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An of-the-moment discussion of the most cutting-edge research in the field of polyimide dielectrics
In Functional Polyimide Dielectrics: Structure, Properties, and Applications, distinguished researcher Dr. Jun-Wei Zha delivers an up-to-date and authoritative discussion of the latest advancements in the application of polyimide dielectrics (PIDs) in electrical and electronic equipment. Readers will find coverage of the application of PIDs in aerospace, high-temperature energy storage capacitors, corona-resistant motors, new energy power equipment, packaging, and more.
The book serves as a valuable reference for improving the performance and expanding the application of existing PIDs, as well as designing and developing new PIDs. It also provides a current overview of research and frontier applications that will prove useful to students and researchers from multiple disciplines.
Functional Polyimide Dielectrics also contains:
- A thorough introduction to the different types of polyimide dielectrics, including their synthesis, composites & blends, processing-property relationships, processing strategies and technologies, properties and applications
- Comprehensive explorations of a wide range of PID applications in a variety of special applications, like energy storage, insulation, thermal conductivity, intelligence, and more
- Practical discussions of the environmental aspects of the technology, as well as life cycle assessment sustainable alternative possibilities
- Complete treatments of smart polyimide dielectrics
Perfect for materials scientists, electronics engineers, polymer chemists, physicists, and mechanical engineers, Functional Polyimide Dielectrics: Structure, Properties, and Applications will also benefit professionals, engineers, and scientists working in the chemical industry.
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Veröffentlichungsjahr: 2025
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
1 Overview of Polyimide Dielectrics
1.1 Introduction
1.2 Structure Design of Polyimide
1.3 Fabrication of Polyimide
1.4 Applications of Polyimide
1.5 Summary and Outlook
References
2 Structures and Properties of Polyimide Dielectric Materials
2.1 Introducing Polar Groups
2.2 Introducing Non‐Conjugated Structures
2.3 Introducing Non‐Coplanar Structures
2.4 Introducing Crosslinking Structures
2.5 Summary and Outlook
References
3 Processing Technology of Polyimide Dielectrics: From Experimental to Industrial Scale
3.1 Introduction
3.2 PID Synthetic Chemistry
3.3 Processing Technologies for PIDs
3.4 Summary
Acknowledgments
Declarations
References
4 Polyimide Dielectrics for High‐Temperature Energy Storage
4.1 Introduction
4.2 Key Characteristics and Mechanisms of Polyimide Dielectrics for Energy Storage
4.3 Intrinsic Polyimide Dielectrics
4.4 All‐Organic Polyimide Composite Dielectrics
4.5 Polyimide‐Based Nanocomposite Dielectrics
4.6 Self‐Healing Polyimide Dielectrics
4.7 Conclusion and Perspective
References
5 Polyimide Dielectrics with Low Dielectric Permittivity
5.1 Fundamental Principles for Low Dielectric Permittivity (Low‐
k
)
5.2 Intrinsic Low‐
k
PI
5.3 Low‐
k
PI Composites
5.4 Porous Low‐
k
PIs
5.5 Multifunctional Low‐
k
PIs
5.6 Conclusion and Prospects
References
6 High Thermally Conductive Polyimide Dielectrics
6.1 Introduction
6.2 Intrinsic Thermally Conductive Polyimide
6.3 Filled Thermally Conductive Polyimide
6.4 Multifunctionalization of Thermally Conductive Polyimides
6.5 Conclusions and Outlook
References
7 Polyimide Nanocomposites for Electromagnetic Interference Shielding
7.1 Introduction
7.2 Theoretical
7.3 Nanofillers Used for PI Electromagnetic Shielding
7.4 Ternary Composites
7.5 Structural Design
7.6 Future Development and Prospects
7.7 Summary
References
8 Colorless Transparent Polyimides with Balanced Dielectric and Optical Properties
8.1 Introduction
8.2 Synthesis and Molecular Structure Design of PIs
8.3 Characterization and Evaluation of Properties
8.4 Advanced Design Strategies for Fabricating High‐Transparency Low‐Dielectric PIs
8.5 Summary and Outlook
References
9 Polyimide Dielectrics in Extreme Environment
9.1 Atomic Oxygen Erosion Mechanism of Polyimide
9.2 Atomic Oxygen Protection Technology for Polyimide
9.3 Example of Atomic Oxygen Erosion Protection
9.4 Summary
References
10 Smart Polyimide Dielectrics
10.1 Introduction
10.2 Shape Memory Polymer
10.3 Heat‐driven SMPI
10.4 Remote‐driven SMPI
10.5 Self‐healing Polyimide
10.6 SMPI‐based Flexible Optical/Electronic Devices
10.7 Smart Polyimide Foam
10.8 Atomic‐Oxygen‐Resistant Polyimide Films with Shape Memory Function
References
Index
End User License Agreement
List of Tables
Chapter 2
Table 2.1 Permittivity and polarization response of benzonitrile‐containing...
Table 2.2 Summary of key data for representative polyimide dielectric mater...
Chapter 3
Table 3.1 Chemical structures of typical TPI dielectrics.
Chapter 7
Table 7.1 PI‐based binary composites for EMI shielding.
Table 7.2 PI‐based binary composites for EMI absorbing.
Table 7.3 PI‐based hybrid materials for EMI shielding.
Table 7.4 PI‐based hybrid materials for EMI absorption.
Chapter 8
Table 8.1 Recent noteworthy review articles on PIs.
Table 8.2 Chemical structures of reported aliphatic/alicyclic‐containing di...
List of Illustrations
Chapter 1
Figure 1.1 Intermolecular and intramolecular charge transfer (CT) in PI.
Figure 1.2 Schematic diagram of the thermal imidization process of PAA.
Figure 1.3 Applications of polyimide.
Chapter 2
Figure 2.1 Structures of benzonitrile‐containing PIs, dielectric properties ...
Figure 2.2 Structure of 2CN‐BTDA, relevant
T
g
values and dielectric properti...
Figure 2.3 Structures of nitrile‐containing diamine BCA and relevant PIs, en...
Figure 2.4 (a) Structures of sulfonyl‐containing PIs based on ODPA and DSDA....
Figure 2.5 (a) Structures of PAA and PI based on PMDA/
m
‐BAPS, discharged ene...
Figure 2.6 (a) Structures of PIs containing bipyidine and relevant dielectri...
Figure 2.7 (a) Structures of semi‐aromatic PIs containing short‐chain alipha...
Figure 2.8 (a) Structures of cycloaliphatic PIs based on CBDA, calculated ba...
Figure 2.9 Structures of two selected PI (A3‐B1) and PI (A3‐B7) with the mos...
Figure 2.10 (a) Structures of aromatic PIs and small aromatic molecules, sch...
Figure 2.11 Structures of traditional and spiro‐structured PIs, schematic di...
Figure 2.12 Structure of PEPA‐terminated PEI (BPADA/4,4′‐ODA) and crosslinki...
Figure 2.13 (a) Structures of NA‐terminated PI (ODPA/4,4′‐ODA), dielectric l...
Chapter 3
Figure 3.1 Development of polymer dielectrics and their maximum usable tempe...
