Core–Shell Structured Biopolymer@BaTiO3 Nanoparticles for

Article pubs.acs.org/JPCC

Core−Shell Structured Biopolymer@BaTiO3 Nanoparticles for Biopolymer Nanocomposites with Significantly Enhanced Dielectric Properties and Energy Storage Capability Yanyan Fan, Xingyi Huang,* Guanyao Wang, and Pingkai Jiang Department of Polymer Science and Engineering, Shanghai Key Laboratory of Electrical Insulation and Thermal Aging, Shanghai Jiao Tong University, Shanghai 200240, China S Supporting Information *

ABSTRACT: Flexible high-dielectric-constant (high-κ) nanocomposite dielectrics comprising polymer matrix and ceramic nanoparticles have important applications in the fields of electrical insulation and energy storage. However, most of the flexible high-κ nanocomposites are fabricated by using nonbiodegradable polymers as matrixes, which may not meet the increasing demands of society for environmental sustainability. In this study, using biodegradable polylactic acid (PLA) as a matrix and core−shell structured BaTiO3 (BT) nanoparticles as high-κ filler, we report the preparation and structure− property relationship of environmentally friendly flexible high-κ polymer nanocomposites. Two types of core−shell structured high-κ nanoparticles [polydopamineencapsulated BT (BT@PDA) and PLA-encapsulated BT@PDA (BT@PDA@PLA)] as well as as-prepared BT nanoparticles were used as filler of the PLA-based high-κ nanocomposites. It was found that, compared with the as-prepared BT nanocomposites, the core−shell nanoparticle-based composites show enhanced dielectric constant, suppressed dielectric loss tangent, and enhanced breakdown strength. In addition, the BT@PDA@PLA nanocomposites have much higher dielectric constant and energy density. The nanoparticle−PLA compatibility and its influence on the dielectric and energy storage properties of the nanocomposites were also investigated. Because the polymer matrix is environmentally friendly and the preparation process of the core−shell nanoparticles is facile and nontoxic, the nanocomposites reported here may be used in the next generation of environmentally friendly high-performance energy storage devices.

■

INTRODUCTION Recently, flexible high-dielectric-constant (high-κ) materials have attracted increasing interest because of their importance in dielectrics and energy storage devices, such as capacitors, electric stress control systems, and thin-film transistors.1−5 Taking electrostatic capacitors as an example, ideal flexible high-κ dielectrics should not only have high dielectric constant but also have high enough breakdown strength, low dielectric loss, and ease of processing.6,7 Traditional ceramic materials (e.g., BaTiO3, hereafter BT) can exhibit high dielectric constant and low dielectric loss, but usually have low breakdown strength, poor processability, and poor flexibility.8 Polymers can have excellent flexibility, good processing ability, high breakdown strength, and low dielectric loss, whereas their dielectric constant is usually not high enough for high-κ dielectric and energy storage applications.9 Polymer nanocomposites filled with high-κ ceramic nanoparticles combine all of the advantages of both components, displaying a great potential for use in the next generation of high-performance energy storage devices. So far, numerous ceramic nanoparticle-based high-κ polymer nanocomposites have been documented.10−17 However, the majority of polymer matrixes utilized are nonbiodegradable materials, which cannot meet the increasing demands of society for environmental safety. With the rapid development of © XXXX American Chemical Society

consumer electronic products, every year the electronic waste generated is growing dramatically, which may cause serious environmental problems.18 Biopolymers produced from biomass may be the favorable alternative for use as an eco-friendly polymer matrix because of their biodegradability and renewability.19−21 In this work, for the first time, a biodegradable polylactic acid (PLA) was used as polymer matrix to fabricate high-κ nanocomposites. PLA is a compostable, biodegradable polymer, and it is made from sustainable sources. These features make it a promising candidate material for reducing the electronic waste disposal problem.22−27 In addition, excellent dielectric properties, such as high breakdown strength and low electrical conductivity, make PLA a potential material for dielectric applications.28,29 However, the dielectric constant of PLA is lower than 4, which cannot fulfill the requirement of high-κ applications. The introduction of BT nanoparticles should be effective in enhancing the dielectric constant of PLA nanocomposites because BT has dielectric constant much higher than that of PLA. Received: October 1, 2015 Revised: November 10, 2015

A

DOI: 10.1021/acs.jpcc.5b09619 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. (a) Schematic illustration for preparation process of core−shell structured BT@PDA@PLA nanoparticles and (b) TEM images of asprepared BT, BT@PDA, and BT@PDA@PLA nanoparticles.

