Polyaniline Assisted Uniform Dispersion for Magnetic Ultrafine Barium

May 24, 2017 - †Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, and ‡Research Center for T...
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Polyaniline Assisted Uniform Dispersion for Magnetic Ultrafine Barium Ferrite Nanorods Reinforced Epoxy Metacomposites with Tailorable Negative Permittivity Hongbo Gu,*,† Hongyuan Zhang,† Chao Ma,† Shangyun Lyu,† Feichong Yao,† Chaobo Liang,§ Xiaomei Yang,*,‡ Jiang Guo,∥ Zhanhu Guo,∥ and Junwei Gu*,§ †

Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, and ‡Research Center for Translational Medicine at East Hospital, School of Life Science and Technology, Tongji University, Shanghai 200092, People’s Republic of China § MOE Key Laboratory of Space Applied Physics and Chemistry, Shaanxi Key Laboratory of Macromolecular Science and Technology, Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’ an, Shaanxi 710072, People’s Republic of China ∥ Integrated Composites Laboratory (ICL), Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37966, United States S Supporting Information *

ABSTRACT: In this paper, a special dielectric property with tailorable negative permittivity is attained in the polyaniline assisted magnetic ultrafine barium ferrite (BaFe12O19) nanorods reinforced epoxy metacomposites. The ultrafine BaFe12O19 nanorods have been prepared through a citrate assisted sol−gel method and a self-propagating combustion combined with high temperature annealing. A thin polyaniline (PANI) layer is introduced on the surface of BaFe12O19 nanorods by in situ polymerization method under the sonication combined with the mechanical stirring in order to improve the dispersion quality and interfacial compatibility between BaFe12O19 and epoxy matrix. The enhanced tensile strength with maximum value of 90.7 MPa for the cured epoxy nanocomposites filled with 10 wt % loading of PANI modified BaFe12O19 (s-BaFe12O19) nanorods was obtained in the tensile tests compared with that of the cured pure epoxy (75.2 MPa) and cured epoxy nanocomposites filled with the same loading of bare BaFe12O19 (u-BaFe12O19) nanorods (74.4 MPa). In the cured s-BaFe12O19/epoxy nanocomposites, the uniform BaFe12O19 distribution has been observed by scanning electron microscope (SEM), which contributes to the improved tensile strength. A negative real permittivity is achieved in both of u-BaFe12O19 and s-BaFe12O19/ epoxy nanocomposites at the low frequency range (∼103 Hz). After further increasing frequency, the real permittivity was changed from negative to positive. The permittivity has no u-BaFe12O19 loading dependency for the u-BaFe12O19/epoxy nanocomposites. However, it is decreased with increasing s-BaFe12O19 loadings in the s-BaFe12O19/epoxy nanocomposites, which exhibits the component rather than structure controlled epoxy metacomposites.

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INTRODUCTION Metamaterials with negative index of refraction, as their smart properties for the capability of manipulating electromagnetic waves provided by artificially designed structures,1,2 have drawn great interests due to their wide applications in the antennas, perfect superlens, optical cloaking, and sound filters.3 Especially, owing to the flexibility, cost-effective processability and lightweight, the polymer-based metacomposites have been intensively studied in recent years.4 Epoxy is an important thermosetting structural material in industry and employed in the applications such as electronic devices, marine system, aerospace parts, and coatings because of its excellent environmental stability and electrical insulating property, high tensile strength, and Young’s modulus.5,6 Moreover, epoxy is currently recognized as one of the most commonly used high voltage © 2017 American Chemical Society

(HV) apparatus for microelectronic packaging, generators, and integrated printed circuit boards (PCB),7 in which the dielectric property is the most important property for designing epoxy insulating materials8 since establishing the electrical property (permittivity) and loss characteristics (frequency-dependent property) of epoxy is necessary in order to design devices for these appropriate applications.9 Normally, the incorporation of various nanoparticles into epoxy can make a great difference in properties from the pure epoxy resin.10,11 Therefore, recently, many researchers have paid more and more attention to the preparation of epoxy with nanoparticles, including silica, Received: April 16, 2017 Revised: May 24, 2017 Published: May 24, 2017 13265

