PAPER www.rsc.org/softmatter | Soft Matter

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Magnetic resonance imaging study of a soft actuator element during operation

Leila Naji,a John Alexander Chudekb and Richard Thornton Baker*a

Received 10th March 2008, Accepted 28th May 2008

First published as an Advance Article on the web 4th July 2008

DOI: 10.1039/b804033h

The physical structure and the actuation mechanism of a Nafion-based soft actuator – a Pt-containing

ionic polymer–metal composite (IPMC) – were investigated using in situ magnetic resonance imaging

(MRI) during application of four different electrical regimes. Importantly, the rawMRI data were used

to generate spatial maps of both proton density (PD) and proton spin–spin relaxation time (T2) across

the sample. These were successfully employed to study changes in the distribution and chemical

environment of water molecules absorbed within the operating actuator device. The IPMC sample was

mapped in this way during the application of a small d.c. potential across its thickness. Three main

phenomena were observed in the results: initial rapid increase in T2 at both electrodes, without an

observed change in PD; slower formation of a region of high T2 and high PD at the IPMC cathode; and

contraction of the polymer along the anode and its expansion along the cathode, giving rise to bending

actuation. Reversing the polarity of the applied potential resulted in the reversal of the direction of the

bending deformation of the IPMC sample and of the distribution of PD and T2 within it. These

phenomena were explained in terms of the unusual structure of Nafion and its interaction with host ions

and the electric field. Up to 20% of the total water content of the IPMC was found to be involved in

long-range electro-diffusion.

1. Introduction

Soft actuators are devices constructed of polymer or gel materials

that are able to undergo dramatic and reversible shape defor-

mation in response to the application of an electrical potential or

chemical stimulus. One class of soft actuator is made from

elements of ionic polymer, such as cation-exchanged Nafion†,

soaked with a solvent, usually water, and provided with high

surface area metal electrodes on opposite faces. Such IPMCs are

of great scientific and technological interest because of their

unique properties and wide range of potential applications in

medical, mechanical, electrical and aerospace engineering.1

IPMC materials are relatively inexpensive, light, compact and

flexible, and can be cut to any desired size and shape. They can

undergo very large bending deformations, require only very

small applied potentials (typically around 5 V) compared to

piezoelectric ceramic (several kV)2 or polymer actuator

technologies (tens of hundreds of V),3 and exhibit relatively short

(from ms to s) response times. Induced strains are also several

orders greater in IPMCs than in piezoelectric ceramics.4 These

properties make IPMCs suitable for potential applications in

artificial valves and muscle in medicine as well as in actuators

for manipulation of fragile objects in robotics and as micro-

actuators in MEMS devices.

A typical IPMC consists of a thin, cation-exchanged ionic

polymer membrane with metal electrodes deposited chemically at

both faces. The best electrodes are made up of small,

aEaStChem, School of Chemistry, University of St Andrews, St Andrews,Fife, UK KY16 9ST. E-mail: rtb5@st-andrews.ac.uk; Fax: +44 1334463808; Tel: +44 1334 463899bCollege of Life Sciences, Sir James Black Centre, University of Dundee,Dundee, UK DD1 5EH

† Trade mark of the Du Pont company

This journal is ª The Royal Society of Chemistry 2008

interconnected metal particles, generally platinum or gold, that

penetrate into the polymer membrane. A further metal layer of

gold or platinum is often deposited onto each electrode to

improve the electrical conductivity at the polymer surfaces. In

order to function, the IPMC must be impregnated with an

electrolyte solution, which is usually aqueous. Nafion is a suit-

able ionic polymer membrane for use in IPMC actuators. Nafion

has good chemical stability when immersed in the required

electrolyte solution,5 remarkable mechanical strength,6 good

thermal stability,7 high ionic conductivity8,9 and high selectivity

to the desired ionic species, even in high salt concentration.10,11

The Nafion group of materials are ionic polymers prepared as the

copolymer of tetrafluoroethene and perfluorinated monomers

containing sulfonic acid groups (Fig. 1). All of the H+ on the

sulfonate groups in the protonated Nafion membrane can be

exchanged for other cations, including Li+. The molecular

backbone forms hydrophobic regions in this ionic polymer and

largely determines the mechanical strength of the membrane. The

sulfonic acid groups at the ends of the short side chains are

thought to group together forming ionic, hydrophilic regions,

known as clusters.12 Cluster diameters are estimated to be about

4–10 nm, depending on the water content of the membrane.13,14

Nafion is able to absorb large amounts of water and other protic

liquids into these hydrophilic regions (clusters) and changes

volume as it does so. It has been reported that clusters may

Fig. 1 Chemical structure of Li+-exchanged Nafion.

