Synthetic Metals, 58 (1993) 309-324 309

Investigations on the effect of 5-sulfosalicylic acid on the properties of polyaniline

Dinesh Chandra Tr ivedi and Sundeep Kumar Dhawan Centre for Studies in Conducting Polymers, Central Electrochemical Research Institute, Karaikudi-623 006 (TN) (India)

(Received August 24, 1992; in revised form November 6, 1992; accepted November 11, 1992)

A b s t r a c t

Results of the investigation of the chemical and electrochemical polymerization of aniline in 5-sulfosalicyiic acid medium and its characterization by electrochemical and spectro- scopic techniques are presented. The investigation reveals that 5-sulfosalicylic acid as a dopant not only enhances electrochemical stability of polyaniline at higher potentials but also yields a polymer which is soluble to the extent of 11 g/l in DMSO. The characterization of conducting polyaniline thus obtained was carried out by electronic and vibrational spectra, ~H NMR in DMSO-d6, X-ray powder diffraction, thermal analysis and by electrochemical techniques.

1. I n t r o d u c t i o n

Conduc t ing po lymer s have b e c o m e foci in po lymers and mater ia ls sc ience due to the i r m a n y known and envisaged technologica l appl icat ions, such as ene rgy s to rage [1, 2], e l ec t romagne t ic in te r fe rence (EMI) shielding [3, 4], e l ec t roch romic dev ices [5, 6] and sensors [7].

Polyani l ine (PAn), the po lymer resul t ing f rom oxidat ive po lymer iza t ion of aniline, has b e e n known since 1862 [8]. Then, af ter a gap of near ly 100 years , the e lec t ron ic conduct iv i ty and its d e p e n d e n c e on r ed o x level, acidity and hydra t ion was r e p o r t e d by Surville et al . [9] in 1968. Polyanil ine has a roused grea t in te res t over the last few years due to its potent ia l for appl ica t ions in var ious op to-e lec t ron ic devices [10, 11] and as an antistat ic mater ia l [ 12 ] in spi te o f its unprocessab i l i ty by usual techniques . Polyani l ine is par t icular ly chal lenging because its conduct iv i ty and solubili ty in polar organic so lvents d e p e n d s no t only on the oxidat ion state but also on the deg ree o f p r o t o n a t i o n and dopant . Polyanil ine is built up f rom r ed u ced (B-N H- B - - NH) and oxidized ( B - N = Q = N - ) r epea t units, where B deno te s benzeno id and Q deno t e s quinoid ring. Thus, the ratio of amine to imine yields var ious s t ruc tu re s such as l eucoemera ld ine ( r educed form), emera ld ine base (50% oxid ized fo rm) and pernigrani l ine (fully oxidized form). Unlike o the r pheny lene -based conduc t ing polymers , polyanil ine has a react ive - N H group in a p o l y m e r chain f lanked on e i ther side by a pheny lene ring which

0379-6779/93/$6.00 © 1993- Elsevier Sequoia. All rights reserved

310

gives it a very high chemical flexibility such as protonation and deprotonation in addition to adsorption through nitrogen which has a lone pair of electrons. The very presence of the -NH group is responsible for interesting chemistry and physics with technological applications [1-7, 13]. Though the envisaged technological applications are many, the unprocessible nature of doped polyaniline has restricted its use.

Protonation in polyaniline not only involves the ingress of protons but is also accompanied by ingress of anions to maintain the charge neutrality. The electrochemical behaviour of polyaniline not only depends on the pH condition but also on the counter ion of the Br6nsted acid used for doping. In this paper, we present our results on the polyaniline--5-sulfosalicylic acid system and compare its characteristic properties with other organic acid-PAn systems reported earlier [14-16], without entering into the controversy of the disputed mechanism of transport of protons and anions being migration or diffusion controlled.

