Surface grafting control of PEGylated poly(vinylidene fluoride) antifouling membrane via surface-initiated radical graft copolymerization

Journal of Membrane Science 345 (2009) 160–169

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Surface grafting control of PEGylated poly(vinylidene fluoride) antifouling membrane via surface-initiated radical graft copolymerization Yung Chang a,∗ , Chao-Yin Ko a , Yu-Ju Shih a , Damien Quémener b , André Deratani b , Ta-Chin Wei a , Da-Ming Wang c , Juin-Yih Lai a a b c

R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan University, Jhong-Li, Taoyuan 320, Taiwan Institut Européen des Membranes, UMR CNRS-ENSCM-UM2, Université Montpellier 2, CC 047, 2 Place Eugéne Bataillon, 34095 Montpellier Cedex 5, France Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan

a r t i c l e

i n f o

Article history: Received 6 July 2009 Received in revised form 21 August 2009 Accepted 26 August 2009 Available online 1 September 2009 Keywords: Poly(vinylidene fluoride) Poly(ethylene glycol) methacrylate Surface-initiated radical graft copolymerization Surface grafting structure Antifouling

a b s t r a c t This work describes the surface grafting control of poly(vinylidene fluoride) (PVDF) membrane with poly(ethylene glycol) methacrylate (PEGMA) via three different modification approaches of surface-initiated radical graft copolymerization, including thermal-induced radical polymerization, surface-initiated atom transfer radical polymerization (ATRP), and low pressure plasma-induced graftpolymerization. Two different surface grafting structures of PEGylated layer, brush-like PEGMA and network-like PEGMA, on PVDF membrane surface were achieved in this study. The chemical composition and microstructure of the various surface-modified PEGylated PVDF membranes were characterized by Fourier transform infrared spectroscopy (FT-IR), contact angle, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) measurements. Antifouling property of the modified PVDF membranes was evaluated according to the amount of protein adsorption and the filtration test for BSA solution in this study. Results show that the amount of adsorbed proteins on the modified PVDF membranes not only depends on the surface hydrophilicity and hydration capacity but also associates with the surface grafting structures of PEGylated layers on PVDF membrane surface. This study not only introduces different practical modification approaches to achieve a hydrophobic PVDF membrane grafting hydrophilic PEGMA, but also provides a fundamental understanding of various PEGylated grafting structures governing the performance of antifouling properties. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Poly(vinylidene fluoride) (PVDF) becomes one of the attractive membrane materials for the use in microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF) regarding its outstanding properties, including excellent chemical resistance, good thermal stability and mechanical properties [1–4]. However, serious biofouling problem on the PVDF membrane surface usually limits its applications to biomedical processes [5–7]. It is generally acknowledged that hydrophilic surfaces are more likely to reduce the nonspecific adsorption of biomolecules, when living systems encounter membrane surfaces. Therefore, an ideal antifouling membrane should possess the excellent mechanical bulk properties of a hydrophobic material, such as PVDF, and the antifouling characteristics of a hydrophilic surface on the membrane pores and surface. The hydrophilic materials such as poly(ethylene glycol)

∗ Corresponding author. Tel.: +886 3 265 4190; fax: +886 3 265 4198. E-mail address: [email protected] (Y. Chang). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.08.039

(PEG)-based material have been shown to be the ideal choice of surface functional moiety with antifouling characteristics [8–10]. PEG-based materials are the most commonly used synthetic materials to effectively resist nonspecific protein adsorption. Therefore, PVDF membranes with hydrophilic PEGylated surface could extend their range of application, particularly in bioseparation or bioreactors. Surface modification is an effective approach to incorporate PEG-based functionalities, such as poly(ethylene glycol) methacrylate (PEGMA), to the existing or commercial PVDF membranes through proper grafting approach, while keeping its bulk properties. It is usually challenge to graft the highly polar PEGMA on the chemical inert and hydrophobic surface of PVDF membrane. Several strategies have been adopted to graft PEGMA polymer brushes from PVDF surfaces through surface activated pretreatment, such as activation by electron beam, gas plasma, ozone, or UV treatment, and then using surface-initiated polymerization in the following process [3,11–18]. Previous works have performed fluoride-based membranes with brush-like structure of PEGMA polymer using the combination of ozone-pretreatment

