Facile Fabrication of Composite Membranes with Dual Thermo- and

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Facile Fabrication of Composite Membranes with Dual Thermo- and pH-Responsive Characteristics Bing Ma, Xiao-Jie Ju, Feng Luo, Yu-Qiong Liu, Yuan Wang, Zhuang Liu, Wei Wang, Rui Xie, and Liang-Yin Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b02427 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 16, 2017

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ACS Applied Materials & Interfaces

Facile Fabrication of Composite Membranes with Dual Thermo- and pH-Responsive Characteristics Bing Ma,† Xiao-Jie Ju,*,†,‡ Feng Luo,† Yu-Qiong Liu,† Yuan Wang,† Zhuang Liu,† Wei Wang,†,‡ Rui Xie,†,‡ and Liang-Yin Chu*,†,‡,§

†

School of Chemical Engineering, Sichuan University, No. 24, Southern 1 Section, Yihuan

Road, Chengdu, Sichuan 610065, P. R. China ‡

State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu,

Sichuan 610065, P. R. China §

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing,

Jiangsu 211816, P. R. China

KEYWORDS Responsive membranes; Composite membranes; Dual thermo-/pH-response; Self-assembly; Nanogels

1 Environment ACS Paragon Plus


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ABSTRACT Facile fabrication of novel functional membranes with excellent dual thermo- and pHresponsive characteristics has been achieved by simply designing dual-layer composite membranes. pH-Responsive poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymers and polystyrene blended with thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) nanogels are respectively used to construct the top layer and bottom layer of composite membranes. The stretching/coiling conformation changes of the P4VP chains around the pKa (~3.5-4.5) provide the composite membranes with extraordinary pHresponsive characteristics, and the volume phase transitions of PNIPAM nanogels at the pore/matrix interfaces in the bottom layer around the volume phase transition temperature (VPTT, ~33

o

C) provide the composite membranes with great thermo-responsive

characteristics. The microstructures, permeability performances and dual stimuli-responsive characteristics can be well tuned by adjusting the content of PNIPAM nanogels and the thickness of the PS-b-P4VP top layer. The water fluxes of the composite membranes can be changed in order of magnitude by changing the environment temperature and pH, and the dual thermo- and pH-responsive permeation performances of the composite membranes are satisfactorily reversible and reproducible. The membrane fabrication strategy in this work provides valuable guidance for further development of dual stimuli-responsive membranes or even multi stimuli-responsive membranes.


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INTRODUCTION Stimuli-responsive membranes, which can change their permeability and selective permeation characteristics according to environmental chemical and/or physical stimuli, are drawing more and more considerable attention.1-3 Such stimuli-responsive smart membranes have

promising

applications

in

numerous

fields,

including

water

treatments,4-6

chemical/biological separations,7,8 sensors,9,10 drug controlled release,11-13 and so on. So far, versatile stimuli-responsive membranes, which respond to different external stimuli including pH,14,15 temperature,16,17 light,18,19 electric field20,21 and specific molecules/ions,22,23 have been designed and fabricated. Compared with single-stimulus-responsive membranes, the dual- and multi-stimuli-responsive membranes that can respond to more than one environmental factors possess more regulation modes and broader applications.24,25 Among various environmental stimuli, temperature and pH are two of the most common and easily controlled ones.26

Therefore, it is of great significance to develop smart membranes

possessing dual thermo- and pH-responsive characteristics. Up to now, various strategies have been developed to prepare dual thermo- and pHresponsive membranes. categories.

The reported methods can be generally classified into three

The first approach is to introduce thermo-responsive and pH-responsive

functional polymers into membrane materials by grafting strategy after the membrane formation. The second approach is to introduce both thermo- and pH-responsive domains during the membrane formation. The third approach is that the two different responsive domains are respectively introduced into membranes during and after the membrane formation. The first grafting approach is the most commonly used strategy. Lee et al.27 prepared the dual thermo- and pH-responsive polyamide membranes by grafting poly(acrylic acid-co-N-isopropylacrylamide) copolymers on the surface of porous polyamide membranes using plasma polymerization technique, and the results showed that the thermo-responsive



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gating coefficient and the pH-responsive gating coefficient were both less than 2. Surfaceinitiated atom transfer radical polymerization (SI-ATRP) is another commonly used method to fabricate dual stimuli-responsive membranes by sequential grafting thermo- and pHresponsive polymeric domains. Various kinds of block copolymers were grafted onto the membranes via consecutive twice SI-ATRP, such as poly(N-isopropylacrylamide)-blockpoly(2-(N,N-dimethylamino) ethyl methacrylate) (PNIPAM-b-PDMAEMA), poly(acrylic acid)-block-poly(N-isopropylacrylamide)

(PAAc-b-PNIPAM),

methacrylate)-block-poly(N-isopropylacrylamide)

poly(2-hydroxyethyl-

(PHEMA-b-PNIPAM)

and

poly(N-

isopropylacrylamide)-block-poly(methacrylic acid) (PNIPAM-b-PMAA).28-31 The thermoand pH-responsive gating coefficients of these smart membranes were in the ranges of 2 ~ 10 and 2 ~ 18, respectively. More recently, Lee et al.32 developed a novel kind of dual thermoand pH-responsive anodic aluminum oxide (AAO) membranes with PNIPAM and PAAc chains on upper and lower surfaces of the membranes respectively using SI-ATRP method. When located in the environment of 20 oC or pH 6, the membrane pores could be closed well, exhibiting excellent responsive characteristics.

