Artificial Muscles - American Chemical Society

Article pubs.acs.org/cm

Artificial Muscles: A Tool To Quantify Exchanged Solvent during Biomimetic Reactions Toribio F. Otero* and Jose G. Martinez Center for Electrochemistry and Intelligent Materials, Universidad Politécnica de Cartagena (UPCT), ETSII, Campus Alfonso XIII, Aulario II, E- 30203, Cartagena, Spain S Supporting Information *

ABSTRACT: Artificial muscles (bending or linear) from films of conducting polymers are faradic and biomimetic gel motors consuming the same charge to move through the same space in a specific electrolyte. The driven volume variation of the film requires the exchange with the electrolyte of a number of counterions, defined by the charge, and solvent molecules during reactions. Working in aqueous solution with different anions using bilayer (polypyrrole/ tape) bending artificial muscles, the charge consumed to describe a constant angle changes with the anion. Volume variations in the polypyrrole film due to the anionic exchange are calculated from the involved charge and the crystallographic radius of every anion. A parallel exchange of water molecules is required to explain the described angles. The number of solvent molecules exchanged between the polymeric membrane and the electrolyte at the same time that an anion (apparent solvation number) or when an electron was extracted from the chains (apparent hydration number) during the reaction was calculated. The use of artificial muscles (bending or linear) as a tool for the quantification of the number of solvent molecules accompanying the electrochemical reactions in p-doping or n-doping conducting polymers is proposed. KEYWORDS: artificial muscle, reaction, solvent exchange, apparent solvation number, apparent hydration number

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varies between (Pol*) and [(Pol)n+(A−)n(S)m]gel driven by reaction 1:9−12 the volume variation is proportional to the driving charge. Both amplitude and rate of the movement are controlled by the consumed charge per mass unit of conducting polymer or by the applied current (charge per unit of time), respectively.6−8 The reactive nature of the device originates that any physical or chemical variable acting on the rate of the electrochemical reaction is sensed by the device while working,11,13 mimicking brain−muscles feedback communication. Basic equations from the electrochemical kinetics allow a full theoretical description of this unique simultaneous actuating−sensing behavior of the device.14 Generation and destruction of free volume in the reactive polymer film is required for the insertion/expulsion of two species: counterions required for charge balance and solvent molecules required for osmotic pressure and chemical balance. In a similar way to the parallel processes taking place in the sarcomere of mammal muscles, the conformational movements of the macromolecules driven by the chemical reactions controls volume changes. Today’s methodologies do not allow uncontroversial determinations of volume variations in conducting polymer films. Only length variations in films15−22 under constant stress or thickness23−26 changes of solid coated

INTRODUCTION Conducting polymers are reactive materials: they can be oxidized/reduced in a reversible way in the presence of electrolytes (solid or liquid). The composition of the reactive material mimics that of the biological systems: reactive macromolecules, ions, and water. Those reactions drive the reversible change of biomimetic electrochemical properties (some of them by several orders of magnitude) as color,1 conductivity,2 porosity,3 stored charge, stored chemicals,4,5 or volume. For any conducting polymer exchanging anions with the electrolyte during p-doping and p-dedoping as polypyrrole doped with ClO4− during generation, the reversible electrochemical reaction driving changes of the above-mentioned properties can be written as: ox

(Pol*)s + n(A−)sol + m(S) XooY [(Pol)n + (A−)n (S)m ]gel + ne− red

(1)

where the different subindexes mean s, solid, and sol, solution; Pol* represents the active chain centers (understood as those points of the polymeric chains where a positive charge will be present after oxidation); A− represents the anions exchanged with the electrolyte to keep the film electroneutrality; and S represents exchanged solvent molecules forming a dense polymer gel (indicated by the sub index gel). Artificial muscles are faradic motors6−8 based on reversible volume variations produced when the material composition © 2012 American Chemical Society

Received: June 26, 2012 Revised: September 27, 2012 Published: September 27, 2012 4093

dx.doi.org/10.1021/cm302847r | Chem. Mater. 2012, 24, 4093−4099

Chemistry of Materials

Article

electrodes (keeping a fixed bottom and generating lateral stress) are possible. The volume variation, ΔVox, related to the entrance of the counterions during oxidation (exit during reduction) of any faradic motor can be determined by the number of ions exchanged (so by the charge consumed during oxidation, Qox, divided by the number of charges of each ion) and the volume of one counterion, which can be estimated from the crystallographic radius, r, of the ion when a spherical ion is considered: ΔVox =

Q ox zF

4 NA πr 3 3

function of the reactive gel is used here to try to clarify a new aspect of the driving reaction.

