Structural, thermal and electrical characterizations of PVA:DMSO:NH4SCN gel electrolytes

Solid State Ionics 178 (2007) 951 – 958 www.elsevier.com/locate/ssi

Structural, thermal and electrical characterizations of PVA:DMSO:NH4SCN gel electrolytes Arvind Awadhia ⁎, S.L. Agrawal SSI Laboratory, Department of Physics, A.P.S. University, Rewa — 486003, India Received 28 December 2006; received in revised form 12 March 2007; accepted 10 April 2007

Abstract An attempt has been made in the present work to cast stable polymeric gel electrolytes by drying as-synthesized gel electrolytes which were prepared by sol gel process. The polyvinyl alcohol (PVA) has been used as matrix in the gel to hold ammonium thiocyanate (NH4SCN) dissolved in dimethyl sulfoxide (DMSO). DSC studies reveal the presence of pristine materials along with PVA–NH4SCN complexes. The FTIR spectral studies affirm complex formation. Ionic conductivity of gel electrolytes shows maxima at 6 wt.% of PVA. The conductivity response has been correlated to complexation behaviour. Temperature dependence of ionic conductivity revealed the existence of liquid like conduction at lower wt.% of incorporated PVA whereas contribution to conductivity by polymeric complexes enhanced at higher wt.% of polymer. © 2007 Elsevier B.V. All rights reserved. Keywords: Polymer electrolytes; Polymer gel electrolytes; Ionic conduction; Protonic conduction; Ionic transference number; FTIR; DSC

1. Introduction Within the realm of ionic conductors, polymer electrolytes have become materials of great interest for applications in different electro-chemical devices due to their distinct properties like mouldability in various physical shapes, light weight, good electrode–electrolyte contact and favourable mechanical properties [1,2]. Conceptually, they form a bridge between conventional liquid electrolyte and solvent free ceramic or molten electrolyte [3]. The ionic conductivity of solvent free polymer salt complex (SPE) is associated to segmental motion of polymers and concentration of charge carrier [4,5] and this limits its utility in electro-chemical devices like solid state batteries and fuel cells. Various methods have been employed to improve the ionic conductivity of polymer electrolytes which have led to development of new routes of preparation of polymer electrolytes. Recently, incorporation of inorganic/organic filler, like ceramic [6,7] and dual phase block copolymer [8] has become an interesting route which leads to formation of composite polymer electrolytes (CPE). However, in these electrolytes, particle size ⁎ Corresponding author. Tel.: +917662230532, +919425363553. E-mail address: [email protected] (A. Awadhia). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.04.001

and concentration of insulating matrix decide their conductivity. The conductivity of polymer electrolytes has long been viewed to be confined mostly in amorphous phase and the crystalline phases are considered as poor conductivity phase. Thus, attempts have also been made to improve the amorphous phase by introduction of a plasticizer in small amounts which may be a low molecular weight substance [9]. An alternative approach to enhance ionic conductivity comparable to that of liquid electrolyte is the formation of polymer gel electrolyte or hybrid electrolyte [9–14]. In most cases, polymer is reported to be not taking part in the conduction process rather acting as stiffener. In contrast to solid polymer electrolytes, gel electrolytes provide liquid like medium for fast ion transport and thus are able to improve ionic conductivity by an order of magnitude approaching that of liquid electrolytes [9,15]. On account of their high ionic conductivity, gel electrolytes have attained precedence over conventional polymer electrolytes. In recent years, few reported gel electrolytes are polymer network of polymethyl methacrylate (PMMA) [12,16,17], polyacrylonitrile (PAN) [18], polyvinylidenefluoride (PVdF) [16,19] and polyethylene oxide (PEO) [15,16]. Literature survey has revealed that most of the work reported on gel electrolytes concerns lithium based polymer batteries [10,11,13,16]. Proton conductors are equally important class of

