New insights into structural development in natural rubber during uniaxial deformation by in situ synchrotron X-ray diffraction

Molecular orientation and strain-induced crystallization of vulcanized natural rubber during uniaxial deformation were studied via in situ synchrotron wide-angle X-ray diffraction (WAXD). The high intensity of synchrotron X-rays and new image analysis methods made it possible to estimate mass fractions of the strain-induced crystals and the amorphous chains in both oriented and unoriented states. Contrary to the conventional conception, it was found that, in highly stretched natural rubber, most chains remained unoriented in the amorphous phase; only a few percent of the amorphous chains were oriented and the rest of the chains were in the crystalline phase. This indicates that stress induces a network of microfibrillar crystals that is responsible for the elastic properties. The new information has prompted us to reconsider the relationships of molecular orientation, induced crystallization and mechanical behavior in natural rubber.