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PVA hollow fiber membranes
Desalination 250 (2010) 530–537
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Preparation and properties of PVDF/PVA hollow fiber membranes Nana Li, Changfa Xiao ⁎, Shulin An, Xiaoyu Hu The Key Laboratory of Hollow Fiber Membrane Materials and Membrane Process, Ministry of Education, Tianjin Polytechnic University, Tianjin 300160, China
a r t i c l e
i n f o
Article history: Accepted 17 October 2008 Available online 6 November 2009 Keywords: PVDF/PVA hollow fiber membrane Hydrophilicity Mechanical property Interface microvoid Flux Rejection ratio
a b s t r a c t On principle of polymer blend phase separation, PVDF/PVA hollow fiber membranes were prepared using phase inversion method. The membrane morphology and performance varied with the blending ratio. The PVDF/PVA blends showed incompatibility by the results of dynamic mechanical analysis (DMA) and infrared attenuated total reflection (FTIR-ATR) sampling technique. Based on bursting pressure and tensile strengths results, we suggest that the mechanical properties of PVDF/PVA blend membranes are worse than that of PVDF membrane. PVA can improve the hydrophilicity of PVDF/PVA hollow fiber membranes, which could be illuminated by the decrease in contact angle, the increase in equilibrium water content (EWC) and the variety in dynamic moisture regain. The pure water flux increases while the rejection ratio decreases with PVA content increasing. Moreover, PVA can improve the anti-fouling property of PVDF/PVA hollow fiber membranes, which could be illuminated by the result of increase coefficient of resistance. © 2009 Elsevier B.V. All rights reserved.
1. Introduction As a semi-crystalline polymer, poly(vinylidene fluoride) (PVDF) is fit for membrane material due to its excellent chemical resistance, physical and thermal stability, high strength and flexibility [1,2]. However, the hydrophobicity of PVDF remains a problem and limits its application, thus, hydrophilic modification of PVDF membranes is one of the hotspots in membrane science [3–5]. Blending is often used to change the properties of polymeric materials [6]. Blends of PVDF/PEG, PVDF/PMMA, PVDF/PAN, PVDF/ Nylon 6, PVDF/SPS, PVDF/PVAc, PVDF/CA, PVDF/PEO and PVDF/PU have been used in the preparation of ultrafiltration membranes [7– 17]. It is well known that poly(vinyl alcohol) (PVA) is one of the polymeric materials for making ultrafiltration membranes. It possesses good hydrophilicity and other advantages [18]. Some papers have indicated that PVA is a polymer miscible with PVDF [19–21], and a high degree of intermolecular interaction presents between PVDF and PVA chains [22]. Although both miscibility and morphology of PVDF/PVA blends have been extensively studied, the effect of blending with PVA on the properties of PVDF membrane has not been reported yet. In this study, PVDF/PVA blend hollow fiber membranes were prepared using phase inversion method. Blending with PVA cannot only improve the hydrophilicity of PVDF membrane, but also form the interface microvoid through polymer blend phase separation. The morphology and performance of these membranes were investigated
⁎ Corresponding author. Department of Materials Science, Tianjin Polytechnic University, Tianjin, China. Tel.: +86 22 24528138. E-mail address: [email protected] (C. Xiao). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.10.027
by using dynamic mechanical analysis (DMA), scanning electron microscope (SEM), ultrared infrared attenuated total reflection (FTIRATR) and contact angle goniometer. 2. Experimental 2.1. Materials The PVDF (W#1300) kindly provided by Kureha Chemical Industry Co., Ltd., was dried for 24 h in a vacuum oven prior to use. The PVA (2099) was purchased from Shanxi Sanwei Co., Ltd., which was rinsed and dried prior to use. Reagent-grade dimethyl sulfoxide (DMSO) and polyethylene glycol (PEG) of molecular weight 600 supplied from Tianjin Fuchen Chemical Reagent Plant were used without further purification. 2.2. Preparation of PVDF/PVA hollow fiber membranes The hollow fiber membranes were prepared with the phase inversion method. The PVA was washed thoroughly in water and then dried in the vacuum oven for approximately 48 h at 50 °C to remove its water content. The PVDF was dried directly in the vacuum oven for approximately 20 h at 90 °C to remove its water content. Then, the spinning solutions were prepared by dissolving polymer in DMSO at various blending ratios of PVDF/PVA/PEG. The total polymer content was 20 wt.%, and PEG content was 6 wt.%. After stirring strongly for 12 h at 95 °C, the resulting homogeneous solution was transferred to a stainless steel reservoir and was then degassed overnight at 70 °C under vacuum. The hollow fiber spinning apparatus and detail spinning procedures have been described elsewhere [23]. In spinning, the spinneret with the
N. Li et al. / Desalination 250 (2010) 530–537
orifice diameter of 2.0 mm and the inner tube diameter of 1.2 mm was used. The fibers were spun by the wet process (air-gap= 20 cm). The temperature of environment was 20 °C and relative humidity was 65%. The take-up velocity was 7–8 m/min. The solution extrusion rate was in the range 2–2.5 mL/min and the internal coagulant flow rate was in the range 2–2.5 mL/min. Water was used as internal coagulation bath and external coagulation bath for all spinning runs. The prepared hollow fibers were immersed in fresh water for at least 2 days and then examined. The PVDF/PVA (90/10, 80/20, 70/30 wt/wt) hollow fiber membranes were labeled as PVDF9, PVDF8, and PVDF7 respectively. Before SEM tests, we put the resulting membranes in glycerol–water solutions (3 parts glycerol to 2 parts water) and then dried in air, which can retain the porous structure. Before the measurements of contact angle and mechanical property tests, we put the resulting membranes in a vacuum for 12 h at 50 °C to gain dry membranes. 2.3. Characterization analysis 2.3.1. Compatibility study In order to examine the compatibility of PVDF/PVA simply, the casting polymer solutions were prepared by dissolving polymer in DMSO at various blending ratios of PVDF/PVA without PEG. The mechanical properties were determined using DMA (DMA242C, NETZSCH, Germany) from 150 to 300 K at a heating rate of 5 K/min, and the frequency was 10 Hz. The FTIR-ATR (TENSOR37, BRUKER, Germany) sampling technique could be used as another means to estimate the compatibility of PVDF/PVA blends. 2.3.2. Morphology examination The structure and morphology of membranes were observed by SEM (Quanta 200, FEI, Netherland). The cross-section of membranes were freeze-fractured under liquid nitrogen. The membrane samples were gold sputtered and analyzed using SEM.
