Solvent-responsive coloring behavior of colloidal crystal films consisting of cross-linked polymer nanoparticles

Cross-linked poly(ethyl acrylate-co-methyl methacrylate) (P(EA-MMA)) nanoparticles were prepared by soap-free emulsion polymerization. Size of the isolated nanoparticles in various organic solvents was largely swollen when the solubility parameter of the solvent was similar to that for P(EA-MMA). Well-ordered colloidal crystal assembly film of the P(EA-MMA) nanoparticle was prepared by drying the as-prepared aqueous suspension. The transparent and colorless film exhibited structural color when it was swollen in the organic solvent. Wavelength of the structural color revealed linear relationship with the size of the isolated nanoparticle in the same solvent. This solvent-responsive color-changing behavior explained that the solvent swelling of the nanoparticle increased the repeating distance of the colloidal crystal and the wavelength of the reflected light from the ordered structure. These results suggest that the structural color of the swollen film can be tuned by the solvent quality for P(EA-MMA).