Improving the processability of conductive polymers: The case of polyaniline

Improving the Processability ofConductive Polymers: The Caseof Polyaniline

J. RUIZGaiker Technology Centre, Parque Tecnologico de Bizkaia, Edificio 202, Zamudio 48170, Spain

Departamento de Quımica Fısica, Facultad de Ciencia y Tecnologıa, Universidad del Paıs Vasco(UPV/EHU), Apartado 644, Bilbao E-48080, Spain

B. GONZALO, J. R. DIOSGaiker Technology Centre, Parque Tecnologico de Bizkaia, Edificio 202, Zamudio 48170, Spain

J. M. LAZA, J. L. VILAS, L. M. LEONDepartamento de Quımica Fısica, Facultad de Ciencia y Tecnologıa, Universidad del Paıs Vasco(UPV/EHU), Apartado 644, Bilbao E-48080, Spain

Received: July 21, 2011Accepted: December 17, 2011

ABSTRACT: Polyaniline (PANI) is one of the most widely used conductivepolymers because of its ease of synthesis in addition to its good electricalproperties. However, the difficulty in its processability limits its potentialapplications. In this work, conductive emeraldine salt (i.e., one of the differentoxidation states of PANI) was synthesized by oxidative chemical polymerization.Different techniques such as Fourier transform infrared spectroscopy, scanningelectron microscopy, thermogravimetric analysis, differential scanningcalorimetry, and dynamical mechanical thermal analysis were used tocharacterize the synthesized PANI. The processability of PANI has been improved

Correspondence to: L. M. Leon; e-mail: [email protected].

Advances in Polymer Technology, Vol. 00, No. 0, 1–9 (2012)C© 2012 Wiley Periodicals, Inc.

IMPROVING THE PROCESSABILITY OF CONDUCTIVE POLYMERS

by processing it, both by compression and extrusion methods, with differentthermoplastic matrices such as polycaprolactone and polybutylene terephthalate.The obtained compounds have not only better processability but also improvedthermal and mechanical properties. However, their conductivity decreases withrespect to PANI, to a greater extent, for the compounds synthesized by theextrusion method. C© 2012 Wiley Periodicals, Inc. Adv Polym Techn 00: 1–9,2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.21261

KEY WORDS: Compounding, Conducting polymers, Extrusion, Polyaniline,Processing

Introduction

C onductive polymers consisting of conjugatedπ -bonds are being intensively investigated

for their potential uses in organic optoelectronicdevices such as light-emitting diodes,1−5 pho-tovoltaic cells,6−11 electrochromic circuits,12−17 orsensors.18−22 At low scale, these conductive poly-mers have also been incorporated as fibers into yarnsand tissues to improve their antistatic properties.23

Although the conductive polymer term was al-ready used to describe black products of ani-line obtained from the oxidation of aniline,24 thestudy of conductive organic polymers began whenMacDiarmid et al.25 discovered in 1976 that the con-ductivity of polyacetylene increased by six orders ofmagnitude when it reacts with iodine (from 10−4 to102 S cm−1). Finally, the Nobel Prize in Chemistryfor 2000 was awarded jointly to them “for the dis-covery and development of conductive polymers.”For this reason, polyacetylene became the most stud-ied conductive polymer both from the scientific andpractical point of view. However, its low stability inthe presence of oxygen has restricted its applicationto the scientific field.

Other polymers that possess an intrinsic con-ductivity are polyfluorene, poly(p-phenylene), poly-thiophene, polypyrrole, and polyaniline (PANI),among others. There are numerous studies that re-late physical properties to chemical properties.26,27

Some of them reported that any minimal disturbancein the molecular structure of these polymers dramat-ically changes their conductivity, electrochemical be-havior, and spectroscopic properties.28 However, de-spite of their good electro–conductive properties,these polymers have a limited number of applica-tions because of their difficulty in processing. Thislack of processability is related to the rigidity of thepolymer chain, which is a direct consequence of the

high level of conjugation of the polymer, that in turnaffects the molecular conductivity.

Additional problems related to conductive poly-mers are their low thermal stability (the synthesizedelectroconductive polymer exists in an energy statethat is susceptible to attack by oxygen) and low solu-bility in common solvents. Several research groups29

have attempted to solve these drawbacks throughthe introduction of chemical modifications to thepolymers.

