Improvement of Antifouling Properties of Polyvinylidene Fluoride

Jan 22, 2014 - Evdokia K. Oikonomou , Szilvia Karpati , Sana Gassara , André Deratani , François Beaume , Olivier Lorain , Sylvie Tencé-Girault , S...
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Improvement of Antifouling Properties of Polyvinylidene Fluoride Hollow Fiber Membranes by Simple Dip Coating of Phosphorylcholine Copolymer via Hydrophobic Interactions Shu Nishigochi,† Toru Ishigami,† Tatsuo Maruyama,† Yan Hao,† Yoshikage Ohmukai,† Yasuhiko Iwasaki,‡ and Hideto Matsuyama*,† †

Center for Membrane and Film Technology, Department of Chemical Science and Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan ‡ Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Osaka 564-8680, Japan S Supporting Information *

ABSTRACT: We present a simple surface modification method for improving the antifouling properties of polyvinylidene fluoride (PVDF) hollow fiber membranes for water treatment. Membranes were dip coated in a block copolymer of 2methacryloyloxyethyl phosphorylcholine (MPC) and butyl methacrylate (BMA) (poly(MPC-co-BMA)) aqueous solution. Membranes coated with poly(MPC-co-BMA) at various coating concentrations exhibited higher antifouling properties than bare and MPC homopolymer-coated membranes, while showing higher water permeabilities after fouling. Fluorescence observation revealed the effect of coating concentration on poly(MPC-co-BMA) distribution within the hollow fiber membranes. The results of quartz crystal microbalance measurements showed that almost no bovine serum albumin was adsorbed onto the poly(MPCco-BMA) coating, whereas it was highly adsorbed onto bare and MPC homopolymer coatings. We quantified the amount of poly(MPC-co-BMA) on the membrane before and after cleaning, using fluorescence microscopy. The poly(MPC-co-BMA) coating layer used in the hydrophobic interaction between BMA moieties and the PVDF membrane surface was quite stable. pyrrolidone),14 poly(vinyl alcohol),15 poly(ethylene glycol),16 and zwitterionic molecules11 have been reported. Among these hydrophilic molecules, the 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer has attracted significant attention because of its high biocompatibility. Ishihara et al. showed that an MPC polymer-coated surface significantly reduced protein adsorption compared with other well-known hydrophilic molecules.17,18 This effect was attributed to the hydration of the phosphorylcholine groups, forming a strongly bound lubrication hydration layer.19 Akhtar et al. and Reuben et al. coated the surfaces of flat sheet polymer membranes by grafting MPC.20,21 Although grafting methods ensure long-term stability, these methods are limited by multistep operations and it is difficult to uniformly modify a hollow fiber membrane surface. Ishihara et al. prepared polysulfone and polyethylene membranes containing an MPC polymer by the blend method.22,23 However, the blend method requires a large amount of MPC polymer because hydrophilic MPC polymer may easily leak from the dope solution during porous membrane preparation based on phase separation methods. This coating method is quite simple for modifying the membrane surface and requires only small amounts of additives. However, MPC coatings on a hydrophobic substrate such as PVDF are considered unstable, because MPC may not

1. INTRODUCTION Polyvinylidene fluoride (PVDF) hollow fiber membranes have been widely applied in ultrafiltration and microfiltration because of their high chemical and thermal resistance and mechanical strength, with a high packing density. However, PVDF membranes are easily fouled by accumulation, deposition, and adsorption of hydrophobic foulants to membrane pores because of their high hydrophobicity. When fouling occurs, the filtration performance decreases and surplus pump power is required to supply the feedwater. Thus, to save energy, it is necessary to suppress the fouling. Research on membrane fouling has shown repeatedly that serious membrane fouling is caused by natural organic matter (NOM) such as humic substances, polysaccharides, and proteins.1−3 Among many types of foulants, proteins are strongly adsorbed onto a PVDF membrane surface and are hardly desorbed because of hydrophobic interactions between proteins and the surface.4−6 Hashino et al. reported that bovine serum albumin (BSA) adsorbed to a PVDF hollow fiber membrane surface undergoes a conformational change and is only slightly desorbed by backwashing.7 Thus, it is important to introduce antifouling properties to PVDF membranes. A number of researchers to date have carried out surface hydrophilic modification to suppress membrane fouling.8−16 Many surface modification methods, such as graft polymerization10,11 and polymer blending during membrane preparation,12,13 have been investigated to impart antifouling. Furthermore, the effects on fouling of many types of hydrophilic modifier on PVDF membranes, such as poly(vinyl © 2014 American Chemical Society

