Chinese Chemical Letters

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1 Chinese Chemical Letters 25 (2014) Contents lists available at ScienceDirect Chinese Chemical Letters journal homepage: Original article Lithium ion conduction and ion-polymer interaction in poly(vinyl pyrrolidone) based electrolytes blended with different plasticizers K. Kesavan, Chithra M. Mathew, S. Rajendran * School of Physics, Alagappa University, Karaikudi, Tamilnadu , India ARTICLE INFO ABSTRACT Article history: Received 7 February 2014 Received in revised form 25 April 2014 Accepted 19 May 2014 Available online 13 June 2014 Keywords: Polymers Thermogravimetric analysis (TGA) X-ray diffraction Luminescence Transport properties Poly(ethylene oxide), poly(vinyl pyrrolidone) (PEO/PVP), lithium perchlorate salt (LiClO 4 ) and different plasticizer based, gel polymer electrolytes were prepared by the solvent casting technique. XRD results show that the crystallinity decreases with the addition of different plasticizers. Consequently, there is an enhancement in the amorphousity of the samples responsible for the process of ion transport. FTIR spectroscopy is used to characterize the structure of the polymer and confirms the complexation of plasticizer with host polymer matrix. The ionic conductivity has been calculated using the bulk impedance obtained through impedance spectroscopy. Among the various plasticizers, the ethylene carbonate (EC) based complex exhibits a maximum ionic conductivity value of the order of Scm 1. Thermal stability of the prepared electrolyte films shows that they can be used in batteries at elevated temperatures. PEO (72%)/PVP (8%)/LiClO 4 (8%)/EC (12%) has the maximum ionic conductivity value which is supported by the lowest optical band gap and lowest intensity in photoluminescence spectroscopy near nm. Two and three dimensional topographic images of the sample having a maximum ionic conductivity show the presence of micropores. ß 2014 S. Rajendran. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. 1. Introduction Polymer blending is one of the most important contemporary ways for the development of new polymeric materials and a useful technique for designing materials with a wide variety of properties [1]. The significant advantages of polymer blends are that the properties of the final product can be tailored to the requirements of the applications. The manifestation of superior properties depends upon the miscibility of the blend. Blending can, however, have profound and sometimes unexpected effects on thermal stability, which cannot simply be predicted on the basis of the behavior of the components and their relative proportions [2]. A common approach is to add low molecular weight plasticizers to the polymer electrolyte system [3]. There has been significant work carried out on the pristine and plasticized PEO lithium salt systems [4 7]. The plasticizers impart salt-solvating power and high ion mobility to the polymer electrolytes. Poly(ethylene oxide) is the most interesting of the base materials because of its high chemical and thermal stability. PEO is a semicrystalline polymer, possessing both amorphous and crystalline * Corresponding author. address: sraj54@yahoo.com (S. Rajendran). phases at room temperature. It can solvate a wide variety of salts even at very high salt concentrations [8]. On the other hand, poly(vinyl pyrrolidone) (PVP) is a second polymer, which deserves special attention among the conjugated polymers because of its good environmental stability, easy processability, moderate electrical conductivity and rich physics in charge transport mechanism. The presence of the carbonyl group (C5O) in PVP, leads to the formation of a variety of complexes with various inorganic salts that exhibit high T g with good environmental, thermal and mechanical stability. Furthermore, PVP is highly soluble in polar solvents, such as alcohol, and the local modification of the chemical structure induces drastic changes in electronic properties [9]. This paper reports on the gel polymer electrolytes based on the PEO/PVP/LiClO 4 complex by the addition of different plasticizers using a well known solvent casting technique. The structural and complex formation features of the prepared electrolyte samples have been confirmed by XRD and FTIR analyses, respectively. The ionic conductivity of the prepared gel polymer electrolytes has also been studied. 2. Experimental Poly(ethylene oxide) (PEO), M w =8 10 3, Sigma Aldrich and poly (vinyl pyrrolidone) (PVP), M w = , Sigma Aldrich /ß 2014 S. Rajendran. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

