Journa l o f Sc ienti fic & Ind ustr ia l Resea rch
Vo l. 6 1, September 2002. pp 680-689
Melt/Solution Processable Polyaniline and Blends
Raji K Paul and C K S Pillai *
Regional Research La boratory (CS I R). T hiru vananthapuram 695 0 19
Conducting polyanil inc ( l'i\ N I ) has bee n at tractin g great att enti on in rece nt tim es clue to its elec tri cal. elc:ct rochemical ilncl
opti ca l propert ies as wc ll as its environmental stab ili ty. U nfortunatcl y. the li mitations in syn thcti c proced ures and poor proeessabi lity
of l'i\N I ha ve res tr·ictecl it s comme rcialization. It opened up vast avenues in resca rch for deve lop ing m.ve l r.1ct hods and tec hniques o f both scientifi c and indu strial importa nce for makin g conducti ng poly mers fu sib le m so lu ble. Durin g the pas t few yea rs.
many attempts have bee n rn acle to irnprovc proccssabilit y o f PA I such as eloping wi th functi onali sed .Jopant >. blending w ith
co nv..: nti onal po ly rn crs. nov..: ! sy nth..:tic routes f:>r th e preparat ion or PAN I and it s modifi cation w ith va rious ri ng or N-substituellls . Among th e: methods so far clcvclopecl. th e proto nation o f PA !J wit h funct iona li sccl dopa nt s and blending\ •ith therm op lasti cs arc th e strat eg ies bei ng w idely adopted. Func ti onali sed dopant s possess ing plas tici sin g c 11111 protonat ing abi liti es . sy nthesized from natura lly ex i stin g matcria ls. an.: also rcpo rt cd to be promi sing. Thus. the usc o r fun cti onali secl dopant-; possess ing
plasti cisi ng c11111 proton atin g abiliti es co uld be o f value in prepa rin g melt/so lution processab le blends o r thermoplasti cs/elastome ri c polymers w ith PA N ! and mi ght pave way for so lution s to probh: m;. such as proccssa bilit y. o:ost and reli abi lit y on
comme rciali sation o r the co nducting po ly m..: rs and it s blend s.
Introduction
Polymeric material s, i n ge neral , are co nsidered electrically non-conductin g. Interes tin g electro nic and optica l properties arc, hovvcvc r, obse rved in organi c polymers w ith co njuga ted stru ctures th at give ri se to conductivity clu e to uniqu e deloca li zcd TI-e lcc tron sys tems.
Th ese properties are be in g exp lored ror practi cal applications in elec tronic and op toelectron ic dev ices such as
rechargeable batteri es, EM I shi eld ing. ligh t em itting eli odes, biosensors ancl cat hodic prot ec ti ons or meta lli c
struc turcs1· 5 The obv ious attracti on is to comb ine in one
ma terial th e elec tri ca l properti es and th e val ue added
appl icat ions of a semi co nductor or a metal w ith attractive mechani cal properti es and process ing scm icond uctor or a metal wi th attr•tctive mechanic al prope rti es and
processing ad va nt ages o r a po l yme 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 an d impact
resi stance in combina ti on w ith lo w cost, place pol y mers
definitely on an advantageous pos iti on over se mi co nductors and metal s1 ' ·
T he first and stimulatin g success in th e f ield of electr ica ll y conducting poly mers took place around 1977
w hen it was clemonstratccl by Shirakawa el ol. that
polyacety lene . an intrinsica ll y insulat or polymer. could
''' Au th or for
cor re~pondc n cc
become hi gh ly conducti ng on treatment with ox idi zing
or reducing age nt s<·- ~ . Since th e pub lica tion of th e ori gina l observa ti on in 1977, th ere has hecn an ex pl os i ve
grow th o f research in to th e wl~ o l e ran ge o r conjugated
polym er stru ctures'1· 10 • Among the intri nsicall y co nducting poly mers (Fi gure I gi ves struc tu res of repeating units
or several conj uga te poly mers), PA I has been attrac ting significant interest in elec tronic app licat ions because
or th eir w ide ran ge or electrica l, elec trochem ical and
optic al properties as well as th eir good enviro nmental
s tabilit y'l· 1 ~ . PA I can also undergo chemicalmodiricati ons on th e ring and nilrogen atotm to give a va riety or
substitut ed derivati ves that can ex i. ti n several dirferent
ox idati on stat es w hi ch can be eloped either by protonati on with a protoni c acid or by ch· rge tran sfer with an
oxi di sing age nt. Additio nall y, the elec tronic ancl op tical
properti es may be contro lled rc ve rsibi I ity by varying th e
dopin g level. For practical appli e<Jt ions, a co nduct in g
poly mer mu st 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 luti on or the me lt. Unrortunatel y.
