Acryloylmorpholine-Grafted PVDF Membrane with Improved Protein

Article pubs.acs.org/IECR

Acryloylmorpholine-Grafted PVDF Membrane with Improved Protein Fouling Resistance Jie Liu, Xiang Shen, Yiping Zhao, and Li Chen* State Key Laboratory of Hollow Fiber Membrane Materials and Processes, School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China ABSTRACT: This work provides a novel approach to improve the fouling resistance of PVDF membrane. An amphiphilic graft copolymer (PVDF-g-PACMO) having poly(vinylidene fluoride) (PVDF) backbones and polyacryloylmorpholine (PACMO) side chains was synthesized using the radical polymerization method, and then the copolymer was cast into a flat membrane via immersion phase inversion. The results indicate that the PACMO chain was successfully grafted onto PVDF main chains, and the grafting degree of PACMO in PVDF-g-PACMO copolymer increases with the increase of the monomer concentration in reaction solution. The structure and performance of as-prepared membranes were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), contact angle measurement, static protein adsorption, and filtration experiments. It is found that a higher grafting degree of PACMO endows the copolymer membrane with larger membrane surface micropores and a better hydrophilicity. The improved hydrophilicity provides the copolymer membrane with the resistance of protein adsorption to the membrane surface and a high flux recovery.

1. INTRODUCTION Poly(vinylidene fluoride) (PVDF) membrane has been widely used for pervaporation, distillation, ion exchange, ultrafiltration (UF), and microfiltration (MF) because of its excellent thermal resistance, excellent chemical resistance, well-controlled porosity, and good mechanical properties. 1−5 However, the membrane fouling caused by its hydrophobic nature will decrease permeation flux with time, resulting in the increase of energy consumption and operational cost in the filtration of aqueous solution containing proteins and other organic components.6,7 As a result, it is necessary to produce a PVDF membrane surface with excellent fouling resistance. Many works have been conducted to improve the antifouling property of PVDF membrane.8−11 Chang et al.12,13 investigated the fouling resistance and hydration capability of PEGylated poly(vinylidene fluoride) (PVDF) microporous membranes. It was found that the PEGylated PVDF membranes could resist protein adsorption to the membrane surface. Recently, our group reported the construction of zwitterionic PVDF membrane surface via the quaternization of N,N-dimethylamino-2-ethylmethacrylate (DMAEMA) with 1,3-propane sultone (1,3-PS).14 The zwitterionic membrane surface was shown to resist protein adsorption and bacterial adhesion because of the improved hydrophilicity induced by the zwitterionic structure. Obviously, hydrophilic modification is a feasible method to mitigate PVDF membrane fouling. The hydrophilic PVDF membrane surface can prevent the adsorption or deposition of foulants since the hydrophilic groups bind with a significant amount of water molecules to form hydrated layers.7 Acryloylmorpholine (ACMO) is hydrophilic, nontoxic, nonantigenic, and biocompatible, and it shows a repellent property for protein adsorption;15−17 it was widely used to synthesize cross-linked networks for gel-phase synthesis of peptides, semipermeable membranes for plasma separation, polymeric supports for gel chromatography and capillary electrophoresis, and drug delivery applications.18−23 ACMO © 2013 American Chemical Society

