Electrical conductivity of poly(ethylene terephthalate)/expanded graphite nanocomposites prepared by in situ polymerization

Electrical Conductivity of Poly(ethylene terephthalate)/Expanded

Graphite Nanocomposites Prepared by In Situ Polymerization

S. Paszkiewicz,1 A. Szymczyk,2 Z. Spitalsky,3 M. Soccio,4 J. Mosn�acek,5

T. A. Ezquerra,4 Z. Rosłaniec1

1Institute of Material Science and Engineering, West Pomeranian University of Technology, Piastow Av. 19,

PL-70310 Szczecin, Poland

2Institute of Physic, West Pomeranian University of Technology, Piastow Av. 19, PL-70310 Szczecin, Poland

3Polymer Institute, Slovak Academy of Sciences, D�ubravsk�a cesta 9, 845 41 Bratislava 45, Slovakia

4Insituto de Estructura de la Materia, Consejo Superior de Investigaciones Cientificas, IEM-CSIC, Serrano 119-121,

28006 Madrid, Spain

5Polymer Institute, Centre of Excellence FUN-MAT, Slovak Academy of Sciences, D�ubravsk�a cesta 9,

845 41 Bratislava 45, Slovakia

Correspondence to: S. Paszkiewicz (E-mail: [email protected])

Received 26 June 2012; revised 22 August 2012; accepted 28 August 2012; published online 28 September 2012

DOI: 10.1002/polb.23176

ABSTRACT: Nanocomposites based on poly(ethylene tereph-

thalate) (PET) and expanded graphite (EG) have been

prepared by in situ polymerization. Morphology of the

nanocomposites has been examined by electronic micros-

copy. The relationship between the preparation method, mor-

phology, and electrical conductivity was studied. Electronic

microscopy images reveal that the nanocomposites exhibit

well dispersed graphene platelets. The incorporation of EG to

the PET results in a sharp insulator-to-conductor transition

with a percolation threshold (/c) as low as 0.05 wt %. An elec-

trical conductivity of 10�3 S/cm was achieved for 0.4 wt % of

EG. The low percolation threshold and relatively high electri-

cal conductivity are attributed to the high aspect ratio, large

surface area, and uniform dispersion of the EG sheets in PET

matrix. VC 2012 Wiley Periodicals, Inc. J Polym Sci Part B:

Polym Phys 50: 1645–1652, 2012

KEYWORDS: conducting polymers; electrical conductivity;

expanded graphite; in situ polymerization; nanocomposites;

nanoparticles; polyesters; Raman spectroscopy; TEM

INTRODUCTION In recent years, polymer-based nanocompo-sites reinforced with expanded graphite (EG) have shownsubstantial improvements in mechanical, electrical conductiv-ity, and barrier properties over the unmodified polymer. Theplausible reason for this enhancement is the sheet-like struc-ture of natural graphite where the atoms are stronglybonded on a hexagonal plane but weakly bonded perpendic-ularly to that plane.1 If these sheets/layers could be sepa-rated down to a nanometer thickness, they would form highaspect ratio (200–1500) and high modulus (�1 TPa) graph-ite nanosheets.1,2 Furthermore, graphite nanosheets couldhave an enormous surface area (up to 2630 m2/g) consider-ing both sides of the sheets are accessible.3 The dispersionof such nanosheets in a matrix plays a key role in theimprovement of both physical and mechanical properties ofthe resultant nanocomposite.4 To obtain electrically conduc-tive systems with high conductivity and acceptable process-ing properties, a reduction of the concentration of conductivefillers is required. The shape and size of the filler are veryimportant parameters to the conductive network.4,5 This can

be achieved by a combination of both synthesis and process-ing techniques that produce complete exfoliation and gooddispersion of graphene particles in the matrix. The synthesisof EG as well as graphite nanosheets is well documented inthe literature.6 Natural graphite is first converted to interca-lated or expandable graphite through chemical oxidation inthe presence of concentrated H2SO4 and HNO3 acids. EG isthen obtained by rapid expansion and exfoliation of expanda-ble graphite in a furnace above 600 �C. The obtained EG canbe further chemically modified to enhance affinity of carbonmaterial with a polymer matrix. For this reason, the electri-cally conductive polymeric composites can have potentialapplications, among other functional applications, in lightemitting devices, batteries, electromagnetic shielding, anti-static, and corrosion resistant coatings.4,7

