Effect of additives on the fabrication of poly(vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP) asymmetric microporous hollow fiber membranes

Journal of Membrane Science 315 (2008) 195–204

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Effect of additives on the fabrication of poly (vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP) asymmetric microporous hollow fiber membranes Lei Shi a,b,c , Rong Wang b,∗ , Yiming Cao a , David Tee Liang b , Joo Hwa Tay b a

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Institute of Environmental Science & Engineering, Nanyang Technological University, Innovation Center (NTU), Block 2, Unit 237, 18 Nanyang Drive, 637723 Singapore c Graduate School of Chinese Academy of Sciences, Beijing 100049, China b

a r t i c l e

i n f o

Article history: Received 3 January 2008 Received in revised form 14 February 2008 Accepted 15 February 2008 Available online 10 March 2008 Keywords: PVDF-HFP Microporous hollow fiber membrane LiCl and glycerol as additives Permeation Hydrophobicity

a b s t r a c t The effects of two typical additives, lithium chloride (LiCl) and glycerol, on the fabrication of poly(vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP) asymmetric microporous hollow fiber membranes were investigated in terms of membrane morphology, structure, permeation performance, hydrophobicity and mechanical property. The addition of the additives into the dope solution altered the morphology and structure of the resultant membranes, which was believed to be associated with the change of the thermodynamic and kinetic properties of the system in the phase inversion process. The membranes with improved pure water permeability (PWP) from 6.908 × 10−5 (7) to 4.835 × 10−4 (49), 3.256 × 10−4 (33) and 5.003 × 10−4 L/h m2 Pa (51 L/h m2 atm) were obtained when 2 wt.% LiCl, 4 wt.% LiCl and 10 wt.% glycerol was used as the additive, respectively. Among these, the membrane made from the dope containing 4 wt.% LiCl possessed the highest retention capability of 40 kDa molecular weight cut-off (MWCO). The LiCl or glycerol addition into the polymer dope also made the membranes exhibit a narrow pore size distribution. Moreover, the membrane hydrophobicity was affected less by LiCl and glycerol than by poly(vinyl pyrrolidone) (PVP). The addition of 4 wt.% LiCl into the dope could form the membrane with four times stronger strain strength than the poly(vinylidene fluoride) (PVDF) commercial membrane while keeping similar rigidity. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Poly(vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP) is a copolymer which attracted attention as a potential membrane material recently. Compared with poly(vinylidene fluoride) (PVDF) homopolymer, PVDF-HFP possesses lower crystallinity and higher free volume due to the incorporation of an amorphous phase of hexafluropropylene (HFP) into the main constituent vinylidene fluoride (VDF) blocks. Addition of HFP group also increases the fluorine content and makes PVDF-HFP more hydrophobic than PVDF. Thus, for some applications where the hydrophobicity of membrane material is crucial, PVDF-HFP is a potential candidate. In recent years, there is an increasing intention to use PVDF-HFP material to prepare membranes for membrane distillation [1], pervaporation [2] or as membrane contactors.

∗ Corresponding author. Tel.: +65 6794 3764; fax: +65 6792 1291. E-mail address: [email protected] (R. Wang). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.02.035

Most of works on PVDF-HFP membrane preparation reported in the literature were about flat sheet membranes [3–5]. PVDF-HFP asymmetric microporous hollow fiber membranes were successfully fabricated using non-solvent induced phase inversion method by our group only lately [6]. It was found that the pure water flux of the hollow fiber membranes made by PVDF-HFP/N-Methyl2-pyrrolidone (NMP) dope solutions without an additive was quite small, even though a low polymer concentration (15 wt.%) was used. The addition of poly(vinyl pyrrolidone) (PVP) into the dope solution can promote the formation of macrovoids in the membrane and the pure water flux was thus increased. It seems that it is necessary to adopt an additive as a pore former to improve the permeation performance of the PVDF-HFP microporous hollow fiber membrane via adjusting the membrane structure. Additives that have been used in the fabrication of PVDF hollow fiber membranes by far can be categorized into (a) polymeric additives such as PVP [7–9] and poly(ethylene glycol) (PEG) [10,11]; (b) low-molecular-weight chemicals, including salts such as lithium chloride (LiCl) [12–14] and lithium perchlorate (LiClO4 ) [14,15],

