- Email: [email protected]
Preparation and characterization of PVDF–PFSA blend hollow fiber UF membrane
Journal of Membrane Science 288 (2007) 123–131
Preparation and characterization of PVDF–PFSA blend hollow fiber UF membrane Wan-Zhong Lang a , Zhen-Liang Xu a,b,∗ , Hu Yang a , Wei Tong a a
b
Membrane Science and Engineering R&D Center, Chemical Engineering Research Center, Shanghai 200237, China Key Laboratory for Ultrafine Materials of the Ministry of Education, East China University of Science and Technology (ECUST), 130 Meilong Road, Shanghai 200237, China Received 3 October 2006; received in revised form 5 November 2006; accepted 8 November 2006 Available online 18 November 2006
Abstract Polyvinylidene fluoride (PVDF)–perfluorosulfonic acid (PFSA) blend hollow fiber ultrafiltration (UF) membranes were prepared by wetspinning method using pure water as an external coagulation solution. Coagulation kinetics of PVDF–PFSA–DMAc system was measured by light transmittance experiment. Using PEG10000 (Mw = 10,000), lysozyme (Mw = 14,400), PEG20000 (Mw = 20,000) and BSA (Mw = 67,000), the separation performances of PVDF–PFSA blend hollow fiber UF membranes were obtained. Based on the experimental results, the delayed demixing process was observed for the dope solution M-1 without PFSA-H while the instantaneous demixing process was observed for the others with PFSAH in the dope solutions M-2 to M-6. The precipitation rate increased with the increment of the PFSA-H concentration in PVDF–PFSA dope solution. The bore fluid solution with 95:5 DMAc:H2 O resulted in single finger-like voids for PVDF–PFSA-H UF membrane. The pure water permeation flux of PVDF–PFSA-H UF membranes firstly increased from <0.1 × 10−5 to 119.1 × 10−5 L m−2 h−1 Pa−1 (<0.1 to 119.1 L m−2 h−1 bar−1 ) for M-1 to M-4(1) and 137 × 10−5 L m−2 h−1 Pa−1 (137 L m−2 h−1 bar−1 ) for M-4(2), and decreased from 119.1 × 10−5 to 79 × 10−5 L m−2 h−1 Pa−1 (119.1 to 79 L m−2 h−1 bar−1 ) for M-4(1), M-5 and M-6 as the PFSA-H concentration changed from 0% to 5% and total PVDF–PFSA-H concentration was 20 wt.%. The molecular weight cut-off (MWCO) of PVDF–PFSA blend hollow fiber UF membranes spun from 20 wt.% PVDF–PFSA-H concentration with 3–5 wt.% PFSA-H was about 20,000 while that spun from 22 wt.% PVDF–PFSA-H or PVDF–PFSA-Na concentration with 5 wt.% PFSA-H or PFSA-Na was about 10,000. As the WPFSA-H /WPVDF increases from 2/18 to 5/15 for M-3–M-6, JBSA /Jw increases from 0.53 to 0.79. The anti-fouling property of PVDF–PFSA-H blend UF membrane is superior to PVDF–PFSA-Na blend membrane. Based on FTIR spectra, PFSA-H existed in PVDF–PFSA blend hollow fiber UF membranes. There is no obvious variation of the intensity of the characteristic peak at 983 cm−1 assigned to C–O–C group of PFSA-H molecules. This also illustrated that PVDF–PFSA-H blend hollow fiber UF membranes were stable. © 2006 Elsevier B.V. All rights reserved. Keywords: Polyvinylidene fluoride; Perfluorosulfonic acid; Hollow fiber; Ultrafiltration
1. Introduction Polyvinylidene fluoride (PVDF) has a superior thermal, chemical resistance and oxidation resistance among polymeric membrane materials. PVDF ultrafiltration (UF) membrane has been extensively investigated in the last 10 years and has been widely used for pervaporation [1,2], ∗
Corresponding author at: Key Laboratory for Ultrafine Materials of the Ministry of Education, East China University of Science and Technology (ECUST), 130 Meilong Road, Shanghai 200237, China. Tel.: +86 21 64252989; fax: +86 21 64252989. E-mail address: [email protected] (Z.-L. Xu). 0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.11.009
membrane distillation [3], ultrafiltration [4] and other fields [5,6]. However, PVDF UF membrane is easily contaminated by protein and other bio-molecules because of its hydrophobic property. Usually, the antifouling property of hydrophobic membrane can be improved by creating a hydrophilic surface though chemical bonding and physical blending methods. Physical blending method is simple and blend membrane usually combines the both properties of two or above polymers. Up to now, many hydrophilic polymers have been used in the PVDF dope solution to prepare blend membranes with favorable performances, such as sulfonate polysulfone (SPS), polyacrylonitrile (PAN), polystyrene sulfonic acid (PSSA), and polyelectrolyte,
124
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131
and so on [7–9]. Inorganic material was also used as the second component to prepare an organic–inorganic PVDF hybrid membrane [10,11]. Perfluorosulfonic acid (PFSA) consists of a polytetrafluoroethylene (PTFE) backbone and regular spaced long perfluorovinyl ether pendant side chains terminated by a sulfonate ionic group. Its chemical formula is shown as follows.
where H+ can be exchanged with Na+ or other cation ions. PFSA has excellent chemical, thermal and mechanical properties [12] and is extensively used in chlor-alkali industry [13], fuel cells [14,15], other applications [16,17]. PFSA shows a good compatibility with PVDF, their blend has been prepared as proton exchange membrane for fuel cell [18,19]. In spite of PFSA acting as a good membrane material, few investigations were found for PFSA used in UF membranes. PFSA has excellent chemical and mechanical properties and potential of extensive applications. However, the highly costly PFSA impedes its large-scale applications except chlor-alkali industry. Besides, the deteriorated ion-change membranes in chlor-alkali industry could not be reused and wasted by burning or dumping. Therefore, PFSA in the deteriorated ion-exchange membranes were recovered and reused in the modification of PVDF UF membrane in this study. The objectives were mainly focused on the improvement of the permeation flux of PVDF UF membrane using the recovered PFSA from the deteriorated ionchange membranes in chlor-alkali industry and the reduction of PVDF UF membrane fouling. 2. Experimental 2.1. Materials PFSA in H+ form (PFSA-H) or Na+ form (PFSA-Na) in granular form was recovered from the deteriorated ion-exchange membrane F-8020 (the product of ASAHI Glass Company, Japan) used in chlor-alkali industry. F-8020 membrane is a PFSA/PFCA (perfluorocarboxylic acid) composite flat membrane reinforced by PTFE fibers. The recovery procedure is described as follows. Firstly, the deteriorated F-8020 membrane was regenerated with 8% hydrochloride solution at 80 ◦ C for 2 h. Secondly, the dissolution was carried out by putting the regenerated membrane into an autoclave with certain amount of water and isopropanol (IPA) (1:1 ratio), then raised to 230 ◦ C and kept for 4 h with extensively stirring. Finally, the solution was cooled to room temperature and filtrated to get clear liquid, after evaporation PFSA powder was obtained. Counter-ions can be transformed by ion-exchange method. PVDF (FR904) powder was purchased from Shanghai San Ai Fu Co. Ltd. (PR China). PEG (Mw = 10,000 and Mw = 20,000) from Shanghai Chemical Agent Company, bovine serum albumin (BSA) (Mw = 67,000) and lysozyme (Mw = 14,400)
from Shanghai Bio Life Sci and Tech Co. Ltd. and N,Ndimethylacetamide (DMAc) from Shanghai Chemical Agent Company were obtained, respectively. 2.2. Preparation of PVDF–PFSA hollow fiber UF membrane Firstly, PFSA-H or PFSA-Na powder was dissolved into DMAC at 80 ◦ C for overnight in triangle flask with strong stirring. Then, a desired weight of PVDF powder was added into PFSA-DMAC solution, the mixture was stirred at 80 ◦ C again for a week to get a homogenous PVDF–PFSA/DMAC solution for spinning at room temperature. The viscosity of the dope solutions was determined by a rotary viscometer (NDJ-79 model, Shanghai Changji Geological Instrument Co. Ltd., China) in Table 1. PVDF–PFSA blend hollow fiber UF membranes were spun by wet-spinning method at room temperature, described in elsewhere [20–23]. The dope solution and bore fluid solution passed through the spinneret with orifice diameter/inner diameter of the tube of 0.9/0.5 mm at the pressure of N2 and constant-flow pump, respectively. The dope solutions were firstly kept in the tank for overnight to degas before spinning. The compositions of the dope solutions and bore fluid solution are also summarized in Table 1. 2.3. Light transmittance measurement The coagulation kinetics of PVDF–PFSA solutions were studied by light transmittance experiment. The schematic diagram of the light transmittance equipment is shown in Fig. 1. The light source of laser was directly exposed to a cast dope solution, about 20 cm above it. The cast dope solution was immersed into the water bath used as coagulation fluid. The transmitted light was detected with an optical detector. The accepted signal was then A/D converted, amplified and recorded by a computer. The intensity of the transmitted light through the cast dope solution was measured as a function of time.
