STUDIES ON STRUCTURAL, THERMAL, OPTICAL ...

IJP, Vol. 5, No. 2, July-December 2012 pp. 159-72

STUDIES ON STRUCTURAL, THERMAL, OPTICAL AND ELECTRICAL PROPERTIES OF PEO+PVP POLYMER FILMS WITH AND WITHOUT Li+ AND Ag+ K. Naveen Kumar1*, S. Uthanna1 and S. Buddhudu1 1Department

of Physics, Sri Venkateswara University, Tirupati-517502, A.P., India. ( *E-mail: [email protected])

Abstract: Solid polymer electrolyte PEO+PVP films comprising lithium, silver and also with both the ions have been prepared using the traditional solution casting technique. Analysis on these polymer films has been carried out from the measurement of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman and optical absorption spectra. Thermal analysis has been done from TG-DTA profiles. Electrical conductivities have been measured using an impedance analyzer in the frequency range of 1Hz - 1MHz and in the temperature range of 300K-373K. The maximum condutivity has been noticed from the Li + ion containing polymer film and it is 1.29x10-4 S/cm at 373K. The morphological studies have been done using the tools of SEM with EDS for all the polymer films studied in the present work. Keywords: Polymer electrolyte films - characterization.

1. INTRODUCTION Solid polymer electrolyte (SPE) systems have attracted a great deal of interest because of their suitability in the development of a wide variety of solid-state electrochemical devices such as batteries, sensors, super capacitors, gas sensors and electro chromic display devices. The main advantages of polymer electrolytes are their mechanical properties, easy to fabricate them in the form of thin films of desirable sizes and also their suitability to ensure the electrode / electrolytes contact in electrochemical devices [1-2]. Addition of alkali metal ions, could further enhance electrical conductivities from such polymer electrolytes and also their crystallinity nature are reported in literature as more promising and encouraging [3-6]. In the present work, it main objectives are to investigate XRD patterns and also conductivities of (PEO+PVP) polymer films with and without Li+, Ag+ ions each separately and also in mixed form (Li+ + Ag+) to validate that Li+ ion singly doped blended polymer films are more promising and encouraging optical systems.

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2. EXPERIMENTAL STUDIES 2.1. Synthesis of Electrolyte Blended Film

The films of polymer electrolytes were prepared using a single solvent, by means of the solution-casting method. From M/S Sigma-Aldrich, Chennai, India both polyethylene oxide (PEO) (MW=6X105) and polyvinylpyrrolydone (PVP) (MW=13X105) were purchased that are in the chemical formulae of:

Films of (thickness ~ 100nm) pure PEO+PVP and also salts such as LiNO3, AgNO3. These salts each separately were added to the host polymer matrix in weight percentage ratios of (0.5 PEO+0.5 PVP), (0.45 PEO+0.45 PVP+0.1 LiNO3), (0.45 PEO+0.45 PVP+0.1 AgNO3), and (0.45 PEO+0.45 PVP+0.05 LiNO3+0.05 AgNO3) by solution casting technique using triple distilled water as the solvent. PEO, PVP and dopants were dissolved in triple distilled water and stirred at the room temperature (~300K) for 10-12 h to get a homogeneous mixture. The solution was transferred onto polypropylene dishes and were allowed to evaporate slowly at room temperature. The final product was dried thoroughly to remove all traces of the solvent. The dried composite polymer films were obtained from the polypropylene dishes and that are preserved in desiccators. 3. CHARACTERIZATION MEASUREMENTS In order to investigate the structure of polymer electrolyte films, X-ray diffraction scans were made on SEIFERT, XRD 3003TT, X-ray diffractometer with CuK� radiation (� =1.540Å) at room temperature. The diffraction patterns were recorded at Bragg’s angle (2�) in the range of (10-700) with scan rate of 50 per minute. FTIR absorption spectra of the films were recorded using Thermo Nicolet IR200 spectrometer. Raman spectrum was carried out on BRUKER RFS 27: Stand alone FT-Raman Spectrometer with an Nd: YAG laser source (1064 nm). Optical absorption spectra were carried out on Perkin Elmer Lambda 950, UV-VIS-NIR Spectrometer. For all samples, TG/DTA measurements were carried out on a Model SDTQ 600 TA Instrument, (specimens were scanned in the nitrogen atmosphere from 30-60oC at a heating rate of 10oC/min). The impedance measurements were performed using a computer controlled phase sensitive multimeter (PSM1700) in the frequency range of 1HZ-1MHZ at different temperatures. Scanning electron microscopy (SEM-ZEISS MA15) and energy dispersive X-ray spectrometry (EDAX) analysis attached to the SEM were used to investigate the morphology and carried out the elemental analysis of the sample studied.

