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Structural, electrical and electron spin resonance properties of CoCl2-filled polyvinylidene fluoride films
Polymer Testing 19 (2000) 865–878
Material Properties
Structural, electrical and electron spin resonance properties of CoCl2-filled polyvinylidene fluoride films A.H. Oraby Department of Physics, Faculty of Science, Mansoura University, Mansoura 35516, Egypt Received 21 May 1999; accepted 30 July 1999
Abstract Polyvinylidene fluoride (PVDF) films, filled with mass fractions wⱕ15% of CoCl2, were prepared. The X-ray diffraction (XRD) scans evidenced semicrystalline structures containing a- and/or b-PVDF phases. IR spectra confirmed these findings, and revealed some structural defects such as mono- and di-fluorinated alkenes and head-to-head segments. The optical absorption spectra suggested the presence of: (a) two optical gaps (one of them, Eg2, depends on W); (b) tetrahedral Co(II) coordination for all of the filling levels (FL); and (c) octahedral Co(II) forms at 15% FL. The electrical resistivity results are discussed on the basis of the modified interpolaron hopping model of Kuivalainen et al., Phys Rev 1985;B31:7900. The temperature and FL dependences of the calculated hopping distance (Ro) are discussed. It is implied that the difluorinated alkenes exert a significant influence on Eg2 and Ro. The electron spin resonance (ESR) spectra confirmed the optical absorption implications. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Four probable crystalline phases were detected, singly or as two mixed phases, in partially crystalline PVDF [1]. The b- and a-polarized crystalline PVDF phases are the most important due to their high piezo and pyro activity [2]. One of the technical features of PVDF is its response to electromagnetic waves (EMW). This feature has been extensively discussed from an electrical point of view [3]. Filling PVDF with a transition metal halide is thought to enhance its EMW sensing feature due to its increased magnetic activity. In the present work, CoCl2 was selected as a filler for PVDF. Besides its interesting magnetic properties Co(II) may exist in different structural forms (with fourfold or sixfold coordinations): tetrahedral, octahedral and isolated forms [4]. The physical properties of polymers usually exhibit non-monotonic filler level (FL) dependence [5]. Thus, it seems important to study the structural, electrical and electron spin resonance (ESR) properties of various CoCl2 filler levels in PVDF films. 0142-9418/00/$ - see front matter. 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 9 ) 0 0 0 5 7 - 4
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Fig. 1. X-ray diffraction scans of various CoCl2 filling levels for PVDF films.
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2. Experimental The films studied were prepared by a casting method. PVDF powder (SOLEF 1008) was dissolved in diemthylformamide (DMF). CoCl2 was also dissolved in DMF. The solution of CoCl2 was added to the dissolved polymer at a suitable viscosity. The mixture was cast in a glass dish and kept in a dry atmosphere at 303 K for 2 weeks to ensure the removal of solvent traces. The thickness of the obtained films was in the range of 0.1–0.5 mm. PVDF films of the following mass fractions (W) of CoCl2 were prepared 0.1, 0.5, 1.0, 5, 10 and 15%. X-ray diffraction (XRD) scans were obtained using a Siemens type F diffractometer with CuKa radiation and a LiF monochromator. An infrared spectrophotometer (Perkin Elmer 883) was used for measuring the IR spectra in the wave number range of 200–4000 cm⫺1. UV/VIS absorption spectra were measured in the wavelength range of 200–800 nm using a spectrophotometer (Perkin Elmer UV/VIS). The electrical resistivity was measured using an insulation tester (level type TM14) of accuracy ±0.2%. The ESR spectra were recorded on a JEOL spectrophotometer (type JES-FE2 XG) at a frequency of 9.45 GH2, using 1,1-diphenyl-2-pierylhydrazyl (DPPH) as a calibrant. 3. Results and discussion 3.1. X-ray diffraction Fig. 1 depicts the XRD scans for various CoCl2 FLs in PVDF films. The observed spectra indicate the presence of semicrystalline structures. The assigned crystalline peaks are listed in Table 1. These peaks are due to the a- and b-PVDF crystalline phases [1], except for the sample of 0.1% FL which contains a-phase only.
