Improved mechanical properties and hydrophilicityof electrospun nanofiber membranes for filtrationapplications by dopamine modification

Liwei Huang, Jason T. Arena, Seetha S. Manickam, Xiaoqiang Jiang, Brian G. Willis,Jeffrey R. McCutcheon n

Department of Chemical and Biomolecular Engineering, Center for Environmental Sciences and Engineering, University of Connecticut, 191 Auditorium Rd.Unit 3222, Storrs, CT 06269-3222, USA

a r t i c l e i n f o

Article history:Received 15 October 2013Received in revised form4 January 2014Accepted 23 January 2014Available online 30 January 2014

Keywords:ElectrospinningMechanical propertiesHydrophilicityPolydopaminePolyacrylonitrilepolysulfone

a b s t r a c t

Electrospun nanofiber membranes (ENMs) are an emerging platform for membrane filtration; however,widespread applications of ENMs are hindered by poor mechanical strength attributed to their highporosity, intrinsically low, random fiber orientations and weak interactions between fiber junctions. Inaddition to suitable mechanical properties, most water-based filtration processes require membranes tobe hydrophilic in order to resist fouling and enhance water flow. In this study, we demonstrate a simplechemical modification capable of improving the mechanical properties of polyacylonitrile (PAN) andpolysulfone (PSu) ENMs. The chemical modification involves the polydopamine (PDA), a hydrophilicpolymer. PDA has the dual benefit of hydrophilization and strengthening of ENMs to improve theirwettability and tolerance to operational and handling conditions. When deposited onto the fibers, PDApromotes bonding between fibers by coating junction points throughout the nonwoven. The coatednonwoven membranes showed a 100 to 300% increases in tensile strength and Young's Modulus with nodecrease in flexibility while retaining their porous structure and high water permeability. For hydro-phobic PSu ENMs, the hydrophilicity was also significantly improved after coating.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Eletrospinning is a versatile technique that can create non-woven mats containing sub-micron scale fibers. Recent researchefforts have used this technique to fabricate nanofibers for variousapplications ranging from membrane filtration to tissue engineer-ing and electrochemical applications [1–3]. Electrospun nanofibermembranes (ENMs) have a great potential in membrane filtrationtechnology due to several attractive attributes, such as a highlyporous and interconnected pore structure, submicron pore sizes,and a large surface area to volume ratio. These characteristicsallow ENMs based filter media to be highly permeable while stillhave high filtration efficiency. ENMs have been successfullycommercialized in air filtration applications [4] and have beenshown to be effective at removing 1–10 μm size particulates [1,5]in aqueous filtrations and thus can be used for treatment of wastewater prior to the treatment by ultrafiltration, nanofiltration andreverse osmosis. Recently, ENMs have been considered as layersin thin film composite membranes in which the polymers are

electrospun onto a non-woven fabric support and then coatedwith a thin barrier layer [6]. This novel three-tier structure hasbeen applied in ultra [6,7] and nanofiltration [8] as well as forwardosmosis [9–12].

One drawback to the use of ENMs in filtration is their lack ofmechanical integrity relative to conventional cast or nonwovenmembranes. Typical pressure-driven water filtration applicationsin particular require membranes to possess sufficient mechanicalstrength to withstand operational conditions and handling duringmodule fabrication. ENMs are relatively weak due to their highlyporous structure as well as weak bonding between the nanofibers[13,14]. The intrinsic fiber modulus and small fiber diameter alsocontribute to the low strength of these materials [14]. Previousstudies have reported several methods to improve the mechanicalstrength of ENMs by altering the nonwoven nanofiber mat intoself-bundled fiber yarns [15] or reinforcing the single nanofiberstrength by adding carbon nanotubes [15,16], layered silicates [17]or graphite nanoplatelets [18] into the polymer solution; however,these approaches either change the non-woven and highly porousnature of ENMs or complicate the electrospinning process. Recentstudies have also demonstrated improvement in individual fibermodulus with reduction in fiber diameter due to improvedpolymer chain orientations in ultrafine fibers [19,20]. However,

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Journal of Membrane Science

0376-7388/$ - see front matter & 2014 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.memsci.2014.01.045

n Corresponding author. Tel.: þ1 860 486 4601; fax: þ1 860 486 2959.E-mail address: jeff@engr.uconn.edu (J.R. McCutcheon).

