This paper introduced the research and application progress on green solvents in the preparation of polyimide from the views of the classification, manufacturing procedures, solvent composition of polyimide films, and the application status of green solvents. Emphatically, the current research status of biobased solvents, including γ-valerolactone (GVL), dimethyl isosorbide (DMI), and dihydrolevoglucosenone (Cyrene^TM) and their applications on the preparation of PI films were presented. The future developing trends of the applications of green solvents in the manufacturing of PI films were prospected.

Firstly, a Y-shape diamine monomer 4-(4′-tertbutyl)phenyl-2,6-di(4′-aminophenyl)pyridine (TPAPP) containing pyridine ring structure and tertbutyl group was synthesized. Then, it was polymerized with 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FAP) and 4,4′-(4,4′-isopropyldiphenyloxy)bis(phthalic anhydride) (BPADA), and a series of copolyimide (PI) films were prepared by thermal imidization. The structure and optical, mechanical, and thermal properties as well as solubility of the PI films were investigated by infrared spectroscopy, X-ray diffraction, UV visible spectra, tensile tests, dynamic thermo-mechanical analysis, thermogravimetric analysis, and solubility tests. The results show that as the molar ratio of TPAPP increases, the optical transparency and glass transition temperature of the copolyimide films exhibit an increasing trend. When the molar ratio of diamine and dianhydride is 5:5, the copolyimide film shows the best mechanical properties, with the tensile strength of 144.9 MPa, the Young's modulus of 3.4 GPa, and the elongation at break of 9.3%. In addition, these copolyimides demonstrate good solubility in aprotic polar solvents.

With the constant updating of flexible display devices, the researches on colorless transparent polyimide (CPI) films as substrate materials have attracted considerable attention. In order to solve the inherent contradictions among optical performance, high-temperature resistance, dimensional stability, and mechanical properties of CPI, we synthesized a series of semi-aromatic CIP films with side chains containing benzimidazole by copolymerization using cyclobutanetetracarboxylic dianhydride (CBDA) as the dianhydride monomer, 2,2′-bis(trifluoromethyl)-4-diaminobiphenyl (TFMB) and 2-(3,5-diaminophenyl)-benzimidazole (BBIA) as diamine monomers. The influence of hydrogen bonding, free volume, and main chain structure on the properties of films was systematically investigated by the Materials Studio software simulation and experimental results. The results show that the prepared CPI films have excellent optical transparency, thermal resistance, and mechanical properties. The average transmittance of CPI films in wave length of 380-780 nm is higher than 85%, the glass transition temperature (T_g) is above 400℃, the linear coefficient of thermal expansion (CTE) is 15×10^-6-17×10^-6 K^-1, the maximum tensile strength is 165 MPa, and the modulus is in the range of 3.7-5.2 GPa.

In order to improve the solution processability of traditional thermoplastic polyimide (TPI) and enhance its adhesion and heat resistance, several soluble thermoplastic polyimide (TPI) resins were prepared from different commercial dianhydride monomers and self-made diamine monomers with pyridine and diphenyl ether structures and active phenol side groups or benzene side group in the main chain through the two-step method. And then the corresponding TPI films and flexible copper clad laminates (FCCLs) were prepared. The properties of TPI resins were analyzed by solubility test and gel permeation chromatography (GPC). The structure, water absorption, mechanical properties, and dielectric properties of TPI films were tested, and the related properties of the FCCLs were also tested. The results show that all the TPI resins can dissolve in strong polar organic solvents such as NMP, and the glass transition temperature (T_g) and 5% thermal weight loss temperature (T_5%) of the corresponding films are in the range of 236.8-325.6℃ and 508.7-553.7℃, respectively, and the residue rate at 800℃ (R₈₀₀) is higher than 64%. The coefficient of thermal expansion (CTE), tensile strength, elongation at break, and water absorption is in the range of 56.36×10^-6-78.30×10^-6℃^-1, 64.93-109.18 MPa, 9.09%-24.60%, and 0.74%-3.75%, respectively. The dielectric constant and dielectric loss of TPI-4 with better comprehensive properties are also lower than that of other samples, and the peeling strength of the corresponding FCCL reaches 0.95 N/mm, but it can only pass the floating welding test at 288℃ for 10 seconds. In addition, the polar phenol side group in TPI-4 not only ensures its organic solubility, but also significantly enhances its heat resistance and mechanical strength, and also reduce its CTE. The existence of reactive phenol side groups provides great convenience for the subsequent chemical modification of TPI. By adding a small amount of cyanate ester resin (CE01) to modify the TPI, the peel strength and the floating welding resistance of the responding FCCLs can be obviously improved.

