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Spectroscopic studies on the effect of doping with CoBr2 and MgCl2 on some physical properties of polyvinylalcohol films
ARTICLE IN PRESS
Physica B 403 (2008) 2137–2142 www.elsevier.com/locate/physb
Spectroscopic studies on the effect of doping with CoBr2 and MgCl2 on some physical properties of polyvinylalcohol films E.M. Abdelrazek Department of Physics, Faculty of Science, Mansoura University, Mansoura 35516, Egypt Received 28 June 2007; received in revised form 26 November 2007; accepted 28 November 2007
Abstract Filled and unfilled polyvinylalcohol (PVA) films were prepared by the casting technique. Films of equal amounts with various concentrations of two fillers (CoBr2 and MgCl2) were prepared. Spectroscopic, structural and some physical properties of these films were studied with different techniques. Fourier-transform infrared (FTIR) revealed that the syndiotacticity structure of the PVA samples causes dense molecular packing in the crystal and also stronger intermolecular hydrogen bonds, which are responsible for the disappearance of the molecular motion. X-ray diffraction (XRD) scans evidenced the presence of some semicrystalline structure of PVA films. The optical absorption spectra suggested the presence of an optical gap (Eg), which depends on filler concentration for all the filling levels. Differential thermal analysis (DTA) suggests that the segmental mobility of an amorphous pure PVA increases as a result of the addition of mixed fillers, becoming less-rigid segments. This indicates that the mixed fillers act as plasticizers. r 2007 Elsevier B.V. All rights reserved. Keywords: X-ray; FTIR; Optical absorption; DTA; ESR
1. Introduction Considerable effort has been recently devoted to preparation and characterization of polymers. This may be because, these materials are considered to be important due to their useful and important applications due to the role of OH group and hydrogen bonds [1]. Furthermore, it can be used as a medical material due to its compatibility to the living body [2]. Like a hydrogen-bonded polymer, polyvinylalcohol (PVA) is used in gas sorption and diffusion [3]. Moreover, PVA can selectively adsorb metal ions such as copper, palladium and mercury [4]. PVA is a polymer with a carbon chain backbone attached with hydroxyl groups. These OH groups can be a source of hydrogen bonding and hence assist in the formation of polymer complexes [5,6]. The change in the degree of crystallinity and lamellar thickness distribution of PVA during dissolution process can be determined by differential thermal analysis (DTA) and Fourier-transform infrared (FTIR) studies [7]. Recent neutron diffracTel.: +20 106192689; fax: +20 502246781.
E-mail address: [email protected] 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.11.029
tion analysis and computer modelling proposed more detailed crystal structure for PVA including intermolecular and intramolecular hydrogen bondings [8]. Irradiation with X-ray diffraction (XRD) and g-ray radiation has a significant effect on the polymer properties and some physical properties that are usually modified. In addition, the electron spin resonance (ESR) study of g-irradiated PVA was investigated and the spectra were assigned [9]. The metalion-doped polymers represent a new class of organic materials. Inorganic additives such as transition metal salts have a considerable effect on the structural, optical, electrical and magnetic properties of the polymer [10]. The present work is mainly focused on the effect of equal amounts of Mg and Co ions dopant addition on the structural, optical, thermal and ESR of PVA films.
2. Experimental 2.1. Samples preparation The PVA films used in this work were obtained in pellet form from Merck, Germany and had molecular weight of
ARTICLE IN PRESS E.M. Abdelrazek / Physica B 403 (2008) 2137–2142
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14 000. PVA films with different equal amounts of CoBr2 and MgCl2 dopant were prepared by the solvent-casting technique. A known quantity of PVA pellets was added to doubly distilled water and kept for 48 h to swell the pellets, with stirring the solution at 70 1C for complete dissolution. CoBr2 and MgCl2 were also dissolved in doubly distilled water. The solution of the mixed fillers was added to the polymer at a suitable viscosity. The solution was poured onto cleaned Petri dishes and dried in an oven at 50 1C. After drying, the films were peeled from the Petri dishes and kept in vacuum desiccators until use. The thickness of the films was in the range of 100–150 mm. PVA films were doped with equal mass fractions (W) of CoBr2 and MgCl2 of 0, 5, 10, 15, 20, 25 and 30 wt%.
