Antifouling Membranes Prepared by a Solvent-Free Approach via Bulk

Article pubs.acs.org/IECR

Antifouling Membranes Prepared by a Solvent-Free Approach via Bulk Polymerization of 2‑Hydroxyethyl Methacrylate Jinming Peng, Yanlei Su, Wenjuan Chen, Xueting Zhao, Zhongyi Jiang,* Yanan Dong, Yan Zhang, Jiazhen Liu, and Xiaochen Fan Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China ABSTRACT: In this study, a solvent-free approach to antifouling membranes is tentatively presented. The membrane bulk matrix is formed through bulk polymerization of liquid monomer 2-hydroxyethyl methacrylate (HEMA) in the absence of an organic solvent. The membrane pores are formed via removal of silica particles generated from the sol−gel process of tetraethyl orthosilicate. The membranes have a homogeneous structure and acquire water flux around 1020 L m−2 h−1 at 0.1 MPa. The rates of rejection of the yeast and oil droplet for all the polyHEMA membranes are above 97.0 and 99.0%, respectively. The polyHEMA membranes exhibit strong hydrophilic and excellent fouling resistance properties as a result of hydrophilic HEMA. The flux recovery ratio is as high as 91.0% using protein as a foulant. After the fluorine-containing additive dodecafluoroheptyl methacrylate (DFHM) had been blended in a casting solution, the poly(HEMA/DFHM) membranes possess an amphiphilic surface and exhibit excellent fouling resistance as well as fouling release properties. The flux recovery ratio is >90%, and the total flux decline ratio is as low as 11.7% using an oil/water emulsion as the foulant.

1. INTRODUCTION Membrane separation technology has emerged as a highly competitive candidate in the water treatment, bioseparation, food, and pharmacy industries because of its inherent advantages such as its high process efficiency, its low level of energy consumption, and its negligible environmental impact. Membrane fouling is the main bottleneck limiting the efficient and wide application of the membranes. The membrane fouling is caused by the undesirable accumulation of foulants, such as biomacromolecules, microorganisms, hydrocarbons, particles, and colloids, resulting in a large flux decline and an increase in the energy and operational cost. The strategies for the preparation of an antifouling membrane are based on manipulating the physicochemical structure of the membrane surface, weakening the interactions between foulants and the membrane surface, and thus inhibiting the adsorption of foulants or the settlement of foulants on the membrane surface.1,2 Recently, three major types of membrane surfaces, hydrophilic, superhydrophobic, and amphiphilic, were widely used to confer antifouling properties upon membranes. The objective of creating a hydrophilic surface is to prevent the foulants from attaching to the membrane surface, which is known as “fouling resistance”. Poly(ethylene glycol) (PEG)-based polymers and zwitterionic polymers are the most widely used hydrophilic modifiers.3−8 Other hydrophilic polymers such as poly(N-vinyl2-pyrrolidone) (polyNVP) and poly(2-hydroxyethyl methacrylate) (polyHEMA) are also usually employed in antifouling modifications.9,10 The antifouling mechanism of hydrophilic modifiers can be mainly ascribed to strong repulsive hydration forces of the tightly bound water layer around the polymer chains caused by hydrogen bonds or electrostatic attraction. The superhydrophobic surface aims to weaken the interfacial bonds so that the attached foulants are more readily removed © 2013 American Chemical Society

by hydraulic shear forces, not prevent foulants from attaching, which is known as “fouling release”. The superhydrophobic surface is based on nonpolar, low-surface energy superhydrophobic polymers, such as poly(dimethylsiloxane) (PDMS) elastomers and fluoro polymers,11−13 taking their inspiration from marine antifouling strategies.14,15 The antifouling mechanism of the superhydrophobic membrane surface is one in which polar molecules (including adhesive protein) barely adhere to the membrane surface because of the reduced frequency of hydrogen bonding and polar interactions between foulants and the membrane surface. The amphiphilic surface comprising hydrophilic domains and superhydrophobic domains on the membrane surface incorporates the benefits of both hydrophilic and superhydrophobic surfaces. The primary motivation for the creation of the amphiphilic surface is to combine the well-known fouling resistance property of the hydrophilic domains with the fouling release property of superhydrophobic domains.16−19 At present, two categories of approaches have been developed to confer membranes with antifouling properties: (1) postmodification on existing membranes such as surface grafting and (2) in situ modification during membrane formation such as surface segregation.5,16,20−23 In the first approach, the modifier polymers are incorporated onto the membrane surface via chemical grafting. Two distinct grafting techniques are employed for membrane modification. The “grafting to” technique is grafting the preformed modifier polymers onto the membrane with initiator sites, while the “grafting from” technique is a surface-initiated polymerization Received: Revised: Accepted: Published: 13137

May 20, 2013 August 13, 2013 August 15, 2013 August 15, 2013 dx.doi.org/10.1021/ie401606a | Ind. Eng. Chem. Res. 2013, 52, 13137−13145

Industrial & Engineering Chemistry Research

Article

Figure 1. Preparation of polyHEMA membranes. (a) The casting solution was confined between two glasses and subjected to thermal treatment. (b) After thermal treatment for 24 h, the casting solution solidified to an inorganic particle-containing dense membrane. (c) The membrane spontaneously peeled off the substrate, and silica particles were removed.

