polyaniline-tailored electromechanical responses of the silver/epoxy conductive adhesive composites

Polyaniline-Tailored Electromechanical Responses of the Silver/Epoxy

Conductive Adhesive Composites

Sarang P. Gumfekar,1 Behnam Meschi Amoli,1 Alex Chen,2 Boxin Zhao1

1Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue

West, Waterloo, Ontario, Canada N2L 3G12Celestica Inc., Toronto, Canada M3C 1V7

Correspondence to: B. Zhao (E-mail: [email protected])

Received 22 March 2013; revised 29 June 2013; accepted 1 July 2013; published online 29 July 2013

DOI: 10.1002/polb.23351

ABSTRACT: In this article, the electromechanical properties of

silver-in-epoxy conductive adhesives with the polyaniline (PANI)

micron particles as cofillers have been investigated. PANI is

a conductive polymer and has a moderate conductivity in

between those of silver and epoxy. It was found that PANI can

be used to tailor both the adhesive’s electrical contact resistance

and its relaxation behavior; however, the effects of adding PANI

were complex. The addition of small amount of PANI (2 wt %)

dramatically increased the contact resistance; it might block the

electrical contacts among silver flakes and was not able to form

a continuous path among themselves. The addition of more

PANI showed a moderate increase in contact resistance, which

increased with the weight fraction of PANI from 6 to 15 wt %.

Interdependent behavior of compressive strain and relaxation in

electrical contact resistance is characterized to evaluate the ori-

gin of this relaxation. The addition of PANI made the relaxation

in electrical contact resistance more sensitive to the compres-

sive strain and the electromechanical coupling to deviate from

the linear relationship. These research findings provide insights

into the way to use PANI to tailor the electromechanical proper-

ties of the adhesive bonds or joints in the development of

advanced functional devices. VC 2013 Wiley Periodicals, Inc. J.

Polym. Sci., Part B: Polym. Phys. 2013, 51, 1448–1455

KEYWORDS: adhesives; conducting polymers; polyaniline; struc-

ture-property relations

INTRODUCTION Electrically conductive adhesives (ECAs) arepolymer composites filled with micron-sized conductive fill-ers. The polymer matrix gives strength and bonding abilityto ECAs, whereas the fillers facilitate with electrical conduc-tivity. ECAs find extensive applications as interconnect mate-rials for device assembly to meet the increasing demand ofminiaturization of electronic products and portable devi-ces.1–4 Adhesive bonding technology requires milder process-ing conditions, fewer processing steps, lower process costthan the conventional soldering technology, and has the finepitch capability because of the availability of small size con-ductive fillers.2–4 Silver microflakes are commonly used incommercial conductive adhesives because they have a highconductivity and relatively stable contact resistance. Inrecent years, there is an increasing interest in the develop-ment of composites composed of two or more filler compo-nents to improve the performance and to reduce cost.1–3,5

Mixture of microscale and nanoscale particles or mixture ofmetallic and conductive polymer fillers has been used toimprove the electrical conductivity of ECAs and to reducecost.1,5,6 Conductive polymer composites have also beenused in the development of smart materials and structuresin which the conductive fillers can response to the external

(electric and magnetic) fields by aligning to the field direc-tion; the resultant anisotropic structures have a pronouncedforce-dependent conductivity or piezoresistivity.7

As illustrated in Figure 1, engineering applications of ECAsinvolve a compressive pressure (also called as compressivestress or Fz) and/or heat during the bonding of components,where the conductivity of the resultant bonds significantlydepends on the pressure and heat. Figure 1 also depicts thetypical morphology of conductive fillers: silver flakes, polyani-line (PANI) microparticles, and their chemical structure. Thereliability of conductive bonding in terms of its contact resist-ance is critical for precise packaging of the functional compo-nents.8,9 However, ECA bonds suffer from a relatively poorreliability in comparison with the conventional soldering andwire bonding. One reason might be related to the distinctivecomposite nature of ECAs; their conductivities depend oncooperative behavior of interacting fillers to form conductivepaths, which are sensitive to the mechanical stress. Despite alarge number of potential applications and many fundamentalstudies, the understandings of the electromechanical behav-iors of ECAs are still very limited; the reliability of ECAs isstill a major concern in the practical applications.2,3,10,11

VC 2013 Wiley Periodicals, Inc.

