Article pubs.acs.org/Macromolecules
Polymer-Fe3O4 Composite Janus Nanoparticles Xiaohui Yao, Jingyun Jing, Fuxin Liang, and Zhenzhong Yang* State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *
ABSTRACT: A Fe3O4 nanoparticle (NP) based on composite Janus NP with single polymer chain is prepared by termination of the modified Fe3O4 NP with the anionic living polymer chain. The requisite that the polymer chain should be sufficiently large over the NP diameter determines the grafting of single polymer chain. From the opposite side of the NP surface, functional species can be selectively grown for example grafting responsive PNIPAM by ATRP. Besides simple combination of the thermal and magnetic responsive performances of different components, the PS−Fe3O4−PNIPAM composite Janus NP shows additional interactive performance such as NIR-triggered Janus/hydrophobic transition at low surrounding temperature below LCST ∼ 32 °C.
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complexation.10 However, this method is highly size and chemistry selective. The middle block should strongly interact with the NP. In the case of large NP, many polymers or ligands are randomly grafted onto the NP surface. On the other hand, intramolecular cross-linking of desired block of copolymers is a straightforward way to achieve a polymeric NP with single polymer chain onto one side or two chains onto the opposite sides. Functional species can be preferentially grown within the polymeric NP toward functional composite NP. In order to avoid chain entanglement and, thus, intermolecular crosslinking, polymer concentration should be extremely low, commonly at 10−3 percentage magnitude or below. A scalable synthesis remains challenging.11 Grafting-onto approach is effective to link polymer chains onto a NP surface. As an example, carboxylic acid ended PS chains can be grafted onto an oleic acid capped Fe3O4 NP via ligand exchange. PS chains are present at the surface via coordination of the carboxylic acid with Fe3O4. The ligand exchange is rather slow after long time for example 48 h.12 It is understandable that the carboxylic acid head at the PS chain end should experience numerous attempts to penetrate the oleic acid layer. Number of the polymer chains onto the NP surface is controlled by PS molecular weight. In the optimum case, two chains are tethered onto the NP surface when the polymer coil is larger than or comparable with the NP diameter. No single chain tethered composite NP is observed. Moreover, the PS chains are linked onto the NP surface by coordination thus metastable. It is also questionable if the polymer chains can be strongly grafted onto a NP surface by covalent bonding. Herein, we present a simple way to covalently graft single polymer chain onto a Fe3O4 NP surface to achieve the
anoparticles (NP) with a diameter of 1−10 nm have gained attention for their diversify applications. Their shape, size and composition thus performance are well controllable.1,2 NPs can self-assembly into giant superstructures to achieve some cooperative properties.3−5 Spatial arrangement of NPs over multiple length scales in a nanocomposite can be further controlled via “bottom-up” technique by block copolymer self-assembly. The approach is advantageous over conventional lithographic “top-down” technique in more precise tuning interparticle separation.6,7 Moreover, synthesis of anisotropic nanoparticles is important to derive some new asymmetric superstructures due to more package modes and “directional” interaction. It is rational to asymmetrically graft polymer chains onto NP surface to generate the anisotropic feature, e.g., Janus NPs. When the Janus NPs are amphiphilic, they can serve as functional solid surfactants toward a much wealthy diagram of superstructures. Molecular NPs (such as C60, POSS, and POM) less than 1 nm become “giant surfactants” after polymer chains are asymmetrically tethered, which can display new self-assembly behaviors.8 We focus on the Janus NPs with polymer chains onto large NPs above 1 nm. It is important to develop methods to prepare anisotropic polymer composite NPs. In particular, it will be more interesting to graft single polymer chain onto a functional NP surface to achieve the anisotropic Janus NPs. Although disassembly of copolymer supramolecular structures toward polymeric Janus NPs is effective, sufficient narrow distribution of molecular weight and strict processing conditions are required. 