With the development of electronics and microelectronics technology, thermally conductive polyimide (PI) film is facing new application requirements. The control of thermal conductivity and preparation of PI film have drawn much attention. However, there is a lack of systematic research on the analytical methods of its thermal conductivity. In this paper, the analytical methods of thermal conductivity for PI film at home and abroad were summarized. The basic principles, main features, and application scope of the transient methods, steady-state methods, and temperature wave analysis were introduced in detail. In addition, the comparison results of thermal conductivity along the out-of-plane direction and in-plane direction and thermal diffusivity of PI films tested by different methods were reviewed. The existing problems and future development trends of the thermal conductivity analytical methods for PI films were summarized and prospected.

In recent years, flexible electronic devices have developed rapidly. As a flexible substrate material and dielectric insulating material, polyimide (PI) film has been widely used in the preparation of flexible electronic devices and flexible circuit boards. However, its high thermal expansion coefficient reduces its dimensional stability in the process of variable temperature processing. Therefore, it is necessary to adjust its thermal expansion coefficient to match with other materials of electronic devices. In this paper, the patent status of low expansion polyimide films at home and abroad, the preparation and application research progress of low expansion polyimide composite films were introduced, and the general trend of synthesis, modification and application research of low expansion PI was forecasted.

Dielectric polymer materials with high breakdown, low loss, high flexibility, and low cost play an important role in film capacitor industry. However, low energy storage density and poor thermal stability limit its applications in high-temperature working environments. In this paper, the polyimide-based dielectric energy storage materials and the research methods to improve energy storage property were introduced emphatically, including the influences of the structure and morphology of inorganic fillers with high dielectric constant and high insulation properties or multifunctional composite fillers on the performance of composite films, and the studies on the property of interface micro-area. The future research direction of interface design for high-temperature medium energy storage composite materials was discussed.

As a special engineering plastic, polyimide is widely used in electrical insulation, electronics, and other fields because of its excellent dielectric properties, mechanical properties, and thermal stability. The polyimide films with excellent thermal stability and dielectric properties can be obtained by adjusting the molecular chain structure of polyimide through molecular structure design and monomer optimization. In this paper, the molecular structure design strategies for adjusting the dielectric properties of polyimide and the influencing mechanism of polyimide structure on its dielectric properties were reviewed, and the research direction of dielectric properties adjusting was prospected.

Since the American DuPont company improved the polyimide (PI) synthesis technology to produce membrane material and applied it to industry for the first time in 1960, PI had shined in the information age, and its related research had been developed by leaps and bounds. In the fields of gas separation industry and new energy, cross-linking modification is an effective way to improve the performance of PI membranes. In this paper, the latest research progress in the PI cross-linking modification for membranes in recent years was reviewed, which included thermal cross-linking, ultraviolet cross-linking, and chemical cross-linking, and the future research direction of cross-linked PI membranes was prospected.

According to the application requirements of low thermal expansion (low-CTE) for colorless and transparent polyimide (CPI) films, the structural design and synthesis of methyl-substituted aromatic diamine monomers containing rigid amide bonds in the main chain, including 2-methyl-4,4′-diaminobenzanilide (MeDABA) and 2,3′-dimethyl-4,4′-diaminobenzanilide (MMDABA), were performed. Firstly, the dinitro compounds were prepared by the amide reactions using the methyl-substituted p-nitrobenzoic acid or methyl-substituted p-nitroaniline as raw material. Then, a series of new diamine compounds were obtained by the reduction of hydrazine hydrate under the catalysis of Pd/C. The chemical structures of the diamine monomers were characterized by differential scanning calorimetric analysis (DSC), Fourier infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), mass spectrometry (MS), and elemental analysis (EA). The results show that the aromatic diamine monomers with expected structure are prepared successfully.

Two novel diamines contained N-phenyl substituted benzimidazole were synthesized, and their difference was that the ortho-position of N-phenyl was substituted by methyl and fluorine atom, respectively. The novel diamines and commercially available dianhydrides 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 4,4′-oxydiphthalic anhydride (ODPA) were used to prepare poly(benzimidazole-imide) (PBII) films by two-step thermal imidization. The properties of new diamine monomer and PBII films were tested and characterized. The results show that the PBII films exhibit good thermal properties (glass transition temperature T_g=341-381℃) and mechanical properties (σ=95-135 MPa). The N-phenyl groups with different ortho units destroy the effective molecular packing, which improves the solubility and optical transmittance of the PBII films.

