Bonding and debonding on demand 

Something needs a quick fix?  No problem, superglue will help. Taking parts that are already bonded cleanly apart, however, might prove to be more difficult. To circumvent this problem, AMI researchers are investigating supramolecular polymers as adhesives that allow bonding and debonding on demand.

Whether it is the poorly fixed soup bowl, a defective dental implant, or a temporary fixture used in a microfabrication process, there are many examples of adhesive applications where being able to separate bonded parts would be beneficial. AMI researchers have shown that this is possible by using adhesives based on supramolecular polymers. Unlike conventional polymers, which in many cases consist of long, chain-like molecules that comprise thousands of covalently connected atoms, supramolecular polymers are composed of much smaller molecules. These building blocks contain chemical binding motifs that are designed to bind with each other and promote the assembly into longer, polymer-like chains (see graphic), even though no permanent chemical bonds are formed between the individual molecules. Materials made on the basis of such supramolecular assembly schemes behave in many ways like normal polymers. But when heated, the binding motifs readily disassemble and the material be-comes liquid. Upon cooling, the process is reverted and the original supramolecular polymer is reformed.

Working initially with previously studied, rubbery supramolecular polymers based on linear building blocks that carry just one supra-molecular binding motif at each end, AMI research-errs demonstrated the usefulness of such materials as reversible adhesives. Under ambient conditions, the materials readily bond glass, metal, and other substrates, but lose their adhesive proper-ties within seconds upon heating.  The temperature at which the debonding occurs can be programmed via the choice of the bind-in motif. Exploiting a light-heat conversion process, it was also possible to utilize ultraviolet light to trigger the debonding. This feature appears to be particularly attractive for ap-plications in which the bonded parts are difficult to access or in situations where heating the surrounding area is undesirable.

With the aim of tailoring the properties of similar supramolecular adhesives for possible applications and to develop a fundamental understanding of the relations governing the structure of such materials, AMI researchers are continuing to apply the design approach to other structures. These include supramolecular networks, composed of building blocks comprised of more than two binding motifs. Some of the materials made this way are glassy and therefore much harder and stronger than the materials initially studied.

Reference: Heinzmann, C.; Coulibaly, S.; Roulin, A.; Fiore, G.L.; Weder, C. Light-Induced Bonding and Debonding with Supramolecular Adhesives, ACS Applied Materials and Interfaces, 2014, 6, 4713– 4719.