Light‐controlled switchable underwater adhesive

Despite extensive efforts in designing and preparing switchable underwater adhesives, it is not easy to regulate the underwater adhesion strength locally and remotely. Here, we design and synthesize photoreversible copolymer of poly[dopamine methacrylamide‐co‐methoxyethyl‐acrylate‐co‐7‐(2‐methacryloyloxyethoxy)‐4‐methylcoumarin]. Due to the dynamic formation and breaking of chemical crosslinking networks within the smart adhesives, the material shows widely tunable adhesion strength from ∼150 to ∼450 kPa and long‐range reversible maneuverability under orthogonal 254 and 365 nm ultraviolet light stimulation via the coumarin dimerization and cycloreversion. Moreover, the adhesive exhibits good circulation performance and stability in an acid–base environment. It also demonstrated that the bolt can be coated with the smart adhesive material for on‐demand bonding. This design principle opens the door to the development of remotely controllable high‐performance smart underwater adhesives.

0][31][32] In spite of the exciting progress, it is still difficult to remotely regulate underwater adhesion and achieve large-span changes in adhesion strength during attachment and detachment.
Herein, we report a reversible underwater adhesive remotely controlled by ultraviolet (UV) light with strong and switchable adhesion strengths from ∼150 to ∼450 kPa (Figure 1A).Strong adhesion was achieved under 365 nm UV light irradiation, while the adhesion was weakened under 254 nm UV light irradiation.Under the irradiation of 365 nm UV light, the adhesive polymer backbone installed with coumarin groups can be crosslinked by forming coumarin dimers, thereby enhancing the cohesion and hydrophobicity of the polymer.After being exposed to 254 nm UV light, the coumarin dimers are cleaved, resulting in the recovery of the cross-linked segment.What is more, the copolymer adhesive exhibited robust adhesion strengths after soaking in a wide pH range of water environments.Due to the switchable adhesion, the smart adhesive material can be applied on the bolt surface to realize on-demand bonding.

| EXPERIMENTAL SECTION
2.1 | Nuclear magnetic resonance spectroscopy 1 H and 13 C nuclear magnetic resonance (NMR) spectra were recorded in dimethyl sulfoxide-d 6 using a 400 MHz spectrometer (Bruker AM-400) at 25 °C.

| UV irradiation
Long wave UV (365 nm) irradiation was performed using a commercial UV lamp with an intensity of 160 mW/cm 2 (measured with LS125 UV Light Meter calibrated at 365 nm).Short wave UV (254 nm) irradiation uses a selfassembled UV lamp with 50 LED bulbs (2 W each.Its light intensity is 160 mW/cm 2 (measured with UVX-254 UV Light Meter calibrated at 254 nm).

| Oscillatory storage modulus rheology
Oscillatory storage modulus rheology was performed on an oscillatory rheometer (RS6000 HAAKE) operating at 20 °C with a 35 mm flat plate geometry.Frequency sweeps were from 0.1 to 10 Hz.

| Tensile strength test
The tensile strength is tested by an electronic universal testing machine (EZ-test; Shimadzu) with a 500 N force transducer.Use a standard fixture to hold the sample.

| Contact angle measurement
Static water contact angle (CA) was measured by using a DSA-100 optical CA meter (Kruss Company Ltd.) at 25 °C.

