development of charged membranes for nanofiltration

Doctoraatsproefschrift nr. 1031 aan de faculteit Bio-ingenieurswetenschappen van de K.U. Leuven

DEVELOPMENT OF CHARGED MEMBRANES FOR NANOFILTRATION

Pejman Ahmadiannamini

Supervisor: Prof. I.F.J. Vankelecom, KU Leuven Prof. B. Meesschaert, KU Leuven/KH Brugge-Oostende Members of the Examination Committee: Prof. J. Delcour, KU Leuven (Chairman) Prof. B. Van der Bruggen, KU Leuven Prof. L. Pinoy, KU Leuven/KaHo Saint-Lieven Prof. P. Pescarmona, KU Leuven Prof. R. Hoogenboom UGent

May 2012

Dissertation presented in partial fulfillment of the requirements for the degree of Doctor of Bioscience Engineering

© 2012 Katholieke Universiteit Leuven, Groep Wetenschap & Technologie, Arenberg Doctoraatsschool, W. de Croylaan 6, 3001 Heverlee, België Alle rechten voorbehouden. Niets uit deze uitgave mag worden vermenigvuldigd en/of openbaar gemaakt worden door middel van druk, fotokopie, microfilm, elektronisch of op welke andere wijze ook zonder voorafgaandelijke schriftelijke toestemming van de uitgever. All rights reserved. No part of the publication may be reproduced in any form by print, photoprint, microfilm, electronic or any other means without written permission from the publisher. ISBN 978-90-8826-241-8 Wettelijk depot D/2012/11.109/20

Acknowledgments At the end of my master’s degree, my objective was to start my career as a chemical engineer. The last thing on my mind was to spend 4 more years at a university. However I shall never regret the decision to come to Leuven. It has been a great experience. For this reason I thank all of you. First and foremost, I would like to extend my sincere appreciation to my promoter, Prof. Ivo Vankelecom for providing me the opportunity to work on this project as well as for his professional and personal support throughout this PhD. His thoughtful insight and passion into my research has changed the course of my scientific, as well as my personal life. I would also like to thank my co-promoter, Prof. Boudewijn Meesschaert for his commitment to perfection and devotion to scientific endeavor. I greatly thank his constant guidance, encouragement and help during my PhD. I also thank all the other members of my PhD jury for their time to read the dissertation and valuable inputs to improve the quality of this work. I am indebted to Prof. Xianfeng Li for his tremendous support. He has always been an invaluable resource and I would like to offer my sincere gratitude and appreciation to him. I would like to thank my parents for making sure I got to where I am now. They made sure to instill in me a set of values and beliefs that helped me become the person I am today. Through advice, love, and understanding, they made me believe that I can do anything and get anywhere if I put my mind to it. I don’t think I can ever repay them, thus I would like to dedicate the summary of my four years of PhD to them. I would like to thank my wife and better half, Ala Yasaghi, for standing by my side through the rough periods of my PhD. She has been an example to follow. Not many words can describe my thanks to her or how much I appreciate her presence in my life. Thank you. I would like to thank my brother, Peyman, for his emotional support. His patience and lively spirit are an example to what I aspire to be. Thank you for being who you are. I would also like to thank my parents-in-law, sisters-in-law and brother-in-law for welcoming me into the family and their support, love, and prayers. I wish to extend my appreciation to Dr. Asim Khan, Agnieszka Holda, Dr. Chalida Klaysom and Izabela Struzynska-Piron for all their support and encouraging pep-talks. I owe a debt of gratitude to all my current and former lab mates in membrane group: Pieter, Angels, Subhankar, Roil, Xinxin, Anna, Aga, Marjan, Katrien V., Tom Dedroog, Katrien H., Sanne, Hubei, , Xia, Yanbo, Gergo, Soon Chien, Nithya, Parimal, Roy, Aylin, Valerie, Waqas, Louise, Jiangshui, Tom Depuydt. It has always been a great pleasure to work with all of you. I would also like to thank all nice and kind COK people especially Lieve, Hilda, Ines, Inge, Birgit and Annelies for their incredible help and support. Finally, I would like to thank my fantastic friend, Hamed, for all his support and kindness over past years and especially during the last few months. I doubt that I will ever be able to convey my appreciation, but I owe him my eternal gratitude. I would like to thank another incredible friend, Siavash, who has always been supportive to me. I

Acknowledgments

In conclusion, my experience at K.U.Leuven was unique. It made a big change in my life. I learned more and more, and I think now I am ready for my next step. Pejman Leuven, June 2012

II

List of Publications Articles in internationally reviewed scientific journals P. Ahmadiannamini, X. Li, W. Goyens, B. Meesschaert, W. Vanderlinden, S. De Feyter, Ivo. F.J. Vankelecom, "Influence of polyanion type and cationic counter ion on the SRNF performance of polyelectrolyte membranes", Journal of Membrane Science, 403-404, (2012) 216-226. P. Ahmadiannamini, X. Li, W. Goyens, N. Joseph, B. Meesschaert, I.F.J. Vankelecom, "Multilayered polyelectrolyte complex based solvent resistant nanofiltration membranes prepared from weak polyacids", Journal of Membrane Science, 394–395, (2012) 98-106. P. Ahmadiannamini, X. Li, W. Goyens, B. Meesschaert, I.F.J. Vankelecom, "Multilayered PEC nanofiltration membranes based on SPEEK/PDDA for anion separation", Journal of Membrane Science, 360 (1-2) (2010) 250-258. X. Li, W. Goyens, P. Ahmadiannamini, W. Vanderlinden, S. De Feyter, I. Vankelecom, "Morphology and performance of solvent-resistant nanofiltration membranes based on multilayered polyelectrolytes: Study of preparation conditions", Journal of Membrane Science, 358 (1-2) (2010) 150-157. Papers at international conferences and symposia P. Ahmadiannamini, X. Li, B. Meesschaert, I. Vankelecom, "Chlorine-resistant polymeric membranes for desalination" ICOM 2011, 23-29 July, 2011, Amsterdam, The Netherlands, Abstract No. ICOM1261. P. Ahmadiannamini, B. Meesschaert, I. Vankelecom, "New biodegradable hybrid membranes for phosphate removal" NAMS 2010, 17-22 July, 2010, Washington DC, USA. P. Ahmadiannamini, X. Li, W. W. Goyens, B. Meesschaert, I.F.J. Vankelecom, "Multilayered polyelectrolyte complexes as nanofiltration membranes for anion separation", EUROMEMBRANE 2009, 6-10 September, 2009, Montpellier, France.