vacuum pump oil (GS-1) with a density of 807 kg/m3 and an average viscosity of 100 Pa s at room temperature was purchased from Beijing Sifang Special Oil Co. (Beijing, China). Bovine serum albumin (BSA) was purchased from the Institute of Hematology, Chinese Academy of Medical Science (Tianjin, China). Dried yeast was obtained from Tianjin Chine-British Health Food Co. Ltd., washed thoroughly with water, and then dispersed in deionized water prior to being used. The commercial CA membranes with a pore size of 1.0 μm and a thickness of ∼150 μm were purchased from Shanghai Mili Membrane Separation Technology Co. Ltd. (Shanghai, China). Deionized water was used throughout the experiments. Other chemicals were all of commercially analytical grade. 2.2. Membrane Preparation. The casting solutions containing 2.0 g of HEMA, 80 μL of HDODA, 0.1 g of BPO, 1.0 g of TEOS, and different amounts of DFHM were stirred supersonically for 5 min to ensure homogeneous mixing. nDFHM/nHEMA was 0, 2.5, 5.0, 7.5, and 10.0%. Nitrogen was injected into solutions to remove oxygen. Then, 20 μL of each casting solution was dripped onto a glass (length of 75 mm and width of 25 mm), spread out onto the whole glass, and covered by another glass of the same size (Figure 1). The load with a weight of 0.25 kg was put on the top slide glass to confer external pressure (1.30 kPa). Thermal treatment at 80 °C was conducted for 24 h in an oven. After being cooled to room temperature, the two glasses were uncoupled. Subsequently, the membranes attached to glass were quickly peeled off by being immersed in a dilute sodium hydroxide solution (pH 10.0) for 4 h and then transferred into a hot concentrated sodium hydroxide solution (40 °C, pH 13.0) for 8 h to remove silica particles. The membranes were washed in ethanol and stored in deionized water before being use. The membrane prepared via bulk polymerization of HEMA with additive DFHM was denoted as the poly(HEMA/DFHM) membrane, while polyHEMA membranes were the membranes prepared without DFHM. 2.3. Membrane Characterization. A field emission scanning electron microscope (FESEM, Nanosem 430) was utilized to investigate the surface and cross-section morphologies of the membranes. The membrane samples frozen in liquid nitrogen were broken and sputtered with gold to produce electric conductivity prior to scanning electron microscopy (SEM). The surface chemical compositions of PES membranes were probed by X-ray photoelectron spectroscopy (XPS) instruments (PHI-1600) using Mg Kα radiation (1254.0 eV) as the radiation source (the takeoff angle of the photoelectron was set at 90°). Survey spectra were collected over a range of 0−1100 eV. The water and oil contact angle was employed to evaluate the hydrophilicity and oleophobicity of the membranes. The measurement was taken at room temperature with a contact

process whereby the modifier polymer chains grow from the functional monomer on the membrane surface.24 In the latter approach, the modifier polymers are presynthesized and then blended with membrane matrix materials to make casting solutions. During the membrane formation process, the modifier polymers segregate to the membrane surface, rendering a membrane with antifouling properties. The surface grafting method is subjected to the intrinsic permeability reduction because of partial blocking of membrane pores as well as the increasing cost and requirement of complicated equipment caused by the extra grafting procedure.25 The in situ surface segregating method can prevent the extra procedure but often requires a complicated presynthesis of the modifier copolymer and may suffer from insufficient stability during long-term operation because of the weak interaction between the modifier polymers and the membrane matrix. Herein, we tentatively propose a solvent-free approach for preparing antifouling membranes. The casting solution comprised the liquid monomer 2-hydroxyethyl methacrylate (HEMA), the initiator benzoyl peroxide (BPO), cross-linking agent 1,6-hexanediol diacrylate (HDODA), pore-forming agent tetraethyl orthosilicate (TEOS), and/or additive dodecafluoroheptyl methacrylate (DFHM). Initiated by thermal decomposition of BPO, free radical polymerization of HEMA occurred in the confined space created by two closely placed glasses. The polyHEMA chains were cross-linked by HDODA, developed into the polymer network, and finally precipitated to a solid membrane. After that, membrane pores were obtained by removing the silica particles, generated via the sol−gel process from TEOS. This in situ approach not only prevents the extra grafting modification procedure along with the presynthesis of the modifier copolymer but also yields the long-period stability via the strong covalent bonds between the modifier polymers and the membrane matrix. Contact angles of the membranes indicate that the polyHEMA membrane exhibits strong surface hydrophilicity and the poly(HEMA/DFHM) membranes display an amphiphilic surface. Filtration experiments show that the polyHEMA membrane manifests strong fouling resistance while the poly(HEMA/DFHM) membranes exhibit both fouling resistance and fouling release properties.