9 One simple case to derive polymer−inorganic anisotropic Janus NPs is to graft single polymer chain onto one side of a given NP or two different chains onto the opposite side. As for sufficiently small NPs, two polymer chains of a triblock copolymer can be segregated onto the opposite sides owing to a steric constraint while the individual NP is wrapped with the middle block via © XXXX American Chemical Society
Received: September 12, 2016 Revised: November 9, 2016
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DOI: 10.1021/acs.macromol.6b02004 Macromolecules XXXX, XXX, XXX−XXX
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Key: (a) A chloromethylphenyl-group capped Fe3O4 NP is prepared by ligand exchange with the hydrolyzed silane from the oleic acid capped NP; (b) a single chain of anionic living polystyrene is grafted onto the NP surface by a rapid elimination of Cl from the NP surface with Li anion of the polymer; (c) PNIPM is grafted from the opposite side by ATRP. freeze-dried. A chloromethylphenyl-capped Fe3O4 NP was achieved, denoted as Fe3O4@Cl NP. 2.4. Synthesis of Anionic Living PS Chains. An anionic living PS chains was polymerized following the procedure. 10.0 mL of anhydrous cyclohexane and 1.0 g of St were charged in a vacuum-dried flask. A desired amount of the n-C4H9Li solution was added. The reaction was performed at 50 °C for 0.5 h. Anionic living PS chains with varied molecular weight were obtained according to the initiator amount. 2.5. Synthesis of PS−Fe3O4@Cl Composite Janus NP. A 10.0 mg sample of the freeze-dried Fe3O4@Cl NP was dispersed in 1.0 mL of anhydrous THF under argon atmosphere. The anionic living long PS chain solution was injected into the Fe3O4@Cl NP dispersion under stirring at 0 °C until the mixture turned to orange. This implied that the living PS was excessive. A small amount of methanol was injected to terminate the residual living PS for 20 min. The PS− Fe3O4@Cl composite Janus NP was collected with a magnet and washed with toluene. Similarly, in the case of short PS chain solution, a PS(Cl)@Fe3O4 core/shell NP was prepared. 2.6. Quaternization of PS−Fe3O4@Cl Composite Janus NP. A 10.0 mg sample of the as-prepared PS−Fe3O4@Cl Janus NP was redispersed in dichloroethane. The quaternization was performed after adding 100.0 μL of triethylamine at 80 °C for 2 h. The quaternized PS−Fe3O4@Cl composite Janus NP was obtained after washing and separation with a magnet. 2.7. Synthesis of PS−Fe3O4−PNIPAM Composite Janus NP. PNIPAM was grafted by ATRP from the PS−Fe3O4@Cl Janus NP. 10.0 mg of the as-prepared PS−Fe3O4@Cl Janus NP, 10.0 mg of Me6TREN, 0.2 g of NIPAM and 2.0 mL of DMF were mixed in a glass tube. The mixture was degassed after three cycles of freeze−pump− thaw. In a frozen state, 5.0 mg of CuBr was added under nitrogen. After another cycle of freeze−pump−thaw, the mixture was stirred at 70 °C for 12 h for the ATRP. The polymerization was terminated after exposure to air. The PS−Fe3O4−PNIPAM composite Janus NP was collected with a magnet and washed with DMF. 2.8. Emulsification with the Janus NP. A 100.0 μg sample of the as-prepared Janus NP was used to emulsify 100.0 μL of toluene and 400.0 μL of water. An emulsion formed by vigorously shaking the mixture for 1 min. 2.9. Characterization. Fourier transform infrared (FT-IR) spectra were recorded on the sample/KBr pressed pellets using a Brüker Equinox 55 instrument at room temperature. Carbon-coated copper grids were immersed and lifted from the dispersions and dried at ambient temperature. The samples were observed by transmission electron microscopy (TEM, JEOL 100CX operating at 100 kV). Gel permeation chromatography (GPC) in DMF was conducted on a system comprised of a Waters 515 HPLC pump, and a Waters 2414 RI detector equipped with four Waters Styragel columns (HT 2, HT 3, HT 4, and HT 5). Polystyrene standards were used for the calibration. Size distribution and ζ potential were measured using a Zeta-sizer (Nano Series, Malvern Instruments) at 25 °C. The crystallinity was characterized by X-ray powder diffraction (XRD) using Rigaku D/ max-2500. The magnetic properties were measured using vibrating sample magnetometer (VSM). UV−vis−NIR absorption spectra of the dispersions in trichloromethane were recorded by PerkinElmer
