Dually Responsive Janus Composite Nanosheets - Macromolecules

May 29, 2015 - They are dually responsive to pH and temperature since wettability of the two sides is tunable accordingly. The nanosheets can serve as...
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Dually Responsive Janus Composite Nanosheets Ziguang Zhao,†,‡ Fuxin Liang,*,† Guolin Zhang,‡ Xuyang Ji,†,‡ Qian Wang,† Xiaozhong Qu,† Ximing Song,‡ and Zhenzhong Yang*,† †

State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Liaoning Provincial Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, Liaoning University, Shenyang 110036, China S Supporting Information *

ABSTRACT: Janus composite nanosheets of PNIPAM/silica/PDEAEMA are synthesized by sequential ATRP grafting two polymers from the corresponding sides of the Janus silica nanosheets. They are dually responsive to pH and temperature since wettability of the two sides is tunable accordingly. The nanosheets can serve as a responsive solid emulsifier. Type and stability of the emulsions are triggered by simply changing pH and temperature.

sheets.8 It is noted that only one individual responsive species is grafted on one side, while the other alkyl-group-terminated side preserves hydrophobicity. It will be more interesting to selectively graft two responsive polymers onto the both sides, which can be triggered with multiple stimuli. Herein, we report a facile approach to synthesize pH and temperature dually responsive Janus composite nanosheets by ATRP (Scheme 1). ATRP is advantageous to precisely tune composition and thickness of functional polymer brushes.9 The synthesis involves three main steps: (1) Silica Janus nanosheets are synthesized by crushing the Janus hollow spheres. Different from the previous alkyl/amine silica Janus ones, the other side is terminated with Si−OH group instead of an alkyl group which facilitates a second termination of ATRP agent thereby. (2) An ATRP agent is conjugated onto the amine-groupterminated side. A thermal responsive polymer PNIPAM is selectively grafted by ATRP.10 Afterward, the Br group is deactivated with NaN3. (3) After a second conjugation of ATRP agent onto the Si−OH side, another ATRP polymerization is performed to graft a pH-responsive PDEAEMA.11 The dually responsive Janus nanosheets are fabricated. Wettability of the two sides can be reversibly tunable between hydrophilic and hydrophobic by changing either temperature or pH. The Janus composite nanosheets serve as a responsive solid emulsifier; type and stability of the emulsions can be adjusted by simply changing pH or (and) temperature.

1. INTRODUCTION Janus materials with two compositions distinctly compartmentalized onto the same object have attracted considerable attention due to their anisotropic characteristics in composition and shape. They have shown diversified promising performances and potential applications.1 In terms of amphiphilic performance, Janus materials can serve as solid surfactants to stabilize emulsions more effectively than the homogeneous ones due to the Pickering effect.2 In comparison with the spherical particles, Janus nanosheets can lead to more stable emulsions since their rotation at an interface is highly restricted.3 Polymeric Janus nanodiscs are synthesized by disintegration of partially cross-linked supramolecular structures of block copolymers.4 However, they are easily swollen and thus deformable in solvents. We have previously proposed a simple way to synthesize silica Janus nanosheets by crushing the corresponding Janus hollow spheres.5 Compared with the previous methods such as lithography etching silicon substrates to prepare robust Janus sheets,6 our approach is advantageous in that the Janus nanosheets can be fabricated on a large scale. Besides asymmetric shape, composition control is another key concern to determine their performance. Especially, the Janus performance and thus aggregation of the materials will be triggered under external stimuli if the compositions are responsive. Recently, some responsive Janus particles have been reported by grafting the corresponding polymers onto one side, whose shape and surface wettability are tuned accordingly.7 Meanwhile, we have reported on grafting responsive polymers or ionic liquid moiety onto the aminegroup-terminated side of the alkyl/amine silica Janus nano© XXXX American Chemical Society

Received: February 19, 2015 Revised: April 28, 2015

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DOI: 10.1021/acs.macromol.5b00365 Macromolecules XXXX, XXX, XXX−XXX

