Self-strengthening plastics and artificial bone made from synthetic polymer may be one step closer to reality thanks to a breakthrough in chemistry that allows mechanical force to induce chemical reactions.
Typically, light, heat, electricity and chemical catalysts are used to activate such reactions. Mechanical force has rarely been used in the past, and primarily to pull apart long chains of polymers.
Now a team led by Jeffrey Moore at the University of Illinois at Urbana-Champaign in the US have found that by applying ultrasound to a solution of specially prepared molecules can induce reactions that break apart one atomic bond at a time.
The team attached two polymer strands to a molecule called benzocyclobutene (BCB) and bombarded the strands with ultrasound. This generated tensile forces that tugged on the connected molecules and, with the proper amount of ultrasound force, they were able to stretch it until one of its bonds broke, forcing the molecule to reform.
'Fundamental discovery'
BCB has two slightly different flavours - called isomers - that usually reform into different shapes when broken. In their experiment, the researchers were able to make both isomers produce the same shape, a feat previously unattainable (see image).
"Before, you could not take and pull a molecule in a very specific way," says Virgil Percec of the University of Pennsylvania, US. "It's a very fundamental discovery. It's chemistry at its best."
When the BCB bonds are broken, they become unstable and cross-link with other similarly broken bonds. This leads the researchers to believe they might ultimately be able to make self-healing materials.
Polymer bonds that reform in this way could perhaps repairs crack or breaks, and might be stronger than those of the initial material. The researchers believe it might even be possible to make a polymeric substance that mimics living bone in this way.
Bone making
"The mechanical properties of bone are altered over time depending on use history," says Moore. "They thicken and harden. With polymeric materials we currently have no ability to do that."
Moore also believes he can find ways to make such reactions reversible, so that objects could be made to harden, or become suppler, depending on the stresses placed on them.
Such substances will be years in the making. In the meantime, Moore and his colleagues hope to produce colours using stress-induced reactions, which could be used to warn that an object is weakening.
"One obvious example would be a parachute cord that turns red when bonds begin breaking" to warn that the chord is weakening, Moore says.
Journal reference: Nature (vol 446, p 423)
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