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Microwave studies on ZnS–HSR ionomers
Materials Chemistry and Physics 79 (2003) 187–190
Microwave studies on ZnS–HSR ionomers K.T. Mathew a,∗ , S. Biju Kumar a , Anil Lonappan a , Joe Jacob a , Thomas Kurian b , Jacob Samuel b , Thommachan Xavier b a
b
Department of Electronics, Cochin University of Science and Technology, Kochi 682022, India Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Kochi 682022, India
Abstract Ionic polymers or ionomers are emerging as important commercial polymers. They are cross-linked materials with thermally reversible behavior. The investigations on the dielectric behavior of a class of ionomers at microwave frequencies are performed employing cavity perturbation technique. Dielectric parameters such as the complex permittivity, conductivity, absorption coefficient and heating coefficient are estimated and the results are presented. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Conducting polymers; Microwaves; Cavity perturbation technique; Complex permittivity
1. Introduction Ionic polymers generally known as ionomers basically contain a small percentage of ionic groups attached to a hydrocarbon backbone. The ionic interaction between the groups result significant changes in the mechanical properties [1,2]. The ionomer has an intermediate position between purely organic structure and purely inorganic structure. The thermally reversible behavior and enhanced mechanical properties make it an ideal material for many industrial applications. Investigations on the dielectric behavior of these materials at microwave frequencies are of importance considering the potential use as electromagnetic interference (EMI) shielding and absorbing materials. Some studies on the absorption and shielding of conducting polymers and composite materials at microwave frequencies have been reported [3,4]. A detailed study on the dielectric parameters at different microwave frequencies for different zinc contents on ionomer base sample is carried out using cavity perturbation technique and reported in this paper.
2. Sample preparation Samples were prepared by sulphonation of high-styrene resin (HSR) using the sulphonating agent, acetyl sulphate, generated in situ, from acetic anhydride and concentrated sulphuric acid. The HSR sulphonic acid produced was neu∗ Corresponding author. Tel.: +91-484-576418; fax: +91-484-575800. E-mail address: [email protected] (K.T. Mathew).
tralized by a solution of zinc acetate in methanol. The zinc sulphonated HSR was washed several times with water and then it was vacuum dried at 50 ◦ C. This ionic polymer is hereafter represented as x·y ZnS–HSR, where x·y shows the number of meq. of sulphonic acid per 100 g of HSR. Five samples were considered, they are I0 , I10 , I20 , I30 and I40 . I0 represents the pure HSR. I10 , I20 , I30 and I40 indicate 10.6, 20.4, 34.38 and 42.7 meq. of sulphonate per 100 g of HSR, respectively. Quantity of sulphonate is analyzed using X-ray fluorescence (XRF) method. Samples were prepared in the form of thin sheets, cut into thin strips and then introduced into the cavity resonator for measurement.
3. Experimental set up and procedure The cavity resonator is constructed with a portion of a transmission line (waveguide or coaxial line) with one or both ends closed. The resonator can be either transmission or reflection type. There is a coupling device (iris) to couple the microwave power to the resonator. The length of the resonator determines the number of resonant frequencies. The transmission type resonator used in this experiment is excited with TE10P mode by connecting it to an HP8510 C Network Analyzer. The resonant frequencies and the corresponding quality factors of the unloaded cavity resonators are given in Table 1. The resonant frequency fo and the corresponding quality factor Qo of each resonant peak of the empty cavity are first determined. Now after selecting a particular resonant frequency, the dielectric sample is introduced into the
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 2 9 1 - 2
188
K.T. Mathew et al. / Materials Chemistry and Physics 79 (2003) 187–190
Table 1 The characteristic features of rectangular cavity resonators Type of cavity S-band cavity (TE103 –TE107 )
Resonant frequency (GHz)
Unloaded Q-factor
2.4397 2.6833 2.9692 3.2853 3.6237
4879 5366 3711 2986 2001
C-band cavity (TE103 –TE107 )
5.0992 5.6335 6.2497 6.9282 7.6580
3399 5123 2718 2887 2470
X-band cavity (TE105 –TE109 )
8.5725 9.3279 10.1460 11.0130 11.9210
2090 1635 1921 1917 1993
which is associated with the dielectric loss of the material. Vs and Vc are the volumes of the sample and the cavity resonator, respectively.
