(10) Design of Hybrid Optical Materials Based on Nanoparticle

Organic materials and polymers have widely been the essential materials sustaining our modern society owing to their many advantages of processability, flexibility, light weight, low cost, and so on. There have been substantial demands for inventing materials having better physical, electrical, mechanical, thermal, and/ or optical properties, which is often difficult to achieve using materials composed of only a single or pure component. In this context, hybridization of inorganic nanoparticles（NPs） into organic polymers has been in the spotlight over the past two decades. In all types of hybridizations, one of the most challenging and advanced issues is the pursuit of favorable optical characteristics such as transparency, refractive index, chromatism, and thermal-optical coefficient. This is because light scattering that significantly increases the turbidity of a material is unavoidable when each component, having a different refractive index, is mixed in a dispersion state. Zirconium oxide（ZrO2） has recently been highlighted because of its high refractive index（nD = 2.1–2.2）, chemical inertness, physical strength, and transmissivity in the UV–NIR range. Therefore, ZrO2 NP is a candidate for an optical reinforcing agent for the creation of hybrid materials with high refractive index and transmissivity. Several studies on the hybridization of ZrO2 NPs with polymers have been reported in the last decade. In this review, a versatile and promising organic-inorganic hybridization method for the fabrication of highly transparent organic-inorganic hybrid bulk materials containing highly crystalline ZrO2 nanoparticles（NPs） is described. Two key technologies that have been developed are the surface-treatment, hydrophobization, and functionalization of ZrO2 NPs originally dispersed in water and their nanodispersion into a polymer continuous phase without any coagulation and/or agglomeration. A unique and fascinating surface treatment method is demonstrated in which the hydrophobization and phase transfer of ZrO2 NPs from water to toluene is simultaneously achieved. Transparent surface-modified ZrO2 nanodispersions in toluene are obtained by a gentle solvent exchange from a ternary solvent mixture composed of water, toluene, and methanol. The addition of carboxylic acids having more than 4 carbon atoms as the surface treatment agent enables this hydrophobization. The surface-treated ZrO2 NP powders after drying possess the ability of redissolution or nanodispersion in several organic solvents and vinyl monomers. Further, optically transparent PSt and PMMA hybrid bulk materials with thicknesses of 10 mm are fabricated by the copolymerization of styrene（ St） or methyl methacrylate（ MMA） as a representative vinylic monomer with methacrylate functionalized ZrO2 NPs as the multivinyl crosslinking agent. Furthermore, the technique is applied to the hybridization of ZrO2 NPs and epoxy-based polymers. Their refractive indices and Abbe’s numbers are quantitatively described by the Lorentz-Lorenz effective medium expansion theory. Their transmissivity is also reasonably explained using Fresnel refraction, Rayleigh scattering, and the Lambert-Beer theories. The present method provides promising candidates for different transparent hybrid materials consisting of inorganic and organic materials.