■

EXPERIMENTAL SECTION Materials. BT nanoparticles with an average diameter of 100 nm were purchased from Shandong Sinocera Functional Material Company (China). Commercially available PLA (4032D) were purchased from Natural Works (United States), which was used as the matrix of the nanocomposites. Lactide (99%) and 4-dimethylaminopyridine (DMAP) were supplied by Acros. Dopamine hydrochloride (98%) was purchased from Aladdin (China). N,N-Dimethylformamide (DMF), toluene, and other solvents were supplied by Shanghai Reagents Co. Ltd. (China). Preparation of Core−Shell BT@Polymer Nanoparticles. The preparation of core−shell structured nanoparticles was performed in two steps. The first step was the preparation of polydopamine-encapsulated BT (BT@PDA) nanoparticles. The BT nanoparticles were dispersed into 2 g L−1 dopamine hydrochloride aqueous solution under stirring for 24 h at 60 °C. The pH of the dopamine aqueous solution was buffered to 8.5 by adding 10 mM Tris-HCl. The BT@PDA was obtained by centrifugation and then washed with toluene three times. The second step was introducing PLA onto the surface of BT@ PDA. The BT@PDA nanoparticles were refluxed in toluene with a Dean−Stark apparatus for 2 h at 135 °C. Then lactide, triethylamine, and DMAP were added into the BT@PDA/ toluene mixture. The mixture were refluxed for an additional 16 h at 135 °C. Finally, the functionalized nanoparticles were collected by centrifugation followed by washing with toluene three times. The obtained nanoparticles were named as BT@ PDA@PLA. Fabrication of PLA-Based Nanocomposite Films. Typically, PLA was dissolved in DMF and then the proposed nanoparticles were added. First, the mixture was treated by ultrasonication for 30 min and then stirred vigorously for 24 h at 65 °C. The solution was dripped into a glass plate and dried at 100 °C for 2 h. Subsequently, the nanocomposite film was peeled off the glass and dried for another 4 h in a vacuum oven to remove the solvent completely. Thin films were prepared by hot-pressing at 180 °C with a pressure of about 20 MPa. Thin films containing different types of nanoparticles (BT, BT@ PDA, and BT@PDA@PLA) were prepared separately. Characterization. Fourier transform infrared (FT-IR) spectroscopy measurements were carried out by a PerkinElmer

When nanocomposites are prepared by the addition of nanoparticles into a polymer, incompatibility of the guest nanoparticles with the host polymer may cause serious nanoparticle aggregation and interfacial debonding between the nanoparticles and the polymer matrix, which in turn results in weak property enhancement of the polymer nanocomposites. To date, many efforts have been made aiming to improve the nanoparticle dispersion and the nanoparticle−polymer compatibility. Surface functionalization has been widely utilized, such as introducing surfactants, coupling agents, or polymer bushes onto the nanoparticle surfaces.30−33 Recently, the preparation of core−shell structured high-κ nanoparticles by surfaceinitiated in situ polymerization has received great interest because of its unique merits: (i) The nanoparticles can be well encapsulated by thickness controllable polymer layers, resulting in significant reduction of surface energy of the nanoparticles.4,34 (ii) The property of the nanocomposites can be tuned by tailoring the physical properties of the polymer shell and/or the interaction between the polymer shell and the polymer matrix.4,35 Because of the advantages of core−shell strategy, PLAencapsulated high-κ BT nanoparticles were suggested to enhance the dielectric properties and energy storage capability of the PLA nanocomposites. Controlled living polymerization methods such as atom transfer radical polymerization (ATRP) and reversible addition−fragmentation chain transfer (RAFT) polymerization have been widely used to prepare core−shell structured BT@polymer nanoparticles.32,35−37 However, the radical polymerization in nature makes the preparation of PLAencapsulated nanoparticles impossible. Herein, surface initiated ring-opening polymerization (ROP) was used to prepare core− shell structured BT@PLA nanoparticles. PLA nanocomposites were fabricated by introducing the BT@PLA nanoparticles into a PLA matrix. The dielectric properties and energy storage capability of the BT@PLA-based PLA nanocomposites were investigated. It was found that the PLA nanocomposites show significantly enhanced dielectric properties and energy storage capability in comparison with the pure PLA, displaying high potential for environmentally friendly dielectric and energy storage applications. B

DOI: 10.1021/acs.jpcc.5b09619 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 2. (a) FT-IR spectra and (b) TGA curves of different types of nanoparticles.

Figure 3. High-resolution core-level C 1s and O 1s XPS spectra of BT@PDA and BT@PDA@PLA nanoparticles.

Paragon 1000 instrument over the range of 4000−450 cm−1. Thermogravimetric analysis (TGA) of BT, BT@PDA, and BT@PDA@PLA nanoparticles was performed by a TG209 F3 instrument (NETZSCH, Germany) at a heating rate of 20 °C min−1 (50−800 °C) in nitrogen flow (20 mL min−1). The chemical compositions of the surface of the BT@PDA and BT@PDA@PLA nanoparticles were determined by X-ray photoelectron spectroscopy (XPS) using an Axis Ultra spectrometer (Kratos Analytical, UK) with a monochromated Al Kα source. Data analysis and curve fitting were performed by using XPSPEAK41 software with a Gaussian−Lorentzian product function and a nonlinear Shirley background subtraction.38 Characterization of the morphology of nanoparticles and thin films was performed by scanning electron microscopy (SEM, Nova NanoSEM 450, FEI, USA) and/or transmission electron microscopy (TEM, JEM-2010, JEOL, Japan). The BT, BT@PDA, and BT@PDA@PLA nanoparticles were prepared by dropping a few drops of nanoparticle solution on the carbon-coated copper grids. Novocontrol Alpha-N high-

resolution dielectric analyzer (GmbH Concept 40, Germany) was used to measure dielectric properties of thin films at the frequency range from 1 Hz to 1 MHz at room temperature. A layer of gold was evaporated on both surfaces of the samples to serve as electrodes. DC breakdown strength was characterized by a dielectric strength tester with a 10 mm ball-to-ball stainless electrode system (DH, Shanghai Lanpotronics Co., China). Electric displacement−electric field (D−E) loops were collected in a Precision Multiferroic Materials Analyzer equipped with Precision 10 kV HVI-SC and Trek MODEL 609B (Radiant Inc., United States). The samples for D−E loop measurement were also sputtered by gold, and the electrode diameter is 3 mm.