DOI: 10.1021/acs.jpcc.7b03580 J. Phys. Chem. C 2017, 121, 13265−13273

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self-propagating combustion method. First, the Ba(NO3)2 and Fe(NO3)3 with a molar ratio of 1:12 were added into 10 mL of deionized water and magnetically stirred for 10 min. Then the citric acid (molar ratio of citric acid/Ba(NO3)2/Fe(NO3)3 = 13:1:12) was mixed with above solution for further magnetic stirring until the citric acid was dissolved. Second, the ammonia was dripped until the pH value was adjusted around 7. Then the solution was magnetically stirred at 80 °C for 1−2 h in a water bath to form the sol−gel solution. After that, the product was heated to 200−220 °C while self-propagating combustion reaction happened. Finally, this burnt sample was grinded and put into a tube furnace to be calcined at 900 °C for 10 h to obtain the well-crystallized barium ferrite nanorods. The schematic for the preparation of barium ferrites is shown in Scheme S1. Fabrication of PANI-Modified BaFe12O19. BaFe12O19 nanorods were surface-modified with PANI via in situ polymerization method. Briefly, BaFe12O19 nanorods (6.69 g), PTSA, and APS (molar ratio of PTSA/APS = 15:9) were added into 80 mL of deionized water in an ice−water bath under a 1 h mechanical stirring (Shanghai Mei Yingpu electric stirrer D2004W, 300 rpm) combined with sonication (model: SK 3200H, frequency value: 50 kHz). Then the aniline solution (18 mmol) was mixed with the above BaFe12O19 nanorods suspension and then mechanically stirred under sonication for an additional 1.5 h in an ice−water bath for further polymerization of aniline. The product was vacuum filtered and washed with deionized water until pH value was about 7. The precipitant was further washed with methanol to remove any possible oligomers. The final powders were dried at 60 °C overnight. The pure PANI was also synthesized following the aforementioned procedures without adding BaFe12O19 nanorods for comparison. Preparation of BaFe12O19/Epoxy Nanocomposites. Epon resin suspension containing 5, 10, 15, and 20 wt % of the as-received BaFe12O19 (u-BaFe12O19) nanorods and surfacemodified BaFe12O19 (s-BaFe12O19) nanorods were prepared. Both u-BaFe12O19 and s-BaFe12O19 nanorods were immersed in Epon resin overnight without any disturbance so that the resin could wet the nanorods. Then the solution was mechanically stirred for 1 h at 600 rpm (D2004W, Shanghai Mei Yingpu instrument and Meter Manufacturing Co., Ltd.). All the procedures were carried out at room temperature. The curing agent was added into Epon monomers or the above-prepared BaFe12O19 nanorods suspended Epon resin solutions with a monomer/curing agent weight ratio of 100/26.5 for 1 h mechanical stirring (200 rpm). Then the solution was mechanically stirred at 60 °C for 2−3 h in a water bath at the same speed (200 rpm) combined with sonication. The BaFe12O19 nanorods with strong magnetic property were easy to sedimentation during the curing process. Therefore, sonication could help the BaFe12O19 nanorods disperse in the epoxy matrix. Finally, the solutions were poured into silicon molds, cured at 120 °C for 5 h, and then cooled down to room temperature naturally. The size and shape for the cured epoxy and epoxy nanocomposites are shown in Figure 1. Characterizations. The crystalline structure of the uBaFe12O19 and s-BaFe12O19 was proceeded through powder Xray diffraction (XRD) analysis using a Bruker AXS D8 Discover diffractometer with GADDS (General Area Detector Diffraction System) manipulating with a Cu Kα radiation source filtered with a graphite monochromator (λ = 1.5406 Å). The chemical structures of u-BaFe12O19, s-BaFe12O19, and pure

