Electrical conductivity and dielectric properties of PMMAexpanded graphite composites

Electrical conductivity and dielectric properties ofPMMA/expanded graphite compositesWenge Zheng,Shing-Chung Wong *School of Materials Engineering,Nanyang Technological University,Nanyang Avenue,639798SingaporeReceived 23January 2002;received in revised form 27August 2002;accepted 29August 2002AbstractPMMA/expanded graphite (EG)composites were prepared by direct solution blending of PMMA with the expanded graphite filler.Electrical conductivity and dielectric properties of the composites were measured by a four-point probe resistivity determiner and a dielectric analyzer (DEA).Interestingly,only 1wt.%filler content was required to reach the percolation threshold ( )of transition in electrical conductivity from an insulator to a semiconductor using PMMA/EG.The thickness of the interlayer of the expanded graphite was shown to be close to the nanometer scale.The reported filler content was much lower than that required for conventional PMMA/carbon black (8wt.%carbon)and PMMA/graphite (3.5wt.%graphite)composites.The improvements in both electrical conductivity and structural integrity were attributed to the difference in filler geometry (aspect ratio and surface area)and the formation of conductive networks in the composites.#2002Elsevier Science Ltd.All rights reserved.Keywords:Electrical properties;PMMA;Graphite;Nanocomposite1.IntroductionRecent interest in low-K dielectric packaging materi-als and conductive polymers have created new incen-tives in blending an engineered proportion of functional and light-weight mechanical components to form unique material systems.Conductive composites offer functional applications in addition to mechanical improvement for load-bearing applications.In the dif-ferent composite systems under consideration,nano-composites give rise to excellent physical properties in addition to conductive and mechanical performance.Nanocomposites are a new class of materials containing at least one filler dimension in the nanometer range [1].Based on the filler geometries,nanocomposites can be classified into three primary categories.Fumed silica dioxide and nanometallic powder are particles,which are characterized by three dimensions in the nanometerrange [2,3].Carbon nanotube and whiskers possess two dimensions in the nanometer range [4,5],whereas clay,mica and expanded graphite layered structural fillers [1,6–8]possess only one dimension in the nanometer range.Among the latter,smectite clay in platelet form has been widely studied [1]because the natural materials are easily available and the intercalation chemistry is reasonably well understood in the literature.The nano-composites containing layered silicates exhibit markedly superior mechanical,thermal and barrier performance in comparison with conventional microcomposites [1,6–10].Unfortunately,nanoclay reinforced polymers do not possess electrical conductivity,photonic and dielec-tric properties that are as good as some functional polymers such as graphite-containing polymers.Different conductive fillers such as carbon black (CB)and metallic powder have been extensively explored for composite components.These fillers effectively improve the conductivity of polymers [3,11–13].Usually rather high filler content is needed to increase the conductivity because the filler size is only in the micrometer range.The significant improvement in electrical conductivity arising from the increase of filler content was observed for most composites and it was explained0266-3538/02/$-see front matter #2002Elsevier Science Ltd.All rights reserved.P I I :S 0266-3538(02)00201-4Composites Science and Technology 63(2003)225–235/locate/compscitech*Corresponding author.Present address:Department of Mechan-ical Engineering and Applied Mechanics,North Dakota State Uni-versity,111Dolve Hall,Fargo,ND 58105,USA.Tel.:+1-701-231-8840;fax:+1-701-231-8913.E-mail address:josh.wong@ (S.-C.Wong).by the percolation transition of the conductive network formation.The percolation values for a critical transi-tion in filler-based composites are 8wt.