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Structural, electrical and magnetic properties of polystyrene films filled with AgNO3–FeCl3 mixed fillers
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 283 (2004) 199–209 www.elsevier.com/locate/jmmm
Structural, electrical and magnetic properties of polystyrene films filled with AgNO3–FeCl3 mixed fillers A. Tawansi, A. El-Khodary, A.E. Youssef Faculty of Science, Physics Department, Mansoura University, Mansoura 35516, Egypt Received 22 March 2004; received in revised form 15 May 2004 Available online 15 June 2004
Abstract The effect of various filling levels (FLs) of mixtures of two transition compounds (X)AgNO3 (10X) FeCl3, on structural, electrical and magnetic properties of polystyrene (PS) films was investigated. The X-ray diffraction (XRD) displayed an unexpected appearance of a number of crystalline peaks beside the two main peaks characterizing the virgin film. The crystalline peaks were attributed to the cluster formation. The infrared (IR) analysis was used to detect the most notably PS peaks and to clarify the structural variations due to the filling. Certain IR peaks were taken as an evidence for the formation of polaron and/or bipolaron bound states in the polymeric matrix. The direct current (DC) electrical conduction studies revealed a linear temperature dependence of the hopping distance R0 for various FLs. This is an indication that the conduction mechanism can be attributed to one-dimensional intrachain type based on the phonon-assisted charge carrier interpolaron hopping model. The DC magnetic susceptibility results, in the temperature range 90–235 K, followed the Curie–Weiss behavior. The positive and negative values of the paramagnetic Curie temperature ðyp Þ indicate the possibility of ferromagnetic and antiferromagnetic exchange interactions between the magnetic centers at low temperatures, respectively. The electron spin resonance (ESR) spectra at the lower values of X depicted a broad signal superimposed on it a narrow one as well as a deformed signal. The ESR investigations at the lower values of X indicated the presence of aggregated Fe+3 confirming the XRD implications about the cluster formation. On the other hand and at higher values of X, an appearance of unresolved five lines of fine structure character was noticed. r 2004 Elsevier B.V. All rights reserved. PACS: 71.20:70; Rv.:76.30.v Keywords: Polystyrene; Mixed AgNO3 and FeCl3 fillers; X-ray diffraction; Infrared; Electrical resistivity; Magnetic susceptibility; Electron spin resonance
Corresponding author. Tel.: +20-50-2242388; fax: +20-50-2246781.
E-mail address: [email protected] (A. El-Khodary). 0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.05.021
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1. Introduction Polymeric materials have attracted considerable attention due to their spectacular growth of interesting potential applications. Among such materials PS which is a preferred material because it has good stiffness, optical, electrical and mechanical properties [1,2]. In addition PS is a non-polar, low loss polymer and hence has desirable electronic properties for use in the field of insulating materials [3,4]. PS and its blends have been widely used for producing materials with improved physical and mechanical properties [5–7]. The introduction of the filler into PS can markedly influence the properties of the polymer. This depends on the chemical nature of the filler and the way in which it interacts with the chains of the polymeric matrix. One can choose the desired filler and the suitable FL to get a filled polymer with the controllable physical and chemical properties for specific technological applications. Recently transition metal ions have been extensively used as fillers in organic polymers because of the partially filled d-electron shells of very interesting properties [3,4,7–16]. Ferric chloride, FeCl3, with trivalent iron ions of 3d5 electron configuration was selected as a filler due to its magnificent magnetic and electrical properties [3,4,8–10,16]. Moreover, FeCl3 has been reported to give the most stable material, an aspect of importance when considering applications [16]. AgNO3 was also chosen as a filler due to its interesting good optical as well as electrical properties. AgNO3 and FeCl3 mixed fillers were used as a trial to achieve a monotonic behavior of FL dependence of certain desired properties in a fair FL range. The aim of the present work is devoted to investigate the effect of the mixed fillers AgNO3 and FeCl3 of various FLs on the physical properties of the filled PS films.
2. Experimental work 2.1. Sample preparation PS was obtained in the form of pellets of molecular weight 100,000 from Aldrich chemical
company. Metal compounds AgNO3 and FeCl3 were provided in solid-state form. The used solvents were cyclohexanone, and dimethylformamide (DMF). The studied PS films filled with transition metal compounds (X)AgNO3 (10X) FeCl3 were prepared by casting methods. Cyclohexanone was used to dissolve the polymer. The transition metal compounds AgNO3 and FeCl3 were dissolved in dimethylformamide (DMF). The polymer and the transition metal compounds were cast to glass dishes and kept in a dry atmosphere at T ¼ 333 K for about ten days to remove the solvent traces. Various concentrations of the fillers were obtained where X ¼ 0:0, 0.1, 0.5, 1.0, 2.5, 5, 7.5, 9.0, 9.5, 9.9 and 10.0 wt%. The film thickness was in the range of 150–250 mm.
2.2. Tools of analysis X-ray diffraction scans were carried out using a Seimens type F diffractometer with CuKa radiation and LiF monochromator. The IR spectra of the different films at room temperature in the wavenumber range 4000–600 cm1 were recorded by the IR spectrophotometer (Perkin Elmer 883). It is remarkable that we will present the IR spectra in the range 2000–600 cm1, in which the main characterizing spectral features were found. The DC electrical resistivity was measured using insulation tester (type TM 14) and autoranging multimeter (Keithley 175 A) of accuracy 70.2% [7]. The DC magnetic susceptibility (w) was measured, in the temperature range of 90–280 K, using a Faraday pendulum balance [17]. The accuracy of the measurements was better than 73.5%. Diamagnetic corrections were done. The ESR spectra were recorded at room temperature on a BRUKER (EMX) spectrometer operating in the X-band frequency (E9.7 GHz) with a field modulation frequency of 100 KHz. The microwave power and modulation amplitude were 10 mW and 0.1 mT, respectively. Standard sample of MgO doped with Mn+2 was used as a calibrant.
