pani wo3 metacomposites(1)

PAPER www.rsc.org/materials | Journal of Materials Chemistry

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Polyaniline-tungsten oxide metacomposites with tunable electronicproperties†

Jiahua Zhu,a Suying Wei,*b Lei Zhang,a Yuanbing Mao,c Jongeun Ryu,d Amar B. Karki,e David P. Younge

and Zhanhu Guo*a

Received 2nd July 2010, Accepted 5th October 2010

DOI: 10.1039/c0jm02090g

Polyaniline (PANI) nanocomposites reinforced with tungsten oxide (WO3) nanoparticles (NPs) and

nanorods (NRs) are fabricated via a facile surface-initiated-polymerization (SIP) method. Scanning

electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations reveal the

uniform coating of polymer on the filler surface and a good dispersion of the nanofillers within the

polymer matrix. Unique negative permittivity is observed in pure PANI and its nanocomposites. The

switching frequency (frequency where real permittivity switches from negative to positive) can be easily

tuned by changing the particle loading and filler morphology. Conductivity measurements are

performed from 50�290 K, and results show that the electron transportation in the nanocomposites

follows a quasi 3-d variable range hopping (VRH) conduction mechanism. The extent of charge carrier

delocalization calculated from VRH well explains the dielectric response of the metacomposites.

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) reveal an enhanced

thermal stability of the nanocomposites with the addition of nanofillers as compared to that of pure

PANI.

1. Introduction

Polyaniline (PANI) is one of the most promising conjugated

polymers which has been investigated for their wide potential

applications such as gas sensors,1,2 solar cells3,4 and electro-

chromic devices.5,6 With the goal of further improving the

performance, lots of efforts have been made to synthesize PANI

with different nanostructures, such as nanofibers (NFs),7,8

nanoparticles (NPs),9 and nanotubes (NTs).10 Though significant

enhancements of the material performances have been achieved

owing to their large specific surface area and unique physico-

chemical properties, they still can not meet the modern require-

ments, that materials are endowed with versatile functionalities.

More recently, researchers are getting more interested in polymer

nanocomposites (PNCs) by the incorporation of metallic or

semiconductive nanomaterials into the conductive polymers,

which may provide a system with interesting functionalities.11–13

Pillalamarri et al.14 have decorated PANI NFs with noble metals

(Au or Ag) synthesized by g radiolysis and obtained the electrical

conductivity (s) of the PNCs 50 times higher than that of pure

aIntegrated Composites Laboratory (ICL), Dan F Smith Department ofChemical Engineering, Lamar University, Beaumont, TX, 77710, USA.E-mail: [email protected]; Tel: +1 409 880-7654bDepartment of Chemistry and Biochemistry, Lamar University,Beaumont, TX, 77710, USA. E-mail: [email protected]; Tel:+1 409 880-7976cApplied Sciences laboratory, Washington State University, Spokane, WA,99210, USAdDepartment of Mechanical & Aerospace Engineering, University ofCalifornia Los Angeles, Los Angeles, CA, 90095, USAeDepartment of Physics and Astronomy, Louisiana State University, BatonRouge, LA, 70803, USA

† Electronic supplementary information (ESI) available: Supplementarymaterials. See DOI: 10.1039/c0jm02090g

342 | J. Mater. Chem., 2011, 21, 342–348

PANI fibers. Tseng et al.15 have also fabricated a nonvolatile

PANI/Au nanocomposite fiber memory device based on an

electric field induced charge transfer from PANI NFs to Au NPs.

Other metals, such as Ni16 and Pd17 are also introduced in PANI

matrix to extend the usage of such materials in various fields such

as microwave absorption.16 Semiconductive nanomaterials

(mostly, metal oxides) are also studied in the conductive PANI

owing to their unique magnetic, optical and catalytic properties,

which could have wide applications on electronics, optics, etc.

