Preparation of Polyvinylidene Fluoride (PVDF) Membranes via

Article pubs.acs.org/IECR

Preparation of Polyvinylidene Fluoride (PVDF) Membranes via Nonsolvent Induced Phase Separation Process using a Tween 80 and H2O Mixture As an Additive Ping-Yun Zhang, Hu Yang, and Zhen-Liang Xu* State Key Laboratory of Chemical Engineering, Membrane Science and Engineering R&D Lab, Chemical Engineering Research Center, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China ABSTRACT: Polyvinylidene fluoride (PVDF) membranes were fabricated by the nonsolvent induced phase separation (NIPS) process using Tween 80 and H2O as a mixture additive from both 60 °C and room-temperature (RT) casting solution. Resultant PVDF membranes revealed improved pure water flux (PWF), enlarged mechanical properties, and well Bovine serum albumin (BSA) and Dextran rejection as a result of addition of water into the PVDF−DMAc−Tween 80 system. The improved performance was attributed to the existence of nonsolvent, which was solubilized by polar head groups of Tween 80 reverse micelle to form the water pool. Further, the interaction between polar head of surfactant and water provided a balance resistance to the interconnection between PVDF and hydrophobic chains of surfactant, which enhanced the thermodynamics stability of casting solution. During demixing process, water diffusion from the interior of casting solution, increased the precipitation rate and led to the insufficient crystallization process of polymer. Finally, the Tween 80 reverse micelle confined the movement of water in solution, making the fingerlike structure slim and confined, with the wall structure between macrovoids. chloride (LiCl).14−16 In several reports, surfactants such as Tween 20 and Tween 80 have been chosen as additives in the membrane preparation process.17−19 Surfactants are special molecules with both hydrophobic hydrocarbon chains and hydrophilic chains (polar head groups), which tend to form reverse micelle structure in polarity solution and such structure could contribute to porous structure formation. However, the reported improvement properties of the membranes prepared using surfactant as a single additive was not obvious, and the reason for this was that the micelles structure had instability in thermodynamics. Tween 80 is a water-soluble nonionic surfactant with a strong affinity with water. H2O molecules will be solubilized by reverse micelle when water is added into the DMAc−Tween 80 mixing medium and to form the water pool of polar head groups of Tween 80 reverse micelle; see Figure 1.

1. INTRODUCTION Because of its competitive mechanical properties, thermal and chemical stability, radiation resistance, etc., PVDF, as one of the most popular materials for membranes preparation, has earned lots of attention.1 PVDF membranes are mainly fabricated via the nonsolvent induced phase separation (NIPS) process, that is immersing the casting solutions into a nonsolvent coagulation bath to induce phase separation. First the diffusive exchange between solvent and nonsolvent introduces liquid−liquid demixing process, and then, the successive liquid−solid phase separation produces a porous structure.2,3 According to the bimodal demixing membrane formation mechanism, the nucleation and growth of PVDF starts from the lean phase; subsequently, the solidification of the PVDF rich phase, or the crystallization of PVDF phases, fixes the membrane morphology. Usually the precipitation process is controlled by both kinetic and thermodynamic factors, and the temperature of the casting solution can also influence the precipitation process, since high temperature contributes to a diffusion process of solvent and polymer, which tends to form a thinner skin layer.4 PVDF membranes with high permeability and good mechanical property are always highly sought after, and membranes with high pure water flux (PWF), improved rejection, and fouling resistance, reasonable mechanical strength, and narrow pore size distribution have been fabricated.5,6 For their good mechanical property, membranes with interconnected bicontinuous structure seem to more prevail over those with fingerlike structure.7 So methods and technology to improve the mechanical property without loss of the filtration property will be appreciated. During the preparation process of a good performaning PVDF membrane, additives play an important role. There are several kinds of widely used additives, such as polymeric additives, like polyvinylpyrrolidone (PVP),8 poly(ethylene glycol) (PEG),9 weak solvent like glycerol,10 and inorganic salt like TiO2,11 Al2O3,12 ferrous chloride,13 and lithium © 2012 American Chemical Society

Figure 1. H2O solubilization process of Tween 80 reverse micelles in DMAc medium.

