Modification of TiO2 Nanoparticles with Oleyl Phosphate via Phase Transfer in the Toluene–Water System and Application of Modified Nanoparticles to Cyclo-Olefin-Polymer-Based Organic–Inorganic Hybrid Films Exhibiting High Refractive Indices

Oleyl-phosphate-modified TiO2 nanoparticles (OP_TiO2) were prepared via phase transfer from an aqueous phase containing dispersed TiO2 nanoparticles to a toluene phase containing oleyl phosphate (OP, a mixture of monoester and diester), and employed for the preparation of OP_TiO2/cyclo-olefin polymer (COP) hybrid films with high-refractive indices. The modification of TiO2 by OP was essentially completed by reaction at room temperature for 8 h, and essentially all the TiO2 nanoparticles in the aqueous phase were transferred to the toluene phase. The infrared and solid-state 13C cross-polarization and magic-angle spinning (CP/MAS) NMR spectrum of OP_TiO2 showed the presence of oleyl groups originating from oleyl phosphate. The solid-state 31P MAS NMR spectrum of OP_TiO2 exhibited new signals at −1.4, 2.1, and 4.8 ppm, indicating the formation of Ti–O–P bonds. CHN and inductively coupled plasma analyses revealed that the major species bound to the TiO2 surface was tridentate CH3(CH2)7CHCH­(CH2)8P­(OTi)3. These results clearly indicate that the surfaces of the TiO2 nanoparticles were modified by OP moieties via phase transfer. OP_TiO2/COP hybrid films exhibited excellent optical transparency up to 19.1 vol % TiO2 loading, and the light transmittance of the hybrid films with 19.1 vol % TiO2 loading was 99.8% at 633 nm. The refractive index of these hybrid films rose to 1.83.