Construction And Functional Application Of Polyoxometalate Supramolecular Water-based Adhesives

The research of artificial water-based adhesives is based on the mimicry and improvment of natural adhesive proteins secreted by underwater marine organisms.Currently,the building blocks in artificial water-based adhesives are mainly polymers which incorporate interfacial bonding units such as dopamine or other polar functional groups.These polymers further cross-link or entangle into form a network structure through covalent or non-covalent interactions(Ionic bonds,hydrogen bonding,host-guest and coordination,etc.).However,the synthesis steps of those polymers are complicated.Simultaneously,they can result in unavoidable environmental pollution.In addition,most of these water-based adhesives are only employed at dry or wet conditions,which cannot be used directly under water line.What's more,most of the reported adhesives exhibited single adhesion,which limit the application area.In this paper,we chose low-cost,simple and readily available small molecules as the building blocks,and polyoxometalate clusters as bridging units to construct a new type of water-based adhesive through a one-step water co-assembly method.It is expected that this strategy can avoid tedious steps and environmental pollution in the synthesis process of adhesives.Then the formation mechanism,adhesion behavior and functional applications were studied.First,catechol residues are widely used as interfacial adhesion motifs in the adhesion proteins secreted by mussel and sand worm,and the catechol groups are easily oxidized to form dimers,which can be used to enhance the cohesion.Herein,dopamine(DA)is used as a starting materials.The DA can be partially oxidized by Na IO4 into covalently linked oligomers,then polyoxometalates(HSi W,KP2W18)are mixed with the partial oxidized DA solution,resulting in the formation a crosslinked network structure,the synergistic effect between covalent and non-covalent interactions play a centered role in stabilized the network structure.The average shear strength tests show that the adhesive exhibits clear adhesion properties at wet and dried state.Rheological tests show that the adhesive has good viscoelasticity and self-healing properties.1H NMR and ESI-MS spectrum indicate the adhesive matrix include both DA monomer and their oligomers.FT-IR and 183 W NMR demomstrate that the topology structure of HSi W remains intact.However,the adhesives loss their adhesion when exposing to fresh water because the catechol groups are easily oxidized to form their quinone counterparts.To solve this problem,We selected aromatic amino acid small molecules with stable chemical feature and strong hydrophobicity as building blocks,and heteropoly acids(HPAs)as bridging units.A new class of underwater adhesives through multiple non-covalent interactions were successfully prepared.First,COO-of amino acid is converted into COOH during solid phase grinding of aromatic amino acid and heteropolyacids,due to the strong acidicity of HPAs.Then the main chain of the aromatic amino acid can interact with HPAs througth ionic bonds,the strong ionic interaction can bring the aromatic unit and HPA close to each other,thereby generating charge transfer.Among them,aromatic rings act as electron donors and heteropolyacids serve as acceptors of electrons.When the grinded powder mixing with water,clusters and clusters are connected to each other by ?-? stacking and hydrophobic interactions to form a cross-linked network structure.The FT-IR,31 P NMR spectra proved the stability of the heteropolyacids.The solid 13 C NMR spectra and ESI-MS proved that the aromatic amino acids did not dissociate or polymerize.FT-IR,XPS demonstrated the protonation of COO-.Solid UV-vis,EPR and XPS datas demonstrated the existence of charge transfer between HPA and aromatic unit of amino acid.Shear strength tests showed that the formed adhesive exhibited good adhesion properties both in dried and underwater state.Rheological tests showed a certain extent viscoelasticity.Importantly,this as-prepared organic/inorganic hybrid adhesive can be directly utilized as electrode coating via soft printing technology.The resultant patterned coating showed spontaneous color change after connecting with aluminum foil in aqueous electrolyte.As a proof-of-concept,a self-powered electrochromic battery was constructed to light up a red light-emitting diode.Subsequently,the reversible discharge and charge process of the battery corresponding to the switchable color change in the patterned coating was demonstrated with the aide of H2O2.As the soft patterned coating also exhibited excellent toleration of mechanical deformation under water line,this hybrid underwater adhesive was further deposited on flexible carbon paper to fabricate the function integrated power platform,which will offer additional benefit for detecting the charging state of a flexible battery via naked eye-visible change in color.In summary,small molecules,including dopamine or aromatic amino acids,were used as building blocks,and polyoxomatalates were used as multivalent cross-linking points,a series of water-based or underwater adhesives have been successfully prepared using multiple non-covalent or covalent and non-covalent synergistic strategies.In particular,adhesives prepared using aromatic amino acids and polyoxometalates can be used as underwater coatings for the construction of patterned electrode materials,and further utilize the electrochromic properties of polyoxometalates to assemble discolored aqueous solution batteries,which expands the functional application of water-based adhesives.