Characterization of membranes prepared from PVDF/PAN blends and their modification with hydrolysis

Abstract

In this study, the melt-spinning and stretching processes were used to prepare polyvinylidene fluoride (PVDF)/polyacrylonitrile (PAN) blend hollow-fiber membranes. These blend membranes have a higher tensile strength than those prepared via the wet phase inversion method. The influence of stretching temperature and draw ratios on the membrane microstructure, pure water flux, and porosity of the membranes was studied. The draw ratio and the stretching temperature can greatly affect the morphology and the permeation performance of the membranes. The Fourier transform infrared spectra demonstrate the differences in the PVDF/PAN blend membranes before and after hydrolysis. The water flux of the hydrolyzed PVDF/PAN blend membranes is sensitive to pH value and salt ionic strength. The effects of different salt ions on the permeability of the hydrolyzed membranes were also investigated.

Keywords

PVDF/PAN blending melt spinning hydrolysis hollow-fiber membrane

Introduction

Based on the lessons from nature, scientists have been designing innovative polymer systems that are environmentally responsive to stimuli in some way. These polymers can respond to external chemical or physical stimuli, such as changes in pH,¹ ^–3 ionic strength,⁴ ^,5 temperature,⁶ and so on. Because of a curious property, smart membranes attract considerable attention on applications in a variety of fields. In the cited researches, pH-sensitive membranes were prepared by grafting or coating the pH-sensitive polyelectrolytes into microporous membranes. The membrane structure was found to be strongly dependent on the dissociation degree of the functional groups of the polyelectrolyte anchored in the membrane pores. When the functional groups were uncharged, the polymer chain coiled down, resulting in the ‘opening’ of the membrane pores. Once the functional groups dissociated, the polymer chain extended, thus filling the pores more uniformly, resulting in the ‘closing’ of the pore.² Then the water flux of the membranes increased or decreased as the pH changed.

Polyvinylidene fluoride (PVDF) and polyacrylonitrile (PAN) are sorts of macromolecule materials with superior properties, and thus have been widely used as separation membrane materials. With the development of membrane applications, multiplicative applied situations need to meet different requirements; thus, obtaining a membrane with diverse properties becomes necessary. PAN is the raw material of commercial acrylic fiber, the polyacrylic acid (PAA)/polyacrylamide (PAAM) chemical structure of which is easy to obtain by alkaline hydrolysis. PAA is one of the most familiar pH-sensitive polyelectrolytes for smart membranes. For instance, Song et al.⁷ prepared a PVDF–PAA membrane through a thermally induced graft polymerization technique. Ying et al.⁸ studied the temperature-sensitive microfiltration membranes that were prepared from the PVDF-g-PNIPAAM copolymers (PNIPAAM: poly(N-isopropylacrylamide)). Yu and Gu⁹ prepared a smart hydrogel fiber based on the hydrolysis of PAN-blend-gelatin, which was in response to pH change. Zhang et al.¹⁰ studied the hydrolysis differences of PAN using different alkaline species. Kobayashi et al.¹¹ studied four types of PAN-charged membranes and found that a compact conformation of the amphiphilic polyionic segments in the membrane causes stable filtration properties. Yang and Tong¹² also studied the ultrafiltration of proteins using a hydrolyzed PAN hollow fiber. A sharp decrease in hydraulic permeability was observed between pH 5 and 6, possibly owing to the swelling of the hydrolyzed layer. Therefore, as the pH changed, the variation in the membrane structure played the key role in determining membrane performance.

The blending of two polymers, one to give the membrane sufficient chemical and thermal stability and the other to provide the membrane with environment-sensitive properties, is an alternative approach for the preparation of smart membranes.¹³ It is well known, however, that hydrolysis of the PAN membrane would lead to a reduction in mechanical strength.¹⁴ Hence, by blending PVDF with PAN, better membrane performance would hopefully be achieved.

