Journa l o f Sc ienti fic & Ind ustr ia l Research

Vol. 6 1, September 2002. pp 680-689

Melt/Solution Processable Polyaniline and Blends

Raji K Paul and C K S Pillai * Regional Research Laboratory (CS IR). T hiru vananthapuram 695 0 19

Conducting polyanil inc ( l'i\N I ) has been at tracting great att ention in recent times clue to its electri cal. elc:ct rochemical ilncl

opti ca l propert ies as wc ll as its environmental stabili ty. Unfortunatcl y. the li mitations in synthcti c procedures and poor proeessabi lity of l'i\N I ha ve res tr·ictecl it s commercial ization. It opened up vast avenues in resca rch for deve lop ing m.ve l r.1ct hods and tech­

niques o f both scientifi c and industrial importance for making conducti ng polymers fusib le m so luble. During the past few yea rs. many attempts have been rn acle to irnprovc proccssability o f PA I such as eloping wi th functi onali sed .Jopant>. blending w ith conv..:nti onal po ly rncrs. nov..: ! synth..:tic routes f:>r th e preparat ion or PAN I and it s modifi cation w ith va rious ri ng or N-substitu­

ellls . Among th e: methods so far clcvclopecl. the protonation o f PA !J wit h funct iona li sccl dopants and blending\ •ith thermop las­tics arc the strateg ies bei ng w idely adopted. Functi onali sed dopants possess ing plas tici sing c 11111 protonat ing abi liti es . synthe­

sized from natura l ly ex isting matcria ls. an.: also rcpo rtcd to be promi sing. Thus. the usc o r functi onali secl dopant-; possess ing

plasti cisi ng c11111 protonating abiliti es could be o f value in preparing melt/so lution processab le blends o r thermoplastics/elasto­meri c polymers w ith PA N ! and mi ght pave way for so lutions to probh: m;. such as proccssabilit y. o:ost and reli abi lit y on commerciali sation o r the conducting po lym..: rs and it s blends.

Introduction Polymeric material s, in general , are considered elec­

trically non-conducting. Interes ting electronic and opti­

ca l properties arc, hovvcvcr, observed in organi c poly­

mers w ith conjugated stru ctures th at give ri se to con­

ductivity clue to unique deloca li zcd TI-e lcc tron systems.

Th ese properties are be ing exp lored ror practi cal appli­cations in elec tronic and optoelectron ic dev ices such as rechargeable batteries, EM I shield ing. l igh t em itting eli ­odes, biosensors ancl cathodic protec ti ons or meta lli c struc turcs1

·5 The obv ious attracti on is to comb ine in one

material the elec tri ca l properti es and the val ue added

appl icat ions of a semi conductor or a metal w ith attrac­tive mechanical properti es and process ing scm iconduc­tor or a metal wi th attr•tctive mechanical properti es and processing ad va ntages o r a po lyme r. The ease o r

proccssab ilit y and rabrication coupled w ith attractive mechan ical properti es, c~pcc i al l y l'lcx ibil ity and impact resi stance in combina ti on w ith low cost, place pol ymers definitely on an advantageous pos ition over semiconduc-

tors and metal s1 ' ·

T he f irst and stimulatin g success in the f ield of elec­

tr ica ll y conducting poly mers took place around 1977 when it was clemonstratccl by Shirakawa el ol. that polyacety lene. an intrinsica ll y insulator polymer. could

''' Au thor for corre~pondcncc

become high ly conducti ng on treatment with ox idi zing or reducing agents<·- ~ . Since the pub lication of the ori gi­

na l observa ti on in 1977, there has hecn an ex plos ive

growth o f research in to the wl~o l e ran ge or conjugated polymer stru ctures'1·

