dc.description.abstract | The objective of this research is to prepare a transparent and high refractive hybrid material, with a target refractive index of 1.8. We choose TiO2 as the inorganic component, due to its high refractive index (above 2.5). The oganic component may be epoxy or PMMA resin, which has a refractive index of 1.5~1.6. To achieve the desired refractive index, it was calculated that we needs about 28~35 % TiO2 by volume. Based on the surface area to volume ratio, such solid content can only be accomplished if the titania particles were about 30 nm. Therefore, the first step toward the objective is to synthesis uniform titania particles in the appropriate size range.
For the preparation of nano-sized titania, we started from Ti(OH)4 precursors obtained by the hydrolysis and condensation of TiCl4 at designed pH condition. After washing away the salt, the precipitates could be peptized either in acid or alkaline. The peptized sol, although transparent, contains only nuclei of ~ 5 nm in size. Further hydrothermal reaction at ~200 oC was needed to grow the crystals to the desired size.
It turned out that crystal growth only occurred if the hydrothermal was done at high alkalinity, particularly when tertiary amine was used. After 200 oC/ 4h of hydrothermal reaction, the initially 5 nm nuclei grew to about 30 nm as we hope. Unfortunately, the product was an aggregate in most of the cases with particle size about 40~100 nm. We find TiO2 concentration and TPAOH ration were the most important factors on the particle and grain size. Only in one occasion were we able to control the grain size to 13 nm and the particle size about 40 nm.
The blending of inorganic particles into organic resin requires the modification of its surface. This can be done with silane as coupling agent. However, nano-sized TiO2 is known to be photocatalyst. The common practice is to passivate the surface by coating with inert such as Al2O3 or SiO2. The coating has to be complete, for incomplete coating might enhance the photocatalytic effect instead of reducing it.
After obtaining titania from the hydrothermal reaction under TPAOH, they were washed and dispersed in ethanol and grafted with silane (GPS or MPS). Only in the case of MPS grafting were it able to form 1 wt% stable white-colloid in MEK, which is a comparable solvent for future incorporation of polymer. | en_US |