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subjected to another freeze−pump−thaw, the mixture was stirred at 30 °C for 24 h. The polymerization was terminated after exposure to air. The composite nanosheets were centrifugated. After discarding the top solvent, the sediments were redispersed in ethanol and subjected to further centrifugation. This purification cycle was repeated for three times and vacuum-dried at 30 °C for 24 h. The PNIPAM/silica Janus composite nanosheets were synthesized. 50 mg of the PNIPAM/silica Janus composite nanosheets was dispersed in 10 mL of dry DMF, and 25 mg of sodium azide was added. The mixture was stirred at 90 °C for 24 h. The product was washed with water and centrifugated at 12 000 rpm for three times. The azide-group-terminated PNIPAM/silica Janus composite nanosheets were obtained. 2.5. Synthesis of PNIPAM/Silica/PDEAEMA Janus Composite Nanosheets. 50 mg of the azide-terminated PNIPAM/silica Janus composite nanosheets was dispersed in 15 mL of ethanol, and 25 mg of 3-aminopropyldimethylethoxysilane was added. The reaction was performed at 70 °C for 24 h under nitrogen. The other silica side was thus terminated with amine group. After centrifugation at 12 000 rpm for 5 min and washing with ethanol for three times, the amine-groupterminated PNIPAM/silica composite nanosheets were dried. After 50 mg of the amine-group-terminated Janus nanosheets was dispersed in 20 mL of dry dichloromethane, 0.4 mL of triethylamine and 0.5 mL of 2-bromoisobutyryl bromide were added. The reaction was performed at 25 °C for 24 h. After centrifugation and washing with dichloromethane for three times, the product was vacuum-dried at 40 °C for 12 h. The ATRP-agent-terminated PNIPAM/silica Janus composite nanosheets were synthesized. After 50 mg of the ATRP-agent-terminated PNIPAM/silica Janus composite nanosheets was dispersed in 6 mL of methanol in a glass tube, 0.5 g of DEAEMA and 24 mg of PMEDTA were added. The mixture was degassed after three cycles of freeze−pump−thaw. In a frozen state, 14 mg of CuBr was added under nitrogen. The tube was subjected to another freeze−pump−thaw and the reaction at 70 °C for 24 h. The reaction was terminated after exposure to air at room temperature. After centrifugation and washing with THF, the product was vacuum-dried at 30 °C for 24 h. 2.6. Labeling Azide-Terminated PNIPAM by Click Reaction. 10-Undecynoic acid (0.73 g, 4.0 mmol) and NHS (0.70 g, 8.2 mmol) were dissolved in 20 mL of dry DMF. EDC·HCl (1.16 g, 8.24 mmol) was added at 0 °C. The solution was stirred at room temperature for 12 h. After 0.30 g of the amine-group-capped silica NPs was dispersed in 10 mL of DMF, the mixture was added. The reaction was performed for 20 h. After centrifugation and washing with THF, the alkynegroup-capped silica NPs were vacuum-dried at 30 °C for 24 h. 50 mg of the PNIPAM/silica/PDEAEMA Janus composite nanosheets and 25 mg of the alkyne-group-capped silica NPs were mixed in 6 mL of DMF under unltrasonication. 12 mg of PMEDTA was added. The mixture was degassed after three cycles of freeze− pump−thaw. In a frozen state, 7 mg of CuBr was added under nitrogen. The tube was subjected to another freeze−pump−thaw and reaction at 25 °C for 24 h. The Janus composite nanosheets were centrifugated at 5000 rpm for 5 min and redispersed in ethanol. The PNIPAM/silica/PDEAEMA Janus composite nanosheets were labeled with the silica NPs onto the azide-terminated PNIPAM side by click reaction. 2.7. Emulsification with the Janus Composite Nanosheets. 0.003 g of the PNIPAM/silica/PDEAEMA Janus composite nanosheets was added in a mixture (0.8 mL of water and 0.8 mL of toluene). After vigorously shaking, the mixture stood there forming emulsions at varied pH and temperature. 2.8. Characterization. In order to avoid aggregation of the Janus nanosheets, ethanol was used as a dispersant. Scanning electron microscopy (SEM) measurement was performed with a HITACHIS4800 apparatus operated at an accelerating voltage of 15 kV. The samples were ambient dried and vacuum sputtered with Pt. Morphology of the ultramicrotomed samples was characterized using transmission electron microscopy (JEOL 100CX operating at 100 kV). Thickness of the samples was measured by AFM with Bruker Multimode 8. Morphology of the emulsions stabilized with the Janus