Fig. 1. Schematic diagram of set-up for coagulation kinetics.
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131
125
Table 1 Parameters of PVDF–PFSA-DMAc dope solutions and bore fluid solutions Membrane no.
Dope solution
Bore fluid solution
Viscosity of dope solutions (×10−3 Pa s (cP))
M-1 M-2 M-3 M-4(1) M-4(2) M-5 M-6 M-7 M-8
PVDF/PFSA-H/DMAc 20/0/80 PVDF/PFSA-H/DMAc 19/1/80 PVDF/PFSA-H/DMAc 18/2/80 PVDF/PFSA-H/DMAc 17/3/80 PVDF/PFSA-H/DMAc 17/3/80 PVDF/PFSA-H/DMAc 16/4/80 PVDF/PFSA-H/DMAc 15/5/80 PVDF/PFSA-H/DMAc 17/5/78 PVDF/PFSA-Na/DMAc 17/5/78
DMAc/H2 O 0/100 DMAc/H2 O 0/100 DMAc/H2 O 0/100 DMAc/H2 O 0/100 DMAc/H2 O 95/5 DMAc/H2 O 0/100 DMAc/H2 O 0/100 DMAc/H2 O 0/100 DMAc/H2 O 0/100
9700 9800 9600 9400 – 8300 7000 – –
2.4. Membrane characterization by SEM and FTIR The morphology of PVDF–PFSA blend hollow fiber UF membrane was examined by a scanning electron microscopy (SEM) (JEOL Model JSM-6360 LV, Japan). The fibers were firstly immersed into liquid nitrogen for a few minutes, then broken and deposited on a copper holder. All samples were coated with gold under vacuum before testing. FTIR spectra were obtained on a spectrometer Nicolet 380 with a resolution of 4 cm−1 . PVDF–PFSA-H membrane spectra were carried out by ATR method. 2.5. Permeation flux (J) and rejection (R) of PVDF–PFSA hollow fiber UF membranes The permeation flux and rejection of PVDF–PFSA blend hollow fiber UF membranes were measured by an UF experimental equipment. As shown in Fig. 2, PVDF–PFSA blend hollow fiber UF membrane modules were self-prepared (outside feeding), about 8 mm in diameter, and eight hollow fibers with an effective length of 22.5 cm were composed into a module. PEG10000, PEG20000, bovine serum albumin (BSA) and lysozyme were chosen as solutes. All feed concentrations always were 1000 ppm. All experiments were conducted at room temperature and a feed pressure of 1.0 × 105 Pa (1.0 bar). The newly prepared PVDF–PFSA blend hollow fiber UF membranes were pre-pressured at 1.0 × 105 Pa (1.0 bar) using the pure water for 1 h before measurement, then the pure water permeation (Jw ) was measured, finally the permeation flux and rejection for the
Fig. 2. Schematic diagram for UF experimental equipment.
different solutions were measured. The concentrations of BSA and lysozyme in the permeate and feed were determined by an UV-spectrophotometer (Shimadzu UV-3000, Japan) and the concentrations of PEG in the permeate and feed were determined by a TOC analyzer (Shimadzu TOC-VCPH , Japan). The permeation flux (J) and rejection (R) were defined as formula (1) and (2), respectively. Q P · A CP R= 1− × 100% CF
J=
(1) (2)
where J is the permeation flux of membrane for each solution or pure water (×10−5 L m−2 h−1 Pa−1 (L m−2 h−1 bar−1 )), Q is the volumetric permeation rate of each solution or pure water (L h−1 ), P is the transmembrane pressure (Pa (bar)) and A is the effective area of the membranes (m2 ). R is the rejection of the solutes (%), CP and CF are the permeate and feed concentration, respectively (wt.%). 3. Results and discussion 3.1. Coagulation process of PVDF–PFSA-H hollow fiber UF membranes Light transmittance experiment was used for the investigation of the precipitation kinetics of six PVDF–PFSA-H dope solutions. The pure water was used as the coagulation solution. The delay time is defined as the time needed before light scattering is induced, detected by a light transmission decrease. Based on different delayed times, the precipitation process can be divided into instantaneous demixing and delayed demixing. The instantaneous demixing process is characterized by an instantaneous light transmission decrease with the generation of many scattering nuclei [24–26]. In Fig. 3, the delayed demixing process was observed for the dope solution M-1 without PFSA-H, while the instantaneous demixing process was observed for the others with PFSA-H in the dope solutions M-2 to M-6. In Fig. 3, the initializing decreasing rates of the light transmittance for M-2–M-6 were similar. However, some differences appear during the period of 25–100 s. A higher precipitation rate is observed in Fig. 3 with a higher mass fraction of PFSA-H in the dope solution except M-6. It is generally accepted that addition
126
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131 Table 2 Hydration properties of Nafion membrane with different counter ions (EW1200) [28]
Fig. 3. Effects of mass ratio of PFSA-H in dope solution on coagulation kinetics (using pure water as coagulation fluid at room temperature).
of hydrophilic substance in the dope solution leads to an acceleration of solvent and non-solvent exchange and is advantageous to form a porous structure [24]. Since the hydrophilic PFSA-H absorbs water quickly than hydrophobic PVDF, it promotes the demixing process of the system. Moreover, precipitation rate is also related to the viscosity of the dope solutions. A lower viscosity will increase the precipitation rate of the solutions. The addition of PFSA-H in the dope solution also decreased the viscosity of the dope solutions in Table 1. That is favorable for the diffusion of water and solvent and results in an increase of the precipitation rate with the amount of PFSA-H increasing in the dope solutions. The reason for the deviation of M-6 from the whole trend is not clear, but we apt to think the non-solvent tolerance with the increase of PFSA slow the precipitation rate of dope solution M-6 [27]. 3.2. Membrane morphologies of PVDF–PFSA hollow fiber UF membranes As shown in Fig. 4, a double finger-like void structure is presented for M-2–M-6 with the addition of PFSA-H in the dope solutions except M-4(2). The cavity size firstly increases, and then goes to be stable when PFSA-H concentration increases. Also, the sponge-like structure in the middle layer of the hollow fiber changes to be thinner and gradually disappears with the addition of PFSA-H from M-1 to M-4. In general, a delayed phase separation forms the membrane with a spongelike structure while an instantaneous phase separation results in the membrane with a finger-like structure. Because of the hydrophilic property of –SO3 − groups and its counter-ions of PFSA-H, the addition of PFSA-H favors water ingression and facilitates water diffusion into the dope solution (Fig. 3). Therefore, PFSA-H acts as like other polyelectrolyte (SPEEK) during the phase inversion process [28]. The effect of the bore fluid solution on the membrane morphology was also investigated for M-4. The dope solution for M-4(2) is the same as that of M-4(1). The only difference is that
Counter ion
Hydration per charge
H+ Na+
22.4 14.0
95:5 DMAc:H2 O solution was used as the bore fluid solution for M-4(2). As shown in Fig. 4, a single finger-like structure for M-4(2) presented in the cross-section. The internal surface layer for M-4(2) disappeared and the internal surface of M-4(2) was porous by the comparison with that of M-4(1) (Fig. 5). M-7 and M-8 was spun from the dope solutions with the compositions of 17:5:78 PVDF:PFSA-H:DMAc and 17:5:78 PVDF:PFSA-Na:DMAc, respectively. The morphologies of the cross-section, external surface and internal surface are shown in Figs. 6 and 7. As seen from Fig. 6, the double finger-like structures of the cross-sections for M-7 and M-8 exist. Moreover, M-7 has a better finger-like structure and a higher porosity than M-8. The different hydration property of PFSA-H and PFSA-Na may be responsible for the different morphologies of M-7 and M-8. Based on Gierke’s investigation (Table 2) [29], H+ form Nafion has a higher hydration per charge than that of Na+ form Nafion. A higher hydration of PFSA-H is more favorable for the absorption of water and induces the separation phase. From Fig. 7, the external and internal surfaces of M-7 containing PFSA-H seem to be smooth while those of M-8 containing PFSA-Na seem to be a little rough. 3.3. Permeation flux (J) and rejection (R) of PFSA-PVDF hollow fiber UF membranes The pure water permeation flux (Jw ) of the prepared hollow fiber membranes are summarized in Table 3. As seen in Table 3, with an increase of PFSA-H concentration from 0 to 5 wt.% Jw firstly increased from <0.1 × 10−5 to 119.1 × 10−5 L m−2 h−1 Pa−1 (<0.1 to 119.1 L m−2 h−1 bar−1 ) for M-1 to M-4(1) and 137 × 10−5 L m−2 h−1 Pa−1 (137 L m−2 h−1 bar−1 ) for M-4(2), and decreased from 119.1 × 10−5 to 79 × 10−5 L m−2 h−1 Pa−1 (119.1 to 79 L m−2 h−1 bar−1 ) for M-4(1), M-5 and M-6. Generally, the pure water permeation flux is related to the porosity, interconnection of cavity, surface pore size and hydrophilic property of membrane. The addition of PFSA-H will increase the porosity of the membranes and hydrophilic property of blend membrane. It will result in an increase of Jw . On the other hand, the addition of PFSA increases the precipitation rate of the dope solution and a quicker demixing rate leads a lower surface pore size, which increases the hydrodynamic resistance and is unfavorable for Jw increasing. Browen et al. [28] and Wang et al. [30] also proved the similar conclusion for PEI/SPEEK and PAN-Co-AMPS/PES-C blend membranes. In the low concentration range of PFSA-H (M-1–M-4), the porosity and hydrophilic property increasing dominates the variation of Jw . As the concentration of PFSA-H is higher, the second factor acts as the major influence on Jw . By the
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131
127
Fig. 4. Membrane morphologies of the cross-sections of PVDF–PFSA hollow fibers for M-1–M-6 (magnification ×250).