corresponding composite Janus NP as illustrated in Scheme 1. Fe3O4 NPs are selected as a model, which are promising due to strong paramagnetic performance and photo hyperthermia effect.13 Chloromethylphenyl-capped Fe3O4 NPs are prepared by ligand exchange of the oleic acid capped ones with the hydrolyzed silane. After a rapid elimination of Cl from the NP surface by combination with Li anion of the living polymers for example polystyrene, PS chains are thus grafted onto the Fe3O4 NP. It is key that in order to ensure grafting single polystyrene chain onto the NP surface, the polymer coil should be larger than or comparable with the NP diameter to gain sufficient steric hindrance. From the opposite side of the PS−Fe3O4@Cl Janus NP, the residual chloromethylphenyl-group can initiate an ATRP to graft another polymer for example thermal responsive PNIPM. We will focus on structural characterization and investigation of Janus performance. In particular, the polymer−Fe3O4 composite Janus NPs will exhibit interactive performances in between the polymers and the functional NPs.
2. EXPERIMENTAL SECTION 2.1. Materials. Ferric chloride (FeCl3), sodium oleate, copper bromide (CuBr) were purchased from Sinopharm Chemical Reagent. Oleic acid and styrene (St) were purchased from Beijing Chemical Reagent. Tris(2-dimethylaminoethyl)amine (Me6TREN) and 4(chloromethyl)phenyltrimethoxysilane (CMPTMS, 90%) were purchased from Alfa Aesar. Triethylamine, N-isopropylacrylamide (NIPAM), n-butyllithium solution in hexane (n-BuLi, 1.6 M) were purchased from J&K Chemical. Styrene was dried over calcium hydride overnight and distilled under reduced pressure, then stored at −10 °C prior to use. Cyclohexane and tetrahydrofuran (THF) were treated with sodium and diphenyl ketone until the solution became dark blue and then distilled under argon atmosphere. CuBr was purified in acetic acid and washed with ethanol and then recrystallized in toluene and hexane. All other reagents were used as received. 2.2. Synthesis of Oleic Acid Capped Fe3O4 NP.14 100.0 mL of sodium oleate aqueous solution (0.2 M) was mixed with 100.0 mL of iron chloride aqueous solution (0.2 M). The brown iron oleate complex precipitate was filtered and washed with distilled water, dried in a desiccator. The complex was added into 20.0 mL of ethanol containing 2.0 mL of oleic acid at room temperature. The mixture was transferred in a Teflon-lined stainless steel autoclave and heated to 180 °C for 5 h. After the autoclave was cooled down, the black oleic acid capped Fe3O4 NP was washed with ethanol and separated with a magnet. The product was redispersed in toluene at a concentration of 10 mg/mL. 2.3. Synthesis of Chloromethylphenyl-Capped Fe3O4 NP. 4(Chloromethyl)phenyltrimethoxysilane (1%, v/v) was added to the oleic acid capped Fe3O4 NP dispersion in toluene at a concentration of 0.5 mg/mL containing trace amount of acetic acid (0.01%, v/v), and stirred at ambient temperature for 24 h. The black product was precipitated and washed with ethanol, separated with a magnet and B
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Figure 1. TEM images of (a) the oleic acid capped Fe3O4 NP and (b) the chloromethylphenyl-capped Fe3O4 NP; (c) size distribution of the two NPs by DLS (a, the oleic acid capped Fe3O4 NP; b, the chloromethylphenyl-capped Fe3O4 NP); both NPs are dispersed in toluene.
Figure 2. (a) GPC traces of four representative PS samples; (b) calculated molecular weight values (solid line) and the measured values (solid square) as a function of the ratio of monomer weight to initiator volume; (c) DLS size distribution of the four PS samples dissolved in toluene at 10 mg/mL; (d) calculated polymer coil hydrodynamic diameters (solid line) and the measured values by DLS (solid square) as a function of the PS molecular weight.
