Cross-linked silica-based aerogels with polymeric materials, as well as incorporating a flexible linkage into the underlying metal oxide, have been proven to improve strength and resilience over their native, or non-cross-linked, counterparts without adversely affecting porosity and density. In this invention, high-temperature, stable, all-organic polyimide aerogels are prepared as reacting linear polyimide chains with a functional monomer to create branchings that are further room-temperature-cured with multifunctional isocyanate to form a three-dimensional network.
Silica aerogels are highly porous materials that consist of mostly air. They are potential candidates for many aerospace applications including insulation for spacesuits and multipurpose structures. However, their use is restricted due to their inherent fragility and environmental instability. Efforts in conformal coating of the skeletal structures of the aerogels with a different polymer, as well as incorporating flexible linkage into the underlying structures, have shown improvements in mechanical strength and recovery without adverse effect on density, % porosity, shrinkage, and surface area. However, it is most desirable to have a flexible, foldable, high-temperature material for various applications including inflatable re-entry vehicles.
Previous studies have proven that organic polyimide aerogels could be prepared from a linear polymer in which the three-dimensional network is formed by intermolecular physical bonding. However, it is anticipated that these thermoplastic polymer gels could collapse under higher-temperature conditions or in the presence of certain solvents, losing the pore structure.
To meet the requirements for hightemperature, flexible, and foldable materials, branched polyimides with amine endcaps are further reacted with multifunctional isocyanate and cured at room temperature to generate a 3D, covalently bonded network with flexible urea linkages. With the addition of a multifunctional isocyanate, mechanical properties of these highly cross-linked materials also improved. Addition of different organic clays also shows an increase in mechanical properties, as well as an increase in viscosity, enhancing their ability in casting thin films. The cross-linked polyimide aerogels are 500 times stronger than traditional silica aerogels, offer 2 to 10 times improved performance over polymer foams in ambient conditions, and offer up to 30 times improved performance in vacuum conditions. They are moisture-resistant, do not shed dust particles, and are heat-resistant to 200 – 300 ºC. They can be formed into whatever configuration is required, providing an advantage over aerogels that exist in block form and must be modified or chemically altered to function as a formfitting insulation.
The unique feature of this innovation is to further react branched polyimides with a multifunctional isocyanate to provide a strong, flexible, and foldable polyimide/urea thin film or a strong, resilient monolith via urea linkage while providing higher cross-linking units throughout the three-dimensional network. In addition to tougher, more resilient, and flexible material, the final product can be obtained at room temperature without further reaction.