Figure 3.2 Classification of PI dielectrics according to the forms of applic...
Figure 3.3 Common synthesis chemistry for PI dielectrics.
Figure 3.4 Theoretical polymerization reactivity predicted by the molecular ...
Figure 3.5 Processing illustration for the PAA‐type of dielectric varnish at...
Figure 3.6 Processing technology for PID films with industrial scale. (a) PA...
Figure 3.7 Processing technology for PI nanodielectric films based on cSiO
2
...
Figure 3.8 Processing technology for PI molding parts dielectrics based on t...
Figure 3.9 Processing technology for crosslinking PI aerogel dielectrics at ...
Figure 3.10 Dielectric breakdown of PI aerogel dielectrics under high voltag...
Chapter 4
Figure 4.1 Structure and application of PI‐based dielectrics for high‐temper...
Figure 4.2 (a) Schematic diagram of electric polarization of dielectric capa...
Figure 4.3 Polarization types and their frequency dependencies of dielectric...
Figure 4.4 Schematic diagram of (a) breakdown types [31], (b) electric break...
Figure 4.5 Schematic diagram of self‐healing process of metalized dielectric...
Figure 4.6 (a) Left: Chemical structures of TP‐PI and STP‐PI and digital pho...
Figure 4.7 (a) The relationship between structure with
E
g
and
T
g
[61]. (b) S...
Figure 4.8 (a) Structures of PI and PEI, and schematic diagrams of interchai...
Figure 4.9 (a) Breakdown strength, (b) energy storage properties, (c) curren...
Figure 4.10 (a) Schematic of the fabrication process of impregnated PI films...
Chapter 5
Figure 5.1 Basic strategies for reducing dielectric permittivity.
Figure 5.2 Low‐
k
PIs with rigid anthracene‐based structures.
Figure 5.3 (a) Schematic diagram of electrostatic potential of phenyl POSS/P...
Figure 5.4 (a) Schematic of the preparation of t‐FG/FPI films. (b) Variation...
Figure 5.5 Different types of POSS‐diamines: (a) aminopropylisobutyl‐POSS ...
Figure 5.6 Schematic diagram of the spatial structure of COF and the crossli...
Figure 5.7 (a) Internal network structures of single and double crosslinked ...
Figure 5.8 (a) Schematic molecular and spatial structure of IL‐CaF
2
/PI compo...
Chapter 6
Figure 6.1 Heat transfer mechanism of polymers.
Figure 6.2 Schematic diagram of molecular chains and molecular structure: (a...
Figure 6.3 (a) Preparation schematic, (b) polarizing microscope image, (c) i...
Figure 6.4 Schematic diagram of the thermal conductivity network.
Figure 6.5 (a) Preparation diagram of PI/BNNS/CNT@αPVA composite film; (b) s...
Figure 6.6 (a) Motion magnetic field induction and heat conduction mechanism...
Figure 6.7 (a) Schematic of the preparation of M@GNS/PI composite films; (b)...
Figure 6.8 (a) Structure diagram of PI composite film with layered structure...
Figure 6.9 (Fe
3
O
4
/PI)‐Ti
3
C
2
T
x
‐(Fe
3
O
4
/PI) composite films with (a) preparatio...
Chapter 7
Figure 7.1 Schematic illustration of microwave interaction with matter.
Figure 7.2 (a) Iron‐based magnetic components and (b) dielectric absorber of...
Figure 7.3 Porous rGO/PI composite foams with closed cell structure.
Figure 7.4 Schematic illustration of the synthesis of CF@NiCo and the fabric...
Figure 7.5 Preparation of gradient‐conductive PI aerogel frame with integrat...
Figure 7.6 (a) Schematic illustration of the synthetic strategy for the FMAP...
Figure 7.7 (a) Schematic of the EMI of Ag@PI aerogel fabric. (b–c) digital p...
Figure 7.8 PI/SiO
2
aerogel fabricated by freeze‐drying method.
Figure 7.9 PI composites with (a) sandwich and (b) multilayer structure for ...
Figure 7.10 PI‐based composites with inner structures.
Chapter 8
Figure 8.1 Frequency response of the dielectric mechanisms [23].
Figure 8.2 Molecular design of highly fluorinated aromatic diamines and uniq...
Figure 8.3 The derived PI films obtained by adjusting the molar ratio of TFM...
Figure 8.4 The synthesis of partially alicyclic PIs based on TB demonstrates...
Figure 8.5 Three aromatic diamines (PPy, mBPPy, mTPPy) with a rigid nonplana...
Figure 8.6 Synthetic route to BATFPX (3) and PIs 5a–5e [93].
Figure 8.7 High thermally stable anthracene‐based PIs with low dielectric co...
Figure 8.8 Preparation of PI nanocomposites with OAPS [103].
Figure 8.9 Synthesis process of CPI/18 CE6 composite films [109].
Figure 8.10 The four paradigms of science: empirical, theoretical, computati...
Figure 8.11 (a) Experimental ε as a function of the frequency, along with th...
Chapter 9
Figure 9.1 Atomic oxygen erosion processes.
Figure 9.2 Surface SEM image of Kapton sample after atomic oxygen irradiatio...
Figure 9.3 Surface AFM image of Kapton sample after atomic oxygen irradiatio...
Figure 9.4 Sesquicarbazone molecular structure formula (POSS).
Figure 9.5 International space station PI usage.
Figure 9.6 Interplate cables with silicone protective coating deposited on t...
Chapter 10
Figure 10.1 Deformation process of SMP: Shape programming and shape recovery...
Figure 10.2 Shape memory process of unidirectional SMP: (a) The shape memory...
Figure 10.3 Shape memory effect curve of graphene‐reinforced polyimide.
Figure 10.4 Shape recovery process of shape memory polyimide.
Figure 10.5 Shape memory properties of PIs during different cycle times.
Figure 10.6 (a) Digital photos and (b) infrared thermal images of shape memo...
Figure 10.7 (a) The chemical structures of reactive monomers and cross‐linke...
Figure 10.8 (a–f) Preparation process. (g) Composition structure of prepared...
Figure 10.9 (a) Repair process of microcapsules; (b) the anti‐corrosion appl...
Figure 10.10 Classification of intrinsic self‐healing materials.
Figure 10.11 Effect of self‐healing time on repair performance of semi aroma...
Figure 10.12 SHSMPI perforation and scratch self‐healing process diagram....
Figure 10.13 (a) Schematic diagram of self‐healing mechanism of PU and (b) P...
Figure 10.14 Schematic diagram and reaction equation of the healing mechanis...
Figure 10.15 Schematic diagram of Mimosa smart thin‐film principles.
Figure 10.16 Schematic diagram of RSPI film synthesis and dissociation proce...
Figure 10.17 (a) Stretchable shape memory cycles of CSMPI substrate. (b) Cur...
Figure 10.18 (a) Optical microscope and (b) laser confocal microscope image ...
Figure 10.19 Application of smart polyimide foam.
Figure 10.20 Polyimide‐based electrode based on a highly efficient palladium...