However, nowadays, the most popularly used ultafiltration and micofiltration hollow-fiber membranes that are composed of skin layer and support layer are usually prepared via the immersion–precipitation method.¹⁵ In previous researches,¹⁶ PVDF/PAN blend membranes were prepared via the wet phase inversion technique commonly due to the easy cyclization of PAN at elevated temperature. Normally, these hollow-fiber membranes have good permeability but low mechanical endurance. PVDF hollow-fiber membranes made by the melt-spinning and stretching process have been studied extensively.¹⁷ The melt-spinning and stretching process does not involve large amounts of organic solvents and endows the membranes with high mechanical properties.¹⁸ However, the PVDF/PAN hollow-fiber membranes made by the melt-spinning and stretching process have seldom been reported.

The melt-spinning and stretching process was used to prepare PVDF/PAN blend hollow-fiber membrane in this study; PVDF and PAN as the polymeric materials, dimethyl sulfoxide (DMSO) as the solvent, and a composite powder as a pore-creating agent were used. Such modification was expected to cause a wide-ranging application of the blend membranes, although the mechanical strength of the blend membranes would be poor after hydrolysis. The membrane was then hydrolyzed by immersing into the alkaline solution. The hydrolyzed PVDF/PAN blend membrane demonstrated properties of pH and ion sensitivity.

Experimental section

Materials and method

PVDF (W#1300 powder) was obtained from Kurehachemical Industrial Co. Ltd (Tokyo, Japan), and PAN (molecular weight of 50,000) was purchased from the Qilu Petrochemical acrylic factory (Shandong, China). DMSO (>99%) was obtained from the Institute of Membrane Science and Technique, Tianjin Polytechnic University, China. The pore-forming agent (a mixture of nanosized potassium chloride (KCl) and silica (SiO₂) particles) was obtained from Tianjin Motian Membrane Engineering and Technology Co. Ltd (Tianjin, China).

The PVDF, PAN, DMSO, and pore-forming agent were frozen and mixed to obtain the PVDF/PAN blends. The mixture was spun into hollow fibers via the melt-spinning method by a twin-screw spinning machine. Figure 1 shows the melt-spinning apparatus. Table 1 shows the spinning conditions. The membranes were stretched with different ratios in water at about 80°C and 100°C. Some of the stretched membranes were hydrolyzed by immersion in the aqueous solution of sodium hydroxide (NaOH). The suitable hydrolysis condition was at the concentration of aqueous NaOH solution of 8% and 50°C for 1 h. After being hydrolyzed for a predetermined period, the membranes were taken out and rinsed with distilled water until the pH value of the rinsed water reached 7.0.

Figure 1.

The scheme of the melt-spinning apparatus.

Table 1.

Spinning parameters of PVDF/PAN hollow-fiber membranes.

	PVDF/PAN hollow-fiber membranes
Composition (wt/wt)	PVDF/PAN/DMSO/pore forming agent: 42.5%/7.5%/17.5%/32.5%
Dope solution temperature (°C)	140°C
Bore fluid	N₂
Extra coagulation	Water, 25 ± 5°C
Air gap (cm)	4
Posttreatment	24 h stored in tap water

DMSO: dimethyl sulphoxide; PVDF/PAN: polyvinylidene fluoride/polyacrylonitrile.

Characterization of membrane

Morphology examination

The morphology of the membranes was observed using field emission scanning electron microscope (SEM; Quanta 800, FEI, the Netherlands). The samples were frozen in liquid N₂, followed by fracturing to expose their cross-sectional areas. Thereafter, they were sputtered with gold and recorded by SEM.

Pure water flux

The pure water flux of the membranes was determined by equation (1). The pressure difference across the membrane was 0.1 MPa.

J = \frac{V}{S \times t}

where J is the pure water flux (in liters per square meter per hour (L m⁻² h⁻¹)), V is the quantity of the permeate (in liter (L)), S is the membrane area (in square meter (m²)), and t is the testing time (in hour (h)).

Porosity determination

Membrane porosity (∊) was defined as the pore volume divided by the total volume of the porous membrane. It can be expressed by equation (2)

ϵ = \frac{W_{1} - W_{2}}{V \times ρ}

where W ₁ is the wet membrane weight, W ₂ is the dry membrane weight, ρ is the water density, and V is the polymer volume.

