Article pubs.acs.org/Langmuir
Zwitterionic Modifications for Enhancing the Antifouling Properties of Poly(vinylidene fluoride) Membranes Antoine Venault,† Wen-Yu Huang,† Sheng-Wen Hsiao,† Arunachalam Chinnathambi,‡ Sulaiman Ali Alharbi,‡ Hong Chen,§ Jie Zheng,*,§ and Yung Chang*,† †
R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan Christian University, Jhong-Li, Taoyuan 320, Taiwan ‡ Department of Botany and Microbiology, College of Science, King Saud University, Riyadh 11451, Kingdom of Saudi Arabia § Department of Chemical & Biomolecular Engineering, The University of Akron, Akron, Ohio 44325, United States S Supporting Information *
ABSTRACT: The development of effective antibiofouling membranes is critical for many scientific interests and industrial applications. However, the existing available membranes often suffer from the lack of efficient, stable, and scalable antifouling modification strategy. Herein, we designed, synthesized, and characterized alternate copolymers of p(MAO−DMEA) (obtained by reaction between poly(maleic anhydride-alt-1-octadecene) and N,N-dimethylenediamine) and p(MAO−DMPA) (obtained by reaction between poly(maleic anhydride-alt-1-octadecene) and 3-(dimethylamino)1-propylamine) of different carbon space length (CSL) using a ring-opening zwitterionization. We coated these copolymers on poly(vinylidene fluoride) (PVDF) membranes using a self-assembled anchoring method. Two important design parametersthe CSL of polymers and the coating density of polymers on membranewere extensively examined for their effects on the antifouling performance of the modified membranes using a series of protein, cell, and bacterial assays. Both zwitterionicmodified membranes with different coating densities showed improved membrane hydrophilicity, increased resistance to protein, bacteria, blood cells, and platelet adsorption. However, while p(MAO−DMEA) with two CSLs and p(MAO−DMPA) with three CSLs only differ by one single carbon between the amino and ammonium groups, such subtle structural difference between the two polymers led to the fact that the membranes self-assembled with MAO−DMEA outperformed those modified with MAO− DMPA in all aspects of surface hydration, protein and bacteria resistance, and blood biocompatibility. This work provides an important structural-based design principle: a subtle change in the CSL of polymers affects the surface and antifouling properties of the membranes. It can help to achieve the design of more effective antifouling membranes for blood contacting applications.
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INTRODUCTION
process. Antifouling materials can be generally classified into two classes: hydrophilic and zwitterionic materials. Among hydrophilic antifouling materials,1−4 poly(ethylene glycol) (PEG) and PEG-derived materials are well-known for their low fouling capability against protein/cell/bacteria attachment because PEG groups promote the establishment of a strong hydration layer via hydrogen bonding that prevents biomolecules from approaching the membrane surface. However, PEG groups are readily subject to oxidative degradation and enzymatic cleavage in complex media.5 Zwitterionic-based materials can bind water molecules even more strongly than PEG chains via electrostatically induced hydration. As a result, zwitterionic materials exhibit extremely high surface resistance to protein/cell/bacterial adhesion and biofilm formation in long-term tests. Coupling with other desired properties,
Membrane fouling is a long-term critical issue, particularly in membrane-related applications for membrane separation/ purification/filtration and water treatment applications for drinking water production, wastewater treatment, and seawater desalination. Among membrane materials, poly(vinylidene fluoride) (PVDF) and PVDF-based membranes have been commercially used in chemical or food industry and biomedical fields because of their advantageous characteristics including high mechanical strength, thermal stability, and chemical resistance. However, PVDF membranes are prone to fouling, mainly caused by the unintended accumulation of biomolecules and (micro)organisms, which consequently leads to the decreased performance and efficiency of a membrane system (e.g., reduction in flux, salt rejection impairment, and shortened membrane lifetime). A number of studies have shown that surface modification of PVDF and other membranes with antifouling materials can be a promising strategy to effectively reduce or delay biofouling © XXXX American Chemical Society
Received: March 11, 2016 Revised: April 2, 2016
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DOI: 10.1021/acs.langmuir.6b00981 Langmuir XXXX, XXX, XXX−XXX
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Langmuir Scheme 1. Molecular Structures of p(MAO−DMEA) and p(MAO−DMPA) (n Is in the Range 85−142)