%for PMMA/CB [11],6.2wt.%for PP/CB [11]and 9wt.%for nylon 6/CB [12].In most cases,relatively large quantities of fillers were needed to reach the critical percolation value.Natural graphite flakes provide good electrical con-ductivity (106S/m at ambient temperature)and layered structure with a c-axis lattice constant,which indicates interplanar spacing,of 0.66nm [14].Since there are no reactive ion groups on the graphite layers,it is difficult to prepare the polymer/graphite nanocomposites via ion exchange reaction in order to intercalate the monomers into the graphite sub-layers.The expanded graphite,however,contains abundant multi-pores ranging from 2nm to 10m m.Average size of the pores is about 2m m.The graphite maintains a layered structure similar to natural flake graphite but with larger layer spacing [14–17].Recently,it was reported that markedly lower volume fraction of expanded graphite was able to reach the percolation threshold of conductivity in nylon 6/graphite and PS/graphite nanocomposites by in situ polymerization of polymer matrix [7,8].In these gra-phite-based nanocomposites,the monomer was first introduced into the pores of the expanded graphite to be followed by polymerization.The expanded graphite has a higher volume expan-sion ratio than that of regular graphite.Furthermore,the multi-pores,functional acids and the OH groups will facilitate physical and chemical adsorption between the graphite and the polymer solution [14,16–17].In this paper,we prepared the PMMA/graphite nanocompo-sites by the solution blending method.After dissolving the polymer,the expanded graphite was mixed with the polymer solution.The polymers were then locked into the pores of the expanded graphite and remained in it after solvent extraction.The morphologies of the mate-rials were examined using SEM.The electrical anddielectric properties of the graphite composites were determined and discussed as a function of expanded graphite content.The mechanical properties of the polymer/graphite composites were investigated by dynamic mechanical analysis (DMA).2.Experimental work2.1.Preparation of expanded graphite (EG)The natural flake graphite (from BEISHU graphite Co.Shandong Province of China)was dried at 80 C in a vacuum oven for 24h.It was mixed and saturated with acids consisting of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 4:1for 24h to form the graphite intercalated compound (GIC).Nitric acid serves as an oxidizer and sulfuric acid is an intercalant [14].The mixture was carefully washed and filtrated with deionized water until the pH level of the solution reached 6.After being dried at 80 C in a vacuum oven for 24h,the GIC was rapidly expanded at 900 C for 15s in a muffle furnace to form expanded graphite (EG).The schematic illustrating the prepara-tion of EG from natural flake graphite is shown in Fig.1.2.2.Preparation of PMMA/graphite composites The PMMA/EG composites were prepared by the solution blending method.PMMA pellets were dried at 60 C in a vacuum oven for 24h.The PMMA pellets were first dissolved into solution with chloroform and then mixed with fillers in different weight fractions of graphite and expanded graphite in a flask by stirring aided by a sonicator.The solvent was evaporated at 60 C and the blends were dried at the same temperature in a vacuum oven for 24h.The composites were hot pressed into film specimens with a thickness of 500m m fortesting.Fig.1.A schematic showing the formation of expanded graphite (EG)from natural flake graphite.226W.Zheng,S.