Reliability of High Strain Ionomeric Polymer Transducers Fabricated
Using the Direct Assembly Process

Barbar Akle, Saila Nawshin, and Donald Leo

Center for Intelligent Material Systems and Structures

Department of Mechanical Engineering

Virginia Tech, Blacksburg, Virginia

**Abstract**

Ionomeric polymer transducers have received considerable attention in
the past several years. These actuators, sometimes referred to as
artificial muscles, have the ability to generate large bending strain
and moderate stress at low applied voltages. As sensors, ionic polymer
transducers generate an electric response due to a mechanical strain.
Typically, ionic polymer transducers are composed of Nafion-117
membranes with platinum electrodes and are saturated with water
diluents. Recently the authors have developed a novel fabrication
technique named the Direct Assembly Process (DAP), which allowed
improved control of the electrode morphology and composition. The DAP
consists of spraying two high surface area metal-ionomer electrodes on a
Nafion membrane. The benefits of the DAP process over previous methods
is the ability to control the thickness of the electrode, the ability to
control the composition of the electrode layer of the transducer, and
the ability to be used with a wide variety of diluents. In past work we
have demonstrated that platinum, ruthenium dioxide, and single-walled
carbon nanotubes can be used as the electrode material with diluents
such as water, formamide, and ionic liquids. In this work we will
present a reliability study of transducers fabricated using the DAP.
Water-hydrated transducers dehydrate and stop moving within 5 minutes
while operating in air under the application of +/-2V. Ionic liquid
based transducers are demonstrated to operate in air for over 400,000
with little loss in performance, and are reliable up to 1 million cycles
with a performance loss of less than 43%. The main source of degradation
is the adhesion of the conductive surface to the high surface area
electrode. This is enhanced in this study by using a PUU linking polymer
that has good adhesion properties to gold. Large voltage and large
strain are proven to decrease the life of the transducer. Formamide
based samples are stable for 3 days under a 1V actuation signal, while
only reliable for 3 h to 4 h under a 2V actuation signal. Solvent
evaporation is the main reason for degradation in formamide samples and
it is increased at 2V indicating some electrochemical activity at such
high voltages. Finally the initial drop in performance and the
fluctuation in the generated strain are shown to be due to the loss of
humidity absorbed from ambient air and the fluctuation in this ambient
humidity respectively.

1.  Ionic Polymer Transducer, Direct Assembly Process, Reliability,
    Actuator

Introduction

Ionic polymers membranes are materials that exhibit ionic conductivity
and ion selectivity. Several researchers have shown that these polymer
membranes can function as electromechanical actuators and sensors
\[1-3\]. An ionic polymer transducer is made of an ionomeric membrane
(typically Nafion, a product of DuPont) sandwiched between two
conductive electrodes. In this state the membrane will bend toward the
anode when a voltage (typically less than 5 V) is applied across its
thickness. Similarly, the polymer will generate charge when deformed.
Ionic polymer transducers have the advantage of being able to generate
large strains \[1, 4\] under small applied voltages, are compliant and
thus compatible with conformal structures, and have a very high
sensitivity to motion when used in charge sensing mode.

Transducers fabricated from ionomeric polymers suffer from several key
limitations, however. Most notable is dehydration and the corresponding
loss in performance of these materials when operated in air. Also, the
process for plating the membranes with the platinum and gold electrodes
is relatively expensive, time-consuming, and produces inconsistent
transducers. Efforts to overcome these issues have been hindered by the
fact that the performance of the membranes is intimately linked to the
morphology of the metal electrodes \[5\]. Bennett and Leo \[6\] have
previously shown that the hydration problem can be overcome by using
ionic liquids as the solvent in these transducers.

Recently Akle et al.\[7\] developed a method called the Direct Assembly
Process (DAP) for fabricating transducers with controlled electrode
dimensions and morphology. In this method large surface area electrodes
are painted on an ionomer membrane. An alternative method that has been
used by a number of researchers, the impregnation-reduction approach, is
a method by which the metallic electrode is chemically deposited on the
membrane \[8\]. Although this method is effective, the process does not
enable direct control on the electrode morphology and limits the type of
electrode particulate that can be placed on the membrane \[4\]. In
contrast, the DAP method allows direct control over the thickness of the
electrode and enables a larger variety of electrode compositions and
transducer diluents \[provide reference\]. Transducers fabricated using
the direct assembly process have been shown to generate strains on the
order of 10% under a +/-2V potential, compared to less than 1% for
transducers fabricated using the impregnation--reduction method \[7\].