A novel dianhydride monomer containing alicyclic ring and amide group was synthesized and further polymerized with several diamine monomers to prepare a series of transparent polyimide films. The properties of the films were tested and characterized. The results show that the synthesized polyimide films have excellent optical properties (T₅₅₀>89%), low coefficient of thermal expansion (CTE<17×10^-6 K^-1), and higher glass transition temperature (T_g>320℃) because of introducing trans cyclohexane and amide into the structure of dianhydride at the same time. The introduction of alicyclic structure decreases the formation of charge transfer complex, which increases the transparency of the polyimide films. On the other hand, the introduction of amide structure decreases the coefficient of thermal expansion.

A polyamide acid was prepared using 4,4′-diamino-2,2′-bistrifluoromethyl benzene (TFMB) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) as raw materials. The azocyclic quinoline (QL) was used to promote its imidization at lower temperature, and the amount of QL, maximum imidization temperature, and curing time were optimized. The imidization degree of polyimide (PI) film was determined by infrared spectroscopy. The results show that when the molar addition of QL is twice that of BPDA, the polyamide acid is only cured for 4 h at 200℃, the imidization degree is more than 99%. After treated at 250℃ for 0.5 h to remove the residual solvent and QL, the thermal stability of the PI is improved greatly, while its light transmittance is basically unchanged. Compared with the PI film imidized at 300℃, the 5% weight loss temperature (T_5%), glass transition temperature (T_g), and tensile strength of the PI film whose imidization promoted by QL decrease slightly, but the elongation at break increases, and the light transmittance at 400 nm increases from 4.5% to 34.4%.

According to the application requirements of thermoplastic black polyimide film in the field of advanced flexible copper clad laminate (FCCL), three organo-soluble polyimide (SPI) resins were prepared from an aromatic diamine monomer containing chromogenic imine (-NH-) group, 4,4′-diaminodiphenylamine (NDA) and various dianhydrides, including 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA), and hydrogenated 3,3′,4,4′- biphenyltetracarboxylic dianhydride (HBPDA) by polymerization. Then PI films were prepared from SPI/DMAc solution at relatively low temperature (80-250℃), and the effects of the above characteristic groups on the optical properties, thermal properties, and electrical properties of PI films were studied systematically. The results show that the SPI resin has good solubility in polar aprotic solvents such as N-methylpyrrolidone (NMP) and N,N-dimethylacetamide (DMAc). The SPI films show intrinsically deep color, the transmittance value at 550 nm of wavelength (T₅₀₀) is lower than 5%, and the lightness (L^*) is below 60. The PI films have good thermal stabilities, the glass transition temperatures (T_g) is up to 375.9℃ and the 5% weight loss temperatures (T_5%) is over 500℃ in nitrogen. Besides, the PI films exhibit good electrical insulating properties, the volume resistivities (ρ_v) is higher than 10¹⁵ Ω·cm.

A series of black polyimide (PI) films were prepared by using 4,4′-diaminodiphenyl ether (ODA) as diamine, pyromellitic dianhydride (PMDA) as dianhydride, and carbon black treated with different types of dispersants as shading filler, respectively, and the effects of dispersants on the surface morphology and properties of the carbon black/PI films were investigated. The results show that the dispersibility of carbon black treated with dispersants in PI matrix and the organic-inorganic interface compatibility are improved, the agglomeration degree of carbon black decreases, and the properties of the films increase. When the carbon black is treated by YK-3 (modified block polymer), the surface morphology and properties of the film are the best, its electric strength, tensile strength, and elongation at break are 145 kV/mm, 146 MPa, and 38%, respectively, and the light transmittance is close to 0.

According to the application requirements of high-temperature resistant polymers with low dielectric constant (low-D_k) and low dielectric loss factor (low-D_f) for the development of high frequency communication technology, two key diamine monomers for fluoro-containing poly(imide-benzoxazole)s (PIBO), including 2,2-bis[3-(4-aminobenzamide)-4-hydroxylphenyl] hexafluoropropane (p6FAHP) and 2,2-bis[3-(3-aminobenzamide)-4-hydroxyphenyl] hexafluoropropane (m6FAHP) were synthesized. The dinitro compounds containing bis (o-hydroxy substituted benzamide) groups in molecular structure were first prepared by the low temperature reactions of the nitro-substituted benzoyl chloride and 2,2-bis(3-amino-4- hydroxyphenyl)hexafluoro-propane (6FAP) in polar aprotic solvent. Then the diamine monomers were obtained by the reduction of hydrogen under the catalysis of Pd/C. The melting points of the diamines were measured by DSC. The chemical structures of the diamines were characterized by ATR-FTIR, NMR, and elemental analysis (EA). The results show that the aromatic diamine monomers with expected structures are prepared successfully.