2 K. Kesavan et al. / Chinese Chemical Letters 25 (2014) were used without further purification. The salt LiClO 4 was purchased from Sigma Aldrich Chemicals Limited, USA. The obtained PEO, PVP and LiClO 4 were dried at 55 8C for 4 h to remove moisture. The plasticizers and the solvent, methanol, used in this study were purchased from Sigma Aldrich and E-Merck, respectively, and were used as received. The gel polymer electrolytes of constant proportions of PEO (72 wt%)/pvp (8 wt%)/liclo 4 (8 wt%)/x (12 wt%), where X = EC, PC, GBL, DMC and DEC, were prepared using a solvent casting technique. The structural analysis of the prepared electrolyte was analyzed by X pert PRO PANalytical X-ray diffractometer. Complex formation between the salt and polymer complex was obtained by using SPECTRA RXI, Perkin Elmer spectrophotometer in the range of cm 1. The gel polymer electrolytes of constant ratios of PEO:PVP:LiClO 4 :X, where X = EC, PC, GBL, DMC and DEC, were prepared with a view to identify the suitable plasticizer for high ionic conductivity. Conductivity studies were carried out with the help of stainless steel blocking electrodes using a computer controlled microauto lab type III Potentiostat/Galvanostat in the frequency range of 100 Hz 300 khz over the temperature range K. Thermal stability of the polymer electrolyte was carried out by thermogravimetric analysis (TG/DTA) using PYRIS DIAMOND from r.t. to 700 8C. Roughness parameter was observed by AFM with the help of AFM (A100SGS). The photoluminescence studies were performed by Carry Eclipse Fluorescence Spectrophotometer. The optical band gap energies were calculated by UV vis analysis using Shimadzu UV Results and discussion 3.1. XRD analysis Fig. 1 represents the X-ray diffraction patterns of pure LiClO 4, PEO, PVP and the prepared complexes respectively. The XRD pattern of pure LiClO 4 salt shows well defined sharp peaks at 20.9, 22.92, 26.56, and corresponding to the lattice planes (1 0 1), (1 1 0), (2 0 0), (2 0 1) and (2 1 0) which indicate its crystalline nature. However, these peaks disappear in the polymer blend films. The characteristic peaks at 2u = 19.2 and corresponding to the reflection of (1 2 0) and (0 1 0) plane, [(Fig._1)TD$FIG] Fig. 1. XRD patterns of the pure and prepared complexes. respectively, confirm the semicrystalline nature of PEO. No such well defined, sharp peaks attributable to PVP could be observed. Instead a broad peak was observed around at 22.78, which suggests the amorphous nature of PVP. From Fig. 1, it is observed that with the addition of lithium salt and different plasticizers 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 complex. The percentage of degree of crystallinity (x) for the prepared sample has been calculated from the relation [10]. x ¼ S S o 100 where S is the sum of the areas of all the crystalline peaks and S o the total area under the diffractogram. For the prepared samples D1, D2, D3, D4 and D5, the degree of crystallinity is found to be 16%, 22%, 24%, 27%, and 29%, respectively. This shows that the degree of crystallinity of the prepared samples increases with different plasticizers, with the EC based electrolyte having reduced crystallinity showing higher ionic conductivity. This is due to the existence of large free volume and flexibility of total chain segmental motion in the polymer electrolyte [11] and hence the higher conductivity. These observations apparently revealed that the polymer undergoes significant structural reorganization upon adding different plasticizers. The plasticizers may have induced significant disorder into the original polymers and this is attributed to the interactions between the polymer and the solvents, which resulted in polymer electrolytes with much lower crystallinity. Hence, the absence of peaks pertaining to lithium perchlorate salt in the complexes indicates the complete dissolution of the salt in polymer matrix FTIR studies FTIR transmittance spectra of pure and prepared complexes are presented in Fig. 2. The vibrational bands at 1343, 1282 and 1236 cm 1 are assigned to CH 2 bending, CH 2 asymmetric stretching and CH 2 symmetric twisting respectively of pure PEO [12]. The bands corresponding to the wave numbers 1343 and 1236 cm 1 are shifted to (1350, 1348, 1347, 1346 and 1350 cm 1 ) and (1242, 1244, 1244, 1248 and 1243 cm 1 ) respectively, whereas the vibrational peak at 1282 cm 1 is present in the complexes D1 and D3 without any change, and shifted in the complexes D2, D4 and D5 to the wave numbers 1283, 1283 and 1284 cm 1 respectively. The absorption band of PEO at 1799 cm 1 represents the