Crack-free Mesoporous Transparent Silica Monoliths
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Transparent mesostructured silica monoliths have been prepared using amphiphilic block polyethylene oxide-polypropylene oxide-polyethylene oxide copolymer as a structure-directing agent. The presence of the block copolymer combined with slow evaporation of the solvent leads to the formation of a crack-free mesoporous silica monolith, however this method requires a long processing time. Zhao et al. recently introduced a relatively fast method for the preparation of well-ordered crack-free mesoporous silica monoliths, greatly reducing the time required to produce a crack-free monolith to 8 hours by using liquid paraffin as a morphology protector. Using Zhao's method the ease and reproducibility of producing crack-free mesoporous silica monoliths was determined. Pluronic P123 and Pluronic F127, both proven structure directing agents, were used as the structure directing agents in this experiment. We report here that Zhao's method was able to produce crack-free silica monoliths, however successful reproducible attempts were only reported at a longer processing time of 18 hours and required smaller vessel sizes (~1.5 cm diameter) than reported by Zhao. A noticeable difference was seen between Pluronic P123 and Pluronic F127, with Pluronic F127 producing crack-free silica monoliths on a more consistent basis. Successful construction of crack-free transparent monoliths was heavily dependent upon the size and shape of the vessel and upon the desired thickness of the silica monolith.