Journal of Colloid and Interface Science 434 (2014) 175–180
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Preparation and characterization of pH-sensitive and antifouling poly(vinylidene fluoride) microfiltration membranes blended with poly(methyl methacrylate-2-hydroxyethyl methacrylate-acrylic acid) Junping Ju a,b, Chao Wang a, Tingmei Wang a,⇑, Qihua Wang a a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 14 May 2014 Accepted 1 August 2014 Available online 12 August 2014 Keywords: Poly(vinylidene fluoride) Microfiltration membrane pH-sensitivity Hydrophilicity Antifouling property
a b s t r a c t Functional terpolymer of poly(methyl methacrylate-2-hydroxyethyl methacrylate-acrylic acid) (P(MMA– HEMA–AA)) was synthesized via a radical polymerization method. The terpolymer could be directly blended with poly(vinylidene fluoride) (PVDF) to prepare the microfiltration (MF) membranes via phase separate process. The synthesized polymers were characterized by Fourier transform infrared (FTIR), the nuclear magnetic resonance proton spectra (1H NMR). The membrane had the typical asymmetric structure and the hydrophilic side chains tended to aggregate on the membrane surface. The surface enrichment of amphiphilic copolymer and morphology of MF membranes were characterized by Fourier transform infrared attenuated total reflection spectroscopy (FTIR–ATR) and scanning electron microscopy (SEM). The contact angle (CA) and water uptake were also tested to assess the hydrophilicity and wetting characteristics of the polymer surface. The water filtration properties were measured. It was found the modified membranes showed excellent pH-sensitivity and pH-reversibility behavior. Furthermore, the hydrophilicity of the blended membranes increased, and the membranes showed good protein antifouling property. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Nowadays, many types of stimuli-responsive microfiltration membranes that respond to external change such as temperature, pH, magnetic and electric field have been reported [1–4]. Compared with other external stimuli, the pH is the most commonly used stimulus in environmentally responsive polymer systems [5]. Carboxyl and pyridine groups are the most widely used pHresponsive functional groups [6,7]. As a microfiltration material, poly(vinylidene fluoride) (PVDF) has earned lots of interest due to its excellent thermal stability, chemical stability and radiation resistance, etc. [8–11]. Stimuliresponsive membrane based on PVDF has gained increasing popularity in the recent years and played an important role in many applications including selective permeation, wastewater treatment and delivery of drugs. The past several years have witnessed an expansion of various methodologies directed at preparing stimuli-responsive microfiltration membrane [12]. For the modification of PVDF and PVDF-based membranes, there are three methods: (1) ⇑ Corresponding author. E-mail address: [email protected] (T. Wang). http://dx.doi.org/10.1016/j.jcis.2014.08.007 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
bulk modification of PVDF material, and then to prepare modified membrane; (2) surface modification of prepared PVDF membrane and (3) blending, which is a simply important way to modify the membranes [13]. For example, Zhai et al. [14] successfully prepared pH-sensitive microfiltration membranes using poly(2-vinylpyridine) and poly(4-vinylpyridine)-graf-poly(vinylidene fluoride) copolymers. Molecular modification by graft copolymerization prior to membrane fabrication is demonstrated a simplified, efficient and economical method for fabricating membranes with pH-sensitive flux properties. Dickson et al. [15] developed and extensively investigated pore-filled pH-sensitive flat-sheet membranes by in situ cross-linking polymerization of acrylic acid inside hydrophobic PVDF microporous membranes. The higher hydrophobicity nature of the PVDF results in the membrane easy fouling by biomolecules and other organic proteins and limits its practical application in the separation process. Thus, many researchers have focused on developing pH-sensitive and protein antifouling membranes. It is generally accepted that fouling decreases with an increase in hydrophilicity of the polymeric material. Zhao et al. [16] synthesized the functional terpolymer of poly(methyl methacrylate-acrylic acid-vinyl pyrrolidone) through a conventional radical polymerization and used as addi-
