Leaching of PVP from Polyacrylonitrile/PVP Blending Membranes: A Comparative Study of Asymmetric and Dense Membranes LING-SHU WAN, ZHI-KANG XU, ZHEN-GANG WANG Institute of Polymer Science, Zhejiang University, Hangzhou 310027, People’s Republic of China

Received 16 December 2005; revised 25 February 2006; accepted 27 February 2006 DOI: 10.1002/polb.20804 Published online in Wiley InterScience (www.interscience.wiley.com).

Poly(N-vinyl-2-pyrrolidone) (PVP) has been often used as an additive to improve the structure and performances of asymmetric membrane. In this work, we examined the leaching of PVP from polyacrylonitrile/PVP asymmetric membranes regarding the effect of leaching time, PVP content, and the molecular weight of PVP. Also, comparative studies of dense membranes were performed. It was found that the water contact angle on the dense membrane surface is very low compared with that on the asymmetric membrane surface. Results from X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy-attenuated total reflection (FTIRATR) showed that more amount of PVP exists at the surface layer of the dense membrane than at the asymmetric one. If the dense membrane was immersed into water for several hours and then dried in the air, the water contact angle increases and closes to that on the asymmetric membrane surface. Although leaching time was extended from 2 h to 15 days, PVP leaches out little from the asymmetric membrane. The leaching of PVP mainly occurs during the phase-inversion process. Furthermore, the surface features were examined by atomic force microscopy and field emission scanning electron microscopy, respectively. In comparison with PVP K30, more PVP K90 remains in the asymmetric membrane based on the FTIR-ATR spectra. However, it can be concluded from the results of XPS that at the most outer surface of the asymmetric membrane (e.g., in depth about 1
2 nm), the residual PVP K90 is almost C 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: the same with PVP K30. V

ABSTRACT:

1490–1498, 2006

Keywords:

additives; blending; membranes; surfaces; water-soluble polymers

INTRODUCTION Poly(N-vinyl-2-pyrrolidone) (PVP) is a watersoluble polymer and has been widely used as pore-forming agent for the preparation of asymmetric membrane by phase-inversion process. PVP is often used to modulate the structure of polymer membranes.1–5 Nouzaki et al.2 prepared Correspondence to: Z.-K. Xu (E-mail: [email protected]. edu.cn) Journal of Polymer Science: Part B: Polymer Physics, Vol. 44, 1490–1498 (2006) C 2006 Wiley Periodicals, Inc. V

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polyacrylonitrile (PAN) ultrafiltration membrane for wastewater treatment using PVP as an additive. They discussed, in detail, the effects of the content of PVP in the casting solution and the molecular weight of PVP on the membrane performances. Kang et al.3,4 studied the effects of molecular weight of PVP on the formation of asymmetric PAN and polyimide membranes. They found that the use of high molecular weight PVP in the preparation of PAN membrane had a strong effect on the depression of macrovoids formation. However, the addition of PVP showed a complex influence on the membrane morphologies for the prepara-

PVP LEACHING FROM PAN/PVP BLENDING MEMBRANES

tion of polyimide membrane, which depended on the solvent used (c-butyrolactone or N-methyl-2pyrrolidone). Recently, Jung et al.5 prepared PAN/PVP asymmetric membranes on which hypochlorite treatment was conducted. They also examined the residual PVP by Fourier transform infrared-attenuated total reflection (FTIR-ATR) and proposed that the residual PVP increased with the content and the molecular weight of PVP added. Furthermore, to improve the membrane performances, PVP has been widely used in the fabrication of hollow fiber membrane.6–12 PVP is also biocompatible. For example, polysulfone dialysis membranes show excellent biocompatibility in clinical use mainly because of the blending with PVP. Hayama et al.13 carefully studied the residual PVP amount in the membranes and the membrane morphologies by using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). They proposed that both the amount of PVP and the membrane surface structures induced by PVP should be responsible for the biocompatibility of polysulfone membranes. PVP can also be grafted on the membrane surface. It has been reported that the immobilization of PVP can improve the biocompatibility of polypropylene microfiltration membrane.14,15 On the other hand, PVP may endow membrane with better antifouling properties, which is very important for the applications of membrane.16–18 However, PVP in the membrane can leach out during hydraulic permeation, which results in a gradual deterioration of the membrane performances or even pollutes the filtrates (e.g., blood).19 Therefore, it is interesting that many efforts has been made to immobilize PVP by radiation or chemical crosslink,19–22 while post-treatments for removing PVP out were also employed to improve the transport characteristics of asymmetric membranes at the same time.5,8–10,12,23 Although it is generally accepted that the substantial amounts of PVP were lost during membrane preparation because of solvent–nonsolvent exchange or during hydraulic permeation,24 the leaching of PVP from membrane has not been studied systematically. Since it is difficult to monitor the PVP loss in the process of asymmetric membrane preparation, on the basis of the corresponding dense membranes, we carefully studied the behaviors of PVP leaching from the PAN/PVP asymmetric membranes by water contact angle measurements, FTIR-ATR, XPS, and AFM. Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb

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EXPERIMENTAL Materials PAN was synthesized by water-phase precipitation polymerization in our laboratory (Mv is about 220,000 g/mol).25 Dimethylsulfoxide (DMSO) was commercially obtained from Shanghai Chemical Agent Co. (China) and purified by distillation before use. PVP with different molecular weights (40,000 g/mol for K30 and 360,000 g/mol for K90) were obtained from Fluka and used as received. Membrane Preparation Asymmetric and dense PAN/PVP blending membranes were prepared by the following procedures. PAN with a certain amount of PVP was dissolved in DMSO at about 80 8C for 6 h with vigorous stirring. After air bubbles were removed completely, the solutions containing 8 wt % of PAN were cast onto clean glass plates, using a casting knife with a 150 lm gate opening. The glass plates with the nascent membranes were placed in the air for 10 min and then immersed into 30 6 0.5 8C ultrafiltrated water bath to solidify into asymmetric membranes. On the other hand, the glass plates with the nascent membranes were dried at 80 8C for 24 h under vacuum to obtain dense membranes. The thickness of the dense membranes was approximately 18 6 2 lm. The asymmetric membranes were dried at 40 8C for 24 h under vacuum for further characterization, while the dense membranes were used directly. According to the contents of PAN and PVP, those membranes were denoted as PAN, PAN5K30, PAN8K30, PAN12K30, PAN15K30, and PAN8K90, where 5, 8, 12, and 15 mean the content of PVP in weight percentage. The dense PAN/PVP blending membranes were rinsed in de-ionized water for 6 h at room temperature to leach PVP out from the outmost surface. To examine the behaviors of PVP leaching, the asymmetric membranes were kept in ultrafiltrated water for different times (2 h, 8 h, 15 days) at about 30 8C. Characterizations Static water contact angles of the membranes were measured by sessile drop method at room temperature with a contact angle goniometer (Dataphysics, OCA20, Germany) equipped with video camera. Typically, a water drop (
2 lL) was

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added on a dry membrane sample in the air, then the image was recorded after 5 s and a static water contact angle was calculated from the image with software. At least 10 measurements of different water drops were averaged to get a reliable value. FTIR-ATR spectra were acquired with a Vector 22 FTIR spectrometer (Brucker Optics, Switzerland) equipped with an ATR accessory (KRS-5 crystal, 458). The membranes were thoroughly dried before FTIR-ATR measurements. All spectra were taken by 32 scans at a nominal resolution of 4 cm1. Data analysis was carried out using the OPUS software (5.0 Build: 5, 0, 53) provided by Brucker Corp. Transmission FTIR spectra were also performed for the transparent dense membranes. However, the absorption of carbonyl groups in PVP was more than 100% because of the high content of PVP and the strong absorption of carbonyl groups in PVP. XPS experiments were carried out on a PHI5000C ESCA system (PerkinElmer, USA) with Al Ka radiation (1486.6 eV). In general, the X-ray anode was run at 250 W and the high voltage was kept at 14.0 kV with a detection angle at 458. The pass energy was fixed at 93.9 eV to ensure sufficient sensitivity. The base pressure of the analyzer chamber was about 5  107 Pa. The survey spectra (0–1200 eV) and the core spectra with much high resolution were both recorded. Binding energies were calibrated using the containment carbon (C1s ¼ 284.6 eV). Data analysis was carried out with the PHI-MATLAB software provided by PHI Corp. The surface morphologies of the original and water-rinsed asymmetric membranes were examined by field emission scanning electron microscopy (FESEM, Serion, FEI, USA). For this purpose, membrane samples were wetted and replaced with a water-ethanol-hexane sequence, dried at room temperature, and then sputtered with gold before FESEM observation. Accordingly, the topographic features of the original and water-rinsed dense membranes were measured by atomic force microscopy (AFM, SPI3800N, Seiko Instruments Inc., Japan) in a tapping mode under ambient conditions.