However, these reported modification

strategies by grafting thermo- and pH-responsive domains on membrane materials after membrane formation involve lots of complex reactions and tedious preparation steps, which are difficult to control. These problems limit the further scale-up applications of these methods.

In comparison, the second approach that introducing the thermo- and pH-

responsive domains during the membrane formation is relatively simple.

Ying et al.33

prepared dual thermo- and pH-responsive poly(vinylidene fluoride) (PVDF) membranes by blending the poly(acrylic acid)-graft-poly(vinylidene fluoride) (PAAc-g-PVDF) copolymers and PNIPAM polymers.

The trans-membrane flux results indicated that the thermo-

responsive gating coefficient was about 6, but the pH-responsive gating coefficient was less than 3. Chen et al.34,35 fabricated two types of dual thermo- and pH-responsive PVDF


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membranes, in which one was blended with smart poly(N-isopropylacrylamide-co-acrylic acid) P(NIPAM-co-AAc) microgels, and the other one was blended with thermo-responsive PNIPAM nanogels and pH-responsive PAAc nanogels.

However, the thermo- and pH-

responsive gating coefficients of these two types of membranes were all less than 5. Taking advantage of dual thermo- and pH-responsive properties of PDMAEMA, different kinds of dual stimuli-responsive membranes were prepared by using PVDF-g-PDMAEMA, poly(styrene)-block-poly[2-(N,N-dimethylamino) ethyl methacrylate] (PS-b-PDMAEMA) and polyethersulfone (PES) blended with F127-b-PDMAEMA as membrane-forming materials.36-39 The thermo-responsive gating coefficients of these membranes ranged from 2 to 5, and the pH-responsive gating coefficients ranged from 3 to 30. In summary, up to now the reported dual thermo- and pH-responsive membranes cannot achieve satisfactory thermoand pH-responsive characteristics at the same time. The main reason might be that most membranes that constructed using the second strategy were formed through liquid-induced phase separation (LIPS), and the membrane formation process is so fast that the responsive domains cannot migrate to the membrane pore surfaces. As a result, the responsive gating performances are not satisfactory.

Such limited temperature- and/or pH-responsive

regulating range bring certain limitations to the membrane applications. Some researchers also used the third approach to fabricate dual thermo- and pH-responsive membranes. Clodt et al.40 firstly prepared the pH-responsive isoporous membranes based on the self-assembly of poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymers. Then, the PSb-P4VP membranes were coated with polydopamine and then further modified with PNIPAM polymers by Michael addition reaction. The membranes exhibited both excellent thermo-responsive and pH-responsive performances, i.e., the membrane pores could be closed well under low pH and low temperature conditions.

However, such three-step

preparation process and time-consuming reaction might be the obstacles for the further



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industrial applications.

To date, facile fabrication of dual thermo- and pH-responsive

membranes with both excellent thermo- and pH-responsive gating characteristics is still a challenge. Here we report on a facile method to fabricate novel functional membranes with both excellent thermo- and pH-responsive gating characteristics by constructing dual-layer composite membranes with pH-responsive and thermo-responsive domains in the top and bottom layers respectively. The composite membranes are fabricated by a simple twicecasting method combining one-step non-solvent-induced phase separation (NIPS) process (Figure 1a, b1, b2). The top layer of the composite membrane is formed by the self-assembly of PS-b-P4VP block copolymers, which displays ordered nanoporous structure in the topsurface41. The stretching/coiling conformation changes of P4VP chains around their pKa (~3.5-4.5) provide the composite membranes with excellent pH-responsive gating property. At pH lower than the pKa, the P4VP chains are stretched due to the protonated pyridine groups, resulting in the closing of the nanopores in the top-layer (Figure 1b3). While, at pH higher than the pKa, the pyridine groups are deprotonated and the P4VP chains coil down, resulting in the opening of the nanopores in the top-layer (Figure 1b4).42,43 The polystyrene (PS) membrane-matrix blended with PNIPAM nanogels is used to construct the bottom layer of the composite membranes. Due to the significant volume change around the volume phase transition temperature (VPTT), the PNIPAM nanogels at the membrane pore/matrix interfaces can serve as the thermo-responsive gates of the membranes (Figure 1b5-b6).44 At temperatures lower than the VPTT, the PNIPAM nanogels are in swollen state, so the membrane gates in the bottom layer are closed; while, at temperatures higher than the VPTT, the PNIPAM nanogels are in shrunken state, so the membrane gates are open. In addition, the compatibility between PS and PS-b-P4VP leads to inter-diffusion of the two corresponding casting solutions before immersing in water, and finally results in tight


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adhesion of these two membrane layers.45

Such a fabrication strategy of dual-layer

composite membranes ensure the respective and independent regulations of thermo- and pHresponsive characteristics. The proposed method introduces the thermo- and pH-responsive domains during the membrane formation, which is facile for further scale-up applications. The results in this study provide valuable guidance for further development of dual stimuliresponsive membranes or even multi stimuli-responsive membranes.

