Journal of Membrane Science 280 (2006) 957–968

Preparation and characterization of poly(phthalazinone ethersulfone ketone) hollow fiber ultrafiltration membranes

with excellent thermal stability

Yongqiang Yang a, Daling Yang a,b, Shouhai Zhang a,b, Jun Wang a, Xigao Jian a,b,∗a College of Chemical Engineering, Dalian University of Technology, Dalian 116012, PR China

b Liaoning High Performance Polymer Engineering Research Center, Liaoning Province 116012, PR China

Received 30 December 2005; received in revised form 7 March 2006; accepted 7 March 2006Available online 27 March 2006

Abstract

Hollow fiber ultrafiltration membranes were prepared successfully from poly(phthalazinone ether sulfone ketone) (PPESK) with a dry/wet phaseinversion technique. Ethylene glycol methyl ether, diethylene glycol (DegOH) and methyl ethyl ketone were used as non-solvent additives andNostiisatfl©

K

1

aop[

apcaw

0d

-methyl-2-pyrrolidone used as a solvent in membrane preparation. The effects of PPESK concentration, the type of additives and the concentrationf DegOH in casting solution on the morphology and performance of hollow fiber ultrafiltration membranes were investigated, respectively. Thetructures of PPESK hollow fiber membranes including the cross-section, the inner/outer edge (details of the cross-sections at the inner/outer edge ofhe membrane), and the external surface were characterized by scanning electron microscope (SEM). It was found that the membrane performances consistent and agrees with the membrane morphology. With the increase of PPESK concentration in the casting solution, the viscosity stronglyncreases and it becomes shear-rate dependent. The morphologies of hollow fiber membranes changed from fingerlike structure to sponge-shapetructure, and the properties of ultrafiltration membrane showed that the pure water flux was about 159 L m−2 h−1, and PEG10,000 rejection wasbove 95% under the operating pressure of 0.1 MPa. PPESK hollow fiber ultrafiltration membranes prepared in this work was also investigated thehermal stability at different operating temperature. As a result, when the temperature of feed solution was raised from 15 to 100 ◦C, the permeationux increased more than three times without significant change of rejection.2006 Elsevier B.V. All rights reserved.

eywords: Hollow fiber; Ultrafiltration membrane; Morphology; Performance; Thermal stability

. Introduction

Ultrafiltration as a separation technology of high efficiencynd low energy consumption has widely been applied in vari-us industries, such as water purification, industrial enzymes,rotein products and a variety of food and beverage products1,2].

Generally, most of the membrane separation system requiresn efficient heat exchanger to have the membrane influent tem-erature below typically 50 ◦C. Such a requirement means extraapital and operating cost when dealing with a hot stream, suchs the effluents from a textile bleaching and dyeing industryhose temperature can range from 40 to 95 ◦C [3,4]. Although

∗ Corresponding author. Tel.: +86 411 83653426; fax: +86 411 83639223.E-mail address: jian4616@dl.cn (X. Jian).

numerous polymers such as polysulfone (PSF), polyethersulfone(PES), polyetherimide (PEI) and polyetheretherketone (PEEK)could be used to prepare ultrafiltration membranes (flat or hollowfiber) by phase inversion method, application of these polymericmembranes is still stringent to the membrane influent temper-ature [5]. Hence, it is necessary to develop high temperatureresistant membranes.

A series of poly(phthalazinone ether sulfone ketone)(PPESK) copolymers, containing different ratios of diphenyl-sulfone and diphenylketone units, were previously synthesized[6–8] in our laboratory. Among the great variety of polymers(such as PES, PSF, PEI, etc.) used in the preparation of mem-branes, PPESK is a novel membrane material, whose chemical,mechanical and thermal characteristics make it very compet-itive with other similar polymers. Fig. 1 gives the chemicalstructure and simulated three-dimensional structure of the novelmonomer 4-(4-hydroxybenyl)-2,3-phthalazin-1-one (DHPZ). It

376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2006.03.023

958 Y. Yang et al. / Journal of Membrane Science 280 (2006) 957–968

Fig. 1. Chemical structure and three-dimensional structure of DHPZ.

can be seen from Fig. 1 that DHPZ has an aromatic hetero-cyclic twisted non-coplanar structure, which results in PPESKhaving well solubility in a variety of solvents (such as NMP,DMF, DMAc and CHCl3), superior mechanical strength, chem-ical resistance and very high glass transition temperature (Tg), inthe range of 263–305 ◦C. Besides, flat-sheet membranes madefrom PPESK have also shown good separation and permeationproperties for the separation of gases and liquids [9,10]. It hasalso been demonstrated that PPESK is a potential membranematerial for high temperature gas separation [11].

