Metal diolates as precursors towards hierarchically organized nanostructured materials - Novel synthesis protocols combined with in-situ-SAXS characterization techniques

Aiming at a better understanding of the formation process of nanostructured materials, we will investigate phase separation phenomena in sol-gel systems on the nano- and micrometer length scale via in-situ small angle X-ray scattering. Highly porous oxides, for example silica, titania, alumina, etc. or mixed silica-metal oxide materials prepared via a novel synthesis route applying metal diolates and comprising a porous network with different levels of hierarchy in the pore sizes will serve as model systems. It is envisioned not only to expand the range of oxidic materials from silica to other transition metal oxides, but also to induce a certain degree of anisotropy into the pore domains, e.g. by mechanical shearing of the sol. Thus, the proposed research comprises three inter-related elements from chemistry and physics: 1.) deliberate network design by tailor-made precursor molecules; 2.) innovative fabrication methods and 3.) detailed characterization of the network development emphazising on in-situ (small angle) X-ray scattering and diffraction. From a chemical point of view, we will develop novel synthetic routes towards highly porous, hierarchically organized transition metal oxide monoliths by applying metal diolates. Metal diolates have the advantage of being processable in purely aqueous solution and thus being compatible with lyotropic liquid crystal phases that will serve as structure-directing agents in the synthesis. As starting compounds the respective alkoxides, e.g. for titania, titanium tetraisopropoxide will be glycolated to give bis(2- hydroxyethyl)titanate as a stable and acid-soluble precursor. This precursor will be processed in the presence of a preformed aqueous lyotropic liquid crystalline phase to yield the oxide with a deliberately designed pore structure. We will demonstrate the high potential of this synthetic approach towards materials with a multimodal pore size distribution by the application of analogous (mixed) metal precursors. From a physical point of view, one focus lies on the structural investigation of the final materials obtained from the glycolated precursor molecules. In addition, previous experiments have shown that contrary to what was expected the lyotropic liquid crystalline phase is not directly templated, but that reorganisation processes occur upon addition of the glycolated precursor. The final network structure is strongly influenced by the mechanism of formation. Therefore, the second focus is on in-situ SAXS measurements, following the structural evolution in the mixture from the sol to the final gel to allow for a deeper understanding of the underlying phase separation processes. Since external parameters such as stirring speed, centrifugation, temperature, etc. have a strong influence on the pore structure, additional measurements are planned comprising shear-induced alignment of the pore system and measurements under different external experimental parameters.

The objective of the project was to develop and design novel multifunctional materials for a wide field of application in our daily lives, e.g. as filters, catalysts, optical sensors or as self-healing materials, and to characterize their nanostructure in order to improve their chemical and physical properties. For improving the macroscopic properties of the materials, a deeper insight on effects on the mesoscale (2 to 50 nanometer) is crucial. The focus of the project was laid on sol-gel derived mixed-metal oxides: The main challenge in the preparation of mixed-oxides is to overcome the problem of phase-separation into two metal-oxide rich domains due to the different reaction rates of the individual metallic compounds. One of the most effective methods in preparing mixed-metal oxides is the application of so-called single-source precursor molecules, where the metallic centres are already linked via an organic spacer. Within the frame of the project, three different mixed-metal oxide systems have been developed and extensively investigated with respect to structural and functional properties and the results have been published in peer reviewed journals.The main experimental technique used in this project to characterize the sol-gel materials with respect to structure and structural development is small angle X-ray scattering (SAXS). SAXS is a non-destructive and an accurate method which does not only allow following the structural evolution during the synthesis and post-treatment, but also determining parameters and time scales guiding the structure formation and/or transformation process. Ex-situ SAXS experiments for the final materials were combined with temperature dependent in-situ experiments during processing, following the structural evolution during synthesis. The knowledge of the rise and time scales of specific structural processes enables the controlled synthesis and optimization of the sol-gel materialsWe extended the investigation of mixed-metal oxides to complex polymeric materials: Polymeric ionic liquids (POILs) could serve as self-healing polymers in materials engineering. When a polymeric material is exposed to an external force, structural damage might occur, possibly leading to material failure. One of the major challenges of macromolecular chemistry is the development of an autonomic process, which enables self-healing of materials, prior to failure. Optimizing the self-healing process requires deep understanding of the ongoing chemical processes within molten/solid polymers and their reaction kinetics. During the project, in-situ SAXS is used as the main investigation technique to probe the structure, the kinetics and the reversibility of phase transformations of materials. By this, polymeric ionic liquids can be characterized with respect to their ability to act as self-healing agents.