Separation and Purification Technology 98 (2012) 298–307

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Separation and Purification Technology

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Synthesis of sulfonated polyphenylsulfone as candidates for antifoulingultrafiltration membrane

Yang Liu, Xigui Yue, Shuling Zhang, Jiannan Ren, Lilong Yang, Qinhong Wang, Guibin Wang ⇑College of Chemistry, Engineering Research Center of High Performance Plastics, Ministry of Education, Jilin University, Changchun 130012, PR China

a r t i c l e i n f o

Article history:Received 5 March 2012Received in revised form 20 June 2012Accepted 20 June 2012Available online 28 June 2012

Keywords:Sulfonated polyphenylsulfoneUltrafiltrationAntifoulingMembrane morphology

1383-5866/$ - see front matter Crown Copyright � 2http://dx.doi.org/10.1016/j.seppur.2012.06.031

⇑ Corresponding author. Tel./fax: +86 431 8516 888E-mail address: wgb@jlu.edu.com (G. Wang).

a b s t r a c t

This work focused on the synthesis of a series of sulfonated polyphenylsulfone (SPPSU) random copoly-mers with various controlled sulfonation levels by using sulfonated monomer in direct copolymerizationmethod and the preparation of antifouling SPPSU ultrafiltration membranes via the conventional immer-sion precipitation phase inversion method. The chemical structures of the SPPSU copolymers were con-firmed by using Fourier transform infrared spectrometer (FTIR) and their thermal properties werethoroughly characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis(TGA). The morphologies of the SPPSU membranes were investigated by scanning electron microscopy(SEM), and the morphology changes of these resultant membranes had been detailedly explained andverified by Hansen solubility parameters (HSPs) from thermodynamic theory. In addition, the surfacehydrophilicity and charged property of the SPPSU membranes were studied by water contact angleand membrane potential measurements, and the results indicated that the sulfonation of membranematerial is really an effective way to enhance the hydrophilicity and negatively charge the membrane.The pure water flux and protein solution permeation through the prepared membranes were increasedwith the increase of the degree of sulfonation. The cycle ultrafiltration experiments for protein solutionrevealed that nonspecific protein adsorption, especially irreversible protein adsorption, for the SPPSUmembranes was significantly reduced, suggesting superior antifouling performance.

Crown Copyright � 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction

Membrane technology, especially ultrafiltration (UF), as aneffective and powerful technique has in recent years supersededother traditional separation processes in industry for removal ofdispersed or dissolved contaminants from water and as it involvesa number of attractive features mainly low energy consumption,mild operating conditions, no additive requirements, no phasechange and environmentally friendly [1]. However, the use ofultrafiltration in most applications is often limited by membranefouling caused by inorganic and organic substances which can ad-sorb on the surfaces or adhere to the pores, resulting in progressivedeterioration of membrane performance, alteration of membraneselectively and increase in costs of energy and membrane replace-ment [2]. Therefore, it is critical to develop advanced antifoulingultrafiltration membranes that have high chemical and biologicalstability, high resistance to fouling, and tailored separation abilityto meet various demands.

The state-of-the-art materials used for manufacturing of com-mercial ultrafiltration membrane are generally made from poly-

012 Published by Elsevier B.V. All

9.

mers [3], such as cellulose [4], poly(vinylidene fluoride) [5],polyetherimide [6], polysulfone [7] and polyethersulfone [8].Among other potential membrane materials, polyphenylsulfone(PPSU) is an amorphous high performance engineering polymerthat is reported to offer more toughness, strength, rigidity, hydro-lytic stability, chemical and mechanical resistance than other poly-mer materials, and capacity to operate at high temperatures. Itsglass transition temperature of 220 �C is higher than that of poly-sulfone (190 �C) and almost equal to that of polyethersulfone(225 �C). However, PPSU has a higher solvent resistance than otherpolymers [9,10], which should be paid more attention for its poten-tial application in some solvent-sensitive membrane separationprocesses. The excellent chemical and physical characteristics ofPPSU resin make it an ideal candidate for the preparation of ultra-filtration membranes. However, the PPSU membrane application inaqueous phase separation is limited by its inherent hydrophobicproperty. On the one hand, the hydrophobicity restricts the perme-ate flux that is a key parameter in defining the membrane perfor-mance; on the other hand, when feed solutions containingsubstances like proteins are filtered, the hydrophobic interactionbetween the PPSU membrane and protein molecules often causesnonspecific adsorption and deposition of proteins on the mem-brane surface or in pores, and results in serious membrane fouling[11,12]. Although the membrane fouling is a very complicated pro-

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Y. Liu et al. / Separation and Purification Technology 98 (2012) 298–307 299

cess, many studies have demonstrated that an increment in thehydrophilicity of membrane material could possibly inhibit theprotein hydrophobic adsorption and thus significantly reduce themembrane fouling caused by the non-specific protein adsorptionand deposition [13,14]. Therefore, the chemical structure of PPSUmust be altered to induce controlled levels of hydrophilicity to en-able high rate of water transport, and to suppress the membranefouling.