10• Among the intri nsicall y conduct­

ing polymers (Figure I gives structu res of repeating units

or several conj ugate polymers), PA I has been attrac t­ing significant interest in electronic app licat ions because

or their w ide range or electrica l, electrochem ical and

optical properties as well as their good environmental s tabilit y'l · 1 ~ . PA I can also undergo chemicalmodirica­

ti ons on the ring and nilrogen atotm to give a va riety or

substituted derivati ves that can ex i. ti n several dirferent

ox idati on stat es which can be eloped either by protona­ti on with a protonic acid or by ch· rge transfer with an oxi di sing agent. Additionall y, the elec tronic ancl optical properti es may be contro lled rc ve rsibi I ity by varying the doping level. For practical appli e<Jt ions, a conduct ing polymer must be cost-cllcctivc, mu. t have good chemi­cal and elec trical sta bi li ty and be able to be easily pro­cessed from either so lution or the me lt. Unrortunatel y. the li m itati ons in syn th eti c procedure~ and poor proccssab ilit y o r PAN I have l·e. tri ctecl it s

commerciali sati on11. Thi s has opened up vas t avenues

for research for deve loping novel methods and tech­niq ues or both sc ientific and industri al importance ror making conducting po lymers fusib .e or sol uble. During

PA UL & PILLA!: MELT/SOLUTION PROCESSA BLE POLY AN ILIN E 68 1

the past few years many attempts have been made to

improve the processability of PAN I such as eloping with functionali secl dopants, blending with conventional pol y­mers, novel synthetic routes for the preparation of PAN!

and modification of the polymer w ith vari ous ring or N­substituents. Among the methods so far developed, the protonation of PAN! with functi onali sed dopants and blending with thermop las ti cs are the strategies being widely adopted . It would be worth whi le to revi ew the

area giving particular emphas is to the methods for prepa­ration of melt/so lution processable protonatecl PAN ! us­ing functi onali sed dopants and other blending methods.

Solution Processability

Doping of P!\ NI Willi Funclionali::ed Dopan/s

When PAN I is doped to the emeraldine salt form the

polymer undergoes an insu lator-to-metal tran sition with

a concomitant conformational change (from compact coi I to expanded co il ) in the polymer backbone to accommo­date the renewed electronic transformati on. The result of the conformational change is a reduction or structural defects along the PAN! chain (e g. twist ing, buckling), which increases then-orbital overlap between the phe­

nyl n-e lect rons and nitrogen p-e lectrons. I t. in tu rn , in­crea. es both conjugation of the chain backbone and the po larons demorali za ti on length. However, due to the

conformati ona l change and grea ter polarit y o r th e ionomeri c form. the so lubility and processabi lity or sa lt form is marked ly reduced. The runctional group present in the dop ing ac id, its structure and orientation play an important role in solub ili zing and processing the con­ducting form of PAN !. Recen t ideas of doping induced

proce . .-.mbilil\', where functiona li sccl dopan ts having po­lar and nonpolar groups that promote compatibil ity be­tween the pol ymer and the so lvent, thus imparting solu­ti on processab ility by micell ar act ion to PAN I paveclthc way for a breakthrough 1 ~. A num ber or recent publica­tions describe the usc or functionali scd proton ic acids(•.­

such as can1phor sulphonic acid (CSA) 1.116. dodccy lbcnzenesu lph oni c acid ( DBS/\ ) 1 ~ · 15· 17 . p­

toluenesulphonic acid 17·1x, octylbenzcncsulphonic ac id 17 ,

po ly(s tyrc ne)sulphonic acid 1'1.21, sulphami c acid 22· 21 .

sulphosa li cylic acid 22· 2~. bcnzencsulphon ic acicl 1x.

