Cross-linked Natural Rubber Research Articles

A universal strategy for decoupling stiffness and extensibility of polymer networks.

Since the invention of polymer networks such as cross-linked natural rubber in the 19th century, it has been a dogma that stiffer networks are less stretchable. We report a universal strategy for decoupling the stiffness and extensibility of single-network elastomers. Instead of using linear polymers as network strands, we use foldable bottlebrush polymers, which feature a collapsed backbone grafted with many linear side chains. Upon elongation, the collapsed backbone unfolds to release stored length, enabling remarkable extensibility. By contrast, the network elastic modulus is inversely proportional to network strand mass and is determined by the side chains. We validate this concept by creating single-network elastomers with nearly constant Young's modulus (30 kilopascals) while increasing tensile breaking strain by 40-fold, from 20 to 800%. We show that this strategy applies to networks of different polymer species and topologies. Our discovery opens an avenue for developing polymeric materials with extraordinary mechanical properties.

Super-tough, super-elastic, temperature-responsive, and tunable viscoelastic elastomer enabled by embedding nanosized liquid metal droplets

Liquid metal (LM) based composites are playing irreplaceable roles in many emerging fields such as stretchable and wearable electronics, soft robotics. However, it is still challenging to facilely fabricate the LM-based elastomers with nanosized and well-dispersed LM domains. Herein, the LM droplets filled elastomer nanocomposites (ENCs) with temperature-responsive, super-tough, super-elastic and tunable viscoelastic properties are introduced. By employing the conventional rubber processing method, LM is fragmented into nanoscale droplets and dispersed uniformly in cross-linked natural rubber (NR) without compromising the soft and highly stretchable properties of the matrix. In addition to the remarkable enhancement in tear resistance, the toughness of the resulting composites is strikingly improved as lowering the applied temperature, which is attributed to the phase transition and the simultaneous volume expansion of LM droplets. Surprisingly, for the viscoelasticity, this LM-based ENCs exhibit almost the same dynamic hysteresis with the pure NR system at the service condition of automobile tires, which is remarkably reduced compared to the traditional ENCs filled with rigid nanoparticles. Furthermore, this material also shows a good damping property for noise attenuation in the case of submarine covering. Collectively, this work opens a new avenue for the next generation of high-performance and multifunctional ENCs equipped in low-temperature working conditions.

Construction of a reduced graphene oxide/polyvinyl alcohol (PVA)-natural rubber (NR) porous rubber sponge toward solar-driven sustainable water purification

Solar-driven evaporation is an environmentally benign and sustainable technology to address the global water crisis. The abundant and renewable solar energy as its sole input energy, thus leading to zero-carbon emission and minimum environmental impact. Thus, in this work, a hydrophilic reduced graphene oxide (rGO)/polyvinyl alcohol (PVA)-natural rubber (NR) porous rubber sponge was obtained by emulsion ice template and vulcanizing at high temperature. The cross-linked NR rubber and the robust PVA endow rGO/PVA-NR sponge with excellent flexibility integrated outstanding mechanical robustness. The water evaporation rate of the resultant rGO/PVA-NR evaporator can reach up to 2.11 kg·m−2·h−1 under 3 sun illumination. Moreover, benefitting from the multi-scale porous structure and hydrophilicity of the rGO/PVA-NR sponge, water is continuously transported from the bulk water to the evaporation surface in whole evaporation process, ensuring a continuous water supply and salt self-cleaning. Moreover, the results of practical solar-driven interfacial evaporation under outdoor conditions indicated that rGO/PVA-NR evaporator has overall multifunctional purification for inorganic ions and wastewater contaminated with organic dyes. This investigation provides a convenient and environmentally friendly strategy toward developing high-performance solar steaming technologies based on rubber sponge for effective water purification to address the global water crisis.

