Fabrication And Properties Of Polyimide Aerogel Fibers

Aerogel fibers,which combine the high specific surface area,high porosity,and low density of aerogels with the outstanding mechanical properties and good flexibility of fibers,have attracted growing attentions in many fields,such as thermal insulation and protection,infrared stealth,electromagnetic shielding,etc.Although aerogel fibers based on graphene,silk fibroin,cellulose,polyvinyl alcohol and nano aramid fiber have been developed,the poor mechanical properties arising from the defects of porous structure and weak chemical structure of the matrix materials have restricted their applications.In addition,the reported complex spinning processes limit the continuous and scalable preparation of these aerogel fibers.Polyimide（PI）,one of high-performance aromatic polymer,is an ideal matrix material for preparing aerogel fibers due to their outstanding combination of properties,which could be attributed to their stable chemical structures.The synthetic route of PI depends on the chemical structures of the target product.In general,PIs with rigid-rod structure,which are insoluble and infusible,are synthesized from the soluble precursor by an imidization process;PIs with bulky side groups or flexible molecular chain,which show good solubility,can be synthesized directly.In our present work,the“wet spinning-chemical imidization”and“hydrogel-aerogel”strategies are proposed to prepare aerogel fibers from PIs with rigid structure or bulky side groups,respectively,which have solved the problems of the spinnability of spinning dope and the difficulty in obtaining both porosity and mechanical properties of aerogel fibers.Moreover,the PI aerogel fibers could be enhanced by controlling their microstructure.A series of functional PI aerogel fibers and their textiles were prepared to expand their applications.This work mainly includes the following four aspects:（1）A“wet spinning-chemical imidization”strategy was designed to prepare PI aerogel fibers with biphenyl structure.In detail,the PAAS solutions with chemical structure of BPDA-PDA were injected into an acetone bath containing chemical imidization agents,and the PI gel fiber would be achieved directly.After the following supercritical CO₂ drying,continuous PI aerogel fibers with lightweight were prepared.PI aerogel fibers prepared by“wet spinning-chemical imidization”showed a perfect three-dimensional network structure.Moreover,a 10 h reaction process would guarantee a high imidization degree of the PI aerogel fibers.The micromorphology of PI aerogel fibers could be adjusted by controlling the spinning precursors’concentration（6-12 wt%）,and the obtained flexible PI aerogel fibers performed low density（180～285 mg/cm³）,high porosity（80%～87%）,large specific surface area（45～137 m²/g）and good tensile strength（0.6～4.6 MPa）.（2）For enhancing the PI aerogel fibers with biphenyl structure,aramid nanofibers（ANFs）were used as the reinforcement to prepare composite aerogel fibers via the“wet spinning-chemical imidization”strategy.Firstly,the ANFs were prepared through a feasible“top-down”approach.The resulting ANFs were detected to form intermolecular hydrogen bonding with PAAS molecular chains and dispersed well in the compound for their abundant proton acceptors and donors.Among all formulations,the PI aerogel fiber with 2 wt%ANFs（PAF-2）,where a sub-network formed by ANFs could be observed,exhibits the highest specific surface area（108m²/g）and good mechanical properties（5.3 MPa）.Meanwhile,the fabric based on PAF-2 performs excellent thermal insulation properties,and the temperature difference between the PAF-2 fabric（with a thickness of 3.04 mm）and the heat source（300℃）can reach up to 179℃.Furthermore,the PAF-2 fabric coated with PTFE exhibits excellent hydrophobic properties and performs durable thermal insulation under the hygrothermal environment.（3）A ionic PI hydrogel fibers（PHF）with trifluoromethyl groups derived from an organic-soluble PI salt（PIS）with the structure of 6FDA-TFMB have been prepared by an efficient“hydrogel-aerogel”strategy.The resulting PHF was then freeze-dried into PI aerogel fiber（PHAF）directly.During the spinning processes,Ca Cl₂ aqueous solution was used as a“sol-gel”transition bath.The resultant PHAFs show a three-dimensional network structure.In addition,the concentration of Ca₂Cl and the time treated by KOH both significantly affected on the PHFs and PHAFs’properties.In all formulations,PHAF-48,which was obtained from a 0.1 mol/L Ca Cl₂coagulation bath,exhibited the largest specific surface area（360 m²/g）and highest tensile fracture strength（9.4 MPa）,as well as good flexibility and stitchability.Thanks to the high porosity,fabric based on PHAF-48 exhibits a low thermal conductivity（44.5 m W/（m·K））and shows great potential in infrared stealth applications.（4）A novel PI aerogel fiber containing carboxyl groups with dense shell morphology were prepared by wet spinning method combined with the freeze-drying technique.Firstly,PI solution with the chemical structure of 6FDA-TFMB-DABA was synthesized by the high-temperature“one-pot”method.Attributed to the unique dense shell,the CPAFs performed outstanding mechanical properties with the tensile strength of 191～265 MPa and initial modulus of 6.6～7.9 GPa,as well as good flexibility.Meanwhile,the high specific surface area（298～464 m²/g）and high porosity（81～89%）render CPAF and their fabrics with excellent thermal insulation and heat preservation properties within a wide temperature range（-190～320℃）.Additionally,a series of functionalized aerogel fibers and their fabrics,including a phase-change fabric with a thermoregulation function and Ag nano-particles coating textiles with a high EMI shielding efficiency,have been prepared for expanding their potential applications in aerospace industry.