Nonsolvent induced phase separation (NIPS) is the most common approach to produce polymeric membranes. Unfortunately, NIPS relies heavily on aprotic organic solvents like N-methyl-pyrrolidone. These solvents are unsustainable, repro-toxic for humans and are therefore becoming increasingly restricted within the European Union. A new and sustainable method, aqueous phase separation (APS), is reported that eliminates the use of organic solvents. A homogeneous solution of two polyelectrolytes, the strong polyanion poly(sodium 4-styrenesulfonate) (PSS) and the weak polycation poly(allylamine hydrochloride) (PAH), is prepared at high pH, where PAH is uncharged. Immersing a film of this solution in a low pH bath charges the PAH and results in a controlled precipitation, forming a porous water-insoluble polyelectrolyte complex, a membrane. Pore sizes can be tuned from micrometers to just a few nanometers, and even to dense films, simply by tuning the polyelectrolyte concentrations, molecular weights, and by changing the salinity of the bath. This leads to excellent examples of microfiltration, ultrafiltration, and nanofiltration membranes. Polyelectrolyte complexation induced APS is a viable and sustainable approach to membrane production that provides excellent control over membrane properties and even allows new types of separations.

Polymeric membranes are used on very large scales for drinking water production and kidney dialysis, but they are nearly always prepared by using large quantities of unsustainable and toxic aprotic solvents. In this study, a water-based, sustainable, and simple way of making polymeric membranes is presented without the need for harmful solvents or extreme pH conditions. Membranes were prepared from water-insoluble polyelectrolyte complexes (PECs) via aqueous phase separation (APS). Strong polyelectrolytes (PEs), poly(sodium 4-styrenesulfonate) (PSS), and poly(diallyldimethylammonium chloride) (PDADMAC) were mixed in the presence of excess of salt, thereby preventing complexation. Immersing a thin film of this mixture into a low-salinity bath induces complexation and consequently the precipitation of a solid PEC-based membrane. This approach leads to asymmetric nanofiltration membranes, with thin dense top layers and porous, macrovoid-free support layers. While the PSS molecular weight and the total polymer concentrations of the casting mixture did not significantly affect the membrane structure, they did affect the film formation process, the resulting mechanical stability of the films, and the membrane separation properties. The salt concentration of the coagulation bath has a large effect on membrane structure and allows for control over the thickness of the separation layer. The nanofiltration membranes prepared by APS have a low molecular weight cutoff (<300 Da), a high MgSO 4 retention (∼80%), and good stability even at high pressures (10 bar). PE complexation induced APS is a simple and sustainable way to prepare membranes where membrane structure and performance can be tuned with molecular weight, polymer concentration, and ionic strength.

The global society is in a transition, where dealing with climate change and water scarcity are important challenges. More efficient separations of chemical species are essential to reduce energy consumption and to provide more reliable access to clean water. Here, membranes with advanced functionalities that go beyond standard separation properties can play a key role. This includes relevant functionalities, such as stimuli-responsiveness, fouling control, stability, specific selectivity, sustainability, and antimicrobial activity. Polyelectrolytes and their complexes are an especially promising system to provide advanced membrane functionalities. Here, we have reviewed recent work where advanced membrane properties stem directly from the material properties provided by polyelectrolytes. This work highlights the versatility of polyelectrolyte-based membrane modifications, where polyelectrolytes are not only applied as single layers, including brushes, but also as more complex polyelectrolyte multilayers on both porous membrane supports and dense membranes. Moreover, free-standing membranes can also be produced completely from aqueous polyelectrolyte solutions allowing much more sustainable approaches to membrane fabrication. The Review demonstrates the promise that polyelectrolytes and their complexes hold for next-generation membranes with advanced properties, while it also provides a clear outlook on the future of this promising field.

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In this work, polyelectrolyte mixing ratio is studied as a tuning parameter to control the charge, and thus the separation properties of polyelectrolyte complex (PEC) membranes prepared via Aqueous Phase...

iii applications, however, these membranes not being mechanically stable enough for filtration tests emphasizes the importance of the selection of PEs for material and membrane properties.In order to obtain PEC membranes with the pH-switch approach, at least one of the PEs must be a weakly charged one. This allows one to control the charges of the PE with pH and to obtain homogeneous casting solutions. In Chapter 5, it is shown that complexation-induced APS is not limited to pairs with one weak PE. Here, the salinity-switch approach is discussed for membranes prepared with two strong PEs, PSS and PDADMAC. Tuning the ionic strength allows control over complex formation by screening the PE charges. Homogeneous casting solutions are obtained by mixing these PEs at high NaCl concentrations and membrane formation is triggered by coagulation baths at low salinity. This approach not only permits the preparation of membranes with any polyanion-polycation pair, but it also provides much milder conditions for membrane formation than the earlier discussed extreme pH changes. The chapter starts with a discussion on the optimum casting solution properties for this PE pair where 200 kDa PSS and 15.4 wt% PE concentration are considered to be the best working condition. In the second part of the chapter, the effect of coagulation bath salinity is investigated. Increasing the salt concentration in the coagulation bath hinders the diffusion of salt from the casting solution to the bath which gives membranes with different structures. Increasing salt concentration increases the skin layer thickness which is inversely proportional to pure water permeance values. All membranes obtained in this chapter are suitable for nanofiltration applications. They have < 300 Da molecular weight cut-off and approximately 80% MgSO₄ retention, in addition, these membranes are stable against 10 bar transmembrane pressure for aqueous filtrations and maintain their integrity at pH conditions such as pH 0 and pH 14 for 40 days.Chapter 6 is a continuation of the investigation on the PSS-PDADMAC pair and it is focused on the effect of PE monomer ratio on membrane charge. PSS to PDADMAC monomer ratio is varied between 1.0:0.8 to 1.0:1.2. All membranes were of asymmetric type and they can be utilized for nanofiltration applications with below 400 Da molecular weight cut-off. Permeance values are approximately 1 L•m −2 •h −1 •bar −1 except the membrane prepared with 1.0:1.2 ratio solution which has more than 20 L•m −2 •h −1 •bar −1 permeance. Both positively and negatively charged membranes were obtained depending on the mixing ratio, which is reported for the first time in literature. There is a tendency for PSS-PDADMAC complexes to be positively charged as often reported. Salt type is very crucial for PECs due to the fact that it alters both the doping degree of the PEC and the mass transfer during phase separation (i.e. salt and PE diffusion). Therefore, it is hypothesized that the NaCl used here allows for the formation of negatively charged membranes un...