In the past decade, PPESK has been attracted much attentionas the promising membrane materials because of its compre-hensive properties as well as good separation properties. PPESKasymmetric membranes are mainly prepared from dry/wet phaseinversion method. To attain a desired membrane morphologyand performance, the phase inversion process must be con-trolled carefully [12]. Some researchers [9,13,14] have focusedon the preparation of flat PPESK ultrafiltration and nanofiltra-tion membrane using traditional phase inversion technique. Forexample, Jian et al. [9] successfully prepared PPESK flat ultrafil-tration and nanofiltration membranes, and discussed the effectsof polymer concentration, the type and concentration of addi-tives in the casting solution on membrane performance. In thepreparation of flat composite nanofiltration membranes, somefactors such as the SPPESK concentration, organic non-solventadditives and curing treatment temperature and time on the mem-be

abeltttpaa

cttipmt

solution on membrane performance were investigated by Daiet al. [10,16]. Su et al. [17] also reported that chloromethy-lated PPESK was prepared into the membrane by phase inver-sion method, and then chloromethylated PPESK membrane wasimmersed into an aqueous trimethylamine solution to introducequaternary nitrogen groups into the membrane. The resultingmembranes have positive charges, and have dramatic increasein the pure water flux and the rejection of dyes and MgCl2 thanchloromethylated PPESK.

Up to now all researchers about PPESK used to fabricatehigh performance membranes material were focused on thepreparation of flat-sheet ultrafiltration and nanofiltration mem-brane. However, there were no previous reports about fabricatingPPESK hollow fiber ultrafiltration membranes through dry/wetspinning technique. Moreover, asymmetric hollow fiber mem-branes that were prepared by the immersion precipitation pro-cess have been widely applied to separation process owing topossess many advantageous characteristics than flat-sheet ones[18–24]. The passage from flat membranes to hollow fiber mem-branes is neither simple nor immediate because of numerousparameters [25–27] that play a prominent role in the formationof the latter type of membrane: for example, the geometry ofthe spinneret, the flow rate and the rheology of the casting solu-tion inside the spinneret, the flow rate of the internal coagulationfluid, the presence of a double-coagulation process, the air gap,etc.

retmmitf

2

2

miffalnmwtCoC(al

rane performance were studied by Dai et al. [13] and Zhangt al. [14].

Wei et al. [15] also used PPESK ultrafiltration membranes the support material of the thin film composite (TFC) mem-ranes by interfacial polymerization and investigated the influ-nce of polymer structure of the support layer and the activeayer on thermal stability of TFC membrane. It was concludedhat with the same fully aromatic polyamide (PA) active layer,hermal stability of PA/PPESK TFC membrane is better thanhat of PA/PSF. On the other hand, with the same PPESK sup-ort layer, thermal stability of TFC membranes with differentctive layer follows such sequence: fully aromatic > alicyclicromatic > aliphatic aromatic.

In addition, other researchers [10,16,17] reported the modifi-ation of PPESK with fuming sulfuric acid and trimethylamineo improve the hydrophilicity and the ionic character, respec-ively. In order to prepare the thermally stable polymers hav-ng hydrophilicity and potentially fouling-resistance, sulfonatedoly(phthalazinone ether sulfone ketone) (SPPESK) was usedembrane material, and the influences of SPPESK concentra-

ion, the type and the concentration of additives in the casting

The primary objectives of this study focus on the prepa-ation of PPESK hollow fiber ultrafiltration membranes withxcellent thermal stability. The effects of PPESK concentration,he type and the concentration of non-solvent additives on the

orphology and the performance of hollow fiber ultrafiltrationembranes were studied extensively. Finally, the thermal stabil-

ty of PPESK hollow fiber ultrafiltration membranes prepared inhis work was also investigated using a method of raising theeed solution temperature from 15 to 100 ◦C.

. Experimental and methods

.1. Materials and instruments

PPESK in powder form was obtained from Dalian Poly-er New Materials Co. Ltd. (PR China). Its chemical structure

s shown in Fig. 2. PPESK used in this research has a sul-one/ketone molar ratio of 1:1. It was dried at 100 ◦C in vacuumor 12 h before use. N-methyl-2-pyrrolidone (NMP) was useds solvent and ethylene glycol methyl ether (EGME), diethy-ene glycol (DegOH) and methyl ethyl ketone (MEK) used ason-solvent additives (NSA) in the preparation of hollow fiberembrane. The polyethylene glycol (PEG) with a moleculareight of 10,000 was used as probe molecule for solute rejec-

ion test obtained from Shenyang Chemical Agent Company (PRhina). Glycerin and Towen80 were used to the post-treatmentf membrane, which purchased from Shenyang Chemical Agentompany (PR China) and Tianjin Chemical Agent Company

PR China), respectively. Deionized water was used to makell solutions. All reagents used in the experiments are ana-ytical reagents. Spectrophotometer-751 (Shanghai Instrument,

Y. Yang et al. / Journal of Membrane Science 280 (2006) 957–968 959

Fig. 2. Chemical structure of PPESK (S:K = 1:1, mol ratio).

