Strong adhesion in wet environments is crucial for medical and marine applications. However, the hydration layer between adhesives and surfaces significantly impedes adhesion. Conventional adhesives like cyanoacrylates, epoxy resins, and polyurethanes perform well in dry conditions but poorly underwater. Bioinspired phenolic polymers, mimicking mussel byssal threads, offer a promising alternative. Mussel adhesive proteins contain catechol groups (3,4-dihydroxyphenylalanine, DOPA) that enable penetration of hydration layers on diverse substrates. Catechol-functionalized polymers have been extensively studied, focusing on backbone structure, topology, co-monomers, molecular weight, and coacervation. Despite advancements, their adhesion strength remains significantly weaker than dry adhesives. The gallol group, with an additional hydroxyl group compared to catechol, shows enhanced underwater adhesion. This research hypothesizes that non-canonical phenolic groups with four or five hydroxyl groups will further improve adhesion, exceeding the capabilities of naturally occurring phenolic compounds. This study aims to synthesize and characterize these novel polymers, investigating their adhesion strength, substrate versatility, and mechanism of action, opening up opportunities for creating superior underwater adhesives.

Extensive research focuses on bio-inspired adhesives, primarily mimicking the catechol-rich proteins found in mussel byssal threads. Studies have explored various aspects of catechol-functionalized polymers, including backbone structure modifications, topology optimization, and the incorporation of different co-monomers to enhance adhesion. However, the adhesion strengths achieved by these bio-inspired adhesives generally fall short of those observed in dry adhesives. Recent research has highlighted the gallol group, a naturally occurring phenolic compound with superior adhesion compared to catechol. This suggests that increasing the number of hydroxyl groups on the phenolic ring could significantly improve underwater adhesion. This work extends the study beyond catechol and gallol, investigating non-canonical phenolic groups with four and five hydroxyl groups, aiming to create significantly stronger underwater adhesives.

The researchers synthesized non-canonical phenolic monomers, 2,3,4,5-tetramethoxystyrene (TMS) and 2,3,4,5,6-pentamethoxystyrene (PMS), using cost-effective methods. Free radical polymerization with styrene (S), followed by deprotection, yielded non-canonical phenolic copolymers (P4HS and P5HS). For comparison, copolymers with one, two, and three hydroxyl groups (P1HS, P2HS, P3HS) were also synthesized. The molecular weight (Mn) of the copolymers was optimized for underwater adhesion. Adhesion strength was measured using a load cell, testing the adhesive bond between two metal rods submerged in water. The influence of molecular weight, number of hydroxyl groups, and hydroxyl group position on adhesion was systematically investigated using copolymers with varying compositions. Quartz crystal microbalance (QCM) was employed to quantify the adsorption of homopolymers on various substrates. Molecular dynamics (MD) simulations were used to study the adsorption energy of individual phenolic units on a Fe surface. The composition of copolymers, with varying amounts of phenolic and styrene units, was also explored to understand the influence of hydrophobicity on water uptake and adhesion strength. Techniques such as scanning electron microscopy (SEM), contact angle measurements, X-ray photoelectron spectroscopy (XPS), 1H NMR, UV-Vis spectroscopy, and Fourier-transform infrared (FT-IR) spectroscopy were used to characterize the polymers, their oxidation states, and surface properties. Finally, backbone modifications were explored to reduce steric hindrance and improve cohesive interactions. The recyclability of the adhesives was evaluated by assessing their removal from substrates using common organic solvents.

The study found that increasing the number of hydroxyl groups on the styrene monomer unit linearly increases underwater adhesion strength up to four hydroxyl groups. P(4HS-co-S) and P(5HS-co-S) demonstrated ultrastrong underwater adhesion, reaching ~7 MPa after 72 hours and sustaining this strength for at least one month. The position of hydroxyl groups significantly impacts adhesion; isomers with hydroxyl groups at positions 3, 4, and 5 exhibited superior adhesion compared to those with hydroxyl groups at positions 2 and 6. Copolymers with a small percentage (5-10%) of non-canonical phenolic groups displayed remarkably strong underwater adhesion, surpassing existing bio-inspired adhesives. Analysis revealed that the spatial confinement of phenolic groups, similar to what is seen in mussel proteins, plays a critical role in enhancing adhesion by creating a hydrophobic microenvironment that repels water and minimizes oxidation. Backbone modifications using methacrylamide resulted in even stronger adhesion, reaching >10 MPa for P(4HMA-co-S) and P(5HMA-co-S). The non-covalent nature of the adhesion allowed for facile removal and reuse of the adhesives using common organic solvents, a significant advantage over conventional, irreversibly crosslinked adhesives. XPS analysis showed limited oxidation of the phenolic groups during underwater adhesion, indicating a different mechanism than oxidation-induced crosslinking observed in bio-inspired adhesives.

This research demonstrates a significant advancement in underwater adhesive technology by developing ultrastrong, versatile adhesives using non-canonical phenolic polymers. The systematic investigation of the number, position, and spatial arrangement of hydroxyl groups provides critical insights into the mechanism of underwater adhesion. The findings highlight the importance of creating a hydrophobic microenvironment to repel water and prevent oxidation, thereby enhancing the stability and longevity of the adhesive bond. The achievement of >10 MPa adhesion strength, surpassing all previously reported underwater adhesives, opens doors to numerous applications in diverse fields. The facile recyclability of the adhesive, achieved without compromising adhesion strength, addresses a major limitation of existing underwater adhesives.

This study successfully synthesized and characterized ultrastrong non-canonical phenolic polymers demonstrating exceptional underwater adhesion. The systematic investigation revealed the importance of the number, position, and spatial arrangement of hydroxyl groups for optimal performance. The achieved adhesion strength significantly surpasses existing commercial and bio-inspired alternatives. Future research should focus on exploring living polymerization techniques for precise molecular weight control and developing organic solvent-free formulations for biomedical applications.

While this study demonstrates remarkable underwater adhesion, further research is needed to fully understand the long-term stability of the adhesives under various environmental conditions. The current synthesis methods utilize organic solvents, limiting their immediate applicability in certain biomedical settings. Exploring alternative solvent-free synthesis methods, such as coacervation or emulsion techniques, would expand the applications of these adhesives.