polyaniline actuators

Synthetic Metals 151 (2005) 25–42

Polyaniline actuatorsPart 1. PANI(AMPS) in HCl

Elisabeth Smelaa,∗, Wen Lub, Benjamin R. Mattesb

a Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USAb Santa Fe Science and Technology, Inc., Santa Fe, NM 87505, USA

Received 27 December 2004; accepted 14 February 2005Available online 17 May 2005

Abstract

Drawn polyaniline films and fibers doped with 2-acrylamido-2-methyl-propane-1-sulfonic acid, PANI(AMPS), were electrochemicallycycled in HCl and their material properties and actuation performance comprehensively characterized. The Young’s modulus was obtained asa function of applied voltage. Actuator figures of merit were derived from isotonic and isometric measurements, including strain, stress, work,p re studied,a actuator fort ine.©

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ower, creep, and efficiency. The effects of sample length, solution pH, electrochemical driving method, frequency, and load wes well as the response of current to applied load for sensing applications. This work presents a complete picture of a polyaniline

he first time. The behavior of the actuator is discussed in terms of the changes in the oxidation and protonation states of polyanil2005 Elsevier B.V. All rights reserved.

eywords:Polyaniline; Fiber; Actuator; Strain; Modulus; Metrics

. Introduction

Polyaniline (PANI) has attracted considerable interest asn electroactive polymer actuator (EAP)[1–35]. Actuatorsave been fabricated from both chemically and electro-hemically synthesized polyaniline, as well as substitutedolyanilines. Various material and actuator properties haveeen studied, primarily in HCl, although other acids and

onic liquids have also been used. Because of the disparateaterials, preparation methods, and cycling conditions, a

ull picture of the performance metrics and behavior of oneaterial–electrolyte system has been unavailable. In thisaper, we present a thorough characterization of the mechan-

cal and actuation properties of highly conducting stretchedolyaniline films and fibers doped with 2-acrylamido-2-ethyl-propane-1-sulfonic acid, PANI(AMPS). To allow

omparison with most previously published studies, theamples were cycled in 1 M HCl. This work should allowotential actuator users to evaluate the merits of this material,

∗ Corresponding author. Tel.: +1 301 405 5265; fax: +1 301 314 9477.

as well as to understand some of the limitations of polyanactuators.

The currently accepted model for the molecular strucand oxidation levels of polyaniline is represented inFig. 1,showing explicitly for the first time all the pathways that grise to actuation. Electronic and ionic charges and proare transferred when the pH and oxidation levels are cha[36–39]. Solvent transfer also occurs, but this is not yetficiently understood to be included in the model.

Polyaniline is electroactive in acids below pH 3–4which it can undergo electrochemical reduction and oxida(redox) between three states: leucoemeraldine (an insuemeraldine salt (a conductor), and pernigraniline (an ilator). The cyclic voltammogram therefore has two pairpeaks corresponding to the two transitions (seeFig. 4a).

In aqueous electrolytes, the extent of protonation depon the solution pH (increasing left to right inFig. 1). Forleucoemeraldine, the pKa is between 0 and 1[36]: pro-tonation begins at approximately pH 2 and is complat pH −1 [40,41]. The pH at which the polymer protnates/deprotonates depends on the acid, however[42]. Innaphthalene sulfonic acid, for example, leucoemeraldi
E-mail address:[email protected] (E. Smela).
379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2005.03.009

26 E. Smela et al. / Synthetic Metals 151 (2005) 25–42

Fig. 1. Electrochemical oxidation states of polyaniline. In the fully protonated states (left), the polymer comprises onlyx units, but in most acids the chains aremixtures ofx andy units, the ratio depending on pH. During electrochemical oxidation in aqueous electrolytes, the gain or loss of anions and protons, and thusthe strain, depends on solution pH. (Anion and proton counts are taken from thex units to the emeraldine salt.)

fully protonated even at pH 2[40]. The pKa of the emeraldinestate is approximately 3[36], while that of pernigraniline isbelow 0. Fully protonated chains consist only of segments la-beledx in Fig. 1, but in most acids the chains are mixtures ofx andy units, the ratio depending on pH. Only the fully pro-tonated form of emeraldine salt is shown in the left columnof Fig. 1; this is the lone state of the six that is electricallyconducting[43]. The two structures shown for this salt arechemically equivalent resonance structures called polarons.

Fig. 1 illustrates why the volume of polyaniline changeswith solution pH: because ions are transported into and outof the polymer to maintain charge neutrality. Polyanilinein the emeraldine salt state contracts upon immersionin bases (going left to right in the center row of thefigure) because of the egress of anions[1]. Althoughpolyaniline undergoes this volume change as the pH ofthe solution is varied (i.e. chemical actuation), volumechange is usually instead controlled electrochemically[2,3,5–8,10–13,15,18,22–24,26,29,31,33]. The addition orremoval of electronic charge from the polymer backbone(going up and down in the figure, respectively) is alsoaccompanied by ion transport, and because of differences inthe pKa’s of the various states, also by proton transport.

Since the pH used during actuation has usually been be-tween 0 and 2, the redox reactions usually follow paths 1 and2 (LB → ES→ PB). During oxidation from leucoemeraldinet an-i es

[36,46]. However, in going from emeraldine to pernigranilinealong path 2 the polymer typically contracts[6,11,13,47,48],and the mass decreases[36,46]. The transition from leu-coemeraldine to emeraldine has most often been used foractuation because pernigraniline is unstable in many com-monly used aqueous acids[41,49–52].

In organic electrolytes, no proton transfer can take placebetween the polymer and the solvent during redox[53], so thereaction follows paths 1 and 3, with anions inserted duringbothreactions[41]; the polymer expands throughout the en-tire range, which is advantageous for practical applications.

Water transport, although it is not included inFig. 1, playsa major role in conjugated polymer actuation[41,54,55], butno consensus has yet been reached on a model. The ionsare typically solvated, particularly in aqueous electrolytes[36,56–65]. Due to changes in polymer-solvent interactionsand osmotic pressure, there can also be a simultaneous in-dependent flow of solvent in or out of the polymer[48,54].Counter-directional outflow of water when ions enter can besubstantial: up to 10 water molecules per anion has been mea-sured[48]. Water transport also depends on acid type and pH[41].

Although the volume change in conjugated polymers isthought to be due primarily to this mass transport of ionsand/or solvent[54,56,60–62,66–91], other factors may alsoplay a role. For example, in the oxidized state the conjugatedp edo

o emeraldine along path 1, polyaniline expands due toon insertion[2,5–7,11,14,17,44,45], and the mass increas

olymer backbone is straighter and stiffer due to increas�rbital interactions, also not included in the model ofFig. 1.

E. Smela et al. / Synthetic Metals 151 (2005) 25–42 27

Depending on the polymer and the electrolyte, therefore,various actuation mechanisms may be observed in a givenexperiment.

2. Experimental

2.1. Materials and sample preparation

Two types of samples were investigated: films and fibers,both drawn (stretched) before use. Film formation and fiberspinning were performed as previously described[23,31,34].The electrical conductivity of the films was approximately400 S/cm and that of the fibers was 1000 S/cm.

