Effects of Coagulation Bath Temperature on the Separation

(1, 8, 10) Generally, the effect of CBT on NIPS process is of major concern. .... Survey spectra are collected over a range of 0−1100 eV, and high-r...
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Effects of Coagulation Bath Temperature on the Separation Performance and Antifouling Property of Poly(ether sulfone) Ultrafiltration Membranes Jinming Peng, Yanlei Su, Wenjuan Chen, Qing Shi, and Zhongyi Jiang* Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, P. R. China

Poly(ether sulfone) (PES) ultrafiltration membranes are fabricated via nonsolvent-induced phase separation by blending with hydrophilic homopolymer additive poly(ethylene glycol) (PEG) or amphiphilic block copolymer Pluronic F127. The effects of coagulation bath temperature (CBT) on membrane structure, separation performance, and antifouling property are investigated in detail. All the membranes display an asymmetric morphology. PES/PEG membranes possess only fingerlike pores of support layer, while there is a spongelike sublayer between skin layer and the fingerlike pores for PES/Pluronic F127 membranes. The thickness of the spongelike sublayer for PES/Pluronic F127 membranes is remarkablely decreased with the increase of CBT. For all the membranes, pure water flux increases substantially with the increase of CBT. The rejection of PES/PEG membrane for bovine serum albumin (BSA) is above 95%. However, the rejection of PES/Pluronic F127 membrane for BSA molecules is decreased sharply form 95.3% to 10.2% with the increase of CBT from 20 to 60 °C. At higher CBT, the antifouling capabilities of PES/Pluronic F127 membranes are slightly weakened mainly because of the lower surface coverage of amphiphilic copolymer. 1. Introduction Most of polymer membranes widely utilized in microfiltration, ultrafiltration, and nanofiltration are fabricated via nonsolventinduced phase separation (NIPS). In the NIPS process, casting solution containing polymer and solvent with or without an additive is cast on a solid substrate and then immersed in a coagulation bath of nonsolvent (usually water). At the interface, the solvent diffuses out to coagulation bath, while the nonsolvent transfers into the casting solution. This exchange of solvent and nonsolvent leads to a liquid-liquid phase separation. The whole solution is quickly divided into two different phases: the continuous polymer-rich phase and the dispersive polymer-lean phase. After that, the polymer-rich phase congregates, develops, and solidifies into the membrane matrix, whereas small droplets of polymer-lean phase dispersing in the polymer-rich phase give rise to the pores. Finally, an asymmetric membrane with dense skin layer and porous support layer is formed. The NIPS is a thermodynamic nonequilibrium process. Thermodynamic factors and kinetic factors, such as solvent/ nonsolvent pairs, the type of additive, the composition of coagulation bath, the pre-evaporation, viscosity of casting solution, and so on,1–7 play a significant role in membrane formation. Thermodynamic factors are related to the phase equilibrium of each component, and kinetic factors are related to the mutual effective diffusivities. Coagulation bath temperature (CBT), as both thermodynamic factor and kinetic factor, can greatly influence the phase separation process and membrane morphology.1,8–11 On one hand, higher temperature leads to enhanced intersolubility. Therefore, in the ternary phase diagrams the one-phase region expands, and liquid-liquid demixing region shrinks at higher CBT. The polymer concentration decreases in polymer-rich phase and increases in polymer-lean phase when phase separation occurs, which leads to a looser membrane. On the other hand, at the higher CBT, the diffusion rates of solvent and nonsolvent molecules become faster and the phase inversion process is accelerated. The membrane * To whom correspondence should be addressed: E-mail: zhyjiang@ tju.edu.cn. Tel: 86-22-2350 0086. Fax: 86-22-2350 0086.