possible to obtain an antibiofouling membrane with controlled surface porosity. As it is well-established at the laboratory scale, this method is ideal to investigate (i) the efficiency of novel antibiofouling polymers, provided that hydrophobic interactions are established between the polymer and the membrane, and (ii) the role of molecular design including, for instance molecular weight, hydrophilic/lipophilic balance, or separation distance between charges carried by the polymer on membranes’ antibiofouling properties. Fundamentally, both intrinsic structure and chemistry of zwitterionic materials (i.e., molecular structure, chemical groups, and chain length) as a membrane coating are equally important for antifouling performance. It is not likely that a single structural or chemical factor is solely responsible for antifouling performance, but rather a combination of many. Specifically, a number of studies have recently investigated the structural dependence of polymers on antifouling performance of poly(N-hydroxyalkylacrylamide) and poly(1-carboxy-N,Ndimethyl-N-(3′-acrylamidopropyl)methanaminium inner salt).1,16,17 Small variations in the carbon space length (CSL) between positively and negatively charged moieties of the zwitterionic materials lead to a large change in polymer− polymer and water−polymer interactions, which may in turn affect polymer conformation and flexibility and consequently their water binding ability linked to different hydration characteristics. However, the CSL-induced combination effects on surface hydration and antifouling behaviors remain unclear and unexamined when incorporating zwitterionic materials into a hydrophobic PVDF membrane system. To address these concerns mentioned above, we synthesized a novel series of zwitterionic polymers based on copolymerization of maleic anhydride-alt-1-octadecene (MAO) groups with either N,N-dimethylenediamine (DMEA) or 3-(dimethylamino)-1-propylamine (DMPA) groups by ring-opening reaction, between the cyclic anhydride and the diamine, resulting in the formation of a zwitterionic molecule of
zwitterionic-based materials are considered as the most promising candidates for the preparation of nonfouling surfaces, alternative to PEG. From a surface-coating/modification viewpoint, different methods such as physical coating, in situ modification, or chemical grafting (UV-treatment, plasma polymerization, etc.)6−8 have been developed to modify PVDF membranes for enhancing their antifouling properties. Some interfaces created from PVDF substrate were poly(vinylidene fluoride)graf t-poly(hydroxyethyl methacrylate) (PVDF-g-PHEMA), poly(vinylidene fluoride)-graf t−poly(oxyethylene methacrylate) (PVDF-g-POEM), poly(vinylidene fluoride)-graf t-poly(sulfobetaine methacrylate) (PVDF-g-PSBMA), and self-doped sulfonated polyaniline (PVDF-g-SPANI) membranes.9−13 Even though surface modifications of PVDF by chemical surface reaction are very popular, it is difficult to produce controllable surface density and uniform porosity at atomic level using these modifications methods because of some damages to the bulk properties and membrane structures under polymerization conditions. From a practical viewpoint, even though these methods are effective for small scale development in laboratories, their scale-up has proved to be challenging. On the other hand, physical modification of a PVDF material by coating (dip coating, thermal evaporation coating) leads to selfassembling and self-organization of the surface-modifying molecules onto the PVDF chains, does not damage the bulk properties of the membranes, and can be fairly well-controlled at the nanoscale, provided a fine-tuning of process parameters (coating time, concentration in the coating solution) and chemical composition of the surface-modifier (nature of hydrophobic/hydrophilic moieties, hydrophilic/hydrophobic balance).14,15 Therefore, their scale-up is less challenging. Surely, a two-step process is involved in this case, but the method is readily carried out, the surface density of the polymer is optimal, and the reduction of porosity observed after coating of a virgin membrane can be well-controlled so that it remains B
DOI: 10.1021/acs.langmuir.6b00981 Langmuir XXXX, XXX, XXX−XXX
