ARTICLE pubs.acs.org/Langmuir
Ionic Conductivity through Thermoresponsive Polymer Gel: Ordering Matters Saurabh S. Soni,*,† Kishan B. Fadadu,† and Alain Gibaud‡ † ‡
Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar - 388 120, Gujarat, India Laboratoire de Physique de l’Etat Condense, UMR CNRS 6087, Universite du Maine, Le Mans, Cedex 09, France
bS Supporting Information ABSTRACT: Thermoreversible polymer gel has been prepared using PEO PPOPEO block copolymer (Pluronic F77) which self-assembles into different microcrystalline phases like cubic, 2D-hexagonal, and lamellar. Addition of electrolyte (LiI/I2 ) converts the gel into a polymer gel electrolyte (PGE) which exhibits microphase-dependent ionic conductivity. The crystalline phases have been identified by SAXS as a function of the polymer concentration. It is found that the optimum value for the ionic conductivity (≈1 103 S.cm1) is achieved in the Im3m phase due to faster diffusion of ions through the 3D-interconnected micellar nanochannels. This fact is further supported by FTIR study, ionic transference number, and diffusion coefficient measurements.
1. INTRODUCTION Amphiphilic block copolymers, both diblock and triblock, are known to form a variety of self-assembled aggregate structures and various ordered structures in dilute solutions.1,2 These structures range from cubic, 2D- and 3D-hexagonal, lamellar to a complex bicontinuous phase. This type of structure depends basically upon both the relative volume of the different blocks and the concentration of the polymer at a given temperature.3 Because of wide range of applications like detergents, emulsifier, defoaming agents, wetting agents, templating agents, dispersants, and vectors in pharmaceutical formulations, poly(ethylene oxide)poly(propylene oxide) poly(ethylene oxide) (PEOPPOPEO) has received a lot of attention in the past two decades.4 A remarkable structure polymorphism has been observed in PEOPPOPEO type block copolymer system in the presence of water (or other polar solvent) by varying the copolymer concentration and temperature.5 The polymer electrolyte (PE) has attracted due attention because of its promising applications in the advanced devices like Li ion batteries, supercapacitors, dye-sensitized solar cells, sensors, electrochromic devices, etc.6 The advantages of PE are no leakage, good mechanical properties, low cost, and avoiding hazardous chemicals which are used in liquid electrolytes. However, the low ionic conductivity (at ambient temperature) of the PE is of main concern. The ionic conductivity of PE is due to the ionic motion in polymer matrix containing amorphous phase above the glass transition temperature, Tg.7 Many kinds of PE such as poly(acrylonitrile), poly(methyl methacrylate), r 2011 American Chemical Society
poly(vinyl chloride), poly(ethylene oxide) (PEO), etc., with or without cross-linking agents have been studied. Out of all these, high molecular weight poly(ethylene oxide) (PEO) is the most widely used system for fundamental studies and practical applications.8 But practical use of PEO-based solid polymer electrolyte is limited because of their low conductivity at room temperature (≈107 S cm1) and poor mechanical properties. This is due to the formation of crystalline domains which interfere with the motion of cation transport.9 To alleviate these drawbacks, considerable research effort has been adopted to increase the amorphous domain in the polymer host by lowering Tg.10,11 The latest reported conductivity for such kind of electrolytes is in the order of 105 S cm1 at 30 °C.11a,b Scrosati et al. have reported a PEO (PEO swollen in ethyl carbonate/diethyl carbonate as solvent) based membrane electrolyte having conductivity 2.1 103 S cm1 at 20 °C.12a They also used this gel electrolyte membrane in Li ion battery.12b On the other side, polymer gel electrolytes (PGE) has received wide attention because of their unique hybrid structure, having cohesive properties of solids and diffusive properties of liquids.13 But due to the increase in viscosity upon gelation, ionic motion is restricted. If we could achieve continuous nanochannels of one domain of polymer through which ions can diffuse easily, we could enhance the conductivity of polymer electrolyte even in gel phase. However, the fabrication of large size single Received: July 13, 2011 Revised: November 8, 2011 Published: November 09, 2011 751
