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Changes in electrical properties of polystyrene films under the influence of electrical discharges in gases.

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Die Angewandte Makromolekulare Chemie 12 (1970) 137-144 (Nr. 149)
From the Institute of Physics, Academy of Sciences of the Azerbaijan SSR, Prospekt Narimsnova 33, Baku, USSR
Changes in Electrical Properties of Polystyrene Films
under the Influence of Electrical discharges in Gases
By M. A. BAQIROV,
S. A. ABAsov and v. P. MALIN
(Eingegangen am 4. November 1969)
SUMMARY:
This paper describes changes of the following electrical properties of polystyrene
films : dielectrical permeability, E , dielectrical loss, tan 6, and conductivity under
the influence of electrical discharges in air and nitrogen.
It is shown, that under the influence of electrical discharges there occurs an increme in conductivity and dielectrical permeability ; furthermore, there appears a
new range of losses, tan 6, and a shift of dipole-elastic losses, tan 6 (alphaprocess),
to higher temperatures, the latter meaning an increase in the relaxation time of the
kinetic fragments.
The changes observed in the electrical properties are due to changes in the
polymer structure resulting from simultaneously occurring processes of crosslinking, degradation and oxidation of polystyrene macromolecules. These proceases
also lead to changes in mechanical strength and molecular weight of the polymer.
The presence of oxygen strongly aocelerates these proccesses that proceed on polystyrene films under the influence of electrical discharges.
ZUSAMMENFASSUNG :
I n dieser Arbeit werden die h d e r u n g e n elektrischer Eigenschaften von Polystyrolfilmen (dielektrische Konstante E, dielektrischer Verlustfaktor tan 6 und
Leitfiihigkeit) unter dem Einflulj elektrischer Entladungen in Luft und in Stickstoff beschrieben.
Es zeigte sich, da13 unter der Einwirkung elektrischer Entladungen die dielektrische Konstante und die Leitfiihigkeit zunehmen, und daB im tan 6-Verlauf uber der
Temperatur ein neues Absorptionsgebiet auftritt und daa Maximum des dipolelastischen Verlustes sich zu h6heren Temperaturen verschiebt ; letzteres bedeutet,
dalj die Relaxionszeit der kinetischen Segmente zunimmt.
Diese Veriinderungen der elektrischen Eigenschaften werden durch h d e r u n g e n
der Struktur des Polymeren bedingt, die durch gleichzeitig verlaufende Vernetzungs-, Abbau- und Oxydations-Prozesse der Polystyrol-Makromolekulehervorgerufen werden. Diese Prozesse fiihren auch zur h d e r u n g der mechanischen Festigkeit
und des Molekulargewichtes.Die Anwesenheit von Sauerstoff beschleunigt die h d e rungen der Eigenschaften von Polystyrollilmen unter der Einwirkung von elektrischen Entladungen erheblich.
137
M.A. BAQIROV,
S. A. ABASOVand V. P. MALIN
It is'known that one of the principal reasons for the worsening of electrical
properties of polymeric isolation in use is the effect of the electric discharges
developing in the aerial inclusions and in the interlayers inside the isolation
itself.
In accordance with this we have investigated the changes of electrical properties of one of the most common polymers-i. e. polystyrene-by measuring
the dielectric power factor (tangent of dielectric loss angle), tan 6, the dielectric
constant (permeability), E , and the specific volumetric resistivity, p v ( h = l / y ,
where y is conductivity), under the effect of electrical discharges in air and
nitrogen. Commercial polystyrene (PS) films 20 micron thick made of amorphous block polystyrene were used for the experiments.
Exprinzental
The effect of electrical dischaxges on PS films was investigated in a specially
designed test cell (Fig. 1) by changing the voltage and duration of the electric
current (a.c., frequency of 50 clsec). The electricel discharges were generated in the
gas gap between the glass plate and the polymer sample. A detailed description of
the test cell waa given'.
Hv
3
2
6
c
Fig. 1. Test cell. 1. metallized covering; 2. glass plate; 3. gas gap; 4. polymer
sample; 5. metall electrode; 0. glass casket.
138
Electrical Prop~rtiesof Polyatyrene F i l m
After the electrical discharge treatment the sample was washed in ethanol and
dried to weight constancy. tan 6 and E were measured on 8 capacity bridge of the
MLE-I type in the temperature range of 20 to 160°C and in the frequency band of
0.4 + 10 Kc/-.
ev was measured with an E-6-3 type termhmmeter feeding the
sample with d. c. voltage of the order of 100 V. To provide a close contact of the
meeauring electrodes with the surface of sample, the latter, before measuring, was
covered with a thin layer of silver powder with the aid of a thermopowderingunit.