The electrochemical stability of the PAn film at higher potential is essential for its possible use as electrochromic material and as catalytic electrodes. Similarly, the soluble PAn is also required for many applications to facilitate post synthesis processing. The solubilization can be achieved by two methods: (i) pre-functionalization of aniline as suggested by Geni~s and Noel [ 17 ]; or (ii) by introducing bulky anionic dopant [ 15 ]. Since PAn has two acid functions, a strong and a weak one, it is possible to achieve a complexation or rather an electrostatic interaction between the charges on the polymer and doping anion to yield a ternary system, whereby it is possible for the doped polymer to interact with the dipole end of the solvent to yield a solvated polymer [18].

Our present study aims at achieving solubility of doped polyaniline of good conductivity and electroactivity in an organic solvent so that thin films on insulating substrates can be obtained either by diping or spraying without affecting the basic molecular geometry of polyaniline.

2. Exper imenta l

Aniline (Fluka, 99.5%) and dimethyl sulfoxide (99%) were distilled under vacuum and kept under nitrogen in the dark. All other chemicals were analytical grade. Before use all solutions were thoroughly purged with At.

Platinum electrodes were polished with increasingly finer grades of alumina (0.05 ftm). The indium-tin oxide (ITO) was used without any such treatment, but was rinsed thoroughly in acetone before use.

Polymerization of aniline in aqueous sulfosalicylic acid solution (SSA) was carried out by two methods: (i) chemical oxidative polymerization using ammonium persulfate; and (ii) anodic polymerization at platinum (Pt), ITO (conducting indium-tin oxide) and stainless-steel (s.s.) electrodes. The polymerization and characterization were carried out at 30_+ 1 °C.

311

2.1. Chemical polymeriza t ion The chemical oxidative polymerization of 0.1:1 mole ratio of aniline and

sulfosalicylic acid was carried out by adding 0.1 mole of ammonium persulfate solution (drop by drop) (for s toichiometry 0.2 moles are required). The stirring of the react ion mixture was continued for 2 h to ensure the completion of the reaction which was indicated by the stabilization of temperature of the reaction mixture. The reaction mixture was then filtered and washed repeatedly with distilled water and finally equilibrated in sulfosalicylic acid for 2 h to achieve maximum doping. The as-obtained polymer was dried under vacuum at 50 °C for 24 h.

2.2. Electrochemical preparat ion The electrochemical polymerization was carried out from 0.1 M monomer

in 1.0 M aqueous sulfosalicylic acid solution. Prior to polymerization, the solution was deoxygenated by passing argon gas for 30 min. The polymerization was carried out at 0.75 V versus saturated calomel electrode (SCE) (charge passed Q = 0 . 3 2 C/cm 2) on platinum, indium-tin oxide (ITO)-coated glass plate (resistivity 2 0 - 2 5 ~ cm) or stainless-steel electrodes. The polymer film growth was also studied by sweeping the potential between - 0 . 2 to +0 .8 V at a scan rate of 50 mV/s.

2.3. Characterization The characterization of polyaniline was carried out after thorough washing

with twice distilled water and methyl alcohol/acetone and dried under vacuum for three days at 50 °C. However, for electrochemical measurements, the electrodeposi ted polymer was used after thorough washings with water and electrolyte. The following methods were used to characterize PAn.

2.3.1. Electronic spectra Electronic spectra of the solution as well as on the electrode surface

were recorded in the range 250 to 900 nm using a Hitachi-U-3400 UV-Vis-near- IR spect rophotometer .

2.3.2. Vibrational spectra Infrared spectra in KBr pellets were recorded in the 4000 to 400 c m -

range on a Nicolet F'r-IR spectrometer .

2.3.3. 1H NMR of soluble PAn Using DMSO-d6 as a solvent and TMS as standard, the IH NMR spectra

of doped and undoped PAn and 5-sulfosalicylic acid were recorded using a Bruker NMR spect rometer at 270 MHz, pulse width 5 /zs, spinning rate 20 Hz at room temperature.

2.3.4. X-ray diffraction powder pat terns X-ray diffraction patterns for PAn-SSA, PAn-neutral and PAn-H2SO4

were recorded using Cu Ka radiation (A= 1.5418 /~) on a JEOL JDX 8030 X-ray diffractometer.

