Strain-induced crystallization of natural rubber as detected real-time by wide-angle X-ray diffraction technique

In the past, the stress-strain behavior of natural rubber has been studied by a variety of mechanical analytical techniques in order to understand the role of induced crystallization by deformation. But, this was generally do ne in a sequential manner by stretching to a certain extent and examining t he stretched part separately. This type of sequential experiment may not fo llow the exact dynamics of the process and thus may lead to erroneous concl usions regarding the evolution of microscopic mechanisms. It is therefore a dvantageous to measure the mechanical response simultaneously with the stru ctural response to map the structural evolution onto the stress-strain beha vior. In this paper, simultaneous measurements of the stress-strain behavio r and X-ray scattering intensity of vulcanized natural rubber were made con tinuously by a specially designed instrument during elongation and subseque nt retraction. These studies revealed three new characteristics of the stra in-induced crystallization. 1. The strain-induced crystallization starts at around 400% strain at 25 degrees C, then increases steeply, although seque ntial measurements reported that natural rubber starts strain-induced cryst allization at 200%. 2. At the onset of retracting, the degree of crystallin ity continues to increase, although a drastic decrease of stress occurs. We suggest the morphology of the strain-induced crystallites changes such tha t do not contribute to stress. 3. Strain-induced crystallites disappear at the strain where the stress-strain loop is closed during retracting. The hy steresis of the stress-strain curve might be attributed to strain-induced c rystallization. (C) 2000 Elsevier Science Ltd. All rights reserved.