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branes were swollen at 20 °C for 12 h and the weights were measured at room temperature. The free liquid on the surface of the swollen membrane was padded dry with filter papers before weighing. The dynamic moisture regain of the membrane is defined as: Wi ð%Þ =
Gi −Gdry × 100 Gdry
ð3Þ
where Gdry is the weight of the dry membrane, and Gi is the dynamic weight of the membrane in the process of absorbing moisture. The dry membranes were suspended in laboratory at 20 °C and relative humidity of around 65%. The samples were weighed every 3 min before 1 h, and then were weighed every 10 min until 3 h. The last weighing is at 4 h. 2.3.6. The permeation performance experiments The hollow fiber membranes were characterized by pure water flux and rejection ratio of egg albumen. The pressure difference across the membrane is 0.1 MPa. The flux was calculated by Eq. (4): J=
V S×t
ð4Þ
where V is the total permeation (m3), S is the total permeation area (m2), and t is the total permeation time (s). The rejection ratio of the membrane was measured with 2 g/L egg albumen solution. The rejection ratio was calculated by Eq. (5): R ð%Þ =
1−
2Cp Cf + Cr
× 100
ð5Þ
where Cp, Cf and Cr are the concentrations in the permeate, the feed and the remaining solution respectively.
2.3.3. The mechanical properties The tensile strength of dry and wet membranes was measured with a universal mechanical testing machine (3369, Instron, USA). The measurements were taken at 20 °C and relative humidity of around 65% with the crosshead speed of 10 mm/min. 2.3.4. Porosity determination The porosity (ε) of the sample was calculated according to Eq. (1). ε ð%Þ =
ρ 1− b × 100 ρt
ð1Þ
where ρb is the bulk density of membrane and ρt is the true density of membrane. 2.3.5. Hydrophilicity measurement The hydrophilicity of the membrane was studied based on the contact angle, equilibrium water content and dynamic moisture regain of the membrane. The contact angle of membrane was measured using a contact angle goniometer (JY-820, Chengde Testing Machine Co., Ltd., China). The contact time was 10 s, and each value was averaged from eight measurements. FTIR-ATR could be used as another means to estimate the membrane hydrophilicity, and it also illuminated the existence of delamination. The equilibrium water content of the membrane is defined as: EWC ð%Þ =
Gwet −Gdry × 100 Gwet
ð2Þ
where Gdry and Gwet are respectively the weights of the dry membrane and the swollen membrane in equilibrium with water. The mem-
Fig. 1. DMA thermograms of pure PVDF, pure PVA and PVDF/PVA blend membranes.
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where μ is the viscosity of feed, Rm is the resistance of membrane, and ΔP is the pressure drop. After fouling by 2 g/L egg albumen solution, the pure water flux was expressed as Eq. (7): J2 =
ΔP μðRm + Rf Þ
ð7Þ
where Rf is the resistance coming from fouling. Assuming μ is constant in the experiment, the increase coefficient of resistance, therefore, can be written as m=
Rf J −J = 0 2 Rm J2
ð8Þ
where m is the increase coefficient of resistance. The anti-fouling properties of blend membranes were characterized by the increase coefficient of resistance. The larger the value of m is, the worse antifouling membrane is. 2.3.8. The stability of blend membrane The mass loss rate was calculated by Eq. (9): F ð%Þ =
m0 −m1 × 100 m0
ð9Þ
where F is the mass loss rate, m0 is the dry weight of the membrane without swelling in water, and m1 is the dry weight of the membrane swelling in water for 10 days. 3. Results and discussion 3.1. Compatibility Hansen thought that the molecular interaction was composed of dispersive interaction, dipolar interaction and hydrogen bond [24]. Therefore the solubility parameter can be expressed by Eq. (10): Fig. 2. The FTIR-ATR spectra of membranes (a) PVA; (b) PVDF; (c) PVDF9 and (d) PVDF7.
δ = δd + δp + δh
2.3.7. The anti-fouling property experiments According to Darcy's Law, the pure water flux was expressed as Eq. (6):
where d, p, and h as subscripts are dispersive interaction, dipolar interaction and hydrogen bond respectively. Hildebrand found that the thermodynamic compatibility among polymers could be represented by solubility parameter (δ), and given ΔHm by Eq. (11) [25]:
J0 =
ΔP μRm
ð6Þ
2
2
2
2
ð10Þ
2
2
2
ΔHm = φ1 φ2 ½ðδd1 −δd2 Þ + ðδp1 −δp2 Þ + ðδh1 −δh2 Þ
Fig. 3. The photograph of PVDF membrane surface (a) and PVDF7 blend membrane surface (b).
ð11Þ
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where φ1 and φ2 are the volume fraction of blends component respectively, the subscripts 1 and 2 referred to two polymers respectively. According to the solution theory, the compatibility of the two polymers is mainly determined by the magnitude of ΔHm, and the lower value of ΔHm is, the better compatibility of the two polymers will be. In polar polymers system, the blends have excellent compatibility when δd, δp and δh are very similar respectively between two polymers. Through calculating, the δd, δp and δh of PVDF and the δd, δp and δh of PVA can be obtained as follows: δd(PVA) = 16.3, δp(PVA) = 19.2, δh(PVA) = 3.3; δd(PVDF) = 16, δp(PVDF) = 14.3, δh(PVDF) = 23.9. The difference between δh(PVA) and δh(PVDF) indicates that PVDF and PVA are incompatible.
