P(VDF-co-HFP) blend hollow fiber membranes for DCMD

Author’s Accepted Manuscript Fabrication of Novel PVDF/P(VDF-co-HFP) Blend Hollow Fiber Membranes for DCMD Peng Wu, Lan Ying Jiang, Biao Hu

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S0376-7388(18)31246-8 https://doi.org/10.1016/j.memsci.2018.09.015 MEMSCI16460

To appear in: Journal of Membrane Science Received date: 5 May 2018 Revised date: 6 July 2018 Accepted date: 1 September 2018 Cite this article as: Peng Wu, Lan Ying Jiang and Biao Hu, Fabrication of Novel PVDF/P(VDF-co-HFP) Blend Hollow Fiber Membranes for DCMD, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.09.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Fabrication of Novel PVDF/P(VDF-co-HFP) Blend Hollow Fiber Membranes for DCMD

Peng Wu1, Lan Ying Jiang1, 2, *, Biao Hu1

1

School of Metallurgy and Environment, Central South University, Changsha, China 2

National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha, China

*Corresponding author: School of Metallurgy and Environment, Central South University, Changsha 410083, Hunan, China; Phone: (86)-731-88716206, Fax: (86)-731-88710171; Email: [email protected]

Abstract

In this research, porous hollow fiber membranes for DCMD desalination were fabricated by non-solvent induced phase inversion. The membrane materials were PVDF 6010 and its blend with P(VDF-co-HFP), expected to combine the advantages of each material. It was found that with higher P(VDF-co-HFP) content in the blend, the membrane materials acquired stronger hydrophobicity. The maximum tensile 1

strength was obtained with hollow fibers having 30% P(VDF-co-HFP), which was 2.86MPa. Cloud point test showed that the one phase region of the solution with neat P(VDF-co-HFP) was closer to the polymer-solvent axis in the phase diagram, indicating a lower thermodynamic stability of this polymer solution. The varied degree of the liquid/liquid demixing and the solid/liquid demixing induced by changing P(VDF-co-HFP) content might be the underlying factor causing the dissimilarities in the above membrane structural and mechanical characteristics. DSC was employed to corroborate the crystallinity of the membranes. The DCMD tests using NaCl aqueous solution showed that membranes with higher PVDF 6010 content provided apparently higher flux. Stable salt rejection above 99% were obtained with all the membranes under the operation conditions investigated.

Key Words: Membrane distillation; Blend; P(VDF-co-HFP); Mechanical strength

1. Introduction

Fresh water supply is one of the key elements for the sustainable development of every society in modern world [1]. With increasing population and intensified human activity in agriculture, industry etc., the demand on fresh and clean water cannot be satisfied solely by utilizing conventional sources [2]. Seawater, the largest part of the hydrosphere becomes the choice and its desalination has changed the water demand 2

and supply relationship, particularly for arid zones [1-3].

The two major categories of commercialized methods to perform seawater desalination are thermal method (e.g. multi-stage distillation (MSE), multi-stage flash (MSF), mechanical vapor compression (MVC)) [4] and membrane method (e.g. reverse osmosis (RO)) [5]. An emerging technology is membrane distillation (MD) [6-7]. As the name implies, MD is in principle a type of distillations and draws support from thermal resource to drive the mass transfer. With vapor pressure difference across the membrane being the factual driving force, the volatility of the feed components determines the separation efficiency theoretically. Unlike conventional distillation, the application of the porous membranes in MD constrains the space occupied by vapor and lowers the footprint of the overall system [8]. Another important advantage of MD is that it can be operated at feed temperature considerably below the boiling point and has the potential to capitalize on low quality heat resource, such as solar energy, geothermal energy, waste heat [6-7]. As a membrane process, MD is also featured with easy scale-up and large flexibility. Further advanced levels of automation and remote control can be more conveniently achieved with MD.

To achieve efficient operation of a MD process in desalination, membrane properties based on materials and structures (exterior and interior) are very essential [6-7]. The most intensively investigated directions addressing DCMD membranes are the 3

improvement of transmembrane water vapor flux and acquisition of high and stable salt rejection. Those factors counted on heavily for enhancing flux are porosity and membrane thickness [9-10]. As for desired salt rejection, the membrane should not be wetted or penetrated by liquid water, while only allows water vapor transport across it. The prerequisite for avoiding spontaneous wetting of a membrane by aqueous solution is the hydrophobicity of membrane material [11-12]. Additionally, the Laplace-Young equation reveals that the critical or the minimum pressure to push the feed liquid into a hydrophobic membrane is related to the maximum surface pore size, the interfacial tension and the contact angle of the liquid on the membrane surface [12]. For a practical MD process, another important aspect of the membrane is the mechanical durability, which is very critical for avoiding membrane failure under a certain level of pressure and/or by continuous vibration during separation [13]. There are two aspects principally considered concerning membrane mechanical property. Firstly, macrovoids in membranes are generally undesirable. It was reported that membrane with a full sponge-like structure exhibited better resistance than that with finger-like one towards the same pressurization condition. The reason is that the former dissipates the stresses more efficiently [14]. However, macrovoids has been found to facilitate higher membrane flux, which makes it very critical to deal with mechanical strength via other approaches. Zuo et al. prepared a PVDF/polyetherimide (PEI) composite hollow fibers for vacuum membrane distillation (VMD) used in seawater desalination. Due to the PEI support, the membranes exhibited excellent mechanical strength of 30.1MPa [15]. In Zhao et al.’s work [16], a type of 4

hydrophobic porous activated carbon (AC) was mixed with PVDF polymer matrix to prepared membranes for VMD application. A load of 0.12wt% AC delivers hollow fibers with tensile strength of 3.1MPa, up 29% over that of the neat membranes. Crosslinking was also found as an efficient method to modify the mechanical property of PVDF membranes [17]. The hollow fibers with a 3-hour reaction time in the casting solution showed a significant improvement of about 40% in Young’s Modulus as compared with the membrane without diamine treatment. The transmembrane flux was also improved in the research. In Wang and Chung’s work [18], it was found that the bursting pressure of PVDF multi-bore hollow fiber (MBF) membrane for VMD was obviously higher than that of the single-bore hollow fiber (SBF); the explanation is that the larger overall cross-section area of the MBF membrane with its lotus-root-like structure. Blending is yet another approach relied on to develop polymeric materials with new properties [19, 20]. It is a method suitable for large-scale preparation and applicable to a variety of membrane configurations. Polydimethylsiloxane (PDMS) and PVDF blend was used in Kang’s work to produce porous membrane for VMD [21]. The addition of PDMS resulted in higher hydrophobicity (max. contact angle 125.90o) and enhanced mechanical strength (max. tensile strength 2.12 MPa) along with higher flux (max. 125.3L/m2-hr).