fabricated at higher CBT would probably exhibit relative thinner skin layer and higher porosity.1,8,10 Generally, the effect of CBT on NIPS process is of major concern. There are a few reports focusing on the effect of CBT on membrane performance. Saljoughi et al. investigated the effect of PVP concentration and CBT on morphology, permeability, and thermal stability of cellulose acetate membranes.8 Wang et al. reported that gelation is the dominate membrane formation mechanism at lower CBT while liquid-liquid demixing is the dominate mechanism at higher CBT for the water/DMAc/PVDF system.10 For membrane fabrication, a hydrophilic homopolymer, such as poly(ether glycol) (PEG) and poly(vinylpyrrolidone) (PVP), and an amphiphilic block copolymer additive, such as triblock PEO-PPO-PEO Pluronic copolymer and hyperbranched star copolymer of polyester-g-methoxylpoly(ethylene glycol) (HPEg-MPEG), are often required in order to form pores and reduce fouling.5,12–19 However, the roles of these two kinds of additives are quite different. Hydrophilic homopolymer molecules possess weak interactions with membrane material. When phase inversion occurs for membrane preparation, most of hydrophilic homopolymer molecules are released and dissolved into surrounding coagulation bath. But for amphiphilic copolymer, the amphiphilic copolymer molecules tangle with membrane matrix because of the strong interactions between the hydrophobic segments of amphiphilic copolymer and membrane material. On the other hand, amphiphilic molecules aggregate and form micelles. The tangled amphiphilic molecules segregate to interface and play a role of surface modifier, while the micelles dissolve into coagulation bath and play a role of pore-form agent. Because of the different behavior of these two kinds of additives in NIPS, the effect of CBT on two kinds of membranes is of great difference. However, the systematic influence of CBT on the structure and performance of membranes with hydrophilic homopolymer or amphiphilic block copolymer additive for the preparation of antifouling membranes is not explored. In the present work, poly(ether sulfone) (PES) ultrafiltration membranes are fabricated via NIPS at coagulation bath temperature spanning from 20 to 60 °C. PEG, as a classic hydrophilic homopolymer, and Pluronic F127, a PEO-PPO-PEO

10.1021/ie9018963  2010 American Chemical Society Published on Web 04/23/2010

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amphiphilic triblock copolymer, are utilized as additives. Scanning electron microscope (SEM) is used to observe the cross-section morphologies of the membranes. Water contact angles are measured to evaluate the hydrophilicity of membrane surface. The surface chemical compositions of PES membranes are probed by X-ray photoelectron spectroscopy (XPS) instruments. Ultrafiltration experiments are conducted to obtain antifouling and permeation property of PES membranes. 2. Experimental Section 2.1. Materials. PES (6020P, Mw ) 29 000) purchased from BASF Co. (Germany) is dried at 110 °C for 12 h before use. Pluronic F127, a PEO-PPO-PEO triblock copolymer, with a molecular weight of 12 600 and a PEO content of 70 wt % is purchased from Sigma. Pluronic F127 is represented by the formula EO100-PO65-EO100 based on the molecular weight and chemical composition. Poly(ethylene glycol) (PEG, Mw ) 2000) is purchased from Damao Chemical Reagent Co. (Tianjin, China). N,N-Dimethylformamide (DMF) is purchased from Kewei Chemicals Co. (Tianjin, China). Bovine serum albumin (BSA, Mw ) 67 kDa) is purchased from Institute of Hematology, Chinese Academic of Medical Sciences (Tianjin, China). Other chemicals are of commercially analytical grade and used without further purification. 2.2. Preparation of PES Membranes. PES blend membranes are prepared via nonsolvent-induced phase separation.20,21 PES is the membrane matrix, and PEG or Pluronic F127 is added as the additive. The mixture of 3.60 g of PES and 3.60 g of PEG (or 3.60 g of Pluronic F127) is dissolved in 12.80 g of DMF and stirred at 60 °C for about 4 h to ensure the homogeneous mixing and then left for 5 h to allow bubbles release completely. After being cooled to room temperature, the solutions are cast on glass plates with a steel knife and then immediately immersed into the coagulation bath of deionized water. The coagulation baths are kept at 20, 30, 40, 50, and 60 °C by a controlled-temperature water bath. Subsequently, the pristine membranes are peeled off and washed thoroughly with deionized water to remove residual solvent. The as-prepared membranes have an average wet thickness of about 250 µm and are kept in deionized water before use. 2.3. Characterization of the PES Membranes. The characterization details of PES ultrafiltration membranes have been reported in our previous articles.14–16 Scanning electron microscope (SEM, Philips XL30E) is utilized to investigate the crosssection morphologies of the membranes. The membrane samples frozen in liquid nitrogen are broken and sputtered with gold for producing electric conductivity prior to SEM observation. The thickness of spongelike sublayer of PES/Pluronic F127 membranes is determined according to the SEM images, which is obtained by measuring three different locations of each membrane. The average value is adopted, and the errors are marked. In order to evaluate the hydrophilicity of blend membranes, the water contact angles of the membranes are measured at room temperature by a contact angle goniometer (JC2000C contact angle meter, Powereach Co., Shanghai, China). Membranes are lyophilized for 3 h and then pressured with a rolling machine. To minimize the experimental error, the contact angle is measured at five different locations for each membrane, and the average is reported. The surface chemical compositions of PES membranes are investigated by X-ray photoelectron spectroscopy instruments (PHI-1600) using Mg KR (1254.0 eV) as radiation source (the takeoff angle of the photoelectron is set at 90°). Survey spectra