Table 3 Permeation flux (Jw ) and rejection (R) of PVDF–PFSA blend hollow fiber UF membranes Membrane no.
M-1 M-2 M-3 M-4(1) M-4(2) M-5 M-6 M-7 M-8
Jw (×10−5 L m−2 h−1 Pa−1 (L m−2 h−1 bar−1 ))
<0.1 2.4 58.2 119.1 137.1 98.7 78.9 67.9 60.7
R (%) BSA
Lysozyme
PEG20000
PEG10000
99.1 99.4 97.1 99.8 96.3 99.5 99.5
89.6 – 90.1 89.6 – 91.5 90.9
97.9 98.8 94.1 96.8 97.8 98.5 97.4
59.2 51.0 – 38.1 55.4 90.1 97.3
128
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131
Fig. 5. External and internal surface morphology of M-4(2) (magnification ×10,000).
Fig. 6. Membrane morphologies of the cross-sections of PVDF–PFSA hollow fibers for M-7 and M-8 (magnification ×250).
comparison with Jw values of M-7, M-8 and M-4(1), An excessive PFSA concentration will produce a negative influence on Jw value. To evaluate the separation performance of PVDF–PFSA blend hollow fiber UF membranes, PEG10000 (Mw = 10,000), lysozyme (Mw = 14,400), PEG20000 (Mw = 20,000) and BSA (Mw = 67,000) were chosen as solutes. The rejections of PVDF–PFSA blend hollow fiber UF membranes for different solutes are summarized in Table 3. The rejections of
PVDF–PFSA blend hollow fiber UF membranes (M-3 to M-6) could be 38.1–59.2% for PEG10000, 89.6–90.1% for lysozyme, 94.1–98.8% for PEG20000 and 96.3–99.8% for BSA. And the rejections of M-7 and M-8 are more than 90% for PEG10000, lysozyme, PEG20000 and BSA. Besides, the pure water permeation flux of M-7 with PFSA-H is higher than that of M-8 with PFSA-Na in Table 3. Therefore, the molecular weight cut-off (MWCO) of PVDF–PFSA blend hollow fiber UF membranes spun from 20 wt.% PVDF–PFSA-H concentration with
Fig. 7. Membrane structures of the external surfaces and internal surfaces of M-7 and M-8 (magnification ×10,000).
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131
129
Table 4 Effect of PFSA concentration in PVDF–PFSA-H-DMAc dope solution on the flux of solute aqueous solution Membrane no.
WPFSA /WPVDF
M-1 M-2 M-3 M-4(1) M-5 M-6 M-7 M-8 Ref. [27]
0/20 1/19 2/18 3/17 4/16 5/15 5/17 5/17
Jw (×10−5 L m−2 h−1 Pa−1 (L m−2 h−1 bar−1 ))
JBSA (×10−5 L m−2 h−1 Pa−1 (L m−2 h−1 bar−1 ))
JBSA /Jw
2.4 58.2 119.1 98.7 78.9 67.9 60.7 72.2
30.9 71.5 78.0 62.0 49.2 38.6 43.6
0.53 0.60 0.79 0.79 0.73 0.64 0.60
3–5 wt.% PFSA-H was about 20,000 while that spun from 22 wt.% PVDF–PFSA-H or PVDF–PFSA-Na concentration with 5 wt.% PFSA-H or PFSA-Na was about 10,000. The antifouling properties of PVDF–PFSA blend hollow fiber UF membranes can be evaluated by the ratio of protein solution flux (JBSA ) and Jw . For the high antifouling UF membrane, the addition of protein in feed solution will cause a little flux loss and the ratio (JBSA /Jw ) will be higher. JBSA /Jw data are summarized in Table 4. As the WPFSA-H /WPVDF increases from 2/18 to 5/15 for M-3–M-6, JBSA /Jw increases from 0.53 to 0.79. JBSA /Jw for M-5 and M-6 is 0.79, which is evidently higher than that of PVDF/PVP hollow fiber membranes [31]. The JBSA /Jw data were plotted as a function of WPFSA-H /WPVDF in Fig. 8. Besides, the JBSA /Jw data of M-7 and M-8 are shown in Table 4. The anti-fouling property of PVDF–PFSA-H blend UF membrane is superior to PVDF–PFSA-Na blend membrane.
Fig. 9. FTIR spectra of M-4(1).
3.4. FTIR characterization of PVDF–PFSA-H blend hollow fiber membrane To investigate the composition variations of PVDF–PFSAH blend hollow fiber UF membranes during UF process, FTIR spectra of M-4(1), PVDF and PFSA-H particles are measured in Figs. 9 and 10. Three types of M-4(1) for 0, 4 and 8 h
Fig. 8. The plot of JBSA /Jw as a function of WPFSA-H /WPVDF in the dope solutions.
Fig. 10. FTIR spectra of PVDF and PFSA-H powders.
130
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131
were conducted in the pure water experiment. By the comparison of the spectra of PVDF particles, PFSA-H particles and M-4(1), it could be found that the characteristic absorption peaks of PVDF at 1043, 1072, 879, and 840 cm−1 [32–34] and the peak at 983 cm−1 assigned to C–O–C group of PFSA molecule[35] were shown in M-4(1). The absorption peak of M-4(1) at 1063 cm−1 assigned to –SO3 − of PFSA molecules [12,35] is overlaid by the peak at 1072 cm−1 of PVDF molecules and only shows a shoulder (pointed out with an arrow) in Fig. 9. It proved the existence of PFSA-H and PVDF in blend membrane. In Fig. 9, the spectra of M-4(1) for 0, 4 and 8 h conducted in the pure water experiment shows that the variation of the intensity at 983 cm−1 was not obvious. This also illustrates that PVDF–PFSA-H blend hollow fiber UF membranes are stable. 4. Conclusions Using PFSA-H and PFSA-Na as the second polymer composition, PVDF–PFSA blend hollow fiber UF membranes were prepared by wet spinning technique. Coagulation kinetics of PVDF–PFSA-DMAc system was measured by light transmittance experiment. Using PEG10000 (Mw = 10,000), lysozyme (Mw = 14,400), PEG20000 (Mw = 20,000) and BSA (Mw = 67,000), the separation performances of PVDF–PFSA blend hollow fiber UF membranes were obtained. Based on the experimental results, the delayed demixing process was observed for the dope solution M-1 without PFSA-H while the instantaneous demixing process was observed for the others with PFSA-H in the dope solutions M-2 to M-6. The precipitation rate increased with the increment of the PFSA-H concentration in PVDF–PFSA dope solution. The bore fluid solution with 95:5 DMAc:H2 O resulted in single finger-like voids for PVDF–PFSA-H UF membrane. The pure water permeation flux of PVDF–PFSA-H UF membranes firstly increased from <0.1 × 10−5 to 119.1 × 10−5 L m−2 h−1 Pa−1 (<0.1 to 119.1 L m−2 h−1 bar−1 ) for M-1 to M-4(1) and 137 × 10−5 L m−2 h−1 Pa−1 (137 L m−2 h−1 bar−1 ) for M-4(2), and decreased from 119.1 × 10−5 to 79 × 10−5 L m−2 h−1 Pa−1 (119.1 to 79 L m−2 h−1 bar−1 ) for M-4(1), M-5 and M-6 as the PFSA-H concentration changed from 0% to 5% and total PVDF–PFSA-H concentration was 20 wt.%. The molecular weight cut-off (MWCO) of PVDF–PFSA blend hollow fiber UF membranes spun from 20 wt.% PVDF–PFSA-H concentration with 3–5 wt.% PFSA-H was about 20,000 while that spun from 22 wt.% PVDF–PFSA-H or PVDF–PFSA-Na concentration with 5 wt.% PFSA-H or PFSA-Na was about 10,000. As the WPFSA-H /WPVDF increases from 2/18 to 5/15 for M-3–M-6, JBSA /Jw increases from 0.53 to 0.79. The anti-fouling property of PVDF–PFSA-H blend UF membrane is superior to PVDF–PFSA-Na blend membrane. Based on FTIR spectra, PFSA-H existed in PVDF–PFSA blend hollow fiber UF membranes. There is no obvious variation of the intensity of the characteristic peak at 983 cm−1 assigned to C–O–C group of PFSA-H molecules. This also illustrated that PVDF–PFSA-H blend hollow fiber UF membranes were stable.
