Macromol. Rapid Commun. 2002, 23, 659–663 659

Highly Conducting Polyaniline Gels

Ignacio Gonz�lez,1 M. Eugenia Mu�oz,1 Antonio Santamar�a,* 1 J. Adolfo Pomposo,2 Hans Grande,2

Javier Rodr�guez-Parra2

1 Polymer Science and Technology Department and Polymer Institute POLYMAT, Faculty of Chemistry, UPV/EHU,P.O. Box 1072, 20080 San Sebasti�n, SpainE-mail: popsaiba@sq.ehu.es

2 CIDETEC, Paseo Mikeletegui 61, 18, 20009 San Sebasti�n, Spain

Keywords: conducting polymers; polyaniline gels; rheology; viscoelastic properties;

IntroductionResearch carried out during the last decade has led to thedevelopment of highly electrically conducting polymers(above 102 S/cm), such as poly(p-phenylene), polyacetyl-ene, polyaniline and polypyrrole, with promising applica-tions in electronic equipment, antistatic formulations, cor-rosion protection and others. One of the main problems isthat intrinsically conducting polymers are semirigid,resulting in intractability unless they are plasticizedusing, for instance, alkyl side chains. Several routeshavebeen proposed to overcome this difficulty, including fiberwet spinning and film casting from solutions, as well asblending the conducting polymer with thermoplasticpolymers. In particular, polyaniline/solvent systems yieldhighly conducting solid films when the solvent isremoved,[1–4] as well as gel networks,[5–11] although in thelatter materials the reported conductivities are only in therange 10–6–100 S/cm. We would like to demonstrate herethat, using suitable conditions for the doping of polyani-line and for the preparation of polyaniline solutions and

subsequent gelation, viscoelastic gels of conductivitiesabove 102 S/cm can be obtained.

Experimental PartPolyaniline emeraldine base (PANI), supplied by PanipolLtd., and the dopant, camphor sulfonic acid (CSA), wereboth dried at 608C in a vacuum oven for 24 h. The solvent,m-cresol, was dried with 0.4 nm molecular sieves. PANI wasdry mixed with CSA at a PANI/CSA ratio of 1:0.5 mol/mol(with respect to the PhN repeat unit of PANI), following theprocedure described by Reghu et al.[12] Solutions were pre-pared by adding PANI/CSA complex (doped polyaniline) tom-cresol. Contrarily to studies in which magnetic and ultra-sonic stirrers were used to prepare the doped polyanilinesolutions[6–11, 13] (Table 1), mechanical stirring is proposed asan improved method to obtain solutions that lead to conduct-ing gels. Mechanical stirring was carried out for 90 min at80 8C and 2000 r.p.m in a special blade-vessel systemdesigned to impart high shear rates to the sample, as well asto avoid solvent evaporation and humidity absorption. The

Communication: An improved method, based onmechanical stirring and a controlled solvent evaporationprocess, is proposed to obtain PANI(CSA)0.5/m-cresolsolutions, which lead to gels with conductivities in therange 3 to 150 S/cm, preserving the elasticity associatedto the gel network. Such high conductivity values haveonly been found for brittle films cast from PANI solu-tions. According to our experimental results, there is nojustification for an analogy between the elastic modulusand the conductivity of physical networks.

Macromol. Rapid Commun. 2002, 23, No. 12 i WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2002 1022-1336/2002/1208–0659$17.50+.50/0

Double logarithmic plot of the conductivity measured atroom temperature versus PANI(CSA)0.5 concentration. Theline corresponds to the scaling law R V c1.6.