Polyimide films with high porosity were prepared using 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 4,4′-diamino-2,2′-dimethyl-biphenyl (mTB) as monomers. During the preparation process, pore-forming agent was added into the system, and the high-temperature thermal imidization of polyamide acid was controlled by the principle of thermo-induced phase separation. The micropore morphology, mechanical properties, and dielectric properties of the films were analyzed. The results show that the porosity of porous films reach 50%-60%. Under the frequency of 10 GHz, the dielectric constant and the dielectric loss factor of the sample PI-1-250-4 h is 2.14 and 0.002 8, respectively, while maintaining good mechanical properties and heat resistance. Due to the introduction of rigid biphenyl building blocks, the porous film still maintains a low coefficient of thermal expansion. The adhesive-free double-sided copper clad laminate, which prepared by combining the porous film with thermoplastic polyimide (TPI) and then pressing copper foil at high temperature, has good interface adhesion and solder resistance, and can apply to high-frequency and high-speed flexible copper clad laminates.

Three fluoro-containing polyimide (PI) resins (PI-II_a, PI-II_b, and PI-II_c) were prepared by two-step chemical imidization procedure using a fluoro-containing dianhydride 9,9-bis(trifluoromethyl)xanthene-2,3,6,7-tetracarboxylic dianhydride (6FCDA, II) and three kinds of aromatic diamines, including 2,2ʹ-bis(trifluoromethyl)-4,4ʹ-diaminodiphenylether (6FODA, a), 1,4-bis[(4-amino-2-trifluoromethyl)phenoxy]benzene (6FAPB, b) and 2,2-bis[(4-aminophenoxy) benzene]propane (BAPP, c), respectively. For comparison, the referenced PI resins (PI-I_a, PI-I_b, and PI-I_c) were prepared with the same procedure except that 6FCDA was replaced by 4,4ʹ-(hexafluoroisopropylidene)diphthalic anhydride (6FDA). And then six kinds of PI films were prepared by high temperature curing process, and their properties were studied. The results show that the solubility of PI-II_a, PI-II_b, and PI-II_c resins in organic solvents is lower than that of PI-I_a, PI-I_b, and PI-I_c resins. Compared with PI-I_a, PI-I_b, and PI-I_c films, PI-II_a, PI-II_b, and PI-II_c films show higher glass transition temperature (T_g), lower linear coefficient of thermal expansion (CTE), little lower optical transparency, and relatively higher dielectric constant (D_k). The PI-II_a film has the optimal comprehensive properties, its T_g and CTE is 362.5℃ and 39.6×10^-6 K^-1, respectively, its optical transmittances at the wavelength of 450 nm (T₄₅₀) is 83.9%, and the D_k at the frequency of 10 GHz is 3.00.

Four kinds of copolymerized polyimide (PI) membranes were prepared through thermal imidization method after solution polymerization between two kinds of self-made diamine monomers 9,9′-bis[4-(4-amino-3-hydroxyphenoxy) phenyl] fluorene (BAHPPF) and 4-(4′-tertbutyl)phenyl-2,6-di(4′-aminophenyl)pyridine (TPAPP) and 4,4′-(hexafluoroisopropene)phthalic anhydride (6FDA), and the molar ratio of BAHPPF and TPAPP is 9∶1, 8∶2, 7∶3, and 6∶4, respectively. Then the corresponding copolymerized thermal rearrangement (TR) membranes were obtained after heat treatment at 450℃. The thermal, mechanical, and gas separation properties of the membrane materials were measured. The results show that the glass transition temperature of the copolymerized PI membranes is around 350℃. As the molar ratio of TPAPP increases, the tensile strength of the copolymerized PI membranes decreases. After thermal rearrangement, the mechanical properties of the copolymerized TR membranes decrease significantly, while the d values increase. When the molar ratio of BAHPPF and TPAPP is 8∶2, the copolymerized TR membrane exhibits the best gas permeability, and the permeation coefficients for H₂, CO₂, O₂, and N₂, is 346.4, 304.5, 72.43, and 13.34 Barrer respectively. Meanwhile, the O₂/N₂ separation performance of the four TR membranes is close to the Robeson upper limit in 2008.

