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Effect of one-dimensional phenomena on electrical, magnetic and ESR properties of MnCl2-filled PVA films
Physica B 254 (1998) 126—133
Effect of one-dimensional phenomena on electrical, magnetic and ESR properties of MnCl2-filled PVA films A. Tawansi!,*, A.H. Oraby!, H.M. Zidan", M.E. Dorgham! ! Department of Physics, Faculty of Science, Mansoura University, P.O. Box 19, Mansoura 35516, Egypt " Department of Physics, Faculty of Science of Damietta, Mansoura University, Egypt Received 15 January 1998; received in revised form 29 April 1998
Abstract Polyvinylalcohol (PVA) films, filled with various mass fractions w(10~3—15%) of MnCl , were prepared by the casting 2 method. Infrared analysis was used to clarify the structural variations due to filling. The assigned conjugated double bonds suggested the presence of polarons and/or bipolarons in the polymeric matrix. The filling level dependence of certain IR absorption peaks were correlated with the obtained physical parameters characterising the other properties. The effect of polaron distribution (along the PVA chain) on the electrical conduction, DC magnetic susceptibility and electron spin resonance was discussed. An intrachain one-dimensional interpolaron hopping conduction mechanism was applied. The magnetic results were attributed to the Ising one-dimensional exchange antiferromagnetic interactions between the Mn2` ions distributed along the PVA chain. ESR spectra suggested the distribution of Mn2` ions in isolated and/or aggregated modes within the PVA matrix. The filling level dependence of ESR parameters was discussed. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: MnCl -filled PVA; Infrared; Electric properties; Magnetic properties; Electron spin resonance properties 2
1. Introduction The importance of the semicrystalline polyvinylalcohol (PVA) polymer [1] arises from the role of OH group and the hydrogen bonds [2]. It had been noted as a medical material due to its compatibility to the living body [3]. PVA amidoxin chelate fibers can selectively adsorb metal ions such as copper, palladium and mercury [4]. As a hydrogen-bonded polymer PVA is used in gas sorption
* Corresponding author. Fax: #20 2050 346 781; e-mail: [email protected].
and diffusion [5]. The solubility and selectivity of certain gasses to the polymer can be enhanced by controlling chain rigidity through aryl-halogenation of the polymer backbone [6]. It had been found that [7] the electrical resistivity of PVA is photostable against UV irradiation. However, Takeda et al. [8] demonstrated that a methyl-red (MR) doped PVA film works as a spatial light modulator, which encodes an optical input image and transforms it into other forms. This was attributed to the induced macroscopically optical anisotropy (due to the trans to cis change in structural forms of MR) with a linearly polarized intense writing beam.
0921-4526/98/$ — see front matter ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 4 1 4 - 1
A. Tawansi et al. / Physica B 254 (1998) 126—133
127
Due to their promising features (such as multivalent states, optical activity and one-dimensional properties) [9,10] manganese ions are considered as a key material for enhancing the one-dimensional magnetic interactions, one-dimensional electric conduction and electromagnetic response of some polymers. This work was devoted to investigating the effect of one-dimensional phenomena on the infrared absorption spectra, electrical, magnetic and electron spin resonance of PVA films filled with various mass fractions of MnCl . 2 2. Experimental The samples were prepared by the casting method. PVA and MnCl were separately dissolved 2 in distilled water. A mixture of the two solutions, having the proper mass fraction of MnCl , was cast 2 on a glass plate and dried in an oven at 50°C for one week. Films, 0.2—0.4 mm thick, having MnCl 2 filling levels up to w"15 (wt%) were obtained. An infrared spectrophotometer (Perkin Elmer 883) was used for measuring IR spectra in the wave number range of 200—4000 cm~1. The electrical resistivity was measured using an electrometer (Keithley 617) with an accuracy $0.2%. The films were in the form of circular discs of 1.6$0.001 cm diameter. The contacts were of highly conductive coated silver layers, with an area of 1 cm2. The magnetic susceptibility was measured using Faraday’s pendulum balance technique with an accuracy better than 3%. Diamagnetic corrections were applied. The electron spin resonance spectra were recorded on a JEOL spectrophotometer (type JES-FE2xG) at frequency 9.45 GHz, using DPPH as a calibrant.
