Development of electrical conductivity in MWCNT polystyrene composites

Development of electrical conductivity with minimum possible percolation threshold in multi-wall carbon nanotube/polystyrene compositesNilesh K.Shrivastava,B.B.Khatua*Materials Science Centre,Indian Institute of Technology,Kharagpur 721302,IndiaA R T I C L E I N F O Article history:Received 1February 2011Accepted 20June 2011Available online 24June 2011A B S T R A C TA method is reported that involves the bulk polymerization of styrene monomer in the presence of multi-wall carbon nanotubes (MWCNTs)and polystyrene (PS)beads,for the preparation of MWCNT/PS conducting composites with a significantly lower (0.08wt.%MWCNT)percolation threshold than previously reported.Thus,the conductivities of 7.62·10À5and 1.48·10À3S cm À1were achieved in the MWCNT/PS composites through homogeneous dispersion of 0.08and 0.26wt.%CNTs,respectively in the in situ polymerized PS region by using 70wt.%PS beads during the polymerization.The extent of dispersion and location of the MWCNTs in the PS matrix has been investigated with a scanning and transmission electron microscopy.The conductivity of the composites was increased with increasing wt.%of the PS beads at a constant CNT loading,indicating the formation of a more continuous network structure of the CNTs in PS matrix.Ó2011Elsevier Ltd.All rights reserved.1.IntroductionSince their discovery,multi-wall carbon nanotubes (MWCNTs)have attracted considerable attention to the researchers for developing electrical conductivity in insulating polymer ma-trix at a very low loading (<2vol.%)of carbon nanotubes (CNTs),as compared to the conventional conductive fillers such as car-bon black (CB)[1–3].This exceptionally superior performance of the CNTs in polymer composites is achieved,not only by using the inherent properties of the CNT,but more essentially by optimizing their dispersion and distribution or individuali-zation in the matrix polymer and interfacial adhesion to take advantage of the enormous surface area per unit volume that nanotubes have.However,a major challenge for their use in polymer matrix is to achieve homogeneous dispersion of the CNTs that exist in the form of aggregates due to strong van der Waals forces originating from their small size and large surface area [4].Three different techniques,such as:(i)in situ polymerization of the monomers in the presence of CNTs [5],(ii)solution casting where both the matrix polymer and CNTs are dispersed in a common solvent followed by evaporation [6],and (iii)melt processing which involves mechanical mixing of the molten polymers and CNTs [7],are often used to prepared the CNT/polymer composites.Moreover,for better dispersion and interfacial adhesion of the CNTs in host poly-mers,chemical modification or functionalization of the CNTs is needed that increases their compatibility with the polymers [8–13].Polystyrene (PS)is widely used in the electronics industry,packaging applications,etc.Several research groups [14–26]have investigated the electrical properties of CNT/PS compos-ite,prepared by various methods.For instance,Kim et al.[14]reported dc conductivity in the range of 10À5S cm À1at 1wt.%CNT loading in the bulk polymerized MWCNT/PS composites.Yu et al.[15]showed conductivity value of $10À3S cm À1at 1.9wt.%CNT loading in MWCNT/PS composite,prepared by latex technology.Safadi et al.[16]reported the surface con-ductivity of $10À4S cm À1with 5wt.%of MWCNT in solution0008-6223/$-see front matter Ó2011Elsevier Ltd.All rights reserved.doi:10.1016/j.carbon.2011.06.070*Corresponding author:Fax:+913222255303.E-mail address:khatuabb@matsc.iitkgp.ernet.in (B.B.Khatua).casted MWCNT/PS composites.Chang et al.[17]observed DC conductivity of8·10À13S cmÀ1in solution blended single-wall carbon nanotubes(SWCNT)/PS composite with0.5wt.% of CNT.Grossiord et al.[18]reported the DC conductivities of%10À2and%10S cmÀ1at0.2and2wt.%vertically-aligned (in-house grown,high structural quality)MWCNT(VGCNTs) loading in the latex blended CNT/PS composite.However,in the composites prepared with the commercial MWCNT,the percolation threshold(conductivity%10À12S cmÀ1)was reached at about0.85wt.%of CNTs.Hermant et al.[19]re-ported the conductivity of10À7S cmÀ1at0.5wt.%CNT load-ing in the emulsion polymerized SWCNT/PS composite. Tchoul et al.[20]reported the conductivity of10À6at 1.5wt.%functionalized CNT loading in solvent evaporated SWCNT/PS composites.Ha et al.[21]reported the conductiv-ity of10À7S cmÀ1at3wt.%CNT loading in the styrene–iso-prene SWCNT/copolymer composite.Dufresne et al.[22] showed the conductivity of10À2S cmÀ1at15wt.%of CNT in MWCNT/poly(styrene-co-butyl acrylate)latex composites. Wang et al.