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Journal of Membrane Science 350 (2010) 130–138

Contents lists available at ScienceDirect

Journal of Membrane Science

journa l homepage: www.e lsev ier .com/ locate /memsci

reparation, characterization and performance studies of ultrafiltrationembranes with polymeric additive

. Arthanareeswarana,b,∗, D. Mohanb, M. Raajenthirenb

Membrane Research Laboratory, Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli 620015, IndiaDepartment of Chemical Engineering, Anna University, Chennai 600025, India

r t i c l e i n f o

rticle history:eceived 4 September 2009eceived in revised form5 December 2009ccepted 15 December 2009

a b s t r a c t

In the present work, the effect of additive, polyethylene glycol 600 (PEG 600), on the ultrafiltration perfor-mance of the polysulfone (PSf)/sulfonated poly(ether ether ketone) (SPEEK) blend flat sheet membranesprepared in a polar solvent of N,N′-dimethylformamide (DMF) by phase separation process was investi-gated. The effects of additive concentration on the ultrafiltration performance such as compaction, purewater flux, water content, hydraulic permeability and morphology were investigated. The addition of

vailable online 23 December 2009

eywords:olyethylene glycolhase separation

PEG 600 in the dope solution increases the exchange rate of additive and non-solvent during the mem-brane formation process, resulting in the appearance of the macrovoids formation as visualized undermicroscope. The pure water flux and Equilibrium water content of the blend membranes were increasedwhile hydraulic permeability was decreased.

embrane separationdditiveltrafiltration

. Introduction

For the past several decades, synthetic polymeric membranesre used for a wide variety of liquid separations such as microfil-ration, ultrafiltration, reverse osmosis, nanofiltration. A common

ethod for the preparation of polymeric membranes is the phaseeparation process [1,2]. At the same time, blending of two dif-erent polymers offers attractive opportunity for the developmentf novel materials with valuable combination of properties [3]. Inecent years, several polymer blends have been developed for suc-essful application in ultrafiltration separation [4–6]. Alongside, theddition of organic or inorganic additives as the third componento the blend polymers has been one of the important proceduressed in membrane preparation to control the morphology and per-ormance of membranes. Studies have been conducted by addingdditives such as polyethylene glycol (PEG), hyperbranched polyg-ycerol (HPG) polyvinylpyrrolidone (PVP) in the casting solution

o improve the membrane performance [7–9]. Idris and Yet [8]ound that the presence of PEG in the dialysis membrane enhancedhe membrane performance exhibited by the high urea clear-nce and high urea permeability. Sivakumar et al. [9] investigated

∗ Corresponding author at: Department of Chemical Engineering, National Insti-ute of Technology, Thanjavur Road, Tiruchirappalli 620015, India.el.: +91 431 2503118; fax: +91 431 2500133.

E-mail addresses: arthanaree10@yahoo.com, arthanareeg@nitt.eduG. Arthanareeswaran).

376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2009.12.020

© 2009 Elsevier B.V. All rights reserved.

the effects of PVP on CA/PSf blend membranes and showed thatincrease in the concentration of PVP in casting solution resultedin the improved performance of membranes. Polysulfone (PSf) iswidely used as an ultrafiltration, reverse osmosis and pervapora-tion membrane since polysulfone has good thermal and chemicalstabilities [10]. However, hydrophobic surface of polysulfone mem-branes has a severe fouling during ultrafiltration processes [11].Therefore, modifying the PSf membrane, including structure andcross-section morphology, hydrophilic properties, is significant forpractical applications. Sulfonated PEEK was first applied in mem-brane formation to improve hydrophilic properties of relativelyhydrophobic polymers [12].

In our previous study [13], SPEEK was introduced to modifyCA membrane in order to obtain CA/SPEEK blend UF membrane.Introduction of SPEEK in the CA membrane resulted in improvedUF performance. Further, we prepared polysulfone and sulfonatedpoly(ether ether ketone) blend membranes and characterized theirultrafiltration performance [14]. It was found that the extentof blend compatibility of polymer compositions is 85/15 wt% ofPSf/SPEEK. Phase separation formed beyond 15 wt% of SPEEK intothe PSf/SPEEK polymer blend. Further, 85/15 wt% of PSf/SPEEKblend membrane shows highest pure water flux, water content, andporosity compared to other membranes. Su et al. [15] selected five

additives such as butanone (BO), ethyl ether (EE), ethylene glycolmonomethylether (EGME), diethylene glycol (DEGC) and polyethy-lene glycol and studied the rejection performance of PEG withmolecular weight of 2000–20,000 Da through poly(phthalazinoneether sulfone)-bisphenol (PPES-B) UF membranes. It focused on

of Mem

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G. Arthanareeswaran et al. / Journal

he effects of type and concentration of additives on PPES-B mem-rane and the membranes with PEG400 as additive exhibited bettererformance than other four as additives.

In sight of this, the main objective of this work is to investigatehe effect of PEG 600 on the performance of PSf/SPEEK blend ultra-ltration membrane in terms of pore statistics, compaction, pureater flux (PWF), equilibrium water content (EWC) and hydraulicermeability (Pm). In the initial experiments, effects of various addi-ives were studied, and PEG 600 was chosen as suitable additive forhis investigation. It has been shown that the introduction of PEG00 beyond 7.5 wt% in the total polymer composition of PSf/SPEEKesulted in heterogeneous solution leading to phase separationuring membrane formation. Hence, we selected a maximum of.5 wt% of PEG 600 in the PSf, PSf/SPEEK blend composition and

ntroduced PEG 600 by reducing the composition of DMF as solvent.he MWCO of the prepared membranes have been determined bysing different molecular weight of proteins. The morphology ofhe resulted membranes has also been studied by scanning electron

icroscope (SEM).