3. Results and discussion 3.1. IR analysis Fig. 1 depicts the IR transmission spectra of PVA filled with various fractions of MnCl . The 2 PVA most notably characterising frequencies, observed in the present spectra, are assigned and
Fig. 1. Infrared spectra of the investigated MnCl -filled PVA 2 films.
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A. Tawansi et al. / Physica B 254 (1998) 126—133
listed in Table 1. The PVA structural deformations due to filling can be resembled by plotting the intensity of the absorption peaks, at 1760 and 3040 cm~1, as a function of log w, as shown in Fig. 2. These peaks will be correlated with the electrical and magnetic properties, respectively. The curve corresponding to vibrations at 1760 cm~1 (which were assigned to C"C stretching of dichlorinated alkenes [11]) exhibits a minimum value at w"0.5% and a sharp descent starting at w"5%. On the other hand, the curve representing the vibrations at 3040 cm~1 (due to RHC"CH 2 alkenes [12]) exhibits two maxima at w"0.1% Table 1 Assignments of the IR characterising peaks of PVA system Frequency (cm~1)
Assignment
850 916 1096 1144 1326 1376 1430 1710 1760 2910 2940 3040
l (CC) l (CH ) 3 2 l (CO) l (C—O—C) d (CH#OH) c (CH ) 8 2 d (CH ) 2 C"O (residual acetate) l (C"C) l (CH ) 4 2 l (CH ) ! 2 RHC"CH 2
and 10%, and a minimum value at w"5%. The absorption peaks at 1620—1680 cm~1 were assigned to C"C stretching, while the peaks at 1720— 1745 cm~1 were assigned to the C"O mode. It is remarkable that the present double-bond segments are considered as suitable sites for polarons and/or bipolarons [11]. If I and I are the peaks at 1440 and 1343 cm~1, 1 2 respectively, the ratio I /I (denoted by I ) repres1 2 OH ents the relative bending mode of the OH group. The influence of filling level on I is demonstrated OH in Fig. 3. It is noted that the filling levels of w"0.01% and 10% divide the curve into three regions, indicating three different OH-bending modes.
3.2. DC electrical conduction The DC electrical resistivity o, was measured in the temperature (¹) range of 298—365 K for the present PVA system. Recalling the presence of polarons and/or bipolarons (in the present polymeric matrix, evidenced by IR analysis) we can use the Kuivalainen et al. [13] modified interpolaron hopping model to discuss our results. In this model the conduction is attributed to phonon-assisted hopping between polaron and/or bipolaron found states in the polymer. Thus the electrical resistivity
Note: l"stretching, d"bending, s"symmetric; a"asymmetric, R"radical.
Fig. 2. Filling level dependence of the absorption peaks at: (r) 1760 cm~1 and (v) 3040 cm~1.
Fig. 3. Filling level dependence of the ratio (I ), of the peak at OH 1440 cm~1 to the peak at 1343 cm~1, representing the OHbending mode.
A. Tawansi et al. / Physica B 254 (1998) 126—133
129
is expressed as o"[k¹/A e2c(¹)(R2/m][(y #y )2/(y )] 0 1 "1 "1 1 ]exp(2B R /m), (1) 1 0 where k is Boltzmann’s constant, A "0.45; B " 1 1 1.39; y and y are the concentration of polarons 1 "1 and bipolarons, respectively; and R "(3pC )1@3 4 *.1 0 is the typical separation between impurities whose concentration is C ; m"(m m2 )1@3 is the average *.1 , M decay length of a polaron and bipolaron wave function; and m and m are the decay lengths paral, M lel and perpendicular to the polymer chain, respectively. According to the calculations of Bredas et al. [14], polarons and bipolarons induce defects of the same extension. The electronic transition rate between polaron and bipolaron states can be expressed as c(¹)"c (¹/300 K)n`1, (2) 0 where n is a constant &10, and the prefactor, c " 0 1.2]1017 s~1, was estimated by Kivelson [15]. The order of magnitude of o in the present work was adjusted with the impurity concentration C , *.1 which actually was the fitting parameter. The parameter m "1.06 nm, while m +0.22 nm [16], , M which depends on the interchain resonance energy and the interchain distance [17]. Taking y "y 1 1" for simplicity, which is an acceptable approximation [14], and using Eqs. (1) and (2), we can obtain the values of the hopping distance R . 0 Fig. 4 depicts the temperature dependency of R at various filling levels of PVA. We found that 0 Eq. (3) is the best fit to the plots in Fig. 4, R (w, ¹)"A exp[!B ¹], (3) 0 (8) (8) where A and B are parameters, of values listed (8) (8) in Table 2, and of filling level dependence presented in Fig. 5. Comparing Fig. 5 with Fig. 2 we note that the curve representing A is, to a good extent, (8) the mirror image of the curve of the filling level dependence of IR vibrations at 1760 cm~1 (due to dichlorinated alkenes, which are suitable sites for polarons and/or bipolarons). This supports the successful application of the present conduction model. It is noteworthy that the obtained values of R for the present system are in the range of 1.2— 0
Fig. 4. The temperature dependence of the charge carrier hopping distance R for various filling levels. 0
3.8 nm. Thus the minimum hopping distance is about five monomer unit lengths, where this unit length is &0.25 nm [16]. Hence the present conduction mechanism is of an intrachain one-dimensional hopping type.
