Materials Today Communications 23 (2020) 100908 Contents lists available at ScienceDirect Materials Today Communications journal homepage: www.elsevier.com/locate/mtcomm Role of polyethylene oxide content in polypyrrole linear actuators a,b a,b c d Roshan Khadka , Peikai Zhang , Ngoc Tuan Nguyen , Tarmo Tamm , Jadranka Travas-Sejdica,b, Toribio F. Oteroe, Rudolf Kieferf,* T a Polymer Electronics Research Center, School of Chemical Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand The MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington 6140, New Zealand Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, 700000 Viet Nam d IMS lab, Institute of Technology, University of Tartu, Nooruse 1, 50441 Tartu, Estonia e Centre for Electrochemistry and Intelligent Materials (CEMI), Universidad Politécnica de Cartagena, Aulario II, Paseo Alfonso XIII, E-30203, CartagenaMurcia, Spain f Conducting polymers in composites and applications Research Group, Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Viet Nam b c A R T I C LE I N FO A B S T R A C T Keywords: PPy-PEO/DBS composites EIS mSICM ECMD Actuator Current research on conducting polymer composite actuators has been mainly focused on achieving increased linear actuation with improved electrical, physical and chemical properties. Polyethylene oxide (PEO) has been shown to enhance both mechanical and electrical properties of conducting polymers at certain concentrations. However, as some of these effects peak off, the optimal concentration is difficult to determine. In this study, polypyrrole (PPy)-PEO composite films, doped with dodecylbenzene sulfonate, were electropolymerized from solutions with different concentrations of PEO. The obtained films were studied by atomic force microscopy (AFM), scanning electron microscopy (SEM), Raman, FTIR and energy dispersive X-ray spectroscopy (EDX). Electrochemical impedance spectroscopy (EIS) and modified scanning ionic conductance microscopy (mSICM) allowed the determination of electronic and ionic conductivities of the samples. Their electro-chemo-mechanical deformations (ECMD) were investigated under cyclic voltammetry. In aqueous electrolyte, the samples showed expansion/contraction during reduction/oxidation, respectively (cation-driven), while opposite behaviour expansion/contraction during oxidation/reduction (anion-driven) behaviour was observed in propylene carbonate solutions. These films obtained from solutions with a PEO content of 5% showed the highest deformations (strain and stress). They also presented the highest ionic and electronic conductivities and redox charge density. The ECMD deformation per unit of redox charge was much higher in PC solutions than in aqueous solutions: more PC molecules are exchanged for osmotic balance per unit of redox charge. 1. Introduction Under electrochemical oxidation/reduction in liquid electrolytes, films of conducting polymers become dense polymer gels constituted by electrochemical molecular machines, ions and solvent [1]: a model material of the intracellular mater from muscle cells [2]. The synthesis and electrochemical characterization of new conducting polymers is advancing both, our basic knowledge about the behaviour of bio-replicating reactive materials and the development of soft, wet and reactive actuators or artificial muscles suitable to construct more intelligent, efficient and compliant tools and robots. As a feedback, they provide new quantitative hypotheses about how muscles from coldblooded animals may work, how the fatigue state may increase during the muscle work, how the nerve pulses with different information packages may be generated from the muscle to inform the brain or why ⁎ Corresponding author. E-mail address: rudolf.kiefer@tdtu.edu.vn (R. Kiefer). https://doi.org/10.1016/j.mtcomm.2020.100908 Received 12 September 2019; Accepted 7 January 2020 Available online 08 January 2020 2352-4928/ © 2020 Elsevier Ltd. All rights reserved. most of the biological functions are asymmetric (work in only one way) [3]. Linear actuators or linear artificial muscles based on the reversible expansion/contraction of conducting polymer films under electrochemical control in liquid electrolytes have been extensively studied [4]. In the past, one of the main goals was to improve the attained electro-chemo-mechanical strains and stresses using different conducting polymers each synthesised and controlled in different organic or aqueous electrolytes. Maximum strain and stress were reported for PPy/TFSI films in the range of 26% [5] and 49 MPa [6], respectively. Most of the linear actuators expand during oxidation (anion-driven) while some special cases such as PPy/DBS films expand on reduction and contract on oxidation (cation-driven) [7]. A maximum strain (10–12 %) was reported for that polymer during the first potential cycle in aqueous electrolyte with exchange of cations [8], decreasing very Materials Today Communications 23 (2020) 100908 R. Khadka, et al. % and 20 wt%) in 1:1 ratio of EG: water solution. The deposited films were peeled off from the stainless-steel working electrode, washed in ethanol to remove excess of pyrrole monomer and in Milli-Q to remove NaDBS residues. The films were dried in a vacuum oven (40 °C, 2 mbar, 24 h). fast during subsequent cycles. Cation-driven systems can often be desirable, as the material is less prone to collapse and loss or activity, especially at significant reduction levels. Recently it has been reported that PPy/DBS films can expand during oxidation (anion-driven) in an organic electrolyte due to un-dissociated cation-DBS pairs [9]. By including a new component during the electrogeneration of PPy/ DBS, such as carbon based material [10], polyoxometalates [11], the stability of the attained linear strain in aqueous electrolyte can be increased [12]. The PEO content increases the ionic conductivity of its polymer composites [13,14]. This fact is being applied to develop polymer lithium batteries [15], ion conductive membranes [16], composites of conducting polymers with rising ionic conductivities [17] and ionic transport between the conducting polymer layers in a trilayer actuator [18,19]. Recent research has revealed that changing the solvent in PPy-PEO (10 wt.