The Role of Lanthanum Stearate on Strain-Induced Crystallization and the Mechanical Properties of Whole Field Latex Rubber.

Natural rubber (NR) is extensively utilized in numerous industries, such as aerospace, military, and transportation, because of its exceptional elasticity and all-around mechanical qualities. However, commercial NR made using various techniques typically has distinct mechanical characteristics. For instance, whole field latex rubber (SCR-WF) cured with accelerator 2-Mercaptobenzothiazole exhibits poor mechanical properties. This work attempts to enhance the mechanical property of SCR-WF via the addition of lanthanum stearate (LaSt). The influence of LaSt on strain-induced crystallization (SIC) and the mechanical properties of SCR-WF were investigated. The results of crosslinking density measured by the equilibrium swelling method demonstrate that the presence of LaSt significantly increases the crosslinking density of SCR-WF with lower loading of LaSt. The results of the mechanical properties show that the introduction of LaSt can enhance the tensile strength and fracture toughness of SCR-WF. To reveal the mechanism of LaSt improving the mechanical properties of SCR-WF, synchrotron radiation wide-angle X-ray diffraction (WAXD) experiments were used to investigate the SIC behaviors of SCR-WF. We found that the LaSt leads to higher crystallinity of SIC for the strain higher than 3.5. The tube model indicates the contribution of LaSt in both crosslinking and topological constraints. This work may provide an instruction for developing SCR-WF with superior mechanical properties.

Octylamine regulating the mechanical robustness of natural rubber by involving in the construction of crosslinking network.

The formation of three dimensional network structure is critical in determining mechanical properties of natural rubber (NR). Consequently, it is vital to regulate crosslinking network of NR by controlling vulcanization process. Inspired by our previous studies on contribution of non-rubber components (NRCs) to the excellent properties of NR, we find octylamine in NRCs decreases the activation energy (Ea) of vulcanization from 82.73 kJ/mol to 44.34 kJ/mol, thereby reducing vulcanization time from 18.67 min to 2.71 min. From microscopic perspective, octylamine tends to coordinate with zinc ions to improve dispersion of ZnO in NR. And octylamine promotes ring-opening reaction of S8 to favor formation of polysulfide intermediates. Therefore, the incorporation of octylamine remarkably improves vulcanization efficiency, which contributes to the formation of a more homogeneous network with higher crosslinking density, enhancing remarkably the strength and toughness of NR. As a result, the tensile strength and fracture energy of samples are as high as 31.15 MPa and 68.88 kJ/m2, respectively. In addition, even with a 60 % reduction in ZnO content, the NR samples still maintain high vulcanization efficiency and excellent mechanical properties after the addition of octylamine, which provides a green and feasible way to alleviate the environmental pollution caused by ZnO.

Toward Mechanically Robust Crosslinked Elastomers through Phase Transfer Agent Tuning the Solubility of Zn2+ in the Organic Phase.

Zinc oxide (ZnO), which is toxic to aquatic organisms, is widely used as an activator in the rubber industry. The reduction of ZnO content is one of the efficient ways to tackle ecological environment impacts induced by ZnO. However, the incompatibility between Zn2+ and organic matrix inhibits the solubility and activity of Zn2+ in the organic matrix, causing the heavy use of ZnO. This work develops a phase transfer agent with Zn2+-philic structure and oleophilic structure to increase the solubility of Zn2+ in the organic matrix. The phase transfer agent and Zn2+ form coordination interactions, while the hydrophobic chains of phase transfer agent and organic matrix form hydrophobic interactions. The above two interactions improve the solubility and activity of Zn2+ in the organic matrix, contributing to the formation of crosslinking network. Through the phase transfer agent strategy, we obtain the mechanically robust elastomers, and the samples with low ZnO content still maintain the superior properties. This work provides an efficient way to reduce ZnO content without sacrificing the performance of elastomers.

Microstructure and Lamellae Phase of Raw Natural Rubber via Spontaneous Coagulation Assisted by Sugars.

Natural rubber (NR) as a renewable biopolymer is often produced by acid coagulation of fresh natural latex collected from Hevea brasiliensis. However, this traditional process is facing a huge economic and environmental challenge. Compared with the acid coagulation, spontaneous or microorganism coagulation is an ecofriendly way to obtain NR with excellent performance. To clarify the influence of different sugars on NR quality, several sugars were used to assist the coagulation process. Influence of different sugars on microstructure and cold crystallization were examined by 1H NMR, DSC, etc. The results indicated that sugars exhibit different biological activity on terminal components of fresh field latex and can influence the resultant molecular structure and basic properties. Brown sugar exhibits higher metabolic activity and is inclined to decompose the protein and phospholipids crosslinking compared with other sugars. The larger molecular weight of sugar molecule is beneficial for the formation of the stable α lamellae phase and higher overall degree of crystallization.

Mechanically Robust Elastomers Enabled by a Facile Interfacial Interactions-Driven Sacrificial Network.

