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Dielectric relaxation properties of poly(ethylene-terephthalate)–polyaniline composite films
Synthetic Metals 98 Ž1998. 157–160
Short communication
Dielectric relaxation properties of poly žethylene-terephthalate/ –polyaniline composite films A.A. Korzhenko a b
a,)
, M. Tabellout a , J.R. Emery a , A.A. Pud b, S. Rogalsky b, G.S. Shapoval
b
Laboratoire de Chimie et Physique des Materiaux Polymeres, ´ ` CNRS 6515, UniÕersite´ du Maine, BP535, 72085 Le Mans Cedex 9, France Institute of Bioorganic Chemistry and Petrochemistry, Ukrainian Academy of Sciences, 50 KharkoÕskoye Shosse, 253160, KieÕ, Ukraine Received 1 September 1998; accepted 1 October 1998
Abstract The dielectric relaxation measurements on polyŽethylene-terephthalate. ŽPET. containing adsorbed aniline and polyaniline ŽPAni., prepared by polymerization into the PET matrix, have been performed between 173 and 403 K in the frequency range 1 to 10 7 Hz. The influence of aniline and PAni on the b-relaxation of PET and the effect of the presence of PAni on the a-relaxation of PET have been studied. For the films PET–PAni doped with HCl, two relaxation processes around 0.5–5 kHz and 0.2–1 MHz were observed. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Dielectric spectroscopy; PolyŽethylene-terephthalate. ŽPET.; Polyaniline; Dielectric relaxation
1. Introduction PolyŽethylene-terephtalate. –polyanyline ŽPET–PAni. composites with transparent and surface conductive properties have a great potential in various electronic and electro-optical devices w1,2x, owing to surface conductive properties, chemical stability and low price. Varying the conditions of the preparation procedure and dopants it is possible to obtain the film with large spectrum of the properties of the conductive surface layer w1,2x. An investigation of this system by Dielectric Relaxation Spectroscopy ŽDRS. would be very helpful to optimize the properties of the PET–PAni composite for the concrete purposes and for testing of model devices as well. The present work is directed on the investigation of the dielectric relaxation processes of PET, PET with adsorbed aniline and PET containing PAni which are studied demonstrating the possibilities of DRS method for characterizing the dielectric–conductor systems.
parent matrix. The PET films were dipped in neat aniline which was further polymerized in an oxidant solution w2x. Both PET-Aniline and the polymerized composite ŽPET– PAni. were used for DRS measurements. The deepness of the penetration for the aniline is around 4–5 mm and the thickness of the surface layer, contained polyaniline is approximately 2–4 mm, as estimated by Raman Spectroscopy. DRS measurements were made with Broadband Dielectric Spectrometer ŽNovocontrol. in the frequency range 0.1 Hz to 10 MHz at the temperature 153 to 433 K, precision of the temperature stabilization being "0,1 K. In order to quantify the mean relaxation time in the range of present interest the frequency domain impedance analysis is the method of choice. Experimental dielectric spectrum ´ )Ž v . is fitted by the Havriliak–Negami ŽHN. function w3x: D´
´ ) Ž v . s ´` q
a
Ž 1 q Ž i vrv 0 . . 2. Experimental The PET double axially stretched films with crystalinity degree 69% and thickness 25 mm have been used as a )
Corresponding author
b
where v s 2p f ; ´` denotes the high frequency limit of the permittivity D ´ is the difference between the low and high frequency limits of ´ X over the relaxation to which the HN function applies, D ´ is also proportional to the area below the ´ Y relaxation peak; ´` is the unrelaxed value of permittivity; a and b are shape parameters.
0379-6779r98r$ - see front matter q 1998 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 8 . 0 0 1 7 9 - 9
A.A. Korzhenko et al.r Synthetic Metals 98 (1998) 157–160
158
Fitting of the experimental dielectric spectra ´ )Ž v . were made with WINFIT 2.4 Ž1996. software of Novocontrol GmbH.