[23]reported DC conductivity of4.5·10À7S cmÀ1 at1wt.%SWCNT loading in suspension polymerized SWCNT/PS composites.Zhang et al.[24]showed the DC con-ductivity of 3.98·10À6S cmÀ1at5wt.%CNT loading in MWCNT/PS composites prepared through in situ polymeriza-tion technique.Ramasubramaniam et al.[25]reported the conductivity of6.89·10À2S cmÀ1at7wt.%of SWCNT loading in solution casted noncovalently functionalized SWCNTs/PS composites.Bhatia et al.[26]observed the DC conductivity of10À2S cmÀ1at1wt.%loading of MWCNT in solution casted MWCNT/PS composites.In summary,reports on the CNT/PS composites revealed that the development of electrical conductivity in the PS ma-trix at very low percolation threshold of MWCNT is a major concern without chemical modification of the CNTs.Here, for thefirst time,we report an innovative,straightforward, industrial scalable and environmentally safe process(nonsol-vent,no liquid waste/solvent effluent is generated),which combines the advantage of in situ polymerization of the sty-rene monomer in the presence of CNTs and commercial PS beads,for the preparation of electrically conductive CNT/PS composites at an extremely low loading of MWCNT without any modification.2.Experimental2.1.Materials detailsStyrene monomer used in this study was of synthesis grade and procured from Merck,Germany.Benzoyl peroxide(BP), used as polymerization initiator,was obtained from Sigma–Aldrich,Inc.General purpose,noncross-linked,crystal PS (grade:500B;M w:215,010;density: 1.04g/cm3;MFI:14g/ 10min at200°C and5kg load)pellets(average diameter %2.95mm and length%3.25mm)were obtained from Total Petrochemicals Inc.,USA.MWCNT employed in this study was of industrial grade(NC7000series;average diameter of9.5nm and length 1.5l m;surface area250–300m2/g; 90%carbon purity),purchased from Nanocyl S.A.,Belgium. The MWCNTs were used as received,without any chemical modification.2.2.Preparation of MWCNT/PS composites through bulk polymerizationThe styrene monomer(250ml)was taken in a500ml separat-ing funnel and20ml of5%NaOH solution was added into it. The mixture was shaken for15min and the purified styrene was decanted into a250ml beaker.This process was contin-ued forfive times.Finally,after washing with de-ionized water,the purified styrene was collected.Desired amount(0.25g)of MWCNT was dispersed in40ml purified styrene monomer in a250ml round shape three-neck glass reactor.The MWCNT/styrene dispersion was ultrasoni-cated for2h at room temperature.Then the reactor,connected with the refluxing condenser and nitrogen inlet to one of the three necks,was immediately placed on a hot plate magnetic stirrer.Required amount(1wt.%)of the benzoyl peroxide,as polymerization initiator,was added to the MWCNT/styrene dispersion in the reactor under constant stirring and the reac-tor temperature was gradually increased to%85°C.During the progress of polymerization reaction(after%45min of the reac-tion when MWCNT/styrene mixture started developing viscos-ity),22g of commercial PS bead was added into the reactor. Addition of the PS beads into the styrene monomer at the initial stage of polymerization would result in swelling of the PS beads and thus dispersion of some CNTs inside the PS beads.The reaction was continued for5h under nitrogen atmosphere with constant temperature and stirring.The MWCNT/PS com-posites thus obtained through in situ bulk polymerization of MWCNT/styrene in the presence of PS beads was air dried andfinally dried in an air oven at60°C for24h.From the weight (%55g)of thefinal product,calculated loadings of the CNT and PS bead in the MWCNT/PS composites were0.45and40wt.%, respectively.The MWCNT/PS composites with lower CNT load-ings(0.26and0.08wt.%)and higher amount of the PS bead(50, 60,70and80wt.%)were also prepared through the same poly-merization route,by varying the amounts of the styrene mono-mer,MWCNT and the PS bead during the polymerization reaction.Through GPC analysis,the weight average molecular weight(M w)of the bulk polymerized neat PS was found to be %36,390.The schematic representation for the preparation of the composites is illustrated in Fig.1.2.3.Molding of the MWCNT/PS compositesThe bulk polymerized MWCNT/PS composites with various amounts of the PS bead and CNT loading were compression molded at210°C in a hot press under constant pressure (10MPa)and the molded parts were air-cooled to room tem-perature for further characterization.During compression, meltflow of the highly viscous PS beads(M w%215,010)inside the mold was expected to be significantly less as compared to the bulk polymerized PS(M w%36,390).3.Characterizations3.1.Electrical conductivityThe direct current(DC)electrical conductivity measurements