Journal of Membrane Science 249 (2005) 21–31

Surface modification of polypropylene microfiltration membranes by the immobilization of poly(N-vinyl-2-pyrrolidone): a facile plasma approach Zhen-Mei Liua , Zhi-Kang Xua,∗ , Ling-Shu Wana , Jian Wub , Mathias Ulbrichtc a

Institute of Polymer Science and RIMST, Zhejiang University, Hangzhou 310027, PR China b Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China c Institut f¨ ur Technische Chemie, Universit¨at Essen, 45117 Essen, Germany Received 19 July 2004; accepted 4 October 2004 Available online 7 January 2005

Abstract This paper describes a facile approach for the surface modification of polypropylene microfiltration membrane (PPMM) by poly(N-vinyl-2pyrrolidone) (PNVP), which involved the physical adsorption of PNVP, followed by a plasma treatment to immobilize PNVP on the membrane surface. Chemical and morphological changes of the membrane surface were characterized in detail by attenuated total reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and water contact angles measurements. Results reveal that both the plasma treatment time and the adsorbed PNVP amount have remarkable effects on the immobilization degree of PNVP. Pure water contact angle on the membrane surface decreases with the increase of PNVP immobilization degree, which indicates an enhanced hydrophilicity for the modified membranes. Static platelets adhesion experiment on the membrane surface was conducted to characterize the hemocompatibility of the PNVP-modified PPMM. The statistical amounts of adhered platelets on unit membrane area decrease significantly, which to a certain degree demonstrates that the hemocompatibility of the PNVP-modified membrane has been improved. Finally, permeation fluxes of pure water and bovine serum albumin solution were measured to evaluate the antifouling property of the PNVP-modified membranes, the results of which have shown an enhancement of antifouling property for the PPMMs. In a word, this pre-adsorption-plasma approach was found to be facile and useful in improving the hemocompatibility and the antifouling property of the PPMMs. © 2004 Elsevier B.V. All rights reserved. Keywords: Polypropylene microfiltration membrane; Poly(N-vinyl-2-pyrrolidone); Plasma treatment; Surface modification; Hemocompatibility; Antifouling property

1. Introduction For many membrane processes such as ultrafiltration and microfiltration, flux decline resulted from protein adsorption, concentration polarization, pore blocking, and gel layer formation, etc., is a repugnant problem, which to a great extent prevents the wide-scale applications of membrane separation processes in aqueous solution treatment and bioseparation. A typical case is the filtration of complex fluids in biotechnology and food industries, from which proteins ad∗ Corresponding author. Tel.: +86 571 8795 2605; fax: +86 571 8795 1773. E-mail address: [email protected] (Z.-K. Xu).

0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.10.001

sorb onto the surface and deposit within the pores of membrane, resulting in biofouling and/or flux reduction for the membrane [1]. It is reported recently that the adsorptive fouling could account for up to 90% of permeability losses [2]. Therefore, much attention has been paid in the past 20 years to find out the mechanism of protein adsorption, and it is now known that the electrostatic forces and the hydrophobic interactions between certain domains in a protein molecule and the hydrophobic membrane surfaces are the main factors [3–5]. Up to now, it has been generally accepted that hydrophilic materials are less sensitive to protein adsorption than hydrophobic ones, the principle behind which is that hydrophilic surface preferentially adsorb water rather than solutes, leaving the membrane surface with protein-resistance