%)/DBS films from initially aqueous to aprotic can enhance the ionic conductivity in the aprotic solvent such as propylene carbonate [20]. The effect of PEO concentration for an electro-chemo-mechanically active composite with PPy has, so far, not been clearly established, as there are several – potentially contradicting – aspects to consider. In addition to the enhanced ion-conductivity, one should take into account the potential plasticizing effect as well as the reduced content of electroactive material inside a composite film. Moreover, changing the composition of the synthesis bath may have an effect on the electropolymerization process and the structure of the resulting material. Here we present the electrochemical synthesis of PPy-PEO/DBS films from aqueous solutions of pyrrole, DBS and different weight fractions of PEO (5 wt.%, 10 wt.%, 15 wt.% and 20 wt.%). The electrochemical and electro-chemo-mechanical characterization of the generated composite films was performed in bis(trifluoromethane) sulfonamide lithium aqueous solutions (LiTFSI-aq) and propylene carbonate solutions (LiTFSI-PC). AFM and SEM measurements were carried out to characterize the morphology of these composite films. Modified scanning ionic conductance microscopy (SICM) measurements were performed on the surface of PPy-PEO/DBS films to evaluate the charging/discharging behaviors in micro drops. Raman, FTIR and energy dispersive X-ray (EDX) spectroscopy of PPy-PEO/DBS films were executed to characterize the physicochemical properties of the composite films. The effect of PEO was also investigated by means of EIS measurements to evaluate the change in ionic conductivities of the films. Cyclic voltammetry measurements were conducted in parallel to ECMD measurements to obtain deeper insight into the actuation response. 2.3. Linear actuation studies PPy-PEO/DBS films were cut to strips of 15 mm x 1 mm. The strips were connected to the force sensor (TRI202PAD, Panlab) with a fixed arm that served as a working electrode in a linear muscle analyzer set up [21]. Before and after actuation cycles, the elasticity modulus was determined by moving the linear actuation stage for 1 μm recording the response in a certain amount of force (mg). A platinum sheet was used as the counter electrode and Ag/AgCl (3 M KCl, 0.23 V vs SHE) as the reference electrode in the measurement cell containing 0.2 M solution of LiTFSI in aqueous (LiTFSI-aq) and propylene carbonate (LiTFSI-PC) solutions. The initial length of the films between the clamps was 1 mm. The films were immersed in one of the electrolytes, pre-stretched in range of 4% (equivalent to 1000 mg) and left immersed in the electrolyte for 4 h before the test. Then the film was submitted to consecutive potential cycles between 0.65 V to -0.6 V at a scan rate of 5 mV s−1. The film length changes (isotonic: constant force of 9.8 mN) and force change (isometric, length between clamps 1 mm) were followed under the applied electrical signal with in-house software [21]. The strain ε was obtained from the formula ε (%) = ΔL/L*100%, where ΔL = L – L1 is the length difference, with L being the initial film length and L1 the length obtained under the isotonic ECMD measurements. 2.4. Material and electrochemical characterization The surface morphology and the composition of the synthesized PPy-PEO/DBS films were analyzed by scanning electron microscopy (Hitachi TM-3000) and energy dispersive X-ray spectroscopy (Helios NanoLab 600, FEI). The surface morphology and roughness of the films were studied by atomic force microscopy (MFP-3D Origin). Modified SICM measurements of PPy-PEO/DBS film surfaces at applied potential range 0.65 to -0.6 V (square wave potential steps at frequency 0.0083 Hz) in LiTFSI-aq and LiTFSI-PC were performed. Single-barrel, micro pipettes with tip diameters of 5−10 μm were fabricated using a Sutter Instruments P-2000 laser puller from 100 mm long, 2 mm outside diameter borosilicate glass capillaries (Harvard Apparatus product number 30-0117). The working electrode (WE) was directly connected to the polymer in a three electrode cell setup, with a platinum wire as the counter electrode (CE) inside the micro pipette and an Ag/AgCl wire as the reference electrode connected to the samples. The conductivity of the film was determined by four-point probe method with a surface resistivity meter (Guardian SRM). Electrochemical impedance spectroscopy (EIS) of PPy-PEO/DBS in dry condition, in LiTFSI-aq and LiTFSI-PC electrolytes were performed using PARSTAT 2273 instrument. Raman spectroscopy (Renishaw System 1000 microprobe, 785 nm excitation line) and Fourier transform infrared spectroscopy (Nicolet 8700, Ge crystal) were made from PPy-PEO/DBS films after electropolymerization. 