Received: Revised: Accepted: Published: 2491

December 3, 2013 January 16, 2014 January 22, 2014 January 22, 2014 dx.doi.org/10.1021/ie404094t | Ind. Eng. Chem. Res. 2014, 53, 2491−2497

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(BA-0, Imoto Co., Kyoto, Japan) based on our previous report.28 PVDF, GTA, and glycerol (PVDF/GTA/glycerol: 30/ 60/10 wt %) were mixed for 1 h at 210 °C in a vessel equipped with a stirrer and then degassed for 1 h at 190 °C. The homogeneous PVDF solution was fed to a spinneret consisting of an inner tube diameter of 0.83 mm and an outer tube diameter of 1.58 mm. The PVDF solution and a bore liquid were extruded from the outer and inner channels of the spinneret, respectively, and were passed through an air gap of 5 mm into a water bath at 0 °C, where phase separation occurred. The hollow fiber membrane was wound onto a take-up winder. The solvent remaining inside the obtained membrane was extracted using water and ethanol. The membrane was kept in DI water until use. 2.3. Surface Modification. The dip coating of poly(MPCco-BMA) and MPC was performed on the outer surface of the PVDF hollow fiber membrane. Both ends of the hollow fiber membrane were sealed and the membrane was then immersed in the poly(MPC-co-BMA) aqueous solution with mixing for 1 h. The poly(MPC-co-BMA) concentration was changed in three steps; 0.01, 0.1, and 0.5 wt %. The coated membranes were immersed in DI water, with mixing for 10 min, and then kept in DI water until use. 2.4. Characterization of Poly(MPC-co-BMA)-Coated Membranes. 2.4.1. FE-SEM Observation. The membrane morphologies were observed by a field-emission scanning electron microscope (FE-SEM, JSF-7500F; JEOL, Tokyo, Japan). To obtain dry membranes, the membranes were placed in a freeze-dryer (FD-1000, EYELA, Tokyo, Japan). The dry membranes were fractured in liquid nitrogen and then sputtered with Pt/Pd. The images for membranes were obtained at an accelerating voltage of 50 kV. 2.4.2. Filtration Experiment. Filtration experiments for 1000 mg·L−1 BSA solution were conducted using a module at room temperature, as shown in Figure 2. A single hollow fiber

be strongly immobilized as it is highly hydrophilic. To immobilize the hydrophilic MPC polymer on the surface, water-soluble amphiphilic copolymers of MPC that can be adsorbed onto a hydrophobic surface in an aqueous solution have been proposed. Iwasaki et al. modified the inner surface of a polysulfone hollow fiber membrane for hemodialysis using an inner solution containing a MPC copolymer during hollow fiber spinning.24 Ye et al. prepared flat sheet and hollow fiber cellulose acetate membranes coated with a copolymer of MPC and butyl methacrylate for a biomedical application and the prepared membranes showed significantly suppressed protein adsorption.25−27 However, to the best of our knowledge, there have been no reports on the surface modification of a PVDF hollow fiber membrane with an MPC copolymer by the dipcoating method for water treatment. This is probably because water treatment processes in practice require long-term stability of the coating layer. It has been considered that a coating layer immobilized by physical adsorption such as hydrophobic interaction may become detached from the membrane surface. In this study, we present an MPC copolymer-modified PVDF hollow fiber ultrafiltration membrane with antifouling properties. We prepared a water-soluble block-type copolymer of MPC and butyl methacrylate (BMA) (poly(MPC-co-BMA)) in aqueous solution and then modified the membrane surface via hydrophobic interaction between BMA moieties and the PVDF surface by immersing the membrane into the solution. We also investigated the effect of coating concentration on the antifouling properties and stability of the coating layer.