alumina, barium titanate, and so on, to form epoxy nanocomposites for studying their differences in dielectric property.12,13 However, most of these epoxy nanocomposites exhibit the positive permittivity, thus, it will be academically interesting and technically important to prepare the epoxy metacomposites with tunable negative permittivity. The magnetic materials are vital materials for modern information technology such as magnetic random access memory (MRAM) for computer, magnetic recording media with high storage densities, and electronic devices.14 Especially, the ultrafine barium ferrites (BaFe12O19), as permanent magnets, are widely used hexagonal hard materials in the high density recording media due to their relatively large saturation magnetization (Ms), high coercivity (Hc), excellent chemical stability, and corrosion resistivity, which could reduce the medium noise and increase the information storage capacity.15 The dielectric property of barium ferrite is also significant in its applications.16 Unfortunately, the dielectric property of barium ferrite/epoxy nanocomposites has been rarely studied. In addition, the strong magnetic property and magnetic dipole−dipole interaction make them hard to disperse uniformly in epoxy matrix, which severely limit their applications for fabricating epoxy nanocomposites. Polyaniline (PANI) is a conducting polymer consisting of alternating reduced (amine groups) and oxidized (imine groups) repeating units.17 It is reported that the existence of amine groups on the polymer backbone of PANI could serve as the coupling agents for the preparation of epoxy nanocomposites, which could help the nanofillers be uniformly dispersed within epoxy matrix and obtain the enhanced mechanical and dynamic mechanical properties.18 However, there is no related report on the PANIassisted dispersion of magnetic BaFe12O19 within epoxy matrix as well as their mechanical properties. Based on the aforementioned part, in the present work, a PANI-assisted uniform dispersion for the ultrafine BaFe12O19 nanorods/epoxy metacomposites with adjustable negative permittivity and improved tensile strength have been reported. The ultrafine BaFe12O19 nanorods have been fabricated by using a citrate assisted sol−gel method and self-propagating combustion combined with high temperature annealing. A thin PANI layer with weight percentage of 12% is formed on the surface of BaFe12O19 nanorods by in situ polymerization under the sonication combined with the mechanical stirring methods. The magnetic property and dielectric property of these epoxy nanocomposites are systematically investigated. The interaction between PANI and epoxy matrix is also explored by Fourier transform infrared spectroscopy (FT-IR). The epoxy nanocomposites filled with pure BaFe12O19 nanorods have also been fabricated for comparison.

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EXPERIMENTAL SECTION Materials. Barium nitrate (Ba(NO3)2, ≥99.5%), iron nitrate (nonahydrate) (Fe(NO3)3·9H2O, ≥98.5%), ammonia (NH3· H2O, 25−28 wt %), citric acid (C8H8O7·H2O, ≥99.5%), ammonium persulfate (APS), ((NH4)S2O8, ≥98.0%), p-toluene sulfonic acid (PTSA) (monohydrate) (C 7 H 8 O 3 S·H 2 O, ≥99.0%), and aniline (C6H7N, 99.5%) were provided by Sinopharm Chemical Company, Inc. Epon 862 (bisphenol F epoxy) and corresponding curing agent EK3402 were purchased from Hexion Inc. All the chemicals were used asreceived without any further treatment. Preparation of Barium Ferrite (BaFe 12O19). The precursor for the barium ferrite was prepared by sol−gel and 13266

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measured in a Physical Properties Measurement System (PPMS) by Quantum Design at the temperature of 25 °C. The tensile tests were conducted in a unidirectional tensile test machine (CTM2100 electronic universal testing machine of Shanghai Xieqiang Co.). The samples (dog-bone shaped) were projected according to the ASTM standard request and prepared as described for epoxy nanocomposites in the molds. A crosshead speed of 1 mm min−1 was used and the strain (mm mm−1) was calculated by dividing the jogging displacement with the initial gage length (about 26 mm). For the dielectric measurement, the experiment was carried out on Agilent Technology Company LCR meter (Agilent, E4980A) with the dielectric test fixture (type Agilent, 16451B) at room temperature within the frequency range of 20−2 × 106 Hz.

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Figure 1. Digital images of the prepared BaFe12O19/epoxy nanocomposite. The round sample is for the dielectric measurement, the rectangular sample is for the microwave absorption tests, and the dogbone shaped sample is for the tensile tests.

