Preparation and characterization of ZnS-polymer nanocomposite films with high refractive index

Novel ZnS-poly(urethane-methacrylate macromer) (PUMM) nanocomposite films with high refractive index were prepared by incorporating thiophenol (PhSH)-4-thiomethyl styrene (TMSt)-capped ZnS nanoparticles into a urethane-methacrylate macromer (UMM), followed by spin-coating and ultraviolet radiation initiated free radical polymerization. PhSH-TMSt-capped colloidal ZnS solution was synthesized by reacting zinc acetate with H2S in N,N-dimethylformamide. UMM macromer was synthesized by reacting 2-hydroxyethyl methacrylate (HEMA) with isocyanate group terminated polythiourethane oligomer which was obtained from polyaddition of 2,2'-dimercaptoethyl sulfide (MES) with 2,4-tolylene diisocyanate (TDI). Thiol-capped ZnS nanoparticles were characterized and found to be 2.0-5.0 nm in diameter with a cubic phase structure. The chemical composition of these ZnS nanoparticles was determined to be Zn2+ : S2- : RS- = 1 : 0.6 : 0.6 using chemical analysis methods. The weight fraction of ZnS nanoparticles in the films was measured by TGA, and it accords well with that of theoretical calculation. FTIR and DSC studies show that ZnS nanoparticles were successfully immobilized into the polymer matrix and the resulting nanocomposite films have a good thermal stability. TEM images indicate that the ZnS nanoparticles were uniformly dispersed in the polymer matrix and the particles remained their original size (2-5 nm) before incorporation into the polymer matrix. The XPS depth profiling technique demonstrates that the ZnS nanoparticles were also dispersed homogeneously in the depth scales of the polymer matrix. The nanocomposite films show high optical transparency in the visible region (T > 95% at 550 nm) and high refractive index in the range of 1.645-1.796 at 632.8 nm as the content of thiol-capped ZnS nanoparticles linearly increased from 0 to 86 wt%.