50th Anniversary Perspective: Dielectric Phenomena in Polymers and Multilayered Dielectric Films

Slight-Crosslinking: Opening a New Journey for the Intrinsic Elastification of Ferroelectric Polymers

Elastic electronic materials are essential components in the next generation of wearable devices and soft robots. However, the development of elastic ferroelectric materials has lagged behind that of conductive and semiconductive materials. The successful realization of intrinsically elastic ferroelectrics through slight-crosslinking has opened a new path for making ferroelectric materials elastic. Given the critical need for materials that combine excellent elasticity with robust ferroelectric properties in the field of elastic electronics, we explore various crosslinking techniques and their performance characteristics. We believe that this cutting-edge research direction holds substantial value for both fundamental studies and practical applications.

Unveiling Synergistic Interface Effects on Charge Trapping Regulation in Polymer Composite Dielectrics through Multiscale Modeling

Interface design is a promising strategy to enhance the dielectric strength in polymer composites through regulating the charge transport process. However, the targeted exploitation of interface effects is limited due to a lack of fundamental understanding of the underlying mechanisms involving elementary electronic processes and details of the intricate interplay of characteristics of molecular building blocks and the interfacial morphology - details that cannot fully be resolved with experimental methods or commonly used band transport models. Here, we instead build a proper theoretical framework for polymer dielectrics based on charge hopping and employ a multiscale modeling approach linking the quantum properties of the charge carriers with nano- and mesoscale structural details of complex interfaces. Applied to a prototypical application-proven cellulose-oil interface system, this approach demonstrates that charges are trapped in the disordered region. Specifically, it unveils this trapping as a synergistic effect of two transport-regulating interface mechanisms: back-transfer to the oil region is suppressed by energetic factors, while forward-transfer to the crystalline cellulose is suppressed by low electronic coupling. The insight into the molecular origins of interface effects via dual-interface regulation in the framework of charge hopping offers new development paths for developing advanced energy materials with tailored electrical properties.

Anomalously High Dielectric Strength and Capacitive Energy Density of Thin Entangled Glassy Polymer Films

The influence of high-intensity electric fields on the stability of polymeric materials is a problem of interest in the design of next-generation energy storage and electronic devices, and for understanding the limits of stability of polymer films exposed to large electric fields generally. Here, we show that the dielectric strength of entangled glassy polymer films increases as an inverse power-law of the film thickness h for "ultrathin" films below a micron in thickness. The dielectric strength enhancement in these polymer films becomes as large as ≈2 GV/m in films thinner than 100 nm, but in this thickness regime, the increase of the dielectric strength depends strongly on the polymer mass, sample aging time, and the method of film preparation. The enhancement of the dielectric breakdown strength is attributed to the mechanical instability of the elastic film subjected to sufficiently large electric fields and a large, but not generally well-understood, enhancement of effective stiffness of entangled glassy polymer films subject to large deformations, an effect that has previously been observed to become greatly enhanced when such films are made thinner. As a proof of principle regarding applications, we utilize ultrathin glassy polymer films of the type studied in our paper to fabricate polymeric nanocapacitors having ultrahigh discharge energy densities (U d max) as large as 27 J/cm3 and having efficiencies greater than 80%. These efficiency values at comparable charge densities are significantly higher than those of competing ferroelectric polymer materials, and we anticipate that our observations will inspire the creation of practical high-energy density nanocapacitor devices for advanced energy storage applications.

Innovative Blown Multi-Micro-Nano-Layer Coextrusion: Insights into Rheology and Process Stability

The present study introduces an innovative blown coextrusion die technology designed to address a critical gap in the production of multilayer films. Unlike conventional systems, this novel die allows for the creation of films with a high number of layers, ensuring layer integrity even in the micro-nano scale. A key advancement of this die is its ability to increase the number of layers without extending the residence time since it does not require an additional multiplier element. The risk of thermal degradation can, thus be, minimized. The die can easily be combined with existing cast coextrusion technologies, making it very versatile. Stability maps were developed to define processability and, in association with rheological analysis, optimal processing windows were determined. This study highlights the potential of enhancing material efficiency by increasing the number of layers while reducing the need for high percentages of EVOH. The produced multilayer films exhibited strong layer adhesion without the use of tie layers, thus improving recyclability and supporting sustainability goals.

A Comprehensive Review of Mxene-Based Emerging Materials for Energy Storage Applications and Future Perspectives

MXenes is a rapidly emerging class of two-dimensional (2D) materials. It exhibits unique properties that make it suitable for a wide range of applications. This review provides a comprehensive overview of the synthesis and processing techniques for MXenes including both bottom-up and top-down approaches. The synthesis of MXene-based composites is explored in detail focusing on Mxene-carbon composites, Mxene-metal oxides, Mxene-metal sulfides, Mxene-polymer composites and MXene-ceramic composites. Key properties of MXenes are examined including structural, electrical, morphological, optical, mechanical, chemical stability, electrical and thermal properties, conductivity, magnetic properties, dielectric charge and catalytic properties. Characterization techniques used to study these properties is also reviewed. Their 2D structure provides a high surface area and unique interlayer spacing, making MXenes ideal for applications in energy storage devices (like supercapacitors and batteries) where surface area and ion transport are critical for performance. The diverse applications of MXenes are presented emphasizing their use in batteries, catalysis, sensors, environmental remediation and supercapacitors. Special attention is given to the supercapacitor applications of MXenes of their potential in energy storage devices. Due to their high capacitance, fast charge/discharge rates, and excellent stability, MXenes are used in supercapacitors, lithium-ion batteries, and sodium-ion batteries. They can store energy more efficiently than many other materials, making them valuable in the quest for efficient, sustainable energy solutions. The progress in MXene supercapacitor devices is providing insights into the latest advancements and future prospects. MXenes are highlighted as versatile materials with significant potential in various technological fields particularly in energy storage. Future research directions and challenges are also outlined for ongoing and future studies in this dynamic area of materials science.

