Stress-Strain Relation and strain-Induced Crystallization in Rubber

Rubber is composed of flexible chains and network points. Theory of rubber elasticity succeeds to elucidate stress-strain relation of rubber using the inverse Langevin equation of entropy modulus. However, actual rubber is much different from ideal networks composed of ideal rubber chains. Network points may not distribute homogeneously and the molecular weight between two network points may show wide distribution. Flexible chains show strain-induced crystallization. Recent synchrotron X-ray and simultaneous stress-strain measurements reveal that strain-induced crystallization reduces the stress by increasing the length of molecules along the stretching direction. Also, strain-induced crystals are created not at the middle of the network points, but at the close location to the network points. The hybrid structure of strain-induced crystallites and network points may be stronger than network points alone. Therefore, strain induced crystallization may increase the tensile strength of rubber by two mechanisms, they are, increase of elongation at break and reinforcement of network points. Natural rubber has biotic network points in nature. After vulcanization, the biotic network may contribute the superior toughness of NR, comparing to IR. Carbon filled NR also shows strain induced crystallization. In order to acquire high tensile strength, molecules should have higher flexibility to perform strain induced crystallization by selecting a kind of carbon blacks, an accelerator and a curing condition.