2. EXPERIMENTAL SECTION 2.1. Materials. The block copolymer, 2-methacryloyloxyethyl phosphorylcholine and butyl methacrylate, poly(MPCco-BMA) (MPC: BMA = 0.3:0.7 (mole fraction), Mw = 100 kDa) shown in Figure 1 and the MPC homopolymer (Mw =

Figure 1. Chemical structure of poly(MPC-co-BMA).

100 kDa) were kindly supplied by NOF Corporation (Tokyo, Japan). Fluorescein isothiocyanate-poly(MPC-co-BMA) (FITC-poly(MPC-co-BMA)) with a molar ratio of 1:99 (FITC: poly(MPC-co-BMA)) was also kindly supplied by NOF Corporation. PVDF (Type 6020, Mw = 322 kDa) was supplied by Solvay Soiexis K.K (Tokyo, Japan). Other chemicals were purchased from Wako Pure Chemical Industries (Osaka, Japan). All chemicals were used without further purification. The water used was high-quality deionized water (DI water, > 15 MΩ·cm−1) produced by an Elix-5 system (Millipore, Molsheim, France). For the filtration and quartz crystal microbalance experiments, a phosphate buffer solution at pH 7.0 was prepared from sodium dihydrogen phosphate and disodium hydrogen phosphate. A bovine serum albumin (BSA) solution was prepared by mixing BSA with the phosphate buffer solution. 2.2. Preparation of Hollow Fiber Membranes. The PVDF hollow fiber membranes were prepared by the thermally induced phase separation method using a batch-type extruder

Figure 2. Schematic of the filtration experiment using a single hollow fiber membrane module.

membrane 110 mm in length was packed into a module.7 Cross-flow filtration was carried out from the outside to the inside of the hollow fiber membrane. The flow rate of the feed solution and the transmembrane pressure were set to 15 mL· min−1 and 50 kPa, respectively. Before filtration experiments, DI water was permeated as a feed and the pure water permeability, J0 [L·m−2·h−1·atm−1], was measured. Then, a BSA solution was ultrafiltered and the time variation of the permeability, J [L·m−2·h−1·atm−1], was tracked. BSA rejection, R [%], is defined as

⎛ Cp ⎞ R = ⎜1 − ⎟100 Cf ⎠ ⎝

(1)

where Cf and Cp represent the BSA concentration in feed and permeate solution, respectively. The BSA concentrations in the 2492

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value of surface relative to the center plane, Ra, is calculated as follows