Reducing Dielectric Loss of High-Dielectric-Constant Elastomer via Rigid Short-Chain Crosslinking

High-dielectric-constant elastomers have broad applications in wearable electronics, which can be achieved by the elastification of relaxor ferroelectric polymers. However, the introduction of soft long chains, with their high mobility under strong electric fields, leads to high dielectric loss. Given the relatively low modulus of relaxor ferroelectric polymers, elastification can be realized by introducing short-chain crosslinkers. In this work, a molecular engineering design is employed, utilizing a rigid short-chain crosslinker to create crosslinks with relaxor ferroelectric polymer, resulting in intrinsic elastomers characterized by a high dielectric constant but low dielectric loss. The obtained intrinsic ferroelectric elastomer possesses a high dielectric constant (35 at 1 kHz and 25 °C) and a low dielectric loss (0.09). Furthermore, this elastomer exhibits stable ferroelectric response and relaxor characteristics even under strains up to 80%. The study supplies a simple but effective method to reduce the dielectric loss of high-dielectric-constant intrinsic elastomers, thereby expanding their application fields in wearable electronics.

Enhanced high-temperature energy storage performances in polymer dielectrics by synergistically optimizing band-gap and polarization of dipolar glass

Polymer dielectrics play an irreplaceable role in electrostatic capacitors in modern electrical systems, and have been intensively studied with their polarization and breakdown strength (Eb) optimized for high discharged energy density (Ud) at elevated temperatures. Small molecules have been explored as fillers, yet they deteriorate thermal stability of matrix which limits their optimal loading to ~1 wt%. Herein, we develop a polymer blend dielectric consisting of common polyimide and a bifunctional dipolar glass polymer which are synthesized from two small molecule components with wide band-gap and large dipole moment. The bifunctional dipolar glass with large molecular weight not only maintains thermal stability of polymer blends even at a high loading of 10 wt%, but also induces substantial enhancement in polarization and Eb than any of individual components does, achieving an ultrahigh Ud of 8.34 J cm-3 (150 °C) and 6.21 J cm-3 (200 °C) with a charge-discharge efficiency of 90%.

High-Density Capacitive Energy Storage in Low-Dielectric-Constant Polymer PMMA/2D Mica Nanofillers Heterostructure Composite

The ubiquitous, rising demand for energy storage devices with ultra-high storage capacity and efficiency has drawn tremendous research interest in developing energy storage devices. Dielectric polymers are one of the most suitable materials used to fabricate electrostatic capacitive energy storage devices with thin-film geometry with high power density. In this work, we studied the dielectric properties, electric polarization, and energy density of PMMA/2D Mica nanocomposite capacitors where stratified 2D nanofillers are interfaced between the multiple layers of PMMA thin films using two heterostructure designs of the capacitors, PMMA/2D Mica/PMMA (PMP) and PMMA/2D Mica/PMMA/2D Mica/PMMA (PMPMP). The incorporation of a 2D Mica nanofiller in the low-dielectric-constant PMMA leads to an enhancement in the dielectric constant, with ∆ε ~ 15% and 53% for PMP and PMPMP heterostructures at room temperature. Additionally, a significant improvement in discharged energy density was measured for the PMPMP capacitor (Ud ~ 38 J/cm3 at 825 MV/m) compared to the pristine PMMA (Ud ~ 9.5 J/cm3 at 522 MV/m) and PMP capacitors (Ud ~ 19 J/cm3 at 740 MV/m). This excellent capacitive and energy storage performance of the PMMA/2D Mica heterostructure nanocomposite may inform the fabrication of thin-film, high-density energy storage capacitor devices for potential applications in various platforms.

Polymer nanocomposite dielectrics for capacitive energy storage

Owing to their excellent discharged energy density over a broad temperature range, polymer nanocomposites offer immense potential as dielectric materials in advanced electrical and electronic systems, such as intelligent electric vehicles, smart grids and renewable energy generation. In recent years, various nanoscale approaches have been developed to induce appreciable enhancement in discharged energy density. In this Review, we discuss the state-of-the-art polymer nanocomposites with improved energy density from three key aspects: dipole activity, breakdown resistance and heat tolerance. We also describe the physical properties of polymer nanocomposite interfaces, showing how the electrical, mechanical and thermal characteristics impact energy storage performances and how they are interrelated. Further, we discuss multi-level nanotechnologies including monomer design, crosslinking, polymer blending, nanofiller incorporation and multilayer fabrication. We conclude by presenting the current challenges and future opportunities in this field.

Effect of Glass Transition Temperature on Enhanced Dielectric Breakdown Strength and Lifetime of Multilayer Polymer Films

High temperature, high energy density, and low loss dielectric films are promising candidates for miniaturized capacitors in electric vehicles and high-speed trains. However, single-component polymers could not achieve these desired properties simultaneously. Polymer multilayer films (MLFs), which combine a high dielectric constant polymer [e.g., poly(vinylidene fluoride) (PVDF)] and a high breakdown/low loss polymer [e.g., polycarbonate (PC)] in a unique layered structure, have the potential achieve them at the same time. In this work, the effects of PC glass transition temperature (Tg) on the dielectric insulation properties (breakdown strength and lifetime) were investigated at high temperatures of 100-150 °C. Three PC materials had Tg values of 145 (PC1), 165 (PC2), and 185 °C (PC3), respectively. It is observed that MLF-PC3 with the highest Tg of PC exhibited the highest Weibull direct/alternating current (DC/AC) breakdown strength and the longest DC/AC lifetime, whereas MLF-PC1 with the lowest Tg showed the lowest Weibull DC/AC breakdown strength and the shortest DC/AC lifetime. A high-temperature high-volage leakage current study revealed that MLF-PC3 exhibited the lowest bulk conductivity at all temperatures under different electric fields. The knowledge obtained from this study will help us design better MLFs with high performance for next-generation miniaturized capacitors.

