CO2-Responsive Cellulose Nanofibers Aerogels for Switchable Oil

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Functional Nanostructured Materials (including low-D carbon)

CO2-responsive Cellulose Nanofibers Aerogels for Switchable Oil-water Separation Yingzhan Li, Liqian Zhu, Nathan Grishkewich, Kam C. Tam, Jinying Yuan, Zhiping Mao, and Xiaofeng Sui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22159 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

CO2-responsive Cellulose Nanofibers Aerogels for Switchable Oilwater Separation

Yingzhan Li,a, b, d,

§

Liqian Zhu,a, d,

§

Nathan Grishkewich,b Kam C. Tam,b Jinying Yuan,c Zhiping Mao, a, d * Xiaofeng Sui a, d *

a

Key Lab of Science & Technology of Eco-textile, Ministry of Education, College of Chemistry, Chemical

Engineering and Biotechnology, Donghua University, Shanghai 201620, China. b

Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, 200

University Avenue West, Waterloo, ON, N2L 3G1, Canada. c

Key Lab of Organic Optoelectronics & Engineering Department of Chemistry, Tsinghua University, Beijing

100084 (P.R. China) d

§

Innovation Center for Textile Science and Technology of DHU, Donghua University, Shanghai, 201620, China. These authors contributed equally to this work.

* Corresponding author

Phone: +86 21 67792720; fax: +86 21 67792707; e-mail: [email protected]. Phone: +86 21 67792605; fax: +86 21 67792707; e-mail: [email protected].

Abstract Cellulose nanofibers (CNFs) aerogels with controllable surface wettability were prepared by grafting poly (N, N-dimethylamino-2-ethyl methacrylate) (PDMAEMA) polymer brushes via surface-initiated atom transfer radical polymerization (SI-ATRP). After grafting PDMAEMA polymer, the surface of the aerogel was hydrophobic. While in the presence of CO2, the surface of the aerogel gradually changes from hydrophobic to hydrophilic. The porous structure and CO2-responsiveness of PDMAEMA brushes within the CNFs aerogels allowed for the on-off switching in the oil-water mixture separation process. These CNFs aerogels were recyclable and displayed attractive separation efficiency for oil-water mixture and surfactant-stabilized emulsions. Furthermore, the switchable surface wettability holds an advantage in the avoidance of oil-fouling, which will greatly improve its recyclability. Keywords 1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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CO2-responsive, SI-ATRP, cellulose nanofiber, aerogel, oil-water separation, switchable

Introduction Cellulosic materials are presently undergoing a paradigm shift from traditional pulp and paper industries to advanced and high-value-added applications.1 Among all the cellulose-based materials, cellulose nanofibers (CNFs) aerogels have been used as “green” components in thermal insulation,2-3 catalyst support,4-5 wound dressing,6 and environmental remediation materials.7 Owing to their highly porous structure, high surface area and adjustable surface wettability, CNFs aerogels are also widely used in oil/water separation.7-9 Such as Zhou et al.10 prepared hydrophobic CNFs aerogels by freezing-dry the mixed suspensions of silylated CNFs and silica nanoparticles and the aerogels hold a separation efficiency higher than 99%. But those materials can be used to remove only one phase from the oil/water mixture/emulsions.10-13 Grafting stimuliresponsive polymer brushes on cellulose aerogels make it possible to realize the perfect control of their surface wettability.14 Recently, we grafted thermoresponsive polymer poly(N-isopropylacrylamide) to the surface of gold nanoparticles supported CNFs aerogel and found that the wettability and catalytic activities of the aerogel could be controllable via adjusting the temperature.5 Consequently, grafting stimuli-responsive polymer brushes on CNFs aerogels will possible allow for the on-off switching in the oil-water mixture separation process. Compared with other sources of stimulation, such as UV, acid and alkali, and temperature, gas triggers (CO2 and N2) do not cause chemical accumulation and materials deformation.15-16 They also have clear advantages over other stimuli in industrial applications, since they can be added and removed easily in a large volume operation,17 particularly in highly porous CNFs aerogels. Che et al.14 fabricated CO2 responsive polymethylmethacrylatepoly-co- (N, N-dimethylaminoethyl methacrylate) membranes by electrospinning and the “CO2-modulable” membranes can be used in 2