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assessed by taking the difference in weight per unit surface area of membranes before and after surface modification. Characterization of Self-Assembled Zwitterionic PVDF Membranes. X-ray photoelectron spectroscopy (XPS) analysis was conducted to ascertain the efficiency of surface-modification. To do so, a PHI Quantera SXM/Auger spectrometer with a monochromated Al KR X-ray source was employed. A hemispherical energy analyzer (with pass energy between 50 and 150 eV) allowed measuring the energy of emitted electrons. The photoelectron takeoff angle was 45°. In this study, we focused particularly on the intensity of C 1s, O 1s, and N 1s spectra to evaluate the efficiency and control of surface modification. Analysis software was from Service Physics, Inc. Membranes’ top surfaces were observed by SEM in order to qualitatively characterize the effect of self-assembling surface modification on membranes’ surface porosity. SEM images can also provide evidence of homogeneous surface modification. To do so, a Hitachi S-3000 instrument was employed, for which accelerating voltage was set to 7 keV. Membranes were previously mounted on a SEM holder and sputter-coated with gold for 100 s. Finally, porosity of membranes was assessed according to the procedure reported by Gu et al.18 To evaluate the zeta potential of membranes at pH 7, samples (length: 2 cm; width 1 cm) were place on the adjustable cell of a SurPASS instrument. A phosphate buffer solution was prepared and used to infiltrate the instrument and the membranes. Then the surface zeta potential of the samples could be measured. Characterization of Antibiofouling Properties of SelfAssembled Zwitterionic PVDF Membranes. Water contact angle measurements were assessed with a goniometer (model FTA1000, First Ten Ångstroms Co., Ltd., USA) to evaluate the surface hydrophilicity of the membranes. A 4 μL water droplet was deposited onto the surface of membranes at 10 different sites. The static water contact was then measured after a 5 s contact time. The average of these 10 measurements was taken as the final water-contact angle. Tests were all performed at 25 °C. Hydration capacity (mg/cm2) was assessed to evaluate the overall membrane hydrophilicity. Membrane disks (diameter: 0.65 cm) were dried and weighed, before being immersed in water for 24 h. Afterward, wet membranes were taken off the water bath and surface water gently wiped out before weighing the samples. The hydration capacity was defined as the difference in weight between the dried and the wet membrane, per unit surface area. Five measurements were performed for each membrane, and the average taken as the final hydration capacity. Adsorption of two model proteinsbovine serum albumin (BSA, Mw ≈ 66 000 g mol−1, Sigma) and fibrinogen (FN, Mw = 340 000 g mol−1)was tested. Adsorption tests of BSA were performed according to the experimental method described in Chiag et al.’s work.19 Briefly, membrane disks (diameter: 0.65 cm) were washed with DI water and dried before being placed in a 24-well plate. They were then soaked in 1 mL of 100% ethanol for 30 min. Thereafter, ethanol was removed and disks were immersed in phosphate-buffered saline (PBS) for 2 h at 25 °C. After removing PBS, samples were incubated with BSA (1 mL at 1 mg mL−1 in PBS) for 2 h and at 25 °C. The concentration of BSA remaining into the incubation bath, and so the amount of BSA adsorbed onto the membrane, was assessed by measuring the absorbance of the solution at 280 nm (UV−vis spectrophotometer, PowerWave XS, Biotech). Tests were repeated three times for each membrane. As for the adsorption of fibrinogen, tests were done according to the following experimental protocol. Membranes having a surface area of 1.32 cm2 were placed in individual well of a 24-well plate and immersed in 1 mL of PBS for 2 h at room temperature. Then PBS was removed, and membranes were incubated with 2 mL of a 0.3 mg/mL fibrinogen solution at 37 °C for 2 h. Samples were then rinsed several times with PBS. Afterward, membranes were transferred in a 6 mL solution containing 1 wt % SDS (sodium n-dodecyl sulfate) and sonicated for 5 min. The SDS solution was transferred into individual wells of a 96-well plate, each well containing 150 μL of solution. 150 μL of a bicinchoninic acid reagent was added to each well, and incubation was performed for 2 h at 37 °C. Finally, the absorbance at 562 nm was determined using a UV/vis spectrophotometer, and the quantitative amount of protein
MAO−DMEA or MAO−DMPA (Scheme 1). We then used a thermal-evaporation method to assemble pseudozwitterionic alternate copolymers onto the PVDF membrane system, yielding zwitterionic-coated PVDF membranes. This anchoring method enables to produce uniform membrane porosity and controlled surface density of coating polymers on the membrane. The physicochemical properties of membranes were carefully examined by SEM, zeta potential, and porosity measurement, contact angles and hydration capacity tests while the antifouling performances of both membranes were extensively tested by protein adsorption assay, blood adhesion assay, and bacterial attachment tests. The structure−property correlation among molecular structure, surface hydration characteristic, and antifouling performance of the zwitterioniccoating PVDF membranes, particularly the CSL effect, are then discussed. The polymer associated with the thermal-evaporation coating technology as well as the fundamental knowledge obtained from this work should be helpful in the development of effective antifouling membrane.
