Diamines represent one of the most powerful and versatile class of monomers used in the commercial polymer business. Diamines are widely used in preparing polyimides, polyamides, and virtually all epoxy resins. The use of aromatic diamines is fundamental for preparation of high performance and temperature resistant polymeric materials. For polyimides perhaps one of the best known materials is Kapton® and this material has found widespread use in space applications. For polyamides, Kevlar® represents an aromatic polyamide that has found widespread use in fire-retardant clothing, armor, and many other applications. Thus, aromatic diamines are of great importance for creating thermally-stable/high-performance polymeric materials.
Although aromatic polymers can exhibit resistance to thermo-oxidative processes, it has been demonstrated that in the presence of atomic oxygen (AO) at high kinetic energies that virtually any and all carbon-carbon bonds are destroyed. Thus, Kapton® has been found to rapidly decompose when exposed to AO, both in a simulated environment and in actual low Earth orbit (LEO) testing. While thin coatings of metal oxides and other inorganic materials provide protection from AO erosion, they suffer from defects that occurring during their application along with cracking and failure due to a mismatch in coefficients of thermal expansion (CTE), impact, minor abrasions, or other typical wear and use scenarios. Once a crack appears, undercutting and acute failure of the coating begins leading to catastrophic polymer damage and then material failure.
Oligomeric silsesquioxanes (OS) can be incorporated into a polymer matrix to improve the resistance of the polymer to degradation by AO. In particular, polyhedral oligomeric silsesquioxanes (POSS™) added within a polymer matrix have been shown to greatly enhance resistance of the polymer to attack by AO. There are examples of OS moieties being added/incorporated into high molecular weight and thermally stable polymer materials; however, there are no previous reports where erosion by exposure to AO was completely stopped as in the current invention.
Wright et al. first disclosed the synthesis of a POSS-diamine and its use in preparing polyimide oligomers in the article titled “The Synthesis and Thermal Curing of Aryl-Ethynyl Terminated coPOSS Imide Oligomers: New Inorganic/Organic Hybrid Resins” (Chemistry of Materials 2003, 15(1), pp 264-268). In latter work, Tomzcak et al. used that same POSS diamine to prepare related polyimides that were tested in LEO for durability against AO. Synthesis of the diamine-POSS structure is a multistep process and requires expensive reagents. Furthermore, at high POSS loadings phase separation occurred in the polyimide and unidentified “solids” appeared in the polymer films.
Svejda et al. in U.S. Pat. No. 6,767,930 describes general methods for incorporating POSS structures into polymer matrices using both reactive methods and guest-host approaches. In this patent the authors demonstrate the value of OS in slowing polymer degradation when exposed to AO.
Lichtenhan et al. in U.S. Pat. No. 6,933,345 put forth several scenarios for POSS incorporation into polymeric materials, both as guest-host and as part of a polymer structure. Specific examples and experimental conditions are not given as well as polymer properties (expected or measured). No methods are given for synthesizing an aromatic OS-diamine or POSS-diamine.
In work by Leu et al., “Synthesis and Dielectric Properties of Polyimide-Tethered Polyhedral Oligomeric Silsesquioxanes (POSS) Nanocomposites via POSS-diamine” (Macromolecules 2003, 36, pp 9122-9127) they describe making POSS-polyimides and at relatively low levels of incorporation phase separation (self-assembly) occurred in the polyimide leading to a significant drop in glass transition, increase in coefficient of expansion, and a significant decrease in the material strength.
Leu et al. also describe an approach in “Polyimide-Side-Chain Tethered Polyhedral Oligomeric Silsesquioxane Nanocomposites for Low Dielectric Film Applications” Chemistry of Materials 2003, 15, pp 3721-3727) by which the POSS structure is incorporated by modifying a fluorinated-polyimide backbone. This requires the polyimide be soluble in organic solvents so lower molecular weight (Mn of 15,000) materials must be utilized. This work was also reported in US patent application 2006/0122350 by Wei et al.
Wright et al. documented new methodology for tethering octahedral silsesquioxanes to soluble polyimides in the article “Chemical Modification of Fluorinated Polyimides: New Thermally Curing Hybrid Polymers with POSS” (Macromolecules 2006, 39, pp 4710-4718). In this paper the authors demonstrate very high loadings of POSS to low molecular weight polyimides and demonstrate that in lab testing films show resistance to erosion by AO. The work requires soluble polyimides, multiple separations of polymer/reactants, and utilizes a specialty monomer that is not commercially available.
Poe and Farmer in U.S. Pat. No. 7,619,042 (2009) describe a new method for modifying a soluble polyimide material of low molecular weight similar to that described by Wright et al. In the patent they provide a single example of a fluorinated low molecular weight polymer with no characterization of the OS-containing polymeric materials with regard to purity, physical, mechanical, and/or AO resistance.
It is to be understood that the foregoing is exemplary and explanatory only and are not to be viewed as being restrictive of the invention, as claimed. Further advantages of this invention will be apparent after a review of the following detailed description of the disclosed embodiments, which are illustrated schematically in the accompanying drawings and in the appended claims.