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are collected over a range of 0-1100 eV, and high-resolution spectra of the C 1s peak are also collected. There are two types of carbons in these membranes: hydrocarbons (CH3- and -CHdCH-) and ether carbons (-C-O-C-); the XPS signal observed at the C 1s region at 284.7 eV is attributed to hydrocarbons, and that at 286.0 eV is attributed to ether carbons.22 The surface coverage of amphiphilic copolymer, φs, is calculated by the following equation: MC-O ) a(1 - φs) + bφs MC-O )

AC-O AC-O + AC-O

(1)

(2)

where MC-O is the ether carbon molar ratio, AC-O is the area of the peak for alkyl carbon, a is the theoretical mole ratio of C-O in PES molecule, 0.333, and b is the theoretical mole ratio of C-O in Pluronic F127 molecule, 0.748. Liquid-liquid displacement porosimetry experiments are employed to estimate the pore size of membranes.23–26 Isopropanol is used as wetting liquid and a mixture of isopropanol and water (1/1, v/v) as intrusion fluid. The flux of the isopropanol-water mixture through both the isopropanol-wetted membrane and the fully intruded membrane is measured as a function of transmembrane pressure. The relationship between the pore size and corresponding pressure is given by the Cantor equation: ∆P )

2γ r

(3)

where ∆P is the transmembrane pressure and γ ) 0.4 dyn/cm based on data for similar alcohol-water system. 2.4. Ultrafiltration Experiments. The ultrafiltration experiments of PES membranes are performed as previously described.14–16 A dead-end stirred cell filtration system connected with a nitrogen gas cylinder and solution reservoir is designed to characterize the separation performance of membranes. The system consists of a filtration cell (model 8200, Millipore Co.) with a volume capacity of 200 mL and an inner diameter of 62 mm. The effective area of the membrane is 28.7 cm2. The feed side of the system is pressed by nitrogen gas. All the ultrafiltration experiments are carried out at a stirring speed of 400 rpm and a temperature of 25 ( 1 °C. Each membrane is initially pressurized at 0.15 MPa for 30 min, and then the pressure is lowed to the operating pressure of 0.1 MPa. The water flux Jw1 (L/(m2 h)) is calculated by the following equation: Jw1 )

V A∆t

(4)

where V (L) is the volume of permeated water, A (m2) is the membrane area, and ∆t (h) is the permeation time. In the following step, the stirred cell and solution reservoir are emptied and refilled rapidly with protein solution (1.0 mg/mL BSA solution, pH is kept at 7.0 with 0.1 M phosphate buffer solution (PBS)). The flux for protein solution Jp (L/(m2 h)) is measured based on the water quantity permeating the membranes at the same pressure (0.1 MPa). The rejections (R) of BSA are calculated by the following equation:

(

R) 1-

)

Cp × 100% Cf

(5)

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Figure 2. Water contact angles for PES/PEG/Pluronic F127 membranes as a function of CBT.

