Separation and Purification Technology 118 (2013) 604–611

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Separation and Purification Technology

journal homepage: www.elsevier .com/locate /seppur

Separation performance of PVDF membranes poled in intense electricfields

1383-5866/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.seppur.2013.07.043

⇑ Corresponding author.E-mail address: hans.coster@sydney.edu.au (H.G.L. Coster).

M.T. Darestani, T.C. Chilcott, H.G.L. Coster ⇑School of Chemical and Biomolecular Engineering, University of Sydney, Sydney, NSW 2006, Australia

a r t i c l e i n f o

Article history:Received 20 February 2013Received in revised form 26 June 2013Accepted 26 July 2013Available online 6 August 2013

Keywords:Electrical polingPoly(vinylidene fluoride)MicrostructureFiltrationMorphology

a b s t r a c t

In this study, PVDF membranes were electrically poled under controlled conditions. Microstructure anal-ysis using scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that electricalpoling changed the microstructure and surface topography of the membranes. Filtration tests usingsodium alginate were performed to investigate the effect of electrical poling on the separation perfor-mance of the membranes. Poled membranes were found to have a lower molecular weight cut-off anda somewhat reduced hydraulic permeability.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Piezoelectric PVDF membranes were fabricated by electricallypoling pre-fabricated membranes. The piezoelectric membranesvibrate when energized by an electric signal and it has been shownthat this delays fouling [1–4]. Further, it has been shown that pol-ing in an intense electric field can change the crystalline structure,microstructure, and electrical properties of PVDF microfiltrationmembranes [2–5]. This suggests that the filtration properties ofthese membranes can also be affected by electrical poling.

Most of microfiltration membranes are porous polymer filmsmade via phase separation techniques. Thermally induced phaseseparation (TIPS), evaporation induced phase separation (EIPS), va-pour induced phase separation (VIPS) and NIPS (non-solvent in-duced phase separation) are some of the processes used forpreparing commercial microfiltration (MF) membranes. Control-ling the pore size distribution of the membranes is one of thekey objectives in membrane manufacturing [6]. In order to controlthe microstructure of the membranes, sophisticated manufactur-ing variants have been developed, including the combination ofvarious phase separation mechanisms [7–9]. Techniques such asmechanical deformation have also been reported to affect themicrostructure of membranes [10–12].

In theory, the mechanical elongation of membranes can changethe configuration of the polymer chains and thus, this techniquecan be used to control the morphology of membranes. However,

the aligned polymer chains usually relax back to their initial con-figuration after the mechanical stress has been removed. Moreover,mechanical stretching is practical only in the direction parallel tothe surface of the membrane and not in the perpendicular direc-tion, which is the direction in which separation takes place. Toovercome this limitation, the current study used the technique ofelectrical poling in which a potential difference is applied acrossthe two sides of the membrane under controlled thermal condi-tions to induced stresses in the normal direction.

The orientation of macromolecules by an electric field is a well-known phenomenon. It has been used in various processing tech-nologies such as electro-spinning for the fabrication of nanofibres[13] or nano-fibrous filters [14] and electro-spraying for ultrathinfilms or nano-particles [15]. AC signals have been used also to de-crease the number of structural defects during the fabrication ofmembranes [16]. However, the orientation of macromolecules bya DC electric field has not been used to change the microstructureof pre-fabricated commercial membranes. In this study, we inves-tigate the effect of electrical poling on the filtration properties andseparation performance of PVDF micro-filtration membranes usingsodium alginate in water as a model feed solution. In these studiesthe poled membranes, although they were rendered piezo-electri-cally active, were not electrically energized to set them intovibration.

2. Materials and methods

The membranes used in this study were PVDF microfiltrationmembranes (Pall Fluoro Tran� W) supplied by PALL Life Sciences

(a) Un-poled membrane (b) Poled membrane

Poling direction

Fig. 1. SEM micrograph of a cross-section of a PVDF membrane before poling (a) and after poling (b) at different magnifications.

M.T. Darestani et al. / Separation and Purification Technology 118 (2013) 604–611 605

(Australia). The thickness and nominal pore size of the membraneswere 123 and 0.22 lm, respectively. These membranes were thenmodified using an intense electric field.