. Experimental

.1. Materials and methods

Commercial grade polysulfone Udel P-3500 (PSf) was procuredrom Amoco Polymers Inc., USA. Commercial grade poly(ether etheretone) (PEEK) (Victrex PEEKTM 450 PF) was supplied by Abhi-an Polymers, India. Analytical grade N,N′-dimethylformamide

DMF) and sodium lauryl sulfate (SLS) were obtained from Quali-ens Fine Chemicals, Glaxo India Ltd., India. Polyethylene glycolPEG) 600, oxalic acid, maleic acid and acetone were procured fromRL Ltd., India. Disodium hydrogen orthophosphate anhydrous andonosodium dihydrogen orthophosphate heptahydrate were pro-

ured from CDH Chemicals Ltd., India. Bovine serum albumin (BSA),w = 69 kDa, pepsin, Mw = 35 kDa, trypsin, Mw = 20 kDa were pur-

hased from SRL Chemicals Ltd., India and used as received. Egglbumin (EA), Mw = 45 kDa was obtained from CSIR Bio Chemicalentre, New Delhi, India. Deionized and distilled water was used

or the preparation of protein feed solution and for the gelation bathreparation.

.2. Preparation of blend membranes

Sulfonation of PEEK by using concentrated sulfuric acid isxplained in our earlier reported procedure [13]. The casting solu-ions (17.5 wt%) were prepared by blending of PSf and SPEEKolymers of different compositions in presence of different con-entrations of PEG 600 in DMF under constant stirring for 4 h at0 ◦C. A series of such polymer solutions were prepared by varyinghe composition of PSf, PSf/SPEEK with an increment of 2.5 wt% ofEG 600, as shown in Table 2. The obtained homogeneous solutionas allowed to stand for at least 3 h to eliminate the air bubbles. In

he membrane preparation process, two types of phase separationan be distinguished. The dry phase inversion takes place in thetmosphere by evaporation of the solvent. The wet phase inver-ion is carried out by immersing the polymer solution film into aoagulation bath of a non-solvent, where an exchange of solventnd non-solvent takes place and the membrane is formed. In thistudy, we prepared membranes by a combination of both processes.

The polymer solution was poured over a smooth glass plate and

pread with a metal rod with side runners to produce 0.22 mmhickness of the membrane. The size of the casting area was approx-mately 29 cm × 25 cm. For those membranes prepared using aolvent evaporation period, the glass plate was placed inside aumidifying chamber. During casting, the relative humidity (RH)

brane Science 350 (2010) 130–138 131

of the casting chamber was maintained at 50 ± 2% and the tem-perature was kept at 20 ± 2 ◦C. Prior to casting, a gelation bath of 2 lconsisting of 2.5% (v/v) DMF and 0.2 wt% SLS in distilled water (non-solvent) was prepared and the bath was ice-cooled to 4 ◦C. Thirtyseconds was the maximum solvent evaporation time that allowedthe preparation of membrane sheets, longer times resulted in holeswithin them. After the solvent evaporation period at humidifyingchamber, the glass plate was immersed into ice-cooled gelationbath at 4 ◦C for at least 1 h. The hardened membranes were eventu-ally peeled from the glass plate and after the gelation, membraneswere stored at 5 ◦C in distilled water until they were tested. Everymembrane sheet was inspected with a light-box to detect defects,such as thin spots and pinholes; defective membranes were dis-carded. Prepared membranes (76 mm diameter) were cut from theflat sheets for the ultrafiltration experiments.

2.3. Membrane characterization

2.3.1. Morphological studiesThe morphological characterization of all the blend membranes

was studied using scanning electron microscopy (SEM) (Leica Stere-oscan440, UK). The membranes were cut into pieces of varied sizesand mopped with filter paper and were immersed in liquid nitro-gen and the frozen membranes were preserved in desiccator forSEM studies. The top surface and cross-sectional view of the mem-branes, was observed by SEM under very high vacuum at 25 kV andat different magnifications.

2.3.2. Pore statisticsThe average pore size and surface porosity of membranes were

determined by the ultrafiltration of protein solutions of differentmolecular weights. The molecular weight of the solute that has asolute rejection (SR) above 80% was used to evaluate the averagepore size, R̄, of the membranes using the following equation [16,17]:

R̄ = 100(

˛

%SR

)(1)

where R̄ is the average pore size (radius) of the membrane (Å), and˛ is the average solute radius (Å). The %SR is a percentage soluterejection of proteins (from Eq. (6)). The average solute radii, alsoknown as the Stoke radii, were obtained from the plot of solutemolecular weight versus solute radius in aqueous solution, whichwas developed by Sarbolouki [17].

The surface porosity, ε, of the membrane was calculated by theorifice model given below assuming that only the skin layer of themembrane is effective in separation.

ε = 3��Jw

�PR̄(2)

where � is the viscosity of the permeate water in Pa s, Jw is the purewater flux of the membrane in m3/m2 s, R̄ is the average pore radiusin Å and �P is the transmembrane pressure in Pa.