Figure 1. Cross-sectional SEM asymmetric morphology of PES membranes at CBT of 20, 40, and 60 °C: (a) PES/PEG 20 °C, (b) PES/PEG 40 °C, (c) PES/PEG 60 °C, (d) PES/Pluronic F127 20 °C, (e) PES/Pluronic F127 40 °C, and (f) PES/Pluronic F127 60 °C.

where Cp and Cf are the protein concentration of permeate and feed solutions, respectively. After ultrafiltration of BSA solution, the membranes are washed with deionized water 20 min, and then the water flux of cleaned membranes Jw2 (L/(m2 h)) is measured again. The flux recovery ratio (FRR) is calculated using the following expression: FRR )

( )

Jw2 × 100% Jw1

(6)

The higher value of FRR, the better antifouling property of the ultrafiltration membrane. 3. Results and Discussion 3.1. Cross-Section Morphologies of PES Membranes. PES has drawn remarkable attention as a membrane material due to its excellent mechanical property and high chemical and biological stabilities. PES ultrafiltration membranes have been widely used to concentrate or fractionate protein solutions. During ultrafiltration, the molecules, smaller than the pores on skin layer, are driven to flow through the membrane, while the bigger protein molecules are mostly rejected. In the present work, PES ultrafiltration membranes are prepared at CBT spanning from 20 to 60 °C. The cross-section morphologies of the prepared PES membranes are observed with SEM, and the SEM images are exhibited in Figure 1. All of the membranes display the typical asymmetric morphology with a thin skin layer on top and a porous bulk at the bottom. The skin layer is responsible for the permeation and rejection of solutes whereas the bulk plays a role as a mechanical support. In Figure 1a-c, the porous bulk of PES/PEG membranes includes a fingerlike

intermediate layer and fully developed macrovoids. However, the cross-section morphologies of PES/Pluronic F127 membranes in Figure 1d-f are different from those of PES/PEG membranes. There exists a spongelike sublayer between skin layer and fingerlike pores for PES/Pluronic F127 membranes. According to nucleation and growth mechanism, if the affinity between solvent and nonsolvent is strong enough, under conditions in which the out-diffusion rate of solvent is much higher than the in-diffusion rate of nonsolvent, the skin layer is very dense, thus lowering the diffusion rate for nonsolvent into the sublayer. These result in fewer nuclei in sublayer and possibly form a fingerlike structure. On the other hand, if the affinity between solvent and nonsolvent is weak, a spongelike structure can be formed.27–30 Compared to homopolymer PEG, the introduction of less hydrophilic PPO segments in amphiphilic Pluronic F127 molecules influences both driving force and relative diffusion rate of DMF and water. The affinity between DMF and water is lowered, and the in-diffusion rate of nonsolvent is accelerated. Accordingly, PES/Pluronic F127 membranes contain a spongelike sublayer while PES/PEG membranes possess only fingerlike pores. As shown in Figure 1, when the CBT increases, there is no significant change of the cross-section morphology for PES/ PEG membranes; however, for PES/Pluronic F127 membranes, the spongelike sublayer domain shrinks. The thicknesses of the spongelike sublayer for PES/Pluronic F127 membranes are 36.8 ( 2.1, 20.2 ( 1.6, and 12.9 ( 1.0 µm at CBT of 20, 40, and 60 °C, respectively. This reduced thickness of the spongelike layer of PES/Pluronic F127 membranes can be interpreted that the affinity between DMF and water becomes stronger at higher CBT. On this condition, the accelerate degree of out-diffusion rate is higher than that of in-diffusion rate. Thus, a thinner spongelike sublayer is generated for PES/Pluronic F127 membranes. 3.2. Surface Characterization of PES Membranes. The water contact angle is employed to investigate the membrane surface hydrophilicity. The membrane surface with higher hydrophilicity presents possess lower water contact angle. As shown in Figure 2, PES/PEG membranes possess rather high water contact angles, which indicate that PES/PEG membranes are relative hydrophobic. There is a slight decrease of the water contact angles of PES/PEG membranes from 77.2 to 72.5° with the increase of CBT from 20 to 60 °C.

