Desalination 295 (2012) 26–34

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Desalination

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Preparation and characterization of modified polyethersulfone hollow fibermembranes by blending poly (styrene-alt-maleic anhydride)

Tao Xiang a, Min Tang a, Yeqiu Liu b, Huijuan Li a, LuLu Li a, Wenyue Cao a,Shudong Sun a, Changsheng Zhao a,⁎a College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, Chinab School of Traffic and Transportation Engineering, Changsha University of Science and Technology, Changsha, 410076, China

⁎ Corresponding author. Tel.: +86 28 85400453; fax:URL's:URL: zhaochsh70@scu.edu.cn, zhaochsh70@16

0011-9164/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.desal.2012.03.021

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 December 2011Received in revised form 22 March 2012Accepted 24 March 2012Available online 23 April 2012

Keywords:PolyethersulfoneHollow fiber membranePoly (styrene-alt-maleic anhydride)pH-sensitivitypH-reversibilityHysteresis

In this study, pH-sensitive polyethersulfone (PES) hollow fiber membrane was prepared by blending a copol-ymer of poly (styrene-alt-maleic anhydride) (PSMA). The PSMA alternating copolymer was synthesized by atraditional synthetic route, and was characterized by Fourier transform infrared spectroscopy (FTIR) analysis,nuclear magnetic resonance (1H NMR) and gel permeation chromatography technique (GPC). The PES/PSMAmembrane was then transformed to carboxylic PES/PSMA-H carboxylic membrane using sodium hydroxidesolution, by which the anhydride groups were transformed to carboxyl groups, and the modified membranesshowed excellent pH-sensitivity and pH-reversibility. Furthermore, the alternating copolymer modified PEShollow fiber membranes showed evident hysteresis of water flux, for which it would take several hours toreach the equilibrium state.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The application of membranes has become one of the mostsought-after techniques in separation processes. Among the mem-branes, pH-sensitive membranes have been extensively studied asimportant environmentally sensitive materials. Though the IEC valuesmay be smaller than those for commercial ion exchange membranes,the membranes could be used in many respects. In recent years, pH-sensitive membranes have been widely used in drug delivery [1,2]and separation processes [3–6], including desalination, water purifi-cation, separation of ethanol–water solution and so on. Hsueh et al.[4] prepared poly (acrylonitrile-co-acrylic acid) membranes for theseparation of ethanol–water solution by steady-state pervaporation.Water clustering and strong water–alcohol interaction within the co-polymer membrane could be used for interpreting the observed per-vaporation and selectivity.

Polymeric pH-sensitive membrane could be prepared by blending[7–10], grafting [11], and pore-filled methods [5,12], and so on. Theblending method is the simplest but most important method to pre-pare pH-sensitive membranes. Nam et al. [7] prepared pervaporationmembranes of chitosan and poly (acrylic acid) (PAA) by blendingthese two polymer solutions in different ratios. The variation of PAAcontents in the polyelectrolyte complex membranes affected the

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membrane swelling behavior and the pervaporation efficiency ofwater–ethanol mixture.

Many polymers could be used to prepare pH-sensitive mem-branes [7–10,13,14], such as poly (acrylic acid), poly (acrylic acid-co-acrylonitrile), poly (4-vinylpyridine) and so on. The functionalgroups in these polymers are carboxyl and pyridine groups. At differ-ent pH values the configuration of the groups could be changed, andthus the solution permeability and solute rejection could be regulat-ed. Acrylic acid and methacrylic acid are the most widely usedmonomers to prepare pH-sensitive membranes.

Poly (styrene-alt-maleic anhydride) (PSMA) alternating copolymerowes special structure like ~~~~M1M2M1M2M1M2M1M2M1M2M1~~~~with regular alternating M1 and M2 units. Random copolymer is the co-polymer consisting of alternating segments of the M1 and M2 units ofrandom length, including single molecule. PSMA can act as a dispersantin soluble formdue to the alternating structure, and usually as an additivein blends or composites to achieve desirable interfacial effects [15–17].The theory of the copolymerization of styrene (St) and maleic anhydride(MA) in N, N-dimethylformamide had been investigated in detail [16]. Inrecent decades, many studies focused on poly (styrene-alt-maleic anhy-dride) alkylamide copolymers for drug delivery. Scott et al. [18] foundthat alkylamine derivatives of PSMAwere capable of destabilizing biolog-ical membranes at acidic pH values and revealed how this activity couldbe modulated for use in intracellular drug delivery applications. Richardet al. [19] investigated the controlled release of paclitaxel (PTx) fromstent coatings comprising an elastomeric polymer and a styrene maleicanhydride (SMA) copolymer.

