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Characterization, electrical and magnetic properties of PVA films filled with FeCl3–MnCl2 mixed fillers
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Journal of Magnetism and Magnetic Materials 320 (2008) 1739–1746 www.elsevier.com/locate/jmmm
Characterization, electrical and magnetic properties of PVA films filled with FeCl3–MnCl2 mixed fillers A. El-Khodary, A.H. Oraby, M.M. Abdelnaby Physics Department, Faculty of Science, Mansoura University, P.O. Box 55, 35516 Mansoura, Egypt Received 21 October 2007; received in revised form 14 January 2008 Available online 24 January 2008
Abstract Polyvinyl alcohol (PVA) films filled with different filling levels (FLs) of XFeCL3(15X)MnCl2 were studied. The DSC thermograms exhibited an increase in the melting temperature with filling, indicating better thermal stability of the filled polymer of interesting industrial applications. The amorphous feature of the filled polymer was depicted using XRD scans. Vibrational studies displayed significant structural deformations. The FL dependence of certain IR absorption peaks was discussed. The dc electrical conduction mechanism was interpreted on the basis of the modified interpolaron hopping model. The present results of the dc magnetic susceptibility (w) suggested the temperature dependence of Curie–Weiss behavior characterized by localized magnetic moments. The effective paramagnetic moment (meff) was estimated; its dependence on the FL exhibited a non-linear character. The electron spin resonance (ESR) study revealed unresolved broad distorted signals characterized by the hyperfine structure. The ESR parameters were evaluated. A correlation between the above-mentioned studies was discussed to relate the structural, electrical and magnetic properties of the filled PVA polymer. r 2008 Elsevier B.V. All rights reserved. PACS: 70; 71.20.Rv; 76.30.v Keywords: Polyvinyl alcohol; FeCl3 and MnCl2 mixed fillers; Characterization; Electrical resistivity; Magnetic susceptibility; Electron spin resonances
1. Introduction Polymers have received much experimental attention in an attempt to synthesize organic polymers alternative to conventional inorganic materials. The electrical resistivity of polymers [1,2] such as polyvinyl alcohol, polyvinyl chloride, polystyrene and polyethylene would normally be classified as insulators possessing specific resistivities of the order 1015 O cm or greater. Fleming and Ranicar concluded that the electrical conduction in polyvinyl chloride occurs via a hopping mechanism [1]. The electrical properties of the polymeric materials can be investigated with filling, and a model is required to interpret the electrical conduction mechanism. Transition metal halides of magnificent electrical and magnetic properties have been used as fillers. Different Corresponding author.
E-mail address: [email protected] (A. El-Khodary). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.01.030
concentrations of the fillers were applied in an attempt to modify the physical properties of the polymer under study and to obtain the desired filled polymer with distinct potential technological applications [3–11]. Moreover, the magnetic properties of the polymeric materials significantly depend on the polymer, the type of the filler and the filling level. The magnetic susceptibility widely varies from the temperature-dependent magnetic susceptibility that follows the Curie–Weiss law or the Ising law, the temperature-independent Pauli parama gnetic (TIPP) susceptibility of the itinerant character to the temperature-independent diamagnetic susceptibility [5,6,8–10,12]. Furthermore, electron spin resonance (ESR) spectroscopy provides a sensitive magnetic technique for detecting the unpaired electrons in the polymeric materials as well as determining the origin of the magnetic centers [6–13]. Polyvinyl alcohol (PVA) has attracted extensive interest in industrial as well as medical and pharmaceutical
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applications. This is due to the important combination of thermal, structural, mechanical, optical, electrical and magnetic properties [4,7–9,11–15]. Motivated by the magnificent electrical and magnetic properties of the transition metal halides, FeCl3 and MnCl2 were selected as fillers of interesting partially filled d electron configuration. The present work on Fe and Mn ions is quite fundamental to the understanding of the large ionic clusters of these materials, such as the single-molecular magnets Fe8 and Mn12. The latter topic makes this work quite relevant to modern magnetism, from both the fundamental and applied aspects [16–18]. The aim of the present work is to study the physical properties of PVA films filled with different filling levels (FLs) of the mixed fillers XFeCL3(15X)MnCl2 as well as to find a correlation between the investigated structural, electrical and magnetic properties. 2. Experimental technique 2.1. Samples preparation PVA in the form of grains of molecular weight 17,000 was provided by Avondale laboratories Beaumont Close Banbury Oxon, England. The samples were prepared by a casting method. PVA, FeCl3 and MnCl2 were dissolved separately in distilled water. A mixture of the dissolved polymer and the fillers of their mass fractions (FeCl3 and MnCl2) were cast on a glass dish and kept in a dry atmosphere at 323 K for about 1 week. PVA films with different mass fractions FLs of the fillers XFeCl3(15–X) MnCl2 were obtained, where X=0, 1, 5, 7.5,10, 14, 15 wt%. The thickness of the films was in the range of 100–150 mm. 2.2. Tools of analysis Differential Scanning Calorimetry (DSC) thermograms were carried out using a thermoanalyser (Shimadzu DSC50) with a measuring temperature range of 293–573 K and a heating rate of 5 K/min. The infrared (IR) spectra were recorded in the wavenumber range of 4000–400 cm1 using a spectrophotometer (PerkinElmer 883). X-ray diffraction (XRD) scans were obtained using a Siemens type F diffractometer with Cu Ka radiation and a LiF monochromator. The dc electrical resistivity was measured using an insulation tester type TM 14 and an autoranging multimeter (Keithley 175 A) of accuracy 70.2% [5]. A Faraday pendulum balance technique was used in measuring the dc magnetic susceptibility with an accuracy better than 73.0%. 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.