Soft Matter, 2008, 4, 1879–1886 | 1879

Fig. 2 Schematic diagram of the bending actuation in an IPMC. (a) No

potential applied; (b) with potential applied.

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contain up to 100 sulfonate groups, with up to 20 water mole-

cules around each.13,14

Fig. 2 is a schematic diagram of the bending deformation of an

IPMC. On application of a small d.c potential across its thick-

ness, the IPMC actuator bends dramatically and reversibly

toward its positively charged surface. This is a result of the

electrically induced movement of cations – together with their

solvation shell of water molecules – through the ionic polymer

network, towards the negatively charged electrode (cathode).

The anionic sulfonate groups are tethered to the polymer and so

are unable to move in the applied electric field. The resulting net

movement of water towards one face of the polymer is sufficient

to cause the polymer to swell at this face and to contract at the

opposite face, so giving rise to a dramatic, reversible bending

motion. The magnitude and speed of deformation depend on the

nature and the ionic conductivity of the ion-exchange polymer,

its thickness, the structure and capacitance of the electrodes, the

level of hydration, the charge on the cations, the mobility of the

cations and the nature of the interface between the electrode and

the polymer.

Several models have been proposed since 1950 for the actua-

tion mechanism of IPMCs and these are described in an extensive

review by Shahinpoor and Kim.15 These models all consider

the sulfonate groups to be permanently attached to the

perfluorinated molecular backbone, whereas the counter cations

(in our case, Li+) are free to move through the polymer towards

the cathode, under the influence of an applied electric field.1 The

hydraulic model1 states that the electrically induced redistribu-

tion of the cations, with their solvation shell of water molecules,

within the IPMC contributes to volumetric swelling stresses

within the Nafion membrane and consequently causes the

bending deformation of the IPMC element. The electrostatic

model1 states that, in addition to volumetric swelling stresses, the

redistribution of cations, under application of an electric field,

produces internal stresses affecting the perfluorinated polymer

backbone. This model suggests that these stresses cause the

perfluorinated molecular backbone to relax and contract at

the anode (cation-depleted region) and to further extend at the

cathode (cation-rich region). The Yamagami–Tadokoro

model16,17 considers the volume change caused by changes in

water content and by the electrostatic force generated by ionic

migration in the membrane. The model presented by Nemat-

Nasser and Li,18 includes all the features of these models in

addition to the effect of electro-osmotic forces. This model

suggests that the redistribution of cations in the cathode- and

1880 | Soft Matter, 2008, 4, 1879–1886

anode-boundary layers, which causes the hydration-induced

pre-tension in the polymer, is accompanied by an osmotic pres-

sure differential within the membrane.

In the work presented here, multi-echo MRI is employed as

a powerful, non-destructive and non-invasive technique to image

an IPMC device during in situ application of an electrical

potential. The aim was to further understand the actuation

mechanism. This MRI method gives rise to images with high

signal-to-noise ratios because it is very insensitive to inhomoge-

neities in the static magnetic field. It is important to image the

electrically induced diffusion of water molecules across the

thickness of the Nafion membrane if we are to learn more about

the processes involved. Because of the limited spatial resolution

of the MRI technique, Nafion elements with a thickness of

approximately 2 mm were cast and prepared as IPMC actuators

to allow clear imaging of the changes occurring in proton

density, PD, and T2 relaxation time across their thickness.

2. Experimental

2.1 Casting of Nafion membrane

Thick Nafion membranes were prepared using a 5 wt% Nafion

solution (equivalent weight 1100) in a mixture of lower aliphatic

alcohols and water (45 wt%) (Aldrich).19 120 ml of the Nafion

solution were heated to remove excess solvent and reduce the

volume to 60 ml. To this was added 15 ml of dimethylformamide,

DMF, (Aldrich) to prevent the formation of cracks in the

membrane on drying.20 The mixture was poured into a Teflon

mould and left at room temperature for five days to dry. The

sample was heated in an oven at 70 �C for 24 h and ramped to

150 �C, the temperature wasmaintained at 150 �C for 2 h and then

allowed to cool. The product of this process was a 5 cm � 5 cm

sheet of Nafion membrane with a yellowish, translucent appear-

ance and dry thickness of 1.65 mm. This yellowish colour was

removed during an extensive cleaning process modified from that

of MacMillan et al.21 The clean, cast Nafion membrane was

stored in deionised (DI) water for use in IPMC preparation.

