Indian Joj,Jrnal of ChemistryVol. 33AJJune 1994, pp, 524-539

Recent trends in conducting polymers: Problems and pro:mises

Sukumar Maiti

Materials Science Centre, ~ndian Institute of Technology, Kharagpur 721 302Rd:eived 19 November 1993

Conducting polymers exhibit one of the most versatile behaviour of polymer materials. This re­vi reports right from the synthesis t the applications of these new electronic materials and dis­eu ses the associated challenges and sol tions. These polymers may be synthesized by conventionaleh 'n and step polymerization techniqu s as well as a few other special processes including electro­ch mical synthesis. The problems of pol mer synthesis in general and synthesis of these el,ectroactivepo ymers in particular have been discus ed. The incorporation of oxidizing or reducing agents intoth se polymers, known as 'doping', type of doping agents used, the mechanism of doping and finallyth influence of the nature and level of opants incorporated on the conductivity and otht~rpropert­ies of polymers have been discussed. T conductivity of these polymers also depends on a numberof actors, viz., technique of synthesis a d processing of polymers, their crystallinity and molecularstr cture, and temperature of the used nvironment. Since the dopants are small molecular weightrn erials and are required in some cas s as high as 30-50% of the polymers, the macromolecularpr perties are thus compromised by d ping. A new type of polymers known as narrow bandgappo mers has therefore been developed which does not require any external dopants. This may blac ieved by reducing the aromaticity or y increasing the content of quinoid form or by incorporationof mpty p-orbitals in the polymer chai , Stability and processability problems of doped polymersan various means to overcome these h ve also been discussed. By introducing ionizable groups inthe polymer chain, intrinsic conducting olymers may also be obtained which do not require exter­nal dopants, Finally, some of the poten ial areas of applications of these new electronic materialsan their promises have been discussed.

Introdu

The versatility of polymers ha neverbefore en manifested so acutely as in th deve-lopment of conducting polymers, a class 0 mate­rials tha is traditionally well-known and widelyused as ·nsulators. Commodity as well as specia­lity poly ers are insulators because the at ms inthe poly er chain are covalently bonded. Metalsare con ucting due to the presence of etallicbonds, i which the valence electrons ar com­pletely elocalized and form an electron cloudaround e metal atoms. In the covalent ondedmolecul of saturated carbon compounds here isno scop of dclocalization of the valenc elec­trons, a d consequently neither charge arriersnor path for their movement are available Sincein the onjugated molecule of a carbo com­pound, elocalization of electrons may occurthrough the interaction of .n-bonded ele trons,such mo ecules may be conducting. Thus it wasthought hat a long-chain conjugated m lecule,such as he polymer of acetylene,t-CH = Ht-n,may pro e to be conducting. In fact, it wz s pro-

posed, purely from theoretical consideration, thatproperly substituted polyacetylene moleculewould exhibit even superconducting behaviour atroom temperature 1,

While Natta and coworkers2 first prepared 1'0­lyacetylene as a dark coloured powder completelyinsoluble in organic solvents, Hatano and cowork­ers3 reported for the first time the electrical con­

ductivity of the order of 10 - 5 Sf em of their poly­acetylene sample. On exposure to air polya­cetylene lost its conductivity, and colour of thesample changed from greenish-black to pale­orange. It now appears that these workers hadprepared the trans-isomer of polyacetylene.

Subsequently, particularly in the seventies, vigo­rous research activities led to the discovery ofpolysulphur nitride4, (SN)x' having metallic con­ductivity and superconductivity5 at 0.24 K. Thisconducting polymer, known as synthetic metal,however, remains an academic curiosity becauseof its brittleness and explosive nature.

Although Hatano and coworkers reported asearly as 1961 the discovery of conducting polya.

I I

MAITI: PROBLEMS & PROMISES OF CONDUCTING POLYMERS 525

cetylene, their product was powdery and insolublein organic ~olvents, and therefore fabrication wasextremely difficult. A decade later, Ikeda, Ito andShriakawa in Japan6-9 succeeded in the synthesisof free-standing polyacetylene films-one coppercoloured with room temperature conductivity of1.7 x 10-9 S/cm, and the other, silver colouredwith a conductivity of 4.4 x lO-s S/cm.

The next significant milestone in conductingpolymer research was the observation that poly­phenylacetylene, like polycyclic aromatic hydro­carbons 10, on exposure to halogen vapours suchas bromide and iodine, showed an eight-fold in­crease in conductivity 11. Similar observation wasreported earlier by Berets and Smith 12by expos­ing polyacetylene pellets to vapours of BF3 andBCI3. Following these earlier examples, Shiraka­wa, McDiarmid, Heeger and coworkers exposedthe free-standing polyacetylene film to vapours ofchlorine, bromine, iodine, arsenic pentafluorideand sodium, and reported an increase in conduc­tivity up to twelve orders of magnitude 13- 1S.In­creasing the conductivity of polyacetylene bytreatment with chemicals was termed as doping,although the traditional doping method and theconcentration of dopant present in inorganic sem­iconductors are quite different.

Very soon it was realized that polyacetylene,because of its processing difficulty and rapid fallin conductivity when exposed to air, has limitedcommerdal applicability. In the late seventieselectrochemical synthesis offered a convenientroute to obtain conducting polymers that arestable to ambient environments16. Thus a few newpolymers have been added to the list of conduct­ing polymers such as polypyrrole16, polythio­phene17, polyparaphenylene18, polyphenylene sul­fide19, polyaniline20, polyphenylene vinylene21, pol­yisothianaphthene22, etc.

Synthesis of conducting polymersConducting polymers may be prepared by

chain or step polymerization process, electro­chemical synthesis, photochemical route and byother methods.

