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.

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.

Towards a Supertough Thermoplastic Polyisoprene Elastomer Based on a Biomimic Strategy.

Natural rubber is one of most famous self-reinforced rubbers thanks to the phenomenon of strain-induced crystallization. It is usually used in a vulcanized form to enhance the mechanical strength but this results in recycling issues. Herein a thermoplastic analogue of vulcanized natural rubber is obtained as a structural mimic. Terminally functionalized polyisoprene rubber B-4A-PIP was prepared by using tetra-analine as physical crosslinking units. The strong binding of tetra-analine groups gave B-4A-PIP a high tensile strength (15 MPa) and breaking strain of 890 %, which is much higher than those of undecorated copolymer B-OH-PIP. B-4A-PIP has a similar onset strain of crystallization and crystallization index to vulcanized natural rubber. Randomly functionalized polyisoprene R-4A-PIP showed a much lower mechanical strength and SIC properties although R-4A-PIP and B-4A-PIP possessed similar molecular weights and amounts of tetra-analine groups.

Synergistic effect of CB and GO/CNT hybrid fillers on the mechanical properties and fatigue behavior of NR composites.

In this paper, graphene oxide (GO) and carbon nanotube (CNT) hybrid fillers were used to replace partial carbon black (CB), and GO/CNT/CB/NR composites were prepared with excellent crack growth resistance, low heat build-up and superior mechanical properties. Mechanical testing revealed a significant synergistic reinforcement between GO/CNT and CB in NR composites. The improved dispersion of GO/CNT hybrid fillers and CB in the NR matrix was characterized by transmission electron microscopy (TEM). Through the fatigue test, the GO/CNT/CB/NR composites showed excellent fatigue crack growth resistance and low heat build-up compared to CB/NR composites. These properties provide the NR composites with better applications in industry.