III

Abstract Polymeric charged nanofiltration (NF) membranes have received a growing interest for water desalination and purification. Selective properties of charged NF membranes can be based upon not only the “sieving effect” but also the “charge effect”. Thus, the surface charge density of the membranes and the structure of the functional groups in the selective layer are imperative factors to determine their performance. In this dissertation, two different approaches for preparation of charged NF membranes were employed; polyelectrolyte multilayer (PEM) membranes and charged mosaic membranes (CMMs). The first part of this dissertation focuses on build-up of PEMs prepared from different PE pairs and their NF properties in aqueous media as well as in organic solvents. In the second part, the preparation and separation performance of a new type of CMMs was studied. Alternate layer-by-layer (LbL) deposition of polyelectrolytes (PE) of opposite charges has emerged as a versatile and inexpensive method to prepare thin films in the nanometer range for different applications. A number of parameters, such as PE type and molecular weight of the PE, ionic strength and pH of the PE solutions can influence the structure of the PEM films deposited by the LbL method. Films prepared via this method are in particular attractive materials with controlled thickness and surface properties for membrane separations. In the first section of the PEM study, poly(diallyldimethylammonium) chloride (PDDA) and sulfonated poly(ether ether ketone) (SPEEK) based PEM membranes with different number of bilayers were prepared via the LbL method from solutions with varying ionic strength. The NF performance of these PDDA/SPEEK membranes was studied for anion separation in aqueous solutions. The range of NF applications has recently been broadened to organic feeds in solventresistant NF (SRNF). A more widespread use of SRNF requires solvent-resistant membranes that preserve their separation characteristics under more aggressive conditions of aprotic or strongly swelling solvents and elevated temperatures. Proven potential of PEM membranes for use in SRNF, especially in aprotic solvents which are really troublesome in SRNF applications, initiated the detailed study of different PE pairs. The charge density of PEs and the thickness of deposited PEMs can be affected by a lot of factors, such as pH and salt concentration in the PE solutions. Thus, a weak PE, polyacrylic acid (PAA), was selected to prepare PEM membranes under different conditions. The influence of the salt concentration and of the pH of the PE solutions on the properties of the membranes was studied in detail and linked to the membrane performance in SRNF. Besides the polymer charge density and the ionic strength of the solution, also the type of polyion, electrolyte and counterion affect the structure of the PEMs and their separation performance. This is why in the last section of the PEM study, new types of PEM membranes were investigated for SRNF applications using PDDA as polycation; and poly(vinylsulfonic acid sodium salt) (PVS) and the more rigid poly(sodium 4-styrenesulfonate) (PSS) as PEs with strong acidic groups. The specific effect of the cationic counterion on the construction and performance of the prepared PEM membranes was examined. Membrane processes can effectively remove all the undesirable species from waste water. However, one challenge encountered in membrane separation processes is to separate V

Abstract

salts from water-soluble organic substances. It would allow in many cases to recycle wastes and reuse into value-added products. This is often a complex separation, especially if the organic compound is charged as well. CMMs contain small domains of opposite charges, set parallel with each other inside the membrane. Charged domains arrange bicontinuous arrays from one membrane surface to the other, such that the membranes can be permeable to both anions and cations and induce their concurrent migrations along the respective fixed charges. Thus, preferential permeabilities of salts over organic matter and a consequential separation between electrolyte and nonelectrolyte can be expected. In the second part of this dissertation, a novel type of CMM was developed from a polymer blend of poly(methyl methacrylate) (PMMA) and poly(vinylbenzyl chloride) (PVBC). Desired surface phase separation occurred via annealing followed by polymer functionalization to introduce charged groups. Separation performance of the resulting CMMs was studied using different salts and organic compounds.

