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Proton-conducting polymer electrolyte films based on chitosan acetate complexed with NH4NO3 salt
ARTICLE IN PRESS
Physica B 355 (2005) 78–82 www.elsevier.com/locate/physb
Proton-conducting polymer electrolyte films based on chitosan acetate complexed with NH4NO3 salt S.R. Majid, A.K. Arof Physics Department, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia Received 7 August 2004; received in revised form 13 October 2004; accepted 13 October 2004
Abstract Chitosan acetate-ammonium nitrate (NH4NO3) films have been prepared by the solution-cast technique. Fourier transform infrared spectroscopy (FTIR) showed that complexation has occurred. FTIR exhibited shifts in amine and carbonyl bands from 1553 to 1520 cm1 and 1636 to 1617 cm1. A new peak was also observed at 1746 cm1. XRD shows that all complexes are amorphous. The highest conductivity at room temperature is 2.53 105 S cm1 for the film containing 45 wt% NH4NO3. The conductivity of the samples is dependent on the number of mobile ions and mobility. r 2004 Elsevier B.V. All rights reserved. PACS: 66.10.Ed; 71.20.Rv; 74.25.Fy Keywords: Chitosan; NH4NO3; Conductivity; Infrared spectroscopy; Mobility
1. Introduction Polymer–salt complexes have received a lot of attention due to their possible application as solid electrolytes in electrochemical devices [1–3]. However, there are not many studies on proton conductors because of the difficulty in establishing the transportation of protons [4]. Polymers that have been used in the making of proton-conductCorresponding
author. Tel.: +60 3 7967 4085; +60 3 7967 4146. E-mail address: [email protected] (A.K. Arof).
fax:
ing films include polyethylene oxide (PEO) [5–7], polyacrylic acid (PAA) [8] and polyvinyl alcohol (PVA) [9]. These have been complexed with various salts, which provide the ions for conduction. Several ammonium salts such as NH4I [5], NH4SO3CF3 [6] and (NH4)SCN [7] were used as doping salts. In this work, chitosan has been used as the base polymer for the studies on proton-conducting films. Chitosan is stable in neutral conditions since the amine and hydroxyl groups on the glucosamine unit can form strong inter- and intramolecular hydrogen bond to crystallize [10].
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.10.025
ARTICLE IN PRESS S.R. Majid, A.K. Arof / Physica B 355 (2005) 78–82
CH2OH O O
OH
NH2
n
Fig. 1. Chitosan.
XRD, FTIR and conductivity studies were carried out to characterize the chitosan–salt complexes and an electrochemical cell was fabricated using the chitosan–salt complex as the electrolyte of the cell. Fig. 1 illustrates molecular structure of segments of chitosan.
2. Experimental 2.1. Sample preparation Chitosan films were prepared from highly viscous powder supplied by Fluka with MW=600,000 g mol1 by the following method: 1 g of chitosan was dissolved in 100 ml of 1% acetic acid solution. The chitosan acetate solution was then cast into plastic Petri dishes and left to dry at room temperature to obtain chitosan acetate (CA) films. To prepare chitosan films complexed with NH4NO3, the salt was added to the CA solution. The salt concentration was varied from 10 to 50 wt%. After casting the solutions were left to dry at room temperature to form the film. 2.2. Characterization 2.2.1. Fourier transform infrared spectroscopy (FTIR) FTIR spectra were recorded on a Perkin Elmer Rx-1 Spectrophotometer with 1 cm1 resolution in the transmission mode from wave numbers 400 to 4000 cm1. FTIR was performed to confirm the occurrence of complexation.
79
2.2.2. X-ray diffraction (XRD) XRD patterns were recorded on a Siemens D5000. Samples were cut into a suitable size and then adhered onto a glass slide. The glass slide was then placed in the sample holder of the diffractometer and the samples were directly scanned at 2y angles between 51 and 801. XRD was performed to determine the nature of the films whether crystalline, amorphous or both. 2.2.3. Electrical impedance spectroscopy (EIS) Electrical impedance spectroscopy was measured using the HIOKI 3522-01 LCR Hi-Tester that was interfaced to a computer in frequency range 50 Hz–1 MHz. When the films were formed they were cut into a suitable size and placed between the blocking stainless steel electrodes of a conductivity cell which are connected by leads to a computer. The bulk impedance (Rb ) value was obtained from the plot of negative imaginary impedance versus real part of impedance and the conductivity of the sample was calculated from the equation below s¼
t : Rb A
Here A is the area of film–electrode contact and t is the thickness of the film.