On one hand, the H-bond formed between the polar head groups and water improved the stability of reverse micelle structure. Received: Revised: Accepted: Published: 4388

August 27, 2011 February 22, 2012 February 28, 2012 February 28, 2012 dx.doi.org/10.1021/ie201806v | Ind. Eng. Chem. Res. 2012, 51, 4388−4396

Industrial & Engineering Chemistry Research

Article

Li et al.20 A collimated laser was directed on the glass plate immersed in the coagulation bath. The light intensity was captured by the light detector and then recorded in the computer. The precipitation rate of the PVDF casting solution in the coagulation bath could be characterized by the curve of light transmittance to immersion time. 2.3. Preparation of PVDF Asymmetric Flat Membranes. Different compositions of polymer dopes were prepared, after PVDF was completely dissolved at 60 °C, and the standing time of the casting solution was at least 24 h to eliminate the bubbles inside. The homogeneous PVDF casting solutions were prepared at 60 °C and at RT, respectively. The casting solutions were cast onto a glass plate at 25 °C and 60% relative humidity by means of a glass rod, and then were immersed into a coagulation bath (deionized water at 25 °C), immediately. The pristine membranes were kept in fresh water for 1 week, and the deionized water was changed twice one day to ensure complete removal of the residual solvent from the membranes. The preparation conditions of the membranes were summarized in Table 1.

On the other hand, the interconnection between hydrocarbon chain of reverse micelle and hydrophobic polymer could improve the thermodynamics stability of the casting solution and lessen the aggregates of polymer (incipient precipitation) caused by nonsolvent. In the present work, we try to investigate the influence of different concentration nonsolvent H2O on PVDF−Tween 80 solution and membrane morphology. The solution property was revealed by dynamic light scattering (DLS) to provide more direct information. The influence of two different casting solutions temperatures on membranes properties in terms of filtration, morphology, hydrophilicity, mechanical property, and crystallization was also investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. Polyvinylidene fluoride (PVDF, Solef 6010) was purchased from Solvay Advanced Polymers, L.L.C (Alpharetta GA, USA). DMAc was purchased from Shanghai Sinopharm Chemical Reagent CO. LTD (China). Tween 80 (HLB-value 4.3) was supplied by Jiangyin Huayuan Chemical Co. LTD (China), and its chemical structure is shown in Figure 2.

Table 1. Resultant PVDF Membranes and Their Corresponding Preparation Conditions

Figure 2. Structure of Tween 80.

Bovine serum albumin (BSA) (MW 67K) and Dextran (MW 70K) were purchased from Lianguan Biochemical Reagent Company of Shanghai and Sigma-Aldrich CO., respectively. Deionized water was prepared by our own lab. All chemicals used in the experiments were analytically grade without further purification. 2.2.1. Measurements of Casting Solutions. Micelle Diameter Size in the PVDF−DMAc−Tween 80−H2O System. The morphology of the DMAc−Tween 80−water solution was detected by DLS (Zatasizer Nano ZS, Malvern Instruments Ltd., U.K). Then a 1.0 wt % PVDF solution was measured to give detailed information of the PVDF−DMAc− Tween 80−H2O system. DLS measurements were performed using a noninvasive backscatter (NIBS) apparatus with a constant 173° scattering angle at a temperature of 25 ± 0.1 °C (room-temperature, RT) and 60 ± 0.1 °C, respectively. The sample volume used for analysis was 2 mL. A total of 15 scans with an individual duration of about 5 min was obtained for each sample. Measurement for each set of aliquots was performed in triplicate, and the lowest polydispersity index (PdI) value was taken. 2.2.2. Surface Tension. The surface tension of the casting solution was measured by JK99C Automatic Surface and Interface Tension Measure Instrument (Shanghai Zhongcheng Digital Technology Apparatus Co. Ltd., China). 2.2.3. Viscosity. The viscosities of PVDF casting solutions were obtained with a DV-II+PRO Digital Viscometer (Brookfield, USA) at 298 and 313 K, controlled by a water bath. The report data was the viscosity at a shear rate of 50 s−1. 2.2.4. Light Transmittance. The light transmittance experiment was carried out by a self-made device as described by

membrane no.