the li m itati ons in syn th eti c procedure ~ and poor
pr occssab ilit y o r PAN I have l· e. tri ctecl it s
comme rciali sati on 11 . Thi s has opened up vas t avenues
for research for deve loping no ve l meth ods and techniq ues or bo th sc ientific and industri al i mportance ror
makin g co nd uc tin g po ly mers fusib .e or sol ubl e. Durin g
PA UL & PILLA!: MELT/SO LUTION PROCESSA BL E POLYAN ILIN E
the past few years man y attempts have been made to
improve the process ability of PAN I such as elopin g with
functi onali secl dopants, blending with co nventional pol ymers, novel sy nthetic routes for the preparation of PAN!
and modificati on of the poly mer w ith va ri ous rin g or Nsubstituents . Among the methods so far developed, the
protonation of PAN! wit h functi onali sed dopants and
blending with th erm op las ti cs are th e strategies being
w idel y adopted . It wo uld be worth whi le to revi ew th e
area giving particular emphas is to the methods for preparation of melt/so luti on process abl e protonatecl PAN ! using functi onali sed dopants and oth er bl ending methods .
Solution Processability
_Lnl
-tot
l~~
Polyacctylcnc (tran s)
n
Polythi ophene
Polypyrrole
PPyr
PA
PT
-rot
II
Poly( p-phen y lcnc)
PPA
Polya nilinc
PA 'I
to-1
11
Poly(p-phcn ylc nc vi ny lcne)
PPV
Dop ing of P!\ N I Willi Funclionali::ed Dopa n/s
Figure !- Struct ures
Wh en PAN I is doped to th e emeraldin e salt form th e
polymer undergoes an insu lator-to-metal tran siti on w ith
a concomitant conformational change (from co mpact co i I
to expanded co il ) in th e polymer backbone to accommodate the renewed electronic tran sformati on. Th e result
of th e co nformational change is a reducti on or stru ctural
defects along th e PA N! chain (e g. twist ing, bucklin g),
whi ch increases then-orbital overlap between th e ph eny l n-e lect rons and nitrogen p-e lectrons. It. in tu rn , increa. es both conjugation of the chain backbone and the
po larons demorali za ti on leng th. However, due to th e
co nformati ona l change and grea ter polarit y o r th e
ionomeri c form. th e so lubilit y and processabi lity o r sa lt
form is marked ly reduced. Th e runctional group present
in th e dop ing ac id, its structure and orientation play an
important role in solub ili zing and processing th e conducting form of PAN !. Recen t ideas of dop in g induced
proce..-.mbilil\', where functiona li sccl dopan ts ha v ing polar and nonpolar groups that promote compatibil ity between the pol ymer and the so l ve nt, thus impartin g soluti on proce ssab ilit y by micell ar act ion to PAN I paveclthc
way for a breakthrough 1 ~. A num ber or recent publications describe th e usc or functionali scd proton ic acids(•.such as can1phor sulphonic acid (CSA) 1.116 .
