This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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2 Journal of Membrane Science 463 (2014) Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: A study on ionic interactions in chitosan oxalic acid polymer electrolyte membranes I.A. Fadzallah, S.R. Majid, M.A. Careem, A.K. Arof n Centre for Ionics University of Malaya (CIUM), Department of Physics, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia article info Article history: Received 31 July 2013 Received in revised form 30 January 2014 Accepted 17 March 2014 Available online 25 March 2014 Keywords: Crosslinked membrane Proton conductivity Chitosan xalic acid abstract Solid polymer electrolyte (SPE) membranes consisting of chitosan and oxalic acid (C 2 4 H 2 ) were prepared by the solution casting method with various amounts of oxalic acid (A) from 10 wt% to 50 wt%. The ionic interaction between chitosan and oxalic acid in each electrolyte with different A concentrations is explained using the Fourier Transform Infrared (FTIR) spectroscopy in the wavenumber range between 1400 and 1800 cm 1. The ionic conductivity of the electrolytes is found to increase with A content up to 40 wt% (A40 membrane) and decrease on further addition of A. The increment in conductivity with A content can be associated with the increase in number of ions. From the study of conductivity variation with frequency, the value of fractional exponent, s is observed to approach 1. X-ray diffraction (XRD) analyses indicate that the A40 sample is the most amorphous with only 13% degree of crystallinity. Thermogravimetric analyses (TGA) show that membranes with wt% A are stable and preserve their compositions up to 168 1C. & 2014 Elsevier B.V. All rights reserved. 1. Introduction The application of biopolymers such as starch [1], chitosan [2] and cellulose [3] in electrical devices is important for environmental safety. The production cost of the polymer electrolyte membranes can be reduced on using biopolymers instead of synthetic ones such as sulfonated aromatic polymers [2]. Among the biopolymers mentioned above, chitosan has been extensively used due to its high percentage of nitrogen content compared to synthetically substituted cellulose. Besides, chitosan exhibits excellent biocompatibility and biodegradability, non-toxicity and adsorption properties. Chitosan is the N-deacetylated derivative of chitin, a naturally abundant biopolymer that can be found in the exoskeleton of crustaceans and insects. Due to the presence of two functional groups amine and hydroxyl in the structure, chitosan has a high potential for modification for various applications [4]. Chitosan has been used as a proton conducting membrane [5 10]. Mukoma et al. [5] reported that the partially hydrated sulfuric acid crosslinked chitosan exhibits a proton conductivity of Scm 1 at ambient temperature, while Smitha et al. [6] reported that the crosslinked chitosan cast on porous polysulfone exhibits a proton conductivity of Scm 1.Theincorporationoftwodifferent salts: NH 4 N 3 and LiCF 3 S 3 with addition of ethylene carbonate n Corresponding author. address: akarof@um.edu.my (A.K. Arof). (EC) plasticizer in chitosan acetate membranes resulted in conductivities of Scm 1 and Scm 1 respectively [7,8]. Chitosan has also been used as a polymer host to prepare solid polymer electrolytes (SPE) for batteries [11 15] and proton exchange membranes (PEM) for fuel cells [5]. The fuel cell PEMs should have high proton conductivity, good fuel barrier properties (minimal gas or methanol crossover), high mechanical strength, low cost, thermal and chemical stability and negligible electronic conductivity. ne of the most widely used membranes is Nafion s 117 due to its good thermal stability, high current density and high proton conductivity [5]. However, the relatively high cost of Nafion s 117 membrane is the significant drawback. The low proton conductivity in other polymer electrolyte membranes generally limits their use in fuel cells and other practical applications and an increase in conductivity to a value approaching 10 3 Scm 1 at room temperature is desirable. Also a clear understanding of the conduction mechanism is essential to improve the conducting properties of the polymer membranes. Missan et al. [9] have reported that poly(vinylidenefluorideco-hexafluoropropene) (PVdF-HFP)/oxalic acid based proton conducting polymer electrolytes achieved a maximum conductivity of Scm 1 with wt% A. They have improved the conductivity to Scm 1 with the addition of 43 wt% of dimethylacetamide (DMA) as a plasticizer. In this study we attempt to prepare a low cost chitosan oxalic acid based proton conducting membrane for possible use in fuel cells. In the present work, studies were carried out to understand the effect of various amounts of oxalic acid on the conductivity of /& 2014 Elsevier B.V. All rights reserved.

