Dielectric polymers with novel chemistry, compositions and architectures

Progress in Polymer Science 80 (2018) 153–162

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Trends in Polymer Science

Dielectric polymers with novel chemistry, compositions and architectures Yali Qiao a , Xiaodong Yin a , Tianyu Zhu a , Hui Li b , Chuanbing Tang a,∗ a

Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208, United States School of Chemistry and Chemical Engineering, Shandong Key Laboratory of Fluorine Chemistry and Chemical Materials, University of Jinan, Jinan, 250022, Shangdong, China b

a r t i c l e

i n f o

Article history: Received 8 March 2017 Received in revised form 26 December 2017 Accepted 26 January 2018 Available online 31 January 2018 Keywords: Dielectric polymer Energy storage Macromolecular architecture Permittivity

a b s t r a c t Dielectric capacitors have attracted ever-increasing interest in recent decades for numerous applications in modern electronic and electrical power systems due to their fast charge/discharge speed and high energy density. Novel dielectric materials are highly sought for these capacitive applications. Polymer dielectrics are attractive as they can offer high dielectric strength, low dielectric loss, and light weight, however, a few challenges still exist. For examples, the state-of-the-art polymer dielectric, biaxially oriented polypropylene (BOPP), has low dielectric permittivity, while polyvinylidene fluoride (PVDF) has high dielectric loss. These hurdles require developing next-generation polymers as dielectric materials with new chemistry and unique architectures that are tunable in compositions, flexible in mechanical properties and stable at high temperature. In this short review, we begin with some theoretical considerations for the rational design of dielectric polymers with high performance. In the guidance of these theoretical considerations, we review recent progress toward dielectric polymers based on two major approaches, in terms of macromolecular architectures, namely main-chain and side-chain dielectric polymers, and various chemistry and compositions are discussed within each approach. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Novel dielectric materials are highly desirable due to their applications in a broad range of advanced energy harvesting and storage systems, such as portable electronic devices, hybrid electric vehicles (HEVs), and pulse power systems [1–5]. Compared to inorganics, polymers are attractive materials as dielectrics [6–12] as they allow for easier processing techniques and lower densities, leading to the formation of lightweight, flexible films suitable for devices with limited size and space. One of the state-of-the-art polymer dielectrics, biaxially oriented polypropylene (BOPP), exhibits a high breakdown strength (Eb > 600 MV m−1 ) and an inherently low dielectric loss (tanı ∼ 0.0002). However, its energy density is limited by a low dielectric constant (εr = 2.2) and an operating temperature below 100 ◦ C (Tm < 140 ◦ C) [13]. The other benchmark polymer dielectric, polyvinylidene fluoride (PVDF), possesses high dielectric constant (εr > 10), which can be significantly enhanced (εr > 50) by proper defect modification, and achieve a very high energy density (>20 J cm−3 ) [8,14–16]. However, PVDF-based ferroelectric poly-

∗ Corresponding author. E-mail address: [email protected] (C. Tang). https://doi.org/10.1016/j.progpolymsci.2018.01.003 0079-6700/© 2018 Elsevier B.V. All rights reserved.

mers suffer from large polarization hysteresis loss due to the strong dipole–dipole interactions and limited operating temperature below 125 ◦ C (Tm < 140 ◦ C).[13,17] Therefore, next-generation polymer dielectrics simultaneously possessing high dielectric constant (>5–10), low loss, high efficiency as well as high operating temperature (>150 ◦ C) are highly sought. A capacitor with a dielectric layer (area A, dielectric thickness d) between two metallic electrodes is applied a voltage V to store energy, which can be described with volumetric energy density by: W=

1 Ad



 Vdq =

EdD

(1)

where E, D, and q = DA are the electric field, electric displacement, the free surface charge, respectively. Eq. (1) shows that the increase of E to some reasonable fractions of the breakdown field Eb can maximize the stored energy density. Moreover, since the electric displacement is related to the polarization P by D = P + ε0 E, high polarization favors high displacement, ultimately leading to enhancement of the stored energy density. For linear dielectric materials with D = ε0 εr E, the energy density is changed to: W=

1 εr ε0 E 2 2

(2)

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Fig. 1. Chemical structures of (meta)aromatic polyurea and polythiourea.

