Crystallization and melting processes in vulcanized stretched natural rubber

We have studied, by simultaneous force and WAXS measurements, crystallization and melting properties of stretched natural poly cis-isoprene, vulcanized at different rates, in static and dynamic deformations. The overall effects of increasing N-C, the number of monomers between cross-link bridges, is to slow the kinetics of crystallization and to decrease the melting temperature, crystallites sizes, crystallinity, and mechanical hysteresis. The origin of these properties is discussed. The morphologies of vulcanized rubbers during static and dynamic deformations are very similar. The process of crystallization (and melting) occurs during these two types of deformation by nucleation (and disappearance) of crystallites with constant sizes. The role of the affine deformation of the cross-link network on the crystallites dimension is pointed out. During cyclic deformations, real time measurements during stretching and recovery permit one to conclude that mechanical hysteresis is due only to the chains crystallization or more exactly to the supercooling (difference between melting and crystallization temperatures). During stress hardening, the form of the stress-strain curve sigmasimilar tolambda(2) is explained following the Flory idea. Each new crystallite formed during stretching is considered as a cross-link. The Flory stress-induced crystallization model is discussed. In the Appendix, we describe the new effect called "inverse yielding" observed in weakly cross-linked rubbers.