3. Results and discussion 3.1. FTIR spectroscopy Fig. 2 shows the spectra of CA-NH4NO3 complexes in region from 400 to 2000 cm1. In the present work, the amine band of pure chitosan acetate film appears at the 1553 cm1 and the carbonyl band at 1636 cm1 [11–13]. When chitosan has dissolved in acetic acid, a chitosan–acetic acid salt or chitosan acetate is formed. It has been reported [14] that the cation of the acetic acid will interact with the nitrogen atom of the amine group. This will shift the amine and other bands as well. In some reports, the band at 1553 cm1 is assigned to asymmetrical COO stretching of the carboxylic group in the acetic acid [11,15]. The assignment of the 1553 cm1 to the asymmetrical
ARTICLE IN PRESS S.R. Majid, A.K. Arof / Physica B 355 (2005) 78–82
80
1746
(e)
(c)
Intensity (a.u)
1520
1617
Transmittance (a.u)
1746
(d)
(f) (e) (d)
(b)
(c)
1636 1553
(b) (a)
(a)
5
30
55
80
2 theta 2000
1600
1200
800
400
Wave number (cm-1)
Fig. 2. FTIR spectra of (a) pure CA film (b) CA+35 wt% AN (c) 40 wt% AN (d) 45 wt% AN and (e) 50 wt% AN.
COO stretching indicates that chitosan and acetic acid have formed a chitosan–acetic acid salt. Upon addition of NH4NO3 salt, the gap between the carbonyl and amine bands is observed to increase up to 97 cm1. A new peak is observed at 1746 cm1. This peak is observed to grow with increase in NH4NO3. Hashmi et al. [4] has proven that in the PEO-NH4ClO4 system, the conducting species is H+ ion. The H+ ion originates from the ammonium ion. The conduction occurs through structure diffusion (Grotthus mechanism), i.e., the conduction occurs through the exchange of ions between complexed sites. 3.2. X-ray diffraction (XRD) Fig. 3 shows the XRD patterns of salted CA complexes. Clearly, Figs. 3(a–e) shows that chitosan/ NH4NO3 form amorphous electrolytes.
Fig. 3. XRD patterns of (a) pure CA film (b) CA+35 wt% AN, (c) CA+40 wt% AN (d) 45 wt% AN (e) 50 wt% AN and (f) pure NH4NO3.
3.3. Electrical impedance spectroscopy (EIS) Figs. 4(a) and (b) show the variation of room temperature conductivity, sRT and the activation energy, E a ; as a function of NH4NO3 concentration. It can be observed that the conductivity at room temperature is 2.53 105 S cm1 with 45 wt% salt. The ionic conductivity in a polymer is generally linked to the number of ions and the mobility of conducting species in the polymer complexes. The Rice and Roth model [16] hypothesized that in an ionic conductor there is energy gap E above which conducting ions of mass m can be thermally excited from localized ionic states to free ion- like states in which the ion propagates throughout the solid with velocity, n. Such an excited free ion-like state has a finite life time t. The velocity is given by n ¼ ð2E=mÞ1=2 and the ‘mean free path’ or distance from one
ARTICLE IN PRESS
Conductivity (Scm-1)
S.R. Majid, A.K. Arof / Physica B 355 (2005) 78–82
81
1.00E-04
complexed site to another, ‘; is given by ‘ ¼ nt: The ionic conductivity, s, can be expressed by
1.00E-05
s ¼ ½2ðZeÞ2 =3kTmnEt expðE=kTÞ:
1.00E-06
1.00E-07
1.00E-08
1.00E-09 0
10
(a)
20
30
40
50
60
Salt content wt. % 0.9
According to Okuyama et al. [17], the distance between two adjacent chitosan monomers is 109 m. Using ‘ ¼ 109 m; the value of t can be calculated. From the Arrhenius plot of ln sT versus 103/T, the activation energy can be obtained from the slope and the pre-exponential factor can be obtained from the intercept at the vertical axis. Thus the number density of the mobile ions, n, can be calculated. Table 1 lists some of the transport parameters in the CA complexes. The ionic mobility is defined as m ¼ s=nq
0.8
(2)
and the diffusion coefficient is given by D ¼ ðkT s=ne2 Þ:
Ea (eV)
0.7
0.6
0.5
0.4 0
(b)
(1)
10
20
30
40
50
60
(3)
The values of m calculated from the Rice and the Roth model lies between 108 and 106 cm2 V1 s. The value of mobility for proton conductors in the PEO+NH4ClO4 system [18] obtained by transient ionic current measurement is between 106 and 104 cm2 V1 s. The value of diffusion coefficient lies between 105 and 107 cm2 s1. Conductivity is therefore controlled by both number of mobile ions and mobility.
salt content (wt.%)
Fig. 4. (a) Variation of conductivity as a function of salt content at room temperature. (b) Variation of activation energy as a function of salt content.