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switchable oil/water mixture separation. However, the smart membranes cannot be used in oil/water emulsions separation. Since CNFs aerogels have been widely used oil/water emulsions separation. So grafting CO2 responsive polymer brushes on the surface of CNFs aerogels can achieve the switchable and high efficiency oil/water mixture and emulsions separation. 18 In this contribution, CO2-switchable surface wettability CNFs aerogels were demonstrated by polymerizing N, N-dimethylamino-2-ethyl methacrylate (DMAEMA) monomers from the surface of CNFs aerogels via Surface-initiated atom transfer radical polymerization (SI-ATRP). Poly (N, N-dimethylamino-2-ethyl methacrylate) (PDMAEMA) possesses tertiary amine groups and has both pH- and temperature responsive properties.19-20 The protonation of tertiary amine groups will make PDMAEMA hydrophilic and exist in an extended chain conformation.21-22 Consequently, the solubility of PDMAEMA will be influenced by the protonation degree of PDMAEMA. However, PDMAEMA has good stability and it can be only partial protonated in the presence of water.23-25 The protonation of PDMAEMA can be significantly enhanced in the presence of dissolved CO2 and the polymer chains will become hydrophilic and exist in an extended chain conformation.14 When N2, air or Ar are introduced, the dissolved CO2 will be removed, the deprotonation of PDMAEMA will result in the polymer chain maintains a compact structure and becomes less hydrophilic.14 By introducing CO2 or N2 to these aerogels, oil-water mixture, the selective permeation through could be manipulated. When PDMAEMA polymer brushes were grafted from the surface of CNFs aerogels, the aerogel surface transformed from hydrophilic to hydrophobic character in the presence of dissolved CO2. These smart aerogels could serve as dual oil-water on-off switches, triggered by the introduction of CO2 or N2. The superior environmental friendliness, decent stability and abundant source of cellulose and the convenient switchable wettability of the CNFs aerogels make them have great potential for larger scale application.

Results and Discussion Flexible CNFs aerogels were constructed by cross-linking CNFs with 3



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glycidoxypropyltrimethoxysilane (GPTMS) and branched polyethyleneimine (b-PEI) according to our previous protocol.26 The high density of amine and hydroxyl groups on the surface of the aerogels offer a versatile platform for secondary reactions to impart the desirable functionality to suit a specific application.27 As shown in Scheme 1, αbromoisobutyryl bromide (BiBB) initiator was introduced to the surface of GPTMS and b-PEI modified CNFs aerogel (CNF-PEI), and SI-ATRP was conducted to grow PDMAEMA polymer brushes from the surface of BiBB modified CNFs aerogel (CNFBr).

Scheme 1. Schematic illustration of the preparation of CNF-g-PDMAEMA aerogel.

FT-IR and 13C NMR confirmed the presence of PDMAEMA polymer brushes as shown in Figure 1. A new peak of C=O groups appeared at 1740 cm-1 in the CNF-Br (Figure 1a), which indicated that BiBB was conjugated to the surface of CNFs aerogels.28 The appearance of an ester bond stretching at 1727 cm-1 confirmed the formation of CNF-g-PDMAEMA.29 By comparing the

13C

NMR

spectroscopy of CNF-PEI and CNF-Br, the new peak in CNF-Br at around 172 ppm (C15, 16) was assigned to the carbonyl carbon of the grafted BiBB.30 The new peaks at around 21, 53 and 59 ppm were attributed to the carbon of methyl groups (C18) and the carbon connected with Br (C14 and C17). For the

13C

NMR

spectroscopy of CNF-g-PDMAEMA (Figure 1b), the new peak at around 177 4


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ppm (C19, C25 and C29) was assigned to the carbonyl carbon of the grafted PDMAEMA and BiBB ester groups.29, 31 The increase of saturated carbon signal at 16~24 ppm was due to the carbon of methylene (C22) and methyl groups (C21 and C24) of the polymer brushes. In addition, the peaks at 45, 53 and 58 ppm were attributed to the methylene connected with O (C20 and C26) and N (C27), and the methyl groups connected with N (C28). The above confirmed the successful grafting of PDMAEMA polymer brushes on the CNFs aerogels.

