Magnetically Responsive Polymer Network Constructed by Poly

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Magnetically Responsive Polymer Network Constructed by Poly(acrylic acid) and Holmium Michinari Kohri,*,† Kenshi Yanagimoto,† Kotona Kohaku,† Shohei Shiomoto,§ Motoyasu Kobayashi,† Akira Imai,∥ Fumiyuki Shiba,‡ Tatsuo Taniguchi,† and Keiki Kishikawa†

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Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, and ‡Department of Materials Science, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan § Department of Applied Chemistry, Graduate School of Engineering, and School of Advanced Engineering, Kogakuin University, 2665-1 Nakano-cho, Hachioji, Tokyo 192-0015, Japan ∥ Technical Services Department, Quantum Design Japan, Inc., Nishiikebukuro Fujita Bldg. 1F, 1-11-16 Takamatsu, Toshima-ku, Tokyo 171-0042, Japan S Supporting Information *

ABSTRACT: Holmium (Ho), one of the lanthanide elements, shows a high magnetic moment. Here we present a simple, yet highly potential approach for preparing polymer-based magnetic materials from a three-dimensional polymer network composed with poly(acrylic acid) and Ho showing trivalent nature. We have successfully prepared a magnetic polymer network that reacts directly to a magnet by three-dimensionally immobilizing Ho in the polymer matrix. The present method allowed a preparation of wide range of magnetic materials using polymeric scaffolds, e.g., polymer-grafted particles and cross-linked polymer gels. As a result of the high Ho content, these materials responded quickly to the magnet. The discovery of a method to prepare magnetic materials will provide flexibility in materials design and greatly expand the scope of application of magnetic materials.

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INTRODUCTION Within the field of materials chemistry, stimuli-responsive materials based on polymers, which respond to temperature, pH, light, CO2, or ultrasonics, have been of interest due to their potential use in active materials.1−3 Magnetically responsive materials are a topic of intense research due to their potential breakthrough applications such as biomedical imaging, medical diagnostics, and memory devices.4,5 Polymerbased magnetic materials are usually prepared by the doping of magnetic nanoparticles, e.g., magnetite (Fe3O4) or maghemite (γ-Fe2O3) particles.6,7 For example, Takahara et al. demonstrated the preparation of polystyrene-grafted Fe3O4 particles for a nanofiller using surface-initiated living radical polymerization techniques.8 Müller et al. used the cylindrical shape of brushes with diblock copolymer side chains composed of poly(acrylic acid)-b-poly(tert-butyl acrylate) as a template for in situ formation of γ-Fe2O3 particles, and composite materials obtained showed a superparamagnetic behavior.9 Pyun et al. obtained a one-dimensional ordered composite material from ferromagnetic colloids, such as cobalt particles, stabilized by polystyrene-based surfactants under a magnetic field.10 As described above, most of the magnetic composite materials contain magnetic nanoparticles as a main component, but it is often difficult to composite magnetic nanoparticles into existing materials. © XXXX American Chemical Society

The lanthanide elements exhibit unique functions such as photoluminescence and magnetic behaviors, since the energy of the 4f orbital is lower than the energy of the 5d orbital, and the electrons in the 4f orbital are shielded.11 The photoluminescence behavior of the lanthanide complex was well investigated and found some applications such as lighting, lasers, optical telecommunications, and medical diagnostics.12−14 Magnetic properties of lanthanide compounds are important issues as well as photoluminescence properties, and many studies on lanthanide single-molecule magnets have been reported.15,16 Additionally, it is also important to develop lanthanide element composite materials that respond to a magnet. Holmium (Ho) shows the highest magnetic moment (10.6 μB) of any naturally occurring element.17 Recently, Eastoe et al. reported that by introducing a magnetic surfactant, prepared by combining commercially available surfactants and holmium(III)-based counterions by electrostatic interaction, on the surface of biomolecules or colloidal particles, it is possible to produce magnetic materials.18,19 We demonstrated that the structural colors can be changed by the magnetic field by covering the surface of melanin-like Received: July 19, 2018 Revised: August 7, 2018

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DOI: 10.1021/acs.macromol.8b01550 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic representation of the preparation of a 3D polymer network via Ho and their dynamic behavior response to a magnet. Magnetic materials using (b) PAA-grafted SiO2 particles and (c) PAA-based gel materials as a scaffold for carrying Ho.

