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

This paper describes a procedure that uses opaline arrays of spherical part icles (with diameters greater than or equal to 100 nm) as templates to fabr icate porous membranes having three-dimensional interconnected networks of air balls. An aqueous dispersion of monodispersed polystyrene (or silica) b eads was injected into a specially designed cell and assembled into an opal ine array under external gas pressure and sonication. After drying, the voi d 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. Su bsequent solidification of the precursor and dissolution of the particles p roduced a well-defined porous membrane having a complex, three-dimensional architecture of air balls interconnected by a number of small circular "win dows". The porous structure of this kind of membrane can be easily tailored by using colloidal particles with different sizes: when spherical particle s 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 simi lar to D/4, and the diameter of circular holes on the surfaces of the membr ane 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 oxid e ceramics such as SiO2 or TiO2. The permeabilities of these porous membran e films for a number of commonly used solvents were tested with a PAMC memb rane 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 engineerin g the porous membranes described in this paper should also find application s in fabricating diffractive sensors and photonic band gap (PBG) materials due to their three-dimensional periodic structures.