Covalent Adaptive Networks for Enhanced Adhesion: Exploiting Disulfide Dynamic Chemistry and Annealing during Application

During the curing process of traditional thermoset adhesives/sealants, it is impractical to avoid all nonideal conditions, e.g., preassembly reactions, trapped voids, and polymerization-induced shrinkage, which may lead to the accumulation of internal stress and poor surface wetting and result in severe, permanent detrimental effects on adhesive/sealant performance. In order to address these issues and improve performance, we have developed a simple method using dynamic chemistry to achieve adaptable thermoset adhesives/sealants. Specifically, by incorporating dynamic chemistry into the polymer network structure, the resulting thermoset adhesives/sealants gain the capability to change their shape upon a stimulus, e.g., heat, to fill voids and release internal stresses generated during the curing process, leading to enhanced surface wettability and superior adhesive performance. We describe in detail an example of such materials based on simple disulfide dynamic chemistry and application methods. After annealing, the dynamically cross-linked adhesives exhibited a factor of ∼4 increase in lap shear strength in comparison with as-cured samples that were not annealed and a factor of 2 increase in lap shear strength when compared to an annealed control adhesive system utilizing permanently cross-linked materials. These results indicate that adaptive thermoset adhesives can be utilized to overcome major problems associated with traditional thermoset adhesives, e.g., unsatisfactory surface wetting and accumulated internal stresses. Given the simplicity of incorporating dynamic chemistries, adaptive thermoset adhesives are promising candidates for future adhesives/sealants with superior performance.