SEMINAR TERM-PAPER Submitted By : Sidharth Razdan

Based on the presentation by Dr. William A. Philip

Topic: Sem Functional Membrane Materials: Opportunities in Water Treatment

IntroductionDr. William A. Philip is an Assistant Professor in the Department of Chemical and Bimolecular engineering at University of Notre Dame. He is heading the W.A.T.E.R. (Water purification and Advanced Transport Engineering Research) laboratory that was established in 2011, with the goal of developing next-generation membranes capable of enhancing chemical separations at the water-energy nexus and serving as a novel platform for mediating mass transfer in advanced membrane applications.W.A.T.E.R. labs research basically focuses on three areas : 1. Membranes Fabricated from self-assembled block copolymers 2. Chemically selective membrane separation 3. Solute permeation through polymeric thin filmsA membrane is broadly defined as a discrete interface between two phases that mediates the transfer of chemical species between these phases. Human life relies on naturally occurring membranes (e.g., the cell wall) and man-made membranes are essential to modern life, as we know it. Polymeric membranes enable the production of fresh water from seawater, the purification of therapeutic pharmaceutical compounds, and the separation of nitrogen gas from air.Man-made membranes are also a novel platform for applications such as chemical sensing, energy generation, and drug delivery. The unique properties of these mass transfer devices arise from the physical structure and chemical functionality of the membranes and their interactions with the permeating chemical species.Dr. Philips research in membrane science attempts to understand how the chemistry and nanostructure of polymeric membranes affects the transport of solutes and solvents across the membranes, and to apply this understanding to the development of next generation membranes with novel properties for use in chemical separations at the water-energy nexus, as controlled release devices, and as protective coatings for perishable products.

MEMBRANES FABRICATED FROM SELF-ASSEMBLED BLOCK COPOLYMERSFiltration is one of the oldest separation processes known to man, with evidence of its use dating back to the ancient Egyptians. Due to the development of polymeric membranes within the last century, solutes with sizes ranging all the way from 1 nm to 1 mm can now be filtered from solution (i.e., nano-filtration to microfiltration). However, current filtration membranes are hindered by a wide pore size distribution. This wide distribution is a direct result of the non-solvent induced phase separation (NIPS) process used to fabricate current membranes. Despite efforts to reduce the spread of this distribution by modifying the NIPS process, it remains a major hindrance to the deployment of membranes in advanced applications. Therefore, new methods and materials need to be developed in order to produce membranes with a single, well-defined pore size. Block co-polymers are an intriguing class of materials that consist of two or more chemically incompatible polymers covalently bonded together. The balance between the enthalpic desire of the constituent polymers (i.e., the blocks) to phase separate like oil and water and their entropic desire to avoid stretching results in these systems self-assembling into a variety of useful structures with characteristic dimensions on the order of 10-100 nm. One of these structures, hexagonally closed packed cylinders, provides an ideal template for producing membranes that have a high density of pores with a single, well-defined size.W.A.T.E.R. group is interested in developing a facile, scalable technique to fabricate membranes from block copolymers using a combined self-assembly and non-solvent induced phase separation (SNIPS) technique. Their aim is to design functional copolymers in order to fabricate nano-structured, high performance membranes with chemically tailored pore walls that enhance membrane filtration and enable advanced membrane applications, such as membrane chromatography and drug delivery.

Figure 1. NIPS and SNIPS

CHEMICALLY SELECTIVE MEMBRANE SEPARATIONSSize selective membrane separations have demonstrated exemplary energy-efficiencies. The success of seawater desalination by reverse osmosis (RO) is a clear example of the energy savings and sustainability that can be realized by replacing traditional separation processes with a size-selective membrane separation. At its inception, RO desalination consumed almost three times more energy than equivalent thermal desalination methods such as multistage flash distillation (MSF). However, over the past forty years, due to fundamental technological advances, the energy demand of seawater RO has fallen dramatically; and it now requires about half the energy of MSF. Unfortunately, the reliable performance of chemically selective membranes lags behind significantly.

This gap in performance needs to be addressed if sustainable membrane separations are to be used to isolate similarly sized molecules (e.g., chiral molecules) or to remove dilute contaminants (e.g., endocrine disrupting chemicals) from drinking water. In order to accomplish this, the W.A.T.E.R. group is focusing on the fundamental thermodynamic and transport phenomena of chemically selective permeation through polymeric membranes before applying that knowledge toward engineering a high performance membrane for directed applications. By looking closely into the structure-property-performance relationships of model systems with a systematic combination of transport testing and materials characterization, they will be able to answer key fundamental questions about what physical and chemical membrane properties (e.g., solute-membrane affinity and membrane pore size) enable the chemically selective transport of solutes through polymeric membranes.

