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Effect of PEG additive on the morphology and performance of polysulfone ultrafiltration membranes
Desalination 272 (2011) 51–58
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Effect of PEG additive on the morphology and performance of polysulfone ultrafiltration membranes Yuxin Ma a,b,⁎, Fengmei Shi c, Jun Ma c, Miaonan Wu a, Jun Zhang a, Congjie Gao b a b c
College of Architecture and Civil Engineering, Heilongjiang University, Harbin 150080, PR China College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, PR China State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, PR China
a r t i c l e
i n f o
Article history: Received 4 November 2010 Received in revised form 23 December 2010 Accepted 23 December 2010 Available online 3 February 2011 Keywords: Phase inversion method Polyethylene glycol Polysulfone membrane Protein rejection Scanning electron microscopy
a b s t r a c t Flat sheet asymmetric polysulfone membranes were prepared by phase inversion method. Dimethyl acetamide was used as solvent and water was used as coagulant. Polyethylene glycol (PEG) of different molecular weights or different dosage of PEG 400 was used as the polymeric additives in the casting solution. The morphology of membranes was analyzed by scanning electron microscope and performance of membranes was evaluated in terms of pure water flux (PWF), protein rejection, porosity, contact angle, tensile strength, and elongation at break. Results show that PEG additive is a pore former and the addition of PEG additive can improve the hydrophilicity of membranes. PWF and porosity of membranes increase with increase in molecular weight of PEG or dosage of PEG 400. With increase in molecular weight of PEG from 400 to 20,000, the PWF increases from 340 L m− 2 h− 1 to 1390 L m− 2 h− 1. With increase in dosage of PEG 400 from 0% to 10%, the PWF increases from 0.81 L m− 2 h− 1 to 420 L m− 2 h− 1. The molecular weight cutoff (MWCO) of membrane keeps at 70,000 Da, and the pore size distribution of membrane skin layer becomes narrower. More PEG dosage weakens the mechanical properties of membrane; the tensile strength at break with PEG 1500 is maximum and the value is 5.62 MPa. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Phase inversion is one of the most important processes for preparing asymmetric polymer porous membranes such as microfiltration, ultrafiltration (UF), nanofiltration, reverse osmosis and supports for composite membrane [1–5]. It is not only a thermodynamic but also a kinetic process and plays an important role in membranes' structure formation. Thermodynamics determine whether a polymer solution is homogeneous and stable or inhomogeneous and likes to phase separate. The starting situation is normally a homogeneous solution, which is forced to phase separate by changing its composition when immersed in a non-solvent bath. Kinetic processes play a key role in this transition. The exchange process between non-solvent in coagulation bath and solvent in polymer dope which is determined by their concentrations governs the ultimate membrane structure [6]. Phase inversion involves conversion of a homogeneous polymer solution consisting of two or more components to a two-phase system, the solid polymer rich phase and the liquid polymer poor phase. The solid phase forms the membrane structure while the liquid phase forms the membrane pores. Reuvers et al. [7] developed a ⁎ Corresponding author. College of Architecture and Civil Engineering, Heilongjiang University, Harbin 150080, PR China. Tel./fax: +86 451 86604021. E-mail address: [email protected] (Y. Ma). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.12.054
model to explain instantaneous demixing or delayed demixing taking place during the phase inversion process. Membranes formed by instantaneous demixing generally show a highly porous substructure (with macrovoids) and a finely porous, thin skin layer. Membranes formed by a delayed demixing mechanism show a porous (often closed-cell, macrovoid-free) substructure with a dense, relatively thick skin layer. Structure and properties of membranes prepared by phase inversion method depend upon many factors. Additives are one of the major factors and play vital role in the formation of membrane structure by enlarging or preventing of macrovoid formation, enhancing pore formation, improving pore interconnectivity, and/or introducing hydrophilicity [8–13]. The frequently used additives are macromolecular such as polyvinylpyrrolidone (PVP), polyethylene glycols (PEG), polyethylene oxide (PEO), organic compounds such as glycerol, alcohols, dialcohols, inorganic salts such as LiCl and ZnCl2, and water. The additive can be a single component or a mixture. PEG as additive is less frequently used compared to PVP, but it could play a similar role in the formation process, acting as a macrovoid suppressor and giving the membrane a hydrophilic character. Xu et al. [14] showed by a reduction of the glass transition temperature of polyetherimide (PEI)/PEG membranes that PEG 600 remained inside the PEI matrix and therefore gave the membrane a hydrophilic character. A secondary role of the PEG could be a pore former and macrovoid suppressor.
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Kim and Lee [15] investigated the effect of various molecular weights of PEG on the formation of PEI asymmetric membranes and reported that small molecular weights of PEG such as PEG 200 and PEG 400 work as pore reducing agents for PEI membranes. Saljoughi et al. [16] found that low molecular weight PEG additive in the cast solution film can increase porosity/permeability and simultaneously thermal/chemical stability of the prepared cellulose acetate membranes. Shieh et al. [17] reported that PEG, being hydrophilic in nature, is used to improve membrane selectivity as well as a pore forming agent. Idris et al. [18] found that presence of PEG of different molecular weights exhibit significant effect on performance of polyethersulfone (PES) membranes. Chakrabarty et al. [19] investigated the effect of PEG of three different molecular weights (400 Da, 6000 Da, and 20,000 Da, respectively) on the formation of polysulfone (PSf) with N-methyl-2pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc) as solvents and found that PEG could be regarded as a pore former. With increase in molecular weight of PEG, the pore number as well as pore area in membranes increases. Membrane with PEG of higher molecular weight has higher pure water flux (PWF) and higher hydraulic permeability due to high porosity. Kim et al. [20] investigated the effect of PEG additive concentration and molecular weight on the structure formation of PSf membranes and their permeation properties connected with the changes of thermodynamic and kinetic properties by using PSf/NMP/PEG casting solution and water coagulant by phase inversion process. They found that PEG can be used as a pore former. With increase in molecular weight of PEG additive (600 Da, 2000 Da, 6000 Da, and 12,000 Da) or the ratio of PEG additive to NMP, the membrane dope becomes less stable, water flux of membrane increases, and solute rejection decreases. Arthanareeswaran et al. [21] studied the effect of PEG 600 on the ultrafiltration performance of the PSf/sulfonated poly(ether ether ketone) blend flat sheet membranes with N,N-dimethylformamide as a solvent and found that the addition of PEG 600 in the dope solution increased the exchange rate of additive and non-solvent during the membrane formation process, resulting in the appearance of the macrovoids formation; the PWF and equilibrium water content of the blend membranes were increased while hydraulic permeability was decreased. From the above literatures, there were a number of works reported using PEG of different molecular weights or different dosage to better the membrane properties or to study the membrane formation mechanism. In the work of Kim [20], PSf concentration was kept constant and PEG concentration and solvent concentration were kept as a whole (the remainder part). In fact, PEG is a weak non-solvent and cannot be regarded as a solvent. With the increase of PEG concentration, the solvent concentration decreases, while the ratio of polymer to solvent increases and affects the membrane formation process. It is necessary to study the effect of PEG concentration on morphology and performance of PSf membranes when it is added to a premixed PSf/solvent mixture. In the work of Chakrabarty [19], PSf concentration was kept at 12% with PEG of 3 different molecular weights (400 Da, 6000 Da, and 20,000 Da) used. In this work, PEG of 6 different molecular weights (400 Da, 800 Da, 1500 Da, 4000 Da, 10,000 Da, and 20,000 Da) was used as additives in 18% PSf and 82% DMAc system. In the present work, the variations of the morphology and the structure of the PSf membrane prepared by diffusion induced phase separation process were reported. PEG of 6 different molecular weights (400 Da, 800 Da, 1500 Da, 4000 Da, 10,000 Da, and 20,000 Da) was used as additives. Effects of molecular weight of PEG additive and PEG 400 dosage on morphology, the permeation characteristics, and mechanical properties of the prepared membrane were investigated in detail. Morphology of each membrane was analyzed by scanning electron microscopy (SEM). The performance of the membranes was investigated by water permeation and bovine serum albumin (BSA) and pepsin rejection behavior. Finally, mechanical properties were determined by tensile strength at break and elongation at break.
