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Conductivity and electrical properties of corn starch–chitosan blend biopolymer electrolyte

incorporated with ammonium iodide

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2014 Phys. Scr. 89 035701

(http://iopscience.iop.org/1402-4896/89/3/035701)

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Royal Swedish Academy of Sciences Physica Scripta

Phys. Scr. 89 (2014) 035701 (10pp) doi:10.1088/0031-8949/89/03/035701

Conductivity and electrical properties ofcorn starch–chitosan blend biopolymerelectrolyte incorporated with ammoniumiodideY M Yusof1, M F Shukur1, H A Illias2 and M F Z Kadir3

1 Institute of Graduate Studies, University of Malaya, 50603 Kuala Lumpur, Malaysia2 Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 KualaLumpur, Malaysia3 Centre for Foundation Studies in Science, University of Malaya, 50603 Kuala Lumpur, Malaysia

E-mail: mfzkadir@um.edu.my

Received 2 July 2013, revised 30 December 2013Accepted for publication 6 January 2014Published 6 February 2014

AbstractThis work focuses on the characteristics of polymer blend electrolytes based on corn starchand chitosan doped with ammonium iodide (NH4I). The electrolytes were prepared using thesolution cast method. A polymer blend comprising 80 wt% starch and 20 wt% chitosan wasfound to be the most amorphous blend and suitable to serve as the polymer host. Fouriertransform infrared spectroscopy analysis proved the interaction between starch, chitosan andNH4I. The highest room temperature conductivity of (3.04 ± 0.32) × 10−4 S cm−1 wasobtained when the polymer host was doped with 40 wt% NH4I. This result was further provenby field emission scanning electron microscopy study. All electrolytes were found to obey theArrhenius rule. Dielectric studies confirm that the electrolytes obeyed non-Debye behavior.The temperature dependence of the power law exponent s for the highest conducting samplefollows the quantum mechanical tunneling model.

Keywords: solid polymer electrolyte, starch–chitosan blend, ammonium iodide, conductivity,FTIR, conduction mechanism

(Some figures may appear in colour only in the online journal)

1. Introduction

Solid polymer electrolytes (SPEs) are important in creatingmaterials for energy storage devices that require highperformance with thin and flexible form factors, such asbatteries and electrochemical double-layer capacitors [1–3].SPEs can greatly reduce the packaging and sealing of a device,which may increase energy and power density [3, 4]. Incontrast to liquid electrolytes, SPEs possess special propertiessuch as ease of fabrication, good shelf life, cyclability andprocessability [5].

Polymers obtained from natural polymers such asstarch and cellulose derivatives like hydroxyethylcelluloseand hydroxypropylcellulose have attracted attention inrecent works because of their mechanical and electricalproperties [6–8]. Recently, starch has become one of the

most popular, renewable and biodegradable polymers. Starchattracts scientists because of its rich variety and abundance innature. It is composed of repeating 1,4-α-d-glucopyranosylunits: amylose and amylopectin. However, the relativeamounts of these substances depend upon the plant sources [9,10]. Different starches like arrowroot, corn and potatostarches have been widely used by researchers [11, 12].Kumar et al [13] reported that a polymer electrolytesystem prepared by mixing ammonium iodide (NH4I) withpotato starch produced an ambient electrical conductivityof ∼2.40 × 10−4 S cm−1. Starch–NH4NO3 electrolyte wasreported to obtain a conductivity of 2.83 × 10−5 S cm−1 atroom temperature [14].

Chitosan consists of β-(1,4)-linked 2-amino-deoxy-d-glucopyranose that can be prepared by deacetylation ofchitin [15–18]. Chitosan has many functional groups

0031-8949/14/035701+10$33.00 1 © 2014 The Royal Swedish Academy of Sciences Printed in the UK

Phys. Scr. 89 (2014) 035701 Y M Yusof et al

including amino and hydroxyl groups which can serveas electron donors [19]. Chitosan is dissolved in aqueoussolutions of some organic and inorganic acids and becomesa cationic polymer due to the protonation of amino groups onthe C-2 position of the pyranose ring [20, 21]. Since chitosanis non-toxic and biocompatible with the human physiologicalsystem, it has been investigated as a biomaterial in someindustries such as tissue engineering [22–24] and biomedicalapplications [25–27]. The application of chitosan as an SPEin electrochemical devices has received much attention fromresearchers [28–30].