sui phani I ic acicl 2-', methanesul phonic acid 21

', sui phonic acid and phosphoric acid derivatives Dentadecv lohenol27

.32 as runctionaliscd dopan ts for im-

Polyacctylcnc (trans) PA

_Lnl l~~

Polypyrrole PPyr

-rot II

-tot n

Polythiophene PT

Poly(p-phenylcnc) PPA

Polyanilinc PA 'I

to-1 11

Poly(p-phcnylcnc viny lcne) PPV

Figure !- Struct ures or conducting polymers

In a noteworthy contributi on in the development of functional ised dopants, Heeger and coworkers have re­po rt ed that camphorsulphonic acid and dodccy lbenzenesulphonic acid in combination with sec­ondary dopants such as 111 -cresolmake PAN! so luble1u 3

in common organic so lvents. It is possible that the hy­drophob ic groups of th e functionalised dopants weaken

the chain-chain interac ti oP of pol ymer and enhance the polymer-solvent interaction thus faci litating the solubiliy o f PAN I .1~. Heeger, M acDiarmide and co ll eagues 1 ~ n . .1s

found that the conduct ivity of PAN I could be increased upto 400 S.cm·1 in so lution cas t films usi ng CSA as the dopant and Ill -c reso l as the so lven t and upto 250 S.cm·1

with DBSA as the dopants but when cast from chloro­fo rm so lution . CSA behaves as a poor dopant giving conductiv iti es or about 10 1 S.cm·1. Th is counter ion in­duced processability of PA I opened a new possibility

or app li cation or PAN I as rlexi ble I ight emitting diodes 15.

The PANI -CSA rilms cast from 111-creso l is an order of

magnitude higher than that for PAN ! doped w ith con­ventional acids and it exhibits a reduced microscopic

disorder and metalli c like transport properties 36 . The unusuall y high conducti vi ty and other properti es have

been explaiw:d by the concept of secondary dop ing.17. ·18

whe re treatn1ent of PAN I-CSA with 111-creso l induces a change in molecu lar conforma ti on from that of a com­

pact coi I structure to that of an expanded coi 1-1 ike struc­ture. Various substituted pheno ls such asp-cresol. 3-ethvlohcnol. 2-ch lorophenol, and2-fluorophenol also act

682 J SCI IND RES VOL 61 SEPTEMBER 2002

b _JL/~~-~:----~//J

/

400 600 1400SOO 1000Wavelength (nm)

1200 1600

Figure 2-UV-Visible spectra of (a) PANI-SPOA (b) PANI-SPDPAA & (c) PANI- SPOP in l1l-cresol

Table 1- Conductivity of doped PANI films cast from different organic solvents (in S cm')

Solvent PANI-SPOP PANI-SPDA PANI-SPDPAA PANI-PDPPA PANI-PDP(bis)PAm-Cresol 0.69 12.00 2.20 1.25 0.13Chloroform 0.15 0.46 0.34 0.39 0.02Xylene 0.50 0.81 13.00 0.S2 0.06THF 0.65 0.80 3.50 0.19 O.OS

tion effect of resorcinol on PANI-DBSA and PANI-CSAsystems was studied by Yikki et al.39 while Hopkins etal.40 showed that hexafluoro-2-propanol promote an ex-panded coil conformation PANI.

There are several reports regarding the process abilityof PANI achieved by protonation with polymeric dopantssuch as poly (acrylic acid)", poly (styrenesulphonicacid)41.42 etc. PANI doped with macromolecular dopantsis expected to show higher environmental stability thanthat doped with small molecule protonic acids, whichcan evaporate at room temperature or higher tempera-tures causing a depression in the conductivity. In a re-cent publication, Pron et al." reported highly conduct-ing and solution processable PANI films showing con-ductivities of 180 S.cm-I using a new plasticizing dopant,1,2-benzenedicarboxylic acid, 4-sulpho, 1,2-di (2-ethylhexyl)ester. Protonation with squaric acid" is alsoreported to solubilise PANI. PANI protonated withfunctionalised sulphonic acids":" has also been success-fully spun into fibers.

In search for better dopants that enhance the desir-able properties discussed above, it has been observedthat molecular systems based on sui phonic acid and phos-

phoric acid derivatives of3-pentadecylphenol (POP) andrelated structures such as cardanol possess structuralfeatures required for functionalized dopants that will havemultiple functions of protonation, solubilisation andplasticisation27-32. One of the significant features of thestructure of these dopants is that they have a flexible ll-:

alkyl (C1sH31)substituent in the meta-position of the aro-matic ring which makes the doped PANI soluble in com-mon solvents or melt processable. These dopants, thus,render plasticizing ability to PANI so that freestandingflexible films could be prepared by both the conventionalmelt processing techniques and by the solution process-ing techniques.