EXPERIMENTAL SECTION Materials PS-b-P4VP block copolymers (175,000-b-65,000 g/mol) were purchased from Polymer Source. PS (192000 g/mol) was purchased from Sigma-Aldrich. NIPAM purchased from TCI was purified by recrystallization.

N,N’-Methylenebise-bis-acrylamide (MBA),

ammonium persulfate (APS), N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), 1,4-dioxane (DOX) and tetrahydrofuran (THF) were supplied by Chengdu Kelong Chemicals and used without further purification. Deionized (DI) water (18.2 MΩ at 25 °C) from a Milli-Q Plus water puriﬁcation system (Millipore) was used throughout the experiments.

Synthesis and Characterization of PNIPAM Nanogels Monodisperse PNIPAM nanogels were synthesized by precipitation polymerization of NIPAM and MBA, which was initiated by APS.46 First, NIPAM (2.26 g), MBA (0.153 g) and APS (0.08 g) were dissolved in 200 ml deionized water. Then, the solution was bubbled with nitrogen gas to remove dissolved oxygen in the system and kept in a water bath at 70 °C for precipitation polymerization for 4 h. After the reaction, the resultant PNIPAM nanogels were purified by centrifugation at 8000 rpm (D-37520, Thermo Scientific) with deionized



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water for several times to remove the unreacted components. Finally, the purified PNIPAM nanogels were freeze-dried and reserved for further use.

The morphology of PNIPAM

nanogels in dried state was observed by a field-emission scanning electron microscope (FESEM, JSM-7500F, JEOL).

The temperature-dependent hydrodynamic diameters of

PNIPAM nanogels in water were measured by DLS (Zetasizer Nano-ZS; Malvern Instruments). The environmental temperature was changed from 20 to 45 °C. Before each datum collection, the highly diluted PNIPAM nanogel dispersion in DI water was equilibrated for 20 min at each predetermined temperature.

Preparation of Composite Membranes with Dual Thermo- and pH-Responsive Characteristics The dual-layer composite membranes were prepared by a simple twice-casting method. The casting solutions for the bottom-layer of membrane were prepared by dissolving PS (25 wt%) and a certain amount of PNIPAM nanogels in NMP. The nanogel content in NMP solution, which was the mass ratio of PNIPAM nanogels to PS, was varied as 0%, 10% and 15%. The casting solutions for the top-layer of membrane were prepared by dissolving PS-b-P4VP (15 wt%) in a ternary solvent mixture with 28.3 wt% DMF, 28.3 wt% DOX and 28.3 wt% THF.47 At first, the bottom-layer casting solution was casted on a glass plate to get a wet film with a thickness of 300 µm. Then, the top-layer casting solution was casted on the previous wet film. The thickness of the wet top-layer film was varied as 200 µm, 150 µm and 100 µm. After evaporation for 10 s, the dual-layer wet film with the glass plate was immediately immersed in a water bath at room temperature to achieve sufficient solidification to allow the membrane formation. All prepared membranes were rinsed with DI water for 2-3 days to leach out the residual solvent, and then dried in air at room temperature for further characterizations.


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Morphological Characterization of Composite Membranes The morphologies of all prepared membranes were characterized by FESEM (JSM-7500F, JEOL).

The membrane samples were stuck on the sample holders to observe the

microstructures of the upper surfaces. To observe the cross-sections, membrane samples were put into liquid nitrogen for enough time, and then fractured mechanically. To avoid charging problems, all samples were sputter-coated with gold for 60 s before observation.

Hydraulic Permeability Test of Composite Membranes To investigate the thermo- and pH-responsive characteristics of composite membranes, transmembrane water fluxes at different temperatures and different pH values were studied. The water fluxes were tested using a “dead-end” filtration apparatus under a constant transmembrane pressure of 0.1 MPa. The effective diameter of the filtration area was 40 mm. For temperature-dependent flux experiments, the test temperature ranged from 20 °C to 45 °C, which was controlled by a thermostatic unit. For pH-dependent flux experiments, the test pH value ranged from 2.5 to 6.8, which was adjusted by nitric acid and ammonium hydroxide. The pH value of the filtration solution was measured by a pH meter (SevenCompact, Mettler Toledo). Before the test, filtration solutions with prescribed pH value were introduced into the filtration apparatus and were kept at a constant temperature for 20 min. The water flux of each membrane at each temperature and pH was measured more than five times to obtain an average value.

