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 DOI: 10.1177/0954406211424979

1427 originally published online 26 October 2011 2012 226:Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science

D K Biswal, D Bandopadhya and S K Dwivedyactuator

metal composite−Electro-mechanical and thermal characteristics of silver-electroded ionic polymer  

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- Oct 26, 2011 OnlineFirst Version of Record 

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Electro-mechanical and thermal characteristicsof silver-electroded ionic polymer–metal compositeactuatorD K Biswal, D Bandopadhya*, and S K Dwivedy

Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Assam, India

The manuscript was received on 14 May 2011 and was accepted after revision for publication on 8 September 2011.

DOI: 10.1177/0954406211424979

Abstract: The proposed work is in line with the evaluation of electro-mechanical and thermalcharacteristics of silver-electroded ionic polymer–metal composite (IPMC). IPMCs are fabricatedfirst using Nafion-117 as base polymer and non-precious metal silver as surface electrode bychemical decomposition method. Several testings are performed on fabricated IPMC to evaluateits thermo-mechanical and micro-structural properties. The characteristics of the electrode layerand deposited particles on IPMC surface are studied using scanning electron microscope. Thebending experiment of the actuator is conducted by applying direct current potential and the tipdisplacement measured. Thermo-gravimetric analysis and differential scanning calorimetry testare carried out, and thermal stability of the actuator is investigated. The crystal structure of IPMCis investigated by X-ray diffraction analysis. Micro-tensile test of the specimen is carried out toascertain the stress–strain relationship and comparison is made with the base polymer, Nafion.The experimental investigations, characterization, and performance of the IPMC demonstrate itseffectiveness to be used as actuator and artificial muscle materials.

Keywords: ionic polymer metal composite, Nafion, silver electrode, hysteresis, stress–straincurve

1 INTRODUCTION

Artificial muscle actuators mimicking the biological

actuating mechanisms are emerged as a new means

of actuators. Polymeric artificial muscle technolo-

gies are being developed that can produce similar

strains and higher stresses using electrostatic forces,

electrostriction, ion insertion, and molecular con-

formational changes. Of particular interest, ionic

polymer–metal composite (IPMC) actuator that can

be actuated or driven by employing voltage which

results in larger strains to produce forces and

change in displacements. IPMC, a class of polymer

labelled as electro-active polymer is being considered

in applications for both sensing and actuation. The

actuation capability with low activation voltage in

aqueous medium as well as in air environment and

its lightweight make the polymer actuator suitable

for many applications over other smart materials.

The advantages make them particularly attractive

for applications such as multi-link active cathe-

ters, artificial muscles [1], bio-medical devices [2],

micro-pumps [3], and biomimetic robots like jelly

fish in medical, mechanical, electric, and aerospace

engineering [4].

Several theories for the IPMC actuation mechanism

have been proposed. However, the exact bending

mechanism of IPMC under electric potential is still

under research. Qualitatively, a well-accepted bend-

ing mechanism is summarized below. It is well

accepted that the mobility of ion–water cluster

(water pressure gradient) causes swelling near the

cathode electrode and equivalent contraction on the

*Corresponding author: Department of Mechanical Engineering,

Indian Institute of Technology Guwahati, Guwahati, Assam

781039, India.

email: dibakarb@iitg.ernet.in

1427

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other side makes IPMC to bend. The bending

deformation depends on various factors such as mag-

nitude and direction of applied electric potential and

moisture content in the base polymer (Nafion/

Flemion) [5]. Figure 1 shows the schematic configu-

ration of the IPMC actuator in static condition (V¼ 0)

and when voltage is applied across its thickness [6].

An IPMC typically consists of a thin polymeric elec-

trolyte membrane, e.g. Nafion or Flemion, sand-

wiched between a noble metal such as gold or

platinum as surface electrodes [7–9]; however, the

process incurs high manufacturing cost. To reduce

the production cost, use of non-precious metals

such as silver was proposed for the fabrication of

IPMC material [10, 11]. Silver nano-powders were

also used and casting method was followed for the

fabrication of IPMC [12].