-C.Wong /Composites Science and Technology 63(2003)225–2352.3.Electrical properties2.3.1.Conductivity testThe conductivity of PMMA/graphite composites was measured by a dispersible four-point resistivity probe system(SIGNATONE)with a limit of10À8S/cm The hot pressed sample was cut into10Â4Â0.4mm3spe-cimens for testing.For PMMA matrix and the compo-sites with lowfiller content,the conductivity of samples was considerably lower than the detectable range of the four-point resistivity probe system and was determined using a digital model(RP2680)resistivity determiner. Both instruments were sensitive to the prescribed limits and the results obtained were assumed to be comparable.2.4.Dielectric measurementsDielectric experiments were conducted using a TA Instrument DEA2970Dielectric Analyzer in Ceramic Parallel Plate mode.The sample dimensions were25Â25Â0.5mm3.Testing temperature ranged from room temperature to100 C.Nitrogen gas was used to pro-vide an inert environment at aflowing rate of500ml/ min.The ramping rate was3 C/min with multi fre-quencies of1,3,10,30,100,300,1000,3000,and 10,000Hz.2.4.1.Dynamic mechanical analysis(DMA)DMA experiments were conducted using TA Instru-ment DMA2980infilm tension mode at afixed fre-quency of1Hz.The sample dimensions were20Â8Â0.5mm3.The sample was tested with a temperature ranging from room temperature to100 C at atmo-spheric pressure and a heating rate of3 C/min.2.4.2.SEM examinationA scanning electron microscope(SEM,Jeol-3410)was used to examine the pore parameters of expanded gra-phite.Gold coated fracture surface was studied to reveal thefiller morphology and dispersion.3.Results and discussion3.1.Structural characteristics of expanded graphite (EG)In the presence of an oxidizer,which is concentrated nitric acid in this work,the intercalant,which is con-centrated sulfuric acid,is dispersed into the graphite layers to form graphite intercalating compounds(GIC), see Fig.1.After rapid expansion of GIC at a high tem-perature,EG was formed.The expansion volume ratio of EG to natural graphite was50–100.The structure of EG was strongly affected by experimental conditions such as temperature,oxidizer’s concentration,intercalating time,etc.Fig.2shows the SEM photomicrographs of untreated graphite(Fig.2a)and expanded graphite (Fig.2b–d).The differences in microstructures between the two differentfillers can be clearly noted.Fig.2b reveals the loose structures of expanded graphite whereas Fig.2c and d display the photomicrographs of the loose structures in successively larger magnifi-cations.Evidently,loose structures containing multi-pores in the EG could be observed in Fig.2c.For the EG prepared by the chemical method,the average pore diameter is about2m m.The surface area per gram is roughly30–40m2/g and the pore volume is about4–8 ml/g[16,17].Fig.2d also suggests the thickness of the EG layer is less than100nm.For the EG,only the spacing of the graphite layers was expanded;the con-ductive characteristics of each EG sheet layer remains the same as graphiteflakes themselves by nature[18]. The graphite expansion would certainly influence the overall conductivity of the bulk materials.After expan-sion,each graphiteflake can be exfoliated into many EG layers as conductivefillers.In addition,some func-tional groups such as–OH,–COOH groups existed on the surface and pores of the EG after acid and high temperature treatments and they could promote the adsorption of molecular chains and monomers onto the pores[17].To summarize,the high surface area and the prevalent pores in EG facilitate the processing and for-mation of in situ polymer/EG composites.3.2.Electrical conductivity of PMMA/graphite compositesFig.3shows the electrical conductivity of PMMA/ graphite vs.filler content comparing graphite and expanded graphite.At lowfiller content,the electrical conductivity of the composite increases with thefiller content.The conductivity of the materials is about 10À16S/cm in the initial stage and this is consistent with the magnitude of an insulator.A sharp increase,which is known as the percolation transition,emerges when thefiller content reaches a critical content.The con-ductivity levels offafter arriving at this critical value. At this stage,the conductivity of the material is about 10À4S/cm for PMMA/EG and10À5S/cm for PMMA/ graphite,which is nearly consistent with that of a semi-conductor.The difference in conductive behavior between PMMA/EG and PMMA/graphite at higherfil-ler concentration is attributed to the enhanced number of conductive paths in the EG composites.Similar result was reported in HDPE/graphite composites with differ-entfiller size[19].The electrical conductivity of the composites exhibited a pronounced transition with the increase offiller content,from an insulator to nearly a semiconductor.This transition can be satisfactorily explained and described by the percolation theory and the formation of the conductive network in the compositesW.Zheng,S.