casting solution temperature

solution composition (mass ratio) (PVDF/Tween 80/DMAc)

H2O content (wt %)

M-0-60 °C M-0-RT M-1.5-60 °C M-1.5- RT M-2-60 °C M-2-RT M-2.5-60 °C M-2.5-RT M-3-60 °C M-3-RT

60 °C RT 60 °C RT 60 °C RT 60 °C RT 60 °C RT

18/3/79 18/3/79 18/3/77.5 18/3/77.5 18/3/77 18/3/77 18/3/6.5 18/3/76.5 18/3/76 18/3/76

0 0 1.5 1.5 2 2 2.5 2.5 3 3

2.4.1. Membrane Characterization. PWF and Rejection Measurement of Membranes. The PWF of the membranes were measured by self-made UF experimental equipment.21 All experiments were conducted at 25 °C and 0.1 MPa. PVDF membranes were precompacted at 0.1 MPa during pure water filtration for 0.5 h, then the pure water permeation flux (JF) was measured. The permeation flux JF was calculated from the following equation:

JF =

V At

(1)

Where JF is the membrane flux (L/m2·h), V is the permeate volume (L), A is the membrane area (2.289 × 10−3 m2), and t is the microfiltration time (h). The rejection of BSA was defined as R = ((Cf − Cp)/Cf), where R is the solute rejection, Cf is the feed concentration (mg/L), and Cp is the permeate concentration (mg/L). The concentration of the feed and permeation were measured by a total organic carbon analyzer (TOC, TNM-1, SHIMADZU). 2.4.2. Porosity (ε) and Mean Pore Size (γm). The membrane porosity (ε) is determined by gravimetric method, measuring 4389

dx.doi.org/10.1021/ie201806v | Ind. Eng. Chem. Res. 2012, 51, 4388−4396


Article

the weight of ethanol contained in the membrane pores,22 see eq 2.

ε=

(m1 − m2) ρG (m1 − m2) m + 2 ρG ρp

× 100% (2)

Where m1 is the weight of the wet membrane (g), m2 is the weight of the dry membrane (g), ρG is the ethanol density (0.789 g/cm3), ρP is the polymer density (1.765 g/cm3). The mean pore size (γm) of the membranes was defined by the following equation: γm =

(2.9 − 1.75ε) × 8ηlJF εAΔP

(3)

Figure 3. Surface tension of casting solutions with different H2O content at RT.

−4

Where η = 8.9 × 10 Pa·s (viscosity of pure water at 278 K), l is the thickness of the membranes, and ΔP is the operation pressure (0.1 MPa). 2.4.3. SEM. The fiber samples were immersed in liquid nitrogen to fracture, after sputtering with gold; the crosssection and top-surface structures of PVDF membranes were observed with a scanning electron microscopy (JEOL, JSM5600LV). 2.3.4. Dynamic Contact Angle. The dynamic contact angles (θ) of PVDF membranes at RT were measured with JC2000D1 (produced by Shanghai Zhongcheng Digital Technology Apparatus Co. Ltd., China). The machine was coupled with a camera enabling image capture at 10 frames/s, and the contact angle was determined from these images. A water droplet of 0.2 μL was dispersed on the membranes surface, and the contact angle was determined using the system software. The advantage of the software was that it was easy to take the actual measurement and to obtain the dynamic contact angle curve with respect to time. 2.4.5. Mechanical Property. Mechanical property was conducted using QJ210A (Shanghai Qingji Instrument Technology Co. Ltd., China) at RT. The flat sample of settled width of 15 cm was clamped at both ends and pulled in tension at a constant elongation speed of 50 mm/min with an initial length of 25 cm. Break strength, elongation and Young’s modulus were obtained from the stress−strain curves through the average of at least 5 measurements. 2.4.6. XRD. XRD results were obtained with a D/max-rB diffractometer (Rigaku, Japan) equipped with graphite monochromated Cu Kα radiation (λ = 0.15405 nm) operated at 100 mA and 40 kV from 10−80°. All measurements were taken at RT.

