A critical component of the structured gel is a material that expands
or contracts when exposed to certain stimuli. Those changes in the
thickness of the gel cause it to change color, through the entire
range of the visible spectrum of light.
Objects that reflect different colors depending on which way you look
at them already exist, but once those objects are manufactured, their
properties can't change. The MIT team set out to create a material
that would change color in response to external stimuli.
�We wanted to develop something that was 'tunable,'� said Thomas, who
is head of MIT's Department of Materials Science and Engineering.
To do that, they started with a self-assembling block copolymer thin
film made of alternating layers of two materials, polystyrene and
poly-2-vinyl-pyridine. The thickness of those layers and their
refractive indices determine what color light will be reflected by the
resulting gel.
By keeping the thickness of the polystyrene layer constant and
altering the thickness of the poly-2-vinyl-pyridine layer with
external stimuli such as pH and salt concentration, the researchers
were able to change the gel's color in fractions of a second.
�This is an ingenious and easy-to-implement method for making photonic
materials whose optical properties can be readily tuned over a wide
range," said Andrew Lovinger, director of the Polymers Program at the
National Science Foundation, which funded this research.
The key to manipulating the thickness of the poly-2-vinyl-pyridine
(2VP) layer is to give the nitrogens on each segment of the 2VP block
a positive charge, yielding a polyelectrolyte chain that can swell to
more than 1,000 percent its volume in water.
If the charges along the chain's backbone are electrically shielded
from each other, for example by adding a high concentration of salt
ions to the water that has permeated the gel, the 2VP chains collapse
into disordered tangles, like balls of string. When the salt ions are
washed away, the 2VP positive charges again repel each other and the
chain extends, causing each 2VP layer to expand and the material to
reflect a different color.
Because the diblock polymer film is a one-dimensional periodic stack,
swelling is limited to one dimension, yielding a color shift of 575
percent in the reflected wavelength, a dramatic improvement over
earlier color-changing gels that are made of charged colloids in a 3D
lattice structure. Those gels expand in three dimensions, giving a
much smaller range of color change.
The new gels are also sensitive to changes in pressure, humidity and
temperature. �You can use mechanical or chemical forces to get really
big responses, going through the entire range of light from
ultraviolet (300 nanometers) to infrared� (1600 nm), Thomas said.
The research team is also working on a gel that changes color in
response to applied voltages.
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