A commercial low viscosity sodium alginate powder (Sigma Al-drich, 180947) was used as a model feed solution to examine theperformance of the PVDF microfiltration membranes. The viscosityof this grade sodium alginate is 20–40 cps (for 2 g/l solution inwater) [17] which is very low compared to the sodium alginatesused in some membrane fouling studies, with typical viscosity of�250 cps (for 2 g/l solution in water) [18].

2.1. Electrical poling

For electrical poling, the membrane specimens were sand-wiched between two electrodes and then heated from room tem-perature to 90 ± 10 �C. A DC power supply (Stanford ResearchSystems, model PS350) was used to generate the desired electricpotential difference across the electrodes. The voltage applied be-tween the electrodes was gradually increased from 0 to 2 kV at arate of about 50 V/min. The electric field strength at a potential dif-ference of 2 kV was 16.3 � 106 V/m and this field was maintainedfor 2 h at the elevated temperature.

Next, the sample was slowly cooled to room temperature (20–25 �C) whilst still in the applied electric field (maintained by the2 kV potential difference). During the last stage of poling, the volt-age was gradually decreased to 0 at a rate of about 50 V/min. Sincethe top electrode was smaller than the bottom electrode to preventelectrical arcing between the electrodes when the high voltage wasapplied, not the entire area of the membrane was poled. For theexperiments the poled area therefore was excised from theremaining membrane.

2.2. Scanning electron microscopy (SEM)

A field emission scanning electron microscope (Zeiss, ULTRAplus) equipped with a Schottky field emission gun (10 kV) wasused to analyze the morphology of the membranes. To carry outthis analysis, the membrane specimens were coated with a15 nm gold layer using a sputtering technique.

2.3. Atomic force microscopy (AFM)

The surface topography of the membranes was characterizedusing a Nanoscope III multimode atomic force microscope (Veeco,Santa Barbara, CA). The microscope was operated in contact modeat a scan rate of 1.05 Hz. Nanoscope software (version 530r3sr3)

was used to analyze the sections of the images and characterizethe roughness of the membrane surfaces.

2.4. Filtration performance

The membranes were pre-wetted before the filtration experi-ments. For this, the membranes were soaked in ethanol for 24 hfollowed by a 2 h immersion in distilled water and then a finalthorough rinse in distilled water.

The filtration experiments were performed using a dead-end fil-tration module. A plastic chamber with a 19.6 � 10�4 m2 piece ofmembrane was exposed to 10 g/l sodium alginate solution at55 kPa. It needs to be pointed out that in this range of pressure alower concentration of high viscosity sodium alginate is requiredto study the fouling of this material [19]. The solution was storedin a 1500 ml feed reservoir installed between the pressure sourceand the filtration cell. Pressure was obtained by compressed airand maintained at a constant value using a pressure regulator. Amagnetic stirrer was used in the filtration chamber to reduce con-centration polarization effects. To ensure a consistent feed concen-tration, the feed solution stored in the reservoir was also agitatedusing a large magnetic stirrer. The agitation rate was 150 rpmand 300 rpm for the filtration chamber and feed reservoir, respec-tively. The permeate was collected in a reservoir placed on an elec-tronic balance (Shimadzu Corp.). The permeate flux was calculatedfrom recordings of the mass as a function of time.

2.5. Rejection and molecular weight cut-off analysis

After the filtration test, the filtrate (permeate) solution wasdried under vacuum and the dry content was weighed and retainedfor molecular weight analysis. The concentration of the solute inthe filtrate was calculated by dividing the dry content by the totalvolume of filtrate.

The molecular weight cut-off (MWCO) of the membrane wasdetermined by comparing the average molecular weights of thesolute in the feed and permeate solutions.

The molecular weight of sodium alginate itself was determinedfrom intrinsic viscosity measurements (g) using an Ubbelohde cap-illary viscosimeter ASTM size 2C (VWR International, France). Solu-tions of sodium alginate in chloroform at different concentrationswere prepared. The plunge times of the solutions through a stan-dard length of capillary tube in the Ubbelhode viscometer were re-corded. The intrinsic viscosity of sodium alginate was thencalculated by extrapolating the data to a concentration of zero.The average molecular weights (MW) of the alginate were then

(a) (b)

0

5,000

10,000

15,000

20,000

25,000

0 30 60 90 120 150 180 210 240

Fre

quen

cy

Un-poled

poled

Pore Polymer

(c)

(d) (e) Fig. 2. SEM micrographs of the surface of an un-poled (a and d) and poled (b and e) PVDF membrane. Graph c compares the histogram of images a and b. Images d and e showthe surface of the un-poled and poled membranes, respectively, at a higher magnification (50,000 times).