The current state-of-the-art procedure to fabricate

this artificial muscle of superior performance is

expensive as the metal electrodes are made of plati-

num or gold. It is anticipated that changing process-

ing and operating conditions may reduce the reliance

upon these metals. There is also serious limitation in

arresting moisture which results in poor performance

during experiment. The proposed research objective

is to use non-precious metal, silver (Ag) as surface

electrode which has shown to be research-worthy

for improving the performance of IPMC. This also

reduces the cost of fabrication of IPMC to be used

as an actuator and sensor for biomedical and even

in artificial prosthetics application. In the current

method of fabrication of silver-electroded IPMCs,

several steps of trial-and-error method were carried

out for optimizing the process parameters as

detailed procedure was not mentioned in the previ-

ous work [10, 13]. Chemical decomposition method

is followed to fabricate the proposed IPMC. Further,

the compositions of chemicals used in this study

are varied from those used in the literatures

[10, 13]. The microscopic and morphological analy-

ses are carried out and it is observed that silver (Ag)

particles penetrate the Nafion membrane surface

considerably. Bending experiment is done by apply-

ing direct current (DC) potential and the tip displace-

ment of the actuator measured. Many experiments

are conducted to assess the thermal stability of the

actuator by thermo-gravimetric analysis (TGA) and

differential scanning calorimetry (DSC) tests. The

crystal structure of the IPMC is investigated by

X-ray diffraction (XRD) analysis. Micro-tensile test

of the IPMC specimen is carried out to ascertain the

stress–strain relationship and comparison made with

the base polymer, Nafion. It is observed that the

developed IPMC exhibits large bending deformation

and generates large tip force with few input voltage.

The experimental investigations, characterization,

and performance of the IPMCs demonstrate its

effectiveness to be used as actuator and artificial

muscle materials.

2 MORPHOLOGICAL STUDY

As a part of this study, IPMCs are first fabricated

following the chemical decomposition method in

the laboratory. A Nafion membrane with an equiva-

lent weight 1100 and thickness 0.183 mm purchased

from Ion Power, Inc., New Castle, Delaware 19720,

USA is used as the base polymer for fabrication.

The process comprises multi-steps, including

pretreatment, adsorption, reduction, and develop-

ment. Pretreatment includes: (a) surface preparation

(roughening of the Nafion surface using metallogra-

phy silicon carbide paper). Surface roughening is

done to increase the surface area, i.e. opening up

more sites for arresting the silver ions. (b) Treatment

with HCl (2 N solution); (c) treatment with deionized

water. Adsorption includes the preparation of

Sodium Hydroxide (NaOH) which is used to provide

Naþ cations into the membrane; Silver Nitrate

(AgNO3) with dilute Ammonia solution (NH3) is used

to prepare the Diamminesilver (I) Hydroxide, and

Ag (NH3)2OH–silver complex solution is used for

Fig. 1 (a) Schematic diagram of the IPMC actuator and (b) its actuation mechanism under electricpotential

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providing the cations (Agþ). In reduction, Dextrose

Anhydrous GR (C6H12O6) is used as an agent to

reduce the silver cations into silver metal; finally,

deionized/distilled water (purchased from Merck

Specialities Private Limited, India) is used for rinsing

and washing. Finally, IPMC strip is obtained after

trimming the membrane from all sides to avoid any

shorting between two surfaces. Figure 2 shows the

photograph of the fabricated Ag-IPMC.

2.1 Topography and microstructure analysis

Before the experiment, IPMC is kept immersed into

ammonium hydroxide solution for sufficient time to

prevent oxidation problem, if any, as the ammonium

hydroxide dissolves silver oxide. The distribution of

the deposited silver particles and depth of deposition,

i.e. cross-sectional view of IPMC are investigated

under scanning electron microscope (SEM). Crystal

structures of the Nafion and IPMC membranes are

studied using XRD analysis. A sample of 10 mm2 is

prepared and examined under SEM (LEO 1430VP)

employing high voltage (15 kV) with 1.56 kX times

magnification. The equipment is integrated with an

energy-dispersive X-ray spectrometer for analysing

the chemical composition of the sample.

Figure 3(a) and (b) shows the SEM micrographs

and chemical composition of the IPMC in transverse

direction, respectively. Morphological study reveals

that the average diameter of the Ag particle is

around 0.5–0.6 �m. Further, the quality of the silver

and its derivative composition on the surface are

investigated by energy-dispersive X-ray (EDX) at

three different points to a few micron depths starting

from outside to inside of the surface and the results

summarized in Table 1. As shown in Fig. 3(b), it is

clearly observed that the Ag particles penetrated

the surface of the Nafion membrane up to a range

8–9mm. A detailed procedure about the fabrication

process is outlined in the study of Biswal et al. [14].

Figure 4 shows the schematic diagram of the IPMC

actuator, where base polymer (Nafion) is sandwiched

between the Ag electrodes on both sides.

