Dielectric Polymer Materials with High Thermal Stability

Dielectric Polymer Materials with High Thermal Stability

11 Dielectric Polymer Materials with High Thermal Stability Guozheng Liang, Longhui Zheng, Ningning Zhu and Aijuan Gu Soochow University, Suzhou, Chin...

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11 Dielectric Polymer Materials with High Thermal Stability Guozheng Liang, Longhui Zheng, Ningning Zhu and Aijuan Gu Soochow University, Suzhou, China

11.1

Introduction

Dielectric materials with high permittivity (called high-ε dielectric materials) are of great interest due to their diverse applications in many civilian and military applications [1]. Owing to the rapid developments taking place in electrical and electronic industries, there is an urgent demand for high-ε materials that are light, strong, and can withstand high temperature encountered during the manufacturing of electrical/ electronic devices. Compared with metals and ceramics, polymer dielectrics are one special type of dielectrics, which have unique advantages including being lightweight, good processing features, mechanical flexibility, higher breakdown strengths, and greater reliability [24]. However, polymer dielectrics are limited to relatively low working temperatures and relatively low dielectric constants [5]. To combine the advantages of polymers and ceramics or metals, ceramic/polymer and conductor/polymer composites have been developed, which have been proved to be the right alternatives and offer excellent material characteristics, such as flexibility, machinability, low-temperature processability, and tunable dielectric properties [6]. For polymer composites, thermal stability is dependent on many factors, of which a polymer matrix is the main aspect because polymers usually have lower thermal stability than metals and ceramics. Therefore, to develop thermally stable dielectric polymer materials, polymers (thermoplastics and thermosetting ones) with thermal stability should be used. This chapter focuses on recent research progress in high-ε dielectric polymer materials with high thermal stability, including polymers and polymer matrix composites. On the one hand, fundamental aspects of high thermal stability and their main effect factors are summarized and Dielectric Polymer Materials for High-Density Energy Storage. DOI: https://doi.org/10.1016/B978-0-12-813215-9.00011-7 © 2018 Elsevier Inc. All rights reserved.

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used to explore strategies of achieving high thermal stability of dielectric polymer materials. On the other hand, the relationship between dielectric properties and thermal stability of dielectric polymer materials is discussed, and the main processes for producing high-ε dielectric polymer materials with high thermal stability are introduced. Finally, the application for high-ε dielectric polymer materials with high thermal stability is addressed.

11.2 Fundamental Aspects of High Thermal Stability 11.2.1 Definition, Performance Index, and Evaluation Method of Thermal Stability The most popular definitions of high thermal stability include the following five aspects: they are (1) long-term durability (. 10,000 hours) at 177 C; (2) the initial decomposition temperature (Tdi) . 450 C (e.g., the temperature of 5% weight loss as measured by dynamic thermogravimetric analysis at a heating rate of 2.50 C/min); (3) low weight loss rates at high temperatures (e.g., 0.05 wt% per hour at 450 C); (4) high heat deflection temperature ( . 177 C, the temperature where 10% deflection occurs on a polymer specimen under a load of 1.52 MPa); and (5) high glass transition temperature (Tg . 200 C) and high mechanical properties. The first three definitions appear to fit most materials including sealants (elastomers), whereas the last two have been directed primarily towards polymers for structural applications [7]. Dielectric properties are sensitive to relaxation of groups and segment/chains of polymers. Tg represents the capability of the chain segment motion of polymers. At temperatures approaching Tg, polymers lose their dimensional and electromechanical stability and display large variations in dielectric constant (ε) and dissipation factor (DF) with temperature, so it is general to use Tg and Tdi as the performance indexes [811]. Dynamic mechanical analysis is usually used for the qualitative assessment of the viscoelastic properties of the cross-linked polymers, and Tg is often considered to be the peak of tan δ temperature curves [12]. Considering the potential application in electronic industries, the temperature at which the weight loss of the sample reaches 5 wt% is regarded as Tdi according to the Test Methods Manual IPC-TM-650 published by the Association Connecting Electronic Industries.

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There are not exact valves for Tg and Tdi that are the criteria for evaluating high thermal stability or not. Generally, Tg . 200 C and Tdi . 450 C at a heating rate of 2.50 C/min may be considered as high thermal stability polymers.

11.2.2 Influences of Main Factors on the Thermal Stability 11.2.2.1 Structural Characteristics of Polymers With regard to thermal stability of polymers, both chemical structure and aggregation structure play important roles. The former mainly refers to the macromolecular backbone, while the latter mainly includes free volume, intermolecular interaction, crystalline phases, and crosslinking density [12,13]. Heterocyclic rings and a high level of aromatic characters are the general chemical features for polymers with high thermal stability, which, however, also make the polymers exhibit poor processing characteristics, great brittleness, and big internal stress. Many significant structural modifications have been made to improve the processability without decreasing thermal stability of polyimide (PI). Typically, introducing flexible units with thermally stable groups such as O, CO, SO2, S, and C(CF3)2 improves the toughness [14]; while disrupting chain symmetry and regularity, and/or preventing ordering of molecules improves solubility through incorporating large pendant groups along the backbone [14], kinks in the PI chain backbone through ortho or meta catenation [15,16], or co-polyimide units [17]. 11.2.2.2 Filler Types In addition to the enhancement of thermal stability, a remarkable improvement in ε is also observed after the incorporation of different types of nanofillers in the polymer matrix [18]. The addition of nanofillers into polymers generally increases the thermal stabilization because of following one or more reasons: (1) Knotting and interlocking of molecules restrict segmental mobility of the polymer chain. (2) Forming crystal with better thermal stability; e.g., the presence of reduced graphene oxide (rGO)-ZnO makes the formation of polyvinylidene fluoride (PVDF) with β-phase, which has a higher thermal stability than α-form due to the better packing of the

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zigzag β PVDF chain than that of the Trans-Gauche-Trans-Gauche α-PVDF chain [19]. (3) Nanoparticles act as a “mass transport barrier” within the polymer matrix to suppress the degradation of polymers [20]. (4) Filler particles act as a heat sink [21]. Therefore, the improved dispersion of fillers as well as the strong interaction between fillers and matrix can reduce free volume and segmental mobility, and thus improve Tg [1,22]. On the other hand, the above reasons also explain why the influence of fillers on the thermal stability is also related to the loading of fillers. Specifically, Liu’s group fabricated silver (Ag)-polyarylene ether nitrile (PEN) nanocomposite films [23]. The Tg initially increases and reaches the maximum value when the content of Ag reaches 2.0 wt%, and then decreases as the loading of Ag increases. This is because Ag nanoparticles get into the interspace of the polymer matrix and decrease the segmental motion of the polymer chain segment, leading to the increment of Tg. However, the size of Ag nanoparticles increases with more Ag introduced, leading to the increase of free volume, which disrupts the packing of the PEN chain segment and decreases the Tg of the composites. Note that, there are some exceptions; i.e., the addition of fillers reduces the thermal stability of polymers, reported due to following reasons: (1) Break of the polymer-polymer interaction [24]. (2) Reduction of crystallinity of thermoplastics. For example, the crystallinity of PVDF reduced with the addition of Na0.5Bi0.5Cu3Ti4O12, leading to more susceptibility to thermal decomposition [25]. (3) Photocatalytic activity of BaTiO3 (BT) [21] and titanium dioxide (TiO2) [26] on the thermal decomposition of polymer chains. 11.2.2.3 Interface Characteristics As is well known, the thermal stability of the composites is closely related to the interfacial interaction between the organic matrix and inorganic filler [10]. A suitably high interfacial interaction is beneficial to obtain polymer composites with high thermal stability; this is mainly because of restrained chain mobility caused by the filler surface [2729]. According to the model of Tsagaropoulos [30], each filler is surrounded by its own tightly and loosely bound layers, which contain chains with restrained mobility. Formation of interfacial bonds between filler surface and surrounding polymer chains is a prerequisite for the two layers to build up. With the existence of interfacial bonds, an adsorption

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Figure 11.1 Polymer chains adsorbed onto the filler surface through interfacial bonding. Reprinted with permission from Y. Yao, G.Q. Lu, D. Boroyevich, K.D.T. Ngo, Effect of Al2O3 fibers on the high-temperature stability of silicone elastomer, Polymer 55(16) (2014) 4232-4240. Copyright 2014 Elsevier Ltd.

layer (the tightly bound layer in the model of Tsagaropoulos) containing chain units nearest to the filler surface is built up. In this layer, there are polymer chains with multiple polymer-filler contacts (solid lines) and that with only a few contacts (dashed lines) (Fig. 11.1). Due to multiple polymer-filler contacts, the mobility of polymer chains in the adsorption layer is strongly restricted [31].