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inorganic acids (acetic acid, phosphoric acid) [16] and organic acids (propionic acid) [17]; (c) weak co-solvents such as ethanol and acetone [14,16]; (d) weak non-solvents such as glycerol [1,18], ethylene glycol [10] and (e) strong non-solvent, water, as an additive in a tiny amount [18]. Nevertheless, the roles of different additives vary in different polymer/solvent/non-solvent systems. The addition of PVP was found to favor macrovoid formation in the fabrication of PVDF membranes [7]. The occurrence of macrovoids was associated with the instantaneous demixing mechanism as PVP addition increased the dope’s thermodynamic instability in reaction with the non-solvent water. On the other hand, the viscosity of the dope solution increased with the amount of the additive, which hindered the diffusions among the components in the phase inversion process (kinetic effect). Consequently, the presence of a relatively large amount of PVP also resulted in macropores’ suppression [19]. Fontananova et al. and Curcio et al. reported that PVP was an effective permeation flux promoter, and the thermodynamic effect played a dominant role during the flat-sheet membrane formations of both PVDF homopolymer and PVDF-HFP copolymer [20,21]. In contrast with PVP, small molecular additives can easily diffuse out during the membrane formation and late washing process. Highly permeable PVDF hollow fibers were able to be prepared. It was observed that the presence of LiCl in the dope solution tended to enhance the coagulation rate and produce membranes with good interconnectivity and porosity, but the enhanced membrane permeation performance was gained at a price of reduction in its mechanical strength [12,13]. Fontananova et al. [20] and Curcio et al. [21] found that the addition of LiCl in the PVDF/DMAc dope worked as permeate flux enhancer at a low concentration of 2.5 wt.% (thermodynamic effect), but it suppressed macrovoid formation at a high concentration of 7.5% and resulted in a decrease of permeation flux. For the PVDF-HFP/DMAc system, LiCl in the entire concentration range (2.5–7.5 wt.%) functioned as a permeation flux enhancer. For an additive with a weak non-solvent nature such as glycerol, in general, its presence in the dope solution brought the initial composition of the casting solution closer to the binodal [22], which increased the tendency of the solution to form macrovoids. According to Yeow et al. [18], the presence of glycerol suppressed gelation induced by crystallization of PVDF and favored the liquid–liquid demixing during the phase inversion, leading to the membrane with regular finger-like macrovoid structure. Feng et al. [1] reported that extended cavities and macrovoids were observed in PVDF-HFP flat-sheet membranes when glycerol was adopted as an additive in the casting solution. In our previous work, PVDF-HFP asymmetric microporous hollow fiber membranes were made from the PVDF-HFP/NMP/PVP dope solution as a first trial [6]. As a continuation of our former effort, the present study focused on the effects of different additives on the formation of PVDF-HFP asymmetric hollow fiber membranes. Other two typical additives, LiCl and glycerol, which used to be applied in the PVDF hollow fiber preparation, were adopted to make PVDF-HFP hollow fibers. With the incorporation of an amorphous phase of HFP into the main constituent VDF blocks, the effect of the same additive on the formation of PVDF-HFP membrane presents variations. Their impacts on the resultant membranes were also compared with the effect of PVP, which was studied in our previous work [6], in terms of membrane structure, morphology and performance. The mechanical strength and hydrophobicity of PVDF-HFP membrane prepared with the two additives were determined and compared with that of a commercial PVDF membrane. It is believed that this work could provide a better understanding of the roles of different additives on the formation of PVDF-HFP asymmetric microporous hollow fiber membranes.

2. Experimental 2.1. Membrane material and chemicals The membrane material, the commercial polymer poly (vinylidene fluoride-co-hexafluoropropylene) (Kynar® powder flex 2801, 12 wt.% HFP) was purchased from ARKEMA, Singapore. N-Methyl-2-pyrrolidone (NMP, >99.5%, CAS#872-50-4, Panreac, Spain) was used as a solvent. Glycerol (99.0%, Cica-Regent, Japan) and lithium chloride (LiCl, 99%, CAS#7447-41-8, UNILAB, Australia) were adopted as additives into the dope solution. Some dextran (C6 H10 O5 )n samples with different molecular weights (molecular weight from 1500 to 400,000 Da, CAS# 9004-54-0, Sigma) were used to characterize the molecular weight cut-off (MWCO) of hollow fiber membranes. All the reagents were used as received. 2.2. Spinning dope preparation, cloud point and dope rheology measurements To prepare dope solutions for spinning, a desired amount of pre-dried PVDF-HFP polymer powder with/without non-solvent additive was dissolved into NMP using jacket flasks. The dope solution was then subjected to continuous stirring for 3 days. Prior to spinning, the dope prepared was degassed under vacuum at ambient temperature overnight. Titrimetric method was employed to determine the cloud point of the PVDF-HFP/NMP/H2 O ternary system at the temperature of around 25 ◦ C. A series of PVDF-HFP/NMP solutions with different polymer concentrations (5, 10, 13, 15, 17 and 20 wt.%) were selected and stored in jacket flasks. Milli-Q ultrapure water as a non-solvent was added into the solution by a buret. After each small quantity of addition, the solution was stirred vigorously to make it homogeneous and transparent again. The process was repeated and the cloud point was recognized when visual turbidity permanently occurred in the solution. The corresponding amounts of polymer, NMP and H2 O were recorded. A RheoStress 300 rheometer (HAAKE Instruments Inc.) was used to determine rheological characteristics of the spinning dopes. The experiments were carried out using a 25 mm cone-plate at 25 ◦ C and the steady-state shear rate was altered in the range from 10 to 300 s−1 . 2.3. Spinning of PVDF-HFP hollow fiber membranes and post-treatment Asymmetric PVDF-HFP hollow fiber membranes were fabricated by dry–jet wet spinning processes. Basically, the dope was pressurized through a spinneret by N2 at a controlled rate, and went through a certain air gap before immersing into a coagulation bath. External coagulant used was tap water, while the mixtures of MilliQ Water and NMP with different ratios were used as the bore fluid. The nascent hollow fiber was taken up by a roller at a free falling velocity and stored in a water bath to remove residual solvent for at least 2 days. The detailed hollow fiber spinning conditions are listed in Table 1. A post-treatment was performed to alleviate the membrane shrinkage during drying process at ambient condition. The membrane was immersed in turn into water/1-propanol (1:1), 1-propanol, 1-propanol/n-hexane (1:1) respectively. Each stage of the solvent exchange lasted 24 h. This process allowed the reagents with a lower surface tension to replace gradually the water in the membrane pores. The membranes were subsequently dried at room temperature before characterization tests.