5. Results and discussion 5.1. Relative complex permittivity (εr∗ )
cavity and the position is adjusted for maximum perturbation (i.e. maximum shift of resonant frequency towards the low frequency region, with minimum amplitude for the peak) and the corresponding change in the frequency of the resonant peak is observed. The new resonant frequency fs the corresponding 3 dB bandwidth and the quality factor Q are determined. The procedure is repeated for other resonant frequencies and the measurements are carried out for all the samples. Knowing the volumes of the sample and the cavity resonator the dielectric parameters can be evaluated. A very precise determination of the volume of the sample is to be ensured, to get accurate values for the parameters.
4. Theory The basic principle of the cavity perturbation technique is that a small dielectric sample introduced into a cavity produces a slight variation (perturbation) of the resonant frequency of the cavity. The dielectric parameters of the sample are evaluated from the resonant frequency and the quality factor of the empty cavity resonator and the loaded cavity [5]. fo − f s V c εr − 1 = (1) 2fs Vs Vc Qo − Qs εr = (2) 4Vs Qo Q s
Fig. 1 shows that the relative dielectric constant (εr ) of ionomer increases with increase in the ionic content. The polarization is caused by the alternating accumulation of charges at interfaces [6] between different phases of the material. Due to the orientation polarization of the dipoles, the possibility of dielectric relaxation (so also dielectric loss) cannot be ruled out at higher frequencies. This might result in the decrease of εr with frequency [7]. This dipole polarization may be related to the “frictional” losses caused by the rotational displacement of molecular dipoles under the influence of the alternating electrical field. The improvement in the dielectric constant with ionic content may be due to the increase in the polar centers such as unassociated ionic moieties, multiplets, ionic clusters and zinc metal ions in the polymer matrix. The microwave responses of these charge centers may be contributing towards the dielectric constant. As frequency decreases, the dipoles move more and more with an associated loss of energy. Fig. 2 shows the variation of imaginary part of complex permittivity with frequency for different ionomer samples. The reorientation of the unassociated groups, because of their high dipole moment, is believed to be a major contributor to the dielectric loss (εr ). At higher frequency, the rotatory motion of the molecules may not be sufficiently rapid for the attainment of equilibrium with the field. The polarization then acquires a component out of phase with the field, and the displacement current acquires a conductance component in phase with the field, resulting in thermal dissipation of energy. When this occurs, dielectric losses will be generated. At higher ionic content, the molecular interactions would lose the flexibility of the polymer chains. The
The effective conductivity (σ e ) is given by: σe = ωε = 2πfs εo εr
(3)
Here, ε¯ r = εr −j εr , ε¯ r is the relative complex permittivity of the sample, εr the real part of the relative complex permittivity, which is usually known as dielectric constant and εr the imaginary part of the relative complex permittivity,
Fig. 1. Variation of the real part of complex permittivity with frequency.
K.T. Mathew et al. / Materials Chemistry and Physics 79 (2003) 187–190
189
Fig. 2. Variation of imaginary part of the complex permittivity with frequency.
Fig. 4. Variation of effective conductivity with frequency.
dipole attached to a flexible chain can reorient more easily than one attached to a stiff chain. The loss tangent is defined as tan δ=εr /εr . It has been found that the tan δ increases with the increase in the ionic concentration as shown in Fig. 3. The tan δ is commonly employed as a direct measure of the dielectric loss, which in turn provides a measure of the conductivity.