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EXPERIMENTAL SECTION

Pyrrole (Fluka) was purified before use by distillation under vacuum using a diaphragm vacuum pump MZ 2C SCHOTT and stored under nitrogen atmosphere at −10 °C. Anhydrous lithium perchlorate salt LiClO4 (Aldrich), trisodium phosphate 12-hydrate Na3PO4·12H2O ( P a n r e a c ) , d i s o d i u m h y d r o g e n p h o s p h a t e 1 2 - h y d r a te Na2HPO4·12H2O (Panreac), sodium peroxodisulphate Na2S2O8 (Panreac), sodium chloride NaCl (Panreac), sodium perchlorate NaClO4 (Sigma-Aldrich), sodium carbonate Na2CO3 (Merck), sodium nitrate NaNO3 (Merck), sodium iodide NaI (Sigma), and acetonitrile (Panreac, HPLC grade) were used as received. Ultrapure water was obtained from Milli-Q equipment. All the electrochemical studies were performed using an Autolab electrochemical workstation (PGSTAT-100 potentiostat/galvanostat) attached to a personal computer and employing GPES electrochemical software. The reference electrode was a Crison Ag/AgCl (3 M KCl) electrode. All potentials reported in this work are referenced to this electrode. A stainless steel electrode (6 cm2) was used as counter electrode. All the experiments were performed at 25 °C (room temperature). For the electrogeneration of the polypyrrole (pPy) films, three stainless steel electrodes having a surface area of 6 cm2 were used, one of them as working electrode placed between two parallel counterelectrodes located at a distance of 1 cm. Two polypyrrole films (one by the side of the steel plate) were synthesized from a 0.2 M pyrrole and 0.1 M LiClO4 acetonitrile solution having 1% water content. The films were electrodeposited by consecutive square potential waves10,50,51 from −0.322 V, kept for 2 s, to 0.872 V, and kept for 8 s, during the time required to consume a total polymerization charge (anodic minus cathodic charges) of 50 C getting an average thickness of 38 μm. After reduction at −0.322 V for 5 min, the coated electrode was rinsed with water and dried in air. The two films (one by side) were peeled off from the metal, determining the mass of the samples using a Sartorious SC2 balance with a precision of 10−7 g. Bilayer artificial muscles were constructed by adhesion of a nonconducting adhesive tape from 3 M to one side of the polypyrrole film (Figure 1). A transversal strip of isolating and nonreactive paint

(2)