27T. Xiang et al. / Desalination 295 (2012) 26–34

However, the copolymer PSMA cannot be directly used to preparepH-sensitive membrane for its poor mechanical property when it wastransformed to hydrolyzed PSMA (PSMA-H). Meanwhile, as a kind ofspecial engineering plastics with outstanding oxidative and wide pHtolerance properties, polyethersulfone (PES) membrane is widelyused in separationfields [20–24]. Therefore, PES has always been select-ed as a polymeric matrix to prepare modified PES membranes with pH-sensitivity and antifouling property by different modification methods.

In our recent studies [8–10], a simplified method was developedto prepare functional PES membranes with pH-sensitivity and ion ex-change capacity by blending with polymers of poly (acrylic acid), poly(acrylic acid-co- acrylonitrile) and poly (acrylonitrile-acrylic acid-N-vinyl pyrrolidinone), respectively. The modified PES hollow fibermembranes showed excellent pH-sensitivity and pH-reversibility,and the copolymers were all random copolymers. Till now, there isno report on the modification of PES hollow fiber membrane byblending alternating copolymers. Thus, in this study, styrene (St)and maleic anhydride (MA) were chosen as the monomers to preparealternating copolymer poly (styrene-alt-maleic anhydride) (PSMA),which was then used to modify PES hollow fiber membrane. The an-hydride groups in the synthesized alternating copolymer PSMA werethen transformed to carboxyl groups by using sodium hydroxide, andthe pH-sensitivity and pH-reversibility of the modified membraneswere investigated.

2. Experimental

2.1. Materials

Polyethersulfone (PES, Ultrason E6020P, BASF) was the polymericmatrix to prepare hollow fiber membrane. Free-radical initiator azo-bis-isobutyronitrile (AIBN) was recrystallized from dried methanol.N, N-Dimethylformamide (DMF) was dried over molecular sieveType A4. Maleic anhydride (MA) was recrystallized from chloroformand then dried in a vacuum oven at room temperature. Styrene (St)was washed with 10% sodium hydroxide solution and then was dis-tilled under reduced pressure before use. All the chemicals exceptPES were purchased from Chengdu Kelong Chemical Reagent Compa-ny, China, and were used without further purification unless other-wise noted.

2.2. Synthesis of poly (styrene-alt-maleic anhydride)

Poly (styrene-alt-maleic anhydride) (PSMA)was synthesized throughfree-radical polymerization of styrene (St) and maleic anhydride (MA)

Fig. 1. General synthetic scheme, (1) synthesis of PSMA alternating copolymer, and

was initiated by azo-bis-isobutyronitrile (AIBN), the synthetic route isshown in Fig. 1 (1). In a typical procedure, MA (0.1 mol) and St(0.1 mol) were dissolved in adequate solvent N, N-Dimethylformamide(DMF) in a 100 mL flask, and then AIBN was added with the amount of0.5 mol% relative to the total monomers. After bubbling for 30 min withnitrogen, the flask was sealed, and the polymerization was carried outat 60 °C for 24 h.

The product was precipitated by cold diethyl ether, and then dis-solved in acetone; the solution was filtrated to remove the polysty-rene (the by-product of the polymerization). After evaporating theacetone from the filtrate by a rotary evaporator, the product wasdried under vacuum at room temperature.

2.3. Preparation of polyethersulfone hollow fiber membranes

A dry–wet spinning technique was used to prepare PES hollowfiber membranes and the device was the same as mentioned in theliterature [25]. PES and PSMAwere dissolved in N-methyl pyrrolidone(NMP) to form homogeneous solution. Then the polymer solutionwas filtrated and degassed. The internal and external diameters ofthe spinneret were 0.5 and 1.18 mm, respectively. In the preparationprocess, water was used as the coagulant, and the air gap was 15 cm.The prepared hollow fiber membranes were incubated in water bathfor 24 h to remove the residual NMP.

