Journal of Membrane Science 593 (2020) 117444

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Journal of Membrane Science

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Nanofiltration membranes with hydrophobic microfiltration substrates for robust structure stability and high water permeation flux T

∗∗ ∗ Xi Zhanga,b, Chang Liua,b, Jing Yanga,b, , Cheng-Ye Zhua,b, Lin Zhangc, Zhi-Kang Xua,b, a MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Hangzhou, 310027, China b Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, China c Key Laboratory of Biomass Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027, China

ARTICLE INFO ABSTRACT

Keywords: Traditional nanofiltration membranes (NFMs) suffer from ultrafiltration substrates with low porosity, small pore Nanofiltration membrane size and relatively poor solvent stability. Herein, NFMs have been fabricated on a series of hydrophobic polymer Microfiltration substrate microfiltration substrates to address these issues. Polyphenol-based coatings of tannic acid/diethylenetriamine Surface wettability (TA/DETA) were co-deposited on the hydrophobic substrates to improve their surface wettability and to make Water permeation flux them appropriate for interfacial polymerization. The spreading behaviors of aqueous solutions, which are of Structure stability significant importance to the formation of defect-free polyamide layers, were directly visualized by laser confocal microscopy. The influences of TA/DETA coatings on the interfacial polymerization were further demonstrated by both dynamic molecular simulation and nanofiltration performance evaluation. The as-prepared NFMs exhibit higher water permeation flux compared with traditional ones because of the large pore size and high porosity of the microfiltration substrates, as well as the relatively low cross-linking degree of polyamide layers. Internal stress during the nanofiltration process was calculated by the theory of thin plates and the results claim good pressure resistance for these NFMs. Therefore, the as-prepared NFMs can be steadily used under high operation pressures even up to 0.9 MPa, which are in accordance with the theoretical calculation. Furthermore, these NFMs also present good solvent resistance since the chemical stability of the no-polar hydrophobic substrates.

1. Introduction size and porosity of the porous substrates because of the reduced trans- membrane resistance and shortened water permeation pathway Nanofiltration membranes (NFMs) have drawn growing attention [18–20]. Some commercialized non-polar polymer membranes, such as over the past years due to their high water permeation flux, low op- polypropylene, polyethylene and poly(vinylidene fluoride) microfiltra- eration pressure and high rejection to multivalent ions [1–3]. They have tion membranes (PPMM, PEMM and PVDFMM), thereby emerge as been widely used in waste-water treatment, biological engineering, satisfactory candidates for support substrates of NFMs due to their medicine/food industry, and seawater desalination [4–7]. Currently, advantages of high solvent resistance, good mechanical strength, low these NFMs usually consist of a polyamide selective layer and a porous cost for raw materials, large pore size and high porosity [21,22]. support substrate [8–11]. Interfacial polymerization is one successful However, it is difficult to directly fabricate a polyamide selective method to fabricate NFMs with such structures [10]. Most of the layer on these microfiltration substrates by interfacial polymerization commercialized NFMs use polysulfone (PSF), polyethersulfone (PES) or because aqueous solutions cannot be well spread on them due to their polyacrylonitrile (PAN) ultrafiltration membranes as porous substrates poor surface wettability [23]. Improving the surface hydrophilicity of due to their proper surface hydrophilicity for carrying out the inter- these hydrophobic substrates is required for carrying out the interfacial facial polymerization [12–14]. However, these substrates exhibit low polymerization. There are many available methods to hydrophilize the chemical resistance to ketones, esters and alcohols, limiting the struc- surfaces of hydrophobic membranes, such as UV, plasma, or ozone-in- ture stability of the NFMs [15–17]. Moreover, It has been reported that duced grafting polymerization and chemical treatment [24–31]. For NFMs can obtain improved water permeability by increasing the pore example, PPMMs were hydrophilized by UV-induced grafting or