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domains of the homo or block copolymer structure is not possible. To overcome this problem, we have focused on the use of amphiphilic block copolymers which self-assemble into different microphases in which nanomicellar channels alignment facilitate ion transportation. Moreover, most of the advanced devices are composed of nonaqueous electrolytes but they are increasingly criticized owing to their flammability which creates many safety and handling issues in case of improper use such as overcharging or short circuit. Furthermore, the special cell assembly technology, the necessity of dry atmosphere during manufacturing, and the costly organic solvents make the devices comparatively expensive. Such environmental and economic problems unnecessarily hamper the further development of safer, less expensive, and green materials and devices. Because of these reasons, many researchers across the globe are working on replacement of organic solvents by water in the advanced devices like Li ion batteries,14 DSSC,15 etc. Here we report a polymer gel of Pluronic F77 (BASF) triblock copolymer which self-assembles into different microstructures in polar solvent like water depending upon the temperature and polymer concentration. Addition of electrolyte (LiI/I2) to this gel creates a polymer gel electrolyte (PGE) in which ionic conductivity shows its dependence on the microcrystalline phases formed by F77.
Figure 1. (a) SAXS profiles and (b) binary phase diagram of F77/water in the presence of LiI/I2 at T = 25 °C. L1 = isotropic micellar phase, I1 = cubic, H1 = 2D-hexagonal, Lα = lamellar. (potential at which diffusion processes occur) was applied to the system under study for 150 s in order to ensure steady state. Transference number was estimated from the potentiostatic polarization method using Ag as blocking electrodes. A +300 mV potential step was applied onto the test cell, and the resulting current was monitored as a function of time. Each measurement was repeated three times, and its average value was tabulated herein. All the measurements were carried out in an argon atmosphere.
2. MATERIALS AND METHODS A F77 (E52P35E52, MW = 6600 g/mol) block copolymer was obtained as gift sample from BASF. LiI (99.9%) and iodine (99.9%) were purchased from Sigma-Aldrich and used as received. Water used in this study was Milli-Q grade. All the samples were prepared in sealed vials by adjusting mass of F77 block copolymer, water (Milli-Q) + LiI/I2 at each compositions. The concentrations of electrolyte solutions were prepared by dissolving LiI and iodine in Milli-Q water, and the amount of electrolyte was maintained constant throughout the study; i.e., each composition contains 4.34 mg of LiI and 1.65 mg of I2 per gram of gel. Solutions were then stored at ambient temperature for several weeks before the measurements. SAXS measurements were carried out using the Rigaku SAXS diffractometer of LPEC (University of Maine, Le Mans) equipped with a Gabriel 2D wire detector (sample-to-detector distance 830 mm and beam wavelength, λ = 0.154 nm). Measurements were monitored as a function of the wave vector transfer, q (q = 4π sin θ/λ) after radial averaging with a typical time acquisition of 10 000 s. Fluid liquid mixtures were inserted in 1 mm quartz capillaries, whereas gels were mounted in an aluminum cell having transparent windows. Thermoreversibility of compositions having cubic phase has been confirmed by optical observation of fluidity as a function of temperature (heating rate = 0.5 °C min1). DSC was recorded on Pyris, Perkin-Elmer. Conductivity was determined by a SOLARTRON 1260 (impedance and gain phase analyzer) using homemade cell having stainless steel electrodes and Teflon-coated cell for solid and liquid, respectively, at different temperatures by performing heatingcooling cycles. Infrared spectra were recorded using an ABB FTIR spectrometer. Electrochemical measurements were carried out using a SOLARTRON 1287 (electrochemical analyzer). A platinum disk ultramicroelectrode (radius 2.5 μm) was used as working electrodes while platinum wire (radius 0.25 mm) served as reference electrode in chronoamperometry. The distance between WE and RE was maintained close enough (≈0.2 mm) to reduce the iR drops. Cyclic voltammetry of each gel was recorded prior to the polarization experiment by applying potential from 0.3 to 0.3 with 20 mV/s scan rate in order to find the potential at which diffusion-limiting processes occurs. During chronoamperometry, a constant positive potential
