Hierarchic Nano-Structures in Natural Rubber by Synchrotron X-Ray and Optical Microscopy

Natural impurities in natural rubber (NR) create not only the naturally occurring network but also multi-scaled nano-structures. The naturally occurring network in unvulcanized NR induces strain-induced crystallization and enhances modulus and tensile strength. Strain-induced crystallization and stress-strain relation in un-vulcanized NR were measured simultaneously using synchrotron wide angle X-ray diffraction (WAXD). Multi-scaled microstructures in un-vulcanized NR induced by natural impurities (i.e., proteins, phospholipids, carbohydrates) were investigated by synchrotron small-angle Xray scattering (SAXS), WAXD and optical microscopy. These nano-structures include large aggregates (size less than 50 =m), well-defined crystals (size less than a few 10 =m) and micelles (size much less than 10 =m). In un-vulcanized NR samples, even though the concentrations of natural impurities are relatively low, the dispersion of these nanostructures significantly affects the mechanical properties. Peroxide vulcanized NR show the effects of the naturally occurring network significantly on the stress-strain relations. Chemical network strengthens stress-strain relation and accelerate strain-induced crystallization, however, strain-induced crystallization does not increase the stress and tensile strength. Strain-induced crystallization in peroxide vulcanized polyisoprene saturates to the certain level of crystallinity (30%) below the maximum strain (the limit of  extensibility). Restrained heating experiments on vulcanized polyisoprene show the weak endurance of naturally occurring network against temperature and stress. The superior mechanical properties in NR owe to natural impurities as the naturally occurring network and multi scale nano-structures.