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tives in the preparation of polyethersulfone hollow fiber membranes via phase inversion process. The terpolymers significantly altered the membrane performance, and the blended hollow fiber membranes showed evident pH sensitivity and protein antifouling property. 2-Hydroxyethyl methacrylate (HEMA) is neutral, hydroxyl-rich monomer and PHEMA can exhibit excellent ability to resist protein adsorption from different single-protein solution [17]. PAA is the most commonly used pH-responsive polymer [18–20]. In the present work, we developed pH-sensitivity and antifouling property microfiltration membranes by blending PVDF with poly(methyl methacrylate-2-hydroxyethyl methacrylate-acrylic acid) (P(MMA– HEMA–AA) via phase separation [21–23]. The P(MMA–HEMA–AA) was synthesized via a free radical solution polymerization. The chemical composition and morphology of the modified membrane surface were investigated in detail by FTIR, 1H NMR, SEM, respectively. Water contact angle (CA), water flux, pH sensitivity, pH-reversibility and antifouling evaluation were also carried out for membrane hydrophilicity and permeability. 2. Experimental 2.1. Materials Poly(vinylidene fluoride) (PVDF) was obtained from Shanghai 3F New Materials. 2-hydroxyethyl methacrylate (HEMA) (99%) was supplied by Aladdin Reagent Co., Ltd. (Aladdin, Germany). Bovine serum albumin (BSA, Mn = 68,000) was kindly provided by Beijing Solarbio Science & Technology Co., Ltd. Methyl methacrylate (MMA) and acrylic acid (AA) were dried over CaH2 and distilled under reduced pressure. N,N-dimethylacetamide (DMAc, 99%), N-methyl pyrrolidone (NMP (99%)) and poly(ethylene glycol) (PEG, 20,000) were purchased from Tianjin Chemical Reagents Company. The presence of PEG was as pore former. Azo-bis-isobutyronitrile (AIBN) was recrystallized three times before use. Deionized water was obtained from a self-made reverse osmosiselectrodeionization (RO-EDI) system and used throughout the experiment. 2.2. Synthesis of P(PMMA–PHEMA–AA) terpolymer The terpolymer was obtained via a radical polymerization method. A typical solution polymerization procedure was shown as follows. A mixture of MMA (0.05 mol), HEMA (0.029 mol), AA (0.052 mol), AIBN (0.00039 mol) and DMAc was placed in a dry round-bottom flask with stirring bar. After bubbling for 30 min with nitrogen, the reaction was allowed to proceed at 80 °C for 24 h. After cooling, the reaction mixtures were poured into a large amount of diethyl ether and the resulting precipitate of poly (MMA–HEMA–AA) was collected by filtration. The synthesized copolymers were characterized by 1H NMR in d6-DMSO and FTIR. Gel permeation chromatographic (GPC) analyses were performed using a Waters GPC system. Tetrahydrofuran was used as an eluent and the results were calibrated with respect to polystyrene standards.
2.4. Membrane characterization 2.4.1. Fourier transform infrared attenuated total reflection spectroscopy (FTIR–ATR) The FTIR–ATR spectroscopy was recorded using a Bruker IFS 66 V/S FTIR spectrometer. The spectra were carried out in the standard wavenumber range of 400–4000 cm1 with a spectral resolution of 1 cm1 and 32 scans. 2.4.2. Contact angle measurement The hydrophilicity of membrane was characterized by measuring the water contact angle on the membrane surface with a contact angle instrument (DSA100, Krüss, Germany) at ambient temperature. The average CA values were determined by the average of at least five measurements taken at different positions on each sample. 2.4.3. Water uptake measurement Water uptake was used to evaluate the adsorption of water to microfiltration membranes. MF membranes rinsed with deionized water were dried at 60 °C under vacuum for 24 h, and its weight (W1) was taken. After that, the dried membranes were immersed in deionized water with pH value of 1, 3, 5, 7, 9 and 11 for at least 24 h and the weight of the wetted membrane (W2) was attained. The water content of microfiltration membrane was calculated by the following relationship.
Water uptake ð%Þ ¼
W2 W1 100 W1
2.4.4. Morphology study FE-SEM (JSM-6701F) characterization of membrane was performed to observe the microstructures. To obtain the cross-sectional image, membrane was immersed in liquid nitrogen and fractured before measurement. Membrane surfaces and cross-sections were coated with gold by sputtering prior to observation. The membrane porosity (e) was calculated according to the equation. ðW 2 W 1 Þ
e ¼ ðW 2 Wq1GÞ qG
1 þW q
100%
P
where W1 is the weight of the wet membrane (g), W2 is the weight of the dry membrane (g), qG is the water density, qP is the polymer density. 2.4.5. pH-sensitive permeation experiment For the pH-sensitivity experiment, the pH value of the feed solution was adjusted by adding 0.1 M HCl or 0.1 M NaOH solution. A dead-end stirred cell filtration device (Shanghai Mo Su Scientific Equipment Co., Ltd, China) connected with a nitrogen gas cylinder was used to measure pure water flux. The effective surface area of the cell was 44.156 cm2 and pre-compacted at 0.02 MPa for a certain time until the flux maintained. Pure water flux (J0) was calculated by the following equation:
J0 ¼
V A Dt
ð1Þ
2.3. Membrane preparation PVDF and the terpolymer were dissolved together in NMP with vigorous stirring. A clear, homogeneous solution was obtained, which was immediately cast on glass plates with a doctor blade. Before the NMP could evaporate, the plates were immersed in a bath of deionized water. The polymer film coagulated in the water and was stored underwater until needed. The experimental details for membranes are shown in Table 1.
Table 1 MF membranes with different terpolymer contents. No.