5



Figure 1. (a) FT-IR spectra and (b) solid-state nanocellulose-based aerogels.

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13C

NMR spectra of

With the successfully grafting of the polymer brushes from the surface of CNFs aerogel, the morphology of CNFs aerogels was examined by SEM. As shown in Figure 2a, the aerogel possessed a porous network structure after the grafting of polymer brushes on its surfaces. In high magnification, the surface of the CNFs aerogel became 6


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rougher, which indicated that there was a layer of polymer brushes on the surface of the aerogel. The grafting ratio was 55% as determined by the change of the weight before and after grafting PDMAEMA brushes (Table S1). Nitrogen adsorption and desorption isotherms were used to examine the evolution of the specific surface area and pore size distribution. As revealed in Figure 2c, the adsorption volume of CNF-g-PDMAEMA was much higher than CNF-PEI at P/P0 > 0.8, indicating the presence of mesopores with the pore size ranging from 2 to 50 nm.3233

Furthermore, the specific surface area of CNF-g-PDMAEMA was 12.72 m2/g, which

was 3 times higher than CNF-PEI (4.30 m2/g) as the polymer brushes increased the surface roughness of CNF-g-PDMAEMA. Additionally, the Barrett-Joyner-Halenda (BJH) pore size distribution of CNFs aerogels (Figure 2d) revealed that there were more pores in the range of 2-10 nm and 60-120 nm in CNF-g-PDMAEMA. In summary, functional CNFs aerogels with polymer brushes can increase its specific surface area and mesoporous structure. The mechanical properties of CNF-PEI and CNF-g-PDMAEMA were studied using the compression strain-stress measurements. Figure 2b indicates that the compression stress of CNF-g-PDMAEMA (61 kPa) was much higher than that of CNF-PEI (30 kPa) when the aerogels were compressed to 50% its thickness. The observed difference is associated with the bulk density, which has the comparatively large influence on the compression strength of aerogels, which increased from 37.17 mg/cm3 for CNF-PEI to 57.58 mg/m3 for CNF-g-PDMAEMA. Although the dimension recovery characteristics of CNF-g-PDMAEMA (~70%) was lower than CNF-PEI, it could recover to its original height in 15 min, demonstrating that the polymer brushes modified CNFs aerogels had the capacity to restore the shape of the aerogel.

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Figure 2. (a) SEM images of CNF-PEI and CNF-g-PDMAEMA, (b) compression stress-strain curves of CNF-PEI and CNF-g-PDMAEMA, (c) nitrogen adsorption and desorption isotherms and (d) Barrett-Joyner-Halenda (BJH) pore size distribution of CNF-PEI and CNF-g-PDMAEMA. Water contact angle measurements were used to confirm the switching capability of the hydrophobic/hydrophilic nature of the aerogels. As shown in Figure 3a, the water contact angle maintained at 130o even after 50 s. It demonstrated that the CNFs aerogel was hydrophobic in the absence of CO2 as the PDMAEMA polymer brushes were in their deprotonated form. After exposing to CO2, the surface of the aerogel gradually switched from hydrophobic to hydrophilic as the PDMAEMA brush became protonated and acquired a positive charge. Figure 3b and Video 1 (SI, Video 1) show the evolution process of the contact angle, which decreased from 130o to 0o within 50 s, confirming that the surface wettability the CNFs aerogel displayed CO2 stimuli-responsive property. The CO2 switchable aerogel was used in the oil-water selective separation. As shown in Figure 3c and Video 2 (SI) the CNFs aerogel could be used to separate 8


