© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1551 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Adv. Mater. 2012, 24, 1551–1557 Feng Yan, Ailin Ding, Míriam Gironès, Rob G. H. Lammertink, Matthias Wessling, Lars Börger, Klaus Vilsmeier, and Werner A. Goedel* Hierarchically Structured Assembly of Polymer Microsieves, made by a Combination of Phase Separation Micromolding and Float-Casting Prof. F. Yan, [+] Dr. A. Ding, [++] Prof. W. A. Goedel University of Ulm Organic & Macromolecular Chemistry - OC3, Ulm, Germany E-mail: werner.goedel@chemie.tu-chemnitz.de Dr. A. Ding, M. Wessling, L. Börger, [¥] Prof. W. A. Goedel Chemnitz University of Technology Physical Chemistry Straße der Nationen 62, 09111 Chemnitz, Germany http://www.tu-chemnitz.de/physchem/ phone/fax : +49 371 531–21240/… 21249 Prof. M. Gironès, Prof. R. G. H. Lammertink, [‡] Prof. M. Wessling [§] University of Twente Membrane Science and Technology Mesa +, Enschede, The Netherlands Dr. L. Börger, K. Vilsmeier BASF SE, Polymer Physics, Ludwigshafen, Germany [+] Present Address: Soochow University, School of Chemistry and Chemical Engineering, Suzhou, China [++] Present Address: Curtin University, Department of Chemical Engineering, Perth, Australia [‡] Present Address: University of Twente, Soft matter, Fluidics and Interfaces, Enschede, The Netherlands [§] Present Address: RWTH Aachen University Chemical Process Engineering, Aachen, Germany [¥] BASF SE, Ludwigshafen, Germany DOI: 10.1002/adma.201104642 Microsieves are porous filtration media that are characterized by a uniform pore size, a high density of pores and a thickness smaller than the width of the holes. [1–3] These characteristics give rise to a high flux through the sieve [4] and a sharp size selectivity. Furthermore, the pores have a negligible tortuosity and the surface of microsieves usually is comparatively smooth. Thus, clogging and filter cake formation can be prevented by either cross-flow filtration or backpulsing. [1,5–12] Microsieves were initially made from purely inorganic materials using photolithography, which facilitates the preparation of hierarchi- cally structured microsieves via repeated cycles of lithographic structuring. The repertoire of methods to produce inorganic microsieves comprises furthermore non photo lithographic ele- ments like the use of block copolymers as etch masks, [13] the embedding of sacrificial particles as templates for pores, [14] track etching, [15] electroforming [16,17] and micromolding. [18] Polymeric microsieves have been prepared by photolithography using directly the structured resist instead of transferring its struc- ture into an underlying substrate. [19] As in the case of inorganic microsieves a repetition of lithographic procedures can lead to hierarchical structures. [20] In our groups we recently developed two methods to pro- duce microsieves made out of organic polymers via casting techniques: phase separation micromolding [21–24] and float- casting. [25–29] In phase separation micromolding an inorganic master con- sisting of pillars of several micrometers in height and diameter that protrude vertically from a flat solid substrate is used as reusable mould for the casting of microsieves from solutions of suitable polymers. Separation between master and polymeric microsieve is facilitated by phase separation and shrinkage induced in the casting process. This gives rise to a polymer layer with pores slightly larger than the diameter of the pillars which is separated from them by a low viscosity liquid and thus can easily be removed from the mould. In float-casting, a mixture of sacrificial hydrophobized silica particles and a non-water soluble, non-volatile, poly- merizable liquid monomer is applied to a water surface. This mixture forms spontaneously a monolayer of particles on the water surface, that are embedded in a layer of the liquid mon- omer in such a way that they protrude from the top as well as the bottom interface of this layer. Subsequently, the liquid monomer is solidified and cross-linked by photopolymerization and the particles are removed. Both methods to produce polymeric microsieves have their merits and shortcomings: the first method is a robust single step process that furthermore allows to easily position the pores at will and to integrate further reinforcing structures within the microsieves. [23] However, the lower limit of the pore size can hardly be made smaller than a micrometer. The second method is preferentially suitable to prepare membranes with sub-mi- crometric pores, [27] however, does not easily allow to choose the spacing between pores and to integrated supportive structures within the microsieve. Thus, microsieves prepared by float- casting need to be supported. Here, we show that one can combine the advantageous aspects of both above mentioned methods to prepare micro- sieves with submicroscopic pores and hierarchically structured support and use them in filtration applications. The preparation of the hierarchical assembly of microsieves is schematically depicted in Figure 1. Polymeric microsieves comprising pores of approximately 6 μm diameter were pre- pared by casting a solution of polyethersulfone in N-methyl pyr- rolidone onto a microfabricated mould and immersing it into water according to refs. [9,23] The pores of this membrane are