Poly(ethylene terephthalate) (PET) is a semicrystalline poly-mer which has good mechanical properties, chemical resist-ance, thermal stability, low melt viscosity, and spinnability(ability to be spun, e.g., in the form of fibers). PET has been

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used in several fields such as food packaging, film technol-ogy, automotive, electrical, beverages and containers, andtextile fibers, and even in the biomedical field as Dacron.8 Away to further improve the properties of this commoditypolymer is through the preparation of nanocomposites bythe addition of nanoclays, carbon nanotubes, or other nano-structures. Nanocomposite materials often possess a combi-nation of physical properties that are not present in conven-tional polymers. Many polymeric nanocomposites using EGas conducting fillers4 have been extensively reportedrecently. However, there are no many reports on preparativemethods of PET/EG nanocomposites by in situ polymeriza-tion and their influence on electrical and physical properties.

In this article, we present a procedure to synthesize nano-composites based on PET and EG by in situ polymerization.The influence of nanofiller content on the resulting PET/EGnanocomposites morphology, nanostructure, and on electricalconductivity was investigated. Our process is suitable toobtain nanocomposites with appropriate EG dispersion onthe PET matrix with relatively high electrical conductivity.

EXPERIMENTAL

MaterialsFor the PET synthesis, the following chemicals were used: di-methyl tereftalate (DMT) (Sigma-Aldrich); ethan-1,2-diol(Sigma-Aldrich), zinc acetate Zn(CH3COO)2 (Sigma-Aldrich)as an ester exchange catalyst; antimony trioxide Sb2O3

(Sigma-Aldrich) as a polycondensation catalyst; Irganox 1010(Ciba-Geigy, Switzerland) as a thermal stabilizer. EG wasprepared by thermal expansion (SGL Carbon SE, Germany);average thickness of the expanded agglomerates was 450–560 nm. Graphene platelets size ranged from 16 lm to 46lm (99%). Before adding the EG to the reaction mixture, itwas combined with ethandiol in order to split agglomeratesand to improve further exfoliation.

Preparation of PET/EG NanocompositesNanocomposites PET/EG were prepared by in situ polymer-ization using a polycondensation reactor (Autoclave Engi-neers, PA) with a capacity of 1000 cm3. Before polymeriza-tion, the EG was dispersed in ethandiol using high-speedstirrer (Ultra-Turax T25) and sonicator (Homogenizer HD2200, Sonoplus, with frequency of 20 kHz and power 200W) in both cases for 30 min. Additionally, to improve thedispersion/exfoliation of EG in ethandiol, an ultra-powerlower sonic bath (BANDELIN, Sonorex digitec, with fre-quency of 35 kHz and power 140 W) was applied for 20 h.

The polymerization process was conducted in two stages. Inthe first stage, transesterification reactions took placebetween DMT and ethandiol under nitrogen flow at atmos-pheric pressure and in a temperature range of 160–180 �C.The ethandiol was used in 50 mol % excess over thedimethyl ester. The methanol formed during the transestrifi-cation was distilled off and collected. The first stage of thepolymerization process was finished when the amount offormed methanol was close to the theoretical one. Then, thesecond stage was begun: the pressure was gradually lowered

to about 0.1 hPa and the polycondensation was carried outat temperature of 275 �C and under continuous stirring (stir-rer speed 40 min�1). The progress of the polymerizationwas monitored by measuring the changes of viscosity of thepolymerization mixture, that is, an increase in torque stirrervalues during the polycondensation. The reaction was con-sidered complete when the viscosity of the system increasedto 14 Pa s. The obtained polymer/nanocomposite wasextruded from the reactor under nitrogen flow in the formof polymer wire. Subsequently, it was injection-molded usinga Baby Plast model 6/10 (Cronoplast S.L. Comp) injectionmolding machine into long pieces with a rectangular crosssection of 2 � 4 mm2. The injection-molding parameterswere as follows: injection pressure 50 bar, melt temperature265–279 �C, mold temperature 26 �C, hold time 10 s, andcool time 15 s. Nanocomposite films with thickness of about0.2 mm were obtained by compression molding at 260 �Cfor 3 min. Two cooling procedures were followed: (i)fast cooling (FC) by quenching in ice water and (ii) slowcooling (SC) under constant pressure of 5 bar at cooling rateof �65 �C min�1.

