Enhanced antifouling ability of a poly(vinylidene fluoride) membrane functionalized with a zwitterionic serine-based layer

Abstract

In this work, an antifouling poly(vinylidene fluoride) (PVDF) membrane was fabricated through covalent surface immobilization of a zwitterionic serine-based layer via facile free radical cross-linking polymerization of serine methacrylate (SerMA) on the membrane surface. The unique network structure along with the zwitterionic nature of serine was expected to endow the prepared membranes with excellent wetting and antifouling abilities. Upon modification, wettability of the membranes was improved significantly with the water contact angle decreased to as low as 32°. Protein adsorption on the membrane surface was suppressed remarkably to as low as 4.8 μg cm⁻². Filtration experiments further demonstrated that the modified membranes showed higher flux (both water and BSA solution), higher BSA rejection ratio and much better antifouling properties. These results suggest that the zwitterionic serine-based layer on the membrane surface provided an ideal barrier between the membrane substrate and the pollutants.

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1. Introduction

Poly(vinylidene fluoride) (PVDF) membrane has been widely applied in water treatment and purification, protein separation, food industry, biomedical processes etc., due to its outstanding chemical resistance, high mechanical strength and thermal stability.^1,2 However, a serious fouling problem exists during the process of filtration, resulting in decreased service life and increased operation cost.^3,4 It is widely acknowledged that improved hydrophilicity of a membrane surface offers the membrane a better fouling resistance.^5,6 Water molecules are preferably adsorbed on the hydrophilic surface to form a protective water layer, which prevents the adsorption and deposition of hydrophobic contaminants on membrane surface.⁷

To improve the hydrophilicity of membranes, poly(ethylene glycol) (PEG), oligo(ethylene glycol) (OEG) and their derives are frequently employed for the fabrication of antifouling membranes via grafting, coating or blending methods. However, a major limitation for these materials is their susceptibility to oxidative degradation. Other materials for the antifouling modification include polyacrylamide,^8,9 poly(2-hydroxyethyl methacrylate),^10–12 poly(acrylic acid),¹³ polysaccharide,¹⁴ Nanoparticles^15,16 and zwitterionic-based materials,^17–20 etc. Among these materials, zwitterionic polymers, such as poly(sulfobetaine), poly(carboxybetaine) and poly(phosphobetaine), have received growing attention as one kind of new generation of antifouling materials due to their excellent hydration capacities.

As natural zwitterions, amino acids have also received growing attention in recent years for the designing of new zwitterionic antifouling polymers due to their biomimetic and zwitterionic nature. It was reported that natural amino acids showed comparable protein adsorption resistivity to PEG-based polymers. Poly(lysine methacrylamide) and poly(ornithine methacrylamide), derived from lysine and ornithine respectively, were grafted onto gold via surface initiated photoiniferter-mediated polymerization, which showed suppressed nonspecific adsorption.²¹ Lysine was directly grafted onto flat sheet PAN-based ultrafiltration membrane via carbodiimide chemistry, which showed superior protein fouling ability.²² Flux recovery ratio of PES ultrafiltration membranes was also enhanced via grafting of glycine-functionalized PVA.²³

Three-dimensional (3D) cross-linked networks or hydrogels are able to hold large amounts of water while remaining insoluble in water, which have been widely applied in biomedical and pharmaceutical materials.²⁴ 3D networks have also attracted much attention for the modification of membranes.^25,26 The hydrophilic layer on membrane surface can act as an ideal physical barrier between membrane surface and the pollutants. The hydrophilic layer can also preferably adsorb water molecules to form another protective hydration layer. However, little efforts have been devoted to the combination of 3D networks and amino acids for the fabrication of antifouling membranes.

In this work, novel zwitterionic serine-based networks were covalently incorporated on membrane surface as hydrophilic and protein resistant layer to improve the antifouling properties of PVDF membrane via free radical cross-linking polymerization of serine methacrylate. Due to the excellent hydration capacities of zwitterionic amino acids and 3D networks, a hydration layer of tightly bounded water molecules was expected to be formed on membrane surface, which acted as a physical and energetic barrier that pollutants have to overcome before they can adsorb onto membrane surface. Upon modification, the membrane morphologies, wettability, permeability/selectivity as well as fouling resistant properties of the prepared membranes were studied in detail.

