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.
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