Sustainable polyelectrolyte complex membranes produced via aqueous phase separation
Due to the COVID-19 crisis the PhD defence of Muhammad Baig will take place (partly) online.
The PhD defence can be followed by a live stream.
Muhammad Baig is a PhD student in the research group Membrane Science & Technology (MST). His supervisor is prof.dr.ir. W.M. de Vos from the Faculty of Science and Technology (S&T).
In Chapter 1, we started this journey by discussing ‘sustainability’, especially in these crucial times when our relationship with the environment is strained. One way to repair our relationship with the environment is to develop, encourage and financially support sustainable manufacturing techniques. This can be achieved by implementing and utilizing the 12 principles of Green Chemistry as discussed in Chapter 1. Greener technologies and processes are the future and really the only way forward for the humankind and the Earth in the long run. One such effort to pursue a greener production of polymeric membranes has been presented in this thesis. We have successfully demonstrated that membranes can be prepared without using any unsustainable organic solvents. The polyelectrolyte complexation (PEC) based ‘Aqueous Phase Separation (APS)’ technique described in this thesis relies solely on water to produce polymeric membranes. This new technique provides a great deal of control over membrane structure and properties, just like the traditional organic-solvent based approaches. PEC membranes with tunable pore sizes have been prepared and optimized for a multitude of applications. The following sections summarize the major conclusions of the research work conducted in this thesis.
In Chapter 2 we explored polyelectrolyte complexation as one of the routes to obtain sustainable APS membranes. Here, the phase separation is achieved by changing the pH of the casting solution containing a strong polyanion i.e. poly(sodium 4-styrenesulfonate) (PSS) and a weak polycation i.e. poly(allylamine hydrochloride) (PAH). The casting solution is prepared by mixing the two polyelectrolytes in a monomer molar mixing ratio of 1:2 (PSS:PAH) at pH ~14. Immersing a thin film of this solution in a pH ~1 bath causes PAH to acquire charge and immediately form a polyelectrolyte complex with PSS. It was found that the molecular weights of the polyelectrolytes and the concentration of the casting solution are crucial parameters that have a significant impact on the membrane structure and properties. Utilizing polyelectrolytes having lower molecular weights typically resulted in porous PEC membranes that are well suited for microfiltration applications. The pore size of these membranes could be controlled from 0.2 µm down to 80 nm by simply varying the casting solution concentration, thereby affecting the kinetics of phase separation. On the other hand, utilizing higher molecular weight polyelectrolytes resulted in membranes having average pore sizes in the range of 2 – 5 nm, perfectly suited for tight ultrafiltration applications. Additionally, the kinetics of phase separation were controlled by adding NaCl to the precipitation bath, allowing the polyelectrolyte chains to rearrange into dense complexes. Nanofiltration type membranes, well suited for organic micropollutant (Mw 250 – 630 Da) removal, were obtained in this way. We successfully demonstrated that APS is indeed a versatile technique that provides great control over the membrane pore size and structure resulting in membranes that can be used for different applications. This greener approach to membrane fabrication completely eliminates the use of organic solvents while providing excellent membranes.
In Chapter 3, we explored various additional factors influencing the thermodynamics of solution and the kinetics of phase separation of the PSS-PAH based APS membranes. Since PAH is a weak polyelectrolyte, it exists in its charged state at neutral and acidic pH. Therefore, in order to prepare a casting solution, NaOH needs to be added to the PAH solution before mixing it with the PSS. The amount of added NaOH is critical as it determines the pH of the solution and as a result, the charge on PAH. A clear homogeneous solution cannot be obtained if the amount of added NaOH in PAH is not enough to completely remove its charge. However, adding significantly larger amounts of NaOH results in salting-out of PAH. Therefore, it is important to carefully balance the amount of NaOH added to PAH to obtain a clear PSS-PAH casting solution. We also find that the monomer mixing ratio of PSS:PAH significantly influences the mutual interactions of the polyelectrolytes in the casting solution thereby affecting the dynamic viscosity. Consequently, the kinetics of phase separation are severely impacted by the monomer mixing ratios resulting in membranes with different morphologies. Since this version of APS relies on the change in pH to induce phase separation, the pH of the coagulation bath becomes critically important since it is the driving force for complexation. It was found that if the driving force for complexation/phase separation is high, such as from solution pH of ~14 to coagulation bath pH ~0.5, the membranes formed instantaneously having dense and brittle structures. On the other hand, at higher bath pHs (pH > 1) the resultant membranes precipitated slower, had porous structures, and were mechanically weak. The pH of the coagulation bath, therefore, also controls the kinetics of polyelectrolyte complexation and the subsequent phase separation, and hence can be fine-tuned to obtain desired membrane morphology. Another important parameter that was found to affect the membrane structure and properties is the amount of crosslinking agent. Glutaraldehyde (GA) was added to the coagulation bath to crosslink the amine groups of PAH. It was revealed that the pore size of the membranes could be tuned from 2 nm through to 450 nm by varying the amount of GA in the coagulation bath. The stability of the PSS-PAH membranes was evaluated by immersing the membranes in highly saline solutions i.e. 1 M NaCl and 1 M KBr for 1 week. The results revealed that the morphology and performance of the membranes remain unaffected after exposure to saline solutions. Additionally, the membranes showed chemical stability against 1200 ppm sodium hypochlorite solution which is typically used as the cleaning agent for polymeric membranes. The results presented in Chapter 3 thus reveal the versatility of pH shift induced APS technique where numerous tuning parameters are available to produce PSS-PAH membranes with various structures, properties, and performances.
