Structure-Property Relationships in the Design of High Performance Membranes for Water Desalination, Specifically Reverse Osmosis, Using Sulfonated Poly(Arylene Ether Sulfone)s

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Virginia Tech


Over 30% of the world's population does not have access to safe drinking water, and the need for clean water spans further than just for human consumption. Currently, we use freshwater for growing agriculture, raising livestock, generating power, sanitizing waste, mining resources, and fabricating consumer goods. With that being said, the world is beginning to feel pressure from the excessive freshwater withdrawal compared to the current freshwater supply. This water stress is causing a water crisis. Places including Australia, South Africa, and California in the United States, just to name a few, are beginning to run out of fresh water to support daily societal demands. This is a phenomenon that is indiscriminately observed in all ranges of economically and politically developed countries and environments. However, it is important to note that less politically and economically developed countries especially those in arid climates, experience higher water stress than countries without such qualities.

With only 2.5% of the world's water being freshwater and 30% of it being accessible as either ground or surface water, freshwater is a scarce resource, especially with the growing population and society's demand for water. Since the remaining 97.5% of water is composed of either brackish or seawater (saline water sources), one way to overcome the water stress would be to convert saline water into freshwater. As a result, various desalination techniques have been developed in the last 80 years that employ either membrane technology or temperature alterations to desalinate either brackish or seawater.

One of the fastest growing methods for producing freshwater is reverse osmosis. Reverse osmosis uses an externally applied pressure, in the form of a cross flow back pressure, to overcome the osmotic pressure produced by the saline gradient across a semi-permeable membrane. The semi permeable membrane commercially consists of an interfacially polymerized aromatic polyamide thin film composite with a polysulfone porous backing that allows water to pass through while barring the transport of salt ions.

This research focuses on the development of sulfonated poly(arylene ether sulfone) derivatives with differing amounts of sulfonation and with the ions placed at different structural positions. Previously, such materials were tested as potential high performance fuel cell membranes, but they are also of interest as potential high performance water desalination membranes, specifically for reverse osmosis.

Two different methods were used to synthesize the sulfonated polysulfone derivatives: direct polymerization and post-modification of a non-sulfonated active polysulfone. The polysulfones from direct polymerization incorporated specialty sulfonated monomers, which were stoichiometrically controlled during the polymerization. Sulfonated polysulfones that were synthesized from post sulfonation incorporated biphenol and hydroquinone monomer units randomly throughout the polysufone backbones. These units could be sulfonated selectively because of their activation towards electrophilic aromatic substitution with sulfuric acid.

Each of the polymers were cast into films ranging between 20-100 microns in thickness and tested for water uptake, hydrated uniaxial tensile properties, crossflow water and salt transport properties, and for crosslinked samples, gel fractions. The water uptakes from all the polysulfones were tuned by the degree of sulfonation or disulfonation present in the polymer. This was either controlled via the presence of a sulfonated monomer or a monomer that was active toward electrophilic aromatic substitution after polycondensation of the polysulfone. All polymers exhibited increases in their water uptake as the degree of sulfonation increased. We also observed a decreasing trend in the hydrated mechanical properties of the films for all the high molecular weight linear polymers as the water uptake was increased. The directly polymerized sulfonated polysulfones were found to have high hydrated elastic moduli ranging between 400 and 1000 MPa, while the post sulfonated counterparts (with either hydroquinone or biphenol incorporated in their structures) exhibited elastic moduli ranging between 1000 and 1500 MPa. It is important to note that the structures of the polymers were slightly different from one another because of the technique used to synthesize them. Thus, the increases in hydrated moduli among polymers synthesized via different routes may have influences from differences in chemical structures.

Some of the polymers with higher degrees of sulfonation were synthesized as amine terminated oligomers with varying controlled molecular weights. The two targeted molecular weights were 5 and 10 kDa. Those oligomers were then crosslinked with a tetra-functional epoxide agent. The increases in sulfonation allowed for increases in water uptake and in theory, the water throughput through the sulfonated polysulfone membrane. Decreases in hydrated mechanical performance of the crosslinked networks with increasing degrees of sulfonation were also observed, similar to their high molecular weight linear counterparts. The directly polymerized crosslinked networks had salt permeabilities that plateaued at 70% disulfonation for both the 5 and 10 kDa polymers. Thus, we expect disulfonation content greater than 70% would lead to higher water throughput without significant increases in salt transport.



Sulfonated Polysulfones, Post-Sulfonation, Polycondensation, Water Desalination, Reverse Osmosis, Poly(Arylene Ether Sulfone)s