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Polymer Testing 32 (2013) 862–869

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Polymer Testing

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Material properties

Electrical conductivity of poly(vinylidene fluoride)/polyaniline blends under oscillatory and steady shearconditions

Johnny N. Martins a, Michaela Kersch b, Volker Altstädt b,Ricardo V.B. Oliveira a,*

a Post-Graduate Program in Material Science, Institute of Chemistry, Universidade Federal do Rio Grande do Sul (UFRGS),Av. Bento Gonçalves 9500, 91501-970 Porto Alegre, RS, BrazilbDepartment of Polymer Engineering, Faculty of Engineering Science, University of Bayreuth, 95447 Bayreuth, Germany

a r t i c l e i n f o

Article history:Received 20 January 2013Accepted 4 April 2013

Keywords:Poly(vinylidene fluoride)PolyanilineElectrical conductivityRheological measurementsBlends

* Corresponding author. Universidade Federal do RBento Gonçalves 9500, 91501-970 Porto Alegre, RS,3308 7199; fax: þ55 (51) 3308 6304.

E-mail addresses: ricardo.oliveira@iq.ufrgs.br, rhotmail.com, ricardo.vb.oliveira@ufrgs.br (R.V.B. Oli

0142-9418/$ – see front matter � 2013 Elsevier Ltdhttp://dx.doi.org/10.1016/j.polymertesting.2013.04.0

a b s t r a c t

Blends of poly(vinylidene fluoride) (PVDF) and polyaniline (PAni) were prepared throughmelt blending in a batch mixer. The morphology, rheological behavior and electricalconductivity were investigated through transmission electron microscopy (TEM) andcombined electro-rheological measurements. Through TEM analysis, it was possible toobserve that all blends showed typical phase separation with the presence of conductivepolymer aggregates. Deformations imposed during a strain sweep caused, not onlydisturbance of the linear viscoelastic behavior, but also changes in electrical conductivity.The oscillatory shear altered the morphology, breaking the PAni domains into smaller ones.This effect increases the distance between them and, consequently, resulted in a decreaseof the electrical conductivity. The measurements under quiescent conditions and steadyshear proved that the disturbance in morphology for PVDF/PAni system is non-recoverable. Through combined electrical and rheological measurements, it was possibleto achieve good correlation between the electrical and flow behavior of PVDF/PAni blends.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Blends of intrinsically conductive polymers (ICP) andthermoplastics constitute a new class of semi-conductiveand conductive materials. These systems combine goodmechanical properties with electrical conductivity andexcellent processability [1–3]. New potential applicationsare being found for thesematerials such as electromagneticshielding [4], electromechanical sensors [5], chemicalsensors [6,7], biosensors [8] and conductive adhesives [9].

io Grande do Sul, Av.Brazil. Tel.: þ55 (51)

icardo.oliveira.ufrgs@veira).

. All rights reserved.01

Among the available ICPs, polyaniline (PAni) is found tobe the most promising [10]. The thermal stability of PAni issuperior to other ICPs, and its processability and conduc-tivity are also equally good. Further, aniline monomer isless expensive than the other ICPs monomers, making thisan advantage for larger scale production. The synthesis ofPAni is very simple, properties can be tuned easily and itpresents numerous applications possibilities. All thesefactors contribute to the superiority of PAni compared tothe other ICPs [11].

On the other hand, polyaniline is usually immisciblewhen blended with thermoplastics, and gross phase sepa-ration may restrict the formulation of compatible materials[12]. However, the compatibility of conductive polymerblends can be enhanced through inserting counter ionsonto the ICP backbone [13]. Generally, functionalized

J.N. Martins et al. / Polymer Testing 32 (2013) 862–869 863

protonic acids are used as doping agents in polyanilinechains, making it possible to form compatible blends withan insulating polymer. Among them, dodecylbenzene-sulfonic acid (DBSA), which has long alkyl chains, isfrequently used. It increases the solubility of PAni.DBSA insolvents and acts as a surfactant, inducing compatibilitywith insulating polymer matrix [14].

In this area, blends of poly(vinylidene fluoride) andpolyaniline have been investigated in literature recently.PVDF is an important engineering polymer and has beenextensively studied in recent years. PVDF has excellentmechanical properties, resistance to chemicals such assolvents and acids, high dielectric permittivity, and uniquepyroelectric/piezoelectric properties, as well as its poly-morphism [15,16]. Due to these remarkable properties, it isused to prepare conducting polymeric materials [17,18].