ether oxygen group which is shifted to the wave numbers 1794, 1794, 1794, 1792 and 1794 cm 1 respectively. The characteristic band at 1100 cm 1 is assigned to C O C (symmetric and asymmetric stretching) of PEO which is shifted in all the complexes [12]. Apart from this, the mode responsible for the band at 845 cm 1 is primarily due to the CH 2 rocking motion with a little contribution from C O stretching motion of PEO, while band at 947 cm 1 originates primarily from the C O stretching motion with some CH 2 asymmetric rocking motion [13]. On the other hand for pure PVP, a vibrational band at 2900 cm 1 is attributed to aliphatic C H stretching which is shifted to 2887, 2886, 2884, 2892 and 2899 cm 1, respectively [14]. The C5O stretching and CH 2 wagging mode of vibration of pure PVP observed at 1651 and 1451 cm 1 are shifted in the complexes with the wave numbers (1652, 1655, 1655, 1653 and 1655 cm 1 ) and (1462, 1462, 1462, 1459 and 1461 cm 1 ), respectively. The C N stretching mode of pure PVP at 1232 cm 1 is shifted to 1230, 1229, 1231, 1233 and 1230 cm 1, respectively [15]. The C C bending

3 [(Fig._2)TD$FIG] 1430 K. Kesavan et al. / Chinese Chemical Letters 25 (2014) [(Fig._3)TD$FIG] Fig. 3. Room temperature complex impedance plots of the prepared samples. Fig. 2. FTIR spectra of (a) pure LiClO 4 ; (b) pure PEO; (c) pure PVP; (d) D1; (e) D2; (f) D3; (g) D4; (h) D5. mode of vibration observed at 1080 cm 1 of PVP is shifted in all the complexes respectively to the wave numbers 1083, 1079, 1097, 1092 and 1072 cm 1 [16]. The following changes in the spectral features have been observed after comparing the pure and their complex spectra. The ether oxygen band at 1799 cm 1 is found to broaden gradually with the addition of various plasticizers and salt in PEO content, which suggests that PEO is distrupted by them. The cm 1 region is assigned to C O C stretching, which decreases in its intensity with the addition lithium perchlorate and different plasticizers due to the interaction between the Li + cations and ether oxygen atoms in PEO. From Fig. 2, it is observed that the intensity of the peak corresponding to C5O (1651 cm 1 ) in PVP is decreased with the addition of plasticizers and lithium salt. This indicates the strong interaction of Li + ions with the carbonyl group. The band at 625 cm 1 corresponds to the ClO 4 anion [17] which is present in the prepared complexes. The absorption bands at 1807, 1173, 894 and 778 cm 1 of EC based complex; 1342 and 953 cm 1 of PC based complex; 1769, 1646, 1036 and 675 cm 1 of GBL based complex and 2991, 1639, 1379 and 1272 cm 1 of DEC based complexes are found to be shifted in their respective complexes [18]. The shifting and absence of the band, in addition to the formation of new bands in the electrolyte systems indicate the polymer, salt and plasticizer interactions in the PEO/PVP based gel polymer electrolytes Conductivity studies Fig. 3 shows the room temperature complex impedance plots of the prepared samples. According to the theoretical analysis of Watanabe and Ogata [19], two semicircles appear in the impedance spectrum for a symmetric cell, i.e., one at high frequencies corresponding to the bulk electrolyte impedance and the other at low frequencies related to the interfacial impedance. It has been also reported that the high frequency semicircle does not appear in practical impedance plots for plasticized polymer electrolyte membranes [20,21]. This feature indicates that the conductivity is mainly due to the ions. The values of the ionic conductivities are determined using the formula, s ¼ l R b A where l is the thickness of the film and A is the surface area of the film. The intercept on the real axis provides the value of the bulk resistance (R b ) of the electrolyte. The highest room temperature ionic conductivity value is found to be Scm 1 for the EC based complex (D1). Table 1 shows the conductivity values of the complexes in the temperature range of K. From Table 1, we can understand that as the temperature increases, the ionic conductivity also increases for all the complexes and this behavior is in agreement with the theory established by Armand et al. [22]. This can be rationalized based on the free volume model [23]. The effect of plasticizers on the polymer segmental motion and ionic mobility depends on the specific nature of the plasticizer, including viscosity, dielectric constant, polymer plasticizer interaction and ion plasticizer interaction. From Table 1, it is concluded that EC based complex exhibits maximum ionic conductivity of the order of Scm 1 as compared to other prepared gel electrolytes which is mainly due to the higher dielectric constant (89.6 at 40 8C) of ethylene carbonate (EC). The plasticizer would dissolve enough charge carriers and provide more mobile medium for the ions so as to enhance the conductivity behavior of the Table 1 Temperature dependent ionic conductivity values of the prepared complexes. Sample code Complex compositions of PEO (72 wt%)/pvp (8 wt%)/liclo 4 (8 wt%)/x (12 wt%) Dielectric constant Ionic conductivity values (s) ( 10 4 Scm 1 ) At different temperatures 303 K 313 K 323 K 333 K 343 K 353 K D1 X = EC D2 X = PC D3 X = GBL D4 X = DMC D5 X = DEC

