Controllable Preparation And Properties Of Multi-functional Polyimide Composite Aerogels

Polyimide（PI）aerogels have unique properties such as low dielectric properties,high mechanical strength,high-temperature resistance,high specific surface area and low thermal conductivity,making them a class of aerogel materials with excellent overall performance.Therefore,PI aerogels are widely used in electrical and electronic,aerospace,mechanical and chemical industries.At present,people mostly use the supercritical drying-chemical imidization process to prepare PI aerogels,but there are still problems such as high production cost and harm to the environment and human body,which seriously hinders the development of PI aerogels.The freeze-drying-thermal imidization process developed in recent years is a more desirable process for the manufacture of PI aerogels because of its simplicity of operation,environmental friendliness,and low production costs.However,the problems such as uncontrollable pore structure,relatively single function and poor weather resistance of PI aerogels have seriously hindered the high performance and wide application of PI aerogel.Therefore,in this thesis,hydrophilicity polyamide acid（PAA）and electrospun PAA nanofibers（PAANF）were used as the main matrix materials and graphene oxide（GO）,MXene and lignin sulfonate（LS）were selected as functionalized building blocks to prepare PI composite aerogels with high mechanical strength,controllable pore structure and multifunctionality through sol-gel,freeze-drying,and thermal imidization processes.GO,MXene and LS have good water dispersibility/water solubility,abundant surface active functional groups,excellent mechanical,thermal and electrical properties,and can be compounded with PAA through physical/chemical interactions and efficiently construct functionalized PI composite aerogels.Surface adsorption,hybrid cross-linking,intercalation cross-linking and wrapping cross-linking were used to combine PI substrates with functional materials respectively,and the way of compounding between the constructed substrates and functional materials was systematically studied.In addition,the internal pore structure of PI composite aerogel was regulated by different freezing methods（single directional freezing,radial radiation freezing）,and PI composite aerogels with layered,radial radiation,layer columnar and multiple pore structures were successfully prepared.The relationship between the freeze-forming method,microscopic pore structure and comprehensive performance of PI composite aerogels was investigated.Finally,the performance of PI composite aerogel in electromagnetic wave absorption,thermal insulation,piezoresistive sensing,and photothermal evaporation was studied in detail,which laid a theoretical and practical foundation for its application in electromagnetic protection,water purification,thermal insulation,and flexible sensing.In summary,the main work of this thesis is as follows:（1）PI/r GO composite aerogel with multilayer impedance gradient structure（PI-GP_x-r GO）was precisely designed and controllably constructed by using vacuum impregnation method to load r GO/PI_x and r GO sequentially in layers,combined with freeze-drying-thermal imidization process.By adjusting the content of r GO in r GO/PI_x,the microstructure and dielectric properties of the composite aerogel could be effectively regulated to obtain an optimized impedance match.The successful construction of the layered impedance gradient structure effectively enhances the interfacial polarization relaxation and internal multiple reflection/scattering of electromagnetic waves within the aerogel skeleton,thus conferring excellent electromagnetic wave absorption performance.The results showed that the PI-GP_1/3-r GO composite aerogel prepared at a mass concentration ratio of 1:2(4.00 mg m L^-1:8.00 mg m L^-1)of r GO to PI in the second r GO/PI_x layer has the most excellent electromagnetic wave absorption performance.The RL_min value of PI-GP_1/3-r GO composite aerogel was up to-32.87d B at 10.39 GHz with an effective absorption bandwidth of 6.22 GHz（8.26 to 14.48 GHz）at a thickness of 4 mm.In addition,the PI/r GO composite aerogel demonstrated excellent thermal stability(T_5wt%>500°C),good thermal insulation ability and high mechanical strength.This work provides an effective strategy for the fabrication of multifunctional composite aerogels with light weight,high strength,high thermal stability and efficient electromagnetic wave absorption performance.