A Study on Performance and Durability of Polyamide-based Membranes Fabricated by Molecular Layer-by-Layer Assembly and Interfacial Polymerization for Desalination

Abstract

Most available reverse osmosis (RO) membranes are based on the thin film composite (TFC) aromatic polyamide (PA) membranes fabricated by interfacial polymerization (IP). However, they have several disadvantages including low resistance to fouling, low chemical and thermal stabilities and limited chlorine tolerance. To improve these problems of PA membrane, new fabrication approach such as Molecular Layer-by-Layer (MLbL) assembly have been introduced for RO membrane. The following projects had accomplished from my research. The first project of this study was to systematically characterize the PA membrane fabricated via IP. The surface morphologies of commercial and a lab-made aromatic PA membrane were investigated in order to understand the PA membrane properties via IP using scanning electron microscopy (SEM) and atomic force microscopy (AFM) etc. The crosslinked IP-PA membranes indicated a high level of roughness and ridge-and-valley morphology which is prone to fouling and need to be washed periodically using oxidants. However, the PA-based membranes can be degraded by exposure to those agents, especially chlorine, for controlling biofouling. Therefore, this study evaluates the change on durability of PA membrane caused by chlorination. The performance variation of PA membrane via IP was investigated by chlorination in both pressurized and unpressurized modes. This study proposed a mechanism that can explain the performance discrepancy between the two modes: pressurized and unpressurized chlorination. Chlorination in an unpressurized mode showed a flow increase at high pH and a flux decline at low pH due to the compaction and swelling of the PA chains, respectively. On the other hand, chlorination performed in a pressurized mode decreased the water in both acidic and alkaline conditions, showing that compaction is overwhelming compared to swelling. The permeability of HOCl, a dominant species at low pH, through the PA membrane was pH independent and almost similar to the system recovery, but the permeability of OCl-, which is dominant at high pH, was maxima at a neutral pH. The different performance behaviors of membranes chlorinated at various pH conditions in the presence or absence of applied pressure could be explained by the permeability of chlorine species and compaction/swelling of polymer chains after chlorination. The effect of membrane chlorination on the chemical property changes at the two different modes was confirmed using attenuated total reflection Fourier transform infrared analysis, and a conceptual model of performance change was proposed to explain the performance discrepancy between the two chlorination modes. The secondary research was to demonstrate the successful fabrication of MLbL-assembled PA-based membrane for desalination. With the selective layer itself, the interface between the selective layer and the support layer is known to affect the subsequent membrane performance and therefore, understanding the effect of interlayer is critically important to achieve the desirable separation performance. The MLbL-assembled PA membrane formed directly on both the pristine and the hydrolyzed polyacrylonitrile (PAN) supports exhibited undesirable separation performance mostly due to the deposition of monomers within open pores of the support, necessitating the introduction of the interlayer. The MLbL layers were fabricated on supports coated by three different interlayers including interfacially polymerized polypiperazine (PIPA), crosslinked poly(ethyleneimine) (PEI) (xPEI), or the polyelectrolyte bilayer of PEI and poly(acrylic acid) (PAA) (PEI/PAA). Although the presence of the PIPA interlayer greatly reduced the MLbL cycle number to reach the plateau rejection value compared to the case without interlayer, the maximum attainable NaCl rejection was still unsatisfactory for RO application. Meanwhile, the PEI/PAA of PEI-based interlayers attributed to produce the MLbL-assembled PA selective layer with smooth morphology and high performance applicable to desalination. The tertiary research reported on the successful design, construction, and performance of MLbL membranes and demonstrated that these materials exceed the performance of membranes synthesized through conventional IP. I conducted MLbL assembly using traditional monomers in RO membrane fabrication (i.e., MPD and TMC), which are rigid aromatic monomers that display relatively low fractional free volume upon network formation. The MLbL process produced highly selective PA layers with precisely-controlled thickness, minimal surface roughness, and well-defined chemical composition. As a result, only 15 cycles of MLbL assembly were needed to achieve the targeted NaCl rejection while the flux was about 75% greater than a traditional interfacially polymerized PA membrane. The high salt rejection demonstrates that the structure of the MLbL selective layer is sufficiently similar to traditional IP, while the reduced thickness of the selective layer equated to a reduced hydraulic resistance and shorter diffusive path length for water to pass through the membrane. Additionally, I showed that the reduced surface roughness and chemical homogeneity achieved by MLbL mitigated membrane fouling. The fourth work demonstrated the broad applicability of MLbL in fabricating a variety of PA-based water desalination membranes with nanoscale control of the selective layer thickness and roughness independent of the specific PA chemistry. This research demonstrated that MLbL enables fabrication of various PA-based membranes with well-defined and deconvoluted intrinsic and extrinsic properties. The MLbL approach provides a rational and tailor-made approach to designing water desalination membranes that can satisfy the performance requirements for a particular application. One other subtle but important attribute of MLbL is that it is an all organic solvent synthesis, which implies that MLbL is not limited to the current set of triacid chloride and diamine monomer chemistries. Scalability remains the primary drawback for MLbL since the slow growth rate of the selective layer is not practical for commercial implementation. Work is currently underway to explore the use of multifunctional monomers that can significantly improve the growth rate of the selective layer while maintaining the key attributes of MLbL.