2. Experimental 2.1. Materials PSf (Udel P-3500, obtained from its manufacturer) was used as the base polymer in the membrane casting solution. DMAc (AP, Tianjin Bodi Chemicals Co., Ltd., PR China) was used as solvent. Poly(ethylene glycol) (PEG; CP, Tianjin Kemio Chemicals Co., Ltd., PR China) (average molecular weight 400 Da, 800 Da, 1500 Da, 4000 Da, 10,000 Da, and 20,000 Da) was used as the non-solvent pore forming additive in the casting solution. Deionized water was used as the main non-solvent in the coagulation bath. BSA (Mw = 67,000 Da) and pepsin (Mw = 35,000 Da) were used in the solute rejection test. 2.2. Membrane preparation Flat sheet PSf membranes were prepared by phase inversion method. Certain amount of PEG 400 or PEG with different Mw was added to a premixed 18% PSf/82% DMAc mixture and dissolved at 60 °C. The solution was magnetically stirred for at least 12 h to guarantee complete dissolution of the polymer. These prepared solutions were kept for at least 24 h without stirring at room temperature to remove air bubbles in the solution. The homogeneous casting solutions were cast uniformly onto a glass substrate by means of a hand-casting knife with a knife gap set at 200 nm and immediately immersed into a water bath. An overview of the experimental conditions was reported in Table 1. 2.3. Characterization of membranes 2.3.1. Permeation flux (PWF) and rejection (R) The permeation flux and rejection of the prepared membranes were measured by a UF cross flow filtration experimental setup fed with distilled water at a transmembrane pressure of 100 kPa after prepressurized for 30 min at 200 kPa. The schematic of UF cross flow filtration experimental setup is presented in Fig. 1. The permeation flux were defined as formula (1). PWF =
V A×t
ð1Þ
where PWF is the pure water flux (L m–2 h–1), V is the permeate volume (L), A is the membrane area (m2), and t is the time (h). Rejection was characterized with 200 mg/L BSA aqueous solution and 200 mg/L pepsin aqueous solution after the membrane was previously filtered with pure water until flux was steady. The concentrations of BSA and pepsin in permeate and feed were determined by a UV spectrophotometer (Shimadzu UV-2450; Japan). It was calculated according to formula (2). R = 1−
Cp Cf
ð2Þ
Table 1 Composition of the casting solution. Membranes
Mw of PEG
PEG⁎ (wt%)
DMAc (wt%)
PSf (wt%)
PSf PSf PSf PSf PSf PSf PSf PSf PSf PSf PSf
400 400 400 400 400 400 800 1500 4000 10,000 20,000
0 2 4 6 8 10 8 8 8 8 8
82 82 82 82 82 82 82 82 82 82 82
18 18 18 18 18 18 18 18 18 18 18
1-0 1-2 1-4 1-6 1-8 1-10 2-8 3-8 4-8 5-8 6-8
⁎ PEG additive dosage was based on mass of 18% PSf/82% DMAc mixture.
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of the cross-section photographs presented in this article is the skin layer of the membrane (that is to say the side of the membrane in direct contact with water in the coagulation step). 2.3.4. Tensile strength and elongation at break Tensile strength and elongation at break of membranes were determined by a universal electronic strength measurement (AGS-J; Shimadzu). Measurements were carried out at room temperature and a strain rate of 20 mm/min was employed. The reported values were the averages of at least eight samples. Membrane
2.4. Cloud point and ternary phase diagram determination
Permeat
1. Feed tank
2. Booster pump
5. Flat membrane cell
3. Buffering tank 6. Flow meter
4. Pressure gauge 7.Valve
Fig. 1. Schematic diagram of UF cross flow filtration experimental setup.
where Cp and Cf are the concentrations of protein in permeate and initial feeds, respectively. 2.3.2. Porosity (P) and contact angle (CA) Membrane porosity was measured in the method of dry–wet weight. The membrane maintained in distilled water was weighed after mopping superficial water with filter paper. Then, the wet membrane was placed in an air-circulating oven at 60 °C for 24 h and then further dried in a vacuum oven at 80 °C for 24 h before measuring the dry weight. From the two weights (wet sample weight and dry sample weight), the porosity of membrane was calculated using formula (3) as P ð%Þ =
Ww −Wd × 100 ρw × A × δ
ð3Þ
where P is the porosity of membrane, Ww is the wet sample weight (g), Wd is the dry sample weight (g), ρw is the density of pure water (g/cm3), A is the area of membrane in wet state (cm2), and δ is the thickness of membrane in wet state (cm). In order to minimize experimental error, each membrane was measured for three times and average was calculated. The contact angle measurements were carried out with a contact angle meter (DSA100; KRÜSS). A water droplet was placed onto a flat homogeneous membrane surface and the contact angle of the droplet with the surface was measured. The reported values were the averages of the contact angles of five droplets. 2.3.3. Scanning electron microscopy (SEM) The morphologies of the membranes were observed with a JEOL JEM-6700F SEM and a HITACHI S-3400N SEM. The observations were carried out on the cross-section and upper surface of membranes broken in liquid nitrogen and coated with gold by sputtering. The top
Cloud point determination method was reported in the study of Blanco [22]. In this study, PSf/DMAc mixtures were kept 100%; PEG additives dosage were based on the PSf/DMAc mixtures. DMAc solutions with different polymer contents (5, 10, 15, and 20 wt%) and different dosages of PEG 400 (0, 2, 4, and 8 wt%) or 8% PEG with different Mw (400, 4000, 10,000, and 20,000) were placed in tubes at a constant temperature (30 °C). Small volumes of a water/DMAc mixture (10/90 in weight) were added to the tubes until turbidity occurred (detected by visual observation). The tubes were heated to 70 °C to dissolve the formed phase then cooled down to 30 °C. The cloud point composition was calculated from the mass balance in the system corresponding to the added volume at which turbidity started to form upon cooling. The protocol was chosen because phases often separate locally at the spot where the non-solvent mixture hits the polymer solution; if the system becomes limpid after a heating–homogenization–cooling sequence, then the cloud point is not reached at the system composition and temperature was studied. Another volume of non-solvent mixture was added to the polymer dope and the temperature sequence was repeated until observation of a persistent turbidity. Such a protocol is valid for mixtures presenting an upper critical solution temperature, as those in the present study. 3. Results and discussion 3.1. Morphological study SEM analysis is an important technique to study the membrane morphology and qualitative information regarding surface and crosssectional morphology of the membranes can be obtained. Fig. 2 shows the SEM image of the cross-section of different membranes prepared with different PEG 400 dosage. It may be seen from Fig. 2 that membranes are having asymmetric structure consisting of a dense top surface layer (skin layer, air side), a porous sublayer (support layer), and a small portion of sponge-like bottom surface layer (glass side). The skin layer acts as a separation layer and the support layer provides the mechanical strength. The sublayer seems to have finger-like cavities beneath the top surface layer and large voids near the bottom surface layer. When the dosage of PEG 400 increases, the size and the
Fig. 2. Cross-section SEM images of PSf membrane prepared with different PEG 400 dosages.
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number of finger-like pores are found to increase. At the same time, the length of finger-like cavities is found shorter, but the large voids are found larger and prominent. It may be inferred from Fig. 2 that the PEG 400 was a pore former in the structure formation of membranes. The change in the size of pores due to the presence of PEG 400 in the polymer structure can be explained as follows. Due to high mutual affinity of DMAc for water, instantaneous demixing results in the formation of finger-like cavities in the sublayer of the prepared membranes [23]. The existence of PEG in the membrane casting solutions has two effects. (1) The dissolution of PEG consumes some of the solvent and leads to higher polymer concentration and higher viscosity of membrane casting solutions. The membrane dope becomes thermodynamically less stable, which results in rapid instantaneous demixing when the membrane dope is immersed into the coagulation bath. (2) PEG is hydrophilic in nature. The hydrophilicity of PEG in membrane dope affects the exchange rate of solvent and non-solvent during phase inversion process and influences the precipitation kinetics and the formation of resulting membrane morphology consequently. The changes in the phase border lines (binodal curves) in PSf/DMAc/ water system with different dosages of PEG 400 are shown in Fig. 3. Fig. 3 shows that, due to the hydrophilicity of PEG additives, the path taken by the polymer solution of fixed starting composition to reach the phase border line will be a little shorter for a more PEG 400 dosage sample. The longer the solvent exchange through the skin formed when the dope is immersed in water, the more developed the processes of polymer lean phase growth and coalescence, thus the larger the finger-like pores. Indeed, the result in Fig. 2 is contrary to it. This can be explained by the effect of PEG 400 on the exchange rate of solvent and non-solvent. In fact, the increase of dosage of PEG 400 will increase the inflow rate of water diffusion in the polymer solution film because of its hydrophilicity and leads to more larger finger-like pores. The result is same to the study of Arthanareeswaran. Fig. 4 shows the SEM image of the upper surface of different membranes prepared with different molecular weights of PEG. The surface of these membranes is very dense. The pores of these membranes are too small to be seen in 30,000 magnifications. The pore size of the UF membranes is usually characterized by their molecular weight cutoff (MWCO), a loosely defined term which is generally taken to mean the molecular weight of the globular protein molecule that is 90% rejected by the membranes. This type surface
0.20
PS
f
0.80
0.85
0.15
0.90
0.10
0.95
DM
Ac
0.05
1.00 0.00
0.05
0% PEG 400 2% PEG 400
0.10
0.00 0.15 Water 0.20
4% PEG 400 8% PEG 400
Fig. 3. Ternary phase diagram with cloud points for PSf/DMAc/water system with different PEG 400 dosages.
