with zwitterionic or charged polymers

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Modification of polysulfone (PSF) hollow fiber membrane (HFM) with zwitterionic or charged polymers Peng Wan, Matthew T. Bernards, and Baolin Deng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01542 • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 18, 2017

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Industrial & Engineering Chemistry Research

Modification of polysulfone (PSF) hollow fiber membrane (HFM) with zwitterionic or charged polymers Peng Wan a, Matthew Bernards a,c, Baolin Deng a,b,d* a

Department of Chemical Engineering, University of Missouri, Columbia, MO 65211, USA

b

Department of Civil & Environmental Engineering, University of Missouri, Columbia, MO 65211, USA

c

Current address: Chemical & Materials Engineering, University of Idaho, Moscow, ID 83844, USA

d

School of Environmental Science and Engineering, South University of Science and Technology of China,

Shenzhen, China

Corresponding Author. Tel: +1 573 882 0075, Fax: +1 573 882 4784 E-mail addresses: [email protected]

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Abstract

Membrane fouling is a critical problem limiting membrane performance and application lifetime. Zwitterionic polymers are highly resistant to irreversible membrane foulants because of their hydrated structure and charge neutral characteristics. The objectives of this study were to improve membrane antifouling performance with zwitterionic polymer modifications and to gain a better understanding of the factors affecting the process. We used polysulfone hollow fiber, instead of flat-sheet membranes, for the study because of its common industrial applications for ultrafiltration. Different characterization methods including scanning electron microscopy, nuclear magnetic resonance, attenuated total reflectance Fourier-transform infrared spectroscopy, contact angle goniometry and cyclic filtration tests of deionoized water and bovine serum albumin (BSA) were applied to characterize the membrane and evaluate the performance of pristine and modified HFMs. Results showed that membrane fouling was significantly decreased by grafting the zwitterion carboxybetaine methacrylate (CBMA) via atom transfer radical polymerization (ATRP). Positively charged [2-(acryloyloxy)ethyl] trimethylammonium chloride (TMA) with a quaternary amine end group and negatively charged 2-carboxyethyl acrylate (CAA) with a carboxyl end group were grafted separately as charged polymer controls. These charged polymers improved the membrane antifouling property, but not to the same extent as the membrane with the zwitterionic polymers. Compared with the pristine PSF HFMs, all modified membranes exhibited enhanced hydrophilicity and antifouling property. Keywords: Hollow fiber membrane; Membrane fouling; Zwitterionic; CBMA; TMA; CAA;


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1. Introduction Membrane filtration is a separation technology widely used for water and wastewater treatment 1, food processing 2 and production of pharmaceuticals 3. Membrane technologies can be categorized into microfiltration, ultrafiltration, nanofiltration, and reverse osmosis, based on the membrane pore size selected for specific applications. Ultrafiltration membranes, with a pore size normally ranging from 2 nm to 100 nm 4, are those used to reject large sized colloidal particulates, fats, bacteria, and proteins while allow soluble ions and other low molecular weight molecules to pass. A desirable ultrafiltration membrane should not only have high performance (i.e., high water permeability and high rejection of particular materials), but also good antifouling characteristics. Fouling is often the key issue limiting the lifetime of membranes for ultrafiltration. Membrane fouling by natural organic matter (NOM) and proteins are significant operational issues in membrane applications. Protein fouling is a complex process because of the myriad of interactions that occur between proteins, membrane surfaces, and other protein molecules5. Nonspecific protein adsorption on the membrane surface and on the membrane pore walls can cause severe membrane fouling. Many studies have been conducted to improve membrane antifouling performance by introducing hydrophilic polymers or nanoparticles to the membrane surface or the membrane matrix 6-8. One example is the use of grafted polyethylene glycol (PEG) that significantly enhances the antifouling property 9, 10. PEG tends to autoxidize, however, especially in the presence of oxygen and transition metal ions 11, 12. Another approach is to embed or coat various nanoparticles for fouling control but the challenge is the leaching of particles out of the macroporous structure of ultrafiltration membranes, due to the poor adhesion and retention of nanoparticles within the polymer matrix13, 14. Recently, zwitterionic materials have drawn much attention because of their hydrophilic nature and overall charge-neutral characteristics15-17. Zwitterionic materials have been investigated for 3 ACS Paragon Plus Environment