∗ Corresponding author. MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Hangzhou, 310027, China. ∗∗ Corresponding author. MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Hangzhou, 310027, China. E-mail addresses: [email protected] (J. Yang), [email protected] (Z.-K. Xu). https://doi.org/10.1016/j.memsci.2019.117444 Received 2 May 2019; Received in revised form 29 July 2019; Accepted 4 September 2019 Available online 05 September 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved. X. Zhang, et al. Journal of Membrane Science 593 (2020) 117444 oxidation with chromic acid solution and then they were used as sub- Table 1 strates for NFMs [23,28,29]. PPMMs and PVDFMMs could also be hy- Average pore diameter, porosity and water contact angle for three kinds of drophilized by plasma treatment for the fabrication of NFMs [30,31]. hydrophobic microfiltration membranes used in this study. However, these methods usually have complex operation process and Substrate Average pore diameter (nm) Porosity (%) Water contact angle (°) high energy cost. The substrate structure may even be destroyed and pore blockage will occur, resulting in the increased trans-membrane PPMM 338.3 81.14 144.25 resistance and reduced water permeation flux [22,23,29,30]. Another PEMM 143.6 52.29 116.56 PVDFMM 307.8 61.81 126.65 disadvantage is the lack of university, which means one can only hydrophilize one kind of hydrophobic substrate with a specific condition, limiting the selection of substrates for NFMs. Therefore, it is still a phenylenediamine (MPD, 99%), tannic acid (TA, Analytical Reagent) challenge to fabricate NFMs on hydrophobic microfiltration substrates and fluorescein sodium (Analytical Reagent) were bought from Aladdin with desirable nanofiltration performances. Chemistry Co. Ltd. (China). Acetone, ethanol, hexane, sodium chloride Polyphenols have strong solid-liquid interface activities and can (NaCl), sodium sulfate (Na2SO4), magnesium chloride (MgCl2), mag- form coatings on various polymer materials [32–35]. Recently, we have nesium sulfate (MgSO4), hydrogen chloride (HCl) and sodium hydro- demonstrated a novel type of polyphenol coating by simple co-deposi- xide (NaOH) were all analytical reagents and obtained from Sinopharm tion of tannic acid (TA) and diethylenetriamine (DETA), which merits Chemical Reagent Co. Ltd. (China). All chemicals were used as received the advantage of simplicity, versatility and university [32,34,36]. It without further purification. Bicine buffer (pH = 7.8) was prepared should be noticed that such coating is particle-free and uniform, thus from bicine and NaOH as reported in our previous work [34]. Ultrapure exhibits a negligible influence on the surface structures of microfiltra- water (18.2 MΩ) was produced by an ELGA Lab Water System (France). tion substrates.34 Herein, we used the TA/DETA coatings to hydrophilize PPMM, PEMM and PVDFMM substrates according to our pre- 2.2. Dynamic molecular simulation vious work [34], making them proper substrates for directly carrying out the interfacial polymerization (schematically shown in Fig. 1). The Materials Studio 2017 R2 was used to carry out dynamic molecular interfacial polymerization was then conducted on these microfiltration simulation. Fig. 2 shows the molecular structures of PIP, MPD, PP and substrates to prepare NFMs. Laser confocal microscopy (LSCM) was TA/DETA coating. The Forcite module with a task of geometry opti- used to visualize the spreading behaviour of aqueous solution on the mization was used to optimize all the structures applied in the dynamic studied substrates. Water can well spread on these TA/DETA co-de- molecular simulation. Especially, PP molecule with 20 repeat units was posited substrates while it shows a de-wetting phenomenon on the used to simplify the simulation. Mixing energies of PIP-PP, PIP-TA/ nascent ones, corresponding to whether the interfacial polymerization DETA, MPD-PP and MPD-TA/DETA were calculated by the Blends can be carried out or not. The resulting NFMs show improved water module. permeation flux compared with traditional ones with ultrafiltration substrates while maintaining a high rejection to divalent ions (> 95% 2.3. Preparation of NFMs for Na2SO4). Internal stress on the polyamide selective layer was calculated to confirm the good pressure resistance of such NFMs. On the The hydrophobic substrates were co-deposited by TA/DETA coating other hand, our NFMs show stable nanofiltration performances against for hydrophilization. Briefly, TA was dissolved in bicine buffer to pre- ethanol and acetone treatment, demonstrating high solvent resistance pare a solution with a concentration of 2 g/L. Then DETA was added which is profited from the chemical stability of the hydrophobic sub- into the freshly prepared TA solution with a TA/DETA mass ratio of 1/ strates. 10. Substrates were pre-wetted in ethanol for 10 min and then quickly immersed into the freshly prepared TA/DETA solution for co-deposi- 2. Experimental tion. The substrates were shaken for desired time at 30 °C and then washed by ultrapure water three times. Subsequently, the TA/DETA co- 2.1. Materials deposited substrates were dried in a vacuum oven overnight to a constant weight. PPMM, PEMM and PVDFMM were obtained from Membrana GmbH Interfacial polymerization was carried out on the TA/DETA co-de- (Germany), Hebei Jinli Plastic Materials Factory (China) and Haining posited substrates to prepare NFMs. First, the aqueous solution of PIP or Chuangwei Filter Equipment Factory (China), respectively. Typical MPD and the hexane solution of TMC were prepared with a con- properties, including average pore diameter, porosity, as well as surface centration of 2.0 g/L and 1.0 g/L, respectively. The substrates were held wettability, are listed in Table 1 for all these hydrophobic microfiltra- in a module (Fig. S1 in Supporting Information) and the upper side of tion substrates. The samples were cut into round pieces with a diameter substrates was immersed in aqueous solution for the interfacial poly- of 4 cm. Then, the substrates were washed by acetone to remove ad- merization. The aqueous solution was poured off after immersion for sorbed impurities and dried in a vacuum oven overnight to a constant 5 min and the residual solution on the substrate surface was subse- weight before use. Polyethersulfone microfiltration membrane quently drained off in air. The aqueous solution saturated substrate was (PESMM) was bought from Haiyan Xindongfang plastic Co. Ltd. (China) then immersed in the hexane solution of TMC for 2 min to form a and used as received. Trimesoyl chloride (TMC, > 99%) was purchased polyamide selective layer via interfacial polymerization. After removing from Qingdao Sanlibennuo Co. Ltd. (China). Piperazidine (PIP, 99%), the excess hexane solution by air-drying, the as-prepared NFM was N,N-bis(2-hydroxyethyl) glycine (bicine, > 99.5%), N-(2-aminoethyl)- placed in an oven at 60 °C for 30 min to stabilize the membrane 1,2-ethylenediamine (diethylenetriamine, DETA, Chemically Pure), m- structure. The as-prepared NFM was then washed by ultrapure water

Fig. 1. Schematic illustration of the preparation process of NFMs with hydrophobic microﬁltration substrates.