3 66 I.A. Fadzallah et al. / Journal of Membrane Science 463 (2014) chitosan oxalic acid polymer electrolyte membranes. xalic acid is a dicarboxylic acid (C C) with a low dissociation constant capable of forming weak acid base interactions in polymer electrolytes that can facilitate proton conduction [9]. The ionic interactions, degree of crystallinity and thermal properties of the chitosan oxalic acid polymer membranes were investigated using Fourier Transform Infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and thermogravimetric analysis (TGA) respectively. The deconvolution of FTIR spectra was done in order to investigate the ionic interaction in detail. The crystallinity of chitosan and the polymer electrolyte membranes was also obtained by deconvoluting the XRD diffractograms to separate the crystalline peaks. 2. Experimental 2.1. Sample preparation Chitosan with more than 75% degree of deacetylation procured from Aldrich and oxalic acid (C 2 H 2 4 H 2 ) from R&M Chemicals was used for the preparation of the polymer electrolyte by the solution casting method. The membrane samples have been prepared according to the procedure described by Fadzallah et al. [16]. Five polymer membranes were prepared with 10%, 20%, 30%, 40% and 50% oxalic acid contents and the samples were named as A10, A20, A30, A40 and A50 respectively. Fig. 1. FT-IR spectra of (a) pure chitosan (b) A10 (c) A20 (d) A30 (e) A40 (f) A50 and (g) pure A Characterizations Fourier Transform Infrared (FTIR) data for the membranes were obtained from 4000 to 650 cm 1 using the Thermoscientific Nicolet is10 spectrophotometer operating at a resolution of 4 cm 1.Impedance measurements on the polymer electrolyte membranes were performed using the HIKI LCR Hi-Tester impedance analyzer interfaced to a computer as a function of frequency ranging from 50Hzto1MHzinthetemperaturerangefrom300to373K at5k intervals. The samples were sandwiched between two stainless steel electrodes with 1 cm diameter. The conductivity was calculated using the following equation: s ¼ d ð1þ R b A where s is the conductivity (S cm 1 ), d is the thickness of the sample (cm), R b is the bulk resistance value (Ω) andais the cross-sectional areaofthemembranesample(cm 2 ). X-ray diffraction (XRD) patterns were recorded on a Bruker model, D8 Advance X-Ray Diffractomer operated with a Cu target at 40 kv and 40 ma. Each sample was cut into 4 cm 2 cm and then placed in the sample holder of the diffractometer. The diffraction angle varied from 51 to 601. Thermal Gravimetric Analyses (TGA) were performed in order to determine the thermal stability and weight loss with a Thermo Scientific analyzer from 24 1C to5001cunder nitrogen gas at a heating rate of 10 1C min Results and discussion 3.1. FT-IR study FTIR spectra of chitosan, oxalic acid and the polymer electrolyte membranes are shown in Fig. 1. Parts of the same spectra on expanded scales are shown in Fig. 2 (from 3700 to 3000 cm 1 ) and Fig. 3 (1800 to 1400 cm 1 ). For chitosan (Fig. 2(a)), the broad peak around 3425 to 3390 cm 1 is assigned to the overlapping of NH and stretching bands. The symmetric and asymmetric Fig. 2. FTIR spectra in the range between 3700 and 3000 cm 1 for (a) pure chitosan (b) A10 (c) A20 (d) A30 (e) A40 and (f) A50. stretching of C H can be seen as the doublet peaks at 2929 cm 1 and 2878 cm 1, the band due to