particles 20,21 with Ho-carrying magnetic surfactants. 22 Although it was convenient for the preparation of magnetic materials without Fe3O4 or γ-Fe2O3 nanoparticles, it was difficult to obtain materials that react quickly to a magnet due to the low amount of Ho, which were introduced in a single layer on the material surface. Here, we demonstrated the preparation of magnetic materials based on a three-dimensional (3D) polymer network via Ho (Figure 1a). By combining poly(acrylic acid) (PAA) and Ho showing trivalent nature,17 Ho are three-dimensionally introduced in polymer matrix, and we succeeded in the creation of a new category of the magnetic polymer in which the polymer itself responds to a magnet. Ho centers will be clusterized in the PAA matrix through a lanthanide-centered covalently bond.23,24 In addition, polymeric scaffolds such as polymer-grafted particles and cross-linked polymer gels made it possible to produce magnetic materials that respond quickly to the magnet (Figure 1b,c). This simple and novel process of using a 3D polymer network via Ho can be used in basic research on magnetic materials and holds a great potential for technological applications.

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energy dispersive X-ray spectrometry (EDS) mapping were performed on a Hitachi H-7650 operated at 100 kV. The dynamic light scattering (DLS) measurements were performed with a Malvern Zetasizer Nano ZS. Atomic force microscopy (AFM) was performed using a JPK Instruments NanoWizard 3 system. Rectangular-shaped silicon cantilevers (tip radius: 7 nm; HyperDrive PPP-NCHAuD; Nanosensors (NanoWorld Group)), with a spring constant of 42 N m−1 and resonance frequency of 135−280 kHz, were used for imaging. The root-mean-square (rms) roughness (Rq) was calculated by the equation Rq =

1 L

∫0

L

z(x)2 dx

(1)

where L and z(x) are the scanning distance and height variation function of the surface along the scanning direction (x), respectively. X-ray photoelectron spectroscopy (XPS) measurement was performed by a Physical Electronics Quantum 2000 Scanning ESCA Microprobe system at 1 × 10−9 Torr using a monochromatic Al Kα Xray source operated at 100 W. XPS spectra were collected at a takeoff angle of 45°, and a low-energy (25 eV) electron flood gun was used to minimize sample charging. The survey spectra (0−800 eV) and highresolution spectra (narrow scan) of the C1s, O1s, Si2p, and Ho4d were acquired at an energy step of 1.0 and 0.125 eV, respectively. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis was performed using a PerkinElmer Optima8300 for Ho and lanthanum (La) determinations. Magnetization curves measurements were performed using superconducting quantum interference device sample magnetometer (MPMS(R)3, Quantum Design, Inc.). Samples were fill into hard gelatin capsules and measured at 300 K in applied magnetic field up to 5570 kA m−1 (70 kOe). These results were obtained by conventional magnetic hysteresis loop measurement technique. The magnetic properties of magnetic gels have been measured (χT vs T plots) in the 1.8−300 K temperature range in a field of 795.8 kA m−1 (10 kOe). Photographs and movies of the samples were taken using an Olympus OM-D digital camera. Preparation of a 3D Magnetic Polymer Network onto Si Plate. The PtBA-modified Si plate was prepared using SI-ATRP according to published literature.25 TFA (5 mL) was added to the PtBA-modified Si plate obtained in DCM (5 mL) and stirred at room temperature. After 24 h, the solvents and regents were removed, and the plate was dried, producing PAA-modified Si plate. Then, Ho was immobilized onto the PAA brushes. Holmium(III) chloride hexahydrate (3.8 g, 100 mmol) was added to the PAA-modified Si plate obtained in NaOH(aq) (20 mL, pH 12) and stirred at room temperature. After 24 h, the plate was dried with H2O. The plates