2.2. Physical measurements The FTIR measurements were carried out using the single-beam FTIR Spectrometer (FTIR-430, Jascow, Japan). The FTIR spectra of the samples were obtained in the spectral range of 400–2000 cm1 with scanning speed of 2 mm/s. The XRD scans were obtained using DIANO corporation—USA equipped with CuKa radiation (l ¼ 1.79026 A˚, the tube operated at 30 kV, the Bragg angle (2y) in the range of 5–501, step size ¼ 0.1 and step (CoBr2 + MgCl2)%
30
time 1 s). Ultraviolet and visible (UV/vis) absorption spectra were measured in the wavelength region of 200–600 nm using a spectrometer (Perkin–Elmer UV/vis). The DTA for the prepared films was carried out using an equipment type (GDTD16-Setaram) with measuring temperature range from room temperature to 300 1C and heating rate 5 1C/min. 3. Results and discussion 3.1. FTIR analysis To identify the different structure groups in the polymer matrices, FTIR studies have been carried out. The FTIR spectra of pure PVA and the PVA treated with two mixed fillers CoBr2 and MgCl2 are shown in Fig. 1. A slight variation was observed in the absorption bands of the filled PVA compared with pure PVA. The assignment of these bands for all samples under study is given in Table 1. Table 1 Assignment of the IR characterizing peaks of the PVA system [11,15] Frequency (cm1)
Assignment
1715 1662 1563 1438 1385 1240 1110 925 879 803 575 465
ns(CQO) Absorbed H2O O–H and C–H bending d(CH2) nw(CH2) nw(CH) ns(CO) nr(CH2) ns(CC) ns(CH2) C–Br Mg–O
ns=stretching; d=bending; nw=wagging and nr=rocking.
Absorption %
25
20
Isy (a. u.)
15 10
5
0 2000
1600
1200 Wavenumber Fig. 1.
800 (cm-1)
400
0
5
10
15 W (wt%) Fig. 2.
20
25
30
ARTICLE IN PRESS E.M. Abdelrazek / Physica B 403 (2008) 2137–2142
The vibrational peaks found in the range 800–600 cm1 can be attributed to n(C–Cl) and M–O, where M ¼ Co or Mg, which indicate that the mixed fillers are complexed with the polymer matrix [11]. The appearance of a band at 1563 cm1 is indicative of the formation of small conjugated polyene sequences, which are presumably responsible for the color of the mixed fillers-treated PVA. These conjugated polyene sequences are suitable sites for the polaron and bipolaron formation [12]. The absorption peak at about 925 cm1 was found to be characteristic of the syndiotactic structure of the prepared films. The PVA
Intensity (a. u.)
(CoBr2 + MgCl2)%
0
structural deformations due to the filling can be calculated by plotting the intensity (Isy) of the absorption peak at 925 cm1 and can be taken as a measure of the syndiotacticity of the PVA [13]. The filling-level (FL) dependence of Isy is shown in Fig. 2. It is clear that Isy increases as W increases up to 15%. This indicates an increase in syndiotacticity of PVA. Nagura et al. [14] reported that the increase in the syndiotacticity of the PVA samples causes dense molecular packing in the crystal and also stronger intermolecular hydrogen bonds, which are responsible for the disappearance of the molecular motion. Beyond W ¼ 15%, Isy decreases as W increases. It is noticed that the syndiotacticity decreases. From the IR spectra, it is observed that upon increasing the mixed fillers CoBr2 and MgCl2 concentrations some of the peaks are shifted and some of them disappear with respect to the pure PVA. These results manifested the conclusion about the specific interaction in polymer matrices and hence the occurrence of complexation [16]. 3.2. XRD The XRD scans for PVA films, filled with various mixed filler fractions (W) of equal amounts of CoBr2 and MgCl2, are presented in Fig. 3. The observed spectra characterize a semicrystalline polymer possessing crystalline peaks at a scattering angle of (211p2yp231) corresponding to a (1 0 1) spacing [17]. The intensity of the (1 0 1) peak can be taken as a measure of the degree of crystallinity (Cn) and plotted as a function of the FL, as shown in Fig. 5. It is clear that Cn exhibits a minimum at W ¼ 10% and a maximum at W ¼ 15%. This finding creates, to some extent, a tuning event that a maximum syndiotactic structure (see Fig. 2) corresponds to a higher degree of crystallinity (Fig. 5) that occurs at same value of the filler concentration W ¼ 15%.
5 10 15 20 25 30
10
15
20
25 30 2θ (degree)
35
40
45
50
Fig. 3.
300
Pure CoBr2
Intensity
Pure MgCl2 (wt.%)
Intensity (a.u.)
5
2139
200
100
10
20 2θ (Degree)
30
40
Fig. 4.
10
20 30 2θ θ (Degree)
40
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7 6
Cn (a. u.)
5 4 3 (CoBr2 + MgCl2)%
2
0
0
10
20 W (wt%)
30
40
Fig. 5.