Malmonge and co-workers [19] were one of the first tostudy this system. They prepared blends using the solutioncasting method and studied the electrical conductivity,dielectric and electrochemical response. They achieved alow percolation threshold and concluded that a continuousconductive pathway was formed through the PVDF matrix.Bliznyuk et al. [20] modified the structure and volume ofPAni and PVDF through differentmethods. They studied theeffects of these modifications on the microstructure andelectrical properties of the blends prepared through solu-tion casting. The authors found good correlation betweenmicrostructural changes and the electrical characteristics ofthe blends.

Recently, Malmonge et al. [21] have prepared blends ofPVDF and PAni using direct aniline polymerization in amixture of PVDF and N,N dimethylformamide. They studiedthe thermal and mechanical behavior of the blends as afunction of PAni doping level and composition. The blendspresented good thermal stability and increase in tensilestrength and Young’a modulus when compared to the neatPVDF. Alternatively, Ray et al. [22] prepared a ternary blendof PVDF/PMMA/PAni through melt blending and studiedthe microstructure, morphology, mechanical and electricalproperties. The authors found good correlation betweenmicrostructure, morphological changes and the macro-scopic properties. They concluded that PVDF and PMMAcontribute to the mechanical upgrade, whilst PAni con-tributes to the improvement in the conductivity of theblends.

In most studies about this subject blending was done insolution. This method offers higher conductivity withlower PAni content in the PVDF matrix. This is because inthis method PAni is not subjected to heat or shear forceswhich cause loss of dopant and decreasing of electricalconductivity. However, compared to solution blending,melt processing is more industrially suitable because of itslow cost and high productivity [23]. It is does not requirethe use of solvents and a solvent removal step, which raisethe ecological and health hazard concerns.

Due to these issues, melt processing has been chosen toblend PVDF and PAni in this study. The final morphology ofblends is usually formed during melt processing. It isimportant to understand the behavior of the phases underdefined deformation in the molten state. Combined rheo-logical and electrical measurements are of high interest in

investigations regarding thermoplastics/ICP blends. Theconductivity of these blends reacts very perceptivelyto changes of the structure induced by mechanicaldeformation.

In this work, we investigated poly(vinilidene fluoride)(PVDF)/polyaniline (PAni) blends through morphologicalanalysis and electro-rheological measurements. The elec-trical and viscoelastic properties were simultaneouslymeasured and studied as a function of PAni content anddeformation conditions. The influence of PAni content andthe deformation conditions on the conductivity duringoscillatory and steady-shear experiments was investigated.To the best of our knowledge, such investigation has notpreviously been conducted on PVDF/PAni blends in theliterature. Thus, the aim of this study was to correlate therheological response in oscillatory and shear flow to theblend structure and, consequently, to the conductivity.

2. Experimental

2.1. Materials

Aniline monomer (Ani) (analytical grade, Nuclear Brasil)was distilled under vacuum and stored under nitrogen in arefrigerator before polymerization reactions. Ammoniumpersulfate (APS) (analytical grade, Merck) and dode-cylbenzenesulfonic acid (DBSA) (technical grade, Carlo ErbaReagentes do Brasil) were used as received. PVDF (Solef6008 specific mass 1.78 g cm�3 melt flow index8 g 10 min�1) used in this work was kindly supplied byAPTA Resinas Company in pellet form.

2.2. Synthesis of PAni.DBSA in aqueous media

The PAni.DBSA was prepared by one step polymeriza-tion in aqueous media according to the Gospodinova pro-cedure [24]. Briefly, aniline monomer (100 mmol) wasmixed in a solution of DBSA (100 mmol) in 250 mL ofdeionized water. Then, 25 mL of an aqueous solution of100 mmol ammonium persulfate was added to the stirredsolution of Ani/DBSA. The polymerization was done at 0–5 �C for 6 h. After the reaction, the dispersionwas poured inacetone and a dark green powder was obtained. The pow-der was filtered, then washed with acetone and deionizedwater and dried under vacuum for 24h at 50 �C.

2.3. Blends preparation

The blends were prepared through melt blending in abatch mixer (torque rheometer equipped with a mixingchamber). The experiments were done at 190 �C, 100 rpmfor 8 min. This time period makes it possible to approachthe steady-state condition for all mixtures under theexperimental conditions. The PAni contents were 0, 10, 20,25 and 30 wt% in PVDF.

2.4. Morphological analysis – TEM

Transmission electron microscopy (TEM) analysis wascarried out in a ZEISS EM-922 OMEGA microscope oper-ating at an acceleration voltage of 80 kV. The samples were

J.N. Martins et al. / Polymer Testing 32 (2013) 862–869864

ultra-thin sections prepared in an ultramicrotome LeicaUC7 at �150 �C using a diamond knife.

