Anion-Responsive Poly(ionic liquid)s Gating Membranes with Tunable

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Anion-responsive poly(ionic liquid)s gating membranes with tunable hydrodynamic permeability Xiang Zhang, Sheng Xu, Jukai Zhou, Weifeng Zhao, Shudong Sun, and Changsheng Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08740 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 3, 2017

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ACS Applied Materials & Interfaces

Anion-responsive Poly(ionic liquid)s Gating Membranes with Tunable Hydrodynamic Permeability Xiang Zhanga, Sheng Xua, Jukai Zhoua, Weifeng Zhaoa,* Shudong Suna, and Changsheng Zhaoa,b,*

a College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, People’s Republic of China b National Engineering Research Center for Biomaterials, Sichuan University, Chengdu 610064, People’s Republic of China *

E-mail

address:

[email protected],

[email protected]

(C.S.

Zhao);

[email protected] (W.F. Zhao).

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Abstract: Novel anion-responsive “intelligent” membranes with functional gates are fabricated by filling polyethersulfone (PES) microporous membranes with poly(ionic liquid)s (PILs) gels. The wetting properties of the PILs could be controlled by changing their counteranions (CAs), and thus the filled PILs gel gates in the membrane pores could spontaneously switch from “closed” state to “open” one by recognizing the hydrophilic CAs in the environment and vice versa. As a result, the fluxes of the “intelligent” membranes could be tuned from a very low level (0 mL/m2mmHg for Cl-, Br- and BF4-) to a relatively high one (430 mL/m2mmHg for TFSI). The anion-responsive gating behavior of the PILs filled membranes is fast, reversible and reproducible. In addition, the “intelligent” membranes are sensitive to contact time and ion concentrations of the hydrophobic CA species. The proposed anion-responsive “intelligent” membranes are highly attractive for ion-recognizable chemical/biomedical separations and purifications.

Keywords: “intelligent” membranes; poly(ionic liquid)s; anion-response; hydrodynamic permeability control


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1 Introduction Nowadays, membrane technologies have got increasing attention and rapid development.1 The membrane separation and mass transfer show significant advantages including eco-friendly, energy-saving, high efficient, simple in operation, etc.2-3 Therefore, membrane technologies have been widely applied in various fields such as environment, energy, chemical engineering and biotechnologies.4-7 The performances of membranes, typically the permeability and selectivity, are key parameters for membrane technologies; and they are usually dictated by the intrinsic chemical properties and pore structure of the membranes.8-9 Thus, the membrane performances usually remain unalterable, which may restrict the wide and efficient applications in extended fields to a certain extent. As a result, the development of “intelligent” or “smart” membranes with regulated permeability and selectivity is highly attractive. Stimuli-responsive membranes are important branch of “intelligent” membranes.3 Figuratively speaking, the stimuli-responsive membranes are the membranes which have been installed smart gates artificially; while the environmental stimuli are like teleswitches of the gates, which can control the gates on the membranes wide-open, half-open or closed. Numerous fabrication methods and applications of stimuli-responsive membranes have been proposed. Among the methods, chemically/physically incorporating stimuli-responsive materials, the so-called “smart” gates, into commercial porous membranes are widely applied.10-13 Up to now, commonly used stimuli-responsive polymers, such as poly(acrylic acid), poly(N-isopropylacrylamide), azobenzene and zwitterionic polymers have been employed to build the smart gates on membranes,14-17 so the pore geometries and the surface 3 ACS Paragon Plus Environment


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chemical properties of the prepared membranes can exhibit changes with the corresponding stimuli, i.e., pH, temperature, light and ionic strength. As a result, these “smart” gates can be self-controlled when the membranes are applied under given conditions.9 The permeability and selectivity will change when the membranes are stimulated, and return to the original state when the stimuli are gone. In other words, continuous stimuli are needed to maintain the changes of the pore geometries and the surface chemical properties of the membranes. However, the changes cannot be “memorized” when the external conditions return to normal. Alternatively, the membranes with special stimuli-responsive behavior are developing: one stimulus can open/close the gates on the membranes, another similar but different one will close/open them, and the states of the gates will be maintained when the stimuli are removed. Distinguishing from the fabrication methods mentioned above, the membranes are mostly formed from block or random copolymers directly.8, 18-20 The porous membranes formed by the copolymers can offer responses to certain solvents: they will swell in good solvents, and shrink in poor solvents.21 Rely on the selective swelling or shrinking, the pore structures of the membranes can be adjusted artificially, led to the adjustments of the permeability and selectivity;22 meanwhile, the adjustment results can maintain if the membranes are on long contact with the given solvents. Such stimuli-responsive membranes have potential applications, while some inevitable flaws also emerge. For instance, since they are formed from copolymers directly, the membranes may be not robust enough. The membranes usually endure repeated solvent corroding to operate the gates, so the morphologies of the membranes and pores are difficult to be controlled.23 In addition, volatile organic solvents are chosen as the stimuli and sometimes the process of stimulating requires heating, and this may 4 ACS Paragon Plus Environment