monomers have identified general features related to low affinity for proteins: (i) they are hydrophilic, (ii) they contain hydrogen bond acceptors, (iii) they lack hydrogen bond donors, and (iv) they are electrically neutral.24 Inspired by the typical characteristics of ACMO, the introduction of the ACMO polymer to the PVDF membrane by the preparation of acryloylmorpholine-grafted PVDF copolymer (PVDF-gPACMO) should be a very useful method to improve protein fouling resistance. The introduction of hydrophilic PACMO chains in copolymer would contribute to the formation of large membrane pores during the membrane formation process.25 The large pore size membrane always exhibits significant flux decline due to membrane fouling.26,27 The major fouling mechanism of the large pore size membrane is pore blocking followed by cake formation since many more solute molecules would easily permeate through the membrane pore channels.28 In spite of some negative effects induced by the large membrane pores, the tethered hydrophilic PACMO chains onto the surface of membrane and membrane pore channels could bind with a significant amount of water molecules,29 and then the hydrated layers can resist the adsorption or deposition of protein to the surface of membrane and membrane pore channels, thereby mitigating the formation of pore blocking and reducing membrane fouling. Unfortunately, few investigations focused on the incorporation of ACMO into the PVDF membrane to improve the antifouling properties.30 The aim of this work is to improve the fouling resistance of PVDF membrane through the preparation of PVDF-g-PACMO copolymer membrane. The schematic illustration for the copolymer synthesis and membrane formation is shown in Received: Revised: Accepted: Published: 18392

October 14, 2013 December 5, 2013 December 5, 2013 December 5, 2013 dx.doi.org/10.1021/ie403456n | Ind. Eng. Chem. Res. 2013, 52, 18392−18400

Industrial & Engineering Chemistry Research

Article

copolymers (C1, C2, C3) with different weight ratios of alkaline-treated PVDF powder and ACMO in the reaction system were synthesized (shown in Table 1). The graft

Figure 1. The PVDF-g-PACMO copolymer was synthesized via radical copolymerization. The copolymerization process could

Table 1. Characteristics of Copolymers and Fabricated Membranes copolymer membrane M0 (PVDF) M1 (PVDF-gPACMO) M2 (PVDF-gPACMO) M3 (PVDF-gPACMO)

type

(weight ratio) treated PVDF/ACMO

grafting degree (%)

mean pore size (nm)

C1

6/2

5.22

6.7 27.8

C2

6/3

6.86

33.2

C3

6/6

13.52

95.1

polymerization proceeded in a water bath for 12 h at 70 °C. The copolymer was precipitated by excess methanol, filtrated, and washed thoroughly with distilled water to remove the residue solvent, monomers, and homopolymers. The resulting copolymer was fully dried in an oven overnight for further characterization and membrane preparation. The chemical structure of the copolymer was investigated by Fourier transform infrared (FT-IR) spectroscopy (Tensor37, Bruker, Germany). To prepare FT-IR samples, the copolymer was cast on a potassium bromide (KBr) disk with a thickness of about 0.8 mm, and then dried by an infrared light. The FT-IR spectrum was obtained with a microsampling IR spectrometer. To inspect the chemical composition of the copolymer, nuclear magnetic resonance proton (1H NMR) spectroscopy was performed on a Bruker ARX 300 instrument with deuterated dimethyl sulfoxide (DMSO) as the solvent. 2.3. Preparation of Membrane. The flat membranes were prepared via the immersed phase inversion method. A 16 g sample of dried PVDF (or PVDF-g-PACMO) and 8 g of PEG were dissolved in 76 g of DMF to prepare the cast solution. The solution was left for 12 h at 60 °C to allow complete release of air bubbles. Then the solution was cast on a glass plate with a casting knife and subsequently immersed into a coagulation bath (distilled water) at 25 °C. After peeling from the glass plate, the membrane was rinsed with water and then stored in water before use. 2.4. Characterization of Membrane. The membrane surface composition was analyzed by X-ray photoelectron spectroscopy (XPS, Geneis 60S, EDAX Inc., Mahwah, NJ, USA) with Al Kα excitation radiation (1486.6 eV). The binding energies were referenced to the C 1s hydrocarbon peak at 284.7 eV. In order to perform the test, the membrane samples were cut with a sharp razor and mounted on standard sample studs using double-sided adhesive tape. The hydrophilicity of membranes was determined by measuring the contact angle of the membrane top surface. The contact angle change with the drop age was recorded using a contact angle instrument (DSA 10 MK2, Krüss GmbH, Hamburg, Germany). A water drop (5 μL) was dropped onto the membrane surface, and the whole process was recorded by a high-speed video camera. The sessile drop method was employed to calculate the contact angle values. The morphologies of membranes were observed by means of scanning electron microscopy (SEM, s-4800, Hitachi, Tokyo, Japan). The cross section samples were fractured in liquid