Journal of Membrane Science 460 (2014) 241–249

a decrease in fiber diameter may result in an decrease in porediameter which may or may not be desired. The most effectiveapproach to increase mat strength is to enhance the bindingstrength between the fibers at their junction points throughoutthe fiber mat, thus promoting bonding between fibers [21,22], byfor instance applying a thermal treatment [1,23–25]. A possibledrawback of heating the mat is the likelihood of shrinkage causedby entropic relaxation of stretched polymer chains [21]. Chemicalmodifications can also be applied to enhance fiber–fiber bondingthrough cross-linking [26,27]. This method is usually adopted byhydrophilic ENMs to improve their integrity in water.

Furthermore, for aqueous filtration applications, a hydrophilicmembrane surface is usually favored. For instance, a hydrophilicmembrane surface helps to reduce protein adhesion and bio-foulingin micro- and ultra-filtration [28]. In forward osmosis applications,improved wettability of a porous support material in thin filmcomposite membranes reduces internal concentration polarizationand improves water flux [29–31]. These applications warrant the useof hydrophilic polymers. This may include polyacrylonitrile (PAN),nylon, and cellulose acetate amongst others. However, these poly-mers have a tendency to plasticize in aqueous environments. Swel-ling of these polymers, especially in their nanofibrous form, could bedetrimental to membrane performance, increase fiber size andchange membrane dimensions, and lead to a reduction in strengthwhile in the swollen state. There are a number of hydrophobicpolymers that do not swell in water, including conventional mem-brane materials like polysulfone (PSu), polyethersulfone (PES), poly-vinylidene fluoride (PVDF), polypropylene (PP), amongst others.However, these polymers are likely to adsorb foulants. Hydrophobicmaterials can be surface modified to increase their hydrophilicity byvarious methods, including plasma treatment [32], chemical oxida-tion and grafting [24,33], or by blending with hydrophilic polymers[34–36]. Often, only minimal modifications are required becauseextensive functionalization can also introduce undesirable effects,such as pore blocking/collapsing.

In this study, a gentle approach to surface modification is usedto both increase hydrophilicity of a nanofiber mat and bind itsfibers together for increased strength. Both benefits are realizedwithout greatly changing the ENMs' morphology and pore struc-ture. The surface modification is accomplished through thedeposition of polydopamine (PDA), a bio-inspired polymer sharingsimilar properties to the adhesive secretions of mussels, through-out the mat. PDA is capable of adhering to virtually all types ofinorganic and organic surfaces, including classically adhesion-resistant materials such as poly(tetrafluoroethylene) (PTFE)[37–39]. It is known that PDA is formed by a polymerization/precipitation reaction using low concentrations of dopamine in anaerated aqueous solution at pHs between 7.5 and 9.5 [40], thedetailed mechanism of PDA formation is still undergoing investi-gation though. Various PDA structures [38,41–44] and reactionpaths [38,44,45] have been proposed. Recently, PDA has been usedfor numerous membrane modification applications because of thecapability to increase the hydrophilicity of a surface. One suchmethod was reported by McCloskey who hydrophilized microfil-tration, ultrafiltration, and reverse osmosis membrane selectivelayers for enhanced fouling resistance to oil/water emulsions and

protein mixtures [28,46]. PDA has was also used to modify thesupport layer of commercially available thin film composite (TFC)reverse osmosis membranes to enhance hydrophilicity and reduceinternal concentration polarization for use in both forward andpressure retarded osmosis [30]. Other applications involve usingPDA as a primer layer for further functionalization. In one suchapproach, PDA modified membranes were further modified withheparin to enhance permeability and biocompatibility of mem-branes used in biomedical and blood-contacting applications [47].PDA films have also been deposited hydrophobic membranes toact as a substrate for the synthesis of a chitosan selective layer forpervaporation [48] and have served as selective layers for nanofiltra-tion, pervaporation, and dehumidification membranes [40,49,50].PDA has also been used to modify PSu substrates prior to FOmembrane synthesis [51], as anti-fouling coatings for TFC selectivelayers used in oil/water separations [52] and as modification for feedspacers for biofoulant adhesion resistance [53].

