Study On Modification And Control Of Functionalized Polyimide Materials For Space And Vacuum Surface Insulation

The development of China’s aerospace industry is flourishing and has achieved worldrenowned success.Polyimide has been widely used in the aerospace industry due to its excellent resistance to high and low temperatures and electrical insulation properties.However,with the gradual increase in spacecraft power requirements,extreme space environments such as rapid changes and gradient distributions in high and low temperatures,high-energy charged particles and plasma environments can cause discharge damage to polyimide insulation materials,seriously threatening the safe operation of spacecraft in orbit.Therefore,in order to address the shortcomings of traditional polyimide materials in the extreme space environment,there is a need to improve the overall performance of polyimide by means of functionalised modifications,and to provide modified polyimide materials with optimised performance synergies for spacecraft,thus further improving the operational reliability of spacecraft in orbit.In this paper,a platform for the preparation of modified polyimides has been developed and the existing preparation process in the laboratory has been rationalised.Using a two-step process,polyimides were synthesised using diamine ODA and dianhydride PMDA.Octa-p-aminophenyl-caged polysesquioxane(POSS)and 4,4’=(hexafluoroisopropylidene)diphthalic anhydride(6FDA)were selected as modifying materials and 1%,3%and 5%molar content of POSS and 10%,30%and 50%molar content of 6FDA were introduced to design and prepare the relevant polyimide modification optimisation scheme.The basic physicochemical properties of the polyimide composite film materials were characterised.The results show that all polyimide composites are fully imidated.The introduction of POSS resulted in a blue shift in the UV spectrum,a weakening of the charge transfer complexation and an increase in light transmission;a slight increase in thermal conductivity,a slight inhibition of the continuous thermal decomposition of the polyimide phase by the phenyl group contained in POSS and the cubic silicon core structure,and an increase in thermal stability;and a decrease in mechanical properties but still with some molecular flexibility.The introduction of 6FDA also resulted in a blue shift in the UV spectrum and an increase in light transmission;the thermal conductivity improved considerably,however,the quaternary carbon structure of 6FDA is susceptible to thermal decomposition and leads to the dissociation of the free radicals contained in the material,thus reducing the thermal stability;the electron induction effect of trifluoromethyl and the space-site resistance effect make the molecular chain less regular and the mechanical properties are reduced.The effect of vacuum and electrode spacing on the flashover voltage was investigated using a vacuum surface flashover experimental platform built in our laboratory,and the vacuum surface insulation properties of the polyimide composites were subsequently tested.On the one hand,the introduction of POSS leads to a reduction in the resistivity of the material,resulting in a suppression of surface charge build-up.On the other hand,the inorganic silicon-oxygen structure formed on the surface of the composite material effectively reduces the depth of the trap energy level when exposed to high temperatures such as corona.6FDA content of 50%of the FP5 composite material has a flashover voltage of 21.1 kV along the vacuum surface,which is 57.4%higher than that of pure polyimide.As the 6FDA content increases,the material resistivity shows a decreasing trend.The introduction of the-CF3 group causes a change in the trap distribution on the surface of the material,the strong electronegativity of fluorine inhibits the development of electron collapse and the high bond energy of the C-F bond makes the emission of secondary electrons more difficult,thus inhibiting the occurrence of flashover along the face of the insulating material.Considering the synergistic modulation of the thermal properties and the vacuum surface insulation properties of the above polyimide composites,polyimide composites with the introduction of both POSS and 6FDA were designed and prepared,and their properties were tested and analysed on all sides.This paper shows that the simultaneous introduction of POSS and 6FDA can further synergistically modulate the thermal properties and vacuum surface insulation of polyimide composites,making them more suitable for use in aerospace environments.The study may provide a reference for further research on the optimisation of spacecraft insulation materials.