EXPERIMENTAL SECTION

Materials. Holmium(III) chloride hexahydrate, tetrahydrofuran (THF), tert-butyl acrylate (tBA), and acrylic acid (AA) were obtained from Kanto Chemical Co., Inc. 2,2′-Azobisisobutyronitrile (AIBN), dichloromethane (DCM), trifluoroacetic acid (TFA), lanthanum(III) chloride heptahydrate, and sodium hydroxide (NaOH) were obtained from Wako Pure Chemical Industries, Ltd. 4-Cyano-4[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid (CSPA) was purchased from Sigma-Aldrich Co., LLC. 1,4-Dioxane was purchased from Tokyo Chemical Industry Co., Ltd. Deionized water with a resistance of 18.2 MΩ·cm was obtained by passing through a Millipore Simplicity UV system. Silica (SiO2) particles (MP-1040) were supplied by Nissan Chemical Industries, Ltd., and were purified by stirring in concentrated nitric acid with subsequent washes with THF. PAA gels (DSC 30) were supplied by Sanyo Chemical Industries, Ltd., and were used as received. All of the other chemicals and solvents were of reagent grade and were used as received. Neodymium magnets were purchased from NeoMag Co., Ltd. Measurements. The Fourier transform infrared (FT-IR) spectra were recorded using a JASCO FT/IR-420. The static contact angle against water (5 μL) measurements were performed by Meiwafosis P200A. Transmission electron microscopy (TEM) observation and B

DOI: 10.1021/acs.macromol.8b01550 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules obtained were characterized by static contact angle, FT-IR, XPS, and AFM measurements. Preparation of Magnetic Particles. CSPA-modified SiO2 particles (2.46 groups nm−2) were prepared by according to published literature.26 AA (0.34 g, 4.7 mmol), free CSPA (6.3 mg, 0.016 mmol), AIBN (0.8 mg, 0.005 mmol), and CSPA-modified SiO2 particles (0.2 g) dispersed in 1,4-dioxane (15 mL) were placed in a 50 mL flask. The mixture was deoxygenated by freeze−deaeration, and then the RAFT polymerization was performed at 70 °C. After 18 h, the particles were separated and purified repeatedly by centrifugation (8000 rpm for 10 min) and redispersion, yielding PAA-modified SiO2 particles (0.18 chains nm−2). Then, Ho was immobilized onto the PAA brushes. Holmium(III) chloride hexahydrate (0.037 g, 0.08 mmol) was added to the PAA-modified SiO2 particles obtained in NaOH(aq) (20 mL, pH 12) and stirred at room temperature. After 1 h, the particles were separated and purified repeatedly by centrifugation (8000 rpm for 10 min) and redispersion, producing holmium-bearing SiO2 particles. Particles obtained were characterized by FT-IR, DLS, XPS, TEM-EDS, ICP-AES, and magnetization measurements. Preparation of Magnetic Gels. Commercially available PAAbased micrometer-sized gel particles were using as carrier. Holmium(III) chloride hexahydrate (1.2 g, 1.39 mmol) was added to the PAA gel particles (0.1 g) in NaOH(aq) (10 mL, pH 12) and stirred at room temperature for 24 h. Gels were purified repeatedly by decantation and dialysis against to H2O for 3 days, producing Hobearing gels. The La-bearing gels were also prepared under the same conditions using lanthanum(III) chloride heptahydrate (1.4 g, 1.39 mmol). Gels obtained were characterized by ICP-AES, TEM-EDS, and magnetization measurements.

polymer network through glass slide plate with thickness of 1 mm, and then almost the same area was measured with a magnet placed. As shown in Figure 2a, little change in the

Figure 2. (a) AFM height profiles of surface of Si plates having Hocarrying polymer brushes in the absence or presence of a magnet under a dry condition at atmospheric pressure. (b) AFM height profiles of bare PAA polymer brush in water.

thickness of polymer network film was observed (ca. 54 nm) under dry conditions. Some of the present authors reported the usability of AFM measurements in a wet state for the evaluation of polymer brush thicknesses.27 Thus, one drop of water was added to the sample, and the AFM measurement was performed in a wet state in which the polymer network layer was swollen in water (Figure 3a). (The samples of