Film samples were cut from a sheet with a thicknessof 15�m; areas were typically either 0.1 cm× 3 cm or0.3 cm× 3 cm. Approximately 1 cm of the strip was im-mersed into the electrolyte during electrochemical cycling.We assumed that only the immersed portion was electro-chemo-mechanically active. The lower 1 cm was kept in theelectrolyte for 10–15 min to ensure that the sample was com-pletely protonated before starting the experiments.

Fiber samples were cut from a length of spun material witha diameter of 105�m. Fibers were completely immersed inthe electrolyte (seeFig. 2), and these were also kept in theHCl for several minutes before the start of cycling.

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model 300B, which has an upper force limit of 50 g (0.5 N)and a step response time of 1.3 ms. This system is able to mea-sure both the force on, and the displacement of, a lever armusing an electromagnet. The position of the arm can be heldfixed for isometric measurements, with the current requiredto do this being a measure of the force applied downward tothe tip of the arm by the actuator. Alternatively, a constantcurrent can be applied to the electromagnet to provide a givenforce for isotonic measurements, with the system measuringrotation of the lever arm. The arm position can be ramped up-ward at a given rate to exert a tensile force on the sample, andthe sample response measured to obtain force-displacementdata that are converted to stress–strain curves, which yieldthe Young’s modulus.

National Instruments acquisition (NI AT-MIO-16E10) andconnector (NI BNC-2090) boards were used to communicatewith the Aurora 300B, and a custom-written LabView pro-gram controlled the instrument. (This software is availablefrom Santa Fe Science and Technology upon request.) Thesystem was based on the one developed by F. Giuntini with D.De Rossi[15,81,92]. Voltage and current data from the out-put port of the EcoChemie pgstat30 were collected simulta-neously with the mechanical data, allowing precise time cor-relation of all the information. The data were viewed in realtime during the experiments and saved as tab-delimited text.

The samples were mounted in a custom electrochemicalc acidsa andp hata ver-t allyc mallPt armwM us oft that ad usest k, in-d linkw eri-a Thes hat itw

ied tot sured( gtho ectiono thef ents( ngei etrics ngesw avep ed ofr

.2. Electrochemistry

An EcoChemie pgstat30 potentiostat/galvanostatsed to control the electrochemical experiments. The

rolyte was 1 M HCl (pH 0) unless otherwise specified.orking electrode was the polyaniline film or fiber, the rence electrode was Ag/AgCl, and the counter electrodePt foil. Samples were contacted on one end, at the bof the electrochemical cell, with a gold electrode.

.3. Mechanical measurements

Mechanical measurements were done with an Aurorantific force/strain transducer, the dual mode lever sy

ig. 2. Experimental set-up for (a) isotonic and (b) isometric measuremhe bottom clamp made electrical contact. The top of the film was atta

o the measuring arm via a nylon thread.

ell designed to allow measurements in both aqueousnd organic electrolytes. The cell was made of Teflonoly(vinyl chloride) (PVC), and it had a glass window tllowed sample visualization. The film or fiber was held

ically in the cell, clamped at the bottom and electricontacted there. The top of the sample was held in a sVC clamp attached to the lever arm via a 120�m diame-

er nylon thread that was glued to the tip of the leverith polystyrene dissolved in methylene chloride (CH2Cl2).odulus measurements were corrected for the modul

he nylon thread. The reason for this arrangement wasead weight cannot be placed on the lever arm (this ca

he system, tuned for fast response, to go into feedbacucing a strong vibration). The clamp and the compliantere made from light-weight, inert, non-corrodible matls to allow them to be immersed into the electrolytes.ample was subject to a small initial stress to ensure tas straight and slightly taut.In isotonic measurements, a constant force was appl

he sample while the displacement of one end was meaFig. 2a). The strain was calculated from the original lenf the sample, and the stress was based on the cross-sf the dry film or fiber. This was essentially equivalent to

ree strain if the force was small. In isometric measuremFig. 2b), the length of the sample was fixed and the chan force was measured during actuation. Cyclic voltammcans (CVs) were used to correlate length and force chaith particular applied potentials, whereas square wotential (SWP) stepping was used to obtain the speesponse.


A spreadsheet calculated performance metrics automati-cally from the data. The energy input was calculated at eachdata point by multiplying current, time interval, and potential.The net energy was calculated by summing all the energies,while the gross energy was calculated by summing the abso-lute values of all the energies. The net energy assumed thatall the charge on the return scan could be recovered usingan energy storage system, and is the most optimistic numberfor calculation of efficiency. The gross energy did not takeinto account the battery-like nature of the actuator and is themost pessimistic value for calculating efficiency. For isotonicmeasurements, work was calculated asW=mgh, or the forcetimes the stroke, wherem is the mass,g the gravitationalconstant, andh the stroke.

2.4. EDS

Energy dispersive spectroscopy (EDS) elemental analysiswas performed with a Noran Instruments system on an FEIenvironmental scanning electron microscope (ESEM). Theaccelerating voltage was 12 KeV.

3. Results and discussion: films

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The film lengthened reversibly by 1.5% during oxida-tion. The strain was relatively small because of the stretch-alignment of the films: since expansion is due primarily toinsertion of ions between the chains, aligning the chains alongthe fiber direction reduces the strain along the draw axis[1,8].

During the very first cycle, an irreversible 1% contractionwas superimposed over the actuation strain, which we assignto the expulsion of the large AMPS− ions. AMPS is mobilein polyaniline and soluble in water, which has allowed us toreplace this ion with others for applications requiring ionsthat are soluble in organic solvents[31]. Energy dispersivespectroscopy showed that AMPS was largely replaced byCl merely by soaking the fiber in HCl for several minutes;cycling enhanced that process. The net contraction continuedto a smaller extent in the subsequent few cycles. (This was notcreep due to the load, which would have caused the polymerto elongate, not contract.)

The length was closely correlated with the redox charge(Fig. 3b), as has been firmly established in numerous previ-ous studies (see for example[1,6,15]): the polymer expandedwhen anodic current was drawn and contracted when cathodiccurrent was drawn. There was, however, a small contractionabove 0.35 V and re-expansion upon scan reversal. This wasassociated with partial oxidation into the pernigraniline state,as was confirmed when the upper potential limit was extendedto 1 V (Fig. 4a). Upon complete oxidation to the pernigrani-l o thei

del( ud-i nce( ntso ingt lightc rec-t ig-i ine[ se thes con-t ding

F isotoni ss (thinb d to aid o.I

.1. Isotonic strain

Isotonic measurements yield important metrics, sucork density and efficiency. Such measurements taken

ng cyclic voltammetry allow one to correlate actuation strhich is the�L/L resulting from the electrical signal, wiotential and charge, and thus the oxidation level. Theseelations provide insight into the actuation mechanisms

A small load (5 g, for a stress of 3.3 MPa for these sampas applied to the films to keep them taut and straight. P

ials were initially limited to between−0.25 and +0.5 V verus Ag/AgCl to avoid the pernigraniline state and thus preegradation in the HCl. Oxidation and reduction between

eucoemeraldine and emeraldine states was stable, aenced by cyclic voltammograms with overlapping trace

ig. 3. (a) Cyclic voltammogram (light gray line) and correspondinglack line) from a different sample is also shown, with the axis reverse

nitial load 5 g (3.3 MPa), scan rate 5 mV/s.