Acknowledgment The authors wish to thank National Key Fundamental Research Development Plan (“973” Plan, No. 2003CB615705) for giving financial supports in this project. References [1] R.Y.M. Huang, R. Pal, G.Y. Moon, Pervaporation dehydration of aqueous ethanol and isopropanol mixtures through alginate/chitosan two ply composite membranes supported by poly(vinylidene fluoride) porous membrane, J. Membr. Sci. 167 (2000) 275. [2] K. Jim, P.N. Pintauro, Asymmetric PVDF hollow-fibre membranes for organic/water pervaporation separations, J. Membr. Sci. 135 (1997) 41. [3] M. Khayet, K.C. Khulbe, T. Matsuura, Characterization of membranes for membrane distillation by atomic force microscopy and estimation of their water vapor transfer coefficients in vacuum membrane distillation process, J. Membr. Sci. 238 (2004) 199. [4] J. Kong, K. Li, Oilremoval from oil-in-water emulsions using PVDF membranes, Sep. Purif. Technol. 16 (1999) 83. [5] C.R. Tavares, M. Vieira, J.C.C. Petrus, et al., Ultrafiltration/complexation proxess for metal removal from pulp and paper industry wastewater, Desalination 144 (2002) 261. [6] N.H. Lee, C.W. Frank, Separation of chiral molecules using polyprptidemodified poly(vinyldene fluoride) membranes, Polymer 43 (2002) 6255. [7] L.Q. Sun, Y. Qian, S.X. Liu, Study of polyvinylidene difloride sulfonate polysulphone co-blend ultrafiltration membrane, Membr. Sci. Tech. (China) 21 (2001) 1. [8] Z.H. Yu, Y. Qian, L. Fu, Research on the PVDF/PAN blend ultrafiltration membrane, Membr. Sci. Tech. (China) 20 (2000) 10. [9] T. Uragami, M. Fujimoto, M. Sugihara, Studies on syntheses and permeabilities of special polymer membranes. 28. Permeation characteristics and structure of interpolymer membranes from poly(vinyldene fluoride) and poly(styrene sulfonic acid), Desalination 34 (1980) 311. [10] A. Bottino, G. Capannelli, A. Comite, Preparation and characterization of novel porous PVDF-ZrO2 composite membranes, Desalination 146 (2002) 35. [11] D.S. Kim, H.B. Park, Y.M. Lee, Preparation and characterization of PVDF/Silica hybrid membranes containing sulfonic acid groups, J. Appl. Polym. Sci. 93 (2004) 209. [12] H.W. Carla, Recent advances in perfluorinated ionmomer membranes: structure, properties and applications, J. Membr. Sci. 120 (1996) 1. [13] R.S. Yeo, J. Mcbreen, G. Kissel, et al., Perfluorosulphonic acid (Nafion) membrane as a separator for an advanced alkaline water electrolyser, J. Appl. Electrochem. 10 (1980) 741. [14] Z.Q. Ma, P. Cheng, T.S. Zhao, A palladium-alloy deposited Nafion membrane for direct methanol fuel cells, J. Membr. Sci. 215 (2003) 327. [15] Z.G. Shao, X. Wang, I.M. Hsing, Composite Nafion/PVA membranes for the direct methanol fuel cell, J. Membr. Sci. 210 (2002) 147. [16] J.S. Jiang, D.B. Greenberg, J.R. Fried, Pervaporation of methanol from a triglyme solution using a Nafion membrane: 1. Transport studies, J. Membr. Sci. 132 (1997) 255. [17] M. Krishnan, J.R. White, J.R. Fox, et al., Integrated chemical systems:photocatalysis at semiconductors incorporated into polymer(Nafion)/mediator systems, J. Am. Chem. Soc. 105 (1983) 7002. [18] M.K. Song, Y.T. Kim, J.M. Fenton, Chemically modified Nafion/poly(vinylidene fluoride) blend iomers for poton exchange membrane fuel cells, J. Power Sources 117 (2003) 14. [19] M.Y. Cho, J.Y. Eom, M.Y. Jung, Characteristics of PVDF copolymer/Nafion blend membrane for direct methanol fuel cell (DMFC), Electrochim. Acta 50 (2004) 583. [20] Z.L. Xu, T.S. Chung, Y. Huang, Effect of polyvinylpyrrolidone molecular wights on morphology, oil/water separation, mechanical and thermal properties of polyetherimide/polyvinylpyrrolidone hollow fiber membranes, J. Appl. Polym. Sci. 74 (1999) 2220.
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131 [21] J. Xu, Z.L. Xu, Poly(vinyl chloride) (PVC) hollow fiber ultrafiltration membranes preparation from PVC/additives/solvent, J. Membr. Sci. 208 (2002) 203. [22] T.S. Chung, Z.L. Xu, Asymmetric hollow fiber membranes prepared from miscible polybenzimidazole and polyetherimide blends, J. Membr. Sci. 147 (1998) 35. [23] Z.L. Xu, F. Alsalhy Qusay, Polyethersulfone (PES) hollow fiber ultrafiltration membranes prepared by PES/non-solvent/NMP solution, J. Membr. Sci. 233 (2004) 101. [24] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, 1991. [25] J.S. Kang, Y.M. Lee, Effects of molecular weight of polyvinylpyrrolidone on precipitation kinetics during the formation of asymmetric polyacrylonitrile membrane, J. Appl. Polym. Sci. 85 (2002) 57. [26] J.H. Kim, K.-H. Lee, Effects of PEG additives on membrane formation by phase inversion, J. Membr. Sci. 138 (1998) 153. [27] J.S. Kang, Y.M. Lee, Effects of molecular weight of polyvinylpyrrolidone on precipitation kinetics during the formation of asymmetric polyacrylonitrile membrane, J. Appl. Polym. Sci. 85 (2002) 57–68. [28] W.R. Browen, S.Y. Cheng, T.A. Doneva, et al., Manufacture and characterization of polyetherimide/sulfonated poly(ether ether ketone) blend membranes, J. Membr. Sci. 250 (2005) 1.
131
[29] T.D. Gierke, G.E. Munn, F.C. Wilson, The morphology in Nafion perfluorinated membrane products, as determined by wide- and small-angle X-ray studies, J. Polym. Sci., Phys. Edn. 19 (1981) 1687. [30] M. Wang, L.-G. Wu, J.-X. Mo, et al., The preparation and characterization of novel charged polyacrylonitrile/PES-C blend membranes used for ultrafiltration, J. Membr. Sci. 274 (2006) 200. [31] Z.L. Xu, X.D. Zhai, G.E. Chen, Study on high-porosity poly(vinylidene fluoride) hollow fiber ultrafiltration membrane, Membr. Sci. Technol. (China) 20 (2000) 10. [32] Y. Bormashenko, R. Pogreb, O. Stanevsky, et al., Vibrational spectrum of PVDF and its interpretation, Polym. Testing 23 (2004) 791. [33] T. Boccaccio, A. Bottino, G. Capannelli, et al., Characterization of PVDF membranes by vibrational spectroscopy, J. Membr. Sci. 210 (2002) 315. [34] V. Sencadas, R. Gregorio Filho, S. Lanceros-Mendez, Processing and characterization of a novel nonporous poly(vinilidene fluoride) films in the b phase, J. Non-Cryst. Solids 352 (2006) 2226. [35] A. Gruger, A. T´egis, T. Schmatko, et al., Nanostructure of Nafion® membranes at different states of hydration An IR and Raman study., Vib. Spectrosc. 26 (2001) 215.