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RESULTS AND DISCUSSION To investigate the responsiveness of the designed polymers to a magnet, the 3D polymer network composed with PAA and Ho were constructed on a silicon (Si) plate (Figure 1a). PAA brushes were constructed on a Si plate by the surface-initiated atom transfer radical polymerization (SI-ATRP) of tBA and a subsequent hydrolysis reaction. Then, the polymer network was constructed by reacting holmium(III) chloride hexahydrate under basic conditions. The wettability of the plates was measured using a static contact angle measurement, which is a typical method for characterizing the surface. The static contact angle for water was 15° on the bare Si plate surface, which was cleaned with plasma treatment. After immobilization of the ATRP initiator on the surface, the value was increased to 63°. While the contact angle of the PtBA-modified Si plate surface increased to 86°, that of PAA-modified Si plate surface decreased to 23° due to the hydrophilic nature of PAA, which is in agreement with a previous study.25 The increased hydrophilicity observed after SI-ATRP and deprotection of carboxylic acid groups of the sample is consistent with the successful introduction of PAA onto the surface. After immobilization of Ho in the PAA brushes, the value changed slightly and then remained at 23°. Detailed analysis of the polymer network carrying Ho is discussed later. The magnetic response of the polymer network from Hocarrying PAA brushes was investigated. The magnetic behavior of the polymer network was measured by atomic force microscopy (AFM) observation under a dry condition at atmospheric pressure. To obtain the AFM height profiles of polymer network films, a portion of the polymer film was scratched with a stainless-steel needle to allow easy measurement of the film thickness. The topography images at boundary area of scratched polymer network and Si plate were first measured by AFM. Next the neodymium magnet (477 mT) was placed under the Si plate covered by the

Figure 3. (a) Schematic diagram of AFM measurement in water. (b) Typical AFM images at the boundary area of partially scratched polymer brushes in water in the presence (right) and absence (left) of the magnet. (c) Frequency distribution of height data observed by cross-sectional high profile of polymer brush (entry 1 in Figure 3d) at boundary area. (d) Cross-sectional profiles at the boundary region of magnetic polymer brushes that had been partially scratched with a needle to estimate the brush thickness in water in the absence (black lines) and presence (red lines) of the magnet.

polymer brushes used under dry and wet conditions were different.) Figure 3b shows the typical topographic AFM images of the polymer network carrying Ho in the absence or presence of a magnet. The AFM analysis of the surface revealed that the resulting films without or with a magnet are homogeneous and relatively smooth, with a root-mean-square C

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Macromolecules roughness of ≈2.8 nm (without magnet) and 1.3 nm (with magnet) in 1 μm2. A detailed derivation method of the average polymer film thickness is shown in Figure 3c. The frequency distribution of the height information within the measurement range (5 μm × 5 μm) was analyzed to calculate the average thickness of polymer network film. The height at which the frequency becomes the maximum in each area of the Si plate and the polymer network film was calculated, and the average polymer film thickness was obtained from the difference between them. Figure 3d shows the AFM height profiles of magnetic polymer film with different thicknesses in the absence (Figure 3d, black lines) and presence (Figure 3d, red lines) of the magnet. The average thickness of polymer network film without a magnet, calculated according to the above method, was approximately 8.0, 29, and 64 nm, respectively. On the other hand, under a magnetic field, the film thickness decreased to approximately 7.2, 25, and 59 nm. The shrinkage ratio of the polymer network film thickness for three samples was approximately 11, 15, and 8.7%, which was relatively constant regardless of polymer brush thickness. In contrast, the thickness of PAA brush in water did not change in the presence of a magnet (Figure 2b), clearly indicating that the introduction of Ho is important for response to the magnet. It was observed that the polymer network from Ho-carrying PAA brushes directly responded to the neodymium magnet in water and shrank dynamically (Figure 1a). While Langer et al. reported dynamic changes in polymer brushes responsive to electric potentials,28 polymer brushes responding to magnets have not been reported. To our best knowledge, this is the first observation of the dynamic behavior of the lanthanide element-containing polymer network layer in response to the neodymium magnet. PAA-grafted silica (SiO2) particles were used as a scaffold to produce submicrometer-sized magnetic particles (Figure 1b). When SiO2 particles grafting PAA brushes were prepared using SI-ATRP in the same manner as for a Si plate, the particles, unfortunately, aggregated during deprotection of the tert-butyl groups. In contrast, PAA-grafted SiO2 particles, prepared using surface-initiated reversible addition−fragmentation chain transfer (SI-RAFT) polymerization of AA, were successfully obtained without aggregation. Hereinafter, experiments were conducted using particles prepared using SI-RAFT polymerization. The samples were characterized by Fourier transform infrared (FT-IR) spectroscopy. As shown in Figure 4a, the rather sharp peak occurring at 1718 cm−1 can be attributed to the stretching vibration of the CO group in PAA. The redshift to 1557 cm−1 in Ho-bearing SiO2 particles suggests the coordination of the carbonyl group to Ho cations. Cao et al. reported a similar tendency that the peaks of the CO groups were red-shifted after binding the lanthanide elements, such as gadolinium or europium to β-diketone groups.29 The volume average diameters of obtained particles in water, measured by DLS, are shown in Figure S1a. The diameter of PAA-grafted SiO2 particles was ca. 152 nm, which was larger than the 100 nm of the core SiO2 particles, indicating that core−shell particles have hydrated PAA layer of ca. 26 nm thickness. The diameter of particles after Ho introduced increased to ca. 280 nm. While the obtained particles were well dispersed in a solvent, formation of secondary particles by Ho was suggested (Figure S1b). Magnetic particles obtained were measured by transmission electron microscopy (TEM) analysis. Figure 4b shows the TEM image of magnetic particles together with the energy dispersive X-ray spectrometry (EDS) mapping data of