A standard sample of MgO doped with Mn2+ was used as a calibrant. 3. Results and discussion 3.1. Characterization 3.1.1. Differential scanning calorimetry DSC Virgin PVA films exhibit glass transition temperature TgE85 1C, crystallization temperature Tc in the range of 90–110 1C, melting temperature Tm at about 202 1C and degradation temperature TdE270 1C [2,19]. The DSC thermograms, in the temperature range from room temperature to 573 K, of PVA films filled with different mass fractions of XFeCl3(15X)MnCl2 are shown in Fig. 1. In the present study, the spectra are characterized by three main peaks, two endothermic peaks at about 190–200 and 278–282 1C and an exothermic one at 281–294 1C. The first peak can be attributed to the melting process, while the second and third were assigned to the degradation process, respectively. Moreover, an appearance of small exothermic peak in the temperature range E90 to 30 1C was noticed at the lower and higher values of X, respectively. This peak can be assigned to the crystallization process. The decrease in the crystallization temperature at higher values of X can be attributed to the important role of the FeCl3 filler. The dependence of the main peaks on the FLs is noticed in Table 1. It is interesting to notice that the spectra are thermally stable up to E280 1C. The existence of two degradation Td peaks as shown in Fig. 1 implies that the polymer degrades in two stages, indicating the existence of two different phases [19]. Moreover, Table 1 depicts that the melting temperature Tm increases with increase in the filling level X, suggesting the significant role of the FeCl3 filler. The increase in the melting temperature Tm(186–207 1C) with filling represents an important improvement of the thermal stability of the filled polymer under investigation, leading to interesting technological applications. 3.1.2. XRD spectra The X-ray diffraction spectra of PVA films filled with different mass fractions of the mixed fillers XFeCL3 (15X)MnCl2 is shown in Fig. 2. The main peaks of the virgin polymer were recorded, suggesting the amorphous feature of PVA of a short-range order. It is well known that the scattering pattern of amorphous polymers usually has two rings. In this study, a broad peak was noticed at 2yE19.51 that can be assigned to the Van der Wall distance of 4.5E5 A˚. This peak may be attributed to the intermolecular distances between neighboring chains. A small hump was observed at 2yE421, corresponding to an approximately hexagonal ordering of the molecular chains [20].
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Fig. 2. XRD scans of PVA films filled with various mass fractions of XFeCl3(15X)MnCl2. Fig. 1. The DSC XFeCl3(15X)MnCl2.
thermograms
of
PVA
films
filled
with
The important decrease in the intensity of the main peak of the virgin polymer at 2yE19.51 for the different values of X may represent a decrease in the lamellar structure of the virgin polymer due to filling. Moreover, the absence of the peaks characterizing FeCl3 and MnCl2 crystals in the present spectra of the filled polymer suggests that the spectra are either free from FeCl3 and MnCl2 crystals or contain too few traces of these crystals to be detected by the present XRD technique. This can be taken as an evidence of the random distribution of the fillers within the polymeric matrix [8,15]. 3.1.3. IR analysis The IR transmission spectra of PVA films filled with different FLs of the mixed fillers XFeCL3(15X)MnCl2 are Table 1 The FL dependence of the main peaks X(%)
Tc (exo)
Tm (endo)
Td1 (endo)
Td2 (exo)
1 5 7.5 10 14
90 – 90 37 32
186 187 197 200 207
278 282.06 281.8 280 277
285 293 294 290 281
Temperatures in centigrade.
shown in Fig. 3. The main PVA peaks were observed. For simplicity, the range of the wavenumber 41800 cm1 is not shown here. The assignment of the most notable characterizing frequencies [21] is demonstrated in Table 2. Certain structural deformations can be identified by investigating the FL dependence of the IR absorption peaks. Fig. 4 displays the PVA structural deformations of the intensity of the absorption peaks at 1090 and 1380 cm1 as a function of the FL. The above-mentioned two peaks exhibit two maxima at 5% and 10% as well as two minima at 7.5% and 14%. These peaks will be correlated with both the electrical and magnetic properties in this study. The non-linear behavior of the FL dependence of the above-mentioned IR absorption peaks clarifies the important role of the interaction of the fillers with the different modes of vibrations. It is remarkable to note that the absorption peak at 1680 cm1, which was assigned to the CQC stretching mode of vibration, can be considered as a suitable site for polarons and/or bipolarons [8,22–24]. 3.2. Electrical properties The dc electrical resistivity r of PVA films filled with different FLs of the mixed fillers XFeCl3 (15–X) MnCl2 was measured in the temperature range of 330–400 K. The goal of this electrical study is to explain the present experimental results and to investigate its corresponding
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Table 2 The most notably characterizing frequencies Frequency (cm1)
Assignment
950 1090 1380 1430 1680 2902 3300
n (CH2) n (CO) g (CH2) d (CH2) ns (CQC) ns (CH2) n (OH)
n ¼ stretching, g ¼ wagging, d ¼ bending and s ¼ symmetric.
40
I (a.u)
30
20
10
Fig. 3. IR transmission spectra of PVA films filled with various FLs of XFeCl3(15X)MnCl2.
electrical conduction mechanism. Different models, such as the Mott and Gurney hopping model, T1/4 and T1/2 variable range hopping models [23,25–27], can be applied. Furthermore, Fleming and Ranicar [1] concluded that the electrical conduction in polyvinyl chloride occurs via a hopping mechanism. The observed linear dependence of ln r on ln T, in a fair temperature range of 330–390 K, which is not shown here, allowed us to rule out the abovementioned conduction models and apply the interpolaron hopping model [8,22,23,28] previously discussed and modified by Kuivalainen et al. [22]. Owing to the quasi one-dimensional conduction model, the resistivity can be expressed as r ¼ ½kT=A1 e2 gðTÞðR2o =zÞ½ðY p þ Y bp Þ2 =Y p Y bp expð2 B1 Ro =zÞ
(1)
where A1 and B1 are constants, A1 ¼ 0.45; B1 ¼ 1.39; Yp and Ybp are the concentrations of polarons and bipolarons, respectively; Ro ¼ (3/4p Cimp)1/3 is the typical separation between impurities whose concentration is Cimp; z ¼ (z||z2?)1/3 is the average decay length of a polaron and
0 0
4
8 x (wt%)
12
Fig. 4. The FL dependence of the IR peaks at (J) 1090 cm1, and (K) 1380 cm1.