Y. Chang et al. / Journal of Membrane Science 345 (2009) 160–169

or plasma-pretreatment with thermal-induced copolymerization and surface-initiated atom transfer radical polymerization (ATRP) [3,12–14,16–20]. Brush-like structure of PEGMA polymer on other membrane systems was also introduced via surface-initiated ATRP [21–23]. Previous work has performed PVDF membranes grafted with network-like structure of PEG polymer using plasma-induced immobilization [24]. It is reported that the PEGylated PVDF membrane provides good resistance of protein adsorption, while surface grafting of PEG layer is fully covered, indicating that surface packing of PEG chains on the membrane induces different biofouling behavior [3,13–16,24]. Although there were many reports demonstrating the reduction of protein adsorption on the surface coverage of PEGylated PVDF membranes by varying PEG grafting amounts, it is still unclear in our knowledge from the reviewed literatures about the systematic illustration of protein reduction behavior associated with surface grafting structures of PEGylated layer on the PVDF membranes. In this work, two grafting structures of PEGMA layers grafted on PVDF membrane (PVDF-g-PEGMA) were achieved. Grafting structure of brush-like PEGMA on the surface of PVDF membranes was prepared via surface-initiated thermal polymerization, and surface-initiated ATRP. The other grafting structure of network-like PEGMA was prepared via plasma-induced graft-polymerization at low pressure. The chemical composition, hydrophilicity, morphology, and structures of the surface grafting layer of PVDF-g-PEGMA were characterized by Fourier transform infrared spectroscopy (FTIR), water contact angle, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Membrane biofouling properties on the prepared membranes were also evaluated by the amounts of protein adsorption using the method of Bradford and the percentage of protein fouling using ultrafiltration tests. This work is aimed at addressing two important issues of PEGMA layers grafted on PVDF membrane, i.e., (i) evaluation of surface grafting qualities of PVDF-g-PEGMA prepared via different approaches of surface-initiated radical graft copolymerization; and (ii) investigation of the biofouling property of PVDF-g-PEGMA governed with different surface grafting coverage and structures of PEGylated layer.

2. Materials and methods 2.1. Materials PEGMA monomer with a molecular weight of about 500 and an average number of ethylene glycol units of about 10 were purchased from Aldrich. Copper(I) bromide (99.999%), 2bromoisobutyryl bromide (BIBB, 98%), 2,2 -bipyridine (BPY, 99%), and triethylamine (99%), poly(ethylene glycol) (PEG8000, 99%) were purchased from Sigma–Aldrich in United States. PVDF powder having a molecular weight of 370,000 g/mol was obtained from the commercial products of Kynar 760 and was washed with acetone before use. N,N-Dimethylacetamide (DMAc, 98%) for preparing the membrane casting solution was also obtained from Sigma–Aldrich. Isopropyl alcohol (IPA, 99%) was obtained from Sigma–Aldrich and was used as a solvent for the ozone treatment. Tetrahydrofuran (THF, 99%) was obtained from Sigma–Aldrich and was used as a solvent for the reaction of BIBB with hydroxyl groups on the PVDF membrane surface. Methanol (99%) was obtained from Sigma–Aldrich and was used as a solvent for the surface-induced ATRP. Bovine serum albumin (BSA) was purchased from Sigma Chemical Co. Phosphate buffer saline (PBS) was purchased from Sigma–Aldrich. Water used in experiments was purified using a Millipore water purification system with a minimum resistivity of 18.0 M m.

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2.2. Surface-initiated copolymerization The PVDF ultrafiltration (UF) membranes were prepared by the wet phase inversion from a DMAc solution containing 15 wt% PVDF powder with pore-forming agent poly(ethylene glycol) (PEG8000, PVDF/PEG weight ratio = 5.7:1). The casting solutions were stirred for 24 h at 40 ◦ C and were left 6 h to allow complete release of bubbles. After casting the solutions with a casting knife of 300 ␮m on a glass plate, the plate was immediately immersed in a coagulation bath of double distilled water. The UF membrane was formed via phase inversion at 4 ◦ C. The formed UF membranes were washed with deionized water for 24 h to completely remove the residual solvent and pore-forming agent. A schematic illustration of surface-initiated copolymerization is shown in Fig. 1. The prepared PVDF UF membrane of about 15.2 cm2 in surface area was pretreated with a continuous stream of O3 /O2 mixture. The O3 /O2 mixture was bubbled through 50 mL isopropanol (IPA) solution with a flow rate of 6 L/min for 3 min and ozone concentration of about 46 g/L at 25 ◦ C which was generated from a custom-built ozone generator (Model OG-10PWA, Ray-E Creative Co., Ltd, Taiwan). After ozone treatment, the reactor flask was cooled quickly in an ice box at 4 ◦ C, and purged by argon for 10 min, and keep in IPA before reaction. Then, three approaches of surface-initiated copolymerization were used for the ozonetreated PVDF membranes. A low PEGMA content about 5 wt% in the reaction solution was used to demonstrate the limitation of the grafting efficiency for each approach of surface-initiated copolymerization. For the surface modification of thermally induced graft copolymerization as shown in Fig. 1(a), the preparation of the PEGMA grafted PVDF membrane (PVDF-g-PEGMA#1) followed the process using a method published previously [16]. For the surface modification of surface-initiated ATRP as shown in Fig. 1(b), the ozone-treated PVDF membrane was placed into a 50 mL distilled water at 80 ◦ C for 2 h and was then dried at room temperature under a vacuum resulting in PVDF-OH. PVDF-OH membranes were immersed in 30 mL of BIBB solution (5 wt%) in dried THF [14]. After the slowly addition of 0.5 g triethylamine at 4 ◦ C for 15 min, the system was reacted at room temperature for 18 h. PVDF membranes were draw out, washed with dried THF and distilled water to give PVDF–Br membranes. The surface copolymerization of PEGMA macromonomer on the PVDF–Br membrane was prepared via surface-initiated ATRP. PVDF–Br membranes were placed into a 30 mL of 5 wt% PEGMA macromonomer solution in methanol with specific reaction time adjusted from 0.5 to 12.0 h to achieve the desired grafting density of PEGMA. A purified argon stream was introduced to degas the solution in a single-necked round-bottom flask for about 10 min. 22.7 mg Cu(I)Br and 50.0 mg 2,2 -bipyridine were added sequentially to the solution. The reactor flask with solution was purged in a dry box with purified nitrogen at 25 ◦ C under constant stirring. After the reaction, the PEGMA grafted PVDF membrane (PVDF-g-PEGMA#2) was transferred into purified methanol and was then extracted with double distilled water and methanol. The residue solvent was removed in a vacuum oven under reduced pressure. In this study, all membranes on the surface modification were performed under the same washed conditions. For the low pressure plasma treatment of plasma-induced graftpolymerization, a schematic illustration is shown in Fig. 1(c). The PVDF membrane of about 10 cm2 in surface area was first incubated in a methanol solution containing 5 wt% of PEGMA macromonomer. The membrane coated with PEGMA macromonomer was then treated by low pressure plasma with argon flow rate of 30 sccm and input power of 150 W controlled by a 13.56 MHz RF generator (Cesar 136, Dressler). After plasma treatment, the PEGMA grafted PVDF membrane (PVDF-g-PEGMA#3) was transferred into purified methanol and was then extracted with double distilled water

