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
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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.%.
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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).
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Materials Today Communications 23 (2020) 100908
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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
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Materials Today Communications 23 (2020) 100908
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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