3. Results Dielectric spectra have been obtained for PET, PET adsorbed aniline monomer Ž10% in mass. and PET film with polyaniline in nonconductive form at one side of the film. At temperatures below the glass transition we can see the b-relaxation peak of PET in dielectric spectrum. As we can see in Fig. 1, the presence of aniline leads to a decrease of the relaxation strength and the loss peak shifts slightly towards high frequencies. For the PET–PAni the tendencies are less pronounced. The activation energies Ea of relaxation process were obtained from Arrhenius equation: ln Ž t . s ln Ž t 0 . q
Ea k BT
The values of the activation energies and relaxation parameters of the dielectric spectra at 233 K, obtained by fitting with HN function are presented in the table. Actually, the value Ea presents averaged relaxation energies because of a broad distribution function of activation energies as it has been shown by dielectric w4x and TSDS w5x measurements. At higher temperatures, above 373 K a broad loss peak appears which is attributed to a-relaxation process w6x. Fig. 2 illustrates this peak, obtained at 403 K for PET and PET–PAni Žfor the PET-Aniline sample we have a complicated picture caused by noncontrolled thermal polymerization of the aniline monomer inside the PET.. From this figure we can see that the presence of PAni, even in thin surface layer only, leads to the considerable shift of the loss peak to high frequencies. The activation energies of
Fig. 1. Dependencies of the imaginary part of dielectric permittivity on frequency at 233 K:1-PET, 2-PETqaniline, 3-PETqPAni. The spline curves are the HN fit results.
Y Fig. 2. Dependencies of ´ on frequency at 403 K for PET Ž1. and PET contained PAni Ž2.. The spline curves are the HN fit results.
the a-relaxation process were obtained using Vogel– Fulsher–Tamman–Hesse ŽVFTH. equation w7–9x:
t Ž T . s t 0 exp
Ea k B Ž T y Tv .
,
where Ea is constant being related to the activation energy, t 0 —the relaxation time at infinite temperature, k B — Boltzmann’s constant and Tv temperature at infinite relaxation time. For both PET and PET–PAni samples the magnitude of Ea is practically the same Ž0.20 " 0.05 eV.. The values of glass transition temperatures ŽTg . 365 and 378 Ž"2 K., for PET and PET–PAni respectively, were obtained from VFTH equation at t s 100 s. The rising of the imaginary part of permittivity at low frequencies, observed in Fig. 2, can be associated with ionic conductivity, that has the magnitudes 8.6 = 10y1 6 and 1.5 = 10y14 Srcm Žat 403 K. for PET and PET–PAni respectively. The PET–PAni sample has electroconductive properties at the surface after doping. The magnitude of the surface
Y
Fig. 3. Dependencies of ´ on frequency for PET contained PAni doped by HCl. The spline curves are the HN fit results.
A.A. Korzhenko et al.r Synthetic Metals 98 (1998) 157–160
Fig. 4. The Arrhenius plots for the low- and high-frequency relaxation peaks observed at Fig. 3.
conductivity of our sample doped by HCl is 6 = 10y5 S Žmeasured by four probe method.. This film gives two specific loss peaks, as we can see in Fig. 3. The intensity of these peaks are considerably higher than the ones associated with a- and b-relaxation processes described above, and their position does not change strongly with temperature. The low frequency peak is of Debye type Žthe symmetry constants of HN equation a s b s 1., for the high frequency peak a changes from 0.33 to 0.45 and b from 0.45 to 0.55 in the temperature region 203 to 303 K. Arrhenius plots are linear for both relaxation times up to 333 K ŽFig. 4., the values of the activation energies, obtained from the Arrhenius plots are 0.088 and 0.051 eV for low and high frequency peak respectively.
