Journal of Membrane Science 277 (2006) 157–164

A novel process for the post-treatment of polyacrylonitrile-based membranes: Performance improvement and possible mechanism Ling-Shu Wan, Zhi-Kang Xu ∗ , Xiao-Jun Huang, Ai-Fu Che, Zhen-Gang Wang Institute of Polymer Science, Zhejiang University, Hangzhou 310027, PR China Received 25 July 2005; received in revised form 20 September 2005; accepted 20 October 2005 Available online 28 November 2005

Abstract To obtain much better performance, post-treatment on microporous membrane is often carried out. Treatment with hypochlorite has normally been used to increase the flux of polymer membrane containing poly(N-vinyl-2-pyrrolidone) (PVP). However, the membrane retention sharply decreases after the treatment. In this paper, a novel post-treatment method, which endowed the membrane with both high flux and relatively high retention, was reported. Blending membranes from polyacrylonitrile with PVP were fabricated and their morphologies, fluxes, and BSA retentions were characterized. Treating the membranes with ammonium persulphate (APS) at 60 and 40 ◦ C were performed and, for comparison, those with hot water and sodium hypochlorite were also carried out. It was found that treatment with APS led to a remarkable increase of flux and only a slight decrease of BSA retention for the membranes. Changes on the pore size were confirmed by field emission scanning electron microscopy. The untreated and treated membranes were carefully studied with FT-IR-ATR. It was proposed that, not only the leaching out of PVP induced by chain scission but also the hydrolysis and cyclization of cyano in polyacrylonitrile took place during the process of APS treatment on the studied membranes. © 2005 Elsevier B.V. All rights reserved. Keywords: Polyacrylonitrile; Poly(N-vinyl-2-pyrrolidone); Membrane; Post-treatment; Ammonium persulphate

1. Introduction Polyacrylonitrile (PAN) exhibits good mechanical properties and has been widely used as separation membrane materials [1–10]. However, due to some inherent disadvantages, such as brittleness, relatively low hydrophilicity and poor biocompatibility, modifications on PAN or PAN-based membranes must be made to meet the requirements of the increasingly extended applications, which include blood contacting devices. Therefore, many efforts have been paid to create a friendly surface for the improvement of the hydrophilicity and biocompatibility of PANbased membranes [1–9]. Photo-induced graft polymerization surface modifications for the preparation of PAN ultrafiltration membrane were reported by Ulbricht et al. [11] and recently by Frahn et al. [5] to fabricate aromatic hydrocarbon selective composite membranes. Biomacromolecules such as human serum albumin, chitosan and heparin were immobilized onto PAN-



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0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.10.037

based membranes to improve their hemocompatibility [12,13]. Incorporating various comonomers including N-vinylimidazol [6], glycidyl methacrylate [7], N-vinyl-2-pyrrolidone [14–16], ␣-allyl glucoside [8], and phospholipid moieties [17], into PAN to improve its hydrophilicity and/or biocompatibility was also widely explored. In spite of some defects such as the leaching out of additives during hydraulic permeation [18], blending is a facile and versatile method to fabricate PAN-based microporous membranes. As we know, in a multi-component polymer system, one component can migrate to the outmost surface or imbed into the bulk of the blend [19–21]. Not only the structure but also the biocompatibility of a membrane can be improved by blending with some additives. Therefore, blending membranes using PVP as an additive were widely described in literatures. For example, the influence of PVP addition on the morphology of asymmetric polyimide membranes fabricated by phase inversion method was studied by Yoo et al. [22]. Moreover, polysulfone dialysis membranes were usually hydrophilized by blending PVP to obtain excellent biocompatibility. Hayama et al. [23] examined four kinds of commercially available polysulfone dialysis

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membranes and found that the biocompatibility was dependent on both the PVP amount and the membrane surface structure. Since both suitable pore structure and biocompatibility should be achieved for polymer membranes using PVP as additive, sometimes post-treatments are necessary. Annealing was often used to decrease the flux and increase the retention for a membrane. Results reported by Kim et al. [24] and Nouzaki et al. [25] confirmed this point. Recently, Jung et al. [10] proposed that the flux decrease of annealed PAN membrane was due to the rearrangement of the amorphous and crystalline domains of PAN. On the other hand, Qin et al. [26] reported a kind of hydrophilic hollow fiber ultrafiltration membranes made from cellulose acetate/PVP/N-methyl-2-pyrrolidone/water systems. Water fluxes for the hypochlorite-treated membranes were three times higher than that of the untreated membrane while the retentions of the treated membranes were much lower. A high flux ultrafiltration polysulfone membrane with low retention was also obtained by them [27]. Jung et al. [10] fabricated asymmetric PAN membranes using PVP as additive and treated the membranes with hypochlorite also. They carefully studied the effect of the molecular weight of PVP on the membrane performance. Sodium hypochlorite treatment on the membrane increased the flux while decreased the retention. Herein, to obtain much better performance, PVP was used as an additive to modulate the morphologies of PAN-based membranes, and a novel post-treatment procedure was performed to generate membranes with high flux and relatively high retention. To explore the possible mechanism, treating the membranes with ammonium persulphate (APS) at 60 and 40 ◦ C were performed and, for comparison, those with hot water and sodium hypochlorite were also carried out.

