The Pressure Dependence of the Low Temperature Thermal Properties of an Amorphous Polymer
Grace, Jeremy Matthew
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https://hdl.handle.net/2142/77426
Description
Title
The Pressure Dependence of the Low Temperature Thermal Properties of an Amorphous Polymer
Author(s)
Grace, Jeremy Matthew
Issue Date
1988
Doctoral Committee Chair(s)
Anderson, A.C.
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
Abstract
The low-temperature behavior of glasses has been described as anomalous for its marked differences from crystalline behavior and as universal for its apparently weak dependence on chemical composition of the glass. For temperatures below 1 K the phenomemological Tunneling Model and the existence of phonons can account for the observed behavior, which is indicative of localized two-level excitations. For temperatures in the range 1 K-10 K there is no widely accepted model to explain the existence of an additional set of vibrational modes or the apparent sharp decrease in phonon mean free path. Some microscopic models for specific glasses imply that the 1 K-10 K regime and the sub-Kelvin regime are related. There is also a controversial theory that ascribes the behavior above 1 K to a universal phonon-fracton crossover in glasses. In order to gain some understanding of the nature of the two-level excitations and the excitations in the 1 K-10 K temperature range, the low-temperature thermal properties of Scotchcast-8 epoxy, an amorphous polymer, were used to probe glassy behavior as a function of pressure. The thermal measurements were performed over the range 0.3 K-10 K at pressures up to roughly 4 kbar. The low-temperature specific heat was observed to drop with pressure. The percent changes were rather uniform over the entire temperature range. For the thermal conductivity, the measurements reveal increased conductivity with pressure for temperatures above 1 K and indicate decreases with pressure for temperatures below 0.3K. From these measurements it is found that the energy density of two-level excitations decreases with pressure, while the coupling of these excitations to phonons increases. The measured changes in the 0.3K-1 K regime indicate that the density of two-level systems depends on the mass density $\rho$ and the Debye temperature $\Theta\sb{\rm D}$ as $\rho$/$\Theta\sb{\rm D}\sp3$. The magnitude of the pressure-induced changes from 1 K-10 K suggests that the excess excitations and strong phonon scattering in this regime are most likely not related to structural length scales in the glass. Finally, the similar changes with pressure over the entire temperature range suggest that all the excitations, namely phonons, two-level systems, and the additional modes above 1 K, are related. These results are discussed with regard to the Tunneling Model, two microscopic models, and the controversial fraction theory.
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