Versatile Landscape of Low-k Polyimide: Theories, Synthesis, Synergistic Properties, and Industrial Integration.

The development of microelectronics and large-scale intelligence nowadays promotes the integration, miniaturization, and multifunctionality of electronic and devices but also leads to the increment of signal transmission delays, crosstalk, and energy consumption. The exploitation of materials with low permittivity (low-k) is crucial for realizing innovations in microelectronics. However, due to the high permittivity of conventional interlayer dielectric material (k ∼ 4.0), it is difficult to meet the demands of current microelectronic technology development (k < 3.0). Organic dielectric materials have attracted much attention because of their relatively low permittivity owing to their low material density and low single bond polarization. Polyimide (PI) exhibits better application potential based on its well permittivity tunability (k = 1.1-3.2), high thermal stability (>500 °C), and mechanical property (modulus of elasticity up to 3.0-4.0 GPa). In this review, based on the synergistic relationship of dielectric parameters of materials, the development of nearly 20 years on low-k PI is thoroughly summarized. Moreover, process strategies for modifying low-k PI at the molecular level, multiphase recombination, and interface engineering are discussed exhaustively. The industrial application, technological challenges, and future development of low-k PI are also analyzed, which will provide meaningful guidance for the design and practical application of multifunctional low-k materials.

High-toughness, extensile and self-healing PDMS elastomers constructed by decuple hydrogen bonding.

Elastomers are widely used in traditional industries and new intelligent fields. However, they are inevitably damaged by electricity, heat, force, etc. during the working process. With the continuous improvement of reliability and environmental protection requirements in human production and living, it is vital to develop elastomer materials with good mechanical properties that are not easily damaged and can self-heal after being damaged. Nevertheless, there are often contradictions between mechanical properties and self-healing as well as toughness, strength, and ductility. Herein, a strong and dynamic decuple hydrogen bonding based on carbon hydrazide (CHZ) is reported, accompanied with soft polydimethylsiloxane (PDMS) chains to prepare self-healing (efficiency 98.7%), recyclable, and robust elastomers (CHZ-PDMS). The strategy of decuple hydrogen bonding will significantly impact the study of the mechanical properties of elastomers. High stretchability (1731%) and a high toughness of 23.31 MJ m-3 are achieved due to the phase-separated structure and energy dissipation. The recyclability of CHZ-PDMS further supports the concept of environmental protection. The application of CHZ-PDMS as a flexible strain sensor exhibited high sensitivity.

Dual-Effect Coupling for Superior Dielectric and Thermal Conductivity of Polyimide Composite Films Featuring "Crystal-Like Phase" Structure.

To match the increasing miniaturization and integration of electronic devices, higher requirements are put on the dielectric and thermal properties of the dielectrics to overcome the problems of delayed signal transmission and heat accumulation. Here, a 3D  porous thermal conductivity network is successfully constructed inside the polyimide (PI) matrix by the combination of ionic liquids (IL) and calcium fluoride (CaF2 ) nanofillers, motivated by the bubble-hole forming orientation force. Benefiting from the 3D thermal network formed by IL as a porogenic template and "crystal-like phase" structures induced by CaF2 - polyamide acid charge transfer, IL-10 vol% CaF2 /PI porous film exhibits a low permittivity of 2.14 and a thermal conductivity of 7.22 W m-1 K-1 . This design strategy breaks the bottleneck that low permittivity and high thermal conductivity in microelectronic systems are difficult to be jointly controlled, and provides a feasible solution for the development of intelligent microelectronics.

Dynamic Covalent Adaptable Polyimide Hybrid Dielectric Films with Superior Recyclability.