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Fig. 1. Schematic illustration of the preparation process of the PVDF-g-PEGMA UF membranes via surface-initiated radical graft copolymerization: (a) ozone surface activation followed by thermal-induced radical polymerization; (b) ozone treatment and bromide activation followed by surface-initiated ATRP; (c) low pressure plasma treatment for plasma-induced graft-polymerization.

and methanol to strip off PEGMA homopolymers and unreacted monomers. The residual solvent was removed in a vacuum oven under reduced pressure. In this study, all membranes after plasma treatment were cleaned under the same post-wash procedures. 2.3. Surface characterization The chemical composition of surface-modified PVDF membranes with PEGMA polymer (PVDF-g-PEGMA#1-3) was characterized using FT-IR spectrophotometer (PerkinElmer Spectrum One) and using a ZnSe as an internal reflection element. Each spectrum was captured by averaged 32 scans at a resolution of 4 cm−1 . The surface composition of the membranes was also characterized by XPS. XPS analysis was performed using a PHI Quantera SXM/Auger spectrometer with a monochromated Al KR X-ray source (1486.6 eV photons). The energy of emitted electrons is measured with a hemispherical energy analyzer at pass energies ranging from 50 to 150 eV. All data were collected at the photoelectron takeoff angles of 45◦ with respect to the sample surface. The binding energy (BE) scale is referenced by setting the peak maximum in the C 1s spectrum to 284.6 eV. High-resolution C 1s spectrum was fitted using a Shirley background subtraction and a series of Gaussian peaks. Data analysis software was from Service Physics, Inc. The grafting weight of PEGMA on the PVDF membrane was determined by the extent of weight increase compared to the virgin PVDF membrane. Prior to the weight measurements, the membranes were dried overnight in a vacuum oven at 50 ◦ C. Water contact angles were measured with an angle-meter (Automatic Contact Angle Meter, Model CAVP, Kyowa Interface Science Co., Ltd, Japan) at 25 ◦ C. The DI water was dropped on the sample surface at 10 different sites. The surface grafting structure of virgin and these surface-modified PVDF membranes and films were examined by AFM. All AFM images were acquired with a JPK Instruments AG multimode NanoWizard (Germany). The instrument is equipped with a NanoWizard scanner and operated in air. For tapping-mode AFM, a commercial Si cantilever

(TESP tip) of about 320 kHz resonant frequency from JPK was used. The relative humidity was less than 40%. 2.4. Protein adsorption on the membranes The adsorption of BSA (99%, purchased from Sigma–Aldrich) onto the prepared PVDF UF membranes was evaluated using the method of Bradford according to the standard protocol of the BioRad protein assay. The membrane with 1 cm2 of surface area was rinsed with 20 mL of ethanol for 30 min and transferred into a clean test tube, followed by the addition of 0.5 mL of PBS solution for 30 min. Then, the membrane was soaked in 0.5 mL of 1 mg/mL BSA in 0.1 M PBS solution (pH 7.4) for 24 h at 37 ◦ C respectively. The solution was then followed by the addition of 1 mL dye reagent containing Coomassie Brilliant Blue G-250. After incubation for 5 min, the absorbance at 595 nm was determined by a UV–vis spectrophotometer. The average of the measured values from three independent membranes for each modified membrane. 2.5. Ultrafiltration experiments A dead-end cell filtration system connected with a nitrogen gas cylinder and solution reservoir was designed to characterize the filtration performance of the prepared membranes. The system consisted of a filtration cell (Model F200-HS, Ray-E Creative Co., Ltd, Taiwan) with a volume capacity of 200 mL and an inner diameter of 44 mm. Before the filtration experiments, the virgin or modified membranes were incubated and pressurized with double distilled water for 30 min at 1.5 atm. All the ultrafiltration experiments were operated at a pressure of 1.0 atm, a temperature of 25 ◦ C and a stirring speed of 300 rpm. The initial permeation flux (Jw0 or JP0 ) was checked from time to time until steady and calculated by the following equation: Jw0 =