In current research, PVDF/poly (vinylidene fluoride-co-hexafluoro propylene) (P(VDF-co-HFP)) blend will be used to prepare membrane for direct contact membrane distillation (DCMD). PVDF has been a very popular material for 5

fabricating porous MD membrane. The PVDF selected in current research is Solef 6010 with a molecular weight (Mw) of 320,000 Da [16, 20]. As found in previous researches, it is a type of PVDF characterized of good mechanical strength. However, one point to be raised is that the hydrophobicity of PVDF is not high enough [22]. Compared to PVDF, P(VDF-co-HFP) is more hydrophobic, due to the incorporation of an amorphous phase of fluoropropylene (HFP) into the main constituent vinylidene fluoride (VDF) blocks [23-27]. Additionally, P(VDF-co-HFP) is also featured with good tensile strength [23]. Several researches reported the fabrication of P(VDF-co-HFP) hollow fiber membranes for DCMD [24-26]. The problem with this material is that its membrane performance in terms of DCMD flux is generally not high, as compared with many PVDF materials [25, 26]. A combination of PVDF and P(VDF-co-HFP) is expected to provide a flexible way to produce membranes with good performance in mechanical strength and DCMD performance. The membrane will be formed by non-solvent induced phase inversion (NIPS). The effect of the blending ratio will be investigated in terms of its effect on membrane microstructure, mechanical properties and DCMD performance. A series of characterizations will be used to assist the analysis about the mechanisms underlying the variation of membrane physiochemical properties. 2. Experiments 2.1. Materials

The supplier for polyvinyldenefluoride (PVDF 6010, Mw=322,000Da, density 6

1.77g/cm3) was Wuxi United Hengzhou Chemical Co. Ltd., China and P(VDF-co-HFP) (Mw=400,000Da, Mw/Mn=3.07, density 1.78g/cm3, 25wt.% HFP) was purchased from Sigma-Aldrich (Shanghai) Co. Ltd. The unit structures of the two polymers are shown in Fig. 1. Polymers were dried in vacuum oven (DZ-2A, Tianjin Taisite Instrument Co. Ltd.) at 60°C for 10 hours prior to use. N-methyl-2-pyrrolidone (NMP) from Sinopharm Chemical Reagent Co. Ltd. was used as the solvent to prepare the dopes. All the other chemicals (i.e. glycerol as non-solvent additives in the polymer dope, AR grade ethanol for external coagulant bath) were purchased from Hunan Huihong Reagent Co. Ltd. Besides, deionized water was used as a component in internal coagulant. All the chemicals were used as received.

2.2. Fabrication of PVDF/P(VDF-co-HFP) Hollow Fiber Membranes

Nonsolvent induce phase separation, also known as wetting spinning, was used to prepare the hollow fiber membranes. Table 1 shows the parameters of dope composition and conditions of spinning. The polymers were dried directly in the vacuum oven for approximately 10 h at 60°C to remove its water content. Then, the spinning solutions were prepared by dissolving polymer in N-methyl-pyrrolidinone (NMP) at various blending ratios of PVDF/P(VDP-co-HFP). The total polymer content was 15wt.% and glycerol content was 10 wt.%. Homogeneous solutions were prepared by magnetic stirring for about 24 h at 60°C. The solution was then transferred to the feed tank of spinning machine and degassed overnight at ambient 7

temperature. The hollow fiber spinning apparatus and detailed spinning procedures have been described elsewhere [28]. The newly fabricated hollow fiber membranes were soaked in pure water to remove the remaining solvent for three days. Finally, the fibers were treated in the freezer dryer (SCIENTZ-16N, Ningbo Scientz Biotechnology Co. Ltd.) for 6 h to obtain dry membranes.

2.3. Preparation of Membrane Module

To fabricate a hollow fiber module, both ends of a PFA plastic tube with an inner diameter of 6 mm and a specific length were joined with a union tee. A piece of fiber was placed in the tube with both ends protruded from the union tee outlets. The space between the hollow fiber and the union tee was then sealed with fast epoxy and dried overnight. Epoxy was applied twice to ensure complete sealing. The effective length of the fibers was about 15 cm.

2.4. DCMD Performance Experiment

The schematic of the testing set-up was illustrated in Fig. 2 [29]. The hollow fiber membranes were characterized by pure water flux and rejection ratio of sodium chloride (NaCl) which concentration was 5 wt.%. The pressure difference across the membrane is 0.1MPa. For pure water, the experimental conditions of the four batches of membrane are the same, flow rate on the feed side was 250mL/min, temperature of 8

feed and permeate were 65°C and 17°C, respectively. While NaCl, we used the control variable method to test the effects of temperature and feed flow rate on the results. When the feed temperature is controlled at 60°C, feed side flow is set to 18L/ h, 36L/h, 54L/h and 72L/h by the peristaltic pump (BT-600EA, Jieheng Peristaltic Pump Co. Ltd., Chongqing). When feed flow is controlled at 18L/h, feed temperature is set to 50°C, 60°C, 70 °C and 80°C by oil bath (DF-101S, Yuhua Instrument Co. Ltd.). After the temperatures of both sides reached stable, the weight change of distillate solution was recorded by electronic balance. An analytical electric conductivity meter (DDS-11A, Shengci Instrument Co. Ltd, Shanghai) was used to measure the electronic conductivity of the distillation solution. The flux was calculated by Eq. (1): - =

0 6´W

(1)

where, M is the total permeation (kg), S is the effective membrane area (m2), and t is the total permeation time (h).

The salt rejection R was calculated by combining Eq. (2) and Eq. (3): R=1− !

=

 

"#" $%& #& '" ('&

(2) (3)

Where, Cd (wt.%) is the concentration of the distillate, Cf (wt.%) is the concentration of the feed solution. C1 and C2 are the salt concentrations, and V1 and V2 are the volumes of liquid in the distillate reservoir at time t1 and t2 (t2-t1=t), respectively. The concentration of the distillate side solution was deduced according to a 9

conductivity-concentration standard curve.

2.5. Characterizations 2.5.1. Ternary Phase Diagram

The cloud point curve, which is considered as the experimentally binodal curve, represents the composition where the solution is not thermodynamically stable and phase

transition

occurs

[30].

The

ternary

phase

diagram

of

a

polymer/solvent/non-solvent system was constructed using the cloud-point titration method [31*27]. Specific amounts of polymer range from 5 wt.% to 25 wt.% for both PVDF and P(VDF-co-HFP) were firstly dissolved in NMP solvent and stirred 60°C until a homogeneous solution was obtained. Mixed liquor of 90% deionized water and 10% ethanol (non-solvent) was slowly titrated into the polymer solution, with a magnetic stirrer vigorously mixing the solution. The titration process stopped when the solution became turbid or showed signs of gelation and re-started when the solution became homogeneous again.

