In situ studies of strain dependent transport properties of conducting polymers onelastomeric substratesVenugopalan Vijay, Arun D. Rao, and K. S. Narayan Citation: Journal of Applied Physics 109, 084525 (2011); doi: 10.1063/1.3580514 View online: http://dx.doi.org/10.1063/1.3580514 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/109/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Highly clear conductive polymer electrode films hybridized with gold nanoparticles Appl. Phys. Lett. 99, 233304 (2011); 10.1063/1.3665612 Structure and properties of polymer core-shell systems: Helium ion microscopy and electrical conductivity studies J. Vac. Sci. Technol. B 28, C6P59 (2010); 10.1116/1.3504589 Direct observation of localized conduction pathways in photocross-linkable polymer memory J. Appl. Phys. 105, 124516 (2009); 10.1063/1.3153980 Conducting Ni nanoparticles in an ion-modified polymer J. Appl. Phys. 98, 066101 (2005); 10.1063/1.2014938 X-ray photoemission spectroscopy studies of conducting polymer-substrate interfaces: Interfacial electrochemicaldiffusion J. Appl. Phys. 81, 7313 (1997); 10.1063/1.365353

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In situ studies of strain dependent transport properties of conductingpolymers on elastomeric substrates

Venugopalan Vijay, Arun D. Rao, and K. S. Narayana)

Jawaharlal Nehru Centre of Advanced Scientific Research, Jakkur P.O., Bangalore-560064, India

(Received 18 December 2010; accepted 16 March 2011; published online 28 April 2011)

We report the changes in the surface electrical resistance, R, of conducting polymer, Poly

(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) films coated on appropriate

flexible substrates in stretched conditions. These studies are important in the context of flexible

organic electronic applications. In situ conductivity measurements on pristine PEDOT:PSS thin

films on elastomeric substrates upon stretching reveal a minima in R as a function of strain, x, prior

to the expected increase at higher strain levels. The studies emphasize (i) role of substrates,

(ii) stress-induced anisotropic features, and temperature dependence of R (iii) in comparison of R(x)

in polymer films to that of conventional metal films. The stress induced changes is modeled in terms

of effective medium approximation. VC 2011 American Institute of Physics. [doi:10.1063/1.3580514]

I. INTRODUCTION

Organic electronics based devices have been pursued in

energy,1,2 display,3,4 and sensor5,6 related applications. The

polymeric nature of the active electronic component and the

possibility of fabricating devices on soft substrates open up a

whole range of sensors that can be integrated to our surround-

ings. The additional possibility of incorporating stretching and

bending capabilities of such products introduces valuable fea-

tures that can be incorporated in novel design concepts.3–6

Substantial efforts have been taken toward this direction in the

recent past.1–8 An essential aspect in the development of the

next generation of flexible devices is the prerequisite knowl-

edge of microscopic aspects of the changes in the structure

and associated electrical transport as a function of the me-

chanical parameters.

Flexibility, a combination of bending and stretching, can

lead to deterioration of both the polymer bulk and electrode

electronic properties along with changes in the mechanical

integrity of the active layers, interfaces, and substrates.9,10

Stretching studies of blend composites consisting of conduct-

ing polymer incorporated in polyurethanes have demon-

strated flexible electrode characteristics.11 However, there

are no reports related to layered-stretchable composite struc-

tures. Critical components in these devices are the conformal

substrates, which have the right combination of surface fea-

tures, optical, and mechanical properties. Besides the novelty

aspect of flexible devices, there are considerable advantages

in the device fabrication and processing conditions upon

introduction of such substrates. A possible candidate for

such a substrate is the styrene ethylene butadiene styrene tri-

block copolymer (SEBS). SEBS elastomer obtained from

Kraton Polymers G1721M, (having a tensile stress of �2

MPa and the elongation break at 200% strain) is traditionally

used in footwear and engineering plastic applications as syn-

thetic replacements for rubber, specifically due to properties

such as flexibility, high traction, and sealing abilities, with

increased resistance to heat, weathering, and chemicals.