-

ine state, the length had decreased all the way back tnitial value in the leucoemeraldine state.

This contraction was expected at pH 0 from the mopath 2) inFig. 1 as well as from previous radiotracer stes [47] and electrochemical quartz crystal microbalaEQCM) work[48]. However, previous strain measuremen polyaniline in 1 M HCl had shown expansion in go

o the pernigraniline state for unstretched films, but a sontraction for stretched films parallel to the draw diion [7]. In bilayers, polyaniline contracted by half the ornal expansion in 2 M HCl upon oxidation to pernigranil7,9]. These differences in reported behavior arise becautrain changes from expansion at very low pH (path 3) toraction at higher pH (path 2), with the magnitude depen

c strain (heavy line) of a PANI(AMPS) film in 1 M HCl. Isometric strecomparison. (b) Potential, strain, and charge vs. time during cyclic vltammetry


Fig. 4. (a) Strain vs. potential during cyclic voltammetry at 50 mV/s to the pernigraniline state. (The strain is substantially smaller than it was at 5mV/s,Fig. 3a.)(b) Strain vs. time at increasing scan rates. Time has been normalized to that of a single cycle. Load 5 g (1.1 MPa).

on the anion, alignment, sample preparation, and other vari-ables in ways that have still not been completely elucidated.

The scan rate was varied to probe the speed of responseof the films. The actuation strain decreased from approxi-mately 0.75% at 10 mV/s, to 0.55% at 50 mV/s, to 0.25% at200 mV/s (seeFig. 4b, in which the time axis has been nor-malized to allow the traces to be overlaid). These data reflectthe dependence of response time on mass transport.

At the slowest scan rate of 10 mV/s, the sample broke at+0.9 V. In the pernigraniline state, the film was too weak tosustain even a 1.1 MPa load. Measurements on fibers thatestablish the dependence of modulus and ultimate tensilestrength on applied potential are described below.

Square wave potential stepping is a more direct measureof the speed of response. In stepping experiments, the strainrate is only limited by the switching time of the film (as wellas by any system RC time constants, see for example[93]),whereas during cyclic voltammetry the scan rate is also rate-limiting.

Results from stepping between−0.25 and +0.5 V areshown inFig. 5. The strain was 1.1%, which was smaller thanthe 1.5% during cyclic voltammetry because the period was

F ppingb rent( tress( h thes

only 60 s compared with 300 s during the CVs. The strainupon oxidation overshot the final value by a small amountduring the first several seconds as the fiber passed throughthe emeraldine state before ending in a partly mixed emeral-dine/pernigraniline final state.

Elongation upon oxidation took only 3 s, but contractionupon reduction took more than 30 s. This was somewhat sur-prising because the ion insertion step has usually been re-ported in the literature to take longer than the ion expulsionstep, since ion transport is easier in the expanded state. Onehypothesis is that the load pulling on the film hinders thecontraction, but this is contradicted by data on fibers, below.

The strain actually led the charge during oxidation, butlagged it during reduction. This effect was also present in thecyclic voltammograms (Fig. 3) and was shown by Qi et al.[34]. The reasons for the strain leading the charge are un-clear, but other actuation mechanisms have been proposedthat may be responsible for the lag during reduction. For ex-ample, conformational relaxation of the polymer backbonesduring reduction needs to be taken into account[28,94,95],and this can happen on a longer time scale. In addition, wa-ter is transported that is not directly associated with the ions[54], occurs at a longer time scale[28], and may go in thecounter direction[48,72,84,96,97].

To try to overcome the slow reduction process, the filmwas driven galvanostatically (constant current rather thanconstant potential) at±5 mA (14 A/cm3) [98]. Duringthe anodic half-cycle, oxidation from leucoemeraldine toemeraldine states was completed in 18 s (after this thevoltage increased to 0.65 V to bring the polymer to thepernigraniline state). However, to reduce the polymer fromthe emeraldine state required−5 mA for 32 s, twice as long.This was because the potential dropped to−0.4 V, at whichhydrogen was evolved: the polyaniline could not be reducedat the 5 mA rate, so the potential went to a voltage at whichanother reaction could take place to supply the current. Nev-ertheless, if galvanostatic cycling at±5 mA was performedthen strain was linear with time (equivalent to charge) witha zero intercept until the oxidized or reduced states were

ig. 5. Strain (heavy line) vs. time during square wave potential steetween−0.25 V (30 s) and 0.50 V (30 s). Initial load 5 g (3.3 MPa). Curdashed light gray line), charge (solid light gray line), and isometric sthin black line) from a different sample are shown for comparison, wittress axis reversed.


reached. The electrochemical strain coefficient, a measureof the strain per unit charge density, was 2× 10−8%/(C/m3).

3.2. Isometric stress

The isometric stress generated by the drawn PANI(AMPS)films fixed at constant length was measured during cyclicvoltammetric cycling to be 5 MPa (Fig. 3a; note that theforce axis has been reversed to aid comparison with thestrain and charge). The force response preceded the strainby 70 mV upon oxidation, but upon reduction it overlaid thestrain curve. Skaarup et al.[99] have recently reported thatstress was also faster than strain in polypyrrole films, whichthey attributed to stiffness changes.

The isometric stress was 3.3 MPa during square wave po-tential stepping, as shown inFig. 5. Upon oxidation the forcedropped initially as the film expanded, but after a few secondsit increased again somewhat; this overshoot due to oxidationto the pernigraniline state was larger than that in the strainmeasurement. As inFig. 3, the force led the strain. The forceresponse during reduction was slower than during oxidation,as it was in the strain measurements. It should be noted thateven though the current flowing during reduction 5 s afterswitching was almost zero, it was nevertheless quite impor-tant, as the charge continued to accumulate and the forcecontinued to increase. The values for isometric stress werec

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square wave potential cycling, and a different scan rate duringcyclic voltammetry). The only two metrics that can be directlycompared with our study are the strain, 0.8% versus 1.5%,and the electrochemical strain coefficient, 6× 10−9 %/C/m3

in both cases. One can also, but with less legitimacy, comparetheir data in HClO4 with ours in HCl: strains were 0.6% ver-sus 1.5%, blocked stresses were 4 MPa versus 5 MPa, workdensities were 5 kJ/m3 versus 50 kJ/m3, and power densitieswere 9 W/m3 versus 330 W/m3 (this difference is largely dueto their slower scan rate). Herod and Schlenoff[1] reportedstrains of approximately 1.5% parallel to the draw axis in 1 MHCl in acid-doped polyaniline, Kaneto et al. 2.5%[6], and Qiet al. 1.1%[34]. Although all of the same order, there are sig-nificant differences between these measurements. Since allthe samples were cycled in the same electrolyte, the differ-ences must be due to preparation, with degree of alignmentlikely being the strongest factor.