Solution-Processed Polymer Dielectric Interlayer for Low-Voltage, Unipolar n-Type Organic Field-Effect Transistors

The integration of organic electronic circuits into real-life applications compels the fulfillment of a range of requirements, among which the ideal operation at a low voltage with reduced power consumption is paramount. Moreover, these performance factors should be achieved via solution-based fabrication schemes in order to comply with the promise of cost- and energy-efficient manufacturing offered by an organic, printed electronic technology. Here, we propose a solution-based route for the fabrication of low-voltage organic transistors, encompassing ideal device operation at voltages below 5 V and exhibiting n-type unipolarization. This process is widely applicable to a variety of semiconducting and dielectric materials. We achieved this through the use of a photo-cross-linked, low-k dielectric interlayer, which is used to fabricate multilayer dielectric stacks with areal capacitances of up to 40 nF/cm2 and leakage currents below 1 nA/cm2. Because of the chosen azide-based cross-linker, the dielectric promotes n-type unipolarization of the transistors and demonstrated to be compatible with different classes of semiconductors, from conjugated polymers to carbon nanotubes and low-temperature metal oxides. Our results demonstrate a general applicability of our unipolarizing dielectric, facilitating the implementation of complementary circuitry of emerging technologies with reduced power consumption.

Intrinsic-designed polyimide dielectric materials with large energy storage density and discharge efficiency at harsh ultra-high temperatures

Polymer dielectric materials with excellent temperature stability are urgently needed for the ever-increasing energy storage requirements under harsh high-temperature conditions. In this work, a novel diamine monomer (bis(2-cyano-4-aminophenyl)amine) was successfully synthesized to prepare a series of cyano-containing polyimides (CPI-1-3), which possessed excellent dielectric properties and high thermostability. The maximum dielectric permittivity was up to 5.5 at 102 Hz for CPI-3, being 2.5 times higher than that of commercially used BOPP. In comparison, the CPI-1 exhibited an outstanding breakdown strength of 433 MV m-1 and a high energy density of 2.5 J cm-3 even at 250 °C, which was the highest value reported under the same conditions. The synthesized CPIs through such an intrinsic approach are potential candidate materials for energy storage and even other applications under simultaneously harsh electrical and thermal conditions.

Functional composites by programming entropy-driven nanosheet growth

Nanomaterials must be systematically designed to be technologically viable1-5. Driven by optimizing intermolecular interactions, current designs are too rigid to plug in new chemical functionalities and cannot mitigate condition differences during integration6,7. Despite extensive optimization of building blocks and treatments, accessing nanostructures with the required feature sizes and chemistries is difficult. Programming their growth across the nano-to-macro hierarchy also remains challenging, if not impossible8-13. To address these limitations, we should shift to entropy-driven assemblies to gain design flexibility, as seen in high-entropy alloys, and program nanomaterial growth to kinetically match target feature sizes to the mobility of the system during processing14-17. Here, following a micro-then-nano growth sequence in ternary composite blends composed of block-copolymer-based supramolecules, small molecules and nanoparticles, we successfully fabricate high-performance barrier materials composed of more than 200 stacked nanosheets (125 nm sheet thickness) with a defect density less than 0.056 µm-2 and about 98% efficiency in controlling the defect type. Contrary to common perception, polymer-chain entanglements are advantageous to realize long-range order, accelerate the fabrication process (<30 min) and satisfy specific requirements to advance multilayered film technology3,4,18. This study showcases the feasibility, necessity and unlimited opportunities to transform laboratory nanoscience into nanotechnology through systems engineering of self-assembly.

Energy Storage Performance of Polymer-Based Dielectric Composites with Two-Dimensional Fillers

Dielectric capacitors have garnered significant attention in recent decades for their wide range of uses in contemporary electronic and electrical power systems. The integration of a high breakdown field polymer matrix with various types of fillers in dielectric polymer nanocomposites has attracted significant attention from both academic and commercial sectors. The energy storage performance is influenced by various essential factors, such as the choice of the polymer matrix, the filler type, the filler morphologies, the interfacial engineering, and the composite structure. However, their application is limited by their large amount of filler content, low energy densities, and low-temperature tolerance. Very recently, the utilization of two-dimensional (2D) materials has become prevalent across several disciplines due to their exceptional thermal, electrical, and mechanical characteristics. Compared with zero-dimensional (0D) and one-dimensional (1D) fillers, two-dimensional fillers are more effective in enhancing the dielectric and energy storage properties of polymer-based composites. The present review provides a comprehensive overview of 2D filler-based composites, encompassing a wide range of materials such as ceramics, metal oxides, carbon compounds, MXenes, clays, boron nitride, and others. In a general sense, the incorporation of 2D fillers into polymer nanocomposite dielectrics can result in a significant enhancement in the energy storage capability, even at low filler concentrations. The current challenges and future perspectives are also discussed.