Chain polymerization processPolyacetylene was prepared by polymerization

of acetylene by Ziegler-Natta catalysts. Free stand­ing films of polyacetylene prepared by the Shira­kawa process involves the passage of acetylenegas over TiCVAl(C2Hsh catalyst solution in n­heptane at O°C, when the polymer is obtained asa film on the surface of the catalyst solution. Theratio of the two geometric isomers of polyacety-

Table 1-cis-trans ratio of polyacetylene isomers obtained un­der various conditions

Polymerization Catalyst cis/transcondition

Toluene, - 78°C

Ti(0- n - C4H9)1 A1(C2Hs h98/2

(1:4)Toluene, 18°C

Ti(O - n- C4H9)1 A1(C2Hsh59/41(1:4)Ethanol, - 78°C

Co(N03h/NaBH4 50150

Toluene, - 78°CMoCl/H20 (1 :0.5)24/76

N-Heptane,O°C

"'iCV A1(C2Hsh20/80

Tetrahydrofuran,

Ni(acetylacetonate h5195100·C dark

n-Hexadecane,

Ti(O -n - C4H9)1 A1(C2Hs)30/100150°C

(1:~)

~

lene formed by this 'process depends on the cata­lyst system and the polymerization conditionsused23 (Table 1).

n(CH == CH) Ziegler/Natt~ 1-CH = CH -t"Catalyst

Benzene and naphthalene have been polyme­rized in arsenic trifluoride solvent using AsF s ascatalyst24,2S.

n@ ~ LI(5U-Cat ~

n<8 A.F~ • ~Oo Cot 0 n

Similarly, acetylene derivatives have been po­lymerized by chain polymerization process usingZiegler type and other types of catalystsystems26-28 (Table 2).

Heterocyclic monomers, viz., pyrrole and thio­phene have also been synthesized by chain po­lymerization process using metal salts such asperchlorates and chlorides of iron andcopper29 - 31.

Step polymerization processThis process, unlike the chain polymerization

process, involves, in general, a condensation-typereaction between the reactive groups of two dif­ferent molecules, although self-condensation isnot ruled out.

Polyphenylene sulfide is prepared by the poly-

.....

-{CH=CH-li

n Br -('&-oNa (~. 0 \- + NoBr~:Y ~ )n

PolyparapheIl¥lene is synthesized by the poly­condensation reaction between p-dibromobenzeneand magnesium in ether in the presence of nickelchloride bipyridylcatalyst37.

Polyphenylene oxide has been synthesized byUllmann condensation of sodium salt of p­bromophenoP8.

Photochemical synthesisReceritly pyrrole has been photopolymerized

using a ruthenium (II) complex as photosensitiz­er45. Under photo-irradiation, Ru(II) is oxidized toRu(ill) and the polymerization is initiated by thisone-electron transfer oxidation process. Polypyr­role (PPy) films may be obtained by photosensi­tized polymerization of pyrrole using a coppercomplex as the photosensitizer46. Photopolymeri­zation of benzol C] thiophene has been carried outin acetonitrile using CCL4 and tetrabutyl ammoni­um bromide47.

Electrochemical SynthesisElectrochemical synthesis of conducting polym­

ers is similar to the electrddeposition of metalfrom an electrolytic bath; the polymer is deposit­ed on the electrode surface and also in the in situdoped form39,4o.

Polypyrrole, poly thiophene and other polymershave been prepared by this route:. For example,pyrrole in aqueous acetonitrile solvent containingtetraethylammonium tetrafluoroborate was .elec­tropolymerized in a two-electrode lelectrochemicalce1l16.The conducting polypyrrole containing BFiion (as dopant) was obtained as a film depositedon the platinum electrode surface.

Similarly a copolymer of thiophene and pyr­role has been prepared by electrochemical polym­erization of 2,2'-thienylpyrrole41. Aniline42, ben­zene, phenol43 and other aromatic hydrocarbons44have been electrochemically polymerized.

Miscellaneous processesConducting polymers have also been synthes­

ized by other methods. Polyacetyllene has beenprepared by an indirect method, not involving theacetylene monomer48 - 50. Polyvinyl chloride ondehydrochlorination by a base in a polar solventyields polyacetylene.

t-BuOK ~DMF/N2

-f- CH - CH ----L2. 1tCI

INDIAN J CHEM, SEe. A, JUNE 1994

THF

(i) 0-5°C

(ii) Room temp.

Ziegler atta

Catalysts containing Homog neous

Pd, W, Mo, Ni, Co Modifi d-Ziegler

MoCI),Mo(Co)6' WCI) Homog neous

Modifi -ZieglerCationi

Modifie -ZieglerAnionic

Free-ra ical

BF)

MoCl), WCl)

Butyllithium

Benzoyl peroxide,

.di-tert-butyl peroxide

y-radiation

of para-dichlorobenzene wi~h sodi-

Cl+0 Na2S- *S*+20 NaCI

-I)+n(U - P- Li)IC6H5

526

C6HS -SjCECH(CH3hSi-C=CH

CICH2-- =CHR-C=C

(R =, alkyl)

Monosl<~b*tuted acetylene .CH3-C~CH AlR/TICh

d acetylene

CHl - C =Ic - nC)H7 MoCls, WCI6

TaBrs

CHI - C =Ie - CoHs MOCIS' MoC16

WCl6

CFj7C '" q- CF3 y-ray

T1ble 2- Polymerization of acetylene derivativfsMonomer Catalyst Polymefuationsystem

Simila ly polyvinylene sulfide33 and P~lythio­phene s lfide34 have been prepared by po ycond­ensation between appropriate dichloro com oundsand anh rous sodium sulfide.

In the author's laboratory a new POlYme~ poly­

ethynyll ulfide, has been synthesized by a similarpolycon ensation technique. The reacti n in­volves t polycondensation between diiod acety­lene35 an sodium sulfide36.

n(l--C=

NMPn(1-- C =Ic- 1)+nNa S----'

2 12Doc

-t-C=C-S-in +12n NaI

Maiti 1nd coworkers have also reported lor the

first tim a phosphorus containing polyme whichis solubl and semi-conducting in the virgO state.The reac ion may be shown as follows35:

MAITI: PROBLEMS & PROMISES OF CONDUCTING POLYMERS 527

Sometimes a precursor polymer was used to pre­pare the conducting polymer. For example, poly(2,5-thienyl vinylene) was prepared by heating theprecursor polymer5l•

The same polymer may be obtained from a dif­ferent precursor polymer52:

High quality polyacetylene films have been pre­pared by the Durham route53,54.

