Liquid single crystal elastomer/conducting polymer bilayer composite actuator: modelling and experiments

Soft Matter

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aCenter for Micro-BioRobotics @SSSA, Istitu

Piaggio 34, 56025 Pontedera, Italy. E-mail:

iit.it; Fax: +39 050 883402; Tel: +39 050 88bDipartimento di Chimica e Chimica Industr

35, 56126 Pisa, Italy. E-mail: valentin@dcccIstituto di Biorobotica, Scuola Superiore Sa

Pontedera, ItalydJozef Stefan Institute, Jamova cesta 39, SI-1

† Electronic supplementary informatioLSCE/PEDOT:PSS actuation via heat10.1039/c3sm51153g

Cite this: Soft Matter, 2013, 9, 11405

Received 26th April 2013Accepted 14th October 2013

DOI: 10.1039/c3sm51153g

www.rsc.org/softmatter

This journal is ª The Royal Society of

Liquid single crystal elastomer/conducting polymerbilayer composite actuator: modelling andexperiments†

Francesco Greco,*a Valentina Domenici,*b Andrea Desii,ac Edoardo Sinibaldi,a

Blaz Zupancic,d Bostjan Zalar,d Barbara Mazzolaia and Virgilio Mattoli*a

In order to integrate electroconductive properties in a Liquid Single Crystal Elastomer (LSCE) and to test direct

actuation of the LSCE by Joule heating, we present a new bi-layered all-organic composite actuator based on

the coupling of a nematic LSCE with a conductive polymer. The bending actuator is fabricated by depositing a

thin conductive polymer layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) over

the surface of a polysiloxane-based monodomain nematic LSCE film. Mechanical properties of

PEDOT:PSS, better matched with LSCE ones compared with metals or inorganic nanoparticles used

in other approaches, allowed us to develop an all-organic reliable millimetre-scale actuating

composite. The thermally induced elongation/compression of the LSCE over 30% is exploited for

the fabrication of bending actuators with curvature up to k ¼ 0.64 mm�1. The LSCE and the

composite material are characterized as regards their thermo-mechanical and electrical properties.

A model is introduced to describe bending of the composite as a function of the thermo-

mechanical properties of the LSCE, and the model is assessed by comparing the model results with

the experimental findings. Bending actuation via direct Joule heating of the composite is also

assessed by supplying the necessary current (50 mA at 1.3 V) through wires connected to the

composite. These results open new possibilities for the application of LCEs in the micro and soft

robotics fields, as well as in the biomedical field.

1. Introduction

In recent years, research in smart polymers has led to thedevelopment of new active materials as well as new actuatorsystems.1 In particular, much effort has been focused on so,compliant materials, because of the interest in developing newsolutions for articial muscles to be used in so robotics2,3 andmicrorobotics.4,5 Among them, Liquid Crystal Elastomers(LCEs) are very promising due to the large changes in theirmicroscopic and macroscopic properties encountered aroundthe isotropic-to-liquid crystal phase transition, and especiallyregarding the very large and reversible actuation strain associ-ated with this transition.6

to Italiano di Tecnologia, Viale Rinaldo

[email protected]; virgilio.mattoli@

3417

iale, Universita di Pisa, Via Risorgimento

i.unipi.it

nt'Anna, Viale Rinaldo Piaggio 34, 56025

000 Ljubljana, Slovenia

n (ESI) available: Video of Bilayering and Joule effect. See DOI:

Chemistry 2013

LCEs are polymer networks including mesogenic units eitherin the polymer chain (main-chain LCEs) or as pendant mono-mers attached to the main polymer chain (side-chain LCEs).6

The most studied LCEs show a nematic phase when decreasingthe temperature from the isotropic phase. The nematic LCEs arecharacterized by a local orientational order due to the align-ment of the mesogens along a preferred direction, namely thelocal nematic director, n. The coupling between the orienta-tional order of the mesogens and the elasticity associated withthe polymer network gives rise to peculiar properties,6–8 such asreversible and controlled shape change by varying tempera-ture.9 For this reason, LCEs are also regarded as “shapememory” systems. Despite great interest in understanding thechemo-physical properties and the molecular origin of theirthermo-mechanical behaviour, currently there are few success-ful cases in which liquid crystalline elastomers have been usedfor non-thermal types of actuation-prototypes,10–13 which are themost attractive for technological applications.14 Light-drivenactuation is oen implemented in order to allow remoteaddressing, by exploiting the photomechanical effect arisingin LCEs incorporating azobenzene derivatives, as both dissolvedor covalently bonded dyes in an elastomer matrix.15,16 A photo-induced contraction is observed in these materials upon

Soft Matter, 2013, 9, 11405–11416 | 11405

http://dx.doi.org/10.1039/c3sm51153g

http://pubs.rsc.org/en/journals/journal/SM

http://pubs.rsc.org/en/journals/journal/SM?issueid=SM009047

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irradiation with UV light, due to the change in the orienta-tional order of mesogens (decrease in the order parameter)caused by trans–cis isomerization of the azobenzene moie-ties.17 Several examples exist regarding bending of LCE lmsalong different directions by using linearly polarized UV orvisible light18 with rather fast actuation times.19 All-polymermicroactuators based on LC networks and driven by light havebeen developed using ink-jet printing technology, as in thecase of microactuating aps mimicking the motion of naturalcilia of microorganisms.20

In order to induce a shape variation by applying an externalstimulus, such as an electric or magnetic one, a possiblestrategy that has been recently adopted relies on the prepara-tion of LCE-based composite materials using either inorganicnanomaterials21–28 or organic polymers.29,30 In the rst case,inclusion of ferroelectric or dielectric nanoparticles in the LCEmatrix determined a change in the dielectric behaviour whenthe samples were elongated or an external stress was applied.31

Moreover, the use of nanoparticles and nanowires embedded inLCE lms has proved to give additional properties due to theinduced alignment of the nanomaterials.25,26 In the case ofcarbon nanotubes included in LCEs27,31–33 this effect was evenhigher, since the new composite materials showed an unusualand new behaviour in the presence of infrared radiation.

