Advances in membrane development based on electrically conducting polymers

Advances in MembraneDevelopment Based onElectrically ConductingPolymers

JAVED ALAM, LAWRENCE AROCKIASAMY DASS,MANSOUR SALEH ALHOSHANKing Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia

ABDUL WAHAB MOHAMMADDepartment of Chemical and Process Engineering, Faculty of Engineering and Built Environment,University Kebangsaan Malaysia, 43600 UKM-Bangi, Selangor, Malaysia

Received: November 23, 2011Accepted: January 8, 2011

ABSTRACT: The use of electrically conducting polymers for membranedevelopment is an interesting research area of membrane science and technology.Electrically conducting polymers have been identified as a promising class ofmaterials for membrane development owing to their unique electroactiveproperties, reversible redox behavior, and the potential for ease of processing. Inthe present review, an attempt has been made to describe the advances in the useof electrically conducting polymers to membrane development and tounderstand their salient features and how these features can be optimized toexhibit advanced or novel functions in membrane development. This article alsopresents significant literature detailing the scientific interest in electrically

Correspondence to: Javed Alam; e-mail: [email protected], [email protected].

Contract grant sponsor: King Abdullah Institute for Nanotech-nology, King Saud University.

Advances in Polymer Technology, Vol. 00, No. 0, 1–9 (2012)C© 2012 Wiley Periodicals, Inc.

ADVANCES IN MEMBRANE DEVELOPMENT BASED ON ELECTRICALLY CONDUCTING POLYMERS

conducting polymers for membrane development. C© 2012 Wiley Periodicals,Inc. Adv Polym Techn 00: 1–9, 2012; View this article online atwileyonlinelibrary.com. DOI 10.1002/adv.21262

KEY WORDS: Electrically conducting polymers, Electroactive properties,Membrane development, Novel functions, Reversible redox behavior

Introduction

T oday, the field of electrically conducting poly-mers has become an interdisciplinary area of

worldwide research and development that has beengrowing explosively, from novel science to new tech-nology, in the past couple of decades.1−7 Electri-cally conducting polymers are rapidly gaining at-traction in new applications because they exhibitan extraordinary range of material properties (suchas electrical and electrochemical properties (likemetals), ease of processing (like polymers), andthe possibility of both chemical and electrochem-ical synthesis.8−12 The use of electrically conduct-ing polymers for membrane development is an in-teresting but less explored area of membrane sci-ence and technology. Electrically conducting poly-mers are used as membrane materials because oftheir barrier properties, which may be tuned byelectrochemical control of the polymer’s oxidationstate, and the versatility of synthesis (as freestandingfilms, composites, layer structures, or as thin filmson conventional substrates).13−19 Also, researchersare applying electrically conducting polymers formembrane development, by taking advantage oftheir reversibly switchable wettability and selectiv-ity, both of which are vital to meet the challenge ofmembrane development.20−27

SCOPE AND DEVELOPMENTS

Separation by selective permeation through poly-mer membranes is one of the fastest growingbranches of membrane technology. Therefore, stronginterest exists in the synthesis of new polymersthat exhibit both higher permeabilities and se-lectivities than presently available polymers.28−31

Electrically conducting polymers are an extremelyfascinating class of polymers with a number of in-teresting features that make them potentially impor-tant materials for the development of membrane,and interestingly they are well known for their se-

lective mass transport characteristics, owing to theirchemical tunability, their stimuli-responsive behav-ior, and their remarkable chemical and electrochem-ical properties. Several electrically conducting poly-mers (such as polyaniline and polypyrrole) havebeen significantly employed to demonstrate howtransport of liquids and gases can be regulated us-ing their dynamic properties (thereby altering theoxidation state of the polymer).32−42

As a new class of materials, electrically conduct-ing polymers are very important from the view-point of design and development of membranesfor a wide range of separation applications (Fig. 1).For the development of membrane, the followingadvances are reported in the use of conductingpolymers:

� thin-film composite membrane,� ion-exchange membrane, and� mixed-matrix nanocomposite membrane.

