Synthesis and characterization of low-refractive-index fluorinated silsesquioxane-based hybrids

Novel low-refractive-index silsesquioxane-based hybrids were synthesized via hydrolytic condensation of fluorinated triethoxysilane precursors, which were prepared by reacting 3-aminopropyl triethoxysilane with acrylates containing fluoroalkyl groups, 1H,1H,5H-octafluoropentyl acrylate (OFPA) and 2,2,2-trifluoroethyl acrylate (TFEA). The hydrolytic condensations proceeded as a homogeneous system in acetone in the presence of aqueous HF solution (3.2%) at 30 °C. The products were soluble in a variety of organic solvents, including CHCl3, THF, and acetone, but were insoluble in hexane and water. The structures of the products were confirmed by 1H NMR, 13C NMR, 19F NMR, and FT-IR spectroscopy. The low polydispersities and reasonable molecular weights of the resulting fluorinated silsesquioxanes (Mn = 5800, Mw/Mn = 1.01; and Mn = 4700, Mw/Mn = 1.04 for the OFPA- and TFEA-based products, respectively) were confirmed by size-exclusion chromatography. Scanning force microscope (SFM) measurements indicated the formation of spherical hybrids having relatively narrow size distributions (average particle diameter < 3.0 nm) without aggregation. The sizes of the hybrids were also confirmed by X-ray diffraction (XRD). The refractive indexes of the TFEA- and OFPA-based silsesquioxane hybrids were 1.43 and 1.40, respectively. These results indicate the formation of novel fluorinated silsesquioxane-based hybrids having good solubility, narrow size distribution, and a low refractive index. Co-condensation of the TFEA- and OFPA-based triethoxysilane precursors affords a series of fluorinated hybrids whose refractive index and various other properties can be manipulated by varying the composition of the feed.