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polycarbonate blend ultrafiltration membranes for water treatment
Journal of Membrane Science 534 (2017) 18–24
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Polyvinyl chloride/polycarbonate blend ultrafiltration membranes for water treatment
MARK
⁎
A. Behboudia,b, Y. Jafarzadeha,b, , R. Yegania,b a b
Faculty of Chemical Engineering, Sahand University of Technology, Tabriz, Iran Membrane Technology Research Center, Sahand University of Technology, Tabriz, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyvinyl chloride Polycarbonate Blend membrane Fouling
In this study, PVC/PC blend membranes were prepared via NIPS methods. Characterization techniques including FESEM, XRD, DSC, contact angle measurement, mechanical properties, abrasion test, stability test, pure water flux and filtration of BSA solution were applied to investigate the effects of PC content on the structure and performance of blend membranes. It was shown than PVC and PC are compatible and the results of FESEM, XRD and DSC analyses confirmed their compatibility. The pore size distribution of membranes shifted toward smaller pores as the content of PC in the membranes increased up to 50% and then shifted back toward larger pores. In addition, hydrophilicity, tensile strength and abrasion resistance of the blend membranes were improved. However, chemical stability of membranes against NaOH solution after 10 days decreased by increasing PC content. Pure water flux and BSA rejection as the performance criteria of membranes improved due to the presence of PC. It was found that antifouling properties of membranes increased with increasing PC content. The results indicated that PVC/PC blend membranes were high performance and fouling resistant membranes in comparison with neat PVC membrane.
1. Introduction Nowadays, membrane processes such as microfiltration, ultrafiltration and reverse osmosis are widely used to purify water for different uses. The major limitation in application of polymeric membranes in water and wastewater treatment processes is fouling problem [1]. Most of polymeric membranes are hydrophobic or less hydrophilic therefore these membranes undergo fouling by proteins and other natural organic matters (NOMs) during water treatment. To overcome this problem, many efforts have been made and it has been accepted that hydrophilic membranes are less susceptible to fouling. Therefore, increasing membrane hydrophilicity is a suitable and effective manner to decrease membrane fouling [2]. During last decades, different strategies have been adopted to mitigate fouling phenomenon which can be classified into four main categories: feed pretreatment, optimization of operating conditions, cleaning procedures and membrane modification [3]. The latter has attracted lots of attentions and several studies have been performed to enhance antifouling properties of polymeric membranes. Modification of membrane surface [4–6], incorporation of inorganic nanoparticles into membrane matrix [7–10] and blending of different polymers [11–13]are three approaches which have been used to prepare anti-
⁎
fouling membranes. Among these approaches, blending of polymers is one of the most convenient and practical ways for improving the antifouling properties and performance of polymeric membranes because it is the most convenient method in operation and the least expensive from economical point of view [14–16]. Among different polymers which are used in fabrication of polymeric membranes, polyvinyl chloride (PVC) has attracted lots of attentions due to its characteristics such as good mechanical strength, abrasion resistance, chemical stabilization, thermal properties, low cost and corrosion resistance [16,17]. However, the use of PVC membrane in water and wastewater treatment processes has been limited because it is easily fouled due to the strong hydrophobicity. To increase hydrophilicity and antifouling properties of PVC membrane, several modification methods have been explored. For example, surface of PVC membrane was modified using surface coating [18] and surface grafting [19] techniques which resulted in hydrophilic and fouling resistant membranes. In surface modification methods, however, internal pores of the modified membranes are barely concerned. Moreover, surface modification methods involve an extra step in membrane fabrication process [14]. Therefore, modification of the bulk of membranes seems to be an alternative method to overcome this problem. Some researchers have investigated embedding inorganic nanopar-
Corresponding author at: Faculty of Chemical Engineering, Sahand University of Technology, Tabriz, Iran. E-mail address: [email protected] (Y. Jafarzadeh).
http://dx.doi.org/10.1016/j.memsci.2017.04.011 Received 28 January 2017; Received in revised form 14 March 2017; Accepted 7 April 2017 Available online 07 April 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved.
Journal of Membrane Science 534 (2017) 18–24
A. Behboudi et al.
solution was prepared by dissolving certain amounts of PEG, PVC and PC in NMP at 35 °C under constant stirring for at least 24 h until a homogeneous solution was obtained. The homogeneous solution was then allowed to degas overnight. Then the solution was cast onto a glass plate using an automatic casting knife (CoaTest, Taiwan). The plate and film were immediately immersed in distilled water as non-solvent to induce phase separation until formed membranes were detached freely from the plate. The water bath was refreshed at least three times to complete NMP exchange. Finally, the membrane was dried and stored at 5 °C. In this research, six membranes were prepared and polymer wt % in all casting solutions was set to 17%. The wt% of PC in the polymer was 0 (neat PVC), 20%, 40%, 50%, 60% and 70%.
ticles such as SiO2 [20], ZnO [21] and TiO2 [17] into PVC membranes. The resultant membranes in these works showed improved water flux and better antifouling properties in comparison with pristine PVC membrane. However, particle agglomeration at higher contents of nanoparticles is an obstacle for organic/inorganic membranes which limits their application. Another bulk modification method is polymer blending, an interesting, inexpensive, versatile and easy method for preparation of polymeric membranes with improved properties [16,22]. In recent years, different polymers have been blended with PVC to prepare blend polymer membranes with enhanced flux and antifouling properties. PVC/PVB (poly vinyl butyral) blend membranes with PVC/PVB ratios ranging from 0:10 to 9:1 were prepared by Peng and Sui and the results showed that water flux and hydrophilicity of the membranes increased by addition of PVB but rejection of egg white protein decreased slightly [22]. Krishnamoorthy et al. prepared PVC/ CA blend ultrafiltration membranes at the presence of polyethylene glycol 600 and found that by increasing the content of PVC in the dope solution, water flux of membranes increased whereas rejection decreased [23]. In another study, PVC/Polystyrene blend membranes were prepared by Alsalhy and it was shown that the rejection of blend membranes were higher than that of pristine PVC membrane even though pure water flux decreased from 112.5 LMH for PVC membrane to 95.2 LMH for PVC/PS blend membrane with 14:6 ratio [24]. Zhou et al. investigated that blending PVC with poly(vinyl chloride-copolyethylene glycol methyl ether methacrylate) copolymer (poly(VCco-PEGMA)) resulted in hydrophilic and fouling resistant membranes in comparison with neat PVC membranes [14]. In another work, PVC/poly (methyl methacrylate-g-polyethylene glycol methacrylate) blend membranes were prepared in water and ethanol coagulation baths and the results showed that hydrophilicity and antifouling properties of blended membranes were higher than that of PVC membrane [13]. The higher hydrophilicity of blended membranes in two last works was attributed to the PEG segments of copolymer [13,14]. Polycarbonate (PC) is a commercial polymer with excellent mechanical properties and chemical stability which is widely used in different applications [25,26]. PC membranes are usually prepared by track-etching technique even though some PC gas separation and microfiltration membranes have been also fabricated by phase inversion method [27,28]. Due to the presence of an oxygen atom doublebonded to a carbon atom in the PC chains, it seems that the blending of PVC and PC results in a hydrophilic membrane with enhanced antifouling property. Therefore, the aim of the present research is to prepare and characterize PVC/PC blend membranes. Membranes with various PVC/PC ratios were prepared via NIPS method and characterized using FESEM, DSC, XRD, contact angle, mechanical tensile, stability test, abrasion test and pure water flux. Moreover, performance and antifouling properties of membranes were evaluated by filtration of BSA protein solution.