4. Conclusions We have been able to prepare a proton conductor based on chitosan and NH4NO3.
Table 1 Transport parameter of CA complexes Sample
sRT (S cm1)
t (s)
m ðcm2 V1 sÞ
D ðcm2 s1 Þ
E a (eV)
Z (cm3)
E1 E2 E3 E4 E5 E6
9.43 109 4.34 108 4.81 107 6.02 106 2.53 105 3.75 106
8.16 1014 8.38 1014 8.84 1014 1.03 1013 1.08 1013 9.11 1014
3.50 108 8.00 108 7.50 107 4.00 106 5.30 106 2.10 106
9.00 1010 2.10 109 1.90 108 1.00 107 1.40 107 5.50 108
0.79 0.74 0.67 0.49 0.45 0.63
1.704 1018 3.40 1018 4.00 1018 9.50 1018 3.00 1019 1.10 1019
ARTICLE IN PRESS 82
S.R. Majid, A.K. Arof / Physica B 355 (2005) 78–82
The highest conductivity obtained was 2.53 105 S cm1 at room temperature. The transport parameters such as mobility, diffusion coefficient and number of mobile ions have been calculated using the Rice and Roth model. The conductivity is dependent on the number of mobile ions and mobility. Acknowledgements S.R. Majid thanks Skim Pascasiswazah for the scholarship awarded and IPPP for vot-f F0131/ 2003B which helped to support the work. References [1] J.R. Mac Callum, C.A. Vincent (Eds.), Polymer Electrolyte Reviews, Elsevier, Amsterdam, 1987. [2] N.M. Morni, N.S. Mohamed, A.K. Arof, Mater. Sci. Eng. B 45 (1997) 140. [3] D.S. Reddy, M.J. Reddy, U.V. Subba Rao, Mater. Sci. Eng. B 78 (2000) 59. [4] S.A. Hashmi, A. Kumar, K.K. Maurya, S. Chandra, J. Phys. D: Appl. Phys. 23 (1993) 1307.
[5] K.K. Maurya, N. Srivastava, S.A. Hashmi, S. Chandra, J. Mater. Sci. 27 (1992) 6357. [6] A.M.M. Ali, N.S. Mohamed, A.K. Arof, J. Power Sources 74 (1998) 135. [7] N. Srivastava, A. Chandra, S. Chandra, Phys. Rev. B 52 (1) (1995) 225. [8] A. Bozkurt, W.H. Meyer, G. Wegner, J. Power Sources 123 (2003) 126. [9] M.A. Vargas, R.A. Vargas, B.E. Mellander, Electrochim. Acta 45 (2000) 1399. [10] F. Mi, H. Sung, S. Shyu, C. Su, Polymer 44 (2003) 6521. [11] C. Peniche, C. Elvira, J.S. Roman, Polymer 39 (25) (1998) 6549. [12] X. Qu, A. Wirsen, A.C. Albertsson, Polymer 41 (2000) 4841. [13] H.Y. Kweon, I.C. Um, Y.H. Park, Polymer 42 (2001) 6651. [14] M. Rinaudo, G. Pavlov, J. Desbrieres, Polymer (1999) 7029. [15] C. Peniche, W.A. Monal, N. Davidenko, R. Sastre, A. Gallardo, J.S. Roman, Biomaterials 20 (1999) 1869. [16] M.J. Rice, W.L. Roth, J. Solid State Chem. 4 (1972) 29. [17] K. Okuyama, K. Noguchi, M. Kanenari, T. Egawa, K. Osawa, K. Ogawa, Carbohydr. Polym. 41 (2000) 237. [18] K.K. Maurya, S. A. Hasmi, S. Chandra, in: B.V.R Chowdari, S. Chandra, S. Singh, P.C. Srivastava (Eds.), Solid State Ionics: Materials and Applications, World Scientific, Singapore, 1992, pp. 573–577.