It is well known that the sulfonation of membrane materials is astraightforward and effective method to improve the membranewater permeability and meanwhile charge the membrane [15].To data, many researches relating to sulfonated materials havebeen reported and used successfully to improve the membraneresistance toward fouling in several membrane separation pro-cesses, such as ultrafiltration [15–17], nanofiltration [18] and re-verse osmosis [19,20]. However, previous sulfonated membranematerials were prepared by post-polymerization sulfonationmethods [15–17]. In general, the sulfonation of membrane materi-als with strong acids (sulfuric acid and chlorosulfuric acid) is aharsh chemical treatment, which can lead to undesirable side reac-tions, chain cleavage and cross-linking. Especially, control of thesulfonation level is very difficult since the sulfonation occurs onthe activated ring, which may deteriorate thermal stability of thefinal polymer [20]. Although a sulfonated polymeric material witha high degree of sulfonation is considerably beneficial to ultrafiltra-tion process, the highly sulfonated material readily compromisesthe mechanical properties of membrane because the material isdifficult to precipitate and solidify in water or other commonlyused non-solvent during the membrane formation [15].

Therefore, increasing need for the precise control of the degreeof sulfonation has driven the new development of sulfonationmethods to overcome the synthetic challenges of traditional meth-od. In contrast, an alternative route was employed to synthesizesulfonated polymer using sulfonated monomer in direct copoly-merization, which is fundamentally different approach from thesulfonation of polymer mentioned earlier. In the sulfonated mono-mer approach, the modification of monomer makes feasible controlof the molecular structures. Meanwhile, this method completelycircumvents those disadvantages of post-polymerization sulfona-tion, and leads to highly reproducible sulfonated materials[20,21]. Most importantly, due to the degree of sulfonation is easilycontrolled by adjusting the ratios of the sulfonated monomer tonon-sulfonated monomer, the direct copolymerization methodcan provides a good opportunity to improve the membrane perfor-mance without sacrificing mechanical strength.

The present study reports on the synthesis of a series of randomsulfonated polyphenylsulfone (SPPSU) with various tailorable sul-fonation levels, which were used in the preparation of antifoulingultrafiltration membrane. The chemical structures and thermalproperties of the SPPSU copolymers were thoroughly characterizedby Fourier transform infrared spectrometer (FTIR), differentialscanning calorimetry (DSC) and thermogravimetric analysis(TGA). The morphology, surface hydrophilicity and charged prop-erty of the SPPSU membranes were studied by scanning electronmicroscope (SEM), water contact angle and membrane potentialmeasurements. The membrane performances during BSA cycleultrafiltration experiments were evaluated and the antifouling per-formances of the SPPSU membranes were examined in detail. Itwas expected that the SPPSU membranes provide a good opportu-nity to improve antifouling property.

2. Theory

The nature of membrane material in casting solution is believedto affect membrane performance. Like for all the mixing processes,

for mutual solubility of membrane material and solvent, their freeenergy of mixing, DGm, should be negative. DGm is defined as

DGm ¼ DHm � TDSm ð1Þ

where DSm describes the entropy of mixing, which is usually posi-tive, DHm is the enthalpy of mixing, which can be correlated withthe Hildebrand solubility parameter (d), as

DHm ¼ /1/2Vmðd1 � d2Þ2 ¼ /1/2VmðDdIÞ2 ð2Þ

where the subscripts 1 and 2 denote membrane material and sol-vent, respectively. / is the volume fraction and Vm denotes the mo-lar volume. It is clear that, to reaching DGm negative, the differencein Hildebrand solubility parameter between d1 (membrane mate-rial) and d2 (solvent), DdI, should be as small as possible.

However, Hildebrand solubility parameter was fit only for non-polar solvents. In fact, the solubility parameter theory was true notonly for non-polar systems, but also for polar systems includingpermanent dipole–permanent dipole and hydrogen bonding inter-actions. Consequently, Hansen extended the original Hildebrandsingle solubility parameter theory to the concept called Hansensolubility parameters (HSPs) in 1967 [22]. In the HSP theory, thetotal solubility parameter is divided into three separate parts:the non-polar/dispersion force, dd, the permanent dipole–perma-nent dipole force, dp, and hydrogen bonding force, dh [21–24].

d2 ¼ d2d þ d2

p þ d2h ð3Þ

HSP are intrinsic physicochemical property of a substance, whichprovide an easy numerical method for fast predicting the basicproperties of substances and estimating interactions between dif-ferent substances, which appears to be more precise. According toHansen’s interpretation of solubility parameter, different sub-stances with sufficiently close three-dimensional solubility param-eters are likely to be compatible and miscible when they mixtogether, which reflect strong interaction between different sub-stances. The difference in Hansen solubility parameters (DdII) be-tween membrane material and solvent was calculated using theequation:

DdII ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðd1;d � d2;dÞ2 þ ðd1;p � d2;pÞ2 þ ðd1;h � d2;hÞ2

qð4Þ

3. Experimental

3.1. Materials

Commercially available 4,40-dichlorodiphenylsulfone (DCDPS)and 4,4-biphenol (BP) were purchased from Yanji Chemical plant,China, and dried under vacuum at 60 �C for 24 h prior to use. Thesulfonated monomer, 3,30-disulfonate-4,40-dichlorodiphenylsulf-one (SDCDPS) was synthesized from DCDPS according to a proce-dure (Scheme 1) described in literature [25]. Bovine serumalbumin (BSA, pI = 4.8, Mw = 67,000) and phosphate buffer solution(PBS, 0.1 mol/L, pH 7.4) were both purchased from Dingguo Bio-technology Co., Ltd. (Beijing, China). Coomassie brilliant blueG250 was purchased from Aldrich. Dimethyl formamide (DMF)was purchased from Shanghai chemical company, China. Other sol-vents and reagents were obtained from Beijing chemical company,China. The polymerization solvent, N-methyl-2-pyrrolidinone(NMP) and toluene were dried as follows: NMP was dried over-night over calcium hydride with a nitrogen purge and distilled atreduced pressure; toluene was dried over molecular sieves priorto use. Anhydrous potassium carbonate was dried under vacuumat 110 �C for at least 24 h before use. Poly(vinylpyrrolidone) (PVP30 K), which was used as pore-former, was purchased from Fluka

Scheme 1. Synthesis of 3,30-disulfonate-4,40-dichlorodiphenylsulfone (SDCDPS).