THIN-FILM COMPOSITES MEMBRANE

Electrically conducting polymers, as a new classof polymers, open up a venue for the growth of the

FIGURE 1. Scope of electrically conducting polymer tomembrane technology.

2 Advances in Polymer Technology DOI 10.1002/adv


thin-film composites membrane. Electrically conduct-ing polymers are used as a part of the thin-film com-posite membrane because of their ion selectivity aswell as their ease of processibility. They can be eas-ily deposited as the coating medium on a wide vari-ety of supporting materials, such as porous metals,filter paper, ion-exchange membranes, microfiltra-tion membranes, fibers, stainless steel, and polymermembranes. Also, they may be directly synthesizedon the membrane by an electrochemical depositionor by chemical polymerization. Electrically conduct-ing polymer coated membranes exhibit unique prop-erties such as alteration of transport of ionic speciesacross the membrane by switching on and off appro-priate electrical potential and the ability to changefrom an oxidized state to a reduced state, which re-sults in controlling the transport rate and the selec-tivity of the membrane. In addition, the chemicaland physical properties of the electrically conduct-ing polymer film can be manipulated in situ by theapplication of the electrical stimuli and doping pro-cess. This in turn allows modulation of the trans-port properties and selectivity of the membrane. Theconcept of using conducting polymers for the devel-opment of thin-film composite membranes was ini-tially reported by Burgmayer and Murray,38 wherethey demonstrated that a polypyrrole-coated mir-croporous gold membrane could act as an ion gate.The polypyrrole was electrochemically depositedonto the gold mircroporous substrate, and the ox-idation state of the polypyrrole film was electro-chemically altered before placing the membrane ina transport cell. They also demonstrated that alter-ing the oxidation state of the polypyrrole influencesthe permeability of several ionic species through thismembrane.

Thin-film composite membranes are generallycharacterized by a nonuniform structure compro-mising a dense selective active top layer or skin sup-ported by a porous sublayer (see Fig. 2).43 Such a typeof noble membrane provides fast flow, low-pressuredrops, and highly consistent flow rates in separationapplications. They are different from conventionallycast nano- or microporous membranes because thelarger pores on the upstream side of the membraneact as a prefilter whereas the absolute-rated down-stream side, or the exclusion zone, acts as an absolutecutoff layer. This is in contrast to traditional nano-or microporous materials that have comparable poresizes on both upstream and downstream sides of themembrane.44,45

In the literature, the selectivity of both gases andliquids separation and ion exchange in electronically

FIGURE 2. Morphology of thin-film compositemembrane.

conducting polymers, coated polymers such as poly-carbonate, cross-linked polystyrene, polypropylene,and polyamide porous substrates are reported to beconsiderably higher than that of the originally un-coated ones.28−31

ION-EXCHANGE MEMBRANE

It is well known that electrically conducting poly-mer films can act as ion-exchange membranes dur-ing reversible oxidation and reduction. Polyanilineand polypyrrole are two conducting polymers thatundergo ion exchange for maintaining charge bal-ance in the polymer. The separation in these mem-branes is based on the electrical potential and/ormagnetic field “barrier” generated by passing cur-rent through the membrane, whereas the sepa-ration in a conventional ion-exchange membranerelies on the relative ionic affinity of the com-pounds being separated. It is this characteristicthat not only provides a unique opportunity for anew class of separation systems but can also pro-pose many applications such as drug release, sen-sors, rechargeable batteries, and controlled potentialseparation.46,47

Ion-exchange membranes are generally fabri-cated from polymeric materials containing poreswith diameters of less than 20 A. The transport prop-erties of ions in these membranes are attributed tostrong interactions between the permeating speciesand the molecular structure of the polymer. Thisinteraction is attributed to the presence of ion-exchange groups in the membrane, which allowthe membrane to discriminate between permeat-ing or migrating ions by virtue of their specificcharge. However, the nature of separation mech-anism in electrically conducting polymer mem-branes differs from the conventional ion-exchangemembrane. The transport of charge-compensatingions that accompany the doping–dedoping process