where (Qox/Fz) NA is the number of balancing counterions, z being the ion charge and NA Avogadro’s number (number of ions in one mole), and (4/3)πr3 is the volume of one ion, r being the crystallographic radius. Moreover, counterions in the reaction promote solvent exchange that can be originated by different driving forces: (i) Donnan equilibrium to maintain equal chemical potentials inside and outside the film; (ii) solvation of the charged counterions; or (iii) osmotic pressure balance.27−32 Some solvent molecules may remain trapped in the polymeric membrane even in the reduced state.33 The solvent is proposed to have a strong influence in both morphology34−36 and behavior37−39 of the conducting polymer membranes. The final result is that in conducting polymers, as in most of the biological reactions in cells involving reactive macromolecules, the reaction drives solvent and ion exchange with the surroundings. In order to quantify this solvent exchange all together for anions, or cations when this is the prevalent exchanged ion,13 different methods have been used: ac electrogravimetry,40 electrochemical quartz crystal microbalance (EQCM),41−45 or electrochemical impedance spectroscopy (EIS).43 The influence of solvation in conducting polymers was studied by Frommer.46 Anions and solvent exchange can be estimated for different oxidation steps by molecular dynamics simulation.47−49 All those methodologies require complex experimental setups, expensive equipment, high qualified staff, and controversial models to quantify the solvent exchange. Understanding and quantifying simultaneous water and ion exchange at the molecular level for biological or biomimetic reactions responsible for biological functions constitutes a basic goal for the subsequent development of chemical predictive models for the quantitative description of biological functions, for states of both health and sickness. Here bilayer (conducting polymer/isolating tape) bending artificial muscles are used as faradic tools for the determination of the solvent exchanged by dense gels of conducting polymers during the polymeric reactions in electrolytes with different anions. We assume that the change in modulus of the pPy film during reaction 1 is similar in each of the studied electrolytes having in all cases a negligible effect compared with the effects of the volume changes (especially if the film thickness is much smaller compared with rest of dimensions) in the polymer. So, when the results from different electrolytes are compared (this manuscript), the effect of this change in the comparative study is negligible. Here bilayer (conducting polymer/isolating tape) bending artificial muscles are used as faradic tools for the determination of the solvent exchanged during reaction. The biomimetic

Figure 1. Scheme of the electrochemical cell.

(black nail polish from Deliplus) on the polypyrrole film allows a good determination of the polypyrrole surface area (and mass) that will be immersed in solution and will take part of the reaction driving the actuator movement. The paint strip also hinders the movement of the electrolyte by capillarity through the polypyrrole film fraction over the electrolyte, the direct contact electrolyte clamp (used for electric contact), and electrolyte reactions on the clamp under current flows. 4094



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The bending movement of the muscles was recorded employing a video camera EVI-D31/B (Sony) connected to a computer by means of a Matrox Meteor II video acquisition card.

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gram of conducting polymer) consumed during the reaction: the mass of the electroactive polypyrrole working (by oxidation/reduction) in each electrochemical actuator (mpPy)active used in this paper was determined. After electrogeneration the polypyrrole films, still adhered to the steel electrode, were reduced at −0.322 V during 300 s, rinsed, and dried. Once a reduced film was peeled off from the metal, its mass (mpPy)film was obtained. The polypyrrole/tape bilayer was then constructed as indicated above. The bilayer actuator was immersed, keeping the transversal paint strip above (touching) the electrolyte meniscus (Figure 1). The mass of electroactive polypyrrole, below this strip, was estimated from the total mass of the reduced film, the length of the film, lfilm, and the length of the active (immersed) film, lactive, bellow the lacquer strip in contact with the solution:

METHODOLOGY

Bending Movement. The flow of an anodic current through the bilayer artificial muscle promotes the oxidation of the immersed polypyrrole film and the continuous bending movement of the bilayer. The initial and the final position of the movement and the described angle were defined as

(mpPy )active =

lactive (mpPy )film lfilm

(5)

The active polypyrrole masses of the artificial muscles used to check the different electrolytes are presented in Table 1. Table 1. Masses of the Reduced, Rinsed, and Dried Polypyrrole Film Fractions Immersed in Every Studied Electrolytea, Specific Oxidation Chargesb, Slopes, rad C−1 g, Obtained from Figure 4c, and Slopes, rad g C−1, from Supporting Information Fig. S1

Figure 2. Electrochemical cell and angle described by the muscle under flow of a constant current for a constant time: the two pictures related to the initial and final position from the recorded video were overlapped.

indicated in Figure 2. Described angles and curvature are related through a linear relationship:52