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Figure 3. TEM images of three PS/Fe3O4 composite NPs with varied molecular weight of PS: (a) 87.9k, (b) 40.1k, and (c) 5.5k, where all the samples are stained with RuO4. (d) Size distribution of two PS/Fe3O4 composite NPs with PS chains: 87.9k (curve a), 40.1k (curve b), and 5.5k (curve c). Lambda 950. The droplets were pendent in water to measure interfacial tension using KRÜ SS DSA30 instrument. A magnet about 0.5 T was used to manipulate the droplets. The sample dispersions or emulsions were irradiated under 808 nm laser (0.5 W/cm2) to monitor the temperature change with the infrared camera (FLIR i7).
emu/g (Figure S2a). The new NPs remain well dispersible in toluene forming a transparent dispersion. After drying the dispersion, the NPs are individual under TEM (Figure 1b). DLS measurement indicates that the hydrodynamic diameter becomes slightly larger to 12 nm from the original 11 nm (Figure 1c). The fact that both NPs are individual without aggregation in the dispersions is also verified by the DLS results. The chloromethylphenyl-group onto the Fe3O4 NP surface can rapidly terminate the anionic group of a living polymer chain. An easy approach is thus provided to highly effective graft living anionic polymer chains onto the NP surface. Especially, when the polymer coil is larger than the NP diameter, grafting single polymer chain onto the NP surface is guaranteed in principle. The second large chain will be excluded far from the NP surface due to a strong steric repulsion. In this case, the Fe3O4 NP is covalently linked with a single polymer chain. An anionic living PS is selected as the model since it is easy to synthesize uniform PS chains with tunable molecular weight by anionic living polymerization. The polymerization is performed at a fixed monomer concentration of 0.1 g/mL and varied volume of n-BuLi solution in hexane (1.6 M). Four samples are synthesized with narrow molecular weight distribution (Figure 2a). The number-average molecular weight of PS follows the equation:
3. RESULTS AND DISCUSSION Oleic acid capped Fe3O4 NPs are synthesized by high temperature decomposition of the iron/oleate complex. Crystalline Fe3O4 is confirmed by XRD pattern (Figure S1). They are paramagnetic with a saturation magnetization of 57.8 emu/g (Figure S2a). The Fe3O4 NPs are well dispersible in solvents such as toluene. The dispersion is brown yet transparent. After drying the dispersion, the NPs are individual under TEM (Figure 1a), implying that no aggregation occurs in the dispersion. The mean diameter is 10 nm. After the oleic acid capped Fe3O4 NPs are treated with CMPTMS, ligand exchange occurs since the hydrolyzed Si−OH can form stronger interaction with the NP surface. The NP surface is terminated with chloromethylphenyl-groups, which is confirmed by FT-IR spectroscopy. In the oleic acid capped Fe3O4 NPs, strong bands at 2922, 2852, and 1462 cm−1 are assigned to the −CH2− group of oleic acid (Figure S3a). The characteristic peaks at 1543 and 1407 cm−1 are assigned to carboxylate group. After CMPTMS modification, the new bands around 1000−1150 cm−1 are assigned to Si−O−Si vibration (Figure S3b). Meanwhile, the characteristic peaks of −CH2− group of oleic acid become dramatically weaker. Chloromethylphenyl-capped Fe3O4 NPs are derived. Chloromethylphenyl-groups are strongly tethered after formation of the Si−O−Si network onto the NP surface. The saturation magnetization value of the Fe3O4@Cl NP is measured 55.6
M̅ n =
106W V [C]
W represents monomer mass (g), V and [C] volume (μL) and molar concentration (mol/L) of the initiator solution. The measured molecular weight values follow the equation well with a correlation coefficient of 0.9996 (Figure 2b). Hydrodynamic D
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Figure 4. (a) DLS results of the quaternized PS−Fe3O4@Cl Janus NP after dispersion in toluene (curve a) and water (curve b). (b, c) TEM and inset SEM images of the supermicelles after drying the Janus NP dispersions in toluene and water, the samples are stained with RuO4. (d) Left: PS− Fe3O4@Cl Janus NP contained trichloromethane drop pendent in water. Right: quaternzied PS−Fe3O4@Cl Janus NP contained trichloromethane drop pendent in water. NP solid concentration in trichloromethane is fixed at 1 mg/mL.
