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Article Cite This: Chem. Mater. 2018, 30, 1102−1112

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Rational Design and Modification of High‑k Bis(double-stranded) Block Copolymer for High Electrical Energy Storage Capability Jie Chen,† Yuxin Wang,‡ Hongfei Li,† Huijing Han,† Xiaojuan Liao,† Ruyi Sun,† Xingyi Huang,*,‡ and Meiran Xie*,† †

School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200241, China Shanghai Key Laboratory of Electrical Insulation and Thermal Aging, Shanghai Jiao Tong University, Shanghai 200240, China

‡

S Supporting Information *

ABSTRACT: High dielectric constant (high-k) polymers have important application in advanced electronic devices such as energy storage, wearable electronics, artificial muscles, and electrocaloric cooling because of their excellent flexibility and ease of processing. However, most of the commercially available polymers have low-k values and the designed strategies for enhancing k are usually at the cost of the increase of dielectric loss. In this work, novel high-k and low loss bis(double-stranded) block copolymers, containing the ionic-conjugated hybrid conductive segments (HCS) with narrow band gap and the insulating segments with wide band gap, were synthesized by tandem metathesis polymerizations. The novel copolymers exhibited enhanced dielectric constant of 33−28 accompanied by low dielectric loss of 0.055−0.02 at 102−106 Hz, and thus greatly increased stored energy density of 9.95 J cm−3 was achieved at relatively low electric field of 370 MV m−1, which is significantly higher than that of the commercial biaxially oriented polypropylene (BOPP) (about 1.6 J cm−3 at 400 MV m−1). In addition, by doping with I2, the k values of the HCScontained block copolymer can increase further to 36.5−29 with low dielectric loss of 0.058−0.026, and the stored energy density maintained at a high level of 8.99 J cm−3 at 300 MV m−1 with suitable I2 content. The excellent dielectric and energy storage capability were attributed to the unique macromolecular structure and well-defined nanomorphology, which not only enhanced the dipolar, electronic, and interfacial polarizations but also significantly suppressed the leakage current and increased the breakdown strength by wrapping the narrow band gap segments in the wide band gap segments.

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INTRODUCTION Polymers with high dielectric properties and good processability are of great interest because of their various applications in energy-storage devices1−5 and organic thin film transistors.6−11 However, for most commercial polymers, the low dielectric constant (k < 10) and capacity density raise problems for practical applications because they are difficult to meet the requirement of high energy density or miniaturization of the energy storage devices.12,13 A simple method to increase the k value is to embed high-k nonconducting (barium titanate, strontium titanate, calcium titanate, etc.)14−17 or conductive (graphene, carbon black, metal, etc.)8,18−21 nanoparticle fillers to polymers. Composites incorporating the nonconducting nanoparticles typically require high filler loading to achieve significant increase in k value,14,22,23 while the nonconducting fillers easily reduce the polymers’ breakdown field strength due to the large k mismatch between the high-k filler and low-k polymer matrix,15 although this kind of composite is mostly considered in energy-storage devices. Alternatively, by use of the conducting fillers, the composites display high-k value at low filler loading in comparison to those containing nonconducting fillers because the electron conduction within the dispersed fillers leads to free charge buildup and interfacial polarization.8,19,24,25 The dielectric loss and leakage current can also be effectively © 2018 American Chemical Society

inhibited by designing the gradient and layered nanostructures when the content of conducting filler is at a low level.26,27 Nonetheless, the highly conductive fillers usually incur a serious leakage current when their loading is close to the percolation threshold, resulting in the unacceptable high dielectric loss and extremely low breakdown voltage, which is not suitable for use in energy storage devices. To resolve these issues such as high dielectric loss, low breakdown strength, suboptimal stored energy density, and efficiency of inorganic filler-contained polymer composites, some kinds of “all-polymer” approaches have been explored.28−32 Most attention has been focused on the use of various polar groups such as −F,33 −CN,34 and sulfone-containing moiety,7,9 which can significantly improve the dipole polarization of the nonferroelectric polymers and increase k value. Especially, when the polar groups can be aligned coherently in one orientation, the dipole moments avoid being counteracted,35,36 which is beneficial to preparation of high-k polymers. Importantly, the enhancement in k value is attained without any compromise in increasing dielectric loss and lowering breakdown strength. In addition to Received: December 2, 2017 Revised: January 17, 2018 Published: January 19, 2018 1102