11.3 Recent Progress in Dielectric Polymer Materials with High Thermal Stability 11.3.1

Dielectric Polymers with High Thermal Stability

11.3.1.1 Thermoplastic Polymers Thermally stable thermoplastics that are often used mainly include polycarbonate (PC), polyphenylene sulfide (PPS), PEN, polyetheretherketone (PEEK), modified PEEK, poly(tetrafluoroethylene-co-vinylidene difluoride-co-hexafluoropropylene), polyetherimide (PEI), and PI. Compared with any other materials, PIs are more desirable to be electrical insulation materials owing to their mechanical property and outstanding thermal and chemical stability [32,33]. They are thermally

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stable at temperatures exceeding 250 C, twice the operating temperature of most common dielectric materials. PIs exhibit insignificant weight loss up to 500 C in nitrogen, their Tdi in nitrogen is 542 C [34]. In addition, PIs maintain their dielectric stability even at a high temperature; however, most common polymers used as dielectric materials exhibit severe decrease in dielectric strength around 70 C due to phase transition of these polymers [17]. Note that wholly aromatic PIs have poor processing characteristics, such as high softening/melting temperatures and an insoluble nature in most of the common organic solvents, so aromatic PI structures are often modified without decreasing thermal stability for achieving commercial applications [17]. PEI is an amorphous engineering thermoplastic with special physical, chemical, electrical, and processing characteristics [35], of which the molecular structure comprises alternating aromatic imide and ether groups. Imide groups provide strength at high temperatures, while the flexible ether group linkages support relatively easy processing. Owing to its excellent mechanical strength, high thermal stability, good molding, and workability, PEN shows great potential applications in fabricating functional polymer composites for many cuttingedge fields, including military, automotive, and electronics industries [36]. Especially, PEN has a relatively high ε (about 5) due to the large number of polar nitrile groups on the aromatic ring, which enhances the polarizability of the polymer compared with that of polyarylether and makes it more desirable for use in polymer-based dielectrics [37,38]. PVDF and its copolymers have been widely investigated because of their high-ε, low DF, excellent thermal stability, exceptional piezo- and pyro-electric properties, good mechanical properties, and chemical resistance [39]. PVDF has five different crystalline phases [40], which show different dielectric properties, so controlling the aggregation structure is important for fabricating composites based on PVDF. Compared with ferroelectric polymers, nonferroelectric polymers have more stable dielectric properties over large frequency and temperature ranges, and such polymers, e.g., PEEK, poly(phthalazinone ether ketone) (PPEK), and polypropylene with polar side groups (PPOH copolymers) exhibit a very low DF and higher breakdown strengths [41]. 11.3.1.2 Thermosetting Polymers Compared with epoxy (EP) resins, bismaleimide (BD) belongs to the class of PI with higher temperature resistance (up to 290 C) and similar

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processability [42]. Its ε also decreases as the aging temperature increases, except for the 350 C case. An increase in DF is observed for aging temperatures above 150 C at frequencies above 105 Hz [43]. Cyanate ester (CE) resin is one of the most important thermosetting resins, which exhibits good mechanical properties, excellent moisture resistance, low volume shrinkage, and very low ε and DF over wide frequency and temperature ranges [44]. These unique properties make CE resins excellent candidates for many cutting-edge applications, such as high frequency printed circuit boards, radoms, advanced structural composites for aircrafts, and high temperature encapsulation [44].

11.3.2 High-ε Polymer Matrix Composites with High Thermal Stability Thermally stable polymers should be used to fabricate high-ε polymer matrix composites with high thermal stability. 11.3.2.1 High-ε Ceramic/Polymer Composites High-ε ceramic/polymer composites in early research were prepared by adding traditional ferroelectric ceramics with common shapes, such as Pb(Zr,Ti)O3 [45], BT [46], Pb(Mg1/3Nb2/3)O3-PbTiO3 [47] and CaCu3Ti4O12 (CCTO) [48]. The dielectric properties of these composites can be adjusted through changing types, surface nature, and loading of ceramic [49,50], and the surface treatment of ceramics is good for obtaining higher ε [51,52]. Recently, some special ceramics have been used for fabricating highε composites: (1) Ferroelectric ceramics with unique shapes, such as flower-like TiO2 [53], BT fiber [54], etc. (2) New ceramics with high-ε, such as FeTiNbO6 [6] and (Bi0.5Na0.5)(Fe0.5Nb0.5)O3 [55]. (3) Coreshell ceramics, e.g., core-shell structured [email protected] PEN nanoparticles [56]. (4) Environmentally-friendly ceramics. A leadfree ferroelectric/polymer 03 composite was prepared by using barium sodium titanate niobate compounds as ceramics; the 03 connected composite shows a ε of 32 and a DF of 0.04 at 1 kHz and 300K [57]. 11.3.2.2 High-ε Conductor/Polymer Composites Although ceramic particles with high-ε can increase the ε of polymer, a large ceramic loading (greater than 50 vol%) tends to deteriorate the mechanical properties of the material.

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Conductor/polymer composites overcome the above shortcomings of ceramic/polymer composites, and various conductors (metals, carbon conductors, polymer conductors) were used to prepare high-ε materials. However, the biggest problem with these composites is high DF. Therefore, one research focus is on high-ε conductor/polymer composites reducing DF without significantly affecting percolation threshold (fc). The thermal stability of a conductor/polymer composite may be lower or higher than the thermal stability of its polymer matrix; this is dependent on the effect of the presence of the conductor on the polymer structure.

11.3.2.2.1 Core-shell Structured Conductors Li et al. [58] prepared carbon coated silver nanowire (AgNW), which was then used to produce PI hybrid with high Tdi (. 500 C). The maximum ε of hybrid films is 126 at 102 Hz, 39 times higher than that of PI, while the DF of that still remained at a low value (c. 0.12).

11.3.2.2.2 Doped Carbon Nanotubes Bose’s group synthesized various nitrogen-doped (N-doped) multiwalled carbon nanotubes (MWCNTs) by varying the synthesis temperature (650 C, 750 C, and 850 C) [59]. Nitrogen doping is adopted to generate numerous polarizable centers in MWCNTs, which has a significant impact on structure and thermal and electrical properties of MWCNTs. The nanocomposites containing self-polarizable MWCNTs show significantly low DF, exhibiting good charge storage ability at a given concentration of MWCNTs.