2. EXPERIMENTAL SECTION 2.1. Materials. 2-Hydroxyethyl methacrylate (HEMA), tetraethyl orthosilicate (TEOS), benzoyl peroxide (BPO), glycerol, and diiodomethane were purchased from Kewei Chemical Reagent Co. (Tianjin, China). Sodium hydroxide and sodium dodecyl sulfonate (SDS) were purchased from Guangfu Chemical Reagent Co. (Tianjin, China). 1,6-Hexane dioldiacrylate (HDODA) was purchased from Aladdin Reagent Database Inc. (Shanghai, China). Dodecafluoroheptyl methacrylate (DFHM) was purchased from XEOGIA FluorineSilicon Chemical Co. Ltd. (Haerbin, China). High-speed 13138

dx.doi.org/10.1021/ie401606a | Ind. Eng. Chem. Res. 2013, 52, 13137−13145


Article

solution reservoir were emptied and refilled rapidly with a foulant solution. The BSA solution and oil/water emulsion were used as model foulants. The flux for the BSA solution or oil/water emulsion Jp (liters per square meter per hour) was measured on the basis of the water volume permeating the membranes at the same pressure (0.1 MPa).

angle goniometer (JC2000C Contact Angle Meter, Powereach Co., Shanghai, China). Membranes were lyophilized for 3 h and then pressured with a rolling machine. The contact angle was measured at five different locations for each membrane, and the average was adopted. The surface free energy (γ) was calculated using the threeliquid Lifshitz−van der Waals acid−base model with the following equations:26 γi = γiLW + 2 γi+γi

Jp =

(1)

γL(1 + cos θ ) = 2 γSLWγLLW + 2 γL+γS +

γS+γL

(1 + cos θ )γL 2

=

(2)

⎛ Cp ⎞ R = ⎜1 − ⎟ × 100% Cf ⎠ ⎝

(3)

γLABγSAB +

γLLWγSLW

(4)

(Wwet − Wdry )/ρwater (Wwet − Wdry )/ρwater + Wdry /ρHEMA

FRR =

Vw AΔt

Jw2 Jw1

× 100% (9)

The higher the FRR, the better the fouling resistance property of the membrane. The fouling release property was evaluated by the total flux decline ratio (DRt), the reversible flux decline ratio (DRr), and the irreversible flux decline ratio (DRir), which were calculated as follows:

× 100% (5)

where P is the porosity (percent), Wwet and Wdry are the wet weight and dry weight of the polyHEMA membranes, respectively, and ρwater and ρHEMA are the density of water and polyHEMA, respectively. 2.4. Membrane Filtration Experiments. A dead-end stirred cell filtration system connected to a nitrogen gas cylinder and a solution reservoir was designed to characterize the separation performance of membranes. The system consisted of a filtration cell (model 8003, Millipore Co.) with a volume capacity of 3 mL. The effective area of the membrane was 0.9 cm2. The feed side of the system was pressured with nitrogen gas. All the filtration experiments were conducted at a stirring speed of 400 rpm and a temperature of 25 ± 1 °C. The membranes were transferred on a substrate of nonwoven fabric. The sample was initially pressurized at 0.15 MPa for 30 min, and then the pressure was lowered to the operating pressure of 0.1 MPa. The water flux Jw (liters per square meter per hour) was calculated by eq 6: Jw =

(8)

where Cp and Cf are the BSA, yeast, or oil droplet concentration of permeate and feed solutions, respectively. The fouling resistance property was tested by a sequential filtration of water, a BSA solution or oil/water emulsion, and water again. The fouling resistance property was evaluated by the flux recovery ratio (FRR), which was calculated by eq 9:

The superficial water of wet polyHEMA membranes was first removed with dry filter paper, and then weights of the wet polyHEMA membranes were measured. Finally, the membranes were dried in an oven at 75 °C for 24 h. The weights of the dry membranes were measured again. The membrane porosity (P) was defined as the volume of the pores divided by the total volume of the membrane. The porosity was calculated by eq 5: P=

(7)

AΔt

where Vp (liters) is the volume of permeation of the foulant solution, A (square meters) is the membrane area, and Δt (hours) is the permeation time. The BSA concentration in solution was 1.0 mg/mL. The oil concentration of the oil/water emulsion was 1000 ppm with 100 ppm sodium dodecyl sulfate as an emulsifier. After ultrafiltration of the mixture solution, the membranes were washed with deionized water for 20 min, and then the water flux of cleaned membranes Jw2 (liters per square meter per hour) was measured again. The BSA/yeast suspension and oil/water emulsion were used to investigate the rejections of membranes. The rejection (R) of solute was calculated by eq 7:

where θ is the contact angle, i denotes either a solid (S) or a +, − liquid (L) phase, and γLW i , γi and γi (millijoules per square meter) are the Lifshitz−van der Waals, acid, and base components, respectively. Two polar liquids (water and glycerol) and one polar liquid (diiodomethane) were selected as test liquids. The surface free energy (γS) was calculated using the three-liquid Lifshitz−van der Waals acid−base model, as AB well as dispersive (γLW S ) and polar components (γS ) expressed by Owens and Wendt27 using the following two equations:

γS = γSLW + γSAB

Vp

⎛ Jp ⎞ ⎟⎟ × 100% DR t = ⎜⎜1 − Jw1 ⎠ ⎝

(10)

where DRt is the degree of total flux decline caused by total fouling. DR r =

DR ir =

Jw2 Jp Jw1

× 100%

Jw1Jw2 Jw1

(11)

× 100% (12)

Obviously, DRt was the sum of DRr and DRir. The lower the DRt, the better the fouling release property of the membranes.