feed and permeate solution were calculated from the absorbance at 280 nm using a UV spectrophotometer (U2000, Hitachi Ltd., Tokyo, Japan). 2.4.3. Visualization of Coating Layer. To visualize the coating layer on the hollow fiber membrane, we observed the cross-section of a FITC-poly(MPC-co-BMA)-coated membrane using a fluorescence microscopy technique developed by Hao et al.29 The aqueous coating solution was prepared by dissolving FITC-poly(MPC-co-BMA) and poly(MPC-coBMA) (FITC-poly(MPC-co-BMA)/poly(MPC-co-BMA): 1/ 99 wt %) in DI water. The dip coating was carried out in the same manner as described above. A membrane sample was freeze-dried and then fixed into a polyethylene capsule (Okenshoji, Tokyo, Japan) using a coagulated mixture of EpoFix resin and hardener (with a ratio of 25:1, Struers, Denmark). The sample was sliced with 10 μm of thickness by a microtome (EMS-150S, ERMA Inc., Tokyo, Japan). Finally, the sample (cross-section of hollow fiber membrane) was observed using a fluorescence microscope (Olympus IX 81) at an exposure time of 500 ms. 2.4.4. Stability Test. After the bare PVDF hollow fiber membrane was dip coated into a 0.5 wt % FITC-poly(MPC-coBMA) aqueous solution in the same manner, ultrasonic cleaning was carried out on the coated membrane for 1 h in a 1 wt % SDS aqueous solution and ethanol as cleaning agents, using an ultrasonic cleaner (USD-4R, frequency 28 kHz, power 240 kW; As One Co., Osaka, Japan). SDS solution has been used as a cleaning agent for fouled membranes because of its amphiphilic property,30 and it has been shown that poly(MPCco-BMA) is highly soluble in ethanol.31 The coated membrane after cleaning was dissolving at 1 wt % into a mixture solvent of DMAc and ethanol (90/10: wt%) containing 0.1 wt % triethanolamine. The fluorescence intensity of the obtained solution was measured using a fluorescence spectrophotometer (FP-8200 Fluorescence Spectrophotometer; Jasco, Tokyo, Japan). Excitation and emission wavelengths were 513 and 533 nm, respectively. The sensitivity of the apparatus was set at low. The standard curves for FITC-poly(MPC-co-BMA) in the PVDF solution are shown in Figure S1. The FITC-poly(MPCco-BMA) mass fraction in the coated membrane before and after cleaning could be determined by the measured fluorescence intensity. 2.5. Characterization of Poly(MPC-co-BMA)-Coated Film Surface. To investigate the properties, such as the surface roughness and protein fouling capability, of the poly(MPC-co-BMA)-coated surface in detail, a PVDF film was used as a substrate for atomic force microscopy (AFM) observations and quartz crystal microbalance measurements, because the inhomogeneous morphology of a porous PVDF membrane surface makes investigation of its surface properties difficult. 2.5.1. AFM Observation. A homogeneous PVDF solution at 20 wt % was prepared by dissolving PVDF into DMAc and then cast on a glass plate with a thickness of 254 μm. A PVDF film was obtained by drying the sample in a vacuum oven at room temperature overnight. The dip coating of poly(MPC-coBMA) onto the film was conducted in the same manner as the hollow fiber membranes. The surface morphology of the poly(MPC-co-BMA)-coated film was observed with an AFM (SPM-9600; Shimadzu, Kyoto, Japan) in noncontact mode. AFM measurements were carried out in DI water. To quantitatively determine the surface roughness, the mean

Ra =

1 LxLy

Lx

∫0 ∫0

Ly

| f (x , y )| d x d y (2)

where f(x, y) is surface relative to the center plane and Lx and Ly are surface dimensions. 2.5.2. Quartz Crystal Microbalance. A quartz crystal microbalance (QCM) was used to measure the adsorbed amounts of BSA onto a PVDF film and the coated surfaces. A sensor composed of gold-coated AT-cut quartz crystals (QSense AB, Gothenburg, Sweden) was spin coated with a DMAc solution containing 0.5 wt % PVDF, at 2000 rpm for 60 s. The PVDF film on the sensor was obtained by drying at 80 °C for 30 min. A QCM measurement was carried out in a flow chamber equipped with the PVDF-coated sensor. Poly(MPCco-BMA) coating was carried out by supplying 0.5 wt % poly(MPC-co-BMA) aqueous solution to the PVDF-coated sensor for 1 h. The adsorption experiments were begun by verifying the stability of the resonance frequency using phosphate buffer. BSA solution (1000 mg·L−1) was then supplied to the sensor and the frequency change was tracked until reaching a steady state. Finally, the BSA-adsorbed sensor was rinsed by supplying PBS buffer. Each experiment was performed at 298 K under static conditions. An increase in adsorbed mass on the QCM sensor surface induces a decrease in resonance frequency. The adsorbed mass, Δm [ng·cm−2], is related to the frequency shift, Δf [Hz], by the Sauerbrey equation32 as follows: Δm = −C