Polyimides (PIs) used in advanced electrical and electronic devices can be electrically/mechanically damaged, resulting in a significant waste of resources. Closed-loop chemical recycling may prolong the service life of synthetic polymers. However, the design of dynamic covalent bonds for preparing chemically recyclable crosslinked PIs remains a challenging task. Herein, new crosslinked PI films containing a PI oligomer, chain extender, and crosslinker are reported. They exhibit superior recyclability and excellent self-healable ability owing to the synergistic effect of the chain extender and crosslinker. The produced films can be completely depolymerized in an acidic solution at ambient temperature, leading to efficient monomer recovery. The recovered monomers may be used to remanufacture crosslinked PIs without deteriorating their original performance. In particular, the designed films can serve as corona-resistant films with a recovery rate of approximately 100%. Furthermore, carbon fiber reinforced composites (CFRCs) with PI matrices are suitable for harsh environments and can be recycled multiple times at a non-destructive recycling rate up to 100%. The preparation of high-strength dynamic covalent adaptable PI hybrid films from simple PI oligomers, chain extenders, and crosslinkers may provide a solid basis for sustainable development in the electrical and electronic fields.

Rising of Dynamic Polyimide Materials: A Versatile Dielectric for Electrical and Electronic Applications.

Polyimides (PIs) are widely used in circuit components, electrical insulators, and power systems in modern electronic devices and large electrical appliances. Electrical/mechanical damage of materials are important factors that threaten reliability and service lifetime. Dynamic (self-healable, recyclable and degradable) PIs, a promising class of materials that successfully improve electrical/mechanical properties after damage, are anticipated to solve this issue. The viewpoints and perspectives on the status and future trends of dynamic PI based on a few existing documents are shared. The main damage forms of PI dielectric materials in the application process are first introduced, and initial strategies and schemes to solve these problems are proposed. Fundamentally, the bottleneck issues faced by the development of dynamic PIs are indicated, and the relationship between various damage forms and the universality of the method is evaluated. The potential mechanism of the dynamic PI to deal with electrical damage is highlighted and several feasible prospective schemes to address electrical damage are discussed. This study is concluded by presenting a short outlook and future improvements to systems, challenges, and solutions of dynamic PI in electrical insulation. The summary of theory and practice should encourage policy development favoring energy conservation and environmental protection and promoting sustainability.

Dynamic Sustainable Polyimide Film Combining Hardness with Softness via a "Mimosa-Like" Bionic Strategy.

Dielectric polyimides (PIs) are ubiquitous as insulation in electrical power systems and electronic devices. Generally, dynamic polyimide is required to solve irreversible failure processes of electrical or mechanical damage, for example, under high temperature, pressure, and field strength. The challenge lies in the design of the molecular structure of rigid polyimide to achieve dynamic reversibility. Herein, a low-molecular-weight polyimide gene unit is designed to crosslink with polyimide ligase to prepare the smart film. Interestingly, due to the variability of gene unit and ligase combinations, the polyimide films combining hardness with softness are designed into three forms via a "Mimosa-like" bionic strategy to adapt to different application scenarios. Meanwhile, the films have good degradation efficiency, excellent recyclability, and can be self-healable, which makes them reuse. Clearly, the films can be used in the preparation of ultrafast sensors with a response time ≈0.15 s and the application of corona-resistant films with 100% recovery. Furthermore, the construction of polyimide and carbon-fiber-reinforced composites (CFRCs) has been verified to apply to the worse environment. Nicely, the composites have the property of multiple cycles and the non-destructive recycle rate of carbon fiber (CF) is as high as 100%. The design idea of preparing high-strength dynamic polyimide by crosslinking simple polyimide gene unit with ligase could provide a good foundation and a clear case for the sustainable development of electrical and electronic polyimides, from the perspective of Mimosa bionics.

Low-Permittivity Copolymerized Polyimides with Fluorene Rigid Conjugated Structure.

As the miniaturization of electronic appliances and microprocessors progresses, low-permittivity interlayer materials are becoming increasingly important for their suppression of electronic crosstalk, signal propagation delay and loss, and so forth. Herein, a kind of copolyimide (CPI) film with a "fluorene" rigid conjugated structure was prepared successfully. By introducing 9,9-Bis(3-fluoro-4-aminophenyl) fluorene as the rigid conjugated structure monomer, a series of CPI films with different molecular weights were fabricated by in situ polymerization, which not only achieved the reduction of permittivity but also maintained excellent thermodynamic stability. Moreover, the hydrophobicity of the CPI film was also improved with the increasing conjugated structure fraction. The lowest permittivity reached 2.53 at 106 Hz, while the thermal decomposition temperature (Td5%) was up to 530 °C, and the tensile strength was ≥ 96 MPa. Thus, the CPI films are potential dielectric materials for microelectronic and insulation applications.