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low-k materials. When the dipolar and ionic groups were incorporated into the double-stranded PNBE or double-stranded PA, the k of block copolymers increased obviously, and the dielectric loss was also down to a low level as the block copolymers selfassembled into a well-defined nanostructure, whereas their energy density is still low (less than 3.0 J cm−3 at 245 MV m−1).58,59 Therefore, it is still a challenging task to develop the all-polymeric molecular composites for matching the requirement of polymeric dielectrics with high performance and reliability. Herein, we demonstrated a new type of all-polymeric molecular composites with an unprecedented bis(double-stranded) polymeric structure possessing the combined dipolar, electronic, and interfacial polarizations with the well-defined nanomorphology, in which the ionic-conjugated conductive segments with narrow band gap were effectively wrapped by the insulating segments with wide band gap to avoid the breakdown strength reducing for the narrow band gap, thereby exhibiting excellent dielectric (k > 33 and dielectric loss 7 J cm−3 at 600 MV m−1), low dielectric loss (93%).40 Recently, a class of “molecular composites” has been developed in which the conductive π-conjugated segments are directly covalent-bonded to polymer chains,29,41−44 and the charge displacement within the conductive segments can produce interfacial polarization in electric field, ultimately resulting in large dielectric responses. For instance, 0−10 wt % of the oligoaniline octamer moiety was attached to the end of ferroelectric poly(vinylidene fluoride) for preparing a “dumbbell-shaped” copolymer to significantly increase the k value from 12 to 85 at 103 Hz;44 when the addition of oligoaniline octamer exceeded 10 wt %, the dielectric loss increased rapidly because of the electron conduction across the film probably. An all-polymeric nanodielectrics of polymethacrylate carrying terthiophene oligomers as side chains were reported by Tang, and these polymers displayed k of 11, dielectric loss of 0.01−0.03, and energy density of 1.56 J cm−3 at 200 MV m−1,30 in which the side chains could self-assemble into conjugated, nanoscale, and electrically conductive domains dispersed in an insulating polymer matrix, leading to strong electronic and interfacial polarizations in the electric field. As a result, the dielectric properties, especially the dielectric loss and breakdown strength of all-polymeric molecular composites, are better than the traditional composites containing inorganic filler, whereas the limited improvements are yet unsatisfactory. For the polymeric dielectrics, their dielectric properties can be predicted by using density functional theory in a highthroughput manner.45 Remarkably, Ramprasad has reported the relationship between the band gap and the electronic and ionic k. With the decrease of band gap, the electronic part of k value increased obviously, while the ionic part of the k value is immune to the above trend, and the total k value is improved.46−49 The electronic part of k is suitable for applications in the sudden charge−discharge device due to the short time scales of this response; however, the too narrow band gap usually leads to poor insulators in the high electric field, and the device is not safe. Therefore, it is a main task to find an optimal trade-off between the total k value and the band gap, and the ideal band gap is about 3 eV.45,49 Polyacetylene (PA) is a kind of unique conducting polymers with a narrow band gap of about 2 eV, and its electrical conductivity can be systematically and continuously tuned over a range of 11 orders of magnitude when doped with a controlled amount of the halogens chlorine, bromine, or iodine (I2), and arsenic pentafluoride,50−54 which may have great application potential in all-polymeric molecular composites when it is wrapped in the wide band gap insulating segments. Doublestranded polynorbornene (PNBE)55,56 and double-stranded PA57 with rigid multilayer planar oligoaryl linker have been synthesized, which have greater resistance to irradiation and thermal and chemical degradation in comparison to their single-stranded counterparts, while they are usually known as

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RESULTS AND DISCUSSION Synthesis and Characterization of Bis(doublestranded) Block Copolymers. To further enhance the k and energy density, we designed the novel bis(double-stranded) block copolymer containing two different double-stranded ionic PNBE and conjugated PA segments, and successfully prepared the conductive diblock copolymer PBNPF30-b-PBHPF30, and the derived conductive-insulating tetrablock copolymers PBNPF 30 -b-PBHPF 30 -(b-PTNP 100 ) 2 and PBNPF 30 -bPBHPF30-(b-PTNP200)2, via the third-generation Grubbs catalyst56-initiated tandem ring-opening metathesis polymerization (ROMP) of N-3,5-bis(trifluoromethyl)biphenyl-norbornene pyrrolidine (TNP)36 and bis[(norbornene pyrrolidinium hexafluorophosphate)phenyl]perylene 2,3,6,7-tetra[bis(trifluoromethyl)phenyl]bisimide (BNPF),58 as well as metathesis cyclopolymerization (MCP) of bis(4-methoxyl-1, 6-heptadiyne) perylene 2,3,6,7-tetra[bis(trifluoromethyl)phenyl]bisimide (BHPF)59 by precisely tuning the structure of each monomer and optimizing the polymerization conditions (Scheme 1). The synthetic procedures and structure characterization of block copolymers are fully presented in the Supporting Information, Table S1, and Figures S1−S4. The results indicated that monomers were completely converted into the anticipated bis(double-stranded) polymer structures. More importantly, 13 C NMR analysis showed that the signals of acetylenic carbons on 1,6-heptadiyne of BHPF disappeared entirely after MCP. If the second double-stranded PBHPF skeleton was not formed effectively by MCP, the inevitable signals of acetylenic carbons should appear in NMR spectra, while the corresponding signals of acetylenic carbons were not found in Figures S3c,d, meaning that there was no triblock polymer structures formed as depicted in Scheme S1. In this new type of all-polymeric molecular composites, the ionic PBNPF, conjugated PBHPF, and insulating PTNP segments were covalently bonded in one polymeric backbone, and the conjugated PBHPF can be further modified by doping with I2. Therefore, the block copolymers possessing strong dipolar, electronic, and interfacial polarizations are envisioned to achieve high k, low dielectric loss, high breakdown strength and energy density, and good charge−discharge efficiency. 1103