morphology may be due to the instantaneous demixing of membrane casting solution and rapid precipitation of polymer matrix. Fig. 5 shows the SEM image of the cross-section of different membranes prepared with different molecular weights of PEG. The structure and morphology changes are similar to the membranes prepared with different PEG 400 dosages. It may also be seen from Fig. 5 that the addition of PEG of different molecular weight causes significant enlargement of the finger-like cavities in the sublayer and large voids near the bottom surface. With increase in molecular weight of PEG, the macrovoids' structure in the membrane crosssection becomes more prominent. The changes in the phase border lines (binodal curves) in PSf/ DMAc/water system with different molecular weight of PEG are shown in Fig. 6. Although the effect of molecular weight of PEG is larger than the dosage of PEG 400, the principle of effect is same to that of PEG 400. The path taken by the polymer solution of fixed starting composition to reach the phase border line will be shorter for a sample with higher molecular weight PEG additive. Increase in molecular weight of PEG will increase the inflow rate of water diffusion in the polymer solution film because of its hydrophilicity and lower mobility and leads to larger finger-like pores. 3.2. Effect of PEG additives on membrane permeability and solute rejection Pure water flux (PWF) and solute rejection are considered to be the key specification factors for any membrane. PWF and solute rejection have a direct relationship with the number of pores and the pore size on the membrane surface (top layer porosity) [13]. The effect of PEG 400 dosage and molecular weight of PEG on PWF and protein rejection (R) are shown in the Figs. 7 and 8. It can be seen from Fig. 7 that PWF increases with increase in the dosage of PEG 400; BSA rejection has no obvious changes and pepsin rejection increases. When there is no PEG 400 addition, the PWF is 0.81 L m− 2 h− 1; the protein rejection is difficult to be determined because of lower flux of protein solution. When the dosage of PEG 400 is 10%, the PWF is 420 L m− 2 h− 1. The results illustrate that the top layer porosity increases with increase in the dosage of PEG 400 and the PEG 400 is a pore former. From the BSA rejection and pepsin rejection results, it can be inferred that the MWCO of membrane keeps at 70,000 Da, but the pore size distribution of membrane skin layer becomes narrower with increase in the dosage of PEG 400. The number of pores increases and the pore size becomes more uniform. The PWF result is same to the studies of Arthanareeswaran and Kim, but the rejection behavior is different. This may be because the PEG of lower Mw is easier to transport with the solvent in the phase separation process and results in the homogeneous skin layer and higher rejection. Another reason may be that PEG 600 used in their studies occupies part of solvent and increases the PSf/solvent ratio and affects the exchange process. The rapid exchange of solvent and non-solvent brings about increase of the volume fraction of polymer in the surface layer and spinodal decomposition occurs consequently. The formation of the top surface is possibly due to spinodal demixing of the casting solution by means of nucleation and growth of the polymer rich phase, i.e., the solid phase, and the demixing leads to much better interconnected pores [24,25]. Polymer lean phase forms the pores and the polymer rich phase forms the membrane matrix. When there is no PEG addition, polymer lean phase (consisting of DMAc and water) is fewer in the surface layer which leads to fewer pores and lower flux of membranes. More PEG addition will increase the ratio of polymer lean phase in the surface layer which leads to more pores formation and higher flux. When the PEG dosage attains certain value, a continuous polymer (PSf) lean phase is intertwined by a continuous polymer (PSf) rich phase which forms the membrane matrix and interconnected pores are formed and the distribution of pore size becomes much narrower. The formation of surface layer restrains the exchange of solvent in the sublayer and nonsolvent in the bath. Part of the PEG in polymer poor phase is dissolved
Y. Ma et al. / Desalination 272 (2011) 51–58
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Fig. 4. Upper surface SEM images of PSf membrane prepared with PEG of different molecular weights (8 wt% PEG dosage based on mass of 18% PSf/82% DMAc mixture).
into the non-solvent, which accelerates the exchange rate of nonsolvent and solvent. PEG in the polymer poor phase in the sublayer has a mutual function with DMAc which restrains the outflow of DMAc. The hydrophilicity of PEG facilitates the inflow of water, accelerates the precipitation of PSf in the sublayer, and results in significant enlargement of the finger-like cavities in the sublayer and formation of macrovoids near the bottom layer. With increase in PEG dosage, the inflow rate of water increases, more finger-like cavities and macrovoids are formed because more polymer lean phase exists, and the porosity of membrane is also increased. Fig. 8 shows that PWF increases with increase in molecular weight of PEG at the same dosage. PWF increases from 340 L m− 2 h− 1 for PEG 400 to 1390 L m− 2 h− 1 for PEG 20,000. BSA rejection and pepsin rejection decrease which means that the pore size of skin layer becomes larger. The addition of PEG with higher molecular weight leads to formation of macrovoids in the skin layer. The PWF and rejection results are same to the studies of Arthanareeswaran and Kim. The molecular weight of PEG might have a substantial role on the precipitation rate [26,27]. With increase in the molecular weight of PEG, more PEG molecules are permanently trapped in the membrane because of their lower mobility after immersion in the coagulation bath and contact angle decreases. At the same time, with increase in
molecular weight of PEG at the same dosage, the molecular quantity of PEG decreases. Polymer lean phase becomes less in quantity and larger in size which results in the formation of larger pores in the skin layer. It can be used to explain the increase in water flux and decrease in the protein rejection of membrane.
3.3. Effect of PEG additive on membrane hydrophilicity and porosity Hydrophilicity and porosity of the membrane are two important parameters in membrane permeation and separation process and have a close relationship with PWF and the morphology of membranes. The contact angle (CA) is often used to describe the surface hydrophilicity [28,29]. In general, membrane hydrophilicity is higher while its contact angle is smaller. The contact angle (CA) and porosity of membrane with different PEG 400 dosages and different molecular weights of PEG are shown in Figs. 9 and 10. It can be seen from Fig. 9 that, with increase in the dosage of PEG 400, contact angle decreases and porosity increases. The contact angle is 87.72° for pure PSf membrane and 79.83° for membrane with 10% PEG 400. This is mainly due to the entrapment of PEG 400 in the membrane which leads to the decrease of contact angle. Porosity is 18.69% for pure PSf membrane and 48.36% for PSf
Fig. 5. Cross-section SEM images of PSf membrane prepared with PEG of different molecular weights (8 wt% PEG dosage based on mass of 18% PSf/82% DMAc mixture).
Y. Ma et al. / Desalination 272 (2011) 51–58
0.20
0.85
Rejection (%)
f PS
0.80
0.15
0.90
0.10
90
1400
80
1200
70
1000 PWF
60
RPepsin
800
RBSA
50
600
40
400
30 0.95
200
20 100
1000
Ac
0.05
10000
DM
MwPEG
0.05
0.10
0.15
PEG 400 PEG 4000
Water
PEG 10000 PEG 20000
Fig. 6. Ternary phase diagram with cloud points for PSf/DMAc/water system with PEG of different molecular weights.
membrane with 10% PEG 400 dosage. The result is uniform to the PWF results and SEM results. Fig. 10 shows that, with increase in molecular weight of PEG, contact angle decreases while porosity increases. The contact angle and porosity are 80.24° and 45.16% for PEG 400 and 68.22° and 61.35% for PEG 20,000, respectively. When the molecular weight of PEG is larger than 4000, the increase of porosity and decrease of contact angle are very remarkable. It is mainly due to that high molecular weight additives are mostly entrapped in the membrane matrix because of their lower mobility in coagulation bath and the membrane hydrophilicity is improved consequently. On the other hand, the hydrophilicity of PEG entrapped in the membrane can facilitate the inflow rate of non-solvent and the porosity of membrane increases. In the polymer rich phase, the growth in concentration of PSf will profoundly increase the viscosity of this phase until vitrification occurs which is generally considered as the end of the structure formation process. At the moment of vitrification, the equilibrium composition has not yet been reached and part of the PEG molecules is permanently trapped in the PSf matrix. Through the residual presence of a certain (small) amount of PEG molecules, a membrane owns a hydrophilic character [9], that is, the entrapment of PEG improves the
500
90
400
80
300
PWF RPepsin
200
RBSA
70
100
hydrophilicity of membrane and more PEG addition and increase in the molecular weight of PEG lead to more residual of PEG and decrease of contact angle.