Advances in Polymer Technology DOI 10.1002/adv 3


(chemical and electrochemical redox switching ofelectrically conducting polymers between conduc-tive and insulating states) elicits the interest ofusing conducting polymer as ion-exchange ma-terials. A number of studies have reported thedevelopment of an electrochemically switchable ionexchanger based on conducting polymers in whichpolypyrrole is one of the most commonly investi-gated electrically conducting polymers for the devel-opment of ion-exchange membranes.48−53 A reviewof the literature revealed that polypyrrole synthe-sized in solutions with small dopants such as Cl−,ClO−

4 , and NO−3 mainly exhibits anion-exchanger be-

havior because of the high mobility of these ionsin the polymer matrix.54−58 Under certain condi-tions, cation exchange was also found to take placewith large dopants such as polyvinylsulfonate andpolystyrenesulfonate, as a result of immobility ofthese ions in the polymer matrix. Furthermore, itwas well established that polypyrrole prepared byboth chemical and electrochemical polymerizationreactions usually carries charges in the polymer, i.e.,some of the nitrogen atoms in polypyrrole are posi-tively charged. To maintain charge neutrality, someof the exchangeable counteranions present in thepolymerization solution are incorporated into thegrowing polymer during polymerization. The exis-tence of positively charged nitrogen atoms in thepolymer provides a good prospect for its applica-tion in the adsorption of anions such as fluorideions.59,60

MIXED-MATRIX NANOCOMPOSITEMEMBRANE

Another breakthrough in the use of electricallyconducting polymers in membrane developmentis the idea of making a mixed-matrix nanocom-posite membrane. Recent advances in the field ofmembrane science and technology strongly sug-gest that many of the current problems involv-ing water quality can be addressed and potentiallyresolved by the use of the mixed-matrix nanocom-posite membrane.61−64 Mixed-matrix nanocompos-ite membranes are a new generation of mem-branes that provide several potential advantagesover bulk membranes. A mixed-matrix nanocom-posite is normally defined as the incorporation ofa nanomaterial (dispersed) phase into a polymermatrix. Generally, inorganic nanomaterials are usedin membrane preparations to improve their limitedchemical, mechanical, and thermal resistance. As re-

ported in the literature, polysulfone, polyethersul-fone, and polyvinylidene fluoride are among thehighest employed polymers in membrane fabrica-tions because of their excellent membrane-formingabilities. They can also be used as matrices withinorganic nanoparticles such as ZrO2, SiO2, TiO2,Al2O3, and zeolites. However, inorganic nanomate-rials are very difficult to disperse into these mem-brane matrices; therefore, various nanostructures(such as nanofibers, nanotubes, and nanoparticles)of electrically conducting polymers have been in-vestigated to determine their properties and possi-ble applications in membrane development. Electri-cally conducting polymers are newly available or-ganic nanomaterials that are expected to make acritical impact on fabricating higher performancemembranes with increased permeability, selectivity,and resistance to fouling.65−68 The biggest advan-tage of these polymers is their processibility. Com-pared with inorganic nanomaterials, electrically con-ducting polymer nanomaterials have more func-tional groups. Thus, they could be easily boundwith polymers or be firmly fixed on substrate mem-branes through different chemical reactions. Apartfrom these qualities, nanostructured conductingpolymers play two important roles in membranedevelopment as both structurally strong memberswith greater corrosion, erosion, chemical, and foul-ing resistance, which provide operational energysaving, and as separation agents, which enhancethe membrane separation figures of merit in termsof selectivity and permeability. Several membranesystems based on conducting polymer nanomate-rials have been demonstrated to have improvedperformance with higher permeability as well asselectivity.69−76

Special Features of ElectricallyConductive Polymers

Electrically conductive polymers have been thesubject of study for past few decades as a possiblemembrane material. A facile and reversible electro-chemistry, where the polymer can be oxidized andreduced with simultaneous change in surface prop-erties, allows electrically conducting polymers notonly to act as an appealing alternative for specificmaterials currently employed for the fabrication ofmembranes but also to distinguish them from other



conventional membranes. Moreover, some more im-portant features that promote electrically conduct-ing polymers are as follows.

