“Mechanism of the self-reinforcement of cross-linked NR generated through the strain-induced crystallization”

The author proposed new models and concept for the self-reinforcement of NR. The first model indicates that general rubber vulcanizate consists of the heterogeneous structure, partially continuous cross-linked phase (75%) and continuous uncross-linked phase (25%). In addition, the author proposed other new models and concept for the strain-induced crystallization in vulcanized NR, in which the strain-induced crystallization takes place in the uncross-linked phase in cross-linked rubber. In the uncross-linked phase under large extension, molecular flow and orientation occur due to the very high compressive, shear and tensile stresses generated by the surrounding hard cross-linked phases, which makes the strain-induced crystallization possible in the uncross-linked phase. As macroscopic extension increases, the crystallization spreads over the whole uncross-linked phases, thus the uncross-linked phase changes its character from original soft rubber to the strong super network consisting of a bundle of extended molecules interconnected at the crystals. The characteristic phenomena observed in the stress–strain relation of NR such as the stress-upturn, high tensile strength and large stress-softening (Mullins effect) can be reasonably explained using these models and concepts.