3. RESULTS AND DISCUSSION The phase diagram (Figure 1) of F77 with water in the presence of salts was constructed at 25 °C using small-angle X-ray scattering (SAXS) measurements. The concentration of LiI/I2 mixture was kept constant throughout the phase diagram. Figure 1a shows SAXS profile for the F77/water mixture in the concentration range 35% (w/w) to 80% (w/w). Below 25% (w/w) (and above cmc) F77/water block copolymer concentration, an isotropic micellar solution is formed in which a hydrophobic core consisting of the PPO blocks is surrounded by an outer shell or corona of hydrophilic PEO (hydrated) blocks.16 With increasing concentration of F77, micelles self-assemble into different crystalline phase, viz., cubic, 2D-hexagonal (2D-hex), lamellar. The sequence of Bragg reflections defined by their Miller indices (hkl) is used to identify the nature of each phase. Note that the cubic phase exhibits a 3D order while the 2D phase, a 2D order and the lamellar one a 1D order. This implies that the Miller indices of these phases are respectively 3, 2, and 1 since each phase is respectively defined by 3, 2, and 1 lattice parameters. Above 32% (w/w) F77/water mixture, we have observed √ a cubic phase√with Bragg reflections following the sequence 2, √ √ 2, 6, 8, 10, .... in q space. This sequence is typical of an Im3m phase with spherical micelles in shape. The possibility of a face-centered cubic phase was ruled out because secondary peak √ at 4/3q was missing. Since in this region solvent amount is higher than in the 2D-hex and lamellar phases, one can infer that the curvature of this phase is more pronounced. F77 aqueous 752
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Table 2. Conductivity, σ, Activation Energy, Ea, and Gelation Temperature of PGEs at 30 °C σ, 103, S cm1
Ea, kJ mol1
polymer, % [O]/[Li]
Table 1. Structural Parameters for Various Phases Extracted from SAXS Measurements Lc, nm
d, nm
Δ/d
35
210.6
9.46
0.067
50
251.7
9.90
0.063
70
232.6
9.87
0.065
solutions 2545% (w/w) (in cubic phase) have interesting characteristics of reversible thermal gelation; i.e., they are liquid at low temperature but becomes gel upon warming, and this gelation process is reversible. Inset of Figure 2 shows the vessel upside down with no fluidity (for 35% (w/w) F77 at T > 26 °C). This is due to the fact that, at lower temperature in the aqueous solution of F77, the PEO is hydrated with large amount of water through H-bonding. As temperature increases, the dehydration of PEO and PPO interface takes place, which leads to increment in hydrophobic interaction between PEO blocks.17 As a result of this, spherical micelles come closer to each other until the volume fraction reaches a value close to the one of a close-packed cubic structure.18 In order to know the degree of ordering in crystalline phase, various structural parameters were extracted from the SAXS measurements (Table 1).19 Lc is the coherence length and Δ/d gives the degree of disorder in the structures, which were derived from the following relations: Lc ¼
λ βs cos θ
1 Δ=̅d ¼ π
rffiffiffiffiffiffiffi βs d λ
ratio
sol
gel
sol
35
1.7
1.00
1.55
40.7
50 70
2.5 3.4
0.54 1.11
gel
temp, °C
19.3
25.7
31.2 24.1
2