2.3. Characterization of membranes 2.3.1. Morphology The morphology of the prepared membranes was visualized with a field emission scanning microscope (FESEM; MIRA3 FEG-SEM, Tescan). Samples of membranes were fractured in liquid nitrogen and coated with gold by sputtering before observation. Pore size distribution and pore density (number of pores per unit area) were estimated by analyzing the surface FESEM images of membranes using Digimizer Image Analysis software. 2.3.2. DSC The compatibility between PVC and PC and also thermal behavior of blend membranes were studied via differential scanning calorimetry (DSC 204 F1 Pheonix, Netzsch) in nitrogen atmosphere. The samples were heated from 25 to 160 °C and held at 160 °C for 5 min, followed by cooling back to 25 °C at 10 °C/min. 2.3.3. XRD X-ray diffraction study of the prepared membranes was conducted by using a diffractometer (D8 Advance, Bruker) equipped with monochromatic Cu-Kα radiation (λ=0.154 nm). All samples were analyzed in continuous scan mode with the 2θ ranging from 10° to 80°. 2.3.4. Contact angle Hydrophilicity of the membranes was evaluated by a contact angle goniometer (PGX, Thwing-Albert Instrument Co., USA) in a sessile drop model at room temperature. The average of 5 measurements was reported to minimize the experimental errors. 2.3.5. Pure water flux, filtration experiments and rejection Pure water flux of membranes was measured via a self-made deadend filtration system. The pre-wetted membranes were compacted for 30 min at 2.5 bar, the pressure was then reduced to 2 bar and after reaching steady state permeation, water flux was calculated using following equation:
2. Experimental 2.1. Materials
J0 = PVC (MW=90000) was supplied by Arvand Petrochemical Company, Iran. PC (MW=190000) was supplied by Khouzestan Petrochemical Company, Iran. Polyethylene glycol (PEG) with molecular weight of 200 Da and 1-methyl 2-pirrolidone (NMP) were purchased from Merck and used as pore former and polymer solvent, respectively. Bovine serum albumin (BSA), sodium hydroxide (reagent grade) and silicon carbide particles (42–47 µm) were purchased from Sigma-Aldrich.
M At
(1)
where J0 is pure water flux (PWF), M is collected mass of water, A is membrane area and t is the time. After measuring J0 , the membrane modulus was connected to another dead-end filtration system containing 1.0 g/L BSA solution. The flux of membranes during filtration was measured for about 4 h and then the modulus was again connected to pure water flux system to measure PWF after fouling (J1). Afterward, the humic acid cake layer formed on the membrane was gently removed mechanically by a sponge and the membrane was rinsed and backwashed by deionized water. Subsequently, the clean membrane was again connected to pure water flux system and PWF after cleaning (J2 ) was measured. Fouling properties of the membranes were evaluated using the following equations:
2.2. Preparation of membranes PVC/PC blend membranes were prepared via non-solvent induced phase separation (NIPS) method. For a specific membrane, the dope 19
Journal of Membrane Science 534 (2017) 18–24
A. Behboudi et al.
TFR =
J0 − J1 J0
(2)
RFR =
J2 − J1 J0
(3)
IFR =
J0 − J2 J0
(4)
FR =
J2 J0
(5)
where TFR, RFR, IFR and FR are total fouling ratio, reversible fouling ratio, irreversible fouling ratio and flux recovery, respectively. These ratios give useful information about fouling phenomenon in the membranes as mentioned elsewhere [2,29]. The humic acid rejection was calculated by:
R (%) = (1−
Cp Cf
) × 100
(6)
where Cp and Cf are the concentrations of humic acid in permeate and feed, respectively. 2.3.6. Mechanical strength, abrasion resistance and stability tests Mechanical strength of prepared membranes was evaluated using a tensile testing machine (Santam STM-5, Iran) at an extension rate of 10 mm/min. Five samples with length of 50 mm and width of 10 mm of each membrane were tested and the average values were reported. The abrasion resistance of the membranes was examined using a method described elsewhere [17]. Briefly, samples of membranes were secured and immersed into 10 wt% silicon carbide slurry. The slurry was then stirred at least 7 days to allow the membranes to be in contact with abrasive silicon carbide particles. Weights of the samples before and after abrasion test were measured to determine weight loss due to the abrasion. In addition, mechanical strength of membranes after abrasion test was evaluated following the similar method mentioned previously. The long term stability of the blend membranes was investigated by submerging the membranes in 1.5 M NaOH solution of pH=12. The solution was shaken slowly for 10 days and after that, the membranes were removed and dried at 60 °C for 12 h. Then, the mechanical strength of them was determined by tensile testing machine to find out the effect of NaOH solution on the stability of the membranes. 3. Results and discussion 3.1. Morphology, compatibility and thermal stability The FESEM images of surface and cross section of prepared membranes are represented in Fig. 1. It can be seen that all the membranes consist of three layers: top layer with a dense skin, middle layer with large macrovoids, and bottom layer with finger-like and/or sponge-like structure. The thickness of bottom layer seems to be increased as the content of PC increased in the dope solution and its structure changed from finger-like to sponge-like. As the content of PC increases in the casting solution, the viscosity of the solution increases because molecular weight of PC is higher than that of PVC in this research (190000 and 90000, respectively). Consequently, higher viscosity leads to the delayed demixing and phase separation which results in sponge-like and denser structure. In addition, the number of surface pores increased with increasing PC content even though surface morphology of neat PVC, 20% and 40% PC membranes are alike. However, the results from image analyzer software, which are depicted in Fig. 2 and Table 1, revealed that the average size of the surface pores decreased slightly with increasing PC content up to 50% PC membrane and then increased. On the other hand, the statistical results from image analyzer software revealed that the number of surface pores are 178, 211, 223, 656, 672 and 667 for
Fig. 1. Surface (left) and cross section (right) FESEM of prepared membranes. (a) Neat PVC, (b) 20% PC, (c) 40% PC, (d) 50% PC, (e) 60% PC and (f) 70% PC blend membranes.
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Fig. 3. Pore size distribution of the prepared membranes. Table 2 Solubility parameters of PVC and PC [30].
Contact angle (deg.)
Neat PVC 20 wt% PC 40 wt% PC 50 wt% PC 60 wt% PC 70 wt% PC
3.56 ± 0.4 3.44 ± 0.4 3.27 ± 0.3 2.85 ± 0.1 4.02 ± 0.3 4.22 ± 0.1
88.6 88.2 81.6 77.2 73.4 66.4
δp (MPa−1)
δh (MPa−1)
δsp (MPa−1)
PVC PC
17.6 18.10
7.80 5.90
3.40 6.90
19.54 20.24
δsp 2 = δD 2 + δH 2 + δP 2
Table 1 Mean pore radius of surface pores estimated by image analyzer software and contact angle of prepared membranes. Mean pore radius (nm)
δd (MPa−1)
polymers (substances) are said to be miscible if the difference between their solubility parameters (δsp ) is less than 1.02 MPa0.5 [22]. The components of solubility parameters of PVC and PC are shown in Table 2 in which δD , δH and δP are dispersion cohesion (solubility) parameter, hydrogen bonding (solubility) parameter and polar cohesion (solubility) parameter, respectively [30]. Having these components on the hand, solubility parameter is calculated using the following equation:
Fig. 2. Estimation of the size and the number of pores on the membranes’ surface by image analyzer software. (a) Neat PVC, (b) 20% PC, (c) 40% PC, (d) 50% PC, (e) 60% PC and (f) 70% PC blend membranes.
Membrane
Polymer
(8)
According to Table 2, it can be seen that δsp PC − δsp PVC =0.7MPa0.5 indicating that PVC and PC are miscible because in the solubility theory when δspi − δsp j <1.02 MPa0.5, then two polymers are compatible. The DSC curves of prepared membranes are shown in Fig. 4. In this figure, DSC curve of neat PC was also added to compare the results. It can be seen that glass transition temperature (Tg) of neat PVC membrane is about 83.5 °C and with increasing the PC content, Tg of blend membranes increased continuously up to 126.4 °C which means that thermal stability of the membranes increased with increasing PC content. Moreover, a single Tg in the curves indicates that there was no secondary phase transition in blend membranes. This means that PVC and PC form compatible and miscible membranes without any segrega-
subsequent membranes which indicates that number of pores per unit area (pore density) increased as the content of PC increased. Fig. 3 shows that pore size distribution curves of membranes shifted towards smaller pores with increasing PC content up to 50% and then shifted back towards larger pores for 60% and 70% PC membranes. This means that blending PVC and PC with PC contents up to 50% results in membranes with not only smaller pores, but also membranes with larger number of pores in comparison with neat PVC membrane. In addition, total area of the pores, calculated by multiplying the number of pores in the mean area of them was 99235, 103596, 115674, 245280, 255184, and 279473 pixels for subsequent membranes, respectively. If we suppose that all of the surface pores counted by image analyzer software are open end, then we may expect that the water permeation of the prepared membranes will increase as the content of PC increases. Moreover, we expect that the rejection of the membranes increases as the mean pore size of the membranes shift to smaller pores and the results in the next sections confirm it. It should be noted that PVC and PC are compatible and this can be shown based on the solubility parameter theory. In this theory, two
Fig. 4. DSC curves for the prepared membranes.
