Preparation and characterization of crosslinked poly(ethylene glycol) diacrylate membranes with excellent antifouling and solvent-resistant properties

Journal of Membrane Science 318 (2008) 227–232

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Preparation and characterization of crosslinked poly(ethylene glycol) diacrylate membranes with excellent antifouling and solvent-resistant properties Guodong Kang a,b , Yiming Cao a,∗ , Hongyong Zhao a,b , Quan Yuan a a b

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e

i n f o

Article history: Received 28 November 2007 Received in revised form 22 January 2008 Accepted 10 February 2008 Available online 4 March 2008 Keywords: Poly(ethylene glycol) diacrylate Prepolymerization solution Antifouling Solvent resistance

a b s t r a c t A series of crosslinked poly(ethylene glycol) diacrylate (PEGDA, MW = 302 g/mol) membranes were prepared via UV-induced polymerization by adding various ethanol in prepolymerization solution. Membrane surface morphology, interior structure, chemical composition and mechanical property were investigated with scanning electron microscope (SEM), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and tensile tester. The results suggested that decreasing PEGDA content in prepolymerization solution could gradually enlarge the pore size in the surface and the bulk of membranes, resulting in a higher water uptake and permeability, but a lower bovine serum albumin (BSA) rejection. Meanwhile, the mechanical property decreased. Specifically, when the PEGDA content in prepolymerization solution changed from 100 wt% to 45 wt%, the pure water flux of membranes increased from zero to 340 L/m2 h at 0.1 Mpa. Finally, the BSA fouling and preliminary solvent soaking experiments also indicated that such membranes exhibited excellent antifouling and solvent-resistant properties. © 2008 Elsevier B.V. All rights reserved.

1. Introduction As a low-pressure membrane process, ultrafiltration has received considerable attention in recent years and it has been widely used in many fields including water purification [1], protein concentrate [2–3] and reverse osmosis pretreatment [4]. However, its numerous applications are adversely impacted by fouling from proteins, other biomolecules and organic matters [5–6]. Membrane fouling can cause significant loss of performance, imposing a negative influence on the efficiency and economics of membrane process [7]. Although the property of fouled membrane can be partially recovered through cleaning, this procedure accordingly decreases its use life and increases the operation cost at the same time. An effective method to mitigate this problem and promote the application of ultrafiltration technology is to improve the antifouling abilities of membranes [8–11]. By far, numerous ways had been examined including surface modification [12–13], incorporation of additives [5,14,15] and preparation of membranes with antifouling materials [16]. Most of these methods were focused on the improvement of membrane surface properties with hydrophilic modifiers. Among various modifiers, polyethylene glycol (PEG) or polyethylene oxide (PEO) and their derivatives were widely investigated. PEG is an uncharged

∗ Corresponding author. Tel.: +86 411 84379053; fax: +86 411 84379329. E-mail address: [email protected] (Y. Cao). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.02.045

polymer with the chemical formula of HO (CH2 CH2 O)n H. Its extraordinary antifouling ability, especially to resist protein adsorption, has been proved due to the hydrophilicity, flexible long chains, large exclusion volume and unique coordination with surrounding water molecules in an aqueous medium [17]. In fact, many researches including coating, incorporating or grafting PEG chains on ultrafiltration membrane surfaces have been performed and had achieved promising results in recent years [18–21]. Polyethylene glycol diacrylate (PEGDA, CH2 CHCO(OCH2 CH2 )n OCOCH CH2 ) is a derivative of PEG with repeated ethylene oxide (EO) units and active end groups. PEGDA can be crosslinked using a UV light source and a photoinitiator. Freeman and coworkers prepared several crosslinked PEO rubbers with PEGDA as a main material for gas separation [22–25]. However, the properties of crosslinked PEGDA membrane in water treatment rarely reported. Until recently, they reported a research on crosslinked PEO fouling resistant coating materials for oil/water separation, and the results were promising [26]. In this study, a series of crosslinked PEGDA membranes with various ethanol contents in the prepolymerization solution were prepared and characterized. The monomer and diluent were different with those in Reference [26]. Surface morphologies and interior structures of these membranes were observed by scanning electron microscope (SEM). The surface chemical composition was analyzed by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Their antifouling properties to bovine serum albumin (BSA) solution were studied in cross-flow filtration model.