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[2]. Therefore, there is considerable interest in developing hydrophilic membrane materials [6,7]. Unfortunately, most inherently hydrophilic polymers are not suitable to fabricate into membranes since such polymers are normally susceptible to chemical and thermal impacts in their applications. Furthermore, inherently hydrophilic polymers readily swell by water and thus the corresponding membrane structures and properties can be changed dramatically. Therefore, in recent years, there has been much interest in developing surface treatment techniques to alter the chemical and physical properties of hydrophobic membrane surface [8–24]. To obtain a hydrophilic membrane surface with antifouling property, several methods have been investigated, which can be divided into two classes: physical and chemical modifications. Up to now, it is well known that adsorbing suitable hydrophilic polymers on the membrane surface alleviates protein fouling, while grafting the hydrophilic polymers on is expected to provide a much more stable and long-standing surface layer [8–13,17,18]. For those polymers with functional groups in either back-bonds or side chains, numerous grafting methods are suitable, while in the case of those hydrocarbon polymers with no active side chains or endgroups (for example, polyethylene or polypropylene), the membranes have to be activated prior to the graft polymerization. To generate active sites on the polymeric substrates for further reactions, plasma treatment is an effective way [8–15] as well as UV [16–18], ␥-ray [19,20] and electron beam [21,22] irradiations. In fact, plasma-initiated grafting is a versatile technique for modifying the membrane surface without affecting the bulk properties [8,10–13]. Exposing a membrane to plasma will generate active groups on the membrane surface, which makes it possible to modify the surface by graft copolymerization when contacts with monomers, this is the so-called plasma initiated surface grafting and can be performed in various gases [23]. The other major processes about plasma technique are either plasma polymerization on, in which a cross-linked thin polymeric layer is deposited on the substrate surface, or plasma treatment, in which intensive oxidation or crosslinking is introduced on the surface region of the substrate [14,24]. As a polymer soluble in both water and organic solvents, poly(N-vinyl-2-pyrrolidone) (PNVP) has been the focus of numerous applications including additives, cosmetics, coatings and biomedicines. For example, Robinson and Williams [25] reported that PNVP could be simply adsorbed on silica particles to inhibit protein adsorption and even to remove adsorbed proteins from the particle surface. This versatile polymer was most recently immobilized on poly(ethylene terephthalate) (PET) film by Meinhold et al. [22] to fabricate supported-hydrogels. The principal reason for successful PNVP applications is its excellent biocompatibility with living tissues and extremely low cytotoxicity. PNVP has also been used to modify the surface properties of polymeric membranes. By low-temperature plasma treatment, N-vinyl-2-pyrrolidone was graft polymer-

ized on poly(ether sulfone) ultrafiltration membrane, which resulted in higher filtration performance with less total and irreversible fouling [8]. Belfort and coworkers [17] had also successfully photochemically modified poly(ether sulfone) ultrafiltration membrane with N-vinyl-2-pyrrolidone to increase the surface wettability and decrease the adsorptive fouling. On the other hand, Higuchi et al. [26] covalently conjugated PNVP on the surface of polysulfone membrane with a multiple chemical process. It was reported that PNVP-modified polysulfone membrane gave lower protein adsorption from a plasma solution and much suppressed number of adhered platelets than original polysulfone and other surface-modified membranes. Most recently, Kang et al. [27] cross-linked PNVP on microporous chlorinated poly(vinyl chloride) membranes to improve their hydraulic permeation behavior. However, among all these literatures concerning the graft polymerization of N-vinyl-2pyrrolidone and/or the tethering of PNVP on the membrane surface, there is scarce report about the grafting/tethering of this versatile polymer on the surface of polyolefin microfiltration membranes. For the widely used hydrophobic membranes such as polypropylene microfiltration membrane, one persistent problem causing performance decline has been “membrane fouling” as mentioned above. Furthermore, the poor hydrophilicity and biocompatibility for this type of membrane limit their further applications in aqueous solution treatments, enzyme-immobilized membrane bioreactors, bioseparations and biomedical devices. Therefore, a series of work were carried out by our group to find out a better way to modify the PPMMs with low irreversible adsorptive fouling and enhanced biocompatibility [28–31]. The goal of this study was to evaluate a new method for the immobilization of PNVP on the surface of polypropylene microfiltration membrane (PPMM). In our previous work [28], the graft polymerization of N-vinyl-2-pyrrolidone on hydrophobic PPMMs using UV photo-assisted and ␥ray preirradiation processes was discussed. However, the grafting polymerizations of N-vinyl-2-pyrrolidone induced by UV irradiation were not efficient as expected, while ␥-ray preirradiation processes and plasma-initiated graft polymerization always results in undesired homopolymer and wastes the starting monomers [8,13,15]. Keep these in mind, in the present work, an efficient and facile method was developed, which included the adsorption of PNVP on the surface of PPMM following with air plasma treatment. Attenuated total reflectance Fourier transform infrared (FT-IR/ATR) and X-ray photoelectron spectroscopies were adopted to investigate the chemical composition changes on the modified membrane surface. Water contact angle measurements were also performed since the wettability is a good state identification of the modified surface. Platelet adhesion, protein adsorption together with filtration experiments were conducted to evaluate the hemocompatibility and the antifouling property of the PNVP-modified membranes.

