Complex Permittivity and Microwave Absorption Studies of Polypyrrole Doped Polyvinylchloride Films

Transcription

1 Advance in Electronic and Electric Engineering. ISSN , Volume 4, Number 4 (214), pp Research India Publications Complex Permittivity and Microwave Absorption Studies of Polypyrrole Doped Polyvinylchloride Films Vaishali Bhavsar 1 *and Deepti Tripathi 2 1 General Department, Institute of Diploma Studies, Nirma University, Ahmedabad, Gujarat. 2 Department of Physics School of Sciences, Gujarat University, Ahmedabad, Gujarat. Abstract Electromagnetic interference is a cause of concern when emitted electromagnetic fields interfere with operation of other electronic equipments. Given rapid development in commercial, military, scientific electronic devices and communication instruments operating in microwave frequency range (GHz), there has been an increased interest in development of such materials that could shield against electromagnetic radiation to protect interference. Conducting polymers can be used as microwave absorbing materials with or without fillers. In the present paper we report the complex permittivity measurements of pure PVC and PVC doped with varying concentrations of PPy films at microwave frequency of 9.37 GHz ( X-band) using microwave bench at room temperature. Further the microwave absorption properties of these films of various thicknesses have been investigated. The maximum absorption coefficient of 118 m -1 was obtained for film of average thickness 85 µm and with 16% concentration of PPy in PVC films. Keywords: Polyvinylchloride, Polypyrrole, Complex Permittivity, Absorption coefficient, Skin depth. 1. Introduction Conducting polymers constitute a family of organic conductors whose electric properties have been studied by many researchers. These materials are still the object of many investigations on the level of their potential applications such as electrochromic devices, corrosion inhibitors, compact capacitors, antistatic coatings

2 418 Vaishali Bhavsar & Deepti Tripathi and electromagnetic interference (EMI) shielding of electronic devices.emi shielding occurs by the reflection and/or absorption of electromagnetic radiation by material which acts against the penetration of radiation through it. Application of appropriate microwave absorbing material in electronic equipments help to control excessive self emission of electromagnetic waves and ensures undisturbed functioning of the equipment in the presence of external electromagnetic radiations (Abbas et al,26). The composite materials having conductive filler with small unit size are attractive for shielding. But conducting polymers do not require any conductive filler in order to provide shielding. Conducting polymers suffer from the limitations of poor processability and mechanical strength. These limitations can be improved by incorporating conducting polymer filler with an insulating polymeric matrix which underlies persistently promising aim towards processability of conducting polymers simultaneously giving mechanical strength (Deshmukh et al, 28).Now a days with proliferation of commercial, military, scientific electronic devices and equipments in high frequency (GHz) ranges,electromagnetic shielding becomes more of a problem. To protect against incoming and potentially disturbing radiation from penetrating into the equipment, electronic devices must be shielded (Abbas et al, 26). To answer some industrial and military (from 8 GHz) shielding applications, it is very important to study the absorption coefficient and penetration depth of microwave absorbing materials. Among all conducting polymers, Polypyrrole (PPy) has gained much interest due to some of its properties like environmental stability, low cost, relatively high conductivity and commercial availability. So Polypyrrole has been selected as conducting polymer in the present work. On the other hand Polyvinylchloride (PVC) which has good resistance to weathering and good mechanical strength has been selected as insulating polymeric matrix. In the present study free standing Polypyrrole (PPy) incorporated PVC films were prepared by a technique called mixing oxidative polymerization (John and others). Pure PVC and PPy doped PVC films with different proportions of PPy with varying thicknesses were prepared by solution cast method. The average thicknesses of these films lie in the range of 263µm-85µm. The complex permittivity of these films were measured at microwave frequency of 9.37 GHz (X-band) using microwave bench and technique developed by Dube and Natrajan (Tanwar et al, 26). Microwave absorbing properties of these films such as absorption coefficient and skin depth were derived which are important parameters for applications like electromagnetic shielding. The effect of film thickness on absorption coefficient and skin depth is analyzed. 2. Experimental 2.1 Material Polyvinylchloride (PVC) (molecular weight 62, gm/mol) supplied by Sigma Aldrich was used as an insulating matrix. Pyrrole monomer supplied by Spectrochem, India, was used as received. Tetrahydrofurran purchased from HPLC was used as

