Protonation of polyaniline with lightly sulfonated
polystyrene
Yiwpirlg
Fii,
K. A. Weiss'3
Polymer Science Program and Department of Chemical Engineering.
University of Connecticut. Storrs. CT 06269-3 136, U.S.A.
(Received: March 29. 1996)
SUMMARY
Protonation of polyaniline base with lightly sulfonated polystyrene in polar solvents
such its dimethyl sulfoxide and N-methyl-2-py~olidonewas investigated by UV-Vis
absorption spectra. As the molar ratio of sulfonated polystyrenelpolyaniline increases,
the conversion from polyaniline base form to salt form is observed owing 10 incrensed
protonation. The isosbestic point clearly shows that quinoid unit and semiquinoid unit
are in equilibrium. They are functions of the sulfonic acid concentration and solvent
medi ii.
Introduction
In the past fifteen years, significant progress has been made on organic conducting polymers with novel electrical, optical and electrochemical properties '.-').
Among these conducting polymers, polyaniline (PANI) shows very promising industrial application, because of its good environmental stability and facile synthesis". lo).
Polyaniline can be reversibly doped by protonation of the imine nitrogen atoms
through acid-base chemistry, and this is accompanied by a ten order of magnitude
increase in conductivity. It can be synthesized by the oxidative chemical or electrochemical polymerization of aniline (AN) in aqueous acidic solution ' I .I3'. The resulting PANI salt (conducting form) is generally not soluble in any organic solvent. and
is not fusible. However, PANI base (insulating form)? which can be obtained by
treating the conductive form with aqueous N k O H . is soluble in N-methyl-2-pyrrolidone (NMP). N,N'-dimethylpropylencurca (DMPU), and slightly soluble i n dimethyl
su I fox i de (DMS0).
Recently. significant progress has been made in the processing of polyaniline.
Cao et al. 'I used functionalized protonic acids, such as dodecylbenzenesulfonic
acid (DBSA), to dope PANI base and simultaneously render the resulting PANI coniplcx soluble in common organic solvents. The use of surl'actant counter-ions to
induce co-solubility and compatibility with bulk polymers enables the synthesis of
highly conducting polyblends with a variety of commercial polymers l5I. Tzou et
al. I('' also reported a similar approach by using organic acid its dopants. Others used
polymeric acids, such as poly(styrene sulfonic acid) 17-'31, and poly(acrylic
acid)ll-?71. Polyaniline doped with these polyelectrolyte polymers exhibited water
solubility 'x-2i'.
We are interested in preparing conductive polymer blends with improvcd processability and better controlled conductivity. Theoretically. the conductivity of polyani0 1996. Hiithig & Wept' Verlag. Zug
ccc 1022- 1336/96/502.50
488
Y. Fu, R. A. Weiss
line can be varied from 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 "). Here, lightly sulfonated polystyrene (HSPS) was chosen as the
polymer matrix, because the randomly placed sulfonic acid groups on the polystyrene chains act as a dilute acid in an organic medium. We expect HSPS 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 communication, we present direct evidence of doping PANI through protonation by sulfonic acid groups of
lightly sulfonated polystyrene in non-aqueous solutions and its dependence on the
solvent environment.
Experimental part
Aniline (99%)was obtained from Fisher and distilled before use. Spectroscopic grade
dimethyl sulfoxide (DMSO), and N-methyl-2-pyrrolidone (NMP) were also obtained
from Fisher. PANI hydrochloride powder was synthesized by the oxidative polymerization of aniline in 1.0 M aqueous HCI with ammonium peroxydisulfate (APS) as the oxidant, as described previously''). The powder was converted to PANI base (emeraldine
base) by treatment with 0.5 M aqueous ammonia solution followed by drying under
= 100000, aw
= 280000) was sulfonated in dichloroethane at
vacuum. Polystyrene (an
50°C with acetyl sulfate following the procedure of Makowski et a1.29) The sulfonated
polystyrene was recovered by steam striping, washed with methanol and dried under
dynamic vacuum. The sulfonic acid concentration was determined to be 5.3 mol-% by
titration of the polymer in toluenehethanol mixed solvent (vol. ratio 90/10) with NaOH
in methanol.
PANVHSPS blends with different molar ratios were prepared by mixing appropriate
volumes of two solutions, 1 mM PANI base (based on the approximate aniline repeating
unit -C,H,NH-) in DMSO or NMP and 10 mM HSPS (based on the styrene unit) in the
same solvent, to give a clear solution. UV-Vis absorbance spectra were recorded with a
Perkin-Elmer Lambda 6 UVNIS spectrophotometer. For solution measurements, the
concentration of PANI was kept constant at 0.06 mM, while the concentration of HSPS
was varied to get the desired [-HSO,H]/[AN] ratios.