Characterization of PET/EG NanocompositesThe inherent viscosity [g] of the polymers was determinedat 30 �C using a capillary Ubbelohde type Ic (K ¼ 0.03294),according to the procedure described elsewhere.9,10 Toensure that the inherent viscosity will not be affected bypresent EG, the polymer nanocomposite solution was filteredthrough 0.2-lm pore size polytetrafluoromethylene filter(Whatman; membrane type TE 35). After filtration, the poly-mer was precipitated and redissolved. The concentration ofthe polymer in phenol/1,1,2,2-tetrachloroethane mixture(60/40 by weight) was 0.5 g/dl. The Mark–Houwink rela-tionship ½g� ¼ 2:29� 10�4 �M0:73

v was used to calculate theviscosity average molar mass of PET homopolymer.11 Thenumber average molar mass (Mn) and weight average molarmass (Mw) were determined by size exclusion chromatogra-phy using Waters GPC instrument equipped with a ShimadzuLC-10AD pump, a WATERS 2414 differential refraction indexdetector (at 35 �C) and a MIDAS auto-injector (50 mL injec-tion volume). 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) wasused as an eluent at a flow rate of 1.0 mLmin�1. PSScolumns (2 � PFG-lin-XL, 7 mm, 8 � 300 mm2) were used.Molar mass and dispersity were calculated relative to PMMAstandards (Polymer Standards).

Morphology and NanostructureThe structure of nanoparticles and nanocomposites wasobserved by scanning electron microscopy (SEM) (JEOL JSM6100 SEM). The samples were cryofractured in liquid nitro-gen, and then vacuum coated with a thin gold film beforethe test. Transmission electron microscopy (TEM) (JEOLJEM-1200 EX Electron Microscope) micrograph was obtainedusing an acceleration voltage of 80 kV. For TEM measure-ments, ultrathin sections were prepared with a ReichertUltracut R ultramicrotome using a diamond knife. Wideangle X-ray scattering (WAXS) measurements were per-formed by means of a Seifert XRD 3000 h/h diffractometerusing Ni-filtered Cu Ka radiation (k ¼ 0.154 nm) at a

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scanning speed of 0.02�/s. Raman spectroscopy experimentswere performed by using a Renishaw Raman InVia ReflexSpectrophotometer, with excitation at 785 nm (diode laser),and a resolution of 2 cm�1.

Broad Band Electrical ConductivityCircular gold electrodes (10 mm or 20 mm in diameter)were deposited by sputtering the metal onto both free surfa-ces of the sample film. The complex permittivity e� ¼ e0 � ie00,where e0 represents the permittivity and e00 the dielectricloss, was measured as a function of frequency (10�1 Hz < F< 106 Hz, being F the frequency of the applied electric field)and temperature (�150 �C up to 150 �C) by using a Novo-control broadband dielectric spectrometer. Temperature con-trol was obtained by a nitrogen jet (QUATRO from Novocon-trol) with a temperature error, during every single sweep infrequency, of 0.1 K. Electrical conductivity was derived byrðFÞ ¼ e02pFe00 where e0 is the vacuum permittivity.

RESULTS AND DISCUSSION

Morphology of NanocompositesEG can be obtained by exfoliating and reducing graphite ox-ide (GO) as the result of the expansion of the graphenesheets from loosely bonded graphene stacks. EG used in thiswork consists of worm-like agglomerates with a size ofabout 200 lm as revealed by SEM images (Fig. 1). Asreported elsewhere, these worm-like particles consist on flat-tened balloon-like units made themselves of elementarygraphite sheets.6 Here relatively big stacks of densely packedEG particles are present. The dispersion of EG in ethandiolproduced good exfoliation as shown in the SEM images (Fig.2) where individual EG particles, indicated by arrows, areclearly visualized embedded in the PET polymer matrix. Ithas been well documented that the effectiveness of nanoad-ditive dispersion strongly depends on both the method andthe process time. As revealed by the SEM images, the use ofa sonicator device seems to be rather effective in order tosplit the ‘‘as received’’ existing worm-like agglomerates intographite sheets and to distribute them in the entire volume

of ethandiol. SEM images also indicate that the EG nano-sheets were encapsulated by the PET matrix. This suggests astrong interaction between graphene sheets and PET matrix.