4 [(Fig._4)TD$FIG] K. Kesavan et al. / Chinese Chemical Letters 25 (2014) polymers and the characteristics of the viscous matrix are emphasized [26] TG/DTA analysis resultant samples [24]. The conductivity is obtained in the decreasing order of PC, GBL, DMC and DEC based electrolytes which are in accordance with their dielectric constants 64.4, 39.1, 3.12 and These results are in accordance with those reported earlier using different plasticizers like EC, PC, GBL, DMC and DEC [18,25]. The temperature versus log ionic conductivity plots of all the prepared complexes are shown in Fig. 4. The non-linearity of Arrhenius plots indicates that ion transport in polymer electrolytes is dependent on polymer segmental motion. Hence, temperature dependent ionic conductivity plots seem to obey a Vogel Tamman Fulcher (VTF) relation which describes the transport properties in a viscous matrix. It supports the concept that the ion moves through the plasticizer-rich phase because the conducting medium, i.e., plasticizer-rich phase involves the plasticizer, salt, [(Fig._5)TD$FIG] Fig. 4. Temperature dependent ionic conductivity plot of the prepared samples. Fig. 5(a and b) shows the TG/DTA curves of the prepared electrolytes and the results are given in Table 2. It can be seen from the TG curves that initial weight loss occurs from 73 8C to 81 8C with a weight loss of 2 5 wt% for all the samples due to solvent evaporation. It is observed that the major weight loss occurs at 306, 341, 340, 337 and 340 8C for EC, PC, GBL, DMC and DEC based complex systems, respectively, with the corresponding weight loss of about wt%. This may be due to the structural decomposition of the polymer blends. Around 350 8C, a small weight loss step has been observed for the plasticized samples. This may be due to the formation of the unstable residue from undecomposed polymer. Similar results have been reported by Wang et al. for PC based polymer electrolyte system [27] and Nasef et al. for EC/DEC based system [28]. It is also found that the complete decomposition of the sample takes place between 415 and 426 8C with the corresponding weight loss of about wt%. After 430 8C, no appreciable weight loss is observed. In DTA curve, the first exothermic curve occurs in the range of C and the second exothermic curve occurs in the range of C for all the prepared electrolytes. This indicates the decomposition of the polymer film is in good agreement with the observed TG results UV vis analysis Absorption versus wavelength: UV vis spectra in the wavelength range nm of the polymer systems are shown in Fig. 6. In the UV region, the band around 194 nm has been observed for all the prepared samples. This absorption peak may be attributed to the n! s + transition, which is very sensitive to Fig. 5. TG (a) and DTA (b) analysis of the prepared complexes respectively. Table 2 TG/DTA results of the prepared samples. Sample code Complex compositions of PEO (72 wt%)/pvp (8 wt%)/liclo 4 (8 wt%)/x (12 wt%) Decomposition temperature (8C) Percent weight loss of the samples Exothermic peaks (8C) I II III I II III I II III D1 X = EC D2 X = PC D3 X = GBL D4 X = DMC D5 X = DEC

5 [(Fig._6)TD$IG] [(Fig._8)TD$IG] K. Kesavan et al. / Chinese Chemical Letters 25 (2014) Fig. 6. Optical absorption for the prepared samples. Fig. 8. (ahg)1/2 versus photon energy for the prepared polymer electrolyte samples. hydrogen bonding. The absorption band around 226 nm may be assigned as p! p*, which originate from unsaturated bonds, mainly C5 5O [29]. Fig. 6 shows a shift in absorption edge toward the higher wavelength. These shifts in the absorption edge may be assigned to the presence of Li-ions in the prepared samples. These bands reflect the variation of the energy band gap, which arises due to the variation in crystallinity within the polymer matrix [30]. Determination of optical band gap (Eg): In the absorption process, an electron is excited from a lower to a higher energy state by absorbing a photon of known energy. The changes in the transmitted