Strengthened adipic acid cross-linked epoxidized natural rubber by cellulose nanocrystals

Strengthened adipic acid cross-linked epoxidized natural rubber by cellulose nanocrystals

Preparation of A Dynamically Vulcanized Nanocomposite Thermoplastic Based on PLA/NR and Cellulose Nanocrystal (CNC)

Dynamically vulcanized nanocomposite thermoplastic based on poly (lactic acid) (PLA)/natural rubber (NR) is prepared through a dynamic curing process. The cross-linked NR phase exhibits a continuous network structure embedded in the PLA phase, which differs from the conventional plastic/rubber system with an "island-sea" morphology in which vulcanized rubber particles are dispersed in the plastic matrix. Its favorable internal interfacial compatibility was confirmed by SEM analysis. Also, CNC has been used as a sustainable additive to improve the mechanical properties of PLA/NR thermoplastic vulcanizate (TPV). To achieve homogeneous dispersion of CNC, the melt mixing method was used. The mechanical properties were considerably enhanced by CNC addition, which increased from 14% of the unfilled TPV to 70% of the sample with CNC (1.5 phr relative to PLA). The "green" raw materials and high mechanical properties of PLA/NR/CNC prepared from the dynamic curing process are expected to open up a wide range of potential applications in the fields of smart medicine.

Strain hardening in biaxially stretched elastomers undergoing strain-induced crystallization.

We reveal strain hardening due to strain-induced crystallization (SIC) in both cross-linked natural rubber (NR) and its synthetic analogue (IR) under planar extension, a type of biaxial stretching where the rubber is stretched in one direction while maintaining the dimension in the other direction unchanged. Utilizing a bespoke biaxial tensile tester, planar extension tests were conducted on geometrically designed and optimally shaped sheet specimens to achieve a uniform and highly strained field. Evident strain hardening due to SIC was observed in both stretching (x) and constrained (y) directions when the stretch (λx) exceeded a critical value λx,c. The λx,c value aligned with the onset stretch of SIC in planar extension, as determined by wide-angle X-ray scattering measurements. Interestingly, the nominal stress ratio between the constrained (σy) and stretching (σx) axes as a function of λx exhibited a distinct minimum near λx,c. This minimum signifies that the increment of σx induced by an increase in λx surpasses that of σy before strain hardening (λx < λx,c), while the relationship is reversed in the strain hardening region (λx > λx,c). The λx,c value in planar extension (4.7 for IR and 4.5 for NR) was slightly lower than that in uniaxial extension (5.7 for IR and 5.2 for NR). This difference in λx,c values can be explained by considering a single mechanical work required for strain hardening, owing to the relatively small dissimilarities between the two stretching modes. This investigation contributes significantly to the understanding of SIC phenomena in biaxial stretching, and provides valuable insights for predicting the mechanical response of SIC rubber under various deformation conditions.

Elastocaloric effect: Impact of heat transfer on strain-induced crystallization kinetics of natural rubber

When natural rubber is rapidly loaded and unloaded there occurs a variation of its temperature called the elastocaloric effect. The objective of this work was to determine whether this temperature variation could be responsible for a delay in strain-induced crystallization (SIC) kinetics and stress relaxation. To this end, the SIC kinetics of cross-linked natural rubber samples with different thicknesses was studied at constant elongation after adiabatic stretching using thermal, mechanical and in-situ WAXS characterizations. The coupling of these techniques revealed that, at a fixed elongation λ (between 5 and 6), the crystallization kinetics and stress relaxation were faster in the thinner sample (with a short heat exchange time constant), thus providing evidence that heat transfer affected the SIC kinetics and should be considered when interpreting experimental results.

Homogenizing Blends of Cross-linked Polymers by Interfacial Exchange Reactions.

Obtaining homogeneous blends of two covalently cross-linked polymers by mechanical mixing is not possible due to their permanent network topologies. Here, we demonstrate an effective route to prepare polymer blends from the common cross-linked epoxidized natural rubber (ENR) and epoxy resin vitrimer (EV) by mechanical mixing. Interfacial exchange reactions between these two networks occur by a dynamic transesterification. The as-prepared ENR-EV blends show excellent mechanical strength, extensibility, and thermal stability. Moreover, they also show typical vitrimeric properties, including self-healing, welding, and reprocessability. This work demonstrates a large-scale preparation of vitrimeric materials with high performance and versatility from commercially available polymers and affords a promising strategy to recycle both waste rubbers and resins.