For example, a multicolor tungsten oxide (WO3)/PANI nano-

composite film is fabricated using a double-pulse electrodeposi-

tion technique and the color can be adjusted between the anodic

coloration of PANI and the cathodic coloration of WO3.18

Enhanced microwave absorption property is observed in the

PANI PNCs containing TiO2 NPs.19 Core-shell structured metal

oxide (CuO, Fe2O3, In2O3)/PANI PNCs as well as PANI hollow

capsules are also fabricated with tremendous potential

applications.20

Metamaterials are of great interest due to their unique negative

physical properties such as permittivity, which can be applied in

cloaking, superlens, wave filters and superconductors.21–23 It is

well recognized that the unusual dielectric property is arising

from the special structures rather than the composition of the

materials. Materials with special structures, such as metallic

mesostructures22 and nanostructures,24 and nano apertures25 are

widely studied for obtaining negative permittivity. WO3 has been

of great interest owing to its favorable electronic properties

including the small electrical potential difference between W(V)

and W(IV)26,27 and its potential wide applications, such as elec-

trochromic ‘‘smart’’ windows, optical modulation devices, and

gas sensors.27–29 Various WO3 nanostructures, including NPs,30

nanodisks,31 NRs,32 nanowires33 and NFs34 have been synthe-

sized. Most of the current research work concentrates on the

This journal is ª The Royal Society of Chemistry 2011

http://dx.doi.org/10.1039/C0JM02090G

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synthesis of WO3 with different nanostructures and on the

exploration of their extensive applications in electrochromic

devices, gas sensors and catalysts with photoelectronic activities.

However, the electrical and dielectric properties of PNCs filled

with WO3 are rarely reported, especially for their conjugated

PNCs.

In this work, we report on the fabrication of WO3/PANI

nanocomposites using a surface initiated polymerization (SIP)

method with different nanostructural WO3, particle and rod.

Unique dielectric properties with tunable negative permittivity

are observed at low frequencies and these polymer nano-

composites (PNCs) are termed as ‘‘metacomposites’’. The filler

morphology effects on the electrical, dielectric and thermal

properties of the metacomposites are systematically studied.

2. Experimental

2.1 Materials

Aniline (C6H7N), ammonium persulfate (APS, (NH4)2S2O8),

p-toluene sulfonic acid (PTSA, C7H8O3S), camphorsulfonic acid

(CSA) and m-cresol ($98%) are purchased from Sigma Aldrich.

All the chemicals are used as-received without any further

treatment. WO3 NPs with an average diameter of 30 nm are

obtained from Nanostructured & Amorphous Materials, Inc.

The WO3 NRs used in this study are synthesized as previously

reported.35 Typically, HCl (3 mol/L) is slowly dropped into an

aqueous solution (19 mL) of Na2WO4$2H2O (0.825 g) and NaCl

(0.290 g) with stirring until its pH value reaches 2.0. Subse-

quently, the solution is transferred into a Teflon-lined autoclave

(45 mL capacity) and hydrothermally processed at 180 �C for

24 h in an oven. After that, the white precipitate is repeatedly

washed with deionized water to obtain the WO3 NRs. The

morphologies of the WO3 NPs and NRs are shown in Fig. 1.

XRD analysis, Fig. S1, ESI,† confirms the formation of the

highly crystalline structures of the NPs and the NRs. However,

NPs are monoclinic WO334,36–39 and NRs are unorthodox struc-

tured WO3.35 High-resolution TEM observation of WO3 NRs

indicates a favorable growth direction along the (0002).35

2.2 Synthesis of WO3 NPs (NRs)/PANI nanocomposites

The WO3 NPs are initially mixed with the solution of PTSA and

APS with a fixed ratio of 60 mmol: 36 mmol in 400 ml deionized

water, following by one hour sonication in an ice-water bath. The

aniline solution (72 mmol in 100 ml deionized water, molar ratio

of PTSA:APS:aniline ¼ 5 : 3 : 6) is mixed with the above WO3

NPs suspended solution at 0 �C and then sonicated for additional

Fig. 1 SEM images of the (a) WO3 NPs and (b) WO3 NRs.


one hour in an ice-water bath for further polymerization. The

product is vacuum filtered and washed with deionized water until

the filtrate reaches pH ¼ 7. The precipitant is washed with

methanol to remove any possible oligomers. The obtained

powders are dried completely at 50 �C. PANI PNCs with

a particle loading of 5, 10, 20, and 30 wt% are fabricated. For

comparison, pure PANI is synthesized following the above

procedures without adding any nanofillers. PANI PNCs with

a rod loading of 10 wt% are also fabricated to illustrate the

nanofiller morphology effect.

2.3 Characterization

Fourier transform infrared spectroscopy (FT-IR, Bruker Inc.

Vector 22, coupled with an ATR accessory) is used to charac-

terize pure PANI and its WO3/PANI PNCs in the range of 500 to

4000 cm�1 at a resolution of 4 cm�1.

