Influence of additives on the properties of casting nafion membranes and SO-based ionic...

Influence of Additives on the Properties of CastingNafion Membranes and SO-Based Ionic Polymer–MetalComposite Actuators

Yanjie Wang,1,2 Hualing Chen,1,2 Yongquan Wang,1 Bin Luo,1 Longfei Chang,1 Zicai Zhu,1,2 Bo Li1,2

1 School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an, People’s Republic of China

2 State Key Laboratory for Strength and Vibration of Mechanical Structure, Xi’an Jiaotong University, Xi’an,People’s Republic of China

As a typical smart material, ionic polymer metal com-posite (IPMC) has a sandwich structure, which con-sists of a base membrane and two thin metallicelectrodes on both sides of the base membrane. Theproperties of the base membrane, Nafion as the mostused base material, strongly affect the performance ofIPMC actuator. This paper reports the effects of differ-ent additives, such as ethylene glycol (EG), dimethylsulfoxide (DMSO), N, N0-dimethyl formamide (DMF),and N-methyl formamide (NMF), on the performancesof the casting membranes and SO-based IPMC actua-tors. Studies have shown that the microstructures ofthe casting membranes with EG and DMSO as addi-tives are more loose and amorphous, leading to higherwater contents and thus higher conductivity than thosewith DMF, NMF, and Nafion 117. Among the castingmembrane-based IPMC actuators, EG-based IPMCactuator has larger deformation and blocking force,higher strain energy density and conversion efficiencyat 2 V DC voltage, whose electromechanical propertiesare most close to that based on Nafion 117. POLYM.ENG. SCI., 00:000–000, 2013. VC 2013 Society of PlasticsEngineers

INTRODUCTION

Ionic polymer–metal composites (IPMCs), as one of

electroactive polymers (EAPs), are very suitable for

actuators and sensors because of their electromechanical

and mechanoelectrical response to applied voltage and

mechanical deformation, respectively [1, 2]. When

applied as actuators, IPMCs have many particularly

advantages, such as large deflection, low-activation

voltage, high compliance, lightness, softness, and so on.

Hence, they are considered as an attractive smart material

and have a large quantity of potential applications in

robotics, aerospace, biomedicine, and so on [3–9]. How-

ever, there are still some defects, such as slow response,

back relaxation and low-blocking force, which mainly

limit the IPMC actuator as a practical actuator in

variety of applications and need to be addressed urgently

[10, 11].

Generally, a typical IPMC has a sandwich structure,

which consists of a base membrane, usually NafionTM

membrane (by DuPont), and two thin metallic electrodes

on both sides of the membrane. The membrane is made

of perfluorinated ionomers, varying in the length of back-

bones (I) and branches (II) and in the nature of the ionic

side group, usually sulfonate anions (III), whose chemical

formula is given in Fig. 1. A certain amount of cations

(R1) inside the base membrane, which balance the elec-

trical charge of the anions fixed on the backbone of the

membrane, could provide actuating ability when the volt-

age is on-load. This coupled electrical–chemical–mechan-

ical properties of IPMCs highly depend on several

factors, such as the microstructure of the ionomer back-

bone, type of the cations, the morphology and conductiv-

ity of the electrodes, and so on.

One of the most important factors that seriously affects

the blocking force of IPMCs is the base membrane, which

provides the backbone and ion transport medium of

IPMCs during actuation process. At present, most of the

studies on IPMCs are limited to the base membrane of

Nafion series, for example, N117, N1110, which have a

relatively poor thickness range from 50 to 250 lm result-

ing in low-blocking force. Apparently, to enhance block-

ing force, the easiest and the best way is to increase the

thickness of the base membrane. So far there are two

ways to form thick base membrane, namely, hot pressing

[13] and solution casting [14], developed by Lee and

Kim, respectively. The former integrates multilayers of

Correspondenc to: Yongquan Wang; e-mail: [email protected]

Contract grant sponsor: National Natural Science Foundation of China;

contract grant number: 51290294; contract grant sponsor: Natural Sci-

ence Basis Research Plan in Shaanxi Province of China; contract grant

number: 2012JM7002.

DOI 10.1002/pen.23634

Published online in Wiley Online Library (wileyonlinelibrary.com).

VC 2013 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—2013

commercial Nafion membranes by hot pressing, but it is

easy to give rise to delamination for repeating actuation

of IPMC [15]. The latter can avoid this drawback and

fabricate the membrane with arbitrary thickness.

However, this work does not give further indications

about the properties of casting membrane. Some research-

ers [16–18] have revealed that the formation of outstand-

ing casting membrane was affected by some factors, for

example, temperature, curving time, and the choice of

additive. The temperature and curving time have been

researched a lot for fuel cell and chlor–alkali application

[19, 20] and the influences of temperature and curving

time on the properties of casting membrane have been

confirmed very well. As for the additives, a few decades

ago, Moore et al. [21] indicated that casting membrane

had a poor mechanical properties and high solubility in

the absence of high-boiling additives. Recently, some

researchers [22–24] begin to focus on the effect of addi-

tives and find that additives seriously affect the formation

of the morphology of casting membranes during the

solution-casting process. In addition, the key properties of

casting membranes, for example, water content, conduc-

tivity and modulus, have been changed, which also bring

significant influence on the performances of SO-based

IPMCs. Unfortunately, their works were not extended to

IPMCs.