300 Y. Liu et al. / Separation and Purification Technology 98 (2012) 298–307

Chemika, Switzerland. Deionized water was used through out thisstudy.

3.2. Polymers synthesis

Sulfonated polyphenylsulfone (SPPSU) random copolymerswith various disulfonation levels were prepared via the direct poly-merization method by aromatic nucleophilic substitution copoly-merization (Scheme 2) as reported previously [25,26], and thesereferences provide background on general synthesis of poly(aryl-ene ether)s.

3.3. Membranes preparation

The procedure of preparing ultrafiltration membranes by theconventional immersion precipitation phase inversion methodwas as follows. First, SPPSU with various disulfonation levels weredried at 80 �C under vacuum for at least 24 h before use, and SPPSUand PVP were dissolved in DMF to form homogeneous casting solu-tions. The mass ratio of polymer, PVP, and DMF were 17:8:75. Afterfiltering and degassing, the polymer solution was cast on non-wo-ven fabric support with a casting knife with a nominal thickness of200 lm at room temperature. Then, after exposing in air for 30 s,the formed membrane with fabric support was immerged in awater bath (20 �C) to form the asymmetric membrane structure.During the preparation of the membranes, the relative humiditywas about 55%. Finally, the membrane was kept in deionized waterfor at least 48 h until all of solvent and water-soluble polymer wereremoved, the flat sheet ultrafiltration membranes were visually in-spected for defects and good areas were chosen for ultrafiltrationmembrane experiments.

3.4. Characterization

3.4.1. Polymer characterizationFourier transform infrared spectroscopy (FTIR, Bruker Vertex

80V) was used to investigate the chemical structures of the SPPSUcopolymers via the KBr pellet method by using a Bruker Vertex 80VFTIR spectrophotometer. All the spectra were baseline corrected.

Scheme 2. Synthesis of sulfon

Thermal transitions of the SPPSU copolymers were studied byusing a modulated DSC (Model Mettler DSC821e) instrument at aheating rate of 20 �C/min under a nitrogen flow of 200 mL/min.The second heat was used to assess glass transition temperatures(Tg).

The thermogravimetric analysis (TGA) was employed to assessthe thermal stability of the copolymers with a Perkine Elmer Pyris1 analyzer under nitrogen atmosphere (100 mL/min) at a heatingrate of 10 �C/min.

The inherent viscosity measurements were carried out with anUbbelohde viscometer at a concentration of 0.5 g/dL in DMF at25 ± 0.1 �C.

All samples used for chemical structure and thermal stabilitycharacterization were dried under vacuum at 80 �C for 24 h.

3.4.2. Membrane characterizationThe cross-section morphologies of the SPPSU membranes were

examined by a scanning electron microscope (SSX-550, Shimadzuequipped with energy dispersive X-ray (EDX) spectroscopy). Themembranes were frozen in liquid nitrogen and fractured to avoiddestroying the structure of the cross-section, and sputtered withgold prior to SEM observation.

The static water contact angles of the membranes were esti-mated by sessile drop method with a contact angle goniometerfrom Drop Shape Analysis (DSA 100 KRUSS GMBH, Hamburg) atroom temperature. About 4 lL of deionized water was droppedonto the membrane surface with a microsyringe, and the value ofwater contact angle was recorded after 3 s. At least five measure-ments in different locations of the membrane samples were carriedout and averaged to yield the contact angles.

3.5. Membrane potential measurements

The membrane potentials of the SPPSU membranes were mea-sured in KCl solutions through a method as described in literatures[27,28]. The membranes (area of 10.4 cm2) were clamped betweentwo self-made polyamide half-cells with volume of 95 cm3 byusing silicone rubber rings. KCl solutions with the concentrationsof 1 � 10�3 mol/L, 2 � 10�3 mol/L, 3 � 10�3 mol/L, 4 � 10�3 mol/L,5 � 10�3 mol/L, 6 � 10�3 mol/L and 7 � 10�3 mol/L were used.

ated polyphenylsulfone.

Y. Liu et al. / Separation and Purification Technology 98 (2012) 298–307 301

Each compartment was filled with 88 mL of KCl solution, whichwas circulated through its compartment by a peristaltic pump ata rate of 80 mL/min. The bottom surface of the membranes wascontacted with the solution of 1 � 10�3 mol/L. The top surface ofthe membranes was contacted with other KCl solutions.

The electrical potential difference of the cell (Ecell) was mea-sured with two Ag/AgCl electrodes which were inserted directlyinto the bulk solutions. In order to eliminate the effect of the asym-metry potential, the potential difference was measured byexchanging the electrodes in the two compartments and the aver-age of two values was calculated. The membrane potential Em isdefined as follows:

Em ¼ Ecell �RTF

lnc2

c1ð5Þ

where R is ideal gas constant, T is absolute temperature, F is Faradayconstant, c2 and c1 is the concentration of Cl� ions in bulk solutions(c1 > c2), respectively.