Table 1 The assigned characterizing XRD peaks for a- and b-crystalline PVDF phases W (%)
2q (degree)
Assignment
0.1 0.5
39.5 20.5 39.8 18.5 20.5 39.5 18.5 20.5 39.5 20.5 39.5 20.5 39.5
(002)a (110)(200)b (002)a (020)a (110)(200)b (002)a (020)a (110)(200)b (002)a (110)(200)b (002)a (110)(200)b (002)a
1.0
5.0
10.0 15.0
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Fig. 2.
IR transmission spectra for variously filled PVDF films.
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Fig. 3. (a) The FL dependence of the intensity of the IR absorption peaks at (*) 510, (쎲) 610 and (䊊) 1750 cm⫺1. (b) The FL dependence of Ir1 and Ir2 relative peaks. (c) The FL dependence of peaks at (䊊) 665 and (*) 1670 cm⫺1.
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3.2. IR analysis The IR transmission spectra are shown in Fig. 2. The main PVDF characterizing peaks are noted. In the present spectra, a-phase is evidenced by the peaks most notable at 490, 530, 610, 796 and 976 cm⫺1, while b-phase is characterized by the peaks at 445, 470 and 510 cm⫺1 [6]. The peaks at 510, 610, 665, 1670 and 1750 cm⫺1 are selected as indicators for the structure deformation due to CoCl2 filling. The peaks at 510 cm⫺1, belonging to b-phase, are assigned to CF bending and wagging modes, while the peaks at 610 cm⫺1 are due to the CF2 bending and skeletal bending of C(H)–C(F)–C(H) with negative phase relative to the symmetry coordinates of the a-phase. The bands at 665 cm⫺1 are taken as a measure for the concentration of the headto-head (CF2CF2) units, which is ⬇20% in the present system [6]. The bands at 1670 cm⫺1 are assigned to the C=C stretching in the monofluorinated alkenes, while the bands at 1750 cm⫺1 are due to the C=C stretching of the difluorinated alkenes [7]. Fig. 3(a) depicts the FL dependence of the intensity (I) of the peaks at 510, 610 and 1750 cm⫺1. The three curves exhibit a sharp decrease as FL increases up to 1.5%. Beyond this value the rate of decay of I is slow for the peaks at 510 and 610 cm⫺1, while the peak intensity at 1750 cm⫺1 increases slightly as FL increases from 1.5 to 15%. Let Ir1 and Ir2 represent the ratios of I at 1750 cm⫺1 to I at 510 cm⫺1 and I at 1750 cm⫺1 to I at 610 cm⫺1, respectively. The FL dependence of Ir1 and Ir2, shown in Fig. 3(b), indicates minimum values at 1.5% FL. This indicates that the FL dependence of the relative content of the
Fig. 3. (continued)
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difluorinated alkenes (as a structural defect), with respect to the content of the a- or b-crystalline phase, changes its behaviour at W=1.5%. Fig. 3(c) shows that the FL dependence of the monofluorinated alkenes (represented by the band at 1670 cm⫺1) exhibits three critical FLs at W=0.5, 1.0 and 10.0%. It is also clear that the role of the head-to-head defect decays as W increases. 3.3. Optical absorption Fig. 4 displays the UV/VIS absorption spectra of various FLs in PVDF films. A weak (and broad) visible absorption band is noticed at 530 nm for 15% FL. This band [4] is due to the 4 T1g(F)→4T1g(P) transition for the hexaquocobalt(II). Moreover, three visible absorption bands are noticed at 630, 666 and 690 nm for 0.5%ⱕWⱕ15%, characterizing the high spin tetrahedral species [4]. The FL dependencies of the intensity of these three bands are shown in Fig. 5. It is clear that the 1% CoCl2 corresponds to the minimum absorption intensity indicating minimum tetrahedral content, while the 10% CoCl2 content exhibits the maximum absorption intensity, indicating maximum tetrahedral content. Using the experimental data of Fig. 4 and Eq. (1) the absorption coefficient (a) was calculated at various photon frequencies (n) [8]
Fig. 4. The optical absorption spectra for variously filled PVDF films.
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Fig. 5.