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Fig. 7. Weight loss per unit area of the membranes after 7 days abrasion test with silicon carbide particles.
Fig. 5. XRD diffraction patterns of the prepared membranes.
tion. The compatibility of PVC and PC was also confirmed by XRD analysis as shown in Fig. 5. In XRD diffraction pattern of neat PVC membrane, there is a broad peak in the 2θ range of about 10–30° which indicates the amorphous nature of PVC [31]. In addition, the small peak observed at 2θ of 38° represents the semicrystallinity of PVC [32]. On the other hand, a broad peak in the 2θ range of about 13–24° was observed in XRD pattern of neat PC membrane indicating that PC is also amorphous. It can be seen that all blend membranes have almost the same XRD diffraction patterns and as the content of PC increased in the membranes, the broadness of the peak decreased slightly and the maxima shifted toward smaller 2θ. These results confirm the compatibility of PVC and PC.
increase in PWF can be related to hydrophilicity and surface pores of the membranes. As mentioned, hydrophilicity of the membranes increased with increasing PC content which favors water permeation. Moreover, as shown in FESEM images, the number of surface pores as well as total area of pores increased as the content of PC increased which provides more paths for water permeation even though pore size distribution of membranes shifted to smaller pores with increasing PC content up to 50%.
3.3. Abrasion test, stability test and mechanical properties Most of polymeric membranes are used in MF and UF pretreatment processes to separate solid particles from seawater prior to reverse osmosis. Solid particles are abrasive materials which reduce the durability of polymeric membranes. Therefore, the effect of abrasive materials on the polymeric membranes is an important issue in membrane technology and preparation of polymeric membranes with abrasion-resistance property seems to be interesting. Fig. 7 shows the weight loss per unit area of the membranes after abrasion test. It can be observed that weight loss of membranes decreased with increasing PC content. This indicates that blending PVC with PC resulted in more abrasion-resistant membranes in comparison with neat PVC membrane. Mechanical properties may also be used to determine abrasion resistance of membranes. In this study, tensile strength and elongation at break for all membranes were measured before and after abrasion test and the results were summarized in Table 3. It can be seen that the tensile strength of the membranes increased continuously as the PC content increased up to 50% and no remarkable increase was observed at higher content of PC. This means that mechanical strength of PVC membrane was improved at the presence of PC which can be attributed to the rigidity of PC chains. Moreover, tensile strengths of the membranes decreased after abrasion test but the tensile reduction is more severe for neat PVC membrane. In other words, tensile reduction (difference between tensile strengths before and after abrasion divided by tensile strength before abrasion) of PVC membrane after abrasion is 44% whereas this ratio for subsequent membranes is 28%, 18%, 4%, 8% and 14%, respectively. Therefore, it can be seen that the membrane containing 50% PC exhibited the highest abrasion resistance. Table 3 also shows that elongation at break for prepared membranes increased slightly with increasing PC content. The higher elongation of blend membranes may be attributed to the degree of crystallinity of them. PVC and PC are amorphous polymers with high rigid chains. This capability is due to bulky side groups which results in high Tg values. On the other hand, rigid polymers with higher molecular weight exhibit more elongation in comparison with those of the lower molecular weight. Therefore, PC has more elongation than PVC due to its higher molecular weight. However, the semi-crystallinity structure of PVC (as
3.2. Hydrophilicity and pure water flux Table 1 shows that the contact angle between membranes and water droplet decreases continuously with increasing PC content in the membranes. The reduction of contact angle indicates that the hydrophilicity of the membranes increased at the presence of polycarbonate. As mentioned, there is an oxygen atom double-bonded to a carbon atom in the PC chains which increases the interaction with water molecules and subsequently, enhances the hydrophilicity. It was shown that the contact angle of neat PC membrane prepared by track etching method is about 72° [33] and as shown in Table 1, the contact angle of neat PVC membrane in the present work is 88.6° which indicates that decrease in contact angle is due to the presence of hydrophilic PC in the blend membranes. The pure water flux of PVC/PC blend membranes are represented in Fig. 6. It can be seen that the PWF increased from 403 kg/m2 h for neat PVC membrane to about 1260 kg/m2 h for 70% PC membrane. The
Fig. 6. Pure water flux of the prepared membranes.
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Table 3 Mechanical properties of the membranes before and after abrasion and stability tests. Before Tests
After abrasion test
After stability test
Membrane
Tensile, MPa
Elongation, (%)
Tensile, MPa
Elongation, (%)
Tensile, MPa
Elongation, (%)
Neat PVC 20 wt% PC 40 wt% PC 50 wt% PC 60 wt% PC 70 wt% PC Neat PC
4.3 ± 0.1 4.7 ± 0.1 6.2 ± 0.2 7.4 ± 0.2 7.4 ± 0.1 7.7 ± 0.2 11.3 ± 0.2
33 ± 3 33 ± 3 35 ± 2 41 ± 2 42 ± 2 42 ± 3 48 ± 3
2.4 ± 0.4 3.4 ± 0.5 5.1 ± 0.4 7.1 ± 0.2 6.8 ± 0.4 6.6 ± 0.4 10.8 ± 0.5
24 ± 4 28 ± 4 30 ± 2 35 ± 2 36 ± 2 37 ± 2 43 ± 2
4.3 ± 0.3 4.7 ± 0.2 6.1 ± 0.1 7.1 ± 0.3 7.3 ± 0.2 7.5 ± 0.1 9.7 ± 0.1
32 ± 2 33 ± 4 34 ± 2 39 ± 3 39 ± 2 39 ± 2 31 ± 2
70 min. For blend membranes, however, the initial as well as final fluxes are higher than that of neat PVC membrane. The flux declines for blend membranes were about 30%, 24%, 19%, 24% and 21% for subsequent membranes, respectively, which indicates that fouling resistance of PVC/PC blend membranes is higher than that of neat PVC membrane and 50% PC membrane had the least decline in flux. It has been accepted that antifouling properties of the membranes increase with increasing hydrophilicity and as shown previously, the contact angle of the membranes decreased with addition of PC in the dope solution due to the presence of carbonate group in the PC chains. Another reason for this result may be related to the surface pore size of the membranes. It has been shown that the membranes with larger pore sizes at the surface fouled easily [34] and as shown previously, pore size distribution of the membranes shifted towards smaller pores as the content of PC increased. More information about antifouling properties of the membranes can be obtained from fouling parameters including TFR, RFR, IFR and FR which were defined in Section 2.3.5. These parameters were estimated based on the pure water flux and filtration flux of BSA solution and the results were depicted in Table 4. It can be seen that TFR of the membranes decreased from 60.33% for neat PVC membrane to 31.51% for 50% PC membrane and then increased for membranes with higher contents of PC but all blend membranes had lower TFR than neat PVC membrane. TFR is a degree of total flux loss due to the fouling and the less TFR value indicates better antifouling property. Therefore, the results show that PVC/PC blend membranes are more fouling resistant than neat PVC membrane and the membrane containing 50% PC has the lowest fouling during 4 h filtration of BSA solution. In addition, RFR value for neat PVC membrane is 30.33% which shows that 50% of total fouling by BSA for this membrane is reversible (remember that TFR=RFR+IFR). It can be observed that with increasing PC content, reversible portion of fouling (RFR/TFR) increased and reached to its maximum for 50% PC membrane and then decreased slightly. This means that blending PVC with PC results in membranes with higher reversible portion of fouling which can be cleaned easily by conventional cleaning methods. Table 4 also shows that FR of all of blend membranes had higher than that of neat PVC membrane and the maximum FR belonged to 50% PC membrane. FR is an index indicating the degree of recovery of water flux after cleaning a fouled membrane and the higher FR value is, the better antifouling property is. These results suggest that PVC/PC blend membranes are more fouling resistant in comparison with neat PVC membrane. This may be because of improved hydrophilicity due to the presence of PC in the membranes. In this study, the rejection test of prepared membranes was carried out by measuring the concentration of BSA in the permeated water and the results are represented in Table 4. As can be observed, the rejection of the membranes increased with increasing PC content up to 50% and then decreased again for subsequent membranes. However, the neat PVC membrane had the least rejection whereas 50% PC membrane showed about 99% rejection. The trend of rejection is in accordance to the results of pore size distribution for the membranes and confirm the fact that blending of PVC and PC results in membranes with small pores
revealed by XRD diffraction patterns, the small peak observed at 2θ of 38° for PVC represents the semi-crystallinity of it) is yet another aspect of PVC which results in lower elongation. These results indicate that blending PVC and PC results in membranes with higher elongation and tensile in comparison with neat PVC membrane. After abrasion test, elongation of neat PVC membrane decreased significantly up to 30% of its initial value due to abrasive effect of particles. However, there was no remarkable decrease of tensile and elongation among blend membranes as they exhibit excellent strength even after abrasion test. It should be noted that increasing PC content improved blend membranes mechanical strength and increased their mechanical behavior in favor of neat PC. Moreover, there was a very little decrease in mechanical properties of PC. In order to investigate long term stability of prepared membranes, they were submerged in 1.5 M NaOH solution of pH=12 for 10 days. After that, mechanical properties of them were determined and the results were summarized in Table 3. As can be seen, tensile and elongation of neat PC membrane decreased remarkably after 10 days being submerged in NaOH solution. Moreover, mechanical properties of blend membranes after stability test decreased as the content of PC increased. However, mechanical properties of PVC membrane remained almost the same which can be attributed to good chemical resistance of PVC. These results mean that even though PVC/PC blend membranes possess good mechanical properties against abrasive materials, addition of more PC to PVC results in membranes with low chemical stability. Therefore, the best blend ratio should be chosen based on the optimized conditions.