RESULTS AND DISCUSSION Morphology and Thermo-Responsive Behaviors of PNIPAM Nanogels The FESEM image of PNIPAM nanogels in dried state is shown in Figure 2a. The PNIPAM



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nanogels present good spherical shape and monodispersity. The diameter of the air-dried PNIPAM nanogels is about 395 nm.

Figure 2b shows the temperature-dependent

hydrodynamic diameter variation of PNIPAM nanogels. With increasing the temperature from 20 °C to 45 °C, the average hydrodynamic diameter of PNIPAM nanogels in water varies from 937 nm to 441 nm, and a dramatic diameter change around 33 °C is observed. Such superb thermo-responsive volume phase transition characteristics, which will play a good gating role in the membranes, are in accordance with previously reported works.44,46

Morphology of Composite Membranes The FESEM images of different kinds of composite membranes are shown in Figure 3. Combining the non-solvent-induced phase separation process and the self-assembly of PS-bP4VP in the top-layer membrane, the top surfaces of all composite membranes present ordered nanopores (Figure 3a1-e1). On account of the compatibility of PS-b-P4VP and PS, excellent binding between the top layer and the bottom layer is observed (Figure 3a2-e2). In high magnifications, the interfaces between the two membrane layers are visible (Figure 3a3e3). In the FESEM images of membrane cross-sections, the top layers present sponge-like structures (Figure 3a2-e2, a3-e3). For the microstructures of the bottom layers, the regions near the interfaces between two layers present cellular-like structures while the regions away from the interfaces present finger-like structures (Figure 3a2-e2, a3-e3), which are totally different from the typical finger-like structures of the PS single-layer membrane prepared by NIPS process (Figure 4). After immersing the dual-layer wet films into the non-solvent (i.e. water bath), the liquid-liquid exchange between non-solvent and the solvent of top-layer casting solution (i.e., a mixture of DMF, DOX and THF) occurs firstly and the top layer is solidified. Then, the aqueous solution containing dissolved DMF, DOX and THF contacts the bottom-layer casting solution and exchanges with the NMP solvent. DMF, DOX and


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THF are all good solvents for PS but water is a non-solvent for PS; therefore, PS can coagulate in such a solvent/non-solvent mixture. According to the research reported by Strathmann et al.48, the coagulation in non-solvent/solvent mixtures will slow down the rate of precipitation, resulting in more cellular-like membrane structures. Based on the above reasons, cellular-like structures are formed in the regions near the interfaces between two layers during the phase separation process. The effects of the content of PNIPAM nanogels in the bottom layer on the microstructures of composite membranes are investigated. The thicknesses of the top-layer wet films are kept constant at 200 µm. The microstructures of composite membranes prepared with 0%, 10% and 15% PNIPAM nanogels are shown in Figure 3a-c. For the composite membrane without PNIPAM nanogels, the region in the bottom layer and near the interface between two layers presents typical cellular-like morphology, in which the closed pores are enveloped in the polymer matrix (Figure 3a3).

After blending PNIPAM nanogels into the bottom-layer

casting solutions, some changes occur on the cellular-like structures in the composite membranes. The PNIPAM nanogels assemble at the pore/matrix interfaces (Figure 3b3-c3). With increasing the content of PNIPAM nanogels from 0% to 10% (Figure 3a3-b3), the cellular-like pores become larger and some open pores appear in the upper region of the bottom layer. The reason for this phenomenon is due to the presence of the top layer, which leads to delayed phase separation of PS. During the phase separation process, the hydrophilic PNIPAM nanogels at the growing pore/matrix interfaces would absorb more water into the growing pore spaces. As a result, the membrane pores with nanogels at the interfaces are enlarged and interconnected with each other (Figure 3b3). When the content of PNIPAM nanogels is further increased to 15%, more open pores are generated and the pores become more interconnected with each other (Figure 3c3). Because the existence of the top-layer can affect the phase separation process of the



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bottom-layer, the thickness of the top-layer wet film is further adjusted to investigate its influence on the microstructure of composite membranes.

The composite membranes

prepared with 150-µm-thick and 100-µm-thick top-layer wet films are fabricated, in which the contents of PNIPAM nanogels are kept constant at 15%. As shown in Figure 3c2-e2, the thickness of the final top layer decreases with reducing the thickness of the top-layer wet film. At the same time, with decreasing the thickness of the top-layer wet film, less solvent (DMF/THF/DOX) contains in the top-layer wet film. Thus, during the precipitation of PS, the proportion of solvent in non-solvent/solvent coagulation bath decreases, and the effects of delayed phase separation get weakened, which finally leads to a more figure-like structure. As a result, more interconnected pores appear in the upper region of the bottom layer (Figure 3c3-e3). To summarize, dual-layer structures are achieved in the composite membranes, in which the upper-layer surfaces exhibit nanoporous structures. The cross-sections of top layers show sponge-like structures, and the cross-sections of bottom layers present cellular-like and finger-like structures simultaneously. With increasing the PNIPAM nanogel content from 0% to 15%, some enlarged cellular-like pores and interconnected pores appear in the bottom layer near the interface of two layers. With decreasing the thickness of top layer, more obvious macroporous structure can be achieved.