L. Shi et al. / Journal of Membrane Science 315 (2008) 195–204 Table 1 Spinning parameters for the PVDF-HFP hollow fibers Spinneret dimension Dope solution flow rate (g/min) Coagulation composition Coagulation temperature (◦ C) Bore fluid composition (NMP/water) (w/w%) Bore flow rate (mL/min) Air gap (cm) Take-up speed Ambient temperature (◦ C) Room humidity (%)

2.6. Membrane morphology observation, dynamic contact angle and mechanical property tests o.d. = 1.7 mm, i.d. = 0.7 mm 5.5 Water 25 ± 1 0/100, 50/50, 80/20 2.5 0.5, 20 Free falling velocity 25 ± 1 80 ± 3

2.4. Measurements of hollow fibers’ pure water flux and MWCO 15 pieces of fibers were sealed into a glass tube to make the labscale modules with 25 cm effective length. Milli-Q ultrapure water was circulated through the shell side of the membrane modules under 9.869 × 10−6 L/h m2 Pa (1 atm) pressure. The specific flux of the membranes was calculated using the formula: J=

Q Q = P × A nDl P

(1)

where J is the specific flux of the hollow fiber membrane (L/m2 h Pa); Q the water flux reading (L/h); P the pressure difference between the feed side and the permeation side of the membrane (Pa); A the effective membrane surface area (m2 ); n the number of fibers in the module; D the outer diameter of hollow fiber (m) and l represents the effective length of hollow fibers (m). The MWCO of asymmetric hollow fiber membranes is defined as the molecular weight at 90% rejection. It was tested using a 1500 ppm dextran aqueous solution with a molecular weight distribution from 1500 to 400,000 Da. The detailed procedure was described in reference [23]. In short, a Waters gel permeation chromatography (GPC) system (the ultrahydrogel columns 120, 250, 500) was used to measure the dextran molecular weight distribution in the feed and permeate solutions. The rejection of hollow fiber membranes was calculated according to R(a) =

197

Cf − Cp Cf

(2)

where Cf and Cp are the concentrations of dextran molecule with diameter a in the feed and permeate solutions, which can be represented by the GPC spectrum intensity of dextran molecules. 2.5. Pore size and pore flux distributions of hollow fiber membranes

A JEOL JSM-5310LV Scanning Electron Microscope (SEM) was used to observe the cross-section and inner surface morphologies of asymmetric hollow fiber membranes. Prior to the test, the dried membrane samples were broken in liquid nitrogen and then sputtered with a thin layer of gold using SPI-Module sputter coater. A tensiometer (DCAT11 Dataphysics, Germany) was employed to determine the hydrophobic properties of the fibers. A sample fiber was held on the arm of an electro balance, and then immersed 5 mm long into Milli-Q water and successively emerged out at an interfacial moving rate of 0.2 mm/min to complete a cycle. In the loop, the weight difference was continuously recorded by the electrobalance and contact angle was calculated based on the Wihelmy method. Three immersion-emersion cycles were carried out for each specimen, and each run was repeated three times for all the hollow fiber samples. Tensile strength test of the membranes was carried out using an Instron 5542 tensile test machine at room temperature. The sample was clamped at the both ends and pulled in tension at constant elongation velocity of 50 mm/min with an initial gauge length of 25 mm. Tensile stress at break, tensile strain and Young’s modulus were obtained. 3. Results and discussion 3.1. Effect of the additives on the isothermal phase diagram The phase diagram of PVDF-HFP/NMP/water system with/ without additives at 25 ◦ C is depicted in Fig. 1 based on the cloudpoint experiment. The linearized cloud-point (LCP) correlation for the system without an additive was also drawn in the figure. Similarly to the cases of PVP as an additive, the addition of LiCl or glycerol into the dope solution brought the envelope of the one-phase homogeneous region (the binodal line) closer to the polymer–solvent axis. It indicated that less amount of water (nonsolvent) was needed to interfere the system equilibrium and induce the copolymer precipitation if the dope contained the additive. The binodal line shifted further towards the polymer–solvent axis with an increase of the additive amount. In comparison with the polymeric additive, PVP, It was found that glycerol reduced significantly the overall non-solvent tolerance of the polymer solution, as the introduction of glycerol made the phase boundary move closest to the polymer–solvent axis. The dope solution turned turbid when it contained 15 wt.% glycerol. However, LiCl presented a similar extent of impact on reducing the