Solution state 1H, 13C and 19F NMR (Bruker Avance, 300 MHz)

was carried out on the cleaned, cast Nafion membranes. The

membranes in acid form were swollen with DI water for 1H and13CNMRandwith absolute ethanol for 19FNMRmeasurements.

No evidence of impurities was observed in the resulting spectra.

2.2 Preparation of IPMC elements

IPMC elements were prepared from the cast Nafion by adapting

the technique reported by Pak et al.22 The membrane surfaces

were roughened with abrasive paper, to increase the interfacial

area between the electrodes and the polymer, and cleaned using

the method described above. Pt electrode layers were deposited

onto each face of the polymer by soaking the membrane in

a 4.5 M solution of [Pt(NH3)4]Cl2 (Aldrich) and reducing the

absorbed cations to the metallic state in a 5 wt% aqueous

solution of NaBH4 (Aldrich). To improve the surface conduc-

tivity of the electrodes a thin (�10 nm) Au layer was deposited

over the Pt-impregnated surfaces of the cast Nafion by physical

vapour deposition (PVD). The protons on the –SO3� groups

were exchanged for Li+ cations by storing the IPMC elements in

saturated LiOH(aq) at room temperature for four weeks.

This journal is ª The Royal Society of Chemistry 2008

Fig. 3 Electrochemical assembly used in the in situ MRI studies.

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2.3 IPMC morphology

IPMC elements were cut, set in resin blocks and the exposed

surfaces were polished to allow analysis of their cross-sections in

an environmental SEM instrument (Philips XL30) with a field

emission gun, maximum spatial resolution of 2 nm at 30 keV and

incorporating energy dispersive X-ray spectroscopy (EDX)

analysis. Back-scattered electron (BSE) imaging and EDX were

employed to study the morphology and chemical composition of

the IPMC elements and of the Pt-impregnated electrode layers in

particular.

2.4 Magnetic resonance imaging

The multi-echo MRI imaging method used in this work was

based on the Carr–Purcell–Meiboom–Gill (CPMG) pulse

sequence,23 and simultaneously produces multiple echoes with

the same phase encoding.24 In this method, the multi-echoes

generated are detected and stored in separate data sets. There-

fore, a series of base images are produced which differ in spin

echo time, TE. The amplitude of the spin echo at a point (x,y),

Mx,y, and so the intensity of the point in the image, decreases

exponentially from its initial value, Mo, at a rate determined by

the spin–spin relaxation time, T2, and TE (eqn 1).

Mx;yðTEÞ ¼ M0 exp

�� TE

T2

�(1)

The contrast of the images obtained frommulti-echo imaging is

determined by the spin density (in this work, proton density, PD)

and the spin–spin relaxation time,T2, in the sample. Images of the

spatial variation of T2 and PD over the sample can be obtained

from the ‘‘raw’’ images by appropriate choice of the parametersTE

and TR, the repetition time. These images provide information

about the local state and physical properties of the sample. The

brightness in a T2 image is proportional to the length of T2. For

protons located in a regionwhere themolecules that contain them

can freely tumble, i.e.with high rotational mobility,T2 tends to be

longer. For protons located in a region where rotational mobility

is significantly restricted, T2 tends to be shorter. The brightness in

a PD image is directly proportional to the concentration of the

proton-containing species (in this paper, water) in the sample, any

change in grey level (measured in arbitrary units, a.u.) being

proportional to changes in the water concentration.

The MRI data presented here were acquired on a Bruker

AVANCE FT NMR spectrometer with a 38 cm bore and 7.1

Tesla superconducting magnet and 25 mm birdcage resonator

probe. After initial calibration, the standard multi-echo pulse

sequence, MSME (multi-slice multi-echo), from the Bruker

Paravisionª library, was used to collect a train of 64 echoes, each

after a 180� pulse, with a TE of 5 ms and TR of 5 s. The T2 and PD

images extracted from the raw MRI images had a matrix size of

128 � 128 pixels. The slice thickness of the region of interest

(ROI) was 1.0 mm.

2.5 MRI of hydrated and dehydrated Li+-exchanged cast

Nafion

An 8 � 8 mm sample of hydrated Li+-exchanged cast Nafion

membrane was rinsed with DI water to remove residual LiOH

and was sandwiched between two similar-sized clean glass slides.

This journal is ª The Royal Society of Chemistry 2008

This assembly was arranged horizontally within a 25 mm o.d.

