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DDeakin Research Online Research Online This is the authors final peer reviewed version of the item published as: Håkansson, Eva, Amiet, Andrew and Kaynak, Akif 2006-07-01, Electromagnetic shielding properties of polypyrrole/polyester composites in the 1–18 GHz frequency range, Synthetic metals, vol. 156, no. 14-15, pp. 917-925. Copyright : 2006, Elsevier B.V.

1

Electromagnetic shielding properties of polypyrrole/polyester

composites in the 1- 18 GHz frequency range

Eva Håkanssona, Andrew Amietb and Akif Kaynak1a

a School of Engineering and Technology, Deakin University, Geelong 3217,

Australiab Defence Science and Technology Organisation, P.O. Box 4331, Melbourne 3001,

Australia

Abstract

The dielectric characteristics of conducting polymer coated textiles in the

frequency range 1-18 GHz were investigated using a non-contact, non-destructive free

space technique. Polypyrrole coatings were applied by solution polymerization on

fabric substrates using a range of concentrations of para-toluene-2-sulfonic acid

(pTSA) as dopant and ferric chloride as oxidant. The conducting polymer coatings

exhibited dispersive permittivity behaviour with a decrease in real and imaginary

components of complex permittivity as frequency increased in the range tested. Both

the permittivity and the loss factor were affected by the polymerization time of the

conductive coating. It was found that the total shielding efficiency of these conductive

fabrics is significant at short polymerization times and increases to values exceeding

80 % with longer polymerization times. The reflection contribution to electromagnetic

shielding also increases with polymerization time.

Keywords: Conductive textiles, electromagnetic shielding, dielectric properties.

1 Corresponding author. Tel.: +61 3 5227 2909; fax: +61 3 5227 2539.

E-mail address: akaynak@deakin.edu.au (A. Kaynak).

2

1. Introduction

1.1. Conducting polymers

Conducting polymers have applications in the fields of electroluminescence,

microelectronics, textiles, sensors and electromagnetic shielding [1-8]. The nature of

conducting polymers makes them not only conductive but also electroactive.

Conductivity becomes useful in applications such as antistatic materials, heat

generation and electromagnetic shielding [6, 9-17], whereas electroactivity is utilized

in applications such as electrical displays, light emission devices (LED), actuators and

biosensors [18-20].

The main limitations for the use of conducting polymers in commercial

applications are the poor processability and brittleness of films formed by chemical or

electrochemical synthesis methods. By polymerizing conducting polymers directly

onto textiles, a thin, homogeneous coating is achieved on the surface of the textile.

This enables combination of a wide range of structural and mechanical properties of

textiles with the electrical properties of the conducting polymers, providing infinite

possibilities in the design of conductive composites. The thickness of a conducting

polymer film formed during chemical synthesis is dependent on the synthesis time

and reactant concentrations and is in the submicron range.

1.2. Utilization of conducting polymers in electromagnetic interference shielding

Proliferation of electronic equipment and wireless systems over the past few

decades has increased the vulnerability to electromagnetic interference (EMI). This

interaction between electronic systems is undesirable and can lead to malfunctioning

in the involved components. To protect electronic equipment from EMI, electrically

conductive materials are used. Shielding from EMI using conducting polymer-coated

textiles has some advantages including flexibility, access to a wide range of

structures, reduced weight and lower cost. Shielding can also be achieved by addition

of conductive fillers, such as carbon black, carbon fibers, metal flakes or powders to a

polymer matrix or to a woven structure. Vapour deposited thin metal coatings have

poor abrasion resistance but are very effective shields.

3

The use of conducting polymers on textile substrates for shielding applications

would have certain advantages such as light weight, flexibility, drape and price. Also,

conducting textiles provide an absorption dominant interaction with the microwave

radiation. An example of a potential application of conducting polymers in EMI

interference is control and modification of indoor wireless channel for minimization

of interference by fabricating frequency selective absorbers and shields based on

multilayer conducting polymer composites. These products could have specific

absorption and reflection properties intended for radiation shields for electronic

equipment.

