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Polymer Znternational41(1996)463-471
Structure and Some Physical Properties
of Electropolymerized Polyacrylonitrile
F. M. Reicha
Physics Department, Faculty of Science, Mansoura University, Mansoura, Egypt
(Received 2 January 1996; revised version received 11 June 1996; accepted 15 July 1996)
Abstract: Electropolymerization of acrylonitrile in an aprotic medium leads to
the formation of two different structural forms of polyacrylonitrile (PAN). First,
PAN having a striated structure and highly cross-linked was condensed on the
cathode. This type of material exhibits unique physical properties, namely that
its conductivity is about
(Qcm)-' and is independent of the frequency.
The second type was gathered from the medium, and elemental analysis, optical
microscopy, X-ray, UV, IR and other investigations showed that the polymer has
an amorphous structure with the presence of interchain cyclization, which
spreads as the current density of electropolymerizationincreases. The chemical
nature of the chromophore developed in PAN prepared by this method is attributed to conjugation of C z N . The structural changes were accompanied by an
increase in conductivity. The conductivity was about
(Q cm)-' with an
activation energy of 1.48eV for a sample prepared at a current density of
0*417rnA~m-~,
increasing to about 8 x lO-'(Qcm)-' with an activation energy
of 0.93 eV for a sample prepared at 2.5 mA cm-' or higher.
Key words: acrylonitrile,electropolymerization,characterization, conductivity.
I NTRO DUCTlO N
In the mid-l960s, fibres of outstanding strength were
obtained by heating polyacrylonitrile (PAN) through a
carefully controlled temperature programme, in a controlled atmosphere, to temperatures in excess of 1000°C.
This discovery made PAN an appealing target for
investigation into the methods of preparation and
thermal degradation, as well as physical and mechanical
properties. The properties of the carbon fibres depend
upon the PAN from which it is prepared and the
thermal treatment to which it is subjected.
Some controversies over the structure of PAN have
been discussed in comprehensive
Differences
in the physical properties of PAN homopolymer can be
found in publications from different laborat~ries,'~-'
as a result of differences in sample p rep arati~ n.'~ -'~
In spite of an enormous number of reports on the
methods of PAN preparation, very little attention has
been paid to its synthesis by the electrochemical technique.6 The technique reported was performed under
carefully controlled physical and chemical conditions
and under a very pure argon atmosphere, which would
make commercial mass-production difficult.
This paper describes the electrochemical polymerization of acrylonitrile (AN) monomer, in an aprotic
medium with different electrode potentials, under
normal laboratory conditions of temperature and atmosphere.
EXPERIMENTAL
The electrochemical polymerization was carried out as
follows. Two platinum strips (2cm x 3cm) were suspended at a distance of 2.5 cm from each other by platinum wires in an electrolytic solution consisting of the
AN monomer (concentration 1.5 M) and a supporting
electrolyte (tetramethyl ammonium bromide, concentration 0.05 M) dissolved in the aprotic solvent
(dimethylformamide, DMF), and connected to a suitable variable resistor, microammeter and constantcurrent power supply (Gelman Sciences, Inc.). The
463
Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain
F. M . Reicha
464
electrolytic current density was fixed for each preparation period (4h), and ranged from 0.25 to
3.33 mAcm-'. The temperature was held constant at
25°C using a water bath, and measured by a copperconstantan thermocouple. The products are classified
into two types: (a) the product deposited on the
cathode, called cathodic electropolymerized acrylonitrile (CEPAN); (b) the product collected from the
medium, called medium electropolymerized acrylonitrile
(MEPAN).
CEPAN was collected directly from the cathode,
while MEPAN was separated from the electrolytic solution, which was added to redistilled water and collected
by filtration, then dried for about 1 week at 40°C in a
vacuum oven. The yield was found to be greater than
75% based on total monomer initially present. All
chemicals were of Analar Grade (BDH) and used
without further purification.