Studies on Structural, Thermal, Optical and Electrical Properties of…

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4. RESULTS AND DISCUSSION 4.1. X-ray Diffraction Analysis

X-ray diffraction studies were carried out to examine the structure of the polymer electrolytes and to investigate the occurrence. Figure 1 represents the X-ray diffraction of pure and doped (PEO+PVP) polymer blends with different dopants such as Li+, Ag+. Pure PEO+PVP blend polymer film exhibits three crystalline peaks of PEO, a maximum intensity peak at 19.20 next maximum intensity peak at 23.60 and relatively less intense peak at 27.10. For PVP no such well defined peak was observed at 2� region of 11-180. Which can be associated with the amorphous nature of pure PVP [7]. These observations confirm that the present polymer blend system possess multiphase, possessing both crystalline and amorphous regions. However, on the addition of the dopants to the polymer blend, the intensity of these peaks decreases gradually and becomes relatively broader, suggesting a decrease in the degree of crystallinity of the blend polymer film. They reported that the intensity of the XRD pattern decreases as the amorphous nature increases with the addition of dopants. This amorphous nature results in greater ionic diffusivity and high ionic conductivity, which can be observed in amorphous polymer blend having flexible backbone [8].

Figure 1: XRD Patterns of PVP+PEO Blended with Li+, Ag+ ions

4.2. SEM Analysis

Figure 2 shows SEM images of PEO+PVP polymer films doped with and without Li+, Ag+ ions. The smooth morphology is closely related to the reduction of PEO crystallinity in the presence of the salt. The reduction of PEO cryastallinity arises from random distribution

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and dissociation of salt which could possibly introduce to topological disorder in the electrolyte film. The reduction of crystalline produces more amorphous phase in the electrolyte system, which makes the electrolyte more flexible. The miscibility of these two polymers has also been confirmed from FTIR and Raman analysis [9]. Figure 8(c) shows PEO+PVP and silver salt complex with pores like shapes presence giving rise to ionic conductivity of this sample and silver particles are also observed in this polymer [10]. To verify the chemicals in the material, an EDAX profile has also been as shown in Figure 2. to confirm the presence of C, O and Ag ions in the samples, however Li which is being with lesser atomic number, that could not be observed in the EDS analysis.

Figure 2: Scanning Electron Microscopic Images of (a) PEO+PVP, (b) Li+: PEO+PVP, (c) Ag+: PEO+PVP and (d) Li++Ag+: PEO+PVP Polymer Films and their Corresponding EDS Spectrum


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4.3. Thermal Analysis

TG/DTA is an effective tool for identification of the thermal stability and the transition behavior of the electrolyte materials. Figure 3 shows TG/DTA thermograms of weight loss as a function of temperature for pure PEO+PVP blend and their dopant complexes with a heating rate of 10 0C/min in the temperature range from room temperature to 7000C. It is clear that the initial weight loss for all the samples occurs at 63±2 0C due to the moisture evaporation and it is stable up to 155±2 0C, above which the solvent evaporated. The major weight losses are observed in the range of 300-4250C for all prepared samples. This could be corresponds to the structural decomposition of the polymer blend and their complexes. In this figures the prepared samples are stable over 4650C and are preferred in lithium batteries. These results have proved that the thermal stability of the Ag+ doped polymer is higher than other samples. In DTA curves, endothermic peaks are observed at the temperature 400C for pure PEO+PVP sample, 400C, 1120 C, 3890C for lithium doped, 400C, 1820C, 2310C for silver doped, 410C, 3720C for lithium and silver doped polymer films as well as exothermic peaks are observed at 820C, 4300C, 2930C for pure PEO+PVP sample, 860C, 3780C, 4200C for lithium doped, 700C, 1680C, 2090C, 4290C for silver doped, and 860C, 2190C, 3590C, 4280C for both lithium and silver doped polymer films. There is a sharp and large exothermic peak at 4240C concurrent with an appreciable weight loss of about 65-82%. This indicates complete decomposition of the film, which is in agreement with the films, which is in agreement with the TG curve. From the above discussion, it is concluded that the thermal stability limit of the polymer electrolyte 3360C [11, 12].

