Macroporous Materials with Uniform Pores by Emulsion Templating
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process that takes place in the continuous (external) phase of the emulsion. Emulsions of equallysized droplets can be produced through a repeated fractionation procedure [7]. Such monodisperse droplets undergo a spontaneous transition to an ordered colloidal crystalline phase [8]. The sol-gel process is then used to produce a space-filling gel of one of a range of possible solid materials. This gel encapsulates the droplets, thus forming 'imprints' of the them in the resulting material. The major advantage of templating emulsion droplets (as opposed to solid microparticles) is that the droplets can easily be removed by evaporation or dissolution prior to drying of the gel. Since drying is accompanied by significant shrinkage we found that this feature is essential in order to obtain uncracked materials. A further advantage is that the deformability of droplets allows emulsion volume fractions in excess of the close packing limit for hard spheres (74%). This enables the fabrication of porous materials with filling fractions lower than 26%. Such materials have a connected pore system. One of the problems that has to be overcome to make this strategy work is that sol-gel processes in general make use of metal alkoxides dissolved in an alcohol and hydrolyzed by the controlled addition of water. Conventional emulsions, however, are aqueous systems and are thus incompatible with most metal alkoxides. We therefore first developed a suitable nonaqueous emulsion [9] in which the water is replaced by another polar liquid, namely formamide. This led to stable oil-in-formamide emulsions. Then, the sol-gel process was adapted so that it would work in the highly polar formamide as the solvent instead of the usual lower alcohols. We demonstrate the emulsion templating process by making ordered macroporous titania, silica, and zirconia. It should, however, not be difficult to adapt the process to making a range of other metal oxides, binary metal oxides, and even organic polymer gels. 167
Mat. Res. Soc. Symp. Proc. Vol. 497 0 1998 Materials Research Society
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Figure I. Schematic of the emulsion templating process. EXPERIMENT The emulsion templating process is shown schematically in Figure I and is further detailed below. Emulsion preparation In ref. [9] it was found that stable nonaqueous emulsions can be prepared with formamide (FA) as the continuous phase. A surfactant that gives optimal stability was the symmetric triblock copolymer (ethylene glycol)20-(propylene glycol) 70-(ethylene glycol) 20. Its molecular weight is 5800 and it contains 30% by weight ethylene glycol monomer. For the emulsion templating we used oil that consisted of 99% iso-octane and 1% silicone oil. The silicone oil (or another high molecular weight oil) is needed to prevent droplet coarsening due to Ostwald ripening [9]. In a typical recipe 20 g of surfactant was dissolved in 1 liter of formamide. Then 150 ml of oil
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