The most common form of the two-parameter log-normal distribution function, f(d), was taken to describe the pore size distribution of the membrane:



1 exp − f (d) = √ 2 ln()d 2 1



ln(d/D∗ ) ln()

2 

(3)

the pore flux distribution function ff (d) was given as follows: ff (d) =

 d 4 D∗



1 exp − √ 2 ln()d 2 1



ln(d/D∗ ) ln()



2 − 8(ln )

2

(4)

where D* ,  are the geometric mean diameter and the geometric standard deviation, respectively. Based on the pore size distribution parameters, the retention coefficient of a solute R(a) can be derived theoretically. D* and  were then obtained by minimizing the differences between the theoretical and experimental R(a) values. The detailed information could be found in reference [24].

Fig. 1. Phase diagram of PVDF-HFP/NMP/H2 O system at 25 ◦ C (() without additive; (—) LCP correlation; () 2 wt.% LiCl; () 4 wt.% LiCl; () 5 wt.% glycerol; (䊉) 10 wt.% glycerol; () 5 wt.% PVP [6]; () 10 wt.% PVP [6]).

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(1720 × 10−3 Pa s (1720 cP)) of the PDVF-HFP/NMP dope solution without any additive. The same phenomenon was found by Wang et al. [14] for the PVDF/LiCl/DMAc system. It was attributed to the formation of acid-base complexes between LiCl and DMAc, as well as the interaction between Li+ cation and electric donor of the polymer molecule. According to Kong and Li [17], a much higher viscosity increase was observed when NMP was used as the solvent with the same amount of LiCl added, implying that LiCl exhibited a stronger interaction with NMP than with DMAc. The introduction of glycerol into the dope solution also increased the viscosity of the dope solution, but the extent of increase was similar to the dope containing the same amount of PVP [6]. An increase in viscosity was also reported previously for the PVDF-HFP/NMP/glycerol system [1], probably due to the formation of bridge-complexes among glycerol, polymer macromolecules and NMP, which deteriorated the polymer chains flexibility or caused a decrease in distributive freedom of the polymer in the dope solution.

Table 2 Compositions of the spinning dopes used

PVDF-HFP (wt.%) NMP (wt.%) Additive (wt.%) a

Plain

LiCl

F15a

2La

4La

5Ga

Glycerol 10Ga

15 85 –

15 83 2

15 81 4

15 80 5

15 75 10

Code.

non-solvent tolerance of the system. The strong effect of glycerol on the thermodynamic stability of the PVDF-HFP/NMP/water system might be attributed to its weak non-solvent feature and good affinity with NMP, which reduced considerably the polymer solubility in NMP [25]. 3.2. Effect of the additives on the viscosity of the dope solutions According to our previous work [6], a dope solution containing 15 wt.% PVDF-HFP polymer was suitable to prepare hollow fibers, and the addition of PVP increased the viscosity of the dope solution. In the present work, the same polymer concentration of 15 wt.% was adopted with LiCl and glycerol as the additives. Table 2 lists the compositions of various spinning dopes used and their corresponding codes. Viscosity of the polymer dope is a key parameter to affect the kinetics of the phase inversion, which affects the structure and performance of resultant membranes. Table 3 gives the viscosity of the polymer dopes measured at 25 ◦ C. It can be seen that although a small amount of LiCl (2–4 wt.%) was added into the dope solution, there was a significant increase in viscosity (3500 × 10−3 (3500 cP) and 8600 × 10−3 Pa s (8600 cP) corresponding to 2 and 4 wt.% LiCl added, respectively) compared to the value

3.3. Effect of the additives on the membrane morphology Fig. 2 shows the morphology of the hollow fiber membranes prepared from the spinning dope without an additive, and the morphology of the hollow fibers made from the dope containing LiCl as the additive is presented in Fig. 3. When pure water was used as the internal coagulant, the presence of LiCl favored the formation of thinner fiber skins. Larger and longer macrovoids under the outer skin can be observed in the fibers spun with 2 and 4 wt.% LiCl additives (Fig. 3, 2L-a and 4L-a) in comparison with that without an additive (Fig. 2, F15-a). The effect of LiCl addition on the membrane morphology could be seen more clearly when the NMP concentration in the bore fluid reached 50 wt.%. A sponge-like structure at the inner side of the fiber spun without an additive (Fig. 2, F15-b) was

Table 3 Viscosity of the dope solutions at shear rate 20 s−1 Dope solution F15  (Pa s (cP)) a

2L −3

1720 × 10

(1720)

4L −3

3500 × 10

(3500)

5G −3

8600 × 10

(8600)

5Pa

10G −3

2100 × 10

(2100)

−3

4340 × 10

(4340)

10Pa −3

2400 × 10

(2400)

3100 × 10−3 (3100)

5P and 10P stand for 5 and 10 wt.% PVP addition into a 15 wt.% PVDF-HFP/NMP dope solution, respectively [6].

Fig. 2. Cross-section morphology of the hollow fibers spun from the PVDF-HFP/NMP dopes without an additive (bore fluid: (a) water; (b) 50/50 NMP/water; (c) 80/20 NMP/water).

