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Cheap polymer opals enable new applications

13 Sep 2007

A new method for fabricating polymer opals could lead to new applications that could benefit from their color changing properties.

A method for mass producing polymer opals that allow novel color-tuning characteristics has been developed by researchers in the UK and Germany. What's more, the researchers believe that they have found a way to make the materials more strongly colored which could open up a range of decorative, sensing, security and photonic applications. (Optics Express 15 9553)

"We have discovered a way to make these structures cheaply in large volume, but also a new way to make strongly colored materials," Jeremy Baumberg, a researcher at the University of Southampton, told optics.org. "We have produced polymer opal sheets 100m long and 1m wide and see no problem in scaling this up."

Baumberg's polymer opals are a type of photonic crystal consisting of a 3D lattice of plastic spheres, each a few hundred nanometers across that are "glued" together with a more flexible polymer material. The lattice structure reflects light in such a way as to produce a color that depends upon the angle of reflection.

So far, a manufacturing process to produce polymer opals has eluded scientists. "Normally, synthetic opals are produced by sedimentation where glass spheres are allowed to settle in a vial which can take months to make a well-ordered structure," commented Baumberg. "Other methods such as spinning precursor solutions in a solvent have proved hard to control."

Baumberg's Southampton team work with colleagues at the Deutsches Kunststoff-Institut, Germany to demonstrate an industrial method of producing 3D ordered structures using conventional plastics. They have also enhanced the scattering of light by adding nanoparticle sized dopants into the lattice.

"Although the difference in refractive index between the spheres and surrounding medium is small, adding tiny nanoparticles in between the spheres leads to an enhancement of color," explained Baumberg. "We use transparent polymers, and add a small amount of carbon nanoparticles in order to get intense metallic greens, oranges, reds and blues."

One of the more interesting applications exploits the flexible nature of the films. "The films can stretch across edges and around corners," commented Baumberg. "Leading airplane manufacturers have expressed interest in coating tailplanes with these striking color solutions."

Security applications could also benefit from the hard-to-replicate appearance of these materials, such as for banknotes, software, credit cards and technology products.

"There are applications such as smart packaging or sensing in which the films can be 'chemically programmed' to respond visually to low levels of environmental targets," added Baumberg. "Food packaging could change color to indicate if the food inside it is spoiling."

The team has a $2m grant from the EPSRC (UK research council) to look at the physics, applications and production of the polymer opals. This involves a number of partners including Merck, Degussa, Unilever, Kodak, the UK military and others. "In this research, the intricate interplay between understanding the new optical physics and the nature of self-assembly in polymer systems produces a wonderfully rich playground that will open up new application possibilities," concluded Baumberg.

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