bipolaron wave function; and z|| and z? are the decay lengths parallel and perpendicular to the polymer chain, respectively. The rate of the electron transition between polaron and bipolaron states can be estimated using the following equation: gðTÞ ¼ go ðT=300 KÞ11 ,
(2)
where the pre-factor go was evaluated by Kivelson and found to be 1.2 1017 s1 [29]. The complete details of this model were discussed elsewhere [8,22–25,29–31]. A linear temperature dependence of Ro for various FLs is shown in Fig. 5. The range of Ro varies between 0.95 and 2.0 nm, which corresponds to E4–8 monomer units, where the monomer unit length E0.25 nm [23,25]. Fig. 6 displays the FL dependence of Ro estimated at temperature T ¼ 365 K. This dependence depicts two maxima at X ¼ 7.5% and 14%, as well as two minima at 1% and 10%. On the other hand, the FL dependence of
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certain IR peaks demonstrates two minima at X ¼ 7.5% and 14% and two maxima at 5% and 10%, (Fig. 4), which reasonably represents the mirror image behavior for that of Ro. This suggests that the local sites exhibiting the modes of vibrations of the two IR peaks at 1090 and 1380 cm1 may be considered as suitable hopping centers for the charge carriers of the electrical conduction mechanism [5,8,22–24].
3.3.1. DC magnetic susceptibility In the present study, the PVA films filled with different mass fractions of XFeCL3 (15–X) MnCl2 were magnetically investigated. It is worthwhile to make a comparison between the present results and the magnetic studies of PVA films filled with CoBr2 as well as MnCl2 previously discussed [8,9]. The latter possessed dc magnetic susceptibility that could be interpreted owing to the infinite one-dimensional lattice of Ising spin character [8,9,32] described by the following equation: X Is ¼ ðNg2 b2 =4kTÞ= expðJ=kTÞ,
2.20 xFeCl3 (15-x) MnCl2 x(wt%) 2.00
1.80
Ro(nm)
1.60
1.40
1.20
15.0 14.0 10.0 7.5 5.0 1.0 0.0
1.00
350
360 370 T(K)
1.6
1.2
0.8 0
5
10
15
x (wt %) Fig. 6. The FL dependence of Ro at temperature T ¼ 365 K.
380
390
experimental results depicted a temperature dependence of the Curie–Weiss behavior characterized by localized magnetic moments [5,28,32–34]: w ¼ C=ðT yp Þ,
400
Fig. 5. The temperature dependence of the hopping distance Ro for different FLs.
(4)
where C is the Curie constant, T is the temperature in K and yp is the paramagnetic Curie temperature, which is the temperature above which the spontaneous magnetization vanishes; it separates the disordered paramagnetic phase at T4yp from the ordered ferromagnetic phase at Toyp. The values of yp were estimated for different FLs using Eq. (4) and plotted in Fig. 8. The positive values of yp for all FLs can be taken as an evidence of the predominance of ferromagnetic exchange interactions between the magnetic centers at low temperatures. Fig. 8 depicts a significant dependence of the paramagnetic Curie temperature yp on the FL. Two maxima were recorded at 7.5% and 14%, as well as two minima at 1% and 10% FL. This agrees quite well with the FL dependence of Ro and reasonably exhibits the mirror image behavior of that of certain IR peaks. These dependences reveal that there are certain energy states having a common contribution to vibrational, electrical and magnetic properties of the present study. Moreover, the effective paramagnetic moment meff was evaluated according to the following equation: meff ¼ 2:828½wm ðT yp Þ1=2 ,
0.80 340
2.0
(3)
where N is the Avogadro’s number, g is the Lande´ splitting factor, b is the Bohr magneton, k is the Boltzmann’s constant, T is the absolute temperature and J is the antiferro-magnetic exchange coupling constant. In this work, the dc magnetic susceptibility w was measured for PVA films filled with XFeCL3(15X)MnCl2 in the low-temperature range of 90–270 K. A relationship between the reciprocal magnetic susceptibility (1/w) for different FLs and the temperature was investigated. A linear behavior was noticed as shown in Fig. 7. The
330
2.4
Ro(nm)
3.3. Magnetic properties
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(5)
where wm is the molar susceptibility of the filled polymer. The dependence of meff on the FL exhibited a non-linear character, as depicted in Fig. 9, indicating that the experimental results do not follow the magnetic dilution behavior. The order of magnitude of meff agreed reasonably with the divalent and trivalent states of Mn2+ and Fe3+, respectively. Furthermore, the Curie–Weiss behavior
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indicated that the energy band diagram of this filled PVA system was characterized by magnetic localized energy states [5,28,32–34]. From the above discussion, it could be concluded that PVA films filled with different fillers exhibited a temperature-dependent magnetic susceptibility. The various results could be interpreted by different magnetic models according to the types of the fillers as well as their concentrations. The present study revealed that the experimental results followed well the temperature-dependent dc magnetic susceptibility of the Curie–Weiss behavior. The important 0.200 xFeCl3(15-x)MnCl2 x(wt %)
1 / χ (106g /emu)
0.160
0.120
0.080 15.0 14.0 10.0 7.5 5.0 1.0 0.0
0.040
0.000 50
100
150
200
250
300
T (K) Fig. 7. The temperature dependence of the reciprocal magnetic susceptibility (1/w) for different FLs.
