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Watching Polymers Heal

NCCR Bio-Inspired Materials researchers at the Adolphe Merkle Institute (AMI) have developed a method to monitor the healing process of polymers and 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.

On a nanometer scale, the healing process requires individual molecules at damaged sites to move across the severed interfaces and re-mix, until the original structure and properties of the material are restored. The fundamental importance of the underlying processes has long been established, but monitoring them on the nanoscale proved to be extremely challenging. NCCR researchers led by Professor Christoph Weder and Dr. Stephen Schrettl (both AMI) has now reported that they can 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 how the movement of molecules across the healed interface affects the recovery of the mechanical properties. As it turns out, a much wider interface than previously assumed is required for complete healing to occur.

Structural Conditions for Thermal Healing

To achieve this, the team investigated a pair of virtually identical supramolecular polymers that form through metal-ligand interactions and heal in response to heat, much like related healable or self-healing materials. The two polymers have identical properties, but the different metal ions have different fluorescence colors and they can also be monitored with high spatial resolution using X-ray spectrum imaging. “Having a Lego-like system in which two polymers that fluoresce red and blue merge and ‘mix’, allowed us to study these processes by looking at the colors, while the movement and fit of the different blocks is identical and unaffected by the color,” says NCCR and AMI alum Dr. Laura Neumann, who was the lead researcher of the study. This simple experimental trick, which the researchers exploited in a similar manner with X-ray imaging, allowed the monitoring of the mass transport across the healed interfaces with high precision.

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, reported in the leading journal Science Advances, 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. “Visually monitoring the healing can be a great aid in the development stage of new materials in which mechanical integrity after healing is important, for example in load-bearing applications,” adds Neumann.