Watching polymers heal
Researchers from AMI’s Polymer Chemistry and Materials group have developed a method to monitor the healing process of polymers. They discovered that a much thicker interphase is required for these materials to recover their original mechanical properties than previously thought.
Self-healing or healable polymers can recuperate their function after sustaining physical damage. Such materials have many interesting applications, including automotive paints, display covers, and varnishes for floors or furniture. As such, this class of materials has attracted considerable interest and first systems are transitioning from research labs into technological applications.
The healing of damaged polymers is often possible by heating them just above the melting temperature, so that separated interfaces can weld together. The process itself is quite complex and involves multiple effects that occur at different length scales. On the macroscopic and microscopic levels, the surfaces need to rearrange, approach each other, and weld. On a nanometer scale, individual polymer molecules must move across the severed interfaces and re-mix, until the original structure and properties of the material are restored. The fundamental importance of the processes occurring on the nanoscale has long been recognized, but monitoring them proved to be extremely challenging. A team of AMI researchers led by Professor Christoph Weder and Dr. Stephen Schrettl has shown that is possible to precisely track the healing process while it happens. In a collaborative effort involving researchers at EPF Lausanne, ETH Zurich, and Martin Luther University Halle-Wittenberg, Germany, the team succeeded in imaging the movement of molecules across the healed interface and was able to correlate this process with the recovery of the material’s mechanical properties. As it turns out, a much wider interface than previously assumed is required for complete healing to occur, at least in the particular materials studied.
The team carried out their studies with a pair of virtually identical supramolecular polymers. Unlike conventional polymers, which consist of long chain-molecules that are formed through irreversible chemical bonds between monomeric building blocks, supramolecular polymers are comprised of chain-like assemblies in which the monomers are connected through weak, reversible interactions. The healing of such materials is greatly accelerated, because the supramolecular assemblies disassemble into smaller molecules when the material is heated or irradiated with ultraviolet light. When the healing process is complete, the material automatically reassembles.
The two supramolecular polymers used by the AMI researchers were based on the same building blocks, which were however assembled with the help of two different metal ion complexes. The two materials exhibit very similar properties, but the two metal ions can be distinguished. “The possibility to heal interfaces between two practically identical, yet easily distinguishable polymers allowed us to study the healing process by monitoring the positions of the metal ions with high spatial resolution using fluorescence imaging and X-ray spectrum imaging, while the movement of the different blocks is identical and unaffected by the nature of the metals,” says AMI alum Dr. Laura Neumann.
The researchers discovered that the original properties are only restored after the thickness of the healed interphase exceeds 100 nanometers. This value is about ten times higher than previously reported for conventional glassy polymers. The findings suggest that relatively straightforward microscopic techniques should be suitable to uncover previously unobservable aspects of the healing process in a wide range of materials, guiding the design of new polymers with improved healing characteristics.
With the objective to move healable polymers closer to application, PhD student Franziska Marx is currently investigating how the findings can be used to modify commercially used materials in order to render them healable. “Our goal is to develop a readily scalable approach to create materials that offer a combination of efficient healing at elevated temperature and mechanical characteristics at normal usage temperature that are comparable to those of currently used polymers” says Marx.
Neumann, L. N.; Oveisi, E.; Petzold, A.; Style, R.; Thurn-Albrecht, T.; Weder, C.; Schrettl, S.; Dynamics and Healing Behavior of Metallosupramolecular Polymers; Science Advances 2021, 7, eabe4154.