Table 1 Composition of the casting solution for membrane preparation Sample

PAN (wt.%)

PVP (wt.%)

DMSO (wt.%)

PAN PAN5K30 PAN8K30 PAN12K30 PAN15K30 PAN8K90

8 8 8 8 8 8

0 5 8 12 15 8

92 87 84 80 77 84

150 ␮m gate opening. The glass plates with the nascent membranes were placed in the air for 10 min and then immersed into 30 ± 0.5 ◦ C ultrafiltrated water for 24 h. After this, the porous membranes were preserved in 5 vol.% formaldehyde aqueous solution. 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, as shown in Table 1. 2.3. Post-treatments on the blending membranes The blending membranes were firstly washed three times with de-ionized water before post-treatments. Post-treatments with 5 wt.% of APS aqueous solution and de-ionized water were carried out at 60 ◦ C for 6 h, respectively. The membranes were also treated by 5 wt.% of APS aqueous solution at 40 ◦ C for 6 h. Treatment with sodium hypochlorite (2000 ppm, 6 h, pH was adjusted to 7.0 by H2 SO4 ) was performed at room temperature (about 25 ◦ C) according to the procedure reported by Qin et al. [26].

2. Experimental

2.4. FT-IR-ATR characterization

2.1. Materials

FT-IR-ATR spectra were acquired with a Vector 22 FT-IR (Brucker Optics, Switzerland) equipped with an ATR accessory (KRS-5 crystal, 45◦ ). The membranes were thoroughly dried before FT-IR-ATR measurements. All spectra were taken by 32 scans at a nominal resolution of 4 cm−1 . The data analysis was carried out using the OPUS software (5.0 Build: 5, 0, 53) provided by Brucker Corporation.

Polyacrylonitrile (PAN) was synthesized by water phase precipitation polymerization in our lab (Mv is about 220,000 g/mol) [28]. Dimethylsulfoxide (DMSO) and ammonium persulphate (APS) were commercially obtained from Shanghai Chemical Agent Co. (China) and purified before use. Poly(Nvinyl-2-pyrrolidone) (PVP) with different molecular weight (40,000 g/mol for K30 and 360,000 g/mol for K90) was purchased from Fluka and used as received. Bovine serum albumin (BSA, pI = 4.8, Mw = 66 kDa) was purchased from SinoAmerican Biotechnology Co. and used as received. Sodium hypochlorite (NaClO), ethanol, n-hexane and other chemicals were analytical grade and used as received without further purification. 2.2. Membrane preparation Porous PAN/PVP blending membranes were prepared by the following procedure. Homogeneous dope solutions as listed in Table 1 were formed at about 80 ◦ C with vigorous stirring for 6 h. After air bubbles were removed completely, the solutions were cast onto clean glass plates using a casting knife with a

2.5. Water contact angle measurement 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 ␮L) was 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. 2.6. Morphology evaluation The surface and cross-section morphologies of the original and treated membranes were examined by field emission

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scanning electron microscopy (FESEM, Sirion, FEI). 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.

where Cp and Cf are the BSA concentration in permeate and feed, respectively.

2.7. Permeation experiments

Membrane solvent resistance characterization was performed according to Ref. [7]. Briefly, the thoroughly dried membranes were cut into pieces of 3 cm × 3 cm and placed into acetone after weighted accurately. Being kept in acetone for 48 h, the membranes were taken out and dried in vacuum, and then weighted carefully. Weight loss was reported by the average of three times measurements.