Figure 10.21 Schematic illustration for fabricating carbon foams derived fro...
Figure 10.22 (a) Schematic diagram of polyimide‐derived laser‐induced graphe...
Figure 10.23 (a) Schematic of electronic skin made of PI/CNT foam. Optical p...
Figure 10.24 The production of AO and its erosive effect to polymer.
Figure 10.25 Surface morphology (a) and surface Si map (b) of PI‐SiO
2
‐15; su...
Figure 10.26 Schematic illustration of shape memory behaviors.
Figure 10.27 A typical SEM image of the AO‐exposed surface (a) polyimide, (b...
Guide
Cover
Table of Contents
Title Page
Copyright
Begin Reading
Index
End User License Agreement
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Functional Polyimide Dielectrics
Structure, Properties, and Applications
Edited by Jun-Wei Zha
Editor
Prof. Jun‐Wei Zha
University of Science and Technology Beijing
No. 30 Xueyuan Road
Haidian District
Beijing 100083
China
Cover Images: © Eugene Mymrin/Getty Images, Courtesy of Jun‐Wei Zha
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1Overview of Polyimide Dielectrics
Mengyu Xiao and Jun‐Wei Zha
University of Science and Technology Beijing, School of Chemistry and Biological Engineering, Department of Chemistry, 30 Xueyuan Road, Haidian District, Beijing, 100083, P. R. China
1.1 Introduction
The advancements in aerospace and new energy technologies have accelerated the demand for high‐temperature‐resistant and high‐performance polymer dielectric materials. Researchers have developed a range of specialty engineering plastics such as polyphenylene sulfide (PPS), polytetrafluoroethylene (PTFE), poly(ether ether ketone) (PEEK), polyethersulfone (PES), and polyimide (PI) [1]. PI is a class of polymer that contains an imide structure in the molecular backbone. The rigid imide ring gives PI excellent high‐temperature resistance; thus, only cyclic polyimides are of practical application. PI was first reported in 1908, [2] and DuPont commercialized Kapton films in the mid‐1960s. Since then, PI has entered an era of booming growth.
PI has been fully developed as a promising polymer, especially in the field of insulating and functional materials. The reason for the considerable interest in PI, compared to other high‐temperature‐resistant polymers, is due to its high structural designability, numerous synthesis and processing methods, as well as its outstanding comprehensive performance and wide range of applications.
This chapter provides a concise overview of the design, fabrication, and application of PIs. Firstly, the PI is introduced from a structural perspective in order to provide an overview of the general relationship between structure and function, which will then be used to guide the subsequent design. Then, the synthesis of PI and the preparation of different types of PI dielectric materials are discussed. Subsequently, the various properties of PI and the corresponding application areas are summarized. Finally, the key points of PI dielectrics are summed up and the future development is prospected.
1.2 Structure Design of Polyimide
The process of designing a polyimide usually involves careful planning of its molecular structure to ensure that the resulting material has the desired properties. PI consists of alternating electron donors (diamines) and electron acceptors (dianhydrides). Therefore, designing PI can be realized by changing the structure of diamine and dianhydride. More than 1000 diamines and more than 400 dianhydrides are currently used to synthesize PIs, resulting in thousands of PIs with different structures. Here, only some common structure–property relationships are briefly described, such as thermal, mechanical, and electrical properties.
For PI, during aromatic heterocyclic polymerization, the glass transition temperature (Tg) is mainly related to the relevant chain length of the macromolecule and the intermolecular forces. PI has a higher Tg and thermal stability than the corresponding polyether or polyester due to intermolecular forces other than van der Waals forces. Intramolecular or intermolecular charge transfer interactions are induced between the electron acceptor and electron donor of the PI, as shown in Figure 1.1. The charge transfer effect is related to the electron affinity of the dianhydride (or the hole affinity of the diamine) and the conformation of the molecular chain. It can be qualitatively argued that increasing the electron affinity of the dianhydride enhances interchain interactions, i.e., increases Tg. Intramolecular charge transfer is greatest when the electron donor and the electron acceptor are co‐planar, but least when they are perpendicular. The planar structure between the electron donor and acceptor can be disrupted by making neighboring substitutions to the C—N bond. For example, with neighboring methyl substitution of diamines, the site‐blocking effect disrupts the coplanar structure and increases the rigidity of the molecular chain, resulting in an improved Tg.
Figure 1.1 Intermolecular and intramolecular charge transfer (CT) in PI.
Source: Ref. [3]/Springer Nature.
The mechanical properties of polymers are mainly influenced by the molecular structure, which determines the intramolecular chemical bonding forces and intermolecular forces. For example, increasing the polarity of the PI or creating hydrogen bonds can increase the chemical bonding and intermolecular forces of the main chain, resulting in increased tensile strength. In addition to the molecular structure, the mechanical properties of PI depend on the synthesis method and processing conditions. The thermal history during the formation of PI affects its aggregate state structure, which ultimately has an impact on the mechanical properties.
The electrical properties of polymers refer to the behavior of polymers under the action of an applied voltage or electric field and the various physical phenomena they exhibit. PI is a typical linear dielectric with excellent dielectric and insulating and properties. The dielectric permittivity of PI is mainly contributed by dipolar polarization of their polar groups. Therefore, the introduction of polar groups into the PI molecular chain, or an appropriate increase in free volume can improve the dielectric permittivity of PI. For instance, Liu et al. [4] introduced polar groups ‐COOH and ‐CO‐NH‐ into the PI main chain by controlling the degree of imidization of PAA (Figure 1.2), resulting in a dielectric permittivity up to 4.59@1 kHz. However, the promotion strategies of dielectric permittivity usually lead to a higher dielectric loss. Thus, systematic consideration of the molecular/structure design according to the polarization mechanisms is necessary to improve the dielectric loss of PIs. The insulating properties of PIs depend mainly on the bandgap and carrier traps. Wang et al. [5] found that the high‐temperature insulation performance would experience diminishing marginal utility as the bandgap increases beyond a critical point (∼3.3 eV) through the study of some series of PI derivatives. Therefore, it is essential to ensure a wide bandgap while constructing deep traps to enhance the insulation performance of PI [6]. There are three strategies for constructing carrier traps: adjusting the intrinsic structure of polymers, preparing inorganic/polymer composites and all organic polymer composites.
Figure 1.2 Schematic diagram of the thermal imidization process of PAA.
Source: Adapted from Ref. [4].
1.3 Fabrication of Polyimide
PI boasts of numerous synthetic pathways and can be tailored for diverse applications, making it unparalleled among other polymers [7]. PIs are generally synthesized by a two‐step process. Firstly, polyamic acid (PAA) is obtained by low‐temperature polycondensation of dianhydride and diamine in a polar solvent (N, N‐dimethylacetamide, N, N‐dimethylformamide, and N‐methylpyrrolidone). Subsequently PAA can be thermally or chemically dehydrated to obtain PI. PI can also be synthesized in one step, where the dianhydride and diamine are polycondensed by heating in a high boiling solvent (phenolics). The monomer ratio, dosing sequence, and reaction temperature are the core parameters affecting the PI polymerization reaction. For example, the ring‐opening polymerization of dianhydride and diamine is an equilibrium reaction and has a high equilibrium constant, with the reaction heavily biased in favor of the product [8]. The polymerization process is significantly exothermic, and a proper lowering of the reaction temperature is more conducive to a positive reaction.