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electrolyte due to their applications in fuel cells, ECDs and other smart devices [20]. Within the family of proton conductors, work on PEO based gel electrolytes with strong acids like H3PO4, H2SO4 and HCl [15,21] and PMMA based gel electrolytes containing benzoic and dicarboxylic acids [16–22] have been reported in recent years. PVA is yet another polymer for development of gel electrolyte on account of its good solvent holding capability and wide temperature window [23]. Moreover, it is known to form good gels for medical applications [24]. Thus in the present investigation, an attempt has been made to synthesize and characterize PVA based gel electrolyte with ammonium salt for possible proton conduction. Generally, when most of the studied gel electrolytes are retained for a long period, exudation of liquid from gel lump occurs which may cause instability in device performance. The as-synthesized gel electrolytes of PVA:DMSO:NH4SCN also show similar behaviour with aging. Therefore, with anticipation of better stability under ambient conditions, as-synthesized gel electrolytes were dried up to some extent at the expense of small value of conductivity [25]. In our earlier studies [25] it was found that formation of few new complexes, definitely, plays a significant role in conductivity behaviour. Thus, in present work spectroscopic studies have been performed to realize the type of complexes that are responsible for charge transport and the effect of drying on these complexes. Attempt has also been made to correlate DSC and FTIR spectroscopic results. The correlative study of conductivity of as-synthesized and dried gel electrolytes has been further made in view of earlier results [26,27] and FTIR, DSC and transference number measurements performed for present investigation. 2. Experimental In the present investigation, polyvinyl alcohol (PVA) (average molecular weight 124,000–186,000), Aldrich make, ammonium thiocyanate (NH4SCN), AR grade, sdfine chem make and aprotic solvent dimethyl sulfoxide (DMSO) AR grade, sdfine chem make, were used for the synthesis of proton conducting gels. PVA was dispersed in different weight ratios in 0.2 M solution of NH4SCN electrolyte prepared in DMSO. Addition of polymer to liquid electrolyte was carried out very slowly for formation of homogeneous gel. After syneresis, gels in form of thick films were taken out. These as-synthesized gel samples were cut into two parts. One part of sample was dried at ∼ 313 K for a week to obtain stable gel electrolytes. Few of these dried gels were scratched with the help of knife edge and a small amount of scratched fine samples were pelletized with KBr. The FTIR spectra of these pellets were recorded by Perkin Elmer Spectrum Rxl in the range 3800 cm− 1–500 cm− 1. The dried gel samples along with pristine materials were also thermally characterized through differential scanning calorimetry (DSC) on Dupont Series 2000 Thermal Analyzer, USA with scanning rate of 5 K min− 1 in a nitrogen purged cell. The scanning temperature range was 298–523 K. Both parts of polymer electrolyte samples (i.e. as-synthesized gels and dried gels), having a thickness around 0.1 mm– 0.15 mm, were sandwiched between two platinum electrodes