2.5. Simultaneous electrical and rheological measurements

The combined electrical and rheological measurementswere done in an Advanced Rheometric Expansion System(ARES)-RDA III rheometer. A plate–plate geometry withelectrically isolated tools with a plate diameter of 30 mmwas used. The gap between the plates was set to 2 mm. AKeithley electrometer model 6517A was used to measurethe high resistivities. Low resistivities weremeasured usinga four-point test fixture combined with a Keithley digitalmultimeter 2100. The same set-up and tools were also usedby Zeiler at al. [25]. The electrical conductivity s wascalculated by the following equation:

s ¼ ½ð4$hÞ�=½p$Rðd2a � d2

i Þ� (1)

where h is the thickness of the sample, R is the measuredresistance and da and di are the outer and internal diameterof the ring shaped electrodes, respectively. Before startingany experiment, an equilibration time of 5 min in themeasuring chamber at the chosen temperaturewas allowed.

Firstly, the nonlinear behavior was investigated by per-forming strain sweep tests. The measurements were made

Fig. 1. TEM micrographs of PVDF/PAni a) 90/10 b) 80/20 c) 7

from from0.1 to 100% strain (g0) at an angular frequency (u)of 0.1 rad s�1 and at a constant temperature of 190 �C. Thelinear viscoelastic behavior was determined by frequencysweep tests between 0.1 rad s�1and500 rad s�1 at a tem-perature of 190 �C. In order to investigate the temporalstability of the blends, time sweep tests were performed atan angular frequency of 300 rad s�1 at a temperature of190 �C. Thus, all frequency sweep and time sweep testswereperformed within the linear viscoelastic regime.

Rotational shear experiments were also performed inorder to study the destruction and a possible recovery ofthe conductive pathway. The conductivity was firstmeasured under quiescent melt conditions for 500 s withno shear applied. After that, a small steady shear defor-mation (0.05 s�1) was applied for 600 s. Then, the con-ductivity was again measured under quiescent meltconditions. All these three stages were done at a constanttemperature of 190 �C. This kind of experiment can befound in literature regarding thermoplastic/carbon nano-tubes composites [26].

3. Results and discussion

Fig. 1 shows the morphologies analyzed by TEM forall PVDF/PAni blends. The morphology of all blends

5/25 d) 70/30 (PAni domains are pointed by arrows).

Fig. 3. Electrical conductivity vs time measured together with the strainsweep.

J.N. Martins et al. / Polymer Testing 32 (2013) 862–869 865

showed typical phase separation with the presence ofconductive polymer aggregates, as indicated by arrows. Thismorphology suggests immiscibility of the blend compo-nents. It can be clearly observed that in the blendwith 10wt% (Fig. 1a) the PAni presented good phase dispersion. ThePAni phase is dispersed through the PVDFmatrix in the formof small conductive islands. The blend with 20 wt% pre-sented, as expected, larger conductive domains. The PAniphase remained well distributed through PVDFmatrix withno sign of agglomeration. In the blend with 25 wt% PAniwe observed the beginning of formation of a coarsemorphology. This coarse morphology is even more pro-nounced in the blend with 30 wt% PAni content. We couldobserve larger PAni interconnected domains through thePVDF matrix in this composition.

Fig. 2 shows the curves for the storage (G0) and lossmodulus (G0 0) as a function of strain for the neat PVDF andfor PVDF/PAni blends. As PAni content increases we couldobserve an improvement of both moduli in the measuredstrain range. An exception to this behavior was observed forthe blend with 10 wt% PAni content. The values for bothmoduli were smaller than the ones for pure PVDF.

The neat PVDF was found to be within the linearviscoelastic regime up to strain rates near 100%. As the PAnicontent increased, the blends presented less stablebehavior regarding the linear viscoelastic region. The blendPVDF/PAni (70/30) was the least stable one. This blendpresented a deviation of linearity at low strain rates. Thisbehavior is due to the fact that the nonlinear viscoelasticregime is closely related to a change in morphology andmicrostructure of the material [27–29]. The blends withhigher PAni contents presented a coarser morphology, dueto the immiscibility of the PVDF/PAni system. This fact canexplain the deviations of linear viscoelastic behavior atlower strain rates than neat PVDF.

