Aug 30, 2013 - ABSTRACT. In this work, core-shell structured BaTiO3/polystyrene nanoparticles (BT-PS) with different thickness of PS shell were synthesized ...
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D. Wang et al.: Dielectric Properties of Polystyrene Based Composites Filled with Core-Shell BaTiO3/Polystyrene
Dielectric Properties of Polystyrene Based Composites Filled with Core-Shell BaTiO3/Polystyrene Hybrid Nanoparticles Dongrui Wang, Meiyan Huang, Jun-Wei Zha, Jun Zhao, Zhi-Min Dang Department of Polymer Science and Engineering, School of Chemistry and Biological Engineering University of Science and Technology Beijing Beijing, 100083, China and Zhongyang Cheng Materials Research and Education Center Auburn University Auburn, AL 36849, USA
ABSTRACT In this work, core-shell structured BaTiO3/polystyrene nanoparticles (BT-PS) with different thickness of PS shell were synthesized through atom transfer radical polymerization and the influence of their shell thickness on dielectric properties of BTPS/PS composites was studied. Two types of BT-PS with the PS shell of 3 nm or 12 nm were obtained by controlling the polymerization time. The structure of BT-PS was carefully characterized by infrared spectroscopy, X-ray diffraction, thermal gravimetric analysis, and transmission electron microscope. The results confirmed the successful preparation of core-shell BT-PS nanoparticles. Compared to pristine BT, the core-shell particles can be more homogeneously dispersed into PS matrix. As a result, higher dielectric constant, higher breakdown strength, and lower dielectric loss were achieved in BT-PS/PS composites. Moreover, the dielectric constant of BT-PS/PS composites displayed frequency independent behavior. In addition, the composites filled by BT-PS with 3 nm of PS shell showed better dielectric properties than those filled by BT-PS with 12 nm of PS shell. A maximum energy density as large as 4.24 J/cm3 was obtained in BT-PS/PS films. Index Terms — Dielectric properties, energy density, composite, core-shell structure, polystryrene.
1 INTRODUCTION DIELECTRICS with high energy densities have drawn considerable interest due to their important applications in advanced electric power systems and electronic devices [1-4]. Current commercial organic dielectrics are based on polymer films, such as polypropylene (PP) and polystyrene (PS), suffering from the low dielectric constant (2-4 for most of nonferroelectric polymers). The energy density of a parallel plate capacitor is defined as ε0εEb2/2, where ε0 is vacuum dielectric constant, ε is the relative dielectric constant, and Eb is the breakdown strength [4]. Therefore it is clear that a higher energy density can be achieved through improving the ε or Eb. Over past few years, many researchers have focused on polymer-based nanocomposites filled with high dielectric permittivity ceramic nanoparticles, expecting to obtain dielectrics which combine the high dielectric permittivity of ceramics and the high breakdown strength of polymers. Of widely investigated ceramic fillers, barium titanate (BaTiO3, BT) may be the most important due to Manuscript received on 30 August 2013, in final form 5 December 2013, Accepted 25 December 2013.
its ferroelectric property and high dielectric permittivity at room temperature [5-11]. However, the realization of high energy storage via this strategy is still a severe challenge until now. Introducing ceramics into polymer matrix often causes a decrease in dielectric strength which offsets the advantages offered by the improvement of dielectric permittivity [4]. The deterioration of breakdown strength in composites is closely associated with the interface incompatibility between the ceramic particles and the polymer matrix, which can lead to the agglomeration of ceramic particles, the generation of micropores and gas voids, etc. Surface modification of fillers is the most effective way to improve the interface compatibility and enhance the dielectric properties [12-16]. Proper organic molecules bonded on ceramic surfaces can strengthen the interactions between inorganic ceramics and polymers, resulting in better dispersion of fillers and decrease of interfacial defects. For example, Kim et al have reported that BT nanoparticles modified with phosphonic acids can be well dispersed in polycarbonate (PC) and poly(vinylidene fluoride) (PVDF) and enhance the energy densities of resultant nanocomposites [15, 16]. With the progress of controlled radical polymerization (CRP) techniques, the design and synthesis of
DOI 10.1109/TDEI.2013.004329
IEEE Transactions on Dielectrics and Electrical Insulation
Vol. 21, No. 4; August 2014
high permittivity core-shell structured hybrid nanoparticles, in which ceramic cores are covalently bonded by hairy polymer chains, has become a new route to resolve the interface incompatibility between fillers and matrix [17, 18]. Recently, Jiang et al have prepared a series of polymer-coated BT nanoparticles through CRP and found that the hybrid materials show increased dielectric constants, relatively low dielectric loss, and weak frequency dependence [19, 20]. The in-situ polymerization method is very promising for developing nanocomposites with high energy density. In this work, we prepared a series of core-shell structured hybrid nanoparticles BT-PS through atom transfer radical polymerization, in which inorganic BT particle was the core and organic PS was the shell. The shell thickness was carefully tuned by controlling the polymerization time. Two types of BT-PS nanoparticles with the shell thickness of 3 nm or 12 nm were obtained. PS based composites filled with such hybrid nanoparticles were fabricated through solution casting method and the influence of shell thickness on dielectric properties of resultant composite films was investigated. We found that the grafting of PS shell can enhance the interactions between BT and PS host and improve the dispersion behavior of BT fillers. Dielectric properties of the composites were greatly improved due to the introduction of PS shell. Meanwhile, we found that the composites filled by BT-PS particles with the shell thickness of 3 nm displayed better dielectric properties than those filled by BTPS with the shell of 12 nm. The preparation and characterization of the core-shell BT-PS nanoparticles, and the morphology and dielectric properties of BT-PS/PS composites are discussed in detail in the following sections.