（2）Inspired by the xylem tissue of dicotyledonous plants,the PI/MXene composite aerogel（RPIMX）with a radially radiating porous structure was controllably prepared by a freeze-drying-thermal imidization process using the interfacial enhancement between PAA and MXene and the radial ice template method.During the sol-gel process,PAA was tightly bound to MXene by hydrogen bonding and formed a stable PAA/MXene composite sheet.During radial radiation freezing,the PAA/MXene composite sheet gradually formed a radial distribution under the pushing and squeezing effect of ice crystal radial growth,and finally a PI aerogel with a biomimetic radial porous structure was successfully constructed by solvent removal and heat treatment.This centrosymmetric radial laminar structure and directional channels of RPIMX composite aerogels contributed to the excellent reversible compressibility,high electrical conductivity,and efficient water transport capability exhibited.As a piezoresistive sensor,RPIMX composite aerogel exhibited high sensitivity(4.30 k Pa^-1),fast response/recovery time（100 ms/80 ms）and excellent fatigue resistance（1000 cycles at 30%strain）in the 0.5%to 80%strain range（60.00 Pa to 73.09 k Pa）,and was capable of real-time monitoring of various human movements and object pressure distributions（5×5 array）.In addition,RPIMX exhibited excellent photothermal evaporation performance with a water evaporation rate of 14.40 kg m^-2h^-1 under 4 times solar irradiation.Such excellent comprehensive performance makes the bionic RPIMX composite aerogel promising as an ideal material for flexible piezoresistive sensors and photothermal evaporators.（3）A lightweight,highly compressible and highly conductive PI nanofiber/MXene composite aerogel（PINF/MA）with a“laminated column”microscopic porous structure was successfully prepared by electrostatic spinning of PAA into PAA nanofibers（PAANF）and assembled with MXene after freeze-drying and thermal imidization treatment.PAANF and MXene were interconnected by hydrogen bonding to form a large number of strongly cross-linked leafy PAANF/MXene lamellae layers.During the freeze-forming process,the PAANF/MXene lamellae were arranged in a parallel and orderly manner by the pushing and squeezing effect of ice crystal growth,and form a solid 3D“layer column”structure with the PAANF between the layers.The PINF/MA composite aerogel obtained after freeze-drying-thermal imidization treatment exhibited excellent piezoresistive sensor performance with a high sensitivity of 22.32 k Pa^-1,an ultra-low detection limit of 0.1%,and a very strong cycling stability（Stable cycle 1500 times at 30%strain）over a wide pressure range of 10.00 Pa～7.93k Pa（0.1%～50%strain）,and can be used for real-time monitoring of various human movements and pressure distributions of different masses（3×3 array）.In addition,the PINF/MA composite aerogel had outstanding electromagnetic wave absorption performance.The PINF/MA composite aerogel showed an RL_min value of-40.45 d B at 15.19 GHz with a thickness of 2.5mm and an effective absorption bandwidth of 5.66 GHz（12.34 to 18.00 GHz）.These outstanding multifunctional performances make PINF/MA composite aerogel an ideal candidate for flexible/wearable piezoresistive sensors and electromagnetic wave absorbers.（4）In this work,lightweight,compressible and highly conductive PINF/LS carbon aerogels（PLCA）with multiple pore structures were prepared by interfacial assembly of PAANF and LS,single-directed ice template method and annealing process controllable by using water-soluble sulfonated lignin（LS）as cross-linking agent.LS and PAANF were wrapped and cross-linked by hydrogen bonding,which effectively ensured the mechanical stability of the aerogel.the structure and properties of carbon aerogels can be effectively regulated by changing the LS content during single directional freeze forming.When the mass concentration ratio of LS to PAANF is 8.00 mg m L^-1:8.00 mg m L^-1,PLCA carbon aerogels with honeycomb/layered multi-pore structure were successfully prepared.Among them,the laminar porous structure in PLCA carbon aerogel can effectively transfer stress,and the interlayer honeycomb structure facilitates moisture transfer.Therefore,PLCA carbon aerogels can be used for piezoresistive sensing studies.Specifically,high sensitivity(12.14 k Pa^-1)over a wide strain range of 0.1%to 60%（10.00 Pa to 30.50 k Pa）,fast response/recovery time（40ms/60 ms）,and long-term cycling stability（2000 cycles at 30%strain）provide a possibility for real-time monitoring capability of human motion.In addition,PLCA carbon aerogels exhibited excellent photothermal evaporation performance with vapor production rates up to 9.39 kg m^-2h^-1 under 3 times solar light irradiation.These excellent properties make this multifunctional nanofiber carbon aerogel promising for applications in high-performance flexible piezoresistive sensors and high-efficiency photothermal evaporators.