3.4. Effect of PEG additive on membrane mechanical properties Tensile strength and elongation at break are two important parameters to describe the mechanical properties of membranes. Tensile strength and elongation at break of membranes with different PEG 400 dosages and different molecular weights of PEG are shown in Figs. 11 and 12. More dosage of PEG 400 weakens the tensile strength at break of membrane because of the increasing porosity, but elongation at break increases from 28.2% for no PEG 400 dosage to 35.6% for 6% PEG 400 dosage and attains its maximum then decreases to 12.8% of 10% PEG 400 dosage. Elongation at break of membrane increases with increase in molecular weight of PEG, while the tensile strength at break increases from 4.48 MPa for PEG 400 to 5.62 MPa for PEG 1500 and attains the maximum then decreases to 3.16 MPa for PEG 20,000. This is may be due to that appropriate increase in Mw of PEG can suppress the formation of macrovoids and enhance the mechanical strength, but when the Mw of PEG is excessively high, the rapid increase of porosity arising from the increase in molecular weight of PEG addition may weaken the mechanical strength. PEG 1500 can be a suitable additive for making asymmetric membranes with a relatively higher mechanical strength. From above, it can be concluded that mechanical property change of membranes are mainly due to the structure change arising from the change of PEG dosage or molecular weight of PEG. In order to obtain 50
PWF (Lm-2h-1)
100
Fig. 8. Effect of PEG of different molecular weights on the pure water flux and protein rejection of PSf membrane (8 wt% PEG dosage based on mass of 18% PSf/82% DMAc mixture).
60 0 50
88
45 86
40 Porosity CA
35
84
30 25
82
20 80
15 0
2
4
6
8
CA ( o )
0.00
0.00 0.20
Porosity (%)
1.00
Rejection (%)
PWF (Lm-2h-1)
56
10
wPEG 400 (wt%) Fig. 7. Effect of PEG 400 dosage on the pure water flux and protein rejection of PSf membrane.
0
2
4
6
8
10
wPEG 400 (wt%) Fig. 9. Contact angle (CA) and porosity of PSf membrane with different PEG 400 dosages.
Y. Ma et al. / Desalination 272 (2011) 51–58
82
6 20
80 78
58 Porosity CA
56
76
54
74
52
CA ( o )
Porosity (%)
60
72
50 48
70
46
16 4
100
1000
3 100
10000
14 Tensile strength Elongation
68
44
18
5
1000
Elongation (%)
62
Tensile strength (MPa)
64
57
12 10000
MwPEG
MwPEG
Fig. 10. Contact angle (CA) and porosity of PSf membrane with PEG of different molecular weights (8 wt% PEG dosage based on mass of 18% PSf/82% DMAc mixture).
Fig. 12. Tensile strength and elongation at break of PSf membrane with PEG of different molecular weights (8 wt% PEG dosage based on mass of 18% PSf/82% DMAc mixture).
membranes with better mechanical properties, the dosage and molecular weight of PEG should be taken into account.
• In order to obtain membranes with better mechanical properties, PEG dosage and the molecular weight of PEG should be considered. More PEG dosage weakens the mechanical properties of membranes because of the increasing porosity. PEG 1500 can be a suitable additive for making asymmetric membranes having a relatively higher mechanical strength.
4. Conclusion Flat sheet PSf membranes were prepared from casting solutions containing 18 wt% of PSf and 82% of DMAc; using diffusion induced phase separation process. Polyethylene glycol (PEG) of 6 different molecular weights (400 Da, 800 Da, 1500 Da, 4000 Da, 10,000 Da, and 20,000 Da, respectively) or different dosages of PEG 400 was used as additives. Effects of PEG 400 dosage and molecular weight of PEG on the morphology and properties such as porosity, contact angle, and mechanical properties were studied. The permeation performance of the prepared membranes was also evaluated in terms of PWF and rejection efficiency of BSA and pepsin protein. The results can be summarized as follows:
7.0
40
6.5
35
6.0
30
5.5
25
5.0
20
4.5
Elongation (%)
Tensile strength (MPa)
• All the membranes are found to have asymmetric structure as seen from SEM photographs. • The PWF is seen to be enhanced greatly with increase in molecular weight of PEG or dosage of PEG 400 due to high porosity of membranes. Increase in dosage of PEG 400 does not change the MWCO of membranes but can make the pore size distribution of membrane skin layer narrower. So, PEG could be regarded as a pore forming agent rather than a pore reducing agent. • The addition of PEG additive can improve the hydrophilicity of membranes. The hydrophilicity of membranes depends on the PEG dosage and the molecular weight of PEG additive.
15
Tensile strength Elongation
10
4.0 2
4
6
8
10
wPEG 400 (wt%) Fig. 11. Tensile strength and elongation at break of PSf membrane with different PEG 400 dosages.
Acknowledgment The authors are grateful for the financial support by the National Natural Science Foundation of China (grant nos. 50978067 and 50778049), Heilongjiang Postdoctoral Foundation and Shandong Postdoctoral Foundation. References [1] E. Staude, L. Breitbach, Polysulfones and their derivatives: materials for membranes for different separation operations, J. Appl. Polym. Sci. 43 (1991) 559–566. [2] T.S. Chung, K.-C. Loh, H.L. Tay, Development of polysulfone membranes for bacteria immobilization to remove phenol, J. Appl. Polym. Sci. 70 (1998) 2585–2594. [3] Y. Pouliot, M.C. Wijers, S.F. Gauthier, L. Nadeau, Fractionation of whey protein hydrolysates using charged UF/NF membranes, J. Membr. Sci. 158 (1999) 105–114. [4] Y. Xu, R.E. Lebrun, P.-J. Gallo, P. Blond, Treatment of textile dye plant effluent by nanofiltration membrane, Sep. Sci. Technol. 34 (13) (1999) 2501–2519. [5] K. Kosutic, L. Kastelan-Kunst, B. Kunst, Porosity of some commercial reverse osmosis and nanofiltration polyamide thin-film composite membranes, J. Membr. Sci. 168 (2000) 101–108. [6] J.G. Wijmans, J.P.B. Baaij, C.A. Smolders, The mechanism of formation of microporous or skinned membranes produced by the immersion precipitation process, J. Membr. Sci. 14 (1983) 263. [7] A.J. Reuvers, J.W.A. van den Berg, C.A. Smolders, Formation of membranes by means of immersion precipitation. Part I A. Model to describe mass transfer during immersion precipitation, J. Membr. Sci. 34 (1987) 45–65. [8] R.M. Boom, I.M. Wienk, Th. Van den Boomgaard, C.A. Smolders, Microstructures in phase inversion membranes. Part 2. The role of a polymeric additive, J. Membr. Sci. 73 (1992) 277–292. [9] I.M. Wienk, R.M. Boom, M.A.M. Beerlage, A.M.W. Bulte, C.A. Smolders, Recent advances in the formation of phase inversion membranes made from amorphous or semi-crystalline polymers, J. Membr. Sci. 113 (1996) 361–371. [10] D.B. Mosqueda-Jimenez, R.M. Narbaitz, T. Matsuura, G. Chowdhury, G. Pleizier, J.P. Santerre, Influence of processing conditions on the properties of ultrafiltration membranes, J. Membr. Sci. 231 (2004) 209–224. [11] H.T. Yeo, S.T. Lee, M.J. Han, Role of polymer additive in casting solution in preparation of phase inversion polysulfone membranes, J. Chem. Eng. Jpn. 33 (2000) 180–185. [12] B. Jung, J.K. Yoon, B. Kim, H.W. Rhee, Effect of molecular weight of polymeric additives on formation, permeation properties and hypochlorite treatment of asymmetric polyacrylonitrile membranes, J. Membr. Sci. 243 (2004) 45–57. [13] M.J. Han, S.T. Nam, Thermodynamic and rheological variation in polysulfone solution by PVP and its effect in the preparation of phase inversion membrane, J. Membr. Sci. 202 (2002) 55–61. [14] Z.-L. Xu, T.-S. Chung, K.-C. Loh, B.C. Lee, Polymeric asymmetric membranes made from polyetherimide/polybenzimidazole/poly(ethylene glycol) (PEI/PBI/PEG) for oil–surfactant–water separation, J. Membr. Sci. 158 (1999) 41–53. [15] I.C. Kim, K.H. Lee, Effect of poly(ethylene glycol) 200 on the formation of a polyetherimide asymmetric membrane and its performance in aqueous solvent mixture permeation, J. Membr. Sci. 230 (2004) 183–188.