2β 1 = (3) R l where 1/R is the curvature, β is the described angle, and l is the length of the muscle. The difference between any initial and any final angles, α, can also be expressed as a function of the initial (1/R1) and final (1/R2) curvatures, as corroborated by experimental results (see Supporting Information): 1 1 2α − = R1 R2 l (4)

electrolyte

mass of reduced and active polypyrrole, mg

maximum specific charge in NaClO4, C·mg−1

slopes from Figure 1, k, rad C−1 g

slopes from Figure S1, k′, rad g C−1

NaCl NaI NaNO3 NaClO4 Na2S2O8 Na2HPO4 Na3PO4

1.9281 3.3187 3.7197 4.6399 3.8159 1.9016 3.7964

36.74 39.23 39.61 35.83 39.83 38.03 36.81

0.035 0.199 0.580 0.251 0.181 0.212 0.098

1 7 25 19 20 6 2

a

Calculated from eq 5. A new muscle was used in every electrolyte. Obtained from the stationary voltammogram of the control performed for every muscle in the reference electrolyte (0.5 M NaClO4 aqueous solution) between −0.7 and 0.5 V versus Ag/AgCl at 10 mV s−1. cDescribing (eq 6) the movement of the studied artificial muscles in different electrolytes. b

In Figure 2 the pPy film at the beginning of the movement is reduced, which means shrunk. During oxidation by flow of a constant anodic current, anions start to penetrate into the film. The polypyrrole film swells, pushing the nonconducting tape and originating the uniform bending movement, the conducting film in the convex side. The continuous bending movement confirms the prevalent exchange of anions during the electrochemical reaction 1. After attaining the final position, the anodic current is stopped and the original position is recovered every time by flowing a cathodic current (promoting the polypyrrole reduction−shrinking) of the same magnitude during the same time. The procedure was repeated several times (consecutive square waves of current) in order to check the reproducibility of the movement. The full procedure was repeated now by flow of different anodic currents (i) during different times, t (keeping the charge, Q = it, constant) and using different salts of the same cation. Driving Specific Charge. Bending artificial muscles are faradic motors6−8 controlled by the specific charge (charge per

Electroactivity of the Active Masses. Before actuating experiments, the electroactivity of the immersed polypyrrole was characterized. Each artificial muscle was immersed in NaClO4 (reference solution) and submitted to five consecutive cyclic voltammograms between −0.7 and 0.5 V versus Ag/AgCl at 10 mV s−1 in order to get stationary voltammetric responses (Figure 3). By integration of the anodic and cathodic currents, the maximum charges involved in the oxidation or the reduction processes were obtained. The concomitant maximum specific (per unit of polymer mass) oxidation and reduction charges of the used actuators are also shown in Table 1. The artificial muscles consumed a specific charge of 38 ± 2 mC mg−1 (5% of mean variation). Pushing the actuation of the artificial muscles ahead this specific charge (anodic or cathodic) will produce overoxidation or partial degradation of the polypyrrole electroactivity. After any partial degradation, whatever the origin, the maximum specific charge and the maximum described angle decrease. 4095



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Figure 3. Stationary voltammograms obtained from a bilayer polypyrrole/tape after three consecutive potential cycles at 10 mV s−1 in 0.5 M NaClO4 aqueous solution. Oxidation charge (Qox) and reduction charge (Qred) can be obtained by integration of the voltammetric branches.

Figure 4. Angles described by a bilayer polypyrrole/tape as a function of the consumed specific charge (per milligram) by flow of different constant currents (0.25, 0.50, 0.75, 1.00, 1.25, 1.50, 1.75, 2.00, 2.25, and 2.50 mA) for different times (for a constant specific charge) through the artificial muscles in 0.5 M aqueous solutions of the studied electrolytes.

Movement Reproducibility. The reproducibility of the movement was checked by flowing a constant specific charge of 25 mC mg−1, by flow of a constant current of 1 mA during 25 s, through the muscles. The angular displacement generated by consumption of that charge was 24 ± 2° (8% of mean variation). After the experiments performed in one electrolyte, the bilayer was checked in the reference electrolyte by cyclic voltammetry. If both voltammetric response and described angle fit those from the control performed before starting experiments, the muscle was used for the study of a new electrolyte. When the difference (decrease by degradation) between both controls is greater than 5%, a new muscle was prepared and the reproducibility of its movements checked before starting the study of the new electrolyte. Every muscle has allowed the study of one or two different electrolytes.

anions (swelling process) for deeper oxidation processes.53−58 In that case lines from Figure 4 should be parabolas showing a minimum. For a specific electrolyte, eq 6 is kept whatever the electrolyte concentration (Supporting Information). By flow of the same constant charge through the muscle, the same constant angle is described (as corresponds to a faradic process following eq 6) whatever the electrolyte concentration.