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Finally, the solubility and solvent resistance of such membranes were also investigated.

the following equation:

2. Experimental

where V was volume of permeated water (L), A was the effective membrane area (m2 ) and t was the ultrafiltration time (h). BSA rejection test was conducted at 0.1 MPa with 1.0 g/L BSA phosphate buffer saline (pH 7.0). The BSA rejection ratio of membrane, R, was defined as follows:   Cp × 100 (3) R (%) = 1 − Cf

2.1. Materials PEGDA-200 (n = 4, MW = 302 g/mol) and 1-hydroxycyclohexyl phenyl ketone (HCPK) were purchased from Aldrich Chemical Company. BSA was purchased from Aoboxing Biotechnology Limited Company (Beijing, China). All other chemicals were of analytically pure grade. Deionized water was produced in our laboratory. 2.2. Membrane preparation The preparation procedure of crosslinked PEGDA membranes was found in the References [23–26]. The prepolymerization solution was prepared by adding 0.1 wt% photoinitiator (HCPK) into PEGDA. After stirring, the solution was mixed with a predetermined amount of ethanol to form the target composition and then sonicated for about 10 min to eliminate the dissolved oxygen. The mixture was sandwiched between a glass plate and a quartz plate, which were separated by spacers to control membrane thickness. Then the solution was polymerized by exposure to 312-nm UV light for 180 s. Various mass concentrations of PEGDA (100 wt%, 60 wt%, 55 wt%, 50 wt% and 45 wt%) in prepolymerization solution were investigated in present study. The obtained solid membranes were immersed in a large amount of deionized water for at least one week to allow any sol to diffuse out of membranes [23]. The thickness of prepared membranes was 180 ␮m. 2.3. Membrane characterization Surface morphology and interior structure of prepared membranes were observed by SEM (Philips XL30E). The chemical composition was analyzed by FTIR (Eqoinox 55) with an ATR unit (ZnSe crystal, 45◦ ). Water uptake measurement was conducted as follows: the membranes were removed from pure water and weighed immediately after blotting the free water on the surface; then, they were dried in a vacuum oven at 80 ◦ C for 24 h. Water uptake could be calculated by the following equation [27]: Water uptake (%) =

Ws − Wd × 100 Wd

(1)

where Ws and Wd were the weights of swollen and dry crosslinked PEGDA membranes, respectively. Tensile strength ( b ), breaking elongation (εb ) and Young’s module (E ) of wet membranes were measured with a tensile tester AG-2000A (Shimadzu, AUTO graph) at room temperature. Tensile conditions were based on China National Bureau of Standards, QB-13022-91 and samples were measured using a programmed elongation rate of 50 mm min−1 . 2.4. Ultrafiltration experiments 2.4.1. Water permeability and BSA rejection A cross-flow membrane test unit at constant flux mode was used to measure membrane performance. The effective membrane area was 13.85 cm2 fixed in a stainless steel cell. After being pre-pressurized at 0.15 MPa for 30 min, the pure water flux was measured at various operation pressures (0.05 MPa, 0.075 MPa, 0.10 MPa and 0.125 MPa). Pure water flux (Jw ) was calculated by

Jw =

V At

(2)

where Cp and Cf (g/L) were BSA concentrations of permeate and feed solutions, respectively. 2.4.2. BSA fouling and reversibility experiments The experimental protocol for fouling runs and fouling reversibility tests was described elsewhere [2,12,14]. All tests were carried out at 0.1 MPa and a temperature of 25 ± 1 ◦ C. Firstly, steady initial pure water flux (Jw0 ) of prepared membrane was measured. Then the feed solution was switched to 1.0 g/L BSA solution and the flux (Jp ) was measured for a period of time. Finally, the feed solution was changed back to deionized water again and the water flux (Jw1 ) was only recorded after the membrane was hydraulically rinsed for 25 min. 2.5. Solvent-resistant experiments The solubility and performance change of crosslinked PEGDA membranes in organic solvents were preliminarily observed to investigate their solvent-resistant properties. Some common solvents including methanol, ethanol, and several polar solvents such as N-methy-2-pyrrolidone (NMP), dimethylformamide (DMF), tetrahydrofuran (THF) and trichloromethane were used. In addition, after being immersed in these solvents for 1 h, the membranes were transferred into pure water for thorough rinsing; then the permeability was measured again for comparison. 3. Results and discussions 3.1. Characterization of crosslinked PEGDA membranes 3.1.1. Appearance, surface morphology and interior structure It was found that the membrane fabricated using pure PEGDA monomer was colorless and transparent; however, with the increasing addition of ethanol, the samples became gradually opaque after polymerization, which were shown in Fig. 1. The increasing opacity of membranes indicated that they had undergone a polymerization induced phase separation process [23,26]. In order to further observe the differences in microstructure, SEM images were employed. Fig. 2 presented the surface morphologies of these membranes. As shown in Fig. 2(A), the membrane prepared with PEGDA only exhibited an essentially nonporous structure. With the increase of ethanol content (i.e., the decrease of PEGDA content) in prepolymerization solution, a porous structure was gradually formed and the micropore size became bigger [Fig. 2(A)–(E)], which was similar to the results reported by Freeman et al. recently [26]. Moreover, the interior structures of these membranes were also observed. For brevity, only the cross-sectional morphologies of two membranes prepared with pure and 45 wt% PEGDA were shown in Fig. 3. It could be seen that the interior and surface morphologies of these membranes were similar. In other words, they did not exhibit the characteristics of asymmetric membranes, but a uniform structure. As a whole, higher content of ethanol during polymerization decreased the crosslinking density and increased

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Fig. 1. Photographs of membranes prepared with various PEGDA in prepolymerization mixture. (A) 100 wt%; (B) 60 wt%; (C) 55 wt%; (D) 50 wt%; (E) 45 wt%.

the intramolecular crosslink-forming loops [23], causing a different structure described above. Here, it could be forecasted that the prepared membranes may have different properties, which will be discussed in the following sections.