where εr is the dielectric material’s relative permittivity, and ε0 = 8.85 × 10−12 F m−1 is the vacuum permittivity. Eq. (2) indicates that materials with large relative permittivity and breakdown field strength can achieve high energy density. At present, in order to achieve high energy density in polymer dielectrics, one typical strategy is to develop PVDF-related ferroelectric polymers [8,18–29]. Nevertheless, experimental results showed that these modifications to PVDF-based copolymers and terpolymers cannot fully suppress the polarization coupling in these ferroelectric polymers, especially at high electric fields. Therefore, it is still of great challenge to further reduce the ferroelectric loss and high field conduction loss in those ferroelectric polymers. Recently, efforts have started to be devoted to improving the energy density of linear polymers by introducing highly polar molecular groups, which is another promising approach toward novel high-performance polymer dielectrics [6,7,10,30,32–34]. In general, the dielectric permittivity in a linear dielectric polymer can be estimated by using the Fröhlich model [35]: (εrs − εr∞ ) (2εrs + εr∞ ) εrs (εr∞ + 2)

2

=

Ngu2 9ε0 kT

(3)

which implies εr ∼ Ngu2 . Here εrs and εr∞ are the dielectric constants at low frequency and optical frequency, respectively; N is the volumetric dipole density; g is the correlation factor; u is the dipole moment; k is the Boltzmann constant; and T is the temperature. Therefore, high volumetric density of highly polar molecular groups with proper dipolar coupling is highly beneficial to the achievement of higher dielectric constant, maintenance of relatively low loss, in turn leading to high energy density in such linear dielectric polymers. In this short review, we intend to explore the recent progress on approaches to developing novel dielectric polymers. In particular, we focus on linear dielectric polymers, based on incorporation of highly polar functional groups and/or molecules via two major approaches, in terms of macromolecular architectures, namely main-chain and side-chain dielectric polymers, and various chemistry and compositions are discussed within each approach. This short perspective is certainly not inclusive for all interesting development over last two decades. For work on dielectric polymeric composites, readers are referred to publications cited herein [11,36–64].

investigated aromatic polyurea (ArPU) (Fig. 1) as dielectric thin films for possible high-temperature, high-energy density capacitor applications. Relatively high dielectric constant (∼4.2), high breakdown strength (690 MV m−1 ), and very low leakage current have been demonstrated in high-quality aromatic polyurea thin films fabricated through vapor deposition polymerization of monomers 4,4 -diamino-diphenylmethane (MDA) and 4,4 -diphenylmethanediisocyanate (MDI) [70,71], resulting in a high energy density (9 J cm−3 ) and very high charge-discharge efficiency (> 95%). These values decrease only slightly from room temperature to 180 ◦ C. The design of direct connection of highly polar urea functional group with highly hydrophobic aromatic rings not only favors the formation of delocalized electrons to enhance the dielectric response, but also suppresses the dipole–dipole coupling to maintain relatively low loss, making aromatic polyurea a polar linear dielectric exhibiting higher dipole moment than PVDF without ferroelectricity. Shortly, the same group reported the development of aromatic polythiourea (ArPTU) (Fig. 1) with greatly improved dielectric performance, especially at high electric fields. ArPTU was synthesized via microwave-assisted polycondensation of 4,4 -diphenylmethanediamine (MDA) with thiourea in N-methyl2-pyrrolidone (NMP) with p-toluenesulfonic acid (p-TSOH) as a catalyst [72]. Due to slightly higher dipole moment of thiourea than that of urea, ArPTU exhibits a moderate increase in dielectric constant (∼4.5 in ArPTU versus ∼4.2 in ArPU). Additionally, ArPTU films demonstrate an extremely high dielectric breakdown strength (>1.1 GV m−1 ), resulting in a maximum electrical energy density up to 24 J cm−3 and very high charge-discharge efficiency of 92.5% at 1 GV m−1 . The dielectric properties do not change significantly with temperature up to 150 ◦ C. Unlike the strongly coupled dipolar polymers such as PVDF that are semicrystalline, ArPTU is an amorphous, glass-phase polymer (Tg > 200 ◦ C), eliminating band conduction and avoiding ferroelectric loss even at the highest measured electric field (1.1 GV m−1 ). The large dipole moments of thiourea functional groups can provide strong scattering to the electrons and ions, thereby further reducing the conduction loss. The weak coupling among the dipoles due to the separation of thiourea groups via aromatic rings is favorable to achieving low dielectric loss. Moreover, the ultrasmooth surface plays a great role in ensuring very high breakdown strength while maintaining low loss [73,74]. They further investigated meta-aromatic polyurea (meta-ArPU) [75] and corresponding meta-aromatic polythiourea (meta-ArPTU) [76] (Fig. 1) with a higher volumetric dipole density and thus

