ABSTRACT
Flory elucidated the mechanism of the occurrence of SIC from thermo-mechanics, that is, SIC is caused by the rise of the melting temperature (Tm) due to a decrease in entropy by forced deformation. Therefore, SIC is accelerated at lower temperature because of super cooling conditions. On the other hand, it is known that un-vulcanized NR preserved in a refrigerator shows whitening due to crystallization. Since the Tm is around room temperature, and the crystallization temperature
1 INTRODUCTION
The superiority in mechanical properties of Natural Rubber (NR) as compared to synthetic rubbers has been considered to originate from Strain-Induced Crystallization (SIC) in NR. In 1925, Katz discovered crystals in stretched NR (both un-vulcanized and vulcanized) by Wide Angle X-ray Diffraction (WAXD). In the early to mid decades between 1930’s to 1950’s, the crystal structure of stretched and frozen NR was studied intensively and different models of the unit cell were proposed.. In 2004, Takahashi and Kumano modified Bunn’s model slightly and reported a monoclinic unit cell (β = 93.1°) with space group P21/a. Rajkumar et al. proposed an orthorhombic unit cell with a space group P212121 and a glide plane symmetry. These researchers measured un-vulcanized NR and unvulcanized De-Proteinized NR (DPNR) at low temperature such as −25°C or −50°C. The samples were stretched 5-8 times its original length and then set to a sample fixture. This sample with the fixture was frozen and exposed to X-rays for several hours in order to acquire strong X-ray diffraction intensities measured by conventional X-ray instruments. On the other hand, the mechanical properties and strain dependency of SIC has been studied extensively since the 1940s. Gent reported that the stress of peroxide vulcanized NR, after stretching to a certain strain, relaxes to zero in a
(Tc) is around −10°C, the preserved un-vulcanized rubber shows crystallization that can be called as Temperature-Induced Crystallization (TIC). Previous researcher’s samples may be affected by TIC, since TIC could be occurred in the sample after the stress relaxes to zero. The initiation of SIC at a certain strain is very fast, such as 60 millisecond and around 100 millisecond. Therefore, SIC might be completed at a certain strain during extension.
