Composites: Part B 52 (2013) 211–216

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Composites: Part B

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Structural, electrical and electrochemical parameters of PEO–NaClO3

composite for battery applications

1359-8368/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesb.2013.04.004

⇑ Corresponding author. Tel.: +91 9739115847.E-mail address: Aparveen981@gmail.com (A. Parveen).

D. Rajendra a, Aashis S. Roy b, Ameena Parveen c,⇑a Department of Physics, CMJ University, Shillong, Meghalaya, Indiab Department of Materials Science, Gulbarga University, Gulbarga, Karnataka, Indiac Department of Physics, Govt. First Grade College, Gurmitkal, Yadgir, Karnataka, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 December 2012Received in revised form 27 February 2013Accepted 7 April 2013Available online 17 April 2013

Keywords:A. Polymer–matrix composites (PMCs)B. Electrical propertiesD. Surface analysisE. Slip casting

Polyethylene oxide–NaClO3 composite have been prepared by solution casting technique with differentweight percentages as a polymer electrolyte for battery application. The prepared composites were char-acterized by various tools like XRD, FTIR and SEM. The X-ray diffraction analysis shows the complexationof polymer with salt and existence of both crystalline and amorphous phases. From FTIR spectra confirmsthe formation of PEO–NaClO3 composites. SEM images shows the grains are highly agglomerated and itsaverage size increases with increase in salt ratio. Frequency dependence of dielectric property and acelectrical conductivity of polymer electrolytes were studied within the frequency range of 50 Hz to5 MHz using complex impedance analysis technique. Ionic conductivity follows Arrhenius type behavioras a function of temperature. The fabricated cell of 25 wt.% of PEO–NaClO3 composites generated highcurrent of 1.79 A.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The secondary solid state battery is one of the most importantapplications of ionic conducting polymers. The study of ionic con-ducting polymers as a polymer electrolyte material for secondarybattery was reported by Miller [1]. Several conducting polymerssuch as polyaniline, polypyrole and polythiophene have beenproved to be promising materials for the electrode of secondarybattery because they are stable in air and have good electrochem-ical properties but these materials decreases ionic conductivity ofthese composites [2]. Chen and Lin reported the study of the dopedpolyaniline with a new class of ionic salt such as LiClO4, LiBF4 andZn (ClO4)2 [3]. The doping was carried out by dissolving polyanilineand the ionic salt in N-methyl-2-pyrrolidone (NMP) as a solventand then casted this solution into film. Yang et al. studied Li-polyaniline cell in which polyaniline was the emeraldine salt formdoped with protonic acid [4].

The studies on these existing polymer electrolytes are basedmostly on PEO and its copolymers because of their strong solvatingability with a variety of mono valent, divalent and transition metalsalts [5]. Particular attention has been focused on the study of PEObased polymer electrolytes complexed with lithium and sodiumsalts because of their direct application in high energy densityrechargeable batteries. Almost all the polymer electrolytes possess

some inherent drawbacks which impose limitations on their utilityin the electrochemical devices. These include their (i) partial crys-talline nature which hinders their overall ionic conductivity (ii)poor thermal/mechanical properties and (iii) electrochemicalinstability at the electrode–electrolyte interfaces. In order to over-come these problems many modifications have been made by add-ing of plasticizer, copolymerization, composite with metal oxide,metal chlorate etc. [6]. From previous studies it has been reportedthat the plasticizer, metal chlorate, metal oxides have improvedsignificantly the properties like electrical, mechanical and thermalbehavior properties [7,8]. Sodium chlorate is easily dissolved inPEO with organic solvent to form polymeric electrolyte. NaClO3

based PEO composites is highly environmental as well as chemi-cally stable of the ether functional group strengthens the superior-ity of PEO. Therefore, it could be used as host materials for a solidpolymer electrolyte [9].

In our earlier we reported the synthesis, characterization andcell parameters of PEO/SnCl2/polyaniline composite (PSP compos-ites) at different weight percentages were prepared using anhy-drous acetonitrile media. It is found that the open-circuit voltageof 0.83 V and a short-circuit current of 912 lA for PSP (70:30:10)composite. It is also interesting to note that in these compositesthe ionic property due to the presence of polyaniline [10]. In thispresent work, we report the synthesis of polyethylene oxide–sodium chlorate (PEO–NaClO3) composites at different weightpercentages using volatile organic solvent. The transport mecha-nism is revealed by two probe technique and it is found the ionic

212 D. Rajendra et al. / Composites: Part B 52 (2013) 211–216

conductivity is more predominant. Hence it could be use as poly-mer electrolyte. Electrochemical cell parameter shows the generat-ing current is high and stable for longer time.

