Fouling control using temperature responsive membranes composed of N-isopropylacrylamide (NIPAAm) and iron oxide nanoparticles

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1 The University of Toledo The University of Toledo Digital Repository Theses and Dissertations 2015 Fouling control using temperature responsive membranes composed of N-isopropylacrylamide (NIPAAm) and iron oxide nanoparticles Sneha Arun Chede University of Toledo Follow this and additional works at: Recommended Citation Chede, Sneha Arun, "Fouling control using temperature responsive membranes composed of N-isopropylacrylamide (NIPAAm) and iron oxide nanoparticles" (2015). Theses and Dissertations This Dissertation is brought to you for free and open access by The University of Toledo Digital Repository. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of The University of Toledo Digital Repository. For more information, please see the repository's About page.

2 A Dissertation entitled Fouling Control Using Temperature Responsive Membranes composed of N- isopropylacrylamide (NIPAAm) and Iron Oxide Nanoparticles by Sneha Arun Chede Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Chemical Engineering Dr. Isabel Escobar, Committee Chair Dr. Maria Coleman, Committee Member Dr. Yakov Lapitsky, Committee Member Dr. Saleh Jabarin, Committee Member Dr. Geoffrey Bothun, Committee Member Dr. Patricia R. Komuniecki, Dean College of Graduate Studies The University of Toledo December 2015

3 Copyright 2015, Sneha Arun Chede This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author.

4 An Abstract of Fouling Control Using Temperature Responsive Membranes composed of N- isopropylacrylamide (NIPAAm) and Iron Oxide Nanoparticles by Sneha Chede Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Chemical Engineering The University of Toledo December 2015 Membrane fouling occurs when there is reversible or irreversible accumulation of macrosolutes present in the water on the membrane surface. Membrane cleaning and eventual replacement due to fouling can add to the operating costs of membrane systems. Reversible fouling can be minimized by crossflow operation and/or backflushing. On the other hand, irreversible fouling cannot be minimized during operation, often requires chemical cleaning, and may result in permanent flux decline. Among irreversible foulants, natural organic matter (NOM) is considered to be a major contributor. NOM is composed of a wide range of hydrophilic and hydrophobic components; hence, any stagnant hydrophobic or hydrophilic membrane has the potential to become fouled. Therefore, a dynamic membrane able to alternate between being more or less hydrophilic would be expected to decrease fouling. The purpose of this study was to cast stimuli responsive membranes to control fouling made of cellulose acetate (CA) and N- isopropylacrylamide (NIPAAm). NIPAAm is a stimuli-responsive polymer, which offers the potential to reversibly collapse or expand the membrane as a function of changes in temperature. Membranes were cast using phase inversion, were characterized chemically iii

5 and morphologically, and were used in filtration experiments using bovine serum albumin (BSA), lipase and humic acid solutions. Flux studies were conducted at alternating cold and hot temperature cycles. CA-NIPAAm membranes displayed on average higher fluxes during operation, along with lower protein and humic accumulation on the membrane surface as compared to regular CA membranes. CA-NIPAAm membranes also showed higher flux recoveries as compared to CA membranes. Temperature activation of temperature-responsive membranes can be energy extensive since it requires heating entire housing and feed streams. Furthermore, heat transfer resistances within the housing can hinder the temperature response time of the membranes. To address these, heating was localized within the membrane matrix by embedding superparamagnetic iron oxide (SPIO) nanoparticles within the temperatureresponsive NIPAAm polymer film. Nanoparticles were chemically attached to polynipaam, and the resultant product was added to the dope solution. Membranes were fabricated and tested for their response to the RF heating. An alternating current (AC) electromagnetic field was used to activate the temperature responsive membrane via electromagnetic heating caused by nanoparticles. Membranes with nanoparticles were studied with and without RF heating, and the results suggested that during RF heating, CA-NIPAAm membranes with nanoparticles became less hydrophilic as compared to without RF heating. CA-NIPAAm membranes with and without SPIO nanoparticles were subjected to RF heating to investigate the effect of the presence of nanoparticles on water infiltration. Higher temperatures were recorded near the membranes with nanoparticles as compared to the membranes without nanoparticles suggesting that an oscillating magnetic field has the potential to be used for temperature activation. iv

6 Finally, the performance of the membranes fabricated at the laboratory-scale and at production scale was evaluated. Since laboratory-scale doctor's blade method could become challenging during the scale up of the membranes, a well-developed pre-metered method, slot die extrusion, was used for continuous casting of liquid films. The feasibility of processing cellulose acetate membranes using a doctor s blade versus a slot die extrusion was examined. The effects of processing methods, conditions, and substrate on the morphology and on the flux of CA membranes were studied. Membranes were tested for their performance under the same process conditions. Overall, membranes fabricated at the laboratory-scale and scale up were similar. v

7 To my parents, Abhishek, Vaishu, and Anuj! vi

8 Acknowledgements My deepest gratitude is to my advisor, Dr. Isabel Escobar, for her continuous support, patience, and motivation during the past five years, and making this a thoughtful and rewarding journey. I have been amazingly fortunate to have an advisor who gave me the freedom to explore on my own, and at the same time, provided the guidance to stay in the right direction. I admire her immense knowledge, commitment to the highest standards, and down-to-earth nature, which makes her a true mentor and a role-model. I could not have imagined a better advisor for my PhD study. I would like to thank my committee members: Drs. Maria Coleman, Yakov Lapitsky, Saleh Jabarin, and Geoffrey Bothun for their encouragement and insightful comments throughout my research journey. I truly appreciate their suggestions, which led me to explore different aspects of material development in my research work. My sincere thanks goes to Dr. Schall and the staff members, Renee and Chanda, for their understanding and help since my first day at the university. Surviving graduate school would not have been possible without the support and care of my friends! In particular, Priyesh, Abhishek, Anup, Maisha, Michael, Samira, and Sunitha, who helped me to stay sane through difficult times and made this journey memorable. I greatly value your friendship. I would also like to acknowledge Udaka, Yuhang, and Nandu for helping me with analytical instruments. Most importantly, none of this would have been possible without the love and patience of my family. I am indebted to my parents, Kamal and Arun Chede. Your strong encouragement for education and belief in my decisions made me achieve my goals in life. You always provided the best environment for us in spite of all the hardships and never let us worry about anything other than our education. I would like to express my heart-felt gratitude to my parents, my brother, Abhishek, and my sister-in-law, Vaishali, for their endless love, concern and support during all these years. I love you all. And finally, I would like to thank Anuj for always being there to cheer me up, keep me focused, and standing by me through all the good and the bad times. Your intellect, wisdom, direction, and kindness have changed me forever! vii

9 Table of Contents Abstract... iii Acknowledgements... vi Table of Contents... viii List of Tables...xv List of Figures... xvii List of Abbreviations... xxi 1 Introduction Literature Review Membrane Theory Membrane Material Membrane Fouling Colloidal Fouling Organic Fouling Inorganic Fouling Biofouling Post-synthesis Modification for Fouling Control Stimuli Responsive Polymers Superparamagnetic Iron Oxide (SPIO) Nanoparticles for Temperature Activation...25 viii

10 2.7 Membrane Fabrication Phase Inversion Membrane Fabrication During Scale Up Research Objectives Materials and Methods Materials Chemical Reagents Glassware and Labware Experimental Work Hydrogel Preparation Hydrogel Characterization Preparation of CA and CA-NIPAAm Membrane at Lab Scale Membrane Preparation for Scale Up Studies Flux Measurement Contact Angle Measurement Pore Size Determination Solute Rejection Membrane Dope Development Introduction Polymer Dope Solutions Determination of Optimal Cellulose Acetate Percentage in Dope Solution Preparation of NIPAAm Membrane...53 ix

11 Casting Method Determine Amount of NIPAAm in the Polymer Studying Effect of Components of the Dope on Temperature Activation Studying the Effect of Different Reaction Time Scanning Electron Microscopy (SEM) Fourier Transform Infrared (FTIR) Spectroscopy Filtration Studies Result and Discussion Determination of Cellulose Acetate Percentage Preparation of NIPAAm Membrane Determine Amount of NIPAAm in the Polymer Studying Effect of Components of the Dope on Temperature Activation Different Reaction Times Conclusion Fouling Control Using Temperature Responsive N-isopropylacrylamide (NIPAAm) Membranes Introduction Materials and Methods Materials Preparation Methods Hydrogel Preparation...73 x

12 Preparation of NIPAAm Membrane Characterization Fourier Transform Infrared (FTIR) Spectroscopy Hydrogel Characterization as a Function of Temperature, Time and Ionic Strength Pore Size Determination Flux Flux Recovery Contact Angle Scanning Electron Microscopy (SEM) Results and Discussion Hydrogel Characterization FTIR Characterization Effect of Temperature Function of Time Effect of Ionic Strength Membrane Characterization Fourier Transform Infrared (FTIR) Spectroscopy Pore Size Determination Contact Angle of Membranes Filtration Studies Charge Analysis Membrane Dynamic Behavior...92 xi

13 Flux Recovery Rejection Analysis Fouling Conclusion Acknowledgements Temperature Responsive Membranes Composed of N-isopropylacrylamide (NIPAAm) and Superparamagnetic Iron Oxide Nanoparticles Introduction Related Studies Performed at University of Rhode Island Studies Performed at The University of Toledo Materials Preparation Methods Preparation of CA-NIPAAm-NP Membranes Membrane Characterization Results and Discussion Scanning Electron Microscopy (SEM) in Backscattering Mode: The University of Toledo RF Heating of the Membranes: University of Rhode Island Conclusion Scale Up Studies of Cellulose Acetate Membranes Introduction Lab Scale Studies Performed at The University of Toledo Materials xii

14 8.2.2 Methods Dope Solution Preparation Membrane Fabrication at Lab Scale Membrane Characterization Scale Up Studies Dope Preparation at The University of Toledo Slot Die Casting at Georgia Institute of Technology Contact Angle of the Substrate and the Tooling Dope Solution Characterization Studies Membrane Fabrication During Scale Up Results and Discussion Contact Angle Studies: Georgia Institute of Technology Dope Solution Characterization: Georgia Institute of Technology Surface Tension Viscosity PET Substrate Results Fabrication in Casting Window Filtration Studies: Performed at The University of Toledo Environment Scanning Electron Microscopy (ESEM) Conclusions Conclusions and Recommendations Conclusions Recommendations xiii

15 References A Flux Data for Membrane Dope Development B Temperature Responsive CA-NIPAAm membranes C Scale Up Studies xiv

16 List of Tables 5.1 PNIPAAm peaks NIPAAm-MBAA crosslinking peaks Filtration solutes for pore size determination PNIPAAm peaks NIPAAm-MBAA crosslinking peaks Rejection of different solutes by membranes Contact angle of CA-NIPAAm membrane below and above LCST Flux recovery analysis after humic filtration Protein rejection Flux Recovery after protein filtration A.1 Temperature activation of 18%CA+4%NIPAAm A.2 Temperature activation of 18%CA+8%NIPAAm A.3 Effect of components of the dope on temperature activation A.4 Data for different reaction times B.1(a) Swelling ratio at different temperature B.1(b) Standard deviation in swelling ratio B.2 Effect of time on the weight of NIPAAm hydrogel B.3 Effect of ionic strength B.4 Protein filtration of CA-NIPAAm membrane of thickness μm xv

17 B.5 Protein filtration of CA membrane of thickness μm B.6 Humic filtration of CA-NIPAAm membrane of thickness μm B.7 Humic filtration of CA membrane of thickness μm B.8 Actual data of 18% CA+2% NIPAAm membranes B.9 Actual data of 18% CA membranes C.1 Advancing contact angle vs. contact line velocity C.2 Shear rate vs. solution viscosity behavior C.3(a) Calculations for casting window C.3(b) Process parameter calculations for set thickness C.4 Flux data for BSA protein filtration C.5 Flux recovery after backwash C.6 Flux data for lipase protein filtration C.7 Flux recovery after backwash C.8 Replicated data for membranes fabricated at laboratory scale C.9 Replicated data for membranes fabricated during scale up C.10 Replicated flux recovery data after lipase filtration xvi

18 List of Figures 1-1 Global physical and economic water scarcity Schematic of pressure driven membrane process The filtration spectrum Membrane schematic (a) dead-end filtration (b) cross flow filtration Cellulose acetate polymer Colloidal fouling of NF membrane during dead end filtration Organic cake layer formation on membrane Membrane fouling during filtration of humic acid solution Possible resistances to the flow during filtration SEM image of biofouling layer on membrane surface Structure of NIPAAm and PNIPAAm NIPAAm in aqueous medium NIPAAm crosslinked hydrogel Mechanism of formation of phase inversion membrane Schematic of slot die coating Casting window NIPAAm hydrogel (a) completely dried and (b) completely swollen Set up for hydrogel studies Membrane casting steps...44 xvii

19 4-4 Dead-end filtration Contact angle measurement Flux profiles at pressure 2.76 bars (40 psi) of membranes for different CA concentration and different thickness (a) CA membrane, (b) CA-NIPAAm membrane; method 1, (c) CA-NIPAAm membrane; method FTIR analysis of CA and CA-NIPAAm membranes prepared by Method FTIR of NIPAAm crosslinked hydrogel FTIR analysis for CA and CA-NIPAAm membranes prepared by Method (a) SEM of cross section of (i) 18% CA, (ii) 18% CA+2% NIPAAm and (iii) 18% CA+4% NIPAAm membranes (b) SEM of cross sections of (i) 18% CA+2% NIPAAm, (ii) 18% CA+4% NIPAAm and (iii) 18%CA+8%NIPAAm membranes Temperature activation of membranes Effect of components of the dope on temperature activation FTIR analysis of membranes with different reaction time Normalized flux profiles of membranes with different reaction time Products of polymerized reactions in CA-NIPAAm dope solution preparation FTIR of NIPAAm crosslinked hydrogel Change in the swelling ratio of gel as a function of temperature Hydrogel (a) at 15 C and (b) at LCST 34 C Change in the weight of gel as a function of time from completely dried till completely swollen state...86 xviii

20 6-6 Effect of ionic strength: concentration of NaCl vs temperature plot FTIR analysis for CA and CA-NIPAAm membranes (a) Surface charge analysis of CA membranes (b) Surface charge analysis of CA-NIPAAm membranes Temperature activation during protein filtration CA-NIPAAm membrane and CA membrane after filtration experiment SEM of cross section of the membranes Temperature activation during humic filtration SEM of clean CA and CA-NIPAAm membranes before filtration SEM images of the membranes after protein filtration SEM images of the membranes after humic solution filtration Response of PNIPAAm coated on magnetite nanoparticles to the temperature increase above LCST Magnetic field generated when current flows through a coil Collapse of NIPAAM coatings on grafted SPIO nanoparticles using standard and 2x EDC reaction conditions PNIPAAm and nanoparticles coupling reaction Membrane placed at the center of the coil for RF heating (a) studies on membranes with and without RF heating (b) four drops of water with dye SEM image of CA-NIPAAm membrane with embedded nanoparticles EDS analysis to check the presence of nanoparticles Membranes after 900 sec of RF heating xix

21 7-10 Membranes after 1460 sec of RF heating Time for complete infiltration with and without RF heating Infiltration time of membranes with and without SPIO NPs during RF heating Membrane casting steps Schematic of the experimental setup (RFIS) Advancing contact angle vs. contact line velocity Shear rate vs. solution viscosity behavior (a) casting using PET substrate (b) casting using glass substrate (a) PET, or substrate side surface (b) polymer side surface Defect free cellulose acetate membrane fabricated using slot die extrusion Flux profiles during BSA protein filtration Flux data during lipase protein filtration ESEM images of CA lab scale and scale up membrane before filtration ESEM images of CA lab scale and scale up membrane after backwash B-1 Replicated data of 18% CA+2%NIPAAm membranes B-2 Replicated data of 18% CA membranes C-1 Plot of Q' vs uw for varying film thickness xx

22 List of Abbreviations CA...Cellulose Acetate CAN...Cerium Ammonium Nitrate DI...Deionized EDCH...N-(3- Dimethylaminopropyl)-N -ethylcarbodiimide hydrochloride EDS/EDX...Energy Dispersive X-ray Spectroscopy ESEM...Environmental Scanning Electron Microscopy FTIR...Fourier Transform Infrared MBAA...Methylene bisacrylamide MES...2-(N-Morpholino)ethanesulfonic acid MF...Microfiltration MWCO...Molecular Weight Cut Off NF...Nanofiltration NHS...N-Hdroxysuccinimide NIPAAm...N-isopropylacryamide NMP...N-methyl pyrrolidinone NOM...Natural Organic Matter NP...Nanoparticle PET...Polyethylene Terephthalate RFIS...Roll-Feed Imaging System RO...Reverse Osmosis SEM...Scanning Electron Microscopy SPIO...Superparamagnetic Iron Oxide UF...Ultrafiltration UV...Ultraviolet xxi

23 Chapter 1 Introduction Fresh clean water is a key resource for human health, balance in the ecosystem, agriculture and industrial expansion [1, 2]. Population growth, coupled with industrialization and urbanization, result in a continuing increase in demand for fresh water. Figure 1-1 shows regions of physical and economic water scarcity. Physical scarcity is a physical limitation in the access of clean water, which could be either due to natural causes, such as dry land, or man-made causes, such as overuse and lack of management [3]. On the other hand, economic scarcity exists when the population lacks the proper means for the usage of water. Hence, water scarcity is a relative concept between the availability and the actual use of water. Source: h p:// Figure 1-1: Global physical and economic water scarcity [3] 1

24 Increasing human and industrial activities has lead to an increase in the demand for fresh water and pollution of existing fresh water sources [4]. Hence, efforts have been made over the past few decades to discover sustainable sources of freshwater, such as seawater and wastewater. Although several water purification techniques are available, their implementation is often affected by cost and practicality of scale-up [5]. Membrane technology has been proven to be effective in recent years due to its promising benefits of significant reduction in the equipment size, simple operational parameters, remote control, high production output, and automation [2, 6, 7]. Membranes can also be used in the place of some of the traditional water treatment applications. For example, nanofiltration (NF) membranes are used to remove hardness instead of conventional water softeners, such as lime softening [8]. MF and UF membranes can replace traditional pretreatments methods, such as clarification of suspended solids, and removal of macromolecules required prior to NF or RO processes [2]. One of the most significant issues affecting membrane processes is membrane fouling [9, 10]. Fouling occurs due to specific intermolecular interactions between macrosolutes present in the feed water and the membrane surface [10, 11], and can happen even in the absence of filtration. Fouling affects the performance of membrane processes via permanent flux decline and reduction in separation efficiency [12]. The most common techniques to prevent fouling are to pretreat the water to remove potential foulants and to clean the membranes during operation. Conventional methods of pretreatment, such as disinfection and particle removal, involve intensive chemical treatments [4]. These treatments also introduce ammonia and chlorine compounds, coagulants, corrosion and scaling controlling chemicals to the feed water. Cleaning of 2

25 membranes to remove accumulated foulants from the surface is done by physical processes, such as backflushing and cross flushing [13], and chemical cleaning using alkaline solutions, surfactants, chelating agents, citric acid and salt solutions [14-16]. Chemical pretreatment to remove fouling can be effective, but must meet safety guidelines with respect to chemicals used [16]. When foulants, such as organic or inorganic particles, colloids and microorganisms, cannot be removed by physical methods of cross-flow or backflushing, it results in permanent decline of flux and shortened membrane lifespan [14, 17-19]. Many methods have been studied to prevent or decrease fouling by changing the membrane surface chemistry [20, 21]. Generally, increase in hydrophilicity has been proven to offer better fouling resistance since many foulants, such as proteins, are hydrophobic in nature [22]. Since hydrophobic membranes are more chemically and mechanically stable, while hydrophilic membranes are less prone to fouling, many of these methods include increasing hydrophilicity of hydrophobic membrane surfaces by physical adsorption or grafting of hydrophilic agents on the surface [20]. However, the addition of such polymer brushes on membrane surfaces has disadvantages, such as increase in the resistance to the flow, uneven polymerization on the surface and difficulties in achieving desired thickness and pore size [23, 24]. Although hydrophobic foulants cause a significant amount of fouling, many studies have shown that hydrophilic components present in natural water sources were mainly responsible for initial flux decline [25, 26]. Hydrophilic components were found to cause pore blocking in both, hydrophilic membrane [25] and hydrophobic membrane having modified hydrophilic 3

26 surface [26]. Hence, it is believed that having a stagnant hydrophilic or hydrophobic membrane eventually leads to a decrease in the membrane performance. This study is focused on the fabrication of a membrane capable of reversibly switching its hydrophilicity in the presence of an external stimulus. By achieving such dynamic behavior, it was hypothesized that solute accumulation on the membrane surface could be reduced, which in turn, would result in the reduction in irreversible fouling. 4