2.5.2. Morphology Examination

The structural characteristics of the membranes were observed by field emission scanning electron microscope (FESEM) (FEI Electron Optics B.V./Nova Nano SEM 230). The cross-section of the hollow fibres was freeze-fractured inside liquid 10

nitrogen. The membrane samples were coated by gold. The current and time duration for spraying gold were 120s and 20mA, respectively.

2.5.3. Porosity and Pore Size Distribution

Weighing method was employed to characterize membrane bulk porosity. Hollow fiber sample of 5cm of hollow fibres by a blade and measure their mass through an analytical balance (FA1104N, Minqiao Precision Scientific Instruments Co. Ltd., Shanghai). The porosity (ε) is obtained using the following equation:

ε = 1-

Vs m/ρ s = 1Vm Vm

(4)

where V is the volume (m3); m is mass of sample and ρ density (g/cm3); subscripts m and s indicate membrane and solid polymer matrix, respectively.

Liquid-liquid displacement method is used to measure the hollow fiber membrane outer and inner surface pore size distribution [32]. Before the test, water and n-butyl alcohol (n-BuOH) mixed solution at the ratio of 1:4 was prepared, thoroughly mixed and sat still for 12 hours at room temperature. After the liquid got stratified, the upper layer of dominant alcohol was poured out, and water rich phase was added into the liquid tank. The lower end of the membrane module was sealed by epoxy resin, water was squeeze out by nitrogen gas under pressure from the top to the lumen side of the membrane. The effective length of the membrane module is 10 cm. Membrane under test soak in the alcohol phase for 2 h before the test to make the fiber is involved by 11

alcohol completely. Water is squeeze out to the outer side of the membrane under the action of pressure. As the rate of effusion of water is related to the entry pressure, the relation between membrane aperture radius ri (m) and hydraulic pressure pi (Pa) is as follows: ri =

2e cos q pi

(5)

Where, e is interfacial tension of n-BuOH and water (N/m2); θ is n-BuOH contact angle on membrane surface, which is assumed to be 0°.

According to the membrane pore size ri (m) and the transmembrane liquid flow velocity Qi (ml/min), the relationship between membrane pore size distribution function, f(r), and pore radius can be obtained by the following formula:

f (r ) =

pi (pi -1Q I - pi Q I-1 ) m p (ri-1 - ri )pi-1 åi=1 i (p i-1Q I - pi Q I-1 ) pi-1

(6)

Liquid entry pressure by water (LEPw) was measured according to the method reported in Racz et al.’s work [33]. The setup was the DCMD device, which was thoroughly washed by deionized (DI) water for 1 hour before the test. The feed side was then fed with 5% NaCl aqueous solution and the distillate side DI water. After circulation started at both sides, the pressure at the feed side increased every 5 minutes, until the ion conductivity at the distillate side shows abrupt increase or the fibers collapsed (indicator: distillate could not be circulated anymore). The pressure was regulated by controlling the valve located near the outlet of feed side. 12

2.5.4. Other Characterizations

The falling ball method was used to compare the viscosity of the polymer solutions (Song ZW). A steel ball with the diameter being 1 cm was put into the polymer solution at ambient temperature. The time for the ball to travel the vertical distance of 8 cm from solution surface to cylinder bottom was recorded and used as an indicator for the viscosity.

The contact angle of the porous flat sheet membranes was photographed by a high-speed camera (Y3M, Integrated Design Tools Inc.) at room temperature. The photos were then opened by Polypro Software that can mark the angle and measure its value. The coagulant for preparing the flat membrane was the same as the external coagulant of hollow fiber spinning. Contact angles at ten different spots of the same piece of sample were measured and an average value with standard deviation was reported.

As for the thermal properties of the hollow fibers, they were evaluated by Differential Scanning Calorimetry (DSC) (STA449F3, NETZSCH Scientific Instruments Trading (Shanghai) Ltd.). Samples of about 5~10 mg were subjected to a heating at a rate of 15°C/min in the range of 50~300°C.

13

A tensile machine (WDW-100N, Jilin Tanhor Testing Machine) was employed to analyze the tensile strength of the hollow fibers and flat dense membrane of two polymers. The two ends of a fiber or a membrane (Wh/ 1cmh3cm) were clamped and fixed without causing bending or extension of the sample. The effective length of hollow fiber was set at 10 cm and the test mode involved stretching at a rate of 10 mm/min along the axial direction until fiber failure. The fiber elongation and tensile strength at break were recorded. The tests were carried out at room temperature (25°C) and 40% relative humidity. The dense membranes for mechanical strength tests were cast following the solvent evaporation method reported in Jiang et al.’s work [34]. The solvent for PVDF and P(VDF-co-HFP) was DMAc and acetone, respectively. Thereafter, the naturally dried membrane was vacuum treated for 8 hours and annealed in vacuo at 100oC for another 12 hours.

The wide-angle X-ray diffraction (WAXD) patterns were detected by an X-ray diffractometer (XRD, West Germany's Siemens Company, D500). The sample was mounted on an aluminum sample holder and the scanning angle was varied from 1° to 80° with a scanning rate of 2° per min. All the spectra were taken at ambient temperatures (25±2°C).

3. Results and Discussions 3.1 Compatibility of PVDF and P(VDF-co-HFP)

14

Polymer blend is severely limited by the incompatibility between components. In general, an incompatible polymer mixture with a low-entropy is associated to the lack of specific interactions between polymers [35]. The compatibility between PVDF and P(VDF-co-HFP) is determined from the value of enthalpy ∆Hm given by Schneier Formula shown below [36]:

{

1 2 2

ΔHm = X1M1ρ1 (δ 1 δ 2 ) [X 2 / (1 - X 2 ) M 2ρ 2 + (1 - X1 ) M1ρ1 ] 2

}

(4)

Where, ∆Hm is the enthalpy of mixing (cal/mol), X1 and X2 are mass fractions of the polymers 1 and 2 (%) (X1+ X2 = 1), M is the Mw of the unit structure (g/mol), ρ is the polymer density (g/mol) and δ is the solubility parameter of the polymer (cal/cm3)1/2. ∆Hm<10×10-3 cal/mol indicates that the blend is compatible, while the system is incompatible if ∆Hm!10×10-3 cal/mol.