After decades of development, PI can be processed by methods suitable for most polymers. For example, PAA solutions can be utilized for cast film formation, spin coating, and spinning. Because inorganic salts are not generally produced during the synthesis of PI, there is no need for an additional purification step, which is very favorable for the preparation of insulating materials. PI can also be thermo‐compressed, extruded, and injection molded by melt processing. Moreover, it is possible to utilize the easy sublimation of dianhydride and diamine for vapor‐phase deposition. The diverse range of processing technologies employed by PI enables the production of a multitude of materials, including films, fibers, foams, and adhesives.
1.4 Applications of Polyimide
PI, a high‐performance polymer, enjoys widespread utilization across various industries owing to its exceptional structural properties, facile synthesis process, and adaptability to diverse processing techniques. Its unique chemical and physical characteristics have allowed it to demonstrate outstanding performance in a broad range of applications, as shown in Figure 1.3[9]. Each of these applications, from aerospace and automotive components to medical implants and semiconductor devices, has benefited from PI's ability to deliver superior performance in challenging environments. Consequently, PI continues to be a highly sought‐after material for a diverse range of applications worldwide.
Figure 1.3 Applications of polyimide.
Source: Ref. [9]/John Wiley & Sons.
1.4.1 Capacitive Energy Storage
PI has a very broad application prospect in the field of dielectric energy storage. There is now a high demand for polymer dielectrics with outstanding high‐temperature capacitance performance [10]. Because in many areas, such as hybrid electric vehicles, underground oil/gas exploration, and aircraft electrification, film capacitors typically operate at high temperatures (>150 °C) [11]. However, the maximum operating temperature of commercially available biaxial‐oriented polypropylene (BOPP) should not exceed 105 °C [12]. PI is considered to be the most promising material for high‐temperature energy storage. Therefore, many researchers have carried out studies in order to improve the usage of PI in high‐temperature energy storage applications [13, 14]. In addition to physical energy storage, PI can be used in the new energy field to manufacture high‐performance battery separators and fuel‐cell bipolar plates.
1.4.2 Electronic Devices
As an important interlayer dielectric material for flexible copper‐clad laminate, PI film plays a crucial role as the structural and functional basis for flexible printed circuit boards. However, the dielectric permittivity of PI is about 3.4, which is no longer sufficient to meet the needs of integrated circuits process development. In order to reduce the dielectric permittivity of PI, researchers have carried out a lot of modification work on PI from the Clausius–Mosotti equation. The dielectric permittivity of PIs can be effectively reduced by introducing fluorine atoms, bulky side groups, or porous structures [15]. Moreover, designing dielectrics with low permittivity and high thermal conductivity is crucial to improve the signal transmission quality of smart terminals and effective heat dissipation of devices [16].
1.4.3 Electrical Engineering
PI is particularly suitable for use as a flexible printed circuit board substrate. For example, transparent polyimide films can be used to make flexible solar cell substrates. Moreover, PI is widely used as an insulating material for high‐frequency transformers, inverter motors, and wind power generation equipment. However, the poor corona resistance of PI at high‐frequency voltages seriously shortens the service life of electrical equipment. The addition of appropriate amounts of nanoparticles has been shown to be effective in improving the corona resistance of PI [17]. DuPont has developed a sandwich structure of 100 CR films (top and bottom layers are Al2O3‐doped PI composite films and the middle layer is a pure PI film), which provides higher corona resistance at high‐frequency voltages, up to 10 times that of a pure PI film (100 HN) [18].
1.4.4 Medical Materials
PI has low toxicity and can be used as a multifaceted medical material. Certain PIs have high compatibility with blood and tissues, which is the reason for their interest in biocompatible material applications. For instance, PI can be used to make artificial joints, implantable medical devices, and biodegradable sutures.
1.4.5 Environmental Protection
The heat and organic solvent resistance of PI is particularly important in the separation of organic liquids and gases. High‐temperature filtration materials prepared from PI fibers are widely used for exhaust gas treatment in cement production, steel, and other industries. In addition, PI has excellent stability and durability and maintains its original performance even in extreme environments, so it can be used for long periods of time without much maintenance or replacement. However, high Tg PI molecular chains are difficult to move at room temperature and cannot be actively recycled and self‐healed after destruction. Self‐healable and recyclable PIs have great practical engineering application value and environmental significance, promoting the development of a sustainable society. In recent years, a small amount of studies have been published to modulate the dynamic behavior of PI such as self‐healable, recyclable, and degradable abilities by adjusting the structure of molecular chains and the composition of monomers [19, 20].
1.5 Summary and Outlook
In summary, PI can be used in many applications through the choice of structural design and synthetic processing methods. These areas include high‐temperature energy storage, low dielectric, corona resistance, thermal conduction, electromagnetic shielding, optical transparency, corrosion resistance, and dynamics.
The primary reason that the PI has not become a more prevalent breed over the past half‐century is its high cost. But from a chemical standpoint, the cost of PI can be reduced. There are two main ways to reduce the cost, one is to develop new monomer synthesis and polymerization methods, and the other is to use PI to modify other polymers. As preparation technology continues to evolve and costs are reduced, PI is poised to assume a more prominent role in the field of materials in the future.
It can be predicted that the future development direction of PI will mainly focus on high performance, multifunctionality, environmental protection and sustainability, and the expansion of application areas. This will bring new opportunities and challenges for the development of polyimide materials, and at the same time will promote the progress and development of related fields.
References
1
Feng, Q.K., Zhong, S.L., Pei, J.Y. et al. (2022). Recent progress and future prospects on all‐organic polymer dielectrics for energy storage capacitors.
Chemical Reviews
122: 3820–3878.
2
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2Structures and Properties of Polyimide Dielectric Materials
Lei Zhai1, Yi‐Kai Wang1,2, and Lin Fan1,2
1Key Laboratory of Science and Technology on High‐tech Polymer Materials, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2, Beijing, 100190, PR China
2University of Chinese Academy of Sciences, School of Chemical Sciences, No.1 Yanqihu East Rd, Huairou District, Beijing, 101408, PR China
Film capacitor technology has rapidly developed and is widely applied in various fields, such as power electronic equipment, high‐voltage transportation, oil and gas deep‐well drilling, high‐frequency coupling circuits, and military direct‐energy weapons [1–5]. As the key material for thin‐film capacitors, polymer dielectric films possessing many advantages, including good dielectric properties and versatile capacitor designs, face increasingly stringent performance requirements. Biaxially oriented polypropylene (BOPP) film currently used in energy storage displays high breakdown strength up to 700 MV/m and low dielectric loss, but its temperature resistance and energy density are poor, making it unable to meet the increasing demand for high‐temperature applications [6–10]. Polymer dielectric materials that can operate under high temperatures, such as polycarbonate (PC), polyvinylidene fluoride (PVDF), polyphenylsulfide (PPS), polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE), poly‐(oxa)‐fluoronorbornene (POFNB), and polyimide (PI) [11–19], have attracted great attention from domestic and international researchers.
