Fabrication And Properties Of Polyimide Composite Aerogels

Aerogels are ultra-light and highly porous nanostructured materials with low thermal conductivity,low dielectric constant,high specific surface area,and other interesting properties due to their fine pore structures,which show extremely high-value applications in the fields of heat,electricity,acoustics and so on.The use of inorganic aerogels represented by silica aerogels has been greatly restricted on account of its poor mechanical properties arising from the defects in the material’s pearl-necklace-like skeletal framework;while organic aerogels are expected to replace some brittle inorganic aerogels in many fields due to their light weight,good flexibility and high mechanical strength.Consequently,the development of polymer aerogels with excellent mechanical strength,ultrahigh thermal stability and desirable flexibility is of great significance to the wider applications of aerogels.As a member of high-performance polymers,polyimides（PI）are widely used due to their high thermal stabilities and good mechanical properties.The aerogels made of polyimides will undoubtly inherit the inherent excellent heat resistance and mechanical properties of polyimide materials.However,as a new emerging materials in recent years,there are still some problems to be solved urgently for polyimide aerogels,for examples,the intrinsic water absorption of polyimides with conventional structure is relatively high;the capillary force caused by water absorption of nanopores under humid and hot conditions can easily lead to the collapse of the pore structure;the dimensional stability under high temperature is still much worse than that of inorganic aerogels and so on,which limits the wide applications of polyimide aerogels.Therefore,a series of polyimide aerogels with excellent comprehensive performance were prepared by adjusting the chemical composition of polyimide backbones,the type of multifunctional cross-linking agents,the hybridization with inorganic nanomaterials.The major contents are as follows:1.Conventional polyimide aerogels made from biphenyl-3,3’,4,4’-tetracarboxylic dianydride（BPDA） and 4,4’-oxidianiline（ODA） exhibit reduced dielectric properties in high-temperature and humid environment due to their poor hydrophobicity.The diamine,2,2’-bis-（trifluoromethyl）-4,4’-diaminobiphenyl（TFMB）,is introduced into BPDA-ODA backbones to obtain polyimide aerogels with adjustable chemical structures.The structures and physicochemical properties of obtained polyimide aerogels were systematically characterized.In addition,with the incorporation of TFMB,the hydrophobicity of polyimide aerogels is dramatically enhanced as the water absorption decreasing from 415%for homo-BPDA-ODA to 13% for the polyimide aerogel at the ODA/TFMB molar ratio of 7/3.Dielectric constant and dielectric loss of the aerogels fall in the range of 1.29 to 1.33 and-0.02 to 0.03,respectively,and more TFMB fractions results in a slightly decrease of dielectric constant and loss tangent.Excellent hydrophobicity and low dielectric properties are expected to expand the application of these polyimide aerogels in high-tech fields such as electronics and wave-transmission materials.All of formulations of aerogels exhibit high absorption capacities for oils,meanwhile,mechanical properties and thermal stability of the polyimide aerogels are also raised to varying degrees due to the rigid-rod biphenyl structure introduced by TFMB.2.Polyimide aerogels were prepared from anhydride end-capped polyamide acid（PAA）oligomers cross-linked by low-cost lab-made amine-functionalized hyperbranched polysiloxane macromers（NH₂-HBPSi）.NH₂-HBPSi was synthesized based on the hydrolysis-condensation reaction between g-aminopropylmethyldiethoxysilane（APDES）,tetraethoxysilane（TEOS） and phenyltrimethoxysilane（PTMS）.To determine the effects of the NH₂-HBPSi crosslinker on the formation of aerogels,two different polyimide oligomers were investigated,BPDA-ODA and BPDA-2,2’-dimethylbenzidine（DMBZ）.Moreover,the thermal insulation,dielectric and CO₂ uptake performances were also investigated.The resultant polyimide aerogels showed low density（0.12 to 0.20 g/cm³）,high specific surface area（279 to 460 m²/g）,low thermal conductivity(29-32 m W·（m·K）^-1),and excellent mechanical properties.The Young’s modulus and specific Young’s modulus of the obtained BPDA-ODA aerogels are15-18 MPa and 75-120 N·m/g,respectively;while the Young’s modulus and specific Young’s modulus of the obtained BPDA-DMBZ aerogels are 22-25 MPa and 150-210 N·m/g,respectively.3.Dimensional instability has been regarded as one of the issues that limits the wide applications of polymer aerogels.Therefore,natural rod-like minerals,namely attapulgite（AT）nanorods,were introduced into polyimide aerogels to obtain the reinforced polyimide/AT composite aerogels with improved dimensional stability,through the strong hydrogen-bonding interaction.In composite aerogels,AT nanorods act as the rigid skeleton supporting the framework of aerogels,the resultant composite aerogels exhibit improved mechanical properties with the increased AT loadings （0-10 wt%）.The compression Young’s and specific modulus increase by 100% and 105%,respectively,for the composite BPDA-ODA aerogels containing 10 wt%AT compared to the pristine polyimide aerogels.Furthermore,the AT nanorods show strong supporting effects in maintaining the structural integrity of aerogels,which present a significant effect on reducing the shrinkage of composite aerogels at high temperatures and retaining their excellent thermal insulation performance,especially for BPDA-ODA aerogels.The detailed investigation reveals that the AT is an effective and low-cost additive for preparing high performance polymer nanocomposite aerogels with improved mechanical properties and thermal dimensional stability.4.There is limited space for the improvement of CO₂ adsorption performance of polyimide aerogels.Although zeolitic imidazolate frameworks（ZIF）are widely regarded as promising materials for CO₂ adsorption and storage because of their high specific surface areas and ordered pore structures,as a fine powder it is often challenging to use them efficiently in practical applications.Therefore,we explored a facile direct mixing,supercritical CO₂ drying,and carbonization process for preparing a novel porous materials based on ZIF/polyimide（ZIF/PI） composite aerogels.The organometallic frameworks are unstable and could collapse as mixing with the polyamic acid solution due to the strong complexation between carboxyl group in polyamic acid chains and metal ions.This complexation shows a great effect on the microporous structure of ZIF/PI composite aerogels and the resultant carbon aerogels,displaying a complex hierarchical porous structure,which can be altered via ZIF loading amounts（up to 20 wt%） and type of ZIF used （lab-made ZIF-8 and commercial ZIF-67 are selected）.The CO₂ adsorption performance of aerogels after carbonization was significantly improved at concentrations of ZIF above 10 wt%,reaching up to 50.0 cm³/g（2.23 mmol/g,298K）.Overall,we reported a direct process for preparing hierarchical polyimide aerogels which can be further carbonized for enhanced CO₂ adsorption performance,demonstrating their practical functionality for CO₂ adsorption.