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3. Results and discussion 3.1. XRD The XRD scans for PS films filled with various mass fractions of (X)AgNO3 (10X)FeCl3 are depicted in Fig. 1. Two main peaks at 2y 19:51 and 101, characterizing the virgin sample were noticed. The first peak at 2y 19:51, which is the most intense one, is the amorphous peak and it corresponds to the Van der Waals distance [18–20]. In amorphous linear polymers the chains are approximately parallel to one another leading to a distance between neighbors equals to about ( [18]. This value is characterizing the 4.5–5.0 A mean distance between adjacent chains. The second peak at 2y 101 is the polymerization peak which can be attributed to the intermolecular backbone–backbone correlation as well as to the size of the side group which corresponds to an approximately hexagonal ordering of the molecular chains [20]. At X ¼ 1%, an unexpected appearance of crystalline peaks at 2y 281,
201
32.51, 44.51 and 46.51 was observed. At X ¼ 9%, an appearance of a crystalline peak at 2y 381 was also noticed. These peaks are not easily assigned to the formation of bulk crystalline materials. It is well-known that the polymer matrix is complex and the behavior of the added fillers, X AgNO3 (10X)FeCl3, is difficult to predict. These make the identification of the produced XRD crystalline peaks more difficult. It is remarkable to mention that microclusters display curious crystallographic anomalies, which cannot be found in the bulk [21]. However, the peaks at 2y 281 and 46.51 may be attributed to AgCl (1 1 1) and (2 2 0), respectively, while the peak at 2y 32:51 can be attributed to AgCl (2 0 0) and/or Ag2O (1 1 1) and the peak at 2y 38:51 can be attributed to Ag (1 1 1) and/or Ag2O (2 0 0) [19]. The small peak (hump) at 2y 44:51 can be attributed to Ag (2 0 0) [19]. All the above peaks almost disappeared at X ¼ 10% except a small peak (hump) at 2y 38:51 which can only be attributed to the filler AgNO3. The above assigned crystalline peaks can be attributed to the formation of clusters. This
Fig. 1. X-ray diffraction scans of PS filled with various mass fractions of (X)AgNO3 (10X)FeCl3, (X) AgCl, ( ) Ag2O and (K) Ag.
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explanation agrees quite well with the ESR interpretation of the formation of aggregated Fe3+ and the absence of isolated Fe3+ [10,13,22–27]. Generally, it is clear that at X ¼ 1%, the aggregation (clusters) is basically due to FeCl3 filler, while at X ¼ 9%, the aggregation (clusters) is basically due to AgNO3 filler. On the other hand, the change of the intensity of different peaks at different values of X can be attributed to the change of the contents of the two fillers FeCl3 and AgNO3. Finally, at X ¼ 10% all the above peaks almost disappeared except the hump at 2y 381 which can be attributed only to the filler AgNO3. 3.2. IR spectroscopy Fig. 2 reveals the IR transmission spectra for PS films filled with various mass fractions of a mixture
of AgNO3 and FeCl3. The most notably PS characterizing peaks are assigned. The aromatic ring (C6H5) gives a rise to the group of bands above 3010 cm1 which are not shown here. The unsaturated C–H stretching above 3000 cm1 as well as the distinctive double bond CQC stretching at about 1600 cm1 identify polymers containing aromatics [28,29]. The C–C stretching mode of phenyl rings at 1480 cm1 and the C–H out of plane deformation modes of monosubstituted phenyl rings at 765 and 695 cm1 are observed [16,28,29]. The present results indicate that the IR transmission decreases with increasing the doping content in the whole studied frequency range. This could be due to free or weakly bound charge carriers [16]. The peaks at 1530, 1275 and 1180 cm1 (solid arrows) are interesting, since Shacklette et al. [30,31] found dopant induced
(X)AgNO3 (10-X)FeCl3
(X)AgNO3 (10-X)FeCl3
X (wt %) 10.0
X (wt %) 2.5
9.9
Transmittance (a. u.)
Transmittance (a. u.)
1.0
0.5
0.1
9.5
9.0
0.0
7.5
pure
5.0
2000
(a)
1800
1600
1400
1200
1000
Wavenumber (cm-1)
800
600
2000
(b)
1800
1600
1400
1200
1000
Wavenumber (cm-1)
Fig. 2. The IR transmission spectra of PS filled with various FLs.
800
600
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3.3. DC electrical conduction The DC electrical resistivity (r) was measured in the temperature range 293–390 K for PS filled with various mass fractions of (X)AgNO3 (10X)FeCl3. The double bond CQC and the deformation in the benzene ring (detected by the IR analysis), may evidence the formation of polarons and/or bipolarons in the polymeric matrix. Therefore, the present results can be discussed on the basis of the Kuivalainen et al. [16] modified interpolaron hopping model, in which the conduction mechanism can be attributed to phonon-assisted one-dimensional charge carrier hopping between polaron and/or bipolaron bound states in the polymeric matrix. According to this model the electrical resistivity can be formulated as
where n is a constant 10, and the pre-factor, g0 ¼ 1:2 1017 s1, was estimated by Kivelson [36]. Using a computer-aided program, the order of magnitude of r in the present work was adjusted with the impurity concentration Cimp, which actually was the fitting parameter. The parameters zJ ¼ 1:06 nm and z? ¼ 0:22 nm [37], depend on the interchain resonance energy and the interchain distance. As an acceptable approximation, and for simplicity put Y p ¼ Y bp [38], and using Eqs. (1) and (2) the values of the separation between impurities which are considered as the hopping distance R0 can be calculated. Owing to Eqs. (1) and (2) the plotting of ln r versus ln T should give a straight line if the measured rðTÞ is dominated by the interpolaron hopping [16,38]. Fig. 3 depicts this plot for variously filled PS films. The linear behavior of these plots corresponds to the temperature ranges in which the interpolaron hopping proceeds [16,38]. The linear temperature dependence of the obtained values of the hopping distance R0 for various FLs is plotted in Fig. 4. This supports the previous postulate that the conduction mechanism 28
24 22 20
16 5.7
(1)
5.75
(a)
ð2Þ
5.8
5.85
5.9
5.95
Ln (T,K) 29.5
(X)AgNO3 (10-X)FeCl3
27.5
Ln (ρ, Ohm.cm)
where A1 ¼ 0:45; B1 ¼ 1:39; Y p and Y bp are the concentration of polarons and bipolarons, respectively, R0 ¼ ð3=4pC imp Þ1=3 is the typical separation between impurities whose concentration is Cimp; z ¼ ðzJj z2? Þ1=3 is the average decay length of a polaron and bipolaron wave function; and zJ and z? are the decay lengths parallel and perpendicular to the polymer chain, respectively. The electronic transition rate between polaron and bipolaron states can be expressed as gðTÞ ¼ g0 ðT=300 KÞnþ1 ;
Pure X=0% X = 0.1 % X = 0.5 % X=1%
18
r ¼ ½kT=A1 e2 gðTÞðR20 =zÞ ½ðY p þ Y bp Þ2 =Y p Y bp expð2B1 R0 =zÞ;
(X)AgNO3 (10-X)FeCl3
26
Ln (ρ, Ohm.cm)
peaks at exactly the same wavenumbers in AsF5doped polyparaphenylene (PPP). Also these peaks were found in doped polyacetylene (PA) [32] and FeCl3-doped PPP [16]. However, Kuivalainen et al. [16] and Fincher et al. [32] stated that these modes of vibrations are not due to specific vibration of the dopant molecules but are intrinsic vibrations of the polymer chain. According to Kuivalainen et al. [16], the mode at 1275 cm1, which also appears at 1298 cm1 according to Racovics et al. [33], is related to the deformations of the benzene ring structure which is typical of the polaron and bipolaron defect states [34,35].
203
25.5
X = 2.5 % X=5% X = 7.5 % X=9% X = 9.5 % X = 9.9 % X = 10 %
23.5 21.5 19.5 17.5 5.725
(b)
5.775
5.825
5.875
5.925
Ln (T,K)
Fig. 3. Ln r against ln T plots for variously filled PS.