Then the following question arises: From the perspec-

tive of IPMCs, is there a relation between the choice of

additives and the properties of the casting membranes?

Does the choice of additives exert particularly effects on

the performance of the SO-based IPMCs?

Aiming at solving these problems, we prepared the cast-

ing membranes with different additives and analyzed the

effects on the performance of SO-based IPMCs. First, the

membranes were prepared by solution-casting process with

different additives, including ethylene glycol (EG), dimethyl

sulfoxide (DMSO), N,N0-dimethyl formamide (DMF), and

N-methyl formamide (NMF). Next, as the key factors

affecting the IPMC performances, the properties of casting

membranes were investigated, such as water content, bend-

ing modulus, ion-exchange capacity (IEC), and ionic con-

ductivity. Then IPMCs were fabricated by the identical

electroless plating process with casting membranes, as

depicted in Ref. [25]. The morphological features of casting

membranes and SO-based IPMCs were observed by using

atomic force microscopy (AFM) and scanning electronmicroscope (SEM), respectively. In addition, the glass-

transition temperature (Tg) and melting point (Tm) of

casting membranes and IPMCs were analyzed by differen-

tial scanning calorimetry (DSC). Finally, to evaluate the

performances of SO-based IPMCs, blocking force and

actuation displacement were tested under 2 V DC voltage,

and strain energy density and efficiency were calculated

and compared by deformation.

PREPARATION OF SAMPLES

Materials

The 5 wt% NafionVR

dispersions (D520) were purchased

from Dupont in USA. Water was first deionized and then

passed through a water purification system (Hi-tech, China).

Concentrated nitric acid (69.5 wt%) and sodium hydroxide

were obtained from Ciron Chemical Co., in China. Four

additives for casting membranes were purchased from

TianLi Chemical Co., in China, including EG, DMSO,

DMF, and NMF. Commercially available NafionVR

117

membrane (N117), purchased from Dupont in USA, was

used as a reference.

Solution Casting of Membranes

A typical solution-casting process of membranes was

performed as follows, namely, first, 50 mL of 5 wt%

NafionVR

dispersions were pipetted separately into four

glass vessels. Next, each one of the four additives, includ-

ing EG, DMSO, DMF, and NMF, was added to four glass

vessels with the quantities of 5 mL in sequence, respec-

tively. And then ultrasonic treated for 0.5 h to make the

solutions mixed uniformly. To obtain a clear uncracked

membrane with good mechanical properties, solutions

were heated up at 80�C for 0.5 h.

Because the casting membrane undergoes morphologi-

cal change above glass-transition temperature (Tg), the

viscous solution was heated up and cured in an oven

under vacuum condition at 120�C for 6 h. After that, the

casting membrane was peeled off from glass vessels by

immersing in boiling distilled water for 5 min. The acid

form of the membranes were obtained by immersing in

boiling 5% H2O2, 0.5M H2SO4 solution, and deionized

water for 1 h, respectively, and finally stored in deionized

water in a climate-controlled environment, out of direct

sunlight, maintained at 10�C–30�C, and 30–70% relative

humidity. The overall casting procedure can be divided

into several steps as shown in Fig. 2.

The membrane thicknesses in dry and wet states were

measured by a micrometer. The casting membranes, pre-

pared with different additives, such as EG, DMSO, DMF,

and NMF, were marked as EG, DMSO, DMF, and NMF,

respectively. The procedure for preparing casting mem-

branes has mainly considered additives as a variable.

Some other parameters such as the precursors, mixed

ratio, and heat treatments employed from This point had

been reported by Ma et al.the literature [26], as illustrated

in Table 1.

FIG. 1. A typical chemical formula of NafionVR

series [12]. [Color fig-

ure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

2 POLYMER ENGINEERING AND SCIENCE—2013 DOI 10.1002/pen

Preparation of SO-based IPMCs

SO-based IPMCs with palladium electrodes were fabri-

cated by electroless-plating method developed in our lab-

oratory. First, the casting membranes were roughened by

sandpaper (class 1200) and boiled in acid solution (2

mol=L HCl) and the deionized water, respectively. Then

the palladium electrode was primary painted on both sides

of the membrane for three times and then chemically

plated for two times. Finally, the IPMCs were obtained

and soaked in 0.2 mol=L HCl for two times (2 h each

time) to exchange the cations in the membrane with H1

cation. In order to acquire the actuation capability, the

IPMCs were immersed in 0.2 mol=L NaOH solution for

two times (2 h each time) to absorb Na1 as mobile ions.