1t CF'~:JL-J-.:{"CFJ WCII/S:HI4~ ~CFJ n

oCFJ + ...J:.-. ALVcF, , tv" W In

The same film may also be obtained by thecopolymerization route55.

Highly ordered polyacetylene may be synthes­ized by the template polymerization technique,viz., polymerization of acetylene within the poresof a membrane containing Ziegler-Natta catal­yst56•57• Such ordered polymer exhibits higherconductivity .

Solid state polymerization has been used for thesynthesis of conducting polymers such as polysul­fur nitride58,59 and polydiacetylene60•

Problems in polymer synthesisPolymerization reactions differ in many respects

from ordinary chemical syntheses. High purity ofmonomers (usually> 99.9%) and other chemicals,solvems, etc. is a prerequisite for obtaining highpolymers. The polymerization conditions shouldbe strictly controlled. Many catalyst systems, e.g.,Ziegler-Natta catalysts and ionic catalysts, arehighly sensitive to moisture, oxygen and other po-

lar chemicals. Thus inert and dry environmentsare required for polymerization. A slight variationin the polymerization condition or in the compo­sition of the catalyst significantly alters the natureof the polymer obtained. Some polymers undergoisomerization reaction under light and/or heat.Such polymers should be carefully stored in adark, cool and dry place.

Doping of polymersDoping in polymeric semiconductors is differ­

ent from that in inorganic or traditional semicon­ductors61. Inorganic semiconductors have a three­dimensional crystal lattice and on incorporationof specific dopants, n-type or p-type in ppm le­vels, the lattice is only slightly distorted. The do­pant is distributed along specific crystal orient­ations in specific sites on a repetitive basis. Thedoping is usually quantitative and the carrier con­centration is directly proportional to the dopantconcentration.

Doping of conducting polymers involvesrandom dispersion or aggregation of do­pants in molar concentrations in the disorderedstructure of entangled chains and fibrils. The do­pant concentration may be as high as 50% (ref.62). Also incorporation of the dopant moleculesin the quasi one-dimensional polymer systemsconsiderably disturbs the chain order leading toreorganisation of the polymer63. Thus the ultimateconductivity in polymeric semiconductors de­pends on many factors, viz., nature and concen­tration of dopants, homogeneity of doping, carriermobility, crystallinity and morphology of polym­ers.

Doping of inorganic semiconductors generateseither holes in the valence band or electrons inthe conduction band. On the other hand, polymerdoping leads to the formation of conjugational de­fects, viz., solitons, polarons, or bipolarons in thepolymer chain64• X-ray diffraction study on io­dine-doped polyacetylene shows that the C - Cbond length of polyacetylene chain increases withdonor doping but decreases on acceptor doping65•

Types of doping agentsDoping agents or dopants are either strong re­

ducing agents or strong oxidizing agents66,67. Theymay be neutral molecules and compounds or in­organic salts which can easily form ions. Thus do­pants may be classified as (a) neutral dopants, (b)ionic dopants, (c) organic dopants and (d) polym­eric dopants (Table 3). Neutral dopants are con­verted into negative or positive ions with or with­out chemical modifications during the process of

528 INDIAm J CHEM, SEe. A, JUNE 1994

'PA: Po1:ra%tylene, PPS: Polyphenylene sulfide,PPP: Pol araphenylene, PPY: Polypyrrole, PTh: Polythio­phene, P : Polyaniline, P3MT: Poly( 3-methyl thioPhJne).

•

..

Doping techniquesDoping of polymers may be canied out by the

following methods: (a) gaseous doping, (b) solu­tion doping, (c) electrochemical doping, (d) self­doping (e) radiation induced doping and (f) ion­exchange doping.

Of these, the first three methods are widelyused because of convenience and low cost. In the

gaseous doping process, polymers are exposed tothe vapours of the dopant under vacuum67• Thelevel of dopant concentration in polymers may beeasily controlled by temperature,.' vacuum, andtime of exposure. Solution doping involves theuse of a solvent in which all the products of dop­ing are soluble. Toluene. acetonitrile, tetrahydro­furan, nitromethane, and other similar polarsolvents are used as solvents. The polymer istreated with the dopant solution.

Simultaneous polymerization andl doping gener­ally occurs in the electrochemical doping tech­nique68• But sometimes this method is used fordoping polymers obtained by other methods also.In this process only ionic type dopants are usedas the electrolyte in polar solvents such as ni­tromethane, acetonitrile, dichlorornethane, tetra­hydrofuran, etc .

Self-doping does not require any external dop­ing agent. In the polymer chain the ionizablegroup, for example, sulfonate group of poly[3(2­ethane sulfonate) thiophene] acts as the dopantfor the polymer69•

High energy radiations such as gamma-ray,electron beam and neutron radiation are used fordoping of polymers by neutral dopants. For exam­ple, gamma-ray irradiation in the presence of SF6gas70 or neutron radiation in the presence of 12

(ref. 71) has been used to dope poly thiophene. Itis assumed that the neutral molecules, SF6 or 12,

first decompose to active dopant species underhigh energy radiation70.