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oil from the oil-water mixture (v: v=1:1) and the oil flux was ~4000 Lm-2h-1. After exposing to CO2 for 15 min, the switchable of the aerogel allows it could separate water from the oil-water mixture (SI, Video 3) and the water flux was ~1000 Lm-2h-1. The oil flux was much higher than water flux may due to the change of pore size. After exposing to CO2, the charged and extended PDMAEMA would form hydrogen bonds with H2O, which would reduce the effective pore size.34 When the absence of CO2, the polymer chains of PDMAEMA maintained a compact structure, the CNFs aerogel had a higher effective pore size. Interestingly, a complete opposite separation process was observed after the removal of CO2 from the CNFs aerogel by putting the aerogel in ethanol and bubbling N2 for 15 min. The on-off switch was maintained even after 5 cycles with no reduction in the oil and water flux (Figure 3d). The average total organic carbon (TOC) contents of the feed oil-water mixture and the corresponding filterable water were measured to assess the separation efficiency. The TOC content of the hexane-water mixture was 258375.0 mg/L. For separate oil from oil-water mixture and separate water from the oil-water mixture, the TOC contents of filterable water were 10.2 and 1.2 mg/L, respectively. This means that the separation efficiency of the CO2 responsive aerogel for oil-water mixture was higher than 99.96%. Furthermore, CNF-g-PDMAEMA also exhibited switching ability for other oil-water mixture, such as hexane-water mixture and dichloromethane-water mixture (Figure S1). After separating oil from the dichloromethane-water mixture, the aerogel was placed in water and bubbling CO2, the aerogel turned from hydrophobic to hydrophilic and there was some dichloromethane was desorbed from the aerogel (Figure S2). Therefore, the switchable surface wettability holds an advantage in the avoidance of oilfouling, which will greatly improve its recyclability.35 These results demonstrated that CNF-g-PDMAEMA could be used as a smart responsive separation medium for the selective oil-water separation by introducing and withdrawing CO2. 9



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Figure 3. (a-b) Wettability of CNF-g-PDMAEMA in different conditions, (c) photographs of the hexane-water mixture separation process. (d) variations in the flux of oil and water in the absence and presence of CO2, respectively. The separation efficiency of CNF-g-PDMAEMA was further evaluated using surfactant stabilized emulsions including water-in-petroleum ether emulsion and petroleum ether-in-water emulsion. The separation of as-prepared emulsions was driven by gravity. Under natural conditions, CNF-g-PDMAEMA was capable of separating oil from the water-in-oil emulsion (Figure 4a). When the water-in-oil emulsion passed through the CNF-g-PDMAEMA, demulsification was induced resulting in the coalescence of the oil droplets, and the oil phase could pass through, while the water phase remained above the aerogel.36 After bubble CO2 for 15 min, CNF-g-PDMAEMA could be used to separate water from the oil-inwater emulsion (Figure 4b). The separation process could also be reversed after bubbling N2 from 15 min. Compared to the original milky white feed emulsion, the collected filtrates are clear and clarified. The microscopic images of these emulsions and filtrate are shown in Figure 4a, b. No oil droplets were found in the filtrate. The TOC contents of water-in-petroleum emulsion ether and petroleum ether-in-water emulsion were 480261.0 and 5525.6 mg/L, respectively. The TOC contents of corresponding filterable water were 50.3 and 10


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22.3 mg/L. The higher TOC contents of the filterable water possibly because there was some surfactant dissolved.37 This separation efficiency of the CO2 responsive aerogel for surfactant stabilized emulsions was than 99.6%. All those results signified that CNF-g-PDMAEMA showed outstanding responsive properties and possessed excellent separation efficiency for surfactant-stabilized emulsions.

Figure 4．Optical microscopy images and photographs of surfactant stabilized emulsions before and after filtration: (a) water in oil emulsion, (b) oil in water emulsion. Conclusions In summary, grafting CO2-responsive PDMAEMA polymer blushes from the surface of CNFs aerogel affords tunable surface wettability of CNFs aerogels by simply bubbling CO2 and N2. In the ambient environment (pH of 7, and 25 °C), the surface of the aerogel was hydrophobic, and it could separate oil from the oilwater mixture. After exposing to CO2 for 15 min, the surface of the aerogel changed from hydrophobic to hydrophilic and the separation process was completely reversible. Moreover, the CO2-responsive also show favourable separation efficiency for surfactant-stabilized emulsions. By demonstrating the effectiveness of using CO2-responsive polymer brushes modified CNFs aerogels to realize the controllable surface wettability, this work points out the great potential of developing smart CNFs aerogel for applications requiring tunable

11



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surface ability. We believe that the strategy reported in this work is promising to scale up.