were done on the compression molded specimen bars of dimensions30·10·3mm3.The electrical conductivity of4572C A R B O N49(2011)4571–4579the conducting composite was measured with a two-probe technique.A minimum offive tests were performed for each sample and the average data was reported.The alternating current(AC)electrical conductivity mea-surements were performed across the thickness of the com-pression molded disc samples having15mm diameter and 1mm thickness.The electrical conductivity was measured un-der2V application in the frequency range of40–3·106Hz using a computer-controlled impedance analyzer(PSM1735). All the scanning was done at room temperature.3.2.Optical microscopyHigh resolution optical microscope(HROM,Carl Zeiss Vision GmbH)was used to investigate the distribution of MWCNT in the PS matrix.Image of the compression molded sample was taken in the monochromatic light with different resolution.3.3.Transmission electron microscopy(TEM)analysisThe extent of dispersion of the MWCNT in the PS matrix was studied by a transmission electron microscope(HRTEM, JEM-2100,JEOL,Japan)operated at an accelerating voltage of 200kV.The MWCNT/PS composites sample was ultra-microtomed under cryogenic condition with a thickness of 50–80nm.Since the CNT has much higher electron density than the polymers,it appeared dark in the TEM images.3.4.Field emission scanning electron microscope(FE-SEM) studyThe phase morphology of the MWCNT/PS composites was studied with afield emission scanning electron microscope (FE-SEM,Carl Zeiss-SUPRATM40),operated at an accelerating voltage of10kV.The specimens were carefully broken under liquid nitrogen atmosphere.The fractured surface of the sam-ples was coated with a thin layer of gold to avoid electrical charging,and SEM images were taken on the fracture surface.4.Results and discussion4.1.Electrical propertyThe room temperature DC conductivity of the MWCNT/PS composites with different weight percent(40–80wt.%)of the PS bead and MWCNT loading(0.08,0.26and0.45wt.%)is shown in Fig.2a.As observed,the conductivity of the MWCNT/PS com-posites was increased with increasing the PS bead content (wt.%)in the composites and a combination of70wt.%PS bead and30wt.%in situ polymerized MWCNT/PS in the composites showed the maximum conductivity value.For instance,the MWCNT/PS composites without any PS bead did not exhibit any conductivity at0.08wt.%CNT loading.Interestingly,the DC conductivity of5.31·10À7S cmÀ1was achieved when the composites was prepared by in situ polymerization of MWCNT–styrene in the presence of40wt.%PS bead.We assumed that the presence of PS beads increased the effective concentration of the CNT in the in situ polymerized PS phase of the composites.The PS beads in the matrix can be regarded as‘excluded volume’in which the CNTs cannot penetrate(evidence by optical micrograph in Fig.7a),raising the CNT concentration in the freshly polymerized continuous PS phase around the PS beads.Thus,the increase in conduc-tivity was a result of increasing the CNT–CNT contact points and entanglements though the overall CNT loading in the composites remain same.However,above70wt.%of the PS bead,the electrical conductivity of the composites showed a decreasing trend.This might be due to the blockage of con-ducting paths as a result of increase in non-conducting PS bead content in the composites.Fig.1–MWCNT/PS composites manufacturing process.The plot of DC conductivity vs.MWCNT loading in the MWCNT/PS composites with 70wt.%PS bead is shown in Fig.2b.As evident,the composite showed a dramatic increase in the conductivity by several orders of magnitude from 10À11to 10À4S cm À1by varying the CNT content between 0.02and 0.29wt.%.This tendency of increase in conductivity clearly indicates the formation of continuous network structure of MWCNTs in the composites,which is well known percolation network.The conductivity of the composites was gradually in-creased on further addition of CNT in the PS matrix.The con-ductivity of the MWCNT/PS composites with 70wt.%PS bead was measured to be 4.67·10À3S cm À1at 1wt.%CNT loading.To estimate the percolation threshold concentration (p c ),the experimental data were fitted using the power law equation (Eqs.