Multilayered Functional Triboelectric Polymers for Self-Powered Wearable Applications: A Review

Multifunctional wearable devices detect electric signals responsive to various biological stimuli and monitor present body motions or conditions, necessitating flexible materials with high sensitivity and sustainable operation. Although various dielectric polymers have been utilized in self-powered wearable applications in response to multiple external stimuli, their intrinsic limitations hinder further device performance enhancement. Because triboelectric devices comprising dielectric polymers are based on triboelectrification and electrostatic induction, multilayer-stacking structures of dielectric polymers enable significant improvements in device performance owing to enhanced interfacial polarization through dissimilar permittivity and conductivity between each layer, resulting in self-powered high-performance wearable devices. Moreover, novel triboelectric polymers with unique chemical structures or nano-additives can control interfacial polarization, allowing wearable devices to respond to multiple external stimuli. This review summarizes the recent insights into multilayered functional triboelectric polymers, including their fundamental dielectric principles and diverse applications.

Polymer-Grafted Nanoparticles with Variable Grafting Densities for High Energy Density Polymeric Nanocomposite Dielectric Capacitors

Designing high energy density dielectric capacitors for advanced energy storage systems needs nanocomposite-based dielectric materials, which can utilize the properties of both inorganic and polymeric materials. Polymer-grafted nanoparticle (PGNP)-based nanocomposites alleviate the problems of poor nanocomposite properties by providing synergistic control over nanoparticle and polymer properties. Here, we synthesize "core-shell" barium titanate-poly(methyl methacrylate) (BaTiO₃-PMMA) grafted PGNPs using surface-initiated atom transfer polymerization (SI-ATRP) with variable grafting densities of (0.303 to 0.929) chains/nm² and high molecular masses (97700 g/mL to 130000 g/mol) and observe that low grafted density and high molecular mass based PGNP show high permittivity, high dielectric strength, and hence higher energy densities (≈ 5.2 J/cm³) as compared to the higher grafted density PGNPs, presumably due to their "star-polymer"-like conformations with higher chain-end densities that are known to enhance breakdown. Nonetheless, these energy densities are an order of magnitude higher than their nanocomposite blend counterparts. We expect that these PGNPs can be readily used as commercial dielectric capacitors, and these findings can serve as guiding principles for developing tunable high energy density energy storage devices using PGNP systems.

Fluoropolymer ferroelectrics: Multifunctional platform for polar-structured energy conversion

Ferroelectric materials are currently some of the most widely applied material systems and are constantly generating improved functions with higher efficiencies. Advancements in poly(vinylidene fluoride) (PVDF)-based polymer ferroelectrics provide flexural, coupling-efficient, and multifunctional material platforms for applications that demand portable, lightweight, wearable, and durable features. We highlight the recent advances in fluoropolymer ferroelectrics, their energetic cross-coupling effects, and emerging technologies, including wearable, highly efficient electromechanical actuators and sensors, electrocaloric refrigeration, and dielectric devices. These developments reveal that the molecular and nanostructure manipulations of the polarization-field interactions, through facile defect biasing, could introduce enhancements in the physical effects that would enable the realization of multisensory and multifunctional wearables for the emerging immersive virtual world and smart systems for a sustainable future.

Exploration of Breakdown Strength Decrease and Mitigation of Ultrathin Polypropylene

Polypropylene film is the most important organic dielectric in capacitor technology; however, applications such as power electronic devices require more miniaturized capacitors and thinner dielectric films. The commercial biaxially oriented polypropylene film is losing the advantage of its high breakdown strength as it becomes thinner. This work carefully studies the breakdown strength of the film between 1 and 5 microns. The breakdown strength drops rapidly and hardly ensures that the capacitor reaches a volumetric energy density of 2 J/cm³. Differential scanning calorimetry, X-ray, and SEM analyses showed that this phenomenon has nothing to do with the crystallographic orientation and crystallinity of the film but is closely related to the non-uniform fibers and many voids produced by overstretching the film. Measures must be taken to avoid their premature breakdown due to high local electric fields. An improvement below 5 microns will maintain a high energy density and the important application of polypropylene films in capacitors. Without destroying the physical properties of commercial films, this work employs the ALD oxide coating scheme to augment the dielectric strength of a BOPP in the thickness range below 5 μm, especially its high temperature performance. Therefore, the problem of the reduction in dielectric strength and energy density caused by BOPP thinning can be alleviated.

An Advanced Strategy to Enhance TENG Output: Reducing Triboelectric Charge Decay

The Internet of Things (IoT) is poised to accelerate the construction of smart cities. However, it requires more than 30 billion sensors to realize the IoT vision, posing great challenges and opportunities for industries of self-powered sensors. Triboelectric nanogenerator (TENG), an emerging new technology, is capable of easily converting energy from surrounding environment into electricity, thus TENG has tremendous application potential in self-powered IoT sensors. At present, TENG encounters a bottleneck to boost output for large-scale commercial use if just by promoting triboelectric charge generation, because the output is decided by the triboelectric charge dynamic equilibrium between generation and decay. To break this bottleneck, the strategy of reducing triboelectric charge decay to enhance TENG output is focused. First, multiple mechanisms of triboelectric charge decay are summarized in detail with basic theoretical principles for future research. Furthermore, recent advances in reducing triboelectric charge decay are thoroughly reviewed and outlined in three aspects: inhibition and application of air breakdown, simultaneous inhibition of air breakdown and triboelectric charge drift/diffusion, and inhibition of triboelectric charge drift/diffusion. Finally, challenges and future research focus are proposed. This review provides reference and guidance for enhancing TENG output.