3.1.2. ATR-FTIR spectroscopy ATR-FTIR measurement was used to investigate surface chemical composition of the prepared membranes. The samples were thoroughly dried in vacuum oven before analysis. Fig. 4 presented

Fig. 2. Surface SEM images of membranes prepared with various PEGDA in prepolymerization mixture. (A) 100 wt%; (B) 60 wt%; (C) 55 wt%; (D) 50 wt%; (E) 45 wt%.

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Fig. 3. Cross-sectional SEM images of membranes prepared with various PEGDA in prepolymerization mixture. (A) 100 wt%; (B) 45 wt%.

Fig. 5. Water uptake of membranes prepared with various PEGDA in prepolymerization mixture. The film thickness of each sample was 180 ␮m. Fig. 4. ATR-IR spectra of monomer and membranes prepared with various PEGDA in prepolymerization mixture. These spectra have been displaced vertically for clarity.

the ATR-FTIR spectra of PEGDA monomer and crosslinked membranes with different PEGDA content in prepolymerization mixture. It could be seen that all spectra were mostly identical and the characteristic peaks for the acrylate groups (i.e., those at about 812 cm−1 , 1190 cm−1 and 1410 cm−1 ) [23,28] practically disappeared for all polymers, which indicated that they had similar chemical composition. Meanwhile, the reaction conversion of acrylate groups for all membranes was nearly 100% under present experimental conditions. 3.1.3. Water uptake measurement The increasing addition of ethanol during polymerization enhanced the formation of porous structure in prepared membranes, which would render a higher water uptake. As shown in Fig. 5, the value was only 15 wt% for membrane prepared with pure PEGDA, while it was up to about 150 wt%, ten times higher, when the PEGDA content in prepolymerization solution decreased to 45 wt%. In summary, water uptake values of crosslinked membranes were

significantly depended on the PEGDA content in prepolymerization solution. A lower mass concentration represented a higher water uptake, implying a higher permeability for the prepared membranes. 3.1.4. Mechanism characterization Table 1 compared the mechanical performance of wet membranes prepared from different composition. The decrease of PEGDA content in prepolymerization solution decreases the tensile strength ( b ) and Young’s module (E ) of the prepared membranes, but increased the breaking elongation (εb ) slightly. The presence of ethanol decreased the crosslinking density and jeopardizing the mechanical performance finally. 3.2. Permeability and BSA rejection Ultrafiltration experiments were conducted to investigate the permeability and BSA rejection of membranes prepared from the prepolymerization mixture with various PEGDA contents. After being pre-pressurized at higher pressure, the steady flux of pure

Table 1 Compared the mechanical performance of membranes prepared with various PEGDA in prepolymerization mixture PEGDA content (wt%)

Tensile strength  b (MPa)

Breaking elongation εb (%)

Young’s module E (MPa)

100 60 55 50 45

2.82 1.66 0.87 0.53 0.42

6.84 7.45 7.94 8.56 9.87

87.35 14.21 8.25 4.74 2.43

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Table 2 BSA rejection as a function of filtration time for membranes prepared with various PEGDA in prepolymerization mixture PEGDA content (wt%)

60 55 50 45

Fig. 6. Pure water flux of membranes prepared with various PEGDA in prepolymerization mixture at various operation pressures.

water and the rejection ratio for BSA of each membrane were evaluated. Pure water permeability as a function of various operation pressures was plotted in Fig. 6. Increasing transmembrane pressure resulted in enhanced water fluxes, which was a characteristic of ultrafiltration membrane. Meanwhile, water permeability significantly increased as the PEGDA content in prepolymerization mixture decreased. It should be noted that, the water permeability of membrane polymerized with pure PEGDA was not observed under the pressure range investigated. When PEGDA content decreased to 60 wt%, the prepared membrane showed a low water permeate flux (about 6 L/m2 h at 0.1 MPa), further the flux increased up to around 340 L/m2 h as PEGDA content in prepolymerization mixture decreased to 45 wt%. These results were clearly in good agreement with the membrane structures observed by SEM images and water uptake results measured above. The effects of PEGDA content in prepolymerization mixture on rejection property of prepared membranes were investigated with 1.0 g/L BSA phosphate buffer saline (pH 7.0) at 0.1 MPa. As the results shown in Fig. 7, a compromised relation could be observed. A membrane with a higher flux exhibited a lower rejection ratio for BSA (measured after filtration for 30 min). The rejection ratio decreased from about 60% to 20% as PEGDA content decreased in range explored. Moreover, the BSA rejection as a function of filtration time was also investigated, which was shown in Table 2. The membrane prepared with 60 wt% PEGDA content in prepolymerization mixture exhibited a nearly constant BSA rejection. However, with the increase of ethanol in prepolymerization solution, the prepared membranes gradually showed a relatively higher increase of BSA rejection after 150 min filtration, implying a greater pore blocking and a cake formation. This was contributed by the difference

