Materials science

Sound switches material from insulator to semiconductor

Force-activated polymer ‘unzips’ into conductive configuration when exposed to ultrasound.

Scientists have designed a polymer that can rearrange its molecular structure in response to force, altering properties including colour and conductivity.

Conjugated polymers contain alternating single and double bonds that create continuous ‘π bonds’, which allow delocalized electrons to move freely. This conductivity can be greatly increased when impurities are introduced into the polymers, making them useful in electronics.

Yan Xia and his colleagues at Stanford University in California designed an insulating polymer containing fused four-membered rings in a ladder-like arrangement. When this non-conducting and colourless polymer is exposed to ultrasound, it experiences a force that ‘unzips’ the ladder to create a molecule with a backbone of alternating single and double bonds, irreversibly forming the conjugated polymer polyacetylene, which is semiconducting and dark blue in colour.