dodccy l bcnzenesu lph o ni c acid ( DBS/\ ) 1 ~ · 15 · 17 . ptoluenes ulphonic acid 17· 1x, octylbenzcncsulp honic ac id 17 ,
po l y(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-', meth anesul phonic acid 21', sui pho ni c
acid and
phosphoric
acid
derivatives
Dentadecv lohenol 27 . 32 as runctionaliscd dopan ts for im-
68 1
or conducting pol ymers
In a noteworthy contributi on in th e development of
functional i sed dopants, Heeger and coworkers ha ve repo rt ed
that
c amphorsulphonic
acid
and
dodcc y lbenzenesulphoni c acid in co mbination with secondary dopants such as 111 -cresolm ake PAN! so lubl e1u 3
in co mmon organi c so lvents. It is possible th at th e hydrophob ic groups of th e fun ctionalised dopants weaken
th e chain-chain interac ti oP of pol ymer and en hance the
poly mer-so l vent interacti on thus faci litating th e solubiliy
o f PAN I .1~. Heeger, M ac Diarmide and co ll eag u es 1 ~ n . .1s
found th at th e co nduct i vity of PAN I co uld be increased
upto 400 S.cm ·1 in so luti on cas t films usi ng CSA as the
dopant and Ill-c reso l as the so l ven t and upto 250 S.cm ·1
with DBS A as the dopants but w hen cast from chlorofo rm so luti on . C SA beha ves as a poor dopant giving
co nductiv iti es o r about 10 1 S.cm·1. Th is counter ion induced proce ssability 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 con ve ntional acids and it ex hibits a reduced microscopic
disorder and met alli c like transpo rt properties 36 . The
unusuall y hi gh cond ucti vi ty and other properti es have
been explai w:d by th e co ncept of secondary dop ing.1 7. ·18
whe re treatn1ent of PAN I-CSA with 111-creso l induces a
change in molecu lar co nforma ti on from that of a co mpact coi I structure to that of an expanded coi 1-1 ike structure. Vario us substituted pheno ls such asp-c resol. 3ethv lohcnol. 2-ch lorophenol, and2-fluorophenol also act
682
J SCI IND RES VOL 61 SEPTEMBER 2002
/~~-~:----~
b
/
_JL
/
/J
400
Figure 2-UV-Visible
600
SOO
1000
Wavelength (nm)
1200
1400
1600
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
m-Cresol
Chloroform
Xylene
THF
PANI-SPOP
0.69
0.15
0.50
0.65
PANI-SPDA
12.00
0.46
0.81
0.80
PANI-SPDPAA
2.20
0.34
13.00
3.50
tion effect of resorcinol on PANI-DBSA and PANI-CSA
systems was studied by Yikki et al.39 while Hopkins et
al.40 showed that hexafluoro-2-propanol
promote an expanded coil conformation
PANI.
There are several reports regarding the process ability
of PANI achieved by protonation with polymeric dopants
such as poly (acrylic acid)", poly (styrenesulphonic
acid)41.42 etc. PANI doped with macromolecular
dopants
is expected to show higher environmental
stability than
that doped with small molecule protonic acids, which
can evaporate at room temperature or higher temperatures causing a depression in the conductivity. In a recent publication, Pron et al." reported highly conducting and solution processable PANI films showing conductivities of 180 S.cm-I using a new plasticizing dopant,
1,2-benzenedicarboxylic
acid, 4-sulpho,
1,2-di (2ethylhexyl)ester.
Protonation with squaric acid" is also
reported to solubilise
PANI. PANI protonated
with
functionalised sulphonic acids":" has also been successfully spun into fibers.
In search for better dopants that enhance the desirable properties discussed above, it has been observed
that molecular systems based on sui phonic acid and phos-
PANI-PDPPA
1.25
0.39
0.S2
0.19
PANI-PDP(bis)PA
0.13
0.02
0.06
O.OS
phoric acid derivatives of3-pentadecylphenol
(POP) and
related structures such as cardanol possess structural
features required for functionalized dopants that will have
multiple functions of protonation,
solubilisation
and
plasticisation27-32. One of the significant features of the
structure of these dopants is that they have a flexible ll-:
alkyl (C1sH31)substituent in the meta-position of the aromatic ring which makes the doped PANI soluble in common solvents or melt processable. These dopants, thus,
render plasticizing ability to PANI so that freestanding
flexible films could be prepared by both the conventional
melt processing techniques and by the solution processing techniques.
Table 1 gives the conductivity
values of films cast
from different organic solvents. Figure 2 shows the UVvisible spectra of some of the protonated PANT in mcresol. But, in contrast to CSA-PANI-/11-cresol systems,
a localized polaron peak exists at about 900 nm. This is
because of the fact that the bulky nature of dopant prevents its easy diffusion into the polymer backbone compared to CSA and therefore the interaction with the solvent is comparatively less. However, bulky nature of the
dopants increases its solubility in all these solvents compared to the other reported systems.
PAUL & PILLAI: MELT/SOLUTION
PROCESSABLE
POLYANILINE
683
Synthetic Approaches: Polymerisation
Table 2 -
Another alternative method to obtain solution
processable PANI is by adopting different synthetic
routes such as template polyrnerisation, enzymatic
polymerisation, emulsion polymerisation and suspension
polymerisation to obtain soluble PANI. The work of
Viswanathan et al.47.51 merit mention for conducting
elaborate studies on the template polymerisation of
aniline by the use of water soluble poly-electrolytic
dopants such as lignosulphonic acids. Use of water as
the solvent makes it environmentally stable and the polymer can easily be switched from one conducting state to
another without changing the amount of the dopant.