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2. Experimental 2.1. Materials and chemicals PPMM with porosity of 45–50% and an average pore diameter of 0.07 ␮m was prepared in our laboratory with the melt-extruded/cold-stretched method [29,30]. To remove any chemicals adsorbed on the membrane surface, nascent membrane was presoaked and then washed with acetone, and dried in a vacuum oven at room temperature for 4 h before the initial mass was determined. Poly(N-vinyl-2-pyrrolidone) (PNVP, K-30, the average molecular weight is about 40,000 g mol−1 ) was of analytical grade purity and purchased from Shanghai Chemical Agent Co., China. Bovine serum albumin (BSA, Mw = 67,000 Da) was a commercial product of SinoAmerican Biotechnology Co. and used as received. The solvents were all of analytical grade and used without further purification. 2.2. Immobilization of PNVP on the membrane surface The process used in this work is schematically described in Fig. 1. In our previous work [30], this process was used to graft a sugar-containing polymer onto the PPMM surface. Similar method was also described by Terlingen et al. [32] to immobilize the physically adsorbed layer of sodium dodecyl sulfate, a surfactant, onto a polypropylene film. As shown in Fig. 1, before air plasma treatment, PPMM was soaked in PNVP/methanol solutions with different concentrations (1–20 wt.%) for 20–24 h. After that, the membrane was taken out from the PNVP solutions and dried under reduced pressure to constant weight. The adsorption degree (AD) of PNVP on the membrane was calculated as following: AD =

Wa − W0 × 100% W0

(1)

23

where Wa is the weight of PPMM after the adsorption of PNVP and W0 is the weight before that. The immobilization of PNVP was carried out in a glowdischarge plasma reactor connected with a diffusion pump and two rotary pumps. The plasma reactor used in this work was purchased from Beijing KEEN Co., China. Tubular type Pyrex reactor (10 cm × 150 cm) was rounded with a pair of copper electrodes. These two electrodes were powered through a matching network by a 13.56 MHz radio-frequency (rf) generator. On the basis of systematic experiments considering membrane etching and immobilization degree (ID) induced by plasma, 30 W was chosen as the applied rf power for all the experiments described here. The PPMM with adsorbed PNVP was mounted on a poly(vinylidene fluoride) frame which was put in the middle of the plasma chamber so that both sides of the membranes were subjected to plasma treatment. The chamber was vacummized to 10 Pa and kept constant at this value by a pressure regulator connected with the plasma chamber. Then, plasma was generated and the membrane was exposed to the air plasma for a predetermined period of time. After plasma treatment, the membrane was taken out from the chamber, washed intensively with methanol, aqueous sodium hypochlorite (NaOCl) solution, and de-ionized water, respectively. The NaOCl solution was adopted here because it was reported to be a useful solvent for the removal of NVP homopolymers [33]. At last the membrane was washed ultrasonically in de-ionized water for 30 s, then dried under reduced pressure. The immobilization degree of PNVP on the membrane surface was determined gravimetrically according to: ID =

Wt − W0 × 100% W0

(2)

where W0 represents the weight of the nascent PPMM, and Wt is the weight of the PNVP-modified one.

Fig. 1. Schematic representative of the approach used for the surface modification of PPMM.

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The grafting yield was designed as: Yield =

immobilized degree × 100% adsorption degree

averaged to get a reliable value, the standard deviation was within ±5%. (3)

2.3. Characterization To investigate the chemical changes between the original membrane and the PNVP-modified membranes, Fourier transform infrared spectroscopy (Bruker Vector 22 FT-IR) with an ATR unit (Attenuated Total Reflection, KRS-5 crystal, 45◦ ) was used. Prior to the measurements, the samples were dried under vacuum at room temperature for 36 h in the presence of P2 O5 . Data were recorded between 700 and 3100 cm−1 with 32 scans. Spectra of X-ray photoelectron spectroscopy (XPS) were recorded on a PHI 5000C ESCA system (PHI Co., USA) employing Al K␣ excitation radiation (1486.6 eV). The X-ray source was run at a power of 250 W (14.0 kV). A pass energy of 93.9 eV was used when obtaining the survey spectra. Prior to the measurements, the samples were dried under vacuum at room temperature for 48 h in the presence of P2 O5 . The pressure in the analysis chamber was maintained at 10−6 Pa during measurements. To compensate for surface charging effect, all survey spectra were referenced to the C1S hydrocarbon peak at 284.6 eV. Surface morphologies of PPMM were taken through SEM or AFM (AFM images were included in supporting information). Membrane samples were sputtered with gold and examined using a Stereoscan 260 scanning electron microscope (Cambridge, UK). The accelerating voltage used was 20 kV. A Seiko instrumental SPA 400 AFM system was used to examine the surface topography of the membranes. AFM images were acquired in the tapping mode with silicone tip cantilevers having a force constant of 20 mN cm−1 . Static water contact angles of the membrane surface were measured by both sessile drop method and captive bubble ¨ method at 25 ◦ C with a contact angle goniometer (KRUSS DSA10-MK2, Germany) equipped with video camera. Prior to the measurements, the membranes were conditioned in a desiccator for at least one month in the presence of P2 O5 to eliminate the effect of polar groups (such as C O and C OH) generated by plasma treatment [30]. In a typical sessile drop method, a water drop (∼10 ␮L) was added on a dry membrane sample in air, the image was recorded immediately (less than 3 s) and a water contact angle was determined from the image with the imaging software. For the captive bubble method, air/water/membrane interfaces were formed by immersing small membrane panel in a glass observation cell containing de-ionized water and releasing an air bubble beneath the membrane surface with a curved syringe. A camera fitted with a video screen provided a magnified image of the bubble, which was then recorded and used to calculate the contact angle. Unless otherwise noted, contact angle measurements were made one month later of the modification [14,30] and at least 10 contact angles were