Taking 1,3,5-tri(4-aminophenoxy) benzene (TAPOB) as crosslink agent, a micro-branched crosslinked structure was constructed in 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 4,4′-diaminodiohenyl ether (ODA) polyimide (PI) film, and a series of PI films with different TAPOB content were prepared. The effects of TAPOB content on the mechanical properties, thermomechanical properties, dielectric properties, and water absorption were studied. The results show that the addition of TAPOB can improve the comprehensive properties of BPDA/ODA PI films significantly. The existence of crosslinked structure is conductive to improve the mechanical properties, decrease the coefficient of thermal expansion (CTE) and water absorption of the film, and the micro-branched structure has a certain effect on reducing the dielectric constant.

At First, a poly(amic acid) (PAA) based on pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) was prepared by polycondensation procedure, and the PAA/SiO₂ composite solutions were obtained by adding different mass fraction of colloidal SiO₂/N,N-dimethylacetamide (DMAc) during the polymerization. Then a series of PI/SiO₂ composite films were prepared by thermal imidization of the PAA/SiO₂ composite solutions in a clean oven from room temperature to 350℃ in nitrogen. The PI composite films were characterized by the attenuated total reflectance Fourier transform infrared (ATR-FTIR), scanning electron microscopy (SEM), thermal-gravimetric analysis (TGA), dynamic mechanical analysis (DMA), and thermo-mechanical analysis (TMA), and the effect of SiO₂ on the dielectric properties of composite film was tested by the impedance analysis measurement. The results show that the composite films with expected structure are prepared, and the SiO₂ disperses in PI matrix uniformly. The incorporation of SiO₂ improves the heat resistance and dimensional stability at high temperature of the PI composite films. The 5% weight loss temperature (T_5%) and 750℃ residual weight ratio (R_w750) of the PI-25 film with 25% of SiO₂ are 611.3℃ and 73.1%, respectively, which are 14.7℃ and 9.2% higher than those of the PI-0 film (without silica). The PI composite films exhibit stable dielectric constant (D_k) and dielectric loss factor (D_f) in the frequency range of 10³-10⁶ Hz. The incorporation of nano-silica slightly increases the D_k of composite films, and the D_k of the PI-25 film at 1 MHz is 3.58, which is a bit higher than that of the PI-0 film (D_k is 3.20).

In order to prepare polyimide (PI) films with high order degree and stable thermomechanical properties, pyromellitic dianhydride (PMDA) monomer was introduced into biphenyltetracarboxylic acid dianhydride-p-phenylenediamine (BPDA-PDA) system, and PI films were prepared by random copolymerization method. The aggregation structure and physical properties of the PI films were analyzed by XRD, TMA, DMA, TGA, prism coupling instrument, and universal testing machine. The results show that different dianhydride composition has a significant effect on the aggregation structure and thermal mechanical properties of PI films. With the increase of PMDA-PDA segment composition, the molecular chain order degree increases, and the molecular chain spacing decreases from 0.469 nm to 0.436 nm. The birefringence value shows an upward trend in the range of 0.18‒0.22. The glass transition temperature (T_g) decreases at first and then increases in the range of 379.86‒439.05℃. The thermal expansion coefficient (CTE) increases at first and then decreases in the range of 0‒7×10^-6 K^-1. The tensile strength and 5% thermal decomposition temperature (T_5%) show a downward trend in the range of 166‒225 MPa and 576.8‒590.4℃, respectively. When the mole fraction of PMDA is 60%, the PI film has excellent comprehensive properties with 0.22 of birefringence, 439.05℃ of T_g, 0.012 5×10^-6 K^-1 of CTE, 302.5℃ of heat resistance index(T_HRI) , and 576.8℃ of T_5%.