2.1.1 XRD analysis

The crystal structures of the Nafion and IPMC mem-

branes are investigated using a Bruker D8 Advance

X-ray diffractometer (Cu-Ka radiation at 40 kV and

40 mA) instrument. Figure 5 shows both the XRD

patterns of Nafion and IPMC membranes. Three

major diffraction peaks of Ag-IPMC are observed at

2�¼ 38.15 �C, 44.3 �C, and 64.45 �C which are corre-

sponding to the crystal faces of (1 1 1), (2 0 0), and

(2 2 0). This observation confirms that deposited

silver film is polycrystalline in nature without any

unwanted phase formation. In fact, it shows the char-

acteristics of face-centred cubic structure which

validates that silver-coated IPMCs have perfect con-

ductivity property. The characteristic diffraction peak

of Nafion membrane at 2�& 18� is in good agreement

with the previous results [15].

Fig. 2 Photographs of fabricated IPMC: (a) surface (top view) and (b) along the thickness

Fig. 3 (a) SEM micrograph and (b) EDX at a cross-section of IPMC

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3 THERMAL AND MECHANICAL

CHARACTERISTICS OF IPMC

Characterization and investigation of thermal and

mechanical properties of IPMC are indispensable

for various applications as sensor and actuator in an

environment subjected to temperature fluctuation.

3.1 TGA and DSC tests

TGA and DSC analyses are carried out to investigate

the thermal behaviours and also to study the effect of

temperature on the Nafion and IPMC membranes.

TG and DSC measurements are carried out on a

NETZSCH STA 449 F3, Jupiter� instrument. Nafion

and IPMC films are cut into small pieces to provide

a suitable sample size of about 5–10 mg. TG and

DSC curves are obtained for Nafion in acidic form

(Nafion Hþ) and IPMC membrane at a heating rate

of 10 �C/min in an Argon gaseous environment

(60 mL/min) while temperatures are maintained at

650 �C and 550 �C, respectively.

Figure 6(a) and (b) shows the TG curves obtained

for both Nafion and IPMC separately. Nafion-117

is a perfluorosulfonate ionomer consisting of a

polytetrafluoroethylene backbone with side-chains

terminated with a sulfonate group. Based on the

Fig. 6 TG (solid line) and DTG (dotted line) curves of: (a) Nafion and (b) IPMC membrane.TG: thermo-gravimetric; DTG; differential thermogravimetry

Fig. 5 XRD patterns of the Nafion and Ag-IPMC mem-brane. IPMC: ionic polymer–metal composite

All dimensions are in mm

Ag - Surfaceelectrode layer

Base polymerNafion - 117

h2

hh1

1 2h h 8 – 9 mm, h = 183 mm ≈ ≈

Fig. 4 Schematic diagram of IPMC showing basepolymer (Nafion) sandwiched between the Agelectrodes on both sides

Table 1 EDX analysis of an Ag-IPMC sample, as shown

in Fig. 3(a)

Points C O F Ag Total

1 5.62 2.07 27.53 64.78 1002 14.78 7.80 18.91 58.51 1003 18.67 — 48.29 33.04 100Maximum 18.67 7.80 48.29 64.78Minimum 5.62 2.07 18.91 33.04

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observation (differential thermogravimetry (DTG)

curve, Fig. 6(a) and (b)), decomposition of Nafion

and IPMC membranes can be categorized into four

different stages with respect to temperature which

are listed in Table 2. The result obtained for Nafion

(Fig. 6(a)) is in good agreement with the experimental

results of Stefanithis and Mauritz [16]. The experi-

mental study (Fig. 6(b)) clearly demonstrates that

content of Ag particles significantly changes the ther-

mal stability of IPMC than that of pure Nafion. It is

observed that at higher temperatures (470–575 �C),

the Nafion membrane as well as the silver particles

starts degradation. Table 2 summarizes the percent-

age of mass loss with possible causes at various tem-

peratures. It is also observed that Nafion and IPMC

membranes are thermally stable below 270 �C and

350 �C, respectively.

Figure 7(a) and (b) demonstrates the DSC thermo-

graphs of Nafion and IPMC membranes, respectively.