PA,PPS,PPP

PA(trans)

PA,PPS,PPPPA

PPP

PAn

PAPA

PP

PPY,PTh

PA(trans)

PA, PPY, P3M'

PPY,PTh

PPY,PTh

PPY,PThPTh

PA(cis)PA

PPY,PAn

PPY,PAnPPY

Active species Polymer"for doping

FeCli

SnCli

AlCl4

ClO.;­Na+

CIOi

BFiCF3SO)

PF6

(CH3)4W

CIOi

AsF6

SO)SO)COo-

Table 3-Dopants for polymers

Neutrald

12

Br2

AsF2

Na

KH2S

FeCI3

SnCI4

Alel3

Ionic do

Polymer~

PVS

PPS

PS-Co~M'A

Dopant

Organic

C~C$OH COO- PPY

CF3S 3Na SO) PPY

JrCH C6H4S03H SO) PPY

doping. onic dopants are either oxidized or re­duced b an electron transfer with the olymerand the ounter ion remains with the pol mer tomake th system neutral. Another type f ionicdopants . volves the anion derived from e dis­sociatio of the dopant molecule, which eutral­izes thle ositive charge of the polymer du . g theelectroc emical doping process. Organic opantsare anio .c dopants generally incorporat d intopolymer from aqueous electrolytes during anodicdeposili n of the polymer. Polymeric dop ts arefunction . ed polymer electrolytes cont .phiphilic .ons.

Mechanism of doping in polymersSince dopants are strong oxidizing or reducing

agents, on doping positive or negative charge car­riers are developed in the polymers. This may berepresented by the following simplified scheme ofreaction66,67,72:

OxidationPolymer +Dopant )(Doped polymer) +

+Dopant-

ReductionPolymer +Dopant ~ (Doped polymer)-

+Dopant +

Ii ,Ii 1111, I '11'14111.H.,~lllllllli

MAITI: PROBLEMS & PROMISES OF CONDUCTING POLYMERS 529

Doping is not merely an oxidation or reductionreaction .. It was found that doping results in rear­rangement of polymer chains and thereby new or­dered structures are formed. The C - C bond

length of polyacetylene on doping decreases orincreases depending on the acceptor or donortype of the dopant used.

In the doped polymers charged solitons areformed. These are charged defects with no spin. Areducing, i.e., donor type, dopant introduces anelectron to the polymer chain which couples witha neutral defect resulting in a negative solitonwith zero spin. Similarly an oxidizing (acceptortype) dopant abstracts an electron from the po­lymer chain and a positive spinless soliton isformed. This may be illustrated by the doping ofpolyacetylene66•

trans .. POlYOCftyl.n.

I Phon A·)

trons- Polyoc.tyl.n.

IPhos. B )

ferent structures, processing of polymers, degreeof crystallinity, and temperature. It has been ob­served that conductivity of cis-polyacetylenedoped with AsFs after connecting to electrode is2-3 times higher than when the contacts are ap­plied after dopingll .

Influence of doping on conductivityThe extent of enhancement of electric conduc­

tivity of a polymer primarily depends on thechemical reactivity of the dopant with the polym­er. The same dopant cannot be effective for dif­rerent polymers. Iodine, for example, enhancesthe conductivity of polyacetylene by 10-12 ordersof magnitude, but it fails to dope polyphenylenesulfide or polyparaphenylene because of its weakoxidizing ability. AsFs, being a stronger oxidizingdopant, can successfully dope polyacetylene, poly­phenylene sulfide or polyparaphenylene. Electricconductivity of conducting polymers both in vir­gin and doped conditions is shown in Table 4.

N.utrol soliton Ta1Jle 4-Electrical conductivity of polymers in virgin anddoped state

Conductivity(S/cm)

4.4 x lO-s

1.6 X 102

0.4

4x 102

80

[C6H4(AsFs)0.4]

[C6H4Ko.6]

[C6H4(AlCI4)0.23]

1.7 X 10-9

5.5 X 102

1.1 X 103

9.7 X 102

10-12

10-s<5 X 102

20

104

10-16

lO-s

5

10-10

110-8

[C4H3N(CI04b) 2 x 102

[C4H3N(BF4)0.nl 102

[C4H3N(CF3C03)0.121!1210-7

10-20

10-20[C4H2S(CI04)0.3]

[C4H2S(BF4)0.3]

Dopant Composition ofdoped polymer

CIOiBFiCF3C03None

CIOi

BFi

None

12 [CHIld

Br2 [CHBrO.23]

AsFs [CH(AsFs)O.I]

Na-naph-!lCHNao28]thalide

Polymer

trn.ns- Polyace­

tylene

Polyaniline

Polythiophene

cis-Polyace- None

tylene 12 [CHIo.3]

AsFs [CH(AsFs)o.tl

Bu4NCI04 [CH(CI04)0.064]

Polyparapheny- None

lene Ii

AsFsKAlCI3

Polyphenylene None

sulfide 12

AsFsNone

H2S04

Polypyrrole None

Dopant 1(Donor)

Nogoti •• sol iton

E&

~G>

Bipotaron

AAAA@

Polaron

Doping-

Doping•Neutral soliton

Dopant L (Ace.ptor I

Positi", soliton

The above sequence of electronic event occursin the polymer chain at a very low doping level.But as the doping level is increased, formation ofa polaron is thought to take place:

With increase in the doping level more andmore polarons interact to form bipolaron, whichis a dication. Existence of bipolarons has been re­ported in various doped highly conducting polym­ers such as polyparaphenylene, polypyrrole, poly­thiophene, poly(3-methyl thiophene) (P3MT) andin polyacetylene. These are regarded as the majorcharge carriers in conducting polymers.

Electrical behaviour

Electrical conductivity of polymers, though pri­marily depends' on doping, is influenced by manyfactors, viz., method of synthesis resulting in dif-

-----~-

530 INDIAN J CHEM, SEe. A, JUNE 1994

77

78

79

79

Ref.Condu(:tivityS/cm

1.1 x 103

3.0x 103

1.6 X 102

1 X 104

Unstretched

Stretched

Unoriented

Oriented

cis-PNAsFs

Table 6-Influence of stretching on the conductivity of po­lymers

Polymer/Dopant Conditionsystem

Table 5-Influence of film thickness on tllle conductivity ofelectrochemically prepared poly( 3-methyl thiophene)

Film thickness Substrate surface Conductivity(A) (S/cm)

2,000 Irtdium-tin oxide 1,975

3,800 "" 1,470

simultaneous mechanical stretching and heattreatment. It bas been reported that by stretchingpolyacetylene film, higher order in chain structurecan be induced which in turn enhances the con­ductivityn-79 (Table 6).

