PPy versus PEDOT versus PEDOT--PPy composites

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In search of better electroactive polymer actuator materials: PPy versus PEDOT versus PEDOT–PPy composites

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Smart Mater. Struct. 22 104006 (http://iopscience.iop.org/0964-1726/22/10/104006) View the table of contents for this issue, or go to the journal homepage for more

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IOP PUBLISHING

SMART MATERIALS AND STRUCTURES

Smart Mater. Struct. 22 (2013) 104006 (16pp)

doi:10.1088/0964-1726/22/10/104006

In search of better electroactive polymer actuator materials: PPy versus PEDOT versus PEDOT–PPy composites Rauno Temmer1 , Ali Maziz2 , Cédric Plesse2 , Alvo Aabloo1 , Frédéric Vidal2 and Tarmo Tamm1 1

IMS Lab, Institute of Technology, University of Tartu, Nooruse 1, 50411 Tartu, Estonia Laboratoire de Physicochimie des Polymères et des Interfaces, Institut des Matériaux, Université de Cergy-Pontoise, 5 mail Gay Lussac, Neuville sur Oise, F-95031 Cergy, France

2

E-mail: [email protected]

Received 30 November 2012, in final form 8 March 2013 Published 19 September 2013 Online at stacks.iop.org/SMS/22/104006 Abstract A comparative study of metal-free air-operated polypyrrole and PEDOT based trilayer actuators is presented. Actuators made of both pure and combined conducting polymers are considered. Trilayer bending actuators, synthesized in similar conditions, are characterized in terms of the structure, electrochemical and electro-chemo-mechanical properties. The characterization was carried out using two popular electrolytes: LiTFSI in propylene carbonate and a room-temperature ionic liquid EMIm TFSI. The results reveal that structure and actuation properties of the synthesized actuators depend on both the polymer chosen for the chemically synthesized electrode layer as well as the electrochemically synthesized working layer. (Some figures may appear in colour only in the online journal)

1. Introduction

to date have been focused on creating layered bending type actuators. During the electrochemical processes, the oxidation of the first electrode is concomitant with the reduction of the second electrode. The resulting electromechanical ECP deformations are opposite, which means that the expansion of one layer occurs at the same time as the contraction of the other layer. This is the principle of actuation of the most described concept, the so-called ‘trilayer’ device [1]. Polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) are among the most popular materials for preparing ECP-actuators. PPy is a well-studied actuator material, available for large-scale production. PPy actuators are known to produce large strains and speeds, acceptable for a number of applications. The most notable drawbacks are high rigidity, low ion diffusion speed and the risk of over-oxidation. PEDOT has received particular attention in recent years in the design of electrochemical actuators [2–4]. It has been described as a chemically and thermally stable polymer exhibiting a particularly high electrochemical stability [5–7] and good conductivity in the doped state. PEDOT has

In recent decades, the development of new materials and devices able to imitate the performance of natural muscles in the direct conversion of electric energy into mechanical energy has been a great challenge for the materials community. In this field, electronic conducting polymers (ECPs) have attracted great attention, notably because they have some important advantages over other electroactive polymer materials such as dielectric elastomers and ferroelectric polymers: for example, low driving voltage, inherent conductivity, catch-states, well-reproducible properties and, especially, the dimensional volume changes due to the ion and solvent expulsion/insertion during a reversible oxidation/reduction process. For an ECP based electrochemical actuator operating in air, as for every electrochemical device able to work in the open air, the presence of two electroactive electrodes and an ion reservoir from which to draw ions is essential. Most efforts 0964-1726/13/104006+16$33.00

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c 2013 IOP Publishing Ltd Printed in the UK & the USA

Smart Mater. Struct. 22 (2013) 104006

R Temmer et al

Table 1. Designations of the different combinations of PPy and PEDOT used for preparation of electrodes.

