Dielectric investigations in PVA based gel electrolytes

Progress in Crystal Growth and Characterization of Materials 52 (2006) 61e68 www.elsevier.com/locate/pcrysgrow

Dielectric investigations in PVA based gel electrolytes Arvind Awadhia, S.K. Patel, S.L. Agrawal* Solid State Ionics Laboratory, Department of Physics, A.P.S. University, Rewa, Madhya Pradesh 486003, India

Abstract Keeping in view the role played by dielectric properties in ion conduction behaviour of electrolytes, the present paper deals with dielectric investigations in PVA based gel electrolytes. In the present work, dielectric properties of the gel electrolytes based on PVA and PVAePEG prepared in NH4SCN (ammonium thiocyanate) solution have been studied in the temperature range 273e373 K over the frequency range 40 Hze100 KHz. Conductivity (s), segmental (a) and dipolar (b) relaxation have been observed in temperature and frequency dependent studies of dielectric loss. The relaxation time for these processes seems to follow Arrhenius nature with energy of activation being quite low in comparison to solvent free PVAe NH4SCN complexes. Ó 2006 Elsevier Ltd. All rights reserved. PACS: 66.10.Ed; 77.22.Ch; 77.22.Gm; 82.45.Gj; 82.45.Wx; 83.80.Kn; 83.80.Tc; 81.20.Fw Keywords: A1. Dielectric behaviour; B1. Gel electrolytes; B1. Polymer electrolytes; B1. Polymer composite electrolytes; B2. Fast ion conductors; A3. Sol-gel technique

1. Introduction In recent years, polymer gel electrolytes have become prominent electrolyte materials for different electrochemical device applications (supercapacitors, fuel cells, batteries and ECDs, etc.) owing to their distinct favourable properties like high ionic conductivity approaching to that of liquid electrolyte and wide temperature window [1,2]. Though a variety of polymeric gel electrolytes have been developed over the years, most of the reports have been focussed * Corresponding author. E-mail address: [email protected] (S.L. Agrawal). 0960-8974/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pcrysgrow.2006.03.009

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around polymers, polymethylmethacrylate (PMMA) and polyvinylidene fluoride (PVdF) [3e 6]. In these electrolyte systems, conductivity behaviour has been either explained on the basis of breathing chain model proposed by Chandra et al. [3] or by Callion-Caravanier et al. [7] model. Deepa et al. [8] and Sekhon et al. [9] have considered dielectric permittivity term in explaining ionic conductivity of PMMA based acid doped gel electrolytes though the role of dielectric constant and loss has not been extensively studied to ascertain their impact on conductivity. Further, it should be noted that the study of dielectric relaxation processes in polymeric systems helps us in understanding the molecular motions and their interactions which are immensely affected by chemical composition, molecular structure and morphology of the sample being examined [10]. Extensive and intensive survey of literature revealed that very little work has been done on dielectric studies of polymeric gel electrolyte in understanding their electrical properties [11]. Hence, an attempt has been made here to investigate dielectric properties of PVA and PVAe PEG blend based gel electrolytes in NH4SCN solution. 2. Experimental Polyvinyl alcohol (Av. MW 124 000e186 000, Aldrich make), polyethylene glycol (Av. MW 1000, Aldrich make), ammonium thiocyanate (NH4SCN) (AR grade, Sd Fine chem. make), and dimethyl sulfoxide (AR grade, Sd Fine chem. make) were used for the preparation of polymer gel electrolytes. The polymeric gel electrolytes of PVA and PVAePEG were prepared in ammonium thiocyanate solution in DMSO by the well known solegel process. Concentration of polymer was fixed to 10 wt% in the gel electrolytes. The blend ratio of PEG:PVA was fixed at 60:40. As synthesized gel electrolytes were sandwiched between two platinum electrodes and impedance measurements were performed on a Hioki make LCR meter (Model 3520) in the frequency range 40 Hze100 kHz at various temperatures ranging between 293 K and 358 K. The dielectric constant and dielectric loss were evaluated from the impedance data. 3. Results and discussion 3.1. Dielectric permittivity studies Fig. 1(a) shows the variation of dielectric constant with frequency at three different temperatures for PVAeNH4SCNeDMSO gel electrolytes. It is observed from the figure that 30 decreases with frequency, the fall being rapid at lower frequencies and saturation appears in the higher frequency range e a feature typically observed in polymer electrolytes [11,12]. The decrease in dielectric permittivity with increasing frequency can be associated to the inability of dipoles to rotate rapidly leading to a lag between frequency of oscillating dipole and that of applied field. Further, the curve exhibits high magnitude of dielectric permittivity e a feature which is expected in gel systems and has also been reported by Browen et al. [13] on gel samples collected from shark electro-sensors. Such high dielectric constant values in polymeric gel samples can be attributed to high ionic conductivity on account of the presence of trapped liquid electrolyte within the polymer matrix and interaction of inorganic salts with PVA. When the temperature is raised, dielectric constant also enhances due to facilitation in orientation of dipoles in the polar dielectric PVA [14]. It is observed from Fig. 1(b) that blending of PEG with PVA enhances the dielectric permittivity by an order of magnitude in comparison to pristine gel electrolytes. However, the fall in 30