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Scheme 1. Synthetic Protocol (a) and Graphic Representations for Conductive-Insulating Structure (b) of Bis(double-stranded) Block Copolymers

Nanostructure and Morphology of Block Copolymer Assembly. The block copolymer films made from the solution in different solvents possess various morphologies of nanostructures, and the regular morphology may endow the block copolymer films with better dielectric and energy storage properties than the irregular morphology of block copolymer.58−63 The block copolymer containing conductive-insulating segments with regular morphology would have the potential to produce substantially higher breakdown strength and better insulation properties because the formed regular conductive nanodomain can possess larger interacting areas with the insulating matrix than microsized domain,28−30 offering greater possibilities to tailor and optimize the properties of all-polymeric molecular composites. Therefore, the morphology of the block copolymer films formed by the solvent evaporation in air was investigated by transmission electron microscopy (TEM) analysis, and used to evaluate the dielectric and energy storage properties of copolymer. TEM image showed that the copolymers PBNPF30-b-PBHPF30 and PBNPF30-b-PBHPF30-(b-PTNP100)2 mostly gathered to form the morphology of irregular nanoshapes with the diameters of 100−300 nm and 60−150 nm in CHCl3 (Figure 1a,c and Figure S5a,c), respectively. In contrast, by changing the solvent to THF, PBNPF30-b-PBHPF30 formed the regular solid nanosphere morphology with an average diameter of 95 nm (Figure 1b and Figure S5b), and PBNPF30-b-PBHPF30-(b-PTNP100)2 formed a regular hollow nanosphere morphology with an average diameter of 40 nm (Figure 1d and Figure S5d), which was attributed to the self-assembly of block copolymers driven by both the solubility discrepancy of different segments (Table S2) and the π−π stacking interaction between the bis(double-stranded) PBNPF-b-PBHPF blocks with large aromatic PBI linkers close

to each other. As a result, the partially soluble hybrid conductive segments (HCS) of ionic PBNPF and conjugated PBHPF with narrow band gap (2.08−1.92 eV) were away from the solvent due to poor solubility in THF and self-assembled into the unique “hybrid conductive domain (HCD)” as the core; meanwhile, the good soluble insulating PTNP segments with wide band gap (3.26 eV) self-assembled as the shell, and finally the hollow nanosphere morphology was generated to lower the surface energy, where the HCD core was wrapped by the insulating shell, as shown in Scheme S2. Importantly, comparing the irregular nanoshape in CHCl3 to the regular hollow nanosphere in THF for PBNPF-b-PBHPF-(b-PTNP)2, the latter should provide a beneficial effect that the HCD was isolated between the insulated segment assembly, which effectively decreased the leakage current and dielectric loss. It is worth noting that some block copolymers performing the induced self-assembly process in selective solvents do not need any post-treatments, and various nanostructures can be predesigned to acquire optimal dielectric properties by changing the solvent or composition of chain segments, which is a very convenient method for preparing the required nanodielectric polymers in comparison to preparation of conventional nanocomposites by the in situ polymerization of monomers on initiator-functionalized inorganic nanoparticle surfaces.15 Further insight into the nanostructures of self-assembled copolymer formed in solution was conducted by dynamic light scattering (DLS) technique, and verified the aggregates existed in solution with different number-average hydrodynamic diameter (Dh). All the size values of copolymers with the self-assembled nanostructures were listed in Table S3. Details on the selfassembly behavior and the self-assembled nanostructure size 1104

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Figure 1. TEM images of PBNPF30-b-PBHPF30 (a,b) and PBNPF30-b-PBHPF30-(b-PTNP100)2 (c,d) in CHCl3 (a,c) and THF (b,d) at 0.01 mg mL−1.