2. Polymers containing dipolar groups in main-chain architectures 2.1. Main-chain (aromatic) urea- and thiourea-functionalized polymers Polyurea and polythiourea, as highly polar polymers, have been well-known for their interesting dielectric, pyroelectric, piezoelectric, and ferroelectric properties [65–68]. The dipole moments of urea and thiourea [69] are 4.56 Debye and 4.89 Debye, respectively, which are much larger than most polymer materials, such as PP, PET, PC, and PVDF (2.1 Debye). The Zhang group initially

Fig. 2. Chemical structures of a series of polythiourea with long and flexible chains.

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Fig. 3. A two-step method for synthesizing polyimides (PIs), and chemical structures of some typically used dianhydrides and diamines for developing high-performance PI dielectrics.

enhanced dielectric properties through replacing the two aromatic rings with one ring, which is good enough to limit the dipole–dipole interaction and prevents the ferroelectric loss. Traditionally, the aromatic polyurea was synthesized via polycondensation of aromatic diamine and aromatic diisocyanate. Meta-PU was successfully synthesized by a green synthetic route, free of any isocyanate, solvent, and catalyst, via polycondensation of m-phenylenediamine and diphenyl carbonate. As a result, the meta-ArPU and meta-ArPTU exhibit considerably increased dielectric constant (∼5.6 in meta-ArPU versus ∼6 in meta-ArPTU) while maintaining low loss and high breakdown strength. Compared with the aromatic PU/PTU, the meta-aromatic PU/PTU has a higher energy density at the same electric field because of the enhanced dielectric properties. These results demonstrate the great potential of strongly dipolar materials with small dipolar coupling to achieve relatively higher dielectric constants than the widely used non-polar or weakly dipolar polymers, as well as a much lower loss than that of nonlinear high dielectric constant polymer dielectrics or polymer–ceramic composites, paving a new way for future dielectric materials with ultrahigh electric-energy density, low loss at high applied fields, and ultrahigh breakdown strength. The groups of Sotzing and Ramprasad presented a systematic study on polythioureas with longer and more flexible chains as promising dielectrics for high energy density capacitor applications [77]. Notably, the design and synthesis was guided by implementing a high throughput hierarchical modeling [31,78] with combinatorial exploration and successive screening, followed by an evolutionary structure search based on density functional

theory (DFT). Specifically, a series of polythiourea (Fig. 2) were prepared, which are based on condensation polymerization of one constant monomer, para-phenylene diisothiocyanate (PDTC), with various diamine monomers, including 4,4 -oxydianiline (ODA), bis(4-aminophenyl)methane (MDA), 1,4-diaminobenzene (PhDA), hexane-1,6-diamine (HDA), and Jeffamine HK511. The usage of various chain segments, such as aromatic, aliphatic and polyether, realizes the fine tuning of dielectric properties via introducing additional permanent dipoles, varying conjugation length, controlling morphology, etc. A dielectric constant of ∼4.5 and a corresponding energy density of ∼10 J cm−3 were achieved in accordance with Weibull characteristic breakdown voltage of ∼700 MV m−1 . These results demonstrate that rational strategies combining computational and experimental procedures are a powerful approach to the development of new dielectric materials. 2.2. Main-chain polyether-containing and nitrile-functionalized polyimides Polyimides (PIs) are a class of polymers with macromolecular repeating units containing a functional imide group, which are used in a diverse range of applications owing to their superior thermal stability, high mechanical properties as well as good chemical resistance. Specifically, as attractive dielectric materials for capacitor applications, polyimides have a more polar backbone than polyolefins and thus offer a higher dielectric constant. More importantly, they rank among the most heat-resistant polymers and are thermally stable at operating temperatures up to 250 ◦ C, much higher than that of most common dielectric polymers (exhibiting