However, one application that takes advantage of a high value of loss tangent is high frequency dielectric heating. In this application, the efficiency of heating is usually compared [8] by means of a comparison coefficient J, which is defined as: 1 J = (4) εr tan δ
5.2. Effective conductivity (σ e ) The effective conductivity is found to increase as the zinc content increases gradually from 0 to 42.7 meq. of sulphonate per 100 gm of the base material. The initial increase of the conductivities with the ion content may be expected to be related to the fact that polar ionic groups form nanoclustering which act as centers of conductances [7]. Fig. 4 gives the variation of conductivity for different frequencies. 5.3. Microwave heating coefficient (J)
The Fig. 5 shows the variation of J with ionic content for different microwave frequencies. Higher the J value, poorer will be the polymer for dielectric heating purposes. Of course, the heat generated in the polymeric material comes from the loss tangent, but that loss may not come entirely from the relaxation loss. Rather, conductivity of the polymeric material may also contribute to the tan δ. This situation may be compared with ohmic heating of metals: the charge carriers are electrons; where as those in dielectric polymeric materials may be ions. 5.4. Absorption coefficient (α)
Practically all applications of polymers in electrical and electronic engineering require materials with a low tan δ.
Materials can be classified in terms of their transparency of wave through it, which in turn specify the absorption of electromagnetic waves when it passes through the medium. The transparency is defined by the parameter, absorption
Fig. 3. Variation of loss tangent with frequency.
Fig. 5. Variation of heating coefficient with frequency.
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K.T. Mathew et al. / Materials Chemistry and Physics 79 (2003) 187–190
[10]. More investigations in this direction are yet to be needed. Acknowledgements Authors, K.T. Mathew, Anil Lonappan and Joe Jacob are thankful to Department of Science and Technology, Government of India for the financial assistance through the project SP/SO/D-22/98. One of the authors S. Biju Kumar thankfully acknowledges Council of Scientific and Industrial Research, Government of India for providing Senior Research Fellowship. Fig. 6. Variation of absorption coefficient with frequency.
References
coefficient [9], which is defined by the equation: α=
π ε f nc
(5)
√ where n is the real part of the complex refractive index ε ∗ , c the velocity of light. A plot of the absorption coefficient of the materials with frequency is shown in Fig. 6.
6. Conclusion An exhaustive study of the dielectric characteristics of ZnS–HSR ionomers at a range of microwave frequencies is carried out in this paper. The results show that these materials have conductivity range comparable with semiconductors. The variation of heating coefficient and absorption coefficient with frequency and concentration is presented. The study indicates that the material may qualify as good candidate for EMI shielding. Some of the samples are competent enough to be considered in microwave monolithic integrated circuits (MMIC) and other microwave devices
[1] D.Y. Chao, J. Kuo, N.H. Wang, C.L. Lee, K.H. Yang, Study of the synthesis and physical properties of fire-resistant polyurethane ionomers, J. Appl. Polym. Sci. 67 (1998) 19–26. [2] J. Samuel, T. Xavier, T. Kurian, lonomers, Prog. Rubber Plastics Technol. 16 (2000) 1–15. [3] Van-Tan Truong, John G. Teman, Complex conductivity of a conducting polymer composite at microwave frequencies, Polymer 36 (1995) 905–909. [4] P. Chandrasekhar, K. Naishadham, Broadband microwave absorption and shielding properties of a poly(aniline), Synth. Metals 2 (1999) 115–120. [5] K.T. Mathew, U. Raveendranath, Sensors Update, vol. 7, Germany, 1999, p. 185. [6] E.J. Frommer, R.R. Chance, Electrical and electronic properties of polymers: state of he art compendium, encyclopedia reprint series, in: I.J. Kroschwitz (Ed.), Wiley/Interscience, New York, 1988, pp. 101–178. [7] T.A. Ezquerra, F. Kremmer, G. Wegner, AC electrical properties of insulator conductor composites, in: A. Priou (Ed.), Dielectric Properties of Heterogeneous Materials: Progress in Electromagnetic Research, vol. 6, Elsevier, New York, 1992. [8] C.Ku Chen, Raimond Liepins, Electrical Properties of Polymers: Chemical Principles, Hanser Publishers, Munich, 1987 (Chapter 3). [9] Bradford L. Smith, M. Carpentier, The Microwave Engineering Handbook, vol. 3, Chapman & Hall, London, 1993. [10] F. Larry, C. Thompson, Grant Wilson, Seiichi Tagawa (Eds.), Polymers for Microelectronics-Resists and Dielectrics, Oxford University Press, Oxford, 1994.