Results and discussion
The UV-Vis absorption spectra of different ratios for PANIMSPS 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 n-n* transition of benzenoid
rings, while the absorption peak at 640 nm is due to the n-x* transition of quinoid
rings on the PANI chain. In the PANI base form, half of the N atoms exist as amine
groups and the other half as imine groups. Since only the imine groups can be doped
by acid, the maximum doping level of the PANI is 0.5 equivalent protons. When
Protonation of pol yaniline with lightly sulfonated polystyrene
489
0.6
0
c
0
a
8 0.5
u1
n
Q
Fig. I . UV-Vis absorption
spectra of PANI base at different concentration of
HSPS in DMSO, concentration of PANI is kept constant at 0.06 mM,
(a) [-SO,H]/[ANI = 0;
fb) [-SO,H]/[AN] =0.75:
(c) J-SO,H]/[AN] = I . 1
0.L
0.3
0.2
0.1
"
320 LOO
L80
560
610
720 800
880
Wavelength in nrn
PAN1 base solution was mixed with HSPS in DMSO solution, a clear solution
resulted. This indicates that the HSPS can induce solubility of the polyaniline salt in
the same way as an organic acid dopant. As the molar ratio of [HSPSj/[PANI] was
increased, the color of the solution gradually changed from blue to green due to
increased protonation of the imine sites of PANI. The UV-Vis spectra showed a new
absorption peak at 820 nm due to protonation of the imine sites of PANI. The
absorption peak originates from a polaron band transition, which is also observed by
protonation with organic acid in solution I").
As the molar ratio of [HSPSJ/[PANI]increased, the intensity of the 820 nm also
increased and correspondingly, the intensity of the quinoid absorption peak at
640 nm decreased. A clear isosbestic point at 730 nm was observed, which indicates
that the sulfonic acid groups directly protonate the imine nitrogen and transform the
quinoid units into semiquinoid units as shown below:
This conversion between quinoid and semiquinoid units is reversible and quantitative. The amount of' sulfonic acid required to reach the maximum doping level,
however, was greater than the theoretical 0.5 equivalent. This reduction of effective
protonation of' quinoid rings of PANI may be due either to conformation hindrance
of the polymeric sulfonic acid groups andor to solvent effects. The influence of the
490
Y. Fu, R. A . Weiss
polymeric nature of acid was evaluated by comparing its effectiveness at protonating
PANI base with that of a low molecular weight organic acid, dodecylbenzenesulfonic acid, in DMSO solution. Little improvement in protonation was found for the
latter acid, especially for [-SO,H]/[AN] ratios of less than 0.5, which indicates that
conformation hindrance of the polymeric sulfonic acid had little effect on protonation of PAN1 for these dilute concentrations.
The protonation of PANI base with HSPS was also studied in NMP solution to
assess the solvent effect on doping behavior. Fig. 2 shows the UV-Vis absorption
spectra for different molar ratios of [HSPS]/[PANI] in NMP. All the absorption
peaks for PANI base were blue-shifted in NMP compared to those in DMSO. The
absorption peak due to the R-X* transition of benzenoid rings on PANI chain shifted
from 335 nm to 325 nm, while the absorption peak due to the R-R* transition of quio,
u
0.5
Fig. 2. UV-Vis absorption
spectra of PANI base at different concentration of
HSPS in NMP, concentration of PANI is kept constant at 0.06 mM,
(a) [-SO,H]/[AN] = 0;
(b) [-SO,H]/[AN] = 1.1;
(c) [-SO,H]/[AN] = 2.1 ;
(d) [-SO,H]/[AN] = 3.2
0.3
0.2
0.1
0
320 400
480 560 640 720 800
880
Wavelength in nrn
noid rings shifted from 640 nm to 585 nm. 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 PANI 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. Chen et al.
studied the doping of PANI by poly(acry1ic acid) (one equivalent) in NMP. They did
not observe the semiquinoid peak in solution, though, they did observe the semiquinoid peak for a thin film cast from the same solution. They attributed this phenomenon to strong hydrogen bonding between the carbonyl group of NMP and the carboxylic acid. Our results indicate that a similar competition of the NMP solvent for
hydrogen bonds with HSPS also decreased the PANI-protonation efficiency of the
polymeric acid, though in our case, we still observe protonation in solution. That
suggests that the semiquinoid peak might also be observed for poly(acry1ic acid)/
PANI complexes in NMP solution if the concentration of poly(acry1ic acid) were
increased to 2-3 equivalents.
49 I
Protonation of polyaniline with lightly sulfonated polystyrene
The intensity of the UV-Vis serniquinoid absorption is proportional to the concentration of the protonated state of PANI, and the intensity of the quinoid absorption is
proportional to the concentration of the neutral state of PANI. The ratios of absorption intensity of the protonated states and the neutral states (A~L',lliql,i,lOid/Aql,l~l~ill
) are
plotted against the ratio of the sulfonic acid group and the aniline repeating unit concentrations, [-SO,H]/[AN], in DMSO and NMP solutions in Fig. 3 . When
[--SO,H]/[ AN] is less than 0.5, A,e,l~iyu,n~,id/Aqu,l,~,id
is essentially zero in both solvents.
which indicates that no significant amount of the protonated state of PANI exists at
low [--SO,H]/[AN] ratios. Once the [--SO,H]/IAN] ratio is above 0.5, however, the
A,c,,i,~l,llll~,id/A~,l,inoid
ratio in DMSO increases dramatically, while the A,,,,,i c,,,,, lo,d/Aql,inr,id
Fig. 3. Protonation of PANI base with
HSPS as function of [-SO,H[/[AN]
ratios in (m) DMSO and (a) NMP
1.0
0
0
1.0
2.0
30
I-SO,Hl/ [AN]
ratio in NMP increases at a much slower rate. The competing hydrogen bonding
between the carbonyl group of NMP and the sulfonic acid may suppress the protonation of PANI base, since it effectively reduces the concentration of available free
acid groups that may interact with the PANI. 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 HCI concentration on the protonation of PANI film was also reported by Wan '('I.