Among the above polymer materials, PI dielectric films typically have become a research hotspot and focus in recent years due to their excellent thermal stability, outstanding mechanical properties, rich structural designability, and high feasibility to scale up for industrial production, which is considered the preferred polymer dielectric choice for the development of high‐temperature film capacitors [20–24]. Polyetherimide (PEI), a commercial PI film product developed by GE company (currently owned by Sabic), is known as a high‐temperature polymer dielectric film, and a number of attempts have been made to validate such film for application in the film capacitor. The molecular chain of PEI is composed of alternating ether bonds and imide rings, which is polymerized by aromatic monomers using 2,2‐bis[4‐(3,4‐dicarboxyphenoxy)phenyl]propane dianhydride (BPADA) as the dianhydride and m‐phenylenediamine (m‐PDA) as the diamine. This structure endows PEI with outstanding comprehensive properties. The PEI‐based capacitor was also found to have excellent thermal stability and insulation resistance up to 150 °C or even higher [21]. In addition, compared with the BOPP‐based capacitor, it is proved that PEI film shows 50% higher dielectric constant and can yield a higher volumetric advantage. In general, such PEI film have relatively lower dielectric loss of about 0.003, good dielectric strength (500–650 kV/mm), and energy density above 3.5 J/cm3. Yang et al. prepared PEI films using Ultem 1000 resin as the raw material supplied by GE company. The resultant PEI film energy density is around 3.2 with efficiency of 70% at 150 °C [25]. On the other hand, Zeng et al. also prepared this PEI film with same molecular structure through laboratory synthesis, but better experimental results were obtained [26]. The energy storage density can reach to 3.8 at 150 °C, and the charge–discharge efficiency is increased to 87%. It is expected that further process improvement will lead to mass production of high‐temperature capacitor‐grade PEI films to meet upcoming market demand.
Although it has shown great potential for application, the energy storage characteristics of traditional PI including PEI films still face many challenges, making it difficult to meet practical application requirements. Previously developed PI materials are not strictly designed for the field of energy storage; thus, it is necessary to reconsider the structure design of PI dielectrics for high‐temperature energy storage. Researchers have paid a lot of attention to designing novel PI dielectric films with different molecular structures, and mainly improve the dielectric constant and energy band gap (Eg) of materials by introducing polar groups, non‐conjugated main chain, non‐coplanar unit, and chemical crosslinking into the molecular structure. In this work, the recent research and development status of PI dielectric films is systematically summarized, with a focus on the relationship between their structural design and dielectric performance. By elucidating the influence of polymer multi‐level structures on energy storage characteristics, we hope to provide guidance for the development of high‐temperature polymer dielectric materials and accelerate the application of PI dielectrics in film capacitors.
2.1 Introducing Polar Groups
On the basis of maintaining the aromatic structure units of PI main chain to achieve good heat resistance and thermal stability, introducing polar functional groups into the molecular main chain or side chain can effectively increase the molecular polarization, which helps to improve the dielectric constant of PI film and in the meantime enhance its energy storage performance. However, the improvement of polarizability is often accompanied by more polarization loss, leading to an obvious decrease in charging and discharging efficiency. Thus, how to balance the energy storage density and efficiency is the main challenge, especially for the evaluation at high temperatures.
2.1.1 Nitrile Group
Nitrile groups (—C≡N) have a large dipole moment, and the introduction of polar nitrile groups can significantly enhance the polarizability of polymers, thereby increasing their dielectric constant. Wang et al. synthesized an asymmetric diamine (3,4‐APBN)‐containing nitrile group and polymerized it with five commercial dianhydrides including 6FDA, DSDA, OPDA, BTDA, and PMDA to obtain a series of aromatic PIs with structures shown in Figure 2.1[27]. Compared with symmetrical diamines such as 3,3‐APBN and 4,4‐APBN, the solution viscosity of poly(amic acid) (PAA) synthesized using 3,4‐APBN is much lower. This is mainly due to the increased conformational entropy of the asymmetric chain, making it difficult for PAA chains to stack with the reduction of interaction between polymer chains. Through the wide‐angle X‐ray diffraction (WAXD) analysis, it was found that asymmetric PI derived from 3,4‐APBN/PMDA was mostly amorphous, while symmetric PI based on 3,3‐APBN/PMDA and 4,4‐APBN/PMDA exhibits semi‐crystallinity. As displayed in Figure 2.1, the dielectric properties of these nitrile‐containing PI films were investigated. By introducing nitrile groups, the dielectric constants of PI films range from 3.08 to 3.62 at 10 kHz, and the results are higher than those of PI without benzonitrile groups in the structure, which shows a dielectric constant of 2.92. For asymmetric PI synthesized through 3,4‐APBN, their dielectric constants are higher than those of symmetric series, due to higher polarity of adjacent nitrile groups caused by asymmetric repeating units. In addition, the polarization response near glass transition temperature (Tg) of PI with asymmetric structure is significantly higher than that of symmetric PI, indicating that under high‐temperature conditions, the enhancement effect of asymmetric structure on dielectric constant becomes more remarkable.
Figure 2.1 Structures of benzonitrile‐containing PIs, dielectric properties of APBN‐based PIs, and temperature‐scan dielectric spectra of m,m‐3BCN/OPDA including the real (εr′) and the imaginary (εr″) parts of relative permittivity [27, 28].
Wang et al. further synthesized three special diamines (m,m‐3BCN, p,p‐3BCN, and p,m‐3BCN) containing three benzonitrile groups, which were connected by meta‐substituted ether bonds [28]. A series of PI films were prepared from such diamines and four aromatic dianhydrides (6FDA, OPDA, BTDA, and PMDA), and these highly polar groups were incorporated into the molecular structure in order to increase the dielectric constant and thereby improve its energy storage performance. As listed in Table 2.1, the dielectric constant results of PIs containing three adjacent nitrile groups can be significantly enhanced that are measured by D‐E loops at the temperatures of 23 °C, 100 °C and 190 °C, respectively, while maintaining a lower dielectric loss than conventional PI film products such as Kapton and Ultem. In addition, the test results of broadband dielectric spectroscopy indicate that molecular chain motion around Tg, especially the dipole motion of polar groups assigned to β transition, can effectively increase the dielectric constant of PIs with the maximum up to 4.9 while retaining relatively low dielectric loss.
Table 2.1 Permittivity and polarization response of benzonitrile‐containing PIs and relevant structures.
Source: Adapted from Ref. [28] with permission of Wiley.