2. Materials and methods 2.1. Materials Sodium dodecylbenzenesulfonate (NaDBS, 99%), poly (ethylene oxide) (PEO, average Mw ca. 100,000 g/mol), ethylene glycol (EG, 99.8%), propylene carbonate (PC, 99%) and bis(trifluoromethane) sulfonamide lithium salt (LiTFSI, 99%) were purchased from SigmaAldrich and used as received. Pyrrole (Py, ≥ 98%) from Sigma-Aldrich was distilled at reduced pressure prior use and stored at low temperature under Ar atmosphere. Deionized water (18.2 MΩ cm at 25 °C, MilliQ+) was used to prepare the aqueous solutions. 3. Results and discussion 2.2. Electropolymerization 3.1. Electro-polymerization of PPy-PEO/DBS films Galvanostatic polymerization (0.1 mA cm−2, 40,000 s, −18 °C) was applied in a three electrode compartment using stainless steel (6 cm2) as working electrode, platinum woven mesh/gauze (25 × 25 mm) as counter electrode and Ag/AgCl wire as reference electrode (0.52 V vs. SHE) controlled by a CH Instruments electrochemical workstation (model CHI440). The monomer solution consisted of 0.2 M pyrrole, 0.2 M NaDBS and different PEO weight fractions (5 wt%, 10 wt%, 15 wt The chronopotentiometric responses recorded during the polymerization processes and the EDX spectroscopy data of the attained PPy-PEO/DBS films, prepared from solutions with different amounts of PEO (5 wt.%, 10 wt.%, 15 wt.% and 20 wt.%) are shown in Fig. 1. The electropolymerization responses presented in Fig. 1a indicate that the deposition potential of PPy-PEO/DBS films decreases with increasing PEO content in the solution. The most significant drop was 2 Materials Today Communications 23 (2020) 100908 R. Khadka, et al. Fig. 1. a Chronopotentiograms from the galvanostatic polymerization (0.1 mA cm−2, 40,000 s at −18 °C) for PPy-PEO/DBS with different amounts of PEO in the polymerisation solution (5 wt% black line, 10 wt% red dotted, 15 wt% green dashed and 20 wt% blue dash dot) in a three electrode cell (Ag/AgCl wire reference, stainless steel working electrode and platinum gauze counter electrode). b: EDX spectra of PPy-PEO/DBS films with different amounts of PEO (5 wt% black line, 10 wt% red dotted, 15 wt% green dashed and 20 wt% blue dash dot).The films were measured after the electropolymerization (in oxidized state). Inset: the oxygen peak. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). on the surface morphology and roughness of the synthesised PPy-PEO/ DBS films. Figs. 2a-d present the attained results. The related SEM images of the film surfaces are shown in Figure S1a-d. From the AFM images and the surface roughness profiles in Fig. 2ad, a general trend can be observed: the surface roughness increases with the increasing fraction of PEO in the PPy-PEO/DBS films. It is also noted that the typical ‘cauliflower’ morphology obtains larger structures (more swollen) with increasing PEO content. The SEM surface images of PPy-PEO/DBS films (Supporting Information, Figure S1a-d) also show more prominent and larger cauliflower structures [24] with the concomitant increase of the surface roughness for higher PEO fractions. Table 1 shows the electronic conductivities of PPy-PEO/DBS films determined as indicated above from the dry films or in aqueous and PC solutions. between 5 and 10% PEO. The effect of the increased PEO content in the solution could be related to the increase of both, the solution ionic conductivity [22] and the film ionic conductivity. The EDX spectra in Fig. 1b show the typical signals of carbon (C) at 0.27 keV, oxygen (O) at 0.52 keV, sodium (Na) at 1.04 keV and sulfur (S) at 2.32 keV. Higher loads of PEO in PPy-PEO/DBS films lead to an increase in the carbon, oxygen and sulfur peaks, as expected. Fig. 1b inset presents the increasing content of oxygen, related to incorporation of increasing amounts of PEO into the PPy film [23]. The sodium peak found in all of the samples indicates the presence of residual electrolyte from the polymerization. 3.1.1. Surface characterization AFM images were obtained to evaluate the effect of the PEO content Fig. 2. The AFM height images (scale 4 μm), 3D images showing surface morphology and the roughness profiles of PPy-PEO/DBS films prepared with a: PEO 5 wt.%, b: PEO 10 wt.%, c: PEO 15 wt.% and d: PEO 20 wt.%. 3 Materials Today Communications 23 (2020) 100908 R. Khadka, et al. Table 1 Electronic conductivities and thickness before (dry state) and after actuation (200 cycles) of PPy-PEO/DBS films (oxidized state) in LiTFSI-PC and LiTFSI-PC electrolytes. Linear PPy-PEO/DBS films 5 wt.% PEO 10 wt.% PEO 15 wt.% PEO 20 wt.% PEO Conductivities [S cm−1] and thickness [μm] Thickness After actuation in LiTFSI-aq After actuation in LiTFSI-PC 34 38 36 35 16 ± 1.4 (40 ± 2 μm) 11.8 ± 1.1 (42 ± 3 μm) 7 ± 0.6 (44 ± 4 μm) 6.6 ± 0.5 (38 ± 3 μm) 19.1 ± 1.1 (39 ± 3 μm) 14 ± 1.3 (41 ± 4 μm) 9.3 ± 0.8 (42 ± 3 μm) 8.4 ± 0.7 (37 ± 3 μm) ± ± ± ± 2 μm 1 μm 3 μm 2 μm the ratio found 0.984 for PPy-PEO(5 wt.%)/DBS, 0.999 for PPy-PEO (10 wt.%)/DBS, 1.001 for PPy-PEO(15 wt.%)/DBS and 0.9 for PPy-PEO (20 wt.%)/DBS films. From the interpretation of these ratios, a clear trend can be observed that the content of dications (bipolaron) increased with increased PEO content until 15 wt.% while a strong decrease was found for 20 wt.