interpenetrating polymer networks based on acrylic elastomers and plasticizers with improved...

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Research ArticleReceived: 5 July 2009 Revised: 30 November 2009 Accepted: 3 December 2009 Published online in Wiley Interscience: 3 February 2010

(www.interscience.wiley.com) DOI 10.1002/pi.2784

Interpenetrating polymer networks based onacrylic elastomers and plasticizers withimproved actuation temperature rangeHan Zhang,a Lukas During,b Gabor Kovacs,b Wei Yuan,a Xiaofan Niua

and Qibing Peia∗

Abstract

The VHB 4910 acrylic elastomer, already exhibiting great efficacy at room temperature, was modified to enhance its actuationperformance via two approaches. To eliminate the requirement of high pre-strain, interpenetrating polymer networks (IPNs)based on poly(trimethylolpropane trimethacrylate) were formed in the original acrylic network. Furthermore, plasticizingadditives were used to reduce the glass transition temperature, broaden the operating temperature range, lower the elasticmodulus (hence the electric field required for actuation) and decrease the viscoelastic loss (to increase response speed).Dibutoxyethoxyethyl formal (DBEF) was found to be a highly effective plasticizing agent. For a relaxed IPN film containing 40wt% DBEF actuated at −40 ◦C with an applied voltage of 6.5 kV, a large deformation (215% in areal expansion) is demonstrated.c© 2010 Society of Chemical Industry

Keywords: dielectric elastomer; interpenetrating polymer network; additive; strain; electroactive polymer

INTRODUCTIONActuators based on electroactive polymers (EAPs) have attractedincreasing attention for a variety of applications including mini-and micro-robots, adaptive structures, prosthetic devices, pumps,valves, heel-strike generators and large-strain sensors.1 – 9 Amongthe various types of EAP, dielectric elastomers based on achemically crosslinked acrylic ester copolymer manufactured forpressure sensitivity bonding – 3M VHB 4910 – have repeatedlyexhibited the most promising properties. These include large-area strains as high as 300%, high energy density (3.5 MJ m−3

calculated from the dielectric strength) and electromechanicalcoupling efficiency (93.7% has been reported).10

Despite the eminent features of these novel actuator materials,there are several constraints that limit their applicability andperformance. First, large mechanical pre-strain, which enhancesperformance by increasing the dielectric breakdown strength ofthe acrylic copolymer films,2,11 is usually required to achievelarge reversible electromechanical strain. However, the rigidpre-strain-supporting structures are given as a reason for thelarge performance gap between active materials and packagedactuators.10 Efforts have been made to mitigate the requirementof mechanical pre-strain. We have previously reported chemicallymodified acrylic copolymer films with interpenetrating polymernetworks (IPNs) achieved by adding a trifunctional additive,trimethylolpropane trimethacrylate (TMPTMA), to the acryliccopolymer network.10,12 – 14 TMPTMA polymerizes and crosslinkswithin the original acrylic network, forming a second elastomericnetwork – poly(TMPTMA) – that interpenetrates with the acrylicnetwork. It has been shown that the additive network caneffectively support the large mechanical pre-strain introducedto the original acrylic network and hence successfully eliminatethe need for external supporting structures.

The IPN films exhibit actuation strain and specific energy densitycomparable to highly pre-strained acrylic films.10 Modest strainhas also been reported in other polymers without pre-strain.These include thermoplastic SEBS copolymers impregnated withaliphatic mineral oil and certain silicone elastomers with or withoutplasticizing additives.15 – 20 These materials show generally lowerdielectric strength and actuation energy density compared tothe IPN films. IPN films of acrylic rubber network modified byinterpenetrating a small amount of silicone rubber using a swellingmethod have also been reported to enhance the tensile strengthand actuating performance of the pristine acrylic rubber.21

Second, conventional acrylic and IPN elastomers exhibitexceedingly small strain at low temperatures, translating to astrong temperature dependence of elasticity.20 At 0 ◦C and below,as the temperature approaches the glass transition temperature(Tg) of the acrylic, the polymer chains begin to ‘freeze’ and canonly move around with difficulty within the polymer matrix.Blending with plasticizing additives has provided an efficientavenue to broaden the operation temperature range, both foracrylic and silicone elastomers.6,20 In the work reported in thispaper we explored the use of plasticizers to broaden the operating