From selected CEPAN and MEPAN samples the
content of carbon, hydrogen, nitrogen and oxygen was
determined by elemental analysis (Cairo University,
Egypt).
Density measurements of both CEPAN and MEPAN
samples were carried out using a density gradient
column (Davenport, London) comprising a mixture of
n-heptane and carbon tetrachloride, because no swelling
takes place between PAN and this mixture.16 Polymer
discs were prepared under constant pressure
(20toncm-') for 30min, and then kept in a vacuum
oven at 40°C for 24 h prior to density measurements.
Relative viscosity measurements for MEPAN were
carried out for 0.5(w/v) solutions in DMF at 30°C
using an Ubbelohde viscometer.
Infrared spectra for both CEPAN and MEPAN
samples were recorded on a Mattson 5000 FTIR
spectrometer by embedding 20mg of the polymer
powder in 200mg KBr, followed by compression of
40mg quantities of this mixture to make pellets.
UV spectra for MEPAN samples were recorded on a
Unicam UV2-100 UV/visible spectrometer V3-32 using
a constant concentration of 2 x i0-4gmi-1 for all
samples, with dimethylsulphoxide,(DMSO) as solvent.
X-ray diffraction studies of both CEPAN and
MEPAN were carried out on a Phillips PM992/05 PW
1840 diffractometer using Cu-Ka irradiation.
The degree of polymer homogeneity of MEPAN
samples was checked by thin layer chromatography
(TLC) using silica gel G F 254 (Merck) with benzene and
acetone solution in a ratio of 9 :1.
Electrical conductivity measurements were performed
at temperatures ranging from room temperature to
423 K, by using a two-probe technique.' The sample
discs (0.1 cm thick and 1.25 cm diameter) were made by
compressing the powdered sample at a constant pressure of 10 ton cm-' for 10min. The common circular
area of diameter 8.0mm on the disc surfaces was silvercoated to achieve better electrode contact. To avoid
N
surface charge effects, another ring, 2.0mm wide, on one
face close to the disc edge was also coated and connected to earth. The specimen was kept in a thermostatically controlled chamber stable to & 0.1"C. The
temperature of the specimen was measured by a thermocouple kept close to the specimen. The chamber was
evacuated to a pressure lower than 10-3torr to minimize moisture effects.
RESULTS AND DISCUSSION
Cathodic electropolymerized acrylonitrile (CEPAN )
It is worth mentioning that CEPAN materials prepared
with different electrolytic current densities usually have
the same physical and chemical properties.
The polymers formed were found to be insoluble in
common organic solvents indicating that they have a
highly cross-linked structure. The average density of
~ 3WC,
CEPAN samples was 1.194 + 0 . 0 0 5 g ~ m - at
which is higher than that reported for PAN prepared by
free-radical polymerization in DMF (1-1685 and
1.1776g~m-~)'','~or in NaCNS (1.1863g ~ m - ~ ) . ' ' , ' ~
This difference in density may be attributed to the high
molecular packing due to cross-linking. Molecular
packing in PAN is mostly due to the strong dipoledipole interaction between the nitrile groups of the
adjacent chain^,^ which increases in the case of crosslinking.
Optical microscopy studies of CEPAN showed that
the condensed film exhibited a striated structure. Figure
1 shows a typical fibril structure of CEPAN prepared at
a current density of 0.417mA cm-'.
X-ray diffraction studies indicate that CEPAN
samples have a partially crystalline structure independent of the current density used for preparation. Figure
2 represents the X-ray diffraction spectrum of samples
prepared at a current density of 0-417mA cm-'. X-ray
peaks appeared at 28 values of 39.1", 42.2", 46.2" and
54.6" after using an average moving point filter for 10
scans. These values are quite different from that reported for PAN prepared by free-radical polymerization
(28 = 16.5" and 29.5").8,10*16,'8This difference in X-ray
diffraction may be ascribed to the high cross-linking
and/or the presence of conjugated structure. .