Figure 4. (a) FT-IR spectra of PAA-grafted SiO2 particles (black line) and Ho-bearing SiO2 particles (red line). (b) TEM image and TEMEDS mapping data of Ho-bearing SiO2 magnetic particles. (c) Narrow-scan XPS spectrum of magnetic particles. (d) Photographs when a magnet is placed next to a glass bottle containing a magnetic particle dispersion (1 wt % in water).

silicon (Si), oxygen (O), and Ho. Because the images of the three element mappings overlapped, the formation of Hocarrying particles was evaluated. The X-ray photoelectron spectroscopy (XPS) spectrum of Ho-carrying SiO2 particles are shown in Figure 4c. The Ho4d spectrum of the samples clearly indicated the introduction of Ho.30 The immobilized Ho onto SiO2 particles surface was recovered by acid treatment (0.1 M hydrochloric acid), and the amount of Ho was determined to be ca. 0.29 mmol-metal/g-sample from inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis, indicating the successful creation of the 3D polymer network via Ho onto the particle surface. The magnetically responsive behavior of Ho-carrying SiO2 particles was investigated. The neodymium magnet (1 T) was placed beside a glass bottle containing the particle dispersions (1 wt %) and was allowed to stand. Some particles having Ho-containing magnetic polymer network were attracted 3 min after placing the neodymium magnet, and after 10 min, the collection of the most particles was observed around the side of the glass bottle where the magnet was placed (Figure 4d). However, some particles remained in the aqueous phase. We believe that all particles did not respond to the magnet due to nonuniformity of Ho introduction amount onto the particles. After removal of the magnet, the collected samples were once again dispersed in solution, suggesting their reversible dispersibility. To demonstrate the versatility of the present method, micrometer-sized magnetic gels were prepared using commercially available PAA-based gel particles as a scaffold (Figure 1c). Holmium(III) chloride hexahydrate was added to the PAA gel particles under basic conditions to prepare magnetic gels. To investigate the importance of Ho regarding the magnetic behavior, gel materials bearing La were also prepared using lanthanum(III) chloride heptahydrate instead of holmium(III) chloride hexahydrate. While Ho is an f-block element with the electronic configuration [Xe] 4f105d16s2, La is a d-block D

DOI: 10.1021/acs.macromol.8b01550 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules element with the electronic configuration [Xe] 5d16s2, suggesting weak magnetic behavior.31 From the TEM-EDS mapping, the introduction of La or Ho was confirmed (Figure 5a,b).

Powders of magnetic materials were encapsulated in hard gelatin capsules, and magnetization curves measurements of samples were conducted at 300 K in applied magnetic field up to 5570 kA m−1. Because it is assumed that the polymer-based magnetic composite materials are mainly used near room temperature, the mass magnetization at 300 K was measured. Results were obtained by conventional magnetic hysteresis loop measurement technique. As shown in Figure 6a, both the

Figure 6. (a) Magnetization curves measurements of Ho-carrying magnetic particles (blue line) and magnetic gels (red line) at 300 K in applied magnetic field up to 5570 kA m−1 (70 kOe). (b) Temperature dependence of the χT product at 795.8 kA m−1 (10 kOe) for magnetic gels. Figure 5. TEM image and TEM-EDS mapping data of (a) La-carrying gels and (b) Ho-carrying gels. To analyze by TEM measurements, samples were deliberately broken. Photographs of a magnet placed on the side of a glass bottle containing water and (c) La-carrying gels and (d) Ho-carrying gels. The behavior of the Ho composite gel material in response to the magnet in real time is shown in Movie S1.