2.4. Platelet adhesion evaluation Experiments were carried out with fresh platelet-enriched plasma (PRP) bought from the hospital. Firstly, the membrane was placed onto a piece of flat glass; a 20 ␮L PRP was carefully dropped on the membrane. After incubation for 30 min at room temperature, the membrane was rinsed several times by phosphate-buffer solution (PBS, Na2 HPO4 4.32 g, KH2 PO4 1.18 g, de-ionized water 1000 mL, pH 7.4). Adhered platelets on the membrane were fixed with 2.5% glutaraldehyde/PBS solution for 0.5 h, followed by dehydration procedure using a series of ethanol–water mixtures (0, 30, 40, 50, 60, 70, 80, 90, 100 vol.% of ethanol) for 30 min, respectively. At least five SEM photographs with magnification of 1000 were taken randomly from each sample surface after gold sputtering, and the amount of platelets adsorbed on unit membrane surface was counted. 2.5. Filtration experiment This process was conducted according to the report of Chen and Belfort [8]. The transmembrane pressure was kept constant at 0.1 MPa. In a typical run, the solution reservoir was initially filled with de-ionized water, and the membrane was precompacted for 30 min during the filtration of de-ionized water, then the water flux was measured every 5 min until the flux remained constant for at least three successive readings, the average of five readings was recorded as J0 . Next, the reservoir was emptied and a BSA solution was poured into it. The BSA solution was prepared by carefully dissolving BSA powder in a phosphate-buffer solution at room temperature, and the buffer solution (4.56 g NaH2 PO4 , 23.00 g Na2 HPO4 , 149.76 g NaCl, 4.02 g KCl, de-ionized water 1000 ml, pH 6.9) was obtained by dissolving preweighed quantities of the salts in the desired amount of de-ionized water. For each cycle of experiment, the concentration of BSA solution was kept constant as 1.0 g L−1 . The flux of BSA filtration (J1 ) was recorded until the flux did not change any longer, which usually needed at least 40 min. After that, the membrane was rinsed with 0.05 N NaOH for 30 min followed with 30 min permeation of de-ionized water. Then, the membrane was removed and placed upside down, the filtrations of 0.05 N NaOH solution and of de-ionized water were repeated each once. Finally, the membrane was flipped back to its original orientation and the de-ionized water flux was measured which was designed as J2 . The effect of static BSA adsorption on the permeation performances of the membranes was also measured. The membrane was previously immersed in 1.0 g L−1 BSA solution for 24 h at 30 ◦ C with constant vibration of 100 rpm. After the equilibrium of adsorption, the water flux (J3 ) of the membrane was measured according to the procedure mentioned above.

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3. Results and discussion 3.1. Immobilization of PNVP on the PPMM surface Plasma is one of the most widely used techniques for the surface modification of polymeric membranes, the efficiency of which can be greatly influenced by such factors as power, treatment time and gas atmosphere. As schematically described in Fig. 1, simply treating the PNVP-presoaked PPMM with air plasma will readily immobilize PNVP or cross-link PNVP on the hydrophobic membrane surface. In this case, one of the most effective factors is the plasma treatment time. Therefore, the influence of plasma treatment time on the immobilization degree of PNVP was examined and the typical results are listed in Table 1. It can be seen that, with nearly similar amounts of adsorbed PNVP, the immobilization degree increases as the plasma treatment time prolongs from 10 to 30 s. While after the plasma treatment time exceeded 30 s, the immobilization degree declines slowly. The calculated grafting yield, which is an illustration of the plasma treatment efficiency, follows the same tendency as immobilization degree. These results can be explained that, as the plasma treatment time increases, more and more active sites are produced both on the membrane surface and the adsorbed PNVP, the combination of these active sites will chemically bind PNVP on the membrane surface. On the other hand, plasma treatment would make bond break, which to some extent leads to the surface deterioration (the so-called plasma etching) [24], and the polypropylene membrane would be lightened after long-time plasma treatment. These two factors determine the resulted immobilization degree mentioned above. For the pre-adsoption-plasma treatment method discussed here, the amount of adsorbed PNVP is another key factor in relating to the immobilization degree of PNVP. As can be seen from Fig. 2, with the increase of the adsorption degree, the immobilization degree increases at first and then decreases remarkably. When the adsorption degree is higher than 50 wt.%, however, the immobilization degree of PNVP seems to be constant. These observations might be ascribed