3.6. Protein adsorption experiments

The amount of proteins adsorbed on membrane is one of themost important evidence in evaluating the fouling resistant abilityof membranes, and BSA was used as model protein to evaluate theanti-protein adsorption performance of three investigated mem-branes in phosphate buffered saline (PBS, pH 7.4). The membranesamples were cut into a round shape with a diameter of about30 mm, and treated by ultrasonication for 30 min in 0.1 M PBSsolution for cleanness. Then the pre-treated membranes were im-mersed into PBS solution containing BSA (1.0 mg/mL) at 25 �C for4 h. After adsorption, each membrane was rinsed three times inthe fresh PBS by gentle shaking. Then these membranes weretransferred into a well-plate filled with PBS solution, and the pro-tein adsorbed on the membrane surface was completely desorbedby ultrasonic treatment at room temperature for 3 min. The ob-tained PBS solution was dyed with Coomassie Brilliant Blue andmeasured by a UV–vis spectrophotometer (UV3600, Shimadzu)to determine the total amount of adsorbed protein. The final re-sults were averaged from three specimens for each membrane.

3.7. Ultrafiltration experiments

The performances of flat sheet ultrafiltration membranes weretested using a stirred dead-end filtration cell at room temperature,and the effective area of the membranes was 12.6 cm2. At first,each membrane was compacted at 0.2 MPa for 1 h prior to per-forming the ultrafiltration experiments. Then the pressure waslowered to 0.1 MPa and all the ultrafiltration experiments werecarried out at this pressure. After compacted, deionized waterwas passed through the membrane to obtain the beginning purewater flux (Jw1, L/m2 h), and the flux was measured every 5 min.After 1 h of water filtration, a BSA solution with a concentrationof 1.0 mg/ml in PBS (pH 7.4) was filtrated for 2 h, and the flux dur-ing protein filtration was recorded which called Jp. After BSA solu-tion filtration, the membrane was washed thoroughly and passedthrough deionized water for another 30 min (the washing timewas not counted in the filtration cycle). Thereafter, the pure waterflux was measured again within 1 h for the membrane, which wasrecorded as Jw2. The flux (Jw and Jp) of the membrane was deter-mined by direct measurement of permeate volume, which was cal-culated by the following equation:

J ¼ VAt

ð6Þ

where V was the volume of permeation, A was the effective mem-brane area and t was the permeation time. In order to evaluate

the recycling property of these membranes, the flux recovery ratio(FRR) during the filtration cycle was calculated using the followingexpression:

FRRð%Þ ¼ Jw2

Jw1

� �� 100 ð7Þ

The higher FRR value means the better the antifouling property ofthe membrane. The membrane rejection ratio (R) was calculatedby using the following equation:

Rð%Þ ¼ 1� Cp

Cf

� �� 100 ð8Þ

In which Cp (mg/L) is the permeate concentration and Cf (mg/L) isthe feed concentration. The solute concentration of permeationwas measured by a UV–vis spectrophotometer (UV3600, Shima-dzu).To study the antifouling property in more detail for theseinvestigated membranes, the degree of total flux loss caused by to-tal protein fouling in the filtration cycle, Rt, was defined as

Rtð%Þ ¼Jw1 � Jp

Jw1

� �� 100 ð9Þ

A high value of Rt corresponds to a large flux decay and seriousmembrane fouling. The total flux loss was caused by both reversibleand irreversible protein fouling. Rr was calculated by followequation:

Rrð%Þ ¼Jw2 � Jp

Jw1

� �� 100 ð10Þ

which was the reversible fouling ratio caused by reversible fouling,and could be eliminated by hydraulic cleaning. And Rir was calcu-lated by follow equation:

Rirð%Þ ¼Jw1 � Jw2

Jw1

� �� 100 ¼ Rt � Rr ð11Þ

which was the irreversible fouling ratio caused by irreversible foul-ing, and can only be eliminated by chemical cleaning or enzymaticdegradation [12,29–31]. Rt was the sum of Rr and Rir.

4. Results and discussion

In this work, sulfonation of the dihalide monomer, DCDPS, re-sults in sulfonic acid functionalization on both deactivated phenylrings ortho to the chlorine moiety and allows for two sulfonic acidgroups per repeat unit after copolymerization. In the sulfonatedmonomer approach, the sulfonic acid groups are located at inher-ently more chemically stable positions than those obtained by postpolymerization sulfonation, which sulfonates the most reactive,least stable positions of the polymer chain. Since no post-polymer-ization synthesis steps are required, these disadvantages of post-polymerization sulfonation are completely eliminated with themonomer sulfonation route. Additionally, use of sulfonated mono-mers affords precise control of the ionic group concentration in thefinal polymer, which leads to make the sulfonated membranematerial with reproducible properties and excellent membraneperformance without sacrificing mechanical properties.

4.1. SPPSU copolymer synthesis and characterization

A series of sulfonated polyphenylsulfone (SPPSU) randomcopolymers with various disulfonation levels described hereinwere synthesized by direct polymerization of a sulfonated mono-mer (SDCDPS) and other monomers (DCDPS and BP), and the de-gree of sulfonation (DS) was controlled by varying the molarratio of SDCDPS to DCDPS. The molar percentages of the monomersfor the copolymers preparation are listed in Table 1, and the

Table 1Fundamental properties of the SPPSU copolymers.