Figure 1. Schematic illustration for the copolymer synthesis and membrane formation.

be divided into two steps. First, the carbon−carbon double bond (CC) on the PVDF main chains could be formed with the treatment of potassium hydroxide (KOH). Second, the double bonds (CC) on the PVDF main chains would react with ACMO monomers to form the graft copolymer (PVDF-gPACMO) having PVDF backbones and PACMO side chains by using 2,2′-azobisisobutyronitrile as initiator. The chemical structure of copolymer was investigated by Fourier transform infrared (FT-IR) spectroscopy and nuclear magnetic resonance (1H NMR) spectroscopy. Subsequently, the copolymer flat membrane was cast via immersed phase inversion. The fouling resistance of as-prepared copolymer membrane was evaluated by water contact angle measurement, static protein adsorption, and cycle filtrations. This work will be helpful for the development of hydrophilic membrane.

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(vinylidene fluoride) (PVDF) powder was purchased from Solvey Company of Belgium. Acryloylmorpholine (ACMO) was obtained from Jiaxing Sicheng Chemicals Co., Ltd., of China. 2,2′-Azobisisobutyronitrile (AIBN, 99%) was supplied by Shanghai Shisihewei Chemical Co., Ltd., of China and was recrystallized from ethanol. N,NDimethylfomamide (DMF) and poly(ethylene glycol) (PEG) (Mn = 10 000) were kindly provided by Tianjin Kemiou Chemical Co., Ltd., of China. Bovine serum albumin (BSA) was supplied by Beijing Solarbio Science & Technology Co., Ltd., of China. All other reagents were of analytical grade and used as received. 2.2. Synthesis of PVDF-g-PACMO Copolymer. The synthetic scheme of PVDF-g-PACMO copolymer is shown in Figure 1. The PVDF powder was first treated with potassium hydroxide (KOH). Briefly, PVDF was immersed in a 10 wt % KOH solution containing 0.05 wt % ethanol, and then the solution was stirred for 10 min at 60 °C. The precipitate was collected by filtration and then washed four times with distilled water. A 6 g sample of alkaline-treated PVDF powder was dissolved in 72 mL of N,N-dimethylfomamide (DMF) at 70 °C. After the complete dissolution, the monomer ACMO and 0.0984 g of AIBN were added to the system under a N2 atmosphere with continuous magnetic stirring. Three types of 18393

dx.doi.org/10.1021/ie403456n | Ind. Eng. Chem. Res. 2013, 52, 18392−18400


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nitrogen. Both the surface and cross section of the samples were sputtered with gold film for 3 min before observation. The membrane pore sizes were determined by static volumetric measurements of nitrogen adsorption using the Barrett−Joyner−Halenda (BJH) method.31 A specific surface area and pore size analyzer (JW-BK112, Beijing JWGB Science & Technology Co., Ltd., Beijing, China) was used to measure the nitrogen adsorption−desorption isotherms. Nitrogen was admitted into the membrane chamber in controlled increments. After each dose of adsorptive, the pressure was allowed to equilibrate and the volume of nitrogen adsorbed was calculated. By extending this process to pressure conditions that condense the gas into the pores, pore sizes can be evaluated. After reaching high enough pressures to ensure gas condensation, the adsorptive gas pressure was reduced incrementally, thereby evaporating the condensed gas from the system. 2.5. Protein Adsorption. The antifouling properties of PVDF-g-PACMO membranes were evaluated through characterization of static protein adsorption and dynamic antifouling properties. Bovine serum albumin (BSA) solution was chosen as a model protein to investigate the adsorption of protein on the membrane surface. In a typical adsorption measurement, the membrane samples were cut into small pieces (2.5 cm × 2.5 cm) and incubated in BSA (0.5 g/L) solution at 25 °C for 24 h to reach adsorption equilibrium. The amount of adsorbed protein (Q) was calculated by the following equation: Q=