Fig. 2. Effect of adsorption degree on the immobilization degree of PNVP; () and yield (). Plasma treatment time: 30 s.

to the effective depth of plasma treatment. It is well known that low-temperature plasma techniques are very surface selective, and the effective depth of plasma treatment is usually ˚ [34]. It is obvious that the reported to be about 1–1000 A thickness of the adsorbed PNVP layer increases with the increase of the adsorption degree. When the thickness is upon the effective depth of plasma treatment, the reactive sites in the atmosphere are difficult to attack directly on the membrane surface (or the polypropylene bulk molecules). Therefore, the number of active sites generated on the membrane surface for PNVP immobilization decreases. In that case, most of the adsorbed PNVP only react with each other. This part of PNVP can be washed away after plasma radiation, thus the immobilization degree of PNVP on the membrane surface decreases. In our previous paper [28], PNVP was also tethered onto PPMM through the UV-assisted or ␥-ray induced surface grafting of N-vinyl-2-pyrrolidone monomer. Typical results from the UV-assisted graft polymerization are compared in Table 2 with the data obtained from the direct immobilization method described in this work. It can be seen that the plasmainduced immobilization of PNVP is relatively simple, and

Table 1 Effect of plasma treatment time on the immobilization degree of PNVP Plasma treatment timea (s)

Adsorption degreeb (wt.%)

ID (wt.%)

10 20 30 40 50 90 120 480 720 900

28.86 ± 2.18 26.97 ± 1.24 29.03 ± 2.27 31.46 ± 3.54 26.59 ± 1.32 30.04 ± 3.09 28.48 ± 2.17 25.66 ± 1.21 29.81 ± 2.46 28.46 ± 2.38

6.41 7.59 15.52 14.18 10.12 5.38 6.73 2.54 1.70 1.93

a b c

Other plasma treatment conditions: pressure 10 Pa, 50 Hz, 30 W, room temperature. PNVP/methanol concentration (wt.%) was 5.00%. Yield was calculated from AD/ID × 100%.

± ± ± ± ± ± ± ± ± ±

0.21 0.13 1.16 1.09 0.71 0.11 0.21 0.09 0.15 0.08

Yieldc (wt.%) 22.14 31.86 53.46 45.07 38.06 17.91 23.63 9.90 5.70 6.78

± ± ± ± ± ± ± ± ± ±

1.39 2.37 4.43 3.63 3.53 1.20 1.38 0.30 0.61 0.46

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Table 2 Comparison of two methods used for the surface modification of PPMM with NVP or PNVP Concentration of NVP or PNVP (wt.%)

Method

ID (wt.%)

Yield (wt.%)

10.36 20.63 30.83 40.94 18.52 23.82 29.03 37.54

UV-assisted, 20 min UV-assisted, 20 min UV-assisted, 20 min UV-assisted, 20 min Plasma treatment, 30 s Plasma treatment, 30 s Plasma treatment, 30 s Plasma treatment, 30 s

1.35 1.68 1.37 1.68 6.41 7.44 15.52 8.13

13.03 8.14 4.44 4.10 34.93 31.26 53.46 21.65

Data for the UV-assisted surface grafting of NVP were cited from [28], and in this situation, the yield was calculated from the weight increase of the membrane divided with the total monomer weight.

the immobilization degree can be easily control through the adjustment of PNVP pre-adsorption amount. 3.2. Characterization of the PNVP-modified membranes The membrane surface was characterized by FT-IR/ATR spectroscopy and the typical spectra are depicted in Fig. 3. Compared the PNVP-modified membranes with the nascent one, a new peak appears at 1660 cm−1 , which is corresponding to the typical absorption of carbonyl group in the pyrrolidone ring of PNVP. Another new peak at about 1260 cm−1 is ascribed to the stretching vibration of C N bond. Moreover, with the increase of PNVP immobilization degree, the intensity of these peaks become stronger. To further verify the chemical changes underwent on the membrane surface, X-ray photoelectron spectra (XPS) for both the PNVP-modified and nascent PPMMs were taken. It can be seen from the survey spectrum in Fig. 4(a) that, there