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The ultimate objective of our research is to develop low-costand efficient conductive adhesives by using novel conductivefillers to the conventional silver-filled epoxy adhesives. Ourprevious work explored the use of PANI as a cofiller insilver-filled epoxy.5 We used PANI to dope the epoxy matrixto reduce the cost by reducing the percolation threshold ofsilver-filled epoxy5 in the context that the large-scale synthe-sis of PANI has been demonstrated to be possible for itspotential use in the electrically conductive composites.11,12

Note that the conductive PANI have also been reported toact as the matrix for nanofillers to make organic/inorganicnanocomposite materials.13 In this study, we investigated thePANI as a potential cofiller material in ECA to tune themechanical and electrical properties of the ECAs and pro-vided a detailed analysis of the electromechanical propertiesof silver-epoxy (Ag-epoxy) and silver-PANI-epoxy (Ag-PANI-epoxy) system in both partially cured and fully cured states.PANI has a moderate conductivity in between those of silverand epoxy. The addition of PANI in the ECAs may increasethe electrical resistance of the ECA composite but have amuch less impact on the electrical properties than the use ofnonconducting fillers such as silica fillers; thus, it may beable to provide balanced electrical and mechanical propertiesand to improve the reliability of ECAs. The change in electri-cal contact resistance under varied compressive loads and itsevolution over contact time were investigated. In contrast tomany studies focusing only on the cured system, the studyof partially cured or an intermediate stage in the curing pro-cess allowed us to obtain fundamental insights into theestablishment of conductive network and the particular roleof PANI in tailoring the electromechanical response of theECAs.

EXPERIMENTAL

Materials and Adhesive Composite PreparationWe used silver flakes (�10 lm, resistivity 1.59 lX cm, den-sity 10.49 g/cm3; Sigma-Aldrich) as a main filler and PANI(3–100 lm, resistivity 0.25 X cm, density 1.36 g/cm3; Sigma-Aldrich) as a cofiller in epoxy resin (DER 322; Dow Chemical).Triethylenetetramine (TETA) containing crosslinking agent(DEH 24; Dow Chemical) was used for curing the epoxy. Irreg-ularly shaped PANI was in emeraldine salt phase (doped withsulfonic acids) and has a dark green color. Two differentmethods were used to prepare the Ag-PANI-epoxy composites.The first method was to add silver fillers into liquid epoxyresin, PANI, and then the curing agent. The second methodwas to add PANI into liquid epoxy resin, silver flakes, andthen the curing agent. The dispersion process involved thehigh shear mixing using a vortex mixer for 45 min followedby 30 min of ultrasonication in an ultrasound bath. The afore-mentioned dispersion process was followed for both of themethods. The entire mixture was degassed under vacuum forhalf an hour, after adding the curing agent; and then it wasspin-coated on an area of 1 3 1 cm2 smooth copper substrate(36 gauge; Basic Copper, IL) at a speed of 1100 rpm. As thethickness of ECA films at the constant RPM changes with thecomposition of ECAs, spinning time was adjusted to have aconsistent film thickness. The ECA films were partially curedat 40 �C for 30 min to have �46% of cure and were fullycured at 150 �C for 2 h; the degree of cure was measured bya modulated temperature differential scanning calorimeter(MTDSC; TA Instrument) with a heating rate of 3 �C/min. Thethickness of the cured adhesive coating was measured to be4506 7 lm and was measured using Vernier caliper.

FIGURE 1 Concept of application of conductive adhesive in microelectronic packaging along with SEM image of the composite

containing silver and polyaniline and structure of the polyaniline. [Color figure can be viewed in the online issue, which is avail-

able at wileyonlinelibrary.com.]

