FTIR studies of chitosan acetate based polymer electrolytes

Transcription

1 Electrochimica Acta 48 (2003) 993/999 FTIR studies of chitosan acetate based polymer electrolytes Z. Osman, A.K. Arof * Physics Department, University of Malaya, Kuala Lumpur, Malaysia Received 29 November 2002 Abstract Chitosan is the product when partially deacetylated chitin dissolves in dilute acetic acid. As such, depending on the degree of deacetylation, the carbonyl, C/O/NHR band can be observed at /1670 cm 1 and the amine, NH 2 band at 1590 cm 1. When lithium triflate is added to chitosan to form a film of chitosan acetate /salt complex, the bands assigned to chitosan in the complex and the spectrum as a whole shift to lower wavenumbers. The carbonyl band is observed to shift to as low as 1645 cm 1 and the amine band to as low as 1560 cm 1. These indicate chitosan/salt interactions. Also present are the bands due to lithium triflate i.e. /761, 1033, 1182 and 1263 cm 1. When chitosan and ethylene carbonate (EC) are dissolved in acetic acid to form a film of plasticized chitosan acetate, the bands in the infrared spectrum of the films do not show any significant shift indicating that EC does not interact with chitosan. EC/LiCF 3 SO 3 interactions are indicated by the shifting of the C/O bending band from 718 cm 1 in the spectrum of EC to 725 cm 1 in the EC/salt spectrum. The Li /EC is also evident in the ring breathing region at 893 cm 1 in the pure EC spectrum. This band has shifted to 898 cm 1 in the EC/salt spectrum. C/O stretching in the doublet observed at 1774 and 1803 cm 1 in the spectrum of pure EC has shifted to 1777 and 1808 cm 1 in the EC/salt spectrum. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: FTIR; Chitosan; Ethylene carbonate; Lithium triflate; Acetic acid 1. Introduction Plasticized polymer /lithium salt electrolytes have been given a lot of attention [1/5], due to their potential application in solid state batteries [5 /7]. The addition of plasticizers such as propylene carbonate (PC) and ethylene carbonate (EC) to polymer electrolytes reduces the glass transition temperature, T g [8,9]. The reduction in T g softens the polymer backbone and increases segmental motion resulting to an enhancement in conductivity. The presence of ion pairs and clusters in solid polymer electrolytes has been confirmed by Raman and Fourier Transform Infrared (FTIR) spectroscopic * Corresponding author. Tel.: / ; fax: / address: akarof@um.edu.my (A.K. Arof). measurements [3,8 /12]. The spectrum of lithium triflate consists of free ions, monodentate ion pairs i.e. LiTf, LiTf 2 and LiTf 2 3 and also Li 2 Tf aggregates. The effect of coordination on the SO 3 symmetric and asymmetric stretching modes of the triflate anion can be substantial. Ion association occurs at the SO 3 end of the anion [13/15]. Chitosan is a biopolymer and has variety of uses [16/ 18]. The amine, NH 2 group in its structure can act as electron donors and interact with inorganic salts. Interaction between a lithium cation and a nitrogen donor has been proven by X-ray photoelectron spectroscopy [19]. EC has been used as a plasticizing agent in chitosan-based electrolytes to improve electrical conductivity [20]. The FTIR spectrum provides information through band properties, frequencies and intensities and can, therefore, be used to predict chemical processes, identify species and determine the increase in the /02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. doi: /s (02)