Results and Discwrswn
Polystyrene is a non-polar polymer and prior to subjection to electrical discharges, it features tan 6 4 10-4,E = 2.4 and
= 1017 ohm cm. Under
ordinary conditions in the investigating range of frequencies and temperatures
tan 6 and E of PS do practically not depend on the frequency and temperature of
measuring excluding the zone of dipole-elasticloss ( N 110- 140"C for the given
range of frequencies of measuring). The zone of the loss is (according to2) connected with the motion of the kinetic segments of the polystyrene macromolecules and is due either to the-though relatively small-polarity of the
PS links or to polar impurities weakly connected with the chain.
It was established3 that under the influence of electrical discharges in air
tan 6 of the PS films exhibits a new zone of loss within the temperature range
of 25-45°C (low-temperature loss) and that tan
of the dipole-elastic loss
increases while simultaneously being shifted towards higher temperatures
(Fig. 2). The temperature intervals of the maxima of the low-temperature loss
do practically not depend on the frequency of measuring (in the given frequency band), with the value of the maximum decreasing considerably both by
washing of the polymer with ethanol and by heating of it. It may be presumed
that these losses are due to the low-molecular-weightcompounds being produced a t the PS film surface by degradation under the inthence of electrical
discharges.
The value of the dipole-elastic loss increases both with the energy of the
discharge4, and with the temperature3 of the gaseous medium.
Under the effect of electrical discharges in introgen atmosphere, only a small
shift of the dipole-elasticloss is observed (Fig. 2).
Under the influence of electrical discharges in air the dielectric permeability
of the PS film increases with the duration of the discharge treatment (Table 1).
N
Table 1. The dependence of
t
Po-I
&
E
-
on the duration of discharges (U = 10 kV).
0
6
2.44
2.74
12
2.82
20
3.07
139
M.A. BAQIROV,
S. A. ABASOVand V.P. MALJN
Fig. 2.
Temperature dependence of tan 6 of the polystyrene films subjected to
electrical discharges and of their products in different media at t = 8h.
U = 9 kV; 30°C, f = 1 Kc/sec.
1. initial polystyrene 6lm; 2. in air, the discharge effects; 3. in air,the products; 4. in oxygen, the discharge effects; 5. in oxygen, the products;
6. in nitrogen, the discharge effects.
Measurement of the specific volumetric resistivity
showed (Fig. 3) that
under the influence of electrical discharges in air the following is observed:
1. a general decrease in eV (that is an increase in conductivity y ) :
2. a sharply expressed maximum of eV appears a t temperatures of20-45°C
corresponding to the temperature range where the low-temperature loss of
tan S is observed.
It should be noted that eV increases if the PS film is heated prior to its subjection to electric discharges.
The experimental results obtained may be explained on the basis of studies
of the chemical and structural changes occwing in the polymer. The infra-red
spectra of the PS films subjected to the effect of electrical discharges in air5
140
Electrical Propert& of Polystyrene Film8
18
$ 17
E
c
0
6
-m
16
15
2.75
3 .O
3.25
3.5
I
- * 105
T
2. for
Fig. 3. Temperature dependence of lg eV. 1. for the initial polystyrene h;
the PS h effected by electrical discharges in air at U = gkV, 0 = 30°C,
t = 5 h; 3. for the w e film but pre-heatedfor 2 hours at 60 "C.
reveal large quantities of new groups and compounds, such as compounds containing C=O groups (1.720 cm-I), OH, C=C bonds (1.680 cm-1) and others.
The intensity of the absorption bands of ethers, C-0-C (1.278cm-1), and of
methyl end groups, CH3 (1.376 cm-I), increase considerably. The spectrum of
PS film subjected to electrical discharges in nitrogen, on the other hand, did
not reveal any significant change.
As is known, C - 0 groups are very polar (their dipole moment is2.8D).They
apparently cause an increase in tan6 and E of the PS film. The increase in
abundance of CH3 groups indicates destruction of the polymeric chains under
the influence of electrical discharges. The destruction process, together with
the process of ionic oxydation, may lead to the formation of low-molecularweight compounds, with ions causing the increase in conductivity of the polymer. Fig. 4 shows a correlation between the minimum of lg and the intensity
of absorption of the C=O groups, i. e., it may be supposed that the minimum
of lg eV, and, accordingly, the low-temperature loss of tan 6 are due to the lowmolecular-weight compounds containing C=O groups. These compounds are
partly removed by washing or heating; so that the low-temperature loss of
tan 6 of the polymer decreases and eV increases.