the CQ of carboxamide appears at 1647 cm 1 and N H amine band appears at 1580 cm 1. ther peaks at 1416 cm 1 (C N stretching coupled with N H plane deformation), 1376 cm 1 (symmetric angular deformation of CH 3 ), 1320 cm 1 (C N stretching of the amino group) and at 1026 cm 1 (stretching vibration of C C) can also be observed in Fig. 1(a) [17 19]. The moisture in oxalic acid was eliminated by heating. The FTIR spectrum obtained for oxalic acid is depicted in Fig. 1(f). The characteristic bands of oxalic acid appear at 1718 cm 1 for free

4 I.A. Fadzallah et al. / Journal of Membrane Science 463 (2014) cm 1 [21]. This might be due to the increasing occurrence of hydrogen bonding between the hydroxyl group of chitosan and the free proton of the dissociated A as concentration is increased (Fig. 4). The deconvolution of the characteristic bands between 1400 and 1800 cm 1 are displayed in Fig. 5. The absorption peaks of NH 2 deformation and the carbonyl stretching amide group are observed to downshift from 1575 to 1559 cm 1 and from 1647 to 1640 cm 1 respectively. ther absorption peaks at 1526 cm 1 for C from oxalic acid dissociation, 1600 cm 1 for NH 3 þ, 1720 cm 1 for free carboxylic group from oxalic acid are also observed. These peak assignments are in agreement with those of Yalcinkaya et al. [19] and Ritthidej et al. [22]. The areas under the deconvoluted IR absorption peaks of NH 3 þ and NH 2 plotted against oxalic acid concentration are shown in Fig. 6. The areas are given as percentages with respect to the total Fig. 3. FTIR spectra in the range between 1800 and 1400 cm 1 for (a) pure chitosan (b) A10 (c) A20 (d) A30(e) A40 and (f) A50. oxalic acid, 1219 cm 1 for asymmetric stretching, and 712 cm 1 for C scissoring asymmetric stretching [20]. The polymer electrolyte membranes (Fig. 1(b e)) show new peaks at 1700 cm 1 and 1190 cm 1 and the peak of N H at1416cm 1 become less intense as the amount of oxalic acid is increased [17 19]. Absorbance (a. u.) Deconvolution and band fitting of IR absorptions The complexation between oxalic acid (A) and functional group of chitosan resulted in band shifting. The overlapping of NH and stretching bands (Fig. 2) were investigated by deconvoluting the infrared absorption between 3000 and 3700 cm 1 using the MNIC software. The characteristic bands between 1400 and 1800 cm 1 (Fig. 3) were also deconvoluted. The absorbance peaks were fitted to a straight baseline using the Gaussian Lorentzian function and the area of the deconvoluted bands were calculated. The deconvolution for absorption area at higher wavenumber regions of the infrared spectroscopy is important in order to study the effect on hydroxyl and amine group with increasing A concentration as depicted in Figs. 2 and 3. The absorption band of free for chitosan at 3543 cm 1 is downshifted to 3531 cm 1 as A concentration is increased. However the symmetric and asymmetric stretching bands for NH 2 of chitosan shift to higher region from 3401 and 3268 cm 1 to 3420 and 3260 cm 1 respectively. These shifts are due to the interaction between H þ from A and the amine group in chitosan. The absorption band for hydroxyl () group of chitosan at 3153 cm 1 is downshifted to Wavenumber (cm -1 ) Fig. 4. Deconvolution and band-fitting of IR spectra between 3700 and 3000 cm 1 for (a) chitosan (b) A10 (c) A20 (d) A30 (e) A40 and (f) A50.