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pollute the environment.24 The last but not the least, templates are needed during the preparation of some membranes, and commercial porous membranes are also needed as the supports before they are applied, thus the preparation process becomes complicate.19 So is it possible to fabricate such smart membranes by more simple and practical way, and also the operations of the gates are more facile and green? By the aid of poly(ionic liquid)s (PILs), it can be done. PILs, also called polymerized ionic liquids, are polyelectrolytes that comprise a polymeric backbone and an ionic liquid (IL) species in monomer repeating units.25 Recently, intensive attention is paid to this novel class of polymers because they combine the properties of ILs and specific polymers; and the designs of PILs-based materials (including gels, films and membranes) with stimuli-responsive functions have been extensively studied.26-27 For instance, Yuan et al. fabricated PILs-based porous membranes via electrostatic complexation, and the pores in the membranes could be opened and closed by isopropanol and water, respectively.28 Yan et al. synthesized dual-stimuli responsive PIL membranes via photo-crosslinking, the transmittance and microstructure of the membranes could be altered in response to temperature and/or pH.29 Meanwhile, many studies showed that ion-exchange capability, one of the most important performances of PILs, could also be utilized to design stimuli-responsive materials.30-31 When asymmetric organic cations of ILs are polymerized into PILs, the inorganic anions of the ILs (counteranions) can be replaced by ion-exchange easily. Many properties of PILs (such as wetting and dewetting properties) strongly depended on the nature of the counteranions. Thus, after modifying with PILs, the wettability of the prepared materials becomes tunable by ion-exchange.32-33 For instance, Texter et al. 5 ACS Paragon Plus Environment


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synthesized reversible solvent-induced polymers using a surfactant based on the PILs. The resulting copolymer materials could be tuned between a hydrogel and a porous structure by changing the anions in the PIL segments.34 In this work, novel anion-responsive membranes were fabricated by facile filling polyethersulfone (PES) microporous membranes with PILs gels. The PILs gels in the membrane pores served as the smart gates, and they can be operated by exchanging the counteranions: the gels will swell in water when the counteranions are hydrophilic, so the membrane pores will be filled completely; and they will shrink when the counteranions are exchanged to hydrophobic ones, then the membrane pores will emerge again. The ion-exchange process can implemented by dipping the membranes into certain salt solutions, or drive the solutions through the membranes. The water fluxes of the PILs filled membranes with hydrophilic and hydrophobic CAs were tested. Meanwhile, the reversibility and repeatability of the membranes, the influences of the CA species, ion concentrations and exchanging times on the hydrodynamic permeability adjustment were investigated. 2 Experimental Section 2.1 Materials Porous polyethersulfone (PES) membranes (average pore size 0.22 µm) were supplied by Haining Jinzheng Filter Co. Ltd, China. 1-Vinylimidazole (VIM, 99%), bromoethane (EBr, 99%),

2,

2’-azobis

(2-methylpropionamidine)

dihydrochlorid

(AIBA,

99%),

N, N’-Methylenebisacryl amide (MBA, 99%), bis (trifluoromethane) sulfonimidate (LiTFSI, 99%), potassium hexafluorophosphate (KPF6, 99%), sodium fluoroborate (NaBF4, 99%) and potassium thiocyanate (KSCN, AR, 98.5%) and phosphate buffer powder (PBS, pH 7.2-7.4) 6 ACS Paragon Plus Environment

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were obtained from Aladin reagent Co. Ltd. (Shanghai China). Sodium chloride (NaCl, AR), potassium perchlorate (KClO4, AR), calcium carbonate (CaCO3, AR), magnesium sulfate (MgSO4, AR), sodium nitrate (NaNO3, AR), NaCl injection (normal saline, NS), acetone (AR) and ethylene glycol (EG, AR) were obtained from Changzheng chemical reagent Co. (Chengdu, China). All the reagents were used without further purification. Deionized water (DI water) was obtained from a pure water production system and used throughout the studies. The mimetic industrial/domestic water (I/D) was prepared according to China national standards GBT19923-2005 and GB5749-2006. It contained 300 mg/L NaCl, 550 mg/L CaCO3, 300 mg/L MgSO4 and 20 mg/L NaNO3. 2.2 Preparation of poly(ionic liquid) (PILs) gels and ion-exchange procedure In order to observe the swelling and shrinkage behaviors intuitively, PILs gels in macro-scale were prepared by free radical polymerization. To avoid volatilization of the low boiling EBr, a pre-reaction was carried out by adding VIM, EBr and EG into an airtight round-bottom flask, and the reaction mixture was stirred at 45 ºC for 12 hours. After that, the cross-linker MBA and the initiator AIBN were added into the flask and stirred until they were completely dissolved. Then, given amount of the reaction products was put into special moulds, and placed into an oven with 65 ºC for 12 hours. After the reaction, the prepared PILs gels were taken out from the moulds and dipped in fresh DI water to remove residual solvent and unreacted chemicals. The details of the sample names and chemical dosages are shown in Table 1 (for example, “G-20-5” represented the monomer concentration was 20 wt. % and the cross-linker concentration was 5 mol. %). For the ion-exchange, the gels were soaked in 20 mL 0.1 M NaCl or LiTFSI solutions for 12 hours, and then soaked in fresh DI 7 ACS Paragon Plus Environment


