SiO2 Nanoparticles in Polyurethane Colloidal Matrix for Color-Adjustable Photonic Coatings

Co-assembly of SiO2 nanoparticles (NPs) with polymers can fabricate photonic coatings with the advantages of one-step fabrication, large-scale production, good adhesion, and less periodic structure defects. However, it is difficult to adjust the color of photonic coatings by this method because the complex interaction between SiO2 NPs and polymers could damage the original periodic structure. In this study, we utilized soft polyurethane (PU) colloidal NPs as a matrix to blend with hard SiO2 NPs for co-assembly. After solvent evaporation, these soft PU NPs are filled between the space of SiO2 NPs and then merged together to change the effective refractive index (RI) and interplanar spacing of coatings. By delicately tuning PU mass ratio, the color of coatings changes in a large range from blue (459 nm) to yellow (512 nm) and yellow (560 nm) to red (636 nm) using 187 and 235 nm SiO2 NPs, respectively. Moreover, the 180° peeling strength increased from 48.48 to 406.06 N/m and the shear strength increased from 0.02 to 0.32 MPa, indicating the addition of PU enhanced the durability of coatings. This one-pot co-assembly strategy enables continuous color regulation and simplifies the fabrication process of color-adjustable photonic coatings, demonstrating potential applications in the areas of pigments, display, and anti-counterfeiting.