Rheological percolation in polystyrene composites filled with polyaniline-coated multiwall carbon nanotubes

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Synthetic Metals 194 (2014) 109–117

Contents lists available at ScienceDirect

Synthetic Metals

jo ur nal home p age: www.elsev ier .com/ locate /synmet

heological percolation in polystyrene composites filled witholyaniline-coated multiwall carbon nanotubes

li Sarvi, Uttandaraman Sundararaj ∗

epartment of Chemical & Petroleum Engineering, University of Calgary, 2500 University Dr. NW, Calgary, AB, Canada T2N 1N4

r t i c l e i n f o

rticle history:eceived 19 February 2014eceived in revised form 3 April 2014ccepted 29 April 2014vailable online 22 May 2014

eywords:ercolation thresholdANi coating

a b s t r a c t

Polyaniline (PANi)-coated multiwall carbon nanotubes (MWCNTs) and uncoated MWCNTs were addedto a polystyrene (PS) matrix using solution and melt mixing. Electrical and rheological measurementswere carried out on polymer composites to study the mechanism of conductive network formation. Thiswork reveals the effect of nanofiller’s coating on network formation using both rheological and electricaltechniques. PANi-coated MWCNTs exhibit a much lower electrical percolation threshold (0.4 wt%) thanuncoated MWCNTs (0.7 wt%), due to better dispersion of PANi-coated MWCNTs in a PS matrix. Disper-sion of nanofillers in a polymer matrix was studied using optical microscopy and differential scanningcalorimetry (DSC). Rheological characterizations of composite samples revealed a percolation behavior at

′ ′′

lectrical conductivityanofiller dispersion

higher concentration than that found electrical percolation. The inverse of damping factor (G /G ) was thebest parameter to elucidate the percolation threshold in a polymeric composite (i.e. PANi-coated MWCNTin PS had electrical percolation of 0.40 wt% and rheological percolation 0.52 wt%). A better affinity of PANiwith PS led to better dispersion of nanofillers and lower rheological percolation for PS/MWCNT-PANicomposites than those seen for PS/MWCNT composites.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Electrically conducting polymer composites, due to their cor-osion resistance, light weight, low cost and ease of processing,ave received significant attention for the replacement of metalsnd inorganic materials for sensors, actuators, supercapacitors andlectromagnetic interference (EMI) shields [1–5]. High electricalapacitance makes these composites suitable for use as embeddedapacitors and supercapacitors [6–11]. Composites with tunableonductivity may be used as chemisensors, as well as differentypes of electronic pressure sensors and switches [11–14].

Multiwall carbon nanotubes (MWCNTs) are high aspect ratioanofillers that demonstrate high conductivity, excellent mechan-

cal properties and corrosion resistance. Different kinds ofonductive nanofillers, such as MWCNTs, metal nanowires andraphene, can be used to make conductive composites. Among

onductive nanofillers, MWCNTs have been most extensivelynvestigated, since they provide high electrical conductivity in

∗ Corresponding author. Tel.: +1 403 220 5900/+1 403 220 5751;ax: +1 403 284 4852.

E-mail addresses: [email protected] (A. Sarvi),[email protected] (U. Sundararaj).

ttp://dx.doi.org/10.1016/j.synthmet.2014.04.031379-6779/© 2014 Elsevier B.V. All rights reserved.

polymer composites at low filler loading and thus, are very relevantto industry [2,15].

Polymer composites, through a gradual addition of MWCNTs,begin to form a long-range connectivity which is called percolationthreshold. Below this threshold, no connected network exists in anypart of the composite, while above this threshold, a giant networkon the order of the system size appears. Dispersion of nanofiller in apolymer matrix is an important parameter to decrease percolationconcentration to its minimum value. Other parameters that affectpercolation concentration are filler shape, density and aspect ratio.High aspect ratio MWCNT, with excellent electrical conductivity,is an outstanding candidate to achieve a conductive composite ata relatively low electrical percolation concentration. However, dueto high van der Waals interactions between nanotubes, significantagglomeration occurs and good dispersion is hard to attain withoutsurface functionalization and/or coating [16–18].