It is remarkable, in Fig. 3, that there is a small scattering peak at 2y ¼ 9.61 for W ¼ 25%. This peak belongs neither to PVA nor to CoBr2 and/or MgCl2 crystals. Fig. 4(a, b) depicts the X-ray scans for CoBr2 and MgCl2 crystals. It is clear that the peaks of Fig. 4(a, b), characterizing CoBr2 and MgCl2 crystals, are not observed in the spectra of Fig. 3. This indicates that the filled PVA films either are free of CoBr2 and/or MgCl2 crystals or contain only traces of these crystals for them to be detected by X-ray analysis (of the present technique). However, this peak may indicate the appearance of a new crystalline phase of the filled samples [18]. The present results imply that the XRD profile is substantially influenced by the FL (Fig. 5).
Absorbance (a.u.)
1
30
25
20
15 10 5 0 200
250
300
3.3. UV/vis optical absorption
500
550
600
Fig. 6.
The room-temperature optical absorption spectra, in the UV/vis region (200–600 nm), of the unfilled PVA and PVA filled with various concentrations of MgCl2 and CoBr2 are shown in Fig. 6. A sharp visible absorption peak is noticed at about 208 nm, indicating the presence of unsaturated bonds, of the type CQO and/or CQC mainly found in the tail–head of the polymer [19]. The band appearing as a shoulder at about 277 nm for Wp15% arising from (CH3)2CQO [20], is attributed to the electronic transition 4 A2g-4T1g(P) [21]. In other words, the band that appeared at 277 nm, with different absorption intensities, may be assigned to p–p*, which comes from unsaturated bonds, mainly CQO and/or CQC. A weak (and broad) visible absorption band is noticed at around 490 nm for 10%pWp30%. This band is due to the 4T1g(F)-4T1g(P) transition for the hexaquocobalt(II) [22]. Fig. 7(a, b) presents the photon energy (hn) dependence of (ahn)1/2, for PVA films with various filling levels, where h is Planck’s constant, u is the photon frequency and the absorption coefficient a(n) was calculated [23] using aðnÞ ¼ ln½t1 ðnÞ=t2 ðnÞ=ðd 2 d 1 Þ;
350 400 450 Wavelength (nm)
(1)
where t1, t2 and d1, d2 are transmittances and thicknesses of both the sample and reference sample, respectively. It is noticed that most of the curves of Fig. 7(a, b) are characterized by linear regions, indicating optical gap Eg of values given by [24] aðn ¼ÞBðhn E g Þ2 =hn,
(2)
where B is a constant. The filling level dependence of Eg is plotted in Fig. 8. Eg experiences a peak value around W ¼ 15%, assuming that Eg to be influenced by the induced states due to MgCl2 and/or CoBr2 filling of PVA. This may be attributed to the change of the filling mode. The most remarkable change in Eg was found at W415%. The decrease of Eg during incorporation of the inorganic compounds into the photopolymer matrix on the interfaces, there occur the nano- sheets forming additional nano-confined states [25], which substantially influence the observed optical features of the materials. In particular, there occurs substantial electron–phonon anharmonic interaction, which change the observed susceptibilities. It is remarkable that there is a coincidence between the filling
ARTICLE IN PRESS E.M. Abdelrazek / Physica B 403 (2008) 2137–2142
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0% 5% 10% 15%
40 35 (αhυ)1/2
30
2141
(CoBr2 + MgCl2)% 30
25 25
20 15 10 5
20 0
1
2
3 4 hυ (eV)
5
6
7 exo
0
15
60 10
endo
20% 25% 30%
50
(αhυ)1/2
40 5
30 20 10 0
0
1
2
3 4 hυ (eV)
5
6
0
7
Fig. 7.
100 5.2
5 Eg (eV)
300
Fig. 9.
5.1
4.9 4.8 4.7 4.6
200 Temperature °C
0
5
10
15 20 W (wt%)
25
30
35
Fig. 8.