Later, IPMCs were cut into small pieces with needed

sizes for testing the performances.

CHARACTERIZATION AND METHOD

Morphology Characterization

The microstructure of the casting membranes were

examined by means of AFM using Dimension Icon model

from Bruker in USA. The tapping mode was choosen to

probe the topography of membrane, with resonance fre-

quency of 380 kHz. The AFM observation was conducted

at room temperature and room humidity. Under these

conditions, the membrane did not show any surface modi-

fication within 24 h after the sample was mounted on a

silicon substrate.

The surface and cross-sectional morphology of

SO-based IPMCs were observed by SEM (TESCAN-

VEGAnXMU VG3210677). SEM was performed at an

accelerating voltage of 20.0 kV. All sample cross sections

were obtained by low-temperature cracking, being placed

in liquid nitrogen for 5 min and then broken into pieces.

Water Content Measurement

The membranes were cut into 5 mm 3 35 mm and

immersed in deionized water for 12 h, and then taken out

and the moisture on the surface was removed carefully

with a tissue paper, and immediately weighed with a

digital balance (w1). The membranes were dried in an

oven at 80�C for 12 h to completely remove the inside

water for the measurement of dry weight (w2). All sam-

ples were tested at room temperature and room humidity.

The water content of the membrane was calculated by the

following equation:

l ¼ w12w2

w2

3 100% (1)

Ion-Exchange Capacity (IEC) Measurement

IECs of the membranes were measured by titration

method. The samples after full acidification were

immersed in 0.1M NaCl solution for 12 h with continuous

stirring. To insure the complete ion-exchange, the sam-

ples were immersed again in another 0.1M NaCl solution

for additional 12 h. The two solutions were mixed and

the displaced H1 ions were titrated with a standard con-

centration of 0.1M NaOH using phenolphthalein as indi-

cator. The volume of applied sodium hydroxide solution

V was recorded to calculate the mole numbers of H1 in

solution according the following equation:

IEC ðmeq=gÞ¼ ðV3NÞNaOH

WeightðPolymerÞ (2)

where N is normality of sodium hydroxide solution.

FIG. 2. Casting procedure using NafionTM solution. [Color figure can be viewed in the online issue, which

is available at wileyonlinelibrary.com.]

TABLE 1. Precursors, additives, mixed ratio, and heat treatments used

for different casting membrane samples.

Samples

NafionVR

precursor Additives

Mixed

ratio

Heat

treatment

(�C, h)

EG-01 D 520 EG 10:1 120, 6

DMSO-02 D 520 DMSO 10:1 120, 6

DMF-03 D 520 DMF 10:1 120, 6

NMF-04 D 520 NMF 10:1 120, 6

Nafion 117a — — — —

aData from Nafion 117 commercial membrane are also tested as a

comparison.

DMF, N,N0-dimethyl formamide; DMSO, dimethyl sulfoxide; EG,

ethylene glycol; NMF, N-methyl formamide.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—2013 3

Conductivity Measurement

The resistivities of the membranes R were measured by

a low-resistivity meter with measurement range 10–2–106X(the Loresta-EP handheld unit with a four-pin probe) and

thus the conductivities were 1=R. Membranes were cut

into discs with the size of radius 10 mm and immersed in

deionized water for 24 h prior to the testing. The same

sample from the same batch was tested for six times and

the average value was recorded. All operations were car-

ried out at room temperature and room humidity.

Differential Scanning Calorimetry (DSC)

DSC thermal analysis of the membranes and SO-based

IPMCs were performed on a Thermal Analysis Model

DSC1 purchased from Mettler Toledo in Switzerland. The

weights of samples were 4�8 mg. Before DSC observa-

tions, the surface of each sample was dried in an oven

(80�C, 2 h). The heating rate of DSC was set at

10�C=min and the N2 flow rate was 80 mL=min.

Electromechanical Properties Testing

To evaluate the effect of additives on the casting mem-

branes, the electromechanical properties of IPMCs, mainly

including elastic modulus, deformation and blocking force,

are measured in fully hydrated state. The performance of

IPMC based on Nafion 117 is tested for comparison. Fig-

ure 3 shows the schematic of the electromechanical testing

device. IPMC strip 35 mm in length and 5 mm in width is

clamped by two copper disks. The displacement at the

point 25 mm away from the fixed point is measured by a

laser-displacement sensor (Keyence, Japan). The blocking

force is detected by a microforce sensor (Transducer Tech-

niques, USA) fixed to the free end point of IPMC strip at

the equilibrium position. The applied voltage and current

are simultaneously measured. The tip displacement w of

SO-based IPMC actuators can be calculated from the

measured displacement d by the following equation:

w ¼ 2Rsin 2 l

2R

� �(3)

where l is the length of the free part of IPMC strip. The

radius of curvature R is evaluated from the measured dis-

placement by the following equation:

1

R¼ 2d

d2 þ d2(4)

where d is the distance between the measuring point and

the fixed point.

