MnCl2 mixed fillers

Physica B 406 (2011) 766–770

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Structural, optical and some physical properties of PVDF films filled with LiBr/MnCl2 mixed fillers E.M. Abdelrazek a,n, Rudolf Holze b a b

Department of Physics, Faculty of Science, Mansoura University, Mansoura 35516, Egypt Institut f¨ ur Chemie, AG Elektrochemie, Technische Universit¨ at Chemnitz, 09107 Chemnitz, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 May 2010 Received in revised form 20 November 2010 Accepted 23 November 2010 Available online 9 December 2010

Films of polyvinylidene fluoride (PVDF) filled with (X)LiBr(20  X)MnCl2 mixture, where X¼ 0, 1, 2, 8, 16 and 20 wt%, were prepared by casting method and studied by ultraviolet/visible optical absorption (UV), differential scanning calorimetry (DSC), X-ray diffraction (XRD), infrared transmission (IR) and electron spin resonance (ESR). The optical absorption spectra suggested the presence of an optical gap (Eg) which depends on filler concentration (W) and arises due to the variation in crystallinity within the polymer matrix. Melting and degradation temperatures were identified using DSC. XRD implied a semicrystalline structure (containing a- and b-PVDF phases for all films). Conjugated double bonds and the role of dimethylformamide with a PVDF chain were detected by IR spectra. The ESR analysis revealed the existence of both isolated and aggregated Mn2 + ions within the PVDF matrix. Published by Elsevier B.V.

Keywords: Polyvinylidene fluoride MnCl2 LiBr Optical absorption DSC XRD FT-IR and ESR

1. Introduction Polymers have attracted the attention of scientific and technological researchers because of their wide applications. Polyvinylidene fluoride (PVDF) as a polycrystalline polymer started drawing scientific interest in the seventies, because of its extraordinary pyro- and piezo-electric properties. These properties combined with both high elasticity and processing ability lend this material numerous technological applications [1]. Recently, PVDF has been used widely in biotechnology [2], photorecording [3], microwave modulation [4] and in rechargeable lithium batteries [5]. Filling PVDF enhances its electromagnetic wave sensing feature due to its increased magnetic activity [6]. Another feature that distinguishes PVDF from other polymers is its polymorphism, that is, it may present at least four crystalline phases, namely a, b, d and g [7]. Moreover, an increasing interest has been devoted to PVDF as electric and/or magnetic field sensors. For this application, PVDF was filled with transition metal halides [8]. Fillers are used in polymers for a variety of reasons: cost reduction, improved processing, density control, optical effect, thermal conductivity, control of thermal expansion, electrical properties, magnetic properties, flame retardancy and hardness, and tear resistance

n

Corresponding author. E-mail addresses: [email protected], [email protected] (E.M. Abdelrazek). 0921-4526/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.physb.2010.11.077

[9]. For example, in optical, electrical and magnetic applications, fillers such as LiBr and MnCl2 are used to provide better optical, electrical and magnetic properties. Manganese is well known as a magneto-active multivalent element; thus its halides can be used as fillers to modify the electric conduction, the optical absorption and magnetic properties of PVDF. On the other hand, MnCl2 is considered as a good candidate for one- or two-dimensional phenomena [10] and for optical memory device [11]. Spectroscopic and physical properties of metal halide doped polymers are extensively studied. But research on a mixture of LiBr and MnCl2-containing PVDF is limited, to our knowledge. Therefore, the present study is devoted to shed some light on the role of filling PVDF with a mixture of LiBr and MnCl2 on optical, structural and electron spin resonance properties.

2. Experimental work The studied PVDF films filled with (X)LiBr(20  X)MnCl2(wt%) were prepared by casting method. Dimethylformamide (DMF) was used to dissolve the used materials. The solution of mixed halides was added to the dissolved polymer at a suitable viscosity. The mixture was cast to a glass dish and kept in a dry atmosphere at 323 K for 2 weeks to ensure the removal of solvent traces. Different concentrations of the fillers were obtained, where X¼0, 1, 2, 8, 16 and 20 wt%. The thickness of films was in the range of 0.15–0.3 mm. Ultraviolet/visible (UV/Vis) absorption spectra were measured in