| RESULTS AND DISCUSSION
The photoresponsive adhesive of poly[dopamine methacrylamide-co-methoxyethyl acrylate-co-7-(2-methacryloyloxyethoxy)-4-methylcoumarin] [p(DMA-co-MEAco-CoumMA)] was synthesized through free-radical polymerization, in which DMA served as the adhesive moiety, MEA as the hydrophobic matrix, CoumMA as the photoresponsive moiety (Figure 1B).DMA component endows the adhesive with excellent underwater viscosity because the catechol group as an adhesive block in mussel foot proteins is well known for its robust underwater adhesion ability.MEA enhances the adhesion property of DMA by improving the hydrophobicity of the copolymer.To reversibly regulate the wet adhesion property, we introduced the coumarin derivative component.Coumarin and its derivatives are often used as crosslinking groups in polymer materials because the conditions required for dimerization and repeated cycloaddition/ cycloinversions are mild.Under the irradiation of longwave UV (365 nm), coumarin undergoes photo-[2 + 2] cycloaddition reaction to form cyclobutane linked dimer, and this process can be reversed upon exposure to shortwave UV light (254 nm). 33The chemical structure of the copolymer was confirmed by the 1 H NMR and 13 C NMR (Supporting Information: Figures S1-S8), all of which proved the successful synthesis of p(DMA-co-MEA-co-CoumMA).Long and viscous fibers can be easily drawn from the uncrosslinked adhesive.This is mainly because the uncrosslinked linear molecular chains are prone to slip under the tensile force, resulting in weak cohesion of the copolymer.Once cross-linked, the copolymer molecular chains form a three-dimensional network, which reduces the slippage between the molecular chains and increases the cohesive force (Figure 1C). Figure 1D schematically illustrates the mechanism of the smart adhesive for the firm adhesion of adherends.Interface adhesion mainly comes from van der Waals interaction, hydrogen bond, hydrophobic interaction, and π-π interaction.Cohesive force benefits from hydrogen bonds, π-π stacking, and covalent interaction, where covalent interaction is the main contributor.As shown in Figure 1E, the covalent interaction can be formed and broken dynamically via coumarin dimerization and cycloreversion.The dimerization of coumarins forms a chemically crosslinked network within the adhesive glue to enhance its cohesion force, while their cycloreversions decrease the cohesion.The formation and breaking of reversible covalent bonds endow underwater adhesive materials with switchable adhesion properties.
Adhesion measurements were performed by employing a universal material testing machine.The composition of p(DMA-co-MEA-co-CoumMA) copolymer was also optimized with different molar ratios of MEA to CoumMA (1:0.384,1:0.512, and 1:0.64).The representative adhesion strength-displacement curves of the adhesives can be used to investigate the effect of molar ratios between MEA and CoumMA of copolymer on the underwater adhesion strength (Figure 2A-C).It can be found that under a certain preload, preload time, and long-wave UV irradiation time, with the increase of the proportion of coumarin derivative in the polymer, the adhesion strength of the polymer increased first and then decreased.This is because the viscosity of the polymer is a trade-off between cohesion energy and interface adhesion.Only when the two are closer, the adhesion strength of the polymer can reach the best state. 34The role of coumarin derivatives is a kind of smart crosslinking agent.When its proportion was relatively small (Figure 2A), the cohesion energy of the polymer was also small, and the polymer broke from the inside in the tensile test.After the content of the coumarin content increased to a certain value (Figure 2B), the cohesion energy of the polymer matched with the interface adhesion, and the adhesion strength reached its peak.However, in the case of excessive coumarin content (Figure 2C), the cohesion energy of the polymer was further increased and the modulus of the polymer also increased.When the polymer was in contact with the matrix, it was affected by excessive internal stress, which damaged the sufficient contact between the polymer and the matrix, reduced the interfacial adhesion, and finally showed a decrease in adhesion strength.Another piece of evidence to support the above conclusion comes from Figure 2A-C.It can be seen that their debonding displacements decreased in turn, which also proved that with the increase of coumarin derivative content, the cohesion energy and internal stress of the polymer increased.These changes were finally reflected in the change of debonding energy, which first increased and then decreased.The debonding energy of the threecomponent polymers is 102.850,106.392, and 27.423 J/ m 2 , respectively, which is consistent with our previous conclusion that the apparent adhesion strength of the polymer is the trade-off between its cohesion energy and interface adhesion.The effect of the contact time on the adhesion strength of the adhesive glue underwater was investigated.As shown in Figure 2D, with the increase in contact time, the adhesion strength of the copolymer gradually increased and finally showed a stable trend.The main reason is that with the increase in contact time, the orientation of adhesive functional groups on the polymer molecular chain tends to be constant, resulting in full contact between the polymer and matrix, and finally reaching equilibrium. 35Figure 2E shows that with the increase of preload, the adhesion strength of the polymer also increased because the increase of preload promoted the full contact between the polymer and the matrix at the same contact time.We also investigated the effect of long-wave UV (365 nm) irradiation time on the adhesive strength of the copolymer under the condition of given coumarin component proportion, the same preload, and contact time (Figure 2F).It was found that with the increase of irradiation time, the adhesion strength of the polymer also increased first and then decreased.When irradiated with a long-wave UV (365 nm) light source (light intensity is 160 mW/cm 2 ) for 60 min, the cohesive energy of the polymer and the interface adhesion strength would reach a balance, and the apparent adhesion strength would be optimal.In conclusion, long-wave UV makes coumarin components form coumarin dimer to form cross-linking, which improved the cohesion energy of the polymer.As a result, the cohesion force of the polymer increased with the increase of irradiation time.When the cohesion force of the polymer is weak, the polymer exhibits low modulus (soft) and adhesion strength.With the increase of cohesion force, when the cohesion energy is close to the adhesion force at the polymer interface, the adhesion strength achieves the strongest.However, after the further improvement of cohesion force, the interface adhesion is finally damaged, resulting in the decrease of interface adhesion strength of the polymer after longtime irradiation.
To further study the stability of the adhesive copolymer in an acid-base environment, we prepared hydrochloric acid and sodium hydroxide solutions whose pH values were equal to 4 and 10, respectively.The adhesive glue was applied to a piece of polymethyl methacrylate (PMMA) with a 2 cm 2 adhesive layer and immersed in deionized water, acid, or base solution.