In this work, we report coating highly porous ENMs with PDAin order to improve overall mat strength and, in the case ofhydrophobic polymers, impart a hydrophilic character to the mat.We demonstrate this using PAN and PSu ENMs. These twopolymers are commonly used in membranes, but their use islargely limited to cast films for standalone use or as components ofcomposite membranes [54–56]. Nanofibers of PAN and PSu havealso been recently considered to perform similar functions[8–10,57]. In addition, PAN is hydrophilic while PSu is a hydro-phobic polymer. A side-by-side study may allow us to understandhow PDA modification affects wettability and flux performancedifferently depending on hydrophilicity of the substrate. Improvedstrength and, in the instance of PSu, improved wettability of theENMs, make them more suitable for aqueous filtration.

2. Experimental

2.1. Materials

Polyacrylonitrile (PAN, Mw¼150,000) was purchased fromScientific Polymer Products (Ontario, NY). Polysulfone (PSu, UDEL3500, Mw¼80,000–86,000) was obtained from Solvay AdvancedPolymers (Alpharetta, GA). N,N-Dimethylformamide (DMF) andN-methyl-pyrrolidinone (NMP) were purchased from Acros Organ-ics (Geel, Belgium) and Fisher Scientific (Pittsburgh, PA), respec-tively as the solvents. The dopamine–hydrochloride was purchasedfrom Sigma-Aldrich (St. Louis, MO). The Tris–HCl was purchasedfrom Fisher Scientific (Pittsburgh, PA). Isopropyl alcohol (IPA) wasobtained from J.T. Baker (Phillipsburg, NJ) as the wetting agent forPSu ENMs before polydopamine (PDA) coating. The water used forsample preparation and membrane tests was ultrapure Milli-Qwater produced by a Millipore Integral 10 water system (MilliporeCorporation Billerica, MA).

2.2. Fabrication of PAN and PSu ENMs.

PAN and PSu ENMs were prepared using a custom-builtelectrospinning setup. Details for the electrospinning system have

Table 1Electrospinning conditions used for this study.

Polymer Solvent Concentration(wt%)

Voltage(kV)

Flow rate(ml/h)

Collector drumrotating rate (rpm)

PAN DMF 10 28 1.0 70PSu DMF/NMP (ratio:7/3) 25 28 1.0 70

L. Huang et al. / Journal of Membrane Science 460 (2014) 241–249242

been described elsewhere [58]. Solutions of 10% PAN in DMF and25% PSu in DMF and NMP (mixed in a 7:3 ratio) were prepared bycontinuously stirring to polymer in solvent(s) at 60 1C for 24 h. Thesolutions were electrospun for 4 h and the produced ENMs weresubstrates for PDA modification. The chosen spinning parametersfor this study are listed in Table 1. The electrospinning took placeat 20 1C and 5–20% relative humidity.

2.3. Coating PAN and PSu ENMs with PDA

The process for PDA coating both the PAN and PSu ENMs isillustrated in Fig. 1. The coating process began by carefullyremoving the samples from substrate (they are spun onto alumi-num foil). They were placed in a holding frame to keep the mat flatduring coating in the coating bath. For coating of the hydrophobicPSu fibers (contact angle �1451) a pre-wetting step was added toensure saturation during modification.

The wetting procedure starts with completely immersing thesamples in IPA and soaking them for 1 h. The IPA is then rinsed outof the membranes using a series of three deionized (DI) waterbaths of 1 L volume for 1 h per water bath.

The coating step takes place at room temperature with theelectronspun mats (both PAN and wetted PSu) completelyimmersed in the PDA coating solution, consisting of two compo-nents: 750 mL of a pH 8.7 Tris–HCl buffer and approximately 1.5 gof dopamine. A short coating time of 1 h to create a relatively lowthickness of PDA layer and a long coating time of 18 h to approacha maximum coating thickness were used in this study. Previousstudies suggest that the thickness of the PDA layer increasedrapidly within first 10 h and remained almost unchanged after20 h, probably due to the self-termination of polymerizationreaction as the depletion of dopamine monomer [40]. Immediatelyfollowing removal from the coating solution the samples wererinsed in two DI water baths and dried before characterization.

2.4. Morphology characterization and property measurements

Changes in surface morphology of between the as-spun baseENMs and treated membranes using both methods were observedby FEI Phenom desktop scanning electron microscope (SEM) (FEICompany, USA). ImageJ software was used to determine theaverage fiber diameter. Forty measurements on each sample werecarried out when making these measurements.

