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Polymeric Janus Nanosheets by Template RAFT Polymerization Yijiang Liu,†,‡ Xinyu Xu,‡ Fuxin Liang,† and Zhenzhong Yang*,† †

State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province China S Supporting Information *

ABSTRACT: We report a general method to synthesize polymeric Janus nanosheets (PJS) by sequential RAFT grafting from a template particle surface. Layer number and composition of the PJS are tunable by feeding sequence and type of monomers. The cPNIPAM−PS PJS is flexible and thermal responsive, which can form a scrolled superstructure. A dually responsive cPAA−PNIPAM PJS is derived by hydrolysis of cPtBA−PNIPAM. Accordingly, stability of the emulsion with the cPAA−PNIPAM PJS is triggered by alternation of pH or/and temperature.

species can be wrapped within an individual Janus nanosheet.16 Onto the exterior surface of the Janus membrane supported particle, the amine group is further terminated using another silane. The amine/acid composite Janus nanosheet is pH responsive.17 The flexible responsive Janus composite nanosheet can experience reversible scrolling and unscrolling transformation, which is used for controlled release of drugs such as doxorubicin by remote NIR irradiation.18 The reversible foldability of the Janus nanosheets thus controlled release of loaded components can be driven by exchanging solvents.19 The flexible Janus nanosheets are also promising in foldable electronic devices,20 optical materials,21,22 and catalysis.23 Polymers are promising due to their diverse functional groups and thus performances. It is significant to develop methods for synthesis of functional polymeric Janus nanosheets. Janus nanosheets or nanodiscs can be prepared by disassembly of a partially cross-linked lamellar supramolecular structure from a block terpolymer.24 Similarly, a Janus composite nanosheet is derived from the lamellar bulk of a gellable terpolymer.25 However, the contour is irregular in shape. Recently, we reported the preparation of Janus nanodiscs of diblock copolymers by disassembly of an alternatively discstacked particle of PS-b-P4VP. The Janus nanodiscs are uniform in thickness and regular in contour.26 The disassembly approach involves many steps and requires special crosslinkable copolymers with a narrow molecular weight distribution. By cross-linking the self-assembled monolayer of a reactive polymeric surfactant at an oil/water interface, polymeric Janus nanosheets are synthesized.27 Narrow molecular weight distribution is unnecessary for this approach. By coassembly of a gellable terpolymer and PEO-b-PS followed by a sol−gel process at an emulsion interface, a Janus mesoporous nanodisc

1. INTRODUCTION Since termed by de Gennes in 1991, Janus objects with two different compositions and thus properties onto the same surface have gained broad interest. They are highly attractive in diverse areas such as functional solid surfactants, building blocks toward superstructures, and self-propelled nanomotors.1−8 In addition to the composition of a Janus object, shape is also significant to determine their performances. Among the shapes, platelet (sheet or disc) has gained lots of attention as a special surfactant.9 The emulsions will become more stable by mosaicking the Janus platelets onto the emulsion interface. It is important to synthesize Janus platelets, especially responsive flexible ones. Based on inorganic Janus nanosheets, polymeric/inorganic composite Janus nanosheets are derived by a favorable growth of polymer chains onto one side of the nanosheets.10−12 Pickering emulsion interfacial grafting is usually used to prepare Janus platelets with the original shape preserved. As an example, when the initiator-modified kaolinite plates are anchored at the Pickering emulsion interface, simultaneous “grafting from” the corresponding sides can lead to Janus platelets.13 However, these Janus nanosheets are rigid. Recently, a flexible Janus graphene with halogen and aryl/oxygen functional groups on each side is prepared via a two-step surface covalent functionalization by a PMMA-mediated transfer approach.14 In order to increase the synthesis yield, Pickering emulsion interfacial synthesis is employed to achieve a Janus GO nanosheet by selective polymerization of PMMA at one side.15 In order to avoid overlapping between the thin GO nanosheets at the emulsion interface, the interface coverage density should be low. We have previously reported the synthesis of thin Janus silica nanosheets (∼3.5 nm) by the sol− gel process of a self-assembled monolayer of a maleic anhydride-terminated silane onto CaCO3 particle surface. The flexible Janus nanosheets with two different sides are significant in many applications. As a flexible solid emulsifier, desired © XXXX American Chemical Society

