Nanoporöse Trennmembranen aus mikrophasenseparierten Blockcopolymeren
Conventional polymeric membranes based on standard polymers such as cross-linked aromatic polyamide for nanofiltration (applied to brackish water desalination or water softening) have reached their performance limits, and they have also insufficient chemical resistance against often used oxidative disinfectants which leads to a more complicated process. Significant improvements could be achieved if the random packing of segments like in classical amorphous polymers would be replaced by controlled microphase-separated structures in “tailor-made” membrane polymers. This work makes a contribution to the development of novel microphase-separated block copolymers to obtain “nanoporous” membranes with barrier pore diameters less than 2 nm in aqueous filtration applications. This is realized by combining corresponding telechelic macromonomers (fluoride-terminated and hydroxyl-terminated, with different chain lengths) obtained by a molecular-weight controlled polycondensation according to Carothers´ equation, and a subsequent second polycondensation step. The obtained poly(arylene ether sulfone) multiblock copolymers are then functionalized in a block-selective manner to create amphiphilic block copolymers with a tendency for microphase separation. The focus is on anchoring anion-exchange groups on the main chain of these polymers, because research on the potential of such materials for nanofiltration is seldom performed when compared with cation-exchange polymers. In addition to the linear amphiphilic ion exchange polymers (PAES-Amin), amphiphilic graft copolymers have also been synthesized. The side chains were polymerized block-selectively with a controlled radical polymerization (ATRP) to the main chains of the above-mentioned base polymers. After a polymer-analogous functionalization reaction, grafted cation exchange polymers (PAES-PDMAEMA+) could be obtained, which also served as base material for membranes. An important part of this work is the characterization of polymer films and thin-film composite membranes obtained via evaporation-induced phase separation. Obtained relationships between polymer synthesis and processing on the one hand, and obtained membrane structure and properties on the other hand will help to improve separation performance of nanoporous membranes by macromolecular design. Some combinations of different telechelic building blocks at different ion exchange group densities showed interesting property profiles in terms of molar mass, water solubility and their potential for the preparation of thin polymer films. Results with thin-film composite membranes show water permeabilities and retention of various salts and neutral substances in the nanofiltration range. Based on hydrodynamic radii of the used salts and neutral substances, barrier pore diameters in the range from 0.65 nm to 0.85 nm are to be expected. The membrane with overall best separation performance has a permeability which is 67% higher compared to the commercial SW30HR membrane while NaCl retention of about 97% is somewhat lower than that reference. However, the new membrane shows higher stability against oxidizing agents.