The mechanical properties of all the PAN and PSu ENMs weremeasured by using a TA Instruments Dynamic Mechanical Analyzer

at 25 1C. A minimum of three strips measuring 40 mm�5.5 mmwere cut from each ENM, specimen thickness was about 50 μm.

2.5. Surface analysis for PDA coated and non-coated ENMs

To investigate the changes of surface chemistry of PDA coatedand non-coated PAN and PSu ENMs, x-ray photoelectron spectro-scopy (XPS) was performed using a monochromated Al K-α sourceinstrument (Kratos, AXIS 165; U.K.). A normal sample position of01 to detector direction was applied. Pass energy of 160 eV and20 eV were used for survey and high-resolution spectroscopy. Nochemical degradation of the surface membrane was found over theexposure to x-ray. All spectroscopy were calibrated to hydrocarbonC 1s peak at 284.6 eV. Sensitivity factors of C 1s, N 1s, O 1s, and S2p from manufacture were used for quantitative calculation.

The change of contact angles before and after PDA coatingprocess was measured using the sessile drop method on a CAM101 series contact angle goniometer (KSV Company LinthicumHeights, MD). The values were taken as an average of at least fivepoints with a volume of 1071 μL. Since hydrophilic (i.e. PAN)ENMs usually give an immeasurable contact angle (water dropletabsorbed by the substrate) due to their highly porous hydrophilicsurface, contact angle measurements may not be a good indicatorof surface hydrophilicity of ENMs. For this reason, contact anglemeasurements were also carried out on PDA modified andunmodified PAN and PSu cast films. The non-coated cast filmswere prepared by casting the same polymer solutions used forelectrospinning onto a glass plate followed by dried in air to avoidpore formation. The casted films were coated by dopamine usingthe same coating procedure for ENMs.

2.6. Membrane characterizations

A mercury intrusion porosimeter (MIP) (AutoPoreIV, Micro-meritics) was used to characterize the pore structure (specific porevolume and pore size) before and after modification with poly-dopamine. The Washburn equation was used to calculate porediameters from the intrusion pressure.

Pd¼ �4γ cos θ

In the equation, P is the intrusion pressure in MPa, d is the porediameter in mm, γ is the surface tension of mercury (485 dynes/cm)and θ is the contact angle of mercury (a value of 1301 wasassumed) with the sample. An assumption of cylindrical poreswas made when using the above equation. The sample was testedin the pressure range of 0–20,000 psi; however no pores havebeen detected when pressure was beyond 2000 psi. It is to benoted that the intrusion technique can detect both through andblind pores but not closed pores; however neither blind nor closedpores contribute to flow through filter media.

A Millipore Amicon bioseparations stirred cell (model 8200,Millipore Corporation Billerica, MA) with active filtration area of28.7 cm2 was used to evaluate the membrane performance. Thepre-wetted, dry, and PDA modified membranes were tested. Topre-wet the membrane, the dry membrane was first placed in themembrane holder then rinsed with 70% Isopropyl alcohol forseveral times and then immersed in DI water for 30 min to ensurethat the membrane was completely wet. The tested membranewas placed in the filtration cell which was then filled with 120 cm3

of DI water. The applied pressure was increased gradually and thecorresponding water flux was measured. As this study concernsmechanical properties of ENMs the measured water fluxes wereconverted to permeability coefficients by dividing the measuredflux by applied pressure.

Fig. 1. Schematic diagram of PDA modification process of PAN and PSu ENMs.

L. Huang et al. / Journal of Membrane Science 460 (2014) 241–249 243

3. Results and discussion

3.1. Influence of PDA coating on membrane surface morphology.

Unlike reported PDA coated dense membranes [40,48,50], theSEM images in Fig. 2 show that there is no PDA skin layer on the topsurface of the electrospun substrate. This suggests that the coatingoccurs throughout the mat and onto the individual fibers. However,there was no measurable change in average fiber diameter, evenwhen coating for 18 h. The average fiber diameter for the unmodi-fied, 1 h PDA modified and 18 h PDA modified were 280765,280760, and 270755 nm for PAN ENMs, respectively, and4607240, 4707240, and 4507210 nm for PSu ENMs, respectively.Unlike dense membranes, the highly porous and inter-connectedstructure of electrospun networks helps the dopamine monomerquickly diffuse into the mat and form a uniform ultrathin PDA layer(invisible from the SEM) on each fiber In addition, unlike literatureon PDA coating hydrophobic membranes [40,48,50] where dopaminemonomer was concentrated on the surface, this study either usedhydrophilic PAN or pre-wetted PSu as coating substrates, whichpromotes dopamine monomer diffusion through the water path intothe wetted pore network. Since the error range of the fiber diametermay overlap the thickness of the PDA layer, it is hard to prove thePDA coating occurs or quantify the thickness of PDA layer merelybased on fiber diameter measurements.

