Determination of degree of crosslinking in natural rubber vulcanizates. Part IV. Stress‐strain behavior at large extensions

Abstract Departures from behavior predicted by the statistical theory of rubberlike elasticity, which occur at high extensions, are ascribed to the finite extensibility of network chains. By use of a non‐Gaussian statistical theory, a relation is obtained between the extension ratio λ * , the extension at which departures are small but significant and the degree of crosslinking. Experimental results on the dependence of λ * on the degree of crosslinking and on the degree of swelling are in good agreement with theoretical predictions. It is shown that the additional determination of λ * permits the calculation of both the degree of crosslinking and the extent of network flaws due to chain ends, from a single simple extension stress‐strain curve. The calculation of these two basic parameters has hitherto been unattainable from measurements on vulcanized rubbers alone. This development provides a new technique of particular value in studies of the degradation of vulcanized networks.