To further investigate the dispersion of EG in the PET matrix,TEM measurements were also performed. Figure 3 showsthe microstructure of PET nanocomposites with 0.05 and 0.2wt % of EG. Figure 3(a, b) shows the presence of single gra-phene layers or graphene stacks. Figure 3(c) shows that the

FIGURE 1 Scanning electron micrograph (SEM) of expanded

graphite as received. Here, relatively big stacks of densely

packed EG particles forming worm-like aggregates6 are

visualized.

FIGURE 2 Scanning electron micrographs (SEM) of PET/EG

nanocomposites with EG content of: (a) 0.05 wt %, (b) 0.2 wt

%, and (c) 0.4 wt %. Selected EG particles are indicated by the

arrows.



graphene layers, visualized as dark flakes, seem to be homo-genously dispersed in the PET matrix with a small amountof agglomeration. The nanosheets appear to be completelyembedded in the polyester matrix indicating an exfoliatedstructure. The high dispersion level of EG sheets can beattributed to a strong interaction between some residual po-lar groups on the surface of graphene and the polar groupsof PET.

This assumption can be corroborated by Raman Spectros-copy experiments. Figure 4 shows the Raman spectra of theEG sample compared with oxidized graphite, GO, its precur-sor. GO exhibits the characteristic bands of carbon-basedmaterials,12 namely the disorder-induced D-band, at �1350cm�1, and the G-band at higher wave number values. Theprominent D-band in GO is consistent with the existence of agreat amount of carboxylic or hydroxyl groups present in thegraphene surface. In contrast, EG exhibits a decrease in the

D-peak intensity as a consequence of the reduction processon GO that leads to EG, its conducting form. Nevertheless theless intense, although detectable, D-band indicates a residualamount of surface groups, which may be responsible for anenhanced interaction with the PET matrix. This effect sug-gests certain level of compatibility between the graphenesurface and the PET matrix enabling an appropriate EG dis-persion, which facilitates the formation of a compact contin-uous network of EG particles throughout the PET matrix asobserved by SEM [Fig. 3(c)].

Characterization of PET/EG NanocompositesThe molar mass of PET during the synthesis was controlledby monitoring the maximum viscosity that can be obtainedprevious to the extrusion the polymer from the reactor. Inour method, the maximum viscosity of the system was set to14 Pa s (at a constant temperature of 275 �C and at a stirrerspeed of 40 min�1). PET prepared by polymerization withoutEG had a number-average molar mass of about 19500 g/mol. Chen et al.13 reported that, in polymeric nanocompo-sites, the viscosity increases only after a certain range of vol-ume fraction implying that at low volume fraction (less than0.4 vol %), nanofluids have lower viscosity than correspond-ing base fluid, due to lubricating effect of nanoparticles.Accordingly, the molar mass of PET is expected to be de-pendent on the EG content in the reaction mixture. In Table1, the values of intrinsic viscosities and molar masses of PETand PET-EG nanocomposites are presented. Both the intrinsicviscosities and molar masses of PET nanocomposites with anEG content lower than 0.2 wt % was higher than those ofneat PET. Thus, the results in Table 1 suggest that eventhough the viscosity of reaction mixture increased with EGcontent, for EG loadings below 0.2 wt %, the viscosity waslower than the viscosity of reaction mixture without EGnanofiller, in agreement with Chen et al. observation.13 Themolar mass of PET prepared in the presence of the higheststudied EG content, 0.4 wt %, was 15,600 g/mol. PET withFIGURE 3 Transmission electron microscopy of PET/EG nano-

composites with: (a) 0.05 wt % of EG; � 75,000, (b) 0.2 wt % of

EG; � 75,000, (c) 0.2 wt % of EG; � 25,000.

FIGURE 4 Raman spectra of expanded (EG) and oxidized

graphite (GO). Raman intensities have been normalized to that

of the G band.