2.3.3. Equilibrium water contentEquilibrium water content is considered to be an important

characterization parameter as it indirectly indicates the degree ofhydrophilicity or hydrophobicity of a membrane [18]. Also it isrelated to the porosity of a membrane. Membranes were weighedin an electronic balance in wet state after mopping the surfacewater with a clean tissue paper. The wet membranes were driedby putting in a vacuum oven for 24 h at a temperature of 50–60 ◦Cand again they were weighed in dry state. The EWC at room tem-

perature is calculated as follows [18]:

EWC (%) = wet membrane weight − dry membrane weightwet membrane weight

× 100 (3)

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J

wa

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P

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32 G. Arthanareeswaran et al. / Journal

.4. Permeation experiment

.4.1. Ultrafiltration experimentsThe ultrafiltration (UF) experiments were carried out in a 400 ml

atch type stirred cell (ultrafiltration cell – S76-400-Model, Spec-rum, USA) fitted with a Teflon coated magnetic paddle. Theffective membrane area available for ultrafiltration was 38.5 cm2.he solution filled in the cell was stirred at 400 rpm using a mag-etic stirrer. All the experiments were carried out at 30 ± 2 ◦Cnd compaction at 414 kPa transmembrane pressure while otherermeation studies at 345 kPa transmembrane pressure. The per-eating solution was collected from the bottom of the cell. The cellas pressurized using a nitrogen cylinder.

.4.2. Membrane compactionBefore using each fresh membrane, it was compacted. The mem-

ranes were initially pressurized with deionized water for 5 h at aransmembrane pressure of 414 kPa which is higher than the max-mum operating pressure designed for the present experiments.he initial water flux was taken about 30 min after the pressuriza-ion in the test cell, at 414 kPa. The pure water flux was measuredfter every hour in order to monitor the compaction behavior. Theembranes were compacted until a steady flux was observed. Pureater flux (Jw) was calculated over measured time intervals using

he following equation. The compact membranes were washedhoroughly before placing them in the membrane batch cell.

w = Q

A �T(4)

here Q is permeating of pure water (l); A, effective membranerea (m2) and �T, the sampling time.

.4.3. Pure water flux and hydraulic permeabilityThe compacted membranes were subjected to pure water flux

tudies and hydraulic permeability was calculated under steady-tate run at a transmembrane pressure of 345 kPa using Eq. (4).embrane hydraulic permeability has got significance particu-

arly for membranes used in pressure-driven separation processes.embrane permeability was determined by allowing deionizedater to pass through the compacted membrane. Flux values

f pure water at different transmembrane pressures (ranging–414 kPa) were measured under steady-state condition using Eq.4). From the slope of the linear relationship between the pureater flux (Jw) and transmembrane pressure, hydraulic permeabil-

ty was calculated as

m = Jw�P

(5)

here Pm is hydraulic permeability (l m−2 h−1 kPa−1); �P is trans-embrane pressure (kPa).

.4.4. Molecular weight cut-off (MWCO)UF experiments were conducted in the batch kit cell mentioned

reviously to study the influence of concentration of pore formerEG on molecular weight cut-off (MWCO) and solute rejection ofhe prepared membranes. Therefore, proteins of different molecu-ar weights such as trypsin (20 kDa), pepsin (35 kDa), EA (45 kDa)nd BSA (69 kDa) were chosen for the estimation of MWCO andolute rejection. All the protein solutions were prepared individu-lly at a concentration of 0.1 wt% in phosphate buffer (0.5 M, pH 7)sing deionized water and used as standard solutions. The UF cell

as filled with protein solution and pressurized at a constant pres-

ure of 345 kPa and stirred at 400 rpm throughout the experimentso minimize fouling. During ultrafiltration, the permeate solutionsf corresponding membranes were collected over a period of timen a graduated tube and were analyzed for the concentration using

brane Science 350 (2010) 130–138

UV-Visible spectrophotometer (Shimadzu, Model UV-160A) at awavelength of 280 nm. From the feed and permeate concentrations(Cp and Cf) the percentage rejection was calculated using Eq. (6)[15].

%SR = 1 −(

Cp

Cf

)× 100 (6)

The main objective of the ultrafiltration experiment was toinvestigate the rejection behaviour of proteins by different mem-branes prepared in the present work under constant pressureconditions. Molecular weight cut-off is an attribute of pore size ofthe membranes and is related to the rejection of a spherical soluteof given molecular weight. The molecular weight has a linear rela-tionship with the pore size of the membrane [19]. In general, themolecular weight cut-off of the membrane is determined by iden-tifying an inert solute of different molecular weights that have asolute rejection of 80–100% in steady-state UF experiments [20].Therefore, proteins of different molecular weights were chosen forthe estimation of MWCO.

3. Results and discussion

The membrane structure and its properties are influenced by thecomposition of the polymer solution (concentration, solvent, addi-tives). The addition of some pore forming additives to the castingsolution is very effective in changing their properties. In order tofind the suitable additive for preparing blend UF membranes withsuitable pore size, the effect of different organic additives as poreformer on pure water flux of PSf membrane was attempted. Thus,the additives such as maleic acid, oxalic acid and polyethylene gly-col 600 were chosen. The additives concentration in the castingsolution of blend membranes was maintained at 5 wt%. The effectof various additives on pure water flux of PSf membrane is givenin Table 1. From Table 1, it is evident that pure PSf has the purewater flux of 16.8 l m−2 h−1. Further, it is evident that PEG 600 hassignificantly enhanced the water flux to 57.8 l m−2 h−1, comparedto other additives (pore formers). In addition, it is well known thatPEG 600 acts as a better pore former compared to maleic acid andoxalic acid in PSf/SPEEK blend system. Zhu et al. suggested thatwhen PEG is entrapped by membrane materials as additive, PEGshould increase hydrophilicity, and results in important change inthe performance of ultrafiltration membrane such as solute reten-tion and fouling [21]. Further, the 600 molecular weight PEG waschosen, because solutions prepared with higher molecular weightsare thermodynamically less stable and coagulation values becomemuch lower [22]. Hence, for further work, PEG 600 was used as apore former additive in PSf/SPEEK blend membranes and the cast-ing compositions of membranes are shown in Table 2.