Electrical conductivity measurements of CEPAN
showed that DC conductivities are less than
(0cm)- ', while AC measurements exhibited no
response to frequency in the range 10 to 105Hz. Likewise there was no observed temperature effect from
room temperature to 423 K. These results clearly indicate that the C=N groups, even the unconjugated
groups, form a highly cross-linked structure.
Elemental analysis of CEPAN samples prepared at
different electrolytic current density showed approximately the same carbon, hydrogen and nitrogen perPOLYMER INTERNATIONAL VOL. 41, NO. 4, 1996
465
Structure and properties of PAN
Fig. 1. Optical micrograph of CEPAN prepared at electrolytic current density of 0 . 4 1 7 m A ~ m -(magnification
~
x 1200).
centages (C 67.86, H 5.69 and N 26.27 (9942%)). These
values are in agreement with those reported16 for PAN
treated up to 200°C, and coincide with those calculated for pure acrylonitrile (calculated percentage, C
67-91,H 5.709, N 25.40).
IR spectra of these samples showed no significant difference with current density. Figure 3 illustrates the
identical IR spectrum of CEPAN samples. This spectrum is comparable with that reported6 for electropolymerized PAN, and that having amorphous
structure.* The absorption peaks appearing in the range
1000-1600 cm- indicate the presence of conjugated
C=N g r o ~ p s . ~ * *Th
*~
e ~band
- ~ ~ around 2200 cmgenerally attributed" to the presence of C=N,
appeared in these spectra and its relative height to the
band at 2240cm-' (which is ascribed to the presence of
'
',
4
3.5
3
2.5
2.5
e
-3
v
2
C
1.5
1.5
1
0.5
'
0
-10
15
20
25
30
35
ze. 0
40
45
50
55
1
0
60
Fig. 2. X-ray diffraction of CEPAN prepared at electrolytic
current density of 0-417mAcm-'; dashed curve, as produced;
the smooth curve is the average of 10 scans.
POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996
C 3 N ) remained constant for all samples prepared
under different current densities. Therefore it can be
concluded that the general structure of the CEPAN prepared is usually independent of the electrolytic current
density used for sample preparation. The structure of
this CEPAN sample is highly cross-linked, striated, partially crystalline and has a few cycles of single conjugated C=N bonds.
Medium electropolymerized acrylonitrile (MEPAN )
MEPAN products formed at various electrolytic
current densities were found to be highly soluble in
DMF in contrast to CEPAN. They exhibited colour
variations from pale yellow to brown, changing to dark
brown with increasing electrolytic current density. Their
densities were found to be 1-1676, 1.1681, 1.683 and
1.1689 g ~ m -for~ samples prepared at electrolytic
current densities 0.417, 0.83, 1.67 and 2-5mA cm-',
respectively. The slight differences in density may be
attributed to the degree of cyclization, which increases
with applied current density. Moreover, these values of
density are lower than the reported values,12'16which
may be due to lower molecular weights of MEPAN.
Optical microscopy investigations showed no characteristic features for all the samples prepared. Also, X-ray
diffractographs of MEPAN indicated an overall amorphous pattern in the region 28 = 5-60", One can conclude from this that at low electrolytic current density
this method of preparation can be used for synthesizing
amorphous PAN, previously obtained by chemical
methods.8*22In order to investigate the chain configuration of this amorphous PAN, the IR spectrum of
MEPAN samples prepared at an electrolytic current
density of 0.25 mA cm-' was compared with that of
466
F. M. Reicha
c
C
P
!
W
V
ztz
5z
a
+
[L
8
WAVE NUMBER CM-'
Fig. 3. IR spectra for CEPAN prepared at current density of 0.417 mAcm-2.
samples prepared chemically* (Fig. 4). No significant
differences between these two spectra were observed.