Δf n

(3)

where C denotes the mass sensitivity constant (17.7 ng·cm−2· Hz1− for a 5 MHz crystal), and n (1, 3, 5, ...) is the overtone number. The seventh overtone (15 MHz) was chosen when the Sauerbrey equation was employed in the data analysis.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Poly(MPC-co-BMA) Coating on Hollow Fiber Membrane Surfaces. 3.1.1. Observation of Membrane Surfaces. Figure 3 shows SEM images of the cross sections and the outer surfaces of the poly(MPC-coBMA)-coated and bare PVDF hollow fiber membranes. The dip coating was performed using 0.5 wt % poly(MPC-co-BMA) aqueous solution. It was observed that the prepared hollow fiber membrane was composed of a spherulitic structure and a skin layer formed on the outer surface. No significant differences were observed in the cross-sectional images of the bare (Figure 2A1) and poly(MPC-co-BMA)-coated (Figure 2B1) membranes. Conversely, the outer surface of the poly(MPC-co-BMA)-coated membrane had a rib-like rough surface (Figure 2B2) and was quite different to that of the bare membrane (Figure 2A2), indicating that the poly(MPC-coBMA) layer was coated on the PVDF membrane surface. As can be seen in the magnified outer surfaces (Figure 2A3,B3), the pore sizes of both membranes were around 20 nm and did not markedly differ. It should be noted that the observed surface structure may differ from the structure under filtration conditions, because SEM observation is carried out under vacuum. Thus, we conducted AFM observations in DI water and investigated the change in surface morphology with the 2493

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Figure 3. SEM images of bare and poly(MPC-co-BMA)-coated PVDF hollow fiber membranes: (A) uncoated (bare PVDF hollow fiber membrane); (B) coated in an aqueous solution of 0.5 wt % poly(MPC-co-BMA). (1) cross-section; (2) outer surface; (3) magnified outer surface.

poly(MPC-co-BMA) coating in more detail, as explained in section 3.2. 3.1.2. Filtration Experiments. Figure 4a shows the time variations of water permeability for the bare, MPC homopolymer-coated, and the PVDF membranes coated with poly(MPC-co-BMA) at different coating solution concentrations, obtained from filtration experiments with BSA solution. Rapid decreases in water permeability of the bare PVDF membrane occurred within the first 0.01 m3·m−2 permeation volume per unit area of testing, indicating that severe membrane fouling was caused by BSA adsorption. However, water permeabilities of the poly(MPC-co-BMA)-coated membranes only gradually decreased, although the initial water permeabilities were lower. Importantly, the final water permeabilities of all poly(MPC-coBMA)-coated membranes were higher than that of the bare membrane. The water permeability decline was suppressed and the final water permeability became higher as the poly(MPCco-BMA) concentration in the coating solution was increased. The water permeabilities of the poly(MPC-co-BMA)-coated membranes were higher than that of the MPC homopolymercoated membrane. Based on these observations, it was concluded that the poly(MPC-co-BMA) coating was effective at improving the antifouling properties of the PVDF hollow fiber membrane. Figure 4b shows the time variations of the relative water permeability of these membranes. Relative permeability J/J0 is defined as the water permeability normalized by the initial water permeability. The final relative permeabilities of the poly(MPC-co-BMA)-coated membranes were much higher than that of the bare membrane. The relative permeabilities in

Figure 4. Time variations of (a) water permeability, (b) relative water permeability, and (c) rejection of the bare, MPC homopolymercoated, and 0.01, 0.1, and 0.5 wt % poly(MPC-co-BMA)-coated PVDF hollow fiber membranes with 1000 mg·L−1 BSA solution.

the cases of 0.1 wt % and 0.5 wt % coating concentrations showed almost the same behaviors and were higher than that for a 0.01 wt % coating concentration. Figure 4c shows the time variation of the BSA rejection of these membranes. Significant increases in rejection of the bare PVDF membrane occurred, while the rejections of the poly(MPC-co-BMA)-coated membranes gradually increased with time. The rejection of the bare PVDF membrane was higher than those of poly(MPC-co-BMA) membranes. These tendencies were related to the fouling. In addition, the initial value of rejection in the case of 0.01 wt % coating concentration was almost zero, indicating that BSA can go through the membrane. The minimum value increased with increasing coating concentration, implying that the effective pore size decreased. To further investigate the effect of the poly(MPC-co-BMA) concentration in the coating solution on fouling during the filtration, we observed the cross-section of the hollow fiber 2494