6
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204
14
9.5 Pure X=0% x=0.1% x= 0.5% X=1% x= 2.5%
(X)AgNO3 (10-X)FeCl3
8.5 7.5
14
(X)AgNO3 (10-X)FeCl3 12
12
10
10
8
8
6
6
4
4
5.5 4.5 3.5 2.5 1.5 300
320
340
(a)
360
380
400
T (K)
(X)AgNO3 (10-X)FeCl3
8
X=5 % x=7.5% x=9% x=9.5% x=9.9% x=10%
7
2
Ro (nm)
6
0
1
2
3
4
5 6 X (wt%)
7
8
9
Ro (nm)
Log (ρ, Ohm.cm)
Ro (nm)
6.5
2 10
Fig. 5. The FLs dependence of (J) log r and (K) R0 at 348 K.
5 4 3 2 300
320
(b)
340
360
380
400
T (K)
Fig. 4. Temperature dependence of R0 for variously filled PS.
is predominated by the interpolaron hopping model [16]. The FLs dependence of the measured log r and the calculated R0 at T ¼ 348 K are shown in Fig. 5. Minimum values of log r and R0 are observed at X ¼ 2:5%, 7.5%, while maxima are observed at X ¼ 0:5%, 5% and 9.5%. This clarifies that the FL affects the distribution of the hopping sites. 3.4. DC magnetic susceptibility The study of the DC magnetic susceptibility (w) of PS films filled with various mass fractions of (X)AgNO3 (10X)FeCl3 was performed in the temperature range 90–235 K. The temperature dependence of the reciprocal magnetic susceptibility is plotted in Fig. 6. These plots are linear and follow the Curie–Weiss law in which w ¼ C=ðT yp Þ;
ð3Þ
where C is Curie constant and yp is the paramagnetic Curie temperature. It is important to note that ferromagnetic materials behave as normal paramagnetics at high
temperatures and as T approaches the magnitude of yp of positive value there is a marked increase in w. The critical temperature obtained for this process is called the Curie temperature. Below this temperature Eq. (3) is meaningless because the sample can have zero-field moment. Once the material becomes ordered, the susceptibility behave in a very complicated way and no longer has a unique value for a given field strength. On the contrary, antiferromagnetic materials behave also as normal paramagnetics at high temperatures and as T approaches the magnitude of yp of negative value w becomes small and it attains its maximum value at certain critical temperature called Ne´el temperature. Below this temperature the spins have antiparallel orientations [39,40]. In the present study the values of yp were calculated for different values of X using Eq. (3) and Fig. 6. These values were plotted in Fig. 7. Positive values of yp were found by suggesting the predominance of ferromagnetic exchange interaction between the magnetic centers at low temperatures. On the other hand, a negative yp value was found at X ¼ 1% indicating the possibility of an antiferromagnetic exchange interaction between the magnetic centers at low temperatures. In addition, the effective paramagnetic moment (meff ) was estimated at T ¼ 100 K, using the following equation: meff ¼ 2:828½wm ðT yp Þ1=2 ;
ð4Þ
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0.3 0.25
pure x=0 x=0.1 x = 0.5
0.2 0.15 0.1 0.05 0 75
125
175
(a)
225
T (K)
0.7
(X)AgNO3 (10-X)FeCl3
x =1 x=2.5 x=5 x=7.5 x=9
0.6 1/χ (106 g/emu)
0.5 0.4 0.3 0.2 0.1 0 75
125
175
(b) 0.35
(X)AgNO3 (10-X)FeCl3
0.3 1/χ (106 g/emu)
225
T (K)
0.2 0.15 0.1 0.05 0
75
90
105 T (K)
(c)
120
Fig. 6. Temperature dependence of the reciprocal magnetic susceptibility for variously filled PS.
(X)AgNO3 (10-X)FeCl3
100 80
θp (K)
60 40 20 0 -20
0
1
2
3
4
5
6
7
8
9
of meff indicate that the fillers affect the magnetic response of PS. Furthermore, the order of magnitude of meff agrees quite well with the trivalent state of Fe3+. The non-linear FL dependence of meff reveals that the present system does not obey the magnetic dilution mechanism [41]. In this study the obtained Curie–Weiss behavior suggests that the energy band diagram of the present filled PS system is characterized by magnetic localized energy states [40], which could be induced by the fillers. The FLs dependence of both yp and meff reveals a nearly broad plateau region (2.5–9%) of interesting magnetic technological applications. It is interesting to mention that an aim of using the mixed fillers is seeking for a fair FL range of an almost monotonic behavior. Moreover, the negative value of yp of antiferromagnetic interaction character noticed at X ¼ 1%, can be correlated with the unexpected appearance of the crystalline peaks from XRD study at the same FL. 3.5. ESR
x=9.5 x=9.9 x=10
0.25
205
10
X (wt %)
-40 -60
Fig. 7. The FLs dependence of yp .
where wm is the molar susceptibility of the filled polymer. The estimated values of meff were plotted as a function of FL in Fig. 8. The significant values
Pure PS film exhibits an unresolved complicated ESR spectrum as shown in Fig. 9. This spectrum can be attributed to the hyperfine structure resulting from the interaction of the unpaired electrons with the nuclei of the virgin sample [10]. The ESR spectra of PS films filled with various mass fractions of (X)AgNO3 (10X)FeCl3 are depicted in Fig. 10. At X ¼ 0:1%, the spectrum characterizing the virgin sample is totally distorted and the actual spectrum has mainly the feature of a broad signal (superimposed on it a narrow signal) characterized by g 2:02 and DH pp 300 G as well as a deformed signal at the lower part of magnetic field E1700 G of g 4:3400. This suggests that the filler affected the magnetic properties of the polymeric matrix. The broad signal can be attributed to the high concentration of FeCl3 filler while the narrow one due to the low concentration of AgNO3 filler. Moreover, the broad signal can be explained by the absence of isolated Fe3+ and the presence of Fe3+–Fe3+ exchange interaction leading to the existence of aggregated Fe3+ due to the proximity of iron ions [10,13,22–27]. Consequently, the ESR
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206 2
(X)AgNO3 (10-X)FeCl3
µeff (BM)
1.5
1
0.5
0 0
1
2
3
4
5 6 X (wt %)
7
8
9
10
Fig. 8. The FLs dependence of meff at 100 K.