27 Chapter 2 Literature Review 2.1 Membrane Theory A membrane is a partition or a barrier between two phases, and on the application of a driving force, it separates the phases. Figure 2-1 shows a schematic of a pressuredriven membrane process. The feed is forced to flow through the membranes by applying pressure as a driving force. The feed is separated into two streams, the concentrate and the permeate. Figure 2-1: Schematic of pressure driven membrane process [27] 5

28 Membranes are generally rated based on their pore size or molecular weight cutoff (MWCO), which is the molecular weight of a solute that has a rejection coefficient of 90% [28]. A typical pore size distribution and respective rejection characteristics of different membrane processes are shown in Figure 2-2. Porous membranes act as a boundary between two phases, whereas non-porous membranes allow controlled and selective transfer of one species from one phase to another phase [29-31]. Porous membranes have the ability to remove colloids, suspended particles, and microorganisms by a sieving mechanism according to the size of the membrane pores and the size of the matter to be removed [16]. On the other hand, non-porous membranes separate molecules as the result of the difference between solubility or diffusivity [32]. Figure 2-2: The filtration spectrum [33] 6

29 Based on the typical removal capability, membrane processes are categorized as: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). MF membranes generally have pore size in the range μm, whereas UF membranes have pore size ranging μm [16]. NF membranes have an average pore size of approximately μm [34]. Separation by MF and UF membranes is governed by size exclusion or sieving mechanism [35], while separation by RO membrane is due to diffusion [32, 36]. In the case of NF membranes, depending upon the molecular weight cut-off, separation is governed by either size exclusion or diffusion, or by both [37]. NF and RO membranes are important for desalination [38, 39]. UF and MF membranes are used for a wide range of operations due to their low operational parameters and size exclusion range [8, 39]. They are used for pretreatment prior to NF or RO process, oil-water separations, milk processing, fruit juice clarification, potable water production, paint industry, purification of pharmaceutical products, and wastewater treatment [8]. Depending upon the feed direction with respect to the membrane surface, filtration mode is referred as dead-end mode or crossflow mode (Figure 2-3). Dead-end filtration mode is very common in the case of low concentration feeds, while in case of high concentration solute feeds, cross-flow mode is preferred to maintain a stable flow rate [8]. In case of cross-flow filtration, due to the shear force induced by cross-flow velocity, solute particles in the feed stream roll off the membrane surface [40] reducing the reversible fouling. 7

30 (a) (b) Figure 2-3: Membrane schematic (a) dead-end filtration (b) cross-flow filtration When a membrane is made of a single selective layer, it is called symmetric and homogeneous and if a membrane is made of more than one layer, it is called asymmetric and non-homogeneous [41]. Asymmetric membranes are made of a thin polymer layer responsible for selective filtration, and this layer is in turn supported by a thick porous support layer that gives mechanical strength to the membrane [42]. Asymmetric and composite membranes are found to have better performance than symmetric membranes because the selective layer of asymmetric membranes is thinner, hence reducing the resistance [41]. 2.2 Membrane Material Various inorganic and polymeric materials are used to prepare membranes. For purposes of this study, polymeric materials are the focus of the discussion. Membranes are prepared using a variety of materials, such as cellulose acetate (CA), polyvinylidene 8

31 fluoride (PVDA), cellulose diacetate (CDA), cellulose triacetate (CTA), polyethersulfone, polyetherurea, polyamide (PA), polyetheramines, and polypropylene [43]. Membrane properties, such as surface charge and hydrophobicity, and process parameters depend on the polymer material used. Electrokinetic potential, also known as zeta potential, reflects the electric charge acquired by the membrane surface when it is brought in contact with an aqueous electrolyte medium [41]. Several membranes have functional groups on their surfaces, which are responsible for the surface charge. Functional group such as carboxylic (R-COO - ), amine (R-NH + 3 ), and sulfonic (R-SO - 3 ) make the membrane surface charged [41]. Interactions between the membrane surface and particles leading to fouling depend upon the charge of the membrane surface and charge of the particles [15]. Hence, various studies are done for the measurement of zeta potential of membranes in different solutions [41, 44, 45]. In aqueous media, membranes exhibit attractive or repulsive behavior towards water [45]. Membranes can be hydrophilic or hydrophobic depending on their composition or their surface chemistry. Cellulose acetate membranes are extremely hydrophilic in nature. Membranes of polymers from the polysulfone (PS) family and polyacrylonitrile (PAN) are considered to have intermediate hydrophilicity, and materials like polyethylene and polypropylene are more hydrophobic [41]. Hydrophilic membranes are preferred for aqueous filtrations since they are less susceptible to fouling [46]. Water molecules in order to access passage through the membrane push other compounds; therefore, hydrophilic membranes give higher flux values [11]. On the other hand, hydrophobic membranes are susceptible to fouling by hydrophobic materials present in the feed water [4, 11, 18, 31]. Since hydrophobic components present in water tend to 9

32 aggregate, they form clusters to lower their free energy [47] and attach to the hydrophobic surface of membrane resulting in fouling. However, hydrophobic membranes possess advantage of robustness, and chemical and mechanical stability [48], and these properties are desirable. Cellulose Acetate Cellulose acetate (CA) is a modified natural polymer, which offers a wide range of properties for preparation of variety of consumer products [49-51]. CA is obtained by reacting cellulose with acetic anhydride and acetic acid in the presence of sulfuric acid [52]. Figure 2-4 shows structure of cellulose acetate polymer. Figure 2-4 Cellulose acetate polymer CA was one of the first polymer materials, which was used specifically for aqueous separation processes [53]. CA membranes posses desired qualities for separation applications, such as high hydrophilicity, good desalting, high potential flux, and low cost [51, 53]. Hence, they are used for reverse osmosis (RO), ultrafiltration, microfiltration 10

33 membrane fabrication. These membranes have widespread application in water and wastewater treatment, food processing, and pharmaceutical industry. Cellulose acetate has a high hydrophilicity due to the presence of three active hydroxyl groups in each repeating unit, which in turn prevents its dissolution in ordinary solvents [41, 48]. However, according to Richards et al. [48], in case of cellulose acetate membranes, cellulose was degraded during the regeneration process and resulted in the loss of the membrane s ability to withstand strong acids, alkali and organic solvents. CA membranes have good fouling resistance because their hydrophilicity prevents binding of proteins [53, 54]. Bai et al. [53] performed studies with CA membranes for treatment of municipal sewage. They blended CA with polyethylene glycol (PEG) 600 additive, which improved the permeability of the CA membrane. Since CA membranes have low oxidation resistance, chemical resistance, and poor mechanical strength; therefore, their use has been declining. Modification of CA membranes prior to application is an important area of research to address its disadvantages [54]. 2.3 Membrane Fouling Membrane fouling occurs due to the reversible and irreversible attachment of macrosolutes present in the feed water to the membrane surface or into the membrane pores [15, 18, 32]. This attachment leads either to an increase in trans-membrane pressure during constant flux mode or a decrease in flux when filtration is operated at constant pressure mode [17, 25, 55]. Reversible fouling can be minimized by the mode of operation, such as with crossflow operation, and by backflushing during filtration, and it ceases once filtration stops. On the other hand, in case of irreversible fouling, a fraction 11

34 of the initial flux cannot be recovered after backflushing [18], and it results in a permanent flux decline. A permanent decline in flux leads to increases in the energy consumption to maintain constant flux and in cleaning frequencies, which in turn increases the operating cost. Chemical and physical pretreatment along with cleaning are often required to control irreversible fouling. Chemical pretreatment/cleaning agents, such as chlorine, cleaning surfactants and other proprietary agents, such as anti-scalants, may have specific storage requirements and may be require to meet the disposal guidelines [16]. There are four main types of fouling, which are colloidal, organic fouling, inorganic, and biofouling [15, 18]. Different mechanisms explain membrane fouling: pore blocking, cake formation, concentration polarization, organic adsorption, inorganic precipitation, and biological fouling [25, 32, 55, 56] Colloidal fouling Colloids interact with the exterior surface or active layer of the membrane rather than with interior pores [41]. Accumulation of colloidal matter results in fouling by different mechanisms [56], including pore blocking, cake formation, concentration polarization, and accumulation [9, 57] on the surface. Figure 2-5 shows an SEM image of colloidal fouling of a nanofiltration (NF) membrane. 12

35 Figure 2-5: Colloidal fouling of NF membrane during dead end filtration [58] Henry et al. [59] explained the four phenomena by which colloidal fouling can take place, which are introduction or deposition of particles by flow, resuspension of particles, agglomeration, and clogging. Their studies confirmed that colloidal fouling starts with deposition of a single particle followed by generation of layers of those depositing particles resulting into a cluster. Eventually this can lead to the point where these clusters can cause blockage of the flow Organic fouling In natural water filtration processes, dissolved natural organic matter (NOM) plays a major role in membrane fouling [10, 42, 58]. NOM is composed of variety of components and a major part of NOM consists of humic substances (HS). HS is a combination of complex macromolecular product of animal residue and plat degradation [10], including lignin, carbohydrates, and proteins. Figure 2-6 shows SEM images of cake layer formation on membrane during organic fouling. 13

36 Figure 2-6: Organic cake layer formation on membrane [60] Components of NOM can be categorized as hydrophobic and hydrophilic. Hydrophobic components include strong hydrophobic acids (fulvic and humic acid) and weak acids (alkyl mono and dicarboxylic acids) [19]. These acids constitute a large fraction of NOM. Humic substances are anionic macromolecules, mainly composed of carboxylic functional groups, and as a result, they are negatively charged at a neutral ph range [37]. Hydrophilic component of NOM includes polysaccharides, alkyl alcohols, amides and bases [17, 19]. NOM-membrane interactions are highly affected by NOM properties (NOM concentration, fractions of humic components and charge), membrane properties (hydrophobicity and charge) and operating conditions (permeate flux, crossflow velocity, and turbulence promoters, such as spacers) [61, 62]. A significant amount of research has been done to explain the role of NOM in fouling of water filtration membranes, and the mechanisms by which fouling takes place. Yoon et al. [19] studied the factors controlling NOM fouling of ultrafiltration membranes. Their studies showed that hydrophobic interactions between NOM and membranes, NOM size and charge density, and 14

37 membrane pore size and surface charge were the major factors. Many studies have concluded that hydrophobic effects influence the interactions between NOM and membranes. Different fractions of NOM (hydrophilic and hydrophobic) depending upon their amount in raw water have different propensities for fouling. A typical flux-time curve during NOM fouling in UF applications shows stages of rapid initial flux drop, which is then followed by a long period of gradual flux decline [48, 63]. The rapid initial flux drop can be attributed to membrane pore blocking since pores are clean and open at the beginning of the filtration. Pore blocking is a rapid and quick process because less than a layer of particles is sufficient for the blockage of the pore [48, 63, 64]. Whereas after pore blockage, the amount of retained particles is increased layer by layer, which results into the cake formation. Since this occurs gradually over the time, flux decrease is also gradual with time [64]. Figure 2-7 shows a scanning electron microscopy (SEM) image showing fouling of a microfiltration membrane due to aggregates of humic acid. Figure 2-7: Membrane fouling during filtration of humic acid solution [65] Formation of gel layer, pore blocking and adsorption during NOM fouling are depicted in Figure 2-8. During filtration, the retained solute can result into a region of 15

38 high concentration near the membrane surface as compared to the bulk solution. This region of high concentration is less permeable to the solvent (usually water), and termed as concentration polarization [66, 67]. Concentration polarization can result into a gel layer formation when the concentration in the immediate vicinity of the membrane surface reaches the solubility limit of the solute [67, 68]. Adsorption is caused by interactions between the membrane and a solute, and this can happen even in the absence of flow through the membrane [67, 69]. Pore blocking occurs when size of the solute and membrane pore size are similar. A cake layer is formed when the solute dimensions are larger than membrane pore, and as a result, solute deposits on the membrane surface, layer by layer [69]. Layer formation on the membrane surface offers added resistance to the flow through the membrane and causes flux decline. concentration polarization gel layer formation membrane adsorption pore blocking Figure 2-8: Possible resistances to the flow during filtration [66] Yuan et al. [10] studied the fouling of microfiltration membrane in the presence of humic acids from river water. Their studies suggested that humic acid fouling was 16

39 governed by deposition of large aggregates of humic matter on the membrane surface. This was due to the intermolecular hydrophobic interactions between humic acid aggregates and membranes. Hydrophobic fraction of NOM contributes significantly towards fouling via formation of hydrophobic clusters, and hence, hydrophobic membranes are highly susceptible to the fouling [4, 11, 18, 31, 70]. However, many studies suggested that the hydrophilic fraction of NOM might be responsible for fouling via the formation of a gel layer on the surface [55, 61, 71]. Feed solutions containing proteins are also known to form a gel layer on membrane surface [66]. The hydrophilic fraction of NOM is considered to foul membranes by gel layer formation, while hydrophobic components foul membranes by adsorption within the pores [71]. Maximous et al. [63] determined that the hydrophilic fraction of NOM contributed towards the initial flux decline of both hydrophilic and hydrophobic membranes via gel layer formation. The hydrophobic fraction of NOM was responsible for pore plugging and eventually cake layer formation, which was prominent in case of hydrophobic membranes. Although, presence of NOM affected the performance of both types of membranes, hydrophilic membranes were found to be more advantageous over hydrophobic membranes due to their resistance to cake formation. The presence of divalent cations also influences fouling. Hong et al. [37] investigated chemical and physical interactions in NOM fouling in the presence of divalent cations. They concluded that in the presence of divalent cations, mainly calcium and magnesium, severe NOM fouling took place since complex of divalent cations led to enhanced bridging between NOM macromolecules and the membrane surface to form a dense fouling layer on the membrane. 17

40 2.3.3 Inorganic fouling Inorganic fouling occurs due to the presence of inorganic salts, such as CaSO 4, CaCO 3, SiO 2 and BaSO 4 [32, 72] that can scale on the membrane surfaces. There are two major mechanisms for inorganic fouling, which are crystallization and particulate fouling [32, 55]. Crystallization occurs due to the precipitation of ions whereas deposition of colloidal particles from bulk to membrane surface leads to particulate fouling. Since inorganic scaling cannot be completely removed with physical methods, chemical cleaning is conventionally used to treat inorganic fouling [16]. Kang et al. [22] stated that properties of membrane surface such as hydrophilicity, surface roughness, and surface charge were the major parameters influencing inorganic fouling. Ning el al. [73] studied controlling inorganic fouling of RO systems and concluded that instead of removing foulants with pretreatments, cleaning using antifoulants was a effective method. Other than chemical cleaning, membrane modification methods such as surface coating, grafting [22] and increasing hydrophilicity of existing membranes [74] to develop antifouling surfaces are also considered as effective ways to treat inorganic fouling Biofouling Biofouling is the unwanted growth of microbial cells leading to development of biofilm on the membrane surface [75, 76]. Microorganisms are present in almost all water systems and tend to colonize on many surfaces [75]. In aqueous filtration processes, microorganisms grow and multiply on the membrane surface by using nutrients present in the water stream. These attached colonies of microorganism excrete extracellular polymeric substance (EPS) and in turn form a biofilm on the membrane 18

41 surface. Similar to fouling caused by accumulation, biofouling also offers resistance to flow by developing an additional layer on the membrane. Figure 2-9 shows biofilm formation during biofouling of RO membrane. Figure 2-9: SEM image of biofouling layer on membrane surface [77] Several strategies have been proposed to control the biofouling, which include pretreatment to remove bacteria and inactivation of bacteria using chemicals or UV rays [75]. Other studies include modification of the membrane surface with antimicrobial molecules, such as silver nanoparticles and carbon nanotubes [75, 78]. Hausman et al [76] developed copper charged polypropylene feed spacers to control biofouling. 2.4 Post-synthesis modifications for fouling control There are three major processes to incorporate different polymer films on membranes [42, 79], which are 1. incorporating a desired polymer or copolymer in the membrane dope and casting the membranes by phase inversion 2. in situ polymerization, and 3. post-synthesis surface modification of existing membrane. Much research has been 19

42 performed to develop fouling resistant membranes through post-synthesis modifications, during which the surface of an existing membrane is modified [74]. Modification has been done using several methods, such as ultraviolet [80] and ion beam irradiation [42], plasma treatment, and grafting of a polymer film on a membrane surface [20, 80, 81]. Using a hydrophilic polymer brush or a gel to reduce fouling is widely reported in research for post synthesis modifications. Many researchers have proposed that if the membrane surfaces were sufficiently covered by such polymer brush, adsorption would be eliminated by steric interactions from the brushes and solute finding its way to the membrane surface [82]. A commonly used grafted polymer is polyethylene glycol (PEG). PEG is well known for its ability to disrupt the interactions between membrane and NOM or proteins, which can foul the membrane [20, 81]. In case of cellulose acetate membranes, membrane surfaces have been grafted with PEG to improve their stability during protein interactions [20]. However, due to its high solubility, PEG can undergo oxidative degradation in aqueous media [74] and there is a possibility of loss of a small portion of grafted PEG from the surface during prolonged periods of filtration [74]. Li et al. [83] found out that although PEG improved the protein resistance of the membrane, it increased the surface roughness, which had potential to offer additional sites for attachment of biofoultants. Abednejad et al. [84] observed that a high degree of PEG grafting could damage the membrane surface, and that it could also lead to pore blocking of the membrane. They also suggested that the PEG molecules can readily fragment under low plasma power conditions and this can lead to the loss of its molecular structure [74]. 20

43 Stimuli responsive or smart polymers have been gaining attention in recent years due to their unique ability of changing conformation in the presence of external stimuli [85]. Due to their unique property they have been widely used in variety of biological applications such as controlled drug delivery, tissue engineering, cell culture, sensors, and bioseparations [86]. 2.5 Stimuli responsive polymers Stimuli responsive polymers undergo reversible changes in their conformation due to a change in different parameters of the surrounding environment; i.e., the stimulus. The stimulus can be ph, temperature, ionic strength or electric or magnetic fields [87]. Polymers, like poly(n-isopropylacryamide) and poly(n,n-diethylacrylamide), change their conformation in the presence of temperature changes [79, 87]. Polymers, such as poly(acrylic acid) and poly(methacrylic acid), respond to ph changes, and for poly(styrene sulfonate) ionic strength is a major stimulus [79]. Gorey et al [21, 23] grafted a responsive polymer film using polymer N- isopropylacrylamide (NIPAAm) on a existing membrane surface. They grafted this polymer film in order to make a fouling resisting stimuli responsive layer; however, the grafted layers entangled on the surface creating patches of NIPAAm brush during activation resulting in a minimized response. Wandera et al [88] used poly (Nisopropylacryamide) (PNIPAAM)-block-poly(oligoethylene glycol methacrylate) (PPEGMA) to graft on commercial membrane surface. They found that by grafting temperature responsive PNIPAAM-b-PPEGMA nanolayers on surface, attachment of foulants was reduced. It also helped to achieve a chemical free way to remove any 21

44 attached foulants. However, due to the added resistance to flow, there was a decrease in the overall flux, and the salt rejection was also poor. Ali el al [89] proposed to use the swelling and deswelling property of NIPAAm hydrogel for desalination of brackish water. They prepared a polymeric network from thermo-responsive co-monomer based on NIPAAm and sodium acrylate. Bordawekar et al [82] proposed the use of hydroxypropyl cellulose (HPC), which is a temperature-stimulated polymer gel to create a brush on membrane. They developed membranes with HPC gel brush on CA membranes and studied their response under alternating temperature conditions. Although the irreversible fouling of a yeast solution was significantly reduced during temperature fluctuations, they concluded that the heating of the feed water for polymer activation was not a viable commercial option. Despite challenges associated with stimuli responsive brushes on the membrane surfaces, these studies offered motivation for pursuing development of stimuli-responsive polymer membranes. N-isopropylacryamide (NIPAAm) N-isopropylacryamide (NIPAAm) is responsive towards temperature changes in the surrounding environment. NIPAAm has a lower critical solution temperature (LCST) of 34 o C [90]. LCST is the phase transition temperature of a thermo-sensitive polymer [86]. Because of the closeness of its LCST to human body temperature, NIPAAm polymers and its crosslinked hydrogels are useful in the area of controlled drug delivery systems [87, 91]. Figure 2-10 shows the structure of NIPAAm monomer and NIPAAm polymer (PNIPAAm). 22

