Light-Triggered Responsive Janus Composite Nanosheets

Oct 2, 2015 - We report the synthesis of light-triggered Janus composite nanosheets and their Janus performance. Onto the amine-group terminated side ...
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Light-Triggered Responsive Janus Composite Nanosheets Ziquan Cao,†,‡ Guojie Wang,*,† Ying Chen,‡ Fuxin Liang,‡ and Zhenzhong Yang*,‡ †

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

‡

S Supporting Information *

ABSTRACT: We report the synthesis of light-triggered Janus composite nanosheets and their Janus performance. Onto the amine-group terminated side of silica Janus nanosheets, a photo-responsive spiropyran-containing polymer (PSPMA) brush has been prepared by ATRP, while the other side terminated with hydrophobic octyl groups is preserved. Upon UV irradiation, the hydrophobic PSPMA side becomes hydrophilic since the hydrophobic spiropyran changes to the hydrophilic zwitterionic merocyanine form (or vice versa with visible light). Consequently, the PSPMA/silica composite nanosheets become Janus from hydrophobic or vice versa. The Janus composite nanosheets can serve as a responsive solid emulsifier, thus the stability of the emulsions can be remotely triggered with light. Unlike those pH- or temperature-responsive Janus materials, the light-triggering process requires no additional input of chemicals or thermal energy.

1. INTRODUCTION Janus materials with two different compositions distinctly compartmentalized on their surface have witnessed great advances due to their diversified promising performances and potential applications.1 In terms of amphiphilic performance, Janus materials can serve as solid surfactants to emulsify immiscible liquids.2 Over the past 20 years, the controlling shape of Janus materials is the main concern, including particle,3 rod,4 and disk.5 More recently, rendering responsive performances to Janus materials has gained more attention, whose Janus performance is tunable in response to environmental stimuli.6 For example, pH-responsive bicomponent polymer Janus silica particles are synthesized by “grafting” approach.7 Amphiphilicity of the pH-responsive Janus particles can be switched due to the changes in shape and chemistry upon changing pH.8 Dually pH- and temperature-responsive mushroom-like Janus polymer particles are synthesized which can serve as a dual responsive particulate surfactant.9 Compared with Janus particles, Janus nanosheets are more efficient to stabilize emulsions due to their highly confined rotation at interfaces.10 We have recently developed a facile approach to large scale production of silica Janus nanosheets by crushing the corresponding silica Janus hollow spheres.11 After selectively grafting pH-responsive poly(2-(diethylamino) ethyl methacrylate) or thermally responsive poly(N-isopropylacrylamide) onto one side of the silica Janus nanosheets, the corresponding responsive Janus nanosheets are derived. Accordingly, stability of the emulsions emulsified with the Janus composite nanosheets can be easily triggered by changing pH or temperature.12 It is noted that the conventional pH or temperature response is usually realized by changing the physicochemical parameters of the continuous phase. Addi© XXXX American Chemical Society

tional input of chemical agents or thermal energy is required. Thus, it will be more attractive to develop optical responsive Janus nanosheets, which can be triggered by directly changing physicochemical parameters of the Janus materials. The parameters of the continuous phase are less influenced. It is allowed to control the Janus performance spatially and temporally. It is rational to employ photo switchable molecules for example spiropyran (SP).13 Upon UV irradiation, the nonpolar closed SP form should transform into polar open isomer of zwitterionic merocyanine (MC) via photochemical cleavage of the C−O bond. The MC form can be transformed back to the original SP form by exposure to visible light. The transformation is reversible. Accordingly, the wettability can be switched reversibly between the hydrophobic SP form and hydrophilic MC form. Herein, we report the synthesis of light-triggered Janus composite nanosheets as illustrated in Scheme 1a. Amine-group terminated silica Janus nanosheets are prepared by crushing the corresponding Janus hollow spheres by emulsion interfacial self-organized sol−gel process.11,14 ATRP agent is selectively conjugated onto the amine-group terminated side, while the other hydrophobic side is preserved. Onto the ATRP agent conjugated nanosheets, a photochromic monomer spiropyran SPMA is polymerized by atom transfer radical polymerization (ATRP). Both hydrophobic octyl-group and SP moiety are distinctly compartmentalized onto the corresponding sides of the PSPMA/silica composite nanosheets. Wettability of the PSPMA side is reversibly tunable between hydrophobic and Received: June 10, 2015 Revised: September 22, 2015