■

RESULTS AND DISCUSSION Preparation and Characterization of BT@PDA and BT@PDA@PLA Nanoparticles. Figure 1 demonstrates the synthesis process of BT@PDA@PLA. First, a PDA layer was C

DOI: 10.1021/acs.jpcc.5b09619 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 4. TEM (left column) and SEM (right column) images of BT/PLA (a, b), BT@PDA/PLA (c, d), and BT@PDA@PLA/PLA (e, f) nanocomposites. All the samples contain 10 vol % BT nanoparticles.

introduced onto the BT surface by air oxidation of dopamine.39,40 Then the free amino groups on the surface of PDA were used to initiate ring-opening polymerization of lactide. Herein, PDA-encapsulated BT (BT@PDA) nanoparticles were treated with lactide and DMAP in toluene. In this case, nucleophilic reactive groups on the surface of BT@ PDA initiated the ring-open polymerization, resulting in covalent bonding between the growing PLA chains and the nanoparticles. Figure 1b shows the TEM images of BT, BT@ PDA, and BT@PDA@PLA nanoparticles. One can see that both BT@PDA and BT@PDA@PLA nanoparticles exhibit a core−shell structure, whereas the shell thickness (about 5−15 nm) of BT@PDA@PLA is much higher than that (about 2−3 nm) of BT@PDA, indicating that the ring-open polymerization of lactide was successfully initiated by the surface groups of BT@PDA nanoparticles. FT-IR spectroscopy was also used to confirm the successful preparation of BT@PDA@PLA. Figure 2a shows the FT-IR

spectra of BT@PDA and BT@PDA@PLA. One can see that, compared with the FT-IR spectrum of BT@PDA, a new peak at 1750 cm−1 appeared in the FT-IR spectrum of BT@PDA@ PLA, which should be attributed to CO stretching vibration of ester groups of PLA. In addition, new absorption bands were also found at 1090−1270 cm−1, indicating the symmetric stretching vibration of C−O−C groups of PLA.41 TGA curves shown in Figure 2 provide further evidence for the successful coating of PDA and PLA onto the BT nanoparticles. The weight loss of nanoparticles shows the order of BT < BT@PDA < BT@PDA@PLA. According to the weight loss shown in Figure 2b, a calculation shows that the average thickness values of the polymer layers are 2.5 and 7.8 nm for BT@PDA and BT@PDA@PLA, respectively, which are consistent with those observed from TEM images (Figure 1). XPS spectroscopy was utilized to analyze the surface chemical composition of the core−shell BT@PDA and BT@ PDA@PLA nanoparticles. The high-resolution spectra of C 1s D

DOI: 10.1021/acs.jpcc.5b09619 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 5. Frequency dependence of dielectric constant and dielectric loss tangent of (a, b) BT/PLA, (c, d) BT@PDA/PLA, and (e, f) BT@PDA@ PLA/PLA nanocomposites at room temperature.

PLA is quite strong, indicating a quite thick layer of PLA on the surface of nanoparticles. This result is consistent with the TEM observation shown in Figure 1. Microstructure of PLA-Based Nanocomposites. Figure 4 shows TEM and SEM images of BT, BT@PDA, and BT@ PDA@PLA nanocomposites. TEM images clearly reveal the dispersion state of the three types of nanoparticles in the PLA matrix. One can see that the as-received BT and BT@PDA exhibit similar dispersion ability in PLA. Although some small aggregates can be found, most of the BT or BT@PDA nanoparticles show a good dispersion in the PLA matrix. Surprisingly, the BT@PDA@PLA nanoparticles exhibit a relatively poor dispersion in the PLA matrix, and the nanoparticles mainly exist in the form of microscale aggregates. TEM images with higher magnification are provided in Figure

and O 1s of BT@PDA and BT@PDA@PLA nanoparticles are shown in Figure 3. The C 1s spectra of BT@PDA could be curve-fitted with three components. Namely, binding energies at ∼285 eV for C−C/C−H bonds, ∼286 eV for C−O/C−N bonds, and ∼288.2 eV for CO/N−CO bonds of PDA.42 Moreover, the O 1s binding energies of BT@PDA near 529.4, 531.1, and 532.6 eV should be attributed to the bulk BaTiO3, CO bonds from PDA, and C−O bonds from PDA, respectively.43 C 1s (Figure 3c) and O 1s (Figure 3d) spectra of BT@PDA@PLA indicate that there are O−CO bonds at 299.0 and 533.7 eV, respectively. This result suggests that the PLA layer was successfully introduced by ring-open polymerization reaction of lactide initiated by amino moieties of PDA on the surface of BT@PDA nanoparticles. It should be noted that the intensity of the band ascribed to O−CO group from E

DOI: 10.1021/acs.jpcc.5b09619 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 6. (a, b, c) Weibull plots of breakdown strength and (d) characteristic breakdown strength of the nanocomposites.