(overall diameter) NMR tube. In order to maintain a constant

water vapour pressure inside the tube, a water-soaked paper plug

was enclosed within it and the tube was sealed with a gas-tight

cap. The tube was inserted in the resonator probe of the MRI

instrument. MRI images were recorded and T2 and PD images

were extracted from these, as described above. The field of view,

FOV, was 15� 15 mm (spatial resolution, 117 mm per pixel). The

used sample was dried at 70 �C for 24 h in order to completely

dehydrate it. This sample was re-placed in the NMR tube as

before, but with a dry paper plug, to prevent the sample

absorbing moisture from the surroundings, and the tube sealed.

MRI data were collected as above.

2.6 In situ MRI of IPMC elements

An 8 � 8 mm sample of hydrated Li+-exchanged IPMC was

rinsed with DI water and was placed in the in situ electrochemical

assembly, as shown in Fig. 3. This assembly was sealed inside

a 25 mm o.d. NMR tube with a water-soaked paper plug, the

tube was inserted into the resonator probe and this in turn placed

in the MRI instrument. Electrical contact was made to each

electrode of the IPMC sample and wires were run from these out

of the MRI instrument to a variable d.c. power supply. MRI

images of the sample were accumulated as described above. The

FOV in these images was 20� 20 mm (spatial resolution, 156 mm

per pixel). The acquisition time for each dataset was approxi-

mately 21 min. Four electrical regimes were applied to a single

sample in sequence to study the behaviour of the water molecules

within the IPMC sample: (a) no potential applied; (b) application

of a continuous 3 V d.c. potential; (c) application of a continuous

3 V d.c. potential of reversed polarity and (d) application of

a continuous 3 V d.c. potential after a second reversal of polarity.

T2 and PD images were extracted from the raw MRI images

obtained during these four electrical regimes. This experimental

sequence was repeated several times with similar samples and

gave rise to qualitatively similar behaviour.

3. Results

3.1 MRI of hydrated and dehydrated Li+-exchanged Nafion

Fig. 4 shows the three-dimensional numerical plots, or ‘‘maps’’,

of T2 and PD throughout the hydrated Nafion membrane. Since

the Li+-exchanged Nafion contains no protons itself, the PD and

T2 maps of all samples studied in this work are assumed to relate

uniquely to the protons present in the water molecules absorbed

in the sample. The maps display uniform values of both T2 and

PD across the sample of about 300 ms and 2100 a.u., respectively.

MRI of the fully dehydrated sample failed because no 1H signal

Soft Matter, 2008, 4, 1879–1886 | 1881

Fig. 4 Three-dimensional plots of (a) relaxation time, T2 (ms), and (b)

proton density, PD (arbitrary units), in a sample of hydrated Li+-

exchanged cast Nafion membrane. Fig. 5 (a) Cross-sectional BSE image of the IPMC at low magnification,

showing the line scanned in EDX. (b) Cross-sectional EDX line-scan plot

of normalised signal intensities for Pt, C, F and S (see text). An abrupt

drop in C signal and increase in Pt signal marks the interface between the

sample and the resin. (c,d) Higher magnification cross-sectional BSE

images showing Pt particles at an electrode region of the IPMC.

Fig. 6 T2 (left column) and PD (right column, primed labels) images of

the IPMC sample obtained: (a, a0) (i) with no potential applied; (b, b0) 29

min after application of a 3 V d.c. continuous potential in the sense

indicated; (c, c0) 0 min after first reversal of the polarity of the 3 V d.c.

potential. The ‘‘�’’ and ‘‘+’’ symbols indicate the cathode and anode sides

of the sample, respectively. These maps were extracted from sets of MRI

images as described in Section 2.4 of the text.

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was detected by the MRI scanner. This confirms that the protons

seen inMRI of the hydrated samples were uniquely related to the

water molecules absorbed within the Nafion membrane.

When measured under the same conditions as used here, bulk

deionised water and bulk saturated LiOH solution gave T2 values

of 411 and 377 ms, respectively. The effect of incorporation of

water into the cast Nafion sample caused a drop in T2 to around

300 ms. Since we relate T2 to the rotational mobility of the water

molecules, this can be interpreted as a modest decrease in rota-

tional freedom and an increase in the strength of interactions

with the surroundings of the water molecules. Therefore, the

water, in general, retains considerable freedom in the cast

Nafion. Thermogravimetric analysis (TGA) experiments (not

reported here) indicated a maximum water content for these

samples of 85 wt%. Although this is not a direct calibration, the

PD value of 2100 a.u. can be taken to represent a water content

approaching this level. These high water contents and the fact

that the cast Nafion is less dense than commercial Nafion, would

explain the high mobility indicated by the long T2 values.

