Multifunctional weak polyelectrolyte multilayers for membrane applications

Thin polymer coatings in the form of polyelectrolyte multilayers (PEM) are an interesting class of coatings for the surface modification of materials to provide functional interfaces. These layers are prepared by layer-by-layer (LbL) assembly, an attractive technique with control over the thickness and the properties of the multilayer. This LbL technique involves the buildup of PEM's by alternate exposure of a charged substrate to solutions of polycations and polyanions, in combination with a rinsing step between each deposition step to remove weakly associated polymer chains. LbL is a versatile technique where control over multilayer properties such as thickness, charge and density, can be easily achieved simply by the choice of polyelectrolytes, the number of coating steps and the coating conditions (pH, ionic strength etc.). As the layers are very thin, usually in the order of nanometers, they need to be characterized precisely in detail on model surfaces prior to coating on the desired substrate. LbL is a suitable surface modification technique for any size and shape of the substrate, and if the substrate is porous such as an ultrafiltration (UF) membrane support, then the PEM can act as a membrane separation layer for nanofiltration (NF) or solvent resistant nanofiltration (SRNF) applications. Additionally these multilayers can be useful to alleviate the problem of fouling in membrane technology, as these multilayers can work as antifouling layers and can also make cleaning of fouled membrane easier by sacrificial removal with some trigger (e.g., salt, pH, surfactant etc.). To provide these functionalities to membranes one suitable choice of PEs is a pair of weak polyelectrolytes such as poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH). Since the charge density is not fixed in these weak polyelectrolytes, control over adsorption, desorption and separation performance of multilayers for membrane applications can be achieved by simple alterations of the pH of the coating solutions. In this thesis, the focus is to investigate the use of weak PEM's on UF membrane supports to make antifouling and easy to clean hollow fiber NF membranes for micropollutants removal from water and for stable SRNF membranes. In Chapter 1, we present a general introduction to the topic and introduce the potential of weak polyelectrolyte multilayers to make multifunctional membranes for liquid applications (NF and SRNF), including their respective adjustable parameters. The different aspects of PEM membranes that can be beneficial for membrane technology are highlighted. In Chapter 2, we demonstrate that multilayers of weak PEs (PAH/PAA) can be used to provide a dual function to membranes i.e., as a NF separation layer and as a sacrificial coating that allows easy membrane cleaning. In order to optimize the coating and sacrificial removal conditions for the applied PEM, adsorption and desorption studies of these multilayers are performed on a model surface (silica) and monitored via fixed angle optical reflectometry. This allows us to understand the buildup behavior and removal of the multilayers at different conditions, something that cannot be precisely monitored on the membrane itself. Subsequently, tight hollow fiber UF membranes were coated with PEM's under identical conditions via a dip coating method. After each deposited layer, a clear increase in membrane resistance against pure water permeation and a switch of the zeta potential confirms the successful membrane modification. Moreover, these PEM based membranes, show rejection performance of solutes in a range typical for NF membranes i.e., hardly rejecting monovalent ions (NaCl) ( 60% of the divalent ion SO42- and of the neutral organic molecule sulfamethoxazole (SMX). Subsequently, the applied PEMs were investigated as sacrificial layers for the cleaning of membranes fouled with silica nano-particles. A rinse and a low pressure backwash with a suitable trigger solution (pH 3, 3 M NaNO3) allowed the successful sacrificial cleaning of the fouled membrane, bringing the resistance of the fouled membrane back to that of a pristine membrane. Recoating the clean membrane support with the same PEM system brings the resistance of the coated membrane equal to the resistance of the original polyelectrolyte multilayer membrane, demonstrating the re-usability of this approach. Chapter 3, focuses on the utilization of these PAH/PAA based weak multilayers to create hollow fiber NF membranes for micropollutant removal from water. The role of the pH of the coating solution is investigated and coupled to membrane performance in terms of rejection for salt ions and micropollutants. We show the easy preparation of low pressure hollow fiber NF membranes by coating PEMs on a UF support. The newly formed PEM NF membranes allow for the selective removal of micropollutants while allowing passage to most salt ions. It is also shown that variation in the pH of the coating solutions leads to a large degree of control over the membrane performance. For example denser membranes are formed at pH [6.0/6.0] of PAH/PAA and shows the highest micropollutants retention (60-80%) while low ion retentions for this membrane indicate that it is not strongly charged. The rejection mechanism of the investigated membranes was found to be mainly determined by size exclusion, but influenced by Donnan exclusion. This study shows a simple way of preparing hollow fiber NF membranes for micropollutants removal from water. In Chapter 4, our weak PEM system is investigated to prepare membranes for solvent resistant nanofiltration (SRNF) applications. SRNF membranes with a PEM separation layer were prepared on a UF hydrolyzed polyacrylonitrile (PAN-H) support by the LbL method while varying the pH of the coating solutions to tune the charge density and structure of PEs, and thus further control the morphology and performance of the resulting membranes for specific dye molecules from organic media. It is found that membranes with more free acid and amine groups are more susceptible to dye adsorption, leading to a film densification, and a low permeance, but in some case excellent dye retentions. Moreover, the prepared PEM membranes are found to be stable under long term-filtrations in organic solvents including troublesome aprotic solvents of industrial importance such as isopropyl alcohol (IPA), acetonitrile (ACN), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF). Results of this study show a simple and versatile way of using pH during coating to tune membrane performance for specific SRNF application. To bring PEM coating and especially the sacrificial cleaning approach into maturity and to coat existing modules we present in Chapter 5, the simplification of the LbL procedure for hollow fibre membranes by employing "dynamic coating" or "active coating". This involves flushing the PE's solutions through the lumen of the hollow fibre membrane. It is shown that dynamic deposition is a simple way to modify the surface of existing hollow fiber UF support to make versatile NF membranes with low fouling propensity (if terminated with PAA) and easy to clean by utilizing the sacrificial layer approach. Moreover this coating method with its reproducibility and homogeneity, has the potential to coat existing UF membrane modules. Rejection performance of solutes (salts and organic micropollutants) by dynamically coated membranes is found to be in the typical NF performance range (50-70% retention). Chapter 6 presents the conclusion and reflections of knowledge obtained from this work. Finally some recommendations and ideas are presented for future research to contribute to the continuity of investigations into sacrificial cleaning and the management of fouling in membranes and possible functionalities that can be introduced to PEM membranes.Chapter 1 Introduction 1 Chapter 2 Sacrificial polyelectrolyte multilayers for membranes 39 Chapter 3 Weak polyelectrolyte multilayers based NF membranes for micro-pollutant removal 69 Chapter 4 Weak polyelectrolyte multilayers for solvent resistant nanofiltration membranes 103 Chapter 5 Dynamic layer by layer (LbL) assembly of multifunctional polyelectrolyte multilayers for membranes 137 Chapter 6 Conclusions and future perspective 173 Summary 185nrpages: 215status: publishe