Membrane about 9.08 cm2 was installed into a permeation cell. The cell was flushed thoroughly with de-ionized water. Then, de-ionized water at 25 ◦ C was forced to permeate through the membrane at a constant permeation pressure of 0.12 MPa. After 30 min, the pressure was lowered to the operation pressure, 0.10 MPa, and then the flux was measured. For every membrane sample, at least three pieces of membrane were measured to get a reliable flux value. After the permeation of de-ionized water at 0.12 MPa for 30 min, de-ionized water was changed to 1.0 g/L BSA phosphate buffered saline solution (PBS, pH 7.4). The BSA solution was filtrated for 2 min at 0.10 MPa, and then 10 mL of filtrate was collected at the same pressure. The BSA concentrations in the feed and filtrate were analyzed using a spectrophotometer (Shimadzu, UV2450) at 280 nm. The solute retention (R) was calculated by: R (%) = (1 − Cp /Cf ) × 100

2.8. Membrane solvent resistance characterization

3. Results and discussion 3.1. Membrane preparation and the permeation properties The phase inversion process was used to fabricate asymmetric membranes from PAN/PVP blends. Composition for the casting solution is summarized in Table 1. The cross-section morphologies of the membranes were observed by FESEM. Typical images are shown in Fig. 1. It can be seen from the FESEM micrographs that, membranes with low content of PVP (PAN, PAN5K30 and PAN8K30) consisted two distinct cross-section morphologies—a selective dense surface layer supported by a

Fig. 1. Micrographs for the cross-sections of polyacrylonitrile-based membranes: (a) PAN, (b) PAN5K30, (c) PAN8K30, (d) PAN12K30, (e) PAN15K30, and (f) PAN8K90.

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Fig. 2. Pure water flux and BSA retention of polyacrylonitrile-based membranes. Fig. 3. Water flux ratio of the water-treated (60 ◦ C, solid square) and ammonium persulphate-treated (60 ◦ C, hollow square) membrane to the original membrane.

much thicker highly porous layer. The size of the macrovoids decreased with the increase of the PVP content from 0 to 8 wt.%. When the content of PVP reached 12 wt.%, the porous layer disappeared and the macrovoids were completely suppressed. If PVP K30 was replaced by PVP K90, no macrovoids was found for the membrane with 8 wt.% additive. It seems that the molecular weight of PVP showed a great influence on the morphology of the blending membrane. This result is in accord with those reported by Yoo et al. [22] and Jung et al. [10]. The effects of PVP addition on the water flux and retention of the membrane are shown in Fig. 2. It can be seen that, with the increase of PVP content, the water flux increased firstly and then decreased sharply. As we know, the dense surface layer is important in membrane filtration and it has been widely accepted that the flux decreases with the thickness of the dense surface layer. Besides the morphology, it would be expected that the residual PVP in the membranes plays a crucial role in the permeation of water. Jung et al. [10] carefully studied the effects of the content and molecular weight of PVP on the water flux of the PAN membrane. Similar results were reported by them. The water fluxes of membranes with low content of PVP might be promoted by the existence of PVP. However, the low flux for membrane with high PVP content or PVP K90 was due to the thicker top layer and the remained PVP in membrane, as proposed by Jung et al. [10]. As shown in Fig. 2, the BSA retention of the blending membrane varied from about 68% to 88%.

3.2. Effects of APS treatment on the membrane properties As we know, the flux of membrane is of importance and high flux means large ability of filtration. It can be seen from Table 2 that, the blending membranes with high content of PVP showed low water flux, especially the membrane using PVP K90 as additive. Thus, APS was chosen to treat the membranes. As shown in Fig. 3, all of the water flux ratios for the APS-treated membrane to the original membrane were more than 1.0. In other words, all of the membranes were increased the fluxes by APS treatment at 60 ◦ C. It is noticeable that, with the increase of PVP content in the blend, the ratio increased accordingly. Especially, the flux of the membrane using PVP K90 as additive showed a remarkable increase. When higher molecular weight of PVP was remained, the selective removal of residual PVP in the PAN membranes was more effective. Reaction between APS and pyrrolidone ring of PVP might occur and caused chain scission of PVP molecules [10]. On the other hand, the small flux of the original membrane should also be responsible for the large water flux ratio for the flux of treated PAN8K90 membrane was still smaller than that of membranes containing PVP K30. In conclusion, the flux of the blending membrane can be effectively increased by APS treatment and the increase of the flux was mostly dependent on the molecular weight of PVP. Nevertheless, the flux of PAN membrane had also been increased by APS treatment. Then, how APS treatment increased the flux?

Table 2 Fluxes (L/m2 h) of the original and treated blending membranes Sample

Original

Water-treated

APS-treated at 60 ◦ C

APS-treated at 40 ◦ C

NaClO-treated

PAN PAN5K30 PAN8K30 PAN12K30 PAN15K30 PAN8K90

269.8 321.1 378.9 231.8 142.1 54.8

160.6 134.8 126.6 87.8 55.5 11.9

550.2 417.2 608.3 485.1 392.0 333.9

283.7 335.4 323.3 317.8 270.9 388.3

405.9 479.4 580.0 485.5 432.7 407.0

L.-S. Wan et al. / Journal of Membrane Science 277 (2006) 157–164

Fig. 4. Water contact angles of the original (blank column), ammonium persulphate-treated (gray column) and water-treated (shadow column) membranes.