Received: July 23, 2017 Revised: November 8, 2017

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DOI: 10.1021/acs.macromol.7b01558 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules is synthesized.28 PCL-b-PAA can form uniform PAA−PCL− PAA sandwiched single crystals. After a selective cross-linking of PAA at both sides, the single crystal is disintegrated to achieve a polymeric Janus nanosheet.29 Formation of the single crystals requires specific composition of the block copolymer by self-seeding method. In general, composition of polymeric Janus nanosheets by disassembly of superstructures is restricted within the given copolymers. We have recently reported the synthesis of maleic acid moiety contained polymeric Janus nanosheets by free radical polymerization of monomers against the maleic anhydride-coated sucrose particle surface.30 The Janus nanosheets are restricted in composition. It is required to develop a general method to synthesize polymeric Janus nanosheets with tunable composition. It is noted that reversible addition−fragmentation chain transfer (RAFT) polymerization has been proved powerful to precisely tune composition and linkage sequence of polymers,31 which can be employed to tune composition and thus performance of the polymeric Janus nanosheets. Herein, we report a general approach to synthesize polymeric Janus nanosheets by sequential RAFT polymerization of monomers from a template particle surface (Scheme 1). A

Self-Assembled Monolayer of RAFT Reagent Coating onto CaCO3 Particle. CaCO3 particle (1.0 g) was dispersed in 12.0 mL of ethanol, the RAFT reagent of 4-cyano-4-(thiobenzoylthio)pentanoic acid (10.0 mg) was added under stirring at 400 rpm for 24 h at room temperature. The RAFT reagent coated CaCO3 composite particle (CaCO3@RAFT) was achieved after washing with ethanol and vacuum drying. Synthesis of Cross-Linked PNIPAM from the CaCO3@RAFT Particle Surface. The CaCO3@RAFT composite particle (1.0 g), 8.0 mg of monomer NIPAM, 2.0 mg of cross-linker MBA, 0.01 mg of initiator AIBN, and 6.0 mL of toluene were mixed. After degassing by freeze−evacuate−thaw and sealing under vacuum, the polymerization was performed under stirring with a magnetic bar at 70 °C for 12 h. After quenching in ice water, the crude product was washed with ethanol and dried under vacuum. A CaCO3@cPNIPAM composite particle was achieved. Synthesis of the CaCO3@cPNIPAM−PS Composite Particle. The CaCO3@cPNIPAM composite particle (1.0 g), St (30.0 mg), 0.03 mg of AIBN, and 6.0 mL of methanol were added to a glass tube equipped with a magnetic stir bar. The mixture was degassed by freeze−evacuate−thaw and then sealed under vacuum. The polymerization was performed at 60 °C for 12 h. After quenching in ice water, the crude product was washed with ethanol and dried under vacuum. The CaCO3@cPNIPAM−PS composite particle was achieved. CaCO3 was removed by dissolution after adding excess 1 M aqueous HCl solution to the composite particle dispersion under ultrasonication at 50 °C for 0.5 h. After cooling to room temperature, the polymeric Janus nanosheet of cPNIPAM-PS was obtained by centrifugation and freeze-drying. Synthesis of the CaCO3@cPtBA Composite Particle. The CaCO3@RAFT composite particle (1.0 g), 18.0 mg of tBA, 2.0 mg of EGDMA, 0.02 mg of AIBN, and 6 mL of toluene were added to a glass tube equipped with a magnetic stir bar. The mixture was degassed by freeze−evacuate−thaw and then sealed under vacuum. The polymerization was performed at 70 °C for 12 h. The sample was quenched in ice water, washed with ethanol, and dried under vacuum. Synthesis of the CaCO3@cPtBA−PNIPAM Composite Particle. The CaCO3@cPtBA particle (1.0 g), NIPAM (30.0 mg), 0.03 mg of AIBN, and 6.0 mL of hexane were added to a glass tube equipped with a magnetic stir bar. The mixture was degassed by freeze−evacuate−thaw and then sealed under vacuum. The polymerization was performed at 70 °C for 12 h. The sample was quenched in ice water, washed with ethanol, and dried under vacuum. Synthesis of the cPAA−PNIPAM PJS. After acidic dissolution of CaCO3 from the composite particle with aqueous HCl, the Janus nanosheet of cPtBA−PNIPAM (20.0 mg) was obtained by centrifugation. TFA (50.0 mg) was added to the Janus nanosheet dispersion in methanol (5.0 mL) under stirring at 25 °C to selectively hydrolyze PtBA for 48 h. After centrifugation and washing with methanol/water, the cPAA−PNIPAM PJS was achieved. Responsive Performance of the cPNIPAM−PS PJS. The cPNIPAM−PS (10.0 mg) PJS was dispersed in some typical solvents, e.g., toluene, THF, and n-heptane (1.0 mL) under untrasonication. Afterward, the dispersions stood for observation. The cPNIPAM−PS (50.0 mg) PJS was dispersed in 1.2 mL of water at two temperatures, e.g., 25 and 50 °C. After ultrasonication for 5 min, the system stood for observation. 10.0 mg of the cPNIPAM−PS PJS was dispersed in a mixture of water/toluene (4/1, v/v). A trace of Sudan Red dye was added in toluene for easy observation. After vigorously shaking the mixture, an oil-in-water emulsion formed. The emulsion was heated to 50 °C for de-emulsification. Responsive Performance of the cPAA−PNIPAM PJS. At 25 °C, the cPAA−PNIPAM (20.0 mg) PJS was dispersed in water at two pHs, e.g., 2 and 8. The dispersions were heated to 50 °C for observation. At 25 °C, the cPAA−PNIPAM (20.0 mg) PJS was dispersed in a mixture of toluene/water at pH = 2. After vigorously shaking the mixture, an oil-in-water emulsion formed. De-emulsification occurred after heating to 50 °C. Similarly, the PAA−PNIPAM (20.0 mg) PJS

