Mussel-Inspired Catechol-Functionalized EVA Elastomers for Specialty Adhesives; Based on Triple Dynamic Network

Marine mussels secrete adhesive proteins that enable them to bind to various surfaces (like rocks, ship’s hull, piers, etc.) in saline and wet environments. One of the main structural components in these adhesive proteins is the presence of a unique catechol-based amino acid, l-3,4-dihydroxyphenylalanine (DOPA), which is responsible for interfacial binding and the rapid solidification of the proteins. Mimicking this functionality in synthetic polymers yields high-strength adhesives that can be attached to both dry and wet surfaces. In this study, a commercially available epoxy-functionalized elastomer, poly(ethylene-co-vinyl acetate-co-glycidyl methacrylate) (EVA-GMA), was modified with dihydroxycaffeic acid (HCA) via transesterification reaction to prepare catechol-functionalized EVA elastomer which in combination with diboronic acid showed self-healing and recyclable characteristics. The catechol-functionalized elastomers can self-heal both dry and underwater, as the catechols can form hydrogen bonding and coordination complexes with metal ions in both dry and wet environments. The tensile study demonstrated a significant improvement in the mechanical strength due to hydrogen bonding and boronic eater cross-linking. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses unequivocally confirm the successful formation of covalent boronic ester cross-links along with metal coordination. The stress relaxation behavior of the boronic ester cross-linked EVA-GMA elastomer was examined at various temperatures to assess its dynamic properties. Moreover, boronic ester cross-linked elastomers showed significant self-healing properties (with a healing efficiency of ∼86%) and were monitored by using optical microscopy and tensile analysis. By incorporating boronic ester-based dynamic covalent bonds, metal coordination, and H-bonding into polymer networks, we achieved strong adhesion and their dynamic nature for fast and reversible adhesion. Interestingly, this catechol-functionalized EVA-elastomer-based adhesive showed excellent bonding characteristics toward different surfaces, as analyzed via lap shear test in both dry and wet environments.