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.

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.

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.

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.

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.

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.

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.

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.

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.

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).