45 Figure 2-10: Structure of NIPAAm and PNIPAAm Below the LCST, polymer-water interactions are predominant (Figure 2-11). Water acts a good solvent and results into extensive hydrogen bonding between polymer and water [23, 91]. Below the LCST, NIPAAm polymer chains are in expanded or in a linear conformation, and it is in a more hydrophilic state [92]. As the temperature is increased above LCST, the polymer interchain collapses (Figure 2-11) leading to a coiled conformation and a less hydrophilic state. LCST is considered as an inherent characteristic of a polymer molecule, which depends on the balance of hydrophobic and hydrophilic species within the monomeric units in the manner, which is not yet fully understood. This thermo-responsiveness is believed to be due to the presence of both a hydrophobic carbon backbone and a side chain isopropyl group, and a hydrophilic amide group along the side chain in each monomer unit [93]. 23

46 Coil conformation Linear conformation Amide-water H-bonding Amide-amide H-bonding Figure 2-11: NIPAAm in aqueous medium [92] Similar to coil to linear conformation of PNIPAAm polymer chains, NIPAAm crosslinked hydrogels have the ability of thermally reversible swelling and deswelling. Hydrogels are crosslinked, three dimensional hydrophilic polymeric networks [89]. Their phase transition is considered to be the competitive result of hydrophobic interaction between the isopropyl groups and backbones, and association of hydrogen bonding between amide groups and water molecules [91]. Figure 2-12 shows the structure of NIPAAm crosslinked hydrogel. Compositions and preparation methods of NIPAAm hydrogel are covered in details in Chapter 4. 24

47 Figure 2-12: NIPAAm crosslinked hydrogel The LCST can be altered by addition of additives to the backbone or to the environment [94]. Effect on the LCST by the addition of solute to the environment of hydrogel is discussed in Chapter 6. The thermo-responsive property of NIPAAm offers the advantage of reducing membrane fouling when NIPAAm is incorporated in the membrane matrix. When the temperature conditions are altered as below and above the LCST of NIPAAm, the membrane is believed to change to a more hydrophilic and to a less hydrophilic state, respectively. By continuously activating the membrane, it is hypothesized that a dynamic stimuli-responsive membrane can prevent foulants from attaching to the membrane surface. 2.6 Superparamagnetic iron oxide (SPIO) nanoparticles for temperature activation Temperature activation of the membrane can be energy intensive since it requires heating of the entire housing and feed streams. Furthermore, heat transfer resistances within the housing can hinder the temperature response time of the membranes. It is believed that these challenges could be eliminated if the heating were localized within the 25

48 membrane matrix. This can be achieved by embedding superparamagnetic iron oxide (SPIO) nanoparticles within temperature responsive NIPAAm polymer film. Superparamagnetism In most atoms, electrons exist in pairs having their spin in opposite directions such that opposite spin cancels their magnetic field. Alternatively, materials with unpaired electrons exhibit a net magnetic field and respond to an externally applied field [95]. Paramagnetic materials, such as aluminum and platinum, due to some unpaired electrons show small magnetic susceptibility to magnetic field. When the external field is removed, they do not retain their magnetization [96]. Ferromagnetic materials, such as iron and nickel, show strong magnetic susceptibility. They have magnetic regions in their matrix known as magnetic domains. Magnetic domains are made up of large number of atoms having dipoles aligned in the direction of applied field. When an external field is removed, the material retains its magnetic properties [95]. Superparamagnetic nanoparticles are ferromagnetic materials existing as nanoscale size particles. When the magnetic particles are sufficiently small, i.e. diameter 3 to 30 nm, they exhibit the phenomenon of superparamagnetism [97-99]. In case of superparamagnetic nanoparticles, each nanoparticle itself is a single magnetic domain [95, 100]. Nanoparticles respond to an external magnetic field similar to paramagnetic materials, hence the term paramagnetic. However, their magnetic susceptibility to an external field is much larger than paramagnetic materials, and hence they are called superparamagnetic nanoparticles [95, 98]. 26

49 An electromagnetic field can be used to induce a magnetic field in superparamagnetic nanoparticles when an external alternating current is applied [101]. As a response to the external electromagnetic field, dipoles of superparamagnetic particles align themselves in the direction of external field via Neel and Brownian relaxations motions. Neel relaxations are caused due to the rotation of internal magnetization vector inside the particle, whereas Brownian relaxations are caused due to the rotation of the particle in the carrier matrix [102, 103]. These relaxation motions generate heat in the surrounding medium. When these SPIO nanoparticles are present in the membrane, an alternating current (AC) electromagnetic field could be used to generate the heat in the membrane matrix. This heat in turn could be used to cause increase in the temperature and continuously activate the temperature responsive membrane. 2.7 Membrane fabrication Numerous methods have been used to fabricate porous polymeric membranes. Generally, a laboratory scale approach, such as knife over blade or solution casting, is used to cast a thin sheet of the polymeric membrane [104, 105]. Then, the final form of the membrane is prepared by sintering, stretching, or phase inversion [106]. However, this method is not always the most suitable for the industrial scale because viscosity and pressure of the surrounding air can affect the casting solution by development of air bubbles [107]. The performance of the membrane is greatly influenced not only by the material but also by the processing methods and conditions because they affect the morphology of the membrane [108]. Slot die extrusion is a well-developed pre-metered approach that 27

50 allows for continuous casting of liquid film, which minimizes changes in liquid properties [107, 109] Phase Inversion In phase inversion, the polymer film is transformed from liquid to solid state [48]. Phase inversion is a versatile technique that offers a wide range of morphologies to the membranes [48, 110]. Figure 2-13 shows a continuous phase of a solvent-polymer mixture (a), which upon phase separation, develops into a homogeneous solutions consisting of suspended micelles dispersed in a continuous phase (b). Eventually, a gel skin is developed in the air-solution interface (c). Figure 2-13: Mechanism of formation of phase inversion membrane [110]. The phase inversion mechanism consists of three stages [110]. In first stage, the dope solution containing the polymer, solvent and non-solvent exists only in one phase (Figure 2-13(a)). In the next stage, by increasing the concentration of non-solvent, the heterogeneous solution (i.e. phase consists of two interdispersed liquid phases) is obtained (Figure 2-13(b)). In the third stage, the phase with high concentration of polymer in the heterogeneous solution solidifies to form a solid matrix, or the membrane (Figure 2-13(c)). 28

51 2.7.2 Membrane Fabrication During Scale Up The slot die coating process is a commonly used process for the manufacturing of plastics and polymer films, as well as coatings and optical films for liquid crystal displays in industry. The polymer solution is suspended with a set volumetric flow rate, Q in, through the slot die on a substrate, as shown in the Figure The substrate is moved with speed of u w in the x-direction. The solution suspended on the substrate results in the formation of a film of constant thickness at the downstream of the die. This thickness is called as the wet thickness (h). Figure 2-14: Schematic of Slot Die Coating [109] Wet thickness or polymer film thickness (h) is a function of the volumetric flow rate per unit width across the die (Q') and substrate velocity (u w ). Film thickness can be expressed as 29

52 To produce polymer films, a limited range of operating parameters exists, which are defined by the casting window, as shown in Figure When a polymer film is casted outside the casting window, various types of defects arise, such as air entrainment, dripping and break lines [109]. It is important to note that in Figure 2-15, Q in values were normalized, and they in fact represent the two dimensional flow rates (Q'). The casting window is the bounded area in which defect free films can be processed. Figure 2-15: Casting window [109] As shown in Figure 2-15, when the flow rate is very high relative to the substrate speed, solution accumulates behind the upstream die, and this is known as dripping. This results in uncontrolled thickness of the film. Another major defect is air entrainment involving high viscosity polymers. Coating speed, as stated before is an important factor for the production of polymer films during scale up. However, if the substrate speed is very high (Figure 2-15), air bubbles are trapped/entrained between the substrate and the polymer film. Air bubbles stay within the films, or sometimes their formation could 30

53 extend through to the top layer of the film creating a pinhole. Since air entrainment puts limitations on the coating speed, it is considered as one of the major defects during the slot die casting process [107]. When the coating speed exceeds the values for air entrainment (Figure 2-15), it results in the formation of break lines. In this case at the increasing coating speed, the original straight contact line breaks into a sawteeth like structure. Ruschak introduced a model to predict theoretically the coating window for Newtonian fluids [111], which was later extended to include viscosity effects for Newtonian materials by Higgins and Scriven [112]. Bhamidipati et al. further examined the development of casting windows for relatively high viscosity, non-newtonian solutions, such as polybenzimidazole (PBI) membrane solutions [109, 113]. This work led to the development of a semi-empirical model to predict the onset of bubbles (air entrainment) for solutions that meet certain material properties and capillary characteristics. As explained earlier, film thickness and substrate speed are inter-related. Maximum wet thickness (h max ) can be achieved at the minimum substrate speed (u w,min ), and the minimum wet thickness (h min ) can be achieved at the highest substrate speed (u w,max ). Minimum wet thickness is also proportional to the capillary number. Since capillary number is a function of viscosity, substrate speed, and surface tension, all these parameters were used to develop the equation: 31

54 (1) Where, u w,max = maximum velocity that the liquid can be cast Q k n H σ = two-dimensional flow rate = consistency index = power law index = gap between the tooling and the substrate = surface tension Equation 1 was used to obtain a first approximation for forming defect-free CA membranes using slot die extrusion. Details on calculations to develop process parameters are presented in Appendix B. 32

55 Chapter 3 Research Objectives The overarching goal of this study is to prepare a stimuli responsive membrane from a dope composed of NIPAAm and CA and achieve better temperature activation/response. Hypotheses 1. A temperature responsive polymer can be used in polymer dope solution in order to fabricate temperature responsive membranes. 2. Alternating hot and cold conditions of temperature could trigger a collapse and swelling behavior (less and more hydrophilic states) of the membrane, which in turn could reduce irreversible fouling. 3. SPIO nanoparticles can be embedded in the membrane matrix for localized heating. 4. Alternating current electromagnetic heating of the SPIO can trigger temperature activation of the membrane. 33

56 In order to investigate above hypotheses, the objectives of this study are the following: Objective 1. Characterize temperature responsive NIPAAm crosslinked hydrogels 1.1 Crosslinked hydrogels using N-isopropylacryamide (NIPAAm) and Methylene bisacrylamide (MBAA) crosslinker were prepared to study crosslinked NIPAAm properties. 1.2 NIPAAm hydrogel responses to the time, changes in temperature and ionic strength were studied. FTIR spectrometry was used to identify chemical characteristics, which were later used as reference for membrane characterization. Objective 2. Identify desirable dope composition and membrane thickness 2.1 Polymer solutions with different concentrations of cellulose acetate (CA) were prepared with different thickness. 2.2 Flow rate profiles for different CA membranes were created to determine the optimum CA polymer concentration. 2.3 Filtration studies and morphological analyses at alternating temperatures were conducted to determine the concentration of NIPAAm required for optimum temperature activation. 2.4 Effect of presence of crosslinker, polymerization initiator, and different reaction times on temperature activation the membranes were investigated. 34

57 Objective 3. Characterize temperature responsive NIPAAm crosslinked hydrogels 3.1 CA-NIPAAm membranes were prepared by phase inversion process.3.2 Following characterization studies were performed on CA and CA- NIPAAm membranes: Membranes were characterized chemically and structurally by Fourier Transform Infrared (FTIR) spectroscopy Pore size of the membranes was determined by filtering latex beads and dyes of different sizes Hydrophobicity of CA and CA-NIPAAm membranes were studied by measuring contact angle of the surface at different temperature conditions Membrane surface and cross section morphologies were studied by scanning electron microscopy (SEM) before filtration studies. 3.3 Evaluation of CA and CA-NIPAAm membranes were performed as follows: Filtration experiments with CA and CA-NIPAAm membranes were done using the following solutes: BSA protein solution, lipase protein solution and humic solution The dynamic nature and temperature activation of CA-NIPAAm membranes was studied by performing filtration at alternating temperature cycles. 35

58 3.3.3 Detection of fouling due to NOM was performed using SEM imaging Rejection of proteins was determined using UV-visible spectroscopy Objective 4. Develop a temperature responsive membrane composed of CA, NIPAAm, and superparamagnetic iron oxide (SPIO) nanoparticles. 4.1 Chemically attach SPIO nanoparticles to PNIPAAm using carbodiimidecoupling chemistry. Use PNIPAAm coupled with NPs along with CA and MBAA to prepare polymer dope solution. 4.2 Characterization of CA and CA-NIPAAm membranes with embedded SPIO nanoparticles was done as follows Determination of NP presence using energy dispersive x-ray spectroscopy (EDS) analysis of CA-NIPAAm-NP membranes The response of the nanoparticles embedded NIPAAm membranes during electromagnetic heating was studied. Objective 5. Scale up studies 5.1 Polymer films of different thickness were coated on PET and glass plates as substrates for scaled production. 36

59 5.2 Following characterization studies were performed on lab scale and scaled up CA membranes Hydrophobicity of CA membranes, laboratory-scale and scaled up, was studied by measuring contact angle of the surface at different temperature conditions Morphological studies of the clean membranes were performed using environmental scanning electron microscopy (ESEM). 5.3 Evaluation of CA membranes was performed as follows: Filtration experiments were performed using BSA protein and lipase protein solutions Characterization of fouling due to proteins was performed using ESEM imaging after filtration experiments Flux recovery was evaluated after performing backwash. 37

60 Chapter 4 Materials and Experimental 4.1 Materials Chemical Reagents NIPAAm (97%, Sigma Aldrich) was purified to >99% by crystallization using hexanes followed by vacuum drying prior to use. Cellulose acetate (average Mn ~ 30,000) was purchased from Sigma Aldrich (St. Louis, MO) and used as received. Methylene bisacrylamide (Polysciences Inc. Warrington, PA ), ammonium cerium (IV) nitrate (Aldrich), sodium metabisulfate (Sigma Aldrich), and ammonium persulfate (Sigma) were used as received. Other chemicals included bovine albumin serum (BSA) 98% (Sigma Aldrich), lipase (Sigma Aldrich), humic acid (Acros), NaCl (Fisher Scientific, Pittsburgh, PA), CaCl 2 (Fisher Scientific), and NaHCO 3 (Fisher Scientific) were used as received. In order to embed nanoparticles in the membrane matrix, nanoparticles were attached to PNIPAAm by EDC coupling reaction. Carboxylic acid group terminated polynipaam (PNIPAAm-COOH) was purchased from Sigma Aldrich (average Mn ~ 2000) and used as received. Amide functionalized iron oxide nanoparticles of size 25 nm were purchased from Ocean NanoTech LLC and were used as received. Components of 38

61 the EDC coupling reactions were MES (Sigma Aldrich), N-(3- Dimethylaminopropyl)- N -ethylcarbodiimide hydrochloride (Sigma Aldrich), N-Hydroxysuccinimide (Sigma Aldrich), and sodium hydroxide (Fisher Scientific) were used as received. Membrane pore sizes were determined by the rejection of latex beads and dyes. Latex beads of sizes 0.1 m, 0.05 m, and 0.03 m were purchased from Sigma Aldrich and diluted in DI water. Brilliant Blue R (pure, ACROS Organics) and Methyl Orange (Certified ACS) dyes were purchased from Fisher Scientific and diluted in DI water to make 10 ppm solutions Glassware and Labware All the glassware i.e., beakers, measuring cylinders, vials, bottles, flasks were cleaned in the laboratory sink by a detergent wash followed by acetone wash, acid wash and minimum of three rinses of DI water. Acid wash solution was consisted of 4 ml of hydrochloric acid per 250 ml of DI water. After the cleaning procedure, glassware were wrapped in aluminum foil and dried in Thermolyne Furnace (Thermo Scientific) oven at 550 C for 5 hours to remove any traces of organic contaminants. The reactant mixture was kept in a nitrogen atmosphere and Aldrich Atmosbag (Sigma Aldrich Atmosbag Z EA) was used for that purpose. Once the polymer dope solution was ready, it was kept in Cole Palmer 9981 Sonicator (Vernon Hills, IL) to degas. The sonicator was used also to ensure even suspension of nanoparticles and prevent aggregation of nanoparticles. ORION STAR A111 ph meter (Thermo Scientific) was used to prepare BSA protein solutions of different ph. All permeates were collected in Nalgene cryogenic sterile vials purchased 39

62 from Thermo Fisher Scientific (Rochester, NY, USA), which were further used for analysis. 4.2 Experimental Work Hydrogel Preparation In order to cast a dynamic NIPAAm membrane, characterization studies had to first be performed using NIPAAM hydrogels. Hydrogels were prepared using the monomer (NIPAAm), a crosslinking agent (N,N-methylenebisacrylamide (MBAA)), initiators (ammonium persulfate and sodium metabisulfate) and a solvent (deionized water (DI)). The solution was flushed with nitrogen, and the reaction was carried out at room temperature in an atmosbag maintaining nitrogen atmosphere. Once ready, the hydrogel was removed and washed thoroughly with DI water prior to use [114] Hydrogel Characterization The NIPAAm hydrogel responses as a function of time, ionic strength, ph and temperature were studied. To study the hydrogel response as a function of time, hydrogel samples were vacuum dried to ensure complete shrinkage and removal of water as shown in Figure 4-1 (a). Dried hydrogel sample was then placed in DI water and its weight was measured as a function of time till it achieved steady state signifying equilibrium swelling (Figure 4-1(b)). 40

63 Figure 4-1: NIPAAm hydrogel (a) completely dried and (b) completely swollen Figure 4-2(a) shows the assembly for hydrogel studies for response of temperature, which includes a glass vessel filled with DI water, stirring hot plate and a thermometer. The hydrogel sample was held in the holder shown in Figure 4-2(b). At the LCST, the hydrogel sample starts to turn opaque as shown in 4-2(c). Figure 4-2: Set up for hydrogel studies 41

64 4.2.3 Preparation of CA and CA-NIPAAm Membrane at Lab Scale Preparation of polymer dope solution for CA Membranes To create a performance baseline for CA-NIPAAm membrane, 18% CA membrane was prepared using a polymer solution of 18% CA in NMP solvent. For scale up studies 20% CA polymer solution was prepared by dissolving CA in NMP solvent. Preparation of polymer dope solution for CA-NIPAAm Membranes A dope solution of 18% cellulose acetate (CA) and 2% NIPAAm was prepared using a crosslinker (MBAA) and an initiator (ammonium cerium (IV) nitrate (CAN)) (0.03 g per 10 ml NMP) in solvent (1-Methyl-2-pyrrolidinone (NMP)). Dope solution preparation was done by two different methods: Method: 1 In the initial preparation procedure, all the reactants were added at the same time to the solvent NMP. Membranes were casted through a phase inversion process. Method: 2 First, CAN was added to NMP and kept for 10 minutes. NIPAAm and MBAA were then added to the solution, which was stirred and kept in nitrogen atmosphere for 20 minutes. CA was added to this solution and kept in an atmosbag maintaining nitrogen atmosphere for 24 hours. Polymerization was carried out in an atmosbag maintaining nitrogen atmosphere. 42

65 Membrane Fabrication by Phase Inversion Phase inversion technique is explained earlier in Chapter 2 Section One of the different techniques used for phase inversion is immersion precipitation. A multicomponent solvent system, which is composed of a solvent, polymer and a nonsolvent agent, is used for phase inversion method. A solvent is the component, which has strong interaction with the polymer. Non-solvent component posseses affinity with polymer and solvent molecules to the greater or a lesser degree, and can cause precipitation of the polymer from the solution [110]. In this technique, a dope solution of polymer and solvent is cast on a suitable support followed by immersion of the film in a non-solvent coagulation bath [48]. A dope solution of CA-NIPAAm-NMP was poured onto a glass plate as shown in Figure 4-5(a) and spread using a doctor s blade to ensure even spreading and thickness of the film in the range of µm as shown in Figure 4-5(b) and 4-5(c). In this study, NMP was the solvent and DI water was the non-solvent agent. The glass plate was immediately immersed in DI water bath as shown in Figure 4-5(d), where exchange between solvent and non-solvent took place. Once the membrane separated from the glass plate, it was transferred to another container of fresh DI water to remove excess solvent. Membranes were soaked in this container at least for 24 hours prior to use. PET substrate was also used for membrane fabrication following similar procedure as glass substrate. Membranes were characterized to confirm the chemistry and hypothesized reactions. 43

66 Figure 4-3: Membrane casting steps (a) pouring dope solution of glass plate (b) making a polymer film using doctor s blade (c) polymer film (d) membrane after phase inversion Membrane preparation for scale up studies performed at Georgia Institute of Technology Membranes were prepared using slot die extrusion. Polyethylene terephthalate (PET) and glass substrates were used for membrane fabrication. Details of slot die extrusion process are presented in Chapter Flux Measurements As defined by EPA s membrane filtration guidance manual [16], flux is throughput of pressure driven membrane filtration system measured as flow per unit of membrane area. Dead-end filtration mode was used in filtration experiments for this study. Filtration experiments were performed using an Amicon filtration cell (Amicon 44

