SUNGLASSES THAT CHANGE COLOUR INSTANTLY!?

Polymers that use electrical current to change colour now have a broader palette for potential uses, such as in sunglasses and window-tinting.

By developing electro-chromic polymer materials in a range of primary and secondary colours and combining them in specific blends, the researchers have covered the colour spectrum.

The materials could be used to make sunglasses that change from tinted to clear in a matter of seconds, at the press of a button. Even greeting cards could change colour through the application of low-voltage electrical current.

“Anything that you would want to have change color at the push of a button would be an application for these”

“We’ve demonstrated the ability to create virtually any color we want by mixing different electrochromic polymers, just like mixing paint,” says Anna Österholm, a research scientist in Georgia Tech’s School of Chemistry and Biochemistry and the paper’s first author.

“Using a simple coating method or even inkjet printing, we can create films that change colour with the application of a voltage.”

COLOUR COMBOS, INCLUDING BROWN

The many colors that have been developed by the team from Professor John Reynolds’ lab over the years include magenta, cyan, yellow, orange, blue, and green polymers that can be dissolved in common solvents. In addition, blends of these polymer solutions can be predictably mixed to target specific colors.

To demonstrate the capabilities, the researchers created brown lenses for sunglasses using a five-layer sandwich of materials, including a film of the electrochromic material, a charge storage layer, and a UV-curable electrolyte, with a cathode and anode layer on either side.

The lenses can be switched between a colored and colorless state by applying a brief pulse of electrical current and do not need a continuous power supply. To maintain the colorless state, a brief refresh pulse needs to be applied approximately every 30 minutes; however, the colored state can be stable for up to several days.

The materials can switch from about 10 percent transmittance to 70 percent transmittance—and back—in a few seconds.

The team has even created four shades of brown, a particularly difficult color combination. To do so, they combined cyan and yellow primary colors with orange and periwinkle-blue secondary colors.

QUICKER CHANGES

Photochromic sunglasses, which darken in response to light using a silver halide reaction, are already on the market. But many of these lenses respond to ultraviolet wavelengths that are filtered out by automobile windshields, require several minutes to transition, and can’t be controlled by users. The passive switching time can be problematic for pilots, drivers, security officers, or others who move quickly between light and dark environments.

“In contrast, by using electrochromic polymers, we can create devices that by pushing a button, can be converted from dark to clear,” says Österholm. “They are completely user-controlled, and it doesn’t matter whether they are being used indoors or outdoors, in a vehicle or an aircraft.”

The electrochromic materials rely on a reduction-oxidation (redox) reaction triggered by the application of an electrical potential provided by a simple coin battery: a positive one volt causes the glasses to be clear, while a minus one volt switches to the colour.

“Essentially, we are just charging and discharging the device, which is what causes the colour change,” explains Eric Shen, a postdoctoral fellow in the Georgia Tech School of Chemistry and Biochemistry.

‘THE PUSH OF A BUTTON’

The electrochromic materials represent years of work by the Reynolds Laboratory to synthesize polymers whose repeat-unit structures provide the desired palette of colors. Because they can be dissolved in the same solvents, additional colours can be created by combining specific quantities of the primary and secondary colours.

“Anything that you would want to have change colour at the push of a button would be an application for these,” says Shen. “We have shown that we can switch them on and off thousands of times, and that we can shine strong light on them without causing degradation of the color.”

The researchers have used simple spray and blade-coating techniques to create films of the colorful materials. They now are using ink-jet printing to create patterns and mix the polymers to create colors.

“The ink-jetting is very versatile when you want to make patterns or very fine features with these materials,” Shen says. “The fact that the polymers are so soluble makes it quite easy to process them using anything that would spread an ink.”

Supported by BASF, the research appears in ACS Applied Materials & Interfaces. The research was done in the laboratory of John Reynolds, a professor in the School of Chemistry and Biochemisty and the School of Materials Science and Engineering at the Georgia Institute of Technology.