New polymer signals red when damaged, then fixes itself

A new plastic turns red when scratched and then repairs itself when exposed to heat or light. Several research groups have previously made self-healing materials, but the new material is the first to combine both sensing and repair capabilities. Marek Urban, a polymer science professor at the University of Southern Mississippi in Hattiesburg described the plastics at the American Chemical Society meeting in San Diego on 26 March.
 
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. ''In principle, it could do so infinitely,'' Urban says. ''But of course there's some fatigue factor involved. When you break polymer chains, 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.
 
Past efforts to make self-healing plastics have involved two different approaches. One is to embed plastics or composites with capsules that contain a healing agent. When the material is damaged, the capsules break open and release the agent, filling in cracks. The other approach, which is more repeatable, is based on polymers that reform chemical bonds under heat or light.
 
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. Most self-healing mechanisms are triggered by heat, light or other stimuli, he adds. ''Sensing and healing is an especially good combination because you have to apply external stimuli.''
 
Urban and his colleagues 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 degrees celsius, 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 copolymer system works amazingly well as repeatably healing material under mild irradiation,'' says Krzysztof Matyjaszewski, a chemistry professor at Carnegie Mellon University. ''It will be interesting to further explore how the molecular architecture of the copolymers, for example making nanostructured block copolymers or nanocomposites, can affect performance.''