BT@PDA@PLA in PLA is unexpected. It is believed that the ring-open polymerization technique used to prepare the BT@ PDA@PLA is the main reason for the poor dispersion of BT@ PDA@PLA in PLA. In other words, aggregates rather than single nanoparticles were formed during the preparation process of BT@PDA@PLA via surface-initiated ring-opening polymerization. Further investigation is needed to clarify the mechanism. Dielectric Properties of PLA-Based Nanocomposites. The dielectric constant of a material indicates the ability to store energy (e.g., charges) when the material is subjected to an electric field, whereas dielectric loss tangent measures the ratio of dissipated energy to the stored energy by a dielectric in a cyclic field. Figure 5 shows the frequency dependence of dielectric constant and dielectric loss tangent of PLA-based nanocomposites at different volume fractions. All three types of nanocomposites show a weak frequency dependence of dielectric constant.44 Compared with the pure PLA, all the nanocomposites show enhanced dielectric constant, and the dielectric constant gradually increases with the increase of nanoparticle loading. Apart from the nanocomposites with 5 vol % nanoparticles where the BT@PDA/PLA exhibit the highest value of dielectric constant, the dielectric constant of the nanocomposites generally follows the order of BT@PDA@ PLA/PLA > BT@PDA/PLA > BT/PLA. When the nanoparticle loading is 20 vol %, the dielectric constants at 1 kHz are 7.52, 8.10, and 8.74 for the nanocomposites with BT, BT@ PDA, and BT@PDA@PLA, respectively, which are all more than two times the dielectric constant (3.04 at 1 kHz) of pure PLA. The increase of dielectric constant in the nanocomposites

S1 (see Supporting Information) to show the aggregation state of nanoparticles in the PLA matrix. One can see that the small aggregates were closely surrounded by the PLA matrix in the nanocomposites of BT/PLA and BT@PDA/PLA. In the BT@ PDA@PLA nanocomposites, however, some holes can be observed inside the large aggregations. These results indicated that large aggregates are the original state of BT@PDA@PLA nanoparticles, which were formed during the preparation process. The PLA matrix cannot be filled into the large aggregates, resulting in the formation of holes inside the aggregates. Although TEM images are powerful tools for characterizing the dispersion state of nanoparticles in a polymer matrix, SEM images are more suitable for revealing the nanoparticle− polymer compatibility. Figure 4 shows the SEM images of the fractured surfaces of the three types of nanocomposites. The SEM images reveal the same dispersion characteristics as those observed from the TEM images. Regarding the compatibility between the nanoparticles and the PLA matrix, we can see that many nanoparticles are uncovered in the nanocomposites with as-received BT, whereas the number of the uncovered nanoparticles was slightly reduced in the BT@PDA nanocomposites. In the BT@PDA@PLA nanocomposites, although most of the BT@PDA@PLA exist in the form of aggregates, a good compatibility between BT@PDA@PLA aggregates and PLA matrix can be recognized. Therefore, the compatibility between the nanoparticles and the PLA matrix follows the order of BT@PDA@PLA/PLA > BT@PDA/PLA > BT/PLA. Considering that the BT@PDA@PLA nanoparticles were well encapsulated by a layer of PLA, the poor dispersion of F

DOI: 10.1021/acs.jpcc.5b09619 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 7. Electric displacement−electric field (D−E) loops of (a) BT-PLA, (b) BT@PDA−PLA, and (c) BT@PDA@PLA nanocomposites.

mainly originates from the electric field enhancement in the polymer matrix, which is induced by the large dielectric constant mismatch between BT and PLA.44 Because both BT and BT@PDA show similar dispersion state (Figure 4) in the PLA matrix, the higher dielectric constant of BT@PDA/PLA should be mainly attributed to the introduction of the PDA layer on the BT nanoparticle surface, which is considered to have higher dielectric constant in comparison with PLA. Compared with the BT@PDA/PLA, the higher dielectric constant of BT@PDA@PLA/PLA should be attributed to the aggregation of the BT@PDA@PLA in the PLA matrix. In this case, the large BT@PDA@PLA aggregates result in high electric field enhancement in the PLA matrix. Regarding the dielectric loss tangent, one can see that the BT/PLA composites always exhibit higher values in comparison with the corresponding BT@PDA/PLA and BT@PDA@PLA/ PLA composites. The lower dielectric loss tangent of the core− shell nanoparticle nanocomposites should be attributed to the improved interfacial compatibility between the nanoparticles and the PLA matrix, which suppresses the dipolar polarization loss of the PLA matrix.45,46 On the other hand, the surface moisture and other impurities may be removed during the introduction of PDA layer onto the BT nanoparticle surface, which can result in reduced electrical conduction loss and/or interfacial polarization loss. Compared with the corresponding BT@PDA nanocomposites, the composites with BT@PDA@ PLA loading higher than 10 vol % show lower dielectric loss tangent, whereas the composite with 5 vol % BT@PDA@PLA exhibit higher dielectric loss tangent. These results indicate the suppressed dipolar polarization of the PLA matrix is the main mechanism for the decrease of dielectric loss tangent in the BT@PDA@PLA nanocomposites. BT@PDA@PLA has excellent interfacial compatibility with the PLA matrix, resulting

in stronger suppression of dipolar polarization of the PLA matrix; thus, the nanocomposites exhibit lower dielectric loss. At low loading of BT@PDA@PLA, the suppression of dipolar polarization is limited because of the aggregation of BT@ PDA@PLA; therefore, the corresponding BT@PDA@PLA exhibits higher dielectric loss tangent. In conclusion, the coating of PDA and PLA layers plays a crucial role in improving the dielectric properties of the nanocomposites by increasing dielectric constant and decreasing the dielectric loss tangent. Electric Breakdown Strength of PLA-Based Nanocomposites. Breakdown strength is a crucial factor for practical application because it determines the highest electric field that the dielectrics can withstand and the maximum energy storage density of dielectrics. The characteristic breakdown strengths of PLA-based nanocomposites were obtained by analyzing the data of each sample using a two-parameter Weibull statistic distribution function (eq 1)47,48 P(E) = 1 − exp[−(E /E0)β ]