VI

Samenvatting Het gebruik van polymeergebaseerde geladen nanofiltratie (NF) membranen voor het ontzilten en zuiveren van water kent een groeiende belangstelling. Selectieve eigenschappen van geladen NF membranen komen tot stand door zowel scheiding op basis van grootte als op basis van lading. Bijgevolg zijn de oppervlakteladingsdichtheid van een membraan en de structuur van de functionele groepen in de selectieve laag belangrijke factoren voor het bepalen van de performantie. In dit proefschrift werden twee verschillende benaderingen voor de synthese van geladen NF membranen uitgewerkt: polyelektrolyt-multilaag (PEM) membranen en geladen mozaïek membranen (‘charged mosaic membranes’ of CMMs). Het eerste deel van dit proefschrift focuste op de opbouw van PEMs aangemaakt via verschillende polyelektrolyten (PE) en hun NF-eigenschappen in zowel water als organische solventen. In het tweede deel werden de synthese en scheidingsperformantie van een nieuw type CMMs bestudeerd. Afwisselende “layer-by-layer” (LbL) afzetting van PEs met tegengestelde ladingen is een veelzijdige en goedkope methode voor de synthese van dunne films met nanometerdimensies voor uiteenlopende toepassingen. Een aantal parameters, zoals PE-type en moleculair gewicht van het PE, evenals de ionische sterkte en pH van de PE-oplossingen kunnen de structuur van de PEM-films aangebracht via de LbL-methode beïnvloeden. Films bereid via deze methode zijn interessante materialen met een controleerbare dikte en oppervlakte-eigenschappen voor membraanscheidingen. In het eerste deel van de PEM studie werden poly(diallyldimethylammonium) chloride (PDDA) en gesulfoneerd poly(ether ether keton) (SPEEK) gebaseerde membranen met een verschillend aantal dubbellagen aangemaakt via de LbL-methode, vertrekkend van oplossingen met variërende ionische sterkte. De NF-performantie van deze PDDA/SPEEKmembranen werd bestudeerd voor de scheiding van anionen in waterige oplossingen. Toepassingen van NF in waterig milieu werden recent uitgebreid met filtraties van organische mengsels in solventresistente NF (SRNF). Voor gebruik in SRNF zijn solventresistente membranen vereist die hun scheidingskarakteristieken behouden onder agressievere condities, zoals filtraties van aprotische of sterk zwellende solventen en bij hoge temperaturen. Het bewezen potentieel van PEM-membranen voor gebruik in SRNF, meer specifiek in aprotische solventen, gaf de aanleiding tot een gedetailleerde studie van gebruik van verschillende PE-paren. De ladingsdichtheid van de PEs en de dikte van de afgezette PEMs kan beïnvloed worden door verschillende factoren, zoals pH en zoutconcentratie in de PE oplossingen. Een zwak PE, poly(acrylzuur) (PAA), werd geselecteerd voor de synthese van PEM-membranen onder verschillende condities. De invloed van de zoutconcentratie en de pH van de PE-oplossingen op de membraaneigenschappen werd in detail bestudeerd en gelinkt aan de membraanperformantie in SRNF. Naast de ladingsdichtheid van het polymeer en de ionische sterkte van de oplossing, hebben ook het type polyion, elektrolyt en tegenion een invloed op de structuur van de PEMs en de bijhorende scheidingsperformantie. Hiervoor werden in het laatste deel van de PEMstudie nieuwe types PEM-membranen onderzocht voor SRNF-toepassingen met PDDA als polykation, en poly(vinylsulfonzuur natrium zout) (PVS) en het meer rigide poly(natrium 4styreensulfonaat) (PSS) als PEs met sterk zure groepen. Het specifieke effect van het kationisch tegenion op de structuur en performantie van de aangemaakte PEM membranen werd eveneens onderzocht. VII

Samenvatting

In het laatste deel van deze doctoraatsthesis werd de uitdaging aangegaan om membranen te bereiden die in waterstromen zouten van de wateroplosbare organische componenten zouden kunnen scheiden. Het gaat hier vaak om een complexe scheiding, vooral wanneer de organische component eveneens een lading bezit. Beschikbaarheid van dergelijke membranen zou in veel gevallen kunnen leiden tot mogelijk hergebruik van de aanwezige polluenten. CMMs bestaan uit kleine domeinen met tegengestelde ladingen, parallel aan elkaar ingebouwd in het membraan. De geladen domeinen zijn hierbij geordend als twee doorlopende fasen die zich uitstrekken van het ene membraanoppervlak naar het andere, zodat de membranen permeabel zijn voor zowel anionen als kationen door gelijktijdige migratie langs de respectievelijke zones met verschillende lading. Bijgevolg kunnen preferentiële permeabiliteiten van zouten over organische materie verwacht worden. In het tweede deel van dit proefschrift werd daarom een nieuw type CMM ontwikkeld, vertrekkend van een polymeermengsel van poly(methyl methacrylaat) (PMMA) en poly(vinylbenzyl chloride) (PVBC). De gewenste fasescheiding in het membraan werd bekomen via “annealing” gevolgd door functionalisering van het polymeer om geladen groepen te vormen. De scheidingsperformantie van de bekomen CMMs werd bestudeerd, gebruik makende van voedingsmengsels met verschillende zouten en organische componenten.

VIII

List of Abbreviations and Symbols List of Abbreviations 1

HNMR ACN AF AFM ATR-FTIR BL BTB CA CMM CS DEX d-LbL DMF ELSD FO FTIR HA HFO HF-PAN HPLC IPA LbL MB MP NF P4VP

PAA PAH PAMPSA PAN PAN-H PC PDDA PE PEC PEEK PEI PEM PEMFC PES PET PLL PMMA

proton nuclear magnetic resonance acetonitrile acid fuchsin atomic force microscopy

attenuated total reflection fourier transform infrared bilayer bromothymol blue cellulose acetate charged mosaic membrane chitosan dextran sulfate dewetting layer-by-layer N,N-dimethylformamide evaporative light scattering detector forward osmosis fourier transform infrared spectroscopy hyaluronic acids hydrated iron oxide hollow fiber membrane

high-performance liquid chromatography isopropyl alcohol layer-by-layer methylene blue

methanol permeability nanofiltration poly(4-vinylpyridine) poly(acrylic acid) poly(allylamine hydrochloride) poly(2-acrylamido-2-methyl-1-propane sulfonic acid) poly(acrylonitrile) hydrolyzed polyacrylonitrile proton conductivity poly(diallyl dimethyl ammonium chloride) polyelectrolyte polyelectrolyte complex poly(ether ether ketone) poly(ethylenimine) polyelectrolyte multilayer proton exchange membrane fuel cell polyethersulfone poly(ethylene terephthalate) poly(L-lysine) poly(methyl methacrylate) IX

List of Abbreviations and Symbols

PSS PSS-H PSS-Na PV PVBC PVIm PVS PVS-H PVS-Na PVSu RB RO SEM SPEEK sPPO SRNF THF UF

poly(sodium 4-styrenesulfonate) poly(styrene sulfonate) in acid form poly(styrene sulfonate) in sodium form pervaporation poly(vinylbenzyl chloride) poly(vinylimidazole) poly(vinylsulfonic acid sodium salt) poly(vinyl sulfate) in acid form poly(vinyl sulfate) in sodium form poly(vinylsulfonate) rose Bengal reverse osmosis scanning electron microscopy sulfonated poly(ether ether ketone) sulfonated poly(2,6-dimethyl 1,4-phenylene oxide) solvent resistant nanofiltration tetrahydrofuran ultrafiltration

