Molecular Structure Design,Preparation And Process Improvement Of Crosslinked Polyimide Aerogel

As a special engineering plastic,polyimide(PI)has excellent mechanical,corrosion resistance,dielectric and other properties,and is widely used in aerospace,electronic packaging,liquid crystal display,photoelectric switch and other fields.The PI aerogel derived from the combination of aerogel and polyimide has both the thermal insulation performance of aerogel and the excellent comprehensive performance of polyimide.However,the current PI aerogel still has the problems of low compression modulus and high drying cost,which limit its practical application.Therefore,the PI aerogel with high modulus was prepared by molecular structure design to meet the dual requirements of pressure bearing and heat insulation,and the optimization of the preparation process to reduce the cost is of great significance for the large-scale application of PI aerogel.Aiming at the disadvantage of low compression modulus of PI aerogel,which is difficult to be used in pressure bearing occasions,this paper designed the molecular structure of PI aerogel by using monomers with different structures as raw materials,improved the mechanical properties of PI aerogel,and prepared PI aerogel with high modulus.On this basis,PI aerogel with similar performance to Supercritical drying samples was prepared at a lower cost by optimizing the drying process.The main achievements are as follows:(1)Firstly,a highly rigid molecular chain with a small free volume was designed by adjusting the molar fractions of meta toluidine(DMBZ),pyromellitic anhydride(PMDA),and biphenylenedicarboxylic anhydride(BPDA)as monomers.The effects of PMDA content in the molecular chain,cross-linking agent type,degree of polymerization and cross-linking agent excess ratio on the performance of PI aerogel were investigated.In terms of mechanical properties,when trimesoyl chloride(BTC)was used as crosslinking agent,the compression modulus of PI aerogel was significantly increased with the increase of PMDA mole fraction in dianhydride.When the PMDA mole fraction in dianhydride was 100%,the compression modulus reached 9.2 ± 0.7 MPa;When the polymerization degree of PI aerogel is 100,the compression modulus of PI aerogel is 32.2± 0.9 MPa when OAPS is used as crosslinking agent;The decrease in polymerization degree leads to a decrease in yield stress and a decrease in compressive modulus;When excessive crosslinking agent is used,the compression modulus of PI aerogel is positively correlated with the amount of BTC added,and the maximum is 20.9 ± 0.7 MPa;On the contrary,increasing the amount of OAPS added leads to a decrease in modulus.At the same time,the obtained PI aerogel can withstand132422 times of its own weight and only 2.1% strain.After being treated in liquid nitrogen and 110 ℃for 1h,its compression modulus remains unchanged,indicating that it has mechanical property stability at high and low temperatures.In terms of optical properties,the yellowness of PI aerogel increases with the increase of PMDA mole fraction in dianhydride.When PMDA mole fraction in dianhydride is 100%,PI aerogel changes from opaque to transparent;Different cross-linking agents have great influence on the transparency of PI aerogel.When 1,3,5-tris(4-aminophenoxy)benzene(TAB)is used as the cross-linking agent and the polymerization degree is 100,the transmittance of5.4 mm thick PI aerogel at 800 nm reaches 76.8%.When octaaminophenyl siloxane(OAPS)was used as the crosslinking agent and the polymerization degree was 20,PI aerogel had the highest transparency,and the transmittance of the 6.5 mm thick sample at 800 nm reached 82.3%.This kind of aerogel with high transparency and high modulus is expected to be used in the aviation field as optical glass.(2)Secondly,the drying process of PI aerogel was optimized,and the traditional Supercritical drying method was replaced by freeze-drying method to dry PI wet gel obtained by chemical imidization method.We prepared PI wet gel with low freezing point dimethyl sulfoxide(DMSO)instead of the traditional non-polar solvent dimethyl acetamide or dimethylformamide as the reaction solvent.At the same time,the ethanol solvent used in the traditional solvent replacement was replaced with dimethyl sulfoxide,tert butyl alcohol,and water.After freeze-drying,the density and porosity of the polyimide aerogel obtained were close to those obtained by Supercritical drying.The pore diameter of PI aerogel obtained by freeze-drying is 1 μ M below,close to the sample obtained by Supercritical drying;In terms of performance,the thermal conductivity of PI aerogel obtained by two drying methods is between 65～73 m W m-1 K-1;At the same time,on a 200 ℃ hot bench,the temperature difference between the upper and lower surfaces of PI aerogel obtained by the two drying methods is close to 100 ℃,with similar thermal insulation.When DMSO is used as the solvent,the uneven gel results in irregular macro shape of wet gel.Therefore,we also use dimethylformamide as the reaction solvent,and use tert butyl alcohol for replacement and freeze drying to prepare PI aerogel with regular shape.Compared with the samples obtained by Supercritical drying,the micro morphology of PI aerogel obtained by freeze-drying is similar to that obtained by freeze-drying,and the pore diameter is less than 100 nm.The difference between the thermal conductivity of the samples obtained by the two drying methods is within 7 m W m-1 K-1.We calculated the preparation cost and found that the economic cost of PI aerogel obtained by freeze drying of tert butyl alcohol was far lower than that of Supercritical drying.All characterizations showed that the structure and properties of polyimide aerogel obtained by freeze-drying were similar to those of Supercritical drying samples,and freeze-drying had lower economic costs and easier operation,which provided a useful exploration for large-scale preparation of high-performance polyimide aerogel.