Preparation, Characterization And Properties Of Polyimide/SiO₂-Al₂O₃ Nanohybrid Films

At present, fillers such as TiO₂, SiO₂ and montmorillonite have been hybridized with polyimide matrix. Moreover some of the favorable results have been obtained. But previous studies about polyimide/inorganic hybrids were concentrated on the binary systems, in which exists a sort of inorganic portion. Due to the key challenge for complicated techniques and preraration ,there were few reports about ternary systems.In this paper it investigates the influencing factors ,such as solid content, reaction temperature and time, time interval of adding material , which affect the molecular weight and its distribution of poly (amic acid) in virtue of orthogonal design method. Molecular weight of poly (amic acid) on different raction conditions is tested by gel permeation chromatography. The results indicates the condition for synthesizing the maximal molecular weight poly (amic acid) is that solid content is 8wt%, reaction temperature is 15℃; time interval is 9 minutes ; reaction time is 3 hours.Polyimide/silica(PI/SiO₂),polyimide/alumina(PI/Al₂O₃)and polyimide/silica- alumina (PI/SiO₂-Al₂O₃) hybrid films are prepared by ultrasonic-mechanical method. Struture and morphology of hybride films are characterized by Fourier transform infrared apparatus and atomic force microscope. Thermal stability, transmittance of UV-Vis, mechanical properties and corona-resistance of polyimide hybrid films are tested. Fourier transform infrared spectra analysis show imidization of hybrid films is incomplete. It is effective for the dispersion of nano-SiO₂ particles to use ultrasonic. Nanoparticles distribute uniformly in the polyimide matrix and the average diameter of the particles is about 70 nm. Compare with pure polyimide films ,corona-resistance time of the hybrid films is longer, moreover with the increasing of the content of nanoparticles it reveals aggrandizement, but the increased range is unconspicuous.The longest corona-resistance time of hybrid films reaches four times than pure polyimide films.Compare with pure polyimide films, thermal stability of hybrid ones show degressive trend, tensile strength and elongation of them are slightly enhanced, transmittance of UV-Vis spectra are decline.PI/SiO₂, PI/Al₂O₃ and PI/SiO₂-Al₂O₃ hybrid films are prepared by sol-gel method. It discusses that morphology characterization and properties ,including struture and morphology , uniformity and dispersity of inorganic phase as well as thermal stability , breakdown strength ,corona-resistance and dielectric properties, transmittance of UV-Vis spectra ,mechanical properties of polyimide hybrid films ,are affected by some factors .And influencing factors are composed of the contents and weight ratio of inorganic components ,hybridized technology as well as imidization process.Imidization process is analysed by Fourier transform infrared spectra, UV-Vis spectrophotometer, atomic force microscope, differential scanning calorimeter and so on .It reveales that with the increasing of imidization temperature inorganic phase size and transmittance was increases at first, then decreases. Nevertheless it appeares peak value at about 140℃. Mixed sol makes reaction heat of poly (amic acid) reduce. At the same time imidization process is accelerated.In sol-gel method according to different modes charged sol ,a series of experiments are done. Firstly,after preparation of poly (amic acid) different sols are added in poly (amic acid) solution.As has been seen from Fourier transform infrared spectra,it indicates complete imidization of hybrid flims. Si-O-Si and Al-O-Al bonds exsiste in the films hybridized with silica and alumina sols.There are linkages between polyimide matrix and nanoparticles. Also this suggestes the existence of Si-O-Al bonds.In response to the results of scanning electron microscope there are spherical shaped and club shaped particles in the films hybridized with silica and alumina sols.Analysis of spot element reveales there are silicon and aluminium element in this point, which proves the form of Si-O-Al bond ulteriorly.Secondly, during preparation of poly (amic acid) different sols are added in poly (amic acid) solution,and the method using ultrasonic-mechanical method compared with nonuse of it. On the base of atomic force microscope and scanning electron microscope, it reveales the average diameter of the particles are nanometer in the hybrid films which sols are added in during the reaction. Also size of nanoparticles is much smaller. However making use of ultrasonic-mechanical method is beneficial to uniform distribution of inorganic nanoparticles.As has been seen from X-ray diffracition spectra results, it shows crystallinity exsisted in pure polyimide film. Inorganic phase is amorphous phase. Furthermore crystallinity of pure polyimide film is damaged by inorganic components. Coupling agents or the variety of imidization techonolgy may change crystallinity of polyimide film. Coupling agents DB-550 changes the structure of hybrid films.Results of diversified test indicate inorganic contents and weight proportion of inorganic components influences UV-Vis transmittance , thermal stability , breakdown strength, corona-resistance and dielectric properties , mechanical properties. All kinds of properties of hybrid films using ultrasonic-mechanical method during preparation of poly (amic acid) mixe solution are better than in other methods.The properties of ternary systems precedes that of binary systems. In comparision with the binary systems, some properties will be probably regulated in the ternary systems.When the inorganic content of the hybrid films added coupling agents DB-550 is 4wt% for SiO₂/Al₂O₃ (4/1), its integrated property is the most outstanding. In comparison with pure polyimide films, decomposition temperature of the hybrid film increases about 18℃; Breakdown strength of it increases percent 20; Corona-resistance time is 16 times than pure polyimide film;Tensile strength increases percent 50.Relation of dielectric constant and dielectric loss angle with frequency in the hybrid films accordes with Debye relaxation polarization mechanism. In comparison with plain polyimide films, the change of dielectric constant and dielectric loss angle of hybrid films is anomalistic. Effect factors of dielectric constant and dielectric loss angle of hybrid films will be discussed ulteriorly.