The role of crystallization and crosslinking in the hysteresis loss of vulcanized Eucommia ulmoides gum

The interplay between crosslinks and crystals is critical for the mechanical properties and applications of Eucommia ulmoides gum (EUG). In this work, EUG samples with different crosslink densities are prepared by sulfur vulcanization. The effect of crosslink density on the crystallization, melting, and mechanical properties are systematically investigated. As expected, crosslinks suppress the crystallization of EUG, resulting in a decrease in melting/crystallization temperature, crystallinity, modulus, and yield stress. Interestingly, vulcanized EUG can gradually crystallize during annealing at room temperature, showing a nonequilibrated state and slow crystallization kinetics. Correspondingly, the modulus increases with the annealing time. Cyclic deformation tests show that EUG vulcanizates have a lower hysteresis loss in the first cycle but a much higher hysteresis loss for subsequent cycles, showing the potential to have low-frequency damping properties. In situ wide-angle X-ray diffraction demonstrates that the hysteresis loss of EUG depends significantly on strain-induced crystallization/melting.