Looking at organic electrolytes, strains in propylene car-bonate (PC) 1 M LiClO4 were reported to be nearly 4 timessmaller, and stresses were more than 10 times smaller[31],than we found in HCl, whereas in aqueous 1 M HClO4 strainswere only half the size of those in HCl[34]. This shows theimportance of solvent transport[41] and ion size, which arenot included in the model inFig. 1. Moreover, polyanilineswitches to cation-transporting in ionic liquids with smallanions and large cations, so there are still other interactionst d.

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omparable to those reported in the literature, 2–4 MPa[20].

.3. Actuator performance metrics

Performance metrics for these films are summarizeable 1. Mazzoldi et al.[19] have also published figureserit for polyaniline actuators in HCl, although under diffnt cycling conditions (square wave coulometry rather

able 1sotonic actuation performance of stretched PANI(AMPS) films in 1Cl; potential limits−0.25 to 0.50 V vs. Ag/AgCl with a load of 3.3 MP

etric Cyclic voltammetry(5 mV/s)

Square wave potentia(30 s at each potentia

requency (Hz) 0.003 0.017ysteresis (Eox −Ered)(V)

0.10 NA

tress (force/area) (MPa) 3.3 3.3ass lifted:mass sample(ratio)

35000 35000

train (%) 1.50 1.30lectrochem. strain coeff.(%/C/m3)

6.4× 10−9 8.8× 10−9

ork/volume (kJ/m3) 49 43pecific work (J/kg) 51 44ower/volume (kW/m3) 0.33 1.40pecific power (W/kg) 0.34 1.46fficiency w/o recovery(%)

0.024 0.037

fficiency w/chargerecovery (%)

0.16 0.12

ote: Dimensions of the PANI(AMPS) strip in solution: 1 cm (l)× 1 mmw) × 15�m (t).

hat play a role in actuation which are not yet understoo

. Results and discussion: fibers

In this section we present results on the elechemical actuation and mechanical properties of stretANI(AMPS) spun fibers. We again performed isotond isometric mechanical measurements using both coltammetry and square wave potential stimulation, butooked at the effects of fiber length, solution pH, cycrequency, and load. We end with an examination of theendence of Young’s modulus on applied potential and aussion of performance under load.

.1. Isotonic strain and isometric stress

The isotonic strain for a fiber during cyclic voltammeaken under the same conditions as those for the film inFig. 3,s shown inFig. 6a. The 2% strain and the behavior wirtually identical, with the same correlation between stnd current.

Isotonic strain (1%) and isometric stress (3 MPa) duquare wave potential stepping are shown inFig. 6b. Thesere again quite similar to the film results,Fig. 5, except tha

here was little overshoot in the stress and none in the sossibly because the fiber did not go as far into the pernig

ine state. The strain again led the charge upon oxidationhe stress led both. The strain and charge were closely cated upon reduction, with the stress again responding f


Fig. 6. (a) Cyclic voltammogram (light gray line) and corresponding isotonic strain (heavy line) of a PANI(AMPS) fiber in 1 M HCl. Initial load 3 g (3.4 MPa),scan rate 5 mV/s. (b) Strain (heavy line) vs. time during square wave potential stepping between−0.20 V (50 s) and 0.50 V (50 s). Initial load 3 g (3.4 MPa).Current (dashed gray line) and charge (solid gray line) are shown for comparison, together with isometric stress (thin black line), measured subsequently andwith the stress axis reversed.

There was one significant difference from the film: the fiberhad a high reduction speed. The process was mostly com-pleted in approximately 10 s, compared to 30 s for the film,despite the fact that the fiber was 7 times thicker.

Fig. 7 shows the data inFig. 6 replotted as stress andstrain versus charge. These results are quite different fromthose reported for 200�m thick electrochemically synthe-sized polyaniline films cycled in 1 M HCl[30], in which novolume expansion took place until after the peak in the cyclicvoltammogram. Those curves were interpreted as showingproton expulsion until half the anodic charge had been con-sumed, followed by anion ingress, but since this delay wasnot observed in our fibers under essentially identical cyclingconditions, and since the volume change in[30] had not sta-bilized prior to scan reversal, perhaps this effect was a resultof water transport and a scan rate that was too fast.

4.2. Effect of fiber length on performance

Ideally, one would like to use the polyaniline fiberswithout adding a metal backing, since that complicatesfabrication and introduces delamination as a potentialproblem. Unfortunately, although highly conducting for aconjugated polymer at 1000 S/cm, the PANI(AMPS) fibers

still had enough resistance to cause the applied potential todrop along the length of the fiber, thus making the oxidationlevel dependent on distance from the metal contact. Todetermine how serious a problem this was for the actuators,the strain during square wave stepping was measured fordifferent lengths of fiber, and the results are plotted inFig. 8.The strain was substantially smaller in longer samples.

Thus, a metal contact is required along the entire length ofthe fiber to optimize performance, even for relatively highlyconducting material. These results are consistent with thoseof Bay et al. showing active lengths on the order of millime-ters[100]. (There have been a number of reports in the recentliterature claiming that the problem of electrode delaminationcan be “solved” by eliminating the metal. Examination of theshapes of such actuators during operation, however, showsthat in those devices the strain also drops with distance, sothis is not actually a viable solution.)

4.3. Effect of pH on performance

The relative proportion of proton versus anion transport,which depends on pH[33,36,41,48,101](Fig. 1), is impor-tant because only anion (and solvent) exchange is believedto result in significant actuation strain. Nevertheless, the role

s a func g.
Fig. 7. Strain (heavy line) and stress of a PANI(AMPS) fiber a tion of charge during (a) cyclic voltammetry and (b) potential steppin


Fig. 8. Strain vs. length of the fiber during SWP in 1 M HCl, holding 20 s ateach potential (−0.2 and +0.5 V) (stress = 3.4 MPa).

of pH in the actuation of the polyaniline fibers has still notbeen clearly established. Mass change during redox has beenstudied as a function of pH using electrogravimetry[41]and electrochemical quartz crystal microbalance (EQCM)[17,36,48]. The mass gain upon oxidation from leucoemeral-dine to emeraldine was found to increase with increasing pHeven though the current dropped. However, when Kanekoand Kaneto[17] looked at bilayer bending of polyaniline inHCl, they observed no dependence of strain on pH betweenpH −2 and 2.5, in contradiction to the EQCM result and themodel inFig. 1. More recent results from this group did showa dependence of strain on pH[33]. (Interestingly, this studyalso showed that polyaniline actuators could operate in 1 MNaCl at neutral pH if an embedded mesh electrode was usedto provide conductivity to the material.) In the related ma-terial poly(o-methoxyaniline), free-standing films cycled inHCl had a strain that increased from 2% below pH 0 to 4%above pH 2[21].