The FL dependence of the visible absorption peaks at (왕) 630, (쎲) 666 and (䊊) 690 nm.
a(n)⫽ln[t1(n)/t2(n)]/(d2⫺d1)
(1)
where t1 and t2 are the transmittances of two films (of the same FL) with thicknesses d1 and d2. The photon energy (hn) dependence of (ahn)1/2 for various FLs, where h is Plank’s constant, were plotted. The plots, which are not presented here, exhibit two linear parts, indicating two optical energy gaps (Eg1, Eg2), each gap can be calculated using Eq. (2): a(n)⫽B(hn⫺Eg)2/hn,
(2)
where B is a constant. Fig. 6 displays the FL dependence of the optical gaps. It is clear that Eg1 changes slightly while Eg2 exhibits a maximum value at W=0.5% then a minimum value at W=1.5% and a maximum value at W⬇5%. The 5% critical FL agrees with that noticed in Fig. 3(a) for the FL dependence of the IR absorption peak at 1750 cm⫺1 due to C=C stretching in the difluorinated alkenes. Thus, it is thought that the difluorinated alkanes, as a structural defect, affect the Eg2 gap. 3.4. DC electrical conductor The dc electrical resistivity (r), was measured in the temperature (T) range 298–403 K. The mono- and/or difluorinated alkenes and the head-to-head defects in the polymeric matrix of the present system, which were evidenced by IR spectra, can be considered as suitable sites for polarons and/or bipolarons. This allows one to use the modified interpolaron hopping model of
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Fig. 6. The FL dependence of Eg1 and Eg2 optical gaps.
Kuivalainen et al. [9] to interpret the present resistivity results. According to this model, the conduction is attributed to phonon-assisted hopping between polaron and/or bipolaron bound states in the polymer. The model suggests the following formula for the resistivity: r⫽[kT/A1e2g(T)(R2o/z]x[(yp⫹ybp)2/(ybp)]x exp(2B1Ro/z),
(3)
where A1=0.45, B1=1.39, yp and ybp are the concentration of polarons and bipolarons, respectively, and Ro=(3/4pCimp)1/3 is the typical separation between impurities whose concentration is Cimp, z=(z11zⲚ2)1/3 is the average decay length of a polaron and bipolaron wave function, and z11 and zⲚ are the decay lengths parallel and perpendicular to the polymer chain, respectively. Bredas et al. [10] reported that polarons and bipolarons induce defects of the same extension. The electronic transition rate between polaron and bipolaron states can be expressed as [11] g(T)⫽1.2⫻1017(T/300K)11
(4)
In the present work the order of magnitude of r was adjusted, using a computer aided program. The parameter z11=1.06 nm, while zⲚ=0.22 [12], which depends on the interchain resonance energy and the interchain distance [13]. Taking Yp=Ybp for simplicity, which is an acceptable approximation [14], and using equations (3) and (4) we can obtain the values of the hopping distance, Ro. A linear temperature dependence is noticed for Ro in Fig. 7. The dependence of 1/Ro on log (W) presented in Fig. 8 exhibits minimum and maximum values at W=0.5 and 5%, respectively. The 5% critical FL agrees with that noticed in Figs. 3(a) and Fig. 6 for the C=C stretching
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Fig. 7. The temperature dependence of Ro for the FLs: (a) (1) 0.01, (2) 0.1, (3) 0.5, (4) 1 and (5) 5%; (b) (6) 10 and (7) 15%.
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Fig. 8. The dependence of 1/Ro on log (W).
mode due to difluorinated alkenes and for Eg2, respectively. This reveals that the difluorinated alkenes induce energy levels of a significant twofold influence as: (1) it changes the Eg2 optical gap; and (2) these induced energy level acts as hopping sites. It is remarkable that the obtained values of Ro are in the range of 2.2–7.7 nm, which is 8.8 to 30.8 times the monomer length (苲0.25 nm [15]). This indicates that the electrical conduction is an intrachain one dimensional hopping type. 3.5. ESR The ESR spectra for various FLs of the present system are shown in Fig. 9. It is clear that these spectra are complex and they exhibit an absorption line at g=4.5009±0.0001 arising from Co(II) tetrahedral coordination [16]. It is observed that the g=4.5 line width is constant for the various Fls, which indicates constant dipolar interactions between Co(II) ions [17]. On the other hand Fig. 10 reveals that the symmetry factor of this line is an FL-dependent function, exhibiting a peak value and a minimum at W=0.5 and 10%, respectively. Comparing Fig. 10 with Fig. 5 we notice that the minimum symmetry is found at the maximum tetrahedral content. It is noteworthy that the spectrum of 15% FL in Fig. 9 contains six lines due to the hyperfine structure of the cobalt nucleus with an unpaired electron, indicating isolated Co2+ ions in the ionmer and evidencing the presence of the Co2+ octahedral form. This confirms the findings of the optical absorption spectra of Fig. 4.
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Fig. 9. The ESR spectra for variously filled PVDF samples.
4. Conclusions The present system is of a semicrystalline structure containing a- and b-PVDF phase, except for the 0.1% FL sample which contains a-phase only. The presence of mono- and di-fluorinated alkenes, as structural defects, were considered as suitable sites (on the polymeric chain) for polarons and/or bipolarons. The head-to-head defect concentration was found to be ⬇20%. A tetrahedral Co(II) coordination was detected for all of the FLs, while the 15% FL contains both the
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Fig. 10. The FL dependence of the symmetry factor.
tetrahedral and octahedral forms. Two types of optical energy gaps (Eg1 and Eg2). Eg1 changes slightly as FL changes, while Eg2 depends on FL significantly and it was correlated with the difluorinated alkanes. The electrical conduction proceeds through the phonon-assisted hopping, of charge carriers, between polaron and/or bipolaron bound states in the polymeric chain. The calculated hopping distance depends linearly on temperature and non-monotonically on FL. The ESR spectra were complex. However, the clear absorption line at g⬇4.5009±0.0001 confirmed the presence of the Co(II) tetrahedral form. Moreover, the hyperfine six lines, observed at 15% FL, confirmed the presence of the octahedral form. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Davis GT, McKinney JE, Broadhust MG, Roth SC. J Appl Phys 1978;49:4998. Hattori T, Masashi K, Hiroji O. J Appl Phys 1996;79:2016. Beaulien R, Lessarad RA, Clin SL. J Appl Phys 1996;79:833. Cotton FA, Wilkinson G. Advanced inorganic chemistry. London: John Wiley and Sons, 1962. Tawansi A, Zidan HM, Oraby AH, Dorgham ME. J Phys D: Appl Phys 1998;31:3428. Kobayashi M, Tashiro K, Tadokoro H. Macromolecules 1975;8:158. Williams DH. Spectroscopic methods in organic chemistry. England: McGraw–Hill, 1966. Davis EA, Mott NF. Phil Mag 1970;22:903. Kuivalainen P, Stubb H, Isotlo H, Yli P, Holmstrom C. Phys Rev 1985;B31:7900. Bredas JL, Chance RR, Silbey R. Phys Rev 1983;B26:5843.
878 [11] [12] [13] [14] [15] [16] [17]
A.H. Oraby / Polymer Testing 19 (2000) 865–878 Kivelson S. Phys Rev 1982;B25:3798. Mott NF, Gurrey RW. Electronic process in ionic crystals. London: Oxford University Press, 1940. Kivelson S. Phys Rev Lett 1981;46:1344. Tawansi A, Abdel Kader HI, Elzalabany M, Abdelrazek EM. J Mater Sci 1994;29:3451. Hazegawa R, Takahashi Y, Chatwi Y, Tadakoro H. J Polym 1972;13:600. Satendra JK, Bhagwan GS, Yudhvir BK. Trans Met Chem 1986;11:89. Burzo E, Chipara M, Ungur D, Ardelean I. Phys Stat Sol (b) 1984;124:k117.
Material Properties
Structural, electrical and electron spin resonance properties of CoCl2-filled polyvinylidene fluoride films A.H. Oraby Department of Physics, Faculty of Science, Mansoura University, Mansoura 35516, Egypt Received 21 May 1999; accepted 30 July 1999
Abstract Polyvinylidene fluoride (PVDF) films, filled with mass fractions wⱕ15% of CoCl2, were prepared. The X-ray diffraction (XRD) scans evidenced semicrystalline structures containing a- and/or b-PVDF phases. IR spectra confirmed these findings, and revealed some structural defects such as mono- and di-fluorinated alkenes and head-to-head segments. The optical absorption spectra suggested the presence of: (a) two optical gaps (one of them, Eg2, depends on W); (b) tetrahedral Co(II) coordination for all of the filling levels (FL); and (c) octahedral Co(II) forms at 15% FL. The electrical resistivity results are discussed on the basis of the modified interpolaron hopping model of Kuivalainen et al., Phys Rev 1985;B31:7900. The temperature and FL dependences of the calculated hopping distance (Ro) are discussed. It is implied that the difluorinated alkenes exert a significant influence on Eg2 and Ro. The electron spin resonance (ESR) spectra confirmed the optical absorption implications. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Four probable crystalline phases were detected, singly or as two mixed phases, in partially crystalline PVDF [1]. The b- and a-polarized crystalline PVDF phases are the most important due to their high piezo and pyro activity [2]. One of the technical features of PVDF is its response to electromagnetic waves (EMW). This feature has been extensively discussed from an electrical point of view [3]. Filling PVDF with a transition metal halide is thought to enhance its EMW sensing feature due to its increased magnetic activity. In the present work, CoCl2 was selected as a filler for PVDF. Besides its interesting magnetic properties Co(II) may exist in different structural forms (with fourfold or sixfold coordinations): tetrahedral, octahedral and isolated forms [4]. The physical properties of polymers usually exhibit non-monotonic filler level (FL) dependence [5]. Thus, it seems important to study the structural, electrical and electron spin resonance (ESR) properties of various CoCl2 filler levels in PVDF films. 0142-9418/00/$ - see front matter. 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 9 ) 0 0 0 5 7 - 4
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Fig. 1. X-ray diffraction scans of various CoCl2 filling levels for PVDF films.