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DISCUSSION Figure 4 shows that by flow of a constant specific charge across the muscle in electrolytes having different anions, the largest amplitude of the angular displacement takes place in NaNO3 aqueous solution. For a consumed specific charge of 50 mC mg−1 the sequence of the described angles is NaNO3> NaClO4> Na2HPO4> NaI> Na2S2O8 > Na3PO4> NaCl. The anions from the studied electrolytes have different charges and different ionic crystallographic radii. As an initial hypothesis it can be assumed that (a) the volume variation responsible for the movement during the reaction is only that required for lodging the balancing counterions inside the polypyrrole film; (b) the strain perpendicular to the surface is identical to that in the plane of the surface the last being the origin of the bending motion; and (c) the changes along the reaction in both Young’s modulus and stiffness of the polypyrrole and tape films are similar in every electrolyte. The volume variation due to the entrance of anions is given by eq 2. From eqs 2 and 6: zΔV 3 FzΔVox α = kQ ox = k = k′ 3 ox 3 4 NAπr r (7)

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RESULTS Angles are described by consumption of the same specific charge in aqueous solutions with different anions. Figure 4 shows the experimental results attained in different electrolytes. Data dispersion is obtained for the same described angle under flow of different currents (0.25, 0.50, 0.75, 1.00, 1.25, 1.50, 1.75, 2.00, 2.25, and 2.50 mA), keeping constant the consumed charge by varying the time of current flow. Those results corroborate that bending artificial muscles are faradic devices:6−8 a linear relationship exists between the specific charge Qox (C g−1) consumed during the movement and the described angle α (rad) (or between the applied current and the bending rate, see Supporting Information): α (rad) = k (rad C−1 g) Q ox (C g −1)

(6)

where k is a constant characteristic of every system (conducting polymer of the muscle and electrolyte). Every electrolyte gives a different slope, which means a different k from eq 6; see Table 1. Those linear relationships and similar results from the literature for anion or cation exchange polymers6−13 contradict results from the literature using thin films on metals in claiming the presence of prevalent exchange of cations (shrinking process) at the beginning of the oxidation (the anions should be trapped in the film) followed by a prevalent exchange of

and ΔVox =

1 αr 3 k′ z

(8)

where k′ = (3/4)(kF/(NAπ)). Equation 8 correlates for a constant consumed specific charge (Qox): volume variation (ΔVox), described angle (α), 4096



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Table 2. Relative Magnitudes (in 0.5 M aqueous solutions of the studied salts related to those from NaCl), Angles Described by the Muscle (Figure 4) for a Constant Specific Charge of 20 mC mg−1; and Crystallographic Radii and Crystallographic Volumesa salt NaNO3

NaClO4

Na2S2O8

NaI

Na2HPO4

Na3PO4

NaCl

9.1

4.0

2.7

3.0

3.4

1.6

1

r(anion) r(Cl−)

1.2

1.3

1.7

1.3

1.3

1.4

1

v(anion) zv(Cl−)

1.7

2.4

2.5

2.0

1.0

0.9

1

α(anion) α(Cl−)

a

In NaCl aqueous solution the muscle describes the lowest angle for the same driving charge: it was considered as reference.