Intensity
PS
500
1000 1500
2000 2500
3000 3500
4000 4500 5000 5500
Magnetic Field (G)
Fig. 9. The ESR spectrum of pure PS film.
demonstrations imply the formation of Fe3+ cluster in the polymeric matrix [10,24,25]. In addition, the unexpected appearance of the crystalline peaks noticed by XRD investigations in the present work also confirms the formation of Fe3+ cluster. It is well-known that the broadening of the line width ðDH pp 300 GÞ depends on the relaxation time of the spin state under study. The two possible relaxation processes are the spin–spin and the spin–lattice relaxation. The spin–spin interaction is usually very efficient unless the sample is extremely dilute. The spin–lattice relaxation is efficient at room temperature, but becomes progressively less at reduced temperature [42]. At the lower part of the magnetic field E1700 G a deformed signal characterized by g 4:3400 can be noticed. This deformed signal can be attributed to the presence of an octahedral distribution of Fe3+ ions in the PS matrix [43–45]. The evolution
of the ESR spectra with X X5% depicts an increase in the intensity of the deformed signal with respect to the broad signal. An appearance of unresolved five lines has also been observed, which can be explained by the fine structure due to isolated Fe3+ ions with electron configuration 3d5 of five parallel spins. This fine structure can be attributed to the dipolar spin–spin coupling, the spin–orbit coupling and the internal electric field within the material. The net result of the above three energy perturbations can be considered as zero-field splitting since they produce an energy level shift in the absence of an external field. Generally, if there are n parallel spins, there will be n equally spaced resonances in the ESR spectrum [42]. The calculated values of g with different FLs are depicted in Fig. 11. A minimum value of g has been noticed at X ¼ 1% which can be correlated with the negative yp value of antiferromagnetic exchange interaction between the magnetic centers from magnetic susceptibility results at the same FL. This minimum value of g can also be correlated with the unexpected appearance of the crystalline peaks from XRD study at the same value of X. This suggests that there are certain energy states having a common contribution to both the structural and magnetic properties of the present system. The Lande´ g factor in this study has values that differ from the g factor for a free electron of 2.0023. In some cases, considerably different g factor values between about 0.2 and 8 have been reported [42]. This can be attributed to one of the following two important points. (a) The internal field of the sample is weak. (b) The paramagnetic electron is well shielded from the field. In this case the orbital angular momentum (L) and the spin angular momentum (S) couple to produce a resultant (J) which itself precesses about the applied magnetic field giving rise to g values, which differ from that of a free electron and it is not easily predictable theoretically [42]. It is worthy to note that the magnetic centers responsible for the observed broad and narrow ESR signals can basically be assigned to localized and delocalized polarons, respectively, formed
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207
(X)AgNO3 (10-X)FeCl3
(X)AgNO3 (10-X)FeCl3
X(wt %) 1.0
0.1
Intensity (a. u.)
Intensity (a. u.)
X(wt %)
0.0
9.9
9.5
9.0
5.0
500
1500
(a)
2500
3500
4500
5500
500
(b)
Magnetic field (G)
1500
2500
3500
4500
5500
Magnetic field (G)
Fig. 10. The ESR spectra of PS filled with various mass fractions of (X)AgNO3 (10X)FeCl3.
polaron and/or bipolaron bound states in the polymeric matrix.
2.14
(X)AgNO 3 (10-X)FeCl 3
2.12 2.1
g
2.08 2.06
4. Conclusion
2.04 2.02 2 0
1
2
3
4
5
6
7
8
9
10
X (wt %) 4.36
(X)AgNO 3 (10-X)FeCl 3
4.32
g
4.28 4.24 4.2 4.16
0
1
2
3
4
5
6
7
8
9
10
X (wt %)
Fig. 11. The FLs dependence of the g values.
during filling [25,46]. Moreover, the formation of polarons is also confirmed from the present DC electrical conduction results where the conduction mechanism was interpreted on the basis of the phonon-assisted charge carrier hopping between
The XRD scans depicted an unexpected appearance of certain crystalline peaks beside the two main peaks characterizing the virgin sample in the FL range 1–9%. The assignment of the above peaks was explained on the basis of cluster formation. This assumption was confirmed from ESR studies. The XRD studies revealed the possibility of obtaining materials of crystalline structure form from amorphous one, in a fair FLs using mixed fillers, that leads to important technological applications. The IR transmission spectra clarified the most notably PS characterizing peaks as well as the structural variations due to the filling. Certain IR peaks were taken as an evidence for the formation of polaron and/or bipolaron bound states in the polymeric matrix. The present DC electrical conduction results were discussed on the basis of the interpolaron
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hopping model. A linear temperature dependence of the hopping distance R0 for various values of X was observed. The FLs dependence of log r and calculated R0 at 348 K was studied. It was implied that the FLs affected the distribution of the hopping sites. The magnetic susceptibility studies implied that w obeyed Curie–Weiss law. The positive values of the paramagnetic Curie temperature yp suggested the predominance of a ferromagnetic exchange interaction between the magnetic centers at low temperatures. The negative value of yp at X ¼ 1% indicated the possibility of an antiferromagnetic exchange interaction. The ESR spectra at the lower values of X depicted a broad signal characterized by g 2:02 and DH pp 300 G superimposed on it a narrow one as well as a deformed signal characterized by g 4:34. The ESR investigations revealed the presence of aggregated Fe+3 indicating the formation of Fe+3 clusters confirming XRD implications about the formation of Fe+3 clusters. On the other hand and at higher values of X, an appearance of unresolved five lines of fine structure character was noticed.
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[13] A. Tawansi, A. El-Khodary, H.M. Zidan, S.I. Badr, Polym. Test. 21 (2002) 381. [14] J. Kuljanin, M. Vuckovic, M.I. Comor, N. Bibic, V. Djokovic, J.M. Nedeljkovic, Eur. Polym. J. 1 (38) (2002) 1659. [15] R. Feyerherm, J. Magn. Magn. Mater. 256 (2003) 328. [16] P. Kuivalainnen, H. Stubb, H. Isotlo, Phys. Rev. B 32 (1985) 7900. [17] R.L. Dutta, A. Syamal, Elements of Magnetochemistry, S. Chand & Company Ltd., New Delhi, 1982 p. 42. [18] A. Tager, Physical Chemistry of Polymers, Mir publishers, Moscow, 1972. [19] R. Hammel, W.J. Mackinght, F.E. Karassz, J. Appl. Phys. 46 (1975) 10. [20] C. Ayyagari, D. Bedrov, G.D. Smith, Macromolecules 33 (16) (2000) 6194. [21] T. Halicioglui, C.W. Bauschlicher Jr, Rept. Prog. Phys. 51 (1988) 883. [22] A. Tawansi, A.H. Oraby, H.M. Zidan, M.E. Dorgham, Physica B 254 (1998) 126. [23] H.M. Zidan, J. Appl. Polym. Sci. 88 (2003) 104. [24] A.E. Youssef, Effect of Filling with Some Transition Metal Compounds on Some Physical Properties of Polystyrene Films, M.Sc. Thesis, Mansoura University, Egypt, 2003. [25] A. Tawansi, A. El-Khodary, A.H. Oraby, A.E. Youssef, J. Appl. Polym. Sci., submitted for publication. [26] S. Geschwind, Electron Paramagnetic Resonance, Plenum Press, New York, 1972. [27] A. El-Khodary, Physica B 344 (2004) 297. [28] R.M. Silverstein, G.C. Bassler, T.C. Morrill, Spectroscopic Identification of Organic Compounds, Wiley, New York, 1973. [29] I. Fleming, D.H. Williams, Spectroscopic Methods in Organic Chemistry, McGraw-Hill, New York, 1996. [30] L.W. Shacklette, R.R. Chance, D.M. Ivory, G.G. Miller, R.H. Baughman, Synth. Met. 1 (1980) 307. [31] L.W. Shacklette, H. Eckhadt, R.R. Chance, G.G. Miller, D.M. Ivory, R.H. Baughman, J. Chem. Phys. 73 (1980) 4098. [32] C.R. Fincher, M. Ozaki, A.G. Mac Diarmid, Phys. Rev. B 19 (1979) 4140. [33] D. Racovics, I. Bozovic, S.A. Stepanyan, L.A. Gribov, Solid State Commun. 43 (1982) 127. [34] J.L. Bredas, R.R. Chance, R. Silbey, Mol. Cryst. Liq. Cryst. 77 (1981) 319. [35] J.L. Bredas, R.R. Chance, R. Silbey, Phys. Rev. B 26 (1982) 5843. [36] S. Kivelson, Phys. Rev. B 25 (1982) 3798. [37] N.F. Mott, R.W. Gurrey, Electronic Process in Ionic Crystals, Oxford University Press, London, 1940. [38] A. Tawansi, H.I. Abdelkader, M. Elzalabany, E.M. Abdelrazek, J. Mater. Sci. 29 (1994) 3451. [39] R.J. Myers, Molecular Magnetism and Magnetic Resonance Spectroscopy, Prince-Hall, Inc., Englewood Cliffs, New Jersey, 1973. [40] D. Jiles, Introduction to Magnetism and Magnetic Materials, Chapman and Hall, London, 1991.