Angelopoulos and group" and others 53. 54 prepared water soluble and doped conducting complex of PANI
(PanAqas) where a polyelectrolyte (styrenesulphonic
acid) was used as a template for the oxidative assisted
chemical polymerisation of aniline. The method of dry
spinning was employed by Samuelson et al." to produce PANI fibers from an enzymatically synthesized="
water soluble highly conducting polyaninline-sulphonated polystyrene complex.
Dopant
SPDP
SPDA
SPDPAA
PDPPA
PDP(bis)PA
Plastification threshold for PANI protonated
with different dopants
Dopant/PANI ratio at the at
the plastification threshold
0.30
0.35
0.35
0.30
0.15
electrical conductivity is decreased instead due to an
increase in electron localization or reduction in the ttconjugation. Thus, use of these substituent groups induces distortions in the chain, reducing the zr-ccnjugation and increasing the chain flexibility.
Melt processability of PANl
Although thermal processing of PANI is highly desirable, only a very few reports could be found in literature. Laska et al. 88-91
have recently shown that phosphoric acid diesters can serve as solubility inducing protonating cum plasticizing agents for PANI. It has been
Several authors adopted an in situ doping emulsion
shown+" in this laboratory that the mono- and di-phospolymerisation route using dopants which can function
phoric acid esters as well as sulphonic acid derivatives
as emulsifying cum protonating agents to obtain
from cardanol and pentadecyl phenol act as very good
processable and conducting PANp9-61.Although the proplasticising cum protonating agents for PANI and free
tonated PANI was still intractable and insoluble, it exstanding flexible films could be prepared by the hot presshibited an exceptional degree of crystalline order and
ing techniques. In these systems the plastcization of PANI
had high molecular weight. Improvements in solubility
is taking place at a very low percentage of the plasticizand processability could be achieved by using sulphonic
ing dopant itself and the problems associated with the
acid dopants containing long alkyl side chains27-32.62.
solution processing could be avoided. Plastification
Thus, the in situ doping emulsion polymerization of
thresholds determined for PANI protonated with differaniline in the presence of these dopant provides doped
ent dopants sulphonic acid of 3-pentadecylphenoxy acePANI exhibiting high molecular weight, high conductic acid (SPDPAA), sulphonic acid of3-pentadecylanisol
tivity, an exceptional degree of crystalline order and ori(SPDA), sulphonic acid of 3-pentadecylphenol (SPDP)
entation and thermally stable. A number of similar and
etc. synthesized in this laboratory possess hydrophobic
related techniques are reported to prep pare processable
groups that plasticises PANI so that free-standing flexPANI. Interesting reports have appeared later on emulible films of protonated PANI could be prepared by the
sion polyrnersiation'v",
conventional melt processing techniques are listed in
Table 2. Figure 3 represents the log conductivity against
Synthetic Approaches: Substitution
the temperature of pressing for these protonated polymers. A maximum conductivity value of 65 S.cm-I was
Another commonly accepted route to obtain
obtained for the PANI-SPDPAA film pressed at 140°C.
processable PANI is by substituting the ring with
The PANI-SPDAfilm pressed at 140°C gave a conductivsulphonic acid groups and to make it water soluble'v".
Recently Epstein group76.77reported very highly sulphoity value of 42 Sicm'. But for PANI-SPDP film the conductivity is comparatively less because of the presence
nated PANI, using the most reduced form of PANI
of a hydroxyl group. All these protonated polymers are
leucoemeraldine base, as the starting material. But, the
J SCIIND
RES VOL (ll SEPTEMBER
~()O~
100...,--------------------------,
II)
..s
:L
0,1
1),1 II
-I----,c----,--~-,-----r-~____r---,__--_,__---1
SI)
11111
i
I ~O
ro
I ()O
Pn.'ssin},! tcmpc raturc
ISO
200
lie
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 conducting films by the melt processing method, The conduct ivi ty value is increased upto 1-1-0"C for the xulphonic
acid protouatcd polymers and up to 120 "C for the phosphoric acid proton.ucd polymers bcc.iuxc ofthe increase
in polaron dclocaliz.uion
with temperature due 10 the
thermal activation, While decrease olconductivitv
from
1-1-0 to 2()0 "C is axsociatcd with the decrease of polaron
dcloculizution due to the over compensation oj' the thermal activation effect so that loss or xomc polarons rake
place?". It is suggested that in proton.ucd PAl I systems
the conductivity values were decreasing with increasing
the temperature from 1-1-0 to 200 "C due to cross linking,
therrnal undoping and ring distoniou'". But in the PA\lJSPDPAA systems it was clear from the DSC analysis
that an cxothcrrn is there in the temperature range 1-+0200 "C and so the decrease of' conductivity
is because of
the changes in the chain structure of' ]>,-\NI due to the
cross Iink i ng as suggested by groups or researchers'); '".
