Behaviors of the positive temperature coefficient of resistance of poly(styrene-co-n-butylacrylate)...

Behaviors of the Positive Temperature Coefficient ofResistance of Poly(styrene-co-n-butylacrylate) Filledwith Ni-Plated Core-Shell Polymeric Particles

JASON CHANG, ALBERT HO, WEI-KUO CHIN

Department of Chemical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, Republic of China

Received 29 May 2006; accepted 25 September 2006DOI: 10.1002/polb.21028Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Conductive composite films of poly(styrene-co-n-butylacrylate) copolymersfilled with low-density, Ni-plated core-shell polymeric particles were prepared andtheir behaviors of positive temperature coefficient of resistance (PTCR) were investi-gated. When the conductive fillers in the composite film were loaded beyond the criti-cal volume, 10 up to 25 vol %, composite films exhibited a unique electrical resistanttransition behavior, which the electrical resistance rapidly increased by severalorders of magnitude at the critical temperature. The PTCR transition temperature,in general, occurred before the glass transition temperature of polymer matrix. Fur-ther increased the conductive filler loading to 30 vol %, the overpacked conductionpaths were formed in the entire composite and the PTCR effects became blurred.While the composite film treated with thermal cycle several times from room temper-ature up to 120 8C, the electrical resistivity increased accompanied with the shift ofthe PTCR transition to lower temperature. The reason might have been caused bythe formed interfacial cracks within the composite film. VVC 2006 Wiley Periodicals, Inc.

J Polym Sci Part B: Polym Phys 45: 322–329, 2007

Keywords: Ni-plated polymeric particle; poly(styrene-co-n-butylacrylate); positivetemperature coefficient of resistance (PTCR)

INTRODUCTION

The positive temperature coefficient (PTC) therm-istor acts as a resettable fuse that prevent devicesfrom damage caused by unexpected outrageousvoltage, currency, or heat. PTC thermistors havebeen widely used in the electrical device industry-wise including safety heater, thermostat element,transient current generator, ambient thermalstate indicator, and so forth. Moreover the build-inPTC resistors as standard protection in the inte-grated circuit boards are more frequently to befound in the recent consumer electronic products.

Generally, there are two categories of materialswith the PTC of resistance (PTCR): ceramics andconductive polymer composites (CPC). The donor-doped barium titanates in the polycrystallineforms, patented by Haayman et al. in 1955,1 werefound to have several orders increment of resistiv-ity beyond a certain temperature, the so-called Cu-rie temperature. The Curie temperature can becontrolled in a wide range as barium titanatedoped with minor amount of either tetravalentdopants (such as scandium, yttrium, lanthanum,or almost any one of the lanthanide-series ele-ments) or pentavalent dopants (such as niobiumor antimony).2–5

The PTCR phenomenon found in CPC wasfirstly discovered by Frydman in 1945,6 and rep-resented by Kohler in 1961.7 Since then, the

Correspondence to: W. K. Chin, (E-mail: [email protected])

Journal of Polymer Science: Part B: Polymer Physics, Vol. 45, 322–329 (2007)VVC 2006 Wiley Periodicals, Inc.

322

polymeric PTCR materials have attracted wideinterests due to the advantages of polymer na-ture in light weight, easy processing and matrixvariety for applications.8–12 Metals with the goodelectrical conductivity, such as silver or nickel, arecommonly used as conductive fillers in the CPC.However, the dispersion and sedimentation prob-lems need to be addressed as the blending ofhigher density metal fillers is in the relativelylower density polymer matrix. Hence, conductivefillers with low density such as carbon black and/or conductive inorganic filler are used in the poly-meric PTCR materials, even though their electri-cal conductivities are not as good as pure met-als.13–19 Unlike the ceramic based PTCR materialswhose Curie transition is triggered by the phasetransition of polycrystalline, the resistivity surgeof CPC is mainly because of the mismatch of ther-mal expansion between the conductive filler andpolymeric matrix, particularly when the phase ormorphology change occurs, such as the melt ofcrystalline polymer or the transition from theglassy to the rubbery state of amorphous polymer.Moreover, the resistivity feature, including ‘‘Curietransition’’ of CPC, can be effected by: (1) thenatures between conductive filler and polymer ma-trix; (2) the dispersion quality of conductive fillerin composite; (3) the interfacial character inbetween conductive filler and matrix; (4) the mis-match of thermal expansion coefficient betweenconductive filler and matrix and (5) the processingcondition and thermal histories of CPC.

