Perparation Of High Elastic And Flame-retardant Polyimide Nanofiber/silica Composite Aerogels And The Property Investigation

The rapid development of aerospace technology has put forward higher requirements for the comprehensive properties of materials and the extreme environment of deep space exploration has brought new challenges to materials.Flexible thermal protection materials have been widely studied and studied as aerospace special engineering materials because of their high flexibility and excellent mechanical properties,while maintaining excellent heat insulation and flame retardant properties.Silica（Si O₂）aerogel has relatively high porosity,low thermal conductivity and refractive index,and is one of the most representative lightweight porous materials.Because of its very good heat insulation,it has been widely used in aerospace,construction and many other fields.But this material is brittle,so it is difficult to process molding,not conducive to later development.Compared with inorganic aerogels,polymer-based aerogels are easier to prepare,and can be scientifically and effectively controlled accurately.Besides,they have excellent flexibility and mechanical properties,which can effectively solve the brittleness problem of inorganic aerogels.However,the poor flame retardancy of most polymers materials has hampered their widespread use.Therefore,it is necessary to develop ultra-light aerogel with excellent flexibility and heat resistance.Polyimide（PI）is a polymer material with excellent comprehensive performance,especially in high thermal stability,excellent mechanical properties,high insulation,because of its unique rigid aromatic heterocyclic structure,whether as a structural material or as a functional material,can be applied to a variety of scenarios.As an advanced fiber material,PI nanofibers obtained by electrostatic spinning technology can effectively solve the disadvantages of low mechanical strength and high brittleness of traditional aerogel due to its outstanding flexibility,low density,strong mechanical strength and high aspect ratio.As a kind of drying technology which is very safe,does not produce high energy consumption and is easy to use,vacuum freeze-drying has been favored in the preparation of such materials.Polyimide nanofiber aerogel（PINFAs）prepared by electrospinning and vacuum freeze-drying technology combines the chemical structure advantages of PI nanofibers such as flexibility,high mechanical strength,high and low temperature resistance,chemical corrosion resistance and other physical structure characteristics of gas gel materials such as low density,high porosity and high specific surface area.It can be used as one of the important research directions of flexible thermal protection materials for aviation.Combined with the above background,the work carried out in this paper involves the following aspects:（1）The precursor polyamide acid（PAA）of PI was synthesized by pyromellitic dianhydride（PMDA）and 4,4’-diaminodiphenyl ether（ODA）,and then PAA nanofibers were prepared by electrospinning technology,and then PAA nanofiber aerogel was obtained by vacuum freeze-drying technology.Finally,PINFAs were obtained after high temperature therimide treatment.The prepared PINFAs samples were yellow in color,with complete structure,volume shrinkage rate of 0.97%,density of 10.23 mg·cm^-3,porosity of 99.27%,thermal conductivity of 31.1 m W/（m·K）at room temperature,showing excellent heat insulation performance;（2）On the basis of PINFAs and without the addition of crosslinking agent,by taking advantage of the designability of PI material,the water-soluble PAA glue solution chemical crosslinking and thermal melting PI nanofiber curing crosslinking and the"double crosslinking"method combining the two methods are adopted.The"self-crosslinking"of nanofibers is achieved by the process of high temperature theriminization to strengthen the aerogel skeleton structure.Uncrosslinked PINFAs,water-soluble PAA crosslinking,thermal melting PI nanofiber crosslinking and"double crosslinking"PINFAs samples were prepared,and the effects of several crosslinking methods on the macroscopic morphology,microstructure,compressive properties,recovery rate and other mechanical properties of PINFAs were analyzed and compared.The elastic PINFAs of"double crosslinking"are obtained.Under the conditions of50%and 80%compression strain,the compression stress is 4 times and 2.4times that of uncrosslinked PINFAs samples,and the recovery rates are up to98.3%and 92.6%,showing excellent compressive resilience;（3）Ceramic nanoparticles fumed Si O₂ is further introduced into the elastic PINFAs system.A series of polyimide nanofiber/silica composite aerogels（Si O₂/PINFAs）were prepared by changing the mass fraction of Si O₂,and the effects of Si O₂ particle content on the macroscopic morphology,microstructure,compression properties and thermal properties of Si O₂/PINFAs samples were investigated.Elastic Si O₂/PINFAs were obtained.The thermal decomposition temperature rose from 511℃to 551℃,and the thermal conductivity at room temperature was about 31 m W/（m·K）.The thermal conductivity at high temperature and the stable temperature of long time heating were lower than that of PINFAs samples,and the decrease was more with the increase of Si O₂ content.Compared with the severe carbonization and volume shrinkage of PINFAs samples,Si O₂/PINFAs samples only showed surface carbonization in the part of contact fire source in the combustion test,and still maintained complete structure and good rebound after combustion,showing excellent flame retardant performance.