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Table 2 The values of A and B parameters of Eq. (3) w (%)
A (8)
B (8)
0.001 0.01 0.1 0.5 1 5 10 15
325 1185 1765.6 4494 3394 135.8 97.553 2610.5
0.015 0.019 0.021 0.023 0.022 0.013 0.012 0.022
Fig. 6. The temperature dependence of the magnetic susceptibility for various MnCl filling levels. 2
experimental data fit well the theoretical curves according to the infinite one-dimensional lattice of Ising spins [17], in which the susceptibility is expressed in terms of the antiferromagnetic exchange coupling constant J (between nearest neighbours, along the polymeric chains, as follows:
Fig. 5. The dependence of the A and B parameters, of Fig. 3, on log w.
(4) s "(Ng2k2 /4k¹ )exp(J/k¹ ), B I4 where N is Avogadro’s number and k the Bohr B magneton. The obtained fitting values of J are plotted as a function of the MnCl filling level, in 2 Fig. 7. It is remarkable that the filling level dependence of J is similar to that of the IR absorption peak at 3040 cm~1, observed in Fig. 2. This may reveal that the Mn2`—Mn2` antiferromagnetic interacting ions are located near the RHC"CH al2 kenes.
3.3. DC magnetic susceptibility 3.4. Electron spin resonance Fig. 6 shows the temperature dependence of the DC magnetic susceptibility of PVA filled with various mass fractions of MnCl . It is found that the 2
The ESR spectrum of unfilled PVA is shown in Fig. 8. It is characterized by three main broad
A. Tawansi et al. / Physica B 254 (1998) 126—133
131
Fig. 7. The dependence of the Ising antiferromagnetic coupling factor (J) on log w.
Fig. 9. ESR spectra of various MnCl filling levels. 2
Fig. 8. ESR spectrum of pure PVA.
signals (due to the pure PVA matrix) with superimposed hyperfine lines (due to free radicals). The average rotational correlation time (q ) of mobile r spins was calculated [18,19] using Eq. (5). q "6.6]10~10¼ [(h /h )1@2#(h /h )1@2], (5) r 0 0 ~1 0 `1 where ¼ is the line width of the midfield signal (in 0 G) and h , h , and h are the peak-to-peak ~1 0 `1 heights of the low-, central-, and high-field signals, respectively. The calculated value of q of the presr ent PVA film is 295 ns. Fig. 9 displays ESR spectra of PVA filled with various mass fractions of MnCl . It is clear that the 2
PVA characterising spectrum disappeared. The spectra at low filling levels consist of six lines superimposed on a broad Lorentzian signal. The six lines are due to the hyperfine structure of manganese nucleus (I"5) with an unpaired electron, indicat2 ing isolated Mn2` in the ionomer. The broad Lorentzian signal is due to the Mn2`—Mn2` exchange interaction, which is caused by the proximity of manganese ions, suggesting [20,21] the presence of aggregated Mn2`. The individual lines of the hyperfine structure became vague on increasing the Mn filling level, indicative of decreasing content of isolated Mn2`, in other words, the increasing content of aggregated Mn2`. Furthermore, the spectrum of PVA films filled with 15% MnCl exhibits a single peak, indicating the ab2 sence of isolated Mn2`. The filling level depenence of the filler local structure can be clarified more explicitly with the aid of the peak-to-peak separation of *H of the main ESR Lorentzian signal and the asymmetry factor (A), which is the ratio between the two halves of this signal. The obtained values of *H and A are plotted, as functions of w, in
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A. Tawansi et al. / Physica B 254 (1998) 126—133 Table 3 The filling level dependence of the ESR parameters (g, q, n and C), calculated using the spectra in Fig. 9 w
g
q
n
C
0 0.1 1 5 15
— 1.9138 1.92599 1.9145 1.9159
0.295 11.1 10.5 10.7 10.7
— 0.19 0.139 0.255 0.275
— 311.69 218.25 441.1 468.15
4. Conclusions
Fig. 10. The dependence of the asymmetry factor (A) and the peak-to-peak field (*H) on log w.