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improving blood compatibility and membrane antifouling properties 18-20. An example is the use of 2methacryloyloxyethyl phosphorylcholine (MPC) to modify a cellulose acetate membrane, to significantly improve the membrane blood compatibility and protein adsorption resistivity 21. For water filtration membranes, sulfobetaine methacrylate (SBMA) and carboxybetaine methacrylate (CBMA) could be good choices because these zwitterionic polymers are much cheaper than MPC and less likely to hydrolyze during long-term applications22. In recent years, CBMA and SBMA have been widely explored as nonfouling surfaces and coatings23, 24. For example, an excellent low-fouling UF membrane was synthesized by grafting SBMA onto a poly(vinylidene difluoride) (PVDF) membrane surface 25. In another study, a polyCBMA layer was created on a PVDF membrane surface via a physisorbed free radical polymerization grafting technique (P-FFPG) 26, resulting in an antifouling membrane with a certain range of CBMA grafting density. To introduce zwitterionic materials to a membrane surface or membrane matrix, atom transfer radical polymerization (ATRP) is proven highly effective, as demonstrated for many well-defined (co)polymers with precisely controlled functionalities, topologies and compositions 27. ATRP is versatile and robust, and can precisely control the density, length and architecture of the antifouling brush layer 28. For example, the ATRP method was utilized to introduce SBMA to a polypropylene (PP) membrane surface resulting in an improved antifouling property 29. The zwitterionic polymer chain length and content were regulated by polymerization time. The surface hydrophilicity of the PP membrane was significantly enhanced by an increasing thickness of grafted poly(SBMA) on the membrane. In another study, various zwitterionic monomers were grafted on a cellulose membrane via surface-initiated ATRP (SI-ATRP) 30. The antifouling ability of the membrane was improved without compromising the cytocompatibility of the substrate. Additionally, the membrane antifouling and blood compatibility were both improved for PSF membranes by grafting SBMA via SIATRP31. Nevertheless, while the studies cited above have clearly demonstrated the potential of zwitterionic modified membranes for fouling control, they normally compare only the membranes modified by


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zwitterionic materials with the conventional pristine membranes. The effect of either positively charged or negatively-charged functional groups in the zwitterionic polymers on membrane fouling has not been fully explored. A recent study by Hadidi and Zydney 32 took a more fundamental approach by examining the fouling behavior of a zwitterionic flat sheet membrane prepared by a conventional covalent attachment, and found the extent of fouling was strongly affected by electrostatic and hydrophobic interactions between the protein and membrane. In this study, we investigated the PSF HFM performance upon grafting of CBMA- zwitterionic polymers in comparison with grafting positively-charged [2-(acryloyloxy) ethyl] trimethyl ammonium chloride (TMA) or negatively-charged 2-carboxyethylacrylate (CAA), with a goal to understand how various surfaces could impact membrane protein fouling. We used polysulfone hollow fiber, instead of flat-sheet membranes, for the study because the PSF HFMs are most commonly-used in industry for ultrafiltration but their modification by CBMA has not been reported. Different ATRP polymerization times were applied, resulting in different grafting thicknesses of the three polymers. Antifouling properties of the membranes against a model protein, bovine serum albumin (BSA), were evaluated by comparing pristine membranes and the membranes modified with one of the three polymer functional groups.

2. Materials and methods 2.1. Materials Copper (II) bromide (CuBr2), 2-(dimethylamino)ethyl methacrylate (99%) and beta-propiolactone (98%) were obtained from Acros Organics (Geel, Belgium) and methanol (99.9%) from Fisher Scientific (Waltham, MA). All of the following chemicals were purchased from Sigma-Aldrich (St. Louis, MO): polysulfone (PSF, MW 35,000 Da), N-methyl-2-pyrrolidone (NMP， anhydrous, 99.5%), polyvinylpyrrolidone (PVP, MW 10,000 Da), CAA (Mn = 144.13 g mol-1), TMA (80 wt% solution in water), chloroform, copper(I) bromide (98%), paraformaldehyde (95%), tin(IV) chloride (98%), 5 ACS Paragon Plus Environment