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CharacterizationsThe electrical properties of the ECAs were measured in thez- or thickness direction by indenting a circular flat copperprobe using a micro/nanomaterial tester (CETR UniversalMicro/Nano Material Tester equipped with an electricalsensor) and in the x–y plane by a typical four-probe setup(Keithley 2440 5A Source Meter; Keithley Instruments).Figure 2(a) shows the schematic of indentation setup tomeasure force-dependant electrical contact resistance. Figure2(b) shows schematic of typical four-point probe techniqueto measure the resistivity of ECAs. The measurement in thez-direction is particularly useful for investigating the cou-pling of electrical and mechanical responses of ECAs. In thisnovel characterization technique, the change of electricalcontact resistance was measured with an increase in contactpressure and time during the loading in viscoelastic state.The circular flat probe of 6.5 mm diameter was used for theindentation experiments instead of the frequently usedpyramidal probes for indentation of various adhesives. Thus,the nominal contact area of 33.2 mm2 between the probeand sample remained constant during the indentation, whichminimized the efforts for the measurement of contact areasand reduced additional variables in the system. The probesurface was examined using an optical profilometer (Wyko1100; Veeco Instruments, Plainview, NY) to determine theroughness of probe surface. Several probes used in the studywere examined using profilometer, and the arithmetic aver-age roughness Ra was measured to be 4.766 1.12 lm. Acompressive force up to 100g was applied on viscoelasticfilms; contact resistance and displacement in z-directionwere recorded as a function of force and time. Furthermore,we explored the relaxation behavior of electrical contactresistance during creeping at a constant compressive forceof 100g for 300 s. Thus, we have investigated the responseof contact resistance of ECA in a partially cured state withthe increase in compressive force and at constant compres-sive force. Scanning electron microscopy (SEM) imaging wasperformed on the cured ECAs for morphological character-izations. As-prepared, cured films of adhesives were surfacepolished to remove the insulating layer of epoxy; gold sput-tering was carried out to ground the electrons and for betterimage quality.

RESULTS AND DISCUSSION

Electromechanical Behavior of PANI-Tailored Silver/Epoxy Conductive CompositesPANI-tailored silver/epoxy conductive composite films wereprepared by two different methods. The first method was toadd silver fillers into liquid epoxy resin, PANI, and then thecuring agent. The second method was to add PANI intoliquid epoxy resin, silver flakes, and then the curing agent.Preliminary experiments showed that the silver flakes wereonly well dispersed in epoxy composite prepared in the firstmethod, resulting in homogenous composite samples whencured. In contrast, silver flakes formed aggregates in epoxycomposites prepared in the second method, giving inhomo-geneous samples. This observation may be explained by thefact that the addition of PANI increased the viscosity of thecomposite, and thereafter, the dispersion of silver flakes wasnot effective. Thus, all the samples used in this work wereprepared by the first method; they have a constant 70 wt %silver fraction and varied fractions of PANI between 0 and15 wt %, relative to the weight of epoxy resin.

In all the ECA samples, the absolute amount of Ag was keptconstant, whereas the amount of PANI was increased. Thechange of overall concentrations of Ag and PANI in the com-posite samples was expected. As shown in Table 1, whenPANI content increases from 0 to 15 wt % (relative to epoxyresin), the volume fraction of PANI in the compositeincreases from 0 to 0.094, which is significant to alter theelectromechanical properties of the composite. The additionof PANI decreases the Ag volume fraction from 0.063 to0.057; this decrease of 0.006 in volume fraction is compara-tively less significant to affect the electromechanical proper-ties of the composite. This situation made it possible to tunethe electromechanical behavior of the composite using PANIcofillers without significantly affecting the contribution of Agfiller to the electromechanical properties of the composite.Figure 3(a) shows the force-dependant contact resistance ofpartially cured ECAs containing varied fractions of PANI as afunction of the compressive force. The partially cured ECAswere viscoelastic pastes, representing an intermediate stagein the curing process. Monitoring the force-dependant behav-iors of these viscoelastic ECAs allowed us to obtain insights

