Isocyanate-derived organic aerogels: polyurethanes, polyureas, polyamides and polyimides
"Aerogels are 3D assemblies of nanoparticles with high open porosity and high surface area, and are pursued for their low density, low thermal conductivity, low dielectric constant and high acoustic attenuation. The foundation for those exceptional properties is their complex hierarchical solid framework (agglomerates of porous, fractal secondary nanoparticles). On the down side, however, aerogels are also fragile materials. The mechanical strength of silica aerogels has been improved by crosslinking the framework with organic polymers. The crosslinking polymer has been assumed to form a conformal coating on the surface of the skeletal framework bridging covalently the elementary building blocks. However, the drawback of this method is the lengthy post-gelation crosslinking process. Since the exceptional mechanical properties of polymer crosslinked aerogels are dominated by the crosslinking polymer, it was reasoned that purely organic aerogels with the same nanostructure and interparticle connectivity should behave similarly. That was explored and confirmed by organic aerogels derived from multifunctional isocyanates through reaction with (a) alcohols (polyurethanes); (b) water (polyureas); (c) carboxylic acids (polyamides); and, (d) acid anhydrides (polyimides). All processes are invariably single-step, one-pot and take place at room temperature or slightly elevated temperatures. The resulting materials are robust, they have very wide range of densities and their nanomorphologies vary from fibrous to particulate or both. By relating the molecular functional group density with the functional group density on the nanoparticle surfaces, this study established that in order for three-dimensional (3D) assemblies of nanoparticles to form rigid nanoporous frameworks, they have first and foremost to be able to develop strong covalent bonding with one another. Thus, all macroscopic properties of an aerogel depend on the surface functionality of the 'growing colloidal particle'. Those findings are relevant to the rational design of 3D nanostructured matter, not limited to organic aerogels. The materials synthesized in this study should have a broad range of applications from flexible thermal and acoustic insulations to ballistic protection"--Abstract, page v.
Mechanically strong aerogels, Microporous carbons, Petal effect superhydrophobic porous materials, Sol-gel chemistry, Structure-property relationships, Chemistry
Missouri University of Science and Technology
Networked Digital Library of Theses and Dissertations