∗ Correspondence to: Qibing Pei, Department of Materials Science and Engineer-ing, Henry Samueli School of Engineering and Applied Science, University of Cal-ifornia at Los Angeles, Los Angeles, CA 90095, USA. E-mail: [email protected]

a Department of Materials Science and Engineering, Henry Samueli School ofEngineering and Applied Science, University of California at Los Angeles, LosAngeles, CA 90095, USA

b Laboratory for Mechanical Systems Engineering, Swiss Federal Laboratoriesfor Materials Testing and Research (Empa), Uberlandstrasse 129, CH-8600Dubendorf, Switzerland

Polym Int 2010; 59: 384–390 www.soci.org c© 2010 Society of Chemical Industry

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Plasticized acrylic elastomer-based IPNs for improved actuation www.soci.org

Figure 1. Chemical structures of plasticizers: (a) bis(2-ethylhexyl) phthalate (DOP) and (b) dibutoxyethoxyethyl formal (DBEF).

temperature range of IPN films based on acrylic–poly(TMPTMA). Aspecific goal was to demonstrate actuation with reasonable strainsat extreme temperatures, which would allow for a greater varietyof applications.

Plasticizers are known for their ability to modify the me-chanical behaviors of polymers. Based on three classes ofcompounds – phthalates, adipates and trimellitates – this class ofmaterials generally comprises weakly polar organic compoundswith Tg in the range of −50 to −150 ◦C and high boiling pointsof the order of 300 ◦C.22,23 The most commonly used plasticizersare based on weakly polar esters, more specifically those ob-tained from phthalic acids including diethyl, dibutyl and n-dioctylphthalates.22 By embedding within and distributing through thepolymer matrix, hence separating polymer chains, plasticizersweaken the intermolecular interaction of the polymer by solvatingaction, increase polymer chain mobility and reduce Tg. The effec-tiveness of a plasticizer depends on three factors: its concentration,its compatibility with the polymer and its viscosity.24

Additionally, acrylic and IPN elastomers, with relatively highmechanical loss factor tan δm, typically have low response speeddue to the inherent viscoelastic loss. Several approaches havebeen employed to reduce hysteresis in EAPs.6,15 – 18,20,23 It hasbeen shown that stress relaxation becomes less pronounced withincreasing plasticizer content in elastomeric matrices.6,23 It shouldalso be noted that higher plasticizer concentration could lead toreduced dielectric constant and storage modulus.23,25

In this paper we present a unique approach to modifying acrylicelastomer films, combining IPNs and plasticizing agents to elim-inate the external rigid pre-strain-supporting structure, broadenthe operating temperature range and minimize the viscoelasticbehavior of the acrylic-based elastomers. Two plasticizers are ex-plored: dibutoxyethoxyethyl formal (DBEF) and bis(2-ethylhexyl)phthalate (DOP). DOP is one of the most common plasticizers avail-able in industry accounting for more than 50% of all plasticizersused.23 DBEF is a less commonly known plasticizer that we havefound capable of greatly improving acrylic polymer propertiesto fit desirable applications. The chemical structures of the twoplasticizers are shown in Fig. 1.

EXPERIMENTALIPN film fabricationFilms of 3M VHB 4910 were biaxially pre-strained 400% by400% (i.e. five times their original length for all four sides).For every 11.796 g of the VHB 4910 film, a mixture – consistingof 1.1443 g of TMPTMA monomer (8.838 wt%) and 0.1040 g ofbenzoyl peroxide (0.882 wt%) as the thermal initiator completelydissolved in 50 mL of ethyl acetate – was uniformly sprayed witha Paasche airbrush onto both sides of the film. The preparedfilms were left to dry overnight, allowing for evaporation of

solvent and full absorption of the sprayed chemicals. Theywere subsequently cured at 85 ◦C for 7 h in vacuum and thenreleased from the supporting structures to remove mechanicalpre-strain, resulting in fully relaxed free-standing films. Because ofthe possibility of higher stress concentration near the edgesof the films still attached to the frames, the edges of thecomposite films were discarded. The preservation of the pre-strain was determined from the differences in distance betweentwo marked dots on the cured film before and after relaxation.All the samples prepared could retain 60–65% of the original400% by 400% pre-strain, which is approximately 260% preservedpre-strain.