Cyclic voltammograms and strains as a function ofpotential are compared inFig. 9 for polyaniline fiberscycled in HCl at pH−1, 0, 1, and 2. (The strains have beencorrected for fiber length according to the linear relationshipin Fig. 8.) As expected, the currents, and thus the chargeexchanged, dropped at higher pH, and the hysteresis (voltageseparation of the oxidation and reduction peaks) increased(Fig. 10). These are signs of increasing resistance with pHd atedu atingu argec

pH,t nionst lity. ItiP n( vedb andi ioni he

Fig. 9. Cyclic voltammograms (5 mV/s) and isotonic strain (under load of3.4 MPa) of a PANI fiber at various pHs in HCl.

fiber contracting first (between−0.2 and +0.1 V) and thenexpanding (+0.1 to +0.6 V). As the pH increases, accordingto Fig. 1, deprotonation becomes less pronounced and anioninsertion becomes more dominant, which is evidenced bythe strains inFig. 9: the upper potential limit for deprotona-tion/contraction decreases with pH (from +0.1 at pH−1, to0 V at pHs 0 and 1, and finally to−0.1 V at pH 2). Becauseof the predominant anion insertion and the disappearance ofanion egress for charge neutrality at high pHs (Fig. 1), higheractuation per unit charge (electrochemical strain coefficient)would be expected as shown inFig. 10. The strain, dueto (charge)× (strain coefficient), therefore peaked at pH0. The combination of a peak in strain with decreasing

ue to deprotonation, which converts conducting protonnits in the emeraldine salt state to unprotonated insulnits in the emeraldine base state, resulting in fewer charriers.

Although the charge during cycling decreased withhe strain coefficient increased as a greater fraction of ahan protons were exchanged to maintain charge neutras interesting to correlate strains at different pHs (Fig. 9) withANI redox mechanisms (Fig. 1). In a strong acid solutiopH −1), charge neutrality upon oxidation may be achiey a mixed mechanism of an initial deprotonation

on egress (−2A−, −4H+, path 4) and a subsequent annsertion (+2A−, path 1). Mechanically, this results in t


Fig. 10. Dependence of charge, hysteresis, strain coefficient, strain, and efficiency on pH.

current (charge) but increasing hysteresis resulted in a nearlyconstant efficiency up to pH 1, above which it dropped inhalf (from 0.02 to 0.01%, assuming charge recovery).

At pHs−1 and 2, the strain curves in consecutive scans didnot overlay each other, but spiraled, with the fiber contractingat pH−1 and lengthening at pH 2. The reasons for this areunclear, but spirals were also seen at pH 0 for scan rates thatwere too fast.

The Young’s modulus of the fibers did not depend on pHin this range. Measurements were performed at pH−1 and1 with no applied potential subsequent to cycling (Young’smodulus depended on potential – Section4.8), while thefibers were still immersed. Both gave ca. 0.5 GPa, consistentwith those in the emeraldine state at pH 0 (see below).

4.4. Effect of period during SWP on performance andmetrics

Standard metrics for determining the response time of con-jugated polymer actuators have not been established in the lit-erature. Some workers use the time it takes to switch betweencompletely oxidized and reduced states, others the shortesttime at which any movement is apparent. Rather than a singlenumber, we suggest that the complete relationship betweencycle time and strain is of more utility.

To obtain this curve, a PANI(AMPS) fiber was stepped in1 M HCl between 0.6 and−0.2 V (versus Ag/AgCl), holdingeach potential for various times. Results for 10, 50, and 200 sare shown inFig. 11a. As for the films, the oxidation reac-tion was faster than the reduction, but the difference was lesspronounced, and, opposite to the films, the difference wassmaller for shorter square wave periods because the initialslope was much larger. Sample preparation therefore plays asignificant, but as yet unelucidated, role in switching speed,since the chemical composition of the two types of sampleswas the same.

Fig. 11b plots the strain as a function of period (corre-sponding to a frequency range of 0.005–1 Hz). Below 50 s,strain was approximately linear with time, but for longer peri-ods, the slope decreased as the film reached its fully oxidizedor reduced state. Comparing the 200-s step time trace with thecurve inFig. 11b, the two superimpose closely, so the strain athigher frequencies can be estimated by a simple examinationof the start of a long-period SWP trace.

The electrochemical strain coefficient was constant atall frequencies (3.5× 10−9 %/C/m3), as was the 0.005%efficiency, again reflecting the tight correlation betweenstrain and charge. (Others, though, have found a dependenceof the strain to charge ratio on frequency in this range[102].)The specific power, simply related to the strain rate, rose


Fig. 11. (a) Strain vs. time for a PANI(AMPS) fiber during SWP stepping with different periods. The curves have been offset for clarity, and the half-period(step time) indicated. (b) Strain vs. step time (1, 2, 5, 10, 50, 200 s). The isotonic load was 1 g (1.1 MPa).

with frequency, from 60 mW/kg for 200-s steps to 430 for1-s steps. These values are low because of the small loadapplied in this experiment; what is important instead fromthese data is the relationship of the metrics to frequency.

4.5. Effect of isotonic force on performance and metrics

By increasing the isotonic force, better performance met-rics can be obtained. For example, for a stress of 11.3 MPa,the work density for the fibers was 133 kJ/m3, the specificwork 74 J/kg, and the power density 1330 kW/m3, all just 10times higher than for a load of 1.1 MPa.Fig. 12shows the

F ariousa ngth10 ) wasr

strain exerted by a PANI(AMPS) fiber under isotonic stressesbetween 1.1 and 34 MPa (measurements were done in the fol-lowing order: 1.1, 5.7, 3.4, 17, 3.4, 3.4, 8.5, 11.3, 22.6, 28,34 MPa).

As with other polymers, creep became evident at higherloads. Polymers are viscoelastic, meaning that when a loadis applied, they continue to stretch over time as the chainsrotate and unfold, with strain proportional to ln(time).

The length of the fibers continuously increased if the stresswas too high, but if the stress was reduced, the samples gradu-ally contracted again, recovering toward their original length.For example, inFig. 12the 3.4 MPa load was repeated twiceafter 17 MPa. The first of these repeated scans (labeled b)shows some contraction, or recovery, after the lengtheninginduced at 17 MPa. The second scan (c) was closer to theoriginal one (a), showing that the recovery process had al-most been completed. Therefore, there was a recoverableviscoelastic component as well as an irreversible plastic de-formation. Recovery has also been observed in polypyrrolefilms [102]. The actuation strain in all three 3.4 MPa curveswas essentially the same, so the small amount of creep thatoccurred at 17 MPa did not significantly affect the magni-tude of the actuation strain when the load was reduced again.These effects not only limit the loads that should be applied,but also complicate actuator control schemes by making loadhistory a factor in present performance. These issues apply,o .

t duet hangei tep,a ining4 , thism n-t tiona n andr e and

ig. 12. Strain vs. time during potential square wave stepping under vpplied loads. The curves have been offset for clarity (fiber active le0 mm, diameter 105�m). Stepping was in 1 M HCl between−0.2 and.5 V, and each potential was held for 50 s. The 3 g load (3.4 MPaepeated three times.

f course, to all electroactive polymer actuators (EAPs)To separate the movement due to actuation from tha

o creep, the actuation strain was approximated as the cn length occurring during the first 10 s after the potential snd the creep as the change in length during the rema0 s. (Above 30 MPa, when the creep became very largeethod of estimation failed.)Fig. 13a illustrates these qua

ities. InFig. 13b, expansion is shown as positive, contracs negative. For the smallest load (1.1 MPa), the oxidatioeduction strains were the same, shown by equal positiv


Fig. 13. (a) Method of estimating actuation strains and creep. (b) Actuation strain and creep vs. applied stress. Dashed lines are guides for the eye. Error barsindicate the variation over four steps.

negative actuation strains, and there was no net expansion,shown by equal positive and negative creeps. (Previous workon aligned polyaniline reported creep at 0.2 MPa perpendic-ular to the stretch direction[8] and at 1 MPa for unorientedfilms [1], but no creep at 1 MPa in the direction parallel tothe stretch direction.) The reduction strain (contraction, per-forming work) became smaller with increasing force, but theoxidation strain (expansion) remained essentially constant atall loads. Net creep became obvious in the slopes ofFig. 12and inFig. 13at loads of 17 MPa and above.