660 I. Gonz�lez, M. E. Mu�oz, A. Santamar�a, J. A. Pomposo, H. Grande, J. Rodr�guez-Parra

concentrations prepared with this procedure range between 5wt.-% and 8.5 wt.-% PANI/CSA. Highly viscous, greenishsolutions were obtained with no visible insoluble particles.The solutions were poured into molds and submitted to acontrolled solvent evaporation process in a climatic chamber.This gave gels containing 5.6 wt.-% to 60 wt.-%PANI(CSA)0.5. A period of 12 d is necessary to obtain themost concentrated gel. Polymer concentration was deter-mined by weight difference. Shrinkage of the gels wasobserved during the solvent evaporation process, which con-stitutes a clear example of so-called “syneresis”, a gel agingphenomenon.[14] The microporosity observed on the gel sur-face is associated with the physical basis of the phenomenon,that is the expulsion of liquid from the pores.The gel state was evidenced by measuring the dynamic

viscoelastic properties of the systems in a controlled stressplate-plate Carri-Med CSL 100 in linear viscoelastic regime.The thickness of the samples was measured under constantpressure to obtain reproducible results. The conductivity wasmeasured using the four point probe method, with a home-constructed device. Special care was taken to avoid exces-sive penetration of the points in the soft gel: a good surfacecontact between the gel (a smooth sample of cylindrical geo-metry) and the points is crucial to obtain consistent results.

Both viscoelasticity and conductivity measurements wereperformed soon after the polymer concentration was meas-ured, thus subsequent changes in gel composition duringexperiments are not expected. This was confirmed during thelongest viscoelastic experiments which lasted 40 min.

Results and DiscussionFrequency sweeps at a constant temperature in lineardynamic viscoelastic regime clearly differentiate polymersolutions from gels. For solutions, the general linear vis-coelastic model[15] predicts that both storage modulus G 9

and loss modulus G 99 scale as G 9 V x2 and G 99 V x,respectively, as frequency x approaches zero. On theother hand, from the definition of a gel, it is deduced thatgel formation is realized when both G 9 and G 99 tend to aconstant value as frequency tends to zero, withG 9 > G 99.[16,17] In Figure 1, the typical responses of a poly-mer solution and a gel are displayed for twoPANI(CSA)0.5/m-cresol systems, at 2 wt.-% and 11.8wt.-% polymer concentrations, respectively. Above apolymer concentration of 5.5 wt.-% all the systems show

Table 1.

Gel preparation PANI concentrationrange

wt.-%

Conductivity range atroom temperature

S/cm

Observation References

Gels are prepared by dissolvingPANI in DBSA using formic acidas a processing medium. At theend formic acid is removed andPANI/DBSA gels are obtained.

(7%–22%) PANI inDBSA, equivalent to(20%–61%) PANI-(DBSA)0.5 in DBSA

3610–3 to 3 The gel is thermoreversible.Magnetic stirring is used toprepare the solutions.

[6, 7, 8]

Camphor sulfonic acid doped PANI,PANI(CSA)0.5, is prepared usingformic acid, which is evaporated atthe end. PANI(CSA)0.5 is dissolvedin m-cresol.

10% PANI(CSA)0.5in m-cresol

10–5 to 10–2 Conductivity increases withaging time. Maximum conduc-tivity is reached after 300 h.Magnetic stirring is used toprepare the solutions.

[8]

PANI is dry mixed with CSA andsolutions are prepared addingPANI/CSA to m-cresol.

(4%–8%) PANI(CSA)0.5in m-cresol

L1 No formic acid is used. Solu-tions are prepared in an ultraso-nic bath. Gel preparing proce-dure is uncertain since poorrepeatability of the viscoelasticand conductivity results isobserved.

[9, 13]

Camphor sulfonic acid doped PANI,PANI(CSA)0.5, is prepared using for-mic acid, which is evaporated at theend. PANI(CSA)0.5 is dissolved in m-cresol (see observations).

(2%–8%) PANI(CSA)0.5in m-cresol

4610–6 to 2610–1 Magnetic stirring followed byultrasonic stirring. Conductiv-ity increases with aging time.The higher conductivity corre-sponds to t = 104 h.

[11]

PANI is dry mixed with CSA andsolutions are prepared adding PANI/CSA to m-cresol. A controlled m-cre-sol evaporation is carried out to obtainhigh polymer concentration gels (seeobservations).