List of Symbols A aq

Cif Cip H h J m

m Mw PolPol+ R(%) Ra RMS V Vm xi y yi α β γ δH ΔP Δt μ X

unit area of membrane [m2] aqueous phase component i concentration in feed component i concentration in permeate film thicknesses film thicknesses permeate flux[kg. m-2 .h-1] multilayer phase permeate mass [kg] molecular weight negative PE positive PE retention average roughness root mean square average roughness permeate volume [m3] molar volume retentate concentration of component i fraction of extrinsic sites permeate concentration of component i separation factor enrichment factor surface free energy [mJ/m2] increase in film thickness pressure difference [bar] Time [h] viscosity

Table of Contents Acknowledgment

I

List of publications

III

Abstract

V

Samenvatting

VII

List of Abbreviations and Symbols

IX

Table of Contents

XI

Chapter 1: General Introduction and Scope

1

1.1. Membrane separation processes

2

1.2. Advantages and limitations of membrane processes

2

1.3. Polyelectrolyte: definitions, properties and applications

2

1.3.1. The layer-by-layer (LbL) deposition method

3

1.3.2. Multilayer structure

4

1.3.2.1. The Zone Model for PEM Films

4

1.3.2.2. The growth modes of PEMs

5

1.3.3. Parameters affecting film growth during the LbL method

6

1.3.3.1. PE type

6

1.3.3.2. PE charge density

7

1.3.3.3. PE molecular weight

7

1.3.3.4. Supporting electrolyte concentration

8

1.3.3.5. Type of supporting electrolyte

8

1.3.3.6. pH

9

1.3.3.7. Temperature

10

1.3.4. Preparation of PEMs

10

1.3.5. Modifications for the enhancement of transport in PEM

11

1.3.5.1. Vibrations

11

1.3.5.2. Electric fields

12

1.3.5.3. Grafting and derivatization with functional groups

12

1.3.5.4. Cross-linking and hybridization

12

1.3.5.5. Annealing of PEMs

13

1.3.6. Applications

14

1.3.7. Membrane application of PEMs

14 XI

Table of Contents

1.3.7.1 Pervaporation

15

1.3.7.2. Nanofiltration

17

1.3.7.3. Solvent Resistant Nanofiltration (SRNF)

19

1.3.7.4. Reverse Osmosis (RO)

19

1.3.7.5. Gas separation

19

1.3.7.6. Fuel cells

20

1.3.7.7. Forward osmosis (FO)

21

1.4. Charged mosaic membranes (CMMs)

22

1.5 Dissertation overview

23

References

25

Chapter 2: PEM NF Membranes for Anion Separation 2.1. Introduction

38

2.2. Experimental

39

2.2.1. Materials

39

2.2.2. Membrane preparation

39

2.2.3. Characterization

39

2.2.3.1. Fourier transform infrared spectroscopy (FTIR)

39

2.2.3.2. Atomic force microscopy (AFM)

39

2.2.3.3. Scanning electron microscopy (SEM)

39

2.2.4. Filtrations 2.3. Results and discussion

40 40

2.3.1. FTIR

40

2.3.2. SEM

41

2.3.3. Film thickness

42

2.3.4. Single salt NF

43

2.3.5. Binary solutions NF

48

2.3.6. Mixed-salt NF

49

2.3.7. pH influence on the performance of the PEC membranes

51

2.4. Conclusions

51

References

52

Chapter 3: Weak Polyacid-Based PEM Membranes for SRNF

XII

37

55

3.1. Introduction

56

3.2. Experimental

57

Table of Contents

3.2.1. Preparation of multilayered membranes

57

3.2.2 Characterisation

59


59


59


59

3.2.3. Filtrations 3.3. Results and discussion

59 60

3.3.1. Membrane thickness

62

3.3.2. Cross section of multilayered PEC membranes

63

3.3.3. Filtration properties of the membranes

64

3.4. Conclusions

68

References

70

Chapter 4: Influence of PE Type & Counter Ion on the SRNF Performance of PEM Membranes

75

4.1. Introduction

76

4.2. Experimental

77

4.2.1. Materials

77

4.2.2. Preparation of membranes

78

4.2.3. Characterization

80


80


80

4.2.3.3. Atomic Force Microscope (AFM)

80

4.2.4. MB absorption studies

80

4.2.5. Filtrations

80

4.3. Results and discussion

81

4.3.1. Cross section of multilayered PEC membranes deposited on PAN-H supports

84

4.3.2. AFM results

85

4.3.3 MB studies

85

4.3.4. Separation properties

86

4.3.4.1. Effect of the positively charged capping layer

89

4.3.4.2. Effect of the ionic cross-linking

90

4.4. Conclusions

92

References

94 XIII

Table of Contents

Chapter 5: Charged mosaic membranes prepared from a polymer blend

99

5.1. Introduction

100

5.2. Experimental

101

5.2.1. Materials

101

5.2.2. Membrane preparation

101

5.2.2.1. Support Preparation

101

5.2.2.2. CMM Preparation

101

5.2.3. Characterization 5.2.3.1.

Attenuated total reflection spectroscopy (ATR-FTIR)

102 fourier

transform infrared 102


102


102

5.2.3.4. HPLC/ELSD Analysis

102

5.2.3.5. Porometer measurement

103

5.2.4. Filtration

103

5.3. Results and discussion

103

5.3.1. ATR-FTIR

103

5.3.2. AFM

104

5.3.3. SEM micrographs of PAN support and CMM

104

5.3.4. Separation properties

106

5.4. Conclusions

111

References

113

General Conclusions

XIV

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3

Weak Polyacid-Based PEM Membranes for SRNF

Abstract

Alternating depositions of oppositely charged polyelectrolyte (PE) complexes (PECs) were recently reported to create stable solvent resistant nanofiltration (SRNF) membranes with high flux and selectivity. The new combination of poly(diallyldimethylammonium chloride) (PDDA) with a weak polyacid (polyacrylic acid, PAA) was investigated in this report, since the charge density of these PEC based membranes can be affected by more factors than with strong polyacids, such as salt concentration and pH. This offers more possibilities to control membrane morphology and performance. Prepared membranes showed more than 90% retention for Rose Bengal (RB) from different solvents, proving the good potential of these membranes in SRNF applications. Keywords: Membrane; Polyelectrolyte complexes; SRNF; Membrane morphology; Layerby-layer method