radiation can decide the types of possible electron transitions. Fundamental absorption refers to band-to-band transition. The fundamental absorption shows a sudden rise in absorption, known as absorption edge, which can be used to determine the optical band gap (Eg = hc/l). Absorption is expressed in terms of a coefficient a (absorption coefficient), which is defined as the relative rate of decrease in light intensity. The absorption coefficient a was calculated from the absorbance (A), where a = A/x and x is the thickness of the polymer film. The absorption coefficient for amorphous materials can be related to the energy of the incident photon as follows [31]. where b is a constant, Eg is the optical band gap energy and the exponent, r, can take the values 1, 2, 3, 1/2 and 3/2 depending on the nature of the electron transitions responsible for the optical absorption. It is well known that r takes the value of 1/2 in the case of direct electronic transition across the direct energy gap in the k space and 2 in the case of indirect electronic transition across an indirect energy gap. Davis and Shalliday [32] reported that near the fundamental band edge, both direct and indirect transitions occur and can be observed by plotting (ahg)2 and (ahg)1/2 versus photon energy (hg). The direct band gap values were evaluated from (ahg)2 versus hg plots (Fig. 7) and the allowed transition energy were determined by extrapolating the linear portions of the curves to zero absorption. The direct and indirect band gap values of the pure PEO/PVP blend are 4.87 and 4.63 ev, respectively [8]. For sample D1, the direct band gap lies at 4.45 ev, while for samples D2, D3, D4 and D5, it lies at 4.55, 4.67, 4.78 and 4.80 ev, respectively. In the case of indirect band gap, the band gap values were obtained from the plots of (ahg)1/2 versus hg at the intercepts on the X axis (energy axis) on extrapolating the linear portion of the curves to zero absorption value (Fig. 8). The indirect band gap values of the samples D1, D2, D3, D4 and D5 are 4.10, 4.31, 4.50, 4.58 and 4.61 ev, respectively. It is clear from the direct and indirect band gap values that the optical band energies were found to increase with the addition of different plasticizers. The increase in the optical band gap may be explained by the charge transfer complexes in the gelled polymer electrolyte. These charge transfer complexes decrease the conductivity by providing additional charges in the lattice [33]. These Fig. 7. (ahg)2 versus photon energy for the prepared polymer electrolyte samples. Fig. 9. Photoluminescence emission spectra of the prepared electrolyte samples. r ðahg Þ ¼ bðhg Eg Þ for hg > Eg (1) ðahg Þ ¼ 0 for hg < Eg (2) [(Fig._7)TD$IG] [(Fig._9)TD$IG]

6 [(Fig._10)TD$FIG] K. Kesavan et al. / Chinese Chemical Letters 25 (2014) Fig. 10. The topography image of the EC based complex, (a) 2D image and (b) 3D image. results are supported by the data obtained from conductivity studies in the present work Photoluminescence studies Photoluminescence emission spectra of the prepared gel polymer electrolytes are shown in Fig. 9 at an excitation wavelength of 370 nm. The emission intensity is directly proportional to the local viscosity of the medium, but the local viscosity is inversely associated with the carrier mobility of the electrolyte [34,35]. Among the various plasticized electrolyte systems, the EC based electrolyte system shows the minimum emission intensity compared to other samples which may be attributed to the maximum dissociation of the ionic charge carriers due to its high dielectric constant. This result is also well correlated with the ionic conductivity studies AFM analysis In the present study, two and three dimensional topographic images of the sample D1 having a maximum ionic conductivity are shown in Fig. 10 (a and b). From the topographic image, the average roughness factor of the prepared sample over the scanned area, 10 mm 10 mm, has been obtained and it is in the order of 36.8 nm. The roughness factor has an important role in the ionic conduction. When the average roughness factor is high, the ionic movement may be slow. But in the present study, the roughness factor is low compared to other polymer electrolyte based systems [36,37]. The images show the minimum number of pores. These small micropores of the gel polymer electrolyte are responsible for the enhancement of ionic conductivity. 4. Conclusion PEO, PVP, LiClO 4 with different plasticizer based, gel polymer electrolytes have been prepared by the solvent casting technique. 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