Reassembled Synthetic Adhesives with High Bonding Strength Based on Dynamic Diels-Alder Bonds

Synthetic adhesives rely on interfacial interactions to bond solid materials. Their molecular structure is highly designable, allowing for tailored properties through component adjustment. However, traditional thermoplastic and thermosetting adhesives face limitations. Thermoplastics are reassembleable but lack strength, while thermosets offer high strength but are non-reassembleable. This research aims to combine the advantages of both by incorporating dynamic covalent bonds into adhesive networks.

Dynamic covalent bonds allow for reversible exchange reactions under external stimuli, enabling network reassembly. We focus on Diels-Alder bonds, which dissociate and reform reversibly, facilitating rapid reassembly. Epoxidized soybean oil (ESO), a sustainable and readily available bio-based material, served as the adhesive substrate. Furyl groups were grafted onto ESO chains, resulting in ESOF, and Diels-Alder bonds were introduced through the reaction with bismaleimide (BMI). The resulting ESOF-Ix network exhibited tunable properties depending on the BMI ratio.

The prepared adhesives showed good solvent resistance and mechanical properties, which could be adjusted through cross-linker concentration. The ESOF-Ix adhesives also demonstrated self-healing capability. Upon heating, scratches on the adhesive surface gradually diminished due to the reformation of Diels-Alder bonds. Moreover, the adhesives showed remarkable recyclability. Repeated hot-pressing cycles did not significantly affect the adhesive's mechanical properties or Tg, confirming the dynamic nature of the network structure.

The adhesives exhibited hydrophilicity, as evidenced by water contact angles below 90 degrees, which is crucial for good adhesion. Torque measurements during reassembly revealed that the viscosity of the adhesive rapidly decreased upon heating due to the dissociation of Diels-Alder bonds, enabling quick reassembly.

In conclusion, this study successfully developed reassembled synthetic adhesives with high bonding strength using ESO and dynamic Diels-Alder bonds. The adhesives exhibited excellent properties, including tunable mechanical properties, self-healing capability, and recyclability, making them highly promising for a wide range of applications.