The increase of flux was really induced by APS treatment, because treatment with hot water at the same temperature decreased the flux remarkably, as shown in Fig. 3. Treatment with hot water could leach out PVP from blending membrane to some extent. However, the membrane became tight by treatment with hot water and then decreased the flux, especially by long time annealing (for example, 6 h), which should be the leading effect for our systems [25]. The decrease of the flux could also be induced by the rearrangement of amorphous and crystalline domains of PAN [10]. The hydrophilicity of the treated and untreated membranes was evaluated by water contact angle measurement. Fig. 4 shows the typical results of the original, APS-treated and water-treated membranes. Generally, the hydrophilized membrane has the tendency to enhance the flux. However, no distinct regularity could be found for the water contact angles. The leaching out of PVP and the porosity of the porous membrane might be responsible for this irregularity. The solvent resistances of the untreated and APS-treated membranes were also examined, as shown in Fig. 5. It was found that the weight loss increased from PAN to PAN15K30 due to increasing PVP loading in the blend membrane. However, all the data of weight loss for the membranes were within 3% except that of the APS-treated PAN15K30. It seems that APS treatment had little influence on the membrane solvent resistance. In order to examine the effect of the treatment temperature on the membrane performance, APS treatment at 40 ◦ C was carried out. Fig. 6 shows the water fluxes of the treated membranes. It can be seen that the membranes with low content of PVP (i.e., PAN, PAN5K30, PAN8K30) did not change their water flux after APS treatment at 40 ◦ C. One can envisage that the effect of annealing on the membrane might equal to that of leaching out of PVP for the membranes with low content of PVP. However, for the membranes with high content of PVP, especially for that with PVP K90, the same behaviors took place for treatment at 40 and 60 ◦ C.

161

Fig. 5. Weight loss of the original (blank column) and ammonium persulphatetreated (shadow column) polyacrylonitrile-based membranes dipped in acetone for 48 h.

3.3. FT-IR-ATR analysis of APS-treated membranes Figs. 7 and 8 show the typical FT-IR-ATR spectra of PAN8K30 and PAN8K90 membranes treated by APS and deionized water at 60 ◦ C and untreated membranes. It is obvious that the blending membranes displayed a complex spectrum attributed to both PAN and PVP. This allowed us to confirm that PVP could not be sufficiently removed by the leaching with water during the formation of porous membrane. Characteristic peak at 2243 cm−1 was due to the stretching vibration of cyano group (–C N). The bands of the asymmetric and symmetric C–H stretching of the polymer main chain at 2850–2980 cm−1 and the C–H deformation vibration at 1452 cm−1 could also be found in these spectra. The peak at 1285 cm−1 was corresponding to the C–N stretching vibration, and the peak at 1416 cm−1 was attributed to the deformation vibration of C–H of the PVP ring, which was shown as a shoulder of the peak at 1452 cm−1 for overlapped with that of the polymer main chain [29]. Furthermore, the strongest peak at 1666 cm−1 arising from

Fig. 6. Water flux for the membranes treated by ammonium persulphate at 40 ◦ C and the water flux ratio for the treated membranes to the original ones.

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L.-S. Wan et al. / Journal of Membrane Science 277 (2006) 157–164 Table 3 Schematic diagrams for the hydrolytic products of PAN8K90 membranes treated by ammonium persulphate at 60 ◦ C No.

1

Fig. 7. Typical FT-IR-ATR spectra of PAN8K30 membranes treated by ammonium persulphate (a, thick line) and de-ionized water (b, thin line) at 60 ◦ C and membrane untreated (c, dot line).

the stretching vibration of carbonyl group (C O) in PVP could be regarded as the characteristic peaks of PVP together with those at 1285 and 1416 cm−1 . Compared with the spectrum (curve (c), in dot line) of the original membrane, almost no changes can be observed for the membrane treated by 60 ◦ C de-ionized water (curve (b), in thin solid line), as shown in both Figs. 7 and 8. That means treatment with hot water changed the physical structures of the membranes (for example, pore size) instead of the chemical structures. However, as shown in the spectrum of the membrane treated by APS at 60 ◦ C, absorption peak at 1666 cm−1 together with those at 1285 and 1416 cm−1 nearly disappeared. Meanwhile, a new small broad absorption peak was found at about 1706 cm−1 , which may be assigned to carboxyl group in carboxylic acid. It can be concluded that, both the chain scission of PVP followed by the leaching out from the blending and the hydrolysis/cyclization of PAN should be responsible for the flux increase of APS-treated membrane. As more clearly exhibited in Fig. 8, several kinds of carbonyl groups appeared for the blending membrane treated by APS at 60 ◦ C, while no changes between the spectra of the orig-