Equation 9 links the volume variation (ΔV) during the oxidation of the electroactive polypyrrole film taking part of the muscle bilayer with the described angle (α) after flow of a constant oxidation charge (Qox), the crystallographic radius (r) of the anion, its charge (z), and the number of exchanged water molecules (n). Assuming again that the anion producing the smaller angle (Cl−) does not trail any water molecule, the number of water molecules incorporated to the polypyrrole film in parallel to every anion is calculated from eq 9:

crystallographic radius (r), and number of charges per anion (z). The number of charges per anion is one for NO3−, ClO4−, I−, or Cl−; two for S2O82− or HPO42−; and three for PO43−. Thus, for the same consumed charge the number of ions exchanged with the muscle is proportional to 1, 1/2, and 1/3, respectively. In order to compare the movement of the different artificial muscles, consuming the same specific charge in electrolytes containing anions with different number of charges, the described angle must be normalized by the number of charges per ion (valence). From Figure 3 the electrolyte giving the smallest angular displacement (NaCl) per consumed specific charge can be considered as a reference for the comparison of the involved magnitudes. Table 2 shows different magnitudes always related to those of Cl−: relative angular displacements by flow of a constant specific charge, α (anion)/α (Cl−); relative crystallographic radius,59 r (anion)/r (Cl−); and relative ionic volume per unit of charge, v (anion)/zv(Cl−). As expected, Table 2 allows the conclusion that a direct relationship between those magnitudes does not exist. The largest displacement of the muscle is obtained in NO3−, but the anion S2O82− has the greatest relative crystallographic radii and the greatest volume per unit of charge. Similar comparisons between anions provides a partial conclusion: despite the faradic nature of the devices described by Figure 4, the relative amplitude of the movement in the different electrolytes must include the volume and charge of the exchanged anions and some other component. Going back to reaction 1, solvent molecules are interchanged to keep chemical and physical balance. The number of molecules of solvent exchanged for every ion is unknown. Nevertheless, the volume variation must be a result of the addition of the volume variations produced by both the number of anions and the number of solvent molecules penetrating in the polypyrrole film during oxidation. The fraction of the volume variation during reaction due to the water exchange will be the number (unknown) of exchanged molecules (n) multiplied by the volume of a water molecule (VH2O) and the total volume can be expressed as ΔV = ΔVox + ΔVw =

1 αr 3 + nVH2O k′ z

n=

yVCl −

Vx z

VH2O

(10) −

where y = α (anion)/α (Cl ) is the angle described by the muscle in the studied salt related to that described in NaCl by flow of the same specific charge (Table 2, first row), VCl is the Cl− crystallographic volume, and Vx is the crystallographic volume of the anion x− (x being NO3−, ClO4−, S2O82−, I−, HPO42−, PO43−, or Cl−). Under the stated hypothesis, the results from Table 3 indicate a very low water exchange during the movement of the Table 3. Theoretical Number (n) of Solvent Molecules Exchanged during Actuation (Reaction) by the Polypyrrole Film of the Muscle in the Studied Electrolytes Assuming That No Water Molecules Are Exchanged in 0.5 M NaCl Aqueous Solution anion

water molecules/anion

water molecules/charge

NO3− ClO4− S2O82− −

4.91 1.07 0.16 0.71 3.22 1.47 0.0

4.91 1.07 0.08 0.71 1.61 0.49 0.0

I HPO42− PO43− Cl−

muscles in S2O82− or Cl−. Three molecules of water per anion are exchanged in PO43−. In HPO42− 3.25 molecule of water moves into the polypyrrole film at the same time that one anion. In I− two water molecules are exchanged altogether three anions; one water molecule per anion in ClO4− and five molecules of water are exchanged at the same time that one anions in NO3−. We can conclude that the apparent solvation numbers of the anions NO3−, ClO4−, S2O82−, I−, HPO42−, PO43−, or Cl−, obtained from the bending movements of artificial muscles in its sodium aqueous solutions during anodic reactions by flow of

(9)

The volume of 1 g of water at 25 °C is 1.0029607 cm3,59 so as 1 g of water is equivalent to 1/18 mol of water, and one mole of water contains 6.022 × 1023 molecules, the volume of a molecule of water is =2.9979 × 10−23 cm3. 4097