Another approach was recently explored by two differentgroups29,30,34 with bilayered composite lms, made of a thickLCE lm covered by a thin layer of organic polymers, thusobtaining a bending actuation or surface wrinkling dependingon the thickness of the polymer layer. The use of conductingpolymers, in particular, is very promising for applications ashigh energy density systems,35 or in elds like robotics, tunableoptics, medical devices36 and space robotics, because it can helpto develop compliant and lightweight articial muscles andbending actuators based on LCEs.10 This could be benecial,especially in biomimetic robotics and microrobotics: indeedLCEs are envisaged as biomimetic materials, due to the fact thattheir typical stress and strain values are similar to those ofnatural skeletal muscle.37,38

From a technological point of view there is an interest inobtaining elongation/contraction or bending of LCE materialsby Joule heating and this has been the focus of some recentresearch.29,39–41 However, fabrication of robust actuators, goodelectrical contacts and durable systems still remain as the maindifficulties to be solved. Indeed, when dealing with electrome-chanical or Joule heating actuation, difficulties typically aroseboth in the fabrication and in the performance of LCE-based(nano)composites or material assemblies. Very oen thesedifficulties are caused by poor processing compatibility andbecause of intrinsically larger stiffness of metal electrodes andinorganic nanocomposite layers, if compared to the very soand compliant LCE materials.14,21–23,25,39–41

In this paper, we present a detailed experimental studyconcerning bilayered all-polymer composite materials based ona monodomain LCE lm and a conductive PEDOT:PSS thinlm. Modelling is also introduced in order to better characterizethe proposed actuator. The polymer matrix used is a specialkind of LCE, usually referred to as a nematic Liquid Single

11406 | Soft Matter, 2013, 9, 11405–11416

Crystal Elastomer (LSCE). The monodomain nematic LSCElms, rst prepared by J. Kupfer and H. Finkelmann,42 arepolysiloxane-based elastomers showing a nematic phase atroom temperature. The particular synthetic procedure adoptedfor their preparation allows one to have lms with a uniaxialsymmetry. The lm is characterized by a uniform alignment ofthe local directors, n, along a preferred direction, which corre-sponds to the main direction of the macroscopic and reversibleelongation/contraction observed at the nematic-to-isotropicphase transition temperature, TN–I. The basic idea of this workwas to incorporate the electroconductive properties in the LSCEmatrix by employing a layer of the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS).The latter, while providing the necessary electroconductivecapability, at the same time acted as an incompressible “skin”over a uniaxially compressing “foundation”; this resulted in thebending of the whole bi-layer structure once a thermal stimuluswas applied around the nematic-to-isotropic phase transition(TN–I � 74 �C). Mechanical properties of PEDOT:PSS (namely itsYoung's modulus E and Poisson's ratio n), are better matchedwith those of the LSCE compared to different materials used inprevious attempts to use Joule heating with the same aim.21,39–41

This allowed us to develop an all-organic composite actuator,based on thermo-responsive properties of the LSCE. ThePEDOT:PSS layer also provided the electrical conductivitynecessary to exploit the Joule heating effect for driving theactuation of the composite. A suitable model for describing thecurvature of the composite as a function of temperature is alsointroduced, based on composite geometry (thickness andlateral dimensions) and mechanical parameters of the constit-uent materials (E and n of both LSCE and PEDOT:PSS) as well ason the thermomechanical behaviour of the LSCE around thenematic-to-isotropic phase transition. An assessment of thismodel is presented by comparing model results with theexperimental data of the actuator bending curvature, asobtained by thermal heating (both external and Joule induced)and quantied by thermal imaging experiments. The obtainedresults encourage us to further pursue the investigation ofLSCE/PEDOT:PSS bilayer actuators, towards applications inmicro and so robotics elds.

2. Results and discussion2.1. Liquid single crystal elastomer thermomechanicalcharacterization

The LSCE used in this work was a poly(siloxane) based elas-tomer with pendant nematic Liquid Crystal (LC) mesogenshaving the molecular structure depicted in Fig. 1a. The pecu-liarity of this material, caused by LC moieties and by the pro-cessing strategy during lm preparation that permitted us toobtain a monodomain structure of the macroscopic system, wasto have a uniform orientation of the local orientational orderdue to alignment of the mesogens along a preferred direction,namely the local nematic director, n (Fig. 1a). This directioncorresponds to the main elongation/contraction direction,observed by varying the temperature around the nematic-to-isotropic phase transition (TN–I). We characterized this

This journal is ª The Royal Society of Chemistry 2013


Fig. 1 (a) Basic design of the bilayer composite with the chemical structure ofthe poly(siloxane) based LSCE containing pendant nematic mesogens and a topconducting layer of PEDOT:PSS (blue). The director n describes the main orien-tation alignment of the mesogens, that is the direction along which the LSCE hasa reversible contraction/elongation through TN–I. By driving a current I in the topconducting layer the actuator bends, thanks to the thermal trigger imposed onthe LSCE by Joule heating. (b) Picture of the bilayer actuator with two wiresembedded at the edges (scale bar 1 mm). (c) Cross sectional view of the bilayer cutalong the red dashed line in (b) (scale bar 100 mm).

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mesophase transition and the reversible elongation/contractionfor the selected material by visually inspecting and videorecording the LSCE sample placed in a climatic test chamber inwhich repetitive thermal cycles were imposed, from roomtemperature (T ¼ 25 �C) up to T ¼ 100 �C, and back to roomtemperature. The sample was placed in the chamber in thevertical position, that is with its long axis (parallel direction tothe n director) along the vertical axis (as seen in Fig. 1a). Ameasurement of the length L of the LSCE sample strip alongthis axis at different temperatures was taken from images andcompared to L0, i.e. the length of the sample in the isotropicphase (i.e. at T ¼ 100 �C). Snapshots of the video taken atselected temperatures during heating and cooling are reportedin Fig. 2a (1–12). The results are reported in Fig. 2b, where theratio L/L0 is shown as a function of temperature, both forheating and cooling and for the rst and h thermal cycles.The reported trends showed a �30% maximum length change(elongation) at room temperature with respect to length L0 inthe isotropic phase, that is for T ¼ 100 �C. The trends reportedin Fig. 2b allowed us to estimate the transition temperature,TN–I � 73 �C, which corresponds to the inection point of thecurves; this nding is in good agreement with DSC measure-ments previously reported for the samematerial (TN–I ¼ 73.7 �C,data not shown here).43 The observed trend of the thermo-mechanical deformation L/L0 (T) is similar to that of othermonodomain LSCEs and it is correlated to the trend of theorientational order parameter S(T), a quantity describing theorientational order in liquid crystals on the molecular level,which is usually determined by means of solid state 2HNMR experiments or by birefringence, FT-IR, SAXS and WAXSmeasurements.44 Indeed, the observed thermomechanicalbehaviour, namely the contraction of the crosslinked


elastomeric network when reaching TN–I, is caused by a decreasein the local orientational order when the temperature isincreased up to the isotropic phase, in which the mesogens arerandomly oriented.