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EXPERIMENTAL METHODS
Materials. Maleic anhydride-alt-1-octadecene (MAO, Mw: 350.54 g/mol, 85−142 repeat units), N,N-dimethylenediamine (DMEA, Mw: 88.15 g/mol) and 3-(dimethylamino)-1-propylamine (DMPA, Mw: 102.18 g/mol) were purchased from Sigma-Aldrich. Tetrahydrofuran (THF) and chloroform (CHCl3) were bought from Tedia and used as solvent. Ethanol was purchased from Echo. Commercial PVDF membranes (pore size: 0.1 μm) were bought from Millipore. DI water with a minimum resistivity of 18.0 MΩ·cm was obtained with a Millipore purification system. Synthesis of Polymers. Macromolecules were synthesized by ring-opening reaction. MAO and either DMEA or DMPA were mixed in THF. Weight contents of MAO and DMEA/DMPA were 10 and 40 wt %, respectively. Species were allowed to react for 20 min at room temperature. Then, the mixture was centrifuged at 5000 rpm for 30 min in order to separate the polymer from the solvent. This operation was repeated three times. The polymer was then dried under vacuum and stored at 4 °C until use. The chemical structures of polymers are presented in Scheme 1. Characterization of Zwitterionic Polymers. 1H NMR spectra of 10 mg/mL zwitterionic polymer solution in chloroform-d (CDCl3) were recorded with a Bruker 500 MHz NMR spectrometer at 500 MHz. The degree of ring-opening polymerization for each functionalized polymer could be determined from results of 1H NMR analysis performed with MestReC software. In addition, a Zetasizer Nano ZS90 instrument (Malvern) was used to evaluate the hydrodynamic diameter as well as the zeta potential of zwitterionic polymer solutions (5 mg/mL in CHCl3). Surface Modification of PVDF Membranes by Thermal Evaporation. Surface modification of membranes was performed by thermal evaporation. We first determined by trial and error the amount (volume) of solvent−chloroform (90% v/v) and ethanol (10% v/v) necessary to entirely cover one surface of a 0.65 cm diameter membrane. We then multiplied the value obtained (20 μL) by 2, as we intended to coat both surfaces, and calculated the amount of copolymer necessary to reach the desired coating density. As for the coating step, 0.65 cm diameter membranes were washed, dried, and put into a 24-well plate. Thereafter, a volume V = 20 μL of polymer solution was deposited onto the surface of membranes. Thermal evaporation was conducted at 25 °C and lasted 60 s. Theoretical coating densities were 0.05, 0.1, 0.5, and 1 mg/cm2. Afterward, membranes were turned upside down, and coating of the second surface was performed similarly. Membranes were then washed in DI water for 1 min and dried in an oven at 45 °C for 1.5 h. Finally, membranes were immersed in a 50 vol % ethanol bath for 5 min and then in DI water for 10 min. Experimental coating densities were also C