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In addition, DMA was evaluated to observe the relationship between Tanδ and temperature. It is well known that the glass transition temperature (Tg) of a polymer is an important criterion for the compatibility of components. As shown in Fig. 1, the Tg value of PVDF is 224.0 K and that of PVA is 337.2 K. These PVDF/PVA blends show two Tg values which are very similar to the individual components. Therefore, PVDF and PVA are incompatible in thermodynamics whereas PVA is a polymer miscible with PVDF in dynamics [26]. Fig. 2 displays the FTIR-ATR spectra of PVDF, PVA, PVDF9 and PVDF7 membranes. Compared with PVDF membrane, the new appearances of the wide bands over 3200–3500 cm− 1 range in PVDF9 and PVDF7, which results from the stretching vibration of O–H. These are due to the presence of hydrophilic PVA. The absorbencies at 2941 and 2943 cm− 1
Fig. 4. SEM graphs of overall cross-sectional view, partial cross-sectional view and sponge-like structure of PVDF hollow fiber membrane (a-1, a-2, a-3), PVDF9 hollow fiber membrane (b-1, b-2, b-3), and PVDF7 hollow fiber membrane (c-1, c-2, c-3).
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Fig. 5. Outer surface SEM graphs of PVDF hollow fiber membrane (a), PVDF9 hollow fiber membrane (b), and PVDF7 hollow fiber membrane (c).
result from the asymmetric stretching vibration of CH2. Furthermore, the strong absorbance at 1172 cm− 1 results from the stretching vibration of C–F, and characteristic frequency of CF2 groups at 1404, 1072 and 878 cm− 1 [27]. The spectra of blend membranes have hardly any shift compared with the spectrum of the two individual components, thus, the interaction between PVDF and PVA is weak [28]. Then the interface microvoids may be produced due to the phase separation in PVDF/PVA blends. The polymer solutions were cast onto a glass plate and placed in room temperature for 8 h, then the PVDF surface and the PVDF7 surface were observed by optical microscope, as shown in Fig. 3. A mass of transparent PVA congregates and disperses in PVDF. The interface between PVDF and PVA is very clear. 3.2. Morphology of PVDF/PVA hollow fiber membranes Fig. 4 shows the SEM results of cross-section structures of hollow fiber membranes. As can be seen, the cross-sectional structures of all the membranes are similar. Near the outer and inner walls of the hollow fiber membranes, long finger-like pores are present, and at the centre of the hollow fiber membranes, sponge-like structures are possessed. The appearance of the fiber structures can be due to the rapid precipitation that occurred at both the outer and inner fiber walls resulting in finger-like pores and to the slow precipitation giving the sponge-like structure at the centre of the fiber. In addition, the cavities are formed in all membranes, and increase with PVA content increasing. The result may be probably due to the hydrophilicity of PVA. The addition of the PVA in the polymer dopes increases the
Fig. 6. Measurement of stress at break of PVDF/PVA hollow fiber blend membranes.
precipitation rate. This may result in a formation of larger cavity structures [29]. Moreover, as PVA content increases, the finger-like pores become larger and the skin layer becomes thinner. The result is also due to the addition of hydrophilic PVA improving water diffusion. Upon further examination at high magnification (Fig. 4(a-3, b-3, c-3)), loose, porous interconnection network structures were observed when PVDF/PVA was 7/3. Fig. 5 shows the SEM results of outer surfaces of hollow fiber membranes. As can be seen, PVDF membrane has a rougher outer surface without obvious pores, and on the outer surfaces of PVDF/PVA blend membranes, interface microvoids appear due to the phase separation in PVDF/PVA. This phenomenon may be probably due to the incompatibility of PVDF/PVA blends.
3.3. The mechanical properties of PVDF/PVA hollow fiber membranes The bursting pressure of PVDF/PVA hollow fiber blend membranes is used as a measure of the toughness of the membrane under pressure. As shown in Fig. 6, the bursting pressure of membrane decreases and the porosity of membrane increases with the increase in PVA content, indicating that the higher the value of porosity is, the worse bursting pressure of membrane will be. Therefore the stress at break is affected not only by composition, but also by the differences in porosity. The tensile strengths of dry and wet hollow fiber membrane are showed in Fig. 7. The tensile strengths of PVDF/PVA blend hollow fiber
Fig. 7. Measurement of tensile strength of PVDF/PVA hollow fiber blend membranes.
N. Li et al. / Desalination 250 (2010) 530–537
Fig. 8. Measurement of contact angle of PVDF/PVA hollow fiber membranes.
membranes were lower than that of PVDF membrane. In the case of PVDF8, the decreasing trend of tensile strength increases appreciably, and then decreases further. The results may be due to the meshwork making of PVA. When PVA content is high, the meshwork breaks probably due to PEG congregating in PVA. In addition, dry membrane has higher tensile strength than wet membrane. This phenomenon was similar to the results of Yang and Tong [30]. The tensile strength of dry membrane was between 3.68 and 2.04 MPa, and that of wet membrane was only between 3.01 and 1.66 MPa. 3.4. The hydrophilicity of PVDF/PVA hollow fiber membranes PVDF possesses a strong hydrophobic property and thus has lower surface energy (7.83 N/m), whereas PVA is a hydrophilic polymer. Accordingly, PVA can be taken to improve the hydrophilicity of PVDF membrane through blending. Fig. 8 displays that the contact angle of hollow fiber membrane decreases as PVA increases. This indicates that PVA can improve the hydrophilicity of the membrane surface. Fig. 9 also shows that the equilibrium water content (EWC) increases with the increase of PVA content in water at 20 °C. As shown in Fig. 10, the dynamic moisture regain of PVDF hollow fiber membrane reaches equilibrium when the membrane is suspended in laboratory at 20 °C and relative humidity of around 65% for 80 min. And the equilibrium time of PVDF9 and PVDF7 are respectively 100 min and 150 min. However the dynamic moisture regain of PVA hollow fiber
Fig. 9. Measurement of equilibrium water content (EWC) of PVDF/PVA hollow fiber membranes.
