Dopant induced solubilization of conducting polyaniline

Indian Jo*rnal of Chemistry VoL 33A, ~une 1994,pp. 552-557

Dopant induced solu~ilization of conducting polyaniline

inesh Chandra Trivedi

Central Electrochemical Research Institute, Karaikudi 623 006, India ived 18 October 1993

This paper discusses the synthetic and ~hYSiCO-Chemicalaspects of solubilization of aromatic sulphonic aCE'doped conducting polyaniline in org nic solvents. The observations of various studies suggest that ar atic sulphonic acids not only influence he charge distribution between aromatic ring and hetl~roatom of pol er chain but also influenceorientatio of poly cation. This solubilization of polyanilineis important for de lopment of many technologies invol ing polyaniline as a new electronic material.

Polyani ne (PAn) refers to a general lass of conduct ng polymer composed of benzen id and quinoid characters connected by n trogen.

Therefo e, it can be visualized that polyanilin is built up fro reduced (B-NH-B-NH) and xidized (-B-N- N-) repeat units, where B enotes benzeno d and Q denotes quinoid rings. hus the ratio of mine to imine yields various structu es, such as leuc meraldine, a reduced form, emeral . e base.

a fifty pe cent oxidized form and a fully oxidi d form known s perni-graniline (Fig.

I).

^{Unlik other}

phenyle e based conducting polymers 1 4, the polyanil ne has a chemically flexibles -NH- roup in its back one which is responsible for int resting cJu:mist and physics5• Electronic condu tion in pohyanil ne not only involves the ingress of rotons but is a so accompanied by ingress of a ions to maintai the charge neutrality. The elect roc emical stability fpolyaniline depends on the pH c ndition as well0 the counter ion of the Br6nsted acid for doping. imilarly the soluble PAn is also req ired for many a plications to facilitate post - s nthesis processi g. The solubilization can be achieve by two methods (i) prefunctionalization of aniline6 ⁷or (ii) by intro ucing functionalized dopant8 -1 . Since PAn has wo acid functionsll a strong and a w ak one, it is poss ble to achieve a complexation or r' ther an electrost tic interaction between the charge on the polymer and dopant to yield a ternary syst m12,13

whereby it is possible for the doped pol mer to interact ith the dipole end of the solvent t yield a solvated polymer.

Proto ation of PAn leads to the form radical c tions by an internal redox reaction

the reor nization of electronic structure to give two semiqui one radical cations (polaronic sta~). The

b. (

~do-..ifQ).

c.

('NJY~~~-CfQ

^{H A-+}

),

ⁿ

d(~2

Fig. I-Various forms of poly aniline (a) h:uco emeraldine, (b) emeraldine base, (c) conducting emeraldine and (d)

pernigraniline.

degree of protonation and resulting electronic conductivity thus becomes the function ofpH. In the protonation process it is essential that ingress of anion occurs to maintain charge neutrality in the resulting doped polymer. This implies that nature of anion (size, crystal structure etc.) should influence the properties of resulting polyaniline"

Materials and Methods

Polyaniline is generally prepared by direct oxidation of aniline using an appropriate oxidant or by anodic oxidations on inert electrodes.

Electrochemical synthesis

Anodic oxidation of aniline on an inert metallic

'1.1

"II '1"'1 I' "I'

TRIVEDI: DOPANT INDUCED SOWBIUZATIONOF POLYANnlNE SS3

(NH4hS20S K2Cr207 KI03

Table I~Redox potentials of polyaniline film in volts versus saturated calomel electrode (SCE)

Pan deposited I Peak II Peak III Peak (Electrochemical) Eo ox E., ox E., ox

mediumt (V) (V) (V)

SSA (>0.8V) 0.140 0.460 - 0.720

«0.8V) 0.140 - 0.720

PTSA 0.130 0.410, 0.5100.710

BSA 0.10 040, 0.480 0.70

SMA 0.125 0.40 - 0.57

SA 0.09 0.36 - 0.625

tSSA = 5-sulphosalicylic acid, PTSA = p-toluene sulphonic acid, BSA = benzenesulphonicacid, SMA = sulphamicacid and SA=sulphuric acid.

Table 2-Effect of oxidizing agent on PAn yield Oxidant Time Yield Oxidant Time Yield

(hr) (%) (hr) (%)

4 37 FeCI3 24 8

12 41 KMn~ 24 6

20 27 KCI03 24 1

Table 3-Effect of protonic acid [Oxidant (NH4hS20SJ

Acid pKa Yield Acid pKa Yield

HCI -7.0 36 HCI04 - 41

H2S04 1.82 30 NH2S03H 1.04 36

HBF4 - 43 BSA 0.2 36

HF 3.17 43 PTSA - 36

CH3COOH 4.75 28 SSA -0.75 39

2.32 11.4

electrode resulted in a clean product and did not need post - synthesis purification. The synthesis was carried out by galvanostatic or potentio~static mode.