RESULTS AND DISCUSSION

Structure Characterization and Functionalization of BaFe12O19. The chemical structure of the prepared barium ferrite is confirmed by XRD and FT-IR spectra. Figure 2A shows the XRD patterns of as-prepared products. It is observed that all the diffraction peaks of as-prepared barium ferrite (Figure 2A-a, u-BaFe12O19) correspond to the typical M type phase of BaFe12O19 (standard JCPDS No. 39−1433), which has a hexagonal space group of P63/mmc (no. 194)19 and lattice parameters of a = b = 5.892 Å, and c = 23.198 Å. This means that the as-prepared u-BaFe12O19 exhibits a uniaxial crystal structure.20 There is no any other phase peaks detected in its XRD pattern, illustrating the high purity and single phase of the fabricated u-BaFe12O19.21 After functionalization of PANI on the surface of BaFe12O19, the XRD patterns are similar to that of u-BaFe12O19, Figure 2A-b, which reveals that the coating

PANI were analyzed by a Fourier transform infrared spectroscopy (FT-IR, Thermo Scientific Inc. Thermo Nicolet NEXUS with an ATR accessory) in the extent of 400−4000 cm−1 with a resolution of 4 cm−1. The morphologies of u-BaFe12O19, sBaFe12O19, and fracture surfaces of the epoxy nanocomposites were observed on a JSM-6700F system with a JEOL field emission scanning electron microscope (SEM). The SEM specimens were prepared by sputtering a thin gold layer coating about 3 nm thick. The thermal stability was performed in a thermogravimetric analysis (TGA, thermal-microbalance apparatus) with a heating rate of 10 °C min−1 and an air flow rate of 60 mL min−1 from 25 to 850 °C. The magnetic characteristics of u-BaFe12O19, s-BaFe12O19, and epoxy nanocomposites were

Figure 2. (A) XRD of (a) u-BaFe12O19 and (b) s-BaFe12O19; (B) FT-IR spectra of (a) u-BaFe12O19, (b) pure PANI and (c) s-BaFe12O19; (C) TGA curves of u-BaFe12O19 and s-BaFe12O19; and (D) room temperature magnetic hysteresis loop of u-BaFe12O19 and s-BaFe12O19. 13267

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Figure 3. SEM images of (A) precursor of u-BaFe12O19, (C) u-BaFe12O19, and (E) s-BaFe12O19. (B), (D), and (F) are the enlarged images for (A), (C), and (E), respectively.

layer of PANI does not lead to phase change in the crystalline structure of u-BaFe12O19.22 The FT-IR spectra of u-BaFe12O19, s-BaFe12O19, and PANI are depicted in Figure 2B. For the FT-IR spectrum of uBaFe12O19, Figure 2B-a, the two characteristic bands located at 530 and 562 cm−1 are associated with the stretching vibrations of Fe−O14 and Ba−O bands,23 respectively. There is no other band observed, suggesting the single powder of u-BaFe12O19. This is consistent with the XRD result (Figure 2A-a). In the FT-IR spectrum of pure PANI, Figure 2B-b, the absorption peaks at 1541 and 1478 cm−1 are assigned to the CC stretching vibration of quinoid and benzenoid rings.24 The peak at around 1290 cm−1 is attributed to the C−N stretching vibration of the benzenoid ring.25 The band of 790 cm−1 is related to the out-of-plane bending of C−H in the substituted benzenoid unit.26 All of these characteristic peaks are accordant with that of PANI in the previous report.18 For the sBaFe12O19, both the characteristic bands from the u-BaFe12O19 and PANI are observed in the FT-IR spectrum, Figure 2B-c. This confirms the formation of PANI functionalized BaFe12O19. However, these characteristic peaks exhibit some shifts relative to that of u-BaFe12O19 and PANI, as indicated in the curve, which demonstrates the interaction between BaFe12O19 and PANI.27