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on the chain result in 5 in (pPy+)n(NO3−)n(H2O)5n, 1.6 in (pPy2+)n(HPO42−)n(H2O)3.2n, 1 in (pPy+)n(ClO4−)n(H2O)n, 0.7 in (pPy+) n(I−) n(H2O)0.7n, 0.5 in (pPy3+) n(PO43−)n(H2O)1.5n, and almost imperceptible in (pPy2+)n(S2O82−)n or (pPy+)n(Cl−)n. Electro-chemo-mechanical artificial muscles are emerging as multifunctional actuators sensing chemical and physical ambient during actuation11,13,14 and are, in addition, a useful tool to quantify the relative number of molecules of solvent that are exchanged with the electrolyte during redox processes.

the same specific charge every time, are 5, 1, 0.1, 0.7, 3.22, 1.47, and 0.0, respectively. Those are apparent solvation numbers because they are the result of the variation of multiple intermolecular (polymer− polymer, polymer−solvent, and ion−solvent) interaction shifts taking place in the polymer film during the driving reaction 1. In this way each of the oxidized materials also could be considered as a reactive polyelectrolyte (polymer−anion) moving through n stoichiometric steps (1 to n counterions per chain) and the exchanged solvent as an apparent hydration number per unit of positive charge present on the oxidized chains of the film, as indicated in Table 3. The resulting apparent hydration numbers per unit of positive charge on the polypyrrole chain should be around 5 in (pPy+)n(NO3−)n(H2O)5n, 1.6 in (pPy2+)n(HPO42−)n(H2O)2.2n, 1 in (pPy+)n(ClO4−)n(H2O)n, 0.7 in (pPy+)n(I−)n(H2O)0.7n, 0.5 in (pPy3+)n(PO43−)n(H2O)1.5n, and almost imperceptible in (pPy2+)n(S2O82−)n or (pPy+)n(Cl−)n. The attained results constitute the base for the accumulation of experimental results for modeling the variation of the intermolecular forces (polymer−polymer, polymer−solvent, anion−polymer, anion−solvent, cation−polymer, and cation− solvent) inside the film along the reaction. With this aim different works are in progress: the study of the influence of new anions, the preparation of muscles exchanging cations and their quantification in electrolytes including different cations, and the influence of the solvent on the muscle movement. A higher apparent hydration number is observed when for the same consumed specific charge larger displacements are attained. Quite similar results were obtained in polypyrrole materials exchanging cations employing EQCM or forcedisplacement measurements.42,44,60

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ASSOCIATED CONTENT

* Supporting Information S

Angular rates measured from a bilayer polypyrrole/tape as a function of the specific applied current. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +34 968 32 54 33. Phone: +34 968 32 55 91. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge financial support from Spanish Government (MCI) Projects MAT2008-06702 and MAT2011-24973, Seneca Foundation Project 08684/PI/08, and ESNAM, the European Scientific Network for Artificial Muscles. J.G.M. acknowledges Spanish Education Ministry for a FPU grant (AP2010-3460).

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CONCLUSIONS Bilayer artificial muscles bending under flow of constant anodic currents in electrolytes containing different anions keeping constant the rest of experimental variables (temperature, electrolyte concentration, cation present in solution and solvent) are electrochemomechanical motors following linear evolutions of the described angle with the consumed specific charge. Keeping constant the driving specific charge (variable that controls the electrochemical process), a significant variation of the angle described by the muscle is observed in electrolytes containing different anions. Different described angles mean different volume variations in the polypyrrole film of the bilayer during the driving reaction. The volume variation during flow of an anodic charge is due to the entrance of anions and solvent (water) molecules. The volume variation due to the exchanged anions is calculated from the number of anions (determined from the consumed charge) and its crystallographic radius. The number of molecules of water were exchanged at the same time that one anion was determined. The apparent solvation numbers of the anions NO3−, ClO4−, S2O82−, I−, HPO42−, PO43−, or Cl− determined from the movement of the artificial muscles in its aqueous solutions related to that obtained in Cl− are 5, 1, 0.1, 0.7, 3.22, 1.47, and 0.0, respectively. The oxidized materials also can be considered as a nonstoichiometric polyelectrolyte (polymer−anion), and the solvent exchange also could be considered as an apparent hydration number per unit of charge present on the oxidized chains of the film. Those hydration numbers per unit of charge

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