Structured gel responds to multiple stimuli in colorful ways

Researchers from the Massachusetts Institute of Technology (MIT; Cambridge, MA) have created a structured gel that can rapidly change color in response to a variety of stimuli, including temperature, pressure, salt concentration, and humidity. According to Edwin Thomas, MIT’s Morris Cohen Professor of Materials Science and Engineering, the photonic gels could yield several novel applications, including colorimetric sensors, active components of simple display devices, and electrically controlled tunable optically pumped lasers, photonic switches, and multiband filters.

Thomas and his colleagues used a self-assembling copolymer thin film made of alternating layers of polystyrene and poly-2-vinyl-pyridine. The thickness of the 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. The key to manipulating the thickness of the poly-2-vinyl-pyridine layer is to give the nitrogens on each segment of the poly-2-vinyl-pyridine block a positive charge, yielding a polyelectrolyte chain that can swell to more than 1000% its volume in water. Because the diblock polymer film is a one-dimensional periodic stack, swelling is limited to one dimension, yielding a color shift of 575% in the reflected wavelength. The gels are also sensitive to changes in pressure, humidity, and temperature. Contact Edwin Thomas at elt@mit.edu.