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Morphology and properties of hollow-fiber membrane made by PAN mixing with small amount of PVDF
Journal of Membrane Science 146 (1998) 179±184
Morphology and properties of hollow-®ber membrane made by PAN mixing with small amount of PVDF Yin Xiuli*, Cheng Hongbin, Wang Xiu, Yao Yongxin Department of Materials Science, Tianjin Institute of Textile Science and Technology, Tianjin, 300160, PR China Received 15 September 1997; received in revised form 17 March 1998; accepted 17 March 1998
Abstract In this paper, the morphological structure and properties such as, miscibility, tensile strength, ¯ux and retention ratio of hollow-®ber membranes manufactured by PAN mixing with small amounts of PVDF have been studied. The hollow ®ber was made from a spinning solution composed of polymer (PAN : PVDF10 : 0, 9 : 1, 7 : 3), additive (PVP, PEG-600) and solvent (DMAC) when immersed in water. The spinnability of blend polymer and the in¯uences of blending on spinning technology have been observed; the morphology of membranes were examined by SEM. The blend membranes possess much higher ¯ux than PAN membrane and fairly good retention ratio especially for the membrane made by PAN : PVDF9 : 1. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Morphology; Hollow ®ber; PAN/PVDF blend; Membrane
1. Introduction Blending is a useful method for modifying the properties of polymer materials, developing new types of materials and it shows great potential for application in various ®elds [1±5]. In recent years, the blend polymer membranes have been an attractive ®eld and several blend membranes have been studied, manufactured and used [6±10]. However, research results on membranes, prepared by polyacrilonitrile (PAN) blended with a small amount of polyvinylidene ¯uoride (PVDF), are so far unreported. PAN is a kind of well-known ®ber material; it is not only very cheap but also possesses good resisting-ageing property and other advantages. However, as hollow ®ber UF mem*Corresponding author. 0376-7388/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0376-7388(98)00107-0
brane material, its spinnability, ¯ux as well as acidand alkali-resistant property need to be improved. In the present study, the hollow-®ber UF membranes were manufactured by PAN blending with a small amount of PVDF and the results demonstrated that PAN and PVDF belong to partially miscible system, by adding small amount of PVDF into the spinning solution, the spinnability of PAN was greatly improved and the hollow-®ber membranes possess much higher ¯ux than pure PAN membrane and at the same time maintains a high retention ratio. 2. Experimental 2.1. Materials and method Polyacrylonitrile (M45000, from Shanghai General Petroleum Chemical Factory), polyvinylidene
180
Y. Xiuli et al. / Journal of Membrane Science 146 (1998) 179±184
Fig. 1. Schematic diagram of the measuring apparatus: (1) module; (2) pump; (3) permeate trough; (4) feed tank; (5) flow meter; (6) PCV; (7) manometer.
absorptance of permeate and original liquids, respectively. SEM photographs of the membranes were obtained on a HITACHI X-650 scanning electron microscope. The hollow ®bers were broken in liquid nitrogen and gold was used as the shadowing agent. The mechanical properties of hollow ®bers were tested by Instron Model-1121. Gage length and Xhead speed were 20 mm and 50 mm/min, respectively. Ten readings for each specimen were taken to evaluate the average value. 3. Results and discussion 3.1. The miscibility of PAN with PVDF
¯uoride ([]0.67, from Shanghai Institute of Organic Fluoric Materials), polyvinylpyrrolidone-K30 (M 30000, from Tianjin Chemical Reagent Supply Station), N,N-Dimethyl acetamide (DMAC, from Shanghai Third Reagent Factory), bovine serum albumin (BSA M67000, from Beijing Chemical Reagent Company), hollow-®ber membranes were formed from a solution composed of different composition blend polymer, additive and solvent by dry-jet wet spinning with tube-in-ori®ce type nozzles, water was used as the coagulation solution which was also injected into the hollow-®ber bores. The spun hollow ®bers were washed well by water, then immersed in 60% glycerine/water solution for ca. 8 h, and thereafter dried at room temperature. 2.2. Measurements Flux and retention ratio of membranes were measured with the apparatus shown in Fig. 1. At ®rst, the membrane was kept at a pressure of 0.2 Mpa for ca. 1 h. then pure water ¯ux was measured. The retention ratios of the membranes were tested with 0.1% BSA solution. The absorptance of original liquids and permeated liquids was obtained by 751G-ultraviolet spectrometer and the ¯ux and retention ratios were calculated from following equations: Ap Qw 100% l=m2 h; A 1 ÿ F Ar t Aol where Qw is the pure water produced, Ar the effective membrane area and t the time, Ap and Aol refer to