Bands appearing at 1500-1600 cm-l were attributed to
the cyclic by-product^,^^ as were the relatively weak
bands around 1420 and 1640cm-'. Most of the bands
were typical of those observed previously and were
ascribed to linear polymerization of nitrile
Samples prepared at high current densities were found
to be more conjugated, as described in the discussion of
elemental analysis and IR spectra.
The changes of weight percentage for carbon, hydrogen and nitrogen in relation to current density are tabulated in Table 1, as are the apparent colours of the
products. We note that the percentage of C, H and N
remains nearly constant on using a current density up
to 2.5 mA ~ m - Furthermore,
~ .
the colour of MEPAN
gradually changes from pale yellow to dark brown as
the current density increases. These results indicate that
the formation of chromophore could be attributed to
the formation of cyclized rings of C S N groups without
liberation of hydrogen from the products (i.e. single
conjugation).
The IR spectra of the products of MEPAN prepared
at different electrolytic current densities are presented in
Fig. 5. These spectra of electropolymerized PAN gathered from the medium, even at low current, have characteristic absorption bands as listed in Table 2. This
drastic change in IR spectra is very similar to those
reported for coloration of radically polymerized acryloWe note that for all
nitrile by pyroly~is.~,'
these samples the band around 2200 cm- l, generally
attributed to C=N, appears in these spectra beside the
1920*21925-27
TABLE 1. Results of elemental analysis and the percentage of the relative intensity
+I,,,,)
of MEPAN samples prepared at different electrolytic current densities and their colours
/,,oo/~(/,200
Current
density
(mA cm-')
0.417
0.83
1 *67
2.50
C
(wt%)
68-10
68.28
67.19
68.22
N
H
(wt%)
(wt%)
Calc
0 (wt%)
5.46
5.51
5.49
5.52
25.64
25.63
25.68
25.71
0.60
0.58
0.64
0.55
(%I
Apparent
colour
33.53
34.58
44.58
45.83
Pale yellow
Light brown
Brown
Dark brown
~2200/~(~22
+012240)
0
POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996
467
Structure and properties of PAN
4000
3600
3;00
2800
2400
2000
2240 cmband. However, the intensity of this
22OOcm-I band shows a systematic increase, while the
nitrile bond stretching ( C e N ) shows a systematic
decrease as the current density increases. Table 1 shows,
for various samples, the percentage of the relative intensity of the enamine band around 2200cm-’, estimated
by the ratio I,,,,/(~ I,,,, + I,,,,), which is the ratio of
the heights of the absorption C=N to the combined
heights of the two bands, 2200 and 2240 cm- l. It can be
seen that this ratio increases with increasing current
TABLE 2. Characteristic IR absorption
bonds
attributed to electropolymerized AN
IR bands
Tentative assignment (absorption modes)
(cm-’)
3 525-3 360
2 940
2 240
2 200
1640
1560
1 520
1450
1420
1385
1170
990
770
stretching (NH)
stretching (CH) (H 7 = y H )
stretching (C=N)
stretching (C=NH)
stretching (C=C)
stretching (C=N),
stretching (C=N)
bending (CH,)
bending in-plane (CH) (H 7 = T H)
bending (CH,) or bending (CH)
stretching (C-N)
bending out-of-plane (CH) (H 7 = y H)
bending out-of-plane (CH) (H 7 =T H)
POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996
1800
1600
1400
l;OO
1630
800
7
600
density up to 2.5mAcm-’ then remains nearly constant. In most previous works, the decrease in the intensity of the 2240cm-’ band was attributed19 to the
decrease in the concentration of CEN groups, which
are not converted to C=N or other chemical structures.
The appearance of a band near 2200cm-’ could be
explained” on the basis of the formation of cyclic structure of conjugated PAN. Therefore, one can conclude
that this ratio reflects the degree of increase of interchain cyclization during the electropolymerization of
AN at various current densities.
The polymer formed reacts with trace amounts of 0,
(present during electropolymerization) to give a
pyrridone-type structure,19 which has a brown colour.