The morphology of the WO3 NPs/NRs and PNCs is charac-

terized with scanning electron microscope (SEM, JEOL field

emission scanning electron microscope, JSM-6700F). The

morphology and the nanofiller distribution in PANI are further

determined by a Philips CM-200 transmission electron micro-

scope (TEM) with a LaB6 filament. The measurement is operated

at an accelerating voltage of 120 kV. Samples for TEM obser-

vation are prepared by drying a drop of WO3/PANI nano-

composite powders ethanol suspension on carbon-coated copper

TEM grids.

The thermal degradation of the PNCs with different particle

loadings is studied with a thermogravimetric analysis (TGA, TA

instruments TGA Q-500). TGA is conducted on pure PANI and

its WO3 NPs/PANI PNCs from 25 to 800 �C with an air flow rate

of 60 ml/min and a heating rate of 10 �C/min.

Differential scanning calorimeter (DSC, Perkin Elmer

Instruments) measurements are carried out under a nitrogen flow

rate of approximately 20 ml/min at 20 �C/min heating rate from

25 to 300 �C.

The dielectric properties are measured by a LCR meter

(Agilent, E4980A) equipped with a dielectric test fixture (Agilent,

16451B) at the frequency of 20 Hz–2 MHz. Pure PANI and its

PNCs powders are pressed in the form of disc pellets with

a diameter of 25 mm by applying a pressure of 95 MPa in

a hydraulic presser and the average thickness is about 0.5 mm.

The same sample is used to measure the electrical conductivity

(s) by a standard four probe method in the temperature range of

50–290 K using a Model 7100 AC transport controller (Quantum

Design). The temperature dependent resistivity is used to inves-

tigate the electron transport mechanism in pure PANI and its

PNCs.

3. Results and discussion

3.1 FT-IR analysis

Fig. 2 shows the FT-IR spectra of pure PANI and its WO3/PANI

PNCs. For pristine PANI, the main peaks at 1572 and 1479 cm�1

correspond to the stretching deformation of quinone and

benzene rings, respectively. The peak at 1295 cm�1 is due to the

C–N stretching in a secondary aromatic amine. The peaks at

1123 cm�1 and 1236 cm�1 correspond to the C–N stretch in

quinoid ring and C–H stretching, respectively. These results are

J. Mater. Chem., 2011, 21, 342–348 | 343


Fig. 2 FT-IR spectra of (a) pure PANI, (b) 5 wt% WO3 NPs/PANI,

(c) 10 wt% WO3 NPs/PANI, (d) 20 wt% WO3 NPs/PANI, and (e) 10 wt%

WO3 NRs/PANI.

Fig. 3 (a) Resistivity vs. temperature and (b) ln(s)�T(�1/4) curve of pure

PANI and its PNCs with different particle loadings and morphology.

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in good agreement with previous spectroscopic characterization

of polyaniline.40,41 The main peaks of WO3/PANI PNCs are

similar to those of pure PANI. However, the peak at 1123 cm�1 in

the pristine PANI spectrum shifts to higher wavenumbers of

1133 cm�1 in the spectra of WO3 NPs/PANI PNCs, indicating an

enhanced degree of charge delocalization on the polymer back-

bone owing to the interfacial interaction between the WO3 NPs

and the PANI molecules,42 which can be further evidenced by the

reduced resistivity of the WO3 NPs/PANI PNCs observed in the

following section. Meanwhile, the peak shift from 1123 cm�1 to

lower wavenumbers of 1112 cm�1 in WO3 NRs/PANI PNCs

corresponds well to the reduced degree of charge delocalization

on the polymer backbone which leads to the increased resistivity.

The peak around 800 cm�1 is attributed to the S–O vibration of

the doping acid (PTSA) on the backbone structure. Peak shift is

also observed, specifically, from 788 cm�1, Fig. 2(a), to 791 cm�1

for 5 wt% WO3 NPs/PANI PNCs, Fig. 2(b), the same shift for

10 wt% WO3 NRs/PANI PNCs, Fig. 2(e), and 803 cm�1 for

10 and 20 wt% WO3 NPs/PANI PNCs, Fig. 2(c&d). The peak

shift toward high wavenumber in this region indicates the

intensified S–O bonding strength of the PNCs as a result of the

protons transferring from the acid group to the polymer back-

bone (often called doping). The larger wavenumber exhibited in

the PNCs may attribute to the higher doping level resulting from

this surface initiated polymerization method.