RESULTS AND DISCUSSION Water contact angle measurement is usually applied to evaluate the hydrophilicity of materials. Figure 1 shows the water contact angles of

Water contact angles of asymmetric (u) and dense (~) membranes with different contents of PVP as indicated in the diagram.

Figure 1.

both asymmetric and dense membranes with different contents of PVP in the casting solutions. It can be seen that blending PAN with PVP does not decrease the water contact angle remarkably for the asymmetric membranes. All of the values for the asymmetric membranes are around 608 except for membrane with high PVP content (PAN12K30 and PAN15K30) which shows slightly low water contact angle (about 528). However, the water contact angle for PAN/PVP dense membrane is very low and decreases slightly with the content of PVP in the range of 20–308. It is noticeable that PAN is of moderate hydrophilicity while PVP is water-soluble. The obvious difference of water contact angles between the asymmetric and dense membranes is interesting. Generally, the porosity or roughness of the membrane surface influence the water contact angle to some extent.26,27 Nevertheless, in our case, the water contact angle of the asymmetric PAN membrane is only about 2.58 higher than that of the dense one. Therefore, XPS was taken to determine the chemical composition of the membrane surface. The binding energy was assigned for each atom, according to the reference by Beamson and Briggs.28 As shown in Figure 2(a), two kinds of nitrogen atoms can be resolved in the XPS spectra of PAN and PVP and they are labeled as N1 and N2. Curve fitting was made to separate the overlapped peaks and a subpeak at 399.57 eV was assigned to the N1 while that at 400.49 eV to the N2. Similarly, the content of PVP can also be obtained from C1s spectra. The content of PVP at the membrane surface can be estimated Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb

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Figure 2. Schematic representation for PAN and PVP (a), and high-resolution XPS spectra of N1s of asymmetric membranes rinsed in water for 8 h: (b) PAN, (c) PAN8K30, (d) PAN8K90 and of dense membranes: (e) PAN8K30 and (f) PAN8K90.

from the areas of subpeaks, which is shown in Table 1. It can be seen that the content of PVP at the dense membrane surface is evidently larger than that at the asymmetric one. Since the penetration depth of XPS analysis is very small (about 1
2 nm), FTIR-ATR was further used to determine the PVP amounts at the asymmetric and dense membrane surfaces. Figure 3 is the typical FTIR-ATR spectra of PAN and PAN8K30 asymmetric membranes. It is obvious Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb

that the PAN8K30 membrane displays a complex spectrum attributed to both PAN and PVP. Characteristic peak at 2243 cm1 is due to the stretching vibration of cyano group (CN), which can be observed in both PAN and PAN8K30 membranes. The strongest peak at 1666 cm1 arising from the stretching vibration of carbonyl group (C¼ ¼O) in PVP can be regarded as the characteristic peaks of PVP. Herein, the area ratio of absorption peak at 1666 cm1 to that at 2243 cm1 was

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Table 1. Contents of PVP at the Surfaces of Asymmetric and Dense Membranes of PAN8K30 and PAN8K90 Calculated from XPS Asymmetric membranes

Dense membranes

PVPa (wt %)

PAN8K30

PAN8K90

PAN8K30

PAN8K90

From N1s From C1s

29.1 27.7

32.2 32.6

50.7 59.2

59.6 66.7

a

The theoretical bulk weight fraction of PVP is 50 wt %.