4. Discussion For the b-relaxation process of PET it has been shown that the local motions which are responsible for it come from the amorphous phase w4x, because the good proportionality between the amorphous proportion of a semicrystalline sample and the b-relaxation strength was observed. Thus, it can be supposed the effect of aniline and PAni presence on b-relaxation is mainly associated with a chain mobility in the amorphous phase of PET. The presence of aniline gives absolutely different result than a plastization effect of the water observed in Ref. w10x: considerable rising of the relaxation strength and decreasing of the relaxation energy. The significant decrease in
159
relaxation strength of the b-relaxation and the increase of activation energy in the presence of absorbed aniline ŽTable 1. can certify an appearance of more complexity of the local motions in PET, that might be explained by a strong interaction between adsorbed aniline and the chains of PET. In the case of PET–PAni we also observe decreasing of the relaxation strength, but the activation energy is less than for PET. The a-relaxation process of PET, which comprises more large-scale motions related to the glass-transition, does not depend linearly on the amorphous content w11x. The presence of the PAni does not change activation energy of the relaxation, but shifts Tg of PET to higher temperature. We can suppose that PAni fills less dense volumes inside the PET matrix, so that Tg of more compact sample is increased. The conductive sample, PET–PAni Ždoped by HCl., gives two relaxation processes ŽFig. 3.. Both peaks are present only when PAni is in conductive form. They are practically Debye type and their relaxation energies have very small values; therefore it can be suggested that the charge carriers are responsible for these phenomena. To give detailed explanation for the origin of these dielectric loss processes further investigations are in progress. However, in presenting these first results we can propose the following assumption. Actually, having such an inhomogeneous system like PET–PAni Žconductive. we can expect the manifestation of the interfacial effect, known under the general name of Maxwell–Wagner effect w3x, resulting in the distributions of Debye-like relaxation times in low frequency region. Thus, the low frequency peak in our experiments ŽFig. 3. can be attributed to interfacial polarization effect between conductive layer containing PAni and PET volume free from PAni. Considering the high frequency peak its similarity with the near-Debye process observed in the case of p–n junction in silicon w3x can be taken into account. This process with the relaxation time around 1 ms Žat 300 K. and activation energy 0.25 eV at high temperatures and 0.075 eV below 100 K. For PAni, a formation of charge carriers, its trapping and recombination are different from that for semiconductors like silicon. In the case of PAni the influence of dopant may be very important. Thus, the relaxation phenomenon we observe around 0.2–1 MHz might characterize the properties of the charge carriers in the layer containing doped PAni.
5. Conclusions Table 1 Parameters of the b-relaxation at 233 K obtained by fitting of the dielectric spectra with HN function Sample
D´
t, s
a
b
Ea , eV
PET PETqAnilin PETqPAni
0.25 0.15 0.17
9.9=10y3 2.9=10y3 7.7=10y4
0.34 0.27 0.28
0.23 0.40 0.55
0.68 1.00 0.58
The DRS study of the b-relaxation process of the PET in the presence of absorbed aniline reveals the difficulties of the local motions in PET because of aniline–PET interaction. This effect is absent in the case of PET–PAni. The presence of the latter leads to increasing the packing density of PET resulting in the shifting of Tg of PET to higher temperatures.
160
A.A. Korzhenko et al.r Synthetic Metals 98 (1998) 157–160
Two relaxation processes observed for the PET containing doped PAni characterize the properties of the conductive layer and charge carriers in it. Further investigation of the nature and properties can have a great importance in the following optimization of the polymer composites with electroconductive properties for utilization. References w1x H. Zhang, C. Li, Synth. Metals 44 Ž1991. 143. w2x A.A. Pud, G.S. Shapoval, V.P. Kukhar, Method of manufacture of
w3x w4x w5x w6x w7x w8x w9x w10x w11x
sensor conducting polymer composite material, Ukrainian claim for invention rights N94010153 of 11.05.93. A.K. Jonscher Dielectric relaxation in solids Chelsea Dielectric Press, 1996. T. Pop, D. Iordashe, A. Jonas, Microelectronic Eng. 33 Ž1997. 337. T. Pop, D. Iordashe, I. Pop, I. Ionescu, Rom. Rep. Phys. 46 Ž1994. 573. T. Tatsum, E. Ito, J. Polymer Sci., Polymer Phys. 30 Ž1992. 701. H. Vogel, Phys. Z. 22 Ž1921. 645. G.S. Fulcher, J. Am. Ceram. Soc. 8 Ž1925. 339. G. Tamman, W. Hesse, Z. Anorg. Allg. Chem. 156 Ž1926. 245. E. Ito, Y. Kobayashi, J. Appl. Polym. Sci. 25 Ž1980. 2145. J.C. Coburn, R.H. Boyd, Macromolecules 19 Ž1986. 2182.