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Fig. 10. Measurement of dynamic moisture regain of PVDF/PVA hollow fiber blend membranes.
membrane cannot reach equilibrium in 480 min. These indicate that PVA can improve the moisture regain of the membrane. All trends of the contact angle, EWC and the dynamic moisture regain indicate that the hydrophilicity of the hollow fiber membrane increases with PVA content increasing. 3.5. The permeation of PVDF/PVA hollow fiber membranes As shown in Fig. 11, the pure water flux increases while the rejection ratio decreases with PVA content increasing. The trend of pure water flux attributed to the finger-like pores becoming larger and the skin layer becoming thinner with PVA content increasing (Fig. 4). Moreover, the interface microvoid structure resulting from phase separation exists in PVDF/PVA blends. The rejection ratio depends more on the denseness of the skin layer than the structure of cross-section. Thus the skin layers of PVDF/PVA blend membranes were less dense than PVDF (in Fig. 5). This result demonstrates that by blending PVDF with PVA, we can prepare membranes of different retention behaviors. 3.6. The anti-fouling properties of PVDF/PVA hollow fiber membranes The anti-fouling properties of blend membranes were characterized by the increase coefficient of resistance. As shown in Table 1, the
Fig.11. Measurement of performance of PVDF/PVA hollow fiber blend membranes.
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Table 1 The increase coefficient of resistance of PVDF and PVDF8. Sample
Pure water flux before fouling (mL cm− 2 h− 1)
Pure water flux after fouling (mL cm− 2 h− 1)
Increase coefficient of resistance
PVDF PVDF8
0.61 4.13
0.43 3.36
0.42 0.23
anti-fouling property of PVDF8 blend membrane is better than that of PVDF membrane. The result may be probably due to the hydrophilicity of PVA. 3.7. The stability of PVDF/PVA hollow fiber membranes The PVDF8 membranes were put into an oven at 120 °C for different time. As shown in Fig. 12, with the time of heat treatment increasing, the mass loss rate of PVDF8 membrane decreases and reaches equilibrium nearly when the time of heat treatment is about 60 min. The pure water flux of PVDF8 membrane also decreases with the increase of the time of heat treatment. The results may be probably due to the enhancement of the degree of crosslinking and the degree of crystallinity after heat treatment [31]. Firstly, the quantity of O–H in the membrane decreases resulting from crosslinking, which makes the hydrophilicity of membrane decrease. Secondly, the macromolecule segment motion occurs in heat treatment, and then the size of porous structure in molecular network decreases. Moreover, some porous structures are extruded to smaller ones by the growing of crystal grain resulting from the crystallization consummated. All of the above make the pure water flux decrease. However, the stability of blend membrane is improved because PVA with hot crosslinking is stable in water. Therefore, the post-crosslinking is an essential step to prepare PVDF/PVA membrane, because it can make PVA stable in the membrane during application. This paper is mainly aimed at studying the morphology and performance of PVDF/PVA membranes with different PVDF/PVA blending ratios, and further research about the post-crosslinking will be carried out in the future.
the phase separation in PVDF/PVA. The bursting pressure and tensile strengths of PVDF/PVA hollow fiber membranes are worse than that of PVDF membrane, suggesting that the blend hollow fiber membrane structure became looser. The hydrophilicity of PVDF/PVA hollow fiber membranes can be improved evidently by blending PVA, which can be illuminated by the decrease of contact angle, the increase in EWC and the variety in dynamic moisture regain. With PVA content increasing, the pure water flux increases while the rejection ratio decreases. This trend can be attributed to the finger-like pores becoming larger and the skin layer becoming thinner with PVA content increasing. Moreover, PVA can improve the anti-fouling property of PVDF/PVA hollow fiber membranes. List of symbols Symbol Definition
Unit
ε ρb ρt EWC Gdry Gwet Gi Wi J V S t R Cp Cf Cr μ Rm ΔP Rf m F M0 M1
% g cm− 3 g cm− 3 % g g g % mL cm− 2 h− 1 mL cm2 H % g/L g/L g/L Pa·s cm− 1 Pa cm− 1 dimensionless % g g
porosity bulk density of membrane true density of membrane equilibrium water content weight of the dry membrane weight of the wet membrane dynamic weight of the membrane dynamic moisture regain pure water flux total permeation total permeation area total permeation time rejection ratio concentration in the permeate concentration in the feed concentration in the remain solution viscosity of feed resistance of membrane pressure drop resistance coming from fouling increase coefficient of resistance mass loss rate dry weight of the membrane without swelling in water dry weight of the membrane swelling in water for 10 days
Acknowledgements 4. Conclusions By blending process, it was possible to produce PVDF/PVA hollow fiber membranes with a hydrophilicity nature. Blending also produces a new porous structure due to phase separation. The results of this study indicate that PVDF and PVA are incompatible, therefore, the interface microvoid is produced resulting from
The authors gratefully acknowledge the financial support of the National Basic Research Program (also called 973 Program of 2006CB708602 and 863 Program of 2007AA030304) and we are grateful to the Ministry of Science and Technology of China. We also thank Professor Shulin An for his theoretical contributions during the early stages of this work. References
Fig.12. The pure water flux and the mass loss rate of PVDF8 membrane after heat treatment.
