Modification of Supramolecular Membranes with 3D Hydrophilic Slide

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Surfaces, Interfaces, and Applications

Modification of supramolecular membranes with 3D hydrophilic sliderings for improvement of antifouling properties and effective separation Sisi Ma, Ligang Lin, Qi Wang, Yuhui Zhang, Honglei Zhang, Yixin Gao, Lin Xu, Fusheng Pan, and Yuzhong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08865 • Publication Date (Web): 12 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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ACS Applied Materials & Interfaces

Modification of supramolecular membranes with 3D hydrophilic slide-rings for improvement of antifouling properties and effective separation Sisi Maa, Ligang Lin*a, Qi Wanga, Yuhui Zhanga, Honglei Zhanga, Yixin Gaoa, Lin Xua, Fusheng Panb, Yuzhong Zhanga aState

Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, P.R.China

bKey

Laboratory for Green Chemical Technology of Ministry of Education, School of

Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China *Corresponding author: Ligang Lin, E-mail: [email protected]

Abstract: A three-dimensional (3D) strategy for the fabrication of ethylene-vinyl alcohol (EVAL) membranes with a dynamic surface was developed based on sliding supramolecular polymer brushes (SSPB). The SSPB with a 3D hydrophilic structure were introduced into the alkyne-EVAL membrane matrix via an azide-alkyne click coupling reaction. The self-mobile hydrophilic slide-rings in the SSPB provided a proactive exclusion system. This resulted in reduced direct contact of the membrane surface with multiple pollutants such as bovine serum albumin (BSA) and oil droplets. The EVAL-SSPB membrane demonstrated increased surface hydrophilicity, underwater oleophobicity, and antifouling properties. More importantly, the abundant hydrophilic rings in the membrane matrix result in supramolecular assembly and efficient hydrophilic sliding channels. This resulted in a dramatic increase in the water 1

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flux (2000 L/m2h), while retaining a 96% rejection of BSA and oil/water emulsions. The results of the study indicate that three effects of the cyclodextrins (CD) rings, i.e., the hydrophilic effect, the exclusion effect, and the sliding effect, enabled the improved membrane performance. The demonstrated 3D fabrication strategy is versatile, facile and scalable, which allows for its application to various other membranes. The fabricated materials possess excellent permeability and separation efficiencies, which make them attractive candidates for use as separation membranes with novel functions. Keywords: membrane, sliding brush, supramolecular, oil/water, separation, antifouling 1. Introduction With the continued development of global industries, the growing amount of wastewater and number of oil spills critically threaten our environment.1-5 In the coming decades, the global demand for clean water is expected to rapidly increase with population growth. For water treatment, membrane separation processes are especially promising due to their clean operation, cost-effectiveness, simplicity, low energy consumption, and for being free of harmful by-products.6,7 However, conventional membrane separation usually suffers from several issues, such as relatively poor permeation and membrane fouling, which result in low filtration efficiency.8 Importantly, in membrane based filtration technologies, there is generally a trade-off between the flux and separation, resulting in unsatisfactory separation 2


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performance.9 Hence, more effective, low-cost, and facile wastewater treatment are still urgently required. Ultrafiltration (UF) membranes, such as those widely applied for water treatment, biological sciences, and chemical engineering, are prepared via the phase inversion method.10 In recent years, many methods to modify membranes have been employed, which include chemical grafting11, surface coating12, and blending with additives13. Unfortunately, while these conventional modifications have been enhancing the membrane antifouling properties, they are often accompanied by the drawback of decreased water flux. High permeation is the key factor for separation efficiency; hence, further surface engineering of the membranes is required to improve both the flux and the membrane’s resistance to foulant deposition and adhesion.14,15 In this work, we aim to effectively balance the trade-off between permeability and selectivity for the development of appropriate membranes for wastewater treatment. Ethylene vinyl alcohol (EVAL) is a hydrophilic membrane material with abundant hydroxyl groups that has excellent hydrophilic, mechanical and antifouling properties.16,17 These hydroxyl groups can be easily modified and grafted. Cyclodextrins (CDs) are a family of macrocyclic supra-amphiphiles with a 3D architecture composed of a hydrophilic rim and an oleophilic cavity.18,19 As a sliding supramolecular polymer brush, polyrotaxane (PR) has a long linear structure with numerous freely sliding and rotating macrocyclic molecules (α-CDs) along the linear polyethylene glycol (PEG) backbone.20-23 To the best of our knowledge, the studies of 3



3D highly hydrophilic membranes with sliding supramolecular polymer brushes (SSPB) for membranes separation applications have not been reported. In this study, we propose new 3D strategies using supramolecular materials for the treatment of wastewater. A novel supramolecular membrane with self-mobile hydrophilic rings was designed to construct a dynamic surface with a highly permeable antifouling layer, as illustrated in Figure. 1.