3.1. Morphological studies

SEM images for the top surface of different membranes are pre-sented in Fig. 1. The top surface and cross-section structures of purePSf and various compositions of PSf/SPEEK blend membranes havebeen reported in our earlier study [15]. It was observed that purePSf membranes have a dense top surface [15]. When PEG is addedto the casting solution, pores appear at the membrane surface.With the increase of PEG content from 2.5 to 7.5 wt% in the castingsolutions, the largest diameter of the micropores at the membranesurfaces increases from about 38.9 to 42.7 Å. At the same time, the

number pores in the surface of membranes also becomes more.Similar results were observed by the other researchers [23,24].Chuanga et al. [22] suggested that the addition of PVP additivesin the system could induce pores in the top layer. However, Wanget al. [24] showed that the size and number of the macrovoids in

G. Arthanareeswaran et al. / Journal of Membrane Science 350 (2010) 130–138 133

Table 1Effect of different additives on PWF of PSf (17.5 wt%) membranes.

Total polymer (wt%) Additives (wt%) Solvent (wt%) Pure water flux at 345 kPa (l m−2 h−1)

PSf Oxalic acid Maleic acid PEG 600 DMF

–––5

flt2mtsampdgda

pAsFsfsF9ficcriatPfmatmw

TE

17.5 – –17.5 5 –17.5 – 517.5 – –

at membrane decreases with increasing LiCl, PVP and PEG con-ent in the PES casting solution. From Fig. 1, it is observed that.5 wt% of PEG 600 in 95/5 wt% PSf/SPEEK blend compositions ofembranes exhibited smaller pores distributed evenly. However,

he pore size increases with 7.5 wt% of PEG 600 concentrations iname blend composition. The blend membrane of PSf/SPEEK/PEGt 85/15/7.5 wt% has the highest surface porosity while the blendembrane of 85/15 wt% at 2.5 wt% PEG has the lowest surface

orosity (Fig. 1). The formation of pores at the top surface is possiblyue to demixing of the casting solution by means of nucleation androwth of the polymer rich phase, i.e. the solid phase [25,26]. Smol-ers et al. [27] reported that appropriate amount of non-solventdditives enhanced the formation of macrovoids.

It is known that the cross-sectional structure of asymmetricolymer membrane depends on the solvent and additive used [28].finger-like structure was formed when DMF/PEG was used as the

olvent/additive pair with various compositions in blend polymers.ig. 2 shows the SEM image of the cross-section of different compo-ition of PEG in PSf and PSf/SPEEK blend membranes. It may be seenrom the figure that membranes so formed are having asymmetrictructure consisting of a dense top layer and a porous sub layer.rom Fig. 2, when PEG 600 content increased from 2.5 to 7.5 wt% in5/05 wt% of PSf/SPEEK blend solution, the sublayer seems to havenger-like cavities as well as macrovoid structure. This finger-likeavities and macrovoid structure was changed with increasing con-entration of PEG. Large macrovoids in the substructure typicallyesult in increased permeability, and therefore, may be desirablen moderate-pressure UF applications. Similar observations werelso found by Strathmann et al. [29] using LiCl as the additive forhe preparation of polyamide membrane. Due to high affinity ofEG with water, instantaneous demixing results, leading to theormation of finger-like cavities in the sublayer of the prepared

embranes [2]. Since decay of the casting solution by adding of PEGdditive somewhat limited the growth of polymer-lean phase, i.e.he growth of macrovoid, dense layer having sponge-like structure

ight be formed. Note that the similar phenomena were observedith the membrane prepared from the various compositions of

able 2ffect of compaction time on PWF of PSf/SPEEK blend membranes with additive concentr

Polymer blend composition(17.5 wt%)

Additive, PEG 600 wt% Pure water flu

PSf SPEEK Time (h)

0

100 0 2.5 9595 5 2.5 11590 10 2.5 12685 15 2.5 150

100 0 5.0 12395 5 5.0 16590 10 5.0 19485 15 5.0 234

100 0 10.0 15895 5 10.0 24490 10 10.0 29785 15 10.0 318

82.5 16.877.5 33.377.5 44.477.5 57.8

PSf/SPEEK/PEG blend casting solution. Since PSf/SPEEK is waterinsoluble, they still remain in the membrane matrix after phaseinversion process. However, part of PEG in the casting solutionmight be removed from the membrane matrix due to PEG dis-solving into the non-solvent (gelation solution) phase during theprocess. The growth of PSf/SPEEK-rich phase might be hinderedduring the phase inversion process and PEG dissolving induced theenlargement of the micropore. Similar observations were reportedfor effect of PEG on PEI gas membranes [30]. Because of this, thefinal morphology of membrane prepared from the PSf/PEEK/PEGsolution is completely different from that of membrane preparedfrom the solution containing PSf and PSf/SPEEK blend.