goal of the present and previous studies is to select the desired magnetic properties of the investigated polymer, by choosing the type of the filler used and its concentration, to obtain the interesting magnetic technological applications. Moreover, different polymers such as poly(vinylidene fluoride) PVDF, polystyrene PS and polymethyl methacrylate PMMA were magnetically studied by Tawansi et al. [5,6,8–10,35]. Various magnetic properties were obtained according to the polymer, the type of the filler and the filling level. The magnetic properties widely vary from the temperature-dependent magnetic susceptibility that follows the Curie–Weis law or the Ising law, the temperatureindependent Pauli paramagnetic (TIPP) susceptibility of the itinerant character to the temperature-independent diamagnetic susceptibility. 3.3.2. Electron spin resonance (ESR) The ESR spectra were recorded at ambient temperature for PVA films filled with various mass fractions of XFeCL3(15X)MnCl2. Virgin PVA films exhibited complicated unresolved ESR signal characterized by three main broad peaks due to the hyperfine structure (Fig. 10a). In the previous study, the average correlation time of mobile spins for the virgin PVA was evaluated and found to be 295 ns [8,36]. The ESR signals of the filled polymer exhibited a total distortion of the virgin signal and unresolved broad distorted signals (Fig. 10b), characterized by the hyperfine structure due to the interaction of the unpaired electrons with the nuclear spins, were recorded. The unpaired electrons responsible for the ESR signals can originate from the polymer and the fillers. It is worthwhile to note that both Fe3+ and Mn2+ ions have the electronic configurations 3d5 of five unpaired parallel spins, leading 0.35 xFeCl3(15-x)MnCl2
100 xFeCl3(15-x)MnCl2
0.30
95
0.25 μeff (BM)
p (K)
90
85
0.20 80
0.15 75
0.10
70 0
5
10 x (wt %)
Fig. 8. The FL dependence of yp.
15
20
0
5
10 x (wt %)
Fig. 9. The FL dependence of meff.
15
20
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x FeCl3(15-x)MnCl2 x (Wt%) 15
Intensity (a.u.)
14
7.5 1
0.0
0.1
0.2 0.3 Magnetic field (T)
0.4
0.5
Intensity (a.u.)
Pure
0.0
0.1
0.2 0.3 Magnetic field (T)
0.4
0.5
Fig. 10. The ESR spectra for (a) pure PVA film, (b) PVA films filled with different FLs.
to important ESR signals. Meanwhile, the nuclear spin of the fillers can be discussed as follows: the manganese nucleus possesses nuclear spin I ¼ 5/2, which leads to hyperfine splitting of 2I+1 ¼ 6 possible magnetic orientations with respect to the applied external magnetic field. On the contrary, the Fe nucleus of zero nuclear spin, I ¼ 0, has no hyperfine splitting [28,37,38]. Moreover, the unresolved spectrum can also be attributed to the fine structure due to the presence of Fe3+ as well as Mn2+ ions having five unpaired parallel spins, where there will be n equally spaced resonances in the ESR signal if there are n parallel spins [39]. Fig. 10b exhibits the FL dependence of ESR spectra. It is worthy to note that the applied magnetic field scale can be divided into two regions owing to the obtained ESR spectra. The first region represents the low magnetic field scale E0–0.25 T, while the second one corresponds to the high field scale E0.25–0.5 T.
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by the peak-to-peak linewidth DHppE30 mT, the asymmetry ratio A/B41 and the Lande´ splitting factor gE4.8 were noticed. These signals can be attributed to the formation of the octahedral structure of the Fe3+ ions [28,40–42]. Meanwhile, in the second region of the field, and at X ¼ 14%, the ESR signal consists of unresolved six lines characterized by the hyperfine structure due to the presence of isolated Mn2+ [7,9,28] with nuclear spin I ¼ 5/2, surrounded by Fe3+ as a dilute material without hyperfine splitting. These unresolved six lines are nearly uniformly spaced, spread over a region of approximately 125 mT and they do not seem to be of equal intensity. The ESR parameters of these lines are the hyperfine coupling constant E16 mT, DHppE10 mT, gE1.4 and A/B41. At X ¼ 1 and 5% FLs (important concentration of MnCl2 filler): In the first region of the field, the broad deformed Lorentzian signals nearly disappeared. This may be attributed to the decrease in the concentration of the FeCl3 filler. Meanwhile, in the second region of the field, the unresolved six lines became vague due to the proximity of Mn2+ ions, leading to Mn2+–Mn2+ exchange interactions confirming the formation of aggregated Mn2+ [7,9,28].
It is remarkable to note that the broadening of the linewidth of the observed signals, DHppE30 mT, in the first region of the applied magnetic field and DHppE10 mT in the second region of the field could be attributed to localized magnetic centers of polaronic type [7,22,28, 41–43]. This confirms the magnetic susceptibility implication of the Curie–Weiss behavior characterized by localized magnetic moments. Since the polarons are charged defects [30,31] and hence bound to the filler, the observed shift in the g factor from that of free electron, gE4.8, in the first region of the applied magnetic field and gE1.3 in the second region of the field due to filling could be caused by a spin–orbit interaction between the polaron and the filling molecule [22,30,31]. It could be concluded that the broadening of the linewidth as well as the shift in the g factor deduced from the above ESR study reveal an important support of the polaronic model. Moreover, the polaron formation was also suggested from both the present IR and the electrical conduction mechanism. Certain IR peaks can be taken as an evidence of the formation of polarons and/or bipolaron-bound states. In addition, the conduction mechanism was interpreted on the bases of the phonon-assisted charge carrier hopping between polaron and/or bipolaron states in the polymeric matrix. 4. Conclusion
At X ¼ 15 and 14% FLs (important concentration of FeCl3 filler): In the first region of the applied magnetic field, broad deformed Lorentzian signals characterized
The present work dealt with the study of the PVA films filled with different FLs of XFeCL3(15X)MnCl2.
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An important increase in the melting temperature of PVA with filling was depicted using the DSC study. This indicated better thermal stability of the filled polymer of interesting industrial applications. The amorphous feature of the filled polymer was revealed using XRD scans. Certain IR absorption peaks could be taken as an evidence of the formation of polaron and/or bipolaronbound states. The dc electrical conduction mechanism might be interpreted on the bases of the interpolaron hopping model. The dc magnetic susceptibility w could be discussed owing to the temperature-dependent Curie–Weiss law characterized by localized magnetic moments. The magnetic centers responsible for the ESR spectra are of polaronic type. The ESR parameters such as the peak-to-peak linewidth DHpp, the asymmetry ratio A/B, the Lande´ splitting factor g and the hyperfine coupling constant were evaluated. A correlation between the vibrational, electrical and magnetic properties supported the polaron formation.