67 Stirred cell ml). Figure 4-6 is a representation of dead-end mode filtration cell. Membrane samples were cut in circular pieces of 4.1 cm 2 to fit into the slot of the stirred cell. The membrane was supported by Whatman TM Filter paper (4, 125 mmø) to prevent cracking under the applied pressure. Constant stirring was maintained during filtration using immersible magnetic stir plate to eliminate concentration polarization effect. Pressure was maintained at 2.76 bars (40 psi) for all the flux experiments using a pressure cap. For a constant membrane area, the time taken to filter a constant volume of feed solution was measured, and flux was calculated as: flux( j) = volume v membrane area a ( ) ( ) time( t) Flux was reported in the units of hr. m 2 and plotted against actual time of filtration period. Figure 4-4: Dead-end filtration 45

68 Filtration experiments were performed at cold (20 C) and hot (45 C) conditions for both CA membranes and CA-NIPAAm membranes. The entire membrane housing was immersed in a water bath to maintain the required filtration temperature. It is important to note that the viscosity of water is temperature dependent and hence flux values were adjusted to account for the viscosity of water as: j 20 C = flux adjusted to reference temperature 20 C j actual = flux recorded at 45 C μ 20 C = viscosity of water at 20 C μ 45 C = viscosity of water at 45 C j 20 C = j actual m20 C m 45 C Contact Angle Measurements Contact angle is defined as the measure of wettability of a surface [18]. Cam-Plus Micro contact angle meter (Tantec Inc., Schaumburg, IL) was used for the contact angle measurement of all the membrane samples in this study. A small drop of water was placed on the membrane surface and resultant angle of the droplet to the surface was measured as shown in Figure 4-7. The higher the hydrophobicity of the membrane, the higher the contact angle is [115]. 46

69 Figure 4-5: Contact angle measurement [116] Pore Size Determination Latex beads of various sizes were filtered through the membranes to determine the pore sizes. Latex bead solutions were prepared by diluting 1 ml of latex bead sample in 100 ml of DI water. After filtration, permeate samples were collected, and the concentrations of beads in feed and permeate were analyzed using Dynamic Light Scattering (DSL), which is used to measure the amount of particulate solute in the solution. Zeta Sizer Nanoseries (Nano-ZS) (Malvern, UK) was used to calculate the concentration of beads in the feed and permeate. Following latex beads filtration, dyes were filtered through the membranes and permeates samples were collected. Dye solutions were prepared in the concentration of 10 ppm in DI water. Different membrane samples from the same sheet of membrane were used for different dye solution feeds. Varian Cary 50 bio UV-Visible Spectrometer (Varian Inc. Palo Alto, USA) was used to measure the absorbance value of dyes present in the feed and permeates. The rejection of the solute was calculated using the difference between the absorbance values of the feed 47

70 and permeate. Rejection of the solutes was used to determine the mean pore size of the membrane. Pore size was taken as the molecular size of the solute which was 90% retained by the membrane [117] Solute Rejection Protein solutions were used as feed during filtration studies. BSA and lipase proteins were used at a concentration of 1g/L in DI water. After filtration, permeate samples were collected in the sterilized vials for further analysis. Rejection of proteins by the membranes was studied by determining the difference between protein concentrations in the feed and permeates samples. UV-visible spectroscopy was used to calculate the concentration of the solutes. Varian Cary 50 bio UV-Visible Spectrometer (Varian Inc. Palo Alto, USA) was used to measure the absorbance value of protein present in the feed and permeates. For all feed solutions, a calibration curve was plotted using the known concentrations of the solute. It was followed by recording the absorbance values of permeates and then calculating the concentration of the solute from the linear equation of the calibration plot. The rejection of the solute was calculated using the difference between the absorbance values of the feed and permeate. All the samples were individually filtered through different membrane sample using dead end filter cell at the pressure 2.76 bar. Filtration with each of the solute was repeated three times to calculate the average. The apparent solute rejection (R) was calculated as the following equation: 48

71 where, C f and C p are concentration of solute in the feed and permeate solutions, respectively. 49

72 Chapter 5 Membrane Dope Development 5.1 Introduction In membrane technology, design of the membrane structure is considered an important task in order to achieve the desired membrane performance [118]. Membrane structure and performance depend on various parameters, such as choice of polymer, polymer concentration, composition of dope solution, evaporation time during fabrication process, and operating conditions [118]. Dope composition and polymer concentration are considered as the most important parameters for tailoring membrane properties [119, 120]. Higher polymer concentrations lead to an increase in chain entanglement, and as a result, microvoid formation in the layers is reduced [119]. As a secondary result, the separation capability of the membrane increases whereas permeability decreases. Hence, it is important to choose the suitable concentration of polymer in order to fabricate membranes for desired application. Dope composition is one of the main factors which influences viscosity of the dope, membrane morphology and pore size of the membrane [120]. As discussed in Chapter 4, the objective of this study was to develop a responsive membrane in order to control natural organic matter (NOM) fouling. Ultrafiltration (UF) 50

73 membranes are widely used for the treatment of surface water and wastewater. NOM is the major foulant of UF membranes during water and wastewater treatment. Therefore, the overarching goal of present study was to develop a temperature responsive UF membrane to control NOM fouling. To achieve this, the first steps involved an investigation of polymer concentration in the dope, membrane thickness and subsequently, the chemistry of NIPAAm addition. For the polymer dope concentration, different cellulose acetate (CA) dope solutions containing different amounts of CA polymer in the dope were tested to determine the suitable concentration of CA to fall in the ultrafiltration (UF) range. The second parameter, membrane thickness affects membrane permeability. Higher membrane thickness results in lower flux profiles due to a higher resistance offered by membrane to the flow [121]. Hence, CA membranes of different thickness were tested for their performance in this study. In order to develop a temperature responsive membrane, NIPAAm polymer was used as the temperature stimulated component in the dope solution. The amount of NIPAAm in the membrane matrix has the potential to influence the temperature activation. High amounts of polymer in a dope solution can affect the membrane structure by narrowing the micro-channels and causing denser matrices [44]. Zhou et al. [122] grafted NIPAAm on zirconium oxide membranes and studied the temperature activation of the membranes at different concentrations of NIPAAm on the membrane surface. They concluded that at increasing amounts of NIPAAm, the thermo-responsive behavior of the membranes gradually decreased due to the narrowing of the pores. Therefore, in the study presented here, it was important to investigate the effect of different amounts of NIPAAm on the activation. Different membranes were prepared with varying amounts of NIPAAm 51

74 in the matrix, and their performance was studied at alternating temperature cycles to determine the suitable amount of NIPAAm to achieve optimum temperature activation. During dope solution preparations, after the addition of NIPAAm-MBAA to NMP solvent, a specific amount of time (reaction time) was allowed before the addition of cellulose acetate. Effect of different reaction times was studied. Since NIPAAm acquire its swelling property through a crosslinked network-like structure, the presence of a crosslinker has an important role in the swelling behavior of NIPAAm [123]. A crosslinker also ensures the presence of NIPAAm in the membrane and prevents NIPAAm from dissolving in the aqueous medium [114]. Therefore, membrane temperature activation in the absence of crosslinker was studied to determine if the NIPAAm was crosslinking. Lastly, ammonium cerium (IV) nitrate (CAN) was added to the dope solutions as polymerization reaction initiator. To study the importance of the initiator, dope solutions were prepared with and without CAN. 5.2 Polymer Dope Solutions Determination of optimal cellulose acetate percentage in dope solution: Different polymer dope solutions with different amounts of cellulose acetate (CA) were prepared in 1-Methyl-2-pyrrolidinone (NMP) solvent as 15% (wt%), 16% (wt%) 18% (wt%), and 20% (wt %) CA with respect to NMP. CA was dissolved in NMP, and the solution was kept stirring using a stir plate and magnetic stirrer for 24 hours. After 24 hours CA in all solutions had completely dissolved in NMP. Membranes were fabricated by the phase inversion process as explained in Chapter 4, Section

75 5.2.2 Preparation of NIPAAm Membrane: Casting Method The chosen CA percentage was 18%, so a dope solution of 18% CA and 2% NIPAAm was prepared using a crosslinker MBAA and an initiator ammonium cerium (IV) nitrate (CAN) in solvent NMP. Dope solution preparation was performed using two different methods. For both methods, membranes were fabricated by phase inversion process as explained in Chapter 4 Section Method: 1 In the initial preparation procedure, all the reactants were added at the same time to the solvent NMP and stirred. The solution was kept in the nitrogen atmosphere for 24 hours until the reactants were completely dissolved. Method: 2 First, CAN was added to NMP and kept for 10 minutes. NIPAAm and MBAA were then added to the solution, which was stirred and kept in a nitrogen atmosphere for 20 minutes. CA was added to this solution and kept in an atmosbag maintaining nitrogen atmosphere for 24 hours Determine Amount of NIPAAm in the Polymer To determine the required amount of NIPAAm to fabricate a temperature responsive membrane, three different dope solutions with solvent NMP were prepared with different amounts of NIPAAm as 1. 18% CA + 2 % NIPAAm 2. 18% CA + 4% NIPAAm 53

76 3. 18% CA + 8% NIPAAm The crosslinker amount in all of the dope solution was maintained as 2% of total NIPAAm used in the dope. The initiator (CAN) was used for polymerization. Dope solutions were prepared by following the procedure as mentioned previously. Membranes were fabricated by phase inversion process as explained in Chapter 4 Section These membranes were characterized to determine the required amount of NIPAAm in the membrane for temperature activation Studying effect of components of the dope on temperature activation Three different dope solutions were prepared as: 1. 18% CA + 2% NIPAAm without crossliner MBAA 2. 18% CA + 2% NIPAAm without initiator CAN 3. 18% CA with MBAA and CAN and without NIPAAm Membranes were again fabricated by phase inversion, and were characterized to study the effects on temperature activation in the absence of one of the components in the membrane Studying the effect of reaction time Six different samples were prepared by adding CAN, NIPAAm and MBAA to NMP solvent, and nitrogen atmosphere was maintained for different time periods: 10, 20, 30, 40, 50, and 60 min. CA was added to all the samples to prepare 18% CA+2% 54

77 NIPAAm dope solutions. Membranes were fabricated by the phase inversion process as explained in Chapter 4 Section Scanning Electron Microscopy (SEM) SEM imaging offers the most direct method to view the membrane at micron or even submicron scale [124, 125]. A Hitachi S-4800 High Resolution SEM (Dallas, Texas) was used to study membrane cross section. To study the cross section of the membrane, a modified direct freeze fracture method was used, where all the membrane samples were frozen in liquid nitrogen for 5 minutes to make them brittle [126]. After 5 minutes the samples were quickly broken and were coated with gold nanoparticles for electron imaging and to prevent charging prior to SEM analysis. Cressington 108auto sputter coater (England, UK) was used for 30 seconds to create a coating of 15nm Fourier Transform Infrared (FTIR) spectroscopy Infrared spectroscopy is used to study the spectral and structural information associated with respective molecular vibrations. This in turn gives the information regarding the sample chemistry. FTIR was used to determine the chemistry of the proposed reactions. It was hypothesized that NIPAAm was homopolymerized and crosslinked in the dope solution. FTIR was used in ATR mode to study the chemical nature of the membranes. FTIR range was 4000 to 400 cm -1, resolution was 4 cm -1 with sensitivity as 1.5. Digilab UMA 600 FT-IT microscope with Germanium (Ge) crystal was used with Micro ATR mode in atmospheric conditions for all the analysis of membrane samples done in this study. 55

78 5.2.5 Filtration Studies Filtration experiments were performed at two pressure conditions, 1.38 bar (20 psi) and 2.76 bar (40 psi), during the studies of CA percentages in the dope. Deionized (DI) water was used as feed. An Amicon filtration cell (Amicon Stirred cell ml) was used for filtration experiments. The membranes were cut in circular pieces of area 4.1 cm 2 to fit into the slot of stirred cell. The membrane was supported by Whatman TM Filter paper (4, 125 mm ø) to prevent cracking under pressure. During temperature activation studies, filtration experiments were performed at alternating temperature conditions as cold (20 C) and hot (45 C) cycles for both CA membranes and CA-NIPAAm membranes. The pressure was kept constant at 2.76 bar (40 psi) for all the flux experiments. In detail, the stirred cell filtration apparatus was placed in a cold water bath until the feed water temperature decreased to 20 C, at which point flux was measured. The stirred cell was then placed in a hot water bath until the feed water reached 45 C, when again flux was measured. It is important to note that the viscosity of water changes at different temperature and hence flux values were adjusted to account for the viscosity of water. Every membrane was precompacted using DI water prior to filtration. Precompaction was followed by the filtration of 1g/L BSA protein solution. 5.3 Result and Discussion Determination of cellulose acetate percentage Figure 5-1 shows flux profiles of different membrane samples with varying thicknesses. Operating pressures for UF membrane are commonly in the range of 2-7 bar 56

79 average flux, L/m 2 hr ( psi) [ ]. Thicknesses in the range of μm are common for UF membranes [127]. Therefore, these were the targets of this investigation. Here, filtration experiments were carried out at 2.76 bar (40 psi) for all membrane samples with thicknesses in the range of μm % CA 16% CA 18% CA 20% CA membrane thickness, μm Figure 5-1: Flux profiles at pressure 2.76 bars (40 psi) of membranes for different CA concentration and different thickness In the case of 15% CA membrane, membrane samples of 120 μm thickness broke during the operation. This suggested that at the lower concentrations of the CA polymer, membranes with desired thicknesses might not be able to withstand the operational pressure. Flux values recorded while using samples of thickness 130 μm and 200 μm were in the MF range. Hence, 15% CA was deemed not suitable for UF operation. Similarly, in case of 16% CA, both the membranes samples of thickness 155 μm and 200 μm displayed high flux values in MF range. Therefore, 16% CA also was not a suitable 57

80 composition. Overall, it was observed that 15% CA and 16% CA were not suitable for UF operation due to either their failure to sustain the operational pressure or resulted into high flux values. On the other hand, 20% CA membranes did not break under the operating pressure; however, they offered the lowest flux values among all the membrane samples. In case of membrane samples of the thicknesses μm and μm, flux values were in the UF range. During the filtration using 18% CA membranes, all the membrane samples with varying thicknesses sustained the target operating pressure. Furthermore, flux values for the 18% CA membranes were in the UF range. The aim of this study was to develop a responsive UF membrane using a dope composed of NIPAAm and CA. Hence, taking into account the presence of NIPAAm in the dope, concentration of CA was maintained such that it could not exceed 20%. For all the experiments, 18% (wt%) CA concentration was selected to develop CA-NIPAAm membranes and 2.76 bar (40 psi) pressure was chosen as filtration pressure Preparation of NIPAAm membrane Casting Method: Two methods of casting the CA-NIPAAm membranes were tested, and involved blending all components together at the same time (Method 1) versus separately (Method 2). CA-NIPAAm membranes (Figure 5-2(a)) obtained by Method 1 showed a different texture than CA membranes (Figure 5-2(b)). Membranes fabricated using Method 1 were analyzed using FTIR, but were not found to show any peaks corresponding to the NIPAAm polymerization and crosslinking taking place (Figure 5-3). This suggested that 58

81 absorbance (a.u) the proposed reactions were not taking place. It is believed that homopolymerization and crosslinking may have faced some hindrance due to bulky groups and the high viscousity of the CA solution. CA-NIPAAm membranes cast by Method 2 did not show any texture and were visually uniform, as seen in Figure 5-2(c). (a) (b) (c) Figure 5-2: (a) CA membrane, (b) CA-NIPAAm membrane; Method 1, (c) CA-NIPAAm membrane; Method 2 18% CA+2% NIPAAm 18% CA wavenumber (cm -1 ) Figure 5-3: FTIR analysis of CA and CA-NIPAAm membranes prepared by Method 1 In order to study presence of crosslinking and homopolymerization of NIPAAm in membrane, it was important to get FTIR data for a crosslinked NIPAAm hydrogel (Figure 5-4), which later will be used as reference for FTIR of CA-NIPAAm membranes. Assignments of major peaks for polynipaam during homopolymerization of NIPAAm 59

82 absorbance (a.u.) are listed in Table 5.1. Cui et al [130] studied the major peaks for NIPAAm-MBAA crosslinking and these are listed in Table wavenumber (cm -1 ) Figure 5-4: FTIR analysis of NIPAAm crosslinked hydrogel Table 5.1: PNIPAAm peaks [131] Wavenumber (cm -1 ) Bond assignment secondary amide N-H stretch CH 3 asymmetric stretching 1650 secondary amide C=O stretching also called as amide I bond 1550 secondary amide C=O stretching called as amide II bond Table 5.2: NIPAAm-MBAA crosslinking peaks [130] Wavenumber (cm -1 ) Bond assignment 3292 secondary amide N-H stretch CH 3 asymmetric stretching 1645 secondary amide C=O stretching also called as amide I bond 1552 secondary amide C=O stretching called as amide II bond 60

83 absorbance (a.u.) Membranes obtained using Method 2 were also subjected to characterization with FTIR, as shown in Figure 5-5. Due to the similar structures of NIPAAm and CA, some overlapping peaks were observed in FTIR of CA-NIPAAm and CA membranes. Therefore, the FTIR of CA-NIPAAm membrane was compared with the FTIR of NIPAAm crosslinked hydrogels (Figure 5-4). The FTIR of CA-NIPAAm membrane showed peaks at 1120 cm -1 for C-N, 1552 cm -1 for secondary amide, C=O stretching called as amide II bond, 1645 cm -1 for secondary amide C=O stretching know as amide I, and 3367 cm -1 for secondary amide N-H stretch, which are responsible for the crosslinking and homopolymerization of NIPAAm. This confirmed the crosslinking and homopolymerization of NIPAAm in CA-NIPAAm membranes % CA 18% CA+ 2% NIPAAm wavenumber (cm -1 ) Figure 5-5: FTIR analysis for CA and CA-NIPAAm membranes prepared by Method 2 61

84 5.3.3 Determine Amount of NIPAAm in the Polymer SEM of cross sections of 3 membrane samples SEM imaging of the cross section of the membranes with varying NIPAAm concentrations were analyzed, as shown in Figure 5-6. More flow channels were observed in the 18%CA+2%NIPAAm membrane structure (Figure 5-6a(ii)) as compared to the 18% CA (Figure 5-6a(i)) and 18%CA+4%NIPAAm (Figure 5-6a(iii)) membrane structures. As shown in Figure 5-6(b), as the NIPAAm amount in the membrane increased, the network-like texture of the cross section appeared to decrease. Membrane with polymer composition of 18%CA+2%NIPAAm (Figure 5-6(b)) appeared to have more micro-channels as compared to 18%CA+4%NIPAAm and 18%CA+8%NIPAAm. As the amount of NIPAAm increased in the dope, the total polymer content was increased. It is believed that increased polymer amount along with dense nature of CA influenced the spongy-network structure of the membrane and caused narrowing of the pores. (i) (ii) (iii) Figure 5-6(a): SEM of cross section of (i) 18% CA, (ii) 18% CA+2% NIPAAm and (iii) 18% CA+4% NIPAAm membranes 62

85 (i) (ii) (iii) Figure 5-6(b): SEM of cross sections of (i) 18% CA+2% NIPAAm, (ii) 18% CA+4% NIPAAm and (iii) 18%CA+8%NIPAAm membranes Flux studies: Filtration studies were performed at alternating temperature cycles under a constant pressure of 2.76 bar (40 psi). In the case of 18% CA+2% NIPAAm membranes, lower average flux values were observed during temperature cycles, as shown in Figure 5-7. However, this membrane composition showed a more significant temperature activation as compared to 18% CA+4% NIPAAm and 18% CA+8% NIPAAm membranes. Average flux profile of 18% CA+2% NIPAAm membrane showed a more significant initial flux decline than the 18% CA+4% NIPAAm membrane and 18% CA+8% NIPAAm membrane. From Figure 5-6(b), the increased polymer percentage resulted in denser matrices narrowing the network structure, and hence temperature activation was decreased. On the other hand, 18% CA+2% NIPAAm membranes showed more channeling, believed to offer effective collapse and expansion, which in turn represented higher temperature activation. Actual data for Figure 5-7 is presented in Appendix A, Tables A.1 and A.2. 63