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water for 12 hours to remove residual ions. Table 1 The chemical dosages and sample names for the PILs gels. Samples

G-20-5

G-20-15

G-30-5

G-30-15

VIM (g)

1

1

2

2

EBr (g)

1.75

1.75

3.5

3.5

MBA (g)

0.081

0.243

0.162

0.486

AIBN (g)

0.017

0.017

0.034

0.034

EG (g)

12.83

12.83

12.83

12.83

2.3 Preparation and characterization of PILs filled membranes The reaction solutions of the PILs gels which were mentioned above were poured into a conical flask and served as pore-filling solutions. The dry porous PES membranes were soaked in the pore-filling solutions for 1 hour and vacuum-pumped 30 minutes to ensure the membrane pores were completely filled. Then the membranes and solutions were transferred to a beaker, sealed with parafilms and placed into an oven of 65 ºC for 12 hours. After the reaction, the PILs gel filled PES membranes were taken out, and the gels on the surfaces were scraped by a knife; then the membranes were dipped in fresh DI water to remove residual solvent and unreacted chemicals. Corresponding to the four different reaction solutions of the PILs gels, the prepared membranes were named as M-20-5, M-20-15, M-30-5 and M-30-15, respectively. The morphology of the membranes was observed by scanning electron microscopy (SEM, JSM-7500F, JEOL). For the SEM observation, the membranes were freeze-dried, cut by a single-edged razor blade after immersing into liquid nitrogen, and then attached to the sample 8 ACS Paragon Plus Environment

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holder, coated with a gold layer. The chemical compositions of the membranes were analyzed by energy dispersive spectrometer (EDS). The EDS spectra were recorded from the cross-sections of the membranes during the SEM observation. 2.4 Characterization of ion-responsive properties of the gel filled porous membranes The ion-responsive properties of the PILs gel filled porous membranes were estimated by comparing the hydrodynamic permeability of the membranes before and after the ion-exchange. The ion-exchange method of the membranes was similar with the gels, each membrane was soaked in 20 mL salt solutions of certain anion species and concentrations for certain time (12 hours unless otherwise specified), and then soaked in fresh DI water for 12 hours to remove residual ions. All the soaking processes were assisted with a temperature-controlled oscillator at a shaking speed of 150 rpm at 25 ºC. In order to measure the hydrodynamic permeability, the membranes were cut and fixed in a dead-end ultrafiltration cell with an effective area of 3.9 cm2. DI water was pumped through the cell with a pressure of 0.05 MPa (the schematic illustration of the measurement is shown in Fig. 1). The membranes were firstly pre-compacted for 30 minutes to get steady filtration, and then the mass of the water coming from the cell was recorded every 5 minutes; the measurement for each membrane was lasted for 25 minutes. Then, the hydrodynamic permeability (F) was determined using equation (1):

F = V / SPt

(1)

where V is the volume of DI water (mL); S is the effective membrane area (m2); P is the pressure applied to the membrane (mmHg); t is the time (h).



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Fig. 1 The schematic illustration of the permeability measurement. 3 Results and Discussion 3.1 The anion-responsive properties of the PILs filled membranes In order to investigate the effects of the monomer and cross-linker concentrations on the responsive properties, four PILs gels with different formulas were filled into the PES membranes. The prepared membranes were named according to the formulas of the PILs gels, for instance, “M-20-5” represented the gel monomer concentration was 20 wt. % and the cross-linker concentration was 5 mol. %. The hydrodynamic permeability could be presented by the water flux of the membranes under given pressure.10 For the ion-responsive property testing, the water fluxes of the PILs membranes with the CA of Cl- or TFSI were tested and the results are shown in Fig. 2. Firstly, the water fluxes of the freshly-prepared PILs membranes with the CA of Cl- were recorded every 5 minutes after pre-compacting. There was no water drop flowed from the ultrafiltration cell even after 25 minutes of permeability tests, and the water flux was thus zero. Then, the membranes were dipped into TFSI solution to accomplish ion-exchange, and rinsed by DI water. The water fluxes increased to about 80 mL/m2mmHg for the [M-30-5][TFSI] and [M-30-15][TFSI], and 430 mL/m2mmHg for the 10 ACS Paragon Plus Environment

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[M-20-5][TFSI] and [M-20-15][TFSI], respectively. When the CA was TFSI, the water fluxes of the membranes increased with the decrease of the monomer concentration. Meanwhile, the cross-linker concentration had no influence on the level of the water fluxes; however, these values decreased gradually as the testing time passed when the cross-linker concentration was 5 mol. %. Afterward, the CA of the membranes was exchanged to Cl-, and the water fluxes of the membranes with the CA of Cl- were recorded after 30 minutes waiting, and the values turned to zero once again. The cycles of “testing-exchanging” were repeated six times (three cycles for Cl- and TFSI exchange), and the water fluxes of the two subsequent cycles agreed well with the first one.

