Thermodynamic analysis of polymer networks

A theoretical description of the changes of volume which accompany the application of stress to a rubber is developed taking into account the anisotropy of the material. Making use of the volume changes the deformational work and the change of internal energy at constant pressure and temperature are calculated. To verify the theoretical predictions stress–strain experiments were carried out in a dilatometer. Additionally the changes of enthalpy were measured in a deformation calorimeter in both extension and compression modes. In the case of butyl rubber the Rotational Isomeric State Theory developed by Flory (1961 ) and Mark (1973 ) predicts a negative contribution of the fraction of the force that is of energetic origin. On the other hand measurements on butyl rubber carried out in the deformation calorimeter yield a positive contribution on the condition that stress-induced crystallisation is prevented. At a temperature of 365 K the energetic contribution to the total force is about 50% at small strains decreasing to 20% at an elongation of 150%. Moreover independently of the sign of intrachain energy changes in filled rubbers it was found that the change of internal energy is always positive due to intermolecular interactions.