Strain-induced Crystallization Of Natural Rubber Research Articles

Temperature Dependence of Rate of Strain-induced Crystallization in Natural Rubber

Temperature Dependence of Rate of Strain-induced Crystallization in Natural Rubber

Prolonging the Fatigue Life of Soft Rubber Actuators by Inhibiting Crack Extension Using the Hysteresis of Strain-Induced Crystallization of Natural Rubber

Prolonging the Fatigue Life of Soft Rubber Actuators by Inhibiting Crack Extension Using the Hysteresis of Strain-Induced Crystallization of Natural Rubber

Strain-Induced Crystallization in Natural Rubber: Flory’s Theory Revisited

In this paper, some aspects of the physical mechanisms involved in strain-induced crystallization (SIC) in cross-linked natural rubber networks are discussed. The theory of SIC as developed by Flory is considered within a somehow innovative perspective, in analogy to the liquid–gas phase transformation. A simple lever rule is proposed to relate the crystallinity index to the local and global draw ratios. Equilibrium properties are considered first. Some simple experiments are discussed within this framework to enlighten some fundamental aspects of SIC. As the order parameter for SIC is the draw ratio of the amorphous phase, the importance of directly measuring this parameter is emphasized. Special emphasis is put on the relaxation of the remaining amorphous fraction that accompanies SIC. The question of crystallization kinetics underlies most aspects of SIC. We show that the actual time dependence of the crystalline content may be related to the mechanism of strain relaxation in a quite simple manner using the lever rule mentioned above. Crystallization kinetics is also fundamental to explain the hysteretic behavior observed in dynamical conditions. Similarities and differences with static SIC are discussed. We transpose the classical theory of nucleation to the case of SIC, and we discuss the kinetics within this framework.

Frustrating Strain-Induced Crystallization of Natural Rubber with Biaxial Stretch.

The supreme mechanical performance of natural rubber (NR) is commonly attributed to strain-induced crystallization (SIC). The SIC of NR during uniaxial stretch has been extensively investigated, whereas that under multiaxial deformation has been rarely reported, which is close to real service conditions (i.e., tire). In this work, the crystallization behavior of NR under biaxial stretch was studied with in situ synchrotron radiation wide-angle X-ray diffraction in combination with a custom-built biaxial stretch machine. It is observed that biaxial stretch frustrates the SIC of NR: within λx/λy < 1.6, where λx and λy are stretch ratios of two mutually perpendicular axes, no crystallization emerges even under large drawing ratio until sample fracture at ambient temperature. This finding challenges the common wisdom of the self-reinforcement mechanism of SIC in NR under multiaxial deformation in real service conditions. A theoretical SIC model is proposed, which can decouple the contributions of conformational entropy reduction ΔSf and amorphous chain orientation f to final Gibbs free energy change (ΔG) during multiaxial deformation. This model quantitatively renders a reproduction of the crystallinity during the biaxial stretch, which is well consistent with experimental results and can be further generalized for flow-induced crystallization of semicrystalline polymers.

The Effect of Epoxidation on Strain-Induced Crystallization of Epoxidized Natural Rubber.

The effect of epoxidation on strain-induced crystallization (SIC) of epoxidized natural rubber (ENR) and mechanism are studied with synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) and polarized infrared spectroscopy (P-IR). WAXD results reveal that appropriate epoxidation, for example, ENR-25 epoxidized with ≈25% isoprene units, can unexpectedly enhance the SIC of natural rubber (NR), resulting in the improvement of tear resistance. On the other hand, exorbitant epoxidation, for example, ENR-40 epoxidized with ≈40% isoprene units, depresses the SIC and weakens the mechanical properties of NR remarkably. P-IR studies reveal that epoxidation can promote the orientation of chain segments along the stretching direction, which plays a determining role on SIC of NR. Accordingly, hierarchical multiscale schematic models are proposed. This insight into epoxidation on SIC of ENR strongly suggests that ENR with appropriate epoxidation degree is a promising candidate material for the fabrication of high-performance engineering rubber products.

Prolonging the fatigue life of the Straight fiber type pneumatic artificial muscle by the strain-induced crystallization of Natural rubber

Pneumatic rubber artificial muscle is attracting attention as an actuator having high cooperativeness with human because of its feature of being flexible and lightweight. The authors have developed Straight fiber type pneumatic artificial muscle (SF-ARM), which is a high power, highly displaced pneumatic rubber artificial muscle. Although SF-ARM has excellent contraction characteristics, it has a problem of short fatigue life. Therefore, in this study, focusing on strain-induced crystallization of natural rubber, the fatigue life of SF-ARM has been extended. First, strain-induced crystallization of natural rubber were confirmed by wide angle X-ray diffraction. Next, analysis of strain concentration in the vicinity of the crack was performed by the finite element method. Finally, a fatigue life test of SF-ARM was carried out to confirm the effect of prolonging the fatigue life by using strain-induced crystallization.