Similarly, under the influence of magnetic fieldof 2-14 KG, high alignment of polyacetylene fi­brils were obtained from nematic liquid crystalsof the polymer which showed very high electricconductivity on doping with iodine 79(Table 6).

Cond ctivity of polymers depends dlr ctly onthe do . g level. For vapour phase dop g, thedoping evel increases with exposure tim of thepolyme to the dopant vapour (Fig. 1). So etimesa sharp rise in conductivity is observed fo a verysmall i rement of the dopant level67. T s sharpincrease may be due to the rapid increase in mo­bility of the charge carriers, which in tu is dueto inter ain interaction.

Effect synthesis, processing and crystal~inity ofpolyme on conductivity

Poly r conductivity also depends on th meth­od of p lymer synthesis73,74, isolation an purifi­cation t chniques, physical treatment of he po­lymer, tc. Presence of oxygen and moi ture inthe: elec olytic medium affects the conduc 'vity ofthe: dop d polymer75. Electrical conductivi alsovalies th the film thickness of the doped polym­er76 (Ta Ie 5). This is probably due to e factthat mo e ordered and defect-free polyme struc­ture is 0 tained in a thin film.

Elect cal conductivity of polymers ~' reases

with the degree of crystallinity (Fig. 2). G nerallydegree f crystallinity may be induced in olym­ers by echanical stretching or in a better ay by

50·(

Fig. 1--

10'3

104

10'5

0123456789

Exposur ••• tim •••I h

nation of conductivity of PPS with leveltime of exposure to AsFs vapours

If doping

Influence of temperature on conductivityOne of the characteristic featun:s of semicon­

ducting materials is the increase in its electricconductivity with increase in temperature. Thisbehaviour is in sharp contrast widl metallic andceramic materials. The influence of temperature(T) on conductivity (a) in s€emiconducting materi­als including conducting polymers, doped or un­doped, follows the relation:

-aaoc e TI/(l +d)

5I

! lJ1H'~'• -5 J-~ Virgin- -10

-15O. 50 100

Crystallinity. %

Fig. 2-V~riation of conductivity of polyacetylene ~ith de­gree of crystallinity

where a is a constant and is proportional to elec­tric activation energy, and d the dimension of.space coordinates. The plot of In a versus T - 0.5.i.e., when d = 1 or T-0.25 (i.e., when d = 3) showsa linear relationship. This indicates the variablerange hopping of charge carriers in one or threedimensions in space80•

Even in conducting doped polymt:rs with metal­lic conductivity (a> 103 S/cm), th'e temperaturedependence of conductivity does not follow themetallic behaviour. It has been neported81 thatFeCl3 doped polyacetylene exhibits a small in­crease in conductivity with decrease in tempera-

I I II t I!II~ , II' i II

MAITI: PROBLEMS & PROMISES OF CONDUCflNG POLYMERS 531

ture up to - 53°C, but thereafter the conductivitydecreases gradually up to - 258°C.

Optical behaviourIt is well-known that absorption of electromag­

netic radiation particularly in the UV to near IRregion of the spectrum provides important infor­mation about the energy bands of inorganic semi­conductors. Optical absorption in conducting po­lymers which are mostly amorphous or paracrys­taIline may be due to the transition of charge car­riers, through a forbidden energy gap, called 'opti­cal gap' or optical band gap. Doping introducesadditional energy bands in this forbidden gap, butit does not interfere with the principal energybands of the virgin polymers (Fig. 3). New ab­sorption peaks appear due to doping of polymers,but again the energy for absorption maximum(Emax) does not depend on the nature of the do­pant82•83•

The number of new absorption peaks on dop­ing depends on the virgin polymer. If the polymeris a degenerate system, e.g. polyacetylene, boththe resonating forms result in doped chains withidentical energy and hence only one new peakwill appear on doping. Polyheterocyclics and poly­phenylene sulfide are non-degenerate systemshaving two different resonating structures, viz.,benzenoid and quinoid, and on doping these twoforms produce new energy states in the optical

gap with dissimilar energy, and hence two newabsorption peaks are produced (Fig. 3).

Attempts have been made to determine the op­tical band gap of polyvinylene sulfide33 using therelation applicable to conventional amorphoussemiconductors84:

ahv oc (hv- Ellf-where J;,~ is the optical band gap, r a const~ant, a optical absorption coefficient andh v the photon energy. The value of r is deter­mined by the curve fitting method and is equal to1/2, 1 or 2. Maiti and coworkers have also usedthe same eqution with r = 2 for determination ofthe optical gap of amorphous polyethynylsulfide85 and poly(p-phenyl phosphoethynedi­yl)86.87.

Narrow band gap conducting polymersThe optical band gap controls the electronic

and optical properties of conducting polymers. Areduction in the optical gap increases the conduc­tivity of the polymers. Attempts have, therefore,been made to reduce the band gap in conductingpolymers by various techniques (Table 7). Onesuch method is the augmentation of the quinoidform in polythiophene by polymerization of iso­thianaphthene, a benzoderivative of thiophene22•

(Cl•••• k! lorml

Polythiophene (Eg=2.1 eV)

(Clulnold I•••••I(A.omolie lorml

o 1 2 3 4 5

Energy, eV

Fig. 3-Effect of d.oping on the optical ab~orption spectra of(a) trans-polyacetylene, 'and (b) polypyrrole. Curves I and 11refer to the corresponding virgin polymers and doped polym-

ers respectively.

~+-

OIl

c:QICClu+­0­o

.~.:. '\.•,,

II ", ;' .., ..•............••... ',

Polyisothianaphthene (Eg = 1.1 eV)

Thus the optical band gap (Eg) is reduced from2.1 eV of polythiophene to 1.1"eV of polyisothia­naphthene.