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chlorotrimethylsilane (99%), methanol (95%), BSA (pI = 4.8, MW = 66,463), acetone (anhydrous) and N,N,N’,N’’,N’’-pentamethyldiethylenediamine (PMDETA). Phosphate buffered saline (PBS, 11.8 mM) solution was prepared in deionized (DI) water. High purity water, produced in a Millipore Synergy system (18.2 MΩ-cm), was used in all experiments. 2.2. Synthesis and characterization of CBMA Carboxybetaine methacrylate (CBMA) was synthesized following a reported method 33, via the reaction of 11.8 mL of 2-(dimethylamino) ethyl methacrylate and 5 mL of β-propiolactone in 50 mL of anhydrous acetone. The reaction mixture was stirred under nitrogen protection at 15 °C for 5 h and then the white precipitate formed was washed with 50 mL of anhydrous acetone and 100 mL of anhydrous ether. The final product was dried under a reduced pressure to obtain the CBMA monomer product and it was stored at 5 °C prior to use. The composition of the CBMA monomer was verified through a 500 MHz 1-H NMR, and the results, as shown in Figure 1, were 6.06 (s, 1H, =CH), 5.68(s, 1H, =CH), 4.55 (t, 2H, OCH2), 3.70 (t, 2H, CH2N), 3.59 (t, 2H, NCH2), 3.10 (s, 6H, NCH3), 2.64(t, 2H, CH2COO) and 1.84 (s, 3H, =CCH3). 2.3. Synthesis of chloromethylated polysulfone (PSF-Cl) and PSF/PSF-Cl membrane Chloromethylation of PSF was accomplished following a method described in the literature 34 and illustrated in Figure 2 (a). Briefly, PSF (10.0 g, 22.5 mmol) dissolved in 250 mL of CHCl3 was first placed in a 500 mL round bottom flask equipped with a reflux condenser under nitrogen protection, and after the addition of paraformaldehyde (6.7 g, 225 mmol), the solution was heated to 55 oC. Then SnCl4 (0.26 mL, 2.25 mmol) and (CH3)3SiCl (28.5 mL, 225 mmol) were added and mixed by stirring at a constant temperature for 18 h. To cast the membranes, the prepared PSF/PSF-Cl (3 g) was dissolved in NMP (16 g) with PVP (1 g) as an additive in a sealed glass bottle, and stirred at 60 oC for 8 h. After doping, the solution was left to


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cool for at least 10 h, and then the PSF-Cl HFM membrane was prepared on a custom-made hollow fiber membrane casting machine with a phase inversion method35. A hollow fiber was shaped when the casting solution was pumped at 40 psi through a spinneret by a nitrogen cylinder and, simultaneously, a bore fluid of DI water was extruded at 0.8 ml/min by a syringe pump. The prepared fibers were immersed in a coagulation bath of tap water. The dimensions of HFMs were controlled by a spinneret with 1000 µm for the outer diameter (OD) and 600 µm for the inner diameter (ID). The fiber collecting velocity was controlled under a constant rate to simplify the spinning process. The collected fibers were cut into lengths of approximately 30 cm and rinsed with DI water thoroughly for further testing. Figure 2 2.4. Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP) of CBMA, TMA and CAA onto the PSF-Cl Hollow Fiber Membrane (HFM) The grafting reactions of CBMA, TMA, and CAA on the PSF-Cl HFM followed a published method 36 as illustrated in Figure 2 (b). Following the modification process detailed in Figure 3, six PSF hollow fibers with the length of 30 cm were rinsed in sequence with ethanol and deionized water at room temperature. The cleaned membranes were tied up at both ends with adhesive tape, then wetted in methanol for 5 min and placed into a dry flask. A fixed concentration of either CBMA (1.00 g, 4.61 mmol), TMA (1.11 g, 4.61 mmol), or CAA (0.66 g 4.61 mmol) were added with CuBr (111.75 mg, 75 mmol), CuBr2 (34.8 mg, 75 mmol), PMDETA (480 μL) into 150 mL degassed DI water: methanol solution (1:1 volume ratio) in a flask under the nitrogen protection at 50 oC. The graft polymerization time for CBMA was set at 40 min, 80 min, or 120 min. Both TMA and CAA were polymerized for 120 min. Following grafting, the membranes were washed with methanol and DI water thoroughly to remove the residual solvent and monomer. Figure 3



