Melting Behavior of Crystallites during High-Speed Shrinkage of Stretched Vulcanizate of Natural Rubber

Natural rubber (NR) exhibits strain-induced crystallization (SIC), whereby the strained NR crystallizes and toughens. To reveal the onset strain of the SIC is one of the central issues, and the dependence of the elongation speed on the onset strain has been investigated by some researchers. On the contrary, no studies have addressed the melting behaviors of crystallites (formed upon the prestretching) during rapid shrinking at strain rates exceeding 100 s–1, likely because the melting processes occur too fast to be experimentally resolved. Due to the tremendous technological improvement of the device, it became possible to measure a series of the wide-angle X-ray diffraction (WAXD) patterns with a 1 ms interval. Therefore, the objective of this study is to clarify the behavior of the melting of the crystallites during the high-speed unloading (shrinking) processes at strain rates of more than 140 s–1 by time-resolved WAXD measurements using synchrotron radiation by utilizing a homemade device for fast contraction of the NR specimens. The results of temporal changes in strain and reflection peak area show that crystallites remained for a few milliseconds after complete shrinkage at high-speed contraction. Compared to the result at very low-speed contraction, the crystallites survived 5 ms more, and it was found that this surviving time is dependent on the shrinking rate. The detailed examination of the results revealed that the (200) and (201) reflection peaks increased in the initial and intermediate stages of the shrinking process, suggesting the increase in the crystallinity. Also, the crystallite size in the [200] and [201] directions increased a bit in a similar stage of shrinking, suggesting that crystallites grow a bit during shrinking. The extent of the orientation of the crystallites hardly decreased, and complete randomization of the orientation was not attained. Actually, the orientation factor remained as high as 0.97. The idea of the critical nucleus is also suggested for melting of the crystallites in the case of the shrinking (unloading) process. Upon melting, the crystallite size still keeps a nonzero value, suggesting the spontaneous melting of a finite-sized crystallite and, in turn, the critical nucleus. Effects of the shrinking speed on the surviving time and the change in the crystallite size were also investigated.