Scheme 1. Illustrative Synthesis of the Polymeric Janus Nanosheets (PJS) by Sequential RAFT Polymerization: (1) a Self-Assembled Monolayer of RAFT Agent of 4-Cyano-4(thiobenzoylthio)pentanoic Acid Forms onto a Template CaCO3 Particle Surface via Complexation; (2) by Sequential RAFT Grafting Two Different Polymers from the Particle Surface and Layered Polymeric Membrane Form; (3) after Acidic Dissolving CaCO3 under Ultrasonication, PJS Are Achieved

representative RAFT reagent of 4-cyano-4-(thiobenzoylthio)pentanoic acid is preferentially absorbed on a CaCO3 particle surface via complexation, forming a self-assembled monolayer with the thiobenzoylthio group exposed. A bilayered polymer membrane is grown from the particle surface by sequential RAFT polymerization. After acidic dissolution of CaCO3 under ultrasonication, the polymeric membrane is detached and disintegrated into flexible Janus nanosheets. Composition and thus performance of the polymeric Janus nanosheets are controllable.

2. EXPERIMENTAL SECTION Materials. Styrene (St), tert-butyl acrylate (tBA), ethylene glycol dimethacrylate (EGDMA), N,N′-isopropylacrylamide (NIPAM), and N,N′-methylenebis(acrylamide) (MBA) were purchased from Alfa Aesar. 4-Cyano-4-(thiobenzoylthio)pentanoic acid was purchased from ACROS Organic. Toluene, methanol, hexane, tetrahydrofuran, nheptane, ethanol, trifluoroacetic acid (TFA), hydrochloric acid (HCl), and 2,2-azobis(isobutyronitrile) (AIBN) were purchased from Sinopharm Chemical Reagent Beijing Co. Calcium carbonate (CaCO3) particle was purchased from Sinopharm Chemical Reagent Shanghai Co. NIPAM was recrystallized with toluene and hexane. AIBN was used after recrystallization with ethanol. St, tBA, and EGDMA were purified over Al2O3 column. All other reagents were used as received. B

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Macromolecules was dispersed in toluene/water at pH = 8 and 25 °C. After vigorously shaking the mixture, no emulsion formed. After further heating to 50 °C, an oil-in-water emulsion formed. 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 and transmission electron microscope (JEOL1011 at 100 kV). The samples for SEM were prepared by vacuum sputtering with Pt. The samples for TEM observation were prepared by spreading the dilute dispersions onto a carbon-coated copper grid. The sample in toluene for cryo-SEM was frozen in liquid nitrogen, fractured, sublimated, and sputtered with Pt using an ACE 600. Cryo-SEM observation was performed on a Hitachi S-4300 at 10 kV equipped with a VCT100 cryo-transmission system. The sample in toluene for cryo-TEM was prepared using a Leica EM GP and observed with a JEOL2011 at 100 kV equipped with a Gatan 626 cryo-sample holder. A bright-field image of the sample in toluene was recorded on a confocal laser scanning microscope (CLSM, Olympus). FT-IR spectroscopy measurement was performed after scanning samples for 32 times using a Bruker EQUINOX 55 spectrometer with the sample/KBr pressed pellets. Polarizing optical micrograph images were recorded using an Olympus optical microscope. AFM images were recorded under ambient conditions using a Digital Instrument Multimode Nanoscope IIIA at a tapping mode.