(2)

where V is the volume of permeated water (L), A is the membrane area (m2), and Δt is the permeation time (h). The BSA rejection ratio (R) was calculated by the following equation:

⎛ Cp ⎞ R = ⎜1 − ⎟ ·100% Cf ⎠ ⎝

(3)

where Cp and Cf are BSA concentrations measured with a UV− vis spectrophotometer in permeate and feed solutions, respectively. In order to test the long-term performance of PVDF-gPACMO membranes, multicycle operations were performed. Each cycle consisted of three stages: water filtration, fouling (BSA filtration, 1 g/L BSA), and water filtration after hydraulic cleaning. For each cycle, the pure water flux Jw was recorded, the flux of BSA solution JB was recorded, and the water flux after hydraulic cleaning Jr was recorded. The filtration experiment was performed as mentioned above. The flux recovery ratio (FRR) was calculated as follows:

FRR =

Jr Jw

· 100% (4)

For further study of irreversible and reversible fouling occurring on the membrane surface, three parameters were defined in detail. One part of membrane fouling that could be eliminated through hydraulic cleaning was defined as reversible fouling (Rr), and the other part of fouling that could not be removed only through hydraulic cleaning was recognized as irreversible fouling (Rir). Rr and Rir were calculated by the following equation:

(C0 − C) × V S

V AΔt

Jw0 =

(1)

where V is the volume of BSA solution; S is the adsorption area of membrane sample; C0 and C are the initial and equilibrium BSA concentrations, respectively. BSA concentrations were obtained by means of the Bradford method relying on the binding of the dye Coomassie Blue G250 to protein,32 measuring the absorbance of the BSA solution at 595 nm using a UV−vis spectrophotometer (TU-1901, Purkinje General Instrument Co., Ltd., Beijing, China). The average of at least three measurements was reported. 2.6. Filtration Experiments of Membrane. Filtration experiments were performed on our homemade equipment. The schematic diagram of the homemade equipment used to measure filtration performances is shown in Figure 2. Each membrane sample was fixed in the filtration cell and initially subjected to pressure for 2 h at 0.2 MPa, and the pressure was then lowered to 0.1 MPa. The steady pure water flux (Jw0) was calculated by the following equation:

Rr = R ir =

Jr − JB Jw

(5)

Jw − Jr Jw

(6)

Rt was the degree of total flux loss caused by total fouling, which was calculated by following equation:

Rt =

Jw − JB Jw

(7)

3. RESULTS AND DISCUSSION 3.1. Characterization of the Synthesized PVDF-gPACMO Copolymer. Hydrophilic monomer ACMO was incorporated to improve the antifouling property of hydrophobic PVDF membrane. The copolymer of PVDF grafted with PACMO was first synthesized via radical copolymerization, and the synthetic process is schematically shown in Figure 1. The effects of ACMO concentration in the reaction solution on the grafting degree of copolymer were considered systematically. Figure 3 shows the FT-IR spectra of pristine PVDF and PVDF-g-PACMO copolymers. It can be seen that all polymers showed the typical characteristic peaks of PVDF, i.e., −CF2 and −CH2 deformation and stretching vibration bonds at 1180 and 1407 cm−1 and amorphous phase at 880 and 842 cm−1. In comparison to the spectrum of pristine PVDF, Figure 3 (C1,

Figure 2. Schematic diagram of the homemade equipment used to measure filtration performances. 18394



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GD (%) =

1 2

(A(a) −

1 A(c) 8 1 A(c) 8

) + 18 A(c)