(1)

where P(E) is the cumulative probability of electrical failure and E and E0 are experimental breakdown strength and characteristics breakdown strength at the cumulative failure probability of 63.2%, respectively. β is the shape parameter which is related to the scatter of the data. Herein, the characteristics breakdown strength is calculated from a fit using Weibull failure statistics across 10 specimens per sample. Figure 6 shows the Weibull plots of breakdown strength of BT, BT@PDA, and BT@PDA@PLA nanocomposites. One can see from Figure 6 that, compared with PLA, all the nanocomposites show decreased breakdown strength after the introduction of BT nanoparticles. This is mainly because of the large electrical mismatch between the matrix and the G

DOI: 10.1021/acs.jpcc.5b09619 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 8. (a) Total stored and (b) discharged energy densities of BT/PLA, BT@PDA/PLA, and BT@PDA@PLA/PLA nanocomposites.

nanoparticles, which causes electric field enhancement in the PLA matrix.4 As the loading of BT nanoparticles increases, the electric field enhancement increases; thus, the breakdown strength of each nanocomposite decreases with the increase of nanoparticle loading. Figure 6d summarizes the characteristic breakdown strength of three types of nanocomposites. Both the BT@PDA and BT@PDA@PLA nanocomposites show enhanced breakdown strength in comparison with the BT nanocomposites. In addition, the BT@PDA and BT@PDA@ PLA nanocomposites have comparable breakdown strength. It is considered that, at a given nanoparticle loading, the breakdown strength of the nanocomposites is mainly determined by two factors: interfacial strength and nanoparticle dispersion. The introduction of BT@PDA and BT@PDA@ PLA results in enhanced interfacial adhesion between the nanoparticles and the PLA matrix, which causes decrease of defects such as voids and pores. This is the main reason for the enhancement of breakdown strength in the BT@PDA nanocomposites. Compared with the BT@PDA nanocomposites, the further improvement of interfacial adhesion between BT@PDA@PLA and PLA matrix does not cause further enhancement of breakdown strength. This is because the positive effect of the enhanced interfacial adhesion on the breakdown strength of the nanocomposites was offset by the aggregation of BT@PDA@PLA in the PLA matrix. Energy Storage Capability of PLA-Based Nanocomposites. The electric displacement−electric field loops of nanocomposites were measured under 100 Hz with varying electric field, and the results are shown in Figure 7. It can be seen from Figure 7 that the maximum electric displacement of each nanocomposite increases with the increase of nanoparticle loading. At a given nanoparticle loading and given electric field, the maximum electric displacement of the nanocomposites follows the order BT@PDA@PLA/PLA > BT@PDA/PLA > BT/PLA. These results indicate that the maximum electric displacement is highly consistent with the dielectric constant of the nanocomposites. On the other hand, the remnant polarization of each nanocomposite also increases with the increase of nanoparticle loading. At a given nanoparticle loading and given electric field, the remnant polarization of the nanocomposites also follows the order BT@PDA@PLA/PLA > BT@PDA/PLA > BT/PLA. These results demonstrate that the introduction of core−shell nanoparticles results in enhanced ferroelectricity of the nanocomposites.

According to the D−E loops shown in Figure 7, the energy density of the nanocomposites was calculated using the equation U = ∫ E dD, where E is the electric field and D is the electric displacement.11,49 The energy densities of the nanocomposites at 120 MV/m are displayed in Figure 8. One can see that the total stored energy density increases with the increase of the nanoparticle loading. Because of the small increase of the maximum electric displacement (Figure 7), the introduction of the BT@PDA results in only slightly enhanced energy density of the nanocomposites. However, the BT@ PDA@PLA/PLA exhibits significantly enhanced energy density because of the significantly enhanced maximum electric displacement. Taking the nanocomposites with 20 vol % nanoparticles as examples, the total stored energy densities of BT/PLA, BT@PDA/PLA, and BT@PDA@PLA/PLA are 0.85, 1.02, and 1.52 J/cm3 at 120 MV/m, respectively. The energy density of the BT@PDA@PLA nanocomposite is almost doubled in comparison with that of the BT nanocomposite. Discharged energy density is a more efficient parameter to measure the effective energy storage capability of a dielectric material.50 As shown in Figure 8, the discharged energy density of the nanocomposites was apparently enhanced by the introduction of the BT@PDA or BT@PDA@PLA. In addition, the introduction of BT@PDA@PLA results in much higher enhancement of discharged energy density when the electric field and the nanoparticle loading are high. Also taking the nanocomposites with 20 vol % nanoparticles as examples, the discharged energy density of the BT/PLA, BT@PDA/PLA, and BT@PDA@PLA/PLA are 0.56, 0.68, and 0.76 J/cm3 at 120 MV/m, respectively. The discharged energy density of the BT@PDA@PLA nanocomposite is still the highest, and it is 36% higher than that of the BT nanocomposite. The energy storage efficiency can be defined as the ratio of discharged energy density to the total stored energy density. Figure 8 indicates that the BT@PDA@PLA nanocomposites exhibit a lower energy storage efficiency in comparison with BT@PDA nanocomposites. The low energy storage efficiency in BT@ PDA@PLA nanocomposites should be attributed to the aggregation of the BT@PDA@PLA, which results in larger remnant polarization and higher dielectric loss under high electric field. Therefore, efforts should be made in the future to improve the dispersion of BT@PDA@PLA nanoparticles in PLA matrix. H

DOI: 10.1021/acs.jpcc.5b09619 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