5 68 I.A. Fadzallah et al. / Journal of Membrane Science 463 (2014) Absorbance (a.u.) Fig. 7. Graph of ionic conductivity of membranes for various A contents at room temperature (300 K) with error bars Wavenumber (cm -1 ) area under the parent peak. The membrane with 50 wt% A (A50) shows the highest area for the NH þ 3 peak, which means that some of the NH 2 group in chitosan has been protonated. Because of this protonation the area under the NH þ 3 peak has increased at the expense of the area under the NH 2 peak Impedance spectroscopy Fig. 5. Deconvolution and band-fitting of IR spectra between 1800 and 1400 cm 1 for (a) chitosan (b) A10 (c) A20 (d) A30 (e) A40 and (f) A50. Fig. 6. Variation of peak areas of NH 3 þ and NH 2 as a function of oxalic acid content. The room temperature (RT) ionic conductivity of the samples was calculated using Eq. (1). The samples were kept in a dessicator prior to the measurements. The Nyquist plots of the chitosan oxalic acid membranes with different amounts of oxalic acid at RT (300 K) and the ionic conductivity values derived from these plots have been reported earlier [16]. The ionic conductivities at RT for all samples plotted against the oxalic acid concentration are shown in Fig. 7. The enhancement of conductivity with increasing amount of oxalic acid is due to the increasing number of protonated amine (NH 3 þ ) of chitosan. The A40 membrane has the maximum room temperature conductivity of Scm 1, which is better than the maximum conductivity value reported for (PVdF-HFP)/A system [9]. At higher oxalic acid content (for sample A50), a decrease in conductivity is observed. The dissolution of oxalic acid in water produces H þ and C which help the dissolution of chitosan powder since chitosan needs a slightly acidic medium to dissolve. The re-association of H þ and C from oxalic acid to form C might be the reason for the conductivity decrement when the A content was increased to 50 wt%. This might be due to the high reactivity property of oxalic acid since it has a low dissociation constant value, pk a (for proton dissociation in A the pk a value is 1.2) which is defined as negative logarithmic of the acidity constant, K a pk a ¼ log K a ð2þ Using a generalized hypothetical acid (HA), the reaction of acid in water can be written as HAþH 2 H 3 þ þa ð3þ and the expression for acidity constant is given by K a ¼ ½H 3 þ Š½A Š ð4þ ½HAŠ Since the concentration of the hypothetical acid is the denominator in Eq. (4) and the concentrations of the dissociated ions are in the numerator, a large value of K a means that the acid is a strong acid and a small value of K a means that the acid is a weak acid. The dissociation constant, pk a is inversely related to K a. Thus the acid will be a strong acid if the pk a value is small [21]. The dissolution of A in water can be represented as 2ðCÞþ2H 2 2H 3 þ þ2c ð5þ xalic acid oxalate ion The high amount of H þ can re-associate with C ion (oxalate ion) to form oxalic acid again since Eq. (5) favours the reaction to proceed to the left hand side. This results in the low amount of free H þ, thus reducing the availability of H þ for proton conduction in

6 I.A. Fadzallah et al. / Journal of Membrane Science 463 (2014) H NH 2 NH 2 n H H 2 Chitosan xalic acid Ionic crosslinking reaction H NH 3 - NH 3 n H NH NH 3 n - Fig. 8. Possible ionic crosslinking reaction scheme of chitosan with oxalic acid. The subscript n denotes the specific number of repeating structural unit of chitosan. the sample A50. The conductivity value is optimized at 40 wt% of A since A40 membrane shows the highest conductivity value compared to those of other A concentration values and this implies that A40 contains the highest amount of protons for ionic conductivity at RT. The oxalic acid is expected to form ionic cross links between chitosan polymer chains, because it contains two carboxylic groups. The possible ionic crosslinking reactions between amine groups of chitosan and carboxylic groups of oxalic acid are schematically shown in Fig. 8. This ionic