In our previous study, a Ni-plated core-shellpolymeric particle with relatively low densitythan the pure metal filler, had successfully devel-oped.20 Figure 1 shows the scanning electronmicrographs (SEM) of the synthesized Ni-plated

core-shell polymeric particles. The average size,nickel content, and density of the conductive par-ticles were 4.03 lm, 85.5 wt %, and 3.06 g/cm3,respectively. Merits of using these conductiveparticles in the CPC are evidences that the elec-trical conductivity remains as good as the metaland the sedimentation of conductive filler in thepolymer matrix can be avoided or minimized dur-ing the processing. In this study, the percolationthreshold and PTCR behaviors of the CPC sys-tem filled with the synthesized Ni-plated core-shell polymeric particles were studied and dis-cussed. To evaluate the effect of the thermalexpansion as well as the glass transition temper-ature, Tg, of the matrix on the PTCR transitiontemperature of the CPC, copolymers of poly(styrene-co-n-butylacrylate) (P(S-co-nBA)) withvaried styrene and n-butylacrylate content wereselected as matrices of CPC.

EXPERIMENTAL

Materials

Styrene and n-butylacrylate (nBA) were pur-chased from 1–Aldrich and Fluka, respectively.Polyvinyl pyrrolidone (PVP) and 2,20-azobis-isobutylronitrile (AIBN) were obtained fromACROS chemical and Showa chemical, respec-tively. The conductive filler used was Ni-platedcore-shell polymeric particles (poly(styrene-co-divinyl benzene), P(S-co-DVB)), synthesized bythe technique has been reported in our previouspapers.20 All other reagents were used as recei-ved, and all aqueous solutions were preparedwith deionized water.

Figure 1. The SEM micrographs of conductive particles. (a) The hollow structure ofP(S-co-DVB) particles: After applying high-pressure grinding to break the particleand (b) the Ni-plated P(S-co-DVB) particles.20

BEHAVIORS OF THE PTCR OF P(S-co-nBA) 323

Journal of Polymer Science: Part B: Polymer PhysicsDOI 10.1002/polb

Synthesis of P(S-co-nBA) Matrix

The dispersion polymerization was used to copo-lymerize styrene and nBA. Three different feedratios of styrene/nBA, 1/0, 9/2, and 8/3, wereused and the resultant polymers were coded asM1, M2, and M3, respectively. Table 1 shows therespective feed compositions for M1, M2, andM3 in the dispersion polymerization process.

Placed the reagents of styrene, nBA, PVP,AIBN, and cosolvent of ethanol/water in a nitro-gen-purged vessel at 5 8C and agitated con-stantly until the AIBN dissolved completely,then cut off the nitrogen and let them reactunder 60 8C for 48 h. The reacted emulsion solu-tion was centrifuged under 6000 rpm for25 min. The segregated product washed by etha-nol several times to remove the residual PVP onthe polymer surface then vacuum dried.

Preparation of Composite Film

P(S-co-nBA) dissolved in the toluene solventwith the result of the solid content of 25 wt %,and then added a controlled amount of Ni-platedpoly(S-co-DVB) particles into the solution. Themixture stirred for 20 min in ice bath, followedby ultrasonic degassing, then coated on a releas-ing film (polyimide, PI) and ambient dried for24 h. The coated film further vacuum dried at40 8C for another 4 h. The thickness of thecoated composite film was around 150–160 lm.

Structural Characterizations

The synthesized P(S-co-nBA)s were structurallycharacterized by FTIR spectroscopy (PerkinElmer), differential scanning calorimetry (DSC2920, TA Instruments), and thermo gravimetricanalysis (TGA 951, DuPont Instruments),

respectively. The heating rate for DSC and TGAanalyses was 10 8C/min. The thermal expansionbehaviors of polymer matrix and composite filmswere verified with TMA 2940 (TA Instruments)in a nitrogen atmosphere in film clamped mode.A constant load of 0.1 N and the heating rate of5 8C/min were applied. The cross-sectional mor-phologies of composite films were investigatedby scanning electron microscopy (SEM, JEOL5600). The electrical resistivity of the compositefilm was measured by a digital Multimeter(HP3457, Hewlett–Packard) with 2-wire-ohmsmethod. The film sample size and heating ratewere 50 � 20 mm2 and 2 8C/min, respectively.