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Fig. 2. Molecular structures of PIP, MPD, PP and TA/DETA coating. for three times and stored in ultrapure water for further characteriza- 7.07 cm2. All the samples were pre-compacted under 0.7 MPa for tions and measurements. 30 min before the evaluation. Both the water and organic solvents permeation flux (F, L/m2⋅h) were calculated by equation (2): 2.4. Characterization Q F = At× (2) The pore size and porosity of microfiltration substrates were mea- fi fi sured using a mercury porosimetry (AutoPore IV 9510, USA). The where Q, A and t represent for the ltrated solvents volume, the l- fi surface wettability of all samples was characterized by measuring the tration area and the ltration time, respectively. Four kinds of salts water contact angle with a drop Meter A-200 contact angle system (Na2SO4, MgSO4, MgCl2 and NaCl) were used to analyze the rejection fi fl (MAIST Vision Insection & Measurement Co. Ltd. China). A droplet of performance with a concentration of 2 g/L and a xed cross- ow rate of 2 μL water was used as the probe in these measurements. The water 30 L/h. And the salt rejection (R) was calculated by equation (3): spreading behavior on the substrate surface was detected by a laser Cp confocal microscopy (LSCM, LSM780, ZEISS, Germany). The substrate R =−(1 ) × 100% Cf (3) was immersed in an aqueous solution of fluorescein sodium with a μ concentration of 5 g/mL for 5 min. It was taken out and the excess where Cp is the concentration of permeation solution and Cf is the aqueous solution on the surface was drained off. Then the substrate was concentration of feed solution. The concentration was proportional to characterized using the LSCM and the excitation wavelength is 442 nm. the conductivity, which was detected by an electrical conductivity The surface and cross-sectional morphologies of membranes were ob- meter (FE30, Mettler Toledo, China). The nanofiltration performance served by a field emission scanning electron microscopy (FE-SEM, was measured under different operating pressures and cross-flow rates. Hitachi, S4800, Japan) and a transmission electron microscopy (TEM, The structure stability of NFMs against organic solvents is examined Hitachi, H-7650, Japan). A UV–vis spectroscope (Shimadzu, Japan) was in two aspects. First, NFMs were immersed in ethanol or acetone for applied to record the UV–vis spectra of MPD solution. The substrate was 24 h. After washing by ultrapure water for three times to remove the first immersed in an MPD solution with a concentration of 2 g/L for excess solvent, the nanofiltration performance of NFMs was measured. 5 min, and the excess solution was drained off from the out surface of The organic solvent permeation flux of NFMs was characterized for 12 h substrate. Then the substrate was immersed in a sample of 5 mL hexane to further analyze their structure stability against organic solvents. The for 5 min. The adsorption of MPD in hexane was simultaneously de- pH stability of NFMs was examined by evaluating the nanofiltration termined. The surface chemistries were analyzed by a Fourier transform performance after immersing them in aqueous solutions with different infrared spectroscope (FT-IR/ATR, Nicolet 6700, USA) equipped with pH values for 24 h. an attenuated total reflectance accessory (ZnSe crystal, 45o). The surface charge properties were measured by an electro kinetic analyzer 3. Results and discussion (SurPASS Anton Paar, GmbH, Austria) using a streaming potential method with KCl (1 mmol/L) as electrolyte solution. The pH of elec- 3.1. Hydrophilization of substrates and fabrication of NFMs trolyte solution was adjusted by HCl and NaOH solutions to investigate the influence of pH on the surface zeta potential. XPS spectra were The hydrophilic coating of TA/DETA can be easily formed by the collected with an X-ray photoelectron spectroscope (XPS, PerkinElmer, cross-linking of TA and DETA via Michael addition reaction [34,37]. USA) using Al Kα excitation radiation (1486.6 eV) at a detected depth This coating is able to adhere onto various materials via hydrogen less than 10 nm. The cross-linking degree (D) of the polyamide layer is bonding, electrostatic adsorption and hydrophobic interaction [33,38]. calculated by equation (1): Therefore, we can simply adjust the surface wettability of the hydro- 42− R phobic substrates by the co-deposition of TA/DETA. Fig. 3(a–c) presents D= ON/ 1 + RON/ (1) dynamic water contact angles for PPMM, PEMM and PVDFMM substrates with different co-deposition times, respectively. PPMM shows a where RO/N is the molar ratios of oxygen to nitrogen determined by XPS water contact angle around 90° when the co-deposition time is 10 min. spectra. There is no decrease tendency along with the measurement time, which means the water droplet cannot spread well on this substrate. Fur- 2.5. Nanofiltration performance measurement thermore, the water contact angle decreases with increasing the co- deposition time, indicating an enhanced hydrophilicity of the substrate A laboratory scale cross-flow flat membrane module was used to surface (Fig. 3(a)). Co-deposition of TA/DETA on PEMM endows the evaluate the nanofiltration performance of the prepared NFMs. Each substrate with nearly the same trend as PPMM. Fig. 3(b) shows that the sample was measured under 0.6 MPa at 30 °C with a filtration area of water contact angle decreases to lower than 90° and the water droplet

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Fig. 3. Dynamic water contact angle of (a) PPMM, (b) PEMM and (c) PVDFMM substrates with different co-deposition times; (d) Water contact angles for different hydrophobic microfiltration substrates before and after co-deposition of TA/DETA for 60 min; (e) 3-Dimensional LSCM images of different microfiltration substrates after wetted by the aqueous solution of fluorescein sodium; (f) Schematic diagrams of the microfiltration substrates after wetted by aqueous solution. can spread well on the substrate surface when the co-deposition time is 10–15 μm (Measured by 2D LSCM images with different focal planes, above 10 min. However, the surface wettability of PVDFMM can be Fig. S2 in Supporting Information), which is significant for carrying out improved slightly compared with PPMM and PEMM. The TA/DETA the interfacial polymerization on them (Fig. 3(f)). coatings are adhered on the hydrophobic substrates due to the hydro- The TA/DETA coatings change the color of the substrates slightly phobic interactions between them [34]. But PVDF has a higher polarity (Fig. S3 in Supporting Information) but maintains their surface compared with PP and PE, making PVDFMM relatively hard to be co- morphologies (Fig. S4 in Supporting Information). Moreover, the pore deposited with TA/DETA [39]. Nevertheless, when the co-deposition size and porosity of the substrates undergo negligible variations before time is 60 min, the PVDFMM substrate shows the water contact angle and after the co-deposition of TA/DETA (Figs. S5 and S6 in Supporting lower than 90° during all the measuring time (Fig. 3(c)). All these re- Information). These features are beneficial to avoid the undesired pore sults indicate that the surface wettability can be promoted from the blockage which is believed to be detrimental to NFMs with low trans- water contact angle of 144°, 117° and 127°–26°, < 20° and 85° by membrane resistance [40]. The TA/DETA coatings are stable at acid or 60 min co-deposition of TA/DETA on the hydrophobic PPMM, PEMM alkaline conditions, which is beneficial for the structure stability of and PVDFMM substrates (Fig. 3(d)), respectively. NFMs (Fig. S7 in Supporting Information). In our case, the prepared The surface wettability has great impact on the spreading behaviors NFMs are assigned as PP-NFM, PE-NFM and PVDF-NFM, respectively of aqueous solution on the substrates, which are important for carrying (Co-deposition time is 60 min in the following parts if there is no ad- out the interfacial polymerization. LSCM was used to visualize the ditional illustration). The monomer concentrations were fixed as 2 g/L spreading behavior of an aqueous solution of fluorescein sodium on the and 1 g/L for PIP and TMC according to our optimization experiments substrates. Fig. 3(e) presents 3D LSCM images from the nascent and TA/ (Figs. S8 and S9 in Supporting Information). Fig. 4 shows typical SEM DETA co-deposited substrates. It can be seen that there is no fluores- and TEM images for the surface morphology, the cross-sectional cence from the nascent substrates, indicating the aqueous solution structure of the polyamide selective layer for PP-NFM, PE-NFM and cannot well spread on the hydrophobic surfaces. On the other hand, the PVDF-NFM. The NFMs show a dense and poreless surface compared TA/DETA co-deposited PPMM, PEMM and PVDFMM substrates are able with original TA/DETA co-deposited substrates (Fig. S6 in Supporting to emit strong green fluorescence from their surface, demonstrating Information) due to the formation of polyamide selective layers. It is homogeneous spreading of the aqueous solution on the hydrophilized worth noting that the dense and poreless polyamide layer cannot be surfaces. These hydrophilized substrates can hold an aqueous layer of formed on the nascent hydrophobic substrates. The thickness is around