·100 (8)

where A(a) and A(c) are the integral areas of the characteristic peak (a) of the PVDF main chain (used as reference peak) and methylene peak (c) of PACMO, respectively. The GD values of PVDF-g-PACMO copolymers calculated from 1H NMR spectra are summarized in Table 1. It is found that the GD of PVDF-g-PACMO copolymer increases from 5.22 to 13.52% with the increase of monomer ACMO concentration. This phenomenon can be explained by the reaction kinetics. A higher monomer concentration provided more chances for the free radicals on PVDF chains to contact with the PVDF chain, and monomer, and thus a higher GD was obtained. 3.2. Chemical Compositions of Membrane Surfaces. The chemical compositions of the membrane surfaces were investigated by XPS analysis. Figure 5 shows the wide-scan XPS spectra and C 1s spectra of as-prepared membranes, respectively. In the wide-scan XPS spectra of PVDF-gPACMO membrane (M1), as compared with PVDF (M0), a new peak with binding energy of 400 eV attributable to N signal can be observed. The content of N 1s increases with the increase of grafting degree of copolymer (shown in Table 2). All of these are consistent with the results in Figures 3 and 4. The peak components for the C 1s peak in the XPS spectrum were further analyzed. The peaks at 286.3 and 290.6 eV corresponding to CH2 and CF2 groups can be observed in C 1s spectrum of PVDF membrane.35 The minor peak with binding energy at about 284.97 eV is attributable to the neutral C−H species, arising from the branching sites and end groups of PVDF chains. The peak area ratio of CH2 to CF2 is about 1.08, which is in good agreement with the real structure of PVDF. In the case of PVDF-g-PACMO membrane (M1), the C 1s spectrum can be curve-fitted into five peak components with binding energies of approximately 284.95, 286.35, 286.92, 288.01, and 290.6 eV, attributed to the C−H, CH2/C−N, C− O, NCO, and CF2 satellite species, respectively. The peak area ratio of NCO to C−O is about 1.17, which is in good agreement with the chemical structure of ACMO. The presence of C−O and NCO peaks were attributed to the PACMO chains on the surface of PVDF-g-PACMO membrane. As seen in Table 2, the peak area percentages of C−O were about 5.45, 5.9, and 7.06% for M1, M2, and M3, and the peak area percentages of NCO were about 6.42, 6.99, and 7.91% for M1, M2, and M3. These results suggest that PACMO chains have been introduced onto the membrane surface and the amount of PACMO chains grafted onto the membrane surface depended on the GD of PVDF-g-PACMO copolymers. These hydrophilic chains on the membrane surface can bind with water molecules to form a hydrated membrane surface, resulting in the prevention of protein adsorption and organic fouling resistance. 3.3. Membrane Morphology. The pristine PVDF and PVDF-g-PACMO copolymer membranes were prepared by the immersed phase inversion method. The cross-sectional and top surface morphologies were investigated by SEM. As-prepared membranes exhibit the typical asymmetric cross-sectional structure (seen in Figure 6), consisting of a skin layer as a selective barrier and a much thicker fingerlike substructure. The formation of this cross-sectional structure is mainly due to the high mutual diffusivity of water and DMF.36 Only slight

Figure 3. FT-IR spectra of PVDF and PVDF-g-PACMO copolymers with different weight ratios in the reaction system.

C2, and C3) shows two additional peaks at 1114 and 1642 cm−1, which are assigned to C−O−C and CO stretching vibrations from the ACMO, respectively, indicating the existence of PACMO side chains in the copolymer. The absorption intensities of C−O−C increase as the ACMO concentration in the reaction system increases. These results demonstrate that the ACMO was successfully grafted onto PVDF powder. The representative 1H NMR spectra of pristine PVDF and the synthesized PVDF-g-PACMO are shown in Figure 4. There

Figure 4. 1H NMR spectra of PVDF and PVDF-g-PACMO copolymers with different weight ratios in the reaction system.

