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Dielectric depolarization spectra of toughened polyvinylchloride.

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Die Angewandte Makromolekulare Chemie 35 (1974) 147-157 ( N r . 513)
From the Research Institute for Plastics, Budapest, Hungary
Dielectric Depolarization Spectra of Toughened
Polyvinylchloride
By PETERHEDVIGand ENIKOFOLDES
(Received 6 July 1973)
SUMMARY:
Toughened PVC systems prepared on the basis of Hostalit H-9970 (Hoechst AG)
were studied by dielectric depolarization spectroscopy. The spectra obtained as a function
of the temperature were compared with dynamic mechanical and dielectric relaxation
spectra and with thermomechanical curves. The dielectric depolarization peaks were
found to correlate with the structural transitions measured by mechanical and dielectric
spectroscopy.
ZUSAMMENFASSUNG:
Auf der Basis von Hostalit H-9970 (Hoechst AG) hergestellte schlagfeste Polymeren-Systeme wurden durch dielektrische Depolarisationsspektroskopie untersucht. Die in Abhangigkeit von der Temperatur aufgenommenen Depolarisationsspektren wurden mit den
dynamisch mechanischen und dielektrischen Relaxationsspektren und mit thermomechanischen Kurven verglichen. Ein enger Zusammenhang wurde zwischen den dielektrischen Depolarisationsmaxima und den durch die mechanische und dielektrische Spektroskopie gemessenen Strukturumwandlungen beobachtet.
lntroduction
The dielectric depolarization method involves measurement of shortcircuit
currents in the absence of external voltage as a function of the temperature
in a previously polarized sample. It is also referred to as thermally stimulated
current or thermocurrent method. For reviews see van TURNHOUT’and
HEDVIG~.
Permanent polarization in a polymeric sample can be induced by storing
it in an electrical field above a structural transition temperature. The sample
is then cooled down in the presence of the field and heated up without
external field to record the current. In such experiments characteristic current
peaks are observed arising from two different sources:
1. Release of trapped monocharges (electrons, ions). The resulting current
will be referred to here as t h e r m o s t i m u l a t e d c u r r e n t .
147
P. HEDVIGand E. FOLDES
2. Change in the effective dipole concentration or dipole orientation. The
resulting current will be referred to here as d e p o l a r i z a t i o n current.
It has been shown earlier3 that in polymers containing polar groups the
current peaks observed during heating the sample after polarization correlate
well with the structural transition temperatures (dispersion regions) measured
by dynamic mechanical or by dielectric relaxation methods. In such systems
depolarization currents are usually much higher than thermostimulated currents. Thermostimulated currents can be measured separately from depolarization currents by irradiating the sample with ionizing radiation at low temperat u r e ~ After
~ . heating up the recorded current peaks do not correlate with
the structural transitions, they arise from the release of charge carriers from
traps.
The fact that depolarization current peaks correlate with structural transitions is used in the present work to study toughened PVC systems. The
depolarization spectra are compared with dielectric and dynamic mechanical
relaxation spectra and with thermomechanical curves.
Experimental
Toughened PVC systems were prepared on the basis of Hostalit H-9970 (Hoechst
AG) with Ongrovil S-165 (Chemical Works, Borsod, Hungary) PVC. The blends were
prepared by rolling between 150°C and 160°C for 10-15 min. The samples were
shaped afterwards by pressing at 150"C. For identification of the depolarization peaks
samples were prepared from the Hostalit compound by extraction with toluene in order
to separate the rubber phase. The polymer was precipitated from the solution with
ethanol. From the extracted polymer samples were pressed at 50°C.
The block scheme of the equipment used for measuring depolarization spectra is
shown in Fig. 1. The sample is equipped with silver-paint electrodes and placed into
a copper block the temperature of which can be programmed. The polarization voltage
is 1-3 kV supplied by a stabilized source. Current is measured by a vibrating capacitor
electrometer and is recorded by an XY recorder. The usual procedure of the measurement
is the following:
1. Heating-up with the field on at a constant speed of 2"C/min to the polarization
temperature, which was chosen above the glass transition of the PVC (110°C).
2. Storing the sample in-field at 110°C for 30 min.
3. Cooling down in the presence of the field at a constant speed of 2 "C/min.
4. After removing the field heating up at a rate of 2"C/min while the current is
recorded.
Depolarization current peaks are found reproducible only if the temperature history
of the sample is fixed.
Dielectric spectra were recorded under similar conditions as depolarization spectra
by using a recording spectrometer described earlier'. This spectrometer is part of
148
Dielectric Depolarization Spectra of Toughened Polyuinylchloride
Fig. I.