As regards the elastic modulus of the studied LSCE, it isimportant to notice that, due to the fact that a phase transitionoccurred during the actuation (i.e. nematic-to-isotropic transi-tion), the elastic modulus E is changing as well, and it is ofcourse a function of temperature. In order to estimate thesemoduli at various temperatures and to extract the parametersused in the model for describing the actuator behavior, stress(s)–strain (3) experiments at selected temperatures were carriedout on the samples, (see Fig. 2c and d).

Stress–strain curves recorded at room temperature clearlyindicate a strong visco-elastic behavior for the LSCE, as high-lighted by the stress hysteresis in subsequent elongation/compression experiments (Fig. 2c). Accurate descriptions of thestress relaxation behaviour of LSCEs exist, in particular, forpolydomain systems and long relaxation times.45–47 However,the mechanical behaviour of monodomain LCSEs can bedescribed with a simplied model when it is strained parallel tothe director. In this case director rotation is not occurring andthe material behaviour is that of a viscoelastic solid for con-cerning creep, recovery and stress relaxation. Stress relaxationoccurs due to an increase in orientational order induced bystrain. The Standard Linear Solid (SLS) is the simplest visco-elastic model that exhibits all these features.48 It can beemployed to obtain mechanical parameters of the LSCE fromstress relaxation experiments. Relaxation curves were ttedusing the exponential function:

sðtÞ ¼ 30

Er þ Ege

�Eg

ht!

(1)

where Er is the elastic modulus of the elastic part (“rubbery”modulus), Eg is the elastic modulus (“glassy” modulus) and h

the viscosity of the uid part of the system. For temperaturesabove TN–I, when the material was in its isotropic phase,complete loss of orientational order (S ¼ 0) made it a pureelastomer, with a linear elastic response and without mechan-ical hysteresis under compression. The elastic behaviour in thisregime was modelled as that of a Hookean solid, s ¼ EH3(Fig. 2d). The breakdown of the SLS model around TN–I wasconrmed by the increase in the error on the tting parametersEr, Eg and h.

2.2. Fabrication of bilayer actuator

The thermomechanical properties of the LSCEs – with suchrepeatable and large actuation strain – are interesting for theiruse as smart, so active materials in the development ofmuscle-like actuators. Nevertheless, to date the need to use athermal trigger for activation severely restricted their practicalapplications, for which electrically-driven control could bemoresuitable. In order to partially overcome this problem one couldconsider coupling LSCEs with metal electrodes or functional-izing them to impart the desired electroconductive properties,for instance by introducing conductive nanoparticles. In this

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Fig. 2 Thermo-mechanical characterization of LSCE. (a) Pictures of the LSCE sample taken at various temperatures during thermal cycling: heating, (1) T ¼ 30 �C, (2)T¼ 45 �C, (3) T¼ 50 �C, (4) T¼ 72.5 �C, (5) T¼ 90 �C, (6) T¼ 95 �C, (7) T¼ 100 �C; cooling, (8) T¼ 90 �C, (9) T¼ 78 �C, (10) T¼ 72.5 �C, (11) T¼ 70 �C, (12) T¼ 30 �C. (b)L/L0 as a function of temperature for various thermal cycles showing reversible compression/elongation of the LSCE sample. (c) Stress (s)–strain (3) hysteresis curves(elongation/compression) of the LSCE samples at T ¼ 28.8, 33.4, 41.2, 46.5, 50.7, 54.1, 58.3, 61.5, 64.5 and 70.0 �C, recorded at a strain rate of 0.02 s�1. (d)Elastic moduli and viscosities of LSCE samples as a function of temperature, calculated from the exponential fit of the stress relaxation curves according to the SLS model(Er, Eg, h) and the Hookean model (EH).

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way, electrically-driven actuation can be obtained via directJoule-heating. Similar approaches have been tested in recentyears by several groups but major drawbacks, mainly related todifficult preparation, electrode cracking and failure, poorcompatibility between llers and LSCEs, and poor conductivity,still exist.14,21–23,25,39–41 Here, we purposely chose a differentapproach by developing an all-organic bilayer actuator in whichthe necessary electroconductive properties are given by athin layer of the conducting polymer poly(3,4-ethyl-enedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Thisconducting polymer exhibits mechanical properties (namely itsYoung's modulus E and Poisson ratio n), that are better matchedwith the LSCE ones, if compared with the cited inorganic llersand metal electrode layers. Indeed, the Young's modulus ofPEDOT:PSS is E ¼ 0.9–2.8 GPa, depending on the relativehumidity, and the Poisson's ratio is n ¼ 0.34, as determined on25 mm-thick samples,49,50 to be compared with E ¼ 78 GPa, n ¼0.42, for gold. The basic actuator design is shown schematicallyin Fig. 1a. When the current I is driven in the top conducting

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PEDOT:PSS lm, the temperature of the sample rises because ofJoule heating and a compressive stress is created inside thebottom LSCE layer, due to orientational rearrangement of thenematic pendant moieties. The temperature-dependentcontraction of the LSCE (Fig. 2a and b) is transformed into abending of the bilayer, due to the almost incompressiblePEDOT:PSS lm, which acts as a mechanically passive layeragainst the activated LSCE. The simplicity of this design ismadepossible because of the good compatibility of the thin lm ofPEDOT:PSS deposited on the polysiloxane-based LSCE. More-over, the addition of a secondary dopant species (dime-thylsulfoxide, DMSO, 5 wt%) in the conducting polymerformulation increased its conductivity51 in order to permit alow-voltage actuation (<10 V), which is one of the importantissues related with this kind of LSCE-based actuator.10,14,41

A simple casting procedure was used for fabricating thebilayer actuators, starting from a commercially availabledispersion of PEDOT:PSS in water, which was dispensed overa LSCE strip and allowed to dry overnight at room temperature.