3 Complex Permittivity and Microwave Absorption Studies of Polypyrrole Doped 419 solvent and Anhydrous Ferric Chloride purchased from Otto Chemicals, was used as oxidant for polymerization of Pyrrole monomer. 2.2 Sample preparation In present work, free standing films of pure PVC and Polypyrrole (PPy) incorporated PVC films with different concentrations of PPy of varied thicknesses have been prepared at room temperature by technique called mixing oxidative polymerization. The films were prepared by solution cast technique at room temperature (24 C). The concentration of PPY in PVC was kept as 6%, 11%,14%,16% and 2%. The thickness of the films was determined using micrometer screw gauge. 2.3 Complex Permittivity Measurements The dielectric measurements in the microwave region (X-band, 9.37 GHz) were carried out using a technique developed by Dube (1984) and Dube and Natrajan (1973). Microwave bench was used for this purpose in which the sample was mounted along the axis of waveguide. With this configuration the field acts in the plane of the film. The advantage of this method is that the thin specimen is placed longitudinally at the centre of the broad side of a hollow rectangular waveguide excited in TE 1 mode, so that the whole specimen remains in the maximum electric field. Measurements of dielectric parameters such as dielectric constant (ε ),dielectric loss (ε ), loss tangent (tanδ) for pure PVC films and PVC films with 6%,11%,14% and 2% concentration of PPY of average thickness 63 µm were done at room temperature. From these data microwave ac conductivity (σ), absorption coefficient (α) and skin depth (δ) were evaluated. Further PVC films with two compositions i.e. 11% and 16% of PPy of different thicknesses (263µm,63 µm and 85µm) were taken up for complex permittivity measurements and the absorption coefficient (α) and skin depth (δ) were further evaluated to analyze the effect of thickness of the polymer films on these parameters. 3. Results and Discussions 3.1 Variation of dielectric properties with concentration of PPy in PVC films of average thickness 63µm Figure 1 shows the variation of dielectric constant (ε ) and loss tangent (tanδ) with varying concentrations of PPy in PVC films. Dielectric constant of PPy doped PVC films is more than that pure PVC films. It also attains maximum value for PPy concentration of 16% in PVC films. Variation of dielectric loss (ε ) and microwave ac conductivity (σ) of these samples are shown in Fig. 2.The dielectric loss shows an increasing trend with the increase of concentration of PPy in PVC upto 16%. The origin of dielectric loss in polymers is attributed to the dipolar absorption dispersion in both crystalline and amorphous polymers (Tanwar et al, 26).Since Conductivity is directly related to the dielectric loss factor, it also shows increasing trend up to 16% concentration and then it decreases for 2% concentration of PPy in PVC.

4 42 Vaishali Bhavsar & Deepti Tripathi % 6% 11% 14% 16% 2% 2.5E+ 2.E+ 1.5E+ 1.E+ 5.E-1.E+ Dielectric Constant Loss tangent Fig. 1: Variation of Dielectric constant and Loss tangent with variation in % concentration of PPy in PVC for films of average thickness 63 µm E-2 3.5E-2 3.E-2 2.5E-2 2.E-2 1.5E-2 1.E-2 5.E-3 Dielectric loss Conductivity (S/cm) % 6% 11% 14% 16% 2%.E+ Fig. 2: Variation of Dielectric loss and Conductivity with variation in % concentration of PPy in PVC for films of average thickness 63 µm. In heterogeneous dielectric due to the accumulation of virtual charge at the interface of two media having different dielectric constants and conductivities, interfacial polarization takes place. This interfacial loss depends on the quantity of weakly polar material as well as on its geometrical shape and its dispersion. The quantity and the geometry of PPY doped PVC films for 16% concentration of PPY is

5 Complex Permittivity and Microwave Absorption Studies of Polypyrrole Doped 421 more favorable for higher interfacial polarization. This may be the reason why maximum conductivity was shown by this composition (John et al, 27). The absorption of electromagnetic waves when it passes through the medium is given by absorption coefficient (α) which is given as " α = Where n = ε and c is the velocity of light. Penetration depth also called the Skin depth (δ) is basically the effective distance of penetration of an electromagnetic wave into the material and is defined as δ = 1/α (John and others). Figure 3 shows the variation of absorption coefficient (α) and skin depth (δ) with % variation in concentration of PPy in PVC. Absorption coefficient is a measure of propagation and absorption of EM waves when it passes through the medium. The dielectric materials can be classified in terms of this parameter indicating transparency of electromagnetic waves passing through it (John et al, 27) Absorption Coefficient (1/m) Skin depth (m) % 6% 11% 14% 16% 2% Fig. 3: Variation of Absorption Coefficient and Skin depth with variation in % concentration of PPy in PVC for films of average thickness 63 µm. The absorption coefficient is dependent on dielectric loss and so it shows the same trend as that of dielectric loss. As revealed by the graph skin depth decreases on increasing the PPy concentration in PVC. When skin depth decreases the material becomes more opaque to electromagnetic radiation. It is clear that absorption coefficient is high for 16% concentration of PPy in PVC and skin depth for this composition is the lowest as skin depth is inversely related to absorption coefficient.