Preparation and characterization of PVDF–PFSA blend hollow fiber UF membrane Wan-Zhong Lang a , Zhen-Liang Xu a,b,∗ , Hu Yang a , Wei Tong a a
b
Membrane Science and Engineering R&D Center, Chemical Engineering Research Center, Shanghai 200237, China Key Laboratory for Ultrafine Materials of the Ministry of Education, East China University of Science and Technology (ECUST), 130 Meilong Road, Shanghai 200237, China Received 3 October 2006; received in revised form 5 November 2006; accepted 8 November 2006 Available online 18 November 2006
Abstract Polyvinylidene fluoride (PVDF)–perfluorosulfonic acid (PFSA) blend hollow fiber ultrafiltration (UF) membranes were prepared by wetspinning method using pure water as an external coagulation solution. Coagulation kinetics of PVDF–PFSA–DMAc system was measured by light transmittance experiment. Using PEG10000 (Mw = 10,000), lysozyme (Mw = 14,400), PEG20000 (Mw = 20,000) and BSA (Mw = 67,000), the separation performances of PVDF–PFSA blend hollow fiber UF membranes were obtained. Based on the experimental results, the delayed demixing process was observed for the dope solution M-1 without PFSA-H while the instantaneous demixing process was observed for the others with PFSAH in the dope solutions M-2 to M-6. The precipitation rate increased with the increment of the PFSA-H concentration in PVDF–PFSA dope solution. The bore fluid solution with 95:5 DMAc:H2 O resulted in single finger-like voids for PVDF–PFSA-H UF membrane. The pure water permeation flux of PVDF–PFSA-H UF membranes firstly increased from <0.1 × 10−5 to 119.1 × 10−5 L m−2 h−1 Pa−1 (<0.1 to 119.1 L m−2 h−1 bar−1 ) for M-1 to M-4(1) and 137 × 10−5 L m−2 h−1 Pa−1 (137 L m−2 h−1 bar−1 ) for M-4(2), and decreased from 119.1 × 10−5 to 79 × 10−5 L m−2 h−1 Pa−1 (119.1 to 79 L m−2 h−1 bar−1 ) for M-4(1), M-5 and M-6 as the PFSA-H concentration changed from 0% to 5% and total PVDF–PFSA-H concentration was 20 wt.%. The molecular weight cut-off (MWCO) of PVDF–PFSA blend hollow fiber UF membranes spun from 20 wt.% PVDF–PFSA-H concentration with 3–5 wt.% PFSA-H was about 20,000 while that spun from 22 wt.% PVDF–PFSA-H or PVDF–PFSA-Na concentration with 5 wt.% PFSA-H or PFSA-Na was about 10,000. As the WPFSA-H /WPVDF increases from 2/18 to 5/15 for M-3–M-6, JBSA /Jw increases from 0.53 to 0.79. The anti-fouling property of PVDF–PFSA-H blend UF membrane is superior to PVDF–PFSA-Na blend membrane. Based on FTIR spectra, PFSA-H existed in PVDF–PFSA blend hollow fiber UF membranes. There is no obvious variation of the intensity of the characteristic peak at 983 cm−1 assigned to C–O–C group of PFSA-H molecules. This also illustrated that PVDF–PFSA-H blend hollow fiber UF membranes were stable. © 2006 Elsevier B.V. All rights reserved. Keywords: Polyvinylidene fluoride; Perfluorosulfonic acid; Hollow fiber; Ultrafiltration
1. Introduction Polyvinylidene fluoride (PVDF) has a superior thermal, chemical resistance and oxidation resistance among polymeric membrane materials. PVDF ultrafiltration (UF) membrane has been extensively investigated in the last 10 years and has been widely used for pervaporation [1,2], ∗
Corresponding author at: Key Laboratory for Ultrafine Materials of the Ministry of Education, East China University of Science and Technology (ECUST), 130 Meilong Road, Shanghai 200237, China. Tel.: +86 21 64252989; fax: +86 21 64252989. E-mail address: [email protected] (Z.-L. Xu). 0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.11.009
membrane distillation [3], ultrafiltration [4] and other fields [5,6]. However, PVDF UF membrane is easily contaminated by protein and other bio-molecules because of its hydrophobic property. Usually, the antifouling property of hydrophobic membrane can be improved by creating a hydrophilic surface though chemical bonding and physical blending methods. Physical blending method is simple and blend membrane usually combines the both properties of two or above polymers. Up to now, many hydrophilic polymers have been used in the PVDF dope solution to prepare blend membranes with favorable performances, such as sulfonate polysulfone (SPS), polyacrylonitrile (PAN), polystyrene sulfonic acid (PSSA), and polyelectrolyte,
124
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131
and so on [7–9]. Inorganic material was also used as the second component to prepare an organic–inorganic PVDF hybrid membrane [10,11]. Perfluorosulfonic acid (PFSA) consists of a polytetrafluoroethylene (PTFE) backbone and regular spaced long perfluorovinyl ether pendant side chains terminated by a sulfonate ionic group. Its chemical formula is shown as follows.
where H+ can be exchanged with Na+ or other cation ions. PFSA has excellent chemical, thermal and mechanical properties [12] and is extensively used in chlor-alkali industry [13], fuel cells [14,15], other applications [16,17]. PFSA shows a good compatibility with PVDF, their blend has been prepared as proton exchange membrane for fuel cell [18,19]. In spite of PFSA acting as a good membrane material, few investigations were found for PFSA used in UF membranes. PFSA has excellent chemical and mechanical properties and potential of extensive applications. However, the highly costly PFSA impedes its large-scale applications except chlor-alkali industry. Besides, the deteriorated ion-change membranes in chlor-alkali industry could not be reused and wasted by burning or dumping. Therefore, PFSA in the deteriorated ion-exchange membranes were recovered and reused in the modification of PVDF UF membrane in this study. The objectives were mainly focused on the improvement of the permeation flux of PVDF UF membrane using the recovered PFSA from the deteriorated ionchange membranes in chlor-alkali industry and the reduction of PVDF UF membrane fouling. 2. Experimental 2.1. Materials PFSA in H+ form (PFSA-H) or Na+ form (PFSA-Na) in granular form was recovered from the deteriorated ion-exchange membrane F-8020 (the product of ASAHI Glass Company, Japan) used in chlor-alkali industry. F-8020 membrane is a PFSA/PFCA (perfluorocarboxylic acid) composite flat membrane reinforced by PTFE fibers. The recovery procedure is described as follows. Firstly, the deteriorated F-8020 membrane was regenerated with 8% hydrochloride solution at 80 ◦ C for 2 h. Secondly, the dissolution was carried out by putting the regenerated membrane into an autoclave with certain amount of water and isopropanol (IPA) (1:1 ratio), then raised to 230 ◦ C and kept for 4 h with extensively stirring. Finally, the solution was cooled to room temperature and filtrated to get clear liquid, after evaporation PFSA powder was obtained. Counter-ions can be transformed by ion-exchange method. PVDF (FR904) powder was purchased from Shanghai San Ai Fu Co. Ltd. (PR China). PEG (Mw = 10,000 and Mw = 20,000) from Shanghai Chemical Agent Company, bovine serum albumin (BSA) (Mw = 67,000) and lysozyme (Mw = 14,400)
from Shanghai Bio Life Sci and Tech Co. Ltd. and N,Ndimethylacetamide (DMAc) from Shanghai Chemical Agent Company were obtained, respectively. 2.2. Preparation of PVDF–PFSA hollow fiber UF membrane Firstly, PFSA-H or PFSA-Na powder was dissolved into DMAC at 80 ◦ C for overnight in triangle flask with strong stirring. Then, a desired weight of PVDF powder was added into PFSA-DMAC solution, the mixture was stirred at 80 ◦ C again for a week to get a homogenous PVDF–PFSA/DMAC solution for spinning at room temperature. The viscosity of the dope solutions was determined by a rotary viscometer (NDJ-79 model, Shanghai Changji Geological Instrument Co. Ltd., China) in Table 1. PVDF–PFSA blend hollow fiber UF membranes were spun by wet-spinning method at room temperature, described in elsewhere [20–23]. The dope solution and bore fluid solution passed through the spinneret with orifice diameter/inner diameter of the tube of 0.9/0.5 mm at the pressure of N2 and constant-flow pump, respectively. The dope solutions were firstly kept in the tank for overnight to degas before spinning. The compositions of the dope solutions and bore fluid solution are also summarized in Table 1. 2.3. Light transmittance measurement The coagulation kinetics of PVDF–PFSA solutions were studied by light transmittance experiment. The schematic diagram of the light transmittance equipment is shown in Fig. 1. The light source of laser was directly exposed to a cast dope solution, about 20 cm above it. The cast dope solution was immersed into the water bath used as coagulation fluid. The transmitted light was detected with an optical detector. The accepted signal was then A/D converted, amplified and recorded by a computer. The intensity of the transmitted light through the cast dope solution was measured as a function of time.