A ReaxFF molecular dynamic study on pyrolysis behavior and sulfur transfer during pyrolysis of vulcanized natural rubber

In this work, ReaxFF molecular simulations were performed to study the pyrolysis behavior of chemical cross-linked natural rubber (NR) under non-isothermal and isothermal conditions. Three different sulfur vulcanized NR models were established and simulated to study the effect of inner sulfur structure on NR decomposition behavior and sulfur evolution in comparison with carbon cross-linked structure. To understand the NR decomposition with temperatures, the non-isothermal simulations were performed between 300 and 3800 K at a 50 K ps−1 heating rate. The results reveal that the decomposition process can be classified into four stages: 1) Structure adjustment; 2) Decomposition of the main carbon chains; 3) Secondary decomposition of heavy tar; and 4) Deep decomposition of light tar. Based on the results of non-isothermal pyrolysis, four different temperatures were selected for the isothermal simulations. Compared with carbon cross-linked NR, sulfur cross-linked structures facilitate the generation of C2H4 and C4H6 in the gas phase at low temperatures. At higher temperatures, more heavy tar is generated. Regarding the sulfur evolution, the sulfur-containing products mainly include H2S, thiophene, sulfide, and thiol. The distribution of sulfur-containing products with temperatures follows the similar pattern with the product distribution of main compounds. At higher temperatures, most sulfur exists in the form of thiophene compounds. In particular, the structure with single CS cross-links facilitates the generation of H2S at low temperatures. The results of this work provide insight into the sulfur transformation and pyrolysis behavior of vulcanized NR.

Bimodal 1H Double Quantum Build-Up Curves by Fourier and Laplace-like Transforms on Aged Cross-Linked Natural Rubber.

The 1H DQ Fourier and Laplace-like spectra for a series of cross-linked natural rubber (NR) samples naturally aged during six years are presented and characterized. The DQ build-up curves of these samples present two peaks which cannot be described by classical functions. The DQ Fourier spectra can be obtained after a numeric procedure which introduces a correction time which depends less on the chosen approximation, spin-½ and isolated CH2 and CH3 functional groups. The DQ Fourier spectra are well described by the distributions of the residual dipolar coupling correlated with the distribution of the end-to-end vector of the polymer network, and with the second and fourth van Vleck moments. The deconvolution of DQ Fourier spectra with a sum of four Gaussian variates show that the center and the width of Gaussian functions increase linearly with the increase in the cross-link density. The Laplace-like spectra for the natural aged NR DQ build-up curves are presented. The centers of four Gaussian distributions obtained via both methods are consistent. The differences between the Fourier and Laplace-like spectra consist mainly of the spectral resolution in the favor of Laplace-like spectra. The last one was used to discuss the effect of natural aging for cross-linked NR.

Vitrimer-like material based on a biorenewable elastomer crosslinked with a dimeric fatty acid

The development of cross-linked epoxidized natural rubber (ENR) that is recyclable, repairable and weldable is presented in this paper. Dimer fatty acid (DFA) was used as cross-linker in ENR50 to generate β-hydroxy ester linkages through epoxide ring-opening reaction between carboxylic groups in DFA and epoxide groups in ENR. Meanwhile, a transesterification catalyst (Zinc catalyst) provided a dynamic covalent network in the cross-linked ENR, which is a vitrimer-like elastomer. The mechanical properties and cross-link density of this vitrimer-like rubber could be tailored by changing the concentration of DFA. Using DFA at approximately 1.5 mmol COOH (~1.23 phr) in ENR50 offered good tensile strength and elongation at break. Stress relaxation showed that the reaction rate of bond exchange in DFA-cured ENR could be significantly accelerated by increasing temperature, suggesting malleable flow of these vitrimers. Moreover, the prepared vitrimer-like elastomer showed repairable, weldable, recyclable and reprocessable characteristics. This technological approach is significant in providing a covalent adaptive network in vulcanized rubber, which has great potential for further development of environmentally friendly rubber vulcanizates.

Designing Supertough and Ultrastretchable Liquid Metal-Embedded Natural Rubber Composites for Soft-Matter Engineering.