Table 1 gives the conductivity values of films castfrom different organic solvents. Figure 2 shows the UV-visible spectra of some of the protonated PANT in m-cresol. But, in contrast to CSA-PANI-/11-cresol systems,a localized polaron peak exists at about 900 nm. This isbecause of the fact that the bulky nature of dopant pre-vents its easy diffusion into the polymer backbone com-pared to CSA and therefore the interaction with the sol-vent is comparatively less. However, bulky nature of thedopants increases its solubility in all these solvents com-pared to the other reported systems.

PAUL & PILLAI: MELT/SOLUTION PROCESSABLE POLYANILINE

Synthetic Approaches: Polymerisation

Another alternative method to obtain solutionprocessable PANI is by adopting different syntheticroutes such as template polyrnerisation, enzymaticpolymerisation, emulsion polymerisation and suspensionpolymerisation to obtain soluble PANI. The work ofViswanathan et al.47.51 merit mention for conductingelaborate studies on the template polymerisation ofaniline by the use of water soluble poly-electrolyticdopants such as lignosulphonic acids. Use of water asthe solvent makes it environmentally stable and the poly-mer can easily be switched from one conducting state toanother without changing the amount of the dopant.Angelopoulos and group" and others 53. 54 prepared wa-ter soluble and doped conducting complex of PANI(PanAqas) where a polyelectrolyte (styrenesulphonicacid) was used as a template for the oxidative assistedchemical polymerisation of aniline. The method of dryspinning was employed by Samuelson et al." to pro-duce PANI fibers from an enzymatically synthesized="water soluble highly conducting polyaninline-sulpho-nated polystyrene complex.

Several authors adopted an in situ doping emulsionpolymerisation route using dopants which can functionas emulsifying cum protonating agents to obtainprocessable and conducting PANp9-61.Although the pro-tonated PANI was still intractable and insoluble, it ex-hibited an exceptional degree of crystalline order andhad high molecular weight. Improvements in solubilityand processability could be achieved by using sulphonicacid dopants containing long alkyl side chains27-32.62.Thus, the in situ doping emulsion polymerization ofaniline in the presence of these dopant provides dopedPANI exhibiting high molecular weight, high conduc-tivity, an exceptional degree of crystalline order and ori-entation and thermally stable. A number of similar andrelated techniques are reported to prep pare processablePANI. Interesting reports have appeared later on emul-sion polyrnersiation'v",

Synthetic Approaches: SubstitutionAnother commonly accepted route to obtain

processable PANI is by substituting the ring withsulphonic acid groups and to make it water soluble'v".Recently Epstein group76.77reported very highly sulpho-nated PANI, using the most reduced form of PANIleucoemeraldine base, as the starting material. But, the

683

Table 2 - Plastification threshold for PANI protonatedwith different dopants

Dopant Dopant/PANI ratio at the atthe plastification threshold

SPDPSPDASPDPAAPDPPAPDP(bis)PA

0.300.350.350.300.15

electrical conductivity is decreased instead due to anincrease in electron localization or reduction in the tt-conjugation. Thus, use of these substituent groups in-duces distortions in the chain, reducing the zr-ccnjuga-tion and increasing the chain flexibility.