Figure 2C shows the TGA curves of u-BaFe12O19, sBaFe12O19 in the air condition from room temperature to 800 °C. There is no obvious weight change during the thermal decomposition process for u-BaFe12O19. In the TGA curve of sBaFe12O19, the obvious weight loss from 300 to 500 °C arises from the large scale degradation of coated PANI polymer layer.13 It is estimated the BaFe12O19 loading in the s-BaFe12O19 from the weight residue of the TGA test is around 88%. The room temperature magnetic hysteresis loops of uBaFe12O19, s-BaFe12O19 are shown in Figure 2D. Since the magnetization of both two samples does not reach saturation at measured magnetic field of 2 T, the saturation magnetization (Ms) is evaluated through the extrapolated saturation magnetization obtained from the intercept of M ∼ H−1 at high field.8 The calculated Ms of u-BaFe12O19 is 74.6 emu g−1, which is a little bit higher than that of bulk barium ferrite (72 emu g−1).28 The obtained Ms of s-BaFe12O19 is 65.5 emu g−1. The decreased Ms for s-BaFe12O19 is attributed to the existence of PANI on the surface of BaFe12O19, from which the weight percentage of BaFe12O19 in s-BaFe12O19 is estimated to be around 87.8%. This is consistent with the TGA result. It is also observed that there is obvious hysteresis loop in the magnetization curves of u-BaFe12O19, s-BaFe12O19. The remnant magnetization (Mr, the residual magnetization after the magnetic field is reduced to zero) for u-BaFe12O19, s-BaFe12O19 is around 30 and 29 emu 13268

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Figure 4. (A) Stress−strain curves of cured (a) pure epoxy and epoxy nanocomposites filled with s-BaFe12O19 (b) 5, (c) 10, (d) 15, and (e) 20 wt % and (B) tensile strength with error bars for the epoxy nanocomposites filled with different loadings of s-BaFe12O19.

g−1, respectively. The coercivity (Hc) of u-BaFe12O19, sBaFe12O19, obtained from the axes crossover as shown in the inset of Figure 2D, is 1.3 and 4.8 kOe, respectively. This means that both highly coercive u-BaFe12O19, s-BaFe12O19 are hard materials and behave as permanent magnets.29 Compared with the u-BaFe12O19, the enhanced Hc of the s-BaFe12O19 may be due to the defects created during the functionalization process, which might act as the pinning centers during the magnetization reversal.30 Figure 3 shows the SEM images for burnt precursor of uBaFe12O19, u-BaFe12O19, and s-BaFe12O19. It is observed that there is no obviously nanosized particulate morphology for the burnt precursor, Figure 3A,B, which forms continuous sheets. In contrast, after annealing at 900 °C. Figure 3C,D, the uBaFe12O19 manifests the nanorod morphology and the surface is very smooth. The average diameter of u-BaFe12O19 nanorods is around 50−80 nm. This demonstrates that the nanorod structure of u-BaFe12O19 is formed during the annealing process due to the high temperature solid state reaction. Meanwhile, the high temperature could also help the formation of highly crystallized structure of BaFe12O19.28 Owing to the strong magnetic dipole−dipole interactions as confirmed in the above magnetization curve, these nanorods are observed to be agglomerated together.31 For the s-BaFe12O19 sample, the nanorod morphology is still well remained, however, the surface of BaFe12O19 becomes rough, as a consequence of the polymerization of PANI on the surface of BaFe12O19.32 This further confirms that the BaFe12O19 has been successfully modified by PANI. Exploration of Epoxy Nanocomposite Formation Mechanisms. It is reported that PANI can serve as the coupling agent for nanofillers and epoxy resins.33 In order to further confirm the interaction between s-BaFe12O19 and epoxy, the FT-IR spectra of cured pure epoxy, epoxy nanocomposites filled with the 20 wt % loading of u-BaFe12O19 and s-BaFe12O19 are shown in Figure S1. Normally, the curing extent of epoxy can be determined by the peak 911 cm−1 in the FTIR spectrum, since this peak arises from the terminal epoxy groups.34 From Figure S1, it is found that after curing procedure, the peak intensity around 911 cm−1 for the cured 20 wt % s-BaFe12O19/ epoxy nanocomposites (Figure S1c) is obviously decreased compared with cured pure epoxy (Figure S1a) and 20 wt % uBaFe12O19/epoxy (Figure S1b), as indicated by purple color, indicating the role of the presence of PANI layer on the surface of BaFe12O19 nanorods. The PANI can promote the curing degree of epoxy and take part in the curing process of epoxy.