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Fig. 4. SEM images for (a–c) the surface and (d–f) the cross-sectional morphologies of PP-NFM, PE-NFM and PVDF-NFM, respectively; TEM images for the polyamide selective layer fabricated on co-deposited (g) PPMM, (h) PEMM and (i) PVDFMM substrates, respectively.

38 nm, 36 nm and 42 nm for these selective layers of PP-NFM, PE-NFM different surface wettability will influence the interfacial polymeriza- and PVDF-NFM (Fig. 4(g–i)), respectively. Moreover, the selective tion and then the nanofiltration performances. Fig. 5(a) shows the na- layers exhibit typical chemical structures of semi-aromatic polyamide nofiltration performances of PP-NFMs based on substrates with dif- films (Fig. S10 in Supporting Information). ferent co-deposition times, which is in accordance with the surface The PP-NFM, PE-NFM and PVDF-NFM were evaluated for their wettability. Polyamide selective layers cannot be fabricated when the nanofiltration performances, including water permeation flux and salt co-deposition time is less than 10 min for PPMM substrates (We signed rejection. One can envisage that the microfiltration substrates with as N/A in the figures) due to the poor surface wettability. The Na2SO4

Fig. 5. Nanofiltration performances of (a) PP-NFM, (b) PE-NFM and (c) PVDF-NFM on substrates with different co-deposition times (The operation pressure is 0.6 MPa); (d) UV–vis spectra of MPD in hexane after diffusing from co-deposited PPMM substrates with different co-deposition times; (e) Mixing energies between different di-amine monomers and substrates calculated by dynamic molecular simulation.

5 X. Zhang, et al. Journal of Membrane Science 593 (2020) 117444 rejection reaches above 95% and maintains a stable value when the co- deposition time is longer than 20 min, demonstrating the formation of a defect-free polyamide selective layer. Moreover, the water permeation flux increases along with the increased co-deposition time. For PE-NFM, the Na2SO4 rejection reaches maximum value of above 95% when the co-deposition time is 15 min while the water permeation flux increases with co-deposition time after that (Fig. 5(b)). However, Fig. 5(c) in- dicates that PVDF-NFM with a Na2SO4 rejection around 95% can only be obtained when the co-deposition time is 60 min, which can be assigned to the much poorer surface wettability of the co-deposited PVDFMM substrate compared with PPMM and PEMM. The pore size and porosity of substrates also influence the final nanofiltration performances of NFMs. As shown in Fig. 5(a)–(c), the water permeation flux has the order of PP-NFM > PVDF-NFM > PE-NFM when co-deposition time is 60 min. Given that the thicknesses of polyamide selective layers are almost the same for these three NFMs (Fig. 4(g)–(i)), Fig. 6. Comparison among NFMs fabricated by interfacial polymerization with fi fi the gradually increased water permeation flux from PE-NFM, PVDF- ultra ltration membranes (UM) or micro ltration membranes (MM) as sub- NFM to PP-NFM can be rationalized to the increased pore size and strates. (The operation pressure is 0.6 MPa for our NFMs). porosity of the substrates (Figs. S4, S5 and S6), which cause the reduced trans-membrane resistance and shortened water pathway [19,20]. Na2SO4 rejection above 95%. Moreover, it has a salt rejection order of PP-NFM was used as a model to further analyze the influences of Na2SO4 > MgSO4 > MgCl2 > NaCl due to the negatively charged substrates on the polyamide selective layers. The thickness of the se- surface of the polyamide selective layer (Figs. S15 and S16 in Sup- lective layers shows no obvious difference for PP-NFMs based on the porting Information). In addition to the decreased cross-linking degree TA/DETA coated substrates with different co-deposition times (Fig. S11 of the polyamide selective layers mentioned above, the high water in Supporting Information). In the meanwhile, the cross-linking degree permeation flux can be ascribed to the large pores and high porosity of of the polyamide selective layers decreases from 0.44 to 0.13 with the the microfiltration substrate than those ultrafiltration ones, which de- increased co-deposition time (Fig. S12 and Table S1 in Supporting In- crease the trans-membrane resistance and shorten the water pathway formation). Therefore, the increased water permeation flux can be ra- during nanofiltration process [19,20]. NFMs prepared with nanos- tionalized to the decreased cross-linking degree, which is along with the tructured materials as interlayers or sacrificial layers show a compar- increased co-deposition time of TA/DETA. We suggest the multiple and able or even higher water permeation flux compared to our PP-NFM strong hydrogen bonding between the TA/DETA coatings and the dia- [54–57]. However, these interlayers or sacrificial layers may impact the mine monomers hinder the diffusion of diamine monomers from the structure stability of NFMs and thus limit their practical applications. aqueous phase to the organic phase. And the increased co-deposition During the pressure-driven nanofiltration process, internal stress time leads to increasing the thickness of TA/DETA coatings on the will occur in both the porous substrate and the selective layer of the substrates, which slows the diffusion of diamine monomers and then NFMs. The internal stress in the selective layer is negligible for tradi- reduces the cross-linking degree of the polyamide selective layers. tional NFMs due to the small pore size of the ultrafiltration substrates. The diffusion of diamine monomers was detected by UV–vis spectra However, this internal stress increases dramatically with enlarging the to prove our speculation. MPD was used as the typical diamine pore size of substrates, which may result in break of the selective layer. monomer because PIP shows very low UV absorption. Fig. 5(d) de- Therefore, it is necessary to take the effects of internal stress on the monstrates the diffusion of MPD becomes slow with the increased co- selective layer into account when microfiltration substrates are used for deposition time of TA/DETA. It should be noted that the same ten- the preparation of NFMs. We calculated the internal stress of selective dencies for water permeation flux, selective layer thickness and cross- layers by the theory of thin plates [58]. As schematically shown in linking degree were observed for PP-NFMs with either PIP or MPD Fig. 7(a), we consider the selective layer as a round thin plate covering based polyamide selective layers (Figs. S12–S14 and Table S1 in Sup- on a single round pore. It will have a micro deformation (M) under the porting Information). Furthermore, Fig. 5(e) presents the mixing en- applied pressure during the nanofiltration process, leading to internal ergies between different diamine monomers and substrates. They are stress in the selective layer. There are two extreme cases: simply sup- 55.71 kJ/mol, 56.31 kJ/mol, −197.72 kJ/mol and − 341.38 kJ/mol for ported structure and built-in supported structure. The maximum tensile PIP-PP, MPD-PP, PIP-TA/DETA and MPD-TA/DETA, respectively. The stress for these two cases can be calculated by equations (4) and (5), results mean that the nascent PPMM substrate has the tendency to respectively. “repulse” diamine monomers. Meanwhile, TA/DETA coating has 3(3+ μPR ) 2 mixing energies with diamine monomers below 0, hindering the dif- σs = 8t2 (4) fusion of diamine monomers. Notably, the absolute value of mixing energy of MPD-TA/DETA is larger than PIP-TA/DETA, which means 3PR2 there are strong interactions between MPD and TA/DETA coating. σb = 4t2 (5) These results match well with the decreasing tendency of cross-linking degrees for MPD and PIP based polyamide selective layers (Table S1 in where σs, σb represent the maximum internal stress of the simply sup- Supporting Information). ported and built-in supported structures, respectively, and P, t, R, μ are pressure applied on the membrane, thickness of the selective layer, 3.2. Nanofiltration performance and structure stability of NFMs average pore diameter of substrate and Poisson's ratio, respectively. In