PVDF (wt.%)
PEG (wt.%)
P(MMA–HEMA–AA) (wt.%)
NMP (wt.%)
Porosity
M-0 M-0.5 M-1.0
14.0 13.5 13.0
2 2 2
0 0.5 1.0
84 84 84
0.7852 0.8532 0.8883
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177
where V is the volume of the permeation water, A is the effective area of the membrane, and Dt is the time of the measurement. The above experiments were carried out at 25 °C. 2.4.6. The pH-reversibility experiment The equipment used in this experiment was the same as above. Then the membranes were alternatively fed with pH 2.0 HCl and pH 11.0 NaOH aqueous solutions with short distilled water rinsing. The permeate solutions were collected after equilibration. The water flux was calculated by using Eq. (1). 2.4.7. Antifouling property The antifouling property of membranes was evaluated by measuring the percentage of water flux recovery after fouling by BSA. Aqueous solution of BSA (1 g/L) was forced to permeate through the membrane at a stirring rate of 600 rpm under 0.03 MPa nitrogen atmospheres for 40 min. After that, the membrane was cleaned by the distilled water for 20 min at room temperature, and then the pure water flux was measured again. The water flux recovery percentage (R) is calculated according to the Eq.
R ðrecoveryÞ ð%Þ ¼
Fig. 2. FTIR–ATR spectra of different membranes prepared in casting solution.
Jx 100 J0
where J0 is the pure water flux of sample membrane and Jx is the pure water flux after fouling, respectively. 3. Results and discussion 3.1. Characterization of the terpolymer The P(MMA–HEMA–AA) was prepared via a radical polymerization method. Fig. 1A shows the FTIR spectrum for the prepared terpolymer. The typical peaks at about 1154 and 1732 cm1 are attributable to the CAOAC and C@O species of PMMA and PHEMA. The peaks near 1732 and 3292 cm1 are ascribed to the C@O and OAH in the carboxyl group of PAA and PHEMA, respectively. In addition, the chemical structure of the terpolymer was further determined by 1H NMR spectroscopy, as shown in Fig. 1B. The chemical shifts at d = 0.560–2.236 (a, b, d, e, m, n) are assigned to the methyl and methylene protons of in the PMMA, PHEMA and PAA. The chemical shifts at d = 3.86 ppm (h) and d = 4.78 ppm (g) can be assigned to ACH2AOA and AOHA peaks from PHEMA, and the intensity ratio of these two peaks is 1:1.02. A weak peak with a chemical shift at d = 12.23 ppm (k) related to the hydroxyl proton unit from the PAA is shown. The FTIR and 1H NMR spectroscopy provided convincing evidence for the successful synthesis of the terpolymer. Moreover, GPC result shows that the Mn of the P(MMA–HEMA–AA) is 36007 g/mol.
Fig. 3. Contact angles of the membranes.
3.2. Chemical composition of the membrane surface The FTIR–ATR spectra of pristine PVDF and blend membranes with different contents of the terpolymer are shown in Fig. 2. It can be seen that the most significant change is the peak at 1730 cm1 for M-0.5 and M-1.0, which indicated the presence of C@O originated from terpolymer in the blended membranes.
Fig. 1. (A) FTIR spectrum of P(MMA–HEMA–AA) and (B) 1H NMR of P(MMA–HEMA–AA) in d6-DMSO.
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Fig. 4. SEM images of top surface (left), cross section (right) morphology: M-0, M-0.5, M-1.0.
Furthermore, the absorbance intensity at 1730 cm1 increases significantly with increasing the content of terpolymer in the membrane. Surface contact angle is mostly used as a measure of the surface hydrophilicity. As shown in Fig. 3, the CA of membranes is a noteworthy decrease toward increasing the content of the terpolymer in membrane. The pure PVDF membrane has the highest water contact angle compared with the modified membranes. By embedding 1.0 wt.% of the terpolymer, the contact angle of the modified membranes reached a lowest value of 59°. By directly blending amphiphilic polymers with PVDF is the simplest, most widely used method to improve membrane property. The hydrophobic polymer (PMMA) could generally miscible with PVDF materials, while hydrophilic polymer (PHEMA and PAA) accumulated spontaneously and self-organized on membrane surface. In addition, the increase of pore size and porosity (as also shown in Table 1) are also important factor for water diffusion more easily. Increasing the content of terpolymer to the PVDF matrix would increase the existence of amphiphilic copolymers in the membrane, which can potentially enhance the surface hydrophilicity and protein fouling resistance.
3.3. Membrane morphology observations Fig. 4 displays surface and cross-sectional SEM images of the pristine PVDF and blend PVDF membrane. All the membranes show a typical asymmetric morphology with finger-like macrovoids. Instantaneous liquid–liquid demixing is thought to provide conditions for macrovoid formation [24,25]. With the addition of P(MMA–HEMA–AA), the modified membrane has larger surface pore size and higher porosity (as also shown in Table 1) than pure PVDF membrane. This might be caused by the increased exchange rate between solvent and nonsolvent during the phase inversion process, which would enhance the formation of macrovoids, with increasing the amount of P(MMA–HEMA–AA) in the casting solution [26].