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However, for PES/Pluronic F127 membranes, the water contact angles are much smaller than that of PES/PEG membranes, which indicate PES/Pluronic F127 membranes possess more hydrophilic surface than PES/PEG membranes. The reason is reported in our previous work that the surface segregation of Pluronic F127 causes a hydrophilic PEO segments enrichment on membrane surface.14–16 The water contact angles of PES/ Pluronic F127 membranes are increased from 51.3 to 68.4° as the CBT increases from 20 to 60 °C. It is speculated that this decrease of water contact angle is caused by the lower surface coverage of Pluronic F127 molecules on the membrane surface. During phase separation process at higher CBT, the molecular thermal motion is accelerated and the affinity between PPO segments and PES chains is weakened. The amount of Pluronic F127 molecules that leave from the membrane surface to nonsolvent is more than those from the membrane bulk to the membrane surface. Therefore, the amount of Pluronic F127 molecules anchored on the membrane surface becomes less at higher CBT. The surface coverage of hydrophilic PEO segments is reduced accordingly. This weakened enrichment of Pluronic F127 molecules on membrane surface is further demonstrated by XPS analysis. XPS analysis is employed to investigate the chemical compositions of PES/Pluronic F127 membrane surface. There are two types of carbons in these membranes: hydrocarbons (CH3- and -CHdCH-) and ether carbons (-C-O-C-). The XPS signal observed at C 1s region at 284.7 eV is attributed to hydrocarbons, and that at 286.0 eV is attributed to ether carbons. Surface coverage of amphiphilic copolymer is calculated based on the area percent of ether carbons. The high-resolution XPS spectra of C 1s for PES/Pluronic F127 membranes at CBT of 20, 40, and 60 °C are presented in Figure 3. The area percent of C-O peak reduces as a function of CBT. The values of φs are 0.514, 0.451, and 0.262 at CBT of 20, 40, and 60 °C, respectively. The decreasing φs value indicates the lower surface coverage of hydrophilic PEO segments and the less hydrophilic membrane surface for PES/Pluronic F127 membranes, leading to a decrease of water contact angle. Conclusively, PES/Pluronic F127 membranes have less hydrophilic surface at higher CBT. 3.3. Permeation Performance of PES Membranes. Ultrafiltration experiments are carried out to investigate the permeability of the PES membranes. The time-dependent fluxes of PES/PEG membranes are presented in Figure 4. The water flux of PES/PEG membrane at CBT of 20 °C is 184.8 L/(m2 h). When CBT is increased to 60 °C, the water flux increases largely to 380.1 L/(m2 h). The increase of water flux may be ascribed to two reasons. One is lower membrane resistance caused by the thinner skin layer; the other is the enlarged pores on the skin layer. Because of the higher CBT, the diffusion rate of solution and nonsolution molecules at the interface is accelerated. Therefore, solvent-nonsolvent exchange remarkably accelerates across the interface, leading to a more rapid precipitation. As a result, the skin layer becomes thinner, which results in less membrane resistance and higher water flux. On the other hand, the increase of flux may be caused by enlarged pores. Liquid-liquid displacement porosimetry experiments are carried to measure the pore size of all fabricated PES membranes. The pore size ranges of PES/PEG membranes are listed in Table 1. The pore size of PES/PEG membranes increases slightly when CBT spanning from 20 to 60 °C. The slightly increase of pore size can be indirectly demonstrated by BSA rejection for PES/ PEG membranes shown in Figure 6. All the R values are above 95%, which means that there is no significant change of pore size. Therefore, the lower membrane resistance of thinner skin