3. RESULTS AND DISCUSSION 3.1. Membrane Preparation. The membrane fabrication process is shown in Figure 1. First, the casting solution encompassing monomer HEMA, cross-linking agent HDODA, initiator BPO, pore-forming agent TEOS, and/or additive DFHM was prepared and evenly dispersed. Casting solutions

(6)

where Vw (liters) is the volume of permeation water, A (square meters) is the membrane area, and Δt (hours) is the permeation time. In the following step, the stirred cell and 13139



Article

Figure 2. SEM images of the (a) surface morphology and (b) cross-section morphology of the polyHEMA membrane. SEM images of the surface morphology of the poly(HEMA/DFHM) membranes with nDFHM/nHEMA values of 5.0 (c) and 10.0% (d).

inversion from the liquid film to the solid membrane in this approach was dependent on the continuously increased viscosity. The polyHEMA network constituted the membrane bulk matrix. Because TEOS could not convert into silica particles without acid or base catalysis, it might remain in the liquid state at the end of the thermal treatment. Then, the slide glasses were immersed in a dilute sodium hydroxide solution (pH 10.0) for 4 h. The film quickly peeled off, and the TEOS converted to silica particles via the sol−gel process in an alkaline environment. After that, films were transferred into a hot concentrated sodium hydroxide solution (40 °C, pH 13.0) for 8 h. The silica particles could convert to silicates and dissolve in the sodium hydroxide solution. After that, the space occupied by the silica particles turned into pores. It is worth mentioning that an organic solvent was absent in the membrane fabrication process. This was because liquid monomer HEMA could dissolve the other components, making a homogeneous casting solution. Another reason might be that the free radical polymerization of the vinyl monomer could proceed without the involvement of an organic solvent.

were spread out on one glass and then covered by another glass. These two glasses offered not only a template for shaping the casting solution into a flat sheet but also an oxygen barrier to prevent free radical polymerization of monomers from oxygen inhibition. The preparation of the polyHEMA membranes involves two reactions, including the free radical polymerization and cross-linking of the monomer and the sol− gel process of the inorganic precursor TEOS into silica particles. The polymerization and sol−gel process played important roles in membrane morphology. At the beginning, all the components in the casting solution were mixed on a molecular scale. The thermal decomposition of BPO triggered the bulk polymerization of HEMA. The primary free radical grabbed one π-electron from the vinyl group of HEMA, generating monomer radicals. After that, monomer molecules continued reacting with monomer radicals and short chain oligoHEMA was generated. With the reactions proceeding, the polyHEMA network developed via further chain growth and cross-linking. The thermal treatment lasted for 24 h to ensure the full development of the polymer network. Unlike the conventional membrane fabrication approaches such as nonsolvent or thermally induced phase inversion, the phase 13140



Article

Figure 3. XPS spectrum (a) and XPS C1s core level spectral resolving results (b) of the polyHEMA membrane. XPS spectrum (c) and XPS C1s core level spectral resolving results (d) of the poly(HEMA/DFHM) membrane with an nDFHM/nHEMA of 10%.

Table 1. XPS Results and Rejections of the PolyHEMA and Poly(HEMA/DFHM) Membranes composition of surface elements (mol %) nDFHM/nHEMA (%)

C−C

C−O

OC−O (C−F)

CF2

CF3

surface coverage of DFHM (%)

rejection of yeast (%)

rejection of BSA (%)

rejection of oil droplet (%)

0 2.5 5.0 7.5 10.0

54.7 50.2 47.9 46.2 44.2

37.0 37.4 36.9 36.5 36.4

8.3 10.1 11.0 11.7 12.1

− 1.5 2.7 3.5 4.6

− 0.8 1.5 2.1 2.7

− 2.9 5.5 7.7 9.9

98.4 97.5 98.6 97.0 98.2

2.8 2.0 2.2 3.1 1.4

99.0 99.3 99.5 99.0 99.7

3.2. Membrane Characterization. SEM is often employed to investigate membrane surface and cross-section morphologies. Figure 2a shows the membrane surface morphologies of the polyHEMA membrane. The membrane pores could be observed. The average pore size of the polyHEMA membrane was ∼1.1 μm. A possible mechanism of mircosized pores is proposed as follows: during polymerization, a TEOS phase might develop from the miscible state to the aggregation state. At the beginning of polymerization, TEOS remains dissolved in the monomer. As polymerization proceeds, the solution is gradually divided into the continuous phase mainly comprising unreacted monomer, oligoHEMA, and HDODA and the dispersed phase comprising TEOS. At the end of the thermal treatment, the continuous phase formed the polymeric network and the dispersed TEOS phase aggregated to a microsized droplet. After dilute and concentrated sodium hydroxide solution treatments, the silica particles were etched and left the microsized pores.

Shown in Figure 2b are the membranes that exhibited homogeneous structure. The thickness of the polyHEMA membrane was ∼2.8 μm. Panels c and d of Figure 2 showed the surface morphology of the poly(HEMA/DFHM) membranes with nDFHM/nHEMA values of 5.0 and 10.0%. The pore size of the poly(HEMA/DFHM) membranes was ∼1.1 μm. In addition, the thickness of the poly(HEMA/DFHM) membranes was similar to that of the polyHEMA membrane. The near-surface compositions of the polyHEMA and poly(HEMA/DFHM) membranes were determined by XPS analysis. Figure 3 shows the typical XPS spectrum and C1s core level spectral resolving results of the polyHEMA membrane and the poly(HEMA/DFHM) membrane with an nDFHM/ nHEMA of 10%. The C1s spectrum was resolved into five peaks at bonding energies of 284.8, 286.3, 288.6, 290.7, and 293.3 eV, corresponding to C−C, C−O, OC−O (C−F), CF2, and CF3, respectively. Because the CF3 signal came from only the DFHM, the surface coverage of DFHM [φS (percent)] was calculated by the following equation: 13141



Article

LW Figure 4. (a) Contact angles and (b) surface free energy parameters, including γS, γAB S , and γS , of the polyHEMA and poly(HEMA/DFHM) membranes as a function of nDFHM/nHEMA.