Short communication
Dielectric relaxation properties of poly žethylene-terephthalate/ –polyaniline composite films A.A. Korzhenko a b
a,)
, M. Tabellout a , J.R. Emery a , A.A. Pud b, S. Rogalsky b, G.S. Shapoval
b
Laboratoire de Chimie et Physique des Materiaux Polymeres, ´ ` CNRS 6515, UniÕersite´ du Maine, BP535, 72085 Le Mans Cedex 9, France Institute of Bioorganic Chemistry and Petrochemistry, Ukrainian Academy of Sciences, 50 KharkoÕskoye Shosse, 253160, KieÕ, Ukraine Received 1 September 1998; accepted 1 October 1998
Abstract The dielectric relaxation measurements on polyŽethylene-terephthalate. ŽPET. containing adsorbed aniline and polyaniline ŽPAni., prepared by polymerization into the PET matrix, have been performed between 173 and 403 K in the frequency range 1 to 10 7 Hz. The influence of aniline and PAni on the b-relaxation of PET and the effect of the presence of PAni on the a-relaxation of PET have been studied. For the films PET–PAni doped with HCl, two relaxation processes around 0.5–5 kHz and 0.2–1 MHz were observed. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Dielectric spectroscopy; PolyŽethylene-terephthalate. ŽPET.; Polyaniline; Dielectric relaxation
1. Introduction PolyŽethylene-terephtalate. –polyanyline ŽPET–PAni. composites with transparent and surface conductive properties have a great potential in various electronic and electro-optical devices w1,2x, owing to surface conductive properties, chemical stability and low price. Varying the conditions of the preparation procedure and dopants it is possible to obtain the film with large spectrum of the properties of the conductive surface layer w1,2x. An investigation of this system by Dielectric Relaxation Spectroscopy ŽDRS. would be very helpful to optimize the properties of the PET–PAni composite for the concrete purposes and for testing of model devices as well. The present work is directed on the investigation of the dielectric relaxation processes of PET, PET with adsorbed aniline and PET containing PAni which are studied demonstrating the possibilities of DRS method for characterizing the dielectric–conductor systems.
parent matrix. The PET films were dipped in neat aniline which was further polymerized in an oxidant solution w2x. Both PET-Aniline and the polymerized composite ŽPET– PAni. were used for DRS measurements. The deepness of the penetration for the aniline is around 4–5 mm and the thickness of the surface layer, contained polyaniline is approximately 2–4 mm, as estimated by Raman Spectroscopy. DRS measurements were made with Broadband Dielectric Spectrometer ŽNovocontrol. in the frequency range 0.1 Hz to 10 MHz at the temperature 153 to 433 K, precision of the temperature stabilization being "0,1 K. In order to quantify the mean relaxation time in the range of present interest the frequency domain impedance analysis is the method of choice. Experimental dielectric spectrum ´ )Ž v . is fitted by the Havriliak–Negami ŽHN. function w3x: D´
´ ) Ž v . s ´` q
a
Ž 1 q Ž i vrv 0 . . 2. Experimental The PET double axially stretched films with crystalinity degree 69% and thickness 25 mm have been used as a )
Corresponding author
b
where v s 2p f ; ´` denotes the high frequency limit of the permittivity D ´ is the difference between the low and high frequency limits of ´ X over the relaxation to which the HN function applies, D ´ is also proportional to the area below the ´ Y relaxation peak; ´` is the unrelaxed value of permittivity; a and b are shape parameters.
0379-6779r98r$ - see front matter q 1998 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 8 . 0 0 1 7 9 - 9
A.A. Korzhenko et al.r Synthetic Metals 98 (1998) 157–160
158
Fitting of the experimental dielectric spectra ´ )Ž v . were made with WINFIT 2.4 Ž1996. software of Novocontrol GmbH.