methylene, −CH of methine groups, and the stretching vibration of −NH of acylamino group in PNIPAM (Figure S2c). Both N and S elements are present in the EDX spectrum (Figure S2d). Microstructure of the cPNIPAM nanosheet is controlled by the cross-linker extent. At a low extent for example 5 wt %, small disc-like particles appear rather than cPNIPAM nanosheet (Figure S3a). At 10 wt %, the nanosheet is achieved (Figure S3b). At an extremely high extent for example 50 wt %, the nanosheet becomes coarse (Figure S3c). The thickness of the nanosheets is less influenced by the crosslinker extent. As an example, the nanosheet synthesized at 10 wt % of the cross-linker is 3.8-nm-thick (Figure S3d), similar to the nanosheet at 20 wt % of the cross-linker. On the other hand, thickness of the cPNIPAM nanosheet is controlled by the monomer amount. When the monomer/CaCO3@RAFT weight ratio increases from 1/100 to 1/50, the cPNIPAM nanosheet becomes 17 nm from 3.9 nm (Figure S3e). At the ratio of 1/20, the nanosheet is 59-nm-thick (Figure S3f). In the following investigation, the cPNIPAM nanosheet is selected as the example which is synthesized at 20 wt % of the cross-linker and the monomer/CaCO3@RAFT weight ratio of 1/100. Styrene is further polymerized from the CaCO3@cPNIPAM composite particle surface by RAFT. Some new characteristic bands appear at 1550 and 675 cm−1, which are assigned to phenyl and para-substituted phenyl of PS (Figure S4). Onto the particle surface, a wrinkled layer is observed under SEM (Figure S5). After dissolution of CaCO3 with aqueous HCl, a highly wrinkled cPNIPAM-PS nanosheet is achieved (Figure 1b). Under TEM, the nanosheet is homogeneous. Both sides of the nanosheet are smooth. The nanosheet is 5.5 ± 0.2-nm-thick by AFM (Figure 1c). The PS side is ca. 1.6 nm thick. One side of the cPNIPAM−PS PJS becomes coarse after absorption of the negatively charged Au nanoparticles (NPs), while the opposite side remains smooth (Figure 1d). In comparison, no Au NPs are found onto both sides of a PS nanosheet (Figure S6a). In contrast, both sides of a PNIPAM nanosheet are covered with Au NPs after absorption (Figure S6b). Therefore, the coarse side of the cPNIPAM-PS PJS after absorption of Au NPs corresponds to cPNIPAM, while the other smooth side corresponds to PS. Janus Performance of the cPNIPAM−PS PJS. The cPNIPAM−PS PJS can be well dispersible in a PS selective solvent such as toluene, forming a stable dispersion. After drying the dispersion, a nanoscroll is observed under SEM (Figure 2a). Under TEM, a multilayered structure of the nanoscroll is discerned (Figure 2b). It is reasonable that the hydrophilic cPNIPAM side curls inwardly while the PS side exposes outwardly to toluene. After the cPNIPAM side is selectively labeled with Au NPs, the interior surface of the nanoscroll indeed becomes coarse while the exterior surface remains smooth (Figure 2c). In order to confirm that the scrolled structure of cPNIPAM−PS nanosheet exists in toluene, the sample morphology in toluene was observed by a cryoelectron microscope. The cryo-SEM image shows the strip structure rather than platelet along the frozen sample cross section, implying the presence of the scrolled structure (Figure 2d). Similarly, multilayer scrolled structure is discerned under cryo-TEM (Figure 2e). The scrolled structure in toluene was also observed with confocal laser scanning microscope at ambient temperature (Figure S7). Therefore, the possibility that the scrolling may occur due to the capillary forces during drying of the solvent is excluded. In a cosolvent such as THF, the cPNIPAM−PS nanosheet is better dispersible, forming a