and electrical conductivity were assessed by impedance measurement on Hioki make LCR meter (model-3520) in the frequency range 40 Hz–100 KHz at various temperatures ranging between 287 K and 373 K. Dielectric spectroscopy was also carried out on dried gel samples using LCR meter at ambient temperature to understand its role on ionic conductivity. Wagner's method [28,29] has been used to assess ionic transference number (ti) and the nature of charge transport. In this technique samples were sandwiched between two blocking electrodes (platinum in the present case) and current monitored as a function of time upon application of dc potential (0.5 V). 3. Results and discussion 3.1. DSC studies It is well established that crystalline and amorphous phases in variable amounts co-exist in most of the polymeric materials [30,31]. PVA is one of the partially crystalline polymer exhibiting both the glass transition (characteristic of amorphous phase) and melting transition (characteristic of crystalline phase) supporting the above argument. This is also evidenced in DSC thermograms of Fig. 1 which represents the thermograms for pure PVA and NH4SCN along with few dried gel electrolytes. The glass transition temperature around 363 K and melting temperature around 489 K obtained for pure PVA (curve b) are in close proximity to the earlier reported results [5,32]. Since polymer gel electrolytes are characterized by salt solution (liquid electrolyte) trapped within the polymer network, thermal behaviour of gel electrolyte is expected to be affected on account of interaction between components and as also evidenced in the present study. When 2 wt.% of PVA was added to 0.2 M ammonium thiocyanate electrolyte to form a gel, the melting endotherm as well as glass transition temperature did not change (curve c) on one hand while the melting endotherm related to NH4SCN appears to broaden up (in comparison to curve a) on the other. The considerable broadness of salt melting peak indicates multiple states of salts i.e. salt aggregates, partially solvated, and partially/fully complexed salt within polymer. Since the salt quantity is comparable to that of polymer and due to oozing of most of the liquidus part during drying process, presence of large number of NH4SCN molecules/aggregates is likely which are yet to be accommodated or otherwise dissociated by polymer gel matrix. Besides, presence of traces of liquidus part also influences the melting of salt resulting in the said broadness. When content of incorporated polymer is enhanced to 4 wt.% in the electrolyte, the fraction of complex shows dominance as evidenced from the existence of broad endothermic transition (curve d) in the temperature range 423–483 K. PVA is known to form proton conducting complexes with ammonium salts and acids [5,29]. Thus the observed broadness can be assigned to co-existence of uncomplexed NH4SCN electrolyte, PVA– NH4SCN complex, PVA:DMSO complex and uncomplexed PVA [26]. The disappearance of salt melting in gel samples prepared with higher polymer content (5.5 wt.% and 8 wt.% PVA viz. curves e and f) revealed full utilization of salt in PVA:

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trolytes prepared with different wt.% of PVA. The peaks have been identified and listed in Table 1. The comparison of FTIR spectrum of DMSO casted PVA film (d) with that of pure PVA (c) and DMSO (b) reveals presence of few new absorption lines at wave-numbers (cm− 1) (i) 752 (ii) 848 (b) (iii) 1352 (iv) 1500 (v) 1598 (vi) 1647 (vii) 2145 (viii) 2235 (ix) 2341 (x) 2367.7 (xi) 2529.4 and (xii) 2729.4. These lines show very small deviation from the lines reported for PVA:DMSO as-synthesized gel electrolytes [27] and correspond to PVA:DMSO complex. The little discrepancy is due to either the sampling method or change in denseness of polymer network. In contrast to FTIR spectra of PVA:DMSO as-synthesized gel electrolytes, some of the lines related to PVA (905, 1091, 1376, 1452 in cm− 1) are retraced which is on account of removal of most of the liquidus part (i.e. DMSO, water, DMS). The otherwise extremely broad υ(OH) absorption peaks became distinctly visible in the present case due to shrinking of broadness. These changes can be attributed to removal of liquidus part (i.e. DMS, water and DMSO) in general and in particular DMSO. Besides, in IR spectrum-d, line related to υ(S_O) is also observed, but it is very weak which reflects still the presence of DMSO in very small fraction and attached to polymer via oxygen atom. Further a new peak at 1058.8 cm− 1 is also present which indicates that some of the DMSO is either free or attached through sulphur atom [33].

Fig. 1. DSC thermogram of (a) NH4SCN (b) pure PVA and gel electrolyte samples containing (c) 2 wt.%, (d) 4 wt.%, (e) 5.5 wt.% and (f) 8 wt.% PVA.