Apparent
ε
r
from
D
‐
E
Loops
ε
r
′ from BDS
Sample ID
a
23 (°C), 10 (Hz)
100 (°C), 10 (Hz)
190 (°C), 1 (kHz)
−150 (°C), 1 (kHz)
190 (°C), 1 (kHz)
Δ
ε
r
′
b
m
,
m
‐3BCN/6FDA
3.7
3.7
4.1
3.13
3.62
0.49
m
,
m
‐3BCN/OPDA
4.0
4.2
4.5
3.58
4.34
0.76
m
,
m
‐3BCN/BTDA
4.0
4.3
4.3
3.42
3.85
0.43
m
,
m
‐3BCN/PMDA
3.8
4.8
4.6
3.42
4.06
0.64
p
,
p
‐3BCN/6FDA
4.3
4.5
4.9
3.45
4.30
0.85
p
,
p
‐3BCN/OPDA
4.0
4.1
4.5
3.33
4.47
1.14
p
,
p
‐3BCN/BTDA
3.9
4.0
4.3
3.56
4.14
0.58
p
,
p
‐3BCN/PMDA
3.7
4.3
4.6
3.42
4.79
1.37
p
,
m
‐3BCN/6FDA
4.1
5.2
4.6
3.25
4.17
0.92
p
,
m
‐3BCN/OPDA
4.0
4.1
4.3
3.63
4.48
0.85
3,4‐APBN/OPDA
4.2
4.5
4.7
3.42
4.06
0.64
3,3‐APBN/OPDA
3.8
4.0
4.0
3.30
3.71
0.41
CP2
3.4
3.1
2.9
2.93
2.98
0.05
Kapton
3.2
3.2
3.1
3.15
3.10
−0.05
Ultem
3.6
3.4
3.0
3.19
3.00
−0.19
a) Chemical strucrures of PIs with benzonitrile are shown in Figure 2.1.
b) Δεr′ = εr′(190 °C) – εr′(−150 °C) at 1 kHz.
Treufeld et al. reported twelve kinds of PIs with one or three nitrile groups in the structures and studied the relationship between dielectric properties and energy storage performance [29]. The results show that introducing highly polar nitrile groups to PI structure can indeed increase the dielectric constant, resulting in an obvious improvement of energy density. The more the content of nitrile groups, the higher the density of energy storage. It is further found that the polar groups have different effects on improving the dielectric constant with the structure change of dianhydrides. For diamines containing the same nitrile group, as the polarity of dianhydride moiety increases, the improvement of dielectric constant gradually decreases, indicating that the dipole moments ascribed to dianhydride and diamine moieties may cancel each other out at the molecular level. The dielectric constant of PI obtained by reaction with PMDA increased the most, while that of PI derived from BTDA changed the least, exhibiting the order of PMDA>ODPA>6FDA>BTDA. In the meantime, PIs based on para‐substituted diamines are more prone to rotation, and this connection method has a higher polarization rate than the meta‐substituted series. According to the results of D‐E loops, it can be seen that such PIs combined with PMDA and para‐substituted diamine with nitrile groups typically exhibit the highest discharge energy density and low losses. However, for most of other PIs containing nitrile groups, it is difficult to avoid an increase in dielectric loss while the dielectric constant is improved, which is not the desired result of structure design.
Zhu et al. designed and prepared PIs with high dielectric constant and low loss by directly introducing two ortho‐substituted nitrile groups into the rigid skeleton, based on two diamines (2CN, 0CN as a contrast) and five dianhydrides (PMDA, BPDA, BTDA, ODPA, and 6FDA) [30]. Compared with diamine 0CN without nitrile groups (1.75 D), the two nitrile groups ortho‐substituted in the diamine 2CN ensure the same dipole moment direction (8.60 D). The direction of dipole moment is ensured to be consistent during polarization, which is beneficial for increasing the overall dielectric constant of PI. In the meantime, the steric hindrance of ortho‐substituted nitrile groups greatly limits its polarization motion at room temperature, thereby reducing the dielectric loss. More importantly, the rigid structure of molecular chain makes it more difficult for the glass transition movement, with Tg close to 325 °C. The rigid structure also facilitates the tight stacking of PI chains and contributes to the increasing density of repeating units, which are beneficial for the further promotion of dielectric performance. As shown in Figure 2.2, the dielectric constant of PI (BTDA/2CN) reached 4.80 and the loss factor was only 0.00157 that is measured at 1 kHz and 25 °C while maintaining high thermal resistance. Moreover, its breakdown strength is 219.4 kV/mm and the maximum discharge energy density reaches 1.023 J/cm3.
Figure 2.2 Structure of 2CN‐BTDA, relevant Tg values and dielectric properties, Weibull breakdown strength, energy density, and charge–discharge efficiency at room temperature.
Source: Ref. [30]/Royal Society of Chemistry.
Tian et al. synthesized a new diamine containing both nitrile groups and imine bond, i.e. bis(2‐cyano‐4‐aminophenyl)amine (BCA) shown in Figure 2.3, and prepared a series of PI films by polymerizing with three dianhydrides (BPADA, ODPA, and PMDA) [31]. It is proved that the introduction of polar nitrile groups can dramatically improve the dielectric constant, mainly due to the high polarity and the formation of intra‐molecular hydrogen bonds, both of which promote dipole polarization and thus increase the dielectric constant. These prepared films exhibited excellent dielectric properties and thermal stability. CPI‐3 (PMDA/BCA) exhibited a substantially improved dielectric constant as high as 5.5 at 102 Hz, which is 2.5 times more than that of commercial BOPP film. Researchers found that the length of repeating units can affect both the density of nitrile groups and the rigidity of molecular chains. CPI‐1 (BPADA/BCA) with longer repeating units shows higher breakdown strength, while CPI‐3 with shorter repeating units has higher dielectric constant. Theoretical calculation results clarified that CPI‐1 has the largest bandgap and trap energy level, which provides a higher charge injection barrier and effectively captures the charge carrier. Thus, its conductivity and dielectric loss can be reduced, and energy storage performance is greatly improved. The CPI‐1 film exhibits excellent breakdown strength of 433 MV/m even at 250 °C and still maintains an energy density up to 2.5 J/cm3, which is one of the highest values reported in this harsh environment.
Figure 2.3 Structures of nitrile‐containing diamine BCA and relevant PIs, energy level distributions, discharged energy density, and charge–discharge efficiency at 150 °C and 250 °C.
Source: Ref. [31]/Royal Society of Chemistry.
2.1.2 Sulfonyl Group
Sulfonyl group (‐SO2‐) is another functional group with a relatively high dipole moment of approximately 4.30 D. The double bond can increase the density of electron cloud and expand the conjugation effect, making the electrons more easily polarizable. To develop the high‐performance PI materials for dielectric applications, it is an effective method to improve the polarizability and dielectric constant by introducing strong polar sulfonyl groups into molecular structure.