Figure. 1. (A) Schematic illustration of EVAL-SSPB membrane, (B)(C) Schematic demonstration of the oil/water separation process by the EVAL-SSPB membrane. Several supramolecular machines have been previously demonstrated for the construction of self-mobile structures such as molecular motors, molecular elevator, and as molecular muscles to complete sophisticated biological tasks.24-27 As a supramolecular membrane, the 3D hydrophilic and self-mobile structure of the SSPB provides greatly aids to the engineering of membrane modifications. Since the 4


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oleophilic cavity structure of the CDs were threaded onto the PEG chains, only the hydrophilic property of the α-CDs rim structure was retained. The anchored SSPB introduces a high density of hydrophilic groups on the surface and pore channels. The α-CDs contain a large amount of self-mobile hydroxyl groups, which are called slide-rings (Figure. 1A). During the phase inversion process, multiple slide-rings concentrate on the outermost membrane surfaces to form dense polymer brushes structure (Figure. 1C). This serves as a steric barrier that prevents the adhesion of oil droplets and proteins on the membrane surface. Unlikely the conventional static and inert membrane separation, a dynamic ‘move and remove’ separation (Figure. 1B) could be effectively exploited.28 Under these supramolecular conditions with hydrophilic slide-rings simultaneously achieves the proactive exclusion of oil molecules and the capture of water molecules. In addition, the achievement of a higher separation is also simultaneously desired. The hydrophilic slide-rings also endows it with self-cleaning properties and efficient removal.29,30 The SSPB inside the membrane changed its morphology during fabrication and provided a smooth transport of water, thus, increasing the water flux. In addition, the SSPB with its unique 3D structure on the membrane surface is also expected to enhance surface roughness (Figure. 1C). Given the above-mentioned benefits, the designed SSPB is expected to show higher selectivity and repulsion due to its hydrophilic/underwater oleophobic properties and dynamic surface. Click chemistry has been a powerful tool for material modification.31-33 The 5



azide-alkyne click coupling was used to prepare the SSPB. The triazole ring structure could be formed by the reaction between azide and alkyne. This triazole ring structure could be effectively connected two different polymers together. The SSPB was designed using azide-alkyne click chemistry into the clickable EVAL membrane matrix to form EVAL-SSPB membranes with a stable cross-linked network structure. The molecular structure and morphology of the membranes as well as the surface wettability and roughness were assessed and compared. Finally, the separation performance and rejection mechanisms were investigated through the filtration of an oil-in-water emulsion. 2. Experimental 2.1. Materials Ethylene vinyl alcohol (EVAL) was purchased from Kuraray chemicals manufacture company. Polyethylene glycol (PEG), cyclodextrins (α-CD, β-CD), sodium lauryl sulfate (SDS), dimethyl sulfoxide (DMSO), propargyl bromide (C3H3Br), triethylamine (TEA), p-toluene sulfonyl chloride (TOSCl, C7H7ClO2S), sodium azide (NaN3) and 3,5-dimethylphenol (DMAP, C7H10N2) were obtained from Sinopharm chemical reagent co. ltd., China. Other reagents were provided from Shanghai aladdin chemistry co. ltd., China. Dichloromethane (CH2Cl2) was dried before use. 2.2. Synthesis of β-CD-N3-OTS The detailed preparation process of β-CD-N3 was provided in our previous

6


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work.34 Briefly, β-CD and C7H7ClO2S were dissolved in deionized water to form a homogeneous solution. Then, aqueous sodium hydroxide was added into the solution dropwise. The filtrate was collected. Then, neutralize the filtrate to pH 8.5 with excessive ammonium chloride (NH4Cl) to induce precipitation and get the β-CD-OTS product. In the next step, β-CD-OTS and NaN3 were dissolved and stirred in deionized water at 80 °C overnight. The filtrate was collected, and the tetrachloroethane was added to induce precipitation. The prepared β-CD-N3 was dried under vacuum at 60 °C. 30 g of β-CD-N3, 6.0 g of p-toluene sulfonyl imidazole and 350 mL deionized water were mixed and stirred in a three-necked round bottom flask at room temperature for 3 h. 21 mL of sodium hydroxide solution (40 wt %) was added in the above solution and the reaction mixture was slowly clarified after cooling. NH4Cl was added to mixture solution to induce white precipitation (β-CD-N3-OTS). 2.3. Synthesis of SSPB (PR-N3) Firstly, 0.5 g of alk-PEG-alk and 5 g of α-CD were ultrasonically dissolved in 50 mL deionized water for 30 min and then stirred for 16 h to form pseudo-PR. Then, 0.94 g of β-CD-N3-OTS (the molar ratio of β-CD-N3-OTS to alk-PEG-alk was 4:1) were dissolved in a solution, which contained CuSO4•5H2O (0.16 mmol), pentamethyl diethylenetriamine (PMDETA, 35 μL) and sodium L-ascorbate (0.32 mmol) under nitrogen atmosphere for 2 h in the darkness. CuSO4•5H2O acts as a catalyst in the azide-alkyne click reaction. After centrifugation, the precipitate was dissolved in 7