Physica B 355 (2005) 78–82 www.elsevier.com/locate/physb
Proton-conducting polymer electrolyte films based on chitosan acetate complexed with NH4NO3 salt S.R. Majid, A.K. Arof Physics Department, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia Received 7 August 2004; received in revised form 13 October 2004; accepted 13 October 2004
Abstract Chitosan acetate-ammonium nitrate (NH4NO3) films have been prepared by the solution-cast technique. Fourier transform infrared spectroscopy (FTIR) showed that complexation has occurred. FTIR exhibited shifts in amine and carbonyl bands from 1553 to 1520 cm1 and 1636 to 1617 cm1. A new peak was also observed at 1746 cm1. XRD shows that all complexes are amorphous. The highest conductivity at room temperature is 2.53 105 S cm1 for the film containing 45 wt% NH4NO3. The conductivity of the samples is dependent on the number of mobile ions and mobility. r 2004 Elsevier B.V. All rights reserved. PACS: 66.10.Ed; 71.20.Rv; 74.25.Fy Keywords: Chitosan; NH4NO3; Conductivity; Infrared spectroscopy; Mobility
1. Introduction Polymer–salt complexes have received a lot of attention due to their possible application as solid electrolytes in electrochemical devices [1–3]. However, there are not many studies on proton conductors because of the difficulty in establishing the transportation of protons [4]. Polymers that have been used in the making of proton-conductCorresponding
author. Tel.: +60 3 7967 4085; +60 3 7967 4146. E-mail address: [email protected] (A.K. Arof).
fax:
ing films include polyethylene oxide (PEO) [5–7], polyacrylic acid (PAA) [8] and polyvinyl alcohol (PVA) [9]. These have been complexed with various salts, which provide the ions for conduction. Several ammonium salts such as NH4I [5], NH4SO3CF3 [6] and (NH4)SCN [7] were used as doping salts. In this work, chitosan has been used as the base polymer for the studies on proton-conducting films. Chitosan is stable in neutral conditions since the amine and hydroxyl groups on the glucosamine unit can form strong inter- and intramolecular hydrogen bond to crystallize [10].
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.10.025
ARTICLE IN PRESS S.R. Majid, A.K. Arof / Physica B 355 (2005) 78–82
CH2OH O O
OH
NH2
n
Fig. 1. Chitosan.
XRD, FTIR and conductivity studies were carried out to characterize the chitosan–salt complexes and an electrochemical cell was fabricated using the chitosan–salt complex as the electrolyte of the cell. Fig. 1 illustrates molecular structure of segments of chitosan.
2. Experimental 2.1. Sample preparation Chitosan films were prepared from highly viscous powder supplied by Fluka with MW=600,000 g mol1 by the following method: 1 g of chitosan was dissolved in 100 ml of 1% acetic acid solution. The chitosan acetate solution was then cast into plastic Petri dishes and left to dry at room temperature to obtain chitosan acetate (CA) films. To prepare chitosan films complexed with NH4NO3, the salt was added to the CA solution. The salt concentration was varied from 10 to 50 wt%. After casting the solutions were left to dry at room temperature to form the film. 2.2. Characterization 2.2.1. Fourier transform infrared spectroscopy (FTIR) FTIR spectra were recorded on a Perkin Elmer Rx-1 Spectrophotometer with 1 cm1 resolution in the transmission mode from wave numbers 400 to 4000 cm1. FTIR was performed to confirm the occurrence of complexation.
79
2.2.2. X-ray diffraction (XRD) XRD patterns were recorded on a Siemens D5000. Samples were cut into a suitable size and then adhered onto a glass slide. The glass slide was then placed in the sample holder of the diffractometer and the samples were directly scanned at 2y angles between 51 and 801. XRD was performed to determine the nature of the films whether crystalline, amorphous or both. 2.2.3. Electrical impedance spectroscopy (EIS) Electrical impedance spectroscopy was measured using the HIOKI 3522-01 LCR Hi-Tester that was interfaced to a computer in frequency range 50 Hz–1 MHz. When the films were formed they were cut into a suitable size and placed between the blocking stainless steel electrodes of a conductivity cell which are connected by leads to a computer. The bulk impedance (Rb ) value was obtained from the plot of negative imaginary impedance versus real part of impedance and the conductivity of the sample was calculated from the equation below s¼
t : Rb A
Here A is the area of film–electrode contact and t is the thickness of the film.