Another PMMA piece was covered on the adhesive layer.After 60 min of long-wave UV irradiation (365 nm), the samples were subjected to shear testing.As shown in Figure 3A, the adhesion strength of the specimens in the acidic solution decreased slightly, while it hardly changed in the alkaline solution.This shows that the designed adhesive has excellent stability in the acid-base environment.To study the durability of the reversible underwater adhesive, the same sample as the above experiment was irradiated with 365 nm UV light for 60 min and then placed in deionized water for 24 and 48 h.The experimental results showed that the reversible underwater adhesive still maintained considerable adhesion strength after prolonged soaking (Figure 3B).
In the above studies, we have demonstrated that the dimerization of coumarins based on the photo-[2 + 2] cycloaddition reaction forms a chemically cross-linked network within the adhesive glue to enhance its underwater adhesion strength.According to the performance of photosensitizer, theoretically, short-wave UV light irradiation can decrease the adhesion strength of the adhesive via the dissociation of cross-linked segments.As expected, the adhesive exhibited high adhesion strength with the contact substrate after exposure to the 365 nm UV radiation but low adhesion after the 254 nm UV light.As shown in Figure 4A, the adhesion strength of the adhesive glue after curing and curing dissociation, from which it can be seen that the adhesion strength of the copolymer after curing dissociation decreased significantly, while the debonding displacement increased.This is because dissociation enhances the toughness of the polymer, although it reduces the crosslinking degree and cohesion energy of the copolymer.The change in the cohesion of the smart adhesive can be further confirmed by its modulus.We used a rheometer to test the change of storage modulus (G′) of the same polymer sample after curing and curing dissociation.As shown in Figure 4B, the storage modulus (G′) after 60 min of UV curing is much higher than after UV dissociation.It shows that the modulus decreased in the process of short-wave UV irradiation.Interestingly, cycloaddition and cycloreversion have a great effect on the wettability of the adhesive copolymer.As shown in Figure 4C, the contact angle of the adhesive copolymer increased after 365 nm UV radiation, which means that the hydrophobicity of the copolymer increases with coumarin dimerization.After the dissociation of short-wave UV light, the contact angle decreased and the hydrophilicity also increased.The reason for the increasing hydrophobicity of copolymer samples after curing is mainly that the cross-linking of molecular chains in the copolymer promotes the hydrophobic MEA fragments to arrange more closely, improving the hydrophobicity of the polymer.The switchable adhesive properties of the adhesive glue rely on the coupled change in polymer cohesion and wettability under 254 and 365 nm UV light irradiation.In addition, as shown in Figure 4D, the effect of short-wave UV dissociation time on the strength of the polymer adhesion was explored.The strength of the adhesion did not decrease significantly until 20 min of irradiation and then decreased rapidly.After 50 min, the trend of the curve tends to be flat.We also measured the cyclic characteristics of the adhesion strength of the samples (Figure 4E).The results show that our underwater reversible adhesive not only has large differences in high and low adhesion states but also has high cyclic stability.In addition, the reversible adhesive has a reversible adhesion effect both underwater and in the air.The adhesive strength in air is higher than underwater without the influence of water molecules on the adhesive and interface (Figure 4F).
According to the characteristics of the smart underwater adhesive, we designed a series of demonstration experiments.A PMMA piece coated with a 6 cm 2 adhesive layer was immersed in deionized water.After 1 min, the adhesive layer with another piece of PMMA was covered, applied a contact pressure of 75 kPa, and held for 10 min.When the adhesive layer was in full contact with the PMMA surface, we removed the preload and then placed the sample under the long-wave UV light source for 60 min.The whole curing process was carried out in deionized water (Supporting Information: Figure S11).After curing, this pair of PMMA bonded by the smart underwater adhesive was used to lift 6 kg kettlebells (Figure 5A).The experimental results show that the adhesive has strong adhesion strength when the cohesion energy matches the interface adhesion strength.Figure 5B shows the pair of PMMA bonded by our reversible underwater adhesive used in the experiment and the size of the coating was 1.5 cm × 4.0 cm.As shown in Figure 5C, the adhesive layer was crosslinked after long-wave UV light, resulting in the increase of cohesion energy, which matched the interface adhesion transition of adhesive.This was the main reason why the adhesive achieved such high adhesion strength.To show the dissociation characteristics of the polymer, we prepared a cured sample in the same way and hung a 20 N weight on it underwater.A short-wave UV light source was placed on the side of the water tank for light dissociation.After 6 min, the sample broke and the weight fell to the ground, indicating that under the continuous short-wave UV irradiation, the cohesion energy of the polymer was continuously weakened.Finally, the sample could not bear the gravity of the weight and broke (Supporting Information: Figure S12).
Interestingly, the adhesive can be used as a smart coating for reversible thread-locking (Figure 5D).In the field of mechanical engineering, applying thread locking compound is a common method to reinforce thread pairs, however, this method is usually long-term and one-time.Once there is a need to disassemble, it often leads to the destruction of the thread pair.Figure 5F shows the optical microscope images of bolts without adhesive coating (upper left) and with adhesive coating (lower left).After dyeing the adhesive layer with rhodamine B, the thickness of the adhesive layer could be easily observed and is about 16 μm.The model and standard of the bolt were GB/T 5783 M8 × 16 (Figure 5E).Through long-wave and short-wave UV irradiation, our reversible adhesive can realize the transformation of high and low adhesion strength as well as the reinforcement/ loosening of thread pairs (Figure 5G). Figure 5H shows the torque required to unscrew the bolt under four different conditions in the experiment, in which the preload applied by tightening the screw was 5 Nm.The experimental results show that when the reversible adhesive was not applied, the torque required to unscrew the bolt was slightly greater than the preload, which was about 6 Nm.After the adhesive was applied, this value increased to about 10 Nm.When cured by long-wave UV light, the cohesion energy of the adhesive gradually increased, which matched the interface adhesion.The torque required to unscrew the bolt sharply increased to 25-30 Nm.This torque has met most application scenarios.The torque of unscrewing the bolt was reduced to about 15 Nm after the dissociation under short-wave UV light, which allowed the bolt to be easily removed.By using the reversible adhesive, the purpose of using a thread tightening agent to reinforce/disassemble bolts without damaging the thread pair on the weak substrate is realized, which has a certain potential application value in the field of mechanical engineering.