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DMSO and reprecipitated in deionized water. The precipitates were separated by filtration and washed with water. PR-OTS powers were dried under vacuum at 65 °C. In the next step, PR-OTS was mixed in 40 mL of DMAC/9%LiCl. 1g of NaN3 was dissolved in 10 mL DMAC and then slowly added in mixed PR-OTS solution at 0 °C under nitrogen atmosphere. The reaction solution was stirred overnight at 25 °C, and

precipitated

in

ethanol.

The

precipitates

were

dissolved

in

N,

N-dimethylformamide and reprecipitated in ethanol and ether. The brown PR-N3 powders were obtained after dried under vacuum. 2.4. Fabrication of EVAL-SSPB membrane As shown in Figure. 2, EVAL-SSPB membranes were fabricated via a chemical coupling technique based on azide-alkyne click chemistry between the azide-treated PR (PR-N3) and the alkyne-treated EVAL (EVAL-alk). Firstly, 5 g of purified EVAL was dissolved in 50 mL DMSO to form a homogeneous solution. Then, 3.11 g of anhydrous potassium carbonate and 1.78 g of a C3H3Br solution were added into the solution. After stirring at 60 °C for 6 h, the EVAL-alkyne solution was obtained. Then, CuSO4•5H2O, PMDETA, PR-N3, and sodium L-ascorbate were subsequently added into the above solution under a nitrogen atmosphere. After stirring at 80 °C for 15 h, the casting solution was obtained. After the removal of air bubbles under vacuum conditions for 10 h, the obtained casting solution was cast onto a glass plate for a film. The film and glass plate were immersed in a pure water bath at 25 °C. During this process, solvent exchange occurred and the solidified membrane formed with porous 8


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structure. To ensure that all of the solvent in the membrane structure was removed, membranes were immersed in the water bath for 24 h.

Figure. 2. Schematic diagram of the SSPB formation strategy via the azide-alkyne click coupling reaction. 2.5. Preparation of various oil-in-water emulsions A milky emulsion of 2 wt % with high stability was prepared using a certain amount of SDS as emulsifier under vigorous mixing for 6 h.35 A series of oil-in-water emulsions containing different oil components such as silicone oil, hexadecane, soybean oil and rapeseed oil were fabricated. The prepared emulsions and filtrate were imaged by optical microscopy (BX51TRF, Japan). 2.6. Characterization Fourier-transform infrared spectroscopy (FTIR, Bruker Daltonics Inc., Germany) and X-ray photoelectron spectroscopy (XPS) spectrometry were used to study the chemical structure and composition. Scanning electron microscopy (SEM, Hitachi 9



S-4800, Japan) was used to observed the surface morphologies and pore channels of the membranes. The mechanical strength was measured by tension machine (LLY-06F) with tensile speed: 20mm/min, temperature: 25 °C and humidity: 45%. Energy-dispersive X-ray spectroscopy (EDX) was employed to characterize the elementary composition and distribution of the membranes. 3D roughness images of the surface of the membranes were recorded by atomic force microscopy (AFM). Hydrophilicity of different membranes was characterized by dynamic contact angle (CA) measurements at more than five different positions of the same sample at 25 °C using the sessile drop method on a contact angle meter (Drop shape analyzer 100, KRÜSS GmbH Co., Germany). Oil/water separation efficiency was measured by total organic carbon (TOC) measurement. 2.7. Permeability, rejection and antifouling measurements The flux and antifouling permeance of the membranes were measured by cross-filtration equipment under the feed pressure of 0.1 MPa supplied by a water pump at 25 °C. The fluxes (Jw1) can be calculated according the Eq. (1).

Where Jw1 is the pure water flux of the membrane (L/m2h), V1 is the permeate volume (L), A is active area of the membranes (m2) and t is the operated time (h). The rejection permeance was performed by a dead-end filtration device under 0.1 MPa. The wastewater was filtered through the membrane, and the filtrate was collected. At the same time, the initial concentration and the concentration of 10


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oil/water emulsion were measured by TOC values. The initial concentration and the concentration of BSA were measured by an UV-spectrophotometer. The fluxes (Jo or JB) and rejection (Ro or RB) were defined using the following Eqs. (2-5), respectively.