2 EXPERIMENTAL 2.1 MATERIALS BaTiO3 nanoparticles with the average diameter of ca. 100 nm were purchased from Aladdin Industrial Corporation(Shanghai, China). Polystyrene (666D) was obtained from Yanshan Petrochemical, China. CuBr (98%, Acros) was first washed with excess acetic acid, followed by ethanol and ether, and it was then dried. Styrene was obtained from Tianjin Jinke Chemicals and distilled before use. All other reagents and solvents were commercial available products and used as received without any further purification. 2.2 PREPARATION OF HYBRID NANOPARTICLES The core-shell BT/PS hybrid nanoparticles were prepared by surface-initiated atom transfer radical polymerization (SIATRP) following procedures similar to the literature [19, 21]. The synthetic route to the nanoparticles is shown as Scheme 1 and the experimental details are described as follows. (1) Surface hydroxylation of BT. BT nanoparticles (4 g) were dispersed in H2O2 (50 mL) through sonication. Then, the mixture was heated to reflux for 6 h. Finally, the hydroxylated BT, BT-OH, was obtained by centrifugation, washed with excess deionized water, and dried under vacuum at 80 oC for 24 h. (2) Surface amination of BT-OH. BT-OH (3 g) was dispersed into toluene (50 mL) through sonication. After KH550 (γ-aminopropyl triethoxysilane, 1.5 mL) was added,
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the mixture was heated to 80 C and kept at this temperature for 24 h under vigorous stirring. The BT-NH2 was isolated by centrifugation, washed with toluene for three times, and then dried under vacuum at 80 oC for 24 h. (3) Synthesis of initiator BT-Br. BT-NH2 (2 g) was dispersed into dichloromethane (25 mL) through sonication and immersed into ice bath. Then a mixture of 2bromoisobutyryl bromide (0.12 g) and triethylamine (0.4 mL) was added into the flask dropwisely under vigorous stirring. The reaction was maintained at 0 oC for 3 h and continued at room temperature for another 20 h. Finally, the BT-Br was isolated by centrifugation, washed with dichloromethane for three times, and dried under vacuum at 80 oC for 24 h. (4) ATRP of Styrene using BT-Br as the initiator. BT-Br (1.816 g) was first dispersed into 30 mL of dimethylformamide (DMF) through sonication and transferred into a Schlenk flask containing CuBr (0.0626 g). After a mixture of styrene (6 mL), N,N,N’,N’,N’’pentamethyldiethylenetriamine (PMDETA, 90.4 μL), and DMF (10 mL) was added in, the flask was degassed by three freeze-pump-thaw cycles and sealed off under nitrogen. After stirring at room temperature for 1 h, the sealed flask was placed in an oil bath at 120 oC for different time intervals. The polymerization was stopped by immersing the flask into an ice bath rapidly and the BT-PS nanoparticles were separated by centrifugation, thoroughly washed with acetone to remove the free polymer, Cu/ligand complex, and any unreacted monomer. The product was finally dried under vacuum at 80 oC for 24. Two types of BT-PS nanoparticles were prepared by controlling the polymerization time to be 3 h or 24 h, which are denoted as BT-PS1 and BT-PS2, respectively. 2.3 FABRICATION OF BT-PS/PS COMPOSITE FILMS The BT-PS/PS composite films were fabricated by a solution cast method. First, certain amount of BT-PS nanoparticles was dispersed into chloroform through sonication for 4 h. Desired amount of PS was then added into the BT-PS dispersion. The amount of PS was calculated to ensure the volume fraction of BT in resultant composites was 10, 20, 30, and 40 %. After mechanical stirring for 6 h, the mixture was casted onto a precleaned glass plate and dried to give thin films with the thickness of 30-40 μm. For comparison, composites filled with pristine BT (denoted as BT/PS) were also prepared through the same procedures. 2.4 CHARACTERIZATION Fourier transform infrared (FT-IR) spectra were measured by using a Nicolet 6700 spectrometer over the range of 4000 to 500 cm-1. X-ray diffraction (XRD) analysis was carried out with a Rigaku D/max 2200 diffractometer using a Cu Kα radiation source. Thermal gravimetric analysis (TGA) of the samples was carried out by using TA instrument TGA 2050 under air environment with a heating rate of 10 oC/min. Transmission electron microscopy (TEM) observation was performed on a JEM-2100 microscope (JEOL, Japan). Scanning electron microscopy (SEM) observation was performed on a Hitachi S-4700 microscope with an accelerating voltage of 20 kV.