Strength and toughness are usually mutually exclusive for materials. The sacrificial bond strategy is used to address the trade-off between strength and toughness. However, the complex construction process of sacrificial network limits the application of sacrificial network. This work develops a facile strategy to construct an interfacial interactions-driven sacrificial network. The authors' group finds that there are the interfacial interactions between arginines (A) aggregates and molecular chains. Such interfacial interactions result in the mechanical properties of samples having a strong dependence on extension rates, which shows that A aggregates construct a network structure by interfacial interactions. The interfacial interactions between A aggregates and chains improve the strength of samples; while the A aggregate network driven by interfacial interactions preferentially ruptures to dissipate large energy for the improvement of fracture toughness, which can be considered as a sacrificial network. Therefore, their designed elastomers have both high strength and high toughness. This work provides an easier strategy for the construction of sacrificial networks, which can promote the industrial application of sacrificial networks in elastomer materials.

Enabling Superior Thermo-Oxidative Resistance Elastomers Based on a Structure Recovery Strategy.

Thermo-oxidative process leads to the structure damage of elastomers, such as the scission of main chains and destruction of crosslinks. The problem that damaged structure brings about the deterioration of mechanical properties has not been solved by the conventional anti-aging methods. Inspired by self-healing process, a structure recovery strategy for recovering the damaged structure induced by thermo-oxidative process is proposed, which endows elastomers with superior thermo-oxidative resistance. The high reactivity between 1,3-diisopropenylbenzene and free radicals realizes high recovery efficiency (from 83% to 118%); the changes in topology structure during recovery process make much more rubber chains bear external stress and improve mechanical properties significantly (from 18.5 to 29.6 MPa). This work paves the way for the development of elastomers with superior thermo-oxidative resistance, meanwhile this work is helpful to push the theoretical research of self-healing to practical application.

MXene Enabling the Long-Term Superior Thermo-Oxidative Resistance for Elastomers.

The ability of long-term thermo-oxidative resistance is very important for elastomers in application. However, many conventional antioxidants are difficult to realize the long-term thermo-oxidative resistance. To overcome this limitation, a design strategy is introduced by combing elastomers with MXene and natural rubber (NR) is chosen as a model material. MXene is efficient in absorbing oxygen and the generated free radicals in the NR matrix and can inhibit the diffusion of oxygen toward the interior. Moreover, MXene, like graphene and carbon black, absorbs molecular chains, inhibiting the migration of MXene toward the surface of the sample. Such characteristics of MXene endow NR/MXene with the long-term outstanding thermo-oxidative resistance. For example, after three days of the thermo-oxidative process for NR/MXene, the tensile strength is 19 MPa and the retention of tensile strength is 63%, which far exceeds the effects of conventional antioxidants. This work not only provides a good guide for the universal design of elastomers with long-term thermo-oxidative resistance but also expands the application of MXene.

The Role of Non-Rubber Components on Molecular Network of Natural Rubber during Accelerated Storage.

Though the non-rubber components have long been recognized to be a vital factor affecting the network of natural rubber (NR), the authentic role of non-rubber components on the network during accelerated storage has not been fully illuminated. This work attempts to clarify the impact of non-rubber components on the network for NR during accelerated storage. A natural network model for NR was proposed based on the gel content, crosslinking density, and the non-rubber components distribution for NR before and after centrifugation. Furthermore, the effect of non-rubber components on the network was investigated during accelerated storage. The results show that terminal crosslinking induced by non-rubber components and entanglements are primary factors affecting the network formation during accelerated storage. By applying the tube model to analyze the stress-strain curves of NR, we found that the contribution of the entanglements to the network formation is larger than that of terminal crosslinking during accelerated storage. The work highlights the role of non-rubber components on the network during accelerated storage, which is essential for understanding the storage hardening mechanism of NR.

Effect of protein on the thermogenesis performance of natural rubber matrix.

Under high-speed strain, the thermogenesis performance of natural rubber products is unstable, leading to aging and early failure of the material. The quality of rubber latex and eventually that of the final products depends among others on the protein content. We found that when the protein is almost removed, the heat generated by the vulcanized rubber increases rapidly. After adding soy protein isolate to the secondary purification rubber, the heat generation of the vulcanized rubber is reduced, and the heat generation is the lowest when the added amount is 2.5-3.0 phr, which on account of protein promotes the construction of a vulcanization network and increases the rigidity of the rubber chain, resulting in a decrease in the potential frictional behavior of the rubber chain during the curl up-extension process.

Mimicking the Mechanical Robustness of Natural Rubber Based on a Sacrificial Network Constructed by Phospholipids.