In order to improve the weak dispersion of two-dimensional nanosheets in polymer matrix and improve the electrical performance of composite materials, a two-dimensional micron expanded layer molybdenum disulfide (E-MoS₂) was prepared by hydrothermal method, and then polyimide (PI) based super-large lattice molybdenum disulfide nanosheets (PI/E-MoS₂) composite films were prepared by in-situ polymerization method. The surface/cross section structure of the composite films was observed by scanning electron microscopy (SEM), the molecular valence bond composition of the composite film was analyzed by Fourier infrared spectroscopy (FTIR), and the phase structure of the composite film was analyzed by X-ray diffraction (XRD). Furthermore, the dielectric constant, DC electrical conductivity, and dielectric loss factor of the composite films were tested, and the effect mechanism of extended lattice on the breakdown and polarization behavior of composite films was studied. The results show that when a small amount of E-MoS₂ nanosheets is added into PI film, there is little agglomeration phenomenon on the surface of PI/E-MoS₂ films, and there is little obvious holes and a small number of structural defects on the cross section, which indicates a good compatibility between fillers and matrix. The incorporation of E-MoS₂ nanosheets can enhance the interfacial polarization of the composite films, improve both the dielectric constant and electric field strength, and decrease the dielectric loss.

In order to improve the solubility and meltability of polyimide (PI), a series of addition thermosetting fluorinated poly(imide-siloxane) copolymer resins (ABIS) were prepared from two kinds of synthesized trifluoromethyl-containing aromatic diamine monomers and 3,3′,4,4′-benzophenone tetracarboxylic acid dianhydride (BTDA), besides, aminopropyl-terminated polydimethylsiloxane (APPS) was chosen as flexible segment, and 3-aminophenyl acetylene (APA) was used as reactive end-capping agent. The structure of ABISs were characterized by proton nuclear magnetic resonance spectroscopy (¹H NMR) and Fourier transformation infrared spectroscopy (FTIR). The solubility, rheological behavior of ABIS and the thermal stability of the cured ABIS were further studied. Moreover, ABIS films were prepared by solution filming, and their physical properties were studied. ABIS resin matrix composites (T300CF/ABIS) were prepared by hot pressing using T300 carbon cloth as reinforcement, and their mechanical properties were also tested. The results show that the introduction of siloxane segment and trifluoromethyl group can significantly increase the flowability and solubility of PI resin systems. The introduction of fluorinated diamine decreases the 5% weight loss temperature (T_d5) of the cured ABIS, but its T_d5 is still higher than 425℃. At the same time, the residual yield at 800℃ (Y_r800℃) of ABIS resins reach 26%. The tensile strength and elongation at break of ABIS films are as high as 15.8 MPa and 65.3%, respectively. In the frequency range of 40 Hz-10⁷ Hz, the dielectric constant (ε) and dielectric loss factor (tanδ) of the PI films are 3.21 and 0.01, respectively, which keep unchanged basically. The contact angle of water on the cured ABIS film is 94°. The flexural strength and interlaminar shear strength (ILSS) of T300CF/ABIS composites can be 137.6 MPa and 16.6 MPa, respectively. The synthesized ABIS resin has promising applications in microelectronic devices and flexible heat-protective composites.

A series of fluorinated polythioetherimide films were synthesized by copolymerization using 4,4′-diaminodiphenyl sulfide (SDA) and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (HFBAPP) as diamine monomers, and 4,4′-biphenylene ether dianhydride (ODPA) as dianhydride monomer. The thermal, electrical, optical properties, and water absorption of the PI films were tested. The results show that the PI films have good thermal, dielectric properties and optical transparency. The glass transition temperature of the film is in the range of 240.7-251.6℃, the 5% and 10% heat loss temperature is above 480℃ and 514℃, respectively, and the residual carbon rate reaches 50% at 800℃. Under the frequency of 1 MHz, the dielectric constant of the PI films are 2.27-2.99, the dielectric loss factors are 0.004 7-0.005 6. The highest transmittance of the films reaches 88.75% in the range of visible light, and the water absorption rate is 0.95%-2.26%.

In order to investigate the improving mechanism of polysiloxane with different substituents on atomic oxygen (AO) erosion resistance of polyimide in space environments, reactive molecular dynamics (MD) simulations was used to analyze the AO erosion resistance of PI composited with polyhedral oligomeric silsesquioxane (POSS) with trifluoromethyl (-CF₃) group and methyl (-CH₃) group. The results show that both the composites exhibit strong AO erosion resistance by forming an SiO₂ layer that blocks AO propagation into the polymer matrix and and heat transfer. The PI/CF₃-POSS shows the best performance, and its normalized mass is 0.83 after 35 ps of AO exposure, while the nomalized mass of the PI/CH₃-POSS composite is 0.78.