Fig. 2 The water flux of the PILs filled membranes with the CA of Cl- or TFSI. The results indicated that the hydrodynamic permeability of the PILs filled membranes could respond to the CAs species; and the water fluxes could be tuned from zero to a relative high level by the ion-exchange between the Cl- and TFSI. Since the electrostatic binding force between the CAs and the quaternary nitrogen heterocycle was relatively strong, the state of the ion response could be “memorized” under normal water environment. The water fluxes decreased gradually for the [M-20-5][TFSI] and [M-30-5][TFSI] along with the time, 11 ACS Paragon Plus Environment


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and the reason might be that the lower cross-linker concentration caused the looser cross-linking structure of the PILs gels. In this case, it would be easier to lose TFSI followed with the stream, and this ultimately caused the reduction of the hydrophobic and resulted in the swelling of the gels. Compared to other methods, the pore-filling was a good option to modify polymeric membranes with PILs gels and made the gels served as the “smart” gates, since the pore-filling method was facile and efficient, and the modification was more uniform for the membranes.35 The cross section SEM images of original PES (M-PES) and M-20-15 membranes with the CAs of Cl- and TFSI are shown in Fig. 3A-C (since the morphologies of other PILs filled membranes observed by SEM were similar, the images were not given). As seen in Fig. 3A, many cavities and pores could be observed in the cross-section of the original membrane. After the pore-filling by the PILs gels with the CA of Cl-, flake-like substances appeared in the cross-section of the [M-20-15][Cl], which were attached to the inner surface of the membranes, and occupied almost all the cavities and pores (as shown in Fig. 3B). Obviously, the appearance of the flake-like substances was caused by the filling of the PILs gels; while in dry environment, the morphologies of the dehydrated gels were collapsed into flake-like structure.36


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Fig. 3 The cross-section SEM images of the membranes: (A) original PES membranes (M-PES); (B) [M-20-15][Cl] and (C) [M-20-15][TFSI], the scale bar is 1 µm; (D) the schematic illustration of the ion response principle. As chemical analysis methods (such as infrared or X-ray photoelectron spectroscopy) for material surfaces could not characterize the chemical components of the pore-filling membranes, energy dispersive spectrometer (EDS) analysis was carried out to qualitatively analyze the cross-section components of the M-20-15, and the results are shown in Table 2. The M-PES was composed by C, O and S elements, and the N and Cl elements were detected in the [M-20-15][Cl], which came from the quaternary nitrogen heterocycles and the CA of Cl-. When the CA was exchanged to TFSI, the cross-section morphology of the membrane was varied. As shown in Fig. 3C, due to the hydrophobic of the gels, the original flake-like structure shrunk and became porous and crisp. The EDS results (as shown in Table 2) showed that the F element from TFSI appeared; instead, the Cl element was not detected, verifying the complete exchange of the CAs. The PILs gel could swell and fill the cavities and pores when the CA was Cl-. In this case, the “gates” of the membranes were closed and the hydrodynamic permeability was very low; 13 ACS Paragon Plus Environment


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when the CA was exchanged to TFSI, the pores of the membranes were appeared again due to the shrinkage of the PILs gels, thus the “gates” opened and the hydrodynamic permeability increased (the schematic illustration is shown in Fig. 3D). Table 2 The atoms percent of the PILs filled membranes, which were calculated by EDS. “-” expressed the element was not detected. C

O

S

N

Cl

F

M-PES

74.47

21.72

3.81

-

-

-

[M-20-15][Cl]

70.38

20.30

3.02

6.00

0.29

-

[M-20-15][TFSI]

67.97

19.99

3.61

4.33

-

4.11

The gels in macro-scale could display the swelling and shrinkage behaviors intuitively. In order to farther understand the effect of the gel behaviors on the membranes permeabilities, the PILs gels in macro-scale were prepared. The contents and the naming method of the gels correspond to the membranes, for instance, “G-20-5” represented the monomer concentration was 20 wt. % and the cross-linker concentration was 5 mol. %. All the gels were soaked in water to reach the equilibrium. Fig. 4A shows the digital photographs of the wet PILs gels with the counteranions (CAs) of Cl- and TFSI, respectively. When the CA was Cl-, all the gels could swell in water, and the sizes and transparency were inversely proportional to their concentrations of the monomer and cross-linker. When the CA was exchanged to TFSI, the sizes and transparency of all the gels reduced obviously. The schematic illustration of the swelling/shrinkage mechanism of the gels is shown in Fig. 4B. The wetting properties of PILs are strongly depended on the nature of their CAs.32 The Cl- is a kind of hydrophilic ion, so the molecular chains of the gels could fully stretch due to the hydrophilicity and the electrostatic 14 ACS Paragon Plus Environment

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repulsion, which were caused by the positive electricity of the quaternary nitrogen heterocycle, and then resulted in the swelling of the PILs gels. Once the CA was exchanged to hydrophobic TFSI,33 the molecular chains of the gels became curly due to the great reduction in wetting property; as a result, the gels shrank and collapsed.