NH4SCN complex formation. Close observation of DSC thermogram (curve d) indicates a strong endothermic transition around 308 K which shifts toward high temperature with increase of polymer concentration. The occurrence of this endotherm is related to interaction of DMSO with PVA. It has been shown [26] that DMSO interacts with PVA and reduces to dimethyl sulfide (DMS). Reduction is further supported by the presence of NH4SCN and hence the occurrence of this endotherm can be attributed to removal of DMS and detachment of DMSO. The detachment of DMSO/DMS becomes difficult upon deterioration in salt fraction or increased packing density of matrix with enhancement of PVA content. Therefore, the shift in endotherm (from 313 K to 340 K) is possibly due to enhancement of physical attachment of DMSO with polar group of PVA. This was also experienced during synthesis i.e. upon enhancement in PVA content, rate of gelification became faster. 3.2. FTIR spectral studies Fig. 2 shows the FTIR spectra of NH4SCN, DMSO, pure PVA, DMSO casted PVA dried film and few dried gel elec-

Fig. 2. FTIR Spectra of (a) NH4SCN (b) DMSO (c) pure PVA (d) DMSO casted PVA film (dried) and dried gel electrolytes prepared with (e) 2 wt.% (f) 5 wt.% (g) 6 wt.% and (h) 8 wt.%.

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Table 1 Assignments of IR absorption bands (in wave numbers) of NH4SCN, DMSO, PVA and dried gel electrolytes PVA

NH4SCN

DMSO

DMSO casted PVA film

3615–3050

– –

3555– 3329 2996

3433– 2870 –

2930 – – – –

– – – – –

2913 – – – 2091

2932 2729 2367.7 2341.4 2139

– – – – 1731 1642 – – – 1446 1437 – 1377 – 1326 1262 1256 1215 1145(sh) 1096 – – – 949 920 850 –

2060 – – – – – – – – – – 1410 – – – – – – – – – – – 952 – – – 752.5 – – – – –

– 1995 – – – 1659 – – – – 1437 1407 – – 1311 – – – – – – 1050 966 – – – – – 700 670 – – –

– – – – – 1647 – 1517 1500 1452.9 1435 1394 1376 1351 – – 1229 – 1135.3 1091 1058 1029.5 – – 905 848 – 752.9 – 658.8 647 617 597.2

– – 640 609 –

Dried Gel Electrolytes with PVA content (wt.%)

Assignments

2

5

6

8

3624– 3285 3285– 2950 2922 2725 2371 2340 2292– 2152 2065 – – 1805 1721.4 – 1598.1 – – – – 1400.8 – 1353 – 1286 1250 – – 1109 – 1023 – 951 – 837 – 763 695.9 – – 585–608 564.2

3624– 3323 3323– 2872 2935 2713 2362.5 2327.5 –

3624– 3060 –

3624– 3060 –

C–H stretch (asym and submethyl)

– 2748 2367.3 2339.2 2222– 2152 2069.5 – 1871 1805 1777 1625.7 1598 – 1488 1462 – 1386 – 1352 – – – – 1146.2 1111.1 1068.2 1024.7 – – – 888.9 818 765.1 – – 643 – –

– 2725 2367.7 2316 –

Sym C–H stretch C–H stretch Asym υ(SH) and acetal overtone Sym υ(SH) and acetal overtone NCS asy. stretch and overtone υ(S_O)

2064 – 1836 1805 1754 1625.7 1598 – – 1456.1 – 1387 – 1351 – 1274 1239 1198.8 1146.2 1111.1 1066.5 1017.5 – – – 894.7 – 764 707 – 640.9 – 696.5

C`N stretch – Combination of COC bend CO stretch CHvib. in vinyl compound C_O and C_N stretch C_O stretch, combination of υ(CH) and υ(CS) and C_C stretch δ(H2O) – COC bend OH and CH bending CH2 bending NH def. vib. and asy δ(CH3) CH2 wagging CH def. vib. CH–OH bending and CH3 inplane def – – CH wagging C–C and C–O stretch C–O stretch and O–H bend and CH def S_O stretch S_O stretch and C–S stretch CH3 rock NCS bending vib. overtone Skeletal Skeletal; C–O stretch – C–S stretch Asym C–S stretch Sym C–S stretch O–H twisting CH wagging –