Dual Stimuli-Responsive Characteristics of Composite Membranes The VPTT of PNIPAM nanogels is about 33 oC and the pKa of P4VP is about 3.5~4.5. To study the dual stimuli-responsive characteristics of the prepared composite membranes, the trans-membrane water fluxes of membranes are measured under different environmental temperatures (in the temperature range of 20-45 oC, which covers the VPTT of PNIPAM nanogels) and pH values (in the pH range of 2.5-6.8, which covers the pKa of P4VP). As


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shown in Figure 5, all composite membranes exhibit excellent pH-responsive characteristics coming from the top-layer membrane. When the temperature is fixed while the pH value increases, water fluxes of the composite membranes increase.

The pH-responsive

characteristics are attributed to synergistic effects of the nanometer-sized pores (about 20 nm) in the top-layer and the pH-dependent conformation changes of P4VP chains.42,43 At pH lower than the pKa of P4VP, the P4VP chains are stretched due to the protonation of the pyridine groups, thus the nanopores in the top-layer can be well closed. At pH higher than the pKa, P4VP chains coil down, resulting in the opening of the nanopores in the top-layer. As a consequence, the fluxes change greatly within pH 2.5-6.8, and excellent pH-responsive characteristics are achieved. The composite membranes prepared with PNIPAM nanogels also exhibit good thermo-responsive characteristics, which are attributed to the temperaturedependent volume change of PNIPAM nanogels in the bottom layer. The PNIPAM nanogels at the membrane pore/matrix interfaces can serve as the thermo-responsive gates of the membranes. When the temperature is lower than the VPTT of PNIPAM, the PNIPAM nanogels are in the swollen state, resulting in that the gates of bottom-layer membrane are in closed state. On the contrary, when the temperature is higher than the VPTT of PNIPAM, the PNIPAM nanogels are in the shrunken state and the membrane gates are open. As a result, the trans-membrane water flux is small at low temperatures and is large at high temperatures, which means that significantly thermo-responsive characteristics are obtained. With the same thicknesses of the top-layer wet films (200 µm), the permeability performances of composite membranes prepared with 0%, 10% and 15% PNIPAM nanogels are different (Figure 5a-c).

For the composite membrane prepared without PNIPAM

nanogels, the trans-membrane water flux is extremely low (Figure 5a) because of the compact cellular-like structure in the bottom-layer membrane (Figure 3a). The slight increase of the trans-membrane water flux of this PNIPAM-free membrane with increasing temperature



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(Figure 5a) is caused by the thermo-induced viscosity decrease of water. With increasing the content of PNIPAM nanogels in the composite membranes, the trans-membrane water flux increases remarkably (Figure 5b,c), which match well with the microstructures of the corresponding membranes (Figure 3b,c). As mentioned above, with increasing the PNIPAM nanogel content, some enlarged cellular-like pores and interconnected pores appear in the bottom layer near the interface of the two membrane layers, which result in the increase of water flux. The more the PNIPAM nanogels are added in the composite membranes as thermo-responsive gates, the more significant the thermo-responsive gating characteristics of membranes (Figure 5b,c). The thickness of the top-layer also affects the trans-membrane water flux. With decreasing the thickness of top-layer wet film from 200 µm to 100 µm (fixing the PNIPAM nanogel content at 15%), although the thermo- and pH-responsive water flux variations are still significant, the maximum water flux increases (Figure 5c-e).

The results are also in

accordance with the microstructures of corresponding composite membranes (Figure 3c-e). Decreasing the thickness of top layer leads to more macropores in the bottom layer near the interface of the two membrane layers, resulting in higher water flux. The effects of PNIPAM nanogel content on the thermo-responsive characteristics of composite membranes are further studied. The water fluxes of composite membranes with different PNIPAM nanogel contents at 20 and 45 oC when pH is 6.8 or 2.5 are shown in Figure 6a and 6c. The water flux of each composite membrane at pH 2.5 is much lower than that at pH 6.8 because of the well stretched P4VP chains. Calculated from the water fluxes, the thermo-responsive gating coefficients of composite membranes with 0%, 10% and 15% PNIPAM nanogels at pH 6.8 (RT45/20, pH6.8), which are defined as the ratios of the water fluxes at 45 oC to those at 20 oC, are 1.50, 1.69 and 9.76, respectively (Figure 6b), and the thermoresponsive gating coefficients of composite membranes with 0%, 10% and 15% PNIPAM


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nanogels at pH 2.5 (RT45/20, pH2.5) are 1.50, 1.60 and 7.04 (Figure 6d). The thermo-responsive gating coefficients of composite membranes without PNIPAM nanogels are match with the change in water viscosity. For composite membranes with PNIPAM nanogels, no matter at pH 6.8 or 2.5, the thermo-responsive coefficients are larger than 1.50 due to the volume phase transition of PNIPAM nanogels. With increasing the content of PNIPAM nanogels from 10% to 15%, more PNIPAM nanogels in the bottom layer act as the thermo-responsive gates, which lead to several-fold increase in the thermo-responsive gating coefficient. Such a sharp increase of water flux and thermo-responsive coefficient is in accordance with the microstructures of the composite membranes. For membranes prepared with 10% PNIPAM nanogels, most of the cellular-like pores are still closed pores (Figure b3), which results in a small water flux and a low thermo-responsive coefficient.