Fig. 4 (A) The digital photographs of the PILs gels with the counteranions of Cl- and TFSI-, the black dotted circles show the size of the moulds and the scale bar is 1 cm; (B) the schematic illustration of the swelling and shrinkage behaviors of the PILs gels; (C) the diameters of the PILs gels. In order to open and close the membrane pores by the swelling and shrinkage of the PILs gels, a key condition must be satisfied: the sizes of the swelled gels should be larger than or equal to the pore sizes of the membrane, and the sizes of the shrunk gels should be smaller than the pore sizes of the membrane. As can be seen from Fig. 4C, all the sizes of the gels with the CA of Cl- were larger than those of the moulds, and became smaller when the CA was exchanged to TFSI. During the pore-filling process, the swelling and shrunk behavior of the filled PILs gels followed the same rule, thus the PILs filled membranes could display anion-responsive property to the hydrodynamic permeability. 15 ACS Paragon Plus Environment


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3.2 The stability, reversible and repeatable properties of the “gates” operation The stability, reversible and reproducible properties of the “smart gates” operation are crucial for the membranes. For ideal stimuli-response, the membranes should equally respond to multiple times of stimuli. Also, the mechanical and chemical properties and morphologies would not be damaged after the stimulating. The M-20-15 was chosen to verify the stability, reversible and repeatable response properties of the PILs filled membranes by increasing the measured pressures and time, and the cycle times of the ion-exchange, since the responsive property of the M-20-15 was obvious and steady. The water fluxes of the [M-20-15][Cl] and [M-20-15][TFSI] under the measured time of 60 minutes and the measured pressures of 0.1, 0.15 and 0.2 MPa are shown in Fig. 5A. The water fluxes of the [M-20-15][Cl] and the [M-20-15][TFSI] were stable within 60 minutes and barely changed when compared with the flux values which were tested under 0.05 MPa: the fluxes kept at zero for the [M-20-15][Cl]; and about 430 mL/m2mmHg for the [M-20-15][TFSI] when the measure pressures increased to 0.1 or 0.15 MPa. The fluxes of the [M-20-15][TFSI] slightly decreased when the measure pressure reached 0.2 MPa, this might because that the membrane was squashed and became wrinkled due to the relative high pressure (as shown in insert in Fig. 5A). The cycles of ion exchange were repeated for three times and the water fluxes of the two subsequent cycles agreed well with the first one, so the data were not given. The water fluxes of the [M-20-15][Cl] and [M-20-15][TFSI] after 2, 4, 6, 8, and 10 cycles of the Cl-TFSI exchanges are shown in Fig. 5B. It also could be seen that the fluxes kept at zero for the [M-20-15][Cl]; and about 430 mL/m2mmHg for the [M-20-15][TFSI], no matter how many times of the ion exchanges are performed. These results indicated that the responsive permeability of the 16 ACS Paragon Plus Environment

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membranes were stable when the measured pressure was lower than 0.2 MPa, and could be perfectly repeated within at least 10 cycles of ion exchange.

Fig. 5 (A) The water fluxes of the [M-20-15][Cl] and the [M-20-15][TFSI] under higher measured pressures (the insert photograph showed the appearance of the membranes before (left) and after (right) the measurement under 0.2 MPa); (B) the water fluxes of the [M-20-15][Cl] and the [M-20-15][TFSI] at different cycles of exchange. Meanwhile, the cross-section morphologies of the membranes during the repeated exchanges were observed by SEM. The SEM images of the membranes for the initial, 5th and 10th times of ion-exchange with the CAs of Cl- and TFSI are shown in Fig. 6A-F. The images indicated that all the [M-20-15][Cl] or [M-20-15][TFSI] exhibited similar morphologies, no matter how many times of the ion exchanges are implemented. For the [M-20-15][Cl], flake-like substances were occupied almost all the cavities and pores of the membranes; while for the [M-20-15][TFSI], the flake-like substances turned to porous and crisp, and the pores of the membranes emerged again. The EDS analysis of the membranes after 10 cycles of exchange (as shown in Fig. 6G and H) was carried out. For the [M-20-15][Cl], only Cl element could be detected among the CAs. As expected, only F element could be detected for the [M-20-15][TFSI]. The EDS results verified that the ion exchange could be well achieved 17 ACS Paragon Plus Environment


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even after 10 cycles of ion-exchange.

Fig. 6 (A-F) The cross-section SEM images of the [M-20-15][Cl] and the [M-20-15][TFSI] at 0, 5 and 10th cycles of exchange, respectively, the scale bar is 1 µm; the atom percent of (G) the [M-20-15][Cl] and (H) the [M-20-15][TFSI] after 10th cycles of exchange. 3.3 The precise hydrodynamic permeability adjustment The adjustment of the hydrodynamic permeability was due to the ion exchange of the PILs gels filled in the membranes. Therefore, by controlling the conditions of the ion exchange process, we can not only open/close the permeability of the membranes, but also adjust the permeability in any desired levels steadily. Herein, the M-20-15 was chosen to verify its precise controllability. As mentioned above, the wetting properties of the PILs gels were depended on the nature of the CAs; common CAs could be selected from a broad range of inorganic and organic anions, including Cl-, Br-, I-, BF4-, PF6-, Tf-, TFSI and so on. Due to their different hydrophilic/hydrophobic natures, various permeabilities of PILs filled membranes could be reached by ion-exchanging with different CA species. The water fluxes of the M-20-15 with various CAs are shown in Fig. 7A. The water flux of 18 ACS Paragon Plus Environment