POLYMER SCIENCE


this molar mass still exhibits suitable rheological propertiesfor extrusion. For the sake of comparison, let us rememberthat textile fiber-grade PET normally has a number-averagemolar mass ranging from 15,000 to 20,000 g/mol, whichcorresponds to an intrinsic viscosity14 between 0.55 and0.67 dL/g. Moreover, it is worth to mention that the polydis-persity values (Mw/Mn) remain at the same level as those ofneat PET.

Nanostructure of the PET/EG NanocompositesFigure 5 shows the WAXS diffractograms of PET/EG nano-composites for selected EG weight % concentrations submit-ted to either FC [Fig. 5(a)] of SC [Fig. 5(b)]. Although FCnanocomposites were essentially amorphous, for SC nano-composites a significant crystallization of the PET matrixwas detected by the appearance of Bragg maxima associatedwith the triclinic crystalline phase of PET.15 In a firstapproach, this effect must be ascribed to the differentquenching process. It is noteworthy that the weak but de-tectable appearance in the FC sample with 0.4 wt % EG ofthe diffraction maxima (�26�) related to the spacing of thegraphite layers in EG. The weakness of this diffraction canbe attributable to two reasons. First, the amount of EG inthe samples is very small, and second, EG contains a signifi-cant fraction of exfoliated graphite. Let us remember thatgraphene should not exhibit this Bragg peak because in thepure graphene no stacks are present. Mass crystallinity wasestimated from the diffraction patterns by following the pro-cedure described elsewhere16,17 and the corresponding val-ues are shown in Table 1. As one can see, the crystallinityincreases with EG content indicating a nucleating effect ofthe nanoadditive.

Electrical ConductivityEG is very attractive for its potential to increase the electri-cal conductivity of insulating polymers at very low concen-tration. In our case,the electrical conductivity of the samplesexhibited a distinct behavior depending on the coolingprocedure. Figure 6 shows the electrical conductivity for thenanocomposites with different amount of EG as a function offrequency for samples submitted to either FC [Fig. 6(a)] orSC [Fig. 6(b)]. The results displayed in Figure 6(a)have been

taken with electrodes of 20 mm in diameter because weobserved that reproducibility of the conductivity results wasimproved under these conditions. Probably the existence of apolymer passive layer in the surface of the nanocompositefilms insulates the metal electrode from the conducting net-work. The probability of contacting the electrode with thenetwork increases with the area of the electrode. In princi-ple, the electrical conductivity of a material should follow alaw as r(F) ¼ rdc þAFs where the presence of a frequencyindependent component, rdc, is characteristic of a conductingbehavior with a significant direct current (dc) conductivity.This general behavior, referred to as the universal dynamic

TABLE 1 Intrinsic Viscosity and Molar Mass of PET Prepared in the Presence of Various EG Loading

Sample [g] dL/g Mv � 104 g/mol Mw � 104 g/mol Mn � 104 g/mol Mw/Mn Xc (%)

PET 0.536 4.13 4.69 1.95 2.41 12

PET/EG 0.025 wt % 0.601 4.82 5.28 2.40 2.21 –

PET/EG 0.05 wt % 0.556 4.34 4.95 2.03 2.44 –

PET/EG 0.075 wt % 0.548 4.26 4.56 1.88 2.43 11.9

PET/EG 0.1 wt % 0.550 4.30 – – – 13.5

PET/EG 0.2 wt % 0.513 3.86 4.17 1.69 2.47 30.3

PET/EG 0.4 wt % 0.501 3.77 3.85 1.56 2.47 34.7

PET2 0.450 3.25 2.98 1.15 2.60 –

Last column includes mass crystallinity values, Xc, for the slow cooled (SC) samples as estimated by wide angle X-ray scattering. Mv, viscosity aver-

age molar mass; Mw, weight average molar mass; Mn, number average molar mass; Mw/Mn dispersity; PET2, additionally synthesized neat PET with

lower molar mass.

FIGURE 5 Wide angle X-ray scattering patterns of selected

PET/EG nanocomposites submitted either to FC or to SC. The

arrow in the upper panel indicates the maximum associated to

the EG nanoadditive. The Bragg maxima have been labeled

according to the Miller indexes of the triclinic unit cell.15



response, was proposed by Jonscher18 and has beenobserved for a great variety of composite materials based oncarbon-black,19 carbon-nanofibers,5 and carbon-nanotubes20

among others. FC nanocomposites with EG content higherthan 0.05 wt % exhibit an absence of frequency dependenceindicating the presence of a rdc contribution significantlyhigher than that of the polymer matrix. For EG content lowerthan 0.05 wt %, the conductivity follows a r(F) � Fs withs ¼ 1 what is characteristic of an insulating material withabsence of rdc component in the measured frequency range.