φS =

A CF3 3/11

× 100%

comparison with the polyHEMA membrane, the poly(HEMA/ DFHM) membranes had much lower γS values. When nDFHM/ nHEMA equalled 2.5%, the γS of the poly(HEMA/DFHM) membrane was substantially reduced to 42.8 mJ/m2. When nDFHM/nHEMA further increased to 10.0%, the poly(HEMA/ DFHM) membrane had the lowest γS (29.6 mJ/m2). With an increase in nDFHM/nHEMA from 0 to 10.0%, the dispersive component (γLW S ) decreased considerably from 44.9 to 20.5 mJ/m2 while the polar component (γAB S ) decreased slightly from 15.2 to 9.1 mJ/m2. These results indicated that the decrease in surface free energy was mainly derived from the reduction in the dispersive component (γLW S ), which could account for the introduction of the nonpolar, low-surface energy DFHM additive. These amphiphilic surfaces with a low surface free energy would be expected to alleviate the adhesion propensity and facilitate the detachment of foulants from the membrane surface under shear force. 3.3. Membrane Separation Properties. Permeability and selectivity are the critical parameters for membrane separation. Filtration experiments were conducted to assess the permeation fluxes and rejections of the polyHEMA and poly(HEMA/ DFHM) membranes. Figure 5 displays fluxes of water, the BSA

(13)

where ACF3 is the area percentage of the CF3 peak and the 3/11 coefficient was derived from the fact that there were three CF3 groups in DFHM containing eleven carbon atoms. The area percentages of peaks and the φS of all the poly(HEMA/ DFHM) membranes are listed in Table 1. The area percentages of the C−O peak for all the poly(HEMA/DFHM) membranes were above 36%, higher than the theoretical value of polyHEMA chains (33.3%). This meant that the membrane surface was enriched with hydroxyl groups. The surface coverage of DFHM on the membrane surface was in accordance with the nDFHM/nHEMA. φS increased from 2.9 to 9.9% when nDFHM/nHEMA increased from 2.5 to 10.0%. These results demonstrate that the amphiphilic membrane surface comprising hydrophilic domains and low-surface energy superhydrophobic domains was successfully constructed by modification of DFHM. Surface hydrophilicity and oleophobicity are important characteristics for the membranes. Strong surface hydrophilicity endows membranes with strong protein adsorption resistance and excellent fouling resistance.6,28 Water and oil contact angles are often employed to evaluate the hydrophilicity and oleophobicity of the membranes. The higher water/oil contact angle represents stronger hydrophilicity and oleophobicity. The water and oil contact angles of the commercial cellulose acetate (CA) membrane were 45.6° and 67.5°, respectively. As shown in Figure 4, the polyHEMA membrane had a water contact angle of 18.9° and an oil contact angle of 28.6°, indicating strong hydrophilicity and weak oleophobicity. With nDFHM/ nHEMA increasing from 2.5 to 10.0%, the water contact angle increased from 59.8° to 84.1° and the oil contact angle increased from 41.5° to 56.3°. These results suggested that because of the blending with DFHM, the membrane surface became less hydrophilic and more oleophobic. The amphiphilic membrane surface was successfully constructed through blending fluorine-containing additive DFHM with hydrophilic monomer HEMA. The surface free energies (γS) of the poly(HEMA/DFHM) membranes calculated using the three-liquid Lifshitz−van der Waals acid−base model are shown in Figure 4b. γS, the AB dispersive component (γLW S ), and the polar component (γS ) of the commercial CA membrane were 71.1, 16.4, and 54.7 mJ/ m2, respectively. The polyHEMA membrane possessed a γS of 2 60.1 mJ/m2 and a dispersive component (γLW S ) of 44.9 mJ/m , AB 2 along with a polar component (γS ) of 15.2 mJ/m . In

Figure 5. Fluxes of water, the BSA solution, and the oil/water emulsion of the polyHEMA membranes as a function of nDFHM/nHEMA.

solution, the oil/water emulsion for the commercial CA membrane, and the polyHEMA and poly(HEMA/DFHM) membranes. The fluxes of water, the BSA solution, and the oil/ water emulsion of the commercial CA membrane were 1526.0, 833.1, and 747.9 L m−2 h−1, respectively. The polyHEMA membranes possessed a water flux of 1171 L m−2 h−1. The 13142



Article

Figure 6. Time-dependent fluxes (a) and antifouling indexes (b) of the polyHEMA and poly(HEMA/DFHM) membranes with BSA as the foulant.

