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.

No abstract

Vulcanizates of natural rubber (NR) and its synthetic analogue (IR) were quickly stretched to 6 times the original length. The post stretch relaxation of tensile stress and the development of strain-induced crystallization (SIC) were studied by simultaneous measurements of the stress and the diffraction intensities using the synchrotron X-ray source. In the range of 8 s, NR crystallized much faster than IR. Accordingly, the origin of the superior toughness of NR was thought to come from the ability of rapid SIC. Time constants of the poststretch crystallization were estimated from the X-ray study. Then the crystallization time constants were used to decompose the contribution of SIC from the total magnitude of the post-stretch relaxation. The contribution of SIC was dominant for the total magnitude of the post-stretch relaxation during several seconds.

No abstract

Relatively fast kinetics of strain-induced crystallization ͑SIC͒ of cross-linked samples with various network-chain densities ͑͒ of natural rubber and its synthetic analog was examined by the fast time-resolved wide angle x-ray diffraction and simultaneous tensile measurements. The lateral crystallite size was almost unchanged with elapsed time, though the crystallization proceeded considerably during the period. The rate of SIC was faster for the samples having the higher during the first tens of seconds. While the development of SIC obviously depends on , progress of relative stress relaxation with time was almost independent of . The different dependence of the experimental results on was explained by assuming coexistence of stretched and relaxed network chains. During SIC at a fixed strain ratio, the intensity of crystalline reflections increased without reducing the intensity of anisotropic amorphous halo on the equator. Accordingly, rather relaxed chains that had shown the off-equatorial scattering were thought to be consumed for the crystal growth.