Properties of Cured Epoxy Nanocomposites. Mechanical Tensile Properties. The representative stress−strain curves of cured pure epoxy and epoxy nanocomposites filled with sBaFe12O19 and u-BaFe12O19 are shown in Figures 4A and S2A. The comparison of the stress−strain curve for the cured epoxy nanocomposites with 10 wt % loading of s-BaFe12O19 and uBaFe12O19 is also depicted in Figure S2B. It turns out that all of these samples are of the typical brittle failure characteristics, for which the samples are broken dramatically after debonding occurs.35 The maximum stress value in the stress−strain curve represents the tensile strength (also called ultimate tensile strength)36 and the obtained corresponding tensile strength is summarized in Figure 4B. It is seen that the tensile strength of epoxy nanocomposites filled with s-BaFe12O19 nanorods is increased as the s-BaFe12O19 nanorod loading increases to 15 wt %, and then decreased with further increasing s-BaFe12O19 nanorod loadings. Even when the s-BaFe12O19 nanorod loading increases to 20 wt %, the average tensile strength (83.2 MPa) is still higher than that of cured pure epoxy (75.2 MPa) and the epoxy nanocomposites filled with the same loading of uBaFe12O19 nanorods, exhibiting the obvious ability of surface functionalization. The highest tensile strength for 10 wt % sBaFe12O19 nanorod loading of epoxy nanocomposites is 90.7 MPa, which is 20.6 and 21.9% higher than that of cured pure epoxy (75.2 MPa) and epoxy filled with the same loading of uBaFe12O19 (74.4 MPa, Figure S2B-b), respectively. The average tensile strength for all of the epoxy filled with s-BaFe12O19 nanorods is higher than that of the epoxy filled with the same loading of u-BaFe12O19 nanorods. For the epoxy filled with uBaFe12O19, Figure S2A, it is found that the tensile strength is decreased with increasing u-BaFe12O19 loadings, which may be resulted from the poor interfacial interaction between uBaFe12O19 and epoxy matrix and incomplete u-BaFe12O19 nanorod dispersions within epoxy matrix.37 The Young’s modulus, determined by the slope of the stress−strain curve in the low strain range, is listed in Table S1. For the epoxy nanocomposites filled with different loadings of s-BaFe12O19 nanorods, the highest value of Young’s modulus is also observed in the 10 wt % loading of s-BaFe12O19/epoxy nanocomposites (2.45 GPa), which is higher than that of cured pure epoxy (2.03 GPa) and 10 wt % loading of u-BaFe12O19/ epoxy nanocomposites (2.36 GPa). Meanwhile, the Young’s modulus for the epoxy filled with s-BaFe12O19 nanorods is also higher than that of the epoxy filled with same loading of uBaFe12O19 nanorods. Both the increased tensile strength and Young’s modulus for the s-BaFe12O19/epoxy nanocomposites relative to those of the cured pure epoxy and u-BaFe12O19/ 13269

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Figure 5. SEM images of fracture surfaces of cured (A) pure epoxy, (B) epoxy nanocomposites filled with 10 wt % s-BaFe12O19, and (C) 10 wt % uBaFe12O19.

within the measured frequency range from 20 Hz to 2 MHz at room temperature. As is shown in Figure 6A, for the cured pure epoxy, the ε′ is almost constant (around 3−4) within the whole measured frequency range as expected, presenting a stable permittivity performance.38 However, for the epoxy nanocomposites, the ε′ demonstrates the obvious frequency dependent property because of the introduced interfacial polarization39 between nanorods and epoxy, Figure 6A,B. Meanwhile, the ε′ is changed significantly after adding of uBaFe12O19 and s-BaFe12O19 nanorods into epoxy matrix, especially at the lower frequency range starting from 20 to 103 Hz. Furthermore, the epoxy nanocomposites exhibits different ε′ values with different loadings of BaFe12O19 nanorods. The amplitude of negative ε′ for epoxy nanocomposites increases with increasing s-BaFe12O19 nanorod loadings and declines as s-BaFe12O19 nanorod loading increases. This is called component rather than structure controlled negative permittivity.3 Similar results were also reported in the polypyrrole/tungsten oxide,40 polypyrrole/carbon nanostructure,41 and polyaniline/multiwalled carbon nanotubes nanocomposites.42 In addition, the ε′ is found to be switched from negative to positive around the frequency of 103 Hz for the epoxy nanocomposites filled with both of u-BaFe12O19 and sBaFe12O19 nanorods. These results are totally different from that of the multiwalled carbon nanotubes/epoxy nano-