3. RESULTS AND DISCUSSION Synthesis of the cPNIPAM−PS PJS. Onto a CaCO3 particle surface (Figure S1a), the RAFT reagent of 4-cyano-4(thiobenzoylthio)pentanoic acid can form a self-assembled monolayer with the thiobenzoylthio group exposed outwardly. A CaCO3@RAFT composite particle forms, whose surface remains smooth after the coating (Figure S1b). After a RAFT polymerization of NIPAM containing 20 wt % of cross-linker MBA from the composite particle surface, a wrinkled layer is present (Figure S2a). After dissolving CaCO3 with aqueous HCl under ultrasonication, a thin cross-linked (denoting as c) cPNIPAM nanosheet is achieved (Figure S2b). It is flexible and easily foldable. The nanosheet is insoluble but dispersible in water at low temperature below the LCST (∼32 °C). Above the LCST, the nanosheet precipitates. The cPNIPAM nanosheet is homogeneous with a thickness of 3.9 ± 0.2 nm (Figure 1a). The characteristic peaks at 1643, 2848, 2921, and 3300 cm−1 are assigned to the vibration of −CO, −CH of

Figure 1. (a) AFM image of the cPNIPAM nanosheet. (b) SEM and inset TEM images of the cPNIPAM−PS PJS. (c) AFM image of the cPNIPAM−PS PJS. (d) The cPNIPAM−PS PJS after being selectively labeled with negatively charged Au NPs onto the cPNIPAM side. C

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Figure 3. SEM images of the cPNIPAM−PS PJS after drying the aqueous dispersion at two temperatures: (a) 25 °C, (b) 50 °C; inset: the optical microscopy images of the dispersions. (c) A toluene-inwater emulsion stabilized with the cPNIPAM−PS PJS at 25 °C. (d) De-emulsification of the emulsion at 50 °C.

surface. After dissolution of the CaCO3 particle under ultrasonication, a flexible cPtBA−PNIPAM nanosheet is obtained (Figure 4a). It is about 7.6-nm-thick (Figure S9c). Figure 2. (a) SEM and (b) TEM images of the cPNIPAM−PS PJS after drying the dispersion in toluene. (c) The cPNIPAM−PS PJS after being labeled with Au NPs. (d) Cryo-SEM and (e) cryo-TEM images of the cPNIPAM−PS PJS in toluene. (f) SEM and inset TEM images of the cPNIPAM−PS PJS after drying the dispersion in THF.

homogeneous dispersion. After drying the dispersion, the nanosheets are individual and flat (Figure 2f). In a poor solvent such as n-heptane, the cPNIPAM−PS nanosheet precipitates, forming thick aggregates (Figure S8). The cPNIPAM−PS PJS should be thermal responsive arisen from PNIPAM with a lower critical solution temperature (LCST ∼ 32 °C). At 25 °C, the cPNIPAM−PS nanosheet is well dispersible in water, forming a stable dispersion. An individual nanoscroll of the cPNIPAM−PS nanosheet forms (Figure 3a). After being heated to a high temperature, for example 50 °C, the dispersion becomes unstable and forms a precipitate. After freeze-drying the dispersion, a heavy aggregate is observed (Figure 3b). The cPNIPAM−PS PJS can serve as a thermal responsive surfactant. Sudan Red dye is added to toluene for easier observation. A toluene-in-water emulsion forms at 25 °C in the presence of the cPNIPAM−PS PJS. The droplets present at the top phase are 5−10 μm in diameter (Figure 3c). The bottom aqueous phase is slightly turbid, implying the existence of smaller droplets. After the emulsion is heated to 50 °C, de-emulsification occurs (Figure 3d). The colored toluene becomes turbid, while the bottom aqueous phase is transparent. This implies that the nanosheet exists in the top toluene phase. Synthesis of the cPtBA−PNIPAM PJS and Derived cPAA−PNIPAM. The sequential RAFT polymerization from the particle surface is a general method to synthesize other PJS with varied composition. As proof of the concept, another PJS of cPtBA−PNIPAM is synthesized, of which the PtBA moiety can be easily hydrolyzed, forming cPAA−PNIPAM. In the first step, a cPtBA nanosheet is synthesized (Figure S9a). It is smooth and ∼5.4-nm-thick (Figure S9b). NIPAM is further polymerized from the CaCO3@cPtBA composite particle

Figure 4. SEM and inset TEM images of the cPtBA−PNIPAM PJS before (a) and (b) after being labeled with the negatively charged Au NPs. (c) SEM image of the derived cPAA−PNIPAM JNS; inset: the TEM image. (d) AFM image of the cPAA−PNIPAM PJS.