Commercial Nafion is compressed on manufacture and had

a maximum water content in the TGA study of 50 wt%. In

previous work,27 T2 values of around 50 ms were found for water

in 0.1 mm thick commercial Nafion samples.

3.2 IPMC morphology

Fig. 5 presents BSE images of a cross-section of the IPMC

sample. In the low magnification image, two bright layers

running parallel to each of the surfaces of the Nafion membrane

are observed and are assigned as the Pt-impregnated regions of

the membrane, the electrodes. In Fig. 5(b), the variations in the

C, Pt, F and S EDX signals (normalised with respect to the F

signal at the sample centre) across the cross-section of the sample

are plotted. Nafion contains C, F and S and the resin gives rise to

the high C signal around the sample. The concentration of Pt is

very high at the surfaces of the membrane but decreases gradu-

ally towards the centre. This can also be seen in the higher

magnification images. There are two distinct Pt layers at each

membrane face: a thin (�1–2 mm), dense layer at the surface and

a broad, porous layer (�320 mm) extending into the membrane.

The Au layer deposited by PVD on the Pt layer is too thin to

1882 | Soft Matter, 2008, 4, 1879–1886

observe in these images. The diameter of the Pt particles

decreases significantly towards the centre of the membrane,

from �50 nm for those in the vicinity of the surface to �10 nm

for those deep inside the membrane.

3.3 In situ MRI of IPMC element

Examples of T2 and PD images extracted from the raw MRI

images of the IPMC sample in electrical regimes (a) to (c) are

presented in Fig. 6. The images represent a cross-sectional slice

through the sample, in the same orientation as in the SEMimage in

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Fig. 5(a), with electrodes at top and bottom of the image. Despite

the samples being restricted by the electrochemical assembly,

electrically induced bending towards the anode is evident.

Fig. 7 presents quantitative information in PD and T2 maps

for all images obtained in chronological order in regimes (a)-(d).

The PD map with no potential applied establishes that the

concentration of water molecules was initially uniform over the

whole sample. On the contrary, the corresponding T2 map shows

that water molecules absorbed at the centre of the IPMC sample

had longer T2 values than those closer to the electrode surfaces.

A gradual fall in T2 from about 140 ms at the centre to about

90 ms at the electrode surfaces is observed. This can also be seen

as a bright horizontal central band in Fig. 6. Since T2 is depen-

dent on the freedom of molecular motion, this reveals that water

molecules are least constrained at the centre of the sample and

become increasingly more constrained as one moves towards the

electrode surfaces, and as Pt concentration increases.

On application of the potential (3 V d.c. continuous),

a number of changes were induced. Considering first the PD

maps (Fig. 7(b)–(d)), these clearly became asymmetric as soon as

the potential was applied, with higher PD values near the

cathode of the IPMC. This implies a relatively long-range net

displacement of water towards the cathode. With time, this

asymmetry became gradually more marked and persisted until

the potential was reversed at the start of regime (c) (Fig. 7(c)). An

asymmetric distribution of PD values was re-established, but in

the opposite sense, with the higher values in the vicinity of the

new cathode. At the start of regime (d), after a further reversal of

the potential, this process began to take place once more. This is

clear evidence of the electro-induced diffusion of water from

Fig. 7 PD (top maps) and T2 (lower maps, primed labels) of the IPMC sampl

89 min after application of a 3 V d.c. continuous potential in the sense indicate

and (d, d0) (i) 0 min after the second reversal of the polarity of the potential

sample, respectively.

This journal is ª The Royal Society of Chemistry 2008

anode to cathode in the working IPMC sample. Indeed, despite

the sample having been held between glass slides, the effect of

actuation – in which the IPMC element became increasingly

convex at the cathode side over time and bent towards the anode

– can be seen clearly. Finally, comparison of these PD maps,

from left to right, also confirms that the sample was dehydrating

through this extended experiment, since their total intensity,

which is proportional to the water concentration, decreased by

a third, from 2100 to about 1400 a.u.

Turning to the T2 maps (Fig. 7(a0)–(d0)), the situation is more

complex because of the initial, underlying non-uniform T2

distribution seen in regime (a). The most obvious phenomenon is

the appearance of very high T2 values (� 300 ms), or spikes, at

both the anode and the cathode. These are most marked when

the potential is first applied and after each reversal of polarity –

i.e. at the beginning of regimes (b), (c) and (d) – their values

diminishing thereafter. Since no similar features were observed in

the corresponding PDmaps, it can be concluded that these spikes

represent a change in the environment of water at these points

but do not correspond to a local change in water concentration.