In the latter method the potential of working electrode was kept around 0.7 V versus saturated calomel electrode, whereas in galvanostatic method the maximum current density of 10 mA cm -²was used. In both the cases polymer film of I Jlm thick was formed.on passing the charge of 0.32 coloumbs. The electrochemically prepared films were compact. The electrochemical characterisation of these film could be carried out in any medium of choice.

Results and Discussion

The redox potentials of the various peaks of the PAn systems in different electrolytes are tabulated in Table

1

indicating the strong dependence of electron transfer pro~ess (I peak) on an anion of the medium which influences the protonation process (peak III) as well.

H H H H

-~--::~ ^-~

-. +~.

H H H H

Kr·1

^I~

- . H-

c •.

^{H :II\:} H-

+ + .•. +.

Fig. 2-Equation explaining cyclic voltammogram peaks of polyaniline.

These shifts in peak potentials can be attributed to an interaction of the bulky dopant with the chemically flexible -NH- of the polymer as is also evident from IH NMR study. Peak I is due to the formation of radical cations and their subsequent oxidation to imine occurs near Peak III. The reaction scheme is represented in Fig.2.

Peak II in cyclic voltammogram can be assigned to adsorption of quinone/hydroquinone generated during the growth of polymer film. The intensity of peak II further increases in the presence of quinone and hydroquinone added externally in the electrolyte conforming adsorption of quinone during synthesis ofP An and is not due to the degradation ofP Anwhich begins only on exceeding potential limit of 0.8 V/vs SCE in the present system.

Chemical method of synthesis

Various oxidants have been used to effect chemical oxidative polymerization of aniline under strong acidic condition at pH I. The yield of polyaniline using various oxidising agents is recorded in Table 2. The perusal of data in Table 2 shows that there is no relationship between the oxidising power of the oxidant and polymer yield. However, the lower the rate of reaction the better is yield of polymer. The rate of reaction is dependent on temperature and hence lower temperature ^0-5°Cis preferred to achieve better yield and better quality of polymer by avoiding the formation of oligomers. The enthalpy of reaction is 372 kJ mol-1 which varies linearly with the concentration of oxidant and lienee a very slow addition of oxidant is advisable. Our study indicates that ammonium persulphate is the best oxidant and polymer so obtained is comparatively free from impurities and secondly the reduced product of ammonium persulphate is ammonium sulphate which is free fromtoxicity. The yield of poly aniline using various ;:l,ids and ammonium persulphate as oxidant is recorded in Table 3.

Polymerization mechanism

The chemical or electrochemical polymerization of

554 INDIAN J tHEM, SEe. A, JUNE 1994

Table 4-1H NMR-data on PAn - organic acids

HCI doped 3.334

3,34 3.34 3.31

PAn Solubility undoped of doped N-H polymer ppm in DMSO

g/I 8.2 8.5 11 8.5

UndoJPed emeralidine bas,~

3.66 3.77 3.89 3.4 PAn doped

N-H ppm

Pan PAn

doped undoped aromatic aromatic proton proton

ppm ppm

7.44 7.0 7.59 7.0

8.1 7.0

8.2 7.0

NMR Assignment BSA PTSA SSA SMA

Table 5_13C NMR data

[SSA, PTSA, BSA, HCI are dopants. concentration X is 0.38 per mole of PAn]

SSA PTSA BSA

Aromatic acid

Charge transport studies

The mechanism for the formation o>fmetallic state and charge conduction in polyaniline is the subject of intensive study. Polyaniline differs from other conducting polymers in the sense that number of electrons on polymer chain are held constant while the number of protons varied.

The salient features of our study on polyaniline organic acid system¹⁵ to ascertain suitable charge transport mechanism are:

(1) The 7t-7t* transition (electronics spectra) in always present and becomes pronounced on aromatic ring and -NH- of polymer backbone. This type of behaviour has also been observed in solid state 13CNMR studies (Table 5). It can be seen from data in Table 4 that organic acid doped PAn which is soluble in organic solvent has comparatively less downfield shift in aromatic carbon not attached to nitrogen than Hel doped PAn. Higher down field shifts are observed in aromatic carbon attached to nitrogen compared to HCl doped PAn. This behaviour possibly points toward the fact that dopant also influences charge distribution on polymer chain. The larger the down field shift for particular atom indicates that better positive charge localization. The influence of orientation of dopant can also be seen in powder X-ray diffraction pattern of PAn synthesized in the presence of various organic acids (Table 6). In all the cases the peaks were sharper than PAn-mineral acids system.

Fig. 3-Reaction mechanism for formation of polyaniline.

aniline to olyaniline is a straight forward rea tion.