2067.5 – – – – 1625 1597 – – 1450 – 1400.1 – 1352.9 – – 1239 1216 1140 1193 1064.3 1027 – 953 900 830–853 – 760.2 – – – – –

The spectra e–h exhibit absorption peaks related to PVA: NH4SCN complex [26]. In addition, an absorption peak at 1721.4 cm− 1 is distinctly observed in spectrum e and related to υ(C_N) which corroborates the complexes reported earlier by Agrawal and Shukla [5]. Simultaneously, in spectrum-e absorption line around 1648 cm− 1 seems to be very weak which reveals that most of OH group of PVA transforms into the PVA:NH4SCN complex instead of oxidizing to form ketones (C_O). Further, the peak at 1721.4 cm− 1 merges with peaks of υ(C_O) and δ(H2O) with enhancement of PVA content in the gel system (spectra f–h). This phenomenon is attributed to enhanced formation of (C_O) group by oxidation of PVA and supported by comparative change in the intensity of line 1351 cm− 1 with respect to line 1400 cm− 1. Similarly, increase in intensity of

N–H and O–H stretch

2333 cm− 1 and 2366 cm− 1 lines are reflections of inter-chain linking by acetal formation which results in loss of flexibility of samples and also evidenced by their physical and mechanical appearance. Besides, spectra (e–h) show strong absorption peak related to NH4SCN at around 1400 cm− 1 and 951 cm− 1. These peaks appear to be more pronounced in comparison to that for assynthesized polymer gel electrolytes. Spectra (e–h) assert the fact that enhancement of PVA content diminishes the intensity of these lines on account of effective solvation by PVA or complexation of remaining salt molecule with PVA. The formation of complex is more probable because υ(C`N) absorption line is also found to diminish with enhancement in PVA content. On the basis of these arguments, the formed complex with possible protonic conduction is foreseen as in Scheme 1.

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Scheme 1.

3.3. Wagner's polarization Fig. 3 depicts the Wagner's polarization curves for two dried gel electrolytes. The ionic transference number were evaluated using the relation tion ¼

Iinitial  Ifinal Itotal  Ielectronic Iionic ¼ ¼ Iinitial Itotal Itotal

conductivity (up to 4 wt.% of PVA content) and dominance of PVA–DMSO–NH4SCN complexes contribution on conductivity at high polymer content. The conductivity response has been explained in the following section. 3.4. Conductivity studies

ð1Þ

where Iionic is current due to ions, Ielectronic (or Ifinal) is current due to electrons and Itotal (or Iinitial) is sum of ionic and electronic contribution to current. Table 2 lists tion of some of these dried gel electrolytes. It is evidenced from this data that the nature of charge transport is similar to solvent free solid polymeric electrolytes [5]. Moreover the charge transport appears to be assisted by mobile ions trapped within the polymer matrix/ complex. It is noticed from the table that tion gradually decreases upon addition of PVA upto 4 wt.% and thereafter increases. Such a behaviour can be associated to loss of ionic

Fig. 4 depicts the variation of ionic conductivity of assynthesized gel electrolytes (curve a) and dried gel electrolytes (curve b) as a function of polymer content incorporated initially. The conductivity of liquid electrolyte (the first point of both curves a and b) was found to be 1.5 × 10− 2 S cm− 1. Dimethyl sulfoxide is a highly aprotic solvent and ammonium thiocyanate is likely to dissociate into respective ions [34], as reflected from high ionic conductivity. The conductivity of as-synthesized gel (curve a) generally decreases linearly with incorporation of PVA upto 12 wt.% (by weight fraction of initial constituents) and very small change is observed in the range of PVA considered in the present investigation. Addition of polymer to liquid electrolyte is expected to significantly change the viscosity resulting in large change in conductivity. However, a decline of conductivity by small degree is observed. This behaviour have been described on the basis of concept of microscopic viscosity, dissolution action by breathing of polymer and formation of proton conducting complex and reported elsewhere [26]. Enhancement of conductivity due to these factors in association Table 2 Ionic transference number (tion) of dried gel electrolytes s.n.