We focus on PEDOT:PSS as a representative conducting

polymer for the active layer. PEDOT:PSS is a technologically

important polymer and is an active component in light emit-

ting diodes,12,13 field effect transistors,14–16 solar cells,17–19

and sensors and plays the role of electrodes or electron block-

ing/hole injecting layers.20 Thin films of PEDOT:PSS are

formed from an aqueous suspension on various rigid or plastic

substrates.21–24 The conducting form of PEDOT:PSS avail-

able as exhibits a conductivity r of 460 6 20 S/cm and a work

function of 5.35 6 0.05 eV. Conductivity of 460 6 20 S/cm

corresponds to a surface resistance (R) value of 140 6 4 0 X/

h which is close to the obtained value in our laboratory of

325 6 70 X/h. However, all elastic substrates are not typi-

cally amenable for solution deposited film formation with

PEDOT:PSS thin films binding well to only those elastic sub-

strates that undergo appropriate substrate-processing proce-

dures in presence of adhesion promoters. Our work primarily

focuses on the studies of electrical transport properties of

PEDOT:PSS films upon deforming the elastomeric substrates.

The mechanical response of the conducting polymer blend

film as the substrates strain, are difficult to track closely,

unlike the electrical response, which can be monitored contin-

uously. Straining basically modifies the surface and the bulk

morphologies, which in turn results in changes of the electri-

cal macroscopic parameters such as the electrode-series resist-

ance. We report specifically the stress induced changes in

PEDOT:PSS thin films coated on SEBS substrates. Interest-

ingly, optical quality PEDOT:PSS film not only maintained

its mechanical integrity in the stretched conditions but exhib-

ited enhancement in electrical conductivity, r, in the stretched

direction over a limited range of strain. However, a decrease

in r was observed in the direction perpendicular to strain,

leading to a strain induced anisotropy. The influence of the

substrate deformation on PEDOT:PSS is unlike that of

changes in conventional metal films where the crack and

defect propagation is clearly observed beyond small threshold

strains. It is well known that for nonhardened free standing

a)Author to whom correspondence should be addressed. Electronic mail:

narayan@jncasr.ac.in.

0021-8979/2011/109(8)/084525/6/$30.00 VC 2011 American Institute of Physics109, 084525-1

JOURNAL OF APPLIED PHYSICS 109, 084525 (2011)

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films, the instabilities from the tensile deformation promote

the film to thin down locally (necking instability) to result in a

rupture. The length scales for the elongation are expectedly in

the order of the film thickness. In case of metal bonded to the

substrate, the strain field is delocalized, and the rupture strain

magnitude exceeds in comparison to the rupture strain of the

free standing film. The stress profile has been speculated to

arise from a formation of multiple necking locations.25,26 It

has been observed that when a metallic film/polymer bilayer

is mechanically loaded, electrical failure likely occurs well

before mechanical collapse because the formation of micro-

cracks on the film surface is more disruptive for the electrical

conductance.27,28 There is no analogous model for defect

propagation in conducting polymer blends adhered to an elas-

tomeric substrate. A model of differential effective medium

approximation is then adapted to explain the nature of the con-

ductivity observables upon straining, which in our case con-

siders electric field scattering from conducting grains

embedded in a matrix.

II. EXPERIMENTAL SETUP AND PROCEDURE

Styrene ethylene butadiene styrene (SEBS) was pro-

cured from Kraton Polymers (G1721M) in pellet form, and

flexible substrate films were obtained in a controlled manner

by solution processing using chloroform as a solvent. The

aqueous dispersion of PEDOT:PSS (Agfa, Orgacon Printing

Ink EL - P3040) was spin coated on SEBS films of 32� 18

mm substrate films at 2000 rpm (30 s), and post annealing

(70 �C for 12 hr) films of �120 nm thickness were obtained.

Gold deposition on all flexible substrates was done using

standard thermal vapor deposition procedure under high vac-

uum conditions.

The setup for stretching and bending was a home-built

arrangement powered by a two stage stepper motor. Bending

of the samples was done by keeping the samples between two

incoming blockades, which bend the sample symmetrically,

and the contacts were taken out from the edges of the samples

[Fig. 5(a)]. For stretching, the film is clamped at two ends and

uni-axially stretched as seen from Fig. 2(a). The free standing

PEDOT:PSS-coated SEBS films were stretched in a controlled

manner for a minimum of two stretch cycles. The electrical

probe assembly consisted of gold-coated neodymium-iron-bo-

ron magnets [two columns of square faced magnets (12 mm)

separated by a distance of 6 mm] and a counter magnetic base

providing the self-alignment required for the studies. The re-

sistance measurements were done at desired stretched ratios.