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[16] E. Saljoughi, M. Amirilargani, T. Mohammadi, Effect of PEG additive and coagulation bath temperature on the morphology, permeability and thermal/ chemical stability of asymmetric CA membranes, Desalination 262 (2010) 72–78. [17] J.J. Shieh, T.S. Chung, R. Wang, M.P. Srinivasan, D.R. Paul, Gas separation performance of poly(4-vinylpyridine)/polyetherimide composite hollow fibers, J. Membr. Sci. 182 (2001) 111–123. [18] A. Idris, N.M. Zain, M.Y. Noordin, Synthesis, characterization and performance of asymmetric polyethersulfone (PES) ultrafiltration membranes with polyethylene glycol of different molecular weights as additives, Desalination 207 (2007) 324–339. [19] B. Chakrabarty, A.K. Ghoshal, M.K. Purkait, Effect of molecular weight of PEG on membrane morphology and transport properties, J. Membr. Sci. 309 (2008) 209–221. [20] J.H. Kim, K.H. Lee, Effect of PEG additive on membrane formation by phase inversion, J. Membr. Sci. 138 (1998) 153–163. [21] G. Arthanareeswaran, D. Mohan, M. Raajenthiren, Preparation, characterization and performance studies of ultrafiltration, membranes with polymeric additive, J. Membr. Sci. 350 (2010) 130–138. [22] J.-F. Blanco, J. Sublet, Q.T. Nguyen, P. Schaetzel, Formation and morphology studies of different polysulfones-based membranes made by wet phase inversion process, J. Membr. Sci. 283 (2006) 27–37.
[23] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, Dordrecht, 1991. [24] A.J. Reuvers, C.A. Smolders, Formation of membranes by means of immersion precipitation. Part II, The mechanism of formation of membranes prepared from the system cellulose acetate–acetone–water, J. Membr. Sci. 34 (1987) 67–86. [25] K. Kimmerle, H. Strathmann, Analysis of the structure-determining process of phase inversion membranes, Desalination 79 (1990) 283–302. [26] H. Strathmann, K. Kock, P. Amar, R.W. Baker, The formation mechanism of asymmetric membranes, Desalination 16 (1975) 179–203. [27] R. Malaisamy, D.R. Mohan, M. Rajendran, Polyurethane and sulfonated polysulfone blend ultrafiltration membranes, J. Colloid Interface Sci. 254 (2002) 129–140. [28] J.J.T.F. Keurentjes, J.G. Harbrecht, D. Brinkman, J.H. Hanemaajer, M.A. Cohen Stuart, H. Van't Riet, Hydrophobicity measurements of MF and UF membranes, J. Membr. Sci. 47 (1989) 333–337. [29] L. Palacio, J.I. Calvo, P. Pradanos, A. Hernandez, P. Väisänen, M. Nyström, Contact angles and external protein adsorption onto ultrafiltration membranes, J. Membr. Sci. 152 (1999) 189–201.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Effect of PEG additive on the morphology and performance of polysulfone ultrafiltration membranes Yuxin Ma a,b,⁎, Fengmei Shi c, Jun Ma c, Miaonan Wu a, Jun Zhang a, Congjie Gao b a b c
College of Architecture and Civil Engineering, Heilongjiang University, Harbin 150080, PR China College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, PR China State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, PR China
a r t i c l e
i n f o
Article history: Received 4 November 2010 Received in revised form 23 December 2010 Accepted 23 December 2010 Available online 3 February 2011 Keywords: Phase inversion method Polyethylene glycol Polysulfone membrane Protein rejection Scanning electron microscopy
a b s t r a c t Flat sheet asymmetric polysulfone membranes were prepared by phase inversion method. Dimethyl acetamide was used as solvent and water was used as coagulant. Polyethylene glycol (PEG) of different molecular weights or different dosage of PEG 400 was used as the polymeric additives in the casting solution. The morphology of membranes was analyzed by scanning electron microscope and performance of membranes was evaluated in terms of pure water flux (PWF), protein rejection, porosity, contact angle, tensile strength, and elongation at break. Results show that PEG additive is a pore former and the addition of PEG additive can improve the hydrophilicity of membranes. PWF and porosity of membranes increase with increase in molecular weight of PEG or dosage of PEG 400. With increase in molecular weight of PEG from 400 to 20,000, the PWF increases from 340 L m− 2 h− 1 to 1390 L m− 2 h− 1. With increase in dosage of PEG 400 from 0% to 10%, the PWF increases from 0.81 L m− 2 h− 1 to 420 L m− 2 h− 1. The molecular weight cutoff (MWCO) of membrane keeps at 70,000 Da, and the pore size distribution of membrane skin layer becomes narrower. More PEG dosage weakens the mechanical properties of membrane; the tensile strength at break with PEG 1500 is maximum and the value is 5.62 MPa. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Phase inversion is one of the most important processes for preparing asymmetric polymer porous membranes such as microfiltration, ultrafiltration (UF), nanofiltration, reverse osmosis and supports for composite membrane [1–5]. It is not only a thermodynamic but also a kinetic process and plays an important role in membranes' structure formation. Thermodynamics determine whether a polymer solution is homogeneous and stable or inhomogeneous and likes to phase separate. The starting situation is normally a homogeneous solution, which is forced to phase separate by changing its composition when immersed in a non-solvent bath. Kinetic processes play a key role in this transition. The exchange process between non-solvent in coagulation bath and solvent in polymer dope which is determined by their concentrations governs the ultimate membrane structure [6]. Phase inversion involves conversion of a homogeneous polymer solution consisting of two or more components to a two-phase system, the solid polymer rich phase and the liquid polymer poor phase. The solid phase forms the membrane structure while the liquid phase forms the membrane pores. Reuvers et al. [7] developed a ⁎ Corresponding author. College of Architecture and Civil Engineering, Heilongjiang University, Harbin 150080, PR China. Tel./fax: +86 451 86604021. E-mail address: [email protected] (Y. Ma). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.12.054
model to explain instantaneous demixing or delayed demixing taking place during the phase inversion process. Membranes formed by instantaneous demixing generally show a highly porous substructure (with macrovoids) and a finely porous, thin skin layer. Membranes formed by a delayed demixing mechanism show a porous (often closed-cell, macrovoid-free) substructure with a dense, relatively thick skin layer. Structure and properties of membranes prepared by phase inversion method depend upon many factors. Additives are one of the major factors and play vital role in the formation of membrane structure by enlarging or preventing of macrovoid formation, enhancing pore formation, improving pore interconnectivity, and/or introducing hydrophilicity [8–13]. The frequently used additives are macromolecular such as polyvinylpyrrolidone (PVP), polyethylene glycols (PEG), polyethylene oxide (PEO), organic compounds such as glycerol, alcohols, dialcohols, inorganic salts such as LiCl and ZnCl2, and water. The additive can be a single component or a mixture. PEG as additive is less frequently used compared to PVP, but it could play a similar role in the formation process, acting as a macrovoid suppressor and giving the membrane a hydrophilic character. Xu et al. [14] showed by a reduction of the glass transition temperature of polyetherimide (PEI)/PEG membranes that PEG 600 remained inside the PEI matrix and therefore gave the membrane a hydrophilic character. A secondary role of the PEG could be a pore former and macrovoid suppressor.