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A brief plasma treatment of the pristine LSCE surface was per-formed just before the solution casting; this procedure modi-ed the LSCE surface energy in order to improve its wettabilityand thus obtain a more uniform coating. Thin wires couldbe embedded in the lm during the casting procedure to providethe necessary connections with the voltage generator. A picture of atypical actuator sample with wires is reported in Fig. 1b.

The thickness of the PEDOT:PSS layer was tuned by modifyingthe volume of the aqueous dispersion dispensed on the LSCE orby performing subsequent steps of casting/drying with identicalvolumes. A careful selection of the thickness of the conductingpolymer layer has been carried out to choose the optimal condi-tions for bending actuation. These were found to be t� 10–15 mmfor the specic sample geometry, corresponding to two subse-quent casting steps, with a volume density of 0.4 ml mm�2 of theaqueous PEDOT:PSS dispersion. A cross-section view of thebilayer actuator is reported in Fig. 1c and allows us to observethe PEDOT:PSS layer thickness (t ¼ 13.7 mm). It is indeed inter-esting to notice that, when thicker lms were fabricated (t $ 100mm) the stiffness of the conducting polymer layer was largeenough to hamper the bending of the bilayer. On the reverse,ultra-thin PEDOT:PSS nanolms with sub-micrometric thicknessshowed reversible surface wrinkling during the elongation/contraction of the LSCE.30

2.3. Bending characterization of bilayer actuator

LSCE/PEDOT:PSS samples were placed on top of a Peltier cell,which provided external heating in order to induce bendingactuation. Simultaneously, temperature distribution wasrecorded with a thermal imaging camera. The PEDOT:PSScoated side was placed facing down. The temperature wasincreased from room temperature up to 100 �C, that is wellabove the nematic-to-isotropic phase transition temperature ofthe LSCE (TN–I ¼ 73.7 �C), causing the bending of the bilayer.Fig. 3 shows the results of this experiment in the case of asample with two layers of PEDOT:PSS (total PEDOT:PSS thick-ness, t ¼ 13.7 mm). The curvature k of the LSCE composite as afunction of temperature is reported in Fig. 4, while Fig. 3ashows images taken at selected temperatures both withconventional (1–4) and IR cameras (thermal imaging, 5–8). It isimportant to notice that, due to the fabrication process of thesebilayer composites, the initial curvature of the samples – i.e.curvature at room temperature, T� 25 �C, prior to all tests – wasnegative: the samples had an initial concave shape on thePEDOT:PSS side of the actuator (Fig. 3a(1 and 5)). As tempera-ture was increased, the curvature k increased with a moderateslope and a “at” conguration with k ¼ 0 was obtained aroundT ¼ 55 �C (Fig. 3a(2 and 6)). A further increase in temperaturecaused a steep increase in curvature, up to positive k values, i.e.samples became bent towards the LSCE side of the bilayer(Fig. 3a(3 and 7)). Maximum steepness of the curvature vs.temperature trend was obtained at around TN–I, while completebending and nal stable conguration with k ¼ 0.64 mm�1 wasreached for temperatures T $ 80 �C (Fig. 3a(4 and 8)). As roomtemperature was restored, the samples returned to their initialpositions and multiple repeated bending/relaxation cycles


could be operated without signicant variation in performance.The observed trend, in optimal accordance with the ndings oflinear strain vs. temperature obtained for LSCE only (Fig. 2),validated the design of a working bending actuator by employ-ing linear contraction upon heating of an LSCE. An increase inPEDOT:PSS thickness, as obtained by piling up multiple castlayers, resulted in a decrease in the nal curvature of the LSCE/PEDOT:PSS bilayer (k at T $ 80 �C), as expected. Typical valueswere k¼ 0.75, 0.64, 0.56 and 0.31mm�1 for samples obtained bythe deposition of 1, 2, 3 and 4 PEDOT:PSS layers, respectively.

2.4. Bilayer actuation via Joule effect

While validation of the basic concept of bending actuation withthe bilayer composite was obtained with external heating, themain focus of our research was to develop an electro-conductivecomposite actuator that could be driven by Joule heating. Tothis end, similar samples and setup were used as in the case ofactuation with external heating, with the exception that thincopper wires were embedded in the PEDOT:PSS layer duringpreparation of composites, to allow us to pass a current throughit. This addition of course changed themechanical properties ofthe composite material at its opposite edges along its length,that is where the metal wires were placed. Indeed, they acted asrigid parts that locally prevented LSCE contraction and partiallyhampered the overall composite bending. At the same time, thebending motion of the LSCE/PEDOT:PSS composite suspendedon these wires and placed in a horizontal position (PEDOT:PSScoated layer is up) also resulted in a vertical movement, due tothe constraints imposed by the wires (see ESI†). Nevertheless,curvature k of the composite, while being reduced with respectto the case of the free-moving composite tested with externalheating, was still notable, as shown in Fig. 3b.