Where Jo or JB is the oil/water emulsion or BSA flux (L/m2h), V2 and V3 are the permeate volume (L) of oil and BSA, respectively. TOC1 is the initial concentration, TOC2 is the concentration of oils after filtration. C1 and C2 are the concentration of BSA before and after experiment, respectively. To determine the antifouling properties of the membranes, the flux recovery rate (FRR) and decline rate (FDR) were calculated by Eq. (6) and Eq. (7).

Where JW1 and JW2 are the pure water flux of the membranes before and after fouling experiment, respectively. 2.8. Porosity and pore size measurements The porosity (ε) of the membranes was measured by the sample gravimetric method. The dry sample was weighed and immersed in pure water for 30 h. Then, the sample was wiped with filter paper lightly to remove residual water on the membrane 11



surface. Finally, the wet membrane was quickly weighed. The porosity (ε) value was calculated according to the Eq. (8): ε=

𝑊𝑎 −𝑊 𝑏 𝑆𝑑𝜌

(8)

× 100%

Where ε is the porosity of the membrane, Wa is the mass of membrane under wet state (g), Wb is the mass of dry membrane (g), S, d and ρ are the membrane sample area (cm2), thickness (μm) and the water density (1 g/mL), respectively. 3. Results and discussion 3.1 Characterization of the clickable functional brushes (PR-N3) The FT-IR spectra of β-CD and the clickable functional brushes (PR-N3) are shown in Figure. 3. Compared to the raw β-CD (Figure. 3A), the characteristic peaks observed at 1365 cm-1 and 1175 cm-1 can be assigned to the S=O groups. The peak at 1599 cm-1 and 814 cm-1can be attributed to the C=C, =C–H groups in the benzene ring. These new peaks indicate the introduction of the methyl benzene sulfonic groups in β-CD-OTS. From Figure. 3B, the peaks corresponding to the stretching vibration of the azide groups (-N3) at 2040 cm-1 and 2108 cm-1 can be clearly observed in β-CD-N3. From Figures. 3A and B, it can be seen that most of the characteristic peaks in the β-CD-OTS framework i.e., 1365 cm-1 (S=O), 1175 cm-1 (S=O), 1599 cm-1 (C=C), and 815 cm-1(=C-H), are retained in the β-CD-N3-OTS. These results indicate that the synthesis of the clickable β-CD-N3-OTS was successful. As shown as Figure. 3C, for the PR-OTS spectrum, the peaks for the S=O, C=C, and =C–H groups were clearly observed. These were typical characteristic peaks of the methyl benzene 12


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sulfonic groups. Compared with PR-OTS for PR-N3, a new strong peak at 2103 cm-1 could be ascribed to the stretching vibration of the azide groups (-N3). This indicated that the clickable functional brushes (PR-N3) were successfully synthesized. Compared with the unmodified EVAL, EVAL-alk can be clearly seen from Figure. 3D that the peaks at 2193 cm-1 and 2252 cm-1 correspond to the stretching vibration of the alkyne groups (-alk). The peak at 1112 cm-1 can be attributed to the stretching vibration of C-O-C groups in EVAL-alk. This shows that an esterification reaction between the hydroxyl groups of EVAL and the propargyl bromide groups takes place.

Figure. 3. Synthesis of the clickable PR-N3functional brushes: FT-IR spectra of (A) β-CD and β-CD-OTS; (B) β-CD-N3 and β-CD-N3-OTS; (C) PR, PR-OTS and PR-N3; (D) EVAL and EVAL-alk. 13



To give further analysis on the synthesis of the clickable functional brushes (PR-N3), the characteristic peaks of PEG, β-CD and α-CD in the SSPB are presented in Figure. 4. And the peaks are assigned names as follows for clear analysis: 5.71 (s, C2OH, C3OH of β-CD, α-CD), 4.47 (d, C1H of β-CD), 4.45 (s, C6OH of α-CD, β-CD), 3.30–3.60 (m, overlapped C3H, C6H, C5H, C2H, C4H of β-CD and α-CD, CH2CH2O of PEG). 36

Figure. 4. 1H NMR spectra of SSPB (PR-N3) 3.2 Characterization of the membranes 3.2.1 FT-IR and XPS analysis of the EVAL-SSPB membrane In order to confirm the successful synthesis of the EVAL-SSPB membrane, its chemical composition was characterized by FT-IR and XPS. The EVAL polymer is expected to form the membrane matrix. To fabricate the membranes with SSPB through efficient click chemistry, the EVAL was activated by introducing alkyne groups onto the polymer chains. The FT-IR and XPS results for the unmodified 14


