Temperature-Induced Coloration and Interface Shell Cross-Linking for the Preparation of Polymer-Based Opal Films

The formation of colloidal crystals and their use as photonic materials are of high interest for various technologies in the field of sensing applications, as templates, absorber materials, catalysts, and membranes. In this study, core-shell particles consisting of a cross-linked poly(methyl methacrylate) core featuring a (polyacrylonitrile-co-styrene) shell are synthesized by starved-feed emulsion polymerization. The resulting particles are investigated with respect to size and monodispersity, as well as the core-to-shell ratio, by means of dynamic light scattering and transmission electron microscopy. Optimized particle sizes are 218 nm for the cores and 276 nm for the core-shell particles. For the formation of highly ordered and free-standing opal films, the particles are processed by the melt-shear organization technique. The resulting films show angle-dependent reflection colors, while reflected colors can be tailored by the design of the core-shell particles. As a focus of this work, polyacrylonitrile as part of the copolymer particle shell is advantageously used both for particle opal film stabilization and for tailoring the reflection colors of the opal films. It is shown that the cyclization reactions at the interface of the particles and within the matrix material significantly influence the optical properties of the opal films upon thermal treatment at 240 °C and for different heat holding times. For instance, the change of color can be simply set from red to blue upon defined thermal treatment conditions. Via this convenient protocol, brilliant reflection colors can thus be obtained based on the insights into the structure-property relationships of the underlying particle architectures and interface reactions. The scalable opal films will pave the way to functional colored materials as interesting candidates for a manifold of sensing applications and temperature-responsive polymeric materials.