Polyimide Fibrous Aerogels Prepared By Freezing-Extraction/Vacuum-Drying And The Investigation On Their Properties

Due to the low density,high porosity,excellent mechanical properties,thermal insulation,fire retardance and thermal stability,polyimide aerogel（PI aerogel）,has a wide prospect of practical application in the field of fire-proof and heat-proof material for light-weight building construction,aerospace and aviation industries as a sort of emerging polymer aerogel.At present,freeze-drying/thermal imidization and chemical imidization/supercritical-drying are common methods preparing polyimide aerogels.However,these two methods require freeze dryer and supercritical instrument which are special instruments.Meanwhile,time-consuming of freeze-drying and high cost,complex operation,severe volume shrinkage of supercritical-drying are shortcomings,which limits their further practical applications.To overcome the drawbacks of the two methods referred above,PI short fibers are used as supporting skeleton,combining freezing-extraction/vacuum-drying-thermal imidization method,which is more facile,cost-,time-saving to prepare a series of PI aerogels of different densities without special drying instruments in this dissertation.Moreover,the as-obtained PI aerogels are mod-ified with polysilazane,and the microstructure,mechanical properties,thermal insulat-ing and fire retardant properties of PI-based aerogels are investigated.Specifically,the research of this paper mainly contains two parts as follows:（1）The PI fibrous aerogels possess 3D network are prepared according to the fol-lowing steps:liquid nitrogen freeze-shaping,solvent exchange curing,and vacuum drying,named as freezing-extraction/vacuum-drying method,followed by thermal im-idization using PI short fibers as supporting skeleton and PAA as glue.The as-obtained aerogels have low density(39.1 mg cm^-3-52.8 mg cm^-3),high porosity（96.2%-97.2%）,high mechanical strength（49-189 k Pa）,excellent thermal insulation(40.4-44.0 m W m^-1 K^-1),which is comparable to that of commercial polyurethane sponge and polyi-mide aerogel prepared via freeze-drying.（2）Polysilazane is a fire-proof coating which could improve the thermal stability of carbon-and polymer-based materials.To further improve the thermal stability and fire retardance and to avoid the probable agglomeration induced by the addition of in-organic particle,the polysilazane modified PIFA（Pszm PIFA）are obtained by immers-ing PIFAs into polysilazane tetrahydrofuran solution named as a facile dip-coating method,followed by ambient-drying and thermal treatment,and a series of Pszm PIFAs of different modified times are obtained.Compared with PIFA,Pszm PIFA possess more outstanding mechanical properties,thermal insulation and thermal stability.