Thermoelasticity of Rubbers

Gough in 1805 found that the temperature of unvulcanized natural rubber sample increases during quick stretching. Also, it contracts when it is heated under a constant load. Joule confirmed this on vulcanized rubber about fifty years later. Now this phenomenon is known as Gough-Joule Effect. It is observed in the case of certain synthetic rubbers also. This is in contrast to the behavior of most of the solids which expand on heating.

Thermoelastic Experiment

The thermoelastic experiment is used to explain the molecular origin of elastic force. When a metal is stretched its internal energy is increased due to the vibration of the constituent particles. When the applied forces are released, retraction occurs due to the decrease in energy. Hence the elasticity is called energetically derived elasticity or energy elasticity. When metal under constant loaded is heated , expansion occurs because of the increased oscillation of constituent particles.

In the case of rubbers, the entropy is decreased during stretching because of the orientation of molecular chains. When the deforming forces are released, retraction occurs because of the tendency of the system to increase the entropy towards the maximum value it had in the undeformed state. Hence the elasticity is known as entropically derived elasticity or entropy elasticity. On heating the molecular motion is increased and thus system becomes more random. This causes the rubber piece to shrink under constant force at high temperature.