Figure 3: TG/DTA Plots for Pure PEO+PVP, Li+: PEO+PVP, Ag+: PEO+PVP and Li++ Ag+: PEO+PVP Polymer Films at Room Temperature

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4.4. FTIR Studies

FTIR spectroscopy has been used to analyze the interactions among atoms or ions in electrolyte system. These interactions can induce changes in the vibrational modes of the molecules in the polymer electrolyte. Figure 4 shows the FTIR spectrum of pure PEO+PVP, Li+: PEO+PVP, Ag+: PEO+PVP, Li++ Ag+: PEO+PVP complexes in the wavenumber range 400-4000 cm-1.

Figure 4: FTIR Spectrum of PVP+PEO Blended Doped with Li+, Ag+

A large broad band between 2975 and 2814 cm-1 and two narrow bands of lower intensity 2585 and 2518 cm-1 are inherent bands of asymmetric (CH) stretching of CH2 of PEO [13]. The characteristic PEO bands at 2378, 2238, 2161 and 1964 cm-1 are observed. The band region in the range of 1480-1410 cm-1 is for CH2 scissoring mode of PEO with in this region. The band at 1489 cm-1 represents the C-H bending of CH2 in PEO. A very small intensity band at 1794 cm-1 corresponds to the ether Oxygen group of PEO. The bands at around 1366 and 1348 cm-1 are indicating the CH2 wagging and CH2 bending, respectively which are characteristic of PEO. The relatively small band at around 1255 cm-1 is assigned to CH2 symmetric twisting of PEO, and the vibrational band at 1101 cm-1 was assigned to C-O-C stretching of PEO [14, 15]. The two bands near 945 and 846 cm-1 are assigned to CH2 rocking vibrations of methylene groups and are related to helical structure of PEO [16]. The band region between 1346-1246 cm-1, and 1281 cm-1 indicates the –CH wagging motion of PVP. The bands at 1242-1297 cm-1 region correspond to CH2 twisting or wagging


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of both PEO and PVP. The characteristic bands at 1144, 1275, 1352, 1440 cm-1 solid polymer electrolyte system. Observed vibrational band at 2904 cm-1 may also be attributed to aliphatic C-H stretching of PVP [17] and also observed the relatively sharper band at 2380 cm-1 is indicating the presence of PVP in sample. The appearing band regious 1700-1610 cm-1 in all samples corresponding to symmetric and asymmetric of C=O stretching modes of PVP. The CH2 wagging of PVP absorbed band at 1445 cm-1 is observed [18]. Above figure shows the FTIR peaks when various dopants such as LiNO3, AgNO3 were added to the mixture of PEO+PVP. Due to addition of dopants to host matrix, the width of the IR spectra near 1100 cm-1 become slightly broader and wave number slightly decreased. This results from the coordination bonds between Li+ or Ag+ ion and ether units (-O-) of PEO. In this case, wave number decreases as C-O single bond strength decreases. This leads to the interruption of crystallization so that the fraction of amorphous substance increases. Also a peak by NO3- ion appears near 1130 cm-1 which broadens this peak. The reagents with LiNO3, AgNO3 had pyridine peaks near 3550 cm-1. This is because the existence of NO3- ion dissociated from LiNO3, AgNO3 changes pyridine in PVP to dopants. From the results NO3 - ions that cause self discharge are expected to minimize the self-discharge rate by forming bonds with pyridine salts. The region 1160-1080 cm-1 region is the location of C-O-C stretching, due to the interaction of Alkali metal cations with ether oxygen atoms in PEO. C-H bending of CH2 in the both PEO and PVP at 1454 cm-1 is shifted gradually towards 1429 cm-1 when dopants are added to the pure blend film. The band at 1355 cm-1 in pure blend is shifted to 1360 cm-1 in the doped films. The vibrational band at 1069 cm-1of reference electrolyte film is assigned to C-O stretching of PEO which has been found to be shifted to 1090 cm-1 when dopant salts are added to this reference film. These FTIR bands are indicated in Table 2. 4.5. Raman Spectrum Analysis