PR China) was employed for measuring the concentration ofPEG10,000.

2.2. Hollow fiber membrane and membrane modulespreparation

Dry/wet spinning technology was used to prepare PPESKhollow fiber membranes, which has been described elsewhere[28–31]. PPESK was dissolved in a mixture of NMP/NSA,and stirred until the homogeneous casting solution formed. Thescheme of the spinning apparatus for PPESK hollow fiber mem-branes is shown in Fig. 3. In this study, reverse osmosis waterwas used as internal coagulation and ultra-filtrated water wasused as external coagulant. A tube-in-orifice spinneret was usedfor hollow fiber membrane preparation. The casting solution wasextruded into the annulus of the spinneret, and the internal coagu-lant was fed into the inner tube of spinneret with the constringentnitrogen gas. The nascent hollow fiber membranes emerged fromthe tip of the spinneret and passed through a certain air gapdistance before entering the external coagulant (water). All hol-low fiber membranes were not drawn, which means that thetake-up velocity was nearly the same as the free falling velocityof nascent fiber in the coagulation bath. The coagulation bathand bore fluid were maintained at room temperature. Detailsof the parameters of spinning hollow fiber membranes in this

Table 1The spinning conditions of resulting hollow fiber membranes

Parameter Castingflow rate(ml/min)

Fluid flowrate(ml/min)

Tcoagulant

(◦C)Troom

(◦C)R.H. (%)

A1–A4 0.08 0.3 23.4 26.2 74B1–B2 0.07 0.2 24.6 25.8 78C1–C3 0.07 0.2 23.2 25.5 79

paper are listed in Table 1.The resulting hollow fiber membraneswere stored in water bath for 72 h at room temperature to leachout the residual NSA and NMP, and then kept in an aqueousglycerin solution (30 wt.%) and Towen80 (1.5 wt.%) for 12 hto prevent collapse of its porous structure and dried in air atroom temperature for the permeation test of membrane modules.Table 2 summarizes the composition of the casting solution, theouter/inner diameter dimensions of the spinneret and the result-ing membranes.

Membrane modules were prepared to test hollow fiber sepa-ration performance in terms of pure water permeation flux andsolute rejection quantitatively. About 10 fibers with a length ofabout 270 mm were assembled into a glass tube. Then, the twoends of the bundles were sealed with a normal-setting epoxyresin. These modules were left overnight for curing before test.To eliminate the effect of the residual glycerol and Towen80 on

appa

Fig. 3. Schematic diagram of the spinning ratus for PPESK hollow fiber membrane.

960 Y. Yang et al. / Journal of Membrane Science 280 (2006) 957–968

Table 2The composition of the casting solution and outer/inner diameter of the spinneretand the thickness of resulting PPESK hollow fiber membranes

Membraneno.

PPESK/NSA/NMPratio

o.d.(�m)

i.d.(�m)

i.d./o.d.ratio

Thickness(�m)

Spinneret – 1149 753 0.66 198

A1 15/20(DegOH)/65 1174 775 0.66 200A2 17/20(DegOH)/63 1120 750 0.67 185A3 19/20(DegOH)/61 1140 770 0.68 185A4 21/20(DegOH)/59 1080 730 0.68 175B1 19/20(EGME)/61 1120 757 0.68 182B2 19/20(MEK)/61 1150 765 0.67 193C1 19/11(DegOH)/61 1130 757 0.67 187C2 19/14(DegOH)/61 1200 804 0.67 198C3 19/17(DegOH)/61 1130 771 0.68 180

o.d., outer diameter; i.d., inner diameter.

module performance, each module was immersed in water for24 h and run in the test system for 30 min under the pressure of0.2 MPa before any sample collection.

2.3. Viscosity of PPESK casting solutions

The viscosity of the casting solutions with different PPESKconcentrations was measured on a numerical display viscosime-ter (NDG-8S, Shanghai Instrument, PR China). The measure-ments were carried out at the room temperature.