levels at W ¼ 15% corresponding to the maximum values of Eg and the intensities of the IR bands at 925 cm1. This reveals a compatibility between the origin of the optical and IR response of the present PVA system. 3.4. Differential thermal analysis (DTA) PVA consists of two inextricably mixed phases: the crystalline solid and amorphous glass [26]. This results in
quite a complex behaviour when the polymer is heated. In the present work DTA traces of pure and filled PVA with different fractions of CoBr2 and MgCl2 are shown in Fig. 9. From this figure we observe that the pure PVA film displayed four phase transitions at: 50, 83, 200 and 260 1C (Table 2). The transition appears as a shoulder at about 50 1C, preferably attributed to the glass transition (Tg) relaxational process resulting from the micro-Brownian motion of the main chain backbone. It is observed from Table 2 that the position of Tg for the PVA films filled with CoBr2–MgCl2 mixed fillers is shifted toward lower temperatures compared with that of the pure PVA [27]. This suggests that the segmental mobility of an amorphous pure PVA increases as a result of the addition of mixed fillers, becoming less-rigid segments. This indicates that the mixed fillers CoBr2 and MgCl2 act as plasticizers. There is another broad and shallow endothermic peak at about 83 1C which is attributed to the a-relaxation associating the crystalline regions. Similar transitions were seen by Garrett and Gurbb [28] at 85 1C. The magnitude of Tg of the pure PVA is greater than those for the filled samples but the
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Table 2 DTA results of pure PVA and mixed-fillers-doped PVA films W (wt%)
Tg (1C)
Ta (1C)
Tm (1C)
Td (1C)
0 5 10 15 20 25 30
50 – 48 – 44 – 45
83 85 94 96 96 78 73
200 196 192 189 188 185 185
260 254 252 253 248 243 240
a-relaxation temperature Ta is smaller for 5%pWp20%. This may be due to the greater crystallinity or formation of another crystalline phase in filled samples. The greater breadth of the relaxations of filled samples can be attributed to the wider range of crystallite sizes and morphology produced [28]. On the other hand, the magnitude of Ta of the pure PVA is greater than those for the filled samples for W ¼ 25% and 30%. The endothermic peak at 200 1C in pure PVA has been attributed to the melting point of PVA [29–31]. A reduction in the Tm values of PVA crystals was evident with an increase in W of CoBr2 and MgCl2. Also, the magnitude of thermal degradation temperature (Td) of the pure PVA is greater than that of the filled samples. It is suggested that the addition of the mixed fillers to PVA films decreases the thermal stability [32]. 4. Conclusions From an analysis of IR and DTA curves and XRD scans of filled PVA films, it may be said that the structure changes took place after the filling with CoBr2 and MgCl2 as a result of defect formation. From the IR curves, the appearance of a band at 1563 cm1 is indicative of the formation of small conjugated polyene sequences which are presumably responsible for the color of the mixed fillers treated PVA. From the DTA curves, we noticed that an increased filler content resulted in a decrease in Tg, Tm and Td of PVA films, which indicated that the filler acted as a plasticizer. The X-ray analysis showed no significant peaks characterizing CoBr2 and/or MgCl2 crystals. The intensity of the (1 0 1) peak of the main phase, detected by XRD, decreased exponentially with increasing Wp10%. It was observed that the intensity of the (1 0 1) peak of the main phase could be used as a measure of the degree of crystallinity (Cn) with a maximum value at W ¼ 15%, which agreed well with the IR findings. Because of their similarity, the W dependence of the intensity (Isy) of the absorption peak at 925 cm1 can be taken as a measure of the syndiotacticity of the PVA. From the UV/visible spectra, it is remarkable that there is a coincidence between
the filling levels at W ¼ 15% corresponding to the maximum values of Eg and the intensities of the IR bands at 925 cm1. This reveals compatibility between the origin of the optical and IR response of the present PVA system.