of Tween 80 reverse micelle in DMAc medium, and this might improve microstructure adjustment of the molecules in casting solution. The viscosity change with water was shown in Figure 4. The casting solution had lower viscosity at higher temperature, and

Figure 4. Effect of H2O content on the viscosity of 18.0 wt % PVDF casting solution.

the viscosity increased with H2O content in solution both at 60 °C and at RT, respectively. The higher viscosity revealed a stronger interconnection between molecules.23 The reason for that was Tween 80 micelles with solubilization water played an important role in the improvement of the interconnection between molecules of the casting solution. The interconnection between hydrocarbon chains of reverse micelle and polymer would enhance the chain entanglement degree of PVDF, which led to increase of viscosity. To provide more information, the DLS measurement was carried out for low PVDF solution. 3.1.2. DLS Study of the Mixed Solvents and 1.0 wt % PVDF Solution. DLS uses the scattered light to measure the rate of diffusion of the aggregated macromolecules in solution. The technique measures the time-dependent fluctuations in the intensity of scattered light from a suspension of particles undergoing random, Brownian motion. Analysis of these intensity fluctuations allows for the determination of the diffusion coefficients, which in turn yield the particle size through the Stokes−Einstein equation. It has been widely used to measure

3. RESULTS AND DISCUSSION 3.1.1. Basic Physical-Chemical Property of Casting Solution and Conformation of PVDF. Surface Tension and Viscosity of Casting Solution. The surface tension change with H2O content in casting solution was shown in Figure 3. Only the data at RT was measured since at high temperature the result may be inaccurate due to the volatility of DMAc and water. Figure 3 showed that the surface tension decreased slowly with the increase of H2O content in casting solution. Since surface tension was related to the surface arrangement of molecules on the interface between air and liquid, the surface tension decreasing of the solution should be related to solubilization of high polarity water molecules 4390



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the size of the micelle or the morphology of polymer in solution. Due to the sensibility of the equipment, only the solution with a low polymer concentration can be measured. However, it is possible to provide the information about the morphology change in casting solution caused by the addition of water. Figure 5 showed the intensity-weighted micelle size distribution of the DMAc−Tween 80 system at RT and 60 °C, respectively.

The DLS data of 1.0 wt % PVDF solution at RT (A) and 60 °C (B) were showed in Figure 6. PVDF chains were in a lowly coiled state and tended to extend, then to be entangled

Figure 6. Intensity-weighted particle size distribution of 1.0 wt % PVDF in different composition solvents at RT (A) and 60 °C (B).

with neighboring chains in pure DMAc solvent,23 and its corresponding Dh at RT was 248.5 nm. However, Figure 6A showed that the Dh of micelles significantly increased due to the formation of large aggregates (incipient precipitation) when H2O was taken as single additive.27 While the Dh of micelles decreased when Tween 80 was chosen as single additive, which contributed to the further enhancement of lowly coiled state and better entangled with neighboring chains of PVDF caused by the interconnection between reverse micelle and polymer. Besides, the Dh of micelles increased when further addition of water as mixture additive with Tween 80. Pure PVDF and PVDF−H2O species with 60 °C were detectable with Dh ranging from a few nanometers to >2 μm, indicating that the large aggregates formed at high temperature and the two systems were not in a thermodynamic equilibrium state.28 The slow increased Dh of micelles of 76:3:3:1 system at 60 °C revealed the improved thermodynamic stability of the PVDF− DMAc−Tween 80−H2O system when compared with other samples. The reason behind that was the interaction between polar head of surfactant and water, providing a balance resistance to the interconnection between PVDF and hydrocarbon chains of surfactant.

