| The aerospace field has high requirements for structural materials.For example,the future supersonic aircraft requires the thermal oxygen stability of structural parts to reach 177-250℃.Polyimide has excellent heat resistance and mechanical properties due to the presence of a large number of aromatic heterocycles in the molecular chain,which can fully meet the requirements of structural components in the aerospace industry.Thermosetting polyimide can withstand a high temperature of400℃,because it is a cross-linked polymer,it has a long-term problem of processing difficulties.Traditional phenylene polyimide can be used for a long time under the environment of 200-300℃,and has excellent dielectric properties,dimensional stability,insulation and radiation resistance.However,due to the huge molecular chain rigidity and large inter molecular forces,they are also difficult to dissolve or melt process.The long-standing processing problems have greatly restricted the application range of polyimides.Therefore,it is extremely important to develop thermoplastic polyimides with high temperature resistance and high thermal oxygen stability.In this paper,3,3’,4,4’-benzophenone tetracarboxylic dianhydride（BTDA）is used as dihydrine,3,3’-diaminodiphenylsulfone（3,3’-DDS）as diamine to reduce the rigidity of the polyamide molecular chain,by introducing a non-coplanar structure of2,3’,3,4’-benzophenonetetracarboxylicdianhydride（a-BTDA）,or9,9-bis（4-amino-phenyl）fluorene（BAFL）and 2,2-bis（3-amino-4-hydroxyphenyl）hexafluoropropane（6FAP）with bulky groups,further breaks the regulation of polyimide molecular chain,weakens the force between molecular chains,and synthesizes a series of thermoplastic polyimides with high temperature resistance and high thermal oxygen stability.The polyimides we synthesized were characterized and tested,and the results are as follows:the glass transition temperature of the thermoplastic polyimide we prepared is between 292℃and 302℃,and the temperature at 5%of thermal decomposition is between 530.45℃and 558.98℃.The carbon residue rate at 800℃is greater than 50%,the thermal expansion coefficient is below 55 ppm.K-1,it can be dissolved in NMP,DMAc,DMF and DMSO at room temperature,the processing viscosity is as low as 9210 Pa.s,and it has excellent resistance thermal and processing properties.However,the shear strength at 250℃decreases more severely than that at room temperature,which indicates that the thermal oxygen stability of polyimides we synthesized is poor and cannot meet the requirements of the aerospace field for structural components such as future supersonic aircraft.In order to improve the thermal oxygen stability,we decided to still use BTDA as the dianhydride,and use 2,2’-bis（trifluoro-methyl）diamino-biphenyl（TFMB）with greater bond energy as the diamine to increase the thermal oxygen stability and solubility of the system,by introducing the non-coplanar structure of a-BTDA,or BAFL、6FAP with bulky groups,further breaks the re gulation of polyimide molecular chain,weakens the force between molecular chains,and synthesizes a series of thermoplastic polyimides.The polyimide we synthesized was characterized and tested,and the results were as follows:The glass transition temperature of the thermoplastic polyimide we synthesized was between 335℃and 358℃,the temperature of thermal decomposition was between 561.60℃and 569.78℃,the residual carbon rate of800℃was greater than 54%,and the coefficient of thermal expansion was below 55ppm.K-1,can be dissolved at room temperature in NMP,DMAc,DMF and DMSO,processing viscosity as low as 8547 Pa.s,250℃can still maintain a high shear strength.It shows that we have successfully prepared thermoplastic polyimide with excellent heat resistance and thermal oxygen stability,as well as excellent solubility and thermal processing,which fully meets the requirements for the use of structural components in the aerospace field. |