2.4. Membrane performance measurement

Fig. 4 shows a schematic diagram of solute-water separationmembrane unit designed for hollow fiber membrane character-ization in our laboratory. Deionized water was used to measurethe pure water flux, and 100 ppm PEG10,000 aqueous solutionswas performed for the solution rejection of each membrane mod-ule. Two modules were parallel tested, and the average of theirperformance was reported.

At the transmembrane pressure 0.2 MPa for 30 min, the purewater flux and the rejection of PEG10,000 were measured at thepressure 0.1 MPa and room temperature, respectively. Deionized

Fb(

water was fed at a constant pressure of 0.1 MPa from the innerlumen to the outer surface of hollow fiber membranes and wascollected and measured. Pure water fluxes (PWF) were obtainedas follows:

PWF = Q

At(1)

where Q is the total volume of the permeation water during theexperiment, A represents the effective membrane area of themodule and t denotes the measuring time.

The solute rejection of membrane was determined with anaqueous solution containing 100 ppm solute, which was fed atconstant flow from the lumen to the outer surface of the mem-branes. The membrane solute rejection R (%) was calculated byEq. (2):

R (%) =(

1 − Cp

Cf

)× 100% (2)

where Cf and Cp represent the solute concentration in feedand permeate solution, respectively. The concentration ofPEG10,000 in the feed and the permeation was analyzed withspectrophotometer-751 at the wavelength of 510 nm, respec-tively.

2.5. Structure of asymmetric membrane

(bm(scs

3

ft

3P

ctsv2tFcobt

ig. 4. Schematic diagram of the properties evaluation of hollow fiber mem-rane. (1) Feed reservoir; (2) tank; (3) recirculation pump; (4) pressure gauge;5) membranes module; (6) rotor flowmeter; (7) circumfluence pressure valve.

The morphologies of the cross-section, the inner/outer edgedetails of the cross-sections at the inner/outer edge of the mem-rane), and the external surface of hollow fiber asymmetricembranes were observed with a scanning electron microscope

SEM; JSM-5600L, JEOL, Japan). After the ethanol–hexaneolvent exchange step, hollow fiber membrane samples wereryogenically fractured in liquid nitrogen and then coated withilver to obtain an adequate contrast of membrane fracture.

. Results and discussion

A series of hollow fiber membranes were prepared under dif-erent operating conditions, varying polymer concentration, theype of non-solvent additives and the concentration of DegOH.

.1. Effect of PPESK concentration on the viscosity ofPESK casting solutions

The phase separation process during membrane formation is aomplex mix of thermodynamic and kinetic phenomena. One ofhe main parameters influencing the kinetics of the phase inver-ion is the viscosity of the polymer solution [32]. Therefore, theiscosity of PPESK/DegOH/NMP casting solution containing0 wt.% DegOH was measured as a function of polymer concen-ration. The results are shown in Fig. 5. It can be concluded fromig. 5 that increasing PPESK concentration, the viscosity of theasting solution increased accordingly. The reason is that the sizef polymer network was increased by enhancing the interactionetween the polymer molecules at the expense of the interac-ion between polymer and solvent, which suggests that polymer

Y. Yang et al. / Journal of Membrane Science 280 (2006) 957–968 961

Fig. 5. Viscosity of PPESK casting solution in NMP as a function of the polymerconcentration at a constant shear rate.

chains are interpenetrated, indicating that the strength of inter-action between polymer and solvent molecule is slightly greaterthan that of polymer molecules. The increasing size of polymernetwork owing to polymer chain interpenetration increases theviscosity of the casting solution [33].

The viscoelastic properties of the casting solution are veryimportant for the spinning process. The shear rates generated inthe spinneret might have an influence on the morphology andon the final transport properties of the membranes. Therefore,Tasselli et al. [32] investigated the effect of the shear rate onthe viscosity of the polymer solutions, and they concluded thatwith an increase of polymer concentration the viscosity increasesstrongly, and it becomes shear-rate dependent. This indicatesthat some orientation of the dissolved polymer chains may occurduring the spinning process. For the flow of Newtonian fluidsthrough an annular tube, the shear rate is highest at the innerwall of the annulus and it is given by Eq. (3):

γR = 4Qs(k ln(1/k) − (1 − k2)/2k)

πR3((1 − k4)/ ln(1/k) − (1 − k2)2)(3)

where γR is the shear rate at the inner wall of the fiber, Qs is theflow rate of the casting solution, R is the outer diameter of theannulus and k is the ratio of inner/outer diameter of the annulus[25].