The compatibility is intrinsically determined by the molecular interaction due to dispersive force, dipolar force and hydrogen bond [37]. The solubility parameters (ˡ) calculated based on the structures of PVDF and P(VDF-co-HFP) shown in Fig. 1 are 7.50 and 7.18 (cal/cm3)1/2, respectively. The ΔHm values of PVDF/P(VDF-co-HFP) blend with different mass ratios were obtained and presented in Fig. 3. As shown, when PVDF/P(VDF-co-HFP) ratio was higher than 50:50 (w/w) or lower than ~4:96 w/w, the ΔHm of PVDF/P(VDF-co-HFP) blend was <10×10-3cal/mol. In other words, PVDF and P(VDF-co-HFP) are compatible in these two range of the mass ratios, whereas incompatible with other blending ratios. The ratios of PVDF/P(VDF-co-HFP) chosen for preparing blend hollow fiber membranes are 90:10, 80:20, 70:30. Shown 15

in Fig. 3 is also the pictures of polymer solutions using NMP as solvent. Apparently, the solutions with the selected ratios all exhibited a transparent state, indicative of homogeneity and compatibility; this observation is consistent with theoretical prediction.

3.2. Polymer Solution Characterizations

The isothermal ternary phase diagram for P(VDF-co-HFP)/NMP/nonsolvent system is plotted in Fig. 4, based on the cloud point experiment. The solvent is NMP and the non-solvent is composed of water and ethanol with a mass ratio of 90:10. The region between the polymer/solvent axis and the binodal line in the phase diagram is the one-phase homogeneous region; and the nonsolvent tolerance of system before phase separation occurrence is generally indicated by the width of this region [24]. The cloud point for PVDF/NMP/nonsolvent was obtained from Yeow et al.’s work [38]. In our experiment, it was found PVDF solution formed gelation easily after mixed with non-solvent and the cloud point was not determined finally. Comparison of the two sets of data shows that the curves of PVDF (Solef 6010)-NMP-non-solvent were the farthest, while that of P(VDF-co-HFP)-NMP-non-solvent the closest to the polymer/solvent axis; This means the non-solvent tolerance in terms of thermodynamic equilibrium for P(VDF-co-HFP) is the lowest; in other words, less amount of nonsolvent was needed to induce the phase inversion of this dope solution. On the other hand, more water is required to generate liquid-liquid demixing for neat 16

PVDF (Solef 6010) solutions. Generally, the stronger affinity between polymer and solvent makes it more difficult for the solvent to move away from the polymer, and consequently greater amount of nonsolvent is needed to disturb the thermodynamic equilibrium and induce phase inversion of the solution [38]. The solubility parameter for NMP is 8.31 (cal/cm3)1/2. Comparison considering this value and the solubility parameters for the two polymers mentioned above implies that P(VDF-co-HFP) has slightly lower affinity towards NMP. Therefore, it requires less water for phase inversion to take place when P(VDF-co-HFP) containing solution is concerned.

Viscosity is another factor influencing solvent and nonsolvent interdiffusion during coagulation and consequently affecting the kinetics of phase inversion in membrane formation [25, 38]. Fig. 5 reveals that the time for the ball falling in polymer solution with higher level of P(VDF-co-HFP) is obviously longer, which suggests higher dope viscosity with improving the content of this co-polymer. This is due to that P(VDF-co-HFP) has significantly higher molecular weight. As widely reported in literature [20], the viscosity of concentrated solutions is a positively related with Mw. The enhanced viscosity will result in higher resistance towards nonsolvent/solvent interdiffusion during membrane evolution [25].

3.3. Membrane Structural Characterizations

Fig. 6 shows the SEM images of the hollow fiber membrane cross-section. As can be 17

seen, the overall structural characteristics of these batches of membranes are similar; the outer and inner annulus near the membrane skin have a layer of finger-like macrovoids. The appearance of the finger-like pores is generally due to the diffusion mechanism with the aid of solutocapillary convection and the local surface instability and skin rupture accompanied by solvent intrusion [39]. Further examination reveals that the length of the cavities near the outer skin gradually decreases as P(VDF-co-HFP) increases in content. Referring to phase diagram shown in Fig. 4 indicates that addition of P(VDF-co-HFP) enhance the thermodynamic instability of solution; nevertheless, the viscosity of the solution also increases. When the effect of the latter outrun the former, delayed demixing has more chance to take place for solution with higher P(VDF-co-HFP) content, which favors formation of spongy like structure over macrovoids. For membrane structures near the inner skin, the macrovoids are much larger than those evolved from the external surface. The outer skin experienced a 15-cm air gap, which allows the solvent removal by evaporation and polymer chain tightening by stretching; the thickening of the solution may delay the solvent intrusion/diffusion and growth of the macrovoids. It was also found that the relationship between macrovoids sizes in the inner lumen annulus and polymer composition is not evident. The influence of inner coagulant intrusion is stronger than that of external coagulant due to the constrained space of the lumen channel with some compression effect. Its balance with dope viscosity and phase stability might generate almost the same macrovoids at the inner side of different fibers [40]. Fig. 7 shows the SEM pictures of outer and inner surfaces of the hollow fiber membranes. 18

The two surfaces for the four batches of membranes are all porous. No apparent difference is identified regarding the outer skin. Concerning inner skin, those of fibers A with only PVDF and the fibers D with the highest P(VDF-co-HFP) content looks like more porous. Additionally, some pores with size approaching hundreds of nm occurs. This is likely due to the partial shrinking of the inner skin, which cause the stretching and breaking at some positions.

The cumulative pore size and probability density function of the hollow fiber membranes’ outer and inner surfaces were measured by the liquid–liquid displacement porosimetry method and plotted in Fig. 8. Generally, all the pores were in the range from 20~60nm, which indicates that the extremely large pores on the inner skin of fibers D shown in Fig. 7 are not representative or exist in minor amounts. Found is also that the curves for pore size of the outer skins are located slightly closer to x-axis origin point, as compared to those of inner skins. The maximum radius of the pore observed on inner surface of hollow fibers A to D is 62.9, 52.0, 42.9 and 54.9, respectively, whereas on outer surface 48.2, 45.9, 33.9, 42.9, respectively.

The curves for measuring LEPw were shown in Fig. 9. Fibers C and D exhibited slightly higher resistance towards pressure and tested up to 0.16MPa. The distillate ion conductivity for testing using fiber A increased obviously faster that when other fibers were concerned. This is probably due to its relatively lower hydrophobicity (Table 3) and simultaneously bigger pore size (Fig. 8). The sharp increase occurred at 19

pressure of 0.14MPa for fiber A. The difference among fibers B, C and D was not obvious.