reality, the maximum tensile stress (σmax) of the selective layer is be- Fig. 6 shows a comparison of the nanofiltration performance among tween σs and σb. In this work, the maximum applied pressure P is NFMs prepared under optimized conditions in this work and other ones 0.9 MPa, μ is 0.3 and the average pore diameters of substrates are reported in literatures [15–17,36,41–53]. Most of the traditional NFMs shown in Table 1. Thus, σmax of PP-NFM, PE-NFM and PVDF-NFM can using ultrafiltration membranes as substrates and their water permea- be calculated and the values are shown in Fig. 7(a). These values are tion flux are usually less than 15 L/m2 h bar. Our PP-NFM exhibits a definitely much lower than the tensile strength of polyamide films water permeation flux up to 33 L/m2⋅h⋅bar and maintains a high (usually 70–100 MPa), indicating that the selective layers cannot be

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Fig. 7. (a) Schematic diagrams for the calculation of the internal stress of the selective layer; nanofiltration performances of PP-NFM, PE-NFM and PVDF-NFM under different operation pressures (b–d) and different cross-flow rates (e–g), respectively. (Normalized water permeation flux (F/F0) and salt rejection (R/R0) were used, where F0 and R0 represent for the water permeation flux and salt rejection for NFMs measured with an operation pressure of 0.6 MPa and a cross-flow rate of 30 L/h.). broken during the nanofiltration process even if the applied pressure is traditional NFMs, can be easily swollen in organic solvents. For the as high as 0.9 MPa. In practice, we measured the nanofiltration per- meanwhile, the polyamide selective layer can be hardly swollen in such formances of PP-NFM, PE-NFM and PVDF-NFM under different applied conditions. These different swelling behaviors will cause the detaching pressures. Normalized water permeation flux and salt rejection are of the selective layers from the porous substrates and deteriorates the shown in Fig. 7(b)–(d). For all of our NFMs, the water permeation flux nanofiltration performances of NFMs [16,17]. Typically, PPMM, PEMM increases with the applied pressure while the Na2SO4 rejection main- and PVDFMM are all solvent-resistant substrates [29,31,59]. The sol- tains almost the same values throughout the whole measurements, vent resistance of PP-NFM, PE-NFM and PVDF-NFM were assessed by demonstrating that the selective layer cannot be fractured with en- immersing them in water, ethanol and acetone for 24 h and then hancing the operation pressure. Therefore, we can conclude that our measuring their nanofiltration performances. Fig. 8(a) demonstrates NFMs with microfiltration substrates possess a sufficient structural ro- that PP-NFM, PE-NFM and PVDF-NFM show nearly the same water bustness for the pressure-driven nanofiltration process. It should also be permeation flux and Na2SO4 rejection, indicating their good solvent noted that the as-prepared NFMs have a good adhesion between poly- resistance. For comparison, the water permeation flux increases while amide layers and substrates for nanofiltration evaluation. As shown in salt rejection decreases obviously after 24 h ethanol treatment for PES- Fig. 7(e)–(f), PP-NFM, PE-NFM and PVDF-NFM all maintain a stable NFM (NFMs prepared on PESMM substrates), demonstrating the nanofiltration performance with varying the cross-flow rate from 30 to structure damage of such NFM. Moreover, PESMM was completely 100 L/h (The cross-flow rate reported in literatures is usually 10–30 L/h dissolved in acetone and the structure of PES-NFM is completely de- for testing nanofiltration performances [36,55,57]), indicating that the stroyed, making it can no longer be used in nanofiltration process. polyamide layers cannot be peeled off from the substrate under such a Moreover, PP-NFM, PE-NFM and PVDF-NFM also exhibit stable organic wide range of the cross-flow rate during the tests and confirming a solvent permeation flux (ethanol and acetone) for 12 h operation robust adhesion between the polyamide layers and substrates. (Fig. 8(b) and (c)), which further confirm their good solvent resistance The solvent resistance is equally important for NFMs in industrial and structure stability. Furthermore, these NFMs show stable water applications. However, polysulfone and polyethersulfone ultrafiltration permeation flux and salt rejection after they are immersed in aqueous membranes, which are commonly used as the porous substrates in solution in the pH range of 3–11 for 24 h (Fig. S17 in Supporting