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a severe decrease in dielectric strength around 70 ◦ C), providing them with better thermal management capability. So far, most studies of polyimides have focused on ways to decrease the dielectric constant of the materials in order to match the requisites as insulating materials in electronic and microelectronic applications [79–83]. However, approaches to increasing dielectric constant of these polymers are highly sought as dielectric materials for capacitor applications. PIs are commonly made through a two-step approach [84]. In the first step, a nucleophilic attack of amine groups toward carbonyl groups in a dianhydride produces a precursor poly(amic acid) (PAA); in the second step, the cyclodehydration reaction yields a final polymer of PI upon the thermal/chemical imidization. The chemical scheme of a general two-step method for the preparation of PIs is shown in Fig. 3. With no catalyst and sole production of water in the synthesis procedure, polyimide with a higher purity minimizes impurity ion-induced migrational loss, which is inherent for PVDF prepared from suspension polymerization. The groups of Sotzing and Ramprasad [85] have recently explored the polyimide chemical space by developing a novel class of polyether-containing polyimides as potential dielectric materials for energy storage applications. A series of polyimides (Fig. 3) were firstly prepared by polymerization of a common aromatic dianhydride, pyromellitic dianhydride (PMDA), with various shortchain aliphatic diamines, including 1,2-diaminopropane (1,2-DAP), 1,3-diaminopropane (1,3-DAP), and some Jeffamines (EDR-104, D230, and HK511), to achieve a high density of imide functional group in the polymer backbone as well as to improve solubility. Homopolymers and copolymers exhibit dielectric constants ranging from 3.96 to 6.57, which is twice of BOPP and therefore leads a twofold increase in capacitance while maintaining low dissipation factors. Also, they have operating temperature well above that of BOPP. The groups of Sotzing and Ramprasad [86] further investigated a series of polyimides by polymerization of different aromatic dianhydrides, extending from PMDA to 3,3 , 4,4 -benzophenone tetracarboxylic dianhydride (BTDA), 4,4 -oxidiphthalic anhydride (ODPA), and 4,4 -(hexafluoroisopropylidene) diphthalic anhydride (6FDA), with various aliphatic diamines, comparing Jeffamines (polyetheramine) with diaminopropane and diaminohexane (alkyl diamine) (Fig. 3). Incorporation of different functional groups such as carbonyl and ether in the bridge of the dianhydride moiety, together with the polyether segment in the diamine moiety, can yield additional permanent dipole moments, resulting in great enhancement of the polar group density in the polymer backbone, and thus a further improvement of the dielectric constant. Among all polyimides, polyimide B4, having the longest cross-conjugated system (BTDA) as well as the longest polyether segment (Jeffamine HK511), exhibits the highest dielectric constant of 7.8, while maintaining low dissipation factors (<1%) and achieving a high energy density around 15 J cm−3 . Notably, the synthesis was guided by high-throughput density functional theory calculations for rational design in terms of a high dielectric constant and band gap, and the relationship between chemical functionalities and dielectric properties were well established by correlations of experimental and theoretical results. These results demonstrated the importance of an increased dipole volume from the ether linkage as well as the longer conjugation length when a carbonyl spacer was inserted between the benzene rings on the dielectric constant, and provided insights into polyether-containing polyimides as promising dielectric materials for achieving high energy density. Tan et al. [87] expanded the polyimide chemical space in another approach via incorporation of polar nitrile groups directly attached to the main chain as potential dielectric materials for energy storage applications. Firstly, PIs containing a single nitrile group per repeating unit based on polymerization of an asymmetric diamine, 2-(3-

Fig. 4. Synthesis and chemical structures of Sn ester-functionalized polymers.