2 994 Z. Osman, A.K. Arof / Electrochimica Acta 48 (2003) 993/999 Table 1 The bands in the ethylene carbonate (EC) spectrum Vibrational modes Wavenumbers (cm 1 ) References C/O stretching 1810/1871; 1776 and 1803; 1773 and [23,37,39] 1798 CH 2 bending 1480 [23] CH 2 wagging 1394 and 1420 [38] Skeletal stretching 970, 1076, 1180 [23] Ring breathing 1067, 890 [38] C/O bending 717 [24] number of certain entities from the increase in the area of the band. It is the intention of this work to use FTIR spectroscopy to prove complexation between chitosan and the lithium salt and to investigate the effect of the plasticizer, EC on the chitosan /lithium triflate based electrolytes with the hope to gain some insight on the conductivity enhancement of the electrolyte. 2. Experimental 2.1. Sample preparation Chitosan (1 g, 6/10 5 g mol 1, Fluka) was dissolved in 100 ml 1% acetic acid solution. Lithium triflate (LiCF 3 SO 3 ) and EC were added accordingly. After complete dissolution, the solutions were cast in petri dishes and left to form films of pure chitosan acetate (CA), CA/EC, CA/LiCF 3 SO 3 and CA/EC/LiCF 3 SO 3 at room temperature. Pure chitosan film was prepared by immersing the CA film in NaOH solution and then continuously washed with distilled water. The films were then transferred into a desiccator for continuous drying FTIR spectroscopy measurements Infrared spectra exhibited in this work were taken with a MAGNA-IR550 Spectrophotometer-Series II in the wavenumber region between 4000 and 400 cm 1. The films used in this work were cut into suitable sizes and placed in the specimen holder of the spectrophotometer. The spectra shown in this paper was the result of 20 scans at the speed of 1 scan per 2 s. In the present work, the infrared spectra of LiCF 3 SO 3, EC, pure CA and pure chitosan were also taken to serve as reference. The resolution of the spectrophotometer was 1cm 1. Fig. 1. The FTIR spectra of (a) pure chitosan and (b) pure CA. 3. Results and discussion In this work, the bands due to lithium triflate are obtained at 766 cm 1 (d s (CF 3 )), 1033 cm 1 (y s (SO 3 )), 1182 cm 1 (y as (CF 3 )), 1230 cm 1 (y s (CF 3 )) and 1263 cm 1 (y as (SO 3 )). The detailed assignment of these bands has been discussed in the literature [9,10,15,21,22]. The bands in the EC spectrum as obtained from literature are listed in Table 1. In the present work, the bands in the EC spectrum are obtained at 975, 1078, 1185, 1395, 1480, 1774, 1803 cm 1. In order to determine the existence of interactions among the components in chitosan-based electrolytes, the FTIR spectra of pure chitosan, pure CA, CA/EC, CA/LiCF 3 SO 3 and CA/ EC/LiCF 3 SO 3 were recorded and analyzed, respectively. Fig. 1 depicts the spectra of pure chitosan and pure CA in the region between 1400 and 1700 cm 1. This is the region where the carbonyl, C/O/NHR, amine, NH 2 and ammonium, NH 3 bands are situated. The carbonyl, C/O/NHR band observed at 1650 cm 1 [26/31],

3 Z. Osman, A.K. Arof / Electrochimica Acta 48 (2003) 993/ Fig. 2. The FTIR spectra of CA with (a) 10 wt.% LiCF 3 SO 3 ; (b) 23 wt.% LiCF 3 SO 3 ; (c) 33 wt.% LiCF 3 SO 3 in the region 1500/2000 cm 1. the amine, NH 2 band at 1590 cm 1 [26,28/34] and the ammonium NH 3 band appears as a small shoulder at /1514 cm 1 [15,32,35,36]. The absence of the NH 3 band in the pure CA spectrum is probably due to interaction between NH 3 of chitosan and the /COO/ of the acetic acid to form the O/C/NHR band. This reaction is expected to happen when the CA film was stored in a desiccator for some time prior to infrared experiment. This observation and its explanation have been reported by Ritthidej and co-workers [15]. In this work, the carbonyl band of the chitosan spectrum can be observed at 1667 cm 1 and the amine (NH 2 ) band at 1590 cm 1, Fig. 1(a). The carbonyl band has shifted to 1653 cm 1 and the amine band to 1570 cm 1 in the chitosan-acetic acid salt film (CA), Fig. 1(b). The shift in the pure CA spectrum indicates that some interaction have occurred between the acetic acid and the nitrogen donors of the chitosan polymer. Fig. 2 depicts the carbonyl and amine bands of salted CA with various concentrations of LiCF 3 SO 3 ((a) 10 wt.%, (b) 23 wt.% and (c) 33 wt.% salt). The amine band has shifted about 10 cm 1 (to 1560 cm 1 ) on addition of salt compared with pure CA. The carbonyl band has further shifted by /7 cm 1 (to 1646 cm 1 ) compared with pure CA. In the spectrum of the sample containing 10 wt.% salt, the full width at half maximum (FWHM) for the amine band is slightly wider than that of the carbonyl band (the maximum height is taken from the peaks of the carbonyl and amine bands, respectively, to their meeting point at 1875 cm 1. On addition of 23 wt.% salt, the peak of the carbonyl band is higher than that of the amine band. The FWHM for the amine band is still greater than that of the carbonyl band. The same situation is still observed in Fig. 2(c) and while these observations indicate salt /chitosan interaction, interaction prefers to take place at the amine site. Fig. 3 shows

