Dependence of elastic properties of vulcanized rubber on the degree of cross linking

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Abstract
Natural rubber and GR‐S synthetic rubber have been vulcanized to varying degrees of cross linking through the use of disazodicarboxylates. These compounds react readily with the above unsaturated polymers in a manner which assures the formation of one cross linkage for each molecule of the disazo compound. Elastic properties of the vulcanizates so produced are similar to those of rubber specimens vulcanized in the conventional manner with sulfur and an accelerator. Their stress‐strain curves deviate from the form prescribed by the statistical theory of rubber elasticity in the same manner as observed by Gee for sulfur vulcanizates. The equilibrium force of retraction at 100% elongation has been measured over a 30‐fold range in the degree of cross linking; equilibrium swelling measurements extend over an 80‐fold range. The magnitude of the force of retraction over most of the range covered agrees remarkably well with the explicit predictions of the statistical theory. The significance of the deviations which are observed is discussed.