ELSEVIER
Synthetic
Metals
84 (1997)
103-104
Protonation of polyaniline with lightly sulfonated polystyrene
Yueping Fu, R. A. Weiss
Polymer Science Program and Department of Chemical Engineering
UniversiQ of Connecticut, Storrs, CT 06269-3136, UsA
Abstract
Protonation of polyaniline base with lightly sulfonated polystyrene in polar solvents such as dimethyl sulfoxide (DMSO) and
N-methyl pyrrolidone
(NMP) was investigated by UV-Vis absorption spectroscopy. The isosbestic point clearly shows that
quinoid unit and semiquinoid
unit are in equilibrium
and is a function of the sulfonic acid concentration.
The protonation of
polyaniline is retarded in NMP compared to DMSO due to prevalent hydrogen bonding.
keywords:
Polyaniline,
sulfonated
polystyrene,
UV-Vis
absorption
1. Introduction
Polyaniline
(PANI) has very promising industrial applications because of its good environmental stability and facile
synthesis [I, 21. Polyaniline
doped with organic protonic
acids [3], and polymeric acid [4] shows improved solubility
and processibility.
We are interested in preparing conductive polymer blends
with improved processability
and better controlled conductivity. Theoretically,
the conductivity of polyaniline
can be
varied from lo-lo to 10 S/cm as a function of doping level. In
reality, the conductivity of polyaniline increases dramatically
at low doping level and levels off at ca. 10% doping [.5]. Here,
lightly sulfonated polystyrene
(HSPS) was chosen as the
polymer matrix, because the randomly placed sulfonic acid
groups on the polystyrene chains behave as a dilute acid. It
will protonate the imine nitrogen sites of polyaniline,
while
possibly retaining the processability
of polystyrene.
This
molecular protonation should promote compatibility
between
polyaniline
and polystyrene
within
the blends,
and
simultaneously
transform
the insulating
polyaniline
base
form to the metallic conducting form, thereby rendering the
blends conductive.
The use of polymeric dopants may significantly improve the stability of the resulting polymer blends,
because small molecule dopants tend to migrate out of the
polymer matrix.
In this paper, we present direct evidence of doping PAN1
through protonation by sulfonic acid groups of lightly sulfonated polystyrene
in solutions and its dependence on the
solvent environment.
2.
Experimental
PANI was synthesized by the oxidative polymerization
of aniline
in 1.0 M aqueous
HCl with ammonium
peroxydisulfate (APS) as oxidant, as described previously [6].
Polystyrene (M, = 100,000, M, = 280,000) was sulfonated to
5.3 mol % in dichloroethane
at 50 OC with acetyl sulfate
following the procedure of Makowski et al [7].
PANI/HSPS
solutions with different molar ratios were
prepared by mixing appropriate volumes of two solutions, 1
mM PAN1 base (based on the approximate aniline repeat unit
-CgHqNH) in DMSO or NMP and 10 mM HSPS (based on
0379-6779/97/%17.00
0 1997
PII SO3794779(96)03857-X
Elswier
Science
S.A. All
rights
reserved
styrene unit) in the same solvent, to give a clear solution,
keeping the concentration of PANI constant at 0.06 mM. UVVis absorbance spectra were recorded on Perkin-Elmer Lambda
6 UV/VIS spectrophotometer.
3. Results
and
discussion
The UV-Vis
absorption spectra of different ratios of
PANI/HSPS in DMSO solution are shown in Fig. 1. The PANI
base has two absorption peaks at 335 nm and 640 nm. The
absorption peak at 335 nm is due to the rc-TC* transition of
benzenoid rings, while the absorption peak at 640 nm is due
to the X-Z* transition of quinoid rings on the PAN1 chain.
“I
‘II
-I’
I”
I”
“I
‘*.
I-’
“I
0.5
g 0.4
B 0.3
5
JJ 0.2
0.1
0.0
300
400
500
600
Wavelength
700
800
900
(nm)
Fig. 1. UV-Vis absorption spectra of PANI base at different
concentration of HSPS in DMSO,
concentration of PAN1 is
kept constant at 0.06 mM, (a) [-S03H]/[AN]
= 0.0;
(b) [-S03H]/[AN]
= 0.75; (c) [-S03H]/[AN]
= 1.1
When PAN1 base solution was mixed with HSPS in DMSO
solution, a clear green solution resulted. This indicates that
the HSPS can induce solubility of the polyaniline salt in the
same way as an organic acid dopant. The UV-Vis spectra
showed a new absorption peak at 820 nm due to protonation of
the imine sites of PANI. This absorption peak originates from
the polaron band transition, which is also observed by protonation with organic acid in solution [8]. As the molar ratio of
HSPS to PAN1 increased, the intensity of the 820 nm peak
Y.Fu, R.A. Weiss/SyntheticMetals 84 (1997) 103-104
104
increased and correspondingly,
the intensity of the quinoid
absorption peak at 640 nm decreased. A very clear isosebestic
point at 730 nm was observed, which implies that sulfonic
acid groups directly
protonate
the imine nitrogen
and
transform the quinoid units into semiquinoid units as shown
below (01 y< 1):
3
2.5
2
1.5
1
0.5
0
1
0
This conversion between quinoid and semiquinoid
units is
reversible and quantitative.