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Figure 3. High-resolution XPS spectra of C 1s for PES/Pluronic F127 membranes at CBT of 20, 40, and 60 °C.

layer for PES/PEG membrane is the dominant reason for higher water flux at higher CBT. The time-dependent fluxes of PES/Pluronic F127 membranes are presented in Figure 5. Compared to PES/PEG membranes, the increase degree of water fluxes for PES/Pluronic F127 membranes is relatively small. The reason is that under the condition of equivalent additive mass the affinity between solvent and nonsolvent in the PES/Pluronic F127 system is weaker than that in the PES/PEG system, owing to the introduction of less hydrophilic PPO segment. At the same time larger molecular weight of Pluronic F127 than PEG makes casting solution more viscous. As a result, solvent-nonsolvent exchange is decelerated, causing thicker skin layers of PES/ Pluronic F127 membranes than that of PES/PEG membranes.

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Figure 4. Time-dependent fluxes of PES/PEG membranes during the ultrafiltration process. The ultrafiltration process includes four steps: pure water ultrafiltration from 0 to 0.5 h, BSA solution ultrafiltration from 0.5 to 1.5 h, water cleaning of 20 min at time of 1.5 h, and pure water flux measurement of the cleaned membranes from 1.5 to 2.0 h. Ultrafiltration was carried out at temperature of 25 °C. The BSA concentration is 1.0 mg/mL in PBS solution.

Figure 5. Time-dependent fluxes of PES/Pluronic F127 membranes during the ultrafiltration process. The ultrafiltration process includes four steps: pure water ultrafiltration from 0 to 0.5 h, BSA solution ultrafiltration from 0.5 to 1.5 h, water cleaning of 20 min at time of 1.5 h, and pure water flux measurement of the cleaned membranes from 1.5 to 2.0 h. Ultrafiltration was carried out at temperature of 25 °C. The BSA concentration is 1.0 mg/mL in PBS solution. Table 1. Surface Pore Size Range of PES Membranes with Additive of PEG and Pluronic F127 CBT (°C) 20 30 40 50 60

PES/PEG 4-25 4-27 6-28 7-29 7-32

pore size range (nm) PES/Pluronic F127 4-28 4-33 5-40 10-51 17-67

Consequently, the water fluxes of PES/Pluronic F127 membranes are much lower than that of PES/PEG membrane. The water flux of PES/Pluronic F127 membrane increases from 102.0 to 245.9 L/(m2 h) with an CBT increase from 20 to 60 °C. As mentioned previously, the increase of water flux at

Figure 6. BSA rejection (R) values of PES membranes as a function of the CBT.

