Strain-induced crystallization (SIC) of natural rubber (NR) is characterized during a cyclic deformation at room temperature and low strain rate (∼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.

Real time Wide-Angle X-ray Scattering (WAXS) measurements during cyclic tensile tests at high strain rates (from 8 s À1 e280 s À1 ) and at room temperature on crosslinked Natural Rubber (NR) are performed thanks to a specific homemade device. From the observed influence of the frequency on the crystallization index at the maximum sample elongation, a characteristic crystallization time is deduced. This is done taking into account the material self-heating during such unusually high strain rates. Two regimes for the dynamic process of strain induced crystallization are evidenced. For the NR tested, the obtained characteristic time is around 20 ms when the material average elongation during the cyclic test is above a critical elongation value l c . l c is the minimum elongation needed to induce crystallization during low strain rate tensile tests. Moreover, a rapid increase of this characteristic time is found when the average elongation decreases below this critical value.