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Membranes

Membranes that are able to change their wetting and / or separation properties (= pore size) upon an external stimulus are very interesting materials for applications like ultrafiltration or the controlled adsorption of "solutes", e.g. unwanted contaminants. Polymers are by far the most important membrane materials, especially because of the relative ease and flexibility of manufacturing a large diversity of effective barrier structures for different membrane processes. Possible pathways toward the design of novel membranes are the modification of already-established membrane structures, an alteration of the preparation techniques, or the use of new building blocks with improved functionalities. Using block copolymers as building blocks represents a facile and straightforward methodology for the simple incorporation of different structural or chemical features into bulk materials. Junctions between two compartments are covalent and therefore thermodynamically, chemically, and mechanically stable.

One of the most important industrial processes for the fabrication of integrally anisotropic ("asymmetric") polymer membranes is non-solvent-induced phase separation (NIPS), where a casted film of a polymer solution is immersed in a precipitation bath. This is a straightforward and fast onestep procedure. Membrane morphology and barrier structure can be controlled by a range of parameters; most important are the mutual interactions between membrane polymer and solvent (or solvent mixture) on the one hand, and between polymer and nonsolvent on the other hand (polymer solvent and nonsolvent must be miscible). The obtained materials typically exhibit a very thin "skin" layer, which strongly determines the separation properties and is mechanically supported by a macroporous substructure. Such "asymmetric" membranes find their applications in pressure-driven separation processes such as ultrafiltration, nanofiltration, or reverse osmosis.

If block copolymers with one stimuli-responsive (pH- and / or temperature) block are used, a transfer of the "switchability" onto the final membrane material can be anticipated. In that way, membranes where the effective pore size is controlled via solution pH or temperature can be prepared. Another key feature of such asymmetric structures as prepared by the NIPS process is that they are self-supporting and their stability basically depends on the employed matrix material. Here, block copolymers represent another measure to influence material performance.

Therefore, we are interested in block copolymers which fulfill the following criteria: high incompatibility between both blocks (well-defined phase boundaries), a mechanically attractive / stable hydrophobic block (matrix material, membrane stability), and a smart and responsive hydrophilic compartment (tunable separation properties).

The images below are from work carried out in collaboration with the group of Mathias Ulbricht (Technical Chemistry II) at the University Duisburg-Essen.

see also: "Self-Supporting, Double Stimuli-Responsive Porous Membranes from Polystyrene-block-poly(N,N-dimethylaminoethyl methacrylate) Diblock Copolymers", Adv. Funct. Mater., 2009, 1040-1045