|A self-healing material that can repair in about a minute under UV light developed. This polymeric material also does not function on the basis of forming chemical bonds between organic compounds for repair. Instead, it relies on localized heating and metal-ligand interactions, as per Nature.
A team of researchers from the Case Western Reserve University, the Army Research Laboratory at Aberdeen Proving Ground and the Adolphe Merkle Institute of the University of Fribourg have developed a polymer-derived substance that can heal on its own when exposed to UV light for not more than a minute. The polymer-based substance is not yet ready for commercial utilization. However in the future, these substances could be utilized in varnishes for furniture and floors, motor vehicle paints and several other applications. The polymers include a Napoleon Complex and the distinct weak molecular interactions offer innovative behavior to these tiny polymers. The molecular layout of the polymers enables them to alter their characteristics when exposed to intense UV light. The new polymer-based substances were developed by means of supramolecular assembly. When compared to traditional polymers, which include extensive, chain-like molecules featuring thousands of atoms, these substances are made of tiny molecules which are grouped into lengthy, polymer-resembling chains by utilizing metal ions as �molecular glue.� The resulting new substances are termed as �metallo-supramolecular polymers� and in several ways show similar behavior as conventional polymers. However when exposed to high dose of UV light, the grouped structures are momentarily separated and the original solid substance is converted into liquid form, which is capable of flowing easily. When radiation is turned off, the substance re-groups into a solid form and the original characteristics are reinstated. The scratches found in the polymers were repaired by using lamps, which are similar to those used by dentists to treat fillings. The researchers found that wherever they moved the UV light, the scratches filled up and vanished, leaving no trace on the surface.
Experiments confirmed that the scientists could continually scratch and restore the substances in the same spot. The enhanced control offered by the UV light enabled them to aim only at the flaw and leave the remaining substance undisturbed. The scientists methodically studied several new polymer-based substances to determine an enhanced combination of mechanical characteristics and curing capability. They determined that metal ions supporting the grouping process through weaker chemical interactions serve as the light-sensitive molecular glue. They also established that the substances that grouped in the most organized microstructures had superior mechanical characteristics. However as structural order reduced, the healing efficiency improved significantly. Understanding these associations is crucial to enhance the lifespan of coatings customized to distinct applications.
The team used rubbery oligomers, poly(ethylene-co-butylene), as the core of their material. They attached ligands, 2,6-bis(1�-methylbenzimidazolyl)pyridine (Mebip), that can bind to metals at the ends of the oligomers. To form long polymers, the researchers added either zinc (Zn2+) or lanthanum (La3+) ions to the solution of oligomers. The metal ions form metal-ligand complexes with the Mebip, linking the oligomers with one another. The team shaped the polymers into films that were 350 to 400 �M thick. They purposefully cut the polymer to about 50-70% of the overall thickness of the film. When the cuts were exposed to two consecutive 30-second rounds of UV light (320 to 390 nm wavelength at an intensity of 950 mW cm-2), the cuts sealed up. The healed material was comparable in toughness to the original polymeric film, and images from atomic force spectroscopy show that the cuts essentially disappeared. This process works because complexes of Mebip with metals are chromophoric, so they can absorb light of a specific wavelength, such as light in the UV range. Once they absorb light, they get into a higher energetic state and then lose that energy by giving off heat. Thus, when the researchers exposed the cuts to UV light, there was heating at the surface of the polymeric film�enough heating to reach over 220�C in 30 seconds. The heat quickly depolymerizes the area around the cut. Once the UV light is turned off, the liquidized area cools, reforms the ligand-metal complexes, and seals up the cut. The healing process can be localized, as you only need to hit damaged areas with UV light. The researchers also show that the healing process would still work if the polymer was under a load of about 8 kPa. They suggest that different ligands could be used to cover a range of absorbable wavelengths. Thus, you could selectively tailor the wavelength of light to heal different types of damaged materials.