To further illustrate the changes occurring inT2 throughout the

IPMC sample under the influence of the four electrical regimes,

the average T2 values within three 10 pixel-wide cross-sections

through the thickness of the sample at its front end, centre and far

end (as viewed in perspective in Fig. 7) were calculated. Fig. 8

shows the averageT2 plots obtained for these three selected cross-

sections of the sample. As a guide to the eye, a moving average

trend-line was added to each plot. For regime (a), the graphs

(Fig. 8(a, a0, a00)) reveal that the distribution ofT2 within the three

selected regions are similar – indicating a uniform physical

e obtained: (a, a0) (i) with no potential applied; (b, b0) (i) 0; (ii) 29; and (iii)

d; (c, c0) (i) 0; (ii) 29 and (iii) 89 min after the first reversal of the polarity;

. The ‘‘�’’ and ‘‘+’’ symbols indicate the cathode and anode sides of the

Soft Matter, 2008, 4, 1879–1886 | 1883

Fig. 8 Average T2 values within three regions of the IPMC sample:

front, centre (primed labels) and back (double primed labels) (referred to

perspective view in Fig. 7) obtained: (a, a0, a00) (i) with no potential

applied; (b, b0, b00) (i) 0; (ii) 29; and (iii) 89 min after application of a 3 V

d.c. continuous potential in the sense indicated. (c, c0, c00) (i) 0; (ii) 29 and

(iii) 89 min after first reversal of the polarity and (d, d00) (i) 0 min after

second reversal of the polarity of the 3 V d.c. potential. The ‘‘�’’ and ‘‘+’’

symbols indicate the cathode and anode sides of the sample, respectively.

Fig. 9 Estimation of the percentage of mobile water in the IPMC sample

during application of 3 V d.c. potential during regime (b).

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structure along the length of the sample – and approximately

symmetrical across the thickness of the sample, with a peak at the

centre. On application of the potential in regime (b), the value of

T2 at the centre decreases slightly and increases at both the anode

and the cathode, but the plots are asymmetrical with a prominent

shoulder of high T2 forming near the cathode. In general, this

shoulder is seen to persist until the reversal of polarity at the start

of regime (c), when a shoulder appeared near the new cathode of

the IPMC. At the start of regime (d), this shoulder disappears in

what can be understood to be the start of the establishment of

a new high T2 region near the new cathode. Throughout this

experiment, these regions of high T2 and those of high PD were

coincident and were associated with the cathode.

To estimate the amount of water molecules that contributed to

the observed bending deformation of the IPMC sample, the

1884 | Soft Matter, 2008, 4, 1879–1886

percentage of water molecules that had moved after application

of the continuous potential was calculated. The PD map of the

sample obtained in regime (a) was subtracted numerically from

all subsequent PD maps in regime (b) to give maps of change in

PD, DPD. The difference in DPD between the anode half and

the cathode half of the IPMC was calculated, divided by two,

normalised with respect to the total PD integrated over the whole

sample, and plotted against time (Fig. 9) as the percentage of

mobile water. This initially increased suddenly and increased

slowly thereafter. The percentage of macroscopically mobile

water molecules was estimated to be about 20% of the total.

4. Discussion

4.1 Effect of the Pt electrodes

Before Pt electrodeswere incorporated, the castNafionmembrane

haduniformvalues of bothPDandT2 (Fig. 4). Incorporationof Pt

electrodes to form the IPMC element caused a slight decrease in

PD (from 2100 to 1800 a.u.), although it remained uniform

throughout the sample (Fig. 7(a(i))). The values of T2, however,

appeared to decrease with increasing Pt concentration within the

sample. It has been established that platinum impregnation of

Nafion polymer at the surfaces leads to an increase in the stiffness

of the polymer25 and that the stiffness of the Nafion membranes

and IPMC samples is inversely related to their water content.18,26

This is because a more rigid sample is less able to expand to

accommodate additional water. Therefore, the IPMC sample

would be expected to have a lower water content compared to the

castNafion sample.The variation ofT2 over the IPMCsamplewas

attributed to the presence of the electrode structures. This was

considered to occur as a result of the increasing stiffness25 of the

membrane from the centre towards the surfaces caused by the

increasing Pt content. This would be expected to cause increasing

restriction of the rotational motion of the water molecules – and

therefore decreasing T2 – towards the surfaces of the IPMC

sample. At the centre, the absence of platinum particles leads to

much less restriction of the rotational motion of water molecules

and to longer T2 values.