1.3. Polymer matrices with conducting polymer fillers

Several research groups have reported electromagnetic shielding behaviour of

conducting polymer-non-conducting material composites. Polyaniline powder is the

most commonly reported filler in thermoplastics such as polyvinylchloride, polyamide

[17], polyurethane [21, 22], polystyrene, polymethylmethacrylate [23], and

thermoplastic-elastomeric matrices such as acrylonitrile-butadiene-styrene (ABS) [24,

25]. Mixtures of polyaniline and elemental silver have been incorporated in

polyethyleneoxide and tested for dielectric properties [26, 27]. Polypyrrole (PPy)

powders have been incorporated in thermoplastics [28], silicon based plastics [29],

and fluoroplastics [30]. The incorporation of conducting polymer powders in non-

conducting matrices gives a shielding effectiveness (SE) value consisting of both

reflection and absorption components. The ability to absorb part of the radiation is an

attribute of these particle filled composites. Problems with a homogenous distribution

of the conducting polymer particles are still present in these composite materials.

4

1.4. Intrinsically conducting films

Poly(p-phenylene-vinylene) and its derivatives [15, 31, 32], poly(p-phenylene-

benzobis-thiazole) [33-35], and polyaniline [36-39] have all been tested for SE and

they generally have shown a high level of reflection due to their high conductivity in

their pure state. Reflection, transmission, absorption and dielectric properties of

polypyrrole films and powders have also been reported [40-43]. Joo and Epstein

made comparisons of a range of conducting polymer films and found a reflection-

dominated shielding effectiveness in all samples [44].

1.5. Conducting polymers coated on textile substrates

Textiles coated with evaporated metals in combination with conducting

polypyrrole were reported to yield high shielding effectiveness [45, 46]. Subjecting a

fabric substrate to metal vapour forms a thin layer of metal on the substrate, which

inherently possesses a high shielding effectiveness. The disadvantage with this

technique is that the metallic coating has poor abrasion resistance. Conducting

polymer coatings generally have better abrasion resistance than metal coatings. In a

study of conducting polymer coatings on textiles [47] poly(3,4-

ethylenedioxythiophene) and poly(vinyl pyrrolidone) on polyester textile substrates

showed high conductivity and hence a high reflection level. M.S. Kim et al [48]

applied polypyrrole onto a polyester substrate by electrochemical method and

achieved high reflection as well as absorption levels up to 35%. In these studies

flanged-coaxial transmission line holders based on the ASTM D4935-89 for the test

of shielding material with dynamic range of 95-120dB in frequency range of 50

MHz-2 GHz were used. Dhawan and co-workers achieved -3 to -11 dB total shielding

effectiveness in the range 8-12 GHz using polypyrrole and polyaniline applied onto

polyester, glass and silica fabrics by vapour phase polymerization [49]. The same

researchers also reported a shielding effectiveness of -35 dB in a silica cloth at 101

GHz [50] using a phase-log oscillator measurement technique. The use of polypyrrole

loaded paper and polyester composites in Salisbury screens, which were subjected to

variable angle incident electromagnetic radiation and measured with a free space

reflectivity arch in the gigahertz range were reported [51-54].

5

In this article we present dielectric properties, including reflection, transmission

and absorption characteristics of polypyrrole coated textiles in the 1-18 GHz

frequency range using a non-destructive free space testing method.

2. Experimental

2.1. Synthesis of conducting polymer

A 0.54 mm thick double sided basket weave Polyester-Lycra® pristine fabric with

a Lycra content of 10% was used. Fabric samples were scoured in Imerol (0.5 g/l) and

sodium carbonate (2.0 g/l) at 80°C to remove any waxes, residual chemicals, oils and

particulates. The samples were dried in a Binder FED 115 Lab oven at 105°C and

allowed to cool to room temperature before being introduced into an aqueous solution

containing para-toluene-2-sulfonic acid monohydrate (pTSA) (98%, Sigma-Aldrich)

as dopant, pyrrole monomer (98%, Aldrich) and a low concentration of wetting agent

(Albegal FFA, Ciba). Ferric chloride hexahydrate (min 98%, Fluka) was added to the

solution and a film of conducting polypyrrole was formed directly on the textile

substrate through oxidative polymerization [55]. An optimized oxidant to monomer

ratio of 1:2.23 was used [12, 56]. The synthesis time was varied between 60 and 180

minutes and the polymerization bath was occasionally stirred to ensure even coating

of conducting polymer on the fabric surface. It has been shown that a dynamic

synthesis process can result in the abrasion of the coating [57].