In situ characterization of strain-induced crystallization of natural rubber by synchrotron radiation wide-angle X-ray diffraction: construction of a crystal network at low temperatures.

Strain-induced crystallization (SIC) of natural rubber (NR) at descending temperatures as low as -60 °C is systematically investigated by in situ synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) measurement. The detailed structural evolution of NR during SIC is studied in the strain-temperature space, where up to four regions are defined depending on the SR-WAXD results. In region I, the molecular chains begin to be oriented under tensile loading. The onset of crystallization happens in the very beginning of region II, and the NR crystal acts as a new physical cross-linking point to form a crystal network, namely the series model. The further increment of crystallinity (> ca. 8%) leads to the transition of the crystal network from the series model to the parallel model in region III. The crystal network is finally accomplished in region IV, where the crystallinity remains almost constant. Interestingly, regions III and IV exist only in the intermediate-temperature zone II (-40 °C to -10 °C), which are missing in zones I (-10 °C to 25 °C) and III (-60 °C to -40 °C). This suggests that sufficient crystallinity (χII-III > ca. 8%) is required to form the parallel model. The new crystal network provides a deep understanding of SIC of NR considering the microscopic features, i.e. oriented amorphous component, the onset of crystallization and crystallinity evolution and its correlation with the macroscopic stress-strain curve.

A physically-based model for strain-induced crystallization in natural rubber. Part II: Derivation of the mechanical model

Despite the numerous experimental investigations performed over the past century and more intensively in the last fifteen years, strain-induced crystallization in natural rubber still remains hardly understood in its precise mechanisms, limiting most of constitutive equations to phenomenological approaches. The present Part II of our work aims to develop a physically-motivated constitutive equation which qualitatively reproduces phenomena observed during deformation. Firstly, the amorphous network is assumed to deform in an equal-force manner, resulting in a representative chain encompassing the inhomogeneity of cross-linking into the chain-length distribution. Then, as proposed in Part I, crystallization and melting conditions are considered, based on classical thermodynamics and on the entangled nature of the polymer network. Finally, a semi-crystallized chain is defined, accounting for both the inhomogeneity of the amorphous phase and the heterogeneity due to the presence of a crystalline phase. This chain is included in a modified full-network model, initially dedicated to amorphous networks. It leads to a constitutive equation which qualitatively reproduces the mechanical response of natural rubber.

A physically-based model for strain-induced crystallization in natural rubber. Part I: Life cycle of a crystallite

Despite the numerous experimental investigations performed over the past century and more intensively in the last fifteen years, strain-induced crystallization in natural rubber still remains hardly understood in its precise mechanisms: a complete theoretical description for crystallization and melting of the involved crystallites is still needed to derive relevant physically-based mechanical constitutive equations. Therefore, the present Part I of our work proposes a coherent theory describing the full nucleation–growth–melting cycle of these crystallites, by using classical thermodynamics of phase transitions and by accounting for the topological constraints due to the network. A graphical representation of crystallite evolution involving strain, temperature, and crystallite size is then introduced, using a physical parameter to express the change of Gibbs free energy due to surface creation for a unit volume of crystalline phase. Finally, experimental results from literature exhibiting shape-memory effects in rubber are elucidated using this crystallite life cycle theory.

Analytical network-averaging of the tube model: Strain-induced crystallization in natural rubber

In this contribution, we extend the analytical network-averaging concept (Khiêm and Itskov, 2016) to phase transition during strain-induced crystallization of natural rubber. To this end, a physically-based constitutive model describing the nonisothermal strain-induced crystallization is proposed. Accordingly, the spatial arrangement of polymer subnetworks is driven by crystallization nucleation and consequently alters the mesoscopic deformation measures. The crystallization growth is elucidated by diffusion of chain segments into crystal nuclei. The crystallization results in a change of temperature and an evolution of heat source. By this means, not only the crystallization kinetics but also the Gough–Joule effect are thoroughly described. The predictive capability of the constitutive model is illustrated by comparison with experimental data for natural rubbers undergoing strain-induced crystallization. All measurable values such as stress, crystallinity and heat source are utilized for the comparison.