2.4. Characterization of the copolymer

To prepare Fourier transform infrared (FTIR) sample, the copoly-mer and the membranes were dissolved in acetone and NMP, respec-tively. Then the solutions were cast on potassium bromide (KBr) diskswith a thickness of about 0.8 mm. The FTIR spectra were measuredwith FT-IR Nicolet 560 (Nicol American).

GPC measurement was performed by the PL220 GPC analyzer(Britain), with tetrahydrofuran (THF) as the eluent. The test was cal-ibrated with polystyrene as the standard polymer.

The composition of the refined copolymer PSMA was determinedby 1H NMR spectroscopy in acetone-d with a Varian Unity Plus 300/54 NMR spectrometer. The characteristic aromatic peaks of the Stand the peaks of the backbone hydrogen from St and MA were usedto determine the composition of the copolymer.

2.5. Transformation of anhydride groups and preparation of filters

To transform poly (styrene-alt-maleic anhydride) (PSMA) to poly(styrene-alt-maleic acid) (PSMA-H), the prepared hollow fiber

(2) transformation of anhydride groups to carboxyl groups to form PSMA-H.

Fig. 2. Diagram for the filtration experiments.

28 T. Xiang et al. / Desalination 295 (2012) 26–34

membranes were incubated in NaOH solution at 50 °C and thesolution was maintained at pH=12.0 for 12 h (Fig. 1 (2)). Then themembranes were placed in double distilled water to achieve the equi-librium, and then were post-treated by 50 wt.% glycerol aqueoussolution for 24 h to prevent the collapse of the porous structureswhen they were dried. After drying in air at room temperature, thehollow fiber membranes were used to prepare filters as described inthe literature [9]. In this study, three kinds of hollow fiber filterswere prepared with the effective area of about 150 cm2, namelyHFM-20-0, HFM-20-0.6 and HFM-20-1.2, respectively. HFM-20-0.6represented that the membranes were prepared from the dope con-sisting of 20% PES and 0.6% PSMA (weight percentage) respectively;and so did the others.

The percent conversion of the anhydride groups to carboxylic acidgroups was measured by weighing the mass change of the hollowfiber membranes before and after the alkaline treatment. The percentconversion (C) was calculated by the following equation:

C% ¼ 101 ma−mbð Þ2mbw

� 100% ð1Þ

where mb and ma are the weight of dried hollow fiber membranebefore and after the alkaline treatment, respectively; w is the weightpercentage of the copolymer in the membrane. In the equation, weassumed that the molar fraction of MA in the copolymer PSMA was50%, which could be confirmed in the following study.

2.6. Scanning electron microscope (SEM) of the hollow fiber membranes

Scanning electron microscopy (JEOL, Japan) was used to study thecross-sections and the surfaces of the membranes. The samples werequenched by liquid nitrogen, coated with a gold layer under vacuumusing a sputter apparatus, and then scanned at the voltage of 20 kV.

2.7. Determination of ion-exchange capacity (IEC)

Tomeasure the IEC, the hollow fibermembrane filter was alternate-ly equilibrated by 0.1 M HCl and 0.1 M NaOH solutions for a couple oftimes, and washed by double distilled water in between. Afterward,enough NaOH solution permeated through the membrane samplewith the inlet pressure of 100 mm Hg and outlet pressure of 80 mmHg, followed by a thorough washing with double distilled water. Theapparatus is shown in Fig. 2. Then, HCl solution was applied to the filterat the same pressure mentioned above, and the amount of HCl wasabout twice the amount required for the theoretical IEC. The theoreticalIECwas calculated by assuming that all the carboxyl groups could trans-form to carboxylate ions and all the carboxylate ions could transform tocarboxyl groups. The vast majority of the feed solution permeated themembranes and was collected and titrated with a standard NaOH solu-tion (0.01 M); a pH meter was used as the indicator. The IEC isexpressed in milliequivalents of proton atoms per gram of the driedmembrane and was calculated by [26]:

IEC mequiv:=gð Þ ¼ VHClNHCl−VNaOHNNaOH

mc� 1000 ð2Þ

where VHCl is the volume of the feed solution prior to the membraneand VNaOH is the volume of the standard titration solution; NHCl is thenormality ofHCl solution (the feed solution), andNNaOH is the normalityof NaOH solution (the standard titration solution); mc is the weight ofthe dried hollow fiber membrane.