References [1] J. Pacansky, S. Schneider, J. Phys. Chem. 94 (1990) 3166. [2] K. Yamaura, N. Kuranuki, H. Suzuki, T. Tanigami, S. Matsuzawa, J. Appl. Polym. Sci. 41 (1990) 2409. [3] M.J. Reimers, M.J. Cibulsky, T.A. Barbari, J. Polym. Sci.—Polym. Phys. 31 (1993) 537. [4] R. Lei, X. Jie, X. Jun, Z. Ruiyum, J. Appl. Polym. Sci. 53 (1994) 325. [5] M. Watanabe, S. Nagona, K. Sanui, N. Ogato, J. Power Sources 20 (1987) 327. [6] M.M. Coleman, P.C. Painter, Prog. Polym. Sci. 1 (1995) 20. [7] S.K. Mallapragad, N.A. Peppas, J. Polym. Sci.—Polym. Phys. 34 (1996) 1339. [8] H.E. Assender, A.H. windle, Polymer 39 (1998) 4303. [9] D.J.T. Zainuddin, T.T. Le Hill, Radiat. Phys. Chem. 62 (2001) 283. [10] A. Tawansi, A.H. Oraby, E.M. Abdelrazek, M. Abdelaziz, Polym. Test. 18 (1999) 569. [11] A.A. El-Zahhar, M.Ismail, H.A. El-Naggar, in: Seventh Arab International Conference on Polymer Science & Technology, 5–9 October, 2003 Cario, Hurghada, Egypt. [12] M. Chakraborty, D.C. Mukherjee, B.M. Mandal, Synth. Met. 98 (1999) 193. [13] S. Rajendran, M. Sivakumar, R. Subadevi, Mater. Lett. 58 (2004) 641. [14] M. Nagura, S. Matsu Zawa, K. Yamamura, H. Ishikawa, Polym. J. 14 (1982) 69. [15] I. Omkaram, R.P. Sreekanth Chakradhar, J. Lakshmana Rao, Physica B 388 (2007) 318. [16] M. Silvakumar, R. Subadevi, S. Rajendran, N.L. Wu, J.Y. Lee, Mater. Chem. Phys. 97 (2006) 330. [17] K.E. Strawhecker, E. Manias, Chem. Mater. 12 (2000) 2943. [18] T. Fahmy, Int. J. Polym. Mater. 50 (2001) 109. [19] K.A.M. Abd El-Kader, S.F. Abdel-Hamied, A.B. Mansour, A.M.Y. El-Lawindy, F. El-Tantaway, Polym. Test. 21 (2002) 847. [20] N.R. Rao, Ultraviolet and Visible Spectroscopy: Chemical Applications, Butterworths, London, 1967. [21] F.H. Abd El-Kader, S.S. Hamza, G. Attia, J. Mater. Sci. 281 (1993) 6719. [22] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, Wiley, London, 1962. [23] G. Chiodlli, A. Magistris, J. Solid State Ionics 18 (1986) 356. [24] E.A. Davis, N.F. Mott, Philos. Mag. 22 (1970) 903. [25] I.V. Kityk, J. Non-Cryst. Solids 292 (2001) 184. [26] J.G. Pritchard, Polyvinyl Alcohol: Basic Properties and Uses, Macdonald Technical and Scientific, London, 1971, pp. 28–44. [27] Y.H. Yu, C.Y. Lin, J.M. Yech, W.H. Lin, Polymer 44 (2003) 3553. [28] P.D. Garrett, D.T. Grubb, J. Polym. Sci. B—Polym. Phys. 26 (1988) 2509. [29] S.K. Mallapragada, N.A. Peppas, J. Polym. Sci. B—Polym. Phys. 34 (1996) 1339. [30] K. Yamamura, N. Kuranuki, M. Suzuki, T. Tanigami, S. Matsuzawa, J. Appl. Polym. Sci. 41 (1990) 2409. [31] M. Tsukada, G. Freddi, J.S. Crighton, J. Polym. Sci. B—Polym. Phys. 32 (1994) 243. [32] H.M. Zidan, J. Appl. Polym. Sci. 88 (2003) 1115.
Physica B 403 (2008) 2137–2142 www.elsevier.com/locate/physb
Spectroscopic studies on the effect of doping with CoBr2 and MgCl2 on some physical properties of polyvinylalcohol films E.M. Abdelrazek Department of Physics, Faculty of Science, Mansoura University, Mansoura 35516, Egypt Received 28 June 2007; received in revised form 26 November 2007; accepted 28 November 2007
Abstract Filled and unfilled polyvinylalcohol (PVA) films were prepared by the casting technique. Films of equal amounts with various concentrations of two fillers (CoBr2 and MgCl2) were prepared. Spectroscopic, structural and some physical properties of these films were studied with different techniques. Fourier-transform infrared (FTIR) revealed that the syndiotacticity structure of the PVA samples causes dense molecular packing in the crystal and also stronger intermolecular hydrogen bonds, which are responsible for the disappearance of the molecular motion. X-ray diffraction (XRD) scans evidenced the presence of some semicrystalline structure of PVA films. The optical absorption spectra suggested the presence of an optical gap (Eg), which depends on filler concentration for all the filling levels. Differential thermal analysis (DTA) suggests that the segmental mobility of an amorphous pure PVA increases as a result of the addition of mixed fillers, becoming less-rigid segments. This indicates that the mixed fillers act as plasticizers. r 2007 Elsevier B.V. All rights reserved. Keywords: X-ray; FTIR; Optical absorption; DTA; ESR
1. Introduction Considerable effort has been recently devoted to preparation and characterization of polymers. This may be because, these materials are considered to be important due to their useful and important applications due to the role of OH group and hydrogen bonds [1]. Furthermore, it can be used as a medical material due to its compatibility to the living body [2]. Like a hydrogen-bonded polymer, polyvinylalcohol (PVA) is used in gas sorption and diffusion [3]. Moreover, PVA can selectively adsorb metal ions such as copper, palladium and mercury [4]. PVA is a polymer with a carbon chain backbone attached with hydroxyl groups. These OH groups can be a source of hydrogen bonding and hence assist in the formation of polymer complexes [5,6]. The change in the degree of crystallinity and lamellar thickness distribution of PVA during dissolution process can be determined by differential thermal analysis (DTA) and Fourier-transform infrared (FTIR) studies [7]. Recent neutron diffracTel.: +20 106192689; fax: +20 502246781.
