Imagine: Your 6-year-old found a nail in the garage and drew pictures across the side of your new car. Gnash your teeth now, but researchers say the fix-up may be cheap and easy to do yourself in the not-too-distant future. They have developed a polymer-based material that can heal itself when placed under ultraviolet light for less than a minute. The scientists envision that re-healable materials like theirs could be used in automotive paints, varnishes for floors and furniture, and many other applications. Their polymers aren’t ready for commercial use, they acknowledge, but prove that the concept works.
“These polymers have a Napoleon Complex: in reality they’re pretty small but are designed to behave like they’re big by taking advantage of specific weak molecular interactions,” said Stuart Rowan, on of the responsables of the project. “Their molecular design allows the materials to change their properties in response to a high dose of ultraviolet light,” said Christoph Weder, researcher. 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, these materials are composed of smaller molecules, which are assembled into longer, polymer-like chains using metal ions as “molecular glue”. The result: the new materials, which the scientists call “metallo-supramolecular polymers”, behave in many ways like normal polymers. But when irradiated with intense ultraviolet light, the assembled structures are temporarily unglued. This transforms the originally solid material into a liquid that flows easily. When the light is switched off, the material re-assembles and solidifies again: the original properties are restored. Using lamps such as those dentists use to cure fillings the researchers repaired scratches in their polymers. Wherever they waved the light beam, the scratches filled up and disappeared, much like a cut that heals and leaves no trace on skin.
Tests showed the researchers could repeatedly scratch and heal their materials in the same location. “We can simply use heat to heal these materials,” Mark Burnworth, a researcher said. “But by using light, we have more control as it allows us to target only the defect and leave the rest of the material untouched.” The researchers systematically investigated several new polymers to find an optimal combination of mechanical properties and healing ability. They found metal ions that drive the assembly process via weaker chemical interactions serve best as the light-switchable molecular glue. They also found the materials that assembled in the most orderly microstructures had the best mechanical properties. But, healing efficiency improved as structural order decreased. “Understanding these relationships is critical for allowing us improve the lifetime of coatings tailored to specific applications, like windows in abrasive environments” Beyer said.
According to Rowan, “One of our next steps is to use the concepts we have shown here to design a coating that would be more applicable in an industrial setting.”