Simplified scheme of the dielectric depolarization spectrometer.
the combined mechanical and dielectric relaxation spectrometer UNIRELAX AS 115
developed in our laboratory. The dielectric permittivity d(T) and the total conductivity
~ ( mT)=oo(T)
,
0
+ 4n
&"(a,
T)
-
were recorded as a function of temperature at constant rate of heating (2"C/min). w
is the fixed angular frequency (1 kHz), oo is the ohmic conductivity, E" is the unreal
part of the permittivity. The ohmic conductivity is an exponential function of the temperature
where 08 is a constant, E is the activation energy of the conduction, k is the Boltzmann
constant. E"(w, T) is a maximum-function representing the dielectric loss peak at the
given frequency. The E'(T) and E"(T) functions are recorded simultaneously.
Dynamic mechanical relaxation spectra were recorded by a forced oscillation setup
developed in our laboratory6. This is also part of the combined spectrometer UNIRELAX AS 115. Signals proportional to the real (G') and unreal (G") parts of the torsional
modulus were recorded simultaneously as a function of the temperature at a fixed
frequency of 5-20Hz. The speed of the temperature program was the same as in
the previous measurements (2 'C/min). The dimension of the sample in the torsional
dynamic mechanical experiments were 45 x 8 x 1 mm.
Thermomechanical measurement was performed by recording the penetration of a
cylindrical rod into the material as a function of the temperature at the same rate
of heating as in the previous measurements. In these experiments the load was applied
to the specimen pneumatically.
149
P. HEDVIG
and E. FOLDES
Results
A typical dielectric depolarization spectrum of a toughened PVC system
prepared by blending Hostalit with PVC is shown in Fig. 2. Two successive
runs are shown. The first curve has been recorded after polarization a t 110"C
for 30 min. The second one has been recorded after cooling down from
160°C to 120°C, switching the voltage on, polarizing for 30 min, cooling
down at a rate of 2"C/min before heating up and recording. Consequently
there is a slight differencein the thermal history of the sample for the subsequent
runs. It is seen that the positions and relative heights of the depolarization
current peaks are fairly well reproduced, only the peak designated by cii
is shifted.
t
lo-" A
Fig. 2. Dielectric depolarization spectrum of a 60:40 blend of PVC and Hostalit H-9970.
Polarization field 8,3kV/cm; time 30 min; temp. 110°C; rates of heating and
cooling 2"C/min. Two subsequent runs. Peaks t12, a; and pz correspond to
Hostalit, those of at and a ; to PVC.
This is generally observed by recording depolarization current spectra:
the peaks are reproducible only if the thermal history is rigorously fixed.
By applying greater cooling rates, by subsequent heating-up the depolarization
peaks are decreased3.
150
Dielectric Depolarization Spectra of Toughened Polyvinylchloride
70-77A
P vc
a.
I
c
c
P
lo-" A
L
u
3
._
E8
c
m
N
._
I
m
Q
0
a6
n
4
2
0
Fig. 3. Comparison of the depolarization spectra of pure PVC, Hostalit and an extractum
from Hostalit in toluene (Extractum). Rates of heating and cooling 2"C/min.
Polarization fields are: 5,9; 7,l; 7,7 kV/cm respectively,polarization temp. 110°C;
time 30 min for all spectra.
151
P. HEDVIG
and E. FOLDES
Fig. 3 shows a comparison of the dielectric depolarization spectra of pure
polyvinylchloride (a), pure Hostalit H-9970 (b), and that of the rubber phase
extracted from Hostalit (c).
I
'
-150
I
I
-100
-50
1
I
0
50
Temperature ( " C )
I
1
100
150
)
Fig. 4. Comparison of the depolarization spectrum Hostalit H-9970 with dielectric
spectra measured at 1 kHz. E' is the real part of the dielectric permittivity,
(r is the total conductivity which contains the ohmic contribution and the
dielectric loss. Rate of heating 2"C/min for all spectra.
152
J
I
50 -100
I
-50
I
0
I
I
50
100
Temperature ("C)
I
I
150
200
>
Fig. 5. Comparison of the depolarization spectrum of Hostalit H-9970 with the dynamic
mechanical relaxation spectra measured at 11,5 Hz. Rate of heating 2"C/min
for all spectra.
153
P. HEDVIG
and E. F ~ L D E S
3t
'i
._
a"
1
Temperature ( " C )
Fig. 6. Comparison of the depolarization spectrum of the elastic phase extracted from
Hostalit in toluene with the thermomechanical curve (penetration) and with
the dynamic mechanical absorption spectrum (loss modulus).
154
Dielectric Depolarization Spectra of Toughened Polyuinylchloride
It is seen that the depolarization current peaks a1 and cc; observed in
the blend correspond to the PVC-component. The peak a1 which is the
glass-rubber transition of PVC also appears in Hostalit, as it contains PVC,
but is absent in the extractum. Peaks a2,a;, a;' and p2 are evidently correlated
to the rubber phase.