70−120 nm were found on the fractured surface (Figures S8a−c), which was basically consistent with the self-assembled nanostructures by TEM (Figure 1c). For the copolymer film made from THF solution at high concentration, relatively regular nanoparticles with the size of 60−80 nm were observed, and the size was slightly larger than that of the hollow nanosphere by TEM (Figure 1d) at low concentration, which was in agreement with the size of the single nanoparticle in solution by DLS (Dh = 100 nm) being somewhat larger than that in dried state tested by SEM. Differently, there were some bright nanoparticles and dark or sunken nanoparticles, and all the nanoparticles were closely aligned on the fractured surface (Figures S8d−f) in which the bright nanoparticles are indicative of hardness and strength for these tightly aggregated regular nanostructures, similar to that of inorganic fiber embedded in the polymer matrix. Although the large self-assembled nanostructures with Dh of 300 nm in THF by DLS were not observed by SEM, this was because the detected large nanostructures by DLS were a combination of several hollow nanospheres, which were loosely aggregated by the π−π stacking interaction, as shown in Scheme S2. Therefore, it could be concluded that the self-assembled nanostructures indeed existed in polymer film and were clearly observed by SEM, which is very important and relevant for the dielectric properties of polymer films. Dielectric Properties of Block Copolymer Films. High k and low loss are important indicators of high-performance dielectric materials. All the k and dielectric loss of block copolymers with different nanostructured morphologies are listed in Table S4. Bis(double-stranded) diblock copolymer PBNPF30-b-PBHPF30 displayed enhanced k of 36−30 in CHCl3 (Figure 2a) or 35.0−31.5 in THF (Figure 2c) compared to the corresponding solely ionic PBNPF (33−26)58 or conjugated PBHPF (17.5) homopolymer,59 but accompanied by high dielectric loss of 0.25−0.15 in CHCl3 (Figure 2b) or 0.27−0.13 in THF (Figure 2d) as frequency varied from 102 to 106 Hz, most likely as serious conduction,17 which made it difficult for HCS PBNPF-bPBHPF to meet the demand for fast discharge film capacitor,

distribution of PBNPF30-b-PBHPF30, PBNPF30-b-PBHPF30(b-PTNP100)2, and PBNPF30-b-PBHPF30-(b-PTNP200)2 in different solvents at varied concentrations (0.005−0.02 mg mL−1) are revealed in the Supporting Information, and the results showed that the copolymers displayed an enhanced self-assembly ability and homogeneous size of nanostructures in THF with the dominating Dh of 42−102 nm compared to those in CHCl3 with the Dh of 67−265 nm, as shown in Figure S6, which were basically larger than those of the self-assembled nanostructure size of copolymers in dried state tested by TEM analysis. The self-assembly capability of selected PBNPF30-b-PBHPF30(b-PTNP200)2 in CHCl3 and THF at the concentration of 150 mg mL−1 was also investigated by DLS, as shown in Figure S7; there was a distinct peak with a dominating Dh of 170 nm for the assembly by π−π stacking in CHCl3, and a small number of single-molecule chains with a Dh of 17 nm still existed. For comparison, there are two peaks with the Dh of 100 nm for single assembly and 300 nm for a combination of several assemblies driven by the π−π stacking and solubility discrepancy between the constituent blocks in THF at a high concentration of 150 mg mL−1, as shown in Scheme S2. The number and size of a combination of several assemblies were significantly higher than those of the assemblies at low concentration because the distance between the assemblies decreased and the π−π stacking interaction kept them close. To prove the morphology of copolymer film was composed of the self-assembled nanostructures in the dried state, the copolymer PBNPF30-b-PBHPF30-(b-PTNP200)2 was selected as a representative to reveal the self-assembled nanostructures at so high concentration of 150 mg mL−1 as the film preparation for dielectric measurement. The scanning electron microscope (SEM) was used to observe the fractured surface to elucidate the morphology of polymer film. The fractured surfaces of the PBNPF30-b-PBHPF30-(b-PTNP200)2 films were obtained by using an external force to break polymer film and their morphologies were shown in Figure S8. For the copolymer film made from CHCl3 solution, the irregular nanoshapes with the diameters of 1105

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Figure 2. Dielectric constant (a,c) and dielectric loss (b,d) of polymer films from CHCl3 (a,b) and THF (c,d) solution.

