Well-healed: Polymers cross-linked with trithiocarbonate (TTC) units were prepared by a RAFT polymerization. The repeatable self-healing systems or macroscopic fusions were accomplished by UV irradiation of the cross-linked polymer in solution and in the bulk state. The macroscopic fusion of completely separated parts was successful (see pictures, BA=n-butyl acrylate). |
Sooner
or later, a cut to the skin or a broken bone will heal on its own;
however, a scratch to a car’s paint or a tear in the wing of an airplane
will not. Materials with self-healing properties could help extend the
durability of products and make repairs easier.
Krzysztof
Matyjaszewski and his co-workers at Carnegie Mellon University
(Pittsburgh, USA) and Kyushu University (Japan) have now developed a
polymer that can repair itself when irradiated with UV light — over and over again. As the scientists report in the journal Angewandte Chemie, this is the first material in which capped covalent bonds repeatedly reattach, even allowing fully separated pieces to be fused back together.
Some
previous solid self-healing materials contain tiny capsules that tear
open to release a chemical agent when the material is damaged and have
been able to repair themselves only one time. Other materials, including
some gels, can repair themselves repeatedly but lack the covalent bonds that increase materials strength and stability.
In
contrast, the new polymeric material produced by the American and
Japanese team is stable and repairs itself again and again. The secret
to their success is that the polymer is cross-linked through
trithiocarbonate units. These are carbon atoms
bonded to three sulfur atoms, two of which use their second bonding
position to attach to another carbon atom. These groups have a special
property: they can restructure under UV light. The light breaks one
carbon–sulfur bond in the trithiocarbonate groups. This produces two
radicals — molecules with a free, unpaired electron. The radicals are
very reactive and attack other trithiocarbonate groups to form new
carbon–sulfur bonds while breaking others to form more free radicals.
The chain reaction stops when two radicals react with each other.
The
researchers were able to heal cut polymer fragments with
irradiation—either immersed in liquid or in bulk. They only had to
firmly press the cut edges together and irradiate them. The edges grew
back together by means of the radical re-organization process described
above.
The self-healing effect goes much further: even shredded polymer
samples could simply be pressed together and irradiated to be fused
into a continuous piece. The resulting object was in the shape of the
cylindrical tube in which the procedure was carried out. This
self-healing process can be carried out repeatedly on the same sample.
The material is thus also interesting as a new recyclable product.
Study
information: Krzysztof Matyjaszewski, Repeatable Photoinduced
Self-Healing of Covalently Cross-Linked Polymers through Reshuffling of
Trithiocarbonate Units,
Angewandte Chemie International Edition
