Speaker
Jeppe Dyre
(Roskilde University)
Description
The talk first gives an overview of “physical aging”, the
highly non-linear very slow changes of material properties
observed, e.g., just below the glass transition. During the
last 50 years material scientists have obtained a good
understanding of physical aging, and a useful
phenomenological theory for describing aging exists, the
1971 Narayanaswamy theory. It is based on the physically
appealing concept of a so-called material time, which may be
thought of as the time measured on a clock with rate that
itself ages. The paper goes on to present data for the
physical aging of organic glasses just below the glass
transition probed by monitoring the following quantities
after temperature up and down jumps: the shear-mechanical
resonance frequency (∼360 kHz), the dielectric loss at 1 Hz,
the real part of the dielectric constant at 10 kHz, and the
loss-peak frequency of the dielectric beta process (∼10
kHz). The setup used allows for keeping temperature constant
within 100 μK and for thermal equilibration within a few
seconds after a temperature jump (both of which are
unprecedented). The data conform to a new simplified version
of the classical Tool-Narayanaswamy aging formalism, which
makes it possible to calculate one relaxation curve directly
from another without any fitting to analytical functions. If
time permits, we derive the new equations.