86 normalized flux % CA + 2% NIPAAm 18% CA + 4% NIPAAm 18% CA + 8% NIPAAm time, hr Figure 5-7: Temperature activation of membranes The compact and denser structure was believed to hinder the expansion and collapse of NIPAAm crosslinked structure within the membrane matrix, which in turn would have the potential to affect activation of 18% CA+4% NIPAAm and 18% CA+8% NIPAAm membranes. These channels along with proposed crosslinking of NIPAAm are believed to offer higher initial flux values in case of 18% CA+2% NIPAAm membranes. Hence, a dope solution of 18% CA+2% NIPAAm composition was selected to develop temperature responsive membranes Studying effect of components of the dope on temperature activation Figure 5-8 shows effect of the presense of different dope components on the responsive behavior of the membrane. Temperature activations in case of no MBAA and no CAN were lower than the regular membrane (i.e. membrane with NIPAAm, MBAA 64

87 normalized flux and CAN). In case of the membranes without initiator CAN, temperature activation was lower than the regular membrane, and it is believed that the absense of CAN could have affected the extent of homopolymerization and crosslinking. In case of the membranes without MBAA, temperature activation was again lower than the regular membrane, and this could be because of the absence of crosslinking. As stated earlier, crosslinking facilitate the extent of swelling, and crosslinking also ensures the presence of NIPAAm in the membrane without dissolving it in the aqueous medium. Temperature activation of the membrane without NIPAAm was the lowest, which was expected because without the temperature responsive element, the membrane did not exhibit the thermo-responsive behavior. Actual data for Figure 5-8 is presented in Appendix A, Table A no CAN no MBAA 18%CA+2%NIPAAm with CAN,MBAA no NIPAAm time, hr Figure 5-8: Effect of components of the dope on temperature activation 65

88 5.3.5 Effect of Reaction Time Figure 5-9 shows FTIR analysis for six samples prepared by allowing different reation times, from 10 to 60 min. FTIR spectra of membrane samples with reaction times of 20, 30, and 40 min showed peaks responsible for homopolmerization and crosslinking of NIPAAm, while samples with reaction time 10, 50, and 60 min showed similar peaks, however with significantly low absorbance value. Filtration studies were done at alternating temperature conditions with all six membranes and their flux profiles are shown in Figure Flux profiles for all six membrane samples showed no sigificant differences. Therfore, it was concluded that allowing different reaction times did not significantly affect the properties of dope solution. Actual data for Figure 5-10 is presented in Appendix A, Table A.4. Figure 5-9: FTIR analysis of membranes with different reaction time 66

89 normalized flux min 20 min 30 min 40 min 50 min 60 min time, hr Figure 5-10: Normalized flux profiles of membranes with different reaction time 5.4 Conclusion 18% (wt%) CA amount was selected for membrane fabrication since with this concentration, membranes operated in the desirable UF pressure range and flux profile. At this concentration, membranes were able to stand a pressure of 40 psi without breaking. NIPAAm amout was selected as 2% (wt%) since dope compositions of 18% CA+2% NIPAAm showed singificant temperature activation during filtration studies, as compared to higher NIPAAm percentages. The presence of a crosslinker MBAA and a polymerization initiator CAN were found to be important for temperature activation. The crosslinker helped facililate the crosslinked spongy network, and hence the significant activation during temeprature cycles. 67

90 Chapter 6 Fouling Control Using Temperature Responsive N- isopropylacrylamide (NIPAAm) Membranes 6.1 Introduction Fouling occurs due to specific intermolecular interactions between macrosolutes present in the feed water [11] and the membrane surface, and can happen even in the absence of filtration. The most common techniques to prevent fouling are to pre-treat the water to remove potential foulants and to clean the membranes during operation. Conventional methods of pre-treatment, such as disinfection and particle removal, involve intensive chemical treatments [4]. These treatments may also introduce ammonia and chlorine compounds, coagulants, corrosion and scaling-controlling chemicals to the feed water. Cleaning of membranes to remove accumulated foulants from the surface is done by physical processes, such as backflushing and cross flushing [13], and chemical cleaning using alkaline solutions, surfactants, chelating agents, citric acid, and salt solutions [14, 15]. Chemical pretreatment to remove fouling can be effective but must meet safety guidelines with respect to chemicals used. When foulants, such as organic or inorganic particles, colloids and microorganisms, cannot be removed by physical methods of cross-flow or backflushing, it results in permanent decline of flux and shortened membrane lifespan [14, 17-19]. 68

91 In aqueous media, membranes exhibit attractive or repulsive behavior towards water [45]. Membranes can be hydrophilic or hydrophobic depending on their composition or their surface chemistry. Hydrophilic membranes, such as cellulose acetate and polysulfone, are preferred for aqueous filtration since they are less susceptible to fouling [46]. Water molecules in order to access passage through the membrane will push other compounds; therefore, hydrophilic membranes give high flux values [11]. Maximous et al. [63] observed that hydrophilic membranes were better able to resist cake formation as compared to hydrophobic membranes, but this did not make hydrophilicity more advantageous with respect to overall fouling tendency. Cellulose acetate has a high hydrophilicity due to the presence of three active hydroxyl groups in each repeating unit, which in turn prevents its dissolution in ordinary solvents [41, 48]. However, according to Richards and Iwuoha [48], in case of cellulose acetate membranes, cellulose was degraded during the regeneration process and resulted in the loss of the membrane s ability to withstand strong acids, alkali and organic solvents. On the other hand, hydrophobic membranes are susceptible to fouling by hydrophobic materials present in the feed water [4, 11, 18]. Hydrophobic matter present in water tend to aggregate, to form clusters to lower their free energy [47], and to attach to the hydrophobic surface of membrane resulting in fouling. However, hydrophobic membranes possess advantages of robustness, and chemical and mechanical stability [48], and these properties are desirable. Many methods have been studied to prevent or decrease fouling by changing the membrane surface chemistry [20, 21]. Since hydrophobic membranes are more chemically and mechanically stable while hydrophilic membranes are less prone to fouling, many of these methods include increasing hydrophilicity of hydrophobic 69

92 membrane surfaces by physical adsorption or grafting of hydrophilic agents on the surface [20]. There are three major processes to incorporate different polymer films on membranes [42, 79], which are 1. incorporating a desired polymer or copolymer in the membrane dope and casting the membranes by phase inversion 2. in situ polymerization to prepare membrane, and 3. post-synthesis surface modification of existing membrane. Much research has been performed to develop fouling resistant membranes through postsynthesis modifications, during which the surface of an existing membrane is modified [74]. Modification has been done using several methods, such as ultraviolet [80] and ion beam irradiation [42], plasma treatment and grafting of a polymer film on a membrane surface [20, 80, 81]. A commonly used grafted polymer is polyethylene glycol (PEG). PEG is well known for its ability to disrupt the interactions between membrane and natural organic matter (NOM) or proteins, which can foul the membrane [20, 81]. In the case of cellulose acetate membranes, to improve their stability during protein interactions, membrane surfaces have been grafted with PEG [20]. The PEG grafted CA membranes showed significant prevention of fouling caused by NOM and lower flux decline during filtration. Gorey et al. [21, 23] grafted a responsive polymer film using polymer N-isopropylacrylamide (NIPAAm) on an existing membrane surface. They grafted this polymer film in order to make a fouling resistant stimuli responsive layer; however, the grafted layers entangled during activation to minimize the response. Therefore, long NIPAAm chains on the membrane surface were detrimental to operational characteristics, and the current study proposes that having NIPAAm embedded in the body of the membrane, as a part of the dope polymer solution, has the potential to lead to improved responsive membranes. 70

93 According to numerous studies, natural organic matter (NOM), which is a major contributor to fouling in water separations applications [17-19, 25, 37, 132], is composed of variety of organic components, such as humic and fulvic acids, and contains both hydrophilic and hydrophobic portions [19]. Hence, having either hydrophilic or hydrophobic static membranes would eventually lead to fouling and membrane degradation. Therefore, a dynamic membrane able to alternate between being hydrophobic and hydrophilic is hypothesized to decrease fouling. Stimuli-sensitive polymers have been gaining attention in recent years due to the unique property that the polymer changes its conformation from a coiled and less hydrophilic state to an uncoiled (or globular) and more hydrophilic state in the presence of a stimulus [21, 85]. By alternating between the comformations, the membrane surface might be made dynamic. Stimuli responsive polymers undergo reversible change in their conformation due to the change in different parameters of the surrounding environment; i.e., the stimulus. The stimulus can be ph, temperature, ionic strength or electric or magnetic fields [87]. Polymers like poly(n-isopropylacryamide), poly(n,ndiethylacrylamide) change their conformation in the presence of temperature changes, while polymers such as poly(acrylic acid), poly(methacrylic acid) response to the ph changes, and for polymers like poly(styrene sulfonate), ionic strength is a major stimulus [79]. N-isopropylacrylamide (NIPAAm) is responsive towards temperature change in the surrounding environment. NIPAAm is a stimuli-responsive polymer that changes its conformation in response to a temperature change, and it has a lower critical solution temperature (LCST) [90] of 34 o C, which makes it interesting since it is close to human 71

94 body temperature. Hence, NIPAAm has applications in biointerfaces and sensors [85], and it serves as an important tool in biomedical applications, such as drug delivery systems [85, 87, 133]. Pelton [134] stated that polynipaam chains have both hydrophobic and hydrophilic groups below and above LCST and temperature dependent behavior is a result of rearrangement of NIPAAm chains due to interaction with opposing phase. Below the LCST, polymer-water interactions are predominant. Water acts like a good solvent and results in extensive hydrogen bonding between the polymer and water [23, 91]. At this point NIPAAm polymer chains are in expanded or linear conformation and it is in a more hydrophilic state. As the temperature is increased above LCST, the polymer interchain collapse leads to the coiled conformation and NIPAAm is in a less hydrophilic state [92]. The purpose of this study was to cast stimuli responsive membranes to control fouling made of cellulose acetate (CA) and NIPAAm. 6.2 Materials and Methods Materials NIPAAm (97%, Sigma Aldrich) was purified to >99% by crystallization using hexanes followed by vacuum drying prior to use. Cellulose acetate was purchased from Sigma Aldrich (average M n ~ 30,000) and used as received. Methylene bisacrylamide (Polysciences Inc.), ammonium cerium (IV) nitrate (Aldrich), sodium metabisulfate (Sigma Aldrich), and ammonium persulfate (Sigma) were used as received. Other chemicals included bovine albumin serum (BSA) 98% (Sigma Aldrich), lipase (Sigma Aldrich), humic acid (Acros), NaCl (Fisher Scientific), CaCl 2 (Fisher Scientific), and NaHCO 3 (Fisher Scientific). Polymerization reaction was carried out in a nitrogen 72

95 atmosphere and Aldrich Atmosbag (Sigma Aldrich Atmosbag Z EA) was used for that purpose. Membrane pore sizes were determined by the rejection of latex beads and dyes. Latex beads of sizes 0.1 m, 0.05 m, and 0.03 m were purchased from Sigma Aldrich and diluted in DI water. Brilliant Blue R (pure, ACROS Organics), and Methyl Orange (Certified ACS) dyes were purchased from Fisher Scientific and diluted in DI water to make 10 ppm solution Preparation Methods Hydrogel preparation In order to cast a dynamic NIPAAm membrane, characterization studies had to first be performed using NIPAAm hydrogels. Hydrogels were prepared using the monomer (NIPAAm), a crosslinking agent (N,N-methylenebisacrylamide (MBAA)), initiators (ammonium persulfate and sodium metabisulfate), and a solvent (deionized (DI) water). The solution was flushed with nitrogen, and the reaction was carried out at room temperature in an atmosbag maintaining nitrogen atmosphere. Once ready, the hydrogel was removed and washed thoroughly with DI water prior to use [114]. The NIPAAm hydrogel responses as a function of time, ionic strength and temperature were studied Preparation of NIPAAm membrane A dope solution of 18% cellulose acetate (CA) and 2% NIPAAm was prepared using a crosslinker (MBAA) and an initiator (ammonium cerium (IV) nitrate (CAN)) in solvent (1-Methyl-2-pyrrolidinone (NMP)). The crosslinker amount was maintained as 73

96 2% (w/w) of NIPAAm, and 0.3 g of CAN were used per 10 ml of NMP. To prepare the dope solution, CAN was added to NMP and kept for 10 minutes. NIPAAm and MBAA were then added to the solution, which was stirred and kept in a nitrogen atmosphere for 20 min. CA was added to this solution and kept in an atmosbag maintaining nitrogen atmosphere for 24 hours. Polymerization was carried out in an atmosbag maintaining nitrogen atmosphere. Two reactions are speculated to occur during the polymerization process along with polymerization of cellulose acetate: crosslinking between NIPAAm and MBAA, and homopolymerization of NIPAAm, as shown in Figure 6-1. (a) (b) (c) Figure 6-1: Products of polymerized reactions in CA-NIPAAm dope solution preparation. (a) cellulose acetate polymerization; hypothesized products are (b) NIPAAm polymerization and (c) NIPAAm-MBAA crosslinking Membranes were casted using the phase inversion method [115]. In phase inversion, the polymer film is transformed from liquid to solid state [48]. One technique used for phase inversion is immersion precipitation. In this technique, a dope solution of polymer and solvent is cast on a suitable support followed by immersion of the film in a 74

97 non-solvent coagulation bath [48]. In this study, NMP was the solvent and DI water was the non-solvent agent. Then, the glass plate was immediately immersed in DI water bath at 20 C, where exchange between solvent and non-solvent took place. Once the membrane separated from the glass plate, it was transferred to another container of fresh DI water to remove excess solvent. Membranes were soaked in this container at least for 24 hours prior to use. Membranes were characterized to confirm the hypothesized reactions Characterization Fourier Transform Infrared (FTIR) spectroscopy Molecules are characterized by their vibrational spectrum and this is considered as a unique property of that molecule [135]. Infrared spectroscopy technique is used to study the spectral and structural information associated with respective molecular vibrations. This in turn gives the information regarding the chemistry of samples. FTIR was used to determine the chemistry of the proposed reactions. FTIR was used in ATR mode to study the chemical nature of the membranes. FTIR range was 4000 to 400 cm -1, resolution was 4 cm -1 with sensitivity as 1.5. Digilab UMA 600 FT-IT microscope with Germanium (Ge) crystal was used with Micro ATR mode in atmospheric conditions for all the analysis of membrane samples done in this study. 75

98 Hydrogel characterization as a function of temperature, time and ionic strength Function of temperature Hydrogels were placed in DI water, and the temperature was gradually increased from 20 C to 50 C. The weight of gel was frequently measured as the temperature was increased to determine the LCST, which is the point where the water content in the hydrogel changes drastically. The swelling ratio is the fractional decrease in the weight of the hydrogel with respect to initial weight of the completely swollen hydrogel sample. Function of time: After washing thoroughly, the hydrogel was cut into pieces, weighed and dried in a vacuum oven. The weight of the completely dried hydrogel was noted. The swelling behavior of NIPAAm hydrogels from a completely dried state to a completely swollen in aqueous medium was observed at room temperature at 20 C which is below LCST. Once a completely dried piece of hydrogel was put in DI water, it swelled by absorbing water, and hence its weight increased. Changes in swelling were then plotted against time. Effect of ionic strength Hydrogels were placed in different concentrations of NaCl and their LCST was recorded as explained earlier. The effect of increasing the concentration of NaCl in DI water on the extent of swelling of the hydrogel was studied in this experiment Pore Size Determination To determine membrane pore size, three different sized latex beads were filtered through the membranes. Latex beads solutions were prepared by diluting 1 ml of latex bead sample in 100 ml of DI water. After filtration, permeate samples were collected, 76

99 and the concentrations of beads in feed and permeate were analyzed using Dynamic Light Scattering (DSL), which is used to measure the amount of particulate solute in the solution. Zetasizer Nanoseries (Nano-ZS) (Malvern, UK) was used to calculate the concentration of beads in the feed and permeate. Following latex beads filtration, dyes were filtered through the membranes and permeates samples were collected. Dye solutions were prepared in the concentration of 10 ppm in DI water. Different membrane samples from the same sheet of membrane were used for different dye solution feeds. Varian Cary 50 bio UV-Visible Spectrometer (Varian Inc. Palo Alto, USA) was used to measure the absorbance value of dyes present in the feed and permeates. The rejection of the solute was calculated using the difference between the absorbance values of the feed and permeate. All the samples were individually filtered through different membrane sample using dead end filter cell at the pressure 2.76 bar. Filtration with each of the solute was repeated three times to calculate the average. Rejection of the solutes was used to determine the mean pore size of the membrane. The filtration solutes are shown in Table 6.1 along with their molecular sizes. Pore size was taken as the molecular size of the solute which was 90% retained by the membrane [117]. Table 6.1: Filtration solutes for pore size determination Solute Latex Beads Brilliant Blue R dye Methyl Orange dye Size (10-6 m)

100 Flux Flux is measured as flow rate of permeate per unit of membrane area. Dead-end filtration mode was used in filtration experiments for this study. Filtration experiments were performed using Amicon filtration cell (Amicon Stirred cell ml). Pressure was maintained as 2.76 bar (40 psi) for all the flux experiments. The membranes were cut in circular pieces of area 4.1cm 2 to fit into the slot of stirred cell. The membrane was supported by Whatman TM Filter paper (4, 125mmø) to prevent cracking under pressure. Every membrane was precompacted using DI water prior to filtration. The time to filter 2 ml of water through the membrane was measured as a function of time to reach a stable flux. For a constant membrane area, the time taken to filter a constant volume of feed solution was measured, and flux was calculated as: flux( j) = volume v membrane area a ( ) ( ) time( t) Flux was reported in the units of L/hr.m 2 and plotted against actual time of filtration period. Various protein and humic solutions were used as fouling media. Two types of BSA protein solutions were prepared i.e. anionic and cationic BSA solutions. BSA is found to have an isoelectric point of ph 4.7 [136]. Hence, BSA carries a net negative charge at above ph 4.7 and net positive charge below ph 4.7. Anionic BSA solution was prepared by dissolving BSA in distilled water and the measured ph was 6.8, while cationic BSA solution was prepared using acetic acid buffer at ph 3 and the measured ph was 3. Lipase solution was prepared with the concentration of 1g/L in DI water. To 78

101 prepare 1 L of humic solution, 4 mg humic acid was added to DI water. To this solution, 0.1 mm CaCl 2 was added as a divalent cation source, 0.1 mm NaHCO 3 was added as buffer system and 1 mm NaCl was added as the background electrolyte [60]. Filtration experiments were performed at alternating temperature conditions as cold (20 C) and hot (45 C) cycles for both CA membranes and CA-NIPAAm membranes. In detail, the stirred cell filtration apparatus was placed in a cold water bath until the feed water temperature decreased to 20 C, at which point flux was measured. The stirred cell was then placed in a hot water bath until the feed water reached 45 C, when again flux was measured. It is important to note that the viscosity of water changes at different temperature and hence flux values were adjusted to account for the viscosity of water Flux recovery Flux recovery is the amount of initial flux that can be recovered after cleaning of the membrane. Backwash, which is a type of physical cleaning method, was performed on the membranes. During backwash, direction of the feed is reversed such that accumulated foulants can be washed off. In present studies, the membrane sample after filtration was placed upside down and DI water was passed through the membrane for the duration of 60 min. After 60 min, membrane was turned and placed back to its original position and precompaction flux was recorded. This flux amount gives the value of recovered flux. This is the reversible portion of the fouling, which can be regained by backwashing. 79

102 Contact angle Contact angle is defined as the measure of wettability of a surface [18]. Cam-Plus Micro contact angle meter (Tantec Inc., Schaumburg, IL) was used for the contact angle measurement of all the membrane samples in this study. A small drop of water was placed on the membrane surface and resultant angle of the droplet to the surface was measured. Contact angle was measured on different locations of the membrane samples, which were held at 20 C and 45 C. The higher the hydrophobicity of the membrane, the higher the contact angle is [115] Scanning Electron Microscopy (SEM) SEM is an important tool to study the morphology of membrane surfaces, and it has been used in various studies to understand membrane formation, pore size and fouling [124]. SEM imaging offers the most direct method to view the membrane at micron or even smaller scale [124, 125]. A Hitachi S-4800 High Resolution SEM (Dallas, Texas) was used to study membrane fouling. Prior to SEM imaging, all the membrane samples were coated with gold nanoparticles for electron imaging and prevent charging. Cressington 108 auto sputter coater (England, UK) was used for 30 seconds to create a coating of 15 nm. To study the cross section of the membrane, a modified direct freeze fracture method was used, where all the membrane samples were frozen in liquid nitrogen for 10 min to make them brittle [126]. After 10 min the samples were quickly broken and were coated with gold nanoparticles for SEM analysis. 80

