Structural Color and Rheology of Self-Assembled Poly(n-Isopropylacrylamide-methacrylic Acid) Microgels in Water

Microgels are excellent building blocks for photonic band gap materials (PBG) in water. Aqueous dispersions of temperature and pH responsive microgels composed by isopropylacrylamide (NIPA, 92 mol%) and methacrylic acid (MA, 8 mol%) show structural color in the concentration range of 44-88 mg/mL. At temperatures below the LCST of pNIPAM in water (similar to 32 degrees C), the dispersions self-organize as an elastic solid, with concentration-dependent refractive index. At 22 degrees C the color changes from red to blue when increasing the concentration. This is due to an increase in the elastic modulus. A transition to a liquid phase (characterized by a loss of the elastic modulus and of the structural color) occurs at a temperature just below the LCST. The color is a result of the diffraction of the visible light that do not propagate within the PBG as shown by the confocal reflectivity images of the microgel dispersions.