Thermotropic color changing nanoparticles prepared by encapsulating blue polystyrene particles with a poly-N-isopropylacrylamide gel

Water-dispersed thermotropic rianoparticles with core-shell structures were synthesized by the in situ polymerization of a lightly crosslinked shell of poly(N-isopropylacrylamide) [poly(NIPAM)] onto blue polystyrene cores. At room temperature, the thermal responsive outer shell is hydrophilic and is in a fully swollen gel state; but as the temperature is raised above 31 degrees C, it becomes increasingly hydrophobic and eventually collapses as the temperature reaches the lower critical solution temperature (LCST) of the poly(NIPAM). Passing through the LCST has a drastic effect on the color of the latex solution, which exhibited an intense blue color at room temperature and gradually pales or lightens as the temperature is raised above 31 degrees C. Analysis using spectroscopic and dynamic light scattering techniques showed that it is a reversible process. Microscopy evaluation of samples dried by the evaporation of water at temperatures above and below the LCST revealed that the swollen/collapse state of the polymer shell influenced the morphology of the dry state. Drying at room temperature resulted in thin films in which only particles of sizes corresponding to the polystyrene core are clearly seen in the microscopy images; but for samples prepared above the LCST, the images revealed a morphology made of much larger particles with diameters of 400-500 nm. (c) 2007 Wiley Periodicals, Inc.