been used in aqueous electrolyte and cation-mobile actuators before to compensate for the lower conductivity of PPy electrodes [8]. It has been shown that PEDOT, as a softer material with higher conductivity, can reduce IR losses along the actuator and introduce higher porosity, allowing better ion movement. No such studies for air-operated actuators have been reported. In most cases, electrochemical deposition is the favoured means for obtaining the working material, due to the much more controllable polymer formation and achievable layer quality. However, therein lies the problem, as the separator membrane has to be electronically non-conductive while the electrodeposition requires an electrode. Separate layer deposition followed by attachment to the membrane or metal coating (sputtering, vacuum evaporation) have been used as solutions. The former is troubled by delamination [9, 10] and the latter hindered by manufacturing costs and cumbersomeness—and sometimes delamination. Since both PPy and PEDOT appear to have potential advantages as actuator materials, it is of interest to directly compare the corresponding air-operated actuators. In addition, one of the goals of the present work is to consider the benefits of combining these polymers together as composites. Metal-free and air-operated actuators are prepared using combined chemical and electrochemical polymerization of PPy and PEDOT, both separately and together as combinations. In all cases, the synthesis process starts with the chemical polymerization in/on the surface layer of the membrane material (polyvinylidene fluoride—PVdF), which is then followed by the electrochemical deposition of additional (same or different) ECP working material. The mobile species for the actuation came from two different types of electrolytes: a room-temperature ionic liquid and lithium bis(trifluoromethanesulfonyl)imide solution in propylene carbonate. The materials and the obtained actuators are characterized under the same conditions (strain difference, strain difference rate, blocking force, etc)—it is the first consistent comparative study of actuators made of these materials (or any two different ECP materials) for air operation.

Conductive electrode

Chemical synthesis

Electrochemical synthesis

PPy/PPy PEDOT/PEDOT PEDOT/PPy PPy/PEDOT PEDOT-FSa PPy-FSa

PPy PEDOT PEDOT PPy n/a n/a

PPy PEDOT PPy PEDOT PEDOT PPy

a

Freestanding film used for reference.

were degassed in an ultrasonic bath and saturated with Ar before use. Commercial 110 µm thick PVdF membranes (Immobilon -P, Millipore, according to product specification: hydrophilic, porosity 70%, pore size 0.1 µm) were used as the electronic separator and electrolyte storage material. 2.2. Preparation of actuators The methodology for the preparation of metal-free airoperated conducting polymer actuators has been described previously [11]. A two-step chemical–electrochemical combined synthesis method was used to prepare the electrode material. The chemically synthesized under-layer forms a conductive electrode surface, allowing the following well-controllable electrochemical deposition of the main working layer of the conducting polymer material while also ensuring good adhesion between the PVdF membrane and the conducting polymers. This synthetic pathway ensures highly electroactive ECP layers with a non-homogeneous distribution of ECP throughout the thickness of the actuator; and the amount of ECP decreases sharply from the outside towards the centre of the film, leading to a trilayer configuration. Combinations of PPy and PEDOT used for the preparation of the actuators are summarized in table 1. 2.2.1. Chemical synthesis. For both conducting polymers, similar oxidative chemical polymerization was carried out simultaneously on both sides of the membrane. Synthesis conditions were optimized to meet two requirements: obtaining sufficiently high electronic conductivity along the surface for subsequent electrochemical polymerization while avoiding the formation of short circuits between the two sides. The PVdF membrane was permeated with pure monomer, the surface was wiped dry with filter paper until no wet reflection. The permeated films were immersed into a hot (60 ◦ C) aqueous oxidant solution (0.075 M APS solution for Py and 1.5 M FeCl3 solution for EDOT) until the membrane had turned dark, which took 15 s for Py and 120 s for EDOT, respectively. The membrane was then washed with cold methanol between fingers in order to terminate the polymerization and to remove the detached remains of the polymer deposit. The obtained conductive membrane was then rinsed repeatedly with methanol and water and dried at 40 ◦ C for 1 h in a vacuum oven.