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2.5. Membrane Characterization and Performance Assessment The chemical composition of the membrane surfaces was characterized by attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy using a Nicolet 4700 FT-IR equipped with a multi-reflection Smart Performers ATR accessory (Thermo Electron Corporation, Waltham, MA). All spectra were captured from 64 scans at a resolution of 2 cm-1. The HFM surface and cross-section were imaged by scanning electron microscopy (SEM) on a Quanta FEG 600 (FEI, Hillsboro, OR). The HFM samples were dried at 25 oC and the cross-section was prepared by freeze-fracturing in liquid nitrogen. The samples were sputtered with platinum to increase the sample conductivity. SEM images were analyzed using ImageJ to determine average pore diameters 37. The membrane contact angle was measured by a video contact angle system (VCA-2500 XE, AST products, Billerica, MA) using the sessile drop method. A cross-flow filtration system was used to investigate the water flux, solute rejection and fouling resistance of the membrane. Prior to each test, DI water was applied to compact the membrane at 5 psi for 3 h. The pure water flux was determined by weighing the amount of permeate water as a function of time, which was accomplished using a LabVIEW automated system (National Instruments LabVIEW 2013 with Ohaus digital balance). For the filtration of BSA from solution, BSA was added into PBS to a concentration of 1.0 g/L. The flux and rejection were calculated by Equation (1) and Equation (2), respectively.

J=

Vp (1)

A⋅t

 Cp R = 1 −  C f 

  × 100 


(2)

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Here J (L/m2h) is the flux of permeate solution, Vp (L) the permeate volume, A (m2) the membrane area, t (h) the filtration time, R the rejection ratio, Cp the concentration of permeate, and Cf the concentrations of feed solution. The protein concentration was determined by UV-visible spectrophotometry at 280 nm. After filtering the BSA solution, the membrane was washed with DI water for 5 min, and the cleaned membrane was evaluated again to determine the pure water flux. The flux recovery ratio FRR is calculated by Equation (3), defined as the ratio of pure water flux after (Jw, i-1) and before (Jw, i) the ith filtration of the protein solution.

FRR, i (%) =

Jw, i × 100 Jw, i − 1

(3)

The fouling-resistance of a membrane was evaluated based on the loss of flux caused by total protein fouling in the ith cycle Rt, i, as calculated by:

Rt, i (%) =

Jw, i − 1 − Jpr , i × 100 Jw, i − 1

(4)

in which Jpr is the water flux of the protein solution. The degrees of flux loss that resulted from reversible fouling (Rr,i) and irreversible fouling (Rir,i) in the ith cycle were calculated from the Equations (5) and (6), respectively:

Rr, i (%) =

Jw, i − Jpr , i × 100 Jw, i − 1

Rir, i (%) =

Jw, i − 1 − Jw, i × 100 Jw, i − 1

3. Results and discussion 9 ACS Paragon Plus Environment

(5)

(6)