Fig. 3. I–t polarization curve of as-synthesized gel electrolytes with (a) 4 wt.% (- - -) and (b) 8 wt.% (—) PVA.

1 2 3 4 5 6 7

PVA weight % incorporated in gel

tion

2 3 4 5 6.6 8 11

0.98 0.60 0.50 0.76 0.80 0.87 0.93

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Fig. 4. Variation of conductivity (in log10) of (a) as-synthesized gel electrolytes and (b) dried gel electrolytes as a function of polymer concentration.

with the concept of microscopic viscosity reduces the effect of enhanced macroscopic or shear viscosity [16,17]. When these as-synthesized gel electrolytes are dried (i.e. dried gels), an interesting conductivity behaviour is observed (curve b). The conductivity initially is seen to drop down significantly till 5 wt.% PVA. Beyond this concentration a small increase in ionic conductivity is noticed to attain an optimum at 6 wt.%. This can be understood from the following discussion. Normally, ionic conductivity of electrolyte depends upon both charge carrier concentration, n and carrier mobility μ as described by the relation r ¼ nd qd l

beside the dissolution of NH4SCN by polymer. Both these factors coupled together appear to cause a significant fall in ionic conductivity of dried gel electrolytes in accordance to Eq. (2). It is worthy to mention here that the liquid uptake ability of gel samples also depends upon the salt concentration in gel system which influences ε as well as conductivity [26,35]. Beyond 5 wt.% of PVA small increase in ionic conductivity is similar to the reports on PMMA based electrolytes [22,36] though at a still lower concentration of polymer. Chandra et al. [22] have tried to explain this rise in ionic conductivity with the help of breathing chain model wherein they showed enhancement in charge carrier concentrations as a result of ion dissociation resulting from folding and unfolding of polymeric chains. In this model, they considered polymeric gel comprising of dissociated ions, ion pairs in the solvent and polymeric chains which may have folded and unfolded and partially folded. The same picture can be visualized up to some extent here to explain our results i.e. dissociation of ions by local pressure fluctuation leading to an increase in the value of n. The high dissociation of salt at higher polymer concentration has already been revealed in DSC as well as FTIR spectral studies wherein melting/ absorbance peak related to NH4SCN seems to disappear. The increase in charge carrier concentration at these polymer contents is further affirmed from dielectric constant values (Table 3) and Eq. (3). It has been shown in earlier studies [5,26,27] that PVA forms complexes with NH4SCN and DMSO which are highly proton conducting materials. Therefore, the formation of

ð2Þ

with q representing the charge of mobile carrier. The charge carrier concentration, n depends upon both the dissociation energy U involved and dielectric constant ε as n ¼ no expðU =ekT Þ

ð3Þ

where k is Boltzmann constant and T the absolute temperature. Upon addition of PVA in the electrolyte, ε was experimentally found to decrease during dielectric measurements (Table 3). This means that the decrease in ε would lead to decrease in value of n. Further, addition of PVA enhances the microscopic viscosity of the system and becomes more pronounced due to drying process. This causes a fall in mobility of charge carriers

Table 3 Dielectric values of gel samples and pristine materials Frequency

500 Hz 1 kHz 10 kHz a b

Dielectric constant, ε, of samples a DMSO

Liquid electrolyte b

Weight percent of PVA in gel 2 wt.%

5 wt.%

6.6 wt.%

8 wt.%

15 7.05 0.7

10771.7 3341.8 61.8

200.6 134.2 17.5

58 31 2.3

250.3 117.1 4.8

1.4 0.7 0.08

ε corresponds to values after division by 103. 0.2 M solution of NH4SCN in DMSO.

▪

Fig. 5. Conductivity as a function of temperature for (a) liquid electrolyte (▴) and gels containing (b) 3 wt.% ( ), (c) 5 wt.% (○), (d) 6 wt.% (–), (e) 6.6 wt.% (+) and (f) 8 wt.% (●) PVA.

