Rubber Elasticity

Rubber is one of the most curious natural materials of all. Its natural elasticity has been a subject of discussion since time immemorial. The logic behind the property of elasticity of rubber or rubber goods is that in a relaxed state, the rubber is attributed with a long, rolled up polymer chains which are interlinked at a couple of points. Now between each pair of links, every single monomer can rotate freely around its neighbor, thus making a way for each section of chain to amass a number of geometries similar to a loose rope attached to fixed points from either ends. At room temperature, rubber holds enough kinetic energy for each section of chain to move frantically, just the way a rope is being shaken violently.

Mr. Werner Kuhn was the one who developed the entropy model of rubber in 1934. During a stretched position, the loose pieces of rope become relatively stiff and thus are not able to oscillate much. Since its kinetic energy is lost as excess heat, the entropy decreases while transiting from relaxed to stretched mode and vice-versa. The change in entropy can be understood by the fact that a relaxed piece of rubber can be folded in many ways as compared to a stretched section of rubber.

Thus, relaxation of a stretched rubber band is actually based on the increased entropy and the transition of thermal energy into kinetic energy. As it is rubber relaxation is an endothermic state, this is precisely the reason why the force exerted by a stretched piece rubber increases with temperature. While during contraction the material experiences adiabatic cooling. This can be tested by stretching a piece of rubber band against your lip and feel the coolness of it.