FIGURE 2 Schematic of (a) indentation setup to measure force-dependant electrical contact resistance and (b) typical four-point

probe technique to measure the resistivity. [Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]


POLYMER SCIENCE



into the establishment of the conductive network. A sharptransition in resistance was observed while applying acompressive force because of the need of a critical or aminimum preload Fzmin to establish the conductive net-

work of the fillers. This critical preload appeared toincrease with the addition of PANI because of increasedviscoelasticity or the yield strength of the composite.After the transition, the contact resistance is constanteven at higher preload, giving a pressure-independentsteady-state value characteristic of the composition of thecomposite.

We noticed that the ECA films became thinner at higherpreload; presumably some of the ECA paste was squeezedout. Furthermore, at high-compressive forces or strains,more epoxy fluids might have moved out than the solid fill-ers so that the local volume fraction of the conductive fill-ers increased to form a highly packed continuousconductive path. The flow of adhesives due to the preloadcould increase the contact area between the probe andadhesives because of the possible contact at the probeedge. To address this effect, we used the indentation forceversus displacement data to estimate the thickness of visco-elastic film that may have been squeezed out to be �0.03mm. Subsequently, we calculated and compared the area ofprobe edge in contact with the film (AE) and the area ofprobe surface (AS; bottom part). The ratio of AE/AS wasfound to be 0.018. Therefore, the possible increase in con-tact area due to the flow of viscoelastic adhesives isinsignificant.

Figure 3(b) shows the steady-state resistance (viscoelasticstate) as a function of the weight percent of PANI. Overall,the steady-state resistance of the composite increased withthe addition of PANI. Because PANI is conductive when com-pared with the insulating epoxy matrix, but its conductivityis far less than that of Ag, the observed increase in thesteady-state contact resistance caused by the addition ofPANI can be attributed to both the increased PANI volumefraction and the decreased Ag volume fraction. We hadexpected a monotonic increase in electrical contact resist-ance with the increase of the weight fraction of PANI. How-ever, the addition of 2 wt % PANI showed a much higherincrease in the steady-state resistance when compared withthe other formulations. To verify this unexpected behavior,ECA films were fully cured after the characterization offorce-dependant resistance and were analyzed for resistivityby a four-point probe technique. Resistivity of the ECAs q iscalculated using the equation q5Rst5 pZ

ln 2

� �VI X cm, where Rs

TABLE 1 Electrically Conductive Adhesive Samples and Their Composition and the Volume Fraction of the Silver (Ag) Filler and

PANI Cofiller

Composition Ag Volume Fraction PANI Volume Fraction

Sample–No PANI Epoxy 1 70 wt % Ag 0.063 0.0

Sample–2% PANI Epoxy 1 70 wt % Ag 1 2 wt % PANI 0.062 0.014




About 13 wt % crosslinking agent triethylenetetramine was added in the epoxy. The weight percentages are relative to the epoxy weight.

FIGURE 3 (a) Force-dependant contact resistance of ECAs of

varied fractions of polyaniline during the loading, and (b) elec-

trical resistivity and contact resistance (viscoelastic state) of

ECAs as measured by four-point probe technique and indenta-

tion, respectively. [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]