higher CBT can be ascribed to lower membrane resistance of thinner skin layer and larger pores of membrane surface. For PES/PEG membranes, the former of lower membrane resistance is the dominant reason. However, for PES/Pluronic F127 membranes, it can be attributed to the cooperation of these two reasons. Similar to PES/PEG membranes, higher CBT leads to a more rapid precipitation and a thinner skin layer for PES/ Pluronic F127 membranes. Simultaneously, the pores on the surface of PES/Pluronic F127 membranes are enlarged at higher CBT, which is given in Table 1. The surface pore size range of PES/Pluronic F127 membranes is 4-28 nm at CBT of 20 °C and is enlarged to 17-67 nm at CBT of 60 °C. Furthermore, the BSA rejection is 95.3% at CBT of 20 °C and decreases sharply to 10.2% at CBT of 60 °C shown in Figure 6, indicating a remarkable enlargement of pore size. Therefore, for PES/ Pluronic F127 membranes, the enlarged pore size is an important factor for increased flux as well as thinner skin layer. The effect of CBT on the pore size of two types of membranes is quite different. It can be interpreted that homopolymer PEG molecules disperse homogenously in casting solution, and higher CBT could not affect the distribution of PEG molecules. After immersing in coagulation bath, solvent and nonsolvent exchanges and phase separation occur. Subsequently, the polymerrich phase containing most PES molecules solidifies, and the polymer-lean phase containing PEG molecules dissolves into coagulation bath. The space once polymer-lean phase occupied turns to pores. Therefore, CBT could not significantly vary the pore size of membrane surface in the PES/PEG system since CBT can hardly influence polymer-lean phase which is full of PEG molecules. However, in the PES/Pluronic F127 system, in order to lower the Gibbs free energy of mixing, amphiphilic Pluronic F127 molecules would self-assemble into micelles. The space of polymer-lean phase containing Pluronic F127 micelles left give rise to pores after polymer-rich phase solidifies. Other researches have reported that the diameter of micelles formed byPluronicmoleculesbecomeslargeratthehighertemperature.31–34 At the higher CBT, after precipitation nonsolvent removes the polymer-lean phase full of larger micelles and finally pore size becomes larger. Protein ultrafiltration is carried out to evaluate the protein permeation property of the membranes. Figure 4 shows the timedependent fluxes of BSA solution for PES/PEG membranes.

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Figure 7. Flux recovery ratio (FRR) values of PES blend membranes in the ultrafiltration experiments.

The flux of protein solution is declined dramatically compared to water flux. BSA molecules in the feed can deposit and adsorb on the membrane surface (cake formation) at the initial BSA ultrafiltration operation. This deposition and adsorption cause an abrupt drop in flux in the first few minutes of the operation. In the stirred cell, the mixing induced by the stirrer may actually sweep protein molecules away from the membrane surface. The deposition and sweeping of protein may reach equilibrium, so that a relatively steady flux for BSA solution is retained in the final operation of BSA solution ultrafiltration. At CBT of 20 °C, the flux of protein solution is 61.1 L/(m2 h). With the CBT increasing to 60 °C, the flux of BSA solution rises to 98.3 L/(m2 h). The fluxes of protein solution for the PES/Pluronic F127 membranes are higher than that of PES/PEG membranes. Figure 5 exhibits the time-dependent fluxes of BSA solution of PES/Pluronic F127 membranes. The fluxes of protein solution are 81.1 L/(m2 h) at CBT of 20 °C and 130.9 L/(m2 h) at CBT of 60 °C. 3.4. Antifouling Properties of PES Membranes. FRR values are calculated in order to assess the efficiency of hydraulic cleaning and the antifouling properties of PES membranes. Higher FRR value means higher efficiency of hydraulic cleaning and stronger antifouling property. FRR values of all the membranes are given in Figure 7. The FRR values of PES/PEG membranes are relative lower than those of PES/ Pluronic F127 membranes. The FRR value of PES/PEG membrane is 72.2% at CBT of 20 °C and drops to 56.7% at CBT of 60 °C. It can be concluded that the protein fouling on PES/PEG membranes is so serious that the fouling can not be removed by hydraulic cleaning. Moreover, the antifouling capacity of PES/PEG membranes is weakened with an increase of CBT based on FRR values. The increased water fluxes of PES/PEG membranes at higher CBT probably induce more BSA adsorption and deposition on the membrane surface which causes more serious membrane fouling. Compared to PES/PEG membranes, PES/Pluronic F127 membranes possess much higher FRR values. The FRR value of PES/Pluronic F127 membrane is 94.6% at CBT of 20 °C, which means a higher efficiency of hydraulic cleaning and a higher antifouling property. The high FRR values caused by high surface coverage of PEO segments due to surface segregation of Pluronic F127.14–16 The high surface coverage of PEO segments leads to a spontaneous rearrangement of the polymer to form molecular brush and strong hydration layer. It is reported that if the water state at the surface is similar to that in an