so it should be improved using insulated segments to decrease the dielectric loss of ionic-conjugated copolymers by inhibiting the ion or electron conduction and mobility.37,38,53 Expectedly, when the insulated PTNP segments bonded to the HCS PBNPF-b-PBHPF forming the derived tetrablock copolymers PBNPF 30 -b-PBHPF 30 -(b-PTNP 100 ) 2 and PBNPF 30 -bPBHPF30-(b-PTNP200)2, they still exhibited high-k values of 32−27.5 and 31−27 in CHCl3 with irregular nanostructure (Figure 2a), as well as 32.5−28.0 and 31.0−27.5 in THF with a regular nanostructure (Figure 2c), respectively, while the dielectric losses decreased obviously to 0.09−0.05 and 0.04−0.03 in CHCl3 (Figure 2b) as well as 0.055−0.05 and 0.035−0.02 in THF (Figure 2d), respectively, due to the contribution from the strong dipolar, electronic, and positive interfacial polarizations28−30 combined with a regular nanostructure morphology by inhibiting the ion and electron conductions, which should be beneficial for practical applications. More importantly, all the HCS-contained block copolymers exhibited higher k than that of any homopolymer,36,58,59 meaning that using the nanoscale HCS composed of both ionic PBNPF and conjugated PBHPF segments with narrow band gap was an effective strategy to increase the k of polymers. The results implied that the block copolymers with the HCD-contained regular nanostructure morphology could improve the dielectric properties because the regular hollow nanosphere morphology provided a beneficial effect that the HCD with narrow band gap was isolated between the insulating shell with wide band gap to decrease the leakage current, and bringing two nanoscale interface polarizations in the electric field, as shown in Figure 3 and Scheme S3, which effectively improved the dielectric performance. The dielectric properties of molecular composites are affected by the conductivity of nanometer domains.29,41−43 The electrical conductivity of PA could be tuned over a range of 11 orders of magnitude by doping.51−53 To further improve the dielectric properties of nanoscale HCD-contained molecular composites, PBNPF30-b-PBHPF30-(b-PTNP200)2 was selected as a representative for doping with various amounts of I2 (Scheme S4) to

Figure 3. Schematic illustrations of the all-polymeric molecular composites PBNPF-b-PBHPF-(b-PTNP)2 capacitor device.

form the conductive PA nanometer domain, which can enhance the electronic and nanointerface polarizations. Figure 4 presented the k and dielectric loss of copolymer PBNPF30-b-PBHPF30-(bPTNP200)2 films with different I2 content. Figure 4a,c clearly shows the increase in k with the raised content of I2. When the copolymer contained various I2 contents from 2.5 to 100 mol %, the k value increased in sequence from 33.5 to 43.0 at 102 Hz, as well as from 27.9 to 33.6 at 106 Hz (Table S5), which were higher than the common commercial polymer dielectric materials. The great enhancement in k for the copolymer is attributed to their fraction of I2 doping. The k increased quickly below 10 mol % I2 and then slowly above 20 mol % I2 (Figure 4c), indicating the I2-doped PA as the HCD content was close to the threshold.1 All I2-doped copolymer films exhibited higher k than that of corresponding undoped copolymer PBNPF30-b-PBHPF30-(bPTNP200)2 (Table S5 vs Table S4), because the electronic and interfacial polarizations had great enhancement when the high conductive PA nanometer domain was constructed after I2 doping. The interfacial polarization displayed mostly the outstanding contribution to increase k. For PBNPF30-b-PBHPF30(b-PTNP200)2 with different I2 content, the main polarizations 1106

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Figure 4. Dielectric constant (a,c), and dielectric loss (b,d) of copolymer PBNPF30-b-PBHPF30-(b-PTNP200)2 films with different I2 contents from THF solution.

Figure 5. Leakage current density of the films made from block copolymers in CHCl3 (a), THF (b), and the PBNPF30-b-PBHPF30-(b-PTNP200)2 film with different I2 content in THF (c) at varied applied electric fields.

low level. As the I2 content increased, more and more I3− was generated. When the I3− ions exceeded the critical value, the shielding effect of the current became weaker and the leakage current became larger, resulting in an increased dielectric loss. Leakage Current Density of Block Copolymers. The insulating behavior is an important characteristic of dielectric materials, and the leakage current density is a great factor to affect the dielectric properties of polymer.15 The PBNPF30-bPBHPF30 film made from CHCl3 solution was easy penetrate at an applied electric field above 40 MV m−1, and its maximum leakage current density exceeded 1.4 × 10−6 A cm−2 at 40 MV m−1 (Figure 5a). As expected, the maximum leakage current densities of the PBNPF-b-PBHPF-(b-PTNP)2 films with mostly irregular shape in CHCl3 decreased to 1.3 × 10−6 and 3.8 × 10−7 A cm−2 at 100 MV m−1 with the elongated insulating PTNP block length due to the lowered mass concentration of HCD. For comparison, the PBNPF30-b-PBHPF30 film with the self-assembled solid sphere nanostructure in THF was penetrated at an electric field above 50 MV m−1 because the ionic blocks were isolated, and the maximum leakage current density was about 1.6 × 10−6 A cm−2 at 50 MV m−1 (Figure 5b); the corresponding PBNPF-b-PBHPF-(b-PTNP)2 films with the self-assembled regular hollow sphere nanostructure in THF