3.4. Membranes swelling behavior The swelling behavior of pristine and modified MF membranes is shown in Fig. 5. The degree of swelling of pristine MF membrane is relatively low. The degree of swelling of modified membrane has a
J. Ju et al. / Journal of Colloid and Interface Science 434 (2014) 175–180
Fig. 5. Degree of swelling of membranes at different pH value.
179
Fig. 7. Cyclic flux test at pH value of 2.0 and 11.0 of the MF membrane.
Fig. 6. Water flux of membranes at different pH value.
noteworthy enhancement with increasing the content of the terpolymer in membrane. The reason is the improved hydrophilicity of the modified membrane, so that more water can be stored in numerous pores. In addition, the swelling of modified MF membrane exhibits pH dependent behavior, which is different from that of pure MF membrane. The pH dependence of the swelling behavior is determined at pH values ranging from acidic to basic, in random sequence. Since AA is a weak acid carboxyl groups dissociate into carboxylate ions at high pH, which results in high charge density in the polymer, causing it to swell. Therefore, the swelling degree of modified membranes is enhanced with increasing of pH value [27–31].
3.5. Filtration property of the MF membranes The membrane water fluxes as a function of pH are shown in Fig. 6. From the comparison of water flux between the pure and modified membrane prepared in different terpolymer contents, it can be seen clearly that the water flux of modified membranes increases observably with the terpolymer content increase. This can be attributed to the fact that the modified PVDF membranes are more hydrophilic, and have larger pore diameter and higher surface porosity. In addition, the water flux of pure PVDF membrane is practically constant as pH varied. However, the modified membrane fluxes show evident pH dependence. For example, the
Fig. 8. Water flux recovery percentage of the different membranes after inner fouling by BSA solution: M-0, M-0.5, M-1.0.
water fluxes of M-0.5 and M-1.0 decrease from about 405 to 127 L/(m2 h) and from 535 to 180 L/(m2 h), respectively, when the pH values change from 2.0 to 11.0. The change in water flux of the blend membranes in response to the variation in pH value is caused by the switch of stretched and collapsed states of PAA at different pH values, which change the apparent size of the micropores in the membranes. Membrane pH reversibility is another parameter used to evaluate pH-sensitive membrane. The pH value of the feed solution was adjusted by adding NaOH or HCl solution. As shown in Fig. 7, when the pH of the feeding solution is changed cyclical, the pure PVDF membrane is not shown reversible pH switchable properties and the flux is practically constant. However, the water flux is reversible between about 383.1 and 138 L/(m2 h) for M-0.5 membrane, and between about 540.8 and 159.2 L/(m2 h) for M-1.0 membrane.
3.6. Antifouling property of membrane One of the major obstacles for the application of MF membrane is the rapid decline of the permeate flux over time as a result of membrane fouling. Hydrophilic polymer additives are mostly used for the blending with PVDF to improve hydrophilicity and antifouling properties of the membrane. In the study, the antifouling behavior of these MF membranes was characterized by measuring
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the water flux recovery after fouling in BSA solution. As shown in Fig. 8, the flux recovery ratios for M-0, M-0.5 and M-1.0 were 57.8%, 66.8% and 78.2%, respectively. M-1.0 shows the best antifouling property. These were because that the PHEMA and PAA easily enriched on the PVDF membrane surface would prevent the protein molecules contacting the hydrophobic PVDF. Therefore, most of the protein molecules depositing on the membrane surface can be easily washed, and the flux recovery ratios increased. But according to Fig. 8, the antifouling property is undesirable. We all know that the flux recovery ratio reached one hundred percent which is not possible. It is usually assumed that fouling decreases with an increase in hydrophilicity of the membrane, but the charge and ionic strength of membranes, morphological properties including surface roughness, porosity, thickness, membrane cleaning and the properties of the solutes should been considered in the future study [32]. Therefore, to maximize the effectiveness of the modified surface, the system approach should been given to improve the fouling resistance of PVDF membranes in practical processes. 4. Conclusion In summary, modified PVDF microfiltration membranes by blending with poly(MMA–HEMA–AA) were successfully prepared. The blended membranes showed both pH sensitivity and pH reversibility response. The changes in the permeation rate of the blend membranes in response to the changes in the pH were attributed to the change in the conformation of the PAA in the membrane. Furthermore, when the terpolymer was blended in the membrane, the water contact angles decreased, and the flux recovery ratios increased. Acknowledgment The authors would like to acknowledge the financial supports from the National Science Foundation for Distinguished Young Scholars of China (Grant No. 51025517) and the National Young Scientists Foundation of China (51403219).