technique at different temperatures (see Supporting Information Figure S1). It should be noted that the gel composition was selected in such a way that each series of measurements corresponds to a different phase, viz. 35% (w/w) (cubic), 50% (w/w) (2D-hex), and 70% (w/w) (lamellar). (Note: the conductivity reported here for 2D-hex and lamellae are through plane.) The conductivity within temperature range from 14 to 52 °C for 35%, 50%, and 70% (w/w) gels shows linear behavior, irrespective of solgel transition and morphology of liquid crystalline phase. Conductivity data were fitted into Arrhenius type equation and plotted as ln σ vs 1/T (Figure 2). This plot shows break in the linearity for 35% (w/w) F77. The point at which the change in slope was observed is defined as gelation temperature (≈26 °C, in good agreement with the obtained from DSC measurements (Figure S2)). The calculated activation energies, Ea, from the slope above and below (only for cubic phase) gelation temperature are depicted in Table 2. For 35% (w/w) gel, Ea is 19.3 and 40.7 kJ mol1, which indicates that ions require less activation energy in gel phase than simple micellar solution. Hence, it can be concluded that transportation of ions in gel is easy compared to simple micellar solution. This might be due to aggregation of polymer chains which restricts the motion of ions in micellar solution. In gel, it is a fact that the increasing viscosity leads to diminished ionic mobility; hence, the conductivity should be decreased upon gelation, but such behavior has not been observed in this case. This can be attributed to the presence of 3D-interconnected micellar nanochannels in cubic Im3m which paves the way for ion conduction. These micellar nanochannels are composed of two domains, i.e., PEO and PPO; due to the low solubility of anion (I3) in aqueous domain,15 it may reside at the PPO/PEO interface while Li+ ions are located in an outer part of the micelle, i.e., corona (hydrated PEO) through ion dipole interactions. The position of the cation in the micelles of PEOPPOPEO triblock copolymer was also studied by Ganguly and co-workers using the small-angle neutron scattering (SANS) technique, and they suggested that Li+ cation resides in the corona region of the micelle by coordinating with the ether oxygen atom of PEO chain.20 All these will lead to better mobility of ions through the polymer backbone. Among all liquid crystalline phases, the optimum value of conductivity, i.e., 1.55 103 S cm1 at 30 °C, was obtained in the cubic crystalline phase (Figure 3). The ionic conductivity values obtained here are much higher (by 24 orders of magnitude) than those reported for polymer electrolytes based on PEO or poly(ethylene glycol) with electrolytes/ionic liquids (≈1 10 5 and ≈1 10 7 S cm 1 for ionic liquid and electrolytes, respectively).21 The conductivity values for pure electrolyte and block copolymer in water at different temperature are given in the Supporting Information (Figure S3). We also
Figure 2. Arrhenius plot of 35%, 50%, and 70% (w/w) PGE.
composition of gel, % (w/w)
gelation
(w/w)
ð1Þ
ð2Þ
where λ is the wavelength of the source used, βs is the full width at half-maximum intensity of the peak (in radians), and d = 2π/qmax. It is clear from this table that the coherence length is somehow higher in the 2D-hex phase than cubic and lamellae which indicates that degree of ordering is higher in case of the former. Degree of disorder in all the case is comparable, and its magnitude indicates that all the structures are well ordered. The ionic conductivity of various concentrations of polymer gel electrolyte (PGE) was recorded via the ac impendence 753
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Figure 3. Conductivity as a function of liquid crystalline phases of F77 at 30 °C. I1 = cubic, H1 = 2D-hex, and Lα = lamellar phases. Figure 5. FTIR of PGE with different concentrations of polymer.
ratio increases (up to certain optimum value) conductivity should increases, but the observed conductivity does not follow this trend. So we can conclude that the [O]/[Li] ratio has nothing to do with the conductivity in gel electrolyte. In order to get a better insight into the ion transport mechanism, the transport number of cations was estimated. It was determined using the potentiostatic polarization method (Wagner’s polarization method) in a symmetrical cell.22,23 The cell consists of Ag/electrolyte/Ag, where Ag was used as blocking electrode for cation. A stimuli of 300 mV was imposed onto the test cell, and generating current was monitored as a function of time for 250 s (Figure 4). The transference number was calculated from eq 3 ti ¼ Figure 4. Plot of current as a function time for different polymer gel electrolytes at 30 °C.