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DOI: 10.1021/acs.macromol.5b01257 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. (a) Illustrative Synthesis of the Light-Triggered Janus Composite Nanosheets and Performancea and (b) Structural Transformation of the SP Form to the MC Form via Photochemical Cleavage of the C−O Bond after Irradiation with UV or Vice Versa with Visible Light

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Onto the amine-group terminated side of the silica Janus nanosheets, a representative optical responsive polymer PSPMA is grafted by ATRP, while the other side with hydrophobic octyl-groups is preserved. Upon UV irradiation, the hydrophobic PSPMA brush becomes hydrophilic. In a reversible way, the hydrophilic performance can be reverted to the hydrophobic performance upon visible light irradiation. water. The desired amount of 2 mol/L aqueous hydrochloric acid was added to adjust pH ∼ 3. Then 10.0 g of n-decane, 5.2 g of TEOS, 1.1 g of APTES, and 1.38 g of OTES were mixed homogeneously as an oil phase. After the oil mixture was dispersed into the aqueous solution, the mixture was homogenized at a speed of 13 000 rpm for 5 min, forming an oil-in-water emulsion. The emulsion stood at 70 °C for 12 h for a self-organized interfacial sol−gel process. Janus hollow spheres were obtained after dissolution of the internal oil phase. The silica Janus nanosheets were prepared by crushing the Janus hollow spheres with a colloid mill. Synthesis of the ATRP Agent Conjugated Silica Nanosheets. First, 100.0 mg of the silica Janus nanosheets was dispersed in 50.0 mL of dry dichloromethane containing triethylamine (0.6 mL) in a flask. Then, 1.0 mL of 2-bromoisobutyryl bromide dissolved in 5.0 mL of dry dichloromethane was added dropwise into flask. The mixture was stirred for 24 h at room temperature. The modified nanosheets were centrifugated and washed with dichloromethane. The ATRP agent conjugated silica nanosheets were obtained. Synthesis of SPMA Monomer. Monomer SPMA was synthesized accordingly.16 Briefly, 0.5 g (1.42 mmol) of SP−OH and 0.25 mL (1.8 mmol) of triethylamine were dissolved in 20.0 mL of dry dichloromethane in a flask. 0.2 mL (2.1 mmol) of methacryloyl chloride dissolved in 5.0 mL of dry dichloromethane was added dropwise under N2 within 30 min. After the mixture was stirred at ambient temperature for 24 h, the solvent was removed using a rotary evaporator. The crude product was purified by silica gel column chromatography (100−200 mesh) using dichloromethane/petroleumether (2:1 v/v) eluent. After vacuum drying at 45 °C for 12 h, the SPMA monomer was obtained with 44.3% yield. 1HNMR (400 MHz, δ ppm, CDCl3): 7.96−8.05 (m, 2H), 7.17−7.23 (m, 1H), 7.09 (d, 1H), 6.89 (q, 2H), 6.73 (d, 1H), 6.68 (d, 1H), 5.86 (d, 1H), 5.56 and

hydrophilic by light irradiation. Upon UV irradiation, the hydrophobic PSPMA side becomes hydrophilic while the other side is maintained hydrophobic. The nanosheets are Janus in wettability. Upon irradiation with visible light, the Janus performance is lost and the composite nanosheets are recovered hydrophobic. The light-triggered transformation between hydrophobic and amphiphilic is reversible. When using the composite nanosheets as a solid emulsifier, stability of the emulsions can be manipulated by simple light irradiation with varied wavelength. As shown in Scheme 1b, nonpolar PSPMA moiety is in the closed SP form without UV irradiation, which is transformed into polar open zwitterionic merocyanine (MC) form via cleaving the C−O bond. The MC form can be transformed back to the original SP form by exposure to visible light.