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Preparation and characterization of pH-sensitive and antifouling poly(vinylidene fluoride) microfiltration membranes blended with poly(methyl methacrylate-2-hydroxyethyl methacrylate-acrylic acid) Junping Ju a,b, Chao Wang a, Tingmei Wang a,⇑, Qihua Wang a a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 14 May 2014 Accepted 1 August 2014 Available online 12 August 2014 Keywords: Poly(vinylidene fluoride) Microfiltration membrane pH-sensitivity Hydrophilicity Antifouling property
a b s t r a c t Functional terpolymer of poly(methyl methacrylate-2-hydroxyethyl methacrylate-acrylic acid) (P(MMA– HEMA–AA)) was synthesized via a radical polymerization method. The terpolymer could be directly blended with poly(vinylidene fluoride) (PVDF) to prepare the microfiltration (MF) membranes via phase separate process. The synthesized polymers were characterized by Fourier transform infrared (FTIR), the nuclear magnetic resonance proton spectra (1H NMR). The membrane had the typical asymmetric structure and the hydrophilic side chains tended to aggregate on the membrane surface. The surface enrichment of amphiphilic copolymer and morphology of MF membranes were characterized by Fourier transform infrared attenuated total reflection spectroscopy (FTIR–ATR) and scanning electron microscopy (SEM). The contact angle (CA) and water uptake were also tested to assess the hydrophilicity and wetting characteristics of the polymer surface. The water filtration properties were measured. It was found the modified membranes showed excellent pH-sensitivity and pH-reversibility behavior. Furthermore, the hydrophilicity of the blended membranes increased, and the membranes showed good protein antifouling property. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Nowadays, many types of stimuli-responsive microfiltration membranes that respond to external change such as temperature, pH, magnetic and electric field have been reported [1–4]. Compared with other external stimuli, the pH is the most commonly used stimulus in environmentally responsive polymer systems [5]. Carboxyl and pyridine groups are the most widely used pHresponsive functional groups [6,7]. As a microfiltration material, poly(vinylidene fluoride) (PVDF) has earned lots of interest due to its excellent thermal stability, chemical stability and radiation resistance, etc. [8–11]. Stimuliresponsive membrane based on PVDF has gained increasing popularity in the recent years and played an important role in many applications including selective permeation, wastewater treatment and delivery of drugs. The past several years have witnessed an expansion of various methodologies directed at preparing stimuli-responsive microfiltration membrane [12]. For the modification of PVDF and PVDF-based membranes, there are three methods: (1) ⇑ Corresponding author. E-mail address: [email protected] (T. Wang). http://dx.doi.org/10.1016/j.jcis.2014.08.007 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
bulk modification of PVDF material, and then to prepare modified membrane; (2) surface modification of prepared PVDF membrane and (3) blending, which is a simply important way to modify the membranes [13]. For example, Zhai et al. [14] successfully prepared pH-sensitive microfiltration membranes using poly(2-vinylpyridine) and poly(4-vinylpyridine)-graf-poly(vinylidene fluoride) copolymers. Molecular modification by graft copolymerization prior to membrane fabrication is demonstrated a simplified, efficient and economical method for fabricating membranes with pH-sensitive flux properties. Dickson et al. [15] developed and extensively investigated pore-filled pH-sensitive flat-sheet membranes by in situ cross-linking polymerization of acrylic acid inside hydrophobic PVDF microporous membranes. The higher hydrophobicity nature of the PVDF results in the membrane easy fouling by biomolecules and other organic proteins and limits its practical application in the separation process. Thus, many researchers have focused on developing pH-sensitive and protein antifouling membranes. It is generally accepted that fouling decreases with an increase in hydrophilicity of the polymeric material. Zhao et al. [16] synthesized the functional terpolymer of poly(methyl methacrylate-acrylic acid-vinyl pyrrolidone) through a conventional radical polymerization and used as addi-
176