Figure 7. Time-dependent fluxes (a) and antifouling indexes (b) of the polyHEMA and poly(HEMA/DFHM) membranes with oil as the foulant.

lower water flux of the polyHEMA membranes might be ascribed to its lower porosity (∼11%) compared to that of the commercial CA membrane (∼75%). The water fluxes of the poly(HEMA/DFHM) membranes were around 1020 L m−2 h−1, lower than that of the polyHEMA membrane, which could be attributed to the less hydrophilic membrane surface. Similarly, the poly(HEMA/DFHM) membranes possessed lower fluxes of the BSA solution (∼690 L m−2 h−1) than that of the polyHEMA membranes (882 L m−2 h−1). However, the fluxes of the oil/water emulsion for the poly(HEMA/DFHM) membranes (∼850 L m−2 h−1) were much higher than that of the polyHEMA membrane (630 L m−2 h−1). This might be caused by the adsorption of less oil on the membrane surface due to the presence of polyDFHM chains with low surface energies. In this study, a BSA/yeast mixture and an oil/water emulsion were selected as a model system for evaluating the selectivity of the membranes. To completely separate the BSA from yeast in the mixture, the pore size of the membranes should be between the diameters of the yeast and BSA so that the yeast, several micrometers, could be rejected while BSA, much smaller, could pass through. The rejections of yeast, BSA, and an oil droplet for the commercial CA membrane were 96.8, 2.3, and 99.2%, respectively. The data in Table 1 show that all the membranes could reject most yeast and let BSA molecules almost penetrate through the membrane. The rejections of yeast were >97.0%. Less than 3.1% of the BSA molecules were retained for all the membranes. In addition, the membranes rejected >99.0% of oil droplets. The polyHEMA and poly(HEMA/DFHM) membranes presented a selectivity comparable to that of commercial CA membranes with similar pore sizes. 3.4. Membrane Antifouling Properties. The fouling behavior of foulants was quite different. Fouling caused by

biomolecules such as proteins adsorbed on the membrane surface through electrostatic attraction and hydrophobic interaction formed a fouling spot on membrane surface, while the oil foulants were prone to coalescing, spreading, and migrating to form a continuous fouling layer on the membrane surface.29,30 The BSA solution and the emulsion of high-speed vacuum pump oil were selected as foulants to test the membrane antifouling performance. Panels a and b of Figure 6 present the time-dependent permeation fluxes of the commercial CA membrane and the polyHEMA and poly(HEMA/DFHM) membranes during the filtration of the BSA solution and the corresponding antifouling indexes, including FRR, DRt, DRr, and DRir. The FRR of the commercial CA membrane was only 72.5%, and the DRt, DRr, and DRir were 57.7, 30.2, and 27.5%, respectively. The FRR of the polyHEMA membrane was as high as 90.4%, while the DRt, DRr, and DRir were at low levels of 24.7, 15.1, and 9.6%, respectively. The strong fouling resistance property of the polyHEMA membranes could be ascribed to a strong repulsive hydration force to repel the adsorption of foulants. It was well-known that the structure and conformation of water molecules near the surface played a crucial role in protein resistance. Strong interaction between water molecules and polyHEMA chains forming through hydrogen bonding induced a compact, continuous, and stable hydration layer on the polyHEMA membrane surface.24 The proteins tend to approach the membrane surface were repelled into the feed by the strong repulsive force. Therefore, the polyHEMA membranes could exhibit an excellent fouling resistance property. When they were blended with DFHM, the initial water flux was reduced to ∼1020 L m−2 h−1. The FRR of the poly(HEMA/DFHM) membrane was around 87%, which was slightly less than that of the polyHEMA membrane. All the poly(HEMA/DFHM) membranes had larger DRt, DRr, and 13143



■

DRir values because of the less hydrophilic membrane surface. However, although the DRt, DRr, and DRir values became larger, they were still at a low level, especially DRir (∼12%). In other words, although the fouling resistance property of the poly(HEMA/DFHM) membranes was weakened because of the amphiphilic surface, the membranes still possessed a strong ability to resist the protein foulants. Panels a and b of Figure 7 present the time-dependent fluxes of the commercial CA membrane and the polyHEMA and poly(HEMA/DFHM) membranes during oil/water emulsion filtration and the corresponding FRR, DRt, DRr, and DRir values. The FRR of the commercial CA membrane was only 64.2%, and the DRt, DRr, and DRir were 45.6, 9.8, and 35.8%, respectively. For the polyHEMA membrane, the permeation flux of the oil/water emulsion remarkably decreased compared with the initial water flux. The DRt value was as high as 46.2%, while the FRR value of the polyHEMA membrane was 91.0%. These results indicated that the polyHEMA membrane exhibited strong fouling resistance but poor fouling release properties. Nevertheless, for the poly(HEMA/DFHM) membranes, the permeation flux decline was much smaller than that of the polyHEMA membrane. The DRt values of all the poly(HEMA/DFHM) membranes were below 20%. When nDFHM/nHEMA equalled 7.5%, the poly(HEMA/DFHM) membrane possessed the lowest DRt value of 11.7%. The FRR values of all the poly(HEMA/DFHM) membranes remained above 90%, suggesting that the introduction of polyDFHM domains did not make the fouling resistance property worse. The strong fouling release property of the poly(HEMA/DFHM) membranes was ascribed to the introduction of low-surface energy polyDFHM chains. The interfacial interactions between oil foulants and the membrane surface were weakened so that the oil droplets could be readily removed by hydraulic shear forces during stirring.16,31

ACKNOWLEDGMENTS

This research was supported by the National Science Fund for Distinguished Young Scholars (21125627), the Research Fund for the Doctoral Program of Higher Education of China (20060056032), the Drug Separation and Purification Project in the Programme for Development of Novel Drug (2009ZX09301-008), and the Program of Introducing Talents of Discipline to Universities (NO B06006).