are two well-known peaks at the chemical shifts of about 2.90 (a) and 2.24 ppm (b) due to head-to-head (hh) and head-totail (ht) bonding arrangements of the PVDF main chain.33 The strong peaks at 3.30 and 2.48 ppm are attributed to residual H2O and the DMSO,33 respectively. Compared to the spectrum of PVDF, an obvious additional peak at 3.6 ppm in the spectrum of all PVDF-g-PACMO copolymers was observed, which was ascribed to −CH2− (c) of the morpholine ring in PACMO graft chains. A weak peak at 2.71 ppm corresponded to CH2CH(CO) of PACMO graft chains.34 The presence of these new peaks also indicates that PACMO has been successfully grafted onto PVDF main chains. The grafting degree (GD) of PACMO (the molar percentage of PACMO in the graft copolymer) can also be calculated from the 1H NMR spectra by using the following equation: 18395



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Figure 5. XPS wide-scan and C 1s core-level spectra of (a, b) pristine PVDF membrane (M0) and (c, d) PVDF-g-PACMO membrane (M1).

Table 2. Chemical Compositions of Membrane Surfaces chemical elements (atom %)

C 1s (atom %)

membrane

C

N

O

F

CH2/C−N

CF2

C−H

C−O

NCO

M0 M1 M2 M3

51.68 56.9 57.43 58.15

− 1.26 1.67 2.21

4.39 4.97 4.98 5.85

43.92 36.85 35.93 33.78

48.63 37.14 37.32 36.76

45.13 32.97 30.87 28.64

6.24 18.02 18.92 19.63

− 5.45 5.9 7.06

− 6.42 6.99 7.91

changes of the fingerlike pore size could be observed from the SEM images, indicating that the introduction of PACMO grafting chains has little effect on the cross-sectional structure of membranes. Figure 6 also shows the surface morphology of membranes. The dense surface structure can be observed for the pristine PVDF membrane, while the micropores are formed on the PVDF-g-PACMO membrane surfaces. Additionally, pore sizes increase with the increase of the grafting degree of ACMO, which is in good agreement with the results of pore size measurement revealed in Table 1. The Barrett−Joyner− Halenda (BJH) method was applied to the isotherms to measure the pore sizes using the Kelvin model of pore filling.31 The average BJH desorption pore diameter increases from 6.7 to 95.1 nm with increasing GD values. These data seem much smaller than the membrane surface pore sizes observed in SEM images. The difference may be explained by the reason that only pore sizes in the range of 2−100 nm could be measured under the conditions used for the adsorption−desorption experiments.31 The increase of membrane pore size for copolymer membranes may be owing to the pore-forming ability of the grafting PACMO chains. PACMO chains of copolymer are hydrophilic, resulting in forming repulsion because of the incompatibility of the hydrophilic chains and hydrophobic PVDF main chains, and reducing the thermodynamic miscibility of the casting solution (the thermodynamic effect). The repulsion and thermodynamic effect force the hydrophilic chains to undergo migration toward the membrane surface, forming membrane pores and increasing the pore size.

3.4. Hydrophilicity of Membrane. Water contact angle is used to evaluate the hydrophilicity of membrane surfaces. Reliable contact angles for porous membranes are difficult to obtain due to capillary forces within pores, contraction in the dried state, heterogeneity, roughness, and restructuring of the surface. However, the relative hydrophilicity (or hydrophobicity) of membranes can be evaluated by comparing the change in the water contact angle with drop age.14,25,37 Figure 7 shows the attenuation curves of the water contact angle with drop age for the pristine PVDF membrane and PVDF-gPACMO membranes. The initial contact angle decreases from 89.5 ± 1.69 to 81.6 ± 1.66, 80.5 ± 1.49, and 78.8 ± 1.12° as the GD of PVDF-g-PACMO copolymers increases, respectively. This result demonstrates that the grafting of PACMO chains brought higher hydrophilicity to the PVDF membrane. The decay rate of the contact angle might be affected by the hydrophilicity of the membrane surface and membrane structure including the surface pore size and wettability of the internal pore channel surface.38 When a water droplet is dribbled on the PVDF-g-PACMO membrane surface, it spreads instantly due to the electrostatic interaction between PACMO chains and water molecules. Meanwhile, the formation of larger membrane surface pores also facilitated the penetration of the water drop into the membrane matrix. The XPS and SEM results show that a higher number of ACMO monomers are distributed on the membrane surface bringing higher hydrophilicity and larger membrane pores are formed as the GD of PVDF-g-PACMO copolymers increases. M3 exhibits a 18396