Using free oscillation attenuation method, the bending

modulus, affected by the composition of the IPMC

including base membrane and metal electrodes, was

measured by bending the sample to appropriate initial dis-

placement. The natural frequency of the cantilever fn was

obtained by a fast Fourier transform of the free vibration

response curve. The stiffness of IPMCs was calculated

according the following Eq. 5, which is derived from the

thin cantilever beam theory of material mechanics.

Eeq ¼2p

3:515

� �2

f 2n

l

Lml3

12

h3t¼ 3:19533

l

L� 12

h3tl3f 2

n m (5)

The parameters h, t, and L represent the thickness, the

width, and the total length of the IPMC strip.

RESULTS AND DISCUSSION

Morphology Analysis

Figure 4 shows AFM topography micrographs of poly-

mer chain conformations of EG (a), DMSO (b), DMF (c),

NMF (d), and Nafion 117 (e) as a reference, respectively.

In all plots, bright is high in topography and high in

phase, and high phase indicates high-stiffness regions in

tapping mode. In addition, the darker regions in height

represent the ionic domains and microporous domains,

which are clearly visible with a diameter of 5–50 nm.

There is a significantly difference in topography among

these samples. It can be noted that Nafion backbones con-

formations are more loose and amorphous in Fig. 1a and

b while the distribution of polymer chain and ionic

domains are more compact and uniform in Fig. 1c–e,

which is responsible for the differences of the compatibil-

ities between Nafion chain molecules and additives.

When evaporated from casting solutions, additives assist

Nafion side chain sulfonic acid groups to move along

FIG. 3. Schematic diagram of the experiment set-up for electromechan-

ical test. [Color figure can be viewed in the online issue, which is avail-

able at wileyonlinelibrary.com.]


with perfluorocarbon backbones so as to form solid mem-

branes. Meanwhile, backbone aggregations in membranes

result in the formation of more sulfonic acid group aggre-

gations, that is, larger ionic clusters. This point had been

reported by Ma et al. [22]. They investigated the change

of Nafion molecular conformations when additives were

evaporated from mixture, and confirmed that the Nafion

molecular conformations in dilute solutions and the mor-

phology of membranes prepared from solution castings

were strongly influenced by solubility parameters and

dielectric constants of solvents. These differences of

Nafion molecular conformations, caused by additives,

will seriously affect the characteristics of the casting

membranes, that is, water content and conductivity.

Figure 5 shows the real optical image of the SO-based

IPMC actuators (a) and the SEM images of Nafion

117-based IPMC actuator (b), (c). It can be seen from

Fig. [5]a and b that the Pd electrode is uniformly grown

on the copolymer surface with the same fabrication pro-

cess. To confirm the thickness of membrane and electro-

des portion, Fig. 5c shows the SEM images of the cross

sections of Nafion 117. From Fig. [5]c, it was clearly

shown that a Pd electrode layer with a thickness of

approximately 15 lm was formed. Notably, Pd grains

FIG. 4. AFM topography images (200nmx200nm). Images (a), (b), (c), (d), and (e) correspond to the topog-

raphy of EG, DMSO, DMF, NMF and Nafion 117, respectively. [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]

FIG. 5. The images of so based IPMCs, Nafion 117 based IPMC as reference. (a)The real optical images,

(b) and (c) are the surface and cross section of Nafion 117 based IPMC, respectively. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com.]


deeply insert into the Nafion copolymer and there is no

clear interface observed between the Pd grains and

Nafion, which indicates that the electrodes adhere very

well on the copolymer. The thicker metal layer with

excellent adhesion should decrease the surface resistance,

increase the current of an IPMC, and enhance the actua-

tion deformation and blocking force.

Analysis of the Water Content

The deformation of IPMC under electric field could

attribute to the ion cluster flux and an electro-osmotic

drag of water induced by ionic migration from the anode

to the cathode direction through the hydrophilic channels

in the perfluorinated sulfonic acid polymer chains, as

illustrated in Fig. 6. Thus, the water content is an impor-

tant factor during the actuation process of IPMC. Cappa-

donia et al. [27] indicated that the conductance of Nafion

membrane is a function of temperature and water content.

If the membrane is too dry, the conductivity is relatively

low, resulting in poor performance. An excess of water

inside the IPMC could lead to cathode flooding problems

and degradation of the performance. A deformation trend

of IPMCs has been obtained with the reduction of water

content, which shows that the reduction of water content

relieves the back relaxation [28] The deformation proper-

ties of IPMCs depend strongly on their water content

while the excess free water is responsible for the relaxa-

tion deformation. Therefore, adequate water content of

membrane is essential to maximize the performance of

IPMC.

The thickness of all casting membranes was measured

to be more than 0.3 mm, as shown in Table 2. The water

contents of the casting membranes are summarized in

Fig. 7. Evidently, EG and DMSO have higher water con-

tents than that of the Nafion 117 in fully hydrated states.

The water contents follow the sequence, namely, EG >DMSO > Nafion 117 > DMF > NMF.