E-mail address: [email protected] 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.11.029
tion analysis and computer modelling proposed more detailed crystal structure for PVA including intermolecular and intramolecular hydrogen bondings [8]. Irradiation with X-ray diffraction (XRD) and g-ray radiation has a significant effect on the polymer properties and some physical properties that are usually modified. In addition, the electron spin resonance (ESR) study of g-irradiated PVA was investigated and the spectra were assigned [9]. The metalion-doped polymers represent a new class of organic materials. Inorganic additives such as transition metal salts have a considerable effect on the structural, optical, electrical and magnetic properties of the polymer [10]. The present work is mainly focused on the effect of equal amounts of Mg and Co ions dopant addition on the structural, optical, thermal and ESR of PVA films.
2. Experimental 2.1. Samples preparation The PVA films used in this work were obtained in pellet form from Merck, Germany and had molecular weight of
ARTICLE IN PRESS E.M. Abdelrazek / Physica B 403 (2008) 2137–2142
2138
14 000. PVA films with different equal amounts of CoBr2 and MgCl2 dopant were prepared by the solvent-casting technique. A known quantity of PVA pellets was added to doubly distilled water and kept for 48 h to swell the pellets, with stirring the solution at 70 1C for complete dissolution. CoBr2 and MgCl2 were also dissolved in doubly distilled water. The solution of the mixed fillers was added to the polymer at a suitable viscosity. The solution was poured onto cleaned Petri dishes and dried in an oven at 50 1C. After drying, the films were peeled from the Petri dishes and kept in vacuum desiccators until use. The thickness of the films was in the range of 100–150 mm. PVA films were doped with equal mass fractions (W) of CoBr2 and MgCl2 of 0, 5, 10, 15, 20, 25 and 30 wt%.
2.2. Physical measurements The FTIR measurements were carried out using the single-beam FTIR Spectrometer (FTIR-430, Jascow, Japan). The FTIR spectra of the samples were obtained in the spectral range of 400–2000 cm1 with scanning speed of 2 mm/s. The XRD scans were obtained using DIANO corporation—USA equipped with CuKa radiation (l ¼ 1.79026 A˚, the tube operated at 30 kV, the Bragg angle (2y) in the range of 5–501, step size ¼ 0.1 and step (CoBr2 + MgCl2)%
30
time 1 s). Ultraviolet and visible (UV/vis) absorption spectra were measured in the wavelength region of 200–600 nm using a spectrometer (Perkin–Elmer UV/vis). The DTA for the prepared films was carried out using an equipment type (GDTD16-Setaram) with measuring temperature range from room temperature to 300 1C and heating rate 5 1C/min. 3. Results and discussion 3.1. FTIR analysis To identify the different structure groups in the polymer matrices, FTIR studies have been carried out. The FTIR spectra of pure PVA and the PVA treated with two mixed fillers CoBr2 and MgCl2 are shown in Fig. 1. A slight variation was observed in the absorption bands of the filled PVA compared with pure PVA. The assignment of these bands for all samples under study is given in Table 1. Table 1 Assignment of the IR characterizing peaks of the PVA system [11,15] Frequency (cm1)
Assignment
1715 1662 1563 1438 1385 1240 1110 925 879 803 575 465
ns(CQO) Absorbed H2O O–H and C–H bending d(CH2) nw(CH2) nw(CH) ns(CO) nr(CH2) ns(CC) ns(CH2) C–Br Mg–O
ns=stretching; d=bending; nw=wagging and nr=rocking.
Absorption %
25
20
Isy (a. u.)
15 10
5
0 2000
1600
1200 Wavenumber Fig. 1.
800 (cm-1)
400
0
5
10
15 W (wt%) Fig. 2.
20
25
30
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The vibrational peaks found in the range 800–600 cm1 can be attributed to n(C–Cl) and M–O, where M ¼ Co or Mg, which indicate that the mixed fillers are complexed with the polymer matrix [11]. The appearance of a band at 1563 cm1 is indicative of the formation of small conjugated polyene sequences, which are presumably responsible for the color of the mixed fillers-treated PVA. These conjugated polyene sequences are suitable sites for the polaron and bipolaron formation [12]. The absorption peak at about 925 cm1 was found to be characteristic of the syndiotactic structure of the prepared films. The PVA
Intensity (a. u.)