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Kim and Lee [15] investigated the effect of various molecular weights of PEG on the formation of PEI asymmetric membranes and reported that small molecular weights of PEG such as PEG 200 and PEG 400 work as pore reducing agents for PEI membranes. Saljoughi et al. [16] found that low molecular weight PEG additive in the cast solution film can increase porosity/permeability and simultaneously thermal/chemical stability of the prepared cellulose acetate membranes. Shieh et al. [17] reported that PEG, being hydrophilic in nature, is used to improve membrane selectivity as well as a pore forming agent. Idris et al. [18] found that presence of PEG of different molecular weights exhibit significant effect on performance of polyethersulfone (PES) membranes. Chakrabarty et al. [19] investigated the effect of PEG of three different molecular weights (400 Da, 6000 Da, and 20,000 Da, respectively) on the formation of polysulfone (PSf) with N-methyl-2pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc) as solvents and found that PEG could be regarded as a pore former. With increase in molecular weight of PEG, the pore number as well as pore area in membranes increases. Membrane with PEG of higher molecular weight has higher pure water flux (PWF) and higher hydraulic permeability due to high porosity. Kim et al. [20] investigated the effect of PEG additive concentration and molecular weight on the structure formation of PSf membranes and their permeation properties connected with the changes of thermodynamic and kinetic properties by using PSf/NMP/PEG casting solution and water coagulant by phase inversion process. They found that PEG can be used as a pore former. With increase in molecular weight of PEG additive (600 Da, 2000 Da, 6000 Da, and 12,000 Da) or the ratio of PEG additive to NMP, the membrane dope becomes less stable, water flux of membrane increases, and solute rejection decreases. Arthanareeswaran et al. [21] studied the effect of PEG 600 on the ultrafiltration performance of the PSf/sulfonated poly(ether ether ketone) blend flat sheet membranes with N,N-dimethylformamide as a solvent and found that the addition of PEG 600 in the dope solution increased the exchange rate of additive and non-solvent during the membrane formation process, resulting in the appearance of the macrovoids formation; the PWF and equilibrium water content of the blend membranes were increased while hydraulic permeability was decreased. From the above literatures, there were a number of works reported using PEG of different molecular weights or different dosage to better the membrane properties or to study the membrane formation mechanism. In the work of Kim [20], PSf concentration was kept constant and PEG concentration and solvent concentration were kept as a whole (the remainder part). In fact, PEG is a weak non-solvent and cannot be regarded as a solvent. With the increase of PEG concentration, the solvent concentration decreases, while the ratio of polymer to solvent increases and affects the membrane formation process. It is necessary to study the effect of PEG concentration on morphology and performance of PSf membranes when it is added to a premixed PSf/solvent mixture. In the work of Chakrabarty [19], PSf concentration was kept at 12% with PEG of 3 different molecular weights (400 Da, 6000 Da, and 20,000 Da) used. In this work, PEG of 6 different molecular weights (400 Da, 800 Da, 1500 Da, 4000 Da, 10,000 Da, and 20,000 Da) was used as additives in 18% PSf and 82% DMAc system. In the present work, the variations of the morphology and the structure of the PSf membrane prepared by diffusion induced phase separation process were reported. PEG of 6 different molecular weights (400 Da, 800 Da, 1500 Da, 4000 Da, 10,000 Da, and 20,000 Da) was used as additives. Effects of molecular weight of PEG additive and PEG 400 dosage on morphology, the permeation characteristics, and mechanical properties of the prepared membrane were investigated in detail. Morphology of each membrane was analyzed by scanning electron microscopy (SEM). The performance of the membranes was investigated by water permeation and bovine serum albumin (BSA) and pepsin rejection behavior. Finally, mechanical properties were determined by tensile strength at break and elongation at break.
2. Experimental 2.1. Materials PSf (Udel P-3500, obtained from its manufacturer) was used as the base polymer in the membrane casting solution. DMAc (AP, Tianjin Bodi Chemicals Co., Ltd., PR China) was used as solvent. Poly(ethylene glycol) (PEG; CP, Tianjin Kemio Chemicals Co., Ltd., PR China) (average molecular weight 400 Da, 800 Da, 1500 Da, 4000 Da, 10,000 Da, and 20,000 Da) was used as the non-solvent pore forming additive in the casting solution. Deionized water was used as the main non-solvent in the coagulation bath. BSA (Mw = 67,000 Da) and pepsin (Mw = 35,000 Da) were used in the solute rejection test. 2.2. Membrane preparation Flat sheet PSf membranes were prepared by phase inversion method. Certain amount of PEG 400 or PEG with different Mw was added to a premixed 18% PSf/82% DMAc mixture and dissolved at 60 °C. The solution was magnetically stirred for at least 12 h to guarantee complete dissolution of the polymer. These prepared solutions were kept for at least 24 h without stirring at room temperature to remove air bubbles in the solution. The homogeneous casting solutions were cast uniformly onto a glass substrate by means of a hand-casting knife with a knife gap set at 200 nm and immediately immersed into a water bath. An overview of the experimental conditions was reported in Table 1. 2.3. Characterization of membranes 2.3.1. Permeation flux (PWF) and rejection (R) The permeation flux and rejection of the prepared membranes were measured by a UF cross flow filtration experimental setup fed with distilled water at a transmembrane pressure of 100 kPa after prepressurized for 30 min at 200 kPa. The schematic of UF cross flow filtration experimental setup is presented in Fig. 1. The permeation flux were defined as formula (1). PWF =
V A×t
ð1Þ
where PWF is the pure water flux (L m–2 h–1), V is the permeate volume (L), A is the membrane area (m2), and t is the time (h). Rejection was characterized with 200 mg/L BSA aqueous solution and 200 mg/L pepsin aqueous solution after the membrane was previously filtered with pure water until flux was steady. The concentrations of BSA and pepsin in permeate and feed were determined by a UV spectrophotometer (Shimadzu UV-2450; Japan). It was calculated according to formula (2). R = 1−
Cp Cf
ð2Þ
Table 1 Composition of the casting solution. Membranes
Mw of PEG
PEG⁎ (wt%)
DMAc (wt%)
PSf (wt%)
PSf PSf PSf PSf PSf PSf PSf PSf PSf PSf PSf
400 400 400 400 400 400 800 1500 4000 10,000 20,000
0 2 4 6 8 10 8 8 8 8 8
82 82 82 82 82 82 82 82 82 82 82
18 18 18 18 18 18 18 18 18 18 18
1-0 1-2 1-4 1-6 1-8 1-10 2-8 3-8 4-8 5-8 6-8
⁎ PEG additive dosage was based on mass of 18% PSf/82% DMAc mixture.
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of the cross-section photographs presented in this article is the skin layer of the membrane (that is to say the side of the membrane in direct contact with water in the coagulation step). 2.3.4. Tensile strength and elongation at break Tensile strength and elongation at break of membranes were determined by a universal electronic strength measurement (AGS-J; Shimadzu). Measurements were carried out at room temperature and a strain rate of 20 mm/min was employed. The reported values were the averages of at least eight samples. Membrane
2.4. Cloud point and ternary phase diagram determination
Permeat
1. Feed tank
2. Booster pump
5. Flat membrane cell
3. Buffering tank 6. Flow meter
4. Pressure gauge 7.Valve
Fig. 1. Schematic diagram of UF cross flow filtration experimental setup.
where Cp and Cf are the concentrations of protein in permeate and initial feeds, respectively. 2.3.2. Porosity (P) and contact angle (CA) Membrane porosity was measured in the method of dry–wet weight. The membrane maintained in distilled water was weighed after mopping superficial water with filter paper. Then, the wet membrane was placed in an air-circulating oven at 60 °C for 24 h and then further dried in a vacuum oven at 80 °C for 24 h before measuring the dry weight. From the two weights (wet sample weight and dry sample weight), the porosity of membrane was calculated using formula (3) as P ð%Þ =
Ww −Wd × 100 ρw × A × δ
ð3Þ
where P is the porosity of membrane, Ww is the wet sample weight (g), Wd is the dry sample weight (g), ρw is the density of pure water (g/cm3), A is the area of membrane in wet state (cm2), and δ is the thickness of membrane in wet state (cm). In order to minimize experimental error, each membrane was measured for three times and average was calculated. The contact angle measurements were carried out with a contact angle meter (DSA100; KRÜSS). A water droplet was placed onto a flat homogeneous membrane surface and the contact angle of the droplet with the surface was measured. The reported values were the averages of the contact angles of five droplets. 2.3.3. Scanning electron microscopy (SEM) The morphologies of the membranes were observed with a JEOL JEM-6700F SEM and a HITACHI S-3400N SEM. The observations were carried out on the cross-section and upper surface of membranes broken in liquid nitrogen and coated with gold by sputtering. The top
Cloud point determination method was reported in the study of Blanco [22]. In this study, PSf/DMAc mixtures were kept 100%; PEG additives dosage were based on the PSf/DMAc mixtures. DMAc solutions with different polymer contents (5, 10, 15, and 20 wt%) and different dosages of PEG 400 (0, 2, 4, and 8 wt%) or 8% PEG with different Mw (400, 4000, 10,000, and 20,000) were placed in tubes at a constant temperature (30 °C). Small volumes of a water/DMAc mixture (10/90 in weight) were added to the tubes until turbidity occurred (detected by visual observation). The tubes were heated to 70 °C to dissolve the formed phase then cooled down to 30 °C. The cloud point composition was calculated from the mass balance in the system corresponding to the added volume at which turbidity started to form upon cooling. The protocol was chosen because phases often separate locally at the spot where the non-solvent mixture hits the polymer solution; if the system becomes limpid after a heating–homogenization–cooling sequence, then the cloud point is not reached at the system composition and temperature was studied. Another volume of non-solvent mixture was added to the polymer dope and the temperature sequence was repeated until observation of a persistent turbidity. Such a protocol is valid for mixtures presenting an upper critical solution temperature, as those in the present study. 3. Results and discussion 3.1. Morphological study SEM analysis is an important technique to study the membrane morphology and qualitative information regarding surface and crosssectional morphology of the membranes can be obtained. Fig. 2 shows the SEM image of the cross-section of different membranes prepared with different PEG 400 dosage. It may be seen from Fig. 2 that membranes are having asymmetric structure consisting of a dense top surface layer (skin layer, air side), a porous sublayer (support layer), and a small portion of sponge-like bottom surface layer (glass side). The skin layer acts as a separation layer and the support layer provides the mechanical strength. The sublayer seems to have finger-like cavities beneath the top surface layer and large voids near the bottom surface layer. When the dosage of PEG 400 increases, the size and the
Fig. 2. Cross-section SEM images of PSf membrane prepared with different PEG 400 dosages.