11.3.2.2.3 Complexes PI-copper complexes (PICuCs) were prepared, which display a highε (c. 133, at 100 Hz), low DF (, 0.08, from 100 Hz to 1000 kHz) and high energy density (8.3111.39 J/cm3). The dielectric properties of PICuCs can be well tuned by varying the amount of Cu complexing with PIs. Moreover, PICuCs show high Tdi (beyond 560 C) and high Tg (309335 C) as well as outstanding mechanical properties, making them promising candidates for polymer film capacitors [60]. Similar dielectric and thermal properties were also found in PI-Yb complexes (PIYbCs) [61]. After complexing with different amounts of

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Yb, Tdi and char yield of PIYbCs increase. The excellent thermal properties of PIYbCs could be attributed to the large amount of rigid bipyrimidine units and benzene rings. 11.3.2.3 Functionalized Conductors Good dispersion of conductors in polymers is important to get composites with desirable dielectric properties. Compared with the chemical (covalent) method [6264], the physical (noncovalent) [6567] technique attracts tremendous interest because noncovalent modifications usually do not destroy the conductor’s electronic structure [68], and thus the excellent properties of conductors can be preserved. Since 2014, several papers have been published on modifying graphene by ionic liquids (IL) due to the unique advantages of IL, such as high ionic conductivity at room temperature, strong polarity, low toxicity, and high chemical and thermal stability [69]. Ding’s group [39] prepared ionic liquid coated GO (GIL) through physical cation-π interaction, and then fabricated PVDF/GIL composites with an enlarged fc (1.86 vol%). Compared with PVDF/graphene composites, PVDF/GIL composites exhibit higher ε that originates from the polar β crystals of PVDF and a remarkable MaxwellWagnerSillars (MWS) effect between PVDF and GIL; moreover, the nonconducting organic segments of the ionic liquid is responsible to the low DF and low conductivity in the PVDF/GIL composites. Sustainability urgently asks for low DF and low fc when developing high-ε conductor/polymer composites. Our group [70] prepared unique hybridized graphene (PIL-TrGO) through decorating epoxy functionalized IL on the surface of thermally-reduced graphene oxide (TrGO) with πcation-π interaction, followed by in-situ polymerization of IL. And then a series of PILTrGO/CE composites were prepared. A large amount of epoxy groups on PIL guarantees good dispersion of the hybridized graphene in CE resin matrix, thus providing the base for transferring outstanding electrical properties of graphene to the composites. The ε and DF at 100 Hz of PILTrGO/CE composites are about 13 and 0.57 times that of TrGO/CE composites, respectively, while the fc of PILTrGO/CE composites is still as low as 0.94 wt%. The mechanism behind this is that PILTrGO/CE composites possess more microcapacitor structures than TrGO/CE composites; moreover, the cationanion charge layers on TrGO surfaces enhance the MWS polarization between PILTrGO hybrid and CE matrix, and then greatly

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increase the ε of composites. PIL coated on graphene surfaces acts as electron insulative layers and thus decreases DF induced by the leakage current between conductive carbon layers. 11.3.2.4 Two Kinds of Conductors or Semiconductors Expanded graphites (EGs) were blended with MWCNTs in a big weight ratio to prepare low-cost EG-MWCNT/CE composites, which have much better dielectric properties than both traditional EG/CE and MWCNT/CE composites [71,72]. Fabricating high-ε conductor/polymer composites with low DF and fc is still a challenge. Hybridized conductor with a “sandwich” structure ([email protected]) and active groups was prepared by introducing polyaniline coated CNT ([email protected]) on the surface of rGO through electrostatic and π-π conjugate forces. And [email protected] hybrids with different loadings of [email protected] were introduced into the EP resin to prepare a series of [email protected]/EP composites [133]. The ε values at 100 Hz of [email protected]/0.75rGO/EP composites are as low as 1020, whereas that of the [email protected]/EP composite with 0.75 wt% [email protected] is as high as 210. Meanwhile, the DF at 100 Hz of [email protected]/EP composite is only 17% of that of 0.75rGO/EP, indicating that the dielectric behavior of [email protected]/EP composites is not originated from a simple addition of basic components, but has an obvious synergistic effect. Chaudhuri’s group prepared PVDF composites filled with Na and Ti codoped NiO (NaTNO) semiconducting nanoparticles. The composites exhibit excellent ferroelectric behavior and tensile strength with a low fc. Specifically, the ε values of composite films are as high 600750, about 6075 times of that of PVDF, while the DF is lower than 0.18 at 1 kHz; besides, the composite films have high dielectric breakdown voltage ( . 140 kV/mm) and saturation polarization (B10 μC/cm2) near fc, showing great potential for capacitor applications in devices [73]. Li’s group [74] prepared PI matrix composites based on solvothermally reduced graphene oxide (STRG). When the temperature is raised from room temperature to 250 C, the ε of PI/STRG composite increases from 33 to 80 due to the promoted polarization of the STRG, while that of PI remains almost constant. In addition, the temperature at 95% weight loss of PI/STRG is 586 C, higher than that of PI (580 C). Jiang et al. [75] fabricated ternary composites through introducing an intermediate CuPc layer on acidified graphite nanosheets (a-GNs), and then embodying a-GNs/CuPc in sulfonated poly(aryl ether ketone)

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(SPAEK). The [email protected]/SPAEK composites have significantly improved dielectric properties compared with a-GNs/SPAEK because of the good dispersion of [email protected] in the SPAEK matrix and effectively suppressed mobility of free charge carriers. Development of high-ε composites with low DF and good flame retardancy is an interesting subject. We prepared CE composites based on multifunctional CNTs (EPHSi-g-MWCNTs) coated with phosphaphenanthrene terminated hyperbranched polysiloxane (EPHSi) [75]. The 2.5EPHSi-g-MWCNT/CE composite with 2.5 wt% EPHSi-g-MWCNTs has the highest ε, which is about 1.4 times of that of 0.7MWCNT/CE composite that has the biggest value among MWCNT/CE composites, while interestingly, the DF at 100 Hz of 2.5EPHSi-g-MWCNT/CE composite is only 5.8 3 1025 times of that of 0.7MWCNT/CE.

11.3.2.5 High-ε Conductor/Ceramic/Polymer Composites These composites were prepared using conductor/ceramic blend [76] or conductor/ceramic with a core-shell structure [77]. Compared with ceramic/polymer composites, the aim of introducing conductors is improving the processing feature through reducing the ceramic loadings, and then it is possible to prepare flexible materials [78,79]; moreover, the composites have higher ε [76]. For conductor/polymer composites, there are two conditions. On the one hand, the presence of ceramics aims at reducing the DF; therefore, ceramic was coated on the surface of the conductor to avoid the contact of conductors [77]. On the other hand, the target may be decreasing fc, so ceramic appears to be a core, which was coated with the conductor. One typical example is that CNT and BT hybrid (H-CNT-BT) with a special core 2 shell structure was used to prepare PVDF composites with extremely low fc and low DF [80]. The good dispersion of CNTs in PVDF is achieved by the double-dispersion mechanism; the first dispersion is carried by BT, and the second one is detached from BT particles. The interfacial action between the electric conductor and the ceramic has a remarkable effect on the dielectric properties of composites. Good interfacial interaction not only consolidates the influence of the physical shear and improves the dispersion of fillers, but also increases the interfacial adhesion among fillers as well as that between fillers and the matrix, and consequently, the resultant composites have much higher ε and lower DF; however, fc tends to be enlarged [81].