2. Experimental details 2.1. Materials Pyrrole (Py, Sigma-Aldrich) and 3,4-ethylenedioxythiophene (EDOT, H C Starck) were freshly distilled at reduced pressure and stored in the dark under inert atmosphere at low temperature. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, ≥99.0%, Sigma-Aldrich) was dried under vacuum at 160 ◦ C for 12 h before use. 4-methyl-1,3-dioxolan-2-one (propylene carbonate, PC, ≥99%, Fluka), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIm TFSI, 99.5%, Chemin de la Loge), ammonium persulfate (APS, (NH4 )2 S2 O8 , ≥98.0%, Sigma-Aldrich), and anhydrous iron trichloride (FeCl3 , 95–100%, Fisher Scientific) were used as received. Ultra-pure MilliQ+ water was used. All solutions 2


R Temmer et al

σq is synthesis charge density (C m−2 ) and F is the Faraday constant. According to John [14], such concentrations yield improved results for TFSI− -doped actuators, as compared to those with less concentrated electrolytes. In order to avoid problems with solvent evaporation, alteration of the solvation, and also in order to obtain higher ionic conductivity, ionic liquids can be used as electrolytes [15, 16]. Among them, EMIm TFSI is a popular candidate due to its stability, high ionic conductivity, non-volatility, hydrophobicity, and low viscosity [17].

Figure 1. Images of space-filling models of EMIm+ , Li+ solvated with four PCs, and TFSI− , respectively.

2.3. Characterization

2.2.2. Electrochemical synthesis and actuator preparation. In order to obtain high-quality ECP electrodes, the electropolymerization was carried out galvanostatically (controlled by PARSTAT 2273) in a one-compartment two-electrode electrochemical cell at a current density 0.1 mA cm−2 and temperature −31.5 ± 1.5 ◦ C for 20 000 s until a total charge 2.0 C cm−2 was consumed. Polymers were deposited on both sides of 30 mm × 30 mm membrane sheets using the chemically synthesized coating as the anode (except for freestanding films, where a AISI316L stainless steel (SS) anode of the same size was used), and parallel SS mesh sheets as cathodes. A synthesis solution with 0.2 M monomer (Py or EDOT) and 0.2 M LiTFSI in PC, with 2 vol% water to facilitate the transport of protons away from the working electrodes was used. After polymerization, the resulting films were washed with methanol and dried (weighted down) in vacuum oven. The edges of the bulk film were then trimmed off, and bending actuators were prepared by cutting the remainder into 4 × 20 mm stripes. Prior to electro-chemo-mechanical characterization, the actuators were immersed in electrolyte (1 M LiTFSI in PC or EMIm TFSI) for at least 48 h. The electrolytes were chosen based on their popularity for actuator applications in the literature, with the added benefit of comparing a non-aqueous solution to a room-temperature ionic liquid. Possessing a common anion, these electrolytes allow the effect of different cation sizes to be investigated. Space-filling model images of corresponding ions are depicted in figure 1. LiTFSI has become a popular electrolyte because of good solubility in many solvents and electrochemical stability. Due to the relatively large anion, TFSI− -doped PPy (PPy/TFSI− ) actuators have showed good performance in LiTFSI-PC electrolyte (Hara, [12]). Li+ ions in propylene carbonate are coordinated with PC molecules (coordination number of the first solvation shell 4.5, [13]); therefore, the mobility of TFSI− dominates. The LiTFSI concentration of 1.0 M was chosen to be about three times higher than the minimum concentration needed to compensate the charge for one fully oxidized electrode at the expected maximum doping level 0.33, calculated (assuming 100% synthesis efficiency) as: c=