All syntheses were carried out at room temperature. After synthesis the samples

were cut to 500 by 500 mm size and stored at a temperature 20°C and a relative

humidity of 65 %.

2.2. Thickness measurement

Thickness measurements were made on each preconditioned sample using a Fabric

Thickness Tester (DGTW01B, Mitutoyo, Japan) in accordance with ISO 9073-2

standard (0.5 kPa). The thickness was averaged over each sample and used in

permittivity measurement.

2.3. Conductivity measurements

6

The surface resistivity of the conducting fabrics was measured using a 34401A

multimeter (Agilent Technologies) in a controlled environment according to AATCC

(American Association of Textile Chemists and Colorists) test method 76-1995,

where two rectangular copper electrodes (20 × 30 mm) are pressed onto the fabric

surface with a 10 N weight. The measurement device was placed on the flat fabric, a

relaxation time of 120 seconds was allowed and 10 resistance measurements were

recorded and averaged for each sample. The surface resistivity RS is given by:

)/( wlRRS = (1)

Where, R is the measured resistance of the fabric, l is the distance between the

electrodes and w is the width of each electrode.

2.4 Free space transmission measurement technique

Free space measurement is used to determine broadband dielectric properties of

larger sized sheet materials. This method involves placing the sample between two

microwave horns and measuring the reflection and/or transmission from the sample.

Figure 1 shows the schematic set-up of signal detection. An Agilent Technology

8510C vector network analyzer system is used to perform all the measurements at

microwave frequencies. The 8510C analyzer is connected to an 8517A S-Parameter

Test Set with an 83651B Synthesized Frequency Source. The frequency range of this

system covers 45 MHz to 50 GHz. The network analyzer system has a dynamic range

of greater than 100 dB, and resolutions of 0.01 dB in magnitude and 0.01 degrees in

phase are readily achievable. An IBM compatible computer controls the system, with

the software written by the second author. Output from the system is usually in the

form of S-parameters, where S11 is the reflected signal (measured at port 1) and S21 is

the transmitted signal (measured at port 2) when an input signal is present at port 1.

These complex quantities can be readily transformed into absolute magnitude and

phase of the reflection and transmission, and the permittivity and permeability of a

material can be extracted when the appropriate formulas are used. The scattering

parameters S11 and/or S21 are recorded at 401 frequency points across the band, with

7

500 readings averaged at each frequency point. The samples were 500 mm square.

The magnitudes of reflection, transmission and absorption were later calculated using

the permittivity values derived from the measured S21 data (i.e. transmission). The

frequency range of the horns was 1 – 18 GHz and the gain of the horn antenna was

between 6 and 16 dB. The distance between the transmission horn and the calibration

plane was set to 310 mm, and the distance between the receive horn and the

calibration plane was set to 165 mm.

Fig. 1. Schematic set-up of free space measurements

One of the main sources of error in free space dielectric measurements is the

diffraction around the sample being tested. The errors caused by diffraction are

reduced when the sample size is sufficiently large. However, a method has been

developed to remove the diffraction from the measurements after a careful calibration

of the system [58]. The diffracted signal can be determined by measuring the signal

reached from port 1 to port 2 when a metal sheet that is exactly the same size as the

test specimens is placed in the sample plane [59, 60]. Since none of the signal travels

through the metal sheet, only the diffracted signal reaches port 2. This is then

removed mathematically from the sample transmission values, thus increasing the

accuracy of the permittivity calculation. The technique is especially useful at lower

frequencies.