used to characterize the relative content of PVP in the membranes. As we know, in the phase inversion process, the exchange of solvent (DMSO) with nonsolvent (water) in coagulation bath takes place and then polymer solution solidifies into asymmetric membrane. Thus, the initial content of PVP in the asymmetric membrane surface layer, that is the content of PVP in asymmetric membrane without contacting with water, cannot be obtained but the theory bulk content of PVP can be calculated. For example, the theory bulk content of PVP in PAN8K30 is 50 wt % because the content of both PAN and PVP are 8 wt % in the casting solution. Figure 4 shows the typical FTIR-ATR spectra of asymmetric membranes with different content of PVP in the casting solution. These membranes were measured after being contacted with water for 2 h in which the phase inversion process occurs. The residual PVP amount in the membrane increases with the content of PVP in the casting solution. Table 2 lists the ratios of the peak area of carbonyl group in PVP to that of

Figure 3. Typical FTIR-ATR spectra of (a) PAN and (b) PAN8K30 asymmetric membranes after being rinsed in water for 2 h.

cyano group in PAN calculated from the FTIRATR spectra of asymmetric and dense membranes. We can see that the amount of PVP at the asymmetric membrane surface layer is far smaller than that at the dense one. The resultant asymmetric membranes were immersed into water for another 6 h or even 15 days to test the leachability of PVP; however, leaching of PVP from these asymmetric membranes is very slow under the studied conditions. It seems that the leaching of PVP mainly takes place during the phase inversion process. As mentioned above, it is difficult to monitor the leaching of PVP during the phase inversion process. But the fabrication of dense membrane, that is evaporating the solvent in casting solution under vacuum, does not cause the loss of PVP, and the PVP amount can be measured by FTIRATR. Therefore, PAN/PVP dense membrane was immersed into water for 6 h to test the leaching of PVP. As shown in Table 2, the PVP amount at the

Figure 4. Typical FTIR-ATR spectra of asymmetric membranes rinsed in water for 2 h. From bottom to top (a) PAN, (b) PAN5K30, (c) PAN8K30, (d) PAN12K30, and (e) PAN15K30. Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb

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Table 2. The Ratio of the Area of Carbonyl Group in PVP to that of Cyano Group in PAN Calculated from FTIR-ATR Spectra of Different Membranes

Dense membranes Asymmetric membranes

No.

PAN5K30

PAN8K30

PAN12K30

PAN15K30

1 2 3 4 5

20.7 10.3 7.4 7.2 6.0

20.1 10.6 7.6 7.6 6.6

23.7 16.8 9.8 9.3 9.2

24.6 19.3 14.8 13.2 11.2

1, 2: Dense membrane on which FTIR-ATR was performed before and after rinsing in de-ionized water for 6 h; 3, 4, 5: Asymmetric membrane on which FTIR-ATR was performed after immersing the nascent membrane into water for 2 h, 8 h, 15 d.

dense membrane surface layer decreases sharply after rinsing the membrane in water, though it is believed that the leaching of PVP from dense membrane is more difficult than from asymmetric one because of its denser structure. Similarly, the water contact angle increases to about 508 for water-rinsed dense membrane, which is close to that of the corresponding asymmetric one (Fig. 5). These results strongly demonstrate the leachability of PVP. The low water contact angle on the dense membrane surface is also confirmed to be induced by the large amount of PVP. AFM was used to confirm the topographic features of the dense membranes before and after being rinsed in water (Fig. 6). It can be seen from these images that the roughness of both dense PAN8K30 and PAN8K90 membranes increase after the membranes being rinsed in water, which is confirmed by the data of root mean square roughness (RMS). The increase of the roughness may be attributed to the leaching of PVP. However, FESEM and AFM measurements reveal that the surface topographies of the asymmetric membrane are affected little by the leaching of PVP. That may be due to the leaching of PVP from the asymmetric membrane mainly takes place during the phase inversion process, that is in the first 2 h. Besides, the surface roughness of asymmetric membrane is larger than that of the dense one, thus the leaching of PVP shows little influence on the surface topographies of the asymmetric membrane. Preliminary results concerning the effect of the molecular weight of PVP on the leaching behaviors has also been obtained. Table 1 shows that PVP K90 preferentially segregates at the dense membrane surface compared with PVP K30, which might be due to both the high molecular weight of PVP K90 and the characteristics of XPS technique. Comparing with the content of PVP at Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb

the surface layer of dense membrane, it can be seen from the results of XPS (Fig. 2 and Table 1) that the leaching of PVP K90 from the asymmetric membrane can also take place. As shown in Figure 6, the leachability of PVP K90 is subsequently confirmed by the changes of the roughness of the dense membranes. Figure 7 shows the FTIR-ATR spectra of PAN8K30 and PAN8K90 asymmetric membranes rinsed in water for 8 h. It is obvious that the remained amount of PVP K90 in PAN8K90 membrane is more than PVP K30 in PAN8K30 membrane. Jung et al.5 also reported that more PVP remained in membrane when higher molecular weight of PVP or more amount of PVP was added. On the one hand, PVP with high molecular weight has longer main chain and then the entanglement is stronger, which is in favor of remaining in membrane. On the other hand, as shown in Figure 8, membrane using PVP with high molecular weight (PVP K90)

Figure 5. Water contact angles of water-rinsed (^) and original (~) dense membranes with different contents of PVP as indicated in the diagram.

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Figure 6. Dynamic-mode AFM images of original dense membranes (a) PAN8K30 and (b) PAN8K90, and dense membranes rinsed in water for 6 h (c) PAN8K30 and (d) PAN8K90.

shows a denser structure in which macrovoids are almost completely suppressed.29 Thus, from the point of the leaching kinetics, the dense structure of membrane may hinder the leaching of PVP. However, if comparing the results from XPS and FTIR-ATR carefully, it was found that they are inconsistent with each other on the point of the residual amounts of PVP K30 and PVP K90 in the asymmetric membranes. Data in Table 1 indicate that the amounts of PVP K30 and PVP K90 remained in the asymmetric membranes are almost the same, while the FTIR-ATR spectra show that more PVP K90 is remained. As we know, the penetration depth of XPS is about 2 nm, while the penetration depth (dp) of FTIRATR is determined by the following equation30: dp ¼ k1 =½2pðsin2 h  n221 Þ1=2 

Figure 7. Effect of the molecular weight on the leaching of PVP illustrated by FTIR-ATR spectra. (a) Thick line for PAN8K30 and (b) thin dot line for PAN8K90 asymmetric membranes after being rinsed in water for 8 h. Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb

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Figure 8. FESEM micrographs of the cross section of (a) PAN8K30 and (b) PAN8K90 asymmetric membranes.

where h is the incident angle of light on the surface of the ZnSe element, k1 is the ratio of the wavelength of incident light to the refractive index of the ZnSe element, and n21 is the ratio of the refractive index of the polymer to that of the ZnSe element. Herein, h is 458; the refractive indices of the polymer and ZnSe are assumed to be 1.50 and 2.4, respectively. When the wavenumber is 2243 cm1 (cyano group), dp, 2243 is
0.89 lm; when the wavenumber is 1666 cm1 (carbonyl group), dp, 1666 is
1.20 lm. Although the penetration depth changes with the wavenumbers for different groups, the penetration depth of FTIRATR is far larger than that of XPS. One can envisage that the leaching of PVP are hindered little by the membrane structure (with many macrovoids or with denser top layer, as shown in Fig. 8) at the most outer surface of the asymmetric membrane. Therefore, the residual amounts of PVP K30 and PVP K90 are the same by XPS.

CONCLUSIONS We found that the water contact angle on PAN/ PVP dense membrane surface is very low. Results from FTIR-ATR and XPS confirmed the large amount of water-soluble PVP existing at the dense membrane surface layer induces the low water contact angle. If the dense membrane was immersed into water for several hours and then dried in the air, the water contact angle increases and closes to that on the asymmetric membrane surface, which demonstrates the leachability of PVP into water. In the case of asymmetric membrane, the leaching of PVP occurs mainly in the process of phase inversion. In comparison with PVP K30, more PVP K90 remains in the asymJournal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb

metric membrane because of the large molecular weight and the corresponding denser membrane structure. For some biomedical usages in which biocompatibility of membrane is required, the addition of PVP with large molecular weight may be helpful. Financial support from the National Natural Science Foundation of China (Grant no. 50273032) is gratefully acknowledged.

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Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb