-
8/9/2019 160080507_ftp
1/7
JOURNAL O F P O L Y M E R S C I E N C E : P A R T A-2 VOL. 8,5739-745 (1970)
Nature of DC Conductivity
in
Polyamides
NI. E. BAIRD, Institute
of
Science and Technology, The U niversity
of
Wales,
Cardiff, Wales,
U . K .
Synopsis
The dc conductivity
of
poly (sebacyl piperazine),
a
polyamide prepared from the
secondary diamine piperazine, in which no N-H groups are present and no hydrogen
bonding can occur, has been examined and compared with that of a normal 610 poly-
amide. The results obtained point clearly to th e conduction in the 610 polyamide being
electronic below about 100C but probably involving protons as well as electrons above
this temperature. This is largely consistent with the findings of earlier work and clarifies
the nature of conduction below about 80C as being almost certainly electronic where
previously it was in doubt. A definite and sometimes marked hysteresis in the con-
ductivity was observed with regard to raising and lowering the temperature of the poly-
amides. This is explained in terms of the space-charge polarization developed in the
materials a t higher temperatures and which becomes clearly evident in their dielectric
behavior. This shows the importance
of
discharging specimens at a sufficiently high
temperature before making conductivity measurements. Th e polarization is
a
bulk
and not an electrode effect, and it will probably depend to
a
marked extent
on
the
morphology of the polyamides.
Introduction
Polyamides (nylons) show a relatively high dc conductivity u, and pre-
vious studies have suggested that two mechanisms might operate. Until
recently, the generally accepted view was that conduction was
with both conduction and a low frequency relaxation arising from the move-
ment
of
protons originating in the hydrogen bonds traditionally thought to
occur between polyamide molecules (Fig. l), but that under certain cir-
cumstances it might be electr~nic.~-~
n
a detailed study of the mech-
anisms and anisotropy of electrical conductivity in nylon
66,
SeanorlOSll
concludes that at temperatures above 120C the conduction involves the
transport
of
both protons and electrons while below this temperature to
8O C,
it
is electronic. At still lower temperatures the nature of conduction
was still in doubt. These studies included the measurements
of
the volume
of gas evolved from the 66 polyamide. Above 120C the gas evolved cor-
responded to about one half of the volume calculated
if
the conduction
process involved only protons. Below 120C the gas evolved corresponded
to a diminishing fraction of the tota l current until below
90C
no evolution
of gas was observed.
The low-frequency relaxation occurring above the glass transition tem-
perature T in polyamides is almost certainly due to a space-charge polar-
739
0
1970 by John Wiley &
Sons,
Inc.
-
8/9/2019 160080507_ftp
2/7
740
BAIRD
Fig. 1. Schematic diagram showing hydrogen bonding in polyamides.
ization since
it
appears to have no counterpart in the dynamic mechanical
spectrum.12J3 Furthermore, the dielectric constant increases continu-
~ u s l y ' - ~ ~ *s the temperature is raised a t constant frequency and shows no
signs of reaching a maximum and then falling off as proportional to the
reciprocal of the absolute temperature as would be expected for orienta-
tion (dipolar) polarization.
In a previous publication14it was pointed out tha t some data were not
consistent with the postulate of proton migration accounting for the low
frequency dielectric relaxation and dc conductivity in polyamides.
A
summary was also given of the work of Cannon,l6-l9 who discussed the im-
portance of secondary forces between molecules in controlling the packing
and configuration of the molecules in the solid state. These facts cast
doubt on the importance of amide protons in the conduction and relaxation
processes. In this publication14 t was also shown that this low-frequency
dielectric behavior occurred to a similar extent in poly(sebacy1 piperazine)
in which no N-H groups were present and no hydrogen bonding could oc-
cur. It was therefore concluded that amide protons could not be the main
cause of this low frequency relaxation in polyamides.
A
detailed comparison of the dc conductivities of
610
polyamide and
poly (sebacyl piperazine) (hereafter called Pip-10 polyamide)
0
,CHz-CH*,
\cH--
CH
-N N- C- (CHJa-C-
II
0
over the temperature range 35-150C has now been made and the results
are given here.
Experimental
Disk specimens approximately 2 in. in diameter with thicknesses in the
range 0.020-0.040 in. were molded from the dry polymers in a laboratory
press under a flow of dry nitrogen to prevent degradation.
These were
-
8/9/2019 160080507_ftp
3/7
DC CONDUCTIVITY IN
POLYAMIDES
741
- 5
r
0
-
- Q U D - - ~ - -
0
0
I0
0 1 2 3 4 5
LOGlo
TIME
N
MINUTES
Fig.
2.
Plots
of
log current (on charge) against log time
for
dry polyamide Pip-10 at
various temperatures: 0)
3 C,
(+)
95 C,
A)
129 C,
0)
48OC,
temperatures sel-
ected in ascending order;
(V) 128 C,
( x )
96 C,
m)
64 C,
temperatures selected in
descending order after previous measurements; (- - - 0 )
64 C,
(- -
X ) 95 C,
in as-
cending order after dischargingfor
46
hr at about
150C.
dried and coated, and the conduction currents were measured exactly as
described in previous papers.14,m The potential difference across the stan-
dard resistance in the electrometer was less than 0.01
v
and the applied
emf was 125
v.
The 610 polymer had impurities not exceeding 0.003% with
a
melting
point of 220C and a (dry) glass temperature T, t about 50C (information
supplied by I.C.I. Ltd, Plastics Division, from dilatometry experiments).
It
had an average molecular weight of about 20,000 as determined from solu-
tion viscosity experiments. The Pip10 polyamide was prepared by inter-
facial polymerization of piperazine and sebacyl chloride, purified by re-
precipitation from benzyl alcohol, and washed free of solvent with acetone.
Its melting point was about 160C and
it
had an ill-defined (dry) glass
transition in the region 80C or higher, (obtained from dilatometry experi-
ments).
It
had an average molecular weight
of
about
33,000,
as determined
from solution viscosity measurements, and a number-average molecular
weight of the order of 16,000.
The density of 610 polymer indicated that
it
was about
40
crystalline*l
while infrared measurements and x-ray powder photographs suggested that
the Pip-10 polyamide was about 60% crystalline. Unfortunately this
latter figure cannot be
EM
reliable
as
that for
a
normal polyamide, because
of a lack of background experience for the Pip-lOpolyamide.
At higher temperatures (63-150C) the behavior of the currents ob
served on charge against time after reaching constant temperature is shown
in Figure 2 for a single specimen of Pip-lO polyamide, and in Figure 3 for
a
single specimen
of
610 polyamide. The specimens were kept on charge a t
each temperature for at least 16 hr (and often longer) to obtain current
levels as reliable as possible, after first being given a thorough discharge a t
-
8/9/2019 160080507_ftp
4/7
74
BAIRD
- 4
-I I 1
0 1 2 3 4
LOG
TIM
IN
MINUTES
10
Fig.
3.
Plots of log current (on charge) against log time
for
dry polyamide 610 at
various temperatures: 0 )63 C,
+)
95OC,
A)
129OC,
0)
48 C, temperatures sel-
ected in ascending order;
(V)
1280CJ(x ) 95 CJ
m )
65OC, temperatures selected in
descending order after previous measurements;
(-
-
-0)
4 CJ
(-
- -x) 95 C, in as-
cending order after discharging for 46 hours at about 150C.
about 150C. The temperatures were selected in ascending order up to
about 150C and then again in descending order. The currents decreased
continuously with time, although usually quite slowly except
at
the higher
temperatures. An important feature of the results is: tha t the currents ob-
served
as
the temperatures are selected in ascending order are always more
than those observed a t the same (or nearly the same) temperature, when
descending.
It
is not due to a process of slowly driving off residual moisture or to some ir-
reversible process in the material because, after dischargingat about 150C
for periods in the range 2448 hr, the currents largely recovered to their
original values,
as
shown by the dotted curves in Figures
2
and 3. The
whole procedure was repeated with a further specimen of Pip-10 polyamide
(batch C, cast from solvent) and the effects were found to be reproducible.
In Figure
4
he conductivities of the two types of polyamide are com-
pared over
a
wide range of temperature (35-150C). The da ta just dis-
cussed for the specimens of 610 and Pip-10 (batches
B
and c ) polyamides
are included for the appropriate larger currents. Data for the lower-
temperature region, 35-7OoC, are also given. Here the conduction currents
were generally obtained as the differences between the currents on charge
and dischargem at corresponding times, these usually showing reasonable
agreement with the currents observed on charge after at least 3 hr. All the
specimens were discharged for about
2
hr at 110C before making measure-
ments. For the 610 and Pip-10 batch
A
materials
a
fresh specimen was
used for each temperature, but for the Pip-10 batch
B
material, the same
specimen was used throughout ensuring a thorough discharge at each
temperature before proceeding to the next one. Some data for nylon 66
This effect occurs with both 610 and Pip-10 polyamides.
-
8/9/2019 160080507_ftp
5/7
DC CONDUCTIVITY IN POLYAMIDES
743
- 6
-
1
L
-9
c
-I0
1 1
3
-I4
>.
k
-I2
-I3
0
U
I
-I
I I
I
2.3
2.4
2.5
26 27 2.8 2.9 3.0 3.1 3.2
3 3
34 3.5
1000
T
OK
Fig. 4. Plots of log dc conductivity against 1000/T for dry polyamides: 0 )610;
0)6 from ref. 11 (Fig.
3,
15 minutes);
+ )
Pip-10, batch A;
A) Pip-10, batch
B;
X
)
Pip-lO, bat,ch
C.
from Seanor (Fig.
3)
are
also
included to show the reasonable agreement
between the conductivities for 66 and 610 polyamides.
At the lowest temperatures the conductivity of the Pip-10 polyamide is
clearly higher than that of the 610 polyamide, bu t a t the highest tempera-
tures, the reverse is true. However the differences in conductivity are not
very large. In the region
75 90C
the curves for the two types of poly-
amide cross over. Differences between the various batches of Pip-10
polyamide are probably due to differences in morphology. In the lower
temperature region, the plots are reasonably linear and the observed acti-
vation energies obtained from the slopes 2.303 Rd(1og a) /d(l /T) were 2.3
eV for 610 polyamide and 2.1 eV for both batches A and
B
of Pip-10 poly-
amide. As the temperature is raised, the slopes of the curves change more
or less continuously, although a t about 80C there is
a
suggestion of a break
in the curves which is probably related to the glass transition of the poly-
mers.
Discussion
The conduction in the pure Pip-10 polyamide must almost certainly be
electronic since there are no
N-H
groups in the repeat units, and any con-
tribution to the conduction current from protons a t the ends of molecules is
likely to be negligible. Assuming even a low dissociation energy of the
order of 1.2 eV (ca. 28 kcal/mole), and Frenkel defects for the proton gives a
-
8/9/2019 160080507_ftp
6/7
744 BAIRD
maximum concentration of terminal protons able to participate in conduc-
tion at 150C of the order of
5 X
1013protons/cc. Taking a figure of
lo-
cm2/V-sec as a realistic mobility for the proton and the electronic charge as
1.60 X
10-19
coulombs gives a likely maximum conductivity from protons
at the ends of molecules as about
10-13
(ohm-cm)-' a t 150C. The con-
ductivity observed at this temperature is about 3 orders greater than this
value. Since there
is
no reason to expect
a
much larger electronic conduc-
tivity in the Pip-10 polyamide than in the normal polyamides, the behavior
of
the 610 and Pip-10 polyamides in Figure
4
shows that the conduction in
normal 610 polyamide is electronic below about lOO C, with no significant
contribution from protons, but above this temperature probably both elec-
trons and (amide) protons contribute to its conductivity, which now be-
comes greater than that of the Pip-10 polyamide. This is largely consistent
with Seanor'sloJ1 indings and clarifies the nature of conduction below
80C
as
being almost certainly electronic, where he considered that the nature of
the conduction was still in doubt. The da ta presented do not allow any
further understanding of the detailed mechanism of conductivity. As
Seanorlovll uggests, this probably involves the electrons of the carbonyl
groups,
as
EleyZ2 uggested for proteins. Conduction will occur along the
system of amide groups rather than along the polymer chain, leading to
anisotropy of conductivity u across molecular chains > u along chains).
A similar anisotropy of charge diffusion in polyamides involving a greater
mobility across the molecular chains than along them has also been re-
portedz3 rom static electrification studies.
The hysteresis observed in the conductivity on ascending and descending
the series of temperatures used is certainly interesting and the most plau-
sible explanation is in terms of the space-charge polarization produced.
At higher temperatures a large space-charge polarization is developed,
and as the temperature is reduced (with the electric field still applied) the
relaxation time for the decay of this polarization becomes very long so that
it persists and decays very slowly. (Electrets can be readily made from
p~lyamides.~)The space charge will produce a marked variation of
electric field and potential in those regions where it accumulates,
so
tha t with a given voltage applied across the specimen the effective field in
the other regions of the material will be lower than i t otherwise would have
been. In certain regions the effective field may be quite low, and the
conductivity as measured could then be much lower than that for the
material without the space-charge polarization or with only the much
smaller polarization built up at the lower temperatures. Unfortunately
virtually nothing is known about the detailed nature of the space charge
accumulation other than that it probably occurs at the boundaries between
crystalline and amorphous regions and depends upon the morphology
of
the
polymer. Capacitance measurements by the author on specimens
of
610
polyamide with a wide range of thicknesses have shown that the polariza-
tion is largely a bulk and not an electrode effect.