Among these conductive polymers, a specialinterest has been shown to PANI, owing to itslow production cost and relatively high electricalconductivity.30 Also, PANI can be synthesized chem-ically or electrochemically31 and can be obtained indifferent oxidation states, as is shown in Fig. 1. Forexample, PANI can be completely reduced to forma structure-type leucoemeraldine, completely oxi-dized to form a structure-type pernigraniline,32,33 orit can be present in an intermediate oxidation statecalled emeraldine salt. Only the last intermediate ox-idation state shows conductivity (approximately 10S cm−1), whereas the other two behave as insulatingmaterials. Of course, each different PANI oxidationstate has its own chemical and physical properties.34

Although the study of PANI began in the early20th century,35 emeraldine salt36 was not discovereduntil 1962 and its electrical conductivity was notdescribed until 1969.37,38 Later on, different studieswere performed to improve PANI’s conductivity byadding fillers such as carbon nanotubes39 at the ex-pense of further decreasing PANI’s processability.

As mentioned earlier, conductive polymers showa series of drawbacks, such as low thermal stabil-ity, processability, and low solubility, as well as poormechanical properties. In this study, a PANI, syn-thesized by oxidative chemical polymerization, wasmixed with two different thermoplastic matrices,such as polycaprolactone (PCL) and polybutyleneterephthalate (PBT), to improve its processability as

2 Advances in Polymer Technology DOI 10.1002/adv


FIGURE 1. Different oxidation states of PANI.

well as thermal and mechanical properties. To thatend, two of the most commonly used processingmethods for polymers, such as compression and ex-trusion, were employed. An improved processabil-ity was obtained at the expense of decreasing theconductivity of the synthesized PANI. Therefore, itis necessary to establish a trade-off between conduc-tivity and processability.

Experimental

MATERIALS

Aniline hydrochloride (99%) and ammonium per-oxodisulfate (APS; 98%), used for PANI synthesis,were supplied by Aldrich (Madrid, Spain) and Fluka(Madrid, Spain), respectively. The common solventacetone (Panreac, Barcelona, Spain) and reactiveas hydrochloric acid (Aldrich; 37%) were used asreceived.

FIGURE 2. Chemical structure of the thermoplasticmatrices employed.

PCL (Union Carbide, Barcelona, Spain) and PBT(BASF, Barcelona, Spain) (Fig. 2) are the thermoplas-tic matrices. They were processed along with PANI,and they were dried in an oven at 50◦C for 48 hbefore use.

PANI SYNTHESIS

Conductive emeraldine salt (Fig. 1) was synthe-sized by oxidative chemical polymerization (Fig. 3)from aniline hydrochloride and APS (1:1.25 molarratio). Aniline hydrochloride (15 g) and APS (33 g)were dissolved separately in the same amount ofMillipore water (290 mL). Then, both solutions weremixed under continuous stirring for 3 min at lowtemperature (0–5◦C) and, finally, the mixture was

FIGURE 3. PANI synthesis.

Advances in Polymer Technology DOI 10.1002/adv 3


TABLE IThermal Properties of PANI, the Thermoplastic Matrices, and the Synthesized Compounds

Glass Transition Melting Temperature, Processing Initial Temperature of Viscositya

Sample Temperature, Tg (◦C) Tm (◦C) Temperature (◦C) Degradation, Ti (◦C) (Pa s)

PANI 107 – – 190b –PCL −60 75 75 353 6PBT 70 235 235 370 10PANI + PCL (20/80) – 122 80 365 12PANI + PCL (40/60) – 126 80 361 85PANI + PBT (20/80) – 244 250 340 12PANI + PBT (40/60) – 220 250 343 12

aMeasured in the twin-screw mini-extruder the during the extrusion processing.bLoss of chloride groups.

allowed to stand for 24 h. During the synthesis, pHremained constant (pH 1). After that, the precipi-tate was filtered and the filtrate was washed threetimes with a solution of hydrochloric acid (0.2 M)and acetone to remove residual and unreacted prod-ucts. Thus, the conductive PANI was obtained as ablack powder (with 70% yield of the overall syn-thetic process) in the emeraldine state after dryingunder vacuum at 50◦C.