In this paper we study the reliability of ionic polymer transducers
fabricated using the direct assembly process. The diluents incorporated
in this study are formamide and 1-ethyl-3-methylimidazolium
trifluoromethanesulfonate (EMI-Tf) ionic liquid. The reliability of the
transducers is analyzed in terms of the peak strain of the sample and
the applied voltage. Surface of degraded transducers are imaged and
analyzed using a Scanning Electron Microscope (SEM). Enhancing the
reliability of the transducers by modifying the fabrication process is
also presented.

Experimental procedures

In this section the Direct Assembly Process fabrication technique is
presented, and the electromechanical characterization setup is
discussed.

**Direct Assembly Process**

The direct assembly process is employed in the fabrication of the hybrid
transducers. In this method, a bare Nafion membrane is first exchanged
from the proton counterion form into one of the alkali metal forms
(Li^+^ was used for this study). This is to prevent charring of the
membrane during the drying step, which has been witnessed when Nafion
membranes in the acid form are heated for extended periods of time.
After ion exchange, the membrane is dried in an oven at 150 C under
vacuum for 12 h. The dry membrane is then immersed in the corresponding
solvent. For ionic liquids the ionomer is placed in a neat EmI-Tf ionic
liquid and heated to 150 C for 4.5 h. The uptake of ionic liquid during
the immersion step is 60 % by the dry weight of the membrane. Formamide
is easier to imbibe in the Nafion membrane; an uptake of more than 100%
is obtained in 1h at 60 C.

![](media/image1.png){width="1.5251968503937008in"
height="1.9236220472440946in"}

**Figure 1: (a) schematic of am ionic polymer transducer assembled using
the direct assembly process. (b) SEM image of the upper electrode of an
ionic polymer transducer.**

After the solvent is imbibed into the membrane, the electrodes are
applied using a conductive powder painting technique. A polymer /
conductor solution is prepared containing a mixture of 5% Nafion
dispersion, isopropyl alcohol (IPA), de-ionized water, and the
conducting powder. In this study RuO~2~ particles are utilized due to
there large suface-area, and good electrical conductivity. Previous work
\[7\] demonstrates that RuO~2~ based transducers generates the most
strain in a repeatable and feasible manner. This solution is painted
directly onto each surface of the solvent containing Nafion membrane.
The alcohol solvents and water are removed by heating the membrane under
an infra-red lamp during the painting process; typically a total of
twenty layers are applied to each side of the membrane. After the
application of the ionomer/conducting powder composite, the sample is
sandwiched between two 100nm thin conductive gold foils and the five
layers of the composite are fused together by a hot-pressing process. A
thin layer of IPA-diluted Nafion solution is applied on both electrodes
to enhance the binding of the gold layer to the actuator -- see Figure
1(a). Later in this study, the Nafion Layer is replaced with a thin
layer linear amine-terminated poly(urethane urea) (PUU) elastomer. For
the membranes that are swollen with the ionic liquid, hot-pressing is
done at 4 MPa and 180 C for approximately 10 to 20 s depending on the
composition of the electrode for ionic liquid solvated samples. For
formamide-solvated transducers a 4 MPa pressure and a temperature of 160
C is applied for less than 5 sec. The gold foils used are a Falcon-Brand
24 carats gold leafs. Incorporating the gold foils on the surface
decreased the surface resistance from greater than 100 kΩ/cm for the
RuO~2~ layers to less than 1 Ω/cm. The samples where freeze cut and
imaged by field-emission scanning electron microscopy (FE-SEM) using a
LEO 1550 microscope operating at 5 kV accelerating voltage. The
degradation of the surface is imaged by the SEM without cutting the
samples. A typical SEM image of the electrode in an ionic polymer
transducer made using the DAP method is shown in Figure 1(b).