Xu et al [31] carried out a study on mechanical properties,water resistance and compatibility of composite films usinga blend of chitosan and starch. Their results suggested thatstarch and chitosan were compatible and can interact witheach other. Sudhakar and Selvakumar [32] reported a roomtemperature conductivity value of 1.20 × 10−7 S cm−1 fora starch–chitosan blend doped with lithium perchlorate(LiClO4). A polymer electrolyte based on a starch–chitosanblend doped with NH4NO3 gives a conductivity value of3.89 × 10−5 S cm−1 at room temperature, as reported by Khiarand Arof [33]. Shukur et al [34, 35] reported that the highestroom temperature conductivity of a starch–chitosan–NH4Clelectrolyte and a starch–chitosan–NH4Br electrolyte is6.47×10−7 and 9.72×10−5 S cm−1, respectively. A compara-tive study by Hema et al [36] shows that a polyvinyl alcohol(PVA)–NH4I electrolyte obtained a higher conductivityof 2.50 × 10−3 S cm−1, compared to PVA–NH4Br (5.70 ×

10−4 S cm−1) and PVA–NH4Cl (1.00 × 10−5 S cm−1). Accor-ding to the authors, the high conductivity obtained byPVA–NH4I is due to the lower lattice energy and largeranionic size of NH4I compared to NH4Br and NH4Cl.In the present work, the choice of NH4I as the dopant inthe starch–chitosan polymer blend is expected to obtaina higher conductivity than starch–chitosan–NH4Br andstarch–chitosan–NH4Cl electrolytes. The chitosan–NH4Ielectrolyte is reported to obtain a room temperatureconductivity of 3.70 × 10−7 S cm−1 [37]. Mohamad et al [38]reported that a chitosan–polyethylene oxide (PEO)–NH4Ielectrolyte obtained the highest room temperatureconductivity of 4.32 × 10−6 S cm−1. The aim of thiswork is to develop a polymer electrolyte system basedon a corn starch–chitosan blend and doped with differentamounts of NH4I, which will provide the charge carriers.The samples will then be characterized by x-ray diffraction(XRD), field emission scanning electron microscopy(FESEM), Fourier transform infrared (FTIR) spectroscopyand electrochemical impedance spectroscopy (EIS) in orderto study the interactions between the polymer host and thecharge carriers as well as the influence of the charge carriersin the conductivity trend and the conduction mechanism.

2. Experimental methods

2.1. Preparation and characterization of variousstarch–chitosan blend films

Different weight percentages (x wt%) of corn starch (Brown& Polson) were dissolved in 100 ml of 1% acetic acid(SYSTERM) and heated at 80 ◦C for 20 min. After the

solutions cooled to room temperature, (100 − x) wt% ofchitosan (viscosity: 800–2000 cP, 1 wt% in 1% acetic acid(25 ◦C), molecular weight of approximately 310 000–375 000with degree of deacetylation >75%, Sigma-Aldrich) wasthen added to the solutions. The mixtures were stirred untilhomogeneous solutions were obtained. All solutions were castonto different plastic Petri dishes and left to dry at roomtemperature, then kept in a desiccator filled with silica geldesiccants for further drying.

XRD measurements of the polymer blend films werecarried out using a Siemens D5000 x-ray diffractometerwhere x-rays of 1.54 Å wavelengths were generated by aCu Kα source. The 2θ angle was varied from 5◦ to 80◦.The morphology of the polymer blend films was studiedusing a Leica 440 scanning electron microscope at 1000×

magnification.

2.2. Preparation of electrolytes

The polymer blend electrolyte system was prepared bydissolving 80 wt% of starch in 100 ml of 1% acetic acid at80 ◦C for 20 min. After the solution was cooled to roomtemperature, 20 wt% of chitosan was added and stirred untilfully dissolved. Different amounts of NH4I (HmbG) wereadded into the solution and stirred until the solutions becamehomogeneous. All solutions were then cast onto plastic Petridishes and left to dry at room temperature. The dried filmswere kept in a desiccator filled with silica gel desiccants forfurther drying.