2.8. pH-sensitivity and pH-reversibility

The apparatus used to test the flux is shown in Fig. 2. All the testswere conducted at room temperature with the feed flux at 20 mL/

min, and the pH value of the feed solution was adjusted by addingHCl or NaOH solution.

Prior to measure the performance of the filters, double distilledwater was pumped to the hollow fiber filters by a peristaltic pumpwith inlet pressure of 100 mm Hg and outlet pressure of 80 mm Hgfor enough time to remove the residual glycerol and other impuritiesfrom the hollow fiber membranes, and to make the functional groupsin the membrane achieve the equilibrium. In fact, distilled water canalso be used to remove glycerin. No other purpose of using doubledistilled water; for convenience, double distilled water was usedthroughout the study.

For the pH-sensitivity experiment, acid–alkali process (the pH ofthe feed solution was changed orderly from 2.0 to 12.0) and alkali–acid process (the pH of the feed solution was changed orderly from12.0 to 2.0) were chosen. The flux response of the hollow fiber mem-brane to pH change was measured by weighing the permeated solu-tion at determined time intervals. For each pH value, the permeatedsolution was collected and the weight was measured till the weightper unit time did not change.

The flux could be calculated according to the following equation:

Flux ml=m2:mm Hg:h

� �¼ V

S:P:Tð3Þ

where V is the permeate volume (ml); S is the effective membranearea (m2); T is the time for collecting solution (h); and P (P=(inletpressure+outlet pressure)/2) is the trans-membrane pressure ap-plied to the hollow fiber membrane filter (mm Hg).

For the reversibility experiment, the test filter was pretreated bydouble distilled water with inlet pressure of 100 mm Hg and outletpressure of 80 mm Hg for about 1 h. Then the filter was alternativelyfed by solutions with pH values of 3.0 and 11.5, and washed bydouble distilled water in between. For each process, the water fluxwas calculated according to Eq. (3).

The empirical permeability, km, an important transmission parame-ter for hollow fiber membrane, was determined from the water flux–pressure tests and calculated by the following equation:

km ¼ QdηAmΔp

ð4Þ

Fig. 3. FTIR spectra for PSMA copolymer (A); PES/PMSA hollow fiber membrane(B); and PES/PMSA-H carboxylic hollow fiber membrane (C).

29T. Xiang et al. / Desalination 295 (2012) 26–34

where Q is the volume flow across the membrane (m3/s); d is themembrane thickness (m); η is the water viscosity (0.00089 Pa.s); Amis the membrane area (m2) and ΔP is the trans-membrane pressure(Pa). The hydrodynamic permeability of the membrane was derivedfrom the slope of the straight line obtained from the relationship of(Qdη/Am) and ΔP. In the study, two typical pH values of 3.0 and 11.5were chosen for each filter (Table 1).

3. Results and discussion

3.1. Characterization of the copolymer

3.1.1. FTIR spectra analysis of the copolymerThe FTIR spectra for the PSMA copolymer, PES/PSMA and PES/

PSMA-H hollow fiber membranes are shown in Fig. 3. As shown inFig. 3 (A), the peaks at 1632, 1495, 1455 cm−1 were assigned to theC\C stretching; the peaks at 734 and 706 cm−1 reflected the C\Hstretching in the mono-substituent aromatic ring; and the peaks at1854 cm−1 and 1780 cm−1 corresponded to the C_O stretching inthe anhydride groups. The FTIR spectra indicated that the copolymerof PSMA was synthesized.

