Fabrication On The Polyimide/Al₂O₃ Composite Films Via Ion Exchange And Its Corona Aging Mechanism

Polyimide has been widely used in aerospace, microelectronics and communication fields due to its excellent properties, which is one of the best polymer materials. But pure polyimide can’t meet the applications of special field with the development of science, so it is indeed necessary to modify polyimide to fabricate functional polyimide composite materials. Combining inorganic nanoparticles with polyimide is an important approach to prepare polyimide composite to realize the fictionalizations of the materials. And these polyimide composite materials have great potential applications. Recently, surface modification and ion exchange method have used to prepare polyimide composite film. This method used the character that polyimide film can be hydrolyzed in alkaline solution to form polyimide salt. Then through the ion exchange process introduce the metal ion in metal salt solution. Finally, metal or metal oxide Composite polyimide films were prepared after re-imidization process. Compare with traditional methods, this method partly simplify the preparation process of composite films, and the internal structure of polyimide film was maintain excellent properties of original polyimide film, and the whole preparation process almost no additional pollutants to discharge and achieve the environmental friendly. Hence, there is good prospect to develop this method for preparing functional polyimide composite materials.Based on the polyimide film of pyromellitic dianhydride/diamino diphenyl as substrate, KOH solution was used to hydrolyze the surface of polyimide films at different time from 30 min to 150 min in order to research the effects of hydrolysis time on the structure, surface morphology and mechanical properties of films. The results indicated that the thickness of the film appeared significantly increased with the increase of hydrolysis time. XRD showed that the hydrolysis decreased order degree of films. FTIR spectra showed that polyimide molecules hydrolyze to amido bond and carboxylic acid bond in KOH solution and amido bond and carboxylic acid formed imide bond by ring-closing reaction after re-imidization process. The results of mechanical test showed that the pretreatment of potassium hydroxide reduces the young’s modulus of the film, and the tensile strength and elongation of re-imidization film increased in 60 min KOH treatment, while more than 60 min hydrolysis treatment time makes the mechanical properties of the films decreased.Polyimide/Al₂O₃ films were prepared by the surface modification with different hydrolysis time, ion exchange technique and heat treatment using polyimide films as the substrates and aluminum chloride as the precursor of Al₂O₃. The morphology, thermal properties and electrical properties of the composite films were characterized and tested. XPS analysis showed that the aluminum oxide was formed on the PI films’ surface and there was no any potassium detected, which indicated that the ion exchange process was carried out rather completely. TEM results showed that the alumina distributed in certain thickness on the surface of the films, there was a clear interface layer between the alumina layer and the substrate. The results also indicated that the introduced content of aluminum oxide increase with the hydrolysis time increasing and the thickness of composite films are further increased. The thermal properties of composite films were better than the pristine film due to introducing aluminum oxide. The breakdown strength of the composite films maintains the excellent properties of the pristen film. Corona-resistant time of composite films was longer than pristine film. The composite film which used KOH treat for 90 min has the longest corona-resistant time 101.2 min, which was almost 10 times longer than pristine film.The corona aging morphology of pure and composite films and the characteristics of the corona breakdown point were compared via scanning electron microscope. And distributions of breakdown point of films were studied. The results showed that the breakdown of pristine film was in short time and the surface morphology become more and more serious with the extension of corona aging time. The damage of composite film is much smaller than that of pure film. The breakdown point of pure film appeared regular circle. The edge of pure film breakdown point is very smooth and has trace of thermal damage and the location of the breakdown occurred in the area of the accumulation of heat. The breakdown point of composite film appeared irregular ellipse. The phenomena of fracture tearing occurring in the edge of composite film breakdown point and the location of the breakdown occurred in the area of the accumulation of the corona damage. Through the analysis of the corona breakdown process showed that the introduction of a alumina film surface to improve the ability of resisting corona damage, on the other hand, has good thermal conductivity of alumina can accelerate the heat loss, reducing the probability of occurrence of thermal breakdown of the thin film, and to improve the film corona resistance of life.According to the above results, Coats-Redfern method was used to calculate and analyze the thermal decomposition activation energy of pristine and composite films, which were 265.80 k J/mol and 231.13 k J/mol, respectively. Meanwhile, the degradation kinetics model of polyimide film was established directly under corona discharge. The degradation activation energy of pristine and composite films under the model were 14.72 k J/mol and 48.37 k J/mol, respectively. The reason why the degradation activation energy of films are smaller than thermal decomposition activation energy is that corona discharge not only caused heating effect but also generate highly reactive gas, high-energy radiation and plasma to degrade film. The composite film’s ability of resist corona degradation was promoted because of the introduction of Al2O3 which can protect the polyimide molecules.