FTIR characterization

Fourier transform infrared (FTIR) spectroscopic measurements were carried out on a Bruker Tensor37 spectrophotometer (Germany). Scans were 32 signals averaged at a resolution of 4 cm⁻¹ from 4000 cm⁻¹ to 600 cm⁻¹.

Tensile strength

The tensile strength of the wet membranes was measured with a tensile tester (YG061-1500, China) with a cross-head speed of 100 mm min⁻¹ at room temperature. Four runs were performed for each specimen.

Results and discussion

Infrared spectral analysis

Figure 2 compares the FTIR spectra of PVDF/PAN blend membranes before and after the hydrolysis of NaOH. The shoulder at around 2243 cm⁻¹ is due to the –CN groups, which is the characteristic absorption of PAN and appears in each spectrum.¹⁹ The band near 1400 cm⁻¹ is assigned to the distortion libration apex of the –CF₂ groups in PVDF, and the 1180 cm⁻¹ band is the flexing libration apex of the –CF₂ groups in PVDF.²⁰ Compared with the spectrum of PVDF, there are few discernible chemical bond changes in PVDF in the blend membranes. The interaction between PVDF and PAN is purely physical. Several different absorption bonds appear on the spectra of the hydrolyzed membranes. Obviously, one is the broad absorption band at around 3300 cm⁻¹, which can be assigned to the stretching vibration of the –OH groups; another is at 1642 cm⁻¹, which is due to the absorption apex of the NH₂ groups in the –CONH₂ groups. In the complete hydrolysis reaction, the hydrolysis degree was only 70%, which showed that 70% of the –CN groups converted into –COO⁻ groups and the remaining 30% of the –CN groups all converted into amide groups.²¹ However, hydrolysis of the cyano group does occur, and the absorption peak of the –C=O groups seems inconspicuous, owing to the low content of PAN in the blend membranes and partial hydrolysis of the –CN groups.

Figure 2.

FTIR spectra of PVDF/PAN blend membranes hydrolyzed by alkaline solution (a) before hydrolysis and (b) after hydrolysis. FTIR: Fourier transform infrared; PVDF/PAN: polyvinylidene fluoride/polyacrylonitrile.

The morphologies of membranes

Figure 3 shows the cross-section morphology of the original PVDF/PAN blend hollow-fiber membrane. In Figure 3(a), the structure of the PVDF/PAN blend membranes is symmetrical and has no obvious skin-core layer. Moreover, macrovoid-free and sponge-like structures can obviously be observed as the magnification is increased. The sponge-like membrane pores are distributed evenly. The pore-forming agent acted as a physical barrier to eliminate contact between polymers and pores created after the KCl was dissolved.²² The high magnification of the SEM in Figure 3(b) shows that the connectivity of the membrane pores is not good, resulting in the poor permeability of the membranes.

Figure 3.

Morphology of cross-section of the original PVDF/PAN membranes ((a) ×300; (b) ×2400). PVDF/PAN: polyvinylidene fluoride/polyacrylonitrile.

Figure 4 shows the inner-side surfaces of PVDF/PAN blend membranes with different draw ratios. Before stretching, the original membrane had a compact conformation, which resulted in a small porosity and few effective micropores. After stretching, the size and the quantity of micropores increased, as clearly seen from Figure 4(b) and (c), respectively. The high-magnification SEM images show so many effective micropores, which increase the permeability of the blend membranes. Compared with Figure 4(d) and (e), with the increase in the stretching temperature, the size and the quantity of effective micropores decrease conversely. The micropores tend to close at the higher stretching temperature, as shown in Figure 4(e).

Figure 4.

Morphology of the inner-side surface of the membranes ((a): original membrane, ×300; (b): 80°C, membrane with two draw ratios, ×300; (c): 100°C, membrane with two draw ratios, ×300; (d): 80°C, membrane with two draw ratios, ×2400; (e): 100°C, membrane with two draw ratios, ×2400).