aminophenoxy)-6-(4-aminophenoxy)benzonitrile (3,4-APBN), and two symmetric diamines, 2,6-bis(3-aminophenoxy)benzonitrile (3,3-APBN) and 2,6-bis(4-aminophenoxy)benzonitrile (4,4-APBN) (Fig. 3), with common dianhydrides (e.g., PMDA, BTDA, DSDA, OPDA, and 6FDA in Fig. 3) were prepared. They exhibited dielectric constants of 3.08–3.62 at 10 kHz, slightly higher than that (2.92) of their analogous PI without nitrile groups. Compared to symmetric structure, the presence of asymmetric structure in each repeating unit can improve the polymer processibility and increase their dielectric constants while maintaining relatively low dielectric loss. In order to further explore the influence of dipole moment density together with molecular asymmetry on the dielectric and other properties (e.g., thermal stability, processability, etc.), PIs containing three nitrile groups in each repeating unit were prepared based on polymerization of three structurally novel diamines (p,p-3CN, p,m-3CN, and m,m-3CN in Fig. 3) with common dianhydrides. Compared to one CN dipole, three CN dipoles can further increase the permittivity (4.9@1 kHz at 190 ◦ C) with relatively low dielectric loss and thus improve electrical energy storage, especially at high temperature [88]. Among three nitrile-containing PIs, the dipole moment of the dianhydride also played an important role. However, unlike the case for polyether-containing PIs, increase in the dipole moment of the dianhydride part (e.g., PMDA < OPDA < 6FDA < BTDA) gradually decreased the enhancement of permittivity. In addition, para–para linkage in the diamine offered a higher permittivity than meta–para and meta–meta linkages between dianhydride and diamine parts due to easier rotation around the para–para bonds. Therefore, PIs with a combination of PMDA dianhydride and a para–para linkage exhibited the highest discharged energy density and a reasonably low loss [89]. These results indicate CN-containing PIs, especially the one with high-CN content, are highly promising dielectric materials for high temperature film capacitors. 2.3. Main-chain organometallic ester-functionalized polymers According to recent computations by Ramprasad et al., incorporation of metal atoms via covalent bonding within a polymer backbone has been demonstrated great promise on improving the dielectric properties of materials [90,91]. Unlike polymer nanocomposites formed by introduction of inorganic nanofillers into polymer matrix, polymers created by direct inserting the metal atom into the polymer backbone are favorable to the fine dispersion of the metal throughout the matrix without undesirable effects from aggregation. Additionally, the existence of metal-heteroatom bonds in the polymer backbone is more beneficial to the enhancement of the dipolar and atomic polarization due to the larger electronegativity difference of metal-heteroatom bonds than that of carbon-heteroatom ones. Correspondingly, an organotin functional group, Sn ester, was chosen for developing new organometallic polymers as potential dielectrics. Sotzing et al. [92] firstly prepared poly(dimethyltin glutarate), or p(DMTGlu) (Fig. 4), by polymerization of dimethyltin

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Fig. 5. Synthesis of side-chain functionalized PP and PE copolymers.

Fig. 6. Synthesis of PMP-NH2 -y and PMP-(NH3 )x A-y copolymers (x = 1, 2, 3, A = Cl− , SO4 2− , PO4 3− , and y = NH3 content) via a catalytic copolymerization process (catalyst: TiCl3 AA and rac-CH2 -bis(3-tert-butyl-indene)ZrCl2 ).

dichloride with glutaric acid, yielding a new polymer with a repeating unit containing a dimethyltin group connected with a carboxylate group on both sides, with a linear chain of three methylene units as the linker. The polymer p(DMTGlu) exhibits good thermal stability up to 235 ◦ C and a high dielectric constant larger than 6, indicating that tin-based organometallic polymers are promising dielectric candidates to combine both high-T capability and a high dielectric constant. Based on the initial success in developing p(DMTGlu), Sotzing et al. [93] further investigated a series of related poly(dimethyltin esters) (Fig. 4), containing the dimethyltin-ester group with various number of methylene units as the linker (0–10), in order to systematically study the effect of the linker length on the dielectric properties. By varying the linker length in the diacid monomers, aliphatic poly(dimethyltin esters) were produced with various weight percentages of tin [92,93]. Generally, the dielectric constant decreased and the bandgap increased when the linker is longer, leading to some “optimal” lengths of the linker (4–7 methylene units) for which both the bandgap and the dielectric constants are high. Among this family, poly(dimethyltin suberate), pDMTSub, with a linker of 6 methylene groups, the dielectric constant can be up to 7 while the bandgap can reach nearly 7 eV. Additionally, the film morphology based on methylene spacer length was responsible for variations in the dielectric constant and the band gap, and can be well tuned by blending with a second homopolymer, such as poly(dimethyltin 3,3,-dimethylglutarate), pDMTDMG (Fig. 4). More importantly, in parallel with the experimental effort, detailed first-principle calculations were performed for this family of organotin polymers. Besides, Sotzing et al. [94] also investigated the effect of incorporating aromatic and chiral groups (Fig. 4) on the dielectric properties of poly(dimethyltin esters), both computationally and experimentally [91,93,95,96]. On one hand, the size and nature of aromatic rings shows some effects on the dielectric properties. As the size of the ring increases, the resulting dielectric constant