FIG. 6. The relation between ionic migration and deformation of IPMC. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]

TABLE 2. Ion-exchange capacity and hydrated/dry thickness values of

prepared samples.

Samples

Ion-exchange

capacity (meq/g)

Thickness (mm)

Dry Hydrated

EG 953 6 14 0.312 0.379

DMSO 978 6 22 0.309 0.369

DMF 996 6 10 0.311 0.358

NMF 941 6 11 0.306 0.347

Nafion 117 1104 6 16 0.178 0.198

DMF, N,N’-dimethyl formamide; DMSO, dimethyl sulfoxide; EG,

ethylene glycol; NMF, N-methyl formamide. FIG. 7. Water content of the membranes.


These results could attribute to the effect of additives

on the conformational formation of the ionic clusters

during the solution casting process and the breakup of

the ionic clusters when the membrane is hydrated.

Nafion molecules have different conformations in differ-

ent solvents, which strongly depend on the dielectric

constant and solubility parameter of the additives. As

solvents were evaporated from casting solutions, the sul-

fonic acid groups on the side chains of Nafion will

move along with perfluorocarbon backbones to form

solid membranes. During this process, various backbone

aggregations in membranes lead to the formation of dif-

ferent sulfonic acid group aggregations. When larger

perfluorocarbon backbones aggregate in membranes,

larger phase separation occurs between perfluorocarbon

backbones and vinylether branched chains, leading to

the formation of larger sulfonic acid group aggregations

on the side chains. In addition, the water content of

membrane is proportional to the degree and sizes of sul-

fonic acid group aggregations [29]. Thus, more sulfonic

acid group aggregations cause higher water content. The

data from Fig. 7 reveal that the water contents of the

casting membranes were also affected by additives. The

experimental results were consistent with that reported

by Ma et al [22].

Furthermore, we investigate the influence of water

content on the mechanical properties of the casting mem-

branes and SO-based IPMCs. Assuming that the casting

membrane was a kind of homogeneous material, the

bending modulus were obtained by Eq. 5 and given in

Table 3. There was great disparity of bending modulus

between the dry and wet states of the casting membranes,

which are also observed in SO-based IPMCs. This result

is mainly caused by the differences of the water content.

He et al. [30] reported that as the thickness of the base

membrane increases, the bending modulus of the mem-

brane and SO-based IPMCs increases. However, for EG

and DMSO, a larger thickness displays a lower bending

modulus. The possible reason may be more water mole-

cules in the casting membranes inside EG and DMSO as

shown in Fig. 7. It is well known that water is a good

plasticizer, even in small quantities because of its low Tg.

In wet state, the bending properties of membrane and

IPMC are diminished because of the disconnecting poly-

mer chains and volume change of the substrate material,

as well as the plasticizer effect of water [31]. Moreover,

using DMF and NMF as additives, there was an increase

of the bending modulus because of less water molecules,

which directly led to the enhancement of interaction

among the perfluorocarbon backbones.

Analysis of IEC and Ionic Conductivity

The IEC of IPMC depends on the number of sulfo-

nate groups in a certain volume of material, which corre-

spond to the number of substituted (non-H1) charge-

balancing cations in the IPMC. Theoretically, the larger

number of sulfonate leads to better performance of the

IPMC. As illustrated in Table 2, the IECs of IPMCs

partly depend on the used additives, and Nafion 117 has

a higher IEC than other samples. Generally, membranes

with higher ICE have more available acidic groups and

consequently higher conductivities. However, the mem-

brane with higher conductivity does not always have

higher IEC.

Figure 8 shows the conductivities of casting mem-

branes in H1 and Na1 forms at room temperature and the

conductivity sequence is EG > DMSO > Nafion 117 >NMF > DMF. The conductivities in H1 form are higher

than that in Na1 form because of stronger interaction

between counterions in Na1 form. Slade [32] and Dimi-

trova [33] investigated the relation between thickness and

conductivity of the casting membranes and proposed three

different hypotheses to explain this phenomenon, they are

(i) differences in the water content; (ii) structural changes

related to the production process; and (iii) layered struc-

ture of the membranes. The results in Fig. 8 are not in

strict accordance with the results reported in [32], because

it is very difficult to confirm predominant factor affecting

TABLE 3. Bending modulus of prepared samples in dry and wet states.

Modulus (MPa)

Samples

Bare membrane IPMC

Dry Hydrated Dry Hydrated

EG 315.3 142.1 367.9 183.7

DMSO 337.6 200.2 380.8 236.2

DMF 767.8 258.3 711.9. 291.8

NMF 635.1 265.1 676.7 310.5

Nafion 117 402.8 164.2 476.1 254.6

DMF, N,N’-dimethyl formamide; DMSO, dimethyl sulfoxide; EG,

ethylene glycol; IPMC, ionic polymer–metal composite; NMF, N-methyl

formamide.