(O-H) (tCH2)

(C-O-C)

2. Materials and methods

2.1. Preparation of composites

Composite of polymer electrolyte films were prepared by astandard solution-cast technique using AR grade precursor materi-als. The commercially available polyethylene oxide (PEO) (M/S Al-drich, M.W. �6 � 105) and NaClO3 were used as received withoutfurther purification. All the precursor materials were vacuum driedat �40 prior to use. An appropriate ratio of PEO and NaClO3 weredissolved in dehydrated methanol and the mixed solution was stir-red thoroughly for 12 h to allow polymer–salt complexation. Theresulting solutions were then poured into the polypropylenepetridishes and allowed to dry slowly at room temperature fol-lowed by vacuum drying as a result PEO/NaClO3 compositesformed at different weight percentages (5, 10, 15, 20 and25 wt.%). At the end, the films were heated at �40 �C for 3–4 h toremove residual solvents if any so as to get freestanding thin filmsof the materials. The entire operation was carried out in an inertatmosphere inside a glove box.

(wCH2)

Fig. 1. (a–f) Shows the FTIR spectra of pure PEO and its composites for variousweight percentages.

Fig. 2. (a–f) Shows the XRD pattern of pure PEO and its composites for variousweight percentages.

2.2. Characterization

The Fourier transmission infrared (FTIR) spectra of the compos-ites were analyzed in a region of 400–4000 cm�1 using a spectralresolution of 4 cm�1 in a Perkin–Elmer model 783 spectrophotom-eter in transmission mode. The XRD patterns of the polymer filmswere recorded by using X-ray diffractometer (Rigaku, Japan). Therecords were taken at room temperature using Cu Ka radiation,for the Bragg angles (2h) varying from 5� to 70� with a scan speedof 5� per minute. In order to evaluate the surface morphology ofPEO and its composites were investigated employing transmissionelectron microscopy (TEM) (JEOL-2010) at 200 keV. For recordingthe SEM image the samples are mounted on an aluminum plateand sputtered conducting gold over that to avoid charging of thesample surface. The grain size was calculated using a linear inter-cept technique, and the average grain size was calculated using thefollowing equation:

Grain size ðPÞ ¼ 1:56C

NMð1Þ

where C is the length of the test line (s), N is the number of inter-cepts, and M is the magnification calculated from the referencescale printed on the micrograph.

The dc conductivities for the PEO and PEO/NaClO3 composites atroom temperature were measured by a two-probe technique usingKeithely model 614 digital multimeters. The resistance data aregeometry dependent. Therefore resistivity (q) was calculated usingthe formula q = RA/I where R, A, and I respectively the resistance,area of the cross-section (pd2/4) and the thickness of the pelletused. The dc conductivity (rdc) can be estimated using the follow-ing equation:

rdc ¼1R

4l

pd2

� �ð2Þ

where l is the thickness of the pellet, d is the diameter of the pelletand R is the resistance.

The dielectric and impedance measurements were carried outby using an impedance analyzer (HIOKI LCR Hi-Tester, model:3532, Japan) a two-probe technique.

3. Results and discussion

We have shown in Fig. 1 FTIR spectra of (a) pure PEO, (b) 5 wt.%PEO: NaClO3 composites, (c) 10 wt.% PEO: NaClO3 composites, (d)15 wt.% PEO: NaClO3 composites, (e) 20 wt.% PEO: NaClO3 compos-ites and (f) 25 wt.% PEO: NaClO3 composites in the wave numberrange 400–4000 cm�1.

All the peaks assigned are shown in the Fig. 1a–f. PEO exhibitscharacteristic bands in the regions 800–1000 cm�1 correspondsto rocking modes, i.e., r(CH2), broad band 1000–1200 cm�1 corre-sponds to stretching mode, i.e. m(C–O–C), 1200–1300 cm�1 corre-sponds to twisting mode, i.e. t(CH2) and band around 1350 cm�1

and 1456 cm�1 corresponds to wagging mode, i.e. w(CH2) andbending mode, i.e. d(CH2) respectively. But in case of polymer saltcomplex apart from this bands another band appears around630 cm�1 corresponds to stretching mode of perchlorate band,i.e. m(ClO3). After addition of salt in polymer, the m(C–O–C) mode

D. Rajendra et al. / Composites: Part B 52 (2013) 211–216 213

becomes broader as compared to the band of pure PEO, indicatingthe complexation of polymer with salt. The peak appears at3000 cm�1 due to the O–H vibration of water molecules. Thereare significant changes in the peak positions, peak intensity andFWHM of the peaks of the polymer electrolyte when salts are beingadded to the polymer [11–13].