crosslinking scheme was based on the scheme proposed by Gümuşoğlu et al. [23] Temperature dependence of conductivity The temperature dependence of conductivity exhibited by the highest RT conducting sample membrane, A40, is shown in Fig. 9 in the form of log s vs. 1000/T. It can be observed that the plots, in the temperature range under consideration, obey the Arrhenius expression given by s ¼ s 0 exp E A kt where s 0 is the pre-exponential factor, E a is the activation energy of ionic conduction, k is the Boltzmann constant and T is the temperature in Kelvin. The calculated activation energy, E A, is 0.61 ev with regression value indicating that the points lie in an almost straight line ac Conductivity The ac impedance technique can be used to obtain the specific dc conductivity of the electrolytes. Fig. 10(a) shows the variations of ac conductivity with frequency for samples with different A contents. Fig. 10(b) shows the variations of ac conductivity with frequency for the A40 sample at different temperatures. The ac conductivity at different frequencies were calculated using the ð6þ Fig. 9. Plot of log s dc vs /T (K 1 ) for A40 membrane with error bars. following equation: Z s r t ac ¼ ð7þ ðz 2 r þz 2 i Þ A where Z r is the real part of the impedance, Z i is the imaginary part of the impedance, t is the thickness of the sample (cm) and A is the cross-sectional area of the membranes (cm 2 ). In both figures, the graphs consist of frequency dependent regions and frequency independent regions that are important characteristics for ion conducting membranes [24]. It can be observed that the edge of the plateaus (the frequency independent regions) in Fig. 10(a) is shifted to higher frequencies as oxalic acid content is increased. The conductivity values obtained from extrapolating the plateau region are not much different to the experimental values determined from the Nyquist plots. Thus it can be inferred that direct current conductivity, s dc, can be evaluated from the extrapolation of

7 70 I.A. Fadzallah et al. / Journal of Membrane Science 463 (2014) Table 1 Comparison of parameters obtained from fit of the experimental data to Eq. (8) for (a) sample membranes with different oxalic acid content at room temperature, 300 K and (b) sample membrane A40 at various temperatures. (a) Membrane r( dc ) A s A A A A A (b) Temperature (K) r (dc) A s Fig. 10. Fit to Eq. (8) of the real part of conductivity vs. frequency for (a) sample membranes with different oxalic acid contents at room temperature, 300 K, and (b) sample membrane A40 at various temperatures. The variation of ac conductivity with frequency obeys Jonsher's universal power law (UPL) given by sðωþ¼s dc þaω s ð8þ where s dc is the frequency independent plateau, A is a temperature dependent term and s is the fractional exponent with value in the range 0oso1. The values obtained for s dc, A and s by fitting Eq. (8) are tabulated in Table 1. The value of s dc shows the maximum value for A40 at room temperature and the value increases with temperature. The value of fractional exponent, s, is related to the degree of correlation between charge carriers. In principle, the value of s should be between 1 and 0.5 indicating the ideal long-range pathway and diffusion limited hopping (tortuous pathway) respectively [27]. The highest s value of 0.88 for sample membranes at room temperature is shown by A40. n the other hand, sample A40 in the temperature range K displays that the fitting s values approach 1. This indicates that long range drift of ions may be responsible for ionic conduction [28] X-ray diffraction (XRD) Fig. 11. XRD patterns of (a) chitosan (b) A10 (c) A20 (d) A30 (e) A40 (f) A50 and (g) pure A (inset). the frequency independent plateau to the vertical axis. The frequency dependent regions at very lower frequencies in both figures corresponded to the electrode polarization. From the ac conductivity at various temperatures in Fig. 11(b), it can be observed that the plateaus shifted from lower to higher frequency regions as temperature is