Microwave studies on ZnS–HSR ionomers K.T. Mathew a,∗ , S. Biju Kumar a , Anil Lonappan a , Joe Jacob a , Thomas Kurian b , Jacob Samuel b , Thommachan Xavier b a
b
Department of Electronics, Cochin University of Science and Technology, Kochi 682022, India Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Kochi 682022, India
Abstract Ionic polymers or ionomers are emerging as important commercial polymers. They are cross-linked materials with thermally reversible behavior. The investigations on the dielectric behavior of a class of ionomers at microwave frequencies are performed employing cavity perturbation technique. Dielectric parameters such as the complex permittivity, conductivity, absorption coefficient and heating coefficient are estimated and the results are presented. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Conducting polymers; Microwaves; Cavity perturbation technique; Complex permittivity
1. Introduction Ionic polymers generally known as ionomers basically contain a small percentage of ionic groups attached to a hydrocarbon backbone. The ionic interaction between the groups result significant changes in the mechanical properties [1,2]. The ionomer has an intermediate position between purely organic structure and purely inorganic structure. The thermally reversible behavior and enhanced mechanical properties make it an ideal material for many industrial applications. Investigations on the dielectric behavior of these materials at microwave frequencies are of importance considering the potential use as electromagnetic interference (EMI) shielding and absorbing materials. Some studies on the absorption and shielding of conducting polymers and composite materials at microwave frequencies have been reported [3,4]. A detailed study on the dielectric parameters at different microwave frequencies for different zinc contents on ionomer base sample is carried out using cavity perturbation technique and reported in this paper.
2. Sample preparation Samples were prepared by sulphonation of high-styrene resin (HSR) using the sulphonating agent, acetyl sulphate, generated in situ, from acetic anhydride and concentrated sulphuric acid. The HSR sulphonic acid produced was neu∗ Corresponding author. Tel.: +91-484-576418; fax: +91-484-575800. E-mail address: [email protected] (K.T. Mathew).
tralized by a solution of zinc acetate in methanol. The zinc sulphonated HSR was washed several times with water and then it was vacuum dried at 50 ◦ C. This ionic polymer is hereafter represented as x·y ZnS–HSR, where x·y shows the number of meq. of sulphonic acid per 100 g of HSR. Five samples were considered, they are I0 , I10 , I20 , I30 and I40 . I0 represents the pure HSR. I10 , I20 , I30 and I40 indicate 10.6, 20.4, 34.38 and 42.7 meq. of sulphonate per 100 g of HSR, respectively. Quantity of sulphonate is analyzed using X-ray fluorescence (XRF) method. Samples were prepared in the form of thin sheets, cut into thin strips and then introduced into the cavity resonator for measurement.
3. Experimental set up and procedure The cavity resonator is constructed with a portion of a transmission line (waveguide or coaxial line) with one or both ends closed. The resonator can be either transmission or reflection type. There is a coupling device (iris) to couple the microwave power to the resonator. The length of the resonator determines the number of resonant frequencies. The transmission type resonator used in this experiment is excited with TE10P mode by connecting it to an HP8510 C Network Analyzer. The resonant frequencies and the corresponding quality factors of the unloaded cavity resonators are given in Table 1. The resonant frequency fo and the corresponding quality factor Qo of each resonant peak of the empty cavity are first determined. Now after selecting a particular resonant frequency, the dielectric sample is introduced into the
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 2 9 1 - 2
188
K.T. Mathew et al. / Materials Chemistry and Physics 79 (2003) 187–190
Table 1 The characteristic features of rectangular cavity resonators Type of cavity S-band cavity (TE103 –TE107 )
Resonant frequency (GHz)
Unloaded Q-factor
2.4397 2.6833 2.9692 3.2853 3.6237
4879 5366 3711 2986 2001
C-band cavity (TE103 –TE107 )
5.0992 5.6335 6.2497 6.9282 7.6580
3399 5123 2718 2887 2470
X-band cavity (TE105 –TE109 )
8.5725 9.3279 10.1460 11.0130 11.9210
2090 1635 1921 1917 1993
which is associated with the dielectric loss of the material. Vs and Vc are the volumes of the sample and the cavity resonator, respectively.