RESULTS AND DISCUSSION

Properties of P(S-co-nBA) and Composite Films

Amorphous copolymer of P(S-co-nBA) was desig-nated to be the matrix of conductive polymer com-posites (CPC). With the control of styrene/nBAfeed ratio in the copolymerization, we are able tomodify the glass transition temperature, Tg, aswell as the thermal expansion coefficient of thematrix, to adjust the PTCR transition temperatureof the CPC. Figure 2 exhibits the FTIR spectra ofP(S-co-nBA), M2, and M3. The absorption bandsat 3026 and 1160 cm�1 represents the C��Hstretch of phenyl group in the polystyrene and thesymmetric stretch of C��O��C group in the poly(nBA), respectively. M3 was synthesized with ahigher nBA content in the feed than M2, therebyhigher absorption intensity at 1160 cm�1 wasobserved comparing to M2.

Table 1. Feed Composition (g) for P(S-co-nBA)Polymerization

Ingredient

Matrix

M1 M2 M3

Styrene 88 72 64n-Butylacrylate 0 16 24PVP 6.4 6.4 6.4AIBN 0.8 0.8 0.8Ethanol 360 360 360Water 40 40 40

Figure 2. FTIR spectra of P(S-co-nBA), M2, and M3.

324 CHANG, HO, AND CHIN


The Tgs’ of M1, M2, and M3, obtained from theDSC, were at 102, 76, and 59 8C, respectively.They proportionally decreased as the content ofnBA in the copolymer increased. The softeningtemperature of copolymer, Ts, obtained from theTMA analysis, was slightly higher than the copol-ymer Tg. The glassy state thermal expansion coef-ficient, ag, of copolymer was proportionallyincreased as the content of nBA increased. Thethermal stability of the matrix was examined byTGA, which revealed no thermal degradationunder 300 8C. Table 2 summarized the physicalproperties of the synthesized copolymers includ-ing Tg, Ts, Td (5 wt % loss temperature), and ag.

Blending Ni-plated core-shell polymeric par-ticles into the copolymer matrix caused its Ts

increase, while in opposite, the ag proportionallydecreased with the increase of filler content, asshown in Figure 3.

Percolation Threshold of Composite Film

Figure 4 shows the volumetric resistivity of thecomposite films at various conductive filler load-ing. When filler loading was below 10 vol %, thevolumetric resistivity was found to be higherthan 108 ohm-cm (instrument upper limit) butdropped down dramatically as the loading wentbeyond 10 vol %.

The percolation phenomenon of a CPC systembeyond its critical volume fraction can be des-cribed by:21–23

qm ¼ ð1=aÞ � ð�� cÞ�c ð1Þ

and

a ¼ 1=ðqdð1� �cÞcÞ ð2Þ

where qm is the measured volumetric resistivityof the composite film, F and Fc are the volume

Table 2. Thermal Properties of Polymer Matrix

Matrix [S]0/[nBA]0

Tg

(8C)Ts

(8C)Td

(8C)ag

(ppm/8C)

M1 1:0 102 110 307 102M2 9:2 76 79 313 115M3 8:3 59 62 310 144

[S]0 – initial concentration of styrene monomer.[nBA]0 – initial concentration of n-butyl acrylate monomer.Tg – the glass transition temperature, determined by DSC.Ts – the softening temperature, determined by TMA.Td – 5 % weight loss temperature, determined by TGA.ag – the glassy state thermal expansion coefficient, deter-

mined by TMA.

Figure 3. The changes of thermal expansion coeffi-cient of composite films under varied conductive fillercontent.

Figure 5. Linear regression plot of the volumetricresistivity of composite film versus F�Fc.

Figure 4. The volumetric resistivity of compositefilm at various conductive filler content.



fraction and critical volume fraction of conduc-tive fillers in the composite, respectively, a and care the characteristic constant and critical expo-nent, respectively, and qd is the volumetric resis-tivity of the most dense pack of conductivefillers, which is 10�3 ohm-cm reported in. ref. 22.At first, an initial value of Fc was assumed andwith the iteration of eqs 1 and 2 the final theo-retical values of Fc ¼ 0.0952 and c ¼ 3.505 wereobtained. Figure 5 shows the logarithmic plot ofqm versus (F�Fc)

c. Comparing to the result inFigure 4, the theoretical estimated critical vol-ume fraction Fc was in good agreement to theexperimental result of 10 vol %.