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Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Preparation and properties of PVDF/PVA hollow fiber membranes Nana Li, Changfa Xiao ⁎, Shulin An, Xiaoyu Hu The Key Laboratory of Hollow Fiber Membrane Materials and Membrane Process, Ministry of Education, Tianjin Polytechnic University, Tianjin 300160, China
a r t i c l e
i n f o
Article history: Accepted 17 October 2008 Available online 6 November 2009 Keywords: PVDF/PVA hollow fiber membrane Hydrophilicity Mechanical property Interface microvoid Flux Rejection ratio
a b s t r a c t On principle of polymer blend phase separation, PVDF/PVA hollow fiber membranes were prepared using phase inversion method. The membrane morphology and performance varied with the blending ratio. The PVDF/PVA blends showed incompatibility by the results of dynamic mechanical analysis (DMA) and infrared attenuated total reflection (FTIR-ATR) sampling technique. Based on bursting pressure and tensile strengths results, we suggest that the mechanical properties of PVDF/PVA blend membranes are worse than that of PVDF membrane. PVA can improve the hydrophilicity of PVDF/PVA hollow fiber membranes, which could be illuminated by the decrease in contact angle, the increase in equilibrium water content (EWC) and the variety in dynamic moisture regain. The pure water flux increases while the rejection ratio decreases with PVA content increasing. Moreover, PVA can improve the anti-fouling property of PVDF/PVA hollow fiber membranes, which could be illuminated by the result of increase coefficient of resistance. © 2009 Elsevier B.V. All rights reserved.
1. Introduction As a semi-crystalline polymer, poly(vinylidene fluoride) (PVDF) is fit for membrane material due to its excellent chemical resistance, physical and thermal stability, high strength and flexibility [1,2]. However, the hydrophobicity of PVDF remains a problem and limits its application, thus, hydrophilic modification of PVDF membranes is one of the hotspots in membrane science [3–5]. Blending is often used to change the properties of polymeric materials [6]. Blends of PVDF/PEG, PVDF/PMMA, PVDF/PAN, PVDF/ Nylon 6, PVDF/SPS, PVDF/PVAc, PVDF/CA, PVDF/PEO and PVDF/PU have been used in the preparation of ultrafiltration membranes [7– 17]. It is well known that poly(vinyl alcohol) (PVA) is one of the polymeric materials for making ultrafiltration membranes. It possesses good hydrophilicity and other advantages [18]. Some papers have indicated that PVA is a polymer miscible with PVDF [19–21], and a high degree of intermolecular interaction presents between PVDF and PVA chains [22]. Although both miscibility and morphology of PVDF/PVA blends have been extensively studied, the effect of blending with PVA on the properties of PVDF membrane has not been reported yet. In this study, PVDF/PVA blend hollow fiber membranes were prepared using phase inversion method. Blending with PVA cannot only improve the hydrophilicity of PVDF membrane, but also form the interface microvoid through polymer blend phase separation. The morphology and performance of these membranes were investigated
⁎ Corresponding author. Department of Materials Science, Tianjin Polytechnic University, Tianjin, China. Tel.: +86 22 24528138. E-mail address: [email protected] (C. Xiao). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.10.027
by using dynamic mechanical analysis (DMA), scanning electron microscope (SEM), ultrared infrared attenuated total reflection (FTIRATR) and contact angle goniometer. 2. Experimental 2.1. Materials The PVDF (W#1300) kindly provided by Kureha Chemical Industry Co., Ltd., was dried for 24 h in a vacuum oven prior to use. The PVA (2099) was purchased from Shanxi Sanwei Co., Ltd., which was rinsed and dried prior to use. Reagent-grade dimethyl sulfoxide (DMSO) and polyethylene glycol (PEG) of molecular weight 600 supplied from Tianjin Fuchen Chemical Reagent Plant were used without further purification. 2.2. Preparation of PVDF/PVA hollow fiber membranes The hollow fiber membranes were prepared with the phase inversion method. The PVA was washed thoroughly in water and then dried in the vacuum oven for approximately 48 h at 50 °C to remove its water content. The PVDF was dried directly in the vacuum oven for approximately 20 h at 90 °C to remove its water content. Then, the spinning solutions were prepared by dissolving polymer in DMSO at various blending ratios of PVDF/PVA/PEG. The total polymer content was 20 wt.%, and PEG content was 6 wt.%. After stirring strongly for 12 h at 95 °C, the resulting homogeneous solution was transferred to a stainless steel reservoir and was then degassed overnight at 70 °C under vacuum. The hollow fiber spinning apparatus and detail spinning procedures have been described elsewhere [23]. In spinning, the spinneret with the
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orifice diameter of 2.0 mm and the inner tube diameter of 1.2 mm was used. The fibers were spun by the wet process (air-gap= 20 cm). The temperature of environment was 20 °C and relative humidity was 65%. The take-up velocity was 7–8 m/min. The solution extrusion rate was in the range 2–2.5 mL/min and the internal coagulant flow rate was in the range 2–2.5 mL/min. Water was used as internal coagulation bath and external coagulation bath for all spinning runs. The prepared hollow fibers were immersed in fresh water for at least 2 days and then examined. The PVDF/PVA (90/10, 80/20, 70/30 wt/wt) hollow fiber membranes were labeled as PVDF9, PVDF8, and PVDF7 respectively. Before SEM tests, we put the resulting membranes in glycerol–water solutions (3 parts glycerol to 2 parts water) and then dried in air, which can retain the porous structure. Before the measurements of contact angle and mechanical property tests, we put the resulting membranes in a vacuum for 12 h at 50 °C to gain dry membranes. 2.3. Characterization analysis 2.3.1. Compatibility study In order to examine the compatibility of PVDF/PVA simply, the casting polymer solutions were prepared by dissolving polymer in DMSO at various blending ratios of PVDF/PVA without PEG. The mechanical properties were determined using DMA (DMA242C, NETZSCH, Germany) from 150 to 300 K at a heating rate of 5 K/min, and the frequency was 10 Hz. The FTIR-ATR (TENSOR37, BRUKER, Germany) sampling technique could be used as another means to estimate the compatibility of PVDF/PVA blends. 2.3.2. Morphology examination The structure and morphology of membranes were observed by SEM (Quanta 200, FEI, Netherland). The cross-section of membranes were freeze-fractured under liquid nitrogen. The membrane samples were gold sputtered and analyzed using SEM.