DOI: 10.1021/acs.langmuir.6b00981 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. 1H NMR spectra of (a) p(MAO−DMEA) and (b) p(MAO−DMPA) polymers. adsorbed onto the membranes could be evaluated using a predetermined calibration curve. Each measurement was carried out six times for each sample. Resistance to macro-biofouling was also evaluated by determining the extent of bacterial attachment onto membranes. Here, we selected Escherichia coli genetically modified with green fluorescent protein, namely E.C. with GFP, in order to facilitate the observation by
confocal microscopy. The steps of the genetic modification as well as those of bacterial culture are detailed elsewhere.20 We performed adsorption tests as follows. First, membranes were washed with PBS and dried. Then, membrane disks (0.65 cm diameter) were disposed in individual wells of a 24-well plate and incubated with 1 mL of solution of E.C. with GFP at 37 °C and for 3 h. Then, we removed the bacterial solution, and washed membranes at least three times with PBS, to D
DOI: 10.1021/acs.langmuir.6b00981 Langmuir XXXX, XXX, XXX−XXX
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Figure 2. C 1s, N 1s, and O 1s core level spectra of self-assembled membranes. remove nonadhering bacteria. Those remaining could then be observed by confocal microscope. Hemocompatibility of Self-Assembled Zwitterionic PVDF Membranes. The adhesion of thrombocytes was assessed using the following experimental protocol. PVDF virgin and self-assembled zwitterionic PVDF membranes having a surface area of 0.4 cm2 were placed in a 24-well tissue culture plate. Then, 1000 μL of PBS was used to equilibrate the wells, and equilibration lasted 2 h at a temperature of 25 °C. After separating thrombocytes from other blood constituents by centrifugation, 200 μL of pure platelet-rich plasma (PRP) or 200 μL of PRP recalcified with calcium (1 M CaCl2, 5 μL) was poured on the membranes and incubated for 2 h at 37 °C. In the latter case, the adhesion of activated thrombocytes was therefore tested. After incubation, membranes were washed twice with PBS (1 mL). Thereafter, they were soaked in a glutaraldehyde solution (2.5% in PBS) maintained at 4 °C and for 48 h. This step allowed fixing the platelets and adsorbed proteins onto the surfaces. Afterward, samples were rinsed twice with PBS before being gradient-dried with ethanol (7 baths, 20 min each, from 0 to 100% v/v in PBS) and eventually dried in air. The adhesion of nonactivated platelets was observed by confocal microscopy (LSCM, A1R, Nikon, Japan) at the following wavelengths: λex = 488 nm/λem = 520 nm. In the case of activated platelets, membranes were fixed onto a special SEM holder using a double-sided adhesive tape, sputter-coated with gold, and platelets adhering onto membranes observed using a JEOL JSM-5410 SEM operated at 7 keV.
