Study on Strain-induced Crystallization of Crosslinked Natural Rubber by Synchrotron Radiation

Abstract

The characteristics of strain-induced crystallization (SIC) are described on sulfur-crosslinked and peroxide-crosslinked natural rubber (NR) and synthetic isoprene rubber (IR). Simultaneous tensile and wide-angle X-ray diffraction measurements using synchrotron radiation systems were carried out in order to elucidate SIC, the rate of which was much faster than so called cold crystallization of NR. Onset strain of SIC of sulfur-crosslinked rubber was almost independent of network-chain density. The stretched molecular chains acted as initiating species of the crystallization, while surrounding chains contributed to the crystal growth. Deformation of crystal lattice with nominal stress was detected, which showed the strain-induced crystallites were responsible for the higher modulus upon stretching. A pantograph model describes the deformation mechanism of the rubber network. The effect of non-rubber components of NR on SIC was clearly detected, where the stress relaxation after high speed stretching gave the significant difference of SIC between NR and IR. Stearic acid did not accelerate SIC of sulfur-crosslinked NR and IR, oppositely with the cold or temperature-induced crystallization of them. The effect of carbon black filler on SIC of sulfur-crosslinked NR is also reviewed. For peroxide-crosslinked NR, SIC occurred when the deformation brought about a definite entropic state, this agrees with the theoretical prediction by Flory and is consistent with the classical theory of rubber elasticity.