Fig. 4. Survey XPS spectra of the membranes: (a) nascent PPMM; (b) 6.73 wt.% PNVP-modified PPMM.

is one peak at 284.6 eV assigned to C1S for the nascent membrane. A very small amount of oxygen at 531.76 eV is also evident, which might be due to the surface contamination of the membrane surface. For the membrane immobilized with 6.73 wt.% of PNVP (Fig. 4(b)), a new peak appears at 399.56 eV, which is designated to N1S in the pyrrolidone ring. Furthermore, the intensity of O1S at 531.76 eV increases dramatically. From the theoretical composition of PNVP, one can understand that the content of N and O atoms should be equal. However, the results in Table 3 reveal that, the amount of O atom is much higher than the N atom for the PNVPmodified membrane. This fact is thought to be due to the groups containing O element on the membrane surface introduced by air plasma treatment. Such groups can also be introduced on the surface when the membrane was exposed to atmosphere after the plasma treatment. The surface morphology of different PPMMs studied in this work was observed with scanning electron microscopy and typical photos are shown in Fig. 5. It can be seen that there are various micropores distributed on the surface with an average pore diameter of about 0.07 ␮m for the nascent PPMM (Fig. 5(a)). Upon plasma treatment, the pore diameters were somewhat enlarged due to the etching effect of plasma (Fig. 5(b)). With the increase of immobilization degree, the PNVP layer spread on the surface and blocked the micropores of the membrane (Fig. 5(c)). 3.3. Surface properties of the PNVP-modified membranes

Fig. 3. FT-IR/ATR spectra of the membranes: (a) nascent PPMM; (b) 6.73 wt.% PNVP-modified PPMM; (c) 10.12 wt.% PNVP-modified PPMM.

Two methods were adopted to measure the water contact angles of the membranes, namely sessile drop method (SDM) and the captive bubble method (CBM), and the results are

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Table 3 Elemental composition of the PPMM surfaces Membrane

Nascent PPMM PNVP modified, DM = 6.73% PNVP theoretical

C

N

O

Relative area

mol%

Relative area

mol%

Relative area

mol%

23114.5 106621.8 –

97.3 83.97 75.00

– 3610.73 –

– 1.69 12.50

1692.7 48056.8 –

2.7 14.34 12.50

shown in Fig. 6. Use of video capture for measuring contact angle of porous materials has been widely used in recent years [12–14,35,36]. Following the reported process, comparison of these values between samples provides a semi quantitative measure of the differences in wettability for porous membranes and, to a certain extent, removes issues asso-

Fig. 5. SEM images of PPMMs with different immobilization degrees of (a) 0 wt.%; (b) 6.73 wt.%; and (c) 8.13 wt.%. Membrane samples were prepared at the same plasma treatment time (30 s) with different PNVP adsorption degrees.

ciated with porous media. For the nascent membrane, the surface is much hydrophobic, the water contact angle measured by SDM was higher than 110◦ . It can be seen from Fig. 6 that the water contact angle declines gradually to 75◦ with the increase of PNVP immobilization. This means that the membrane surface is somewhat hydrophilic after the immobilization of PNVP. As demonstrated by XPS analysis, after the air-plasma treatment, together with the immobilization of PNVP macromolecules, polar groups containing oxygen are also introduced on the membrane surface. Both the immobilized PNVP and these polar groups can change the wettability of the membrane surface. To eliminate the contribution of the small polar groups generated by air-plasma treatment, all membranes to be measured the contact angle were conditioned in a desiccator for at least one month. In that case, the polar groups generated by the plasma treatment can re-orientate in the surface region and bury in the interior of the membrane surface (the so-called “hydrophobic recovery” phenomena of plasma treatment) [14,30]. Therefore, the hydrophilicity increase for the membranes mentioned above is mainly due to the PNVP chains immobilized on the surface. On the other hand, the contact angles measured by CBM show a similar tendency, however, the definite values are lower than those by SDM. It is very interesting that the difference of contact angles measured by SDM and CBM increases gradually from 10◦ for the nascent membrane to about 40◦ for

Fig. 6. Effect of immobilization degree on the water contact angle of the membranes measured by two methods: ( ) sessile drop method; () captive bubble method. The samples were prepared at the same plasma treatment time (30 s) with different PNVP adsorption degrees. Prior to the measurements, the membranes were conditioned in a desiccator for 1 month.