312

2.3.5. Thermal analys is Thermogravimetric and differential thermal analyses were recorded on

a Perkin-Elmer 7 thermal analyser. Nitrogen was used as the purge gas at a flow rate of 100 ml/min. The heating rate was 20 °C/min.

2.3.6. Electrochemical measuremen t s Chronoamperometric studies of polyaniline film obtained by the poten-

tiodynamic method were carried out on a Bio-analytical system (BAS 100 A), USA, by switching the potential between - 0 . 2 and + 0.8 V to - 0 . 2 V.

Cyclic voltammetric studies were carried out using a Tacussel bipad potentiostat coupled with an X-Y recorder (BBL, Model SE 780) and PARC 175 universal programmer.

3. Resul t s and d i scuss ion

The polymerization of aniline to polyaniline in the presence of 5- sulfosalicylic acid which has three different functional groups, viz. -SO2OH, -COOH and phenolic -OH, having pKa, - 0 . 7 5 , pK~ 2.32 and pK~ 11.40 [19], may bring certain possible changes in the properties of polyaniline because PAn involves protonation as well as ingress of counter anions to maintain charge neutrality. A report in the literature suggests that protonation equilibria involve exclusively the quinone diimine segment of the polymer chain having two imine nitrogens with PKa, = 1.05 and pKa2=2.55 [20]. In view of these two pKa values of polyaniline which are close to the pK~ values of 5-sulfosalicylic acid, interesting results should be obtained with regards to properties of polyaniline.

It is generally accepted that protonation of PAn leads to formation of radical cations by an internal redox reaction which causes the reorganization of electronic structure to give two semiquinone radical cations (polaronic state). The degree of protonation and resulting electronic conductivity thus become a function of pH. In this protonation process it is essential that ingress of anions occurs to maintain charge neutrality in the resulting doped polymer. This implies that the nature of the anions (size, crystal structure, etc.) should influence the properties of the resulting polyaniline. Various properties of the PAn systems are given in Table 1.

In the present investigation, SSA-doped PAn was found soluble in dimethyl sulfoxide (DMSO), dimethyl formamide (DMF) and N-methyl-2-pyrrolidinone (NMP). The observations on solubility, spectral IH NMR and electrochemical studies suggest the possible interaction of dopant via electrostatic interaction with the chemically flexible - N H - group of PAn. Therefore, dopant-induced interaction of the polymer with the dipole end of the active solvent cannot be ruled out. Hence, the maximum solubility (11 g/l) is observed in DMSO where the sulfoxide group is capable of interacting with the dopant which is electrostatically attached to the polymer. These types of interactions are less likely in the solvents devoid of active groups, such as hydrocarbons or halogenated hydrocarbons.

TA

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Though the polyanilines obtained by both chemical and electrochemical methods have identical environmental stability, the electrochemical method leads to the formation of thin film on the electrode surface which can find applications in electrochromic displays and for various other electrochemical studies.

3.1. Cyclic voltammetric studies The electrochemical polymerization of aniline in SSA medium was per-

formed using the cyclic potential sweep method by switching the potential from - 0 . 2 to +0 .8 V (SCE) (Fig. l(a)) and - 0 . 2 to 1.4 V (SCE) (Fig. l(b)) at a scan rate of 50 mV/s. In Fig. l(b) the peak appearing at 0.97 V versus SCE in the first cycle corresponds to oxidation of aniline, whereas the corresponding peak in H2SO4 medium appears at 0.8 V versus SCE. This suggests that the generation of anilinum radical cations (essential for poly- merization of aniline) occurs at a higher potential in SSA medium. In subsequent cycles, new oxidation peaks (II and III) appear indicating that these radical cations undergo further coupling and the peak current increases continuously with successive potential scans, indicating the build up of electroactive polyaniline on the electrode surface. Though the peak potential of the peak observed in the first cycle is at 0.97 V (very broad; formation beginning at around 0.74 V), this suggests that even by keeping the potential around this value can lead to formation of polymer at a slow growth rate, which is beneficial to obtaining a more-ordered thin polymer film useful for electro- chromic displays. Figure l(a) shows polymer growth by potential sweep up to 0.8 V.

