Strain-induced Crystallization Of Natural Rubber Research Articles

Crystal and Crystallites Structure of Natural Rubber and Synthetic cis-1,4-Polyisoprene by a New Two Dimensional Wide Angle X-ray Diffraction Simulation Method. I. Strain-Induced Crystallization

A novel two-dimensional simulation method to analyze a wide-angle X-ray diffraction (WAXD) pattern is applied to crystal and crystallites of strain-induced crystallization (SIC) of unvulcanized natural rubber (NR) and synthetic polyisoprene rubber (IR) at −50 °C. The unit cell dimensions of SIC were determined by calculation of scattering factors based on atomic coordinates and symmetry, and were compared with the observed data. The unit cell dimensions are the same in NR and IR, and do not change during extension. The crystallites’ dimensions in NR and IR are almost the same at −50 C. The orientation of crystallites in NR is lower than in IR. The order of crystals in NR is lower than in IR. The crystalline fraction increases with strain because of the increase in the number of crystallites. The onset strain of SIC in NR is smaller than the one in IR. The different behaviors of SIC in NR and IR seem to be caused by the pseudonetwork of NR.

Strain‐induced crystallization of natural rubber with high strain rates

AbstractStrain‐induced crystallization (SIC) of natural rubber (NR) samples with different strain rates at a fixed strain was investigated by synchrotron radiation X‐ray diffraction measurements, which provided the evolution trends of crystal sizes and crystallinity during the SIC process. It was found that the Avrami index was about 1 during the crystallization of NR after the cessation of stretch, which demonstrated that sporadic nucleation occurred during SIC process. The increase of the crystallinity was attributed to the increase of the number of new crystallites rather than the growth of the crystal size. An unexpected relationship between the final crystallinity and the strain rates was observed. The increase of physical crosslink points originated from either entanglement or crystallite was considered as the reason that leads to the nonmonotonic variation of the final crystallinity with strain rates. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Kinetics of Strain-Induced Crystallization in Natural Rubber Studied by WAXD: Dynamic and Impact Tensile Experiments

The time-dependence of strain-induced crystallization in cross-linked natural rubber was studied using synchrotron wide-angle X-ray diffraction (WAXD) with an unprecedented time resolution in the ms range. In-situ dynamic mechanical tests and tensile impact tests were carried out. In tensile impact tests, consisting of a strain step within less than 10 ms, it was found that roughly half of the crystallization process was complete within less than 5 ms, provided the strain is large enough. Afterward, crystallization proceeded on a time scale of a few seconds. This implies that under dynamic loading at frequencies which are typically encountered in the application of rubbers, the degree of crystallinity is considerably lower than under equilibrium conditions. This was directly confirmed by in situ dynamic cyclic experiments at a frequency of approximately 1 Hz. Since crystallization is a major factor contributing to the outstanding mechanical properties of natural rubber, these findings can aid in the inter...

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

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.

A stroboscopic X-ray apparatus for the study of the kinetics of strain-induced crystallization in natural rubber

A stroboscopic X-ray diffraction machine is described that allows studying kinetics aspects of strain-induced crystallization in natural rubber. Besides conventional mechanical cycling in extension, samples may be submitted to periodic extension cycling over a 0.05 Hz–30 Hz frequency range and for various amplitudes of elongation. Pertinent parameters as crystalline content, crystallite orientation and segmental order parameter for the amorphous fraction are determined. The existence of a regulation process of the strain experienced by the chains that remain molten due to the phenomenon of strain-induced crystallization is clearly demonstrated during conventional mechanical cycling at low deformation rate. Melting retardation is shown to lead to an equalization of crystalline content in conditions of periodic cycling of moderate amplitude and below some average elongation. On the other hand, crystallization appears to be a rapid process subsisting up to the maximal cycling frequency corresponding to a stretching time of 17 ms. This is confirmed by periodic cycling of higher amplitude that suggests that strain-induced crystallization may appear within millisecond time. An approximate logarithmic dependence of the crystalline content and elongation of the amorphous fraction upon stretching time is further found.

STRAIN-INDUCED CRYSTALLIZATION OF NATURAL RUBBER: A REVIEW OF X-RAY DIFFRACTION INVESTIGATIONS

Abstract Strain-induced crystallization of natural rubber was discovered in 1925 by the means of x-ray diffraction and has been widely investigated by this technique until today. The studies devoted to the structure of the crystalline phase of natural rubber are first reviewed. This structure is strongly anisotropic and can be related to the exceptionally good strength and fatigue properties of this material. The relationships between strain-induced crystallization of natural rubber and its mechanical response, during static or tension-retraction tests, are also reviewed and discussed; in particular, the hysteresis of the stress-strain curve is mainly explained by strain-induced crystallization. The kinetics of crystallization under both static and cyclic deformation is also discussed, as well as the influence of different factors, depending either on material composition (crosslink density, carbon black fillers) or on external parameters (temperature, strain rate…).