Experimental section Materials A 1.28 wt% aqueous suspension of CNF was purchased from Tianjin Woodelf biotechnology Co. Ltd. α-Bromoisobutyryl bromide (BiBB, >98%), 2-(dimethylamino) ethyl

methacrylate

(DMAEMA,

99.5%,

stabilized

with

MEHQ)

and

poly(ethyleneimine) (PEI, 99%) were purchased from Adamas Reagent Co., Ltd. Dichloromethane (CH2Cl2, super dry, 99.9%) was purchased from J&K Scientific Ltd. Methanol (MeOH, ≥ 99.8%). Acetone (≥99.5%) were purchased from ShangHai Yunli Economic and Trade Co., Ltd. N,N,N',N'',N''-Pentamethyldiethylenetriamine (PMDETA,

99%),

3-glycidoxypropyltrimethoxysilane

(GPTMS,≥98%),

trimethylamine (TEA, ≥99.0%), ethylenediaminetetraacetic acid (EDTA), copper (Ⅱ) bromide (CuBr2, 98.5~100.5%), Span 80 and copper (Ⅰ) bromide (CuBr, ≥98.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Isomeric alcohol ethoxylates (1309) was purchased from BASF (China) Co., Ltd. Deionized water was purified by a Milli-Q purification. CuBr was purified in glacial acetic acid overnight and then washed with methanol several times and dried in vacuum at 40 °C overnight before use. DMAEMA was purified by vacuum distillation. All the other chemicals were used as received. Preparation of cellulose-based aerogel GPTMS (0.064 g, 59.8 μL) was added dropwise into CNF suspension (5 g, 1.28 wt%) under magnetic stirring at ambient temperature. Then 0.32 g 20 wt% PEI aqueous solution was added after 2 h. Thereafter, the suspension was frozen with liquid nitrogen from bottom to the top and the resulting frozen gel was subjected to freeze-drying on a Labconco FD5-3 freeze-dryer at -55 ℃ for 24 h. Consequently, the cellulose-based aerogel was cured for 30 min at 110 ℃ in a vacuum oven to ensure sufficient cross12


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linking followed by rinsing with water to remove unbound materials. Then, the sample was further purified by extracting absorbed water with acetone successively using a Soxhlet extractor. Finally, the sample was dried for 1 h at 60 °C in a vacuum oven. Preparation of bromo-initiator modified aerogel Briefly, CNFs aerogels (200 mg), 30 ml CH2Cl2 and 1.12 ml TEA (0.81 g, 8 mmol) were added to a 50 ml three-necked round-bottomed flask equipped with an addition dropping funnel. BiBB (2.16 g, 10 mmol) and 10 ml CH2Cl2 were added in the funnel. After degassing for 30 min, the BiBB solution was added dropwise into the mixture with constant stirring under nitrogen atmosphere at 0 °C. The reaction was maintained at room temperature for 24 h. Then, the resulted CNF aerogels were rinsed with acetone thoroughly to remove unbound materials and dried in vacuum oven. The obtained aerogel was coded as CNF-Br. Preparation of CNF-g-PDMAEMA Approximately 0.2 g CNF-Br, 4.21 ml DMAEMA and 11.5 mg CuBr2 and 30 ml MeOH/H2O mixed solution (1:1 v/v) were added to a 50 ml three-necked roundbottomed flask. 0.315 ml PMDETA, 0.085 g CuBr and 10 ml MeOH/H2O were added into another flask. After degassing for 30 min, the PMSETA and CuBr solution was transferred into a flask containing CNF-Br and the reaction was kept at 60 °C for 24 h. Consequently, the resulted aerogels were washed with 0.1 M EDTA solution to remove adsorbed copper and rinsed thoroughly with acetone. Finally, the dried aerogel was coded as CNF-g-PDMAEMA. Oil-water separation experiments Oil/water mixture The dried CNF-g-PDMAEMA aerogel was fixed between two glass tubes and a mixture of petroleum ether and water used as the oil-water mixture was poured into the upper glass tube. When the separation was in the stimulation of CO2, the aerogel was immersed in water with CO2 purged for 15 min. When the separation was in the stimulation of N2, the aerogel was placed in ethanol and N2 was bubbled for 15 min to exhaust CO2. At the end of each cycling experiment, the aerogel was washed with 13