would be lower; polymers containing (electron-neutral) benzene rings would have higher average dielectric constants compared to those having (electron-donating) thiophene rings or (electronwithdrawing) pyridine rings. On the other hand, chiral groups can be used to tune the crystallinity of polymers, and in turn, affect the averaged dielectric constant. In all, these polymers exhibit dielectric constants larger than 4.5 with dissipation between 0.1% and 10%. Very recently, p(DMTSub) was chosen for further investigation together with a complementary polymer, poly(dimethyltin 3,3-dimethylglutarate), p(DMTDMG) (Fig. 4), by ways of blending and copolymerization, in order to optimize the dielectric properties of poly(dimethyltin esters) [97]. Films made from both blend BP20 and copolymer CP20 (the concentration of p(DMTSub) in the blend or copolymer is 20%) showed high dielectric constants of 6.6 and loss of approximately 1%. Copolymerization was found to be better suited than blending for energy storage as the energy density of CP20 was calculated at approximately 6 J cm−3 compared to 4 J cm−3 for BP20. The insights from this research could help to improve the feasibility of poly(dimethyltin esters) for use as a next generation polymer dielectric.

3. Polymers containing dipolar groups in side-chain architectures 3.1. Side-chain hydroxyl- and ammonium-functionalized polyolefins As one of the-state-of-art polymer dielectrics, BOPP exhibits high breakdown strength and inherently low energy loss. However, its energy density is greatly limited by a low dielectric constant. Accordingly, it is of great interest to explore effective approaches to increase dielectric properties of PP polymers in order to achieve

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Fig. 7. Synthesis of oligothiophene-containing homopolymers and diblock copolymers.

higher energy density in PP-based capacitors while maintaining high breakdown strength and low energy loss. To this end, Chung et al. [98] systematically investigated a family of isotactic polypropylene copolymers (f-PP) that contain various −(CH2 )n -CH3 , −(CH2 )n -O-Si(CH3 )3 , and-(CH2 )n -OH side groups. These chemically modified PP polymers were prepared by copolymerization of propylene with various comonomers, using both homogeneous Et(Ind)2 ZrCl2 /MAO and heterogeneous TiCl3 AA/Et2 AlCl Ziegler-Natta catalyst systems. As shown in Fig. 5, the borane- and silane-containing PP copolymers could be further interconverted into corresponding OH-containing polymers. All PP copolymer films were fabricated, and the effects of side-chain groups (comonomer units) and associated morphological change to their dielectric properties were studied. It was observed that most of the side chains (comonomer units) reduce PP crystallinity and increase the molecular motion that is not fully reversible in the time scale of measurement, leading to the dielectric sensitivity to frequency, temperature, and applied electric field, as well as large dielectric loss in D-E loops. In contrast, PP-OH copolymers with high molecular weight show high crystallinity with a unique network structure through interchain OH group dimerization (Hbonding), offering not only higher polarizability but also good reversibility as well as yielding nearly consistent dielectric constant over a wide range of temperature (between −20 and 100 ◦ C), frequencies (between 100 and 1 M Hz), and applied electric fields (>600 MV m−1 ). The PP-OH (with 4.2 mol% OH content) based thin film capacitor displays a linear reversible charge storage capacity with high releasing energy density >7 J cm−3 at an applied electric field of 600 MV m−1 , without showing any significant increase in energy loss and remnant polarization at zero electric field. These results demonstrated that the addition of a small amount (2–6 mol%) of OH side groups to isotactic polypropylene can significantly increase (nearly double) the dielectric constant while maintaining a relatively low dielectric loss and displaying minimal impact on the semicrystalline morphology, which was also confirmed by simulations, affording an effective approach to increase the dielectric properties and energy storage capabilities of PP polymers. More recently, inspired by previous experimental results for PP OH, Kumar and Chung et al. carried out experiments and simulations to study the dielectric properties of PE OH (Fig. 5) in order to reveal the mechanism by which the addition of a small number of polar OH groups to a nonpolar polymer can not only greatly increase the dielectric constant, but also maintain a low dielectric loss (<1%) [99]. It was found that a stoichiometric amount of adsorbed water forms tightly H-bonded clusters of water molecules and hydroxyl groups, which inhibit the −OH groups from relaxing and contributing to the dielectric loss within the experimental time scales. It indicates that the approach of functionalizing non-