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2. Experimental The films studied were prepared by a casting method. PVDF powder (SOLEF 1008) was dissolved in diemthylformamide (DMF). CoCl2 was also dissolved in DMF. The solution of CoCl2 was added to the dissolved polymer at a suitable viscosity. The mixture was cast in a glass dish and kept in a dry atmosphere at 303 K for 2 weeks to ensure the removal of solvent traces. The thickness of the obtained films was in the range of 0.1–0.5 mm. PVDF films of the following mass fractions (W) of CoCl2 were prepared 0.1, 0.5, 1.0, 5, 10 and 15%. X-ray diffraction (XRD) scans were obtained using a Siemens type F diffractometer with CuKa radiation and a LiF monochromator. An infrared spectrophotometer (Perkin Elmer 883) was used for measuring the IR spectra in the wave number range of 200–4000 cm⫺1. UV/VIS absorption spectra were measured in the wavelength range of 200–800 nm using a spectrophotometer (Perkin Elmer UV/VIS). The electrical resistivity was measured using an insulation tester (level type TM14) of accuracy ±0.2%. The ESR spectra were recorded on a JEOL spectrophotometer (type JES-FE2 XG) at a frequency of 9.45 GH2, using 1,1-diphenyl-2-pierylhydrazyl (DPPH) as a calibrant. 3. Results and discussion 3.1. X-ray diffraction Fig. 1 depicts the XRD scans for various CoCl2 FLs in PVDF films. The observed spectra indicate the presence of semicrystalline structures. The assigned crystalline peaks are listed in Table 1. These peaks are due to the a- and b-PVDF crystalline phases [1], except for the sample of 0.1% FL which contains a-phase only.
Table 1 The assigned characterizing XRD peaks for a- and b-crystalline PVDF phases W (%)
2q (degree)
Assignment
0.1 0.5
39.5 20.5 39.8 18.5 20.5 39.5 18.5 20.5 39.5 20.5 39.5 20.5 39.5
(002)a (110)(200)b (002)a (020)a (110)(200)b (002)a (020)a (110)(200)b (002)a (110)(200)b (002)a (110)(200)b (002)a
1.0
5.0
10.0 15.0
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Fig. 2.
IR transmission spectra for variously filled PVDF films.
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Fig. 3. (a) The FL dependence of the intensity of the IR absorption peaks at (*) 510, (쎲) 610 and (䊊) 1750 cm⫺1. (b) The FL dependence of Ir1 and Ir2 relative peaks. (c) The FL dependence of peaks at (䊊) 665 and (*) 1670 cm⫺1.
