Supramolecular polymers are assembled from monomeric building blocks through non-covalent, directional interactions such as H-bonding, π-π stacking, and ligand-metal complexation. The nature and strength of useful supramolecular motifs can be varied over a wide range, and the reversible and in many cases dynamic nature of supramolecular binding can lead to stimuli-responsive properties. The possibility to temporarily reduce the molecular weight of the supramolecular assemblies by shifting the equilibrium to the monomer side by exposure to an appropriate stimulus (or alternatively to use systems that are highly dynamic at ambient and require no further activation) can be used to create healable (or self-healing) polymers, as the resulting increase of the chain mobility and decrease of the material's viscosity enable the disassembled material to flow and fill cracks and gaps, before the original material is reformed by shifting the equilibrium back to the assembled state. Examples of thermally healable materials based on this general concept include hydrogen-bonded rubbers based on telechelic poly-(ethylene-co-butylene) functionalized with 2-ureido-4[1H]-pyrimidinone (UPy) units, elastomer networks based on fatty acids, ethylene diamine, and urea, and phase-separated elastomers based on a polystyrene-polyacrylic acid brush copolymer. Optically healable supramolecular polymers, which are advantageous because the stimulus can be applied in a targeted manner, have also been realized, for example on the basis of telechelic poly(ethylene-co-butylene) that was chain-terminated with terdentate ligands and assembled into a polymer with stoichiometric amounts of Zn2+ or Eu3+. However, on account of the dynamic nature of the supramolecular motifs employed, and the use of building blocks with low glass transition temperature virtually all known healable supramolecular polymers exhibit a low resistance to mechanical stress. This problem can to a certain extent be overcome by the introduction of a rigid, reinforcing (nano)filler, but even with this improvement the stiffness (storage modulus of <250 MPa) and strength (tensile strength of <5 MPa) thus far reported are very limited, which represents an obstacle for the exploitation of such materials as a replacement of thermoset resins in coatings, adhesives, and other applications. While molecular glasses represent a well-investigated class of materials, polymeric supramolecular glasses have been rarely observed.
Accordingly, one problem of the present invention was to create supramolecular materials that exhibit relatively high stiffness and yet can efficiently be healed.
Still another problem of the present invention was to provide supramolecular materials that offer excellent coating and adhesive properties, and whose adhesive properties can be altered by an external stimulus such as light or heat to enable (de)bonding on demand.