Polyimides of a recently developed type have an attractive combination of properties, including low solar absorptivity (manifested as low color) when cast into thin films, resistance to atomic oxygen and ultraviolet radiation, solubility in organic solvents, high glass-transition temperatures (Tgs), and high thermal stability. The focus of the development work was on polymers that can endure the space environment and that have specific combinations of properties for use on Gossamer spacecraft. Because of their unique combination of properties, these polymers are also expected to find use in a variety of other applications on Earth as well as in space. Examples of other space applications include membranes on antennas, second-surface mirrors, thermal optical coatings, and multilayer thermal insulation. For both terrestrial and space applications, these polyimides can be processed into various forms, including films, fibers, foams, threads, adhesives, and coatings.
In preparation for synthesizing a polyimide of this type, one makes [2,4-bis(3-aminophenoxy)phenyl]diphenylphosphine oxide (hereafter denoted 'the phosphine oxide diamine'), which is a novel aromatic diamine. In an experiment, this aromatic diamine was made from commercially available starting materials in a two-step process in relatively high yield. Then, in experimental syntheses of polyimides of this type, the phosphine oxide diamine was reacted with aromatic dianhydrides in polar aprotic solvents. In other experiments, copolyimides were synthesized by use of the phosphine oxide diamine in combination with other commercially available aromatic diamines. These various syntheses made it possible to tailor the polymers to reduce costs and obtain hitherto unavailable combinations of properties.
The figure illustrates the example of one synthesis, in which the phosphine oxide diamine was reacted with oxydiphthalic anhydride (ODPA) in N-methylpyrrolidinone (a polar aprotic solvent) at 20 weight percent to form a polyamide acid having an inherent viscosity of 1.01 dL/g. The polyamide acid was converted to the polyimide by reaction with a mixture of acetic anhydride and pyridine. The polyimide was isolated, dried, and dissolved in N,N-dimethylacetamide, then thin films were cast and stage-dried at temperatures up to 250 °C for one hour. A 1.6-mil (≈0.04-mm)-thick film was found to have 85-percent transparency at a wavelength of 500 nm, solar absorptivity of 0.06, thermal emissivity of 0.56, Tg of 212 °C, 5-percent loss of weight upon heating in air at a rate of 2.5 °C/minute up to 461 °C, and the following tensile properties at a temperature of 23 °C: strength of 14.7 kpsi (≈101 MPa), modulus of 410 kpsi (≈2.8 GPa), and elongation of 4.7 percent. A film subjected to atomic oxygen in a ground-based experiment that simulated 6 months in a low orbit around the Earth exhibited 81-percent transmission at 500 nm, only 1.74-percent of mass loss, and excellent retention of properties in general.
In another example, a copolyimide was made from the reaction of ODPA with 75 mole percent of the phosphine oxide diamine and 25 mole percent of 3,4'-oxydianiline. A 1-mil (≈0.025-mm)-thick film of this copolyimide was found to have 88- percent transparency at a wavelength of 500 nm, solar absorptivity of 0.06, thermal emissivity of 0.38, Tg of 218 °C, and the following tensile properties at a temperature of 23 °C: strength of 14.7 kpsi (≈101 MPa), modulus of 460 kpsi (≈3.2 GPa), and elongation of 6.3 percent.