For our spinning apparatus, with inner and outer diameters oft(rflγ

isbrtsopfl

transport properties of the membranes [25]. However, this phe-nomenon was not studied in detail in the present work becausethe casting solution flow rate was kept constant in the experi-ments.

3.2. Effect of PPESK concentration on the morphologiesand properties of hollow fiber membrane

Among the principal effects induced by the variation of poly-mer concentration in the casting solution, the most importantare the variation of the morphology and transport properties ofresulting membrane. Fig. 6 (membrane Al to membrane A4)shows the cross-section, the inner/outer edge (details of thecross-sections at the inner/outer edge of the membrane) andthe external surface structure of hollow fibers prepared fromthe casting solutions with different PPESK contents, respec-tively. As may be observed that a structure (A1-CS/IE/OE)with a double layer fingerlike voids was formed in the cross-section of hollow fiber membrane for 15 wt.% PPESK castingsolution, while a sponge-like morphology (A2-CS/IE/OE andA3-CS/IE/OE) with some fingerlike voids was formed for 17 and19 wt.% PPESK casting solution, respectively. With an increaseof PPESK content in the casting solution, it can be also seenfrom Fig. 6 that the two layer fingerlike structures decreaseboth in size and number, which are replaced by a sponge-likestructure formed under the inner and outer skin layer when thePftoaite

ibwibi(tTwistittKdwcag

he spinneret of 753 and 1149 �m, respectively, k = 0.66 and Eq.3) reduces to γR = 1502.5Qs (Qs in cm3/s). During the prepa-ation process of hollow fiber membrane, the casting solutionsow rate is 0.08 ml/min and thus the maximum shear rate isR = 1502.5 × (0.08/60) = 2.0 s−1. At this value the PPESK cast-

ng solution with the highest concentration clearly demonstrateshear thinning behavior. In reality the maximum shear rate wille even higher than calculated for the Newtonian liquid above,esulting in higher shear rates at the wall. Therefore, it is likelyhat some orientation of the polymer chains occurs inside thepinneret. If the subsequent relaxation is sufficiently slow, therientation may be partly frozen in upon precipitation of theolymer in the coagulation bath or upon contact with the boreuid. This might have some influence on the morphology and/or

PESK concentration increases to 21 wt.%. It can be observedrom Fig. 6 (A1-ES to A4-ES) that with higher PPESK concen-ration, the structures of external surface show the change fromne with a discrete pore distribution of appreciable dimensions ton almost compact surface layer. The external surface becomesncreasingly dense and compact when the polymer concentra-ion increases, and with PPESK concentration of 21 wt.% thexternal surface is almost free of voids.

The reason is that, the viscosity of PPESK casting solutions strongly dependent on the polymer concentration, which cane also seen from Fig. 5. The higher viscosity will reduce theater–NMP exchange rate, which is in favor of delayed demix-

ng in the casting solution. That is to say, the exchange speedetween the mixed solvent solution and water slowed. Besides,n the short interval that hollow fiber passes through the air gapabout 20 mm) it may absorb a small amount of humidity fromhe air. This brings the solution closer to the demixing conditions.herefore, the tendency of the immediate phase separation waseakened, and the size of fingerlike pores decreased, as shown

n Fig. 6. It is well known that the addition of NSA to the castingolution tends to suppress the formation of macrovoids. Indeedhis is most likely to occur in a more concentrated PPESK cast-ng solution because the binodal demixing curve moves closero the solvent/polymer axis at relatively high polymer concen-rations [32,34]. This is in good agreement with the findings ofesting and Frietzsche [35] who reported the formation of aouble layer fingerlike void structure with a PSF dope at 17%,hereas only at 37% of polymer did they obtain a structure

ompletely free of fingerlike voids. At the same time, Qin etl. [36] also found the similar result, who concluded that a fin-erlike structure was formed in PES hollow fiber membranes if

962 Y. Yang et al. / Journal of Membrane Science 280 (2006) 957–968

Fig. 6. Effect of different PPESK content in the casting solution on the morphologies (CS: cross-section; IE: inner edge; OE: outer edge; ES: external surface; PM:part magnified of the cross-section) of hollow fiber membranes (1, 2, 3 and 4 refer to the casting solution containing 15, 17, 19 and 21 wt.% PPESK, respectively).

the dope solution contained 30 wt.% of polymer as comparedwith 37 wt.% polymer concentration which no fingerlike struc-ture formed in the cross-section of hollow fibers. These can alsoexplain our experimental results shown in Fig. 6.