(CoBr2 + MgCl2)%
0
structural deformations due to the filling can be calculated by plotting the intensity (Isy) of the absorption peak at 925 cm1 and can be taken as a measure of the syndiotacticity of the PVA [13]. The filling-level (FL) dependence of Isy is shown in Fig. 2. It is clear that Isy increases as W increases up to 15%. This indicates an increase in syndiotacticity of PVA. Nagura et al. [14] reported that the increase in the syndiotacticity of the PVA samples causes dense molecular packing in the crystal and also stronger intermolecular hydrogen bonds, which are responsible for the disappearance of the molecular motion. Beyond W ¼ 15%, Isy decreases as W increases. It is noticed that the syndiotacticity decreases. From the IR spectra, it is observed that upon increasing the mixed fillers CoBr2 and MgCl2 concentrations some of the peaks are shifted and some of them disappear with respect to the pure PVA. These results manifested the conclusion about the specific interaction in polymer matrices and hence the occurrence of complexation [16]. 3.2. XRD The XRD scans for PVA films, filled with various mixed filler fractions (W) of equal amounts of CoBr2 and MgCl2, are presented in Fig. 3. The observed spectra characterize a semicrystalline polymer possessing crystalline peaks at a scattering angle of (211p2yp231) corresponding to a (1 0 1) spacing [17]. The intensity of the (1 0 1) peak can be taken as a measure of the degree of crystallinity (Cn) and plotted as a function of the FL, as shown in Fig. 5. It is clear that Cn exhibits a minimum at W ¼ 10% and a maximum at W ¼ 15%. This finding creates, to some extent, a tuning event that a maximum syndiotactic structure (see Fig. 2) corresponds to a higher degree of crystallinity (Fig. 5) that occurs at same value of the filler concentration W ¼ 15%.
5 10 15 20 25 30
10
15
20
25 30 2θ (degree)
35
40
45
50
Fig. 3.
300
Pure CoBr2
Intensity
Pure MgCl2 (wt.%)
Intensity (a.u.)
5
2139
200
100
10
20 2θ (Degree)
30
40
Fig. 4.
10
20 30 2θ θ (Degree)
40
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7 6
Cn (a. u.)
5 4 3 (CoBr2 + MgCl2)%
2
0
0
10
20 W (wt%)
30
40
Fig. 5.
It is remarkable, in Fig. 3, that there is a small scattering peak at 2y ¼ 9.61 for W ¼ 25%. This peak belongs neither to PVA nor to CoBr2 and/or MgCl2 crystals. Fig. 4(a, b) depicts the X-ray scans for CoBr2 and MgCl2 crystals. It is clear that the peaks of Fig. 4(a, b), characterizing CoBr2 and MgCl2 crystals, are not observed in the spectra of Fig. 3. This indicates that the filled PVA films either are free of CoBr2 and/or MgCl2 crystals or contain only traces of these crystals for them to be detected by X-ray analysis (of the present technique). However, this peak may indicate the appearance of a new crystalline phase of the filled samples [18]. The present results imply that the XRD profile is substantially influenced by the FL (Fig. 5).
Absorbance (a.u.)
1
30
25
20
15 10 5 0 200
250
300
3.3. UV/vis optical absorption
500
550
600
Fig. 6.
The room-temperature optical absorption spectra, in the UV/vis region (200–600 nm), of the unfilled PVA and PVA filled with various concentrations of MgCl2 and CoBr2 are shown in Fig. 6. A sharp visible absorption peak is noticed at about 208 nm, indicating the presence of unsaturated bonds, of the type CQO and/or CQC mainly found in the tail–head of the polymer [19]. The band appearing as a shoulder at about 277 nm for Wp15% arising from (CH3)2CQO [20], is attributed to the electronic transition 4 A2g-4T1g(P) [21]. In other words, the band that appeared at 277 nm, with different absorption intensities, may be assigned to p–p*, which comes from unsaturated bonds, mainly CQO and/or CQC. A weak (and broad) visible absorption band is noticed at around 490 nm for 10%pWp30%. This band is due to the 4T1g(F)-4T1g(P) transition for the hexaquocobalt(II) [22]. Fig. 7(a, b) presents the photon energy (hn) dependence of (ahn)1/2, for PVA films with various filling levels, where h is Planck’s constant, u is the photon frequency and the absorption coefficient a(n) was calculated [23] using aðnÞ ¼ ln½t1 ðnÞ=t2 ðnÞ=ðd 2 d 1 Þ;
350 400 450 Wavelength (nm)
(1)
where t1, t2 and d1, d2 are transmittances and thicknesses of both the sample and reference sample, respectively. It is noticed that most of the curves of Fig. 7(a, b) are characterized by linear regions, indicating optical gap Eg of values given by [24] aðn ¼ÞBðhn E g Þ2 =hn,
(2)
where B is a constant. The filling level dependence of Eg is plotted in Fig. 8. Eg experiences a peak value around W ¼ 15%, assuming that Eg to be influenced by the induced states due to MgCl2 and/or CoBr2 filling of PVA. This may be attributed to the change of the filling mode. The most remarkable change in Eg was found at W415%. The decrease of Eg during incorporation of the inorganic compounds into the photopolymer matrix on the interfaces, there occur the nano- sheets forming additional nano-confined states [25], which substantially influence the observed optical features of the materials. In particular, there occurs substantial electron–phonon anharmonic interaction, which change the observed susceptibilities. It is remarkable that there is a coincidence between the filling
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0% 5% 10% 15%
40 35 (αhυ)1/2
30
2141
(CoBr2 + MgCl2)% 30
25 25
20 15 10 5
20 0
1
2
3 4 hυ (eV)
5
6
7 exo
0
15
60 10
endo
20% 25% 30%
50
(αhυ)1/2
40 5
30 20 10 0
0
1
2
3 4 hυ (eV)
5
6
0
7
Fig. 7.