2. Experimental

2.1. Materials and reagents

L-serine, methacryloyl chloride, cupric carbonate basic and 8-hydroxyquinoline were purchased from Energy Chemical (Shanghai, China). N,N-methylenebis(acrylamide) (MBAA) and azobisisobutyronitrile (AIBN) were supplied by Aldrich Corp. Bovine serum albumin (BSA), ammonium persulfate (APS), poly(ethylene glycol) (PEG, M_n = 20 000), N,N-dimethylacetamide (DMAc), acetone, ether, chloroform, methanol, ethanol and potassium hydroxide were all obtained from Sinopharm Chemical Reagent (Shanghai, China). Poly(vinylidene fluoride) powder was supplied by Arkema Inc. and dried at 100 °C for 24 hours before use. All other chemicals were used as received without further purification.

2.2. Preparation of serine methacrylate monomer

Serine methacrylate (SerMA) was prepared through the reaction of serine with methacryloyl chloride according to a method published previously.^{27,28 1}H NMR (400 MHz, D₂O, Me₄Si, δ) of SerMA: 6.01 (s, 1H, C=CH₂), 5.61 (s, 1H, C=CH₂), 4.43 (d, 2H, CH₂), 3.97 (t, 1H, CH), 1.77 (s, 3H, CH₃).

2.3. Membrane preparation

The membrane preparation process was shown in Fig. 1. Before modification, pure PVDF membrane was prepared by using the non-solvent induced phase separation method. A mixture of PVDF (10.0 g), PEG (3.0 g) and DMAc (50.0 g) was stirred at 75 °C for 10 h to form a homogeneous solution, sealed and stored at 75 °C overnight. The solution was then casted on a clean glass plate and immersed immediately into water coagulating bath at room temperature. After spontaneous detachment from the glass, the resultant membranes were rinsed extensively with water and kept in water before use.

Fig. 1 Schematic illustration of the preparation process of the graft membranes.

Zwitterionic serine-based networks grafted PVDF membrane was fabricated by an interfacial polymerization based on the cross-linking reaction between SerMA and MBAA on alkaline-treated membrane. Initially, the pure membrane was treated with 2.0 M KOH aqueous solution at 50 °C for 15 min and rinsed extensively with water. The post-treated PVDF membrane with a surface area of approximately 17.0 cm² was then put into a 50.0 mL aqueous solution with a predetermined amount of SerMA monomer. Subsequently, equivalently molar amount of the initiator (i.e. APS) and the cross-linking agent (i.e. MBAA) were added. The feed mole percentage of MBAA was 1.0% of the total monomer. After bubbling of nitrogen for 15 min, the flask was sealed and the polymerization was carried out at 70 °C under constant stirring for 10 h. The resultant membrane was rinsed extensively with water and stored in water before use. The resultant membranes were marked by M0.3, M0.5, and M1.0 corresponding to the various monomer contents (10² g L⁻¹) in the reaction solution. For comparison, the pure PVDF membrane was marked by M0.

2.4. Membrane characterization

Surface chemical composition of the pure and graft membranes was tested using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Nicolet 8700, USA) with zinc selenide as an internal reflection element and X-ray photoelectron spectroscopy (XPS, PHI5000C ESCA, USA). Surface and cross-section morphologies of the membranes were detected by scanning electron microscopy (SEM, Hitachi SU8010, Japan). All samples were freeze-dried for 30 h, spray-coated with gold before SEM observation. For cross-section detection, the samples were fractured in liquid nitrogen. The surface topographies of the membranes were analyzed by atomic force microscopy (AFM, Agilent-5500, USA) with tapping mode. Water contact angles of the membrane surface were measured following the sessile drop method with a telescopic goniometer (Dataphysics OCA40, Germany) at 25 °C to evaluate the wettability of the membranes. At least 5 measurements were conducted for each sample.

