Pingkai Jiang

Core–Shell Structured High‐k Polymer Nanocomposites for Energy Storage and Dielectric Applications

High-k polymer nanocomposites have considerable potential in energy storage and dielectric applications because of their ease of processing, flexibility, and low cost. Core-shell nanoarchitecture strategies are versatile and powerful tools for the design and synthesis of advanced high-k polymer nanocomposites. Recent and in-progress state-of-the-art advancements in the application of core-shell nanoarchitecture strategies to design and prepare high-k polymer nanocomposites are summarized. Special focus is directed to emphasizing their advantages over conventional melt-mixing and solution-mixing methods: first, homogeneous nanoparticle dispersion can be easily achieved even in highly loaded nanocomposites; second, the dielectric constant of the nanocomposites can be effectively enhanced and meanwhile the high breakdown strength can be well-preserved; third, for nanocomposites filled with electrically conductive nanoparticles, dielectric loss can be effectively surpressed, and meanwhile a high dielectric constant can be achieved. In addition, fundamental insights into the roles of the interfaces on the dielectric properties of the nanocomposites can be probed. The last part of the article is concluded with current problems and future perspectives of utilizing the core-shell nanoarchitecture strategies for the development of high-k polymer nanocomposites.

Large dielectric constant and high thermal conductivity in poly(vinylidene fluoride)/barium titanate/silicon carbide three-phase nanocomposites.

Dielectric polymer composites with high dielectric constants and high thermal conductivity have many potential applications in modern electronic and electrical industry. In this study, three-phase composites comprising poly(vinylidene fluoride) (PVDF), barium titanate (BT) nanoparticles, and β-silicon carbide (β-SiC) whiskers were prepared. The superiority of this method is that, when compared with the two-phase PVDF/BT composites, three-phase composites not only show significantly increased dielectric constants but also have higher thermal conductivity. Our results show that the addition of 17.5 vol % β-SiC whiskers increases the dielectric constants of PVDF/BT nanocomposites from 39 to 325 at 1000 Hz, while the addition of 20.0 vol % β-SiC whiskers increases the thermal conductivity of PVDF/BT nanocomposites from 1.05 to 1.68 W m(-1) K(-1) at 25 °C. PVDF/β-SiC composites were also prepared for comparative research. It was found that PVDF/BT/β-SiC composites show much higher dielectric constants in comparison with the PVDF/β-SiC composites within 17.5 vol % β-SiC. The PVDF/β-SiC composites show dielectric constants comparable to those of the three-phase composites only when the β-SiC volume fraction is 20.0%, whereas the dielectric loss of the PVDF/β-SiC composites was much higher than that of the three-phase composites. The frequency dependence of the dielectric property for the composites was investigated by using broad-band (10(-2)-10(6) Hz) dielectric spectroscopy.

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

Core-shell structured BaTiO3/poly(methyl methacrylate) (PMMA) nanocomposites were successfully prepared by in situ atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) from the surface of BaTiO3 nanoparticles. A broadband dielectric spectrometer was used to investigate the temperature dependence of the dielectric properties of the nanocomposites in a frequency range from 0.1 Hz to 1 MHz. It was found that the nanocomposites not only showed a significantly increased dielectric constant when compared with pure PMMA, but also showed the inherent low loss of the base polymer in a wide range of frequencies. Only in the very low frequency/high temperature range, can a higher dielectric loss can be observed in the nanocomposites. It was also found that the effective dielectric constant of the core-shell structured hybrid nanoparticles can be tailored by varying the polymer shell thickness. The dielectric response of beta relaxation of PMMA was also studied and the results showed that the nanoparticles had no influence upon the relaxation activation energy. Fourier-transform infrared spectroscopy (FTIR) and 1H NMR spectra confirmed the chemical structure of the PMMA shell on the surface of the BaTiO3 nanoparticles. Transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) results revealed that the PMMA shell thickness could be well controlled by tuning the feed ratio of MMA to BaTiO3.

Mechanically Flexible and Multifunctional Polymer‐Based Graphene Foams for Elastic Conductors and Oil‐Water Separators

We present a novel strategy for the fabrication of ordered and flexible polymer-based graphene foams by self-assembly of graphene sheets on a 3D polymer skeleton. The obtained graphene foams show excellent mechanical, electrical, and hydrophobic properties, thus holding great potential as elastic conductors and oil-water separators.

Hyperbranched-polymer functionalization of graphene sheets for enhanced mechanical and dielectric properties of polyurethane composites

The incorporation of graphene sheets (GSs) into polymer matrices affords engineers an opportunity to synthesize polymer composites with excellent physical performances. However, the development of high performance GS-based composites is difficult because of the easy aggregation of GSs in a polymer matrix as well as the weak interfacial adhesion between GSs and the host polymer. Herein, we present a simple and effective route to hyperbranched aromatic polyamide functionalized graphene sheets (GS–HBA). The resulting GS-HBA exhibits uniform dispersion in a thermoplastic polyurethane (TPU) matrix and strong adhesion with the matrix by hydrogen-bond coupling, which improve the load transfer efficiency from the matrix to the GSs. Thus, the GS–HBA–TPU composites possess excellent mechanical performance and high dielectric performance. It has been demonstrated that the GS–HBA composite has higher modulus, higher tensile strength and higher yield strength, and remains at nearly the same strain at break when compared with the composites with graphene oxide, ethylene diamine-modified graphene, and hydrazine reduced graphene. In addition, the hyperbranched polymer chains allow construction of a large number of microcapacitors and suppress the leakage current by isolating the GSs in a TPU matrix, resulting in a higher permittivity and lower loss tangent for the GS–HBA composite in comparison with ethylene diamine-modified graphene, or hydrazine reduced-graphene composites.