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Fig. 8. (a) Nanoﬁltration performances of PP-NFM, PE-NFM, PVDF-NFM and PES-NFM after immersed by diﬀerent solvents for 24 h; (Normalized water permeation

ﬂux (F/F0) and salt rejection (R/R0) were used, where F0 and R0 represent for the water permeation ﬂux and salt rejection for NFMs immersed in water.) Normalized

(b) ethanol and (c) acetone permeation ﬂux for PP-NFM, PE-NFM and PVDF-NFM tested for 12 h (F0 represents for the permeation ﬂux of NFMs measured when the operation time is 0 and the operation pressure is 0.6 MPa)

Information). It means that our NFMs own good stability in acid or wettability can be improved by TA/DETA coatings for these substrates alkaline solutions. to carry out the interfacial polymerization. The microfiltration substrates have much larger pore size and higher porosity than the ultrafiltration ones, leading to reduced trans-membrane resistance and 4. Conclusions shortened water pathway. These properties greatly improve the water permeation flux of NFMs compared with traditional ones. The NFMs NFMs were prepared on three kinds of commercial hydrophobic show high structure stability and can be used under a wide range of microfiltration substrates by interfacial polymerization. The surface

8 X. Zhang, et al. Journal of Membrane Science 593 (2020) 117444 operation pressures with a stable nanofiltration performance. Thanks to on the permeance of thin film composite membranes: part I. track-etched poly- the good chemical stability of the substrates, the as-prepared NFMs carbonate supports, J. Membr. Sci. 514 (2016) 684–695. [21] Y.-F. Yang, Y. Li, Q.-L. Li, L.-S. Wan, Z.-K. Xu, Surface hydrophilization of micro- show a superior solvent resistance over traditional ones against organic porous polypropylene membrane by grafting zwitterionic polymer for anti-bio- solvents such as ethanol and acetone. Therefore, they are promising to fouling, J. Membr. Sci. 362 (2010) 255–264. be applied in practical applications because of their excellent nanofil- [22] K. Pan, P. Fang, B. Cao, Novel composite membranes prepared by interfacial polymerization on polypropylene fiber supports pretreated by ozone-induced tration performance and structure stability. polymerization, Desalination 294 (2012) 36–43. [23] K. Pan, H. Gu, B. Cao, Interfacially polymerized thin-film composite membrane on Acknowledgements UV-induced surface hydrophilic-modified polypropylene support for nanofiltration, Polym. Bull. 71 (2014) 415–431. [24] H.-Y. Yu, Z.-K. Xu, H. Lei, M.-X. Hu, Q. Yang, Photoinduced graft polymerization of This work is financially supported by the National Natural Science acrylamide on polypropylene microporous membranes for the improvement of Foundation of China (Grant No. 21534009). We are grateful for the antifouling characteristics in a submerged membrane-bioreactor, Separ. Purif. – support of the Research Computing Center in College of Chemical and Technol. 53 (2007) 119 125. [25] H.-Y. Yu, M.-X. Hu, Z.-K. Xu, Improvement of surface properties of poly(propylene) Biological Engineering at Zhejiang University for assistance with the hollow fiber microporous membranes by plasma-induced tethering of sugar moi- dynamic molecular simulations carried out in this work. eties, Plasma Process. Polym. 2 (2005) 627–632. [26] Y. Wang, J.-H. Kim, K.-H. Choo, Y.-S. Lee, C.-H. Lee, Hydrophilic modification of polypropylene microfiltration membranes by ozone-induced graft polymerization, Appendix A. Supplementary data J. Membr. Sci. 169 (2000) 269–276. [27] Q. Yang, J. Tian, Z.-K. Xu, Photo-induced graft polymerization of acrylamide on Supplementary data to this article can be found online at https:// polypropylene membrane surface in the presence of dibenzyl trithiocabonate, Chin. J. Polym. Sci. 25 (2007) 221–226. doi.org/10.1016/j.memsci.2019.117444. [28] A.P. Korikov, P.B. Kosaraju, K.K. Sirkar, Interfacially polymerized hydrophilic microporous thin film composite membranes on porous polypropylene hollow fibers References and flat films, J. Membr. Sci. 279 (2006) 588–600. [29] P.B. Kosaraju, K.K. Sirkar, Interfacially polymerized thin film composite membranes on microporous polypropylene supports for solvent-resistant nanofiltration, J. [1] A.W. Mohammad, Y.H. Teow, W.L. Ang, Y.T. Chung, D.L. Oatley-Radcliffe, N. Hilal, Membr. Sci. 321 (2008) 155–161. Nanofiltration membranes review: recent advances and future prospects, [30] H.I. Kim, S.S. Kim, Plasma treatment of polypropylene and polysulfone supports for Desalination 356 (2015) 226–254. thin film composite reverse osmosis membrane, J. Membr. Sci. 286 (2006) [2] Y. Du, W.-Z. Qiu, Y. Lv, J. Wu, Z.-K. Xu, Nanofiltration membranes with narrow 193–201. pore size distribution via contra-diffusion-induced mussel-inspired chemistry, ACS [31] E.-S. Kim, Y.J. Kim, Q. Yu, B. Deng, Preparation and characterization of polyamide Appl. Mater. Interfaces 8 (2016) 29696–29704. thin-film composite (TFC) membranes on plasma-modified polyvinylidene fluoride [3] Y. Lv, Y. Du, S.-J. Yang, J. Wu, Z.-K. Xu, Polymer nanofiltration membranes via (PVDF), J. Membr. Sci. 344 (2009) 71–81. controlled surface/interface engineering, Acta Polym. Sin. 12 (2017) 1905–1914. [32] T.S. Sileika, D.G. Barrett, R. Zhang, K.H.A. Lau, P.B. Messersmith, Colorless mul- [4] J.S. Lee, J.A. Seo, H.H. Lee, S.K. Jeong, H.S. Park, B.R. Min, Simple method for tifunctional coatings inspired by polyphenols found in tea, chocolate, and wine, preparing thin film composite polyamide nanofiltration membrane based on hy- Angew. Chem. Int. Ed. 52 (2013) 10766–10770. drophobic polyvinylidene fluoride support membrane, Thin Solid Films 624 (2017) [33] D.G. Barrett, T.S. Sileika, P.B. Messersmith, Molecular diversity in phenolic and 136–143. polyphenolic precursors of tannin-inspired nanocoatings, Chem. Commun. 50 [5] M. Shibuya, K. Sasaki, Y. Tanaka, M. Yasukawa, T. Takahashi, A. Kondo, (2014) 7265–7268. H. Matsuyama, Development of combined nanofiltration and forward osmosis [34] X. Zhang, P.