A Novel Structural Polyimide Material With Phosphorus And POSS Synergistic For Use In Atomic Oxygen Resistance

Currently, most spacecraft orbit from200-700km above Earth’s surface. In thislow Earth orbit (LEO) altitudes, there are many environmental factors impact thespace vehicles’ operation and working life, such as thermal cycling, microgravity,neutral gas, space debris, ultraviolet (UV), radiation and atomic oxygen (AO).Numerous research results have shown that the AO gives particular serious damage tospacecraft external materials in LEO. Therefore, developing both can adapt to theenvironment and can satisfy the performance requirements of the spacecraft aerospacematerials has become crucial to the development of the space industry. As is wellknown, polyimide has been widely used as spacecraft material due to its outstandingproperties, such as high strength, good thermal stability and resistance UV irradiatingstability. However, AO will interact with polyimide in LEO by complex chemical andphysical reactions in the constant process of collisions, eroding the material anddegrading the performance of polyimide. Thus, developing new high efficiencytechniques to resist AO can prolong the spacecraft’s working life and enhance theworking effectiveness of the spacecraft. Researchers have searched a variety ofmethods for AO resistance. It is generally summarized as the following two methods:applying protective coatings on the surface of polyimide and introducing functionalcompounds into polyimide by physical blend or chemical bond. Surface exertprotective coating is an effective means of atomic oxygen erosion resistance, spacematerial common protective coatings include inorganic coating, organic coating andorganic/inorganic hybrid coating. Matrix reinforcement refers to the atomic oxygenerosion resistance good silicon, phosphorus, zirconium, tin and other elements,through graft copolymerization, blending method is introduced into the polymermatrix, the use of these elements in the space atomic oxygen environment has"self-healing" or "self-healing" function to improve the atomic oxygen resistance ofspace material.Atomic oxygen on material erosion rate is a can intuitively reflect theindex of the atomic oxygen erosion resistance materials, surface coating method andreinforcement, matrix, and the key lies in development has lower the atomic oxygen erosion rate of the materialIn recent years, with special structure and function of cage and a half times thesiloxane (POSS) to improve the performance of polyimide of atomic oxygenresistance reported widely attention.Of the system, the researchers examined theintroduced in polyimide main chain and side chain POSS, its atomic oxygenresistance. The results show that, regardless of the POSS in polyimide main chain orside chain, the atomic oxygen resistance were significantly improved. At the sametime, due to the introduction of POSS, increase the free volume of polyimide,effectively alleviate the space high-speed particle impact produced by residual stress,eliminate residual stresses and the synergistic effect of irradiated by atomic oxygen,improve the atomic oxygen tolerance of polyimide materials. But have to mention isthat NASA space exposure experiment LDEF study found that for a long time, apolymer of POSS after irradiated by atomic oxygen for a long time will produce tinycracks, the atomic oxygen erosion can be directly through the crack at the bottom ofthe material, thus affect the operating life of spacecraft. In addition, will be introducedto the benzene phosphorus oxygen groups polyimide chain to improve its resistance toa number of atomic oxygen performance has also been reported. NASA’s space ofphosphorous polyimide exposure experiment MISSE on-orbit test for four years, theresults show that phosphorous polyimide has stronger atomic oxygen resistance.Further study found that when phosphorus polyimide irradiated by atomic oxygen, itssurface will generate a layer of polyphosphate passivation layer, the passivation layeris not react with atomic oxygen, can effectively protect the bottom material from theatomic oxygen erosion, atomic oxygen resistance increased. Therefore, will beintroduced to the phosphorus group and POSS polyimide, can significantly improvethe atomic oxygen resistance, and can make up due to the effects of long-term use ofcracks.In this paper, a series of the polyimide with novel structure phosphorus-halfsiloxane hybrid materials are synthetic.In order to avoid to produce gel, we appliedthe Flory gelation theory.By coaxial source ground simulation device of atomicoxygen exposure experiment was carried out, under the different atomic oxygen cumulative flux, phosphorous and polyimide-half siloxane hybrid materials ofatomic oxygen resistance significantly enhanced, and with the increase of phosphoruscontent, the atomic oxygen erosion rate is lower.In addition, in order to explore thephosphorus with POSS synergy resistance mechanism of atomic oxygen erosion,respectively on the material in the atomic oxygen environment exposure before andafter exposure to changes in surface chemical composition, chemical structure andsurface morphology change is studied, the results showed that phosphorus andpolyimide-half siloxane hybrid materials of phosphorus generated surfacepassivation layer can effectively fill the passivation layer cannot cover the areacontaining silicon, the synergy to protect the bottom material from atomic oxygenfurther erosion.