According to the solution theory, for a system without polar interaction and hydrogen bonding, the miscibility of two polymers is mainly governed by the solubility parameter () of two polymers. The closer the value of is for different polymers, the easier it is for them to mix and dissolve homogeneously. Actually, if |aÿb| were >0.5, making an entirely miscible solution would be dif®cult. The solubility parameter of PAN and PVDF is (PAN)14.39(Cal/cm3)1/2 and (PVDF)15.10(Cal/cm3)1/2, respectively. Judging by the value of solubility parameter, it is impossible for these polymers to be entirely miscible in any proportion. However, analyzing the structure of PAN and PVDF, we found that both PAN and PVDF are polar polymers, one containing a strong polar ±CN group, another having two F atoms; moreover, there must be speci®c interaction between dipole in PVDF and the nitrile grouping in PAN. Provided that a suitable solvent has been chosen, they should be miscible, at least, at some composition. DMAC is a polar non-proton solvent and a very good solvent for polar substances. The ±C=O group in DMAC reveals good, to a certain extent, miscibility of PAN and PVDF. The results in this study proved this judgement. Table 1 lists the results of the mixing test in this research. The results clearly show that, with DMAC as solvent, PAN and PVDF belong to a partially miscible system only when one component is in the continuous phase, in which case a stable spinning solution can be obtained. Besides, the miscibility of polymers is also dependent on both temperature and additives.
Y. Xiuli et al. / Journal of Membrane Science 146 (1998) 179±184
181
Table 1 The miscibility of PAN with PVDF
Table 2 The effect of blending on the mechanical property of hollow fibers
Temperature
Property
PAN : PVDF 10 : 0 9 : 1
7:3
3:7
1:9
0 : 10
3.1 5
2.1 25
1.1 50
1.4 83
2.3 145
PAN : PVDF
(8C)
9:1
7:3
5:5
3:7
1:9
25 40 60 80
x o o o
x x o o
x x x x
x o o o
o o
Tensile strength (Mpa) Elongation (%)
o
Note: additive: 10% PEG-600; test temperature : room temperature; polymer content 15%; spinning temperature 708C.
Note: o ± miscible; x ± divided into two layers; solvent: DMAC; additive: PEG-600 concentration 10%; polymer concentration: 15%.
3.2. Spinnability and tensile strength of hollow fiber In order to prepare useful hollow-®ber membranes, the spinnability of casting solution and the tensile strength of the ®bers are very important. PVDF possesses fairly good spinnability for making hollow ®ber and at the same time with excellent ¯exibility, but low tensile strength. The spinnability of PAN for hollow ®ber is not quite satisfactory. This is because PAN is a hydrophilic polymer, and the coagulating speed in water is much faster than most membrane materials and, after coagulation, it is fragile and very easy to break up at a high take-up speed. Of more serious concern is the fact that hollow ®ber often splits apart in the axial direction. To overcome this disadvantage, we have tried some methods, such as increasing the height of the dry section during spinning, adding solvent into the coagulating bath which slows down the precipitation speed. By using these methods, the spinning technology and ®ber property have shown improvement, but the ¯ux of such hollow-®ber membranes is very low. Nevertheless, owing to its fairly good retention ratio and high tensile strength and, especially, less expensive feature, it still appears worthwhile to develop such membranes. In the present study, we ®nd that by blending small amounts of PVDF into PAN, the precipitation rate of casting solution slows down, i.e. the take-up speed of spinning for blend ®ber decreased by one-third compared with PAN spinning and this is good for controlling spin technology, manufacturing better hollow ®bers and, what is more important, it is found that the ¯ux of the hollow-®ber membranes has increased greatly and at the same time maintained a reasonable retention ratio. However, the tensile strength of blend hollow ®ber is lower than that
2.7 19