2.5. Protein adsorption

As a sticky protein, BSA was used to evaluate the nonspecific protein adsorption on the surface of the prepared membranes. Firstly, FITC-labeled BSA (BSA–FITC) was prepared according to a reported procedure.^29,30 Then a membrane sample with an area of 0.5 × 0.5 cm² was incubated 0.1 g L⁻¹ FITC–BSA solution in phosphate buffer saline (PBS, pH 7.4) and shaken in a dark place at 4 °C for 12 h. After that, the membrane was rinsed with PBS gently to remove the non-firmly adsorbed proteins on the surface and dried with a stream of nitrogen gas. The adsorption of BSA–FITC on membrane surface was observed with a fluorescence microscope (Olympus BX-51, Japan).

To quantitatively characterize protein adsorption properties of the pure and graft membranes, a membrane sample with an area of 1.0 × 1.0 cm² was put into BSA solution (1.0 g L⁻¹, pH 7.4) and incubated at 4 °C for 12 h to reach adsorption equilibrium. Then the membrane was taken out and washed with PBS and deionized water sequentially before immersed into 2.0 wt% sodium dodecyl sulfate (SDS) aqueous solution at 37 °C and shaken for 2 h to remove the adsorbed proteins. The protein concentration of the SDS solution was measured using the Micro BCA™ Protein Assay Reagent Kit (PIERCE) and a UV-vis spectrophotometer (UV-1800, Shimadzu, Japan) at the wavelength of 562 nm. Protein adsorption was determined by the amount of protein on membrane per area.

2.6. Filtration experiments

The permeation and separation properties of the prepared membranes were measured by a home-made dead end unit with the effective filtration area of 12.5 cm². All the filtration experiments were conducted at a pressure of 0.1 MPa. The samples were firstly pressured at 0.15 MPa to reach a steady flux before tests. Water flux and rejection ration of BSA (1.0 g L⁻¹, pH 7.4) was calculated by the following equations:where the parameters of V, S, t, C_p and C_f denoted the pure water permeate volume (L), membrane area (m²), permeation time (h), BSA concentrations in the feed and permeate respectively. The BSA concentrations were determined by a UV-vis spectrophotometer (UV-1800, Shimadzu, Japan) at the wavelength of 280 nm.

2.7. Antifouling assessment

To evaluate the antifouling properties of the prepared membranes, a three-cycle filtration experiment was conducted with BSA as the model pollutant. Initially, the membrane was pressured with water at 0.15 MPa for at least 30 min before water flux was recorded within 30 min at an interval of 5 min. Then the feed was changed to BSA solution (1.0 g L⁻¹, pH 7.4) and BSA flux was recorded within the following 30 min. After that, the membrane was washed with deionized water for 30 min before another cycle was conducted.

On the basis of dynamic fouling experiments, the water flux recovery ratio (FRR), the reversible flux decline ratio (RFR), the irreversible flux decline ratio (IFR) and the total flux decline ratio (TFR) were employed and calculated to further evaluate the antifouling properties of the prepared membranes. The values of FRR, RFR and IFR were calculated by the following equations:^31,32

where the parameters of J₁, J₂ and J_p represented water flux of the first cycle, water flux of the second cycle and BSA solution flux respectively.

2.8. Stability

The long-term stability of the grafted networks on membrane surface was evaluated by a 3 d washing experiment using water as the solvent. The pure membrane i.e. M0 was directly coated with serine-based networks following the process of M1.0, and used as a control (M0-C). Both of the membranes (M0-C and M1.0) were washed with water at 30 °C for 3 d and the water contact angle values were detected at an interval of 0.5 d.

3. Results and discussion

As shown in Fig. 1, novel serine-based zwitterionic networks was covalently immobilized on the surface of PVDF membrane via free radical cross-linking polymerization of SerMA. The pure PVDF membrane was firstly treated with KOH solution to obtain C=C double bonds on membrane surface, which were subsequently employed for the immobilization of zwitterionic PSerMA networks. Upon modification, a protective hydration layer was expected to be formed on membrane surface due to the excellent hydration capacities of the zwitterionic structure and three-dimensional (3D) networks, which protects the membrane from pollutants.