Figure 5 shows the Raman spectra of PEO+PVP system in the range from 200-3500 cm-1. The Raman spectrum of PEO exhibits bands at 1067 cm-1 and very weak features near 1040 cm-1 [19]. And we observed Raman active modes of CH2 rocking and CO stretching modes 861 cm-1, 829 cm-1, 846 cm-1 bands. The spectral region from 932 cm-1 in PEO systems has also been assigned to CO stretching motions mixed with CH 2 rocking vibrations [20]. In the high molecular weight PEO salt complex new bands occur at 952 cm-1, 965 cm-1, as before, the complex should be viewed as a superposition of two phases, with the dominant band at 963 cm-1 in. In pure PEO the two strongest bands in this region at 1343 cm-1 and 1360 cm-1 appear at 1340 cm-1 and 1367 cm-1 respectively. In addition, a new band is seen at 1343 cm-1 due to presence of PEO. Two bands at 1445 cm-1 and 1464 cm-1 is observed in this blended film indicates the presence of PEO. It has been observed that the intensity of the Raman band corresponds to the C-N vibrational bands at 754 cm-1 and C-C stretching vibration at 932 cm-1. The bands at 1235 cm-1 and 1426 cm-1 can be attributed to C-N stretching and C-H bending vibrations of pure PVP respectively. The bands at 1666 and 2930 can be attributed to C=O and C-H stretching

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Figure 5: Raman Spectrum of the PEO+PVP Blended Polymer Film

vibrations [21, 22]. The broadening in Raman Spectra is usually an indication of amorphous nature of the blended polymer electrolyte [23-25]. The amorphous nature of the polymer electrolyte has also been confirmed by XRD analysis. 4.6. Optical Absorption Analysis

Figure 6 shows the UV-VIS absorption spectra of three samples of blended polymer films with two absorption bands at 286 nm due to the NO3 ligand of the Ag cation and a weak band at 420 nm due to SPR of the polymer embedded with Ag [26]. This observation clearly indicates the formation of Ag particles in the PEO+PVP blended matrix. The formation of Ag particles prior to the UV irradiation is due to the reducing action of PVP itself, converting silver ions in to the metallic silver and further enhanced through the Photo-reduction process of remaining silver ions as a result of UV irradiation [27]. 4.7. Impedance Analysis

The samples were vacuum dried at 300K for 1h and the measurements were done by sandwiching the electrolyte film between two aluminum electrodes. Ac impedance spectroscopy has become a powerful method for the investigation of ionic conductivity of solid polymer electrolyte films. Figure 7 shows the complex impedance plane plots (Z’ Vs Z”) of PEO+PVP blended film doped with Li+, Ag+ contained salts with increasing the temperature. From the figure two well defined regions such as a high frequency semicircle related to the parallel combination of a resistor and capacitor and a low frequency spike


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Figure 6: Optical Absorption Spectrum of PEO+PVP Polymer Blend Doped with Li+ and Ag+ ions

Figure 7: Cole-Cole Plots of PVP+PEO Blend Doped with Li+, Ag+ at Different Temperatures

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representing formation of double layer capacitance at the electrode electrolyte interface due to migration of ions at low frequency. The low frequency response appearing as an inclined spike at an angle less than 900 to the real axis indicates the inhomogeneous nature of the electrode-electrolyte interface. By the intersection of semicircle with the real axis we can find the bulk resistance of the polymer electrolytes. The magnitude of the bulk resistance decreases the conductivity of the solid polymer electrolyte increases. The ionic conductivity of the solid polymer electrolyte was calculated by the following formula