Influence of stretching temperature

Figures 5 and 6 show the water flux and the porosity of the membrane, respectively, with different draw ratios at 80°C and 100°C. There is such a marked water flux increase in the stretched membrane as lots of micropores are created during the stretching process. The phase separation of the interface between PVDF and PAN easily happens owing to its partial miscibility characterization, which can lead to stress concentration during the stretching process. Thus, the effective micropores are opening.²³ The permeability of the blend membranes is significantly increased. As the draw ratio increases, the water flux becomes higher. Figure 5 shows that the water flux of the membranes stretched at 80°C is higher than that at 100°C in the same draw ratio. In addition, the porosity of the membranes increases after stretching. There seems to be a drastic change in the permeability of the membranes stretched at 80°C and at 100°C. The stretching temperature of membranes at 100°C is close to the glass transition temperature (T _g) of PAN (T _g is about 92–104°C²⁴). The macromolecular chains of polymers are easy to move at a high temperature and the pore size is smaller. The stability of the pore structure becomes worse and easily deformed. Thus, the water flux is decreased.

Figure 5.

Water flux of the membrane curves with draw ratio at different temperatures.

Figure 6.

Porosity of the membranes curves with draw ratio at different temperatures.

Influence of hydrolysis

Figures 7 and 8 show the water flux and the porosity of the membranes, respectively, with different draw ratios drawn at 100°C before and after hydrolysis. As shown in chemical equation (3), the hydrolysis of the membrane with alkaline solution is based on the conversion of the –CN groups of the PAN surface into –CONH₂, then into the –COO⁻ groups.¹⁰

Figure 7.

Water flux of the hydrolyzed membranes curves with different draw ratios at 100°C.

Figure 8.

Porosity of the hydrolyzed membrane curves with different draw ratios at 100°C.

The tensile strength of the fiber is very important to prepare useful hollow-fiber membranes. Table 2 shows the characterization of the original membranes. The outer and the inner diameters of the PVDF/PAN blend hollow-fiber membranes have a little change after hydrolysis. The PVDF/PAN blend membranes prepared by the melt-spinning method have higher mechanical properties than those prepared using the wet phase inversion method. The tensile strength of the PVDF/PAN blend membranes prepared by the latter method is almost 1.13–4.63 MPa.²⁵ The tensile strength of blend membranes has a slight decrease after hydrolysis. The PVDF/PAN blend membranes were obtained by extruding PVDF/PAN mixtures from a twin-screw extruder. PAN as the dispersed phase could form microfibrils in the PVDF matrix. The chemical structure of PAN polymer molecules undergoes a great change after hydrolysis and leads to some defects in the PVDF/PAN blend membranes. As shown in Table 2, the tensile strengths are 29.83 MPa and 26.85 MPa before and after hydrolysis, respectively. After the tensile property test, these hollow-fiber membranes offer no problem in the commonly used pressure.

Table 2.

Characterization of PVDF/PAN hollow-fiber membranes.

PVDF/PAN (original membranes)	Before hydrolysis	After hydrolysis
OD (ID/mm)	2.11 ± 0.08	2.04 ± 0.02
	1.33 ± 0.05	1.22 ± 0.06
Wall thickness (mm)	0.43 ± 0.01	0.42 ± 0.03
Tensile strength (MPa)	29.83 ± 1.51	26.85 ± 1.27

OD: outer diameter; ID: inner diameter; PVDF/PAN: polyvinylidene fluoride/polyacrylonitrile.

By comparison, the water flux and the porosity of the membrane stretched in water at 100°C and hydrolyzed with the alkali solution were determined, as shown in Figures 7 and 8. Here, it can be seen that the water flux and the porosity of the hydrolyzed membranes have declined so much. These changes are possibly attributed to the swelling of the hydrolyzed layer in the aqueous medium, owing to the hydrolysis reaction of the –CN groups to be the –COO– groups.¹² This process decreases the membrane pore size and porosity, so the water flux becomes lower.