3.2. Influence of PDA coating on membrane surface chemistry.

XPS analysis has been employed to quantitatively determinethe chemical composition of the ENM’s surface before and afterPDA coating. Since PAN contains no oxygen groups, the XPS surveyand the O 1s spectra of PAN ENMs, PAN–PDA 1 h and PAN–PDA18 h. Results are given in Fig. 3. Compared to unmodified PANENMs, the PDA modified O 1s peaks appeared in the spectrum ofPAN–PDA 1 h and PAN–PDA 18 h coated samples, which wereattributed to the oxygen from the PDA layer. Increasing the PDA

Fig. 2. Scanning electron micrographs of (a) non-coated PAN; (b) PAN–PDA 1 h; (c) PAN–PDA 18 h; (d) non-coated PSu; (e) PSu–PDA 1 h; and (f) PSu–PDA 18 h.

Fig. 3. XPS spectra of non-coated and coated PAN ENMs with different coatingtimes: (a) survey; and (b) O 1s.

L. Huang et al. / Journal of Membrane Science 460 (2014) 241–249244

coating time to 18 h the PAN–PDA ENMs display a stronger O 1ssignal. Based on the peak intensity of each element, the percentagechange of each element has been evaluated. As shown in Table 2,The O/C molar ratio of PAN–PDA 1 h ENM surface approached thetheoretical value of pure dopamine (0.286). The amount ofdopamine molecules introduced onto PAN–PDA 18 h ENM is about1.5 times that of the PAN–PDA 1 h sample, as determined by theratio of the O 1s intensity between 18 h and 1 h (15.8/10.3).

PSu contains no nitrogen groups, so for the PSu fibers, the XPSsurveyed the N 1s and S 2p spectra of the PSu, PSu–PDA 1 h andPSu–PDA 18 h ENMs are given in Fig. 4. There is no nitrogen peakfor PSu; however, the nitrogen signal detected on both PSu–PDA1 h and PSu–PDA 18 h at 399.5 eV (Fig. 4(b)), belonging to thenitrogen element within the amino group of the dopaminemolecules [59]. The sulfur signal was measurably weakened aftercoating with PDA modification (Fig. 4(c)), which also is evidencethat the PDA has been successfully introduced onto the PSu ENMs'surface. As shown in Table 3, The N/C molar ratio of PSu–PDA 18 hENM surface approached the theoretical value of pure dopamine(0.125). The amount of dopamine molecules for PSu–PDA 18 hENM is about 1.3 times of the PSu–PDA 1 h modified sample, basedon the ratio of the N 1s intensity between 18 h and 1 h modifica-tion times (7.2/5.4).

3.3. Influence of PDA coating on mechanical properties.

The ultimate tensile strength, Young’s module and elongationat break of the PAN and PSu ENMs were measured both before andafter PDA modification. Results are shown in Figs. 5 and 6. BothPAN and PSu membranes exhibit improved mechanical propertiesafter PDA modification. PAN EMNs exhibit a 100% increase in boththe tensile strength and Young's modulus, indicating a strongerand stiffer material after modification. For PSu ENMs, the ultimatetensile strength and Young’s modulus improvement were found tobe 80% and 210%, respectively. On the other hand, there is nostatistically significant change of elongation at break after mod-ification for both PAN and PSu, suggesting the flexibility ofnanofibers was still retained. Modifying for 18 h shows almostno improvement in mechanical properties when compared to 1 hcoating time for both PAN and PSu.

PDA coated ENMs showed improved mechanical propertiesbecause the PDA layer glues nanofibers together improving inter-connectedness within the mat. When the PDA layer forms in thevoid space between fibers, they glue the fibers at points of overlap.It is important to point out that the mechanical properties mightbe further improved by forming a thicker and denser PDA layeronto the ENMs. This may be possible by optimizing other dopa-mine modification parameters, i.e. altering the dopamine concen-tration, changing the pH or performing multiple coatings.