of pure PAN ®ber as revealed by the results listed in Table 2. After application of the property test, we found that although the tensile strength of blend ®ber is lower than that of PAN ®ber, the toughness of blend ®ber is better than that of PAN ®ber; moreover, at the commonly used pressure, these hollow-®ber membranes offer no problem. In the following discussion, we will concentrate on membranes made by PAN mixing with small amounts of PVDF. 3.3. The morphology of membranes Figs. 2 and 3, show SEM photos obtained in this study. All of the hollow ®bers have sandwich structure, i.e. ®ber inner-layer, middle-layer and outerlayer pores. However, the thickness of each layer, the size and the shape of the pore change with the composition of spinning solution. The cross-sectional SEM photomicrographs of hollow ®bers formed from a solution composed of PAN : PVDF10 : 0, 9 : 1, 7 : 3, respectively; 15% polymer, 5% PVP, and 80% DMAC are presented in Fig. 2. It is clearly seen that a pure PAN membrane and that containing 10% PVDF PAN membrane have similar curved ®nger-type pore structure (Fig. 2(a) and (b)), but the pore size of blend membrane is a little bit larger than that of the PAN membrane, especially for the pores in ®ber inner-layer, whereas the middle-layer is thinner. Fig. 2(c) shows a different and irregular pore structure which was made from blend polymer of PAN : PVDF7 : 3. An additive is a pore-forming agent. Adding some sort of additive into casting solution, will change its composition and the assembled state of macromolecules and, thus, the pore shape, size and distribution. In this research, the spinning solution contained two
182
Y. Xiuli et al. / Journal of Membrane Science 146 (1998) 179±184
Fig. 2. Cross-sectional SEM photos of hollow-fiber membranes spun from solution consisting of 15% polymer, 5% PVP, 80% DMAC. (a) Pure PAN; (b) PAN : PVDF9 : 1; and (c) PAN : PVDF7 : 3.
polymers, therefore, the morphology of membranes is not only governed by additives but also by the ratio of two polymers, which is much more complicated. Nevertheless, we still ®nd that an additive has great in¯uence on the structure of a membrane, as is shown in Fig. 3. where the membranes are formed by changing the additive into 10% PEG-600 and keeping other components as well as the spinning technology constant. Pure PAN membrane has narrow crevice-like inner and outer pore layers with a thick middle layer. But the blend membranes possess typical ®nger-typed pores mixed with numerous small sponge-like pore structures, especially for the membrane of PAN : PVDF7 : 3. Strathmann [11] found a close correlation between membrane structure and precipitation rate. In general, systems with fast precipitation rates tend to form
®nger-typed structure, whereas systems with slow precipitation rate result in sponge-type structure. This phenomenological correlations seems to be valid for this system with PEG-600 as additive but not when PVP is the additive. The reason for this is not quite clear yet. The fact is, no matter which additive is being applied, the precipitation rate obviously slows down in this system, as has already been discussed in Section 3.2. However, as we can see from Fig. 2, there is no sponge-typed pore structure formed. 3.4. The properties of membranes The properties of membranes are determined by the morphological structure which, in turn, is governed by the composition of the casting solution and spinning
Y. Xiuli et al. / Journal of Membrane Science 146 (1998) 179±184
183
Fig. 3. Cross-sectional SEM photos of hollow-fiber membranes spun from solution consisting of 15% polymer, 10% PEG-600, 75% DMAC. (a) Pure PAN; (b) PAN : PVDF9 : 1; and (c) PAN : PVDF7 : 3.
technology. From the foregoing discussion, we can expect that the membranes made by this system will have good properties. The ¯ux (f) and the retention ratio (R) of membranes investigated in this study are summarized in Table 3. The results show that the hollow-®ber membranes made by PAN blending with a small amount of PVDF have greatly improved the membrane ¯ux and the retention ratio was maintained at a reasonable level, especially for the membrane formed by the polymer ratio of PAN : PVDF at 9 : 1. Besides, the blend membranes resist most chemical agents better than a pure PAN membrane; e.g. when immersed in acid, alkali, alcohol, ester, and various salt solution for 10 days the ¯ux and the retention ratio of PAN membrane decreased greatly, but the blend membranes only revealed a slight change in these properties.