3.1.1. Effect of concentration of PEG on average pore size andporosity

The average pore size and porosity of the membranes deter-mined from the protein rejection studies are shown in Fig. 3. Itis evident from these results that the membrane prepared in theabsence of additive has relatively smaller pore size and porosity.PSf in presence of 2.5 wt% PEG additive membrane had the averagepore size value of 38.9 Å and porosity value was 0.74. The averagepore size of PSf in absence of both SPEEK and PEG was determinedand discussed in earlier work [15]. Further, the average pore sizeincreases from 38.9 to 42.7 Å and the porosity increases from 0.74to 1.35 with increase in the concentration of pore former, PEG from2.5 to 7.5 wt% in 100% polysulfone membrane. When increasing PEGconcentration from 2.5 to 7.5 wt%, an upward trend in average poresize from 40.2 to 48.7 Å and 41.2 to 50.5 Å for PSf:SPEEK blend com-positions of 95/5 and 85/15 wt%, respectively, had been observed.Similarly, the porosity increases from 0.89 to 1.46 and 1.42 to 1.79for PSf:SPEEK blend compositions of 95/5 and 85/25 wt%, respec-tively, when increasing the PEG concentration from 2.5 to 7.5 wt%,

respectively. The increase in PEG content in blend may favor the for-mation of larger pores in membranes. Increase in the pore size andporosity, in principle, would lead to the increase in the permeateflux of the membrane. This is in agreement with the results obtainedin this study. Although bigger pore size will favor high permeate

ations.

x (l m−2 h−1) at 414 kPa

1 2 3 4 5

80 71 62 45 45104 85 70 57 57116 102 85 69 69131 122 110 92 92

109 98 79 71 71148 127 101 89 89183 159 127 113 113197 168 145 126 126

153 136 109 96 96189 159 121 110 110218 189 152 128 128234 203 174 141 141

134 G. Arthanareeswaran et al. / Journal of Membrane Science 350 (2010) 130–138

fferen

flsoba[

3

ceiw[toaa

tion from 2.5 to 7.5 wt%, respectively. The SEM pictures showingthe top surface of the membranes with different PEG compositionsevidence this trend (Fig. 1). This trend indicates the leachability ofthe water soluble additive leading to the formation of larger pores

Table 3Effect of additive concentration on equilibrium water content of PSf/SPEEK blendmembranes.

Blend composition(17.5 wt%)

Equilibrium water content (%)

PSf (%) SPEEK (%) PEG 600 (wt%)

2.5 5.0 7.5

Fig. 1. SEM top surface image showing the influence of di

ux, the solute rejection will drastically fall. The membrane bestuited for any application would be the one that has more numberf pores with smaller size. Because smaller size of pores will favoretter retention and higher number of pores will favor high perme-te flux. Similar observation was also reported by Malaisamy et al.31] for cellulose acetate and sulfonated polysulfone membranes.

.1.2. Effect of concentration of PEG on equilibrium water contentEquilibrium water content of the membranes is an indirect indi-

ation of the hydrophilicity and flux behavior of membranes. Theffect of PEG concentration on EWC of PSf/SPEEK blend membraness illustrated in Table 3. The EMC of PSf is 76.5% in absence of PEG,

hich was this result has been also reported in our earlier work

15]. Further, the EMC increases from 77.2 to 80.2% with increase inhe concentration of pore former, PEG, from 2.5 to 7.5 wt% in 100%f PSf membrane. Further, the EWC increases from 82.5 to 87.5%nd 84.3 to 91% for the blend compositions of PSf/SPEEK of 90/10nd 85/15 wt%, respectively, when increasing the PEG concentra-

t composition of PEG 600 in PSf/SPEEK blend membranes.

100 0 77.2 (0.6) 79.3 (0.5) 80.2 (0.5)95 5 80.8 (0.4) 81.7 (0.4) 82.8 (0.4)90 10 82.5 (0.5) 83.7 (0.4) 87.5 (0.3)85 15 84.3 (0.4) 85.2 (0.6) 91.0 (0.5)

Note: Numbers in the parenthesis are standard deviation.

G. Arthanareeswaran et al. / Journal of Membrane Science 350 (2010) 130–138 135

mposition of PEG 600 in PSf/SPEEK blend membranes on the morphology.

[ocSC

3

3

wccatoocP

Fig. 2. SEM cross-section image showing the influence of different co

9]. The EWC actually represents the fraction of water moleculesccupied in the pores of the membrane. An increase in the waterontent indicates that the membrane has become more porous.imilar trends were also observed by Lv et al. [32] for modifiedA membranes with Pluronic F127 in the casting solutions.

.2. Permeation experiments

.2.1. Effect of compaction time on fluxThe effect of SPEEK concentration on PWF of PSf membrane

as discussed in our earlier work [15]. The effect of additive con-entrations on PWF of prepared PSf/SPEEK membranes at variousompaction time is shown in Table 2. In general, the PWF decreasess compaction time increases for all the prepared membranes. In

he polymer casting solution, hydrophilic additive concentrationsf pore former, PEG 600, were varied, from 2.5 wt% in an incrementf 2.5 wt% for all the polymer blend solutions and the maximumompatible additive concentration was found to be 7.5 wt%. TheWF on compaction at every hour for 4–5 h for PSf and PSf/SPEEK

Fig. 3. Effect of concentration of PEG on average pore size and porosity of PSf/SPEEKblend membranes (— porosity, . . . average pore size; × 100/0%,� 95/05%, � 85/15%).

136 G. Arthanareeswaran et al. / Journal of Membrane Science 350 (2010) 130–138

Table 4Initial and final flux ratio during compaction of PSf/SPEEK blend membranes with additive concentrations.