■

Poly(Ethyl Methacrylate) Modified Poly(Vinylidene-co-Trifluoroethylene). Appl. Phys. Lett. 2014, 104, 263901. (7) Qiao, Y. L.; Islam, M. S.; Wang, L.; Yan, Y.; Zhang, J. Y.; Benicewicz, B. C.; Ploehn, H. J.; Tang, C. B. Thiophene PolymerGrafted Barium Titanate Nanoparticles toward Nanodielectric Composites. Chem. Mater. 2014, 26, 5319−5326. (8) Xie, L. Y.; Huang, X. Y.; Huang, Y. H.; Yang, K.; Jiang, P. K. Core@ Double-Shell Structured BaTiO3−Polymer Nanocomposites with High Dielectric Constant and Low Dielectric Loss for Energy Storage Application. J. Phys. Chem. C 2013, 117, 22525−22537. (9) Zhu, L.; Wang, Q. Novel Ferroelectric Polymers for High Energy Density and Low Loss Dielectrics. Macromolecules 2012, 45, 2937− 2954. (10) Gao, L.; He, J.; Hu, J.; Li, Y. Large Enhancement in Polarization Response and Energy Storage Properties of Poly(Vinylidene Fluoride) by Improving the Interface Effect in Nanocomposites. J. Phys. Chem. C 2014, 118, 831−838. (11) Wang, G. Y.; Huang, X. Y.; Jiang, P. K. Tailoring Dielectric Properties and Energy Density of Ferroelectric Polymer Nanocomposites by High-κ Nanowires. ACS Appl. Mater. Interfaces 2015, 7, 18017−18027. (12) Xie, L. Y.; Huang, X. Y.; Yang, K.; Li, S. T.; Jiang, P. K. ″Grafting to″ Route to PVDF-HFP-GMA/BaTiO3 Nanocomposites with High Dielectric Constant and High Thermal Conductivity for Energy Storage and Thermal Management Applications. J. Mater. Chem. A 2014, 2, 5244−5251. (13) Kim, P.; Doss, N. M.; Tillotson, J. P.; Hotchkiss, P. J.; Pan, M.J.; Marder, S. R.; Li, J.; Calame, J. P.; Perry, J. W. High Energy Density Nanocomposites Based on Surface-Modified BaTiO3 and a Ferroelectric Polymer. ACS Nano 2009, 3, 2581−2592. (14) Li, J. J.; Claude, J.; Norena-Franco, L. E.; Il Seok, S.; Wang, Q. Electrical Energy Storage in Ferroelectric Polymer Nanocomposites Containing Surface-Functionalized BaTiO3 Nanoparticles. Chem. Mater. 2008, 20, 6304−6306. (15) Zhang, X.; Shen, Y.; Zhang, Q. H.; Gu, L.; Hu, Y. H.; Du, J. W.; Lin, Y. H.; Nan, C. W. Ultrahigh Energy Density of Polymer Nanocomposites Containing BaTiO3@TiO2 Nanofibers by AtomicScale Interface Engineering. Adv. Mater. 2015, 27, 819−824. (16) Yu, K.; Bai, Y. Y.; Zhou, Y. C.; Niu, Y. J.; Wang, H. Poly(Vinylidene Fluoride) Polymer Based Nanocomposites with Enhanced Energy Density by Filling with Polyacrylate Elastomers and BaTiO3 Nanoparticles. Appl. Phys. Lett. 2014, 104, 082904. (17) Song, Y.; Shen, Y.; Hu, P. H.; Lin, Y. H.; Li, M.; Nan, C. W. Significant Enhancement in Energy Density of Polymer Composites Induced by Dopamine-Modified Ba0.6Sr0.4TiO3 Nanofibers. Appl. Phys. Lett. 2012, 101, 152904. (18) Fromer, N. A.; Diallo, M. S. Nanotechnology and Clean Energy: Sustainable Utilization and Supply of Critical Materials. J. Nanopart. Res. 2013, 15, 2011. (19) Jenck, J. F.; Agterberg, F.; Droescher, M. J. Products and Processes for a Sustainable Chemical Industry: A Review of Achievements and Prospects. Green Chem. 2004, 6, 544−556. (20) Carothers, W. H.; Dorough, G. L.; Natta, F. J. Studies of Polymerization and Ring Formation. X. The Reversible Polymerization of Six-Membered Cyclic Esters. J. Am. Chem. Soc. 1932, 54, 761−772. (21) Miyata, K.; Fujita, S.; Ohki, Y.; Tanaka, T. Comparison of Partial Discharge Resistance among Several Biodegradable Polymers. IEEE Trans. Dielectr. Electr. Insul. 2007, 14, 1474−1476. (22) Auras, R.; Harte, B.; Selke, S. An Overview of Polylactides as Packaging Materials. Macromol. Biosci. 2004, 4, 835−864. (23) Raquez, J. M.; Habibi, Y.; Murariu, M.; Dubois, P. Polylactide (PLA)-Based Nanocomposites. Prog. Polym. Sci. 2013, 38, 1504−1542. (24) Hassan, M. K.; Wiggins, J. S.; Storey, R. F.; Mauritz, K. A. Broadband Dielectric Spectroscopic Characterization of the Hydrolytic Degradation of Carboxylic Acid-Terminated Poly(D,L-Lactide) Materials. Polymer 2007, 48, 2022−2029. (25) Ohki, Y.; Hirai, N. Electrical Conduction and Breakdown Properties of Several Biodegradable Polymers Biodegradable Polymers. IEEE Trans. Dielectr. Electr. Insul. 2007, 14, 1559−1566.