Tong et al. reported a series of sulfonyl‐containing polyimide (SPI) films combined with different flexible linkages between benzene units in the molecular chains, and their chemical structures are provided in Figure 2.4(a) [32]. These SPIs exhibit high thermal stability, with Tg ranging from 244 °C to 304 °C and decomposition temperatures exceeding 500 °C. The relationship between molecular structure of SPIs and dielectric properties was explained, and the results show that introducing sulfonyl groups into the polyimide backbone can significantly increase the dipole density, thereby enhancing the dielectric constant. However, it is worth noting that the connected position of sulfonyl group can affect the rotation and orientation of the dipoles, thus affecting their effectiveness. Sulfonyl groups connected to the benzene rings in diamine (4,4′‐DDS and 3,3′‐DDS) have greater mobility and are more effective than those connected to the benzene‐imide rings in dianhydride (DSDA). Moreover, flexible chains can promote the rotation and orientation of dipoles, resulting in the decrease of dielectric loss factor. For example, SPI‐1 (ODPA/4,4′‐DDS) with para‐para‐linked sulfonyl group in diamine moiety displays higher dielectric constant than the meta‐meta‐linked SPI‐2 (ODPA/3,3′‐DDS), mainly due to the more symmetrical structure of SPI‐1 that makes the energy barrier of free rotation lower and also contributes to greater alignment of dipoles in alternating electric field. SPI‐1 maintains stable dielectric properties up to 150 °C, with the discharged energy density as high as 7.04 J/cm3 and charge–discharge efficiency of 91.3% at 500 MV/m, respectively. Furthermore, SPI‐3 (DSDA/ODA) with a shorter repeating unit shows higher dielectric constant than other SPIs with similar molecular structures because of the higher sulfonyl group density. It proved that high dielectric constant and low loss factor can be achieved simultaneously without sacrificing heat resistance by introducing high polar sulfonyl groups, together with maintaining high dipole density and moderate flexible structure.
Figure 2.4 (a) Structures of sulfonyl‐containing PIs based on ODPA and DSDA.
Source: Ref. [32]/John Wiley & Sons
; (b) structure of sulfonyl‐containing PI (BTDA/BIA/4,4′‐DDS), schematic illustration under charging state, energy density, and charge–discharge efficiency at 25 °C and 150 °C.
Source: Ref. [33]/with permission of Elsevier.
Wu et al. prepared a new kind of sulfonyl‐containing PI dielectric film by copolymerization of BTDA dianhydride with benzimidazole diamine (BIA) and 4,4′‐DDS, and the chemical structures and schematic illustration under charging state are given in Figure 2.4(b) [33]. As the content of sulfonyl‐containing diamine increases, the dielectric constant of film is improved to 4.45 while maintaining a low dielectric loss factor below 0.005. According to the results of molecular dynamics simulation, the introduction of sulfonyl groups leads to an increase in the spacing between molecular chains, which is accompanied with more free volume. It is undoubtedly beneficial for the polarization process, resulting in a synergistic increase in dielectric constant while reducing dielectric loss. In addition, it is found that energy storage density can be increased when the amount of sulfonyl groups is appropriate, but an excess addition of sulfonyl‐containing diamine will reduce the charge–discharge efficiency of film. PI‐15 (BTDA/BIA/4,4′‐DDS) with a mount of 15 mol% 4,4′‐DDS exhibits high energy density of 3.27 J/cm3 at 380 MV/m measured at room temperature and also has good dielectric properties at a high temperature. When the test temperature is raised to 150 °C, this film can maintain a low dielectric loss of 0.026 at 1 kHz and a high breakdown strength, with an energy density of 1.14 J/cm3 at 260 MV/m and the efficiency around 74% as shown in Figure 2.4(b). The above results demonstrate that the sulfonyl‐containing PI dielectrics are possible to be used as one of promising candidates for next‐generation film capacitors, particularly in harsh environmental applications.
2.1.3 Carboxylic Group
Research indicates that PIs containing a certain amount of amide acid can play the same role as the high polar groups due to the carboxylic groups (‐COOH), enhancing the dielectric constant as well as energy storage characteristics. In addition to introducing carboxylic groups through special monomers, the PI film containing carboxylic acid groups can be directly obtained by controlling the film preparation process. PI films are typically prepared from the precursor PAA through thermal imidization method, where cyclization and dehydration occur during the conversion process. Considering the beneficial effect of polar carboxylic groups on achieving a higher dielectric constant and improved energy density, it is possible to retain a portion of amide acid groups in the PI film by controlling the incomplete imidization degree of PAA through drying temperature and time.
Dai et al. prepared PI‐PAA copolymer films with different composition ratios by adjusting the thermochemical process, using PMDA as the dianhydride and m‐BAPS as the diamine with the chemical structures shown in Figure 2.5(a) [34]. The decrease of PAA content in the copolymer is conducive to enhancing the breakdown strength and energy density, but leads to a reduction in the dielectric constant and loss factor. In the meantime, the leakage current density is also reduced and the bandgap becomes narrower as the PAA content decreases. It is mainly because of the transformation from the carboxylic groups in PAA to the imide rings in PI, and low PAA content means less polar groups that are retained in the finally prepared film. Among various PI‐PAA copolymers, the 0.87PI‐0.13PAA film exhibits both the highest breakdown strength of 616 MV/m and the highest energy density up to 8.9 J/cm3 at room temperature. This 0.87PI‐0.13PAA copolymer is further improved by introducing 0.1 vol% boron nitride nanosheets (BNNS), and demonstrates a high discharged energy density of 1.38 J/cm3 and high efficiency over 96% under the test conditions of 150 °C and 200 MV/m, which is much better than the properties of commercial BOPP. Furthermore, the results of 20,000 cycles of charge–discharge test and 35 days high‐temperature endurance test are provided, the charge–discharge performance is proved to be robust, and the energy density or efficiency at 150 °C shows no sign of degradation.
Figure 2.5 (a) Structures of PAA and PI based on PMDA/m‐BAPS, discharged energy density, and charge–discharge efficiency at 25 °C and 150 °C.
Source: Ref. [34]/John Wiley & Sons;
(b) structures of PAA, PI‐PAA, and PI based on ODPA/4,4′‐DDS, discharged energy density, and charge–discharge efficiency at 25 °C and 150 °C.
Source: Ref. [35]/AIP Publishing LLC.
Liu et al. prepared a series of PI‐PAA films with different degrees of imidization (71%, 90%, and 100%) by polymerizing dianhydride ODPA with sulfonyl‐containing diamine 4,4′‐DDS [35]. In addition to introducing polar sulfone groups in the diamine moiety, the polarization rate and dielectric constant of film are further increased by retaining amide acid groups in the partially imidized PI‐PAA structure. Both polar sulfonyl groups and carboxylic groups can act as traps to suppress the migration of charge carriers, thereby enhancing the breakdown strength and energy storage efficiency of film. However, it is also found that an excess of polar functional groups can lead to trap overlap, which in turn reduces the energy barrier. As presented in Figure 2.5(b), PI film with imidization degree of 90% named SO2PI‐2 exhibits the best comprehensive performance, and achieves a discharged energy density of 5.14 J/cm3 and a charge–discharge efficiency of 90% at 25 °C and 535 MV/m. Especially, the discharged energy density of this film can reach 3.29 J/cm3 under conditions of 150 °C and 400 MV/m, with a charge–discharge efficiency of 80%. Based on the above research, it is demonstrated that PI‐PAA copolymer with suitable PAA contents can achieve superior energy storage performance. In view of the simple fabrication process, this special pathway showed the potential for constructing high‐temperature capacitors.