Table 3 The effect of ratio of PAN/PVDF and additives on the flux (f) and retention ratio (R) Additives
PAN : PVDF f (l/m2h)
PVP (5%) PEG-600 (5%) PEG-600 (10%)
R (%)
10 : 0
9:1
7:3
10 : 0
9:1
7:3
99.4 74.5 73.3
287.7 179.4 254.8
271.6 164.5 237.3
92.6 94.6 94.5
88.9 93.5 96.6
87.2 88.1 93.6
Note: Polymer content 15%; spinning temperature 708C.
4. Conclusion 1. PAN and PVDF belong to a partially miscible system; the miscibility depends on both, the ratio of two polymers and the dissolving temperature.
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Y. Xiuli et al. / Journal of Membrane Science 146 (1998) 179±184
Only at fairly high temperature and when one component is in a continuous phase and the other in the dispersive phase that a stable casting solution can be made. 2. The morphology of membranes is governed by both, additives and the ratio of two polymers which can change the precipitation rate. Blend membranes possess better pore structure than PAN membrane, especially when the ratio of two polymers is 9 : 1. 3. By adding small amounts of PVDF into PAN, the flux of membranes increased greatly while maintaining the retention ratio nearly unchanged. The property of blend membranes to resist most chemical agents is much better than that of pure PAN membranes. 4. The results of the present research proved that blending is a prospective method for developing a new type membrane material. References [1] T.M. Malik, Thermal and mechanical characterization of partially miscible blends of poly(ether ether keton) and polyether sulfone, J. Appl. Polym. Sci. 46 (1992) 303.
[2] L.A. Utracki, P. Sammut, Rheological evaluation of polystyrene/polyethylene blends, Polym. Eng. Sci. 28 (1988) 795. [3] T.M. Malik, P.J. Carreau, N. Capleau, Characterization of liquid crystalline polyester polycarbonate blends, Polym. Eng. Sci. 29 (1989) 600. [4] L.A. Utracki, Polymer Alloys and Blends: Thermodynamics and Rheology, Hanser, Munich, 1989. [5] Yin Xiuli, Thermal behavior and mechanical property of PET/PBT blend fiber, Chinese J. Appl. Chem. 12 (1995) 111. [6] B. Bikson, J.K. Nelson, N. Muruganandam, Composite cellulose acetate/poly(methyl methacrylate) blend gas separation membranes, J. Membr. Sci. 94 (1994) 313. [7] J.S. Chiou, D.R. Paul, Sorption and transport of inert gases in PVDF/PMMA blends, J. Appl. Polym. Sci. 32 (1994) 793. [8] Wang Baoguo, Sun Hongliang, PAN/PS blend UF membrane, Techn. Water Treatment 22 (1996) 85. [9] Wu Kaifen, Li Shushen, Research on PES/PDC blend UF membrane, Membr. Sci. Techn. 12 (1992) 23. [10] Yin Xiuli, Deng Hua, Research on blend membrane of PVDF/ PS, Techn. Water Treatment 23 (1997) 131. [11] H. Strathmann, Production of microporous media by phase inversion processes. Materials Sci. of Synthetic Membrane, ACS Symp. Ser. No. 269, American Chemical Society, Washington, DC, 1985, Chap. 8.