polar polymers, such as PP and PE, with a small number of polar groups is a promising pathway to improve dielectric properties and energy storage capabilities of nonpolar polymers and thus worthy of in-depth experimental and computational investigation in the future. Usually, it is believed that ion-containing polymers are not suitable for achieving high energy density and high breakdown strength due to the significant conduction loss at a high electric field. However, Nan et al. [100] recently demonstrated that a series of ion-containing (co)polymers are of great potential as novel polymer dielectrics. Unlike conventional perspectives of ion-containing polymers acting as dielectrics, Nan et al. believed that electric breakdown strength is more strongly affected by the quality of films rather than the ion-induced conductive loss. Ion-containing polymers are more suitable for film stretching due to the enhancement of melt viscosity caused by the ionic association and the improved mechanical properties. Therefore, it is highly attractive to investigate the dielectric properties of novel ion-containing (co)polymers, which are expected to not only increase the energy density and reduce the energy loss, but also achieve high operating temperature and facilitate better thermal management at elevated temperature. Specifically, a new family of poly(4-methyl-1-pentene) copolymers PMP-(NH3 )x A-y (x = 1, 2, 3, A = Cl− , SO4 2− , PO4 3− , and y = NH3 content) containing various NH3 + Cl− , −(NH3 )2 SO4 2− and −(NH3 )3 PO4 3− side groups were synthesized by copolymerization of 4-methyl-1-pentene and bis(trimethylsilyl)amino-1-hexene, using either a traditional Ziegler–Natta or a metallocene catalyst (Fig. 6). As compared to pure PMP films, the dielectric performance and energy storage capacity of [PMP-(NH3 )x A-y] ionomer films can be significantly improved due to ionic cross-linking. Among all [PMP-(NH3 )x A-y] ionomers, the one with −(NH3 )3 PO4 3− side group can attract more positive charges in the polymer chains because of the three negative charges of PO4 3− group, allowing for more favorable formation of the crosslinking network. Consequently, the PMP-(NH3 )3 PO4 3− −4.5 thin film, prepared by stretching after the hot pressing process, showed an enhanced dielectric constant (∼5) with good frequency and temperature stabilities up to 160 ◦ C as well as a high energy density (7.4 J cm−3 ) at high breakdown strength (600 MV m−1 ) with high charge–discharge energy efficiency (93%), all of which make such ion-containing polymers promising for application in dielectric energy storage devices. 3.2. Side-chain oligothiophene-functionalized polymers Conjugated thiophene-containing oligomers and polymers exhibit a high degree of polarization due to the existence of sulfur atoms and extended ␲-electrons, thus considered as a good candidate for dielectric materials [101,102]. The Tang group for

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Fig. 8. Synthesis of oligothiophene-containing (a) polymer brushes and (b) polynorbornenes.

the first time investigated polymers with pendant conjugated oligothiophene in the side chain as potential high performance dielectric materials [103]. Specifically, methacrylate polymers pendant with terthiophene oligomers were synthesized using oligothiophene-containing methacrylate, 2-(2,2 :5 ,2”-terthien-5yl)ethyl methacrylate (TTEMA) as a monomer via reversible addition fragmentation chain-transfer (RAFT) polymerization with cumyl dithiobenzoate (CDB) as a transfer agent (Fig. 7). Formation of extremely small crystalline domains (<2 nm) associated with terthiophene side chains in an amorphous matrix was confirmed by DSC and XRD studies. More importantly, it was demonstrated that the polymers can function as high performance nanodielectrics with high dielectric permittivity (ca. 9–11) and low dielectric loss (<0.02) in an excellent consistency over a wide range of frequency (100 Hz–4 MHz). Besides, linear and reversible polarization-depolarization profiles were obtained with relatively low loss. Moreover, molecular weight has shown a significant effect on the dielectric performance, with higher molecular weight being beneficial to the achievement of higher breakdown strength and higher energy density. It was believed that the high polarizability and fast dielectric response are the most favorable factors, which are attributed to the formation of nanoscale dipoles deriving from nano-crystallites of pedant oligothiophene groups. Motivated by the promising dielectric properties of PTTEMA polymers, a series of diblock copolymers [104] composed of polystyrene and terthiophene-containing methacrylate (PTTEMAb-PS) with varying PTTEMA content were synthesized via RAFT polymerization (Fig. 7). The crystallinity increases with the increase of PTTEMA content (DSC and XRD), which is consistent with previous results and further verifies the presence of terthiophene-rich crystalline domains. As a result, the relative permittivity increases