It also can be seen from the partial magnified picture of theouter edge of Fig. 6 (A4-PM) that the porous network and thenodular structure were formed in the membrane cross-sectionwhen PPESK concentration was 21 wt.%. The reason is that

with the polymer concentration increasing, that the aggregationof polymer molecules is promoted due to entanglement of poly-mer chains. This makes the radius of gyration of the polymercoil smaller. The resulting intermolecular physical cross-linksrestrict chain mobility. In other words, by increasing the concen-tration of PPESK, the coils of the polymer chains are crowdedtogether and as a result become smaller, indicating that the inter-action of the polymer molecules is apparently greater.

Y. Yang et al. / Journal of Membrane Science 280 (2006) 957–968 963

Fig. 7. Effect of PPESK content on the pure water flux and the rejection ofhollow fiber membranes.

Fig. 7 illustrates the permeation and the separation perfor-mance of PPESK hollow fiber membranes fabricated from dif-ferent PPESK casting concentration. It can be seen from Fig. 7that the PEG10,000 rejection increases from 95.0 to 98.2% andthe pure water flux decreases from 392 to 172 L m−2 h−1 whenPPESK concentration in the casting solution changes from 15to 21 wt.%. The reasons for the decrease of pure water flux andthe increase of rejection are mainly because the performance ofmembranes is controlled by the overall porosity and the pore sizein the external surface. Higher polymer concentration results inhigher polymer net density, which means the pore size is smallerand the pore number is fewer in the process of membrane solid-ification.

3.3. Effect of the type of additives on the morphologies andproperties of PPESK membranes

An important factor that determined the phase inversion pathof a membrane-forming system is the composition of the castingsolution and the coagulation media. Usually, NSAs were addedinto the polymer solution to change the phase separation pro-cess [37]. The addition of NSA in the casting solution has beenshown to play an important role in the development of mem-brane structure with improved separation performance. Differentadditives endow the resulting membranes with different mor-pmfN

shefiicmupes

in contact with the internal layer of hollow fiber membranes.Therefore, phase separation occurs quickly on the internal rimand the internal skin layer is created. Under the internal skin,the amount of water diffuses into the polymer solution, andliquid–liquid demixing process occurs. Due to lower polymerconcentration and strong interaction between water and PPESKresults in the growth of thin polymer layer [38] and then the fin-gerlike structures created under the external skin layer and theinternal skin layer.

The sponge-like structure also appeared on the top of the fin-gerlike structures when the hollow fiber membranes B1 (B1-IEand B1-OE) and B2 (B2-IE and B2-OE) prepared from EGMEand MEK additive in the casting solution, respectively. Com-pared with membrane B1 and B2, membrane B3 used DegOHas NSA has a sponge-like structure with large voids in the shapeof spheres or ellipsoids over the entire cross-section of the mem-brane, adjoining each other sparsely. The reason is that the pres-ence of NSA in the casting solution might change the phase sepa-ration path of PPESK membrane-forming system. That is to say,the delayed phase separation occurred at first and the sponge-likestructure formed in the upside of the support layer. As the immer-sion time was prolonged, more water filtered through the surfacelayer and induced the immediate phase separation finally. There-fore, the fingerlike structure formed in the downside of supportlayer. When the concentration of DegOH was 20 wt.% in thecasting solution, the delayed phase separation was occurred.Ts

ochBmtwTatscbspoa

uINmfMpmmo

hology and performance, therefore, hollow fiber ultrafiltrationembranes are prepared with a dry/wet phase inversion method

rom the casting solution, which contains PPESK, NMP andSA (EGME, MEK or DegOH).Fig. 8 shows that the membrane morphologies of the cross-

ection, the inner/outer edge and the external surface of PPESKollow fiber asymmetric ultrafiltration membranes with differ-nt NSA. It can be seen from Fig. 8 that the PPESK hollowber membrane B1 and B2 have the similar structure, exhibit-

ng a two-layer fingerlike structure extended to the middle of theross-section, and there is a layer sponge-like structure at theiddle of cross-section adjoining each other because water was

sed as the inner and outer coagulation. In the dry/wet spinningrocess, the external layer of nascent hollow fiber membranesnters into the external coagulation bath after passing through ahort air gap distance, and the internal coagulant water is always

herefore, the fingerlike pores shortened and the sponge-liketructure formed under the internal and external skins.