L. Shi et al. / Journal of Membrane Science 315 (2008) 195–204

199

Fig. 3. Cross-section morphology of hollow fibers spun with LiCl as additive (bore fluid: (a) water; (b) 50/50 NMP/water; (c) 80/20 NMP/water).

altered to a finger-like structure with the addition of LiCl in the dope solution (Fig. 3, 2L-b and 4L-b). However, the macrovoids seemed to be suppressed in a certain extent by adding 4 wt.% LiCl in the dope (Fig. 3, 4L-b). With a further increase of NMP concentration in the bore fluid to 80 wt.%, by comparing Fig. 3, 2L-c with 4L-c, it was found that the large macrovoid formation was suppressed significantly. Roughly half of the cross section was occupied by the sponge-like structure, which hindered the spread of the finger-like pores developed from the outer surface when the dope contained 4 wt.% LiCl. The above observations were believed to be associated with the change of the thermodynamic and kinetic properties of the system before and after LiCl addition. LiCl has good affinity with water. LiCl addition increased the dope’s thermodynamic instability in reaction with water, which facilitated a rapid phase demixing and resulted in macrovoid formation. On the other hand, LiCl possesses strong interactions with the polymer and solvent, which was supported by the significant increase in viscosity of LiCl added

PVDF-HFP/NMP dope solutions (Table 3). The strong interactions among the components of the spinning dope tended to delay the dope precipitation (the kinetic effect), which partially offset the thermodynamic impact of LiCl addition. As a result, the size of the macrovoids was reduced. It seems that a small amount of increase (from 2 to 4 wt.% LiCl) could change the kinetic property of the system significantly when a very weak internal coagulant was used. The effect of glycerol addition on the membrane morphology is shown in Fig. 4. Compared with the morphology of the membranes prepared using additive-free dope solutions (Fig. 2), larger finger-like pores and less sponge-like structure can be seen on the cross-section of the membranes with 5 wt.% glycerol as the additive. When the glycerol concentration in the dope solution increased to 10 wt.%, the morphology of the hollow fiber was similar to that with 5 wt.% glycerol, using water as the internal coagulant (Fig. 4, 5G-a versus 10G-a). However, the difference in morphology caused by different amounts of glycerol addition (5 and 10 wt.%) can be clearly observed when the NMP/water mixture in a ratio of 50/50 or 80/20

Fig. 4. Cross-section morphology of hollow fibers spun with glycerol as additive (bore fluid: (a) water; (b) 50/50 NMP/water; (c) 80/20 NMP/water).

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Table 4 Dimension and performance of the as-spun membranes Membrane code

Bore fluid composition NMP/water (w/w%)

Air gap (cm)

Mean o.d./i.d. (␮m)

PWP (L/(h m2 Pa) (L/(h m2 atm)))

MWCO (kDa)

F15-a F15-b F15-c 2L-a 2L-b 2L-c 4L-a 4L-b 4L-c 5G-a 5G-b 5G-c 10G-a 10G-b 10G-c

0/100 50/50 80/20 0/100 50/50 80/20 0/100 50/50 80/20 0/100 50/50 80/20 0/100 50/50 80/20

20 0.5 0.5 20 0.5 0.5 20 0.5 0.5 20 0.5 0.5 20 0.5 0.5

933/648 941/656 783/595 966/624 991/646 727/527 1061/677 1093/693 930/606 1004/634 1063/647 861/552 1032/624 1062/689 892/560

6.908 × 10−5 (7) 9.869 × 10−6 (1) 3.947 × 10−5 (4) 4.835 × 10−4 (49) 1.480 × 10−4 (15) 1.677 × 10−4 (17) 3.256 × 10−4 (33) 1.283 × 10−4 (13) 4.934 × 10−5 (5) 1.283 × 10−4 (13) 1.973 × 10−5 (2) 4.934 × 10−5 (5) 5.003 × 10−4 (51) 0.003 (31) 9.869 × 10−5 (10)

10 3 5 90 30 35 40 23 7 20 3 5 100 45 15

was used as the bore fluid. Under the condition of 50/50 NMP/water mixture as the internal coagulant, the fingers formed beneath the outer and inner surfaces of the fiber prepared with 10 wt.% glycerol as the additive propagated at a comparable speed to make the sponge-like structure sandwiched in the middle of the fiber wall (Fig. 4, 10G-b versus 5G-b), whereas the propagation of the finger-like macrovoids beneath the outer surface was so fast that the fully developed fingers spread across the whole cross-section of the membrane fabricated with 10 wt.% glycerol as the additive and 80/20 NMP/water mixture as the internal coagulant (Fig. 4, 10G-c versus 5G-c). As discussed in the Section 3.1, glycerol addition into the PVDFHFP/NMP/water system brought the phase boundary much close to the polymer–solvent axis (Fig. 1). The initial composition of the dope solution was thus in close proximity to the binodal line. Consequently, the phase inversion during the membrane formation was accelerated, which enhanced the tendency of macrovoids’ growth [25]. In terms of the kinetic effect, the presence of 10 wt.% glycerol increased the dope viscosity from 1720 × 10−3 (1720 cP) to 4340 × 10−3 Pa s (4340 cP), and this change was not very significant when compared with the impact of 4 wt.% LiCl addition on the dope viscosity (increased to 8600 × 10−3 Pa s (8600 cP)). The thermodynamic effect due to glycerol addition seems to play a dominant role in determining the membrane structure. Thus, under the condition of 50/50 NMP/water as the internal coagulant, the finger-like pores, which were not observed at the inner side of the fiber made from the dope without an additive, occurred beneath the inner surface and extended towards the middle of the fiber wall due to 5 and 10 wt.% glycerol additions, respectively. When 80 wt.% NMP was mixed with water as the internal coagulant, however, it was too soft that the rapid development of the finger-like pores under the outer surface allowed the fingers to cover the entire cross-section of the membrane.