DSC thermograph for Nafion (Fig. 7(a)) shows a strong

endothermic peak near 88 �C as glass transition

temperature (Tg), and another peak near 143 �C as

crystallization temperature (Tc). Thermal transition

Table 2 Percentage of mass loss for Nafion and

Ag-IPMC

Temperaturerange (�C)

Mass loss,Nafion-117(%)

Mass loss,Ag-IPMC (%) Observation

25–290 5.9 2.89 Loss of moisture290–400 10.16 3.83 Desulfonation400–470 26.25 11.09 Side-chain

decomposition470–575 49.63 59.2 Backbone

decomposition575–650 0.247 0.19 Residue

Fig. 7 DSC thermograph of: (a) pure Nafion and (b) Ag-IPMC. IPMC: ionic polymer–metalcomposite

Fig. 8 (a) Bending experiment setup and (b) schematic configuration showing the IPMC underpotential gradient. IPMC: ionic polymer–metal composite

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above 200 �C may be associated with thermal

degradation of the Nafion membrane. The high exo-

thermic peak near 332 �C indicates the melting

point temperature (Tm) of the Nafion. As shown in

Fig. 7(b), the glass transition (Tg), crystallization

(Tc), and melting point (Tm) temperatures for IPMC

are observed at 96 �C, 375 �C, and 486 �C, respectively.

It is anticipated that the increase in Tg, Tc, and Tm of

IPMC could be due to the presence of silver particles

with the Nafion.

3.2 Mechanical characteristics of IPMC actuator

3.2.1 Bending experiment

Experiment is conducted with an IPMC actuator of

size 20� 5� 0:2 mm3, in cantilever configuration by

applying voltage at the fixed end to calibrate its bend-

ing characteristics. Water is used as the polar solvent.

Copper strips are used at fixed end and voltage is

applied quasi-statically by a DC power supply

(0–32 V, 0–2 A) and subsequently bending tip deflec-

tion of the actuator is measured. The IPMC has been

subjected to input voltage from 0.2 to 1.2 V with an

increment of 0.2 V. For each input voltage after 30 s

(sufficient time is allowed to settle any back relaxa-

tion effect and to attain the steady state), tip deflec-

tion Pðpx , py Þ is measured. Figure 8(a) shows the

photograph of the IPMC actuator subjected to input

voltage 1.2 V, while Fig. 8(b) shows the schematic dia-

gram of the bending configuration under potential

gradient. Fig. 9(a) shows the change in tip position

in transverse direction with various input voltages.

Figure 9(b) demonstrates the tip deflection of IPMC

actuators of various electrode materials, such as

IPMC of size 20� 5� 0.2 mm3 fabricated by casting

method using silver nano-powder [12], gold-plated

IPMC of size 40� 5� 1 mm3 [17], platinum-plated

IPMC of size 35� 7� 0.2 mm3 prepared by manual

polishing [18], and IPMC fabricated by casting

method with platinum as the surface electrode of

size 50 � 15� 0.45 mm3 [19]. It is clearly observed

that the developed IPMC exhibits superior perfor-

mance compared to other surface electrode materials

with less power consumption.

Figure 10(a) shows the change in tip angles of the

IPMC with input voltages. Assuming a constant cur-

vature bending as shown in Fig. 8(b), the following

relationship can be established

R ¼1

�c¼�

lð1Þ

where R is the bending curvature, �c the radius of

curvature, l the length of the IPMC, and � the tip

angle. Figure 10(a) shows the relationship between

voltage and tip angle obtained using linear curve fit-

ting approximation technique with a quality factor

(R2) 0.9729 and is given in equation (2)

� ¼ 1:4011 V � 0:2924 ð2Þ

After measuring the tip deflection for each input

voltage, equation (1) is used to obtain the voltage

versus curvature relationship for both increasing

and decreasing input voltages and plotted in

Fig. 10(b). It is observed that the input–output rela-

tionship exhibits hysteresis.

Fig. 9 (a) Bending deformation and (b) comparison of tip displacement of various electrode mate-rials with this study

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3.2.2 Tip force measurement

An IPMC actuator of size 25� 6� 0:2 mm3 is studied

experimentally to measure the tip force using the

setup, as shown in Fig. 11(a). Voltage is applied at

one end and the other one just allowed to touch the

digital measuring balance (Make: Sartorius, Model:

BSA2245-CW). Voltage is applied gradually (not the

step input) and the tip force measured for an input

1 V. Figure 11(b) shows the force generated at the tip

of the actuator for a time interval of 0–120 s. The data

are collected with an interval of 1 s and plotted. Tip

force generated depends on many factors such as

dimensions (length, width, and thickness), elec-

trode materials, dehydration, and applied voltage.

Comparative study shows that the developed IPMC

generates much more force compared to other elec-

trode materials such as the IPMCs of Ag, Pt, and Au, as

given in Table 3.