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Nano CCTOFe3O4/PI hybrid films were prepared under an external magnetic field. The inclusion of conductive Fe3O4 nanoparticles markedly enhances the interface polarization and spacecharge polarization of nano CCTOFe3O4/PI hybrid films compared with that of nano CCTO/PI films. Meanwhile, the structure of nano CCTOFe3O4 particles effectively suppresses the formation of conductive networks. The enhanced dielectric properties of the hybrid films originate from the large interfacial area and interfacial polarization induced by the external magnetic field [82]. 11.3.2.6 High-ε All-organic Dielectric Composites Jung et al. [83] prepared PI composites with core-shell polypyrrole (PPy). For the composite with 15 wt% PPy in [email protected], its ε is over 100, while the electrical conductivity is 1028 S/cm. As the loading of PPy in the dielectric layer is over 30 wt%, so the composite shows ultrahigh ε (487) and low DF (, 0.18). All-organic polyconjugated ladder structure (PcLS)/PI composite was produced, in which the PcLS was derived from polyacrylonitrile (PAN). The PcLS/PI composite not only presents high dielectric performances of high-ε, low DF, high electrical breakdown strength and high energy density, but also has excellent thermal properties [84]. 11.3.2.7 High-ε Polymer Composites With Novel Space Structures Recently, many attempts have been made to develop high-ε and low DF composites through building a special macrostructure (configuration) based on traditional polymers, ceramics, and/or conductors. The most attractive merit of this method is exploring the possibility of fabricating composites with desirable performances with available raw materials, especially noting the fact that it usually takes a long time to develop and commercialize new raw materials.

11.3.2.7.1 Composites With Layered Structure The purpose of building a multilayer structure is to suppress the generation of the leakage current by enhancing the polarization of the space charge (SCP), and thereby obtaining high ε and low D F .

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Shen’s group [85] prepared topological-structure modulated polymer nanocomposites based on PVDF (or PVDF-TrFE-CFE) and [email protected] nanofibers. Modulation of topological structure induces substantial redistribution of the local electric field among the constituent layers, leading to enhanced electric polarization at a low electric field and increased breakdown strength. These synergistic effects provide an ultrahigh energy density of ca. 12.5 J/cm3 and high discharge efficiency of about 70% at 350 kV/mm. The merit of high energy density at a low electric field endows the nanocomposites with critical significance to make dielectric nanocomposites as viable energy storage devices. PVDF-based multilayered dielectrics containing alternating layers of confined carbon black (CB) were fabricated using a layermultiplying extrusion. The ε of a 256-layer specimen is beyond 60, almost three times the predicted value. Furthermore, the ε of multilayer composites becomes less sensitive to CB loading. Because of the presence of sandwiched insulating PVDF layers, the breakdown strength of a 256-layer specimen is 2.4 MV/m, greatly larger than that of a conventional composite with similar CB concentration and conductivity [86]. Amino-modified-CNT (NH2-MWNT)/PI flexile composite films with a sandwich structure were prepared through step-by-step casting, in which a dielectric layer (NH2-MWNT/PI composites) intercalated between bottom and top PI layers. The sandwich composite films show a high-ε and ultralow DF, and the ε values of the composite films are almost frequency independent between 1 and 1000 kHz. When the NH2-MWNT content of the mid-layer is 10 wt%, the multilayer composite film (P-10-P) shows the highest ε (31.3) at 1 kHz, while the DF of P-10-P is only 0.0016. Besides, the obtained multilayer composite films have high breakdown strength and maximum energy storage density. More importantly, the sandwich structured multilayer composite films can withstand severely high temperatures [87]. We fabricated symmetric and asymmetric layered-structure composites with high-ε and low DF through simultaneously enhancing polarity of space charge and restraining the production of leakage current. Specifically, a asymmetric double-layered material was produced by forming a polyethylene (PE) layer with a thickness of 7 μm [88] or EG/ CE composite layer [89] on MWCNT/CE composite. The interface between the two layers not only brings strong SCP, leading to very high-ε, but also hinders the production of the conduction loss, resulting in significantly reduced DF (Fig. 11.2) [89].

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Figure 11.2 Schematic distributions of MWCNTs, EGs, and space polarized charges in 0.5MWCNT/CE5.0EG/CE composites: (A) pMWCNT , pc, (B) pMWCNT  pc, (C) pMWCNT . pc, (D) interfacial polarized charges between two layers. Reprinted with permission from B.H. Wang, Y.C. Jiao, A.J. Gu, G.Z. Liang, L. Yuan, Dielectric properties and mechanism of composites by superposing expanded graphite/cyanate ester layer with carbon nanotube/ cyanate ester layer, Compos. Sci. Technol. 91 (2014) 8-15. Copyright 2013 Elsevier Ltd.

11.3.2.7.2 Three-dimensional Framework of Ceramics For ceramic-particles filled polymer composite (CPC), very a high content of ceramic (3050 vol%) is generally required to obtain the desired high-ε. Such high ceramic loading often brings deterioration in the preparation process and increase of leakage current [87,90,91]. In addition, the ε values of CPCs reported so far are usually about 4050, and the highest value is less than 80 (100 Hz) [90,9295]. To avoid these disadvantages of available CPCs, our group designed and prepared a new type of CE resin composite based on BT foam (BTF) with a three-dimensional open foam structure [96]. With 33.5 vol % BTF, the resultant BTF/CE composite has a high-ε (141.3 at 100 Hz). Yu et al. prepared a three-dimensional porous BT (3D-BT) with lignocelluloses; the ε of 3D-BT/EP composite reaches 200 when the BT content is 30 vol% [97].

11.3.2.7.3 Orientation of Fillers Freeze casting combined with a vacuum assisted infiltration process was used to fabricate Ni0.5Ti0.5NbO4/CE composites with special structure, where both the Ni0.5Ti0.5NbO4 and the CE resin phases are continuous directional distribution and parallel to the electric field [98]. This

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is very close to the parallel model of dielectrics. With the same Ni0.5Ti0.5NbO4 content, the composite with directional distribution shows much higher ε and much lower DF than the composite with homogeneous distribution in which Ni0.5Ti0.5NbO4 particles are homogeneously distributed.

11.3.2.7.4

Selective Dispersion in Different Polymer Phases

The distribution and contents of conductive fillers have a decisive influence on the dielectric properties of filler/polymer composites. For the same components, composites with different dielectric properties can be fabricated using selectively dispersing conductors in different polymer phases. Cocontinuous structure rather than seaisland structure is often used as the polymer matrix to prepare high-ε composites owing to the double percolation phenomenon [99]. A series of composites (0.4MWCNT/PEI/BD) with different morphologies (sea-island, cocontinuous phase, and phase inversion) was fabricated through embodying a fixed loading (0.4 wt%) of MWCNTs into an incompatible PEI/BD system [100]. MWCNTs prefer to selectively distribute in the BD phase, and tend to enrich around the PEI dense zone and arrange normally to the radius of the PEI sphere zone, so the morphology of MWCNTs and thus dielectric properties of composites can be facilely controlled by adjusting the PEI content (Fig. 11.3). The ε and DF of 0.4MWCNT/PEI/BD composite with 10 wt % PEI, of which the morphology is sea-island, are about 4.5 and 0.1 times of those of 0.4MWCNT/BD composite, respectively, overcoming the critical problem of available conductor/polymer composites. On the other hand, with the aid of the Haake instrument, MWCNTs were stably and uniformly dispersed in PEI continuous phase (Fig. 11.4) [101]. The resultant h-MWCNT/20PEI/BD composite has remarkably higher ε and energy storage density as well as lower DF compared with MWCNT/ 20PEI/BD composites through traditional blending; besides, the fc of the former is as low as 0.35 wt%, only 60% of that of the latter. MWCNT/phenolphthaleinpoly(ether sulfone) (cPES)/CE composites were also prepared through melting blending (Fig. 11.5), of which fc is as low as 0.89 wt%, only about 0.25 times of that of MWCNT/CE. Besides, MWCNT/CE/cPES composites exhibit greatly improved ε, much larger breakdown strength, and greatly decreased DF [102].