3. Results and discussion 3.1. FTIR spectroscopy Fig. 2 shows the spectra of CA-NH4NO3 complexes in region from 400 to 2000 cm1. In the present work, the amine band of pure chitosan acetate film appears at the 1553 cm1 and the carbonyl band at 1636 cm1 [11–13]. When chitosan has dissolved in acetic acid, a chitosan–acetic acid salt or chitosan acetate is formed. It has been reported [14] that the cation of the acetic acid will interact with the nitrogen atom of the amine group. This will shift the amine and other bands as well. In some reports, the band at 1553 cm1 is assigned to asymmetrical COO stretching of the carboxylic group in the acetic acid [11,15]. The assignment of the 1553 cm1 to the asymmetrical
ARTICLE IN PRESS S.R. Majid, A.K. Arof / Physica B 355 (2005) 78–82
80
1746
(e)
(c)
Intensity (a.u)
1520
1617
Transmittance (a.u)
1746
(d)
(f) (e) (d)
(b)
(c)
1636 1553
(b) (a)
(a)
5
30
55
80
2 theta 2000
1600
1200
800
400
Wave number (cm-1)
Fig. 2. FTIR spectra of (a) pure CA film (b) CA+35 wt% AN (c) 40 wt% AN (d) 45 wt% AN and (e) 50 wt% AN.
COO stretching indicates that chitosan and acetic acid have formed a chitosan–acetic acid salt. Upon addition of NH4NO3 salt, the gap between the carbonyl and amine bands is observed to increase up to 97 cm1. A new peak is observed at 1746 cm1. This peak is observed to grow with increase in NH4NO3. Hashmi et al. [4] has proven that in the PEO-NH4ClO4 system, the conducting species is H+ ion. The H+ ion originates from the ammonium ion. The conduction occurs through structure diffusion (Grotthus mechanism), i.e., the conduction occurs through the exchange of ions between complexed sites. 3.2. X-ray diffraction (XRD) Fig. 3 shows the XRD patterns of salted CA complexes. Clearly, Figs. 3(a–e) shows that chitosan/ NH4NO3 form amorphous electrolytes.
Fig. 3. XRD patterns of (a) pure CA film (b) CA+35 wt% AN, (c) CA+40 wt% AN (d) 45 wt% AN (e) 50 wt% AN and (f) pure NH4NO3.
3.3. Electrical impedance spectroscopy (EIS) Figs. 4(a) and (b) show the variation of room temperature conductivity, sRT and the activation energy, E a ; as a function of NH4NO3 concentration. It can be observed that the conductivity at room temperature is 2.53 105 S cm1 with 45 wt% salt. The ionic conductivity in a polymer is generally linked to the number of ions and the mobility of conducting species in the polymer complexes. The Rice and Roth model [16] hypothesized that in an ionic conductor there is energy gap E above which conducting ions of mass m can be thermally excited from localized ionic states to free ion- like states in which the ion propagates throughout the solid with velocity, n. Such an excited free ion-like state has a finite life time t. The velocity is given by n ¼ ð2E=mÞ1=2 and the ‘mean free path’ or distance from one
ARTICLE IN PRESS
Conductivity (Scm-1)
S.R. Majid, A.K. Arof / Physica B 355 (2005) 78–82
81
1.00E-04
complexed site to another, ‘; is given by ‘ ¼ nt: The ionic conductivity, s, can be expressed by
1.00E-05
s ¼ ½2ðZeÞ2 =3kTmnEt expðE=kTÞ:
1.00E-06
1.00E-07
1.00E-08
1.00E-09 0
10
(a)
20
30
40
50
60
Salt content wt. % 0.9
According to Okuyama et al. [17], the distance between two adjacent chitosan monomers is 109 m. Using ‘ ¼ 109 m; the value of t can be calculated. From the Arrhenius plot of ln sT versus 103/T, the activation energy can be obtained from the slope and the pre-exponential factor can be obtained from the intercept at the vertical axis. Thus the number density of the mobile ions, n, can be calculated. Table 1 lists some of the transport parameters in the CA complexes. The ionic mobility is defined as m ¼ s=nq
0.8
(2)
and the diffusion coefficient is given by D ¼ ðkT s=ne2 Þ:
Ea (eV)
0.7
0.6
0.5
0.4 0
(b)
(1)
10
20
30
40
50
60
(3)
The values of m calculated from the Rice and the Roth model lies between 108 and 106 cm2 V1 s. The value of mobility for proton conductors in the PEO+NH4ClO4 system [18] obtained by transient ionic current measurement is between 106 and 104 cm2 V1 s. The value of diffusion coefficient lies between 105 and 107 cm2 s1. Conductivity is therefore controlled by both number of mobile ions and mobility.
salt content (wt.%)
Fig. 4. (a) Variation of conductivity as a function of salt content at room temperature. (b) Variation of activation energy as a function of salt content.
