Polychromic plastics: the magic starts here

Chemists open the doors to new applications for colour-changing polymers.

False fingernails can now be controlled electronically to adjust their colour to any given shade. Moreover, they can be coupled to a camera and made to mimic the background to create chameleon-like invisibility. The Centro de Technologias Electroquimicas, a research centre in San Sebastian, Spain, has developed this technology using electrochromic polymers. While this particular application may find only a limited market, conducting polymers that are able to change colour in response to electronic signals are likely to spread into many applications, from displays to rear-view mirrors, because chemists have now removed the final barriers to their application.

Fred Wudl’s group at the University of California, Los Angeles, US, claims to have developed the first electrochromic polymer that appears green in its neutral form, thereby completing the red-green-blue colour space and making any desired colour accessible by mixing.

While red and blue conducting polymers can be achieved by fine tuning just one absorption band to the desired value, Wudl et al had to tailor their polymer, a pyrazine derivative known as poly-DDTP, such that it has two major absorption peaks, leaving a minimum at around 550 nm which corresponds to the reflected green light. The researchers created polymer films and cycled them between the two differently coloured states more than 10 000 times, without observing any changes in the spectroscopic properties, suggesting that the material is sufficiently robust for use in display technology ¹ .

In a separate project, the same research group also improved the colouring efficiency of an electrochromic display material. Starting from the existing poly(3,4-ethylene-dioxythiophene) [P(EDOT)], the researchers inserted methoxy ethylhexyloxybenzene between the EDOT units to achieve higher mobility of the counterions, and thereby higher doping levels of the resulting compound. By testing the new material in a prototype electrochromic cell, Wudl’s group confirmed that it is purple-red in the neutral state, blue when partially oxidised, and transparent when fully oxidised ².

Apart from fingernails, displays are the obvious first application for electrochromic polymers. But once their fabrication has been optimised and scaled up, there are virtually no limits to polymer-based display options. Think of the moving portraits at Hogwarts School for Witchcraft and Wizardry in J K Rowling’s Harry Potter novels. You don’t need any magic to create such effects when you have a full palette of electrochromic molecules. 

Michael Gross