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ethanol and water. The average TOC contents of the feed solutions and the corresponding filterable water were measured to evaluate the separation efficiency. Oil/water emulsions The preparation of the emulsions To prepare surfactant-stabilized water-in-oil emulsion, Span 80 (0.2 wt%) was used as the emulsifier and the weight ratio of petroleum ether and water was fixed to 99: 1. Then the mixture was under mechanically stirred for 5 min at 8000 r/min. For surfactant-stabilized oil-in-water emulsion, 1309 (0.2 wt%) was used as the emulsifier and the weight ratio of petroleum ether and water was fixed to 1: 99. Then the mixture was under mechanically stirred for 5 min at 8000 r/min. The separation of the emulsions In accord with the separation of oil/water mixture, the dried CNF-g-PDMAEMA aerogel was fixed between two glass tubes and the as-prepared water-in-oil emulsion was poured into the upper glass tube. When the separation was in the stimulation of CO2, the aerogel was immersed in water with CO2 purged for 15 minutes and then the as-prepared oil-in-water emulsion was poured into the upper glass tube. Characterization The graft ratio of the aerogel was evaluated by the weight change after the modification with PDMAEMA. The graft ratio (W, wt %) was calculated according to eq. 1: W (%) 

m  m0 m0

(1)

Where m0 (g) is the dried weight of the CNF-Br and m (g) is the dried weight of the CNF-g-PDMAEMA. ATR FT-IR spectra of the aerogel was carried out on a PerkinElmer Spectrum-Two (American) in the range of 4000-500 cm-1 at a resolution of 4 cm-1. Solid state 13CNMR spectras were performed on an AVANCE II 400MHz spectrometer (Bruker, Switzerland). The surface morphology of the aerogel was characterized by a Hitachi TM-3030 scanning electron microscopy (Japan). The pore structure of the aerogel was 14


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determined through Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption measurements using a V-Sorb 2800P (China). The mechanical properties of the aerogel were evaluated on a universal testing machine (UH6502, China) at a compression speed of 2 mm/min, the maximum compression strain (ε) was set to 50%. Water contact angle measurements were tested by a contact angle goniometer (Theta, Sweden). The average TOC of the feed solutions and corresponding filterable water ware measured by TOC analyzer (Shimadzu, Japan)

Associated content The Supporting information is available free of charge on line. The weight changes of CNFs aerogels before and after grafting PDMAEMA; photographs of hexane-water and dichloromethane-water mixture separation processes; photos of CNF-g-PDMAEMA after separating dichloromethane from dichloromethane-water mixture. In the presence of CO2, the evolution process of the contact angle (Video 1) In the absence of CO2, separate oil from oil-water mixture (Video 2) In the presence of CO2, separate water from oil-water mixture (Video 3)

Author information Corresponding Authors *E-mail: [email protected]. Phone: +86 21 67792720. Fax: +86 21 67792707 (Z.M.). *E-mail: [email protected]. Phone: +86 21 67792605. Fax: +86 21 67792707 (X.S.). Notes The authors declare no competing financial interest.

Acknowledgments This work was financially supported by the Fundamental Research Funds for the 15



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Central Universities (No. 2232018A3-04, No. 2232018-02, and No. 2232017A-05), the National Key R&D Program of China (No. 2016YFC0802802), and the Programme of Introducing Talents of Discipline to Universities (No. 105-07-005735).

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CO2-Responsive Cellulose Nanofibers Aerogels for Switchable Oil

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