Polymer blend composition (17.5 wt%) Additive, PEG 600 (wt%) Solvent, DMF (wt%) Pure water flux at 414 kPa (l m−2 h−1)

PSf (%) SPEEK (%) Initial Final Final/initial ratio

100 0 2.5 80.0 95 45 0.4795 5 2.5 80.0 115 57 05090 10 2.5 80.0 126 69 0.5585 15 2.5 80.0 150 92 0.62

100 0 5 77.5 123 71 0.5795 5 5 77.5 165 89 0.5490 10 5 77.5 194 113 0.5885 15 5 77.5 234 126 0.54

7.07.07.07.0

maT19atmtptisb

b3aPPtl

3

bm

Fb

100 0 7.5 795 5 7.5 790 10 7.5 785 15 7.5 7

embranes with various PEG 600 concentrations was measured,nd the initial and steady-state values are reported in Table 4. Fromables 1 and 4, it is observed that the membrane prepared from00% PSf in presence of 2.5 wt% PEG 600 as additive has a flux of5 l m−2 h−1 and almost remains constant at a value of 45 l m−2 h−1

fter 4–5 h of compaction time. This constant value is indicatedhe completion of compaction. In addition, an increase in pore for-

er concentration to 5 wt%, increased the constant flux from 45o 71 l m−2 h−1. During compaction under hydraulic pressure, theolymeric chains reorganize and this leads to change in the struc-ure with a lowered volume of porosity. This leads to an increasen the hydraulic permeability offered by the membrane, and con-equently lowers the flux. Similar observations have been reportedy Chakrabarty et al. [18].

When 7.5 wt% of PEG 600 was added in the 80/15 wt% PSf/SPEEKlend system in place of solvent DMF, the initial water flux was18 l m−2 h−1. After 4 h of compaction, the water flux decreased andttained a steady-state value of 141 l m−2 h−1. The SEM images ofSf/SPEEK membranes support these observations that at 7.5 wt%EG 600 concentration numerous pores with higher sizes formedhan with the 2.5 wt% of PEG composition (Fig. 1), which is due toeaching out of additives in the gelation bath from the membrane.

.2.2. Effect of concentration of PEG on pure water fluxThe effect of PEG concentration on PWF through PSf/SPEEK

lend membranes is illustrated in Fig. 4. The PWF is the key require-ent of any membrane and must be determined essentially. After

ig. 4. Effect of concentration of PEG on pure water flux of PSf/SPEEK blend mem-ranes at 345 kPa pressure.

158 96 0.61244 110 0.45297 128 0.43318 141 0.44

compaction, the pure water fluxes of all the membranes were mea-sured at a constant sampling period under steady-state conditions,after 30–45 min of stabilization at 345 kPa transmembrane pres-sure. The PWF increases from 38.4 to 76.7 l m−2 h−1 with increasein the concentration of pore former, PEG, from 2.5 to 7.5 wt% in 100%of PSf membrane. Further, when increasing PEG concentration from2.5 to 7.5 wt%, an upward trend in PWF from 56.6 to 108.9 l m−2 h−1

and 77.4 to 118.9 lm−2 h−1 for PSf/SPEEK blend compositions of90/10 and 85/15 wt%, respectively, had been observed. Kim and Lee[22] showed that the PWF increases exponentially with increasingPEG/NMP ratio up to a ratio of 1:1. This may be attributed to theformation of macrovoids by leaching out of PEG from the mem-brane forming system during gelation, which becomes the domainof water molecules. Similar observation has been reported by otherresearchers for blend membranes with PEG as an additive [7,33].

3.2.3. Effect of concentration of PEG on hydraulic permeabilityHydraulic permeability of clean membrane, Pm, is an indication

of tolerance of membrane towards hydraulic pressure. After thePWF reached steady state, Pm is determined by measuring the PWFunder various transmembrane pressures from 69 to 414 kPa. Thelinear proportionality of PWF to applied pressure can be directlyassociated to the transport resistance. The presence of an additivein the casting solution has considerable effect in the hydraulic per-meability. Thus when the PEG concentration was increased from 2.5to 7.5 wt% in the casting solution of PSf, there was a correspondingdecrease in hydraulic permeability from 9.1 to 4.5 l m−2 h−1 kPa−1

as shown in Fig. 5. It is due to the instability of pores which areformed by leaching out the PEG during the coagulation process inmembrane casting technique. Similar trend had also been observedfor blend membranes prepared from 5, 10 and 15 wt% SPEEKwith PSf in presence of additive. Thus, permeability of PSf/SPEEK(95/5 wt%) and PSf/SPEEK (85/15 wt%) is reduced from 9.3 to 3.7and 5.4 to 2.9 l m−2 h−1 kPa−1, respectively, when PEG concentra-tion in blend polymer casting solution increased from 0 to 7.5 wt%.The decreased value of hydraulic permeability may be due to thefact that the addition of pore former in the casting solution results inthe formation of macrovoids on the membrane surface due to ther-modynamical instability, which enhances precipitation and porousnature [29]. Similar observations have also been reported else-where [27,34].

3.2.4. Effect of concentration of PEG on molecular weight cut-off

The molecular weight cut-off (MWCO) of membranes are gen-

erally defined by the membrane manufacturers as the molecularweight of the solute at which above 80% of uncharged moleculesare retained [19]. The rejections of above 80% are found in practice,even for substances of molecular size much larger than the poros-

G. Arthanareeswaran et al. / Journal of Mem

Fig. 5. Effect of concentration of PEG on hydraulic permeability of PSf/SPEEK blendmembranes.