COMPOUNDS PROCESSING

As indicated above, PANI is difficult to pro-cess mainly because of its high viscosity; to im-prove its processability, some thermoplastic matri-ces (PCL and PBT) with low viscosity were added(Table I). Other reasons for choosing these thermo-plastic matrices to mix with PANI are, in the case ofPCL, its low melting temperature, which facilitatesprocessing and, in the case of PBT, its high melt-ing temperature, which is near the temperature atwhich the PANI starts to lose its conductivity. More-over, both thermoplastic matrices have polar charac-ter, which increases the interaction with PANI, withincreasing polar character it improves their mix-ing and processing. These thermoplastic matriceshave been processed in different amounts with PANI(20 and 40 wt% of PANI) to form the correspond-ing compound. All compounds were processed in atwin-screw mini-extruder (Haake Minilab Thermo-Electron Corporation, Marietta, OH) equipped withtwo counterrotating screws (100 rpm for 10 min) anda closed loop for recirculation to study the variationof the viscosity with time at a given temperature(slightly above the corresponding melting temper-ature). The thermoplastic matrices were processedwith the synthesized conductive polymer PANI byextrusion to obtain threads. These samples were also

processed by compression after applying a pres-sure of 40 tonnes. Thus, compounds in the form ofpressed pellet were obtained (13 mm in diameter).

CHARACTERIZATION

PANI was characterized by different analyti-cal techniques such as Fourier transform infraredspectroscopy (FTIR; Perkin-Elmer spectrophotome-ter Spectrum 100; Perkin-Elmer, Shelton, CT) andscanning electron microscopy (SEM; Zeiss EVO 50;Zeiss, Jena, Germany) to verify that the synthesishad taken place successfully.

The thermal stability of all the samples was eval-uated by thermogravimetric analysis (TGA) witha Mettler Toledo TGA 50 thermobalance (MettlerToledo, Greifensee, Switzerland). The experimentswere conducted from 25 to 800◦C, at a heating rateof 10◦C min−1, under nitrogen atmosphere.

To characterize the thermal properties of the sam-ples, differential scanning calorimetry (DSC) testswere performed with a Mettler Toledo DSC 30calorimeter under a constant nitrogen flow (50 cm3

min−1). First, samples were heated twice from –100to 150◦C at a heating rate of 10◦C min−1, followedby two new heating scans from −100 to 350◦C at thesame heating rate. The amount of sample used was6.7 mg.

The thermomechanical properties of the pro-cessed compounds were characterized by dynami-cal mechanical thermal analysis (DMTA) in the shearmode (MKII from Polymer Laboratories, Loughbor-ough, UK). The shear tests were accomplished withthe circular disks obtained during the compressionprocessing (13 mm × 2 mm) at deformation frequen-cies of 1, 3, and 10 Hz and a strain of 64 μm. Runswere carried out from 30 to 250◦C at a heating rateof 2◦C min−1.



FIGURE 4. FTIR spectrum of synthesized PANI.

Conductivity measurements were carried out atroom temperature by the four probe technique (Vander Pauw method) according to ASTM F76-86 normor the twoprobe technique according to UNE 21-303-83 norm. The Van der Pauw method was used forsamples with a resistance less than 1000 �, whereasthe two probe technique was used for samples witha resistance more than 1000 �.

Results and Discussion

Figure 4 shows the FTIR spectrum obtained forthe synthesized PANI. In this spectrum, the peakaround 1590 cm−1 corresponds to the stretching be-

FIGURE 5. Scanning electron micrograph of the PANIpowder.

FIGURE 6. TGA curves of PANI, thermoplastic matricesand synthesized compounds processed by compression.

tween the C atom of the benzene ring and the N atomof the quinoid ring of the PANI molecule, whereasthe peak around 3200–3300 cm−1 is owing to stretch-ing of N H. Figure 5 shows the scanning electronmicrograph of the PANI powder, showing a granu-lar morphology. Both experimental results confirmthe synthesis of the PANI.40

Viscosity measurements for each sample havealso been carried out during the extrusion process-ing in the twin-screw mini-extruder. All the synthe-sized compounds have a low viscosity, making theirprocessing easier, as can be observed from Table I.

THERMAL STABILITY

Figure 6 shows the thermogravimetric curvesboth for raw samples (PANI, PCL, and PBT) andfor the synthesized compounds processed by com-pression. From Table I, the initial temperature ofdegradation (Ti ) can be observed, which is deter-mined as the intersection between the tangent to thebaseline and the inflexion point in the thermogravi-metric curve for PANI, PCL, PBT and the differentcompounds synthesized.