Alternating [1.1.1]Propellane-(Meth)Acrylate Copolymers: A New Class of Dielectrics with High Energy Density for Film Capacitors

Polymer dielectrics with high energy density are of urgent demand in electric and electronic devices, but the tradeoff between dielectric constant and breakdown strength is still unsolved. Herein, the synthesis and molar mass control of three alternating [1.1.1]propellane-(meth)acrylate copolymers, denoted as P-MA, P-MMA, and P-EA, respectively, are reported. These copolymers exhibit high thermal stability and are semi-crystalline with varied glass transition temperatures and melting temperatures. The rigid bicyclo[1.1.1]pentane units in the polymer backbone promote the orientational polarization of the polar ester groups, thus enhancing the dielectric constants of these polymers, which are 4.50 for P-EA, 4.55 for P-MA, and 5.11 for P-MMA at 10 Hz and room temperature, respectively. Moreover, the high breakdown strength is ensured by the non-conjugated nature of bicyclo[1.1.1]pentane unit. As a result, these copolymers show extraordinary energy storage performance; P-MA exhibits a discharge energy density of 9.73 J cm^-3 at 750 MV m^-1 and ambient temperature. This work provides a new type of promising candidates as polymer dielectrics for film capacitors, and offers an efficient strategy to improve the dielectric and energy storage properties by introducing rigid non-conjugated bicyclo[1.1.1]pentane unit into the polymer backbone.

Improving the Energy Storage Performance of All-Polymer Composites By Blending PVDF and P(VDF-CTFE)

Organic film capacitors have incredibly high power density and have an irreplaceable position in pulsed power systems, high-voltage power transmission networks and other fields. At present, the energy storage density and energy storage efficiency of organic film capacitors are relatively low, resulting in excessive equipment volume. The performance of organic film capacitors is determined by polymer materials, so it is crucial to develop a polymer composite with high energy storage density and high charge-discharge efficiency. Poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-CTFE)) is incorporated into the polyvinylidene fluoride (PVDF) matrix by solution blending. The successful preparation of the all-polymer composite material solves the problems of low breakdown electric field strength, low discharge energy density, and low charge-discharge efficiency of high-dielectric ferroelectric materials. The discharge energy density of the PVDF/P(VDF-CTFE) (70/30) film is more than twice that of pure PVDF due to the increase of phases α and γ and the decrease of crystallinity. Under the breakdown electric field (380 kV mm^-1 ), PVDF/P(VDF-CTFE) (70/30) film also has an ultrahigh energy storage efficiency of 64%. The relationship between the structure and properties of composite materials is investigated in this study, which has important implications for the development of capacitors with high energy storage density.

Suppressing the Loss of Polymer-Based Dielectrics for High Power Energy Storage

Polymer-based dielectrics have received intensive interest from academic community in the field of high-power energy storage owing to their superior flexibility and fast charge-discharge ability. Recently, how to suppress the loss of polymer-based dielectrics has been increasingly recognized as a critical point to attain a high charge-discharge efficiency in the film capacitors. Some achievements are made in analyzing the source of loss and suppressing loss via Edison's trial and error method. In this review, the significance of suppressing loss in polymer-based dielectrics is firstly emphasized. Then, different sources of loss are discussed carefully and an in-depth analysis of the related measurements is presented. Next, recent research results in suppressing loss are summarized and discussed in detail according to different strategies. Finally, the challenges and opportunities in the loss suppression research for the rational design of high-efficiency polymer-based dielectrics are proposed.

High-Energy-Density and High Efficiency Polymer Dielectrics for High Temperature Electrostatic Energy Storage: A Review

Polymer dielectrics are key components for electrostatic capacitors in energy, transportation, military, and aerospace fields, where their operation temperature can be boosted beyond 125 °C. While most polymers bear poor thermal stability and severe dielectric loss at elevated temperatures, numerous linear polymers with linear D-E loops and low dielectric permittivity exhibit low loss and high thermal stability. Therefore, the broad prospect of electrostatic capacitors under extreme conditions is anticipated for linear polymers, along with intensive efforts to enhance their energy density with high efficiency in recent years. In this article, an overview of recent progress in linear polymers and their composites for high-energy-density electrostatic capacitors at elevated temperatures is presented. Three key factors determining energy storage performance, including polarization, breakdown strength, and thermal stability, and their couplings are discussed. Strategies including chain modulation, filler selection, and design of topological structure are summarized. Key parameters for electrical and thermal evaluations of polymer dielectrics are also introduced. At the end of this review, research challenges and future opportunities for better performance and industrialization of dielectrics based on linear polymers are concluded.

Advances on Thermally Conductive Epoxy-Based Composites as Electronic Packaging Underfill Materials-A Review

The integrated circuits industry has been continuously producing microelectronic components with ever higher integration level, packaging density, and power density, which demand more stringent requirements for heat dissipation. Electronic packaging materials are used to pack these microelectronic components together, help to dissipate heat, redistribute stresses, and protect the whole system from the environment. They serve an important role in ensuring the performance and reliability of the electronic devices. Among various packaging materials, epoxy-based underfills are often employed in flip-chip packaging. However, widely used capillary underfill materials suffer from their low thermal conductivity, unable to meet the growing heat dissipation required of next-generation IC chips with much higher power density. Many strategies have been proposed to improve the thermal conductivity of epoxy, but its application as underfill materials with complex performance requirements is still difficult. In fact, optimizing the combined thermal-electrical-mechanical-processing properties of underfill materials for flip-chip packaging remains a great challenge. Herein, state-of-the-art advances that have been made to satisfy the key requirements of capillary underfill materials are reviewed. Based on these studies, the perspectives for designing high-performance underfill materials with novel microstructures in electronic packaging for high-power density electronic devices are provided.