5. Results and discussion 5.1. Relative complex permittivity (εr∗ )
cavity and the position is adjusted for maximum perturbation (i.e. maximum shift of resonant frequency towards the low frequency region, with minimum amplitude for the peak) and the corresponding change in the frequency of the resonant peak is observed. The new resonant frequency fs the corresponding 3 dB bandwidth and the quality factor Q are determined. The procedure is repeated for other resonant frequencies and the measurements are carried out for all the samples. Knowing the volumes of the sample and the cavity resonator the dielectric parameters can be evaluated. A very precise determination of the volume of the sample is to be ensured, to get accurate values for the parameters.
4. Theory The basic principle of the cavity perturbation technique is that a small dielectric sample introduced into a cavity produces a slight variation (perturbation) of the resonant frequency of the cavity. The dielectric parameters of the sample are evaluated from the resonant frequency and the quality factor of the empty cavity resonator and the loaded cavity [5]. fo − f s V c εr − 1 = (1) 2fs Vs Vc Qo − Qs εr = (2) 4Vs Qo Q s
Fig. 1 shows that the relative dielectric constant (εr ) of ionomer increases with increase in the ionic content. The polarization is caused by the alternating accumulation of charges at interfaces [6] between different phases of the material. Due to the orientation polarization of the dipoles, the possibility of dielectric relaxation (so also dielectric loss) cannot be ruled out at higher frequencies. This might result in the decrease of εr with frequency [7]. This dipole polarization may be related to the “frictional” losses caused by the rotational displacement of molecular dipoles under the influence of the alternating electrical field. The improvement in the dielectric constant with ionic content may be due to the increase in the polar centers such as unassociated ionic moieties, multiplets, ionic clusters and zinc metal ions in the polymer matrix. The microwave responses of these charge centers may be contributing towards the dielectric constant. As frequency decreases, the dipoles move more and more with an associated loss of energy. Fig. 2 shows the variation of imaginary part of complex permittivity with frequency for different ionomer samples. The reorientation of the unassociated groups, because of their high dipole moment, is believed to be a major contributor to the dielectric loss (εr ). At higher frequency, the rotatory motion of the molecules may not be sufficiently rapid for the attainment of equilibrium with the field. The polarization then acquires a component out of phase with the field, and the displacement current acquires a conductance component in phase with the field, resulting in thermal dissipation of energy. When this occurs, dielectric losses will be generated. At higher ionic content, the molecular interactions would lose the flexibility of the polymer chains. The
The effective conductivity (σ e ) is given by: σe = ωε = 2πfs εo εr
(3)
Here, ε¯ r = εr −j εr , ε¯ r is the relative complex permittivity of the sample, εr the real part of the relative complex permittivity, which is usually known as dielectric constant and εr the imaginary part of the relative complex permittivity,
Fig. 1. Variation of the real part of complex permittivity with frequency.
K.T. Mathew et al. / Materials Chemistry and Physics 79 (2003) 187–190
189
Fig. 2. Variation of imaginary part of the complex permittivity with frequency.
Fig. 4. Variation of effective conductivity with frequency.
dipole attached to a flexible chain can reorient more easily than one attached to a stiff chain. The loss tangent is defined as tan δ=εr /εr . It has been found that the tan δ increases with the increase in the ionic concentration as shown in Fig. 3. The tan δ is commonly employed as a direct measure of the dielectric loss, which in turn provides a measure of the conductivity.
