In order to investigate the effect of relaxation to the stress-strain curves of natural rubber vulcanizates, stress relaxation was measured after the cessation of elongation with various strain rates. Stress relaxation curves obeyed Cotten's equation over a fairly wide time scale.A parameter of Cotten's equation (A) is taken as a measure of the rate of stress decay. It was shown that the relationship between the rate of stress decay and elongation ratio had three distinct regions with respect to elongation ratio, i. e., small, intermediate and large deformations. The ranges of the latter two regions were dependent on temperature, and these regions were indistinguishable at higher temperatures.Density, birefringence and X-ray diffraction measurements revealed that the region of the intermediate deformation was attributable to chain orientation and that the region of the large deformation was attributable to stress induced crystallization.
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