Sample Feed (mol %) Viscosity Tg (�C) T5% (�C) DS

SDCDPS DCDPS BP gsp/c (dL/g)

PPSU 0 100 100 0.59 220.8 547.3 0SPPSU-5 2.5 97.5 100 0.65 226.4 533.8 0.05SPPSU-10 5.0 95.0 100 0.62 228.5 530.2 0.10SPPSU-15 7.5 92.5 100 0.72 233.5 526.5 0.15SPPSU-20 10.0 90.0 100 0.83 234.2 522.5 0.20

Fig. 2. DSC curves of the SPPSU copolymers: (a) PPSU, (b) SPPSU-5, (c) SPPSU-10, (d)SPPSU-15 and (e) SPPSU-20.

302 Y. Liu et al. / Separation and Purification Technology 98 (2012) 298–307

nomenclature is SPPSU-X, where X is the concentration of hydro-philic sulfonic acid groups. The viscosity characterization indicatesthat all obtained copolymers have high molecular weight.

FTIR was used to verify the incorporation of the sulfonic acidgroups and chemical structures of the sulfonated polyphenylsulf-one copolymers. As shown in Fig. 1, two absorption peaks wereclearly observed at 1030 cm�1 and 1103 cm�1 for all sulfonatedcopolymers from SPPSU-5 to SPPSU-20, which could be attributedmainly to symmetric and asymmetric stretchings of the sulfonicacid group. Additionally, the densities of the characteristic peakof the sulfonic acid group symmetric stretching increase withSDCDPS contents. The FTIR spectrum aids in concluding that thesulfonated monomers are introduced into the copolymers success-fully as expected.

The glass transition temperatures (Tg) of the sulfonatedpolyphenylsulfone copolymers had been investigated with differ-ential scanning calorimetry (DSC) under nitrogen. Fig. 2 showsthe DSC thermograms of the SPPSU copolymers as a function ofthe degree of sulfonation. All the investigated copolymers areamorphous polymers. The DSC results showed an increase of Tg

with the increase of sulfonated monomer content. This phenome-non could be explained by the introduction of sulfonic acid groups:firstly, increase the intermolecular interaction between molecularchains by pendant ions; and secondly, increase molecular bulki-ness will result in difficulty in segment movement.

The thermal stability of the SPPSU copolymers had been inves-tigated by thermogravimetric analysis (TGA) measurement undernitrogen. There was only one weight loss step for PPSU above500 �C, which was assigned to the degradation of the polymerchain. However, the SPPSU copolymers showed two weight losssteps. The initial weight loss was assigned to the loss of the sul-fonic acid groups, and the second thermal degradation above500 �C was assigned to the decomposition of the copolymer back-

Fig. 1. FTIR spectrum of the SPPSU copolymers: (a) PPSU, (b) SPPSU-5, (c) SPPSU-10,(d) SPPSU-15 and (e) SPPSU-20.

bone. The theoretical weight percents of the sulfonic acid groups inthe SPPSU copolymers are less than 5%. Therefore, the 5% weightloss temperatures of these copolymers were above 500 �C, indicat-ing their good thermal stability (Table 1). In addition, the weightpercents of the sulfonic acid groups in the SPPSU copolymers in-creased with the increase of the degree of sulfonation, and the ini-tial weight losses of the SPPSU copolymers correspondingincreased, so the 5% weight loss temperature shifted toward lowtemperature with the increase of sulfonated monomer content.

4.2. Morphologies of SPPSU membranes

The scanning electron microscopy (SEM) was employed toinvestigate the morphology changes of the SPPSU membranes,and the SEM photographs are shown in Fig. 3. It is apparent thatthe SPPSU membranes have typical asymmetric structures, whichconsist of the thin dense skin layer and the porous sublayer. Ascan be seen in Fig. 3, the PPSU membrane has a long and thin fin-ger-like structure while the SPPSU-5 membrane shows that the fin-ger-like macrovoid structure become slightly wider. Themorphological change of the SPPSU-5 membrane is rather unper-ceivable, but when the degree of sulfonation is higher, the changeof morphology is quite noticeable. In particular, it is obvious thatthe number of macrovoids gradually decreased with the increaseof the degree of sulfonation (the SPPSU-10, SPPSU-15 and SPPSU-20 membranes). Another obvious difference among these mem-branes is their pore sizes in the inner of sublayer macrovoids. Thisphenomenon may result from the delayed phase separation as thehydrophilic SPPSU precipitates slowly and thus creates more andlarger pores [32,33]. The delayed phase separation mechanismmay be able to explain why the SPPSU-5 membrane shows a largenumber of pores in the inner of sublayer macrovoids, while onlyfew pores can be observed in the inner of sublayer macrovoids ofthe PPSU membrane. When the content of the sulfonated monomerfurther increased, the pore sizes in the inner of sublayer macrov-oids of the SPPSU-10 membrane increased dramatically, and aninterpenetrating network of the pores in the inner of sublayer mac-rovoids formed are observed in the SPPSU-15 and SPPSU-20membranes.