-C.Wong/Composites Science and Technology63(2003)225–235227[8].According to the theory,the percolation threshold ( )corresponds to the onset of the transition from an insulator to a semiconductor.For the PMMA/graphite composites, %3.5wt.%graphite but %1wt.%when the filler is replaced by EG.Clearly,EG composites require a much lower percolation threshold and,in so doing,they are more effective in electrical conductivity in comparison with normal graphite composites.In contrast,PMMA/CB conductive composites require %8wt.%.Fig.3.Electrical conductivity of PMMA/graphite (&)and PMMA/EG (*)plotted as a function of weightfraction.Fig.2.SEM photomicrographs of (a)natural flake graphite,(b)expanded graphite (EG),(c)EG at higher magnification,and (d)abundant multi-pores with an interlayer thickness less than 100nm are revealed in EG .228W.Zheng,S.-C.Wong /Composites Science and Technology 63(2003)225–235The reduction in filler content for PMMA/EG com-posites can be attributed to the filler shape and the sur-face area per gram of filler.Carbon black,for example,is particulate in shape with lower aspect ratio than that of the layering structure of graphite and EG.The nat-ural flake graphite possesses a layering structure.The aspect ratio is about 50–100with a thickness of 10–20m m as observed in the SEM.So for polymer/graphite composites,higher aspect ratio and surface area of fil-lers lead to a lower percolation threshold.As shown in Fig.4,the graphite layer is expanded and it could also be seen in the presence of polymer.The thickness is estimated to be less than 100nm.These graphite layers exhibit the highest aspect ratio and the largest surface area per gram.Dispersion of graphite is also important to the varia-tion of percolation threshold for conductivity transition in the composites.In our experiment,we used long time,intensive stirring and ultrasonic bath to promote fine dispersion for the composites.The sufficient adsorption of the PMMA molecular chains onto various pores of the expanded graphite was also the likely factor con-tributing to good dispersion.If the solution concen-tration is too high,the higher viscosity may hinder the polymer chains from entering the minor pores of the graphite and thus lead to poor dispersion of graphite flakes in thepolymer.Fig.5.e 00from DEA studies of (a)PMMA,(b)PMMA/EG (3wt.%)and (c)PMMA/graphite (5wt.%)at different frequencies:1,3,10,30,100,300,1000,3000and 10,000Hz.Fig.4.SEM photomicrograph of PMMA/EG.W.Zheng,S.-C.Wong /Composites Science and Technology 63(2003)225–2352293.3.Dielectric properties of PMMA/graphite composites In a dielectric analyzer,the loss factor corresponds to the conductivity of the ing parallel plate electrodes,the loss factor(e00)can be calculated as follows: e00¼d=2 RAfe oðÞð1Þwhere A=electrode plate area,R=electrical resistance, d=plate spacing,f=frequency and e o=absolute per-mittivity of free space(8.85Â10À12F/m).So the loss factor is related to the conductance(1/R).Fig.5shows the loss factor,e00,obtained from the PMMA/graphite and PMMA/EG composites as a function of temperature and frequency using the DEA. PMMA alone shows very low e00values(about0.1)at all frequencies as indicated in Fig.5a.The change in e00 with temperature shows the typical characteristics of an insulator.The pattern of the curves is more or lessflat given the relatively small order of magnitude of the Y-axis.The results for PMMA containing3wt.%EG are shown in Fig.5b.The curves show much higher e00 values(about108at1Hz)and they are constant with temperature change.In this case PMMA/EGcomposites Fig.6.e00from DEA studies of PMMA/graphite(&)and PMMA/EG(*)vs.filler content at25 C and a frequency of1Hz.Fig.7.Variation of e00of2wt.%graphitefilled PMMA with temperature at different frequencies:1,3,10,30,100,300,1000,3000and10,000Hz. 230W.Zheng,S.