||The objective of this research is to prepare a high refractive index transparent inorganic/organic hybrid material, by dispersing nanocrystals of titanium dioxide in epoxy resin. In order to achieve a refractive index higher than 1.8, the content of inorganic material must be more than 50 wt%. Various possibilities have been tested. In one approach, commercially available titania nanocrystals were modified by a number of chemicals so that they can be dispersed to a good solvent for the resin. Among the surface modifier tested were different silanes and organic acids. In another approach, the direct reaction of Titanium tetrachloride (TiCl4) and hydroxy ethyl methacrylate (HEMA) was tried.|
The direct reaction of HEMA and TiCl4 produced a transparent precursor (Ti(OMA)4) that can be dispersed in various alcohols and other organic solvents. A transparent film can be cast and either thermal or UV cured. However, due to the coordination of four organic blocks to the titanium ion, the inorganic content of the material produced was rather low. Unless highly reactive titanium alkoxide was used, it is difficult to increase the inorganic content. In order to achieve a high inorganic content, it is necessary to start with nanoparticles of crystalline titania.
To introduce inorganic particles into epoxy, the common wisdom is to modify the surface with epoxy functioned silane coupling agent GPS. Unfortunately, titania is so active that it react with the epoxy group of silane and produces some yellow complex. It was therefore necessary to coat the titania with silica barrier layer to avoid such reaction. After the coating of silica and silne modification, the titania nanocrystals was mixed with epoxy resin to form a clear sol in solvent. A hybrid material is produced upon the removal of the solvent. Finally, the hybrid epoxy resin is cured and its refractive index measured. A relation between the refractive index and the composition is obtained.
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