Fig. 1. Schematic diagram of set-up for coagulation kinetics.
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131
125
Table 1 Parameters of PVDF–PFSA-DMAc dope solutions and bore fluid solutions Membrane no.
Dope solution
Bore fluid solution
Viscosity of dope solutions (×10−3 Pa s (cP))
M-1 M-2 M-3 M-4(1) M-4(2) M-5 M-6 M-7 M-8
PVDF/PFSA-H/DMAc 20/0/80 PVDF/PFSA-H/DMAc 19/1/80 PVDF/PFSA-H/DMAc 18/2/80 PVDF/PFSA-H/DMAc 17/3/80 PVDF/PFSA-H/DMAc 17/3/80 PVDF/PFSA-H/DMAc 16/4/80 PVDF/PFSA-H/DMAc 15/5/80 PVDF/PFSA-H/DMAc 17/5/78 PVDF/PFSA-Na/DMAc 17/5/78
DMAc/H2 O 0/100 DMAc/H2 O 0/100 DMAc/H2 O 0/100 DMAc/H2 O 0/100 DMAc/H2 O 95/5 DMAc/H2 O 0/100 DMAc/H2 O 0/100 DMAc/H2 O 0/100 DMAc/H2 O 0/100
9700 9800 9600 9400 – 8300 7000 – –
2.4. Membrane characterization by SEM and FTIR The morphology of PVDF–PFSA blend hollow fiber UF membrane was examined by a scanning electron microscopy (SEM) (JEOL Model JSM-6360 LV, Japan). The fibers were firstly immersed into liquid nitrogen for a few minutes, then broken and deposited on a copper holder. All samples were coated with gold under vacuum before testing. FTIR spectra were obtained on a spectrometer Nicolet 380 with a resolution of 4 cm−1 . PVDF–PFSA-H membrane spectra were carried out by ATR method. 2.5. Permeation flux (J) and rejection (R) of PVDF–PFSA hollow fiber UF membranes The permeation flux and rejection of PVDF–PFSA blend hollow fiber UF membranes were measured by an UF experimental equipment. As shown in Fig. 2, PVDF–PFSA blend hollow fiber UF membrane modules were self-prepared (outside feeding), about 8 mm in diameter, and eight hollow fibers with an effective length of 22.5 cm were composed into a module. PEG10000, PEG20000, bovine serum albumin (BSA) and lysozyme were chosen as solutes. All feed concentrations always were 1000 ppm. All experiments were conducted at room temperature and a feed pressure of 1.0 × 105 Pa (1.0 bar). The newly prepared PVDF–PFSA blend hollow fiber UF membranes were pre-pressured at 1.0 × 105 Pa (1.0 bar) using the pure water for 1 h before measurement, then the pure water permeation (Jw ) was measured, finally the permeation flux and rejection for the
Fig. 2. Schematic diagram for UF experimental equipment.
different solutions were measured. The concentrations of BSA and lysozyme in the permeate and feed were determined by an UV-spectrophotometer (Shimadzu UV-3000, Japan) and the concentrations of PEG in the permeate and feed were determined by a TOC analyzer (Shimadzu TOC-VCPH , Japan). The permeation flux (J) and rejection (R) were defined as formula (1) and (2), respectively. Q P · A CP R= 1− × 100% CF
J=
(1) (2)
where J is the permeation flux of membrane for each solution or pure water (×10−5 L m−2 h−1 Pa−1 (L m−2 h−1 bar−1 )), Q is the volumetric permeation rate of each solution or pure water (L h−1 ), P is the transmembrane pressure (Pa (bar)) and A is the effective area of the membranes (m2 ). R is the rejection of the solutes (%), CP and CF are the permeate and feed concentration, respectively (wt.%). 3. Results and discussion 3.1. Coagulation process of PVDF–PFSA-H hollow fiber UF membranes Light transmittance experiment was used for the investigation of the precipitation kinetics of six PVDF–PFSA-H dope solutions. The pure water was used as the coagulation solution. The delay time is defined as the time needed before light scattering is induced, detected by a light transmission decrease. Based on different delayed times, the precipitation process can be divided into instantaneous demixing and delayed demixing. The instantaneous demixing process is characterized by an instantaneous light transmission decrease with the generation of many scattering nuclei [24–26]. In Fig. 3, the delayed demixing process was observed for the dope solution M-1 without PFSA-H, while the instantaneous demixing process was observed for the others with PFSA-H in the dope solutions M-2 to M-6. In Fig. 3, the initializing decreasing rates of the light transmittance for M-2–M-6 were similar. However, some differences appear during the period of 25–100 s. A higher precipitation rate is observed in Fig. 3 with a higher mass fraction of PFSA-H in the dope solution except M-6. It is generally accepted that addition
126
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131 Table 2 Hydration properties of Nafion membrane with different counter ions (EW1200) [28]
Fig. 3. Effects of mass ratio of PFSA-H in dope solution on coagulation kinetics (using pure water as coagulation fluid at room temperature).
of hydrophilic substance in the dope solution leads to an acceleration of solvent and non-solvent exchange and is advantageous to form a porous structure [24]. Since the hydrophilic PFSA-H absorbs water quickly than hydrophobic PVDF, it promotes the demixing process of the system. Moreover, precipitation rate is also related to the viscosity of the dope solutions. A lower viscosity will increase the precipitation rate of the solutions. The addition of PFSA-H in the dope solution also decreased the viscosity of the dope solutions in Table 1. That is favorable for the diffusion of water and solvent and results in an increase of the precipitation rate with the amount of PFSA-H increasing in the dope solutions. The reason for the deviation of M-6 from the whole trend is not clear, but we apt to think the non-solvent tolerance with the increase of PFSA slow the precipitation rate of dope solution M-6 [27]. 3.2. Membrane morphologies of PVDF–PFSA hollow fiber UF membranes As shown in Fig. 4, a double finger-like void structure is presented for M-2–M-6 with the addition of PFSA-H in the dope solutions except M-4(2). The cavity size firstly increases, and then goes to be stable when PFSA-H concentration increases. Also, the sponge-like structure in the middle layer of the hollow fiber changes to be thinner and gradually disappears with the addition of PFSA-H from M-1 to M-4. In general, a delayed phase separation forms the membrane with a spongelike structure while an instantaneous phase separation results in the membrane with a finger-like structure. Because of the hydrophilic property of –SO3 − groups and its counter-ions of PFSA-H, the addition of PFSA-H favors water ingression and facilitates water diffusion into the dope solution (Fig. 3). Therefore, PFSA-H acts as like other polyelectrolyte (SPEEK) during the phase inversion process [28]. The effect of the bore fluid solution on the membrane morphology was also investigated for M-4. The dope solution for M-4(2) is the same as that of M-4(1). The only difference is that
Counter ion
Hydration per charge
H+ Na+
22.4 14.0
95:5 DMAc:H2 O solution was used as the bore fluid solution for M-4(2). As shown in Fig. 4, a single finger-like structure for M-4(2) presented in the cross-section. The internal surface layer for M-4(2) disappeared and the internal surface of M-4(2) was porous by the comparison with that of M-4(1) (Fig. 5). M-7 and M-8 was spun from the dope solutions with the compositions of 17:5:78 PVDF:PFSA-H:DMAc and 17:5:78 PVDF:PFSA-Na:DMAc, respectively. The morphologies of the cross-section, external surface and internal surface are shown in Figs. 6 and 7. As seen from Fig. 6, the double finger-like structures of the cross-sections for M-7 and M-8 exist. Moreover, M-7 has a better finger-like structure and a higher porosity than M-8. The different hydration property of PFSA-H and PFSA-Na may be responsible for the different morphologies of M-7 and M-8. Based on Gierke’s investigation (Table 2) [29], H+ form Nafion has a higher hydration per charge than that of Na+ form Nafion. A higher hydration of PFSA-H is more favorable for the absorption of water and induces the separation phase. From Fig. 7, the external and internal surfaces of M-7 containing PFSA-H seem to be smooth while those of M-8 containing PFSA-Na seem to be a little rough. 3.3. Permeation flux (J) and rejection (R) of PFSA-PVDF hollow fiber UF membranes The pure water permeation flux (Jw ) of the prepared hollow fiber membranes are summarized in Table 3. As seen in Table 3, with an increase of PFSA-H concentration from 0 to 5 wt.% Jw firstly increased from <0.1 × 10−5 to 119.1 × 10−5 L m−2 h−1 Pa−1 (<0.1 to 119.1 L m−2 h−1 bar−1 ) for M-1 to M-4(1) and 137 × 10−5 L m−2 h−1 Pa−1 (137 L m−2 h−1 bar−1 ) for M-4(2), and decreased from 119.1 × 10−5 to 79 × 10−5 L m−2 h−1 Pa−1 (119.1 to 79 L m−2 h−1 bar−1 ) for M-4(1), M-5 and M-6. Generally, the pure water permeation flux is related to the porosity, interconnection of cavity, surface pore size and hydrophilic property of membrane. The addition of PFSA-H will increase the porosity of the membranes and hydrophilic property of blend membrane. It will result in an increase of Jw . On the other hand, the addition of PFSA increases the precipitation rate of the dope solution and a quicker demixing rate leads a lower surface pore size, which increases the hydrodynamic resistance and is unfavorable for Jw increasing. Browen et al. [28] and Wang et al. [30] also proved the similar conclusion for PEI/SPEEK and PAN-Co-AMPS/PES-C blend membranes. In the low concentration range of PFSA-H (M-1–M-4), the porosity and hydrophilic property increasing dominates the variation of Jw . As the concentration of PFSA-H is higher, the second factor acts as the major influence on Jw . By the
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131
127
Fig. 4. Membrane morphologies of the cross-sections of PVDF–PFSA hollow fibers for M-1–M-6 (magnification ×250).