Morphology and properties of hollow-®ber membrane made by PAN mixing with small amount of PVDF Yin Xiuli*, Cheng Hongbin, Wang Xiu, Yao Yongxin Department of Materials Science, Tianjin Institute of Textile Science and Technology, Tianjin, 300160, PR China Received 15 September 1997; received in revised form 17 March 1998; accepted 17 March 1998
Abstract In this paper, the morphological structure and properties such as, miscibility, tensile strength, ¯ux and retention ratio of hollow-®ber membranes manufactured by PAN mixing with small amounts of PVDF have been studied. The hollow ®ber was made from a spinning solution composed of polymer (PAN : PVDF10 : 0, 9 : 1, 7 : 3), additive (PVP, PEG-600) and solvent (DMAC) when immersed in water. The spinnability of blend polymer and the in¯uences of blending on spinning technology have been observed; the morphology of membranes were examined by SEM. The blend membranes possess much higher ¯ux than PAN membrane and fairly good retention ratio especially for the membrane made by PAN : PVDF9 : 1. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Morphology; Hollow ®ber; PAN/PVDF blend; Membrane
1. Introduction Blending is a useful method for modifying the properties of polymer materials, developing new types of materials and it shows great potential for application in various ®elds [1±5]. In recent years, the blend polymer membranes have been an attractive ®eld and several blend membranes have been studied, manufactured and used [6±10]. However, research results on membranes, prepared by polyacrilonitrile (PAN) blended with a small amount of polyvinylidene ¯uoride (PVDF), are so far unreported. PAN is a kind of well-known ®ber material; it is not only very cheap but also possesses good resisting-ageing property and other advantages. However, as hollow ®ber UF mem*Corresponding author. 0376-7388/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0376-7388(98)00107-0
brane material, its spinnability, ¯ux as well as acidand alkali-resistant property need to be improved. In the present study, the hollow-®ber UF membranes were manufactured by PAN blending with a small amount of PVDF and the results demonstrated that PAN and PVDF belong to partially miscible system, by adding small amount of PVDF into the spinning solution, the spinnability of PAN was greatly improved and the hollow-®ber membranes possess much higher ¯ux than pure PAN membrane and at the same time maintains a high retention ratio. 2. Experimental 2.1. Materials and method Polyacrylonitrile (M45000, from Shanghai General Petroleum Chemical Factory), polyvinylidene
180
Y. Xiuli et al. / Journal of Membrane Science 146 (1998) 179±184
Fig. 1. Schematic diagram of the measuring apparatus: (1) module; (2) pump; (3) permeate trough; (4) feed tank; (5) flow meter; (6) PCV; (7) manometer.
absorptance of permeate and original liquids, respectively. SEM photographs of the membranes were obtained on a HITACHI X-650 scanning electron microscope. The hollow ®bers were broken in liquid nitrogen and gold was used as the shadowing agent. The mechanical properties of hollow ®bers were tested by Instron Model-1121. Gage length and Xhead speed were 20 mm and 50 mm/min, respectively. Ten readings for each specimen were taken to evaluate the average value. 3. Results and discussion 3.1. The miscibility of PAN with PVDF
¯uoride ([]0.67, from Shanghai Institute of Organic Fluoric Materials), polyvinylpyrrolidone-K30 (M 30000, from Tianjin Chemical Reagent Supply Station), N,N-Dimethyl acetamide (DMAC, from Shanghai Third Reagent Factory), bovine serum albumin (BSA M67000, from Beijing Chemical Reagent Company), hollow-®ber membranes were formed from a solution composed of different composition blend polymer, additive and solvent by dry-jet wet spinning with tube-in-ori®ce type nozzles, water was used as the coagulation solution which was also injected into the hollow-®ber bores. The spun hollow ®bers were washed well by water, then immersed in 60% glycerine/water solution for ca. 8 h, and thereafter dried at room temperature. 2.2. Measurements Flux and retention ratio of membranes were measured with the apparatus shown in Fig. 1. At ®rst, the membrane was kept at a pressure of 0.2 Mpa for ca. 1 h. then pure water ¯ux was measured. The retention ratios of the membranes were tested with 0.1% BSA solution. The absorptance of original liquids and permeated liquids was obtained by 751G-ultraviolet spectrometer and the ¯ux and retention ratios were calculated from following equations: Ap Qw 100% l=m2 h; A 1 ÿ F Ar t Aol where Qw is the pure water produced, Ar the effective membrane area and t the time, Ap and Aol refer to
According to the solution theory, for a system without polar interaction and hydrogen bonding, the miscibility of two polymers is mainly governed by the solubility parameter () of two polymers. The closer the value of is for different polymers, the easier it is for them to mix and dissolve homogeneously. Actually, if |aÿb| were >0.5, making an entirely miscible solution would be dif®cult. The solubility parameter of PAN and PVDF is (PAN)14.39(Cal/cm3)1/2 and (PVDF)15.10(Cal/cm3)1/2, respectively. Judging by the value of solubility parameter, it is impossible for these polymers to be entirely miscible in any proportion. However, analyzing the structure of PAN and PVDF, we found that both PAN and PVDF are polar polymers, one containing a strong polar ±CN group, another having two F atoms; moreover, there must be speci®c interaction between dipole in PVDF and the nitrile grouping in PAN. Provided that a suitable solvent has been chosen, they should be miscible, at least, at some composition. DMAC is a polar non-proton solvent and a very good solvent for polar substances. The ±C=O group in DMAC reveals good, to a certain extent, miscibility of PAN and PVDF. The results in this study proved this judgement. Table 1 lists the results of the mixing test in this research. The results clearly show that, with DMAC as solvent, PAN and PVDF belong to a partially miscible system only when one component is in the continuous phase, in which case a stable spinning solution can be obtained. Besides, the miscibility of polymers is also dependent on both temperature and additives.