are dipolar, electronic, and interfacial polarizations. When the frequency was at a low range of 102−105 Hz, the dipolar, electronic, and interfacial polarizations could respond to the electric field in time and function together; while the frequency was over 105 Hz, the interfacial polarization could not catch up, and the curves of k showed a slump. The dielectric loss values of all copolymer films stayed at a reasonable low level of 0.058−0.026 in a wide frequency range of 102−106 Hz. As the I2 content increased, the dielectric loss value slightly increased from 0.04 to 0.058 at 102 Hz, as well as from 0.026 to 0.03 at 106 Hz (Figures 4b,d). Besides, when the I2 content was less than 60 mol %, the dielectric loss of copolymer films remained at a gentle increase with the increase in I2 content in the frequency range; when the I2 content was over 60 mol %, the loss at 3 × 102−105 Hz increased obviously (Figure 4b), which should be attributed to the serious dark current of excess I3− ions. According to the literature,64 the molecular I2 dissociates in a PA matrix to form I+ and I3− as shown in Scheme S5. The formation of a complex of this I+ with the conjugated double bond stabilized the cations, and the corresponding generated I3− was dispersed around the cations. Because the ions around the PA segment were isolated between the insulating shells, the increase value of dielectric loss was not obvious when the anion content held a 1107

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(Table S5). After the films were doped with different amounts of I2, the maximum polarizations for the copolymer films increased, while their breakdown field was decreased because of their raised conductivity (Figure S9c) and leakage current density (Figure 5c). When the films were doped with 2.5 and 5 mol % I2, the maximum polarizations of copolymer films were up to 5.09 μC cm−2 at 320 MV m−1 and 5.39 μC cm−2 at 300 MV m−1, respectively, which were higher than the undoped copolymer film (4.59 μC cm−2 at 370 MV m−1) even if the breakdown field fell (Figure S14b). Obviously, considering the electric field remained at 300 MV m−1, their polarizations still increased as the doped I2 content increased; in other words, the copolymer films with higher I2 content should have higher stored energy density under the same given electric field. Low remnant polarization and high energy storage efficiency can be expected because of narrower hysteresis loops.7 For energy storage dielectric materials, high energy density is the basic requirement for polymer dielectrics with high electrical energy storage performance, which came from three aspects: high breakdown field, high electric displacement, and last but not least, high energy efficiency.14,15,65,66 The energy density (Ue) of a capacitor was given by the following equation: Ue = ∫ E(dD),12 where E and D were the applied electric field and electric displacement, respectively. According to this equation, the stored energy density (Us) or released energy density (Ur) of polymers as a function of the maximum electric field strength could be obtained by integrating the charge or discharge curve of the D−E loop. The charge−discharge efficiency (η) was given by the following equation: η = (Ur/Us) × 100%. The dissipative energy during the charge−discharge cycle was a disadvantage in applications, and thus needs to be avoided. For the nanoscale HCD-contained block copolymer films with irregular nanostructure in CHCl3, the Us/Ur and η were 0.05/0.02 J cm−3 and 40.0% for PBNPF30-b-PBHPF30 at 30 MV m−1 (Table S4), which were even lower than those (0.29/0.20 J cm−3 and 68.9% at 79 MV m−1) of ionic PBNPF58 because of the severe current leakage of HCD. The Us/Ur and η increased obviously to 4.67/2.97 J cm−3 and 63.6% for PBNPF30-b-PBHPF30-(bPTNP100)2 at 250 MV m−1, as well as 6.83/5.60 J cm−3 and 81.9% for PBNPF30-b-PBHPF30-(b-PTNP200)2 at 330 MV m−1 (Figure 6a−c and Figure S15). For block copolymer films with well-defined nanostructures in THF, the Us and Ur were as low as 0.17 and 0.07 J cm−3 for PBNPF30-b-PBHPF30 at 50 MV m−1 (Table S4). Surprisingly, the Us and Ur were up to 9.49 and 6.18 J cm−3 for PBNPF30-bPBHPF30-(b-PTNP100)2 at 350 MV m−1, and even further to 9.95 and 8.33 J cm−3 for PBNPF30-b-PBHPF30-(b-PTNP200)2 at 370 MV m−1 (Figure 6d−f), which were at a high level in the dielectric polymer family and much higher than those of similar polymers (Us < 3.0 J cm−3 at 245 MV m−1);58,59 their η values were 44.6, 65.1, and 83.7% separately (Figure S15). These data were better than those of corresponding copolymer films with irregular nanostructure in CHCl3 because the narrow band gap segments were isolated by the wide band gap segments in the well-defined nanostructure, resulting in a higher breakdown strength, which displayed a positive contribution to improve Ur and η. Besides, as the insulating PTNP block length increased, the corresponding η was improved, which was in agreement with the lowered dielectric loss of copolymers. For the I2-doped copolymer films, they exhibited decreased Us/Ur and breakdown field of 8.79/7.52 J cm−3 and 320 MV m−1 with 2.5 mol % I2, as well as 8.99/7.70 J cm−3 and 300 MV m−1 with 5 mol % I2 (Figures 7a and Figure S16a,b), respectively,