Based on: P. Ahmadiannamini, X. Li, W. Goyens, N. Joseph, B. Meesschaert, I.F.J. Vankelecom, Multilayered Polyelectrolyte Complex Based Solvent Resistant Nanofiltration Membranes Prepared from Weak Polyacids, Journal of Membrane Science, 394–395, (2012) 98-106.


3.1. Introduction The layer-by-layer (LBL) method is a simple and versatile method to prepare thin films and membranes for different applications, allowing easy control over thickness and surface properties [1-4]. This is normally done via the alternating exposure of a charged substrate to solutions of positive and negative polyelectrolytes (PE). Provided that each adsorption step leads to charge inversion of the surface, the subsequent deposition finally results in a layered complex, a so-called self-assembled PE complex (PEC), stabilized by strong electrostatic forces. Films prepared via this method are in particular attractive materials with controlled thickness in the nanometer range for membrane separations. Nanofiltration (NF) is such a membrane process in which liquid feeds are separated over a membrane by means of pressures between 5 and 20 bars. Large scale applications currently exist in waste water treatment and drinking water production [5-7]. The range of NF-applications has recently been broadened to organic feeds in solvent-resistant NF (SRNF) [8-13]. A more widespread use requires solvent-resistant membranes that preserve their separation characteristics under more aggressive conditions of aprotic or strongly swelling solvents and elevated temperatures. PEC-based membranes have already proven to be promising materials for various membrane separations, namely pervaporation (PV) [14-17], nanofiltration (NF) [18-29], reverse osmosis (RO) [30, 31], forward osmosis (FO) [32, 33] and fuel cells [34-36]. Due to the electrostatic repulsion forces, PECs possess the ability to reject ions with a charge opposite to that of the last deposited layer. Because of the high charge density, this repulsion is even maintained in very open structures. PEC films have been demonstrated to be stimuli-responsive materials since the structure can self-rearrange upon various external stimuli like ionic strength [37, 38], pH [39, 40], temperature [41, 42] and solvents [43-45]. We recently reported the first use of PEC membranes in SRNF [46, 47]. The results demonstrated that PEC membranes show a good potential for use in SRNF, especially in aprotic solvents, like dimethylformamide (DMF) and tetrahydrofuran (THF), which are really troublesome in SRNF applications. Sulfonated poly(ether ether ketone) (SPEEK) was used as negatively charged polymer. The interactions between SPEEK and the positively charged poly(diallyldimethylammonium chloride) (PDDA) formed thin and defect-free membranes. Recently, plenty of work on weak PEs as building blocks for PECs has been carried out. As their charge density can be tuned by changing the pH, these systems thus provide extra levels of control over morphology and performance of PECs [48-56]. A higher charge density is expected to lead to thinner deposited layers. Indeed, the self repulsion of the chains is increased and such polymer chains thus possess a larger cross-sectional area in solution, thus covering a larger surface upon adsorption on the substrate [57]. The structure of PEC films can be rationalized by a model put forward by Ladam et al. [58]. Typically, the cross section of PEC films can be subdivided into three zones. Zone I consists of the first few layers close to the substrate, which are mainly influenced by the substrate, while Zone III is composed of a few layers close to the upper surface of the film. Zone II in between is the “bulk” film, which is not affected by either interface or substrate and the PE charge compensation is achieved by means of intrinsic charge compensation. When the first few layers are deposited, the zones I and III are formed. When the number of layers is further increased, zone II emerges and the multilayer growth takes place in zone II only [59]. In this chapter, a second type of PEC-based membranes is investigated for SRNF56


applications. A weak PE, polyacrylic acid (PAA), was selected to prepare PEC membranes under different conditions. The influence of the salt concentration and pH of the PE solutions on the properties of the membranes was studied in detail and linked to the membrane performance in SRNF. 3.2. Experimental Polyacrylonitrile (PAN, Mw = 150,000 Da) was purchased from Scientific Polymer Product. PDDA (Mw = 200,000-350,000 Da) was obtained from Aldrich as a 20 wt% aqueous solution. PAA (Mw = 5000 Da) was purchased from Acros as a 50% aqueous solution. Isopropyl alcohol (IPA) , THF, acetonitrile (ACN) and DMF were obtained from VWR, Chem Lab, Fisher Chemicals and Sigma-Aldrich, respectively and were used as solvents. Bengal rose sodium salt (RB) (dye content ~ 95%), acid fuchsin (AF) (dye content ~ 70%) and bromothymol blue (BTB) (dye content, 95%) were purchased from Sigma-Aldrich and used as solutes. The characteristics of the solutes used in this chapter are listed in Table 3.1. Table 3.1. Some of the main solute properties. Component (Mw, g/mol)

Charge

Molar volume (cm3/mol)

Rose Bengal (1017)

−2

272.8

Acid Fuchsine (585.50)

−2

246.9

Bromothymol Blue (624.39)

0

281.3

Structure

3.2.1. Preparation of multilayered membranes The PEC-based membranes were prepared by means of an automated dip-coater (HTML, Belgium) [60]. The equipment comprises four separate vessels containing both PE solutions as well as two distillated water washing solutions (Figure 3.1). Dipping time, 57


agitation during dipping via up and down movements, drip time, number of cycles and oscillation speed was directed in a control device.

