Researchers at the Adolphe Merkle Institute have developed a polymer-based material that can bond and debond-on-command when placed under ultraviolet light. Their findings were published as the Editors Choice in a recent issue of ACS Applied Materials & Interfaces. The reversible adhesives could be used to bond and debond materials used in automotive parts, metal and glass surfaces, packaging materials, medical devices, and many other applications. The proof-of-concept studies carried out can now be adapted and further developed for commercial use.

Adhesive: Picture of a supramolecular adhesive bonding two glass slides together and illuminated with UV light. Pictures courtesy Christian Heinzmann for Adolphe Merkle Institute, University of Fribourg, Switzerland.

The new materials were created by a mechanism known as supramolecular assembly.  Unlike conventional polymers, which consist of long, chain-like molecules with thousands of atoms, they are composed of smaller molecules, which are assembled into longer, polymer-like chains using metal ions or hydrogen bonding motifs to link the small components together and create a “quasi-crosslinked” network structure. The result: the new materials, which the scientists call “supramolecular polymers”, behave in many ways like normal polymers. However, when irradiated with intense ultraviolet light, the supramolecular structures disassemble, resulting in a “liquid-like” material state. The supramolecular assemblies reform again as soon as the stimulus is removed.

When used as an adhesive, the material is able to bond objects together under ambient conditions. Upon exposure to an external stimulus – such as ultraviolet light or heat – the liquid-like state of the material possesses no cohesive properties anymore and the bonded objects can be separated easily. As soon as the external stimulus is removed, the original material properties are restored and it can be re-used as an adhesive. The process of bonding and debonding can be performed multiple times without losing the adhesive properties of the original material.

Reversible adhesive: Pictures displaying the bonding and debonding properties of the supramolecular adhesive. Under ambient conditions, the rubbery supramolecular adhesive is able to glue objects together such as glass slides (top). Upon irradiation with UV light, the supramolecular polymer liquefies and loses its adhesive properties (bottom). As soon as the light is removed, the material solidifies again and the original properties are restored. The whole process is fully reversible and can be repeated multiple times. Pictures courtesy Christian Heinzmann for Adolphe Merkle Institute, University of Fribourg, Switzerland.

Schematic supramolecular: Schematic representation of the molecular design of the supramolecular adhesive. The supramolecular assembly consists of comparatively short chain-like molecules (black) with a functional group on each end (blue), which can be connected by a metal ion (grey), extending the chain length. The connected / disconnected states can be influenced, for example with ultraviolet light or heat. Under ambient conditions (left) the supramolecular polymer behaves like a regular rubbery material, whereas when exposed to a stimulus, the material liquefies (right). As soon as the stimulus is removed, the supramolecular assemblies reform again, restoring the original material properties. This process is reversible and can be repeated multiple times. Schematic courtesy Christian Heinzmann for Adolphe Merkle Institute, University of Fribourg, Switzerland.

Read our stories: 

Making supramolecular polymers tougher (2019)

Researchers from the Adolphe Merkle Institute’s Polymer Chemistry and Materials group have developed a new approach to toughen supramolecular polymers and have demonstrated the usefulness of such materials as adhesives that enable bonding and debonding on demand when heated or exposed to intense UV light.

Bonding and debonding on demand (2014)

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.