Only when the content of

PNIPAM nanogels increases to a certain extent, the pore-forming effect of PNIPAM nanogels can display remarkably. Accompanied with the pore-forming effect, the gating function of PNIPAM nanogels can be exerted. Thus, for composite membranes prepared with 15% PNIPAM nanogels, there are more open pores in the upper region of the bottom layer and the pore interconnectivity is much better (Figure c3). As a result, the water flux increases remarkably and more PNIPAM nanogels play a role as thermo-responsive gates, so that the thermo-responsive coefficient suddenly increases (Figure 6). Such a relatively high gating coefficient can provide a wide regulating range of water flux. The effects of top-layer thickness on the pH-responsive characteristics of composite membranes are also investigated. From the water fluxes of composite membranes with different thicknesses of the top-layer wet films at pH 2.5 and pH 6.8 with temperature fixed as 45 oC or 20 oC (Figure 7a and 7c), we can find that the water fluxes of all membranes at 20 o

C are a little lower than that at 45 oC due to the swollen of PNIPAM nanogels. The pH-

responsive gating coefficients of composite membranes with top-layer wet-film thickness of



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200 µm, 150 µm and 100 µm at 45 oC (RpH6.8/2.5, T=45), which are defined as the ratios of the water fluxes at pH 6.8 to those at pH 2.5, are 97.86, 108.26 and 123.69, respectively (Figure 7b). While, the pH-responsive gating coefficients of corresponding composite membranes at 20 oC (RpH6.8/2.5, T=20) are 70.55, 51.96 and 91.67, respectively (Figure 7d). The results show that although the water fluxes of composite membranes are slightly affected by the thickness of top-layer, the composite membranes have similar pH-responsive gating coefficients. The pH-responsive gating coefficients are as high as 100, indicating that the pores on the surface of top-layer are closed well under low pH environment. The trans-membrane water fluxes at 20 oC/pH 2.5 and 45 oC/pH 6.8 and the corresponding dual stimuli-responsive gating coefficients are shown in Figure 8. At 20 oC/pH 2.5, the PNIPAM nanogels are in swollen state and the P4VP chains are all in stretched state, that the pores in the two layers are all closed, resulting extremely small water fluxes. On the contrary, at 45 oC/pH 6.8, the PNIPAM nanogels and the P4VP chains are all in the shrunken state, large water fluxes are obtained due to the open pores in the two layers (Figure 8a,c). The dual thermo-/pH-responsive gating coefficients (R(T45,pH6.8)/(T20,pH2.5)) of composite membranes with 0%, 10% and 15% PNIPAM nanogels are 20.81, 74.59 and 688.52, respectively (Figure 8b). The significant difference of dual thermo-/pH-responsive gating coefficients results from the fact that the water fluxes of composite membranes at 20 oC/pH 2.5 are relatively similar due to the well closed pores, while the water fluxes at 45 oC/pH 6.8 show large differences due to the different porous structures in bottom-layer membranes. Meanwhile, the dual thermo-/pH-responsive gating coefficients (R(T45,pH6.8)/(T20,pH2.5)) of composite membranes with top-layer wet-film thickness of 200 µm, 150 µm and 100 µm are 688.52, 711.84 and 522.10, respectively (Figure 8d). Owing to well closed pores at 20 oC/pH 2.5, these dual thermo-/pH-responsive gating coefficients are similar. These results also show that the composite membranes with 15% PNIPAM nanogels have extraordinary dual stimuli-


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responsive characteristics, whose dual stimuli-responsive gating coefficients are as high as 700. Such dual thermo-/pH-responsive gating coefficients are orders of magnitude higher than those of previously reported dual thermo- and pH-responsive membranes.27-31,33-39 It is obvious that the responsive gating coefficients of all composite membranes triggered simultaneously by pH and temperature stimuli are much larger than those triggered by single pH or temperature stimulus, which demonstrates that the increase in the regulating methods leads to the increase in the regulating range of the composite membranes. As we know, one stimulus factor can produce two different switching states, which are open and close states. So, two stimulus factors can produce four different switching states, which are open-open, open-close, close-open and close-close states. For our proposed dual thermo- and pH-responsive composite membranes, the four switching states are OpenpH– OpenT, OpenpH–CloseT, ClosepH–OpenT and ClosepH–CloseT states. Figure 9a shows the water flux contour map of the composite membrane prepared with 15% PNIPAM nanogels and 200-µm-thick top-layer wet film, and the four switching states are corresponding to the four vertices of the contour map. At 45 oC/pH 6.8, P4VP chains coil down and the pores in the top-layer membrane are open; at the same time, the PNIPAM nanogels are in the shrunken state and the gates in the bottom-layer membrane are also open. So, the water flux at 45 oC/pH 6.8 reaches to 778.03 kg m-2 h-1 bar-1 (Figure 9a,b). At 20 oC/pH 6.8, P4VP chains still coil down and the pores in the top-layer membrane are still open; but the PNIPAM nanogels are in the swollen state and the gates in the bottom-layer membrane are closed. As a result, the water flux at 20 oC/pH 6.8 decreases to 79.72 kg m-2 h-1 bar-1 (Figure 9a,b). Similarly, the water flux at 45 oC/pH 2.5 decreases to 7.95 kg m-2 h-1 bar-1 (Figure 9a,b) due to the stretched P4VP chains and closed pores in the top-layer membrane. At 20 oC/pH 2.5, the pores in the top-layer membrane and the gates in the bottom-layer membrane are all closed, which results in a very small water flux of only 1.13 kg m-2 h-1 bar-1 (Figure 9a,b).