Vw0 VP0 or JP0 = At At

(1)

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Table 1 The reaction conditions for the surface copolymerization of the PVDF membranes. Membranes

Grafting weight (mg/cm2 )

Reaction conditions of grafting PEGMA a

Virgin PVDF PVDF-g-PEGMA#1b PVDF-g-PEGMA#2c PVDF-g-PEGMA#3d

◦

PEGMA content (wt%)

Temperature ( C)

Grafting time

0 5 5 5

– 80 25 –

– 0.5–12.0 h 0.5–12.0 h 5.0–240.0 s

0.00 0.01–0.05 0.11–0.25 0.02–0.76

a A low PEGMA content about 5 wt% in the reaction solution was used to demonstrate the limitation of the grafting efficiency for each approach of surface-initiated copolymerization. b Membranes prepared by thermal-induced grafting copolymerization. c Membranes prepared by surface-initiated atom transfer radical polymerization. The ratio of PEGMA macromonomer/Cu(I)Br/bpy was 15:1:2. d Membranes prepared by plasma-induced grafting copolymerization.

where the parameters: Vw0 , VP0 , A, and t denote the pure water and protein solution permeate volume in the initial stage (in L), membrane area (in m2 ) and permeation time (in h). The pure water flux of the protein fouled membranes (Jw1 ) was measured at steady filtration for 60 min at 25 ◦ C. The average of measured values using three independent membranes for the virgin and each modified membrane was taken as its pure water flux. The degree of flux recovery ratio (RFR ) was then calculated by the following equation: RFR =

J

w1

Jw0



× 100%

(2)

To evaluate the fouling-resistance of a membrane, the degree of flux loss caused by total protein fouling in the ultrafiltration, Rt , was defined as



Rt =

Jw,0 − JP,0 Jw,0



× 100%

(3)

The flux loss was caused by both reversible and irreversible protein fouling in the ultrafiltration (Rr and Rir ). Each of them was defined by



Rr =

Jw,1 − JP,0 Jw,1

 Rir =



Jw,0 − Jw,1 Jw,0

× 100%

(4)

 × 100% = 100% − RFR

(5)

3. Results and discussion 3.1. Surface grafting copolymerization and characterization In order to improve of antifouling property of hydrophobic PVDF membrane, the grafting of hydrophilic PEGMA layer on membrane surface was introduced using surface-initiated radical graft copolymerization. As shown in Fig. 1, three different approaches of surface grafting copolymerization were performed to modify PVDF membrane surface grafted with PEGMA polymer in this study. Fig. 1(a) showed that the membrane of PVDF-g-PEGMA#1 was prepared by the method of thermally induced grafting copolymerization, as reported previously [16]. As shown in Fig. 1(b), the integrated process for surface-initiated ATRP could be divided into three stages. The first stage was to produce hydroxyl groups on the PVDF membrane surface (PVDF-OH) via ozone treatment and heating. The amount of produced hydroxyl groups can be controlled by the ozone treating time. The second stage in the preparation process was subsequent to coupling the initiator of bromide (Br) onto the PVDF-OH membrane surface (PVDF-Br) via dehydration reaction. The following stage in the preparation process for PVDF-gPEGMA#2 was finally to copolymerize the PEGMA macromonomer from the PVDF–Br membrane surface via surface-initiated ATRP. As shown in Fig. 1(c), the first step of the integrated process is the uniform coating of PEGMA macromonomer onto PVDF membrane

Fig. 2. FT-IR spectra of the virgin PVDF membrane and PVDF membranes grafted with PEGMA polymer.

surface and pores in a PEGMA/methanol incubation solution. The amount of coated macromonomer can be controlled by the mass content of monomer in the incubation solution. The following step is the low pressure plasma treatment for PVDF-g-PEGMA#3 which grafts the PEGMA polymer onto the PVDF membrane surface via plasma-induced graft-polymerization. All reaction conditions for the surface copolymerization of the PVDF membranes are summarized in Table 1. The virgin PVDF UF membrane was obtained from the wet inversion process without any surface modification. The membrane of PVDF-g-PEGMA#1 was prepared by surface-induced thermal copolymerization at 80 ◦ C based on our previous study, the membrane of PVDF-g-PEGMA#2 was prepared at 25 ◦ C by surface-initiated ATRP, and the membrane of PVDF-g-PEGMA#3 was prepared at low pressure by plasmainduced grafting copolymerization [16,19,21]. In general, the grafting weight of the copolymerized PEGMA on PVDF membrane surface increase with increasing macromonomer concentration of PEGMA in the reaction solution [16,20]. It is usually challenge to obtain high grafting density of PEGMA chains on the membrane surface at low concentration of PEGMA macromonomer. Therefore, a low PEGMA content of 5 wt% in the reaction solution was introduced to demonstrate the limitation of the grafting control for each approach of surface-initiated copolymerization. To compare the range of achieved grafting weight of PEGMA on PVDF membrane surface, the reaction time for both membranes of PVDFg-PEGMA#1 and PVDF-g-PEGMA #2 was adjusted from 0.5 to 12 h and that for the membrane of PVDF-g-PEGMA#3 was adjusted from 5 to 240 s. FT-IR measurement was used to characterize the chemical composition of the PEGMA modified PVDF membrane and its typical spectrum was shown in Fig. 2. For the grafted PEGMA, the presence of the grafted polymer can be ascertained from the ester carbonyl