IT Ie IT
ð3Þ
where IT denotes the total initial current which is composed of ionic current and electronic current. Ie is the current after time, t, which is mainly due to the electronic current. Ionic transference numbers obtained from the above curve are 0.80, 0.78, and 0.71 for 35%, 50%, and 70% (w/w) polymer gel, respectively. From this data it follows that transference of ion is faster in 35% (w/w) gel than remaining gels. It is also revealed from these values that as the concentration of polymer increases, the transference number of the ion is decreasing which is due to the increase in the [O]/[Li] ratio. Since at higher ratio ordered PEO/Li domains are formed which impedes the transport of cation.2426 To understand this mechanism of interaction between electrolyte and oxygen atoms of PEO chains, the FTIR technique was employed. Figure 5 shows that addition of 30% (w/w) of electrolyte solution to the polymer leads to a shifting in the band attributed to the COC deformation, from 1103 to 1092 cm1. Bonding of cation (Li+) with the ether oxygen atom of COC linkage will decrease bond order of COC linkage. This decreasing bond order of COC group will lead to band shifting toward lower wavenumber (Table 3). This shift suggests an interaction between Li cation and oxygen atoms of PEO (this shift could be partially due to the hydrogen bonding of water molecule, too). At high water
recorded conductivity of the 35% (w/w) by increasing concentration of LiI/I2 at 28 °C. At 100 and 500 mM concentrations of LiI shows conductivities 1.35 103 and 4.96 103 S cm1, respectively, which suggests that there is an increase in conductivity by increasing salt concentration. From Figure 3 it is found that the cubic and lamellar phases have comparable conductivity while a minimum is observed in 2D-hex phase. The enhancement in conductivity in lamellae having highest polymer concentration may be due to the continuous plate like structure in which lower hydration of ions would favor hoping mechanism (segmental motion of Li+ ion through coordination with oxygen atoms of the PEO chain). In solid polymer electrolytes, interaction between PEO chains and Li+ ion is strong due to the close packing of the polymer chains. While in case of gel electrolytes, it is partially applicable since the PEO chains are hydrated with water. But as the concentration of polymer increases, the interaction between PEO chain and Li+ becomes more pronounced which consequently leads to predominance of hopping conductivity of the ion through the PEO chain. [O]/[Li] ratio in the 35% , 50%, and 70% (w/w) gel electrolytes are 1.7, 2.5, and 3.4, respectively. It is fact that as the [O]/[Li] 754
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Table 3. Main FTIR Bands for Pure Polymer and 35%, 50%, and 70% (w/w) Polymer Gel Electrolytes band position, cm1 vibration mode
Pure F77
70% F77
50% F77
35% F77
COC stretch
1103
1092
1084
1084