This is consistent with our observation that, as the
current density increases, the sample colour turns from
pale yellow to light brown then to brown and finally to
dark brown. The UV spectra (Fig. 6 ) for MEPAN
samples prepared at various current densities confirm
this conclusion. The UV spectra show that the absorption intensity increases with increasing preparation
current density, reaching a constant value at a current
density of about 2.5 mA cm-’. The absorption peak at
4.05eV is characteristic of all samples prepared at different current densities. Also, it can be noted that some
absorption peaks near to the characteristic one (4.05 eV)
appeared for samples prepared at current densities
above 2.5rnAcm-’. This may be a result of byproducts due to additional monomer reaction pathways
F. M . Reicha
468
w
7
I,
I
4
3
I
YO
'r
/f
C
r
a
11
s
111
. .
_._-
I
t
t
1
n
C
~ ..
.
Wavenumbers
Fig. 5. IR spectra of MEPAN prepared at current densities: (a) 0*417rnA~m-~;
(b) 0 . 8 3 3 m A ~ m - ~(c)
; 1 * 6 7 m A ~ r n -(d)
~;
2*5rnA~rn-~.
POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996
469
Structure and properties of PAN
5.0-4.8
-
4.61.4-
i90200
20
40
tio
m
300
20
40
m
80
W a n length
400
20
LO
m
80
m
M
40550
(nm)
Fig. 6. UV spectra for MEPAN prepared at current densities: (a) 0-417rnAcm-’; (b) 0.833mAcrn-’; (c) 1*67rnAcm-’; (d)
2.5 d c m - ’ ; (e) 3-33mA cm-’. The peaks below 250nm are artifacts related to the limitations of the detector.
and/or further reaction of the polymer. Therefore one
can conclude that as the current density used for electropolymerization increases, the interchain cyclization
spreads over the polymer chain due to conjugation of
CEN, until the current density reaches about
2.5 mA cm-’, above which side reactions may take
place.
It is interesting to note that the samples prepared at
current densities ranging from 0.25 to 2.5 mA cm-’
possess a homogeneous structure, while those prepared
at current densities above 2.5 mA cm-’ show nonhomogeneity as indicated by TLC. The samples prepared at current densities 0-417 and 2.5 mA cm-’ were
chosen for investigation by TLC. The results indicated
that both samples have homogeneous structure. TLC
showed mean retardation factors of 0.58 and 0.26 for
the two samples, respectively. The retardation factor RP
depends upon molecular weight and/or the dipole
moments. The ratio of the intrinsic viscosities of both
samples was found to be 1 : 1.02, which shows a slight
increase in molecular weight for samples prepared at
current density of 2.5 mA cm-’ over those prepared at
0.417 mA cm-’. This indicates that the molecular
weight is not the major contributing factor. The main
change in R, could be related to the dipole moment as
POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996
follows. The high value of the dipole moment (3.9
debye)28 of C=N causes strong attraction or repulsion
(according to the orientation) of other medium molecules (for example, acetone in the present case) which
also possess a high dipole moment. A strong and rather
specific interaction between pairs of C E N groups is
also possible, which minimizes the possible attraction
with a c e t ~ n e ’ ~leading
. ~ ~ to an increase in values of R,
for samples having more C z N groups without conjugation. This result may therefore be another indication of increasing conjugation with increasing electrolytic current density used for sample preparation.
DC conductivity of MEPAN samples
The current as a function of the applied field (Vm-’) in
the range from lo4 to lO' Vm-l was studied at various
temperatures. Within experimental error the current
exhibits a linear field dependence. From this ohmic relation the conductivity at each temperature was determined.
The temperature variation of DC conductivity for
MEPAN samples (prepared at different current density)
is shown in Fig. 7. The data clearly show that, in the
temperature region investigated, the DC conductivity of
470
F. M . Reicha
-3-.