Figure 2. Chemically selective membrane separation

SOLUTE PERMEATION THROUGH POLYMERIC THIN FILMSThe transport of small molecules through polymeric thin films is critical to a large number of applications. For example, it plays a key role in efficiency and efficacy of membranes designed for gas separations and desalination.Their interest in this area focuses on two specific applications: osmotically driven membrane processes (ODMPs) and edible protective coatings.

ODMPs are a class of water treatment and energy generation technologies that have garnered attention due to their ability to produce useable water from highly impaired water sources and high salinity brines. In ODMPs, two solutions of differing concentrations are placed on opposite sides of a semi-permeable membrane, which induces the permeation of water from the less concentrated feed solution to the more highly concentrated draw solution (i.e., osmosis). Unfortunately, membranes are not perfect barriers and solutes also permeate across the membrane. Due to the unique operating geometry of ODMPs, as solutes from the draw solution move past solutes from the feed solution within the polymeric membrane, they may interact with each other, and alter their permeation rates.W.A.T.E.R. group is investigating how these interactions arise and affect permeation rates as well as the influence that these interactions have on the systems level design of ODMPs.

Polymer films are frequently used to protect produce from external hazards and physical damage during shipping. The biodegradability of edible polymer films provides several advantages over conventional polymer films. However, these edible films need to perform as well as existing technologies if they are going to be adapted. They are working on understanding if edible coatings can be as effective as non-edible coatings in regulating the permeation of oxygen and carbon dioxide to and from produce.

Figure 3. Osmotically driven membrane processes

FUTURE WORK Development of nano-porous membrane reactors which could be used to remove harmful contaminants selectively. Development of proper structure-property-performance relationships to elucidate how all of these myriad parameters in the macromolecular design and SNIPS casting process affect the structure formation. Identifying if a specific chemical interaction or the state of water within the cellulose acetate membrane is responsible for the anion selective transport through cellulose acetate membranes. Regardless of its origins, accounting for the ion exchange that is a result of these solute-solute interactions is essential for the accurate design of ODMPs. Understanding interactions at the molecular level that may lend deeper insights in the permeation of solvents and solutes through polymeric membranes used for more well established membrane processes, such as reverse osmosis.

ReferencesWilliam A. Phillip, Javid Rzayev, Marc Hillmyer, E.L. Cussler, Gas and Liquid Transport through Nanoporous Block Copolymer Membranes,Journal of Membrane Science,2006, 286, 144-152.Liang Chen, William A. Phillip, E.L. Cussler, Marc Hillmyer, Robust Nanoporous Membranes Templated by a Doubly Reactive Block Copolymer,Journal of the American Chemical Society,2007, 129, 13786-13787.William A. Phillip, B. ONeill, Marc Rodwogin, Marc Hillmyer, E.L. Cussler, Self-Assembled Block Polymer Membranes Used for Water Filtration,ACS Applied Materials & Interfaces,2010, 2, 847-853.Rachel Mika Dorin, William A. Phillip, Hiroaki Sai, Joerg Werner, Menachem Elimeleh, Ulrich Wiesner,Designing Block Copolymer Architectures for Targeted Membrane Performance,Polymer,Available online 27 September 2013, ISSN 0032-3861.Gavin J. Irvine , Sahadevan Rajesh , Michael Georgiadis , and William A. Phillip, "Ion Selective Permeation Through Cellulose Acetate Membranes in Forward Osmosis",Environmental Science & Technology,2013, 47 (23), 1374513753.William A. Phillip, E. Martono, Liang Chen, Marc Hillmyer, E.L. Cussler, Seeking an ammonia selective membrane based on nanostructured sulfonated block copolymers,Journal of Membrane Science,2009, 337, 39-46.Nathan T. Hancock, William A. Phillip, Menachem Elimelech, Tzahi Cath, Bidirectional Permeation of Electrolytes in Osmotically Driven Membrane Processes,Environmental Science & Technology,2011, 45, 10642-10651.Yizhou Zhang, Jessica L. Sargent, Bryan W. Boudouris, William A. Phillip, Nanoporous membranes generated from self-assembled block polymer precursors: Quo Vadis?,Journal of Applied Polymer Science