606 M.T. Darestani et al. / Separation and Purification Technology 118 (2013) 604–611

calculated using Mancini et al.’s [20] model (Eq. (1)). This methodis detailed elsewhere [21–25].

g ¼ 1:228� 10�5 ðMWÞ0:936 ð1Þ

Several samples were taken from each solution and measuredusing the capillary viscometer. The results provide an estimate ofthe molecular weights of the alginate aggregates. The viscometerwas immersed in a water bath at 25 ± 0.1 �C to ensure a constanttemperature during the viscosity measurements.

3. Results and discussion

3.1. Microstructure of membranes

SEM images of the microstructure of the membranes before andafter poling are shown in Fig. 1. It can be seen that poling signifi-cantly changed the microstructure of the film. The un-poled mem-brane is essentially isotropic, while the poled membrane isstrongly anisotropic with a layered microstructure. Thus, it appearsthat electrical poling could provide an additional or alternativemodality to fabricate multi-layered membranes [26,27].

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The poling process may also affect the membrane surfaces.Fig. 2 compares typical SEM images of PVDF membrane surfacesbefore and after electrical poling. At first glance, it seems that pol-ing did not change the microstructure of the surface of the mem-brane. However, the topography histograms of the images shownin Fig. 2-c show that the surface of the poled membrane has a dif-ferent microstructure from the un-poled membrane.

Fig. 2-d shows that the pores on the surface of the un-poledmembrane overlap randomly and as a result have irregular shapes.In contrast, as shown in Fig. 2-e, the pores on the surface of thepoled membrane look more aligned and have more regular shapes.

The topography of the membrane surface was examined usingAFM. Large (40 � 40 lm) images from different parts of the mem-brane surfaces were scanned in order to generate reliable data.Fig. 3 shows some typical results.

It is possible to use single parameters, such as average or root–mean square roughness (Rms), to describe membrane roughness[28,29]. However, more information can be obtained from AFMimaging using complementary metrics, such as skewness, whichquantifies the asymmetry and flatness [30,31]. Here, the AFMimages were analyzed using WS �M software [32]. The histogramand other roughness characteristics of the surfaces of the poled andun-poled membranes are compared in Fig. 3-c. The average rough-ness and root–mean square roughness of the un-poled membrane

(c)

Fig. 3. AFM topographic images of the un-poled membrane surface (a) and poled membraThe dashed lines in image c show the expected normal distributions.

surfaces were greater than those of the poled membrane surfaces.This suggests that the surfaces of un-poled membranes are rougherwith a higher occurrence of coarse features.

The negative skewness calculated for poled membranes sug-gests that the features on the surfaces of these membranes aresmaller than a normal distribution. However, the features on thesurfaces of un-poled membranes are greater than predicted froma normal distribution, resulting in positive skewness. Positiveskewness has been commonly reported for micro- and ultra-filtra-tion membranes [33]. However, negative skewness has been re-ported in the literature and is attributed to a smaller number ofmulti-pores [34].

3.2. Flux experiments

Fig. 4 shows typical data for the permeate flux (J) during con-stant pressure (DP = 55 kPa) filtration of distilled water and 10 g/l sodium alginate solution through both un-poled and poled PVDFmicrofiltration membranes.

The permeate using distilled water as the feed for un-poledmembranes had a high initial flux of 2050 ml/m2/s, followed by arapid decline during the first 2 min, and then a steady decline to1650 ml/m2/s. For comparison, the water flux for poled mem-branes had an initial value of 1650 ml/m2/s, which is 20% less than

ne surface (b). The plot in c shows a comparison of the histograms of images a and b.

608 M.T. Darestani et al. / Separation and Purification Technology 118 (2013) 604–611

that of the un-poled membranes. This was followed by a steadydecline to 1523 ml/m2/s, which is only 5% less than that for un-poled membranes.

The water flux decline can be attributed in part to compres-sion of the membrane, some of which is a reversible compress-ibility of polymeric membranes [35]. These results suggest thatthe effect of pressure on decreasing hydraulic permeability isgreater for un-poled membranes than poled membranes. Apossible explanation is that the random porous structure ofun-poled membranes is more compressible than the layeredstructure of poled membranes.

