Fabrication and characterization of porous membranes with highly ordered three-dimensional periodic structures

This paper describes a procedure that uses opaline arrays of spherical particles (with diameters greater than or equal to 100 nm) as templates to fabricate porous membranes having three-dimensional interconnected networks of air balls. An aqueous dispersion of monodispersed polystyrene (or silica) beads was injected into a specially designed cell and assembled into an opaline array under external gas pressure and sonication. After drying, the void spaces among the spheres were filled with a liquid precursor such as a UV-curable (or thermally cross-linkable) prepolymer or a sol-gel solution. Subsequent solidification of the precursor and dissolution of the particles produced a well-defined porous membrane having a complex, three-dimensional architecture of air balls interconnected by a number of small circular "windows". The porous structure of this kind of membrane can be easily tailored by using colloidal particles with different sizes: when spherical particles of diameter D are used, the dimension of air balls in the bulk is similar to D, the size of circular windows interconnecting these air balls is similar to D/4, and the diameter of circular holes on the surfaces of the membrane is similar to D/2. We have demonstrated the fabrication procedure using a variety of materials, including a UV-curable poly(acrylate-methacrylate) copolymer (PAMC), UV-curable polyurethanes, and sol-gel precursors to oxide ceramics such as SiO2 or TiO2. The permeabilities of these porous membrane films for a number of commonly used solvents were tested with a PAMC membrane as the example. Our measurements indicate that the liquid permeability of this porous membrane strongly depends ion the properties of the liquid. In addition to their uses in filtration, separation, and tissue engineering the porous membranes described in this paper should also find applications in fabricating diffractive sensors and photonic band gap (PBG) materials due to their three-dimensional periodic structures.