Joule heating-based actuation was characterized by applyinga voltage (V¼ 0–1.3 V) in a composite actuator with two layers ofPEDOT:PSS (total PEDOT:PSS thickness, t ¼ 15.3 mm), whoseresistance at room temperature Trt ¼ 25 �C was Rrt ¼ 60 U.Simultaneous video recording with a visible and thermalimaging camera allowed us to estimate the bending andtemperature change as the current I was increased. Selectedframes taken at different temperatures are reported inFig. 3b(1–8). Interestingly, it is possible to note that thetemperature of the LSCE layer is almost uniform under thedifferent conditions. This is possibly due to the good thermalconductivity of the PEDOT:PSS that heats the LSCE uniformlyon the top surface, to the relatively small LSCE thickness withrespect to the surface area and to the low thermal conductivityof the air in contact with the free LSCE surface. The trend ofchange in temperature DT ¼ T – Trt, versus the Joule power P ¼VI is reported in Fig. 5a. At the same time, the resistance R wasfound to decrease as the temperature increased, as reported inthe same gure. It is important to notice that the use of amodied formulation of PEDOT:PSS (addition of DMSO as asecondary dopant agent causing a relevant increase inconductivity, see Experimental section) was particularlyimportant for obtaining such low values of electrical resistance.Indeed, due to the fact that larger driving voltages (V > 8–10 V)

Soft Matter, 2013, 9, 11405–11416 | 11409


Fig. 3 Bending actuation of LSCE/PEDOT:PSS bilayer actuators. (a) Direct heating experiment using a Peltier element. (a) Camera frames taken (1) at roomtemperature, (2) at the nematic–isotropic transition onset, (3) during the transition, and (4) at T > TN–I. (5–8) Corresponding thermal camera frames. (b) Joule heatingexperiment. (b) Camera frames taken at (1) room temperature, (2) at the nematic–isotropic transition onset, (3) during the transition, and (4) at T > TN–I. (5–8) Cor-responding thermal camera frames (scale bar 1 mm).

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can cause over-oxidation of PEDOT:PSS, with irreversible loss ofelectrical conductivity,51 it would be impossible to achieve thesame currents (i.e. same Joule heating) in a PEDOT:PSS lmhaving higher electrical resistance, without irreversiblydamaging it. The over-oxidation process occurs when a rela-tively high voltage is applied in the presence of oxygen, causinga reaction with irreversible interruption of the conjugatedp-system on the backbone of the PEDOT polymer.52 As aconsequence, PEDOT:PSS permanently loses its electricalconductivity.

As regards the observed trend of curvature k as a functionof temperature in the case of Joule heating, it is useful tocompare these results with those obtained in the case ofexternal heating (Fig. 4). As visible in the graph reported inFig. 5b, the initial negative bending was approximately thesame, and the two trends are similar as temperatureincreases up to TN–I, with a small positive shi in the onset ofthe transition for the Joule heating case. On the other hand,as temperature reached TN–I, k did not increase any more withfurther heating, with a maximum curvature k ¼ 0.33 mm�1

observed for a supplied current I ¼ 50 mA at V ¼ 1.3 V, cor-responding to a Joule power of 65 mW. The reduced extent ofbending with respect to free-moving actuators (externalheating) is ascribable to the constraints imposed to thecomposite by the wires. Nevertheless, due to the fact that thebending curvature is rapidly changing in the limited range63–73 �C, with k correspondingly changing from 0 to 0.30mm�1, and to the very accurate control of the local temper-ature by means of Joule heating, these results allow us to

11410 | Soft Matter, 2013, 9, 11405–11416

envision a fast, controlled, fully reversible actuation byworking in the mentioned range. One can think of providingthe necessary current to stabilize the actuator at k ¼ 0, andthen precisely tune the bending by ne-tuning the current.Irradiation with an infrared laser source was also investi-gated as the thermal trigger of actuation, and some repre-sentative results are reported as ESI.†

2.5. Bilayer actuator bending model

In light of the aforementioned experimental results and withthe main aim of determining a suitable actuation strategy forthe bending of the bilayered structure at hand, we hereaerintroduce a simple model providing bilayer curvature k as afunction of the input Joule power P. To this purpose, we sepa-rately link k to temperature T (see Section 2.5.1.), and T to theinput power P (see Section 2.5.2.). The former submodel isvalidated by considering the experiment involving the Peltiercell (see Section 2.3.), while the latter one is assessed against theJoule heating experimental data (see Section 2.4.). We prelimi-narily remark that a simplied treatment is deliberatelypursued, so as to get simple – yet physically representative –

relationships among the involved entities, commensuratewith the available experimental data and the necessaryapproximations.

2.5.1. Curvature submodel. Based on the thermally drivenLSCE elongation/contraction experiments (see Section 2.1.) andby considering the relatively slow bilayer deformations, itseemed appropriate to recall the following curvature–strain



Fig. 4 (a) Bending of LSCE/PEDOT:PSS sample, heated by means of a Peltier cell:curvature (k) vs. temperature (T). Experimental data (circles, joined by solid curve)are suitably described by the adopted model (dotted curve). (b) Contours of thebilayer maximum curvature (solid curves) and of the corresponding strain energy(dashed curves), as obtained by varying hs and hf in eqn (2), while adopting atemperature above TN–I, so as to consider the isotropic LSCE phase. Axis scalesare non-dimensionalized by using the experimental thicknesses (hrefs ¼ 285.3 mm,hreff ¼ 13.7 mm); contour values for bothmaximum curvature and strain energy arenon-dimensionalized with respect to the corresponding model predictions basedon the experimental thicknesses (so that level-1 contours pass through the (1,1)grid point).

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relationship, commonly used in the literature for modelling theelastic deformation of bilayer actuators:53

k ¼ 6 3

hfhm

1þ h

1þ hmð4þ 6hþ 4h2 þ h3mÞ : (2)

In this equation, h ¼ hs/hf denotes the ratio betweensubstrate (LSCE, active layer) and lm (PEDOT:PSS, passivelayer) thickness. Moreover, m ¼ Ms/Mf represents the ratiobetween the biaxial elastic moduli (Mi¼ Ei/(1� ni

2), with i¼ s,f),while 3 denotes the mismatch strain at the interface betweenthe two layers. Such an expression generalizes the one originally


derived for active layers that are thin compared to the passiveones, and for innitesimally small deformations. Indeed, eqn(2) also applies to thin passive layers and large deformations.54