displayed the lowered leakage current densities of 1.8−1.2 × 10−7 A cm−2 at 100 MV m−1, implying that the well-defined hollow nanosphere could reduce the leakage current and improve the dielectric properties of block copolymers, which did have an obvious influence on the dielectric loss and energy efficiency. The leakage current densities of copolymers followed a reasonable increasing trend with the raise of applied electric field. The resistivity calculated from the I−V characteristics is 1010−1013 Ω·cm, which was consistent with the conductivities of the films under increased frequency (Figure S7). Namely, they had excellent insulating property and qualified for practical applications. Intuitively, the maximum leakage current densities of the PBNPF30-bPBHPF30-(b-PTNP200)2 films containing 2.5 and 5 mol % doped I2 was 1.7 × 10−7 and 2.6 × 10−7 A cm−2 at 100 MV m−1 (Figure 5c), respectively, which was not significantly higher than that of undoped copolymer. With the I2 content increasing, the maximum leakage current densities of copolymer films containing 10, 20, 40, 60, 80, and 100 mol % I2 were up to 5.8 × 10−7, 8.2 × 10−7, 1.4 × 10−6, 2.9 × 10−6, 5.8 × 10−6, and 9.2 × 10−6 A cm−2 at 100 MV m−1 in sequence, and they were 5−75 times higher than that of undoped copolymer, which did have an obvious influence on the maximum breakdown filed and energy density. So the investigation of the electrical energy storage properties focused on the block copolymer films containing the optimal doped I2 content (≤5 mol %). Electrical Energy Storage Properties of Block Copolymers. The linear polarization behavior and low hysteresis observed for polymers were indicative of the low level of energy dissipation, which was in agreement with the low dielectric loss in the impedence test.1,13,65 Predictably, the hysteresis loop with dipole saturation of HCS PBNPF30-b-PBHPF30 was wide at high electric fields due to the serious leakage current; the ion and electron migrations were responsible for the hysteresis, which was a disadvantage in thin film capacitors, and the maximum polarization of HCS PBNPF30-b-PBHPF30 with a regular solid sphere nanostructure in THF was lowered to 0.45 μC cm−2 at 50 MV m−1. The hysteresis loops of conductive copolymers could be effectively narrowed by lengthening the insulating PTNP blocks because the ion and electron migrations were restrained. As expecting, insulating segments-bonded HCS of PBNPF 30 -b-PBHPF 30 -(b-PTNP 100 ) 2 and PBNPF 30 -bPBHPF30-(b-PTNP200)2 with well-defined hollow nanosphere in THF displayed narrower hysteresis loops, and thus their maximum polarizations were reached up to 4.25 μC cm−2 at 350 MV m−1 and 4.59 μC cm−2 at 370 MV m−1 (Figures S12d−f), respectively. These maximum polarizations had a strengthened change compared to those (0.22 μC cm−2 at 30 MV m−1, 2.95 μC cm−2 at 250 MV m−1, and 4.08 μC cm−2 at 330 MV m−1, respectively) of the corresponding copolymers with irregular nanostructure in CHCl3 (Figures S12a−c), indicating that the well-defined nanostructure affected the maximum polarization and narrowed the hysteresis loops of conductive-insulating copolymers to a certain extent because the hybrid conductive PBNPF-b-PBHPF segments were effectively wrapped by the insulating PTNP segments, and thus decreased the leakage current densities and improved the interfacial polarization, which was in good agreement with low dielectric loss. Figures S13a,b and S14a showed the D−E loops of the selected copolymer PBNPF30-b-PBHPF30-(b-PTNP200)2 films with the optimal doped I2 content measured at 102 Hz under varying electric fields at room temperature. High electric displacement was observed for the copolymer films resulting from their high k, which should be favorable for high energy density 1108

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Figure 6. Energy density versus applied electric field for polymer films from PBNPF30-b-PBHPF30 (a,d), PBNPF30-b-PBHPF30-(b-PTNP100)2 (b,e), and PBNPF30-b-PBHPF30-(b-PTNP200)2 (c,f) in CHCl3 (a−c) and in THF (d−f).