E.M. Abdelrazek, R. Holze / Physica B 406 (2011) 766–770

3. Results and discussion 3.1. UV/visible optical absorption Fig. 1 displays the UV/VIS absorption spectra in the range of 200–800 nm of PVDF films filled with two mixed fillers LiBr and MnCl2. In previous works, our research group investigated the optical absorption of pure PVDF films [12]. This investigation revealed that, the spectrum of the unfilled PVDF is characterized by a sharp absorption edge (SAE) at about 212 nm and there are no absorption peaks at higher wavelengths. The spectra of the filled PVDF films exhibit a shift of the SAE towards higher wavelengths. The shift in absorption edge in the doped PVDF reflects the variation in the energy band gap, which arises due to the variation in crystallinity within the polymer matrix [13]. The small band at about 360 nm in UV spectra is due to crystal field transitions of isolated Mn2 + ions and/or Li ions [14]. The optical energy gap (Eg) for an indirect transition can be determined using Davis and Mott [15]formula :   ahn 1=2 Eg ¼ hn ð1Þ B

where h is Planck’s constant, n is the photon frequency, B is a constant and a is the absorption coefficient, which can determined as a function of photon frequency using the equation:

a ¼ 2:303

A d

ð2Þ

where A is the absorbance and d is the thickness of the sample. The plot of (ahn)1/2 versus the photon energy hn at room temperature is shown in Figs. 2 and 3. The extrapolated of linear portion in the range of 3.4ohn o5.5 indicates an optical energy gap Eg, which can be considered as an evidence for allowed indirect transition. Fig. 4 depicts the dependency of Eg on the LiBr and/or MnCl2 content. It is clear that Eg exhibits a rapid increase up to W¼8 wt%: Then Eg increases slowly as LiBr and/or MnCl2 content increases. The change of Eg may be attributed to the change of induced energy states due to the change of chilation mode at filling level (FL) ¼8 wt% [16].

70 (x) LiBr2 (20-x) MnCl2

60 50

(αhυ)1/2

the wavelength region of 200–800 nm using a Shimadzu UV 2101PC instrument (resolution 0.1 nm). The differential scanning calorimetry (DSC) measurements were performed using (Perkin Elemer-Pyris 1) apparatus in the temperature range 30–420 1C with a heating rate of 10 1C/min. X-ray diffraction (XRD) scans were obtained using (‘‘STOE-STAD IP’’ powder diffractometer with Cu-K (alpha) radiation) the Bragg angle (2y) in the range of 5–801. FT-IR spectra were recorded with a Perkin Elmer FT-IR-1000 spectrophotometer and KBr pellets at 2 cm  1 resolution (8 scans each) were used for measuring the IR spectra in the wavenumber range of 800–3100 cm  1. The ESR spectra were recorded on BRUKER MEX with cavity ER4102 at 100 kHz modulation, 9.65 Ghz, X-band, using 1,1 diphenyl-2-pierylhydrazyl (DPPH) as a calibrant.

767

40 30 20 x=1 x=2 x =8

10

x = 20

0 0

1

2

3

4

5

6

7

6

7

hυ (eV) (x) LiBr2 (20-x) MnCl2

Fig. 2. (ahn)

versus photon energy (hn) for various FLs.

70

X=0

(x) LiBr2 (20-x) MnCl2

60 Absorbance (a. u.)

1/2

X=1

50

(αhυ)1/2

X=2 X=8 X = 16

40 30 20

X = 20

x=0 x = 16

10 0

200

300

400

500

600

700

800

0

1

2

4

5

hυ (eV)

Wavelength (nm) Fig. 1. The UV/vis absorption spectra of FLs in PVDF films.

3

Fig. 3. (ahn)

1/2

versus photon energy (hn) for various FLs.

768

E.M. Abdelrazek, R. Holze / Physica B 406 (2011) 766–770

2.5

2

Eg (eV)

1.5

1

0.5

0 0

5

10

15 W (wt%)

20

25

Fig. 6. X-ray diffraction scans of PVDF filled with various mass fraction of (X) LiBr(20 X)MnCl2.

Fig. 4. Filling level (FL) dependence of an optical energy gap (Eg). Table 1 The assigned characterizing X-ray diffraction peaks for a- and b-crystalline PVDF phases. Sample 0.0% 1.0% 2.0% 8.0% 16.0% 20.0%

2y (degree)

Assignment

20.50 40 20.55 40 20.52 40 20.46 40 20.61 40 20.58 40

(1 1 0)b, (0 0 2)a (1 1 0)b, (0 0 2)a (1 1 0)b, (0 0 2)a (1 1 0)b, (0 0 2)a (1 1 0)b, (0 0 2)a (1 1 0)b, (0 0 2)a

(2 0 0)b (2 0 0)b (2 0 0)b (2 0 0)b (2 0 0)b (2 0 0)b

3.3. X-ray diffraction (XRD)

Fig. 5. DSC curves of PVDF films.