2.3. Electrolytes characterization

The impedance of the films was measured using a HIOKI3532-50 LCR HiTESTER at a frequency range between 50 Hzand 5 MHz from room temperature to 343 K. The electrolytefilms were sandwiched between two stainless steel electrodesof a conductivity holder. The value of bulk resistance Rb

obtained from the measurement was used to calculate theconductivity σ using the following equation:

σ =t

Rb A, (1)

where t is the thickness of the electrolytes and A is theelectrode–electrolyte contact area.

The FTIR studies were performed using a Spotlight400 Perkin-Elmer spectrometer in the wave number rangeof 400–4000 cm−1 at a resolution of 1 cm−1. The objectiveof FTIR was to confirm complexation between polymer andsalt. Zess Auriga FESEM at 5000× magnification was alsoused to study the morphology of the polymer electrolyte. Thisstudy will give an insight into the surface morphology of thesemi-crystalline polymer electrolyte.

3. Results and discussion

3.1. Characteristics of polymer blend films

The choice of a suitable ratio of polymer blend is crucialto its ability to serve as a polymer host in electrolytes.The choice controls the reduction in crystallinity of the

2

Phys. Scr. 89 (2014) 035701 Y M Yusof et al

polymer blend. From the x-ray diffractogram of the purechitosan film (100 wt% chitosan) in figure 1(a), a crystallinepeak appears at 2θ = 23◦. According to Hasegawa et al [39],the diffraction peak of chitosan is at 2θ = 20◦. Two broadcrystalline peaks appear at 2θ = 17.1◦ and 23◦ for the XRDpattern of pure starch film (100 wt% starch) in figure 1(k),which is comparable with the results of Wu et al [40]. Thepeaks due to starch can be clearly observed for 90 wt%starch and 10 wt% chitosan, as shown in figure 1(j). Withthe addition of 20 wt% chitosan, the crystalline peaks dueto starch and chitosan disappear, inferring that the blend iswell formed and miscible, as can be seen in figure 1(i). Theabrupt change in the pattern could be due to the fact thateach polysaccharide has its own crystalline region in the blendfilm and the XRD patterns are expressed as simply mixedpatterns [41]. According to Mathew and Abraham [42], theintramolecular interactions between the NH+

3 and hydroxylgroups in chitosan and starch chains reduce the crystallizationof the film. Further addition of chitosan makes the sampleless amorphous, as indicated by the broad crystalline peakdue to starch and chitosan, as shown in figures 1(b)–(h). Astudy carried out by Sudhakar and Selvakumar [32] suggestedthat a blend comprising 60 wt% chitosan and 40 wt% starchserved as a better host for conduction when doped with1.5 wt% LiClO4 and plasticized with 25 wt% glycerol. In theirwork they claimed that the addition of 50 wt% starch andabove to the blends made the electrolyte brittle due to highintermolecular bonding in starch. Zhai et al [41] reported thatit was difficult to form a homogeneous starch/chitosan mixturewith a higher content of chitosan. Therefore, the present resultis in good agreement with the studies of Zhai et al [41],where the XRD pattern for the blend comprising pure 20 wt%chitosan and 80 wt% corn starch was the most amorphous andthe blend exhibiting the best tensile strength. Hence in thepresent study the polymers were blended at a ratio of 80 wt%starch and 20 wt% chitosan (4:1) as the blend is expectedto host a reasonably fast ionic conduction due to its moreamorphous nature compared with other blend compositions.

SEM analysis was carried out to study the miscibilitybetween the components in the polymer blend.

SEM micrographs of the surfaces of the 100 wt% starchfilm, 100 wt% chitosan film and 80 wt% starch+20 wt%chitosan film are shown in figure 2. It can be observedin figure 2(a) that the micrograph of 100 wt% starch filmexhibits characteristic patterns on the surface. These patternsrepresent the withered ghost granules of starch polymer [43].In figure 2(b), the micrograph of 100 wt% chitosan filmshows a smooth and homogeneous surface. The surface alsoshows a good structural integrity. It can be seen that thesurface of 80 wt% starch+20 wt% chitosan film as shown infigure 2(c) exhibits a similar pattern to the 100 wt% chitosanfilm. Zhai et al [41] reported that the chitosan microdomainsdispersed within the starch matrix in the blend film. Thisobservation is similar to that seen on the film surface of thecellulose/carboxymethylated-chitosan blend [44].