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branes were swollen at 20 °C for 12 h and the weights were measured at room temperature. The free liquid on the surface of the swollen membrane was padded dry with filter papers before weighing. The dynamic moisture regain of the membrane is defined as: Wi ð%Þ =
Gi −Gdry × 100 Gdry
ð3Þ
where Gdry is the weight of the dry membrane, and Gi is the dynamic weight of the membrane in the process of absorbing moisture. The dry membranes were suspended in laboratory at 20 °C and relative humidity of around 65%. The samples were weighed every 3 min before 1 h, and then were weighed every 10 min until 3 h. The last weighing is at 4 h. 2.3.6. The permeation performance experiments The hollow fiber membranes were characterized by pure water flux and rejection ratio of egg albumen. The pressure difference across the membrane is 0.1 MPa. The flux was calculated by Eq. (4): J=
V S×t
ð4Þ
where V is the total permeation (m3), S is the total permeation area (m2), and t is the total permeation time (s). The rejection ratio of the membrane was measured with 2 g/L egg albumen solution. The rejection ratio was calculated by Eq. (5): R ð%Þ =
1−
2Cp Cf + Cr
× 100
ð5Þ
where Cp, Cf and Cr are the concentrations in the permeate, the feed and the remaining solution respectively.
2.3.3. The mechanical properties The tensile strength of dry and wet membranes was measured with a universal mechanical testing machine (3369, Instron, USA). The measurements were taken at 20 °C and relative humidity of around 65% with the crosshead speed of 10 mm/min. 2.3.4. Porosity determination The porosity (ε) of the sample was calculated according to Eq. (1). ε ð%Þ =
ρ 1− b × 100 ρt
ð1Þ
where ρb is the bulk density of membrane and ρt is the true density of membrane. 2.3.5. Hydrophilicity measurement The hydrophilicity of the membrane was studied based on the contact angle, equilibrium water content and dynamic moisture regain of the membrane. The contact angle of membrane was measured using a contact angle goniometer (JY-820, Chengde Testing Machine Co., Ltd., China). The contact time was 10 s, and each value was averaged from eight measurements. FTIR-ATR could be used as another means to estimate the membrane hydrophilicity, and it also illuminated the existence of delamination. The equilibrium water content of the membrane is defined as: EWC ð%Þ =
Gwet −Gdry × 100 Gwet
ð2Þ
where Gdry and Gwet are respectively the weights of the dry membrane and the swollen membrane in equilibrium with water. The mem-
Fig. 1. DMA thermograms of pure PVDF, pure PVA and PVDF/PVA blend membranes.
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where μ is the viscosity of feed, Rm is the resistance of membrane, and ΔP is the pressure drop. After fouling by 2 g/L egg albumen solution, the pure water flux was expressed as Eq. (7): J2 =
ΔP μðRm + Rf Þ
ð7Þ
where Rf is the resistance coming from fouling. Assuming μ is constant in the experiment, the increase coefficient of resistance, therefore, can be written as m=
Rf J −J = 0 2 Rm J2
ð8Þ
where m is the increase coefficient of resistance. The anti-fouling properties of blend membranes were characterized by the increase coefficient of resistance. The larger the value of m is, the worse antifouling membrane is. 2.3.8. The stability of blend membrane The mass loss rate was calculated by Eq. (9): F ð%Þ =
m0 −m1 × 100 m0
ð9Þ
where F is the mass loss rate, m0 is the dry weight of the membrane without swelling in water, and m1 is the dry weight of the membrane swelling in water for 10 days. 3. Results and discussion 3.1. Compatibility Hansen thought that the molecular interaction was composed of dispersive interaction, dipolar interaction and hydrogen bond [24]. Therefore the solubility parameter can be expressed by Eq. (10): Fig. 2. The FTIR-ATR spectra of membranes (a) PVA; (b) PVDF; (c) PVDF9 and (d) PVDF7.
δ = δd + δp + δh
2.3.7. The anti-fouling property experiments According to Darcy's Law, the pure water flux was expressed as Eq. (6):
where d, p, and h as subscripts are dispersive interaction, dipolar interaction and hydrogen bond respectively. Hildebrand found that the thermodynamic compatibility among polymers could be represented by solubility parameter (δ), and given ΔHm by Eq. (11) [25]:
J0 =
ΔP μRm
ð6Þ
2
2
2
2
ð10Þ
2
2
2
ΔHm = φ1 φ2 ½ðδd1 −δd2 Þ + ðδp1 −δp2 Þ + ðδh1 −δh2 Þ
Fig. 3. The photograph of PVDF membrane surface (a) and PVDF7 blend membrane surface (b).
ð11Þ
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where φ1 and φ2 are the volume fraction of blends component respectively, the subscripts 1 and 2 referred to two polymers respectively. According to the solution theory, the compatibility of the two polymers is mainly determined by the magnitude of ΔHm, and the lower value of ΔHm is, the better compatibility of the two polymers will be. In polar polymers system, the blends have excellent compatibility when δd, δp and δh are very similar respectively between two polymers. Through calculating, the δd, δp and δh of PVDF and the δd, δp and δh of PVA can be obtained as follows: δd(PVA) = 16.3, δp(PVA) = 19.2, δh(PVA) = 3.3; δd(PVDF) = 16, δp(PVDF) = 14.3, δh(PVDF) = 23.9. The difference between δh(PVA) and δh(PVDF) indicates that PVDF and PVA are incompatible.