Y. Xiuli et al. / Journal of Membrane Science 146 (1998) 179±184
181
Table 1 The miscibility of PAN with PVDF
Table 2 The effect of blending on the mechanical property of hollow fibers
Temperature
Property
PAN : PVDF 10 : 0 9 : 1
7:3
3:7
1:9
0 : 10
3.1 5
2.1 25
1.1 50
1.4 83
2.3 145
PAN : PVDF
(8C)
9:1
7:3
5:5
3:7
1:9
25 40 60 80
x o o o
x x o o
x x x x
x o o o
o o
Tensile strength (Mpa) Elongation (%)
o
Note: additive: 10% PEG-600; test temperature : room temperature; polymer content 15%; spinning temperature 708C.
Note: o ± miscible; x ± divided into two layers; solvent: DMAC; additive: PEG-600 concentration 10%; polymer concentration: 15%.
3.2. Spinnability and tensile strength of hollow fiber In order to prepare useful hollow-®ber membranes, the spinnability of casting solution and the tensile strength of the ®bers are very important. PVDF possesses fairly good spinnability for making hollow ®ber and at the same time with excellent ¯exibility, but low tensile strength. The spinnability of PAN for hollow ®ber is not quite satisfactory. This is because PAN is a hydrophilic polymer, and the coagulating speed in water is much faster than most membrane materials and, after coagulation, it is fragile and very easy to break up at a high take-up speed. Of more serious concern is the fact that hollow ®ber often splits apart in the axial direction. To overcome this disadvantage, we have tried some methods, such as increasing the height of the dry section during spinning, adding solvent into the coagulating bath which slows down the precipitation speed. By using these methods, the spinning technology and ®ber property have shown improvement, but the ¯ux of such hollow-®ber membranes is very low. Nevertheless, owing to its fairly good retention ratio and high tensile strength and, especially, less expensive feature, it still appears worthwhile to develop such membranes. In the present study, we ®nd that by blending small amounts of PVDF into PAN, the precipitation rate of casting solution slows down, i.e. the take-up speed of spinning for blend ®ber decreased by one-third compared with PAN spinning and this is good for controlling spin technology, manufacturing better hollow ®bers and, what is more important, it is found that the ¯ux of the hollow-®ber membranes has increased greatly and at the same time maintained a reasonable retention ratio. However, the tensile strength of blend hollow ®ber is lower than that
2.7 19
of pure PAN ®ber as revealed by the results listed in Table 2. After application of the property test, we found that although the tensile strength of blend ®ber is lower than that of PAN ®ber, the toughness of blend ®ber is better than that of PAN ®ber; moreover, at the commonly used pressure, these hollow-®ber membranes offer no problem. In the following discussion, we will concentrate on membranes made by PAN mixing with small amounts of PVDF. 3.3. The morphology of membranes Figs. 2 and 3, show SEM photos obtained in this study. All of the hollow ®bers have sandwich structure, i.e. ®ber inner-layer, middle-layer and outerlayer pores. However, the thickness of each layer, the size and the shape of the pore change with the composition of spinning solution. The cross-sectional SEM photomicrographs of hollow ®bers formed from a solution composed of PAN : PVDF10 : 0, 9 : 1, 7 : 3, respectively; 15% polymer, 5% PVP, and 80% DMAC are presented in Fig. 2. It is clearly seen that a pure PAN membrane and that containing 10% PVDF PAN membrane have similar curved ®nger-type pore structure (Fig. 2(a) and (b)), but the pore size of blend membrane is a little bit larger than that of the PAN membrane, especially for the pores in ®ber inner-layer, whereas the middle-layer is thinner. Fig. 2(c) shows a different and irregular pore structure which was made from blend polymer of PAN : PVDF7 : 3. An additive is a pore-forming agent. Adding some sort of additive into casting solution, will change its composition and the assembled state of macromolecules and, thus, the pore shape, size and distribution. In this research, the spinning solution contained two
182
Y. Xiuli et al. / Journal of Membrane Science 146 (1998) 179±184
Fig. 2. Cross-sectional SEM photos of hollow-fiber membranes spun from solution consisting of 15% polymer, 5% PVP, 80% DMAC. (a) Pure PAN; (b) PAN : PVDF9 : 1; and (c) PAN : PVDF7 : 3.