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the [M-20-15][Cl] was recorded firstly, and recorded again after the membrane was completely ion-exchanged with the CAs of different hydrophilic/hydrophobic natures, i.e., Br-, BF4-, SCN-, ClO4-, PF6- and TFSI, respectively. When the CAs were exchanged to Br- and BF4-, the water fluxes were still zero, and increased to about 20, 37, 300 and 430 mL/m2mmHg when the CAs were exchanged to SCN-, ClO4-, PF6- and TFSI, respectively. The cycles of ion exchange were repeated three times and the water fluxes of the two subsequent cycles agreed well with the first one. The difference of the water fluxes could be attributed to the different hydrophilic/hydrophobic natures of the CAs. In water environment, the shrinkage of the PILs gels with hydrophobic CAs led to large pores in the membranes; as a result, the hydrodynamic permeability increased. The order of the hydrophilic/hydrophobic natures of the CAs are shown in Fig. 7B;37-38 and the water fluxes agreed well with this order: the hydrophobicity of Cl-, Br- and BF4- was not enough to “open the gates”, so the membrane pores were closed and no water flux could be detected; while for the other four anions, the water fluxes of the membranes increased with the increase of the hydrophobicity.

Fig. 7 (A) The water fluxes of M-20-15 when exchanging to various CAs; (B) the chemical formulas and hydrophilic/hydrophobic order of the counteranions. 19 ACS Paragon Plus Environment


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The capacity of ion exchange is influenced by various external factors, such as initial ion concentration, temperature, exchanging time, pH and so on.39-41 Meanwhile, the swelling ratios of the PILs gels could be varied by the species and the capacities of the CAs: during the exchange from Cl- to hydrophobic TFSI, the gels will shrink more evidently with the increase of the TFSI amount, leading to the more emerging pores in the membranes. So we controlled the ion exchange capacity of the PILs gels by adjusting two easily-controlled factors (initial ion concentration and exchanging time) during the ion exchange processes, to aim to adjust the hydrodynamic permeability of the membranes. The water fluxes of the [M-20-15][Cl] membrane were recorded firstly, and recorded again after the membrane was ion exchanged with different initial TFSI concentration solutions for 12 hours, or with 0.1 M TFSI solutions for different exchange time. As seen from Fig. 8A and C, the fluxes of the [M-20-15][TFSI] membrane treated with 0.005, 0.02 and 0.1 M TFSI solutions were about 35, 225 and 430 mL/m2mmHg, respectively; while the fluxes with ion exchange times of 10, 30, 60 and 120 minutes were about 180, 275, 370 and 430 mL/m2mmHg, respectively. The cycles of ion exchange were repeated for three times and the water fluxes of the two subsequent cycles agreed well with the first one. The results indicated that the hydrodynamic permeability of the membranes could be adjusted by changing the concentrations of the incubation solution and the time for ion exchange. Such adjustments were reversible and repeatable.


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Fig. 8 (A) The water fluxes of [M-20-15][Cl] and [M-20-15][TFSI] exchanged by different concentrations of TFSI solutions; (B) the water flux - TFSI concentration relation curves of M-20-15; (C) the water fluxes of [M-20-15][Cl] and [M-20-15][TFSI] with different exchanging times; (D) the water flux-exchanging time relation curves of [M-20-15][TFSI] exchanged by different concentrations of TFSI solutions. The average fluxes of the [M-20-15][TFSI] treated with different TFSI concentrations (from 0.005 to 0.4 M) and different exchange times (from 10 to 300 minutes) are shown in Fig. 8B and D. All the fluxes showed a logarithm increase with the increase of the ion initial concentration and exchange time: the increase of the fluxes was dramatic in the beginning of the axis X, then slowed down, and finally reached equilibrium values. The equilibrium could be reached when the initial concentration was higher than 0.2 M or the exchange time was more than 2 hours. The relationship between the fluxes and the ion concentrations (or 21 ACS Paragon Plus Environment