3.2. FTIR analysis

FTIR spectroscopy was used to examine the interactionsof starch–chitosan and starch–chitosan–NH4I. By blending

2θθ (degree)

5 20 35 50 65 80

(a)

(c)

Inte

nsi

ty (a

.u.)

23° 17.1°

23°

(b)

(d)

(e)

(g)

(h)

(k)

(f)

(j)

(i)

Figure 1. X-ray diffraction patterns of (a) pure chitosan film, (b)10 wt% starch+90 wt% chitosan, (c) 20 wt% starch+80 wt%chitosan, (d) 30 wt% starch+70 wt% chitosan, (e) 40 wt%starch+60 wt% chitosan, (f) 50 wt% starch+50 wt% chitosan, (g)60 wt% starch+40 wt% chitosan, (h) 70 wt% starch+30 wt%chitosan, (i) 80 wt% starch+20 wt% chitosan, (j) 90 wt%starch+10 wt% chitosan and (k) pure starch film.

two polymers, there will be more sites for ion hopping andexchange, which will lead to an increase in conductivity [45].The infrared spectra of starch, chitosan and starch–chitosanfilms are presented in figure 3. From figure 3(a), the hydroxylbands in the spectrum of pure chitosan and pure starch filmappear at 3354 and 3280 cm−1, respectively. When two ormore substances are mixed, characteristic spectra peaks willchange to reflect the interactions between physical blends andchemical reactions [46]. The hydroxyl band of the 80 wt%starch +20 wt% chitosan blend film appears at 3288 cm−1.The carboxamide O C–NHR band in the spectrum of thepure chitosan film is located at 1640 cm−1 while the amineNH2 band appears at 1549 cm−1, as shown in figure 3(b)(i).These positions are almost the same as those reported in otherstudies [45, 47]. The position of the carboxamide and aminebands have shifted to 1639 and 1548 cm−1, respectively, inthe spectrum of pure starch–chitosan blend film, as shown infigure 3(b)(ii). This result is also comparable with a reportby Xu et al [31]. Liu et al [48] reported that the position ofthe amide-I band in pure chitosan film shifted from 1638.68to 1638.97 cm−1 in starch–chitosan film. They suggested thatinteractions had taken place between the hydroxyl groups ofstarch and the amino groups of chitosan, even though theintensity increased with little spectra shift.

Figure 4(a) shows the FTIR spectra for polymer–saltcomplexes in the region of 2900–3500 cm−1. The additionof 10 wt% NH4I has shifted the hydroxyl band of the purestarch–chitosan film from 3288 to 3286 cm−1, as shownin figure 4(a)(ii). The hydroxyl band is further shifted tolower wave numbers as the NH4I content increases. On theaddition of 40 wt% NH4I, the hydroxyl band appears at3175 cm−1. However, on addition of 50 wt% NH4I as shownin figure 4(a)(vi), the υas (NH+

4) mode and υs (NH+4) mode

3

Phys. Scr. 89 (2014) 035701 Y M Yusof et al

(c)

(a) (b)

Figure 2. SEM micrographs of (a) pure starch film, (b) pure chitosan film and (c) 80 wt% starch+20 wt% chitosan film.

appear at 3058 and 2985 cm−1, respectively. Further additionof NH4I shifts the υas (NH+

4) and υs (NH+4) modes to lower

wave numbers. This result suggests that the symmetry ofNH+

4 is lowered, and reflects the interaction between thecation and the polymer [49]. On addition of 10 wt% NH4Isalt, the carboxamide and amine bands are shifted to 1635and 1525 cm−1, respectively, as shown in figure 4(b)(ii).The bands are continuously shifted to lower wave numberswith the addition of NH4I salt up to 40 wt%. From these FTIRresults, it can be concluded that complexation has occurredbetween the starch–chitosan blend and NH4I salt.