In this study, MWCNTs were coated with polyaniline, which is anintrinsically conductive polymer with conductivity of up to 10 S/cm(in emeraldine salt form). Coating helped create a better disper-sion of MWCNTs in the PS matrix due to a lower interfacial energyof PANi with PS (calculated as 4.72 dyne/cm and 2.44 dyne/cm

using harmonic and geometric mean equations, respectively) com-pared to interfacial energy of MWCNTs with PS (calculated as13.76 dyne/cm and 7.33 dyne/cm calculated using harmonic andgeometric mean equations, respectively) [19–22].

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110 A. Sarvi, U. Sundararaj / Synthetic Metals 194 (2014) 109–117

CNT (

pautocsemFec(i

m[vlpop

Fig. 1. (A) uncoated MW

The effect of PANi coating on electrical conductivity was com-ared with uncoated MWCNTs mixed with PS, using both solutionnd melt mixing. One of the key objectives of this study was tose rheological techniques to monitor the rheological behavior ofhe polymer composite as we increased the filler loading. Rhe-logical percolation thresholds were characterized by a gradualhange in the material mechanical response from a liquid-like to aolid-like behavior, which is a consequence of the microstructuralvolution of the system [23,24]. One major feature of rheologicaleasurements is their high sensitivity to microstructural changes.

or example, rheology can be used for the measurement of thearly stage of network formation. One way to rheologically monitorrystallization is by means of a dynamic mechanical spectroscopyDMS). In this case, small amplitude oscillatory shear (SAOS) flows applied to the samples with different filler loading.

A more advanced use of SAOS experiments involves the deter-ination of the so-called critical gel point. Winter and co-workers

24] were the first to recognize that polymer crystallization can beiewed as a physical gelation process, where the transition between

iquid-like and solid-like behavior takes place at the critical geloint. Within this framework, we employed physical gelation the-ry to determine MWCNT network properties at its rheologicalercolation concentration.

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Fig. 2. Conductivity of solution-mixed PS/CNT80-PANi20, solution m

B) PANi-coated MWCNT.

2. Experimental

PANi polymer and CNT surfaces do not have a strong affin-ity for each other. Core-shell nanofibers, therefore, cannot becreated by solution mixing. Rather, in situ polymerization of ani-line monomers in the presence of MWCNTs was carried out tocreate core-shell nanofibers [25]. The MWCNTs were obtainedfrom NanocylTM, product no. NC7000, with an average diameterof 9.5 nm, average length of 1.5 �m and specific surface area of250–300 m2/g. Fig. 1 shows the TEM micrographs of MWCNTs andPANi-coated MWCNTs.

2.1. Solution mixing method

After making the MWCNT-PANi core-shell nanofibers, the nextstep was to disperse the core-shell nanofibers into polystyrene (PS)using a solution method [25,26]. The same method was employedfor making PS composites filled with uncoated MWCNTs. PS wasused because of the excellent electrical and mechanical propertiesof its composites with conductive nanofillers. Core-shell nanofibers

were dispersed in dimethylformamide (DMF) which was used asthe medium for mixing with PS. Sonication was applied for an hourto ensure a good dispersion. PS (America Styrenics Styron 666D,

32.521.5NT wt%

MWCNT80-PANi20 (solu�on)

Solu�on Mixing

Melt mixing

ixed PS/CNT and melt mixed PS/CNT as a function of MWCNT.

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thetic Metals 194 (2014) 109–117 111

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A. Sarvi, U. Sundararaj / Syn

I = 7.5, Mw = 200,000 g/mol and Tg = 100 ◦C) was also dissolved inMF in a separate beaker. The nanofibers/DMF mixture was added

o the PS/DMF solution, and the final mixture was magneticallytirred to obtain a homogeneous dispersion of the nanofibers.

Methanol was added as a non-solvent to the mixture and the PSomposite powder precipitated out of solution. The composite washen dried in a vacuum oven at 80 ◦C for 24 h. Compression mold-ng was used to make 1 mm thick samples of the composites forlectrical property measurements. All the samples were preheatednd pressed at 250 ◦C, under a pressure of 40 MPa, for 20 min.

.2. Melt mixing method

Samples were melt-mixed using a novel custom-built miniatureixer: the Alberta Polymer Asymmetric Mixer (APAM) [20].Thisixer has a capacity of about 2 ml. PS granules were dry-mixedith MWCNTs, then melt-mixed in the APAM at 250 ◦C for 15 min.

he rotor speed was set at 80 rpm. Melt-mixed samples wereompression-molded at the same condition of solution-mixed sam-les.