Fig. 7. Effect of PEGDA content in prepolymerization mixture on pure water flux and BSA rejection ratio of membranes prepared. The experimental pressure was 0.1 MPa.

BSA rejection (%) 30 (min)

60 (min)

90 (min)

120 (min)

150 (min)

63.2 40.6 27.3 20.7

63.5 41.2 28.4 21.8

63.5 41.7 29.2 22.5

64.2 42.5 29.8 23.7

64.0 43.1 31.2 25.5

of membrane structure. It is commonly considered that the membrane with bigger surface pore always has a more serious fouling. Therefore, the membrane prepared with less PEGDA content in prepolymerization solution, which had a bigger pore size as shown in Figs. 2 and 3, exhibited a higher change of BSA rejection. Nevertheless, the increase of BSA rejection was slight, which indicated that such membranes might be also suitable for the separation of macromolecules based on molecular weight difference almost without fouling [5]. 3.3. BSA fouling and reversibility To further evaluate the antifouling properties of the crosslinked PEGDA membranes, BSA fouling and reversibility experiments were conducted. Since the membrane prepared with 60 wt% PEGDA in the prepolymerization solution had a very low water permeate flux, only about 6 L/m2 h at 0.1 MPa, it was not employed in this test. The results according to experimental protocol for fouling runs and fouling reversibility tests were plotted in Fig. 8; (A) presented

Fig. 8. Flux behaviors during cross-flow filtration with BSA solution.

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pure water fluxes, BSA solution fluxes and pure water fluxes after hydraulic rinsing of three membranes; (B) presented the relative values. As seen in the data, flux reduced for all membranes during protein filtration. The membrane with higher initial flux showed more distinct flux decline and lower water flux recovery after cleaning, indicating a severer fouling. This observation was consistent with the results above. Although such membranes were fouled inevitably, the flux loss was less than that of common commercial membrane materials such as polyethersulfone through qualitative comparison [15]. Moreover, all membranes recovered about 90% or more of the initial flux after a hydraulic rinsing process, implying a property of “easy to clean” [14,29]. The experimental results further proved the extraordinary antifouling ability of PEG materials, especially to resist protein adsorption. 3.4. Solvent-resistant property The solubility of prepared polymers was investigated. It was found that such membranes could not be dissolved in any common solvent, including methanol, ethanol, and some polar solvents such as N-methy-2-pyrrolidone, dimethylformamide, tetrahydrofuran and trichloromethane, etc. However, the common polymer materials for preparing ultrafiltration membranes, such as polysulfone and poly(ether sulfone), are dissolvable in such solvents. Interestingly, when the white water-swollen membrane was immersed into NMP for a period of time, it gradually changed to colorless and transparent. After being transferred into a large amount of pure water, it finally became white and opaque again. However, it was found that the permeability of treated membrane had no significant change. This result preliminarily indicated a potential application of these membranes in water treatment especially containing polar solvents. 4. Conclusions In this study, a series of crosslinked PEGDA (n = 4, MW = 302 g/mol) membranes were prepared via UV radiation by adding various ethanol in prepolymerization solution. SEM images and ultrafiltration experiments suggested that PEGDA content in prepolymerization solution substantially influenced membrane structure and performance. The membrane prepared with pure PEGDA had a dense nonporous structure and exhibited no water permeability under pressure range explored. However, with the decrease of PEGDA content in prepolymerization solution, an enhancing pore size in membrane surface and bulk was observed, resulting in an increased permeability and water uptake, but a decreased selectivity. BSA ultrafiltration experiments indicated that these membranes exhibited excellent antifouling properties and flux recoveries. Finally, the extraordinary solvent-resistant properties of prepared membrane were also observed, implying a potential application in water treatment especially containing polar solvents. Acknowledgements Financial support from Chinese Ministry of Science and Technology in form of National 973 Program (No. 2003CB615703) is gratefully acknowledged. References [1] R.H. Li, T.A. Barbari, Performance of poly(vinyl alcohol) thin-gel composite ultrafiltration membranes, J. Membr. Sci. 105 (1995) 71–78.

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