Exposure to a 50†Hz magnetic field at 100†µT exerts no DNA damage in cardiomyocytes.

The effects of exposure to magnetic fields (MFs) at electric frequencies (50-60 Hz) on carcinogenicity are still in debate. Whether exposure to MFs affects the heart is also a debated issue. This study aimed to determine whether exposure to extremely low frequency MFs (ELF-MFs) induced DNA damage in cardiomyocytes both in vitro and in vivo Human ventricular cardiomyocytes were exposed to 50 Hz ELF-MF at 100 µT for 1 h continuously or 75 min intermittently. The effects of the treatments were evaluated by DNA damage, redox status changes and relative signal molecular expression. Moreover, ten male Sprague-Dawley rats were exposed to a 50 Hz MF at 100 µT for 7 days, while another 10 rats were sham exposed. The protein levels of p53 and Hsp70 in heart tissue were analyzed by western blot. The results showed that exposure to ELF-MF did not induce DNA damage, changes to cell cycle distribution or increased reactive oxygen species level. No significant differences were detected in p53 and Hsp70 expression level between the ELF-MF and sham-exposure groups both in vitro and in vivo All these data indicate that MFs at power-frequency may not cause DNA damage in cardiomyocytes.This article has an associated First Person interview with the first author of the paper.

Effects of continuous exposure to power frequency electric fields on soybean Glycine max.

With the increasing density of high voltage transmission systems, the potential risks and hazards of environmental electric fields (EFs) generated by these systems to surrounding organisms is becoming a source of public concern. To evaluate the effect of environmental EFs on plants, we used soybean as a model and systematically evaluated the effect of continuous exposure to different intensities (0 kV/m, 2 kV/m, and 10 kV/m) of power frequency EFs on agronomic characters, yield, nutrient contents, protective enzyme activities, and gene transcription. We found that the effects on soybean were more pronounced when plants were exposed to EF during development (especially at the seedling stage) than when they were exposed at maturity. The functional leaf number, stem diameter, plant dry weight, and pod number were largely unaffected by EF, while the germination rate and protective enzyme activities increased with increasing EF intensity. In plants exposed to low-intensity EF (2 kV/m), some agronomic characters, including chlorophyll content, plant height, and bean dry weight, as well as the soluble sugar and total protein contents, were significantly higher than those of plants exposed to high-intensity EF (10 kV/m) and control plants (0 kV/m). Through transcriptome analysis, we found that 2,977 genes were significantly up-regulated and 1,462 genes were down-regulated when plants were exposed to EF. These differentially expressed genes mainly encode ribosome proteins and related enzymes involved in carbon metabolism pathway, providing a novel perspective for understanding molecular mechanisms underpinning the responses to EF stress in soybean.

Significantly enhanced dielectric constant and energy storage properties in polyimide/reduced BaTiO3 composite films with excellent thermal stability.

In this work, reduced BaTiO3 (rBT) particles with a large number of defects sintered in a reducing atmosphere (95N2/5H2) were introduced into polyimide (PI) matrix without using any modifier or surfactant components. The rBT/PI composite films fabricated by an in situ polymerization method showed significantly enhanced dielectric constant and energy storage density. The dielectric constant of the rBT/PI composite with 30 wt% rBT reached up to 31.6, while maintaining lower loss (tg δ = 0.031@1000 kHz) compared to pure PI (ε r = 4.1). Its energy storage density (9.7 J cm-3 at 2628 kV cm-1) was enhanced by more than 400% over that of pure PI (1.9 J cm-3 at 3251 kV cm-1), and was greater than the energy density of the best commercial biaxially-oriented-polypropylenes (BOPP) (1.2 J cm-3 at 6400 kV cm-1). The energy storage efficiency was around 90% due to the linear dielectric performance of rBT/PI composite films. The improved dielectric constant and energy storage density could be attributed to the combined effect of the interface interaction between two phases and the surface defects of rBT induced by the reducing atmosphere. Therefore, rBT/PI composite films with high dielectric constant, energy storage density and storage efficiency may have potential applications in the preparation of embedded capacitors.