In order to investigate the complexation of sodium chlorate saltwith the polymer PEO, XRD studies were performed. Fig. 2 showsthe comparative profiles of XRD pattern of (a) pure PEO, (b)5 wt.% PEO: NaClO3 composites, (c) 10 wt.% PEO: NaClO3 compos-ites, (d) 15 wt.% PEO: NaClO3 composites, (e) 20 wt.% PEO: NaClO3

composites and (f) 25 wt.% PEO: NaClO3 composites. A comparisonof the diffraction spectra of complexed PEO with that of pure PEOand various weight percentages of PEO: NaClO3 composites revealsthe following differences. The diffraction peaks observed for 2h val-ues at 19.1� and 23.3� were found to be less intense in complexedPEO: NaClO3 compared to the pure PEO film. This indicates that theaddition of NaClO3 salt caused a decrease in the degree of crystal-linity of the polymer PEO. This could be due to the disruption of thesemi-crystalline structure of the film by NaClO3 salt. Hodge et al.[14] established a correlation between the intensity of the peakand the degree of crystallinity. Peaks corresponding to the polymer

Fig. 3. (a–f) Shows the SEM image o

PEO were also present in the complexed PEO: NaClO3. This showedthe simultaneous presence of both crystalline complexed anduncomplexed PEO. No peaks corresponding to NaClO3 wereobserved in complexed PEO, indicating the absence of excess salt(uncomplexed) in the complexed polymer films. The peaks exhib-ited a decrease in intensity with broadening at higher concentra-tions of NaClO3 salt in the PEO. This indicated a decrease in thecrystalline phase and a dominant presence of the amorphousphase. This amorphous nature resulted in greater ionic diffusivitywith high ionic conductivity, as amorphous polymers have flexiblebackbone.

Fig. 3a shows the Scanning Electronic Micrograph (SEM) imageof pure polyethylene oxide. The particles are highly agglomeratedand granular in shape. The average grain size was found to be 2–4 lm. The grains are well interconnected with each other whichindicate that they have enough binding energy to combine withneighbor grains or molecules. The SEM image of 5 wt.% of PEO/NaClO3 composite is shown in Fig. 2b. The grains are highlyagglomerated, irregular in shape but they are well interconnectedeach other and the average grain size were found to be 0.22 lm.The SEM image of 15 wt.% of PEO/NaClO3 composite is shown inFig. 2c. It is observed from the image that all grains are irregular

f pure PEO and its composites.

Fig. 5. dc Conductivity of pure polyethylene oxide and its composite at threedifferent temperatures.

214 D. Rajendra et al. / Composites: Part B 52 (2013) 211–216

in structure arranged one above the others. The average grain sizewas found to be 0.43 lm. Fig. 2d shows the SEM image of 25 wt.%of PEO/NaClO3 composites prepared at room temperature. It isfound from the image that the grains are highly agglomeratedand have porosity and good interconnectivity between the parti-cles. The average grain size was found to be 0.53 lm. By comparingthe figure (a–d) it can be concluded that the gradual increase ingranular size and change in morphology helps the transportationof charge particles through the polymer chains.

The variation of conductivity as a function of temperature forpure and different concentrations of NaClO3 in PEO is shown inFig. 4 over a temperature range 40–160 K. In the temperaturerange of study the conductivity increases with temperature andfollowed Arrhenius behavior throughout the three regions (regionsI, II and III) having different activation energies. While regions I andIII might be due to intrachain and interchain segmental motions,region II may be due to the transition of the polymer from the crys-talline to amorphous phase [15,16]. In polymer electrolytes,change of conductivity with temperature is due to the segmentalmotion, which results in an increase in the free volume of thesystem. This increase in free volume would facilitate the motionof ionic charge [17]. As per Arrhenius relation the dependence ofconductivity has the form r = r0 exp(Ea/KT) where r0 is the pre-exponential factor, Ea the activation energy and k is the Boltzmannconstant. These values are found to decrease with increasing con-centration of NaClO3. This may be due to the fact that the additionof small amounts of dopant forms charge transfer complexes in thehost lattice [18]. These charge transfer complexes increase theelectrical conductivity by providing additional charges in the lat-tice, resulting in a decrease of activation energy.