Table 3 Permeation flux (Jw ) and rejection (R) of PVDF–PFSA blend hollow fiber UF membranes Membrane no.
M-1 M-2 M-3 M-4(1) M-4(2) M-5 M-6 M-7 M-8
Jw (×10−5 L m−2 h−1 Pa−1 (L m−2 h−1 bar−1 ))
<0.1 2.4 58.2 119.1 137.1 98.7 78.9 67.9 60.7
R (%) BSA
Lysozyme
PEG20000
PEG10000
99.1 99.4 97.1 99.8 96.3 99.5 99.5
89.6 – 90.1 89.6 – 91.5 90.9
97.9 98.8 94.1 96.8 97.8 98.5 97.4
59.2 51.0 – 38.1 55.4 90.1 97.3
128
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131
Fig. 5. External and internal surface morphology of M-4(2) (magnification ×10,000).
Fig. 6. Membrane morphologies of the cross-sections of PVDF–PFSA hollow fibers for M-7 and M-8 (magnification ×250).
comparison with Jw values of M-7, M-8 and M-4(1), An excessive PFSA concentration will produce a negative influence on Jw value. To evaluate the separation performance of PVDF–PFSA blend hollow fiber UF membranes, PEG10000 (Mw = 10,000), lysozyme (Mw = 14,400), PEG20000 (Mw = 20,000) and BSA (Mw = 67,000) were chosen as solutes. The rejections of PVDF–PFSA blend hollow fiber UF membranes for different solutes are summarized in Table 3. The rejections of
PVDF–PFSA blend hollow fiber UF membranes (M-3 to M-6) could be 38.1–59.2% for PEG10000, 89.6–90.1% for lysozyme, 94.1–98.8% for PEG20000 and 96.3–99.8% for BSA. And the rejections of M-7 and M-8 are more than 90% for PEG10000, lysozyme, PEG20000 and BSA. Besides, the pure water permeation flux of M-7 with PFSA-H is higher than that of M-8 with PFSA-Na in Table 3. Therefore, the molecular weight cut-off (MWCO) of PVDF–PFSA blend hollow fiber UF membranes spun from 20 wt.% PVDF–PFSA-H concentration with
Fig. 7. Membrane structures of the external surfaces and internal surfaces of M-7 and M-8 (magnification ×10,000).
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131
129
Table 4 Effect of PFSA concentration in PVDF–PFSA-H-DMAc dope solution on the flux of solute aqueous solution Membrane no.
WPFSA /WPVDF
M-1 M-2 M-3 M-4(1) M-5 M-6 M-7 M-8 Ref. [27]
0/20 1/19 2/18 3/17 4/16 5/15 5/17 5/17
Jw (×10−5 L m−2 h−1 Pa−1 (L m−2 h−1 bar−1 ))
JBSA (×10−5 L m−2 h−1 Pa−1 (L m−2 h−1 bar−1 ))
JBSA /Jw
2.4 58.2 119.1 98.7 78.9 67.9 60.7 72.2
30.9 71.5 78.0 62.0 49.2 38.6 43.6
0.53 0.60 0.79 0.79 0.73 0.64 0.60
3–5 wt.% PFSA-H was about 20,000 while that spun from 22 wt.% PVDF–PFSA-H or PVDF–PFSA-Na concentration with 5 wt.% PFSA-H or PFSA-Na was about 10,000. The antifouling properties of PVDF–PFSA blend hollow fiber UF membranes can be evaluated by the ratio of protein solution flux (JBSA ) and Jw . For the high antifouling UF membrane, the addition of protein in feed solution will cause a little flux loss and the ratio (JBSA /Jw ) will be higher. JBSA /Jw data are summarized in Table 4. As the WPFSA-H /WPVDF increases from 2/18 to 5/15 for M-3–M-6, JBSA /Jw increases from 0.53 to 0.79. JBSA /Jw for M-5 and M-6 is 0.79, which is evidently higher than that of PVDF/PVP hollow fiber membranes [31]. The JBSA /Jw data were plotted as a function of WPFSA-H /WPVDF in Fig. 8. Besides, the JBSA /Jw data of M-7 and M-8 are shown in Table 4. The anti-fouling property of PVDF–PFSA-H blend UF membrane is superior to PVDF–PFSA-Na blend membrane.
Fig. 9. FTIR spectra of M-4(1).
3.4. FTIR characterization of PVDF–PFSA-H blend hollow fiber membrane To investigate the composition variations of PVDF–PFSAH blend hollow fiber UF membranes during UF process, FTIR spectra of M-4(1), PVDF and PFSA-H particles are measured in Figs. 9 and 10. Three types of M-4(1) for 0, 4 and 8 h
Fig. 8. The plot of JBSA /Jw as a function of WPFSA-H /WPVDF in the dope solutions.
Fig. 10. FTIR spectra of PVDF and PFSA-H powders.
130
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131
were conducted in the pure water experiment. By the comparison of the spectra of PVDF particles, PFSA-H particles and M-4(1), it could be found that the characteristic absorption peaks of PVDF at 1043, 1072, 879, and 840 cm−1 [32–34] and the peak at 983 cm−1 assigned to C–O–C group of PFSA molecule[35] were shown in M-4(1). The absorption peak of M-4(1) at 1063 cm−1 assigned to –SO3 − of PFSA molecules [12,35] is overlaid by the peak at 1072 cm−1 of PVDF molecules and only shows a shoulder (pointed out with an arrow) in Fig. 9. It proved the existence of PFSA-H and PVDF in blend membrane. In Fig. 9, the spectra of M-4(1) for 0, 4 and 8 h conducted in the pure water experiment shows that the variation of the intensity at 983 cm−1 was not obvious. This also illustrates that PVDF–PFSA-H blend hollow fiber UF membranes are stable. 4. Conclusions Using PFSA-H and PFSA-Na as the second polymer composition, PVDF–PFSA blend hollow fiber UF membranes were prepared by wet spinning technique. Coagulation kinetics of PVDF–PFSA-DMAc system was measured by light transmittance experiment. Using PEG10000 (Mw = 10,000), lysozyme (Mw = 14,400), PEG20000 (Mw = 20,000) and BSA (Mw = 67,000), the separation performances of PVDF–PFSA blend hollow fiber UF membranes were obtained. Based on the experimental results, the delayed demixing process was observed for the dope solution M-1 without PFSA-H while the instantaneous demixing process was observed for the others with PFSA-H in the dope solutions M-2 to M-6. The precipitation rate increased with the increment of the PFSA-H concentration in PVDF–PFSA dope solution. The bore fluid solution with 95:5 DMAc:H2 O resulted in single finger-like voids for PVDF–PFSA-H UF membrane. The pure water permeation flux of PVDF–PFSA-H UF membranes firstly increased from <0.1 × 10−5 to 119.1 × 10−5 L m−2 h−1 Pa−1 (<0.1 to 119.1 L m−2 h−1 bar−1 ) for M-1 to M-4(1) and 137 × 10−5 L m−2 h−1 Pa−1 (137 L m−2 h−1 bar−1 ) for M-4(2), and decreased from 119.1 × 10−5 to 79 × 10−5 L m−2 h−1 Pa−1 (119.1 to 79 L m−2 h−1 bar−1 ) for M-4(1), M-5 and M-6 as the PFSA-H concentration changed from 0% to 5% and total PVDF–PFSA-H concentration was 20 wt.%. The molecular weight cut-off (MWCO) of PVDF–PFSA blend hollow fiber UF membranes spun from 20 wt.% PVDF–PFSA-H concentration with 3–5 wt.% PFSA-H was about 20,000 while that spun from 22 wt.% PVDF–PFSA-H or PVDF–PFSA-Na concentration with 5 wt.% PFSA-H or PFSA-Na was about 10,000. As the WPFSA-H /WPVDF increases from 2/18 to 5/15 for M-3–M-6, JBSA /Jw increases from 0.53 to 0.79. The anti-fouling property of PVDF–PFSA-H blend UF membrane is superior to PVDF–PFSA-Na blend membrane. Based on FTIR spectra, PFSA-H existed in PVDF–PFSA blend hollow fiber UF membranes. There is no obvious variation of the intensity of the characteristic peak at 983 cm−1 assigned to C–O–C group of PFSA-H molecules. This also illustrated that PVDF–PFSA-H blend hollow fiber UF membranes were stable.
