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|Title:||n.m.r. Studies of Supercooled Viscous Fluids and Silica Gels (Nmr)|
|Author(s):||Artaki, Iris G.|
|Department / Program:||Chemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||Pulsed N.M.R. techniques are used to study the dynamic behavior of viscous fluids ((eta) > 1 Poise) in the supercooled state. The applicability of the hydrodynamic equations to high viscosity systems is investigated at a molecular level in a series of selectively deuterated compounds of varying molecular symmetry and structure. Pressure is used as an experimental variable to extend the range of observable viscosities, and in addition to separate the effects of kinetic energy and density on the molecular interactions in the supercooled state.
It is found that the rotational-translational coupling parameter exhibits a strong density dependence at high viscosities. The nature and extent of the dependence is primarily determined by molecular structure. The results are interpreted in terms of simple molecular models based on free volume concepts. The coupling parameter is found insensitive to kinetic energy changes. It is concluded that volume rather than kinetic energy plays the decisive role in determining the rotational-translational coupling in supercooled viscous fluids.
The origin of the anomalous discontinuity in the temperature dependence of viscosity is investigated using isopropyl benzene for a model compound. The rotational and translational contributions to the rotating frame relaxation time are separated through an isotopic dilution study. It is shown that the viscosity anomaly of isopropyl benzene is associated with a change in the type of translational rather than reorientational dynamics.
The effect of pressure on the polymerization kinetics of sol-gel processes is investigated at a molecular level using high resolution ('29)Si spectroscopy. The condensation subsequent to hydrolysis of the initiating silicon-alkoxide reagent, Si(OCH(,3))(,4), is monitored as a function of elapsed time at different pressures. It is shown that high pressures have a dramatic accelerating effect on the condensation rate, but do not alter the mechanism via which the polycondensation rate, but do not alter the mechanism via which the polycondensation reaction proceeds. The extent of the condensation rate enhancement is quantitatively evaluated using kinetic principles. Transition state theory is employed to provide a detailed mechanism of the pressure induced acceleration of the gelation process.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1984.
|Date Available in IDEALS:||2014-12-15|