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3.1. Characterization of Membranes The ATR FTIR spectra of the pristine and modified membranes are shown in Figure 4. For the PSF membrane, the characteristic peaks appearing at 1590 and 1488 cm-1 are assigned to aromatic C-C stretching, 1245 cm-1 is assigned to the C-O-C stretching of the aryl ether group and 1150 cm-1 is assigned to the symmetric O=S=O stretching of the sulfone group37. A new peak appeared at 1723 cm-1 for the samples with grafted polymers, and denoted as PSF TMA, PSF CAA, PSF CBMA 40, PSF CBMA 80 and PSF CBMA 120. This new peak is due to the O-C=O stretch from the grafted monomers31. Figure 4 As shown in Figure 5, the water contact angle of the PSF membranes is significantly modified following polymer grafting. The pristine PSF membrane had a contact angle of 68 ,̊ which was reduced to 36.8 ̊ after grafting CBMA for 120 minutes. The contact angle also decreased with increasing CBMA ATRP graft time. The results clearly indicate that when CBMA is grafted on the membrane surface it brought higher hydrophilicity to the membrane due to the hydration effect of the zwitterionic structure. This is consistent with the result by Sui, Y et al.36, who observed that the contact angle of the PVDF membrane decreased with increasing ATRP time and hydrophilic HEMA monomer contents. The water contact angles determined for PSF-TMA and PSF-CAA were lower than the pristine PSF and PSF-Cl membranes, but higher than that found for PSF-CBMA 120. This result may be caused by the more hydrophilic nature of CBMA as compared to TMA and CAA. Another potential explanation may be related to the grafting efficiency. Because both the TMA and CAA monomers are charged, there might be electrostatic repulsion when multiple layers are introduced during the ATRP process, which reduces the grafting efficiency. This is not seen in the case of the charge neutral CBMA monomer. In a previous study, Bernards et al found that charged monomers had shorter polymer brush coatings for single monomer solutions as compared to coatings from mixed charged solutions.38. These differences were also attributed to electrostatic repulsions found in the single monomer systems. 10 ACS Paragon Plus Environment

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Figure 5 Figure 6 shows representative surface and cross-sectional morphologies of the modified and pristine HFMs. The PSF HFM was porous with an average pore size of 10.1 ± 2.4 nm. The PSF-Cl HFM had an average surface pore size of 18.3 ± 6.1 nm. This pore size was reduced by the grafting process, with the PSF-CBMA 120, PSF-TMA, and PSF-CAA samples having pore sizes of 4.6 ± 2.4 nm, 10.9 ± 2.9 nm, and 11.4 ± 3.2 nm, respectively. The images of the cross-sections suggested that the HFMs have a thickness between 95 nm and 190 nm, that was unaffected by the grafting process. The reduction in pore size following the grafting of monomers is consistent with previously reported results 25. Figure 6 3.2. Membrane Performance and Antifouling Properties The membrane water flux was measured in a cross flow system under a constant transmembrane pressure of 5 psi. To assess membrane fouling characteristics, a cyclic filtration test approach was adapted. Briefly, after 3 h of compaction, the membrane was exposed to DI water for 0.5 h to obtain the pure water flux (Jw0), and then the feed solution was switched to the BSA solution for 1.5 h. After the membrane was cleaned with a pure water flush, the membrane was switched to DI water again for another 0.5 h, completing one cycle. The pure water flux and BSA rejection were tested for each of the fabricated membranes including pristine PSF and modified ones (Figure 7). The pure water fluxes of PSF-TMA (68 L/m2.h) and PSF-CAA (74 L/m2.h) were actually significantly higher than the water flux of the original PSF HFM (46 L/m2.h). The PSF-CBMA 80 and PSF-CBMA 120 had comparatively low water flux around 38 L/m2.h. As for the BSA rejection, the PSF-CBMA 40, 80, and 120 all had a rejection at 85±3% , which was lower than those obtained for the pristine PSF, PSF-TMA, and PSF-CAA HFMs. The BSA rejection in the second cycle is slightly higher than in the first cycle.



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The higher water fluxes of PSF-TMA and PSF-CAA compared with the pristine PSF membrane were resulted from the improved water flux of the PSF-Cl membrane, as reported in the literature31. We checked the flux of the PSF-Cl once and it was consistent with the above reported results. This is likely due to the increased pore diameter seen in the PSF-Cl as compared to the pristine PSF HFM. To some extent, the more CBMA grafted onto the membrane structure, the lower the flux due to changes in the pore dimensions26. The pure water flux of PSF-CBMA 80/120 was lower than that for the PSF-TMA and PSF-CAA HFMs because the monomer subunit of CBMA is larger than that of TMA and CAA, causing more pore blockage per unit of monomer. The rejections of BSA for all of the modified membranes were lower than that of the pristine PSF membrane, this could be contributed to the large pore dimensions of PSF-Cl structure 31. Different BSA rejections for various modified membranes were likely due to the different interactions between BSA and membrane surfaces. CBMA is a neutral monomer with minimal BSA adsorption, so its rejection to BSA is low. TMA is a positively-charged monomer capable of adsorbing BSA, which could result in pore blocking and more BSA rejection. CAA is a negatively charged monomer, which could repel the negatively charged BSA via electrostatic interactions, thereby improving the rejection. Figure 7 The duration of grafting CBMA had an impact on the water flux of PSF HFMs as shown in Figure 8 (a). The polyCBMA-modified membranes with different times were substantially different from each other, but all had higher flux than the pristine PSF membrane. The membrane grafted with CBMA for 120 min showed the highest relative permeation flux during the cyclic filtration. It is likely that a longer reaction time resulted in more polyCBMA grafted on the membrane surface, and thus a better antifouling property of the membranes. This could be contributed to the hydrophilic but charge-neutral characteristics of CBMA. It is also consistent with reports using different monomers or membranes where the grafting density of monomers on the membrane surface was found to increase as the grafting time increased31, 36 and an increased density of CBMA led to a better antifouling performance 26. 12 ACS Paragon Plus Environment

