Thermoresponsive Photonic Crystals

Hydrogel nanoparticles that undergo volume phase transitions in response to changes in temperature and pH have been used to assemble colloidal crystal gels with environmentally tunable optical properties. When monodisperse, ∼210-nm-diameter hydrogel particles are close-packed via centrifugation, the resultant viscous polymer pellet displays a bright iridescence in the visible region of the spectrum. This iridescence can then be modulated via temperature changes, which induce the component nanoparticles to undergo thermo-initiated volume phase transitions. More importantly, these crystals undergo a completely reversible order−disorder transition in response to larger temperature fluctuations; the crystal can be processed in its disordered (solution) state and then reformed to the iridescent crystal spontaneously upon cooling. The preparation and initial characterization of these materials are presented.