3.3. Conductivity study

The conductivity of electrolytes depends on the chargecarrier concentration. Hence, when the charge concentrationis changed by doping the polymer with the salt, then theconductivity is also expected to change [13]. The increasein conductivity is also attributed to the increase in thenumber of complexation sites of the polymer electrolyte [45].The variation of room temperature conductivity as a function

of NH4I concentration is presented in figure 5. It canbe seen that the conductivity increases to (3.04 ± 0.32) ×

10−4 S cm−1 as the salt content increases to 40 wt%, whichmay be related to the increase in the number of mobilecharge carriers [14]. Kumar et al [13] reported that thehighest conductivity of the starch–NH4I electrolyte was∼2.40 × 10−4 S cm−1. The higher conductivity value obtainedin this work shows that the conductivity of an electrolyte canbe increased by using a polymer blend as a host. Buraidahand Arof [45] also reported that the conductivity of 55 wt%chitosan+45 wt% NH4I was increased from 3.73 × 10−7 to1.77 × 10−6 S cm−1 when chitosan was blended with PVA.According to the authors, more complexation sites can beprovided by blending two polymers. Hence, there will be moresites for ion migration and exchange to take place, leading toan increase in conductivity [45]. In the present work, furtheraddition of NH4I leads to a decrease in conductivity. Atconcentrations of NH4I above 40 wt%, the distance betweendissociated ions may become too close, and they are able torecombine and form neutral ion pairs that do not contributetoward conductivity [1].

4

Phys. Scr. 89 (2014) 035701 Y M Yusof et al

(a)

Tra

nsm

itta

nce

(a.

u.)

Wavenumber (cm-1)

300032503500

3280

3288

3354 (i)

(iii)

Wavenumber (cm-1)

Tra

nsm

itta

nce

(a.

u.)

(b)

149015901690

1548 1639

1640

1549

(i)

(ii)

Figure 3. (a) FTIR spectra for (i) pure chitosan film, (ii) pure starchfilm and (iii) 80 wt% starch+20 wt% chitosan blend film in theregion of 3000–3500 cm−1. (b) FTIR spectra for (i) pure chitosanfilm and (ii) 80 wt% starch+20 wt% chitosan blend film in the regionof 1490–1690 cm−1.

3.4. Field emission scanning electron microscopy study

The FESEM micrographs of selected samples are shown infigure 6. When 40 wt% NH4I is added, it can be clearlyseen that the surface of the electrolyte exhibits a porousstructure, as seen in figure 6(c), compared to the samplecontaining 10 wt% NH4I, shown in figure 6(a), and 20 wt%NH4I, shown in figure 6(b). According to Xi et al [50],the ionic conductivity of an electrolyte is influenced by theporosity since the pore connectivity is important for thetransportation of the charge carriers in the electrolyte. Thus,the conductivity increment toward the addition of 40 wt% saltis affected by the porous structure of the electrolyte. Whenmore than 40 wt% salt was added, the morphology consistsof solid particles that have suspended out of the surface, asshown in figure 6(d). These solid particles can be attributedto the recrystallization of the salt since the polymer host isunable to accommodate the salt, resulting in recombinationof the ions. This phenomenon contributes to the loss ofa significant amount of charge carriers, which leads to adecrease in conductivity [1]. Thus, FESEM has helped tostrengthen the conductivity studies in the present work.

3.5. Dielectric analysis

Dielectric studies of the polymer electrolyte provide importantinsights into the polarization effect at the electrode/electrolyteinterface, and aid our further understanding of theconductivity trend [51]. The equations for the dielectricconstant εr and dielectric loss εi are shown as follows:

εr =Z i

ωC0(Z2r + Z2

i ), (2)

εi =Zr

ωC0(Z2r + Z2

i ), (3)

where Z i is an imaginary part of impedance, Zr is a real partof impedance, C0 is vacuum capacitance and ω is angular

Wavenumber (cm-1)

(a)

Tra

nsm

itta

nce

(a.

u.)

29003200350029

85

3058

30

57

2983

29

81

3053

3175

(i) 3288

(iii) 3272

(iv) 3179

(ii) 3286

(viii)

(v)

(vi)

(vii)

(b)

148015901700

Tra

nsm

itta

nce

(a.

u.)

