Molucular Design,synthesis And Anti-atomic-oxygen Functionalization Of Polyimide Aerogels

With the development of space technology,the demand of lightweight insulation materials for space structures is becoming more and more urgent.The thermal protection effects of the traditional thermal insulation materials/structures(such as insulation tile/felt and thermal protective bottom structure,etc.)is very good,but the density of the structures is very high and shaping ability is very poor,so it can not meet the requirements of the lightweight space structures.Currently,organic aerogels have been rapidly developed with the advantages of low density,low thermal conductivity,high thermal stability,and so on.Among various polymer aerogels,polyimide(PI)aerogels have attracted considerable attention both from the academic and industrial community due to their excellent combined properties,including high thermal and environmental stability,low density,good dielectric features(low dielectric constant,high dielectric strength,etc.)and structural versatilities.The excellent combined properties make PI aerogels good candidates for space applications.Although the research of PI aerogels has been developed rapidly,it is not yet fully resolved regarding the microstructure,the space environment protection,etc.The first problem is how to reveal the formation of PI aerogels microstructure,and how to adjust the high performance of the crosslinked aerogel materials by controlling the microstructure of cross-linked PI.Secondly,when the traditional PI materials used in the low earth orbit(LEO),the performance of PI will be decreased by strong erosion of the atomic oxygen(AO),so it is necessary to study anti-atomic-oxygen properies for PI aerogels.In order to improve the atomic oxygen resistance of the PI aerogels,a series of work has been conducted in the present work,including the new molecular design of PI aerogels,the optimization of processing conditions for PI aerogels,and the structural modification for the PI aerogels.The main research issues in the present thesis are summarized as below:1.Synthesis and characterization of flexible crosslinked PI aerogelsIt was found that the molecular structure of dianhydride monomer would affect the gelation rate of PI wet gels,while that of diamine would affect the shrinkage of PI wet aerogels.At the same time,the design molecular weight(Mn)of PI gels had great effects on the gelation rate.The Mn value of 15000 g/mol was found to be the most appropriate value for the PI wet gels preparation.The preparation processes of PI aerogels were firstly investigated in details.A series of cross-linked PI aerogels(crosslinked PI aerogels,CPIAs)were prepared with 3,3',4,4'-biphenyltetracarboxylic dianhydride(s BPDA)and2,2'-dimethylbenzidine(DMBZ)as monomers,and house-made octa(aminophenyl)silsesquioxane(OAPS)as the crosslinker via sol-gel procedure followed by supercritical carbon dioxide(Sc CO₂)drying procedure.The prepared CPIAs exhibited excellent thermal stability with a glass transition temperature(Tg)of 330?.The densities of PI aerogels could be controlled by tuning the solid contents for the preparation of PI wet gels.As solid contents for the PI wet gels are increased,the densities and surface areas of final PI aerogels increased,and the pore diameters decreased.PI aerogels developed from the solid content of 5 wt.%exhibited the best comprehensive properties.2.Preparation and characterization of polyimide/nano-sized silica aerogelsA series of polyimide/nano-sized silica nanocomposite aerogels were prepared by using CPIA developed in Chapter 2 as the matrix and silica nanoparticles as the filler.The effects of adding amounts of silica on the physical and chemical properties of the nanocomposite aerogels were investiagted.Scanning electron microscope measurements indicated that the silica particles could be homogeneously dispersed into the CPIA aerogel when their amount were lower than 10 wt%of the whole aerogels.The addition of SiO₂nanoparticles has an important influence on the microstructure and properties of the aerogels.The CPIA aerogel matrix exhibits nano-pore structure which is formed by the fiber filament,the SiO₂ nanoparticles are adhered to the surface of the fiber filament,and the air can be filled in the nano-pore structure.The CPIA/SiO₂ aerogels have mesoporous structure,and the pore size distribution is relatively wide,and the pore size distribution is from a few nanometers to several tens of nanometers.In addition,the introduction of SiO₂nanoparticles enhanced the heat resistance of the aerogel.After the atomic oxygen radiation with the dose of 5.95×10²⁰ atoms/cm²,the nanocomposite aerogels with 7 wt%of silica nanoparticles(CPIA-SiO₂-7)exhibited an atomic oxygen erosion rate of 21%that of CPIA-SiO₂-0 sample without silica nanoparticles,and that is about 5.9%of the Kapton.CPIA/SiO₂ aerogels have protective effect to the atomic oxygen.When the atomic oxygen irradiation dose increased,surface of PI resin is oxidized,the SiO₂ nanoparticles will be exposed on the surface and the continuous atomic oxygen of irradiation will cause aerogel all exposed in the outer surface of the resin oxidized,decomposited,and then a passivation layer,which mainly composed of SiO₂ nanoparticles formed on the surface.The passivation layer can prevent the further penetration of atomic oxygen,so as to provide a protective barrier to the aerogels.3.Synthesis and property of intrinsically atomic oxygen resistant polyimide aerogelsFirst,a diamine monomer with polyhedral oligomeric silsesquioxane(POSS)substituent in the side chain,N-[(isobutyl-POSS)propyl]-3,5-diaminobenzamide(DA-POSS)was synthesized.A series of polyimide aerogels(SPIA-0?SPIA-30)with different POSS(Si₈O₁₂)loadings were prepared with s BPDA,DMBZ and DA-POSS as monomers,and OAPS as the crosslinker via sol-gel procedure followed by supercritical carbon dioxide(Sc CO₂)drying procedure.The effects of POSS loadings on the physical and chemical properties of the polyimide aerogels were studied.It was found that with the increase of the POSS loadings,the initial thermal decomposition temperatures of the aerogels slightly decreased;the average pore diameters in the aerogels gradually increased;and the compression strength and modulus gradually decreased.In order to verify the thermal protection effect of the intrinsically atomic oxygen resistant polyimide aerogels,the high temperature performance of PI gel was studied by using SPIA-30 as the research object.Preliminary results show that the polyimide aerogels can withstand high temperature shock at 300?.Atomic oxygen measurements indicated that with the increase of the POSS loadings,the atomic oxygen resistant ability of the aerogels apparently increased.After the atomic oxygen radiation with the dose of 5.71×10²⁰ atoms/cm²,the SPIA-30 aerogel(DA-POSS:DMBZ=30:70,molar ratio)exhibited an atomic oxygen erosion rate of 11%that of SPIA-0(DA-POSS:DMBZ=0:100,molar ratio),about 8.9%of the Kapton.The XPS test shows that C element content of the SPIA aerogels will significantly reduce after atomic oxygen irradiation,while the content of O and Si elements increase significantly.At the same time,the bonding energy of O and Si elements can also increase,this suggested that a passivated layer containing silicon oxide SiO_x deposited on the surface of the aerogel after atomic oxygen irradiation.When the atomic oxygen irradiation dose continues to increase,SiO_x will continue to be oxidized to SiO₂,which can prevent further permeation of atomic oxygen.The protective layer has the function of“self-repairing”,which will greatly improve the adaptability of the space atomic oxygen environment of the material and extend the service life of the materials.