Table 2 displays the bulk porosity of hollow fiber obtained by weighing method. The mean values for hollow fibers A, B, C and D were 87.6%, 84.7%, 81.2% and 75.3%, respectively. Referring to Fig. 4 regarding phase diagram indicates that addition of P(VDF-co-HFP) reduces the thermodynamic stability of the solution. Faster phase inversion and precipitation or solidification will be resulted, which is characterized of shorter time for porous structure evolution. At the same time, as the surface tension decreases, and more importantly, the viscosity increases for the solution with higher co-polymer, water penetration into the dope is hindered more. Consequently, delayed demixing featured with comparable non-solvent moving front and precipitation front will have more chance to occur. Thus, it is more difficult for large macrovoids to develop. Higher viscosity also reduces the freedom of polymer chains’ movement, which suppresses the pore formation. These were corroborated by the SEM pictures showing smaller macrovoids for hollow fiber D in Fig. 6. Additionally, the relative rate of NMP diffusion out from over water penetration/diffusion into the solution will be enhanced. An important consequence is higher polymer concentration at skin before the occurrence of phase inversion and hence denser surface structure.

Fig. 10 shows the XRD patterns of all the membranes. It is found that they all have similar main diffraction peaks. The two sharp peaks at 2θ of ca. 20o and 38o 20

corresponding for (020) and (202) crystalline peaks of PVDF; in other words, the main crystalline phase in these membranes is principally related with VDF section. This is because the major component in the blend is PVDF. The thermal properties of the four batches of hollow fibers were examined by DSC and the resultant spectra are summarized in Fig. 11. All the thermograms have an endothermic transition peak associated with melting, and the temperature of first melting peak (Tm) were all around 170oC. This value is almost the same as that reported by Figoli et al. for neat Solef6010 membranes [20]. The degree of crystallinity indicated by the area of the peak becomes lower when the copolymer content changes from 0 to 20wt.%, then higher when reaches 30wt.%. This variation pattern may be explained by the interference of P(VDF-co-HFP). Homopolymer PVDF is a polymer more prone to crystallization than the copolymer, which is possibly due to the more obvious structural symmetric feature of the former. With addition of P(VDF-co-HFP), the ordered arrangement leading to crystalline will be disrupted. Nevertheless, further enhancing the copolymer content will strength the effects of delayed demixing attributed to higher surface tension and dope viscosity mentioned above. With this characteristic, the kinetically slower crystallization would have the chance to surpass the liquid-liquid separation dependent on non-solvent content [38, 41] which is controlled by diffusion. As a result, the degree of crystalline increases with 30wt.% of P(VDF-co-HFP) in the polymer matrix. It is also found that the melting temperature shifted slightly first to lower level, then to higher level.

21

3.4 Contact Angle and Mechanical Properties

Table 3 displays the contact angles of the flat membranes with top surface structures being expected to be the same as those of external surfaces of corresponding hollow fiber membranes. The values for hollow fibers A to D were 78.19°, 88.63°, 90.60°, 95.14°, respectively. This is mainly due to P(VDF-co-HFP) with higher hydrophobicity. Incorporation of an amorphous phase of HFP into the main constituent VDF blocks enhances the fluorine content, which makes P(VDF-co-HFP) more hydrophobic than PVDF [12]. The contacts angles of the neat P(VDF-co-HFP) membrane prepared in the same manner as the other flat membranes is 92.21o (SD=4.04). A review of these data would reveal that the contact angles of the blend membranes are higher than the values expected with average mean based on two neat polymeric membranes and blending ratios. One of the possible reasons is that during phase inversion, hydrophobic segments migrated to the membrane external surface. Khayet et al. reported that the hydrophobic surface modification macromolecules can move to the air/polymer solution interface during membrane casting and therefore change its chemical and physical properties [42]. The driving force for the migration of SMMs to the interface is the low surface energy of the fluorinated blocks. The air gap for the hollow fiber spinning in current work is 15cm, which gives chance for the hydrophobic segments to be enriched at dope/coagulant interface in a similar manner.

The tensile strength and elongation at break of the four batches of hollow fibers are 22

demonstrated in Fig. 12. The same properties of neat polymers in dense film state were also characterized for comparison. It was found that Solef 6010 has tensile strength of 13.0MPa (SD1.82) and tensile elongation of 35.7% (SD9.57), while those for P(VDF-co-HFP) 14.1MPa (SD3.2) and 27.5% (SD4.68), respectively. This strength of P(VDF-co-HFP) is lower that reported in Zhu et al.’s work, which is 17MPa [43A]. The discrepancy may be due to the different HFP content and film formation conditions, which are not the focus of this work. Both curves in Fig. 12 regarding hollow fibers took on increasing pattern with higher P(VDF-co-HFP) content. The bursting and compressing pressures of the four batches of hollow fiber membranes are already displayed in Fig. 8 about the pore size characteristics; their variation is roughly in the same order as the tensile strength in Fig. 12. The mechanical properties of a membrane are closely related to its microstructure evolved during membrane formation. One reason for the increase of tensile strength as observed is the correspondingly lower porosity as shown in Table 2 [40]. It is also reported that macrovoid is the weak point detrimental to membrane mechanical strength; whereas, sponge-like structure is stronger as it can lead to more uniform distribution of external stimulus in terms of load or force applied over the structure [14]. As observed in Fig. 6 showing membrane SEM photographs, the size of macrovoids and hence their occupation in overall crosssection diminish as P(VDF-co-HFP) content becomes higher in the membrane matrix. This factor will also likely be in favor of improving the strength of membrane with higher concentration of copolymer. 23

A more detailed examination of the data, however, indicates that the enhancement degree in tensile strength is slight, and not in proportion to the magnitude of change in porosity and macrovoids, which was obvious and significant. The contrast might be balanced by crystallinity change revealed based on DSC spectra aforementioned. PVDF is a semi-crystalline polymer, and its mechanical properties of the samples will depend heavily on the crystallization degree of the fibers [20, 44]. In Sukitpaneenit and Chung’s work, the transformation of microstructure from the spherulitic to cellular state was attributed to a decrease in crystallinity of membranes and led to improvement of the mechanical properties [44]. Concerning tensile elongation, it first increases then decrease with enhance P(VDF-co-HFP) content. Fig. 13 summarizes the mechanical strength VS. porosity for several porous membranes used in DCMD. The table showing the detailed information about the data is given in Appendix A. Although this is just a rough comparison, some observations are obvious. Porous membranes fabricated using more conventionally used PVDF by non-solvent induced phase inversion (NIPS) are located at the lower level in terms of tensile strength and porosity reduction can cause some improvement of this property. One exception is associated with utilization a novel green solvent (triethyl phosphate, TEP) in membrane formation [45]. The reason is that a certain level of TEP in the bore fluid enhanced the strength of coagulant, leading to faster precipitation and solidification of the dope solution. As a result, the bore fluid is hindered more from getting into the dope and the number of macrovoids was reduced. P(VDF-co-HFP) is a material 24

investigated for many years, but with better mechanical property as compared with the majority of PVDF materials. Solef 6012, a relatively new type of PVDF, delivered membranes with tensile strength higher than 3MPa, a much better performance than aforementioned PVDF [46]. The membranes of Solef 6010 and its blending with P(VDF-co-HFP) reported in current work also exhibited better strength than most other PVDF membranes shown in Fig. 14 [45-50]. An extraordinary achievement is due to the application of combine electrospinning (ES) and hot pressing in forming P(VDF-co-HFP) membranes [50]. The tensile strength is up to 12.6MPa for a membrane with porosity as high as 80%.