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Figure 8 (b) shows the antifouling performance of the membranes grafted with CMBA in comparison with TMA and CAA. All of the monomers were grafted on the PSF HFM for 120 min. The PSF CBMA 120 showed a much better antifouling property than both PSF-TMA and PSF-CAA. The relative flux of PSF-TMA was lower than the flux for both PSF-CAA and PSF-CBMA 120 after 30 min of pure water filtration. BSA is a negatively-charged protein at neutral pH as indicated by its isoelectric point of approximately 4.840. The positively charged quaternary amine terminal group of TMA facilitates the adsorption of BSA due to electrostatic attractions.38 Therefore, fouling occurred more easily on the PSF-TMA membrane in the presence of BSA, and the pure water flux recovery was about 60%. The negatively charged carboxyl terminal group of CAA is repulsive to BSA at pH 7.4, so the flux recovery was higher, and approximately 70% of the original flux was recovered. Even though there is some electrostatic repulsion, surfaces grafted with carboxyl groups could still adsorb some BSA at neutral pH41. Finally, when the membrane surface was grafted with CBMA, the membrane showed the highest antifouling property against BSA, as indicated by the pure water flux recovery values of 85-90% of the original flux. The reversible, irreversible, and total fouling resistance are plotted in Figure 9 for the unmodified and modified membranes for the first filtration cycle with BSA solution as a feed. The flux loss due to protein adsorption, as expressed by the total protein fouling (Rt,i, Equation (4)), was caused by both reversible fouling and irreversible fouling. The reversible fouling (Rr,i), as calculated by Equation (5), represents the portion that could be recovered by a membrane wash. The irreversible fouling (Rir,i), calculated by Equation (6), resulted from permanent protein adhesion and pore blockage. At the initial stage of BSA solution filtration, the permeation flux decreased dramatically because of protein adsorption to the membrane. The water flux (Jpr,i) tended to reach a steady state after the BSA adsorption to the membrane surface and pores reached saturation. Jw,i was measured after membrane washing. Then, the Jw,i and Jpr,i could be used for the calculation of the water flux recovery and membrane fouling resistance.



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It is clear from the results presented in Figure 8 that the PSF-CBMA 120 has the lowest levels of irreversible fouling. Figure 9 The pure water flux recovery of the pristine and modified membranes for different cycles is shown in Figure 10. The flux recovery ratio increased with the CBMA grafting time. The PSF-CBMA 120 presented the best flux recovery ratio. There was also no obvious flux recovery improvement found for the PSF-TMA sample as compared to the pristine PSF HFM, while the flux recovery of the PSF-CAA was higher than the pristine PSF HFM. It has been shown that the more CBMA grafting to a membrane surface, the better the antifouling properties26. This was also seen here. The flux recovery of PSF-CAA was enhanced as compared to the pristine PSF HFM because of the improvement in the hydrophilicity of the membrane and the electrostatic repulsion between BSA and the negatively charged membrane surface. However, the negatively charged carboxyl group does not perform as well as the zwitterionic CBMA functional group for preventing protein fouling41. Similarly, even though the PSF-TMA HFM was more hydrophilic than the PSF HFM, the positively charged quaternary amine group promoted the adsorption of BSA onto the membrane structure, reducing the recovery. Figure 10