Figure 5. Intensity-weighted micelle size distribution of different composition solvents at RT (A) and 60 °C (B).

The scattering intensity data was derived by the automeasure software (Malvern Instruments, Malvern UK) to obtained the harmonic intensity weighted average hydrodynamic diameter (Dh), also referred to as the z-average diameter of the reverse micelles.24 At room temperature with the increase of water content, the Dh of the micelle first decreased and then increased. That could be explained by the structure change of micelle after adding water. As shown in Figure 5A, with a small amount of water added, Tween 80 was tightly bound to water molecules, so the micelle hydrodynamic diameter decreased. With the amount of water increasing, the size of the inner core of the micelle composed by water increased, so the Dh of micelle increased.25 At 60 °C as showed in Figure 5B, the micelle hydrodynamic diameter increased with the addition of water. The enhanced Dh micelles at 60 °C was attributed to the thermal movement of molecules at high temperature, and the bounding between surfactant and water became less tight.26 4391



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enhanced the mobility of the polymer chains and solvent, and this improved the diffusion rate of solvent and nonsolvent in casting solution. This, combined with the slightly increased viscosity of the solution after the addition of water as shown in Figure 4, revealed that the viscosity change was not the dominant factor in determining the precipitation rate of the casting solution. 3.3.1. Membrane Performance. Membrane Morphologies and Filtration Property. The top surface and crosssection of resultant PVDF membranes could be seen in Figure 8. The cross-section of all membrane presented the morphology of a skin layer followed by full development finger-like macrovoids in the sublayer. The feature indicated that the precipitation was dominated by instantaneous liquid−liquid demixing. Examined in detail, differences of membrane morphology with the addition of nonsolvent H2O could be observed. Without H2O addition, the macrovoids were large, irregular, and less confined, whereas in the case of H2O adding, the finger-like structure became slim and confined, and the wall structure between macrovoids increased. The reason for this was that there were Tween 80 reverse micelles forming in the solution, which confined the movement of water in solution, and thus the formation of the finger-like structure became controlled to some degree with the further addition of water. Membranes prepared with higher casting temperature had less random macrovoids compared with those prepared at RT. The difference lied in the improved mobility of polymer chains at high temperature, which became more flexible, and the pore structure of resultant membranes was adjusted by their movement. The filtration properties of resultant membranes were listed in Tables 2 and 3. The PWF of membranes increased with H2O content increasing in PVDF casting solution, and the membranes prepared with higher casting temperature had lower PWF at same fabrication condition. The influence of H2O content on porosity of membranes coincided with that of PWF change. Membranes prepared with high loading amount of H2O have higher flux, which further supported the above standpoint that micelle structure and nonsolvent water were favorable to the formation of porous structure. Finger-like macrovoids of membranes always resulted in higher porosity. Table 2 revealed that the overall porosity of the resultant membranes was in the range of 78−88%. The BSA and dextran rejection of resultant membranes in Table 3 presented the stability and well rejection property of PVDF membranes. 3.3.2. Dynamic Contact Angles of Membranes. The dynamic contact angles were characterized by continuously tracking and measuring the contact angles of water on PVDF membrane surface over time as shown in Figure 9. The results illustrated that the addition of nonsolvent H2O in casting solution enhanced both the start contact angle (θs) and equilibrium contact angle (θE) of PVDF membranes. Besides, it also could be found that the larger of H2O content was in the casting solutions, the bigger the θs was. However, the contact angle decreased with time and trended to θE in finite time, and the difference between θs and θE was not very much with different H2O concentrations in casting solutions. Moreover, it also revealed that high preparation temperature favored high θs and θE of membranes at the same fabrication condition of resultant membranes. Combined with the mean pore size of membranes as shown in Table 4, this demonstrated that the hydrophibility surface structure change of membranes was not