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Journal of Magnetism and Magnetic Materials 283 (2004) 199–209 www.elsevier.com/locate/jmmm
Structural, electrical and magnetic properties of polystyrene films filled with AgNO3–FeCl3 mixed fillers A. Tawansi, A. El-Khodary, A.E. Youssef Faculty of Science, Physics Department, Mansoura University, Mansoura 35516, Egypt Received 22 March 2004; received in revised form 15 May 2004 Available online 15 June 2004
Abstract The effect of various filling levels (FLs) of mixtures of two transition compounds (X)AgNO3 (10X) FeCl3, on structural, electrical and magnetic properties of polystyrene (PS) films was investigated. The X-ray diffraction (XRD) displayed an unexpected appearance of a number of crystalline peaks beside the two main peaks characterizing the virgin film. The crystalline peaks were attributed to the cluster formation. The infrared (IR) analysis was used to detect the most notably PS peaks and to clarify the structural variations due to the filling. Certain IR peaks were taken as an evidence for the formation of polaron and/or bipolaron bound states in the polymeric matrix. The direct current (DC) electrical conduction studies revealed a linear temperature dependence of the hopping distance R0 for various FLs. This is an indication that the conduction mechanism can be attributed to one-dimensional intrachain type based on the phonon-assisted charge carrier interpolaron hopping model. The DC magnetic susceptibility results, in the temperature range 90–235 K, followed the Curie–Weiss behavior. The positive and negative values of the paramagnetic Curie temperature ðyp Þ indicate the possibility of ferromagnetic and antiferromagnetic exchange interactions between the magnetic centers at low temperatures, respectively. The electron spin resonance (ESR) spectra at the lower values of X depicted a broad signal superimposed on it a narrow one as well as a deformed signal. The ESR investigations at the lower values of X indicated the presence of aggregated Fe+3 confirming the XRD implications about the cluster formation. On the other hand and at higher values of X, an appearance of unresolved five lines of fine structure character was noticed. r 2004 Elsevier B.V. All rights reserved. PACS: 71.20:70; Rv.:76.30.v Keywords: Polystyrene; Mixed AgNO3 and FeCl3 fillers; X-ray diffraction; Infrared; Electrical resistivity; Magnetic susceptibility; Electron spin resonance
Corresponding author. Tel.: +20-50-2242388; fax: +20-50-2246781.
E-mail address: [email protected] (A. El-Khodary). 0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.05.021
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1. Introduction Polymeric materials have attracted considerable attention due to their spectacular growth of interesting potential applications. Among such materials PS which is a preferred material because it has good stiffness, optical, electrical and mechanical properties [1,2]. In addition PS is a non-polar, low loss polymer and hence has desirable electronic properties for use in the field of insulating materials [3,4]. PS and its blends have been widely used for producing materials with improved physical and mechanical properties [5–7]. The introduction of the filler into PS can markedly influence the properties of the polymer. This depends on the chemical nature of the filler and the way in which it interacts with the chains of the polymeric matrix. One can choose the desired filler and the suitable FL to get a filled polymer with the controllable physical and chemical properties for specific technological applications. Recently transition metal ions have been extensively used as fillers in organic polymers because of the partially filled d-electron shells of very interesting properties [3,4,7–16]. Ferric chloride, FeCl3, with trivalent iron ions of 3d5 electron configuration was selected as a filler due to its magnificent magnetic and electrical properties [3,4,8–10,16]. Moreover, FeCl3 has been reported to give the most stable material, an aspect of importance when considering applications [16]. AgNO3 was also chosen as a filler due to its interesting good optical as well as electrical properties. AgNO3 and FeCl3 mixed fillers were used as a trial to achieve a monotonic behavior of FL dependence of certain desired properties in a fair FL range. The aim of the present work is devoted to investigate the effect of the mixed fillers AgNO3 and FeCl3 of various FLs on the physical properties of the filled PS films.
2. Experimental work 2.1. Sample preparation PS was obtained in the form of pellets of molecular weight 100,000 from Aldrich chemical
company. Metal compounds AgNO3 and FeCl3 were provided in solid-state form. The used solvents were cyclohexanone, and dimethylformamide (DMF). The studied PS films filled with transition metal compounds (X)AgNO3 (10X) FeCl3 were prepared by casting methods. Cyclohexanone was used to dissolve the polymer. The transition metal compounds AgNO3 and FeCl3 were dissolved in dimethylformamide (DMF). The polymer and the transition metal compounds were cast to glass dishes and kept in a dry atmosphere at T ¼ 333 K for about ten days to remove the solvent traces. Various concentrations of the fillers were obtained where X ¼ 0:0, 0.1, 0.5, 1.0, 2.5, 5, 7.5, 9.0, 9.5, 9.9 and 10.0 wt%. The film thickness was in the range of 150–250 mm.