% PEO. A similar relation between the peaks can be observed for the 957 cm−1 (ring deformation associated with radical cations) and 926 cm−1 (ring deformation associated with dications), where the ratios showed the same tendencies. The shoulder at 854 cm−1 was associated to −CH2 rocking band and CO stretching motions of PEO [29]. The electronic conductivity (Table 1) decreases with increasing PEO film content, the highest conductivity, 16 ± 1.4 S cm−1 and 19.1 ± 1.1 S cm−1, were attained from the PPy-PEO(5 wt.%)/DBS film measured in LiTFSI-aq and LiTFSI-PC electrolytes, respectively. In general, all films had higher conductivity in the organic electrolyte, agreeing to previous research [20]. 3.1.2. Characterization by Raman and FTIR spectroscopy To analyse the chemical structure of the PPy-PEO/DBS films, FTIR and Raman spectra of PPy-PEO/DBS samples (oxidized state) were obtained, and are presented in Fig. 3a and b, respectively. The FTIR spectra of PPy-PEO/DBS films (Fig. 3a) show a broad peak at 3400 cm−1 that refers to NeH stretching [24] with double peaks at 2925 cm−1 and 2850 cm−1, which correspond to aliphatic −CH3 and −CH2 groups of immobilized sulfonate anions of DBS-, the presence of the latter also indicated by the SO2 stretching vibration at 1640 cm-1 [24]. The typical PPy ring vibrations can be found at 1550 – 1527 cm−1 and 1450 cm-1 as in previous research [23]. The absorption of the polaron/bipolaron bands are found at 1034 cm−1 – 1190 cm−1 (here 1180 cm−1) and 900 cm-1 [25]. The 900 cm−1 line couldn’t be identified in PPy-PEO (20 wt%)/DBS films. In previous work [14], PEO and PEO-AgCF3SO3 films investigated using FTIR confirmed PEO with typical CeOeC stretching vibration (1145 cm−1, 1095 cm−1 and 1059 cm−1) with strongest signal at 1095 cm−1, here observed as a small shoulder for PPy-PEO(5–15 wt.%)/DBS (Fig. 3a) but absent in PPy-PEO(20 wt.%)/DBS films. The intensity of the 960 cm−1 peak representing CeOeC vibration mode [14], decreased from PEO 5 wt% to PEO 15 wt.%. Interestingly, the PPy-PEO(20 wt.%)/DBS film made with the highest amount of PEO in the electropolymerization solution showed the least signal strength of PEO in the FTIR studies. The Raman shifts shown in Fig. 3b of PPy-PEO/DBS composites present the PPy C]C backbone stretching [26] located at 1570 cm−1 indicating the oxidation of PPy-PEO/DBS composite films. The peak at 1483 cm-1 [26] is assigned to the skeletal band and the double peaks at 1310 cm−1 and 1380 cm−1 describe the CN stretching mode of PPy [27]. The 1234 cm−1 broad peak represents the C–H in plane bending of PPy [28]. The double peaks at 1044 cm−1 (CeH in-plane bending, associated with radical cations) and 1074 cm−1 (CeH in-plane bending associated with dications) are assigned to the polaron/bipolaron content [27]. Taking their ratio gives information about the doping state, 3.1.3. Characterization by Electrochemical impedance spectroscopy (EIS) The ionic conductivities of PPy-PEO/DBS films generated from different PEO (5 wt.%, 10 wt.%, 15 wt.% and 20 wt.%) content in the polymerization solution were characterized by EIS as dry films or in LiTFSI-aq and LiTFSI-PC electrolytes. Figs. 4a-c present the attained results. From the Nyquist plots presented in Fig. 4a-c and using the Randles equivalent circuit, the charge transfer resistance (RCT equivalent to Zre) can be obtained [30] (Fig.S2). Perhaps somewhat surprisingly, the resistivity of the material increases with increasing PEO wt.%, both in dry and wet state. The conductivities [31] were calculated using Eq. 1, with A the PPy-PEO/DBS film dimensions: length 1.5 cm * width 0.5 cm (7.5 10−5 m2), the thickness (w) of the samples shown in Table 1. σi = ω Zre * A (1) Table 2 reveals that the PPy-PEO (5 wt.%)/DBS films exhibit the highest conductivity in both as dry samples (98.7 μS cm−1) or in each of the studied solutions, 50.2 μS cm−1 in LiTFSI-aq solution and 37.1 μS cm−1 in LiTFSI-PC solution. When the PEO content increases the conductivity decreases. Comparing the results from the two studied electrolytes the PEO (10−20 wt%) samples shows the highest conductivities in LiTFSI-PC electrolyte. As reported previously [17] the conductivity of PPy -PEO composites with low PEO content (2.5 g/l) increases significantly (up to 50%) with the increase of the relative humidity. Here, the conductivity is higher in organic electrolyte compared to an aqueous one. Higher ionic conductivities of the films will influence any of the electrochemical properties of the material: Fig. 3. PPy-PEO/DBS films (oxidized state, at 0.6 V) with different PEO amounts of 5 wt.% (black), 10 wt.% (red), 15 wt.% (green) and 20 wt.% (blue) obtained from a: FTIR spectroscopy (4000-500 cm−1) and b: Raman spectroscopy (800 – 1800 cm−1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 4 Materials Today Communications 23 (2020) 100908 R. Khadka, et al. Fig. 4. Nyquist plots of PPy-PEO/DBS with different wt.% load of PEO in polymerisation solution (5 wt.% black, □, fitted line; 10 wt.% red, ○, fitted dotted; 15 wt.% green, ◇, fitted dashed and 20 wt.% blue, star;, fitted dashed dot) in a: dry state with inset PEO 15 wt.% and PEO 20 wt.%, and in b: LiTFSI-aq and c: LiTFSIPC electrolytes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). LiTFSI-aq and LiTFSI-PC electrolytes. Figure S3 presents the attained stationary chronoamperometric responses. The involved charges, which determine the number of involved Li+ ions (Faraday law, n = Q/e−, with e− the electron charge: each electron flowing through the system requires the exchange of one positive ion between the film and the electrolyte, reaction 1), were attained by integration of the concomitant anodic and cathodic branches. Fig. 5a and b present the results. Fig. 5a presents the film oxidation charge, Q, versus different PEO content in the original electropolymerization solution. The PPy-PEO (5 wt.