Fig. 10. At w+1% maximum values are noted for both *H and A, indicating a most symmetric Mn2` local distribution within the PVA matrix. Moreover, the filling level dependence of A is the mirror image of the corresponding dependence of each of the antiferromagnetic coupling factor J (in Fig. 7) and the IR absorption at 3040 cm~1 (in Fig. 2). This confirms that the location of the Mn2`—Mn2` exchange antiferromagnetic interacting ions is near to the RHC"CH alkene site. Using the present 2 ESR spectra, the following parameters were calculated and listed in Table 3: the g-factor, the spin relaxation time q (in ms), the number of unpaired electrons n (per Mn atom), and the spin density C. The deviation of the g-value from 2.00 is partly ascribable to the contribution of orbital angular momentum to the magnetic moment of Mn2` [22]. Table 3 demonstrates that the filling level dependence of g, q, n and C parameters are nonmonotonic. This may be ascribed to the influence of structural defects, Cl—H replacement and formation of MnCl 2 crystallites [23].
Various types of conjugated double bonds were assigned in the IR spectra, evidencing the distribution of polarons and/or bipolarons along the PVA chains. The IR analysis revealed three different OH-bending modes proceeding in the three filling regions of 0.01%)w)10%. The linear distribution of polarons resulted in one-dimensional features of electric conduction (attributed to intrachain interpolaron hopping) and magnetic susceptibility (based on the Ising antiferromagnetic interaction). The ESR was compatible with the findings of the other investigated properties, and it confirmed the predominant role of Mn2`—Mn2` spin interactions which were significantly affected by the electronic orbital angular momentum. The present MnCl -filled PVA system is a good candi2 date for one-dimensional technical (electric and/or magnetic) applications.
References [1] Y. Takahashi, J. Polym. Sci. B. 35 (1997) 193. [2] J. Pacansky, S. Schneider, J. Phys. Chem. 94 (1990) 3166. [3] K. Yamaura, N. Kuranuki, M. Suzuki, T. Tanigami, S. Matsuzawa, J. Appl. Polym. Sci. 41 (1990) 2409. [4] R. Lei, X. Jie, X. Jun, Z. Ruiyun, J. Appl. Polym. Sci. 53 (1994) 325. [5] M.J. Reimers, M.J. Cibulsky, T.A. Barbari, J. Polym. Sci. B. 31 (1993) 537. [6] R.T. Chern, F.R. Sheu, L. Jia, V.T. Stannett, H.B. Hopfenberg, J. Membr. Sci. 50 (1990) 285. [7] A. Tawansi, M.D. Migahed, M.I.A. Elhamid, J. Polym. Sci. B 24 (1986) 2631. [8] T. Takeda, K. Nakagawa, H. Fujiwara, Nonlinear Opt. 7 (1994) 295.