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Figure 11.3 Schematic morphologies of different composites: (A) 0.4MWCNT/ BD, (B) 0.4MWCNT/5PEI/BD and 0.4MWCNT/10PEI/BD, (C) 0.4MWCNT/15PEI/ BD, and (D) 0.4MWCNT/20PEI/BD). Reprinted with permission from Y.C. Jiao, L. Yuan, G.Z. Liang, A.J. Gu, Facile preparation and origin of high-k carbon nanotube/poly(Ether Imide)/bismaleimide composites through controlling the location and distribution of carbon nanotubes, J Phys. Chem. C 118(41) (2014) 24091-24101. Copyright 2014 American Chemical Society.

Figure 11.4 Schematic diagrams of solution mixing and Haake melt-mixing. Reprinted with permission from Y.C. Jiao, L. Yuan, G.Z. Liang, A.J. Gu, Dispersing carbon nanotubes in the unfavorable phase of an immiscible reversephase blend with Haake instrument to fabricate high-k nanocomposites with extremely low dielectric loss and percolation threshold, Chem. Eng. J. 285 (2016) 650-659. Copyright 2015 Elsevier B.V.

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Figure 11.5 Schematic change in microstructure during the preparation process of MWCNT/CE/cPES composites. Reprinted with permission from L. Zhao, L. Yuan, G.Z. Liang, A.J. Gu, Significantly enhanced dielectric properties and energy storage density for high-k cyanate ester nanocomposites through building good dispersion of pristine carbon nanotubes in a matrix based on in situ noncovalent interaction with phenolphthalein poly(ether sulfone), RSC Adv. 5(115) (2015) 94635-94644. Copyright 2015 Royal Society of Chemistry.

11.3.2.7.5

Gradient Distribution of Functional Fillers

Permittivity gradient (ε-G) composites with extremely low DF based on surface treated MWCNTs (eMCNTs) and CE resin were developed by the gravity sedimentation method. The surface treatment of MWCNTs is necessary to form ε-G composites because the good dispersion of nanotubes in the resin matrix as well as the attractive interfacial adhesion between nanotubes and the matrix are key aspects for guaranteeing the gradient distribution of the concentration of nanotubes in the composites. With the same content of nanotubes, the gradient composites have similar ε but remarkably lower DF and 25% lower fc than traditional eMCNT/CE composites, where eMCNTs are uniformly dispersed in the matrix [103]. A general method that can simultaneously increase the dielectric constant by ten times and decrease the dielectric loss by five orders of magnitude was setup by building a three-layer material consisting of two MWCNT/CE composite layers and one polyethylene (PE) thin film through layer-by-layer casting. The dispersion and distribution of MWCNTs in CE resin was precisely controllable through adjusting the prepolymerization time of MWCNTs and CE blends [104].

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11.4 Relationship Between Thermal Stability and Dielectric Properties of Dielectric Polymer Materials 11.4.1 Influence of Thermal Stability on Dielectric Constant There are internal and external factors that determine the dielectric properties of polymers and related composites. Chemical and physical structures are internal factors, while temperature (especially Tg) and frequency are external ones. The nature of the influence of these factors is that dielectric properties originate from electronic, ionic, dipole, and interfacial polarizations in the applied electric field. The ε of a material has been determined by the ability of the polarizable units to orient as fast as in the direction of the applied AC signal. The occurrence of electronic and ionic polarizations is very fast, while it takes a long time for dipole polarization, so dielectric behaviors often show frequency-dependent and temperature-dependent polarization mechanisms [105]. The ε of most dielectric materials decreases as frequency increases because dipoles in macromolecules tend to orient themselves in the direction of the external alternating electric field. In the low-frequency region, the degree of interfacial polarization is very high compared to that in the high-frequency range. However, in the high-frequency range, the dipole will hardly be able to orient itself in the direction of the applied field due to less time. At high frequencies, the occurrence of periodic alternation of the applied electric field results in the drastic reduction in the diffusion of dipoles in the field direction and space charge accumulation. Dipole molecules almost cannot orient themselves at the temperature region below Tg because the structure is glassy at these temperatures [106], and the thermal energy absorbed by the dipoles is small, so only a restricted number of dipoles can rotate within a small angle. When the temperature increases, the dipoles get more thermal energy and then the dipole orientation is accelerated. The side groups of side-chain macromolecules are known to be the most sensitive to temperature change, so β-relaxation is generally associated with local noncooperative motion of side groups or parts of them. When the temperature falls within the range of Tg, both ε and DF suddenly increase because the chain mobility of the polymer increases, and thus polar groups can

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move in response to the applied electric field [107]. This kind of relaxation is treated as α-relaxation [108]. When the temperature further increases, the thermal oscillation is intensified, and the degree of orientation is diminished [109]; besides, the pronounced effect of thermal expansion of the matrix leads to some decrease in charge density (net charge/volume) [21], as a result, the ε decreases to some degree. The relaxation in cross-linked polymers mainly depends upon the polymerfiller interactions, which leads to the formation of an interphase. The thickness of the interphase is inversely proportional to the interfacial tension between viscoelastic polymer and solid filler phases. At low frequencies, the ε of the samples reaches a higher value with an increase in temperature. It is also observed that the ε value is gradually decreased with an increase in frequency at all filler loadings throughout the testing temperatures [110]. 0 The variation of the real part of ε (ε ) and the imaginary part of ε (εv ) with temperature is different for polar and nonpolar polymers. For 0 nonpolar polymers, their DK and DvK are independent of temperature; however, materials having permanent dipoles show significant variation of ε with temperature [111], which is due to the following two effects on the polarization of dipolar. One effect is weakening intermolecular forces and thereby increasing the orientation vibration, while the other effect is increasing thermal mobility, and thus the orientation vibration is strongly affected [112]. Generally, the dipoles move randomly without any external electric field, but when an electric field is applied, these dipoles undergo polarization (alignment with the field) [21]. To study the temperature dependence of the dielectric properties, a temperature coefficient (Temp-Coef) representing the maximum derivation of ε from its median value is defined as shown in Eq. (11.1) [113]. Temp 2 Coef 5

MaxðεÞ 2 MinðεÞ MaxðεÞ 1 MinðεÞ

(11.1)

where Max(ε) and Min(ε) are the maximum and minimum ε at a constant frequency over the temperature range, respectively.

11.4.2

Influence of Thermal Stability on Dielectric Loss

The DF is defined as the ratio (or angle in a complex plane) of the loss reaction to the electric field in the curl equation to the lossless reaction, as shown in Eq. (11.2), so the DF involves polarization dependent loss and leakage loss.

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At room temperature, the polarization dependent loss plays the domain role. As the temperature increases, DF mainly originates from the leakage loss because the orientation of dipoles is accelerated [114]. DF 5

ωεv 1 γ ωε0

(11.2)

where ε0 represents the familiar lossless permittivity given by the product of the free space permittivity; εv is the imaginary component of permittivity attributed to bound charge and dipole relaxation phenomena; ω is frequency, and γ is conductivity. In polymers and their composite, DF is a function of electrical conductivity, which depends on the number of charge carrier mobility. The main affecting factors are polarity of polymer, polymer morphology [115], degree of crystallinity of polymer, and molecular weight, etc. The DF values of polar and nonpolar polymers exhibit different dependences on temperature. The ε values of polar polymers increase quickly at the temperature above Tg due to the mobility of polymer chains; however, the chain relaxation of nonpolar polymers such as polypropylene and poly(m-phenylene) (PMP) do not lead to an increase in DF. If nonpolar polymers contain residual assistant agents, e.g., catalysts, then the mobility above Tg of the host polymer may lead to significantly higher DF, particularly at low frequencies.

