Preparation And Thermal Safety Performance Of Polyimide Composite Aerogel Based On Green Crosslinker

Aerogel is an ultra-light solid material with a highly porous nano-interconnected network structure which,due to its unique porous structure,has a high specific surface area,low thermal conductivity,low dielectric constant and other properties that make it highly valuable in thermal,electrical and acoustic applications.The bead chain skeleton structure of inorganic aerogel,represented by silica,leads to its poor mechanical properties,which greatly limits its widespread use;while organic aerogel is expected to replace some of the brittle inorganic aerogel in many fields due to its light weight,flexibility and high strength,etc.Therefore,the development of polymer aerogels with good mechanical strength,thermal stability and low thermal conductivity is important for the expansion of this type of material.Therefore,the development of polymer aerogels with good mechanical strength,thermal stability and low thermal conductivity is important for the expansion of these materials.As a member of high performance aromatic polymers,polyimide(PI)has received much attention for its excellent high temperature stability and good mechanical properties.The polyimide aerogel material will undoubtedly inherit the characteristics of polyimide material such as high intrinsic heat resistance and excellent mechanical properties.However,as a new class of materials emerging in recent years,polyimide aerogels still have many problems to be solved,for example,the synthesis process of crosslinking agent for conventional structure polyimide is complicated,and the synthesis process requires a large amount of environmentally unfriendly organic solvents;the thermal conductivity of polyimide aerogels is still higher than that of inorganic aerogels;the dimensional stability at high temperature still has a large gap with that of inorganic aerogels,etc.,which limits the wide application of this aerogel.This limits the wide application of this aerogel.Therefore,in this round,I intend to develop green cross-linker coached polyimide-silica composite aerogel,reduce the thermal conductivity of polyimide aerogel,enhance the thermal insulation ability,and improve the thermal dimensional stability and service temperature.The main research contents are as follows.(1)Green crosslinker crosslinked polyimide aerogel.Two kinds of polymer crosslinkers,aliphatic polymer and silicone polymer,were prepared by radical polymerization reaction and hydrolytic condensation reaction of silane coupling agent,respectively,which simplified the preparation process of crosslinker,shortened the synthesis cycle of crosslinker,reduced the use of organic solvents in the synthesis process of crosslinker molecules,made the polyimide aerogel easy to operate in the preparation process,and shortened the polyimide preparation cycle.Through MM2 simulation,it is found that PSMAin the polyimide solution is in the state of rigid helical chain,which is not conducive to the dispersion of cross-linker molecules in the solution,and the side bases of the hesitant rigid molecular chain will be partially obscured by the reaction,resulting in the relatively low strength of the aerogel prepared by it.The crosslinker PMN molecules show a free walking state in the polyimide solution,which is conducive to its rapid crosslinking reaction with polyimide,resulting in its higher mechanical strength.(2)Preparation and thermal insulation enhancement mechanism of PI-SiO2 nanosphere hybrid aerogel.The ball-chain structure of organic cross-linked polyimide hybrid aerogel was prepared,and the introduction of the ball-chain structure can effectively reduce the shrinkage of the hybrid aerogel during the drying process.The SiO2 added into the ball-chain structure are small-sized nanoparticles,which can significantly increase the specific surface area of the aerogel(up to 752 m2/g)after forming the aerogel.The increase of the specific surface area further reduces the thermal conductivity of the aerogel(down to 0.027 W/(m·K))and enhances the thermal insulation property of the aerogel.By simplifying the fiber structure of PI-SiO2 hybrid aerogel into a nanoscale cubic skeleton,an equivalent heat transfer model of PI-SiO2 sphere-chain structure was established based on the series-parallel thermal resistance theory of steady-state heat transfer process,and the upper and lower limit values of the theoretical thermal conductivity of the hybrid aerogel were calculated,and it was found that the thermal conductivity of the hybrid aerogel was affected by several factors such as SiO2 addition,SiO2 particle size and fiber curvature,and the thermal conductivity of the hybrid aerogel was reduced with the increase of SiO2 specific surface area.The thermal conductivity of the heterogeneous aerogel was found to be influenced by several factors such as SiO2 addition,SiO2 particle size and fiber curvature.(3)Preparation and thermal safety of PI-SiO2 interpenetrating aerogel with high temperature resistance.The formation of SiO2 aerogel network inside the polyimide by Si(OH)4 and then hybridized with the polyimide fiber network to form a double network interpenetrating chemical structure can effectively reduce the shrinkage during the drying of the double network interpenetrating aerogel.Due to the good dispersion and high content of Si(OH)4 independent network structure in the polyimide molecule,it can microscopically separate the dispersed phase at the molecular level,which can prevent the dimensional change caused by the structural change of polyimide at high temperature,and then significantly improve the stability of the pore structure of the sample for long-term use in high temperature environment.The porous structure formed by the addition of SiO2 aerogel itself can significantly increase the porosity of the composite aerogel,which in turn protects the polyimide fibers penetrating through it,enabling it to maintain a low internal temperature when the external environment is high,enhancing the stability of the pore structure of the sample under high temperature environment.The interpenetrating polyimide fibers provide effective support for the double mesh interpenetrating aerogel,and the two synergistically improve the mechanical strength of the sample.At the same time,the increased porosity leads to a further reduction of the thermal conductivity(down to 0.022 W/(m·K))of the doubleweb interpenetrating aerogel,which provides better thermal insulation properties.