FIG. 8. Conductivities with H1 and Na1 form of casting membranes

and NafionVR

117 at room temperature.


the conductivity of the membrane. Nevertheless, the

results clearly show that the EG and DMSO have better

conductivities than other membranes.

In addition, the additives have great effect on the water

content of the membranes (Fig. 6) and the EG has the

highest value. These results reveal that the conductivities

FIG. 9. (a) DSC thermograms of EG(a), DMSO(b), DMF (c), NMF (d), and NafionVR

117 (e) as a reference,

respectively. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]


of the samples are more related to the water content

rather than IEC properties of the membranes.

Analysis of DSC

In order to assess the influence of the additives on

thermal behavior of the casting membranes and IPMCs,

DSC was performed from 0�C to 300�C, and the results

are shown in Fig. 9. The state change of samples could

be divided into two main stages of temperature range.

The first stage was located close to 110�C and exhibits

an intense endothermic effect because of the glass transi-

tion of the polymeric matrix, while the second stage was

located close to 230�C, in which the small endothermic

peak is assigned to the melting of crystalline regions [34].

By measuring the enthalpy change, we find it is very little

for the quantity of crystalline regions inside the casting

membranes. The two endothermic stages represent the Tg

and Tm, respectively, the temperature points of which are

determined by the point of inflection in our study. From

Fig. 9, the Tg of EG, DMSO, DMF, NMF, and Nafion

117 were found to be 104.90, 131.37, 114.81, 109.39, and

117.58�C, respectively, all of which show a similar Tm

value. DMSO shows the highest Tg among samples.

These differences of Tg may result from the change of the

chain structure and ion clusters of the polymeric matrix.

According to Gierke et al. [29], a change in the micro-

structure of the casting membranes simultaneously leads

to a different number of sulfonic groups per aggregate

and a change in the size of the aggregate, thus various

organized clusters and cohesive interactions are expected.

Consequently, different energy is required to overcome

the ionic interactions and to cause higher mobility of the

chains. The Tm of the casting membranes is almost the

FIG. 9. (Continued).


same as in DSC results, only a small gap among them. In

addition, this could be attributed to the same curing tem-

perature with the result of forming the same proportion of

crystalline region in the copolymer. The variation of the

Tm values may be mainly because of errors as well as to

possible mass changes during experiment process.

In addition, the data revealed that the Tg of SO-based

IPMC is lower than that of the corresponding casting

membrane while the Tm has a contrary result, which all

samples comply with. So, the Pd electrodes help to

expand the temperature range from Tg to Tm of the mem-

branes. The above conclusion has been confirmed by

rerunning more samples. Thermal behaviors with errors

of casting membranes and SO-based IPMCs are summar-

ized in Table 4.

Actuation Displacement

Figure 10 represents the electromechanical perform-

ance of the EG-, DMSO-, DMF-, NMF-, and Nafion

117-based IPMC actuators as a function of time at 2.0 V

DC voltages. Figure 10a shows the actuation displace-

ments. From Fig. 10a, the actuators immediately actuated

toward the anode side at 2.0 V DC voltages because of

the movement of the water molecules and hydrated cati-

ons toward the cathode. After 2 s, the actuators started to

slowly relax back toward the cathode, whereas EG- and

DMSO-based IPMC actuators display a faster back relax-

ation than others, which is attributed to the higher IEC

and higher water content of the casting membranes. SO-

based IPMC actuators show the tip displacement of 4.02,

3.22, 3.17, 1.1, and 6.9 mm, respectively, at a voltage of

2.0 V DC voltages. It should be noted that the Nafion-

based IPMC actuator attained the highest value of actua-

tion displacement with time among all samples. Because

the elastic modulus of the casting membrane increases

with the thickness, which makes IPMC to have larger

rigidity, thus Nafion 117-based IPMC exhibits larger dis-

placement as the thickness decreases with the same input

voltage. However, as the thickness increases, EG-based

IPMC still represent a larger value of tip displacement

than other thicker IPMCs. Although there are approximate

thicknesses among the thicker IPMCs, the water content

of EG-based IPMC is higher of all samples. Therefore,

this result could attribute to the interaction of the water

content and thickness of the casting membranes.

Figure 10b shows the blocking forces of SO-based

IPMCs, which display back relaxation just like the dis-

placements. As can be seen, the thicker IPMCs exhibit a

considerably higher blocking force compared to the thin-

ner Nafion 117-based IPMC. At 2 V DC voltage input,

the blocking forces for the thicker IPMCs reach up to

1.536, 1.19, 1.0, and 0.873 mN, respectively, whereas the

forces for Nafion 117-based IPMC is up to 0.377 mN.

The outstanding performance of the thicker IPMCs may

be because of the larger thickness and higher water con-

tent inside the base membranes. As was found from the

thickness and water content measurements (Table 3 and

Fig. 6), the increase of thickness enhanced the bending

modulus while the excess water improved an electro-

osmotic drag of water induced by ionic migration from

the anode to cathode direction. It can be concluded that

TABLE 4. Thermal analysis (Tg; Tm) of Nafion, modified Nafion, and

SO-based IPMCs.

