It is a matter of importance to separate the elastic force into entropic and energetic
contributions. A stress acting on the rubber network will stretch out and orient the
chains between the crosslink joints. This will decrease the entropy of the chains and
hence give rise to an entropic force. The change in chain conformation is expected to
change the intramolecular internal energy. The packing of the chains may also change
affecting the intermolecular-related internal energy. Both the intra- and
intermolecular potentials contribute to the energetic force. The following
thermodynamic treatments yield expressions differentiating between the entropic and
energetic contributions to the elastic force