Ho-carrying particles and gels showed paramagnetic nature. While the magnetic response of gelatin capsules (background: data not shown) was negligibly small compared with the sample, the mass magnetization of the magnetic particles (blue line) and gels (red line) at 5570 kA m−1 were ca. 0.34 and 5.7 A m2 kg−1, respectively. The magnetic properties of magnetic gels have been measured (χT vs T plots) in the 1.8−300 K temperature range under a field of 795.8 kA m−1 (Figure 6b). The room temperature χT value is found near 0.31 m3 K kg−1, and this remains practically unaltered over a wide temperature range (100−300 K). In addition, the results also suggested that the sample showed paramagnetic properties. Although the paramagnetic centers are far apart from each other, and there is no long-distance magnetic order, the Ho-carrying polymer network material has high mass magnetization and exhibited excellent properties to rapidly respond to the neodymium magnet at room temperature.

The magnetically responsive behavior of the gel materials was investigated. The obtained gels and water were added to the glass bottle. (The micrometer-sized gel materials did not disperse in the solvent and precipitated at the bottom of the bottle.) Then, the neodymium magnet (477 mT) was placed on the top side. As shown in Figure 5c, the La-introduced gels (1.99 mmol-metal/g-sample determined by ICP-AES) were not responded to the magnet and were not moved. In contrast, Ho-introduced gels were attracted and were moved toward the magnet, indicating the importance of Ho on magnetic behavior (Figure 5d). Additionally, the Ho-carrying gels were moved quickly according to the movement of the magnet (Movie S1). It is believed that the faster and stronger response of the magnetic gels compared to that of the magnetic particles is due to the amount of immobilized Ho. From the ICP-AES analysis, it was found that the amount of Ho immobilized into the magnetic gels was ca. 1.57 mmol-metal/g-sample, which is larger than the amount carried in the magnetic particles (vide supra). Ho could be efficiently introduced to the gel material, which is a cross-linked body of PAA gels, because Ho can be immobilized not only on the surface but also inside the materials. Although the isolated Ho center was not expected to be attracted to the magnet at room temperature, as a result of supporting a large amount of holmium using polymeric scaffolds, we successfully prepared materials that respond to the bulk magnet. Recently, Allouche et al. reported a similar phenomenon; while the dysprosium (Dy) molecular precursor did not exhibit magnetic resonance, the materials carrying Dy on the surface of the SiO2 particles showed magnetic resonance.32

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CONCLUSION In conclusion, we demonstrated the preparation of Ho-carrying PAA-based magnetic polymers and found an unprecedented phenomenon that the 3D magnetic polymer network built on a plate shrink in response to a magnet. It was shown that submicrometer-sized particles coated with a magnetic polymer network also respond to a magnet in a solvent dispersed state. Additionally, it was found that micrometer-sized gels, which can introduce a large amount of Ho inside, can move quickly corresponding to the movement of the magnet. Because many studies have been conducted on techniques for introducing polymer brushes into various substrates and techniques for manufacturing polymer composite materials, our strategy will allow for the preparation of magnetically responsive materials from a wide variety of polymeric scaffolds, which enable the flexibility and processability of materials. The results shown here are promising steps in the development of lanthanide element composite magnetic materials. E

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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.8b01550.

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DLS measurements of obtained particles (PDF) Movie S1: magnetically responsive behavior of magnetic gels (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.K.). ORCID

Michinari Kohri: 0000-0003-1118-5568 Motoyasu Kobayashi: 0000-0001-8349-4365 Fumiyuki Shiba: 0000-0002-6503-1081 Keiki Kishikawa: 0000-0002-7539-568X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.K. acknowledges the support of JSPS KAKENHI (Grant 16K14072), the Mazda Foundation, and the Mitsui Chemical Company Award in Synthetic Organic Chemistry, Japan. We thank Prof. Kyoichi Saito and Ms. Shoko Naruke of Chiba University for ICP-AES measurements. We acknowledge Nissan Chemical Industries, Ltd., and Sanyo Chemical Industries, Ltd., for providing reagents.

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REFERENCES

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DOI: 10.1021/acs.macromol.8b01550 Macromolecules XXXX, XXX, XXX−XXX