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.