The variation of conductivity (r) as a function of various weightpercentage of NaClO3 concentration in PEO with various weightpercentages (5, 10, 15, 20 and 25 wt.%) at three different tempera-tures is shown in Fig. 5. The data reveals that the conductivity ofpure PEO at 50 temperature the increased value is found to be1.53 � 10�5 Scm�1 for 5 wt.% NaClO3, at temperature 100 itincreases to 3.03 � 10�5 Scm�1and at 150 temperature the conduc-tivity is found to be 6.0 � 10�5 Scm�1. The increase in conductivitybecomes flatter on further addition of NaClO3 to the polymer. Theincrease in conductivity at lower dopant concentrations of NaClO3

is attributed to the formation of charge transfer complexes or de-crease in the crystallinity [19], while the slow increase at higherdopant concentrations is due to the formation of ionic aggregates.These ionic aggregates impede the conduction process and de-crease the conductivity. The ionic conductivity of PEO and

40 60 80 100 120 140 160

0.0

2.0x10-5

4.0x10-5

6.0x10-5

8.0x10-5

1.0x10-4

σ dc S

/cm

Temperature in oC

PEO 05 wt % 10 wt % 15 wt % 20 wt % 25 wt %

Fig. 4. dc Conductivity of pure PEO and its composites.

(PEO + NaClO3) composite is calculated from the relation r = I/RbAwhere I is the thickness of the film, A, the area of the film and Rb,the bulk resistance of the material which is obtained from theintercept on the real axis at the high frequency end of the compleximpedance [20].

Fig. 6 shows the thermal coefficient of PEO/NaClO3 compositeswith different weight percentages. It is observed that the negativethermal coefficient increases with increase in the chlorate in thepolyethylene oxide matrix. This is due to the chlorate ions whichact as circuit breakers when applied voltage is removed or dis-charge starts in electrochemical cell. It is examined that a value in-creases from�0.19 to�0.14 because polyethylene oxide comprisesionic conductivity and crystalline organic polymer matrix that isloaded with sodium chlorate particles to make it conductive. Whilecharging, polymer is in a crystalline state with the sodium chlorateforced into the regions between crystals forming many conductivechains. Therefore thermal coefficient increases significantly withincreasing the sodium chlorate.

The frequency dependence of ac conductivity for pure polyeth-ylene oxide and its composites doped with NaClO3 as a function offrequency at various weight percentages (5, 10, 15, 20 and 25 wt.%)is shown in Fig. 7. The ac conductivity, rac of the material is calcu-lated using measured dielectric data using equation rac = xere0 tand where x = 2pf is the angular frequency, e0 the vacuum permittiv-

Fig. 6. Variation of thermal coefficient (a) of pure polyethylene oxide and itscomposites.

0 1x106 2x106 3x106 4x106 5x1060.0

5.0x10-6

1.0x10-5

1.5x10-5

2.0x10-5

2.5x10-5

3.0x10-5

3.5x10-5

Frequency in Hz

PEO 05 wt % 10 wt % 15 wt % 20 wt % 25 wt %

σ ac S

/cm

Fig. 7. Ac Conductivity of pure PEO and PEO–NaClO3 composites.

102 103 104 105 106

0.0

2.0x104

4.0x104

6.0x104

Rea

l per

miti

vity

(ε')

Frequency in Hz

PEO 05 wt% 10 wt % 15 wt % 20 wt % 25 wt %

Fig. 8. Real part of permittivity of pure polyethylene oxide and its composites as afunction of frequency.

102 103 104 105 106

0.00

0.05

0.10

0.15

0.20

tanδ

Frequency in Hz

PEO 05 wt % 10 wt % 15 wt % 20 wt % 25 wt %

Fig. 9. Variation of tangent loss for pure polyethylene oxide and its compositesdoped with NaClO3 as a function of frequency.

Fig. 10. Cell structure of poly(ethylene oxide)/NaClO3 composites covered withtaflon.

Fig. 11. Variation of current for pure polyethylene oxide and its composites dopedwith NaClO3 as a function of time.

0 20000 40000 60000 80000 100000

2.0

1.5

1.0

0.5

0

Volta

ge (V

)

Time in second

PEO 05 wt % 10 wt % 15 wt % 20 wt % 25 wt %

Fig. 12. Variation of voltage for pure polyethylene oxide and its composites dopedwith NaClO3 as a function of time.