Melt processability of PANlAlthough thermal processing of PANI is highly de-

sirable, only a very few reports could be found in litera-ture. Laska et al.88-91have recently shown that phospho-ric acid diesters can serve as solubility inducing proto-nating cum plasticizing agents for PANI. It has beenshown+" in this laboratory that the mono- and di-phos-phoric acid esters as well as sulphonic acid derivativesfrom cardanol and pentadecyl phenol act as very goodplasticising cum protonating agents for PANI and freestanding flexible films could be prepared by the hot press-ing techniques. In these systems the plastcization of PANIis taking place at a very low percentage of the plasticiz-ing dopant itself and the problems associated with thesolution processing could be avoided. Plastificationthresholds determined for PANI protonated with differ-ent dopants sulphonic acid of 3-pentadecylphenoxy ace-tic acid (SPDPAA), sulphonic acid of3-pentadecylanisol(SPDA), sulphonic acid of 3-pentadecylphenol (SPDP)etc. synthesized in this laboratory possess hydrophobicgroups that plasticises PANI so that free-standing flex-ible films of protonated PANI could be prepared by theconventional melt processing techniques are listed inTable 2. Figure 3 represents the log conductivity againstthe temperature of pressing for these protonated poly-mers. A maximum conductivity value of 65 S.cm-I wasobtained for the PANI-SPDPAA film pressed at 140°C.The PANI-SPDAfilm pressed at 140°C gave a conductiv-ity value of 42 Sicm'. But for PANI-SPDP film the con-ductivity is comparatively less because of the presenceof a hydroxyl group. All these protonated polymers are

J SCIIND RES VOL (ll SEPTEMBER ~()O~

100...,--------------------------,

II)

:L..s 0,1

SI) 11111 I ~O i ro I ()O

Pn.'ssin},! tcmpc raturc lie

1),1 II -I----,c----,--~-,-----r-~____r---,__--_,__---1ISO 200

Fiuurc -'~C:Ol1dllClj\'ily I',l pr<:,,,in).'temperature or (a) l'A:\I-SI'DPA.-\(h) 1'.-\l\:I-SI'D,\ 8: (L) 1',\:\1- SPJ)[> rilim,

thermally stable up to 200 "C for preparing highly con-ducting films by the melt processing method, The con-duct ivi ty value is increased upto 1-1-0"C for the xulphonicacid protouatcd polymers and up to 120 "C for the phos-phoric acid proton.ucd polymers bcc.iuxc ofthe increasein polaron dclocaliz.uion with temperature due 10 thethermal activation, While decrease olconductivitv from1-1-0to 2()0 "C is axsociatcd with the decrease of polarondcloculizution due to the over compensation oj' the ther-mal activation effect so that loss or xomc polarons rakeplace?". It is suggested that in proton.ucd PAl I systemsthe conductivity values were decreasing with increasingthe temperature from 1-1-0to 200 "C due to cross linking,therrnal undoping and ring distoniou'". But in the PA\lJ-SPDPAA systems it was clear from the DSC analysisthat an cxothcrrn is there in the temperature range 1-+0-200 "C and so the decrease of' conductivity is because ofthe changes in the chain structure of' ]>,-\NI due to thecross Iink ing as suggested by groups or researchers'); '".The conducii \ it)' or the proronutcd PAN I dccrcuscx ahovc200 "C as in the CISl' or PAl'\I-DBSA and other sulphonicacid systcms'" because at higher temperature thermaldegradation is a dominant factor, It is posxible thatchanges in the chain structure due to cross-linking canalso take place at these higher temperatures,

The low temperature conductivity measurcmcnt-: orthe protonutcd PANI films by the melt processing tech-niques show the development or homogeneous conduct-ing PANI and it indicates that these systems are close iothe transition from a disordered metal to Fermi glass in-sulator, Studies show that In of normalized resistance i-,proportional to Till in the temperature range ISO-SO I(

indicating three dimensional variable runge hopping con-

ductiou. The data were anal yscd for Mous variable rangl'hopping conduction ill three dimensions in till' tempera-ture range ISO-.')O K. The temperature dependence orresisti\'ely is comparatively weaker. decreasing hv onlya small factor on cooling the sample from room tern-perature to ~S K. This observation or the metallic tern-perature dependence of resistivity above 200 K and apower-law dependence below I()() K. demonstrated thattill' thermally proccxsuhlc films of ]>i\NI-SPD;\ andP!\i\I-SPDPi\r\ are almoxt precisely on the mctul-insu-later boundary as in the ease of P,-\:\ I-CS;\ solutionproccssuhlc films!':" Thus, it is clear from the abovediscussions that the transport properties o,'P:\\1 arc ah()improved by these bulky dopants,