The results of electrical conductivity as a function oftime measured together with the strain sweeps are shownin Fig. 3. Overall, the electrical conductivity increased as afunction of PAni loading, as expected. Also, all blendsshowed a decrease in the electrical conductivity as afunction of time. This was due to the fact that increasing thestrain rate caused a change in the morphology of theblends. Under these conditions, the distance between PAni

Fig. 2. a) Storage modulus vs strain for the PVDF/PAni blend

domains was enlarged with deformation, which reflects inthe electrical conductivity. The blend PVDF/PAni (90/10)showed higher stability in terms of electrical conductivitywhen compared to the other blends. This fact corroborateswith the strain sweep measurements which showed thisblend with a more stable response. The blend PVDF/PAni(90/10) has a phase dispersed morphology and the PAnidomains are far from each other. Hence, the effect of theoscillatory shear on the electrical conductivity is lesspronounced.

The other blends presented a sharp decrease in theelectrical conductivity in the whole measuring range. Theblend with 30 wt% PAni showed the sharpest declinealthough it had the highest conductivity values. This fact isrelated to the disruption of the coarse morphology pre-sented in this system. The electrical conductivitydecreasing over time can also be related to the dopingagent. The DBSA should be degrading due to the time ofexposition to high temperature. Polyaniline can also beundergoing some sort of degradation due to exposure tohigh temperature.

s. b) Loss modulus vs strain for the PVDF/PAni blends.

Fig. 5. Complex viscosity vs frequency for PVDF/PAni blends.

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Fig. 4 shows the storage (G0) and loss modulus (G0 0) as afunction of frequency for neat PVDF and all blends. Theblend PVDF/PAni (90/10) had similar values for bothmodulias the neat PVDF. The other blends showed higher valuesfor storage (G0) and loss modulus (G0 0) when compared tothe neat PVDF. The polyaniline phase was probably brokeninto smaller droplets due to the oscillatory shear. Thiscaused an increase in the PVDF/PAni interface and, conse-quently, improved the elasticity in the molten state. Thisbehavior was also evidenced in literature for manyimmiscible blends [27]. The enhancement of interfacesbetween PVDF-PAni, may provide, besides the increase inthe elastic response (solid like behavior), also a rise of theenergy dissipation component (G0 0), which explains theincrease in both moduli.

The complex melt viscosity as a function of frequencyfor the PVDF/PAni blends is shown in Fig. 5. The viscositywas proportional to the PAni loading. The effect of PAni onthe viscosity is more pronounced at low frequencies andthis behavior weakened as frequency increases due to theshear thinning effect. Also, the viscosity of pure PVDF andthe blend with 10 wt% PAni presented a Newtonian plateauwhich characterizes liquid-like behavior. Nevertheless, inthe blends with 20, 25 and 30 wt% PAni this plateau dis-appeared and the curves started to present a steep slope atlow frequency. This is characteristic of a pseudo solid-likebehavior of this system. Also, the viscosities were ordersof magnitude higher, mainly at low frequencies. This resultcorroborated with the rise in the elasticity verified throughthe storage moduli. This viscosity effect can also beexplained by the increase of PVDF/PAni interface as well asby the presence of more PAni in the blend.

In order to clarify these outcomes, electrical conduc-tivity was measured as a function of time (Fig. 6) along thefrequency sweep experiments. As expected, the overallincrease in electrical conductivity was proportional to thePAni loading in the blend. However, we observed a gradualdecrease of electrical conductivity as a function of time andfrequency for all blends. Therewas a sharp drop after 10 s ofmeasurement which corresponds to a frequency of300 rad s�1. This trend may be related to a time and tem-perature effect or even to a frequency effect on blendmorphology regarding some degradation of PAni chains orloss of dopant.

Fig. 4. a) Storage modulus vs frequency for the PVDF/PAni blend

The drop in electrical conductivity as a function of timemay be associated with two distinct factors. There was aloss of doping agent molecules throughout the experimentdue to the temperature effect. It could have affected theelectrical response negatively since the presence of adoping agent makes the electrical conductivity higher inconductive polymers [30]. Another factor to take intoconsideration is the influence of the oscillatory shear. Itcaused a change in morphology, breaking the PAni domainsinto smaller ones. This effect enlarges the distance betweenthen and, consequently, causes a decrease in the electricalconductivity. Also, at low frequencies, the deformation ofthe dispersed phase domains is more pronounced inpolymer blends [31].

In order to investigate the temporal stability of theblends under oscillatory shear, time sweep tests were done.The results are shown in Fig. 7 and helped to elucidate theeffects of time and frequency on the rheological and elec-trical response. As can be seen, the storage modulus (G0)values increased as a function of PAni content in the blend.This behavior was also demonstrated by the other rheo-logical measurements made in this work. The G0 valueswere constant within the time interval measured and

s. b) Loss modulus vs frequency for the PVDF/PAni blends.

Fig. 6. Electrical conductivity vs time measured together with the frequencysweep.