Wavenumber (cm-1)

1639 1558

1621

16201512

1627 1520

1514

1525 1635

(i)

(ii)

(iii)

(iv)

(v)

Figure 4. (a) FTIR spectra for (i) pure starch–chitosan film with (ii)10 wt% NH4I, (iii) 20 wt% NH4I, (iv) 30 wt% NH4I, (v) 40 wt%NH4I, (vi) 50 wt% NH4I, (vii) 60 wt% NH4I and (viii) pure NH4Isalt in the region of 2900–3500 cm−1. (b) FTIR spectra for (i) purestarch–chitosan film with (ii) 10 wt% NH4I, (iii) 20 wt% NH4I, (iv)30 wt% NH4I and (v) 40 wt% NH4I in the region of1480–1700 cm−1.

NH4 I content (wt% )

Con

duc

tivi

ty σσ

(S c

m-1

)

1.E-10

1.E-08

1.E-06

1.E-04

1.E-02

0 10 20 30 40 50 60 70

Figure 5. Effect of NH4I content on conductivity of electrolyte atroom temperature.

frequency. The frequency dependence of εr and εi are shownin figures 7(a) and (b), respectively. From figures 7(a) and (b),it is observed that the variation of εr and εi follow the same

5

Phys. Scr. 89 (2014) 035701 Y M Yusof et al

12

(a)

(c) (d)

(b)

Figure 6. FESEM micrograph of starch–chitosan blend film containing (a) 10 wt% NH4I, (b) 20 wt% NH4I, (c) 40 wt% NH4I and (d)50 wt% NH4I.

trend as the conductivity result, as the highest conductingsample has the highest values of εr and εi. There are variousfactors that influence ionic conductivity; namely the numberof charge carriers, the concentration of mobile ions and theavailability of a connecting polar domain as the conductionpathway [52]. In this work, the increase in conductivity can beattributed to the increment of charge carriers, while the declinein conductivity value can be explained by the formation ofion pairs producing neutral species, which reduces number offree ions and in turn decreases the conductivity [33]. Khiarand Arof [33] also reported that the dielectric study for thestarch–chitosan–NH4NO3 system follows the same trend astheir conductivity result.

Both εr and εi rise toward low frequencies due to theelectrode polarization and space charge effects, confirmingthe non-Debye behavior of the polymer electrolytes [14]. Theperiodic reversal of the electric field occurs rapidly at high

frequencies where there is no excess ion diffusion in thedirection of the field [53]. The polarization is then decreased,leading to the decrease in the values of εr and εi.

3.6. Temperature dependence of ionic conductivity

Figure 8 represents the temperature dependence of ionicconductivity for all compositions of starch, chitosanand NH4I. The increase in conductivity with increasingtemperature can be attributed to the increase in numberdensity and mobility of ions [1, 54]. The polymer chainacquires faster internal modes where bonding rotationsproduce motions to favor inter- and intra-chain ionhopping [55]. This phenomenon leads to the increase inconductivity. The conductivity–temperature plot in figure 8shows that all electrolytes are Arrhenian, hence the

6

Phys. Scr. 89 (2014) 035701 Y M Yusof et al

(b)

εε i

Log f (Hz)

0

5000

10000

15000

20000

25000

30000

35000

1 2 3 4 5 6 7

10

2030

4050

60

wt% NH4I

wt% NH4I wt% NH4I wt% NH4I

wt% NH4I wt% NH4I

(a)

Log f (Hz)

ε r

0

5000

10000

15000

20000

25000

30000

35000

1 2 3 4 5 6 7

10

20

30

40

50

60

wt% NH4I

wt% NH4I wt% NH4I

wt% NH4I

wt% NH4I wt% NH4I

Figure 7. Frequency dependence of (a) εr and (b) εi at roomtemperature.

conductivity can be expressed as

σ = σ0 exp

[−Ea

kT

], (4)

where σ0 is a pre-exponential factor, Ea is the activationenergy of conduction and k is the Boltzmann constant. Theactivation energy decreases with increasing conductivity, asshown in table 1. The highest conducting sample has thelowest activation energy of 0.20 eV. Buraidah and Arof [45]also reported that the highest conducting sample in thechitosan–PVA–NH4I system with a conductivity of 1.77 ×

10−6 S cm−1 has the lowest activation energy of 0.38 eV. Thisresult implies that the ions in highly conducting samplesrequire lower energy for migration. Lower activation energyalso resulted from the short distance between transit sitesprovided by the blended polymers [45].