Ferroelectric polymer/silver nanocomposites with high dielectric constant and high thermal conductivity

Ferroelectric polymer nanocomposites with silver (Ag) nanoparticles as inclusions were prepared and the dielectric properties and thermal conductivity were studied. The results showed that the nanocomposites have high dielectric constant and high thermal conductivity. When the loading level of Ag nanoparticles is 20.0 vol %, the dielectric constant and thermal conductivity of the nanocomposites were 120 at 103 Hz and 6.5 W/mK, respectively. Our results also showed that there is no percolation in the nanocomposites when Ag loading range is within 20.0%.

Permittivity, thermal conductivity and thermal stability of poly(vinylidene fluoride)/graphene nanocomposites

Poly(vinylidene fluoride) (PVDF)/graphene nanocomposites were prepared by solution blending. The incorporation of graphene sheets (GSs) increased the permittivity, thermal conductivity and thermal stability of PVDF, resulting in a transition from electrical insulator to semiconductor with a percolation threshold of 4.5 wt%. The composite containing 7.5% GSs had a permittivity higher than 300 at 1000 Hz, which is about 45 times that of pure PVDF. The thermal conductivity of the composite with 0.5% GSs was increased by approximately a factor of 2 when compared with the pure PVDF. The addition of 0.05% GSs produced an increase in the maximum decomposition temperature of PVDF of over 20°C.

Fabrication of two-dimensional hybrid sheets by decorating insulating PANI on reduced graphene oxide for polymer nanocomposites with low dielectric loss and high dielectric constant

Novel polyaniline decorated reduced graphene oxide (rPANI@rGO) two-dimensional (2D) hybrids sheets were successfully prepared by in situ polymerization of aniline on graphene oxide (GO) sheets and successive reduction by hydrazine. PANI is heavily reduced, thus it is electrically insulating. The hybrid sheets were used as a novel filler for high performance poly(methyl methacrylate) (PMMA) nanocomposites. Our results show that, when compared with the PMMA/rGO composites, the PMMA/rPANI@rGO nanocomposites not only show a high dielectric constant but also have low dielectric loss. For example, at 1000 Hz, a dielectric constant of 40 and a dielectric loss of 0.12 were observed in the PMMA/rPANI@rGO nanocomposite with rGO/PMMA volume ratio of 6%, whereas the dielectric constant and dielectric loss of PMMA/rGO composite with rGO/PMMA volume ratio of 6% are about 20 and 1250, respectively. More importantly, the dielectric properties of PMMA/rPANI@rGO nanocomposites can be tuned by controlling the addition of the hybrid sheets. The improved dielectric properties in PMMA/rPANI@rGO nanocomposites should originate from the isolation effect of rPANI on the rGO in PMMA matrix, which not only improves the dispersion of rGO but also hinders the direct electrical contact between rGO. This research sets up a novel route to polymer composites with high dielectric constants and low dielectric loss, and also expands the application space of graphene-based fillers.

Core-shell structured hyperbranched aromatic polyamide/BaTiO3 hybrid filler for poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) nanocomposites with the dielectric constant comparable to that of percolative composites.

Polymer nanocomposites with the dielectric constant comparable to that of percolative composites are successfully prepared by using core-shell structured hyperbranched aromatic polyamide grafted barium titanate (BT-HBP) hybrid nanofiller. Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TrFE-CFE) was used as the polymer matrix because of its high intrinsic dielectric constant and easy processability. The BT-HBP hybrid nanofiller were prepared by a solution polymerization of diaminobenzoic acid on the surface of amino-funcationalized BT nanoparticles. Nuclear magnetic resonance ((1)H NMR) and transmission electron microscopy (TEM) were used to verify the chemical structure of the hyperbranched aromatic polyamide and core-shell structure of the hybrid filler, respectively. It was found that the nanocomposite with 40 vol % BaTiO3-HBP had a dielectric constant of 1485.5 at 1000 Hz, whereas the corresponding nanocomposite sample with untreated BaTiO3 only showed a dielectric constant of 206.3. Compared with classic percolative composites, the advantage of the PVDF-TrFE-CFE/BaTiO3-HBP nanocomposites is that the composites show high enough breakdown strength and high dielectric constant simultaneously. An enhanced interfacial polarization mechanism between the BT-HBP and the polymer matrix was suggested for understanding the observed unusually high dielectric constant.

Alumina-coated graphene sheet hybrids for electrically insulating polymer composites with high thermal conductivity

Graphene has attracted considerable attention as a promising candidate to improve the thermal conductivity of polymers owing to its extremely high intrinsic thermal conductivity (∼5300 W m−1 K). However, graphene-based composites show a high electrical conductivity even with a low loading of fillers, which greatly limits their applications in electronic devices. Herein, we present a new class of fillers of alumina-coated graphene sheet (GS@Al2O3) hybrid fillers via an electrostatic self-assembly route. This unique structural design combines the advantages of both the GS and Al2O3, resulting in PVDF/GS@Al2O3 composites that show not only high thermal conductivity, but also retain high electrical insulation. For instance, the thermal conductivity of PVDF composites with 40 wt% GS@Al2O3 is up to 0.586 W m−1 K and the volume resistivity is above 4 × 1014 Ω cm. Moreover, this self-assembly route is a simple and scalable strategy for fabricating high performance thermally conductive materials.