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.