103 absorbance (a.u.) 6.3 Results and Discussion Hydrogel Characterization FTIR Characterization In order to study presence of crosslinking and homopolymerization of NIPAAm in membrane, it was important to get FTIR data of NIPAAm crosslinked hydrogel, which later will be used as reference for FTIR of CA-NIPAAm membranes wavenumber (cm -1 ) Figure 6-2: FTIR of NIPAAm crosslinked hydrogel Assignments of major peaks for polynipaam during homopolymerization of NIPAAm are listed in Table 6.2. Cui et al[130] studied the major peaks for NIPAAm- MBAA crosslinking and these are listed in Table

104 Table 6.2: PNIPAAm peaks Wavenumber (cm -1 ) Bond assignment secondary amide N-H stretch CH 3 asymmetric stretching 1650 secondary amide C=O stretching also called as amide I bond 1550 secondary amide C=O stretching called as amide II bond Table 6.3: NIPAAm-MBAA crosslinking peaks Wavenumber (cm -1 ) Bond assignment 3292 secondary amide N-H stretch CH 3 asymmetric stretching 1645 secondary amide C=O stretching also called as amide I bond 1552 secondary amide C=O stretching called as amide II bond FTIR showed strong peaks at 1120 cm -1 for C-N stretch [135] 1550 cm -1 for secondary amide C=O stretching, called as amide II bond, 1650 cm -1 for secondary amide C=O stretching know as amide I, and cm -1 for secondary amide N-H stretch, which are observed when polynipaam polymerization and NIPAAm-MBAA crosslinking takes place [130, 131, 135]. 82

105 Effect of Temperature: A swelling ratio is a comparison of weight of the hydrogel at particular temperature against the initial weight at 15 C, when the hydrogel is completely swollen. Figure 6-3 shows the change in the swelling ratio of three different hydrogel samples as a function of temperature. NIPAAm contains hydrophilic amide group and hydrophobic isopropyl group [137]. Below the LCST, due to extensive hydrogen bonding between amide group of NIPAAm and water molecules, the hydration shell around the NIPAAm chains remains stabilized giving it the swollen state. Here, water is a good solvent and polymer-solvent interactions are stronger than polymer-polymer interactions. When the temperature is raised above LCST, rapid dehydration of NIPAAm chains leads to destabilization of the hydration shell [138]. From Figure 6-3, a drastic decrease in the swelling ratio of hydrogel was observed at 34 C, representing a drastic change in the water content of the hydrogel. This agreed with literature values for the LCST of NIPAAm as 34 C [23, 87, 139]. Actual data for Figure 6-3 is represented in Appendix B, Tables B.1(a) and B.1(b). 83

106 swelling ratio temperature, C Figure 6-3: Change in the swelling ratio of gel as a function of temperature Figure 6-4(a) shows a hydrogel at 15 C in its completely swollen condition whereas Figure 6-4(b) the visual drastic decrease in the water content of collapsed hydrogel signifying the drastic decrease in hydrogel swelling ratio. 84

107 Figure 6-4: Hydrogel (a) at 15 C and (b) at LCST 34 C Function of time: Changes in swelling of hydrogel were then plotted against time, as shown in Figure 6-5. At time 0, a dried piece of hydrogel was introduced in water. As the time progressed polymer-water interactions were increased since below LCST water acts a good solvent. Once a piece of hydrogel was completely swollen, there was no change in the weight, and this weight matched with the initial weight of hydrogel before drying. This procedure was repeated in aqueous medium by alternating temperatures below and above LCST, and it confirmed that the swelling behavior was fully reversible and that repeated cycles of heating and cooling did not affect the swelling ratio of the hydrogels. Actual data for Figure 6-5 is represented in Appendix B, Table B.2. 85

108 weight, g time, min Figure 6-5: Change in the weight of gel as a function of time from completely dried till completely swollen state Effect of Ionic Strength Salt addition reduces the chemical potential of water and this process encourages dehydration of the polymer leading to a collapse of the hydrogel [87, 138]. Many studies have found that ionic strength can shift the NIPAAm LCST [138, ]. Anions are found to have greater effect on the hydration shell than cations [138, 142]. As shown in Figure 6-6, as the concentration of NaCl increased, the LCST of NIPAAm was shifted to lower temperatures. In 1 M NaCl solution, the LCST was dropped to at 24 C. Actual data for Figure 6-6 is represented in Appendix B, Table B.3. 86

109 concentration, mm temperature, C Figure 6-6: Effect of ionic strength: concentration of NaCl vs temperature plot Salts have an influence on the water structure around NIPAAm polymer by ordering and disordering it [138, 141], and the above observations can be explained by phenomenon named salting out effect. Burba et al. [138] used NMR spectroscopy to quantitatively measure the thermodynamic parameters of the hydration and dehydration processes of NIPAAm. Their studies show the influence of salts like NaCl and CaCl 2 on the phase transition behavior of NIPAAm in heating and cooling cycles. Their results concluded that dehydration of hydrogels above LCST occurs in a two-stage process, where formation of hydrophobic bonds takes place in stage I and stage II is related to dissociation energy of hydrogen bonds. Enthalpy and entropy of formation for stage I is reduced by the addition of salts. NMR studies done by Pastoor et al [140] also showed that the increase in the concentration of salts like NaCl strongly affected the thermodynamics of stage I by gradually decreasing the Gibb s free energy. 87

110 Infrared spectroscopy studies done by Paz et al. [141] show that below LCST, there was extensive hydrogen bonding between amide groups of polymer and water molecules, while above LCST, polymer intra bonding increased resulting in a collapsed state. They further concluded that the presence of salt ions reduced the barrier for interchain collapse more efficiently due to structural changes in the hydration shell surrounding the hydrophobic moieties. The resulting dehydration caused a decrease in the distance between polymer chains followed by increased interaction between hydrophobic moieties even at lower temperature. This phenomenon is known as the salting out effect Membrane Characterization Fourier Transform Infrared Spectroscopy (FTIR) Figure 6-7 shows the FTIR analysis for CA and CA-NIPAAm membranes. Due to the similar structures of NIPAAm and CA, some overlapping peaks were observed in FTIR of CA-NIPAAm and CA membranes at. Therefore, the FTIR of CA-NIPAAm membrane was compared with the FTIR of NIPAAm crosslinked hydrogels as shown in Figure 6-2. The FTIR of CA-NIPAAm membrane showed similar peaks as 1120 cm -1 for C-N, 1550 cm -1 for secondary amide, C=O stretching called as amide II bond, 1650 cm -1 for secondary amide C=O stretching know as amide I, and cm -1 for secondary amide N-H stretch, which are responsible for the crosslinking and homopolymerization of NIPAAm. The comparison confirms the crosslinking and homopolymerization of NIPAAm in CA-NIPAAm membranes. 88

111 absorbance (a.u.) % CA 18% CA+ 2% NIPAAm wavenumber (cm -1 ) Figure 6-7: FTIR analysis for CA and CA-NIPAAm membranes Pore Size Determination Rejection values of latex beads and dyes are showed in Table 6.4. CA-NIPAAm membrane showed 90% of the rejection of latex bead of size of 0.03 μm. Furthermore, CA-NIPAAm membrane pore size was smaller than the pore size of CA membrane, which was expected since the CA membranes were cast with 18% polymer while CA- NIPAAm membranes had 18% CA and 2% NIPAAm. Table 6.4: Rejection of different solutes by membranes Solute type Size (10-6 m) Rejection, % CA-NIPAAm CA Latex bead ± ± 1.7 Latex bead ± ±

112 Latex bead ± ± 3.8 Brilliant Blue R ± ± 0.4 Dye, Methyl ± ± 0.6 Orange Contact Angle of Membranes Table 6.5 shows the contact angle values of membranes below and above LCST temperature. Contact angle of CA-NIPAAm membrane at 20 C was significantly lower than contact angle at 45 C, which showed that below LCST temperature CA-NIPAAm membrane was more hydrophilic and above LCST temperature it was less hydrophilic. Table 6.5: Contact angle of CA-NIPAAm membrane below and above LCST CA-NIPAAm membrane CA membrane 20 C (below LCST) 47 ± 1 o 64 ± 2 o 45 C (above LCST) 56 ± 2 o 62 ± 1 o Filtration Studies Charge Analysis Membrane surface charge analysis was done by filtering anionic and cationic BSA protein solutions. Separate filtration analysis was done with CA-NIPAAm and CA membranes using both the solutions as shown in Figure 6-8(a) and Figure 6-8(b). 90

113 normalized flux normalized flux Elimelech et al. [41] studied cellulose acetate membrane surface charge in water within a ph range of 3 to 11 and different NaCl concentrations, and found that at ph = 7, CA membranes have a low negative charge tending towards neutral [41, 44, 45, 143] % CA anionic BSA cationic BSA time, hr Figure 6-8(a): Surface charge analysis of CA membranes 12 18% CA + 2% NIPAAm anionic BSA cationic BSA time, hr Figure 6-8(b): Surface charge analysis of CA-NIPAAm membranes 91

114 In order to verify membrane charge during filtration, anionic and cationic BSA solutions were filtered through the membranes. Since CA membranes tend to neutrality, it was hypothesized that the filtration of anionic and cationic BSA solutions through CA membranes would not show any significant flux differences, which was observed in Figure 6-8(a). If CA-NIPAAm membranes displayed a more charged surface, the two BSA solutions would interact more with the membrane surface due charge attraction and repulsion. One of the BSA solutions would then foul the membrane faster (charge attraction) and would show a flux decline, while the other BSA solution would show a more constant flux (i.e. little fouling due to charge repulsion). No significant differences in the flux profile were observed during the anionic and cationic BSA filtration experiments using CA-NIPAAm membranes (Figure 6-8(b)). Thus, it is believed that as with CA membranes, CA-NIPAAm membrane charge can be assumed to tend towards neutral Membrane Dynamic Behavior To confirm the dynamic nature of CA-NIPAAm membranes (i.e., the response to a temperature change), flux was measured at alternating temperature cycles below the LCST (20 C) and above the LCST (45 C) using first, deionized (DI) water as feed to create a baseline, and followed by BSA and lipase filtration. The alternating temperature cycles were obtained by placing the stirred cell filtration apparatus in cold or hot water baths until the feed water reached the desired temperature. Figure 6-9 shows the plot of temperature-adjusted flux versus time. CA-NIPAAm membrane precompaction with DI water was performed for approximately 8 hours. Precompaction was followed by 92

115 temperature adjusted flux, L/(m 2 hr) filtration with BSA and lipase solutions, each for approximately 8 hours. To compare the performance of CA-NIPAAm with CA membranes, similar filtration studies were performed on CA membranes with constant volume filtration. Actual data for Figure 6-9 is presented in Appendix A, Tables A.1 and A % CA+2% NIPAAm 18% CA Average flux: 18%CA+2%NIPAAm Average flux: 18% CA time, hr Figure 6-9: Temperature activation during protein filtration. The circles symbolize CA membrane filtration, while the diamonds symbolize CA-NIPAAm membrane filtration. For both membrane curves, flux values were taken at 20 C (below LCST) and 45 C (above LCST); hence, the zigzag nature. Average flux lines were drawn to show the overall average flux during filtration. Thickness of CA and CA-NIPAAm membranes were maintained at 150 m ± 3 m. Initial precompaction flux values for CA-NIPAAm membranes were higher as 93

116 compared to those for CA membranes. At the end of filtration experiments, as shown in Figure 6-10 heavier deposition of proteins was observed on CA membranes as compared to CA-NIPAAm membranes, which showed no visible deposition. It is proposed that the visible deposition of proteins on the CA membrane surface explained the lower average flux values and longer experiment duration for constant volume filtration. CA CA-NIPAAm Figure 6-10: CA-NIPAAm membrane and CA membrane after filtration experiment When the cross section of both the membranes were analyzed by SEM, as shown in Figure 6-11, a more sponge-like structure was observed in the CA-NIPAAm membrane as compared to the CA membrane. This more sponge-like structure is hypothesized to provide more flow pathways, and along with proposed crosslinking of NIPAAm are believed to offer higher initial flux values in case of CA-NIPAAm membranes. 94

117 CA-NIPAAm membrane CA membrane Figure 6-11: SEM of cross section of the membranes Average flux values for CA-NIPAAm membranes remained higher than for CA membranes; that is, time duration for constant volume filtration was shorter for CA- NIPAAm membranes than CA membranes. The hypothesized reasons for this behavior are the presence of more sponge-like structure of the CA-NIPAAm membranes (Figure 6-11), the increased hydrophilicity of the CA-NIPAAm membrane as compared to CA membranes (Table 6.5), and the prevention of cake formation [11, 18, 20, 32]. Figure 6-12 shows temperature adjusted flux profiles for both membranes during humic solution filtration alternating between 20 C and 45 C. The alternating temperature cycles were obtained by placing the stirred cell filtration apparatus in cold or hot water baths until the feed water reached the desired temperature. Average flux of CA-NIPAAm membrane remained higher than CA membrane during precompaction and humic solution filtration. As mentioned earlier, humic solution is a mixture of hydrophilic and hydrophobic components and by having a stagnant hydrophilic CA membrane surface, it significantly contributed to the fouling. On the other hand, the dynamic nature of CA- 95

118 NIPAAm membranes contributed towards the decrease in fouling, which it is believed to explain the higher average flux or shorter time duration for constant volume filtration process. Actual data for Figure 6-12 is presented in Appendix A, Tables A.3 and A.4. Further confirmation was provided by comparing the hydrophobicity (measured by contact angle) as temperature changed. The contact angle at above the LCST was higher than at below the LCST, as shown in Table 6.5, which suggests that the CA- NIPAAm membrane became less hydrophilic above its LCST. Contact angle measurements confirmed that CA-NIPAAm membranes at cold conditions were more hydrophilic (contact angle = 47 ± 1 ) than at hot conditions (contact angle = 56 ± 2 ), which again supports the dynamic nature of membranes. 96

119 temperature adjusted flux, L/m 2 hr % CA+2% NIPAAm 18% CA 18%CA+2%NIPAAm- average flux 18%CA- average flux time, hr Figure 6-12: Temperature activation during humic filtration. The triangles symbolize CA membrane filtration, while the diamonds symbolize CA-NIPAAm membrane filtration. For both membrane curves, flux values were taken at 20 C (below LCST) and 45 C (above LCST); hence, the zigzag nature. Average flux lines were drawn to show the overall average flux during filtration Flux recovery After filtration, membranes were subjected to a backwash cycle to estimate the flux recovery; that is, the reversible flux. As mentioned earlier, backwash is a physical cleaning of the membrane to remove fouling. Backwash was performed by pure water filtration in the opposite direction. After backwash, DI water flux was calculated to determine the recovered flux. Table 6.6 shows flux recovery values after filtrations. Flux 97

120 recovery after protein filtration was 90% ± 2% for CA-NIPAAm membrane and 84% ± 3% for CA membrane. Backwash after humic filtration resulted into 88% ± 1% flux recovery for CA-NIPAAm membrane and 78% ± 2% for CA membrane. In both cases, flux recovery for CA-NIPAAm remained significantly higher than CA membranes. The reason behind the higher flux recovery is proposed to be associated with the increased hydrophilicity of CA-NIPAAm membranes as compared to CA membranes (Table 6.5). Table 6.6: Flux recovery analysis after filtration. Flux recovery after protein filtration Flux recovery after humic filtration 18% CA + 2% NIPAAm 90% ± 2% 88% ± 1% 18% CA 84% ± 3% 78% ± 2% Rejection Analysis The fouling solution used was 1 g/l BSA and 1 g/l lipase prepared in DI water. As shown in Table 6.7, there was no significant difference in the protein rejection values for CA-NIPAAm and CA membranes. Table 6.7: Protein rejection BSA average rejection (%) Lipase average rejection (%) CA-NIPAAm membrane 92.7 ± ± 5.2 CA membrane 91.2 ± ±

121 6.3.4 Fouling Scanning Electron Microscopy (SEM) SEM analysis for both CA and CA-NIPAAm membrane samples was done before and after filtration studies. Figure 6-13 shows SEM images of CA and CA-NIPAAm membranes before filtration experiment. After filtration at alternating temperature cycles, SEM imaging was done on the same membrane sample to study fouling due to proteins and due to humic solution. Figure 6-14 shows SEM of membranes at the end of BSA and lipase proteins filtration experiment, while Figure 6-15 for after humic solution filtration. SEM images of CA membrane after the protein filtration showed more texture than CA membranes before filtration. On the other hand CA-NIPAAm membranes appeared to have similar texture as before protein filtration. SEM images of CA membrane after humic filtration showed more texture than CA-NIPAAm membrane, which we believe due to the heavier deposition of humic components on CA membrane surface. CA membrane before filtration CA-NIPAAm membrane before filtration Figure 6-13: SEM of clean CA and CA-NIPAAm membranes before filtration. 99

122 CA membrane CA-NIPAAm membrane Figure 6-14: SEM images of the membranes after protein filtration. CA membranes seem to show more peaks and ridges, which could be associated with more fouling. CA membrane CA-NIPAAm membrane Figure 6-15: SEM images of the membranes after humic solution filtration. CA membranes seem to show more peaks and ridges, which could be associated with more fouling. 100

123 6.4 Conclusions The purpose of this study was to create a membrane made of cellulose acetate and a temperature-sensitive polymer (N-isopropylacrylamide (NIPAAm)). The presence of polymerized and crosslinked NIPAAm in membrane was supported by FTIR analyses of membranes. Filtration experiments were performed at alternating temperature conditions using DI water, protein and humic solutions. The data confirmed the temperature activation of CA-NIPAAm membrane. Contact angle values at different temperatures further confirmed the dynamic response of the membrane. CA-NIPAAm membranes showed higher average flux values as compared to CA membranes of similar thickness; in other words, for the constant volume filtration process, time duration of CA-NIPAAm membrane was shorter. SEM studies showed decreased fouling in case of CA-NIPAAm membranes. The CA-NIPAAm membranes developed here displayed similar surface charge and similar protein rejection as CA membranes; along with higher hydrophilicity, higher initial flux values, higher flux recoveries, and, hence, lower irreversible fouling. Results support the hypothesis that by developing a dynamic temperature responsive membrane, irreversible fouling can be decreased and thus, the membrane lifespan has the potential to be improved. 6.5 Acknowledgements The authors would like to acknowledge both USGS 104(b) and the National Science Foundation for partial funding of this project. 101

124 Chapter 7 Temperature Responsive Membranes Composed of N- isopropylacrylamide (NIPAAm) and Superparamagnetic Iron Oxide Nanoparticles 7.1 Introduction Stimuli-sensitive polymers have been gaining attention in recent years due to the interest in the unique property that the polymer changes its conformation from a coiled state to a globular one in the presence of a stimulus [85, 86]. The stimulus can be ph, temperature, ionic strength or electric or magnetic fields [87, ]. Polymers, like poly(n-isopropylacryamide) and poly(n,n-diethylacrylamide), change their conformation in the presence of temperature changes [79, 87]. Polymers, such as poly(acrylic acid) and poly(methacrylic acid), respond to the ph changes, and for poly(styrene sulfonate) ionic strength is a major stimulus [79]. Temperature stimulated polymers have the advantage over other responsive polymers of easy manipulation and adjustment of the stimulus (i.e., temperature) [44]. When temperature is the stimulus, changes in the surrounding temperature can cause phase change in certain polymers. The phase change arises from the existence of a lower critical solution temperature (LCST) such that the polymer precipitates from aqueous solution as the temperature is increased. Among temperature-stimulated polymers, N-isopropylacryamide (NIPAAm) has received much attention since its transition temperature can be modulated near human body 102

125 temperature [147]. A temperature decrease causes NIPAAm polymer to expand into a hydrophilic state, while a temperature increase causes it to collapse into a hydrophobic state. The thermo-responsive property of NIPAAm offers the advantage of reducing membrane fouling when NIPAAm is incorporated in the membrane matrix. When the temperature conditions are altered as below and above the LCST of NIPAAm, the membrane is believed to change to a more hydrophilic and to a less hydrophilic state, respectively. By continuously activating the membrane, it is hypothesized that a dynamic stimuli-responsive membrane can prevent foulants from attaching to the membrane surface. Zhou et al. [44] developed a ceramic membrane by grafting NIPAAm on the surface. They conducted filtration experiments at temperature below and above LCST of NIPAAm. They observed during their studies that when temperature is raised above the LCST, water flux was increased. They concluded that above LCST, PNIPAAm chains dehydrated to form a compact hydrophobic structure in water and membrane pores were more open facilitating more passage for water. By activating the membrane surface via temperature cycles, they achieved high flux recoveries, which they attributed to a reduction in fouling. Ali el al. [89] prepared a crosslinked polymeric network by copolymerization of sodium acrylate with comonomer NIPAAm. They proposed to use the swelling and deswelling property of NIPAAm hydrogel for desalination of brackish water. Bordawekar et al. [82] developed a brush of temperature-stimulated polymer hydroxypropyl cellulose (HPC) on CA membranes, and studied its response under alternating temperature conditions. They observed that irreversible fouling of a yeast 103