A. Tawansi et al. / Physica B 254 (1998) 126—133 [9] A. Tawansi, H.M. Zidan, A.H. Eldumiaty, Polym. Testing (1998), in press. [10] A. Tawansi, A.H. Oraby, E.M. Abdelrazek, M.I. Ayad, M. Abdelaziz, J. Appl. Polym. Sci. (1998), in press. [11] A. Tawansi, H.M. Zidan, Y.M. Moustafa, A.H. Eldumiaty, Phys. Scripta 55 (1997) 243. [12] I. Fleming, D.H. Williams, Spectroscopic Methods in Organic Chemistry, McGraw-Hill, New York, 1966, p. 56. [13] P. Kuivalainen, H. Stubb, H. Isotalo, P. Yli, C. Holmstro¨m, Phys. Rev. B 31 (1985) 7900. [14] J.L. Bredas, R.R. Chance, R. Silbey, Phys. Rev. B 26 (1982) 5843. [15] S. Kivelson, Phys. Rev. B 25 (1982) 3798.
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[16] N.F. Mott, R.W. Gurney, Electronic Processes in Ionic Crystals, OUP, London, 1940, p. 34. [17] M. Latour, K. Anis, R.M. Faria, J. Phys. D 22 (1989) 806. [18] D. Kivelson, J. Chem. Phys. 33 (1960) 1094. [19] T. Uhlich, M. Ulbricht, G. Tomaschewski, R. Stosser, J. Appl. Polym. Sci. 58 (1995) 1587. [20] S. Yano, H. Yamashita, M. Matsushita, K. Aoki, J. Yamauchi, Colloid Polym. Sci. 259 (1981) 514. [21] H. Toriumi, R.A. Weiss, H.A. Frank, Macromolecules 17 (1984) 2104. [22] A. Tawansi, H.I. Abdelkader, E.M. Abdelrazek, J. Mater. Sci. Technol. 13 (1997) 194. [23] I. Yamauchi, S. Yano, Macromolecules 15 (1982) 210.
Effect of one-dimensional phenomena on electrical, magnetic and ESR properties of MnCl2-filled PVA films A. Tawansi!,*, A.H. Oraby!, H.M. Zidan", M.E. Dorgham! ! Department of Physics, Faculty of Science, Mansoura University, P.O. Box 19, Mansoura 35516, Egypt " Department of Physics, Faculty of Science of Damietta, Mansoura University, Egypt Received 15 January 1998; received in revised form 29 April 1998
Abstract Polyvinylalcohol (PVA) films, filled with various mass fractions w(10~3—15%) of MnCl , were prepared by the casting 2 method. Infrared analysis was used to clarify the structural variations due to filling. The assigned conjugated double bonds suggested the presence of polarons and/or bipolarons in the polymeric matrix. The filling level dependence of certain IR absorption peaks were correlated with the obtained physical parameters characterising the other properties. The effect of polaron distribution (along the PVA chain) on the electrical conduction, DC magnetic susceptibility and electron spin resonance was discussed. An intrachain one-dimensional interpolaron hopping conduction mechanism was applied. The magnetic results were attributed to the Ising one-dimensional exchange antiferromagnetic interactions between the Mn2` ions distributed along the PVA chain. ESR spectra suggested the distribution of Mn2` ions in isolated and/or aggregated modes within the PVA matrix. The filling level dependence of ESR parameters was discussed. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: MnCl -filled PVA; Infrared; Electric properties; Magnetic properties; Electron spin resonance properties 2
1. Introduction The importance of the semicrystalline polyvinylalcohol (PVA) polymer [1] arises from the role of OH group and the hydrogen bonds [2]. It had been noted as a medical material due to its compatibility to the living body [3]. PVA amidoxin chelate fibers can selectively adsorb metal ions such as copper, palladium and mercury [4]. As a hydrogen-bonded polymer PVA is used in gas sorption
* Corresponding author. Fax: #20 2050 346 781; e-mail: [email protected].
and diffusion [5]. The solubility and selectivity of certain gasses to the polymer can be enhanced by controlling chain rigidity through aryl-halogenation of the polymer backbone [6]. It had been found that [7] the electrical resistivity of PVA is photostable against UV irradiation. However, Takeda et al. [8] demonstrated that a methyl-red (MR) doped PVA film works as a spatial light modulator, which encodes an optical input image and transforms it into other forms. This was attributed to the induced macroscopically optical anisotropy (due to the trans to cis change in structural forms of MR) with a linearly polarized intense writing beam.