The conducii \ it)' or the proronutcd PAN I dccrcuscx ahovc
200 "C as in the CISl' or PAl'\I-DBSA and other sulphonic
acid systcms'" because at higher temperature thermal
degradation
is a dominant factor, It is posxible that
changes in the chain structure due to cross-linking can
also take place at these higher temperatures,
The low temperature conductivity measurcmcnt-: or
the protonutcd PANI films by the melt processing techniques show the development or homogeneous conducting PANI and it indicates that these systems are close io
the transition from a disordered metal to Fermi glass insulator, 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' temperature range ISO-.')O K. The temperature dependence or
resisti\'ely is comparatively
weaker. decreasing hv only
a small factor on cooling the sample from room ternperature to ~S K. This observation or the metallic ternperature dependence of resistivity above 200 K and a
power-law dependence below I()() K. demonstrated that
till' thermally proccxsuhlc films of ]>i\NI-SPD;\
and
P!\i\I-SPDPi\r\
are almoxt precisely on the mctul-insulater boundary as in the ease of P,-\:\ I-CS;\ solution
proccssuhlc films!':" Thus, it is clear from the above
discussions 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 yIller in conventional [ilrn tonning mauiccs hy hlending
h~ls been adopted to pl"Ucess inuinvicallv
conducting
polymers into coatings and films, The doped P:\\I blends
wirh hulk polymcr« are expected to h~I\L' a number or
advantages such as (i ) control of conductivity. (ii) mechanical properties (iii) cost considcr.n ions. (i v) transparency. (v) colorabi Iity and (vi) process: ng proper: ics'".
Hancricc and Mundal'" showed that ann doping and
comple\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 having a surfactant group led to the preparation of polyblcnd
with polymers such as poly (alkylmcthucryl.ucs)'".
poly-
PAUL & PILLA!: MELT/SOLUTION
PROCESSABLE
POLYANILINE
685
W 7 -1---...---,-----r----r---.----r------.--.,...---1
o
10
20
30
40
Weight of PANI, per cent
Figure 4 -
Conductivity vs PANI content in PYC (a) PAN I (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 (ethylene-co-vinyl
acetatej'?' etc. Conductive composites of
PANI with polyurethanes'?'
and thermoplastics
such as
nylon 105-\09
and polyvinyl chloride I10were also fabricated.
Actually the main advantages of doped PANT blends with
bulk polymers are expected to be (i) control of conductivity, (ii) mechanical properties (iii) cost considerations,
(iv) transparency,
(v) colorability
and (vi) processing
properties III.
Solution processed blends are, however, said to have
problems such as compatibility!'?
and formation of aggregates'!'. Use of high boiling and acidic solvents such
as m-cresoII13-115(it is also suspected to be a cancer causing substance) are not industrially favourable 112.113,116,
Therefore, significant research effort has been directed
towards the thermal processing of conducting polymer
blends where there is possibility of formation of continuous network formation of conducting polymers in
the matrix polymer. III,117·124
In order to reduce the percolation threshold, a heat treatment process of the blends
before the processing is adopted. The preparation of
PANT-PYC blends is also describedv'": 126by a dry mixing of the polymers.
It has been127.130that addition of compatibilizers
such
as, for example, selected esters of gallic acid /phosphoric acid esters favours the formation of a continuous,
percolating
PANInetwork
in thermally
processed
polyaniline-polyolefin
blends. Functionalised
dopants
possessing solubilising and compatibilising groups might
do better!" and hence the dopants such as SPDPAA,
SPDPA etc. possessing' plasticizing
cum protonating
abilities could be used to prepare thermoplastic and elastomeric blends of PAN I 132.133.These dopants facilitate
the mixing of the components of the blend where the
dispersion of PANT grains in the matrix polymer was
considerably enhanced by the presence of the plasticizers, which apparently loosened the PANI grain-grain
adhesion forces. This is expected to give a very low percolation
threshold.
