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42086-AC5
Synthesis and Diffusion Properties of Ordered Nanoporous Silica Membranes with Controlled Macroscopic Morphology
Our research effort in the past year was focused on the synthesis of ordered mesoporous silica membranes by a counter-diffusion self-assembly (CDSA) method in polycarbonate track etch membranes. In the CDSA approach, a porous support is placed at the interface separating a silica source and water phase (water, surfactant and acid). Here water, surfactant and silica precursor slowly counter-diffuse into the pores of the support, allowing the silica precursor to hydrolyze and condense. In the presence of a surfactant, fibrous silica containing a large number of ordered mesopores (~3 nm) are aligned in either straight or helical fashion across the fiber axis.
We studied the growth of ordered mesoporous silica plugs within the pores of both hydrophilic and hydrophobic track-etched polycarbonate supports. These supports have large non-connected pores with diameters of 5ƒÝm and 8 ƒÝm respectively. We tried two modifications of the original method, Method A, to allow for evaporation induced self assembly. In Method B, the height of the silica source on the support was decreased so as to allow for evaporation. In Method C, the support filled with the silica precursor is placed on the water phase where it floats for the entire duration of the experiment. The supports before and after being coated with silica were characterized by XRD, SEM and helium gas permeation. Since these supports have extremely high gas permeance, a modified permeation system measuring oxygen permeation with constant transmembrane pressure was built to characterize the quality of the membranes.
SEM micrographs taken of the membranes synthesized by Method A showed that silica plugs filled in all the pores of both supports. The plugs ran the entire length of the support pore and the morphology is typical of MCM-41 fibers. XRD studies revealed no ordered peaks. The lack of ordered peaks could be due to the small amount of silica (as the supports have less than 15% porosity), which is insufficient to be deflected by X-rays. In addition, if the pores are aligned perpendicular to the support, we would not see ordered deflection peaks, or this could be indicative of the disordered nature of the silica plugs. The as-synthesized membrane showed a two-order of magnitude decrease in permeance (4.27x10-6mol/m2.Pa.s) as compared to the support (1.2x10-4 mol/m2.Pa.s), indicating good quality plugging. CDSA synthesis by Method B in the hydrophobic supports exhibited 60-70% pore plugging. The XRD pattern for the hydrophobic membrane confirms the presence of hexagonally ordered silica. In some cases, there is deposition of particles on the support and not within the pores of the support. This is likely due to improper placement of the polycarbonate membrane support at the surface of the water phase during synthesis. Hydrophobic membrane showed a permeance of 7.35x10-6 mol/m2.Pa.s, indicating plugging, but a lower quality membrane than produced by Method A. CSDA synthesis on the hydrophilic supports did not yield good quality silica plugs in the pores.
SEM micrographs of the hydrophobic membranes grown by Method C reveal that 80-90% of the pores are plugged. The membranes also have a thin film on the surface of the polycarbonate support on the silica precursor side. The presence of mesoporous silica is verified by small angle XRD data. The hydrophilic support exhibits heavy particulate deposition without plugs. The absence of plugging of all the pores of the hydrophobic support can be attributed to the incomplete saturation of pores of the support with the organic silica precursor when being placed at the interface or due to the accidental removal during preparation of sample for SEM studies. The permeance of these membranes was about 5.68x10- 6 mol/m2.Pa.s.
Results for the straight pore polycarbonate track etch membranes have shown that good quality ordered mesoporous silica membranes can be synthesized with the CDSA growth approach. Macroporous straight pore supports facilitate diffusion of the hydrophobic silica precursor and the amphiphilic surfactant leading to formation of ordered mesoporous structure within the support pores. The structure of the silica plugs formed is influenced by the morphology of the pore.
SEM micrographs of all the membranes reveal that there are gaps between the silica plugs and the support pore. The gaps were most likely formed due to shrinkage of the pores in the plugs upon drying. At the present the research efforts are focused at sealing these gaps. We have tried a counter diffusion deposition of amorphous silica. The membranes with silica plugs modified with the this method exhibit a two-time decrease in permeance. We are currently in the process of adjusting the reaction conditions to allow better sealing of the gaps.