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tives in the preparation of polyethersulfone hollow fiber membranes via phase inversion process. The terpolymers significantly altered the membrane performance, and the blended hollow fiber membranes showed evident pH sensitivity and protein antifouling property. 2-Hydroxyethyl methacrylate (HEMA) is neutral, hydroxyl-rich monomer and PHEMA can exhibit excellent ability to resist protein adsorption from different single-protein solution [17]. PAA is the most commonly used pH-responsive polymer [18–20]. In the present work, we developed pH-sensitivity and antifouling property microfiltration membranes by blending PVDF with poly(methyl methacrylate-2-hydroxyethyl methacrylate-acrylic acid) (P(MMA– HEMA–AA) via phase separation [21–23]. The P(MMA–HEMA–AA) was synthesized via a free radical solution polymerization. The chemical composition and morphology of the modified membrane surface were investigated in detail by FTIR, 1H NMR, SEM, respectively. Water contact angle (CA), water flux, pH sensitivity, pH-reversibility and antifouling evaluation were also carried out for membrane hydrophilicity and permeability. 2. Experimental 2.1. Materials Poly(vinylidene fluoride) (PVDF) was obtained from Shanghai 3F New Materials. 2-hydroxyethyl methacrylate (HEMA) (99%) was supplied by Aladdin Reagent Co., Ltd. (Aladdin, Germany). Bovine serum albumin (BSA, Mn = 68,000) was kindly provided by Beijing Solarbio Science & Technology Co., Ltd. Methyl methacrylate (MMA) and acrylic acid (AA) were dried over CaH2 and distilled under reduced pressure. N,N-dimethylacetamide (DMAc, 99%), N-methyl pyrrolidone (NMP (99%)) and poly(ethylene glycol) (PEG, 20,000) were purchased from Tianjin Chemical Reagents Company. The presence of PEG was as pore former. Azo-bis-isobutyronitrile (AIBN) was recrystallized three times before use. Deionized water was obtained from a self-made reverse osmosiselectrodeionization (RO-EDI) system and used throughout the experiment. 2.2. Synthesis of P(PMMA–PHEMA–AA) terpolymer The terpolymer was obtained via a radical polymerization method. A typical solution polymerization procedure was shown as follows. A mixture of MMA (0.05 mol), HEMA (0.029 mol), AA (0.052 mol), AIBN (0.00039 mol) and DMAc was placed in a dry round-bottom flask with stirring bar. After bubbling for 30 min with nitrogen, the reaction was allowed to proceed at 80 °C for 24 h. After cooling, the reaction mixtures were poured into a large amount of diethyl ether and the resulting precipitate of poly (MMA–HEMA–AA) was collected by filtration. The synthesized copolymers were characterized by 1H NMR in d6-DMSO and FTIR. Gel permeation chromatographic (GPC) analyses were performed using a Waters GPC system. Tetrahydrofuran was used as an eluent and the results were calibrated with respect to polystyrene standards.
2.4. Membrane characterization 2.4.1. Fourier transform infrared attenuated total reflection spectroscopy (FTIR–ATR) The FTIR–ATR spectroscopy was recorded using a Bruker IFS 66 V/S FTIR spectrometer. The spectra were carried out in the standard wavenumber range of 400–4000 cm1 with a spectral resolution of 1 cm1 and 32 scans. 2.4.2. Contact angle measurement The hydrophilicity of membrane was characterized by measuring the water contact angle on the membrane surface with a contact angle instrument (DSA100, Krüss, Germany) at ambient temperature. The average CA values were determined by the average of at least five measurements taken at different positions on each sample. 2.4.3. Water uptake measurement Water uptake was used to evaluate the adsorption of water to microfiltration membranes. MF membranes rinsed with deionized water were dried at 60 °C under vacuum for 24 h, and its weight (W1) was taken. After that, the dried membranes were immersed in deionized water with pH value of 1, 3, 5, 7, 9 and 11 for at least 24 h and the weight of the wetted membrane (W2) was attained. The water content of microfiltration membrane was calculated by the following relationship.
Water uptake ð%Þ ¼
W2 W1 100 W1
2.4.4. Morphology study FE-SEM (JSM-6701F) characterization of membrane was performed to observe the microstructures. To obtain the cross-sectional image, membrane was immersed in liquid nitrogen and fractured before measurement. Membrane surfaces and cross-sections were coated with gold by sputtering prior to observation. The membrane porosity (e) was calculated according to the equation. ðW 2 W 1 Þ
e ¼ ðW 2 Wq1GÞ qG
1 þW q
100%
P
where W1 is the weight of the wet membrane (g), W2 is the weight of the dry membrane (g), qG is the water density, qP is the polymer density. 2.4.5. pH-sensitive permeation experiment For the pH-sensitivity experiment, the pH value of the feed solution was adjusted by adding 0.1 M HCl or 0.1 M NaOH solution. A dead-end stirred cell filtration device (Shanghai Mo Su Scientific Equipment Co., Ltd, China) connected with a nitrogen gas cylinder was used to measure pure water flux. The effective surface area of the cell was 44.156 cm2 and pre-compacted at 0.02 MPa for a certain time until the flux maintained. Pure water flux (J0) was calculated by the following equation:
J0 ¼
V A Dt
ð1Þ
2.3. Membrane preparation PVDF and the terpolymer were dissolved together in NMP with vigorous stirring. A clear, homogeneous solution was obtained, which was immediately cast on glass plates with a doctor blade. Before the NMP could evaporate, the plates were immersed in a bath of deionized water. The polymer film coagulated in the water and was stored underwater until needed. The experimental details for membranes are shown in Table 1.
Table 1 MF membranes with different terpolymer contents. No.