-C.Wong/Composites Science and Technology63(2003)225–235derived from Eq.(1)display comparable conductivity to a semiconductor.Similar trends are shown for PMMA/graphite composites at higher filler content (5wt.%)in Fig.5c .However,the e 00values for unexpanded PMMA/graphite are slightly lower than those pertain-ing to PMMA/EG.This indicates that PMMA/EG exhibits great resemblance to a semiconductor at a lower filler content.Fig.6shows the variation of e 00with different filler content for PMMA/graphite and PMMA/EG at 1Hz.Clearly,the same conclusion can be drawn from e 00trends in Fig.6as that drawn the elec-trical conductivity in Fig.3.PMMA/EG reaches the percolation threshold at a markedly lower filler content than that for PMMA/graphite.The variation of e 00for PMMA/graphite composites with temperature at a given filler content is shown in Fig.7.A sharp increase in e 00takes place at around 80 C at different frequencies.The increase in the electrical conductivity of the composite as temperature increases above the T g of the PMMA matrix,as revealed in Fig.10a ,can be caused by the possible flocculation of conductive fillers.Such a sharp transition was not observed for the PMMA/EG under our experimental conditions.As noted earlier,the PMMA/EG reached the percolation threshold at 1wt.%at room tempera-ture.As a result,we were not able to observe a similar phenomenon for PMMA/EG even though it could have taken place at a lower EGcontent.parison of G 0from DMA studies between (a)PMMA/graphite and (b)PMMA/EG as a function of temperature.Different filler wt.%is given.W.Zheng,S.-C.Wong /Composites Science and Technology 63(2003)225–2352313.4.Dynamic mechanical propertiesDynamic mechanical analysis(DMA)measures the cyclic response of a material as a function of the tem-perature.The storage modulus(G0),loss modulus(G00) and tan of PMMA/graphite and PMMA/EG from room temperature to100 C are shown in Figs.8–10, respectively,as a function offiller content.The results for the PMMA/graphite indicated that G0(Fig.8a)and G00(Fig.9a)are close to that of pure PMMA.But the glass transition temperature,T g,of the composites is shifted to higher temperatures than that of pure PMMA,as indicated by the shift of tan in Fig.10a.This observation is ascribed to the restricted segmental movement upon the addition offillers[20].The results appear consistent with the behavior of otherfilled poly-meric systems[20,21].For PMMA/EG composites,however,the DMA results in Figs.8b and9b reveal a completely different pattern from PMMA/graphite composites.First,G0of the PMMA/EG exhibits an apparent increase compared to that of pure PMMA.G0increases from2.8GPa for pure PMMA to 3.4GPa for PMMA/EG even at a relatively lowfiller weight fraction(2wt.%).Evidently, EG at the nanometer scale can improve the stiffness of the materials even at relatively lowfiller content,which parison of G00from DMA studies between(a)PMMA/graphite and(b)PMMA/EG as a function of temperature.Differentfiller wt.%is given.232W.Zheng,S.-C.Wong/Composites Science and Technology63(2003)225–235is consistent with some other nanocomposites[1].Sec-ond,G00,which is indicative of energy dissipation in viscoelastic deformation,of PMMA/EG is also greatly improved upon EG addition.G00increases from 250MPa of PMMA to380MPa of PMMA/EG.The results suggest that the mechanical integrity of PMMA/ EG is superior to those of PMMA alone and PMMA/ graphite composites.The tan of PMMA/EG compo-sites as shown in Fig.10b indicates a shift of T g to higher values by as much as10 C in comparison with pure PMMA.The improvement in T g is significant given the relatively smallfiller content being explored. Fig.11plots the G0for PMMA/graphite and PMMA/ EG as a function offiller content at room temperature. Interestingly,G0increases with EG whereas it decreases as unexpanded graphite content increases at lowerfiller content.The decrease could be due to many possible reasons including the weak interface between the bulk graphiteflakes and the PMMA matrix and transgra-nular cleavage in the graphite.The dramatic increasein parison of tan from DMA studies between(a)PMMA/graphite and(b)PMMA/EG as a function of temperature.Differentfiller wt.