It is well known that the morphology and performance of theresulting membrane strongly depend on both the kinetics andthermodynamics of the phase separation process. The former is re-lated with the precipitation kinetics and the exchange rate be-tween solvent and nonsolvent, and the latter with the polymer–

Fig. 3. SEM images of the cross-sectional of the SPPSU membranes: (a) PPSU, (b) SPPSU-5, (c) SPPSU-10, (d) SPPSU-15 and (e) SPPSU-20.

Y. Liu et al. / Separation and Purification Technology 98 (2012) 298–307 303

solvent interactions, solvent-nonsolvent interactions and interfa-cial stability [34]. It is actually a diffusion-induced phase separa-tion process, which involves conversion of a liquid polymersolution of two or more components into a two-phase system:the polymer-rich phase and the polymer-lean phase. When theconcentration of nonsolvent exceeds a certain threshold, complete

precipitation occurs and the structure is fixed, the polymer-richphase forms the membrane structure while the polymer-leanphase forms the membrane pores. The precipitation rate can beconsidered to determine the amount of the available for the nucle-ation and growth of masses of polymer-lean phase within a matrixof polymer-rich phase after phase separation is induced [35,36].

304 Y. Liu et al. / Separation and Purification Technology 98 (2012) 298–307

Simply put, a lower precipitation rate provides more time beforeprecipitation is reached, enabling nucleation and growth of thepolymer-lean phase to progress further, so that larger pores canform. The exchange rate between solvent and nonsolvent duringthe phase inversion process plays a crucial role in controlling theprecipitation rate, and further governing the resulting membranemorphology [37,38]. The viscosity of the casting solution can ham-per the exchange between components during the demixing pro-cess, thus, influencing the resulting membrane morphology. FromTable 1, it is seen that the viscosity of the SPPSU copolymers in-crease with the increase of the degree of sulfonation. Therefore,the viscosity of the casting solution is raised with higher DS ofthe SPPSU copolymers, which plays an important role by hinderingseverely the exchange rate between solvent and nonsolvent duringthe phase inversion process. There is a greater amount of time be-tween when phase separation is induced and when precipitation isreached, nucleation and growth of the polymer-lean phase is ableto process further. The sizes of pore and macrovoid therefore bothbecome larger when the degree of sulfonation is higher. On thewhole, the kinetics of the phase separation process explains therelationship between the change of the membrane morphologyand the degree of sulfonation of the SPPSU copolymers.

It is commonly accepted that the morphological change of theSPPSU membranes is also correlated to the interaction betweenmembrane material and solvent. The relative strength of poly-mer–solvent interaction determines the morphology and perfor-mance of the final membrane. To study the morphology changein more detail from theory point, affinity of solvent to membranematerial can be estimated by the introduction of HSP theory. How-ever, Hildebrand and Hansen solubility parameters have beendetermined experimentally for a limited number of polymers. Formost polymers, several useful prediction methods based on molec-ular structures have been proposed to estimate HSP values by Vankrevelen (1976), Hoy (1985) and Beerbower (1984) [22,39]. In thiswork, the Van krevelen group contribution method was applied forcalculating HSP values of the SPPSU polymers. This method re-quires decomposition of the repeat unit of the polymer into func-tional groups, and cohesion parameters listed in Table 2 are usedin the calculation of HSP values. And HSP values were calculatedby the following equations [22]:

dd ¼RFdi

Vgið12Þ

dp ¼

ffiffiffiffiffiffiffiffiffiRF2

pi

qVgi

ð13Þ

dh ¼ffiffiffiffiffiffiffiffiffiREhi

Vgi

sð14Þ

where Vgi is the molar volume and Fdi, Fpi, Ehi are the dispersionforce, the dipole force, the hydrogen bonding force components ofthe solubility parameter, respectively.

Table 2Group contributions to Hansen (Vgi, Fdi, Fpi and Ehi) solubility parametersa[22,40].

Group Frequency Vgi (cm3/mol)

Fdi (J1/2 cm3/2/mol)

Fpi (J1/2 cm3/2/mol)

Ehi (J/mol)

4-2DS 65.5 1270 110 0

2DS 65.5 1270 110 0

–O– 2 10.0 100 400 3000–SO2– 1 + 2DS 31.8 587 1455 11347–OH 2DS 9.7 210 500 20000

a The –SO3H group can further be divided in to –SO2– and –OH groups.

The summary of the components of the Hansen solubilityparameters for the SPPSU copolymers with different DS and theDd values between DMF and SPPSU is presented in Table 3. Asfor Hansen solubility parameters, it is evident from Table 3 thatthe values increase with an increase in DS of polymer. This increaseis not the same for all parameters, and the maximum increase isobserved for dh, which is not surprising considering that the sul-fonic acid groups are very electrophilic and have a strong tendencyto form hydrogen bonds. It is well known that a smaller value of Ddleads to a stronger interaction. It can be noticed that the intensityof the interaction between SPPSU and DMF increases with the in-crease of the sulfonated monomer content. When the polymersolution contacts with water in a phase inversion process, thestrong interactions between SPPSU and DMF restrict the diffusionprocess of water and suppress the exchange rate of DMF and water,which increase the time to reach precipitation and induce the de-layed liquid–liquid dimixing process take place. This means thatthe liquid–liquid dimixing rate decrease with the increase of thedegree of sulfonation. This thermodynamically indicates that theaddition of the sulfonic acid groups can induce the more weakenedcoagulation. Generally speaking, a highly porous sublayer (withmany finger-like macrovoids) and a finely porous, thin skin layerare formed when the precipitation process is fast due to instanta-neous liquid–liquid dimixing process, whereas the slow precipita-tion rate (the delayed liquid–liquid dimixing process) results in aporous sublayer (often fewer macrovoids) with a dense, relativelythick skin layer [41,42]. In sum, the experiment results reflect thequalitative relationship between the morphological change and thedegree of sulfonation, and the relationship has been explained andverified by Hansen solubility parameters from theory discussion.

