Crystallization Of Natural Rubber Research Articles

A cryo-bulge apparatus for in situ weather balloon crystallization capturing during blowing by synchrotron radiation x-ray scattering.

A cryo-bulge apparatus, which can be directly installed in the synchrotron radiation x-ray scattering beamline, is designed and manufactured. Using the cryo-bulge apparatus, the crystallization of natural rubber during blowing can be captured in situ. For mechanical measurements, the rubber film is tightly clamped at the periphery of a circular window. A low temperature measurement is achieved by the presence of a large iron block, which ensures low temperature variation (<±2 °C in 1 h) during x-ray data acquisition. Since the incident x-ray beam passes through the top-most position of the rubber film, the information obtained by the current equipment is essentially under an equibiaxial deformation mode. Owing to precisely controlled internal pressure and temperature, the crystallization of rubber can be observed in situ by wide-angle x-ray scattering. The onset of crystallization is observed at a temperature T < 0 °C with an internal pressure P > 21 kPa. This suggests that the crystallization of rubber during blowing can occur under the equibiaxial deformation condition at low temperatures. The power scaling law is found to be 0.52%/kPa. The cryo-bulge apparatus is capable of clarifying the microstructural evolution of rubber during multi-dimensional deformation, which can provide guidance for the optimization of a weather balloon.

Cellulose nanocrystals as nucleating agents for the strain induced crystallization in natural rubber.

Vulcanized natural rubber (NR)/cellulose nanocrystals (CNC) composites with a CNC content of up to 5 wt% using physical blending and dicumyl peroxide crosslinking were prepared. The tensile properties were investigated at slow and high strain rates. The slow strain rate tests revealed an increase of the elastic modulus concomitant with a decrease of strain at the crystallization onset while increasing the CNC fraction. The high strain rate tests performed near adiabatic conditions demonstrated the ability of the CNC to improve the elastocaloric properties of the NR matrix, with an increase of 30% and 15% of heating and cooling capacities, respectively, in the presence of 3 wt% CNC. Such results were ascribed to (i) a higher thermoelastic effect, due to strain amplification in the NR matrix in the presence of CNC and (ii) a nucleating effect of the CNC on strain induced crystallization. This series of materials can be proposed as a promising eco-friendly alternative to conventional carbon black filled rubber as potential green elastocaloric materials (heating pump, cooling machines).

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

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

Thermodynamics of strain-induced crystallization in filled natural rubber under uni- and biaxial loadings, Part II: Physically-based constitutive theory

To understand thoroughly the strain-induced crystallization in natural rubbers, conventional mechanical measurements are inadequate because they only provide a macroscopic relation between stress and strain. In this second part, a physically-based constitutive model for filled natural rubbers is coupled with the infrared thermography-based quantitative surface calorimetry (Part I) to shed new light on multiaxiality of strain-induced crystallization. In contrast to previous works, the kinetics of phase transition outside thermodynamic equilibrium is discussed. By introducing only two additional parameters (compared to the equilibrium crystallization theory), underlying mechanisms of nonequilibrium strain-induced crystallization can be well interpreted. To capture multiaxiality of strain-induced crystallinity, the analytical network-averaging is utilized for the calculation of kinematic internal variables. Model predictions are then compared with comprehensive testing data (Part I) and demonstrate good agreement with these experiments.

Strain-induced self-assembly of crystallites in elastomers.

We propose a model of strain-induced crystallization in elastomers, combining Flory's original work with a description of crystallization adopted from the theory of micellar solutions. The experimentally observed hysteresis appears in the model due to a continuous, unidirectional change of the free energy difference between straightened polymer sections which occur isolated and those which are aggregated. The model yields good qualitative and even semiquantitative agreement with measurements of crystallization in natural rubber at variable cross-link density, strain amplitude, and temperature. The attendant description of the stress hysteresis is less good but still qualitatively correct.

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.

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.

Thermodynamics and mechanics of stretch-induced crystallization in rubbers.

The aim of the present paper is to provide a quantitative prediction of the stretch-induced crystallization in natural rubber, the exclusive reason for its history-dependent thermomechanical features. A constitutive model based on a micromechanism inspired molecular chain approach is formulated within the context of the thermodynamic framework. The molecular configuration of the partially crystallized single chain is analyzed and calculated by means of some statistical mechanical methods. The random thermal oscillation of the crystal orientation, considered as a continuous random variable, is treated by means of a representative angle. The physical expression of the chain free energy is derived according to a two-step strategy by separating crystallization and stretching. This strategy ensures that the stretch-induced part of the thermodynamic crystallization force is null at the initial instant and allows, without any additional constraint, the formulation of a simple linear relationship for the crystallinity evolution law. The model contains very few physically interpretable material constants to simulate the complex mechanism: two chain-scale constants, one crystallinity kinetics constant, three thermodynamic constants related to the newly formed crystallites, and a function controlling the crystal orientation with respect to the chain. The model is used to discuss some important aspects of the micromechanism and the macroresponse under the equilibrium state and the nonequilibrium state involved during stretching and recovery, and continuous relaxation.