The arrangement consisted of Au-coated magnetic probes on

an arm that can be gradually introduced on the film along with

a counter magnetic plate on the other side of the film that pro-

vided the support and the alignment [Fig. 2(a) schematic].

The base magnetic plate–magnet interaction enabled accurate

positioning of the electrodes on the same region at different

stretching ratios. The base-magnet interaction contact

strengths where adjusted to a magnitude so as to minimize

any variation in the resistance values. The probe induced

stress was kept to a minimal level, and the integrity of the film

surface was maintained during the course of measurements.

The conductance values using this arrangement were in close

agreement with four-probe measurements using standard

thermally evaporated Au-coated electrodes for unstretched

samples, indicating negligible contact resistance in the mea-

surement setup. It is to be noted that the stretching/straining

was done with the probe arm lifted off, and successive meas-

urements are taken by placing the probes on the strained film at

the same spatial position.

III. RESULTS AND DISCUSSION

Variety of substrates including SEBS, PDMS, PMMA,

and natural and nitrile rubbers were used to deposit

PEDOT:PSS films. Uniform films were obtained on these

substrates using appropriate processing conditions. A mild

strain (2� 5%) of the substrate, however, resulted in a cer-

tain degree of nonuniformity and inhomogeneities in the film

on all the substrates except on SEBS layer [Figs. 1(a)–1(c)].

This observation can be interpreted on the basis of lower ad-

hesive interaction of PEDOT:PSS on the non-SEBS sub-

strates. The PEDOT:PSS film’s rupture, which appears in the

form of cracks and delamination, was prevalent for a wide

range of thickness (120 nm to 1 lm). The electrical resist-

ance measurements followed the trends observed in the

structure in the form of a large increase in R beyond strain

values of 2� 5%.

FIG. 1. (Color online) Choice of sub-

strates: (a) and (b) depict stretching and

delamination of a typical PEDOT:PSS

film coated on natural rubber substra-

tes.(c) indicates cracks developed on

stretched PEDOT:PSS coated on nitrile

rubber. (d) shows surface of PEDOT:PSS

on stretched SEBS substrates.

084525-2 Vijay, Rao, and Narayan J. Appl. Phys. 109, 084525 (2011)

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In case of SEBS substrates (thickness: �1 mm), the

PEDOT:PSS films were able to maintain their integrity over

a wider range of strain [Fig. 1(d)]. The textured surface of

SEBS creates local fluctuations in surface energy and contact

area; this promotes the adhesion of PEDOT:PSS films to sub-

strates through van der Waals interactions. Moreover, the

addition of ethanol, ethylene glycol, and other surfactants to

PEDOT:PSS dispersion decreases the polymer substrate

interfacial tension and provides better binding capabilities.

The surface of PEDOT:PSS films on strained/unstrained

SEBS were quite uniform. SEM and atomic force micros-

copy (AFM) studies on pristine samples showed no indica-

tions of any preferred orientation. r was also measured along

different directions on the film surface prior to stretching.

The surface r did not exhibit any signs of anisotropy, and

the magnitude did not have any directional dependence. The

absence of inhomogeneities on the semitransparent

PEDOT:PSS surface is seen in Fig. 1(d). SEM studies on

nonstretched and the stretched samples (up to 20% strain)

did not reveal any apparent surface discontinuities, cracks, or

signs of delamination in the micrometer regime of the

PEDOT:PSS film. Discernible features between the stretched

and the unstretched films were also not present in the higher

magnification SEM and AFM observations.