the large macrovoid formation was suppressed because of the enhanced kinetic effect. On the other hand, the membranes spun from the 5 wt.% glycerol contained dope solution exhibited a small improvement in water permeation. The further addition of glycerol to 10 wt.% improved the membrane PWP remarkably. For instance, the PWP of 10G-a membrane was four times higher than that of the 5G-a membrane. The PWP of the membrane as a function of the bore fluid composition can be seen clearly in Fig. 5. The pure water flux of the membrane made from the dope with the same LiCl or glycerol addition decreased with an increase of NMP fraction in the internal coagulant, whereas the membrane made from the PVP contained dope solution presented an opposite trend of PWP increasing with the increase of NMP fraction in the bore fluid [6]. Normally, when the internal coagulant became soft, the precipitation at the inner side of the nascent fiber was delayed, which lead to the removal of the inner skin layer and the PWP might increase accordingly [26,27]. However, with the addition of different additives into the dope solution, the phase inversion process became more complicated. The final membrane morphology and structure were affected by the combined effects of the bore fluid composition and the additive. For instance, under the condition of 80/20 NMP/water as the internal coagulant, the delayed precipitation rate at the inner side of the fiber became even slow due to the kinetic effect of 4 wt.% LiCl addition. The sponge-like structure became tighter (Fig. 3, 4L-c), which might contain pores with poor interconnectivity and/or considerable dead volumes. It was confirmed from the inner surface morphology of the 4L-c membrane when com-

3.4. Effect of the additives on the membrane performance Table 4 gives the dimension, pure water flux and MWCO of the as-spun hollow fibers made at different conditions. For the case of water as the internal coagulant, the pure water permeability (PWP) of the membrane spun from the dope containing a low concentration (2 wt.%) LiCl was seven times higher than that of the counterpart without an additive. When the amount of LiCl increased to 4 wt.%, however, the PWP of the membrane decreased but the retention capability was improved (40 kDa of MWCO). This result was consistent with the membrane morphology change with different amounts of LiCl addition, which was discussed in Section 3.3. At a relatively high LiCl concentration in the dope solution,

Fig. 5. Pure water permeation of the PVDF-HFP membranes spun with the bore fluids containing different NMP fractions (() F15; () 2L; () 4L; () 5G; (䊉) 10G; () 10P [6]).

L. Shi et al. / Journal of Membrane Science 315 (2008) 195–204

201

Fig. 6. Inner surface morphology of the membranes prepared using 80/20 NMP/water as the bore fluid at 10,000 magnification.

pared with the membranes prepared with 2 wt.% LiCl as the additive and without an additive, shown in Fig. 6 (4L-c versus 2L-c and F15-c). It was noticed that when 50/50 and 80/20 NMP/water were used as the internal coagulant, the water permeation flux of the membrane made from the dope containing 5 wt.% glycerol as the additive (the 5G membranes) was very close to the values of the counterpart without an additive (the F15 membrane). This could be correlated with the fact that, unlike other kinds of additives such as PVP and LiCl, the glycerol’s function as a pore former was mainly derived from its thermodynamic effect. Glycerol has good affinity with NMP and a minor interactivity with PVDF-HFP macromolecules [1,25]. The addition of glycerol in the dope reduced the polymer solubility in NMP. Thus, when more NMP was presented in the internal coagulant, the dissolution of the polymer in NMP was recovered to a certain extent and the glycerol’s thermodynamic effect was offset partially. In this situation, the demixing process at the inner side of the fiber might be similar to that without an additive, which was reflected from the similar morphologies of the inner surfaces of the 5G-c membrane and the F15-c membrane, shown in Fig. 6.

3.5. Effect of the additives on the membrane pore size and pore flux distributions Fig. 7 shows the pore size distribution of the membranes prepared with LiCl and glycerol as additives, and corresponding pore size distribution parameters are listed in Table 5. The pore size distributions of the membranes 5P-a and 10P-a, which were made using 5 and 10 wt.% PVP as an additive in the previous work [6], respectively, were also characterized and presented in the figure for comparison.

