(701b) Investigating the Application Gutter Layers in Polyamide Thin Film Composite Membranes for Water Treatment

Polyamide membranes are very prevalent in the membrane desalination field and are used for nanofiltration (NF) and reverse osmosis (RO) membranes. These membranes are made possible by a 40-year old technology known as interfacial polymerization (IP). This is the process in which a diamine and an acyl chloride react at the interface of two immiscible liquid phases and form a thin and densely cross-linked layer in-situ on porous substrates. However, the IP process cannot directly control thickness and chemistry of the structure and relies heavily on the support membrane to determine the characteristics and performance of the membrane. The substrate porosity, pore spacing, and surface chemistry all can impact membrane performance in unpredictable ways. In the past, gutter layers have been used in gas separation to overcome support dependence and improve permeance, but they are not well explored in desalination. A gutter layer is a thin layer between the substrate and the selective layer and acts as a barrier between the two to lessen disruptive flow. It is able to ease the geometric constraints of the conventional polyamide membrane structure by providing paths of least resistance. This would improve the permeability performance of the overall membrane without creating a significant loss in selectivity. Smooth gutter layers no longer limit the selective layer to the properties of the substrate, solving the issue of support dependence as well. To form thin gutter layers, electrospray 3D printing offers exceptional control of thickness and surface smoothness. This method utilizes an electric field to atomize solvent droplets which are then sprayed onto the substrate and react at its surface to form membrane structures. Through this technique, many chemistries could be evaluated as a potential material for gutter layers, particularly piperazine (PIP), ethylenediamine (EDA), and Polyvinyl Alcohol (PVA). The specific parameters of each chemistry can be more directly controlled through electrospray 3D printing, allowing a more thorough investigation of the most optimal structures of each material. These were chosen based on their various characteristics and applications. PIP is a widely used monomer in NF membranes and its hydrophilic nature improves the water permeance capabilities of membranes. EDA has also been applied to the NF field and can form loose structures, creating membranes with high permeance and low selectivity, ideal for gutter formations. PVA is a highly hydrophilic polymer, also lending to its potential for high water permeance. All of these must be optimized, especially PVA. This polymer must be cross-linked to prevent dissolution in water, but not too much such that water permeance would be too low to efficiently act as a gutter layer. The analysis of these chemistries was taken further with an investigation of resistances of each layer of the membrane structure. It is known that resistance is inversely proportional to the thickness and by conducting these studies, the resistance contributions of each layer can be better understood. Through this, the current and future membrane structures can be honed for water and salt performances. Improvement in membrane performance via gutter layers would advance future membrane-based desalination technologies, allowing for routes for new membrane chemistries that are otherwise not able to be formed into thin film composite membranes.