In case of the electrical conductivity r(x), a clear pattern

is observed upon analyzing the trend from an extensive set

of measurement on about 50 films. The variation in (x)

observed for the stretching measurement can partly be

accounted by experimental factors including minor film

inhomogeneities across the 6 mm film span and residual

clamping strain adjustments. As will be seen later, the varia-

tions can also be attributed to the local size distribution of

the PEDOT domain in the polymer blend. Nevertheless, a

general trend in all the cases is the initial increase of film

conductance in the strained direction and the average esti-

mate for the decrease in R||(x) is �15 6 4% for strain

x� 14%. Here || indicates the direction of strain. Figure 2

shows the percentage change in surface resistance (DR||) with

strain for a representative five samples. Figure 2(b) shows

the change in surface resistance (DR\) normal to strain. It is

to be noted that the dimensional aspect, that is, interelectrode

distance, electrode length, and film thickness (L, w, t) of the

region of measurement was unchanged. The interelectrode

distance L was maintained in a carefully implemented Au-

coated magnetic probe setup as described in the measure-

ment description section. The relative constancy (<5%) in

thickness in the applied strain regime was monitored by

interferometry and microscopy measurements. Nevertheless,

the thinning down of films only leads to a marginal change

(increase) in the assessment of r magnitude in these films.

Hence the improved values of r(x) cannot be attributed to

the errors arising from the estimates in the dimensions.

Beyond a range of Dx � 14 6 4%, the R||(x) decreases line-

arly. The trends remained similar with varying strain rates of

(0.01 – 100 mm/s). Strains beyond 30% were not possible

due to the noncompliance of the thin SEBS substrates (�0.5

mm) that were utilized. It can be stated that PEDOT:PSS

could withstand the strain without appreciable deterioration

in its structural and electrical properties.

In comparison, for thermally evaporated Au films on

SEBS (L, w, t – dimensions of 20 mm, 10 mm, 150 nm,

respectively), the end to end resistance marginally varies at

low-initial strains and drastically increases and diverges

beyond x � 7 6 3% strain. Cracks of few micrometer sizes

that develop at low strain levels do not disrupt the conduct-

ing pathways, and continuity is maintained. However beyond

7%, which can be taken as a percolation threshold for this

configuration, the interelectrode conductance diverges. The

low thresholds reflect the weak adhesive strength between the

two layers25,28,29 and the compliant nature of the substrate,

which fails to impede strain localizations.25,28 Figure 3(b)

depicts the cracks observed on Au layers coated on SEBS at

10% strain. It is to be noted that it is possible to strain conven-

tional electrodes (Au) to much higher levels on tailored sub-

strates with additional procedures.30,31 In this context, the

conducting polymer films on SEBS completely differs in the

electrical response in the low strain regime, where the rincreases in (0< x< 15% range). Even for x � 20%, the

strained PEDOT:PSS on SEBS shown in Fig. 3(b) did not

have any evidence of crack or discontinuity formation. These

trends appear as a smooth profile without the presence of a

divergence in R(x) (Fig. 2).

A useful way to portray the stretch induced phenomena

in low dimensional electrical systems is in terms of the mac-

roscopic anisotropy; [r||(x)/r\(x)]. r|| is the film conductivity

in the direction of strain and r\ is the conductivity in the

direction perpendicular to strain and in the plane of the film.

As depicted in (Fig. 4), the anisotropy increases with strain

in the low strain regime (0< x< 10%) where dR||(x)/dx< 0,

indicative of alignment processes and a regime where r||(x)

and r\(x) have competing trends. Beyond �10%, a plateau

in r||(x)/r\(x) is observed as shown in Fig. 4 that basically

corresponds to the regime where similar trends in the r||(x)

and r\(x) response are observed. The initial plateau regime

FIG. 2. (Color online) Conductivity changes on straining: The plot of per-

centage change in surface resistance with strain in the direction of stretch for

a representative five sets of samples. Inset (a) is a schematic of the uni-axial

stretching setup and the self aligning Au-magnet electrodes. Inset (b) shows

the response of film surface resistivity to strain in the direction perpendicular

to the strain.

084525-3 Vijay, Rao, and Narayan J. Appl. Phys. 109, 084525 (2011)

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corresponds to the onset of crack and defect formation

processes.