polymers, therefore, the morphology of membranes is not only governed by additives but also by the ratio of two polymers, which is much more complicated. Nevertheless, we still ®nd that an additive has great in¯uence on the structure of a membrane, as is shown in Fig. 3. where the membranes are formed by changing the additive into 10% PEG-600 and keeping other components as well as the spinning technology constant. Pure PAN membrane has narrow crevice-like inner and outer pore layers with a thick middle layer. But the blend membranes possess typical ®nger-typed pores mixed with numerous small sponge-like pore structures, especially for the membrane of PAN : PVDF7 : 3. Strathmann [11] found a close correlation between membrane structure and precipitation rate. In general, systems with fast precipitation rates tend to form
®nger-typed structure, whereas systems with slow precipitation rate result in sponge-type structure. This phenomenological correlations seems to be valid for this system with PEG-600 as additive but not when PVP is the additive. The reason for this is not quite clear yet. The fact is, no matter which additive is being applied, the precipitation rate obviously slows down in this system, as has already been discussed in Section 3.2. However, as we can see from Fig. 2, there is no sponge-typed pore structure formed. 3.4. The properties of membranes The properties of membranes are determined by the morphological structure which, in turn, is governed by the composition of the casting solution and spinning
Y. Xiuli et al. / Journal of Membrane Science 146 (1998) 179±184
183
Fig. 3. Cross-sectional SEM photos of hollow-fiber membranes spun from solution consisting of 15% polymer, 10% PEG-600, 75% DMAC. (a) Pure PAN; (b) PAN : PVDF9 : 1; and (c) PAN : PVDF7 : 3.
technology. From the foregoing discussion, we can expect that the membranes made by this system will have good properties. The ¯ux (f) and the retention ratio (R) of membranes investigated in this study are summarized in Table 3. The results show that the hollow-®ber membranes made by PAN blending with a small amount of PVDF have greatly improved the membrane ¯ux and the retention ratio was maintained at a reasonable level, especially for the membrane formed by the polymer ratio of PAN : PVDF at 9 : 1. Besides, the blend membranes resist most chemical agents better than a pure PAN membrane; e.g. when immersed in acid, alkali, alcohol, ester, and various salt solution for 10 days the ¯ux and the retention ratio of PAN membrane decreased greatly, but the blend membranes only revealed a slight change in these properties.
Table 3 The effect of ratio of PAN/PVDF and additives on the flux (f) and retention ratio (R) Additives
PAN : PVDF f (l/m2h)
PVP (5%) PEG-600 (5%) PEG-600 (10%)
R (%)
10 : 0
9:1
7:3
10 : 0
9:1
7:3
99.4 74.5 73.3
287.7 179.4 254.8
271.6 164.5 237.3
92.6 94.6 94.5
88.9 93.5 96.6
87.2 88.1 93.6
Note: Polymer content 15%; spinning temperature 708C.
4. Conclusion 1. PAN and PVDF belong to a partially miscible system; the miscibility depends on both, the ratio of two polymers and the dissolving temperature.
184
Y. Xiuli et al. / Journal of Membrane Science 146 (1998) 179±184
Only at fairly high temperature and when one component is in a continuous phase and the other in the dispersive phase that a stable casting solution can be made. 2. The morphology of membranes is governed by both, additives and the ratio of two polymers which can change the precipitation rate. Blend membranes possess better pore structure than PAN membrane, especially when the ratio of two polymers is 9 : 1. 3. By adding small amounts of PVDF into PAN, the flux of membranes increased greatly while maintaining the retention ratio nearly unchanged. The property of blend membranes to resist most chemical agents is much better than that of pure PAN membranes. 4. The results of the present research proved that blending is a prospective method for developing a new type membrane material. References [1] T.M. Malik, Thermal and mechanical characterization of partially miscible blends of poly(ether ether keton) and polyether sulfone, J. Appl. Polym. Sci. 46 (1992) 303.
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