Jenekhe synthesized a series of heteroaromaticconjugated polymers containing alternating aro­matic and quinoid structures in the main chainhaving the followingrepeat unit8 8 •

532 I INDIAJ CHEM, SEe. A, JUNE 1994

Table 7-N

ow bandgap conducting polymers

Polymer

I tructureBandgap, Eg Reasons for small EgRef.

(eV)

Polyisothi~aphthene&t g'1.13

Introduction of quinoid22,

5 n 5 in

structure40

n "II , Ll01121

Poly[a-( 5'1-tetrathioPhenediYI)

~,;>t- rl¥~1:1.1-0.75Increase of quinoid88

benzylide ]

content

m=ol, n.l,2.3

Poly(CYcl0xen~a[2.1-b; 3,4-b'] dithio~

(~\

0

Occupancy of frontier1.2 141

phene4-o e) ~~orbitals by :incorpora-5 5 n

tion of empty orbitals

Poly(dithiellO (3,4-b: 3',4'-d]-thiophene)

(~_/tloOPlanar quinoid142

backbone

R-

R

tR

R

Poly(2,3-dlhexyl thieno( 3,4-b ]-pyrazine)

KHlo95

Planar quinoid143N N NN

{-a),backbone

\-~~ s nR,

IPolycrocoqaines -k:2f:rx?-i0.5

Strong donor-acceptor144

+, ~ n

interaction

Rl 0Rl = CH), R2=n-C12 1125Poly(2,6-PJridine dicarbonylsulfide)

t 085

Strong donor-acceptor89

{'rrJ§lrr si~ ~~J§l~/slinteraction and occu-

pancy of frontier orbi-o 0 0 0

tals by incorporation ofempty p-orlbitals - ,

the b nd gap measured by the optical bsorp­tion sp tra of the polymers having X sulfur,R =, phe yl and m = 1, n = 1, 2 or 3 in th abovestmctur shows gradual reduction in valu s from1.1 eV 0 0.83 eV and finally to 0.75 e whenthe fract on of the quinoid segments, i.e. (m +n)was inc eased from 0.5 to 0.66 and . ally to0.75. !

Anoth r approach for lowering of the opticalband ga in semiconducting polymers is b sed onthe reclu tion of their aromaticity. Thus cy lopen­ta[2,1-b; 3,4-b']-dithiophene-4-one having he fol­lowing t 0 resonating structures has a r duceclaromatic ty in the polymerized form clue to the

poly( 4-oxo-4 H -cyclopenta[ 2, 1-b; 3,4-b']-dithio-phene-2,6-cliyl with E g = 1.2 eV (ref. 48).

Incorporation of empty p-orbitals in the polym­er chain by effective variation in tht~ occupancy offrontier orbitals is another technique for loweringor band gap in conducting polymers such as po­ly(2,6-pyridinedicarbonylsulfide)89. This is due tothe possibility of formation of canonical resonat­ing forms of the polymer:

"I

MAITI: PROBLEMS & PROMISES OF CONDUCTING POLYMERS 533

Stability and processability problemsConducting polymers in general exhibit poor

thermal and environmental stability. Morevoer,due to extended conjugated chain structure, thesepolymers are insoluble and infusible and hencenot easily processable. Polyacetylene, for example,shows electric conductivity in the semiconductingrange. But on exposure to atmosphere its conduc­tivity falls rapidly90,91.

Conducting polymers, on doping, become moreunstable in environmental conditions. The rate of

reduction in conductivity of iodine-doped polya­cetylene under room temperature is higher thanthat of its virgin state92. However, in dry oxygen,iodine-doped polyacetylene loses its conductivitymore slowly than its undoped state. Polypheny­lene sulfide (PPS) which is stable in the virginstate, is unstable in the presence of moisture inthe doped state80. But in dry air or oxygen, dopedPPS is stable. Polyparaphenylene is very stable inair in the presence of moisture but when dopedwith oxidizing dopants it is unstable in moisture93.But polythiophene and poly( 3-methyl thiophene)are stable both in the doped and undoped statesin the presence of moisture or air and oxygen94.The stability of these polymers is attributed to thedelocalization of electrons in the chain throughparticipation of the non-bonding pair of electronsof the sulfur atom. This principle of stabilizationof conducting polymers has also received supportfrom polyethynyl sulfide36,85 and poly(P­phenylphosphoethynediyl)86,95. These two polym­ers do not show any reduction in conductivityeven after one year of storage in normal environ­ment.

Influence of dopants on the stability of conductingpolymers

The nature of dopants plays an important role inthe stability of conducting polymers. For example,perchloric acid doped polyacetylene is not sensitiveto water and oxygen. Similarly, electrochemical dop­ing of polyacetylene with sodium fluoride makes itmore resistant to-oxygen96.When poly( 3-methyl thio­phene) is doped with S03CFi it develops stability inatmospheric air94.

It may be noted that conjugated chain polymers arereactive to oxygen and atmospheric air resulting

in the oxidation and crosslinking of the chainwhile heterocyclic polymers are not so reactivewith oxygen or air.

Stabilization of the conducting polymers hasbeen tried by two routes, viz., incorporation ofantioxidants such as benzoquinone and hinderedphenols or by using radical traps such as azobisi­sobutyronitrlle. Another method is ion implanta­tion91.

Thermal stabilityThermal stability of most of the conducting po­

lymers is poor excepting the heterocyclic polym­ers. The rate of thermal degradation is very muchdependent on the environment. Polyacetylene, forexample, degrades at room temperature in air oroxygen atmosphere, but in helium its degradationstarts only at 320°C (ref. 97). Similarly, polythio­phene and its derivatives are stable up to 200­250°C in air, but do not decompose up to 700°Cin inert atmosphere9J. Nature of the dopant alsoaffects the thermal stability of the polymers. -Poly­pyridine, for example, doped with arylsulfonatesis only stable up to 80°C in humid environment,but when doped with BFi it retains its stabilityup to 150°C (ref. 98).