■

4. CONCLUSIONS A facile solvent-free approach to preparing antifouling membranes was developed through bulk polymerization of HEMA, with inorganic particles as the pore-forming template. The polyHEMA membrane manifested hydrophilic, while the poly(HEMA/DFHM) membranes exhibited an amphiphilic surface. Because of the introduction of DFHM, the surface energy of the poly(HEMA/DFHM) membrane substantially decreases to 29.6 mJ/m2. The water fluxes of the membranes were around 1020 L m−2 h−1 under 0.1 MPa. All the membranes could reject more than 97.0% of yeast and 99.0% of the oil droplet. Although the poly(HEMA/DFHM) membranes possess a lower protein flux, they present much better fouling resistance to proteins than the commercial CA membrane. Moreover, the poly(HEMA/DFHM) membranes showed higher permeation fluxes and better fouling release properties in the presence of oil foulants.

■

Article

■

LIST OF SYMBOLS Jw = water flux (L m−2 h−1) Jm = flux of the mixture of BSA and yeast (L m−2 h−1) V = volume of permeated water (L) A = membrane area (m2) Δt = operation time (h) R = rejection of solute (%) Cp = solute concentrations of the permeation solution (mg/ mL) Cf = solute concentrations of the feed solution (mg/mL) FRR = flux recovery ratio (%) Jw1 = initial water flux (L m−2 h−1) Jw2 = water flux after clean (L m−2 h−1) DRt = degree of total flux decline (%) DRr = degree of reversible flux decline (%) DRir = degree of irreversible flux decline (%) θ = contact angle (deg) γ = surface free energy (mJ/m2) = Lifshitz−van der Waals component of the surface free γLW i energy (mJ/m2) γ+i = acid component of the surface free energy (mJ/m2) γ−i = base of the surface free energy (mJ/m2) γS = surface free energy of the solid phase (mJ/m2) γLW S = dispersive component of the surface free energy (mJ/ m2) 2 γAB S = polar component of the surface free energy (mJ/m ) φS = surface coverage of DFHM (%) ACF3 = area percentage of the CF3 group (%) REFERENCES

(1) Rana, D.; Matsuura, T. Surface modifications for antifouling membranes. Chem. Rev. 2010, 110 (4), 2448. (2) Banerjee, I.; Pangule, R. C.; Kane, R. S. Antifouling coatings: Recent developments in the design of surfaces that prevent fouling by proteins, bacteria, and marine organisms. Adv. Mater. 2011, 23 (6), 690−718. (3) Ostuni, E.; Chapman, R. G.; Liang, M. N.; Meluleni, G.; Pier, G.; Ingber, D. E.; Whitesides, G. M. Self-assembled monolayers that resist the adsorption of proteins and the adhesion of bacterial and mammalian cells. Langmuir 2001, 17 (20), 6336−6343. (4) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.; Mayes, A. M. Science and technology for water purification in the coming decades. Nature 2008, 452 (7185), 301− 310. (5) Hester, J. F.; Banerjee, P.; Mayes, A. M. Preparation of proteinresistant surfaces on poly(vinylidene fluoride) membranes via surface segregation. Macromolecules 1999, 32 (5), 1643−1650. (6) Wang, Y. Q.; Wang, T.; Su, Y. L.; Peng, F. B.; Wu, H.; Jiang, Z. Y. Remarkable reduction of irreversible fouling and improvement of the permeation properties of poly(ether sulfone) ultrafiltration membranes by blending with pluronic F127. Langmuir 2005, 21 (25), 11856−11862. (7) Jiang, S.; Cao, Z. Ultralow-Fouling, Functionalizable, and Hydrolyzable Zwitterionic Materials and Their Derivatives for Biological Applications. Adv. Mater. 2010, 22 (9), 920−932.

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-22-27406646. Fax: +86-22-27406646. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 13144



Article

purification in the coming decades. Nature 2008, 452 (7185), 301− 310. (26) Kwok, D. The usefulness of the Lifshitz−van der Waals/acid− base approach for surface tension components and interfacial tensions. Colloids Surf., A 1999, 156 (1), 191−200. (27) Owens, D. K.; Wendt, R. Estimation of the surface free energy of polymers. J. Appl. Polym. Sci. 1969, 13 (8), 1741−1747. (28) Ostuni, E.; Chapman, R. G.; Holmlin, R. E.; Takayama, S.; Whitesides, G. M. A survey of structure-property relationships of surfaces that resist the adsorption of protein. Langmuir 2001, 17 (18), 5605−5620. (29) Tuteja, A.; Choi, W.; Ma, M.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Designing superoleophobic surfaces. Science 2007, 318 (5856), 1618−1622. (30) Dickinson, E. Interfacial structure and stability of food emulsions as affected by protein−polysaccharide interactions. Soft Matter 2008, 4 (5), 932−942. (31) Zhao, X.; Su, Y.; Chen, W.; Peng, J.; Jiang, Z. pH-responsive and fouling-release properties of PES ultrafiltration membranes modified by multi-functional block-like copolymers. J. Membr. Sci. 2011, 382 (1), 222−230.