While one side of the nanosheet is coarse, the other side is smooth. Onto the smooth side, the negatively charged Au NPs are preferentially absorbed, and then both sides become coarse (Figure 4b). It is thus confirmed that the smooth side is PNIPAM. After selective hydrolysis of the cPtBA side using TFA, the cPAA−PNIPAM PJS is derived (Figure 4c). Under TEM, the nanosheet is homogeneous while the coarse wrinkles are lost. The cPAA−PNIPAM PJS is 7.3-nm-thick (Figure 4d). The degree of hydrolysis is measured by FT-IR spectra. For pure cPtBA (curve a, Figure S10), the characteristic bands at 1740 and 1370 cm−1 are assigned to −CO and tert-butyl groups. After grafting of PNIPAM, the new peak appears at 1560 cm−1, which is assigned to the deformation vibration of −NH of the acrylamino group (curve b, Figure S10). After hydrolysis, the peak at 1370 cm−1 becomes remarkably weaker D

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Macromolecules (curve c, Figure S10). This indicates that almost all tert-butyl groups are hydrolyzed. Meanwhile, the peak at 1560 cm−1 is less influenced, implying PNIPAM is well preserved after the hydrolysis. Furthermore, while the N element detected by EDX spectrum keeps almost the same, the O element content increases after hydrolysis (Figure S11a,b). Dually Responsive Performance of the cPAA−PNIPAM PJS. In combination with pH-responsive PAA and thermal responsive PNIPAM, the cPAA−PNIPAM PJS should be dually responsive. At pH = 2 and T = 25 °C, a transparent aqueous dispersion of the cPAA−PNIPAM PJS forms. After drying the dispersion, a slightly scrolled structure of the nanosheet is observed under SEM (Figure 5a). This is consistent with a

Figure 6. (a) A toluene-in-water emulsion stabilized with the cPAA− PNIPAM PJS at pH = 2 and T = 25 °C. (b) Destabilization of the emulsion when heating to 50 °C at pH = 2. (c) The toluene/water biphasic system in the presence of cPAA−PNIPAM PJS at pH = 8 and T = 25 °C. (d) Upon heating to 50 °C at pH = 8, a toluene-in-water emulsion forms again.

hydrophobic PNIPAM. A toluene-in-water emulsion forms again (Figure 6d).

4. CONCLUSIONS In summary, a general method is proposed to synthesize polymeric Janus nanosheets (PJS) by sequential RAFT from a template particle surface. The first example PJS of cPNIPAM− PS is thermal responsive and capable to scroll into a superstructure by changing temperature. In the case of another dually responsive PJS of cPAA−PNIPAM, dispersion and aggregation in water are triggered simply by alteration of pH or (and) temperature. Accordingly, switchable emulsification and de-emulsification of two immiscible liquids can be achieved using the dually responsive PJS. The current report provides a simple way to synthesize PJSs with tunable layer numbers and compositions. The corresponding composite PJSs will be derived by a favorable growth of the species within a desired layer.

Figure 5. Dually responsive performance of the cPAA−PNIPAM PJS in water. (a) At pH = 2 and T = 25 °C. (b) At pH = 2 and T = 50 °C. (c) At pH = 8 and T = 25 °C. (d) At pH = 8 and T = 50 °C.