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membrane coated with FITC-poly(MPC-co-BMA) by fluorescence microscopy. Figure 5 shows the cross-sectional

surfactant. Conversely, in the case of ethanol, the poly(MPCco-BMA) mass fraction decreased to around 50% after cleaning, probably because poly(MPC-co-BMA) is more soluble in ethanol than in water.31 The fluorescence method we describe here is useful for investigating adsorption of coating layers on a porous membrane surface with complex geometry. To investigate the effect of hydrodynamic force on coating stability, cross-flow filtration was continued for 18 h. The Reynolds number in the feed channel defined by the mean velocity and the equivalent diameter (the inside diameter of the tube − the outer diameter of the hollow fiber membrane) was set to 76.4. Figure 7 shows the time variation of water

Figure 5. Cross-sectional fluorescence microscope images of poly(MPC-co-BMA) adsorption to PVDF membranes: (A) 0.01 wt %; (B) 0.5 wt % poly(MPC-co-BMA) coating concentrations.

microscope images of poly(MPC-co-BMA) adsorbed on the PVDF membrane at different poly(MPC-co-BMA) concentrations in the aqueous solution. In the case of a 0.01 wt % poly(MPC-co-BMA) concentration, we observed the FITCpoly(MPC-co-BMA) coating layer near the outer surface of the membrane (Figure 5A). FITC-poly(MPC-co-BMA) penetrated inside the membrane at increased coating concentrations (Figure 5B). This was probably because FITC-poly(MPC-coBMA) diffusion through membrane pores was increased during dip coating at increased coating concentration. Thus, the FITCpoly(MPC-co-BMA) adsorption occurred not only on the membrane surface but also on the pore surface, explaining the higher antifouling property of the membrane coated with poly(MPC-co-BMA) at 0.5 wt % than that for the membrane coated at 0.01 wt %, as seen in Figure 4b. Although molecules becomes hydrophobic due to the FITC modification in some cases, the degree of FITC labeling was quite low and did not affect the nature much under the present condition. 3.1.3. Stability of the Poly(MPC-co-BMA) Coating Layer. Figure 6 shows the poly(MPC-co-BMA) mass fraction

Figure 7. Time variation of water permeability of a PVDF hollow fiber membrane coated with a 0.5 wt % poly(MPC-co-BMA) aqueous solution during filtration of DI water for 18 h.

permeability for a 0.5 wt % poly(MPC-co-BMA)-coated hollow fiber membrane. The water permeability remained almost constant throughout the experiment. Under the conditions used, the coating layer appeared to remain largely attached the membrane. Based on these results, it was concluded that the coating layer was quite stable because of strong hydrophobic interactions between BMA moieties and the PVDF membrane surface. 3.2. Characterization of Poly(MPC-co-BMA) Coating Layer on Film Surface. To investigate the effect of the poly(MPC-co-BMA) coating on the PVDF surface on the interfacial properties such as the roughness and protein adsorption characteristics in more detail, we used a PVDF film instead of a PVDF hollow fiber membrane. Investigation using the film allowed us to characterize the poly(MPC-coBMA)-coated surface in the water phase. Figure 8 shows the AFM images of the PVDF films coated with poly(MPC-co-BMA) at different coating concentrations, compared with the bare PVDF film surface. In the case of the bare PVDF surface, a smooth surface morphology was observed (Figure 8A). For the cases of the bare and 0.01 wt % coating concentrations (Figure 8B), the surface morphologies did not differ markedly. However, in the case of the 0.5 wt % coating concentration, a spherical morphology was observed, as shown in Figure 8C. This was probably because amphiphilic poly(MPC-co-BMA) molecules formed a micellar structure on the surface.33 Table 1 shows the relationship between coating concentration and the mean roughness, Ra, of membranes coated with poly(MPC-co-BMA). Ra was high in the case of a 0.5 wt % coating concentration, while the bare and 0.01 wt % poly(MPC-co-BMA)-coated surfaces showed little roughness. These results indicate that the high antifouling

