High refractive index thin films of ZnS/polythiourethane nanocomposites

High refractive index optical thin films of nano-ZnS/polythiourethane (PTU) composites have been prepared via immobilization of thiophenol (PhSH)/mercaptoethanol (ME)-capped ZnS nanoparticles into a PTU matrix. PhSH/ME-capped ZnS particles with a diameter of 2-6 nm were synthesized by reacting zinc acetate with H2S in N,N-dimethylformamide. The nanocomposite films with high refractive index were prepared by adding an isocyanate group terminated PTU oligomer, which was synthesized by polyaddition of 2,2'-dimercaptoethylsulfide (MES) and isophorone diisocyanate (IPDI), to a DMF solution containing colloidal ZnS particles, and followed by spin-coating and multi-step heat curing. FTIR studies show that the ZnS nanoparticles were successfully immobilized into the polymer matrix. TGA results indicates that the nanocomposite films have a good thermal stability and the weight fraction of ZnS particles in the films is well in accordance with that of theoretical calculations. TEM studies suggest that the ZnS nanoparticles with a diameter of 2-6 nm were well dispersed in the PTU matrix and maintained their original size before immobilization. The refractive index of the transparent ZnS/PTU nanocomposite films was in the range from 1.574 to 1.848 at 632.8 nm, which linearly increased with the content of PhSH/ME-capped ZnS from 0 to 97 wt%.