%)/DBS film exhibits the highest oxidation/reduction charge, which gradually decreases with increasing PEO content (the film synthesized from PEO 20 wt.%) shows the lowest charge. This behavior corelates to the electronic conductivities presented in Table 1: the PPyPEO(5 wt.%)/DBS sample also presented the highest conductivity, decreasing with increasing PEO content. The same tendency was found in LiTFSI-PC solutions with higher involved charges (180 μC to 60 μC). Previously, it has been discovered [7,9], that in aprotic solvents, such as propylene carbonate, the ion couples Li+DBS− do not dissociate, the positive charges generated upon oxidation on the PPy chains get compensated by the entrance of TFSI− anions and solvent (for osmotic balance) from the solution (anion-driven), represented in Eq. 3: charging/discharging of batteries and supercapacitors, the actuation of artificial muscles, ionic diffusivity and ionic migration through artificial membranes, and so on. 3.1.4. Characterization by modified scanning ionic conductance microscopy (mSICM) The mSICM was used to determine the current evolution in single micro-drops on the PPy-PEO/DBS film surfaces. The mSICM [32,33] is basically electrochemical measurements in a micro-drop located on the sample surface using a micro-pipette containing the electrolyte where the counter electrode included (Pt wire) and the reference electrode near the sample (Figure S3). The expected behaviour of the PPy-PEO/ DBS films in aqueous electrolytes is the reversible oxidation/reduction by exchange of Li+ ions with the electrolyte (cation-activity): PPy− PEO(DBS −Li+)n (S)n ⇆ PPy n + − PEO(DBS −)n + nS+ nLi+ + ne− (2) To evaluate the charging/discharging properties of PPy-PEO/DBS films (Reaction 1, Eq. 2, forwards/backwards, respectively) the samples were submitted to consecutive square potential waves (using the modified SICM methodology) between 0.65 and-0.6 V at 0.0083 Hz in both, Table 2 Conductivities σI calculated using Eq. 1, and RCT (charge transfer resistance) from EIS measurements with the thickness ω of the PPy-PEO/DBS films in dry state, in LiTFSI-aq and LiTFSI-PC electrolyte. Average values and standard deviations were calculated from three separate measurements performed for each samples. Parameters RCT (Zre) (Ω) σI (μS cm−1) RCT (Zre) (Ω) σI (μS cm−1) RCT (Zre) (Ω) σI (μS cm−1) PPy-PEO (5 wt.%)/DBS Dry state 51.3 ± 4.3 98.7 ± .8.3 LiTFSI-aq 106.2 ± 9.4 50.2 ± 4.4 LiTFSI-PC 140.0 ± 11.3 37.1 ± 3.0 PPy-PEO (10 wt.%)/DBS PPy-PEO (15 wt.%)/DBS PPy-PEO (20 wt.%)/DBS 90.5 ± 8.3 50 ± 4.6 950.4 ± 76.2 5.05 ± 0.4 3100.2 ± 176.4 1.51 ± 0.1 1005.5 ± 87.3 5.6 ± 0.5 1403.2 ± 132.5 4.2 ± 0.4 1714.1 ± 112.5 2.9 ± 0.2 232.3 ± 22.5 23.5 ± 2.3 1094 ± 42.1 5.1 ± 0.4 1216.2 ± 112.5 4.1 ± 0.4 5 Materials Today Communications 23 (2020) 100908 R. Khadka, et al. Fig. 5. Square wave potential step measurements of PPy-PEO/DBS films using modified SICM. The potential range of 0.65 to -0.6 V and frequency of 0.0083 Hz were applied, showing the charge accumulation at oxidation (Qox, ◼) and reduction (Qred, ★) against different loads of PEO (wt.%); a: LiTFSI-aq and b: LiTFSI-PC. For each PPy-PEO/DBS film, mean values and standard deviation were calculated using three separate samples. PPy− PEO(DBS −Li+)n (S)n + mS+ nTFSI− ⇆ PPy n + (TFSI−)n − PEO(DBS −Li+)n (S)n + m + ne−, experiments, Fig. 6a, c) and stress (attained under isometric experiments, Figures S4a and S4c) were obtained for the PEO 5 wt.% samples. In aqueous solution, the material expands on reduction and contracts during oxidation (Fig. 6a) which indicates a “cation-driven” actuation: the charge of the trapped DBS− ions is compensated by the incorporation of solvated Li− ions during reduction, being expelled to the solution on oxidation, following Reaction 1 (Eq. 2). The strain variations show cyclic (closed cycles) and stable responses being a consequence of the great stability and reproducibility of the voltammetric and coulovoltammetric (Figure S4b) responses, indicating the absence of irreversible reactions in that potential range. The asymmetry of the strain/potential responses (different strain on the anodic and cathodic scans at the same potential) in Figs. 6a and 6c were attributed to the asymmetry of the reaction-driven conformational and structural (shrinking-compaction vs relaxation-swelling) changes [34] together the asymmetric exchange of solvent: the entrance by osmosis follows the faradaic entrance of ions while the electrophoretic solvent expulsion is pushed by the ions expelled through the shrinking gel. The cyclovoltammetric responses shown in Fig. 6b for LiTFSI-aq electrolyte revealed for PEO 5 % and 10 % broad oxidation waves at 0.0 V and reduction waves at -0.3 V. Further increase of PEO in PPy/DBS decreased the current density, also suppressing the redox peaks. The (3) with S here a PC molecule. 3.2. Electro-chemo-mechanical characterization To investigate the linear actuation of the synthesised PPy-PEO/DBS films, isotonic and isometric electro-chemo-mechanical (ECMD) deformation measurements were conducted under voltammetric cycles. 