3.1.2. 1H NMR spectrum of the copolymerThe chemical structure of the copolymer PSMA was analyzed by

1H NMR, as shown in Fig. 4. The characteristic aromatic peaks of thestyrene subunits (δ=6–7.5 ppm, 5H) (Fig. 4a) and the peaks of thebackbone hydrogen from styrene and maleic anhydride (δ=0–3 ppm, 2H from maleic anhydride R\CH\COO\, 3H from styreneR\CH2\R′, R\CH\Ar) (Fig. 4b and c) were the markers to deter-mine the copolymer composition. The NMR results coincided withthose in reference [18]. NMR analysis showed that the copolymersare composed of 50% styrene and 50% maleic anhydride in molarratio, as expected from the tendency of the monomers to polymerizetogether as one unit. [16]

3.1.3. GPC measurementThrough GPC measurement, the molecular weight and the molec-

ular weight distribution of the prepared alternating copolymer PSMAwere obtained. The number-average molecular weight (Mn), weight-average molecular weight (Mw), and Z-average molecular weight(Mz) were 3.00×104, 1.72×105, and 4.40×105, respectively; andthe molecular weight distribution was 5.73. The molecular weightof the copolymer was relatively large, while the dispersity waswide. The reason might be the gel effect and chain transfer [27,28].In solution polymerization, the gel effect was significant as the mono-mer concentration was considerably large, so the molecular weightand its dispersity increased. Meanwhile, solution polymerizationcould increase chain transfer which could decrease the molecularweight and its dispersity. The gel effect and chain transfer occurredat the same time, leading to the results above (Fig. 5).

3.2. Preparation and characterization of the hollow fiber membrane

3.2.1. Preparation of the hollow fiber membraneIn this study, PES/PSMA hollow fiber membranes were fabricated

by a dry–wet spinning technique, which was the common method

Table 1Ion exchange capacity of PES/PSMA-H blended membranes.

Membrane no. Titrated IEC (mequiv./g) Calculated IEC(mequiv./g)

HFM-20-0 0 0HFM-20-0.6 0.21 0.26HFM-20-1.2 0.39 0.50

to prepare hollow fiber membrane [25]. The alkaline treatment wasa critical procedure to prepare the pH-sensitive hollow fiber mem-brane. It is simple and convenient to realize that the transformationfrom anhydride groups to carboxyl groups for the copolymer PSMAand the percent conversion was nearly 100%. Thus, alkaline treatmentwas used to realize the transformation. However, it was impossible toprepare hollow fiber membrane by blending PSMA-Hwith PES directly,for its poor miscibility with PES in NMP. Thus, PSMA was blended withPES first to prepare PES/PSMA hollow fiber membranes, and then wastreated with sodium hydroxide solution to transform to PES/PSMA-Hcarboxylic hollow fiber membranes.

3.2.2. FTIR spectra of the membraneThe FTIR spectra of the matrix and carboxylic hollow fiber mem-

branes are shown in Fig. 3 (B) and (C), respectively. The characteristicpeaks 1854 cm−1 and 1780 cm−1 of the anhydride groups were notobvious in Fig. 3 (B), since the content of PSMA was very low inPES/PMSA hollow fiber membrane.

A new peak at 1758 cm−1, the characteristic peak for C_O stretch-ing of the carboxyl group, was observed after the alkaline treatment.Though the peak was not obvious in FTIR diagram after the alkalinetreatment, however, it was the evidence for the anhydride conversion.Furthermore, the following pH-sensitivity experiment could illustratethe existence of the carboxyl groups in the membrane for the obviouspH-sensitivity of PSMA/PES membrane compared with PES membrane.

Fig. 4. 1H NMR spectrum of PSMA.

Fig. 5. GPC curve of PSMA.

30 T. Xiang et al. / Desalination 295 (2012) 26–34

However, the percent conversion cannot be confirmed by FTIR, andwillbe discussed in the following section.

3.2.3. Percent conversion of the anhydride groupsThe percent conversion of the anhydride groups to carboxyl groups

was determined according to themass change of the hollowfibermem-branes before and after the alkaline treatment. According to reference[18] the anhydride groups in the copolymer PSMA can be measuredby titration. However, in this study, PSMAwas blendedwith polyether-sulfone which was insoluble in the solvent for titration. In fact, theweight percentage of the copolymer in themembrane can be calculatedaccording to the composition of the spinning polymer solution. Mean-while, the copolymer composition could be determined by 1H NMRspectrum. Thus, the weight of the anhydride groups in the dried mem-branes could be calculated. By the alkaline treatment, anhydride groupstransformed to carboxyl groups, and theweight of the copolymer or thedriedmembrane changed. Thus, the percent conversion can be calculat-ed through the weight change of the dried hollow fiber membranebefore and after the alkaline treatment according to Eq. (1). The calcu-lated percent conversions of the anhydride groups were 91.5% (molarpercentage) and 89.6% for HFM-20-0.6 and HFM-20-1.2, respectively.Therefore, most of the anhydride groups had been transformed to car-boxyl groups.