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aqueous solution or the free water fraction on the surface is sufficiently high, the protein adsorption can be reduced substantially.35,36 After modification by Pluronic F127, a PEO layer which holds large amount of free water is generated on the PES membranes surface. The fouling caused by protein molecules adsorption and deposition on the membrane surface or in pores is reduced dramatically for PES/Pluronic F127 membranes. The FRR value of the PES/Pluronic F127 membrane is gradually decreased from 94.6 to 80.6% with an increase of CBT from 20 to 60 °C. As CBT increases, the degree of surface coverage of PEO segments on the PES/Pluronic F127 surface is decreased and surface hydrophilicity is lowered, which worsens the antifouling property. Meanwhile, the higher water flux and larger pore size PES/Pluronic F127 membrane at higher CBT may bring protein blockage in the membrane pores. The trapped protein molecules in the membrane pores cannot be washed out easily. Therefore, the PES/Pluronic F127 membranes display lower FRR values at higher CBT. Conclusively, the fabrication of PES/Pluronic F127 membranes with better antifouling property should be carried out at lower CBT. 4. Conclusions The influence of CBT on PES membranes fabricated using PEG and Pluronic F127 as additives is studied. At higher CBT, the pore size of skin layer is slightly increased for PES/PEG membranes and is significantly enlarged for PES/Pluronic F127 membranes. Meanwhile, at higher CBT, the water permeation of two types of PES membranes is enhanced but the antifouling property is decreased. The lower surface coverage of Pluronic F127 molecules at higher CBT is the main reason for the decreased antifouling property of PES/Pluronic F127 membrane. Acknowledgment This research is supported by the Research Fund for the Doctoral Program of Higher Education of China (20060056032), Drug Separation and Purification Project in Programme for Development of Novel Drug (No. 2009ZX09301-008), and Project supported by State key laboratory of precision measuring technology and instruments (Tianjin University). Literature Cited (1) Shin, S.; Kim, J.; Kim, H.; Jeon, J.; Min, B. Preparation and characterization of polyethersulfone microfiltration membranes by a 2-methoxyethanol additive. Desalination 2005, 186, 1–10. (2) Torrestiana-Sanchez, B.; Ortiz-Basurto, R. I.; Fuente, E. B. Effect of nonsolvents on properties of spinning solutions and polyethersulfone hollow fiber ultrafiltration membranes. J. Membr. Sci. 1999, 152, 19–28. (3) Lin, D. T.; Cheng, L. P.; Kang, Y. J.; Chen, L.; Young, T. Effects of precipitation conditions on the membrane morphology and permeation characteristics. J. Membr. Sci. 1998, 140, 185–194. (4) Wu, L. S.; Sun, J. F.; Wang, Q. R. Poly(vinylidene fluoride)/ polyethersulfone blend membranes: Effects of solvent sort, polyethersulfone and polyvinylpyrrolidone concentration on their properties and morphology. J. Membr. Sci. 2006, 285, 290–298. (5) Susanto, H.; Ulbricht, M. Characteristics, performance and stability of polyethersulfone ultrafiltration membranes prepared by phase separation method using different macromolecular additives. J. Membr. Sci. 2009, 327, 125–135. (6) Conesa, A.; Tania, G.; Cristina, P. Membrane thickness and preparation temperature as key parameters for controlling the macrovoid structure of chiral activated membranes (CAM). J. Membr. Sci. 2007, 287, 29–40. (7) Blanco, J. F.; Sublet, J.; Nguyen, Q. T.; Schaetzel, P. Formation and morphology studies of different polysulfones-based membranes made by wet phase inversion process. J. Membr. Sci. 2006, 283, 27–37. (8) Saljoughi, E.; Amirilargani, M.; Mohammadi, T. Effect of Poly(vinyl pyrrolidone) concentration and coagulation bath temperature on the

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ReceiVed for reView December 2, 2009 ReVised manuscript receiVed March 17, 2010 Accepted April 13, 2010 IE9018963