3. Results Dielectric spectra have been obtained for PET, PET adsorbed aniline monomer Ž10% in mass. and PET film with polyaniline in nonconductive form at one side of the film. At temperatures below the glass transition we can see the b-relaxation peak of PET in dielectric spectrum. As we can see in Fig. 1, the presence of aniline leads to a decrease of the relaxation strength and the loss peak shifts slightly towards high frequencies. For the PET–PAni the tendencies are less pronounced. The activation energies Ea of relaxation process were obtained from Arrhenius equation: ln Ž t . s ln Ž t 0 . q
Ea k BT
The values of the activation energies and relaxation parameters of the dielectric spectra at 233 K, obtained by fitting with HN function are presented in the table. Actually, the value Ea presents averaged relaxation energies because of a broad distribution function of activation energies as it has been shown by dielectric w4x and TSDS w5x measurements. At higher temperatures, above 373 K a broad loss peak appears which is attributed to a-relaxation process w6x. Fig. 2 illustrates this peak, obtained at 403 K for PET and PET–PAni Žfor the PET-Aniline sample we have a complicated picture caused by noncontrolled thermal polymerization of the aniline monomer inside the PET.. From this figure we can see that the presence of PAni, even in thin surface layer only, leads to the considerable shift of the loss peak to high frequencies. The activation energies of
Fig. 1. Dependencies of the imaginary part of dielectric permittivity on frequency at 233 K:1-PET, 2-PETqaniline, 3-PETqPAni. The spline curves are the HN fit results.
Y Fig. 2. Dependencies of ´ on frequency at 403 K for PET Ž1. and PET contained PAni Ž2.. The spline curves are the HN fit results.
the a-relaxation process were obtained using Vogel– Fulsher–Tamman–Hesse ŽVFTH. equation w7–9x:
t Ž T . s t 0 exp
Ea k B Ž T y Tv .
,
where Ea is constant being related to the activation energy, t 0 —the relaxation time at infinite temperature, k B — Boltzmann’s constant and Tv temperature at infinite relaxation time. For both PET and PET–PAni samples the magnitude of Ea is practically the same Ž0.20 " 0.05 eV.. The values of glass transition temperatures ŽTg . 365 and 378 Ž"2 K., for PET and PET–PAni respectively, were obtained from VFTH equation at t s 100 s. The rising of the imaginary part of permittivity at low frequencies, observed in Fig. 2, can be associated with ionic conductivity, that has the magnitudes 8.6 = 10y1 6 and 1.5 = 10y14 Srcm Žat 403 K. for PET and PET–PAni respectively. The PET–PAni sample has electroconductive properties at the surface after doping. The magnitude of the surface
Y
Fig. 3. Dependencies of ´ on frequency for PET contained PAni doped by HCl. The spline curves are the HN fit results.
A.A. Korzhenko et al.r Synthetic Metals 98 (1998) 157–160
Fig. 4. The Arrhenius plots for the low- and high-frequency relaxation peaks observed at Fig. 3.
conductivity of our sample doped by HCl is 6 = 10y5 S Žmeasured by four probe method.. This film gives two specific loss peaks, as we can see in Fig. 3. The intensity of these peaks are considerably higher than the ones associated with a- and b-relaxation processes described above, and their position does not change strongly with temperature. The low frequency peak is of Debye type Žthe symmetry constants of HN equation a s b s 1., for the high frequency peak a changes from 0.33 to 0.45 and b from 0.45 to 0.55 in the temperature region 203 to 303 K. Arrhenius plots are linear for both relaxation times up to 333 K ŽFig. 4., the values of the activation energies, obtained from the Arrhenius plots are 0.088 and 0.051 eV for low and high frequency peak respectively.