2. EXPERIMENTAL METHODS Materials. n-Octyltriethoxysilane (OTES) and 2-bromoisobutyryl bromide were purchased from ACROS Organic. γ-Aminopropyltriethoxysilane (APTES), copper(I) bromide (CuBr), tris [2(dimethylamino)ethyl] amine (Me6TREN), and methacryloyl chloride were purchased from Alfa Aesar. 1′-(2-Hydroxyethyl)-3′, 3′-dimethyl6-nitrospiro (2H-1-benzopyran-2, 2′-indoline) (SP−OH) was purchased from TCI. Tetraethyl orthosilicate (TEOS), paraffin, cyclohexane, ethanol, tetrahydrofuran (THF), dimethylformamide, petroleumether and dichloromethane were purchased from Sinopharm Chemical Reagent Beijing. Hydrolyzed styrene−maleic anhydride (HSMA) copolymer was synthesized as described previously.15 Synthesis of Silica Janus Nanosheets. A 15.0 mL aliquot of 10 wt % HSMA copolymer aqueous solution was dissolved in 75.0 mL of B

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Macromolecules 6.07 (d, 2H), 4.29 (t, 2H), 3.36−3.61 (m, 2H), 1.92 (s, 3H), 1.28 (s, 3H), 1.16 (s, 3H) (Figure S1). Synthesis of the PSPMA/Silica Composite Nanosheets by ATRP. First, 50.0 mg of the ATRP agent conjugated silica Janus nanosheets, 0.4 g of SPMA, 15.0 mg of CuBr, 28.0 μL of Me6TREN, and 2.0 mL of dimethylformamide were dissolved in Schlenk flask of 10.0 mL. After the mixture was deoxygenated eight times using the freeze−pump−thaw procedure, the reaction was performed at 80 °C for 48 h. The PSPMA/silica composite nanosheets were purified after four cycles of centrifugation in tetrahydrofuran. Emulsification with the PSPMA/Silica Composite Nanosheets. First, 5.0 mg of the hydrophobic PSPMA/silica composite nanosheets were dispersed in 2.0 mL of cyclohexane. The dispersion was exposed to UV light for 20 min to ensure that the hydrophilic open MC form was dominant. Afterward, 0.5 mL of water was added. A water-in-oil emulsion formed after the mixture was vigorously shaken. At 70 °C, 10.0 mg of the PSPMA/silica composite nanosheet dispersion in 1.0 g of paraffin (Tm: 52−54 °C) was exposed to UV light for 20 min. Afterward, 1.0 mL of water was added. After the mixture was shaken, a water-in-melt paraffin emulsion formed. Subsequently, the emulsion was naturally cooled down to room temperature. Characterization. Structure and morphology of the samples were characterized using scanning electron microscopy (Hitachi S-4800 at 15 kV) equipped with an energy dispersive X-ray (EDX) analyzer. The samples for SEM observation were prepared by vacuum sputtering with Pt after being ambient dried. 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. Fourier transform infrared (FT-IR) spectra were obtained on a PerkinElmer spectrophotometer with the sample/KBr pressed pellets at room temperature. Thermogravimetric analysis (TGA) was performed using the PerkinElmer Pyris 1 TGA under air at a scanning rate of 10 °C/ min. Polarizing optical microscopy images were recorded using Olympus optical microscope. UV/vis absorption spectra were recorded using a JASCO V-570 spectrophotometer. The UV light from a high-pressure mercury lamp (365 nm, 500 W nominal powers) was used. Both the UV and visible light intensities were fixed at 10 mW/cm2. The two light sources were held at the same distance from the samples.

selectively conjugated onto the amine-group terminated side of the silica Janus nanosheet via amidation reaction between bromide (−COBr) and amine- (−NH2) groups. Both sides preserve smooth (Figure S2e). The ATRP agent conjugated nanosheets are thick 74 nm measured by AFM (Figure 1a).

Figure 1. (a, b) AFM images of the ATRP agent conjugated Janus nanosheets and the PSPMA/silica composite Janus nanosheets; (c) SEM image of the PSPMA/silica composite nanosheets after drying the dispersion in cyclohexane; (d) SEM image of the PSPMA/silica composite nanosheets after drying the dispersion in water upon UV irradiation for 20 min, the polymer side is selectively labeled with the trisodium citrate capped Fe3O4 nanoparticles.