THE CONCEPT OF DOPING

The concept of “doping” is the unique, cen-tral, and underlying theme in conducting polymers,which was found to cause significant concurrentchanges in the membrane morphology and surfaceproperties, which controlled/enhanced permeabil-ity in gases and liquids. Doping is the process oftransforming a polymer to its conductive form viachemical oxidation or reduction.

Polymer + Dopant = (Doped polymer)+ + Dopant−

Polymer + Dopant = (Doped polymer)− + Dopant+

During the doping process, positive or negativecharge carriers are developed in the polymers bymeans of doping agents (e.g., either strong reducingagents or strong oxidizing agents). The doping pro-cess plays a very interesting role in membrane devel-opment; for example, it was found that some degreeof “nanoporosity” could be induced in the mem-brane by the electrically conducting polymers, dop-ing, dedoping, and redoping process.31,62−64,77−79

This can be explained in such a way that the con-ducting polymer membranes are doped with dif-ferent dopants before they were cast. Subsequent re-moval of dopants from the membrane by immersionin a strong base induces nanoporosity in the mem-branes. Thus, the dopants (large-sized acids) act assoft templates, creating nanoporosity in membranes.The doping process also enables:

� change the nature of surface such as reversibleswitchable wettability from hydrophobic tohydrophilic,80−84

� switch on/off ion selectivity,85

� control morphology,86,87 and� induce ion-exchange property.88

EASE TO PROCESS

Although early electrically conducting poly-mers were generally insoluble and infusible, greatprogress has been made in producing processibleelectrically conducting polymers via polymerizationfrom soluble precursors or by chemical modifica-tions (the inclusion of functional side groups as well

as substitution of the intractable backbone with dif-ferent types of substituents), which render the poly-mer itself soluble. As a matter of fact, electricallyconducting polymers show poor mechanical prop-erties that limit their large-scale industrial appli-cations, but the superiority of organic conductivematerials over their counterpart inorganic materialsresults in their tremendous architectural flexibility,their inexpensiveness, and ease of processing andfabrication. So far, they have been investigated as

� freestanding films/membranes,� nanomaterials in membrane host for the de-

velopment of mixed-matrix nanocompositemembranes,

� layer structures, and� deposited film for the formation of thin-film

composite membranes.

ANTIFOULING PROPERTIESPROSPECTIVE

Fouling is the most serious problem affectingsystem performance, which is generally attributedto the hydrophobic nature of membrane materials.The materials, which are commonly used in com-mercial membrane fabrication, belong to hydropho-bic materials including polysulfone, polyethersul-fone, and polyamides. Thus, they easily suffer se-rious membrane-fouling problems. In fact, electri-cally conducting polymers are not hydrophilic ma-terials, but interest has developed in their smartsurfaces.81,82 Electrically conducting polymers ex-hibit the reversible switching process betweenthe doped and undoped states with a simulta-neous change in surface properties (such as re-versibly switchable wettability, i.e., switch on/offhydrophilicity or hydrophobicity, surface charge,and dynamic nontoxic surface). This dynamic sur-face properties perspective promoted electricallyconducting polymers as new antifouling membranematerials, and they have proved as effective and apotential alternate for hazardous antifouling agentsto control the fouling of membrane. Some of themembrane systems based on electrically conduct-ing polymers have been employed to demonstratehow fouling can be regulated using electrically con-ducting polymer surface properties.80,89 In addition,conducting polymers are used as membrane mate-rials because of their ability to withstand extremetemperatures and harsh chemical conditions.



Scientific Interests inElectrically Conducting Polymersfor Membrane Development

The electroactivity, reversible redox properties,and their tremendous architectural flexibility of con-ducting polymers have attracted great interest inmembrane development. Many excellent researchpapers and reviews are published concerning the useof electrically conducting polymers for membranedevelopment, which promoted them as promisingmembrane materials. Because the literature relatedto application of electrically conducting polymers toseparation is vast, Table I presents only the insightfulreviews and research articles.