**Electromechanical Characterization**

The materials fabricated in this work are characterized as
electromechanical transducers using a custom built characterization
setup. The samples used for these tests are 3 mm x 30 mm rectangular
transducers held in a cantilevered configuration in a clamp with fixed
gold electrodes (see Figures 2 (a) and (b)). The free length of the
transducer is approximately 19 mm. The thickness of the transducer
varies between approximately 0.15 mm and 0.25 mm depending on the
thickness of the applied electrode and that of the membrane. A potential
V~i~ is applied across the ionic polymer transducer in the form of a
sine wave of Frequency 0.2 Hz up to 1 Hz. The magnitude was also varied
from 1V up to 3V. The displacement *x(t)* at any point along the length
is measured with a Canon PowerShot G6 camera. A grid with a spacing of
0.6mm is placed under the tip of the moving tip. Short movies where
taken for 30sec and the number of grid points are counted between the
peak-peak displacement of the polymer. Interpolating between the grid
points the overall resolution of the system is calibrated to be 0.3mm
(see Figure 2(a)). The current *i(t)* induced in the polymer due to the
applied voltage *v(t)* is also measured by measuring the voltage drop
across a small (0.1 Ω) resistor---see Figure 2(a). Monitoring the
current provides us a mechanism to detect the source of degradation in
performance. This is done by monitoring the capacitive and resistive
properties of the transducers with number of cycles.

![](media/image3.png){width="2.12992125984252in" height="2.5in"}

**Figure 2: (a) Electromechanical characterization setup, attached to a
current measuring circuit (b) Sample overlay of two frames**

The free deflection *x(t)* (in the time domain) is used with equation
(1) to compute the bending strain as a function of time or frequency. In
equation (1), ε is strain, *h* is the thickness of the sample, and
*L~f~* is the free length of the sample. This equation assumes that the
transducer bends with a constant curvature; experimental results
demonstrate that this is an appropriate assumption for the experiments
performed in this paper \[4\].

$\varepsilon(t) = \frac{x(t)h}{L_{f^{2}}}$ (1)

The gain of the current measuring circuit shown in Figure 2 is computed
according to

$v_{o}(t) = \frac{R_{2}}{R_{1}}0\text{.}1 \ast i(t)$. (2)

The impedance is computed by measuring the applied voltage and induced
current. Using a Fourier transform the ratio of the voltage and current
are computed in the frequency domain according to

$Z(\omega) = \frac{v(\omega)}{i(\omega)}$. (3)

Results and Discussion

The experimental results of the transducers built using the direct
assembly process discussed earlier are presented in this section. First
we will present and compare the reliability results of Formamide and
EMI-Tf saturated samples. Next, the reliability as a function of
operating frequency, peak strain, and magnitude of the potential are
also presented and discussed. SEM images of the surface are obtained and
are used to analyze the source of deterioration. Finally the linking
polymer is changed to PUU, and reliability results are compared to
similar sample with Nafion Linking polymer.

**Reliability of Formamide solvated transducers**

![](media/image4.wmf){width="4.446527777777778in"
height="2.652083333333333in"}**Figure 3: Actuation strain of formamide
saturated samples under 1 V and 2 V actuation and operated at a
frequency of 0.2Hz.**

A possible mechanism of degradation of actuation performance in ionic
polymer transducers is the reduction in ionic conductivity that occurs
due to loss of solvent. This degradation mechanism causes the loss of
actuation in water hydrated samples when operated in air \[6\]. Water
hydrated samples lose most of its performance in less than 1000 cycles
when operated in air under a frequency of 1Hz and a potential of 2V. In
this study we measure the reliability of formamide saturated ionic
polymer transducers. Formamide samples degraded relatively fast compared
to ionic liquids, but kept moving around 1000 cycles longer than that of
water samples for the small number of actuators tested. Under 2V and
0.2Hz formamide samples started to significantly degrade after around
2000 cycles, which corresponds to 3h of actuation as shown in Figure 3.
A formamide sample tested with 1V potential actuated until 25000 cycles
without any significant loss in performance. This corresponds to
approximately 35 h of actuation.

Solvent loss due to actuation was confirmed by measuring the weight of
the transducer at the beginning of the experiment and at the conclusion
of the experiment. At the start of the study the weight of the sample
which is tested under 1V dropped from 0.0423g to 0.0263g, while the 2V
tested sample dropped from 0.403g to 0.0235g. This is a weight loss of
approximately 38%.