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Journal of Magnetism and Magnetic Materials 320 (2008) 1739–1746 www.elsevier.com/locate/jmmm
Characterization, electrical and magnetic properties of PVA films filled with FeCl3–MnCl2 mixed fillers A. El-Khodary, A.H. Oraby, M.M. Abdelnaby Physics Department, Faculty of Science, Mansoura University, P.O. Box 55, 35516 Mansoura, Egypt Received 21 October 2007; received in revised form 14 January 2008 Available online 24 January 2008
Abstract Polyvinyl alcohol (PVA) films filled with different filling levels (FLs) of XFeCL3(15X)MnCl2 were studied. The DSC thermograms exhibited an increase in the melting temperature with filling, indicating better thermal stability of the filled polymer of interesting industrial applications. The amorphous feature of the filled polymer was depicted using XRD scans. Vibrational studies displayed significant structural deformations. The FL dependence of certain IR absorption peaks was discussed. The dc electrical conduction mechanism was interpreted on the basis of the modified interpolaron hopping model. The present results of the dc magnetic susceptibility (w) suggested the temperature dependence of Curie–Weiss behavior characterized by localized magnetic moments. The effective paramagnetic moment (meff) was estimated; its dependence on the FL exhibited a non-linear character. The electron spin resonance (ESR) study revealed unresolved broad distorted signals characterized by the hyperfine structure. The ESR parameters were evaluated. A correlation between the above-mentioned studies was discussed to relate the structural, electrical and magnetic properties of the filled PVA polymer. r 2008 Elsevier B.V. All rights reserved. PACS: 70; 71.20.Rv; 76.30.v Keywords: Polyvinyl alcohol; FeCl3 and MnCl2 mixed fillers; Characterization; Electrical resistivity; Magnetic susceptibility; Electron spin resonances
1. Introduction Polymers have received much experimental attention in an attempt to synthesize organic polymers alternative to conventional inorganic materials. The electrical resistivity of polymers [1,2] such as polyvinyl alcohol, polyvinyl chloride, polystyrene and polyethylene would normally be classified as insulators possessing specific resistivities of the order 1015 O cm or greater. Fleming and Ranicar concluded that the electrical conduction in polyvinyl chloride occurs via a hopping mechanism [1]. The electrical properties of the polymeric materials can be investigated with filling, and a model is required to interpret the electrical conduction mechanism. Transition metal halides of magnificent electrical and magnetic properties have been used as fillers. Different Corresponding author.
E-mail address: [email protected] (A. El-Khodary). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.01.030
concentrations of the fillers were applied in an attempt to modify the physical properties of the polymer under study and to obtain the desired filled polymer with distinct potential technological applications [3–11]. Moreover, the magnetic properties of the polymeric materials significantly depend on the polymer, the type of the filler and the filling level. The magnetic susceptibility widely varies from the temperature-dependent magnetic susceptibility that follows the Curie–Weiss law or the Ising law, the temperature-independent Pauli parama gnetic (TIPP) susceptibility of the itinerant character to the temperature-independent diamagnetic susceptibility [5,6,8–10,12]. Furthermore, electron spin resonance (ESR) spectroscopy provides a sensitive magnetic technique for detecting the unpaired electrons in the polymeric materials as well as determining the origin of the magnetic centers [6–13]. Polyvinyl alcohol (PVA) has attracted extensive interest in industrial as well as medical and pharmaceutical
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applications. This is due to the important combination of thermal, structural, mechanical, optical, electrical and magnetic properties [4,7–9,11–15]. Motivated by the magnificent electrical and magnetic properties of the transition metal halides, FeCl3 and MnCl2 were selected as fillers of interesting partially filled d electron configuration. The present work on Fe and Mn ions is quite fundamental to the understanding of the large ionic clusters of these materials, such as the single-molecular magnets Fe8 and Mn12. The latter topic makes this work quite relevant to modern magnetism, from both the fundamental and applied aspects [16–18]. The aim of the present work is to study the physical properties of PVA films filled with different filling levels (FLs) of the mixed fillers XFeCL3(15X)MnCl2 as well as to find a correlation between the investigated structural, electrical and magnetic properties. 2. Experimental technique 2.1. Samples preparation PVA in the form of grains of molecular weight 17,000 was provided by Avondale laboratories Beaumont Close Banbury Oxon, England. The samples were prepared by a casting method. PVA, FeCl3 and MnCl2 were dissolved separately in distilled water. A mixture of the dissolved polymer and the fillers of their mass fractions (FeCl3 and MnCl2) were cast on a glass dish and kept in a dry atmosphere at 323 K for about 1 week. PVA films with different mass fractions FLs of the fillers XFeCl3(15–X) MnCl2 were obtained, where X=0, 1, 5, 7.5,10, 14, 15 wt%. The thickness of the films was in the range of 100–150 mm. 2.2. Tools of analysis Differential Scanning Calorimetry (DSC) thermograms were carried out using a thermoanalyser (Shimadzu DSC50) with a measuring temperature range of 293–573 K and a heating rate of 5 K/min. The infrared (IR) spectra were recorded in the wavenumber range of 4000–400 cm1 using a spectrophotometer (PerkinElmer 883). X-ray diffraction (XRD) scans were obtained using a Siemens type F diffractometer with Cu Ka radiation and a LiF monochromator. The dc electrical resistivity was measured using an insulation tester type TM 14 and an autoranging multimeter (Keithley 175 A) of accuracy 70.2% [5]. A Faraday pendulum balance technique was used in measuring the dc magnetic susceptibility with an accuracy better than 73.0%. 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.