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substantial increase in surface grafting amount of PEGMA chains on the PVDF membrane surface compared to surface-initiated ATRP. Thus, it provides a significant practical insight that the surface modification via plasma treatment can be performed at a short grafting time with high grafting efficiency. 3.2. Surface grafting structures of PEGMA layer on the prepared PVDF membranes

Fig. 3. Changes in the PEGMA grafting weight of PVDF-g-PEGMA#1, PVDF-gPEGMA#2 and PVDF-g-PEGMA#3 as a function of grafting time during surface copolymerization, respectively.

groups attributable to the band for C O stretch at 1727 cm−1 , which obviously appears on the modified PVDF membranes. For the membrane of PVDF-g-PEGMA#1, the FT-IR characterization has been shown in previous report [16]. For the membrane of PVDF-gPEGMA#2 and PVDF-g-PEGMA#3, the ratio of the intensity of the absorption band for O–C O at 1727 cm−1 to that of the adsorption band for C–F at 1400 cm−1 ([O–C O]/[C–F] ratio) reflects the relative graft amount of PEGMA on the PVDF membrane surface. It was found that both the intensity of the O–C O absorption at 1727 cm−1 and the [O–C O]/[C–F] ratio increased obviously as the starting reaction time was increased. The result indicates that the growth of the grafted PEGMA polymer is dependent on the control of the grafting time during the reaction. The intensity of the O–C O absorption and the [O–C O]/[C–F] ratio of PVDF-g-PEGMA#3 is higher than that of PVDF-g-PEGMA#2, which indicate the grafting amount of PEGMA on PVDF membrane using plasma-induced grafting copolymerization is more than that using surface-initiated ATRP. Fig. 3 shows the dependence of grafting weight of the modified PVDF membranes on the grafting time during the surface copolymerization for all membranes of PVDF-g-PEGMA#1, PVDFg-PEGMA#2, and PVDF-g-PEGMA#3. The grafting weight (mg/cm2 ) of surface-modified PVDF membrane is defined as the difference in weight between the modified PVDF membrane and the virgin PVDF membrane divided by the total surface area of the virgin PVDF membrane. For the membranes of PVDF-g-PEGMA#1 and PVDF-g-PEGMA#2, the grafting weight of PEGMA on the PVDF membrane surface increased as the grafting time increased from 0 to 12 h with the low content of 5 wt% PEGMA in methanol solution. After the copolymerization time of 3 h, the grafting weight of the both prepared membranes approach the limitation of about 0.05 and 0.25 mg/cm2 , respectively. It was found that grafting efficiency of PEGMA brushes on PVDF membrane surfaces using surface-initiated ATRP is higher than that using surface-induced thermal copolymerization. For the membranes of PVDF-g-PEGMA#3, the grafting weight of PEGMA on the surface of PVDF membranes increased as the plasma treating time increased from 5 to 240 s. Results showed that grafting weight of the PVDF-gPEGMA#3 membrane reaches above the value of 0.25 mg/cm2 via plasma-induced graft-polymerization just for 60 s, indicating the

In general, the uniformity of the grafted polymer onto the modified membrane surface is usually crucial to control the filtration efficiency of a prepared membrane. The surface structure of the modified membranes was observed by AFM. In Fig. 4, the surface morphology and roughness of the PVDF membrane surface revealed an obvious change associated with the preparation approach. For the membrane of PVDF-g-PEGMA#1, the surface roughness of the modified PVDF membrane via thermal-induced polymerization performed less surface uniformity than that of the virgin PVDF membrane. The grafted PEGMA polymer on the PVDFg-PEGMA#1 membrane showed an inhomogeneous distribution and an obvious increase of the surface RMS roughness of about 52.2 ± 3.9 nm. However, for the membrane of PVDF-g-PEGMA#2, it was found from the AFM images that the grafted hydrophilic PEGMA polymer on the PVDF membrane surface was homogeneous distribution via surface-initiated ATRP. Further, the surface RMS roughness had almost no change of about 40.9 ± 3.4 nm that is close to a level comparable with the surface RMS roughness of the virgin PVDF membrane (∼39.4 nm), indicating the formation of the uniform grafted surface for the prepared membrane of PVDF-g-PEGMA#2. For the membrane of PVDF-g-PEGMA#3, the surface roughness of the modified PVDF membrane via plasmainduced graft-polymerization performed an obvious increase of about 78.8 nm obtained from the plasma treating time of 30 s, which the grafting weight is almost the same to PVDF-g-PEGMA#2 obtained from the reaction time of 12 h. The result indicates the grafting structures of PEGMA layer on PVDF membrane surface are associated with the different preparation approach using surfaceinitiated ATRP and plasma-induced graft-polymerization. The AFM results suggest that surface-initiated ATRP is a controlled process to prepare hydrophilic PVDF membranes with a uniform distribution of grafted PEGMA polymer. To further demonstrate the structures of PEGMA layer on PVDF membrane surface, the grafted PEGMA was identified by XPS measurement. Fig. 5(a) shows the XPS spectrum of the PVDF membrane coated with PEGMA macromonomer (PVDF-c-PEGMA). The C1s core-level spectrum of the PVDF-c-PEGMA was curve-fitted with peak components for the [C–C, C–H], [C–O], and [C O] species, at the binding energies of about 284.6, 286.5 and 288.8 eV, respectively. The characterization results of PEGMA layer on the PVDF membranes are summarized in Table 2. By XPS analysis of the elemental compositions of PVDF-c-PEGMA membranes, the peak component area ratio of [C–C, C–H]/[C–O]/[C O] for the chemical structure of PEGMA macromonomer used in this study is about 2.86:19.40:1, which is in good agreement with the theoretical ratio of 3:20:1 for the chemical structure of PEGMA with an average number of ethylene glycol of about 10 units. According to the reaction mechanism of thermal-induced radical polymerization and surface-initiated ATRP, PVDF-g-PEGMA#1 and PVDF-g-PEGMA#2 could keep the structure of ethylene glycol units in the grafted PEGMA layer that have been reported in previous studies [3,12–14,16–23]. Thus, reasonable grafting structure of brush-like PEGMA on the surface of PVDF-g-PEGMA#1 or PVDFg-PEGMA#2 is illustrated as shown in Fig. 6(a). The C1s core-level spectra of the PVDF-g-PEGMA#3 using plasma treatment of 30 and 240 s possessed three components of the [C–C, C–H], [C–O], and [C O] species via curve fitting as shown in Fig. 5(b) and (c). From