126 solution was significantly reduced during temperature fluctuations. However, heating of the feed water for polymer activation was not a viable commercial option. While temperature-responsive membranes have shown significant promise, they suffer from two challenges. First, bulk thermal heating is energy intensive and would require heating of the entire membrane housing and feed streams. Second, heat transfer resistances within the housing would affect heating and cooling rates and this would hinder the temperature response time of the polymer. These challenges could both be eliminated if membrane heating was localized and intensified within the matrix or surface coating. Magnetite (Fe 3 O 4 ) has gained attention due to its magnetic properties along with biocompatibility, stability and low cytotoxicity and hence, superparamagnetic iron oxide (SPIO) nanoparticles have existing and potential applications in biomedical applications [148, 149]. AA single nanoparticle is able to locally and selectively raise the temperature of the molecule and/or matrix in which the nanoparticle is embedded [150]. Figure 7-1 shows response of PNIPAAm to the temperature increase above its LCST. This temperature increase was caused due to the heat generation via magnetite nanoparticles. Figure 7-1: Response of PNIPAAm coated on magnetite nanoparticles to the temperature increase above LCST [148] 104

127 When exposed to an electromagnetic field, magnetic nanoparticles can absorb energy from the field and release this energy in the form of heat to the surrounding medium. Using this property, a focused and concentrated heating of materials can be achieved at nanoscale [149]. A highly localized heating can be achieved within the volume of nanoparticles and under most conditions, temperature remains homogeneous over the entire volume of the material that contains the nanoparticles [150]. NIPAAm/SPIO nanoparticle composites have been prepared previously, but primarily for biomedical or materials applications. Hoare et al. [151] used 12 nm diameter SPIO nanoparticles (magnetite, Fe 3 O 4 ) to collapse a NIPAAm hydrogel drug reservoir supported within an ethyl cellulose membrane and control drug release under AC EMF heating. NIPAAm latexes have also been prepared with physically entrapped SPIO nanoparticles, and the presence of nanoparticles was shown to not affect the LCST [152]. Results have also demonstrated that NIPAAm hydrogels containing SPIO nanoparticles are stable over multiple AC EMF heating cycles and retain their native LCST and expansion/collapse characteristics [153]. Himstedt et al. [143] grafted a hydrophilic polymer brush on polyamide composite nanofiltration membrane and then end-capped it with magnetite (Fe 3 O 4 ) SPIO nanoparticles. They used an oscillating magnetic field to activate the nanoparticles such that nanoparticles experienced a magnetic force and torque. As a result, the polymer brush caused micro-mixing directly above the membrane surface, and this resulted in a significant decrease in the concentration polarization in boundary layer and improved membrane performance. Recently, Ng et al. [154] functionalized magnetite (Fe 3 O 4 ) nanoparticles on the surface of a commercial microfiltration membrane in order to reduce accumulation of foulants on its 105

128 surface. They carried out crossflow filtration studies with the activation of magnetite nanoparticles using an external electromagnetic field. Due to the oscillating magnetic field, nanoparticles generated torque effects at the surface, which helped in preventing the accumulation of foulants on the surface. However, functionalization of nanoparticles required the addition of a polymer layer to the existing membrane surface, which offered resistance to the flow. In this study, temperature activation was hypothesized to be possible by embedding small superparamagnetic iron oxide (SPIO) nanoparticles within temperaturesensitive membranes. An alternating current (AC) electromagnetic field (EMF) was used to generate heat in the membrane matrix via nanoparticles. Magnetic field produced by a coil: When current is passed through a conductive coil, a magnetic field is generated that flows through the center of the coil and circles back around the outside of the coil, as shown in Figure 7-2. This magnetic field is concentrated at the center of the coil and is relatively weaker outside. Concentrated magnetic field at the center of the coil can be used to induce magnetism in ferromagnetic materials [155]. 106

129 Figure 7-2: Magnetic field generated when current flows through a coil [155] 7.2 Related studies performed at University of Rhode Island In order to develop a temperature responsive membrane, studies were performed on NIPAAm-coated magnetic iron oxide nanoparticles for check the response of NIPAAm. These studies were performed at the University of Rhode Island by the Bothun Research Group, and their analysis of results are shown below. Hydrogel with SPIO nanoparticles preparation Thirty nm anionic (carboxyl) in 50 mm borate buffer (obtained at 7.6x10 14 nanoparticles/ml; Ocean NanoTech, Springdale, AL) were used for modifications. Carboxylated NIPAAm (NIPAAm-COOH) was attached to iron oxide magnetic nanoparticles of size 30 nm using carbodiimide (EDC) chemistry by modifying standard EDC procedures [156, 157]. NIPAAm with terminal hydroxy group was prepared by telomerization of monomer NIPAAm using HESH as chain transfer agent. Reactants NIPAAm, hydroxyethanethiol, and benzoyl peroxide were dissolved in tetrahydrofuran (THF) followed by degasification by freeze thaw cycle and were sealed in vacuum. After 107

130 removal of a majority of THF by distillation under vacuum, the reactants were precipitated in diethyl ether from acetone, and then dried in vacuum. The obtained polymer was named PNIPAAm-OH. PNIPAAm-OH, succinic anhydride, and dimethylaminopyridine (DMAP) were dissolved in CH 2 Cl 2. The polymerization was carried out at 25 C followed by evaporation of CH 2 Cl 2.The reactant were precipitated in diethyl ether from THF, and then dried in vacuum. The obtained polymer was named PNIPAAm-COOH. Then, the solutions of PNIPAAm-COOH and EDC were prepared in buffers (sodium or potassium phosphate). The reaction mixtures were shaken for a period of 24 hr at room temperature. After the incubation period, samples were placed on the top of a bar magnet where sedimentation of magnetic particles occurred within 2 min. Effects of temperature activation on coated SPIO NPs The plot of hydrodynamic diameter vs. temperature shows how the particle diameter changes due to NIPAAm shrinkage with increasing temperature (Figure 7-3). By crossing the LCST, the volume of NIPAAm coating reduced by ~70%. 108

131 Hydrodynamic Diameter (nm) x EDC Standard Raw NPs Temperature ( o C) Figure 7-3: Collapse of NIPAAM coatings on grafted SPIO nanoparticles using standard reaction conditions and doubling the reaction conditions (2x EDC) Increase in temperature was caused due to the heat generated during relaxation motions of SPIO nanoparticles as a response to the AC induced magnetic field. Change in the hydrodynamic diameter of NIPAAm coated nanoparticles was due to the collapse of NIPAAm at the temperature above its LCST. This suggests that, SPIO NPs can cause temperature increase in surrounding medium during external applied AC field. 7.3 Studies performed at The University of Toledo Materials Cellulose acetate was purchased from Sigma Aldrich (average M n ~ 30,000) and used as received. NIPAAm (97%, Sigma Aldrich) was purified to >99% by crystallization using hexanes followed by vacuum drying prior to use. Methylene bisacrylamide (Polysciences Inc.) and ammonium cerium (IV) nitrate (Aldrich) were 109

132 used as received. Carboxylic acid group terminated polynipaam (PNIPAAm-COOH) was purchased from Sigma Aldrich (average M n ~ 2000) and used as received. Iron oxide nanoparticles (in borate buffer, 1 mg/1 ml) of size 25 nm with amide group attached were purchased from Ocean NanoTech LLC (Springdale, AR) and were used as received. Components of the EDC coupling reactions were 2-(N-morpholino)ethanesulfonic acid (MES) (Sigma Aldrich), N-(3-dimethylaminopropyl)-N -ethylcarbodiimide hydrochloride (Sigma Aldrich), N-hydroxysuccinimide (Sigma Aldrich), and sodium hydroxide (Fisher Scientific) used as received Preparation Methods Preparation of CA-NIPAAm-NP membranes Coupling of PNIPAAm-NP PNIPAAm and NP were coupled using N-ethyl-N(3-Dimethylaminopropyl) carbodiimide (EDC) chemistry. EDC chemistry was used in this study to attach SPIO nanoparticles to NIPAAm (Figure 7-4), where EDC facilitates the reaction between carboxylic and amine functional groups [158]. PNIPAAm, with functional COOH groups, and iron oxide nanoparticles, with functional -NH 2, were used to prepare the product PNIPAAm-NP. In the first step, 0.1 g MES was added to 10 ml DI water, which was followed by the addition of g EDCH and g NHS. Solution ph was maintained at 6 by adding NaOH. This was followed by the addition of 0.01 g PNIPAAm-COOH. The solution ph was then adjusted to 7 by adding NaOH. To this mixture, 1 ml of nanoparticles solution with functional amino group (NP-NH 2 ) was 110

133 added as the final step. The reaction mixture was then vacuum dried to obtain the product PNIPAAm-NP. Figure 7-4: PNIPAAm and nanoparticles coupling reaction Addition of PNIPAAm-NP to membrane matrix Dope solutions of cellulose acetate (CA), NIPAAm, MBAA, and PNIPAAm-NP were prepared using CAN as initiator and MBAA as crosslinker in NMP solvent. As discussed in Chapter 5, Section 5.3.3, the presence of crosslinked NIPAAm plays an important role in temperature activation. Since nanoparticles were attached to PNIPAAm, NIPAAm monomer was added along with the crosslinker (MBAA) to the dope solution. The amount of NIPAAm and PNIPAAm-NP together was maintained as 2% (wt %) and the ratio of PNIPAAm-NP to NIPAAm was 1:2 for the preliminary studies. Crosslinker (MBAA) amount was maintained as 2% (wt%) of the amount of NIPAAm and 0.03 g of CAN was added to every 10 ml of NMP solvent. CA amount was maintained as 18% (wt%) with respect to NMP solvent. To prepare the dope solution, CAN was added to the solvent and kept for 10 minutes. NIPAAm, MBAA and PNIPAAm-NP were then added to the solution and kept in a nitrogen atmosphere for 20 minutes. CA was added to this solution and kept in an atmosbag maintaining nitrogen atmosphere for 24 hours. 111

134 Membrane Fabrication The key to achieving an even distribution of nanoparticles within the film is to insure that the nanoparticles were well dispersed in dope solution and remain dispersed during casting [159]. Prior to membrane fabrication, dope solutions were sonicated using a Cole Palmer 9981 Sonicator (Vernon Hills, IL) for 15 min to prevent the aggregation of nanoparticles and to ensure that nanoparticles were evenly suspended in the dope solution. Membranes were fabricated by phase inversion process. Phase inversion process is explained in detail in Chapter 4, section Membrane Characterization Scanning Electron Spectroscopy (SEM) - Energy Dispersive X-ray Spectroscopy (EDS): performed at The University of Toledo In the SEM, a beam of high-energy electrons is focused at a point on a surface and scanned in raster pattern across the surface of the sample. When the electrons hit the surface, they enter the layers of the surface atoms and get scattered. Scattering can be elastic or inelastic based on the amount of energy those electrons carry. Inelastically scattered electrons lose significant amount of energy during collision, so they carry a low energy when they escape the surface and are termed as secondary electrons. Elastically scattered electrons lose very little or no energy during collision and carry a high amount of energy when they escape the surface. They are known as backscattered electrons and generally do not provide significant topographical information. The amount of backscattered electrons is more for the material with higher average atomic number as compared to the material of lower atomic number. 112

135 Hence, when high atomic number regions are present in relatively low atomic number surface, backscattering generates images with atomic number contrast. In other words, the regions of the higher atomic numbers appear bright as compared to rest of the surface in the SEM image. Since backscattering mode is more suitable for samples with metal oxides, SEM analysis of membranes was performed in backscattering mode. A Hitachi S-4800 High Resolution SEM (Dallas, Texas) was used in backscattering mode to study the distribution of nanoparticles on the membrane. Prior to imaging, a modified direct freeze fracture method [126] was used, where membrane samples were frozen in liquid nitrogen for 5 minutes to make them brittle. After 5 minutes, the samples were quickly broken and were coated with gold nanoparticles for SEM analysis. Along with the SEM imaging, energy dispersive x-ray spectroscopy (EDS) analysis of the membrane sample was done to confirm the presence of iron oxide nanoparticles. EMF Heating of the membrane: performed at University of Rhode Island Membranes prepared in the Section were tested for temperature activation via nanoparticles. Radio frequency (RF) heating was achieved by sending current through coil. In present studies, current of 250 A and frequency 173 khz was carried through a copper coil for RF heating purpose. Membrane samples were cut and taped on a plastic support which was in turn place at the center of the coil as shown in Figure

136 Figure 7-5: Membrane placed at the center of the coil for RF heating Another membrane sample without RF heating was also studies to create a reference as shown in Figure 7-6 (a). Carboxyfluorescein green dye was dissolved in water to prepare 0.2 mm solution, which was used to study infiltration in membrane samples. Four Drops of 15 μ each were placed on each of the membrane (Figure 7-6 (b)). (a) (b) Figure 7-6: (a) studies on membranes with and without RF heating (b) four drops of water with dye 114

137 7.4 Results and Discussion: Scanning Electron Spectroscopy (SEM) in backscattering mode: University of Toledo Prior to the SEM imaging, membrane samples were coated with gold nanoparticles for imaging and to prevent charging. It is important to note that the size of gold nanoparticles was approximately 20 nm, which closely compared to the average size, 25 nm, of the SPIO NPs. Therefore, gold nanoparticles had the potential to mask the SPIO NPs present on the surface, which could affect the detection of the SPIO NPs added to the membrane. Twenty kv voltage was used to obtain SEM images of the membranes. As previously stated, during backscattering, regions of higher atomic number appear in brighter contrast relative to the surface of lower atomic number in which the higher atomic number material is present. Hence, it is proposed that the brighter regions in Figure 7-7 are of SPIO NPs (marked with arrows). Figure 7-7: SEM image of CA-NIPAAm membrane with embedded nanoparticles 115

138 Following SEM imaging, EDS analysis was performed on the highlighted (arrow pointed) spots to check the presence of Fe 3 O 4 nanoparticles. Since, it was difficult to focus on the smaller spots during EDS analysis, analysis for larger aggregates was performed. As explained earlier, during backscattering mode, materials of higher atomic numbers appear in different contrast as compared to the surrounding materials of lower atomic numbers. Since iron from nanoparticles has a higher atomic number than rest of the elements from the cellulose acetate membrane, it was believed that all iron oxide nanoparticles and their aggregates would appear in contrast against the membrane surface. Since the aggregates had same contrast against the surface of lower atomic number (Figure 7-7), it was assumed that they were all of the same material. Figure 7-8 shows EDS analysis of one of the larger aggregates. EDS showed the presence of iron and oxygen in the aggregate, which suggested that the aggregate could be iron oxide nanoparticles. Since there were no other sources of iron in the membrane samples, the presence of iron in EDS analysis confirmed the presence of SPIO NPs in the membrane matrix. 116

139 Figure 7-8: EDS analysis to check the presence of nanoparticles RF heating of the membranes: University of Rhode Island RF heating was achieved by sending a current through the coil. The temperature was recorded using a LASER temperature thermometer (i.e., infrared thermometer gun). Notably, the recorded temperature was from the region directly above the membrane. The temperature near the vicinity of the nanoparticles in the membrane matrix is believed to be higher [149, 150]. Green dyed water droplets were placed on each membrane sample, 117

140 and were monitored for changes during the heating period. The droplets were infiltrated in the membrane samples, leaving either diffused or tight green color rings on the membrane surface. It is believed that when the temperature was below the LCST, due to the more hydrophilic state of the membranes, water could spread through the membrane matrix in shorter time periods. On the other hand, when the temperature was above the LCST, due to the lower hydrophilicity of the membrane, the infiltration time was longer. Due to evaporation, there could be some water loss This resulted in water leaving tight and intact dye rings on the membrane surface. Importantly, the loss of water from the droplets was a result of a combination of reduced hydrophilicity and evaporation. However, evaporation of the water was not quantified in the studies. After 900 sec, green dyed water droplets were visible on the membranes with and without RF heating, as shown in Figure 7-9. In the case of the membrane without RF heating, diffused green rings were formed around the droplets, suggesting partial infiltration of water in the membrane. Without RF heating, the recorded temperature was C, which is below the LCST of NIPAAm. Hence, the hydrophilic state of NIPAAm membrane below the LCST could explain diffusion or partial infiltration of water droplets. The membrane samples were then heated with RF heating. During RF heating, the temperature was C, which is close to the LCST of NIPAAm. Because of this the membrane was believed to be in a less hydrophilic state. This could be the reason why water droplets did not infiltrate and there was no diffused-ring formation around the droplets after 900 sec of RF heating. 118

141 no RF heating with RF heating Figure 7-9: Membranes after 900 sec of the experiment After 1460 sec, all the droplets on the membrane kept without RF heating had infiltrated completely. Large green rings left by the dye in the water can be seen in Figure As previously explained, the surface temperature of the sample was C, which is below the LCST of NIPAAm. The infiltration of water in the membrane could be a simple filtration due to the hydrophilicity of the surface. The membrane sample that was heated using RF heating to a temperature of C showed droplet infiltration without the presence of green diffusing rings, suggesting infiltration of water leaving the dye on the surface. 119

142 Figure 7-10: Membranes after 1460 sec of RF heating Figure 7-11 shows a plot of the amount of time that it took for complete infiltration of water through the membrane with and without RF heating. Without RF heating, the infiltration time recorded was lower than the time recorded for infiltration during RF heating. Longer time of infiltration suggests that the membrane was less hydrophilic due to the temperature conditions above the LCST. This in turn was hypothesized to be due to the RF heating via SPIO nanoparticles. 120

143 time for complete water infiltation, sec no RF RF Figure 7-11: Time for complete infiltration with and without RF heating RF heating was then performed using membranes without nanoparticles for comparison (Figure 7-12). It is believed that more heat generation was caused in membranes with SPIO NPs as compared to the membranes without nanoparticles. It was observed that the temperature rise in the membranes with SPIO NPs was higher as compared to the membranes without nanoparticles. The temperature recorded during RF heating at the end of infiltration in the case of membranes with SPIO nanoparticles was C, while that for membranes without nanoparticles was C. Since SPIO nanoparticles generate locally intensified heat, it is believed that the local temperature in the membrane was higher than the recorded temperature. Therefore, it was concluded that the presence of SPIO nanoparticles caused significant heating and led to an increase in temperature to around the LCST. 121

144 time for complete water infiltration, sec 1800 RF heating NP no NP Figure 7-12: Infiltration time of membranes with and without SPIO NPs during RF heating 7.5 Conclusion Superparamagnetic iron oxide nanoparticles were incorporated in the membrane by chemically attaching them to PNIPAAm. Membranes with embedded nanoparticles were characterized to verify the presence of nanoparticles and to check the response of nanoparticles to the alternating current. SEM-EDS confirmed the presence of nanoparticles in the membrane matrix. Membranes with nanoparticles were studied with and without RF heating. Surface temperature was increased during RF heating. Less wetting was apparent from the green rings during RF heating, which suggests that during RF heating, membrane was less hydrophilic as compared to without RF heating. Membranes with and without nanoparticles were subjected to the RF heating to investigate the effect of presence of nanoparticles on water infiltration. Temperature increase achieved with SPIO nanoparticles present in the membrane was higher than the 122

145 membrane without nanoparticles. In conclusion, oscillating magnetic field can be used to achieve temperature activation of the membrane. 123

146 Chapter 8 Scale Up Studies of Cellulose Acetate Membrane 8.1 Introduction Optimization and scale up are considered as two of the most important components of process development [160]. Optimization focuses on the improvement of separation performance, which includes maximization of separation factor, mass transfer rates, and membrane lifetime, whereas scale up is fabricating a membrane of large enough area to process large volumes. Scale up includes designing of a membrane system to determine the area requirements and to integrate membranes into cost effective membrane modules [160]. This study focused on the challenges during scale up, and the effect of process parameters on the morphology and flux of cellulose acetate (CA) membranes. CA membranes are one of the most researched polymers for separation and barrier applications because of its many useful characteristics including relatively low cost, good toughness, high biocompatibility, and high potential flux [ ]. Due to these characteristics, CA membranes have been widely used for ultrafiltration, microfiltration, nanofiltration, and reverse osmosis. There have been many methods used to fabricate porous polymeric membranes. Generally, a laboratory scale approach, such as knife over blade or solution casting, is 124

