First evidence and characterization of strain-induced crystallization heterogeneity in natural rubber under homogeneous strain states

Abstract

This paper provides the first evidence of strain-induced crystallization heterogeneities at the macroscopic scale, accompanied with strong memory effects during uniaxial cyclic tensile loadings in unfilled natural rubber. Specimens with two different sulphur amounts, 1.2 phr and 1.6 phr, and two different geometries, small and large widths, have been considered. The crystallinity has first been evaluated in the middle of each specimen thanks to a quantitative surface calorimetry approach based on infrared thermography. The calorimetric approach was then used to simultaneously access the crystallinity at different points of the tested specimens and to investigate spatial variations in the crystallinity along a profile at the middle of each specimen. SIC was found to occur heterogeneously along the profile, i.e. at the macroscopic scale, whatever the geometry and the sulphur amount are. Moreover, crystallization and melting initiate in the same zones during load-unload cycles. Several families in terms of heat sources due to SIC have been identified.