3.4. Antifouling properties and rejection Fig. 8 shows the flux-time curves for the prepared membranes during filtration of 1.0 g/L BSA solution. It can be seen that the flux of neat PVC membrane was the least among the membranes and its value decreased from 337 kg/m2 h to about 190 kg/m2 h (ca. 44%) after
Fig. 8. Changes in permeate flux of the prepared membranes during 4 h filtration of 1.0 g/L BSA solution.
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Table 4 Fouling parameters and BSA rejection of prepared membranes. Membrane
TFR (%)
RFR (%)
IRF (%)
FR (%)
Rejection (%)
Neat PVC 20 wt% PC 40 wt% PC 50 wt% PC 60 wt% PC 70 wt% PC
60.33 ± 0.7 55.21 ± 0.5 38.67 ± 0.6 31.51 ± 0.5 36.34 ± 0.3 44.87 ± 0.3
30.33 ± 0.7 30.64 ± 0.5 20.14 ± 0.6 21.02 ± 0.5 20.44 ± 0.3 19.88 ± 0.3
30.00 ± 0.7 24.54 ± 0.5 18.53 ± 0.6 10.57 ± 0.5 15.91 ± 0.3 24.99 ± 0.3
70.00 ± 0.6 75.46 ± 0.5 81.47 ± 0.6 89.50 ± 0.5 84.07 ± 0.3 75.01 ± 0.3
84.94 ± 0.04 91.81 ± 0.04 97.61 ± 0.02 98.92 ± 0.02 96.08 ± 0.04 93.14 ± 0.02
mium (VI) removal, Carbohydr. Polym. 153 (2016) 582–591. [12] S. Li, Z. Cui, L. Zhang, B. He, J. Li, The effect of sulfonated polysulfone on the compatibility and structure of polyethersulfone-based blend membranes, J. Membr. Sci. 513 (2016) 1–11. [13] L.-F. Fang, B.-K. Zhu, L.-P. Zhu, H. Matsuyama, S. Zhao, Structures and antifouling properties of polyvinyl chloride/poly(methyl methacrylate)-graft-poly(ethylene glycol) blend membranes formed in different coagulation media, J. Membr. Sci. 524 (2017) 235–244. [14] Z. Zhou, S. Rajabzadeh, A.R. Shaikh, Y. Kakihana, W. Ma, H. Matsuyama, Effect of surface properties on antifouling performance of poly(vinyl chloride-co-poly(ethylene glycol)methyl ether methacrylate)/PVC blend membrane, J. Membr. Sci. 514 (2016) 537–546. [15] M. Hossein Razzaghi, A. Safekordi, M. Tavakolmoghadam, F. Rekabdar, M. Hemmati, Morphological and separation performance study of PVDF/CA blend membranes, J. Membr. Sci. 470 (2014) 547–557. [16] S. Jiang, J. Wang, J. Wu, Y. Chen, Poly(vinyl chloride) and poly(ether sulfone)-gpoly(ether glycol) methyl ether methacrylate blend membranes with improved ultrafiltration performance and fouling resistance, J. Appl. Polym. Sci. 132 (2015) 41726. [17] A. Behboudi, Y. Jafarzadeh, R. Yegani, Preparation and characterization of TiO2 embedded PVC ultrafiltration membranes, Chem. Eng. Res. Des. 114 (2016) 96–107. [18] S.M. Hosseini, S.S. Madaeni, A. Zendehnam, A.R. Moghadassi, A.R. Khodabakhshi, H. Sanaeepur, Preparation and characterization of PVC based heterogeneous ion exchange membrane coated with Ag nanoparticles by (thermal-plasma) treatment assisted surface modification, J. Ind. Eng. Chem. 19 (2013) 854–862. [19] R. Patel, M. Patel, S.H. Ahn, Y.K. Sung, H.-K. Lee, J.H. Kim, J.-S. Sung, Bioinert membranes prepared from amphiphilic poly(vinyl chloride)-g-poly(oxyethylene methacrylate) graft copolymers, Mater. Sci. Eng.: C 33 (2013) 1662–1670. [20] Z. Yu, X. Liu, F. Zhao, X. Liang, Y. Tian, Fabrication of a low-cost nano-SiO2/PVC composite ultrafiltration membrane and its antifouling performance, J. Appl. Polym. Sci. 132 (2015) 41267. [21] H. Rabiee, V. Vatanpour, M.H.D.A. Farahani, H. Zarrabi, Improvement in flux and antifouling properties of PVC ultrafiltration membranes by incorporation of zinc oxide (ZnO) nanoparticles, Sep. Purif. Technol. 156 (2) (2015) 299–310. [22] Y. Peng, Y. Sui, Compatibility research on PVC/PVB blended membranes, Desalination 196 (2006) 13–21. [23] L. Krishnamoorthy, P.M. Arif, R. Ahmedkhan, Separation of proteins from aqueous solution using cellulose acetate/poly (vinyl chloride) blend ultrafiltration membrane, J. Mater. Sci. 46 (2011) 2914–2921. [24] Q.F. Alsalhy, Hollow fiber ultrafiltration membranes prepared from blends of poly (vinyl chloride) and polystyrene, Desalination 294 (2012) 44–52. [25] M. Di Luccio, R. Nobrega, C.P. Borges, Microporous anisotropic phase inversion membranes from bisphenol A polycarbonate: effect of additives to the polymer solution, J. Appl. Polym. Sci. 86 (2002) 3085–3096. [26] M. Di Luccio, R. Nobrega, C.P. Borges, Microporous anisotropic phase inversion membranes from bisphenol-A polycarbonate: study of a ternary system, Polymer 41 (2000) 4309–4315. [27] R. Kumar, Kamakshi, M. Kumar, K. Awasthi, Functionalized Pd-decorated and aligned MWCNTs in polycarbonate as a selective membrane for hydrogen separation, Int. J. Hydrog. Energy 41 (2016) 23057–23066. [28] Y. Zhang, J. Zhu, Composite photocatalytic membrane prepared by embedding porous SiO2 shell/void/TiO2 core particles into polycarbonate for photodegrading and removing pollutant from water, Chem. Eng. Sci. 126 (2015) 390–398. [29] Y. Jafarzadeh, R. Yegani, Thermal, mechanical, and structural properties of zno/ polyethylene membranes made by thermally induced phase separation method, J. Appl. Polym. Sci. 132 (2015) 42338. [30] C.M. Hansen, Hansen Solubility Parameters: A User's Handbook, CRC Press, 2000. [31] W.E. Mahmoud, Morphology and physical properties of poly(ethylene oxide) loaded graphene nanocomposites prepared by two different techniques, Eur. Polym. J. 47 (2011) 1534–1540. [32] F. Ma, N. Yuan, J. Ding, The conductive network made up by the reduced graphene nanosheet/polyaniline/polyvinyl chloride, J. Appl. Polym. Sci. 128 (2013) 3870–3875. [33] D. Çökeliler, Enhancement of polycarbonate membrane permeability due to plasma polymerization precursors, Appl. Surf. Sci. 268 (2013) 28–36. [34] X. Fu, T. Maruyama, T. Sotani, H. Matsuyama, Effect of surface morphology on membrane fouling by humic acid with the use of cellulose acetate butyrate hollow fiber membranes, J. Membr. Sci. 320 (2008) 483–491.