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EVAL, the prepared clickable EVAL-alk, and the EVAL-SSPB membranes are shown in Figure. 5A. The characteristic peaks at 1650 cm-1 and 1036 cm-1 were assigned to the symmetrical stretching vibration of the N=N and C-N groups in triazole, respectively. The presence of the triazole peaks confirmed the successful fabrication of the EVAL-SSPB membrane through the azide-alkyne click reaction. The XPS full-scan spectra shown in Figure. 5B confirm the results of the surface elemental analysis of the membranes. Strong XPS peaks can be clearly observed at 285.1 eV (C 1s) and 531.9 eV (O 1s), which can be attributed to the typical groups on the EVAL chains. Compared with the unmodified EVAL membrane, the EVAL-SSPB membrane presented a new peak at 400.1 eV (N 1s), which could be assigned to the triazole structure from the azide-alkyne click reaction. From Table 1, we can see that the content of nitrogen (N 1s) increased to 2.54%, indicating that SSPB successfully grafted onto the EVAL chains. The XPS results were consistent with the FT-IR results.

Figure. 5. (A) FT-IR and (B) XPS spectra of (1) EVAL membrane, (2) EVAL-alk membrane and (3) EVAL-SSPB membrane. 15



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Table 1 The XPS results for the EVAL, EVAL-alk, and EVAL-SSPB membranes Membrane C1S[mol%] O1S[mol%] N1S[mol%] nN/nO [mol%]/[mol%] EVAL

74.42

24.32

0

0

EVAL-alk

76.29

21.69

0

0

EVAL-SSPB

71.62

23.94

2.54

0.11

3.2.2. EDX analysis on EVAL-SSPB membrane surface EDX mapping was used to help characterize the elemental distribution of the modified EVAL membranes. Figure. 6B shows the EDX mapping of the membrane surface. More N elements are present through the triazole ring attached to the “adhesive” membrane surface during the azide-alkyne click reaction. These results are consistent with the FT-IR and XPS analysis. The higher O elements content stems from the large number of α-CDs attached to the SSPB. This can clearly be observed by comparing the surface of the EVAL-SSPB membrane to the EVAL membrane. The results further indicate the successfully modification of the surface of the EVAL membranes with SSPB. The increasing O content is believed to play an important role in improving the hydrophilic properties of the membrane.

16


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Figure. 6. (A) SEM and (B) EDX mapping images of the EVAL surface and the EVAL-SSPB membranes. Red and green dots indicate the signals for N and O, respectively. 3.2.3. Morphological and mechanical characterization The morphologies of the membranes were investigated by SEM and AFM. As shown in Figures. 7A1-C1, a coarse surface with micro/nano-particles was observed on the EVAL-SSPB membrane. Due to this surface is hydrophilic, hydrated water can be filled in the “valleys” on the surface. Appropriate uneven surface can enhance membrane surface roughness to achieve underwater superoleophobic properties. This morphology is favorable for reducing the adsorption of oil foulants on the membrane surface. The differences between the bottom surface of the EVAL and EVAL-SSPB membrane can clearly be seen in Figures. 7A2, C2. In this work, octanol is chosen added to introduce pores into EVAL-SSPB membrane structure. Higher porosity was shown in Figure. S1 with the addition of octanol. As shown in Figure. 7C1, EVAL membrane exhibits a typical finger-like pore channel structure due to the high mutual 17



diffusivity of the coagulation bath (water) and the DMSO during phase inversion process. The magnified cross-sectional images (Figure. 7C3) of the EVAL-SSPB membrane show that the 3D honeycomb porous channels are composed of many nanopores, which provides the necessary smooth channels for feed through the membranes. The results show that the SSPB has an influence on the structure and size of the membranes pore.

Figure. 7. Morphological analysis of (A) EVAL and (B) EVAL-SSPB membranes. (A1–B1, top surface; A2–B2, bottom surface; A3–B3, cross-section; C1–C3, high magnification). The pore size distribution of EVAL and EVAL-SSPB membranes were detected by Mercury intrusion porosimeter (MIP, Auto Pore IV 9510, America), as presented 18


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in Figure. 8. The pore size of EVAL membrane was about 0.71 μm. It adequately decreasing to 0.28 μm and have microcavities (which ranges from 1-4 μm) after modified with octanol and SSPB. It likely fabricated by the synergistic effects of octanol and SSPB due to their solubility difference in the coagulation bath (water) during phase inversion process. The microcavities in the EVAL-SSPB membrane was due to the pore-forming action of octanol. This phenomenon indicated that EVAL-SSPB membrane pore was composed of 3D honeycomb porous channels with different sizes. The result was consistent with SEM observations. This honeycomb porous structure could lead to greater absorption of water, at the same time, smaller pores have good rejection effect of hydrophobic pollutants. These different finger like sub-structure and sponge pore channels are expected to demonstrate better permeation and a high working efficiency.

