The pressure dependence of the low-temperature thermal properties of an amorphous polymer
Grace, Jeremy Matthew
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https://hdl.handle.net/2142/23926
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 Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
pressure dependence
low-temperature thermal properties
amorphous polymer
glasses
crystalline behavior
phenomemologial Tunneling Model
Language
en
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 Kat 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 0.3 K- 1 K regime indicate that the density of two-level systems
depends on the mass density p and the De bye temperature 8n as p/8n3. 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 fracton theory.
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