with the increase of PTTEMA content in the diblock copolymers, and the permittivity is higher in diblock copolymers with moderate PTTEMA content which possess a higher density of isolated polarizable terthiophene-rich domains. On the other hand, it was also noticed that most films of either PTTEMA homopolymers or PTTEMA-b-PS copolymers are brittle in nature, not only limiting the film-making processes, but also having a negative effect on the dielectric performance (e.g., breakdown strength and energy loss). To this end, the Tang group introduced a polymer brush architecture to prepare oligothiophene-containing polymers [105], which is expected to improve the film quality/processability and in turn optimize the dielectric performance. As shown in Fig. 8, the polymer brushes were synthesized via a “grafting-from” technique by a combination of ROMP and RAFT process. There are still nanoscale crystalline domains (∼2 nm) formed in the polymer brushes, and the film-forming capability and processability were partially improved compared with the corresponding homopolymers because transparent free-standing films were achieved. These results indicate that polymer brush architecture would indeed improve the mechanical robustness of films to some extent. However, these free-standing films did not achieve enough flexibility, and the dielectric properties were mediocre. As a follow-up work, Tang et al. recently reported a concerted macromolecular architecture strategy by synergistically tuning both the polymeric backbone and side chain [106]. This approach employs polynorbornene as a polymeric backbone in conjunction with oligothiophene as pendant polar groups (Fig. 8). This architecture resulted in highly flexible and transparent dielectric polymer films, superior to all methacrylate polymer counterparts. Especially, a facile hydrogenation of the polynorbornene backbone resulted in more than two fold increase in the electric field breakdown strength

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Fig. 9. A summary of selective polymer dielectrics developed in recent years.

with the conservation of low dielectric loss. However, the relative permittivity decrease substantially as well (from ∼10 to ∼4), which is associated with intrinsic amorphous characteristics of the polynorbornence-based films. These results indicated that the packing of the polar group in the side chain dominates dielectric permittivity, and the backbone manipulates thermal/mechanical properties that are essentially correlated to breakdown strength and dielectric loss. There is a trade-off when balancing the contribution from both polymeric backbone and polar conjugated group in the side chains. In order to improve dielectric performance at low electric field while maintaining relatively high breakdown strength and low loss at high electric field, further optimization is under study based on polymers with pendant ␲-conjugated functional polar groups. 4. Concluding thoughts Recent progress toward novel dielectric polymers exhibiting linear behaviors has demonstrated their great potential for developing next-generation film capacitors with high dielectric constant, low loss dielectrics and high operating temperature for energy storage applications. The reasonable choice of highly polar functional groups and/or molecules with proper dipolar coupling, as well as suitable macromolecular architectures, in terms of main-chain and side-chain polymers, is of great importance to enhancing high dielectric properties and energy storage capabilities in such linear dielectric polymers. As shown in Fig. 9, we summarize a few selective polymers developed over the last decade to illustrate the dynamic growth of this field. Moreover, the great advances through synergy of theoretical and experimental studies on dielectric polymers may lay a foundation to in-depth understanding of behaviors of polymer dielectrics and open a feasible pathway to rationally designing and significantly improving polymer dielectric performance as energy storage materials, which could match the ever-increasing requirements for practical applications. In the future, multifaceted efforts need to focus on approaches to developing novel dielectric polymers, espe-

cially linear ones for high energy density and low loss dielectrics. On one hand, further increase in polarization levels in dielectric polymers could enhance the dielectric constant and thus increase the energy density of such polymers. On the other hand, further reduction in dielectric loss and increase in breakdown strength of polymer dielectrics would improve the efficiency and reliability of capacitors, and reduce the demand for thermal management. Although the recent study of novel dielectric polymers provides better understanding of polymer dielectrics with improved electronic properties, there are several obstructions that limit the development and application of dielectric polymers. Firstly, the computational prediction is still limited on study of polarizability and bandgap, while exploring other critical properties including dielectric loss and breakdown strength requires much more detailed understanding of how an electric field impacts nanostructures of polymers. Secondly, most novel dielectric polymers require complicated organic synthesis or well-controlled film fabrication processing, which is far from large-scale manufacturing. One of neglected, but probably critical pieces in the synthesis is the contamination of residual metal catalysts, which could be detrimental toward lifetime of devices. The use of organocatalytic processes is highly desirable toward mechanically robust and good filmforming dielectric polymers. As a result, it is essential to explore not only materials with robust and eco-friendly synthetic strategy, but also new manufacturing methods. Acknowledgments This short review is a result of our work over the last few years supported by the Office of Naval Research (award N000141110191) and the SC NASA EPSCoR (award 22-NE-USC Tang). References [1] Barber P, Balasubramanian S, Anguchamy Y, Gong SS, Wibowo A, Gao HS, et al. Polymer composite and nanocomposite dielectric materials for pulse power energy storage. Materials 2009;2:1697–733.

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