��

l Rb A

Where ‘l’ is the thickness of the polymer electrolyte (cm), A is the area of the blocking electrode (cm2), and Rb is the bulk resistance of solid polymer electrolyte film [28]. At 373K temperature the lithium ion doped PEO+PVP blended polymer electrolyte film exhibit the high ionic conductivity in the order of 1.29x10-4 S/cm than other doped polymer films. The conductivity of these four samples with varying the temperature as follows in Table 1. Table 1 Ionic Conductivity Values of Prepared Samples at Different Temperature Ionic conductivity (Scm-1) Polymer electrolyte film

300K(RT)

313K

333K

353K

373K

PEO+PVP PEO+PVP: Li+ PEO+PVP: Ag+ PEO+PVP: Li++ Ag+

4.90x10-8 4.71x10-5 1.57x10-8 3.62x10-8

9.34x10-8 6.37x10-5 2.74x10-8 5.33x10-8

5.57x10-7 6.90x10-5 1.56x10-7 3.05x10-7

1.47x10-6 1.21x10-4 1.15x10-6 1.18x10-6

2.05x10-6 1.29x10-4 2.66x10-6 2.15x10-6

4.8. Temperature Dependent Ionic Conductivity Analysis

The temperature dependent ionic conductivity plots are shown in Figure 8. The Arrhenious plots suggest that the ion transport in polymer electrolytes depends on the polymer segmental motion. Figure 8 explains that the ionic conduction in the polymer electrolyte system obeys the VTF (Vogel-Tamman-Fulcher) relation, which describes the transport properties in a viscous matrix. The increase of conductivity with temperature is interpreted as being due to a hopping mechanism between co-ordination sites, local structural relaxations and segmental motion of polymers. The increase in conductivity with temperature can be linked to the decrease in viscosity and hence , increased chain flexbility. According to Druger et al. in polymer electrolyte, the change of conductivity; with temperatue can be explained by increase in the free volume of the system that facilitates the migration of ions. For devices operating over a wide temperature range, it is desirable to have a uniform conductivity [29]. 4.9. Dielectric Analysis

In Figure 9 the variation of C-’ (the real part of the complex dielectric permittivity: -* = C - ’-j C - ”) as a function of frequencies at room temperature for the polymer electrolyte C


Figure 8: Arrhenius Plots for all Polymer Electrolyte Films

Figure 9: Dielectric Loss and Dielectric Constants of the PVP+PEO Polymer Blend with Li+, Ag+ Doped Film at Room Temperature

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IJP, 5(2), 2012 Table 2 FTIR Bands Assignment of the PEO+PVP Polymer film Wavenumber (cm-1)

Assignment

a

3550

Pyridine peak

[18]

b

2904

C-H stretching vibrations

[17]

Reference

c

2585, 2518

C-H stretching of CH2

[13]

d

2380

PVP characteristic band

[18]

e

2378, 2238, 2161, 1964

PEO characteristic bands

[14]

f

1794

Etheric oxygen group

[15]

g

1700-1610 C=O

stretching modes

[18]

h

1489

C-H bending of CH2

[15]

i

1480-1410

CH 2 scissoring

[14]

j

1445

CH 2 wagging

[18]

k

1366, 1348

CH 2 wagging & bending

[15]

l

1346, 1246,1281

-CH- wagging motion

[17]

m

1255

CH 2 symmetric twisting

[15]

n

1242, 1297

CH 2 twisting or wagging

[17]

o

1101

C-O-C stretching

[15]

p

945, 846

CH 2 rocking vibrations of methyle group

[16]

samples are shown. As expected, the variation tendency of dielectric constant with frequency is the reverse of electrical conductivity. The C-’ attains high value at low frequency and decreases exponentially with increase in frequency. The decrease of dielectric constant is mainly attributed to the mis-match of interfacial polarization of composites to external electric fields at elevated frequencies. The permittivity enhances near the percolation threshold. It is usually behind that the percolation threshold is an important point at which electrical property varies a lot. There fore study conducting composites in the vicinity of percolation threshold is an effective way to know the electrical transport behavior of composites. The dielectric relaxation parameter of the polymer electrolytes can be obtained from the study of Tan as a function of frequency. The dielectric loss tangent, Tan can be defined by the equation Tan� =��. The variation of Tan with frequency for all the prepared PEO+PVP doped with Li+, Ag+ polymer complexes at 303K is presented in Figure 9. The high values of Tan with decreasing frequency at room temperature may be attributed to face charge build up at the interface between the sample and the electrode (space charge polarization). The electrode polarization (EP) relaxation frequency fEP, which is used to evaluate the EP relaxation time �EP = (2 � fEP)-1 [30, 31]. 5. CONCLUSION PEO+PVP blended films with lithium and silver ions have successfully been synthesized by using solution casting technique. XRD studies have revealed an amorphous nature of