on the surface. If we consider the performance of the membranes in terms of rejection and pure water flux, we would find that all PVC/PC blend membranes possess higher pure water flux and higher BSA rejection than that of neat PVC membrane. 4. Conclusion PVC/PC blend membranes with different contents of PC were prepared and characterized by a set of analyses. The compatibility between PVC and PC was confirmed by FESEM, DSC and XRD analyses as it could be predicted by Hansen solubility theory. The results showed that pore size distribution of blend membranes shifted to small pores as the content of PC in the membranes increased. This led to higher pure water flux and higher BSA rejection for blend membranes. Moreover, hydrophilicity of the membranes increased with increasing PC content which was attributed to the carbonate group in the PC chains. The blend membranes also possessed higher tensile strength before and after abrasion test. However, chemical stability of membranes against NaOH solution after 10 days decreased by increasing PC content. It was shown that antifouling properties of blend membranes were better than neat PVC membrane which was attributed to the hydrophilicity and pore size distribution. In conclusion, blending PVC with PC resulted in high performance membranes with improved antifouling properties and PVC/PC membrane with ratio of 50:50 was chosen as the best membrane in this study. References [1] J.H. Jhaveri, Z.V.P. Murthy, A comprehensive review on anti-fouling nanocomposite membranes for pressure driven membrane separation processes, Desalination 379 (2016) 137–154. [2] Y. Jafarzadeh, R. Yegani, M. Sedaghat, Preparation, characterization and fouling analysis of ZnO/polyethylene hybrid membranes for collagen separation, Chem. Eng. Res. Des. 94 (2015) 417–427. [3] N. Hilal, O.O. Ogunbiyi, N.J. Miles, R. Nigmatullin, Methods employed for control of fouling in MF and UF membranes: a comprehensive review, Sep. Sci. Technol. 40 (2005) 1957–2005. [4] C. Qiu, F. Xu, Q.T. Nguyen, Z. Ping, Nanofiltration membrane prepared from cardo polyetherketone ultrafiltration membrane by UV-induced grafting method, J. Membr. Sci. 255 (2005) 107–115. [5] H.-Y. Yu, M.-X. Hu, Z.-K. Xu, J.-L. Wang, S.-Y. Wang, Surface modification of polypropylene microporous membranes to improve their antifouling property in MBR: NH3 plasma treatment, Sep. Purif. Technol. 45 (2005) 8–15. [6] M.L. Steen, A.C. Jordan, E.R. Fisher, Hydrophilic modification of polymeric membranes by low temperature H2O plasma treatment, J. Membr. Sci. 204 (2002) 341–357. [7] C.Y. Lai, A. Groth, S. Gray, M. Duke, Preparation and characterization of poly (vinylidene fluoride)/nanoclay nanocomposite flat sheet membranes for abrasion resistance, Water Res. 57 (2014) 56–66. [8] S. Balta, A. Sotto, P. Luis, L. Benea, B. Van der Bruggen, J. Kim, A new outlook on membrane enhancement with nanoparticles: the alternative of ZnO, J. Membr. Sci. 389 (2012) 155–161. [9] Y. Jafarzadeh, R. Yegani, S.B. Tantekin-Ersolmaz, Effect of TiO2 nanoparticles on structure and properties of high density polyethylene membranes prepared by thermally induced phase separation method, Polym. Adv. Technol. 26 (2015) 392–398. [10] Z. Xu, T. Wu, J. Shi, K. Teng, W. Wang, M. Ma, J. Li, X. Qian, C. Li, J. Fan, Photocatalytic antifouling PVDF ultrafiltration membranes based on synergy of graphene oxide and TiO2 for water treatment, J. Membr. Sci. 520 (2016) 281–293. [11] T. Riaz, A. Ahmad, S. Saleemi, M. Adrees, F. Jamshed, A.M. Hai, T. Jamil, Synthesis and characterization of polyurethane-cellulose acetate blend membrane for chro-
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Polyvinyl chloride/polycarbonate blend ultrafiltration membranes for water treatment
MARK
⁎
A. Behboudia,b, Y. Jafarzadeha,b, , R. Yegania,b a b
Faculty of Chemical Engineering, Sahand University of Technology, Tabriz, Iran Membrane Technology Research Center, Sahand University of Technology, Tabriz, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyvinyl chloride Polycarbonate Blend membrane Fouling
In this study, PVC/PC blend membranes were prepared via NIPS methods. Characterization techniques including FESEM, XRD, DSC, contact angle measurement, mechanical properties, abrasion test, stability test, pure water flux and filtration of BSA solution were applied to investigate the effects of PC content on the structure and performance of blend membranes. It was shown than PVC and PC are compatible and the results of FESEM, XRD and DSC analyses confirmed their compatibility. The pore size distribution of membranes shifted toward smaller pores as the content of PC in the membranes increased up to 50% and then shifted back toward larger pores. In addition, hydrophilicity, tensile strength and abrasion resistance of the blend membranes were improved. However, chemical stability of membranes against NaOH solution after 10 days decreased by increasing PC content. Pure water flux and BSA rejection as the performance criteria of membranes improved due to the presence of PC. It was found that antifouling properties of membranes increased with increasing PC content. The results indicated that PVC/PC blend membranes were high performance and fouling resistant membranes in comparison with neat PVC membrane.
1. Introduction Nowadays, membrane processes such as microfiltration, ultrafiltration and reverse osmosis are widely used to purify water for different uses. The major limitation in application of polymeric membranes in water and wastewater treatment processes is fouling problem [1]. Most of polymeric membranes are hydrophobic or less hydrophilic therefore these membranes undergo fouling by proteins and other natural organic matters (NOMs) during water treatment. To overcome this problem, many efforts have been made and it has been accepted that hydrophilic membranes are less susceptible to fouling. Therefore, increasing membrane hydrophilicity is a suitable and effective manner to decrease membrane fouling [2]. During last decades, different strategies have been adopted to mitigate fouling phenomenon which can be classified into four main categories: feed pretreatment, optimization of operating conditions, cleaning procedures and membrane modification [3]. The latter has attracted lots of attentions and several studies have been performed to enhance antifouling properties of polymeric membranes. Modification of membrane surface [4–6], incorporation of inorganic nanoparticles into membrane matrix [7–10] and blending of different polymers [11–13]are three approaches which have been used to prepare anti-
⁎
fouling membranes. Among these approaches, blending of polymers is one of the most convenient and practical ways for improving the antifouling properties and performance of polymeric membranes because it is the most convenient method in operation and the least expensive from economical point of view [14–16]. Among different polymers which are used in fabrication of polymeric membranes, polyvinyl chloride (PVC) has attracted lots of attentions due to its characteristics such as good mechanical strength, abrasion resistance, chemical stabilization, thermal properties, low cost and corrosion resistance [16,17]. However, the use of PVC membrane in water and wastewater treatment processes has been limited because it is easily fouled due to the strong hydrophobicity. To increase hydrophilicity and antifouling properties of PVC membrane, several modification methods have been explored. For example, surface of PVC membrane was modified using surface coating [18] and surface grafting [19] techniques which resulted in hydrophilic and fouling resistant membranes. In surface modification methods, however, internal pores of the modified membranes are barely concerned. Moreover, surface modification methods involve an extra step in membrane fabrication process [14]. Therefore, modification of the bulk of membranes seems to be an alternative method to overcome this problem. Some researchers have investigated embedding inorganic nanopar-
Corresponding author at: Faculty of Chemical Engineering, Sahand University of Technology, Tabriz, Iran. E-mail address: [email protected] (Y. Jafarzadeh).
http://dx.doi.org/10.1016/j.memsci.2017.04.011 Received 28 January 2017; Received in revised form 14 March 2017; Accepted 7 April 2017 Available online 07 April 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved.