3.7. Transport parameters

Rice and Roth [56] proposed the hypothesis that in an ionicconductor there is an energy gap above which conductingions of mass m can be thermally excited from localized ionicstates to free ion-like states in which the ion propagatesthroughout the solid with velocity v. Although the Rice

Log

σσ (S

cm

-1)

1000T -1 (K-1)

-10

-8

-6

-4

-2

2.6 2.8 3.0 3.2 3.4

10

20

30

40

50

60

wt% NH4I

wt% NH4I

wt% NH4I

wt% NH4I

wt% NH4I

wt% NH4I

Figure 8. Conductivity of starch–chitosan–NH4I system at elevatedtemperature.

and Roth equation was formulated for superionic conductorstypified by (i) cationic disorders, (ii) β-alumina structuresand (iii) defects, the equation can be applied to provide anestimation of the number density of mobile ions that assistthe ion conduction in polymer electrolytes. This is becausethe equation given is dependent on Ea, which can be obtainedfrom the conductivity–temperature relationship, and l, whichcan be considered as the distance between two coordinatingsites or two atoms with the lone pair electrons across whichthe ions may hop [57]. According to the Rice and Rothmodel [56], conductivity can be expressed as

σ =2

3

[(Ze)2

kTm

]nEaτexp

(−Ea

kT

), (5)

where Z is the valency of conducting species, e is the electroncharge, m is the mass of the charge carrier, n is the numberdensity of ions and τ is the traveling time of the ions. Thevalue of τ is obtained using the equation

τ =l

v. (6)

According to Khiar and Arof [14], l is the distancebetween two repeating units of amylose fiber and taken tobe around 10.40 Å, which is used in the present study. Thevelocity v of mobile ions is obtained using

v =

√2Ea

m. (7)

The value of τ can be used to determine the value of nusing equation (5). The ionic mobility µ was calculated usingthe value of n

µ =σ

ne. (8)

In polymer electrolytes, there are two possible mobileionic species: cations and anions. The type of cationresponsible for the ionic conductivity in the polymerblend–ammonium salt system has been identified to beH+ [58]. Hashmi et al [59] reported that in PEO complexedwith ammonium perchlorate (NH4ClO4), the conducting

7

Phys. Scr. 89 (2014) 035701 Y M Yusof et al

Table 1. Transport parameters of starch–chitosan–NH4I system at room temperature.

NH4I (wt%) σ (S cm−1) Ea (eV) τ (s) n (cm−3) µ (cm2 V−1 s−1)

10 (1.71 ± 1.24) × 10−9 0.43 1.15 × 10−13 1.64 × 1017 6.50 × 10−8

20 (5.31 ± 0.58) × 10−8 0.37 1.23 × 10−13 5.31 × 1017 6.24 × 10−7

30 (2.57 ± 1.70) × 10−6 0.29 1.39 × 10−13 1.29 × 1018 1.25 × 10−5

40 (3.04 ± 0.32) × 10−4 0.20 1.68 × 10−13 5.50 × 1018 3.45 × 10−4

50 (2.51 ± 0.83) × 10−6 0.29 1.39 × 10−13 1.26 × 1018 1.25 × 10−5

60 (1.49 ± 1.14) × 10−7 0.35 1.27 × 10−13 7.03 × 1017 1.32 × 10−6

species is the H+ ion. The H+ ions of NH4I will be coordinatedwith the N atom of the amine group in chitosan [59]. TheNH+

4 cations have an ideal tetrahedral structure where one ofthe four hydrogen atoms is weakly bound and can dissociateeasily under the influence of an electric field. This H+ ioncan hop from one site to another leaving a vacancy whichwill be filled by another H+ ion from a neighboring site.This structural diffusion is known as the Grotthus mechanism,which is also applicable in this work. Majid and Arof [60]have also inferred that H+ is the conducting species in achitosan acetate–NH4NO3 sample.