epoxy nanocomposites indicate the improved interfacial interaction between s-BaFe12O19 and epoxy matrix. Microstructure of Fracture Surface. Figure 5 shows the SEM microstructures of fracture surface for the cured pure epoxy and epoxy nanocomposites filled with 10 wt % loading of s-BaFe12O19 and u-BaFe12O19. In Figure 5A,B, the cured pure epoxy displays a smooth fracture surface with a river-like pattern. However, for the cured epoxy nanocomposites, Figure 5C−F, owing to the presence of BaFe12O19, the fracture surface becomes rough and the crack direction is randomly distributed. For the 10 wt % loading of s-BaFe12O19/epoxy nanocomposites, Figure 5C,D, the s-BaFe12O19 nanorods are observed to be uniformly dispersed within epoxy matrix as marked with blue color. Yet, for the 10 wt % loading of uBaFe12O19/epoxy nanocomposites, Figure 5E,F, it is seen that the u-BaFe12O19 nanorods are severely agglomerated together. These differences may lead to the improved tensile strength for the s-BaFe12O19/epoxy nanocomposites and the decreased tensile strength in the u-BaFe12O19/epoxy nanocomposites. This is in accordance with the results obtained in the tensile tests as aforementioned. Permittivity. Figure 6 shows the real permittivity (ε′) and dielectric loss tangent (tan δ) as a function of frequency for the cured pure epoxy and epoxy nanocomposites filled with different loadings of u-BaFe12O19 and s-BaFe12O19 nanorods 13270

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Figure 6. (A) Frequency-dependent ε′ for cured (a) pure epoxy, and epoxy nanocomposites filled with (b) 5, (c) 10, and (d) 20 wt % loading of uBaFe12O19; (B) Frequency-dependent ε′ for cured (a) pure epoxy and epoxy nanocomposites filled with (b) 5, (c) 10, and (d) 20 wt % loading of sBaFe12O19; (C) Frequency-dependent tan δ for (a) pure epoxy and epoxy nanocomposites filled with (b) 5, (c) 10, and (d) 20 wt % loading of uBaFe12O19; (D) Frequency-dependent tan δ for (a) pure epoxy and epoxy nanocomposites filled with (b) 5, (c) 10, and (d) 20 wt % loading of sBaFe12O19.

composites18 and barium titanium/epoxy nanocomposites,13 in which only positive permittivity is obtained within the measured frequency range. This switching frequency is normally related to the plasma frequency (ωp),3 which is an intrinsic frequency of materials. As the angular frequency (ω) of incident light is lower than ωp, the ε′ is negative, otherwise, the ε′ is positive. The ε′ at higher frequency range of 103∼105 Hz for the cured pure epoxy and epoxy nanocomposites is plotted in the inset of Figure 6A,B. It is seen that ε′ tends to decrease at the higher frequency of ∼105 Hz due to the wellknown dielectric relaxation phenomenon, in which the response of dipoles to the frequency is delayed in an alternating electric field.43 Generally, the adding of nanofillers into polymer matrix might introduce the interfacial imperfection, including holes or voids, which could result in the increased dielectric loss, even the interfacial modification between nanofillers and polymer matrix has been made.44 In Figure 6C,D, it is shown that the tan δ of the epoxy nanocomposites is indeed increased by adding of both u-BaFe12O19 and s-BaFe12O19 nanorods relative to the pure epoxy (around 0.004). Especially the tan δ for uBaFe12O19/epoxy nanocomposites is higher than that of sBaFe12O19/epoxy nanocomposites due to the poor interfacial interaction between nanorods and epoxy matrix as aforementioned. Moreover, some peaks are observed in the tan δ curves of epoxy nanocomposites, which does not appear in that of the cured pure epoxy. It is reported that these peaks might be caused by the relaxation occurred in the nanocomposite systems.45 After the introduction of u-BaFe12O19 and sBaFe12O19 nanorods into epoxy matrix, the interfaces between nanorods and epoxy matrix give rise to the interfacial polarization, which leads to the occurrence of relaxation.46 Magnetic Property. Figure 7 shows the room temperature hysteresis loop of epoxy nanocomposites filled with 10 wt %