However, one application that takes advantage of a high value of loss tangent is high frequency dielectric heating. In this application, the efficiency of heating is usually compared [8] by means of a comparison coefficient J, which is defined as: 1 J = (4) εr tan δ
5.2. Effective conductivity (σ e ) The effective conductivity is found to increase as the zinc content increases gradually from 0 to 42.7 meq. of sulphonate per 100 gm of the base material. The initial increase of the conductivities with the ion content may be expected to be related to the fact that polar ionic groups form nanoclustering which act as centers of conductances [7]. Fig. 4 gives the variation of conductivity for different frequencies. 5.3. Microwave heating coefficient (J)
The Fig. 5 shows the variation of J with ionic content for different microwave frequencies. Higher the J value, poorer will be the polymer for dielectric heating purposes. Of course, the heat generated in the polymeric material comes from the loss tangent, but that loss may not come entirely from the relaxation loss. Rather, conductivity of the polymeric material may also contribute to the tan δ. This situation may be compared with ohmic heating of metals: the charge carriers are electrons; where as those in dielectric polymeric materials may be ions. 5.4. Absorption coefficient (α)
Practically all applications of polymers in electrical and electronic engineering require materials with a low tan δ.
Materials can be classified in terms of their transparency of wave through it, which in turn specify the absorption of electromagnetic waves when it passes through the medium. The transparency is defined by the parameter, absorption
Fig. 3. Variation of loss tangent with frequency.
Fig. 5. Variation of heating coefficient with frequency.
190
K.T. Mathew et al. / Materials Chemistry and Physics 79 (2003) 187–190
[10]. More investigations in this direction are yet to be needed. Acknowledgements Authors, K.T. Mathew, Anil Lonappan and Joe Jacob are thankful to Department of Science and Technology, Government of India for the financial assistance through the project SP/SO/D-22/98. One of the authors S. Biju Kumar thankfully acknowledges Council of Scientific and Industrial Research, Government of India for providing Senior Research Fellowship. Fig. 6. Variation of absorption coefficient with frequency.
References
coefficient [9], which is defined by the equation: α=
π ε f nc
(5)
√ where n is the real part of the complex refractive index ε ∗ , c the velocity of light. A plot of the absorption coefficient of the materials with frequency is shown in Fig. 6.
6. Conclusion An exhaustive study of the dielectric characteristics of ZnS–HSR ionomers at a range of microwave frequencies is carried out in this paper. The results show that these materials have conductivity range comparable with semiconductors. The variation of heating coefficient and absorption coefficient with frequency and concentration is presented. The study indicates that the material may qualify as good candidate for EMI shielding. Some of the samples are competent enough to be considered in microwave monolithic integrated circuits (MMIC) and other microwave devices
[1] D.Y. Chao, J. Kuo, N.H. Wang, C.L. Lee, K.H. Yang, Study of the synthesis and physical properties of fire-resistant polyurethane ionomers, J. Appl. Polym. Sci. 67 (1998) 19–26. [2] J. Samuel, T. Xavier, T. Kurian, lonomers, Prog. Rubber Plastics Technol. 16 (2000) 1–15. [3] Van-Tan Truong, John G. Teman, Complex conductivity of a conducting polymer composite at microwave frequencies, Polymer 36 (1995) 905–909. [4] P. Chandrasekhar, K. Naishadham, Broadband microwave absorption and shielding properties of a poly(aniline), Synth. Metals 2 (1999) 115–120. [5] K.T. Mathew, U. Raveendranath, Sensors Update, vol. 7, Germany, 1999, p. 185. [6] E.J. Frommer, R.R. Chance, Electrical and electronic properties of polymers: state of he art compendium, encyclopedia reprint series, in: I.J. Kroschwitz (Ed.), Wiley/Interscience, New York, 1988, pp. 101–178. [7] T.A. Ezquerra, F. Kremmer, G. Wegner, AC electrical properties of insulator conductor composites, in: A. Priou (Ed.), Dielectric Properties of Heterogeneous Materials: Progress in Electromagnetic Research, vol. 6, Elsevier, New York, 1992. [8] C.Ku Chen, Raimond Liepins, Electrical Properties of Polymers: Chemical Principles, Hanser Publishers, Munich, 1987 (Chapter 3). [9] Bradford L. Smith, M. Carpentier, The Microwave Engineering Handbook, vol. 3, Chapman & Hall, London, 1993. [10] F. Larry, C. Thompson, Grant Wilson, Seiichi Tagawa (Eds.), Polymers for Microelectronics-Resists and Dielectrics, Oxford University Press, Oxford, 1994.