Indian Journal of Pure & Applied Physics Vol. 51, February 2013, pp. 112-117

Electrostatic charge dissipation and electromagnetic interference shielding response of polyaniline based conducting fabrics

Parveen Saini1,2* & Veena Choudhary2

1Polymeric & Soft Materials Section, CSIR-National Physical Laboratory, New Delhi 110 012, India 2Center for Polymer Science & Engineering, Indian Institute of Technology, New Delhi 110 016, India

*E-mail: pksaini@nplindia.org

Received 7 May 2012; revised 24 August 2012; accepted 3 December 2012

Aqueous dispersions of poly(acrylic acid) (PAA) or polystyrene sulphonic acid (PSS) doped polyanilines (PANIs) designated as PANI-PAA and PANI-PSS, respectively were prepared by in-situ polymerization of aniline in the presence of above acids (PAA and PSS). These dispersions were coated over cotton fabrics by dip coating technique so as to impart electrical conductivity making them useful for antistatic and electromagnetic interference (EMI) shielding applications. XRD of these fabrics display the characteristic peaks of polyaniline and cotton substrate, whereas SEM images show the presence of conducting PANI coating over individual fibers as well as in the inter-weave regions. There fabrics show rapid (within 0.2 sec) dissipation of static charges and conform to antistatic criteria of 10%-cutoff decay time of less than 2.0 sec. In addition, moderate EMI shielding response was also observed with effectiveness (SE) value of −5.2 dB and −5.7 dB for four layered shields made up of PANI-PAA and PANI-PSS coated cotton fabrics, respectively.

Keywords: Electromagnetic interference shielding, Electrostatic dissipation, Conducting polymers, Polyaniline, Electrical conductivity, Antistatic fabrics

1 Introduction

Electromagnetic interference (EMI) and electrostatic dissipation (ESD) are most undesirable byproducts of explosive growth of electronics and miniaturization of devices1-5. EMI is becoming an alarming issue which tries to disturb the usual performance of electronic appliances and may even adversely affect the human health. This demands effective solutions to ensure disturbance free working of sensitive electronic components and devices1-3. Similarly, electrostatic dissipation (ESD) is a critical issue especially with handling of sensitive electronic items (e.g. data storage devices, IC chips, space, defense or medical instruments) due to their susceptibility towards electrostatic discharge3-5. Generally, conventional polymers used for packaging of electronic items are electrically insulating and failed to dissipate the static charges. Furthermore, instrument housings are transparent towards EM radiations which result failure to prevent the EMI effects resulting in degradation of device performance. In principle, electrical conduction is the most important requirement for safe and rapid dissipation of static charges and for efficient EMI shielding action, but compared to EMI (�~10−3 to 1.0 S/cm), the level of required conductivity2,3,6,7 for

ESD is much lower (�~10−5 to 10−9 S/cm). In the light of above scenario, it is a challenging task to convert inherently insulating thermoplastic to a product that would provide an effective solution to ESD or EMI. In the past, several attempts have been made to introduce the electrical conductivity driven antistatic properties in conventional polymers e.g. by addition of antistatic agents8, incorporation of conducting additives like metal particles, fibers or whiskers, carbon black, graphite powder or carbon nanotubes9-

13. However the functioning of antistatic agents is critically dependent on the relative humidity3,14

whereas the metal and carbon filled materials suffered from the problems like bleeding, poor dispersion and irreproducible results15. In addition to filled composites, surface coating of metals can also be done by electroplating, electroless plating or vacuum deposition techniques. However, coatings tend to suffer from their poor wear or scratch resistance16,17. In this consideration, conducting polymers with tunable conductivity and good processability can offer an attractive solution1,3,6,18-21. Among various candidates, polyanaline (PANI) is one of the most promising conducting polymers due to its good environmental/thermal stability, high electrical conductivity, facile processing and low/nominal cost.