2.2. Tools of analysis X-ray diffraction scans were carried out using a Seimens type F diffractometer with CuKa radiation and LiF monochromator. The IR spectra of the different films at room temperature in the wavenumber range 4000–600 cm1 were recorded by the IR spectrophotometer (Perkin Elmer 883). It is remarkable that we will present the IR spectra in the range 2000–600 cm1, in which the main characterizing spectral features were found. The DC electrical resistivity was measured using insulation tester (type TM 14) and autoranging multimeter (Keithley 175 A) of accuracy 70.2% [7]. The DC magnetic susceptibility (w) was measured, in the temperature range of 90–280 K, using a Faraday pendulum balance [17]. The accuracy of the measurements was better than 73.5%. Diamagnetic corrections were done. The ESR spectra were recorded at room temperature on a BRUKER (EMX) spectrometer operating in the X-band frequency (E9.7 GHz) with a field modulation frequency of 100 KHz. The microwave power and modulation amplitude were 10 mW and 0.1 mT, respectively. Standard sample of MgO doped with Mn+2 was used as a calibrant.
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3. Results and discussion 3.1. XRD The XRD scans for PS films filled with various mass fractions of (X)AgNO3 (10X)FeCl3 are depicted in Fig. 1. Two main peaks at 2y 19:51 and 101, characterizing the virgin sample were noticed. The first peak at 2y 19:51, which is the most intense one, is the amorphous peak and it corresponds to the Van der Waals distance [18–20]. In amorphous linear polymers the chains are approximately parallel to one another leading to a distance between neighbors equals to about ( [18]. This value is characterizing the 4.5–5.0 A mean distance between adjacent chains. The second peak at 2y 101 is the polymerization peak which can be attributed to the intermolecular backbone–backbone correlation as well as to the size of the side group which corresponds to an approximately hexagonal ordering of the molecular chains [20]. At X ¼ 1%, an unexpected appearance of crystalline peaks at 2y 281,
201
32.51, 44.51 and 46.51 was observed. At X ¼ 9%, an appearance of a crystalline peak at 2y 381 was also noticed. These peaks are not easily assigned to the formation of bulk crystalline materials. It is well-known that the polymer matrix is complex and the behavior of the added fillers, X AgNO3 (10X)FeCl3, is difficult to predict. These make the identification of the produced XRD crystalline peaks more difficult. It is remarkable to mention that microclusters display curious crystallographic anomalies, which cannot be found in the bulk [21]. However, the peaks at 2y 281 and 46.51 may be attributed to AgCl (1 1 1) and (2 2 0), respectively, while the peak at 2y 32:51 can be attributed to AgCl (2 0 0) and/or Ag2O (1 1 1) and the peak at 2y 38:51 can be attributed to Ag (1 1 1) and/or Ag2O (2 0 0) [19]. The small peak (hump) at 2y 44:51 can be attributed to Ag (2 0 0) [19]. All the above peaks almost disappeared at X ¼ 10% except a small peak (hump) at 2y 38:51 which can only be attributed to the filler AgNO3. The above assigned crystalline peaks can be attributed to the formation of clusters. This
Fig. 1. X-ray diffraction scans of PS filled with various mass fractions of (X)AgNO3 (10X)FeCl3, (X) AgCl, ( ) Ag2O and (K) Ag.
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explanation agrees quite well with the ESR interpretation of the formation of aggregated Fe3+ and the absence of isolated Fe3+ [10,13,22–27]. Generally, it is clear that at X ¼ 1%, the aggregation (clusters) is basically due to FeCl3 filler, while at X ¼ 9%, the aggregation (clusters) is basically due to AgNO3 filler. On the other hand, the change of the intensity of different peaks at different values of X can be attributed to the change of the contents of the two fillers FeCl3 and AgNO3. Finally, at X ¼ 10% all the above peaks almost disappeared except the hump at 2y 381 which can be attributed only to the filler AgNO3. 3.2. IR spectroscopy Fig. 2 reveals the IR transmission spectra for PS films filled with various mass fractions of a mixture
of AgNO3 and FeCl3. The most notably PS characterizing peaks are assigned. The aromatic ring (C6H5) gives a rise to the group of bands above 3010 cm1 which are not shown here. The unsaturated C–H stretching above 3000 cm1 as well as the distinctive double bond CQC stretching at about 1600 cm1 identify polymers containing aromatics [28,29]. The C–C stretching mode of phenyl rings at 1480 cm1 and the C–H out of plane deformation modes of monosubstituted phenyl rings at 765 and 695 cm1 are observed [16,28,29]. The present results indicate that the IR transmission decreases with increasing the doping content in the whole studied frequency range. This could be due to free or weakly bound charge carriers [16]. The peaks at 1530, 1275 and 1180 cm1 (solid arrows) are interesting, since Shacklette et al. [30,31] found dopant induced
(X)AgNO3 (10-X)FeCl3
(X)AgNO3 (10-X)FeCl3
X (wt %) 10.0
X (wt %) 2.5
9.9
Transmittance (a. u.)
Transmittance (a. u.)
1.0
0.5
0.1
9.5
9.0
0.0
7.5
pure
5.0
2000
(a)
1800
1600
1400
1200
1000
Wavenumber (cm-1)
800
600
2000
(b)
1800
1600
1400
1200
1000
Wavenumber (cm-1)
Fig. 2. The IR transmission spectra of PS filled with various FLs.
800
600
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3.3. DC electrical conduction The DC electrical resistivity (r) was measured in the temperature range 293–390 K for PS filled with various mass fractions of (X)AgNO3 (10X)FeCl3. The double bond CQC and the deformation in the benzene ring (detected by the IR analysis), may evidence the formation of polarons and/or bipolarons in the polymeric matrix. Therefore, the present results can be discussed on the basis of the Kuivalainen et al. [16] modified interpolaron hopping model, in which the conduction mechanism can be attributed to phonon-assisted one-dimensional charge carrier hopping between polaron and/or bipolaron bound states in the polymeric matrix. According to this model the electrical resistivity can be formulated as
where n is a constant 10, and the pre-factor, g0 ¼ 1:2 1017 s1, was estimated by Kivelson [36]. Using a computer-aided program, the order of magnitude of r in the present work was adjusted with the impurity concentration Cimp, which actually was the fitting parameter. The parameters zJ ¼ 1:06 nm and z? ¼ 0:22 nm [37], depend on the interchain resonance energy and the interchain distance. As an acceptable approximation, and for simplicity put Y p ¼ Y bp [38], and using Eqs. (1) and (2) the values of the separation between impurities which are considered as the hopping distance R0 can be calculated. Owing to Eqs. (1) and (2) the plotting of ln r versus ln T should give a straight line if the measured rðTÞ is dominated by the interpolaron hopping [16,38]. Fig. 3 depicts this plot for variously filled PS films. The linear behavior of these plots corresponds to the temperature ranges in which the interpolaron hopping proceeds [16,38]. The linear temperature dependence of the obtained values of the hopping distance R0 for various FLs is plotted in Fig. 4. This supports the previous postulate that the conduction mechanism 28
24 22 20
16 5.7
(1)
5.75
(a)
ð2Þ
5.8
5.85
5.9
5.95
Ln (T,K) 29.5
(X)AgNO3 (10-X)FeCl3
27.5
Ln (ρ, Ohm.cm)
where A1 ¼ 0:45; B1 ¼ 1:39; Y p and Y bp are the concentration of polarons and bipolarons, respectively, R0 ¼ ð3=4pC imp Þ1=3 is the typical separation between impurities whose concentration is Cimp; z ¼ ðzJj z2? Þ1=3 is the average decay length of a polaron and bipolaron wave function; and zJ and z? are the decay lengths parallel and perpendicular to the polymer chain, respectively. The electronic transition rate between polaron and bipolaron states can be expressed as gðTÞ ¼ g0 ðT=300 KÞnþ1 ;
Pure X=0% X = 0.1 % X = 0.5 % X=1%
18
r ¼ ½kT=A1 e2 gðTÞðR20 =zÞ ½ðY p þ Y bp Þ2 =Y p Y bp expð2B1 R0 =zÞ;
(X)AgNO3 (10-X)FeCl3
26
Ln (ρ, Ohm.cm)
peaks at exactly the same wavenumbers in AsF5doped polyparaphenylene (PPP). Also these peaks were found in doped polyacetylene (PA) [32] and FeCl3-doped PPP [16]. However, Kuivalainen et al. [16] and Fincher et al. [32] stated that these modes of vibrations are not due to specific vibration of the dopant molecules but are intrinsic vibrations of the polymer chain. According to Kuivalainen et al. [16], the mode at 1275 cm1, which also appears at 1298 cm1 according to Racovics et al. [33], is related to the deformations of the benzene ring structure which is typical of the polaron and bipolaron defect states [34,35].
