Biomimetic polymeric membranes for water treatment (2015)

This project is about the interplay of the three major components of aquaporin based biomimetic polymeric membranes (ABPMs): Aquaporins (AQPs), amphiphilic block copolymers, serving as a vesicular matrix for the hydrophobic AQP exterior (proteopolymersomes) and a polymeric membrane as embedment for the proteopolymersomes and mechanical support. To reach maximal functionality of ABPMs, the interplay of each component needs to be optimized. The optimization of AQPs and amphiphilic block copolymers was investigated by mixing bacterial Aquaporin Z (AqpZ) with polybutadiene polyethylene oxide (PB-PEO) diblock copolymers, where molecular weight (Mn) and hydrophilic volume ratio ( f ) were systematically varied to study the eect of incorporation eciency on these molecular parameters. The incorporation was characterized using freeze fracture transmission electron microscopy (FF-TEM), fluorescence correlation spectroscopy (FCS), small-angle x-ray and neutron scattering (SAXS/SANS), stopped-flow light scattering (SFLS) and Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Polymersomes as separate systems were further characterized on their molecular parameters, on formation and analysis methods. The interplay of proteopolymersomes and polymeric mesh support (in this case polyethersulfone, PES) was examined via integration of proteopolymersomes in an active layer (AL) formed by interfacial polymerisation between a linker molecule in aqueous phase and another in organic phase on top of the PES. The resulting thin-film composite (TFC) membrane was analyzed via cross-flow forward osmosis (FO), scanning electron microscopy (SEM), fourier-transformed infrared spectroscopy (FTIR), as well as in the non-supported form over FTIR and a specialized microfluidic visualization approach. Where no clear dierences between proteopolymersomes and polymersomes could be obtained within FF-TEM, SAXS showed that the incorporation of AQPs in a polymersome bilayer changed its property to a more smooth and well-defined shape, as well as a to a shift to more vesicular structures, as compared to coexisting micellar structures in the case of polymersomes. FCS revealed a protein-to-vesicle-ratio of 2.78, when AQP10 was incorporated in polymersome of a low Mn PB-PEO polymer. With regard to the molecular parameters of the polymersomes, FF-TEM and DLS revealed that the size and polydispersity of polymersomes is expressed in three main regions of Mn and f : Low Mn/high f (mixed sizes/polydisperse), low f (small size/monodisperse) and high Mn/high f (large size/monodisperse). This could be related to a less determined free energy function for polymersome formation with several minima for the first region. With regard to polymersome formation, polymersomes formed with detergent-mediated film dehydration (FR) were found to be less brittle than the ones formed by solvent evaporation (SE). This was revealed by SFLS, where a higher signal was obtained for FR prepared polymersomes and by DLS. The brittleness of SE prepared polymersomes was significantly increased from polymers of 3.75 kg/mol Mn and lower as compared to to 3.8 kg/mol and higher, where it was slightly increased above polymer concentrations of 5 mg/ml. FR prepared polymersomes were furthermore modified using biobead removal and dialysis at varying temperature and time. No dierence in polymersome size and configuration was obtained between any of the variations. With regard to polymersome analysis, 17 dierent analysis techniques were used to obtain polymersome size, lamellarity, bilayer thickness or surface charge. Novel methods like atomic force microscopy and nanoparticle tracking analysis turned out to give reliable information on polymersome size like known techniques such as Cryo-TEM. Cryo-TEM gave as well reliable information about lamellarity, SAXS/SANS about bilayer thickness. SEM, FTIR and microfluidic experiments on the interaction between all three components of ABPMs revealed that (proteo)polymersomes could be integrated successfully in the AL for the one AL linker couple (polyhedral oligomeric silsesquioxane, POSS and trimesoyl chloride, TMC) where for the other couple (polyethyleneimine, PEI and cyanuric chloride, CC), the AL was almost not formed in presence of the proteopolymersomes. The modest membrane performance of all membranes revealed however defects in the AL, which could be due to the micrometer sized pores of PES. ABPMs could provide a smart solution to one of the worlds greatest challenges in the next decades: the scarcity of clean water and sanitation access.