.3. Characterizations and measurements

The electrical resistivity measurements were done on 1 mmhick rectangular samples. All the sample surfaces were cleanedith ethanol prior to measurement.

Two different resistivity measurement machines were used toover the range of volume resistivity from insulating materials toonducting materials. Volume resistivity measurements were per-ormed according to ASTM 257-75 standards, employing a LorestaP resistivity meter (MCP-T610 model, Mitsubishi Chemical Co.,

apan) for the samples with volume resistivity less than 104 � cm. four-pin probe was used, so that the effect of contact resistanceid not distort the measurement. To measure the volume resisti-ity of materials with resistivities higher than 104 � cm, a HirestaP resistivity meter and UR type probe were used at 100 V.

Rheological measurements were performed using a strain/stressontrolled rheometer, MCR-302 Anton Paar. Disk-like samples,ith 25 mm diameter and 1 mm thickness, were placed between

he cone and plate of the rheometric fixture, with 25 mm diame-er and angle of 1◦. Applied strain was set to be constant at 0.1%.requency sweeps from 0.1 to 300 Hz were performed to measureoss and storage modulus. The measurement temperature was sett 250 ◦C.

The optical micrographs were obtained using an Olympusicroscope fitted with a Mettler Toledo hot stage and connected to

Olympus DP80 digital camera. The differential scanning calorime-ry (DSC) tests were carried out on a TA Instruments Q100, using aeating and cooling rates of 10 ◦C/min.

. Result and discussion

The conductivity measurements of the PS matrix filled withifferent concentrations of PANi-coated MWCNTs and uncoatedWCNTs are plotted in Fig. 2. A significant decrease in the electri-

al percolation concentration is obtained for MWCNT80-PANi20/PSomposites (0.4 wt%), when compared to the solution-mixedWCNT/PS composites (0.7 wt%) and melt-mixed MWCNT/PS

omposites (0.8 wt%). The lower percolation threshold of solution-ixed samples, compared with melt-mixed samples, is due to

etter dispersion of carbon nanotubes for composites made usingolution mixing method. Surprisingly, the percolation threshold

or polystyrene composites filled with PANi-coated MWCNTs is

uch lower than the two other samples. Our earlier investiga-ions showed that the coating layer helped with better dispersionf MWCNTs [25]. Since PANi is an intrinsically conductive polymer,

Fig. 3. Optical Micrographs of (A) Solution-mixed PS/MWCNT80-PANi20, (B)solution-mixed PS/MWCNT and (C) melt mixed PS/MWCNT. In all cases, concen-tration is 0.5 wt% of nanofillers.

the coating layer filled the insulative gap between MWCNTs at theircontact spots, which significantly reduced the contact resistanceand enhanced the conductivity of polymer composite.

Optical micrographs of three different types of polymer com-

posites support the electrical conductivity results. Fig. 3A showsthat PANi coated-MWCNTs are well-dispersed and well-distributedin polymer. Nanofiller clusters, with dimension size of onehundred nanometers, were able to form a conductive network

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80

84

88

92

96

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rans

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Tem

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MWCNT wt%

Melt

Solu�on

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ee sam

(mt

mnawpmscacAlfrcd

dtmattaTtipbantfro

Fig. 4. Glass transition temperature for thr

� = 6.29 × 10−4 S/m) at 0.4 wt% of nanofiller loading. Solution-ixed and melt-mixed samples without PANi were insulative at

his filler level.Fig. 3B shows bigger clusters of MWCNTs in the range of a few

icrons to hundreds of micrometers. The ultrasonication tech-ique, employed during the solution mixing process, resulted in

good distribution of MWCNTs. Fig. 2C shows that there is aeak dispersion and also a weak distribution of MWCNTs in theolymer matrix. As a result, the percolation threshold for melt-ixed samples was significantly higher than that for the other

amples. Decreasing percolation threshold of conductive polymeromposites is important, due to the cost and weight of product,nd processability of the compound. A low percolation thresholdan be achieved by properly dispersing and distributing nanofillers.

perfect distribution, however, does not mean a minimized perco-ation threshold. In the case of conductive polymer composites, theormation of only a few paths is sufficient to transport charge car-iers through the sample. A designed structure inside the polymeromposite, with an optimum amount of nanofiller, can significantlyecrease the electrical percolation concentration [27–29].