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The different switching states of composite membranes can result in several orders of magnitude change in water flux, which demonstrate that we can feasibly change the pH or temperature to get a permeability we need (Figure 9a). In addition, it is clearly to see that the pH-responsive characteristics of the composite membranes are more significant than the thermo-responsive gating characteristics. As we know, the stimuli-responsive characteristics are mainly determined by the close degree of membrane pores.

For the top-layer of

membrane, the nanopores on the surfaces can be well blocked by the stretching P4VP, and the water flux at low pH can reach to 0.42,43 However, for the bottom-layer of membrane, because the increased size of PNIPAM nanogels is not close enough to the pore size, the swollen PNIPAM nanogels can only reduce the pore size but not completely block the pores.49 In summary, the close degree of the pores caused by the stretched P4VP chains is greater than that caused by the swollen PNIPAM nanogels, leading to more remarkable pHresponsive characteristics. In order to ensure the stability of the composite membranes, we also investigate the effects of trans-membrane pressure on the trans-membrane water flux. As shown in Figure 9c, at the four different switching states, the water fluxes of the composite membrane all increase linearly with increasing the operation pressure, which means that the PNIPAM nanogels assembled at the pore/matrix interfaces are stable enough without collapse to resist the operation pressure in a certain range.

Reversible Thermo- and pH-Responsive Characteristics of Composite Membranes To verify the reversibility and durability of the thermo- and pH-responsive permeation performances of composite membranes, the permeation experiments are carried out repeatedly by alternatively changing environmental temperature across the VPTT of PNIPAM nanogels or changing pH across the pKa of P4VP. As shown in Figure 10, the


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thermo- and pH-responsive characteristics of the composite membrane with 15% PNIPAM nanogels and 200-µm-thick top-layer wet film are satisfactorily reversible and reproducible even after five rounds of operation, suggesting that the PNIPAM nanogels in the membrane could maintain their thermo-responsive swelling/shrinking properties under repeated temperature changes and the P4VP chains in the membrane could maintain their pHresponsive stretching/coiling properties under repeated pH changes.

CONCLUSION We have developed a facile method to fabricate a novel type of dual thermo- and pHresponsive membranes with excellent dual thermo- and pH-responsive characteristics by exquisitely designing composite dual-layer membranes, in which pH-responsive PS-b-P4VP block copolymers are used to construct the top-layer of membrane and PS bended with thermo-responsive PNIPAM nanogels is used to construct the bottom-layer of membrane. The compatibility between PS and PS-b-P4VP results in tight adhesion of the two layers in the membrane. Based on such unique architecture, the microstructures, water fluxes and dual thermo- and pH-responsive properties of composite membranes can be flexibly regulated. With increasing the content of PNIPAM nanogels, the water fluxes and the dual stimuliresponsive gating coefficients increase. With decreasing the thickness of top-layer, the water fluxes increase while the dual stimuli-responsive gating coefficients change a little. Under different temperatures and pH values, the water fluxes of composite membranes change in the order of magnitude and the dual thermo-/pH-responsive gating coefficients can reach up to 700, which are much higher than those of most previously reported membranes. The thermoand pH-responsive permeation performances of the composite membranes are stable and reproducible, which make the composite membranes as good candidates for further investigations and industrial applications. The methods and results in this study provide



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valuable guidance for further development of multi stimuli-responsive and multi-functional membranes via simply constructing multi-layer membrane structures with various materials.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (L.-Y. Chu); [email protected] (X.-J. Ju). Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge support from the National Natural Science Foundation of China (21490582, 21622604), the Program for Changjiang Scholars and Innovative Research Team in University (IRT15R48) and State Key Laboratory of Polymer Materials Engineering (sklpme2014-1-01).