3.5 Membrane DCMD Performance 3.5.1 Effect of Temperature

Fig. 14 illustrates the DCMD performance of the four batches of the hollow fiber membranes as a function of feed side temperature. The feed solution is 5wt.% NaCl aqueous solution. The inlet temperature of the feed chosen for the investigation include 50, 60, 70 and 80oC and the inlet temperature for the distillate was maintained at approximately 15°C. The feed flow rate is controlled at 54L/h, whereas 20L/h for the distillate. As observe, higher temperature gives rise to higher flux, due to the enhance feed side vapor pressure. The flux for different fibers is in the order of A>B>C>D. Some factors aforementioned account mainly for this trend, which are the surface and bulk porosity. Yet another aspect is the structure property due to 25

crystallinity. Generally, free volume situated inside amorphous region for penetrant transfer will be diminished while tortuosity of membrane will be increased upon polymer crystallization [9].

Fig. 15 made a comparison of DCMD flux found in different researches inclusive current work. As is well-known, flux in DCMD is highly dependent on membrane thickness and porosity. Therefore, 3D plot with porosity and thickness of the membranes as independent variables is given. Table B1 in Appendix B displays the references, membrane structure and formation technology for the data in Fig. 15. Obviously, trade-off between flux and thickness and porosity is present, which follows the transport mechanism in DCMD. As the operation condition may be different, the relationship is not absolutely strict. The flux of P(VDF-co-HFP) membranes by conventional NIPS in several works is low due to the limited porosity [51-52]. Blending it with Solef 6010 has promoted the flux to an intermediate level as mentioned previously by improving the porosity via adjusting the phase inversion mechanism. Further observation of Fig. 15 tells that ES with hot pressing could generate membranes with thinner wall and high porosity, since the technology lifts away the bottleneck or constraint on membrane strength [53]. Consequently, the flux reached higher than 20kg/m2h [54-55].

3.5.2 Effect of Feed Flow Rate

26

Four distinct velocity ranges were selected, while maintaining feed temperature 60°C using 5wt.% NaCl as the feed solution. A moderate feed temperature range of 60°C was used in order to obtain average concentration effect and clearly observe the effect of scaling. Membrane modules were cleaned out and dried for each run. Fig. 16 compares the relative flux and membrane performance at inlet feed flow of 18 L/h, 36 L/h, 54 L/h and 72 L/h in DCMD.

The feed solution was concentrated, and the flux and conductivity were monitored. As illustrated in Fig. 16, the permeate flux decline trend showed a similar pattern of for all the flow velocity setting. It was found that higher distillate fluxes are produced at higher feed flow rates. This result indicates that the effect of the temperature on the permeate flux is more pronounced than the effect of flow rate which is reflected from the high values of flux obtained at the higher temperature and flow rate studied.

4. Conclusions

In this study, laboratory prepared PVDF/P(VDF-co-HFP) blend hollow fiber membranes were developed for DCMD. The two polymers were compatible in most range of blending ratio and 10, 20 and 30wt.% for P(VDF-co-HFP) in the polymer matrix were selected. The hydrophobicity of PVDF/P(VDF-co-HFP) membranes was higher due to the more content of fluorine in the copolymer, which was confirmed by the increase of contact angle. Additionally, there might exist the preferential migration 27

of hydrophobic segment to the dope/coagulant surface during phase inversion. All the fibers exhibited mechanical strength better than membranes made from several other types of PVDF membranes. This was attributed to the intrinsic properties of Solef 6010 and P(VDF-co-HFP). With P(VDF-co-HFP) content increasing, membrane porosity became lower. The higher viscosity and lower surface tension due to addition of the copolymer were thought to cause delayed mixing favoring more compact structure on membrane surface and fewer macrovoids in the bulk. These structural features were closed related with the following finding regarding DCMD performance.

Acknowledgement

The authors would like to thank National Natural Science Foundation of China (Project no. 21176265), Hunan Provincial Science and Technology Plan (Project no. 2014GK3106) and Chinese National Project for Overseas Experts in Culture, Education and Public Health.

Appendix A

Table A1. Mechanical strength of membranes using PVDF-based materials. 28

Research

Polymer/Formation technology*

Tensile

Tensile

strength

elongation

(MPa)

(%)

0.89

1.28

185

/

2.08

/

0.89

0.93

143

0.89

1.44

77

0.81

3.43

252

0.68

1.62

120

/

1.03

174

Porosity

K. Y. Wang et al., Mixed Matrix PVDF Hollow Fiber Membranes with Nanoscale Pores for Desalination through Direct

PVDF (Kureha

Contact Membrane Distillation. Industrial

1300)/TIPS

& Engineering Chemistry Research, 48 (2009) 4474-4483. Y. Su et al., Preparation of PVDF Membranes via TIPS Method: The Effect of Mixed Diluents on Membrane Structure and Mechanical Property. Journal of

PVDF/TIPS

Macromolecular Science: Part A Pure and Applied Chemistry 440 (2007) 305-313. M. M. Teoh et al., Development of Novel Multichannel Rectangular Membranes with Grooved Outer Selective Surface for

PVDF (Kureha

Membrane Distillation. Industrial &

1300)/NIPS

Engineering Chemistry Research, 50 (2011) 14046-14054 K. J. Lu et al., Novel PVDF membranes comprising n-Butylamine functionalized graphene oxide for direct contact membrane distillation. Journal of

PVDF (Kynar®HSV 900)/NIPS

Membrane Science 539 (2017) 34-42. E. Drioli et al., Novel PVDF hollow fiber membranes for vacuum and direct contact membrane distillation applications. Separation and Purification Technology

PVDF

(Solef

6012)/NIPS

115 (2013) 27-38. Y. D. Tang et al., Effect of spinning conditions on the structure and performance of hydrophobic PVDF hollow fiber membranes for membrane

PVDF (FR-904)/NIPS

distillation. Desalination 287 (2012) 326-339. N. A. Hashim et al., Stability of PVDF hollow fibre membranes in sodium hydroxide aqueous solution.

PVDF (Kynar 761)/NIPS

29

Chemical Engineering Science 66 (2011) 1565-1575. Chang et al., Using green solvent, triethyl phosphate (TEP), to fabricate highly porous PVDF hollow fiber membranes for

0.79

2.7

153

0.89

0.76

106

0.8

12.6

25

0.53

5.4

267

PVDF (Kynar®HSV 900)/NIPS

membrane distillation. Journal of Membrane Science 539 (2017) 295-304. B. S. Lalia et al., Nanocrystalline cellulose reinforced PVDF-HFP membranes for

P(VDF-co-HFP)

membrane distillation application.