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Furthermore, ability to modulate its conductivity as well as processability by careful control of oxidation state and protonation level makes PANI an ultimate choice for EMI or ESD applications1,2,6,19-21. However, despite all the advantages, the main drawback of PANI is its poor processability. Interestingly, the direct coating of conducting polymers on the surface of textiles via in-situ polymerization route eliminates the need of thermal or solution processability3,22-24. Most importantly, this approach combines the mechanical properties, stitching-ability and flexibility of insulating fabrics with electrical conductivity of �-conjugated polymers. Herein we report the synthesis of aqueous dispersions polyaniline (PANI) by in-situ polymerized of aniline (AN) monomer in the presence of polymeric dopant such as poly(acrylic acid) (PAA) and polystyrene sulphonic acid (PSS). Subsequently, cotton fabrics were immersed in these dispersions to form surface coating of conducting polymers. These fabrics were characterized by instruments like XRD, SEM and semiconductor characterization system. The static charge dissipation (or ESD) and EMI shielding in 12.4-18.0 GHz frequency range capabilities were also measured.

2 Experimental Details

Aniline (Loba Chemie) was freshly double distilled before use. Poly(acrylic acid) (PAA, regenerated from Na-salt, Sigma-Aldrich) and polystyrene sulphonate (PSS, Sigma-Aldrich), ammonium persulphate (APS, Merck) were used without further purification. Millipore water (resistivity>106 Ohm-cm) was used for synthesis and washings. The dispersions of doped PANI were prepared by polymerization of aniline monomer in the presence of polymeric dopants like poly(acrylic acid) (PAA) and polystyrene sulphonate (PSS). In a typical reaction, 2.0 mol of dopant (PAA or PSS) was dissolved in 1.0 L of water to which 0.1 mol of aniline monomer was added. The reaction mixture was then transferred to beaker and maintained under constant stirring. The polymerization was initiated by drop wise addition of 0.1 M aqueous solution of APS and allowed to proceed at room temperature (23°C). After completion of polymerization (6 h), a dark green dispersion of doped PANI was formed that was stored for further use. The cotton fabrics were coated by immersion in above-mentioned PANI-PSS and PANI-PAA dispersions followed by drying and designated as PSC and PAC, respectively.

The XRD patterns were recorded using Bruker D8 Advance X-ray diffractometer in the diffraction (2�) range of 10-40o at a scan rate of 0.02°/s, slit width of 0.1 mm using CuK� line (�= 1.540598Å) as radiation source. Morphologies were observed using a scanning electron microscope (SEM, Leo-440, UK) whereas room electrical conductivity measurements were performed using Keithley-4200 semiconductor characterization system. Shielding effectiveness (SE) was measured using a vector network analyzer (VNA, Model: E8263B, Agilent Technologies) by placing coated fabric samples inside a sample holder connected between the flange of Ku-band (12.4-18.0 GHz) coaxial-to-waveguide adaptors.

3 Results and Discussion

It is expected that the presence of conducting polyaniline over cotton substrate should give superimposed XRD peaks of cotton and PANI especially at peak at 2θ~25°

that represent conducting form of doped PANI. Figure 1 shows the XRD patterns of uncoated cotton as well as cotton fabrics coated with PANI-PSS and PANI-PAA i.e. Cotton, PSC and PAC, respectively. Results show that pure cotton gives characteristic peaks at 2θ values of 14.9°

(d=5.94 �), 16.5° (d=5.37 �), 22.8° (d=3.90 �) and a

broad hump (weak) at 2θ =34.2° (d=2.62 �), which

represents the characteristic signature of cellulosic structure25 and match with its standard XRD pattern (JCPDS No. 50-2241). However, coating of cotton fabric with dispersions of PANI-PAA or PANI-PSS results in appearance of weak but observable bands of coated polymers that were masked by pronounced 2θ~23o peak of cotton substrate. Therefore, diffraction

10 15 20 25 30 35 40Diffraction angle (2θθθθ, deg.)

PAC

PSC

Cotton

Inte

nsit

y (

a.u

.)