0921-4526/98/$ — see front matter ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 4 1 4 - 1
A. Tawansi et al. / Physica B 254 (1998) 126—133
127
Due to their promising features (such as multivalent states, optical activity and one-dimensional properties) [9,10] manganese ions are considered as a key material for enhancing the one-dimensional magnetic interactions, one-dimensional electric conduction and electromagnetic response of some polymers. This work was devoted to investigating the effect of one-dimensional phenomena on the infrared absorption spectra, electrical, magnetic and electron spin resonance of PVA films filled with various mass fractions of MnCl . 2 2. Experimental The samples were prepared by the casting method. PVA and MnCl were separately dissolved 2 in distilled water. A mixture of the two solutions, having the proper mass fraction of MnCl , was cast 2 on a glass plate and dried in an oven at 50°C for one week. Films, 0.2—0.4 mm thick, having MnCl 2 filling levels up to w"15 (wt%) were obtained. An infrared spectrophotometer (Perkin Elmer 883) was used for measuring IR spectra in the wave number range of 200—4000 cm~1. The electrical resistivity was measured using an electrometer (Keithley 617) with an accuracy $0.2%. The films were in the form of circular discs of 1.6$0.001 cm diameter. The contacts were of highly conductive coated silver layers, with an area of 1 cm2. The magnetic susceptibility was measured using Faraday’s pendulum balance technique with an accuracy better than 3%. Diamagnetic corrections were applied. The electron spin resonance spectra were recorded on a JEOL spectrophotometer (type JES-FE2xG) at frequency 9.45 GHz, using DPPH as a calibrant.
3. Results and discussion 3.1. IR analysis Fig. 1 depicts the IR transmission spectra of PVA filled with various fractions of MnCl . The 2 PVA most notably characterising frequencies, observed in the present spectra, are assigned and
Fig. 1. Infrared spectra of the investigated MnCl -filled PVA 2 films.
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A. Tawansi et al. / Physica B 254 (1998) 126—133
listed in Table 1. The PVA structural deformations due to filling can be resembled by plotting the intensity of the absorption peaks, at 1760 and 3040 cm~1, as a function of log w, as shown in Fig. 2. These peaks will be correlated with the electrical and magnetic properties, respectively. The curve corresponding to vibrations at 1760 cm~1 (which were assigned to C"C stretching of dichlorinated alkenes [11]) exhibits a minimum value at w"0.5% and a sharp descent starting at w"5%. On the other hand, the curve representing the vibrations at 3040 cm~1 (due to RHC"CH 2 alkenes [12]) exhibits two maxima at w"0.1% Table 1 Assignments of the IR characterising peaks of PVA system Frequency (cm~1)
Assignment
850 916 1096 1144 1326 1376 1430 1710 1760 2910 2940 3040
l (CC) l (CH ) 3 2 l (CO) l (C—O—C) d (CH#OH) c (CH ) 8 2 d (CH ) 2 C"O (residual acetate) l (C"C) l (CH ) 4 2 l (CH ) ! 2 RHC"CH 2
and 10%, and a minimum value at w"5%. The absorption peaks at 1620—1680 cm~1 were assigned to C"C stretching, while the peaks at 1720— 1745 cm~1 were assigned to the C"O mode. It is remarkable that the present double-bond segments are considered as suitable sites for polarons and/or bipolarons [11]. If I and I are the peaks at 1440 and 1343 cm~1, 1 2 respectively, the ratio I /I (denoted by I ) repres1 2 OH ents the relative bending mode of the OH group. The influence of filling level on I is demonstrated OH in Fig. 3. It is noted that the filling levels of w"0.01% and 10% divide the curve into three regions, indicating three different OH-bending modes.
3.2. DC electrical conduction The DC electrical resistivity o, was measured in the temperature (¹) range of 298—365 K for the present PVA system. Recalling the presence of polarons and/or bipolarons (in the present polymeric matrix, evidenced by IR analysis) we can use the Kuivalainen et al. [13] modified interpolaron hopping model to discuss our results. In this model the conduction is attributed to phonon-assisted hopping between polaron and/or bipolaron found states in the polymer. Thus the electrical resistivity
Note: l"stretching, d"bending, s"symmetric; a"asymmetric, R"radical.