4 996 Z. Osman, A.K. Arof / Electrochimica Acta 48 (2003) 993/999 Fig. 3. The FTIR spectra of CA with (a) 10 wt.% LiCF 3 SO 3 ; (b) 23 wt.% LiCF 3 SO 3 ; (c) 33 wt.% LiCF 3 SO 3 in the region of CF 3 deformation mode. the band /761 cm 1. This band is attributed to CF 3 deformation mode of the salt. This band increases in intensity as more salt is added and shifts to higher wavenumbers. This again provides evidence of CA /salt interaction. The increase in intensity implies the increase in the number of free ions from the salt, which could explain the increase in conductivity of samples. Fig. 4 represents the spectra of pure CA and plasticized CA films with increasing content of EC (up to 57 wt.%) in the region between 700 and 1850 cm 1. There is hardly any noticeable shift in peaks of the plasticized CA films as presented in this figure. Hence, the CA/EC system is a mixed phase with no interaction with one another. Fig. 5(a) represents the infrared spectrum of pure EC and EC mixed with salt in the weight ratio 1:1. It can be observed that the C/O bending band which appears at 718 cm 1 in the spectrum of pure EC has shifted to 725 cm 1 in the spectra of EC-salt. This is assigned to the interaction between Li of the salt and C/O bending band of the EC molecule. This result is in well agreement that reported by Chintapalli [24]. Fig. 5(b) depicts the Li /EC interaction, which is also evident in the ring Fig. 4. The FTIR spectra of (I) pure CA and CA with (II) 10 wt.% EC (III) 33 wt.% EC (IV) 57 wt.% EC. breathing region at /893 cm 1 in the pure EC spectrum, and has shifted to 898 cm 1 in the EC/ LiCF 3 SO 3 spectrum. This in agreement with the result of Starkey and Frech [25] where it was predicted that the Li coordinates to the three oxygen atoms in PC. The doublet bands due to the C /O stretching, Fig. 5(c) observed at 1774 and 1803 cm 1 in the spectrum of pure EC have shifted to 1777 and 1808 cm 1, respectively, in the EC/LiCF 3 SO 3 spectra. The infrared spectra of CA/EC/LiCF 3 SO 3 are shown in Fig. 6. The features have been described before are also observed in the spectra of these samples containing different concentrations of salt. At low salt concentrations, 5 wt.% (Fig. 6(a)) the doublet peaks of

5 Z. Osman, A.K. Arof / Electrochimica Acta 48 (2003) 993/ Fig. 5. The FTIR spectra of (I) pure EC and (II) EC mixed with LiCF 3 SO 3 in the region of (a) C/O bending; (b) ring breathing; and (c) C/O stretching bands. EC at 1774 and 1803 cm 1 are not readily observed. As the salt content added reaches 20 wt.% (Fig. 6(c)) small bands are observed at 1773 and 1802 cm 1. The intensity of these bands increases as the salt concentration increases to 40 wt.%, Fig. 6(e) at 1774 and 1803 cm 1. Likewise the band at 761 cm 1 only exist as a small band in Fig. 7(a) the intensity increases with salt content as can be observed in Fig. 7(b/e). This again could be evidence for the increase in the number of free ions, which are responsible for ionic conductivity. wavenumbers. There is no interaction between plasticizer, EC and chitosan polymer but there is interaction between the salt and the plasticizer at the C/O bending and stretching modes and the ring breathing region. The increase in intensity of the CF 3 deformation mode of the salt in the spectrum implies that the number of free ions due to salt dissociation has increased which resulted in the increase of conductivity on addition of the plasticizer. 4. Conclusions The lithium triflate salt interacts with the chitosan polymer to form a CA/LiCF 3 SO 3 complex as shown by the shift in the carbonyl and amine bands to lower Acknowledgements Z.O. would like to thank the Ministry of Science, Technology and Environment for the scholarship awarded and A.K.A. thanks the government of Malaysia for the vote

6 998 Z. Osman, A.K. Arof / Electrochimica Acta 48 (2003) 993/999 Fig. 6. The FTIR spectra of (a) CA/EC/5 wt.% LiCF 3 SO 3 ; (b) CA/EC/15 wt.% LiCF 3 SO 3 ; (c) CA/EC/20 wt.% LiCF 3 SO 3 ; (d) CA/EC/30 wt.% LiCF 3 SO 3 ; (e) CA/EC/40 wt.% LiCF 3 SO 3 in the region of C/O stretching band. Fig. 7. The FTIR spectra of (a) CA/EC/5 wt.% LiCF 3 SO 3 ; (b) CA/EC/15 wt.% LiCF 3 SO 3 ; (c) CA/EC/20 wt.% LiCF 3 SO 3 ; (d) CA/EC/30 wt.% LiCF 3 SO 3 ; (e) CA/EC/40 wt.% LiCF 3 SO 3 in the region of CF 3 deformation mode.

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