Molecular orientation and structural development in vulcanized polyisoprene rubbers during uniaxial deformation by in situ synchrotron X-ray diffraction

Molecular orientation and strain-induced crystallization of vulcanized natural rubbers (by sulfur and peroxide) and synthetic polyisoprene rubber (by sulfur) during uniaxial deformation at 0 °C were studied by in situ synchrotron wide-angle X-ray diffraction. The high intensity of synchrotron X-rays and new image analysis methods made it possible to estimate the mass fractions of strain-induced crystals and amorphous chains in both oriented and unoriented states. Most of the polymer chains (∼75%) were found to be in the random coil state even at large strains (>5.0). Only about 5% the amorphous chains were oriented, whereas the rest of the chains (∼20%) were in the crystalline phase. Sulfur vulcanized and peroxide vulcanized natural rubbers did not exhibit notable differences in structure and property relationships. In contrast, synthetic polyisoprene rubber showed a different behavior of deformation-induced structural changes, which can be attributed to the difference in cross-link topology. Our results indicated that strain induces a network of microfibrillar crystals in both natural and synthetic polyisoprene rubbers due to the inhomogeneity of cross-link distribution that is responsible for their elastic properties.