/ES with hot press

Desalination 332 (2014) 134–141. L. Shi et al., Fabrication of poly(vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP) asymmetric microporous hollow fiber membranes.

P(VDF-co-HFP)/NIPS

Journal of

Membrane Science 305 (2007) 215–225.

*ES: Electrospinning; NIPS: Non-solvent induced phase inversion; TIPS: thermally induced phase inversion

Appendix B

Table B1. DCMD Flux of the P(VDF-co-HFP) membranes reported in other researches. Structure/F Thickness/bulk

Feed

Flux

porosity/mean

Tempera

(kg/h·

surface pore size

ture(℃)

m)

ormation Research technology *

2

30

S. Fadhil et al., Novel PVDF-HFP flat sheet membranes prepared by triethyl phosphate (TEP) Single

46-65μm/0.78-0.8

layer/NIPS

3/0.06-0.08μm

solvent for direct contact

60

13-16

70

4.0

membrane distillation. Chemical Engineering and Processing 102 (2016) 16–26 S. A. Rahman et al., Improvement of PVDF-co-HFP Hollow Fiber Membranes for

Single

Direct Contact Membrane

layer/NIPS

/ Distillation Applications. Indian Journal of Science and Technology M. C. Garcia Payo et al., Effects of PVDF-HFP concentration on membrane distillation performance and

Single

55-94μm/0.76-0.7

1.4-0.1 45

structural morphology of

layer/NIPS

0/44-114nm

1

hollow fiber membranes. Journal of Membrane Science 347 (2010) 209– 219 B. S. Lalia et al., Fabrication and characterization of polyvinylidenefluoride-co Single -hexafluoropropylene layer/ES

110μm/0.58/0.26

with hot

μm

(PVDF-HFP) electrospun

68

22

59

9

membranes for direct pressing contact membrane distillation. Journal of Membrane Science 428 (2013) 104–115 B. S. Lalia et al.,

Single

Nanocrystalline cellulose

layer/ES

280μm/0.80/0.38

reinforced PVDF-HFP

with hot

μm

membranes for

pressing

31

membrane distillation application. Desalination 332 (2014) 134–141 M. Khayet et al., Experimental design and optimization of asymmetric flat-sheet

Single

membranes prepared for

layer/Phas

210μm/0.71/76n 65

3.9

63

20

m direct contact membrane

e inversion

distillation. Journal of Membrane Science 351 (2010) 234–245 L.D. Tijing et al., A novel dual-layer bicomponent Dual-layer electrospun nanofibrous (hydrophili membrane for c PAN

80μm/0.9/0.6-2.5

support)/E

μm

desalination by direct contact membrane S with hot distillation. Chemical pressing Engineering Journal 256 (2014) 155–159

Singl e layer B. S. Lalia et al., A facile approach to fabricate superhydrophobic membranes with

125μm/0.6/0.2μ

6

m

7

e

63-145μm/0.74-0

4

0.72

layer

.81/73-88nm

5

-1.4

13

/ES low contact angle hysteresis. Journal of Membrane Science 539 (2017) 144–151 with coati ng Singl M. C. García-Payo et al., Water desalination by membrane distillation using PVDF-HFP hollow fiber membranes. Membrane Water Treatment 3 (2010)215-230

/NIPS Singl e layer F. E. Ahmed et al., Membrane-based detection of wetting phenomenon in direct

/ES

contact membrane distillation. Journal of Membrane Science 535 (2017) 89–93

with

6 400μm/ - /0.4μm

12 5

hot press ing

32

Singl L. G. Fernández et al., Mechanism of formation of hollow fiber membranes for e

96μm/0.78/0.7μ

8

layer

m

0

13

membrane distillation: 1. Inner coagulation power effect on morphological characteristics. Journal of Membrane Science 542 (2017) 456–468 /NIPS Singl L. G. Fernández et al., Hollow fiber membranes with different external corrugated e surfaces for desalination by membrane distillation. Applied Surface Science 416

8 /

layer

16 0

(2017) 932–946 /NIPS Singl L. G. Fernández et al., Effects of mixed solvents on the structural morphology and e

113-180μm/0.65-

8

4.8-

layer

0.71/65-69nm

0

6.3

membrane distillation performance of PVDF-HFP hollow fiber membranes. Journal of Membrane Science 468 (2014) 324–338 /NIPS

* ES: Electrospinning; NIPS: Non-solvent induced phase inversion; TIPS: thermally induced phase inversion

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40

Table 1. Parameters and conditions of the hollow fiber spinning. Parameters

Conditions Polymer (P(VDF-co-HFP)/PVDF) (15%)

Dope composition

A

0/10

(wt%)

B

1/9

C

2/8

D

3/7

Glycerol

NMP

10%

75%

External Ethanol/Water = 10/90 coagulant (wt%) Bore fluid (wt%)

Ethanol/Water = 10/90

Air gaps (cm)

15

Take up speed 7 (m/min) Dope fluid rate 4 (ml/min) Bore fluid flow 2 rate (ml/min) 41

External coagulant

25 o

temperature ( C)

Table 2. The porosity of the hollow fiber membranes measured by weighing method. ID

A

B

C

D

Mean Value

87.32

83.28

81.87

75.28

Standard Deviation

0.326

0.795

0.277

0.710

42

Table 3. Contact angles of the flat membranes with different PVDF/P(VDF-co-HFP) blending ratio. Membrane

A

B

C

D

79.05

87.57

89.64

96.37

0.69

1.92

0.77

3.60

ID Contact angle image Average o

value ( ) Standard deviation o

()

43

Highlights 1. Blend of PVDF and PVDF-co-HFP were employed to fabricate hollow fiber membranes for DCMD. 2. The contact angle increases with higher P(VDF-co-HFP) content; LEPw for fibers with only PVDF is the lowest. 3. The bulk porosities of the membranes were all higher than 75%; the mechanical strength was 2.86MPa for fibers with 30% copolymer. 4. Analysis based on phase diagram, viscosity, XRD and DSC were given for the physicochemical properties variation.

44

x

F 2C

P(VDF-co-HFP) (HFP 25wt.%)

PVDF-HFP

CF2

n

F

C

CF3

y

Fig. 1. Unit structures of the two polymers for preparing the hollow fiber membranes.

H2C

CF2

PVDF

H2C

1

PP 3

P

5 P

T

6 T

4

P

PP

7 PP

T

PP

P

10

T

9

PP

Fig. 2. Set-up for the DCMD experiment. 

