Molecular insights into the adhesion of mussel-inspired copolymers to dissimilar materials

We investigated the adhesion mechanisms of mussel-inspired adhesive copolymers to dissimilar materials, specifically aluminum (Al) and polypropylene (PP), focusing on how galloyl-containing monomers, which mimic catechol groups, affect adhesive properties. Tensile tests, confocal Raman microscopy for cross-sectional analysis, and sum-frequency generation (SFG) spectroscopy for surface and interfacial analysis were employed to examine the molecular structure-adhesion relationship. The adhesion mechanism was found to be highly substrate-dependent. Specifically, the adhesion to Al is primarily driven by interfacial adsorption of galloyl groups, whereas adhesion to PP is governed by intermolecular hydrophobic interactions. The differences in adhesive strength among the copolymers were attributed to their bulk cohesive strength, which was influenced by the aggregation morphology of the galloyl moieties. In contrast, for PP, although a micrometer-scale interphase formed via mutual diffusion, its thickness showed no correlation with adhesive strength. This finding, combined with SFG analysis of the fracture surfaces, suggests that adhesion is inferred from our spectroscopic and mechanical evidence to be dominated not by strong molecular chain entanglement but by intermolecular forces, likely hydrophobic interactions involving the non-polar alkyl chains in the copolymers. These findings highlight that designing effective bio-inspired adhesives for dissimilar materials requires optimizing both interfacial interactions and bulk aggregation morphology.