Fig. 4 shows the glass transition temperature (Tg) for threeifferent composites. As the graph indicates, the glass transitionemperature decreased as more nanofiller was added to the poly-

er particularly in solution-mixed samples. Once nanoparticlesre dispersed, they can interfere with unperturbed radius of gyra-ion or size of the polymer chain and increase the free volume andhus, act like a plasticizer [30]. The more nanoparticles that inter-ct with the polymer chain, the more free volume that is created.he increase or decrease in Tg of polymer, however, is related tohe type of interaction between the polymer and nanofiller. A goodnteraction leads to increased Tg; a bad interaction, as in our sam-le results in a decreased Tg. Whether the interaction is good orad, the more change in Tg, the more the interfacial interaction, as

result of higher surface areas provided by nanofillers. It is alsooticeable, from Fig. 4, that Tg for melt-mixed samples is higher

han for solution mixing and PANi-coated MWCNTs. The lower Tg

or PS samples filled with the same nanofiller loading may be aesult of nanofiller dispersion in the polymer. The better dispersionf nanofillers resulted in more interaction between nanofillers and

ple types as function of MWCNT loading.

polymer chains due to the increased area which, in turn, resultedin a lower Tg. Surface area in melt-mixed samples is very low, sothere is little to no effect on Tg.

Viscoelastic properties of PS/MWCNT80-PANi20 nanocompos-ites are presented in Fig. 5 for a range of nanofiller concentrations.Fig. 5 provides evidence that nanofiller clusters have a dramaticeffect on the rheological behavior, even at loading as low as 0.2 wt%.As the loading increases, both storage modulus and loss modulusincrease, although this increase is more evident at low frequencies.The damping factor, however, demonstrates a fundamental changein its behavior at low frequencies as loading is increased.

At 250 ◦C, at low frequencies, PS chains are fully relaxed andexhibit typical homopolymer-like terminal behavior with the scal-ing property of approximately G′ ∼ ω2 [31]. At nanofiller loadinghigher than 2 wt%, however, this terminal behavior disappeared, asthe data in Fig. 5A and B shows. As nanofiller loading increased, thepolymer composites tend to deviate further from homopolymer-like behavior. Storage modulus due to network formation byadding nanofillers became more resistant to the shear at low fre-quencies (frequency-independent). Rheological parameters at lowfrequencies are very sensitive to any physical/chemical networkin polymer. Rheological measurements have been carried out tostudy the early stage of crystallization in crystalline polymers,where relative crystallinity barely reaches 2% of the ultimate crys-tallinity [23,24]. Rheological measurements, therefore, can be usedas direct evidence to investigate the initial formation of the net-work of nanofillers. Fig. 5C indicates a change in behavior of thesample from liquid-like to solid-like as the nanofiller increased. ForPS/MWCNT80-PANi20 nanocomposite samples, it is observed thatat filler loading near 0.5 wt%, a plateau in damping factor (tan(ı))occurs at low frequencies. This plateau at low frequencies is an indi-cation of network formation in polymer composites, which can beconstrued as the rheological percolation threshold [23,24].

Similar to the electrical percolation threshold, the rheologicalpercolation threshold is correlated with power-law dependence

of the rheological parameter with the filler loading [32]. Tounderstand which rheological parameter describes the percolationbetter, storage modulus, loss modulus and inverse of damping fac-tor (G′/G′′) were plotted and are shown in Fig. 6. The normalized

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0.1

1

10

100

1000

10001001010.10.01

Dam

ping

Fac

tor,

Tan(

δ)

Frequency, ω (rad/s)

0 wt%0.2 wt%0.5 wt%1 wt%1.2 wt%2.5 wt%

C

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1.E+00

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1.E+03

1.E+04

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Stor

age

Mod

ulus

, G' (

Pa)

Frequency, ω (rad/s)

2.5 wt%

1.2 wt%

1 wt%

0.5 wt%

0.2 wt%

0 wt%

A

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

10001001010.10.01

Loss

Mod

ulus

, G"

(Pa)

Frequency, ω (rad/s)

2.5 wt%

1.2 wt%

1 wt%

0.5 wt%

0.2 wt%

0 wt%

B

Fig. 5. (A) Storage modulus, (B) Loss modulus (C) Damping factor of PS/MWCNT80-PANi20 versus frequency at different filler concentrations; T = 250 ◦C and Strain 0.1%.