On filtration of a solution containing 10 g/l of sodium algi-nate, the flux for un-poled membranes declined by about 90%after 20 min of filtration as is shown in Fig. 4-b. By comparisonthe flux for poled membranes under the same conditions de-creased by only about 70%. The flux data for each membranecould be fitted by a simple exponential function (see Fig. 1-b).In spite of the fact that the initial flux for the un-poled mem-branes was 2.4 times greater than that for poled membranes,the exponential fitting shows that the flux for the membraneswould be the same after approximately 35 min of filtration. Thistype of behaviour is characteristic of the effect of average poresize [36], suggesting that un-poled membranes with larger poressize, on average, had a higher flux than poled membranes and,hence, its flux declined faster.

1400

1500

1600

1700

1800

1900

2000

2100

0 10 20 30

Flux

(g/m

2 /s)

Filtrtion time (min)

Poled

Un-poled

(a) Flux of distilled water

J un-poled = 951e-0.108tR² = 0.9638

J poled = 395e-0.08tR² = 0.95180

100

200

300

400

500

600

700

800

900

1000

0 10 20 30 40

Flux

(g/m

2/s)

Filtrtion time (min)

Un-poled membrane

Poled membrane

(b) Flux of sodium alginate

Fig. 4. Permeate flux as a function of filtration time during the filtration of (a)distilled water and (b) a solution of 10 g/l sodium alginate through an unpoled andelectrically poled PVDF membrane electrical poling.

Four classic filtration laws for dead filtration [37] have beenused to explain the flux performance of micro-filtration and ul-tra-filtration membranes [19,38–40]. Cake filtration describes thedeposition of particles larger than the membrane pore size ontothe membrane surface. Standard blocking describes the depositionof particles smaller than the membrane pore size onto the porewalls, thereby reducing the pore size. Intermediate blocking andcomplete blocking are terms used to explain the occlusion of poresby particles with or without particle superimposition. The follow-ing simplified linear Eqs. (1)–(4) are used to explain the flux behav-iour of cake filtration, standard blocking, intermediate blocking,and complete blocking, respectively:

t=V ¼ aV þ b ð2Þ

t=V ¼ at þ b ð3Þ

1=J ¼ at þ b ð4Þ

lnðJ=J0Þ ¼ at þ b ð5Þ

In each equation, V is the cumulative volume of permeate at time t, Jis the flux, J0 is the initial flux, and a and b are the modelparameters.

(t/V) Un-poled = 0.039t + 0.5171

R² = 0.9985

0

0.5

1

1.5

2

2.5

3

0 500 1000 1500 2000

t/V

t

Un-poled

Poled

(a) Standard pore blocking model

(t/V)Poled = 3.0285v + 0.8359

R² = 0.9927

0

0.5

1

1.5

2

2.5

3

0 0.2 0.4 0.6 0.8 1 1.2

t/V

v

Un-poledpoled

(b) Cake filtration model

Fig. 5. Standard pore blocking (a) and cake filtration (b) models for un-poled (�)and poled (4) PVDF membrane filtration performance compared with the exper-imental data.

M.T. Darestani et al. / Separation and Purification Technology 118 (2013) 604–611 609

The standard blocking model fits the flux data of the un-poledPVDF membranes well, as can be seen in Fig. 5. This is in agreementwith previous reports on the performance of micro-filtration PVDFmembranes for the filtration of sodium alginate [19]. However, forthe poled membranes, the standard blocking model is not a good fit(as shown in Fig. 5-a).

For the poled membranes, the filtration data fits more closelywith the cake filtration model, as shown in Fig. 5-b. This modelcould also accommodate the filtration behaviour of the un-poledmembranes, although the standard blocking model gave a margin-ally better fit in agreement with previous reports [19]. It is possiblethat no single model applies to the entire filtration process andthat the operation moves from one type to another as filtrationproceeds. However, the results of this modelling suggest that pol-ing changes the fouling behaviour of PVDF membranes.

3.3. Rejection and molecular weight cut-off (MWCO) of membranes

In the filtration experiments, the concentration and averagemolecular weights in both the feed and permeate were determinedusing the poled and un-poled PVDF membranes. The permeatesample was taken at the end of 20 min filtration experiment. Theresults are shown in Fig. 6. The concentration and average molec-ular weight of the permeate solution filtered using the un-poledmembranes were only slightly lower than those for the feed solu-tion. The low rejection of the un-poled PVDF microfiltration mem-brane to the sodium alginate with an average molecular weight of100–150 kDa is in agreement with previous reports [39].