As shown in Fig. 6, the thermal degradation ofPANI takes place in four stages. First, between 50and 100◦C, a small mass loss (∼10%) occurs as a re-sult of vaporization of water and solvents employedin the PANI synthesis (hydrochloric acid and ace-tone). Then, there is a second degradation step (200–290◦C) attributed to the loss of chloride groups inthe conductive emeraldine salt (A− = Cl− in Fig. 1).This fact has already been reported previously by



different researchers.41−45 Despite this, to verify thisfact, the conductivity of the PANI both at room tem-perature and at high temperatures (250◦C) was mea-sured. The conductivity decreases significantly withan increase in the temperature because of the lossof chloride groups, which are responsible for thePANI’s conductivity. Finally, the decomposition ofthe PANI takes place at approximately 570◦C, and itresults from the breakdown of the amino group (thethird step between 470 and 570◦C) and the degra-dation of the aromatic ring (the fourth step between650 and 800◦C).

Figure 6 also shows the thermal stability of thetwo thermoplastic matrices employed in this study(PCL and PBT). As can be seen, they show a greaterstability than PANI (numerical data of Ti are pre-sented in Table I). This higher thermal stability ofthe thermoplastic matrices is reflected in an increasein thermal stability of the prepared compoundswith respect to PANI, as can also be observed fromTable I and Fig. 6. Therefore, it can be concluded thatmixing PANI with a thermoplastic matrix not onlyreduces its viscosity, thereby increasing its process-ability, but also results in a material with a higherthermal stability than the initial polymer. In addi-tion to this, the thermal stability is high enough thatit is possible to use these materials for a number ofapplications.

THERMAL PROPERTIES

Figure 7 shows the thermal properties of the syn-thesized PANI. In the first scan, an endothermic peak

FIGURE 7. DSC curves of synthesized PANI.

(∼100◦C) related to the evaporation of the water andother solvents employed in the PANI synthesis canbe observed. No peaks were observed in the secondscan, whereas in the third one there is a new en-dothermic peak (∼250◦C) as a result of the elimina-tion of the chloride groups of the PANI molecule.46

Finally, the glass transition temperature (Tg = 107◦C)can be observed from the fourth heating scan shownin Fig. 7.

Table I presents the glass transition temperature(Tg) and the melting temperature (Tm) of the sam-ples measured by DSC. This table also presents theprocessing temperature of each sample. It can be ob-served that the processing temperature is equal tothe melting temperature of the thermoplastic ma-trices, slightly less than the PCL compounds andslightly more than the PBT compounds. Anyway, asignificant improvement in the PANI’s processabil-ity was achieved.

DYNAMIC-MECHANICAL PROPERTIES

Dynamic-mechanical properties of the com-pounds processed by compression were studied byDMTA. As can be observed from Fig. 8, PCL hasnot been measured because of its low melting tem-perature (75◦C). However, PANI does not undergoany remarkable processing until temperatures rise toabove 250◦C where, as observed from DSC and TGA,the loss of chloride groups begins from the PANIstructure. The synthesized PCL compounds beginthe fusion process at approximately 70◦C, which cor-responds to the PCL’s melting temperature while

FIGURE 8. tan δ versus temperature curves obtainedfrom DMTA for PCL compounds processed bycompression.



FIGURE 9. tan δ versus temperature curves obtainedfrom DMTA for PBT compounds processed bycompression.

still retaining to a certain extent the thermal and me-chanical stability up to 140◦C. Therefore, it appearsthat the mixture of PANI and PCL improves not onlythe processability of the compounds obtained butalso their dynamic-mechanical properties.

Figure 9 shows the dynamic-mechanical proper-ties of the PBT compounds. For PBT thermoplasticmatrix, tan δ shows two peaks at 70 and 230◦C, whichcorrespond to the glass transition temperature andthe melting temperature, respectively. Likewise, thesynthesized PBT compounds also present two peaksof tan δ, which correspond to the same PBT charac-teristic temperatures, but they also have a small peakat about 250◦C because of the loss of the chloridegroups from the PANI molecule. In this case, as a re-sult of the good thermal and dynamic-mechanicalproperties of the two starting components (PANIand PBT), no improvement has been observed inthese properties with the formation of compound.However, a better processability has been noticed inthe same way as that for the PCL compounds.