(8) Zhao, Y. H.; Wee, K. H.; Bai, R. Highly hydrophilic and lowprotein-fouling polypropylene membrane prepared by surface modification with sulfobetaine-based zwitterionic polymer through a combined surface polymerization method. J. Membr. Sci. 2010, 362 (1), 326−333. (9) Taniguchi, M.; Belfort, G. Low protein fouling synthetic membranes by UV-assisted surface grafting modification: Varying monomer type. J. Membr. Sci. 2004, 231 (1), 147−157. (10) Li, L.; Yan, G.; Wu, J. Modification of polysulfone membranes via surface-initiated atom transfer radical polymerization and their antifouling properties. J. Appl. Polym. Sci. 2008, 111 (4), 1942−1946. (11) Kim, D.-G.; Kang, H.; Han, S.; Lee, J.-C. Dual effective organic/ inorganic hybrid star-shaped polymer coatings on ultrafiltration membrane for bio-and oil-fouling resistance. ACS Appl. Mater. Interfaces 2012, 4 (11), 5898−5906. (12) Chen, W.; Su, Y.; Peng, J.; Dong, Y.; Zhao, X.; Jiang, Z. Engineering a Robust, Versatile Amphiphilic Membrane Surface Through Forced Surface Segregation for Ultralow Flux-Decline. Adv. Funct. Mater. 2011, 21 (1), 191−198. (13) Zhao, X.; Chen, W.; Su, Y.; Zhu, W.; Peng, J.; Jiang, Z.; Kong, L.; Li, Y.; Liu, J. Hierarchically engineered membrane surfaces with superior antifouling and self-cleaning properties. J. Membr. Sci. 2013, 441, 93−101. (14) Martinelli, E.; Suffredini, M.; Galli, G.; Glisenti, A.; Pettitt, M. E.; Callow, M. E.; Callow, J. A.; Williams, D.; Lyall, G. Amphiphilic block copolymer/poly(dimethylsiloxane) (PDMS) blends and nanocomposites for improved fouling-release. Biofouling 2011, 27 (5), 529−541. (15) Lejars, M. n.; Margaillan, A.; Bressy, C. Fouling release coatings: A nontoxic alternative to biocidal antifouling coatings. Chem. Rev. 2012, 112 (8), 4347−4390. (16) Chen, W. J.; Su, Y. L.; Peng, J. M.; Dong, Y. N.; Zhao, X. T.; Jiang, Z. Y. Engineering a Robust, Versatile Amphiphilic Membrane Surface Through Forced Surface Segregation for Ultralow FluxDecline. Adv. Funct. Mater. 2011, 21 (1), 191−198. (17) Park, D.; Weinman, C. J.; Finlay, J. A.; Fletcher, B. R.; Paik, M. Y.; Sundaram, H. S.; Dimitriou, M. D.; Sohn, K. E.; Callow, M. E.; Callow, J. A. Amphiphilic surface active triblock copolymers with mixed hydrophobic and hydrophilic side chains for tuned marine fouling-release properties. Langmuir 2010, 26 (12), 9772−9781. (18) Callow, J. A.; Callow, M. E. Trends in the development of environmentally friendly fouling-resistant marine coatings. Nat. Commun. 2011, 2, 244. (19) Imbesi, P. M.; Gohad, N. V.; Eller, M. J.; Orihuela, B.; Rittschof, D.; Schweikert, E. A.; Mount, A. S.; Wooley, K. L. NoradrenalineFunctionalized Hyperbranched Fluoropolymer−Poly(ethylene glycol) Cross-Linked Networks As Dual-Mode, Anti-Biofouling Coatings. ACS Nano 2012, 6 (2), 1503−1512. (20) Chen, H.; Belfort, G. Surface modification of poly(ether sulfone) ultrafiltration membranes by low-temperature plasma-induced graft polymerization. J. Appl. Polym. Sci. 1999, 72 (13), 1699−1711. (21) Shi, Q.; Su, Y. L.; Chen, W. J.; Peng, J. M.; Nie, L. Y.; Zhang, L.; Jiang, Z. Y. Grafting short-chain amino acids onto membrane surfaces to resist protein fouling. J. Membr. Sci. 2011, 366 (1−2), 398−404. (22) Chang, Y.; Ko, C. Y.; Shih, Y. J.; Quémener, D.; Deratani, A.; Wei, T. C.; Wang, D. M.; Lai, J. Y. Surface grafting control of PEGylated poly(vinylidene fluoride) antifouling membrane via surfaceinitiated radical graft copolymerization. J. Membr. Sci. 2009, 345 (1), 160−169. (23) Asatekin, A.; Kang, S.; Elimelech, M.; Mayes, A. M. Anti-fouling ultrafiltration membranes containing polyacrylonitrile-graft-poly(ethylene oxide) comb copolymer additives. J. Membr. Sci. 2007, 298 (1−2), 136−146. (24) Chen, S.; Li, L.; Zhao, C.; Zheng, J. Surface hydration: Principles and applications toward low-fouling/nonfouling biomaterials. Polymer 2010, 51 (23), 5283−5293. (25) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Mariñas, B. J.; Mayes, A. M. Science and technology for water 13145


Antifouling Membranes Prepared by a Solvent-Free Approach via Bulk

Recommend Documents