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Fig. 1. Variation of dielectric constant with frequency at 303 K ( ), 323 K (O) and 353 K (-) for (a) PVA and (b) PVA:PEG based gel electrolyte.

is relatively sharper with increasing frequency for blend based gel system. PEG is a well known plasticizer [15] on one hand and on the other PPG, PEG and other polyethers have been reported to form complexes with salt [16,17]. It has been shown that the oxygen atom present on polyether chain acts as an electron acceptor. The cation of salt forms transient cross-links with the chain segment by coordinating with oxygen atom on polymer backbone and thus leads to polymeresalt interaction. Similarly, PVA has been shown to interact with ammonium salts through coordination of anion group [18]. Thus NH4SCN is expected to interact with both PEG and PVA and thereby cause steric hindrance in rotation of dipoles as a result of cross-linking. Both these results tend to increase 30 value, but with a sharp fall with increasing frequency. The permittivity of polar dielectric materials typically increases on increasing the temperature because of decrease in viscosity of the system [19]. Further, in case of crystalline and semicrystalline polymeric materials, the crystalline phase dissolves progressively into amorphous phase with increase in temperature. This in turn influences the polymer dynamics and thus the dielectric behaviour. Fig. 2(a) and (b) shows the variation of dielectric constant with temperature of PVA and PVAePEG blend based gel electrolytes at 40 Hz. Similar variation was

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Fig. 2. Variation of dielectric constant with temperature at 40 Hz for (a) PVA and (b) PVA:PEG based gel electrolyte.

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noticed at higher frequencies as well. In Fig. 2(a) the change of dielectric constant for pristine gel electrolyte is similar to that of PVAeNH4SCN solvent free complex reported by Agrawal and Shukla [20] with the only difference in magnitude and shift in peak position. As against the earlier report, permittivity peaks in PVAeNH4SCNeDMSO gel electrolytes appear at 317 K, 339 K and 353 K, which tend to diminish with the increase in frequency. These peaks can be correlated to partial complexation of PVA and NH4SCN. Agrawal and Shukla [20] have correlated the increase of dielectric constant of PVAeNH4SCN solid polymer electrolytes to the reduction in the fraction of cross-links as a result of interaction between the components. Similar arguments can be applied here to explain large changes in dielectric permittivity. However, the presence of liquid within the polymer matrix further accentuates 30 value. Further the appearance of peaks in permittivity at 317 K and 339 K can be associated with decrease in viscosity of the system. The peak around 353 K can be related to the structural change around glass transition temperature of uncomplexed PVA. When PEG was added to pristine gel electrolytes for the formation of blend based electrolyte, only two permittivity peaks could be noticed in the dielectric spectra of Fig. 2(b) around 343 K and 363 K. Since Tg of PEG is 233 K and no peak related to this transition could be observed, it indicates the interaction of PEG with PVA and NH4SCN. Due to this miscibility, permittivity peak could be noticed at an intermediate temperature. 3.2. Dielectric loss studies Fig. 3(a) and (b) shows the variation of dielectric loss with frequency at three different temperatures for both PVA and PVAePEG based gel electrolytes. In Fig. 3(a) dielectric loss peak is observed around 266 Hz at 353 K, which shifts towards lower frequency regime as temperature progressively goes down. On enhancement of temperature, the motion of charge carriers becomes easier and thus can be relaxed even at higher frequency. Normally in ion conducting polymers, conductivity relaxation peak appears well above Tg or at very low frequency and is slow in nature due to repeated step diffusion of carriers for relaxing the field [21]. The observed relaxation peak seems to satisfy this argument and so can be correlated to s relaxation. This peak seems to disappear in PVAePEG blend based gel electrolytes probably due to shift of

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Fig. 3. Variation of dielectric loss with frequency at 303 K ( ), 323 K (-) and 353 K (O) for (a) PVA and (b) PVA: PEG based gel electrolytes.

