A novel polymer that signals red when damaged, then fixes itself

Several research groups have previously made self-healing materials, but a new material is the first to combine both sensing and repair capabilities. A new plastic that turns red when scratched and then repairs itself when exposed to heat or light has been introduced by Marek Urban, a polymer science professor at the University of Southern Mississippi. The plastic is a block co polymer, which is composed of different polymers linked together. When the material is scratched, it turns from clear to red along the damaged area. The crack disappears and the material reverts to its clear colour when it is exposed to heat or light. It can change colour and repair itself over and over again. Some fatigue factor is involved- as when the polymer chains break, some oxidise and there are typical degradation times of polymer. The material could be used to make scratch-free laptops and cell phones, and paints and coatings that mend themselves. Automobile and aircraft parts made of the material would alert users to injuries. Mechanics could then decide whether to repair the damage with light or replace the damaged part altogether. The team made the block copolymer with three different polymer segments: methyl methacrylate (MMA), n-butyl acrylate (nBA), and spironapthoxazine (SNO). The researchers suspend nanoparticles of the three polymers in water and then dry the suspension on a substrate. As the water evaporates, the polymer units self-coalesce into a block copolymer film. When the material is scraped, the ring-shaped SNO units open up, which changes their optical properties and leads to the red color. The copolymer, now made of the MMA and nBA units, takes on a stretched configuration. When exposed to visible light or a high temperature of 95°C, the SNO rings close and go back to their original color. The MMA and nBA backbone collapse back from its stretched state, pulling in neighboring copolymers to fill in the scratch.

The ability to sense stress by changing colour and the ability to heal with appropriate stimuli have been shown separately before, but the new work ''combines the two in a unique way,'' says Michael Kessler, a materials science and engineering professor at Iowa State University. ACS goes on to report that as of now there are two strategies to develop self-healing polymers. One depends on compounds with �healing� properties being released when a plastic becomes �injured.� Others rely on exposure to a certain type of light to catalyze the reaction. Urban believes those healing through light exposure are more beneficial as they can repair multiple times, whereas those relying on compounds that can heal only once. This new class of plastic is designed to overcome a critical flaw of plastics: Once damaged, plastics are very hard to fix and often just have to be replaced. For this reason, self-repair plastics have been one active area of research in materials science. The plastics created by Urban�s group have small molecular links that stretch across the long chemical chains that make up the plastic. When a scratch or cut happens, these links break and change their shape. The shape change generates a visible color change as a red patch around the defect. When the damaged plastic is exposed to sunlight or light from a light bulb, pH changes or temperature, the molecular links connect up again, fix the damage, and remove the red mark. Urban says the new class of plastics can self-repair repeatedly. He adds that the new plastics are also more environmentally friendly than most other plastics because its production is based in water rather than organic solvents. The Urban team now focusing on making self-repair plastics that can tolerate heat.