the single polymer chain (17 nm). A PS−Fe3O4@Cl Janus NP is thus achieved. It is reasonable that the chloromethylphenylmoiety should be exposed at the opposite side. The saturation magnetization is measured 39.7 emu/g (Figure S2b). In the case of another long PS chain (M̅ n = 40.1k), the composite NP remains the similar parachute shape but with a smaller PS part of 10−15 nm (Figure 3b). The average hydrodynamic diameter is 22 nm. The saturation magnetization is measured 48.3 emu/ g. When using a shorter PS chain (M̅ n = 5.5k), no parachute shape is observed but a core/shell structure with a PS shell thick about 2−5 nm (Figure 3c). The average hydrodynamic diameter of the core/shell NP is 19 nm (Figure 3d). The saturation magnetization is measured 47.4 emu/g. On the basis of the VSM results (Figure S2), grafting number of polymer chains onto the NP surface can be estimated (appendix Figure S2). It is reasonable to assume N = 1 in the longest PS chain case (M̅ n = 87.9K) since the polymer chain size is much larger than the NP size. The grafting number of another longer PS chain (M̅ n = 40.1K) is calculated 0.83, implying that single polymer chain is grafted. The results are consistent with the parachute shape of the composite NPs under TEM. In the shorter chain case (M̅ n = 5.5K), the calculated grafting number is 6.9, implying many chains are grafted. This result is also consistent with the core/shell structure rather than a parachute shape. After quaternization of the example PS−Fe3O4@Cl Janus NP (as shown in Figure 3b), the NP becomes positively charged with a zeta potential of 51 mV. In comparison, the original Janus NP is nearly neutral with a zeta potential of −9 mV. The magnetization is less influenced by the quaternization (Figure S4). The quaternized Janus NP becomes dispersible in both
diameter (Dh) of the PS chain coils in toluene is measured by DLS (Figure 2c). The values are closely consistent with the calculation Dh = 0.01645 × M̅ w0.5936(Figure 2d).15,16 For example, hydrodynamic diameter of the longest PS chain (87.9k) is large 18 nm, while hydrodynamic diameter of the shortest PS chain (5.5k) is 4 nm. A living anionic PS chain solution is slowly injected into the Fe3O4@Cl NP dispersion in anhydrous THF under stirring at 0 °C. The red PS solution immediately becomes colorless upon adding into the dispersion. The termination occurs rapidly. When additional small amount of the solution is further added, the dispersion appears orange color. This implies that the termination is fully accomplished. The residual living PS chains are deactivated by adding methanol. The composite Fe3O4 NPs are separated with a magnet and washed with cyclohexane. The characteristic bands of PS appear at 700 cm−1 and 2800−3100 cm−1 (Figure S3c). The Fe3O4 NP is discerned under TEM. PS is distinguished after staining with RuO4. When a long living PS chain (M̅ n = 87.9k) is used, the composite NP displays a parachute shape with condensed single PS chain about 20−25 nm asymmetrically present onto one side of the Fe3O4 NP. All the composite NPs display the same parachute structure at all the regions under TEM observation. This implies that only single polymer chain is grafted. If this was not the case, for example grafting two chains, the chains would be present at opposite sides of the NP surface due to steric repulsion of the chains during the grafting. The composite NPs would be symmetric with two parachutes. In fact, such structure has never been observed under TEM. DLS measurement gives the average hydrodynamic diameter of the parachute Janus NP 29 nm (Figure 3d), larger than the Fe3O4@Cl NP (11 nm) and E
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Figure 5. (a) Quaternized PS−Fe3O4@Cl Janus NP dispersion in water and collected with a magnet. (b) manipulation of the oil drop containing the Janus NP with a magnet at a large distance (b1) and a short distance (b2). (c) Toluene/water emulsion stabilized with the Janus NP, magnetic collection of the emulsion droplets, de-emulsification under a stronger magnetic field while the Janus NPs are recycled. Green dye is added in toluene for easier observation.