The amount of sulfonic acid
required to reach the maximum doping level was greater than
the theoretical 0.5 equivalent.
This reduction of effective
protonation of quinoid rings of PAN1 may be due either to
conformation hindrance of the polymeric sulfonic acid groups
or to solvent effects, or both.
go’
2 0.
5
s 0.
6
0.
0.
Wavelength (nm)
Fig. 2. UV-Vis absorption spectra of PAN1 base at different
concentration of HSPS in NMP, concentration of PAN1 is kept
constant at 0.06 mM, (a) [-S03H]/[AN]
= 0.0; (b) [S03H]/[AN] = 1.1; (c) [-S03H]/[AN]
= 2.1; (d) [-SOgH]/[AN] =
3.2.
The protonation of PAN1 base with HSPS was also run in
NMP solution to assess the solvent effect on doping behavior.
Fig. 2 shows the UV-Vis absorption spectra of different molar
ratios of HSPS/PANI in NMP solution. The absorption peaks
for PAN1 base were blue shifted in NMP compared to those in
DMSO. As the [HSPS]/[PANI]
ratio increased, a new
absorption peak at 830 nm was observed due to the transition
from semiquinoid rings. Fig. 2 clearly shows that doping of
PAN1 by HSPS in NMP solution
does occur, but the
protonation
is less effective than in DMSO, and the acid
concentration has to be more than doubled to reach the same
amount of doping level.
The semiquinoid
absorption intensity is proportional to
the concentration of the protonated state of PANI, and the the
quinoid absorption intensity is proportional
to the concentration of the neutral state of PANI. The ratios of absorption
intensity of the protonated
states and the neutral states
(Asemiquinoid/Aquinoid)
are plotted against the concentration
ratio of the sulfonic acid group and the aniline repeat unit,
0.5
1
1.5
2
2.5
3
3.5
[-S03H] / [AN]
Fig. 3. Protonation of PAN1 base with HSPS as function of
[-S03H]/ [AN] ratios in DMSO and NMP.
[-S03H]/[AN],
in DMSO and NMP solutions in Fig. 3. When
[-SO3H]/[AN]
is less than 0.5, Asemiquinoid/Aquinoid
is
essentially zero in both solvents.
This indicates that no
significant amount of the protonated state of PAN1 exists at
low [-SOgH]/[AN]
ratios. Once the [-S03H]/[AN]
ratio is
above 0.5, the Ascmiquinoid/Aquinoid
ratio in DMSO
increases dramatically, while the Asemiquinoid/Aquinoid
ratio
in NMP increases at a much slower rate. The prevalent
hydrogen bonding between the carbonyl group of NMP and the
sulfonic acid significantly
retards the protonation
of PAN1
base, because hydrogen bonding may reduce the available free
acid groups. We recently also observed undoping of PANI-CSA
in NMP solution during dilution with the color change from
green to blue. The transition from an insulating form to a
metalic conductive form of PANI is evidenced by a steep
increase in the concentration
of protonated material.
A
similar observation for the effect of HCl concentration on the
protonation of PAN1 film was also reported by Wan [9].
Acknowledgment
We gratefully acknowledge
work by Connecticut Innovations,
financial
Inc.
support
for this
References
[ 11 E. M. Genies, A. Boyle, M. Lapkowski and C. Tsintavis,
Syrtth. Met., 36 (1990), 139.
[2] F. Lux, Polymer, 35 (1994), 2915.
[3] Y. Cao, P. Smith and A. J. heeger, Synth. Mef., 48
(1992), 91.
[4] M. Angelopoulos and N. Patel, Polym. Muter. Sci. Eng.,
71 (1994), 222.
[S] A. G. Macdiarmid, J. Chiang, M. Halpern, W. Huang, S.
Mu, N. L. D. Somasiri, W. Wu and S. I. Yaniger, Mol.
Cryst. Liq. Cryyst., 121 (1985), 173.
[6] G. E. Asturias, A. G. MacDiarmid, R. P. McCall and A. J.
Epstein, Synth. Met., 29 (1989), E157.
[7] H. S. Makowski, R. D. Lundberg and G. H. Singhal, U.S.
Pat. 3,870,841, (1975).
[X] K. Tzou and V. Gregory, Synth. Met., 53 (1993), 365.
[9] M. Wan, J. Polym. Sci., Polym. Chern. Ed., 30 (1992),
543.