There is a need in x-ray astronomy for an oxygen-less polymeric filter. This is because the mass absorption coefficient of oxygen is greater than that of carbon at every energy in the soft x-ray region except for a narrow band between the oxygen k-edge at 543.1 eV and the carbon k-edge at 284.2 eV. First generation polymeric filter materials included polypropylene, poly(p-xylylene), and the polycarbonates. Polycarbonates were stronger than their contemporary materials but had the disadvantage of containing nearly 20% by weight oxygen. As second generation materials, polyimides are significantly stronger than polycarbonates, but by chemical definition they too contain oxygen. The ideal filter material for use outside the band would be an oxygen-less version of a polyimide. These polymers exist, and among the proposed "third generation" materials are the polybenzimidazoles and the polyphenylquinoxalines. In a preliminary study, the soft x-ray transmission of a 2500 A polybenzimidazole film was measured at the Stanford Synchrotron Radiation Laboratory. The polybenzimidazole was 10 to 20% more transmissive than the polyimide DuPont 2610D and the polycarbonate Lexan. Another benefit of this invention is the ease with which polybenzimidazole films release from the substrates onto which they were deposited. Polybenzimidazole is proposed as a release agent for any x-filter material.A commercial application of the innovation includes current or future spacecraft flights involving extreme ultraviolet or soft x- ray optics and/or instrumentation. Such a filter would have been appropriate on any of the past missions using Lexan, such as AXAF- S, the Extreme Ultraviolet Explorer, Solar A, or ROSAT. The ease of fabrication of a polybenimidazole filter will lower its cost relative to a polyimide-based filter. Oxygen containing analogs of the polybenzimidazoles (such as the polybenzoxazoles) are appropriate for the so-called "water window" of an x-ray microscope. Additional applications are pressure windows at synchrotron facilities and gas separation membranes.
Keywords: Phase_I, NASA, Abstract, FY94