increased indicating that the conductivity increases with increasing temperature. The low conductivity values at low frequency (at lower temperature) regions are related to the accumulation of ions due to the slow periodic changes of the electric field [25,26]. The XRD patterns of polymer electrolyte membranes with different amounts of oxalic acid are shown in Fig. 11. Chitosan oxalic acid salt or chitosan oxalate can be formed when chitosan was dissolved in oxalic acid. In order to study the XRD patterns of chitosan powder and the crosslinked membranes, the XRD diffractograms were deconvoluted using some non-linear least squares software to separate the crystalline peaks from the continuous scattering background. Fitting of multi-peaks using Gaussian distribution was done after carrying out baseline correction for a particular diffractogram. XRD has been used to relate the crystallinity with degree of deacetylation (DD) of chitin by Zhang et al. [28]. They have reported that a peak of maximum intensity at 2θ91 reflection is diminished with the increase of DD and shifted to a higher angle. A second intensive peak at 2θ191 also diminished with the increase in DD. The positions of the peaks reported by Zhang et al. [28] are in agreement with those reported by Pawlicka et al. [29]. Since the degree of deacetylation of chitosan used in this research is higher than 75%, the crystalline peaks should be observed at higher 2θ angles when compared to those of the original peaks in chitin. In our study, for pristine powder chitosan, three sharp peaks and two broad peaks are observed at 2θ angles of 9.71, and and and respectively [29,30]. The XRD diffractograms of chitosan powder and the ionic crosslinked membranes with their deconvoluted peaks are shown in Fig. 12. The first and second crystalline peaks in the crosslinked

8 I.A. Fadzallah et al. / Journal of Membrane Science 463 (2014) Intensity (a. u) Fig. 13. TGA curves of chitosan powder, polymer electrolyte membrane with different amounts of A and pure A. It can be seen that A40 membrane shows the lowest degree of crystallinity of 13% (corresponding to the highest degree of amorphousness) giving the highest conductivity value. In comparison, the low conducting chitosan powder has a higher crystallinity degree of 17% Thermogravimetric analysis (TGA) 2θ (degree) Fig. 12. The deconvolution of XRD patterns of (a) chitosan (b) A10 (c) A20 (d) A30 (e) A40 and (f) A50. Table 2 Degree of crystallinity values of chitosan and the crosslinked membranes. Sample Chitosan powder 17 A10 16 A20 15 A30 14 A40 13 A50 14 Crystallinity degree, χ c (%) membranes are found to be at 8 131, and at respectively. A new peak is observed for the sample A50 at 2θ361 suggesting the presence of local ordering, due to the excess content of oxalic acid. Thus the lowest crystallinity is expected for the A40 membrane. The degree of crystallinity, χ c, can be estimated by using the model of two-phase approach as depicted in the formula below χ c ð%þ¼ c ð9þ cþa where c is the crystalline part of the membrane, and a is the amorphous part of the membrane. The estimated χ c values for all membranes are tabulated in Table 2. The thermogravimetric traces of chitosan, oxalic acid and ionic crosslinked chitosan oxalic acid membranes are shown in Fig. 13. The oxalic acid undergoes a sharp decomposition at 125 1C and pure chitosan powder decomposed rapidly at 291 1C. A10 membrane shows a three step weight loss at 50 1C, 216 1C, C. A20 shows a three step weight loss at around 50 1C, 208 1C and C. A30 undergoes weight losses at 50 1C, 192 1C and C. A40 shows a four step weight loss at around 50 1C, 168 1C, 191 1C and C. A50 undergoes weight losses at 50 1C, 125 1C, 180 1C and 300 1C. The first small weight loss observed in all samples including pure A and pure chitosan at around 50 1C is due to the removal of absorbed moisture in the materials. The last weight loss at around C is due to the degradation of polymer. The weight losses observed at intermediate temperatures from 168 to 201 1C in the