-F. Ren, H.-C. Yang, L.-S. Wan, Z.-K. Xu, Co-deposition of tannic acid process for production of ethanol from pretreated rice straw, Bioresour. Technol. and diethlyenetriamine for surface hydrophilization of hydrophobic polymer 235 (2017) 405–410. membranes, Appl. Surf. Sci. 360 (2016) 291–297. [6] Y. El-Rayess, M. Mietton-Peuchot, Membrane technologies in wine industry: an [35] H. Wang, J. Wu, C. Cai, J. Guo, H. Fan, C. Zhu, H. Dong, N. Zhao, J. Xu, Mussel overview, Crit. Rev. Food Sci. Nutr. 56 (2016) 2005–2020. inspired modification of polypropylene separators by catechol/polyamine for Li-ion [7] D. Zhou, L. Zhu, Y. Fu, M. Zhu, L. Xue, Development of lower cost seawater desa- batteries, ACS Appl. Mater. Interfaces 6 (2014) 5602–5608. lination processes using nanofiltration technologies — a review, Desalination 376 [36] X. Zhang, Y. Lv, H.-C. Yang, Y. Du, Z.-K. Xu, Polyphenol coating as an interlayer for (2015) 109–116. thin-film composite membranes with enhanced nanofiltration performance, ACS [8] S. Karan, Z. Jiang, A.G. Livingston, Sub–10 nm polyamide nanofilms with ultrafast Appl. Mater. Interfaces 8 (2016) 32512–32519. solvent transport for molecular separation, Science 348 (2015) 1347–1351. [37] M. Jastrzebska, J. Zalewska-Rejdak, R. Wrzalik, A. Kocot, I. Mroz, B. Barwinski, [9] M. Zargar, Y. Hartanto, B. Jin, S. Dai, Hollow mesoporous silica nanoparticles: a A. Turek, B. Cwalina, Tannic acid-stabilized pericardium tissue: IR spectroscopy, peculiar structure for thin film nanocomposite membranes, J. Membr. Sci. 519 atomic force microscopy, and dielectric spectroscopy investigations, J. Biomed. (2016) 1–10. Mater. Res. A 78 (2006) 148–156. [10] M.J.T. Raaijmakers, N.E. Benes, Current trends in interfacial polymerization [38] M.A. Rahim, H. Ejima, K.L. Cho, K. Kempe, M. Müllner, J.P. Best, F. Caruso, chemistry, Prog. Polym. Sci. 63 (2016) 86–142. Coordination-driven multistep assembly of metal−polyphenol films and capsules, [11] H.F. Ridgway, J. Orbell, S. Gray, Molecular simulations of polyamide membrane Chem. Mater. 26 (2014) 1645–1653. materials used in desalination and water reuse applications: recent developments [39] A. He, C. Zhang, Y. Lv, Q.-Z. Zhong, X. Yang, Z.-K. Xu, Mussel-inspired coatings and future prospects, J. Membr. Sci. 524 (2017) 436–448. directed and accelerated by an electric field, Macromol. Rapid Commun. 37 (2016) [12] A. Rahimpour, M. Jahanshahi, N. Mortazavian, S.S. Madaeni, Y. Mansourpanah, 1460–1465. Preparation and characterization of asymmetric polyethersulfone and thin-film [40] G. Han, S. Zhang, X. Li, N. Widjojo, T.-S. Chung, Thin film composite forward os- composite polyamide nanofiltration membranes for water softening, Appl. Surf. Sci. mosis membranes based on polydopamine modified polysulfone substrates with 256 (2010) 1657–1663. enhancements in both water flux and salt rejection, Chem. Eng. Sci. 80 (2012) [13] W. Zhang, G. He, P. Gao, G. Chen, Development and characterization of composite 219–231. nanofiltration membranes and their application in concentration of antibiotics, [41] B.-W. Zhou, H.-Z. Zhang, Z.-L. Xu, Y.-J. Tang, Interfacial polymerization on PES Separ. Purif. Technol. 30 (2003) 27–35. hollow fiber membranes using mixed diamines for nanofiltration removal of salts [14] A.L. Ahmad, B.S. Ooi, Properties–performance of thin film composites membrane: containing oxyanions and ferric ions, Desalination 394 (2016) 176–184. study on trimesoyl chloride content and polymerization time, J. Membr. Sci. 255 [42] H. Wu, B. Tang, P. Wu, Optimizing polyamide thin film composite membrane (2005) 67–77. covalently bonded with modified mesoporous silica nanoparticles, J. Membr. Sci. [15] H. Tang, J. He, L. Hao, F. Wang, H. Zhang, Y. Guo, Developing nanofiltration 428 (2013) 341–348. membrane based on microporous poly(tetrafluoroethylene) substrates by bi- [43] Y.-J. Tang, Z.-L. Xu, S.-M. Xue, Y.-M. Wei, H. Yang, A chlorine-tolerant nanofil- stretching process, J. Membr. Sci. 524 (2017) 612–622. tration membrane prepared by the mixed diamine monomers of PIP and BHTTM, J. [16] Y. Li, Y. Su, J. Li, X. Zhao, R. Zhang, X. Fan, J. Zhu, Y. Ma, Y. Liu, Z. Jiang, Membr. Sci. 498 (2016) 374–384. Preparation of thin film composite nanofiltration membrane with improved struc- [44] Y. Kyunghwan, S.H. Benjamin, C. Benjamin, High flux nanofiltration membranes tural stability through the mediation of polydopamine, J. Membr. Sci. 476 (2015) based on interfacially polymerized polyamide barrier layer on polyacrylonitrile 10–19. nanofibrous scaffolds, J. Membr. Sci. 326 (2009) 484–492. [17] L. Huang, N.-N. Bui, M.T. Meyering, T.J. Hamlin, J.R. McCutcheon, Novel hydro- [45] G.S. Lai, W.J. Lau, P.S. Goh, A.F. Ismail, N. Yusof, Y.H. Tan, Graphene oxide in- philic nylon 6,6 microfiltration membrane supported thin film composite mem- corporated thin film nanocomposite nanofiltration membrane for enhanced salt branes for engineered osmosis, J. Membr. Sci. 437 (2013) 141–149. removal performance, Desalination 387 (2016) 14–24. [18] H. Strathmann, Synthetic membranes and their preparation, in: M.C. Porter (Ed.), [46] G.S. Lai, W.J. Lau, P.S. Goh, A.F. Ismail, Y.H. Tan, C.Y. Chong, R. Krause-Rehberg, Handbook of Industrial Membrane Technology, Noyes Publications, Park Ridge, S. Awad, Tailor-made thin film nanocomposite membrane incorporated with gra- New Jersey, 1988. phene oxide using novel interfacial polymerization technique for enhanced water [19] G.Z. Ramon, M.C.Y. Wong, E.M.V. Hoek, Transport through composite membrane, separation, Chem. Eng. J. 344 (2018) 524–534. part 1: is there an optimal support membrane? J. Membr. Sci. 415 (2012) 298–305. [47] J. Zhu, S. Yuan, A. Uliana, J. Hou, J. Li, X. Li, M. Tian, Y. Chen, A. Volodin, [20] L. Zhu, W. Jia, M. Kattula, K. Ponnuru, E.P. Furlani, H. Lin, Effect of porous supports B.V. Bruggen, High-flux thin film composite membranes for nanofiltration mediated