Fig. 1 — XRD patterns of uncoated cotton and PANI coated fabrics (PAC & PSC)

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pattern of PSC shows peaks at 2� values of 14.8o, 16.5o and 22,8o due to cotton substrate and weak 20.3°

(d=4.37 �) and 25.1° (d=3.54 �) reflections due to

coating of PANI-PSS. Similarly, in the XRD pattern of PAC, peaks at 2� values of 15.0o (d=5.90 �), 20.6° (d=4.31 �) and 25.4° (d=3.50 �) due to coating of PANI-PAA were observed in addition to all the characteristic peaks of cotton thereby, confirming the presence of electrically conducting PANI-PAA coating over the cotton fabric. These SEM images of pure (uncoated) cotton fabric [Fig. 2(a & b)] revealed that cotton fibers possess smooth surface with collapsed tubular morphology. In contrast, the micrographs of PANI-PSS and PANI-PAA coated fabrics i.e. PSC (Fig. 2c) and PAC (Fig. 2d), respectively, show the presence of conducting polyaniline both on the surface of individual fibers as well as in the inter-fiber (inter-weave) regions. It can be seen that (Fig. 2c) dip coating of PANI-PSS produces thick and non-uniform coating alongwith large chunks of agglomerated polymer particles whereas PANI-PAA (Fig. 2d) forms relatively uniform coating with less number of agglomerated polymer particles. The above results implied that, although the dip coating using dispersions of PANI-PAA or PANI-PSS offers an easy solution for obtaining conducting coating of over otherwise insulating fabrics, but it is extremely difficult to suppress the secondary growth and achieve coating selectively and exclusively over individual fibers. Nevertheless, the formation of coating is expected to provide the surface conductivity to fabrics

(due to formation of interconnected conducting domains) which is a desirable condition for antistatic or EM radiation shielding applications. The room temperature electrical conductivity of uncoated cotton fabric was ~10−11 S/cm. However, conductivity of PANI coated fabrics i.e. PSC (5.1×10−3S/cm) and PAC (5.9×10−3 S/cm) conforms to the conductivity range specified in the Electronics Industries Association (EIA) standards for ensuring safe, rapid and efficient dissipation of electrostatic charges3,6,7. The simplest way to test the charge retention capability of materials is to put some charge on the material and see how quickly this charge disappears. Corona charging provides a rapid and repeatable non-contact means of depositing charge on any surface, with controllable magnitude and polarity. The schematic of the basic arrangement for the measurement of charge decay profile as well as quantity of deposited corona charge is shown in Fig. 3. The sample of suitable size was sandwiched between the two hinged flat metal plates and placed in such a way so that it completely covers the rectangular cavity in instrument’s base plate (~55 × 64 mm2). The surface is charged by a high voltage corona discharge from the tips of a small conical cluster of fine wires mounted on the underside of a light moveable plate. This plate is moved between the field-meter sensing aperture (45 × 54 mm2) and the material surface exposed through the instrument base plate. A fast response 'field mill' electrostatic field-meter gives fast, sensitive and stable measurement of surface potential.

The actual measurement of antistatic charge decay time involves application of corona voltage of 5000 V and recording the time required for decay of accepted voltage to 10% of its initial strength. This time is known as 10% cut-off limit and the corresponding 50% discharge is referred as 50% cut-off limits. The antistatic material suitable for any commercial applications should show 10% cut-off decay time3,6,7 of less than 2.0 s. The static decay profile of blank cotton (Fig. 4a) shows a peak at 150.09 V which indicates that cotton accepted only ~3.0% of the applied voltage (5.0 kV). Further, the accepted surface voltage decayed to 10% cut-off limit in 3.54 s that was higher than limiting time of 2.0 s. Such a low dissipation rate can be ascribed to the inherent insulating nature of the cotton. However, on coating cotton with conducting PANI-PSS or PANI-PAA, charge acceptability and retention capability drastically decreases (Fig. 4b), due to rapid charge

Fig. 2 — SEM images of pure cotton fabric (a) at low (5.0 X) & (b) high (1.0 kX) magnifications and PANI coated fabrics viz. (c) PSC & (d) PAC

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dissipation. Therefore, PSC accepted 42.3 V of applied corona voltage (5.0 kV), which decays to 10% cut-off within 0.15 s. Similarly, PAC accepted only 36.4 V of applied corona voltage (5.0 kV), which rapidly decays within 0.13 s. The above decay time values are much less than the antistatic criteria of 2.0 s and ensures efficient ESD performance of fabric under service conditions. The good antistatic response of these textiles arises due to coated fiber that forms conducting domains having high aspect ratio accruing from the long lengths of individual fibers. Further, the woven fabric also enhances the charge transport due to the formation of continuous network of interconnected conducting fibers over whole surface of fabric. Electromagnetic (EM) shielding depends on the electrical conductivity (intrinsic property) and thickness (extrinsic property) of the shielding material. As shown in schematically in Fig. 5, a part of incident energy (Ei, Hi) gets reflected (Er, Hr) by