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the reference polymer film. The confirmation bands of PEO and PVP have been done based on the profiles of FTIR spectra and also from Raman spectrum. The surface morphological studies have been done using SEM and EDS studies. The thermal stability studies have been carried out from the profiles of TG/DTA. The lithium doped PEO+PVP polymer blend has exhibited high ionic conductivity in the order of 1.29x10-4 S/cm at 373K compared to other samples and thus we could confirm the fact that Li+ doped PEO+PVP polymer film could be suggested as an ideal optical material for its use in the development of polymer film based devices. Acknowledgement

One of us K. Naveen Kumar, would like to thank the UGC, for awarding him with a project fellowship in SAP-CAS programme in the department. References [1] Angesh Chandra, R. C. Agrawal and Y. K. Mahipal, Ion Transport Property Studies on PEO–PVP Blended Solid Polymer Electrolyte Membranes, J. Phys. D: Appl. Phys., (2009), 42, 135107. [2] M. Ravi, Y. Pavani, K. Kiran Kumar, S. Bhavani, A.K. Sharma, V.V.R. Narasimha Rao., Studies on Electrical and Dielectric Properties of PVP: KBrO4 Complexed Polymer Electrolyte Films., Mater. Chem. Phys., (2011), 130, 442. [3] Amrtha Bhide, J. Hariharan, A New Polymer Electrolyte System (PEO)6:NaPO3., J. Power Sources, (2006); 159(2), 1450. [4] M. B. Armand, Polymer Electrolytes., Annu. Rev. Mater. Sci., (1986), 16, 245. [5] F. M. Gray, Polymer Electrolytes, The Royal Society of Chemistry, England, (1977). [6] S. Ramesh, Tan Winie, A. K. Arof., Investigation of Mechanical Properties of Polyvinyl ChloridePolyethylene Oxide (PVC-PEO) Based Polymer Electrolytes for Lithium Polymer Cells. Eur. Polym. J., (2007), 43, 1963. [7] M. Morita, F. Araki, N. Yoshimoto, M. Ishikawa, H. Tsutsumi., Ionic Conductance of Polymeric Electrolytes Containing Lithium Salts Mixed with Rare Earth Salts., Solid State Ionics, (2000), 136, 1167. [8] P. Balaji Bhargav, V. Madhu Mohan, A. K. Sharma, V.V.R. Narasimha Rao, Investigation on Electrical Properties of (PVA:NaF) Polymer Electrolyte for Electrochemical Applications., Curr. Appl. Phys., (2009), 9, 165. [9] S. A. M. Noor, A. Ahmad, I. A. Talib & M. Y. A. Rahman, Effect of ZnO Nanoparticles Filler Concentration on the Properties of PEO-ENR50-LiCF3SO3 Solid Polymeric Electrolyte., Ionics (2011), 17, 451. [10] M. Ulaganathan & S. Rajendran: Preperation and Characterization of PVAc/ PVDFHFp)-based polymer Blend Electrolytes. Ionics (2010), 16, 515. [11] E. M. Abdelrazak; Curr. Appl. Phys., (2010), 10, 607. [12] S. Rajendran, M. Siva Kumar, R. Subadevi., Effect of Salt Concentration in Polyvinyl Alchohol Based Solid Polymer Electrolytes. J. Power Sources, (2003), 124, 225. [13] S.A.M. Noor, A. Ahmad, I.A. Talib, M.Y.A. Rahman., Morphology, Chemical Interaction, and Conductivity of a PEO-ENR50 Based on Solid Polymer Electrolyte., Ionics (2010), 16, 161.

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