Incorporation of heteroatoms having non-bond­ing electron-pairs in the conjugated chain struc­ture augments thermal stability in conducting po­lymers. Polyethynyl sulfide85 is stable up to about200°C and poly(P-phenylphosphoethynediyl)86 isstable up to 2·50°C.

Processability of conducting polymersDelocalization of electrons requires a conjugat­

ed chain structure which in turn brings insolubil­ity and infusibility to the polymers. Conductingpolymers such as polyacctylene, pulyparapheny­lene, polypyridine, etc., possess therefore poorprocessability. Polyacetylene could not be pro­cessed in the powder form until its free-standingfilm was obtained by the Shriakawa technique.Some unusual solvent systems have been deve­loped for making films from conducting polymers,For example, molten arsenic trifluoride or molteniodine have been used as solvents for polypheny­lene sulfide99,lOo.

A number of general techniques has been deve­loped for improving the process ability of the con­ducting polymers. These include the following: (a)block copolymerization, (b) increase of cham fle­xibility, (c) appropriate substitution in the chain,(d) formation of polymer blends, and , (e) use ofprocessable precursor polymers. It may be notedthat all the above approaches to increase the pro-

534 INDIAN JICHEM, SEe. A, JUNE 1994

r '

cetylene has been deposited on carbon fiber toobtain a composite with high mechanicalstrength 108. Such blending or mixing of conductin,gpolymers with their nonconducting processablecounterparts offers not only enhanced processa­bility but also augments necessary mechanicalstrength, adhesion and environmental stabil­ityI09,1l0.

Processable precursors-Use of soluble precur­sor polymers for generation of high molecularweight conducting polymers is an indirect way toovercome the processability proble:m. This is atwo-step process: first, a high molecular weightsoluble polymer is prepared and a film is castfrom solution; next the film is heated to convert itinto the final insoluble desired polymer. The de­hydrohalogeilation of polyvinyl chloride by treat­ment with a base48 - 50 and the metathesis polym­erization of the Diels-Alder adduct by the Dur­ham route to produce poly acetylene are two ex­amples of application of processable precursorsfor synthesis of insoluble conducting polym­ers53,54.

Intrinsic (self-doped) conducting polymersSince on doping charged species are formed in

the polymer chain it is logical to assume that ifionizable groups are incorporated in the polymerchain, the latter may not require dloping for en­hancement of electric conductivity. These poly­mers containing ionizable species or groups arecalled self-doped, or better termed as non-dopedor intrinsic conducting polymers. The ionizablegroup generally introduced to various conductingpolymers is sulfonic acid group or its salts. Table8 lists a few such intrinsic conducting polymers.

Prospects of conducting polymersRecent research and development activitles

show that conducting polymers exhibit conducti­vity from the semiconducting range (- 10 - 5

S/cm) right up to metallic conductivity (-104S/cm). With this range of electric conductivity andlow density coupled with lower price, lower pro­cessing energy requirement, and above all uniqueversatility of application and design flexibility, po­lymeric conductors pose a serious cfiallenge tothe established inorganic semiconductor technol­ogy. New applications are emerging for thesemodem electronic materials.

Application of conducting polymers in thedevelopment of a rechargeable battery appears tobe feasible and is at the threshold of commerciaii­

zation. A number of conducting polymers suchas polyacetylene, polyaniline and oilier polyheter-

SOiNo+ iOf sOl N-1+

__ --l.~ 'I ~S n

tion of long alkyl sulfonate groute position in thiophene moleculater-soluble polythiophene.

The resence of bulky phenyl SUbstitUlnts inpoly(p-p enylphosphoethynediyl) makes it solublein polar solvents such as DMF, DMAc, MSO,etc.35•95. imilarly, alkyl substitution to the 3-posi­tion in e pyrrole ring makes poly( 3-oc 1 pyr­role) sol ble in common organic solvents105•

Poly r blends-Blends of rigid conduc' g po­lymers .th processable polymers are rep rted tohave im roved processability. Blends of p lyacet­ylene d polyester have been formed 106. Blendsof poly ytrole and polyvinyl alcohol ha e alsobeen pr pared by electro polymerization 107 Polya-

cessabili are achieved at the cost of cond

of the pol ers.Block copolymerization-The rigidity f the

conjugat chain may be reduced and ther by itsprocessa ility improved by block copoly eriza­tion telc .que. A block copolymer of p lyiso­prene ~m polyacetylene with up to 20% of thelatt(~r is oluble in common organic solv nts 101.

Similarly, block copolymerization of 3-me lthio­phene d methyl methacrylate produces poly­mers sol Ie in tetrahydrofuran 102.

Increa in chain flexibility-Chain fle 'bilitycan be mproved by incorporation of exiblecenters flexible linkages in the chain. In rpor­ation of rsenic atoms in the polyacetylene makesthe latte soluble in common organic sol ents24•This is ssible by breaking the regular se uenceof conju tion of polyacetylene by arsenic om inthe back one.

Appro riate substitution-Substitution 01 ether

groups' the 3-position of thiophene m leculemakes e substituted poly thiophene sol ble inthe mix: re of tetrahydrofuran, methylen chlo­rid(~and richloroethane (4:1:1 )103.

R Ro . !A\S ~s~

R= -CH20(CH2hO(CH2hOCH3

Similarly I 2-methoxy aniline on pOlyme~zationyields <ll spluble and stable polymerlO4•

nrQ1 • (rQr--T:-"Yl..OCHl ~i;HjNH2 NH2

I " 'i", I II

MAm: PROBlEMS & PROMISES OF CONDUCTING POLyMERS535

Table 8-Intrinsic conducting polymers

Polymer (structure)

Counter ionConductivityRef.

