Preparation And Properties Of Polyimide/Silica Hybrid Films

With the development of aerospace science and technology,the higher demands to the properties of materials are put forward.Organic/inorganic hybrid materials have obtained more and more attentions because of their present characteristics and properties of both moieties.Polyimide（PI）,as a typical kind of engineering polymer material with highest thermal stability hitherto,has been widely used in the aeronautic and electronic industries owing to its outstanding performances such as excellent thermal and electric properties and mechanical properties.Its high thermal stability and glass transmission temperature are beneficial to the distribution of the nano-particles,accordingly avoiding precipitation among them in the synthesis process of the polyimide.Nevertheless,their relatively high humidity absorption（up to 3-4%）and high thermal expansion coefficient（～5*10-5K-1）still impede their broader applications.To obtain PI/nano inorganic particles hybrid materials,it is feasible to adopt the sol-gel approach.In the paper,polyimide/nano-silica（PI/SiO₂） hybrid films were prepared coming from the traditional sol-gel reaction.Firstly,the polyamide acid（PAA）was synthesized from pyromellitic dianhydride（PMDA）and 4,4’-oxydianiline（4,4-ODA）, dimethylacetamide（DMAc）as solvent,and its stability and Theological behavior were studied.The results indicated that polyamide acid/DMAc solution showed a typical shear thinning behavior,which was stable at room temperature,when the solid content was 10% it had good flowing property and film forming ability.Secondly,a new approach to prepare nano-silica/organic solvent dispersion was presented in the paper,which was the key to obtain the water-free PI/nano-silica hybrid films.Thirdly,polyimide/nano-silica（PI/SiO₂）hybrid films were prepared using in-situ polymerization.A series of testing methods,such as Fourier transform infrared（FTIR）spectrometer,thermo gravimetric analysis （TGA）,dynamic mechanical analysis（DMA）,scanning electron microscopy（SEM）,mechanical performance testing etc,were used to investigated the structure and properties of Pl/nano-silica hybrid materials. The results showed that the imidization of the polyamide acid began at 100℃and completed at 300℃as revealed by FTIR,and the introduction of nano-silica in matrix had no influence on the imidization.SEM observation indicated the homogeneous dispersion of silica particles in polyimide,when the content of silica was lower than 3 wt%,the silica dispersed in scale of about 100nm,but the size of silica particles increased with the increase of silica content owing to the occurrence of aggregation.The tensile strength and elongation at break of PI/nano-silica hybrid films increased and then decreased with increasing nano-silica content,showing a maximum at 3 wt% silica content.But,the elastic modulus increased obviously with the increase of silica content.The thermal stability of PI/nano-silica hybrid films were improved owing to the addition of nano-silica,the thermal degradation temperature was 20℃higher at 8 wt% silica content than that of pure PI.In addition,DMA results showed that the glass transition temperature（Tg）of PI/nano-silica hybrid films increased with the increase of silica content.