3.2 Electrical conductivity

Fig. 3(a) shows the resistivity as a function of the temperature for

pure PANI and its PNCs with a particle loading of 5, 10, 20 wt%

and a rod loading of 10 wt%, respectively. The resistivity is

observed to decrease with the increase of the temperature, indi-

cating a semiconducting behavior of the PNCs in the whole range

of temperature.43 The nanofillers with different morphology are

observed to exhibit extremely opposite electron conduction

properties. The PNCs show enhanced s with the addition of WO3

NPs and higher s is obtained with higher particle loading (pure

PANI: 1.41 � 10�3 S/cm, 5%: 2.31 � 10�3 S/cm, 10%: 2.69 � 10�3

S/cm, 20%: 5.32 � 10�3 S/cm at 290 K), which is different from

our previous observations in the SiC/polypyrrole and

344 | J. Mater. Chem., 2011, 21, 342–348

Fe2O3/polypyrrole PNCs with an optimum particle loading of

10 wt% for obtaining the highest s.44,45 However, a reduced s is

observed when the PNCs are reinforced with WO3 NRs (8.01 �10�4 S/cm at 290 K), even lower than that of pure PANI. This is

due to the high contact resistance between the NRs and the

polymer arising from the lower packing factor for fillers with

a non-sphere shape.

The electron transportation mechanism is investigated by

exploring the relation between ln(s) and T�1/n, where n¼ 2, 3 and

4. The best fits with a standard deviation less than 0.02 for each

sample are obtained with n ¼ 4 in the temperature range of

50�290 K. The linear relation between ln(s) and T�1/4 obtained

from the experimental results, Fig. 3(b), indicates a quasi 3-d

variable range hopping (VRH, the low temperature behavior of

the resistivity in the strongly disordered systems where the states

are localized).45 The temperature dependent VRH conductivity is

given by eqn (1)

s ¼ s0 exp

"��

T0

T

�1=n#

(1)

where, T0 is the characteristic Mott temperature related to the

electronic wave function localization degree and s0 is the

conductivity at infinite high temperature. The value of n assumes

4, 3, and 2 for three-, two-, and one- dimensional systems,

respectively. The T0 and s0 for each sample are summarized in

Table 1. Both T0 and s0 decrease with the increase of particle



Table 1 T0 and s0 for pure PANI and its nanocomposites

Samples To � 107 (K) s0 (S/cm)

Pure PANI 3.02 1151515% WO3 NPs 1.76 1816010% WO3 NPs 1.38 943320% WO3 NPs 1.12 831410% WO3 NRs 3.19 73130

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loading. In addition, the values of T0 and s0 are significantly

higher for the PNCs reinforced with NRs than those in the PNCs

with the same loading of NPs. Generally, a larger T0 implies

a stronger localization of the charge carriers, and thus represents

a lower s.46 However, it is interesting to observe that a bigger T0

corresponds to a higher s0, which is due to the modulated elec-

tronic wave47 in the fibrous nanostructures of these nano-

composites.

3.3 Dielectric properties

Fig. 4 shows the real permittivity (30) of pure PANI and its PNCs

as a function of frequency. 30 of the samples can be classified into

three stages, Fig. 4(a). In the first stage, 30 increases with the

increase of frequency within the range of 20–103 Hz, including

a permittivity transition from negative to positive. In the second

Fig. 4 Real permittivity of pure PANI and WO3/PANI nanocomposites

within different frequency ranges (a) 20–2M Hz, and (b) 20–400 Hz. Inset

shows the real permittivity in the frequency range of 103 Hz to 106 Hz.


stage from 103 to 2� 106 Hz, 30 is almost constant with a value of

30�50 in the PNCs with a particle loading higher than 5 wt%.

However, 30 is significantly higher for the pure PANI (�500) and

5 wt% WO3 NPs/PANI PNCs (�200), which attributes to the

stronger localization of charge carriers as evidenced by the larger

T0, Table 1. After that, 30 experiences a step-like decrease

(dielectric relaxation, i.e., the momentary delay in the dielectric

constant of a material with respect to a change in the electric

field, which is also observed in TiO2/epoxy48 and multiwalled-

CNT poly(vinyldene fluoride) PNCs49) towards higher frequency

in the third stage.