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solution was prepared by dissolving certain amounts of PEG, PVC and PC in NMP at 35 °C under constant stirring for at least 24 h until a homogeneous solution was obtained. The homogeneous solution was then allowed to degas overnight. Then the solution was cast onto a glass plate using an automatic casting knife (CoaTest, Taiwan). The plate and film were immediately immersed in distilled water as non-solvent to induce phase separation until formed membranes were detached freely from the plate. The water bath was refreshed at least three times to complete NMP exchange. Finally, the membrane was dried and stored at 5 °C. In this research, six membranes were prepared and polymer wt % in all casting solutions was set to 17%. The wt% of PC in the polymer was 0 (neat PVC), 20%, 40%, 50%, 60% and 70%.
ticles such as SiO2 [20], ZnO [21] and TiO2 [17] into PVC membranes. The resultant membranes in these works showed improved water flux and better antifouling properties in comparison with pristine PVC membrane. However, particle agglomeration at higher contents of nanoparticles is an obstacle for organic/inorganic membranes which limits their application. Another bulk modification method is polymer blending, an interesting, inexpensive, versatile and easy method for preparation of polymeric membranes with improved properties [16,22]. In recent years, different polymers have been blended with PVC to prepare blend polymer membranes with enhanced flux and antifouling properties. PVC/PVB (poly vinyl butyral) blend membranes with PVC/PVB ratios ranging from 0:10 to 9:1 were prepared by Peng and Sui and the results showed that water flux and hydrophilicity of the membranes increased by addition of PVB but rejection of egg white protein decreased slightly [22]. Krishnamoorthy et al. prepared PVC/ CA blend ultrafiltration membranes at the presence of polyethylene glycol 600 and found that by increasing the content of PVC in the dope solution, water flux of membranes increased whereas rejection decreased [23]. In another study, PVC/Polystyrene blend membranes were prepared by Alsalhy and it was shown that the rejection of blend membranes were higher than that of pristine PVC membrane even though pure water flux decreased from 112.5 LMH for PVC membrane to 95.2 LMH for PVC/PS blend membrane with 14:6 ratio [24]. Zhou et al. investigated that blending PVC with poly(vinyl chloride-copolyethylene glycol methyl ether methacrylate) copolymer (poly(VCco-PEGMA)) resulted in hydrophilic and fouling resistant membranes in comparison with neat PVC membranes [14]. In another work, PVC/poly (methyl methacrylate-g-polyethylene glycol methacrylate) blend membranes were prepared in water and ethanol coagulation baths and the results showed that hydrophilicity and antifouling properties of blended membranes were higher than that of PVC membrane [13]. The higher hydrophilicity of blended membranes in two last works was attributed to the PEG segments of copolymer [13,14]. Polycarbonate (PC) is a commercial polymer with excellent mechanical properties and chemical stability which is widely used in different applications [25,26]. PC membranes are usually prepared by track-etching technique even though some PC gas separation and microfiltration membranes have been also fabricated by phase inversion method [27,28]. Due to the presence of an oxygen atom doublebonded to a carbon atom in the PC chains, it seems that the blending of PVC and PC results in a hydrophilic membrane with enhanced antifouling property. Therefore, the aim of the present research is to prepare and characterize PVC/PC blend membranes. Membranes with various PVC/PC ratios were prepared via NIPS method and characterized using FESEM, DSC, XRD, contact angle, mechanical tensile, stability test, abrasion test and pure water flux. Moreover, performance and antifouling properties of membranes were evaluated by filtration of BSA protein solution.
2.3. Characterization of membranes 2.3.1. Morphology The morphology of the prepared membranes was visualized with a field emission scanning microscope (FESEM; MIRA3 FEG-SEM, Tescan). Samples of membranes were fractured in liquid nitrogen and coated with gold by sputtering before observation. Pore size distribution and pore density (number of pores per unit area) were estimated by analyzing the surface FESEM images of membranes using Digimizer Image Analysis software. 2.3.2. DSC The compatibility between PVC and PC and also thermal behavior of blend membranes were studied via differential scanning calorimetry (DSC 204 F1 Pheonix, Netzsch) in nitrogen atmosphere. The samples were heated from 25 to 160 °C and held at 160 °C for 5 min, followed by cooling back to 25 °C at 10 °C/min. 2.3.3. XRD X-ray diffraction study of the prepared membranes was conducted by using a diffractometer (D8 Advance, Bruker) equipped with monochromatic Cu-Kα radiation (λ=0.154 nm). All samples were analyzed in continuous scan mode with the 2θ ranging from 10° to 80°. 2.3.4. Contact angle Hydrophilicity of the membranes was evaluated by a contact angle goniometer (PGX, Thwing-Albert Instrument Co., USA) in a sessile drop model at room temperature. The average of 5 measurements was reported to minimize the experimental errors. 2.3.5. Pure water flux, filtration experiments and rejection Pure water flux of membranes was measured via a self-made deadend filtration system. The pre-wetted membranes were compacted for 30 min at 2.5 bar, the pressure was then reduced to 2 bar and after reaching steady state permeation, water flux was calculated using following equation:
2. Experimental 2.1. Materials
J0 = PVC (MW=90000) was supplied by Arvand Petrochemical Company, Iran. PC (MW=190000) was supplied by Khouzestan Petrochemical Company, Iran. Polyethylene glycol (PEG) with molecular weight of 200 Da and 1-methyl 2-pirrolidone (NMP) were purchased from Merck and used as pore former and polymer solvent, respectively. Bovine serum albumin (BSA), sodium hydroxide (reagent grade) and silicon carbide particles (42–47 µm) were purchased from Sigma-Aldrich.
M At
(1)
where J0 is pure water flux (PWF), M is collected mass of water, A is membrane area and t is the time. After measuring J0 , the membrane modulus was connected to another dead-end filtration system containing 1.0 g/L BSA solution. The flux of membranes during filtration was measured for about 4 h and then the modulus was again connected to pure water flux system to measure PWF after fouling (J1). Afterward, the humic acid cake layer formed on the membrane was gently removed mechanically by a sponge and the membrane was rinsed and backwashed by deionized water. Subsequently, the clean membrane was again connected to pure water flux system and PWF after cleaning (J2 ) was measured. Fouling properties of the membranes were evaluated using the following equations:
2.2. Preparation of membranes PVC/PC blend membranes were prepared via non-solvent induced phase separation (NIPS) method. For a specific membrane, the dope 19
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TFR =
J0 − J1 J0
(2)
RFR =
J2 − J1 J0
(3)
IFR =
J0 − J2 J0
(4)
FR =
J2 J0
(5)
where TFR, RFR, IFR and FR are total fouling ratio, reversible fouling ratio, irreversible fouling ratio and flux recovery, respectively. These ratios give useful information about fouling phenomenon in the membranes as mentioned elsewhere [2,29]. The humic acid rejection was calculated by:
R (%) = (1−
Cp Cf
) × 100
(6)
where Cp and Cf are the concentrations of humic acid in permeate and feed, respectively. 2.3.6. Mechanical strength, abrasion resistance and stability tests Mechanical strength of prepared membranes was evaluated using a tensile testing machine (Santam STM-5, Iran) at an extension rate of 10 mm/min. Five samples with length of 50 mm and width of 10 mm of each membrane were tested and the average values were reported. The abrasion resistance of the membranes was examined using a method described elsewhere [17]. Briefly, samples of membranes were secured and immersed into 10 wt% silicon carbide slurry. The slurry was then stirred at least 7 days to allow the membranes to be in contact with abrasive silicon carbide particles. Weights of the samples before and after abrasion test were measured to determine weight loss due to the abrasion. In addition, mechanical strength of membranes after abrasion test was evaluated following the similar method mentioned previously. The long term stability of the blend membranes was investigated by submerging the membranes in 1.5 M NaOH solution of pH=12. The solution was shaken slowly for 10 days and after that, the membranes were removed and dried at 60 °C for 12 h. Then, the mechanical strength of them was determined by tensile testing machine to find out the effect of NaOH solution on the stability of the membranes. 3. Results and discussion 3.1. Morphology, compatibility and thermal stability The FESEM images of surface and cross section of prepared membranes are represented in Fig. 1. It can be seen that all the membranes consist of three layers: top layer with a dense skin, middle layer with large macrovoids, and bottom layer with finger-like and/or sponge-like structure. The thickness of bottom layer seems to be increased as the content of PC increased in the dope solution and its structure changed from finger-like to sponge-like. As the content of PC increases in the casting solution, the viscosity of the solution increases because molecular weight of PC is higher than that of PVC in this research (190000 and 90000, respectively). Consequently, higher viscosity leads to the delayed demixing and phase separation which results in sponge-like and denser structure. In addition, the number of surface pores increased with increasing PC content even though surface morphology of neat PVC, 20% and 40% PC membranes are alike. However, the results from image analyzer software, which are depicted in Fig. 2 and Table 1, revealed that the average size of the surface pores decreased slightly with increasing PC content up to 50% PC membrane and then increased. On the other hand, the statistical results from image analyzer software revealed that the number of surface pores are 178, 211, 223, 656, 672 and 667 for
Fig. 1. Surface (left) and cross section (right) FESEM of prepared membranes. (a) Neat PVC, (b) 20% PC, (c) 40% PC, (d) 50% PC, (e) 60% PC and (f) 70% PC blend membranes.