Influence of salt ions on hydrolysis membrane performance

Figure 9 shows that the water flux of the hydrolyzed blend membrane (two draw ratios at 80°C) changes with various concentrations of sodium chloride (NaCl) solution at different pH values. When in acid solution, the water flux first increases, and then decreases as the concentration of the NaCl solution increases. However, the results turn out to be contrary to what happened in the alkaline solution. This phenomenon could be justified by considering two sides of a reversible chemical equation. In acidic solution, the chemical reaction is shown in the equation (4)²⁶

\begin{aligned} {[- {C O O}^{-}]}_{g e l} + {[{N a}^{-}]}_{g e l} + [H^{+}] + [{C l}^{-}] \to \\ {[- {C O O H}^{-}]}_{g e l} + [{N a}^{+}] + [{C l}^{-}] \end{aligned}

The pKa of PAA is 4.28. When the pH is less than the pKa, the H⁺ ionic strength is very high.²⁷ In acidic conditions as pH value at 2.0, the carboxylic acid groups practically cannot be dissociated, and the addition of a little NaCl in the solution could promote the reverse chemical equation (4), which can increase the fixed charges between the polymers. Thus, the electrostatic repulsion between macromolecules becomes larger, the pore size of the membrane becomes smaller, and then the water flux declines. Afterward, the water flux increases with the increase in the NaCl content. The higher concentration of NaCl solution can form the ion atmosphere around the polyelectrolyte that may cause shielding effects on the electrostatic repulsion between macromolecules;²⁸ this causes the size of the membrane pores to undergo bigger changes and then the water flux to increase.

Figure 9.

Water flux of hydrolyzed membranes with different concentrations of NaCl at different pH values solution.

In alkaline solution, the chemical reaction is shown in the equation (5)²⁶

\begin{aligned} {[- C O O H]}_{g e l} + [{N a}^{+}] + [{O H}^{-}] \to \\ {[- {C O O}^{-}]}_{g e l} + {[{N a}^{+}]}_{g e l} + H_{2} O \end{aligned}

When the pH value of the solution comes to 12, the hydrolyzed PVDF/PAN membrane can be interpreted as the polymer cations exchange membrane under fixed pressure, and the fixed ion exchange groups are –COO⁻. Polycarboxylic groups attract cations into the hydrogel to replace the H⁺ ion as the pH rises above its pKa. Generally, such a membrane phenomenon may be attributed to two factors.

First, the pressure difference and temperature under the test conditions remain constant; the water permeation is proportional to the osmotic pressure difference of free ions across the membrane. However, the water permeation reaches maximum and minimum values as the concentration of the filtration solution changes. This phenomenon is known as abnormal osmosis.²⁸ In the dilute solution, the osmotic coefficient of the electrolyte is exceedingly small, so the water permeation is proportional to the concentration of the electrolyte. The electrolyte permeation increases with the increase in the concentration of the solution. When the osmotic coefficient of the electrolyte increases sharply, it cannot be ignored; therefore, water permeation is decreased.

Second, in the dilute solution, the fixed charges of the –COO⁻ groups have strong repulsion to the –OH groups in accordance with the Donnan equilibrium theory. That is, the interface between the ion-exchange membrane and the electrolyte solution is considered to act as a shell, which can only transmit the solvent and a part of ion. This shell²⁹ prevents the ionization tendency of the –COO⁻ groups. Thus, a small amount of the NaCl in the solution is less affected on chemical equation (5), and the free ion atmosphere has a strong shielding effect on the electrostatic repulsion between macromolecules. Afterward, the swelling behavior of the PAA hydrogel becomes weaker, and the membrane pore size and the water flux increase. Then, the water flux curve shifts toward the low number side with the addition of NaCl. With the increase in NaCl concentration, the osmotic pressure difference of free ion across the shell becomes bigger, which accelerates the Na⁺ ion into the hydrogel, which enhances the positive chemical equation (4). Moreover, this promotion effect is greater than its shielding effect. Thus, the electrostatic repulsion between macromolecules increases according to the fixed charges between polymers. The water flux also comes to the low line.

