Guest Editorial: High-performance polyimide dielectric materials

Polyimides are advanced polymeric materials that are well known for their excellent thermal, mechanical, electrical and chemical resistance properties. As an important kind of high-temperature resistant dielectric material, polyimides have been widely used in various applications such as electronics, microelectronics and electrical fields, due to their high thermal stability, high glass transition temperature (T_g), outstanding dielectric and electrical insulating performance at the high electric field or at high frequencies. Polyimide dielectric materials have a rich variety of reactive monomers, which endows the molecular structures with strong designability and facilitates the regulation of material properties. Moreover, polyimides can be compounded with various functional fillers to achieve the multifunctional dielectric materials with low or high dielectric properties, low dielectric loss and high breakdown strength. Polyimide dielectric materials also have an ease of processability that make them patternable for many types of integrated devices. With the ongoing advancements in a wide range of novel electronic, microelectronic and new energy applications, polyimide dielectric materials have gained increasing interest from both fundamental and applied research. High-performance polyimide dielectric materials are essential for the development of new electronic or electrical devices where further considerations are required, including higher temperature resistance and energy storage, lower dielectric constant and dielectric loss, improved thermal conduction management as well as better reliability or flexibility in harsh environments. In order to meet the more stringent application requirements mentioned above, there is an urgent need to develop polyimide dielectric materials with higher comprehensive performance, which requires joint development through new theoretical designs, new structures, methods, processes and other means. A wide variety of research is being conducted to prepare kinds of functional polyimide dielectric materials to address applicable challenges and explore possible opportunities in different fields. This current Special Issue is focused on ‘High performance polyimide dielectric materials’ and their applications in different topics, emphasising the latest innovations in polyimide or polyimide-based dielectric materials and better understanding of deep relationship between their chemical or composition structures and overall performances.

In this Special Issue, four high-quality papers have undergone peer-reviewed and eventually been accepted for publication. These published papers include five original research papers and one review article in the application field of high-performance polyimide dielectric materials. All the papers can be clustered into three main categories related to dielectric materials, namely preparation, molecular design and measurement or simulation. (1) The first category of paper offers simple and effective strategies to prepare crosslinked polyimide aerogel with a microporous structure and functional fluorinated graphene/polyimide (FG/PI) composites, respectively. The effects of manufacturing process on thermal insulation property or thermal conductivity, as well as permittivity and electrical insulating performance, are well studied. The papers in this category is of Qiu L and Zha J W et al., (2) The second category of paper exhibits novelties in the molecular design of high-performance polyimide dielectrics with significant enhancement of dielectric properties. Some special polyimides containing functional structures or groups are designed and synthesised, such as PIS series with siloxane segments in the main chain and SPI series with sulfonyl groups in the side chain. This paper is of Tong H and Li X M et al. (3) The last category proposes new simulation and measurement methods for polyimide nanocomposite dielectrics or polyimide films, rendering them suitable for high-temperature energy storage and flexible solar wing applications. An energy storage and release model considering the charge trapping effects is constructed and electrical properties as well as stress-strain characteristics of films under different tensile stresses are respectively analysed in detail. These papers are of Min D M et al., Qin S C and Zhang J W et al. A brief presentation of each of the paper in this special issue follows.

Qiu L and Zha J W et al. present a freeze-drying strategy combined with regulation of microstructure to prepare a series of crosslinked polyimide aerogels by introducing crosslinking agents into a linear structure. The thermodynamic, thermal insulation and dielectric properties are achieved by controlling the rigidity and composition of polymerised monomers. It is proved that the increased rigidity of a molecular structure is beneficial for the formation of denser micro-pores, thus obtaining excellent comprehensive properties. This work provides a new pathway to construct polyimide aerogels with ultralow permittivity and good thermal insulating, promoting a wider application in the field of integrated circuits or aerospace exploration.

Tong H and Li X M et al. designed and synthesised a kind of novel diamine with sulfonyl-containing side chain. The corresponding polyimide dielectrics (SPI) were further prepared to investigate the influence of the polar side chain on dielectric and energy storage properties. Moreover, molecular simulation was adopted to analyse the correlation relationships of molecular structure, microscopic parameters and properties. It is found that the introduction of the strong polar sulfonyl group in the side chain can effectively enhance dielectric and energy storage properties, and the dipolar moment density calculated from molecular simulation is closely correlated to permittivity measured from experiments. This work offers an effective combined method of molecular simulation and experiments to assist in the molecular design of high-performance polyimide dielectrics.

Min D M et al. introduce an energy storage and release model that is constructed after considering the charge trapping effects and simulate the high-temperature energy storage and release properties of polyimide nanocomposite dielectrics with different charge injection barriers and trap parameters at 150℃. A triangular voltage is applied to the electrodes at both sides of the polyimide nanocomposite dielectrics. The electric displacement-electric field loop is simulated, and the discharged energy densities and energy efficiencies are calculated. It is found that increasing the charge injection barrier, deep trap energy and deep trap density can effectively reduce the charge injection and carrier mobility, thereby improving the discharged energy densities and energy efficiencies of dielectric capacitors. This work provides a theoretical and model support for the improvement of the high-temperature energy storage performance of nanocomposites in the fields of aerospace or pulse power.

Qin S C and Zhang J W et al. explore measurement methods of electrical properties and stress-strain characteristics under different tensile stresses for polyimide films applied in flexible solar wings. The polyimide films used in flexible solar wings are subjected to irradiation in space and tensile mechanical stress, which makes the charge accumulation effect and results to electrostatic discharge. To solve this problem, it is necessary to establish a test method for charge transport and accumulation characteristics of polyimide under tensile stress. The mechanical properties of polyimide films under tensile stress were investigated experimentally, and a model for predicting the film thickness was developed using the micro-element method. Finally, a method for conducting conductivity measurements under tension was further discussed. This work provides a basis for investigating the electrical conductivity mechanism and stress-strain characteristics of polyimide films under different tensile stresses, thus promoting the formulation selection and performance improvement of polyimide for flexible solar wings of spacecraft.

All of the papers selected for this Special Issue fully display that polyimide or polyimide-based dielectric materials with high performance are significantly moving forward. The demand for innovative and diverse polyimide materials with superior thermal, mechanical, electrical insulating and dielectric properties has greatly increased, and related research has attracted more and more attention. The study of relationship between structures, fabrication and performance of polyimide dielectric materials is the focus of basic research in the future. Meantime, it is urgent to develop relevant measurement methods, theoretical analyses and multiscale modelling. The application research of polyimide dielectric materials in the fields of high-temperature energy storage, high-frequency communication, advanced electronics or microelectronics needs to make breakthrough progress.