In order to investigate the arc resistance of coating modified polyimide fabric, a waterborne polyurethane was used as matrix to prepare a barrier thermal insulating coating with SiO₂ aerogel and a reflective thermal insulating coating with TiO₂ filler, and then two coatings were applied on the polyimide fabric, respectively. The effect of filler addition amount and the matrix on the thermal protection performance and rupture performance of polyimide fabric under the effect of fault arc was studied. The results show that the thermal protection performance of the modified polyimide fabric increases with the increase of filler addition, the thermal protection performance of polyimide modified by thermal insulating coating with 6% of SiO₂ and 6% of TiO₂ increases by 34.79% and 21.78%, respectively. Waterborne polyurethane matrix has no effect on the thermal protection performance of polyimide fabrics, but it can make their rupture performances increase by 20%.

Photosensitive polyimide (PSPI) is a kind of high-performance polyimide with special structures, which is widely used in semiconductor packaging, integrated circuits, optical display, and other fields due to its excellent heat resistance, mechanical properties, insulating properties, and lithographic processability. In this paper, the PSPI patents applied in China were taken as object, the overall situation of these patents, including changes in the number of patent applications, applicants, and their source countries was analyzed systematically. Especially, the patent applicants inside and outside China and the patent subject distribution since 2011 were discussed. The difference of patent technology and protection priorities of enterprise applicants inside and outside China was compared in detail. The patent technology characteristics and development trends of PSPI were explored.

Photosensitive polyimide (PSPI) is widely used in electronics, microelectronics, optical display, and other fields. Foreign companies attach great importance to the patent protection of PSPI material formulas and applications. In this paper, the PSPI patents applied by Toray Industries Inc. in China were taken into research, and the changes in the number of patent applications, legal status, technical subject distribution, and other key information were analyzed in detail. Through combining with Toray's representative PSPI products and their applications, the technical development, patent protection strategy, key points of patent technology of Toray were clarified. Based on the above analysis, the technology development trends and application direction of PSPI materials were explored and presented.

In order to study the influence of different interface pressure on the electric field distortion of the composite interface of crosslinked polyethylene/silicon rubber (XLPE/SR) insulation in the cold shrinkable cable intermediate joint, a simulation model of 35 kV cold shrinkable cable intermediate joint was established by the finite element simulation software. The electric-thermal-force field of the composite interface without defects and with metal impurities, circumferential scratches, and longitudinal scratches was simulated and calculated. The results show that the interface pressure has different effects on the joint deformation and electric field strength distortion under different conditions. The joint deformation with longitudinal scratch is more significant, and the field strength distribution at the defect is more obviously affected by the interface pressure. The field strength of the shielding tube is inversely proportional to the interface pressure, and the field strength of the stress cone in the joint with longitudinal scratches is directly proportional to the interface pressure. The overall field strength of the insulation composite interface except for the defects decreases with the increase of the interface pressure.

The insulation status of the cable intermediate joint plays an important role in the safe and stable operation of cable. In view of the immaturity of terahertz imaging technology and the limitation of finite element analysis method, an ultrasonic signal characterization method for typical internal defects of silicone rubber in cable accessories was proposed in this paper. At first, different types of typical defect in silicone rubber were designed by simulation. Then the time-domain characteristic quantity and correlation analysis were carried out on the reflected echoes received in the ultrasonic detection simulation, and the correlation coefficient between the time-domain waveform characteristic quantity and the size and angle of defects was obtained. Finally, an ultrasonic test platform was built on the basis of pulse echo method to carry out ultrasonic test on the flat silicone rubber samples with defects. The results show that the size and angle of the internal defects of silicone rubber have a good correlation with the amplitude peak of the first echo, and the detection results of crack defects are more affected by the angle than that of bubble defects, and the amplitude peaks of defects at different angles are significantly different. It is concluded that the ultrasonic signal can effectively characterize the typical internal defects of silicone rubber in cable accessories.

Metal particle pollutants inside gas-insulated metal enclosed switchgear (GIS) equipment can cause severe electric field distortion, leading to surface discharge of insulators. This paper proposed a method of dielectrically functionally gradient insulator for suppressing metal particles. The electric field distribution around the basin insulator and the motion characteristics of metal particles were simulated. The results show that metal particles released from the shell are affected by the axial electric field force and tend to move towards the insulator. The permittivity distribution of the laminated functionally gradient (ε_L-FGM) insulator decreases in the radial direction, which can homogenize the electric field distribution along the insulator surface and drive away the metal particles by reversing the electric force with the axial electric field. However, the ε_L-FGM insulator enhances the electric field on the surface of the shell, and the jump height of metal particle is 2.7 times higher than those around the uniform insulator. Compared with ε_L-FGM insulator, the ε_T-FGM insulator with a spatial permittivity gradient, which designed based on the topology optimization, reduces the jump height of metal particles and performs better effect in regulating the electric field and expelling the particles.