3.2.1. Actuation under cyclic voltammetry A strip from each of the synthesised materials was submitted to consecutive potential cycles between 0.65 V and -0.6 V at 5 mV s−1 (200 cycles for each applied electrolytes) in each of the two studied electrolytes: LiTFSI-aq and LiTFSI-PC solution. The ECMD response was recorded to evaluate the generated strain. Fig. 6 presents the attained results. The coulovoltometric (charge consumed by Reaction 1 vs. potential) responses and the parallel stress evolutions from the PPy-PEO/ DBS films are shown in Figure S4. From the isotonic and isometric cyclic voltammetric experiments of PPy-PEO/DBS films, the largest strain, 5–6% (attained under isotonic Fig. 6. Voltammetric responses (b and d) (scan rate 5 mV s−1) and the accompanying strain evolution (a, c) from PPy-PEO/DBS films in LiTFSI-aq and LiTFSI-PC solutions, respectively. The PPy-PEO/DBS films were electrogenerated from solutions with different PEO content (as indicated in the Figures). 6 Materials Today Communications 23 (2020) 100908 R. Khadka, et al. Table 3 Strain, stress, charge densities and elastic moduli before and after actuation (200 cycles) for PPy-PEO/DBS films in LiTFSI-aq electrolyte. For each experiment, three separate films were measured, the obtained values were averaged. PPy-PEO/DBS LiTFSI-aq Strain [%] Stress [MPa] Charge density [C cm−3] Elastic Modulus before actuation [kPa] Elastic Modulus after actuation [kPa] PEO PEO PEO PEO 5.2 4.8 4.3 4.0 0.12 0.15 0.15 0.16 95 96 60 67 88 49 51 67 44 29 42 93 5 wt.% 10 wt.% 15 wt.% 20 wt.% ± ± ± ± 0.4 0.5 0.4 0.3 ± ± ± ± 0.01 0.01 0.01 0.01 ± ± ± ± 7 9 5 4 ± ± ± ± 9 5 4 6 ± ± ± ± 4 3 4 8 state – for anion active films, the reduced state means a collapsed structure – both ions and solvent has left. For cation-active films, on the other hand, the oxidized state is mostly the same as in case of anionactivity, while in the reduced state cations and solvent enter. One should also not forget the significantly lower elastic modulus – the material offers less resistance to deformation – partly related to the reference state. Whatever the PEO content, in PC the elastic modulus of the films decreased after actuation, demonstrating the plasticizing effect of PC. Also, in PC, increasing PEO content resulted in decreasing strain and elastic modulus for the whole range. In summary, as conducting polymer actuators are faradaic devices [[36],37], charge density should be related to the magnitude of actuation. As PPy is the only electroactive material in the composite film, adding too much PEO will reduce both electronic conductivity and the charge density achievable in the same timeframe. Adding in the higher stiffness of higher-PEO-content films, the lower strain response is far from surprising. While the actuation direction in PC is opposite to that in aqueous, the charge efficiency is significantly higher – less charge creates more mechanical response (stress and strain). cyclic voltammetric curves of PPy-PEO/DBS films in LiTFSI-PC (Fig. 6d) were mostly featureless, the overall current densities were less than half of those in the aqueous solution, also no clear dependence between current density and PEO content could be identified. The evolution of the charge densities Q with the potential E shows (Figure S4b) a closed loop whatever the studied PPy-PEO/DBS sample, indicating “steady state” responses and no presence of irreversible reactions such as water electrolysis or overoxidation in the applied potential range [35]. Table 3 compares the strain, stress, charge densities and elastic modulus of the PPy-PEO/DBS films obtained by 200 actuation cycles driven by cyclic voltammetry in LiTFSI-aq electrolyte. The elastic moduli were determined before and after actuation cycles. Recent research [10] has shown that for PPy/DBS with carbide-derived carbon particles, the elastic modulus before and after actuation cycles was different. While the charge density shown by 5% and 10% PEO containing PPy-PEO/DBS films (Table 3), the former showed higher stain and the latter higher stress. Otherwise, the strain decreases with increasing PEO content (and decreasing charge density) while the stress increases. Again interestingly, the 20% PEO films showed the lowest elastic moduli, both before and after actuation. Apparently, there is an optimum in the plasticizing effect there. While for most materials, the elastic moduli decreased after actuation, the PPy-PEO(20 wt.%)/DBS samples shows an opposite behaviour, where the elastic modulus increased during actuation. This effect can be explained by the lower conductivity, leading to slower and less thorough response to potential cycles. Naturally, the elastic modulus has an effect on all actuation behaviour. In case a mechanically very stable response is required, relatively higher (between 15 and 20%) PEO content should be applied to minimize the effects of elastic modulus change. Charge efficiency is also clearly higher at higher PEO content, as slightly less strain is obtained at significantly lower charge consumption. In LiTFSI-PC electrolyte, the film expands under oxidation and contracts on reduction following the so called “anion-driven” actuation: the electrochemical actuation of the PPy-PEO/DBS films was driven by reaction 2 (Eq. 3) being opposite to the actuation in LiTFSI-aq electrolyte (reaction 1, Eq. 2) [36]. Table 4 compares strain (Fig. 6c), stress (Figure S3c), charge densities and elastic moduli (before and after actuation) of PPy-PEO/DBS films. Also in PC, the highest charge density, strain and stress were reached by the PPy-PEO (5 wt.