D × σq , p × h × (D + 2) × F

For all characterization experiments, separate samples were used in order to minimize the influence of the order of experiments. 2.3.1. Scanning electron microscopy and energy dispersive x-ray spectroscopy. Scanning electron microscopy (SEM) of the actuators was performed using a Hitachi TM3000 (acceleration voltage 15 kV, back-scattered electron detector) and CARLZEISS AG-ULTRA 55 GEMINI (15 kV, secondary electron detector). The distribution of sulfur along the cross-section (broken under liquid nitrogen) of the membrane was mapped on the micrographs by means of an energy dispersive x-ray spectrometer (EDX) (Hitachi TM3000 equipped with SwiftED 3000, accelerating voltage 15 kV). 2.3.2. Electronic and ionic conductivity. In order to compare the electronic conductivities of electrodes made of different polymers on different substrates with different spatial distributions in a consistent manner, using familiar bulk material conductivity measurement methods and units comparable to the literature, the effective thickness and effective conductivity terms are introduced. Effective thickness is defined here as the thickness of the ECP material, assuming 100% synthesis efficiency and uniform density (depends only on polymer type and synthesis charge). The effective thickness is equal to the real thickness (and the effective electronic conductivity equal to real electronic conductivity of the polymer) if the conductivity of the polymer is measured on a non-conductive substrate and its spatial distribution and composition are uniform. The effective electronic conductivity of electrochemical ECP layers in the dry state was measured along one side of the actuator with an in-house 4-point probe, according to Smits equation [18]: σe = I × (4.532 × w × V)−1 ,

(2)

where σe is the effective electrical conductivity, w is the effective thickness of the electrochemically synthesized electrode layer, I is the applied constant current between the outer contacts of the probe, and V is the measured voltage between the inner contacts. The ionic conductivity was measured by means of impedance spectroscopy using a VSP potentiostat (Biologic SA). The experiments were performed in a

(1)

where c is the concentration, D is the doping level, p is the porosity of the membrane, h is the thickness of the membrane, 3


R Temmer et al

temperature range from 20 to 100 ◦ C in steps of 10 ◦ C, in the frequency range of 0.01 Hz–100 MHz. PVdF membranes, permeated with electrolyte were fixed in a cell with pressure contact SS electrodes. The ionic conductivity σi is calculated using the equation [19]: σi =

h , Z×S

micrometre after swelling in electrolyte), and L is the distance from the fixed end of the actuator to the projection of the laser beam to the plane parallel to the middle position of the actuator. A linear approximation is applicable in the low-voltage region to the functions of displacement [14, 20, 23], and blocking force [24] on voltage, enabling the representation of the results in units of % V−1 and mN V−1 , respectively. These values are calculated from the frequency-wise division of the measured responses’ Fourier transform by the Fourier transform of the exciting voltage [14, 20, 21]. Assuming uniform curvature of the actuator, the strain difference rate εr is calculated analogously to Madden et al [25] as an average over a full bending cycle:

(3)

where Z is the real part of the complex impedance, h is the thickness of the sample, and S is the sample area. 2.3.3. Electrochemical measurements. Cyclic voltammetry was carried out using a PARSTAT 2273 potentiostat. A two-electrode configuration was adopted, in which the two surfaces of the chemically or chemically–electrochemically coated membrane (clamped between gold plates) were the electrodes. In order to exclude the effect of evaporation of solvent, the electrochemical measurements were only performed using EMIm TFSI as the electrolyte. The calculation of the doping levels was based on the weight measurements, assuming 100% efficiency of the synthesis, and the following relations. The total amount of the oxidized repeating units in polymer after synthesis (mol): Pmol =

Qs , F × (D + 2)

εr = 4 × ε × f ([11]),

where ε is the strain difference and f is the frequency of applied voltage and the constant four is the number of movements (passages) between neutral and extreme position during the full cycle corresponding to frequency f . For step response, displacement frequency response, and blocking force frequency dependence, simultaneous measurements of voltage, current, and displacement or blocking force were performed using an in-house setup consisting of a National Instruments PCI-6036E analog input DAQ, laser displacement meter LK-G82/LK-G3001P (Keyence) and isometric force transducer MLT0202 (ADInstruments).