However, polymers having different properti scan be obtain d by changing experimental condi ions.

This obs vation has led to multitude of poly- merizatio mechanisms. Presently the most idely accepted echanism is that in which anili e on oxidation .ves radical cation which after elimi ation of a prot n and further oxidation gives nitr nium cation. Ni renium cation in subsequent coupli g and oxidation reactions leads to the formati n of polyanilin . The reaction sequence can be wri ten as in Fig.3.

Dopant in uced solubility of polyaniline

It is gen rally acepted that PAn has a sol ubi ityof less than ne per cent in common organic sol ents.

However, the undoped form can be dissol ed in strong suI huric or acetic acids. Using these tw acids it is possi Ie to spincast PAn but, is not possi Ie to obtain thi transparent films of PAn for fabrica ion of electronic devices. Therefore, the solubilizat on of dopt:d P has aroused greater interest in or er to facilitate ost synthesis processing. Use of 0 ganic acid as a d pant favours the dissolution of dope PAn in solvent like DMSO, by forming ternary s stem compose of polymer, dopant and solven . The formatio of dopant induced ternary syst m is reflected n IH NMR studies (Table 4). A close examinati n of Table 4 data reveals that dow field shift of ar matic and N-H proton is large in se of SSA dope polymer which has a solubility of I gm per Iitre.

This dolwn field shift in proton NMR reflects that the dopants influence the charge distribution bdtween

Aromatic133128120133

C-H Aromatic

165 170 159137 C-N

138 157

'I " "~,II I'

TRIVEDI: DOPANT INDUCED SOWBILIZA110N OF POLYANILINE 555

Table 6-X-ray diffraction pattern [Cu-Ko.= 1.5418A]

PAn-SSA PAn-BSA PAn-PTSA PAn-SMA Emeraldine base

d 4.643 4.514 4.308 26

19.10 19~65 20.60 d

4.655 3.652 3.559 3.457 29

19.5 24.35 25.00 25.75 d

4,951 4.79 4.548 3.44 29

17.9 18.5 19.5 25.85 d

6.4 5.86 3.6 3.531 29

13.8 15.1 24.7 25.2 3.524

d 4.84 25.25

29 18.3 d

4.779

14.60 4.525

24.55 3.623

25.35 3.511

Electrochemically synthesised PAn - organic acids tho 26 lies between 8' to 10' showing better ordered architect essentially due to rod-like sturcture of the dopant.

29 18.55

Table 7--Optical absorption peak positions in nm. Table 8-Solubility and optical data Dopant level

BSAPTSA SMASSA

H2SO4System organicConductivityUV visibleSolubility X/mole

acid doped (S em-I)

(g/I) DMSO 0

525 518315.615 320,606

620x =O.5/mole solution

0.25

327.839308,627 spectra peak

- ^-

position in

0.38

329,447 322,416 305,620

418,670 nm

634,934

62230 0.02 J?olyanisidine

326,427,729 0.5

328;447 416,622307,449 419,672

1:1 Copolymer0.1 of 320,441,86125

655,942

623 anisidine-aniline 0.5 Electro- 328,447

418.680 422,720

419,682 320,420

Polytoludine18 0.5 313,750

chern cell 686,963 820

0.5 DMSO

343,453 335,449 317,449 332,417

Insoluble1.1 Copolymer25 0.2 320,800

Solution

662 646 649

639 aniline

Poly

0.05 434,82725

(O-phenetidine)

increasing dopant concentration. The band at 2 eV is present invariably in all the cases (Table 7).

(2) The long wavelength absorption due to free carrier is absent at moderate doping level.

(3) The de magnetic susceptibilities are in the range ofl x 1O-4emu/mol-z x 10-4 emu/mole which is higher than other known systems.

(4) The X band ESR spectra can be resolved into Lorenzian and Gaussian line shapes. For the electrochemically synthesised polymers, ESR signals were absent at room temperature, however, a weak broad line appeared at 77 .K.

(5) The thermopower(s) is practically indepen- dent of temperature.

(6) The very large thermopower in the order of

60-70

¹¹V/K.is observed at higher doping levels which varies linearly with T at high temperatures but is independent of T at low temperatures.

The dopant induced solubilization in polyaniline which increases the charge on -NH- must also be influencing the molecular orientation of polymer chain by influencing the torsional angIe between two adjacent phenyl rings. Therefore, we thought it fit to compare our results of dopant induced and substituent induced solubilization of polyanilines.

Table 8 gives the solubility and optical data on substituted polyanilines. This comparative study shows lot of similarities in optical absorption data of pure susbstituted polymers like polyanisidine, polytoluidin and polyphenetedine where we find that absorption due to free carrier does not extend to infrared region indicating that probably the bipolarons are not the charge carrier species However, on copolymerization the situation is quite different and band due to bipolarons are observed.