3.2. Differential scanning calorimetry (DSC) Fig. 5 shows the DSC thermograms for the thermal behavior of PVDF films filled with (X)LiBr(20 X)MnCl2, where X¼0, 1, 2, 8, 16 and 20 wt%, was studied by DSC in the temperature range of 30–450 1C. A small endothermic peak is observed at about 37 1C, which is attributed to the small amount of water that is always present in the conventional polymer unless it is carefully vacuum dried [17]. An exothermic peak is observed at about 172 1C, which is attributed to the PVDF melting temperature [18]. Endothermic peaks observed in the temperature range 378–391 1C are assigned to the PVDF degradation [19]. The PVDF films filled with different filling levels samples was slightly shifted towards higher temperatures. This indicates that the addition of LiBr increases the thermal stability.

The XRD scans of various (LiBr+MnCl2) filler levels in PVDF films is shown in Fig. 6. The general feature of the observed spectra indicates the presence of a semicrystalline structure. Table 1 lists the assigned crystalline peaks. These peaks characterize the a- and bPVDF crystalline [20] phases. The area under the peaks (Aa, Ab and AT ¼Aa +Ab) can be taken as a measure of the degree of crystallinity (of a and b phases, respectively), where Aa/AT and Ab/AT are the relative a- and b-phase contents, respectively. Fig. 7 depicts the influence of (LiBr+MnCl2) content on Aa/AT and Ab/AT. It is clear that for Wr8%, the relative a-content decreases (while the relative b-content increases) and W48%, the relative a-content increases (while the relative b-content decreases). It is remarkable (in Fig. 6) that there is a small scattering peak at 2y ¼371. This is due to increasing the LiBr content resulted due to increase in intensities of the peak, and this indicates that LiBr molecules greatly affected the PVDF structure.

3.4. IR analysis IR absorption spectra (800–3100 cm  1) were conducted for X¼0, 1, 2, 8, 16 and 20 wt% for the present system is shown in Fig. 8. The structural modification can be identified by investigation of the FL dependence of certain IR absorption peaks 950–1550 cm  1

E.M. Abdelrazek, R. Holze / Physica B 406 (2011) 766–770

769

(X) LiBr (20-X)MnCl2

X (wt%)

Intensity (a. u.)

0

1

8 16

0

2000

4000 6000 Magnetic field (Gauss)

Fig. 7. The FLs dependence of Aa/AT and Ab/AT.

8000

Fig. 9. The ESR spectra of the PVDF filled with (X)LiBr(20  X)MnCl2 mixed halides.

1.2 A H

1

H (KG), A

0.8

0.6

0.4

Fig. 8. IR absorption spectra of PVDF filled with various FLs.

depicted in Fig. 8. The intensities of the bands at 950 cm  1 that refers to the g-phase and 1550 cm  1 that was assigned to the CQC stretching in the difluorinated alkenes [21] revealed maximum intensity at X¼0 wt% and minimum intensity at X¼ 20 wt%. The peaks noted at 1905, 2195, 2280 and 2400 cm  1 may be assigned to the role of dimethylformamide [22] with a PVDF chain.

3.5. Electron spin resonance (ESR) Fig. 9 displays the ESR spectra, measured at about T¼293 K, for samples of X ¼0, 1, 8 and 16 wt%. The characterizing spectrum of pure PVDF reported in Ref. [23], is not observed here. It is clear that the spectra at high X consist of six lines and a broad component overlapping the six lines. The six lines can be ascribed to the hyperfine structure of the manganese nucleus (I¼5/2) with an

0.2

0 0

5

10 W wt%

15

20

Fig. 10. The FL dependence of A (K) and H (’).

unpaired electron, which indicates the presence of isolated Mn2 + in the ionomer. Moreover, the broad signal of the Lorentzian shape suggests the presence of an aggregated form of Mn2 + . The individual lines of the high FL, X ¼8 and 16 wt%, can be ascribed to the hyperfine structure of the manganese nucleus (I ¼5/2) with an unpaired electron and can be assigned to the presence of an isolated Mn2 + [24]. The FL dependence of the peak-to-peak separation (H) of the main ESR Lorentzian signal and the

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E.M. Abdelrazek, R. Holze / Physica B 406 (2011) 766–770

asymmetry factor (A), which is the ratio between the two halves of this signal, is shown in Fig. 10. It is clear that A decreases as X increases, which indicates that Mn2 + loses its symmetrical local distribution. Conversely and there is maximum value for A at X¼8 wt%. This confirms the findings of the optical absorption, moreover, Fig. 10 depicts a maximum value for H around X ¼8% indicating a maximum of dipolar interactions between the magnetic ions.

