Formation of self-supporting porous membranes and tiny ceramic rings

02-Oct-2003

Wetting is the magic word behind the clever new method developed by Hui Xu and Werner A. Goedel of the University of Ulm and BASF AG. The researchers used their technique to produce self-supporting membranes with highly uniform pores and, in a second step, microscopically small ceramic rings. These rings are potentially useful as structural elements for microelectronic devices. Thin membranes with precisely controlled pores are commonly made by dissolving away one component of a structured matrix. Until now, the problem with this has been that the membranes, which are formed on a solid support, are then difficult to separate from it. Xu and Goedel decided to use a water surface as their support. Water surfaces cannot usually be covered by organic substances; nearly all non-water-miscible organic liquids form drops (fat globules), not an even layer, on a water surface. This is where a new principle, "particle-assisted wetting", comes into play. When the organic liquid is mixed with tiny hydrophobized silica gel particles, the particles bind to the water surface, pulling the organic liquid along to form a thin, mixed layer on the surface of the water. The researchers use particles whose diameter allows them to stick out above and below the layer. The liquid, an acrylate, is then cross-linked (polymerized) by irradiation with light. Hydrofluoric acid vapors then dissolve the embedded silica spheres out of the layer, leaving behind a polymer membrane that is thin and highly porous, while being robust enough to be transferred from one support to another or to be stretched, whilst remaining self-supporting, over porous substrates. The uniformity of the pore size makes this new type of membrane suitable for use in separation techniques such as ultra- or sterile filtration. The membrane can also serve as a "ring-mold". When the polymer membrane on a solid support is dipped into a solution of an organic titanium compound and dried, and the titanium compound is converted into titanium oxide by a chemical reaction with moist air, a thin ceramic layer is formed on the walls of the pores. Subsequent heating removes the membrane, leaving behind lots of little rings, whose outer diameter corresponds to the size of the pores in the membrane. The secret behind this technique lies in the wetting. The dipping solution selectively wets the interior of the pores, but not the outer surface of the membrane. The reason for this may be that the silica gel particles "stamp" a different chemical structure onto the pores' surfaces during the cross-linking of the acrylate. If the polymer membrane is dunked into the solution without a solid support, the rings end up firmly attached to each other, because the slowly deteriorating membrane deforms the rings such that they can be sintered together. The result is a stable ceramic membrane made of connected rings.

https://www.chemeurope.com/en/news/29206/formation-of-self-supporting-porous-membranes-and-tiny-ceramic-rings.html