| The altitudes of 200 to 1000Km above the Earth's surface is called low Earthorbit (LEO), where many satellites around the Earth travel. At LEO altitudes thehazardous environment towards polymers includes atomic oxygen (AO), UVradiation, ionizing radiation, ultra high vacuum, thermal cycles. AO is formed byphoto-dissociation of molecular oxygen by short wavelength solar ultravioletradiation and it is considered to be the most severe hazard for polymer-basedmaterials on orbiting spacecrafts because of its strong oxidative. Exposure to LEOenvironment leads to significant changes of many spacecraft surface materials. Theinteraction between the energetic atomic oxygen and polymer components mayresult in materials surface erosion and changes in chemical composition, surfacemorphology, optical properties and thermal properties. The effects of AO on variousmaterials have been widely investigated in recent years.The polyimide that is a kind of special engineering plastics has been used mainlyas aerospace structural adhesives and interlayer dielectric materials inmicroelectronics, because of their excellent thermal properties, good mechanicalproperties and low dielectric constant. But polyimide materials which used onaircraft were prone to be eroded by atomic oxygen in low earth orbit. In the meantime, we found that the organic compound of silicone acted with atomic oxygen tobecome a layer of SiO2 which attached on the surface of the materials. Otherwise theorganic compound of silicone was used on aircraft because of its excellent adhesiveproperties, resistance to humidity and characteristic electrical properties.Researchers want to blend the organic compound of silicone with polyimidematerials to improve the resistant to the atomic oxygen. But the system wasseparated into each other because of their different properties. Later researchersdesigned a kind of polyimide-poly(dimethylsiloxane)(PDMS) block copolymer.They found that the PDMS was prone to migrate to the surface because of its lowsurface energy in the system. However, these materials separated into each otherwhen used on aircraft.In order to solve this problem, a novel kind of PI composite film containingdifferent content on each surface was prepared from poly(imidesiloxane) copolymerand polyimide. First, poly(imidesiloxane) copolymer was synthesized from4,4'-Oxydianiline(ODA), α,ω-aminopropylpoly(dimethylsiloxane)(PDMS) and 3,4,3', 4'-biphenyltetracarboxylic dianhydride(s-BPDA). Then, a THF solution of thecopolyamide acid and a DMAc solution of the polyamide acid were mixedthoroughly, doctored on a glass plate and placed at R.T. for 1h,dried at 40 oC,60oCfor 3h. Then the composite film was obtained with thermal imidization. Allcharacterization results from X-ray photoelectron spectroscopy (XPS), Massspectrum (MS) and Contact angle measurements showed that the PDMS hassegregated to the air surface and predominated in it. Moreover, compared withnormal PI films, the composite film has nearly the same excellent thermal propertiesand mechanical properties.In low earth orbit (LEO) environment, the most important factor which affectsspacecraft life time is that its surface acts with atomic oxygen. So the properties ofthe composite films used on spacecraft on its orbit had been given. It proved that thecomposite films have better resistant to the atomic oxygen than PI. |
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