Hierarchal multi-scaled structures in natural rubber

Studies by synchrotron X-ray and optical microscope revealed hierarchal multi-scale structures in natural rubber (NR). NR is composed of rubber component and natural impurities such as proteins, phospholipids, carbohydrates and others.  The functional groups of the ends of rubber chains react with natural impurities and make networks. At the same time, natural impurities make micelles, agglomerates and crystals. The superior mechanical properties in NR to synthetic analogue (IR) are owed to hierarchical multi-scaled structures. In the case of un-vulcanized state, the different mechanical properties and strain-induced crystallization of NR and IR are quite obvious since naturally occurring network aligns rubber chains to induce crystallization and contributes to elastic responses of NR. After the vulcanization, chemical network govern elastic properties in both NR and IR mainly. However, naturally occurring network contributes to the temperature dependency of stress relaxation and elongation at break in vulcanized NR. Although the contents of natural impurities are low, the sizes of agglomerates and crystals suggest that they may change the course of crack propagation in tensile strength and tearing resistance.