203
25.5
X = 2.5 % X=5% X = 7.5 % X=9% X = 9.5 % X = 9.9 % X = 10 %
23.5 21.5 19.5 17.5 5.725
(b)
5.775
5.825
5.875
5.925
Ln (T,K)
Fig. 3. Ln r against ln T plots for variously filled PS.
6
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204
14
9.5 Pure X=0% x=0.1% x= 0.5% X=1% x= 2.5%
(X)AgNO3 (10-X)FeCl3
8.5 7.5
14
(X)AgNO3 (10-X)FeCl3 12
12
10
10
8
8
6
6
4
4
5.5 4.5 3.5 2.5 1.5 300
320
340
(a)
360
380
400
T (K)
(X)AgNO3 (10-X)FeCl3
8
X=5 % x=7.5% x=9% x=9.5% x=9.9% x=10%
7
2
Ro (nm)
6
0
1
2
3
4
5 6 X (wt%)
7
8
9
Ro (nm)
Log (ρ, Ohm.cm)
Ro (nm)
6.5
2 10
Fig. 5. The FLs dependence of (J) log r and (K) R0 at 348 K.
5 4 3 2 300
320
(b)
340
360
380
400
T (K)
Fig. 4. Temperature dependence of R0 for variously filled PS.
is predominated by the interpolaron hopping model [16]. The FLs dependence of the measured log r and the calculated R0 at T ¼ 348 K are shown in Fig. 5. Minimum values of log r and R0 are observed at X ¼ 2:5%, 7.5%, while maxima are observed at X ¼ 0:5%, 5% and 9.5%. This clarifies that the FL affects the distribution of the hopping sites. 3.4. DC magnetic susceptibility The study of the DC magnetic susceptibility (w) of PS films filled with various mass fractions of (X)AgNO3 (10X)FeCl3 was performed in the temperature range 90–235 K. The temperature dependence of the reciprocal magnetic susceptibility is plotted in Fig. 6. These plots are linear and follow the Curie–Weiss law in which w ¼ C=ðT yp Þ;
ð3Þ
where C is Curie constant and yp is the paramagnetic Curie temperature. It is important to note that ferromagnetic materials behave as normal paramagnetics at high
temperatures and as T approaches the magnitude of yp of positive value there is a marked increase in w. The critical temperature obtained for this process is called the Curie temperature. Below this temperature Eq. (3) is meaningless because the sample can have zero-field moment. Once the material becomes ordered, the susceptibility behave in a very complicated way and no longer has a unique value for a given field strength. On the contrary, antiferromagnetic materials behave also as normal paramagnetics at high temperatures and as T approaches the magnitude of yp of negative value w becomes small and it attains its maximum value at certain critical temperature called Ne´el temperature. Below this temperature the spins have antiparallel orientations [39,40]. In the present study the values of yp were calculated for different values of X using Eq. (3) and Fig. 6. These values were plotted in Fig. 7. Positive values of yp were found by suggesting the predominance of ferromagnetic exchange interaction between the magnetic centers at low temperatures. On the other hand, a negative yp value was found at X ¼ 1% indicating the possibility of an antiferromagnetic exchange interaction between the magnetic centers at low temperatures. In addition, the effective paramagnetic moment (meff ) was estimated at T ¼ 100 K, using the following equation: meff ¼ 2:828½wm ðT yp Þ1=2 ;
ð4Þ
ARTICLE IN PRESS A. Tawansi et al. / Journal of Magnetism and Magnetic Materials 283 (2004) 199–209 (X)AgNO3 (10-X)FeCl3
0.3 0.25
pure x=0 x=0.1 x = 0.5
0.2 0.15 0.1 0.05 0 75
125
175
(a)
225
T (K)
0.7
(X)AgNO3 (10-X)FeCl3
x =1 x=2.5 x=5 x=7.5 x=9
0.6 1/χ (106 g/emu)
0.5 0.4 0.3 0.2 0.1 0 75
125
175
(b) 0.35
(X)AgNO3 (10-X)FeCl3
0.3 1/χ (106 g/emu)
225
T (K)
0.2 0.15 0.1 0.05 0
75
90
105 T (K)
(c)
120
Fig. 6. Temperature dependence of the reciprocal magnetic susceptibility for variously filled PS.
(X)AgNO3 (10-X)FeCl3
100 80
θp (K)
60 40 20 0 -20
0
1
2
3
4
5
6
7
8
9
of meff indicate that the fillers affect the magnetic response of PS. Furthermore, the order of magnitude of meff agrees quite well with the trivalent state of Fe3+. The non-linear FL dependence of meff reveals that the present system does not obey the magnetic dilution mechanism [41]. In this study the obtained Curie–Weiss behavior suggests that the energy band diagram of the present filled PS system is characterized by magnetic localized energy states [40], which could be induced by the fillers. The FLs dependence of both yp and meff reveals a nearly broad plateau region (2.5–9%) of interesting magnetic technological applications. It is interesting to mention that an aim of using the mixed fillers is seeking for a fair FL range of an almost monotonic behavior. Moreover, the negative value of yp of antiferromagnetic interaction character noticed at X ¼ 1%, can be correlated with the unexpected appearance of the crystalline peaks from XRD study at the same FL. 3.5. ESR
x=9.5 x=9.9 x=10
0.25
205
10
X (wt %)
-40 -60
Fig. 7. The FLs dependence of yp .