Scheme 1. Illustrative Synthesis of the Dually Responsive Janus Composite Nanosheets: (a) Onto the Amine-GroupTerminated Side of the Janus Silica Nanosheets, ATRP Agent Is Selectively Conjugated; (b) Selective Grafting of Thermal Responsive PNIPAM by ATRP and Deactivation of the End Active Group with NaN3; (c) Grafting pHResponsive PDEAEMA via a Second Selective ATRP from the Other Side

2. EXPERIMENTAL METHODS 2.1. Materials. 3-Aminopropyltrimethoxysilane (APTMS), 2bromoisobutyryl bromide, triethylamine, pentamethyldiethylenetriamine (PMDETA), and ethylene dimethacrylate (EGDMA) were purchased from ACROS. Tetraethyl orthosilicate (TEOS), n-decane, tetrahydrofuran, 2-propanol, toluene, dichloromethane, N,N-dimethylformamide (DMF), hydrochloric acid solution (36−38%), trisodium citrate, ferric chloride hexahydrate (FeCl3·6H2O), ferrous chloride (FeCl2), sodium azide (NaN3), and copper bromide (CuBr) were purchased from Sinopharm Beijing. Tris(2-dimethylaminoethyl)amine (Me6TREN) and 2-(dimethylamino)ethyl methacrylate (DEAEMA) were purchased from TCI. Hydrolyzed styrene−maleic anhydride (HSMA) copolymer was synthesized.12 N-Isopropylacrylamide (NIPAM), 10-undecynoic acid, N-hydroxysuccinimide (NHS), and N-(3-(dimethylamino)propyl)-N′-ethylcarbodiimide hydrochloride (EDC·HCl) were purchased from J&K Chemical. They were recrystallized with toluene and hexane. DEAEMA and dichloromethane were distilled under reduced pressure. Copper bromide was purified in acetic acid and washed with ethanol for three times. All other reagents were used as received without further purification. 2.2. Synthesis of Silica Janus Nanosheets. The silica Janus nanosheets were synthesized by a self-organized sol−gel process at an emulsion interface.5 15 mL of 10 wt % aqueous HSMA was dissolved in 75 mL of water. Hydrochloric acid aqueous solution (2 mol/L) was added to adjust pH ∼ 2.5. 10 g of n-decane, 1.1 g of APTMS, and 5.2 g of TEOS were mixed as an oil phase. After the oil phase was added into the aqueous phase and homogenized at a speed of 13 000 rpm for 5 min, an oil-in-water emulsion formed. The emulsion stood at 70 °C for 12 h for a self-organized interfacial sol−gel process forming a silica shell. After the resultant emulsion was cooled down to room temperature, the silica Janus hollow spheres were obtained by a sequential filtration and washing with water for three times. The silica Janus nanosheets were prepared by crushing the hollow spheres with a colloid mill. 2.3. Synthesis of ATRP-Agent-Terminated Silica Janus Nanosheets. 50 mg of the silica Janus nanosheets was dispersed in 20 mL of dry dichloromethane containing 2% (v/v) triethylamine. 0.5 mL of 2-bromoisobutyryl bromide was added under stirring. The reaction was carried out at 0 °C for 0.5 h and room temperature for another 24 h to allow a selective termination of the amine group of the silica Janus nanosheets. After centrifugation and washing with dichloromethane, the ATRP agent terminated Janus silica nanosheets were obtained as a light yellow product. 2.4. Synthesis of Azide-Terminated PNIPAM/Silica Composite Janus Nanosheets. 50 mg of the ATRP agent terminated silica Janus nanosheets, 30 mg of Me6TREN, 0.6 g of NIPAM, and 6 mL of 2-propanol were mixed in a glass tube. The mixture was degassed after three cycles of freeze−pump−thaw. In a frozen state, 15 mg of copper bromide (CuBr) was added under nitrogen. After the tube was B

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Figure 1. (a) SEM image of the Janus silica hollow spheres. (b) SEM image of the silica Janus nanosheets after crushing the Janus hollow spheres. (c, d) SEM and AFM images of the ATRP-agent-terminated silica Janus nanosheets.