11.4.3 Influence of Thermal Stability on Breakdown Strength The electrical breakdown field strength is an important parameter for selecting appropriate dielectric materials to avoid short circuit, especially under operation at room temperature [116]. Only a few literatures reported the influence of thermal stability on breakdown strength. For BT/PI [46] or BT/PVDF [117] composites, they have decreased breakdown strengths compared to PI or PVDF because of the lower dielectric strength of BT. Breakdown strengths decrease with increasing temperature for all BT/PI and BT/ PVDF nanocomposites with different loadings of BT (1020 vol%), and this tread is enhanced as the loading of BT increases. This is because more ultrafine ceramic fillers bring more surface effect and internal stresses.

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11.5 Strategies of Achieving High Thermal Stability of Dielectric Polymer Materials 11.5.1

Modification of Polymers

To achieve high thermal stability, building structure with heterocyclic rings and a high level of aromatic characters are effective methods; however, this also make the polymers exhibit poor processing characteristics, great brittleness, and big internal stress. To overcome the above problems, researchers used a physical method by mixing the toughening modifiers with thermosets directly. The thermoset-thermoplastic interpenetrating polymer network (IPN) is a blend modification technology, exhibiting characteristic properties, such as synergistic effect, interface interpenetrating, and forced mutual capacitance; in addition, other properties of IPN materials are more outstanding than the single component [118]. The Liu group prepared 2,2-bis[4-(3,4)-dicyanophenoxy phenyl]propane (BAPh)/PEN-OH systems with IPN structure through chemical reaction, which show good processability and excellent thermal stabilities due to flexible PEN-OH chains, BAPh, and heterocyclic rings wounded around the framework of each other as well as a heterocyclic ring produced from the polymerization between BAPh and PEN-OH. Compared with BAPh, the addition of PEN-OH can increase the ε to some degree; however, the ε shows little dependence on various contents of PEN-OH. On the other hand, BAPh/PEN-OH polymers have lower ε than PEN-OH, this may be attributed to the occurrence of polymerization reaction and the formation of a nonpolar phthalocyanine ring and symmetrical triazine ring [119]. A series of thermally stable poly(arylene ether ketone)s (PAEKs) (Fig. 11.6) bearing the benzimidazole structure in the main chains were synthesized, which show high Tg (157319 C) and excellent thermal stability (Tdi . 438 C in air) as well as low ε at 25 C (less than 2.66, 0.150 kHz) [120]. Tong introduced benzocyclobutene (BCB) moieties into the backbone of PMPs to produce thermally cross-linked resin. Cured PMP-BCB shows very high thermostability, and has very high char yield at 1000 C in N2 [121]. Cyclodextrin microsphere (M-CDP) with high thermal stability and low content of hydroxyl groups was synthesized, which was then used to modify CE resin. Although M-CDP has lower Tdi than CE resin, modified CE resins with suitable contents of M-CDP have higher Tdi

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Figure 11.6 Synthesis of poly(arylene ether ketone-co-x%benzimidazole). Reprinted with permission from J. Liao, Y. Chu, J. Wang, M. Zhou, Y. Cao, Dielectric and gas transport properties of the films of thermally stable poly (arylene ether ketone)s containing content-tunable benzimidazole moiety, J. Appl. Polym. Sci. 132(3) (2015) 41289. Copyright 2014 Wiley Periodicals, Inc.

values than CE resin. This is because the active groups of M-CDP increase the concentration of triazine rings formed, and also provide chemical bonding between M-CDP and CE resin [11]. The thermooxidative stability of PEI is improved by blending with poly-(vinylidene fluoride-co-hexafluoro propylene) [122], which is due to the presence of strong C 2 F bonds. To improve the thermal stability of polyurethanes, chemical modification with heterocyclic groups through copolymerization or blending with thermally stable polymers has been suggested. It has been proven that incorporation of imide, amide, or ester units into the polyurethane chain significantly improves the thermal stability. Especially, the imide group has superior thermal stability over the amide and ester groups [123].

11.5.2

Syntheses of New Dielectric Polymers

Through capping phthalonitrile groups at the ends of linear PEN, the obtained self-cross-linked PEN film shows excellent thermal resistance (Tdi 5 524.3 C in N2 atmosphere, 533.6 C in O2 atmosphere), stable ε, and low DF over wide frequency (100200 kHz) and temperature (25380 C) ranges. In addition, the cross-linked PEN film has

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excellent reversibility and robustness of dielectric properties by cycling heating and cooling from room temperature to 400 C up to ten times or heating at 300 C for 12 hours. Therefore, the cross-linked PEN film would be a promising candidate for high performance film capacitors and electronic devices used at high temperature [124]. Iron phthaocyanine (FePc) polymer (Fig. 11.7) was prepared via polymerizing phthalonitrile with ferrous chloride. The cured FePc polymer has high Tdi (508 C) and high char yield (77.8 wt% at 800 C). The formation of cross-linked network structure effectively facilitates the improvement in thermal stability of FePc polymer. In addition, the dramatic transition of conductivity, dielectric, and magnetic properties appears when the annealing temperature is 550 C because of the formation of the turbostratic carbon, α-Fe phase, and cemetite, and thereby enhancing electrical conductivity and magnetic properties [125].

Figure 11.7 Structure of FePc polymer. Reprinted with permission from Z. Wang, K. Jia, X. Liu, Effect of elevated annealing temperature on electrical conductivity and magnetic properties of iron phthalocyanine polymer, J. Polym. Res. 23(3) (2016) 48. Copyright 2016 Springer Science 1 Business Media.

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The key to enhance high temperature performance is to incorporate thermostable main-chains. Inorganic or inorganic/organic hybrid mainchains could be promising candidates. BCB/vinylphenyl-introduced polycarbosilanes (PVBCS) were synthesized by H2PtCl6 catalyzed ringopening copolymerization of 4-(1-methylsilacyclobutyl) BCB and 1methyl-1-(4-vinylphenyl) silacyclobutan. UV and thermally cured polycarbosilanes exhibit high temperature performance (Tdi 5 473 C). The high temperature performance could be attributed to the main-chain structure of polycarbosilane and the BCB-based cross-linked structure [126]. Three co-poly(arylene ether)s (PAE) with very high thermal stability in air (Tdi 5 406421 C) and high Tg (215228 C) were synthesized by incorporating cardo fluorene moiety and bulky trifluoromethyl pendant groups in the polymer backbone as well as building a higher degree of aromaticity [127].

11.5.3

Design of Novel Types of Functional Fillers

Hydrangea-like flowers or clusters comprising MoS2 nanosheets were synthesized to prepare composites with significantly improved electrical energy storage capability; even the loading of MoS2 superstructures is very low. The enhanced energy density of composites is mainly ascribed to the increase of ε induced by the presence of MoS2 superstructures [128]. Similarly, PVDF composites filled with hydrangea-like zinc oxide (ZnO) superstructures (Fig. 11.8) were prepared [129]. Hydrangea-like ZnO shows a marginal influence on the microstructure of PVDF, but has significant enhancement effects on thermal conductivity, thermal stability, and ε of the composites as well as slightly lower breakdown strength. Compared with commercial ZnO nanoparticles, hydrangea-like ZnO superstructures endow composites with much higher thermal conductivity and ε due to the formation of percolation-like structure in the hydrangea-like ZnO composites. The enhancement of thermal stability of the composites may be ascribed to two factors. One is that ZnO acts as a barrier to retarding the formation and escape of volatile byproducts during the pyrolysis; the other is that the thermal motion of PVDF segments near ZnO surfaces is restricted due to physical interlock and chemical interaction. The former occurs when the polymer chains are bound with the ZnO superstructure, whereas the latter originates from the existence of hydrogen bonding between the PVDF matrix and the hydroxide groups on ZnO surfaces.

