Comparative Study on Strain-Induced Crystallization Behavior of Peroxide Cross-Linked and Sulfur Cross-Linked Natural Rubber

• Published in: Macromolecules Vol. 41; no. 15; pp. 5876 - 5884
• Main Authors: Ikeda, Yuko, Yasuda, Yoritaka, Hijikata, Kensuke, Tosaka, Masatoshi, Kohjiya, Shinzo
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 12.08.2008
• Subjects: Applied sciences; Exact sciences and technology; Natural polymers; Natural rubber; Physicochemistry of polymers; Vulcanization; Vulcanizate; Mechanical properties; Natural rubber; Tensile property; Experimental study; Sulfur; Isothermal crystallization; Comparative study; Organic peroxide; Elongation (mechanics)
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma800144u

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.