Figure 2. (a) SEM image of the PNIPAM/silica Janus nanosheets (+: PNIPAM side; −: silica side). (b) Cross-section TEM image of the PNIPAM/ silica Janus nanosheets. (c) SEM image of the PNIPAM/silica/PDEAEMA Janus nanosheets. (d) Cross-section TEM image of the PNIPAM/silica/ PDEAEMA Janus nanosheets. (e) AFM image of PNIPAM/silica/PDEAEMA Janus nanosheets. (f) Selective labeling PNIPAM side of the PNIPAM/silica/PDEAEMA Janus nanosheets with the alkyne-group-capped silica NPs by click reaction. C

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Figure 3. SEM images of the nanosheets after freeze-drying aqueous dispersions of PNIPAM/silica/PDEAEMA Janus composite nanosheet. Inset: the corresponding dispersions at varied pH and temperature: (a) pH = 10 and T = 50 °C; (b) pH = 2 and T = 50 °C; (c) pH = 10 and T = 25 °C; (d) pH = 2 and T = 25 °C. nanosheets was characterized using Leica DMLP microscope. FT-IR spectroscopy was performed with the sample/KBr pressed pellets after scanning samples for 32 times using a Bruker EQUINOX 55 spectrometer. Thermogravimetric analysis (TGA) was performed using the PerkinElmer Pyris 1 TGA under air at a scanning rate of 10 °C/min.

From the ATRP conjugated side, a representative thermal responsive polymer PNIPAM is grafted forming the PNIPAM/ silica Janus composite nanosheets (Figure 2a). The PNIPAM grafted side (+) becomes coarse, while the other side (−) remains smooth. In the FT-IR spectrum, new peaks at 1645 and 1543 cm−1appear (Figure S3). They are assigned to the amide bond of PNIPAM. The PNIPAM to silica weight ratio is 54/46 measured by TGA (Figure S4). The PNIPAM/silica Janus composite nanosheets are thick (50.9 nm) (Figure S5). A bilayered structure of the PNIPAM/silica nanosheets is observed from the cross-section TEM image (Figure 2b). The grafting content is tunable with polymerization time. At early polymerization stage, the grafting content increases rapidly with time (Figure S6). After 24 h, the grafting content reaches a plateau. No remarkable increase is found with further prolonging polymerization time, for example 35 h. In our study, the polymerization time is fixed at 24 h to ensure completion of the polymerization. On the other hand, at the fixed polymerization time of 24 h, the grafting content is also tunable with monomer feeding content. The PNIPAM/silica weight ratio increases linearly with the monomer/silica feeding ratio (Figure S7). Along a similar approach, from other side of the PNIPAM/ silica Janus composite nanosheets, another representative pHresponsive polymer PDEAEMA can be grafted via ATRP. Prior to conjugating the ATRP agent onto the Si−OH side, the active end of PNIPAM is terminated with NaN3 to prohibit a further growth of PDEAEMA onto the PNIPAM side. Br element disappears after the termination (Figure S8). The new peak at 2098 cm−1 indicates the presence of an azide group. No characteristic peak at 1730 cm−1 of PDEAEMA appears after addition of DEAEMA (curve a, Figure S9). In comparison, PDEAEMA can be further grafted onto the PNIPAM side by ATRP if the PNIPAM side is not deactivated with NaN3 (curve b, Figure S9). Only one but thicker polymer layer is observed under cross-section TEM. The other Si−OH side of the azideterminated PNIPAM/silica Janus nanosheets is modified with

3. RESULTS AND DISCUSSION The oil phase of n-decane containing two silanes of APTMS and TEOS is emulsified in water forming an oil-in-water emulsion in the presence of HSMA. At the emulsion interface,an acid catalyzed self-organized sol−gel process occurs forming a Janus silica shell.5 Exterior surface of the shell is terminated with amine group, while the interior surface is terminated with Si−OH group. After removal of the internal oil phase by dissolution, silica Janus hollow spheres are achieved. After drying, they become collapsed (Figure 1a). The exterior surface is smooth. After the Janus hollow spheres are crushed by colloidal milling, silica Janus nanosheets are obtained (Figure 1b). Amine and Si−OH groups are compartmentalized onto the corresponding sides. They are dispersible in water but not in oil. The peaks at 1645 and 1550 cm−1 indicate the presence of amine group (Figure S1). The silica Janus nanosheets are thick 30.0 nm measured by AFM. Onto the amine-group-terminated side, an ATRP agent of 2-bromoisobutyryl bromide is selectively conjugated between amine and COBr groups. Both sides preserve smooth after the conjugation (Figure 1c). The nanosheets become Janus which can be well dispersible in water and oil. The nanosheets become slightly thicker 32.9 nm (Figure 1d). They are uniform in thickness. Prior to the conjugation, no Br element is present in the silica Janus nanosheets (Figure S2a). 1.92% of Br element is detected after the conjugation (Figure S2b). It is confirmed that ATRP agent of 2-bromoisobutyryl bromide has been selectively conjugated onto the amine-terminated side of the silica Janus nanosheets. D