Materials Tg(�C) DH(mJ) Tm(�C) DH(mJ)

EG 104.90 6 2.1 –25.14 224.88 6 2.4 –7.27

EG–IPMC 98.45 6 3.2 –58.29 232.49 6 2.5 –5.29

DMSO 131.37 6 1.6 –29.06 228.85 6 3.2 –1.69

DMSO–IPMC 106.37 6 2.3 –67.24 237.88 6 2.7 –5.11

DMF 114.81 6 2.8 –29.71 233.25 6 4.2 –8.00

DMF–IPMC 106.31 6 1.6 –100.48 237.30 6 2.7 –2.39

NMF 109.39 6 2.2 –30.27 237.37 6 3.2 –5.59

NMF–IPMC 95.91 6 2.6 –53.73 240.95 6 2.5 –4.20

Nafion 117 117.58 6 3.1 –192.92 234.00 6 2.6 –3.51

Nafion 117–IPMC 104.45 6 2.9 –81.28 244.99 6 3.4 –4.41

DMF, N,N0-dimethyl formamide; DMSO, dimethyl sulfoxide; EG,

ethylene glycol; IPMC, ionic polymer–metal composite; NMF, N-methyl

formamide.

FIG. 10. Comparison of step responses with SO-based IPMCs: (a) The

tip displacement and (b) The blocking force. [Color figure can be viewed

in the online issue, which is available at wileyonlinelibrary.com.]


as the thickness and water content increases, the blocking

force increases. The conclusion is consistent with that

reported by Lee et al. [35]. EG-based IPMC exhibits the

largest blocking force of 1.536 mN approximately. Thus,

the thickness and water content of polymer matrix plays a

significant role in improving the blocking force of IPMC.

The comparison of the actuation displacement and

force results of SO-based IPMC actuators showed that the

casting membrane-based IPMC actuators gave lower

actuation displacements with time than the Nafion

117-based IPMC actuator, which mainly depend on the

difference of the thickness. However, among the casting

membrane-based IPMC actuators, EG-based IPMC actua-

tor displays higher actuation force. This might be because

of higher modulus, higher water content, and IEC of cast-

ing membrane with EG as an additive, which provide the

favorable condition of continuous migration of Na1 and

hydrated cations toward the cathode side.

Strain Energy Density and Efficiency

The strain energy density of EG-, DMSO-, DMF-,

NMF-, and Nafion 117-based IPMC actuators was

obtained by measuring the deformation under 2 V DC

voltage. Generally, the generated mechanical energy

includes kinetic energy and strain energy during the

IPMC actuating process. Because the former type of

energy is much smaller than the latter one, the kinetic

energy is ignored and the strain energy resulted from the

bending motion of the IPMC is considered as the gener-

ated mechanical energy in this study. The value of the

strain energy of a cantilever beam (one end rigidly fixed

and the other end free, as shown in Fig .2 as a function

of uniaxial normal stress rx (in length direction of the

strip) can be expressed as follows:

V e ¼1

2

ðrxexdV¼ 1

2

ðr2

x

Eeq

dV (6)

where Eeq is the equivalent bending modulus of IPMC

assuming that IPMC was isotropic.

Considering that the length l and the moment of inertia

I of the cantilever could derive to the value of bending

moment M(t) at time t, the strain energy of IPMC can be

written as follows:

V e ðtÞ ¼ðl

0

½MðtÞ�2

2EeqIdx¼MðtÞ2l

2EeqI(7)

Furthermore, according to the Euler–Bernoulli beam

theory, the moment M(t) can be described by the meas-

ured displacement d(t) and corresponding equivalent force

FeqðtÞ at the free end of the IPMC (x 5 l).

MðtÞ ¼ FeqðtÞl (8)

h ¼ l

RðtÞ ¼MðtÞlEeqI

(9)

RðtÞ ¼

�d2 þ dðtÞ2

�2dðtÞ (10)

where l, h, R(t), and d represent the free length, rotation

angle, radius of curvature, and the distance between the

observation point and the fixed end of IPMC strip,

respectively.

Combining the Eqs. 6–10, the strain energy density at

time t can be expressed as the function of d(t), as in the

following equation:

q e ¼V e ðtÞ

V¼ 2lEeqId2ðtÞ

V½d2 þ d2ðtÞ�2(11)

where V represents the volume of each sample.

Figure 11a shows the measured current at 2 V DC

voltage, and the current responses were capacitive, which

may be attributed to the double layers adjacent to Pd par-

ticles metallic layers. Generally, when Nafion-based

IPMCs using water as a solvent is subjected to a DC volt-

age, it undergoes a fast bending deformation towards the

FIG. 11. Results of the actuation voltage applied to SO-based IPMCs:

(a) Measured current and (b) Calculated strain energy. [Color figure can

be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

F11


anode, followed by a slow relaxation toward the cathode.