| CONCLUSION
In conclusion, we have developed a light controllable underwater reversible p(DMA-co-MEA-co-CoumMA) copolymer via free radical polymerization of 2-methoxyethyl acrylate, 3-methacryloyl dopamine, and coumarin derivative.The as-prepared adhesive has obviously different adhesion strengths from 150 to 450 kPa under 254 and 365 nm UV light irradiation, respectively, which rely on the coupled change of cohesion and wettability.Under 365 nm UV light irradiation, the molecular fragments of coumarin derivatives within the polymers are cross-linked, so as to improve their cohesion and hydrophobicity.Under 254 nm UV light irradiation, the crosslinked molecular fragments are partially dissociated and returned to the uncross-linked state, and the cohesion and hydrophobicity of the polymer decreased.The smart adhesive has stable cyclic reversible adhesion underwater and can still maintain high adhesion stability after being exposed to 365 nm UV light in acid-base environment.It also demonstrated that the smart adhesive material can be coated to bolts for ondemand bonding.It is hoped that the combined properties of strong underwater adhesion and remote control will open doors for innovations in different engineering applications.

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I G U R E 1 Design and synthesis of light-responsive underwater adhesives.(A) Schematic diagram of underwater adhesion of smart adhesives.(B) Molecular formula of adhesion polymer.(C) Photograph of the prepared adhesive.(D) The proposed mechanism of the smart underwater adhesives for gluing adherends.(E) Principle of light response characteristics and photosensitive molecular fragments of the switchable adhesive.