3.2.4. SEM observationThe structure and morphology of the hollow fiber membranes were

observed by scanning electron microscopy (SEM). The SEM pictures ofthe cross-sections and the surfaces of the membranes are shown inFig. 6 (A) and (B), respectively. The wall thickness of the hollow fibermembrane was about 60 μm and the inner-diameter was about500 μm. As shown in Fig. 6 (A), a typical structure was formed duringthe preparation process. Skin layers could be found on both sides ofthe membrane wall, under which was a finger-like structure, similarto the reported results [29].

As shown in Fig. 6 (A), with the increase of the copolymer amounts,macrovoid increased, which agreed with our recent study [9]. Thismight be caused by the poor miscibility between PES and PSMA. Mean-while, the structure of themembrane did not change obviously after thefunctionalization of the anhydride groups. Consequently, the alkaline-solution-dealing process had no obvious influence on the cross struc-ture. Furthermore, there was no significant difference in both theinner and outer surfaces after the alkaline treatment as shown inFig. 6 (B).

3.2.5. Ion exchange capacity (IEC)To investigate the charge property of the blended membrane,

ion exchange capacity (IEC) of the membrane was studied. Thecalculated IECs of the membranes HFM-20-0.6 and HFM-20-1.2 were

0.26 mequiv./g and 0.50 mequiv./g, respectively. While the titratedIECs of the two fibers were 0.21 mequiv./g and 0.39 mequiv./g, respec-tively, which were about 80% of the calculated IECs. The IECs of theprepared membranes were smaller than those of the commercial ion-exchange membranes (1–2 mequiv./g) [30]; thus the prepared hollowfiber membranes could not be used as ion exchange membranes.However, the pH sensitivity of the membrane was notable, and can beused for flux control, whichwould be discussed in the following section.

3.3. Membrane water flux as a function of pH value

The effect of pH values on water fluxes through the filters (HFM-20-0.6 and HFM-20-1.2) was studied and the results are presentedin Fig. 7. As shown in Fig. 7 (A), for the HFM-20-0.6, the water flux de-creased from 53.32 to 11.50 ml/(m2.mm Hg.h) in the acid–alkali pro-cess and increased from 11.91 to 61.36 ml/(m2.mm Hg.h) in thealkali–acid process. For the HFM-20-1.2, the water flux decreasedfrom 96.32 to 11.08 ml/(m2.mm Hg.h) in the acid–alkali process andincreased from 12.53 to 104.47 ml/(m2.mm Hg.h) in the alkali–acidprocess, as shown in the Fig. 7 (B).

For the alkali–acid process, the water fluxes for the HFM-20-0.6and HFM-20-1.2 filters exhibited chemical valve behavior at pH be-tween 4 and 10. In contrast, for the acid–alkali process, the waterfluxes of the HFM-20-0.6 filter exhibited chemical valve behavior atpH between 8 and 10; however, the water flux of the HFM-20-1.2did not show chemical valve behavior and continued to decline.According to literatures [31,32], acrylic acid was well known as apH-sensitive material, exhibiting pKa values of about 4.26 for themonomer [31] and 4.7 for the polymer [32], respectively, whichwere not in agreement with those in Fig. 7. This phenomenon hadalso been observed in our earlier studies for modified PES membrane[33,34]. Cheng et al. [33] prepared blended PES follow fiber mem-brane with remarkable pH-sensitivity and anti-fouling property andexhibited chemical value behavior at pH between 9 and 10. Wang etal. [34] prepared functional carboxylic PES membrane and the mem-branes exhibited chemical value behavior at pH between 5 and 10.According to the article [35], it was proved that the copolymerizationof hydrophobic segments would increase the pKa, for the hydropho-bic segments would decrease the ionization of PAA chains.