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Fig. 4. Tapping-mode AFM of surface RMS roughness of the modified PVDF membranes with PEGMA: (a) virgin PVDF membrane; (b)–(d) PVDF-g-PEGMA#1 with the grafting time of 0.5, 1.0 and 12 h, respectively; (e)–(g) PVDF-g-PEGMA#2 with the grafting time of 0.5, 1.0 and 12 h; (h)-(j) PVDF-g-PEGMA#3 with the grafting time of 30, 90 and 240 s, respectively. The dimensions of the scan images are 30.0 ␮m × 30.0 ␮m with a Z scale of 300 nm.

composition analysis summarized in Table 2, the [C–O]/[C O] ratio of the PVDF-g-PEGMA#3 decreased from 10.05:1 to 6.51:1 as the plasma treating time increased from 30 to 240 s, which indicates the degradation of ethylene glycol structure during low pressure plasma treatment. It was also found that the [C–C, C–H]/[C O] ratio increased as the increase of plasma treating time, which indicates the crosslinking reaction occurs between PEGMA chains or between PEGMA chains and PVDF membrane surface. Therefore, possible grafting structure of network-like PEGMA on the surface of PVDF-g-PEGMA#3 is proposed as shown in Fig. 6(b). 3.3. Correlation of surface hydrophilicity and grafting structure of the prepared PVDF membranes with protein adsorption The antifouling behavior of protein adsorption onto a surface with grafted polymer chains has been extensively studied over the past two decades [25–30]. The formation of the hydration layer on a hydrophilic surface was believed as a crucial issue to repel proteins and made the protein conformations unchanged. In general, grafting density, chain length, and chain conformation of grafted hydrophilic polymer on the surface are the determining factors associated with the surface hydration layer of binding water molecules via hydrogen bonding. It is reported that the PEGylated surface coverage increased with the increase of PEG grafting density and chain length, resulting in the good resistance of protein adsorption on PVDF membranes with grafting PEG [3,13–16,24]. Fig. 7 showed the effect of surface grafting weight of PEGMA on the change of surface contact angle and the hydration capacity of the

modified PVDF membranes. The reduced values of water contact angle in Fig. 7 indicate that the surface hydrophilicity of modified PVDF membranes increases with increasing grafting amount of PEGMA chains on the surface of PVDF membranes. The water contact angle of the PVDF-g-PEGMA#2 achieved as low as about 60◦ , indicating an obvious increase in hydrophilicity compared to the virgin PVDF membrane surface, whose water contact angle was about 80◦ . The decrease of water contact angle along with the increase in surface grafting coverage of the PVDF-g-PEGMA membrane is attributed to the increased surface coverage of hydrophilic PEGMA chains on the hydrophobic PVDF membrane. It was also found that there appeared almost unchanged value of the water contact angle as the grafting coverage above 0.15 mg/cm2 for the PVDF-g-PEGMA#2. This indicates that grafted PEGMA chains on the PVDF membrane surface approaches its respective saturated coverage and surface uniformity with steady water contact angle. In this study, hydration capacity of the prepared membrane was evaluated by the difference in wet weight between the PEGMA grafted PVDF membrane and the virgin PVDF membrane divided by the total surface area of the virgin PVDF membrane. The measured quantity of hydration capacity may undergo the following three types of contribution as illustrated in Fig. 6: (1) trapping water molecules in the porous structure of the virgin PVDF membrane, (2) binding water molecules around the ethylene glycol structure of the PEGMA brushes, and (3) captured water molecules in the confined space between PEGMA chains. Result in Fig. 7 indicated that the hydration capacity of the virgin PVDF membrane is about 2.0 mg/cm2 , which is attributed form trapping water