4. Conclusion The present work tried to explore, more explicitly, the role of most of the microstructural forms on the optical, thermal and magnetic properties of PVDF films filled with (X)LiBr(20  X)MnCl2, in the mass fraction range of 0 r(X)r20 wt%: This mixture of Mn2 + and Li2 + ions enhanced the magnetoactivity and thermal stability of PVDF. Moreover, improvement of the optical response of the films by increasing the optical energy gap (Eg) values. The PVDF films filled with different filling levels samples was slightly shifted towards higher temperatures. This indicates that the increasing of LiBr increases the thermal stability. The asymmetry factor decreases as X increases, which indicates that Mn2 + loses its symmetrical local distribution and indicates the presence of isolated Mn2 + in the monomer.

Acknowledgements Permission to use experimental facilities by M. Mehring (X-ray diffraction) and W.A. Goedel (Thermal analysis) is gratefully

acknowledged. Further support from the Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft is appreciated. References [1] R. Gregono, M. Cestori, N. Chaves, The Polymeric Material Encyclopedia, CRC Press, Boca Raton, FL, 1996. [2] L.P. Cheng, D.J. Lin, C.H. Shih, A.H. Dawar, C.C. Gryte, J. Polym. Sci. 37 (1999) 2079. [3] A. Tawansi, A.H. Oraby, E. Ahmed, E.M. Abdelrazek, M. Abdelaziz, J. Appl. Polym. Sci. 70 (1998) 1759. [4] A. Tawansi, M.I. Ayad, E.M. Abdelrazek, J. Appl. Polym. Sci. 72 (1999) 771. [5] K.M. Kim, N.G. Park, K.S. Ryu, S.H. Chang, Polymer 43 (2002) 3951. [6] M. Abdelaziz, Effect of filling with transition metal chlorides on physical properties of PVDF, some of its blends, Ph.D. Thesis, Mansoura University, 2002. [7] W. Kilee, C. Sikha, Polymer 39 (1998) 7131. [8] A. Tawansi, M.I. Ayad, E.M. Abdelrazek, J. Mater. Sci. Technol. 13 (1997) 124. [9] R.N. Rothon, Particulate Fillers for Polymers, Summary, 1st Ed., ChenTec, UK, 2002. [10] R. Dingle, M.E. Lines, S.L. Holt, Phys. Rev. 187 (1969) 643. [11] Q. Zhang, R.W. Wehatmore, J. Phys. D 34 (2001) 2296. [12] A. Tawansi, A.H. Oraby, S.I. Badr, I.S. Elashmawi, Polym. Int. 53 (2004) 370. [13] R.F. Bhajantri, V. Ravindrachary, A. Harisha, Vincent Crasta, Suresh P. Nayak, Boja Poojary, Polymer 47 (2006) 3591. [14] B. Hannoyer, M. Lenlet, R. Corles, J. Non-Cryst. Solids 151 (1992) 209. [15] E.A. Davis, N.F. Mott, Philos. Mag. 22 (1970) 403. [16] A. Tawansi, A.H. Oraby, H. Zidan, M.E. Dorgham, Physica B 126 (1998) 254. [17] A. Tawansi, M.A. Soliman, N. Kinawy, S.I. Badr, Polym. Bull. 19 (1988) 289. [18] A. Tawansi, A.H. Oraby, H.I. Abdelkader, M. Abdelaziz, J. Magn. Magn. Mater. 262 (2003) 203. [19] Y.D. Wang, M. Cakmak, J. Appl. Polym. Sci. 68 (1998) 909. [20] G.T. Davis, J.E. McKinney, M.G. Broadhust, S.C. Roth, J. Appl. Phys. 49 (1978) 4998. [21] J. Fleming, D.H. Williams, Spectroscopic Methods in Organic Chemistry, McGraw-Hill, New York, 1966. [22] M. Kobayashi, K. Tashiro, H. Tadokoro, Micromolecules 8 (1975) 158. [23] A. Tawansi, E.M. Abdel-Razek, H.M. Zidan, J. Mater. Sci. 32 (1997) 6243. [24] A. Tawansi, A.H. Oraby, E.M. Abdelrazek, A.M. Ayad, M. Abdelaziz, J. Appl. Polym. Sci. 70 (1998) 1437.