D. Rajendra et al. / Composites: Part B 52 (2013) 211–216 215

ity the ac conductivity pattern indicates a constant and progressiverise in ac conductivity on increasing frequency.

Complex impedance spectroscopy is a powerful technique tostudy relaxation phenomena in the materials and also to calculatethe ac electrical conductivity of the materials. Fig. 8 shows the var-iation of real part of permittivity of pure polyethylene oxide and its

composites doped with NaClO3 as a function of frequency at vari-ous weight percentages (5, 10, 15, 20 and 25 wt.%) of PEO. For allthe cases the relative dielectric constant decreases sharply in thelower frequency region and showing a frequency independent va-lue at higher frequency region irrespective of polymer to salt ratio.The decrease of er with increase in frequency may be attributed tothe electrical relaxation processes.

Fig. 9 shows the variation of tangent loss with frequency varia-tion of tangent loss for pure polyethylene oxide and its composites

Table 1Cell parameters.

Sl. no. Cell parameter electrolyte PEO + NaClO3 composites

PEO 05 10 15 20 251 Area of the cell (cm2) 1.20 1.20 1.20 1.20 1.20 1.202 Open circuit voltage (V) 2.37 2.41 2.67 3.11 3.17 3.333 Short circuit voltage (lA) 310 319 337 376 389 3934 tion 0.97 0.941 0.973 0.979 0.98 0.9835 tele 0.008 0.0086 0.0091 0.0096 0.0096 0.00976 Power density (W kg�1) 1.137 1.149 1.135 1.275 1.266 1.3797 Current density (lA cm�2) 437.2 478.4 488.5 558.1 651.2 674.98 Energy density (mW h kg�1) 133.9 157.1 164.2 187.3 189.2 193.79 Discharge time for plateau region (h) 43 121 135 167 170 173

216 D. Rajendra et al. / Composites: Part B 52 (2013) 211–216

doped with NaClO3 at various weight percentages (5, 10, 15, 20 and25 wt.%). The tan d peaks are shifted towards higher frequency sideon increasing frequency. The appearance of peaks for each concen-tration in the loss spectrum (tangent loss) suggests the presence ofrelaxing dipoles in the polymer electrolyte.

4. Battery application

Poly(ethylene oxide)–NaClO3 composites based electrochemicalcell were fabricated as an electrolyte materials. In fabrication so-dium used as cathode and the mixture of iodine, carbon blackand poly(ethylene oxide)/NaClO3 composites as anodic materials(33:33:33). The Na poly(ethylene oxide)–NaClO3composites I2 -+ C + poly(ethylene oxide)/NaClO3 composites cell is developed in-side the glove box (Fig. 10) by covering a Teflon coated completelyair tighten at room temperature. Polymer electrolyte solution isprepared in an acetonitrile and it was drop casted over the anodiclayer. Further, cathode metal layer was fixed by pressing at 50 N.The total thickness of the cell is 1.5 mm and later cover with Teflontaking two outlets of copper connections in order to carry theexperiments [21–24].

The cells have been fabricated based on poly(ethylene oxide)/NaClO3 composites as polymer electrolytes and their dischargecurves determined under a constant load of 100 kX. There is an ini-tial rapid decrease in polarization current as a function of time(Figs. 11 and 12), and this may due to the formation of a thin layerof ions at the electrode interface which resist the study flow of ionsthrough circuit. This suggests that the charge transport in poly(eth-ylene oxide)/NaClO3 composites is predominantly ionic. This indi-cates that the poly(ethylene oxide)/NaClO3 composites have anelectrochemical stability and are thus suitable for application insolid-state batteries. As the NaClO3 content increases in PEO, thereis an increase in the polarization current and in the voltage stabil-ity. With the addition of more than 25 wt.% NaClO3 in PEO matrix,however, it is also observed that as the NaClO3 increases graduallyin matrix it increases the stability of composites [25,26]. The cellparameters were calculated and are shown in the Table 1. Fromthe data, it is clear that the short circuit current (SCC), open circuitvoltage (OCV) and discharge time for the plateau region werefound to be greater in (PEO–NaClO3 composites) cell comparableto others. This may be due to the high ionic conductivity and high-er degree of amorphosity of this system when compared to othersystems. The cell parameters of the present electrolyte systemare comparable with the earlier work reported on different poly-mer electrolyte systems [27,28]. This supports the practical appli-cation of the present electrolyte in solid-state battery applications.