The adopted model was derived under the following assump-tions: thicknesses of both active and passive layers are smallcompared to their lateral dimensions; the passive layer ishomogeneous, isotropic, and linearly elastic, and the activelayer is isotropic; edge effects close to substrate periphery arenegligible and all physical quantities do not change whenmoving parallel to the interface; all stress components in thethickness direction are null throughout the material. Further-more, a uniform curvature is also assumed (so that k in eqn (2)is truly a scalar). Such an assumption, which is not valid forlarge non linear deformations, is also violated when there isinterfacial slip or in-plane force.55 Rigorously speaking, it isextremely hard to match all of the aforementioned assumptionsin real experimental conditions. Moreover, even if LSCEs tend tobehave like isotropic elastomers when stretched/compressedparallel to their director6 as in the considered deformation,some effects perpendicular to the director could be poorlydescribed by the adopted plate model, especially in the nematicrange, since it is based on the Poisson effect classically intro-duced in linear elasticity. Nonetheless, peculiar (i.e. tempera-ture-dependent) LSCE behaviour was directly fed into themodel, as described below. In view of the accepted simplica-tions, model representativeness – at this stage – were regardedas a working hypothesis. We applied eqn (2) by adopting thefollowing parameters (thicknesses were measured on theexperimental sample): hs ¼ 285.3 mm, hf ¼ 13.7 mm, Ef ¼ 2.8GPa, nf ¼ 0.35,49 ns ¼ 0.5. The latter parameter was assumed tobe independent of temperature, and equal to the valuecommonly accepted for materials similar to the consideredLSCE (such as e.g. PDMS), for simplicity. Moreover, Es ¼ Es (T)was dened, as a function of temperature T, by (linearly)interpolating the elastic moduli EH and Er obtained from theexperimental data as described in Section 2.1. (see also Fig. 2d).Finally, the mismatch strain was computed as 3 ¼ (L(T) – L*)/Lrt,where Lrt ¼ 3.46 mm is the bilayer length (i.e. the lateraldimension parallel to the director) at room temperature, whileL(T) denotes the length as a function of temperature T, asobtained by (linearly) interpolating the L/L0(T) experimentalpoints shown in Fig. 2b. In this way, the recorded temperature-dependent LSCE characteristics were directly exploited in themodel. In addition, L* represents an offset introduced for cali-brating the null curvature point. In particular, when consid-ering the Peltier experiment, we chose L* ¼ L(T ¼ 55 �C), sincethe null curvature was observed at 55 �C (see Section 2.3.). Thecurvature provided by eqn (2) is reported in Fig. 4a, plottedalongside the experimental data.

It should be noticed how well the adopted model, despite itsmany inherent simplications, managed to accurately repro-duce the experimental data. This result seems to support(a posteriori) the representativeness of the model. Indeed, notonly was the k–T shape correctly matched (including the sharptransition around TN–I), but also the asymptotic value for largetemperatures, providing k ¼ 0.62 mm�1 (versus the 0.64 mm�1

experimental value). The small discrepancy for the lower T

Soft Matter, 2013, 9, 11405–11416 | 11411


Fig. 5 LSCE/PEDOT:PSS sample actuation by the Joule effect. (a) Temperature increase (DT) as a function of input Joule power (P). Inset: electrical resistance (R/Rrt,normalized with respect to the value at room temperature) vs. temperature increase. (b) Curvature (k) vs. input Joule power. The discrepancy between the model trend(dotted curve) and the experimental data (circles, joined by a solid curve) is due to the constraints imposed by the electrical connections/wiring on the sample.

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values seems to be ascribable to friction effects (during theactuation phase, associated with negative k values, the sampleslides on the supporting plane over a larger contact area) and tosample preparation issues (mainly due to sample positioningon the supporting plane, possibly affected by light pushingclose to the sample boundaries) that are hard to control in arepeatable manner, at least at the present developmental stage.In light of these considerations, it seems reasonable to appre-ciate the obtained agreement between themodel results and theexperimental values, paving the way for further characterizationof the composite actuator under investigation. For instance, eqn(2) predicts a high sensitivity of the curvature with respect tochanges in substrate thickness, to be better characterizedthrough further investigations. For instance, eqn (2) can beused to study the variation of the maximum curvature kmax

(which is a clear performance indicator for the consideredactuators) as a function of both hs and hf, e.g. by adopting atemperature above TN–I, so as to consider the isotropic LSCEphase. Fig. 4b reports some contour (solid) lines of kmax/k

refmax as

a function of hs/hrefs and hf/h

reff , where hrefs ¼ 285.3 mm and hreff ¼

13.7 mm are the experimental thicknesses, and krefmax is obtainedfrom eqn (2) using hrefs and hreff . The considered contours showthat for a xed LSCE thickness, kmax increases by decreasing thePEDOT:PSS thickness, while for a xed PEDOT:PSS thicknessthere is an optimal LSCE thickness. Indeed, a reduced curvatureis obtained both when the LSCE layer is too thin (the active layercontribution is not enough) or too thick (bending stiffness ofthe LSCE layer itself becomes predominant). In particular, theobtained contours show that kmax can be increased by workingwith reduced thicknesses compared to the ones adopted in ourexperiments, yet the validity limits of eqn (2) in the larger-curvature region should be further assessed through experi-ments. Nonetheless, these predictions are fully consistent withthe aforementioned maximum curvature values observed whenvarying hf using multiple PEDOT:PSS layers. Anyway, it isexpected that by using reduced thicknesses, the work

11412 | Soft Matter, 2013, 9, 11405–11416

accomplishable through the actuator would be reduced as well,so broader studies should be carried out in order to optimizeactuator performance. The analysis of the actuator work, whichalso depends on a targeted external load, is beyond the presentscope. Nevertheless, consistent with the adopted simpliedtreatment, we can consider the bilayer strain energy U tocorrelate with the work capability of the actuator. In particular,by considering the isotropic LSCE region (as done for kmax

above) and by recalling classical results from plate theory,56 wecan approximate the strain energy associated with each layer(which has a constant curvature in our approach) as propor-tional to k2D, where D ¼ Eh3/(12(1 � n2)) denotes plate bendingstiffness. Consistently, we can approximate the strain energy atthemaximum curvature as Uf kmax (Ms hs

3 +Mf hf3), and we can

still use eqn (2) to investigate U as a function of hs and hf. Inparticular, Fig. 4b also reports some contour (dashed) lines forU/Uref, where Uref is computed by adopting the referencethicknesses. As expected, the larger curvature region in Fig. 4bis associated with a reduced strain energy, because of thereduced thickness contribution in the considered expressionfor U. Non trivial trends are obtained for the strain energy,because of the negotiation between the indirect effect of thethickness through the equilibrium curvature. For instance, byreducing the PEDOT:PSS thickness, starting from the referenceconditions, it is possible to slightly gain some strain energythanks to the initially prevailing effect of the increased kmax. Yetfurther reduction of the PEDOT:PSS layer implies a reduction ofU, due to the reduced Mfhf