Figure 2 shows the cyclic voltammogram of polyaniline film obtained by potential sweeping in a blank 1.0 M SSA medium (not containing the monomer). It shows two main redox couples at 0.140 V (peak I) and 0.72 V (peak III). In order to explain the electrochemical behaviour of polyaniline, formation of radical cations near peak I which are subsequently oxidized into imines near peak III has been suggested [21, 22] and can be represented in the following equation:

H H H H I I - e - I I

- -N--O--N- '__ -" - - N - - ( ~ - N -- -t-e- -IF-

- e + e

PeQR III H H I I

- - N ~ N - - +.~ ~ .

P e Q k l

H H

315

50

30

10

:3_

-10

(a)

400

300 "~

200

100

0

-100

-200

4 0 o J i L L L i I L I I I t i L 4 2 o o 2 o~ o 6 0.8 l o 12 1~

( b ) E / v

Fig. 1. Electrodeposition of polyaniline in 1.0 M sulfosalicylic acid solution + O. 1 M aniline by potential sweeping between (a) - -0 .2 and 0.8 V, and Co) - 0 . 2 and 1.4 V on Pt electrode (0.25 cm 2) vs. SCE at a scan rate of 50 mV/s .

Peak II in the cyclic voltammogram is essentially due to adsorption of quinone/hydroquinone generated during the growth of the polymer film which gets strongly adsorbed in the polymer matrix and this peak II persists even in the SSA medium (not containing the monomer). The intensity of peak II further increases in the presence of quinone and hydroquinone added externally in the electrolyte and, therefore, is not due to degradation of PAn which

316

40

0

-h0

I I I I I L -Of. 0 0,'-. 0.8

E (V}//SCE

Fig. 2. Cyclic voltammogram of PAn in 1.0 M SSA at a scan rate of 50 mV/s.

TABLE 2

Redox potentials of polyaniline film (vs. SCE)

PAn deposited Peak I Peak II Peak III Ref. Ep,ox Ep,ox Ep,ox (V) Or) O7)

O H (> 1.0 v) 0.140 0.460 0.720 this work

~ 0 0 C - ~ - - 503H (<0.8 V) 0.140 0.720 this work

50~H 0.130 0.410 0.510 0.710 15 H~C

~ ) - ,~0~ H 0.10 0.40 0.480 0.70 15

SOaHNH2 0.125 0.40 0.67 16

H2S04 0.09 0.36 0.625 15

begins only on exceed ing 0.8 V versus SCE in the p resen t system. The fo rma t ion of quinone and hydroqu inone does no t occu r when po lymer is ob ta ined at 0.8 V in SSA medium, whereas the fo rmat ion of qu inone and hyd roqu inone does o c c u r when the po lymer is synthesized at 0.8 V in H2SO4 medium.

The redox potent ia ls of the var ious peaks of the PAn sys tems in different e lect rolytes are tabula ted in Table 2. It is c lear tha t polymer iza t ion potent ial is essential ly dependen t on the electrolyt ic med ium and hence a change in peak potent ia ls o f the r edox couple is observed.

On c o m p a r i n g these results we find that in sulfuric acid med ium the peak potent ia ls o f the PAn film co r re spond ing to anodic peaks I and III lie at 0 .09 and 0 .625 V versus SCE, whereas the co r re spond ing peak potent ials

317

in SSA medium lie at 0.14 and 0.72 V versus SCE. The shift in peak potential can be attr ibuted due to an interaction of the bulky dopant with the chemically flexible - N H - of the polymer as is also evident from 'H NMR study.