A standard sample of MgO doped with Mn2+ was used as a calibrant. 3. Results and discussion 3.1. Characterization 3.1.1. Differential scanning calorimetry DSC Virgin PVA films exhibit glass transition temperature TgE85 1C, crystallization temperature Tc in the range of 90–110 1C, melting temperature Tm at about 202 1C and degradation temperature TdE270 1C [2,19]. The DSC thermograms, in the temperature range from room temperature to 573 K, of PVA films filled with different mass fractions of XFeCl3(15X)MnCl2 are shown in Fig. 1. In the present study, the spectra are characterized by three main peaks, two endothermic peaks at about 190–200 and 278–282 1C and an exothermic one at 281–294 1C. The first peak can be attributed to the melting process, while the second and third were assigned to the degradation process, respectively. Moreover, an appearance of small exothermic peak in the temperature range E90 to 30 1C was noticed at the lower and higher values of X, respectively. This peak can be assigned to the crystallization process. The decrease in the crystallization temperature at higher values of X can be attributed to the important role of the FeCl3 filler. The dependence of the main peaks on the FLs is noticed in Table 1. It is interesting to notice that the spectra are thermally stable up to E280 1C. The existence of two degradation Td peaks as shown in Fig. 1 implies that the polymer degrades in two stages, indicating the existence of two different phases [19]. Moreover, Table 1 depicts that the melting temperature Tm increases with increase in the filling level X, suggesting the significant role of the FeCl3 filler. The increase in the melting temperature Tm(186–207 1C) with filling represents an important improvement of the thermal stability of the filled polymer under investigation, leading to interesting technological applications. 3.1.2. XRD spectra The X-ray diffraction spectra of PVA films filled with different mass fractions of the mixed fillers XFeCL3 (15X)MnCl2 is shown in Fig. 2. The main peaks of the virgin polymer were recorded, suggesting the amorphous feature of PVA of a short-range order. It is well known that the scattering pattern of amorphous polymers usually has two rings. In this study, a broad peak was noticed at 2yE19.51 that can be assigned to the Van der Wall distance of 4.5E5 A˚. This peak may be attributed to the intermolecular distances between neighboring chains. A small hump was observed at 2yE421, corresponding to an approximately hexagonal ordering of the molecular chains [20].
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Fig. 2. XRD scans of PVA films filled with various mass fractions of XFeCl3(15X)MnCl2. Fig. 1. The DSC XFeCl3(15X)MnCl2.
thermograms
of
PVA
films
filled
with
The important decrease in the intensity of the main peak of the virgin polymer at 2yE19.51 for the different values of X may represent a decrease in the lamellar structure of the virgin polymer due to filling. Moreover, the absence of the peaks characterizing FeCl3 and MnCl2 crystals in the present spectra of the filled polymer suggests that the spectra are either free from FeCl3 and MnCl2 crystals or contain too few traces of these crystals to be detected by the present XRD technique. This can be taken as an evidence of the random distribution of the fillers within the polymeric matrix [8,15]. 3.1.3. IR analysis The IR transmission spectra of PVA films filled with different FLs of the mixed fillers XFeCL3(15X)MnCl2 are Table 1 The FL dependence of the main peaks X(%)
Tc (exo)
Tm (endo)
Td1 (endo)
Td2 (exo)
1 5 7.5 10 14
90 – 90 37 32
186 187 197 200 207
278 282.06 281.8 280 277
285 293 294 290 281
Temperatures in centigrade.
shown in Fig. 3. The main PVA peaks were observed. For simplicity, the range of the wavenumber 41800 cm1 is not shown here. The assignment of the most notable characterizing frequencies [21] is demonstrated in Table 2. Certain structural deformations can be identified by investigating the FL dependence of the IR absorption peaks. Fig. 4 displays the PVA structural deformations of the intensity of the absorption peaks at 1090 and 1380 cm1 as a function of the FL. The above-mentioned two peaks exhibit two maxima at 5% and 10% as well as two minima at 7.5% and 14%. These peaks will be correlated with both the electrical and magnetic properties in this study. The non-linear behavior of the FL dependence of the above-mentioned IR absorption peaks clarifies the important role of the interaction of the fillers with the different modes of vibrations. It is remarkable to note that the absorption peak at 1680 cm1, which was assigned to the CQC stretching mode of vibration, can be considered as a suitable site for polarons and/or bipolarons [8,22–24]. 3.2. Electrical properties The dc electrical resistivity r of PVA films filled with different FLs of the mixed fillers XFeCl3 (15–X) MnCl2 was measured in the temperature range of 330–400 K. The goal of this electrical study is to explain the present experimental results and to investigate its corresponding
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Table 2 The most notably characterizing frequencies Frequency (cm1)
Assignment
950 1090 1380 1430 1680 2902 3300
n (CH2) n (CO) g (CH2) d (CH2) ns (CQC) ns (CH2) n (OH)
n ¼ stretching, g ¼ wagging, d ¼ bending and s ¼ symmetric.
40
I (a.u)
30
20
10
Fig. 3. IR transmission spectra of PVA films filled with various FLs of XFeCl3(15X)MnCl2.
electrical conduction mechanism. Different models, such as the Mott and Gurney hopping model, T1/4 and T1/2 variable range hopping models [23,25–27], can be applied. Furthermore, Fleming and Ranicar [1] concluded that the electrical conduction in polyvinyl chloride occurs via a hopping mechanism. The observed linear dependence of ln r on ln T, in a fair temperature range of 330–390 K, which is not shown here, allowed us to rule out the abovementioned conduction models and apply the interpolaron hopping model [8,22,23,28] previously discussed and modified by Kuivalainen et al. [22]. Owing to the quasi one-dimensional conduction model, the resistivity can be expressed as r ¼ ½kT=A1 e2 gðTÞðR2o =zÞ½ðY p þ Y bp Þ2 =Y p Y bp expð2 B1 Ro =zÞ
(1)
where A1 and B1 are constants, A1 ¼ 0.45; B1 ¼ 1.39; Yp and Ybp are the concentrations of polarons and bipolarons, respectively; Ro ¼ (3/4p Cimp)1/3 is the typical separation between impurities whose concentration is Cimp; z ¼ (z||z2?)1/3 is the average decay length of a polaron and
0 0
4
8 x (wt%)
12
Fig. 4. The FL dependence of the IR peaks at (J) 1090 cm1, and (K) 1380 cm1.