In conclusion, protonation of PANI base with lightly sulfonated polystyrene was
demonstrated for DMSO and NMP solutions. A relatively low concentration of sulfonic acid groups on the polymer, 5.3 mol-72, was sufficient for doping PAN1 and
promoting solubility of the resulting macromolecular complexes. The observation of
an isosbestic point clearly shows that the quinoid unit and the semiquinoid unit were
in equilibrium, and it was a function of the sulfonic acid group concentration. The
protonation of polyaniline is retarded in NMP compared to DMSO due to the competition of hydrogen bonding of the NMP solvent to the sulfonic acid group.
Ackrro~c.lc.clgernent:We gratefully acknowledge financial support for this work by Con-
wcticut Imovufio~?.~,
hc.
492
Y. Fu, R. A. Weiss
T. J. Skothei, Ed., Handbook of Conducting Polymers, Marcel Dekker, New York
1986
)' A. 0. Patil, A. J. Heeger, F. Wudl, Chem. Rev. 88, 183 (1988)
3, J. Roncali, Chem. Rev. 92,7 11 (1992)
4, R. H. Baughman, J. L. Bredas, R. R. Chance, R. L. Elsenbaumer, L. W. Shacklette,
Chem. Rev. 82,209 (1982)
5 , W. R. Salaneck, I. Lundstron, B. Ranby, Eds., Conjugated Polymers and Related
Materials, Oxford Univ., New York 1993
6 , P. N. Prasad, D. R. Ulrich, Eds., Nonlinear Optical and Efectroactive Polymers, Plenum, New York 1988
7, J. L. Bredas, R. R. Chance, Eds., Conjugated Polymeric Materials: Opportunities In
Electronics, Optoelectronics, and Molecular Electronics, Kluwer Academic, Dordrecht 1989
8, J. L. Bredas, R. Silbey, Eds., Conjugated Polymers, Kluwer Academic, Dordrecht
1991
9, E. M. Genies, A. Boyle, M. Lapkowski, C. Tsintavis, Synth. Met. 36, 139 (1990)
1'
F. Lux, Polymer 35,2915 (1994)
11) J. P. Travers, J. Chroboczek, F. Deverux, F. Genound, M. Nechtschein, A. Syed, E. M.
Genies, C. Tsintavis, Mol. Cryst. Liq. Cryst. 121, 195 (1985)
12) A. G. Macdiarmid, J. Chiang, M. Halpern, W. Huang, S. Mu, N. L. D. Somasiri,
W. Wu, S. I. Yaniger, Mol. Cryst. Liq. Cryst. 121, 173 (1985)
13) Y. Fu, R. L. Elsenbaumer, Chem. Muter: 6,671 (1994)
14) Y. Cao, P. Smith, A. J. Heeger, Synth. Met. 48,91 (1992)
Is) C. Y. Yang, Y. Cao, P. Smith, A. J. Heeger, Synth. Met. 53,293 (1993)
16) K. Tzou, V. Gregory, Synth. Met. 53, 365 (1993)
17) S. Ghosh, V. Kalpagam, Synth. Met. 60, 133 (1993)
M. Angelopoulos, N. Patel, J. W. Shaw, N. C. Labianca, S. A. Rishton, J. Vuc. Sci.
Technol. B 11,2794 (1993)
19) M. Angelopoulos, N. Patel, Polym. Muter: Sci. Eng. 71, 222 (1994)
20) K. Shannon, J. E. Fernandez, J. Chem. SOC.,Chem. Commun. 643 (1994)
21) Y. Liao, K. Levon, Macromol. Rapid Commun. 16,393 (1995)
22) J. H. Hwang, S. C. Yang, Synth. Met. 29, E271 (1989)
23) Y. Kang, M. Lee, S. Rhee, Synth. Met. 52, 3 19 (1992)
24) J. Liu, S. Yang, J. Chem. Soc., Chem. Commun. 1529 (1991)
25) S. Chen, H. Lee, Synth. Met. 47, 233 (1992)
26) S. Chen, H. Lee, Synth. Met. 55-57, 1040 (1993)
27) S. Chen, H. Lee, Macromolecules 28,2858 (1995)
28) G. E. Asturias, A. G. MacDiarmid, R. P. McCall, A. J. Epstein, Synth. Met. 29, El57
(1989)
29) U.S. Pat. 3,870,841 (1975), H. S. Makowski, R. D. Lundberg, G. H. Singhal; Chem.
Abstr: 81, 153,695C (1974)
30) M. Wan, J. Polym. Sci., Polym. Chem. Ed. 30, 543 (1992)