unloading curves is the energy dissipated inside the material.