After the conjugation, the nanosheets become not dispersible in water. The Janus performance is lost. After treatment with triethylamine, the −Br end group of the ATRP agent becomes quaternized. The nanosheets become well dispersible in both water and cyclohexane, indicating that the Janus performance has been restored. Negatively charged trisodium citrate capped Fe3O4 nanoparticles are used to preferentially label the quaternized −Br side via electrostatic interaction. As a result, one side of the nanosheets becomes coarsened, while the other side remains smooth (Figure S2f). Prior to the conjugation, no Br element is present at the silica Janus nanosheets (Figure S3a). The nanosheets contain 3.31% of Br element after conjugation with the ATRP agent (Figure S3b). The amidation reaction is further confirmed by FT-IR spectra. In comparison with the original silica Janus nanosheets (curve a, Figure S4), the new characteristic peaks at 1645 and 1543 cm−1 appear which are attributed to amide bond (curve b, Figure S4). The peaks at 1456 and 1392 cm−1 reveal the presence of the ATRP agent. From the ATRP agent conjugated side of the silica nanosheets, a PSPMA brush is preferentially grafted by ATRP. The new peak at 1726 cm−1 reveals the presence of ester carbonyl-group of PSPMA (curve c, Figure S4). The new peak at 1460 cm−1 is assigned to benzene rings, and a typical stretching peak of aryl nitro group at 1340 cm−1 is also present. Grafting degree of PSPMA is tunable by alteration of monomer SPMA feeding amount. Two grafting degrees of 3.4 wt % (curve c) and 15.7 wt % (curve d, Figure S5) are achieved. The PSPMA/silica composite nanosheets are only dispersible in oil such as cyclohexane not in water, implying that the composite nanosheets are hydrophobic. The example PSPMA/silica composite nanosheets (15.7 wt % of PSPMA) become thicker to 78 nm from 74 nm of the silica nanosheets (Figure 1b). In

3. RESULTS AND DISCUSSION Starting from the silica Janus nanosheets with two different hydrophobic/hydrophilic pedant groups onto both sides, a family of functional Janus nanosheets can be derived after a favorable growth of desired materials. The silica Janus nanosheets are achieved after crushing the parent Janus hollow spheres.11,14 An oil-in-water emulsion forms in the presence of HSMA as a stabilizer. The oil phase is composed of n-decane, OTES, APTES, and TEOS. At pH ∼ 3.0, an acid-catalyzed selforganized sol−gel process occurs at the emulsion interface to form a Janus silica shell. The hydrophilic amine-group from APTES and hydrophobic octyl-group from OTES are distinctly compartmentalized onto exterior and interior sides of the silica shell, respectively. After dissolution of the internal oil phase, the silica Janus hollow sphere forms (Figure S2a). The silica Janus nanosheets are achieved by crushing the Janus hollow spheres (Figure S2b). Both sides of the silica Janus nanosheets are smooth. After selectively labeling the amine-group terminated side with negatively charged trisodium citrate capped Fe3O4 nanoparticles via electrostatic interaction, the amine-group terminated side becomes coarsened while the other side remains smooth (Figure S2c). While the silica Janus nanosheets are dispersible in water and stacked into a face-to-face bilayered superstructure, the amine-group terminated sides are exposed to the aqueous phase (Figure S 2d). ATRP agent can be C

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Figure 2. (a) Color reversible change of the PSPMA/silica composite Janus nanosheet dispersion in THF by UV and visible light irradiation; (b) SEM image of the PSPMA/silica composite Janus nanosheets after drying the dispersion after UV irradiation for 10 min; (c) UV/vis absorption spectra of the PSPMA/silica composite Janus nanosheet dispersion upon UV irradiation at varied time; (d) UV/vis absorption spectra of the dispersion upon visible light irradiation at varied time. The nanosheet concentration is 0.02 mg/mL. PSPMA grafting degree is 15.7 wt %.