is the resistance, Z is the thickness, I is the applied current,and V is the measured voltage. The resistivity of fully curedsamples is plotted as a function of the weight fraction ofPANI in Figure 3(b). It shows the same trend as the steady-state resistance in viscoelastic state. To explain this behavior,we considered the effect of packing density or concentrationof PANI on electron conduction ability of the ECA. With theaddition of 2 wt % PANI, the PANI particles were dispersedsparsely in the composite and might have blocked the con-tacts between silver flakes by acting as a spacer; meanwhile,the packing density of PANI was low and not sufficient toform a continuous path among themselves. These sparselydispersed PANI particles cannot conduct electrons in theepoxy matrix. At high concentrations, the PANI particlesmight have formed a continuous path that is conductivebecause of the finite conductivity of PANI even though itsconductivity is much less than Ag. To a certain extent, thisconductive path among PANI particles may have compen-sated the reduced conductivity because of simultaneousdecrease of the Ag contents. Morphological analyses wereperformed on all the samples using SEM. Typical SEMimages for samples with 2, 6, 10, and 15 wt % PANI areshown in Figure 4(a–c), in which the PANI particlesappeared darker than the silver flakers. Energy-dispersiveX-ray spectroscopy (EDX) analysis was also carried out;although silver peak was identifiable in EDX, the nitrogen ofthe PANI backbone was not distinguishable from the carbonbecause of their low atomic numbers. We examined manySEM images and found that PANI was able to be distin-guished in its shape and brightness from silver particles (see

Fig. 1). All the SEM images displayed similar morphologies.The filler particle packing density increased with the PANIcontents; the 2 wt % is the least packed [Fig. 4(a)], whereasthe 15 wt % is the most packed [Fig. 4(d)]. Furthermore, wenoticed that silver particles may have sintered during thecuring.

Effects of PANI on the Relaxation in Electrical ContactResistanceWe further examined the change in the electrical resistance ofthe ECA under a constant compressive force of 100g as shown inFigure 5. Note that the curves for each sample presented in Fig-ure 5 are the continuation of the corresponding curves shown inFigure 3(a) with a constant compressive force. As expected, thepartially cured composites crept at that compressive force. Thecompressive strain was calculated by the change in thickness Z

or the displacement d as e5 Z02ZtZ0

5 dZ0, where Z0 is thickness of

the film at t5 0 and Zt is the thickness at time t. Figure 5 showsthe change in both electrical resistance and compressive strainwith time during the creep at the constant compressive force.The contact resistance decreased with time for samples of 6, 10,and 15% PANI, same as the sample without PANI. However, thecontact resistance increased with time for samples of 2% PANI.Obviously, the samples of 2% PANI behaved differently from allother samples, consistent with the observations shown in Figure3. Note that the increased scale of y-axis or the range of resist-ance gives better view of the change in resistance.

To obtain insights about the possible linkage between thechange in contact resistance and that in compressive strain,

FIGURE 4 SEM images of ECAs with (a) 2%, (b) 6%, (c) 10%, and (d) 15% polyaniline.


POLYMER SCIENCE


we normalized the relaxation in contact resistance, same asthat of compressive strain. The following equation definesthe normalized relaxation in contact resistance:

eR5R02Rt

R0; (1)

which is similar to the compressive strain, e5 Z02ZtZ0

. R0 is ini-tial resistance at t5 0 and Rt is resistance at time t. Note thatthe normalized relaxation in contact resistance can also bedefined in terms of the change in electrical conductivitybecause of the reciprocal relationship between the electricalresistivity and conductivity. For the sample of 2 wt % PANI,the electrical contact resistance (Rt) increased with the timeresulting in negative eR; thus, this sample was not analyzedfurther. Figure 6 shows the normalized relaxation in contactresistance as a function of time at varied PANI concentrations.The normalized relaxation in contact resistance increasednonlinearly with time and tended to reach a constant value ifsufficient time was provided. Over the period of 300 s, thenormalized relaxation in contact resistance of the sampleswithout PANI was considerably larger than those with PANI;

the increase in the normalized relaxation in contact resistancewas also faster than the others, particularly within the first150 s. Between 150 and 300 s, the increase in the normalizedrelaxation of the contact resistance was much slower for allthe samples, reaching quasi-equilibrium states. The additionof PANI significantly reduced the quasi-equilibrium normal-ized relaxation in contact resistance of the Ag-epoxy compo-sites. The addition of 6 and 10 wt % PANI showed similarrelaxation curves that lied between the curves of sampleswithout PANI and with 15% PANI. A much denser conductivenetwork may have been formed at 15 wt % PANI fractionthan the other two; it took about 50 s to reach the steady-state normalized relaxation in contact resistance, which ismuch less than the other samples.