For polymer−surfactant solution in terms of micelles size to intensity signal showed the micelle structures in solution were very complex when compared with Figure 5, and both polymer aggregate and micelle structure existed in solution. Mainly two peaks can be observed in Figure 6, the narrower intensity signal of a few hundred nm scale corresponding to the micelle structure, the other corresponding to the aggregated structure of PVDF themselves or mixed with separated surfactant molecules. Above data about the morphology solution of different composition revealed that the interaction between water and surfactant as well the interconnection between hydrocarbon chains of reverse micelle and polymer played an important role in the microstructure adjustment of the solution. Such change surely would be embodied in the membrane formation process. 3.2. Precipitation Kinetics of Casting Solutions by Dynamical Light Transmittance. Precipitation kinetics of the casting solutions in the pure water coagulation bath was investigated by light transmission experiment as shown in Figure 7. The curves showed that with H2O content increasing

Figure 7. Precipitation rate of PVDF casting solutions with different H2O content at RT (A) and 60 °C (B) in a pure water coagulation bath.

in casting solutions, the precipitation rate speeded up both at RT and at 60 °C. The reasons for the above results were list as below. On one hand, the existence of reverse micelle and its solubilization of nonsolvent contributed to the increasing miscibility between casting solution and the coagulant, and this enhanced the precipitation rate of casting solution.17 On the other hand, the reverse micelle structure was favorable to water diffusion from the interior of casting solution, which further increased the precipitation rate. Especially, high temperature 4392



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Figure 8. SEM images of PVDF membrane prepared at 60 °C and RT from various H2O content casting solutions (coagulant deionized water; a top surface; b cross-section).

Table 5, respectively. With the water content increasing in casting solution, the corresponding membrane had higher break strength, enlarged elongation at break, and an improved Young’s modulus. Normally, nonsolvent in solution would lead to a deterioration of the mechanical properties. However, the results in Figure 10 and Table 5 were contrary to the common hypothesis. The reason for this could be deduced from SEM images, and the increased wall structure between macrovoids contributed to the improved mechanical properties. At high temperature, improved mobility of polymer chains led to a better arrangement, so the mechanical properties of membrane were better than that at room temperature. 3.3.4. Membrane XRD Analysis. As a semicrystalline polymer, four different forms, i.e. α-phase, β-phase, γ-phase, and δ-phase existed in PVDF.29 The crystallization of PVDF was affected by additives and casting solution temperatures.7 The peaks at 2θ = 17.8°, 18.6°, and 19.8° were the characteristic of α phase, while the peak at 2θ = 20.6° corresponded to the unresolved (110) and (200) reflections of the β-phase for PVDF.4 Figure 11 was the XRD spectra of PVDF membranes with various fabrication conditions. The results revealed that the range of a very intensity peak was wide, and the characteristic peak such as α and β could not be identified clearly. Besides, the effect of different casting temperatures on crystallization of PVDF was not obvious. No identified intensity peak indicated that the crystal formation of PVDF was

Table 2. Flux and Porosity of PVDF Membranes Prepared at 60 °C and at RT (Operation Pressure 0.1 MPa) PWF (L/m2·h)

porosity (%)

membrane no.

60 °C

RT

60 °C

RT

M-0 M-1.5 M-2 M-2.5 M-3

74.3 86.8 92.8 84.9 125

82.4 95.8 179 189 192

78.7 80.5 80.9 81.5 83.7

80.1 84.5 87.1 86.5 88.2

Table 3. BSA and Dextran Rejection of Resultant PVDF Membranes Rejection (%) BSA solution

dextran solution

membrane no.