Enhanced Dielectric Strength and Capacitive Energy Density of Cyclic Polystyrene Films

The maximum capacitive energy stored in polymeric dielectric capacitors, which are ubiquitous in high-power-density devices, is dictated by the dielectric breakdown strength of the dielectric polymer. The fundamental mechanisms of the dielectric breakdown, however, remain unclear. Based on a simple free-volume model of the polymer fluid state, we hypothesized that the free ends of linear polymer chains might act as “defect” sites, at which the dielectric breakdown can initiate. Thus, the dielectric breakdown strength of cyclic polymers should exhibit enhanced stability in comparison to that of their linear counterparts having the same composition and similar molar mass. This hypothesis is supported by the ∼50% enhancement in the dielectric breakdown strength and ∼80% enhancement in capacitive energy density of cyclic polystyrene melt films in comparison to corresponding linear polystyrene control films. Furthermore, we observed that cyclic polymers exhibit a denser packing density than the linear chain melts, an effect that is consistent with and could account for the observed property changes. Our work demonstrates that polymer topology can significantly influence the capacitive properties of polymer films, and correspondingly, we can expect polymer topology to influence the gas permeability, shear modulus, and other properties of thin films dependent on film density.

Progress on Polymer Dielectrics for Electrostatic Capacitors Application

Polymer dielectrics are attracting increasing attention for electrical energy storage owing to their advantages of mechanical flexibility, corrosion resistance, facile processability, light weight, great reliability, and high operating voltages. However, the dielectric constants of most dielectric polymers are less than 10, which results in low energy densities and limits their applications in electrostatic capacitors for advanced electronics and electrical power systems. Therefore, intensive efforts have been placed on the development of high-energy-density polymer dielectrics. In this perspective, the most recent results on the all-organic polymer dielectrics are summarized, including molecular structure design, polymer blends, and layered structured polymers. The challenges in the field and suggestions for future research on high-energy-density polymer dielectrics are also presented.

Multilayer Dielectric Nanocomposites with Cross-Linked Dielectric Transition Interlayers for High-Temperature Applications

In energy storage and transportation systems, polymer dielectrics are widely applied in smart grids, electric vehicles, and power conditioning owing to their incomparable power density and high reliability. However, the dielectric constant (ε) and breakdown strength (E_b) normally cannot be increased simultaneously, which results in insufficient discharged energy density especially at high temperatures. In this work, enhanced E_b and high energy density are archived in multilayer polymer nanocomposites by introducing cross-linked dielectric transition layers. Specifically, the sandwiched composite achieves a huge discharge energy density of 4.64 J cm^-3 with a charged-discharged efficiency of 84% at 150 °C and 500 MV m^-1. The formation of cross-linked dielectric transition layers between layers of the multilayer nanocomposite could effectively restrain the growth of the electrical tree and greatly increase the E_b. This work presents a strategy for designing high-performance multilayered dielectric polymer nanocomposites by introducing cross-linked dielectric transition layers to reduce the loss from interlayer interfaces.

A Journey from Processing to Recycling of Multilayer Waste Films: A Review of Main Challenges and Prospects

In a circular economy context with the dual problems of depletion of natural resources and the environmental impact of a growing volume of wastes, it is of great importance to focus on the recycling process of multilayered plastic films. This review is dedicated first to the general concepts and summary of plastic waste management in general, making emphasis on the multilayer films recycling process. Then, in the second part, the focus is dealing with multilayer films manufacturing process, including the most common materials used for agricultural applications, their processing, and the challenges of their recycling, recyclability, and reuse. Hitherto, some prospects are discussed from eco-design to mechanical or chemical recycling approaches.

High Energy Density and Temperature Stability in PVDF/PMMA via In Situ Polymerization Blending

Dielectrics with improved energy density have long-standing demand for miniature and lightweight energy storage capacitors for electrical and electronic systems. Recently, polyvinylidene fluoride (PVDF)-based ferroelectric polymers have shown attractive energy storage performance, such as high dielectric permittivity and high breakdown strength, and are regarded as one of the most promising candidates. However, the non-negligible energy loss and inferior temperature stability of PVDF-based polymers deteriorated the energy storage performance or even the thermal runaway, which could be ascribed to vulnerable amorphous regions at elevated temperatures. Herein, a new strategy was proposed to achieve high energy density and high temperature stability simultaneously of PVDF/PMMA blends by in situ polymerization. The rigidity of the amorphous region was ideally strengthened by in situ polymerization of methyl methacrylate (MMA) monomers in a PVDF matrix to obtain PVDF/PMMA blends. The atomic force microscopic study of the microstructure of etched films showed the ultra-homogenous distribution of PMMA with high glass transition temperature in the PVDF matrix. Consequently, the temperature variation was remarkably decreased, while the high polarization response was maintained. Accordingly, the high energy density of ∼8 J/cm³ with ∼80% efficiency was achieved between 30 and 90 °C in PVDF/PMMA films with 39–62% PMMA content, outperforming most of the dielectric polymers. Our work could provide a general solution to substantially optimize the temperature stability of dielectric polymers for energy storage applications and other associated functions.

Melting Temperature Depression of Polymer Single Crystals: Application to the Eco-Design of Tie-Layers in Polyolefinic-Based Multilayered Films

In this paper, we describe a method for determining polymer compatibility, which will aid in establishing the requirements of polyolefinic materials for the eco-design of multilayer films for mechanical recycling while avoiding the use of reactive tie layers. Our ultimate goal is to define the molecular characteristics of the polyolefinic structural layer that improve compatibility with the tie layer during mechanical recycling. We have investigated the melting temperature depression of single crystals of various polyethylenes embedded in commercial polymeric matrices with various functionalities (ester, acrylate, acetate and methacrylic acid sodium ionomer), which can be potentially used as tie layers. We demonstrate how the concentration and molecular architecture of the matrices affect the melting temperature of the embedded single crystals differently depending on the latter's molecular architecture. The main finding indicates that the tie layers are more compatible with linear polyethylene than with branched polyethylenes. Indeed, our results show that the heterogeneous Ziegler-Natta linear low-density polyethylene is incompatible with all of the tie layers tested. The depression of melting temperatures observed are in excellent agreement with the results obtained by investigating the rheological behaviour and morphological features of solution-mixed blends in which segmental interactions between polymeric chains have been, in theory, maximized. Because Ziegler-Natta linear density polyethylene is one of the most commonly used polymers as a structural layer in multi-layer applications, the findings of this study are useful as they clearly show the unsuitability of this type of polyethylene for recycling from an eco-design standpoint. The specific molecular requirements for polyethylene layers (branching content less than 0.5/100 carbon atoms) can be specified for use in packaging, guiding the eco-design and valorisation of recycled multi-layered films containing this material.