(S/cm)$OiPoly(3-thiophene-w-alkylene sulfonate) ¥~tH+,Na+

10-7145'I ~ I n

m-1,'

~

Poly(2,2',5',5" -tetrathienyl- 3' -propylsulfonate)

~ /, 5 nK+

10-2146

(CH2)3ISO)

Poly(N -propylpyrrolesuIfonate)

iOtnN

Fe2+,Fe3+10-6147I

lrnOJso)Poly(3-butylpyrrolesulfonate)

(C~)"

Na+

0.1146

~N n

H

PolyanilinesuIfonate

{@-Z~~*H+

0.1148

Fig. 4-Schematic representation of a rechargeable polymerlithium battery.

vices the same polymer through control of dop­ing, or two different polymers-one polyhetero­cyclics and the other polyaniline- have beenused. Plastic field effect transistor (PET), (Fig. 5)for example, has been fabricated with polythio­phene as the semiconductor and [rtoluene sulfon­ate doped polypyrrole as the source drain elec­trode115• Similarly a thin film [rn junction wasmade by depositing alternate layers of polypyrrole

Si02 Au Gate Ga- In Alloy

Fig. 5-Cross-sectional view of a plasticfield effect transistor(FET)

.j

Anode

Gasket

r Palyaniline{SUS mesh

I----- Separator 20 mm

![

ocyclics has been used as electrode materials for.rechargeable batteries. In fact, polyaniline-lithiumbattery (Fig. 4) has already been marketed 111,112.

Vigorous research activities are being carried outto develop all polymeric solid-state batteriesll3and also to use composite electrodes such as car­bon fiber-polyacetylene composite and alkali me­tal a1loy~for better battery performance114•

A number of electronic devices such as Schott­

ky diodes, plastics transistors, [rn junction, etc.,have been developed using semiconducting/con­ducting polymers. For the fabrication of these de-

J

536 INDIAN J CHEM, SEe. A, JUNE 1994

-----..--...-.......i ..".........,~_-+-.......,,--....r-....,..I I \

Gold Source Drain Gold

Fig. 8-Cross-sectional view of a 'molecullebased' transistorusing polyaniline.

planted in vitro to obtain clinical information forcontrol of diseases like diabetes etc,13s(Fig. 7).

The most exciting prospect of conducting po­lymers has been envisaged in the development ofmolecular electronics in which individual conduct­

ing polymer molecules will act as wires, diodes,transistors and other electronic devices139 (Fig. 8).It is expected that through LB t(~chnique highlyoriented molecular layers may be built up suc­cessively with desired polymers having controlledconductivity and predetermined sequence. Thismay perhaps be the right step to molecular elec­tronics.

However, much work needs to be done beforesome of these exciting ideas can be transformedinto commercial products. Conducting polymershaving adequate mechanical strength and chemi­cal/environmental durability as well as metallicconductivity have not yet been synthesized. Al­though doping can enhance the conductivity tovery high orders of magnitude, the high level ofdoping required negates many of the desirableproperties of conducting polymers. Moreover,complete control of the molecular architecture ofconducting polymers still remains largely unattain­able. However, these limitations would be over­come in near future, and then a new generation ofconducting polymers will offer the most versatileelectronic materials for future technology.

..

r •

Ag

Si02

-Ket.- .. ­Glucose - .._-.- .. ~- -

'-5- - ------ --f - .....'.. 13 N _'-- - .... -:::-- - -_I- ..- -4 .. - - ---- -- -.- - - --\ -5 i Water .. - -\ - -I \

Fig. 7-A schematic diagram of a biosensor

PI -

Polypyrrole _with GlucoseOxidase

Wire to FrontWire to ITO Electrode

'~ITO- .~ MetaIPolymer _J --Glass

Fig. 6-~chema\ic representation of a semiconduct~ngpolym­er light emitting diode (lED)

and pol thiophene on a platinum substr te withcontwll d electrochemical doping to m e thepolypyr ole layer Jrtype and polythiophe e layern-typell . Using chloroform soluble poly 3-hexylthiophe e) a 100-400 nm thick fIlm was cast onindium-' oxide surface and then indi m was

deposit on the surface by CVD techniq e to fa­bricate'Schottky diode having room tern eraturerectifka ion property1l7,lls.

Light mitting diodes (LEDs) have beeRmade

by dep siting a fIlm of semiconducting olymersuch as poly[2-methoxy, 5-(2'-ethyl hexo )-1,4­phenyle e vinylene] on indium-tin oxide coatedglass su ace1l9,120(Fig. 6).

Since the electric conductivity of co ductingpolyme s varies in the presence of differe t chem­ical sub tances, these are widely used as hemicalsensors or gas sensors. For example, thin fIlms ofpolypyr ole show a decrease in conduc ivity inthe pre ence of even 0.1% ammonia in ir, andan iner ase in conductivity in 0.1% nitrog n diox­ide and 0.1% hydrogen sulfide. The sens' ivity ofsuch g sensors is found to be 0.01% as con­centrati n in airl21. These gas sensors a e oper­able at mbient temperature .

Con cting polymers have been used to pre­pare L ngmuir-Blodgett (LB) films. If it s possi­ble to c st a multilayered LB fIlm with th desiredmolec:u r architecture from soluble co ducting

poly me , the scope for miniaturization f elec­tronic: omponcnts such as micro circuit will beremark bly extended. LB films with high olecu­lair ord r have been cast from solutions of poly(al­kyl thio hene)122and poly(alkyl pyrrole)123.

Som of the new fields for application of con­ducting polymers include gas separatio mem­brane 12, photo electrochemical celll25, op ical de­vices126127, ion gates12S.129,memory sto age de­vices 130131,non-linear circuit elements 132 etc. Inbiologi al applications, conducting polym rs, viz.,polypy role are used as biosensors 133 a d con­trolled release devices for pharmaceutic s134-136Bioco patibility of the polymeric conduc ors is aplus pint for fabrication of artificial he rts con­sisting of biological fuel cells and elect odes137.Condu ting polymer-based biosensors ma be im-

MAITI: PROBLEMS & PROMISES OF CONDUCTING POLYMERS 537

AcknowledgementsThe author thanks his research students Drs

Md. S. Rahman, Miss M. Mahapatra and particu­larly Sri S. Kundu for their research and provid­ing valuable information in this exciting field.

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