bipolaron wave function; and z|| and z? are the decay lengths parallel and perpendicular to the polymer chain, respectively. The rate of the electron transition between polaron and bipolaron states can be estimated using the following equation: gðTÞ ¼ go ðT=300 KÞ11 ,
(2)
where the pre-factor go was evaluated by Kivelson and found to be 1.2 1017 s1 [29]. The complete details of this model were discussed elsewhere [8,22–25,29–31]. A linear temperature dependence of Ro for various FLs is shown in Fig. 5. The range of Ro varies between 0.95 and 2.0 nm, which corresponds to E4–8 monomer units, where the monomer unit length E0.25 nm [23,25]. Fig. 6 displays the FL dependence of Ro estimated at temperature T ¼ 365 K. This dependence depicts two maxima at X ¼ 7.5% and 14%, as well as two minima at 1% and 10%. On the other hand, the FL dependence of
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certain IR peaks demonstrates two minima at X ¼ 7.5% and 14% and two maxima at 5% and 10%, (Fig. 4), which reasonably represents the mirror image behavior for that of Ro. This suggests that the local sites exhibiting the modes of vibrations of the two IR peaks at 1090 and 1380 cm1 may be considered as suitable hopping centers for the charge carriers of the electrical conduction mechanism [5,8,22–24].
3.3.1. DC magnetic susceptibility In the present study, the PVA films filled with different mass fractions of XFeCL3 (15–X) MnCl2 were magnetically investigated. It is worthwhile to make a comparison between the present results and the magnetic studies of PVA films filled with CoBr2 as well as MnCl2 previously discussed [8,9]. The latter possessed dc magnetic susceptibility that could be interpreted owing to the infinite one-dimensional lattice of Ising spin character [8,9,32] described by the following equation: X Is ¼ ðNg2 b2 =4kTÞ= expðJ=kTÞ,
2.20 xFeCl3 (15-x) MnCl2 x(wt%) 2.00
1.80
Ro(nm)
1.60
1.40
1.20
15.0 14.0 10.0 7.5 5.0 1.0 0.0
1.00
350
360 370 T(K)
1.6
1.2
0.8 0
5
10
15
x (wt %) Fig. 6. The FL dependence of Ro at temperature T ¼ 365 K.
380
390
experimental results depicted a temperature dependence of the Curie–Weiss behavior characterized by localized magnetic moments [5,28,32–34]: w ¼ C=ðT yp Þ,
400
Fig. 5. The temperature dependence of the hopping distance Ro for different FLs.
(4)
where C is the Curie constant, T is the temperature in K and yp is the paramagnetic Curie temperature, which is the temperature above which the spontaneous magnetization vanishes; it separates the disordered paramagnetic phase at T4yp from the ordered ferromagnetic phase at Toyp. The values of yp were estimated for different FLs using Eq. (4) and plotted in Fig. 8. The positive values of yp for all FLs can be taken as an evidence of the predominance of ferromagnetic exchange interactions between the magnetic centers at low temperatures. Fig. 8 depicts a significant dependence of the paramagnetic Curie temperature yp on the FL. Two maxima were recorded at 7.5% and 14%, as well as two minima at 1% and 10% FL. This agrees quite well with the FL dependence of Ro and reasonably exhibits the mirror image behavior of that of certain IR peaks. These dependences reveal that there are certain energy states having a common contribution to vibrational, electrical and magnetic properties of the present study. Moreover, the effective paramagnetic moment meff was evaluated according to the following equation: meff ¼ 2:828½wm ðT yp Þ1=2 ,
0.80 340
2.0
(3)
where N is the Avogadro’s number, g is the Lande´ splitting factor, b is the Bohr magneton, k is the Boltzmann’s constant, T is the absolute temperature and J is the antiferro-magnetic exchange coupling constant. In this work, the dc magnetic susceptibility w was measured for PVA films filled with XFeCL3(15X)MnCl2 in the low-temperature range of 90–270 K. A relationship between the reciprocal magnetic susceptibility (1/w) for different FLs and the temperature was investigated. A linear behavior was noticed as shown in Fig. 7. The
330
2.4
Ro(nm)
3.3. Magnetic properties
1743
(5)
where wm is the molar susceptibility of the filled polymer. The dependence of meff on the FL exhibited a non-linear character, as depicted in Fig. 9, indicating that the experimental results do not follow the magnetic dilution behavior. The order of magnitude of meff agreed reasonably with the divalent and trivalent states of Mn2+ and Fe3+, respectively. Furthermore, the Curie–Weiss behavior
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indicated that the energy band diagram of this filled PVA system was characterized by magnetic localized energy states [5,28,32–34]. From the above discussion, it could be concluded that PVA films filled with different fillers exhibited a temperature-dependent magnetic susceptibility. The various results could be interpreted by different magnetic models according to the types of the fillers as well as their concentrations. The present study revealed that the experimental results followed well the temperature-dependent dc magnetic susceptibility of the Curie–Weiss behavior. The important 0.200 xFeCl3(15-x)MnCl2 x(wt %)
1 / χ (106g /emu)
0.160
0.120
0.080 15.0 14.0 10.0 7.5 5.0 1.0 0.0
0.040
0.000 50
100
150
200
250
300
T (K) Fig. 7. The temperature dependence of the reciprocal magnetic susceptibility (1/w) for different FLs.