Figure 3.1- Demonstration of the automated dip-coater (HTML, Belgium). PEs were dissolved in aqueous medium at a concentration of 0.2 wt%. Hydrolyzed PAN (PAN-H) was used as support. The PAN support was prepared via phase-inversion. The PAN-H support was obtained by immersing the PAN support in an aqueous NaOH solution. The remaining NaOH was removed by washing with water and followed by immersion of the PAN-H support in an aqueous HCl solution to convert the -COONa into -COOH groups [46].

Scheme 3.1- Preparation of multilayered PEC membranes To adsorb each PEC-layer, the PAN-H support was immersed in the solution of the cationic PE (PDDA), followed by rinsing with water, then immersed in the solution of the anionic PE (PAA) and rinsed again with water. The described procedure was repeated until a maximum of 20 pairs of polycation/polyanion bilayers was adsorbed (Scheme 3.1). Immersion time in the individual solutions was 5 min. The salt concentration in both PEs was 58


controlled by adding different amounts of NaCl. Salt concentrations of 0, 0.1, 0.2 and 0.5 M were selected, referred to as PDDA/PAA-0, PDDA/PAA-0.1, PDDA/PAA-0.2 and PDDA/PAA-0.5. Four different pHs (2, 3, 4 (the natural pH of the PAA solution) and 5) were selected, which are referred to as pH2-PDDA/PAA, pH3-PDDA/PAA pH4-PDDA/PAA and pH5-PDDA/PAA respectively. The pH was adjusted each time by adding HCl or NaOH. No pH adjustment for samples prepared with different salt concentration was done and similarly, no salt was added to the samples prepared from solutions with different pH. A similar method was used for deposition of the reference systems prepared on silicon wafer for AFM and FTIR characterization. 3.2.2 Characterisation 3.2.2.1. Fourier transform infrared spectroscopy (FTIR) FTIR spectra were acquired using a Bruker IFS66V/S spectrophotometer in the transmission mode. 3.2.2.2. Scanning electron microscopy (SEM) SEM (Philips XL FEG30) was carried out to study the cross-section and surface structure of the membranes. The cross-section was obtained after breaking the membranes in liquid nitrogen. The SEM samples were first gold coated before use. 3.2.2.3. Atomic force microscopy (AFM) AFM experiments were performed using a Multimode AFM with a Nanoscope IV controller (Veeco/Digital Instruments, Santa Barbara, USA). Samples were imaged in air in tapping mode with a drive frequency of 200–300 kHz. Silicon nitride oxide-sharpened tips (NCHR, Nanosensors, Germany) were used. The average roughness Ra and the root-meansquare value (Rms) were calculated by eq (3.1).

Ra 

1 N  Zi  Z N i 1

and

RMS 

1 N

N

Z

i

Z

2

(3.1)

i 1

The multilayered films for thickness measurement were prepared under the same conditions but on a silicon wafer. The film thickness was determined by measuring the depth of a scratch after indentation of the film with a sharp knife. The imaging was performed after adjusting the tip to the scratch edge [46]. 3.2.3. Filtrations SRNF-experiments were performed using a high-throughput apparatus (HTML, Belgium) [60] containing 16 filtration cells with 4.53 cm2 membrane area each. The system was pressurized with nitrogen to 40×105 Pa (40 bar). During filtration, the feed solution was stirred at 11.66 Hz (700 rpm) to minimize concentration polarization. Permeate samples were collected in cooled flasks as a function of time, weighed and analyzed. The cells were filled up by about 40 ml of feed solution per membrane and about 5 ml of the permeate was collected for each membrane. The long-term stability of PEC membranes in THF was carried out using a SterlitechTM HP4750 stirred cell with an active area of 14.6 cm2. The cell was 59


filled with 300 ml of feed solution and was pressurized with nitrogen to 0.5×105 Pa (0.5 bar). During filtration, the feed solution was stirred at 16.66 Hz (1000 rpm) to minimize concentration polarization. The retention values were calculated from concentrations of the permeate and of the original feed solutions according to eq (3.2).  c R  1  p  c f 

   100% 

(3.2)

were Cp and Cf are solute concentration in the permeate and the feed, respectively The permeation was stopped when the retention reached a constant value. All measurements were based on at least three samples, and the average values were used. 3.3. Results and discussion Multilayered PEC films were first prepared on a silicon wafer to fundamentally study the interactions between PAA and PDDA. Membranes were prepared at different pH and with addition of different amounts of salt to investigate the changes in membrane morphology and performance. The FITR spectra of the pH4-PDDA/PAA films containing different bilayer numbers are shown in Figure 3.2a. Peaks at 1562 and 1457 cm -1 are attributed to asymmetrical −COO and symmetrical −COO stretchings of PAA. The peak at 1720 cm -1 is attributed to C=O groups stretching [59]. The peaks clearly increase with increasing bilayer number, which is strong evidence for the LBL growth. With the salt concentration increasing from 0 to to 0.1 M, a slight increase in the transmittance peak at 1720 cm -1 was observed. With further increase of the salt concentration to 0.5 M, the transmittance peak was decreased (Figure 3.2b). Normally, by adsorbing polyions from salt solutions with varying ionic strength, the layer thickness can be controlled. Screening the charges by salt ions decreases the electrostatic interactions because the characteristic decay length (Debye length) of an electrical potential is shorter in solutions at higher salt concentration [61]. As a consequence, the radius of gyration of PEs will be smaller due to decreased self-repulsion, resulting in chains with more coils and a more loopy structure. Upon adsorption, this will lead to a lower surface area per chain, hence a larger thickness for membranes with comparable numbers of bilayers [57]. At higher salt concentrations, however, another phenomenon is also taking place: diffusion of oppositely charged polymer into the surface is enhanced by freeing up individual segments from previously deposited layers via competitive ion exchange [62] and thickness of PEC multilayers reduces consequently. Furthermore, dissociation of multilayers can take place at higher salt concentrations, where the remaining intrinsic compensation (polymer/polymer ion pairing) is no longer sufficient to keep the multilayers together. This may cause a decrease in thickness [59, 62]. When increasing the pH from 2 to 4, the characteristic FTIR-peaks of PAA decrease, reflecting a decreased thickness of the PECs. This decrease in thickness of multilayers results from the decreasing segmental population of deposited loops per coating that occurs as the PAA chains become more charged with increasing pH. Indeed, fewer polymer chains are then needed to compensate the earlier deposited charges. When the pH of the solution is further increased beyond 4, a remarkable thickness transition is observed. The intensities of adsorbed PEC increase rather than continuing to decrease with increasing pH. A thickness transition behavior of this type has been previously reported by Rubner et al., also using PAA as polyanion [55, 63].

