Preparation And Properties Of High-temperature-resistant And High-strength Polyimide Aerogels Based On Benzimidazole Diamine

The composite material is a combination of outstanding physical,thermal and mechanical properties.Owing to its fascinating mechanical properties,excellent fatigue resistance,low thermal expansion properties and high dimensional stability,the composites have become attractive materials to replace metals in many fields.The advanced aerospace carbon fiber reinforced composites,which are normally combined with the lightweight structures(foam,aerogel and honeycomb,etc.),should maintain high mechanical properties,thermal-insulating properties and temperature resistance in the extreme environments.For example,the high flexibility and excellent mechanical properties at high temperatures are eagerly desirable to fabricate the heat-insulation and shock-absorption functional layer to protect the sophisticated electronics from destruction caused by the external stimulus.As one of the typical lightweight and porous materials,the aerogels have attracted substantial attentions recently.Unfortunately,there have been few reports on the materials which can simultaneously fulfill the high-temperature thermal insulation,flexibility and mechanical properties.Therefore,it is an urgent need to develop a kind of aerogel which exhibits high mechanical properties,temperature resistance for the aviation and aerospace and maintains the flexibility.In this study,a simple aqueous strategy for constructing hydrogen-bonding networks is used to produce thermoplastic polyimide(PI)aerogels.The chemical structure and synthesis conditions of the PI aerogels are confirmed with FTIR,¹³CNMR,TMA and TGA.The freezing temperature is-40^oC and the highest imidization temperature is 380^oC.Secondly,a series of flexible thermoplastic polyimide(PI)aerogels with exceptional mechanical,thermal-insulating properties,and temperature resistance is synthesized by the green hydrogel strategy.The flexible blocks(4,4'-diaminodiphenylsulfone(DDS))link into rigid polymeric monolayer structures((2-(4-aminophenyl)-1H-benzimidazol-5-amine(APBIA)and3,3',4,4'-biphenyltetracar-boxylicdianhydride(BPDA))to form a thermoplastic structure by using a synergistic effect to construct hydrogen-bonding networks.Based on the backbone chemistry,chain length,a certain amount of DDS and optimal polymer concentration,the influences of morphology and network structure on the mechanical properties,thermodynamic behavior and other properties of these aerogels are evaluated.Thus,a kind of aerogels with a low shrinkage of 8.42%,an ultrahigh compressive modulus of 11.17MPa and a specific modulus of 81.03MPa·cm³·g^-1 is successfully prepared.Benefitting from the formation of ultra-strong network backbones,the aerogels show excellent thermal-insulating properties(0.0481·W·m^-1·K^-1)and temperature resistance.After the heat treatment at 300^?C,the compressive modulus still has a 91%retention rate,thus it can be used as a lightweight heat-insulation composite sandwich structures up to 300^?C.In order to study the high-temperature flexibility and thermal insulation of PI aerogels,the copolymer aerogel obtained by using 30mol%flexible blocks reserves their high flexibility,only 10%dimensional changes after 40000 compression-release cycles at 400^oC are showed.In the thermal insulation test for the material,the surface temperature of the aerogel firstly rises with the increase of heating time,then reaches a peak value of more than 400^oC,and remains constant.In addition,the aerogel temperature at the edge is only about 120^oC,and the upper surface temperature of the sample is only 75^oC.Therefore,these flexible aerogels with ultra-high modulus can be used as insulating materials to provide extended protection under extreme conditions.In aerospace exploration,they have high-temperature lightweight composite sandwich structures for the potential applications such as engine sealing,antenna substrates,friction nano power generator,personal wearable protection devices and light thermal sensors in extreme temperature environments,etc.