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exchange time) was similar with that between the ion exchange capacities and the ion concentrations (or exchange time) in some conditions of ion exchange dynamics (or isotherms) study: the increase of the exchanged ion amounts was also fast in the beginning, then slowed down, and finally reached equilibrium.39, 41 As we speculated above, the PILs gels would shrunk more if the amount of the hydrophobic TFSI was higher, leading to the increase of the water fluxes. Combining the experimental results, the hydrodynamic permeability of the membranes was almost proportional to the amounts of the hydrophobic CAs after the ion exchange from hydrophilic CAs to hydrophobic CAs. These results revealed that we might control the permeability of the membranes to any desired levels by the guide of the ion exchange theory. 3.4 In-situ ion exchange and water fluxes tests As mentioned above, in order to exchange the CAs, the membranes were taken out of the dead-end ultrafiltration cell after the flux test, then dipped into certain salt solutions, and cleaned by fresh DI water. Alternatively, the ion exchange could also be completed by driving certain salt solution through the membranes during the test directly (here the second method of ion exchange was called as in-situ ion exchange). In this section, the M-20-15 was chosen to verify the ion response property by the in-situ ion exchange, and hydrophilic Cl- and hydrophobic PF6- were used. As shown in Fig. 9A, the steps of permeability tests were performed. In step (a), the flux of the fresh prepared [M-20-15][Cl] was tested by pure water, and the flux was zero. In step (b),the pure water was replaced by given concentrations of PF6solutions (0.02, 0.05 and 0.1 M), in this case, the CA was exchanged to PF6- (the [M-20-15][Cl] was turned to [M-20-15][PF6]), and the fluxes rapidly increased to about 380, 22 ACS Paragon Plus Environment

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390 and 440 mL/m2mmHg, respectively. However, in step (c), the PF6- solution was replaced by pure water, and the fluxes of the [M-20-15][PF6] reduced gradually, and were steady at about 290 mL/m2mmHg. Finally, pure water was replaced by given concentrations of Clsolutions (0.02, 0.05 and 0.1 M) in step (d), the CA was exchanged to Cl- again (the [M-20-15][ [PF6] was turned to [M-20-15][Cl]), and the fluxes rapidly reduced to about 18, 18 and 20 mL/m2mmHg, respectively. Once the pure water was used again to test the fluxes of the [M-20-15][Cl], and the flux returned to zero. This test process was repeated for three times and the results of the two subsequent tests were very close to the first one, so the data were not given.

Fig. 9 (A) In-situ ion exchange and water flux results of the M-20-15; (B) the digital photographs of the [G-20-15][Cl] in pure water and 0.2 M NaCl solution; (C) the schematic illustration of ionic strength sensitivity of PILs gels. The results indicated that by the in-situ ion exchange, the response rate of the membrane 23 ACS Paragon Plus Environment


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was very fast (less than 1 minute, seen from step a to b, and step c to d in Fig. 9A). The in-situ ion exchange was actually a dynamic exchange process. In this case, the exchange equilibrium could be reached in a very short time, especially the concentration of the salt solution was relatively low.42-43 Moreover, the fluxes increased with the increase of the salt concentrations (step b and d), and they were higher than pure water testing (corresponding to step c and a). The reducing rates of the fluxes were different when the salt solutions were replaced by pure water. The values for the [M-20-15][PF6] reduced slowly when the PF6solution was replaced by pure water, while the values for the [M-20-15][Cl] reduced quickly. This response behavior was resulted from the ionic strength sensitivity of the PILs gels, and they could respond to the stimulus of the ion concentration of the surrounding environment regardless of the CAs species. Taking [G-20-15][Cl] as an example, the photographs were taken after the gels were dipped in pure water and 0.2 M NaCl solution for enough time (as shown in Fig. 9B), and the size of the gel in NaCl solution reduced slightly (from 1.25 to 0.89 cm). Since the binding between the quaternary nitrogen heterocycles and the CAs was caused by electrostatic interaction, and the CAs would lose in certain amount,37, 44 so the swelling ratio of the gel increased due to electrostatic repulsion. However, when the gel was dipped in NaCl solution, the high concentration ions screened almost all the positively-charged quaternary nitrogen heterocycles, which weakened the electrostatic interaction.45-46 Meanwhile, due to competitive osmosis, the salt ions diffused into the matrix of the gel and replaced water molecules.47 So the swelling ratio of the gel reduced (the schematic illustration is shown in Fig. 9C). Besides, the binding force between the quaternary nitrogen heterocycles and 24 ACS Paragon Plus Environment

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hydrophobic CAs was relatively strong,30, 37 so the elapse rate followed with the stream was slow, and this was the reason why the fluxes of the [M-20-15][PF6] reduced slowly when the PF6- solution was replaced by pure water. As a result, the response behavior of the membranes tested by in-situ ion exchange was not only caused by the wetting property change after the ion exchange, but also due to the ionic strength sensitivity of the PILs. 3.5 The application of the PILs filled membranes Mass transfer of liquids is one of the most important applications and properties of the membrane materials.9 The above studies indicated that the PILs gels filled membranes could be served as “remote control valves” when they were applied in pure water system, and the transfer amounts could be adjusted precisely via the facile way. However, some hydrophilic anions such as carbonate, phosphate and halogen ions are always ubiquitous in industrial and domestic water;48 and the frequently-used mediums (such as phosphate buffer (PBS) and normal saline (NS)) in biomedical fields always contain certain amounts of hydrophilic anions. Thus, the adjustable performance of the membranes might be disturbed when they were applied in these surroundings. To investigate this and further explore the applications of the membranes, the flux tests were carried out as the feed solutions were changed to mimetic industrial/domestic water (I/D, which was prepared according to China national standards GBT19923-2005 and GB5749-2006.), 0.1 M PBS and NS, respectively, and the flux results of the M-20-15 are shown in Fig. 10A. The fluxes of the [M-20-15][Cl] were zero when the feed solutions were I/D and PBS. For the I/D, the flux increased and stabilized around about 420 mL/m2mmHg when the CAs were exchanged to TFSI; while for PBS, the flux decreased obviously during the tests. Meanwhile, the flux values were kept in a relatively low level 25 ACS Paragon Plus Environment