PTCR Behaviors of Composite Films

The temperature effect on the electrical resistiv-ity of composite film was measured in conduc-tive filler loading range from 15 to 30 vol %.Each specimen had been heated from ambient

temperature to 120 8C with a constant heatingrate of 2 8C/min, except for M1c sample whichwas up to 140 8C. Following with natural coolingto ambient temperature and repeated the sameheating cycle for five times. Figure 6 shows thechanges of volumetric resistivity of compositefilm during heatup at various filler loading, bymatrix M3 only. Figures 7 and 8 were the volu-metric resistivity changes during heatup of thecomposite films consisted of different matrix andfiller loading fixed at 15 and 25 vol %, respec-tively. In the range of 15–25 vol % conductive fil-ler loading, the composite film showed uniquePTCR behaviors such that the volumetric resis-tivity rapidly increased by several orders ofmagnitude as the temperature reached a criticalvalue. A valuable finding was that PTCR transi-tion temperature in our composite films could betriggered before matrix Tg. However, the transi-tion temperature would be shifting to the lowtemperature, along with the increase of ambient

Figure 6. The change of volumetric resistivity of composite film during heatup.Polymer matrix was M3 and conductive filler loading was: (a) 15, (b) 20, (c) 25, and(d) 30 vol %.



resistivity, once the thermalcycling was started.The relative equilibrium state was approachedafter four or five cycles. The PTCR transitionbecame obscured when increased the filler load-ing to 30 vol %, as shown in Figure 6(d).

Upon heating, the thermal expansion of poly-meric matrix and conductive fillers are different

which will result the increasing gaps among fill-ers. As long as the gap inbetween conductive fill-ers reach a critical value, the conductive pathswithin the composite became disconnected, thusthe resistivity of CPC increased with severalorders of magnitude. Generally, the PTCR transi-tion of CPC is mainly caused by the sudden

Figure 7. The changes of volumetric resistivity ofcomposite films during heatup. Conductive filler load-ing was 15 vol % and polymer matrix was: (a) M1, (b)M2, and (c) M3.

Figure 8. The changes of volumetric resistivity ofcomposite films during heatup. Conductive filler load-ing was 25 vol % and polymer matrix was: (a) M1, (b)M2, and (c) M3.



increase of matrix volume due to the melting ofcrystallites or the shifting from the glassy state tothe rubbery state.8,24 However, once the thermalexpansion of polymeric matrix during the glassystate is high and enough to break the conductivepath, the PTCR transition can be triggered evenbefore its matrix Tg. This is what we found in oursystems. At the same filler loading, the PTCRtransition temperature of CPC decreased as theag of the matrix increased. In other words, the

lowest PTCR transition temperature was found inthe M3 CPC system because its ag was the highestamong three matrices, as shown in Figure 4.

Several reasons might contribute to the resis-tivity-temperature (R-T) curves shifts in our sys-tem during the thermalcycling test, such asthe orientation of polymer chains, the stressrelaxation, or the interfacial morphology changeinbetween conductive filler and polymer matrix.Figure 9 shows the cross-sectional SEM micro-

Figure 9. The cross-sectional SEM micrographs of composite film before and afterfive times thermal cycles. (a) M1 with 25 vol % of conductive fillers, (b) M2 with25 vol % of conductive fillers, and (c) M3 with 20 vol % of conductive fillers.



graphs of composite films before and after fivethermal cycles. Lots of interfacial cracks aroundconductive particles were found. This was mainlyattributed to the interfacial bonding failureinbetween heterogeneous phases, caused by mis-match of thermal expansion between the matrixand the filler. This phenomenon became evenmore crucial when temperature had beenincreased. As interfacial cracks occurred, the am-bient composite resistivity increased, while PTCRtransitions shifted toward lower temperature,because conductive paths were having higherpossibility of disconnection during heatup. Asbenchmark to Figure 9(a,b), the interfacial cracksshown in Figure 9(c) were less obvious.

CONCLUSIONS

Ni-plated core-shell polymeric particles wereused as conductive fillers in the CPC study. TheCPC film had shown a PTCR transition in fillerloading range from 15 to 25 vol %. However, thetransition became unobservable when loadingwas up to 30 vol %. The PTCR transition tem-perature occurred before the Tg of polymer ma-trix, due to the higher thermal expansion ofpolymer matrix during heating, which exploredanother thinking about the design PTCR andCPC systems. The interfacial cracks betweenthe conductive filler and polymer matrix phasesformed while the composite film was enduredwith several thermal cycles. Therefore, besidesthe component selections, proper stress relaxa-tion process as well as the good compatibility inthe filler and matrix was a crucial factor worthserious consideration while designed an overloadprotection device for industrial applications con-cerning the CPC materials.

This work was supported by the National ScienceCouncil of the Republic of China under grants NSC92-2216-E-007-028.

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