Functional elastomers with incredible toughness and stretchability are indispensable for applications in soft robotics and wearable electronics. Furthermore, coupled with excellent electrical and thermal properties, these materials are at the forefront of recent efforts toward widespread use in cutting-edge electronics and devices. Herein, we introduce a highly deformable eutectic-GaIn liquid metal alloy-embedded natural rubber (NR) architecture employing, for the first time, industrially viable solid-state mixing and vulcanization. Standard methods of rubber processing and vulcanization allow us to fragment and disperse liquid metals into submicron-sized droplets in cross-linked NR without compromising the elastic properties of the base matrix. In addition to substantial boosts in mechanical (strain at failure of up to ∼650%) and elastic (negligible hysteresis loss) performances, the tearing energy of the composite was enhanced up to 6 times, and a fourfold reduction in the crack growth rate was achieved over a control vulcanizate. Moreover, we demonstrate improved thermal conductivity and dielectric properties for the resulting composites. Therefore, this work provides a facile and scalable pathway to develop liquid metal-embedded soft elastomeric composites that could be instrumental toward potential applications in soft-matter engineering.

Green biocomposites of chitosan dispersed in natural rubber matrices, and their urea controlled-release application

A green biocomposite comprising chitosan dispersed in partially cross-linked natural rubber matricesd was prepared. The starting materials and biocomposites were characterized by means of Fourier transform–infrared spectroscopy, thermogravimetric analysis, and scanning electron microscopy. Chitosan was found well dispersed in partially cross-linked natural rubber matrices indicating that natural rubber latex is successfully stabilized by sodium dodecyl sulfate. The swelling study of the biocomposites was carried out in several solvents. The biocomposites were used for the controlled-release of urea. The release behavior was found consistent, controlled, and prolonged over a period of 72 h. The Korsmeyer–Peppas model was applied to the experimental data and the possible release mechanism was found Fickian diffusion, which indicates that the polymer relaxation time is much greater than the characteristic solvent diffusion time.

Encapsulation of acidified chitosan within partially cross-linked natural rubber matrices and their potential slow-release application

Biocomposites comprising acidified chitosan (CTS) encapsulated in partially cross-linked natural rubber (NR) matrices (CTS-e-NR) were prepared and characterized by Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) and field emission scanning electron microscopy (FE-SEM). The results of FT-IR, TGA and FE-SEM revealed that CTS was merely encapsulated within partially cross-linked natural rubber, and no chemical interaction was observed between CTS and NR. Soil burial biodegradation studies indicated that CTS-e-NR was degraded beyond any doubt, and weight loss of 48.98% was observed within 8 months. The release behaviour of CTS-e-NR was investigated in distilled water, where the naphthols (pesticide precursors) 1-hydroxynaphthalene (1-N), 2-hydroxynaphthalene (2-N), 1,4-dihydroxynaphthalene (1,4-N) and 2,6-dihydroxynaphthalene (2,6-N) were used as model pesticides. The percentage released in the first 24 h was found to be 7.38, 4.39, 2.96 and 6.95%, while 62.69, 60.64, 22.68 and 40.985% was released at the 35th day, for 1-N, 2-N, 1,4-N and 2,6-N, respectively. The release of naphthols followed non-Fickian diffusion: the combination of both polymer erosion and diffusion release mechanisms. In addition, the release behaviour was found to be consistently controlled and prolonged over a period of 35 days. Therefore, CTS-e-NR is strongly recommended for the controlled-release application of real pesticides.

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.

Draw Ratio at the Onset of Strain-Induced Crystallization in Cross-Linked Natural Rubber

The first theory of strain-induced crystallization in regular elastomers was laid out some 70 years ago by Flory. He identified the driving mechanism that concurs to the stabilization of the crystalline phase under stretching as the relaxation of the remaining amorphous phase because of the growth of oriented crystallites. However, Flory’s theory is an equilibrium theory and does not apply to dynamic conditions that are kinetically controlled. We presently performed careful measurements of the crystallinity index and draw ratio for the amorphous phase during mechanical cycling and step-wise extension so as to enlighten their intricate interplay. We propose a simple description of the crystallization process that allows establishing a quantitative relation between these two parameters and predicting their time-evolution. This analysis leads to the definition of a master curve for the time-variation of the crystallinity index.

Synergistic Cross-linking and Reinforcing Enhancement of Rubber Latex with Cellulose Nanocrystals for Glove Applications

The effect of cellulose nanocrystals (CNCs) on the reinforcing, cross-linking, and solvent barrier properties of lightly cross-linked natural rubber (NR) latex films for dipped goods applications w...

Design of Dual Hybrid Network Natural Rubber-SiO2 Elastomers with Tailored Mechanical and Self-Healing Properties.