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Figure 4. Emulsification of the immiscible mixture of toluene/water with the PNIPAM/silica/PDEAEMA Janus composite nanosheets. (a) Optical microscopy image of the mixture (inset) at pH = 10 and T = 50 °C, no emulsification occurs, the hydrophobic nanosheets dispersible in toluene; (b) at pH = 2 and T = 25 °C, no emulsification occurs, the hydrophilic nanosheets dispersible in water; (c) a toluene-in-water emulsion forms at pH = 2 and T = 50 °C; (d) a water-in-toluene emulsion forms at pH = 10 and T = 25 °C; (e, f) SEM images of the frozen paraffin droplets stabilized with the Janus nanosheets at two magnifications. A single layer of the nanosheets is present onto the paraffin core surface.

PNIPAM/silica/PDEAEMA Janus composite nanosheets should be dually responsive to pH and temperature. The dual response in water is systematically characterized. At pH = 10 and T = 50 °C, both PDEAEMA and PNIPAM sides are hydrophobic. The composite nanosheets are not dispersible in water but precipitate (Figure 3a). After freeze-drying the system, highly heavy aggregates are found. When aqueous HCl is added at 50 °C to adjust pH ∼ 2, the hydrophobic PDEAEMA side is transformed into hydrophilic while the PNIPAM is preserved hydrophobic. The nanosheets become amphiphilic (Janus), which are dispersible in water (Figure 3b). After freeze-drying the dispersion, a face-to-face stacking bilayered structure is observed. It is reasonable that the PDEAEMA sides should face toward the aqueous phase while the hydrophobic PNIPAM sides are stacked. When the system is cooled down to 25 °C from 50 °C at the fixed pH ∼ 10, the PNIPAM side is transformed from hydrophobic to hydrophilic while the PDEAEMA remains hydrophobic. The nanosheets are amphiphilic to form another bilayered structure by back-toback stacking (Figure 3c). While the system is cooled down to 25 °C from 50 °C, acid is simultaneously added to adjust pH from 10 to 2; both the PDEAEMA and PNIPAM sides are transformed from hydrophobic to hydrophilic. The composite nanosheets are better dispersible in water (Figure 3d). After drying the dispersion, the composite nanosheets are present in the form of individual layer. In a converse way, the dispersion

3-aminopropyldimethylethoxysilane and 2-bromoisobutyryl bromide to conjugate the ATRP agent. 1.34% of Br element is present (Figure S10). After another polymerization of DEAEMA, the PNIPAM/silica/PDEAEMA Janus composite nanosheets are achieved (Figure 2c). Both sides of the Janus nanosheets become coarse. The new peak at 1730 cm−1 is assigned to the carbonyl group of PDEAEMA (Figure S11). TGA result indicates that polymer/silica weight ratio is 74/26 (Figure S12). Based on the PNIPAM/silica weight ratio of 54/ 46, the PDEAEMA/silica/PNIPAM weight ratio is calculated 42/26/32. Triple-layered sandwich structure of the PNIPAM/ silica/PDEAEMA Janus nanosheets is observed from the crosssection TEM image (Figure 2d). The Janus nanosheets become thicker from 50.9 to 70.8 nm (Figure 2e). To further clarify the distinct compartmentalization of the two polymers, the alkynegroup-capped silica NPs (∼20 nm) are used to preferentially label the azide-group-terminated PNIPAM side by click reaction (Figure 2f). The PNIPAM side becomes more coarsening, while no NPs are found on the other side. It is known that PNIPAM is thermal responsive with a LCST ∼ 32 °C.10 After increasing temperature to 50 °C across the LCST, the polymer chains become hydrophobic from hydrophilic. PDEAEMA is pH-responsive with a pKa ∼ 7.2.11 At low pH, for example 2, the protonated polymer chains become cationic and highly hydrophilic. At high pH of 10, the deprotonated polymer chains become hydrophobic. The E