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3.2. IR analysis The IR transmission spectra are shown in Fig. 2. The main PVDF characterizing peaks are noted. In the present spectra, a-phase is evidenced by the peaks most notable at 490, 530, 610, 796 and 976 cm⫺1, while b-phase is characterized by the peaks at 445, 470 and 510 cm⫺1 [6]. The peaks at 510, 610, 665, 1670 and 1750 cm⫺1 are selected as indicators for the structure deformation due to CoCl2 filling. The peaks at 510 cm⫺1, belonging to b-phase, are assigned to CF bending and wagging modes, while the peaks at 610 cm⫺1 are due to the CF2 bending and skeletal bending of C(H)–C(F)–C(H) with negative phase relative to the symmetry coordinates of the a-phase. The bands at 665 cm⫺1 are taken as a measure for the concentration of the headto-head (CF2CF2) units, which is ⬇20% in the present system [6]. The bands at 1670 cm⫺1 are assigned to the C=C stretching in the monofluorinated alkenes, while the bands at 1750 cm⫺1 are due to the C=C stretching of the difluorinated alkenes [7]. Fig. 3(a) depicts the FL dependence of the intensity (I) of the peaks at 510, 610 and 1750 cm⫺1. The three curves exhibit a sharp decrease as FL increases up to 1.5%. Beyond this value the rate of decay of I is slow for the peaks at 510 and 610 cm⫺1, while the peak intensity at 1750 cm⫺1 increases slightly as FL increases from 1.5 to 15%. Let Ir1 and Ir2 represent the ratios of I at 1750 cm⫺1 to I at 510 cm⫺1 and I at 1750 cm⫺1 to I at 610 cm⫺1, respectively. The FL dependence of Ir1 and Ir2, shown in Fig. 3(b), indicates minimum values at 1.5% FL. This indicates that the FL dependence of the relative content of the
Fig. 3. (continued)
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difluorinated alkenes (as a structural defect), with respect to the content of the a- or b-crystalline phase, changes its behaviour at W=1.5%. Fig. 3(c) shows that the FL dependence of the monofluorinated alkenes (represented by the band at 1670 cm⫺1) exhibits three critical FLs at W=0.5, 1.0 and 10.0%. It is also clear that the role of the head-to-head defect decays as W increases. 3.3. Optical absorption Fig. 4 displays the UV/VIS absorption spectra of various FLs in PVDF films. A weak (and broad) visible absorption band is noticed at 530 nm for 15% FL. This band [4] is due to the 4 T1g(F)→4T1g(P) transition for the hexaquocobalt(II). Moreover, three visible absorption bands are noticed at 630, 666 and 690 nm for 0.5%ⱕWⱕ15%, characterizing the high spin tetrahedral species [4]. The FL dependencies of the intensity of these three bands are shown in Fig. 5. It is clear that the 1% CoCl2 corresponds to the minimum absorption intensity indicating minimum tetrahedral content, while the 10% CoCl2 content exhibits the maximum absorption intensity, indicating maximum tetrahedral content. Using the experimental data of Fig. 4 and Eq. (1) the absorption coefficient (a) was calculated at various photon frequencies (n) [8]
Fig. 4. The optical absorption spectra for variously filled PVDF films.
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Fig. 5.
The FL dependence of the visible absorption peaks at (왕) 630, (쎲) 666 and (䊊) 690 nm.
a(n)⫽ln[t1(n)/t2(n)]/(d2⫺d1)
(1)
where t1 and t2 are the transmittances of two films (of the same FL) with thicknesses d1 and d2. The photon energy (hn) dependence of (ahn)1/2 for various FLs, where h is Plank’s constant, were plotted. The plots, which are not presented here, exhibit two linear parts, indicating two optical energy gaps (Eg1, Eg2), each gap can be calculated using Eq. (2): a(n)⫽B(hn⫺Eg)2/hn,
(2)
where B is a constant. Fig. 6 displays the FL dependence of the optical gaps. It is clear that Eg1 changes slightly while Eg2 exhibits a maximum value at W=0.5% then a minimum value at W=1.5% and a maximum value at W⬇5%. The 5% critical FL agrees with that noticed in Fig. 3(a) for the FL dependence of the IR absorption peak at 1750 cm⫺1 due to C=C stretching in the difluorinated alkenes. Thus, it is thought that the difluorinated alkanes, as a structural defect, affect the Eg2 gap. 3.4. DC electrical conductor The dc electrical resistivity (r), was measured in the temperature (T) range 298–403 K. The mono- and/or difluorinated alkenes and the head-to-head defects in the polymeric matrix of the present system, which were evidenced by IR spectra, can be considered as suitable sites for polarons and/or bipolarons. This allows one to use the modified interpolaron hopping model of
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Fig. 6. The FL dependence of Eg1 and Eg2 optical gaps.
