Polyurea aerogels: From nanoscopic to macroscopic properties

The morphology of a material is intrinsically a qualitative property and in order to relate nanomorphology to synthetic conditions, it is necessary to express nano/micro-structure quantitatively. In this context, polyurea aerogels were chosen as a model system with demonstrated potential for rich nanomorphology and being guided by a statistical Design-of-Experiments model, a large array of materials (208) with identical chemical composition, but quite different nanostructures were prepared. By reflecting upon the SEM images, it was realized that our first pre-verbal impression about a nanostructure is related to its openness and texture; the former is quantified by porosity (Π), and the latter is related to the contact angle (θ) of water droplets resting on the material. Herewith, the θ/Π ratio is referred to as the K-index, and it was noticed that all polyurea aerogel samples could be put in eight K-index groups with separate nanomorphologies. The K-index was validated as a morphology predictor by compressing samples to different strains: as porosity decreases, contact angle decreases proportionally, and the K-index remains constant. The predictive power of the K-index was demonstrated with new PUAs prepared in eight binary solvents. Finally, using response surface methodology, K-indexes and other material properties of interest were correlated to synthetic conditions, thus enabling synthesis of materials with prescribed properties at a time. The second part of this dissertation focuses on polyurea aerogels consisting of different arrangements of nanoparticles (1.2 ≤ K-index ≤ 1.5). SAXS, XRD and SEM have demonstrated that these nanostructures consist of similar-size primary particles. A model for the formation of these nanoparticles through Molecular Dynamic simulations is suggested --Abstract, page iv