Figure. 8. Pore size distribution of the membranes As shown on Figure. S2, EVAL-SSPB membrane sustains the tensile strength and its elongation percentage increase to 55%. The obviously increase in elongation is due to form a cross-linked network in EVAL membrane matrix. It shows an 19



outstanding elasticity, which is enough for a typical UF/MF separation. The surface roughness of the EVAL membrane before and after azide-alkyne click coupling were characterized via AFM. The brighter and darker regions show the ridges and valleys on the surface of the membranes, respectively. The AFM images (Figure. 9A and B) show that the surface of EVAL membrane was smoother; the gaps between the valleys and ridges were not obvious. In contrast, the protrusions that appeared on the EVAL-SSPB surface and the obvious ridges and valleys resulted in the high roughness for the membrane surface (Figure. 9C and D). It was obvious that the surface roughnesses of EVAL and EVAL-SSPB membranes were 0.378 and 0.481 μm, respectively. These observations confirmed the SEM results that the “ridges” on the surface were likely covered by SSPB. It proved by above XPS (Figure. 5B) and SEM-EDX element distribution (Figure. 6). The AFM images revealed that the higher roughness of the EVAL-SSPB membrane surface was probably due to the ability of the SSPB to form 3D cross-links with the EVAL chains. The AFM result correlated well with the morphologies observed using the SEM images.

20


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Figure. 9. AFM topographies of (A) (B) EVAL and (C) (D) EVAL-SSPB membranes. 3.2.4. Wetting Behaviors of the membranes Surface wettability plays a key role in determining the separation properties of a membrane, include its oil/water demulsification ability.37 The hydrophilicities of the EVAL, EVAL-octanol, and EVAL-SSPB membranes were characterized by water contact angle (WCA) measurements and is presented in Figure. 10. Generally, the WCA value less than 90° is hydrophilic, otherwise hydrophobic. The WCA of the pure EVAL membrane was 85° (Figure. 10), reflecting its hydrophilic nature of it. Compared with EVAL membrane, it is can be seen that the WCA value for the EVAL-SSPB membrane decreased to 0° in a very short period. When water droplet was contacted with the surface, water droplet can spread due to surface hydrophilic characteristics. Then, water droplet was beginning to permeate the membrane matrix 21



due to its higher hydrophilic pore channel. The WCA values are highly related to the SSPB click coupling content. Since the α-CDs are hydrophilic on the outside, on increasing the SSPB content on the membrane surface, the hydrophilicity can be enhanced. This is due to the abundant -OH hydrophilic groups of α-CDs on the PR structure.38,39 It is well known that the hydrophilicity is related to the surface roughness and the surface chemical properties. According to the theory of Cassic: higher surface roughness will be easier to wet because of its larger area of action and larger surface tension.40 It decreases the WCA, when the WCA of the surface is less than 90°. The higher surface roughness aids the hydrophilicity of the surface.

Figure. 10. Water contact angles (CA) of EVAL, EVAL-octanol, and the EVAL-SSPB membranes. The oleophobicity is an important parameter of the membranes for oil-water separation.41 To investigate the oleophobicity of the membranes, the underwater oil contact angle (OCAs) and oil adhesion forces of the membranes were measured for 22


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the following organic solvents dyed with Sudan red, 1,2-dichloroethane, trichloro methane, and oil (Figure. 11). Interestingly, the EVAL-SSPB membrane wet with water exhibited a rapid slippery behavior when an oil droplet was contacted with the surface (SI Video 1). All the repeated experiments also showed no significant oil adhesion to the EVAL-SSPB surface (Figure. 11A). The detachment of oil droplets from the surface of the EVAL-SSPB membrane by water can be attributed to the low adhesion force between the SSPB and the oil which results in the oil droplets sliding down the surface. In contrast, oil droplets on the EVAL surface maintained a semispherical shape and did not move during the long-time measurement (Figure. 11B). For the EVAL membrane, it was difficult to remove the adhered oil from the surface, even on holding the membrane vertically or by vigorous shaking of the membrane (SI Video 1). It can be seen that the EVAL membrane has an OCA of less than 90° underwater. However, as shown in Figure. 11D, the oil droplet maintains a spherical shape on the EVAL-SSPB surface with the OCA greater than 150°, confirming its underwater superoleophobic properties. A high-water content can be trapped in the rough surface of the EVAL-SSPB membrane. This results in the formation of a repulsive water layer that impedes oil contact with the surface, which significantly reduces the contact area and leads to high OCA values. At the same time, since the oil droplets could not adhere to the surface, they roll back and forth when the membrane was gently shake (SI Video 2). The spontaneous oil/water separation is driven by the high affinity of the EVAL-SSPB membrane surface to water molecules 23