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D. Wang et al.: Dielectric Properties of Polystyrene Based Composites Filled with Core-Shell BaTiO3/Polystyrene
Scheme 1. Synthetic route of the core-shell hybrid nanoparticles.
Dielectric properties of the composites were measured by using an Agilent 4294A impedance analyzer system. The thin film samples with the area of about 1 cm2, onto which silver electrodes had been painted, were tested. The breakdown strength of the composite film was measured in a heat-transfer fluid bath at room temperature with a CS2674A high-voltage amplifier under a ramp rate of 200 V/s. The thin film samples were cut to a size of 5 cm × 5 cm and their thicknesses were in the range of 10-15 μm. Ten specimens were tested for each sample and the number-averaged values were reported.
3 RESULTS AND DISCUSSION 3.1 PREPARATION AND CHARACTERIZATION OF HYBRID BT-PS NANOPARTICLES The synthetic route to the core-shell hybrid nanoparticles BT-PS is shown as Scheme 1. In summary, BT-Br nanoparticles containing bromide groups on surfaces were prepared through hydroxylation, amination, and esterification in turn. Then the obtained BT-Br was used as the initiator to initiate the ATRP of styrene to give hybrid BT-PS. To investigate the influence of shell thickness on dielectric properties of composites, BT-PS hybrid nanoparticles with different PS shell thickness were synthesized by adjusting the polymerization time of styrene. In this work, two different types of BT-PS nanoparticles were prepared by controlling the polymerization time to be 3 h or 24 h, which were denoted as BT-PS1 and BT-PS2, respectively. The FT-IR spectra of intermediate and final products are shown as Figure 1. Compared to pristine BT, a broad absorption band at 3420 cm1 in the spectrum of BT-OH can be clearly observed, indicating the existence of hydroxyl groups on BT surfaces. After reacted with KH550 and 2-bromoisobutyryl bromide, BT-OH was converted to BT-Br. In the spectrum of BT-Br, the peaks at 1650 and 1440 cm-1, corresponding to the amide groups, and the peak at 1110 cm-1, corresponding to the C-Br bonds, clearly reveal that the bromide-containing initiator has been successfully grafted onto the surfaces of BT nanoparticles. In the spectrum of BT-PS1, the characteristic peaks of alkyl groups at 2920, 2850 cm-1, and the characteristic peaks of benzene ring at 3020, 1600, and 1400 cm-1, prove that PS chains have grown from BT surfaces. The surface grafting of PS was further confirmed by XPS characterization. As seen in Figure 2, XPS signals of Ba 3d
Figure 1. FT-IR spectra of (a) BT, (b) BT-OH, (c) BT-Br, and (d) BT-PS1.
Figure 2. XPS spectra of (a) Ba 3d3 and 3d5 and (b) C 1s of BT before and after the surface grafting with PS chains.
were much reduced while the peak of C 1s was enhanced due to the shielding by the PS shell on the surfaces of BT nanoparticles. The core-shell structure of the obtained BT-PS hybrid nanoparticles were observed by TEM. Typical TEM images are shown in Figure 3. It can be clearly seen that a dense and uniform layer is coated around the BT particles. The observations again confirm that the PS chains have been uniformly grafted onto BT surfaces. Meanwhile, the thickness of grafted PS can be tuned by controlling the polymerization time. The thickness of shells was measured to be 3 nm for BTPS1 and 12 nm for BT-PS2. It should be noting that, for BTPS2 particles, the thicker shell leads to the adhesion of different particles and the formation of nanoparticle clusters.We also use a software (ImageJ) to analyze the size and size distribution of BT, BT-PS1, and BT-PS2 in TEM
IEEE Transactions on Dielectrics and Electrical Insulation
Vol. 21, No. 4; August 2014
Figure 3. TEM images of the core-shell hybrid nanoparticles: (a) and (b) are BT-PS1; (c), (d), and (e) are BT-PS2.
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Figure 5. TGA curves of different nanoparticles: (a) BT, (b) BT-Br, (c) BTPS1, and (d) BT-PS2.
Figure 6. XRD patterns of different nanoparticles: (a) BT, (b) BT-Br, (c) BTPS1, and (d) BT-PS2.
Figure 4. Diameter distributions of hybrid nanoparticles obtained from TEM images: (a) BT, (b) BT-PS1, and (c) BT-PS2.
images. The statistical data are shown as Figure 4.The average diameter of BT, BT-PS1, and BT-PS2 were 107 nm, 113 nm, and 131 nm, respectively. According these data, the volume fraction of PS in hybrid BT-PS nanoparticles can be obtained
through geometric calculation, which is calculated to be 15.0 vol% for BT-PS1 and 51.5 vol% for BT-PS2, respectively. Therefore, the weight fraction of PS in BT-PS can also be known, which is calculated to be 2.98 wt% for BT-PS1 and 12.6 wt% for BT- PS2. The amount of PS grafted onto BT surfaces was also analyzed by TGA. Figure 5 gives the TGA curves of pristine BT, BT-Br, BT-PS1, and BT-PS2. The weight loss at 600 oC for each sample is also annotated in the figure. It can be observed that the weight loss at 600 oC increased in the order of BT