’ ACKNOWLEDGMENT This work has been financially supported by DST, New Delhi, India (Ref. No. DST/TSG/PT/2008/23). S.S.S. thanks the Universite du Maine, Le Mans, France, for “Visiting Professorship” to work with Prof. Alain Gibaud. ’ REFERENCES (1) Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52, 32. (2) Hamley, I. W. Physics of Block Copolymers; Oxford University Press: Oxford, England, 1998. (3) (a) Cui, H.; Chen, Z.; Zhong, S.; Wooley, K. L.; Pochan, D. J. Science 2007, 317, 647. (b) Li, Z. B.; Kesselman, E.; Talmon, Y. H. U.; Myer, M. A.; Lodge, T. P. Science 2004, 306, 98. (c) Yabu, H.; Higuchi, T.; Shimomutra, M. Adv. Mater. 2005, 17, 2062. (4) (a) Scmolka, I. R. In Non Ionic Surfactants; Schmick, M. J., Ed.; Marcel Dekker: New York, 1967, Chapter 10. (b) Brinker, C. J.; Lu, Y.; Sellinger, A.; Fan, H. Adv. Mater. 1999, 11, 579. (c) Soni, S. S.; Henderson, M. J.; Bardeau, J. F.; Gibaud, A. Adv. Mater. 2008, 20, 1493. (5) (a) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1998, 14, 2627.(b) Chu, B.; Zhaou, Z. In Non-Ionic Surfactants; Nace, V. M., Ed.; Surf. Sci. Ser.; Marcel Dekker: New York, 1996; Vol.60. (6) (a) Goodenough, J. B.; Kim, Y. Chem. Mater. 2010, 22, 587. (b) Grey, F. M. Solid Polymer Electrolytes; VCH Publishers: New York, 1991. (c) Scrosati, B. Application of Electroactive Polymers; Chapman & Hall: London, 1993. (d) Freitas, F. S.; de Freitas, J. N.; Ito, B. I.; de Paoli, M. A.; Nogueira, A. F. ACS Appl. Mater. Interfaces 2009, 1, 2870. (7) Druger, S. D.; Nitzam, A.; Ratner, M. A. J. Chem. Phys. 1983, 79, 3133. (8) (a) Lightfoot, P.; Mehta, M. A.; Bruce, P. G. Science 1993, 262, 883. (b) Shi, J.; Peng, S.; Pei, J.; Liang, Y.; Cheng, F.; Chen, J. ACS Appl. Mater. Interfaces 2009, 1, 944. (9) Nishimoto, A.; Agehara, K.; Furuya, N.; Watanabe, T.; Watanabe, M. Macromolecules 1999, 32, 1541. (10) (a) Kao, H. M.; Chao, S. W.; Chang, P. C. Macromolecules 2006, 39, 1029. (b) Liang, W. J.; Kuo, P. L. Polymer 2004, 45, 1617. (c) Kumar, B.; Rodrignes, S. J. J. Electrochem. Soc. 2001, 148, A1336. (d) Croce, F.; Appetecchi, G. B.; Persi, L.; Scrosati, B. Nature 1998, 394, 456. (11) (a) Kao, H. M.; Chen, C. L. Angew. Chem. 2004, 116, 998. (b) Wu, H. Y.; Sikia, D.; Lin, C. P.; Wu, F. S.; George, T. K.; Kao, H. M. Polymer 2010, 51, 4351. (c) Chen-Yang, Y. W.; Hwang, J. J.; Hung, A. Y. Macromolecules 2000, 33, 1237. (12) (a) Appetecchi, G. B.; Aihara, Y.; Scrosati, B. Solid State Ionics 2004, 170, 63. (b) Appetecchi, G. B.; Aihara, Y.; Scrosati, B. J. Electrochem. Soc. 2003, 150, A301. (13) (a) Sannier, L.; Bouchet, R.; Rosso, M.; Tarascon, J.-M. J. Power Sources 2006, 158, 564. (b) Stephan, M. Eur. J. Polym. 2006, 42, 21. (14) Luo, J.-Y.; Cui, W.-J.; He, P.; Xia, Y. Y. Nature Chem. 2010, 2, 760. (15) Law, C-H.; Pathirana, S. C.; Li, X.; Anderson, A. Y.; Barnes, P. R. F.; Listorti, A.; Ghaddar, T. H.; O’Regan, B. C. Adv. Mater. 2010, 22, 4504. (16) (a) Chu, B.; Hsiao, B. Chem. Rev. 2001, 101, 1727. (b) Booth, C.; Attwood, D. Macromol. Rapid Commun. 2000, 21, 501. (17) (a) Soni, S. S.; Brotons, G.; Bellour, M.; Narayanan, T.; Gibaud, A. J. Phys. Chem. B 2006, 110, 15157. (b) Ivanova, R.; Lindman, B.; Alexandridis, P. Adv. Colloid Interface Sci. 2001, 8990, 351. (18) Mortensen, K.; Pederson, J. S. Macromolecules 1993, 26, 805. (19) Bronstein, L. M.; Karlinsey, R. L.; Yi, Z.; Carini, J.; Zwangiger, U. W.; Konarev, P. V.; Svergun, D. I.; Sanchez, A.; Khan, S. Chem. Mater. 2007, 19, 6258. (20) Ganguly., R.; Aswal, V. J. Phys. Chem. B 2008, 112, 7726. (21) (a) Benedetti, J. E.; De Paoli, M. A.; Nogueira, A. F. Chem. Commun. 2008, 9, 1121. (b) Kim, Y. J.; Kim, J. H.; Kang, M. S.; Le, M. J.; Won, J.; Lee, J. C.; Kang, Y. S. Adv. Mater. 2004, 16, 1753. (c) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Exnar, I.; Gratzel, M. J. Am. Chem. Soc. 2003, 125, 1166.