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*E-mail: [email protected]; Fax: 86-10-82619206; Tel: 86-1062559373 (Z.Y.).

becomes unstable and the nanosheets precipitate again after simultaneously increasing temperature and pH. This implies that transformation is reversible. The Janus composite nanosheets can serve as a solid emulsifier to stabilize emulsions. The emulsion stability can be triggered with pH and thermal stimuli. Toluene and water (1:1 vol/vol) are selected as a representative immiscible pair. At pH = 10 and T = 50 °C, no emulsion forms. The nanosheets are preferentially dispersible in the top toluene phase (Figure 4a). It is understandable that the nanosheets are hydrophobic. At pH = 2 and T = 25 °C, no emulsion forms. The nanosheets are hydrophilic and preferentially dispersible in the bottom water phase (Figure 4b). When the mixture system is heated to 50 °C while pH is maintained at 2, the PNIPAM side becomes hydrophobic from hydrophilic while the PDEAEMA side remains hydrophilic. The nanosheets are Janus. A toluene-inwater emulsion forms (Figure 4c). After the emulsion is cooled down to 25 °C, de-emulsification occurs, forming two layers similar to the system as shown in Figure 4b. The nanosheets are dispersible in the bottom aqueous phase. At an equal amount of water and toluene, continuous phase of the emulsion is water. This reveals that the hydrophilic PDEAEMA dominates. It is understandable that weight content of PDEAEMA (42 wt %) is much higher than PNIPAM (32 wt %) in the composite nanosheets. On the other hand, when pH is increased to 10 from 2 while temperature is maintained at 25 °C, a water-intoluene inverse emulsion forms (Figure 4d). This reveals that the hydrophobic PDEAEMA is dominant. The de-emulsification occurs after increasing temperature to 50 °C. The nanosheets are dispersible in the top oil phase. In order to observe the orientation of the Janus nanosheets at the oil−water interface, a melt paraffin (Tm: 52−54 °C) in water emulsion forms at high temperature (70 °C) and pH = 2. After the paraffin emulsion is cooled down to ambient temperature, orientation structure of the Janus nanosheets on the paraffin core surface is frozen (Figure 4e). A magnified image clearly indicates that the paraffin core surface is covered with a single layer of the Janus nanosheets (Figure 4f).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by MOST of China (2012CB933200) and NSFC of China (51173191, 51233007, and 51373073). (1) (a) Walther, A.; Müller, A. H. E. Chem. Rev. 2013, 113, 5194− 5261. (b) Hu, J.; Zhou, S. X.; Sun, Y. Y.; Fang, X. S.; Wu, L. M. Chem. Soc. Rev. 2012, 41, 4356−4378. (c) Jiang, S.; Granick, S. Janus Particles Synthesis, Self-assembly and Applications; RSC: London, 2012. (d) Liang, F. X.; Zhang, C. L.; Yang, Z. Z. Adv. Mater. 2014, 26, 6944−6949. (e) Kim, J. W.; Larsen, R. J.; Weitz, D. A. Adv. Mater. 2008, 20, 3239−3243. (2) (a) Binks, B. P.; Fletcher, P. D. I. Langmuir 2001, 17, 4708−4710. (b) Glaser, N.; Adams, D. J.; Böker, A.; Krausch, G. Langmuir 2006, 22, 5227−5229. (c) Fujii, S.; Yokoyama, Y.; Miyanari, Y.; Shiono, T.; Ito, M.; Yusa, S.; Nakamura, Y. Langmuir 2013, 29, 5457−5465. (3) (a) Nonomura, Y.; Komura, S.; Tsujii, K. Langmuir 2004, 20, 11821−11823. (b) Nonomura, Y.; Komura, S.; Tsujii, K. J. Phys. Chem. B 2006, 110, 13124−13131. (4) (a) Walther, A.; Andre, X.; Drechsler, M.; Abetz, V.; Müller, A. H. E. J. Am. Chem. Soc. 2007, 129, 6187−6198. (b) Deng, R. H.; Liang, F. X.; Zhou, P.; Zhang, C. L.; Qu, X. Z.; Wang, Q.; Li, J. L.; Zhu, J. T.; Yang, Z. Z. Adv. Mater. 