Page 24 of 39

and low adhesion of oil to the membrane due to the slide-rings. After the cross-filtration tests, the membranes were cleaned with DI-water for 2 h and the images of the EVAL and EVAL-SSPB membrane before and after oil fouling are shown in Figure. 11C. The EVAL membrane appears to be less clean and shows a large amount of red dyed oil deposited on the surface, while the surface of EVAL-SSPB membrane revealed less fouling. This is because the hydration layer can not only significantly alleviate the interactions between oil and the membrane, but the self-mobile slide-rings also efficiently prevent oil deposition on the surface. The results

mentioned

above

confirm

that

the

SSPB

with

slide-rings

has

hydrophilic/oleophobic and antifouling characteristics; the resulting EVAL-SSPB membrane can be employed for the separation of oil/water emulsions.

Figure. 11. Digital photographs showing the slippage of the oil droplet from the (A) EVAL-SSPB surface and (B) EVAL membrane; photographs of (C) the membranes before and after oil fouling, and (D) the OCA of EVAL-SSPB membrane. 24


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3.3. Separation of BSA and oil-water-emulsion The separation and antifouling performance of the EVAL, EVAL-octanol, and EVAL-SSPB membranes were investigated using bovine serum protein (BSA) and an oil/water emulsion. As shown in Figure. 12A, the FDR values for the EVAL-octanol and the EVAL-SSPB membranes were calculated to be 32.1% and 26.6%, respectively. After cleaning with water to eliminate membrane surface fouling, 81% of the pure water flux was recovered for the EVAL-SSPB membrane while only 67.1% was recovered for the EVAL-octanol membrane. As shown in Figure. 12B, the EVAL-SSPB membrane had a higher BSA solution flux (630 L/m2h) than both the EVAL (54 L/m2h) and EVAL-15%octanol membranes (80 L/m2h). Furthermore, the EVAL-SSPB membrane had a greater rejection approaching 100%. Hence, the results demonstrate that the EVAL-SSPB membranes possess superior antifouling performance. Figure. S3 shows the total organic carbon (TOC) values of the oil concentration before and after separation with the EVAL-SSPB membrane. It can be seen that the color of the filtrate becomes colorless and transparent and the TOC values were close to the value of DI-water, but lower than that of the feed. From the optical microscopy images in Figure. S3, it can be seen that the oil droplets are scattered in the feed; after separation, almost no oil and emulsion droplets could be observed. Compared to the high TOC values in the feed, the oil rejection was still higher than 95%, indicating the effective oil/water separation performance of the EVAL-SSPB membrane. 25



Hydrophilic/underwater superoleophobic layers promote the water molecules pass through the membrane matrix rapidly but reject oil molecular. It is hard for the oil molecules of the emulsion to get through the hydrophilic brush layer, which results in that mass of oil molecules accumulate into larger size, and causes the demulsification of surfactant-stabilized water-in-oil emulsions. The SSPB chains with sliding and rotating rings are helpful in the formation of the unstable and movable regions on the membrane surface, which can reduce the oil molecules adhesion. SSPB contains many hydrophilic/oleophobic rings, it can create more water molecular binding sites and create stronger hydration structure to avoid the direct contact between oil and membrane surface. The polyhydroxy structure is beneficial to the infiltration of water molecules. It simultaneously achieves the proactive exclusion of oil molecules and the capture of water molecules. For the oil/water emulsion separation membrane, the antifouling performance and the flux are of greater important than rejection themselves.42,43 To maintain the high fluxes for the different membranes, EVAL-40% octanol and EVAL-alk membranes were fabricated in this work to the observe oil removal rate and flux variability compared to the EVAL-SSPB membrane (SI Video 3). After filtration, there was no change in the color of the filtrate, suggesting that the oil and SDS still remained in the filtrate (EVAL-octanol and EVAL-alk membranes). At the same time, the fluxes of both membranes exhibited a sharp decline. This can be attributed to the unfiltered oil droplets gradually adhering and depositing on the membrane pore, 26