147 used to cast a small thin sheet of the polymeric membrane dope solution. Then, the final form of the membrane is prepared by sintering, stretching, or phase inversion [106]. The performance of the membrane is greatly influenced not only by the material but also by the processing methods and conditions because they affect the morphology of the membrane [108]. In numerous studies, the primary film casting process was limited to doctor blade extrusion [104, 105]. However, this method is not always the most suitable for the industrial scale because viscosity and pressure or surrounding air can affect the casting solution by development of air bubbles [107]. Alternatively, slot die extrusion is a well-developed pre-metered approach that allows for continuous casting of liquid film in a controlled environment, which also minimizes changes in liquid properties, such as rheology [107, 109]. The slot die coating process is a commonly used process for the manufacturing of plastics and polymer films, as well as coatings and optical films for liquid crystal displays (LCD) in industry. The slot die process offers the advantage of pre-metered and controlled coating thickness by a predetermined flow rate of solution through the slot die and the substrate speed [109]. Thus, in membrane technology, slot die coating is a common process for scale up purposes. There is a constant demand to increase the processing speed of the thin film since the processing speed can increase the production output. At the same time, there is also a constant demand for maintaining low thicknesses of the film considering its applications in coating and polymer industry [107]. However, if these two key factors are not balanced, the polymer film quality cannot be maintained properly. Film quality is dependent upon several factors, such as the solution properties, 125

148 fabrication process and processing parameters, and defects arise when these factors are altered [107, 109]. In this study, the effects of processing method, conditions and substrate on the morphology and on the flux of CA membranes were studied. Laboratory-scale and production-scale processes, i.e. casting knife and slot die extrusion, respectively, were used to cast thin sheets of 20% CA membrane solution. The films were cast on glass and polyethylene terephthalate (PET) surfaces to determine which led to optimal scale up properties. The studies were part of a collaboration with Georgia Institute of Technology. Laboratory-scale studies were performed at The University of Toledo, which included preparation of polymer dope, membrane fabrication and characterization with respect to morphology and performance. Polymer dope solutions prepared at The University of Toledo were used for scale up studies, which were performed at Georgia Institute of Technology. Scale up studies included membrane fabrication via slot die extrusion and characterization of the membranes (i.e. viscosity and contact angle measurements and determination of casting window). These membranes were further characterized for morphology and performance at The University of Toledo. 8.2 Laboratory Scale Studies performed at The University of Toledo Materials Cellulose acetate was purchased from Sigma Aldrich (average M n ~ 30,000) and used as received. One-Methyl-2-pyrrolidone (NMP) with an analytical purity of 99.0+% (Alfa Aesar) was used as the solvent and deionized (DI) water was used as the non- 126

149 solvent agent. Bovine albumin serum (BSA) 98% (Sigma Aldrich) and lipase (Sigma Aldrich) proteins were used for filtration studies. Approximate molecular weights and hydrodynamic radii of BSA and lipase, as provided by the manufacturer, are listed in Table 8.1. Table 8.1: Molecular weights and hydrodynamic radii of proteins Molecular weight Hydrodynamic radius BSA ~ 66 kda ~ 3.5 nm lipase ~ 48 kda ~ 2.2 nm Feed Solutions Constant volume precompaction was performed using DI water as feed. For subsequent constant volume filtration studies, protein solutions prepared with DI water were used as feed solutions. Prior to BSA filtration, membranes were precompacted using 130 ml DI water, which was then followed by filtration of 140 ml BSA protein solution of concentration of 1 g/l. Prior to lipase filtration, membranes were precompacted using 120 ml DI water and precompaction was followed by filtration of 100 ml lipase solution of concentration 1 g/l Methods Dope Solution Preparation To prepare 20% CA polymer dope, CA was added to NMP solvent. The solution was stirred for 24 hours by using magnetic stirrer. When the solution was ready, 127

150 membranes were casted using doctor blade extrusion, as a standard laboratory scale process, and slot die extrusion, as a scalable process Membrane Fabrication at Laboratory Scale In this technique, a dope solution of polymer and solvent are cast on the substrate followed by immersion of the film into a non-solvent coagulation bath [48], leaving a solidified membrane. In this case, either the glass plate or PET film was immediately immersed in DI water bath, where exchange between solvent and non-solvent took place. Once the membrane separated from the substrate, it was transferred to another container of fresh DI water to remove excess solvent. Membranes were soaked in this container at least for 24 hours prior to use. The membrane thicknesses were measured using Mitutoyo (Mitutoyo, IL, USA) thickness gage in multiple locations across and parallel to the direction of casting and the average membrane thickness determined. An illustration of film extrusion, film formation, and the resulting membrane is shown in Figures 8-1(a) - 8-1(d). Phase inversion processed is discussed in details in Chapter 4, Section

151 Figure 8-1: Membrane casting steps (a) pouring dope solution of glass plate (b) making a polymer film using doctor s blade (c) polymer film (d) membrane after phase inversion Membrane Characterization Filtration Studies Flux is defined as the throughput of pressure driven membrane filtration system measured as flow per time and per unit of membrane area [16]. Filtration experiments were performed in dead-end filtration mode using a 10 ml Amicon Stirred cell 8010 (EMD Millipore, Massachusetts, USA). In the present study, 4.8 bar (70 psi) pressure was used for all the filtration experiments. For a constant membrane area, the time taken to filter a constant volume of feed solution was measured, and flux was calculated as: flux( j) = volume v membrane area a ( ) ( ) time( t) 129

152 Flux was reported in the units of L/hr.m 2 and plotted against actual time of filtration period. The membrane was cut in circular pieces of area 4.1 cm 2 to fit into the slot of stirred cell. The membrane was supported by Whatman TM Filter paper (4, 125 mm ø). Every membrane was precompacted using DI water until constant flux readings were obtained. The time to filter 2 ml of water through the membrane was measured. Details on the filtration process are also discussed in Chapter 4, Section Flux Recovery Following the protein filtration, flux recovery was calculated for all the membrane samples. Flux recovery is the amount of the initial flux that can be recovered after membrane cleaning. Backwash, which is a type of physical cleaning method, was performed after filtration. During backwash, the direction of the feed is reversed such that accumulated foulants can be washed off. Here, the membrane samples after filtration was placed upside down, and DI water was passed through the membranes for the duration of 60 min. After 60 min, membranes were turned and placed back to its original position, and pure water flux was recorded. Environmental Scanning Electron Microscopy (ESEM) Environmental Scanning Electron Microscopy (ESEM) was used to study membrane morphology and fouling. ESEM imaging offers the most direct method to view the membrane at micron or even smaller scale [124, 125]. A FEI QUANTA 3D Duel Beam Electron Microscope (FEI at Hillsboro, Oregon USA) was used to analyze membrane morphology and fouling. Prior to ESEM imaging, all the membrane samples 130

153 were coated with gold nanoparticles for 15 sec for electron imaging and to prevent charging. ESEM images of the membranes were taken before and after backwash. 8.3 Scale-Up Studies Dope Preparation at The University of Toledo Polymer dope solution of 20% CA was prepared as explained in Section at The University of Toledo. This solution was used for both laboratory scale, performed at The University of Toledo, and scale up studies, performed at Georgia Institute of Technology Slot Die Casting at Georgia Institute of Technology Contact Angle of the substrate (glass) and the tooling (aluminum) Contact angle quantifies the wettability of a surface [18]. For scale up, the sessile drop method was used to measure the contact angle of the membrane solution with respect to the tooling and substrate, and for membranes with respect to water. Dynamic and static contact angle measurements were carried out on glass and aluminum using a Rame-hart Goniometer model 500-U1, and analyzed using DROP image advanced software. A drop of solution is formed on the substrate. The angle between the solid phase (glass or aluminum) and the liquid phase (20% CA dope solution) is measured by the software during the first 20 min in order to obtain the dynamic contact angle profile. Width of the droplet as it spreads was measured at discrete time increments for 20 minutes. The dynamic advancing contact angle as a function of the contact line velocity was then calculated from the time rate of change of the droplet width during the initial 131

154 spreading of the droplet. The static contact angle was calculated after the droplet stopped spreading and the contact angle came to a steady state Dope Solution Characterization Studies Surface Tension Surface tension of the polymer is an important parameter to take into the consideration in order to fabricate a defect free polymer film. Coating speed depends on surface tension values since solution with lower surface tension can restrict the coating speeds to lower values [107]. Surface tension values were used to determine process parameters in slot die extrusion. A Lauda Drop volume tensiometer, model TVT2, was used to calculate the surface tension based on measuring the volume of the drop of the solution. In this method, drops are produced at the tip of a capillary that is connected to a syringe. At the moment when weight overcomes the holding force, the drop is detached and falls down passing through a light source, which measures the cross section of the drop and the volume is determined. Viscosity Viscosity plays a significant role in determining the processing conditions because fabrication defects, such as air bubble entrapment, highly depends on the solution viscosity [109]. Air entrainment rate is lower at lower solution viscosities [107]. ARES L-S2 rheometer capable of performing dynamic, steady and transient mechanical tests was employed to determine the material functions. The tests were performed using a parallel plate fixture with a diameter of 25 mm and at room temperature. In order to 132

155 generate the flow curve and characterize the behavior of the solution, steady rate sweep tests were carried out in which the viscosity was measured as a function of shear rate, which was varied between 0.1 and 400 1/s Membrane Fabrication During Scale Up Experimental Set Up For membrane casting, a custom designed and built roll-to-roll imaging system (roll-feed imaging system (RFIS)) was used. A schematic of RFIS is shown in Figure 8-4. During slot die extrusion, the CA dope was suspended on a plastic PET film as it moved past the slot die at a given substrate velocity. 133

156 Figure 8-2: Schematic of the experimental setup (RFIS) [165] During the slot die extrusion process, CA dope solution was forced through an aluminum slot die with a slot gap of 90 µm and a gap height of 178 µm above the substrate. The volumetric flow rate into the die was controlled by varying the supply pressure from a nitrogen tank. The 100 µm thick PET substrate was supplied from a feed roller under the slot die and was collected by a take-up roller. For the current RFIS system, 1.3 mm/s was the lower limit for substrate velocity and was used during the fabrication of all the membranes. The motorized roller acted upon the take up roller and controlled the substrate velocity, which was kept at a constant 1.3 mm/s. Sections of the cast CA solution and PET substrate were cut from the PET roll for curing prior to 134

157 traveling over the end roller after the flat platen section. This process was altered slightly for casting on the glass substrate. PET roll was used to convey glass plates under slot die. The PET and glass substrates were cleaned prior to casting with isopropyl alcohol to remove dust and foreign matter, and to ensure consistent wetting properties. After casting, the liquid film was converted into a membrane via the phase inversion method [115]. 8.4 Results and Discussion There were challenges during slot die extrusion to obtain the membrane in the desired thickness range ( μm) due to the current equipment limitations. In order to produce defect free membranes, the substrate velocity has to be low so that the shear forces do not overcome the surface tension forces. The roll-to-roll system could not be reliably set in a range low enough to be inside the coating window. In some cases, this scenario can be overcome by increasing the flow rate, but this has two limitations. First, due to the viscosity of the 20% CA solution, the pressure required to substantially increase the flow rate was high and could exceed the limitations of the system. In order to obtain the desired flow rate, calibration curves for flow rate versus pressure were needed to be created, but they were often not linear or reliable. Second, since the flow rate and substrate speed are related to the target film thickness, increasing the flow rate could set the operating conditions outside the casting window. Another parameter, which could be changed was the offset height of the die from the substrate (H). However, the limitations during the setting of this parameter had the potential to affect the coating window since, 135

158 setting the value of H very high could cause break lines, whereas setting the value very low could cause dripping Contact Angle Studies: Performed at Georgia Institute of Technology For each substrate three drops were used in order to determine the average value. Figure 8-5 shows the dynamic advancing contact angle for NMP-20 wt% CA on aluminum and glass at an ambient temperature of 24.5 C. The static contact angles for aluminum and glass were found to be 57.9 and 27.5, respectively. Since contact angles provides the information about the wettability of a surface, the static contact angle values are used to determine the spreading of a dope solution during fabrication. Contact angle values lower than 90 corresponds to high wettability while larger contact angle correspond to low wettability [166]. Contact angle of the CA dope for aluminum slot die and glass substrate were below 90, which assures better suspension of dope through slot die without trapping air bubbles and spreading on glass substrate. The actual data for Figure 8-5 is presented in Appendix C, Table C

159 θ [deg] Aluminum Glass u [mm/s] Figure 8-3: Advancing contact angle vs contact line velocity Dope Solution Characterization: Performed at Georgia Institute of Technology Surface Tension: The measurements were carried out for four drops of NMP-20wt% CA, which resulted in an average value of 42.3 ± 0.23 mn/m. Surface tension value was used to develop process parameters of slot die extrusion Viscosity As illustrated in Figure 8-6, at strain rate values less than 20 1/s, viscosity appears to remain constant, and increasing or decreasing the shear rate does not have a significant effect on viscosity. This behavior is quite similar to that of Newtonian fluids. However, by further increasing the shear rate, the solution displays, shear thinning behavior and 137

160 η [Pa.s] viscosity drops from 19.6 Pa.s to 4.5 Pa.s at shear rate of 250 1/s, as shown in Figure 8-6. The values obtained from the experiment were used to develop process parameters. The actual data for Figure 8-6 is presented in Appendix C, Table C Strain rate [1/s] Figure 8-4: Shear rate vs. solution viscosity behavior PET substrate results Membranes fabricated on PET substrate were not separated from the substrate and hence were required to be peeled off from PET film manually. Filtration studies were performed with the membranes at a pressure of 2.78 bar (40 psi). The membranes did not withstand pressure condition and breaking of membranes occurred during the operation. Environment Scanning Electron Microscopy (ESEM) was used to study the surface of the membranes fabricated on the PET substrate. As shown in Figure 8-7, 138

161 ESEM image of the membrane surface of the side, which was detached from substrate, showed a rough texture as compared to the membrane fabricated on the glass substrate. (a) Figure 8-5: (a) casting using PET substrate (b) casting using glass substrate (b) The membrane samples were then studied to compare the surface texture of both the sides as shown in Figure 8-8. In order to analyze the overall difference between the textures of the surfaces, a larger area was selected and hence the magnification was reduced. The PET, or substrate, side surface showed significant roughness as compared to the polymer surface. It is believed that roughness was due to the damage caused during the peeling off process. This could also be the reason for breaking of the membranes during pressure driven filtration process. Therefore, a glass substrate was selected to fabricate all the membranes. 139

162 (a) (b) Figure 8-6: (a) PET, or substrate side surface (b) polymer side surface Fabrication in Casting Window Process parameters such as substrate velocity and dope flow rate were determined as explained in Chapter 2, Section It was calculated that to fabricate defect-free CA membranes (Figure 8-9), a two-dimensional flow rate of 0.13 mm 2 /s with a maximum speed of 1.3 mm/s had to be maintained. Actual data for process parameter calculations is presented in Appendix C, Tables C.3(a) and C.3(b). Figure 8-7: Defect-free cellulose acetate membrane fabricated using slot die extrusion 140

163 8.4.5 Filtration Studies: Performed at The University of Toledo Protein filtration Studies Figure 8-10 shows the flux profiles of both laboratory scale and scale up CA membranes during constant volume BSA protein filtration. The thicknesses of the membranes fabricated at the laboratory scale and during scale up were 150 μm and 223 μm, respectively. Prior to BSA protein filtration, both membrane samples were precompacted. Constant volume precompaction was performed by using 130 ml DI water. Following precompaction, 140 ml BSA solution were filtered through both membrane samples. Membranes fabricated at laboratory scale and during scale up showed initial precompaction flux values of 8.1 L/m 2 hr and 6.7 L/m 2 hr, respectively. Differences in membrane thickness are believed to have caused the difference in the initial flux values. Final flux values at the end of the BSA filtration for the membranes fabricated at the laboratory scale and scale up were 5.8 L/m 2 hr and 4.7 L/m 2 hr, respectively. There was no significant difference observed in the flux decline profiles of both membranes during BSA protein filtration. Actual data for Figure 8-10 is presented in Appendix C, Table C

164 flux, L/m 2 hr precompaction-lab scale BSA filtration- lab scale precompaction- scale up BSA filtration- scale up time, hr Figure 8-8: Flux profiles during BSA protein filtration Filtration studies were then performed on different membrane samples using 1 g/l lipase solution as feed for constant volume. Figure 8-11 shows flux profiles for both membranes. The thicknesses of the membranes fabricated at the laboratory scale and during scale up were 120 and 215 μm, respectively; therefore, it was again expected that the laboratory scale membranes would display higher flux values. Prior to lipase filtration, both membrane samples were precompacted using 120 ml DI water. Following precompaction, 100 ml lipase solution were filtered through both membranes. Membranes fabricated at laboratory scale and during scale up showed initial precompaction flux of 10.8 L/m 2 hr and 7.2 L/m 2 hr, respectively. Final flux values at the end of the lipase filtration for the membranes fabricated at the laboratory scale and scale up were 7.8 L/m 2 hr and 5.7 L/m 2 hr, respectively. The flux decline in case of the membranes fabricated at laboratory scale was higher as compared to the membranes 142

165 flux, L/m 2 hr fabricated during scale up. It is believed that extent of fouling during lipase filtration in case of laboratory scale membrane was higher, and thus, it resulted in higher flux decline. To investigate this, flux recovery and ESEM analysis were performed. Actual data for Figure 8-11 is presented in Appendix C, Table C precompaction- lab scale lipase filtration- lab scale precompaction- scale up lipase filtration- scale up time, hr Figure 8-9: Flux data during lipase protein filtration Flux Recovery Flux recovery for all the membranes was determined following backwash. Table 8.1 shows flux recovery values after BSA and lipase filtration. Flux recovery represents the reversible portion of fouling, which can be regained by backwashing. There was no significant difference between flux recovery values after backwash between laboratory scale and scale up membranes in case of BSA filtration. On the other hand, there was a significant difference in the flux recovery values during lipase filtration studies. When 143

166 compared to the flux profiles during lipase filtration, flux recovery values suggested that this could be a due to significant amount of fouling caused by lipase protein solution. To further investigate the effect of the proteins on the membrane performance, all membrane samples were subjected to ESEM analysis. Actual data for Table 8.2 is presented in Appendix C, Tables C.5 and C.7. Table 8.2: Flux Recovery after Protein Filtration Lab Scale Study Scale Up Study Flux recovery after BSA filtration Flux recovery after lipase filtration 76.4% 77.1% 72.7% 80.5% Environment Scanning Electron Microscopy (ESEM) ESEM analysis of CA membrane samples was performed before and after filtration studies. Figure 8-12 shows ESEM images of the laboratory scale and scale up membranes before filtration. Although both membrane samples were clean (i.e., not fouled) and were fabricated from the same dope solution, their textures appeared to be different. This could be because of the different fabrication methods. Membrane fabrication at laboratory scale was performed with a casting knife without any set velocity using a stationary substrate. The dope solution was poured on the substrate manually, without predetermined flow rate and pressure. On the other hand, the slot die experimental set up had predetermined process parameters, such as flow rate of the polymer dope and the pressure applied to achieve the flow rate of the dope. The substrate 144

167 was moving at a set velocity. Therefore, it is believed that different fabrication methods could cause the difference in the membrane appearance as observed during ESEM imaging. Lab scale Scale up Figure 8-10: ESEM images of CA lab scale and scale up membrane before filtration Figure 8-13 shows ESEM images of the membranes after backwash. ESEM of backwashed membranes after BSA protein filtration showed rougher textures as compared to the clean membranes (Figure 8-12). The rougher textures are believed to be due to accumulated matter (i.e., BSA and lipase), which was not removed during the backwash. There was no significant difference in the texture between membranes fabricated at laboratory scale and during scale up. 145

168 Lab scale Scale up after BSA protein filtration followed by backwash after lipase protein filtration followed by backwash Figure 8-11: ESEM images of CA lab scale and scale up membrane after backwash When the results from filtration studies and ESEM imaging were compared together, it was believed that the higher flux decline and lower flux recovery values during lipase solution filtration were due to the fouling caused by lipase molecules. Since the surface texture after the backwash showed no significant difference, it was believed that the fouling was caused mainly by the pore blockage instead of cake formation. Hence, the fouling was not revealed by ESEM. As stated previously, lipase molecules have smaller size than BSA protein molecules. Due to its smaller size and the rougher texture of the laboratory scale membrane [167, 168], lipase was believed to have caused more fouling in case of laboratory scale membrane. 146