CONCLUSIONS Biodegradable PLA-based high-κ nanocomposites were prepared by using core−shell structured BT@PDA and BT@ PDA@PLA nanoparticles as filler. The nanocomposites with each type of core−shell nanoparticles exhibit significantly enhanced dielectric constant, suppressed dielectric loss, and enhanced breakdown strength when compared with the nanocomposites with as-prepared BT. In addition, compared with the BT@PDA nanocomposites, the BT@PDA@PLA nanocomposites exhibit much higher dielectric constant; thus, the nanocomposites have much higher enhanced energy density. It was also found that the nanoparticle dispersion and nanoparticle−PLA compatibility show significant influence on the dielectric properties and energy storage capability of the nanocomposites. Efforts should be made to improve the dispersion of BT@PDA@PLA in the PLA matrix to increase the energy efficiency of the BT@PDA@PLA nanocomposites.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09619. TEM images of BT/PLA, BT@PDA/PLA, and BT@ PDA@PLA/PLA nanocomposites (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

X.H. conceived the idea of the work. Y.F. prepared the samples, performed measurements, and prepared the figures. X.H. wrote the manuscript. All authors commented on the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge support from the National Natural Science Foundation of China (51522703, 51477096, 51277117) and the Special Fund of the National Priority Basic Research of China under Grant 2014CB239503. X.H. thanks the 2013 SMC Excellent Young Faculty Award of Shanghai Jiao Tong University and Shanghai Pujiang Program (PJ14D018) for financial support.

■

REFERENCES

(1) Ortiz, R. P.; Facchetti, A.; Marks, T. J. High-κ Organic, Inorganic, and Hybrid Dielectrics for Low-Voltage Organic Field-Effect Transistors. Chem. Rev. 2010, 110, 205−239. (2) Li, Q.; Chen, L.; Gadinski, M. R.; Zhang, S. H.; Zhang, G. Z.; Li, H. Y.; Haque, A.; Chen, L. Q.; Jackson, T.; Wang, Q. Flexible HighTemperature Dielectric Materials from Polymer Nanocomposites. Nature 2015, 523, 576−579. (3) Dang, Z.-M.; Yuan, J.-K.; Yao, S.-H.; Liao, R.-J. Flexible Nanodielectric Materials with High Permittivity for Power Energy Storage. Adv. Mater. 2013, 25, 6334−6365. (4) Huang, X. Y.; Jiang, P. K. Core-Shell Structured High-κ Polymer Nanocomposites for Energy Storage and Dielectric Applications. Adv. Mater. 2015, 27, 546−554. (5) Tang, H. X.; Lin, Y. R.; Sodano, H. A. Synthesis of High Aspect Ratio BaTiO3 Nanowires for High Energy Density Nanocomposite Capacitors. Adv. Energy Mater. 2013, 3, 451−456. (6) Li, J. J.; Gong, H. H.; Yang, Q.; Xie, Y. C.; Yang, L. J.; Zhang, Z. C. Linear-Like Dielectric Behavior and Low Energy Loss Achieved in I