1. Feed tank; 2. Water bath (heater); 3. Peristaltic pumps; 4. Hollow fiber module; 5. Pressure gauges; 6. Thermometers; 7. Condensate tank; 8. Conductivity meter; 9. Distillate tank; 10. Balance; 11. Conduit

2

1

11

8 PP

2

2.00E-03

4.00E-03

6.00E-03

8.00E-03

1.00E-02

1.20E-02

0.1

0.2

0.4

0.5

0.6

0.7

Mass ratio of PVDF in the blend

0.3

Miscible

Immiscible

0.8

0.9

1 PVDF

Fig. 3. The mixing enthalpy of PVDF/PVDF-HFP blend system.

0.00E+00 P(VDF-co-HFP) 0

Enthalpy of mixing (cal/mol)

3

Fig. 4. Ternary phase diagram of P(VDF-co-HFP) system.

Non-solvent

4

0

2

4

6

8

10

A

4.04

B

4.96

C

6.64

D

7.48

Fig. 5. Viscosity of hollow fiber formation dopes indicated by small ball falling time.

Falling Time (s)

5

C

A

50μm

50μm

50μm

50μm

D

500μm

500μm

50μm

50μm

Fig. 6. SEM graphs of overall cross-sectional view, partial cross-sectional view and sponge-like structure of PVDF hollow fiber membrane.

500μm

500μm

B

6

50μm

50μm

1µm

1µm

1µm

1µm

1µm

1µm

Fig. 7. SEM photographs of hollow fiber membranes’ outer (upper) and inner (lower) surfaces. 7

1µm

1µm

Pore fraction with size less than stated value

0

40

80

0.25

0.10 0.05

0.00

0.10

0.05

0.00

60

0.15

0.15

20

0.20

0.142MPa

0.20

0.25

0

40

80

0.25

0

40

80

0.25

20

60

0

outer skin

0.00

0.05

0.05

60

0.10

0.10

20

0.15

0.15

0.00

0.20

0.195MPa

0.20

Pore size (nm)

0.145MPa

0.00

0.00

0.00

0.00

0.175MPa

0.20

0.20

0.20

0.20

0.172MPa

0.40

0.40

0.40

0.40

0.111MPa

0.60

0.60

0.60

0.60

20

(D)

0.80

1.00 0.80

1.00 0.80

1.00

1.00

1.20

0.80

1.20

(C)

(A)

1.20

(B)

1.20

60

inner skin

40

0.181MPa

0.171MPa

80

Fig. 8. Cumulative pore size (upper) and probability density function (lower) curves for the hollow fiber membranes (the pressure data in the figure indicate the hollow fiber collapsing and bursting pressures during the tests). 8

Probability density function

0

2

4

6

8

10

0.05

0.15

Pressure (MPa)

0.1

0.2

0

2

4

6

8

10

0

0.05

0.15

Pressure (MPa)

0.1

D

C

Fig. 9. Variation of ion conductivity of distillation side as a function of feed side hydraulic pressure.

0

B

A

Ion conductivity (µS/cm)

Ion conductivity (µS/cm)

0.2

9

Fig. 10. XRD of PVDF/PVDF-HFP hollow fiber membranes.

2θ (o)

A

B

C

D

10

Fig. 11. DSC curves (heating cycle) for the hollow fibers membranes.

11

2 0

50

0

30

4

100

10 20 P(VDF-co-HFP) content (wt.%)

6

150

0

8

200

10

Fig. 12. Tensile strength and elongation at break of hollow fiber membranes with different PVDF/P(VDF-co-HFP) blending ratio.

Tensile elongation (%)

250 Tensile strength (MPa)

12

0

5

10

15

0

0.2

ES: electrospinning NIPS: non-solvent induced phase inversion

0.4

Current work

0.6

PVDF/NIPS

Porosity

P(VDF-co-HFP)/NIPS

0.8

PVDF using TEP solvent/NIPS

Solef 6012/NIPS

P(VDF-co-HFP)/ES with hot pressing

1

Fig. 13. Summary of tensile strength vs. porosity for membranes with different PVDF-based materials.

Tensile strength (MPa)

13

Flux (kg/m2-hr)

Flux (kg/m2-hr)

50

SD0.46

50

70

70

Temperature (oC)

60

SD0.32

SD0.30

80

80

SD0.15

Temperature (oC)

60

(C)

SD0.38

90

90

50

60

70

80

90

100

110

50

60

70

80

90

100

Rejection (%) 0

5

10

15

20

25

0

5

10

15

20

25

40

40

50

SD0.23

50

60

SD0.67

70

70

SD0.53

Temperature (oC)

60

SD0.54

80

80

(D)

SD1.13

SD0.17

Temperature (oC)

SD0.69

SD0.86

(B)

90

90

50

60

70

80

90

100

110

50

60

70

80

90

100

110

Fig. 14. Effect of feed temperature on hollow fiber membrane DCMD Flux when treating 5wt.% NaCl aqueous solution.

40

40

SD1.01

SD1.18

SD0.52

(A) Rejection (%)

0

5

10

15

20

25

0

5

10

15

20

110

Flux (kg/m2-hr) Flux (kg/m2-hr)

25

Rejection (%) Rejection (%) 14

Flux (kg/m -h)

T hi

0

5

10

15

20

ckn

ess

100

(um

)

300

0.50

(A) Feed temperature<=60oC

200

ES with hot pressing

ES with hot pressing

y sit ro o P

0.75

1.00

Current work

T hi

0

5

10

15

20

25

ckn

ess

100

(um

)

200 300

80oC

0.50

80oC

r Po

os

0.75

ity

80oC; blend of Solef 6010/P(VDF-co-HFP)

1.00

80oC; Solef 6010

(B) Feed temperature>60oC

80oC

15

Fig. 15. Summary of flux vs. porosity and thickness for DCMD membrane made of in previous researches (P(VDF-co-HFP)) and current work (PVDF and its blending with P(VDF-co-HFP)) .

2

25

2 Flux (kg/m -h)

Flux (kg/m2-hr)

60

80

Flux (kg/m2-hr) Flux (kg/m2-hr)

Rejection (%) Rejection (%)

Feed flow rate (L/h)

50 40

0

20

70

5

0

90

110

10

15

Feed flow rate (L/h)

80

50 60

0 40

70

5

20

90

10

0

110

15

60

80

40

60

80

Feed flow rate (L/h)

50 20

0 0

70

90

110

5

10

15

Feed flow rate (L/h)

50 40

0 20

70

5

0

90

110

10

15

Rejection (%)

Fig. 16. Effect of feed flow on hollow fiber membrane DCMD flux in treating 5wt.% NaCl aqueous solution.

Flux (kg/m2-hr)

Rejection (%) 16