A qualitatively similar behavior is observed for the EG nano-composites prepared by SC [Fig. 6(b)]. Surprisingly, the sam-ple with 0.075 wt % of EG exhibited the higher conductivity.This effect can be visualized in Figure 7, which shows rdc

values as a function of the EG concentration for both fastand slow cooled nanocomposites.

For the sake of comparison, a value of r(F) taken at F ¼ 0.1Hz has been considered for the nonconducting samples. Themost obvious feature is that FC nanocomposites exhibitedsignificantly higher conductivity values than the slow cooledones. In both cases, a characteristic percolative behavior wasobserved. Initially, for low concentrations, the conductivity

remained at the same level as the insulating PET matrix. Ata certain critical concentration around 0.05 wt %, the con-ductivity started a sudden increase. The dc conductivityabove the critical concentration of nanoadditive, /c, can beanalyzed in terms of the percolation theory21 by means ofthe standard scaling law given by rdca(/ � /c)

t, where t is acritical exponent. Although /c depends on the lattice inwhich particles are accommodated, the critical exponent tdepends only on the dimensionality of the system.21 Thislaw can be applied upon considering both volume or weight% concentrations.22 Theoretical calculations, supported by agreat amount of experimental observations, propose valuesof t between 1.6 and 2 for three-dimensional systems.21 Thefitting of the percolation equation to the experimental datais represented in Figure 7 by the continuous lines. For theSC series, the 0.075 wt % EG sample was excluded from thefit. This analysis provides t values of around 1.7 and /c ¼0.05 for the fast cooled samples and of t ¼ 3.5 and with /c

¼ 0.05 for the slow cooled nanocomposites, respectively.Although the t-value for the FC nanocomposites was wellwithin the expectations of the percolation theory, the one forthe SC nanocomposites was much beyond. As mentionedbefore, the SC sample with 0.075 wt % of EG exhibitedhigher conductivity than the rest of the SC nanocompositesand has not been considered for the fitting. In a firstapproach, we attribute this effect to the crystallinity of thenanocomposite which is for this sample of �12% beinglower than those of the rest SC samples. As a matter of fact,the conductivity values for the 0.075 wt % sample are veryclose for both SC and FC. Percolative behavior in compositematerials with high values of the critical exponents was fre-quently reported in the literature.23–25 According to percola-tion theory, the insulator-conductor transition occurred atthe critical concentration at which an infinite cluster of con-nected particles appeared. Two particles are usually consid-ered to be connected when they are in physical contact. Inthis framework, a homogeneous distribution of isotropic

FIGURE 6 (a) Alternating current conductivity, r(F) as a func-

tion of frequency (F) for EG-PET nanocomposites with different

EG concentrations: 0 (n), 0.025 (*), 0.05 (D), 0.075 (^), 0.1 (^),

0.2 (l), and 0.4 (&) wt %, (a) fast cooling samples and (b) slow

cooling samples.

FIGURE 7 Logarithm of the dc electrical conductivity versus

nanoadditive weight concentration for the fast (FC, n) and

slow (SC, &) cooled nanocomposites. The continuous lines are

the predictions of percolation theory.


POLYMER SCIENCE


distribution is considered. Values of t-exponent higher thanthose expected by percolation theory was found in compos-ite materials in which tunnelling conduction was present25

or where anisotropy effects were significant.23 In the latercase, large apparent conductivity exponents should be asso-ciated to anisotropic conducting heterogeneities in an insu-lating medium. In the present case, SC nanocomposites couldpresent two sources of anisotropy induced effects. On theone hand, cooling was performed at such slow rate that crys-tallization of the PET matrix was possible [Fig. 5(b)]. On theother hand, the cooling was accomplished under a certainpressure. Considering the anisotropic nature of the EG nano-additive [Fig. 5(b)], one can suppose that the pressure couldpromote an orientation of the EG platelets imparting certainlevel of anisotropy unlike the case in which FC at zero pres-sure was used. Accordingly, we proposed the semicrystallinematrix and/or the effect of pressure as the main reasons forthe difference in the percolative behavior between SC and FCPET/EG nanocomposites. Because the crystallinity of the SCsamples varies with the concentration, the deviation of the0.075 wt % EG sample from the percolative trend can beattributed to the low level of crystallinity of this samplewhich approaches its value to that of the FC one.