Fig. 8. Temporal stability under oscillatory shear, electrical conductivity vstime.

J.N. Martins et al. / Polymer Testing 32 (2013) 862–869 867

remained in the same order of magnitude for all blends. Wealso observed the same trend for G0 0.

The electrical conductivity as a function of timemeasured at a constant frequency of 300 rad s�1 at 190 �C ispresented in Fig. 8. There was a decrease of the electricalconductivity during the entire time sweep test. This factcleared the assumption that time and temperature areinfluencing the drop of electrical conductivity along therheological measurements. In the blends PVDF/PAni (75/25) and (70/30) we could observe a sharp drop in theelectrical conductivity after about 20 s of exposure toconstant oscillatory shear. This fact implies that in thesecompositions the frequency is also affecting the electricalresponse. This was due to the fact that the oscillatory sheardisrupts the morphology, particularly in these composi-tions where it is not so energetically stable.

In order to achieve a more general understanding of theinfluence of shear on the electrical conductivity of PVDF/PAni blends, we also performed steady shear experiments.We applied a constant “shear pulse” (shear rategsteady ¼ 0.05 s�1) of 600 s duration to all the blends. Wealso investigated the electrical response under quiescentmelt conditions before and after this shear pulse.

Fig. 7. a) Temporal stability under oscillatory shear, storage modulus vs tim

Fig. 9 shows the time-dependent conductivity for allPVDF/PAni blends during quiescent melt and steady-shear conditions. The curves presented three differentregions. In the first region, up to approximately 500 s,the electrical conductivity was measured under quiescentconditions. In the second region, it was measuredunder steady-shear conditions. In the third region, theelectrical conductivity was again measured under quies-cent conditions. Once more, the overall increase of elec-trical conductivity was proportional to the PAni contentin the blend. We observed a drop in the electrical con-ductivity as a function of time for all blends in all thethree regions.

The dropwas not so sharp in the first region of quiescentmelt, which may have been caused by the temperatureeffect on the doping agent molecules, as mentioned before.We observed a more pronounced decrease in the electricalconductivity in region II (steady shear) when compared toregion I. This was due to the effect of shear on themorphology, as evidenced in the other rheological experi-ments. The temperature effect on the doping agent alsocontributed to the decrease of electrical conductivity dur-ing steady-shear conditions. In region III, as in the others,the temperature effect also contributed to the drop in the

e. b) Temporal stability under oscillatory shear, loss modulus vs time.

Fig. 9. Electrical conductivity vs time under quiescent and steady-shearconditions.

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electrical conductivity. We did not observe the recovery ofconductivity which some authors claim [26,32] exists forcertain conductive systems, such as thermoplastic/carbonnanotube composites. This proved the assumption thatin thermoplastic/ICP blends the effect of shear on themorphology is non-recoverable. Also, this steady-shearmeasurement showed that shear induces a change inmorphology in PVDF/Pani blends, which reflects in theelectrical response.

4. Conclusions

PVDF/PAni blends were prepared by melt blendingusing a mixing chamber at a temperature and time scalesuitable for industrial processing. TEM analysis showedthat the morphology of all blends presented typical phaseseparation with the presence of conductive polymer ag-gregates. The blends with 10 and 20wt% PANI showed goodphase dispersion morphology while the ones with 25and 30 wt% presented coarse morphology. Changes inmorphology induced by deformation during strain sweepcaused, not only disruption of the linear viscoelasticbehavior, but also changes in electrical conductivity. Thepresence of PAni changed the rheological behavior fromliquid-like to pseudo solid-like. The oscillatory shearcaused a change in morphology, breaking the PAni domainsinto smaller ones. This effect enlarges the distance betweenthem and, consequently, resulted in a decreasing in theelectrical conductivity. The electrical conductivity overtime measured at a constant frequency decreased duringthe entire time sweep test. The assumption that time andtemperature are influencing the drop of electrical conduc-tivity was proved with these measurements. The experi-ments under quiescent conditions and steady-sheardemonstrated that the change in morphology is non-recoverable for all PVDF/PAni blends. Good correlationbetween the electrical and flow behavior can be achievedfor PVDF/PAni blends through the combined electro-rheological measurements.

Acknowledgements

The authors are thankful to CNPq for financial supportin Brazil and to CAPES for financing the scholarship inGermany (Process n� 5119/11-1). We are also thankful toAPTA Resinas Company Brazil for supplying the PVDF andto Bayer GmbH for lending the electrically isolated tool. Wealso would like to acknowledge the experimental supportof Mrs. Ute Kuhn from the Department of Polymer Engi-neering of University of Bayreuth.

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