Modeling the Normalized Relaxation in ContactResistanceWe noticed that the variations in normalized relaxation incontact resistance with time and PANI concentrations aresimilar in the trend to those of compressive strain for amajority of the samples with the 2% as an outlier. Hence, wefitted the normalized relaxation in contact resistance behav-ior of ECAs using Burger’s micromechanical model. Burger’smodel consists of one Maxwell unit and one Kelvin unit con-nected in series and divides the strain of polymeric or com-posite material into three parts: instantaneous deformationresulting from Maxwell spring, viscoelastic deformationresulting from Kelvin units, and viscous deformation result-ing from Maxwell dashpot. Mathematically, it is a linear com-bination of two exponential functions.

eR tð Þ; e tð Þ5A3eBt1C3eDt ; (2)

where the constants A, B, C, and D are composed of viscousand elastic coefficients of the composite material.14–16 The

FIGURE 5 Relaxation in electrical contact resistance and increase

in compressive strain of ECAs as a function of time and polyani-

line concentration at a constant compressive load (ECR, electrical

contact resistance; •, contact resistance; �, compressive strain).

FIGURE 6 Change in normalized relaxation in contact resistance

as a function of time and polyaniline concentration. Solid line

shows fitted response of normalized relaxation in contact resist-

ance using Burger’s model. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]




Burger’s model has been widely used to characterize theviscoelastic behavior of the materials17–19 and was recentlyapplied by Ding et al.20 to fit the changes in electrical resist-ance of carbon black-filled rubber composite during com-pression. In our study, we used eq 2 to fit the experimentaldata; the resulting fitting parameters (A, B, C, and D) arelisted in Table 2. It can be seen from Figure 6 that the fittinglines from the Burger’s model (the solid line) well match thenormalized relaxation in contact resistance data.

Figure 7 shows the direct relationship between the normal-ized relaxation in contact resistance and compressive strainat different PANI concentrations. For the first approximation,the normalized relaxation in contact resistance and compres-sive strain of Ag-epoxy composite are linearly coupled forthe strain range e 5 0–0.1. It appears that the conductiveadhesive layer behaved like a homogeneous conductor atlow strains. According to the classical electric theory, theelectrical resistance of a conductor R5 qL/A, where q is theresistivity, L is the length, and A is the cross-sectional areaor the contact area of the indenter. In our measurementsetup, A is a constant. The linear proportional relationshipbetween R and L suggested that the resistivity of the adhe-sive layer is constant at low strains. The addition of PANI

significantly reduced the compressive strain but not muchthe normalized relaxation in contact resistance. It is interest-ing to notice that the dependence of normalized relaxationin contact resistance on compressive strain (i.e., the initialslope of eR vs. e) drastically increased with the increasedPANI fractions. The addition of 6 wt % PANI showed the lin-ear coupling having a higher initial slope than that withoutPANI. The addition of 10 wt % PANI also showed a linearcoupling having an even higher initial slope. The addition of15 wt % PANI showed the highest initial slope but has a sig-nificant deviation from the initial linear coupling when thecompressive strain is larger than 0.005. The addition of PANIincreased the resistivity of the Ag composite as the PANI isless conductive than Ag. In other words, the PANI bridgedthe silver flakes, making the adhesive composite more vis-cous or solid-like and more electrically resistant; thus, therelaxation in contact resistance is more sensitive to the com-pressive strain. However, the deviation from the initial linearrelationship for 10 and 15 wt % PANI at high-compressivestrains indicated stronger interaction or larger contact areaamong the filler particles. As PANI particles are softer thansilver flakes, they may undergo plastic deformation so as toincrease the cross-sectional area of conductive path and togive rise to a lower resistivity at high-compressive strains.Similar nonlinear relationship between conductivity andcompressive pressure has been observed for the smart struc-tures of magnetorheological elastomers by Zhu et al.,21

which was also attributed to the increased cross-sectionalarea of conductive paths.

