Polymer Communication Strain-induced crystallization of peroxide-crosslinked natural rubber Yuko Ikeda a, * , Yoritaka Yasuda a , Shinya Makino b , Shinya Yamamoto b , Masatoshi Tosaka c , Kazunobu Senoo c , Shinzo Kohjiya c,1 a Graduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan b Faculty of Engineering and Design, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan c Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Received 4 July 2006; received in revised form 16 December 2006; accepted 8 January 2007 Available online 11 January 2007 Abstract Effect of network-chain density (n) on the strain-induced crystallization of peroxide-crosslinked natural rubber was studied by fast time- resolved synchrotron wide-angle X-ray diffraction. It was observed that the elongation ratio at the onset of strain-induced crystallization (a c ) became smaller with the increase of n. The difference of the entropy between the undeformed and deformed states at each a c was nearly equal in spite of the variation of n. The calculated melting temperature at a c of the samples was also almost the same regardless of their n . These observations mean that the strain-induced crystallization occurred when the deformation brought about a definite entropic state for peroxide- crosslinked natural rubber. They agreed with the prediction by Flory and were consistent with the classical theory of rubber elasticity. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Peroxide-crosslinked natural rubber; Strain-induced crystallization; Network-chain density 1. Introduction Recently, the studies on the strain-induced crystallization (SIC) of crosslinked rubber have been extensively conducted by synchrotron wide-angle X-ray diffraction (WAXD) [1e 14]. The synchrotron radiation system combined with a stretch- ing machine becomes a powerful tool for studying dynamics of deformation behavior of soft materials, because the effect of relaxation of macromolecules can be minimized by setting the irradiation time shorter. Thus, several interesting results on SIC of natural rubber (NR) and synthetic rubber have been re- ported and the observed phenomena have been discussed by comparing with theoretical models. For example, we found that the rate of SIC of sulfur-crosslinked NR was faster for the samples with higher network-chain density (n) [14], which was the opposite trend in the former works conducted for the stretched samples during relatively long-time under small strain [5,15]. The difference would be ascribed to the speed of each WAXD measurement. Thus, the importance of direct observation of structure in real time during deformation of rubber is clear. Among general-purpose rubbers, NR is necessary to pre- pare the high performance pneumatic tires used under severe conditions. One reason for the excellent mechanical properties of NR has been assumed due to its SIC ability. Thus, most of the NR products, so far, have been produced using a sulfur- cure system, and consequently many studies on the SIC of sulfur-crosslinked NR have been conducted [1e12,14,16e23]. However, only a few reports [3,9,13,19] have partially de- scribed the crystallization of peroxide-crosslinked NR upon stretching. Peroxide-crosslinked NR is nowadays utilized more and more owing to their high transparency, lower compression set and improved heat resistance. Therefore, it is important for practical applications to reveal characteristics of SIC for per- oxide-crosslinked NR in situ during deformation. * Corresponding author. Tel.: þ81 75 724 7558; fax: þ81 75 724 7580. E-mail address: yuko@kit.ac.jp (Y. Ikeda). 1 Present address: Visiting Professor, Faculty of Science, Mahidol Univer- sity at Salaya, Phuttamonthon, Nakorn Pathom 73170, Thailand. 0032-3861/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2007.01.006 Polymer 48 (2007) 1171e1175 www.elsevier.com/locate/polymer