Research Advances in Hierarchically Structured PVDF-Based All-Organic Composites for High-Energy Density Capacitors

Polymer film capacitors have been widely applied in many pulsed power fields owing to their fastest energy-released rates. The development of ferroelectric polyvinylidene fluoride (PVDF)-based composites has become one of the hot research directions in the field of high-energy storage capacitors. Recently, hierarchically-structured all-organic composites have been shown to possess excellent comprehensive energy storage performance and great potential for application. In this review, most research advances of hierarchically-structured all-organic composites for the energy storage application are systematically classified and summarized. The regulating strategies of hierarchically structured all-organic composites are highlighted from the perspective of preparation approaches, tailored material choices, layer thicknesses, and interfaces. Systematic comparisons of energy storage abilities are presented, including electric displacement, breakdown strength, energy storage density, and efficiency. Finally, we present the remaining problems of hierarchically structured all-organic composites and provide an outlook for future energy storage applications.

Improving the Energy Density and Efficiency of the Linear Polymer PMMA with a Double-Bond Fluoropolymer at Elevated Temperatures

A variety of applications can be found for high-temperature film capacitors, including energy storage components and pulsed power sources. In this work, in order to increase the energy density (U _e), poly(vinylidene fluoride-chlorotrifluoroethylene-double bond) (P-DB) is introduced into poly(methyl methacrylate) (PMMA) to manufacture composite films by a solution casting process. In the case of the pure PMMA film, there is significant improvement in the polarization (P _max) and breakdown field (E _b) of the composite film. These improvements can effectively increase the U _e of the composite film at room temperature and the elevated temperature. The results show that at an elevated temperature of 90 °C and at 350 MV/m, the U _e of 40 vol % P-DB reaches 8.7 J/cm³, and the efficiency (η) of 77% is also considerable. Compared with biaxially oriented polypropylene (2.0 J/cm³), the proposed film exhibits 4 times enhancement in the energy storage density, meaning that it can be an energy storage capacitor with huge potential at high temperatures.

Recent Advances in Multilayer-Structure Dielectrics for Energy Storage Application

An electrostatic capacitor has been widely used in many fields (such as high pulsed power technology, new energy vehicles, etc.) due to its ultrahigh discharge power density. Remarkable progress has been made over the past 10 years by doping ferroelectric ceramics into polymers because the dielectric constant is positively correlated with the energy storage density. However, this method often leads to an increase in dielectric loss and a decrease in energy storage efficiency. Therefore, the way of using a multilayer structure to improve the energy storage density of the dielectric has attracted the attention of researchers. Although research on energy storage properties using multilayer dielectric is just beginning, it shows the excellent effect and huge potential. In this review, the main physical mechanisms of polarization, breakdown and energy storage in multilayer structure dielectric are introduced, the theoretical simulation and experimental results are systematically summarized, and the preparation methods and design ideas of multilayer structure dielectrics are mainly described. This article covers not only an overview of the state-of-the-art advances of multilayer structure energy storage dielectric but also the prospects that may open another window to tune the electrical performance of the electrostatic capacitor via designing a multilayer structure.

High loading BaTiO3 nanoparticles chemically bonded with fluorinated silicone rubber for largely enhanced dielectric properties of polymer nanocomposites

Integrating high-loading dielectric nanoparticles into polar polymer matrices potentially can profit the intrinsic polarization of each phase and allow for greatly enhanced dielectric properties in polymer nanocomposites. It is however challenging to achieve desirable highly filled polar polymer composites because of the lack of efficient approaches to disperse nanoparticles and maintain interfacial compatibility. Here, we report a versatile route to fabricate highly filled barium titanate/fluorinated silicone rubber (BT/FSR) nanocomposites by "thiol-ene click" and isostatic pressing techniques. The loaded BT nanoparticles (from 82 wt% to 90 wt%) are chemically bonded with FSR in the nanocomposites. The existence of the polar group (-CH₂CF₃) of the polymer matrix does not affect the uniform dispersion of the nanoparticles or the good interfacial compatibility. The 90 wt% BT/FSR nanocomposite shows the highest dielectric constant of 57.8 at 10³ Hz, while the loss tangent can be kept below 0.03. Besides, BT/FSR nanocomposites display higher breakdown strength than BT/SR nanocomposites. This work offers a facile strategy towards superior dielectric properties of polymer nanocomposites.

Challenges and Opportunities of Polymer Nanodielectrics for Capacitive Energy Storage

With the modern development of power electrification, polymer nanocomposite dielectrics (or nanodielectrics) have attracted significant research attention. The idea is to combine the high dielectric constant of inorganic nanofillers and the high breakdown strength/low loss of a polymer matrix for higher energy density polymer film capacitors. Although impressively high energy density has been achieved at the laboratory scale, there is still a large gap from the eventual goal of polymer nanodielectric capacitors. In this review, we focus on essential material issues for two types of polymer nanodielectrics, polymer/conductive nanoparticle and polymer/ceramic nanoparticle composites. Various material design parameters, including dielectric constant, dielectric loss, breakdown strength, high temperature rating, and discharged energy density will be discussed from both fundamental science and high-voltage capacitor application points of view. The objective is to identify advantages and disadvantages of the polymer nanodielectric approach against other approaches utilizing neat dielectric polymers and ceramics. Given the state-of-the-art understanding, future research directions are outlined for the continued development of polymer nanodielectrics for electric energy storage applications.