100 5.2
5 Eg (eV)
300
Fig. 9.
5.1
4.9 4.8 4.7 4.6
200 Temperature °C
0
5
10
15 20 W (wt%)
25
30
35
Fig. 8.
levels at W ¼ 15% corresponding to the maximum values of Eg and the intensities of the IR bands at 925 cm1. This reveals a compatibility between the origin of the optical and IR response of the present PVA system. 3.4. Differential thermal analysis (DTA) PVA consists of two inextricably mixed phases: the crystalline solid and amorphous glass [26]. This results in
quite a complex behaviour when the polymer is heated. In the present work DTA traces of pure and filled PVA with different fractions of CoBr2 and MgCl2 are shown in Fig. 9. From this figure we observe that the pure PVA film displayed four phase transitions at: 50, 83, 200 and 260 1C (Table 2). The transition appears as a shoulder at about 50 1C, preferably attributed to the glass transition (Tg) relaxational process resulting from the micro-Brownian motion of the main chain backbone. It is observed from Table 2 that the position of Tg for the PVA films filled with CoBr2–MgCl2 mixed fillers is shifted toward lower temperatures compared with that of the pure PVA [27]. This suggests that the segmental mobility of an amorphous pure PVA increases as a result of the addition of mixed fillers, becoming less-rigid segments. This indicates that the mixed fillers CoBr2 and MgCl2 act as plasticizers. There is another broad and shallow endothermic peak at about 83 1C which is attributed to the a-relaxation associating the crystalline regions. Similar transitions were seen by Garrett and Gurbb [28] at 85 1C. The magnitude of Tg of the pure PVA is greater than those for the filled samples but the
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Table 2 DTA results of pure PVA and mixed-fillers-doped PVA films W (wt%)
Tg (1C)
Ta (1C)
Tm (1C)
Td (1C)
0 5 10 15 20 25 30
50 – 48 – 44 – 45
83 85 94 96 96 78 73
200 196 192 189 188 185 185
260 254 252 253 248 243 240
a-relaxation temperature Ta is smaller for 5%pWp20%. This may be due to the greater crystallinity or formation of another crystalline phase in filled samples. The greater breadth of the relaxations of filled samples can be attributed to the wider range of crystallite sizes and morphology produced [28]. On the other hand, the magnitude of Ta of the pure PVA is greater than those for the filled samples for W ¼ 25% and 30%. The endothermic peak at 200 1C in pure PVA has been attributed to the melting point of PVA [29–31]. A reduction in the Tm values of PVA crystals was evident with an increase in W of CoBr2 and MgCl2. Also, the magnitude of thermal degradation temperature (Td) of the pure PVA is greater than that of the filled samples. It is suggested that the addition of the mixed fillers to PVA films decreases the thermal stability [32]. 4. Conclusions From an analysis of IR and DTA curves and XRD scans of filled PVA films, it may be said that the structure changes took place after the filling with CoBr2 and MgCl2 as a result of defect formation. From the IR curves, the appearance of a band at 1563 cm1 is indicative of the formation of small conjugated polyene sequences which are presumably responsible for the color of the mixed fillers treated PVA. From the DTA curves, we noticed that an increased filler content resulted in a decrease in Tg, Tm and Td of PVA films, which indicated that the filler acted as a plasticizer. The X-ray analysis showed no significant peaks characterizing CoBr2 and/or MgCl2 crystals. The intensity of the (1 0 1) peak of the main phase, detected by XRD, decreased exponentially with increasing Wp10%. It was observed that the intensity of the (1 0 1) peak of the main phase could be used as a measure of the degree of crystallinity (Cn) with a maximum value at W ¼ 15%, which agreed well with the IR findings. Because of their similarity, the W dependence of the intensity (Isy) of the absorption peak at 925 cm1 can be taken as a measure of the syndiotacticity of the PVA. From the UV/visible spectra, it is remarkable that there is a coincidence between
the filling levels at W ¼ 15% corresponding to the maximum values of Eg and the intensities of the IR bands at 925 cm1. This reveals compatibility between the origin of the optical and IR response of the present PVA system.
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