8

Another source of error in all dielectric measurements, particularly in the case of

conducting polymer coated fabrics, is accurate thickness measurements. In this

investigation the sample thicknesses were averaged over the sample area to improve

accuracy.

Dielectric properties of a wide range of materials have been tested successfully

using free space techniques, including acrylics [61]; Teflon, PVC and microwave

absorbing composites [62-65]; and liquids [66]. Truong et al used this method to

investigate the radiation response of polypyrrole powder in an elastomeric matrix and

mixed in commercial paints [67].

The interaction of a material with electromagnetic fields can be characterized by

complex permeability μ* (related to magnetic component of field), complex

permittivity ε* (related to electrical component of field), and total (ac + dc)

conductivity tot.

The permeability of a material is generally expressed in relative units compared to

that of free space ( 0 = 4π × 10-7 H/m). There are very few materials possessing

permeabilities significantly different from free space permeability at microwave

frequencies, with pure iron and some ferrites the notable exceptions. Since the

conductive fabrics investigated in this work are non-ferrous, the relative complex

permeability is set to that of free space

01 ⋅+= irμ (1)

Permittivity is also usually expressed in relative terms, where the permittivity of

free space 0ε is 8.854 × 10-12 F/m. The permittivity consists of a real part associated

with the energy storing capacity of the material, and an imaginary part related to the

electrically dissipative nature of the material.

rrr iεεε ′′+′= (2)

All permittivity values presented in this work are relative permittivities (i.e. r).

As an electromagnetic wave travelling in free space encounters a material, some of

the incident radiation enters the material (i.e. transmitted or absorbed) and some is

reflected. The reflection coefficient Γ is defined as the fraction of radiation that is

reflected from the front surface and is given by

9

1

1

12

12

+

−=

+−

=Γ

r

r

r

r

ZZZZ

εμ

εμ

(3)

where, Z1 is equal to the characteristic impedance of free space and Z2 is equal to

the characteristic impedance of the medium encountered. The transmission

coefficient, T, is the ratio of transmitted electric field strength to the incident electric

field strength and is given by:

εμωεμωγ

dc

idd eeeT

⎟⎠⎞

⎜⎝⎛−

−−− ===2

(4)

where, is the propagation constant, is the angular frequency and d is the

thickness of the material.

When using the free space technique, it is possible to extract both complex

permittivity and permeability of the sample under test by measuring both S11 and S21.

However, due to the non-magnetic nature of polypyrrole, it is sufficient and

appropriate to calculate the complex permittivity only, using either the transmission or

reflection signal. In practice, the transmission signal is easier to measure with greater

accuracy.

Nicolson and Ross [68] showed that for electrically thin materials the scattering

parameters S11 and S21 can be described as

( ) ( )22

2

11 11

Γ−Γ−

=TTS ω (5)

( ) ( )22

2

21 11

Γ−Γ−

=T

TS ω (6)

Where, is the angular frequency, T is the transmission coefficient and Γ is the

reflection coefficient.

By rearranging Equation 6 and using Equations 3 and 4, the transmission S21

can be expressed in terms of μ and ε, show below in Equation 7.

10

2

2)(

)(

2

1

1)(1

1

1

⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜

⎝

⎛

+

−−

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛

⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜

⎝

⎛

+−

−

=(

−

−

εμεμ

εμεμ

ω

μεω

μεω

dci

dci

21

e

e-1

)S (7)

It is impossible to obtain a form of the equations where the permittivity is given

explicitly in terms of S21. Instead, an approximation technique is necessary to solve

the implicit equation for permittivity. In this case Newton's method was utilized to

approximate the values of the real and imaginary parts of permittivity to an accuracy

of 10-7 [65]. These calculations were carried out at each of the 401 frequencies across

the frequency range tested. The magnitudes of incident reflection and absorption

coefficients were then calculated for all 401 frequencies using equations valid for

normal incidence on a single dielectric slab [69], based upon the calculated

permittivity from the transmission measurements.

