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University of Southampton creates new colour changing material

Published: 15 August 2007

Scientists at the University of Southampton and the Deutsches Kunststoff-Institut (DKI) in Darmstadt, Germany have created a new type of plastic film that allows objects to change their colour.

The researchers, led by the University’s Professor Jeremy Baumberg and Otto Pursiainen, developed an Opal polymer to mimic the gemstone's iridescent properties.

The new material can be used across a range of applications including eye-catching paint that shimmers, food packaging that changes colour if its contents spoil and banknotes that are hard to counterfeit.

The Opal polymer is structured from billions of 200 nanometre (nm) polymer spheres – 200 nm is the amount that nails grow in the blink of an eye. During extrusion, the spheres are jostled together and settle into a regular array which is fixed by the congealing polymer in between them.

When the material is structured in layers, even though it has no pigment, it seems to shimmer like a butterfly wing when it is turned or twisted. As light hits the nano-structured surface of the Opal polymer material it scatters and is split up into different colours. The colours change under different conditions and it’s this quality that makes it very difficult to forge.

Professor Baumberg comments: "The constant spacing between the spheres means certain colours of light are trapped inside the films. Normally this would result in a very specific enhanced reflection if you hold the film up to a light source in the right orientation, but most of the time these films, like many opals or clouds, look milky white. We have put tiny black carbon nanoparticles into the films (without disrupting their order) and these strongly scatter light of only the particular trapped colour – this makes the films go virulent colours.

"As the films are elastic, stretching them changes the sphere spacing, and hence the colour. Applications are in any smart packaging, high-value coatings such as cars, mobile phones and planes, and security. Currently we are trying to develop good theoretical models to predict the strength of the effect, and control the ordering on an industrial scale."

A paper entitled Nanoparticle-tuned structural color from polymer opals has just been published on the Optics Express, The International Electronic Journal of Optics, website and can be accessed at: http://www.opticsexpress.org/abstract.cfm?id=139950

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