slightly hydrophobic PAA at pH = 2. While heating to a high temperature, for example 50 °C, the dispersion becomes turbid progressively. Eventually, a heavy aggregation occurs at the bottom (Figure 5b). At 25 °C, while pH is elevated to ∼8 by adding aqueous ammonia, the cPAA side becomes highly ionized and hydrophilic. The dispersion becomes transparent, and the nanosheet tends to be flat (Figure 5c). This is understandable that both sides of the nanosheet are hydrophilic. Upon heating to 50 °C, the transparent dispersion becomes turbid yet stable (inset Figure 5d), and SEM shows the cPAA−PNIPAM nanosheet is scrolled. When using the cPAA−PNIPAM PJS as a solid surfactant, the stability of the emulsion can be triggered by alteration of pH or (and) temperature. Sudan Red dye is added in toluene for easier observation. At 25 °C and pH = 2, a toluene-in-water (1/4, vol/vol) emulsion forms (Figure 6a). The emulsion droplets in the top layer are 20−50 μm in diameter. The bottom aqueous phase is slightly turbid containing some smaller emulsion droplets. Upon heating to 50 °C, the emulsion destabilizes and the emulsified toluene is released (Figure 6b). Some aggregates are found in both the top toluene phase and the bottom aqueous phase. This is understandable that both sides of the polymeric nanosheets are hydrophobic at 50 °C and pH = 2. At pH = 8 and T = 25 °C, no emulsion forms (Figure 6c). The nanosheet is hydrophilic and preferentially dispersible in the bottom aqueous phase, forming a turbid dispersion. Upon heating to 50 °C at pH = 8, the nanosheet becomes amphiphilic with hydrophilic cPAA and

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01558. SEM images, FI-IR, EDX, and AFM of the representative synthesized nanosheets of cPNIPAM, cPtBA, and cPtBA−PNIPAM PJS. SEM images of monolayer PS and PNIPAM labeled with Au NP; cryo-SEM, cryoTEM, and CLSM images of the cPNIPAM−PS PJS dispersed in toluene; SEM/TEM images and photographs of the cPNIPAM−PS PJS dispersed in n-heptane (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(Z.Y.) E-mail [email protected]; Fax 86-10-82619206; Tel 86-10-62559373. E

DOI: 10.1021/acs.macromol.7b01558 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules ORCID

(21) Li, F.; Song, Q.; Zhang, X. Two-dimensional folded nanosheets lead to an unusual circular dichroism effect in aqueous solution. Langmuir 2014, 30, 6064−6070. (22) Kim, J. H.; Bohra, M.; Singh, V.; Cassidy, C.; Sowwan, M. Smart composite nanosheets with adaptive optical properties. ACS Appl. Mater. Interfaces 2014, 6, 13339−13343. (23) Lin, Y. Y.; Thomas, M. R.; Gelmi, A.; Leonardo, V.; Pashuck, E. T.; Maynard, S. A.; Wang, Y.; Stevens, M. M. Self-assembled 2D freestanding Janus nanosheets with single-layer thickness. J. Am. Chem. Soc. 2017, 139, 13592−13595. (24) Walther, A.; André, X.; Drechsler, M.; Abetz, V.; Müller, A. H. E. Janus discs. J. Am. Chem. Soc. 2007, 129, 6187−6198. (25) Gao, L.; Zhang, K.; Chen, Y. M. Dumpling-like nanocomplexes of foldable Janus polymer sheets and spheres. ACS Macro Lett. 2012, 1, 1143−1145. (26) 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. Janus nanodisc of diblock copolymers. Adv. Mater. 2014, 26, 4469−4472. (27) Wang, Q. G.; Liu, Y. J.; Qu, X. Z.; Wang, Q.; Liang, F. X.; Yang, Z. Z. Janus nanosheets by emulsion interfacial crosslinking of reactive surfactants. Colloid Polym. Sci. 2015, 293, 2609−2615. (28) Jia, F.; Liang, F. X.; Yang, Z. Z. Janus mesoporous nanodisc from gelable triblock copolymer. ACS Macro Lett. 2016, 5, 1344− 1347. (29) Qi, H.; Zhou, T.; Mei, S.; Chen, X.; Li, C. Y. Responsive shape change of sub-5 nm thin, Janus polymer nanoplates. ACS Macro Lett. 2016, 5, 651−655. (30) Zhou, P.; Wang, Q.; Zhang, C. L.; Liang, F. X.; Qu, X. Z.; Li, J. L.; Yang, Z. Z. pH responsive Janus polymeric nanosheets. Chin. Chem. Lett. 2015, 26, 657−661. (31) Moad, G.; Rizzardo, E.; Thang, S. H. Toward living radical polymerization. Acc. Chem. Res. 2008, 41, 1133−1142.

Zhenzhong Yang: 0000-0002-4810-7371 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSF of China (51622308 and 51603177) and the Opening Foundation of Beijing National Laboratory for Molecular Sciences.

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DOI: 10.1021/acs.macromol.7b01558 Macromolecules XXXX, XXX, XXX−XXX