Figure 6. (a, b) TEM images of the PS−Fe3O4−PNIPAM Janus NP after sequential staining with PTA and RuO4; (c) the Janus NP aqueous dispersion at 25 and 35 °C; (d) NIR triggered de-emulsification.
toluene and water. In contrast, the PS−Fe3O4@Cl Janus NP is not dispersible in water. DLS measurement indicates that large supermicells form in the quaternized PS−Fe3O4@Cl Janus NP dispersions (Figure 4a), which are large 73 and 67 nm in
toluene and water, respectively. After drying the dispersion in toluene, a flowerlike supermicelle is achieved (Figure 4b). The Fe3O4 NPs are present in the internal core, while the gray hydrophobic PS chains form the corona. Although the F
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the precipitate is redispersible in water upon shaking. This implies that the thermally triggered Janus/hydrophobic transformation of the composite NP is reversible. The Janus NP can absorb light within a broad wavelength range especially in the NIR range. The absorption intensity increases with the NP content (Figure S6a). At 0.2 mg/mL of the NP, the dispersion can be heated to 42.5 °C from 25.5 °C during the laser irradiation for 10 min (Figure S6b). In the case of 0.1 mg/mL, the dispersion is heated to 36.8 °C. The emulsion stabilized with the Janus NP is stable at room temperature ∼25.5 °C (Figure 6d). While 0.5 mL of the emulsion is irradiated, the emulsion is gradually heated to 29.2 °C, and the emulsion starts to destabilize. Upon removal of the irradiation, the destabilization proceeds until a complete phase separation. Oil and water are separated into two layers. The brown NPs are present in the top oil phase. It is noticed that the maximum temperature is 30.8 °C far below LCST ∼ 32 °C. This NIR triggered Janus performance at low temperature is resulted from the hyperthermia effect of Fe3O4 and a fast thermal energy transfer to PNIPAM. NPIPAM is locally heated above LCST (∼32 °C) while the surrounding temperature is below the LCST ∼ 32 °C.
supermicell is irregular in contour, the surface is smooth (inset Figure 4b). After drying the aqueous dispersion, the PS chains form the core while the hydrophilic Fe3O4 NPs form the shell (Figure 4c). The supermicelle is irregular with a coarsening surface (inset Figure 4c). In order to further prove that the quaternized PS−Fe3O4@Cl Janus NP is amphiphilic, interfacial tension of the Janus NP contained trichloromethane drops pendent in water is measured (Figure 4d). The interfacial tension decreases significantly from 32.4 to 18.4 mN/cm2. No remarkable change of the interfacial tension (31.6 mN/cm2) is observed in the case of PS−Fe3O4@Cl NP. In comparison, the core/shell NP is not dispersible in water even after quaternization. The quaternized chloromethylphenyl-moiety should be embedded beneath the PS shell. Only a slight decrease (29.1 mN/cm2) in interfacial tension is observed in the case of the quaternized core/shell NP (Figure S5). The quaternized PS−Fe3O4@Cl Janus NP dispersed in water can be easily collected with a magnet, leaving a colorless water phase behind (Figure 5a). During measurement of interfacial tension of the oil drops pendent in water, they can be easily manipulated with a magnet. The drops are deformed and deflected toward the magnet at a proper distance e.g. Eight mm. Upon removal of the magnet, the drops can completely recover the original shape (Figure 5b1). When the distance is sufficiently close, e.g., 5 mm, the drops “jump” onto the magnet from the needle. Oil flows down while the NPs are adhered onto the magnet (Figure 5b2). Although some magnetic surfactants of magneto-active metal complex anions contained ionic liquids such as 1-methyl-3-butylimidazolium tetrachloroferrate ([bmim]FeCl4) have been reported, their magnetic response is too weak that only drops are slightly deflected upon applying magnet (0.4 T) at very close distance. The homogeneous mixture of [bmim]FeCl4 in water cannot be separated even using a stronger magnet of 1 T.17 The quaternized PS−Fe3O4@Cl Janus NP is amphiphilic. A stable toluene/water emulsion forms in the