60 °C

RT

60 °C

RT

M-0 M-3

85.2 93.7

87.1 88.3

85.9 86.2

87.3 83.8

related to the mean pore size, but to other reasons. The idea that the hydrophobic hydrocarbon chain of the surfactant might have been imparted during the demixing process could account for this. 3.3.3. Membrane Mechanical Properties. Mechanical properties of membranes in terms of break strength, elongation at break, and Young’s modulus are shown in Figure 10 and 4393



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Table 5. Elongation Ratio at Break and Young’s Modulus of PVDF Membranes at 60 °C and RT elongation at break (%) membrane no. M-0 M-1.5 M-2 M-2.5 M-3

60 °C 12.8 18.0 32.5 35.2 39.1

± ± ± ± ±

1.5 0.9 1.1 3.1 3.3

10.8 32.7 32.3 38.9 42.2

± ± ± ± ±

Young’s modulus (MPa) 60 °C

RT 0.1 0.7 2.1 1.1 2.4

14.0 20.2 22.2 25.3 25.7

± ± ± ± ±

1.8 0.1 1.4 2.1 0.3

RT 15.7 16.9 22.2 35.2 32.2

± ± ± ± ±

0.2 0.5 1.5 3.5 1.1

Figure 9. Dynamic contact angles of PVDF membrane prepared at RT (A) and 60 °C (B) from various H2O content casting solutions.

Table 4. Mean Pore Size of PVDF Membranes Prepared at 60 °C and RT mean pore size (nm) membrane no.

60 °C

RT

M-0 M-1.5 M-2 M-2.5 M-3

21 24 26 26 28

21 28 34 37 35

Figure 11. X-ray diffraction scans of PVDF−Tween 80 membranes fabricated from various conditions: (A) M-1.5-RT, M-1.5-60 °C; (B) M-2.5-RT, M-2.5-60 °C.

a mixture of α and β crystallites, or an undeveloped α or β crystalline phase. Previous studies had reported that there was insufficient time to induced crystallization due to rapid occurrence of demixing process.30

4. CONCLUSIONS Polymer casting composition (i.e., surfactant and different concentration of nonsolvent H2O) and casting temperatures (i.e., 60 °C and RT) were key parameters influencing physicalchemical and precipitation kinetics of casting solution, as well as the resultant morphology and mechanical properties of PVDF membranes. It was the water that changed the physical-chemical

Figure 10. Effect of H2O concentration on break strength of PVDF membranes at RT and 60 °C. 4394



