Preparation And Performance Research Of Polyimide Based Composite Aerogels

With the development of industrialization and the increase of emissions from human activities,the problems of environmental pollution and high energy consumption are becoming more and more serious.CO₂ from burning fossil fuels and airborne particulate pollutants urgently need effective materials for adsorption and filtration,and excessive energy consumption can be alleviated by thermal insulation materials with good mechanical properties and moisture resistance.Polyimide（PI）aerogels have both advantages of aerogels and PI,and have the characteristics of high porosity,high specific surface area,low density,low thermal conductivity,excellent mechanical strength,thermal stability and corrosion resistance,which are widely used in adsorption,heat insulation,air filtration and other fields.Based on this,in this thesis,4,4’-diaminodiphenyl ether（ODA）and 3,3′,4,4′-biphenyltetracarboxylic dianhydride（BPDA）were used as monomers,and different PI-based composite aerogels were synthesized through different routes,and their properties were characterized and analyzed.The following three aspects of work were mainly carried out:（1）Polyethylene glycol（PEG）was introduced into PI aerogel by sol-gel method at low temperature,and PI/PEG composite aerogel with directional lamellar shape was prepared by freezing liquid nitrogen.The effects of PEG content and directional pore structure of PI/PEG composite aerogel on CO₂ adsorption performance were studied.The results showed that,the CO₂ adsorption capacity of L-PI/PEG-4 composite aerogel was 16.76 cm³/g at 25°C and 1 bar,and showed high CO₂/N₂ selectivity and adsorption cycle stability.（2）By adjusting the composition of aging solution,the combination of chemical imidization and freeze-drying was realized.PI/Thermoplastic polyurethane（TPU）composite aerogels were synthesized by this new method,and the comprehensive properties of PI aerogels prepared by this technology were excellent.Interestingly,the volume shrinkage of the composite aerogel was decreased from 24.14%to 5.47%,which leads to low density（0.095g/cm³）and elevated porosity（92.4%）.In addition,strong mechanical strength（1.29 MPa）and high hydrophobicity（123.6°）were achieved.More importantly,the thermal conductivity of PI/TPU composite aerogel at ambient temperatures is as low as 29.51 m W·m^-1·k^-1.（3）PI/TPU composite aerogel film was synthesized by sol-gel method and CO₂supercritical drying.Due to the interaction force between PI and TPU,the molecular chain of PI has a high degree of crosslinking.In addition,the composite aerogel film had a high specific surface area of 286.33 m²/g and good flexibility.In terms of air filtration,composite aerogel film mainly relies on the interception effect to filter sol particles,and its filtration efficiency is at a high level.At the flow rate of 15 L/min,the filtration efficiency of 0.3μm sol particles is98.16%,and the filtration efficiency of 10μm sol particles is as high as 100%.Due to the reduction of contact time between particles and aerogel film,the interception effect is weakened,and the filtration efficiency of samples decreases at high flow velocity.Due to the aggregation of TPU chains and the potential connection between PI and TPU,a certain proportion of macropores appear in the composite aerogel film,which makes it have a lower pressure resistance than that of pure PI aerogel film,which is 600 Pa.