Comparative study on strain-induced crystallization behavior of peroxide cross-linked and sulfur cross-linked natural rubber

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 (alpha(c)) decreased with an increase of network chain density (nu) 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 nu was similar for both P-NR and S-NR. Calculated. entropy differences between the undeformed and the deformed states (Delta S-def) at alpha(c) were equal for P-NR regardless of nu, whereas it became smaller with the increase of nu for S-NR. The SIC behavior of P-NR is in agreement with the prediction or 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 nu 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.