Acknowledgment The authors wish to thank National Key Fundamental Research Development Plan (“973” Plan, No. 2003CB615705) for giving financial supports in this project. References [1] R.Y.M. Huang, R. Pal, G.Y. Moon, Pervaporation dehydration of aqueous ethanol and isopropanol mixtures through alginate/chitosan two ply composite membranes supported by poly(vinylidene fluoride) porous membrane, J. Membr. Sci. 167 (2000) 275. [2] K. Jim, P.N. Pintauro, Asymmetric PVDF hollow-fibre membranes for organic/water pervaporation separations, J. Membr. Sci. 135 (1997) 41. [3] M. Khayet, K.C. Khulbe, T. Matsuura, Characterization of membranes for membrane distillation by atomic force microscopy and estimation of their water vapor transfer coefficients in vacuum membrane distillation process, J. Membr. Sci. 238 (2004) 199. [4] J. Kong, K. Li, Oilremoval from oil-in-water emulsions using PVDF membranes, Sep. Purif. Technol. 16 (1999) 83. [5] C.R. Tavares, M. Vieira, J.C.C. Petrus, et al., Ultrafiltration/complexation proxess for metal removal from pulp and paper industry wastewater, Desalination 144 (2002) 261. [6] N.H. Lee, C.W. Frank, Separation of chiral molecules using polyprptidemodified poly(vinyldene fluoride) membranes, Polymer 43 (2002) 6255. [7] L.Q. Sun, Y. Qian, S.X. Liu, Study of polyvinylidene difloride sulfonate polysulphone co-blend ultrafiltration membrane, Membr. Sci. Tech. (China) 21 (2001) 1. [8] Z.H. Yu, Y. Qian, L. Fu, Research on the PVDF/PAN blend ultrafiltration membrane, Membr. Sci. Tech. (China) 20 (2000) 10. [9] T. Uragami, M. Fujimoto, M. Sugihara, Studies on syntheses and permeabilities of special polymer membranes. 28. Permeation characteristics and structure of interpolymer membranes from poly(vinyldene fluoride) and poly(styrene sulfonic acid), Desalination 34 (1980) 311. [10] A. Bottino, G. Capannelli, A. Comite, Preparation and characterization of novel porous PVDF-ZrO2 composite membranes, Desalination 146 (2002) 35. [11] D.S. Kim, H.B. Park, Y.M. Lee, Preparation and characterization of PVDF/Silica hybrid membranes containing sulfonic acid groups, J. Appl. Polym. Sci. 93 (2004) 209. [12] H.W. Carla, Recent advances in perfluorinated ionmomer membranes: structure, properties and applications, J. Membr. Sci. 120 (1996) 1. [13] R.S. Yeo, J. Mcbreen, G. Kissel, et al., Perfluorosulphonic acid (Nafion) membrane as a separator for an advanced alkaline water electrolyser, J. Appl. Electrochem. 10 (1980) 741. [14] Z.Q. Ma, P. Cheng, T.S. Zhao, A palladium-alloy deposited Nafion membrane for direct methanol fuel cells, J. Membr. Sci. 215 (2003) 327. [15] Z.G. Shao, X. Wang, I.M. Hsing, Composite Nafion/PVA membranes for the direct methanol fuel cell, J. Membr. Sci. 210 (2002) 147. [16] J.S. Jiang, D.B. Greenberg, J.R. Fried, Pervaporation of methanol from a triglyme solution using a Nafion membrane: 1. Transport studies, J. Membr. Sci. 132 (1997) 255. [17] M. Krishnan, J.R. White, J.R. Fox, et al., Integrated chemical systems:photocatalysis at semiconductors incorporated into polymer(Nafion)/mediator systems, J. Am. Chem. Soc. 105 (1983) 7002. [18] M.K. Song, Y.T. Kim, J.M. Fenton, Chemically modified Nafion/poly(vinylidene fluoride) blend iomers for poton exchange membrane fuel cells, J. Power Sources 117 (2003) 14. [19] M.Y. Cho, J.Y. Eom, M.Y. Jung, Characteristics of PVDF copolymer/Nafion blend membrane for direct methanol fuel cell (DMFC), Electrochim. Acta 50 (2004) 583. [20] Z.L. Xu, T.S. Chung, Y. Huang, Effect of polyvinylpyrrolidone molecular wights on morphology, oil/water separation, mechanical and thermal properties of polyetherimide/polyvinylpyrrolidone hollow fiber membranes, J. Appl. Polym. Sci. 74 (1999) 2220.
W.-Z. Lang et al. / Journal of Membrane Science 288 (2007) 123–131 [21] J. Xu, Z.L. Xu, Poly(vinyl chloride) (PVC) hollow fiber ultrafiltration membranes preparation from PVC/additives/solvent, J. Membr. Sci. 208 (2002) 203. [22] T.S. Chung, Z.L. Xu, Asymmetric hollow fiber membranes prepared from miscible polybenzimidazole and polyetherimide blends, J. Membr. Sci. 147 (1998) 35. [23] Z.L. Xu, F. Alsalhy Qusay, Polyethersulfone (PES) hollow fiber ultrafiltration membranes prepared by PES/non-solvent/NMP solution, J. Membr. Sci. 233 (2004) 101. [24] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, 1991. [25] J.S. Kang, Y.M. Lee, Effects of molecular weight of polyvinylpyrrolidone on precipitation kinetics during the formation of asymmetric polyacrylonitrile membrane, J. Appl. Polym. Sci. 85 (2002) 57. [26] J.H. Kim, K.-H. Lee, Effects of PEG additives on membrane formation by phase inversion, J. Membr. Sci. 138 (1998) 153. [27] J.S. Kang, Y.M. Lee, Effects of molecular weight of polyvinylpyrrolidone on precipitation kinetics during the formation of asymmetric polyacrylonitrile membrane, J. Appl. Polym. Sci. 85 (2002) 57–68. [28] W.R. Browen, S.Y. Cheng, T.A. Doneva, et al., Manufacture and characterization of polyetherimide/sulfonated poly(ether ether ketone) blend membranes, J. Membr. Sci. 250 (2005) 1.
131
[29] T.D. Gierke, G.E. Munn, F.C. Wilson, The morphology in Nafion perfluorinated membrane products, as determined by wide- and small-angle X-ray studies, J. Polym. Sci., Phys. Edn. 19 (1981) 1687. [30] M. Wang, L.-G. Wu, J.-X. Mo, et al., The preparation and characterization of novel charged polyacrylonitrile/PES-C blend membranes used for ultrafiltration, J. Membr. Sci. 274 (2006) 200. [31] Z.L. Xu, X.D. Zhai, G.E. Chen, Study on high-porosity poly(vinylidene fluoride) hollow fiber ultrafiltration membrane, Membr. Sci. Technol. (China) 20 (2000) 10. [32] Y. Bormashenko, R. Pogreb, O. Stanevsky, et al., Vibrational spectrum of PVDF and its interpretation, Polym. Testing 23 (2004) 791. [33] T. Boccaccio, A. Bottino, G. Capannelli, et al., Characterization of PVDF membranes by vibrational spectroscopy, J. Membr. Sci. 210 (2002) 315. [34] V. Sencadas, R. Gregorio Filho, S. Lanceros-Mendez, Processing and characterization of a novel nonporous poly(vinilidene fluoride) films in the b phase, J. Non-Cryst. Solids 352 (2006) 2226. [35] A. Gruger, A. T´egis, T. Schmatko, et al., Nanostructure of Nafion® membranes at different states of hydration An IR and Raman study., Vib. Spectrosc. 26 (2001) 215.