Fig. 4 shows a comparison of the dielectric depolarization spectrum of
Hostalit with the corresponding dielectric spectra. It is seen that the transitions
observed in the d(T) spectra and the maxima of the o(T) spectra correlate
well with the depolarization spectra if the shift due to the higher effective
frequency is considered. The dielectric spectra were measured at 1 kHz. Similar
correlation between the dielectric spectra and depolarization current peaks
were observed for the blend and for the extractum as well. The high temperature
transitions could not be detected by the a.c. method because the ohmic
conductivity was too high.
Similar correlation has been observed with the dynamic mechanical spectra.
By this method only the a1 peak could be detected.
In Fig. 5 the mechanical relaxation spectra are shown in comparison with
the dielectric depolarization spectrum. It is seen that the transition a1 is
sharp (the glass transition of PVC), the transition a; is only slightly indicated.
The glass transition of the elastomer phase is not detected.
In Fig. 6 the dielectric depolarization spectrum of the extractum (rubber
phase) is compared with torsional dynamic mechanical resonance spectra
and with thermomechanical curves. The torsional mechanical spectra were
recorded at 11,5 Hz using the same rate of heating (2 OC/min) as by recording
the depolarization spectra. The storage- and loss-moduli were simultaneously
recorded as a function of the temperature.
The thermomechanical curve represents the penetration of a cylindrical
rod of 1 mmdiameter into the sample, recorded as a function of the temperature
at a constant load of 1,5 kp/an2.
It is seen that the thermomechanical and dynamic mechanical curves show
transition at about - 5 "C which is evidently the glass-rubber transition of
the elastomer phase. The a; transition appears as a shoulder in the torsional
dynamic relaxation curve and as a very well resolved separate peak in the
dielectric depolarization spectrum.
Discussion
The depolarization currents are so high that it is very difficult to imagine
that they are simply due to reorientation of dipoles connected to the polymer
155
P. HEDVICand E. FOLDES
chains. It is also known7 that the oscillator force E O - E ~ of the dielectric
dispersion line d(T) increases when the frequency is decreased. Very high
permittivity values are reached at very low frequencies (10- 3 . 10- Hz) which
would correspond to an effective dipole moment over 20 Debye, which is,
of course not possible.
It seems to be more reasonable to assume that the low frequency dielectric
polarization, and correspondingly the dielectric depolarization current peaks
are mainly due to the induced dipole densities formed at the interfaces of
structural inhomogeneities (defects) of the system. Such a mechanism has
been suggested by one of the authors earlier on the basis of measurements
of the effects of thermal pretreatments to the depolarization spectra of polyethylene’ and polymethyl-methacrylate3.
According to this view the depolarization current peaks which correlate
well with the structural transitions are due to the reorganization of the physical
structure of the polymer which involves drastic change in defect concentration
and corresponding change in interfacial dipole polarization. So the decay
of the interfacial polarization is principially not due to conduction, but rather
due to the destruction of the defects (interfaces) themselves at structural
transit ion.
A semiquantitative interpretation of this effect on the basis of ionic exciton
states will be published soon9.
If this interpretation is correct the intensity of the depolarization current
peak should represent the rate of change of the structural defect concentration
at the transition involved. This would explain the result shown in Fig. 4
and 5 that the cr; peak of the elastomer phase appears strong in the depolarization spectra, but is weak in the 1 kHz dielectric and 11,5Hz mechanical
spectra. It has been also observed that this peak is extremely sensitive to
previous thermal treatment. Similarly the or’-peak observed in pure PVC
at about 160“C indicates a structural transition in the visco-elastic liquid
state, which is very difficult to observe by other methods.
J. V A N TURNHOUT,
Polym. J. 2 (1971) 173
P. HEDVIG,in “Magnetic Resonance in Chemistry and Biology”, J. HERAKand K.
J. ADAMIC?
(Ed.), AMPERE Summer School, BaSko Polje, Yugoslavia, 1971, Marcel
Dekker, New York, 1973
CH. SOLOUNOV
and P. HEDVIG,
Proc. 111. Tihany Symposium on Radiation Chemistry,
J. DOBO and P. HEDVIG(Ed.), Publ. House of the Hungarian Ac. Sci., Budapest,
1972, p. 899
P. HEDVIG,in “Nobel Symposium Series No. 22”. PER OLOFKINNEL,B. RANBY
and V. RUNNSTROM
REIO(Ed.)
156
Dielectric Depolarization Spectra of Toughened Polyvinylchloride
’
P. HEDVIG,
J. Polym. Sci. C 33 (1971) 315
P. HEDVIG,
Miianyag Cs Gumi 7 (1970) 283
W. REDDISH,Amer. Chem. SOC.,Polym. Prepr. 6 (1966) 571; Y. ISHIDA,Kolloid
Z. Z. Polym. 168 (1960) 29
P. HEDVIG,Paper presented at the International Microsymposium on Polarization
and Conduction in Insulating Polymers, Bratislava-Harmonia 1972
P. HEDVIG,Paper to be presented at the I. International Conference for Quantum
Chemistry, Menton, France 1973
157
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