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number of finger-like pores are found to increase. At the same time, the length of finger-like cavities is found shorter, but the large voids are found larger and prominent. It may be inferred from Fig. 2 that the PEG 400 was a pore former in the structure formation of membranes. The change in the size of pores due to the presence of PEG 400 in the polymer structure can be explained as follows. Due to high mutual affinity of DMAc for water, instantaneous demixing results in the formation of finger-like cavities in the sublayer of the prepared membranes [23]. The existence of PEG in the membrane casting solutions has two effects. (1) The dissolution of PEG consumes some of the solvent and leads to higher polymer concentration and higher viscosity of membrane casting solutions. The membrane dope becomes thermodynamically less stable, which results in rapid instantaneous demixing when the membrane dope is immersed into the coagulation bath. (2) PEG is hydrophilic in nature. The hydrophilicity of PEG in membrane dope affects the exchange rate of solvent and non-solvent during phase inversion process and influences the precipitation kinetics and the formation of resulting membrane morphology consequently. The changes in the phase border lines (binodal curves) in PSf/DMAc/ water system with different dosages of PEG 400 are shown in Fig. 3. Fig. 3 shows that, due to the hydrophilicity of PEG additives, the path taken by the polymer solution of fixed starting composition to reach the phase border line will be a little shorter for a more PEG 400 dosage sample. The longer the solvent exchange through the skin formed when the dope is immersed in water, the more developed the processes of polymer lean phase growth and coalescence, thus the larger the finger-like pores. Indeed, the result in Fig. 2 is contrary to it. This can be explained by the effect of PEG 400 on the exchange rate of solvent and non-solvent. In fact, the increase of dosage of PEG 400 will increase the inflow rate of water diffusion in the polymer solution film because of its hydrophilicity and leads to more larger finger-like pores. The result is same to the study of Arthanareeswaran. Fig. 4 shows the SEM image of the upper surface of different membranes prepared with different molecular weights of PEG. The surface of these membranes is very dense. The pores of these membranes are too small to be seen in 30,000 magnifications. The pore size of the UF membranes is usually characterized by their molecular weight cutoff (MWCO), a loosely defined term which is generally taken to mean the molecular weight of the globular protein molecule that is 90% rejected by the membranes. This type surface
0.20
PS
f
0.80
0.85
0.15
0.90
0.10
0.95
DM
Ac
0.05
1.00 0.00
0.05
0% PEG 400 2% PEG 400
0.10
0.00 0.15 Water 0.20
4% PEG 400 8% PEG 400
Fig. 3. Ternary phase diagram with cloud points for PSf/DMAc/water system with different PEG 400 dosages.
morphology may be due to the instantaneous demixing of membrane casting solution and rapid precipitation of polymer matrix. Fig. 5 shows the SEM image of the cross-section of different membranes prepared with different molecular weights of PEG. The structure and morphology changes are similar to the membranes prepared with different PEG 400 dosages. It may also be seen from Fig. 5 that the addition of PEG of different molecular weight causes significant enlargement of the finger-like cavities in the sublayer and large voids near the bottom surface. With increase in molecular weight of PEG, the macrovoids' structure in the membrane crosssection becomes more prominent. The changes in the phase border lines (binodal curves) in PSf/ DMAc/water system with different molecular weight of PEG are shown in Fig. 6. Although the effect of molecular weight of PEG is larger than the dosage of PEG 400, the principle of effect is same to that of PEG 400. The path taken by the polymer solution of fixed starting composition to reach the phase border line will be shorter for a sample with higher molecular weight PEG additive. Increase in molecular weight of PEG will increase the inflow rate of water diffusion in the polymer solution film because of its hydrophilicity and lower mobility and leads to larger finger-like pores. 3.2. Effect of PEG additives on membrane permeability and solute rejection Pure water flux (PWF) and solute rejection are considered to be the key specification factors for any membrane. PWF and solute rejection have a direct relationship with the number of pores and the pore size on the membrane surface (top layer porosity) [13]. The effect of PEG 400 dosage and molecular weight of PEG on PWF and protein rejection (R) are shown in the Figs. 7 and 8. It can be seen from Fig. 7 that PWF increases with increase in the dosage of PEG 400; BSA rejection has no obvious changes and pepsin rejection increases. When there is no PEG 400 addition, the PWF is 0.81 L m− 2 h− 1; the protein rejection is difficult to be determined because of lower flux of protein solution. When the dosage of PEG 400 is 10%, the PWF is 420 L m− 2 h− 1. The results illustrate that the top layer porosity increases with increase in the dosage of PEG 400 and the PEG 400 is a pore former. From the BSA rejection and pepsin rejection results, it can be inferred that the MWCO of membrane keeps at 70,000 Da, but the pore size distribution of membrane skin layer becomes narrower with increase in the dosage of PEG 400. The number of pores increases and the pore size becomes more uniform. The PWF result is same to the studies of Arthanareeswaran and Kim, but the rejection behavior is different. This may be because the PEG of lower Mw is easier to transport with the solvent in the phase separation process and results in the homogeneous skin layer and higher rejection. Another reason may be that PEG 600 used in their studies occupies part of solvent and increases the PSf/solvent ratio and affects the exchange process. The rapid exchange of solvent and non-solvent brings about increase of the volume fraction of polymer in the surface layer and spinodal decomposition occurs consequently. The formation of the top surface is possibly due to spinodal demixing of the casting solution by means of nucleation and growth of the polymer rich phase, i.e., the solid phase, and the demixing leads to much better interconnected pores [24,25]. Polymer lean phase forms the pores and the polymer rich phase forms the membrane matrix. When there is no PEG addition, polymer lean phase (consisting of DMAc and water) is fewer in the surface layer which leads to fewer pores and lower flux of membranes. More PEG addition will increase the ratio of polymer lean phase in the surface layer which leads to more pores formation and higher flux. When the PEG dosage attains certain value, a continuous polymer (PSf) lean phase is intertwined by a continuous polymer (PSf) rich phase which forms the membrane matrix and interconnected pores are formed and the distribution of pore size becomes much narrower. The formation of surface layer restrains the exchange of solvent in the sublayer and nonsolvent in the bath. Part of the PEG in polymer poor phase is dissolved
Y. Ma et al. / Desalination 272 (2011) 51–58
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Fig. 4. Upper surface SEM images of PSf membrane prepared with PEG of different molecular weights (8 wt% PEG dosage based on mass of 18% PSf/82% DMAc mixture).
into the non-solvent, which accelerates the exchange rate of nonsolvent and solvent. PEG in the polymer poor phase in the sublayer has a mutual function with DMAc which restrains the outflow of DMAc. The hydrophilicity of PEG facilitates the inflow of water, accelerates the precipitation of PSf in the sublayer, and results in significant enlargement of the finger-like cavities in the sublayer and formation of macrovoids near the bottom layer. With increase in PEG dosage, the inflow rate of water increases, more finger-like cavities and macrovoids are formed because more polymer lean phase exists, and the porosity of membrane is also increased. Fig. 8 shows that PWF increases with increase in molecular weight of PEG at the same dosage. PWF increases from 340 L m− 2 h− 1 for PEG 400 to 1390 L m− 2 h− 1 for PEG 20,000. BSA rejection and pepsin rejection decrease which means that the pore size of skin layer becomes larger. The addition of PEG with higher molecular weight leads to formation of macrovoids in the skin layer. The PWF and rejection results are same to the studies of Arthanareeswaran and Kim. The molecular weight of PEG might have a substantial role on the precipitation rate [26,27]. With increase in the molecular weight of PEG, more PEG molecules are permanently trapped in the membrane because of their lower mobility after immersion in the coagulation bath and contact angle decreases. At the same time, with increase in
molecular weight of PEG at the same dosage, the molecular quantity of PEG decreases. Polymer lean phase becomes less in quantity and larger in size which results in the formation of larger pores in the skin layer. It can be used to explain the increase in water flux and decrease in the protein rejection of membrane.