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which leads to blockage (Figure. 12C). However, using the EVAL-SSPB membrane the filtrate became very clear, demonstrating that the oil and SDS had been successfully removed from the emulsion. Considering that the water flux and oil-water-SDS flux of EVAL-SSPB membrane is higher than other membranes, it can be concluded that EVAL-SSPB membrane shows the best oil/water emulsion separation performance among all the membranes investigated in this work. This indicates that the honeycomb porous structure and surface properties enhanced the flux of EVAL-SSPB membrane obviously with not decreasing the rejection. Namely, EVAL-SSPB membrane could break the ‘‘trade-off effect” effectively. This can be attributed to the preferential adhere of water molecules onto the hydrophilic rotaxanes for the increased water transport, and a dynamic surface with the lower adhesion force between the SSPB and the BSA, oil molecules. The increased hydrophilicity of the modified membrane surface reduced water resistance through the membrane, resulting in an increase of the water flux.44 As described in Figure. 12C, simple water washing leads to an almost complete recovery of the initial flux (FRR = 97–99%). The EVAL-SSPB membrane shows almost no decline in the water flux, better anti-oil-fouling performance and oil rejection (Figure. 12D) during the cyclic oil-water-SDS separation. This attributes to the sliding and rotating rings which can shove the oil and BSA molecules and decrease the membrane fouling. The surface layer has a large excluded volume that reduces the direct contact of the membrane surface with oil and proteins. 27



Page 28 of 39

Figure. 12. Variation of the permeate water flux through different membranes with time during separation (A) BSA and (C) soya oil-water emulsion; (B) BSA flux and BSA rejection of the membranes; (D) soya oil-water emulsion flux and oil rejection of the different membranes. Additionally, the stability of membrane performance plays an important role in practical oil/water separation, and the results are shown in Figure. 13. After 15 cycles, the oil rejection of EVAL-SSPB membrane is still greater than 95 %, and the underwater

super-oleophobic

properties

are

maintained,

demonstrating

the

membrane’s excellent stability and recyclability in separating SDS stabilized oil-in-water emulsion owing to the stability of the azide-alkyne click chemistry and the strong adhesion between EVAL membrane matrix and SSPB. The cross-linked 28


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network structure of the EVAL-SSPB retained the steady membrane performance.

Figure. 13. Reusability performance of EVAL-SSPB membrane for SDS stabilized trichloro methane-in-water emulsion.

Table 2 Performance comparison of oil/water separation materials during the cyclic oil-water-SDS separation. Membrane

Water flux

Rejection FRR

FDR Pressure-driven

Ref

(L/m2h)

(%)

(%)

(%)

(MPa)

PVDF@pDA@SiO2

572

98

-

-

0.08

45

PVDF/PDA/PMEN

1600

99

99

18

0.08

46

GOP(PGS/GO=4)

1800

99

93

38

0.06

47

PVDF-360M1

350

99

87

30

0.07

48

EVAL-SSPB

2600

98

97

14

0.1

This work

The performance of different oil/water separation membranes reported in the literatures are summarized in Table 2 for comparison. EVAL-SSPB in this work 29



shows higher flux and antifouling performance than other membranes, and the separation rejection was at high level. The results indicate that the EVAL-SSPB membrane has great potential applications in the separation of oil-in-water wastewater. 4. Conclusions In summary, a versatile method was developed to design a supramolecular membrane surface with 3D hydrophilic slide-rings via azide-alkyne click coupling with super-hydrophilic SSPB to improve EVAL membrane performance. The EVAL-SSPB membrane exhibited super-hydrophilicity and excellent underwater oleophobicity with an oil contact angle of 159 ± 0.4°. This resulted in membranes with effective oil/water emulsion separation performance. The EVAL-SSPB membrane also exhibited low oil droplet and BSA adherence on the self-mobile dynamic surface. The flux and separation efficiency of the membrane were above 2000 L/m2h and 98%, respectively. Therefore, the click coupling of SSPB demonstrated the ability to simultaneously enhancing the permeability and antifouling properties of EVAL membranes. This study demonstrates a novel method to modify supramolecular membrane surfaces in order to improve their water flux and the oil emulsion separation efficiency.

Acknowledgements This work is supported by National Natural Science Foundation of China (No. 21676199, 21476173). The authors give thanks to the National college students' 30


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innovative entrepreneurial training plan (No. 201910058007). This work is also supported by Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT) of Ministry of Education of China (Grand No. IRT17_R80). Ligang Lin contributed equally to this work and should be considered co-first author.

Supporting Information

The word file contains oil droplets slip phenomenon, additional contrast experiments and the behaviour of oil droplets on the EVAL-SSPB membrane. The Supporting Information is available free of charge on the ACS Publications website.

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Modification of Supramolecular Membranes with 3D Hydrophilic Slide

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