06-11-2012, 03:59 PM
Comparing dielectric measurements on poly(ethylene terephthalate) at constant heating rates with isothermal measurements
[attachment=38322]
Abstract
In this study, dielectric relaxation spectroscopy experiments were carried out during heating in the a relaxation region of PET. The results
for low heating rates were consistent with isothermal measurements. Moreover, the results expressed as a function of temperature were much
clearer than those expressed as a function of frequency. However, the loss peaks obtained in these non-isothermal experiments showed a
systematic shift on the temperature axis when the heating rate increased. A significant but nearly constant thermal lag was observed in
experiments with heating at 1 K min21 when using different equipment. It appears that isothermal data obtained in a well calibrated apparatus
can be used to adjust the temperature axis of dielectric results obtained in ramp temperature experiments. q1999 Elsevier Science Ltd. All
rights reserved.
Introduction
Dielectric relaxation spectroscopy (or dielectric thermal
analysis—DETA) has been widely used to investigate the
segmental mobility of the chains and the phase transitions in
polymers [1, 2]. This technique also provides some relevant
dielectric/electrical information that may be important in
specific applications.
In thiscase, the data manipulation and the theoretical interpretation
of the results is relatively simple. However, similar experiments
can be performed in a heating mode run experiment at
a specific frequency, measuring the complex permittivity as
a function of temperature (1*(T )). In this way, characterization
of the material is less time consuming, which is an
advantage. Moreover, in some cases the various relaxation
regions detected in a polymer may become more evident
when the results are expressed as a function of the temperature,
relative to those obtained in isothermal conditions and
expressed as a function of frequency.
PL equipment
The Polymer Laboratories DETA apparatus consists of a
General Radio 1693 digital RLC bridge, a dielectric spectrometer
head, a temperature controller unit (Model 706) and a
controlling computer with PL software. The two components
of permittivity (1
0 and 1
00) are measurable as purely
capacitative and purely conductive terms on the DETA
bridge.
The dielectric spectrometer cell consists of a lower base
unit, with provision for mounting the sample, and a
demountable temperature enclosure (internal dimensions:
6 cm diameter and 6 cm high). The sample, without any
surface treatment, was kept between two condenser plates
(33 mm diameter). The temperature is measured by a small
platinum resistance thermocouple placed at approx. 5 mm
below the bottom electrode.
Conclusions
It is apparent that dynamic dielectric studies carried out
during heating are a useful tool to study systems whose
relaxations are not well defined on the frequency axis. In
the case of PET and for a specific dielectric equipment it
was demonstrated that this kind of measurement led to a
good agreement with isothermal experiments, with the
advantage of less time-consuming testing. However, some
precautions must be taken, namely, the dT/dt gradient must
be relatively low ( , 2 K min21). When the heating rate is
raised a systematic shift of the a relaxation of PET to lower
temperatures was observed.
[attachment=38322]
Abstract
In this study, dielectric relaxation spectroscopy experiments were carried out during heating in the a relaxation region of PET. The results
for low heating rates were consistent with isothermal measurements. Moreover, the results expressed as a function of temperature were much
clearer than those expressed as a function of frequency. However, the loss peaks obtained in these non-isothermal experiments showed a
systematic shift on the temperature axis when the heating rate increased. A significant but nearly constant thermal lag was observed in
experiments with heating at 1 K min21 when using different equipment. It appears that isothermal data obtained in a well calibrated apparatus
can be used to adjust the temperature axis of dielectric results obtained in ramp temperature experiments. q1999 Elsevier Science Ltd. All
rights reserved.
Introduction
Dielectric relaxation spectroscopy (or dielectric thermal
analysis—DETA) has been widely used to investigate the
segmental mobility of the chains and the phase transitions in
polymers [1, 2]. This technique also provides some relevant
dielectric/electrical information that may be important in
specific applications.
In thiscase, the data manipulation and the theoretical interpretation
of the results is relatively simple. However, similar experiments
can be performed in a heating mode run experiment at
a specific frequency, measuring the complex permittivity as
a function of temperature (1*(T )). In this way, characterization
of the material is less time consuming, which is an
advantage. Moreover, in some cases the various relaxation
regions detected in a polymer may become more evident
when the results are expressed as a function of the temperature,
relative to those obtained in isothermal conditions and
expressed as a function of frequency.
PL equipment
The Polymer Laboratories DETA apparatus consists of a
General Radio 1693 digital RLC bridge, a dielectric spectrometer
head, a temperature controller unit (Model 706) and a
controlling computer with PL software. The two components
of permittivity (1
0 and 1
00) are measurable as purely
capacitative and purely conductive terms on the DETA
bridge.
The dielectric spectrometer cell consists of a lower base
unit, with provision for mounting the sample, and a
demountable temperature enclosure (internal dimensions:
6 cm diameter and 6 cm high). The sample, without any
surface treatment, was kept between two condenser plates
(33 mm diameter). The temperature is measured by a small
platinum resistance thermocouple placed at approx. 5 mm
below the bottom electrode.
Conclusions
It is apparent that dynamic dielectric studies carried out
during heating are a useful tool to study systems whose
relaxations are not well defined on the frequency axis. In
the case of PET and for a specific dielectric equipment it
was demonstrated that this kind of measurement led to a
good agreement with isothermal experiments, with the
advantage of less time-consuming testing. However, some
precautions must be taken, namely, the dT/dt gradient must
be relatively low ( , 2 K min21). When the heating rate is
raised a systematic shift of the a relaxation of PET to lower
temperatures was observed.