3.3. Effect of PEG additive on membrane hydrophilicity and porosity Hydrophilicity and porosity of the membrane are two important parameters in membrane permeation and separation process and have a close relationship with PWF and the morphology of membranes. The contact angle (CA) is often used to describe the surface hydrophilicity [28,29]. In general, membrane hydrophilicity is higher while its contact angle is smaller. The contact angle (CA) and porosity of membrane with different PEG 400 dosages and different molecular weights of PEG are shown in Figs. 9 and 10. It can be seen from Fig. 9 that, with increase in the dosage of PEG 400, contact angle decreases and porosity increases. The contact angle is 87.72° for pure PSf membrane and 79.83° for membrane with 10% PEG 400. This is mainly due to the entrapment of PEG 400 in the membrane which leads to the decrease of contact angle. Porosity is 18.69% for pure PSf membrane and 48.36% for PSf
Fig. 5. Cross-section SEM images of PSf membrane prepared with PEG of different molecular weights (8 wt% PEG dosage based on mass of 18% PSf/82% DMAc mixture).
Y. Ma et al. / Desalination 272 (2011) 51–58
0.20
0.85
Rejection (%)
f PS
0.80
0.15
0.90
0.10
90
1400
80
1200
70
1000 PWF
60
RPepsin
800
RBSA
50
600
40
400
30 0.95
200
20 100
1000
Ac
0.05
10000
DM
MwPEG
0.05
0.10
0.15
PEG 400 PEG 4000
Water
PEG 10000 PEG 20000
Fig. 6. Ternary phase diagram with cloud points for PSf/DMAc/water system with PEG of different molecular weights.
membrane with 10% PEG 400 dosage. The result is uniform to the PWF results and SEM results. Fig. 10 shows that, with increase in molecular weight of PEG, contact angle decreases while porosity increases. The contact angle and porosity are 80.24° and 45.16% for PEG 400 and 68.22° and 61.35% for PEG 20,000, respectively. When the molecular weight of PEG is larger than 4000, the increase of porosity and decrease of contact angle are very remarkable. It is mainly due to that high molecular weight additives are mostly entrapped in the membrane matrix because of their lower mobility in coagulation bath and the membrane hydrophilicity is improved consequently. On the other hand, the hydrophilicity of PEG entrapped in the membrane can facilitate the inflow rate of non-solvent and the porosity of membrane increases. In the polymer rich phase, the growth in concentration of PSf will profoundly increase the viscosity of this phase until vitrification occurs which is generally considered as the end of the structure formation process. At the moment of vitrification, the equilibrium composition has not yet been reached and part of the PEG molecules is permanently trapped in the PSf matrix. Through the residual presence of a certain (small) amount of PEG molecules, a membrane owns a hydrophilic character [9], that is, the entrapment of PEG improves the
500
90
400
80
300
PWF RPepsin
200
RBSA
70
100
hydrophilicity of membrane and more PEG addition and increase in the molecular weight of PEG lead to more residual of PEG and decrease of contact angle.
3.4. Effect of PEG additive on membrane mechanical properties Tensile strength and elongation at break are two important parameters to describe the mechanical properties of membranes. Tensile strength and elongation at break of membranes with different PEG 400 dosages and different molecular weights of PEG are shown in Figs. 11 and 12. More dosage of PEG 400 weakens the tensile strength at break of membrane because of the increasing porosity, but elongation at break increases from 28.2% for no PEG 400 dosage to 35.6% for 6% PEG 400 dosage and attains its maximum then decreases to 12.8% of 10% PEG 400 dosage. Elongation at break of membrane increases with increase in molecular weight of PEG, while the tensile strength at break increases from 4.48 MPa for PEG 400 to 5.62 MPa for PEG 1500 and attains the maximum then decreases to 3.16 MPa for PEG 20,000. This is may be due to that appropriate increase in Mw of PEG can suppress the formation of macrovoids and enhance the mechanical strength, but when the Mw of PEG is excessively high, the rapid increase of porosity arising from the increase in molecular weight of PEG addition may weaken the mechanical strength. PEG 1500 can be a suitable additive for making asymmetric membranes with a relatively higher mechanical strength. From above, it can be concluded that mechanical property change of membranes are mainly due to the structure change arising from the change of PEG dosage or molecular weight of PEG. In order to obtain 50
PWF (Lm-2h-1)
100
Fig. 8. Effect of PEG of different molecular weights on the pure water flux and protein rejection of PSf membrane (8 wt% PEG dosage based on mass of 18% PSf/82% DMAc mixture).
60 0 50
88
45 86
40 Porosity CA
35
84
30 25
82
20 80
15 0
2
4
6
8
CA ( o )
0.00
0.00 0.20
Porosity (%)
1.00
Rejection (%)
PWF (Lm-2h-1)
56
10
wPEG 400 (wt%) Fig. 7. Effect of PEG 400 dosage on the pure water flux and protein rejection of PSf membrane.
0
2
4
6
8
10
wPEG 400 (wt%) Fig. 9. Contact angle (CA) and porosity of PSf membrane with different PEG 400 dosages.
Y. Ma et al. / Desalination 272 (2011) 51–58
82
6 20
80 78
58 Porosity CA
56
76
54
74
52
CA ( o )
Porosity (%)
60
72
50 48
70
46
16 4
100
1000
3 100
10000
14 Tensile strength Elongation
68
44
18
5
1000
Elongation (%)
62
Tensile strength (MPa)
64
57
12 10000
MwPEG
MwPEG
Fig. 10. Contact angle (CA) and porosity of PSf membrane with PEG of different molecular weights (8 wt% PEG dosage based on mass of 18% PSf/82% DMAc mixture).
Fig. 12. Tensile strength and elongation at break of PSf membrane with PEG of different molecular weights (8 wt% PEG dosage based on mass of 18% PSf/82% DMAc mixture).
membranes with better mechanical properties, the dosage and molecular weight of PEG should be taken into account.
• In order to obtain membranes with better mechanical properties, PEG dosage and the molecular weight of PEG should be considered. More PEG dosage weakens the mechanical properties of membranes because of the increasing porosity. PEG 1500 can be a suitable additive for making asymmetric membranes having a relatively higher mechanical strength.
4. Conclusion Flat sheet PSf membranes were prepared from casting solutions containing 18 wt% of PSf and 82% of DMAc; using diffusion induced phase separation process. Polyethylene glycol (PEG) of 6 different molecular weights (400 Da, 800 Da, 1500 Da, 4000 Da, 10,000 Da, and 20,000 Da, respectively) or different dosages of PEG 400 was used as additives. Effects of PEG 400 dosage and molecular weight of PEG on the morphology and properties such as porosity, contact angle, and mechanical properties were studied. The permeation performance of the prepared membranes was also evaluated in terms of PWF and rejection efficiency of BSA and pepsin protein. The results can be summarized as follows:
7.0
40
6.5
35
6.0
30
5.5
25
5.0
20
4.5
Elongation (%)
Tensile strength (MPa)
• All the membranes are found to have asymmetric structure as seen from SEM photographs. • The PWF is seen to be enhanced greatly with increase in molecular weight of PEG or dosage of PEG 400 due to high porosity of membranes. Increase in dosage of PEG 400 does not change the MWCO of membranes but can make the pore size distribution of membrane skin layer narrower. So, PEG could be regarded as a pore forming agent rather than a pore reducing agent. • The addition of PEG additive can improve the hydrophilicity of membranes. The hydrophilicity of membranes depends on the PEG dosage and the molecular weight of PEG additive.
15
Tensile strength Elongation
10
4.0 2
4
6
8
10
wPEG 400 (wt%) Fig. 11. Tensile strength and elongation at break of PSf membrane with different PEG 400 dosages.
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