Coordination-bond-assisted fabrication of robust composite photonic crystal films through melt-compression†

Embedding photonic crystals (PCs) into a polymer matrix and directly promoting the interactions between building blocks through various forces (such as coordination bonds) are effective strategies for fabricating robust PC films. However, owing to the adverse effect of metal salts on electrostatic repulsion during an assembly process, coordination bonds have long been ignored as an effective force for strengthening PC films. Herein, monodisperse PS@PEA-PAA core–shell spheres were prepared through a stepwise emulsion polymerization process, and a complex with ZnCl₂ to serve as a precursor for PC films was formed. Under vertical compression at elevated temperatures, polymer shell melt drove the assembly of PS cores into highly ordered arrays with steric repulsion as the necessary balancing force and locked the whole structure. With 1 mL of AA monomer added to PS@PEA-PAA building blocks and 2 mmol of ZnCl₂ added to form a complex precursor, compared with the results of PS@PEA PC films, coordination bonds in PS@PEA-PAA composite PC films induced a 420.8% increase in Young's modulus, which clearly demonstrates the impressive ability of coordination bonds to promote mechanical properties. Blue, green and red PC films were prepared as the basic elements using 225 nm, 265 nm and 310 nm PS@PEA-PAA spheres as building blocks, respectively, which can be further combined selectively into cyan, violet and yellow PC films following the additive color mixing rule. The robust free-stranding PC films can be further tailored and recombined into PC patterns with versatile designing styles and abundant optional structural colors, demonstrating great value for practical applications.