crosslinked membranes arise due to the delinking and decomposition of oxalic acid. A50 shows an additional weight loss at 125 1C due to the decomposition of free A (excess A) which are not taking part in the ionic linking. n the whole, TGA studies indicate that the chitosan oxalic acid polymer membranes with wt% A are stable and preserve their compositions up to 168 1C. However, the A50 membrane has good stability only up to 125 1C. 4. Conclusions The ionic interaction between chitosan and oxalic acid (A) in solid polymer electrolytes with various A concentrations can be explained best by deconvoluting the IR absorptions in the range between 1400 and 1800 cm 1. The area under the deconvoluted peaks of NH 3 þ is found to increase with oxalic acid concentration due to the protonation of NH 2 in chitosan. The ionic conductivity of the electrolytes is found to increase with A content up to 40 wt % (A40 membrane) and decrease on further addition of A. The increment in conductivity with A content is due to the increase in number of ions and the drop in conductivity beyond the optimized concentration could be attributed to ion re-association due to high concentration of ions. The study of s ac data reveals that there is a high correlation between the charge carriers

9 72 I.A. Fadzallah et al. / Journal of Membrane Science 463 (2014) (protons) since the value of fractional exponent, s, is approaching 1. XRD analyses show that A40 exhibits the lowest crystallinity degree of 13% which favours the ionic conduction. TGA results show that the membranes with wt% A are stable and preserve their compositions up to 168 1C. Acknowledgments The authors would like to express gratitude to University Malaya for research Grant PV A (PPP Grant) and FP A. References [1] N. Wang, X. Zhang, H. Liu, J. Wang, Carbohydr. Polym. 77 (2009) [2] B. Smitha, D. Anjali Devi, S. Sridhar, Int J. Hydrog. Energy 33 (2008) [3] A.S. Samsudin, W.M. Khairul, M.I.N. Isa, J. Non Cryst. Solids 358 (2012) [4] M.N.V. Ravi Kumar, React. Funct. Polym. 46 (2000) [5] P. Mukoma, B.R. Jooste, H.C.M.J. Vosloo, J. Power Sources 136 (2004) [6] B. Smitha, D. Anjali Devi, S. Sridhar, Int. J. Hydrog. Energy 33 (2008) [7] L.S. Ng, A.A. Mohamad, J. Membr. Sci. 325 (2008) [8] A.K. Arof, Z. sman, N.M. Morni, N. Kamarulzaman, Z.A. Ibrahim, M. R. Muhamad, J. Membr. Sci. 36 (2001) [9] H.P.S. Missan, P.P. Chu, S.S. Shekon, J. Power Sources 158 (2006) [10] F. Goktepe, S.U. Celik, A. Bozkurt, J. Non Cryst. Solids 354 (2008) [11] N.S. Mohamed, R.H.Y. Subban, A.K. Arof, J. Power Sources 56 (1995) [12] N.M. Morni, A.K. Arof, J. Power Sources 77 (1999) [13] N.M. Morni, N.S. Mohamed, A.K. Arof, Mater. Sci. Eng. B 45 (1997) [14] R.H.Y. Subban, A.K. Arof, S. Radhakrishna, Mater. Sci. Eng. B 38 (1996) 156. [15] L.S. Ng, A.A. Mohamad, J. Power Sources 163 (2006) [16] I.A. Fadzallah, S.R. Majid, M.A. Careem, A.K. Arof, Ionics (2014), org/ /s z. [17] Z. Cui, Y. Xiang, J. Si, M. Yang, Q. Zhang, T. Zhang, Carbohydr. Polym. 73 (2008) [18] E.J. Baran, Carbohydr. Polym. 74 (2008) [19] S. Yalçinkaya, C. Demetgül, M. Timur, N. Colak, Carbohydr. Polym. 79 (2010) [20] M. Boczara, R. Kurczab, M.J. Wojcik, Vib. Spectrosc. 52 (2010) [21] T.W.G. Solomon, C. Fryle, rganic Chemistry, 8th ed., Wiley International Edition, U.P. Noida, India, [22] G.C. Ritthidej, T. Phaechamud, T. Koizumi, Moist heat treatment on physicochemical change of chitosan salt films, Int. J. Pharm. 232 (2002) [23] T. Gümuşoğlu, G.A. Ari, H. Deligöz, J. Membr. Sci. 376 (2011) [24] R. Cheruku, L.G. Govindaraj, Mater. Sci. Eng. B 177 (2012) [25] A.S.A. Khiar, R. Puteh, A.K. Arof, Physica B 373 (2006) [26] S. Chopra, S. Sharma, T.C. Goel, R.G. Mendiratta, Solid State Commun. 127 (2003) [27] D.K. Pradhan., R.N.P. Choudhary, B.K. Samantaray, Int. J. Electrochem. Sci. 3 (2008) [28] Y. Zhang, C. Xue, Y. Xue, R. Gao, X. Zhang, Carbohydr. Res. 340 (2005) [29] A. Pawlicka, M. Danczuk, W. Wieczorek, E. Zygadlo-Monikowska, J. Phys. Chem. A 112 (2008) [30] F. Hassan, H.J. Woo, N.A. Aziz, M.Z. Kufian, S.R. Majid, Ionics 19 (2013)