Fig. 13predicts a blocking stress, above which there is nostrain, on the order of 40 MPa. However, this is quite differ-ent from the isometric stress, reported above and below, of2.5–5 MPa. Blocking stress and isometric stress are normallytreated as the same thing, so a difference, particularly of thissize, was unexpected. The load data are further discussedbelow.

The metrics obtained under the various loads during iso-tonic stepping are given inTable 2. Some of them simply

scale with strain (such as the electrochemical strain coeffi-cient), others with load (such as the mass lifted to actuatormass ratio, which reached 159,000). Work and power dependon both. Most of the metrics were optimal at 17 MPa; belowthat the stresses were too small although the strains werelarge, and above that the strain fell faster than the force rose.Somewhat surprising was the fact that the anodic charge, andthus the power consumed during SWP, did not depend ap-preciably on load. The relationship between load and energyinput is further explored below.

The results of isometric measurements, which were per-formed after each isotonic test, are shown inFig. 14. Thestress response was much faster than the strain response(Fig. 12): 4 s compared with 10. Unlike the strain, the ac-tuation stress exerted by the fiber did not depend appreciablyon load in this range, but was a constant at approximately2.5 MPa. At the highest initial load (27 MPa), the overallforce gradually dropped over time due to creep, and thenstabilized.

Table 2Isotonic actuation performance of a PANI(AMPS) stretched fiber with a 105�m diameter and 10 mm length in 1 M HCl with various loads upon square wavestepping from−0.2 to +0.5 V vs. Ag/AgCl, holding each potential for 50 s (frequency of 0.01 Hz)

Metric 1 g 3 g 7.5 g 15 g 25 g 30 g

Stress (force/area) (MPa) 1.1 3.4 8.5 17.0 28.3 34.0M 1910 91000S 0.84 5E .56E 9S 15.7 1P .29S 0.16 8A 46.7 4M 0.008 16M 0.015 28

ass lifted:mass sample (ratio) 6400train (%) during reduction (contraction) 0.92lectrochem. strain coeff. (%/C/m3) × 10−9 1.64 1nergy Density (work/volume) (kJ/m3) 10 2pecific work (work/mass) (J/kg) 5.7ower density (kW/m3) 0.10 0pecific power (W/kg) 0.057nodic charge in 1 Cycle (mC) 48.4in. energy efficiency (%) 0.003ax. energy efficiency (%) 0.006

0 47800 95500 159200 10.72 0.61 0.35 0.1

1.35 1.1 0.66 0.2961 104 99 51

33.7 57.1 54.6 28.0.61 1.03 0.99 0.51

0.337 0.57 0.54 0.246.3 46.6 45.6 44.0.018 0.030 0.029 0.00.033 0.055 0.053 0.0


Fig. 14. Isometric stress vs. time during potential square wave stepping withdifferent initial lengths, or baseline loads (y-axis intercept).

4.6. Extended cycling

The performance of a PANI(AMPS) fiber upon extendedcycling at 3.4 MPa is shown inFig. 15. These data were takenfrom the fiber used in the previous section immediately afterthe highest load measurements. The creep recovery, a con-traction of a full 6%, is evident, and it occurred over a time pe-riod of more than 4 h. The strain was unchanged over the 270cycles, even increasing a small amount. At 0.67%, however,it was smaller than the strain during the previous 3.4 MPa cy-cles at 0.84%; the 34 MPa load did therefore seem to affect thestrain performance. The cycling was stopped after 272 cycles.

4.7. Effect of load on current

Takashima et al. showed in 1997 that electrochemical cur-rent can be induced in free standing films of polyaniline by

F uringS

mechanical force[12]. In addition, Otero and Cortes haverecently shown that polypyrrole bimorph actuators can senseforce: when operated at constant current, the voltage in-creased with the load pushed[103]. If the power consumedduring actuation depends on load, then we would expect tosee an increase in the reduction current for higher loads, sincethat is the half of the cycle during which work is performed(contraction).

Two chronoamperograms are shown inFig. 16a, one takenunder a stress of 1.1 MPa and the other under 34 MPa duringthe isotonic measurements presented above. The reductionpeak current did not increase, although the peak did narrowa little. The oxidation peak current, on the other hand, didincrease, and this peak also became narrower. The total chargewas somewhat reduced (Table 2, Fig. 16b).

The oxidation peak currents as a function of load areplotted inFig. 16b. Points from both the 4th and 5th steps areplotted; the two overlay each other, showing that step-to-stepreproducibility was good. It is clear that currents weregenerally higher for higher loads, but the relationship wasnot linear as in the Otero devices[103]. There was a strongdependence on history, indicated by the arrows, which showthe order in which the data were obtained. The point takenwith a 3.4 MPa isotonic stress immediately after 17 MPawas higher, but when this measurement was repeated, thecurrent dropped to the value obtained at this load originally.T kene chainc et al.[

4

ucedso theoa of as filmd meri erebyr Thec pactt ed thed

slyb cted)s ithd e ox-i gpi atterc sible,w herc

ig. 15. Isotonic strain vs. cycle number (3.4 MPa load, 60 s period dWP stepping). Data taken after 34 MPa load ofFig. 12.

he 8.5 MPa point was higher than the 17 MPa point taarlier. These results are not easy to understand, butonformation must play a role, as postulated by Otero104]. Total charge dropped slightly with load.

.8. Effect of oxidation potential on Young’s modulus

The mechanical properties of the oxidized and redtates of conjugated polymers are different[105,106]. Forne thing, the chains are stiffer and more planar inxidized state because of alignment of the� orbitals tollow charge conduction. More importantly, the additionubstantial number of ions and solvent molecules to theisrupts interchain interactions, and introduces ion-poly

nteractions. The ions and solvent act as plasticizers, theducing the Young’s modulus and creep resistance.hange in modulus has been found to significantly imhe actuation behavior[107]. In this section, we report thependence of Young’s modulus on applied potential inrawn PANI(AMPS) fibers.

The Young’s modulus of polyaniline has previoueen reported to be higher in the reduced (contratate[108]. In cation-transporting polypyrrole doped wodecylbenzenesulfonate, the modulus was higher in th

dized (contracted) state[54,109], but in anion-transportinolypyrrole doped with tosylate[105,106]or PF6 [107,110],

t was higher in the oxidized (expanded) state. In the lase, ion crosslinking was suggested to be responith plasticization being less important. Data from otonjugated polymers is also mixed.


