Mechanically Robust, Self-Healable, and Reprocessable Elastomers Enabled by Dynamic Dual Cross-Links

Abstract

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.

Cross-linker control of vitrimer flow

“…For dissociative reversible networks, mixing cross-linkers with the same exchange mechanism but different relaxation rates modulates stress relaxation profiles. 13,46,47 While Dichtel, 48 Chen, 49 and Guo 50 The continuous relaxation spectra were then extracted from the stress relaxation data (Figure 5b). Multiple relaxation modes were observed when the relaxation rates differed by several orders of magnitude.…”

Vitrimers: directing chemical reactivity to control material properties

“…4b). 45 The latter vitrimers were proposed to show a dual crosslinking, based on dynamic boronic ester exchange and by coordinated Zn 2+ ions (via available alcohol moieties from the opened epoxy) as sacricial bonds upon mechanical stress. It must be noted that similar effects could already be at play in Leibler's original Zn(II)-doped polyhydroxyl transesterication vitrimers.…”

Evolution of self-healing elastomers, from extrinsic to combined intrinsic mechanisms: a review

“…Other non-covalent systems were used as ionic interactions combined with DA chemistry in an acrylic copolymer. 211 In the last year, metal-ligand coordination has also been combined with different covalent systems and matrices such as ENR (with boron-based bonds 212 ), PDMS (with imine bonds 213 ), PU (with DA chemistry 214 ) and PUU (with disulfide bonds 215 ), but with very different results that limit establishing yet a clear trend for these combinations.…”