-4 - -
-5..
.--
‘5
<!-
-
-6--
-7-.
b
I’- 8 - -9--
-10
--
-11
-12
2.2
2.4
2.6
2.8
3.0
3.2
(1WOlT)K
3.4
3.6
3.8
4.0
Fig. 7. Variation of loga as a function of temperature for MEPAN samples prepared at current densities: (a) 0-417mA~rn-~;
(b)0~833mAcm-2;(c)1~67mAcm-2;(d)2~5mAcm-2.
these samples obeys the Arrhenius relation :
o = a. exp(-E/K T )
where cro is the conductivity pre-exponential factor, K is
the Boltzmann constant and E is the activation energy
for conduction. Each sample is characterized by a single
activation energy in the temperature interval used here.
The conduction activation energy has been calculated
using least-squares fitting of the data to the above relation. The variation of activation energy E with current
density is given in Table 3. The activation energy for
samples prepared at current density 0.417mA ~ m was
- ~
found to be 1.48eV7 which is in good agreement with
the earlier reported value of 1-5eV for pyrolysed polyTABLE 3. Values of DC electrical resistivity p at
room temperature, activation energy E and preexponential factor a, for MEPAN samples prepared
at different current densities
Current density
(rnA crn-’)
0.41 7
0.833
1.67
2.50
p at 300 K
(ncm)
1.5 x
3.4 x
1.3
5.8 x
10’’
10’0
~10~
107
Log Qo
E
(n-’crn-‘)
(ev)
13.12
11.23
9.1 7
7.68
1-48
1a27
1.09
0.93
a~rylonitrile.~’
As the current density increases the conduction activation energy decreases until the current
density reaches about 2.5 mA ~ m - ~
but, on further
increase in current density there is little change in activation energy values.
Extrapolation of the plot of log o against 1000/T gives
values of the conductivity pre-exponential factor oo.
These are listed in Table 3 together with values of resistivity at room temperature and activation energy E.
From Fig. 7 it is concluded that the electronic conduction in these samples is a thermally activated
process with a single activation energy in the temperature interval used for measurements. It is noticed
from Fig. 7 and Table 3 that E decreases with increasing preparation current density (up to 2-5mA ~ m - ~ ) ,
and the pre-exponential factor oo, which is of the order
of 1013~2-1cm- for samples prepared at current
density
0.417 mAcm-2,
decreases to
about
106C2-’ cm-’ for samples prepared at current density
2*5mAcrn-’ or higher. Also, we note that for all
samples prepared at current densities above
2.5 mA ~ m the
- ~variation of logo against 1000/T
which is not represented in the figure, gives values nearly
identical to that prepared at a current density of
2.5 mA cm-’. From the values of resistivity, the preexponential factor and activation energy one can conclude that the samples behave as semi-conducting
’
POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996
Structure and properties of PAN
materials. Their conductivities increase as the preparation current density increases, which in turn
increases the cyclization in the macrochain. In this way,
the electrical conduction of the polymer samples
(MEPAN) is due to participation of lone pairs of electrons on the conjugated nitrogen atoms with a-bonds in
the macrochain, in which delocalization of electrons is
also possible. As conjugation increases the electron
mean-free path increases, leading to an increase in conductivity and in the pre-exponential factor, while a
decrease in conduction activation energy is possible.
CONCLUSIONS
The results of the present experiments reveal that
MEPAN and CEPAN are different structural types of
PAN formed during its synthesis by the electropolymerization technique. The CEPAN samples most
commonly studied are highly cross-linked, partially
crystalline, have a striated structure and are insulating
R-' cm-'). MEPAN
in nature (conductivity
collected from the solution is completely amorphous,
and its conductivity was found to increase as the
current density used for the polymerization process
increased. This is attributed to the spreading of conjugated C=N groups which occurred during the preparation process as shown by elemental analysis, IR, UV
and other techniques.
-
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