Tear energy and strain-induced crystallization of natural rubber/styrene-butadiene rubber blend

Strain-induced crystallization of natural rubber (NR), dispersed in styrene-butadiene rubber (SBR), was investigated in relation to dimensional feature of a dispersoid and crosslink density of NR by measuring tear energy (G) of crosslinked NR/SBR blends. The crosslinked NR/SBR blends in ratios of 1/9 and 3/7 by weight were prepared by mixing masticated NR and SBR with an internal mixer at a rotor speed of 30 rpm, followed by crosslinking with dicumyl peroxide on a hot press at 444 K for 60 min. The G, measured in wide-ranges of temperature and tear rate, was superposed into a master curve with a Williams-Landel-Ferry shift factor. The G of the NR/SBR(3/7) blend abruptly decreased to a level comparable to that of SBR at about melting temperature of NR crystals formed on straining. The temperature, at which the dramatic decrease in the G occurred, was associated with the dimensional feature of the NR dispersoid and the crosslink density.

Strain-Induced Crystallization of Natural Rubber: Effect of Proteins and Phospholipids

Abstract The effects of proteins and phospholipids in natural rubber (NR) on the strain-induced crystallization behavior during uniaxial deformation were studied by in-situ synchrotron wide-angle X-ray diffraction (WAXD) technique and simultaneous measurements of stress-strain relation. The influences of proteins and phospholipids in NR were evaluated separately by decomposition methods using deproteinization and lipase treatment, respectively. It was found that both components form a naturally occurring network, which is responsible for the strain-induced crystallizability of unvulcanized NR and the corresponding high mechanical property. This network also plays a significant role in strain-induced crystallization of vulcanized natural rubber.

Strain-Induced Crystallization of Crosslinked Natural Rubber As Revealed by X-ray Diffraction Using Synchrotron Radiation

Strain-induced crystallization (SIC) of natural rubber (NR) has been extensively studied even before the advent of macromolecular physics. However, there are still some unsolved basic issues in this field. In this review article, classic studies on SIC of NR are briefly introduced, and then recent results by synchrotron X-ray diffraction studies in separate papers by different authors are categorized and interpreted on the basis of molecular models. Cyclic deformation experiments provided information on partial orientation of the network-chains, on nucleation and morphological changes of crystals and on stress field around the strain-induced crystals. On the other hand, experiments under constant strain provided information on kinetics of SIC, on stress relaxation due to SIC, and so on. The experimental results could be explained under the assumption that the SIC is dominated by strain, and that the crystals are of folded-chain type. However, in order to consistently explain the various experimental results, we have to establish a unified molecular model of the network structure.

The Behavior of Carbon Black-Filled Natural Rubber under High Strain Rates

Abstract Tensile tests were conducted to obtain the deformation and failure characteristics of unfilled natural rubber (NR) and natural rubber with 25, 50, and 75 phr of N550 carbon black filler under quasistatic and dynamic loading conditions. The quasistatic tests were performed on an electromechanical INSTRON machine, while the dynamic test data were obtained from tensile impact experiments using a Charpy impact apparatus. In general, the modulus of the stress-extension ratio curves increases with increasing strain rate up to about 407, 367, 346, and 360 s−1 for unfilled, and 25, 50, and 75 phr for filled NR, respectively. Above these strain rates, the unfilled and filled natural rubber stress-extension ratio curves remained unchanged. The modulus increased with increasing strain rate because there was little time for stress relaxation. Above a critical strain rate, no change in modulus was observed because the time of the experiment was short compared to the lowest characteristic relaxation time of the material. Dynamic stress-extension ratio curves did not have the very sharp upturn at break, which is observed from strain-induced crystallization in natural rubber under quasistatic loading. Strain-induced crystallization appeared to be suppressed at high rates of loading. In fact, the highest dynamic tensile strength for the 25- and 50-phr carbon black-filled natural rubbers was smaller than those under quasistatic loading, while the highest dynamic tensile strength of the 75-phr carbon black-filled NR was greater than that in the static test. This indicated that high amounts of carbon black fillers will impede strain-induced crystallization in natural rubber.

Effect of Network-Chain Length on Strain-Induced Crystallization of NR and IR Vulcanizates

Abstract Strain-induced crystallization of natural rubber (NR) and synthetic isoprene rubber (IR) with various crosslinking densities was investigated by wide angle X-ray diffraction using a synchrotron radiation and simultaneous tensile measurements. The elongation ratio at the onset of crystallization (αc) was almost independent of crosslinking density. IR samples showed larger αc values than NR because of the lower stereoregularity of IR. These results suggest that the onset of crystallization is determined by increased melting temperature by strain due to an entropic reason. The amount of oriented amorphous component changed approximately linearly with strain, and was a little larger in IR than in NR when compared at the same elongation ratio. At small strain (and stress), crystallinity in IR was lower than in NR. These results indicate that, at small strain region, the more stress is assigned to oriented amorphous in IR than in NR.