Preparation of plasticized filmsIPN films prepared as described above were sprayed uniformlywith various quantities of DOP (purchased from Sigma Aldrich)and DBEF (acquired from Sartomer). For every 0.44 g of IPN film,feed ratios of plasticizer : IPN = 1 : 2, 1 : 1 and 2 : 1 corresponding to0.22, 0.44 and 0.88 g of plasticizer were dissolved in 5 mL of ethylacetate. The solutions were then sprayed onto the IPN films, whichwere subsequently left to dry overnight under ambient conditionsto evaporate the ethyl acetate and allow the additives to be fullyabsorbed into the films. By determining the differences in massbefore and after spraying with the plasticizer, it was found that theweight percentages of plasticizers in these films were 25, 40 and44% (corresponding to feed ratios of plasticizer : IPN = 1 : 2, 1 : 1,2 : 1, respectively).

Actuator fabricationTwo types of actuation experiment were conducted. Diaphragmactuators were fabricated to investigate the effect of plasticizer onoperating temperature range. To form a diaphragm, samples weresealed over a pressure chamber supplied with air to maintain apneumatic pressure on one side of the diaphragm, guaranteeingfilm deformation in a spherical geometry. During actuation thesamples experienced out-of-plane strain and deformed outwardsto form a bulge. Thin layers of conductive carbon grease weresmeared onto both sides of the films as electrodes. The setup wasthen placed inside a Thermotron S-1.2 environmental chamberwith a testing range of −73 to 177 ◦C. Figure 2(a) shows thesetup viewed through the window of the chamber. Samples wereactuated using a high-voltage box built in-house from −40 to 80 ◦Cwith a temperature increment of 10 ◦C. A high-speed camera wasused to capture images of the spherical deformation, and imageanalysis was employed to calculate the areal strain.

For small deformations, the biaxial strains of the sphericaldeformation were calculated based on the surface area of a sector.

Polym Int 2010; 59: 384–390 c© 2010 Society of Chemical Industry www.interscience.wiley.com/journal/pi

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www.soci.org H Zhang et al.

(a)

(b)

Figure 2. (a) Diaphragm actuator (RIPN film treated with 40 wt% DBEFtested at −40 ◦C, with a voltage of 6.5 kV) viewed through the window ofan environmental chamber. (b) Testing apparatus used to characterize themechanical properties of the samples.

The surface area S is given by

S = 2πhR = π (h2 + r2)

where h is the height of the bulge, R is the radius of the sector andr is the radius of the arc. The strain in area is given by

εArea = (h2 + r2) − (h20 + r2

0)

(h20 + r2

0)

For large deformations, a different approach to calculate theareal strain was employed because the assumption of a sphericaldeformation would underestimate the strain. Thus instead, thebulge was estimated to be a combination of a cylinder with radiusrc and height hc, and a partial sphere with radius rs and height hs.The surface areas of the cylinder and the sector are 2π rchc andπ (h2

s + r2c ), respectively. If the original area before deformation is

π r20, then the areal strain is given by

εArea = A − A0

A0= [2π rchc + π (h2

s + r2c )] − π r2

0

π r20

It should be noted that the electromechanical response ofsuch diaphragm membranes subjected to an initial inflationpressure might vary depending on the initial inflation state aswell as other parameters.26 Certain experimental conditions couldlead to an overestimation of the areal strain – such as a largelocal deformation in the center of the diaphragm membrane,which would be unreflective of the actual volume change of themembrane due to voltage signal.27

In our experiment, to ensure that the calculated areal strainswere a true representation of the volume enclosed by eachmembrane, the following measures were taken:

1. The pressure difference was kept small.2. The entire membrane was inflated as opposed to a deformation

with a disproportionately large pole displacement.3. The electroelastomer remained stable such that the system

configuration would not be at a state with a much greateror smaller enclosed volume, and the overall stress remainedtensile.28,29

Moreover, at low temperatures, we noticed that the smallpneumatic pressure applied could not easily inflate the membraneas it became much stiffer, but with an electric stimulus themembrane inflated outwards with a large enclosed volumeimmediately – further proof that the electrical component wasthe major contributor to the deformation.