Feed solution

Filtrate of un-poled

membrane

Filtrate of poled

membrane

0

2

4

6

8

10

Con

cent

rati

on o

f SA

(gr

/l)

0

20

40

60

80

100

120

140

160

A v

erag

e m

olec

ular

wei

ght

(kD

a)

Feed solution Filtralte of un-poled membrane

Filtralte of poledmembrane

(a)

(b)

Fig. 6. Concentration (a) and average molecular weight (b) of sodium alginate inthe feed and permeate solutions of poled and un-poled membranes after 2 h offiltration. The points shown are samples from different experiments.

In contrast, filtration using the poled membranes significantlyreduced the concentration and molecular weight of the sodiumalginate in the permeate. The rejection of the poled membranewas about 50% and the average molecular weight of the sodiumalginate was 50–100 kDa. These values are close to the typicalmolecular cut-off (MWCO) of ultra-filtration membranes [19,40].

3.4. Microstructure of membranes after filtration tests

Fig. 7 shows SEM micrographs of the un-poled and poled mem-branes after the filtration tests. The surfaces of both the poled andun-poled membranes were covered with sodium alginate whichformed sticky hydrogel layers due to its adhesive properties. Itcan be seen that the structure of the fouling layer on the surfaceof the un-poled membrane (Fig. 7-a) was porous, whilst the foulinglayer formed on the surface of the poled membrane was much den-ser (Fig. 7-b). The porous fouling layer detected on the surface ofthe un-poled membrane explains the good fit of the filtration datawith the pore blocking model [41].

The high magnification SEM images of the cross-sections ofthe membranes (Fig. 7-e and f) show that the pores of the un-poled membrane are still visible and there is only slight particledeposition. This validates the low rejection and high MWCOachieved using un-poled membranes. In contrast, the tight foul-ing across the cross-section of the poled membrane indicatesthat it was fouled internally. This could be explained by the lay-ered structure of the poled membrane. It is possible that sodiumalginate molecules, which can pass through the porous top layer,were eventually trapped by the dense layer of the poled mem-brane. This multi-layer sieving behaviour of the poled membranecould explain the delayed flux decline to levels below 10–20% ofthe initial flux, compared to the un-poled membrane [19,38–40].

4. Conclusions

The results conclusively show that poling in an intensive elec-tric field significantly changed the microstructure and filtrationperformance of PVDF microfiltration membranes. Poling changedthe isotropic porous structure of the membrane to an anisotropic,layered, structure and made the membrane surface slightlysmoother. The results of the filtration experiments showed thatpoled membranes had a lower flux compared to un-poled mem-branes. The greater rejection and lower MWCO of the poled mem-branes suggested that electrical poling changed the separationcharacteristics of microfiltration membranes to characteristicsmore akin to ultrafiltration membranes.

It should be noted that in this study the poled membranes werenot set into vibration by the application of an AC signal. Althoughthe prime objective of using electrical poling was to impart piezo-electric activity to PVDF microfiltration membranes, the resultsprovide evidence that an electric field can also be used to changethe microstructure of the membranes. The results of this hypothe-sis will be discussed further in our future publications.

Acknowledgments

We acknowledge the assistance of the staff of AMMRF (Austra-lian Microscopy & Microanalysis Research Facility) at the ElectronMicroscope Unit of the University of Sydney. We thank Dr. BogumilEichstaedt for his help in setting up the experiments. M.T.Dacknowledges the support of the University of Sydney and the Aus-tralian Government through an Endeavour International Postgrad-uate Research Scholarship (EIPRS).

enarbmemdeloPenarbmemdelop-nU

Surface Surface

Cross sectionCross section

Cross section Cross section

Feed Side

Feed Side

Filtration Direction Filtration Direction

(a) (b)

(c) (d)

(e) (f)Fig. 7. SEM micrographs of membranes after the filtration of 10 g/l sodium alginate solution at 55 kPa for 25 min: (a) surface of the un-poled membrane; (b) surface of thepoled membrane; (c) edge and cross-section of the un-poled membrane; (d) edge and cross-section of the poled membrane; (e) cross-section of the un-poled at highmagnification; and (f) cross-section of the poled membrane at high magnification.

610 M.T. Darestani et al. / Separation and Purification Technology 118 (2013) 604–611

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