(5%–60%)PANI(CSA)0.5in m-cresol

3 to 150 No formic acid is used.Mechanical stirring under spe-cial conditions is employed.

this work

Highly Conducting Polyaniline Gels 661

the gel behavior of the figure, with both G 9 and G 99 beingconstant. The time necessary to reach the lowest fre-quency of the experiment, 10–2 Hz, is 40 min, which indi-cates that in plate-plate oscillatory flow, the gels arestable with respect to possible aging effects, such asmoisture absorption, m-cresol evaporation or chemicalcrosslinking. From frequency sweeps as those of Figure 1an elastic modulus of the gel, Ge, independent of fre-quency, can be determined for each PANI(CSA)0.5 con-centration.The results of a temperature sweep at a constant fre-

quency of 1 Hz of one of the gels, is displayed in Figure2. Although the actual gel-sol transition cannot be deter-mined,[18,19] for most of the samples, a symptom of gelmelting is detected, since at a certain temperature (708Cin Figure 2) G 99 > G 9, and a liquid-like system is obtained.However, at higher temperatures both moduli start toincrease with temperature and loss tangent decreases.This is probably due to uncontrolled m-cresol evaporationat high temperature, but the possibility of the creation ofa liquid-crystalline solution[20] as the gel network meltsshould also be considered. With increasing the tempera-

ture the isotropization of the liquid-crystalline solutionwould lead to an increase in G 9, G 99, and complex viscos-ity. In any case, the temperature window for liquid-likebehavior is very narrow, and the thermoreversibility ofthe gels cannot be proven.The effect of PANI(CSA)0.5 concentration on elastic

modulus at 108C and on conductivity at room tempera-ture is presented in Figure 3 and 4, respectively. It isworth noting the extraordinarily high conductivitiesfound in our gels. In Table 1 we compare our conductiv-ity results with those reported in the literature for PANI-based gels, we have not included the results of Jana andNandi[10] (conductivity range 10–3 to 2 S/cm2) becausetheir systems are peculiar, defined as “gels without aliquid”. The differences are very significant since con-ductivities above 100 S/cm2 are measured for our highestconcentrations. Such high values have only been obtainedin PANI/CSA brittle films cast from m-cresol.[4,13]

According to Ikkala et al.,[4] during solvent evaporationfor film formation, m-cresol molecules twist the PANIrings into a more planar conformation, thus increasing the

Figure 1. Storage modulus G 9 (open symbols) and loss modu-lus G 99 (solid symbols) as a function of frequency for aPANI(CSA)0.5/m-cresol solution (2 wt.-%) and a PANI(CSA)0.5/m-cresol gel (11.85 wt.-%), at T = 10 8C.

Figure 2. The effect of temperature on dynamic viscoelasticfunctions (taken at 1 Hz) for a PANI(CSA)0.5/m-cresol (5.6wt.-%/94.4 wt.-%) gel.

Figure 3. Double logarithmic plot of the storage modulus mea-sured at 10 8C versus PANI(CSA)0.5 concentration. The line cor-responds to the scaling law G V c2.

Figure 4. Double logarithmic plot of the conductivity meas-ured at room temperature versus PANI(CSA)0.5 concentration.The line corresponds to the scaling law R V c1.6.

662 I. Gonz�lez, M. E. Mu�oz, A. Santamar�a, J. A. Pomposo, H. Grande, J. Rodr�guez-Parra

conductivity. As m-cresol is further evaporated, planarcrystalline domains are formed, giving rise to a polymer/liquid phase separation. This mechanism may be relatedto the aforementioned syneresis observed in our samplessince it is assumed[14] that in polymer gels, this phenom-enon is due to a process of phase separation, in which thepolymer chains are drawn together. Therefore, our resultsseem to demonstrate that the model proposed by Ikkalamay also be valid to explain the formation of highly con-ducting viscoelastic gels. In other words under ourexperimental conditions, crystalline domains (planar dueto m-cresol) would be capable of forming a material withthe conductivity of a solid film, but with the viscoelasticresponse of a gel, like that shown in Figure 1. Thishypothesis corresponds to the commonly assumed rela-tion between the conductivity and the degree of crystalli-nity present in the material. In contrast, studies on PANIfilms protonated with 2-acrylamido-2-methyl-1-propane-sulfonic acid (AMPSA) indicate that conductivity doesnot require a large degree of crystallinity.[21] Within thisframework, our gels can represent a valuable system toinvestigate the correlation between conductivity and crys-tallinity, although this is beyond the scope of this article.In our case, the physical network may inhibit the forma-tion of high order and relatively high conductivity is stillobtained.The scaling laws of percolation systems, G V cf and R V

