Strain-induced crystallization of natural rubber samples with various network-chain densities, ν, was investigated by synchrotron X-ray diffraction measurements. It was found that the onset strain of crystallization was almost independent of ν. Lateral crystallite size and degree of orientational fluctuations of crystallites were also evaluated. These results indicated that stretched molecular chains acted as nuclei while surrounding chains could also contribute to the crystal growth. Deformation of crystal lattice with nominal stress was detected, and the strain-induced crystallites were found to be responsible for the increased modulus upon elongation. The unit cell volume decreased almost linearly with nominal stress. By assuming the deformation mechanism of the rubber network as a pantograph, the reinforcement effect of the crystallites is thought to be brought out not directly by crystallites connected in series but indirectly through the surrounding network chains.

Structure evolution during deformation of unfilled natural rubber ͑NR͒ vulcanizate and filled ones with carbon black or calcium carbonate was investigated by the synchrotron x-ray diffraction. The crystallization onset strain, ␣ 0 , was found to decrease by the inclusion of the filler. However, corrected ␣ 0 values into the effective strain ratio of deformable rubber portion were almost constant between filled and unfilled samples. Accordingly, our model of strain-induced crystallization of unfilled NR vulcanizates, assuming that melting temperature is independent of network-chain length ͑n͒, was applied to the filled samples. The discrepancy between classical theories and experimental results was thought to come from the distribution of n. By the inclusion of filler, the lateral crystallite size was decreased but the orientational fluctuation increased. The lattice of the strain-induced crystallites changed almost linearly with the nominal stress. In addition, the degree of lattice deformation decreased with the filler content, especially in the carbon black-filled system. All these experimental results are consistent with the proposed model.

Strain-induced crystallization of natural rubber (NR) and synthetic isoprene rubber (IR) with various crosslinking densities was investigated by wide angle X-ray diffraction using a synchrotron radiation and simultaneous tensile measurements. The elongation ratio at the onset of crystallization (α c ) was almost independent of crosslinking density. IR samples showed larger α c values than NR because of the lower stereoregularity of IR. These results suggest that the onset of crystallization is determined by increased melting temperature by strain due to an entropic reason. The amount of oriented amorphous component changed approximately linearly with strain, and was a little larger in IR than in NR when compared at the same elongation ratio. At small strain (and stress), crystallinity in IR was lower than in NR. These results indicate that, at small strain region, the more stress is assigned to oriented amorphous in IR than in NR. * Corresponding

Strain-induced crystallization (SIC) behavior of natural rubber (NR) cross-linked by peroxide or sulfur was comparatively studied by time-resolved wide-angle X-ray diffraction measurements at SPring-8. Stretching ratio at the onset of SIC (αc) decreased with an increase of network chain density (ν) for peroxide cross-linked NR (P-NR), while it remained constant for sulfur cross-linked NR (S-NR). But, dependence of relative crystallization rates on ν was similar for both P-NR and S-NR. Calculated entropy differences between the undeformed and the deformed states (ΔS def) at αc were equal for P-NR regardless of ν, whereas it became smaller with the increase of ν for S-NR. The SIC behavior of P-NR is in agreement with the prediction on homogeneous or uniform networks by Flory. Thus, the network structure of S-NR is supposed to be less homogeneous than that of P-NR. The inhomogeneity in S-NR is estimated due to the presence of domains of high ν value embedded in the rubbery network matrix, which is supported by the stress dependences of apparent lateral crystallite size. The mechanical characteristics of S-NR and P-NR are also discussed from the viewpoint of their SIC behaviors on the basis of the network structures.