![](media/image5.wmf){width="3.308267716535433in"
height="2.189763779527559in"}![](media/image6.wmf){width="3.3881889763779527in"
height="2.2216535433070868in"}

**Figure 4: Reliability as a function of (a) peak strain, and (b) as a
function of voltage**

**Reliability of EMI-TF solvated transducers**

Three ionic polymer transducer membranes are prepared for this
experiment. All the samples are made using the same EMI-Tf ionic liquid
saturated Nafion 117 ionomeric membrane. The electrode is sprayed from
the same mix with a sample having an electrode thickness of 30µm
compared to another with a 10µm thick electrode. Table 1 shows the
nomenclature and architecture of these samples. Akle et al \[7\]
demonstrated that the peak strain increase with electrode thickness.
Samples S3 and S4 are cut from the same transducer; S3 is tested under
1.5V while S4 is tested under 2V. Shown in Figure 4(a), is the
reliability of samples S1 and S2 under 2V and 0.2Hz. It is clear from
Figure 4(a) that a larger strain is causing a faster deterioration in
the actuation performance. The applied potential had a major effect on
the reliability --see Figure 4 (b). This might be due to the increased
strain since there is no significant loss in sample weight. Notice that
sample S4 degraded significantly after 15,000 cycles compared to over
100,000 for that of sample S1. This is explained by the fact that the
peak strain in sample S4 is almost double that of S1.

**Table 1: Sample architecture**

  -------- ------------------ ------------------ --------------------------
           Membrane           Electrode          Test Conditions
                              Thickness          

  S1       180 μm             30 μm              2V and 0.2Hz
           Nafion/EMI-TF      RuO~2~/Nafion      

  S2       180 μm             10 μm              2V and 0.2Hz
           Nafion/EMI-TF      RuO~2~/Nafion      

  S3       180 μm             30 μm              1.5V and 1Hz
           Nafion/EMI-TF      RuO~2~/Nafion      

  S4       180 μm             30 μm              2V and 1Hz
           Nafion/EMI-TF      RuO~2~/Nafion      
  -------- ------------------ ------------------ --------------------------

Observing the initial drop in the performance during the first 100
actuation cycles (see Figure 4(b)) motivated us to study the effect of
humidity on these tests. An experiment with controlled humidity is
performed to study this initial drop effect and the fluctuations in the
strain output throughout the experiment. The sample tested in this
experiment is similar in architecture to samples S3 and S4.

![](media/image7.wmf){width="4.695833333333334in"
height="2.490972222222222in"}

**Figure 5: Reliability of an ionic liquid sample under zero humidity
condition.**

In this experiment the testing apparatus is placed in a sealed clear box
with silicon humidity absorbents. The humidity was less than 4% for the
duration of the experiment, and the camera was placed outside the clear
box. The tested sample was operated under 2V and 1Hz.

As shown in Figure 5, the initial drop is significantly lower compared
to the initial drop in all the other open air reliability experiments
(see Figure 4(b)). Also the data is smoother, with much less
fluctuations. This indicates that the initial drop in the reliability
curve is due to the loss of some of the absorbed water, and the
fluctuation is due to the changes in the ambient humidity.

Solvent evaporation of EMI-TF in the transducer is assessed in this
study. Ionic liquid has an immeasurably low vapor pressure. Ionic liquid
samples where dried in the oven under 70C and vacuum for 24 h before the
reliability test started. The weight is measured immediately after the
sample is taken out of the oven. After few weeks of actuation the same
drying process took place to neglect the effect of room humidity, and
the weight is measured again. In all the ionic liquid samples the weight
difference was non significant (\< 1%).

All the results so far demonstrated that the actuators solvated with
ionic liquids exhibited reduced actuation strain after typically 200000
cycles. It was found by visual inspection during the tests that the
actuators developed brown islands on the gold electrodes (see Figure
6(a)). This result motivated us to study the surface properties of the
transducers to correlate the growth of the dark regions on the electrode
with the reduction in actuation strain.

A LEO 1550 field-emission scanning electron microscopy (FE-SEM)
operating at 5 kV accelerating voltage was used to study of an ionic
liquid saturated transducer fabricated using the DAP after being
actuated for 400000 cycles at 2V and 2Hz (see Figure 6(b)).

