Space materials polyimides

Excellent, isotropic electrical properties. Dielectric constants of 2.4 to 2.6 have been demonstrated. Low dielectric constant is crucial as interconnect density increases. As space between conducting lines shrinks, inductance and cross-talk become problematic, but can be mitigated with lower dielectric constant materials. Polyimides, which are used extensively in the industry, exhibit anisotropic electrical properties in-plane dielectric constant can be as high as 4 while out of plane dielectric constant is generally above 3. 

Because of the high functional values that polyimides can provide, a small-scale custom synthesis by users or toU producers is often economically viable despite high cost, especially for aerospace and microelectronic appHcations. For the majority of iudustrial appHcations, the yellow color generally associated with polyimides is quite acceptable. However, transparency or low absorbance is an essential requirement iu some appHcations such as multilayer thermal iusulation blankets for satellites and protective coatings for solar cells and other space components (93). For iutedayer dielectric appHcations iu semiconductor devices, polyimides having low and controlled thermal expansion coefficients are required to match those of substrate materials such as metals, ceramics, and semiconductors usediu those devices (94). 

For many years prior to the development of high-temperature thermoplastics and thermosets, such as the polyimides. polysulfones, and epoxies, phenolic molding material dominated the high temperature-resistant market. This emphasizes their ability to resist temperature degradation in the 400-500°F (204-260 C) range. Because phenolics were found to possess excellent ablative properties, it has been reported that both the American and Soviet space efforts used them in combination with certain other... 

M. Tagawa, M. Matsushita, M. Umeno and N. Ohinae, Laboratory Studies of Atomic-Oxygen Reactions on Spin-Coated Polyimide Films, Proceedings of the 6th International Symposium on Materials in a Space Environment, Noordwijk, The Netherlands, 19-23 September 1994, ESA SpeciaJ Pubhca-tion 368, Ed. T. D. Guyenne (European Space Agency, Paris, Prance, 1994) p. 189. 

The incorporation of POSS nanostructures into polyimides has been shown to significantly extend the lifetime of these materials in LEO. Studies on the effect of a hyperthermal O-atom beam on POSS-PIs have shown the improved oxidation resistance imparted to polyimides by the addition of POSS. XPS data of both the AO-exposed and space-flown POSS-PI materials indicated that the improved oxidation resistance of these materials is due to a rapidly formed silica layer upon exposure of POSS-polymers to high incident fluxes of atomic oxygen. 

In addition to the dibenzodioxin reaction, the more classical polymerization reaction of imide formation has been used to form PIMs. Weber and co-workers [117, 118] firstly reported the synthesis of soluble poly(amide) and poly(imide) using monomers derived from 9,9 -spirobifluorene. Such a spirobifluorene generates a 90° kink per repeating imit and thus prevoits space-efficient chain packing, resulting in microporous materials with high surface areas. For example, by reacting 2,2 -diamino-9,9 -spirobifluorene (Fig. 5.15, Al) with pyromellitic dianhydride (PMDA) (Fig. 5.15, Bl), a soluble polymer with a BET surface area of 551 m g was obtained [118]. Binaphthalene-based polyimide was synthesized by Ritter et al. 

Matrix Materials Polymers used for matrix materials are usually thermosets, with epoxies being the most common (35). Others are polyesters and phenolics. Sometimes thermoplastics are used, principally polyimides, polysulfones, and polyetheretherketone (PEEK). The matrix has several roles in polymer-fiber composites. These include maintaining desired fiber spacing, transmitting shear loads between layers of fiber, and reducing the tendency to transmit stress concentration from broken fibers to intact fibers. 

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