3 contribution (whose relativeimportance is also weighted byMf [Ms). Indeed, based on thecontours in Fig. 4b, it is possible at the maximum to increase Uby 50% while keeping the reference kmax and, reciprocally, toincrease kmax at maximum by 50% while keeping the referenceU. This can be seen, in particular, by the tangency of the relevantcontours in Fig. 4b. This could suggest using a slightly reducedPEDOT:PSS thickness and a slightly increased LSCE thickness(compared to the ones adopted in the experiments), in order to



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increase both curvature and stored energy. However, a moredetailed study also accounting for the power expenditureneeded to activate the considered bending (and thereforesome estimates of the energetic efficiency) is necessary tobetter characterize the considered actuator, so as to properlybenchmark its performance against comparable devices.Nonetheless, the simplied treatment introduced in this workmight provide a starting step for tackling such a quantitativeinvestigation.

2.5.2. Temperature submodel. In light of the experimentaltrend linking Joule power to temperature increase, as discussedin Section 2.4. (cf. Fig. 5a), the following linear submodel wasintroduced:

T � Trt ¼ aP, (3)

where the coefficient a comprehensively accounts for thermalexchange of the bilayer with the surrounding medium (i.e. airfor the considered experiments), at steady state conditions. Alinear tting of the data in Fig. 5a provided a ¼ 0.759 �0.005 �C mW�1 (R2 ¼ 0.998). It is worth remarking that such asimplistic model ts the quasi-static modelling approachadopted in the present study (while the study of the coupled,thermo-viscoelastic dynamic problem is beyond the presentscope). Repeatability of the linear trend in Fig. 5a was veried,so that it is straightforward to obtain the sought k–Pmodel, bycombining eqn (2) and eqn (3). The resulting model wasassessed against the Joule heating experiment data intro-duced in Section 2.4. Relevant parameters for such an exper-iment are: hs ¼ 344.7 mm, hf ¼ 15.3 mm (measured on theexperimental sample); moreover, null curvature was observedat T ¼ 60 �C, so that we chose L* ¼ L(T ¼ 60 �C). The curvatureprovided by the combined model (2–3) is reported in Fig. 5b,against the experimental data.

It should be noticed that the model correctly describes thecurvature transition (still around TN–I), while overestimatingthe asymptotic curvature values. This fact is probably dueto the constraints imposed by electrical connections/wiringon the sample, which detrimentally inuences the achievablecurvature. Nevertheless, this effect seems not to detrimen-tally impinge on the proposed actuator concept, for manifoldreasons. Indeed, the obtained results show that, upon stan-dard calibration, it is possible to identify the Joule powerworking range for effectively exploiting the remarkablecurvature change exhibited by the considered compositeactuator. Consequently, it is possible to tune its geometricalparameters e.g. in order to achieve a targeted curvaturewithin the working range. On the other hand, the imple-mentation itself of the electrical connections/wirings on theconductive substrate was a main enabling step for achievingthe proposed novel actuator, so that it is advisable to opti-mize their conguration in view of the specically pursuedactuation objectives. More rened modelling approaches,also accounting for actual implementation constraints(including wirings), can be pursued, possibly grounded inthe basic results already achieved in the present, deliberatelysimplied context.


3. Experimental3.1. LSCE preparation

In our study, a conventional side chain nematic monodomainLSCE was used. The material, in the form of lm, was synthe-sized using the standard two-crosslinking-steps procedure.42

The polymer backbone was based on a commercial hydrox-ymethyl-polysiloxane, which was crosslinked by 1,4-bis (undec-10-en-1-yloxy) benzene used as cross-linker unit (1). Theside-chain moieties were composed of the usual rod-likemesogens (4-methoxyphenyl 4-(but-3-en-1-yloxy) benzoate) (2).The LSCE sample had a crosslinking percentage of 9% andshowed mesomorphic behaviour, as determined by differentialscanning calorimetry (DSC), with a cooling rate of 10 �C min�1:glass phase – Tg ¼ �8 �C – nematic phase – TN–I ¼ 73.7 �C –

isotropic phase. Synthesis of components 1 and 2 was per-formed as described in ref. 43 and a complete description of thepreparation of LSCE is reported elsewhere.29 LSCE lms wereprepared with lateral dimensions of about 1 � 4 cm2 andthickness of about 350 mm, in which uniform uniaxial align-ment of the nematic local domains was obtained with the localdirector n oriented along the vertical direction (long axis of thesamples).

3.2. LSCE thermo-mechanical characterization

LSCE sample strips (9 � 11 mm2) were used for thermo-mechanical characterization (elongation/compression vs.temperature). The sample was mounted in the vertical positionwith the upper edge attached to a xed frame and the bottomedge to a load (metal ring with weight 1.9 g), by means of pol-yimide adhesive tape (see Fig. 2a). The sample was placed insidea climatic test chamber (CTC 256, Memmert, Germany), whichhad a glass window to allow visual inspection of the sample.Various thermal cycles at controlled humidity (30% RH) wereimposed on the sample from room temperature up to 100 �C,that is well above the nematic-to-isotropic phase transitiontemperature TN–I, and back to room temperature, while simul-taneously recording a video of the elongation/compression ofthe sample.