3.1. Surface characterization

To confirm the presence of zwitterionic PSerMA networks on membrane surface, ATR-FTIR was used to execute comparative evaluation of the pure and graft membranes. As shown in Fig. 2, the peaks around 500–1500 cm⁻¹ ascribed to the structure of PVDF are similar for all the membranes. Compared with the pure membrane, there is clearly a new peak at 1723 cm⁻¹ in the spectra of the graft membranes, which can be attributed to the ester groups of SerMA.³³ The broad band at 3400 cm⁻¹ can be assigned to O–H and N–H stretching variations.³⁴ Moreover, the relative intensity of the above characteristic peaks obviously increases with SerMA concentration in the reaction solution, corresponding to the increasing amount of SerMA on membrane surface. These results indicate that zwitterionic serine-based networks have been successfully grafted on membrane surface.

Fig. 2 ATR-FTIR spectra of the pure and graft membranes.

The chemical compositions of the pure and graft membranes were further analyzed by XPS, as shown in Fig. 3. The new peak at 400.0 eV ascribed to N1s is resulted from the introduction of the zwitterionic networks since SerMA is the sole source of nitrogen. Upon modification, fluoride amount on membrane surface is decreased substantially and almost no fluoride can be detected on the surface of M1.0, indicating that the surface is almost fully covered by the zwitterionic networks.

Fig. 3 XPS wide-scan spectra of the pure and graft membranes.

The effects of the incorporation of the zwitterionic networks on the microstructure of the membranes were studied by using SEM and AFM. As shown in Fig. 4, all of the membranes show relatively dense top surface without large pores or obvious defects. However, with the increasing concentration of SerMA in the reaction solution, the pores of the graft membranes become less and smaller, since more SerMA was cross-linked onto membrane surface resulting in an increase of coverage for surface pores. The three-dimensional AFM surface topography images of the pure and graft membranes were shown in Fig. 5. Compared with the pure membrane, the surface RMS (root mean square) roughness of the graft membranes has a slightly decrease of approximately 8–10 nm. The incorporation of zwitterionic networks on membrane surface makes the surface smoother slightly, resulting from the coverage of membrane pores. However, it is noteworthy that the difference in surface roughness among the membranes is negligible.

Fig. 4 SEM characterization of the surfaces of the pure and graft membranes.

Fig. 5 Tapping-mode AFM of surface roughness of the pure and modified membranes.

3.2. Membrane hydrophilicity

The antifouling properties of the membranes are mainly affected by the surface hydrophilicity, since a hydrophilic surface tends to resist the adsorption of pollutants. Therefore, the hydrophilicity properties of the pure and graft membranes were characterized using water contact angle (WCA) measurements. As shown in Fig. 6, WCA of the pure membrane is as high as 92° in accordance with the poor wettability of PVDF. Upon modification, the WCA value is decreased significantly to as low as 32° for the membrane of M1.0. These results indicate that wettability of the membranes is improved greatly through the incorporation of zwitterionic PSerMA networks on membrane surface, due to the excellent hydration capacities of the positive and negative ions of the zwitterionic structure of serine.

Fig. 6 Water contact angle of the pure and graft membranes.

3.3. Protein adsorption

Nonspecific adsorption on membrane surface plays a dominant role in the process of membrane fouling. BSA, a sticky protein, has been frequently used for the study of nonspecific adsorption as a model pollutant. As shown in Fig. 7, uniform and intense fluorescence can be observed for the pure membrane, corresponding to severe BSA–FITC adsorption on PES membrane surface. Compared with the pure membrane, fluorescence intensities of the graft membranes are decreased significantly and almost no fluorescence can be seen for the membrane of M1.0, indicating reduced BSA–FITC adsorption on the graft membranes.

Fig. 7 Fluorescence microscopy images for the pure and graft membranes.

BSA adsorption amount on membrane surface was shown in Fig. 8. BSA adsorption amount on the pure membrane is as high as 32.0 μg cm⁻² while that of the graft membranes is decreased to as low as 4.8 μg cm⁻², which is a very low value compared with the reported ones.^35,36 These results indicate that the anti-protein adsorption properties of the membranes were improved significantly via the covalently immobilization of zwitterionic PSerMA networks on membrane surface. Similar to PEG and its derives, zwitterionic PSerMA networks can form a hydration layer of tightly bounded water molecules, which acts as a physical barrier on one hand and an energetic barrier on the other hand that pollutants have to overcome before they can adsorb onto membrane surface.

Fig. 8 BSA adsorption amounts on the pure and graft membranes.

