Self-assembled Bioinspired Underwater Adhesive by Host-guest Interaction

Designing and developing adhesives that bond strongly to wet surfaces in humid environments or completely submerged in water has been a real challenge. Curing of underwater adhesives usually requires a long time as well as external energy, which limits the range of applications for underwater adhesives. We overcame these problems by integrating the host-guest interactions of adamantane (AD) and β-Cyclodextrin (β-CD) and the hydrophobic interactions of polydimethylsiloxane (PDMS) into a single system and mimicking the adhesion mechanism of mussel foot proteins. We prepared hyperbranched polymers P1 containing adhesion groups dihydroxyphenylalanine (DOPA) and AD, as well as PDMS polymers P2 capped with β-CD. Through a one-step Michael addition reaction, the adhesion functional group DOPA, the rigid hydrophobic group benzylamine hydrochloride (BENA), and the guest functional unit AD were successfully integrated into the hyperbranched polymers P1. The (β-CD)-capped PDMS host polymers P2 was obtained by reacting the host functional unit β-CD with poly(dimethylsiloxane), diglycidyl ether terminated (PDMS-DGE). The host-guest underwater adhesive was obtained by mixing P1 and P2 polymer solutions and forming a stable inclusion P3 complex of AD with β-CD in water. Strong underwater adhesion was mainly realized through three points: 1. AD and β-CD were adaptively underwater assembled through the host-guest interaction, which squeezed out the water from the inner cavity of the β-CD, realizing the hydrophobic repulsion of molecular chains and increasing the cohesive energy; 2. the catechol group of DOPA formed a large number of hydrogen bonds with the surface of the substrate, resulting in strong interfacial adhesion; 3. hydrophobic PDMS resisted water erosion and protected the internal polymer chain, thus achieving the strength of the underwater adhesion stable for a long period of time. Experimental tests had shown that this rational design endowed the underwater adhesive with exceptional performance. The stabilized inclusion P3 formed through adaptive assembly exhibited strong underwater adhesion strength (brass substrate in pure water 12 h reaches 940.8kPa), which was a significant improvement over the adhesion strength of existing underwater adhesives (200~600 kPa). Significantly, P3 required only one hours to initially cure and adhere the substrate tightly, and reached peak adhesion strength after 12h of curing. Contact angle tests had confirmed the good wettability of P3 adhesives on different substrate surfaces (organic and inorganic). This allowed the adhesive to tend to displace interfacial water when applied to the substrate in an underwater environment, thereby gaining sufficient contact area on the substrate to achieve strong adhesion to a wide range of substrates. In addition to its strong adhesive properties, our underwater adhesive remained stable in a wide range of aquatic environments (ultrapure water, acidic and alkaline solutions, seawater). Even after 15 days of immersion in water, it maintained high adhesion strength. The host-guest adhesive P3 maintained strong adhesion properties after several adhesion-detachment cycles underwater, which proved its reusability. Economically and environmentally, this not only helped to reduce costs, but was also in line with the concept of green chemistry, which protected the environment and helped to reduce the burden on marine. This in-situ underwater adaptive curing strategy greatly improved the ease of application of underwater adhesives in complex environments and provides abundant possibilities for the subsequent design of a new generation of green underwater adhesives that combined high adhesion strength, long-term durability and stability in harsh environments.