Stress–strain curves of LSCE strips were collected using ahome-made measurement platform. Samples were clamped attheir edges to a motor driven translation stage (modelM-126.CG1, Physik Instrumente, Karlsruhe, Germany) at oneend and to a high precision load cell at the other (model LRF400 FSH00259, Futek, USA). The control and conditioningelectronics related to stage and load cell was connected througha multichannel DAQ Board (model USB-6218, National Instru-ments, US) with a dedicated PC. Stage position control and loadcell signal collection were performed using soware developedwith Visual Studio 2008 (Microso, Redmond, CA). Sampleheating was provided by a 30 � 30 mm2 Peltier module (GlobalComponent Sourcing, Hong Kong) in close proximity to thesample. The temperature was monitored by real-time thermalimaging using a thermal camera (A325sc, FLIR Systems,Wilsonville, OR) with a close-up lens (T197415, FLIR Systems)featuring an 8 � 6 mm2

eld of view and a spatial resolution of

Soft Matter, 2013, 9, 11405–11416 | 11413


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25 mm. At selected temperatures, samples underwent two sets ofthree elongation–compression cycles, performed at constantstrain rates (0.004 s�1 and 0.02 s�1), with inversion occurring at3 ¼ 0.12 (0.17 for samples over TN–I). Stress relaxation experi-ments consisted in an elongation reaching 3 ¼ 0.12, a 10 srelaxation and nally a compression back to the starting posi-tion of the stage, performed at a strain rate of 0.02 s�1. Igor Pro6 (Wavemetrics, Lake Oswego, OR) was used for data analysis.

3.3. Bilayer LSCE/PEDOT:PSS composite preparation

The actuators tested in this studyweremade of composite bi-layeredlms in which a LSCE substrate was coated with a conductivepolymer layer of poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The LSCE samples were cut intosmall strips (1.2 � 3.5 mm2) and exposed to air plasma (80 s,power P ¼ 7 W, PDC-32G Plasma Cleaner, Harrick Plasma) inorder to improve the wettability of their surface by aqueoussolutions. 5 wt% dimethylsulfoxide (DMSO, Aldrich) wasadded to a commercially available dispersion of PEDOT:PSS inwater (Clevios� PH1000, 1 : 2.5 PEDOT:PSS ratio; Heraeus,Germany) in order to improve its conductivity,51 and themixture was stirred for at least 2 hours at RT before lmdeposition. Then the solution was ltrated (Minisart�,average pore size 1.20 mm, Sartorius) and deposited with amicropipette dispenser over the LSCE substrate (0.4 ml mm�2).The solution was le to dry at room temperature overnight(drop casting), in order to realize a homogenous lm ofPEDOT:PSS (single layer). By repeating this deposition anddrying procedure multiple times for the number ofPEDOT:PSS layers needed (double layer, multiple layers),samples at larger thickness of PEDOT:PSS could be prepared.In the case of samples for testing actuation with Joule heating,thin copper wires (diameter 50–170 mm) were used in order toprovide electrical contacts for applying voltage to the actuator.The copper wires were rst cleaned with acetone and iso-propanol and then coated with gold (40 mA, 120 s; Q150R ESSputter Coater, Quorum Technologies, United Kingdom) tominimize contact resistance. Two wires for each sample wereplaced on the sample surface at opposite edges along its longaxis, just before the drop casting.

3.4. Thermal bending of bilayer LSCE/PEDOT:PSS actuators

Actuators obtained by the deposition of 1–4 layers ofPEDOT:PSS over an LSCE strip were put over a Peltier elementand heated from room temperature up to 100 �C. Temperaturedistribution and actuator shape were recorded using a thermalcamera with a close-up lens. For each measurement, the samplewas allowed to reach a thermal steady state (usually aer twominutes) before taking the picture. Temperature was controlledby progressively raising the voltage of the source driving thePeltier element. Curvature was evaluated by measuringthe radius of the best circular tting of the bilayer midplane inthermal frames taken on the sample side. Sample videosand pictures during experiments were collected with a uEyeUI-2250-MM CCD camera (IDS Imaging Development Systems,Obersulm, Germany) equipped with a Zoom 6000 zoom lens

11414 | Soft Matter, 2013, 9, 11405–11416

(Navitar, Rochester, NY). Image analysis was carried out withImageJ (http://rsb.info.nih.gov/ij/).

3.5. Joule heating of bilayer LSCE/PEDOT:PSS actuators

Wired actuators were connected to a controlled voltage sourcein series to a shunt precision resistance with a nominal value of100U to monitor the current. The driving voltage Vexc was raisedin 0.2 V increments from 0 to 8 V, corresponding to 1.3 V overthe sample. Current I, voltage drop across the sample V and athermal picture were recorded two minutes aer each incre-ment. Evaluation of the sample curvature was carried out as fordirect heating experiments.

4. Conclusions

In this paper a new bilayer composite based on LSCE and aconductive PEDOT:PSS thin lm was presented. LSCE mate-rial employed in the study has a shape-memory behaviourtriggered by temperature with a sensitive contraction alongone preferred axis by increasing the temperature from roomtemperature up to the nematic to isotropic transitiontemperature, TN–I. Thermo-mechanical characterization of theemployed LSCE over multiple elongation–compression cycleswas presented. Reversible bending actuation resulted in thecase of LSCE/PEDOT:PSS composite, and thermal actuationwas characterized in terms of bending curvature vs. tempera-ture in dedicated experiments by video recording withcameras both in visible light and IR (thermal imaging). Thismethod allowed us to simultaneously appreciate the actuationand the application of stimulus (heating), while providing avisual inspection on homogeneity of temperature. By exploit-ing the good electroconductive properties of PEDOT:PSS andits mechanical properties, better matched with the compliantLSCE with respect to other approaches, it was possible to drivecurrent in the composite and to provide the thermal triggerthrough the Joule heating effect. Actuation by Joule heatingwas demonstrated and the results were compared to the caseof external heating. Finally, the experimental results werecompared with a simple model able to describe the bilayercurvature as a function of a given electrical power. Thesenew composite materials are very promising, since achievingelectrical control of the actuation of LSCE could allow usto envision many practical applications, in the elds ofso robotics, micro-robotics, optics, microuidics andbiomedicine.

Acknowledgements

V. D. is grateful to COST (European Cooperation in Scienceand Technology) in the framework of ESNAM (EuropeanScientic Network for Articial Muscles) – COST ActionMP1003. V. D. thanks the COST ACTION MP1003 for theshort term scientic mission COST-STSM-MP1003-11566. V.D. thanks the Centre of Excellence NAMASTE (Ljubljana) forthe nancial support as visitor professor in 2011. Thecooperation between IIT and DCCI (protocol number 45748)is also acknowledged.



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