Two point bending studies of PEDOT: PSS on SEBS

substrates (�200 lm) were also carried out. The R changes

were measured with varying the inverse of radii of curvature,

r [Fig. 5(a)]. A weak exponential type response in R(r) was

observed as seen from the figure. However, the changes were

two orders of magnitude lower than the stretching case over

the entire regime of bending. These negligible changes are

also evident upon examining the film surface, which remains

relatively unchanged after the bending action. In the context

of large area device applications, the electrical and mechani-

cal changes are not significant for the typical range of envis-

aged bending radii.

The changes in the mechanism of the electrical transport

arising from the stretching induced structural variations were

followed by temperature, T-dependent studies of r. Samples

with different degrees of stretching were studied by transfer-

ring the stretched PEDOT:PSS on SEBS films onto a rigid

support using an adhesive support. The stretched films pre-

served in this manner exhibited a reasonably stable magni-

tude of R over a prolonged duration at room temperature

without any indication of any self-relaxation processes. The

T-dependent studies essentially highlight the clear differen-

ces in the R(T) for different degree of stretching. In general,

a decreasing “exponential character” in the R(T) was

observed with increasing degree of strain (0< x< 20%).

This is clearly seen in (Fig. 6) for the in-stretch direction

response. Similar trend was observed in case of R\(T), which

indicates a common isotropic electrical transport mechanism.

Conduction models applied for high conducting PEDOT:

PSS films, predominantly exhibit a [r¼r0 exp(�To/T)m] de-

pendency where the r0 and To are constants and the exponent

value of m¼ 0.5 has been justified on the basis of a polaronic

island model,32,33 charge tunneling through conducting grains,

1-D variable range hopping model.34,35 However, in the pres-

ent case, an exponent close to 0.55 is obtained only for x¼ 0

(Fig. 6). For the strained sample, a deviation from this form of

r(T) is obtained. r(T) has a weaker T dependence (a near-lin-

ear behavior) for the strained films. It is difficult to conclude

with the present data that there is a change in the transport

mechanism or mere changes in the parameters; r0 and To val-

ues. A model that can incorporate the strain induced changes

has been proposed in the framework of the heterogeneous

model36,37 and has been extensively applied to conducting

polymers.36–38 The general concepts in these models along

with the inclusion of the microscopic aspects of PEDOT:PSS

blend structure can be used to explain the onset of a

FIG. 3. (a) SEM image of PEDOT:PSS

film on SEBS at 20% strain. Inset (b) is

an image of stretched Au films on SEBS

at 10% strain.

FIG. 4. (Color online) r||/r\ as a function of the strain in the PEDOT:PSS

film along with change in R\ and R|| with strain. The dotted lines are fit to

the model.

FIG. 5. (Color online) Response to bending: Inset (a) depicts the schematic

of the bending setup which is used to calculate “r” (cm). Inset (b) is the

measured PEDOT:PSS sample (3.2� 1.8 cm) in symmetrically bent condi-

tion on 200 lm thick SEBS substrate, and the plot depicts the change in sur-

face resistance versus the inverse of radii of curvature (r).

084525-4 Vijay, Rao, and Narayan J. Appl. Phys. 109, 084525 (2011)

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semimetallic type response r(T). However, extensive investi-

gation is required to comprehensively understand the changes

induced by straining.

The bulk and the surface morphology of PEDOT:PSS

has been studied extensively.34,35,39,40 Conducting hydropho-

bic PEDOT globules are thought to be surrounded by longer

hydrophilic PSS chains in an aqueous dispersion, which

upon spin coating and secondary doping form randomly ori-

ented micrometer sized domains containing aligned PEDOT

rich granules (�25 nm).34 The correlations between mechan-

ical stress and electrical conductivity of conducting polymers

from aqueous suspensions coated on elastomeric substrates

are expected to be complex. However, the effect of straining

PEDOT:PSS films can be understood in terms of the orienta-

tion and extension of the granular regions of PEDOT that are

embedded in the PSS matrix. For doing this, we consider an

effective medium model of differential effective medium

approximation (DEMA). In the DEMA model that originated

from the Bruggermann equation, the Maxwell Garnett gener-

alizations and Cohen et al.’s work, one calculates the effec-

tive physical coefficients of a multiphase system through

multiple scattering approximations of various fields such as

the electric field.41–43 Each grain (of a different phase, vary-

ing in shape, size, and di-electric constant e) is incrementally

introduced to the matrix, and the scattering contribution

from the newly added grain is integrated at every step. The

resultant is then used as a background matrix in the next

step, for the next added grain. The effective medium model

has been used successfully to explain the observed optical

and transport features of conducting polymers and other

organic devices.44–47 However, in our case, a differential

form of the model was necessitated to model the dynamical

changes in morphology that are expected upon straining.