Another aspect to be elucidated is about the nature of theconduction mechanism in PET/EG nanocomposites. To godeeper into this issue, r(F) was measured as a function oftemperature for selected samples whose rdc conductivitylied above the percolation threshold. Figure 8 shows rdcdata represented as a function of the reciprocal temperature.In all cases, rdc increased with temperature exhibiting tem-perature-activated character. It is noteworthy that FC nano-composites exhibited a clear discontinuity in rdc upon pass-ing the Tg of the polymer matrix (70 �C) (Fig. 8 verticaldashed line), whereas SC did not. Because in FC nanocompo-sites, the polymer matrix was in the amorphous state, it

could be expected that crystallization took place once Tg wasovercome.26 Thus, the decrease of rdc at T > Tg observed foramorphous FC nanocomposites is consistent with the lowerconductivity levels of SC nanocomposites, which are essen-tially semicrystalline [Fig. 5(b)].

The obtained here percolation thresholds for amorphous andsemicrystalline PET/EG nanocomposites prepared by in situpolymerization are much lower than those presented previ-ously for PET/graphene nanocomposites prepared by meltcompounding.27,28 Li and Jeong28 have shown that the elec-trical conduction path of graphene sheets in PET/EG nano-composites prepared by melt-compounding is formed at �5wt % EG. The presence of carbon nanofillers has the influenceon electrical conductivity of polymer-based nanocomposites,but also tribological29,30 and mechanical10,28,30 properties canbe affected. Further investigation on the effect of grapheneloading on the mechanical properties of PET nanocompositesis in progress and will be published elsewhere.

CONCLUSIONS

By applying an intensive dispersion process consisting of theultrasonication and subsequent ultrahigh-speed stirring ofEG in monomer (ED) followed by in situ polymerization, it ispossible to prepare PET/EG nanocomposites with high exfo-liation degree of graphite nanosheets. With the increase ofEG content, the electrical conductivity of the PET/EG compo-sites shows a transition from an insulator to a conductor.The obtained nanocomposites exhibit conducting behaviorwith a low percolation threshold of �0.05 wt %. A high elec-trical conductivity of 10�3 S/cm of PET nanocomposites wasachieved with only 0.4 wt % of exfoliated EG. The low perco-lation threshold and relatively high electrical conductivitycan be attributed to the high aspect ratio, large surface area,and uniform dispersion of the EG nanosheets in PET matrix.Nanocomposite films obtained in the process of FC (amor-phous films) showed higher conductivity than nanocompo-sites obtained in the SC process (semicrystalline films).

ACKNOWLEDGMENTS

The authors thank the financial support from the PolishNational Science Centre and the Slovak Academy of Sciences inthe frame of ERA-NET project APGRAPHEL and from theSpanish Ministry of Science and Innovation (MICINN), GrantMAT2009-07789. Z. Spitalsky thanks for financial supportby European Reintegration Grant FP7-ERG-213085‘‘ORITUPOCO.’’ M. Soccio is indebted to the JAE-Doc programof CSIC.

REFERENCES AND NOTES

1 Yasmin, A.; Luo, J. J.; Daniel, I. M. Compos. Sci. Technol.

2006, 66, 1182–1189.

2 Viculis, L. M.; Mack, J. J.; Kaner, R. B. Science 2003, 299, 1361.

3 Viculis, L. M.; Mack, J. J.; Mayer, O. M.; Hahn, H. T.; Kaner,

R. B. J. Mater. Chem. 2005, 15, 974–978.

4 Potts, J. R.; Dreyer, D. R.; Bielawski, C. W.; Ruoff, R. S. Poly-

mer 2011, 52, 5–25.

FIGURE 8 Values of rdc as a function of the reciprocal temper-

ature for different PET/EG nanocomposites. The dashed line

indicates the Tg of the PET matrix.