4. Conclusions We have been able to prepare a proton conductor based on chitosan and NH4NO3.
Table 1 Transport parameter of CA complexes Sample
sRT (S cm1)
t (s)
m ðcm2 V1 sÞ
D ðcm2 s1 Þ
E a (eV)
Z (cm3)
E1 E2 E3 E4 E5 E6
9.43 109 4.34 108 4.81 107 6.02 106 2.53 105 3.75 106
8.16 1014 8.38 1014 8.84 1014 1.03 1013 1.08 1013 9.11 1014
3.50 108 8.00 108 7.50 107 4.00 106 5.30 106 2.10 106
9.00 1010 2.10 109 1.90 108 1.00 107 1.40 107 5.50 108
0.79 0.74 0.67 0.49 0.45 0.63
1.704 1018 3.40 1018 4.00 1018 9.50 1018 3.00 1019 1.10 1019
ARTICLE IN PRESS 82
S.R. Majid, A.K. Arof / Physica B 355 (2005) 78–82
The highest conductivity obtained was 2.53 105 S cm1 at room temperature. The transport parameters such as mobility, diffusion coefficient and number of mobile ions have been calculated using the Rice and Roth model. The conductivity is dependent on the number of mobile ions and mobility. Acknowledgements S.R. Majid thanks Skim Pascasiswazah for the scholarship awarded and IPPP for vot-f F0131/ 2003B which helped to support the work. References [1] J.R. Mac Callum, C.A. Vincent (Eds.), Polymer Electrolyte Reviews, Elsevier, Amsterdam, 1987. [2] N.M. Morni, N.S. Mohamed, A.K. Arof, Mater. Sci. Eng. B 45 (1997) 140. [3] D.S. Reddy, M.J. Reddy, U.V. Subba Rao, Mater. Sci. Eng. B 78 (2000) 59. [4] S.A. Hashmi, A. Kumar, K.K. Maurya, S. Chandra, J. Phys. D: Appl. Phys. 23 (1993) 1307.
[5] K.K. Maurya, N. Srivastava, S.A. Hashmi, S. Chandra, J. Mater. Sci. 27 (1992) 6357. [6] A.M.M. Ali, N.S. Mohamed, A.K. Arof, J. Power Sources 74 (1998) 135. [7] N. Srivastava, A. Chandra, S. Chandra, Phys. Rev. B 52 (1) (1995) 225. [8] A. Bozkurt, W.H. Meyer, G. Wegner, J. Power Sources 123 (2003) 126. [9] M.A. Vargas, R.A. Vargas, B.E. Mellander, Electrochim. Acta 45 (2000) 1399. [10] F. Mi, H. Sung, S. Shyu, C. Su, Polymer 44 (2003) 6521. [11] C. Peniche, C. Elvira, J.S. Roman, Polymer 39 (25) (1998) 6549. [12] X. Qu, A. Wirsen, A.C. Albertsson, Polymer 41 (2000) 4841. [13] H.Y. Kweon, I.C. Um, Y.H. Park, Polymer 42 (2001) 6651. [14] M. Rinaudo, G. Pavlov, J. Desbrieres, Polymer (1999) 7029. [15] C. Peniche, W.A. Monal, N. Davidenko, R. Sastre, A. Gallardo, J.S. Roman, Biomaterials 20 (1999) 1869. [16] M.J. Rice, W.L. Roth, J. Solid State Chem. 4 (1972) 29. [17] K. Okuyama, K. Noguchi, M. Kanenari, T. Egawa, K. Osawa, K. Ogawa, Carbohydr. Polym. 41 (2000) 237. [18] K.K. Maurya, S. A. Hasmi, S. Chandra, in: B.V.R Chowdari, S. Chandra, S. Singh, P.C. Srivastava (Eds.), Solid State Ionics: Materials and Applications, World Scientific, Singapore, 1992, pp. 573–577.