Integrating Sacrificial Bonds into Dynamic Covalent Networks toward Mechanically Robust and Malleable Elastomers

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.

Vitrimers: directing chemical reactivity to control material properties

“…For the boronic ester/ZnCl 2 ENR vitrimers, Guo and co-workers also explored 'sacricial' supramolecular bonds based on hydrogen bond formation of dangling amides, in combination with a transesterication catalysed via TBD. 46 Later, they also introduced a benzimidazole group in styrenebutadiene rubber (SBR) to form Zn 2+ -imidazole complexes that induced micro-phase separation and led to signicant enhancement of the mechanical properties. 47 Likewise, Zhang, Sun and co-workers reported a strategy where 2-ureido-4[1H]pyrimidinone (Upy) dimers were combined with boronic ester cross-linkers, to enhance mechanical properties of supramolecular networks by addition of physical cross-links.…”

A catalyst-free and recycle-reinforcing elastomer vitrimer with exchangeable links

“…Although modied llers could act as crosslinking and reinforcing agents to construct a threedimensional reversible crosslinking network, the introduction of modied llers would lead to the retarding of the topology network rearrangement due to the restriction of the macromolecular chain movement. 25,27 Besides, catalysts are inevitably employed to promote the reversible exchange reactions, which could shorten the service lifetime and prevent the recyclability of vitrimers owing to the thermal degradation and leaching of the catalysts. [28][29][30] It is also pointed out that the incorporation of toxic catalysts will potentially limit the widespread applications of these high performance materials.…”

Cyber–Physiochemical Interfaces

“…Researchers have turned to designing materials with compatible mechanical properties via various approaches such as molecular and geometry design, nanomaterial network manipulation, and hydrogel‐based electrodes . Molecular design is a bottom‐up approach, where both elastic and rigid units are rationally designed to dynamically tune the mechanical properties (Figure b) . When exposed to strains, the noncovalent 2,6‐pyridine dicarboxamide moieties with hydrogen bonding dissipate energy via bond breakage and the conjugated backbones in the polymer maintain the high charge transport abilities.…”