obtained after 64 s. This indicates that the SP to MC isomerization is fast which is fully completed within seconds. After removal of UV irradiation, the MC form is maintained in dark. The absorption peak intensity at 582 nm decreases upon visible light irradiation (Figure 2d), corresponding to the MC to SP back isomerization. It takes a longer time (about 20 min) for the recovery to reach the equilibrium. The isomerization kinetics are related with PSPMA grafting degree. At a lower PSPMA grafting degree of 3.4 wt %, the isomerization from SP to MC is fully completed within 5 s (Figure S7a). Similarly, the isomerization from MC to SP form is completed after 3 min visible light irradiation (Figure S7b). A pellet was compressed from the PSPMA/silica composite nanosheets for measuring water contact angles (Figure S8). Before UV irradiation, water contact angle is 108° (above 90°), implying the pellet is hydrophobic. After UV irradiation, water contact angle becomes smaller 83° (below 90°), implying the pellet becomes hydrophilic. Considering the light independent wettability of the octyl-group terminated surface, it is concluded that the hydrophilic/hydrophobic transformation of the pellet is essentially originated from the PSPMA side of the composite Janus nanosheets. In selective solvents, the Janus nanosheets can self-assemble into a bilayered superstructure. In water, the PSPMA/silica composite nanosheets are not dispersible but precipitate at the bottom (inset Figure 3a). The nanosheets are heavily aggregated, which can not be disassembled under sonication (Figure 3a). After UV irradiation for 20 min, the PSPMA side becomes hydrophilic in the MC form while the other hydrophobic octyl-group side is preserved. The composite nanosheets become well dispersible in water (inset Figure 3b), and the dispersion becomes highly purple in color. The highly purple color means that MC form is dominant. In order to observe the “real” structure of the nanosheet aggregates, the very dilute dispersion is freeze-dried. A bilayered superstructure is observed which is face-to-face stacked between the nanosheets (Figure 3c). It is reasonable that the PSPMA sides in the MC form should face toward the continuous

order to measure molecular weight of grafted polymers using gel permeation chromatography (GPC), the grafted PSPMA chains were cleaved from the composite nanosheets by selectively dissolving the silica using aqueous hydrofluoric acid (2.0 wt %). The PSPMA possesses a number-average molecular weight of 1.0 × 104 g/mol with a polydispersity index of 1.26 (Figure S6). The estimated value of grafting density is ca. 0.25 chains/nm2.17 After drying the dispersion in cyclohexane, the nanosheets are present in the form of individual one rather than aggregation (Figure 1c). Both sides are smooth. Upon UV irradiation for 20 min, the PSPMA/silica composite nanosheets become dispersible in water. It is understood that the PSPMA experiences an isomerization from closed SP form to open MC form. The MC form is positively charged at pH = 7.18 Negatively charged trisodium citrate capped Fe3O 4 nanoparticles are added to preferentially label the MC side via electrostatic interaction. As a result, the polymer side becomes more coarsened, while the other side remains smooth (Figure 1d). The smooth sides are face-to-face stacked. Dispersion behavior of the composite nanosheets can be triggered using light. In order to further characterize the isomerization of the PSPMA/silica composite nanosheets with UV/vis absorption spectroscopy, a colorless cosolvent THF is used as the dispersant. Upon UV irradiation for 10 min, the slightly yellow PSPMA/silica composite nanosheet dispersion becomes highly purple (Figure 2a). After drying both the dispersions before and after UV irradiation, the PSPMA/silica composite nanosheets are present in the form of individual nanosheet which is irrelevant with UV irradiation (Figure 2b). Prior to the UV/vis absorption spectroscopy measurement, the dispersion is exposed to visible light for 30 min ensuring that PSPMA chains are in the ring-closed SP form. Upon UV irradiation, a strong absorption band appears at 582 nm (Figure 2c), which is arisen from the zwitterionic MC form. The absorption intensity increases very fast at the initial stage, which becomes slower with prolonging irradiation time After 8 s irradiation, the absorption intensity has reached 80% of the plateau value D

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Figure 3. SEM images of the PSPMA/silica composite nanosheets after naturally drying the aqueous dispersion before (a) and after (b) UV irradiation for 20 min, inset optical images of the dispersions; (c, d) SEM and cross-section TEM images of the PSPMA/silica composite Janus nanosheets after freeze-drying the aqueous dispersion after UV irradiation for 20 min.