where wm is the molar susceptibility of the filled polymer. The estimated values of meff were plotted as a function of FL in Fig. 8. The significant values
Pure PS film exhibits an unresolved complicated ESR spectrum as shown in Fig. 9. This spectrum can be attributed to the hyperfine structure resulting from the interaction of the unpaired electrons with the nuclei of the virgin sample [10]. The ESR spectra of PS films filled with various mass fractions of (X)AgNO3 (10X)FeCl3 are depicted in Fig. 10. At X ¼ 0:1%, the spectrum characterizing the virgin sample is totally distorted and the actual spectrum has mainly the feature of a broad signal (superimposed on it a narrow signal) characterized by g 2:02 and DH pp 300 G as well as a deformed signal at the lower part of magnetic field E1700 G of g 4:3400. This suggests that the filler affected the magnetic properties of the polymeric matrix. The broad signal can be attributed to the high concentration of FeCl3 filler while the narrow one due to the low concentration of AgNO3 filler. Moreover, the broad signal can be explained by the absence of isolated Fe3+ and the presence of Fe3+–Fe3+ exchange interaction leading to the existence of aggregated Fe3+ due to the proximity of iron ions [10,13,22–27]. Consequently, the ESR
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206 2
(X)AgNO3 (10-X)FeCl3
µeff (BM)
1.5
1
0.5
0 0
1
2
3
4
5 6 X (wt %)
7
8
9
10
Fig. 8. The FLs dependence of meff at 100 K.
Intensity
PS
500
1000 1500
2000 2500
3000 3500
4000 4500 5000 5500
Magnetic Field (G)
Fig. 9. The ESR spectrum of pure PS film.
demonstrations imply the formation of Fe3+ cluster in the polymeric matrix [10,24,25]. In addition, the unexpected appearance of the crystalline peaks noticed by XRD investigations in the present work also confirms the formation of Fe3+ cluster. It is well-known that the broadening of the line width ðDH pp 300 GÞ depends on the relaxation time of the spin state under study. The two possible relaxation processes are the spin–spin and the spin–lattice relaxation. The spin–spin interaction is usually very efficient unless the sample is extremely dilute. The spin–lattice relaxation is efficient at room temperature, but becomes progressively less at reduced temperature [42]. At the lower part of the magnetic field E1700 G a deformed signal characterized by g 4:3400 can be noticed. This deformed signal can be attributed to the presence of an octahedral distribution of Fe3+ ions in the PS matrix [43–45]. The evolution
of the ESR spectra with X X5% depicts an increase in the intensity of the deformed signal with respect to the broad signal. An appearance of unresolved five lines has also been observed, which can be explained by the fine structure due to isolated Fe3+ ions with electron configuration 3d5 of five parallel spins. This fine structure can be attributed to the dipolar spin–spin coupling, the spin–orbit coupling and the internal electric field within the material. The net result of the above three energy perturbations can be considered as zero-field splitting since they produce an energy level shift in the absence of an external field. Generally, if there are n parallel spins, there will be n equally spaced resonances in the ESR spectrum [42]. The calculated values of g with different FLs are depicted in Fig. 11. A minimum value of g has been noticed at X ¼ 1% which can be correlated with the negative yp value of antiferromagnetic exchange interaction between the magnetic centers from magnetic susceptibility results at the same FL. This minimum value of g can also be correlated with the unexpected appearance of the crystalline peaks from XRD study at the same value of X. This suggests that there are certain energy states having a common contribution to both the structural and magnetic properties of the present system. The Lande´ g factor in this study has values that differ from the g factor for a free electron of 2.0023. In some cases, considerably different g factor values between about 0.2 and 8 have been reported [42]. This can be attributed to one of the following two important points. (a) The internal field of the sample is weak. (b) The paramagnetic electron is well shielded from the field. In this case the orbital angular momentum (L) and the spin angular momentum (S) couple to produce a resultant (J) which itself precesses about the applied magnetic field giving rise to g values, which differ from that of a free electron and it is not easily predictable theoretically [42]. It is worthy to note that the magnetic centers responsible for the observed broad and narrow ESR signals can basically be assigned to localized and delocalized polarons, respectively, formed
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207
(X)AgNO3 (10-X)FeCl3
(X)AgNO3 (10-X)FeCl3
X(wt %) 1.0
0.1
Intensity (a. u.)
Intensity (a. u.)
X(wt %)
0.0
9.9
9.5
9.0
5.0
500
1500
(a)
2500
3500
4500
5500
500
(b)
Magnetic field (G)
1500
2500
3500
4500
5500
Magnetic field (G)
Fig. 10. The ESR spectra of PS filled with various mass fractions of (X)AgNO3 (10X)FeCl3.
polaron and/or bipolaron bound states in the polymeric matrix.
2.14
(X)AgNO 3 (10-X)FeCl 3
2.12 2.1
g
2.08 2.06
4. Conclusion
2.04 2.02 2 0
1
2
3
4
5
6
7
8
9
10
X (wt %) 4.36
(X)AgNO 3 (10-X)FeCl 3
4.32
g
4.28 4.24 4.2 4.16
0
1
2
3
4
5
6
7
8
9
10
X (wt %)
Fig. 11. The FLs dependence of the g values.
during filling [25,46]. Moreover, the formation of polarons is also confirmed from the present DC electrical conduction results where the conduction mechanism was interpreted on the basis of the phonon-assisted charge carrier hopping between
The XRD scans depicted an unexpected appearance of certain crystalline peaks beside the two main peaks characterizing the virgin sample in the FL range 1–9%. The assignment of the above peaks was explained on the basis of cluster formation. This assumption was confirmed from ESR studies. The XRD studies revealed the possibility of obtaining materials of crystalline structure form from amorphous one, in a fair FLs using mixed fillers, that leads to important technological applications. The IR transmission spectra clarified the most notably PS characterizing peaks as well as the structural variations due to the filling. Certain IR peaks were taken as an evidence for the formation of polaron and/or bipolaron bound states in the polymeric matrix. The present DC electrical conduction results were discussed on the basis of the interpolaron
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hopping model. A linear temperature dependence of the hopping distance R0 for various values of X was observed. The FLs dependence of log r and calculated R0 at 348 K was studied. It was implied that the FLs affected the distribution of the hopping sites. The magnetic susceptibility studies implied that w obeyed Curie–Weiss law. The positive values of the paramagnetic Curie temperature yp suggested the predominance of a ferromagnetic exchange interaction between the magnetic centers at low temperatures. The negative value of yp at X ¼ 1% indicated the possibility of an antiferromagnetic exchange interaction. The ESR spectra at the lower values of X depicted a broad signal characterized by g 2:02 and DH pp 300 G superimposed on it a narrow one as well as a deformed signal characterized by g 4:34. The ESR investigations revealed the presence of aggregated Fe+3 indicating the formation of Fe+3 clusters confirming XRD implications about the formation of Fe+3 clusters. On the other hand and at higher values of X, an appearance of unresolved five lines of fine structure character was noticed.
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