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Figure 11.8 SEM images of (A and B) the hydrangea-like ZnO, (C) a hydrangea flower, and (D) commercial ZnO nanoparticles. Reprinted with permission from L. Fang, W. Wu, X. Huang, J. He, P. Jiang, Hydrangea-like zinc oxide superstructures for ferroelectric polymer composites with high thermal conductivity and high dielectric constant, Compos. Sci. Technol. 107 (2015) 67-74. Copyright 2014 Elsevier Ltd.

P(VDF-HFP)-based composites incorporated with flower-like TiO2 (F-TiO2) particles were developed (Fig. 11.9) [53]. F-TiO2 filled composites show not only much greater ε, but also similar breakdown strengths as the composites with commercial TiO2 (C-TiO2). For instance, the ε of P(VDF-HFP) composite filled with 20 vol% F-TiO2 reaches 83.1 at 100 Hz, in contrast to 43.4 for the composite filled with 20 vol% C-TiO2 and 11.3 for P(VDF-HFP). Meanwhile, the F-TiO2/P (VDF-HFP) composites can withstand a high electric field over 50 MV/ m. The improvement in ε could be attributed to the enhancement of MWS polarization, which originates from the large amount of charge carriers accumulated at the sophisticated interfaces between F-TiO2 particles and the P(VDF-HFP) matrix. Khastgir et al. [21] prepared flexible BT/poly(dimethylsiloxane) (PDMS) nanocomposites, of which BT has multipod-like structure (Fig. 11.10). The thermal stability improves with the increase in the

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Figure 11.9 SEM image of flower-like TiO2 and frequency dependence of ε. Reprinted with permission from N. Xu, L. Hu, Q. Zhang, X. Xiao, H. Yang, E. Yu, Significantly enhanced dielectric performance of Poly(vinylidene fluoride-cohexafluoropylene)-based composites filled with hierarchical flower-like TiO2 particles, ACS Appl. Mater. Interfaces 7(49) (2015) 27373-27381. Copyright 2015 American Chemical Society.

Figure 11.10 FESEM image of multipod-like BT. Reprinted with permission from S. Nayak, T.K. Chaki, D. Khastgir, Development of Flexible Piezoelectric Poly (dimethylsiloxane)BaTiO3 Nanocomposites for Electrical Energy Harvesting, Ind. Eng. Chem. Res. 53(39) (2014) 14982-14992. Copyright 2014 American Chemical Society.

loading of BT, and the maximum rate of mass loss (Tmax) reaches the highest value at 23.08 wt% of BT. This is the combined result of two competitive factors. One is that homogeneously distributed BT particles in the matrix act as a heat sink, the other is that the photocatalytic activity of BT surpasses the thermal stability. The thermal decomposition temperatures of PI composite films based with BT-fiber or BT nanoparticle were compared. It is found that the film with BT fiber has higher thermal stability than that with a BT particle. In addition, the decomposition temperature and char yield at

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750 C of PI/BT-fiber composite films shift to higher values with increasing calcination temperature because BT fibers calcined at a higher temperature have a larger specific surface area, thus improving interfacial fiber/matrix interaction with PI; this is beneficial to restrict the thermal motion of PI chains [54], and consequently BT-fiber/PI composites exhibit improved thermal stability. Specifically, the temperature at Tmax is as high as 628 C when the calcination temperature of BT fibers is 1000 C. NiO nanoparticles (NiO NPs) or amorphous carbon doped NiO nanoparticles (C-NiO NCs) were used to prepare PVDF composite films [130]. Composite films show about 1830 C higher Tdi than PVDF. Note that a significant increase in ε appears in the three phase C-NiO/PVDF system compared to the two phase NiO/PVDF system. For C-NiO/PVDF with 20 wt% C-NiO, its ε is as large as 317.4 at 20 Hz with relatively low DF and good flexibility. These results are thought to be attributed to interfacial polarization at the interface between NiO/C-NiO and PVDF, evolution of conductive network, and formation of microcapacitive structure in these modified PVDF thin films. Nano-Ba(Fe0.5Nb0.5)O3 (BFN) with giant ε and better thermal stability was used to prepare PVDF composite with enhanced ε [131].

11.5.4

Improvement of Interface Characteristics

The addition of fillers into a polymer matrix may modify its properties by causing interphase interactions or the formation of an interface at the boundary between the polymer matrix and filler particles [132]. The interfacial bonds formed between the filler surface and the polymer matrix can be either physical (e.g., van der Waals force and hydrogen bonds) or chemical (e.g., covalent bonds) in nature [31]. Chemical bonding is effective to build good interfacial nature; however, this changes the structure and performance of fillers [133]. This problem does not appear in physical bonding, but it has a special requirement on the structures of both fillers and polymers; i.e., fillers and polymers should have the ability to form physical action.

11.5.5

Others

Any structural features that reduce free volume or segmental motion will increase Tg, so the irradiation technique has been used to improve Tg of polymers. The cross-linking behavior of cinnamyl double bonds

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in side chains restricts segmental motion by forming a threedimensional network and thus enhanced Tg. Compared with polymer film without irradiation, the Tg of the film irradiated for 6 minutes is increased by about 22K [134].

11.6 Main Processes for Producing Dielectric Polymer Materials with High Thermal Stability Several ways to fabricate nanocomposites are solution mixing, in-situ polymerization, and melt processing.

11.6.1

Solid Phase Process

For this process, no solvent is used; instead, melt-blending, melt extrusion (screw extruder), or a combination thereof is used. Biocompatible ternary nanocomposites based on PEEK/PEI blends and bioactive TiO2 nanoparticles were fabricated via ultrasonication, followed by conventional melt-blending without using nanoparticle surface treatments or polymer functionalization [135]. Binary composite based on the clay/PVDF system was prepared by screw extrusion and molding, successively [136]. Typically, approximately 10 wt% clay powders and 90 wt% of PVDF pellets were fed into a twin screw extruder. The temperature of operation was adjusted to 443K, 453K, 463K, and 473K from the feed zone to the die zone of the screw, with a rotational screw speed of 60 rpm. These extruded pellets were shaped by using a compression molding machine at 473K under an applied constant force of 15 tons for 20 minutes. The composite film was cooled naturally to room temperature. PVDF composites based on CCTO and La doped CCTO (LaCCTO) with giant ε were prepared through melt extrusion process [137]. High-ε poly(ether sulfone) wrapped MWCNT/PEEK composite films with superior thermal properties were prepared through meltblending and subsequent melt extrusion [138]. Their Tdi values are higher than 575 C; the mechanism behind it not only contains high conductivity of MWCNTs, the uniform dispersion of the wrapped MWCNTs in PEEK matrix, and strong interfacial adhesion between wrapped MWCNTs and PEEK, but also includes the melt extrusion employed.

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Liquid Phase Process

Liquid phase process is suitable for the polymer that is a liquid, or a solvent is used; this is also the most common method for fabricating polymer nanocomposites because good dispersion can be obtained by the selection of suitable solvent and treatment of the nanocomposites. At the same time, further interfacial modification can be easily achieved [139]. Polymer matrix nanocomposites based on PC and nanosized-cubic/ tetragonal phases of BT were fabricated using a solution method followed by hot pressing [22]. Yang et al. [124] prepared cross-linked flexible PEN film with ultrahigh thermal stability. The film is obtained by solution-casting of PEN terminated phthalonitrile (PEN-Ph) combined with post self-crosslinking at high temperature. NiO nanoparticle/C-NiO nanocomposite embedded flexible PVDF thin films with different amounts of fillers (120 wt%) were prepared via simple solution casting method [130]. Novel polymer blends were prepared from PEN and PVDF via solution mixing [140].