Table 5Solute rejection and MWCO of PSf/SPEEK blend membranes with PEG as additive.

Polymer blendcomposition(17.5 wt%)

PEG 600 (wt%) %SR of proteins MWCO (kDa)

PSf (%) SPEEK (%)

100 0 2.5 83 3995 5 2.5 85 4585 15 2.5 87 45

100 0 5 89 4595 5 5 87 4585 15 5 88 >69

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100 0 7.5 82 4595 5 7.5 85 >6985 15 7.5 82 >69

ty of the membranes. Thus, molecules or solutes having moleculareight larger than the molecular weight cut-off (MWCO) of a mem-

rane will not pass through the membrane. This is undoubtedly dueo polydispersity in the dimensions of the membrane pores. The

WCO of the membranes can generally be estimated using linearolymers such as polyethylene glycol, dextrans or using sphericalolecules such as proteins, etc. It is seen from Table 5 that mem-

rane made from pure PSf showed a MWCO of 39 kDa due to lowerore size. An increase in additive concentration from 2.5 to 7.5 wt%esults in the increase of MWCO of pure CA from 39 to 45 kDa. Itas believed that the incorporation of additive in casting solutionould alter the MWCO of all the PSf and PSf/SPEEK membranes. The

ncrease in MWCO with increasing PEG may be due to fast exchangeate of PEG and water during gelation process, which forms a largeore on membrane surface as also evidenced from SEM results. On

ncreasing the SPEEK content in blend from 5 to 15 wt% in presencef PEG increases the MWCO from 45 to >69 kDa. Similar results havelso been reported by Malaisamy et al. [35].

. Conclusions

The effects of pore forming additive on the morphology, equi-ibrium water content pore statistics and permeation performance

ere investigated. The addition of PEG increases the exchange ratef additive and non-solvent, resulting in the enhancement of the

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brane Science 350 (2010) 130–138 137

macrovoids formation. On addition of PEG in PSf/SPEEK dope solu-tion, a sharp interface between the PEG and non-solvent water wasformed, resulting in increased solvent–non-solvent exchange rate.

The observations can be summarized as follows:

� All the blend membranes were found to have asymmetric struc-ture as seen from SEM photographs.

� PEG 600 can be a suitable additive for making asymmetric mem-branes having a higher PWF.

� It was found that PSf/SPEEK blend polymer solutions havingadditive PEG concentration more than 7.5 wt% did not formmembrane.

� With increase in concentration of PEG in PSf/SPEEK blend, thepore size as well porosity was increased. The hydraulic perme-ability was seen to reduce greatly with increase in concentrationof PEG.

� A significant increase in MWCO of membrane was observedwhen concentration of PEG was increased from 2.5 to 7.5 wt%in the membrane casting solution.

Overall, PWF experiments and pore statistics suggest that themembrane prepared with the addition of PEG as additive demon-strated the best performance; therefore, it should be considered asadditive in practical applications.

References

[1] R.W. Baker, Membrane Technology and Applications, McGraw-Hill, New York,2000.

[2] T. Matsuura, Synthetic Membranes and Membrane Separation Processes, CRCPress Inc., 1994.

[3] M. Zeng, Z. Fang, C. Xu, Effect of compatibility on the structure ofthe microporous membrane prepared by selective dissolution of chi-tosan/synthetic polymer blend membrane, J. Membr. Sci. 230 (2004) 175–181.

[4] W. Zhao, Y. Su, C. Li, Q. Shi, X. Ning, Z. Jiang, Fabrication of antifoul-ing polyethersulfone ultrafiltration membranes using Pluronic F127 as bothsurface modifier and pore-forming agent, J. Membr. Sci. 318 (2008) 405–412.

[5] M. Wang, L.G. Wu, J.X. Mo, C.J. Gao, The preparation and characterization ofnovel charged polyacrylonitrile/PES-C blend membranes used for ultrafiltra-tion, J. Membr. Sci. 274 (2006) 200–208.

[6] W.R. Bowen, S.Y. Cheng, T.A. Doneva, D.L. Oatl, Manufacture and characterisa-tion of polyetherimide/sulfonated poly(ether ether ketone) blend membranes,J. Membr. Sci. 250 (2005) 1–10.

[7] Y.H. Zhao, B.K. Zhu, X.T. Ma, Y.Y. Xu, Porous membranes modified by hyper-branched polymers. I. Preparation and characterization of PVDF membraneusing hyperbranched polyglycerol as additive, J. Membr. Sci. 290 (2007)222–229.

[8] A. Idris, L.K. Yet, The effect of different molecular weight PEG additives on cel-lulose acetate asymmetric dialysis membrane performance, J. Membr. Sci. 280(2006) 920–927.

[9] M. Sivakumar, D. Mohan, R. Rangarajan, Studies on cellulose acetate-polysulfone ultrafiltration membranes. II. Effect of additive concentration, J.Membr. Sci. 268 (2006) 208–219.

10] S. Wu, Polymer Interface and Adhesion, Chapman and Hall, London, 1987.11] S. Mok, D.J. Worsfold, A. Fouda, T. Matsuura, Surface modification of polyether-

sulfone hollow fiber membranes by �-ray irradiation, J. Appl. Polym. Sci. 51(1994) 193–199.

12] M. Peer, UK Patent, GB 2216134 A (1989).13] G. Arthanareeswaran, K. Srinivasan, R. Mahendran, D. Mohan, M. Rajendran, V.

