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)₇CHCH(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.