Plastic changes color in heat

Materials that change color depending on the temperature are potentially very useful. They can be used in sensors, adaptive camouflage, passive environmental controls and changeable paint.

Researchers from Helsinki University of Technology and the University of Helsinki in Finland, the Technical Research Centre of Finland and the University of Groningen in the Netherlands have engineered a plastic that loses its color when heated.

The material is green at room temperature but loses its color when heated beyond 125 degrees Celsius. It could eventually be used to produce relatively inexpensive temperature-based paint.

The material consists of two types of polymers that, because they repel each other, automatically assemble into alternating layers. The layer structure repeats every 117 nanometers, which causes the material to reflect light that has a wavelength of 530 nanometers.

Heating the material breaks hydrogen bonds, which releases stretched-out polymer molecules, allowing them to coil up. This shortens the polymer layers, which causes the material to reflect light that has a wavelength of 370 nanometers. The change in the size lightwave the material reflects is relatively large over a relatively narrow temperature range of about 10 degrees Celsius, according to the researchers.

The materials could be used in practical applications in two to five years, according to the researchers. The work appeared in the November 28, 2004 issue of Nature Materials.