Energy storage due to strain-induced crystallization in natural rubber: The physical origin of the mechanical hysteresis

Strain-induced crystallization is classically assumed to be responsible for the hysteresis loop observed in the mechanical response of cis-1,4-polyisoprene. The aim of the present study is to investigate where does this energy go. Energy balances carried out using infrared thermography have shown that the hysteresis loop is due neither to intrinsic nor thermal dissipation, but is entirely used by the material to change its microstructure. Thus, significant changes in the internal energy accompany SIC. Experiments performed show that the mechanical energy brought to deform the material is stored elastically in the amorphous phase (chain alignment and accumulation of topological constraints in the crystallite vicinity) and is released with a different kinetics during crystallite melting. The demonstration that NR is able to store mechanical energy without converting it into heat is a realistic way to explain its extraordinary resistance to crack growth.

Features of strain-induced crystallization of natural rubber revealed by experiments and simulations

Understanding the strain-induced crystallization (SIC) mechanism of natural rubber (NR) is a prerequisite for comprehending the reinforcement mechanism of NR and for designing new high-performance rubber materials. With the help of new technologies that have enabled more accurate experimental measurement of the microstructure and the use of molecular simulations that can be applied to probe structural changes during stretching in real time, some interesting results have been found. For instance, even at high strains, a very large fraction of the unoriented amorphous phase still remains in the stretched sample with homogeneous or heterogeneous networks. In addition, the onset strain of SIC in peroxide-cured NR decreases with an increasing crosslinking density, while sulfur-cured NR is independent of the crosslinking density, which cannot be explained by conventional theories. The presence of nanofillers, entanglements, non-rubber components and pseudoend-linked networks also results in abnormal phenomena of SIC. Both experimental measurements and molecular simulations were applied to probe the strain-induced crystallization (SIC) of natural rubber in real time, and some interesting results have been found. A very large fraction of unoriented amorphous phase remains, even at high strains. The onset strain of SIC in peroxide-cured NR decreases with increasing crosslinking density, while that in sulfur-cured NR is independent of crosslinking density. This difference may be caused by different network structures. Nanofillers, entanglements, non-rubber components and pseudoend-linked networks also result in abnormal SIC phenomena.

Dominating factor of strain-induced crystallization in natural rubber

The contribution of entropy change due to stretching of polymer chains in promoting crystal nucleation is theoretically derived for strain-induced crystallization of natural rubber. The results of theoretical calculation are compared with experimental results obtained by fast time-resolved wide-angle X-ray diffraction. Usual values of surface free energies corresponding to chain-folded nuclei lead to theoretical results far from experimental measurements. Because the discrepancy comes from the large activation energy of nucleation even after the stretching of polymer chains, additional contribution of reduced surface free energies due to the formation of bundle-like nuclei was taken into account. This treatment allows to faithfully reproduce experimental results and then to conclude that nuclei formed in natural rubber during stretching are of bundle-like type. Moreover, it reveals that surface energies have a greater effect on the decrease of critical free energy than the change in entropy due to deformation.

Observing Nucleation Transition in Stretched Natural Rubber through Self-Seeding.

Potential competition between fringed-micelle nucleation (N1) and folded-chain nucleation (N2) widely exists in strain-induced crystallization (SIC). However, during uniaxial deformation, no in situ observational evidence of nucleation transition from N2 to N1 in SIC of natural rubber (NR) has been reported yet. In this work, self-seeding provides an effective way for this observation. By the introduction of residual TIC (temperature-induced crystallization)-melting crystallites into pure NR system, in situ synchronic WAXD revealed the formation of low-oriented crystal in the initial deformation stage, which gradually evolves into highly oriented crystal at last. The low-oriented crystal is related to secondary folded-chain nucleation (N2) on the surface of residual TIC-melting crystallites (self-seeding), while newly formed highly oriented crystal is associated with N1. For the first time, the concept of "self-seeding" is innovatively applied to SIC process so that NR exhibits clear nucleation transition phenomenon. Further, theoretical computation of nucleation barrier in the special NR system well reflects that self-seeding has the role of both increasing critical strain of nucleation transition and decreasing onset strain of SIC, thus providing conditions for the observation.

Kinetics of strain-induced crystallization in natural rubber: A diffusion-controlled rate law

The kinetics of strain-induced crystallization in natural rubber were studied by synchrotron wide-angle X-ray diffraction (WAXD) in a strain jump setup over a broad temperature range. The increase of crystallinity after a strain jump was observed on a millisecond time scale, until a steady state plateau was approached. A novel model, based on chain dynamics scaling laws for diffusion in polymer networks was developed to interpret the time and temperature dependence of the strain-induced crystallization. In contrast to established thermal crystallization models, this model for the strain-induced crystallization proposes that the formation of stretched crystallizable chain segments takes place before the attachment to a growing crystallite.