This relaxation process could attribute to the slow diffusion

of water molecules from the stiffened cathode to the elasti-

cally softened anode. The energy density curves (Fig. 11b)

are obtained based on the above derivation. It can be noted

that the energy density have similar trends with the bend-

ing deformation of SO-based IPMC actuators that a rapid

rise followed by a slow decline before going back to the

equilibrium position. The Nafion 117-based IPMC actuator

has the largest value of strain energy up to 0.0181 J=mm3

before the relaxation occurs because of larger tip deforma-

tion and smaller thickness. For the casting membrane

based IPMC actuators, the strain energy of the EG-based

IPMC actuators reached up to 0.0113 J=mm3, while the

NMF-based IPMC get a poor value of 0.0012 J=mm3.

From the previous discussion, we can conclude that the

variation of energy densities depends on the combined

influence of a variety of factors, including water contents,

IECs, conductivities, and bending modulus.

In addition, the energy density of SO-based IPMC

actuators gradually increases after going back to the equi-

librium position and the DMSO-based IPMC actuator rise

to 0.0098 J=mm3, which is higher than other SO-based

IPMC actuators. This reverse deformation will result in

high-energy consumption and limit the potential applica-

tions in relevant areas.

In order to estimate the performance more clearly, the

energy efficiencies of SO-based IPMC actuators were cal-

culated by the ratio between generated mechanical energy

and applied electrical energy. The dynamic energy con-

version factor (DCF) and the average energy conversion

factor (ACF) can be evaluated by the following equation:

gDCF ¼V e ðtÞuðtÞiðtÞ (12)

gACF ¼

ðV e ðtÞdtðuðtÞiðtÞdt

(13)

where uðtÞ;iðtÞ; and t represent instantaneous electrical

voltage, current, and loading time, respectively.

The DCF results of the SO-based IPMC actuators were

shown in Fig. 12a. It could be noted that EG-based IPMC

displays larger values (0.69%) of all the casting

membrane-based IPMC actuators, only lower than that of

Nafion-based IPMC actuator. Mohsen Shahinpoor et al.

[36] reported that the efficiency was significantly reduced

by the water leakage out of the surface electrode at low

frequencies. The optimum efficiency values of IPMC are

approximately 2.5–3.0%. In addition, the important sour-

ces of energy consumption for the IPMC actuation had

been summarized in their works. Meanwhile, R. Baugh-

man et al. [37] indicated that the energy conversion factor

is <1%. However, the obtained values still are favorable

compared to other types of bending actuator, that is, con-

ducting polymers and piezoelectric materials at similar

conditions, exhibiting considerably lower efficiencies [38,

39]. The ACF is calculated by the Eq. 13 during the time

before the relaxation occurs. Figure 12b shows the AFC

of SO-based IPMCs. Except for Nafion 117-based IPMC

actuator, the EG-based IPMC actuator displays larger

energy efficiency more than 0.4% as well.

CONCLUSIONS

The effects of different additives on the performances

of casting membranes were analyzed, using a series of

experimental results, as well as the SO-based IPMCs. The

membranes were successfully obtained based on commer-

cial NafionVR

dispersion by solution casting process with

different additives. The surface and cross section of

Nafion 117-based IPMC were observed by SEM images,

and the morphologies of the membranes were character-

ized by AFM topography. There are significantly differ-

ences in topography among these samples. And the

microstructures of the casting membranes with EG and

DMSO are more loose and amorphous, leading to higher

water contents and thus higher conductivity than those

FIG. 12. DCF and ACF of SO-based IPMCs: (a) The dynamic energy

conversion factor (DFC) results and (b) The average energy conversion

factors (AFC) results. [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com.]


with DMF, NMF, and Nafion 117. For the thermal prop-

erties from DSC curves, the Tg of the casting membranes

have been changed by the additives while a similar Tm

results from the same curing temperature, which seems to

have no relation with the additives. Among the casting

membrane-based IPMC actuators, EG-based IPMC actua-

tor has larger deformation and blocking force, higher

strain energy density, and conversion efficiency at 2 V

DC voltage, whose electromechanical property is most

close to that based on Nafion 117. However, our research

shows that all casting membranes with additives lead to

lower energy density and energy conversion factors than

commercially available Nafion 117 membrane. Even so,

there is a feasible improvement in performance of SO-

based IPMC actuators for enhancing the actuation force.

In addition, it is clearl that EG is a more preferable addi-

tive during the casting process for the enhancement of

IPMC performance.

This article has confirmed that additives have great

influences on the mechanical properties of the casting

membranes and consequently electromechanical coupling

of IPMCs. Further research works will focus on exploring

other additives and different methods to improve the per-

formance of the SO-based IPMCs for different application

background.

ACKNOWLEDGMENTS

The authors thank Mr. Jinyan Zhao for his help in

AFM measurements and Mr. Liangliang Zhang for his

help in DSC and SEM measurements.

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