By adjusting the structure and composition of different diamine and dianhydride, we realized the controllable preparation of block copolymerized polyimide films. The effects of different composition structure of polyimide on its mechanical properties, thermal properties, and linear expansion coefficient were studied. The results show that according to the difference of rigid components (such as PDA, PMDA) content, the elastic modulus, elongation at break, and tensile strength of the films change regularly. The thermogravimetric analysis and the study of thermal decomposition kinetics show that due to the high phenyl and structural symmetry of the molecular chain of the prepared polymer, PI has excellent thermal stability, and the carbon residue rate is as high as 50% at 900℃. Moreover, its dimensional stability is very good, the linear expansion coefficient is very close to that of Cu, which shows potential application in copper clad plates and flexible devices.

In order to improve the thermal conductivity of polyimide film and maintain its insulating properties, SiO₂ was used to coat AgNWs to obtain AgNWs@SiO₂ core-shell structure. At First, the AgNWs@SiO₂ was dispersed in polyamide acid (PAA) discharge fiber by electrospinning technology, and the heat conduction path and the dispersity of AgNWs@SiO₂ in PI matrix was planned and improved by electrospinning technology, respectively. Then the PAA glue containing AgNWs@SiO₂ was used to impregnate PAA electrospun film, and an E-AgNWs@SiO₂/PI composite film was obtained after thermal imidization. The effects of filler modification and content on the thermal conductivity and insulating properties of the composite films were studied. The results show that when the mass fraction of filler is 25%, the thermal conductivity of E-AgNWs/PI and E-AgNWs@SiO₂/PI composite films is 2.92 W/(m·K) and 2.80 W/(m·K), which are 14.6 and 14 times higher than those of pure PI films, respectively. The dielectric constant of E-AgNWs@SiO₂/PI composite film decreases to below 5, the dielectric loss factor maintains below 0.015, and the volume resistivity increases to 1.79×10¹³ Ω·m.

A graphene oxide (GO) was synthesized by modified Hummers’ method, and a graphene oxide/polyimide (GO/PI) composite film was prepared by solution blending method. The structure and the properties of the composite film were analyzed by XRD, TMA, TGA. The results show that compared with the PI film without GO, when the mass fraction of GO in composite film is 0.2%, the mechanical properties of the composite film increase obviously, the tensile strength increases by 26.77%, the elongation at break increases by 76.47%, and the elastic modulus changes little basically. When the mass fraction of GO is 0.1%, the maximum value of T₅ and T₁₀ of the composite film are 587.3℃ and 603.3℃, which increase by 2.44% and 1.69% compared with that of the PI film without GO, respectively. It is indicated that the addition of GO with appropriate amount can enhance the mechanical properties and thermal properties of PI films.

LDHNSs dispersion was obtained by ultrasonic stripping of hydrotalcite assisted with intercalation agent. PI/LDHNSS composite films were prepared by in-situ polymerization, and their electrical properties were studied. The results show that the LDHNSs disperse uniformly in PI matrix without obvious agglomeration and stacking, which show good interfacial compatibility with the matrix. Compared with pure PI film, the volume resistivity and electric strength of the composite films decrease slightly to some extent with the addition of LDHNSs. However, it should be note that the corona resistance life of the composite film is improved significantly. When the mass fraction of LDHNSs is 0.5%, the corona resistance life of the composite film is the longest, which is about 8 times longer than that of pure PI, realizing the maximum improvement of corona resistance performance for composite at a relatively low addition amount.

Under different environmental relative humidity (RH), the corona resistance of polyimide film presents significant differences. In order to understand the effects of RH on the surface corona characteristics and ageing process of film, we monitored the corona discharge process, electrical properties, and surface morphology of the film under different RH, and obtained the effects of RH on the corona ageing of film. The results show that with the increase of RH, the changing trends of the discharge activity at positive and negative half cycle are opposite. When the RH is equal or less than 69%, both the discharge quantity and discharge frequency show rapid growth stage, stationary stage, and secondary development stage with corona time, and when the RH attains 84%, both of them increase linearly with time. The changing rate of moisture content (Δwt%) of the film increases significantly at the beginning of corona ageing and tends to be stable at the middle and later periods, and when the RH attains 84%, the Δwt% increases linearly with the ageing time. It is analyzed that the surface electrothermal damage of film caused by corona discharge is the main cause of the rapid decrease of surface resistivity (ρ_s). With the corona erosion, water penetrate into the film gradually, eventually leading to the sharp decrease of ρ_v till breakdown.