3.2. Chronoamperometric studies The polyaniline film in blank sulfosalicylic acid medium exhibits a multiple

colour change (yellow e~green) on switching the potential from - 0 . 2 to +0 .8 V, to - 0 . 2 V versus SCE. (For E>~0.8 V versus SCE, the PAn film turns blue.) These changes correspond to the different oxidation states of PAn that appear upon varying the potential. Figure 3 shows the electrochromic response of a PAn film (c. 0.75 ~m thick, charge passed Q = 0 . 2 1 C/cm2).

The response times are different for the oxidizing (50 ms) and reducing steps (40 ms) which can be explained on the basis of different electrical conductivities of the two states. A life cycle test of the PAn film in 1.0 M SSA medium shows that on lowering the potential switching window to 0.4 V after 105 cycles (cycle duration 1.2 s) only a 5% loss in electrochemical activity of the PAn film is observed, whereas a 60% loss in the electrochemical activity of the PAn film is observed on switching the potential between - 0.2 and 0.6 V, to - 0 . 2 V versus SCE (one complete cycle duration = 1.6 s). We have also observed that on increasing the film thickness the response time

Tso mA -t_

i ~ _ i I I 200

(a) TIME ( m s )

Co) TIME ( m s

Fig. 3. C h r o n o a m p e r o m e t r i c curves for (a) 0.75 tzm thick and (b) 5.9 ~ m thick PAn films in 1.0 M SSA medium. Exp. condit ions: initial E = - 2 0 0 mV; high E = 800 mV; low E = - 2 0 0 mV; pulse w i d t h = 100 m s for (a) and 1500 m s for (b); sample intensity = 100 Us for (a) and 1500 p.s for (b).

318

increases. On passing a charge of 1.81 C/cm e (PAn film thickness - 5 . 9 /zm), the response times are 800 and 550 ms for the oxidizing and reducing steps, respectively (Fig. 3(b))°

3.3. Cyclic vo l tammogram o f PAn f i l m in nard-aqueous m e d i u m The broadening of CV peaks is observed in the cyclic voltammogram

of PAn-SSA film in acetonitrile-LiC104 medium (Fig. 4). However, the CV peaks resume their sharpness on addition of free acid to the medium (Fig. 4, curve b). This behaviour of PAn supports the widely accepted view that protons are essential for the conduction mechanism.

3.4. Cyclic vo l tammogram o f PAn-cast f i l m f r o m DMSO solution The polyaniline film obtained by evaporation of PAn solution in DMSO

(in vacuum at 50 °C) on a platinum electrode (0.25 cm 2, process was repeated twice) gave a cyclic voltammogram (Fig. 4, curve c) having peak potential values at 0.29 and 0.6 V versus SCE. However, the middle peak observed in the usual cyclic voltammogram of PAn due to quinone/hydroquinone was missing. This indicates that DMSO solution-cast PAn film is free from other impurities like quinone/hydroquinone.

3.5. UV-Vis spectra Figure 5 shows the diffuse reflectance spectra of the electrochemically

prepared PAn film at 0.8 V versus SCE on an ITO substrate in SSA medium. A dark green-coloured film (1.0 /zm thick) showed two absorption bands at 422 and 720 nm, whereas a PAn film prepared in H2SO4 medium has absorption bands at 320, 420 and 820 nm [23a]. PAn film prepared at 0.8 V after neutralization gave very broad spectra having one peak centred at 573 nm indicating that the short conjugation has a molecular weight of 30 000 and is almost similar to vapour phase-deposited PAn film in its

150

loo

50

!

-111o i I I I

-0 -2 0 0-2 O~ 0-6 0.8

E ( V ) S C E

Fig. 4. Cyclic v o l t a m m o g r a m s o f (a) PAn fi lm in acetonitri le--LiClO4 m e d i u m ( l .O M), (b) as (a) p l u s 1 m l o f 1.0 M HC104 a n d (c) P An film o n a Pt e l ec t rode (0 .25 c m 2) c a s t by e v a p o r a t i o n o f P A n - D M S O so lu t i on in ] .0 M SSA a t a s c a n ra te o f 50 m V / s vs . SCE.