9 X. Zhang, et al. Journal of Membrane Science 593 (2020) 117444

by a rapid co-deposition of polydopamine/piperazine, J. Membr. Sci. 554 (2018) 518 (2016) 141–149. 97–108. [54] J.-J. Wang, H.-C. Yang, M.-B. Wu, X. Zhang, Z.-K. Xu, Nanofiltration membranes [48] Y. Zhao, Z. Zhang, L. Dai, S. Zhang, Preparation of a highly permeable nanofiltra- with cellulose nanocrystals as an interlayer for unprecedented performance, J. tion membrane using a novel acyl chloride monomer with -PO(Cl)2 group, Mater. Chem. 5 (2017) 16289–16295. Desalination 431 (2018) 56–65. [55] Y. Zhu, W. Xie, S. Gao, F. Zhang, W. Zhang, Z. Liu, J. Jin, Single-walled carbon [49] C. Wang, Z. Li, J. Chen, Y. Zhong, Y. Yin, L. Cao, H. Wu, Zwitterionic functionalized nanotube film supported nanofiltration membrane with a nearly 10 nm thick “cage-like” porous organic frameworks for nanofiltration membrane with high ef- polyamide selective layer for high-flux and high-rejection desalination, Small 12 ficiency water transport channels and anti-fouling property, J. Membr. Sci. 548 (2016) 5034–5041. (2018) 194–202. [56] M.-B. Wu, Y. Lv, H.-C. Yang, L.-F. Liu, X. Zhang, Z.-K. Xu, Thin film composite [50] Z. Tan, S. Chen, X. Peng, L. Zhang, C. Gao, Polyamide membranes with nanoscale membranes combining carbon nanotube intermediate layer and microfiltration turing structures for water purification, Science 360 (2018) 518–521. support for high nanofiltration performances, J. Membr. Sci. 515 (2016) 238–244. [51] F. Xiao, B. Wang, X. Hu, S. Nair, Y. Chen, Thin film nanocomposite membrane [57] Z. Wang, Z.X. Wang, S. Lin, H. Jin, S. Gao, Y. Zhu, J. Jin, Nanoparticle-templated containing zeolitic imidazolate framework-8 via interfacial polymerization for nanofiltration membranes for ultrahigh performance desalination, Nat. Commun. 9 highly permeable nanofiltration, J. Taiwan Inst. Chem. Eng. 83 (2018) 159–167. (2018) 2004. [52] S.-M. Xue, C.-H. Ji, Z.-L. Xu, Y.-J. Tang, R.-H. Li, Chlorine resistant TFN nanofil- [58] S. Timoshenko, S. Woinowsky-Krieger, Theory of Plates and Shells, McGraw-Hill tration membrane incorporated with octadecylamine-grafted GO and fluorine- Book Company, Inc, 1959. containing monomer, J. Membr. Sci. 545 (2018) 185–195. [59] S.-H. Park, S.J. Kwon, M.G. Shin, M.S. Park, J.S. Lee, C.H. Park, H. Park, J.-H. Lee, [53] X. Kong, Y. Zhang, S.-Y. Zeng, B.-K. Zhu, L.-P. Zhu, L.-F. Fang, H. Matsuyama, Polyethylene-supported high performance reverse osmosis membranes with en- Incorporating hyperbranched polyester into cross-linked polyamide layer to enhanced mechanical and chemical durability, Desalination 436 (2018) 28–38. hance both permeability and selectivity of nanofiltration membrane, J. Membr. Sci.