Influence of strain rate and temperature on the onset of strain induced crystallization in natural rubber

Abstract

Strain induced crystallization (SIC) of natural rubber (NR) has been studied in a large range of strain rate (from 5.6×10−5s−1 to 2.8×101s−1) and temperature (from −40°C to 80°C) combining mechanical and thermal analysis. Both methods are used to extend the study of SIC from slow strain rates – performed with in situ wide angle X-rays scattering (WAXS) – to high strain rates. Whatever the temperature tested, the stretching ratio at crystallization onset (λc) increases when the strain rate increases. This strain rate effect is strong at low temperature (close to Tg) and weak at high temperature (much higher than Tg). A theoretical approach derived from the Hoffman–Lauritzen equation has been developed and provides a good qualitative description of the experimental results. At low temperature, the strong increase of λc with strain rate is explained by a too long diffusion time compared to the experimental time. At high temperature, SIC kinetics is rather controlled by the nucleation barrier which mainly depends on the strain energy. When the stretching ratio increases, this nucleation barrier strongly decreases, allowing crystallization even for short experimental time.