4. Conclusions PSF HFMs were successfully modified with zwitterionic or charged polymers via ATRP. The membrane modified with neutrally-charged CBMA showed the best antifouling property when compared to those modified by charged polymers and the pristine PSF HMF. The water flux, protein rejections and antifouling properties of the modified membranes were dependent on the charge state and chemical structure of the monomers. The results suggested that the more zwitterionic monomer grafted to the


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membrane structure, the better the antifouling property of the resulting membrane. The membrane could be modified with charged or neutral polymers for different applications. While grafting positively or negatively charged molecules on the PSF HFM surfaces could improve its antifouling properties, the membrane modified with zwitterionic CBMA has the highest resistance to protein fouling for the ultrafiltration applications.

Acknowledgements We gratefully acknowledge Professor Qingsong Yu in the Department of Mechanical & Aerospace Engineering at MU for providing us access to the contact angle measurement system. We also acknowledge Dr. Wei Wycoff in the Department of Chemistry for completing the NMR analysis. This work was supported by the United States Geological Survey through Missouri Water Resources Research Center and National Science Foundation (Award # 1544794 ).

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Figure 1. Representative NMR spectrum confirming the successful synthesis of CBMA.


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Figure 2. Schematic of the (a) PSF-Cl synthesis and (b) modification of PSF HFM membrane with CBMA, TMA, or CAA.

Fig 3. Grafting procedure for attaching CBMA, TMA, or CAA to PSF-Cl membranes.



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Fig 4. ATR-FTIR spectra for the PSF membrane in both its native and modified states (PSF-Cl, PSFTMA, PSF-CAA, PSF-CBMA 40, PSF-CBMA 80 and PSF-CBMA 120).


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80 70

Contact Angle (degree)

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60 50 40 30 20 10 0 F PS

l A A 20 40 80 F-C -CA -TM S A1 MA MA F F P M B B S S B P P F-C F-C F-C PS PS PS

Fig 5. Pure water contact angles of the pristine PSF and various modified PSF hollow fiber membranes.



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(a)

(b)

(c)

(d)

(e)

(f)


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(g) (g)

(h)

(i)

Fig 6. Representative SEM images of surface and cross-sectional morphologies of hollow fiber membranes. Surface: (a) PSF, (b) PSF-Cl, (c) PSF-CBMA 40, (d) PSF-CBMA 80, (e) PSF-CBMA 120, (f) PSF-TMA, (g) PSF-CAA; Cross-section: (h) PSF, (i) PSF-Cl. The scale bar is 500 nm in each surface image and 300 µm in each cross-sectional image.



100

100

90

Pure Water Flux (L/m2.h)

80

80

70 60

60 50 40

Rejection (%)

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40

30 20 10

20 Pure Water Flux Rejection of the First Cycle Rejection of the Second Cycle

0

0 40 80 20 A1 MA MA M B B B F-C F-C F-C PS PS PS

F PS

MA F-T S P

AA F-C S P

Fig 7. The membrane pure water flux and percentage rejection of BSA during the first and second cycles.


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Fig 8. Time-dependent fluxes through the membranes during a cyclic filtration process at 25 ̊C.



100

Rt,1 Rr,1 Rir,1

80

Fouling Ratio (%)

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60

40

20

0 40 20 80 A1 MA MA M B B B F-C F-C F-C PS PS PS

F PS

A -TM F PS

A -CA F PS

Fig 9. Fouling ratios (Rt,1 Rr,1 and Rir,1) of the pristine PSF HFM and modified membranes in the first cycle using a BSA solution as a feed.


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120

First Cycle Second Cycle 100

Flux Recovery (%)

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80

60

40

20

0 0 0 A A 120 A4 A8 -CA -TM A M M F F M B B S S B P P F-C F-C F-C S PS PS P

F PS

Fig 10. The water flux recovery following the first and second cycle for the PSF, PSF-CBMA 40, PSFCBMA 80, PSF-CBMA 120 PSF-TMA and PSF-CAA HFMs.



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TOC Graphic


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with zwitterionic or charged polymers

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