As shown in the figure, it was also found that the curves for acid–alkali process and alkali–acid process for the same membrane did notoverlap each other, which was different from other reports [8,9], andcould be called the hysteresis of water flux. The flux hysteresis mightbe caused by the hysteresis of the ionization of COOH groups. In ourrecent studies [8–10], the water flux hysteresis was not observed,and there was no obvious difference in the water fluxes at the samepH value between the acid–alkali process and the alkali–acid process.The polymers used to blend with PES in these studies were randomcopolymers. However, PSMA-H was an alternating copolymer withtwo adjacent COOH groups. The characteristic structure might resultin the hysteresis of ionization of COOH groups.

3.4. Confirmation of hysteresis of ionization of COOH groups

To confirm the hysteresis of the ionization of the COOH groups,new experiments were designed to measure the permeability ofpure water of HFM-20-1.2. For the first experiment, the feed solutionwas controlled at pH=3.0 for about 2 h to reach the equilibrium statewith the inlet pressure of 100 mm Hg and outlet pressure of 80 mmHg. Then, the feed solution was switched to double distilled water,and the fluxes were measured until the water flux did not change.For the second experiment, the feed solution was controlled atpH=11.5 for about 2 h, and the following operations were thesame as that for pH=3.0. The results are shown in Fig. 8.

As shown in the figure, it could be found that thewater fluxes finallyreached the same value after about 6 h and the value was 33.50 ml/

Fig. 6. (A) SEM images of the cross-section views of the hollow fiber membranes HFM-20-0 (A), HFM-20-0.6 (B) and HFM-20-1.2 (C). Among them, A-T, B-T and C-T are for the PES/PSMA-H carboxylic membranes. (B) SEM images of the surface views of the hollow fiber membranes HFM-20-1.2. Among them, images of A and B are the inner and outer surface ofthe membrane, respectively. A-T and B-T are for the PES/PSMA-H carboxylic membrane.

31T. Xiang et al. / Desalination 295 (2012) 26–34

(m2.mm Hg.h), which was between the values in acid–alkali processand alkali–acid process in the pH-sensitivity experiment. When thefeed solution changed from pH=3.0 to pH=7.0, the initial water fluxwas 83.55 ml/(m2.mm Hg.h), which was consistent with the flux atpH=3.0 in the pH-sensitivity experiment. When the solution changedfrompH=11.5 to pH=7.0, the initialwater fluxwas 12.00 ml/(m2.mmHg.h), which was consistent with the flux at pH=11.5 in the pH-sensitivity experiment. Meanwhile, the water flux could approach theequilibrium faster when the initial pH was 11.5 than that when the ini-tial pH was 3.0. Thus, the hysteresis of water flux in alkaline conditionwas more severe than that in acidic condition.

Through the experiment, it was proved that it take at least 6 h toreach the equilibrium when the medium environment changedfrom acidic or alkaline condition to neutral condition. The requiredtime was far longer than the test time in the pH-sensitivity experi-ment. Thus, in the pH-sensitivity experiment, the hysteresis ofwater flux was observed when the pH changed, and the hysteresisresulted in the difference in water fluxes at the same pH in acid–alkaliprocess and alkali–acid process.

3.5. Membrane pH-reversibility

To study the membrane flux as a function of environmental pH,the pH reversibility of the membrane was evaluated with the buffersolution at pH 3.0 and 11.5, and the data are presented in Fig. 9. Asshown in the figure, when the solution was alternated, the fluxeswere reversible between about 10.0 and 60.0 ml/(m2.mm Hg.h) forHFM-20-0.6, and 9.5 and 75.0 ml/(m2.mm Hg.h) for HFM-20-1.2, re-spectively. Meanwhile, the attenuation of water flux was not obviousafter going through several circulations and the water fluxes couldreach a relative stable value.

At pH 11.5, the carboxyl groups of PSMA-H could dissociate to car-boxylate ions to provide high charge density in the membranes, andthe copolymer would be swelling, which resulted in the decease ofwater flux. From the figure, it could be found that when the circum-stance changed from acid to neutral condition, the water flux hardlydecreased in 10 min. In contrast, when the circumstance changedfrom alkali to neutral condition, the water flux hardly increased in10 min. These results were different from our previous studies [8,9],

Fig. 7. Water flux as a function of pH values. The water flux for acid–alkali process (△)and alkali–acid process (■). (A) is for HFM-20-0.6 filter; (B) is for HFM-20-1.2 filter.Duplicate experiments showed similar results.

