Effect of chitosan and alcohol solvents on uniform structural color formation in partially deacetylated nanochitin films

Cast films of partially deacetylated nanochitin exhibit structural colors originating from interference of reflected light; however, nonuniform convection during drying often induces uneven film thickness and color irregularities. In this study, the effects of additives and solvents were systematically investigated to establish a facile approach for obtaining uniform structural coloration. Chitosan, an ionic polymer with high viscosity, was employed as an additive to suppress the migration of nanochitin during solvent evaporation. Consequently, homogeneous thin films displaying vivid and uniform structural colors were obtained. UV-Vis spectroscopy, FE-SEM, and AFM analyses confirmed that the coloration originated from light interference between the upper and lower surfaces of the films, similar to soap films. The film thickness decreased with the amount of chitosan added, enabling precise control of the reflected hue. Furthermore, the influence of the solvent type was examined. The addition of alcohols such as ethanol and 2-propanol resulted in the formation of more uniform films, attributable to the generation of counterflow to the Marangoni convection caused by their lower boiling points and surface tensions compared with water. These findings demonstrate that the appropriate selection of chitosan concentration and solvent type enables both uniform structural coloration and tunable hue in nanochitin-based films.