319

5 .d

z

720 PAn doled (SSA)

300 zOO 500 6 0 7 0 800 900

WAVE LENGTH , nm

Fig. 5. Diffuse reflectance spectra of (a) PAn in SSA medium on an ITO electrode and (b) PAn in the neutral form.

15 " - " 1 i"~ c // "-\ i r ~ / '

i i / ' ~ / I : / / i

{; ~f 'x, i i

i- 0 I I I L I " ~ '" . . . . ~

?oo 5o0 700 9oo

h( nrn )

Fig. 6. UV-Vis solution spectra of (a) undoped PAn solution in DMSO, (b) PAn-SSA solution in DMSO and (c) undoped PAn solution containing 1.0 M SSA.

spectra l character is t ic [23b]. Whereas PAn film p repa red at 1.0 V versus SCE has the same spectral features as that of PAn film p repared at 0.8 V versus SCE in H2SO4 medium, PAn film p repa red in SSA med ium has a bet ter e lec t rochemica l stability at h igher potentials . The difference in e lectronic spec t ra may be due to different molecular or ienta t ion or res t r ic ted conjugat ion because of involvement of the bulky coun te r ion.

The electronic spec t ra (Fig. 6) of 25% SSA-doped PAn solut ion in DMSO gave absorp t ion bands at 317, 449 and 646 nm, whereas the absorp t ion bands due to the 7r-Tr* t ransi t ion and metall ic po la ron band are observed at 362, 420 and 832 nm on 50% doping (Fig. 6, curve c). On removal of dopant , the absorp t ion bands are observed at 319 and 611 nm (Fig. 6, curve a). In this s tudy it is not clear why a ba thoch romic shift is observed on increas ing the dopan t level, which is reversible on revers ing the dopan t level. Recently, it has been repor ted that p ro tona t ion o f emera ld ine base in N, No

l

dimethylacetamide by perchloric acid depends upon base concentration and it is also possible that protons added to the solution may not be completely bound [23c]. However, with this experimental detail, it is not possible to draw any conclusions; the probability of change in molecular orientation [24] (c/s or trans) cannot be ruled out because of the bulky nature of the dopant.

3.6. Infrared spectra The principal absorption bands observed in the F'r-IR spectra of the

PAn-SSA system are given in Table 1. In the region 1650-1400 c m - i , bands due to the aromatic ring breathing mode, N-H deformation and C-N stretching are observed. Bands at 1568 and 1487 cm -1 are the characteristic bands of nitrogen quinoid and benzenoid and are present due to the conducting state of the polymer. These bands show a blue shift from 1568 to 1595 cm -I and from 1477 to 1504 cm -1 on removal of dopant from polymer (Fig. 7). These changes are indicative of the conversion of benzenoid rings to quinoid rings in the PAn matIix. The absorption band at 1140 cm-1 in

320

P A n - U n d o p e d

c:~ ~

P A n - doped

m m

I I I I I I L000 3600 3200 2800 2 / ,00 2000 1600 1200 500 " 0 0

W A V E N U M B E R ( c r n - I )

Fig. 7. FT-IR spectra of polyaniline in KBr.

321

the PAn-sulfuric acid system is said to be due to charge delocalization on the polymer backbone [25] because, in the undoped state, the intensity of this band is very weak. In the present system also very strong bands at 1124 and 1147 cm-1 can be assigned to the characteristic mode of B-NH-Q or B-NH-B. The band at 1189 cm-~ in the doped sample can be assigned to the -SO3 stretching vibration which is missing in the undoped sample. This band is found at 1163 cm-~ in pure sulfosalicylic acid; therefore, the shift in -SO3 stretching in the pure dopant and intercalated dopant is ascribed to the strong electrostatic interaction between dopant and polymer. Similarly, an intense band at 1026 cm -1, due to the symmetric -SO8 vibration, and medium intensity bands at 1673 and 1547 cm -1, due to the carboxyl group of the dopant, are observed and these bands vanish on removal of dopant.