%)/DBS films (Table 4). The charge efficiency was several times higher though, as just about half of the charge density (compared to those in aqueous solutions) produced around twice the strain, which can be explained by a different reference 4. Conclusion Films of PPy-PEO/DBS were electrogenerated from solutions with different PEO concentrations (5 wt%, 10 wt%, 15 wt% and 20 wt%), keeping the rest of the components (pyrrole, DBS and salt) constant. Raman, FTIR and EDX spectroscopy revealed the successful incorporation of PEO into the growing PPy network but not directly proportional to the PEO wt.% in the original solution. SEM and AFM pictures indicated that the surface roughness of the films increased with increasing PEO content. Higher PEO content in the films resulted in decreased conductivities both in dry samples and those immersed in aqueous or PC solutions. Lower conductivities manifested themselves in lower charge density stored/released during the film oxidation/reduction, also affected by the reduced content of PPy – the only electroactive component. Significant differences of the electro-chemo-mechanical responses were observed for films with different PEO content and in different solvents. While the lowest PEO content (5 wt.%) gave the largest strain and stress in both electrolytes, the performance and charge-efficiency were significantly higher in the organic solution. The trends of the characteristics, like elastic moduli, stress, etc were all straightforward in PC, while those in aqueous solutions had maxima and minima in the studied PEO content range. For different application scenarios, in different fields, such as in electrodes for soft and wet batteries, sensors or Table 4 Strain, stress, charge densities and elastic moduli before and after actuation (200 cycles) for PPy-PEO/DBS films in LiTFSI-PC electrolyte. For each films, three separate films were measured. PPy-PEO/DBS LiTFSI-PC Strain [%] Stress [MPa] Charge density [C cm−3] Elastic Modulus before actuation [MPa] Elastic Modulus after actuation [MPa] PEO PEO PEO PEO 10.1 ± 0.5 7.8 ± 0.6 5.4 ± 0.4 4.7 ± 0.4 1.12 0.71 0.55 0.55 43.5 ± 4 39.3 ± 3 39.5 ± 4 28 ± 3 0.17 0.26 0.37 0.57 0.04 0.11 0.25 0.40 5 wt.% 10 wt.% 15 wt.% 20 wt.% ± ± ± ± 0.10 0.60 0.05 0.04 7 ± ± ± ± 0.02 0.02 0.04 0.05 ± ± ± ± 0.03 0.01 0.02 0.04 Materials Today Communications 23 (2020) 100908 R. Khadka, et al. actuators for soft robotics, composites of different PEO content could appear optimal, depending on the requirements and the environment. https://doi.org/10.4236/msa.2011.211218. [15] P. Novak, O. Inganas, Self-discharge rate of the polyprrole-polyethylene oxide composite electrode, J. Electrochem. Soc. 135 (1988) 2485–2490. [16] F. Vidal, C. Plesse, G. Palaprat, A. Kheddar, J. Citerin, D. Teyssié, C. Chevrot, Conducting IPN actuators: from polymer chemistry to actuator with linear actuation, Synth. Met. 156 (2006) 1299–1304, https://doi.org/10.1016/j.synthmet. 2006.09.012. [17] Y.C. Liu, Method of evaluating the ionic conductance of polypyrrole films and improvement of ionic conductance of polyethylene oxide-incorporated polypyrrole composite, Mater. Chem. Phys. 77 (2003) 791–795, https://doi.org/10.1016/ S0254-0584(02)00168-2. [18] A. Maziz, C. Plesse, C. Soyer, E. Cattan, F. Vidal, Top-down Approach for the Direct Synthesis, Patterning, and Operation of Artificial Micromuscles on Flexible Substrates, ACS Appl. Mater. Interfaces 8 (2016) 1559–1564, https://doi.org/10. 1021/acsami.5b09577. [19] N. Festin, A. Maziz, C. Plesse, D. Teyssié, C. Chevrot, F. Vidal, Robust solid polymer electrolyte for conducting IPN actuators, Smart Mater. Struct. 22 (2013) 104005, , https://doi.org/10.1088/0964-1726/22/10/104005. [20] R. Khadka, Z. Zondaka, A. Kesküla, M. Safaei Khorram, T. Thien Khanh, T. Tamm, J. Travas-Sejdic, R. Kiefer, C. Minh City, Influence of solvent on linear polypyrrolepolyethylene oxide actuators, J. Appl. Polym. Sci. 46831 (2018) 1–7, https://doi. org/10.1002/app.46831. [21] M. Harjo, T. Tamm, G. Anbarjafari, R. Kiefer, Hardware and software development for isotonic strain and isometric stress measurements of linear ionic actuators, Polymers (Basel) 1054 (2019) 1–14. [22] S.A. Theron, E. Zussman, A.L. Yarin, Experimental investigation of the governing parameters in the electrospinning of polyethylene oxide solution, Polymer (Guildf). 45 (2004) 2017–2030, https://doi.org/10.1109/TDEI.2006.1657971. [23] R. Khadka, N. Aydemir, A. Kesküla, T. Tamm, J. Travas-Sejdic, R. Kiefer, Enhancement of polypyrrole linear actuation with poly(ethylene oxide), Synth. Met. 232 (2017) 1–7, https://doi.org/10.1016/j.synthmet.2017.07.011. [24] V.K. Gade, D.J. Shirale, P.D. Gaikwad, P. Kakde, P.A. Savale, H.J. Kharat, Synthesis and Characterization of Ppy-PVS, Ppy-pTS, and Ppy, Int. J. Polym. Mater. Polym. Biomater. 