Blends of conducting polymers

Cornp.ui hi Iizution of the imractublc conducii \C pol y-Iller in conventional [ilrn tonning mauiccs hy hlendingh~ls been adopted to pl"Ucess inuinvicallv conductingpolymers into coatings and films, The doped P:\\I blendswirh hulk polymcr« are expected to h~I\L' a number oradvantages such as (i ) control of conductivity. (ii) me-chanical properties (iii) cost considcr.n ions. (i v) trans-parency. (v) colorabi Iity and (vi) process: ng proper: ics'".Hancricc and Mundal'" showed that ann doping andcomple\ing PAr>,JIwith functionalizcd sulphonic acids(DBSA and CSA). the PANI call he co-dissolved in Ii/-

cresol in various ratios with an insulating host polymer.ic' g. poly( methyl mcthacrylutc i] which forms robust.transparent conducting films when cast from solution.Since then. till' proronation of PAi\1 with a dopant hav-ing a surfactant group led to the preparation of polyblcndwith polymers such as poly (alkylmcthucryl.ucs)'". poly-

PAUL & PILLA!: MELT/SOLUTION PROCESSABLE POLYANILINE 685

10W 7 -1---...---,-----r----r---.----r------.--.,...---1

o 4020 30Weight of PANI, per cent

Figure 4 - Conductivity vs PANI content in PYC (a) PANI (SPDA)os (b) PANI (SPDPAA)o5 & (e)PANI (PDPPA)os' Pressing temperature, 160 "C; pressing time, 15 min

styrene98,99 poly (vinyl chloride)loo.\o2 and poly (ethyl-ene-co-vinyl acetatej'?' etc. Conductive composites ofPANI with polyurethanes'?' and thermoplastics such asnylon 105-\09and polyvinyl chloride I10were also fabricated.Actually the main advantages of doped PANT blends withbulk polymers are expected to be (i) control of conduc-tivity, (ii) mechanical properties (iii) cost considerations,(iv) transparency, (v) colorability and (vi) processingproperties III.

Solution processed blends are, however, said to haveproblems such as compatibility!'? and formation of ag-gregates'!'. Use of high boiling and acidic solvents suchas m-cresoII13-115(it is also suspected to be a cancer caus-ing substance) are not industrially favourable 112.113,116,Therefore, significant research effort has been directedtowards the thermal processing of conducting polymerblends where there is possibility of formation of con-tinuous network formation of conducting polymers inthe matrix polymer. III,117·124In order to reduce the perco-lation threshold, a heat treatment process of the blendsbefore the processing is adopted. The preparation ofPANT-PYC blends is also describedv'": 126by a dry mix-ing of the polymers.

It has been127.130that addition of compatibilizers suchas, for example, selected esters of gallic acid /phospho-ric acid esters favours the formation of a continuous,percolating PANInetwork in thermally processedpolyaniline-polyolefin blends. Functionalised dopantspossessing solubilising and compatibilising groups might

do better!" and hence the dopants such as SPDPAA,SPDPA etc. possessing' plasticizing cum protonatingabilities could be used to prepare thermoplastic and elas-tomeric blends of PAN I 132.133.These dopants facilitatethe mixing of the components of the blend where thedispersion of PANT grains in the matrix polymer wasconsiderably enhanced by the presence of the plasticiz-ers, which apparently loosened the PANI grain-grainadhesion forces. This is expected to give a very low per-colation threshold. Thus, highly conductingthermopladtic blends of PYC with PANl-SPDA, PANI-SPDPAA and PANI-PDPPA were prepared by mechani-cal mixing at room temperature for an extended periodof time to achieve optimum homogeneity. Yarious plas-ticized PANI/PYC ratios were used. The mixture con-taining PANT and PYC was then hot pressed at 160°Cfor 15 minute to obtain thin films 2729.

