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

Abstract

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 (v) 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 v was similar for both P-NR and S-NR. Calculated entropy differences between the undeformed and the deformed states (ΔSdef) at αcwere equal for P-NR regardless of v, whereas it became smaller with the increase of v 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 v 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. © 2008 American Chemical Society.