56 (2007) 37–41, https://doi.org/10.1080/00914030600735155. [25] H.K. Lim, S.O. Lee, K.J. Song, S.G. Kim, K.H. Kim, Synthesis and properties of soluble polypyrrole doped with dodecylbenzenesulfonate and combined with polymeric additive poly(ethylene glycol), J. Appl. Polym. Sci. 97 (2005) 1170–1175, https://doi.org/10.1002/app.21824. [26] Y. Furukawa, S. Tazawa, Y. Fujii, I. Harada, Raman spectra of polypyrrole and its 2,5- 13C- substituted and C-deuterared analogues in doped and undoped states, Synth. Met. 24 (1988) 329–341. [27] G. Han, G. Shi, Electrochemical actuator based on single-layer polypyrrole film, Sensors Actuators B Chem. 113 (2006) 259–264, https://doi.org/10.1016/j.snb. 2005.02.055. [28] R. Kiefer, N. Aydemir, J. Torop, T. Tamm, R. Temmer, J. Travas-Sejdic, I. Must, F. Kaasik, A. Aabloo, Carbide-derived carbon as active interlayer of polypyrrole trilayer linear actuator, Sensors Actuators B Chem. 201 (2014) 100–106, https://doi. org/10.1016/j.snb.2014.04.093. [29] J.F. Jurado, C.V. Hernández, R.A. Vargas, Preparation of zinc oxide and polyethylene oxide composite membranes and their phase relationship, Dyna 174 (2012) 79–85. [30] A. El Jaouhari, M. Laabd, E.A. Bazzaoui, A. Albourine, J.I. Martins, R. Wang, G. Nagy, M. Bazzaoui, Electrochemical and spectroscopical studies of polypyrrole synthesized on carbon steel from aqueous medium, Synth. Met. 209 (2015) 11–18, https://doi.org/10.1016/j.synthmet.2015.06.017. [31] R. De Levie, The influence of surface roughness of solid electrodes on electrochemical measurements, Electrochim. Acta 10 (1965) 113–130, https://doi.org/10. 1016/0013-4686(65)87012-8. [32] C. Laslau, D.E. Williams, J. Travas-Sejdic, Progress in Polymer Science The application of nanopipettes to conducting polymer fabrication, imaging and electrochemical characterization, Prog. Polym. Sci. 37 (2012) 1177–1191, https://doi.org/ 10.1016/j.progpolymsci.2012.01.002. [33] N. Aydemir, J. Parcell, C. Laslau, M. Nieuwoudt, D.E. Williams, J. Travas-Sejdic, Direct writing of conducting polymers, Macromol. Rapid Commun. 34 (2013) 1296–1300, https://doi.org/10.1002/marc.201300386. [34] T.F. Otero, I. Boyano, Comparative study of conducting polymers by the ESCR model, J. Phys. Chem. B 107 (2003) 6730–6738, https://doi.org/10.1021/ jp027748j. [35] L. Valero, T.F. Otero, J.G. Martinez, J.G. Martínez, Exchanged Cations and Water during Reactions in Polypyrrole Macroions from Artificial Muscles, ChemPhysChem. 15 (2014) 293–301, https://doi.org/10.1002/cphc.201300878. [36] R. Kiefer, A. Kesküla, J.G. Martinez, G. Anbarjafari, J. Torop, T.F. Otero, Interpenetrated triple polymeric layer as electrochemomechanical actuator : Solvent influence and diffusion coefficient of counterions, Electrochim. Acta 230 (2017) 461–469, https://doi.org/10.1016/j.electacta.2017.01.191. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The work was supported by Estonian Research Council Grant IUT20-24 and the University of Auckland, New Zealand. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mtcomm.2020. 100908. References [1] T.F. Otero, From electrochemically-driven conformational polymeric States to macroscopic and sensing artificial muscles, in: P. Gaspard, J.-P. Sauvage (Eds.), From Non-Covalent Assem. to Mol. Mach. Wiley-VCH, Weinheim, Germany, 2011, pp. 443–452. [2] T.F. Otero, J.G. Martinez, Biomimetic intracellular matrix (ICM) materials, properties and functions. Full integration of actuators and sensors, J. Mater. Chem. B Mater. Biol. Med. 1 (2013) 26–38, https://doi.org/10.1039/c2tb00176d. [3] S. Beaumont, T.F. Otero, Chemical sensors from the cooperative actuation of multistep electrochemical molecular machines of polypyrrole: potentiostatic study. Trying to replicate muscle’s fatigue signals, Smart Mater. Struct. 27 (2018) 074001. [4] D. Melling, J.G. Martinez, E.W.H. Jager, Conjugated polymer actuators and devices: progress and opportunities, Adv. Mater. 31 (2019) 1808210. [5] S. Hara, T. Zama, W. Takashima, K. Kaneto, TFSI-doped polypyrrole actuator with 26% strain, J. Mater. Chem. 14 (2004) 1516, https://doi.org/10.1039/b404232h. [6] T. Zama, S. Hara, W. Takashima, K. Kaneto, Comparison of conducting polymer actuators based on polypyrrole doped with BF4-, PF6-, CF3SO 3-, and ClO4-, Bull. Chem. Soc. Jpn. 78 (2005) 506–511, https://doi.org/10.1246/bcsj.78.506. [7] R. Kiefer, J.G. Martinez, A. Kesküla, G. Anbarjafari, A. Aabloo, T.F. Otero, Polymeric actuators : Solvents tune reaction-driven cation to reaction-driven anion actuation, Sensors Actuators B. Chem. 233 (2016) 328–336, https://doi.org/10. 1016/j.snb.2016.04.090. [8] L. Bay, K. West, P. Sommer-Larsen, S. Skaarup, M. Benslimane, A conducting polymer artificial muscle with 12% linear strain, Adv. Mater. 15 (2003) 310–313, https://doi.org/10.1002/adma.200390075. [9] A. Kivilo, Z. Zondaka, A. Kesküla, P. Rasti, T. Tamm, R. Kiefer, Electro-chemomechanical deformation properties of polypyrrole/dodecylbenzenesulfate linear actuators in aqueous and organic electrolyte, RSC Adv. 6 (2016), https://doi.org/ 10.1039/c6ra20766a. [10] Z. Zondaka, R. Valner, T. Tamm, A. Aabloo, R. Kiefer, Carbide-derived carbon in polypyrrole changing the elastic modulus with a huge impact on actuation, RSC Adv. 6 (2016) 26380–26385, https://doi.org/10.1039/c6ra01511e. [11] Z. Zondaka, A. Kesküla, T. Tamm, R. Kiefer, Polypyrrole linear actuation tuned by phosphotungstic acid, Sensors Actuators, B Chem. 247 (2017) 742–748, https://doi. org/10.1016/j.snb.2017.03.061. [12] Z. Zondaka, M. Harjo, A. Khan, T.T. Khanh, T. Tamm, R. Kiefer, Optimal phosphotungstinate concentration for polypyrrole linear actuation and energy storage, Multifunct. Mater. 1 (2018) 14003, https://doi.org/10.1088/2399-7532/aae8a4. [13] N. Kato, Y. Watanabe, M. Sanui, K. Ogata, Ionic Transportnumber of network PEO electrolytes, Solid State Ion. 41/42 (1990) 632–636. [14] N. Gondaliya, Structural and Conductivity Studies of Poly(Ethylene Oxide) – Silver Triflate Polymer Electrolyte System, Mater. Sci. Appl. Chem. 02 (2011) 1639–1643, 8