ct for the effect of polymer concentration c on elastic mod-ulus and electrical conductivity, respectively, are used tofit our experimental results of Figure 3 and 4. The best fitsare obtained for f = 2 and t = 1.6. According to percolationmodels, the scaling exponent f varies from 1.7 to 4,[14] butin most of these models only enthalpic effects are consid-ered,[22] which reduces the applicability of these theories topolymer gels, where entropic effects should be considered.This was done by Daoud and Coniglio[23] who predicte anexponent f = 2.67. On the other hand, Jones and Mar-qu�s[24] relate elasticity to the fractal dimension of theobjects connecting at the junctions of the network. In parti-cular, for a rod structure (like that expected for PANIchains), f varies between 1.5 for pure entropic elasticeffects and 2 for enthalpic elasticity. Considering that ourexperimentally observed f = 2 value is close to the valuespredicted by different models, it is difficult to extractsound conclusions from the application of the theory toour gels. Moreover, experimental values close to f = 2 areobserved for many other polymer gels as well.[16] There-fore, the comparison of our experimental results with theo-retical approaches and other experimental data only leadsus to assert that the elasticity/concentration correlationfound for the present conducting PANI gels satisfies thestandards of polymer gels. This suggests that the increasein elastic modulus denoted in Figure 3, is probably duesolely to the effect of polymer concentration. Within theframework of PANI gels, this is not a trivial result, since it

supports the absence of spurious effects during gel pre-paration and viscoelastic measurements, such as moistureabsorption, crosslinking or phase separation. It is not likelythat such a simpleGV c2 correlation would be found underthe effect of any of these factors.The currently accepted values for conductivity expo-

nent t in percolation theory are 1.1 to 1.3 for two dimen-sions, and 1.65 to 2 in three dimensions,[14,22] which areclose to the value found experimentally (t = 1.6). To ourknowledge only two papers report R-c correlations forpolymer gels. In the work of Fizazi et al.[25] a scaling lawwith an exponent of 2.6 is shown for a polyethylene gelin which a conducting polymer, poly(3-octylthiophene),is incorporated. On the other hand, from the data ofIkkala et al. (Figure 5 in ref.[8]), a very strong dependence,R V c6, beyond the range predicted by any theory, isdeduced for gels of PANI(DBSA)0.5 dissolved in DBSA.Notwithstanding the pioneering work of P.G. de Gen-

nes,[26] in which the exact analogy between conductivityand elasticity is claimed, i.e. R and G should scale in thesame way, more recent studies[27, 28] show different scalingbehaviors for both properties. This difference could alsobe noticed in our work and a similar conclusion can bedrawn from a recent article by Tiitu et al.[11] According torecent theoretical advances,[22] there is no justification foran analogy between the elastic modulus and conductivityof percolation networks.

ConclusionsPolymer gels based on PANI(CSA)0.5/m-cresol solutionsshow conductivities above 100 S/cm preserving the elas-ticity associated to the gel network. A route to a new classof materials has been found, since until now such highconductivity values were only obtained in brittle filmscast from PANI solutions. As could be expected consider-ing the recent advances in percolation models, R versus cand G versus c results show that there is no analogybetween elastic modulus and conductivity in these gelnetworks.

Acknowledgement: Financial support through University ofthe Basque Country UPV/EHU (UPV203.215-G05/99) andSpanish Ministry of Science and Technology MCYT (MAT2000-0047-P4-03) is acknowledged. We wish to thank PanipolLtd. for PANI supply.

Received: April 15, 2002Revised: June 11, 2002

Accepted: June 17, 2002

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