Fig. 8. Typical FT-IR-ATR spectra of PAN8K90 membranes treated by ammonium persulphate (a, thick line) and de-ionized water (b, thin line) at 60 ◦ C and membrane untreated (c, dot line).

Position (cm−1 )

Schematic diagram

a

1650–1680

2

∼1705

3

∼1727

4

∼1770

a Peak at 1650–1680 cm−1 can also be assigned to the carbonyl group of residual PVP; however, the intensity of this peak is far smaller compared with the original membrane.

inal membrane and the water-treated membrane were observed. If carefully analyzing the spectrum of the PAN8K90 membrane treated by APS, at least four peaks can be found in the broad peak around 1700 cm−1 , as listed in Table 3. For treatment with strong oxidant such as APS, not only PVP in the membrane almost sufficiently leached out which was confirmed by the disappear of bands at 1285 and 1416 cm−1 , but also the chemical reactions of cyano groups in PAN took place. Cyano group in PAN can be changed into amide (–CONH2 ) or further carboxylic acid (–COOH), therefore, peak at 1650–1680 cm−1 was assigned to amide (No. 1 as listed in Table 3) and peak at ∼1705 cm−1 was owing to carboxylic acid (No. 2, this peak may also originate from the scission/hydrolysis of the PVP side group). There was still a small separated peak at about ∼1768 cm−1 . According to some other studies [29], peak at 1768 cm−1 was attributed to five-membered cyclic imides (CONHCO, No. 4), which was

Fig. 9. BSA retention for the membrane treated by ammonium persulphate at 60 ◦ C.

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Fig. 10. Micrographs of the surfaces of PAN8K30 membranes: (a) original, (b) treated by ammonium persulphate, and (c) treated by hypochlorite.

generated from the head-to-head structure of PAN. Since headto-tail structure would be generally expected to be overwhelmingly predominant [30], it seems sound for only a very small peak at 1768 cm−1 existing. Consequently, six-membered cyclic imides (No. 3) corresponding to peak at ∼1727 cm−1 might also be produced by the post-treatment. Similar cyclization of PAN by oxidation was studied by Brandrup and Takahagi [31,32].

place during the process of APS treatment on the membrane containing PVP. Acknowledgement Financial support from the National Natural Science Foundation of China (Grant No. 50273032) is gratefully acknowledged.

3.4. Comparison of treatments with NaClO and APS References Treatment by sodium hypochlorite on the membranes was also carried out for comparison. The fluxes of these sodium hypochlorite-treated membranes are summarized in Table 2 and the retentions of APS-treated membranes at 60 ◦ C are shown in Fig. 9. Treatment by sodium hypochlorite really increased the water flux remarkably. However, the BSA retention of the sodium hypochlorite-treated membrane is zero in our situation. Interestingly, water fluxes for the APS-treated membranes increased a lot while the retentions only decreased slightly and lowered from 68–88% to about 50%. This can be confirmed by FESEM characterization. Fig. 10 shows the micrographs of the surfaces of PAN8K30 membranes. Only small pores can be observed for the membrane treated by APS (Fig. 10(b)) at a magnification of 50,000× and large pores can be found for the membrane treated by sodium hypochlorite (Fig. 10(c)) even at a magnification of 10,000×. Unfortunately, membranes treated by sodium hypochlorite or APS showed a slight decrease of mechanical strength and further researches are required. 4. Conclusion Polyacrylonitrile membrane using various contents of PVP as additive was fabricated and a novel method for the posttreatment of the membrane were described. It was found that the macrovoids could be effectively suppressed with the increase of PVP content or using high molecular weight PVP as additive. Treating the membrane with APS showed about 1.5–7 times higher water flux than the untreated one. Meanwhile, the BSA retention decreased slightly from 68–88% to about 50%. However, the BSA retention of the membrane treated by hypochlorite was zero in our situation. The changes of membrane pore size were confirmed by field emission scanning electron microscopy. Based on the FT-IR-ATR analysis, it was proposed that, not only the leaching out of PVP induced by chain scission but also the hydrolysis and cyclization of cyano in polyacrylonitrile took

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