5 Linares, A.; Canalda, J. C.; Cagiao, M. E.; Garcia-Gutierrez, M.

C.; Nogales, A.; Martin-Gullom, I.; Vera, J.; Ezquerra, T. A. Mac-

romolecules 2008, 41, 7090–7097.

6 Celzard, A.; Mareche, J. F.; Furdin, G. Prog. Mater. Sci. 2005,

50, 93–179.

7 Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K.

M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.;

Ruoff, R. S. Nature 2006, 442, 282–286.

8 Cowie, J. M. G. Polymers: Chemistry and Physics of Modern

Materials; Taylor & Francis Group, 2008.

9 Szymczyk, A. Eur. Polym. J. 2009, 45, 2653–2664.

10 Szymczyk, A.; Roslaniec, Z.; Zenker, M.; Garcia-Gutierrez, M.

C.; Hernandez, J. J.; Rueda, D. R.; Nogales, A.; Ezquerra, T. A.

Express Polym. Lett. 2011, 5, 977–995.

11 Samperi, F.; Puglisi, C.; Alicata, R.; Montaudo, G. Polym.

Degrad. Stab. 2004, 83, 3–10.

12 Araujo, P. T.; Terrones, M.; Dresselhaus, M. S. Mater. Today

2012, 15, 98–109.

13 Chen, L.; Xie, H. Q.; Li, Y.; Yu, W. Thermochim. Acta 2008, 477,

21–24.

14 Scheirs, J.; Long, T. E. Modern Polyesters; Wiley, 2003.

15 Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.;

John Wiley & Sons: New York, 1989.

16 Blundell, D. J.; Osborn, B. N. Polymer 1983, 24, 953–958.

17 Denchev, Z.; Nogales, A.; Ezquerra, T. A.; Fernandes-Nasci-

mento, J.; Balta-Calleja, F. J. J. Polym. Sci. Part B: Polym.

Phys. 2000, 38, 1167–1182.

18 Jonscher, A. K. Nature 1977, 267, 673–679.

19 Connor, M. T.; Roy, S.; Ezquerra, T. A.; Calleja, F. J. B. Phys.

Rev. B 1998, 57, 2286–2294.

20 Nogales, A.; Broza, G.; Roslaniec, Z.; Schulte, K.; Sics, I.;

Hsiao, B. S.; Sanz, A.; Garcia-Gutierrez, M. C.; Rueda, D. R.;

Domingo, C.; Ezquerra, T. A. Macromolecules 2004, 37,

7669–7672.

21 Stauffer, D. Introduction to Percolation Theory; Taylor &

Francis: London, 1987.

22 Sandler, J. K. W.; Kirk, J. E.; Kinloch, I. A.; Shaffer, M. S. P.;

Windle, A. H. Polymer 2003, 44, 5893–5899.

23 Carmona, F.; Prudhon, P.; Barreau, F. Solid State Commun.

1984, 51, 255–257.

24 Quivy, A.; Deltour, R.; Jansen, A. G. M.; Wyder, P. Phys.

Rev. B 1989, 39, 1026–1030.

25 Ezquerra, T. A.; Kulescza, M.; Cruz, C. S.; Baltacalleja, F. J.

Adv. Mater. 1990, 2, 597–600.

26 Alvarez, C.; Sics, I.; Nogales, A.; Denchev, Z.; Funari, S. S.;

Ezquerra, T. A. Polymer 2004, 45, 3953–3959.

27 Zhang, H.-B.; Zheng, W.-G.; Yan, Q.; Yang, Y.; Wang, J.-W.;

Lu, Z.-H.; Ji, G.-Y.; Yu, Z.-Z. Polymer 2010, 51, 1191–1196.

28 Li, M.; Jeong, Y. G. Compos. A 2011, 42, 560–566.

29 Brostow, W.; Keselman, M.; Mironi-Harpaz, I.; Narkis, M.;

Peirce, R. Polymer 2005, 46, 5058–5064.

30 Brostow, W.; Broza, G.; Datashvili, T.; Hagg Lobland, H. E.;

Kopyniecka, A. J. Mater. Res. 2012, 27, 1815–1822.


POLYMER SCIENCE


Electrical conductivity of poly(ethylene terephthalate)/expanded graphite nanocomposites prepared by in situ polymerization

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