Structurally colored polymer films with narrow stop band, high angle-dependence and good mechanical robustness for trademark anti-counterfeiting

Abstract

The photonic stop bands of colloidal crystals appear as structural colors, which are potentially useful for display devices, colorimetric sensors, optical filters, paints, and photonic papers. However, low durability and pale colors caused by the undesired scattering of light seriously limit their practical applications. In this article, a polydimethylsiloxane (PDMS)/photonic crystal (PC)/PDMS sandwich structure was designed as a free standing structural colored film with good durability and brilliant color. The monodispersed polystyrene (PS) spheres were self-assembled on the hydrophobic PDMS surface to facilitate the integrity of the assembled structure and then, PDMS with a refractive index of 1.41 was filled in the gaps between the PS spheres (n_PS = 1.59), replacing air (n_air = 1). The surface was finally covered with a thin layer of PS PCs, forming a continuous and free standing PC film. The continuous feature of the composite PC can greatly improve their mechanical properties. At the same time, the lower index contrast results in narrow reflection peaks for the composite films, which indicates that higher color purity and brightness could be achieved. Clearly distinguished, vivid structural colors can be observed between red to green or green to blue by tuning the viewing angle from 5° to 50° for films composed of PS spheres with diameters of 247 nm or 209 nm, respectively. They can also be easily patterned by spraying methods and embedded as a trademark on clothes. Patterns with different structural colors at different angle can be clearly be observed under sunlight, which makes them potentially useful as security materials.