PVDF (wt.%)
PEG (wt.%)
P(MMA–HEMA–AA) (wt.%)
NMP (wt.%)
Porosity
M-0 M-0.5 M-1.0
14.0 13.5 13.0
2 2 2
0 0.5 1.0
84 84 84
0.7852 0.8532 0.8883
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177
where V is the volume of the permeation water, A is the effective area of the membrane, and Dt is the time of the measurement. The above experiments were carried out at 25 °C. 2.4.6. The pH-reversibility experiment The equipment used in this experiment was the same as above. Then the membranes were alternatively fed with pH 2.0 HCl and pH 11.0 NaOH aqueous solutions with short distilled water rinsing. The permeate solutions were collected after equilibration. The water flux was calculated by using Eq. (1). 2.4.7. Antifouling property The antifouling property of membranes was evaluated by measuring the percentage of water flux recovery after fouling by BSA. Aqueous solution of BSA (1 g/L) was forced to permeate through the membrane at a stirring rate of 600 rpm under 0.03 MPa nitrogen atmospheres for 40 min. After that, the membrane was cleaned by the distilled water for 20 min at room temperature, and then the pure water flux was measured again. The water flux recovery percentage (R) is calculated according to the Eq.
R ðrecoveryÞ ð%Þ ¼
Fig. 2. FTIR–ATR spectra of different membranes prepared in casting solution.
Jx 100 J0
where J0 is the pure water flux of sample membrane and Jx is the pure water flux after fouling, respectively. 3. Results and discussion 3.1. Characterization of the terpolymer The P(MMA–HEMA–AA) was prepared via a radical polymerization method. Fig. 1A shows the FTIR spectrum for the prepared terpolymer. The typical peaks at about 1154 and 1732 cm1 are attributable to the CAOAC and C@O species of PMMA and PHEMA. The peaks near 1732 and 3292 cm1 are ascribed to the C@O and OAH in the carboxyl group of PAA and PHEMA, respectively. In addition, the chemical structure of the terpolymer was further determined by 1H NMR spectroscopy, as shown in Fig. 1B. The chemical shifts at d = 0.560–2.236 (a, b, d, e, m, n) are assigned to the methyl and methylene protons of in the PMMA, PHEMA and PAA. The chemical shifts at d = 3.86 ppm (h) and d = 4.78 ppm (g) can be assigned to ACH2AOA and AOHA peaks from PHEMA, and the intensity ratio of these two peaks is 1:1.02. A weak peak with a chemical shift at d = 12.23 ppm (k) related to the hydroxyl proton unit from the PAA is shown. The FTIR and 1H NMR spectroscopy provided convincing evidence for the successful synthesis of the terpolymer. Moreover, GPC result shows that the Mn of the P(MMA–HEMA–AA) is 36007 g/mol.
Fig. 3. Contact angles of the membranes.
3.2. Chemical composition of the membrane surface The FTIR–ATR spectra of pristine PVDF and blend membranes with different contents of the terpolymer are shown in Fig. 2. It can be seen that the most significant change is the peak at 1730 cm1 for M-0.5 and M-1.0, which indicated the presence of C@O originated from terpolymer in the blended membranes.
Fig. 1. (A) FTIR spectrum of P(MMA–HEMA–AA) and (B) 1H NMR of P(MMA–HEMA–AA) in d6-DMSO.
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Fig. 4. SEM images of top surface (left), cross section (right) morphology: M-0, M-0.5, M-1.0.
Furthermore, the absorbance intensity at 1730 cm1 increases significantly with increasing the content of terpolymer in the membrane. Surface contact angle is mostly used as a measure of the surface hydrophilicity. As shown in Fig. 3, the CA of membranes is a noteworthy decrease toward increasing the content of the terpolymer in membrane. The pure PVDF membrane has the highest water contact angle compared with the modified membranes. By embedding 1.0 wt.% of the terpolymer, the contact angle of the modified membranes reached a lowest value of 59°. By directly blending amphiphilic polymers with PVDF is the simplest, most widely used method to improve membrane property. The hydrophobic polymer (PMMA) could generally miscible with PVDF materials, while hydrophilic polymer (PHEMA and PAA) accumulated spontaneously and self-organized on membrane surface. In addition, the increase of pore size and porosity (as also shown in Table 1) are also important factor for water diffusion more easily. Increasing the content of terpolymer to the PVDF matrix would increase the existence of amphiphilic copolymers in the membrane, which can potentially enhance the surface hydrophilicity and protein fouling resistance.
3.3. Membrane morphology observations Fig. 4 displays surface and cross-sectional SEM images of the pristine PVDF and blend PVDF membrane. All the membranes show a typical asymmetric morphology with finger-like macrovoids. Instantaneous liquid–liquid demixing is thought to provide conditions for macrovoid formation [24,25]. With the addition of P(MMA–HEMA–AA), the modified membrane has larger surface pore size and higher porosity (as also shown in Table 1) than pure PVDF membrane. This might be caused by the increased exchange rate between solvent and nonsolvent during the phase inversion process, which would enhance the formation of macrovoids, with increasing the amount of P(MMA–HEMA–AA) in the casting solution [26].