PROMISING CONDUCTING POLYMERS

From the perspective of technological impor-tance, electrically conducting polymers can begrouped into six main families: aniline, pyrrole,thiophene, phenylvinylene, acetylene, and pheny-lene and their derivatives. However, the majorityof membranes fabricated from these polymers have

used either polypyrrole or polyaniline, because theyexhibit novel properties that are not typically avail-able in other electrically conducting polymers.32,72

Polyaniline

Polyaniline is a model electrically conductingpolymer because of its simple acid/base dopingchemistry, chemical and thermal stability, as wellas its processibility. The interest in polyaniline as amembrane material stems from the chemically flex-ible NH group in its backbone, which is respon-sible for an interesting doping/dedoping chem-istry that can potentially enhance its characteristicsfor specific separation applications.85,87,96−100 Ther-mally and chemically stable polyaniline is ratherunique as it is the only polymer that can be dopedby protic acid and can exist in different forms de-pending upon the pH of the medium. In mem-brane development for separation science, polyani-line was used for membranes as supported films,surface layers, and very recently nanofillers (parti-cles and fibers) for applications ranging from gasesto liquids separation. Polyaniline is regarded so faras the best alternative for gas separation, becausethe molecular spacing of polymer chains can be

TABLE IScientific Interest in Electrically Conducting Polymers for Membrane Separation Systems

First Author Article Title

Anderson, M. R. Conjugated polymer films for gas separation13

Anderson, M. R. Gas separation membranes: A novel application for conducting polymers39

Burgmayer, P. Ion gate electrodes. Polypyrrole as a switchable ion conductor membrane90

Daniel, L. Switchable gate membranes. Conducting polymer films for the selective transport of neutral solutionspecies18

Mansouria, J. Novel membranes from conducting polymers15

Ferreira, C. A. Transport of metallic ions through polyaniline-containing composite membranes91

Orlov, A. V. Structure and gas separation properties of composite films based on polyaniline100

Sairam, M. Polyaniline membranes for separation and purification of gases, liquids, and electrolyte solutions32

Illing, G. Toward ultrathin polyaniline films for gas separation93

Pellegrino, J. The use of conducting polymers in membrane-based separations—a review and recentdevelopments23

Pile, D. L. Electrochemically modulated transport through a conducting polymer membrane34

Weidlich, A. Conducting polymers as ion-exchanger for water purification92

Ball, I. J. Pervaporation studies with polyaniline membranes and blends94

Wen, L. Doping-dependent ion selectivity of polyaniline membranes85

Partridge, A. C. Ion transport membranes based on conducting polymers37

Mirmohseni, A. Application of conducting polymer membranes part 1: Separation of nitric and phosphoric acids22

Price, W. E. Development of membrane systems based on conducting polymers17

Martin, C. R. Electronically conductive polymers as chemically selective layers for membrane-based separations86

Mattes, B. R. Morphological modification of polyaniline films for the separation of gases87



controlled by its interesting doping/dedopingchemistry. Therefore, polyaniline membranes havebeen much studied only for gas separation applica-tions by a number of groups, whereas Anderson etal. in 1991 published remarkable results on perme-abilities of various gases through polyaniline films,and highest selectivities were achieved by doping,dedoping, and controlled redoping of polyaniline.13