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the contribution of binding and captured water molecules. Fig. 7 showed that the hydration capacity of PVDF-g-PEGMA#3 is higher than that of PVDF-g-PEGMA#2 at the same PEGMA grafting weight of about 0.25 mg/cm2 , which is due to that the confined space for captured water molecules in the grafting structure of networklike PEGMA layer is more than that of brush-like PEGMA layer as illustrated in Fig. 6. In this study, BSA was selected as a model protein to test antifouling performance for the membranes of PVDF-g-PEGMA#2 and PVDF-g-PEGMA#3. The single protein adsorption was evaluated by immersing the membranes in 1 mg/mL BSA solution and the amount of adsorbed protein was estimated by the method of Bradford. All of the protein adsorption amounts on the membranes were correlated according to the standard calibration curve established using the UV absorbance at 595 nm. Fig. 8 showed the effect of the hydration capacity on the BSA adsorption on the modified membrane surface. It was found that reduction in nonspecific BSA adsorption exhibited a linear correlation with the variation of the hydration capacity of PVDF-g-PEGMA#2. It can be seen that the relative protein adsorption on PVDF-g-PEGMA#2 is effectively reduced to 20% of that on virgin PVDF as the hydration capacity increased to 6 mg/cm2 , indicating that the grafting structure of brush-like PEGMA layer can highly resist nonspecific protein adsorption. However, the relative protein adsorption on PVDF-gPEGMA#3 is only reduced to the limitation at 60% of that on virgin PVDF, even as the hydration capacity increased to 10 mg/cm2 . The results may be related to the types of water molecules in the hydrated layer and the grafting structure of the PEGMA layer on the PVDF membrane surface. As shown in Fig. 6(b), the grafting structure of network-like PEGMA layer on PVDF-g-PEGMA#3 provides more hydration capacity of captured water molecules than that of binding water molecules, as indicated by the degradation of ethylene glycol structure and the crosslinking of PEGMA chains. In general, the binding water molecules around the PEGMA structure decreased with the decrease in the repeated units of ethylene glycol. Based on the current studies of general antifouling mechanism, it is acceptable to consider that the binding water molecules around the pendent groups of the antifouling chains play a key role in providing resistance to protein adsorption [27,28]. Thus, the observed results indicate that the hydrated layer on PVDF-g-PEGMA#2 with more binding water molecules than that on PVDF-g-PEGMA#3. These results also suggest that the evaluation of protein adsorption on the hydrated membrane surfaces should be considered not only the surface hydrophilicity and hydration capacity but also the surface grafting structure of the prepared PVDF membranes. Fig. 5. C1s core-level spectra of the (a) PVDF-c-PEGMA, and PVDF-g-PEGMA#3 from (b) 30 s, and (c) 240 s of plasma treatment.

3.4. Analysis of the ultrafiltration of BSA solution

molecules in the membrane matrix due to the highly porosity in PVDF membranes. In general, the increase in grafting weight associated with the increase in the thickness of the PEGMA layer resulted in the increased quantity of hydration capacity, which is depend on

The biofouling characteristics of the PEGMA grafted PVDF membrane was evaluated by ultrafiltration tests. Fig. 9 showed typical results for the permeation fluxes through four different membrane samples, including the virgin PVDF membrane and three PVDF membranes grafted with PEGMA (PVDF-g-PEGMA#1 and PVDF-g-

Table 2 Surface characterization of PEGMA composition on the PVDF membranes. Samples

PEGMAa PVDF-c-PEGMAb PVDF-g-PEGMA#3, 30 sc PVDF-g-PEGMA#3, 240 sc a b c

Compositions of grafting PEGMA layer (mol%) [C–C, C–H]

[C–O]

[C O]

12.50 12.30 24.28 38.57

83.34 83.40 68.86 53.25

4.16 4.30 6.85 8.18

Theoretical compositions of PEGMA macromonomer. Membranes coated with PEGMA of about 4.5 mg/cm2 . Membranes prepared by plasma-induced grafting copolymerization.

[C–C, C–H]/[C O]

[C–O]/[C O]

3.00 2.86 3.54 4.72

20.00 19.40 10.05 6.51

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Fig. 6. Illustration of the grafting structures of (a) brush-like PEGMA, and (b) network-like PEGMA on the PVDF membrane surface.

PEGMA#2 using the grafting time of 12 h, and PVDF-g-PEGMA#3 using the grafting time of 240 s). To begin the test, pure water was first passed through the membrane until a steady flux (Jw0 ) was obtained. It was found that the Jw0 of the membrane grafted with PEGMA was smaller than that of the virgin PVDF membrane. The reduced flux was probably due to resistance from the layer of grafted PEGMA polymer brushes or networks onto PVDF membrane surface, especially for the case of PVDF-g-PEGMA#3 indicating the grafting structure of network-like PEGMA layer with a strong flux resistance during filtration test. The operated ultrafiltration process could be divided into three phases. The first phase was the passing of pure water. The second phase was the filtration of protein solution. The third phase was a simple membrane cleaning by pure water flushing and then following the passing of pure water. It should also be noted that the rejection ratio of BSA with regard to

Fig. 7. Changes in the water contact angle and degree of hydration for the prepared PVDF membranes as a function of the PEGMA grafting weight on the membrane surface.