Fig. 8 (Bl-ES to B3-ES) shows the external surface structuresf hollow fiber membranes spun from different NSA. It can beoncluded from these pictures that the external surface of PPESKollow fiber membrane looks more smooth except the membrane2-ES taking MEK as NSA with the more discrete appreciableicropores. That is to say, the pore size of membrane B2-ES in

he external surface is much larger than that of membrane B1-ESith EGME and membrane B3-ES with DegOH, respectively.his is because the hydrophilicity of MEK is worse than EGMEnd DegOH. After the nascent hollow fibers were contacted withhe coagulant (water), EGME and DegOH flowed out from theolution surface more quickly than MEK, and thereby MEK con-entration in the nascent fibers surface increased significantlyefore the surface solidification. The experimental results mayuggest that, like the formation of external surface structure, theresence of additives plays an important role in the formationf internal skin structure by altering the precipitation of PPESKnd the exchange of solvents with NSA.

The effect of these additives on the asymmetric hollow fiberltrafiltration membrane performance has been given in Table 3.t can be concluded from Table 3 that the effect of the type ofSAs on the properties of membrane is great. As a whole, theembranes with EGME and DegOH as NSAs had better per-

ormance than that with MEK. EGME is the best additive andEK is the worst one among the three tested additives for the

ure water flux and the rejection. Since the separation perfor-ance of hollow fiber membranes is largely determined by theembrane structure, this can explain the reason why the changes

f membrane performance with different non-solvent additives.

964 Y. Yang et al. / Journal of Membrane Science 280 (2006) 957–968

Fig. 8. Effect of the type of additives on the morphologies (CS: cross section; IE: inner edge; OE: outer edge; ES: external surface) of hollow fiber membranes (1, 2and 3 refer to the type of additives in casting solution being EGME, MEK and DegOH, respectively).

3.4. Effect of DegOH concentration on the morphologiesand properties of PPESK membranes

Fig. 9 gives the morphologies of the membranes preparedfrom PPESK casting solutions with different DegOH content. In

Table 3Effect of the type of additives on the asymmetric hollow fiber ultrafiltrationmembrane performance

Type of additivesa EGME (B1) MEK (B2) DegOH (B3)

PWF (L m−2 h−1) 293 227 159Rejection (%) 96.3 89.6 97.2

a The concentrations of polymer and NSA in the casting solution is 19 and20 wt.%, respectively.

Fig. 9, membrane C1-CS with 11 wt.% DegOH concentration inthe casting solution has a long and narrow fingerlike structureextended from inner surface to outer surface, while membraneC2-CS with 14 wt.% DegOH concentration and membrane C3-CS with 17 wt.% DegOH concentration shows that the fingerlikestructure became less and shorter. The structure of membraneC4-CS with 20 wt.% DegOH content had a sponge-like struc-ture with some voids like sphere as shown in Fig. 9. In a word,membrane morphology changes slowly from thin and long fin-gerlike structure through a wide and short fingerlike structureto the sponge-like structure with some voids as DegOH con-centration in the casting solution increases. Several authors[39,40] reported that appropriate amount of non-solvent addi-tives enhanced the formation of macrovoids while too muchnon-solvent suppressed their formation because the delayed

Y. Yang et al. / Journal of Membrane Science 280 (2006) 957–968 965

Fig. 9. Effect of different additive content on the morphologies (CS: cross section; IE: inner edge; OE: outer edge; ES: external surface) of hollow fiber membranesfrom the casting solution of 19 wt.% PPESK (1, 2, 3 and 4 refer to the casting solution containing 11, 14, 17 and 20 wt.% DegOH, respectively).

demixing in the growth stage is inhibited. This means that themembrane morphology is strongly affected by the content ofnon-solvent additives.

It also can be seen from Fig. 9 (C1-OE to C4-OE) that, withthe increase of DegOH content, the double layer fingerlike struc-ture decreased, and the sponge-shape structure formed in theouter edge of the cross-section of hollow fiber membrane. Inthe inner edge of the cross-section of the hollow fiber, similarchange, but to a small extent, was observed, as shown in Fig. 9(C1-IE to C4-IE). Reason for the morphology change in theouter edge of fibers was that the nascent fibers pass a certain airgap distance before entering the external coagulation bath, how-ever, the lumen of the hollow fibers always contacts with water.Thus, NMP outflow into the internal coagulant would increasethe driving force for the non-solvent/solvent exchange betweenthe coagulant and the casting solution. Rapid exchange betweensolvent and non-solvent would render the PPESK solution super-saturated, and spinodal demixing would take place [41].

In Fig. 9, membrane C1-ES to membrane C4-ES also showthat the external surfaces are denser because the increase ofDegOH content in the casting solution and water is used asa coagulation bath, and the dense skin layer is formed due toinstantaneous liquid–liquid demixing process [5,31,36]. Uponthe addition of non-solvent additive into the polymer solu-tion, the binodal line of the membrane-forming system shiftedto the polymer/solvent axis, less water was needed for thephase separation [42]. Therefore, PPESK solution composi-tion would be located in the unstable region of the phase dia-gram before the phase separation occurred in the metastableregion.