3.4. Membranes swelling behavior The swelling behavior of pristine and modified MF membranes is shown in Fig. 5. The degree of swelling of pristine MF membrane is relatively low. The degree of swelling of modified membrane has a
J. Ju et al. / Journal of Colloid and Interface Science 434 (2014) 175–180
Fig. 5. Degree of swelling of membranes at different pH value.
179
Fig. 7. Cyclic flux test at pH value of 2.0 and 11.0 of the MF membrane.
Fig. 6. Water flux of membranes at different pH value.
noteworthy enhancement with increasing the content of the terpolymer in membrane. The reason is the improved hydrophilicity of the modified membrane, so that more water can be stored in numerous pores. In addition, the swelling of modified MF membrane exhibits pH dependent behavior, which is different from that of pure MF membrane. The pH dependence of the swelling behavior is determined at pH values ranging from acidic to basic, in random sequence. Since AA is a weak acid carboxyl groups dissociate into carboxylate ions at high pH, which results in high charge density in the polymer, causing it to swell. Therefore, the swelling degree of modified membranes is enhanced with increasing of pH value [27–31].
3.5. Filtration property of the MF membranes The membrane water fluxes as a function of pH are shown in Fig. 6. From the comparison of water flux between the pure and modified membrane prepared in different terpolymer contents, it can be seen clearly that the water flux of modified membranes increases observably with the terpolymer content increase. This can be attributed to the fact that the modified PVDF membranes are more hydrophilic, and have larger pore diameter and higher surface porosity. In addition, the water flux of pure PVDF membrane is practically constant as pH varied. However, the modified membrane fluxes show evident pH dependence. For example, the
Fig. 8. Water flux recovery percentage of the different membranes after inner fouling by BSA solution: M-0, M-0.5, M-1.0.
water fluxes of M-0.5 and M-1.0 decrease from about 405 to 127 L/(m2 h) and from 535 to 180 L/(m2 h), respectively, when the pH values change from 2.0 to 11.0. The change in water flux of the blend membranes in response to the variation in pH value is caused by the switch of stretched and collapsed states of PAA at different pH values, which change the apparent size of the micropores in the membranes. Membrane pH reversibility is another parameter used to evaluate pH-sensitive membrane. The pH value of the feed solution was adjusted by adding NaOH or HCl solution. As shown in Fig. 7, when the pH of the feeding solution is changed cyclical, the pure PVDF membrane is not shown reversible pH switchable properties and the flux is practically constant. However, the water flux is reversible between about 383.1 and 138 L/(m2 h) for M-0.5 membrane, and between about 540.8 and 159.2 L/(m2 h) for M-1.0 membrane.
3.6. Antifouling property of membrane One of the major obstacles for the application of MF membrane is the rapid decline of the permeate flux over time as a result of membrane fouling. Hydrophilic polymer additives are mostly used for the blending with PVDF to improve hydrophilicity and antifouling properties of the membrane. In the study, the antifouling behavior of these MF membranes was characterized by measuring
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the water flux recovery after fouling in BSA solution. As shown in Fig. 8, the flux recovery ratios for M-0, M-0.5 and M-1.0 were 57.8%, 66.8% and 78.2%, respectively. M-1.0 shows the best antifouling property. These were because that the PHEMA and PAA easily enriched on the PVDF membrane surface would prevent the protein molecules contacting the hydrophobic PVDF. Therefore, most of the protein molecules depositing on the membrane surface can be easily washed, and the flux recovery ratios increased. But according to Fig. 8, the antifouling property is undesirable. We all know that the flux recovery ratio reached one hundred percent which is not possible. It is usually assumed that fouling decreases with an increase in hydrophilicity of the membrane, but the charge and ionic strength of membranes, morphological properties including surface roughness, porosity, thickness, membrane cleaning and the properties of the solutes should been considered in the future study [32]. Therefore, to maximize the effectiveness of the modified surface, the system approach should been given to improve the fouling resistance of PVDF membranes in practical processes. 4. Conclusion In summary, modified PVDF microfiltration membranes by blending with poly(MMA–HEMA–AA) were successfully prepared. The blended membranes showed both pH sensitivity and pH reversibility response. The changes in the permeation rate of the blend membranes in response to the changes in the pH were attributed to the change in the conformation of the PAA in the membrane. Furthermore, when the terpolymer was blended in the membrane, the water contact angles decreased, and the flux recovery ratios increased. Acknowledgment The authors would like to acknowledge the financial supports from the National Science Foundation for Distinguished Young Scholars of China (Grant No. 51025517) and the National Young Scientists Foundation of China (51403219).
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