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(about 20 mL/m2mmHg) when the feed solution was NS. The PBS contained certain concentrations of phosphate and chloride ions, while the chloride ion concentration of NS was about 0.15 M. So when membrane pores were opened, the CAs TFSI were replaced by hydrophilic anions when the PBS and NS streams were flowed through the membrane, resulting in the close of the pores. The results indicated that the PILs gels filled membranes could be applied in the industrial and domestic water systems; however, the adjustable performance would be disturbed when they were used in PBS system; and due to the relatively high NaCl concentration, the performance would be destroyed when the feed solution was NS. Meanwhile, some studies indicated that the PILs could also respond to some organic solvents, such as acetone and isopropyl alcohol,28, 49 since the compatibility of them could be improved when the CAs were exchanged from hydrophilic to hydrophobic one. To investigate the responsive property of the membranes when they were applied to organic solvent system, the fluxes of the [M-20-15][Cl] and [M-20-15][TFSI] were tested as the feed solutions were mixtures of DI water and acetone with different volume ratios, and the results are shown in Fig. 10B. With the increase of the acetone volume ratios, the fluxes of the [M-20-15][TFSI] decreased obviously, and the fluxes decreased to zero when the acetone volume ratio was higher than 20%. Oppositely, the fluxes of the [M-20-15][Cl] appeared when the acetone volume ratio reached 80%, and increased obviously with the increase of the acetone volume ratios. The results indicated that the membranes could be applied to detect acetone in aqueous solution, or block the aqueous solution polluted by acetone when the ACs were hydrophobic; while when the membranes were in hydrophilic state, the membranes 26 ACS Paragon Plus Environment

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could be applied to detect water in acetone solution, or block the acetone solution contained a certain amount of water.

Fig. 10 (A) The fluxes of [M-20-15][Cl] and [M-20-15][TFSI] when the feed solutions were replaced by I/D, PBS and NS solutions; (B) The fluxes of [M-20-15][Cl] and [M-20-15][TFSI] when the feed solutions were mixtures of DI water and acetone with different volume ratios. Since the permeability of the membrane was adjusted by given species of anions, the anions would leak into water environment inevitably. Although the hydrophilic chloridion is nontoxic, the toxicity of the hydrophobic fluorinated anions such as PF6- and TFSI should raise the concern. Some studies indicated that PF6- could undergo decomposition in water to form HF, while TFSI was more stable,30-31 the fluoride release of TFSI was not high enough to induce a toxic effect.50 Cytotoxicity tests showed that the cytotoxicities of PILs were related with the molecular structure of the cation parts, and most types of ionic liquids with the anion of TFSI did not inhibit cell proliferation.51-52 Meanwhile, the toxicity against activated sludge and zebrafish tests also indicated that TFSI appeared to be the less toxic.52 As a result, the application of PILs filled membranes would barely poison water environment. 27 ACS Paragon Plus Environment


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4 Conclusions In summary, (1) anion-responsive intelligent membranes were fabricated by “pore-filling” method. Poly(ionic liquid)s (PILs) gels were chosen as the smart gates and successfully filled into the porous polyethersulfone membranes. (2) By exchanging the counteranions (CAs) with different hydrophilic/hydrophobic nature, the hydrodynamic permeability of the membranes could be tuned from a very low value to a relatively high one. The adjustment was reversible and repeatable. (3) When the CAs were hydrophobic (the membranes opened to water flow), the fluxes were inversely proportional to the monomer concentrations and the cross-linker concentration. Also, the water fluxes were unstable if the cross-linker concentration was relative low. (4) The degree of permeability adjustment was influenced by various factors: the hydrophilic/hydrophobic nature of the CAs, the initial ion concentrations and the exchange time. The relationship between the fluxes and the effecting factors was explicit, so we could adjust the permeability of the membranes to discretionary levels by controlling the factors. (5) The water flux tests by “in-situ ion exchange” indicated that the response rate of the membranes was rapid and the membranes showed special ionic strength sensitivity. (6) The membranes could also be applied in industrial and domestic water fields and could detect and block the mixtures of acetone and DI water. Thus, the proposed method provides a fresh idea and contributes a modest force to develop “intelligent” membranes. Acknowledgements This work was financially sponsored by the National Natural Science Foundation of China (Nos. 51433007, 51503125 and 51673125), the State Key Laboratory of Polymer Materials Engineering (Grant No.sklpme2015-1-03), and the Sichuan Province Youth Science and 28 ACS Paragon Plus Environment

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Technology Innovation Team (No. 2015TD0001). We would also thank our laboratory members for their generous help, and gratefully acknowledge the help of Ms. Hui Wang, of the Analytical and Testing Center at Sichuan University, for SEM observation.



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Anion-Responsive Poly(ionic liquid)s Gating Membranes with Tunable

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