Strain-induced crystallization of carbon black-filled natural rubber during fatigue measured by in situ synchrotron X-ray diffraction

Abstract

Natural rubber (NR) exhibits great fatigue properties which are usually explained by its ability to crystallize under strain. Nevertheless, strain-induced crystallization of NR in fatigue has never been investigated. We perform original in situ fatigue tests during which the degree of crystallinity, and the number and volume of crystallites are measured by synchrotron wide angle X-ray diffraction. For all loading conditions, the number of crystallites is constant. The evolution of their volume depends on the minimum stretch ratio achieved at each cycle. The results show that cyclic loading conditions modify the macromolecular structure of the material, in particular of its amorphous phase.

Introduction

Natural rubber (NR, cis-1,4-polyisoprene) vulcanizates have the ability to crystallize under strain at room temperature. Strain-induced crystallization (SIC) of NR has been discovered by Katz in 1925 with the help of X-ray diffraction [1]. This technique has since permitted to obtain the crystallographic data of NR [2], [3], to exhibit the existence of stretch ratio thresholds of crystallization and melting of the crystallites [4], [5], to relate SIC to the mechanical hysteresis of the stress–strain response [6], [4], 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 SIC in NR, the reader can refer to the recent review of Huneau [8].

The great majority of the studies on SIC of NR focus on uniaxial quasi-static cycles and relaxation tests. Here, the expression “quasi-static” stands for low strain rates compared to crystallization rate. But NR is often used in engineering applications for its great properties in fatigue such as long fatigue life, even at large strain [9], [10], [11]. The mechanical properties of NR have been thoroughly studied [12], [13], [14]; but studies on the evolution of SIC during fatigue testing of NR are very rare, though it is often accepted that the remarkable fatigue properties are closely related to SIC. This is mainly because the typical frequencies of fatigue tests (1 Hz or more) are not compatible with the long time acquisition required by X-ray diffraction measurements (from a few seconds to an hour). Nevertheless, Kawai succeeded in measuring SIC during fatigue by using a stroboscopic technique to accumulate the weak intensity of the diffracted beam over a large number of cycles [15]. He studied only one set of loading conditions for which both minimum and maximum stretch ratios achieved are 3.5 and 4.5, respectively. In this case, he observed an increase in the degree of crystallinity with the number of cycles. Furthermore, Rouvière et al. recently measured the evolution of crystallinity along fatigue life for different uniaxial loading conditions by performing interrupted fatigue tests [16]. Nevertheless, this method does not allow to separate SIC induced by fatigue from SIC induced by constant stretching during the 45-min acquisition of the X-ray diffractogram.

The aim of the present study is to measure the evolution of the strain-induced crystallinity during fatigue of carbon black-filled NR and to determine the mechanisms that drive this evolution. For this purpose, we developed an innovative experimental method which allows to measure SIC in real time during a fatigue experiment. In-situ wide angle X-ray diffraction (WAXD) measurements are performed with a very short exposure time achieved thanks to synchrotron radiation. Thanks to the results obtained, we highlight the key role of the melting stretch ratio and we propose two mechanisms of SIC in NR fatigue depending on loading conditions.