Fig. 2. Filling level dependence of the absorption peaks at: (r) 1760 cm~1 and (v) 3040 cm~1.
Fig. 3. Filling level dependence of the ratio (I ), of the peak at OH 1440 cm~1 to the peak at 1343 cm~1, representing the OHbending mode.
A. Tawansi et al. / Physica B 254 (1998) 126—133
129
is expressed as o"[k¹/A e2c(¹)(R2/m][(y #y )2/(y )] 0 1 "1 "1 1 ]exp(2B R /m), (1) 1 0 where k is Boltzmann’s constant, A "0.45; B " 1 1 1.39; y and y are the concentration of polarons 1 "1 and bipolarons, respectively; and R "(3pC )1@3 4 *.1 0 is the typical separation between impurities whose concentration is C ; m"(m m2 )1@3 is the average *.1 , M decay length of a polaron and bipolaron wave function; and m and m are the decay lengths paral, M lel and perpendicular to the polymer chain, respectively. According to the calculations of Bredas et al. [14], polarons and bipolarons induce defects of the same extension. The electronic transition rate between polaron and bipolaron states can be expressed as c(¹)"c (¹/300 K)n`1, (2) 0 where n is a constant &10, and the prefactor, c " 0 1.2]1017 s~1, was estimated by Kivelson [15]. The order of magnitude of o in the present work was adjusted with the impurity concentration C , *.1 which actually was the fitting parameter. The parameter m "1.06 nm, while m +0.22 nm [16], , M which depends on the interchain resonance energy and the interchain distance [17]. Taking y "y 1 1" for simplicity, which is an acceptable approximation [14], and using Eqs. (1) and (2), we can obtain the values of the hopping distance R . 0 Fig. 4 depicts the temperature dependency of R at various filling levels of PVA. We found that 0 Eq. (3) is the best fit to the plots in Fig. 4, R (w, ¹)"A exp[!B ¹], (3) 0 (8) (8) where A and B are parameters, of values listed (8) (8) in Table 2, and of filling level dependence presented in Fig. 5. Comparing Fig. 5 with Fig. 2 we note that the curve representing A is, to a good extent, (8) the mirror image of the curve of the filling level dependence of IR vibrations at 1760 cm~1 (due to dichlorinated alkenes, which are suitable sites for polarons and/or bipolarons). This supports the successful application of the present conduction model. It is noteworthy that the obtained values of R for the present system are in the range of 1.2— 0
Fig. 4. The temperature dependence of the charge carrier hopping distance R for various filling levels. 0
3.8 nm. Thus the minimum hopping distance is about five monomer unit lengths, where this unit length is &0.25 nm [16]. Hence the present conduction mechanism is of an intrachain one-dimensional hopping type.
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Table 2 The values of A and B parameters of Eq. (3) w (%)
A (8)
B (8)
0.001 0.01 0.1 0.5 1 5 10 15
325 1185 1765.6 4494 3394 135.8 97.553 2610.5
0.015 0.019 0.021 0.023 0.022 0.013 0.012 0.022
Fig. 6. The temperature dependence of the magnetic susceptibility for various MnCl filling levels. 2
experimental data fit well the theoretical curves according to the infinite one-dimensional lattice of Ising spins [17], in which the susceptibility is expressed in terms of the antiferromagnetic exchange coupling constant J (between nearest neighbours, along the polymeric chains, as follows:
Fig. 5. The dependence of the A and B parameters, of Fig. 3, on log w.
(4) s "(Ng2k2 /4k¹ )exp(J/k¹ ), B I4 where N is Avogadro’s number and k the Bohr B magneton. The obtained fitting values of J are plotted as a function of the MnCl filling level, in 2 Fig. 7. It is remarkable that the filling level dependence of J is similar to that of the IR absorption peak at 3040 cm~1, observed in Fig. 2. This may reveal that the Mn2`—Mn2` antiferromagnetic interacting ions are located near the RHC"CH al2 kenes.
3.3. DC magnetic susceptibility 3.4. Electron spin resonance Fig. 6 shows the temperature dependence of the DC magnetic susceptibility of PVA filled with various mass fractions of MnCl . It is found that the 2
The ESR spectrum of unfilled PVA is shown in Fig. 8. It is characterized by three main broad
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131
Fig. 7. The dependence of the Ising antiferromagnetic coupling factor (J) on log w.