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.

Thermomechanics of strain‐induced crystallization in natural rubber under cyclic loading

AbstractStrain‐induced crystallization in natural rubber has been widely investigated in the last decades by scattering techniques. Due to dissipative processes and structural changes in the material, deformation processes are accompanied with a significant amount of heat production/consumption. Time‐resolved wide‐angle X‐ray diffraction (WAXD) was used to characterize the structural changes in natural rubber and the local degree of crystallinity during mechanical deformation. By the simultaneous use of an infrared camera, the scattering results could be combined with changes in the temperature of the specimen due to dissipation and crystallization resp. melting of the crystallites. (© 2017 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

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.

Effect of strain‐induced crystallization on the tear strength of natural rubber/styrene butadiene rubber blend

AbstractStrain‐induced crystallization of natural rubber (NR) can greatly influence the mechanical properties of NR blends. In this study, we investigated the effect of strain‐induced crystallization on the tear energy (G) of NR/styrene butadiene rubber (SBR) blends with different compositions and cross‐link densities. Using a shift factor, we could superimpose G of an SBR blend and an NR/SBR (1/9) blend onto a single master curve. In contrast, G of NR was split into two master curves. Although the master curve of G for the NR/SBR (3:7) blend was split, increasing the cross‐link density threefold made it possible to superimpose it onto a single master curve. Strain‐induced crystallization of NR in the NR/SBR blend did not occur as a result of decreasing the NR content, increasing the cross‐link density, or removing fatty acids. Together, these findings indicate that strain‐induced crystallization of NR plays an important role in the mechanical properties of the blend.

Filler-induced heterogeneous distribution of stretch-induced crystallization in natural rubber: An in-situ synchrotron-radiation micro-focused scanning X-ray diffraction study

Crystallinity distribution of natural rubber around filled micrometer-size glass beads under different strains are studied with in-situ micro-focused scanning X-ray diffraction (μ-SXRD), where the glass beads are taken to serve as a model of filler. The experimental results suggest, due to the gradient stress field around the glass bead, the oscillatory distribution of crystallinities around the glass bead tends to form soft-hard double network with multi-scale hierarchical structures and spontaneously responds to external strains. The double network spreads stress in larger region and avoids stress concentration, in which the hard domains with high crystallinities effectively sustain the external stress while the soft domains with low crystallinity store elastic energy. According to the equal strain and equal stress conditions, the quantitative calculation on enhancement factor indicates that the soft-hard double network structure can increase the fracture energy to about three orders of magnitude as comparing to that of structure with homogeneous distribution of crystallinity.

Effect of non-rubber components on the mechanical properties of natural rubber

Effect of the nanomatrix structure on mechanical properties of natural rubber was investigated in relation to the strain-induced crystallization. Structure of natural rubber was analyzed through Fourier transform infrared spectroscopy, wide-angle X-ray diffraction measurement and transmission electron microscopy. The nanomatrix of the non-rubber components was found to be inevitably formed in natural rubber, in which natural rubber particles linking to fatty acids were dispersed in the nanomatrix of the proteins and phospholipids. The nanomatrix disappeared after deproteinization of natural rubber with urea. Tensile strength and modulus of natural rubber were reduced by removal of the fatty acids and the proteins, which resulted in disappearance of the nanomatrix structure. The effect of fatty acids on the crystallization of natural rubber in small particles as a dispersoid was proved by tensile test of blend of natural rubber and styrene butadiene rubber. Copyright © 2016 John Wiley & Sons, Ltd.

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.

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.

Influence of strain rate and temperature on the onset of strain induced crystallization in natural rubber

Strain induced crystallization (SIC) of natural rubber (NR) has been studied in a large range of strain rate (from 5.6×10−5s−1 to 2.8×101s−1) and temperature (from −40°C to 80°C) combining mechanical and thermal analysis. Both methods are used to extend the study of SIC from slow strain rates – performed with in situ wide angle X-rays scattering (WAXS) – to high strain rates. Whatever the temperature tested, the stretching ratio at crystallization onset (λc) increases when the strain rate increases. This strain rate effect is strong at low temperature (close to Tg) and weak at high temperature (much higher than Tg). A theoretical approach derived from the Hoffman–Lauritzen equation has been developed and provides a good qualitative description of the experimental results. At low temperature, the strong increase of λc with strain rate is explained by a too long diffusion time compared to the experimental time. At high temperature, SIC kinetics is rather controlled by the nucleation barrier which mainly depends on the strain energy. When the stretching ratio increases, this nucleation barrier strongly decreases, allowing crystallization even for short experimental time.