7 cf ) Impulsive Stimulated Thermal Scattering of Glass - Forming Liquids

Impulsive Stimulated Thermal Scattering is a time domain laser spectroscopy that is applied to studying the relaxation dynamics of liquids in the supercooled temperature regime. Characterizing the relaxation dynamics of supercooled liquids is essential to developing an understanding of the physics of the liquid-glass transition. Mode Coupling Theory is a popular theory that has had some success in explaining experimental observations of the dynamics of certain supercooled liquids. Glass forming liquids are categorized according to fragility. To date, many of the experimental and theoretical studies of glass forming liquids have focused on liquids with fragile characteristics. To extend the field of research, ISTS studies are conducted on glycerol, a glass-forming liquid of intermediate strength. Other experimental studies of glycerol have given conflicting results, and the applicability of Mode Coupling Theory is questioned. ISTS is added to the other experimental techniques to measure the DebyeWaller factor and relaxation times in the regime from a few nanoseconds to hundreds of microseconds. There is no MCT anomaly in the temperature range measured. Also, the shape of the relaxation spectrum is constant, without the enhanced stretching commonly observed at the crossover temperature Tc. ISTS is also applied to characterization of the dynamics of a glass-forming polymer, Polypropylene Glycol. Like many polymers, Polypropylene Glycol shows two features in the slow relaxation portion of the dielectric spectrum. In ISTS, only the feature which is strongly coupled to the density is observed, at least quantitatively. A comparison of the results for two molecular weights shows that the relaxation observed in ISTS is independent of molecular weight, and therefore related to the main a peak in the dielectric spectrum, which is also independent of molecular weight. Some temperature dependence is observed in the Debye-Waller factor and the stretching of the relaxation spectrum. There is no Debye-Waller factor anomaly as predicted by MCT in the temperature range measured. The conclusion reached from ISTS experiments on non-fragile glass forming liquids is that, despite the success MCT has enjoyed in predicting the behavior of relatively simple systems, there is not sufficient detail incorporated in the theory to address the complexity of networked materials (glycerol) and polymers. Thesis Supervisor: Keith A. Nelson Title: Professor of Chemistry