Preparation And Functionalization Of Anisotropic Polyimide Aerogels

Aerogels have received widespread attention in recent years for their excellent performance in heat insulation,catalysis,adsorption purification,aerospace,etc.,due to factors like lightweight,low density,high porosity,and low thermal conductivity.Polyimide aerogels(PIAs)combine great thermal stability,chemical stability and excellent mechanical properties of polyimides,in the meanwhile,they meet the requirements of low-density and high porosity,which make them possess good application prospects.However,common polyimide aerogels are often prepared by using monomers with flexible structures and supercritical fluid drying methods,thereby limiting there application.Besides,few studies have reported polyimide aerogels with both excellent mechanical properties and strain responsiveness at high temperatures.In this thesis,functional anisotropic polyimide aerogels were prepared by controlling the freeze-drying methods of the polyimide precursors and incorporating carbon nanotubes as fillers.The main research progress is as following:In this thesis,starting from the polymer chain structure and pore structure design of the polymer,the commercial rigid structure monomer p-phenylenediamine and3,3',4,4'-biphenyltetracarboxylic dianhydride were selected to prepare poly(amic acid).Afterward,the poly(amic acid)salt solution was prepared.U-PIAs and r-PIAs were prepared by unidirectional and random freezing methods of poly(amic acid)salt solutions followed with thermal imidization treatment,respectively.U-PIAs have tubular pore structures,while r-PIAs exhibit the morphology of near-spherical pores.The PIAs have good thermal properties,5% weight loss temperature is 555 °C,and the glass transition temperature is 379 °C.PIAs have anisotropic mechanical properties.Additionally,different pore structures are evaluated for their influence on the mechanical properties of aerogels.Furthermore,PIAs show good mechanical properties under high temperature,u-PIA-6 exhibits high-temperature compressive strength value more than twice as high as those of commercial polyimide foam at 177 °C.In addition,the anisotropic pore structure imparts anisotropic thermal insulation properties to aerogel,and the sheet layer structure of u-PIAs along the vertical freezing direction results in a low thermal conductivity and good thermal insulation properties.In addition,carbon nanotubes/polyimide composite aerogels(MWCNT/PIAs)were manufactured via unidirectional freezing by introducing carbon nanotubes as conductive fillers with p-phenylenediamine and 3,3',4,4'-biphenyltetracarboxylic dianhydride as monomers.Firstly,the chemical structure and microscopic morphology of the composite aerogels were characterized.The composite aerogels displayed anisotropic tubular pore structures.The composite aerogels have good thermal properties with 5% weight loss temperature range of 507-532 °C and glass transition temperature range of 376-383 °C.The anisotropic mechanical properties of samples are determined by the pore structure.As a result of the introduction of carbon nanotubes,the composite aerogels possess dielectric and electromagnetic shielding properties.Carbon nanotube loading,sample testing direction,and sample thickness were discussed in detail for their effects on composite aerogels' dielectric and electromagnetic properties.Furthermore,the electromagnetic shielding performance of the composite aerogels was evaluated quantitatively based on the shielding mechanism and the attenuation constant derived from the idealized model.Samples with a thickness of 10 mm and 20% carbon nanotube loading had an average of 84.2 d B of EMI SE in the vertical freezing direction of 8.2-12.4 GHz.Finally,anisotropic carbon nanotubes/polyimide aerogels in the field of strain sensors were studied.Anisotropic MWCNT/PIAs were selected as resistive pressureinduced responsive materials based on the research presented in the first two chapters.MWCNT/PIAs exhibit anisotropic compressive responsiveness in part due to the anisotropy conductivity of the carbon nanotubes dispersed within the anisotropic aerogel.Compression responsiveness of the composite aerogels in the vertical freezing direction is higher than corresponding value in the parallel freezing direction.The responsiveness increases when the strain is increased separately.Aerogels in the vertical freezing direction have the highest sensitivity of 1.23 at 10% carbon nanotube loading.When the composite aerogels underwent 1000 compression cycles,the responsiveness showed significant fluctuations in the parallel freezing direction but good cyclic stability in the vertical freezing direction.All samples have good sensitivity in the vertical freezing direction under high temperature conditions(177 and 316 °C),with gauge factors ranging from 1.22 to 1.18(10% MWCNT/PIA).In addition,MWCNT/PIAs are able to maintain their response stability under high-strain(30% and60%)and high-temperature cycling conditions.The composite aerogels also exhibit good step compressing and bending induced strain response behaviors,and can be utilized as a flexible strain alarm in complex environments.