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Figure 6. SEM images of cross-sectional and top surface morphology of pristine PVDF membrane (M0) and PVDF-g-PACMO membranes (M1, M2, and M3).

3.5. Protein Adsorption of Membrane. The fouling resistance of the membrane is found to be related to the protein adsorption. In the present work, the protein adsorption to PVDF-g-PACMO membranes was determined by immersion into the BSA solution. The result is shown in Figure 8. It can be seen that the amount of adsorbed BSA to PVDF-g-PACMO membranes is lower than that to pristine PVDF membrane and

remarkable attenuation tendency due to its large pore size on the membrane surface, as well as improved hydrophilicity. Since the GD of C1 and C2 is not high and the pore sizes on M1 and M2 are not large, the attenuation tendency of M1 and M2 is not obvious. Besides, there is no significant difference about the attenuation tendency between M1 and M2 on account of the close GD values and pore sizes. 18397



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Figure 9. Pure water flux and BSA rejection ratios of pristine PVDF membrane (M0) and PVDF-g-PACMO membranes (M1, M2, and M3).

Figure 7. Water contact angles of pristine PVDF membrane (M0) and PVDF-g-PACMO membranes (M1, M2, and M3).

to 93.8% since increasing the pore size of the membrane also allows the macromolecular solute to permeate through the membrane. 3.7. Antifouling Performance of Membrane. To investigate the fouling behavior in the filtration process, cyclic filtration with protein solution was conducted. BSA solution was filtrated to investigate the trends of protein fouling on membranes. The flux declined with time; flux recovery and degree of protein fouling were studied in detail. Figure 10

Figure 8. Amount of adsorbed BSA to pristine PVDF membrane (M0) and PVDF-g-PACMO membranes (M1, M2, and M3).

BSA adsorption decreases with increasing GD. When the GD increases to 13.52%, the BSA adsorption decreases to 33.37 ± 10.32 μg/cm2, about 28.6% of the BSA adsorption to pristine PVDF membrane. The decreased BSA adsorption could be caused by the introduction of hydrophilic moieties (PACMO chains) with a repellent property for protein adsorption. The mechanism is complex but might include the following factors: shielding electrostatic interactions between the surface and protein due to the hydrophilic layer; van der Waals interactions between the PACMO chains and the water; hydration interactions due to the formation of structured water associated with the hydrophilic PACMO side chains that results in a repulsive force to protein adsorption.39 3.6. Filtration Performance of Membrane. The permeation properties and separation performance of the PVDF-g-PACMO membranes were evaluated by filtration tests. Figure 9 shows the pure water fluxes and BSA rejection ratios of pristine PVDF membrane and PVDF-g-PACMO membranes, respectively. Pure water flux significantly increases, whereas BSA rejection ratio slightly decreases with elevated GD of PVDF-g-PACMO copolymer. Interestingly, the pure water flux of M3 is about 2.5 times than that of the pristine PVDF membrane, reaching 126.59 ± 2.45 L/(m2·h). The permeation flux of a membrane in the filtration of aqueous solution is determined mainly by two factors as follows: one is membrane structure, including membrane porosity and pore size; the other is membrane surface hydrophilicity.40 In the present work, the improvement of surface hydrophilicity and formation of microporous structure have a combined effect on the filtration performance. The rejection ratio for BSA decreases from 99.3

Figure 10. Time-dependent flux of pristine PVDF membrane (M0) and PVDF-g-PACMO membranes (M1, M2, and M3) with three cycles of BSA solution filtration.