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114 A. Sarvi, U. Sundararaj / Synthetic Metals 194 (2014) 109–117

0

20

40

60

80

100

1.210.80.60.40.20

Nor

mal

ized

Log

(pro

pert

y)

MWCNT wt%

Conduc� vity

G'/G"

G'

G"

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ig. 6. The log-normalized values of conductivity, inverse loss tangent, storage moncentration. The rheological parameters were taken at ω = 0.1 rad/s.

og of conductivity shows an onset of percolation before rheologi-al parameters. When conductive nanofiller approaches each other,t close distances (lower than 10 nm) electrical charges can jumprom one conductive segment to the neighbor segment. While forn rheological percolation, at low viscosities, stronger interactionuch as an chain entanglement is needed. Therefore, rheologicalercolation occurs at slightly higher concentration than the electri-al percolation. Among different rheological parameters, log (G′/G′′)s closest to the electrical percolation of nanocomposites. It can beoted that (G′/G′′) is a derived parameter that holds some physicalignificance, as compared to G′ and G′′ that are directly measured32]. Better prediction is also seen using complex viscosity values.he percolation values for the three sample types are tabulated inable 1.

Table 1 shows electrical and rheological percolation values forhe three different samples. As the results indicate, rheological per-olation follows the same trend of electrical percolation throughhe samples. From Table 1 and Fig. 6, it can be concluded that G′

s good choice to report percolation among directly measured val-es, while the inverse of damping factor (G′/G′′) is the best choiceo find the percolation threshold in a polymeric composite. Addinganofiller to polymer causes an increase in both loss and storageoduli; however, at percolation threshold, the increase in storage

odulus is more significant than the increase in loss modulus due

o the factor of network elasticity. Therefore, the ratio of storage tooss modulus (G′/G′′) is more sensitive to network formation. Elec-rical percolation happened when the first interconnected network

able 1lectrical and rheological percolation threshold for melt-mixed PS/CNT, solution-ixed PS/CNT and PS/CNT80-PANi20.

Electrical &rheologicalparameter

Melt mixedPS/MWCNT

Solution mixedPS/MWCNT

PS/MWCNT80-PANi20

� [S/m] 0.8 0.7 0.4G′/G′′ 0.94 0.89 0.52G′ [Pa] 0.96 0.92 0.57G′′ [Pa] 1.02 0.95 0.59�* [Pa s] 0.99 0.94 0.51

s and loss modulus of PS/MWCNT80-PANi20 composites as function of MWCNT

formed. The initial conductive network was weak and brittle, whichwould be out of the sensitivity range of the rheometer. Thus, aspreviously mentioned, rheological percolation happens at slightlyhigher concentration than the electrical percolation.

Differences in rheological responses with the addition ofnanofillers can be observed in the graphs of complex viscosity (�*)versus complex modulus G*. As Fig. 7a shows, a plot of �* vs G*

begins to diverge at concentrations higher than 1 wt%. Rheologicalpercolation for melt-mixed PS/MWCNTs, therefore, occurs some-where between 0.5 wt% and 1 wt% of nanofiller loading. At a fillerloading below 0.5 wt%, a plateau in (�*) at low G* shows a Newto-nian behavior at low frequencies. For solution-mixed PS/MWCNTsand PS/MWCNT80-PANi20 samples, in Fig. 7b and c, complexviscosity deviates from Newtonian behavior at even lower fillerconcentrations. Since G* increases with frequency, we can say thatcomplex viscosity in concentrated samples sharply decreased asfrequency (i.e. increase in G* in Fig. 7) increased. High loading ofnanofillers allowed a strong network to form, which then failedat high frequencies resulting in a sharp drop in viscosity. In otherwords, composites showed a shear-thinning type of behavior.

To study the stiffness of the network, the MWCNT network canbe considered as a physical gel. The infinite network at the gel pointis revealed in a strong coupling of the relaxation modes over awide range of size scales. A power-law relaxation modulus in theterminal zone is seen [23,24]:G′(ω) = St−nwhere t is time, S is thegel stiffness and n is the critical relaxation exponent. Fig. 8 showsthe gel parameters for three different composites in the range ofnanofiller concentration. The decrease in the critical relaxationexponent and the increase in gel stiffness for higher nanofiller load-ing both indicate that the gel becomes harder and relaxes slowerat higher loading. This is consistent with the sharp drop in viscos-ity for an increase in nanofiller concentration (Fig. 7). Therefore,the critical gel is more resistant to the shear forces when fillerloading increased. The relaxation exponent also shows a rheolo-gical percolation via a change in slope of the n versus MWCNT

wt% which is an indication of a change in the gel formation mech-anism. The gel parameters S and n for PS/MWCNT80-PANi20 fallbetween melt-mixed PS/MWCNT and solution-mixed PS/MWCNTcomposites.