6 422 Vaishali Bhavsar & Deepti Tripathi 3.2 Variation of complex permittivity and microwave absorption properties for two compositions of PPY doped PVC films of varying thicknesses. Here two compositions are selected to further investigate the effect of thickness on dielectric loss, microwave ac conductivity, absorption coefficient and skin depth. Fig. 4 shows variation of dielectric loss for two compositions of PPy doped PVC films of thicknesses 263µm, 63µm and 85µm % 16% Dielectric loss of films of average thickness 263micrometer Dielectric loss of films of average thickness 63micrometer Dielectric loss of films of average thickness 85micrometer Fig. 4: Variation of Dielectric Loss and Absorption coefficient for two concentrations of PPy in PVC for films of various thicknesses. It is clear from the figure that the dielectric loss increases with increase in thickness of the films. Conductivity (Fig. 5) being a function of dielectric loss also shows the same trend. 4.E-2 3.5E-2 3.E-2 2.5E-2 2.E-2 1.5E-2 1.E-2 5.E-3.E+ 11% 16% Conductivity of films of average thickness 263micrometer Conductivity of films of average thickness 63micrometer Conductivity of films of average thickness 85micrometer Skin depth for films of average thickness 263 micrometer Skin depth for films of average thickness 63 micrometer Skin depth for films of average thickness 85 micrometer Fig. 5: Variation of Conductivity and Skin depth with for two concentrations of PPy in PVC for films of various thicknesses.

7 Complex Permittivity and Microwave Absorption Studies of Polypyrrole Doped 423 The absorption coefficient (Fig. 4) is found to increase with increasing thickness of the films for both the compositions and showing the maximum value of 118 m -1 for the film of average thickness of 85µm for 16% concentration of PPy in PVC. The skin depth (Fig. 5) being inversely related to the absorption coefficient shows a reduction with increase in thickness of the films. 4. Conclusions For the films of average thickness 63µm, the maximum conductivity of 3.4x1-2 S/cm is obtained for 16% PPy doped in PVC. The PVC films for 16% concentration of PPY are more favorable for higher interfacial polarization hence showing higher conductivity. The same composition shows maximum absorption coefficient of 113 m - 1. The dielectric loss and the microwave ac conductivity were found to increase with increasing thickness of the PPy doped PVC films. The absorption coefficient is also found increase with increasing thicknesses of the films.the skin depth being inversely related to the absorption coefficient shows a reduction with increase in thickness of the films. 5. Future Scope of Work Attempts are being made to investigate the variation in complex permittivity and microwave absorbing properties for other compositions of PPy doped PVC films in the X-band (8-12 G Hz) and to attain the value of absorption coefficient above 2 m -1 so that these films can be used as microwave absorbers. Work is also in progress to study the UV-Visible spectra of these films to analyze their optical properties. 6. Acknowledgements Experimental facilities developed using financial assistance provided through the DST- FIST (Level- I) have been utilized to carry out this work and it is gratefully acknowledged. Authors are thankful to Prof. P.N. Gajjar, Head, Department of Physics, School of Sciences, Gujarat University, and Ahmedabad for his constant encouragement. Authors are also thankful to Dr. V.A. Rana, Associate Prof., and Dept. Of Physics, Gujarat University for his kind support in taking microwave measurements. References [1] A Tanwar, K K Gupta, P J Singh and Y K Vijay (26), Dielectric measurements on PWB materials at microwave frequencies, Bull. Mater Sci., Vol.29, No.2, pp [2] Honey John, Rinku M. Thomas, Rani Joseph, K T Mathew, and Studies on the dielectric behavior of polypyrrole and its semi interpenetrating networks with polyvinylchloride in the microwave field, TA12 Dielectric Property Measurements and Techniques.

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