Influence of the glass transition on the secondary relaxation of an epoxy resin
R. Casalini
INFM and Department of Physics, University of Pisa, Piazza Torricelli 2, 56126 Pisa, Italy
D. Fioretto
INFM and Department of Physics, University of Perugia, via Pascoli, 06100 Perugia, Italy
A. Livi, M. Lucchesi, and P. A. Rolla
INFM and Department of Physics, University of Pisa, Piazza Torricelli 2, 56126 Pisa, Italy
Received 1 July 1996
The dynamics of diglycidyl ether of bisphenol-A DGEBA was studied by employing broadband dielectric
spectroscopy over a wide temperature range extending from below to above the glass-transition temperature.
Dielectric spectra reveal the existence of two relaxation processes: the structural relaxation, slowing down for
decreasing temperature and freezing at T
g
, and a secondary process present in both liquid and glassy phase.
Above the glass transition the temperature behavior of the structural relaxation time is properly described by a
Vogel-Fulcher-Tamman equation with the singularity at T
0
=234 K. The analysis of the variation of the
relaxation strength versus temperature of both processes clearly shows the onset of the structural relaxation at
T
on
350 K. Below this temperature the two relaxations progressively separate their time scales and change
their shapes. The occurrence of the glass-transition phenomenon is markedly evidenced by the changes of the
relaxation strength and of the low-frequency slope of the secondary relaxation. S0163-18299708830-9
INTRODUCTION
The glass transition is no longer considered a distinguish-
ing feature of a few systems but rather a physical phenom-
enon which can occur in every material provided that the
melt is cooled at sufficiently high rate. In fact, as a melt is
cooled, the relaxation processes progressively slow down
and, if the cooling rate becomes faster than the relaxation
rate, nonequilibrium structures can be eventually frozen and
a glassy solid obtained.
In recent years many efforts have been devoted to find an
unified description of the complex behavior of supercooled
liquids going through the glass transition. Much of the theo-
retical approaches, such as Adam and Gibbs’ free volume, or
more recent percolation models
1–3
and the scaling form dis-
cussed by Dixon,
4
deal with the dynamics of the structural
relaxation only, though it is experimentally well known that
secondary relaxations and splitting of the single high-
temperature process into at least two processes a slow co-
operative process and a faster secondary one is a very com-
mon scenario in both glass-forming simple liquids
5,6
and
polymeric systems.
7
Such subglass relaxations are generally
characterized by an Arrhenius temperature behavior of the
relaxation time and have been associated with localized mo-
tions of different origin.
8–11
A special interest in such phenomena was induced by the
mode coupling theory
12,13
MCT which, since its early for-
mulation, predicted the bimodal relaxation as a main feature
of the glass transition.
Moreover, a bifurcation of two relaxation processes was
experimentally found in some systems close to the critical
temperature, T
c
, of the MCT.
14
More generally, recent di-
electric investigations have shown that the decoupling of
structural and secondary relaxations is a crucial feature of the
relaxation of both glass-forming simple liquids and poly-
meric systems for temperatures higher than the glass
transition.
7,15
In this paper we analyze the dielectric behavior of struc-
tural and secondary relaxation of dyglycidyl ether of
bisphenol-A DGEBA epoxy resin, a system which shows a
secondary relaxation strong enough to be easily detected in
the whole region from the decoupling of the structural relax-
ation down to temperatures below the glass transition, in the
glassy state. Dielectric measurements were made in a wide
frequency and temperature interval and the temperature be-
havior of the whole set of dielectric parameters, i.e., relax-
ation times, relaxation strengths, and shape parameters, is
analyzed to have an insight into the splitting phenomenon
and into the characteristics of the dynamics of the system
near the glass transition.
EXPERIMENT
The epoxy resin used in this study was a commercial
sample EPON828 by Shell Co. of a low molecular weight
liquid diglycidyl ether of bisphenol-A DGEBA with an ep-
oxy equivalent weight of about 190. The complex dielectric
constant of the sample was measured over a frequency range
spanning from 10
3
to 10
10
Hz, by using two different tech-
niques. In the low-frequency interval 10
3
– 10
7
Hz the im-
pedance analyzer HP4194A was employed. The apparatus
and the measurement procedure were previously described.
16
In the high-frequency interval 10
7
– 10
10
HZ measurements
were carried out via Hewlett-Packard Network Analysers
HP8753A and HP8720C. Dielectric spectra were collected
for temperatures ranging from 163 to 363 K.
PHYSICAL REVIEW B 1 AUGUST 1997-II VOLUME 56, NUMBER 6
56 0163-1829/97/566/30166/$10.00 3016 © 1997 The American Physical Society