3. Z NMR spectra The 'H NMR spectra of SSA-doped polyaniline gave signals at 7.36 ppm

due to aromatic protons and 3.89 ppm due to N-H of the polymer. These signals are shifted to 7.0 and 3.34 ppm on removal of dopant. These downfield shifts indicate the interaction of dopant with the polymer matrix to yield active sites which are responsible for solubilizing of polymer in its doped state. The influence of the shift with regard to various dopants is tabulated in Table 3, which reveals that the higher the downfield shift in the N-H proton, the more soluble is the conducting form of polyaniline in a solvent like DMSO.

3.8. X-ray diffraction pat tern Figure 8 shows the X-ray diffraction pattern of the emeraldine base,

polyaniline-doped SSA and polyaniline-doped H~.SO4. In all the cases broad- ening occurs, indicating the amorphous nature of material. The 20 and d values are recorded in Table 4. However, in the present study less broadening of peaks than reported for other polyaniline systems [26] has been observed.

3.9. Thermal stability o f the polyani l ine sys tem Figure 9(a) shows the thermogravimetric curve of the emeraldine base,

indicating negligible weight loss (~ 3%) up to 428 °C; from 428 to 584 °C,

TABLE 3

~H NMR data of PAn-organic acid in DMSO-d6 (shift with reference to TMS)

Organic PAn-doped PAn-undoped PAn-doped PAn-undoped Ref. Solubility acid (aromatic (aromatic N-H N-H (g /100 ml

proton) proton) (ppm) (ppm) of DMSO) (ppm) (ppm)

SSA 7.36 7.0 3.89 3.34 this work 1.1 PTSA 7.59 7.0 3.77 3.34 15 0.85 BSA 7.44 7.0 3.66 3.33 15 0.82 Emeraldine

base 7.0 3.3 this work

322

TABLE 4

X-ray diffraction pat tern (Cu Ka, ~ = 1.5418 /~)

PAn-SSA PAn-H2SO4 Undoped PAn

2e d 20 d 20 d

18.55 4.779 19.05 4.655 19.10 4.643 19.60 4.525 24.35 3.652 19.65 4.514 24.55 3.623 25.00 3.559 20.60 4.308 25.35 3.511 25.75 3.457

1 -- PAr= uf~doped

2 - P A n - H 2 S O 4

3 - P A n - S S A

~ 3

_z 2

29

Pig. 8. X-ray diffraction powder pat terns of (1) emeraldine base, (2) PAn-doped H2SO 4 and (3) PAn-doped SSA.

the loss in weight is 30%. Figure 9(b) shows the curve of the SSA-doped polyaniline. It can be seen that the doped polymer is thermally less stable than the undoped polymer. In the doped polymer the weight loss begins at 234 °C and continues up to 320 °C; the weight loss is 40%, corresponding to the weight of the counter ion. Unlike neutral polyaniline, the degradation in doped PAn is continuous, possibly due to the decomposition of the polymeric backbone.

4. C o n c l u s i o n s

It has been shown that it is possible to achieve solubilization of the conducting form of polyaniline using 5-sulfosalicylic acid as dopant. In addition to the solubilizing effect, this dopant gives better crystallinity and electrochemical stability at higher potentials. The polymer also shows elec- trochromic response when cycled between - 0 . 2 and 0.4 V versus SCE with a response time of 50 ms and a drop in electroactivity of about 5% is observed after 1 × 105 cycles.

323

~ool

80

2 40

20

(a)

P A n - L

/

i / / i /

I I I I / I I IOO 300 500 7(30

TEMPERArURE,'C

0 E

-~-o ; >

-2.0~

-3.0~

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-5"0

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u J

20

o I/ , ;o ' ' s;o ' 7;0 Co) TEMPERATURE "C

-~ >~

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-12

Fig. 9. Thermogravimetr ic analysis curves of (a) undoped polyaniline and (b) SSA-doped polyaniline.

A c k n o w l e d g e m e n t

The authors wish to thank Professor S. K. Rangarajan for his keen interest in the electrochemical growth mechanism of conducting polymers.

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