Thermodynamic Description of Strain-Induced Crystallization of Natural Rubber by a Combination of the Tube Model and a Scaling Argument

The addition of nanoparticles remarkably improves the mechanical properties and strain-induced crystallizability of natural rubber. To reveal the underlying mechanism for the enhancement of these physical properties, we analyzed the microstructural changes of rubber due to the inclusion of clay and carbon black by using the tube model. The rubber filled with clay exhibited a stronger confinement effect of entanglements on the chain conformation compared with the pure rubber and that filled with carbon black. As a result, we propose a new thermodynamic approach to the entropic elasticity of entangled rubber by a combination of the blob scaling argument and the tube model. The thermodynamic analysis demonstrated that the restriction effects of both entanglements and the filler network led to a reduction of the conformational entropy in clay-filled rubber, resulting in the decrease of the onset strain of crystallization. These results revealed the mechanism of the strain-induced crystallization and filler reinforcement from the viewpoint of thermodynamics.

Temperature dependence of strain-induced crystallization in natural rubber: On the presence of different crystallite populations

The effect of the temperature on strain induced crystallization (SIC) of natural rubber (NR) is studied by in situ wide angle X-rays scattering (WAXS) experiments performed along different thermo-mechanical paths (cyclic deformation at different temperatures or crystallization and melting in the deformed state). The crystallinity index (CI), the average crystallite size (L200) and the average volume of the crystallites (V) increase (decrease) similarly if the sample is cooled (heated) in the deformed state or stretched (unstretched) at fixed temperature. These experiments are analyzed through a thermodynamic description assuming that SIC is controlled by successive nucleation of crystallite populations. Such description predicts that for a stretching ratio λ above (below) 4 and a temperature above (below) room temperature, crystallization first occurs in chains with the highest (lowest) network chain density, leading to the smallest (largest) crystallites.

Strain-Induced Crystallization of Natural Rubber and Cross-Link Densities Heterogeneities

Strain-induced crystallization (SIC) of natural rubber (NR) is characterized during a cyclic deformation at room temperature and low strain rate (∼10–3 s–1) using in situ wide angle X-rays scattering (WAXS) measurements. The crystallinity index (CI) and average size of the crystallites in the three main directions are measured during loading and unloading. A scenario describing SIC is then proposed, assuming that SIC corresponds to the successive appearance of crystallite populations whose nucleation and growth depend on the local network density. From this scenario, a methodology, coupling experimental observations and thermodynamic description is developed to determine the distribution of the network chain density associated with the size of a corresponding crystallite population. Finally, complex cyclic tests are performed. They suggest the existence of a memory effect in the chains involved in crystallization, which eases the nucleation process of the crystallites.

The impact of strain-induced crystallization on strain during mechanical cycling of cross-linked natural rubber

The improvement of X-ray diffraction techniques since the last ten years allowed a renewal of the study of strain-induced crystallization in natural rubber. In particular real-time measurements are now commonplace. However, due to experimental difficulties, the exploitation of the X-ray data as far as the strain state of the remaining molten fraction is concerned has been left aside. Indeed the knowledge of the local extension of the molten chains is crucial for understanding the peculiar stress behavior observed during the crystallization process. This paper presents a systematic study of this parameter during mechanical cycling performed at various temperatures. It is shown that crystallization limits the amplitude of strain endured by the molten fraction during traction. This process of strain regulation may be one explanation for the protective effect of crystallization against tearing. A precise evaluation of the contribution of the molten fraction to the retraction force during mechanical cycling is made. The role of crystallites as nanofillers may be quantified this way.

A micro-mechanically based continuum model for strain-induced crystallization in natural rubber

Recent experimental results show that strain-induced crystallization can substantially improve the crack growth resistance of natural rubber. While this might suggest superior designs of tires or other industrial applications where elastomers are used, a more thorough understanding of the underlying physics of strain-induced crystallization in natural rubber has to be developed before any design process can be started. The objective of this work is to develop a computationally-accessible micro-mechanically based continuum model, which is able to predict the macroscopic behavior of strain crystallizing natural rubber. While several researchers have developed micro-mechanical models of partially crystallized polymer chains, their results mainly give qualitative agreement with experimental data due to a lack of good micro–macro transition theories or the lack of computational power. However, recent developments in multiscale modeling in polymers give us new tools to continue this early work. To begin with, a micro-mechanical model of a constrained partially crystallized polymer chain with an extend-chain crystal is derived and connected to the macroscopic level using the non-affine micro-sphere model. Subsequently, a description of the crystallization kinetics is introduced using an evolution law based on the gradient of the macroscopic free energy function (chemical potential) and a simple threshold function. Finally a numerical implementation of the model is proposed and its predictive performance assessed using published data.