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these complexes throughout the entire gel matrix in association with presence of DMSO nourishes the conductivity. The two opposing factors i.e. (i) increase in conductivity due to complex formation and increase in charge carrier concentration on one hand and (ii) fall in conductivity due to increase in microscopic viscosity on the other try to counterbalance. Consequently, an increase in conductivity till 6 wt.% of polymer in the gel has been observed. Beyond this concentration, viscosity of the system as well as tortuousity of conduction path increases significantly due to increase in fraction of uncomplexed PVA and therefore there is a considerable fall in conductivity as observed in curve b (Fig. 4).

occupied by polymer for complex formation. The FTIR spectral studies affirm the DSC results along with identification of PVA: NH4SCN and PVA:DMSO complexes. These complexes are seen to influence the conductivity behaviour. The conductivity behaviour of dried gel prepared with lower polymer concentration still exhibit liquid like nature besides the drying of the sample and very small influence is noticed by polymeric matrix. On the other hand, at higher polymer concentration, polymeric nature of conducting phase is clearly observed. The temperature dependence of conductivity reflects VTF behaviour.

3.5. Temperature dependence of conductivity

Authors are thankful to M/s Birla Ericssion Optical Ltd., Rewa and Central Drug Research Institute, Lucknow for providing DSC and FTIR spectroscopy facilities respectively. Authors are also grateful to UGC, N. Delhi for the financial support in the form of minor research project. Thanks are also due to Prof. S. Chandra, BHU Varanasi, Prof. K. Sharma, Govt. Model Sc. College, Rewa and Mr. V. P. Singh, Rewa for their stimulated and fruitful discussion. The help taken by SDBS Web: http://www.aist.go.jp/RIODB/SDBS for IR spectral analysis is also duly acknowledged.

Fig. 5 shows the temperature dependence of conductivity of 0.2 M NH4SCN electrolytes casted in DMSO along with gel electrolytes containing different wt.% of PVA in the gel. The conductivity of liquid electrolyte is seen to change very slowly with temperature (curve a). Such behaviour is obvious because dissociation in liquid is almost complete and as a consequence increase in concentration of mobile ions with temperature is unlikely. The gel electrolytes appear to follow behaviour similar to that of liquid electrolyte, particularly at lower concentration of the polymer in the gel. Though the variation of conductivity with temperature for liquid electrolyte is different from that of dried gel electrolytes, the rate of fall in conductivity with 1/T in the two cases is gradual and within an order of magnitude. This feature is in contrast to PVA–NH4SCN solvent free complexes wherein two arrhenius regions separated by non-linear variation has been reported [37]. Observed difference in slopes of gel electrolytes and liquid electrolyte can be attributed to formation of complexes of polymer and salt as also evidenced in DSC thermograms and FTIR spectra. Similar behaviour has been reported for PMMA and PVdF based gel electrolytes [22,38–40]. Critical examination of curves (b–f) indicate that ionic conduction of dried gel electrolytes can be best described by VTF relation [1] r ¼ ro T 1=2 exp  B=ðT  To Þ

ð4Þ

where all the terms carry their usual meanings. Besides exhibiting VTF behaviour, the temperature dependence of conductivity also depicts significant variation for the gel films containing higher wt.% of PVA i.e. beyond 6.6 wt.% of PVA. This is on account of increased contribution of PVA:DMSO: NH4SCN complexes on conduction mechanism as evidenced by DSC and IR studies. 4. Conclusion Dried polymer gel electrolyte of PVA:DMSO:NH4SCN has been obtained from as-synthesized system and characterized thermally, structurally and electrically. Thermal studies revealed that large fraction of uncomplexed/undissociated salt aggregates are present in the dried gel samples prepared with small amount of PVA whereas at higher wt.% of PVA content all the salt is

Acknowledgements

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