To study the effect of plasticizer on response speed, sampleswere attached to frames, smeared with carbon grease as electrode,and connected to an oscilloscope as well as the high-voltage box toproduce planar strain. LabVIEW software was employed to outputstrain and voltage data.

Mechanical property characterization of modified IPN filmsThe biaxial deformation of clamped films under pneumaticpressure was measured using a simple setup built in-house byEmpa as shown in Fig. 2(b). The sample films were carefullymounted onto one end of a hallow aluminium cylinder structurewhose other end was supplied with air to produce a pneumaticpressure to biaxially stretch the films. Deflection of the central pointof the spherical deformation was photographed and analyzed,which was subsequently used to calculate strain caused by elasticfilm deformation under stress. Voltage outputs correlating to airpressure (10 V corresponds to 400 mbar) were also recorded. Fromthese data the stress–strain behavior of the samples was analyzed.

RESULTSBroadening of functional temperature range of IPN filmsRelaxed IPN (RIPN) films plasticized with DOP were first investi-gated. As shown in Fig. 3, at each tested temperature the sampletreated with DOP shows larger deformation than the untreatedRIPN film across the entire temperature range. The increase instrain is expected because DOP molecules separate the chainsin the polymer network and weaken intermolecular interactions,decreasing the elastic modulus of the resultant elastomer. Thisis confirmed in Fig. 4, which shows the stress–strain behavior ofRIPN films untreated and treated with DOP. The same finding hasbeen recently reported.23

The pseudo-Young’s modulus then can be calculated based onthe theory for large deflection in circular thin plates developed byTimoshenko.30 At small stretch ratios, the Young’s modulus E ofa clamped circulate plate under a uniform applied pressure P isgiven by

E = 3Pa4(1 − ν2)

16h4

[ω0

h+ 0.442

1 − ν2

(ω0

h

)3]

where h, ω0, a and ν are the film thickness, deflection at thefilm center, radius of the plate and Poisson’s ratio (assumed tobe 0.5), respectively. The pseudo-Young’s modulus of a RIPN filmis determined to be 8.5 MPa, in good agreement with the 5 MPa

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Figure 3. Strain response of RIPN films untreated and treated with DOP and DBEF plasticizers, from −40 to 80 ◦C, at a constant voltage of 5 kV.

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

0

5

10

15

20

25

30

35

40

Pre

ssu

re (

kPa)

Stretch Ratio λ

RIPNRIPN w/ 44 wt% DOPRIPN w/ 5 wt% DBEFRIPN w/ 25 wt% DBEFRIPN w/ 40 wt% DBEF

Figure 4. Stress–strain behavior of RIPN films without plasticizer andplasticized with DOP and DBEF.

reported earlier.10 RIPN films treated with 44 wt% DOP have apseudo-Young’s modulus of 2.3 MPa.

Furthermore, in Fig. 3, whereas the unplasticized RIPN filmdeforms significantly less as the temperature is increased ordecreased from ambient temperature, especially exhibiting almostno deformation at −30 and −40 ◦C, the DOP-treated sampleshows significant improvement in strain at elevated and reducedtemperatures. However, the strain at −40 ◦C is only 32% of thestrain at 20 ◦C. DOP does not seem to be as effective a plasticizeras DBEF, whose effectiveness will be demonstrated in subsequentdiscussions.

We attribute the poorer compatibility and higher viscosity ofDOP to its mediocre effectiveness as a plasticizer. The samples

treated with DOP appear turbid, whereas DBEF-treated samplesare completely transparent and clear.

The DOP-treated sample seems to have very small dropletsformed on the film surface as in the case of agglomeration. Theextensive aromatic ring structure and high viscosity of DOP mayprevent the additive from deeply penetrating and being absorbedinto the IPN film.

The temperature versus strain behaviors of free-standing IPNfilms plasticized with 25, 40 and 44 wt% DBEF are also shown inFig. 3. All samples show the same trend: a peak is observed ataround 20–30 ◦C, with a decrease in strain as the temperaturedeviates from 20–30 ◦C.