This type of observation confirms that charge transport becomes faster on formation of block copolymer (cf. with inorganic system where dissimilar oxidation states of metal ions facilitate faster electron transfer across the ligand).

Thus the I: I copolymer of aniline and toluidine has a electrochromic response time of 40illSand 25 ms for oxidation and reduction steps respectively, whereas for polyaniline and poly toluidine response time observed for same thickness of polymer is 120 and 100 ms.

However, in the case of poly aniline the absence of ESR signal indicates that bipolarons are possible charge carriers. The diversity of observation in

556 INDIAN J tHEM, SEC. A, JUNE 1994

CIS

CIS-TRANS

Fig. 4--po~ible orientation of polyaniline in the presFnce of dopants like benzene sui phonic acid (BSA) p-toluene sulphonic

acid PTSA) and 5-sulphosalicyclic acid (SSA).

absorptio spectra and ESR data on PAn-o ganic acid syste .suggest that polyaniline chain is n t rigid as believe but has possibly cis or trans orient tions (FigA). is type of cis or trans orientatio will restrict th formation of bipolar on to give pol ronic metals (fi rromagnetism in polymers). The ecent expe:rime ts in our laboratory do indica e the possibilit of obtaining molecular magnets.

Technolo ical application

.Thoug the understanding of charge transp rt and struttura aspects of polyaniline are far from satisfacto y, it has been predicted to be the fu ristic material r many growing technologies, bec use ( f its greate environmental stability, highly fay urable economi ; easy synthesis and unique cond ction mechanis involving electron transfer follo cd by protonati n of -N- site to enable technologist 0tune its magn tic, optical and electrical prope ies at molecula level.

The fo owing is the brief account of work tarried out at C RI on development of poly aniline s new material. One of the envisaged applicati ns of conducti g polyaniline is in the cont ⁰¹ of electrom gnetic radiation 16 - 18.

A met odology was developed to impartfl ibility to other ise unprocessible and powdery pol aniline by gra ting it on flexible surface like syntheti naturally occurring fibres. These rafted surfaces ave good stability against mecl1anic ¹abuse and are f ee from corrosion and can withstan strong alkaline r acidic fumes. The conductivity of rafted surfaces an be varied to suit the need, for e ample, surfaces having the resistance of more th n 100 ohm/squ re can find application as an antista ics and surfaces of less resistance can be used to ontrol

electro-magnetic radiations in many civil and defence equipments.

Our study indicates that shielding dfectiveness of 50 dB or more can be achieved upto 10:1kHz and 20-40

dB I;1thigher frequency ¹⁰³MHz. The experimental detail for grafting of poly aniline and its application in control of electromagnetic radiation have been described elsewhere 16 - 18.

The study has also been carried out to use pressed pellet of polyaniline as cathode material in a cell containing zinc chloride/ammonium chloride solution, pH 4 to 5.5 and zinc metal or its allo)! as an anode. The life cycle test shows a good stability and power density is comparable to that of lead-acid battery.

Polyaniline based battery system has a very high energy density output under acidic condition because of continued protonation of polyaniline to keep it always under changed condition. However, this system can find application under specific conditions and can replace lead dioxide as a battery material in lead-acid battery system.

Recently we have also shown that polyaniline and substituted polyanilines have a great promise to be used as corrosion inhibitors, they offer inhibition efficiencies of 85-90 per cent at 25 ppm leveP^{9 - 20.}

We have indicated that polyaniline can find application as an electrochromic material in commercial sign boardes22•23, because of highly reversible colour change from green to yellow or blue and vice versa occurs on reversal of applied potential.

Howevers, the response time are quite high (40 ms to 120 ms) to compete with liquid crystal display.

Similarly it can be used aspH sensor and modified electrode24.

As the conduction mechanism involves the protonation which makes polyaniline as a highly stable and electro active material in the presence of acids, it is considered as a suitabk: material for corrosion protection of metal surfaces of ground support equipments and structure used for launching of space vehicles. In this case the corrosion resistance offered by PAn is due to active electronic barrier at the interface, which retards the electron transfer from metal to environment to prevent oxidation (corrosion) of metal.

The other commercial applications envisaged are:

(i) photovoltaic conversion, (ii) electrochemical heating pump, (iii) anti statics garments, (iv) zebra connectors, (v) flexible light emitting diodes of varied colours, (vi) separation of gases like nitrogen and oxygen, (vii) electroplating of metals- coprer for printed circuit boards, (viii) biosensors, _,and (ix) controlled drug delivery system.

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TRIVEDI: DOPANTINDUCED SOLUBILIZATION OF POLYANILINE 557

Acknowledgement

Author wishes to thank all his coworkers whose names have appeared in references for their help during the course of investigations.

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