3.2.3 Evaluation of elastic modulus

Tensile tests are conducted to investigate the stress–

strain behaviour of Nafion and the IPMC using a

micro-tensile tester (Make: Deben UK Ltd., Model:

Microtest 5 kN), as shown in Fig. 12(a), while sample

dimensions are shown in Fig. 12(b). The tensile tests

are performed with a test speed 0.5 mm/min. Figure

13(a) and (b) shows the stress–strain curves obtained

for both Nafion and IPMC in dry and wet conditions

(only for IPMC), respectively. Young’s modulus of

Nafion membrane is found to be 238.5 MPa, which is

Fig. 10 (a) Change in tip angle with input voltage and (b) voltage vs curvature relationship obtainedexperimentally

Fig. 11 (a) Schematic diagram of the experimental setup for force measurement and (b) forcegenerated at the tip of IPMC with time. IPMC: ionic polymer–metal composite

Electro-mechanical and thermal characteristics of silver-electroded ionic polymer–metal composite actuator 1433

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Table 3 Comparison of developed tip force of various electrode materials

Serial number Material and size Tip force References

1 Ag-IPMC (20� 5� 0.23 mm3, 1 V) 0.114 gf Chung et al. [12]2 Pt-IPMC (50� 15� 0.45 mm3, 1 V) 0.6667 gf Peng et al. [19]3 N-DIW (silver-plated Nafion with water

as solvent) (20� 1 mm2, 1 V)0.2 mN Tamagawa et al. [13]

N-H (silver-plated Nafion with HCl assolvent) (20� 1 mm2, 1 V)

0.25 mN

N-SC (silver- and copper-plated Nafionwith water as solvent) (20� 1 mm2, 1 V)

0.55 mN

N-SCN (silver-, copper-, and nickel-plated Nafion with water as solvent)(20� 1 mm2, 1 V)

0.6 mN

4 Au–Ni-Nafion IPMC(11.5� 4.7 mm2, 4 V)

40 mN Siripong et al. [20]

5 Pt-IPMC (4� 0.5 cm2, 3 V) 1.5 mN Saher et al. [21]6 Ag-IPMC (20� 5� 0.2 mm3, 1 V) 0.33 gf &3.3 mN This study

Note: IPMC: ionic polymer–metal composite.

Fig. 12 (a) Micro-tensile testing setup arrangement and (b) sample for micro-tensile test

Fig. 13 Stress–strain curves of: (a) Nafion and Ag-IPMC in dry condition and (b) Ag-IPMC in wetcondition. IPMC: ionic polymer–metal composite

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in good agreement with the results supplied by

DuPont [22]. The Young’s modulus of Ag-IPMC in

dry state is found to be 421.55 MPa, which is higher

than the base polymer Nafion due to the presence of

silver particles, while for fully hydrated IPMC, Young’s

modulus is found to be 81.877 MPa. IPMC of high

Young’s modulus requires more electric potential to

work properly and is not suitable for many applica-

tions. The present results show a moderate Young’s

modulus value compared to the previous results

[13, 20, 23]; hence, it is suitable for application

where low electric potential is required. Figure 14

shows the SEM micrograph of the IPMC after the ten-

sile test and it is observed that significant plastic

deformation occurs and cracks are propagated

before failure. It is observed that for Nafion, crack

starts to form around 135 s after the load is applied,

while for IPMC, it is observed to be 250 s.

4 CONCLUSIONS

The chemical decomposition method is followed for

depositing silver electrode layer over Nafion film

which appears to yield favourable results in the fabri-

cation of IPMCs. Morphological characteristics exam-

ined by SEM of IPMC suggested that there is good

bonding between the metal and polymer layer and a

uniform densely packed distribution of Ag particles

over the polymer surface. DSC and TGA tests of

the IPMC confirm improved thermal stability and ten-

sile test the increase in Young’s modulus. Bending

actuation results show a large deflection; as a result,

relatively large tip forces are achievable from these

IPMCs using low input voltage. The experimental

results suggest that manufacturing low-cost IPMCs

is possible for application as actuator and as well as

artificial muscle materials.

ACKNOWLEDGEMENTS

The authors thank the Department of Science and

Technology (DST), Government of India under

SERC FAST Track Scheme (SR/FTP/ETA-076/2009)

providing partial support. Authors also acknowledge

the support from Central Instrument Facility, IIT

Guwahati, for morphological, DSC, and TGA

analyses.

� Authors 2011

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