Better understanding the synthesis-structure-performance relationships of polyamide membranes prepared via interfacial polymerization

Promovendus/a: Ines Nulens

Promotor(en): Prof. dr. ir. Ivo Vankelecom, Dhr. Alexey Kubarev

Water, the fundament of life on earth, is becoming a scarce resource at an increasing rate due to climate change, population growth, industrialization and contamination of available freshwater resources. The challenge of our time is to secure safe and sufficient water, at affordable costs, that meets the standards for all sorts of applications ranging from municipal and industrial wastewater treatment and reuse, over drinking water, process water to ultrapure water. Membrane-based desalination or reverse osmosis (RO), is a powerful technique to produce high quality water from a variety of feed streams, such as seawater and brackish (ground)water but also all kind of fresh water sources. The current state-of-the-art RO membranes are fully aromatic polyamide (PA) thin film composite (TFC) membranes. PA TFC membranes rely on a thin dense PA top-layer to selectively retain ions and small organic molecules while water can pass. The success of this type of membranes is ascribed to its high water–salt selectivity combined with reasonable water permeance and relatively easy and fast manufacturing. Nevertheless, to ensure future water quality and improve the sustainability of membrane-based water purification, next-generation RO membranes should overcome several challenges. With respect to sustainability, lower fouling tendency, reduced sensitivity to chlorine-induced oxidation and greener membrane fabrication are desired. On the other hand, the trade-off between water permeability and water-salt selectivity needs to be overcome and improved rejection of chemicals of concern regarding human health is needed. Given the various source and product qualities, membrane development needs to evolve towards rational engineering for target separations. In order to do so, distinct synthesis-structure-performance relationships (SSPs) need to be established. This can only be achieved by improved understanding of the detailed structure, functioning and formation mechanism of PA which are hampered by nanoscale dimension of the PA layer and its high formation rate (during interfacial polymerization (IP)). Therefore, innovative research methodologies and measurement techniques are required to allow detailed post-synthesis and in-situ (i.e., real-time during IP) characterization.

Aiming at rational membrane design for water purification, this dissertation focused on the mechanistic understanding of IP to establish SSPs of PA TFC membranes.

Chapter 1 provides an introduction to PA TFC membranes, IP, fluorescence microscopy and microfluidics. The need for SSPs and the key role of in-situ characterization in resolving them, are discussed. In chapter 2, a theoretical framework describing the relation between synthesis conditions and overall PA shape. Two variables were identified to govern transitions from stable to unstable states, leading to morphological shifts. A broad range of synthesis parameters was then interpreted in terms of these two variables. Predictions with respect to the resulting morphology were shown to agree with extensive literature analysis. Chapter 3 investigates the concurrent influence of monomer concentrations and six organic solvents with widely varying characteristics on membrane properties and performance. Following the results, two new descriptors of the SSPs are introduced that describe the monomer supply in the reaction zone. Qualitative synthesis-performance relationships were formulated by analysis of a new extensive dataset completed with a rigorous analysis of literature data.

In the second part of the dissertation, methods for in-situ characterization of IP were developed. To achieve fast and detailed selective real-time probing of IP, fluorescence microscopy was applied. On top of that, fluorescence microscopy was combined with a microfluidic device to allow assessment of film performance. The optimization study of the microfluidic device design and operational protocol (chapter 4) stressed that the key properties for successful microfluidic-based IP are: a stable liquid-air interface prior to IP and separate evacuation of both monomer streams from the microfluidic device. As a result, the interface remains pinned during IP to allow stable in-situ measurements and after IP, filtration experiments can be performed inside the device. Chapter 5 introduces a simple droplet-based in-situ method. Initially, six fluorescent dyes were selected to probe temperature, pH and the monomer reaction as well as two reference dyes. However, analysis of reference experiments show interactions between certain dyes and the biphasic solvent system or the monomers. The dyes that were found to be compatible with IP allowed to measure real-time pH and densification of the forming PA film. Using this methodology, it was derived that immediately upon initiation of IP, the pH drops and molecules are hindered to move towards the organic phase but 30 - 40 s of reaction are required to grow the final film. Next, performance characterization of the formed film that was characterized in-situ was realized by combining fluorescence microscopy with microfluidics (developed in chapter 4). In this way, real-time data could be overlayed with performance data to propose a mechanistic explanation for the SSP for varying TMC concentration.

Overall, this dissertation advances the field of membrane science by deepening the understanding of SSPs of PA-based films prepared via IP. It does so by offering the foundations of a framework for the synthesis-morphology relationship, by introducing two new parameters to assess the synthesis-performance relationship with respect to monomer concentrations and solvent type and by developing new methods for real-time monitoring of IP using fluorescence microscopy (combined with microfluidics).