F I G U R E 2
Characterization of wet adhesion.(A-C) Adhesion strength-displacement curves of adhesives with different photosensitizer content after 1 h of UV radiation.(D) The relationship between the adhesion strength and the contact time with a 25 kPa preload force.(E) Adhesion strength increases first and then decreases with irradiation time.(F) The relationship between adhesion strength and preload with a 10 min preload time.UV, ultraviolet.

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I G U R E 3 Stability of smart adhesive.(A) Adhesion stability of the polymer in acid-base environment.(B) The displacement-adhesion strength curves of smart adhesive after soaking in water for 24 and 48 h.

F I G U R E 4
Reversibility of smart adhesive.(A) The displacement-adhesion strength curves of smart adhesive after curing and curing dissociation.(B) Frequency dependency of storage moduli G′ under the 254 and 365 nm UV light.(C) Changes in the hydrophilicity and hydrophobicity of the adhesive before and after crosslinking.(D) Relationship between 254 nm UV irradiation time and adhesion strength.(E) Reversible underwater adhesion upon alternating short-wave and long-wave UV irradiation.(F) The adhesion strength in water and in air upon alternating 254 and 365 nm UV irradiation.UV, ultraviolet.

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I G U R E 5 Demonstration and application of the strong bonding and switchable adhesion performance of the smart adhesive.(A) Hanging Kettlebell experiment, the Kettlebell weighs 6 kg.(B) Sample photo, adhesive layer area 6 cm 2 , scale bar = 3 cm.(C) The adhesive layer is strengthened underwater after exposure to long-wave ultraviolet light for 1 h.(D) Schematic diagram of bolt tightening test.(E) Component diagram of bolt tightening test, including bolts, tapped holes, and smart adhesives.(F) Optical microscope images before and after coating the adhesive layer, scale bar = 500 μm.(G) Chemical process of high and low torque conversion.(H) Loosening torques under four conditions.