Strain-induced crystallization of natural rubber. III. Reexamination of axial-stress changes during oriented crystallization of natural rubber vulcanizates

Abstract

In order critically to evaluate the previous morphological interpretations of stress relaxation during oriented crystallization, a reexamination was made of the relaxation behavior of the same oriented natural rubber material used for the x-ray investigation in Part II of this series. Our results were found to be in qualitative agreement with previously published reports. Comparison of stress-relaxation rates at low temperatures for samples with strains of 200% or more with crystallization rates at room temperature by others for similar elongations indicates a dual nature in the strain-induced crystallization process. This conclusion is in full agreement with the indication of dual crystalline morphologies, namely, fibrillar and lamellar, for both electron microscopy and x-ray investigations on highly stretched samples reported in Parts I and II. Examination of stress-relaxation data in light of morphological evidence for oriented crystallization indicates that caution must be exercised in attaching morphological significance to the Avrami exponent n obtained from stress-relaxation or crystallization data.