Strains of the unplasticized RIPN film show a steep reduction ateither temperature extreme. In particular, at −30 and −40 ◦C, theunplasticized RIPN film shows almost no response to electric field.We also experimented with unplasticized RIPN samples at −40 ◦Cwith an applied voltage of 6.5 kV and still observed no actuation.

The effect of plasticizer concentration is also confirmed inFig. 3. Not only does higher DBEF concentration result in morepronounced deformation at any given temperature, but also ingreater preservation of room-temperature strain in the lowertemperature regime. Compared to the more gradual decrease instrain with the samples treated with 40 and 44 wt% DBEF, thesample plasticized with 25 wt% DBEF shows a rather precipitousdrop in strain as the temperature is lowered. In fact, we achievedan areal strain as high as 215% at −40 ◦C at an applied voltage of6.5 kV with a sample treated with 40 wt% DBEF, as shown in Fig. 5.

Furthermore, we observe that the films become increasinglysofter as more DBEF is added: films are ‘smooth’ at zero orlow concentrations of DBEF and become more wrinkled withhigher DBEF concentration. The decrease in elastic modulus withincreasing DBEF concentration is also verified in Fig. 4, in whichthe stress–strain behaviors of an untreated RIPN film and IPN filmstreated with 5, 25 and 40 wt% DBEF are shown. Films with zero


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(a)

(b)

Figure 5. Deformations of a RIPN film plasticized with 40 wt% DBEFactuated at −40 ◦C, with an applied voltage of (a) 0 kV and (b) 6.5 kV (215%areal strain).

Table 1. Pseudo-Young’s moduli, calculated based on the Timo-shenko equation, for untreated films and films treated with varyingamounts of plasticizer at a stretch ratio of 1.02 (note that at a stretchratio of 1, the films are 400% biaxially pre-strained)

Composition Pseudo-Young’s modulus (MPa)

RIPN 8.5

RIPN with 44 wt% DOP 2.3

RIPN with 5 wt% DBEF 7.9



and low concentrations of DBEF behave almost the same, withmarkedly smaller deformations than the samples treated with 25and 40 wt% DBEF at a given stress level.

Table 1 gives the calculated pseudo-Young’s moduli (basedon the Timoshenko equation) for films treated under differentconditions. At 0 wt% DBEF, the film is unplasticized RIPN with apseudo-Young’s modulus of 8.5 MPa. As the weight percentageof DBEF increases from 5 to 25 wt% and then to 40 wt%, thepseudo-Young’s modulus decreases from 7.9 to 1.7 MPa and thento 1.5 MPa.

Effect of plasticizer on response speedThe actuation of dielectric elastomers is limited by viscoelasticity.The strain in areal expansion decreases significantly as frequencyincreases. Figure 6 shows the effect of plasticization on theresponse speed. The areal strain of the RIPN film decreases tohalf the original strain at 2 Hz, whereas the decrease in strain forthe DBEF-treated RIPN films is more moderate.

Figure 6. Effect of plasticizer on response speed.

DISCUSSIONFor all samples, the reduction in strain at temperatures aboveroom temperature indicates ideal rubber behavior at elevatedtemperatures. In this temperature regime – well above Tg – theentropic elasticity dominates due to higher molecular and chainmotion with increasing temperatures. Elastic modulus increasesproportionally with increasing temperature in this range, which hasbeen recently confirmed experimentally.25 In the low-temperatureregime, Tg is approached as temperature is lowered, immobilizingpolymer chains and hence resulting in smaller deformationsmacroscopically.

The difference in Tg can explain the steep reduction in strainof unplasticized RIPN films and, in contrast, the pronounceddeformations of plasticized films at low temperatures, as shown inFig. 3. Tg of RIPN has been reported to be approximately −30 ◦C.13

Thermodynamics dictates that the film has little molecular motionin this ‘frozen’ state, and deformation, if any, would be caused bybond stretching and bond angle opening. Plasticized RIPN films,however, have significantly reduced Tg due to an increase in freevolume. At such low temperatures there is still chain mobility,translating to deformation macroscopically.