As shown in Figure 10, the water flux of the hydrolyzed blend membrane (two draw ratios at 80°C) decreases with the addition of different salts in the solution at pH 2.0. The tendency is due to the reverse chemical equation (3), which is effectively improved by the Na⁺, Ca²⁺, and Fe³⁺ cations. Different salts have various contributions to the water flux, as more distinctly exhibited in Figure 11. Compared with the pure water filtrate, the order of the water flux of different salts in the same concentration is NaCl > CaCl₂ > FeCl₃. According to the adsorption effect, the multivalent ion could be much more close to the –COO⁻ groups than the monovalent ion could be because of the greater charge density of multivalent ion, which is named as charge selectivity. Ions such as Na⁺, Ca²⁺, and Fe³⁺ all exist around the polyelectrolyte, and the adsorption capacity of the polyelectrolyte maintains a certain relationship to the concentration of these ions in the solution. Nevertheless, in a strong acidic condition, the dissociation groups in the polymer are reduced. Therefore, the adsorption and complexation effects have limited influence on the membrane pore size.

Figure 10.

Water flux curve with different filtrates at pH value 2.0.

Figure 11.

The differences of water flux compared with pure water.

Obviously, the Ca²⁺ and Fe³⁺ ions possess more charges than the Na⁺ ion, a situation that enlarges the shielding effect, which causes the swollen effect to decrease and the water flux to increase.

Conclusion

The PVDF/PAN hollow-fiber blend membranes prepared by the melt-spinning and stretching process possess high mechanical properties, which were slightly decreased after hydrolysis. The permeability of the membranes increased significantly after the membranes were stretched, and the water flux of the membranes stretched at 100°C was higher than that of the membranes stretched at 80°C with in the same draw ratio. The PVDF/PAN membranes possess more porous membrane structure and higher hydraulic permeability; however, a high stretching temperature could result in the deformation and closing of the membrane pores, which has been observed from the SEM micrographs.

After stretching, the membranes were hydrolyzed using the NaOH solution in this study. The water flux of the hydrolyzed PVDF/PAN blend membranes is sensitive to different pH values and salt ionic strength. In case of the acid solution, the water flux first increases, then it decreases as the concentration of the NaCl solution rises. Nevertheless, the results turned out to be contrary to that in the alkaline solution. Compared with the pure water filtrate, the order of the water flux of different salts in the same concentration is NaCl > CaCl₂ > FeCl₃.

Footnotes

Funding

This work was financially supported by the National Basic Research Development Program of China (973 Program, 2012CB722706), the National Natural Science Foundation of China (51073120), the Science and Technology Plans of Tianjin (10SYSYJC27900), and the Basic Research Program of China National Textile and Apparel Council. The authors have also been helped by rights of the National Natural Science Foundation of China (21274109).

References

Zhou

Fan

Chen

Yang

Yuan

. Separation of NaCl, glycin from collagen solution by the thermal sensitive polyurethane membrane. Desalination 2009; 249: 843–849.

Dickson

. Modelling of the pore structure variation with pH for pore-filled pH-sensitive poly(vinylidene fluoride)–poly(acrylic acid) membranes. J Membr Sci 2008; 321: 162–171.

Mathias

. Photograft-polymer-modified microporous membranes with environment-sensitive permeabilities. React Funct Polym 1996; 31: 165–177.

Lebrun

. Investigation of the solute separation by charged nanofiltration membrane: effect of pH, ionic strength and solute type. J Membr Sci 1999; 158: 93–104.

Hegazy EI-Sayed

Kamal

Khalifa

Mahmoud

GhA.

Ion-exchange membranes for metal ions separation. Iran Polym J 1999; 8: 223–230.

Petrov

Ivanova

Christova

Ivanova

. Modification of polyacrylonitrile membranes with temperature sensitive poly(vinylalcohol-co-vinylacetal). J Membr Sci 2005; 261: 1–6.

Song

Zhang

Song

Gao

. Preparation and characterization of the modified polyvinylidene fluoride (PVDF) hollow fibre microfiltration membrane. J Mater Sci Technol 2007; 23: 55–60.

Ying

Kang

Neoh

Kato

Iwata

. Drug permeation through temperature-sensitive membranes prepared from poly(vinylidene fluoride) with grafted poly(N-isopropylacrylamide) chains. J Membr Sci 2004; 243: 253–262.

. Effects of microstructure, crosslinking density, temperature and exterior load on dynamic pH-response of hydrolyzed polyacrylonitrile-blend-gelatin hydrogel fibers. Eur Polym J 2009; 45: 1706–1715.

10.