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In addition, DMA was evaluated to observe the relationship between Tanδ and temperature. It is well known that the glass transition temperature (Tg) of a polymer is an important criterion for the compatibility of components. As shown in Fig. 1, the Tg value of PVDF is 224.0 K and that of PVA is 337.2 K. These PVDF/PVA blends show two Tg values which are very similar to the individual components. Therefore, PVDF and PVA are incompatible in thermodynamics whereas PVA is a polymer miscible with PVDF in dynamics [26]. Fig. 2 displays the FTIR-ATR spectra of PVDF, PVA, PVDF9 and PVDF7 membranes. Compared with PVDF membrane, the new appearances of the wide bands over 3200–3500 cm− 1 range in PVDF9 and PVDF7, which results from the stretching vibration of O–H. These are due to the presence of hydrophilic PVA. The absorbencies at 2941 and 2943 cm− 1
Fig. 4. SEM graphs of overall cross-sectional view, partial cross-sectional view and sponge-like structure of PVDF hollow fiber membrane (a-1, a-2, a-3), PVDF9 hollow fiber membrane (b-1, b-2, b-3), and PVDF7 hollow fiber membrane (c-1, c-2, c-3).
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Fig. 5. Outer surface SEM graphs of PVDF hollow fiber membrane (a), PVDF9 hollow fiber membrane (b), and PVDF7 hollow fiber membrane (c).
result from the asymmetric stretching vibration of CH2. Furthermore, the strong absorbance at 1172 cm− 1 results from the stretching vibration of C–F, and characteristic frequency of CF2 groups at 1404, 1072 and 878 cm− 1 [27]. The spectra of blend membranes have hardly any shift compared with the spectrum of the two individual components, thus, the interaction between PVDF and PVA is weak [28]. Then the interface microvoids may be produced due to the phase separation in PVDF/PVA blends. The polymer solutions were cast onto a glass plate and placed in room temperature for 8 h, then the PVDF surface and the PVDF7 surface were observed by optical microscope, as shown in Fig. 3. A mass of transparent PVA congregates and disperses in PVDF. The interface between PVDF and PVA is very clear. 3.2. Morphology of PVDF/PVA hollow fiber membranes Fig. 4 shows the SEM results of cross-section structures of hollow fiber membranes. As can be seen, the cross-sectional structures of all the membranes are similar. Near the outer and inner walls of the hollow fiber membranes, long finger-like pores are present, and at the centre of the hollow fiber membranes, sponge-like structures are possessed. The appearance of the fiber structures can be due to the rapid precipitation that occurred at both the outer and inner fiber walls resulting in finger-like pores and to the slow precipitation giving the sponge-like structure at the centre of the fiber. In addition, the cavities are formed in all membranes, and increase with PVA content increasing. The result may be probably due to the hydrophilicity of PVA. The addition of the PVA in the polymer dopes increases the
Fig. 6. Measurement of stress at break of PVDF/PVA hollow fiber blend membranes.
precipitation rate. This may result in a formation of larger cavity structures [29]. Moreover, as PVA content increases, the finger-like pores become larger and the skin layer becomes thinner. The result is also due to the addition of hydrophilic PVA improving water diffusion. Upon further examination at high magnification (Fig. 4(a-3, b-3, c-3)), loose, porous interconnection network structures were observed when PVDF/PVA was 7/3. Fig. 5 shows the SEM results of outer surfaces of hollow fiber membranes. As can be seen, PVDF membrane has a rougher outer surface without obvious pores, and on the outer surfaces of PVDF/PVA blend membranes, interface microvoids appear due to the phase separation in PVDF/PVA. This phenomenon may be probably due to the incompatibility of PVDF/PVA blends.
3.3. The mechanical properties of PVDF/PVA hollow fiber membranes The bursting pressure of PVDF/PVA hollow fiber blend membranes is used as a measure of the toughness of the membrane under pressure. As shown in Fig. 6, the bursting pressure of membrane decreases and the porosity of membrane increases with the increase in PVA content, indicating that the higher the value of porosity is, the worse bursting pressure of membrane will be. Therefore the stress at break is affected not only by composition, but also by the differences in porosity. The tensile strengths of dry and wet hollow fiber membrane are showed in Fig. 7. The tensile strengths of PVDF/PVA blend hollow fiber
Fig. 7. Measurement of tensile strength of PVDF/PVA hollow fiber blend membranes.
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Fig. 8. Measurement of contact angle of PVDF/PVA hollow fiber membranes.
membranes were lower than that of PVDF membrane. In the case of PVDF8, the decreasing trend of tensile strength increases appreciably, and then decreases further. The results may be due to the meshwork making of PVA. When PVA content is high, the meshwork breaks probably due to PEG congregating in PVA. In addition, dry membrane has higher tensile strength than wet membrane. This phenomenon was similar to the results of Yang and Tong [30]. The tensile strength of dry membrane was between 3.68 and 2.04 MPa, and that of wet membrane was only between 3.01 and 1.66 MPa. 3.4. The hydrophilicity of PVDF/PVA hollow fiber membranes PVDF possesses a strong hydrophobic property and thus has lower surface energy (7.83 N/m), whereas PVA is a hydrophilic polymer. Accordingly, PVA can be taken to improve the hydrophilicity of PVDF membrane through blending. Fig. 8 displays that the contact angle of hollow fiber membrane decreases as PVA increases. This indicates that PVA can improve the hydrophilicity of the membrane surface. Fig. 9 also shows that the equilibrium water content (EWC) increases with the increase of PVA content in water at 20 °C. As shown in Fig. 10, the dynamic moisture regain of PVDF hollow fiber membrane reaches equilibrium when the membrane is suspended in laboratory at 20 °C and relative humidity of around 65% for 80 min. And the equilibrium time of PVDF9 and PVDF7 are respectively 100 min and 150 min. However the dynamic moisture regain of PVA hollow fiber
Fig. 9. Measurement of equilibrium water content (EWC) of PVDF/PVA hollow fiber membranes.