![](media/image8.png){width="2.5208661417322835in"
height="1.7236220472440944in"}![](media/image9.png){width="2.247244094488189in"
height="1.9133858267716535in"}![](media/image10.jpeg){width="0.5062992125984253in"
height="2.027165354330709in"}

**Figure 6: (a) Image of a degraded ionic polymer transducer (b) Surface
SEM Image of an ionic polymer transducer after 400,000 actuation cycle
at 2V.**

The light areas in the SEM image shown in Figure 3 represent the gold
and the dark area is the high surface area RuO~2~-Nafion electrode. This
clearly demonstrates that the gold leaf is delaminating from the
electrode. The onset of cracking and delamination of the gold leaf could
be noticed in the part to the left of the SEM image. This is a
mechanical fatigue process due to the repetitive elongation and
contraction of the Nafion underneath the gold leaf. The delaminating
gold could be also noticed in Figure 1(b) which represents the cross
section of a similar transducer. These SEM images indicate that the weak
adhesion between the Nafion and the gold leaf is causing the gold to
delaminate, and hence the surface conductivity of the transducer will
deteriorate until the surface becomes an open circuit when complete
delamination occurs. These results demonstrate that the primary
mechanism for degradation in the actuation strain of the ionic liquid
samples is mechanical fatigue at the interface between the deposited
electrode and the gold leaf.

**Modifying the Linking Polymer for Increased Adhesion**

From the previously mentioned results, it is believed that the reason
for degradation in EMI-Tf ionic liquid saturated samples is the loss of
the gold leaf on the surface of the polymer. This conclusion is
supported by the SEM images and other reliability results, especially
that the peak strain is well correlated with the number of actuation
cycles. Furthermore the EMI-TF ionic liquid is well known to be highly
stable under 2V and this is proven in this paper by measuring the
insignificant weight loss in degraded samples. Linking the gold leaf to
the ionic polymer helps this conductive layer to stick uniformly to the
polymer surface. In this experiment two samples, one with PUU linking
and the other with Nafion where assembled. The samples are originally
cut from the same transducer membrane before the gold is added.

PUU is chosen in this work because it contains amine and amide groups
which react with gold to form a good adhesion \[9\]. The PUU used in
this study is proven to have excellent gold adhesion properties \[10,
11\]. Furthermore, this linear polymer has a melting temperature Tm=120
C, which is below the temperature we heat to press the transducer. This
will enable good adhesion between the PUU and the high surface area
electrode.

![](media/image11.wmf){width="5.811811023622047in"
height="3.1110236220472443in"}

**Figure 7: Tip strain of two ionic polymer transducers one with a PUU
and another with a Nafion linking degrading as a function of cycles.**

Shown in Figure 7 is the reliability of the two samples under 0.2 Hz and
2V. At the beginning of the experiment both PUU linked and Nafion linked
samples started close to 0.78% strain with the PUU linked transducer
performed around 0.02% strain more. This indicated that the addition of
the PUU linking layer didn't degrade the performance of the transducer.
However, the reliability result clearly demonstrates that PUU linked
polymer has much better reliability compared to the Nafion linked
transducer. As soon as actuation starts a noticeable difference of 0.1%
strain is realized only after 1000 cycles. The PUU sample lost less than
43% of its performance in over a million cycles under 0.2Hz and 2V,
while it maintained 50% of its original peak strain after 1.35 million
cycles. The Nafion linked polymer lost 50% of the original transduction
after 70000 cycles. This is an improvement by a factor of 18. Finally,
we note that 1.35 million cycles under 0.2Hz corresponds to
approximately 78 days of continuous operation.

Conclusions

This study demonstrates that ionic polymer transducers made using the
Direct Assembly Process are reliable up to 1 million cycles with a
performance loss of less than 43%. The main source of degradation is the
adhesion of the conductive surface to the high surface area electrode.
This is enhanced in this study by using a PUU linking polymer that has
good adhesion properties to gold. Large voltage and large strain are
proven to decrease the life of the transducer. Formamide based samples
are stable for 3 days under a 1V actuation signal, while only reliable
for 3 to 4 h under a 2V actuation signal. Solvent evaporation is the
main reason for degradation in Formamide samples and it is increased at
2V indicating some electrochemical activity at such high voltages.
Finally the initial drop in performance and the fluctuation in the
generated strain are shown to be due to the loss of humidity absorbed
from ambient air and the fluctuation in this ambient humidity
respectively.

Acknowledgements

The authors thank Serkan Unal and Timothy E. Long, Dept. of Chemistry,
Virginia Tech for providing the PUU materials. This work was supported
by the U.S. Army Research Laboratory and U.S. Army Research Office under
contract / grant number DAAD19-02-1-0275 Macromolecular Architecture for
Performance (MAP) MURI.

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