Power-frequency magnetic fields at 50 Hz do not affect fertility and development in rats and mice.

In the present study, the effects of power-frequency magnetic fields (PF-MF) on fertility and development were investigated in rats and mice. Adult Sprague-Dawley rats and C57BL/6J mice were divided into four groups: a sham exposure group and 30-µT, 100-µT and 500-µT PF-MF exposure groups. The rats were exposed for 24 weeks, and the exposure time for mice ranged from 18 d to 12 weeks, dependent on the different investigated end points. The rats and mice were exposed for 20 h/d. Plasma hormone levels in rats and mice were analyzed. Furthermore, pregnancy rates and implanted embryos were recorded in pregnant mice. Finally, the neonatal growth of mice was evaluated. The results showed that none of the three intensities affected the body weight and paired ovary weight in female rats. Meanwhile, none of the three intensities affected the body weight, weights of paired testes, weights of paired epididymis and sperm count in male rats. Similarly, no significant differences were found in plasma sex hormone levels between the different PF-MF exposure groups and the sham exposure group. In addition, the pregnancy rates and implanted embryos were not significantly different between the four groups. Moreover, PF-MF exposures had no effects on either the number of fetuses in pregnant mice or the growth and development of neonatal mice.

The effects of a 50-Hz magnetic field on the cardiovascular system in rats.

A 50-Hz magnetic field (MF) is a potential health-risk factor. Its effects on the cardiovascular system have not been fully investigated. This study was conducted to explore the effects of long-term exposure to a 50-Hz MF on the cardiovascular system. In the study, an exposure system was constructed, and the distribution of the 50-Hz MF was determined. Sixty-four Sprague-Dawley (SD) rats were exposed to a 50-Hz MF at 100 µT for 24 weeks, 20 h per day, while another 64 rats were sham exposed. During the exposure, blood pressure was measured every 4 weeks. After 24 weeks, echocardiography, cardiac catheterization and electrocardiography were performed. Moreover, heart and body weight were recorded, and haematoxylin-eosin staining and real-time PCR were conducted. The results showed that compared with the sham group, exposure to a 50-Hz MF did not exert any effects on blood pressure, pulse rate, heart rate or cardiac rhythm. Furthermore, echocardiography and cardiac catheterization showed that there were no significant differences in the cardiac morphology or haemodynamics. In addition, histopathological examination showed that exposure to a 50-Hz MF had no effects on the structure of the heart. Finally, expression of the cardiac hypertrophy-related genes did not show any significant differences between the 50-Hz MF exposure group and the sham group. Taken together, in SD rats, exposure to a 50-Hz/100 µT MF for 24 weeks did not show any obvious effects on the cardiovascular system.

Long-term exposure to ELF-MF ameliorates cognitive deficits and attenuates tau hyperphosphorylation in 3xTg AD mice.

Although numerous studies have reported the influence of extremely low frequency magnetic field (ELF-MF) exposure on human health, its effects on cognitive deficits in Alzheimer's disease (AD) have remained under debate. Moreover, the influence of ELF-MF on hyperphosphorylated tau, which is one of the most common pathological hallmarks of AD, has not been reported to date. Therefore, transgenic mice (3xTg) were used in the present study. 3xTg mice, which express an APP/PS1 mutation combined with a tau (P301L) mutation and that develop cognitive deficits at 6 months of age, were subjected to ELF-MF (50Hz, 500µT) exposure or sham exposure daily for 3 months. We discovered that ELF-MF exposure ameliorated cognitive deficits and increased synaptic proteins in 3xTg mice. The protective effects of ELF-MF exposure may have also been caused by the inhibition of apoptosis and/or decreased oxidative stress levels that were observed in the hippocampus tissues of treated mice. Furthermore, tau hyperphosphorylation was decreased in vivo because of ELF-MF exposure, and this decrease was induced by the inhibition of GSK3ß and CDK5 activities and activation of PP2Ac. We are the first to report that exposure to ELF-MF can attenuate tau phosphorylation. These findings suggest that ELF-MF exposure could act as a valid therapeutic strategy for ameliorating cognitive deficits and attenuating tau hyperphosphorylation in AD.