Polymers that can be healed could extend the lifetime of materials in so many applications. Chemists from the US and Switzerland have for the first time developed polymers that can be healed by exposure to ultraviolet light alone.
In the recent years, many strategies have been developed for healing polymers. In many cases, they are healed by heating to the glass transition temperature which transforms the polymer from its hard state into a molten state enabling the polymer chains to reform. Unfortunately, this technique is slow and difficult to use in practice. To overcome the problem, a method was needed to manipulate polymeric structure at the molecular level.
Burnworth et al. used supramolecular polymers which are lower molecular mass polymer units held together in long chains by metal-ligand bonds. These non-covalent bonds are weaker than the bonds that hold hydrogen and oxygen atoms in a water molecule but strong enough to enable the new material to possess polymer-like properties.
|Healed by UV light|
Metal atoms have special affinity to electron rich ligands. This allows metal atoms to form metal-ligand bonds in a polymer with ligand groups present in it its structure.
More importantly, working with these metal-ligand bonds has enabled the researchers to manipulate the bonds at the molecular level with light energy. A polymer sheet deliberately cut to 50% of the film thickness was exposed to UV light in the range of 330 – 390 nm. It was observed (as seen in the picture) that the polymer ‘healed’ by filling up the cut that was made earlier.
Metal-ligand bonds of the kind present in this polymer allow for the conversion of light energy into heat. In this case, the light energy causes the surface of the polymer to rapidly heat up to 220 °C in a very short time. The healing occurs in this state when polymer is allowed to flow and re-arrange. The advantage of using light energy lies in its specificity. Unlike heat energy, it is possible to direct light energy to precisely those areas which require repair.
Also because different metal-ligand complexes absorb light at different wavelengths it should be possible to tune the wavelength of light needed for healing. Thus, one can imagine that it may be possible to heal a broken mobile phone case just by keeping it in sunlight.
Reference: Mark Burnworth et al., Nature, 472, 334.