Microstructural Regulation And Multifunctional Realization Of Flexible Polyimide Aerogels

Polyimide(PI)aerogels are one of the best organic polymer aerogels,which possess superior properties such as high flexibility and highly efficient thermal insulation.PI aerogels have been regarded as one of the key strategic materials,showing wide application in aerospace,energy,shipbuilding,and microelectronics industries.At present,the domestic research on PI aerogels is still in the initial stage,and many technical bottlenecks have not yet been solved,while the existing PI aerogel has the disadvantages of large shrinkage,low flexibility,and low temperature resistance.In this paper,we have proposed a variety of methods such as freeze-drying self-assembly method,intrinsic monomer regulation,nanofiber toughening,organic-inorganic hybridization,surface corrosion combined with phase separation,and surface modification.In addition,we have developed the designation and regulation of the microstructure of PI aerogels.Besides,we have made breakthrough in the preparation technology of temperature-resistant and heat-insulating PI aerogel membranes with high strength,high toughness,and high flexibility.Moreover,the resulted PI aerogels can expand the functionality of PI aerogels and explore the applications of PI aerogels in the fields of oil-water separation,solar energy collection,and photo-thermal conversion.The main contents and results are as follows:(1)"Fiber self-bonding" design is adopted to construct a continuous isotropic fiber network structure by utilizing the bonding behavior during thermal imination of polyamide acid(PAA)nanofibers.Based on the continuous fiber network,hyperelastic polyimide nanofiber aerogel with high hydrophobicity can be prepared,and its microstructure,mechanical properties,hydrophobicity mechanism and oil adsorption ability are studied.Besides,carbon nanofiber aerogel can be obtained by carbonization of polyimide nanofiber aerogel,and its microstructure,mechanical properties,electrical properties and applications in piezoresistive pressure sensors are studied.(2)We have innovatively proposed the method of surface corrosion and phase separation to prepare the PI aerogel membranes with controllable surface pores.Besides,we have systematically studied the influence of phase-separation coagulation bath(water,ethanol,and ethanol aqueous solution)on the microstructure of PI aerogel membranes.The results show that the microstructure of the PI aerogel membranes can be precisely controlled by controlling the surface corrosion time and the ratio of the phase-separated coagulation bath.Surface corrosion method could realize the connection of the upper and lower surfaces of the hierarchical pores,which is the premise of realizing the multifunctional application of the PI aerogel membrane.The PI aerogel membranes crosslinked by MWCNTs are obtained by the cross-linking between a small amount of MWCNTs and PAA precursor.The super-hydrophobicity is achieved by surface modification.The results show that the MWCNTs-cross-linked PI aerogel membrane exhibits hierarchical pore structure.By adjusting the surface corrosion time,the separation of oil-water mixture,light oil emulsion,and heavy oil mixture can be respectively realized.Besides,by adding MWCNTs and surface corrosion,the light absorbance of the MWCNTs cross-linked PI aerogel membrane can be adjusted,which possesses both high-efficiency light absorption ability and fast water transport ability,making it suitable for solar water evaporation.The as-prepared MWCNTs cross-linked PI aerogel membrane is a potential candidate for solar thermal conversion.(3)Based on the sol-gel method and regulation of intrinsic monomers,the high flexible and high transparent PI aerogel membranes are prepared by introducing an asymmetric diamine monomer(OTD).The results show that the transparency and flexibility of PI aerogels are greatly improved with the increase of OTD monomer content.When the OTD content is greater than 25%,PI aerogel can be prepared as PI aerogel membrane.The flexible,light-transmitting,and heat-insulating PI aerogel membrane can be used as a parabolic trough solar collector.The solar collection temperature can reach231.5 °C,which is much higher than that of ordinary flat-plate solar collectors.(4)PI nanofiber reinforced PI aerogel membranes are prepared by ultilizing the crosslinking reaction between PI nanofibers and PI molecular chain,which can further improve the temperature resistance and heat insulation performance of flexible nanoporous PI aerogel membranes.The as-prepared PI nanofiber reinforced PI aerogel membranes can be used as flexible thermal protection materials at high temperature.Besides,based on the micro-nano mechanical testing method and finite element simulation,the single fiber pulling-out experiment and the interfacial strength test between the fiber and aerogel matrix are carried out.Besides,the fibrous pulling-out model in the aerogel matrix with different embedded depths is established by ABAQUS software.The model effectively simulates the change of pulling-out force and displacement during the fiber pulling-out process.Besides,the model shows the damage and failure forms of the aerogel matrix,and reveals the toughening mechanism of PI aerogel composites.(5)The organic-inorganic hybrid PI aerogel membranes with excellent foldable and unfolded properties are successfully prepared by designing the double cross-linking between inorganic nanomaterials and PI aerogel molecules at the nanoscale.MWCNTs grafted with surface functional groups by surface modification are applied as inorganic hybrid materials.The organic-inorganic hybrid reaction,hybridization mechanism,microstructure,mechanical properties,and thermal insulation properties of the organicinorganic hybrid PI aerogel membranes are studied.The prepared MWCNTs crosslinking PI aerogel membrane exhibits excellent folding and unfolding property and possesses the integrated properties of low density,high tensile strength,high fracture toughness,and good thermal insulation properties.The "fiber self-bonding" design,organic-inorganic hybridization,and surface corrosion method proposed in this paper can provide novel ideas for the preparation and characterization of PI aerogel membrane with high flexibility,good mechanical property,efficient thermal insulation,and multi-functionality.This paper have studied the microstructure design and regulation,preparation process,and multifunctional application of flexible PI aerogel membranes,which can provide a comprehensive research basis and guidance for the practical engineering application of PI aerogel membranes.