(4)

where Qs is the total synthesis charge, F is Faraday constant and D is the doping level. The total mass of the synthesized polymer film, where charge is compensated by dopant is then: m = Pmol × (MECP + D × Mdopant ),

3. Results and discussion

(5)

3.1. Membrane properties and ionic conductivity

where m is the mass of the film, MECP is the molar mass of a single PEDOT or PPy repeating unit (monomer—2H), and Mdopant is the molar mass of the dopant anion (TFSI− ). Inserting equation (4) into equation (5) and solving for D yields: D=

2 × F × m − Qs × MECP . Qs × Mdopant − F × m

Ions are an essential factor influencing virtually every aspect of the actuator performance. To characterize the membrane/electrolyte system used for supplying ions to both of the studied conducting polymers, ionic conductivity measurements were carried out using equation (3). At RT, the ionic conductivity of the membrane, permeated with 1 M LiTFSI in PC, was found to be 5.37 × 10−4 S cm−1 and 7.83 × 10−4 if permeated with EMIm TFSI. As in case of ion gels [26] and pure EMIm TFSI ionic liquid [27], the curve of EMIm TFSI (figure 2) in macroporous membrane fits well with Vogel–Fulcher–Tammann (VFT) equation [26]:

(6)

2.3.4. Electro-chemo-mechanical characterization. For the measurement of all electro-chemo-mechanical parameters, the actuators were mounted side-ways (2 mm clamped, free length 18 mm) between flat gold contacts. Displacement and blocking force were measured at 5 mm distance from the clamped end. In order to minimize the effect of solvent evaporation during measurements, the measurement duration was reduced significantly by evaluating the dynamics of the actuator according to John et al [14, 20] and Pillai [21]. The actuators were excited with a mirrored logarithmic sweep sine signal (0.001–65 Hz, 0.8 V, duration 524 s). From the resulting displacement signal, strain difference between the electrodes was calculated according to Sugino et al [22]: ε=

2×d×h , L2 + d 2

(8)

σi = σ0 × exp [(−B/(T − T0 ))] ,

(9)

where σ0 (S cm−1 ), B (K) and T0 (K) are constants. Table 2 summarizes the fitted parameters of the VFT empirical model. Different behaviour of the 1 M LiTFSI electrolyte can be explained by the Li+ dominated ion transport (due to higher diffusion speed as compared to TFSI− ) influenced in turn by the relatively stable PC solvation shell. The sharp increase of ionic conductivity beyond 30 ◦ C in the case of LiTFSI can be explained by the jump in Li+ diffusion coefficient (in PC solutions) starting from around 298–310 K (25–37 ◦ C) [28, 29]. The rapid diffusion coefficient transition close to room temperature together with heat dissipation during actuation may, therefore, have a rather negative impact

(7)

where ε is the strain difference, d is half of the peak to peak displacement, h is the thickness of the actuator (measured with 4


R Temmer et al

After the chemical synthesis of PPy (figure 3(b)), the surface of the membrane appears covered by a dense and rough layer, while the chemical PEDOT structure (figure 3(c)) is quite different, with a globular structure much less uniform than PPy. It is important to mention that no measurable thickness increase of the membrane was observed after the Py or EDOT chemical polymerization step. This means that the porous nature and the void space observed between the globular structures of PEDOT corresponds to a structure opened in the PVdF pores. This difference between the PPy and PEDOT chemical synthesis step can be related to the different solubility of the monomers in water—pyrrole is more soluble in water than EDOT and diffuses out of the pores more quickly than EDOT. Then PPy deposits to the membrane from the outside, while EDOT polymerizes inside the outer layer of the porous membrane, thus hindering the penetration of the oxidant deeper into the membrane. The latter is also indicated by the much smaller tendency of PEDOT (as compared to PPy) to occasionally develop a few localized short circuits between opposite sides of the membrane during chemical synthesis. This structural difference between the two polymers after the chemical synthesis step helps to follow the incorporation of both monomers to the structure during the subsequent electropolymerization. The electrochemically synthesized layer makes up for most of the ECP mass, and it has a much smoother surface (figures 3(d) and (e)). For both the electrochemical PPy and PEDOT, a globular structure resulting from a three-dimensional nucleation growth mechanism is observed and shows clusters of globules. The morphology of PPy is still different from PEDOT—PPy forms a very uniform and compact globular surface (globule