4. Discussion For the b-relaxation process of PET it has been shown that the local motions which are responsible for it come from the amorphous phase w4x, because the good proportionality between the amorphous proportion of a semicrystalline sample and the b-relaxation strength was observed. Thus, it can be supposed the effect of aniline and PAni presence on b-relaxation is mainly associated with a chain mobility in the amorphous phase of PET. The presence of aniline gives absolutely different result than a plastization effect of the water observed in Ref. w10x: considerable rising of the relaxation strength and decreasing of the relaxation energy. The significant decrease in
159
relaxation strength of the b-relaxation and the increase of activation energy in the presence of absorbed aniline ŽTable 1. can certify an appearance of more complexity of the local motions in PET, that might be explained by a strong interaction between adsorbed aniline and the chains of PET. In the case of PET–PAni we also observe decreasing of the relaxation strength, but the activation energy is less than for PET. The a-relaxation process of PET, which comprises more large-scale motions related to the glass-transition, does not depend linearly on the amorphous content w11x. The presence of the PAni does not change activation energy of the relaxation, but shifts Tg of PET to higher temperature. We can suppose that PAni fills less dense volumes inside the PET matrix, so that Tg of more compact sample is increased. The conductive sample, PET–PAni Ždoped by HCl., gives two relaxation processes ŽFig. 3.. Both peaks are present only when PAni is in conductive form. They are practically Debye type and their relaxation energies have very small values; therefore it can be suggested that the charge carriers are responsible for these phenomena. To give detailed explanation for the origin of these dielectric loss processes further investigations are in progress. However, in presenting these first results we can propose the following assumption. Actually, having such an inhomogeneous system like PET–PAni Žconductive. we can expect the manifestation of the interfacial effect, known under the general name of Maxwell–Wagner effect w3x, resulting in the distributions of Debye-like relaxation times in low frequency region. Thus, the low frequency peak in our experiments ŽFig. 3. can be attributed to interfacial polarization effect between conductive layer containing PAni and PET volume free from PAni. Considering the high frequency peak its similarity with the near-Debye process observed in the case of p–n junction in silicon w3x can be taken into account. This process with the relaxation time around 1 ms Žat 300 K. and activation energy 0.25 eV at high temperatures and 0.075 eV below 100 K. For PAni, a formation of charge carriers, its trapping and recombination are different from that for semiconductors like silicon. In the case of PAni the influence of dopant may be very important. Thus, the relaxation phenomenon we observe around 0.2–1 MHz might characterize the properties of the charge carriers in the layer containing doped PAni.
5. Conclusions Table 1 Parameters of the b-relaxation at 233 K obtained by fitting of the dielectric spectra with HN function Sample
D´
t, s
a
b
Ea , eV
PET PETqAnilin PETqPAni
0.25 0.15 0.17
9.9=10y3 2.9=10y3 7.7=10y4
0.34 0.27 0.28
0.23 0.40 0.55
0.68 1.00 0.58
The DRS study of the b-relaxation process of the PET in the presence of absorbed aniline reveals the difficulties of the local motions in PET because of aniline–PET interaction. This effect is absent in the case of PET–PAni. The presence of the latter leads to increasing the packing density of PET resulting in the shifting of Tg of PET to higher temperatures.
160
A.A. Korzhenko et al.r Synthetic Metals 98 (1998) 157–160
Two relaxation processes observed for the PET containing doped PAni characterize the properties of the conductive layer and charge carriers in it. Further investigation of the nature and properties can have a great importance in the following optimization of the polymer composites with electroconductive properties for utilization. References w1x H. Zhang, C. Li, Synth. Metals 44 Ž1991. 143. w2x A.A. Pud, G.S. Shapoval, V.P. Kukhar, Method of manufacture of
w3x w4x w5x w6x w7x w8x w9x w10x w11x
sensor conducting polymer composite material, Ukrainian claim for invention rights N94010153 of 11.05.93. A.K. Jonscher Dielectric relaxation in solids Chelsea Dielectric Press, 1996. T. Pop, D. Iordashe, A. Jonas, Microelectronic Eng. 33 Ž1997. 337. T. Pop, D. Iordashe, I. Pop, I. Ionescu, Rom. Rep. Phys. 46 Ž1994. 573. T. Tatsum, E. Ito, J. Polymer Sci., Polymer Phys. 30 Ž1992. 701. H. Vogel, Phys. Z. 22 Ž1921. 645. G.S. Fulcher, J. Am. Ceram. Soc. 8 Ž1925. 339. G. Tamman, W. Hesse, Z. Anorg. Allg. Chem. 156 Ž1926. 245. E. Ito, Y. Kobayashi, J. Appl. Polym. Sci. 25 Ž1980. 2145. J.C. Coburn, R.H. Boyd, Macromolecules 19 Ž1986. 2182.