Thus,
highly
conducting
thermopladtic blends of PYC with PANl-SPDA, PANISPDPAA and PANI-PDPPA were prepared by mechanical mixing at room temperature for an extended period
of time to achieve optimum homogeneity. Yarious plasticized PANI/PYC ratios were used. The mixture containing PANT and PYC was then hot pressed at 160°C
for 15 minute to obtain thin films 2729.
Figure 4 represents the log conductivity vs. the content of PANI in these blend films. In the case of PANISPDA-PYC polyblend a sample containing 2 wt% fraction of PANI itself is giving a conductivity
value of
I
3.4x l O:' S.cm- with no indication of a sharp percolation
threshold., the percolation threshold is occurring at 5
wt% of PANI content. Laska et at,89 reported that the
percolation threshold is observed for a 25 wt% of the
electroactive component in the case of PANI protonated
with DBSA and their blends with PVe. In the present
case, a lower percolation threshold is obtained because
the plastification of PANI by the dopant strongly facilitates the mixing of the components of the blend. The
6S6
J SCI IND RES
VOL 61 SEPTEMBER
2002
!()r------------55
7S
20
7
0(1, -'50-' liD
[)O----:;(I' .
-250'
11':
TenpiKi
0.25
030
035
0.·+0
T·1M (KI/'1
Figure
5-Plot
of In resistance
norrnalised
iv T'" of PANI-SPDA-PVC
resistance-temperature
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 adhesion forces. Thus, this remarkable reduction in the percolation threshold is achieved by the influence of the
flexible side chains of the dopants, which plasticizes
PANI. An enhancement of compatibility of the polymer
with an increase in the length of the side chains is obviously expected and experimentally
evident.
Figure 5 represents a plot of In of normalised resistance vs Tl/~ for a polyblend film of PANI-SPDA-PVe
system, having 25 wt% of PANI. It shows that In of
norrnaliscd resistance is proportional to Tl/~ in the temperature range 35-300 K indicating a three-dimensional
variable range hopping conduction. It indicates that these
thermoplastic conducting blends exhibit metal like transport properties as suggested by Menon et al. I.1-1 for PAN 1eSA-PMMA
systems and by Wessling!" for thermoplas-
tic blends of PANI.
Another widely accepted technique suggested for the
preparation of processable conducting polymer blends
is the polymerisation
of conducting
polymer into the
matrix polymer. There have been reports on blending of
PANI through electrochemical,
chemical and emulsion
polyrnerisation
of aniline onto polymer substrates!":':".
The inability of the polymerising agent in this method to
diffuse into the matrix polymer might affect the blend
properties. Furthermore,
such blends are not thermally
processable afterwards without affecting the conductiv-
blend with 25 \,1'1 per cent of PAN!. The
plot is shown
in the inset
ity. Melt proccssablc blends of conducting polymers with
rubbery, non-polar host polymers, which are widely used
by the cable industry, have not been developed':". In this
laboratory, the dopant, SPDPAA was successfully employed in an ill situ doping emulsion polyrnerisution
of
aniline on to the elastomeric polymer matrix, poly ethylene-co-vinyl
acetate (EVA)27.''i. The PANI-SPDPAAEVA blend obtained by the emulsion polymcrisation
method was thermally processed to get free standing elasto mcric and highly
conducting
films. The melt
proccssable film having 4.5 I wt(/( of PANI gave a conductivity value of 2x 10' S.cm'. In the case of conducting clastorneric blends, this is the first time that such a
low value of percolation threshold is obtained. As the
content of PANI increase from 4.5 to 2S.5 wt';(, the conductivity increases to (l.S9 S.CIllI and this behaviors is
because of the fact that the plasticising dopants facilitate the uniform dispersion of the homogeneously
protonated PANI into the EVA matrix. The low temperature conductivity measurements
show these elastomeric
blends are obeying the one dimensional variable range
hopping conduction
compared
to the thermoplastic,
where the non-linear temperature dependence of thermoelectric power indicates that these samples are far into
the insulating regime, well away from the M-I transition. The resistivity increases by several orders of magnitude as the temperature is lowered, indicating presence of extensi ve disorder and the formation of inhomogeneous
metallic
islands
due to the emulsion
polyrnerisation method.
