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.

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.

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.

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%.

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.

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.

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.

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.

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.

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.