However, producing the asymmetric polyanilinemembrane with a consistent controlled skin layerthickness can be a challenge; researchers started toemploy polyaniline as a part of the composite mem-branes to exploit the high selectivity that it possessesas a dense membrane. Therefore, Kaner’s group in1997 developed gas separation membranes on thebasis of polyaniline–polyimide blends.13,29,75,87 Thepolymer blends improved pure polyaniline thermalstability and increased the pure gas pair fluxes ascompared with the homopolymer (i.e., based onpure polyimide or pure polyaniline). The separationfactors for all gas pairs were approximately equalor slightly lower than the polyaniline membranes,which was a good attempt because the polyani-line membranes had a greater selectivity as com-pared with polyimide membranes. Also, Orlov etal. had prepared composite polyvinyltrimethylsi-lane (PVTMS) with polyaniline.100 Polyaniline wascoated on the PVTMS film by borderline polymer-ization of aniline. They found that the O2/N2 andCO2/CH4 selectivities of the undoped compositefilms were higher than that of PVTMS. Later, in 2005,Illing et al. developed ultrathin polyaniline mem-brane using polyvinylidene difluoride as a supportmaterial.72 The produced composite membranes hadrelatively significant selectivity of H2/N2 and O2/N2of 120 and 5, respectively. They also mentioned thatthe composite membranes were more flexible andless brittle, which is more practical for commercialand bulk handling. Hellgardt and coworkers contin-ued to explore the composite membranes as well asproducing nanostructured self-supported polyani-line membranes in 2006.65 They found that althoughthe self-supported membranes selectivity for H2/N2(3 4 8) was three times higher than the compos-ite membranes, it decreased the permeation rateby 105 times. Recently, Chatzidaki et al. have con-ducted work similar to an earlier study of blend-ing of polyaniline–polyimide for their hollow fibermembrane fabrication and found that by addingpolyaniline into the polyimide matrix, the pure gasespermeation rates increased by 60–600 times ascompared with polyimide-based membranes.101

Very recently, Hasbullah et al. in 2011 had pre-pared integrally skinned asymmetric hollow fibersmembrane from the emeraldine base form ofpolyaniline to promote the formation of the skinlayer required to ensure good gas permeationperformance.31

Polypyrrole

Since a polypyrrole thin film was first electro-chemically synthesized by Diaz et al.,102 it hasbeen intensively used for many possible appli-cations in sensors, energy storage systems, andplastic electronics. An interesting breakthrough tothe polypyrrole thin film happened in 1982 whenBurgmayer and Murray developed a polypyrrole-deposited mircroporous metallic substrate, whichcould act as an ion gate or ion-exchangermembrane. Polypyrrole-coated membranes exhibitunique properties, including in situ alteration oftransport of ionic species across the membrane byswitching on and off appropriate electrical potential,which results in controlling the transport rate andselectivity of membrane. Polypyrrole has attractedspecial interest in membrane development becauseit can be deposited as thin film on different types ofsubstrates, including gold, glassy carbon, and ink-made carbon composite, and even polymers by us-ing cyclovoltammetric, galvanostatic, and potentio-static deposition methods. As a membrane material,it was first electrochemically deposited on mircrop-orous metallic substrate that could act as an ion-gatemembrane. Since then, several membrane systemsbased on polypyrrole have been significantly inves-tigated for various separation applications, such aselectrotransport of anions or cations, separation ofcopper(II) from mixtures with other cations, recov-ery of proteins, and selective separation of differentanions.47,60,66,95,103,104

Summary

Electrically conducting polymers, a promisingnew class of materials, are very important fromthe viewpoint of design and development of mem-branes for a wide range of separation applications.Thermally and chemically stable electrically con-ducting polymers elicit the possibility of both ex-ploiting the chemical and physical attributes of the



polymer for membrane-based separations and incor-porating their electronic and electrochemical prop-erties to enhance separation efficiencies. They havealso attracted a lot of attention in membrane de-velopment because of their ease of processibility.They can be deposited either as the coating mediumor by the electrochemical process on a wide vari-ety of supporting materials, such as microporousmetals substrate, conventional polymer membranes,filter paper, and inorganic membranes for the for-mation of thin-film composite membranes. Thesemembranes exhibit unique properties, including insitu alteration of transport of ionic species acrossthe membrane by switching on and off appropriateelectrical potential, which results in controlling thetransport rate and selectivity of membrane that mayhave important implications in the development ofadvanced liquid-handling devices as well as drugrelease systems.

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Advances in membrane development based on electrically conducting polymers