Figure 7. Stored energy density versus applied electric field (a) and energy density versus I2 content (b) for I2-doped PBNPF30-b-PBHPF30-(bPTNP200)2 films from THF solution.

compared to the undoped copolymer film (9.95/8.33 J cm−3 and 370 MV m−1) (Table S5). Although the maximum Us/Ur and the breakdown field declined basically with the increase in I2 content, the representative Us/Ur values of I2-doped copolymer films at a certain electric field were still higher than those of undoped copolymer film; for example, they were 7.72/6.63 J cm−3 with 2.5 mol % I2 or 8.99/7.70 J cm−3 with 5 mol % I2 at 300 MV m−1, higher than 5.80/4.89 J cm−3 with 0 mol % I2 at 300 MV m−1 (Figure 7b or Figure S16i), which were at a high level in the polymeric dielectric substance and greatly exceeded the best commercially available biaxially oriented polypropylene (BOPP) dielectrics (1.6 J cm−3 at 400 MV m−1).7,30,62 Importantly, compared to those previously reported molecular composites containing dodecylbenzenesulfonic acid, camphorsulfonic acid, or HCl-doped oligoaniline,29,41,42 the I2 dopant has its distinctive merit for doped copolymer films. Usually, the protonic acid could drastically reduce the breakdown field and η despite low doping content due to the harmful ionic polarization, which limited frequency response of the capacitors, while I2 as a nonproton dopant had little effect on the breakdown field and η at low doping content, which were

mainly affected by the leakage current. Therefore, the main aim could be focused on reducing the leakage current of polymers. The covalent linkage between the conductive and insulating moieties and construction of a unique nanostructure could be of benefit to reduce the leakage current. The conductive I2-doped PA and ions (nanoscale HCD) were wrapped in the core of a hollow nanosphere, and their η values were relatively high, which were 85.6 and 85.7% with the I2 content of 2.5 and 5 mol % (Table S5), respectively. Finally, compared with some previous all-polymeric dielectrics,9,32,33,49,67−75 the energy densities versus efficiencies of the nanoscale HCD-contained copolymer PBNPF30-b-PBHPF30-(b-PTNP200)2 films without or with 2.5 and 5 mol % I2 showed a combination of high energy density and efficiency level (Figure 8), which seem to be better candidates for dielectric materials in comparison to the reported most polymer dielectrics.

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CONCLUSIONS By rational design, novel all-polymeric molecular composite of HCS-contained bis(double-stranded) conductive-insulating block copolymer PBNPF-b-PBHPF-(b-PTNP)2 was synthesized, 1109

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21374030) and the Large Instruments Open Foundation of East China Normal University (No. 20162042). Notes

The authors declare no competing financial interest.

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Figure 8. Energy density versus efficiency of PBNPF30-b-PBHPF30-(bPTNP200)2 at 370 MV m−1, PBNPF30-b-PBHPF30-(b-PTNP200)2 with 2.5 mol % I2 at 320 MV m−1, PBNPF30-b-PBHPF30-(b-PTNP200)2 with 5 mol % I2 at 300 MV m−1, and some other polymer dielectrics at 400 MV m−1 in the previous literature.

and self-assembled into a HCD-contained regular nanostructure with the strong dipolar, electronic, and interfacial polarizations, as well as the good film-forming property (Figure S19). The block copolymer with the regular hollow nanosphere morphology exhibited high-k value of about 30, low dielectric loss of 0.02, and excellent stored/released energy density of 9.95/8.33 J cm−3 with good charge−discharge efficiency of 83.7%, which were superior to those of the same copolymers with irregular nanostructure, and also better than those of any ionic or conjugated polymers. After the films were doped with a suitable amount of I2, the k value of block copolymer films can further reach 34 with low dielectric loss of 0.03, and the stored/released energy density maintained at 8.99/7.70 J cm−3 with better charge−discharge efficiency of 85.7%. Therefore, the proposed HCS polymer structure, multiple polarizations, and well-defined nanomorphology can drastically enhance the dielectric properties and improve the application potential of polymer films in energy storage devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b05042. Full experimental procedures, characterization, and related discussion; tables; schemes; GPC traces; UV− vis, IR, and NMR spectra; DLS diagrams; TEM and SEM images; D−E loops; conductivity, dielectric constant, and loss at different temperatures, energy density, and charge−discharge efficiency curves; TGA and DSC curves; stress−strain curves; photographs (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.X.). *E-mail: [email protected] (X.H.). ORCID

Ruyi Sun: 0000-0001-8913-0431 Xingyi Huang: 0000-0002-8919-6884 Meiran Xie: 0000-0002-7411-3927 Funding

This research was financially supported by the National Natural Science Foundation of China (Nos. 21574041, 21704025, and 1110

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DOI: 10.1021/acs.chemmater.7b05042 Chem. Mater. 2018, 30, 1102−1112

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DOI: 10.1021/acs.chemmater.7b05042 Chem. Mater. 2018, 30, 1102−1112