Control Synthesis And Characterization Of Crack-Free Transparent Mesostructured Silica Monoliths

Transparent mesostructured silica monolith is of great interest for that it is an ideal host material for optical devices, such as lenses, chromophores, laser devices, and nonlinear optical media. However, investigations on the transparent mesostructured silica monolith are in an initial stage. There are many problems under resolvement. The primary problem that has to be overcome is the occurrence of macrocracks through drying due to the large shrinkage that occurs when solvent is removed from the gels; this severely impedes the preparation of crack-free monoliths with large size and limits their practical applications. Therefore, it's a important research subject of materials science to prevent gel from cracking.In this paper, we introduce a way of preparing large-sized crack-free transparent mesostructured silica monoliths. The process was performed under ambient pressure and at a constant temperature that can be changed at the range of room temperature～60℃. The transparent mesostructured silica monoliths were synthesized in the acid medium through solvent evaporation from the silica gels prepared by using amphiphilic triblock copolymer surfactant Pluronic P123 as structure-directing agent, tetraethoxysiliane (TEOS) as silica source, and ethanol as cosolvent. The synthesis process and the processing parameters were systematically studied. It's found that the products had fine transparency, ordered mesostructure, narrow pore distribution, high surface area and high thermostability. Moreover, it can be doped with transition metal ions and organic molecules. The transparent mesostructured silica monoliths can be prepared at a wide range of compositions. The product copied the shape of the reaction vessel. Using vessels with different shapes and changing the quantities of reactants, the shapes and sizes of the monoliths can be varied. To take the monolithic dish as an example, the diameter and thickness were tunable in the ranges of 20～60mm and of 1～8 mm, respectively. Mesoporous materials were obtained by calcination at 550℃for 6 h. The calcined mesoporous silica monoliths became white and lose their previous transparency, which perhaps due to internal micro-cracks that resulted from the decomposition of the template surfactant. In addition, we developed the same method for preparing transparent mesostructured silica monoliths to another templates, such as amphiphilic triblock copolymer surfactant Pluronic F127, and decaethylene glycol hexadecyl ether (Brij56). The mesostructured silica monoliths have been characterized by powder X-ray diffraction (XRD), nitrogen adsorption, FT-IR spectrum and thermal-gravimetric (TG) analysis.The research results of this paper are as follows:(1) The large-sized crack-free transparent mesostructured silica monoliths were successfully prepared by using P123 as structure-directing agent. The whole process was performed at a constant temperature that can be changed from room temperature to 60℃, and the synthesis system was TEOS-H₂O-C₂H₅OH-HCl-P123. The appropriate mass ratio of P123 for preparing crack-free transparent mesostructured silica monoliths was 12.98%～42.72% or even greater. Some important structural parameters of the typical silica monolith were enumerated as follows. The crystal lattice constant and the pore diameter were 7.62 nm and 3.6nm, respectively. Hereby, the pore wall thickness of sample was 4.02nm. The surface area and the pore volume were 608.66m²/g and 0.47cm³/g, respectively.(2) In additon, the large-sized crack-free transparent mesostructured silica monoliths can also be synthesized by using F127 as structure-directing agent and at a constant temperature that can be changed at the range of room temperature～60℃. Compared to P123, F127 has a larger relative molecular mass. It's found that for preparing transparent mesostructured silica monoliths the mass ratio of F127 was tunable in a range of 10.06-30.86% or even wider. The results of characterization showed that the material also had ordered mesostructure and narrow pore size distribution. Based on the characterization results, we gained some structural parameters of the typical material. The crystal lattice constant and the pore diameter were 10.74nm and 3.6nm, respectively. Hereby, the wall thickness of pore was 7.14nm. The surface area and the pore volume were 500.17m²/g and 0.42cm³/g, respectively. In comparison with the transparent mesostructured silica monoliths synthesized by using P123 as structure-directing agent, those materials prepared by using equal mass ratio of F127 had about the same pore diameter, but their pore wall thickness were larger which benefited the improvement of thermostability.(3) As we had expected, the crack-free transparent mesostructured silica monoliths were also prepared with a procedure similar to that used in P123 system by using Brij56 as structure-directing agent and at whichever tempeture from room temperature to 60℃. As a result, the transparent mesostructured monoliths can be prepared when the mass ratio of Brij56 was equal or greater than 15.72%. The characterization results showed that the material had ordered mesostructure and narrow pore size distribution. Based on the characterization results, we gained some structural parameters of the typical material. The crystal lattice constant and mostprobable pore diameter were 4.22nm and 2.0nm, respectively. Hereby, the wall thickness of pore was 2.22nm. The surface area and the pore volume were 450.48m²/g and 0.26cm³/g, respectively.By comparison, the method for preparing crack-free transparent mesostructured silica monolith proposed in this paper had many advantages. The processing temperature can be continuously changed from room temperature to 60℃. The whole process was performed directly at constant temperature and ambient enviroment, so that the preparation process was simple and low-cost. Moreover it had many other advantages, such as short processing time, simple operation procedure, better reproducibility and be applicable to another templates. In addition, block copolymer templates are nontoxic and can be degradated easily, so the process was environment-friendly and low-cost. The way of preparing crack-free transparent mesostructured monoliths had improved some shortcomings in previous works. The product had high transparency and will be a important role in developing optical devices. The method proposed in this paper had been applied for national patent of invention. We have our own intellectual property rights.