Strain-induced crystallization of natural rubber as studied by high-resolution solid-state 13C NMR spectroscopy

A series of high-resolution solid-state 13C NMR experiments were performed on both unstretched and in situ stretched natural rubber samples. From the 13C CP/MAS spectra, it was found that natural rubber does form small crystals at room temperature though the degree of crystallinity is very small. Furthermore, from the 13C DD/MAS spectra, the crystalline signals were found to increase with the increase of draw ratio. 13C spin-lattice relaxation time ( T 1) and 1H spin–spin relaxation time ( T 2) of in situ stretched natural rubber were measured for the first time, which provided further evidences for the conclusion that there exist crystals in both stretched and unstretched natural rubber samples. Quantitative 13C NMR measurements indicated that strain-induced crystallization occurs when the draw ratio reaches about 2.0 and the maximum crystallinity of our natural rubber samples can be as high as 19.3% upon stretching.

Strain-induced crystallization of natural rubber as studied by high-resolution solid-state 13C NMR spectroscopy

A series of high-resolution solid-state 13C NMR experiments were performed on both unstretched and in situ stretched natural rubber samples. From the 13C CP/MAS spectra, it was found that natural rubber does form small crystals at room temperature though the degree of crystallinity is very small. Furthermore, from the 13C DD/MAS spectra, the crystalline signals were found to increase with the increase of draw ratio. 13C spin-lattice relaxation time (T1) and 1H spin–spin relaxation time (T2) of in situ stretched natural rubber were measured for the first time, which provided further evidences for the conclusion that there exist crystals in both stretched and unstretched natural rubber samples. Quantitative 13C NMR measurements indicated that strain-induced crystallization occurs when the draw ratio reaches about 2.0 and the maximum crystallinity of our natural rubber samples can be as high as 19.3% upon stretching.

Strain-induced crystallization of natural rubber as detected real-time by wide-angle X-ray diffraction technique

In the past, the stress–strain behavior of natural rubber has been studied by a variety of mechanical analytical techniques in order to understand the role of induced crystallization by deformation. But, this was generally done in a sequential manner by stretching to a certain extent and examining the stretched part separately. This type of sequential experiment may not follow the exact dynamics of the process and thus may lead to erroneous conclusions regarding the evolution of microscopic mechanisms. It is therefore advantageous to measure the mechanical response simultaneously with the structural response to map the structural evolution onto the stress–strain behavior. In this paper, simultaneous measurements of the stress–strain behavior and X-ray scattering intensity of vulcanized natural rubber were made continuously by a specially designed instrument during elongation and subsequent retraction. These studies revealed three new characteristics of the strain-induced crystallization. 1. The strain-induced crystallization starts at around 400% strain at 25°C, then increases steeply, although sequential measurements reported that natural rubber starts strain-induced crystallization at 200%. 2. At the onset of retracting, the degree of crystallinity continues to increase, although a drastic decrease of stress occurs. We suggest the morphology of the strain-induced crystallites changes such that do not contribute to stress. 3. Strain-induced crystallites disappear at the strain where the stress–strain loop is closed during retracting. The hysteresis of the stress–strain curve might be attributed to strain-induced crystallization.

Morphology of strain-induced crystallization of natural rubber. Part II. X-Ray studies on cross-linked vulcanizates

Abstract The results of wide- and small-angle X-ray studies on bulk vulcanizates of natural rubber are described. At room temperature no discrete X-ray peaks occur at small angles, even for highly strained (700%) specimens for which a significant degree of crystallinity is present at room temperature. Below the isotropic melting point of the network, discrete small-angle X-ray scattering develops for both unstrained and strained specimens. The Bragg periodicity corresponds closely to the longitudinal lamellar spacing from electron micrographs of thin films under corresponding conditions. The X-ray long period exhibits significant thermal reversibility. Upon heating, following initial crystallization at −25°C, the long period continuously increases from about 140 Å at −25°C to about 250 Å at 17°C. After heating, progressive cooling from room temperature results in continuous decreases in long period and at least 80% of the initial increase on heating is recovered. These results in conjunction with electron microscopy data (Part I) strongly suggest that thermally reversible changes in physical lamellar periodicity occur for natural rubber and are discussed in detail in light of current concepts concerning long period reversibility in bulk polymers.

Morphology of strain-induced crystallization of natural rubber. I. Electron microscopy on uncrosslinked thin film

Pt shadowing, Au decoration, OsO4 staining, electron diffraction, and bright- and dark-field electron microscopy have been used to elucidate strained and unstrained thin-film morphology of natural rubber. Unstrained natural rubber exhibits a 100–150-Å nodular morphology in the melt at room temperature. When cooled below room temperature, the original nodular morphology is replaced by an unoriented lamellar morphology. The crystal thickness of the lamellae is about 55 Å at −28°C. When stretched to elongations greater than about 200% the strain-crystallized films show a distinct fibrillar morphology at room temperature. The fibrils appear to be composed of individual crystallites, 120 Å in diameter. When cooled to −25°C, including those films which have been highly stretched (900%), the original fibrillar morphology is replaced by an oriented lamellar morphology. The transformation to lamellar morphology involves a lateral alignment of the room-temperature crystallites and a decrease in crystal thickness. Upon heating above −25°C, the lamellar crystals thicken and lamellar periodicity increases, but eventually the structure reverts to the fibrillar form as room temperature is approached, indicating the reversible nature of the morphological transformation.