Preparation And Properties Of Highly Transparent Polyimide Containing Benzimidazole

Optically transparent polymer films are widely used in the new generation of information industry represented by flexible electronics and flexible display due to their excellent optical transparency and mechanical properties.With the rapid development of electronics and display technology towards high speed,high density,high integration and ultra-thin direction,the requirements of polymer optical film materials are becoming higher and higher,such as high heat resistance,high temperature dimensional stability and water and oxygen resistance characteristics.Traditional optical polymer films（such as polyethylene terephthalate,polyethylene naphthalate two formic acid glycol ester,polycarbonate,etc.）have excellent optical transparency,but the limited glass transition temperature limits their application.High-performance polyimide films have excellent comprehensive properties,such as good mechanical properties,excellent heat resistance and chemical stability,and become one of the preferred materials for the industrial application of optically transparent polymer films in the future.Traditional polyimides（PI）produce new absorption bands in the visible wavelength range resulting in dark color of films due to the strong CT interaction between the alternating electron donor（diamine）and the electron acceptor（dianhydride）.This problem can be improved by designing the structure of the polyimide.Therefore,the asymmetric benzimidazole and bulky substituent were integrated into the polyimide through the molecular design to balance the heat resistance,mechanical properties and optical properties of polymer.The relationship between molecular structure,molecular packing and optical transmittance was explored.The influence of molecular structure on optical properties and so on was clarified essentially,which can provide an effective reference for optimizing the performance of optical polymer films.Firstly,benzimidazole-based dinitro and diamine monomer were synthesized by nucleophilic substitution and reduction reaction with aniline as starting material,and phenyl group was successfully introduced into benzimidazole polyimide.Two kinds of polyimide films were prepared by polymerization of the two diamine monomers with 2,2’-bis（3,4-anhydrodicarboxyphenyl）-hexafluoropropane（6FDA）.It had been proved that the introduction of phenyl group into the polymer backbone chain could disrupt the stacking of molecular chains,improved their free volume,hindered intramolecular and intermolecular CT interaction,and thus improved the optical properties of polymer films.Additionally,the N-phenyl-substituted poly（benzimidazole imide）s maintained high thermal and mechanical properties of traditional poly（benzimidazole imide）s,and its 5%thermal decomposition temperature(T_d5%)is greater than 500℃ and its glass transition temperature（T_g）was up to 374℃.At the same time,these materials have excellent tensile modulus（E＞5GPa）,tensile strength（σ＞140 MPa）and better solubility.In addition,it was also beneficial to improve the optical properties of polymer films by introducing biphenyl as bulky substituted group into the polymer backbone chain.In order to further study the effect of different construction substituted groups on the properties of polymers,two N-biphenyl substituted isomeric diamine monomers were prepared by nucleophilic substitution and reduction reaction,and two polyimides were prepared by one-step polymerization with 6FDA at high temperature.The polyimides could dissolve well in common organic solvents,which proved their good solubility.No thermal decomposition occured before 500℃,and the glass transition temperature was above 330℃ which shown good thermal properties.They also had excellent mechanical properties.The optical test results indicated that the introduction of bulky biphenyl group destroyed the regularity of molecular arrangement,reduced the intermolecular interaction force,and at the same time weakened the intermolecular CT effect.The transmittance of polyimide film was up to 80%at the wavelength of 450 nm.Polyimides with meta-biphenyl groups owned better thermal properties,mechanical properties and optical transmittance.Furthermore,the asymmetric benzimidazole unit,meta-biphenyl group,methyl or three fluorinated methyl group were simultaneously introduced to the diamine monomer,which made the molecules have distorted structure in space.The diamines were reacted with aromatic dianhydride 6FDA and 1,2,4,5-cyclohexanetetracarboxylic acid dianhydride（HPMDA）to achieve four polyimides.The properties of polyimides were studied and the reasons were explained from the molecular point of view.It was confirmed that the introduction of diphenyl group and methyl/trifluoromethyl at different positions in space resulted in the distorted structure of the two diamine monomers,and this distorted spatial configuration resulted in the amorphous structure of the polymer.Different degree of distortion had influence on the performance of the polymer.The distorted structure destroyed the molecular arrangement,reduced the intermolecular forces,and weakened the intermolecular and intramolecular CT effect.The improved optical performance of the polyimide films was as high as 79%at 400 nm,through rate the yellow index was under 4.Meanwhile,all polyimide films had considerable mechanical properties（E＞3.3 GPa,σ＞110 MPa）,thermal properties(T_d5%＞450℃,T_g＞310℃)and solubility.Finally,in order to improve the dimensional stability of the transparent polyimide containing benzimidazole unit,the meta-biphenyl,methyl and amide group were integrated into the diamine monomer so that the monomer molecule had the alternate spatial structure of rigid fragment and flexible fragment.A series of polyimides were prepared by copolymerization with cyclobutane-1,2,3,4-tetracarboxylic dianhydride（CBDA）and HPMDA.The thermal stability of polyimides was decreased(T_d5%＞400℃),but the glass transition temperature was still high（T_g＞350℃）.The results also showed excellent mechanical properties（E＞3.5 GPa,σ＞130 MPa）and solubility in common organic solvents of the polyimides.Introduction of amide structure maked the CTE value of the films as low as 29.5 ppm/K.Moreover,the good optical transmittance（up to 68%at the wavelength of 400nm）indicated that the introduction of amide unit could well balance the optical and thermal properties of the films.In this study,a series of transparent polyimides were prepared by introducing asymmetric benzimidazole unit and different classes of substituent units into polymers.Molecular design was used to gradually weaken the intermolecular forces and block the CT effect of polymers.The relationship between the structure and properties of polymers was studied by combining experimental and molecular simulation methods.In addition,the structure of dianhydride used in polymerization was further optimized,and the effects of the types and positions of substituents on the properties of polymer were researched,which provided theoretical support for the preparation of polyimide films with excellent optical properties.