the virgin PVDF membrane was only 52%, indicating the membrane of wider pore size were formed. Thus, the prepared PVDF membranes could reject about 50% of proteins with molecular weight of about 70.0 kDa. The rejection ratios of BSA for three PVDF membranes grafted with PEGMA were higher than that of virgin PVDF membrane (PVDF-g-PEGMA#1 with the BSA rejection ratio of about 60%, PVDF-g-PEGMA#2 with the BSA rejection ratio of about 70%, and PVDF-g-PEGMA#3 with the BSA rejection ratio of about 80%). It was also found that the BSA rejection ratios increased as the PEGMA grafting weight increased. As shown in Fig. 9, the permeation flux of BSA solution decreased rapidly compared to the flux of pure water (Jw0 ) at the initial stage because of protein fouling and concentration polarization. According to our previous study, the effects of concentration polarization on the permeation flux can be effectively reduced using high speed stirring at 300 rpm for the protein solution during the ultrafiltration experiments [31]. A

Fig. 8. Effect of the hydration capacity of the prepared PVDF membranes on the protein adsorption.

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Fig. 9. Time-dependent flux of the virgin PVDF membrane and three PVDF membranes grafted with PEGMA (PVDF-g-PEGMA#1, #2 and #3) of their respective grafting weight via different modification approaches. Ultrafiltration process was operated at a pressure of 1.0 atm, a temperature of 25 ◦ C and a stirring speed of 300 rpm. The BSA concentration is 1.0 mg/mL in PBS solution.

steady flux (JP0 ) was obtained when the protein adsorption became saturated. To monitor the irreversible membrane fouling, the pure water flux (Jw1 ) was measured after membrane cleaning. The values of Jw0 , JP0 and Jw1 could then be used to calculate water flux recovery, total fouling, reversible and irreversible fouling ratios. Fig. 10 shows these ratios of the PVDF-g-PEGMA#2 membranes grafted different amounts of PEGMA during the ultrafiltration using BSA as the tested protein. The total protein fouling (Rt ) was calculated by Eq. (3) which represents the percentage flux loss because of protein adsorption and retention. The values of Rt decreased as the grafting weight increased, indicating the grafting of PEGMA polymer on the PVDF membrane surface exhibited an efficient reduction of the total protein fouling. The total protein fouling was actually built up by persistent protein adsorption and temporal protein blockage. The flux loss because of temporal protein blockage was reversible and could be recovered by membrane cleaning. Therefore, the reversible fouling ratio, Rr , was defined by the percentage of pure water flux recovered in the third phase from its loss in the second phase. The percentage of unrecoverable flux, which was resulted from the fouling of persistent protein adsorption, was defined as the irreversible fouling ratio, Rir . The water flux recovery

ratio, RFR , defined as the ratio of pure water flux in the third phase to that in the first phase, was exactly equal to 1 − Rir . The antifouling capability of a membrane was usually monitored by the value of Rir or RFR . The higher value of RFR indicated the lower persistent protein adsorption to the membrane operated during the ultrafiltration process. The value of RFR was 55.5% for the untreated PVDF membrane, but it increased to 90.8% as the grafting density increased approach 0.25 mg/cm2 . The Rt value represented the overall effect of fouling on flux loss. The loss is probably due to the contribution of both the loosely attached and the firmly adsorbed proteins. It was observed that the Rt decreased with the increase in PEGMA grafting density from 0 to 0.25 mg/cm2 . The Rr value was contributed by the loosely attached proteins, which represents a light increasing from 10 to 20% with the increase in PEGMA grafting density. This might be due to the creation of physically attached protein attributed from long PEGMA polymer chains. The analysis of BSA filtration indicates that the PVDF membranes grafted with PEGMA brushes effectively reduced the irreversible membrane fouling. 4. Conclusions In this work, PVDF UF membranes were grafted with hydrophilic PEGMA polymer through three different modification approaches of surface-initiated radical graft copolymerization. The effects of the grafting time control for each modification approach on surface grafting qualities, such as surface hydrophilicity, hydration capacity, grafting weight, and uniformity, of the grafted PEGMA layer on the PVDF membrane surface were studied. Results showed that hydration capacity of water molecules inside the grafting structure of network-like PEGMA is more than that of brush-like PEGMA. However, it appears that grafting structure of brush-like PEGMA lead to lower protein adsorption and better antifouling for BSA filtration than that of network-like PEGMA on PVDF membrane surface. These results suggest that the evaluation of protein adsorption on the hydrated membrane surfaces should be considered not only the surface hydrophilicity and hydration capacity but also the surface grafting structure of the prepared PVDF membranes. Acknowledgements The authors express their sincere gratitude to the Center-ofExcellence (COE) Program on Membrane Technology from the Ministry of Education (MOE), R.O.C., to the project Toward Sustainable Green Technology in the Chung Yuan Christian University, Taiwan (CYCU-97-CR-CE), and to the National Science Council for their financial support. References

Fig. 10. Effect of PEGMA grafting density on flux recovery ratio (RFR ) and fouling ratio (R: Rt , Rir , and Rr ) in the ultrafiltration test PVDF-g-PEGMA#2 membranes using BSA as the tested protein.

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