Synthesis and applications of multilayer membranes by layer-by-layer technique

Abstract

Layer-by-layer deposition (LbL) of multilayers has emerged as a simple and versatile technique to prepare thin films in the nanometer range for different applications. The main advantages of this method are the ability to tune the film thickness at the nanometer level and tailor the structure, composition and properties of the multilayer films. Due to its versatility, this LbL technique has been widely used to make nano-structured layers in different membrane separation areas. Two different types of multilayer membranes were studied in this dissertation: polyelectrolyte multilayer membranes (PEMMS) by electrostatic interactions and hydrogen bonded multilayer membranes (HBMMS). The first section of the dissertation focuses on the buildup of polyelectrolyte multilayer membranes for solvent resistant nanofiltration. The second section deals with the fabrication of hydrogen bonded multilayer membranes for aqueous filtrations. PEMMs are considered to be feasible candidates for use in solvent resistant nanofiltration (SRNF). Despite their huge potential in the different fields of membrane separation, such membranes are not commercially available yet due to their time consuming preparation procedure. In the first part of the dissertation, the optimization of the membrane preparation was examined to render it more feasible from a practical view point. Two polyion combinations were investigated in this work: poly(diallyldimethylammoniumchloride) with poly(styrene sulfonate) and poly(diallyldimethylammoniumchloride) with poly(vinylsulfonicacid). The optimized fabrication procedure involved the construction of 3 bilayers in just 12 min without compromising the SRNF properties, thus facilitating the upscaling of these membranes. The next part of the dissertation focused on further simplifying the LbL approach and on enhancing the SRNF capacity of these PEMMs. An ultrathin nanostructured membrane comprised of a 7 nm thick selective layer was successfully fabricated, consisting of one single bilayer of poly(diallyldimethylammoniumchloride) and hyperbranched sulfonated poly(aryleneoxindole). A detailed investigation of the SRNF performance was done on the membrane structure, together with a systematic variation of the structure of the hyperbranched polyelectrolyte, including its linear analogue. These membranes exhibit excellent SRNF performance, superior to that of the commercially avaliable membranes. Moreover, the fabrication of the membranes can be done in a simple 2 step procedure, which further significantly reduced the overall preparation time while realizing excellent filtration properties. In the final part of this dissertation, multilayer membranes were successfully fabricated on a crosslinked polyimide support by the alternate deposition of one tannic acid and poly(2-alkyl-2-oxazoline). The self-assembly is enabled by the hydrogen bonding between the amide groups of the poly(2-alkyl-2-oxazoline) and the multiple phenol groups of tannic acid. The applicability of these HBMMs in aqueous filtrations was demonstrated by optimizing various preparation parameters. It is shown that 7 to 10 bilayers are required to have a defect-free membrane with an optimal separation performance. The promising NF performance, together with the facile and scalable fabrication process, suggests the potential use of these membranes in separation applications.status: publishe