In
view of the effects described here, some of the published information on
dc conductivity of polyamides obtained by cycling over a temperature range
-
8/9/2019 160080507_ftp
7/7
DC CONDUCTIVITY IN POLYAMIDES
745
with
a
fixed applied voltage would appear to be suspect. Specimens should
be thoroughly discharged at
a
sufficiently high temperature before making
measurements
at
any given temperature.
This polarization will also cause transientsz0 o flow after the application
or removal of a step voltage and could give the appearance of time-depen-
dent conductivity. The space charge could also lead to non-ohmic be-
havior of the polyamide. Dipolar orientation may also contribute to the
polarization above the glass transition of the polymers. However, in view
of
the relatively small increase in dielectric constant associated with the
orientation polarization of a normal polyamide,' as compared with the
magnitude
of
the low-frequency relaxation, and because this polarization
will fall off as temperature is raised, dipolar orientation will probably not
make a significant contribution to the polarization at temperatures well
above
T , .
Depolarization
of
the polymers could be speeded up by irradi-
ation with light of an appropriate wavelength which would detrap the space
charge without affecting the orientation polarization, and thus permit
separahg the two contributions to the polarization.
A
discussion of possible mechanisms of the space-charge polarization to-
gether wit,h full details
of
the dielectric data will be given in a later paper.
References
1.
W. A. Yager and W.
0.
Baker,
J.
Amer. Chem.SOC.
4,2171 (1942).
2.
D.
W. McCall and
E.
W. Anderson,
J . Chem. Phys .,
32,237 (1960).
3.
D. D. Eley and D. I. Spivey,
Trans. Faraday SO C.,
7,2280 (196>.
4.
A.
J.
Curtis,
J.
Res. Nat. Bur . Stand.,
65,3 , 185 (1961).
5. L. T. Yu, J.
Phys. ,
24,677 (1963).
6. T.
Nakajima and Y. Matsumoto, Repts. Progr. Polym. Phys. Japan,
6, 241
7. H. Kasica, M. Kryszewski, A. Szymaniski, and M. Wiodarczyk,
J.
Polym.
Sci.
8. Y. Miyoshi and N. Saito,
J. Phys. SO C. apan,
24,1007 (1968).
9. R.
Goffaux, Rev.
Gen.
Elect.,
75,
No.
11, 1250 (1966).
(1963).
A-l ,6,1615 (1968).
10.
D. A. Seanor, in Electrical Conduction Properties of Polymers, J . Polym.
Sci.
C ,
11.
D.
A.
Seanor,
J.
Polym.
Sci .
A-.2,6,463 (1968).
12. S.
Hirota,
S.
Saito and T. Nakajima, Kolloid-Z.,
2
olym.,
213,109 (1966).
13.
M. Takayanagi,
Mem . Fac. Eng. Kyushu Univ.,
23,41 (1963).
14.
M.
E.
Baird, G. T. Goldsworthy, and C. J. Creasey, J. Polym. Sci. B ,
6, 737
15.
C. G. Cannon,
J.
Chem. Ph ys.,
24,491 (1956).
16.
C. G. Cannon,Disc Faraday SO C.
5.59 (1958).
17.
C. G. Cannon
Spectrochim. Acta,
10,341 (1958).
18.
C. G. Cannon, Speetrochim. Ac ta,
16,302 (1960).
19. R. G. C. Arridge and C. G. Cannon,
Proc. Roy.
Soc. (London), A278,91 (1964).
20.
M. E. Baird, Revs. Mo d. Phy s.,
40,219 (1968).
21.
Maranyl Nylons, Technical Service Note
N101,
I.C.I. Ltd., Plastics Division.
22.
D. D. Eley and
D.
I.
Spivey,
Tra ns. Faraduy Soc.,
56,1439 (1960).
23.
R. G. C. Arridge, Bri t .J .A pp1 . Phys . ,
18,1311 (1967).
17),
A.
Rembaum and R. F. Landel, Eds., Interscience, New York, 1967,p. 195.
(1968).
Received May 27 1969
Revised September
11
1969