The specific switching frequency, which corresponds to the

transition of 30 from negative to positive value, is observed in the

frequency range from 40 to 400 Hz, Fig. 4(b). The switching

frequency shows an increase as the particle loading increases, for

examples, from 40 Hz for pure PANI to 60, 70, and 220 Hz for

the PNCs with a particle loading of 5, 10 and 20 wt%, respec-

tively. In particular, it is worthwhile to notice that the PNCs

reinforced with 10 wt% NRs show the highest switching

frequency (280 Hz). The lowest negative permittivity for each

sample is observed at 20 Hz. Generally, the PNCs filled with

WO3 NPs show lower absolute value of negative permittivity

(200�800) than that of pure PANI (�1143). This is different

from the PNCs filled with WO3 NRs, which surprisingly shows

even lower negative permittivity of 1480 at 20 Hz. The negative

permittivity is primarily due to the large plasmon resonance at

relatively low frequencies.50 The obtained theoretical investiga-

tions on the plasmon resonance frequency in various nano-

structures reveals that a larger aspect ratio of a metal nanorod

has more field lines spread out in the dielectric medium for the

field polarization parallel to the rod.34 Thus, a large plasmon

resonance is expected arising from the nanorod structured

composites, which well explains the significantly lower negative

permittivity of the NRs filled PNCs as compared to that of the

PNCs filled with NPs, Fig. 4(b). The unique negative permittivity

observed at low frequencies may shed some light on their

potential applications in the high temperature superconductors

for providing attractive force between similar charges.47

Fig. 5 shows the imaginary permittivity (30 0) of pure PANI and

its PNCs within the frequency ranges of 20–2 � 106 Hz and

20–400 Hz, respectively. Pure PANI exhibits the largest negative

30 0 and shows the highest peak value at 60 Hz. After that, 30 0

decreases gradually towards high frequency. The switching

frequencies for each sample are quite consistent with the

frequency values as shown in 30. It is worth noting that both 30

and 30 0 of the PNCs are significantly lower than those of pure

PANI, Fig. 4(a) and Fig. 5(a). A similar reduced permittivity

with the addition of TiO2 NPs in epoxy resin48 was also reported.

Fig. 6 shows the dielectric loss (tand) curve of the samples as

a function of frequency. The tand decreases sharply by three

orders of magnitude within the frequency range of 20–5� 104 Hz

and it keeps relatively constant within the range from 0.1 to 1

afterwards. The filler morphology effect on tand within

20–400 Hz is compared in Fig. 6(b). All the three samples exhibit

a peak value and the corresponding frequency shifts from

40 (pure PANI) to 60 and 260 Hz for PNCs filled with 10 wt%

WO3 NPs and 10 wt% WO3 NRs, respectively. Though the PNCs

with NRs exhibit the highest peak value of tand, the significantly

lower tand at low frequencies (<125 Hz) provides some essential

J. Mater. Chem., 2011, 21, 342–348 | 345


Fig. 5 Imaginary permittivity of pure PANI and WO3/PANI nano-

compositeswithindifferent frequencyranges (a)20–2MHz,and(b)20–400Hz.

Inset shows the imaginary permittivity at 5� 104–2� 106 Hz.

Fig. 6 Dielectric loss of pure PANI and WO3/PANI nanocomposites

within different frequency ranges (a) 20–2M Hz, and (b) 20–400 Hz.

Fig. 7 SEM of (a) 10 wt% WO3 NPs/PANI (b) 10 wt% WO3 NRs/PANI

and TEM of (c) 10 wt% WO3 NPs/PANI (d) 10 wt% WO3 NRs/PANI

PNCs. Inset figure in (d) shares the same scale bar.

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insights on the advanced applications such as supercapacitors,

which is necessary for obtaining high energy density.51 Moreover,

combining with the negative permittivity within the low

frequency, these materials may also give some hint to serve as

high temperature superconductive materials.47

3.4 SEM and TEM microstructure observation

Fig. 7 shows the microscopic microstructures of the PANI

nanocomposites reinforced with WO3 NPs and NRs, respec-

tively. Nanofiber structures appear to form when the fillers are in

particle shape, Fig. 7(a). The particles are well embedded in the

polymer matrix and dispersed uniformly as justified by TEM

observation, Fig. 7(c). The diameter of the nanocomposite fibers

is about 50–100 nm, which is quite consistent with the inter-

facially grown PANI NFs observed by Huang and Kaner.7

Fig. 7(b&d) shows the SEM and TEM microstructures of the

WO3/PANI PNCs filled with NRs, respectively. The NRs are

observed to be coated with a uniform layer of PANI and the

thickness is about 1.5 times of the diameter of the NRs. All these

results reveal the advantages of this SIP method to fabricate

PNCs with a uniform coating and an improved dispersion.