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Table 4 Average amount of adsorbed platelets on the surface of PNVP-modified PPMMs with different immobilization degree Immobilization degree of PNVPa (wt.%)

Amount of adsorbed platelets (×10−8 m2 membrane)

0 1.07 3.03 6.41 7.44 8.13 10.12 12.38

>200 150 ± 8.5 80 ± 7.6 46 ± 4.5 17 ± 2.5 13 ± 1.2 8 ± 1.1 2 ± 0.3

a The PNVP-modified PPMMs were prepared at the same plasma treatment time (30 s) with different PNVP adsorption degrees.

the membranes immobilized with more than 10 wt.% PNVP. On reason for these results is that the static contact angle measured by SDM is normally close to the dynamic advancing angle while that measured by CBM is similar to the dynamic receding angle. Another possible explain of these results may be that the polar parts of pyrrolidone rings interact with water and this interaction contributes to the decrease of the interfacial free energy of the membrane surface, even though the material itself has a high surface free energy. One should keep in mind that the CBM measurements are performed in an aqueous environment allowing the polymers to absorb water. This adsorption enhances with the increase of the immobilization degree of PNVP, which, in turn, increases the difference of contact angles measured by SDM and CBM, respectively. The wet surface in CBM measurement cannot therefore be directly compared to the dry surface in SDM measurement [37]. The extent of platelet adhesion and the morphology of the adhered platelets are considered to be an early indicator of thrombogenicity of blood contacting biomaterials. Fig. 7 is the SEM micrographs of the studied membranes contacted with platelets for 30 min. The statistical numbers of platelets adhered on the membrane are listed in Table 4. For the nascent PPMM (Fig. 7(a)), a large amount of platelets are adhered on the surface with serious aggregation. Many deformed platelets, such as pseudopodia, can also be observed on the nascent membrane. After surface modification with PNVP (Fig. 7(b) and (c)), the aggregation of platelets and the formation of pseudopodium are suppressed gradually. From Table 4 we can see that the amount of adhered platelets on unit membrane surface area decreases sharply with the increase of PNVP immobilization degree. At higher PNVP immobilization degree, there is scarce platelet adhered on the membrane surface. These results indicate that the hemocompatibility of PPMM can be improved obviously by the immobilization of PNVP, which may be ascribed to the hydrophilicity and the biocompatibility of PNVP. It has been suggested that, the behavior of platelet adhesion on polymeric materials depends strongly on the surface characteristics such as wettability, hydrophilicity/hydrophobicity balance, surface free energy, chemistry, charge density, roughness, micropore diameters

Fig. 7. Platelet adhesion on PPMMs with different immobilization degrees of (a) 0 wt.%; (b) 6.41 wt.%; and (c) 8.13 wt.%.

and mechanical characteristics [38]. For the PNVP-modified PPMMs, the surface becomes more hydrophilic and rougher as revealed by water contact angle measurement and AFM observation (please see the supporting information). When the membrane surface contacts with platelets, the PNVP chains can be hydrated, these hydrated chains on the surface have exerted hydrodynamics and steric hindrance effects to the approaching of platelets, which greatly reduces the adhesion of platelets. 3.4. Permeation and antifouling property of the membranes The filtration results for the nascent PPMM and the PNVPmodified PPMMs with different PNVP immobilization degree are shown in Fig. 8. Due to the relatively low porosity

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Fig. 9. Permeation flux after static BSA adsorption (J3 ), static adsorptive fouling as well as the dynamic adsorptive fouling for the PNVP-modified PPMMs with immobilization degrees of (1) 0 wt.%; (2) 4.23 wt.%; (3) 7.44 wt.%; (4) 10.12 wt.%; (5) 14.18 wt.%.

Fig. 8. Permeation fluxes (a) and flux changes (b) during filtration for the PNVP-modified PPMMs with the immobilization degrees of (1) 0 wt.%; (2) 4.23 wt.%; (3) 7.44 wt.%; (4) 10.12 wt.%; (5) 14.18 wt.%.