aqueous phase, while the octyl-group sides are stacked internally. The bilayered structure is further confirmed by cross-section TEM image (Figure 3d). In another selective solvent cyclohexane, the PSPMA/silica composite nanosheets are well dispersible (Figure S9a). After drying the dispersion, individual nanosheet is observed. After UV irradiation for 10 min, the dispersion becomes highly purple in color. A bilayered superstructure is observed with the hydrophobic octyl-group facing toward the external cyclohexane phase (Figure S9b). In contrast, in cosolvents for example ethanol, no aggregation is observed but individual nanosheet irrelevant with UV irradiation (Figures S9c and S9d). The PSPMA/silica composite nanosheets can serve as a solid emulsifier, and the emulsion stability can be remotely triggered with light. As an example, the PSPMA/silica composite nanosheets (containing 15.7 wt % of PSPMA) are preferentially dispersible in the top cyclohexane phase (Figure 4a1). The bottom water phase is transparent. No emulsion forms after shaking the mixture. After UV irradiation for 10 min, the top PSPMA/silica composite nanosheet dispersion becomes highly purple (Figure 4a2). While the PSPMA side in the MC form becomes hydrophilic, the composite nanosheets become amphiphilic. After shaking the mixture, a water-in-cyclohexane (1/4 vol/vol) emulsion forms (Figure 4a3). The emulsion droplets precipitate after 5 h (Figure S10), which remain stable over one month in darkness (Figure S11a). The droplets are 40−180 μm in diameter (Figure 4b). This implies that the transformation from MC to SP form is rather slow in darkness. Alternatively, visible light irradiation was used to accelerate the MC to SP transformation. After irradiation with visible light for 20 min, droplets start to coalesce and become larger. Some nonspherical larger droplets are observed (Figure 4c). This implies that a partial de-emulsification occurs. In darkness, the state remains stable (Figure S11b). Upon another irradiation with UV for 5 min, the partially de-emulsified mixture can be recovered to the original well emulsified state (Figure S11c). After 40 min visible light irradiation, the de-emulsification is completely achieved. As a result, the mixture is separated into two layers with a distinct phase boundary (Figure 4a4). The nanosheets are preferentially distributed in the top oil phase. No droplets are observed in the bottom water phase. In a

Figure 4. (a) Janus performance of the PSPMA/silica composite nanosheets as a light responsive surfactant: (1) immiscible cyclohexane (top) and water (bottom) and (2) sample after UV irradiation for 10 min; (3) water-in-cyclohexane (1/4 vol/vol) emulsion stabilized with the PSPMA/silica composite nanosheets; (4) de-emulsification after visible light irradiation for 40 min. (b) Optical microscopy image of the water-in-cyclohexane emulsion (a3). (c) Optical microscopy image of the water-in-cyclohexane emulsion after visible light irradiation for 20 min. (d) SEM image of the cavity after water evaporation from the water-in-paraffin emulsion at room temperature, where the orientation of the PSPMA/silica composite nanosheets is frozen at the interface. (e) Magnified SEM image of part d.

reversed way, emulsification occurs to form a stable emulsion again upon UV irradiation. The light-triggered emulsification/ de-emulsification transformation is reversible. It is interesting that the continuous phase remains oil even at high water fraction for example water/oil (4:1 vol/vol) (Figure S12). This implies that the Janus nanosheets are more lipophilic. In order to observe orientation of the PSPMA/silica composite Janus nanosheets at the emulsion interface, a melt-paraffin (Tm: 52− 54 °C) is used instead of cyclohexane. At 70 °C, a water-inparaffin emulsion forms. The paraffin phase is solidified upon cooling to room temperature, orientation of the PSPMA/silica composite nanosheets at the emulsion interface is fixated. At the fracture surface, some cavities are created after water evaporation from the continuous paraffin matrix (Figure 4d). A magnified SEM image of the cavity interior surface reveals that the PSPMA/silica composite nanosheets are present parallel onto the interface (Figure 4e).

4. CONCLUSION In summary, we have proposed a facile approach to synthesize light-triggered Janus nanosheets by ATRP grafting optical responsive spiropyran-containing polymer (PSPMA) brushes onto one side of the silica nanosheets. The hydrophobic/ hydrophilic transformation of the PSPMA side can be triggered in a reversible way using UV and visible light irradiation. As a result, the composite nanosheets can experience a reversible transformation between hydrophobic and Janus. When using E

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the composite nanosheet as a light-triggered solid emulsifier to stabilize an emulsion, emulsification and de-emulsification can be easily triggered using light.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01257. NMR, FTIR, EDX, TGA, GPC, water contact angle, and SEM images of some representative PSPMA/silica composite nanosheets (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(G.W.) Telephone: +86-10-62333619. E-mail: guojie.wang@ mater.ustb.edu.cn. *(Z.Y.) Telephone: +86-10-82619206 Fax: +86-10-62559373. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF of China (51233007, 51373025, and 21074010), Program for New Century Excellent Talents (NCET-11-0582), and Fundamental Research Funds for Central Universities (FRE-TP-12-004B).

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DOI: 10.1021/acs.macromol.5b01257 Macromolecules XXXX, XXX, XXX−XXX