Covalently cross-linked rubbers are renowned for their high elasticity that play an indispensable role in various applications including tires, seals, and medical implants. Development of self-healing and malleable rubbers is highly desirable as it allows for damage repair and reprocessability to extend the lifetime and alleviate environmental pollution. Herein, we propose a facile approach to prepare permanently cross-linked yet self-healing and recyclable diene-rubber by programming dynamic boronic ester linkages into the network. The network is synthesized through one-pot thermally initiated thiol-ene "click" reaction between a novel dithiol-containing boronic ester cross-linker and commonly used styrene-butadiene rubber without modifying the macromolecular structure. The resulted samples are covalently cross-linked and possess relatively high mechanical strength which can be readily tailored by varying boronic ester content. Owing to the transesterification of boronic ester bonds, the samples can alter network topologies, endowing the materials with self-healing ability and malleability.

Covalent cross-linking of rubbers is essential for obtaining high resilience and environmental resistance but prevents healing and recycling. Integrating dynamic covalent bonds into cross-linked rubbers can resolve the trade-off between permanent cross-linking and plasticity. The state-of-the-art elastomer-based dynamic covalent networks require either intricate molecular makeup or present poor mechanical properties. In this work, we demonstrate a simple way to prepare mechanically robust yet healable and recyclable elastomeric vitrimers by engineering dynamic dual cross-links of boronic esters and coordination bonds into a commercial rubber. Specifically, epoxidized natural rubber is covalently cross-linked with a boronic ester cross-linker carrying dithiol through chemical reaction between epoxy and thiol groups. The covalently cross-linked networks are able to alter the topologies through boronic ester transesterifications, thereby conferring them with healing ability and reprocessability. In particular, the mechanical properties can be remarkably enhanced by introducing sacrificial metal–ligand coordination bonds into the networks without compromising the healing ability or reprocessability.

Reinforcing rubbers and expanding their application galleries are two important issues in material science and engineering. In this work, we demonstrate a bioinspired design of high-performance and macroscopically responsive diene-rubber by engineering sacrificial metal−ligand motifs into a chemically cross-linked architecture network. The metal−ligand bonds are formed through the coordination reaction between the pyridine groups in butadiene−styrene−vinylpyridine rubber (VPR) and metal ions. Under external load, the metal−ligand bonds serve as sacrificial bonds that preferentially rupture prior to the covalent network, which dissipates energy and facilitates rubber chain orientation. Based on the function mechanisms, the modulus, tensile strength, and toughness of the samples are simultaneously improved without sacrificing the extensibility, and these properties can be conveniently tuned by varying the structure parameters of the covalently cross-linked network and metal−ligand bonds. Moreover, the dissociation/re-formation of metal−ligand bonds upon heating/cooling can endow VPR with thermally triggered adaptive recovery for shape memory application.

No abstract

Vitrimers are a class of covalently cross-linked polymers that have drawn great attention due to their fascinating properties such as malleability and reprocessability. The state of art approach to improve their mechanical properties is the addition of fillers, which, however, greatly restricts the chain mobility and impedes network topology rearrangement, thereby deteriorating the dynamic properties of vitrimer composites. Here, we demonstrate that the integration of sacrificial bonds into a vitrimeric network can remarkably enhance the overall mechanical properties while facilitating network rearrangement. Specifically, commercially available epoxidized natural rubber is covalently cross-linked with sebacic acid and simultaneously grafted with N-acetylglycine (NAg) through the chemical reaction between epoxy and carboxyl groups, generating exchangeable β-hydroxyl esters and introducing amide functionalities into the networks. The hydrogen bonds arising from amide functionalities act in a sacrificial and reversible manner, that is, preferentially break prior to the covalent framework and undergo reversible breaking and reforming to dissipate mechanical energy under external load, which leads to a rarely achieved combination of high strength, modulus, and toughness. The topology rearrangement of the cross-linked networks can be accomplished through transesterification reactions at high temperatures, which is accelerated with the increase of grafting NAg amount due to the dissociation of transient hydrogen bonds and increase of the ester concentration in the system.