Zhang

Meng

. Hydrolysis differences of polyacrylonitrile support membrane and its influences on polyacrylonitrile-based membrane performance. Desalination 2009; 242: 313–324.

11.

Kobayashi

Nagai

Wang

Fujii

. Charged ultrafiltration membranes of polyacrylonitrile covalently binding amphiphilic quaternary ammonium group: effect of aliphatic hydrophobic groups on the filtration properties. J Membr Sci 1996; 112: 219–228.

12.

Yang

Tong

. Loose ultrafiltration of proteins using hydrolyzed polyacrylonitrile hollow fiber. J Membr Sci 1997; 132: 63–71.

13.

Ying

Kang

Neoh

. Characterization of membranes prepared from blends of poly(acrylic acid)-graft-poly(vinylidene fluoride) with poly(N-isopropylacrylamide) and their temperature- and pH-sensitive microfiltration. J Membr Sci 2003; 224: 93–106.

14.

Vega

Morris

D’Accorso

. PAN chemical modification: synthesis and characterization of terpolymers with 1,2,4-oxadiazolic pendant groups. React Funct Polym 2006; 66: 1609–1618.

15.

Liu

Xie

Wang

. Preparation of PET threads reinforced PVDF hollow fiber membrane. Desalination 2009; 249: 453–457.

16.

Zhang

Song

Liu

. Single-side hydrolysis of hollow fiber polyacrylonitrile membrane by an interfacial hydrolysis of a solvent-impregnated membrane. J Membr Sci 2010; 350: 211–216.

17.

Samon

Schultz

Hsiao

Seifert

Stribeck

Gurke

. Structure development during the melt spinning of polyethylene and poly(vinylidene fluoride) fibers by in situ synchrotron small- and wide-angle X-ray scattering techniques. Macromolecules 1999; 32: 8121–8132.

18.

Zhu

. Structure formation and characterization of PVDF hollow fiber membranes by melt-spinning and stretching method. J Appl Polym Sci 2007; 106: 1793–1799.

19.

Yin

Cheng

Wang

Yao

. Morphology and properties of hollow-fiber membrane made by PAN mixing with small amount of PVDF. J Membr Sci 1998; 146: 179–184.

20.

Awanis Hashim

Liu

. Stability of PVDF hollow fibre membranes in sodium hydroxide aqueous solution. Chem Eng Sci 2011; 66: 1565–1575.

21.

Yan

Sun

. Preparation and characterization of pH-sensitive hydrogel fibers based on hydrolyzed-polyacrylonitrile/soy protein. J Appl Polym Sci 2008; 108: 1100–1108.

22.

Huang

Xiao

. Fabrication and properties of poly(tetrafluoroethylene-co-hexafluoropropylene) hollow fiber membranes. J Mater Chem 2011; 21: 16510–16516.

23.

Chen

Liang

Xiao

. Preparation of pressure responsive hollow fiber membrane by melt-spinning of polyurethane-poly(vinylidene fluoride)-poly(ethylene glycol) blends. Mater Manuf Processes 2010; 25: 1018–1020.

24.

Chou

Yang

. Effect of draw ratio and coagulant composition on polyacrylonitrile hollow fiber membranes. Sep Purif Technol 2006; 52: 380–387.

25.

Yang

Liu

. The permeation performance of polyacrylonitrile/polyvinylidine fluoride blend membranes. J Membr Sci 2003; 226: 119–130.

26.

Fei

Zhang

. Bending behaviour of electroresponsive poly(vinyl alcohol)/poly(acrylic acid) semi-interpenetrating network hydrogel fibres under an electric stimulus. Polym Int 2002; 51: 502–509.

27.

Fei

Zhang

Zhong

PVA/PAA thermo-induced hydrogel fiber: Preparation and pH-sensitive behavior in electrolyte solution. J Appl Polym Sci 2002; 85: 2423–2430.

28.

Tasaka

Nagasawa

. Anomalous osmosis through a model membrane constructed with potassium polystyrenesulfonate solution. Biophys Chem 1976; 4: 305–308.

29.

Helfferich

Ion-exchange kinetics. V. Ion exchange accompanied by reactions. J Phys Chem 1965; 69: 1178–1187.