4.3. Membrane potentials, surface hydrophilicity and proteinadsorptions of SPPSU membranes

In order to investigate the surface charged properties of theSPPSU membranes, the membrane potentials were measured as afunction of concentrations in KCl solutions and their correspondingvalues calculated using Eq. (5), and the results are shown in Fig. 4.It can be seen that the membrane potentials are always negative tothe SPPSU membranes and become more negative with an increasein the DS values. Clearly, the sulfonation of membrane materials isreally an effective way to prepare negatively charged membrane,which may be very useful to suppress the membrane fouling andpromote the protein binding.

Surface hydrophilicity is one of the most important factors indetermining antifouling property and performance of ultrafiltra-tion membrane. The hydrophilicity and wettability of the SPPSUmembranes in this study was evaluated by contact angle measure-ment, which was also used to assess the surface (interfacial) freeenergies of substrate surfaces. It is commonly accepted that thelower contact angle represents the greater tendency for water towet the membrane, the higher surface energy and the higherhydrophilicity. Fig. 5 shows the detailed water contact data fromthe measurements on these different membranes, and the surfacefree energy was also calculated according to the followingequation:

cos h ¼ �1þ 2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffics

cle�bðcs�clÞ

2

rð15Þ

where cs and cl represent the solid and liquid surface free energy,respectively. The value of water surface free energy is 72.8 mJ/m2.b is the constant coefficient related to a specific solid surface andthe value of 0.0001247 is adopted from previously reported litera-ture [43].

Table 3The HSP values of SPPSU and DMF, and Dd between SPPSU and DMF.

dd (MPa1/2) dp (MPa1/2) dh (MPa1/2) d (MPa1/2) DdI (MPa1/2) DdII (MPa1/2)

DMF 17.4 13.7 11.3 24.86 – –PPSU 18.7 5.0 7.4 20.72 4.14 9.62SPPSU-5 18.7 5.2 8.0 20.99 3.97 9.21SPPSU-10 18.7 5.3 8.6 21.25 3.61 8.92SPPSU-15 18.7 5.5 9.1 21.51 3.35 8.59SPPSU-20 18.7 5.6 9.5 21.71 3.15 8.40

Fig. 4. Membrane potential vs. ln(c1/c2) for the SPPSU membranes.

Y. Liu et al. / Separation and Purification Technology 98 (2012) 298–307 305

From Fig. 5, it is seen that the contact angles decreased with theincrease of the degree of sulfonation, and the surface free energiesincreased gradually. The PPSU membrane has the highest contactangle of 83.47� and the lowest surface free energy of 33.26 mJ/m2, indicating the lowest hydrophilicity. The SPPSU membranehas a more hydrophilic surface than the PPSU membrane, and thistendency was attributed to the hydrophilic nature of the sulfonicacid groups. The decrease of contact angle indicated that a highlyhydrophilic surface was created. These results indicated that theintroduction of the sulfonic acid groups could effectively enhancethe hydrophilicity and improve antifouling property of PPSU-basedultrafiltration membrane.

The static protein adsorption is one of the dominant factors indetermining the membrane fouling, and the reduction of protein

Fig. 5. Water contact angles and surface free energies of the SPPSU membranes.

adsorption will enhance the antifouling property of membrane.Herein, BSA was used as the model protein to evaluate the staticprotein adsorption on the surface of the SPPSU membranes. Inmany cases, the nonspecific protein adsorption on the membranesurface due to the inherent hydrophobic characteristic often causesserious membrane fouling. Therefore, the increment in the mem-brane hydrophilicity is a straightforward and effective method toenhance the antifouling property of membrane. As is shown inFig. 6, the adsorption amount for measured protein exhibited thefollowing tendency: the BSA adsorption amount declined drasti-cally with the increase of sulfonated monomer content. Interest-ingly, this tendency is very similar to the tendency found in thestatic water contact angle measurement. The phenomenon canbe explained as follows: on the one hand, the sulfonic acid groupsform a regular hydration layer on the membrane surface via hydro-gen bond, and the protein was excluded from the hydration layerto avoid the substantial entropy loss caused by the entrance oflarge protein molecules into the membrane surface; on the otherhand, the isoelectric point for BSA appears when pH is 4.8, BSAmolecules show negative potential in phosphate buffered saline(PBS, pH 7.4), and the presence of the same charge (negativecharge) on the surface of the SPPSU membrane was believed to re-pel the BSA molecules and decrease the BSA adsorption amount bythe electrostatic repulsive interaction.

4.4. Permeation properties of SPPSU membranes

Ultrafiltration experiments were carried out to investigate theseparation performance of a series of SPPSU membranes. Fig. 7shows the changes in water permeation and protein rejection ofthe different membranes. The water flux was gradually increasedwith an increase of the degree of sulfonation, while BSA rejectionratio was slightly decreased from 98.8% to 92.2% as the DS value in-crease from 0 to 0.2. The pure water flux (Jw1) of the PPSU mem-

Fig. 6. BSA adsorption on the SPPSU membranes.