The system was re-calibrated every 5 minutes during the measurement process in

order to adjust for changes in the ambient temperature and humidity with time.

3. Results and discussion

3.1 Surface resistivity as a function of polymerization time

The polymerization of pyrrole gives a homogenous coating of conducting

polypyrrole on the fabric surface. The originally light coloured samples become black

after the oxidative polymerization due to the high absorption across the visible spectra

of conjugated polymers [55, 70, 71]. Figure 2 displays the decrease in resistivity with

increase in dopant concentration and polymerization time. Surface resistivity of the

samples ranged from 180 to 1300 ohms/square.

11

0.014 0.016 0.018 0.020 0.022 0.024 0.026 0.028

200

400

600

800

1000

1200

t = 60 min t = 90 min t = 120 min t = 180 min

Res

istiv

ity [o

hm/s

q]

Dopant concentration

Fig. 2. The surface resistivity versus para-toluene-2-sulfonic acid concentration for different

polymerization times of polypyrrole coated fabric specimens.

Resistivity of samples coated for longer polymerization times is lower due to

increased film thickness. As the concentration of dopant is increased very slightly,

from 0.015 mol/l to 0.0165 mol/l, there is a large decrease in resistivity, particularly

pronounced at shorter polymerization times. The rate of decrease of surface resistivity

decreases with the increase in the concentration of the dopant. The rate is further

reduced at longer polymerization times since the polymerization yield decreases

exponentially with time.

3.2 Relative Permittivity

The permittivities of polypyrrole coated textiles were evaluated with respect to

coating times of 60 and 180 minutes at para-toluene sulfonic acid (pTSA) dopant

concentrations of 0.015, 0.018 and 0.027 mol/l. The positive values correspond to the

real permittivityε ′ , whilst the negative values refer to the imaginary componentε ′′ .

At shorter polymerization times the real permittivity is relatively low.

Figure 3 shows the variation of permittivity with frequency at a polymerization

time of 60 minutes. There is a slight difference in the real part of permittivity at the

12

low end of the frequency span for the 0.018 mol/l pTSA concentration. Permittivity

values appear to increase with the dopant concentration, but it is not significant for

these concentration ranges and polymerization times.

Fig. 3. Relative permittivity versus frequency of conducting textiles prepared at a polymerization

time of 60 minutes at 0.015, 0.018 and 0.027 mol/l of p-toluene-2-sulfonic acid.

Figure 4 reveals that increasing the polymerization time to 180 minutes results in a

significant increase in the real part of permittivity of the conductive fabrics, indicating

a larger energy storage capacity in the material. The magnitude of the imaginary part

of permittivity has also increased, showing that the conductivity of the material is

greater than those with shorter polymerization times.

13

Fig. 4. Relative permittivity versus frequency of conducting textiles prepared at a polymerization

time of 180 minutes with 0.015, 0.018 and 0.027 mol/l of p-toluene-2-sulfonic acid.

As the frequency increases, magnitudes of both real and imaginary part of

permittivity decrease irrespective of the polymerization times. This is due to the

shorter time available for re-configuration within the material in order to oppose the

applied field before it is reversed.

The values of relative imaginary permittivity is directly proportional to the total

(ac + dc) conductivity of the material and may be expressed as

ωσ

ε toti=′′ (8)

where, tot is the total conductivity of the material and is the angular frequency.

The characteristic shape of the permittivity spectra of Figures 3 and 4 can be seen to

decrease inversely with frequency. Although some of the conducting polymer samples

tested have relatively high conductivity they still have lower conductivity than

metallic conductors. This semi-conducting nature of the materials tested affects the

permittivity since the predominant polarization mechanisms in the present frequency

14

range are the dipole interactions and the migration of free charge carriers in the

material (migration polarization).