Figure 4 represents the log conductivity vs. the con-tent of PANI in these blend films. In the case of PANI-SPDA-PYC polyblend a sample containing 2 wt% frac-tion of PANI itself is giving a conductivity value of3.4x l O:' S.cm-I with no indication of a sharp percolationthreshold., the percolation threshold is occurring at 5wt% of PANI content. Laska et at,89 reported that thepercolation threshold is observed for a 25 wt% of theelectroactive component in the case of PANI protonatedwith DBSA and their blends with PVe. In the presentcase, a lower percolation threshold is obtained becausethe plastification of PANI by the dopant strongly facili-tates the mixing of the components of the blend. The

6S6 J SCI IND RES VOL 61 SEPTEMBER 2002

55

!()r-- -----------7S

20

0(1, -'50-' liD [)O----:;(I' . -250' 11':

TenpiKi7

0.25 0.·+0030 035T·1M (KI/'1

Figure 5-Plot of In resistance iv T'" of PANI-SPDA-PVC blend with 25 \,1'1 per cent of PAN!. Thenorrnalised resistance-temperature plot is shown in the inset

dispersion of PANI grains in the pve matrix was, thus,considerably enhanced by the presence of the plasticizer,which apparently loosened the PANI grain-grain adhe-sion forces. Thus, this remarkable reduction in the per-colation threshold is achieved by the influence of theflexible side chains of the dopants, which plasticizesPANI. An enhancement of compatibility of the polymerwith an increase in the length of the side chains is obvi-ously expected and experimentally evident.

Figure 5 represents a plot of In of normalised resis-tance vs Tl/~ for a polyblend film of PANI-SPDA-PVesystem, having 25 wt% of PANI. It shows that In ofnorrnaliscd resistance is proportional to Tl/~ in the tem-perature range 35-300 K indicating a three-dimensionalvariable range hopping conduction. It indicates that thesethermoplastic conducting blends exhibit metal like trans-port properties as suggested by Menon et al. I.1-1 for PAN 1-eSA-PMMA systems and by Wessling!" for thermoplas-tic blends of PANI.

Another widely accepted technique suggested for thepreparation of processable conducting polymer blendsis the polymerisation of conducting polymer into thematrix polymer. There have been reports on blending ofPANI through electrochemical, chemical and emulsionpolyrnerisation of aniline onto polymer substrates!":':".The inability of the polymerising agent in this method todiffuse into the matrix polymer might affect the blendproperties. Furthermore, such blends are not thermallyprocessable afterwards without affecting the conductiv-

ity. Melt proccssablc blends of conducting polymers withrubbery, non-polar host polymers, which are widely usedby the cable industry, have not been developed':". In thislaboratory, the dopant, SPDPAA was successfully em-ployed in an ill situ doping emulsion polyrnerisution ofaniline on to the elastomeric polymer matrix, poly eth-ylene-co-vinyl acetate (EVA)27.''i. The PANI-SPDPAA-EVA blend obtained by the emulsion polymcrisationmethod was thermally processed to get free standing elas-to mcric and highly conducting films. The meltproccssable film having 4.5 I wt(/( of PANI gave a con-ductivity value of 2x 10' S.cm'. In the case of conduct-ing clastorneric blends, this is the first time that such alow value of percolation threshold is obtained. As thecontent of PANI increase from 4.5 to 2S.5 wt';(, the con-ductivity increases to (l.S9 S.CIllI and this behaviors isbecause of the fact that the plasticising dopants facili-tate the uniform dispersion of the homogeneously pro-tonated PANI into the EVA matrix. The low tempera-ture conductivity measurements show these elastomericblends are obeying the one dimensional variable rangehopping conduction compared to the thermoplastic,where the non-linear temperature dependence of ther-moelectric power indicates that these samples are far intothe insulating regime, well away from the M-I transi-tion. The resistivity increases by several orders of mag-nitude as the temperature is lowered, indicating pres-ence of extensi ve disorder and the formation of inhomo-geneous metallic islands due to the emulsionpolyrnerisation method.