presents the time-dependent flux during filtration. For each cyclic filtration, the process can be divided into three phases. The first 1/2 h in the curve is referred to pure water filtration. The second phase is 1 h filtration of BSA protein solution, and the third phase is the steady pure water filtration after the sample membrane is flushed by pure water. For all membranes, the fluxes decreased in the initial stage of protein ultrafiltration, which were attributed to protein adsorption and/or convective deposition on the membrane surface. After the equilibrium between the deposition and diffusion of protein was reached, a relatively steady flux was retained. The flux decline of the membranes is mostly caused by concentration polarization and pore blocking.41 It is obvious that the BSA flux of pristine PVDF membrane showed a drastic decrease and then became stable in a few minutes. However, PVDF-g-PACMO mem18398



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Figure 11. Rt, Rr, Rir, and FRR of pristine PVDF membrane (M0) and PVDF-g-PACMO membranes (M1, M2, and M3) during multicycle filtration.

branes took more time to reach relatively steady BSA fluxes and PVDF-g-PACMO membrane with a higher grafting degree took more time. Another important phenomenon observed was that the BSA fluxes of PVDF-g-PACMO membranes were higher than the water flux of PVDF membrane. A summary of Rt, Rr, Rir, and flux recovery (FRR) of PVDF and PVDF-g-PACMO membranes is shown in Figure 11. After multicycle filtration of BSA solution, it is found that the FRRs were 68.2, 82.6, 83.5, and 89.2%, corresponding to M0, M1, M2, and M3 membranes, respectively. As the GD increases, the FRR increases significantly. This result is consistent with that of the above static protein adsorption, and PVDF-g-PACMO membranes display a better antifouling property than the PVDF membrane. For each filtration cycle, with the grafting degree of PACMO increasing, Rr increases but Rir and Rt decrease, indicating that part of irreversible fouling has converted into reversible fouling and the membrane fouling has been reduced. Previous works have proved that membrane fouling was more severe for the larger pore size membranes as compared to the smaller pore size membranes and the main type of membrane fouling for the large pore size membranes is pore blocking followed by cake formation.26−28 In the present work, the pore size of asprepared PVDF-g-PACMO copolymer membrane is substantially much higher than that of pristine PVDF membrane. Nevertheless, in comparison with PVDF membrane, the copolymer membrane exhibits a better antifouling property. This result indicates that the incorporation of the PACMO chains on the membrane surface and membrane pore channels effectively migrates membrane fouling induced by the pore blocking. This is ascribed to the hydration of water molecules within the PACMO chains. The hydrate layers can resist the adsorption and deposition of proteins on the membrane surface and membrane pore channels, thereby preventing the formation of pore blocking and leading to lower membrane fouling.

aggressive chemical cleaning procedures, and thereby leading to longer membrane lifetimes and decreased operating costs of filtration systems.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 22 83955013. Fax: +86 22 83955013. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the National Natural Science Foundation of China (Grants 21374078 and 21174103). REFERENCES

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4. CONCLUSIONS Copolymers of PVDF grafted with PACMO were successfully synthesized via radical polymerization. The existence of grafted PACMO in PVDF powder was confirmed by FT-IR spectroscopy and 1H NMR spectra. The membranes were cast from pristine PVDF and PVDF-g-PACMO copolymer via the immersed phase inversion method. Larger micropores were observed on the PVDF-g-PACMO membrane surface as the GD increased. The water contact angle results indicate that the hydrophilicity of PVDF membrane has been improved due to the incorporation of ACMO and the GD of copolymer can significantly affect the hydrophilicity. PVDF-g-PACMO membranes exhibited an excellent antifouling property. The ability to recover relatively high flux with only a pure water rinse was exceptionally promising, potentially eliminating the need for 18399



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Acryloylmorpholine-Grafted PVDF Membrane with Improved Protein

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