UC Davis

A detailed knowledge of the atomic structure of glasses is of enormous significance in understanding the structure-property relationships in these materials. On the other hand, the dynamical behavior of their parent supercooled liquids is intimately linked to the materials’ viability for all stages of industrial glass production. However, the atomistic mechanism of viscous flow and the structural control on the viscoelastic behavior and the associated relaxation phenomena in supercooled inorganic glass-forming liquids remain poorly understood to date. In addition, the structure and properties of a liquid can be quite different as the liquid volume reaches a length scale of several nanometers in confined geometries. These nanoconfined liquids display a plethora of intriguing behavior including the well-known melting point depression and the nonuniversal depression/elevation of the glass transition temperature. Despite extensive experimental and simulation studies on nanoconfined fluids, the atomistic mechanisms responsible for such behavior remain controversial and largely unresolved.This dissertation presents the studies of the viscoelastic behavior of chalcogenide and phosphate glass-forming liquids investigated using a combination of small-amplitude oscillatory and steady shear parallel-plate rheometry and high-temperature nuclear magnetic resonance (NMR) spectroscopy in order to probe the corresponding structural relaxation processes associated with their viscous flow in the bulk state. The rheological spectra of these liquids reveal the presence of a slow and a fast relaxation process in these liquids, which could be attributed to the bond scission/renewal, and the segmental chain motion dynamics, respectively. The existence of the latter process was directly observed in short-chain phosphate liquids by high-temperature 31P dynamical NMR spectroscopy. The network connectivity increases in As-Se and Ge-Se glass-forming liquids upon progressive addition of As/Ge to Se, which is manifested in a floppy-to-rigid transition of the dynamics. On the other hand, the phosphate network becomes increasingly cross-linked as either P2O5 is added to a metaphosphate composition or a high field-strength modifier cation replaces a low field strength cation in the structure of a metaphosphate composition. The attendant compositional evolution of the conformational entropy of Se chain segments in chalcogenides or phosphate chain segments in Na-Zn metaphosphates is shown to be related to the corresponding variation in their fragility indices. The results of calorimetric and 2H NMR spectroscopic studies of the thermodynamic and kinetic transitions of nanoconfined liquids are also reported. Nanoconfined supercooled water displays a strong dynamical heterogeneity with the coexistence of both mobile liquid-like and immobile solid-like water molecules at temperatures lower than the bulk melting point, consistent with a layer-by-layer solidification process. In the extreme case of confined media with bimodal pore structures, the spatial heterogeneity in the dynamics of both water and glass-forming supercooled ortho-terphenyl is preserved over a length scale of several angstroms. Collectively, these results suggest a mechanistic relationship between the structural and dynamical heterogeneities in nanoconfined fluids.