Fig. 9. ESR spectra of various MnCl filling levels. 2
Fig. 8. ESR spectrum of pure PVA.
signals (due to the pure PVA matrix) with superimposed hyperfine lines (due to free radicals). The average rotational correlation time (q ) of mobile r spins was calculated [18,19] using Eq. (5). q "6.6]10~10¼ [(h /h )1@2#(h /h )1@2], (5) r 0 0 ~1 0 `1 where ¼ is the line width of the midfield signal (in 0 G) and h , h , and h are the peak-to-peak ~1 0 `1 heights of the low-, central-, and high-field signals, respectively. The calculated value of q of the presr ent PVA film is 295 ns. Fig. 9 displays ESR spectra of PVA filled with various mass fractions of MnCl . It is clear that the 2
PVA characterising spectrum disappeared. The spectra at low filling levels consist of six lines superimposed on a broad Lorentzian signal. The six lines are due to the hyperfine structure of manganese nucleus (I"5) with an unpaired electron, indicat2 ing isolated Mn2` in the ionomer. The broad Lorentzian signal is due to the Mn2`—Mn2` exchange interaction, which is caused by the proximity of manganese ions, suggesting [20,21] the presence of aggregated Mn2`. The individual lines of the hyperfine structure became vague on increasing the Mn filling level, indicative of decreasing content of isolated Mn2`, in other words, the increasing content of aggregated Mn2`. Furthermore, the spectrum of PVA films filled with 15% MnCl exhibits a single peak, indicating the ab2 sence of isolated Mn2`. The filling level depenence of the filler local structure can be clarified more explicitly with the aid of the peak-to-peak separation of *H of the main ESR Lorentzian signal and the asymmetry factor (A), which is the ratio between the two halves of this signal. The obtained values of *H and A are plotted, as functions of w, in
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A. Tawansi et al. / Physica B 254 (1998) 126—133 Table 3 The filling level dependence of the ESR parameters (g, q, n and C), calculated using the spectra in Fig. 9 w
g
q
n
C
0 0.1 1 5 15
— 1.9138 1.92599 1.9145 1.9159
0.295 11.1 10.5 10.7 10.7
— 0.19 0.139 0.255 0.275
— 311.69 218.25 441.1 468.15
4. Conclusions
Fig. 10. The dependence of the asymmetry factor (A) and the peak-to-peak field (*H) on log w.
Fig. 10. At w+1% maximum values are noted for both *H and A, indicating a most symmetric Mn2` local distribution within the PVA matrix. Moreover, the filling level dependence of A is the mirror image of the corresponding dependence of each of the antiferromagnetic coupling factor J (in Fig. 7) and the IR absorption at 3040 cm~1 (in Fig. 2). This confirms that the location of the Mn2`—Mn2` exchange antiferromagnetic interacting ions is near to the RHC"CH alkene site. Using the present 2 ESR spectra, the following parameters were calculated and listed in Table 3: the g-factor, the spin relaxation time q (in ms), the number of unpaired electrons n (per Mn atom), and the spin density C. The deviation of the g-value from 2.00 is partly ascribable to the contribution of orbital angular momentum to the magnetic moment of Mn2` [22]. Table 3 demonstrates that the filling level dependence of g, q, n and C parameters are nonmonotonic. This may be ascribed to the influence of structural defects, Cl—H replacement and formation of MnCl 2 crystallites [23].
Various types of conjugated double bonds were assigned in the IR spectra, evidencing the distribution of polarons and/or bipolarons along the PVA chains. The IR analysis revealed three different OH-bending modes proceeding in the three filling regions of 0.01%)w)10%. The linear distribution of polarons resulted in one-dimensional features of electric conduction (attributed to intrachain interpolaron hopping) and magnetic susceptibility (based on the Ising antiferromagnetic interaction). The ESR was compatible with the findings of the other investigated properties, and it confirmed the predominant role of Mn2`—Mn2` spin interactions which were significantly affected by the electronic orbital angular momentum. The present MnCl -filled PVA system is a good candi2 date for one-dimensional technical (electric and/or magnetic) applications.
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