Mechanical strength test reveals that prolonging coating timemay not further improve the strength and may be considered asless time and cost efficient. From application point of view, 1 hcoating samples were of greater interest and used in the followingpore structure characterizations and pure water permeability tests.

3.4. Influence of PDA coating on surface hydrophilicity.

The effect of coating on the contact angles of both PAN castfilms and ENMs is shown in Table 4. For PAN, there is no significantchange in contact angle after coating for both cast films and ENMs.A possible reason is that PAN is already hydrophilic and coatingwith another hydrophilic polymer cannot lead to further improve-ments in the hydrophilicity. The reason why PAN ENMs gave animmeasurable contact angle has been discussed above in thematerials and methods session. The highly porous nature of theENMs allows water to quickly penetrate into the electrospun mat.

Table 2Surface elemental composition of PAN ENM and PAN–PDA composite ENMs withdifferent coating times.

Sample Atom percentage (mol%) O/C(calculated)

Theoretical O/Cof dopamine

C N O

PAN 80.0 20.0 0 0 0.286PAN–PDA 1 h 71.7 18.7 10.3 0.145PAN–PDA 18 h 66.4 17.8 15.8 0.238

Fig. 4. XPS spectra of non-coated and coated PSu ENMs with different coatingtimes: (a) survey; (b) N 1s; and (c) S 2p.

L. Huang et al. / Journal of Membrane Science 460 (2014) 241–249 245

Table 3Surface elemental composition of PSu ENM and PSu–PDA composite ENMs with different coating times.

Sample Atom percentage (mol%) N/C (calculated) Theoretical N/C of dopamine

C O N S

PSu 86.3 11.3 0 2.4 0 0.125PSu–PDA 1 h 73.8 19.5 5.4 1.3 0.073PSu–PDA 18 h 74.3 17.5 7.2 1.0 0.097

Fig. 5. Mechanical properties of non-coated and coated PAN ENMs with differentcoating times: (a) tensile strength; (b) Young's Modulus; and (c) elongationat break.

Fig. 6. Mechanical properties of non-coated and coated PSu ENMs with differentcoating times: (a) tensile strength; (b) Young's Modulus; and (c) Elongationat break.

L. Huang et al. / Journal of Membrane Science 460 (2014) 241–249246

The contact angle changes after coating for PSu casting films aswell as ENMs are listed in Table 4. The contact angle of the non-coated PSu ENMs was measured to be 1451, and that of the castfilm 791. The increased hydrophobicity of the PSu ENMs is due toinherent surface roughness and trapped air pockets [60,61]. Aftercoating, both PSu cast films and ENMs showed decreased contactangle due to the introduction of hydrophilic PDA onto substratesurface; however, ENMs appeared to give a much more dramaticdecrease in contact angle than the cast films and this is still attributedto the larger pores of ENMs. The coated PSu ENMs became hydrophilicand quickly absorbed the water droplet into the fiber network.Therefore, the contact angle was not measureable.

3.5. Influence of PDA coating on pore structure

As discussed above, SEM image might not provide enoughinformation on how PDA coating affects the membrane morphol-ogy or pore structure due to large standard deviation of fiberdiameter and limitation to resolve small pores or void space undercertain magnification. In addition, top-surface SEM can onlyprovide 2-Dimensional image. Therefore, MIP was used to betterevaluate the pore structure change before and after PDA coating.The pore volume changes between PDA modified and unmodifiedENMs for both PAN and PSu are shown in Table 5. It was found thatthe pore volume modestly decreased for both PAN and PSu. Thelower pore volume of the coated ENMs can be explained bythe PDA coating of smaller pores and tight junctions between

Table 4Contact angles of coated and non-coated PAN and PSu.

Sample Contact angle (degree)

Cast film ENM

PAN 48.274.0 N/Aa

PAN–PDA 1 h 47.473.0 N/Aa

PSu 78.870.6 145.775.1PSu–PDA 1 h 70.571.3 N/Aa

a Contact angles were unmeasurable due to absorption of the drop into thefiber mat.

Table 5Specific pore volume of coated and non-coated ENMs of PANand PSu.