PAUL & PILLA!: MELT/SOLUTION PROCESSABLE POLYAN ILIN E
Another method used to prepare the bl e nds of conducting polym ers is the direct d ispers io n o f condu cting
polymer powders in the no ncondu ct ing po lymer matri x.
An example is the Versicon 1 M, (A lli ed Signal) w here
hi ghly polar thermoplastics such as po lycaprol actone and
po ly(viny l ch loride) are used to g ive conductive bl e nd s
having me lt processabi lity. But in the di spersio n ro utes
there is the poss ibility of the fo rm ati o n of no n-equilibrium two phase systems 11 2 w ith th e conductive phase
being the di spersed one so that there is tendency to occur in fl occulated structures. Attempt to introduce ino rganic materials to obtain desired properties are being
reported. The work of Tang e t a /. 141 to prepa re conducti ve a nd s up e rpar a m ag ne ti c fi lm s ( ma g net ic
n a noco mp os it es) by in co rp o r a tin g m ag h e mit e
nanoc lus te rs into doped PAN I mi xes is worth menti oning. Similarly, Kim e t a /. 142 have repo rted preparati o n of
nanoco mpos ites of PAN land Na+-mo ntmorill o nite c lay
which are expected to g ive e nhanced pro perti es.
Commercial products based on PANI
The first commercial products incorporating conducting po lymers were actually made a few years ago. In the
late 1980s, th e Japanese Co mpa nies Bridges tone and
Seiko co mmerc iali zed a rechargeabl e butto n-cel l batte ry
that used PAN I for one e lectrode and lithium fo r the other.
The most impo rtant ap pli catio n of the co ndu c tin g
polymer bl e nds is th e use as antistat ic material s (e g,
anti stati c s pray coat in g based o n PA I sol uti o n in water, named Pan-Aq uas by IBM Thomas J. Watson Research Center).
Conduct ive and su perparamagnetic fi Ims (mag net ic
nanocom pos ites) co uld be prepared by incorporating
maghe mite nanoc lu ste rs into doped PAN! mix es. They
are useful in in fo rm at ion storage, clo lour im ag in g, magnet ic refrigeration, Ferro fluid s, etc.
Allied Si g na l' s Versiconn \ a cond uct ive fo rm of
PAN I, is a d ispersibl e powder rath er than a so luti o n and
severa l companies in co rpo rated it into products as pai nts
and coatin gs. But th e maj o r prob le ms for wide app li cations rema in cost and reliability. Ching W. Tang and hi s
co ll eag ues at Eastm an Kodak arc by far leading the way
in bringing o rga ni c-based Light E mitting Di odes to market .
Work by Scientists at NASA with the Los Alamos
National Labo rato ry, and at Zipperling Kess ler in Ham burg, Germany, has show n th at a coatin g of PAN I doped
with hvrlro 2:en chl oride. stops iron ru st ing and s teel ru st-
687
than conventi o nal meth ods. Po ly me r based batteries, fo r
instance, have a lo nger she lf life th an do conve ntion al
o nes, but they have penetrated the market o nly in a limited way. According to Ray H. Baug hman of A lli ed-Si gnal in Mo rri stow n, N . J ., conducting polymers co nstitute a radica l no ve l market area, which is expected to
make g reat "Fo rtun es" .
Conclusions
Processability of PANI has been a great concern till
rece ntl y. A number of strateg ies have bee n discussed for
pre paring me lt/so luti on processabl e PAN!. The use of
functionalised dopants to indu ce so luti o n I m e lt
processa bility in PAN! was indeed a break thro ugh and
many deve lopme nts were repo rted o n furthe r e nh ancing
th ei r capabiliti es. Func ti o nali sed dopants possess ing
plastici sing cum proto natin g abiliti es sy nthesized from
an in ex pe nsive naturally existing material s are reported
to pro mi se. The processa bl e PA !with hi gh co nduct ivity values would be of pote nti a l value scientifica ll y and
indu strially. So, the use of the pl asti c ising dopants might
pav e way to so luti o n s to th e problems s uc h as
processab ility, cost and re li abi li ty on commerc ialisation
of th e co ndu ctin g polymers a nd its bl e nds.
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