All of the transport parameters are listed in table 1.The values of n lies between 1017 and 1018 cm−3 and µ

values are in the range of 10−8–10−4 cm2 V−1 s−1. Generally,conductivity is dependent on the mobility and number ofmobile ions [60]. From table 1, the increasing conductivityvalue is influenced by the increasing mobility and numberdensity of ions. The highest conducting sample with theincorporation of 40 wt% NH4I has the highest n and µ

values of 5.50 × 1018 cm−3 and 3.45 × 10−4 cm2 V−1 s−1,respectively. Shuhaimi et al [61] reported that the n value ofmethyl cellulose doped with 25 wt% NH4NO3 electrolyte was4.86 × 1018 cm−3, which is quite comparable with the presentresult. The addition of 50 and 60 wt% of NH4I decreases thenumber density and mobility of ions, leading to a decrease inconductivity.

3.8. Conduction mechanism

The ac conductivity σac is described as

σac = εoεrω tan δ. (9)

By substituting εr tan δ = εi,

σac = εoεiω. (10)

Generally, σac is analyzed at a high frequency region todetermine the ionic conduction mechanism using Jonscher’suniversal power law [62, 63]

σ(ω) = Aωs + σdc, (11)

where σ(ω) is the total conductivity, A is atemperature-dependent parameter, s is the power lawexponent, σdc is the frequency-independent dc conductivityand

σac = Aωs . (12)

By substituting equation (12) into equation (10), the valueof s is obtained from the following relation:

ln εi = lnA

εo+ (s − 1) ln ω. (13)

ln ωω (Hz)

lnε i

7.8

8.2

8.6

9.0

9.4

12.7 12.8 12.9 13.0 13.1 13.2 13.3

298K303K308K313K318K323K328K333k338k343k

Figure 9. ln εi versus ln ω at different temperatures forstarch–chitosan blend film with 40 wt% NH4I.

T (K)

s

0.00

0.20

0.40

0.60

0.80

1.00

295 305 315 325 335 345

Figure 10. Plot s versus T for starch–chitosan blend film with40 wt% NH4I.

The value of exponent s is obtained from the slope ofthe plot of ln εi against ln ω in figure 9. In the present work,the acceptable frequency range is 12.90 ω 6 ln6 13.21. Thisis because in the high frequency region, there is no orminimal electrode polarization occurring [37]. There arevarious reports that suggest that the acceptable frequencyrange is at the high frequency region [37, 64].

Figure 10 shows the plot of s against temperature forthe highest conducting electrolyte sample. The exponents is observed to be independent of the temperature, and

8

Phys. Scr. 89 (2014) 035701 Y M Yusof et al

the plot can be best represented by the equation of s =

−0.00 008 T + 0.3931. The small gradient of −0.00 008implies that s is almost temperature-independent. From thefigure, quantum mechanical tunneling (QMT) is the mostsuitable model to explain the conduction mechanism ofthe starch–chitosan–NH4I system. According to Majid andArof [65], these ions are able to tunnel through the potentialbarrier that exists between two possible complexation sites.

4. Conclusion

Starch–chitosan blend electrolytes with differentconcentrations of NH4I were successfully prepared usingthe solution cast method. From the XRD results, 80 wt%starch+20 wt% chitosan was found to be the most suitableratio to serve as the polymer blend host. SEM analysisalso shows that blending chitosan with starch improves thestructural integrity. From FTIR studies, the complexation ofstarch–chitosan–NH4I was confirmed by the shift of hydroxyl,carboxamide and amine bands of pure starch–chitosan film.The highest room temperature conductivity is obtained at(3.04 ± 0.32) × 10−4 S cm−1 with the addition of 40 wt%NH4I, which is higher compared to works carried out byprevious researchers. From FESEM analysis, the variationof conductivity is influenced by the morphology of thesamples. The porous structure of the highest conductingelectrolyte proved to create an easy transportation methodfor the charge carriers. Dielectric studies suggest that thesamples in this study show non-Debye behavior. The plot ofconductivity–temperature shows that all electrolytes obeyedthe Arrhenius rule. Employing the Rice and Roth model, thehighest conducting sample has the highest n and µ values of5.50 × 1018 cm−3 and 3.45 × 10−4 cm2 V−1 s−1, respectively.It can clearly be seen that the conductivity variation is mainlydue to the concentration and mobility of free ions. Theconduction mechanism for the highest conducting electrolyteis best represented by the QMT model.

Acknowledgment

The authors would like to thank the University of Malaya forthe financial support provided (grant no. PG050-2012B).

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