Table 5 Pore size distribution parameters for the membranes spun with different additives

D* (nm) 

2L-a

4L-a

5G-a

10G-a

5P-aa

10P-aa

5.78 1.58

6.13 1.50

3.26 1.52

7.85 1.34

5.15 1.76

4.64 1.82

a 5P-a and 10P-a refer to the membrane spun from the 15 wt.% PVDF-HFP/NMP dope solution with 5 and 10 wt.% PVP addition at an air gap of 20 cm, respectively [6].

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It can be seen that the geometric mean diameter of the membrane made with 4 wt.% LiCl as an additive was similar to that made with 2 wt.% LiCl, and the pore size distribution became a bit narrow with more LiCl added into the dope solution. When 10 wt.% of glycerol was added into the dope solution, the mean pore size of the membrane increased sharply from 3.26 nm with 5 wt.% glycerol addition to 7.85 nm. The 10G-a membrane also had the smallest geometric standard deviation () compared with other membranes, suggesting that it possessed the narrowest pore size distribution. As for the membranes using PVP as the additive, these membranes exhibited around 5 nm mean pore size but the widest pore size distribution, which resulted in the least retention capability [6]. For instance, the PWP of the 5P-a membrane was 4.145 × 10−4 L/h m2 Pa (42 L/h m2 atm) with 150 kDa MWCO [6], whereas the 10G-a membrane had a PWP value of 5.003 × 10−4 L/h m2 Pa (51 L/h m2 atm) with 100 kDa MWCO. In fact, the membrane pore size distribution is very important to determine the membrane performance. Fig. 7 also illustrates the pore flux distributions of the membranes using three different additives. The peaks of the pore flux distribution curve ff (d) shifted towards right side of x-axis compared with the pore size distribution curve f(d) in Fig. 7, suggesting that the small portion of big pores made a considerable contribution to the flux. Especially, the pore fluxes of the 5P-a and 10P-a membranes were spread apart much wider than other membranes. That was why the pure water fluxes of the 5P-a and 10P-a membranes were similar to that of 10G-a membrane, though the mean pore sizes of the 5P-a and 10Pa membranes (5.15 and 4.64 nm, respectively) were smaller than that of the 10G-a membrane (7.85 nm). 3.6. Effect of the additives on the membrane hydrophobic and mechanical properties The hydrophobicity of the PVDF-HFP membranes was found to be reduced when PVP was used as the additive for membrane fabrication because of its residues trapped in the polymer matrix [6]. The effect of LiCl or glycerol addition into the dope solution on the

Fig. 7. Pore size and pore flux distributions of the PVDF-HFP membranes spun with different additives and water as the bore fluid (() 2L; () 4L; () 5G; (䊉) 10G; () 5P; () 10P).

Fig. 8. Dynamic contact angle of the PVDF-HFP membranes spun with different additives (* a commercial PVDF membrane, 5P and 10P membrane spun from a 15 wt.% PVDF-HFP/NMP dope solution with 5 and 10 wt.% PVP addition at an air gap of 20 cm, respectively [6]).

resultant membrane’s hydrophobicity was examined in the present work. Since the membrane surface was dry before it was immersed into the DI water at the first loop of measurement, the contact angles produced at the second cycle were more close to the real situation to indicate the membrane hydrophobicity. Thus the data from the second cycle are shown in Fig. 8. The contact angle of PVDF-HFP membranes made from the dope with LiCl or glycerol as an additive was lower than that of the membranes made from the dope without an additive (the F15-a membrane), but the decrease of the contact angle due to LiCl or glycerol addition was smaller compared to the case with PVP addition. It was also noticed that the more LiCl added, the higher reduction of the membrane contact angle, whereas the membrane contact angle was reduced less with more glycerol added into the dope solution. In contrast with macromolecular weight additives such as PVP, the small molecular additive LiCl can diffuse out much easily during the immersion and later washing process. As for glycerol, since it is a weak non-solvent for the dope system, it possesses the lowest affinity with the polymer among the three additives. It can be almost completely leached out during the nascent fiber preparation. Thus the hydrophobic property of the membranes was affected by the three additives in the sequence of PVP > LiCl > glycerol. The strain at break, the tensile stress at break and the Young’s modulus of the PVDF-HFP membranes made with LiCl or glycerol as an additive are depicted in Figs. 9–11, respectively along

Fig. 9. Strain at break of the PVDF-HFP membranes spun with different additives (* same as that in Fig. 8).

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Fig. 10. Tensile stress at break of the PVDF-HFP membranes spun with different additives (* same as that in Fig. 8).