presence of the Janus NP (Figure 5c). The dispersed oil droplets move toward the magnet leaving colorless transparent water phase. Upon removal of the magnet, the emulsion is recovered. After increasing the magnetic field by applying another magnet (∼0.5 T), the emulsion becomes destabilized. Three phases are separated: oil, water, and the Janus NP. The brown Janus NP is strongly adhered onto the magnet while decanting the water and oil phases, respectively. The Janus NP can be recycled for reuses. The Fe3O4 NP also displays a photothermal effect besides paramagnetic performance. An example thermal responsive polymer PNIPAM is grafted from the chloromethylphenylcapped NP surface by ATRP, which is verified by the new peaks at 1645 and 1543 cm−1 (Figure S3d). After staining with PTA, PNIPAM corona is discerned that is present onto one side of the NP (Figure 6a). In comparison, only dark Fe3O4 NP is visible from the composite NP without staining under TEM (inset Figure 6a). After another staining with RuO4, the condensed PS chain is distinguished onto the other side (Figure 6b). PS and PNIPAM are segregated onto the opposite sides of the middle Fe3O4 NP, achieving a PS−Fe3O4−PNIPAM composite Janus NP. The Janus NP is well dispersible in water below LCST ∼ 32 °C forming a brown transparent dispersion (left Figure 6c). Upon the dispersion is heated to 35 °C above the LCST, the PNIPAM becomes hydrophobic. As a result, the composite NP precipitates leaving a colorless transparent water phase (right Figure 6c). At low temperature,
4. CONCLUSION We propose a method to prepare polymer−Fe3O4 composite Janus NPs by grafting a single polymer chain onto a Fe3O4 NP when the polymer coil is larger than the NP diameter. Living anionic polymer chains are easily covalently linked onto the NP surface via a fast elimination of LiCl via termination between the anionic end group (Li+) with Cl of the chloromethylphenylgroup on the NP surface. The residual chloromethylphenylgroups can derive positively charged hydrophilic performance after quaternization and thermal responsive PNIPAM chains by ATRP onto the other side. The Janus NPs are responsive to magnetic and thermal stimuli. Shape and stability of the emulsion droplets can be manipulated under a magnetic field. Thanks to the photo thermal effect of Fe3O4 NP under NIR irradiation, destabilization of the emulsion is triggered by NIR irradiation at low surrounding temperature. This interactive performance is arisen from the hyperthermia effect of Fe3O4 and a fast thermal energy transfer to locally heat PNIPAM above LCST (∼32 °C). In our recent work, the PS−Fe3O4@Cl composite Janus NP with the same microstructure has been prepared at high solid content 10 wt %. This implies that a large quantity of the Janus NP will be available. The synthesis approach can be extended to other functional NPs, polymer chains and responsive species onto the NP surface.
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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.6b02004. XRD pattern, VSM curves, and FT-IR spectra of the representative NPs; photographs of two chloroform droplets pendent in water containing the PS@Fe3O4@Cl core/shell NP and the quaternized one; graph of absorption spectra of the PS−Fe3O4−PNIPAM composite Janus NP aqueous dispersion with varied solid concentrations and their temperature increase with irradiation time at varied solid concentrations (PDF) G
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Surface Properties with Responsive Surfactants. Angew. Chem., Int. Ed. 2012, 51, 2414−2416.
AUTHOR INFORMATION
Corresponding Author
*(Z.Y.) E-mail:
[email protected]. Telephone: +86-1082619206. ORCID
Zhenzhong Yang: 0000-0002-4810-7371 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by Ministry of Science and Technology (2012CB933200) and National Natural Science Foundation of China (51233007, 51173191).
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REFERENCES
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DOI: 10.1021/acs.macromol.6b02004 Macromolecules XXXX, XXX, XXX−XXX