The adhesion of erythrocytes and leucocytes was also tested. As for erythrocytes, red blood cell concentrate was first prepared by centrifuging at 1200 rpm for 10 min 250 mL of blood. Then, 200 μL of blood was incubated with membranes (0.4 cm2) for 2 h at 37 °C. Membranes were then rinsed with PBS and immersed for 10 h in a 300 μL glutaraldehyde solution (2.5% in PBS) maintained at 4 °C. After rinsing several times membrane disks with PBS, erythrocytes adhering on the membranes were observed by confocal microscopy (LSCM, A1R, Nikon, Japan) at λex = 488 nm/λem = 520 nm. Concerning the leucocytes, white blood cell concentrates (200 μL), prepared by centrifuging blood for 30 min at about 2100 rpm, were incubated on membrane disks for 120 min at 37 °C. The procedures to fix and observe leucocytes were similar to those used for erythrocytes adhesion tests. The interaction of whole blood with membranes was also studied. Detailed protocol for this test is presented in a previous work.21 Additionally, the hemolytic activity was assessed to evaluate the extents of cell lysis. The method utilized is also detailed elsewhere.22
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RESULTS AND DISCUSSION Synthesis and Characterization of P(MAO−DMEA) and P(MAO−DMPA) Polymers. MAO and DMEA or MAO and DMPA was initiated and polymerized in THF to synthesize p(MAO−DMEA) or p(MAO−DMPA) using a ringopening reaction. NMR spectra of the polymers are gathered in Figure 1. The reaction between MAO and DMEA or MAO and E
DOI: 10.1021/acs.langmuir.6b00981 Langmuir XXXX, XXX, XXX−XXX
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Langmuir DMPA is complete, as evidenced by calculating the ratio between the area of peak at 2.27 ppm assigned to the methyl protons (six protons h or i′) held by a nitrogen atom and the area of peak at 0.96 ppm corresponding to the terminal methyl protons of the alkyl chain (three protons a or a′). This ratio was 1 in each case. Then, from the knowledge of the molecular weights of DMEA, DMPA, and MAO, it was possible to evaluate the molecular weight of the polymers: for p(MAO− DMEA), it ranged between 37 289 and 62 294 g/mol, while for p(MAO−DMPA), it was in the range 38 418−64 286 g/mol. In addition, the size of polymers was evaluated by DLS. The diameter of p(MAO−DMEA) was found to be 192 ± 43 nm, while it logically increased to 245 ± 58 nm for p(MAO− DMPA) owing to the supplementary −CH2 group. Finally, it has to be noted that the polymers are totally water-insoluble and only soluble in chloroform. Surface Modification and Characterization of PVDF Membranes with Zwitterionic Polymers. The thermal evaporation process was used to coat zwitterionic polymers onto PVDF membranes, making hybrid membranes with hydrophilic p(MAO−DMEA) or p(MAO−DMPA) outer surfaces and hydrophobic bulk. The surface physicochemical compositions of the membranes were characterized and confirmed by SEM and XPS. As shown in Figure 2, compared to the virgin PVDF membrane, significant changes in the modified membranes were observed in several major locations: (i) CF2 peak at 290.8 eV was decreased and even disappeared; this indicates that zwitterionic polymers are able to fully cover the PVDF surface, such that fluorine atoms cannot be detected. (ii) Two new peaks appeared at 284.8 and 288 eV, attributed to the characteristic peaks of the carbon skeleton and the carbonyl group in both MAO and DMPA/DMEA. The intensity of these peaks increased with the polymer coating density. (iii) A decreasing of the intensity of CH2 peak (PVDF) was observed at 286.5 eV.10 (iv) No peaks of N and O elements were observed for the virgin PVDF membrane. In contrast, for the modified PVDF membranes, three new peaks at 399, 401, and 531.5 eV emerged and were assigned to the characteristic peaks of the tertiary amine, quaternary ammonium, and carboxylate groups of p(MAO−DMEA) and p(MAO−DMPA) polymers grafted at the surface of PVDF. Meanwhile, their intensity increased corresponding to the surface enrichment of the zwitterionic polymers. Moreover, it can be seen in Figure 3 that there was no major change of surface morphologies between the polymer-coated PVDF membranes and the virgin PVDF membrane. The membrane surfaces remained homogeneous, suggesting that the thermal evaporation process leading to self-organization of the polymer chains was well controlled. It was evidenced further by the assessment of surface porosity remaining in a narrow range (72−76%), even though the coating density increased from 0.049 to 0.974 mg/cm2 (Table 1). So the coated polymers did not block the pores of the virgin membrane. This was attributed to the facts that (i) polymers were too small to block membrane pores of 0.1 μm, (ii) local hydrophobic interactions between PVDF and polymers were established at the nanoscale, and (iii) no agglomerates of p(MAO−DMEA) or p(MAO− DMPA) were formed. Eventually, the surface-modification process in which p(MAO−DMEA) and p(MAO−DMPA) polymers self-assemble at the membrane surface is particularly useful for preparing uniform membranes while still retaining their porous structures and arising properties.
Figure 3. SEM characterization of self-assembled zwitterionic membranes.