11.6.3

In-situ Polymerization Process

This method is utilized to obtain the homogeneous dispersion of nanoparticles [79]. A series of conductive titanium carbide (TiC) nanoparticles doped PI composite films were prepared via in-situ polymerization. Before their addition, TiC nanoparticles were modified by polyvinylpyrrolidone to improve the dispersion of TiC in the PI matrix [116]. BT/PI nanocomposite films were prepared by in-situ polymerization process. To improve the interfacial compatibility of two phases, BT was modified with 2-phosphonobutane-1,2,4-tricarboxylic acid and acrylicacrylate-amide copolymers [141]. Ag nanorod/PI nanocomposites with high conductivity (an order of magnitude higher than PI), frequency-independent ε (3.8B4.2), and low DF (,0.05) were prepared by an in-situ polymerization process [142]. GO/PI composite films were prepared via in-situ polymerization method. GO sheets were well dispersed in the PI matrix due to the hydrophilic nature of oxygen containing groups in GO sheets.

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Meanwhile, both GO/PI and rGO/PI composite films show better thermal stability than PI film [143]. High-ε PI composite films with CCTO and derivatives were also prepared by in-situ polymerization [144,145].

11.6.4

Others

CoY hexaferrite-filled PEEK composites were fabricated using a high-speed blender for 30 minutes with the help of a lubricant [146]. The composite film was first prepared using solution casting, followed by the hot-pressing process to get final composites. During the hotpressing process, multiple layers of the as-cast films with the same composition are stacked together as the starting materials. It is found that, by using controlled hot-pressing conditions, the ε of the composites can be significantly enhanced [147].

11.7 Applications for High-ε Polymer Materials with High Thermal Stability The ε, DF, AC electric conductivity and thermal stability are the crucial quantities in the design of many electronic devices. Hightechnology electronic devices require new high-ε and high frequency suited materials. High-ε ( . 510), high temperature ( . 150 C), and low DF polymers are attractive dielectric materials for a variety of practical applications such as film capacitors for power-conditioning, power electronics in hybrid electric vehicles, pulsed power, and gate dielectrics for field-effect transistors [148]. Emerging Internet of Things applications require radio frequency dielectrics with desirable sizes and integrated performances (especially those that can stand high thermal stability and a harsh environment) as well as good manufacturability that meets the need for easy deployment in ubiquitous sensing and communication applications [146]. Raj et al. [146] prepared phase-pure Y type planar hexaferrites (Ba2Co2Fe12O22) through solid state reaction route and finely dispersed in the PEEK matrix using a high speed mixer. The composites can attain a maximum ε up to 8 and a DF of 0.005. The composites simultaneously show high-ε and good thermal stability, and hence can be effectively used for miniaturized and embedded radio frequency components and substrates.

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Ni0.5Ti0.5NbO4/CE composites were prepared with ultralow loss, and are suitable for printed circuit boards [52]. The FePc obtained via sintering process at high annealing temperature can be useful as high performance materials, particularly in electronic or magnetic applications where thermal stability was required [125]. Wang et al. [149] prepared boron nitride nanosheets/poly(methyl methacrylate) (BNNS/PMMA) nanocomposites using a solution cast method. The obtained nanocomposites exhibit much higher energy density, enhanced dielectric strength, and lower current density compared with PMMA. Even at 70 C, the nanocomposite with the optimized 12 wt% BNNS content shows a discharged energy density of 0.866 J/ cm3, an efficiency over 97%, as well as a high power density of 0.39 MW/cm3. These results demonstrate that incorporating barrier nanosheets into the polymer matrix can significantly hinder conduction loss and thus improve charge-discharge efficiency. Therefore, BNNS/ PMMA nanocomposites show the potential to be utilized as capacitors with much reduced volume and weight in hybrid electric vehicles and aerospace power electronics operating under high temperatures. MgO-Al2O3-ZnO codoped Ba0.6Sr0.4TiO3 (BMAZ)/PVDF composites were prepared via the solution casting method [114]. A high-ε tunability of 10.9% (under a bias of 1.0 kV/mm) was obtained for the composite with 40 vol% BMAZ, which is higher than those of some BMAZ-based ferroelectric ceramics. BMAZ/PVDF composites are suitable for tunable capacitor and phase shifter applications.

11.8

Concluding Remarks

Dielectric polymer materials with high thermal stability have gained broad attention and attracted great interest due to their importance in many cutting-edge fields, where a high temperature is encountered during the manufacturing of applications. Ceramic/polymer and conductor/ polymer composites are two basic kinds of dielectric polymer materials; their thermal stability is dependent on many factors, including structure of polymers, filler, and interface, among them the polymer matrix is the main aspect because polymers usually have lower thermal stability than metals and ceramics. Note that these factors are generally dependent on each other; especially the structure of the polymer may be changed due to the presence of fillers, so the structure of the polymer matrix in composites may be different from that of pure polymer; as a result, the

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above factors play complicated and competitive roles in the thermal stability of composites. Inorganic filler/polymer composites generally have better thermal stability than polymers as these fillers usually act as a “mass transport barrier” and heat sink. In general, there exists an optimum concentration of fillers in composites to get the highest thermal stability, the thermal stability often declined with further increased loading of fillers mainly due to the aggregation of fillers. The composites containing fillers with high photocatalytic activity, such as BT and TiO2 on the thermal decomposition of polymer chains, often result in degraded thermal stability. To prepare dielectric polymer materials, polymers with high thermal stability should be used. PI, PEN, PVDF, and derivatives are most often used as engineering thermoplastics, while BD and CE resins are typical thermally resistant thermosetting polymers. Unique processing characteristics is one of the biggest merits of thermosetting resins over thermoplastics, which also makes thermosetting composites suitable to employ liquid phase process to fabricate related composites without using solvent. Thermoplastics have good toughness, so they are good candidates for producing electric devices that require high flexibility. Ceramic/polymer and conductor/polymer composites have their special characters (both advantages and disadvantages) in dielectric properties. Recent research progress aims at overcoming disadvantages and further exerting advantages of each kind of composite, and as expected, much effort has been made to prepare multiphase composites containing both ceramics and conductors. A research direction that deserves wide concern is developing high-ε and low DF composites through building special macrostructure (configuration) based on traditional polymers, ceramics, and/or conductors, which provides the biggest possibility to fabricate composites with desirable performance with available raw materials, and also puts forward the intensive study on building the structure-property relationship. As the properties of a material must meet the need of a specific application, so the target and the detailed proposal developed for preparing new and high-performance composite for one application may be different from that for another one. In any case, low DF and high thermal stability are two necessary properties of high performance dielectric materials. The dielectric properties of polar polymers and related composites are closely related to temperature because the molecular motion ability is dependent on temperature, so when the temperature falls in the range of glass transition, the dielectric properties of polymers and composites

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show amazing change. Therefore, it is necessary to improve the thermal stability of polymers to obtain good stability in dielectric performance. To improve the thermal stability of dielectric polymer materials, two main strategies have been utilized. One is synthesizing new polymers and functional fillers with high thermal stability; the other is improving the interface nature between the polymer matrix and fillers. For the former, it is important to avoid possible problems, such as poor processing characteristics, great brittleness, and big internal stress, derived from a building structure containing heterocyclic rings and a high level of aromatic characters. There are three basic processing methods for preparing dielectric polymer materials; composites are usually processed with a similar method as polymer matrices. In-situ polymerization is a special one for treating fillers and preparing composites. Up-to-date, high-ε dielectric polymer materials have made considerable progress; however, those with high thermal stability still need more effort. At present, Tg and Tdi are generally used as the performance indexes for thermal stability; however, they are not complete ones for evaluating thermal stability of dielectric materials in actual applications. Therefore, systematic investigations of integrated performances of dielectric polymer materials should be carried out to meet the needs in a specific application.

Acknowledgments We thank the National Natural Science Foundation of China (51473107, 51173123, 20974076, 50773048) and the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD) for financially supporting this project.

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