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Fig. 3. Pore size distribution of the prepared membranes. Table 2 Solubility parameters of PVC and PC [30].
Contact angle (deg.)
Neat PVC 20 wt% PC 40 wt% PC 50 wt% PC 60 wt% PC 70 wt% PC
3.56 ± 0.4 3.44 ± 0.4 3.27 ± 0.3 2.85 ± 0.1 4.02 ± 0.3 4.22 ± 0.1
88.6 88.2 81.6 77.2 73.4 66.4
δp (MPa−1)
δh (MPa−1)
δsp (MPa−1)
PVC PC
17.6 18.10
7.80 5.90
3.40 6.90
19.54 20.24
δsp 2 = δD 2 + δH 2 + δP 2
Table 1 Mean pore radius of surface pores estimated by image analyzer software and contact angle of prepared membranes. Mean pore radius (nm)
δd (MPa−1)
polymers (substances) are said to be miscible if the difference between their solubility parameters (δsp ) is less than 1.02 MPa0.5 [22]. The components of solubility parameters of PVC and PC are shown in Table 2 in which δD , δH and δP are dispersion cohesion (solubility) parameter, hydrogen bonding (solubility) parameter and polar cohesion (solubility) parameter, respectively [30]. Having these components on the hand, solubility parameter is calculated using the following equation:
Fig. 2. Estimation of the size and the number of pores on the membranes’ surface by image analyzer software. (a) Neat PVC, (b) 20% PC, (c) 40% PC, (d) 50% PC, (e) 60% PC and (f) 70% PC blend membranes.
Membrane
Polymer
(8)
According to Table 2, it can be seen that δsp PC − δsp PVC =0.7MPa0.5 indicating that PVC and PC are miscible because in the solubility theory when δspi − δsp j <1.02 MPa0.5, then two polymers are compatible. The DSC curves of prepared membranes are shown in Fig. 4. In this figure, DSC curve of neat PC was also added to compare the results. It can be seen that glass transition temperature (Tg) of neat PVC membrane is about 83.5 °C and with increasing the PC content, Tg of blend membranes increased continuously up to 126.4 °C which means that thermal stability of the membranes increased with increasing PC content. Moreover, a single Tg in the curves indicates that there was no secondary phase transition in blend membranes. This means that PVC and PC form compatible and miscible membranes without any segrega-
subsequent membranes which indicates that number of pores per unit area (pore density) increased as the content of PC increased. Fig. 3 shows that pore size distribution curves of membranes shifted towards smaller pores with increasing PC content up to 50% and then shifted back towards larger pores for 60% and 70% PC membranes. This means that blending PVC and PC with PC contents up to 50% results in membranes with not only smaller pores, but also membranes with larger number of pores in comparison with neat PVC membrane. In addition, total area of the pores, calculated by multiplying the number of pores in the mean area of them was 99235, 103596, 115674, 245280, 255184, and 279473 pixels for subsequent membranes, respectively. If we suppose that all of the surface pores counted by image analyzer software are open end, then we may expect that the water permeation of the prepared membranes will increase as the content of PC increases. Moreover, we expect that the rejection of the membranes increases as the mean pore size of the membranes shift to smaller pores and the results in the next sections confirm it. It should be noted that PVC and PC are compatible and this can be shown based on the solubility parameter theory. In this theory, two
Fig. 4. DSC curves for the prepared membranes.
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Fig. 7. Weight loss per unit area of the membranes after 7 days abrasion test with silicon carbide particles.
Fig. 5. XRD diffraction patterns of the prepared membranes.
tion. The compatibility of PVC and PC was also confirmed by XRD analysis as shown in Fig. 5. In XRD diffraction pattern of neat PVC membrane, there is a broad peak in the 2θ range of about 10–30° which indicates the amorphous nature of PVC [31]. In addition, the small peak observed at 2θ of 38° represents the semicrystallinity of PVC [32]. On the other hand, a broad peak in the 2θ range of about 13–24° was observed in XRD pattern of neat PC membrane indicating that PC is also amorphous. It can be seen that all blend membranes have almost the same XRD diffraction patterns and as the content of PC increased in the membranes, the broadness of the peak decreased slightly and the maxima shifted toward smaller 2θ. These results confirm the compatibility of PVC and PC.
increase in PWF can be related to hydrophilicity and surface pores of the membranes. As mentioned, hydrophilicity of the membranes increased with increasing PC content which favors water permeation. Moreover, as shown in FESEM images, the number of surface pores as well as total area of pores increased as the content of PC increased which provides more paths for water permeation even though pore size distribution of membranes shifted to smaller pores with increasing PC content up to 50%.
3.3. Abrasion test, stability test and mechanical properties Most of polymeric membranes are used in MF and UF pretreatment processes to separate solid particles from seawater prior to reverse osmosis. Solid particles are abrasive materials which reduce the durability of polymeric membranes. Therefore, the effect of abrasive materials on the polymeric membranes is an important issue in membrane technology and preparation of polymeric membranes with abrasion-resistance property seems to be interesting. Fig. 7 shows the weight loss per unit area of the membranes after abrasion test. It can be observed that weight loss of membranes decreased with increasing PC content. This indicates that blending PVC with PC resulted in more abrasion-resistant membranes in comparison with neat PVC membrane. Mechanical properties may also be used to determine abrasion resistance of membranes. In this study, tensile strength and elongation at break for all membranes were measured before and after abrasion test and the results were summarized in Table 3. It can be seen that the tensile strength of the membranes increased continuously as the PC content increased up to 50% and no remarkable increase was observed at higher content of PC. This means that mechanical strength of PVC membrane was improved at the presence of PC which can be attributed to the rigidity of PC chains. Moreover, tensile strengths of the membranes decreased after abrasion test but the tensile reduction is more severe for neat PVC membrane. In other words, tensile reduction (difference between tensile strengths before and after abrasion divided by tensile strength before abrasion) of PVC membrane after abrasion is 44% whereas this ratio for subsequent membranes is 28%, 18%, 4%, 8% and 14%, respectively. Therefore, it can be seen that the membrane containing 50% PC exhibited the highest abrasion resistance. Table 3 also shows that elongation at break for prepared membranes increased slightly with increasing PC content. The higher elongation of blend membranes may be attributed to the degree of crystallinity of them. PVC and PC are amorphous polymers with high rigid chains. This capability is due to bulky side groups which results in high Tg values. On the other hand, rigid polymers with higher molecular weight exhibit more elongation in comparison with those of the lower molecular weight. Therefore, PC has more elongation than PVC due to its higher molecular weight. However, the semi-crystallinity structure of PVC (as
3.2. Hydrophilicity and pure water flux Table 1 shows that the contact angle between membranes and water droplet decreases continuously with increasing PC content in the membranes. The reduction of contact angle indicates that the hydrophilicity of the membranes increased at the presence of polycarbonate. As mentioned, there is an oxygen atom double-bonded to a carbon atom in the PC chains which increases the interaction with water molecules and subsequently, enhances the hydrophilicity. It was shown that the contact angle of neat PC membrane prepared by track etching method is about 72° [33] and as shown in Table 1, the contact angle of neat PVC membrane in the present work is 88.6° which indicates that decrease in contact angle is due to the presence of hydrophilic PC in the blend membranes. The pure water flux of PVC/PC blend membranes are represented in Fig. 6. It can be seen that the PWF increased from 403 kg/m2 h for neat PVC membrane to about 1260 kg/m2 h for 70% PC membrane. The
Fig. 6. Pure water flux of the prepared membranes.