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A. Sarvi, U. Sundararaj / Synthetic Metals 194 (2014) 109–117 115

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+051.E+041.E+031.E+021.E+01

Com

plex

Vis

cosi

ty, η

* (

Pa

.s)

Compl ex Modulus, G* (Pa)

2.5 w t%1.2 wt%

1 wt%0.5 w t%

0.2 w t%0 wt%

A

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+051.E+041.E+031.E+021.E+01

Com

plex

Vis

cosi

ty, η

* (

Pa

.s)

Complex Modulus, G* (Pa)

2.5 w t%

1.2 w t%

1 wt%

0.5 w t%

0.2 w t%

0 wt%

B

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+051.E+041.E+031.E+021.E+01

Com

plex

Vis

cosi

ty, η

* (

Pa

.s)

Complex Modulus, G* (Pa)

0 wt%0.2 wt%0.5 wt%1 wt%1.2 wt%2.5 wt%

C

Fig. 7. Complex viscosity of (A) melt-mixed PS/MWCNT, (B) solution-mixed PS/MWCNT, (C) solution-mixed PS/MWCNT80-PANi20 versus storage modulus at different fillerconcentrations.

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116 A. Sarvi, U. Sundararaj / Synthetic

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Solu�onMeltMWCNT80-PANi20 (solu�on)

10

100

100 0

10000

100000

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ig. 8. (A) Relaxation exponent and (B) Gel stiffness of melt mixed PS/MWCNT,olution mixed PS/MWCNT and PS/MWCNT80-PANi20 as a function of filler con-entrations.

Gel parameter results indicate that melt-mixed samples haveow stiffness which shows a weak interaction among big clustersf MWCNTs. The gel network, therefore, has a lower resis-ance to applied shear. Fig. 8 also shows that critical gel inS/MWCNT80-PANi20 is ∼40% lower modulus and softer than thatn solution-mixed PS/MWCNT. Several possibilities for the differ-nce between rheological behavior of uncoated and PANi-coatedWCNTs can be considered. One possibility is that the coating pro-

edure reduced MWCNTs bending stiffness by creating defects [33].nother possibility is that adding the coating layer decreased thespect ratio of nanofillers (i.e. due to increased diameter, aspectatio a = l/d decreased). That is, the thickness of coating was mea-ured to be about 15 nm, which reduced the aspect ratio of MWCNTsourfold from 160 to 40 (due to increase in diameter from 9.5 nmo 40 nm). Nevertheless, the increase in steric hindrance also hin-ered the connection between MWCNT-MWCNT and MWCNT-floci.e. flocs are colloids which come out of suspension of MWCNTsn DMF) [33]. The overall elasticity of network decreased by using

WCNT coated with PANi.

. Conclusion

In this study, the effect of PANi coating on dispersion andistribution of nanofillers was investigated. It was found that a

ower percolation threshold was obtained for PANi-coated MWCNTs a result of better dispersion and also large-distance interac-ion of MWCNT-MWCNT and MWCNT-floc. Polystyrene sampleslled with PANi-coated MWCNTs and bare MWCNTs, at different

[

[

Metals 194 (2014) 109–117

concentrations, were prepared using melt mixing and solution mix-ing techniques. It was found that the PANi-coated sample exhibiteda good dispersion and low electrical percolation threshold. Thegood dispersion resulted in low rheological percolation concentra-tion, as well. Rheological percolation was measured using differentrheological parameters. The rheological percolation, defined byindirect rheological parameters such as the ratio (G′/G′′), matcheselectrical percolation behavior. Critical gel parameters revealedthat as the filler loading increased, the critical gel became harderand relaxed more slowly. It was found that due to the loweraspect ratio and weaker interaction between MWCNT-MWCNT andMWCNT-floc, the gel network for PANi-coated MWCNT is softerthan that of uncoated MWCNT.

Acknowledgment

The authors would like to acknowledge Natural Sciences andEngineering Research Council of Canada (NSERC) for their financialsupport.

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