Sample Specific pore volume (cc/g)

PAN 11.3470.28PAN–PDA 1 h 7.3372.30PSu 9.3270.61PSu–PDA 1 h 5.9070.70

Fig. 7. Pore diameter distribution of ENMs: (a) non-coated PAN; and (b) PAN–PDA 1 h. Fig. 8. Pore diameter distribution of ENMs: (a) non-coated PSu; and (b) PSu–PDA 1 h.

L. Huang et al. / Journal of Membrane Science 460 (2014) 241–249 247

the fibers. In addition to decreased pore volume, the PDA mod-ification also seems to result in smaller pores. As can be seen inFig. 7, there was an increase in pore volume contribution for porediameters in the range of 0.1–0.5 mm for the PAN–PDA 1 hcompared to the unmodified membranes. The shift in pore volumecontributions was more obvious for PSu, as shown in Fig. 8. Forpore diameters of 1–5 mm, the pore volume percentage increasedfrom nearly 0% for the unmodified PSu to approximately 30% forthe PSu–PDA 1 h ENMs. On the other hand, for pore diameters of70–100 μm, the percentage decreased from about 45% to 35%.

3.6. Influence of PDA coating on water flux performance.

The flux performance of the PAN pre-wet, PAN dry, and PAN–PDA 1 h ENMs are shown in Fig. 9. It can be seen that the purewater permeability of all three membranes decreased withincreasing applied pressure. This is due to membrane compactionby the applied pressure. The mechanism behind ENMs compactionhas been discussed elsewhere [22], but generally ENMs are proneto compression under higher applied pressure due to the weakinteractions between fibers and highly porous nature. Fiberdeformation may occur thus decreases the membrane thickness,and also reduces the interconnectivity of the pores, decreasing thepore volume and pore size.

Compared to the dry unmodified PAN ENMs, the pre-wettedsample did not show significantly improved water flux. This mightbe due to the hydrophilic nature of PAN ENMs, which already possessgood wettability. For the same reason, there is no permeabilityimprovement for PAN–PDA 1 h ENMs either. Instead, there is a slightdecrease in permeability compared to unmodified ENMs which wouldlikely be due to reduced pore volume and pore diameters of thecoated samples. Nevertheless, for the PAN–PDA 1 h ENMs, theinfluence of pore structure on water flux performance is minimaland the high permeability of ENMs was still retained.

The flux performance of the pre-wet PSu, dry PSu, and PSu–PDA 1 h ENMs are shown in Fig. 10. Similar to the PAN ENMs, thepure water permeability of all three membranes decreased withincreasing applied pressure. Unlike the PAN ENMs the pre-wettedPSu ENMs showed higher water permeability than the drymembrane at low pressures. For instance, there is a 14% increasein permeability for pre-wetted PSu ENMs over dry PSu ENMs atpressures below 3 psi. PSu ENMs are hydrophobic and pre-wettingimproves water transport through the fiber network; however, athigh pressures the pre-wetted PSu ENMs did not show significantdifference compared to dry PSu ENMs. The effect of the membranewetting on pure water permeability at low pressures may be

pronounced, but with increasing pressure the membrane iscompletely wetted out and the water permeability differencesbetween the pre-wetted and dry PSu ENMs is negligible.

Similar to the pre-wet PSu ENMs, the PSu–PDA 1 h ENMs alsoshowed higher flux than the dry PSu ENMs. As discussed earlier,the PDA modification significantly improves the surface hydro-philicity but may also block the smaller pores. The PSu–PDA 1 hENMs increased surface wettability overcomes the influence ofreduced pore volume and pore diameters. The PSu–PDA 1 happroached the flux performance of the pre-wet PSu ENMs.

4. Conclusion

In this study, the PAN and PSu ENMs were modified with PDAto make them more suitable for water filtration applications. ThePDA modified ENMs exhibited higher strength and improvedhydrophilicity without significant morphological changes. Overall,the PDA modification of ENMs showed promise as a way to bothimprove mechanical properties and tune surface chemistry with-out compromising the characteristics which would lead to poorperformance as a membrane. This approach may have additionalbenefits related to fouling resistance or as a primer layer forfurther modification and may also be used in applications ofelectrospun nanofibers beyond the field of membrane filtration.

Acknowledgments

The authors acknowledge funding from the National ScienceFoundation Chemical and Biological Separations Program (CBET#1160098 and CBET #1067564), the Department of Energy, andthe Environmental Protection Agency (#R834872). We thankSolvay for providing polysulfone for this study.

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