In Chapter 4, the applicability of the PSS-PAH membranes was taken one step further by showing that these membranes can also act as excellent support materials for functional coatings. Ultrafiltration type PSS-PAH membranes, obtained in Chapter 2, were coated with polyelectrolyte multilayers using three different polyelectrolyte pairs. It was found that only 4.5 bi-layers were required to transform the membranes into dense nanofiltration types that show a molecular weight cut-off (MWCO) in the range of 210 to 390 Da with salt retentions in excess of 90%. In addition, interfacial polymerization (IP) was performed on the PSS-PAH membranes to obtain nanofiltration membranes that yielded excellent separation performance. There are several benefits of using PSS-PAH membranes as supporting materials. For instance, the natural positive charge on the membranes facilitates the multilayer build up without any pre-treatments. Additionally, the excess amines of PAH provide additional bonding with the polyamide layer during IP, thereby improving the adhesion of the coating. These developments demonstrate that the APS approach allows membranes that can be used as excellent supports for functional coatings.
One major downside of these membranes described so far, is that they require extreme pH environments (from pH ~14 to pH ~1) to induce polyelectrolyte complexation. This problem was alleviated in Chapter 5 where polyethyleneimine (PEI) was used as a weak polyelectrolyte to prepare PSS-PEI membranes. The benefit of using PEI is that it is easily available in its uncharged state which means that a casting solution can directly be prepared by mixing PSS and PEI without adding any NaOH. Consequently, extreme pH, for example pH ~1, is not necessary to induce phase separation. The PSS-PEI solution can be precipitated at a benign pH of 4 to obtain PEC membranes. Similar to the PSS-PAH based APS membranes, the impact of several tuning parameters such as the molecular weight of PEI, the concentration and pH of the coagulation bath, and the amount of crosslinking agent was investigated in detail. Contrary to what was observed for the PSS-PAH membranes, using higher molecular weight polyelectrolytes resulted in microfiltration type PSS-PEI membranes and vice versa. This has to do with the branched nature of higher molecular weight PEI (~750 kDa) which forms a more open polyelectrolyte complex with PSS. On the other hand, excellent nanofiltration type membranes having a MWCO of ~200 Da were obtained using relatively lower molecular weight PEI (~ 25 kDa). PSS-PEI membranes performed even better as compared to the PSS-PAH membranes. In addition, the casting solution viscosity is significantly lower for the former at the same polymer concentration, making it easier to process for flat sheet membrane production. The major benefit, however, remains that these membranes can be prepared at mild pH conditions, making this APS approach even more sustainable and environmentally friendly.
In Chapter 6 an entirely different approach to prepare PEC membranes was taken where instead of pH, salt was used as the driving force for complexation. Here, a mixture solution of two oppositely charged strong polyelectrolytes i.e. PSS as polyanion and poly(diallyldimethylammonium chloride) (PDADMAC) as polycation, was prepared at high salt concentrations where the charges are screened. The solution is then cast as thin film and immersed in deionized water where the salt diffuses out of the solution leading to polyelectrolyte complexation and subsequent phase separation. Asymmetric nanofiltration membranes having MWCO of <300 Da with dense top layers and porous support were prepared via this approach. The effect of various parameters such as the molecular weights of the polyelectrolytes, the polymer solution concentration, and the bath salinity were investigated in detail. It was found that the salinity of the coagulation bath is a critical parameter that determines the rate of precipitation and the resulting thickness of the separation layer. The foremost advantage of this salt-induced APS is that it does not rely on changes in pH at all while producing sustainable nanofiltration type membranes.
In all described chapters above we described producing flat sheet sustainable membranes via the APS technique. An important step to further establish the versatility of APS lies in the production of hollow fiber (HF) membranes.
In Chapter 7 we showed that PSS-PEI based hollow fiber membranes can indeed be prepared via the APS technique. HF membranes for micro- and ultrafiltration applications were prepared by carefully tuning the fiber spinning conditions. This approach opens up a world of possibilities for APS membranes where also other polyelectrolyte systems such as the PSS-PAH and PSS-PDADMAC systems could be used for the production of excellent HF membranes. Additionally, efforts could also be devoted to studying other potential polyelectrolyte pairs to develop APS membranes for specific applications which will be discussed later in the coming sections.