5. Conclusion

The PEO/NaClO3 composites were prepared solvent castingmethod using acetonitrile as solvent and the electrochemical cell

was fabricated in Teflon box. XRD patterns of the (PEO + NaClO3)showed the decrease of intensity of peaks corresponding to purePEO with the increase of salt wt.% ratio suggesting a decrease inthe crystallinity of the complex. The complexation of the NaClO3

salt with PEO polymer was confirmed by FTIR studies. The conduc-tivity was found to increase with increase of dopant concentrationand temperature in pure PEO as well as in all compositions of PEO–NaClO3 composites electrolytes. The ionic transport number datain the (PEO–NaClO3 composites) polymer electrolyte films indicatethat the conduction is due to ions rather than electrons. Usingthese films, electrochemical cells were fabricated and the parame-ters of the cells shows there is increases in the charge density. Asthe NaClO3 content increases in PEO, there is an increase in thepolarization current and in the voltage stability. With the additionof more than 25 wt.% NaClO3 in PEO matrix, it is also observed thatas the NaClO3 increases gradually in matrix it increases the stabil-ity of composites Thus PEO–NaClO3 composites electrolytes arefound to be good candidate for solid-state batteries.

References

[1] Miller JS. Adv Mater 1993;5:671.[2] Chiang JC, MacDiarmid AG. Synth Met 1986;13:193.[3] Osaka T, Momma T, Ito H, Scrosati B. J Power Sources 1997;68:392.[4] Killian JG, Coffey BM, Gao F, Poehler TO, Searson PC. J Electrochem Soc

1996;143:936.[5] Chen SA, Lin LC. Macromolecules 1995;28:1239.[6] Vincent CA. Prog Solid State Chem 1989;88:109.[7] Gray FM. Polymer electrolytes: fundamentals and technological

applications. New York: VCH Publishers; 1991.[8] Shriver DF, Bruce PG. In: Bruce PG, editor. Solid state

electrochemistry. Cambridge Univ. Press; 1995. p. 119.[9] Ballared DGH, Cheshire P, Mann TS, Przeworski JE. Macromolecules

1990;23:1256.[10] Parveen Ameena, Anilkumar Koppalkar R, Patil Shantala D, Roy Aashis S. Ionics

2013;19:91.[11] Fauteux D, Massucco A, McLin M, Buren MV, Shi J. Electrochim Acta

1995;40:2185.[12] Thakur AwalendraK, Hashmi SA. Solid State Ionics 2010;181:1270.[13] Kunteppa H, Roy Aashis S, Koppalkar Anil R, Ambika Prasad MVN. Physica B

2011;406:3997.[14] Hodge RM, Edward GH, Simon GP. Polymer 1996;37:1371.[15] Roy AS, Anilkumar KR, Ambika Prasad MVN. J Appl Polym Sci 2012;123:1928.[16] Roy AS, Anilkumar KR, Ambika Prasad MVN. J Appl Polym Sci 2011;121:675.[17] Kunteppa H, Roy AashisS, Devendrappa H, Ambika Prasad MVN. J Appl Polym

Sci 2012;125:1652–6.[18] Mohamad AA, Mohamad NS, Yahya MZA, Othman R, Ramesh S, Alias Y, et al.

Solid State Ionics 2003;156:171.[19] Macdonald JR. Impedance spectroscopy. New York: John Wiley & Sons; 1987.

p. 23.[20] Subba Reddy CV, Sharma AK, Rao VVRN. Polymer 2006;47:1318.[21] Umadevi C, Mohan KR, Achari VBS, Sharma AK, Rao VVRN. Ionics 2010;16:751.[22] Miyamoto T, Shibiyama K. J Chem Phys 1983;79:3133.[23] Park CH, Kim DW, Prakash J, Sun Y-K. Solid State Ionics 2003;159:111.[24] Achari VBS, Reddy TJR, Sharma AK, Rao VVRN. Ionics 2007;13:349.[25] Rama Mohan K, Achari VBS, Rao VVRN, Sharma AK. Polymer testing

2011;30:881.[26] Thutupalli GKM, Tomlin SG. J Phys D: Appl Phys 1976;9:1639.[27] Chandrasekaran R, Sathiyamoorthi R, Selladurai S. Ionics 2008;14:387.[28] Chandra A, Agrawal RC, Mahipal YK. J Phys D: Appl Phys 2009;42:135107.