Furthermore, from Table 1 it is noted that the pseudo-Young’smodulus of the sample plasticized with 44 wt% DOP is still higherthan that of the sample treated with 40 wt% DBEF, confirmingthat DBEF has better effectiveness as plasticizer than DOP in termsof reducing the elastic modulus. Comparing the curves in Fig. 3,the maximum areal strains at 20 ◦C for samples plasticized with44 wt% DOP and 40 wt% DBEF are 56 and 60%, respectively. Asmaller amount of DBEF can increase the actuation strain moresignificantly than a larger amount of DOP.

The addition of plasticizer has both advantageous and disad-vantageous effects on the mechanical and electrical properties ofpolymers. As seen in the previous discussions, it can reduce Tg

and elastic modulus of the polymer. A significant benefit of thereduction in elastic modulus is that the electric field required toactuate the polymer is lowered. As shown in Fig. 7, at the samestrain the unplasticized RIPN film requires a much larger electricfield to actuate it than the DBEF-treated RIPN film. The finding hereis in good agreement with the results reported by Nguyen et al.23

Also, elastomers that can generate large deformation and attainhigh energy density at low driving voltage are desired. The energy


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Figure 7. Electric field-induced strain (area expansion) for unplasticizedand plasticized RIPNs.

density Us is given by

Us = 1

2Yδ2 = 1

2(ε0εrE2)2 1

Y

where Y is the Young’s modulus, δ is the strain in the thicknessdirection and ε0 and εr are, respectively, the permittivity offree space (8.85 pF m−1) and the relative permittivity (dielectricconstant) of the polymer.

A reduced elastic modulus also leads to higher energy den-sity. Nevertheless, it has been found that the dielectric con-stant of elastomeric films decreases with increasing plasticizerconcentration.23 Moreover, the dissipation factor at low frequen-cies increases with plasticizer concentration, leading to decreasedefficiency and breakdown strength.23 Thus, to modify the acrylic-based elastomeric films with plasticizing agents, the effectivenessof the plasticizer is critical so that the dielectric constant, efficiencyand breakdown strength of the polymer would not be compro-mised due to the presence of an excessive amount of plasticizer,not to mention that at excessively high plasticizer concentrationthe polymer would lose too much of its tear strength and becomeuseless for applications. In the light of this discussion, because amuch higher DOP content is required for significant improvementof material properties, we conclude that DBEF is a better candidateas plasticizing agent to modify the acrylic-based elastomers.

CONCLUSIONSWe have investigated modified acrylic elastomers with IPNsbased on poly(TMPTMA) and the effect of plasticizing addi-tives on the operating temperature range, elastic modulus,response speed and performance of the resultant elastomers.Samples treated with plasticizers become softer, have lowerelastic modulus and reduced Tg, allowing for applications ina broadened temperature range and at higher frequencies. Atroom temperature, strains in areal expansion of the samplestreated with plasticizers are higher than the unplasticized sam-ples due to a reduction in the elastic modulus. At temperaturesbelow 0 ◦C, DBEF-treated samples still have significant deforma-tion (as large as 215% areal strain has been achieved with a

sample containing 40 wt% DBEF tested at −40 ◦C with a con-stant voltage of 6.5 kV), unlike unplasticized RIPN films thatshow little response to electric field at −30 and −40 ◦C. Attemperatures above room temperature, the areal strains of allsamples, both plasticized and unplasticized, decrease with in-creasing temperature. Other benefits of plasticization includea smaller electric field strength required for actuation and re-duced viscoelastic loss (hence increased response speed). DBEFhas been found to have greater effectiveness as a plasticizingagent than DOP.

These findings should narrow the performance gap betweenpackaged dielectric elastomer actuators and the active material.Improved actuators with broad operating temperature range andhigh response speed can be fabricated.

ACKNOWLEDGEMENTSThis work was carried out with financial support from GeneralMotors Corporation and the University of California DiscoveryProgram. We would also like to thank Christopher Henry of HRLLaboratories, Kurt Gantner from the Swiss Federal Laboratoriesfor Materials Testing and Research (Empa) and Qi Chen from theSoft Materials Research Laboratory, UCLA Materials Science andEngineering, for their contributions to the work presented.

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