60


Figure 3.2- FTIR of multilayered PECs based on PDDA/PAA prepared under different conditions (a) Different numbers of PDDA/PAA bilayers (no salt added), (b) 20 bilayers of PDDA/PAA prepared from PE solutions with different salt concentration (c) 20 bilayers of PDDA/PAA prepared from PE solutions at different pH. A thermodynamic model that captures all features observed experimentally was put forth to explain this behavior. In essence, the model indicates that, at a critical high surface charge density, the entropic penalty for spreading a chain in its flat conformation on the surface 61


(molecularly thin layer) is overcome by the enthalpic gain to the free energy of adsorption. As the surface charge density decreases below this point, however, the energy gain for spreading a chain over the surface is not sufficient to overcome the loss in configurational energy, and a sharp transition to a thicker layer with a high segmental population of loops occurs. As observed experimentally, this transition to a thick layer was predicted to occur when the surface charge density dropped somewhat below its fully charged state. 3.3.1. Membrane thickness AFM experiments were performed using a Multimode AFM with a Nanoscope IV controller. The thickness and roughness of the films were investigated by AFM. The PEC thickness, as obtained from AFM scratching on a PEC coated silicon wafer, is shown in Figure 3.3. Obviously, the film thickness increases in all cases with increasing PDDA/PAA bilayer number. With the salt concentration in the PE solutions increasing from 0 to 0.5 M, the thickness of membranes first increases and then decreases except for the 5 bilayer system, where the thickness hardly varies. Possibly the differences in thickness are too small to be measured accurately for these thin layers containing 5 bilayers only (Figure 3.3a). A thickness increase at high ionic strength has been ascribed to the formation of more coiled PEs in solution which occupy a smaller area upon adsorption [59].On the other hand, higher salt concentrations can also lead to decomposition of multilayers, which might cause a decrease in thickness [59, 62].

Figure 3.3- The thickness of multilayered PDDA/PAA PEC membranes prepared under different conditions of (a) salt concentration and (b) pH. The thickness of films prepared from solutions with different pH in Figure 3.3b gives a 62


decreasing tendency with increasing pH from 2 to 4. These changes in thickness can be related to the increased charge density of PAA chains at higher pH value, thus requiring less material to compensate all charges present on the earlier deposited layers [63]. When the pH of the solution is increased to 5, at which PAA chains are nearly fully charged the thickness increases in line with the previously reported results [55]. With addition of salt, the roughness of the resulting membranes decreases significantly (Figure 3.4). This follows from the changed conformation of PAA in solution: the adsorption of loopy structured chains will take place with introduction of salts in the PE solution, making the surface smoother.

Figure 3.4- Roughness of multilayered PDDA/PAA membranes prepared from solutions with different salt concentration (1μm×1μm scan scale). 3.3.2. Cross section of multilayered PEC membranes Figure 3.5 shows the cross section of a (PDDA/PAA-0.2)20 deposited on a porous PANH. A clear top layer image can be seen at higher magnification (bottom), which gives direct evidence for the growth of a homogeneous PEC layer on the membrane support. The thickness of the top layer is in the same range with the AFM scratching result.

Figure 3.5- The cross section of a multilayered PEC membrane on PAN-H support (20 bilayers PDDA/PAA-0.2)

63


3.3.3. Filtration properties of the membranes Figure 3.6 gives the filtration data of the membranes prepared from solutions with different salt concentrations (PDDA/PAA-0, PDDA/PAA-0.1, PDDA/PAA-0.2 and PDDA/PAA-0.5) as a function of bilayer number and using RB as solute in IPA. Logically, the fluxes of the membranes are decreasing with increasing bilayer numbers. The retention values do not significantly change with deposition of more bilayers. According to the threezone model the zones I and III are formed already after a few layers are deposited. Deposition of more layers is proposed to result in the further growth of zone II only, thus decreasing membrane fluxes while keeping retentions intact [57]. As can be seen, the permeance of membranes consisting of the same number of bilayers shows an increase followed by a decrease with increasing salt concentration in the deposition solutions. As described above, adding salt into the PE solutions leads to deposition of more loopy-structured chains which consequently make the material less dense. At higher salt concentrations, however, inter diffusion of oppositely charged PE chains can cause a decrease in flux.

Figure 3.6- The SRNF performance of multilayered PEC membranes prepared from polyanion solutions with different salt concentration. The pH of all solutions was kept at 4. All membranes show RB retentions higher than 70%. The membrane prepared from a 0.5 M NaCl solution is clearly the least selective, due to the loopy polymer chains that lead to looser membrane structures. Further, the dissociation of PECs at high ionic strength might be 64


another factor that affects their retention [59, 62]. For the assembly of weak polyanions, the pH of the dipping solution is an important factor to affect film formation. In this case, pH controls the charge density of the adsorbing polymer, leading to an unprecedented ability to control this process, as compared with the earlier reported SRNF membranes made from SPEEK [46, 47]. Figure 3.7 gives the SRNF-performance of the membranes prepared by varying the pH of the PAA solutions (pH2-PDDA/PAA, pH3-PDDA/PAA pH4-PDDA/PAA and pH5PDDA/PAA). Similar to the membranes prepared at different ionic strength, the retention values do not change with deposition of more bilayers. This trend is in good agreement with the zone model for PECs build-up mechanism in which deposition of further bilayers results in the growth of zone II only [57]. The membranes prepared at pH 5 show the highest permeances despite their higher thickness. The higher thickness is however overruled here by the lower density of these adsorbed layers due to their loopy structure. The membranes prepared at this pH indeed also show lower RB retentions.