CONDUCTIVITY MEASUREMENTS

The conductivity of synthesized PANI was mea-sured using the Van Der Pauw method (the four-probe technique according to ASTM F76-86 norm)and a value of 8.2 S cm−1 was obtained. As the stor-age condition is an important factor that needs tobe considered in the loss of conductivity of the con-ductive polymers, the effect of the storage conditionon the synthesized PANI has been studied. PANI

was stored both in an inert atmosphere of nitrogenand oxidative atmosphere of air for 3 months, afterwhich conductivity of the samples was again mea-sured. It was observed that the conductivity of theair-stored PANI decreased to about half of its initialvalue, whereas the conductivity of vacuum-storedPANI remained constant.

When the conductive polymer was mixed withthe thermoplastic matrices, the conductivity was sig-nificantly reduced; however, its processing capacitywas significantly improved. Therefore, it is neces-sary to establish a trade-off between conductivityand processability. Accordingly, the thermoplasticmatrices have been processed with PANI to form acorresponding compound in different amounts (20and 40 wt% of PANI) because above 40 wt% of PANIthe processing of the samples (by both compressionand extrusion methods) is very difficult.

The conductivities of the samples processed bythe compression method are presented in Table II. Itcan be observed that the conductivities of the PCLcompounds are greater than the PBT ones. This is be-cause the processing of the PCL compounds was car-ried out at temperatures close to 75◦C; below 200◦Cis the temperature (as can be seen by TGA in Fig. 6) atwhich the loss of the chloride groups begins, which isresponsible for PANI’s conductivity. However, PBTcompounds were processed at temperatures close to250◦C (above 200◦C) and, therefore, a partial loss ofchloride groups was observed. Moreover, when theamount of conductive polymer is increased the con-ductivity also increased; therefore, by doubling thepercentage of conductive polymer in the compoundthe conductivity increases by an order of magnitude,as can also be observed from Table II.

The conductivities of the samples processed bythe extrusion method are presented in Table III. Theconductivities of the PCL compounds are again more

TABLE IIConductivity of PANI, Thermoplastic Matrices, andSynthesized Compounds Processed by Compression

Formulation Conductivity (S cm−1)

PANI 8.2PCL 4.9 × 10−13

PBT 3.6 × 10−13

PANI + PBT (20/80) 5.3 × 10−7

PANI + PBT (40/60) 5.1 × 10−6

PANI + PCL (20/80) 3.2 × 10−5

PANI + PCL (40/60) 8.6 × 10−4



TABLE IIIConductivity and Properties of Synthesized Com-pounds Processed by Extrusion

Processing ConductivityFormulation Temperature (◦C) (S cm−1)

PANI + PBT (20/80) 235 4.7 × 10−13

PANI + PBT (40/60) 235 5.4 × 10−12

PANI + PCL (20/80) 75 8.9 × 10−7

PANI + PCL (40/60) 75 1.5 × 10−6

than the PBT compounds for the same reasons ex-plained above. Moreover, when the two types ofprocessing are compared, it is observed that theextrusion compounds have a lower conductivitythan the compression ones. This decrease in theconductivity of the compounds processed by extru-sion is attributed to processing conditions. It mustbe taken into account that in the extrusion processthe samples are subjected to strong shears, whichcan damage the structure of PANI.

On the other hand, PBT compounds are morehomogeneous than PCL compounds; however, thethreads made from them are more rigid because ofthe higher intrinsic rigidity of PBT with respect toPCL.

Conclusions

Oxidative chemical polymerization has been usedas the synthetic route to obtain successfully conduc-tive PANI with relatively high conductivities, usinganiline hydrochloride and APS as precursors.

It has been observed that because of the lack ofprocessability of PANI it is necessary to add somethermoplastic matrices with low viscosity to facili-tate the processing of the compounds both by com-pression and extrusion. The selected thermoplasticmatrices PCL and PBT were used because of theirlow viscosity, processing temperatures, and polarcharacter.

When PANI was mixed with the thermoplasticmatrices, the conductivity was significantly reduced;thus, it is necessary to establish a trade-off betweenthe conductivity and processability. It has been de-termined that not more than 40 wt% of PANI can beadded to the compound.

The compounds processed by compression ex-hibit higher conductivity than extrusion because

in the extrusion method the samples are subjectedto strong shears, which can damage the chan-nels through the circulation of electrons, result-ing in the conductivity of PANI molecule (chloridegroups).

Finally, despite of the more homogeneity and thehigher rigidity of the PBT compounds, the conduc-tivities of the PCL compounds are more than the PBTcompounds because of their lower processing tem-perature, which does not affect the PANI’s structure.

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Improving the processability of conductive polymers: The case of polyaniline