Fig. 16. (a) Chronoamperograms for low and high isotonic loads (5th step). (b) Peak oxidation current vs. isotonic load; arrows indicate sample order(see text)(points shown for both 4th and 5th steps, which are so close that they overlay each other). On the other axis, isotonic and isometric oxidation charge vs. load.

Prior to modulus measurements, each fiber was steppedbetween oxidizing and reducing potentials (−0.2 to +0.5 Vversus Ag/AgCl) 20 times to “break in” the fiber. Then a fixedpotential was applied to the fiber, and the current monitoreduntil it fell essentially to zero and the length of the fiber nolonger changed. To obtain extension-force curves, the fiberswere stretched at a rate of 0.01 mm/s, and the force was mea-sured until the fibers broke. The curves for the two extremesof −0.2 and +0.7 V are shown inFig. 17a. The Young’s mod-uli for each potential are plotted inFig. 17b, together with acyclic voltammogram for reference.

There was a three-fold reduction in modulus uponoxidation from leucoemeraldine to emeraldine, presumablydue primarily to plasticization by the dopants and solvent,which act as lubricants, as well as to a decrease in hydrogenbonding between chains. If the chains can slide past eachother more readily, the material is easier to stretch and themodulus drops. However, the modulus decrease started ata lower potential than the oxidation peak and leveled off atthe peak, so other factors must contribute also.

These moduli of between 1.7 and 0.5 GPa compare withpreviously reported values of 0.23 GPa for undoped and0.18 GPa for doped drawn fibers[108] and 0.7 GPa fordoped drawn fibers[20]. The modulus we measured in thedry state for PANI(AMPS) was 5.1 GPa, which compareswith 0.05 GPa for dry undrawn PANI(AMPS)[111], 4.3and 14 GPa for undoped dry undrawn and drawn polyani-line [112], 3.5 GPa for dry undoped drawn fibers[20], and1.5 GPa for dry, doped, drawn PANI(ClO4) fibers[20].

For PANI(AMPS) fibers cycled in ionic liquid, Lu andMattes[32] reported that the modulus decreased with oxida-tion level from 0.35 GPa in the emeraldine state to 0.2 GPain the leucoemeraldine state. In this electrolyte, however,cations rather than anions are exchanged, so the fibers expandupon reduction. Generalizing, we conclude that in polyani-line the modulus is higher in the more contracted state anddrops upon insertion of ions. The AMPS in this fiber hadbeen exchanged prior to cycling with triflate, and the triflateis believed to stay in the polymer during cycling in the ionicliquid. The modulus for the anion-filled, oxidized state in that

odulu
Fig. 17. (a) Stress vs. strain curves. (b) Young’s m s vs. applied potential. Dashed line is a guide for the eye.


Fig. 18. Theoretical behavior of (a) actuation strain, (b) elastic strain, and (c) creep as a function of load.

study, 0.35 GPa, is approximately the same as that in our ox-idized state, 0.5 GPa. Upon inserting even more ions duringreduction, the modulus dropped still further, supporting thispicture.

The ultimate tensile strength and the strain at break werenot strongly correlated with potential and had significant scat-ter, but they ranged between 20 and 45 MPa and between 4and 26%. Defects in the fibers may have contributed to thescatter. Previously reported values for the ultimate tensilestrength of polyaniline were 50 MPa[111] and 35–50 MPa[20]. There was one exception: the ultimate tensile strengthand strain at break at 0.7 V, at the start of the pernigranilinestate, were always substantially lower. (Recall that this wasalso found in the isotonic measurements of the film samples.)Herod and Schlenoff[1] had also reported rapid mechanicalfailure above the emeraldine state. Toughness of the fibers(the area under the stress–strain curve) ranged from 40 to800 MPa.

4.9. Discussion of performance under load results

The data inFigs. 13 and 14are not straightforward tointerpret. First, the blocked stress and isometric stress werenot equal. Second, the oxidation strain did not vary with load.Third, the isometric stress did not depend on load.

lumec ppliedl t un-k rmale es. Too ationa eenm gh ithp nald ingn

ε

T chedi

isk r cann ess.

They are connected by a line, typically straight. The reduc-tion in strain with load happens regardless of elastic strainand creep. InFig. 13, the reduction strain followed this be-havior. One might also expect the oxidation strain to decreasethe same way if the ion insertion and ejection processes aresymmetric, but this may not be the case under load.

Even if the actuator underwent no volume change dueto mass transport, the fact that the Young’s modulus varieswith oxidation level causes a length difference in a loadedactuator (but not in the free strain condition). This other typeof actuation strain is simply proportional to the difference inmoduli and the load stress[107]:

ε = σ

(1

Yox− 1

Yred

)

Given the difference in moduli we measured, we would haveexpected an elastic strain of +4% at 30 MPa when the actuatorswitched to the emeraldine state, but surprisingly this wasnotobserved.

Finally, a load placed on a polymer will cause it to creep,with the strain rate typically proportional to the stress raisedto a power:

ε ∼ mσn

The power varies with material, and not all polymers fol-l t al la-t hicho do.Tb

o thei at thei h, orl

5

tch-o rs,a mostc everp nter-r ling

The total strain has at least three components: vohange due to mass transport, elastic strain due to the aoad, and creep. There may also be strain due to as yenown effects, such as differences in coefficients of thexpansion (CTE) between the oxidized and reduced statur knowledge, the change in temperature during actund the CTE as a function of oxidation level have not beasured for any conjugated polymer actuator, althouas been reported that both polypyrrole[113] and polythio-hene[114] have a negative CTE in the dry state. The fieformation is found by summing all contributions, assumo interactions:

total = εact + εelastic+ εcreep+ εCTE + εother

he behavior of the first three as a function of load is sketn Fig. 18.

The actuation strain is maximum with no load; thisnown as the free strain. It goes to zero when the actuatoo longer lift the load; this is known as the blocked str

ow this law, but it is often a nonlinear relationship. Aoad of 28 MPa,n=−0.4, but to really determine the reionship, much higher loads would need to be applied, wur system, with an upper limit of 0.5 N, was unable tohe relationship between creep and load stress inFig. 18haseen sketched schematically based on the results ofFig. 13.

The reason that the blocked stress was not equal tsometric stress is obscure, and likewise, the reason thsometric stress did not depend on initial stretched lengtoad.

. Further discussion and conclusions

We have fully characterized the performance of streriented PANI(AMPS), in the form of both films and fibes an actuator material in HCl. This represents theomplete picture of a single actuator in one electrolyteresented, and allows one to begin to correlate the ielationships between various figures of merit and cyc


conditions. The impact of sample length, pH, electrochemi-cal driving method, frequency, oxidation level, and load havebeen elucidated. A number of figures of merit were calcu-lated from isotonic and isometric measurements, includingstrain, stress, work, power, and efficiency; these metrics havebeen summarized in the tables. Our other major findings aresummarized below.

The behavior of the actuators was substantially accountedfor by the well-established model inFig. 1; this figure, for thefirst time, explicitly shows the pathways relevant to actuation.It is clear from the figure why different research groups haveobtained varying results in different acids and pHs, and in or-ganic versus aqueous electrolytes (Fig. 1paths 1 and 2 versus1 and 3). The degree of protonation and ability of the poly-mer to change protonation level control the amount of chargethat can be exchanged and the extent of ion insertion duringelectrochemical cycling, which in turn control the Young’smodulus and strain. This model does not, however, addresskinetic effects, water transport, or changes in the mechanicalproperties of the polymer chains themselves. These also playimportant roles in actuation, but more data is required beforea more comprehensive model can be developed.