Kuivalainen et al. [9] to interpret the present resistivity results. According to this model, the conduction is attributed to phonon-assisted hopping between polaron and/or bipolaron bound states in the polymer. The model suggests the following formula for the resistivity: r⫽[kT/A1e2g(T)(R2o/z]x[(yp⫹ybp)2/(ybp)]x exp(2B1Ro/z),
(3)
where A1=0.45, B1=1.39, yp and ybp are the concentration of polarons and bipolarons, respectively, and Ro=(3/4pCimp)1/3 is the typical separation between impurities whose concentration is Cimp, z=(z11zⲚ2)1/3 is the average decay length of a polaron and bipolaron wave function, and z11 and zⲚ are the decay lengths parallel and perpendicular to the polymer chain, respectively. Bredas et al. [10] reported that polarons and bipolarons induce defects of the same extension. The electronic transition rate between polaron and bipolaron states can be expressed as [11] g(T)⫽1.2⫻1017(T/300K)11
(4)
In the present work the order of magnitude of r was adjusted, using a computer aided program. The parameter z11=1.06 nm, while zⲚ=0.22 [12], which depends on the interchain resonance energy and the interchain distance [13]. Taking Yp=Ybp for simplicity, which is an acceptable approximation [14], and using equations (3) and (4) we can obtain the values of the hopping distance, Ro. A linear temperature dependence is noticed for Ro in Fig. 7. The dependence of 1/Ro on log (W) presented in Fig. 8 exhibits minimum and maximum values at W=0.5 and 5%, respectively. The 5% critical FL agrees with that noticed in Figs. 3(a) and Fig. 6 for the C=C stretching
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Fig. 7. The temperature dependence of Ro for the FLs: (a) (1) 0.01, (2) 0.1, (3) 0.5, (4) 1 and (5) 5%; (b) (6) 10 and (7) 15%.
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Fig. 8. The dependence of 1/Ro on log (W).
mode due to difluorinated alkenes and for Eg2, respectively. This reveals that the difluorinated alkenes induce energy levels of a significant twofold influence as: (1) it changes the Eg2 optical gap; and (2) these induced energy level acts as hopping sites. It is remarkable that the obtained values of Ro are in the range of 2.2–7.7 nm, which is 8.8 to 30.8 times the monomer length (苲0.25 nm [15]). This indicates that the electrical conduction is an intrachain one dimensional hopping type. 3.5. ESR The ESR spectra for various FLs of the present system are shown in Fig. 9. It is clear that these spectra are complex and they exhibit an absorption line at g=4.5009±0.0001 arising from Co(II) tetrahedral coordination [16]. It is observed that the g=4.5 line width is constant for the various Fls, which indicates constant dipolar interactions between Co(II) ions [17]. On the other hand Fig. 10 reveals that the symmetry factor of this line is an FL-dependent function, exhibiting a peak value and a minimum at W=0.5 and 10%, respectively. Comparing Fig. 10 with Fig. 5 we notice that the minimum symmetry is found at the maximum tetrahedral content. It is noteworthy that the spectrum of 15% FL in Fig. 9 contains six lines due to the hyperfine structure of the cobalt nucleus with an unpaired electron, indicating isolated Co2+ ions in the ionmer and evidencing the presence of the Co2+ octahedral form. This confirms the findings of the optical absorption spectra of Fig. 4.
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Fig. 9. The ESR spectra for variously filled PVDF samples.
4. Conclusions The present system is of a semicrystalline structure containing a- and b-PVDF phase, except for the 0.1% FL sample which contains a-phase only. The presence of mono- and di-fluorinated alkenes, as structural defects, were considered as suitable sites (on the polymeric chain) for polarons and/or bipolarons. The head-to-head defect concentration was found to be ⬇20%. A tetrahedral Co(II) coordination was detected for all of the FLs, while the 15% FL contains both the
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Fig. 10. The FL dependence of the symmetry factor.
tetrahedral and octahedral forms. Two types of optical energy gaps (Eg1 and Eg2). Eg1 changes slightly as FL changes, while Eg2 depends on FL significantly and it was correlated with the difluorinated alkanes. The electrical conduction proceeds through the phonon-assisted hopping, of charge carriers, between polaron and/or bipolaron bound states in the polymeric chain. The calculated hopping distance depends linearly on temperature and non-monotonically on FL. The ESR spectra were complex. However, the clear absorption line at g⬇4.5009±0.0001 confirmed the presence of the Co(II) tetrahedral form. Moreover, the hyperfine six lines, observed at 15% FL, confirmed the presence of the octahedral form. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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