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peak towards lower frequency regime which is beyond the range of present study. Further, the loss value of PVAePEG blend based gel electrolyte is two orders of magnitude higher than that of PVA based gel electrolyte indicative of enhanced relaxation. Fig. 4(a) and (b) shows the variation of dielectric loss with temperature at three frequencies. Fig. 4(a) reveals three relaxation peaks around 312.5 K, 332 K and 353 K which seem to shift towards higher temperature in PVA based gel electrolyte. On the contrary only one relaxation peak is noticed around 343 K at 40 Hz (Fig. 4(b)) for PVAePEG based gel electrolytes. Further, the shift is smaller in magnitude for the peak around 353 K. Agrawal and Shukla [18] have shown a decrease in Tg of PVAeNH4SCN pristine complexes due to strong interaction between the components. Keeping this in mind and DSC results on PVAeNH4SCNeDMSO gel electrolytes, 332 K relaxation peak can be associated to structural relaxation i.e. a relaxation. In ion conducting polymers dipolar relaxation, expressed by b relaxation, is observed well below Tg [22]. Such behaviour has also been reported for PVAeCoCl2 and PVAeNH4SCN complexes [20,23]. Therefore, the shoulder relaxation noticed around 313 K can be correlated to b relaxation and attributed to cooperative motion of several repeat units containing dipoles. The high temperature (353 K) peak can be assigned to conductivity relaxation as this occurs at a temperature well above a relaxation and due to low magnitude. It is expected that two polymers having quite different a loss peak, temperature perturb their respective local environment to produce a single loss peak [9]. In case of blend based system the only observed relaxation at 343 K corresponds to a relaxation owing to Tg of the blend as dictated by Fox relation [15]. The presence of single relaxation loss peak around a temperature intermediate to a relaxation of unbounded system can be attributed to good miscibility of polymers. It is also noticed from Fig. 4(b) that when the frequency is increased one additional shoulder peak becomes visible. This might be due to conductivity relaxation process. 3.3. Loss tangent studies Loss tangent curves of Fig. 5(a & c) and (b & d) exhibit the presence of three shoulder peaks for pristine gel electrolytes (313 K, 323 K and 333 K) and two shoulder peaks for blend based electrolytes (328 K and 338 K) respectively. As the frequency of measurement is increased two

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Fig. 4. Variation of dielectric loss with temperature (in K) at 40 Hz (B), 500 Hz (-) and 1 kHz (O) for (a) PVA and (b) PVAePEG based gel electrolytes.

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Fig. 5. Dielectric loss vs temperature (in K) at 40 Hz (O) and 500 Hz (B) for PVA (a & c) and PVAePEG (b & d) based gel electrolyte.

of the peaks (323 K and 333 K in case of pristine system and 328 K and 338 K in case of blend based system) coalesce to form a single broad peak, thereby constituting ab relaxation peak as also reported for PVA based system earlier [20,23]. The other relaxation peaks have already been described earlier in dielectric loss studies. Fig. 6 depicts the variation of relaxation time with temperature for different relaxation processes. As indicated earlier, both PEG and PVA form complexes with NH4SCN. Therefore it is quite likely that PEG gets attached to PVA chain via the pendent group of PVAeNH4SCN complex thereby decreasing molecular mobility of composite gels. As a consequence, the relaxation time becomes more significant as shown in the present study. The curves clearly seem to fit well in Arrhenius relation. t ¼ to expðDU=kTÞ

ð1Þ

where to is pre-exponential factor, DU is activation energy of process, k is Boltzmann constant and T is temperature in Kelvin. 4. Conclusion In the present investigation an attempt has been made to characterize PVA based polymeric gel electrolytes and PVAePEG blend based gel electrolyte through dielectric measurements. It

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-2 -2.5

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Fig. 6. Arrhenius plot of relaxation time for b- (O) and a- ( ) relaxation of PVA and PVA:PEG based gel electrolytes.

is realized that conductivity relaxation for PVAeNH4SCNeDMSO gel electrolytes appears at considerably lower temperature compared to PVAeNH4SCN solvent free polymer electrolytes. The a and b relaxation peaks appear approximately at the same position but with low activation energies. Presence of single relaxation peak based system suggests high miscibility of PVA with high molecular weight PEG.

Acknowledgement Authors are thankful to UGC, New Delhi for providing financial support in the form of research project towards completion of the present work.

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A. Awadhia et al. / Progress in Crystal Growth and Characterization of Materials 52 (2006) 61e68 J. Sandahl, L. Borjesson, J.R. Stevens, L.M. Torell, Macromolecules 23 (1990) 63. S.L. Agrawal, P.K. Shukla, Ind. J. Pure Appl. Phys. 38 (2000) 53. K.P. Singh, P.N. Gupta, R.P. Singh, J. Polym. Mater 9 (1992) 131. S.L. Agrawal, P.K. Shukla, Phys. Stat. Sol. (a) 163 (1997) 247. P.K. Shukla, Ph.D. thesis, A.P.S. University, Rewa, 1997. A. Tager, Physical Chemistry of Polymers, MIR Publishers, Moscow, 1978. M.A. Elshahawy, M.M. Elkholy, Eur. Polym. J. 30 (1994) 259. Dr. S.L. Agrawal is presently working as associate professor at A.P.S. University Rewa. His current research interests are in the area ion and electron conducting polymers, ECD/smart windows, magnetic transducers and solar energy conversion device.

Mr. Arvind Awadhia is part time lecturer at Physics Department, A.P.S. University, Rewa. He has submitted his doctoral thesis on ion conducting polymer gel electrolytes. His research interest are ionics, energy devices and smart windows.

Mr. S.K. Patel is Part time lecturer in Govt. Engg. College, Rewa and pursuing his doctoral degree at A.P.S. University, Rewa in the area of composite polymer electrolytes.