Strain-Induced Crystallization of Natural Rubber and Cross-Link Densities Heterogeneities

Strain-induced crystallization (SIC) of natural rubber (NR) is characterized during a cyclic deformation at room temperature and low strain rate (similar to 10(-3) s(-1)) using in situ wide angle X-rays scattering (WAXS) measurements. The crystallinity index (CI) and average size of the crystallites in the three main directions are measured during loading and unloading. A scenario describing SIC is then proposed, assuming that SIC corresponds to the successive appearance of crystallite populations whose nucleation and growth depend on the local network density. From this scenario, a methodology, coupling experimental observations and thermodynamic description is developed to determine the distribution of the network chain density associated with the size of a corresponding crystallite population. Finally, complex cyclic tests are performed. They suggest the existence of a memory effect in the chains involved in crystallization, which eases the nucleation process of the crystallites.