The preparation of natural rubber (NR)–silica (SiO2) elastomeric composites with excellent mechanical properties along with better self-healing ability remains a key challenge. Inspired by the energy dissipation and repairability of sacrificial bonds in biomaterials, a strategy for combining covalent and noncovalent sacrificial networks is engineered to construct a dual hybrid network. Here, the approach used to fabricate the composites was self-assembly of NR, bearing proteins and phospholipids on its outer bioshell, with SiO2 via metal-ion-mediated heteroaggregation effected by reversible electrostatic and H-bonds. Further, covalent cross-links were incorporated by a silane coupling agent, bis [3-(triethoxysilyl) propyl] tetrasulfide. The intrinsic self-healing ability of the composite at the molecular level was studied by broadband dielectric spectroscopy that unraveled the mechanism of the healing process. The synergistic effect between the molecular interdiffusion of the cross-linked NR chains and the electrostatic and H-bonding interactions imparted an exceptional self-healing characteristic to the liquid–liquid-mixing-prepared NR–SiO2 composites with improved mechanical performance. Specifically, the segmental relaxation dynamics of the healed composite was largely restricted due to increased number of ion–dipole interactions and S–S cross-links at the junction of the cut surface. We envisage that this extraordinary healing property, unreported yet, would be of great importance toward the design of novel NR–SiO2 elastomeric hybrids with superior mechanical properties.

INFLUENCE OF STRAIN-INDUCED CRYSTALLIZATION ON STRESS SOFTENING OF SULFUR CROSS-LINKED UNFILLED GUAYULE AND DANDELION NATURAL RUBBERS

ABSTRACT The stress softening behaviors of sulfur cross-linked unfilled guayule natural rubber (S-GR) and sulfur cross-linked unfilled dandelion natural rubber (S-DR) under cyclic deformation were characterized by simultaneous wide angle X-ray diffraction and tensile measurements under cyclic deformation. The behaviors were found to be affected by their strain-induced crystallization (SIC) phenomena and aggregated nonrubber components. The stress softening degree at stretching ratio = 8.0 of S-DR was almost two times larger than that of S-GR. Additionally, the hysteresis loss and residual strain of S-DR were much higher than those of S-GR in the same cycle. The key factors that caused the increase in their degree of stress softening were the increase in average volume and the decrease in average number of strain-induced crystallites upon cyclic deformation, not the insignificant decrease in crystallinity. The breakage of the aggregated nonrubber components is a main origin of stress softening behaviors for S-GR and S-DR because their SIC behaviors also were significantly influenced by the aggregates of nonrubber components in the rubber matrixes. Both the effects of nonrubber components and SIC on stress softening were more dominant in S-DR than in S-GR, probably owing to the larger amount of aggregated nonrubber components in the former than in the latter. The results will be useful in effectively using guayule and dandelion natural rubbers as alternatives to Hevea natural rubber in the rubber industry.

Strong Strain Sensing Performance of Natural Rubber Nanocomposites.

A detail study concerning the strain (tensile) dependent electrical conductivity of elastomeric composites is reported in this present paper. Multiwall carbon nanotubes (CNT), conducting carbon black (CB), and their combinations were considered as conducting filler in cross-linked natural rubber matrix. The loadings of the fillers were considered from 3 to 11 phr (filler concentration close to their percolation threshold). Without hindering the elastic nature of the composite (reversible stretchability up to several 100%), the change of relative resistance, ΔR/R0 (ΔR is the change in the resistance with respect to strain and R0 is the initial resistance of the sample) of the CB filled composites was found to be as much as ∼1300 at around 120% elongation. This value is much higher than any other reported values obtained from conducting polymeric composites. It was found that CNT offered a strong strain dependent character in the regime 100% to 150% elongation, whereas, the carbon black filled natural rubber showed strong strain dependencies at 50% to 100% elongation strain. The combination of two different fillers could be exploited to tailor and manipulate the sensing operating regime from 50% to 150% strain depending on the ratios of the two filler system. Additionally, after several loading-unloading cycles, the conductivity of the sample was very stable for CB filled system but for CNT filled system the conductivity of the sample was altered. This type of elastic materials could be used in structural health monitoring, sensors in different dynamic elastomeric parts like tires, valves, gaskets, engine mounts, etc.