3.5 Thermogravimetric analysis

Fig. 8 shows the TGA curves of pure PANI and its PNCs with

a particle loading of 5, 10, 20 and 30 wt%, respectively. Two

346 | J. Mater. Chem., 2011, 21, 342–348

weight loss regions are observed in the curves. The slight weight

loss from room temperature to 150 �C is attributed to the elim-

ination of the moisture in the samples. The major weight loss of

all the samples from 150 to 700 �C is due to the degradation of

PANI. With the addition of WO3 NPs, the 20 wt% weight loss

temperature increases from 410.3 �C (pristine PANI) to 423.4,

433.9, 454.2 and 475.6 �C for PNCs with a particle loading of 5,



Fig. 8 TGA curves of pristine PANI and its WO3/PANI PNCs with

various particle loadings.

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10, 20 and 30 wt%, respectively. Though several reports

demonstrate the lowered thermal stability of PANI PNCs with

the incorporation of inorganic fillers due to the weakened

interfacial interaction between the two components.52,53 In the

present work, the thermal stability of the PNCs is improved

because the polymer tightly binds to the nanofiller surface with

this newly developed SIP method. This fabrication method

favors an ordered and compact structure, which delays the

thermal degradation of the PANI.54 The degradation curve of

pure PANI shows an almost complete decomposition at 800 �C,

with only 0.73% weight residue left. However, the weight residue

of the PNCs is much higher as compared to the mass fraction of

the WO3 NPs, i.e., 9.8, 16.0, 28.8 and 41.1% for the PNCs rein-

forced with 5, 10, 20 and 30 wt%, respectively. The PNCs have

a weight residue much larger than that of the initial particle

loading estimation, calculated from the initial weight of particles

and monomers. This is due to the lower conversion of monomers

during the polymerization of aniline, which is also observed in

SiC/polypyrrole PNCs.44

3.6 Differential scanning calorimetry

The DSC curves of pure PANI and its PNCs indicate an endo-

thermic transition starting from 50 �C and centering around

Fig. 9 DSC curves of PANI and PNCs with different WO3 NPs

loadings.


125 �C, Fig. 9. This result agrees with the moisture evaporation,

which are trapped inside the polymer or bound to the polymer

backbone, as evidenced by the first degradation stage of TGA

curve, Fig. 8. Generally, the glass transition temperature (Tg) of

PANI powders is not evident in the thermographs.47 The

exothermic transition observed at 194.1�249.1 �C is believed not

to be Tg. Instead, it would be attributed to a series of chemical

reactions. Basically, bond scissioning followed by a bond

formation55 are involved when the powders are heated. The bond

scissioning is endothermic, which is compensated by the gener-

ated heat by bond formation and shows an exothermic peak at

around 150�300 �C. The increased peak temperatures from

194.1 �C (pristine PANI) to 224.5, 236.7, 246.3 and 249.9 �C for

the PNCs filled with a particle loading of 5, 10, 20 and 30 wt%,

respectively, further demonstrate the ordered polymer structure

as well as a good interfacial interactions between the NPs and the

polymer matrix.

4. Conclusions

Polyaniline nanocomposites with unique negative permittivity

have been fabricated by a surface initiated polymerization

method. Different nanostructured tungsten oxides have been

investigated. Temperature dependent electrical resistance studies

reveal that the electron transportation in the PNCs follows

a quasi 3-d variable range hopping (VRH) conduction mecha-

nism. And the resistivity of the PNCs decreases with the increase

of the WO3 nanoparticle loading. However, the resistivity

increases with the addition of the WO3 NRs owing to the higher

contacting resistance. Dielectric properties show that the

switching frequency can be easily enlarged by increasing the WO3

NPs loadings and the PNCs filled with WO3 NRs exhibit

significantly higher switching frequency as compared to that of

the PNCs with WO3 NPs. TGA and DSC investigations reveal

that the thermal stability of the PNCs is significantly enhanced

with the addition of nanofillers.

Acknowledgements

This work is supported by the research start-up fund from Lamar

University. Partial financial support from Northrop Grumman

Corporation is kindly acknowledged. David P. Young

acknowledges support from the NSF under Grant No. DMR

04-49022. The financial supports from Dan F. Smith Department

of Chemical Engineering and College of Engineering at Lamar

University are kindly acknowledged.

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