32 T. Xiang et al. / Desalination 295 (2012) 26–34

which showed that there was no significant difference between thefluxes when the solution changed from acidic or alkaline condition.The obvious difference of the neutral flux in the reversibility experi-ment was caused by the hysteresis of water flux, as stated in the pre-vious section.

Fig. 8. Water fluxes with time for HFM-20-1.2 when pure water was applied to themembrane equilibrated with the pH values of 3.0(○)and 11.5(●), respectively.

Fig. 9. Water flux for the membrane as the feed solution was exchanged amongpH=3.0, pH=7.0 and pH=11.5. (A) is for HFM-20-0.6 filter; (B) is for HFM-20-1.2filter. Duplicate experiments showed similar results.

3.6. Hydrodynamic permeability

The relationship between the water flux and the trans-membranepressure for the membranes was investigated both in acidic condition(pH=3.0) and in alkaline condition (pH=11.5), as shown in Fig. 10.It was found that the relationship was linear with the correlation coeffi-cient greater than 0.98 in the test range. The permeabilities (km) of theHFM-20-0.6 and HFM-20-1.2 are shown in Table 2. For HFM-20-0.6the km in acidic condition was 3.8 times larger than that in alkaline con-dition. For HFM-20-1.2, the corresponding multiple was nearly 8.

At alkaline condition (pH=11.5), the km of HFM-20-0.6 and HFM-20-1.2 was almost the same. However, at acidic condition (pH=3.0),the km of HFM-20-1.2 was twice larger than that of HFM-20-0.6. It is

Fig. 10. The relationship between water flux and pressure. (A) is for HFM-20-0.6 filter;(B) is for HFM-20-1.2 filter. Duplicate experiments showed similar results.

33T. Xiang et al. / Desalination 295 (2012) 26–34

interesting to be noticed that the content of the PSMA alternating co-polymer in the HFM-20-1.2 was nearly twice larger than that in theHFM-20-0.6. That is to say, when the feed solution was at alkalinecondition, the permeability of the membranes was almost the same,no matter the difference in the copolymer contents. At acidic condi-tion, the multiple of the permeability was almost the same as thepolymer content in the membrane.

When the feed solution was at alkaline condition, the carboxylgroups in the membranes could dissociate to carboxylate ions to pro-vide high charge density, and the swelling of the copolymer causedthe pore shrink. From the above paragraphs we could conclude thatthe pore shrink was almost the same for the membranes containingdifferent quantity of the copolymer. In the same way, when the feedsolution was at acidic condition, the carboxyl groups were spreadand the increase of the polymer content in the membrane led togreater water flux and permeability (km).

Table 2The empirical permeability (km) of PES/PSMA-H blended membranes.

Membraneno.

Empirical permeability (km)

Acidic condition (pH=3) Alkaline condition (pH=11.5)

HFM-20-0.6 3.67×10−18 9.6×10−19

HFM-20-1.2 7.32×10−18 9.2×10−19

4. Conclusion

Poly (styrene-alt-maleic acid) (PSMA) copolymer with alternatingstructure was synthesized by free-radical polymerization of styrene(St) and maleic anhydride (MA). Through blending method, PES/PSMA hollow fiber membrane was prepared. The anhydride groupscould be transformed to carboxyl groups to obtain PES/PSMA-H car-boxylic hollow fiber membrane after alkali treatment. The carboxylicmembranes showed significantly pH-sensitivity and pH-reversibilitydue to the carboxyl groups. Moreover, the water flux curves inacid–alkali process did not overlap with those in alkali–acid processdue to the hysteresis of the ionization of the COOH groups. Mean-while, it was found that the hydrodynamic permeability (km) wasalmost the same for the two membranes when the feed solutionwas at alkaline condition; and at acidic condition, the multiple ofthe permeability was almost the same as the content of the copoly-mer in the membrane.

Acknowledgment

This work was financially sponsored by the National Natural Sci-ence Foundation of China (No. 50973070, 51073105 and 30900691),and State Education Ministry of China (Doctoral Program for HighEducation, No. JS 20100181110031). We should also thank our labo-ratory members for their generous help, and gratefully acknowledgethe help of Ms. X.Y. Zhang and Ms. H. Wang of the Analytical andTesting Center at Sichuan University, for the SEM micrographs.

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