Draw Ratio at the Onset of Strain-Induced Crystallization in Cross-Linked Natural Rubber

The first theory of strain-induced crystallization in regular elastomers was laid out some 70 years ago by Flory. He identified the driving mechanism that concurs to the stabilization of the crystalline phase under stretching as the relaxation of the remaining amorphous phase because of the growth of oriented crystallites. However, Flory's theory is an equilibrium theory and does not apply to dynamic conditions that are kinetically controlled. We presently performed careful measurements of the crystallinity index and draw ratio for the amorphous phase during mechanical cycling and step-wise extension so as to enlighten their intricate interplay. We propose a simple description of the crystallization process that allows establishing a quantitative relation between these two parameters and predicting their time-evolution. This analysis leads to the definition of a master curve for the time-variation of the crystallinity index.