Figure 7. Room temperature hysteresis loop of epoxy nanocomposites filled with 10 wt % loading of s-BaFe12O19 nanorods.

loading of s-BaFe12O19 nanorods. It is found that the Hc, as shown in the inset on the bottom right side of Figure 7, is 4.8 kOe, which is the same as that of the s-BaFe12O19 nanorods. This proves that the nonmagnetic epoxy matrix has no obvious effect on the Hc of s-BaFe12O19 nanorods. The Ms of this epoxy nanocomposite is estimated to be 7.64 emu g−1. Based on the obtained Ms of BaFe12O19 nanorods (74.6 emu g−1), the weight percentage of s-BaFe12O19 nanorods is around 10.2%. The inset in the top left corner of Figure 7 depicts the good magnetic property of dog-bone shaped s-BaFe12O19/epoxy nanocomposites, which can be attracted by the permanent magnet. 13271

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The Journal of Physical Chemistry C

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CONCLUSION In this work, the unique negative permittivity is obtained in the ultrafine BaFe12O19/epoxy nanocomposites, in which the BaFe12O19 nanorods have been fabricated by a citrate assisted sol−gel method and a self-propagating combustion combined with the high temperature annealing, then characterized by XRD, FT-IR, TGA, SEM, and magnetic property test. The presence of 12 wt % of PANI on the surface of BaFe12O19 nanorods could effectively improve the dispersion quality and the interfacial compatibility between BaFe12O19 and epoxy matrix. The epoxy nanocomposites filled with different loadings of u-BaFe12O19 and s-BaFe12O19 nanorods have been prepared. The enhanced tensile strength has been obtained for the cured epoxy nanocomposites filled with s-BaFe12O19 nanorods compared with that of the cured pure epoxy and the cured epoxy nanocomposites filled with the same loading of bare BaFe12O19 nanorods. The SEM microstructure images show a uniform s-BaFe12O19 distribution within epoxy matrix, while a big agglomeration of u-BaFe12O19 nanorods is observed in the u-BaFe12O19/epoxy nanocomposites. A switching frequency, at which the permittivity changes from negative to positive, is obtained in both of the s-BaFe12O19/epoxy and the uBaFe12O19/epoxy nanocomposites. The permittivity in sBaFe12O19/epoxy metacomposites can be tailored by the different loadings of s-BaFe12O19 nanorods. The hysteresis loop indicates that the epoxy matrix has no obvious effect on the Hc of s-BaFe12O19. These materials have the potential applications in some typical sensors for a broader impact.47−49

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Foundation of Innovation and Creation for Graduate Students in Northwestern Polytechnical University.

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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.jpcc.7b03580. Preparation process of barium ferrite, FT-IR spectra of epoxy and its nanocomposites, tensile properties of cured u-BaFe12O19/epoxy nanocomposites, thermal properties, and ε″ of cured BaFe12O19/epoxy nanocomposites (PDF).

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hongbo Gu: 0000-0003-3038-2348 Zhanhu Guo: 0000-0003-0134-0210 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Shanghai Science and Technology Commission (14DZ2261100). H.G. thanks the financial support from the Science and Technology Commission of Shanghai Municipality (No. 15YF1412700) and Young Elite Scientist Sponsorship Program by CAST (YESS, No. 2016QNRC001). J.G. is grateful for the support and funding from the National Natural Science Foundation of China (Nos. 51403175 and 81400765), Foundation of Aeronautics Science Fund (Nos. 2015ZF53074 and 2016ZF03010), and Seed 13272

DOI: 10.1021/acs.jpcc.7b03580 J. Phys. Chem. C 2017, 121, 13265−13273

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