Thermodynamics of rubber elasticity

Abstract

The process of vulcanization, which links the polymer molecules into a three dimensional network, enables reversible thermodynamic measurements to be made on elastomeric materials from which the entropy and energy contributions to the free energy of deformation can be determined. The molecular processes responsible for rubber elasticity are most readily interpreted in terms of the constant volume coefficients (∂ U /∂ L ) V, T and (∂ S /∂ L ) V, T . Taken as a whole the results from thermodynamic studies on lightly cross-linked rubbery polymers support the view that the retractive force is primarily intramolecular in origin. The results also show that in general the retractive force contains a significant energetic contribution. The Gaussian theory of rubber elasticity as reformulated by Flory is able to account satisfactorily for the temperature dependence of the force, but it does not lead to an adequate representation of the pressure depen­dence of the force and hence of the strain induced dilation. Several pheno­menological equations which have been proposed for predicting the dilation behaviour for the case of simple extension are considered.