(45–50% for the nascent PPMM) and hydrophilicity [29], the pure water flux for each PPMM studied is lower than 100 mL m−2 h−1 . However, it can be seen from Fig. 8(a) that almost all modified membranes experience both increases in the pure water and protein solution fluxes. When filtrated with a 1.0 g L−1 BSA solution, the membrane has an enhancing tendency toward antifouling property as the immobilization degree of PNVP increases, which is shown by the decrease of the total flux loss (1 − J1 /J0 ), from 69% for the nascent membrane to 34% for the modified membrane immobilized with 14.18 wt.% PNVP (Fig. 8(b)). Cleaning the membrane with de-ionized water and NaOH solution was intended to remove BSA molecules adsorbed on the membrane surface, the results of washing demonstrate the recovery ability of the membrane fouled by proteins. From the columns shown in Fig. 8(b), it can be seen that the membranes modi-

fied with PNVP are much more easily recovered with washing, and the overall water flux recovery ((J2 − J1 )/(J0 − J1 )) is larger than that of unmodified PPMM. For 10.12 wt.% PNVP-immobilized PPMM, about 69% of the pure water flux loss was recovered after washing, indicating that a great part of BSA adsorptive fouling is reversible and the adhesion between the BSA and the PNVP-modified membranes is small. Nevertheless, for the PNVP-modified PPMMs, even cleaning the membrane with NaOH solution does not restore the flux after BSA filtration. This might be reasonable that PNVP chains are mainly immobilized on the membrane surface and BSA can be adsorbed in the pores during the filtration and then block the pores. Furthermore, the hydrophilicity of PNVP-modified PPMM is not strong enough to prevent the irreversible adsorption of proteins on the membrane surface. The static adsorption of BSA and its effect on the permeation property for the studied membranes were also measured. The water fluxes for the membranes adsorbed with BSA (J3 ) as well as the static adsorptive fouling (1 − J3 /J0 ) are shown in Fig. 9, the data of dynamic adsorptive fouling (1 − J2 /J0 ) are also listed here. Compared the static adsorption of BSA with the dynamic ones, one can clearly see the disparity of membrane properties performing at different conditions. For nascent PPMM, the dynamic adsorptive fouling is much higher than the static one. Upon the immobilization of PNVP macromolecules on the PPMM surface, both the dynamic and static adsorption of BSA decrease. While with the increase of PNVP immobilization degree, the difference between dynamic protein fouling and static one decreases. At high PNVP immobilization degree, the values of dynamic and static adsorptive fouling reach almost the same, meaning there is nearly no difference in adsorptive fouling for membranes permeating or just keeping in touch with BSA solutions. In a summary, all PNVP-immobilized PPMMs have higher pure water fluxes with enhanced flux recovery and reduced

30

Z.-M. Liu et al. / Journal of Membrane Science 249 (2005) 21–31

flux loss from BSA adsorptive fouling. Nevertheless, the fluxes for the PNVP-immobilized PPMM do not change in the same order of immobilized degree. A 4.23 wt.% PNVPmodified PPMM has the highest J0 and J2 , while J1 of the 10.12 wt.% PNVP-modified PPMM is the highest among the five membrane samples. From the SEM images we know that the immobilized PNVP chains may obstruct micropores, leading to the decrease in both pore size and porosity. Meanwhile, the immobilization of PNVP macromolecules onto the PPMM surface produces a more hydrophilic surface. This indicates that at low immobilization degree the hydrophilicity has a major effect on the permeability, while this may be overcompensated by pore blocking due to the immobilized polymer at high immobilization degree.

4. Conclusions Polypropylene microfiltration membranes were modified by the immobilization of poly-(N-vinyl-2-pyrrolidone) on the surface. The modification procedure differs from those previous reports in that poly(N-vinyl-2-pyrrolidone) is bound on the membrane by plasma treatment instead of generating from the corresponding monomer. FT-IR/ATR spectra and XPS results demonstrate the chemical changes occurring on the membrane surface. Results from water contact angle measurements indicate that the hydrophilicity of the modified membranes can be enhanced actually with the increase of immobilization degree. Platelets adhesion experiments reveal that the membranes are obviously hemocompatible after the immobilization of poly(N-vinyl-2-pyrrolidone) chains. Water and BSA solution permeation as well as static BSA adsorption results demonstrate the antifouling property of the membrane is improved.

Acknowledgements Financial supports from the National Nature Science Foundation of China (Grant no. 20074033), the National Basic Research Program of China (2003CB15705), and the High-Tech Research and Development Program of China (Grant no. 2002AA601230) are gratefully acknowledged. Prof. Zhi-Kang Xu also thanks the financial supports both from Deutsche Forschungsgemeinschaft (DFG) and the Education Ministry of China for the visit of a Chinese guest scientist to Germany.

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Surface modification of polypropylene microfiltration ...

sorb onto the surface and deposit within the pores of mem- brane, resulting in ... ing could account for up to 90% of permeability losses [2]. Therefore, much attention ... recent years, there has been much interest in developing sur- face treatment ... resulted in higher filtration performance with less total and irreversible fouling ...

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