goal of the present and previous studies is to select the desired magnetic properties of the investigated polymer, by choosing the type of the filler used and its concentration, to obtain the interesting magnetic technological applications. Moreover, different polymers such as poly(vinylidene fluoride) PVDF, polystyrene PS and polymethyl methacrylate PMMA were magnetically studied by Tawansi et al. [5,6,8–10,35]. Various magnetic properties were obtained according to the polymer, the type of the filler and the filling level. The magnetic properties widely vary from the temperature-dependent magnetic susceptibility that follows the Curie–Weis law or the Ising law, the temperatureindependent Pauli paramagnetic (TIPP) susceptibility of the itinerant character to the temperature-independent diamagnetic susceptibility. 3.3.2. Electron spin resonance (ESR) The ESR spectra were recorded at ambient temperature for PVA films filled with various mass fractions of XFeCL3(15X)MnCl2. Virgin PVA films exhibited complicated unresolved ESR signal characterized by three main broad peaks due to the hyperfine structure (Fig. 10a). In the previous study, the average correlation time of mobile spins for the virgin PVA was evaluated and found to be 295 ns [8,36]. The ESR signals of the filled polymer exhibited a total distortion of the virgin signal and unresolved broad distorted signals (Fig. 10b), characterized by the hyperfine structure due to the interaction of the unpaired electrons with the nuclear spins, were recorded. The unpaired electrons responsible for the ESR signals can originate from the polymer and the fillers. It is worthwhile to note that both Fe3+ and Mn2+ ions have the electronic configurations 3d5 of five unpaired parallel spins, leading 0.35 xFeCl3(15-x)MnCl2
100 xFeCl3(15-x)MnCl2
0.30
95
0.25 μeff (BM)
p (K)
90
85
0.20 80
0.15 75
0.10
70 0
5
10 x (wt %)
Fig. 8. The FL dependence of yp.
15
20
0
5
10 x (wt %)
Fig. 9. The FL dependence of meff.
15
20
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x FeCl3(15-x)MnCl2 x (Wt%) 15
Intensity (a.u.)
14
7.5 1
0.0
0.1
0.2 0.3 Magnetic field (T)
0.4
0.5
Intensity (a.u.)
Pure
0.0
0.1
0.2 0.3 Magnetic field (T)
0.4
0.5
Fig. 10. The ESR spectra for (a) pure PVA film, (b) PVA films filled with different FLs.
to important ESR signals. Meanwhile, the nuclear spin of the fillers can be discussed as follows: the manganese nucleus possesses nuclear spin I ¼ 5/2, which leads to hyperfine splitting of 2I+1 ¼ 6 possible magnetic orientations with respect to the applied external magnetic field. On the contrary, the Fe nucleus of zero nuclear spin, I ¼ 0, has no hyperfine splitting [28,37,38]. Moreover, the unresolved spectrum can also be attributed to the fine structure due to the presence of Fe3+ as well as Mn2+ ions having five unpaired parallel spins, where there will be n equally spaced resonances in the ESR signal if there are n parallel spins [39]. Fig. 10b exhibits the FL dependence of ESR spectra. It is worthy to note that the applied magnetic field scale can be divided into two regions owing to the obtained ESR spectra. The first region represents the low magnetic field scale E0–0.25 T, while the second one corresponds to the high field scale E0.25–0.5 T.
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by the peak-to-peak linewidth DHppE30 mT, the asymmetry ratio A/B41 and the Lande´ splitting factor gE4.8 were noticed. These signals can be attributed to the formation of the octahedral structure of the Fe3+ ions [28,40–42]. Meanwhile, in the second region of the field, and at X ¼ 14%, the ESR signal consists of unresolved six lines characterized by the hyperfine structure due to the presence of isolated Mn2+ [7,9,28] with nuclear spin I ¼ 5/2, surrounded by Fe3+ as a dilute material without hyperfine splitting. These unresolved six lines are nearly uniformly spaced, spread over a region of approximately 125 mT and they do not seem to be of equal intensity. The ESR parameters of these lines are the hyperfine coupling constant E16 mT, DHppE10 mT, gE1.4 and A/B41. At X ¼ 1 and 5% FLs (important concentration of MnCl2 filler): In the first region of the field, the broad deformed Lorentzian signals nearly disappeared. This may be attributed to the decrease in the concentration of the FeCl3 filler. Meanwhile, in the second region of the field, the unresolved six lines became vague due to the proximity of Mn2+ ions, leading to Mn2+–Mn2+ exchange interactions confirming the formation of aggregated Mn2+ [7,9,28].
It is remarkable to note that the broadening of the linewidth of the observed signals, DHppE30 mT, in the first region of the applied magnetic field and DHppE10 mT in the second region of the field could be attributed to localized magnetic centers of polaronic type [7,22,28, 41–43]. This confirms the magnetic susceptibility implication of the Curie–Weiss behavior characterized by localized magnetic moments. Since the polarons are charged defects [30,31] and hence bound to the filler, the observed shift in the g factor from that of free electron, gE4.8, in the first region of the applied magnetic field and gE1.3 in the second region of the field due to filling could be caused by a spin–orbit interaction between the polaron and the filling molecule [22,30,31]. It could be concluded that the broadening of the linewidth as well as the shift in the g factor deduced from the above ESR study reveal an important support of the polaronic model. Moreover, the polaron formation was also suggested from both the present IR and the electrical conduction mechanism. Certain IR peaks can be taken as an evidence of the formation of polarons and/or bipolaron-bound states. In addition, the conduction mechanism was interpreted on the bases of the phonon-assisted charge carrier hopping between polaron and/or bipolaron states in the polymeric matrix. 4. Conclusion
At X ¼ 15 and 14% FLs (important concentration of FeCl3 filler): In the first region of the applied magnetic field, broad deformed Lorentzian signals characterized
The present work dealt with the study of the PVA films filled with different FLs of XFeCL3(15X)MnCl2.
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An important increase in the melting temperature of PVA with filling was depicted using the DSC study. This indicated better thermal stability of the filled polymer of interesting industrial applications. The amorphous feature of the filled polymer was revealed using XRD scans. Certain IR absorption peaks could be taken as an evidence of the formation of polaron and/or bipolaronbound states. The dc electrical conduction mechanism might be interpreted on the bases of the interpolaron hopping model. The dc magnetic susceptibility w could be discussed owing to the temperature-dependent Curie–Weiss law characterized by localized magnetic moments. The magnetic centers responsible for the ESR spectra are of polaronic type. The ESR parameters such as the peak-to-peak linewidth DHpp, the asymmetry ratio A/B, the Lande´ splitting factor g and the hyperfine coupling constant were evaluated. A correlation between the vibrational, electrical and magnetic properties supported the polaron formation.
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