Table 4. Saturation Limiting Currents and Diffusion Coefficient of Gels from Chronoamperometry Measurements polymer concn, % (w/w) PGE
iss, nA
D, 107, cm2 s1
35
32.00
1.66
50
2.76
0.21
70
29.34
1.52
content, strong shifting in COC deformation band is observed due to domination of Hbonding. At 1358 cm1, a band attributed to the CCH3 stretching is also observed. The CH vibration can also be observed at 1466, 2881, and 2970 cm1. Moreover, the diffusion coefficient and saturation limiting current obtained from chronoamperometry measurments27 (see Supporting Information, Figure S4) also suggest that ionic transport is faster in case of 35% and 70% (w/w) gels while it is lower in the case of 50% (w/w) gel (Table 4). Both of these parameters support the fact that ionic mobility in all these gels is solely dependent upon the microcrystalline phases formed by self-assembly of block copolymers.
4. CONCLUSION To summarize, we have demonstrated that the well-ordered microphases are formed by self-assembly of F77 block copolymer with LiI/I2 in aqueous solutions. The 25%45% (w/w) F77/ water (LiI/I2) composition exhibits a thermoreversible phase transition from a sol to a gel. The conductivity data shows that 3D-interconnected tunneling structure in body-centered cubic phase is responsible for high conductivity values. It should be noted that conductivity can be enhanced up to certain extent by increasing the LiI/I2 concentration. The increasing order of ionic conductivity for different types of microcrystalline phases are 2D-hex < lamellar < cubic phase. The advantages of this PGE are simple preparation, no need to add blending agents to decrease crystallinity, reasonably high conductivity, thermoreversibility, etc. These results will open up the possibilities for the use of water-based electrolytes in DSSC using hydrophobic dyes and in Li ion battery with the help of modified Li electrodes. Fabrication of DSSC using this water-based polymer gel electrolyte is in progress. ’ ASSOCIATED CONTENT
bS
Supporting Information. Equation to derive ionic conductivity from impedance measurements and conductivity of various PGE’s at different temperature, DSC thermogram, conductivities of pure electrolytes and polymer at different temperatures, details of chronoamperometry. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 755
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(22) Saikia, D.; Kumar, A. Electrochim. Acta 2004, 49, 2581. (23) Chandra, S. Superionic Solids: Principle and Applications; NHPC: Amsterdam, 1981. (24) Gray, F. M.; Vincent, C. A.; Kent, M. J. Polym. Sci., Part B 1989, 27, 2011. (25) Fu, Y.; Pathmanathan, K.; Stevens, J. R. J. Chem. Phys. 1991, 94, 6323. (26) Gupta, S.; Shahi, K.; Binesh, N.; Bhat, S. V. Solid State Ionics 1993, 67, 97. (27) Kawano, R.; Watanabe, M. Chem. Commun. 2005, 2107.
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