Comparison between theoretical and experimental values of the volume changes accompanying rubber extension

The molecular theory of rubber elasticity assumes the free energy to consist of two parts: a liquidlike free energy that is governed by intermolecular interactions and is independent of strain at constant volume and an intramolecular interaction free energy equal to the sum of the free energies of the chains making up the network. The volume increases of rubber samples as a function of their length were found to be considerably larger than predicted by the molecular theory. Therefore, contrary to common belief, the values of (∂ E /∂ L ) V , T might not be related solely to changes in intramolecular interactions with extension. Also, the usual procedure to obtain values of (∂ E /∂ L ) V , T from measurements of (∂ f /∂ T ) p , L with the aid of the molecular theory is not correct.