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FIGURES

Figure 1. Schematic illustration of the fabrication strategy (a) and dual stimuli-responsive mechanism of the proposed composite membranes (b). In the preparation process, firstly the PS solution is casted on a glass plate to get a wet film (a1-a2); then the PS-b-P4VP solution is casted on the previous wet film (a3-a4); next, after an evaporation time of 10 s (a5), the duallayer wet film together with the glass plate is immersed in a water bath to obtain the final composite membrane by sufficient solidification (a6). As a result, the top-layer membrane is constructed with pH-responsive PS-b-P4VP block copolymer and the bottom-layer membrane is constructed with PS blended with thermo-responsive PNIPAM nanogels (b1-b2). At pH lower than the pKa of P4VP, the P4VP segments stretch due to the protonated pyridine groups, resulting in pore closing in the top-layer membrane (b3), while at pH higher than the pKa of P4VP, the P4VP segments coil down due to the deprotonation of pyridine groups, resulting in pore opening in the upper-layer membrane (b4). When temperature is lower than the VPTT of PNIPAM, the PNIPAM nanogels are in swollen state and the pore gates are closed in the bottom-layer membrane (b5); while, when temperature is higher than the VPTT, the PNIPAM nanogels are in the shrunken state and the pore gates are open in the bottomlayer membrane (b6).



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Figure 2. Morphology (a) and thermo-responsive behavior (b) of PNIPAM nanogels. a) FESEM image of air-dried PNIPAM nanogels. Scale bar is 1 µm. b) Temperature-dependent hydrodynamic diameters of PNIPAM nanogels in water.


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Figure 3. Morphologies of the prepared composite membranes. FESEM images of surfaces (a1–e1), cross-sections (a2–e2) and the magnified cross-sections (a3-e3) of the composite membranes prepared with 0% PNIPAM nanogels and 200-µm-thick top-layer wet film (a), 10% PNIPAM nanogels and 200-µm-thick top-layer wet film (b), 15% PNIPAM nanogels and 200-µm-thick top-layer wet film (c), 15% PNIPAM nanogels and 150-µm-thick top-layer wet film (d), and 15% PNIPAM nanogels and 100-µm-thick top-layer wet film (e). Scale bars are 200 nm in (a1–e1), 50 µm in (a2–e2), and 2 µm in (a3-e3).



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Figure 4. FESEM images of the surface (a), cross-section (b) and magnified cross-section (c) of PS single-layer membrane blended with 15% PNIPAM nanogels. Scale bars are 50 µm in (b) and 2 µm in (a, c).


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Figure 5. Temperature- and pH-dependent hydraulic permeability performances of the composite membranes prepared with 0% PNIPAM nanogels and 200-µm-thick top-layer wet film (a), 10% PNIPAM nanogels and 200-µm-thick top-layer wet film (b), 15% PNIPAM nanogels and 200-µm-thick top-layer wet film (c), and 15% PNIPAM nanogels and 150-µmthick top-layer wet film (d), and 15% PNIPAM nanogels and 100-µm-thick top-layer wet film (e).



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Figure 6. Effects of PNIPAM nanogel content on the thermo-responsive behaviors of the composite membranes. a, b) Water fluxes at 20 and 45 oC (a) and the thermo-responsive gating coefficients (b) of the composite membranes with different PNIPAM nanogel contents when pH is 6.8. c, d) Water fluxes at 20 and 45 oC (c) and the thermo-responsive gating coefficients (d) of the composite membranes with different PNIPAM nanogel contents when pH is 2.5. δ is defined as the thickness of the top-layer wet film.


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Figure 7. Effects of the thickness of top layer membrane on the pH-responsive behaviors of composite membranes. a, b) Water fluxes at pH 2.5 and 6.8 (a) and the pH-responsive gating coefficients (b) of composite membranes with different thicknesses of the top layer when temperature is 45 oC. c, d) Water fluxes at pH 2.5 and 6.8 (c) and the pH-responsive gating coefficients (d) of composite membranes with different thicknesses of the top layer when temperature is 20 oC.



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Figure 8. Effects of the PNIPAM nanogel content and the thickness of top layer on the dual stimuli-responsive behaviors of composite membranes. a, b) Water fluxes at 20 oC/pH 2.5 and 45 oC/pH 6.8 (a) and the dual stimuli-responsive gating coefficients (b) of composite membranes with different PNIPAM nanogel contents. c, d) Water fluxes at 20 oC/pH 2.5and 45 oC/pH 6.8 (c) and the dual stimuli-responsive gating coefficients (d) of composite membranes with different thicknesses of the top layer.


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Figure 9. The hydraulic permeability performances of the composite membrane with 15% PNIPAM nanogels and 200-µm-thick top-layer wet film. a) The water flux contour map of the composite membrane, and four schemes near the four vertices of the contour map are corresponding to four switching states of the composite membrane at different pH values and temperatures. b) Water fluxes of the composite membrane at four switching states. c) Effect of trans-membrane pressure on the water flux of the composite membrane.



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Figure 10. Reversible thermo-responsive (a) and pH-responsive (b) water fluxes of the composite membrane with 15% PNIPAM nanogels and 200-µm-thick top-layer wet film.


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Graphic for TOC


Facile Fabrication of Composite Membranes with Dual Thermo- and

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