The effect of DegOH content on the hollow fiber ultrafil-tration membrane separation performances was examined andthe results were given in Fig. 10. It was found that with theincrease of DegOH content in casting solution, the pure waterflux decreased from 322 to 169 L m−2 h−1 and the PEG10,000rejection is increased from 95.6 to 97.2%.

966 Y. Yang et al. / Journal of Membrane Science 280 (2006) 957–968

Fig. 10. Effect of DegOH content on the pure water flux and the rejection ofhollow fiber membranes.

3.5. The thermal stability of PPESK hollow fiberultrafiltration membranes

Further experiments were conducted to study the thermalstability of PPESK hollow fiber ultrafiltration membrane usingmembrane A4 to separate 100 ppm PEG10,000 aqueous solu-tion at different operating temperature. The results are shown inFig. 11. It can be concluded that as the operating temperaturerose from 15 to 100 ◦C, the permeate flux increases dramaticallyfrom 179 to 651 L m−2 h−1 with a slight decrease of the rejec-tion for the same range of temperature change. The results are ingood agreement with the findings of Dai and co-workers [9] who

Fig. 11. Influence of operating temperature on the rejection and the permeateflux for PPESK hollow fiber membrane.

thought that the increase of permeation flux with temperature isanticipant because water viscosity decreases exponentially withtemperature, and the permeate flux is inversely proportional tothe viscosity of permeate for flow through channels [43]. Inaddition, the slight decrease of rejection is possibly due to theincrease of diffusivity of the solute molecules at higher temper-ature.

In another way, in order to examine the effects of the operatingtemperature on the structures of PPESK hollow fiber ultrafiltra-tion membrane, morphology of the resulting membrane was alsoobserved under the SEM before and after measurement to 100 ◦Csolute–water solution, respectively. Fig. 12 gives the morphol-ogy of the magnified cross-section and the internal surface under

Ft

ig. 12. Effect of operating temperature on the morphology (CS: cross-section; IS: ino the structure before and after measurement to 100 ◦C solute–water solution, respec

ternal surface) of PPESK hollow fiber ultrafiltration membranes (1 and 2 refertively).

Y. Yang et al. / Journal of Membrane Science 280 (2006) 957–968 967

the different operating conditions. It can be seen from these pho-tographs that the structures of the skin layer and the sub-layerwere not apparently changed before and after measurement to100 ◦C solute–water solution. At the same time, the high valueof the rejection also indicates that the morphology of the mem-brane may not change when the operating temperature increasedfrom 15 to 100 ◦C.

Therefore, the permeation flux could be increased greatly byraising operation temperature from 15 to 100 ◦C with a slightdecrease of rejection observed. This also demonstrates that thePPESK hollow fiber ultrafiltration membranes have the excellentthermal stability.

4. Conclusions

High thermal stability of PPESK hollow fiber ultrafiltrationmembranes with various morphologies and performances hasbeen prepared with dry/wet phase inversion spinning methodsuccessfully. Both purified water as coagulant and EGME, MEKand DegOH as a non-solvent additives had effective impacts onmembrane morphologies and properties, respectively. With theincreasing of PPESK concentration and DegOH content in thecasting solution, the morphologies of corresponding membranecross-section was changed from fingerlike structure to sponge-shape structure and the external surface was changed from looser

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to denser. In addition, the viscosity of the casting solution isstrongly dependent on PPESK content, that is to say, with theincrease of PPESK content the viscosity strongly increases andit becomes shear-rate dependent.

PPESK concentration in the casting solution is the mostimportant factor for the performance of the hollow fiber mem-branes: when the concentration of polymer is 18–20 wt.%, andDegOH is taken as NSA, hollow fiber membrane has betterperformances, that is to say, the pure water flux was about159 L m−2 h−1, and the rejection with molecular weight cut-off (PEG10,000) was above 95% under the operating pressureof 0.1 MPa. At the same time the permeation flux could beincreased more than triplicity with a slight decrease of rejectionobserved by raising operation temperature from 15 to 100 ◦Cbecause of the excellent thermal and mechanical stability of themembranes.

Acknowledgements

The authors wish to sincerely thank the financial supportsof the Key Project of Chinese National Programs for Fun-damental Research and Development (“973” plan, Grant no.2003CB615700) and the Chinese National Programs for HighTechnology Research and Development (“863” plan, Grant no.2003AA33G030).

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