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Fig. 10. Measurement of dynamic moisture regain of PVDF/PVA hollow fiber blend membranes.
membrane cannot reach equilibrium in 480 min. These indicate that PVA can improve the moisture regain of the membrane. All trends of the contact angle, EWC and the dynamic moisture regain indicate that the hydrophilicity of the hollow fiber membrane increases with PVA content increasing. 3.5. The permeation of PVDF/PVA hollow fiber membranes As shown in Fig. 11, the pure water flux increases while the rejection ratio decreases with PVA content increasing. The trend of pure water flux attributed to the finger-like pores becoming larger and the skin layer becoming thinner with PVA content increasing (Fig. 4). Moreover, the interface microvoid structure resulting from phase separation exists in PVDF/PVA blends. The rejection ratio depends more on the denseness of the skin layer than the structure of cross-section. Thus the skin layers of PVDF/PVA blend membranes were less dense than PVDF (in Fig. 5). This result demonstrates that by blending PVDF with PVA, we can prepare membranes of different retention behaviors. 3.6. The anti-fouling properties of PVDF/PVA hollow fiber membranes The anti-fouling properties of blend membranes were characterized by the increase coefficient of resistance. As shown in Table 1, the
Fig.11. Measurement of performance of PVDF/PVA hollow fiber blend membranes.
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Table 1 The increase coefficient of resistance of PVDF and PVDF8. Sample
Pure water flux before fouling (mL cm− 2 h− 1)
Pure water flux after fouling (mL cm− 2 h− 1)
Increase coefficient of resistance
PVDF PVDF8
0.61 4.13
0.43 3.36
0.42 0.23
anti-fouling property of PVDF8 blend membrane is better than that of PVDF membrane. The result may be probably due to the hydrophilicity of PVA. 3.7. The stability of PVDF/PVA hollow fiber membranes The PVDF8 membranes were put into an oven at 120 °C for different time. As shown in Fig. 12, with the time of heat treatment increasing, the mass loss rate of PVDF8 membrane decreases and reaches equilibrium nearly when the time of heat treatment is about 60 min. The pure water flux of PVDF8 membrane also decreases with the increase of the time of heat treatment. The results may be probably due to the enhancement of the degree of crosslinking and the degree of crystallinity after heat treatment [31]. Firstly, the quantity of O–H in the membrane decreases resulting from crosslinking, which makes the hydrophilicity of membrane decrease. Secondly, the macromolecule segment motion occurs in heat treatment, and then the size of porous structure in molecular network decreases. Moreover, some porous structures are extruded to smaller ones by the growing of crystal grain resulting from the crystallization consummated. All of the above make the pure water flux decrease. However, the stability of blend membrane is improved because PVA with hot crosslinking is stable in water. Therefore, the post-crosslinking is an essential step to prepare PVDF/PVA membrane, because it can make PVA stable in the membrane during application. This paper is mainly aimed at studying the morphology and performance of PVDF/PVA membranes with different PVDF/PVA blending ratios, and further research about the post-crosslinking will be carried out in the future.
the phase separation in PVDF/PVA. The bursting pressure and tensile strengths of PVDF/PVA hollow fiber membranes are worse than that of PVDF membrane, suggesting that the blend hollow fiber membrane structure became looser. The hydrophilicity of PVDF/PVA hollow fiber membranes can be improved evidently by blending PVA, which can be illuminated by the decrease of contact angle, the increase in EWC and the variety in dynamic moisture regain. With PVA content increasing, the pure water flux increases while the rejection ratio decreases. This trend can be attributed to the finger-like pores becoming larger and the skin layer becoming thinner with PVA content increasing. Moreover, PVA can improve the anti-fouling property of PVDF/PVA hollow fiber membranes. List of symbols Symbol Definition
Unit
ε ρb ρt EWC Gdry Gwet Gi Wi J V S t R Cp Cf Cr μ Rm ΔP Rf m F M0 M1
% g cm− 3 g cm− 3 % g g g % mL cm− 2 h− 1 mL cm2 H % g/L g/L g/L Pa·s cm− 1 Pa cm− 1 dimensionless % g g
porosity bulk density of membrane true density of membrane equilibrium water content weight of the dry membrane weight of the wet membrane dynamic weight of the membrane dynamic moisture regain pure water flux total permeation total permeation area total permeation time rejection ratio concentration in the permeate concentration in the feed concentration in the remain solution viscosity of feed resistance of membrane pressure drop resistance coming from fouling increase coefficient of resistance mass loss rate dry weight of the membrane without swelling in water dry weight of the membrane swelling in water for 10 days
Acknowledgements 4. Conclusions By blending process, it was possible to produce PVDF/PVA hollow fiber membranes with a hydrophilicity nature. Blending also produces a new porous structure due to phase separation. The results of this study indicate that PVDF and PVA are incompatible, therefore, the interface microvoid is produced resulting from
The authors gratefully acknowledge the financial support of the National Basic Research Program (also called 973 Program of 2006CB708602 and 863 Program of 2007AA030304) and we are grateful to the Ministry of Science and Technology of China. We also thank Professor Shulin An for his theoretical contributions during the early stages of this work. References
Fig.12. The pure water flux and the mass loss rate of PVDF8 membrane after heat treatment.
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