4.2 Effect of the potential

The application (or reversal) of a potential was seen to have two

major effects:

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1. The establishment, over time, of a region of high T2 near the

cathode of the IPMC, which was accompanied by an increase in

PD, also near the cathode. It seems logical that the increase in PD

near the cathode is caused by the long-range induced diffusion of

solvated [Li(H2O)x]+ cations in the applied electric field. Protons

in the solvation shell, and perhaps in entrained free water, would

cause the increase in PD. The corresponding increase in

T2 implies that the proton environment is less restrictive

near the cathode. This can be explained by considering the

structure of hydrated Nafion, which is comprised of nanometric

ionic regions, known as clusters, rich in –SO3� groups and

counter-cations, and hydrophobic regions containing only the

perfluorinated polymer backbone. In the absence of a potential,

any water present would be expected to be located in the ionic

regions. Here, hydrogen bonding and electrostatic interactions

will restrict the motion of water molecules resulting in short T2

values. As shown in Fig. 10, when an electrical potential is

applied, some of the [Li(H2O)x]+ cations will be forced to act as

charge carriers and must exit one cluster and enter the hydro-

phobic region in order to pass to the next cluster.27 Because of the

time spent in this environment where interactions with the

surroundings are weak, the time-averaged T2 of the protons

associated with these ions will be longer than normal. Over time,

there will be a net long-range movement of such cations towards

the cathode, causing the rises in both PD and T2 observed here.

Two further related mechanisms may be important in the

increase of T2 and PD near the cathode. Firstly, as a consequence

of the forced increase in water content near the cathode, water

will increasingly be accommodated in less and less ionic

(hydrophilic) regions. That is, as fewer of the more energetically

Fig. 10 Schematic representation of hydrated ionic clusters and

hydrophobic –CF2– containing backbone regions in the Nafion nano-

structure (a) with no potential applied and (b) during application of

a potential, showing movement of a solvated Li+ ion between ionic

clusters.

This journal is ª The Royal Society of Chemistry 2008

favourable ionic sites are available, the water will begin to occupy

less favourable, less ionic regions. Since the length of T2 is related

to the hydrophobic nature of the local chemical environment,

this will lead to an increase in T2, as is seen. Secondly, as more

water enters the area near the cathode, the hydrophilic regions

would be expected to increase in size in general. As they

expanded, one might expect more interaction between the water

within the hydrophilic region and the surrounding polymer

chains as well as less interaction between the water molecules and

the ions within the hydrophilic region. Both these effects would

also result in an increase in T2. The reverse effects would be

expected to lower both PD and T2 at the anode.

2. The initial generation of large ‘‘spikes’’ in T2 at both the

anode and the cathode, which were not accompanied by similar

features in the corresponding PDmaps. This may be explained as

a fast, short-range (undetectable in PD maps) process related to

the model proposed by Nemat-Nasser and Li.18According to this

model, as the electrical field is applied, the hydrated Li+ ions

present in the hydrophilic regions experience an electrostatic

repulsive force from the anode and an attractive electrostatic

force from the cathode, forming a thin boundary layer at each

electrode. The influence of these forces seems to be initially much

stronger at the charged electrode–membrane interfaces since the

largest initial changes in T2 occurred in these regions. It seems

that, at the anode, as the electrical field is applied, the electro-

static repulsive force between the hydrated Li+ ions within the

hydrophilic regions and the positively charged surface forces the

short-range displacement of hydrated Li+ ions out of the clusters

near the anode and away from it, into the hydrophobic phase of

the polymer. The presence of the hydrated Li+ ions in the more

hydrophobic region of the polymer – where interactions between

the water molecule and its immediate surroundings are weaker –

gives rise to the long T2 values observed near the anode. At the

cathode, the negative surface charge attracts hydrated Li+ ions

from the neighbouring hydrophilic regions, pulling them out of

the clusters into the hydrophobic polymer region. Again, this

gives rise to a long T2 relaxation at the cathode also.

Comparison of T2 maps for regimes (b) and (c) reveals that

much greater increases in T2 occurred near the new cathode and

anode as the polarity of the applied potential was reversed at

the start of regime (c), than those observed when the potential

was applied for the first time in regime (b). This can be

explained by considering that at the start of regime (c) the

distribution of cations was not at its equilibrium state. There-

fore, significant self-diffusion of these cations, along this

concentration gradient, would be expected to be superimposed

on the electrically induced diffusion of cations, contributing to

a larger observed change.

