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.

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.

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%.

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.

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.

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.

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%.

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.