�

Absorbance

(A)

Incident Wave

(Ei, Hi)

Transmitted Wave

(Et, Ht)

Transmittance (T)

Reflected Wave

(Er, Hr)

Reflectance (R)

Multiple Internal

Reflection (MIR)

2nd Reflection

2nd Transmission

Shield Thickness (t)

Fig. 5 — Schematic representation of EMI shielding phenomenon showing reflection, absorption and multiple reflection steps

shielding whereas other part was attenuated/absorbed by bulk of the shielding before being transmitted (Et, Ht) from the other face. Besides multiple internal reflections (MIRs) or re-reflection that occurs due to

Fig. 3 — Schematic of JCI static charge decay measurement unit

Fig. 4 — Static decay profiles of different fabrics viz. (a) cotton, (b) PSC & PAC

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reflections between opposite faces of shield can also contribute towards total shielding. The efficiency of shield is measured in terms of quantity called “shielding effectiveness (SET)” and can be expressed1-3,6,19-20 as :

t

10 10 10 ( ) 10 log 20log 20logt t

T

i i i

P E HSE dB

P E H

� � � � � �= = =� � � � � �

� � � � � �

...(1)

where Pi (Ei or Hi) and Pt (Et or Ht) are the power (electric or magnetic field intensity) of incident and transmitted EM waves, respectively. Fig. 6 shows the measured frequency dependence of SET values of the cotton fabrics coated with dispersions of PSS and PAA i.e. PSC and PAC, respectively. The results show that total shielding effectiveness (SET) value of the single layer of PSC (Fig. 6a) and PAC (Fig. 6b) was −1.5 dB and −1.7 dB, respectively. Such low values can be attributed to relatively low conductivity (0.19 S/cm for PANI-PSS and 0.95 S/cm for PANI-PAA) of the PANI doped with polymeric dopants (PSS or PAA) compared to that doped with conventional dopants26,27 (e.g. 2.1 S/cm and 1.1 S/cm for PANI doped with DBSA and LiSIPA, respectively). However, when the thickness was increased to by stacking four layers, the SET values were enhanced to −5.2 dB (PAC) and −5.7 dB (PSC), that represented attenuation level of 69.8% and 73.1%, respectively. The above improvement in SET

values can be attributed to the enhancement of absorption loss which is related to thickness of absorber1,6,19,20. Further, the multiple reflection contribution due to impedance mismatch at interfaces also contributes towards cumulative absorption. In addition, stacking brings additional advantage that microwave-transparent inter-weave spaces (voids) of

one layer are partially or completely covered by subsequent layer(s). 4 Conclusions

Conducting fabrics were successfully synthesized by dip coating of cotton fabrics with aqueous dispersions of poly(acrylic acid) (PAA) and polystyrene sulphonic acid (PSS) doped polyaniline. XRD of these fabrics shows the presence of characteristic peaks of polyaniline dominated by pronounced peaks of cotton substrate. SEM images display the formation of conducting PANI coatings over fabrics (individual fibers as well as in inter-fiber spacings) resulting in establishment of electrical conductivity. These fabrics show rapid dissipation (~0.1 to 0.2 s) of static charges and conform to antistatic criteria of 10%-cutoff decay time of less than 2.0 s. In addition, these fabrics also display finite EM radiation shielding capabilities with shielding effectiveness (SE) value (for a four layered shield) of −5.2 dB and −5.7 dB for PAC (PANI-PAA-cotton fabric) and PSC (PANI-PSS-cotton fabric) respectively. Acknowledgement

Authors wish to thank Director NPL for giving permission to publish the results. Authors also thank Dr N Vijayan for XRD patterns and Mr K N Sood for SEM images. References

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Fig. 6 — Frequency dependence of total shielding effectiveness (SET) for multilayered shield based on PANI-PAA or PANI-PSS coated cotton fabrics i.e. (a) PSC & (b) PAC

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