In films, oxidation (expansion) from leucoemeraldine toemeraldine was significantly faster than reduction (contrac-tion), 3 s versus more than 30. In fibers, however, the reduc-tion was only somewhat slower, if at all. Since both typeso ngthso rea-s otedt filmsw ntf , ei-t t then hichm ues-t ticalf

andfi iso-m yclicv andfi s andp bones duet iledt stilli andi delayb earchw

ingt odea e inmi roacht Bay

et al. [116] show a similar concept applied to planarsamples.

The frequency response curve was essentially equivalentto the strain versus time curve during square wave poten-tial stepping at low frequency, which means that only onemeasurement is required to obtain this information. The fre-quency response is often reported as a single number, withsome authors reporting the time it takes for switching betweencompletely oxidized and reduced states, and others reportingthe maximum frequency at which any movement is apparent.This makes comparison of actuators impossible. One com-plete cycle, given that there are usually differences betweenoxidation and reduction rate, of a low frequency square wavewould provide potential actuator users with a complete pic-ture of the frequency response.

Higher potentials are effective at increasing polyanilinespeed in ionic liquids and organic electrolytes, which havelarge potential windows[23,27]. In an aqueous electrolytelike HCl, however, the response time cannot be improved bysimply switching to current control or by increasing the po-tential because that results in unwanted reactions, such ashydrogen evolution or bringing the polymer to the perni-graniline state (leading to polymer degradation). This is adrawback of working in water.

For loads above 15 MPa, the PANI(AMPS) started tocreep, although upon removal of the load it slowly recoveredt traint ionala ng ani ve toz ilinefi ress,a y thei stics s noto ficest , theb singn re tob nderl

eu-c rtion.S onicl ponc inga . Thep withfi alll anice r thisb learf sn

usedi ing

f sample were of the same material, had immersed lef approximately 1 cm, and had been stretch-aligned, theon for this difference is not clear, although it should be nhat the fibers were more conductive than the films. Theere thinner, 15�m versus 105�m, but this does not accou

or the disparity. Our hypothesis is that their morphologyher the pore size or the degree of crystallinity, differed aano-scale, too small to be imaged with the ESEM. Waterial properties affect switching rate remains a key q

ion for future work, since greater actuator speed is crior many applications.

Stresses and strains were comparable in the filmsbers, ranging from 2.5 to 5 MPa and 1 to 2%. Theetric stress response led the isotonic strain in both c

oltammograms and chronoamperograms, in both filmsbers. This has also been seen in polypyrrole actuatorostulated to be due to changes in the polymer backtiffness[99]. Some authors have postulated that straino changes in polymer backbone conformation, from coo straight, may also play a role in actuation. There isnsufficient data, however, to do more than speculate,t should be noted that others have instead noted aetween the current and the onset of strain. More resould be needed to examine this question carefully.Strain decreased rapidly with sample length, show

hat even for highly conducting material, a metal electrlong the sample is required to optimize performancacro-scale systems. The work of Ding et al.[115] with

ntegral helical electrodes represents one good apphat can be taken, and the corrugated electrodes by

o a considerable extent. Load curves relating actuation so stress were obtained by subtracting the creep. In traditctuators, the blocking stress can be found either by doi

sometric measurement or by extrapolating the load curero strain. However, the blocking stress of the polyanbers was much larger than the isometric actuation stnd the isometric stress was not significantly affected b

nitial load applied to the sample. Furthermore, the elatrain expected from the change in Young’s modulus wabserved. Although taking into account elastic strain suf

o describe the behavior of some polypyrrole actuatorsehavior of the polyaniline fibers was more complex, raiumerous questions. If conjugated polymer actuators ae applied in load-bearing applications, the behavior u

oad will need to be studied more carefully.The Young’s modulus dropped upon oxidation from l

oemeraldine to emeraldine, upon anion and water inseince the modulus has been found to fall even further in i

iquids going from emeraldine to leucoemeraldine, uation insertion, disruption of interchain hydrogen bondnd plasticization can be concluded to be responsibleernigraniline state had a very low tensile strength,bers at that oxidation level breaking readily under sm

oads. Reports are lacking on what happens in orglectrolytes in the pernigraniline state, so the reason forittleness still needs to be elucidated. One thing is c

rom data in methanesulfonic acid[35], and that is that it iot due to degradation.

The mass lifted to actuator mass metric is frequentlyn the literature, but it can be optimized simply by apply


more load to a short sample. Work density or specific worktake into account not only force, but also strain, and so rep-resent better overall measures of actuator performance. Tooptimize power metrics, the frequency should be high, but itmust be balanced against the loss in strain at higher frequen-cies; there will be an optimum value.

Stress on the fibers affected the electrochemistry, althoughprimarily the oxidation (ion insertion, expansion) step. Thiswas seen in the chronoamperograms, confirming prior reportsof sensing capability. Under heavy loads, current peaks weretaller and narrower, and the total anodic charge fell somewhat.Unfortunately from an application perspective, the effect wassensitive to sample history.

Metrics for the film and fiber actuators were similar, aswell as consistent with those previously reported. This isgood news from the point of view that polyaniline actuatormaterials produced by different groups are roughly compara-ble and not extremely dependent on preparation conditions.However, it is bad news from the point of view of obtainingimproved metrics. The numbers reported so far would seemto be the best that one can achieve without fundamentallydifferent approaches.

Acknowledgments

archP areg ntinif stem.W ndfi

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[13] M. Kaneko, K. Kaneto, React. Funct. Polym. 37 (1998) 155–161.[14] M. Kaneko, K. Kaneto, IEICE Trans. Electron. E81-C (7) (1998)

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Mater. Sci. Eng. C 6 (1) (1998) 65–72.[16] D. De Rossi, A. Mazzoldi, Linear fully dry polymer actuators, in:

Y. Bar-Cohen (Ed.), Proc. SPIE’s Sixth Int. Symp. Smart Struc.Mater., vol. 3669, Electroactive Polymer Actuators and Devices(EAPAD), Newport Beach, CA, 1999, pp. 35–44, 1–2 March.

[17] M. Kaneko, K. Kaneto, Synth. Met. 102 (1999) 1350–1353.[18] K. Kaneto, S. Sewa, W. Takashima, Artificial muscles, European

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[23] W. Lu, A.G. Fadeev, B.H. Qi, E. Smela, B.R. Mattes, J. Ding,G.M. Spinks, J. Mazurkiewicz, D.Z. Zhou, G.G. Wallace, D.R.MacFarlane, S.A. Forsyth, M. Forsyth, Science 297 (5583) (2002)983–987.

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This work was funded by the Defense Advanced Reserojects Agency (contract no. MDA972-99-C-0004). Werateful to Prof. D. De Rossi in Pisa and his student F. Giu

or helping us to set up our mechanical measurement sye thank D. Yang and P.N. Adams for polyaniline film a

ber preparation.

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