Stress-induced crystallization and melting of natural and synthetic cis-isoprene are compared by simultaneous Wide Angles X-ray Scattering (WAXS) and mechanical measurements.
Natural (NR) and synthetic polyisoprene (SR) rubbers have the same composition and sulfur crosslink density. At fixed elongation and during cyclic deformation, the properties X of the semi crystalline phase are determined as function of draw ratio λ; X being crystallites dimensions and orientation, half-time of crystallization, melting temperature, crystallinity and crystalline zone dimensions around a crack tip. It is shown that all the curves X(λ) of both types of rubber at room temperature can be superposed by a simple translation δλ along the draw ratio axis. This translation factor of the order of 0.5 to 1 does not depend on the property X. These effects are explained by the decrease of the melting and crystallization temperatures due to the presence of chain defects in synthetic rubber.