Materials that use polystyrene nanospheres rather than toxic dyes to produce colour have been the subject of research by scientists at the University of Southampton, UK. Offering new possibilities for structural colours, with iridescent, prismatic, multi-hue and luminescent tones, the materials have already attracted the interest of Unilever, Kodak, Merck, Mercedes and Degussa for applications ranging from packaging to automotives.

Professor Jeremy Baumberg at Southampton says, ‘With increasing concern for the environment, the use of traditional dyes is becoming problematic. This has opened up opportunities in producing materials at the nanoscale, [using] components that are benign.’

The team has grown core-shell spheres with hard polystyrene centres from tiny seeds until they reach a specified size, such as 200nm. The spheres are then mixed with nanoparticles less than 50nm in diameter, for example black carbon which turns white films green.

The resulting polymer precursor melt is processed, via extrusion or compression moulding, to form films that are hundreds of microns thick, several metres in length and contain ordered spheres. Baumberg says, ‘The ordering of the spheres is much better than natural or artificial opals. The sphere size controls the wavelength at which light is reflected and scattered from the film. Because the polymers can be elastomeric, the films are stretchable, which changes their colour.’

He adds, ‘The key advance is to make films which scatter light into all directions at particular wavelengths, and this comes from adding nanoparticles in regular positions in the periodic structure, that is, in between the spheres. The regular array of spheres multiply reflected light [through] many thousands of layers.’

Further research will focus on the scaleability of the material.