(1)and (2))for the composites conductivity near percola-tion threshold [27,28]:r DC /ðp Àp c Þt for p >p c ð1Þr DC /ðp Àp c ÞÀsfor p >p cð2Þthe nanotubes and hence a large contact resistant.When the MWCNTs are separated by a very thin layer of polymer,tunnelling of the electrons between the neighboring CNTs oc-curs through the polymer.This characteristic of conducting current can be attributed to the tunneling of electrons.In gen-eral,the electrons in a polymer cannot transfer from one elec-trode to another through the insulator due to the existence of an energy barrier.However,when a voltage is imposed be-tween the two,the shape of the energy barrier is changed and there is a driving force for the electrons to move across the barrier by tunneling,resulting in a small current when the distance between neighboring electrodes is sufficiently small so that the electrons in the polymer composites are tun-neling one by one from one MWCNT electrode to the nearest MWCNT electrode and forming an MWCNT–polymer pathway.It induces resistance and limits the conductivity of the composites.In the range of CNT loading from 0.02to 0.29wt.%,the con-ductivity value of the MWCNT/PS composites was drastically increased by several orders of magnitude,from 10À11toFig.2–Conductivity of MWCNT/PS composites:(a)with the wt.%of PS bead in PS matrix,and (b)with MWCNT loading.The inset in (b)represents the log–log plot for r DC vs.(p Àp for the same composites.The straight line in the inset is least-squares fit to the data using Eq.(1)returning the best fit values p c =0.045%and t =2.11.10À4S cm À1.At higher CNT loading between 0.5and 1wt.%,the conductivity stabilized at around 10À3S cm À1,which is best to our knowledge,the highest value ever reported for MWCNT/PS composites at this low level of CNT loading with unaligned,unmodified,commercially available MWCNTs of similar qualities (carbon purity,aspect ratio).Fig.3a compares the conductivity of the MWCNT/PS composites under investi-gation with published data for the CNT/PS composites.Sun et al.[37]reported conductivity near about 10À3S cm À1at 3wt.%MWCNT in MWCNT/solution blended syndiotactic the DC conductivity of 10À7S cm À1with 3.5wt.%of CNT load-ing in MWCNT/PS composite prepared by bulk polymerization of styrene with aqueous dispersion of MWCNT.The low percolation threshold of %0.045wt.%MWCNT loading in the PS matrix prepared by in situ polymerization of styrene in the presence of PS beads can be explained con-sidering the excluded volume of the beads in which the CNTs cannot (or can hardly)penetrate,raising the local CNT con-centration very high in the continuous PS phase,surrounding the PS beads.The advantage of this method is that it gives flexibility to use other polymers with selective dispersion of MWCNTs in the in situ polymerized phase.To compare the conductivity of the MWCNT/PS composites prepared by this new method with conventional methods,we prepared MWCNT/PS composites by solution mixing and melt mixing process with different weight fractions of the MWCNT.The measured electrical conductivities of the solution pro-cessed MWCNT/PS composites are shown in Fig.3b.It can be seen that the conductivity of the composites at 2wt.%of CNT loading was %3.3·10À3S cm À1,which is comparable to the conductivity value (1.3·10À3S cm À1)obtained with this new method even at 0.26wt.%CNT loading in the presence of 70wt.%PS bead during polymerization of MWCNT/styrene.However,the composites prepared by the melt mixing method showed the conductivity %2.3·10À4S cm À1at 3wt.%CNT loading.The high electrical conductivity of the MWCNT/PS composites prepared by this new method can be explained in terms of excluded volume of the beads in the PS matrix.Thus,the CNTs are getting concise in a small region (in situ polymerized PS phase)of the composites which decreased the average distance between the nanotubes.This enhanced physical contacts between the CNTs or electron tunneling through the polymer,leading to a high conductivity even at low CNT loading than the solution casted and melt-mixed MWCNT/PS composites.Fig.4a shows the variation of AC conductivity with the fre-quency at room temperature for 0.08wt.%MWCNT loading in the composite with increasing wt.%of the PS bead.As ob-served,the conductivity of neat PS was increased with increasing the frequency,as expected for an insulating mate-rial.Moreover with increase in wt.%of the PS bead while keeping the CNT loading constant,the conductivity was grad-ually increased.This indicated the formation of more contin-uous network structure of the CNTs with increase in wt.%of the PS bead.Up to a critical frequency the ac conductivity re-mained almost constant showing a plateau region,beyond which an increase in the conductivity was evident with fur-ther increase in the frequency for each composites.The crit-ical frequency value was shifted to the higher region with increase in wt.%of the PS bead.This might be due to the in-crease in effective loading of MWCNT in the bulk polymerized PS phase with increasing wt.%of PS bead that led to the for-mation of more number of network structure of the CNTs.This result is in agreement with the previous reports [41,42]where the critical frequency shifted to the higher frequency region with increase in the CB loading in polymer matrix.Fig.4b shows the frequency dependence of AC conductivity of the composites with variation of CNT loading,keeping a constant wt.%of PS bead.It can be seen that the conductivity of the composites increased drastically with respect to theFig.3–(a)Comparison of our result to published electrical conductivity data,and (b)DC conductivity for the solution casted and melt-mixed MWCNT/PS composites.C A R B O N49(2011)4571–45794575pure PS.On addition of 0.08wt.%CNT into the PS matrix,sharp increase in the conductivity was observed in the PS/MWCNT composites.At lower loading (up to 0.46wt.%)of the CNT,a slight variation in the CNT loading reasonably chan-ged the conductivity value of the composites.However,beyond this CNT loading,a slight increase in the conductivity of the composites was evident.This might be due to the creation of dead arms instead of conducting paths after a certain filler loading.The conductivity of the MWCNT/PS composites with 0.08wt.%CNT loading was constant up to the critical fre-quency,and then an increase in the conductivity was evident with the frequency.Interestingly,at higher loading of CNT,the conductivity of the composites was more or less constant with the change in frequency throughout the entire frequency range.This is the behavior of a highly conducting composite where the flow of current is due to the electron transfer,and not related to the polarization.The variation of dielectric constant of 0.08wt.%MWCNT/PS composites with the frequency at room temperature with increasing wt.%of PS bead and CNT is shown in Fig.5.In bothFig.4–AC conductivity of MWCNT/PS composites vs.frequency at (a)various wt.%of PS bead,and (b)different MWCNT loading.Fig.5–Dielectric constant of MWCNT/PS composites (a)with variation of PS bead at constant CNT loading,and (b)with variation of CNTs at constant PS bead loading.flowing of the beads to some extent during compression mold-ing.The PS beads in the matrix phase where the CNTs failed to penetrate can be considered as‘‘excluded volume’’.This led to increase the effective concentration of the CNT in the in situ polymerized PS matrix,and thus increased the probability of forming more conducting paths in the composites.Fig.7b shows the FESEM image of the cryogenic fractured MWCNT/PS composites containing0.08wt.%CNT and 70wt.%PS bead.The micrograph also clearly shows the dis-tribution of CNT in selective regions of the composite and dis-cernible amount of the CNTs were penetrated inside the PS beads of the composites during molding process.However, major portion of the CNTs were distributed in the in situ bulk polymerized PS phase of the matrix,raising the effective con-centration of CNTs very high in the bulk polymerized PS phase.This led to the formation of more continuous network structure of the CNTs in the in situ polymerized PS phase and thus enhanced the conductivity of the composites.Fig.7c–e shows the TEM images of the0.08wt.%MWCNT/PS composites consisting of70wt.%PS bead.Since,the size of the PS beads in the composite formulation was in millimeter scale, the full size of the bead was not appeared in the TEM image. However,we could capture some portions of the PS beads in the TEM image(Fig.7c and d).As can be seen(Fig.7c and d), the continuous network structure of the MWCNTs was formed through distribution of the CNTs in the selective regions(in situ polymerized PS phase),mostly outside the PS beads,in the PS matrix.The TEM image at higher magnification(Fig.7e)clearly shows good dispersion and individualization of the CNTs in the in situ bulk polymerized PS phase of the composites.To investigate the absolute morphology of the MWCNT/PS composites,we prepared another composites by bulk poly-merization of styrene in the presence of MWCNTs and sus-pension polymerized PS beads(spherical,size:70–110l m). Fig.7f represents the TEM image of compression molded MWCNT/PS composites containing0.1wt.%CNT and 25wt.%suspension polymerized PS bead.As can be seen, the presence of the beads and MWCNTs were evident in PS matrix.The CNTs were mostly located in the in situ polymer-Fig.6–Dielectric loss tangent(tan d)of MWCNT/PS composites(a)with variation of PS bead at constant CNT loading,and(b)with variation of CNTs at constant PS bead loading.ized PS phase and could hardly penetrate inside the PS beads of the composites.5.ConclusionsHere,for thefirst time,we demonstrate an innovative,straight forward method to prepare the MWCNT/PS conducting com-posites with minimum possible CNT loading in the PS matrix. 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