PAUL & PILLA!: MELT/SOLUTION PROCESSABLE POLYAN ILINE 687

Another method used to prepare the blends of con­

ducting polymers is the direct d ispers ion of conducting

polymer powders in the nonconduct ing po lymer matri x. An example is the Versicon1

M, (A lli ed Signal) where

highly polar thermoplastics such as po lycaprol actone and poly(viny l chloride) are used to g ive conductive blends

having melt processabi lity. But in the di spersio n routes there is the poss ibility of the fo rmati on of non-equilib­

rium two phase systems 11 2 with the conductive phase

being the di spersed one so that there is tendency to oc­

cur in fl occulated structures. Attempt to introduce inor­

ganic materials to obtain desired properties are being reported. The work of Tang et a /. 141 to prepare conduc­

ti ve a nd s up e rpara m ag ne ti c fi lm s ( ma g net ic na noco mp os ites) by in co rp o ra tin g m ag he mit e

nanoc lus te rs into doped PAN I mi xes is worth menti on­ing. Similarly, Kim et a /. 142 have reported preparati on of

nanocomposites of PAN land Na+-mo ntmorill onite c lay

which are expected to g ive enhanced properti es.

Commercial products based on PANI The first commercial products incorporating conduct­

ing polymers were actually made a few years ago. In the

late 1980s, the Japanese Companies Bridges tone and Seiko commerc iali zed a rechargeable butto n-cel l batte ry that used PAN I for one e lectrode and lithium for the other.

The most important ap pli cation of the conduc ting

polymer blends is the use as antistat ic material s (e g, anti stati c spray coat ing based o n PA I sol uti on in wa­

ter, named Pan-Aq uas by IBM Thomas J. Watson Re­

search Center). Conduct ive and su perparamagnetic fi I ms (mag net ic

nanocompos ites) could be prepared by incorporating

maghemite nanoclu ste rs into doped PAN! mixes. They are useful in in fo rmat ion storage, clo lour imag ing, mag­

net ic refrigeration, Ferro fluid s, etc. Allied Sig na l' s Versiconn \ a cond uct ive fo rm of

PAN I, is a d ispersible powder rather than a so luti on and severa l companies incorpo rated it into products as pai nts and coatings. But the major proble ms for wide app li ca­tions rema in cost and reliability. Ching W. Tang and hi s co lleagues at Eastman Kodak arc by far leading the way

in bringing o rgani c-based Light Emitting Di odes to mar­ket .

Work by Scientists at NASA with the Los Alamos National Laborato ry, and at Zipperling Kess ler in Ham ­

burg, Germany, has show n that a coating of PAN I doped with hvrlro 2:en chloride. stops iron rust ing and s teel rust-

than conventio nal methods. Po ly mer based batteries, fo r instance, have a lo nger she lf life than do conventional

o nes, but they have penetrated the market only in a lim­ited way. According to Ray H. Baughman of A lli ed-Sig­nal in Morri stown, N. J ., conducting polymers consti­

tute a radica l nove l market area, which is expected to make great "Fortunes" .

Conclusions

Processability of PANI has been a great concern till

recentl y. A number of strategies have been discussed for

preparing melt/so lution processable PAN!. The use of functionalised dopants to induce so luti o n I me lt

processability in PAN! was indeed a break through and many deve lopments were reported on furthe r enhancing

thei r capabiliti es. Func ti o nali sed dopants possess ing plastici s ing cum proto nating abilities synthesized from

an inexpensive naturally existing material s are reported

to pro mi se. The processable PA !with hi gh conduct iv­

ity values would be of potentia l value scientifica ll y and industrially. So, the use of the pl asti c is ing dopants might

pav e way to so luti o ns to th e problems s uc h as processability, cost and re li abi li ty on commercialisation of the co nducting polymers and its blends.

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