141
M.A. BAOIROV,
S. A. ABASOVand V.P. MALIN
Fig. 4. The dependence of the minimum of lg ev and the intensity of absorptionof
the C=O groups depending on the electrical discharge in air: 1. dependence of the maximum of lg eV; 2. dependence of the intensity of absorption of the C=O groups.
The shift of the dipole-elmtic loss of the PS film to the zone of higher temperatures shows that the processes taking place under the effect of electrical discharges lead to a slowing down of the motion of the kinetic segments of the
polystyrene macromolecules. The most probable process is the cross-linking of
macromolecules. The appearance of non-soluble fractions (gel fraction) upon
dissolution of the PS film indicates the occurrance of cross-linking under the
effect of electrical discharges.
It is to be noted that the maximum gel content under the influence of electrical discharges in air (12.6 yo)was nearly 10 times greater than the gel content
under this effect in nitrogen. Elemental chemical analysis showed that the gel
content generated under the influence of discharges in air (C60, H3.7,036.3 yo)
differs only slightly from the gel content produced under the action of ozone
only (C 58.4, H 3.6, 0 38y0), and corresponds to the formula C4H302. Crosslinking due to oxygen is indicated by the increase in intensity of the C-0-C
absorption band.
The occurrance of degradation and cross-linking processes in the polymeric
chains of the PS films under the influence of electrical discharges in air is also
indicated by the changes in mechanical strength, a, and molecular weight M.
Actually, m other investigations showed3.6, under the effect of electrical discharges a decrease or an increaae of a (depending on the experimental conditions) and M takes place, the characters of their changes corresponding to
142
Electrical P r q r t i e e of Polystyrene F i l m
Fig. 5. The dependence of mechanical strength and molecular weight of polystyrene
fYms on the voltage, U, of the electrical discharge applied in nitrogen and in
air. 1. Mechanical strenght u in nitrogen; 2. Mechanical strength u in air;
3. molecular weight M in nitrogen; 4. molecular weight M in air.
each other (Fig. 6).The decrease in mechanical strength a, and in molecular
weight M, testifies to the destruction process, and their increase testifies to the
process of cross-linking of the polymeric macromolecules.
It is as yet difficult to determine exactly the mechanism of the oxidation,
cross-linking and destruction processes of polymers under the effect of electrical discharges. The main reason for this is the diversity of various factors of
the electrical discharge effect : the ionic and electronic bombardment, local
heating of the polymer surface in the points of contact with the channel of the
discharge spark, the action of gaseous products (ozone, nitrogen oxides) etc.
In any case, it is obvious that the presence of oxygen in the zone of the electrical
discharge accelerates considerably the processes of destruction and crosslinking occuring in the polymer which, together with the process of oxidation,
leads to chances in such PS film characteristics as E , tan 6 , PO, a, and M.
1
2
S. A. Asasov, M. A. BAQIROV,
N. V. KLIYOVAand V. P. MALIN,
Iswesetije,Akademii Nauk Aserbaidshanskoi SSR, Sserija Fisiko-Matematitschesskich i Technitschesskich Nauk-8 (1965) 137.
0. BROENS
and F. H. M ~ R Kolloid-Z.
,
140 (1957) 121.
143
M. A. BAQIROV,
S. A. ABASOV
and V. P. MALIN
3
S. A. ABASOV,M. A. BAQIROV,
E. Y. VOLCHENXOV,
T. I. GUSEINOV,N. W. KLIand V. P. Malin, Plaate u. Kautschuk 3 (1968) 184.
M. A. BAQIROV,N. V. KLIMOVAand V. P. MALIN, Iswesstija Akademii Nauk
Aaerbaidschanakoi SSR, Sserija Fisiko-Matematitschesskich i Technitschesskich
Nauk 5 (1966) 66.
M. A. BAQIROV,E. Y. VOLCHENXOV
and V. P. MALIN,
A Collection of papers,
Materials of the Scientific Conference of Young Scientists and Post-Graduate
Research Students of the Azerb. SSR Academy of Sciences, Phys.-techn. and
Mathem. series, Azerb. SSR Academy of Sci., Baku 1966, p. 129.
T. I. GUSEINOV,S. A. ABASOVand M. A. BAQIROV,
Mechanics of Polymers 1
YOVA
4
5
6
(1968) 69.
144
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