Enhanced crystallization behaviors and dielectric performance of poly(vinylidene fluoride) film induced by polyamide-1

Poly(vinylidene fluoride) (PVDF)-based composites attract tremendous attention as dielectric materials. However, their development has been limited due to the raised problem in the in-homogeneous polymer composites. In this work, a novel PVDF-based film incorporated with polyamide-1, containing the highest density of dipole among all polyamides, was prepared to improve the crystallization behaviors and dielectric properties. The results showed that the optimal concentration of polyamide-1 in PVDF was 6 wt.%. The crystallization rate of PVDF was improved in the presence of polyamide-1. Interestingly, the polyamide-1 was conductive to the formation of β form crystal of PVDF, which exhibited great electric performance. The dielectric constant of PVDF increased sharply and loss tangent still kept at a low level of 0.03@100 Hz when the concentration of polyamide-1 was 6 wt.%. This work may provide a new direction to design dielectric materials for PVDF blends.

Enhancing Dielectric Strength of Epoxy Polymers by Constructing Interface Charge Traps

Epoxy polymer-based dielectric materials play a crucial role in advanced electronic devices and power equipment. However, high voltage-stress applications impose stringent requirements, such as a high dielectric strength, on epoxy polymers. Previously reported studies have shown promising material architectures in the form of epoxy polymer-nanoparticle dielectrics, which can restrict the movement of high-energy electrons by the interface charge traps associated with the various interfacial regions. However, these high-energy electrons inevitably traverse the epoxy polymer matrix and destroy the molecular structure, thereby creating a weak link for dielectric breakdown. In this study, a general strategy is developed to improve the dielectric strength by constructing interface charge traps in the molecular structure of the epoxy polymer matrix, using the -CF₃ group in partial replacement of the -CH₃ group. The proposed strategy increases the dielectric strength (39.5 kV mm^-1) and surface breakdown voltage (26.9 kV) of the epoxy polymer matrix by 22.08% and 13.3%, respectively, because the interface charge trap hinders the movement of high-energy electrons. At the same time, the strategy does not degrade the mechanical and thermal properties. The results hold potential for wide application in the manufacturing of advanced future electrical and electronic equipment requiring resilience to high-voltage stress.

Dielectric polymers for high-temperature capacitive energy storage

Polymers are the preferred materials for dielectrics in high-energy-density capacitors. The electrification of transport and growing demand for advanced electronics require polymer dielectrics capable of operating efficiently at high temperatures. In this review, we critically analyze the most recent development in the dielectric polymers for high-temperature capacitive energy storage applications. While general design considerations are discussed, emphasis is placed on the elucidation of the structural dependence of the high-field dielectric and electrical properties and the capacitive performance, including discharged energy density, charge-discharge efficiency and cyclability, of dielectric polymers at high temperatures. Advantages and limitations of current approaches to high-temperature dielectric polymers are summarized. Challenges along with future research opportunities are highlighted at the end of this article.

Recent Advances in the Synthesis of Polymer-Grafted Low-K and High-K Nanoparticles for Dielectric and Electronic Applications

The synthesis of polymer-grafted nanoparticles (PGNPs) or hairy nanoparticles (HNPs) by tethering of polymer chains to the surface of nanoparticles is an important technique to obtain nanostructured hybrid materials that have been widely used in the formulation of advanced polymer nanocomposites. Ceramic-based polymer nanocomposites integrate key attributes of polymer and ceramic nanomaterial to improve the dielectric properties such as breakdown strength, energy density and dielectric loss. This review describes the "grafting from" and "grafting to" approaches commonly adopted to graft polymer chains on NPs pertaining to nano-dielectrics. The article also covers various surface initiated controlled radical polymerization techniques, along with templated approaches for grafting of polymer chains onto SiO2, TiO2, BaTiO3, and Al2O3 nanomaterials. As a look towards applications, an outlook on high-performance polymer nanocomposite capacitors for the design of high energy density pulsed power thin-film capacitors is also presented.

Reversible Stimuli-Dependent Aggregation-Induced Emission from a "Nonfluorescent" Amphiphilic PVDF Graft Copolymer

A poly(vinylidine fluoride) graft random copolymer of t-butyl aminoethyl methacrylate (tBAEMA) and oligo(ethylene glycol) methyl ether methacrylate (OEGMA, Mn = 300) [PVDF-g-P(tBAEMA-ran-OEGMA), PVBO] is synthesized by atom transfer radical polymerization (ATRP), and PVBO is fractionated to get a highly water-soluble fraction (PVBO-1) showing a reversible on/off fluorescence behavior with gradual increase and decrease in pH, respectively, achieving a maximum quantum yield of 0.18 at pH = 12. PVBO-1 dissolved in water shows large multimicellar aggregates (MMcA), but at pH 12, crumbling of larger aggregates to much smaller micelles occurs, forming nonconjugated polymer dots (NCPDs), as supported by transmission electron microscopy and dynamic light scattering study. The reversible fluorescence on/off behavior also occurs with the decrease and increase of temperature. Theoretical study indicates that, at high pH, most of the amino groups become neutral and exhibit a strong tendency to form aggregates from crowding of a large number of carbonyl and amine groups, minimizing the HOMO-LUMO gap, showing an absorption peak at the visible region, and generating aggregation-induced emission.