Overall, the dependence of the relaxation in contact resistanceon the compressive strain suggested that the strain or relaxa-tion in contact resistance comes at least partly from the com-pressive strain induced during the application. Thus, it isimportant to have a precise control of the bonding pressureand time to have the desired electrical conductivity; detailedviscoelastic characterizations of the partially cured adhesivemay be needed in future work to elucidate the relaxationmechanisms. Furthermore, as a monotonic increase in thesteady-state contact resistance from 6 to 15 wt % PANI and amonotonically increase in the slope of the linear couplingbetween normalized relaxation in contact resistance and com-pressive strain were observed, we may conclude that theaddition of PANI provides a way or operating window to tai-lor the desired electrical conductivity of ECA. It is particularlyimportant when well-controlled conductivity of ECAs isneeded for the precision engineering application, for example,in the development of antistatic materials, electromagneticinterference shielding coatings, conducting membrane materi-als, flexible display, and many other functional devices.22–24

CONCLUSIONS

In summary, we performed a detailed electromechanicalstudy of electrically conductive adhesives filled with silverflakes and PANI micron particles in viscoelastic state. Itrevealed complex electromechanical effects of adding PANIto the silver/epoxy conductive adhesive composites. The

TABLE 2 Fitted Values of Parameters in the Burger’s Model for

the Normalized Relaxation of the Electrical Contact Resistance

A B C D

Sample–No PANI 0.7316 20.0001 20.7263 20.0103

Sample–6% PANI 0.1539 0.0006 20.1515 20.0110

Sample–10% PANI 1.0699 20.0018 21.0511 20.0031

Sample–15% PANI 0.0907 0.0009 20.0820 20.0561

FIGURE 7 Direct dependency of normalized relaxation in con-

tact resistance on compressive strain as a function of polyani-

line concentration. [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]


POLYMER SCIENCE



addition of a small amount of PANI (2 wt %) dramaticallyincreased the contact resistance because it might block thecontacts among silver flakes and was not able to form a con-tinuous path among themselves. In contrast to the 2 wt %PANI, the addition of more PANI showed a moderateincrease in contact resistance; and the contact resistanceincreased with the weight fraction of PANI from 6 to15 wt %, suggesting that a continuous PANI pathway wasformed with the addition of PANI particles. The relationshipbetween compressive strain and relaxation of contact resist-ance was further investigated for the samples other than 2wt % PANI; the Burger’s micromechanical model was appliedand found able to fit the change of contact resistance withtime under a constant compressive force. These researchfindings show that the addition of PANI to conventional sil-ver flake-filled epoxy can be used to tailor the steady-statecontact resistance and its dependence on the compressivestrain. Other than reducing the cost, it may provide a novelway to control the conductivity to the adhesive bonds orjoints in the assembly of portable electronic devices and inthe development of advanced functional devices.

ACKNOWLEDGMENTS

The authors thank the Natural Sciences and EngineeringResearch Council of Canada (NSERC) for the financial support.They also thank M. Meshram for the help with the modelingusing Burger’s model, and Geoffrey Rivers for the experimentalassistance with the DSCmeasurements.

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SGML and CITI Use OnlyDO NOT PRINT

Electrically conductive adhesives (ECAs) find extensive applications as interconnect

materials for device assembly to meet the increasing demand of miniaturization of

electronic products and portable devices. However, ECA bonds suffer from a relatively

poor reliability in comparison with conventional soldering and wire bonding. In this

work, polyaniline is investigated as a cofiller in silver-filled epoxy in both partially

cured and fully cured states, providing insights into the establishment of a conductive

network and the role of polyaniline in tailoring the electromechanical response of the

ECAs.

polyaniline-tailored electromechanical responses of the silver/epoxy conductive adhesive composites

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