The system under consideration in our case consists of

spherical conducting grains in an insulating matrix similar to

the morphology exhibited by PEDOT:PSS. A similar model

system where DEMA has been applied successfully is in the

studies of polyelectrolyte systems by Oliver et al.,48 where

enhancement of transport properties in the strain direction

and decrease in the transport coefficients in the normal direc-

tion are interpreted in terms of a two phase DEMA model.

The two phase model by Oliver et al. is modeled as a granu-

lar conducting phase, associated with oriented molecular

structures, which dynamically change from spherical

domains to prolate domains upon straining, and are sur-

rounded by an insulating phase at all times. Upon the model-

ing of these topological effects, the system then exhibits an

increased conductivity in the direction of strain along with a

strain induced anisotropy, which is understood by the

decreased scattering of the field in the r|| direction and an

increased scattering in the r\ direction. This is similar to

what we observed in PEDOT:PSS up to strain levels of

<15%, where an enhancement of r|| and a decrease in r\measurements are observed with increasing strain. The r||(x)

and r\(x) trends obtained are similar to the extrapolated

dependencies at low strain regimes of the model. In our sim-

plified EMA model, the sample is modeled by considering

rectangular conductive unit cells separated by insulating

boundaries in place of a globular model. Upon straining

(along the x direction), the dimensions of these cells change

from dx, dy, and dz to kdx, Hkdy, and Hkdz; where k is the

strain ratio. The area of the face of a unit cell varies as (k)�1

in the strain direction and as Hk normal to the strain. Consid-

ering the unit cell boundary area as the rate determining step

for r and enforcing volume conservation conditions with

strain, the function r(k) is determined to be r|| ! k2 and r\! 1/Hk. These dependencies are plotted in terms of the

strain (x) and resistance (R) in Fig. 4 as dotted lines. A good

match with the experimental data for both r||(x) and r\(x) is

observed in the lower strain regimes (<15%).

However at higher strain regimes (>15%), which can be

taken as a limit strain for application of this model for the

PEDOT:PSS on SEBS, defect nucleation, distribution and

propagation start dominating, and a departure from the trends

predicted from the model are observed in the defect dominat-

ing regime (Fig. 4). At these strain levels, the morphological

variations are expected to be severe where defect dynamics

need to be modeled along with DEMA to arrive at a com-

plete picture at all strain regimes.

IV. CONCLUSION

In summary, the strain dependent electrical transport in

PEDOT:PSS thin films reveal interesting features. We empha-

size the importance of substrates and propose SEBS as a

potential candidate for flexible organic electronics applica-

tions. A novel method to monitor r in situ during the straining

process was devised to implement the studies. Pristine

PEDOT:PSS film stretched to large strains (28%) maintained

its film integrity. A minima in the response of R(x) with uni-

axial strain prior to a monotonic increment was observed.

This response, which indicated a decrease � of 15% in

R magnitude and the strain induced anisotropy in r, was

modeled in the framework of the DEMA model. R(T) exhib-

ited a general trend of an effective decrease in temperature de-

pendence as a function of strain (<20%), suggesting a

FIG. 6. (Color online) Temperature dependence: Surface resistance as a

function of T for PEDOT:PSS films with different degrees of strain. Ln(r) vs

(1000/T0.5) for 0, 10, and 20% strained samples are shown in the insets (a),

(b), and (c), respectively.

084525-5 Vijay, Rao, and Narayan J. Appl. Phys. 109, 084525 (2011)

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lowering of the disorder and an increase in the alignment of

the conducting domains.

ACKNOWLEDGMENTS

We acknowledge the funding support from Department

of Atomic Energy, Govt of India, toward this project. The

authors thank Kasper Wenderich for his assistance in the

measurement setup.

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