Crystallization and Melting Processes in Vulcanized Stretched Natural Rubber

Abstract

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 σ ∼ λ 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.

Role of strain induced crystallization and oxidative crosslinking in fracture properties of rubbers

“…Crystallization at the crack tip increases the crack propagation energy significantly. In other words, SIC improves the fracture strength substantially [12].…”

Characteristics of Strain-Induced Crystallization in Natural Rubber During Fatigue Testing: In Situ Wide-Angle X-Ray Diffraction Measurements Using Synchrotron Radiation

“…The samples are classical flat dumbbell specimen with a 10 mm gauge length and a 2 3 4 mm 2 section. Preliminary quasi-static tests are conducted beforehand to determine (1) the relation between the local stretch ratio k at the center of the sample and the displacements of the grips and (2) the thresholds of crystallization k C and melting k M of the material at room temperature (similar to Trabelsi et al 5 ). Even if the experiments are not conducted at the same time, the same tensile machine is used, and the experimental conditions are set identical: first, a five-cycle accommodation of the sample is performed, then the sample is elongated to 100 mm at the speed of 0.012 mm.s À1 , with the total duration of the corresponding cycle being about 2 h.…”

“…The spectra extracted from I ani (2h, b) and I iso (2h) are classically fitted by a series of Pearson functions. 4,5,21,22 Different characteristics of the crystallized phase of NR are calculated from those quantities.…”

“…The size l hkl of the crystallites in the direction normal to the (hkl) diffraction planes, deduced from the Scherrer formula, 4 in which K is a scalar that depends on the shape of the crystallites (here we adopt 0.78 as proposed by Trabelsi et al 5 ), k is the radiation wavelength, h is the Bragg angle, and FWHM 2h is the full width at half-maximum of the (hkl) reflection in 2h.…”

“…Strain-induced crystallization (SIC) of NR was discovered by Katz in 1925 with the help of X-ray diffraction. 1 This technique has since permitted researchers to obtain the crystallographic data of NR, 2,3 to evidence the existence of stretch ratio thresholds for crystallization and melting of the crystallites, 4,5 to relate SIC to the mechanical hysteresis of the stress-strain response, [4][5][6] and to put into light the effect of fillers such as carbon black. 7 For more details on the use of X-ray diffraction for the investigation of SIC in NR, the reader can refer to the recent review by Huneau.…”