DOI: 10.1021/acs.jpcc.5b09619 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C (26) Liu, M. J.; Chen, S. C.; Yang, K. K.; Wang, Y. Z. Biodegradable Polylactide Based Materials with Improved Crystallinity, Mechanical Properties and Rheological Behaviour by Introducing a Long-Chain Branched Copolymer. RSC Adv. 2015, 5, 42162−42173. (27) Wang, J. H.; Sun, K.; Wang, J. C.; Guo, Y. Preparation of PLACoated Energy Storage Microcapsules and Its Application in Polyethylene Composites. Polym.-Plast. Technol. Eng. 2013, 52, 1235−1241. (28) Matsugasaki, N.; Shinyama, K.; Fujita, S. Dielectric Breakdown and Mechanical Properties of Polylactic Acid of Different Spherulite Sizes. IEEJ Trans. Electr. Electron. Eng. 2013, 8, S106−S107. (29) Amass, W.; Amass, A.; Tighe, B. A Review of Biodegradable Polymers: Uses, Current Developments in the Synthesis and Characterization of Biodegradable Polyesters, Blends of Biodegradable Polymers and Recent Advances in Biodegradation Studies. Polym. Int. 1998, 47, 89−144. (30) Kim, P.; Doss, N. M.; Tillotson, J. P.; Hotchkiss, P. J.; Pan, M. J.; Marder, S. R.; Li, J. Y.; Calame, J. P.; Perry, J. W. High Energy Density Nanocomposites Based on Surface-Modified BaTiO3 and a Ferroelectric Polymer. ACS Nano 2009, 3, 2581−2592. (31) Kim, P.; Jones, S. C.; Hotchkiss, P. J.; Haddock, J. N.; Kippelen, B.; Marder, S. R.; Perry, J. W. Phosphonic Acid-Modified Barium Titanate Polymer Nanocomposites with High Permittivity and Dielectric Strength. Adv. Mater. 2007, 19, 1001−1005. (32) Xie, L. Y.; Huang, X. Y.; Wu, C.; Jiang, P. K. Core-Shell Structured Poly(Methyl Methacrylate)/BaTiO3 Nanocomposites Prepared by in Situ Atom Transfer Radical Polymerization: A Route to High Dielectric Constant Materials with the Inherent Low Loss of the Base Polymer. J. Mater. Chem. 2011, 21, 5897−5906. (33) Zhou, T.; Zha, J.-W.; Cui, R.-Y.; Fan, B.-H.; Yuan, J.-K.; Dang, Z.-M. Improving Dielectric Properties of BaTiO3/Ferroelectric Polymer Composites by Employing Surface Hydroxylated BaTiO3 Nanoparticles. ACS Appl. Mater. Interfaces 2011, 3, 2184−2188. (34) Paniagua, S. A.; Kim, Y.; Henry, K.; Kumar, R.; Perry, J. W.; Marder, S. R. Surface-Initiated Polymerization from Barium Titanate Nanoparticles for Hybrid Dielectric Capacitors. ACS Appl. Mater. Interfaces 2014, 6, 3477−3482. (35) Zhu, M.; Huang, X. Y.; Yang, K.; Zhai, X.; Zhang, J.; He, J. L.; Jiang, P. K. Energy Storage in Ferroelectric Polymer Nanocomposites Filled with Core-Shell Structured Polymer@BaTiO3 Nanoparticles: Understanding the Role of Polymer Shells in the Interfacial Regions. ACS Appl. Mater. Interfaces 2014, 6, 19644−19654. (36) Chen, Z.; Xie, L. Y.; Huang, X. Y.; Li, S. T.; Jiang, P. K. Achieving Large Dielectric Property Improvement in Polymer/Carbon Nanotube Composites by Engineering the Nanotube Surface Via Atom Transfer Radical Polymerization. Carbon 2015, 95, 895−903. (37) Yang, K.; Huang, X. Y.; Zhu, M.; Xie, L. Y.; Tanaka, T.; Jiang, P. K. Combining Raft Polymerization and Thiol-Ene Click Reaction for Core-Shell Structured Polymer@BaTiO3 Nanodielectrics with High Dielectric Constant, Low Dielectric Loss, and High Energy Storage Capability. ACS Appl. Mater. Interfaces 2014, 6, 1812−1822. (38) Shirley, D. A. High-Resolution X-Ray Photoemission Spectrum of the Valence Bands of Gold. Phys. Rev. B 1972, 5, 4709−4714. (39) Mrowczynski, R.; Nan, A.; Turcu, R.; Leistner, J.; Liebscher, J. Polydopamine - a Versatile Coating for Surface-Initiated Ring-Opening Polymerization of Lactide to Polylactide. Macromol. Chem. Phys. 2015, 216, 211−217. (40) Yang, K.; Huang, X. Y.; He, J. L.; Jiang, P. K. Strawberry-like Core−Shell Ag@Polydopamine@ BaTiO3 Hybrid Nanoparticles for High-κ Polymer Nanocomposites with High Energy Density and Low Dielectric Loss. Adv. Mat. Interfaces 2015, DOI: 10.1002/ admi.201500361. (41) Song, W. H.; Zheng, Z.; Tang, W. L.; Wang, X. L. A Facile Approach to Covalently Functionalized Carbon Nanotubes with Biocompatible Polymer. Polymer 2007, 48, 3658−3663. (42) Gaspar, H.; Pereira, C.; Rebelo, S. L. H.; Pereira, M. F. R.; Figueiredo, J. L.; Freire, C. Understanding the Silylation Reaction of Multi-Walled Carbon Nanotubes. Carbon 2011, 49, 3441−3453.

(43) Acres, R. G.; Ellis, A. V.; Alvino, J.; Lenahan, C. E.; Khodakov, D. A.; Metha, G. F.; Andersson, G. G. Molecular Structure of 3Aminopropyltriethoxysilane Layers Formed on Silanol-Terminated Silicon Surfaces. J. Phys. Chem. C 2012, 116, 6289−6297. (44) Badia, J. D.; Monreal, L.; de Juano-Arbona, V. S.; Ribes-Greus, A. Dielectric Spectroscopy of Recycled Polylactide. Polym. Degrad. Stab. 2014, 107, 21−27. (45) Wu, W.; Huang, X. Y.; Li, S. T.; Jiang, P. K.; Toshikatsu, T. Novel Three-Dimensional Zinc Oxide Superstructures for High Dielectric Constant Polymer Composites Capable of Withstanding High Electric Field. J. Phys. Chem. C 2012, 116, 24887−24895. (46) Fragiadakis, D.; Runt, J. Molecular Dynamics of Segmented Polyurethane Copolymers: Influence of Soft Segment Composition. Macromolecules 2013, 46, 4184−4190. (47) Huang, X. Y.; Li, Y.; Liu, F.; Jiang, P. K.; Iizuka, T.; Tatsumi, K.; Tanaka, T. Electrical Properties of Epoxy/POSS Composites with Homogeneous Nanostructure. IEEE Trans. Dielectr. Electr. Insul. 2014, 21, 1516−1528. (48) Huang, X. Y.; Xie, L. Y.; Hu, Z. W.; Jiang, P. K. Influence of BaTiO3 Nanoparticles on Dielectric, Thermophysical and Mechanical Properties of Ethylene-Vinyl Acetate Elastomer/BaTiO3 Microcomposites. IEEE Trans. Dielectr. Electr. Insul. 2011, 18, 375−383. (49) Li, Q.; Han, K.; Gadinski, M. R.; Zhang, G. Z.; Wang, Q. High Energy and Power Density Capacitors from Solution-Processed Ternary Ferroelectric Polymer Nanocomposites. Adv. Mater. 2014, 26, 6244−6249. (50) Khanchaitit, P.; Han, K.; Gadinski, M. R.; Li, Q.; Wang, Q. Ferroelectric Polymer Networks with High Energy Density and Improved Discharged Efficiency for Dielectric Energy Storage. Nat. Commun. 2013, 4, 2845.

J

DOI: 10.1021/acs.jpcc.5b09619 J. Phys. Chem. C XXXX, XXX, XXX−XXX