2.1.4 Pyridine and Urea Group
Peng et al. utilized a newly synthesized diamine (BPBPA) and several commercial dianhydrides (BPDA, PMDA, BTDA, and ODPA) to prepare a series of PIs containing bipyridine units which are provided in Figure 2.6(a) [36]. These PI films exhibit high thermal resistance with Tg ranging from 275 °C to 320 °C and high mechanical properties with tensile strengths between 175 MPa and 221 MPa. Among them, PIs based on BTDA and BPDA show higher dielectric constants compared to those films based on PMDA and ODPA. The PI derived from BTDA/BPBPA has long‐range conjugated units and polar carbonyl groups in structure, resulting in a dielectric constant as high as 7.2 and a dielectric loss factor of 0.038, with the minimal energy loss under high‐frequency operation. This is attributed to the enhanced electronic and dipole polarization within the bipyridine units, where each bipyridine unit has two lone electron pairs in the conjugated system. Under external electric field, the high electron mobility increases the electronic polarization, and the strong polarity of nitrogen atoms further improves the dipolar polarization. The synergistic effect results in a significant increase in dielectric constant, and polarizability followed the decreasing order: benzophenone structure > diphenyl structure > diphenyl ether structure > phenyl structure. The energy density of BTDA/BPBPA film can reach up to 2.77 J/cm3 at room temperature, suggesting a good energy storage performance.
Figure 2.6 (a) Structures of PIs containing bipyidine and relevant dielectric constants.
Source: Ref. [36]/with permission of Elsevier;
(b) structures of PIs containing urea, carbamate, and sulfonyl groups, electrostatic potential distribution, Weibull breakdown strength, discharged energy density, and charge–discharge efficiency at 25 °C.
Source: Ref. [37]/John Wiley & Sons.
Tang et al. prepared different novel PIs containing functional pendant groups in the side chains, including urea groups (BU‐PI), carbamate groups (BC‐PI), and sulfonyl groups (BS‐PI) with structures illustrated in Figure 2.6(b) [37]. The dielectric properties of PIs were compared, and the influence of different dipolar groups on dielectric behaviors was investigated comprehensively. The results show that high dielectric constant and low dielectric loss can be achieved by both the enhanced orientational polarization and the suppressed dipole–dipole interactions of dipolar groups. It is worth noting that the free rotation of polar urea groups can significantly improve the oriented polarization, shown by the strong γ transitions with low activation energies, with the result that BU‐PI has the highest dielectric constant of 6.14 and a relatively low dielectric loss of 0.0097. Moreover, BU‐PI exhibits a stable dielectric constant over a wide temperature range, attributed to the presence of intermolecular hydrogen bonds. BU‐PI shows the discharged energy density of 6.92 J/cm3 with charge–discharge efficiency over 83% at 500 MV/m. Compared with the most commonly used polar sulfonyl groups, this research proves that urea groups as the dipolar pendant group can endow PIs with much better dielectric properties.
2.2 Introducing Non‐Conjugated Structures
Heat‐resistant polymers generally benefit from the conjugated benzene rings in their molecular chains, which ensure good thermal stability. However, the π‐π stacking effect of these conjugated structures makes the Eg narrow, leading to a reduction in discharge energy density and an increase in leakage current. Unlike traditional PIs with the fully aromatic structures, introducing non‐conjugated units into the main chain can disrupt the long‐range conjugation of electrons, which can contribute greatly to the improvement of Eg and dielectric strength. The design strategies for introducing non‐conjugated structures into polymer chain mainly include the incorporation of aliphatic and cycloaliphatic structures, both of which are semi‐aromatic PIs.
2.2.1 Aliphatic Structure
Baldwin et al prepared two series of semi‐aromatic PIs including homopolymers and copolymers by reacting dianhydride PMDA with short‐chain diamines containing different alkyl groups or ether bonds (Jeffamines) with the purpose of producing PIs with high imide density [38]. The chemical structures of these semi‐aromatic PI are displayed in Figure 2.7(a), and the relationship between structure and dielectric properties was investigated, especially analyzing the influence of aliphatic structure. The dielectric constants of such PIs can be increased from 3.96 to 6.57, much higher than that of BOPP. Researchers found that the dielectric constant is directly proportional to the percent functionality of imide groups. The higher density of imide groups corresponds to the greater molecular polarization and leads to the higher dielectric constant. By using short‐chain aliphatic diamines, the density of imide groups containing in the molecular chain can be increased, thereby improving the dielectric constant of PI film. In addition, aliphatic structures such as the longer ether chains or the shorter alkyl groups greatly affect the solubility and thermal performance of PIs. The results show that PIs with long‐chain ether linkages have better solubility but lower Tg, while exhibiting the reduced dielectric constants due to the larger free volume between flexible molecular chains. It is indicated that the dielectric loss of semi‐aromatic PIs is mainly related to their structural rigidity. The rigidity of polymer chain can be increased by introducing short‐chain aliphatic diamines, thereby reducing the loss factor that means less response to the switching electric field. Based on these series of PIs containing aliphatic structures, their experimental dielectric constants are also compared to calculation results through density functional theory (DFT), and it is proved that there is a close relationship between them.
Figure 2.7 (a) Structures of semi‐aromatic PIs containing short‐chain aliphatic diamines, the calculated band gap, and dielectric constant results.
Source: Ref. [38]/John Wiley & Sons;
(b) structures of semi‐aromatic PIs based on different aromatic dianhydrides and aliphatic diamines, DFT‐based initial screening results, and dielectric constants.
Source: Ref. [39]/American Chemical Society.
Ma et al. reported a series of semi‐aromatic PIs by polycondensation reactions with aromatic dianhydrides (PMDA, BTDA, ODPA, and 6FDA) and different aliphatic diamines including 1,3‐DAP, DAH, Jeffamine D230, and HK511 [39]. These semi‐aromatic PIs were synthesized by the guidance of high‐throughput DFT calculations for rational design in terms of a high dielectric constant and Eg. The maximum Tg of such PIs is as high as 174 °C for PI (BTDA/1,3‐DAP). The maximum dielectric constant is up to 7.8 with a dielectric loss of less than 0.01 for PI (BTDA/HK511), and its energy density is notably improved and can reach around 15 J/cm3, which is 3 times that of BOPP. This is mainly ascribed to the increased dipole volume from the ether linkage and the longer conjugation length built on the carbonyl spacer inserted between the benzene rings in dianhydride moiety. When the dianhydride remains unchanged but the diamine is altered, PIs containing polyether chain segments have higher dielectric constants than those containing aliphatic chain segments. It is shown that the arrangement of benzene rings, ether bonds, and other functional groups in dianhydrides moiety has a significant impact on dielectric properties. For example, the prepared PI (BTDA/HK511) with the longest conjugation length and polyether segment exhibits the highest dielectric constant. It is also found that the incorporation of polyether chain segments may also increase the dielectric loss. Furthermore, PIs based on the 6FDA and long‐chain ether diamines show the highest Eg values that are measured by UV absorption, which indicate lower electron mobility. As provided in Figure 2.7(b), the relationship between Eg