Mohan, Studies on cellulose acetate and sulfonated poly(ether ether ketone)blend ultrafiltration membranes, Eur. Polym. J. 40 (2004) 751–762.

14] G. Arthanareeswaran, D. Mohan, M. Raajenthiren, Preparation and performanceof polysulfone-sulfonated poly(ether ether ketone) blend ultrafiltration mem-branes. Part I, Appl. Surf. Sci. 253 (2007) 8705–8712.

15] Y. Su, X. Jian, S. Zhang, C. Yan, Preparation of novel PPES-B UF membrane withgood thermal stability: the effect of additives on membrane performance andcross-section morphology, J. Membr. Sci. 271 (2006) 205–214.

16] J. Abdoul Raguime, G. Arthanareeswaran, P. Thanikaivelan, D. Mohan,M. Raajenthiren, Performance characterization of cellulose acetate andpoly(vinylpyrrolidone) blend membranes, J. Appl. Polym. Sci. 104 (2007)

3042–3049.

17] M.N. Sarbolouki, A general diagram for estimating pore size of ultrafiltrationand reverse osmosis membranes, Sep. Sci. Technol. 17 (1982) 381–386.

18] B. Chakrabarty, A.K. Ghoshal, M.K. Purkait, Effect of molecular weight of PEGon membrane morphology and transport properties, J. Membr. Sci. 309 (2008)209–221.

1 of Mem

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

38 G. Arthanareeswaran et al. / Journal

19] R. Mahendran, R. Malaisamy, D. Mohan, Cellulose acetate and polyethersul-fone blend ultrafiltration membranes. Part I. Preparation and characterizations,Polym. Adv. Technol. 15 (2004) 149–157.

20] B. Jung, Preparation of hydrophilic polyacrylonitrile blend membranes forultrafiltration, J. Membr. Sci. 229 (2004) 129–136.

21] L.P. Zhu, L. Xu, B.K. Zhu, Y.X. Feng, Y.Y. Xu, Preparation and characterization ofimproved fouling-resistant PPESK ultrafiltration membranes with amphiphilicPPESK-graft-PEG copolymers as additives, J. Membr. Sci. 294 (2007) 196–206.

22] J.H. Kim, K.H. Lee, Effect of PEG additive on membrane formation by phaseinversion, J. Membr. Sci. 138 (1998) 153–163.

23] W.Y. Chuang, T.H. Young, W.Y. Chiu, C.Y. Lin, The effect of polymeric additives onthe structure and permeability of poly(vinyl alcohol) asymmetric membranes,Polymer 41 (2000) 5633–5641.

24] G.J. Wang, L.Y. Chu, M.Y. Zhou, W.M. Chen, Effects of preparation conditionson the microstructure of porous microcapsule membranes with straight openpores, J. Membr. Sci. 284 (2006) 301–312.

25] A.J. Reuvers, C.A. Smolders, Formation of membranes by means of immersionprecipitation. Part II. The mechanism of formation of membranes prepared from

the system cellulose acetate–acetone–water, J. Membr. Sci. 34 (1987) 67–86.

26] K. Kimmerle, H. Strathmann, Analysis of the structure-determining process ofphase inversion membranes, Desalination 79 (1990) 283–302.

27] C.A. Smolders, A.J. Reuvers, R.M. Boom, I.M. Wienk, Microstructures in phaseinversion membranes. Part I. Formation of macrovoids, J. Membr. Sci. 73 (1992)259–275.

[

[

brane Science 350 (2010) 130–138

28] I.C. Kim, H.G. Yun, K.H. Lee, Preparation of asymmetric polyacrylonitrile mem-brane with small pore size by phase inversion and post-treatment process, J.Membr. Sci. 199 (2002) 75–84.

29] H. Strathmann, K. Kock, P. Amar, R.W. Baker, The formation mechanism ofasymmetric membranes, Desalination 16 (1975) 179–203.

30] J.J. Shieh, T.S. Chung, R. Wong, M.P. Srinivasan, D.R. Paul, Gas separation per-formance of poly(4-vinylpyridine)/polyetherimide composite hollow fibers, J.Membr. Sci 182 (2001) 111–123.

31] R. Malaisamy, R. Mahendran, D. Mohan, M. Rajendran, V. Mohan, Cellu-lose acetate and sulfonated polysulfone blend ultrafiltration membranes.I. Preparation and characterization, J. Appl. Polym. Sci. 86 (2002) 1749–1761.

32] C. Lv, Y. Su, Y. Wang, X. Ma, Q. Sun, Z. Jiang, Enhanced permeation performanceof cellulose acetate ultrafiltration membrane by incorporation of Pluronic F127,J. Membr. Sci. 294 (2007) 68–74.

33] Y. Liu, G.H. Koops, H. Strathmann, Characterization of morphology controlledpolyethersulfone hollow fiber membranes by the addition of polyethyleneglycol to the dope and bore liquid solution, J. Membr. Sci. 223 (2003) 187–

199.

34] I. Cabasso, Membranes Encyclopedia of Polymer Science and Engineering, vol.9, John Wiley & Sons, New York, 1989, pp. 509–579.

35] R. Malaisamy, R. Mahendran, D. Mohan, Cellulose acetate and sulfonated poly-sulfone blend ultrafiltration membranes. II. Pore statistics, molecular weightcutoff, and morphological studies, J. Appl. Polym. Sci. 84 (2002) 430–444.