Comparison of Stress-Strain Relations of Polybutadiene Vulcanizates in Compression and in Extension

According to elementary statistical theory of rubber elasticity, the modulus of rigidity τ/(λ - λ−2) is constant. The phenomenological theory predicts that this modulus is in first approximation equal to the sum of a constant (2C1) and a deformation-dependent term, 2C2. The dependence on elongation of experimental modulus data of many elastomers agrees more or less with the C2 term. Several attempts have been made to explain this dependence on the basis of molecular phenomena or by refinements of the statistical theory of elasticity. At present there is no consensus of opinion on the origin or interpretation of the deformation-dependent term. Almost all of the data on this dependence have been generated in extension experiments.