%is given.Note the shift of T g asfiller content increases.W.Zheng,S.-C.Wong/Composites Science and Technology63(2003)225–235233G 0as EG increases warrants further investigation of the stiffening mechanisms especially in molecular terms.Note that the variation in G 0,to some extent,corre-sponds to the percolation threshold for transition in conductivity of PMMA/EG.The results suggest that there exists a correlation between the conductive net-work formation and the stiffening effect arising from the EG dispersion in the polymer solution.In general,it is clear PMMA/EG exhibits superior mechanical integrity in comparison with the unexpanded graphite compo-sites.These experimental observations are interesting as composite theory,which only takes into account of the filler volume fraction,cannot explain the opposite trends of the materials of comparable filler volume fractions.Nevertheless,it is understood that the good dispersion of EG in PMMA matrix could enhance the filler aspect ratio and interfacial factors,which could on the whole contribute to potent stiffening effect for the composites.4.ConclusionsConductive PMMA/EG composites were prepared by the direct solution blending method.The electrical con-ductivity and dielectric properties of the materials were measured with resistivity and DEA tests.Similar con-clusions could be inferred from both sets of tests.The PMMA/EG nanocomposites exhibited the lowest per-colation threshold ( =1wt.%)in comparison with the conventional PMMA/CB conductive composites ( =8wt.%)and unexpanded PMMA/graphite composites ( =3.5wt.%).Since the EG possesses abundant pores and the highest aspect ratio,molecular chains of poly-mers were easily intercalated with the pores of the EG by adsorption mechanisms.It was conjectured thatconductive EG fillers in composites were interconnected and readily formed conductive networks that enhanced the conductivity of the composites.Further evidence supporting this conclusion is provided elsewhere [22].DMA investigation of the materials revealed EG imparted higher G 0,G 00and T g to the polymer matrix,where the graphite was well dispersed leading to excel-lent properties.References[1]Alexandre M,Dubois P.Polymer-layered silicate nanocompo-sites:preparation,properties and uses of a new class of materials.Mater Sci Eng 2000;28:1–59.[2]Jana SC,Jain S.Dispersion of nanofillers in high performancepolymers using reactive solvents as processing aids.Polymer 2001;42:6897–905.[3]Saujanya C,Radhakrishnan S.Structure development and crys-tallization behaviour of PP/nanoparticulate composite.Polymer 2001;42:6723–31.[4]Calvert P.Potential applications of nanotubes.In:Ebbesen TW,editor.Carbon nanotubes.Boca Raton,FL:CRC press;1997.[5]Favier V,Canova GR,Shrivastava SC,Cavaille JY.Mechanicalpercolation in cellulose whiskers nanocomposites.Polym Eng Sci 1997;37:1732–9.[6]Theng BKG.The chemistry of clay-organic reactions.New York:Wiley;1974.[7]Chen GH,Wu DJ,Weng WG,Yan WL.Preparation of polymer/graphite conducting nanocomposite by intercalation polymeri-zation.J Appl Polym Sci 2001;82:2506–13.[8]Pan YX,Yu ZZ,Ou YC,Hu GH.New process of fabricatingelectrically conducting nylon 6/graphite nanocomposites via intercalation polymerization.J Polym Sci Part B:Polym Phys 2000;38:1626–33.[9]Kojima Y,Usuki A,Kawasami M.Mechanical properties ofnylon-6-clay hybrid.J Mater Res 1993;6:1185–9.[10]Lagaly G.An Introduction:from clay mineral–polymer interac-tions to clay mineral–polymer nanocomposites.Appl Clay Sci1999;15:1–9.Fig.11.G 0of PMMA/graphite (&)and PMMA/EG (*)plotted as a function of filler weight fraction at 25 C.234W.Zheng,S.-C.Wong /Composites Science and Technology 63(2003)225–235[11]Zois H,Apekis L,Omastova M.Electrical properties and per-colation phenomena in carbon blackfilled polymer composites.Proceedings—International Symposium on Electrets,10th Inter-national Symposium on Electrets(ISE10),1999.pp.529–532. 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