As shown in the previous sections, the changes occurring in T2

and PD at the cathode and anode regions in the T2 and PD

images mostly conform to the Nemat-Nasser and Li model.18

However, according to their model, under application of the

electric field, thin boundary layers are formed at the anode and

cathode, which shield the remaining part of the IPMC from the

influence of the applied electric field and all the processes which

lead to the bending deformation of the IPMC occur at these two

layers. In the present study, the changes observed in T2 near the

cathode and anode as the potential was applied may indicate the

formation of boundary layers. However, the changes observed

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were not limited to thin cathode and anode regions since T2

eventually changed over the whole sample.

5. Conclusions

1. Thick Nafion elements were prepared by casting, and MRI

was used to study the distribution and chemical environment of

water absorbed within them. Both T2 (�300 ms) and PD were

found to be uniform over the sample.

2. The only protons detected in these materials were those

pertaining to water molecules.

3. Pt electrodes were incorporated to make an IPMC device.

The T2 of protons was found to be inversely related to the

concentration of platinum, whereas PD was uniform over

the sample and slightly lower than in the unelectroded sample.

Both effects were explained by the increased rigidity introduced

into the material by addition of the Pt layers.

4. The physical structure and the actuation mechanism of the

IPMC actuator were investigated using in situ MRI during

application of four different electrical regimes. The method used

allowed separate T2 and PD maps of the sample to be obtained.

These were used to study changes in the distribution and

chemical environment of water molecules absorbed within an

operating IPMC actuator device.

5. Application of a small d.c. potential across the thickness of

the IPMC device resulted in an initial rapid increase in T2 at both

electrodes, without an observed change in PD, and the slower

formation of a region of high T2 and high PD at the IPMC

cathode. The bending actuation of the IPMC was also observed.

6. Reversing the polarity of the applied potential resulted in

the reversal of the direction of the bending deformation of the

IPMC and of the distribution of PD and T2.

7. The rapid, dramatic increases, or spikes, in T2 without

accompanying change in PD, on application of the potential,

were attributed to short-range electrostatic interactions between

the electrode surfaces and [Li(H2O)x]+ cations in neighbouring

hydrophilic regions, or clusters. Before application of the

potential, the vast majority of such ions would be accommodated

in these hydrophilic regions where interaction with the

surrounding water molecules and ions would be relatively strong,

giving rise to relatively short T2 values. On application of the

potential, spikes in T2 occurred near both cathode and anode.

This immediate effect was explained as being due to the

[Li(H2O)x]+ ions being removed from the hydrophilic clusters to

the hydrophobic regions of the Nafion, by attractive or repulsive

electrostatic forces at the cathode and anode, respectively.

The hydrophobic regions consist essentially of perfluorinated

polymer chains, and so interaction between this environment and

the water molecules associated with the [Li(H2O)x]+ ions would

be weaker and give rise to longer T2 values.

8. The slower increase in T2 with accompanying increase in PD

at the active cathode of the IPMC sample was attributed to the

long-range electrically-induced diffusion of [Li(H2O)x]+ ions, and

perhaps entrained water, away from the anode and towards the

cathode. This long-range movement of water is the most widely

accepted mechanism of actuation in these devices. The increase in

T2 was attributed to a combination of the following three effects.

1886 | Soft Matter, 2008, 4, 1879–1886

(i) The accumulation of mobile [Li(H2O)x]+ ions near the

cathode. In order to be mobile between clusters these ions would

have to spend a proportion of the time in hydrophobic envi-

ronments between clusters, where interaction with their

environment would be weaker, causing the T2 values of the

associated water protons to be longer.

(ii) As more water was transported to the cathode region, it

would be accommodated in increasingly less favourable, more

hydrophobic environments, resulting again in longer T2 values.

(iii) Finally, as the hydrophilic regions, or clusters, accepted

more water they would expand into less hydrophilic regions,

again increasing the overall T2 associated with the water

molecules contained within them.

9. The amount of water involved in long-range electro-

diffusion was estimated from the PDmaps to be up to 20% of the

total water content of the IPMC.

Acknowledgements

We gratefully acknowledge a Nicholl–Lindsay Scholarship and

a Universities UK ORS award for one of the authors, L.N. SEM

imageswere obtained at theCHIPS facility,University ofDundee.

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