Mechanical strength and toughness are usually mutually exclusive, but they can both appear in natural rubber (NR). Previous studies ascribe such excellent properties to highly cis stereoregularity of NR. To our surprise, after the removal of non-rubber components (NRC) by centrifugation, the strength and toughness of NR decrease dramatically. It is still a challenge for us to make out for the problem of how NRC affect the properties of NR. Our group ascribes the superior mechanical robustness of NR to NRC. To further verify such a viewpoint, we add phospholipids (phosphatidylcholines) into NR without NRC. Phosphatidylcholines construct a sacrificial network, which ruptures preferentially upon deformation to dissipate energy. Moreover, some of phosphatidylcholines participate in the vulcanization reaction, which further improves the mechanical strength and energy dissipation. As a result, the mechanical strength and toughness of samples are as high as 21.1 MPa and 49.6 kJ/m2, respectively, which have reached the same level as that of NR. Therefore, this work not only imitates the excellent mechanical robustness of NR but also further provides a rational design for elastomers with excellent mechanical robustness.

Structure and Properties of Reduced Graphene Oxide/Natural Rubber Latex Nanocomposites.

In this work, the morphology and properties of graphene modified natural rubber (NR) was studied. Graphite oxide was chemically reduced with poly(sodium 4-styrenesulfonate) as a stabilizer, then the obtained graphene was blended with NR using latex technique. It is observed that a stable graphene suspension was fabricated by chemical reduction of graphite oxide in the presence of surfactant, while graphene was dispersed homogeneous in NR matrix as tested by WAXD and TEM. Dynamic mechanical analysis showed that storage modulus enhanced dramatically after the incorporation of graphene due to the high surface area of graphene and the exfoliated structure of graphene in NR. The electrical property and thermal conductive properties of NR/graphene nanocomposites improved significantly following the increase in graphene concentration.

Graphene networks and their influence on free-volume properties of graphene-epoxidized natural rubber composites with a segregated structure: rheological and positron annihilation studies.

Epoxidized natural rubber-graphene (ENR-GE) composites with segregated GE networks were successfully fabricated using the latex mixing combined in situ reduced technology. The rheological behavior and electrical conductivity of ENR-GE composites were investigated. At low frequencies, the storage modulus (G') became frequency-independent suggesting a solid-like rheological behavior and the formation of GE networks. According to the percolation theory, the rheological threshold of ENR-GE composites was calculated to be 0.17 vol%, which was lower than the electrical threshold of 0.23 vol%. Both percolation thresholds depended on the evolution of the GE networks in the composites. At low GE concentrations (<0.17 vol%), GE existed as individual units, while a "polymer-bridged GE network" was constructed in the composites when GE concentrations exceeded 0.17 vol%. Finally, a "three-dimensional GE network" with percolation conductive paths was formed with a GE concentration of 0.23 vol%, where a remarkable increase in the conductivity of ENR-GE composites was observed. The effect of GE on the atom scale free-volume properties of composites was further studied by positron annihilation lifetime spectroscopy and positron age momentum correlation measurements. The motion of ENR chains was retarded by the geometric confinement of "GE networks", producing a high-density interfacial region in the vicinity of GE nanoplatelets, which led to a lower ortho-positronium lifetime intensity and smaller free-volume hole size.

Design of microencapsulated carbon nanotube-based microspheres and its application in colon targeted drug delivery.

The present study aims to prepare and evaluate carbon nanotubes (CNTs)-based colon-specific microspheres using irinotecan as a model of drug. The synthesis of CNTs-based microspheres including attachment of folate-chitosan conjugate and irinotecan to CNTs via non-covalent interaction, followed by microencapsulation with Eudragit S-100 by an oil-in-oil solvent evaporation technique. The obtained samples were characterized in case of surface morphology, drug loading efficiency and particle size. In vitro drug release behavior was studied in different pH medium and the obtained data were subjected to kinetic equations. It was found that the Eudragit-coated microparticles were spherical with smooth surface, and the particle size varied with the core/coat ratio. In vitro drug release shows that the irinotecan released in a slow and sustained fashion from the CNTs-based carriers without coating with Eudragit. No drug release was observed from Eudragit-coated microspheres when the medium pH below 7, while when the pH reached 7.4, the coating layer of Eudragit began to dissolve and a controlled release of irinotecan was observed. The cell viability test indicates that the drug free FA-CS decorated CNTs had no influence on the cell proliferation rates of HT-29 cells, while the irinotecan-loaded CNTs drug system proved to be the most cytotoxic.

Tris(ethyl-enediamine-κN,N')cobalt(III) aqua-tris-(oxalato-κO,O)indate(III).

In the cation of the title compound, [Co(C(2)H(8)N(2))(3)][In(C(2)O(4))(3)(H(2)O)], the Co(III) atom is coordinated by six N atoms from three ethyl-enediamine mol-ecules. The Co(III)-N bond lengths lie in the range 1.956 (4)-1.986 (4) Å. In the anion, the In(III) atom is seven-coordinated by six O atoms from three oxalate ligands and by a water mol-ecule. The cations and anions are linked by extensive O-H⋯O and N-H⋯O hydrogen bonds, forming a supermolecular network.