3.4. Filtration experiment

The permeability and selectivity capacities of the pure and graft membranes were studied with ultrafiltration experiments. As shown in Fig. 9, water flux of the membranes is firstly increased from 155 to 235 L m⁻² h⁻¹ and then decreased to 205 L m⁻² h⁻¹. A similar trend can also be observed for BSA solution flux. However, BSA rejection ratio of the membranes is increased progressively. These results can be ascribed to the pore narrowing effects and the improved wettability of the membranes as discussed above. Generally, surface pore structure and surface hydrophilicity are the two main factors that affect the value of membrane flux. Pore narrowing effects have become the dominant factor for the membranes of M1.0, resulting in decreased water flux. However, the increased hydrophilicity of the graft membranes makes the effective pore size smaller, resulting in increased rejection ratio.

Fig. 9 Water flux, BSA solution flux and BSA rejection ratio of the pure and graft membranes.

3.5. Antifouling assessment

A multi-cycle filtration experiment was conducted to test the antifouling properties of the pure and graft membranes. As shown in Fig. 10, all of the membranes suffer continuous flux decline after the filtration of BSA solution due to the accumulation of protein aggregates and the growing filter cake layer. However, the graft membranes suffer much less flux decline while the pure membrane losses most of its initial flux after three cycles of filtration. Flux recovery ratios of the pure and graft membranes during the filtration process is presented in Fig. 11. Flux recovery ratios of the membranes are increased from 63.0% to 95.5% after the grafting polymerization, and can even reach as high as 99.1% after three cycles of filtration. These results indicate that the antifouling properties of the membranes were improved significantly through the surface incorporation of zwitterionic serine-based networks.

Fig. 10 Time dependent fluxes for the pure and graft membranes.

Fig. 11 Water flux recovery ratios of different cycles for the pure and graft membranes.

To further study the antifouling properties of the membranes, TFR, RFR and IFR were used. It is notable that the higher values of RFR and lower values of TFR and IFR mean better antifouling capacities. As shown in Fig. 12, compared with the pure membrane, all of the graft membranes show higher values of RFR and lower values of TFR and IFR. IFR of the graft membranes is decreased to as low as 4.5%, corresponding to its excellent antifouling properties. The FRR and IFR values of this work are better than most of the reported ones in the literature.^35,37,38 These results can be attributed to the superior hydration capacities of the zwitterionic structure of serine and the cross-linked networks, which bind numerous water molecules around to form a protective water layer on membrane surface. By the surface incorporation of zwitterionic serine-based 3D networks, the surface of PVDF membrane is covered by a dense water layer and the total membrane fouling and irreversible membrane fouling are reduced substantially.

Fig. 12 TFR, RFR, IFR and FRR values during the filtration of BSA solution.

3.6. Stability

The stability of the grafted networks on membrane surface was evaluated by detecting the evolution of WCA values with washing time. The membrane directly coated with the zwitterionic networks (M0-C) was used as a control. As shown in Fig. 13, the WCA of the coated membrane is increased gradually with washing time while the graft membrane maintains its hydrophilicity with WCA around 32°. The high stability of the grafted networks can be ascribed to the covalent bonding between the networks and the surface.

Fig. 13 Evolution of WCA values of the modified membranes.

4. Conclusions

In this study, novel zwitterionic 3D networks based on serine was covalently immobilized on membrane surface via cross-linking polymerization of SerMA to improve the antifouling properties of hydrophobic PVDF membrane. Due to the excellent hydration capacities of the zwitterionic structure and the cross-linked networks, a protective hydration layer of tightly bounded water molecules was proposed to be formed on membrane surface, which acted as a physical and energetic barrier that pollutants have to overcome before they can adsorb onto membrane surface. Upon modification, the graft membranes exhibited better wettability, higher permeability, higher selectivity and better antifouling properties than the pure PVDF membrane. This work provided an effective alternative to the traditional poly(betaine)-based materials, which can be applied in water treatment, protein separation and blood purification.

Acknowledgements

We gratefully acknowledge support from the National Science Foundation of China (No. 21174027), Program for New Century Excellent Talents in University (No. NCET-12-0827) and Program of Introducing Talents of Discipline to Universities (No. 111-2-04).

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