Figure 3.7- The SRNF properties of multilayered PDDA/PAA PEC membranes prepared from polyanion solutions with different pH. Salt was not added to the solutions with different pH.

65


As reported earlier, PEC based membranes are stable in a wide variety of organic solvents. THF was selected for further filtration to demonstrate the chemical stability of the membranes. Since 5 bilayers were enough already to form defect-free membranes, these membranes were selected for this filtration. The membranes show a very high RB retention combined with a good flux, thus showing good potential use of these membranes in this type of solvent (Figure 3.8). Fluxes for THF were much higher than for IPA which can be due to the different physico-chemical properties of the solvents and the interactions between the membrane and the solvent. As quantified by Bhanushali et al. with the Vm/μ parameter, the most important physical properties of the solvents are viscosity (μ) and molar volume (Vm) [64]. THF has a much higher Vm/μ than IPA, which might explain the higher THF flux. However, the interactions between solutes and solvent, solvent and membranes may play an additional role still as well [65].

(a)

(b)

Figure 3.8- The SRNF performance in a RB/THF solution of a PEC membrane prepared from PE solutions containing 5 bilayers with (a) different pH (no salt added) and (b) different salt concentration (pH was kept at 4). To broaden the SRNF applications of prepared membranes, (PDDA/PAA-0)20 was selected to test with harsh organic solvents, such as DMF and ACN. The results are presented in Figure 3.9, showing that the membrane could effectively reject RB from DMF and ACN solutions. Furthermore, long-time stability of the (PDDA/PAA-0)20 was evaluated by filtration of a feed solution containing RB in THF for a period of 48 h. The results are depicted in Figure 3.10, showing very promising long-time stability of PEC membranes in SRNF application. 66


Figure 3.9- The SRNF properties of (PDDA/PAA-0)20 in RB/DMF and RB/ACN solutions.

Figure 3.10- The long-time SRNF properties of (PDDA/PAA-0)20 in a RB/THF solution.

Figure 3.11- The SRNF properties of (PDDA/PAA-0)20 in AF/IPA and BTB/IPA solutions. 67


In addition, filtrations with smaller solutes (AF and BTB) in IPA were carried out using a (PDDA/PAA-0)20 membrane. The results are presented in Figure 3.11, showing a high retention on AF and a lower retention on BTB. The order of retention for different solutes in IPA is RB>AF>BTB, respectively. Observed retentions prove that solute transport through PEC membranes is a function of both charge and density of the active layer, retaining the solutes through Donnan (electrostatic) and sieving effects (steric hindrance), respectively. Comparison of the membranes from the present study with previously reported PEC multilayer membranes in SRNF [46-47] reveals that the membranes containing weak polyanions have RB retentions comparable to those of membranes prepared from strong polyanions. Contrary to the applications in aqueous NF [29, 63], membranes constructed from weak PEs thus do not show lower retentions in SRNF than membranes prepared from strong PEs (Tables 3.2 and 3.3). Permeances generally seem lower for the membranes prepared from weak polyanions. Table 3.2- NF performance of PEC multilayer membranes in aqueous applications. Membrane

Permeance

Retention (%)

Reference

(PAA pH 4.5/PDDA pH 4.5)4.5

(l.m .bar .h ) 12.15 ± 4.34

Cl−4.4 ± 4.3

(PSS/PDADMAC)4.5

20.83 ± 0.86

−15.5 ± 1.4

92.3 ± 0.9

[27]

(PDADMAC/SPEEK)5

15.6 ± 1.04

15.9 ± 4

93.9 ± 1

[29]

-2

-1

-1

SO4224 ± 14

[27]

Table 3.3- SRNF performance of PEC multilayer membranes in solvent applications (RB in IPA). Membrane

Permeance(l.m-2.bar-1.h-1)

RB retention (%)

Reference

(PAA pH 4/PDDA pH 7)5

0.03

97.0 ± 2.1

(PDADMAC/SPEEK)5

0.08

98.2

present work [47]

The conformation of PEs in solution but even after deposition is highly dependent on the solvent quality. In the presence of a good solvent, the polymer chains will try to maximize the polymer/solvent contacts and swell. In the case of a poor solvent, the change of the PE conformation in solution is realized due to progressive screening of the intrachain electrostatic repulsion and the increasing influence of hydrophobic attraction. As a result, the PE chains adopt a more compact conformation in order to reduce polymer/solvent interactions [45]. The lower SRNF permeances for prepared PEC membranes compared with those in aqueous NF applications could be due to the compaction of the membranes upon exposure to IPA. However, swelling of the PEC membranes could be another key parameter to influence the permeability. As shown by Miller et al., films prepared from LBL deposition of different PE combinations exhibited stronger swelling in water than in ethanol [66]. Higher swelling in water can obviously lead to higher fluxes and lower retentions in aqueous NF compared. 3.4. Conclusions Multilayered PEC membranes were successfully prepared from PDDA and PAA polyions by the LBL method. For the first time, the SRNF performance of these membranes was studied in detail. The role of different factors that affect the membrane formation and 68


further membrane performance, like salt concentration during preparation and pH of the PE solutions, were studied. AFM and SEM showed that a thicker membrane surface can be obtained by adding salt to the PE solutions during membrane preparation. As the pH of PE solutions increases, normally thinner layers were deposited, although they underwent a thickness transition at a critical pH. Resulting membranes proved to be very useful for filtrations in organic solvents, including aprotic solvents like THF, for which they showed excellent solvent stability and good retentions in the NF-range.

69


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