As expected, the permittivity values obtained are dispersive in all cases. The data

obtained with the free-space measurement transmission technique is remarkably free

from any jitter, which would indicate problems caused by interference from the

diffraction signal. Diffraction is a major cause of inaccuracy in free space permittivity

measurements using transmission data, and the stability of our results is due to both

the large sample size and an efficient mathematical removal of diffraction from the

measured transmission values. These measures have led to an accurate transmission

data measurement.

3.3. Shielding effectiveness of thin conducting polymer textiles

Although the conducting polymer coating on the fabrics is very thin (less than 500

nm), measurements reveal a high total shielding effectiveness. The shielding

effectiveness increases as the polymerization time is increased, due to a larger amount

of conducting polymer deposited on the surface of the material.

The measurements show that the highest shielding effectiveness was achieved in

the sample with dopant concentration of 0.027 mol/l and a polymerization time of 180

minutes. Second was the sample with pTSA concentration of 0.018 mol/l, coated for

180 minutes. The third highest SE value was achieved by the sample with a dopant

concentration 0.027 mol/l, coated for 120 minutes. Hence, both dopant concentration

and polymerization time affect the total shielding effectiveness and it is not possible

to distinguish either of these two factors as being deterministic of shielding behaviour.

In metallic materials the electromagnetic interference shielding effectiveness is

very high and exclusively attributed to reflection of radiation due to the high

conductivity of the material. In medium level conductivity materials a large part of the

radiation is absorbed in the material and lost as heat. A highly absorptive material will

reduce the retro-reflection of the radiation and can therefore be desirable in shielding

sensitive equipment. The conducting polymers investigated in this work both reflect

and absorb radiation, and generally have larger absorption levels than reflection. The

percent shielding contributions from reflection and absorption of the three samples

mentioned above are shown in Figure 5.

15

Fig. 5. Reflection (bottom three) and Absorption (top three) percentages for 3 selected samples.

pTSA concentrations and polymerization times are indicated.

The levels of absorption were higher than the levels of reflection in all the tested

samples and absorption was relatively even throughout the frequency range for

different concentrations of dopants and polymerization times. Reflection increased

with increase in dopant concentration and polymerization time due to the increase in

conductivity. The reflection values were more dispersive than the absorption and at

the long polymerization times the reflection increased throughout the frequency

range.

The total shielding effectiveness levels are displayed in Figure 6 and are also

dispersive due to the reflection contribution increasing as frequency increases. A

maximum shielding effectiveness is achieved at long polymerization time with a high

concentration of dopant. It is still to be investigated if this trend continues at longer

polymerization times and at higher dopant levels. Maximum shielding effectiveness

greater than 80 percent has been achieved in the best performing conducting textile at

high frequencies.

16

Fig. 6. Total shielding effectiveness for selected pTSA concentrations (0.024 mol/l and 0.027

mol/l) and polymerization times (120 and 180 minutes)

4. Conclusions

Aqueous chemical synthesis gives a homogenous coating on large size textile

substrates. The permittivity of these conducting polymer coated textiles was measured

using a free space measurement technique over the frequency range 1 – 18 GHz. This

technique is broadband, non-destructive and has proven to be useful on thin flexible

samples – measurements that are generally difficult to perform. Stable permittivity

values were obtained for a range of samples and results show that the conducting

polymer coated textiles were lossy over the full frequency range. The measurements

were shown to be relatively free from diffraction effects.

The shielding analysis shows that a higher dopant concentration results in a

higher conductivity and hence a higher transmission loss of the radiation. The

transmission loss of the investigated samples is as high as -8.68 dB, corresponding to

a maximum total shielding effectiveness of around 86.45 percent. The absorption

dominated shielding proves that these conducting polymer composites are good light-

weight candidates for application in shielding materials.

AcknowledgementsThe authors wish to acknowledge Dr. Peter Jewsbury for assisting with the

initiation of this work.

2 4 6 8 10 12 14 16 18 20

70

75

80

85

120 min, 0.027 mol/l pTSA 180 min, 0.024 mol/l pTSA 180 min, 0.027 mol/l pTSA

Tota

l tra

nsm

issi

on lo

ss [%

]

Frequency [GHz]

17

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