In order to further prove that the zwitterionic polymers are coated onto the PVDF membrane, we examined the surface charge of the membranes before and after modification of zwitterionic polymers by measuring the zeta potentials. As shown in Figure 4, compared to the negative zeta potential of the virgin PVDF membrane (−15.6 ± 1.5 eV) which is consistent with that reported in the study of Pasmore et al.,23 zeta potentials of PVDF membranes self-assembled with p(MAO−DMEA) were −9.3 ± 2.7, −5.3 ± 1.2, −0.3 ± 1.57, and −0.1 ± 1.2 eV for experimental coating densities of 0.049, 0.097, 0.48, and 0.97 mg/cm2, respectively, while zeta potentials of PVDF membranes self-assembled with p(MAO−DMPA) were −7.1 ± 1.7, −6.0 ± 1.1, −2.0 ± 1.2, and −0.9 ± 4.2 eV for theoretical coating densities of 0.05, 0.1, 0.5, and 1 mg/cm2, respectively. These results indicate that the zwitterionic polymers were successfully coated on the surfaces of the PVDF membranes. Hydration Properties of Zwitterionic Polymer-Coated PVDF Membranes. Water contact angle is a convenient way to assess the property of the membrane surface, which highly depends on surface chemistry, roughness, porosity, or pore size. Figure 5 shows that both zwitterionic-modified PVDF membranes exhibited significantly decreased water contact angles from 115 ± 1° (virgin PVDF) to 67 ± 10° (PVDF-gMAO−DMEA) and 64 ± 2° (PVDF-g-MAO−DMPA). The water contact angles decreased with polymer coating density, suggesting that the zwitterionic membranes have strong electrostatically induced interactions with water molecules. Meanwhile, as the coating density increased to 1.0 mg/cm2, hydration capacities of zwitterionic membranes reached a F
DOI: 10.1021/acs.langmuir.6b00981 Langmuir XXXX, XXX, XXX−XXX
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Langmuir Table 1. Coating Density and Porosity of Self-Assembled Zwitterionic PVDF Membranes PVDF membranes self-assembled with p(MAO−DMEA) theor coating density (mg/cm2) 0 0.05 0.1 0.5 1
expt coating density (mg/cm2) 0 0.049 0.097 0.484 0.974
± ± ± ±
0.004 0.001 0.000 0.058
PVDF membranes self-assembled with p(MAO−DMPA)
porosity (%)
theor coating density (mg/cm2)
± ± ± ± ±
0 0.05 0.1 0.5 1
76 74 75 72 74
1 1 2 3 1
expt coating density (mg/cm2) 0 0.048 0.094 0.504 0.989
± ± ± ±
0.009 0.004 0.021 0.015
porosity (%) 76 74 76 75 75
± ± ± ± ±
1 1 2 1 1
Figure 4. Evaluation of the surface charge bared by the virgin and surface-modified PVDF membranes.
Figure 5. Hydration properties of self-assembled zwitterionic surfaces.
plateau at 0.75 mg/cm2 for PVDF membranes self-assembled with p(MAO−DMEA) and 0.64 g/cm2 for PVDF membranes self-assembled with p(MAO−DMPA), suggesting the formation of a hydration layer. Hydrophilicity is a prerequisite to nonfouling behavior of an interface. The plateau reached clearly indicates that zwitterionic membranes enabled to form a saturated surface hydration. In
addition, we noted that membrane self-assembled with p(MAO−DMEA) was more hydrated than membrane selfassembled with p(MAO−DMPA) after a 24 h immersion time in water. One may have noted that the values obtained with these membranes for water contact angle are higher than those reported in the literature with other systems (membranes modified with zwitterions). For example, Azari and Zhou G
DOI: 10.1021/acs.langmuir.6b00981 Langmuir XXXX, XXX, XXX−XXX
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Figure 6. Evaluation of nanobiofouling: resistance of self-assembled zwitterionic surfaces to (a) BSA adsorption and (b) FN adsorption. The error limits on data on FN adsorption are very small (