Fig. 8. Time-dependent fluxes of the SPPSU membranes during the proteinultrafiltration experiment.

Fig. 9. Summary of the flux recovery ratio (FRR), the total fouling ratio (Rt), thereversible fouling ratio (Rr) and the irreversible fouling ratio (Rir) of the SPPSUmembranes during the protein ultrafiltration experiment.

306 Y. Liu et al. / Separation and Purification Technology 98 (2012) 298–307

brane is 142.8 L/m2 h, which is lower than that of the SPPSU mem-branes. This result may be attributed to the increased hydrophilic-ity and surface morphology change of the SPPSU membranes.

Fig. 8 presented time-dependent flux during ultrafiltrationoperation. It is apparent that the flux decreased dramatically atthe initial operation of BSA solution ultrafiltration due to mem-brane fouling caused by protein adsorption or deposition on themembrane surface. When the adsorption and deposition of proteinmolecules may reach equilibrium, a relatively steady flux (Jp) wasretained in the final operation of BSA solution ultrafiltration. After2 h of BSA solution filtration, the membranes were washed thor-oughly and passed through deionized water for another 30 min,and the water fluxes of the cleaned membranes (Jw2) were mea-sured again. FRR is introduced to reflect the resistant fouling abilityof the membranes, and higher value of FRR means the higher resis-tant fouling ability. The FRR values were calculated and presentedin Fig. 9. The FRR value is only 55.7% for the PPSU membrane,meaning the existence of serious membrane fouling. The SPPSUmembranes have larger FRR values, suggesting that adsorbed anddeposited protein on the SPPSU membrane surfaces could be easilywashed away. The SPPSU-20 membrane has the highest FRR value(77.8%), which is consistent with the protein adsorption experi-ment results.

The protein molecules adsorbed or deposited on the surface andinside the membrane pores causing membrane fouling, whichcould be further divided into reversible and irreversible fouling.One part of fouling, recognized as reversible fouling, was causedby reversible protein adsorption or deposition and could be elimi-nated through hydraulic cleaning (hydrodynamic method); whilethe other part of fouling, defined as irreversible fouling, couldnot be eliminated only through hydraulic cleaning. More detailedresults of the total fouling ratio (Rt), the reversible fouling ratio(Rr) and the irreversible fouling ratio (Rir) of the all investigatedmembranes are given in Fig. 9. It can be seen that the Rt value ofthe PPSU membrane is larger than that of the SPPSU membranes.The bigger Rt value indicates higher total flux loss, correspondingto more protein adsorption and deposition on the membrane sur-face. Meanwhile, the Rir value of the PPSU membrane is the largestamong the all investigated membranes. It can be concluded thatthe protein fouling on the PPSU membrane is so serious that thefouling cannot be removed by hydraulic cleaning. The SPPSU mem-branes have not only the lower Rt values but also the lower Rir val-ues, and the protein fouling in the SPPSU membrane wassuppressed significantly under the ultrafiltration process. In addi-tion, the SPPSU membranes possess a negatively charged characterexperience electrostatic repulsions between the negatively

Fig. 7. Water fluxes and BSA rejections of the SPPSU membranes.

charged surface and the negatively charged protein, which leadsto a decrease in BSA adsorption on the membrane surface. Gener-ally speaking, when protein molecules contacted with the mem-brane surface, water molecules between protein and themembrane surface would be replaced. The polar sulfonic acidgroups can take up large quantities of free water, which possiblyprevents protein molecules from close contact with the membranesurface. These results indicated obviously that the introduction ofthe sulfonic acid groups efficiently reduces total membrane foul-ing, especially irreversible membrane fouling.

5. Conclusions

This study is aimed at optimizing polymer design by employingsulfonated monomer in direct copolymerization method that pro-vides a good opportunity to improve the resultant membrane per-formance without sacrificing mechanical strength. A series ofsulfonated polyphenylsulfone (SPPSU) random copolymers withvarious well controlled sulfonation levels were synthesized by pre-cise adjusting the ratios of the sulfonated monomer to non-sulfo-nated monomer, and their chemical structures were confirmed

Y. Liu et al. / Separation and Purification Technology 98 (2012) 298–307 307

by using FTIR. The thermal properties of the SPPSU copolymerswere thoroughly characterized by DSC and TGA study. The mor-phologies of the SPPSU membranes were investigated by SEM,and the remarkably change of morphology had been explainedand verified by Hansen solubility parameters from theory pointthat opened a way for establishing a complete qualitative relation-ship between the membrane morphology and polymer structure.In addition, both water contact angle and membrane potentialexperiments confirmed the highly hydrophilic and negativelycharged characteristics of the SPPSU membranes. The cycle ultrafil-tration experiments showed the pure water flux and protein solu-tion permeation through the SPPSU membranes were increasedwith the increase of the degree of sulfonation. Meanwhile, theSPPSU membranes had higher flux recovery ratio and lower extentof membrane fouling, especially irreversible membrane fouling, ascompared to those of the PPSU membrane, which displayed a supe-rior antifouling performance. In sum, this study demonstrated thatthe application of sulfonated PPSU material with the help of directcopolymerization method can effectively enhance the hydrophilic-ity, negatively charge the membrane and significantly improveultrafiltration membrane performance.

Acknowledgements

This work was supported by a Grant from the National HighTechnology Research and Development Program of China (863 Pro-gram) (No. 2012AA03A212).

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