Fig. 11. Young’s modulus of the PVDF-HFP membranes spun with different additives (* same as that in Fig. 8).

with corresponding data of a commercial PVDF membrane and the membranes with PVP addition [6]. All the PVDF-HFP membranes exhibited pronounced ductile behavior better than PVDF commercial membrane no matter what type of additives was added. In terms of other mechanical properties, it was observed in previous work [12,13] that adding a high amount of LiCl into the PVDF dope can enhance the membrane permeation performance, but it was gained at a price of reducing its mechanical strength. However, the PVDF-HFP membrane prepared with 4 wt.% LiCl as the additive was the best, as the membrane’s ductile capability (extension at break), stretch resistance (tensile at break), and rigidity (in the form of Young’s modulus) all exceeded the commercial PVDF membrane. But, it should be noted that at a low concentration of 2 wt.%, LiCl’s presence acting as a macrovoid former jeopardized the mechanical strength of prepared membrane as compared with the case of 4 wt.% LiCl addition. The membrane prepared from the dope with glycerol addition also had a much lower Yong’s modulus than the PVDF membrane, which was similar to the case of 10 wt.% PVP addition [6]. The decreased mechanical stability was attributed probably to the longer and abundant of finger-like macrovoids presented in the cross-section of the PVDF-HFP membrane made with glycerol as an additive. 4. Conclusions In the present work, the effects of two typical additives, LiCl and glycerol, on the fabrication of PVDF-HFP asymmetric microporous hollow fiber membranes were studied and compared with

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the impact of PVP addition. Based on the analyses of the isothermal phase diagram for the PVDF-HFP/NMP/water system without/with an additive and the viscosity of the spinning dope solutions, the way in which the additives affected membrane preparation was investigated in terms of membrane morphology, structure, pure water flux, MWCO, hydrophobic and mechanical properties. It was found that the addition of LiCl or glycerol reduced the thermodynamic stability of the dope solution in reaction with water, similarly to the case of PVP as an additive. The viscosity of the PVDFHFP/NMP dope solution with the additive also increased compared with the dope solution without any additive. Thus, the morphology and structure changes of the resultant membranes caused by the additive were believed to be associated with the change of the thermodynamic and kinetic properties of the system in the phase inversion process. A small amount of LiCl addition (from 2 to 4 wt.%) could change the kinetic property of the system significantly, whereas the thermodynamic effect due to glycerol addition (from 5 to 10 wt.%) plays a dominant role in determining the membrane structure. The membranes with improved pure water permeability from 6.908 × 10−5 (7) to 4.835 × 10−4 (49), 3.256 × 10−4 (33) and 5.003 × 10−4 L/h m2 Pa (51 L/h m2 atm) can be obtained when 2 wt.% LiCl, 4 wt.% LiCl and 10 wt.% glycerol was used as the additive, respectively. Among these, the membrane made from the dope containing 4 wt.% LiCl possessed the highest retention capability (40 kDa of MWCO). The LiCl or glycerol addition into the polymer dope solution also made the resultant membranes exhibit a narrow pore size distribution compared to the effect of PVP addition. Moreover, the hydrophobic property of the membranes was affected by the three additives in the sequence of PVP > LiCl > glycerol. The addition of 4 wt.% LiCl into the dope solution could form the membrane with almost four times stronger strain strength than the PVDF commercial membrane while keeping the similar rigidity. The presence of glycerol or PVP in the dope solution improved the permeation but jeopardized the mechanical strength of resultant membranes. Acknowledgements The authors from Singapore gratefully acknowledge the support of Agency of Science, Technology and Research of Singapore (A*STAR) for funding this research with the grant number of 032 101 0024. The authors also thank Drs. Feng Chunsheng, Zhang Hongyan, Liuming and Yuhui for useful discussion and help. References [1] C.S. Feng, R. Wang, B.L. Shi, G.M. Li, Y.L. Wu, Factors affecting pore structure and performance of poly(vinylidene fluoride-co-hexafluoro propylene) asymmetric porous membrane, J. Membr. Sci. 277 (2006) 55. [2] X.Z. Tian, B.K. Zhu, Y.Y. Xu, P(VDF-co-HFP) membrane for recovery of aroma compounds from aqueous solutions by pervaporation. I: Ethyl acetate/water system, J. Membr. Sci. 248 (2005) 109. [3] J.M. Tarascon, A.S. Gozdz, C. Schmutz, F. Shokoohi, P.C. Warren, Performance of Bellcore’s plastic rechargeable Li-ion batteries, Solid State Ionics 86–88 (1996) 49. [4] V. Arcella, A. Sanguineti, E. Quartarone, P. Mustarelli, Vinylidenefluoridehexafluoropropylene copolymers as hybrid electrolyte components for lithium batteries, J. Power Sources 81–82 (1999) 790. [5] E. Reverchon, S. Cardea, PVDF-HFP membrane formation by supercritical CO2 processing: elucidation of formation mechanisms, Ind. Eng. Chem. Res. 45 (2006) 8939. [6] L. Shi, R. Wang, Y.M. Cao, C.S. Feng, D.T. Liang, J.H. Tay, Fabrication of poly(vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP) asymmetric microporous hollow fiber membranes, J. Membr. Sci. 305 (2007) 215. [7] S. Atchariyawut, C.S. Feng, R. Wang, R. Jiraratananon, D.T. Liang, Effect of membrane structure on mass-transfer in the membrane gas–liquid contacting process using microporous PVDF hollow fibers, J. Membr. Sci. 285 (2006) 272. [8] B.J. Cha, J.M. Yang, Effect of high-temperature spinning and PVP additive on the properties of PVDF hollow fiber membranes for microfiltration, Macromol. Res. 14 (2006) 596.

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