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Table 3 Mechanical properties of the membranes before and after abrasion and stability tests. Before Tests
After abrasion test
After stability test
Membrane
Tensile, MPa
Elongation, (%)
Tensile, MPa
Elongation, (%)
Tensile, MPa
Elongation, (%)
Neat PVC 20 wt% PC 40 wt% PC 50 wt% PC 60 wt% PC 70 wt% PC Neat PC
4.3 ± 0.1 4.7 ± 0.1 6.2 ± 0.2 7.4 ± 0.2 7.4 ± 0.1 7.7 ± 0.2 11.3 ± 0.2
33 ± 3 33 ± 3 35 ± 2 41 ± 2 42 ± 2 42 ± 3 48 ± 3
2.4 ± 0.4 3.4 ± 0.5 5.1 ± 0.4 7.1 ± 0.2 6.8 ± 0.4 6.6 ± 0.4 10.8 ± 0.5
24 ± 4 28 ± 4 30 ± 2 35 ± 2 36 ± 2 37 ± 2 43 ± 2
4.3 ± 0.3 4.7 ± 0.2 6.1 ± 0.1 7.1 ± 0.3 7.3 ± 0.2 7.5 ± 0.1 9.7 ± 0.1
32 ± 2 33 ± 4 34 ± 2 39 ± 3 39 ± 2 39 ± 2 31 ± 2
70 min. For blend membranes, however, the initial as well as final fluxes are higher than that of neat PVC membrane. The flux declines for blend membranes were about 30%, 24%, 19%, 24% and 21% for subsequent membranes, respectively, which indicates that fouling resistance of PVC/PC blend membranes is higher than that of neat PVC membrane and 50% PC membrane had the least decline in flux. It has been accepted that antifouling properties of the membranes increase with increasing hydrophilicity and as shown previously, the contact angle of the membranes decreased with addition of PC in the dope solution due to the presence of carbonate group in the PC chains. Another reason for this result may be related to the surface pore size of the membranes. It has been shown that the membranes with larger pore sizes at the surface fouled easily [34] and as shown previously, pore size distribution of the membranes shifted towards smaller pores as the content of PC increased. More information about antifouling properties of the membranes can be obtained from fouling parameters including TFR, RFR, IFR and FR which were defined in Section 2.3.5. These parameters were estimated based on the pure water flux and filtration flux of BSA solution and the results were depicted in Table 4. It can be seen that TFR of the membranes decreased from 60.33% for neat PVC membrane to 31.51% for 50% PC membrane and then increased for membranes with higher contents of PC but all blend membranes had lower TFR than neat PVC membrane. TFR is a degree of total flux loss due to the fouling and the less TFR value indicates better antifouling property. Therefore, the results show that PVC/PC blend membranes are more fouling resistant than neat PVC membrane and the membrane containing 50% PC has the lowest fouling during 4 h filtration of BSA solution. In addition, RFR value for neat PVC membrane is 30.33% which shows that 50% of total fouling by BSA for this membrane is reversible (remember that TFR=RFR+IFR). It can be observed that with increasing PC content, reversible portion of fouling (RFR/TFR) increased and reached to its maximum for 50% PC membrane and then decreased slightly. This means that blending PVC with PC results in membranes with higher reversible portion of fouling which can be cleaned easily by conventional cleaning methods. Table 4 also shows that FR of all of blend membranes had higher than that of neat PVC membrane and the maximum FR belonged to 50% PC membrane. FR is an index indicating the degree of recovery of water flux after cleaning a fouled membrane and the higher FR value is, the better antifouling property is. These results suggest that PVC/PC blend membranes are more fouling resistant in comparison with neat PVC membrane. This may be because of improved hydrophilicity due to the presence of PC in the membranes. In this study, the rejection test of prepared membranes was carried out by measuring the concentration of BSA in the permeated water and the results are represented in Table 4. As can be observed, the rejection of the membranes increased with increasing PC content up to 50% and then decreased again for subsequent membranes. However, the neat PVC membrane had the least rejection whereas 50% PC membrane showed about 99% rejection. The trend of rejection is in accordance to the results of pore size distribution for the membranes and confirm the fact that blending of PVC and PC results in membranes with small pores
revealed by XRD diffraction patterns, the small peak observed at 2θ of 38° for PVC represents the semi-crystallinity of it) is yet another aspect of PVC which results in lower elongation. These results indicate that blending PVC and PC results in membranes with higher elongation and tensile in comparison with neat PVC membrane. After abrasion test, elongation of neat PVC membrane decreased significantly up to 30% of its initial value due to abrasive effect of particles. However, there was no remarkable decrease of tensile and elongation among blend membranes as they exhibit excellent strength even after abrasion test. It should be noted that increasing PC content improved blend membranes mechanical strength and increased their mechanical behavior in favor of neat PC. Moreover, there was a very little decrease in mechanical properties of PC. In order to investigate long term stability of prepared membranes, they were submerged in 1.5 M NaOH solution of pH=12 for 10 days. After that, mechanical properties of them were determined and the results were summarized in Table 3. As can be seen, tensile and elongation of neat PC membrane decreased remarkably after 10 days being submerged in NaOH solution. Moreover, mechanical properties of blend membranes after stability test decreased as the content of PC increased. However, mechanical properties of PVC membrane remained almost the same which can be attributed to good chemical resistance of PVC. These results mean that even though PVC/PC blend membranes possess good mechanical properties against abrasive materials, addition of more PC to PVC results in membranes with low chemical stability. Therefore, the best blend ratio should be chosen based on the optimized conditions.
3.4. Antifouling properties and rejection Fig. 8 shows the flux-time curves for the prepared membranes during filtration of 1.0 g/L BSA solution. It can be seen that the flux of neat PVC membrane was the least among the membranes and its value decreased from 337 kg/m2 h to about 190 kg/m2 h (ca. 44%) after
Fig. 8. Changes in permeate flux of the prepared membranes during 4 h filtration of 1.0 g/L BSA solution.
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Table 4 Fouling parameters and BSA rejection of prepared membranes. Membrane
TFR (%)
RFR (%)
IRF (%)
FR (%)
Rejection (%)
Neat PVC 20 wt% PC 40 wt% PC 50 wt% PC 60 wt% PC 70 wt% PC
60.33 ± 0.7 55.21 ± 0.5 38.67 ± 0.6 31.51 ± 0.5 36.34 ± 0.3 44.87 ± 0.3
30.33 ± 0.7 30.64 ± 0.5 20.14 ± 0.6 21.02 ± 0.5 20.44 ± 0.3 19.88 ± 0.3
30.00 ± 0.7 24.54 ± 0.5 18.53 ± 0.6 10.57 ± 0.5 15.91 ± 0.3 24.99 ± 0.3
70.00 ± 0.6 75.46 ± 0.5 81.47 ± 0.6 89.50 ± 0.5 84.07 ± 0.3 75.01 ± 0.3
84.94 ± 0.04 91.81 ± 0.04 97.61 ± 0.02 98.92 ± 0.02 96.08 ± 0.04 93.14 ± 0.02
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