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(10) Mansourizadeh, A.; Ismail, A. F. Preparation and characterization of porous PVDF hollow fiber membranes for CO2 absorption: Effect of different non-solvent additives in the polymer dope. Int. J. Green Gas Con. 2011, 5, 640. (11) Yuliwati, E.; Ismail, A. F. Effect of additives concentration on the surface properties and performance of PVDF ultrafiltration membranes for refinery produced wastewater treatment. Desalination 2011, 273, 226. (12) Yan, L.; Li, Y. S.; Xiang, C. B. Preparation of poly(vinylidene fluoride)(pvdf) ultrafiltration membrane modified by nano-sized alumina (Al2O3) and its antifouling research. Polymer 2005, 46, 7701. (13) Ma, J.; Wang, Z.; Pan, M.; Guo, Y. A study on the multifunction of ferrous chloride in the formation of poly(vinylidene fluoride) ultrafiltration membranes. J. Membr. Sci. 2009, 341, 214. (14) Mansourizadeh, A.; Ismail, A. F. Effect of LiCl concentration in the polymer dope on the structure and performance of hydrophobic PVDF hollow fiber membranes for CO2 absorption. Chem. Eng. J. 2010, 165, 980. (15) Elashmawi, I. S. Effect of LiCl filler on the structure and morphology of PVDF films. Mater. Chem. Phys. 2008, 107, 96. (16) Fontananova, E.; Jansen, J. C.; Cristiano, A.; Curcio, E.; Drioli, E. Effect of additives in the casting solution on the formation of PVDF membranes. Desalination 2006, 192, 190. (17) Amirilargani, M.; Saljoughi, E.; Mohammadi, T. Improvement of permeation performance of polyethersulfone (PES) ultrafiltration membranes via addition of Tween-20. J. Appl. Polym. Sci. 2010, 115, 504. (18) Amirilargani, M.; Saljoughi, E.; Mohammadi, T. Effects of Tween 80 concentration as a surfactant additive on morphology and permeability of flat sheet polyethersulfone (PES) membranes. Desalination 2009, 249, 837. (19) Field, R.; Hang, S.; Arnot, T. The influence of surfactant on water flux through microfiltration membranes. J. Membr. Sci. 1994, 86, 291. (20) Li, Q.; Xu, Z. L.; Yu, L. Y. Effects of mixed solvents and PVDF types on performances of PVDF microporous membranes. J. Appl. Polym. Sci. 2010, 115, 2277. (21) Zhang, P. Y.; Wang, Y. L.; Xu, Z. L.; Yang, H. Preparation of poly (vinyl butyral) hollow fiber ultrafiltration membrane via wetspinning method using PVP as additive. Desalination 2011, 278, 186. (22) Li, Q.; Xu, Z. L.; Liu, M. Preparation and characterization of PVDF microporous membrane with highly hydrophobic surface. Polym. Adv. Technol. 2009, 20, 1. (23) Panu, S.; Chung, T. S. Molecular elucidation of morphology and mechanical properties of PVDF hollow fiber membranes from aspects of phase inversion, crystallization and rheology. J. Membr. Sci. 2009, 340, 192. (24) Santiago, P. S.; Moura, F.; Moreira, L. M.; Domingues, M. M.; Santos, N. C.; Tabak, M. Dynamic Light Scattering and Optical Absorption Spectroscopy Study of pH and Temperature Stabilities of the Extracellular Hemoglobin of Glossoscolex paulistus. Biophys. J. 2008, 94, 2228. (25) Falcone, R. D.; Correa, N. M.; Silber, J. J. On the Formation of New Reverse Micelles: A Comparative Study of Benzene/Surfactants/ Ionic Liquids Systems Using UV−Visible Absorption Spectroscopy and Dynamic Light Scattering. Langmuir 2009, 25, 10426. (26) Ghosh, S. Comparative studies on brij reverse micelles prepared in benzene/surfactant/ethylammonium nitrate systems: Effect of head group size and polarity of the hydrocarbon chain. J. Colloid Interface Sci. 2011, 360, 672. (27) Cheng, L. P.; Lin, D. J.; Shih, C. H.; Wang, A. H.; Cryte, C. C. PVDF Membrane Formation by Diffusion-Induced Phase SeparationMorphology Prediction Based on Phase Behavior and Mass Transfer Modeling. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 2079. (28) Loos, K.; Bo1ker, A.; Zettl, H.; Zhang, M.; Krausch, G.; Muller, A. H. E. Micellar Aggregates of Amylose-block-polystyrene Rod-Coil Block Copolymers in Water and THF. Macromolecules 2005, 38, 873.

property and precipitation kinetics of the PVDF−DMAc− Tween 80 system. Water was solubilized by Tween 80 reverse micelles, which favored microstructure adjustment of the molecules, decreasing surface tension and increasing viscosity of casting solution. The interaction between polar head groups of surfactant and water provided a balance resistance to interconnection between PVDF and hydrophobic chains of surfactant, and this enhanced the thermodynamic stability of the casting solution. Besides, the reverse micelle structure was favorable to the diffusion of water in the solution, which increased the precipitation rate and led to insufficient crystallization process of PVDF. All membranes had fingerlike structure with wall structure between macrovoids, and the macrovoid structure was confined by Tween 80 reverse micelles. It could be concluded that PVDF membranes with good rejection of BSA and dextran improved PWF, and enhanced mechanical properties could be fabricated from the PVDF−DMAc−Tween 80−H2O system.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 86-21-64253061. Fax: 86-21-64252989. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Key Technology R&D Program of China (20092X02032) and the Key Program of Science and Technology of Guangdong Province (2011A080403004) for their financial supports. Also we are grateful to Dr. Ling-Feng Han for mechanical property measurement.

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REFERENCES

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Preparation of Polyvinylidene Fluoride (PVDF) Membranes via

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