Poly(4-vinylpyridine) and Poly(methacrylic acid) Particle Architectures for pH-Responsive and Mechanochromic Opal Films

While stimuli-responsive structural colors are commonly found in nature, mimicking these in artificial materials is challenging. Dynamically switchable and tunable coloration, however, is in high demand in widespread fields of applications, including advanced display and monitoring technologies, smart sensing, and anticounterfeiting. This work reports a scalable protocol for the synthesis of tailor-made core-shell particles and subsequent processing to opal films with iridescent, pH-responsive, and mechanochromic structural color. Novel monodisperse core-shell architectures based on hard polystyrene core particles are synthesized via stepwise emulsion polymerization in a starved-feed mode. The incorporation of 4-vinylpyridine and methacrylic acid as functional comonomers in the soft particle shell facilitates pH-responsive swelling and deswelling. Mechanically stable and well-ordered colloidal crystal films are obtained by the self-assembly of the particles during processing with the powerful melt-shear organization technique. Thereby obtained opal films show Bragg-scattering at the colloidal crystalline structure and exhibit brilliant green-turquoise to blue-violet reflection colors, dependent on the angle of view and illumination. Upon changes in the pH value or mechanical deformation, the reflected wavelength shifts by more than 100 nm, leading to intriguing changes in the visible structural color. Excellent reversibility is achieved by the subsequent application of a convenient UV cross-linking strategy, corroborating the high application potential of these advanced functional materials.