2014, 26, 4469−4472. (5) (a) Liang, F. X.; Liu, J. G.; Zhang, C. L.; Qu, X. Z.; Li, J. L.; Yang, Z. Z. Chem. Commun. 2011, 47, 1231−1233. (b) Liang, F. X.; Shen, K.; Qu, X. Z.; Zhang, C. L.; Wang, Q.; Li, J. L.; Liu, J. G.; Yang, Z. Z. Angew. Chem., Int. Ed. 2011, 50, 2379−2382. (6) (a) Bucaro, M. A.; Kolodner, P. R.; Taylor, J. A.; Sidorenko, A.; Aizenberg, J.; Krupenkin, T. N. Langmuir 2009, 25, 3876−3879. (b) Dorvee, J. R.; Derfus, A. M.; Bhatia, S. N.; Sailor, M. J. Nat. Mater. 2004, 3, 896−899. (7) (a) Tanaka, T.; Okayama, M.; Minami, H.; Okubo, M. Langmuir 2010, 26, 11732−11736. (b) Yamagami, T.; Kitayama, Y.; Okubo, M. Langmuir 2014, 30, 7823−7832. (c) Tu, F. Q.; Lee, D. J. Am. Chem. Soc. 2014, 136, 9999−10006. (d) Tu, F. Q.; Lee, D. Chem. Commun. 2014, 50, 15549−15552. (e) Lu, C. L.; Urban, M. W. ACS Macro Lett. 2014, 3, 346−352. (8) (a) Chen, Y.; Liang, F. X.; Yang, H. L.; Zhang, C. L.; Wang, Q.; Qu, X. Z.; Li, J. L.; Cai, Y. L.; Qiu, D.; Yang, Z. Z. Macromolecules 2012, 45, 1460−1467. (b) Ji, X. Y.; Zhang, Q.; Liang, F. X.; Chen, Q. N.; Qu, X. Z.; Zhang, C. L.; Wang, Q.; Li, J. L.; Song, X. M.; Yang, Z. Z. Chem. Commun. 2014, 50, 5706−5709. (c) Yang, H. L.; Liang, F. X.; Chen, Y.; Zhang, C. L.; Wang, Q.; Qu, X. Z.; Li, J. L.; Wu, D. C.; Yang, Z. Z. Macromolecules 2013, 46, 2754−2959. (9) (a) Huang, X.; Appelhans, D.; Formanek, P.; Simon, F.; Voit, B. Macromolecules 2011, 44, 8351−8360. (b) Panahian, P.; Kalajahi, M. S.; Hosseini, M. S. Ind. Eng. Chem. Res. 2014, 53, 8079−8086. (10) (a) Wang, X. H.; Qiu, X. P.; Wu, C. Macromolecules 1998, 31, 2972−2976. (b) Wu, T.; Zhang, Y. F.; Wang, X. F.; Liu, S. Y. Chem. Mater. 2008, 20, 101−109. (c) Zheng, Q.; Pan, C. Y. Eur. Polym. J. 2006, 42, 807−814. (11) (a) Tan, J. F.; Ravi, P.; Too, H. P.; Hatton, T. A.; Tam, K. C. Biomacromolecules 2005, 6, 498−506. (b) Sun, J. T.; Hong, C. Y.; Pan, C. Y. J. Phys. Chem. C 2010, 114, 12481−12486. (c) Lee, S. H.; Lee, D. H.; Lee, W. J.; Kim, S. O. Adv. Funct. Mater. 2011, 21, 1338−1354. (12) Jin, Z. G.; Wang, Y.; Liu, J. G.; Yang, Z. Z. Polymer 2008, 49, 2903−2910.

4. CONCLUSION In summary, a facile and general approach is developed to synthesize dually responsive Janus composite nanosheets. The pH-responsive PDEAEMA and thermal responsive PNIPAM are selectively grafted onto the corresponding sides of the silica Janus nanosheets via ATRP. Wettability of each side can be separately tunable between hydrophilic and hydrophobic by changing pH or temperature. The composite nanosheets can serve as a responsive solid emulsifier to stabilize emulsions, whose stability is triggered by simply changing pH or (and) temperature. The dually responsive Janus nanosheets may be promising in phase transfer catalysis.

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S Supporting Information *

Figures S1−S12. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.macromol.5b00365.

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*E-mail: [email protected]; Fax: 86-10-62559373; Tel: 8610-82619206 (F.L.). F

DOI: 10.1021/acs.macromol.5b00365 Macromolecules XXXX, XXX, XXX−XXX