Figure 6. Coated poly(MPC-co-BMA) mass fractions in PVDF hollow fiber membranes: (1) before cleaning, and after ultrasonic cleaning with (2) an aqueous solution of 1 wt % SDS and (3) ethanol.

evaluated by the fluorescent intensity in the hollow fiber membrane before and after ultrasonic cleaning with SDS solution. The mass fraction of the coating layer after cleaning was almost the same as that before cleaning. This indicates that poly(MPC-co-BMA) adsorbed to the PVDF surface remained adsorbed, despite the presence of ultrasonication and a 2495

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Figure 8. AFM images of surface structures of poly(MPC-co-BMA) coated on PVDF films: (A) bare film surface; (B) 0.01 wt % poly(MPC-coBMA) coated; (C) 0.5 wt % poly(MPC-co-BMA) coated.

the hydrophobic surface faced toward the water phase in the case of the poly(MPC-co-BMA) coating.34 These tendencies are consistent with the water permeability declines obtained in the filtration experiments (Figure 4). The present immobilization method using the hydrophobic interactions between BMA moieties and the PVDF surface is important for showing the high antifouling properties of MPC on the PVDF surface.

Table 1. Effect of Poly(MPC-co-BMA) Coating Concentration on Mean Roughness of the PVDF Film Surfacea

a

membrane

Ra (nm)

bare PVDF surface 0.01 wt % poly(MPC-co-BMA) coated 0.5 wt % poly(MPC-co-BMA) coated

5.3 5.7 114.8

4. CONCLUSIONS In this study, we proposed a simple dip-coating method for PVDF hollow fiber membranes using poly(MPC-co-BMA) by hydrophobic interaction to improve antifouling properties. By simply immersing PVDF hollow fiber membranes into an aqueous poly(MPC-co-BMA) solution, the membrane surfaces were successfully coated with poly(MPC-co-BMA). The poly(MPC-co-BMA) coating layer provided PVDF hollow fiber membranes with high antifouling properties. The effect of coating concentration on the fouling was discussed, and investigation of the stability of the coating layer revealed that that surface immobilization achieved via hydrophobic interactions between BMA moieties and the PVDF membrane surface was quite stable. The present method is a relatively simple way to modify the PVDF hollow fiber membrane surface, while obtaining high stability. If the coating layer became detached with time, further dip coating may recover the antifouling property. Thus, we conclude that dip coating with an aqueous poly(MPC-coBMA) solution is a promising candidate for surface modification to improve the antifouling property of PVDF membranes for water treatment.

Ra: mean roughness

property shown by the poly(MPC-co-BMA) coating in the filtration experiments (Figure 4) was not due to increased surface roughness, because the results of the AFM measurement did not correlate with those of the fouling experiment with BSA solution. The amount of BSA adsorbed onto these films was also investigated using QCM. Figure 9 shows the time variation of

■

ASSOCIATED CONTENT

S Supporting Information *

Standard curve for FITC-poly(MPC-co-BMA) in PVDF solution. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. Time variations of amounts of BSA adsorbed onto uncoated, the 0.5 wt % MPC homopolymer-coated, and the 0.5 wt % poly(MPCco-BMA)-coated PVDF film, obtained by QCM measurements.

■

the amounts of BSA adsorbed on the coated PVDF surfaces. Interestingly, the amount of BSA adsorbed onto the poly(MPC-co-BMA)-coated film remained at almost zero during the experiment, while the amount adsorbed onto the bare PVDF surface sharply increased and then reached an adsorption equilibrium. This indicates that the poly(MPC-coBMA) coating layer suppressed BSA adsorption. Although the MPC homopolymer-coated surface also suppressed BSA adsorption, the amount adsorbed was much higher than that adsorbed onto the poly(MPC-co-BMA)-coated surface. This might be because the hydrophilic phosphorylcholine groups on

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +81-78-803-6180. Fax: +81-78-803-6180. Notes

The authors declare no competing financial interest.

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dx.doi.org/10.1021/ie404094t | Ind. Eng. Chem. Res. 2014, 53, 2491−2497